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VOLUME 49 2014 ISSUE 6

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JOURNAL

OF CHEMICAL TECHNOLOGY

AND METALLURGY

SOFIA

Journal of Chemical Technology and Metallurgy

The Journal of Chemical Technology and Metallurgy started originally in 1954 as Annual Journal of the former Higher

Institute of Chemical Technology. It ran in Bulgarian. In 2000 its name was changed to Journal of the University of

Chemical Technology and Metallurgy. It was published quarterly in English. Since 2013 it has run bimonthly as

Journal of Chemical Technology and Metallurgy.

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R. Dimitrov

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University of Chemical Technology and Metallurgy,

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7BEditorial Board

S. J. Allen

Queens University of Belfast, UK

D. Angelova

University of Chemical Technology and Metallurgy, Bulgaria

M. Bojinov University of Chemical Technology and Metallurgy, Bulgaria

J. Carda

University Jaume I, Castellon, Spain

G. Cholakov University of Chemical Technology and Metallurgy, Bulgaria

V. Dimitrov Bulgarian Academy of Sciences

N. Dishovsky


S.J.C. Feyo de Azevedo Universidade do Porto, Portugal

M. Jitaru

University “Babeş -Bolyai”, Cluj-Napoca, Romania S. Kalcheva


F. Keil Hamburg University of Technology, Germany

T. Konstantinova


M. Kucharski AGH University of Science and Technology, Krakow, Poland

J.M. Le Lann

Institut National Polytechnique, Ecole Nationale Supérieure

des Ingénieurs en Arts Chimiques et Technologiques,

Toulouse, France

A. Mavrova


D. Mehandjiev Bulgarian Academy of Sciences

V. Meško

International Balkan University, Skopje, Macedonia

L. Mörl University “Otto-von-Guericke”, Magdeburg, Germany B. Nath

European Centre for Pollution Research, London, UK

D. Pavlov

Bulgarian Academy of Sciences

L. Petrov Bulgarian Academy of Sciences

D.C. Shallcross

The University of Melbourne, Australia

M. Simeonova


V. Stefanova


D. Stoilova

Bulgarian Academy of Sciences

N. Tsarevsky

Southern Methodist University, Dallas, Texas, USA

I. Turunen

Lappeenranta University of Technology, Finland

S. Vassileva


S. Veleva


L.Vezenkov


Ž. Živković

University of Belgrade, Technical Faculty, Bor, Serbia

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TABLE OF CONTENTS

Effect of the modified solid product from waste tyres pyrolysis on the properties of styrene-butadiene rubber based compositesNedialko Delchev, Petrunka Malinova, Mihail Mihaylov, Nikolay Dishovsky..................................................................525

In-situ preparation of Polyamide-6/polypropylene glycol copolymers with mineral fillers

Petko Krastev, Roza Mateva..........................................................................................................................................535

Effects of temperature and concentration dependent viscosity on onset of convection in porous media Fatemeh Bahadori, Sima Rezvantalab.........................................................................................................................541

Laue functions model vs Scherrer equation in determination of graphene layers number on the ground of XRD dataBeti Andonovic, Misela Temkov, Abdulakim Ademi, Aleksandar Petrovski, Anita Grozdanov, Perica Paunović, Aleksandar Dimitrov........................................................................................................................545

Kinetics of borohydride electrooxidation: revisited Sasha Kalcheva, Ivan Kanazirski...................................................................................................................................551

Electrochemical studies of metal complexes of malonyl dihydrazide

K. Ramana Kumar, A. Raghavendra Guru Prasad, V. Srilalitha, Y.N. Spoorthy, L.K. Ravindranath...........................................559

Electronic and infrared spectral studies on the poly(propyleneamine) dendrimers peripherally modified with 1,8-naphthalimides Desislava Staneva, Ivo Grabchev, Pavlina Mokreva.....................................................................................................569

Sensor activity and logic behaviour of some 2-aminoterephthalic derivativesPolya Miladinova, Nikolai Georgiev...........................................................................................................................577

Compensation effect in the kinetics of thermal aging of semi chemical pulp Rumyana Boeva, Greta Radeva....................................................................................................................................585

Study of the complex equilibrium between titanium (IV) and tannic acidAndriana Surleva, Petya Atanasova, Tinka Kolusheva, Latinka Costadinnova.......................................................................594

Solvatochromism of homodimeric styryl pyridinium saltsStanislava Yordanova, Ivan Petkov, Stanimir Stoyanov................................................................................................601

Electric-arc-carbothermic processing of agglomerated waste dispersal materials from ferrous metallurgyDaniela Grigorova, Maxim Marinov, Rossitza Paunova................................................................................................610

Journal of Chemical Technology and Metallurgy, 49, 6, 2014

Utilization of waste powder and sludge in iron ore sintering processDaniela Grigorova, Tsvetan Tsanev, Maxim Marinov..................................................................................................615

Research of influence equal channel angular pressing on the microstructure of copperSergey Lezhnev, Irina Volokitina, Toncho Koinov...........................................................................................................621

Guide for Authors..........................................................................................................................................................631


Nedialko Delchev, Petrunka Malinova, Mihail Mihaylov, Nikolay Dishovsky

525

Journal of Chemical Technology and Metallurgy, 49, 6, 2014, 525-534

EFFECT OF THE MODIFIED SOLID PRODUCT FROM WASTE TYRES PYROLYSIS ON THE PROPERTIES OF STYRENE-BUTADIENE

RUBBER BASED COMPOSITES

Nedialko Delchev1, Petrunka Malinova2,

Mihail Mihaylov2, Nikolay Dishovsky2

1 Polymermetal Ltd., Haskovo, Bulgaria2 University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected]

ABSTRACT

Being resistant to moisture, oxygen, ozone, solar radiation, microbiological degradation, etc., waste car tyres and their deposition present a great environmental problem. Pyrolysis is an environment-friendly process for their recycling. This paper presents data referring to the study of the effect of the solid product from waste tyres pyrolysis on the properties of styrene-butadiene rubber (SBR) based composites. It is found that the further modification of the pyrolysis carbon black decreases two times the ash content and six times the zinc oxide concentration in the ash but the curing characteristics of the investigated SBR based compounds remain unaffected. The values of the modulus 100, the tensile strength and Shore A hardness of vulcanizates containing modified pyrolysis carbon black increase, while the elongation at break and the residual elongation decrease slightly. The effects observed are related to the decreased ash content of the pyrolysis carbon black following the modification as well as to the increased silicon oxide concentration in the remaining ash.

Keywords: rubber, waste tyre, carbon black, pyrolysis.

Received 11 March 2014Accepted 05 September 2014

INTRODUCTION

Being resistant to moisture, oxygen, ozone, solar radiation, microbiological degradation, etc., waste car tyres and their deposition present a great environmental problem. On the other hand, that waste is flammable and its uncontrolled combustion endangers further the environment.

The different methods for waste tyres reuse involve:l Recovery of the used tyres;l Mechanical grinding/shredding (obtaining of

rubber powder);l Obtaining a rubber reclaim;

l Devulcanization;l Pyrolysis, etc. Pyrolysis is the process of thermal destruction of or-

ganic materials. It proceeds in absence of oxygen at tem-peratures in the range from 500°С to 800°С. The waste tyres subjected to pyrolysis (directly or having under-gone some preliminary treatment) are heated gradually starting from an ambient temperature to 700°С-800°С at constant purging by nitrogen, carbon dioxide and/or water vapour [1]. There is a renewed interest to pyrolysis as a method for recycling and reuse of waste tyres be-cause it is an environmentally friendly process yielding useful products. The latter include pyrolysis oil (40 - 60


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%), solid carbon residue (30 – 40 %), steel and gaseous products [2]. Each of those products can find various industrial applications. For instance, the pyrolysis oil, which is a complex mixture of organic components, can be used as a plasticizer in rubber industry, as diesel oil [2 - 5] or as a carbon black source [6]. The application of the pyrolysis products in the former two cases is hampered by the high carbohydrates concentrations [7 - 10]. The gaseous products contain H2, H2S, CO, CO2, CH4, C2H4, C3H6, etc. Тhеy can be used as the energy source required for the pyrolysis proceeding [2]. The solid product of waste tyres pyrolysis (called also py-rolysis carbon black) includes carbon black (80 - 90 %) and inorganic substances (10 - 20 %) which are always present in the rubber compounds used to manufacture car tyres. The briquetted product can be burnt in industrial furnaces. Though being very caloric, containing ca 20-25% of ash, it is not lucrative. It is hypothesized that the pyrolysis carbon black can entirely or partially replace the conventional carbon black in rubber industry follow-ing a subsequent mechanical or chemical treatment. That product may be quite effective in manufacturing less demanding rubber items such as floorings for various usages, isolation stall mats, hit resistant coatings, etc.

The present paper aims at presenting the investiga-tions on the effect of the modified solid product from waste tyres pyrolysis on the properties of composites based on styrene-butadiene rubber (SBR).

EXPERIMENTAL

MaterialsThe investigations were performed on 1500 type

styrene-butadiene rubber (Ravaflex SBR 1500) sup-plied by Ravago Group. It is a copolymer of styrene (23,5%) and butadiene and is easily processed. The solid pyrolysis product was obtained in accordance with the procedure described in [1].

Modification of the solid pyrolysis productThe solid pyrolysis product was placed in a round

bottom flask and hydrochloric acid (5 %) was added at a ratio of 1:4. The mixture was boiled under a reflux condenser for 20 min. Then the product was washed with distilled water over a buchner funnel till obtaining a clear filtrate, which gave no white residue in presence of silver nitrate. A strong odour of hydrogen sulphide was evolved

during the modification indicating that the sulphur and sulphur-containing compounds were removed from the carbon black. The modification of the solid pyrolysis product was performed to reduce its ash content being one of its drawbacks. The solid pyrolysis product will be further referred to as pyrolysis carbon black.

Rubber compoundsFour SBR based compounds containing virgin and

modified pyrolysis carbon black were prepared. Their formulations are presented in Table 1. Aiming an objec-tive comparison, the added amount of pyrolysis carbon black was estimated on the ground of the carbon (active substance) content, i.e. the compensated ash content which was determined in advance. Thus 60 g and 83.3 g of virgin pyrolysis carbon black in compounds denoted by SBR 1 and SBR 2 correspond to 50 g and 70 g con-ventional carbon black, respectively. While 54.7 g and 76.6 g of modified pyrolysis carbon black in compounds denoted by SBR 3 and SBR 4 correspond to 50 g and 70 g conventional carbon black, correspondingly.

Preparation and vulcanization of the rubber com-pounds

The rubber compounds were prepared using an open two-roll laboratory mill (L/D 320х160 and friction of 1.27) according to a specific recipe and blending regime. The speed of the slow roll was 25 rpm.

The rubber was plasticized on the rolls for several minutes prior to adding the ingredients.

The mill had to be cooled at each blending cycle to avoid compounds sticking onto the rolls. The blend was cut diagonally when the ingredients were absorbed in the rubber matrix and the strap prepared was transferred to the opposite side of the steam-roller. Then the compound was homogenized by making a roll and driving it through a narrow shank. The compounds obtained in the form of a sheet were kept for 24 prior to the vulcanization proceeding.

The specimens were vulcanized on an electrically heated hydraulic press at 10,0 МРа and 160°С accord-ing to the optimums of each rubber compound that were determined using an moving die rheometer (MDR 2000).

Experimental MethodsCharacterization of the virgin and modified py-

rolysis carbon black


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Determination of the losses on heating performed according to ISO 15651/2-91

1. The weighing glass vessel and its lid were dried separately in a drying oven for 30 min at 105 ± 2ºС and subsequently transferred to a dessicator to cool to an ambient temperature. Then they were weighed with an inaccuracy < 0.1 mg.

2. About 2 g of pyrolysis carbon black were weighed in a glass vessel with an inaccuracy < 0.1 mg.

3. The uncovered weighing glass vessel containing the sample and its lid were dried in a drying oven for 1 h at 105 ± 2ºС.

4. The weighing glass vessel was covered with the lid and transferred to the dessicator. Its lid was removed again and it was cooled to an ambient temperature. Then the lid was put on and the vessel was weighed with an inaccuracy < 0.1 mg.

The weight loss on heating was calculated in % ac-cording to the formula:

( )1 2 1 0/ 100, %m m m m− − (1)

where m0 is the weight of the weighing vessel (g), m1 is the total weight of the weighing vessel with the lid and the sample prior to the heating (g) , while m2 is the total weight of the weighing vessel with the lid and the sample after the heating (g).

Determination of the ash content according to ISO 15651/3-91

1. The crucible and its lid were put into a muffle furnace set at 550 ± 25ºС for 1 h, then transferred to a dessicator and cooled to an ambient temperature. It was weighed with an inaccuracy < 0.1 mg.

2. A pyrolysis carbon black sample of a mass > 2 g was dried in a drying oven for 1 h at 105 ± 2ºС.

3. About 2 g of the pyrolysis carbon black sample were put into the preheated crucible and weighed with an inaccuracy < 0.1 mg.

The ash content was calculated according to the formula:

( )2 0 1 0/ 100, %m m m m− − (2)

where m0 was the weight of the crucible with its lid (g), m1 was the weight of the crucible with its lid and

the sample loaded (g), while m2 was the weight of the crucible with its lid and the ash obtained (g).

Determination of dibutyl phthalate absorption (oil absorption number) according to ISO 9665-76

The tested pyrolysis carbon black (0.5 g weighed with an inaccuracy < 0.001 g) was placed in a porcelain mortar and several drops of dibutyl phthalate (DBP) were added using a microburette. The mixture was ground carefully with the pestle till the disappearance of DBP oil stains on the mortar or on the plate surface. In fact the introduction of DBP continued till all carbon black could be gathered on the pestle. The absorption of DBP (Х) in ml/100 g, was calculated according to the formula:

( )/ 100X V G= (3)

where V was the quantity of absorbed DBP (ml), while G was the weight of the pyrolysis carbon black sample (g).

Determination of the iodine number by the titration method according to ISO 15651/1-91

The pyrolysis carbon black sample, already prelimi-nary dried, was weighed and stirred intensively with a certain amount of iodine solution of a predetermined concentration. Then the solid was separated by centrifu-gation, while the residual solution was titrated with a solution of sodium thiosulfate of a certain concentration.

The iodine number IAN per 1 mg iodine, i.e. the iodine adsorbed by 1 g of pyrolysis carbon black (with an inaccuracy < 0.1 mg/g) was calculated by the formula:

( ) ( ) 14 5 1 4 5

5125,9 158,64

CIAN V V C V Vm m

= − = − (4)

where V4 was the volume of sodium thiosulfate solution used to titrate the iodine solution in the blank sample (ml), V5 was the volume of sodium thiosulfate solution used to titrate the iodine solution in the sample (ml), С1 was the concentration of sodium thiosulfate solution (mol/dm3), while m was the weight of pyrolysis carbon black sample (g).

Determination of the specific surface area accord-ing to Klyachko-Gurvich method

The solid body specific surface area was determined


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by Klyachko-Gurvich method. The method is based on the dependence of macromolecular surface on the volume of the gas needed for its formation in m2. The dependence is linear in accordance with the BET theory. The latter shows that a linear section of the adsorption isotherm starts to outline with the formation of a mono-macromolecular layer on the adsorbent surface.

The determination of the specific surface area by this method involves measuring the decrease of the adsorbate pressure (that of the gaseous nitrogen) due to its adsorp-tion on the surface of a given specimen at a constant temperature (in this case -196ºС referring to the liquid nitrogen temperature) in a system of a fixed volume.

Fig. 1 presents the equipment used.The specific surface area was determined in accord-

ance with:

( )00 . PPfS ∆−∆= (5)

where ΔP was the decrease of the adsorbate pressure due to the adsorption of the sample and the ‘dead’ volume of the apparatus at – 196ºС, ΔP0 was the decrease of the adsorbate pressure due to the ‘dead’ volume of the apparatus. It is worth adding that

( )dmfP ,0 =∆ (6)

where m was the sample mass, while d was the sample density. The specific surface area was defined by:

,/0 mSS = m2/g (7)

Characterization of the ash content of the virgin and modified carbon black by a full silicate analysis (an weight analysis, AAS, ICP-OES)

Average samples of virgin and modified carbon black were prepared by grinding to obtain particles of a size < 100 mµ .

Heating losses were determined by weighing the samples placed in platinum crucibles. Analytical scales were used. The crucibles were heated in a muffle furnace at 1000ºС for 1 h. The losses on heating were determined by the balance of the samples weight before and after the heating.

Silicon dioxide amount was determined by the weighing method. The samples were placed in plati-num crucibles and decomposed by melting in presence of potassium-sodium carbonate. The melt still hot was dissolved in dilute hydrochloric acid. The sediment was filtered off from the solution and subsequently washed with dilute hydrochloric acid and water. The sediment and the filters were put in a platinum crucible preheated to a constant weight in a muffle furnace at 1000ºС for 1 h. The residues were in fact pure SiO2. They were weighed and the percentage amount of the component was calculated taking into account that of the reference sample. The solution left after melt removal was trans-ferred to a measuring cylinder to be used in determining the metals content by ICP-OES spectrometric analysis.

The samples were additionally dissolved in a mix-ture of acids (HF, HСlO4, HNO3) aiming the determina-tion of the alkali metals content. They were placed in platinum pans and heated slowly in a sand bath. The dry residue was dissolved in dilute hydrochloric acid and the solution obtained was transferred to a measuring cylinder. The amount of Mg, Ca, Cu, Fe, Zn, Al, Pb, Ti was determined by ICP-OES method using a Prodigy high dispersion ICP-OES spectrometer (Teledyne Lee-man Labs) of dual view configurations (radial/axial) and 0.007 nm resolution. The amount of Na and К was estimated by AAS using a Perkin Elmer 5000 atomic absorption spectrophotometer working in an emission regime (acethylene/air flame, К wavelength of 766.5

Table 1. Formulations of SBR compounds filled with virgin (non-modified) and modified pyrolysis carbon black (phr).

Ingredients SBR 1 SBR 2 SBR 3 SBR 4 1. Styrene-butadiene rubber SBR 1500 100 100 100 100 2. Zinc oxide 3.0 3.0 3.0 3.0 3. Stearic acid 1.0 1.0 1.0 1.0 4. Non-modified pyrolysis carbon black 60.0 83.3 - - 5. Modified pyrolysis carbon black - - 54.7 76.6 6. N-Tert-Butyl-2-Benzothiazole Sulfenamide

(TBBS) 1.0 1.0 1.0 1.0

7. Sulphur 1.75 1.75 1.75 1.75


529

nm, Na wavelength of 589.0 nm).The spectral results referring to the components

concentrations were recalculated as oxides.

Determination of the vulcanization character-istics

The vulcanization characteristics of the rubber compounds were determined on an MDR 2000 moving die rheometer (Alpha Technologies) acording to ISO 3417:2010.

A sample of the rubber compound was placed in a hermetic chamber at pressure lower than that initially determined. It was kept constant at high temperatures. The piston rod penetrated the sample and oscillated at small rotation amplitude (the oscillation angle was 0.5°). That caused shear deformation of the sample. The torque moment required for the disk oscillation depended on the deformation resistance (shear modulus) of the rub-ber. The torque moment was recorded graphically as a function of time.

Determination of the mechanical properties of the vulcanizates obtained

Shore A HardnessShore A hardness of the samples was determined 24

h after the vulcanization on a Mitotoyo portative hard-ness testing machine according to ISO 7619:2012. The hardness was determined by the penetration of a tester into the material under conditions previously set.

Determination of the stress-strain propertiesDouble sided belts were cut from the rubber samples

on a plate punching machine 24 h after the vulcanization. The belts were measured by a micrometer with preci-sion of 0.01 mm. The determination of modulus 100 and modulus 300, the tensile strength, the elongation at break of the vulcanizates studied were determined on a dynamometer at a 500 mm/min speed of the wedge

action jaws according to ISO 37:2008.

Determination of the residual elongationThe residual elongation in the treated part of the

samples was measured at least 1 min after the break. The two parts of the broken belt were reconnected and the length of the treated part was measured. The residual elongation was calculated by the formula:

( )2 0 0/ 100r l l lε = − (8)

where εr was the residual elongation (%), ℓ0 was the initial belt length (before tensioning), while ℓ2 was the elongation observed 1 min after the release.

Determination of accelerated heat ageing resistanceThe accelerated heat ageing resistance was deter-ccelerated heat ageing resistance was deter-heat ageing resistance was deter-ageing resistance was deter- was deter-

mined according to ISO 188:2009 for 72 h at 100°С in a thermochamber ventilated by forced air convection. The aging coefficients were calculated by the formula:

( ) / 100K B A A= − (9)

where В was the parameter value prior to aging, while А was the parameter value found after the aging. It is worth adding that the negative value of the aging coefficient (%) is indicative of aggravated mechanical properties.

RESULTS AND DISCUSSION

Characterization of virgin and modified pyrolysis carbon black

The results from the investigation of the properties of virgin and modified pyrolysis carbon black are sum-marized in Table 2.

The comparison of the properties of pyrolysis carbon black prior to and after the modification reveal that:

- The losses on heating of pyrolysis carbon black

Properties Virgin pyrolysis carbon black

Modified pyrolysis carbon black

Losses on heating, % 1.45 1.78 Ash content, % 16.2 8.6 Dibutyl phthalate absorption (oil number), ml/100 g 120 127 Iodine adsorption (iodine number), mg/g 182 171 Specific surface area, m2/g 137.95 120.85

Table 2. Properties of the pyrolysis carbon black investigated.


530

change slightly upon modification mainly because of moisture and other volatile substances presence. The losses on heating are less than 1.8 %.

- The ash content obtained after the modification is almost two times lower - 16.2 % prior to and 8,6 % after modification, correspondingly. Taking into consideration the main purpose of the treatment, it can be concluded that the goal is achieved.

- The modification does not affect the structure of pyrolysis carbon black because the oil number values of virgin and modified pyrolysis carbon black are close. The same is valid for the specific surface area and the iodine number, i.e. the modification results predominantly in decrease of the ash content of pyrolysis carbon black. The latter was subjected to additional analyses expect-ing further decrease of its content. The results from the weight analysis, AAS and ICP-OES are summarized in Table 3. The latter shows that ZnO content in pyrolysis carbon black is almost six times lower after its treat-ment with hydrochloric acid, which in turn verifies the conclusion that the main purpose of the modification was achieved. The content of SiO2 is the highest in the ash of modified pyrolysis carbon black as it does not react with hydrochloric acid. In fact SiO2 is present as the material subjected to pyrolysis included “green

tyres” as well. It is known that they use silicon dioxide but not carbon black as filler. Another reagent, such as hydrofluoric acid, is required to remove SiO2 but in fact its removal is not a must in case of pyrolysis carbon black as it is active filler. An option would be to introduce some silane coupling agent to the rubber mixture like it is usually done when filling with fresh silicon dioxide is carried out. Silane bifunctional nature improves the elastomer-filler interaction. On the other hand, the sig-nificantly decreased zinc oxide amount in the ash can be considered a significant success. The decrease of zinc oxide concentration in rubber compounds is a challenge worldwide because of zinc ions ecotoxicity. The greatest concentration decrease results from the modification in case of Na2О, К2О, Fе2O3, MgO and CaO. There are no changes in the concentration of TiO2 and PbO.

Curing characteristics of the rubber compounds comprising virgin (non-modified) and modified py-rolysis carbon black

The vulcanization isotherms of the SBR based compounds containing various amounts of virgin (SBR 1 and SBR 2) and modified (SBR 3 and SBR 4) py-rolysis carbon black are shown in Fig. 2. It is seen that they have an identical curve pattern. A wide plateau of

Fig. 1. Scheme of the apparatus used to determine the specific surface area - 1, 2, 3, 4, 5, 6 - taps; 7 - adsorbers with a cryo-genic storage dewar isolation; 8 - vacuum adsorption pump; 9 - mercury nanometer; 10 - glass bulb.


531

vulcatization (equilibrium torque curves) is outlined, which in turn indicates that the compounds are suitable for thick-wall items manufacture. The compounds con-. The compounds con- The compounds con-taining virgin and modified pyrolysis carbon black of an amount equal to 50 phr carbon black show overlapping isotherms, unlike those referring to amounts equal to 70 phr. The most important fact is that the usage of modi-fied pyrolysis carbon black does not worsen the curing

characteristics of the compounds, i.e. there is no reason to exclude the pyrolysis solid product as filler.

The curing characteristics (minimum torque - ML,

maximum torque - ML, as well as the difference between them – ΔM; the thermoplastic interval - ts1; optimum cur-ing time - t90; curing rate – Vс) of the studied SBR based compounds obtained at 160оС are presented in Table 4. The latter shows that the values of the vulcanization pa-

Substances Contnent, %

Virgin pyrolysis carbon black

Modified pyrolysis carbon black

SiO2 38.80 87.32 Al2O3 0.77 0.51 CaO 0.31 0.20 MgO 0.50 0.27 Na2О 2.41 0.49 К2О 5.65 0.74

Fе2O3 0.53 0.26 TiO2 0.03 0.03 ZnO 46.10 7.22 CuO 0.19 0.11 PbO 0.01 0.01

Losses on heating, 1000оС 4.69 2.83

Table 3. Ash content data as obtained from weight, AAS and ICP-OES analyses.

ML, dNm MH, dNm ΔM, dNm tS1, min t90, min Vс, %/min SBR 1 4.54 31.48 26.94 1:44 10:28 12.8 SBR 2 8.06 41.52 33.46 1:02 10:50 10.8 SBR 3 4.56 32.00 27.44 1:44 10:57 11.8 SBR 4 9.13 47.38 38.25 1:05 10:54 10.6

Table 4. Curing characteristics of the studied SBR based compounds obtained at 160оС.

SBR 1 SBR 2 SBR 3 SBR 4 Modulus 100, МРа 3.2 5.2 3.6 6.3 Modulus 300, МРа 12.0 - - - Tensile strength, σ, МРа 12.2 11.8 12.8 12.3 Elongation at break, ε1, % 305 227 284 198 Residual elongation, ε2, % 8 5 7 5 Shore A Hardness, relative units 69 78 71 80

Table 5. Mechanical properties of the SBR based composites filled with virgin and modified pyrolysis carbon black.


532

rameters increase at higher filler amounts, regardless of the presence or absence of modification, i.e. the pyrolysis carbon black has no effect on the vulcanization process.

Mechanical properties of the SBR based compos-ites filled with virgin and modified pyrolysis carbon black

The mechanical properties of the SBR based vul-canizates are summarized in Table 5. It is seen that the values for the modulus 100 (M100) of all studied SBR based vulcanizates increase at higher filler amounts. The increase of М100 values of vulcanizates containing less filler is smaller (11 %) in case of modification, while that of vulcanizates containing higher filler amounts is more pronounced (17,5 %).

Modulus 300 (M300) is achieved only in case of a sample containing virgin pyrolysis carbon black of an amount equal to 50 phr carbon black.

The slight changes in the tensile strength values demonstrate that the modification of pyrolysis carbon black has a beneficial effect on that property.

The elongation at break decreases with the increase of pyrolysis carbon black amount. It is less than 10% for vulcanizates containing virgin and modified pyrolysis carbon black of an amount equal to 50 phr, while it is 13 %, when the amount considered is equal to 70 phr. Hence, the modification of the filler, despite of the slight tendency of decrease, has no significant effect on the values of that parameter.

The residual elongation decreases with the increase

of pyrolysis carbon black amount, but does not change with the filler modification.

Shore A hardness values increase slightly with the filler amount increase for vulcanizates containing the same filler. Identical change is observed in case of filler modification.

The results presented show that the values of modu-lus 100, the tensile strength and Shore A hardness for vulcanizates containing modified pyrolysis carbon black increase slightly when compared to those referring to vulcanizates containing virgin pyrolysis carbon black. Meanwhile, the values of the elongation at break and the residual elongation are slightly lower. This is most probably due to the lower ash content in the modified pyrolysis carbon black and the fact that it consists predominantly of silicon oxide. It should be added that higher values of M100 and M300, of the tensile strength and hardness of the vulcanizates as well as the lower elongation values are characteristic of all compounds containing silicon oxide.

Resistance to heat agingThe heat aging coefficients of the SBR based vul-

canizates are presented in Table 6. The heat aging resist-ance of the vulcanizates containing minimal amounts of virgin and modified pyrolysis carbon black (equal to 50 phr active substance) is improved in regard to the ten-sile strength and the elongation at break. The tendency remains unchanged with increase of pyrolysis carbon black amount.

Fig. 2. Vulcanization isotherms of the studied SBR based compounds.


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The heat aging resistance coefficient with regard to Shore A hardness has positive values for all vulcanizates studied. The modification of pyrolysis carbon black affects the heat aging resistance of SBR based vulcani-zates. The change in Shore A hardness values is smaller in case of modified pyrolysis carbon black. Hence those compounds are more aging resistant.

CONCLUSIONS

Pyrolysis carbon black obtained from waste tyres pyrolysis is successfully modified by a method designed to decrease the ash content and that of zinc oxide in particular. It is found that the modification introduced decreases two times the ash content and six times the zinc oxide concentration in the ash.

The studies on the curing characteristics, the me-chanical properties and the heat aging resistance of SBR based vulcanizates containing virgin and modified pyrolysis carbon black demonstrate that:

• The curing characteristics are not affected by the modification of the pyrolysis carbon black.

• The values of modulus 100, the tensile strength and Shore A hardness for the vulcanizates containing modified pyrolysis carbon black increase, while the elongation at break and residual elongation decrease slightly. The effects observed are related to the decreased ash content in pyrolysis carbon black following the modification as well as to the increased silicon oxide concentration in the remaining ash.

• The modified pyrolysis carbon black has a ben-eficial effect on the heat aging resistance of the vul-canizates.

Additional decrease of the ash content in case of us-

ing pyrolysis carbon black can be achieved by a further modification with another reagent, e.g. hydrofluoric acid. This will result in silicon oxide elimination. Silane can be used instead of the secondary modification as it enhances the interaction between the rubber macro-molecules and silicon dioxide. A reinforcement effect is expected. The latter will be the scope of further studies.

AcknowledgementsThe authors acknowledge the financial support pro-

vided by the Operational Programme Development of the Competitiveness of the Bulgarian Economy 2007-2013, Procedure BG161PO003-1.1.06 - Support for Research and Development Activities of Bulgarian Enterprises; Pri-ority Axis 1: Knowledge and Innovations Based Econom-ic Development (Contract BG161PO003-1.1.06-0034).

REFERENCES

1. L. Ljutzkanov, Method of Processing Carbon Containing Materials, Patent 63594/26.02.2002, Republic of Bulgaria.

2. M. Juma, Z. Koreňová, J. Markoš J. Annus, L’. Jelemenský, Pyrolysis and Combustion of Scrap Tire, Petroleum & Coal, 48, 1, 2006, 15-26.

3. A. M. Cunliffe, P.T. Williams, Composition of Oils Derived from the Batch Pyrolysis of Tires, J. Anal. Appl. Pyrolysis, 44, 2, 1998, 131-152.

4. P.T. Williams, A.J. Brindle, Catalytic Pyrolysis of Tyres: Influence of Catalyst Temperature, Fuel, 81, 18, 2002, 2425-2434.

5. P.T. Williams, A.J. Brindle, Aromatic Chemicals from the Catalytic Pyrolysis of Scrap Tyres, Journal of Analytical and Applied Pyrolysis, 67, 1, 2003, 143-164.

Kσ, % Kε1, % КSh, % SBR 1 -31.1 -54.4 10.1 SBR 2 -23.9 -55.5 2.6 SBR 3 -25.8 -50.0 7.6 SBR 4 -21.0 -47.5 1.3

Table 6. Changes in the mechanical properties of SBR based vulcanizates following their heat aging at 100°C.

*Kσ – aging coefficient with regard to tensile strength; Kε1 – aging coefficient with regard to elongation at break; КSh – aging coefficient with regard to Shore A hardness.


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6. Marek A. Wójtowicz, Rosemary Bassilakis, Michael A. Serio, Carbon Black Derived from Waste Tire Pyrolysis Oil, Advanced Fuel Research, Inc.

7. P.T. Williams, R.P. Bottrill, Sulfur-Polycyclic Aromatic Hydrocarbons in Tyre Pyrolysis Oil, Fuel, 74, 5, 1995, 736-742.

8. I.M. Rodriguez, M.F. Laresgoiti, M.A. Cabrero, A. Torres, M.J. Chomón, B. Caballero, Pyrolysis of Scrap Tyres, Fuel Processing Technology, 72, 1, 2001, 9-22.

9. M.F. Laresgoiti, B.M. Caballero, I. Marco, A. Torres, M.A. Cabrero, M.J. Chomón, Characterization of the Liquid Products Obtained in Tyre Pyrolysis, Journal of Analytical and Applied Pyrolysis, 71, 2, 2004, 917-934.

10. B. Benallal, C. Roy, H. Pakdel, S. Chabot, M.A. Poirier, Characterization of Pyrolytic Light Naphtha from Vacuum Pyrolysis of Used Tyres Comparison with Petroleum Naphtha, Fuel, 74, 11, 1995, 1589-1594.

Petko Krastev, Roza Mateva

535


IN-SITU PREPARATION OF POLYAMIDE-6/POLYPROPYLENE GLYCOL COPOLYMERS WITH MINERAL FILLERS


University of Chemical Technology and Metallurgy8 Kl. Ohridski, 1756 Sofia, BulgariaE-mail: [email protected]

ABSTRACT

Polyamide-6 /Polypropylene glycol (PPG) copolymers were synthesized via anionic polymerization of ε-caprolactam in the presence of inorganic additives - graphite and boron carbide using isophorone diisocyanate end functionalized PPG prepared in-situ as a macroactivator. Sodium caprolactam was used as an initiator. The influence of the graphite and boron carbide fillers on the degree of conversion was investigated. The copolymers obtained were characterized by 1H NMR and FTIR spectroscopies. The charpy impact strength of the composites was evaluated.

Keywords: polyamide-6, copolymers, graphite, boron carbide.

Received 11 April 2014Accepted 05 September 2014

INTRODUCTION

The focus on the sustainable development of indus-try led in the recent years to the necessity of new mate-rials able to meet the most stringent specifications for mechanical, thermal, electrical properties but reducing the environmental footprint. In many cases engineering plastics, the most common one still being Polyamide-6 (PA-6) and Polyamide 6,6 (PA-6,6) are able to match these requirements due to their high tensile strength, stiffness, high melting point, relatively good impact/chemical resistance and lower weight compared to that of metals. As different applications of these materials have different specifications, the need of modifying their properties for certain end-use is critical. This area of polymer modification is explored extensively and a broad range of commercial modifiers is presented on the market. Modifiers based on rubbers, acrylics, silicones, polyurethanes are developed to improve the impact strength of PA-6. Mineral fillers like glass fibers, carbon nanotubes [1], but also aramid types of polymers are incorporated into the polymer matrix in the course of the final stages of the injection molding process aiming

at better tensile strength, temperature properties and dimensional stability. Lubricants like metal stearates or high molecular silicones are usually applied to obtain better processing-melt flow and mold release. Another aspect of properties modification refers to the devel-opment of copolymers or polymer blends, the most popular ones being PA-6 and PA-6,6 copolymer, PA-6,6/Polyphenylene oxide, PA-6 blends with polyolefins, etc. Thus the use of PA-6 in many fields like the production of automotive-under the hood appliances, etc., requir-ing materials able to withstand high temperatures for extended periods of time, has greatly increased. On the other hand, the continuous increase of the energy costs and the raw materials prices, the concept of light weighting (metal replacement) across the industry as well as the usage of renewable material sources drive the interest towards creating new approaches to polymer materials. One of them, the so called reaction injection molding (RIM) was developed by Monsanto in 1970s and further commercialized by DSM [2]. This process al-lows producing the final plastic part from ε-caprolactam (ε-CL), polyether prepolymer and in initiator by one-step process. This concept, applicable to PA-6 in particular,


536

allows significant energy savings. It provides also modi-fication of the properties of the PA-6 aiming to meet certain requirements. This is achieved by introduction to the polymer chain of elastomeric segments such as polybutadiene, styrene-butadiene, polyols, etc. [3, 4]. It is worth adding that the application of the RIM concept to PA-6 copolymerization envisages the formulation of a 3-component system (ε-CL, prepolymer, filler) as PA-6 is usually modified with the introduction of mineral fillers [5, 6]. The behavior of these composite structures has to be investigated as they affect the polymerization process, while their properties depend in turn on the concentrations of the prepolymer and the filler. The present paper reports data on the copolymerization of PA-6 with polypropylene glycol (PPG) in the presence of two mineral fillers- graphite and boron carbide (B4C) in view of their possible participation in the RIM process.

EXPERIMENTAL

MaterialsThe monomer ε-CL (BASF), Mv=113.16 was dried

for 3 days over P2O5 in a vacuum oven at 60oC.The initiator, sodium salt of ε-CL (Na-CL), was synthesized following the procedure described in ref. [7], Mv=424. Isophorone diisocyanate (5-isocyanate-1-isocyanatome-thyl- 1,3,3-trymetylcyclohexane) (IF;Merck), methanol (Fluka), toluene (Fluka) were used as received. PPG (Fluka) (average molecular mass of 2000) was kept in a molecular sieve for 10h at 25oC under vacuum. Graphite of particle size (PS) of 20 µm and B4C (Fluka) of PS of 15-62 µm were heated at 150oC for 2 min prior to use.

SynthesisFunctionalization of PPG with isophorone di-

isocyanateThe PPG (1 mol dissolved in 5 ml toluene) was

placed into a 250 ml flask equipped with a separating funnel, a reflux condenser, a thermometer, a stirrer and N2 inlet. Isophorone diisocyanate (2.2 mol) was added dropwise through the separating funnel at vigorous stirring. The reaction was carried out at 50oC for 4 h. FTIR spectra showed that the NCO group absorption band reached a constant value at υ=2269.9 cm-1 on the 4-th hour in correspondence with previous studies [8].

Synthesis of PA-6/PPG polymers with graphite and B4C

The polymerization was carried out in the bulk at 180oC with the application of:

a) the ampule technique. Functionalized PPG with IF, the monomer ε-CL, additives (Graphite or B4C) and ampules were placed into a glass flask under nitrogen atmosphere and melted for 20 min at 120oC by vigor-ous stirring (Macroactivator preparation in-situ). After mixture homogenization, the temperature was increased to 140oC, then the initiator (Na-CL) was added at vigor-ous stirring and the melt was forced into ampules by N2.

b) the mold casting. The melt was prepared follow-ing the procedure described in (a). It was then transferred to an aluminum mold pre-heated at 180oC.

Block copolymers of different ratios of PPG:IF, graphite and B4C were prepared. The initiator concentra-tion was equal to 1 mol % in reference to the monomer quantity.

AnalysеsDegree of conversionThe polymers obtained were placed into a Sox-

hlet apparatus, while the non-reacted monomers were extracted with methanol for 8h. The residue formed was dried in vacuum at 60oC until constant weight was reached. The degree of conversion was determined gravi-metrically as a ratio of the weights of the polymers prior to and after the extraction. The additives B4C and graphite were not removed from the compound in calculating the values as they being insoluble in methanol would not affect the values of the degree of conversion.

1H NMRThe 1H NMR spectra were recorded on a Bruker

AM400 apparatus operating at 400MHz. The 1H NMR analysis of the purified copolymers was performed in a mixture of HCOOH:CDCl3 (1:1 v/v) using formic acid as an internal standard.

FTIRThe IR spectra of copolymers free from additives and

monomers were recorded in the range of 4000 cm-1-450 cm-1 using a Perkin Elmer 1600 (FTIR) spectrophotom-eter with KBr pellets.

Charpy Impact StrengthThe impact resistance was measured at room tem-

perature by Charpy pendulum on 50 mm x 4 mm x 6 mm specimens with average notch depth of 0.5 mm.


537

The distance between the supports was 40 mm, while the pendulum velocity of impact was 2.9 ms-1.


It is known that ε-CL polymerization is relatively slow and proceeds with a significant induction period. This problem can be overcome by using macroactiva-tors [9]. Thus the induction period is eliminated and the polymerization occurs within several minutes leading to polymer-monomer equilibrium. Isophorone function-alized PPG was used as a macroactivator aiming the formation of PA-6/PPG/Graphite and PA-6/PPG/B4C copolymers. The polymerization mechanism is shown in Fig. 1.

The PPG macroactivator was synthesized in two steps. The first one referred to the functionalization of PPG using isophorone diisocyanate. It was followed by in-situ synthesis of the corresponding N-carbamoillac-tam proceeded.

Several polymerizations of ε-CL varying the PPG-IF and graphite or B4C content were carried out. The ratio of the components in the polymerization system was varied in order to investigate the influence of the fillers and the macroactivator on the degree of conversion. The low molecular weight activator acetyl caprolactam (Ac-CL) was used as a reference. As the copolymeriza-

tion process proceeds with high polymerization rate and degree of conversion, it is attractive to obtain different composite materials of various properties.

The changes in the degree of conversion of PA-6/PPG-IF/graphite copolymers as a function of the PPG-IF and grafite content are shown in Figs. 2 - 4. It is seen that the degree of conversion is very high in most of the cases considered. A lower degree of conversion is obtained for PA-6/PPG-IF/graphite. Its PPG-IF content was the highest.

Similar results were observed for the copolymeri-zation systems based on PA-6/PPG-IF, where B4C was used as filler. The influence of B4C concentration on the degree of conversion is almost negligible. In fact the macroactivator content is the main factor (Fig. 5).

The PA-6/PPG-IF/graphite as well as the PA-6/PPG-IF/B4C copolymers obtained upon extraction of low

Fig. 1. Mechanism of activated anionic polymerization (R =PPG-IF).

Fig. 2. Degree of conversion at different percentage of PPG-IF at 5th min polymerization time.

Fig. 3. Degree of conversion at 10th min of polymerization with different percentage of PPG-IF co-monomer.


538

molecular weight products and fillers are investigated by 1H NMR and FTIR.

The FTIR spectra (Figs. 6 and 7) show the character-istic bands of PA-6. The absorption at 3299cm-1 refers to N-H Amide I, Amide II, that at 3072 cm-1 is determined by the vibrations of N-H bond of Amid II, while those at 1637 cm-1 and 1542 cm-1 correspond to -C=O in Amide I and N-H + C-N in Amide II , correspondingly. The absorption at 688 cm-1 is typical for PA-6. The absorption of the C-O-C stretching vibration at 1119cm-1 is due to the incorporated PPG.

The synthesized copolymers are investigated by 1H NMR as well. The results obtained are illustrated in Fig. 8. Peaks at 1.69 ppm, 1.60 ppm (d), 1.39 ppm (s) (-CH2-), 2.39 ppm (m) (-CH2-CO-), 3.32 (s) (-CH2-NH-) are outlined.

Fig. 4. Degree of conversion at 15th min of polymerization with different percentage of PPG-IF co-monomer.

Fig. 5. Degree of conversion of different copolymers with B4C used as an additive at 15th min of polymerization.

Fig. 6. FTIR spectra of PA-6/PPG-IF copolymers.

Fig. 7. FTIR spectrum of PA-6/PPG-IF copolymer with characteristic absorption bands of PA-6 and PPG.

They are typical for PA-6. The peaks recorded at 1.24 ppm (s) (-CH3), 3.65ppm (s), (-CH2-O), 3.78 ppm (s), (-CH(CH3)-) refer to the PPG block.

All results pointed above provide to conclude that copolymers of PA-6 and PPG are synthesized.

Charpy Impact StrengthOne of the main disadvantages of PA-6 refers to its ef-

fect on the impact strength observed. It is expected that the copolymers obtained will show better mechanical proper-ties. That is why the influence of graphite and B4C additives on the impact strength of the newly prepared copolymers is followed. The results show significantly improved impact strength of the copolymers obtained when compared to that of pure PA-6/PPG ones (Figs. 9, 10). The effect is outlined


539

up to 5 % presence of fillers. No performance change is observed on further increase of the latter. Therefore it can be concluded that the optimal filler concentration is 5 mass % in case 3 % PPG is used as a macroactivator.

CONCLUSIONS

Novel composite materials based on activated ani-onic polymerization of ε-CL and PPG-IF macroactivator are successfully obtained in the presence of mineral fillers - graphite and B4C. The formation of PA-6/PPG copolymers is verified by FTIR and 1H NMR analysis. It is found that the mineral filler incorporation results in

significant improvement of the impact strength. In case of high concentrations of the macroactiva-

tor (10 %), the increase of the graphite content leads to increase of the polymerization rate and yield. This provides to consider it as an active additive, unlike B4C which is practically inert. On the other hand, at certain levels of loading (3 – 5 %), both fillers contribute to the improvement of the composites impact strength.

REFERENCES

1. D.Yan, G.Yang, Synthesis and properties of homo-geneously dispersed polyamide 6/MWNTs nanocom-

Fig. 8. 1H NMR spectrum of PA-6/PPG copolymer.

Fig. 10. Charpy impact strength of PA-6/PPG/B4C co-polymers.

Fig. 9. Charpy impact strength of PA-6/PPG/Graphite copolymers.


540

posites via simultaneous in situ anionic ring-opening polymerization and compatibilization, J. Appl. Polym. Sci., 112, 2009, 3620-3626.

2. R. J. Palmer and Updated by Staff 2005, Polyamides, Plastics, Kirk-Othmer Encyclopedia of Chemical Technology.

3. R. Mateva, R. Filyanova, R. Dimitrov, R. Velichkova, Structure, mechanical, and thermal behavior of nylon 6-polyisoprene block copolymers obtained via anionic polymerization J. Appl. Polym. Sci., 91, 2004, 3251-3258.

4. Y. Chen, S. Chen, Bulk anionic copolymerization of ε-caprolactam in the presence of macroactivators derived from polypropylene glycol, J. Appl. Polym. Sci., 47, 1993, 1721-1729.

5. R. Mateva, O. Ishtinakova, R. N. Nikolov, Ch. Djambova, Kinetics of polymerization of ε-caprolactam

in the presence of inorganic dispersed additives, Eur. Polym. J., 34, 1998, 1061-1067.

6. H.Unal, Morphology and mechanical properties of composites based on polyamide 6 and mineral addi-tives, Mater. Des., 25, 2004, 483-487.

7. J. Stehlíĉek, G. S. Chauhan, M. Znáŝiková, Preparation of polymeric initiators of the anionic polymerization of lactams from polyetherdiols, J. Appl. Polym. Sci., 46, 1992, 2169-2175.

8. P. Petrov, K. Jankova, R. Mateva, Polyamide-6-b-polybutadiene block copolymers: Synthesis and properties, J. Appl. Polym. Sci., 89, 2003, 711-717.

9. J. Brožek, M. Marek, J. Roda, J. Králíček, Polymerization of lactams, 81. Anionic polymerization of lactams ac-celerated with N-iminolactams, 2 ε-caprolactam (6-hex-anelactam), Makromol. Chem., 189, 1988, 11-27.

Fatemeh Bahadori, Sima Rezvantalab

541


EFFECTS OF TEMPERATURE AND CONCENTRATION DEPENDENT VISCOSITY ON ONSET OF CONVECTION IN POROUS MEDIA


Chemical Engineering Department, Urmia University of Technology, P.O. Box: 57166-419, Urmia, IranE-mail: [email protected]

ABSTRACT

The instability theory is applied to diffusion-convection phenomena in porous media, where the viscosity of the oil varies due to gas dissolution. An important application of this theory refers to the case where the diffusion-convection is employed as an EOR (Enhanced Oil Recovery) technique in oil reservoirs.

This paper presents results on the onset of Rayleigh–Bénard and Darcy- Bénard- Marangoni convective motions of a Boussinesq fluid taking into account its temperature dependent viscosity and surface tension. Numerical simulations are carried out using the Volume of Fluid (VOF) model, while that of Darcy is applied to the mathematical formulation in evaluating the flow structure and the heat transfer in a two-dimensional fluid porous layer. The results obtained show the trend observed by Hashim and Wilson.

Keywords: Rayleigh–Bénard (RB) convection, Darcy–Bénard-Marangoni (DBM) convection, onset of convection.

Received 17 June 2014Accepted 30 September 2014

INTRODUCTION

Darcy–Bénard (DB) convection is a type of natural convection determined by the unstable vertical density difference in a horizontal fluid saturated porous layer heated from bellow. The DB convection is extensively studied [1 - 3] because of its importance referring to many scientific, engineering and technological appli-cations such as oil and gas recovery and underground contaminant transport.

The buoyancy-induced flow in a cavity when the heat transfer comes from bellow leads to patterns of convection cells. However, apart from the buoyancy forces, the convective instability can also result from the surface tension at the free surface contact with the gas known as Darcy–Marangoni (DM) convection. The Marangoni convection in a liquid saturated porous media is discussed [4, 5] in details. Rudraiah and Prasad study [6] the effect of Brinkman boundary layer on the onset of convection in a porous layer driven by surface ten-sion gradients. In addition, Nield suggest a composite fluid and porous layer model for the study of Marangoni convection in a porous layer [7]. Desaive, Lebon and Hennenberg investigate [8] the coupled capillary and

gravity driven instability in a fluid layer overlying a porous one. Using the Brinkman’s model to describe the flow in the porous medium they determine the critical Rayleigh and Marangoni numbers for the convection onset. Shivakumara et al. [9] obtain an exact solution for the onset of the surface tension driven convection in superposed fluid and porous layers using the Darcy momentum law.

The aim of this paper is to simulate a porous layer saturated with liquid which is subjected to Darcy–Bé-nard, Darcy–Maragoni and Darcy–Bénard-Maragoni (DBM) convection.

GOVERNING EQUATIONS

A fluid confined in a porous medium is considered with the assumption that the following linear dependen-cies hold for the density and the viscosity:

[ ]0 01 ( )C Cρ ρ α= − −

(1)

00

1 ( )C Cµ βµ

= − − (2)


542

It is assumed that Boussinesq approximation is valid and the fluid density varies linearly with the temperature in the buoyancy force term:

0 0[1 ( )]T T Tρ ρ α= − − (3)where αT is the thermal expansion coefficient and ρ0 is the density at T=T0. The fluid viscosity is assumed to vary linearly with the temperature in correspondence with:

00

1 ( )T T Tµ βµ

= − − (4)

Moreover, the surface tension is also a function of temperature

(5)00

1 ( )T Tσ τσ

= − −

The governing equations for the variable viscosity conditions are formulated [10] as follows:The Continuity equation

0v∇ = (6)

The equations of motion

2v P g v v vKµ ρ µ µ µ= −∇ + + ∇ +∇ ∇ +∇ ∇

(7)

The equation of diffusion2

ev C D C∇ = ∇ (8)Wooding defined [11] the Rayleigh number for a

viscous liquid of variable density in a porous medium. The relation he advances is:

(9)2

e

d gKdRdz Dρ

µ=

where K is the permeability of the porous medium, d is tube diameter, while De is the effective diffusivity of the dissolved material.

NUMERICAL DETAILS

The governing equations were discretized by ap-

plying the second-order upwind scheme for the volume fraction and the power law scheme for the momentum and energy equations. The VOF approach was employed to simulate the heat transfer and the fluid flow in the porous media.

As shown in Fig. 1, the simulation area refers to a liquid saturated porous layer of thickness L and two lateral walls, an impermeable on the left and a sym-metrical on the right. The bottom boundary is rigid, while the top surface, which is in contact with the gas, is treated as a free surface. A temperature difference of ∆T is maintained between the top and bottom boundaries of the porous layer.


The flow structure and the heat transfer due to Darcy-Bénard, Darcy- Maragoni and Darcy-Bénard- Maragoni convection are discussed.

StreamlinesFig. 2 shows the streamlines that are formed due to

DB and DBM convection. As it can be seen, the con-vection loops in case of DBM convection are bigger than those in presence of DB convection, illustrating thus the surface tension effect. In addition, the greater size of DBM convection loops corresponds to a smaller number of loops.

Heat transferFig. 3 shows the average Nusselt number calculated

for a cavity subjected to heat transfer from bellow. As it is seen, the increase of the thermal Rayleigh number results in increase of the average Nusselt number. In ad-dition, it shows that the highest average Nusselt number is obtained in case of DBM convection. Besides, the Nusselt number obtained in presence of DB convection is higher than that obtained for DM convection.

Fig. 1. Schematic presentation of the porous medium and the corresponding coordinate system.


543

Over stable curvesFig. 4 shows typical stability curves for DB and

DM convection. These results correspond to the trend reported by Hashim and Wilson [4]. The latter investiga-tion refers to a case which treats the effect of the surface tension as well.

Fig. 2. Streamlines due to DB (a) and DBM (b) convection.

(b)

(a)

Fig. 3. Nusselt number obtained in case of DB, DM and DBM convection.

Fig. 4. Calculated stability curves referring to DB and DM convection.

CONCLUSIONS

Simulation of the onset of convection in a porous layer with concentration and temperature dependent viscosity is carried out. The Darcy model is employed in the mathematical formulation to evaluate the flow structure and the heat transfer in a two-dimensional fluid porous layer.

List of symbols

a dimensionless wave numberc concentration of diffusing fluidDe effective diffusivity in porous mediumD diffusivity in porous mediumd tube diameterg gravity constantg gravity vectorK permeabilityL fluid layer thicknessNu Nusselt numberp pressureR Rayleigh numberT temperaturew z-component of the velocityW dimensionless wv velocity vectorz vertical coordinateZ dimensionless vertical coordinateα coefficient of concentration expansionαT coefficient of thermal expansionβ viscosity-concentration dependence factor


544

βT viscosity-temperature dependence factorμ fluid viscosityρ fluid densityτ surface tension -temperature dependence factorσ surface tension

Subscriptsc critical propertyproperty at reference concentration

REFERENCES

1. I. Bejan, S. Dincer, A.F. Lorente, A.H. Miquel, A.H. Reis, Porous and Complex Flow Structures in Modern Technologies, Springer, New York, 2004.

2. D.B. Ingham, I. Pop, Transport Phenomena in Porous Media, Pergamon, Oxford, 1988.

3. D.A. Nield, A. Bejan, Convection in Porous Media, (3rd ed.), Springer-Verlag, New York, 2006.

4. I. Hashim, S.K. Wilson, The onset of Bénard-Marangoni convection in a horizontal layer of fluid, Int. J. Eng. Sci., 37, 1999, 643-662.

5. M. Hennenberg, M.Z. Saghir, A. Rednikov, J.C. Legros, Porous media and the Bénard–Marangoni problem, Trans. Porous Media, 27, 1997, 327-355.

6. N. Rudraiah, V. Prasad, Effect of Brinkman bound-ary layer on the onset of Marangoni convection in a fluid saturated porous layer, Acta Mech., 127, 1998, 235-246.

7. D.A. Nield, Modelling the effect of surface tension on the onset of natural convection in a saturated porous medium, Trans. Porous Media, 31, 1998, 365-368.

8. T.H. Desaive, G. Lebon, M.H. Hennenberg, Coupled capillary and gravity driven instability in a liquid film overlying a porous layer, Phys. Rev. E, 64, 2001, 66304-66308.

9. I.S. Shivakumara, S.P., Suma, K.B. Chavaraddi, Onset of surface-tension driven convection in superposed fluid and porous layers, Arch. Mech., 58, 2006, 71-92.

10. F. Rashidi, A. Bahrami, Mathematical Modeling of Onset of Convection in a Porous Layer with Viscosity Variation, J. Thermal Sci., 9, 2000, 141-145.

11. R.A. Wooding, The Stability of a Viscous Liquid in a Vertical Tube Containing Porous Material, Proc. Roy. Soc. A, 252, 1959, 120-125.

Beti Andonovic, Misela Temkov, Abdulakim Ademi, Aleksandar Petrovski, Anita Grozdanov, Perica Paunović, Aleksandar Dimitrov

545


LAUE FUNCTIONS MODEL VS SCHERRER EQUATION IN DETERMINATION OF GRAPHENE LAYERS NUMBER

ON THE GROUND OF XRD DATA

Beti Andonovic, Misela Temkov, Abdulakim Ademi, Aleksandar Petrovski,

Anita Grozdanov, Perica Paunović, Aleksandar Dimitrov

Faculty of Technology and Metallurgy,SS Cyril and Methodius University, Skopje, MacedoniaE-mail: [email protected]

ABSTRACT

The present study reports data referring to the determination of the layers of graphene samples obtained by elec-trolysis in aqueous electrolytes and molten salts using a reverse change of the potential applied. The analysis, based on the calculation of 002 XRD peak intensities, is carried out with the application of the Scherrer equation and the Laue functions model. The latter results differ from those obtained on the ground of Scherrer equation but coincide with data obtained by Raman spectroscopy and other methods. This is attributed to the multi-layer structure of the graphene samples studied.

Keywords: graphene, electrochemical production, XRD analysis, layers, Scherrer equation.

Received 10 June 2014Accepted 05 October 2014

INTRODUCTION

Graphene is the building unit of all carbon allotropes [1]. Mechanical exfoliation of graphite, chemical vapor deposition (CVD) on copper film surfaces [2], nanotubes cutting [3], and different electrochemical methods are used for the production of graphene [4, 5]. It can be obtained as monolayer- bi-layer- and multilayer-flakes or sheets [6] depending on the procedure used. The structural characterization of graphene is of outmost importance because its highly unusual properties are largely determined by its structure.

The number of layers in graphene samples can be estimated using XRD data. The latter can be described by the Scherrer equation which is found adequate [7]. But the application of the Laue functions model presents also definite interest as the results it gives are based on the treatment of the graphene thickness distribution. The present study is aimed at the comparative examination of the data obtained with the application of the Scher-rer equation the Laue functions model using grapheme samples produced electrochemically in aqueous and non-aqueous electrolytes.

Laue functions model in uniform and non-uniform graphene thickness distribution

The XRD pattern is analyzed using the following Laue functions model which includes graphene thickness distribution and certain parameters [8]:

2

0

22 )( ∑=

∝N

j

ijkaj

jefF βθ (1)

where F is a structure factor, N is the number of gra-phene layer, )(θf is an atomic scattering factor which varies from 6.00 to 6.15 e/atom with incident radiation ranging from 2 to 433 KeV, j jka (4 d sin ) /π θ λ= , where jd is a lattice spacing between jth and (j-1)th layer, θ is an angle between the incident ray and the scattering planes, λ is a wavelength of X-ray, and jβ is an occupancy of jth graphene layer. The value of jβ is between 0 and 1. The employed equation parameters

jβ make it possible to calculate the n-layer graphene regions coverage of the graphene samples produced by the two electrochemical procedures.

Hence XRD intensities of the curves in Fig. 1 (a-d) were calculated thereof.

In Fig. 1 (a-d), theoretical XRD curves are shown,


546

with intensities calculated using model (1). The values of employed parameters

jβ provide an insight into oc-cupancies of each of the graphene layers and therefore corresponding coverages. In Fig. 1 (a-c) are shown graphenes with uniform thickness distribution, meaning all jβ parameters are equal and therefore highest layer covering 100 % of the area. Thus the curves represent bilayered graphene with 100 % coverage of the second layer, 3-layered graphene with 100 % coverage of the third layer, and 16 layered graphene with 100 % cover-age of the 16-th layer.

In Fig.1d is shown a theoretical XRD curve of multilayered graphene with non-uniform thickness distribution. The employed jβ parameters enable the calculation of each of the jth layer region occupancy and hence the jth layer dj coverage percentage, where di-1 = βi-1 – βi , i = 2, 3,…, n. The calculations of the coverages are given in Table 1.

From the results given in Table 1, it is clear that the layers are non-uniformly distributed, where the monolayer graphene part covers 60 % of the structure. The average number of graphene layers is calculated as NGL = 3.312.

Scherrer equation and graphene layers number in uniform and non-uniform graphene thickness distribution

The mean dimension of the crystallite perpendicular to the plane of graphene samples 002L can be determined by the familiar Scherrer equation:

θβλ

cos002⋅

=kL

(2)where k 0.94= is the shape factor, β is the full width at half maximum given in radians, λ is a wavelength of X-ray, and θ is the angle between the incident ray and the scattering planes. The number of graphene layers N may be determined from the equation 002002 )1( dNL −= , where

002d is the average distance between graphene planes [7].In Table 2 are given the values for the curves, shown

in Fig. 1 (a-d).In the last two columns in Table 2, one may easily

compare the values which are calculated results for number of graphene layers. In the case of uniform dis-tribution of the graphene layers, the results are in good agreement, whereas in the case of non-uniform thick-ness distribution, the values greatly differ. Studies and

a) b)

c) d)

Fig. 1. Theoretical curves obtained by model (1) calculations: a) Theoretical XRD

representation of uniformly distributed bilayered graphene; b) Theoretical XRD

representation of uniformly distributed 3-layered graphene; c) Theoretical XRD

representation of uniformly distributed 16-layered graphene; d) Theoretical XRD

representation of non-uniformly distributed multilayered graphene.

0 10 20 30 40

Uniformlydistributedbilayeredgraphene

0 10 20 30 40

Uniformlydistributed3-layeredgraphene

0 10 20 30 40

Uniformdistribution of16-layeredgraphene

0 10 20 30 40

Non-uniformdistribution ofmultilayergraphene

Fig. 1. Theoretical curves obtained by model (1) calculations: a) Theoretical XRD representation of uniformly distrib-uted bilayered graphene; b) Theoretical XRD representation of uniformly distributed 3-layered graphene; c) Theoretical XRD representation of uniformly distributed 16-layered graphene; d) Theoretical XRD representation of non-uniformly distributed multilayered graphene.


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analysis that were performed upon graphene samples obtained by two different electrochemical methods, and which are further presented, show that in the case of non-uniform distribution, model 1 is more reliable method for determining the number of graphene layers.

Determining graphene layers number for pro-duced graphene samples with non-uniform graphene thickness distribution

Two graphene samples are analyzed, with the focus

on the determination of the graphene layers number. Scherrer equation method and model 1 are compared for obtaining results for the layers number of the graphene samples studied. The graphene samples were prepared by two different electrochemical methods: high temperature electrolysis in molten salt (graphene sample GMSE2) and electrolysis in aqueous solution (graphene sample GAE1), both using non-stationary current regime.

In Fig. 2a, are shown the theoretical curves calcu-lated using model 1, in the case of graphene sample

Parameter βj:

j=1,2,…,23

Value Occupancy of

the j-th layer in % Layer Share of

the j-th layer in %

β1 1 100 d1 60

β2 0.4 40 d2 10

β3 0.3 30 d3 3

β4 0.27 27 d4 5

β5 0.22 22 d5 2

β6 0.2 20 d6 5

β7 0.15 15 d7 1

β8 0.14 14 d8 1

β9 0.13 13 d9 2

β10 0.11 11 d10 2

β11 0.09 9 d11 2

β12 0.07 7 d12 1

β13 0.06 6 d13 1

β14 0.05 5 d14 1

β15 0.04 4 d15 1

β16 0.03 3 d16 1

β17 0.02 2 d17 1

β18 0.01 1 d18 0

β 19 0.01 1 d19 0.5

β20 0.005 0.5 d20 0

β21 0.005 0.5 d21 0.4

β22 0.001 0.1 d22 0

β23 0.001 0.1 d23 0.1

Table 1. Occupancies and coverages of each of the layers in grapheme, shown in Fig.1d.


548

GMSE2 produced by electrolysis in molten salt at non-stationary current regime. The theoretical curves (1 and 2) are given for comparison to the theoretical curve (3) that exhibits good fitting to the experimental curve. The experimental curve GMSE2 is presented as (4).

In Fig 2b part of the Raman spectrum of sample GMSE2 is given, showing its C-peak. Its position

NC)(Pos is directly connected to the graphene layers number N, and it varies with N as in the formula [9]:

+=

NC N

πµα cos12)(Pos

where 18 312.8 10 Nmα −= × is the interlayer coupling, and 2

277.6 10 kg Aµ−

−= ×

is the graphene mass per unit area.According to the analysis of the XRD 002 peak and

the employed jβ parameters, the j layer region cover-ages are given in Table 3.

The average value for number of graphene layers for graphene sample GMSE2 is calculated as NGL = 2.4 for the dominant structure (above 75 %) and NGL = 7.43 for the overall structure, by calculations from model 1. Using the Scherrer equation for the number of graphene layers determination, was obtained N = 27.7.

According to the C-peak position, which is the only value from Raman spectra that directly points the

number of graphene layers [9], for sample GMSE2 was calculated N = 2.54.

Model 1 stands out as the method giving results in accordance to C-peak calculations for graphene layers number.

In Fig. 3 are shown curves, calculated from the model 1, for graphene sample GAE1 produced by elec-trolysis in aqueous solution at non-stationary current regime, for 1≠jβ , which suggests that the number of graphene layers has a distribution.

The dotted line (1) in Fig. 3 is calculated curve for uniformly distributed monolayer graphene, the line (2)which is narrower than the monolayer graphene line, but broader than the green experimental curve (3) GAE1, is calculated curve for a non uniform distribution of graphene layers number for a 3-layered graphene. The line (4) is calculated curve for a non uniform distribution of graphene layers number for a multi-layered graphene. There is a noticeable discrepancy with the experimental curve due to its asymmetry. However, as the correlation coefficient is 0.92ρ = , it provides an insight with a fair accuracy into j-layer graphene regions share. Accord-ing to jβ parameters, the coverages of n-layer graphene regions are calculated (Table 4).

θ FWHM (in

Deg)

L002 (in nm) N (by L002) N (by model 1)

Fig.1a 12.36 11 7.17 2.9 2

Fig.1b 12.36 8 9.86 3.7 3

Fig.1c 12.36 1.5 51.6 15.3 16

Fig.1d 12.36 1.5 51.6 15.3 3.3

Table 2. Calculated values for curves shown in Fig. 1a-d from Eq. 2 and model 1.

30 32 34 36 38 40

0

Inten

sity

Wave number, cm–1

C=35,72

a) b) C-peak at 35.72 cm-1

Fig. 2. a) Non uniform multilayer distribution for Sample GMSE2 calculated from model 1;

b) C-peak position in Raman spectrum for graphene sample GMSE2.

Fig. 2. a) Non uniform multilayer distribution for Sample GMSE2 calculated from model 1; b) C-peak position in Raman spectrum for graphene sample GMSE2.

a) b)

(1)

(2)

(3)

(4)


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According to these calculations, the dominant structure (above 90 %) is few-layered, and the average value for number of sample GAE1 graphene layers is calculated as NGL = 2.57 for the dominant graphene structure and NGL=4.25 for the overall graphene structure. According to Scherrer equation the number of graphene layers is N = 15.3.

However, considering GAE1 TEM images and GAE1 Raman spectrum analysis results (Fig. 4 a, b), model 1 again stands out as a method producing results which are in accordance with TEM images results and Raman spectrum I2D/IG FWHM ratio. TEM images in Fig. 4 a, clearly indicate high share of few-layer region, particularly monolayer region, whereas from GAE1

Raman spectroscopy in Fig. 4 b, I2D/IG = 1.49, which estimates the number of GAE1 graphene layers as N~4. The obtained number N of graphene layers is in accord-ance with the number obtained by model 1.

CONCLUSIONS

This study shows that the accuracy of Scherrer equa-tion method for determining the number of graphene lay-ers by XRD data decreases as the level of non-uniformity of graphene thickness distribution increases. Therefore, another method is involved for determining the aver-age number of graphene layers by XRD data in the studied graphene samples. It is a Laue functions model

Monolayer region coverage ~ 18.75%

2 layers region coverage ~ 21.25%

3 layers region coverage ~ 3.75%

4-6 layers region coverage ~ 2.5%



> 10 layers region coverage < 25%

Table 3. Coverages of j-layer GMSE2 graphene regions.

Fig. 3. Non uniform multilayer distribution curve for Sample GAE1 calculated from model 1.

Monolayer region coverage ~ 40%

2 layers region coverage ~ 10%

3-6 layers region coverage ~ 15%

7-10 layers region coverage ~ 5%

> 10 layers region coverage < 10%

Table 4. Coverages of j-layer graphene sample GAE1 regions.

(1)

(2)

(3)

(4)


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which includes graphene thickness distribution and the employed parameters allow calculations of graphene j-layer region coverage, thus enabling determination of the graphene layers number.

Graphene samples which are subject of our study are produced by two different methods: high temperature electrolysis in molten salt and electrolysis in aqueous solution, both using non-stationary current regime.

The analysis and comparison of the two methods show that both are in agreement as far as graphene sam-ples that are considered have uniform thickness distribu-tion. However, in non-uniform distribution cases, Laue functions model (model 1) stands out as reliable and in accordance with other methods results. It additionally provides information on graphene samples j-layer oc-cupancies and therefore coverages with a fair accuracy. The results relevant to graphene samples produced by electrolysis in aqueous electrolyte and by electrolysis in molten salts, both using reverse change of the applied potential, have shown that these graphene samples are few-layered.

REFERENCES1. K. S. Novoselov, et al., Two-dimensional atomic

crystals, Proc. Natl Acad. Sci. USA 102, 2005, 10451-10453.

2. A. Ismach, C. Druzgalski, S. Penwell, A. Schwartz-berg, M. Zheng, A. Javey, J. Bokor, Y. Zhang, Direct

500 1000 1500 2000 2500 3000 3500

0

200

400

600

800

1000

1200

1400

1600

1800

Inten

sity,

a.u.

Raman shift (cm–1)

a) b)

Fig. 4. a) TEM images of Sample GAE1 graphene sheets; b) GAE1 Raman spectrum.

Fig. 4. a) TEM images of Sample GAE1 graphene sheets; b) GAE1 Raman spectrum.

chemical vapor deposition of graphene on dielectric surfaces, Nano Letters, 10, 2010, 1542.

3. L. Jiao, L. Zhang, X. Wang, G. Diankov, H. Dai, Nar-row graphene nanoribbons from carbon nanotubes, Nature, 458, 2009, 877.

4. A.T. Dimitrov, A. Tomova, A. Grozdanov, O. Popovski, P. Paunović, Electrochemical production, characterization, and application of MWCNTs, DOI 10.1007/s10008-012-1896-z, J Solid State Electro-chem., 17, 2013, 399-407.

5. C. Schwandt, A.T. Dimitrov, D J. Fray, High-yield synthesis of multi-walled carbon nanotubes from graphite by molten salt electrolysis, Carbon, 50, 2012, 1311-1315.

6. C.T.J. Low, F.C. Walsh, M.H. Chakrabarti, M.A. Hashim, M.A. Hussain, Electrochemical approaches to the production of graphene flakes and their poten-tial applications, Carbon, 54, 2013, 1-21.

7. B.K. Saikia, R.K. Boruah, P.K. Gogoi, A X-ray dif-fraction analysis on graphene layers of Assam coal, J. Chem. Sci., 121, 1, 2009, 103-106.

8. A. Ruammaitree, H. Nakahara, K. Akimoto, K. Soda, Y. Saito, Determination of non-uniform graphene thickness on SiC (0 0 0 1) by X-ray diffraction, Ap-plied Surface Science, 282, 2013, 297- 301.

9. Andrea C. Ferrari, Denis M. Basko, Raman spectros-copy as a versatile tool for studying the properties of graphene, Nature Nanotechnology, 8, 2013, 235.

a) b)

Sasha Kalcheva, Ivan Kanazirski

551


KINETICS OF BOROHYDRIDE ELECTROOXIDATION: REVISITED

Sasha Kalcheva1, Ivan Kanazirski2

1 University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected] University of Mining and Geology “ St. Ivan Rilski”, “Prof. B. Kamenov” Str., Sofia 1700, Bulgaria

ABSTRACT

The initial steps of sodium borohydride oxidation are studied at PtAu alloy electrodes of Au bulk composition of 20, 40, 60 and 80 at. %. Linear sweep voltammetry is applied at low scan rates in 0.01 M NaBH4 in 1.00 M NaOH. The experiments are performed at four temperature values in the range from 293.2 K to 323.2 K. The data obtained are com-pared to those referring to Pt and Au as well. The values of the exchange current density and the total number of electrons exchanged are determined following the effect of the temperature applied. The activation energy for the exchange current density is found dependent on the bulk composition of the alloys studied. The lowest value is expected for an alloy of Au bulk content of ca 50 at. %. The dependence on the surface composition is only a tentative one in view of the sensitivity of the reaction studied. Its proceeding requires the presence of four adjacent active centers on the electrode surface whose heats of adsorption are strongly affected by the alloying.

Keywords: borohydride electrooxidation, electrocatalysis, platinum, gold, PtAu alloys.

Received 29 May 2014Accepted 30 July 2014

INTRODUCTION

Sodium borohydride (NaBH4) has hydrogen content of 10.6 mass % and theoretical H-capacity of 10.8 mass % making it one of the most attractive compounds for chemical hydrogen storage [1]. Hydrogen is released through a hydrolysis reaction proceeding in correspond-ence with:

( )4 2 24NaBH 4H O NaB OH 4H+ → + (1)

The reaction can take place in room conditions but it has to be catalyzed. Sodium borohydride can also be used as a direct fuel [2 - 5] whose hypothetical oxida-tion reaction is:

4 2 2NaBH 8OH NaBO 6H O 8e− −+ → + + (2)

It is worth noting that if the oxidation of hydrogen produced in reaction (1) is considered, the two processes are formally equivalent. But 4BH− poor anodic efficiency of the direct borohydride fuel cell (DBFC) is attributed to reaction (1) and which is why the underlying chal-lenge is to find an electrocatalyst that is active towards oxidation while being inert towards hydrolysis.

A large number of fundamental electrochemical studies of 4BH− oxidation were carried out on Au [6-25] and Pt [7,8, 21, 22, 26-31] of various catalyst morpholo-gies with the application of electrochemical, electro-analytical and surface techniques. Calculations and modeling based on the Density Functional Theory (DFT) were carried out as well. For several years the interest to Au as an anode material in DBFC was much greater


552

than that to Pt as it was believed that it catalyzed pre-dominantly the direct 4BH− oxidation reaction (BOR), while Pt favored the competition between this preferred reaction and the proceeding heterogeneous hydrolysis. It is well recognized that there are uncertainties regarding BOR mechanism on Au. Some of the schemes advanced suggest several EC steps [6, 8, 16], other focus on an overall CE pathway at low reaction overpotentials [11, 17, 22]. It is suggested [12, 22-24, 25] that BOR is lim-ited at low overpotentials by the weak initial adsorption of the 4BH− ion resulting in a very low surface coverage of 4BH− and subsequent reactive intermediates.

There are some uncertainties in connection with BOR mechanism on Pt as well. They are determined by the competition between several electrochemical and chemical steps generating hydrogen as a by-product even at pH 14. It is assumed [21, 22, 26] that BOR at Pt starts with destructive chemisorption of 4BH− to the electrode followed by the ionization of the adsorbed surface hydrogen:

4 3BH Pt Pt BH H 2e− + −→+ − + +← (3) This low potential direct oxidation at Pt precedes

[22] the hydrolysis of 4BH− , which follows CE-type [8] reactions, i.e. the complete oxidation of 4BH− follows stepwise mechanism. The comparative investigation of BOR on bulk Au and Pt electrodes [21] provides physical evidence of the intermediate species and H2 formation as a function of the electrode potential advancing the knowledge that Pt can be Faradaic efficient, unlike Au, i.e. Pt will outperform Au as an anode material in DBFC.

Binary Pt-based electrocatalysts [32 - 40] are found to circumvent some of the major problems referring to borohydride oxidation transformation. All results ob-tained imply synergism in the electrocatalytic activity of PtAu bimetallic systems. The comparative study of the kinetics of borohydride oxidation in the range of the first anodic maximum at Pt, Au and PtAu bulk alloys [40] shows that two-step formation of adsorbed species containing BOH and H proceeds at the alloy surface. The electronic effect determining the performance ob-served is attributed to decrease of B-H bonding strength when compared to that of unalloyed Pt. In view of the perfect control [30, 31, 41] of the surface morphology, texture and structure required for the juxtaposition of the intrinsic activity of Pt and Au, it looks challenging to follow the effect of the alloys surface composition on the behavior of the alloys studied in the range of the first

anodic maximum. This is in fact the aim of the present communication.

EXPERIMENTAL

The kinetic study envisaged was carried out follow-ing the procedure described in [40]. Linear sweep (LSV) voltammetry was applied varying the scan rate from 0.020 Vs-1 to 0.100 Vs-1 and the temperature within the interval from 293.2 K to 323.2 K. The electrochemical measurements were performed with an Autolab PGSTAT 30 with a FRA2 module driven by GPES 4.9 and FRA 4.9 software (EcoChemie, The Netherlands).

Stationary electrodes of PtAu alloy electrodes of Au bulk content of 20, 40, 60 and 80 at. % were used. Com-parative investigations were carried out at polycrystal-line Pt and Au electrodes as well. Pt or Au mesh counter electrodes and Ag/AgCl (KClstd) reference electrode were used. The true electrode surface of the working electrodes was determined at room temperature prior to each experiment in a three-compartment cell containing 1.0 M aqueous solution of NaOH. The working electrode surface had to be pretreated because of the fouling observed [21, 42]. The procedure developed [40] was applied in a second cell of identical configuration and dimensions. The values of the alloys surface composi-tion were within the error range of the method applied.

All experiments were conducted in 0.01M solution of NaBH4 (Merck, p.a.) in 1.0 M NaOH (Merck, p.a.). Bi-distilled water was used. Each of the borohydride solutions was prepared prior to the corresponding experi-ment to exclude any homogeneous hydrolysis. A third three-electrode electrochemical cell was used.


The comparative investigation of the electrodes catalytic performance is carried out on the ground of the exchange current densities observed following the effect of the alloys composition and the temperature applied. The exchange current density, 0j , is the product of the electrochemical specific rate constant and a concentration term containing the activities of the oxidizing and reduc-ing species. It is introduced in electrochemical kinetics [43 - 45] by the rate equation of an electrode reaction:

( ){ ( ) }0j nFv j exp F / RT exp 1 F / RTβ η β η= = − − − (4)


553

where η is the overpotential (η is the difference between the potential E and the reversible potential revE ), β is the symmetry factor, v stands for the chemical reaction rate, while the rest of the symbols have their usual meaning.

At low values of the overpotential ( F / RT 0.1β η ≤ ) Eq. (4) can be put [44] into linear form by expanding the exponent and taking only the first two terms:

( ) 0j FjRT

η η= (5)

This relationship is found valid for a complex electrode reaction including intermediates adsorption. The derivation [44] is done considering the system at a steady state. When several consecutive reactions are involved and the system is at steady state, all steps occur at the same rate ( )e e e e

1 1 2 2 3 3 i iv A v A v A ... v A const= = = =. In fact, although all steps in the sequence proceed at the same rate, the affinities of all steps, except the rate-determining one, are near zero, i.e. these steps are considered to be at equilibrium. The rate-determining step has an exchange rate, which is much lower than that of the other steps and affinity, which is much higher. Assuming that the rate-determining step occurs ν times per act of the overall reaction, its affinity is given by

iA A /ν= . This provides to express the net rate, v , of the reaction close to equilibrium through:

erds

Av vRTν

=

(6)where e

rdsv is the exchange rate of the rate-determining step. Since the affinity A of an electrode reaction is equal to the electrical energy per mole which is applied to remove the system from its equilibrium, the rate of the overall reaction is given [44] by:

oj nFjRTη

ν=

(7)

It is seen that Eq. (7) is in fact identical with Eq. (5) derived for a simple one-electron reaction. Furthermore it can be used in this study as the system behaves revers-ibly at low scan rates. Eq. (7) is applied in the form:

0 0p p,rev p

j nF j nFj E ERT RT

= − + (8)

assuming that 1ν = . Eq. (8) shows that pj depends linearly on the peak potential pE providing to estimate

0j from the slope obtained. The calculations carried out with the application of

Eq. (8) are based on LSV profiles recorded at scan rates

varying in the range between 0.020 Vs-1 and 0.100 Vs-1 at several temperature values. Fig. 1 illustrates some of the scans recorded. It is evident that the peak current density increases, while the peak potential value shifts in positive direction with scan rate increase at any of the temperature values set. Fig. 1 shows as well that the curves cross each-other at potentials prior to that of the peak. It is worth noting that this behavior is observed only in absence of electrode surface fouling which in turn leads to the suggestion that it can be assigned to a non-negligible hydrolysis of 4BH− in correspondence with findings [21, 22]. The latter refer to the behavior of Pt rotating disc electrodes. The fit of the experimental data to Eq. (8) is illustrated in Fig. 2.

The estimation of 0j requires the introduction of

Fig. 1 (b). LSV scans recorded at an alloy of Au bulk con-tent of 60 at. % (Au surface content of 44.9 %) at 299.0 K with scan rates of: (1) 0.020; (2) 0.030; (3) 0.040 and (4) 0.050 Vs-1.

Fig. 1 (a). LSV scans recorded at an alloy of Au bulk content of 40 at. % (Au surface content of 9.7 %) at 308.4 K with scan rates of: (1) 0.020; (2) 0.030; (3) 0.040 and (4) 0.050 Vs-1.


554

the value of n . The latter gives the total number of electrons exchanged in the process investigated. It is determined on the ground [40 and references therein] of the rate dependence of the peak current density. Much higher scan rates are required. They have been applied and some of the values obtained were reported [40]. The

temperature effect on n is followed on the ground of the temperature dependence of the diffusion coefficient D. Theoretically it is given by an exponential equation of the type 0D D exp( E / RT)= − , where 0D is the maximum diffusion coefficient, while E is the activation energy for diffusion usually given varying from 12.6 1kJ mol− to 28.1 1kJ mol− . The values of D reported in [19], i.e. 2.42x10-5 cm2 s-1 at 298 K and 5.00x10-5 cm2 s-1 at 338 K, provide to calculate E in case of 4BH− diffusion. The value found is equal to 15.2 1kJ mol− , i.e. it falls within the range pointed above. The calculations refer-ring to n show that the value for Pt changes from 0.9 at 299.2 K to 1 with temperature increase, while that of the alloy containing 60 at. % is 0.8 and changes to 0.9 at the highest temperature applied. The values found for the alloys containing 40 at. % and 80 at. % of Au stay constant with temperature variation. They are 0.4 and 0.6, correspondingly. That obtained for the alloy of Au bulk content of 20 at. % changes from 1.3 to 1.8 with temperature increase. In fact this is the only electrode material studied whose n value is close to the expected value of 2. It should be added that all values obtained are lower than 1 and 2, correspondingly, because of hydrolysis proceeding [21, 22], but they are in accord with the mechanistic concepts of borohydride oxidation on Pt(111) and Au(111) [23, 24, 29]. They are based on DFT calculated energy diagram of the reactions taking place. Further support is given by investigations [46 - 51] focused on the relation between the surface electronic structure and the reactivity of transition on noble metals and alloys. I t is assumed that in case of Pt [29] and most of the alloys studied [40] the anodic peak is determined by the proceeding of:

4, aqBH 4 BH 3H e− ∗ ∗ ∗ −+ → + + where the symbol (*) denotes adsorption sites or ad-sorbed species. The potential range of reaction (9) proceeding is obviously too low for most of the elec-trodes to provide the occurrence of the second step of the 2e-process, i.e.

aqBH OH BOH H e∗ − ∗ ∗ −+ + ∗→ + + (10)The concepts just outlined are in accord with the first

peak potential values reported in [40]. For the alloys of Au bulk content less than 80 at. % they are lower than this for Pt, which in turn is lower than that for Au [8, 19, 21, 22]. These findings are valid for all temperature values studied. They are in correspondence with the data refer-ring to PtAu bimetallic systems examined [2, 32 - 40].

(9)

Fig. 2 (b). Dependence of the peak current density, pj , on the potential value,

pE , of the first oxidation peak recorded at PtAu alloys of Au bulk content of 60 at. % : line (1) refers to 299.0 K and Au surface content of 44.9 %; line (2) refers to 308.4 K and Au surface content of 44.6 %; line (3) refers to 318.0 K and Au surface content of 44.8 %.

Fig. 2 (a). Dependence of the peak current density, pj , on the potential value, pE , of the first oxidation peak recorded at PtAu alloys of Au bulk content of 40 at. % : line (1) refers to 298.3 K and Au surface content of 11.3 %; line (2) refers to 308.4 K and Au surface content of 9.7 %; line (3) refers to 317.6 K and Au surface content of 8.9 %.


555

The values of 0j determined depend on the surface composition of the electrode materials. Fig. 3 outlines this dependence. It is seen that 0j for three of the al-loys is close to and slightly less than that for Pt at lower temperatures (Fig. 3a). These values are much smaller than that for Pt at higher temperatures (Fig. 3b). The exchange current density referring to the alloy of Au surface content of 30 % is the highest among those ob-tained in this study at all temperatures applied.

The results just reported determine the interest to-wards the temperature effect on 0j . It can be described [52] by:

S0

Ej Ac kT exp4kT

ρπ= − (11)

where A is the frequency factor, c is the concentration of the reacting ion, ρ is the density of the electrode ma-terial’s electronic states, k is the Boltzmann constant, while

SE is the reorganization energy, characterizing the ligands’ charging. In fact SE / 4 is the activation energy [52] for the exchange current density. Eq. (11) shows that it can be estimated on the ground of the slope of the linear dependence of 0ln j vs. 1/ T . Some of the lines obtained in correspondence with Eq. (11) are presented in Fig. 4. The values of the activation energy are calculated. They are found dependent on the bulk as well as on the surface composition of the alloys investigated. The cor-responding dependences are illustrated in Figs. 5 and 6. It is worth pointing out that the dependence on the bulk composition is of the form obtained in [40]. The curve presented in Fig. 5 goes through a minimum at Au bulk

content of ca 50 at. %. Besides, the activation energy obtained for two of the alloys is less than that of Au. The effect of the surface composition is not so well outlined. Fig. 6 illustrates the relation’s tentative character. The curve shown goes through a minimum at Au surface con-tent of 30 %. The latter value is attracting attention as the highest exchange current densities as well as the highest value of the total number of electrons exchanged are found for this particular surface content. The generaliza-tion of all facts obtained provides to conclude that the adsorption sites required for the proceeding of reaction (9) are most favorably distributed in case of Au surface presence of ca 30 %. The requirement of four adjacent

Fig. 3. Dependence of the exchange current density versus the surface composition of the alloy electrodes studied at (a) 298 K; (b) 308 K. The actual temperature values differ slightly but are close to those cited.

Fig. 4. Temperature dependence of the exchange current density found for PtAu alloys of Au bulk content of 40 at. % (line 1); 60 at. % (line 2); 80 at. % (line 3).


556

adsorption centers on the electrode surface combined with the alloying effect on the heats of adsorption of these cites can explain the sensitivity of the oxidation reaction studied and hence the scattering of the points around the tentative curve in Fig. 6.

CONCLUSIONS

The comparative kinetic investigation of the process determining the first anodic peak of borohydride oxida-tion at Pt, Au and Pt-Au alloys of Au bulk content of 20, 40, 60 and 80 at. % provides the determination of the corresponding exchange current density taking into consideration the total number of electrons exchanged in

the peak’s potential range and the temperature applied. The latter is varied in the range from 293.2 K to 323.2 K. The activation energy for the exchange current density is estimated and the effect of the alloys composition is followed. It is found that the alloys activation energy is lower than that of Pt. The lowest value is expected at Au bulk content of ca 50 at. %. The dependence on the alloys surface composition is only tentatively outlined most probably because of the sensitivity of the surface reaction studied. Its proceeding requires the presence of four adjacent active centers whose heats of adsorption are strongly affected by the alloying.

AcknowledgementsThe financial support from the University of Chemi-

cal Technology and Metallurgy, Sofia, through Project No 10977/ 2012 is gratefully acknowledged.

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K. Ramana Kumar, A. Raghavendra Guru Prasad, V. Srilalitha, Y.N. Spoorthy, L.K. Ravindranath

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ELECTROCHEMICAL STUDIES OF METAL COMPLEXES OF MALONYL DIHYDRAZIDE

K. Ramana Kumar1, A. Raghavendra Guru Prasad2, V. Srilalitha2, Y.N. Spoorthy3, L.K. Ravindranath3

1 Malla Reddy Engineering College, Hyderabad, A.P., India2 The ICFAI Foundation for Higher Education, Hyderabad, A.P., India E-mail: [email protected] Sri Krishnadevaraya University, Anantapur, A.P., India

ABSTRACT

The present article deals with the polarographic and cyclic voltammetric investigations on metal (Mn(II), Ni(II), Cu(II), Zn(II), Cd(II), Fe(III) and Co(III)) complexes with malonyl dihydrazide (MAH). The variables that influence the electrode process were extensively discussed. The sites susceptible for reduction were reported by comparing the results obtained from polarography and cyclic voltammetry.

Keywords: malonyl dihydrazide, metal complexes, polarography, cyclic voltammetry.


INTRODUCTION

Hydrazides are potential therapeutic compounds due to their wide range of biological activities [1 - 3]. The -NH-NH- hydrazide fragment [4 - 6] plays an im-portant role in medicinal chemistry. Earlier work has demonstrated that some drugs show increased activity when administered as metal chelates rather in the form of original organic compounds [7, 8]. Such complexes have an important role in bioinorganic chemistry and redox enzyme systems [9 - 12]. The study of structural and binding features of various metal complexes can play an important role in better understanding of the biologi-cal process. Redox properties of a drug can give insight into its metabolic or pharmaceutical activity [13 - 15]. Literature survey reveals that electrochemical studies have been exploited to predict the behavior of ligand and its complexes in biochemistry and medicine [16].

The aim of present work is to study the electrochemi-cal behaviour of Mn(II), Ni(II), Cu(II), Zn(II), Cd(II), Fe(III) and Co(III) metal complexes with malonyl dihydrazide (MAH). The electron transfer mechanism of the metal complexes is investigated with the aid of

polarography and cyclic voltammetry. In earlier studies [17], the authors have reported the physico-chemical aspects the metal complexes of MAH.

EXPERIMENTAL

Materials and methodsAll chemicals used were of analytical reagent grade

obtained from Merck India Limited. pH measurements were made using pH meter Model L1-10 manufactured by ELICO Private Limited, Hyderabad, India. DC Re-cording Polarograph manufactured by ELICO Private Limited, Hyderabad, India was used for polarographic studies. Cyclic voltammetric unit consists of X-Y re-corder (Model RE. 0074), PAR 173 potentiostat, PAR 175 universal programmer. A circulating type thermostat manufactured by Toshniwal, Bombay, India with water as the thermostatic liquid was employed to maintain a constant temperature.

General experimental procedure 10 mL of buffer solution and 2.5 mL of metal com-

plex solution (1×10-2 M), 10 mL of dimethylformamide and remaining volume of distilled water to make the


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total volume to 25 mL were taken into a polarographic/cyclic voltammetric cell. Polarograms/cyclic voltam-mograms were recorded after deaeration of the solution with nitrogen gas. The polarographic maxima in all cases were suppressed by the addition of two drops of 0.001 % gelatin.

RESULTS AND DISCUSSIONPolarographic studiesThe interpretation of polarographic waves of metal

complexes has been discussed by Lingane [18]. The electrochemical behaviour of complexes of Mn(II), Ni(II), Cu(II), Zn(II), Cd(II), Fe(III) and Co(III) with MAH in aqueous dimethylformamide (40 % v/v) is presented in this article.

Polarographic behaviour of MAH complexes of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II)

Metal complexes under study have displayed a sin-gle reduction wave in the pH range 2.1 - 10.1. The half wave potentials were shifted to more negative values with increase in pH. However, the half wave potential values were constant beyond pH 8.1. This indicates that protons were involved in the reduction process. The number of protons (p) was a small non-integer value and thus indicates the surface protonation [19]. The decrease in wave height with increase in pH may be attributed to the decrease in the rate of protonation. This fact further indicates that the protonated form of the electroactive species has undergone reduction. It was observed in solutions of pH 8.1 - 10.1 that the wave heights and the half wave potentials remain constant signifying the electroactive nature of the protonated and unprotonated forms of the depolarizer.

No wave was observed either with buffer solution or with the reagent when they were employed indepen-dently. The half wave potentials of complexes were found to be more negative than the simple metal ions (Table 1). The change in half wave potential (E1/2) with the concentration of the complex may be attributed to the irreversible nature of the electrode process [20].

This fact was further confirmed from the non-integral values of n obtained from the plots of

log )ii(

id −

vs Edme [21].

The plots are shown in Fig. 1. The values of H/h1/2 were fairly constant (Table 2). These facts suggest that the

complexes undergo diffusion controlled reduction [22].Effect of temperatureThe polarograms of the metal complexes were re-

corded at 303, 313, 323 and 333 K at pH 4.1 to study the effect of temperature on the half wave potential and the wave height. The complexes exhibited a well defined single cathodic wave at all temperatures studied. Linear relationship between wave height (H) and h1/2 suggests the diffusion controlled nature of waves at all temperatures.

Wave height increases with increase in temperature and the value of the temperature coefficient lies between 0.762 - 1.38 % deg-1. ana values (α is the transfer coef-ficient and na is the number of electrons involved) were calculated using Tomes criteria [23]. The decrease in ana values (Table 3) with increase in temperature may be due to decrease in a values and the decrease in a values was due to the increasing difficulty in transfer of electrons with increasing temperature. A decrease in ana value [24 - 27] suggests that the system tends to

Fig. 1. Semi log plots of M-MAH; [M-MAH] = 1×10–3M; pH = 4.1; Medium = Aqueous dimethyl formamide (40 % v/v); I and II indicate the first and the second wave, respectively.


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become increasingly irreversible. This was further supported by the fact that the half

wave potential values tend to become more negative with increase in temperature. The thermodynamic parameters were evaluated from the equations proposed by Meites-Israel [28], Oldham-Parry [29] and Gaur Bhargava [30]. The diffusion coefficient value required for the calculation of formal rate constant ‘K

h,f ’ at different temperatures was calculated from Stoke-Einstein [31] equation. A perusal of the data pertaining to thermody-namic parameters showed that the formal rate constant decreases with increase in temperature and signifies that the electrode reaction was rendered increasingly irreversible with the raise in temperature. This observa-tion has further confirmed the conclusions arrived on the basis of ana values. Thermodynamic parameters namely enthalpy of activation (DH*p), heat of activation at con-

stant volume (DH*v) and entropy of activation (DS*) are shown in Table 4. The data pertaining to heterogeneous rate constant K

h,f and activation free energy change, DG* at typical pH values were shown in Table 1. It was observed from the data that the electrode process was becoming increasingly irreversible with increase in pH and the same was evinced by the decrease in K

h,f and

increase in DG* values with increase in pH [32]. Plots of log Kºf,h vs 1/T for the polarographic reduction of metal complexes studied by different treatments are presented in Fig. 2.

The sites susceptible for reduction were metal ion,

amide ( C

O

NH ) and azomethine group (>C=N-NH-) in the enolic form. Since the reagent alone was not undergoing reduction in the potential range of study, the reduction waves observed in the case of complexes

pH -E1/2 V vs SCE

Wave height, H (cm) DE1/2/DpH ana

Number of protons (P)

D×106 cm2 sec-1 I×103 K°f,h cm sec-1 DG* K cal

mole-1

I wave

II wave

I wave

II wave

I wave

II wave

I wave

II wave

I wave

II wave

I wave

II wave

I wave

II wave

I wave

II wave

I wave

II wave

Mn(II)-MAH 4.1 1.61 - 2.7 - 0.04368 - 0.47 - 0.3471 - 1.526 - 1.500 - 1.488×10-14 - 11.281 - 8.1 1.78 - 2.0 - 0.04368 - 0.43 - 0.3471 - 0.838 - 1.111 - 3.999×10-15 - 11.621 - Ni(II)-MAH 4.1 0.72 - 6.7 - 0.0375 - 0.61 - 0.3740 - 9.40 - 3.72 - 2.60×10-8 - 7.56 - 8.1 0.87 - 6.0 - 0.0375 - 0.57 - 0.3740 - 7.54 - 3.33 - 1.07×10-10 - 8.98 - Cu(II)-MAH 4.1 0.18 - 6.3 - 0.05812 - 0.69 - 0.6976 - 8.58 - 3.55 - 1.12×10-1 - 4.20 - 8.1 0.39 - 5.5 - 0.05812 - 0.74 - 0.6976 - 6.57 - 3.11 - 2.61×10-5 - 5.77 - Zn(II)-MAH 4.1 1.28 - 6.3 - 0.04166 - 0.69 - 0.5212 - 8.31 - 3.50 - 1.63×10-15 - 11.85 - 8.1 1.45 - 5.5 - 0.04166 - 0.79 - 0.5212 - 6.34 - 3.06 - 1.33×10-19 - 14.29 - Cd(II)-MAH 4.1 0.78 - 6.8 - 0.03438 - 0.47 - 0.2848 - 9.68 - 3.78 - 1.202×10-7 - 7.16 - 8.1 0.90 - 6.4 - 0.03438 - 0.54 - 0.2848 - 8.58 - 3.55 - 2.091×10-9 - 8.21 - Fe(III)-MAH 4.1 0.33 - 2.8 - 0.03519 0.04277 0.57 0.89 0.333 0.629 7.54 - 1.67 - 7.53 ×10-4 - 4.90 - 8.1 1.23 1.77 3.3 6.5 0.03519 0.04277 0.54 0.84 0.333 0.629 5.66 5.23 1.44 2.78 9.37 ×10-13 1.11 ×10-27 10.21 19.10 Co(III)-MAH 4.1 0.42 - 3.0 - 0.03519 0.04277 0.57 0.89 0.333 0.629 7.54 - 1.67 - 7.53 ×10-4 - 4.70 - 8.1 0.57 1.50 2.6 5.0 0.03519 0.04277 0.54 0.84 0.333 0.629 5.66 5.23 1.44 2.78 9.3 ×10-13 1.11 ×10-27 10.21 19.10

Table 1. Polarographic characteristics and kinetic parameters of M-MAH complex ( 1 × 10-3 M).

h pH 4.1 pH 8.1 pH 4.1 pH 8.1 pH 4.1 pH 8.1 pH 4.1 pH 8.1 pH 4.1 pH 8.1

H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 [Mn(II)-MAH] [Ni(II)-MAH] [Cu(II)-MAH] [Zn(II)-MAH] [Cd(II)-MAH]

80 3.0 0.34 2.2 0.25 7.2 0.81 6.5 0.73 6.9 0.77 6.1 0.68 6.8 0.76 6.0 0.67 7.2 0.80 6.9 0.77 70 2.7 0.32 2.0 0.24 6.7 0.80 6.0 0.72 6.4 0.76 5.6 0.67 6.3 0.75 5.5 0.66 6.8 0.81 6.4 0.76 60 2.5 0.32 1.8 0.23 6.1 0.79 5.6 0.72 6.0 0.77 5.0 0.65 5.8 0.75 5.2 0.67 6.3 0.81 5.8 0.75 50 2.3 0.33 1.7 0.24 5.6 0.79 5.0 0.71 5.5 0.78 4.7 0.66 5.3 0.75 4.7 0.66 5.8 0.82 5.3 0.75 40 2.0 0.32 1.6 0.25 5.1 0.81 4.5 0.71 4.9 0.77 4.3 0.68 4.7 0.74 4.3 0.68 5.2 0.82 4.8 0.76

Fe(III)-MAH Co(III)-MAH pH 4.1 pH 8.1 pH 4.1 pH 8.1

I wave II wave I wave II wave I wave II wave I wave II wave H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2 H H/h1/2

80 3.1 0.35 - - 3.6 0.40 7.0 0.78 3.3 0.37 - - 2.8 0.31 5.4 0.60 70 2.8 0.33 - - 3.3 0.39 6.5 0.78 3.0 0.36 - - 2.6 0.31 5.0 0.60 60 2.6 0.34 - - 3.1 0.40 6.1 0.79 2.7 0.35 - - 2.5 0.32 4.7 0.61 50 2.3 0.33 - - 2.7 0.38 5.4 0.76 2.5 0.35 - - 2.3 0.33 4.4 0.62 40 2.2 0.35 - - 2.5 0.39 5.0 0.79 2.4 0.38 - - 2.1 0.33 4.0 0.63

Table 2. Influence of mercury column height (h cm) on wave height (H cm); Medium: Aqueous DMF (40 % v/v); [M-MAH]:1 × 10-3 M.


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were due to the metal ion present in the complex. The single wave obtained in the pH range of study may be attributed to the reduction of M(II) to M(0).

Polarographic behaviour of Fe(III) and Co(III) complexes of MAH

The polarographic behaviour of Fe(III) and Co(III) complexes of MAH have been studied in the pH range 2.1 to 10.1.

A single well defined wave was observed in the pH range 2.1 - 10.1 and was attributed to the one electron reduction (Table 1) of metal complex. The half wave potential values were shifted to more negative values compared to the shift of half wave potentials of the waves of simple metal ions. The values of H/h1/2 were fairly constant. Linear plots were obtained between the wave height and the concentration of the complex. These facts suggest that the complexes undergo diffusion controlled reduction. The change in half-wave potential (E1/2) with concentration of complex may be attributed to the irreversible behaviour of the electrode process [33]. This fact was further confirmed by the fractional values of n obtained from the log

iii

d − vs Edme plots.

The half wave potentials were shifted to more nega-tive values with increase in pH. As specified earlier, the half wave potentials were not altered beyond pH 8.1. These observations reveal that the protons were involved in the reduction process. However, an increase in wave height with increase in pH may be attributed to the electroactive nature of the unprotonated form of the complex species. The wave height remains constant beyond pH 8.1.

It was observed from the data presented in Table 1 that two waves were observed in pH range 6.1 - 10.1. The absence of second wave in the pH range 2.1 - 4.1 may be due to the formation of a species which can be reduced at more negative potentials. The half wave potentials of the second wave become increasingly negative with the increase in pH and with increase in concentration of the metal complex. n value was found to be a non-integer. These facts suggest that the electrode process was irreversible. Constant values of H/h1/2 and the linear

relationship observed between log ii

i

d − vs Edme suggest

the diffusion controlled nature of the reduction process. The effect of pH on half wave potential and wave height was similar to that observed for the first wave.

Effect of temperatureThe studies pertaining to effect of temperature were

carried out in solutions of pH 4.1. The data given in

Fig. 2. Plot of kof,h versus 1/T for the polarographic reduction

of M-MAH; [M-MAH] = 1×10–3M; pH = 4.1; Medium= Aqueous dimethyl formamide (40 % v/v); X = 10, 4, 16, 11, 19, 7 and 6 for Cd(II), Cu(II), Mn(II), Ni(II), Zn(II), Co(III) and Fe(III), respectively. MI, OP and GB indicate Meites-Israel, Oldham-Parry and Gaur Bhargava treatment, respectively.


563

Table 4 are indicative of similar inferences as in the case of complexes of Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II). The negative DS* values suggest that the activated state has a more stable structure than the initial state. The number of electrons involved in the reduction pro-cess calculated using De Vries and Kroon method [34] was one in the case of first wave and two in the case of second wave. The wave height of the second wave was observed to be double that of the first wave. Hence the sequence of reduction process may be represented as M(III)-SAH

1eM(II)-SAH

M(II)-SAH M(0)2e

Cyclic voltammetric studies The cyclic voltammograms for all the complexes

were recorded in solutions of pH 2.1 - 8.1 at the scan rates of 10, 20, 50, 100 and 200 mV/sec.

Mn(II)-MAH, Zn(II)-MAH and Cd(II) - MAH complexes

The results obtained in the cyclic voltammetric stud-ies on MAH complexes of Mn(II), Zn(II) and Cd(II) are given in Tables 5 and 6.

A sharp cathodic peak in the case of Mn(II)-MAH complex and a broad peak in the case of Zn(II) and Cd(II)-MAH complexes were observed at all scan rates. The absence of anodic peak in the reverse scan indicates that the reduction process was irreversible [35]. The presence of single cathodic peak in all cases suggests that the reduction process involves two electrons and the same was inferred from the polarographic studies.

Peak potentials were shifted to more negative values

M-MAH

Temperature K

Half-wave potential -E1/2 V vs

SCE

Wave height, H (cm)

Temperature coefficient %

deg-1 αna

D × 106 cm2 sec-1

Mn(II) 303 1.56 2.4 - 0.49 1.206 313 1.61 2.7 1.178 0.47 1.527 323 1.67 3.1 1.382 0.45 2.012 333 1.73 3.5 1.214 0.43 2.565

Ni(II) 303 0.66 6.1 - 0.52 7.79 313 0.72 6.7 0.938 0.54 9.40 323 0.77 7.4 0.994 0.57 11.47 333 0.84 8.0 0.780 0.61 13.40 Cu(II) 303 0.15 5.9 - 0.65 7.29 313 0.18 6.4 0.814 0.69 8.58 323 0.21 7.0 0.896 0.74 10.26 333 0.24 7.6 0.822 0.79 12.09 Zn(II) 303 1.25 5.7 - 0.64 6.80 313 1.28 6.3 1.00 0.69 8.31 323 1.32 6.8 0.76 0.73 9.68 333 1.36 7.4 0.84 0.78 11.47 Cd(II) 303 0.72 6.2 - 0.45 8.05 313 0.78 6.8 0.92 0.47 9.68 323 0.84 7.5 0.98 0.49 11.78 333 0.90 8.1 0.77 0.52 13.74 Fe(III) 303 0.30 2.5 - 0.64 5.235 313 0.33 2.8 1.133 0.61 6.567 323 0.37 3.2 1.335 0.57 8.578 333 0.41 3.7 1.452 0.54 11.470 Co(III) 303 0.39 2.6 - 0.61 5.66 313 0.42 3.0 1.43 0.57 7.54 323 0.46 3.5 1.54 0.54 10.26 333 0.52 3.9 1.08 0.52 12.74

Table 3. Effect of temperature on the polarographic characteristics of M-MAH at pH 4.1; Medium: Aqueous DMF (40 % v/v); [M-MAH] = 1 × 10-3 M.


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and the peak currents increase with increase in scan rate. Further, peak potential increase was more pronounced in solutions of pH 8.1 compared to that in solutions of pH 4.1. However a reverse trend was noticed in the case of peak current. The increase in peak current with increase in scan rate indicates that no chemical reaction was taking place in the vicinity of electrode [36].

Nature of the electrode processThe electrode process was found to be diffusion

controlled and irreversible under experimental condi-tions. The diffusion controlled nature of the electrode process was confirmed by following facts.

• ipc/n1/2 values are unaltered [37] (linear plot of ipc

vs n1/2, Fig. 3).

• The peak current increases with increase in con-centration of the depolarizer [38].

The irreversible nature of the reduction process was confirmed by the following facts:

• The cathodic peak potentials were independent of scan rates [39] i.e. ipc/n

1/2 vs n plot was a straight line parallel to the scan rate axis as explained by case I of Nicholson and Shain mechanism related to reversible charge transfer.

• The anodic peak was absent in the reverse scan [38].

• The value of (Epc/2 - Epc) being greater than 56.5 mV

n[38].

M-MAH Parameter Meites - Israel Treatment Oldham - Parry Treatment Gaur - Bhargava Treatment

303K 313K 323K 333K 303K 313K 323K 333K 303K 313K 323K 333K

Mn(II)

Kºf,h × 1016 (cm sec-1) 5.541 6.809 7.823 9.678 5.537 6.941 7.968 10.020 3.112 3.835 4.422 5.527 DH*P ( K cal mole-1) 3.660 3.660 3.660 3.660 3.960 3.960 3.960 3.960 3.696 3.696 3.696 3696 DH*V ( K cal mole-1) 3.058 3.038 3.018 2.998 3.358 3.338 3.318 3.298 3.094 3.074 3.054 3.034 DG* ( K cal mole-1) 12.258 12.605 12.969 13.310 12.257 12.600 12.964 13.299 12.407 12.760 13.128 13.471 −DS* (eu) 30.36 30.56 30.81 30.97 29.37 29.59 29.86 30.03 30.74 30.95 31.19 31.34

Ni(II)

Kºf,h × 109 (cm sec-1) 24.73 5.45 1.12 0.25 24.81 5.49 1.14 0.25 26.86 5.60 1.07 0.27 DH*P ( K cal mole-1) 30.96 30.96 30.96 30.96 30.96 30.96 30.96 30.96 32.30 32.30 32.30 32.30 DH*V ( K cal mole-1) 30.36 30.34 30.32 30.30 30.36 30.34 30.32 30.30 31.70 31.68 31.66 31.64 DG* ( K cal mole-1) 7.65 8.32 9.03 9.75 7.65 8.32 9.03 9.75 7.63 8.31 9.05 9.73 −DS* (eu) 74.95 70.35 65.91 61.71 74.95 70.35 65.91 61.71 79.44 74.66 70.00 65.79

Cu(II)


Zn(II)


Cd(II)

Kºf,h × 109(cm sec-1) 27.43 6.80 1.55 0.24 27.59 6.90 1.56 0.24 29.95 7.05 1.50 0.22 DH*P ( K cal mole-1) 32.18 32.18 32.18 32.18 31.82 31.82 31.82 31.82 33.28 33.28 33.28 33.28 DH*V ( K cal mole-1) 31.58 31.56 31.54 31.52 31.22 31.20 31.18 31.16 32.68 32.66 32.64 32.62 DG* ( K cal mole-1) 7.62 8.26 8.95 9.76 7.62 8.26 8.94 7.60 9.76 8.25 8.95 9.79 −DS* (eu) 79.08 74.44 69.94 65.34 77.89 73.29 68.85 64.26 82.77 77.99 73.34 68.56

Fe(III)


Co(III)


Table 4. Kinetic and thermodynamic parameters of polarographic reduction of M-MAH complex (1×10-3 M) at pH 4.1.

Scan rate Vs-1

pH 4.1 pH 8.1 pH 4.1 pH 8.1 pH 4.1 pH 8.1 -EPC V iPC cm -EPC V iPC cm -EPC V iPC cm -EPC V iPC cm -EPC V iPC cm -EPC V iPC cm Mn(II)-MAH Zn(II)-MAH Cd(II)-MAH

0.010 1.66 2.9 1.83 2.1 1.32 3.7 1.52 3.2 0.82 3.1 0.95 2.8 0.020 1.66 3.3 1.83 2.3 1.32 3.9 1.52 3.3 0.82 3.3 0.95 2.9 0.050 1.69 4.0 1.86 2.5 1.35 4.5 1.55 3.4 0.85 3.8 0.98 3.2 0.100 1.72 4.9 1.89 2.8 1.38 5.1 1.58 3.6 0.88 4.4 1.01 3.5 0.200 1.75 6.0 1.92 3.1 1.41 6.0 1.61 3.9 0.91 5.2 1.04 3.9

Table 5. The effect of scan rate on peak potential and peak current; [M-MAH] = 1×10–3M; Medium = Aqueous dimethyl formamide (40 % v/v).


565

• The shift of Epc value towards more negative potential [38].

Ni(II)-MAH complexThe results obtained are presented in Table 6. The

results indicate that the complex leads to a single ca-thodic peak at low scan rates and two cathodic peaks along with one anodic peak at high scan rates. The

electrode process was found to be diffusion controlled in nature. An inspection of the cyclic voltammograms as well as the polarograms of the complex in solutions of pH 4.1 and 8.1 indicates that the electrode process involves two electrons.

Cyclicvoltammetric results showed two cathodic peaks at high scan rates. The first cathodic peak may be due to the one electron reduction of Ni(II)-MAH com-plex to the Ni(I)-MAH. The second peak may be due to the one electron reduction of Ni(I)-MAH complex. The peak currents increase with increase in scan rate. The peak current values were almost same for each of the two peaks indicating that the reduction involves one electron in each step. An anodic peak was noticed in reverse scan at high scan rates. The fact that the an-odic peak was obtained by the application of potential below the peak potential of the second cathodic peak confirms that the anodic peak was due to the oxidation of Ni(I)-MAH to Ni(II)-MAH. The difference between

the peak potentials (DEp ) in the present investigation was found to be 60 mV. Further it was noticed that the ratio of cathodic to anodic peak currents (ipcI / ipa) was almost equal to one. Hence, the electrode process may be identified as quasi reversible.

Cu(II)-MAH complexThe results pertaining to cyclic voltammetric behav-

iour of Cu(II)-MAH complex are presented in Table 6. Cyclic voltammograms obtained at pH 4.1 and 8.1 show a single cathodic peak in solutions of pH 4.1 and two cathodic peaks along and an anodic peak in reverse scan in solutions of pH 8.1 at high scan rates. In contrast only a single polarographic wave was observed at pH 4.1 and 8.1 in polarographic studies. An inspection of the cyclic voltammograms as well as the polarograms of the complex in solutions of pH 4.1 suggests that the po-larographic reduction wave manifests itself as a cathodic peak in cyclic voltammetric studies. This was attributed to the reduction of Cu(II)-MAH complex to the metal.

The two cathodic peaks observed in solutions of pH 8.1 at high scan rates suggest that the reduction was taking place in two steps. The first cathodic peak was attributed to the one electron reduction of Cu(II)-MAH complex to the Cu(I)-MAH and the second cathodic peak was due to the reduction of Cu(I)-MAH to the metal. The peak current values were almost same. The reduction process involves two steps, each involving one electron. An anodic peak was noticed in the reverse scan at high

Fig. 3. Plot of ipc versus υ1/2 for M-MAH; [M-MAH] = 1×10–3M; Medium = Aqueous dimethyl formamide (40 % v/v); I and II indicate the first and the second cathodic peak, respectively.


566

scan rates. This behaviour was similar to that observed in the case of Ni(II)-MAH complex at pH 8.1.

A value of 0.07 V for DEp and a value one for the ratio ipc / ipa suggest that the electrode process was quasi reversible.

Fe(III)-MAH complexThe results pertaining to Fe(III)-MAH complex are

presented in Table 6.In solutions of pH 4.1, the complex produces a

single cathodic peak and an anodic peak in the reverse scan. It was observed that the anodic peak potential was approximately equal (difference was 10 mV) in mag-nitude to that of cathodic peak potential. The cathodic peak obtained was due to the reduction of Fe(III)-MAH complex to Fe(II)-MAH and the anodic peak formation was due to the oxidation of Fe(II)-MAH to Fe(III)-MAH complex. A ratio of cathodic to anodic peak currents i.e., ipcI/ipa was nearly one. These arguments suggest that the electrode process was quasi reversible in nature. The fact that the Fe(II)-MAH complex formed was not undergoing further reduction was confirmed by carrying out the cyclicvoltammetric experiment with Fe(II)-MAH complex. In this case, it was found that the complex did not undergo reduction, perhaps Fe(II) complex was stabilized and was in electro-inactive form.

In solutions of pH 8.1, two cathodic peaks were observed. The peak potentials in general increase with increase in scan rate. The facts that, cathode peak po-

tentials were independent of scan rate, Epc/2 – Epc values being greater than 60 mV

nmV and the peak potentials were

shifted to more negative values suggest that the electrode process was irreversible. On the other hand, increase in peak current values with increase in concentration of the depolarizer and the linear plots of ipc vs n1/2 have confirmed the diffusion controlled nature of the process. The magnitude of ipcII was almost twice that of ipcI. This suggests that the first cathodic peak was due to the re-duction of Fe(III)-MAH to Fe(II)-MAH and second to the reduction of Fe(II)-MAH to metal.

Based on the above mentioned facts, the reduction processes in different media are given below. pH 4.1: Fe(III)-MAH 1e Fe(II)-MAHpH 8.1Step 1: Fe(III)-MAH 1e Fe(II)-MAHStep 2: Fe(II)-MAH Fe(0)2e

Co(III)-MAH complexThe results pertaining to Co(III)-MAH complex are

presented in Table 6.The explanation for the diffusion controlled nature

of the electrode process was same as that for other complexes.

In solutions of pH 4.1, a single cathodic peak was observed at all scan rates. The peak potential and peak currents increase with the scan rate. Irreversible nature of the process was confirmed by the absence of anodic

Scan rate Vs-1

pH 4.1 pH 8.1 -EPC I

V -EPC II

V -EPa V

-E1/2 V

∆E mV

iPC I cm

iPC II cm

iPa cm

-EPC I V

-EPC II V

-EPa V

-E1/2 V

∆E mV

iPC I cm

iPC II cm

iPa cm

Ni(II)-MAH 0.010 -- 0.84 -- -- -- -- 2.4 -- -- 0.90 -- -- -- -- 2.1 -- 0.020 -- 0.84 -- -- -- -- 3.0 -- -- 0.90 -- -- -- -- 2.5 -- 0.050 0.66 0.87 0.72 0.69 60 3.6 4.0 3.4 0.72 0.93 0.78 0.75 60 1.8 3.3 1.7 0.100 0.69 0.90 0.75 0.72 60 5.2 5.4 2.3 0.75 0.96 0.81 0.81 60 2.2 4.3 1.9 0.200 0.72 0.93 0.78 0.75 60 6.9 7.1 2.9 0.78 0.99 0.84 0.84 60 2.6 5.6 2.3

Cu(II)-MAH 0.010 0.24 -- -- -- -- 2.6 -- -- -- 0.42 -- -- -- -- 2.2 -- 0.020 0.24 -- -- -- -- 2.8 -- -- -- 0.42 -- -- -- -- 2.3 -- 0.050 0.27 -- -- -- -- 3.3 -- -- 0.30 0.45 0.40 0.37 70 2.3 2.5 1.8 0.100 0.30 -- -- -- -- 3.9 -- -- 0.36 0.48 0.43 0.40 70 2.9 2.6 2.1 0.200 0.30 -- -- -- -- 4.6 -- -- 0.39 0.48 0.46 0.43 70 3.1 2.9 2.6

Fe(III)-MAH 0.010 0.39 -- 0.38 0.36 10 1.6 -- 1.5 1.30 1.80 -- -- -- 1.4 2.8 -- 0.020 0.39 -- 0.38 0.39 10 1.8 -- 1.7 1.30 1.80 -- -- -- 1.5 2.9 -- 0.050 0.42 -- 0.43 0.43 10 2.3 -- 2.1 1.33 1.83 -- -- -- 1.8 3.2 -- 0.100 0.45 -- 0.46 0.46 10 2.9 -- 2.6 1.36 1.86 -- -- -- 2.2 3.4 -- 0.200 0.48 -- 0.49 0.49 10 3.7 -- 3.2 1.39 1.89 -- -- -- 2.7 3.8 --

Co(III)-MAH 0.010 0.45 -- -- -- -- 1.4 -- -- 0.60 1.50 -- -- -- 1.0 1.9 -- 0.020 0.45 -- -- -- -- 2.0 -- -- 0.60 1.50 -- -- -- 1.3 2.2 -- 0.050 0.48 -- -- -- -- 3.1 -- -- 0.63 1.53 -- -- -- 2.0 3.0 -- 0.100 0.51 -- -- -- -- 4.7 -- -- 0.66 1.56 -- -- -- 3.0 3.8 -- 0.200 0.54 -- -- -- -- 6.5 -- -- 0.69 1.59 -- -- -- 4.2 5.0 --

Table 6. The effect of scan rate on peak potential and peak current; [M-MAH] = 1×10–3M; Medium = Aqueous dimethyl formamide (40 % v/v).


567

peak in the reverse scan. Hence the reduction process in this medium is given below.

Co(III)-MAH1e Co(II)-MAH

In solutions of pH 8.1, the behaviour of Co(III)-MAH complex was similar to that reported for Fe(III)-MAH complex. Hence the reduction process in this medium may be represented as follows.

Step 1: Co(III)-MAH 1e Co(II)-MAHStep 2: Co(II)-MAH Co(0)2e

CONCLUSIONS

The electrochemical investigations of series of metal (Mn(II), Ni(II), Cu(II), Zn(II), Cd(II), Fe(III) and Co(III)) complexes of MAH were reported. The results obtained in polarography were compared with those obtained in cyclicvoltammetric studies. The effect of height of the mercury column, concentration of the complex and temperature, etc. on electrode process were explored in detail and presented. The redox properties of complexes were extensively investigated by polarogra-phy and cyclic voltammetry. Mn(II), Zn(II), Cd(II) and Co(III) complexes exhibited diffusion controlled and ir-reversible electrode process, where as Fe(III), Cu(II) and Ni(II) complexes have undergone diffusion controlled and quasi reversible electrode process.

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Desislava Staneva, Ivo Grabchev, Pavlina Mokreva

569


ELECTRONIC AND INFRARED SPECTRAL STUDIES ON THE POLY(PROPYLENEAMINE) DENDRIMERS PERIPHERALLY

MODIFIED WITH 1,8-NAPHTHALIMIDES

Desislava Staneva1, Ivo Grabchev2,3, Pavlina Mokreva4

1 University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria2 University of Sofia “St. Kliment Ohridski”, Faculty of Medicine, 1, Koziak Str, 1407 Sofia, Bulgaria3 Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia4 Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

ABSTRACT

In this article we report the results of a study of infrared, absorption and fluorescence spectra of eight novel poly(propyleneamine) dendrimers from second generation, modified with 1,8-naphthalimide units. Special attention has been paid to the investigation of the influence of the nature of the substituents at C-4 position in the 1,8-naphthalimides and thus on the polarization of the dendrimer molecule as a whole. The influence of the dendrimer polarization on the spectral characteristics are discussed.

Keywords: poly(propyleneamine), FT-IR spectroscopy, absorbance, fluorescence, photoactive dendrimers, 1,8-naph-thalimide.

Received 28 May 2014Accepted 05 September 2014

INTRODUCTION

Over the last years dendrimers have been investi-gated intensively as a new polymer architecture [1 - 3]. They are hyperbranched, monodisperse, well defined three-dimensional macromolecules, possessing a very high concentration of different terminal functional groups in their periphery.

The studies on fluorescent dendrimers are focused generally on their synthesis and applications in different fields such as chemistry, biology, medicine and physics. Functionalizing the dendrimers with photoactive groups expands the spheres of their applications. When the periphery is modified, the dendrimer comprises a great

number of closely located fluorophores which could be independent from each other or can interact. In the lat-ter case the dendrimers acquire new properties defined as the “dendrimer effect”. These new compounds have turned out to be meeting appropriately the requirements of “high-technologies”: sensors for environment pollut-ants, optoelectronics, light-harvesting antenna systems for solar energy conversion, as well as those of biology and medicine [4 - 10].

The peripheral modification of PAMAM dendrimers from zero and second generation with 1,8-naphthalimide units has shown that they can be used as sensors for detection of metal ions and protons. It was found that the sensor activity depends strongly of the chemical


570

structure of the substituents at the C-4 position of the naphthalene nucleus (11 - 16).

Poly(propyleneamine) (PPA) dendrimers, as a new class of commercial dendrimers, have been attracting an increasing amount of interest as a result of the different application, envisaged for them [1]. These dendrimers comprise only tertiary amino groups in their core and primary amino groups in the periphery in contrast to the PAMAM dendrimers, where they have also amidic groups. The luminescent characteristics of modified PPA dendrimers can be easily customized by modify-ing the periphery with different types of fluorophores. Fluorescent dendrimers having 1,8-naphthalimide as fluorescent units can be interpreted as a combination of conventional dendrimers and low molecular mass fluorophores [17 - 23].

The aim of this work was to study the influence of the substituents in 1,8-naphthalimide structure on the spectral characteristics of newly synthesized PPA dendrimers by infrared, absorption and fluorescence spectroscopy. The effect that the substituents at C-4 position of the naphthalene ring have upon the main spectral characteristics has been investigated and dis-cussed. These studies are conducted in order to better and complete characterization of the dendrimers.

EXPERIMENTAL

Materials and methodsThe PPA dendrimers from the second generation,

modified with 1,8-naphthalimide derivatives have the structures presented in Scheme 1. Their synthesis and main photophysical characteristic were described ear-lier [17, 20, 21,13]. Infrared analysis of all 1,8-naph-thalimide labeled dendrimers was carried out using an Infrared Fourier transform spectrometer (IRAffinity-1”Shimadzu”) with the diffuse-reflectance attachment (MIRacle Attenuated Total Reflectance Attachment) at a 2 cm-1 resolution. UV/vis spectrophotometric inves-tigations were performed using “Thermo Spectronic Unicam UV 500” spectrophotometer. The fluorescence spectra were taken on a “Cary Eclipse” spectrophotom-eter. All spectra were recorded using 1 cm path length synthetic quartz glass cells. The fluorescence quantum yield was determined on the basis of the absorption and fluorescence spectra, using anthracene Φst = 0.27 and Fluorescein Φst = 0.85 [24].


1,8-Naphthalimides can be treated as a combination

N N

N

N

N

N

N

N

N

N

N

N

N

N

OO

O

O

O

OO

OOO

O

O

O

O

O

O

A

A

A

AA

A

A

A

Scheme 1. Chemical structure of PPA dendrimer from the second generation, modified whit 1,8-naphthalimide in the periphery (A = H (D1); Br (D2); NO2 (D3); HNCH3 (D4); N(CH3)2 (D5); NHCH2CH2N(CH3)2 (D6); NHCH2CH2CH3 (D7); N-methylpiperasine (D8).


571

of two sub-systems; viz. the naphthalene ring and a di-carboximide (-CO-NR-CO-) group in a six membered ring. Their photophysical properties depend mainly on the polarization of the 1,8-naphthalimide molecule. The polarization occurs upon irradiation, resulting from the donor-acceptor interaction between the substituents A at C-4 position, and the carbonyl groups (C=O) from the imide structure of the chromophoric system. Therefore, in the practical application of the dendrim-ers synthesized by us, it is important to know the effect of this substituent on their spectral characteristics. It is well known that only amino substituents in position C-4 give yellow green fluorescence. Thus, the presence of 4-aminosubstituted 1,8-naphthalimides bonded to the commercial PPA dendrimers gives them some new interesting photophysical properties.

Table 1. Experimental infrared wave numbers of D1- D8 dendrimers in cm-1.

Demdrimer nNH nC-H arom

nCH2 aliph

nASC=O nS

C=O nC=C arom

nCH2 aliph

nCNC

dC-H arom

D1 3068 2958 2874

1696 1654 1624 1587

1437 1342 1232 1174

874 777

D2 3066 2951 2808

1697 1655 1616 1570 1458

1435 1342 1231 1188

848 779 748

D3 3078 2947 2808

1706 1660 1624 1452

1439 1330 1239 1188

833 783 760

D4 3356 3078 2951 2839

1678 1637 1614 1573 1456

1435 1354 1246 1189

820 771 756

D5 3068 2953 2870

1684 1643 1614 1576 1450

1425 1354 1242 1207

835 777 758

D6 3360 3063 2958 2928 2832

1674 1636 1612 1573 1458

1431 1192 1242 1350

822 783 756

D7 3369 3076 2957

2932 2871

1679 1638 1613 1579 1460

1431 1191 1243 1354

775 758

D8 3073 2955 282

1684 1640 1611 1578

1432 1189 1241 1397

785 755

1700 1600 1500 1400 1300 1200 110040

50

60

70

80

90

100

T / %

Wavenumber / cm-1

D4 D3

Fig. 1. Infrared spectra of D3 and D4 dendrimers.


572

Infrared spectral characteristics Stretching and deformation vibrations of the main

functions in the infrared region of the spectra of PPA dendrimers from second generation modified with dif-ferent substituted 1,8-naphthalimides D1-D8 are sum-marised in Table 1.

Fig. 1 shows the infrared spectra of D3 and D4 dendrimers in the 1500 - 1750 cm-1 region. The C=O group of 1,8-naphthalmides gives rise to two absorp-tion frequency bands, which are characteristic for the vibrations caused by the asymmetrical and the sym-metrical carbonyl groups from the 1,8-naphthalimide chromophoric system [25]. In this case the high fre-quency C=O stretching vibration is assigned to the asymmetrical mode. These bands for dendrimer D3 with a substituted nitro group are at 1706 cm-1 and at 1660 cm-1. That is due to the electron-accepting nature of the nitro group. After the substitution of the nitro group with donating amino group, the characteristic bands for the 4-methylamino-1,8-naphthalimide-PPA dendrimer (D4) are shifted to the lower frequency region at 1678 cm-1 and 1637 cm-1, respectively. The difference between the symmetric and asymmetric vibrations assigned to the 1,8-naphthalimides, is 38 - 46 cm-1 (structures 1 and 2 in Scheme 2).

In the infrared spectra of 4-nitro-1,8-naphthalimide–PPA dendrimer D3, containing a nitro (NO2) group, the band at 1523 cm-1 is characteristic for the asymmetrical vibrations of the nitro group.

The position of the absorption bands, characteristic for the C=O groups, depends on the nature of substitu-

ents at C-4 position. The spectra of the dendrimers with the donating groups (D4 - D8) have absorption bands shifted hipsocromicaly, if compared to those of the dendrimer with an electron accepting group.

Fig. 2 shows that for the dependence of the two C=O frequency bands of dendrimers D1 - D5 on the Hammett constants (s), a linear correlation is obtained. The analyti-cal form of the dependence is given by Eqs. (1) and (2):

nS (cm-1) = 1651 + 14.2 s (1) nAS (cm-1) = 1693 + 16.9 s (2)

which were derived using the lest squares method with a correlation coefficient R=0.974 (1) and R=0.996 (2), SD = 1.28 -2.46, and N=5 for both C=O groups.

The aromatic naphthalene ring from the 1,8-naph-thalimide units is responsible for the absorptions due to the sp2 C-H stretching vibrations at 3063 - 3078 cm-1.

The characteristic C-H out of plane deformation vibrations of the aromatic naphthalene rings are in the 748 - 783 cm-1 region. As seen from Fig. 3 D1 gives rise to a single peak at 777 cm-1 in the spectra of the

Scheme 2

AAA

__+ +N

OON

OON

OO

R R R

1 2

Fig. 2. Relationship between nSC=O and nASC=O stretch-ing vibrations and Hammett substituent constants s for dendrimers D1-D5.

-0,8 -0,4 0,0 0,4 0,81620

1640

1660

1680

1700

1720

D5

D5

nSC=O

nASC=O

D4

D4D3

D3D2

D2D1

D1

Wav

enum

ber /

cm

-1

sFig. 3. Deformation vibrations (d) of the aromatic C-H bends of dendrimers.

850 800 750 700 65050

60

70

80

90

dC-H (arom)

dC-H (arom)

T / %

Wavenumber / cm-1

D5 D4 D3 D1


573

naphthalene rings non-substituted at C-4 position. In the other cases, dendrimers with substitutents at C-4 have a doublet of peaks in the 771 - 783 cm-1 and in 748 - 758 cm-1 region, which can be explained by the different polarization of 1,8-napthalimide molecules. Small peaks for aromatic C-H vibrations have been also obtained in the 820 - 874 cm-1 region.

All PPA dendrimers under study have aliphatic methylene (-CH2-) groups in their core. The absorption in the 2808 - 2870 cm-1 region for the asymmetric and in the 2951 - 2958 cm-1 region for the symmetric stretching vibrations indicates the presence of a hydrogen atom bonded to a sp3 hybridized carbon atom. Dendrimers D4 - D8 have methyl group in their structure with ab-sorption at 1382 cm-1, characteristic for a sp3 hybridized C-H bond.

Dendrimers D4, D6 and D7 with secondary amino groups at C-4 position of the 1,8-naphthalimide struc-ture, have peaks in the 3356 - 3363 cm-1 and 1300 - 1303 cm-1 regions. Tertiary amines from the entire dendrimer core and 1,8-naphthalimide structure (D5 and D8) do not show bands in these regions.

The absorption bands in the 1330 - 1354 cm-1 region are characteristic for the imide (C-N-C) bonds in the 1,8-naphthalimde structure, bonded to the dendrimer molecules and to C-N from aliphatic tertiary amino groups in the dendrimer structure. In the case of den-drimers D3 these bands are overlapped with the bands characteristic for the symmetrical vibrations of the nitro group. The absorption bands are recorded as a shoulder. The infrared frequencies at 1174 - 1207 cm-1 and at 1231 - 1246 cm-1 are also assigned as C-N bonds.

Spectral characteristics in dichloroethane solu-tion

In Table 2 are collected the basic photophysical characteristics of the dendrimers under study in a dichlo-

roethane solution: the absorption (lA) and fluorescence (lF) maxima, the extinction coefficient (e), Stokes shift (nA - nF), and the quantum yield of fluorescence (FF).

From the data in Table 2 it follows that the nature of the substituent at C-4 position has an important role in the dendrimer photophysical properties. When the sub-stituent A is a hydrogen, bromine atoms and nitro groups, the respective dendrimers absorb in the UV region and emit very weak fluorescence (D1 and D2). The longwave absorption band in the UV region is a charge transfer band due to electron transfer in the transition S1 → S0.

In dichloroethane solution dendrimers D4 - D8 ex-hibit yellow-green colour with absorption maxima at lA= 402 - 430 nm and fluorescence with maxima situated at lF = 500 - 512 nm. The hypsochromic shift of the ab-sorption and fluorescence maxima in the case of D4 and D5 can be explain by the steric interaction between one of the methyl groups of the 4-dimethylamino substituent at C-4 position and the hydrogen atom at the C-5 position (the peri effect) [23]. For the 4-methylamino substituent in the same position at D4, this effect is negligible. Also the absorption and fluorescence spectra of D4 and D6 are very similar which is due to approximately the same donor ability to the secondary amino groups.

The molar extinction coefficient for dendrimers D1 - D8 is approximately eight fold higher than that of the monomeric 1,8-naphthalimide derivatives with the same substituents at C-4 position [26 - 28]. This allows for the suggestion that no ground state interaction occurs between the 1,8-naphthalimide chromophoric units [29].

The Stokes shift indicates the differences in the properties and structure of the dendrimers in the ground state S0 and the first exited state S1. The Stokes shift can be calculated by the equation (3).(nA - nF) = (1/lA - 1/lF) x 10-7 cm-1 (3)

The Stokes shift values are in the 3529-5344 cm-1

Dendrimer lA nm

e l g-1 cm-1

lF nm

(nA-nF) cm-1

FF

D1 338 94200 385 3611 0.001 D2 342 91000 401 4302 0.004 D3 330 92100 - - - D4 430 99500 511 3686 0.590 D5 418 96000 509 4277 0.146 D6 430 101000 506 3493 0.510 D7 427 89000 500 3529 0.840 D8 400 83000 512 5344 0.490

Table 2. Photophysical characteristics of dendrimers D1 - D8 in dichloroethane solution.


574

region which is in accordance with other investigations on similar 1,8-naphthalimide derivatives [26 - 28, 30]. This parameter also depends on the type of substitution in the C-4 position. Perhaps the decisive factor here is the possibility of conformational changes.

The fluorescence efficiency of the dendrimers has been estimated by measuring their quantum yield FF on the basis of the absorption and fluorescence spectra by equation (4).

2

2u st Du

F stst u Dst

S A nS A n

F = F (4)

where the ΦF is the emission quantum yield of the sam-ple, Φst is the emission quantum yield of the standard, Ast and Au represent the absorbance of the standard and the sample at the excited wavelength, respectively, while Sst and Su are the integrated emission band areas of the standard and the sample respectively, and nDst and nDu are the solvent refractive index of the standard and sample, u and st refer the unknown and standard, respectively.

As seen from the data in Table 2, the dendrimers have quantum yield in the large region FF = 0.001-0.840. Blue emitting dendrimers (D1and D2) have smaller FF with comparison to that of the green emitting dendrim-ers, due to the absence of electron donating substituents at C-4 position of the 1,8-naphthalimide units. In the case of the D4 and D5 the FF of D5 is lower, because of possible conformation changes of the 1,8-naphthalimide chromophoric system in the exited state. In dichloro-ethane, dendrimers D6 - D8 emit strong fluorescence with high FF which is due to the specific nature of the substituents in the 1,8-naphthalimide structure where the photoinduced electron transfer dominates [31, 32]. Dendrimer D3 does not emit fluorescence, which is due to the electron accepting nature of the nitro group and the respective influence of this group on the polarization of the 1,8-naphthalimide structure.

CONCLUSIONS

Eight newly synthesized PPA dendrimers from sec-ond generation, modified with 4-substituted-1,8-napthal-imide fluorophores in the dendrimer periphery, have been studied by infrared, absorption and fluorescence spectroscopy. It has been shown that the substitutents at C-4 position of the 1,8-naphthalimide have an impact upon the frequencies attributed to the carbonyl groups

from the 1,8-naphthalimide fragments incorporated into the PPA dendrimer molecule, while the substituent at the imide nitrogen atom affects slightly the polarization. The electron donor or electron acceptor ability of the substituents also plays an important role in the dendrimer photophysical properties and especially - in the quantum fluorescence yield.

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Polya Miladinova, Nikolai Georgiev

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SENSOR ACTIVITY AND LOGIC BEHAVIOUR OF SOME2-AMINOTEREPHTHALIC DERIVATIVES



ABSTRACT

Herein we report on the sensor activity of three blue-emitting triazine derivatives of 2-aminoterephthalic acid. The novel compounds are configured as a “fluorophore-receptor” system. The fluorophores designed can act as a pH-probe via an “off-on-off” fluorescence sensing mechanism due to an internal charge transfer. The sensor activity towards cations and anions in water/DMF (1:1, v/v) media is studied by monitoring the fluorescence intensity changes. The ions tested exert a high quenching effect suggesting that the compounds are sensitive to strong acid (pH<3) and strong alkaline media (pH>8). The logic function XNOR can be performed using an acid and a base as chemical inputs, while a digital comparator can be designed on the ground of pH induced changes in the spectral properties of the novel molecule.

Keywords: 2-aminoterephthalic derivatives, ICT (intramolecular charge transfer), pH molecular sensors, molecular logic gates.


INTRODUCTION

The rapid grow of nanotechnology extends the concept of a macroscopic device to a molecular level and which is why the design and synthesis of (supra)-molecular species capable of mimicking the functions of macroscopic devices are currently of great interest [1 - 3]. Molecular devices operate via electronic and/or nu-clear rearrangements and just like macroscopic devices require energy, provided as chemical, electrical energy, or light for their elements’ operation and communication. The luminescence is one of the most useful techniques to monitor the operation of molecular-level devices [4 - 6]. This determines the interest towards synthesis of novel fluorescence compounds as a considerable and inseparable part of nanoscience development.

Further semiconductors miniaturization reaches its limit. Therefore, the design and construction of molecu-lar systems capable of performing complex logic func-tions is of great scientific interest now [7 - 9]. The logic

gates in semiconductor devices work using binary logic, where the signals are encoded as 0 and 1 (low and high current). This process is executed on a molecular level using different procedures, but the most common are based on the optical properties of the molecule switches. They encode the low and high concentrations of the input guest molecules and the output fluorescent intensities by the binary 0 and 1, respectively [10 - 12]. The first proposal to execute logic operations at a molecular level was made in 1988, but the field developed only five years later, when the analogy between molecular switches and logic gates was experimentally demonstrated by de Silva [13]. The field has recently extended from simple switches to more complex molecular systems which can perform a variety of classical logic functions [14 - 16] and act as a half-adder [16-18], a full-adder [19, 20], a keypad lock [21, 22], a half-subtractor [23, 24], a full-subtractor [19, 20], an encoder-decoder [25, 26], a digital comparator [27, 28]. The fluorescence switching development is attractive to analytical chemistry as well


578

due to the increasing requirements in connection with the environment pollution and the further development of the diagnostic medicine and biology [29-31].

The fluorescence molecular sensors are designed at present using three basic approaches based on intra-molecular charge transfer (ICT), photoinduced electron transfer end energy transfer [32 - 33]. The receptor in the ICT chemosensors is directly attached to the electron-donating/withdrawing unit that is conjugated to the fluorophore electron-withdrawing/electron-donating unit. During system’s excitation the fluorophore un-dergoes donor-acceptor intramolecular charge transfer. The subsequent change in the dipole moment results in a Stokes shift that depends on the microenvironment of the fluorophore. Changes in quantum yields and lifetimes are often observed [34, 35] in addition to these shifts.

The derivatives of 2,5-diaminoterephthalate are organic chromophores that being in solid state exhibit fluorescence of high efficiency in the visible region. They attract increasing attention in the field of functional materials and optoelectronic devices such as organic light-emitting diodes (OLEDs), light-emitting field-effect transistors, semiconductor lasers and fluorescent solid sensors [36]. 2-aminodimethylterephthalates are organic chromophores that exhibit fluorescence in the visible region too. They find application in textiles and polymers production [37 - 39], but there is no informa-tion on their possible use chemosensors. With this in mind, the study of their sensor activity is of interest.

We have recently synthesized six blue-emitting tri-azine derivatives of 2-aminoterephthalic acid [38]. Herein we report on the ability of three of them (Scheme 1) to serve as pH chemosensing materials and molecular logic gates.

EXPERIMENTAL

Materials and methodsA pH meter Metrohm 704 (www. metrohm.com)

coupled with a combined pH electrode was used for the pH measurements. Commercial standard buffers for pH 2, 7 and 10 (Aldrich) were used for calibration. The absorption spectra were obtained using Hewlett Packard 8452A spectrophotometer (www.gclctoronto.com) against water or the buffers described above. The fluorescent spectra were recorded on a Scinco FS-2 fluorescence spectrophotometer (www.scinco.com). The excitation source was a 150 W Xenon lamp. Excitation and emission slits width was 5 nm. The fluorescence measurement was carried out in right angle sample geometry. A 1×1 cm quartz cuvette was used for the spectroscopic analysis. The quantum yields of fluores-cence were calculated using 9,10-Diphenylanthracene (ФF = 0.95 in ethanol) [40] as a standards according to Eq. (1) [41]:

2sample sampleref

F ref 2ref sample ref

S nA

S A nΦ = Φ

(1)

where Aref, Sref, nref and Asample Ssample, nsample stand for the absorbance at the excitation wavelength, the integrated emission band area and the solvent refractive index of the standard and the sample, respectively.

The effect of pH on the absorption and fluorescence properties of compounds 1-3 were studied by multiple addition of amounts of 0.1 M NaOH or 0.1 M HCl to 250 mL of 1x10-5 M of the respective compound. The solution pH as well as the absorption and the fluores-

Scheme 1.


579

cence spectra were recorded following each addition of NaOH or HCl solution. The fluorescence intensity of each mixture was measured at 470 nm. The spectral data were collected using FluoroMaster Plus 1.3 and processed further by OrginPro 6.1 software.

All experiments were performed at room tempera-ture.


The basic spectral characteristics of 2-aminotere-phthalic derivatives depend on the terephthalic mol-ecule polarization due to the electron donor-acceptor interaction occurring between the amino substituent and the carbonyl groups from the chromophoric sys-tem. Thus it can be predicted that the interaction of a guest with the donor or acceptor moiety will change the photophysical properties of the fluorophore. We chose compounds 1-3 because of their remarkable structure containing both amino and amido fragments in their chromophoric system. The amide fragments are widely used functional groups in anion recognition because the acidity of the NH group can be easily tuned by adjusting the electronic properties of neighboring substituents so that it can recognize anions through hydrogen-bonding and deprotonation interactions [42, 43]. Additionally, the 2-amino moiety in the terephthalic fluorophore is a strong electron donating group and it can serve as a cation receptor fragment [44, 45]. Hence compounds 1-3 are expected to detect cations as well as anions. When a cation interacts with the terephthalic fluorophore amino group, the latter reduces its electron-donating character.

Scheme 2

Fig. 1. Absorption spectra of sensor 3 in water/DMF (1:1, v/v) solution at different pH values.

Compound

ФF

pKa

pH 2 pH 7 pH 10 1 0.006 0.011 0.003 2.6

9.9

2 0.012 0.021 0.009 2.5 10.1

3 0.032 0.090 0.021 2.5 10.0

Table 1. Quantum yields and pKa values of compounds 1, 2 and 3.


580

Because of the resulting reduction in polarization, a blue shift of the absorption spectrum is expected, together with a decrease in the quantum yield of fluorescence (Scheme 2). The anion effect is similar. In case of an anion interaction with the terephthalic fluorophore amides, the carbonyl electron-accepting ability decreases due to the amides deprotonation which generates a strong electron density near the carbonyl group. This results in decrease of the ICT efficiency in the 2-aminoterephthalic fluorophores, which in turn leads to blue shifting and lower quantum yield.

This was the reason to investigate the photophysical behaviour of compounds 1-3 in water/DMF (1:1, v/v) solution at different pH values (in presence of protons as cations and a hydroxide as anions). Compounds 1-3

Fig. 2. Fluorescence spectra of sensor 3 in water/DMF (1:1, v/v) solution at different pH values: (A) pH= 2-3.5 and (B) pH= 3.5-13.7.

Fig. 3. Fluorescence intensity of compound 3 at 510 nm in water/DMF (1:1, v/v) as a function of pH.

show the longest-wavelength absorption band in the range of 280 nm - 430 nm in the media pointed above. It is attributed to the typical 2-aminoterephthalic ICT process. The pH effect on the absorption properties of the compounds examined is negligible in a wide pH range. This is illustrated by the absorption spectra of compound 3 at pH 2, pH 7 and pH 10 presented in Fig. 1.

The emission spectra of the compounds under study (1-3) do not show significant pH-dependent changes in the pH window of 3.5-8, since 2-aminoterephthalic fluorophore does not affect the ICT excited state. The cal-culated quantum yields of fluorescence are in the range between 0.011-0.09 (Table 1, pH 7). These values are very low and can be explained taking into consideration that water is an effective fluorescent quencher. Besides, the high hydrophobicity of the compounds examined results a self-quenching effect in aqueous media due to fluorophore aggregation.

pH decrease from 3.5 to 2 results in a blue shifting (ΔλA = 4 nm) and a quenching effect of the fluorescence of probes 1-3 (Fig. 2A) because of protonation of 2-ami-noterephthalic nitrogen, which in turn decreases the ICT efficiency. Fluorescence quenching (FQ) determined as the ratio of the maximum fluorescence intensity Io at pH 3.5 and the minimum fluorescence intensity I at pH 2 is found equal to 1.52, 1.58 and 1.44 for compounds 1, 2 and 3, correspondingly.

At high pH values (pH > 8) the amido groups are de-protonated and an anion is formed. The latter decreases the electron deficiency at the adjacent carbonyl groups which in turn decreases the ICT efficiency. This is related


581

to the lower electron accepting ability of terephthalic carbonyls. As a result, a hypsochromic shift (ΔλA = 13 nm) and fluorescence quenching effect are observed (Fig. 2A). The values of FQ determined as the ratio of the maximum fluorescence intensity Io at pH 8 and the minimum fluorescence intensity I at pH 14 are found equal to 10.88, 11.94 and 11.24 for compounds 1, 2 and 3, correspondingly.

In general, bell-shaped pH titration curves reflecting the “off-on-off” fluorescence response of compounds 1-3 are obtained (Fig. 3). The analysis of the fluorescence changes as a function of pH according to Eq. (2) [47] gives two pKa values for each compound, for the proto-nated and the deprotonated form, respectively.

log [(IFmax – IF) / (IF – IFmin)] = pH - pKa (2)

The significant pH effect on the emission spectra of sensors 1-3 provides to conclude that they can execute logic operation upon addition of H+ and OH- as inputs. The fluorescence of sensors 1, 2 and 3 monitored at 470 nm in the presence of an acid and a hydroxide shows that a XNOR molecular logic gate (Table 2) can be constructed starting off a neutral solution. It is seen that the emission output is low (coded for binary 0) with the input either of an acid or a base (Fig. 4). The simultane-ous input of an acid and a base annihilates each other’s effect and generates high emission (coded for binary 1)

at 470 nm mimicking thus the XNOR logic function.The XNOR logic gate is actually a comparator as

the output is 1 only when both inputs are of an identical value playing thus the role of the mathematical sym-bol “equal”. XNOR cannot however say which input is higher than the other. An additional output value serving as the mathematical symbol “greater than” or “lower than” is required to construct a magnitude digital comparator. In fact the compounds examined are able to act as a magnitude digital comparator in a three-valued logic output mode (the output has three values “low”, “medium” and “high”) due to the different quenching effect of the cations and anions present. For example, the quantum yield of fluorescence of compound 3 in a neutral solution is 0.09 and can be coded as “high”. In presence of NaOH the quantum yield decreases to 0.021

Fig. 4. Fluorescence spectra of compound 3 in presence of a chemical input of 20 µL of 0.1 mol L-1 HCl (Input 1) and of 20 µL of 0.1 mol L-1 NaOH (Input 2) to water/DMF (1:1, v/v).

Input

H +

Input

OH -

Output Fl470

XNOR

Qf

H+ = OH- 0 0 1 0.090 (High)

H+ < OH- 0 1 0 0.021 (Low)

H+ > OH- 1 0 0 0.032 (Medium)

H+ = OH- 1 1 1 0.090 (High)

Table 2. The truth table for the logic behaviour of com-pound 3.


582

to be referred as a “low value”, while the addition of HCl decreases the quantum yield to 0.032 corresponding to a “medium” value, i.e. a value between 0.021 (the “low value”) and 0.09 (the “high value”). When both inputs are equal (H+ = OH-), the output (Fl470) is “high”. In case H+ > OH-, the output (Fl470) refers to the “medium value”, while the addition of a base, i.e. when H+ < OH-, the output refers to the “low” value.

The compounds acting as molecular logic gates of chemical inputs are actually complex chemosensors. This feature is revealed in case the sample analyzed responses to a predefined standard. If the fluorescence output of novel sensors 1, 2 and 3 is 1, then the solution tested is in the pH window of 3-8. If the fluorescence output is 0, then the solution tested has a strong acid (pH < 3) or a strong alkaline (pH > 8) reaction. In other words compounds 1, 2 and 3 are sensors with neither fast response that the analyzed sample is nor strong acid and nor strong alkaline.

CONCLUSIONS

Three new fluorescent pH sensors 2-aminotereph-thalic acid derivatives are presented and their photo-physical behavior in water/DMF (1:1, v/v) media as a function of pH is studied. The compounds studied show low fluorescence emission intensity in alkaline and acid media. It is enhanced upon further acidification or alkalization. The system is in “on” state in pH range of 3-8. The effect is suggested to result from the amine receptor’s protonation, when pH < 3 and deprotonation, when pH > 8. The fluorescence changes indicate that sensors 1, 2 and 3 can act as an efficient “off-on-off” switch for pH determination. The sensors’ logic behavior is also examined using fluorescence as output and H+ and OH− as chemical inputs. The logic function XNOR is achieved and the logic circuit of a digital comparator is demonstrated.

Acknowledgements

The authors gratefully acknowledge the financial support from the Bulgarian National Science Fund (Project DDVU-02/97) and Science and Research Pro-gram of the University of Chemical Technology and Metallurgy, Sofia, Bulgaria.

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Rumyana Boeva, Greta Radeva

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COMPENSATION EFFECT IN THE KINETICS OF THERMAL AGING OF SEMI CHEMICAL PULP



ABSTRACT

The effect of thermal ageing of neutral sulfite semi-chemical pulp derived from poplar wood is followed on the ground of the comparative investigation of bleached and unbleached samples. The brightness reversion observed in the course of the process is studied at 90 °C, 105 °C and 120 °C. The degree of brightness is determined at the 6-th, 12-th, 24-th and the 36-th hour of the process duration. The kinetic analysis carried out shows that the exponential kinetic equation valid for processes taking place on uniformly inhomogeneous surfaces describes the kinetics of the process investigated. The linear relation between the apparent pre-exponential factor and the apparent activation energy, as obtained from an Arrhenius analysis, indicates the existence of a compensation effect.

Keywords: semi chemical pulp, thermal aging, bleaching, exponential kinetic equation, compensation effect.

Received 11 June 2014Accepted 05 August 2014

INTRODUCTION

The low-cost raw forest products and the processes providing the use of wood biomass could solve the problem with the scarce natural resources. The high cellulose and the relatively low lignin content make pop-lars well suited for all treatment methods – mechanical, semi-chemical, sulphate and sulphite. Semi-chemical pulping process is a two-stage method. It combines chemical treatment aimed at lignin partial removal and a mechanical refining step which results in defibring. The chemical treatment in case of semi-chemical pulping can be carried out using sodium sulphite, caustic soda and kraft liquor. The main advantages of the neutral sulfite semi-chemical process refer to the relatively low wood quality required, the high yields and the low consumption of chemicals [1-3]. The semi-chemical pulps produced on the ground of hardwoods have highеr strength but relatively low brightness when compared to that obtained from softwoods. Because of the lignin content the unbleached samples of semi-chemical pulps

age faster than the bleached one. The yellowing starts with the oxidation of the lignin phenolic hydroxyl groups, which in turn leads to the subsequent formation of quinones, quinone-methydes and cyclohexadienes [4-7]. The process rate increases with the temperature increase. The effect considered can be partially avoided with the application of a two-stage bleaching method. The latter is often used for combined bleaching. The oxidizing reagents introduced during the first stage affect strongly the chromophore groups in the lignin and the other colouring compounds present. It is worth noting that the chromophores formation is accompanied by aromatic nuclei attack and demethoxylation but it does result in lignin’s degradation. The subsequent treatment with reducing agents during the second stage provides reduction of the residual chromophores or those addi-tionally formed in the course of the oxidation which in turn results in brightness increase. The process is limited to lignin’s transfer, mainly on the fibers’ surface, from an oxidized to a reduced form. Besides, conjugated (double or triple) bonds play an important role in organic sub- play an important role in organic sub-play an important role in organic sub-


586

stances coloration. Heavy metals ions form also coloured complexes with the pulp and hence decrease the bleach-ing effectiveness and increase the bleaching reagent consumption. Their effect is decreased or eliminated by the introduction of complex formation compounds - chelating agents like ethylenediamine tetraacetic acid (EDTA), polysulphates, etc [8-11].

The study of the kinetics of semi-chemical pulp ther-mal ageing is expected to provide data whose treatment may facilitate to elucidate its mechanism and hence to optimize the process taking also into consideration the advantages of the methods of treatment and storage. The problem is that ageing is a complex process which does not obey formal kinetic principles. The rate of changes in the fiber materials depends on the temperature, on the bleaching sequence, on the process duration, as well as on other factors. Some authors describe the degradation reactions brought about by ageing as first or second order reactions [12-16]. But the equations derived so far as-sume that the processes take place on a homogeneous surface. Other authors used the autocatalytic kinetic model to explain the biomass thermal degradation, but the temperatures used were very high and the aim was to determine the content and the mass of each component that was affected by the volatilization or the oxidation process [17].

The temperature effect on most of the catalytic processes is usually described by the temperature de-pendence of the rate constant k in accordance with the Arrhenius equation:

ERTk Ae

−= (1)

where the apparent pre-exponential factor � and the ap-� and the ap- and the ap-parent activation energy � are non-temperature depend- � are non-temperature depend-are non-temperature depend-ent parameters. There are cases where they both change in the course of the process. Their variation depends on the increase of the extent of reaction, a phenomenon often referred to as the compensation effect [18-21]. The increase of � is usually attributed to energetic hindrances, while the variation of � is connected with the entropy factors effect on the rate of the process ex-amined. In some cases, the changes in A and E display a linear dependence according to the Cremer-Constable relation [21-22].

ln A aE b= +

(2)The latter implies that values of ln A plotted against

those of E fall on a straight line with slope a and intercept b. The compensation effect has been known for almost 100 years. It has been studied in a variety of cases in heterogeneous inorganic and organic catalysis [23-31], but no consensus on its nature is reached. One of the possible explanations refers to an enthalpy-entropy relationship.

This work reports experimental data obtained in a comparative kinetic investigation of the effect of the process duration, the bleaching pretreatment and the temperature increase on the thermal ageing of semi-chemical pulp derived from poplar wood. The analysis accounting of the complex influence of the samples heterogeneity is aimed at elucidating the mechanism of the process.

EXPERIMENTAL

The experiments were carried out with semi-chem-ical pulp (SCP) obtained from rapidly growing poplar wood of increased density (Populus Trichocarpa Xeuram Populus deltoides cultivar Hunneg). After bark removal the wood was chopped into chips of standard dimensions of 15 mm/ 20 mm / 3 mm.

SCP was obtained using 100 g absolutely dry chips. The hydromodule was of 1:5, the treatment temperature was 85°С, while the duration was 90 min. 7 % NaOH and 5 % Na2SO3•7Н2О2 (the concentration are evaluated in respect to the mass of the absolutely dry chips used) were introduced. The same conditions were applied to 10 g of chips aiming yield determination.

The chips, weighed initially, were placed in a ther- chips, weighed initially, were placed in a ther-chips, weighed initially, were placed in a ther-mostated vessel to preserve the temperature applied. Then the solutions of NaOH and Na2SO3 were added in the amounts pointed above. The duration of the treatment followed the technological regime. Then the solution left was removed and the chips were washed to a neutral reaction. They were subse�uently fi bered with the ap- They were subse�uently fi bered with the ap-They were subse�uently fibered with the ap- with the ap-with the ap- the ap-the ap- ap-ap-plication of a disc fibering devicee (Spront-Valdron). The fibering extent reached was 12oSR. It was determined by Schoppet-Riegler method (Bulgarian Standard ISO 5267-1). Then the samples were washed and sorted manually using two sieves.

SCP yield was 90 %. It was determined by the weight method. The application of the latter required to soak the chips for 24 hours in distilled water, to wash them to a neutral reaction and then to dry them to a constant


587

weight. Non-treated samples were investigated as well to outline the bleaching effect on the kinetics of the thermal ageing of SCP.

Bleaching of SCPSCP prepared under the conditions pointed above

was subjected to a two-stage combined bleaching, i.e. bleaching with H2O2 (I stage) followed by bleaching using the reducing agent Rongalyt C (II stage). The first stage was applied to 10 % of SCP at 70°С within 90 min. In this case the medium рН was ca 10.5, while the amount of H2O2 was 2 %. The additives used re-ferred to 2 % NaOH, 5 % Na2SiO3, 0.5 % MgSO4 and EDTA (Na2C10H14O8N2.2H2O) as a complexion agent. All amounts pointed so far were calculated in respect to the absolutely dry fibrous material used. The procedure applied required to place the sample in a polyethylene bag, to bring it to the temperature value envisaged and then to introduce the bleaching solution under constant stirring to provide complete homogenization. Then the sample treated was transferred to the thermostated reac-tion vessel where the stirring continued. Upon the first stage completion the fibrous material had to be washed until reaching a neutral reaction. Rongalyt C (NaHSO2.CH2O.2H2O) and EDTA were introduced during the sec-ond stage of the bleaching process which was carried out for 90 min at 70 °С. The concentration of SCP was 5 %, while the consumption of Rongalyt C and EDTA was 2 % and 0.5 %, correspondingly. The bleaching procedure was analogous to that applied during the first stage. The

bleached fibrous material obtained was finally washed until a neutral reaction was obtained.

Ageing of SCPFiber mass casts of a diameter of about 6 cm were

made. They were dried in absence of light at room tem-perature. Samples of unbleached and bleached SCP were subjected to thermal ageing for 36 hours at temperature of 90°С, 105°С and 120°С. The degree of brightness was determined using E�REPHO – 2000 using a stand- determined using E�REPHO – 2000 using a stand-determined using E�REPHO – 2000 using a stand- using E�REPHO – 2000 using a stand-using E�REPHO – 2000 using a stand- – 2000 using a stand- using a stand-ard light source of D 65 at λ = 457 nm. The degree of brightness prior to and after the ageing was determined following ISO 2470:202. Furthermore, the degree of brightness of the bleached samples was determined at the 6th, 12th, 24th and the 36th hour of the process. This was done to follow the kinetics of the ageing process. A number of experimental series were investigated. Each value reported below is based on three identical one obtained in the course of the study.


The effect of thermal ageing of SCP is followed using bleached and unbleached samples under the condi-tions pointed above. All values obtained are summarized in Table 1.

The data presented in Table 1 shows that the two-stage bleaching leads to 36 % increase of the degree of brightness. At 90°С the brightness reversion is 3.83 % for the unbleached samples, while it is 5.11 % for the

Table 1. Values of the degree of brightness, W %, of unbleached and bleached SCP in the course of the process of ageing h at the temperature values studied.

Time, h

Brightness W %

90°C 105°C 120°C

unbleached bleached unbleached bleached unbleached bleached

0 31.3 48.9 31.9 50.6 32.0 49.2

6 30.6 48.0 31.0 48.8 30.9 46.8

12 30.3 47.2 30.6 47.8 29.8 45.1

24 30.2 46.8 30.1 47.0 29.0 44.8

36 30.1 46.4 29.9 46.2 28.8 43.4

1


588

bleached one. �t 105�С it is �.�� % and �.�� %, respec- one. �t 105�С it is �.�� % and �.�� %, respec-one. �t 105�С it is �.�� % and �.�� %, respec-. �t 105�С it is �.�� % and �.�� %, respec-�t 105�С it is �.�� % and �.�� %, respec-105�С it is �.�� % and �.�� %, respec-�С it is �.�� % and �.�� %, respec-С it is �.�� % and �.�� %, respec- it is 6.26 % and 8.69 %, respec-6.26 % and 8.69 %, respec-.26 % and 8.69 %, respec-% and 8.69 %, respec-and 8.69 %, respec- 8.69 %, respec-8.69 %, respec-%, respec-respec-tively, while at 120°С the values found are 10.00 % and 11.78 %. These observations can be explained with the lower initial brightness of the unbleached samples.

The comparative investigation of the thermal age-ing kinetics is done with the introduction of the kinetic variable, a:

0

0

W WW

a −= (3)

where W0 is the initial brightness value in % ISO, while W in % ISO is the current value connected with the time of the treatment. The variable a can be also considered as an extent of thermal ageing or as a relative decrease of the degree of brightness in the course of the process.

The kinetic curves showing the change of a with time t, h at the three temperature values studied are presented in Fig. 1.

As Fig. 1 shows a increases with time and tem-increases with time and tem-perature for both types of samples. This increase of the kinetic variable corresponds to the increase of the extent of thermal ageing, i.e. to the corresponding brightness reversion of SCP. It is also evident that the tendency towards ageing of the unbleached samples is better outlined after the 24th hour when compared to that of the bleached one. This result is attributed to the fact that the ageing process of the bleached samples is not completed, while the stability to thermal ageing of the unbleached samples is well decreased. This effect is better outlined at 120°С.

The kinetics of thermal ageing is described by the exponential equation (Eq. 4):

aaevv −= 0 (4)

where the current and the initial rate of the ageing

process are designated by dvdta

= and ν0. It is valid for

processes taking place on uniformly inhomogeneous

surfaces where the active centres are distributed linearly

in correspondence with their energy [30-36]. Its applica-

bility is experimentally verified [31-33]. For a surface

of this type the heterogeneous reaction rate decreases exponentially with a increase. The kinetic coefficient of inhomogeneity а in Eq. (4) accounts for the energy and entropy inhomogeneity of the system. All kinetic curves are linearized in coordinates a vs. ln t in correspondence with the approximate integral form of the exponential kinetic equation:

( )01 1ln lnv a ta a

a = + (5)

The linear dependences obtained in correspondence with Eq. (5) are presented in Fig. 2.

The slope value of the lines obtained (Fig. 2) pro-vides the determination of a (Eq. 5). The values of the latter are presented in Table 2. It is seen that a decreases with temperature increase. This tendency is better ex-pressed for the unbleached SCP.

The temperature dependence of a can be described

0 5 10 15 20 25 30 35 40

0,02

0,04

0,06

0,08

0,10

0,12

0,14

a

Time, h

900C 1050C 1200C 900C 1050C 1200C

Fig. 1. Kinetic curves referring to the ageing process studied in case of unbleached SCP (----) and bleached SCP (____).

1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6

0,02

0,04

0,06

0,08

0,10

0,12

0,14

a

lnt

- -900C- -1050C- -1200C

900C 1050C 1200C

Fig. 2. Linearized a vs. ln t kinetic curves for unleached (----) and bleached (____) SCP.


589

by Eq. (6) [37]:

01 aTR

Ba −= (6)

Constant В (kJ mol—1) in Eq. (8) stands for the interval of energy inhomogeneity and takes account of the active centres of different energy, while coefficient а0 stands for the interval of entropy inhomogeneity and is connected with the number of the active centres, their disposition and availability.

The specifics of the process assume in fact the presence of energy and entropy inhomogeneity of the system. The ratio of a0 and В found in the present case is a constant, i.e.

3.01792530

29018690 =

=

=

blunblBa (mol kJ—1).

It depends neither on the temperature, nor on the bleaching. This result leads to the conclusion that the inhomogeneity observed is determined only by the type of the system (i.e. it has a constant value for the particular system studied).

Combining both Eqs. (4) and (5) leads to the fol- both Eqs. (4) and (5) leads to the fol-both Eqs. (4) and (5) leads to the fol- Eqs. (4) and (5) leads to the fol-Eqs. (4) and (5) leads to the fol- (4) and (5) leads to the fol-4) and (5) leads to the fol-) and (5) leads to the fol-and (5) leads to the fol- (5) leads to the fol-5) leads to the fol-) leads to the fol-leads to the fol- fol-fol-lowing expression of the current rate v of the process:

1vat

= (7)

It is calculated at different values of a leading to the conclusion that v decreases with a increase for all samples studied. The dependence obtained is visualized in Fig. 4.

The logarithmic form of the exponential kinetic equation (Eq. 4) can be used as well for the determina-tion of the initial rate, v0 (h

—1), of the process of thermal

ageing (at a→0), i.e.:

0ln lnv v aa= − (8)

The values of n0 are presented in Table 2. The initial rate of ageing of the bleached samples is relatively higher than that of the unbleached one but the latter become more stable to temperature increase and their brightness reversion proceeds more slowly with a increase.

The rate-temperature dependence is used to calculate the apparent activation energy E and the pre-exponential factor ln A, according to the Arrhenius equation:

ERTv Ae

−= (9)

Table �. Values of the kinetic coefficient of inhomogeneity, а, and the initial rate of the process, v0 (h

—1).

T °C Unbleached pulp Bleached pulp

а v0 103

(h—1)

а v0 103

(h—1)

90 100 10.78 64 11.00

105 52 11.33 33 13.56

120 27 13.43 19 20.85

1

2,55 2,60 2,65 2,70 2,75

20

30

40

50

60

70

80

90

100

a

1/(T 103) -1, K-1

unbleached pulp bleached pulp

Fig. 3. Temperature dependence of coefficient of inhomo-geneity a for both types of SCP treatment.

-7,8

-7,5

-7,2

-6,9

-6,6

-6,3

-6,0

-5,7

-5,4

-5,1

-4,8

-4,50,025 0,030 0,035 0,040 0,045 0,050 0,055 0,060

a

lnv

- - 900C- -1050C- -1200C

900C 1050C 1200C

Fig. 4. �inear dependence between ln v and for unbleached (----) and bleached (____) SCP.


590

The values obtained as a function of a are summa-are summa-rized in Table 3. The dependence found for the bleached samples is visualized in Fig. 5. The temperature effect on the thermal aging rate of the unbleached samples shows the same tendency.

As Fig. 5 shows the linear dependences obtained have different slopes, i.e. the values of activation en-ergy E and pre-exponential factor ln A increase with a increase. This means that the surface is energy and entropy heterogeneous.

The activation energy E (kJ mol-1) of the bleached

samples is higher at the beginning of the process, a→0. But in the course of the treatment it becomes lower, most probably because of the larger number of active centres available on the surface. Тhe ageing process is hampered with a increase in case of unbleached pulp. These findings are in accord with the experimental results concerning the decrease of the current rate of the process at the bleached and the unbleached samples

(Fig. 4). The rate of ageing is lower for the unbleached one and hence it can be concluded that the bleached samples are more resistant to thermal aging for a longer time and at higher temperatures. On the other hand, ln A increases also in the course of the process. This indicates that the number of the chromophore groups responsible for brightness reversion increases. Besides, the values

found at a→0 are higher for the bleached samples but the total increase of the entropy factors in the course of the process is greater at the unbleached one.

The dependence of E and ln A on a increase is linear as required by the model of inhomogeneous surfaces [33-37]. It is described by Eq. (10) and Eq. (11), respectively:

aBEE += 0 (10)

0 0ln lnA A a a= + (11)

where �0 (kJ mol—1) and ln �0 are the apparent activa-tion energy and the apparent pre-exponential factor at

a→0 (Table 3), respectively. The values of constant В (kJ mol—1) (Eq. 10) and constant a0 (Eq. 11) coincide with those estimated with the application of Eq. (4). It is worth noting that their ratio, i.e. the ratio between the entropy and the energy heterogeneity of the bleached and unbleached pulp coincides with that previously calculated

3.00 =Ba mol kJ—1.

The dependences of E and ln A on a are presented in Figs. 6 and 7, correspondingly.

It is seen from Figs. 6 and 7 that the linear depend-ences presented intersect at a ≈ 0.013. The time required to reach this point is in the range of 1.5 h at 120 °С and 4.5 h at 90 °С. W0 decreases with about 1.3% during this

-8,0

-7,5

-7,0

-6,5

-6,0

-5,5

-5,0

-4,5

-4,0

2,55 2,60 2,65 2,70 2,75

1/(T 103) -1, K-1

ln v

a=0 a=0.035 a=0.04 a=0.05

Fig. 5. Temperature dependence of the initial (at a→0) and current rate (at a=const) for bleached SCP.

Unbleached pulp Bleached pulp

a 0.0 0.035 0.04 0.05 0.0 0.035 0.04 0.05

E (kJ mol—1) 18.5 104.59 117.72 142.18 24.85 94.44 99.76 120.80

ln A 1.46 26.92 30.78 36.9 3.65 24.45 26.00 32.23

1

Table 3. Values of activation energy E (kJ mol—1) and pre-exponential factor ln A at a = const.


591

time interval. The effect of the energy and the entropy factors on the thermal ageing of the unbleached samples is better expressed at a values greater than that pointed above. This is most probably connected with the exhaus-tion of the available active centers on the surface and the increase of the number of the chromophore groups responsible for brightness decrease. The bleached sam-ples age more slowly as their lignin content is lower.

The process rate is affected both by the apparent ac-tivation energy and the apparent pre-exponential factor. The simultaneous change of these counteracting factors with a increase brings about a compensation effect [23-27]. A linear dependence of lnA on E is obtained: ln A n mE= + (12)

Besides, it is valid for all samples studied. The ap-plicability of Eq. (12) is illustrated in Fig. 8. It is worth noting that the value of the slope found is given by

3.00 ==Bam (mol kJ—1),

while the value of the intercept coincides with constant n described by

00 0ln an A E

B = − .

The correlation obtained indicates a common mechanism of the ageing process of the treated and the non-treated samples. It suggests that the increase of the activation energy slows down the ageing process but not as much as that required to compensate the increase of the number of the chromophore groups responsible for the brightness reversion. This in turn leads to the conclu-sion that bleaching has a positive but limited effect on the ageing, especially at high temperatures.

CONCLUSIONS

The effect of thermal ageing of SCP is followed on the ground of the comparative investigation of bleached and unbleached samples. The kinetics of the process is described by the exponential kinetic equation valid for processes taking place on uniformly inhomogeneous surfaces. The initial rate of bleached samples ageing is relatively higher than that of the unbleached one but the

0,01 0,02 0,03 0,04 0,05

20

40

60

80

100

120

140

E (k

J m

ol -1

)

a


Fig. 6. Linear dependences of the apparent activation en-ergy, E (kJ mol—1) on the extent of process proceeding, a.

0,01 0,02 0,03 0,04 0,05

5

10

15

20

25

30

35

ln A

a


Fig. 7. Linear dependences of the pre-exponential factor, ln A, on the extent of process proceeding, a.

20 40 60 80 100 120 140

5

10

15

20

25

30

35

lnA

E (kJ mol -1)


Fig. 8. The compensation effect: linear dependence of pre-exponential factor ln A vs. activation energy E (kJ mol—1).


592

latter become more stable to temperature increase and their brightness reversion proceeds more slowly. The current rate-temperature dependence is used to calculate the apparent activation energy and the apparent pre-exponential factor of the Arrhenius equation. They are found to increase simultaneously with increase of the extent of the process. The compensation effect found is common for the samples of both types. This indicates that the mechanism of thermal ageing does not depend on the temperature, the process duration and the degree of bleaching.

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14. X. Zou, T. Uesaka, N. Gurnagul: Prediction of paper permanence by accelerated aging: I. Kinetic analysis of the aging process, II. Comparison on the predition with natural aging results, Cellulose, 3, 4, 1996, 243-267.

15. E. Avgerinos, E. Koukios, J.-C. Parajo, F. Girio, Kinetic modeling of shelfcatalyzed hydrolysis of lignocellulosic materials, Proceedings of 3rd Panhellenic Conference in Chemical Engineering, 2, 2001, 637–640.

16. H. Z. Ding, Z. D. Wang, On the degradation evolution equations of cellulose, Cellulose, 15, 2008, 205-224.

17. A. Barneto, J. Carmona, J. Alfonso, �. Alcaide, Use of autocatalytic kinetics to obtain composition of lignocellulosic materials, Bioresour. Technol., 100, 2009, 3963–3973.

18. H. �ynggaard, A. Andreasen, C. Stegelmann, P. Stoltze, P. Analysis of simple kinetic models in heterogeneous catalysis, Prog. Surf. Sci., 77, 2, 2004, 71-137.

19. G. Bond, M. Keane, H. Kral, J. �echer, Compensation phenomena in Heterogeneous Catalysis: General Principles and a possible Explanantion Cat. Rev.- Sci. Eng., 42, 2000, 323-383.

20. T. Bligaard, K. Honkala, A. �ogadottir, J. Orskov, S. Dahl, C. Jacobsen, C. On the compensation effect in heterogeneous catalysis, J. Phys. Chem B, 107, 2003, 9325-9331.

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594


STUDY OF THE COMPLEX EQUILIBRIUMBETWEEN TITANIUM (IV) AND TANNIC ACID

Andriana Surleva, Petya Atanasova, Tinka Kolusheva, Latinka Costadinnova


ABSTRACT

Titanium-tannin (Ti-TA) complex is used in the modern leather technology as an alternative of chromium tannage and as an effective modification of vegetable tanning. The ratio between metal and tanning reagent is a key factor for minimizing the unused quantities of tannic acid that could reach the environment by waste waters and cause damages on aqueous ecosystems. However, a lack of reliable data about Ti-TA complexation limits the possibility for fine tuning of the composition of tanning solutions. This study is aimed at characterization of titanyl-tannin complex: its structure, stoichiometry and stability constant. The data of alpha-coefficient of titanyl-hydroxocomplexation at different pH values were also presented.The results obtained showed that in the concentration interval 5x10-6 – 2x10-5 M and pH 4, the main titanium IV species in titanyl/tannin/formate solutions is titanyl-hydroxo-tannin complex TiO(OH)TA. It was proved by UV-Vis and IR spectrometry that the aromatic carboxylic oxygen (Ar-C=O) and probably phenolic oxygen of tannin are involved in the coordination of titanyl ion. The stability constant of TiO(OH)TA was estimated: lgβ = 16.53. The obtained molar ratio curves and the calculated values of the equilibrium constants could be used in leather technology for selecting an appropriate composition of tanning solutions.

Keywords: tannin, titanium, titanyl, stability constant, complexation, leather tanning.

Received 11 July 2014Accepted 15 September 2014

INTRODUCTION

The titanium-tannin (Ti-TA) complex is a modern tanning-coloring agent used in leather technology re-search in attempts to replace the synthetic dyes with natural ones and to decrease the level of allergic impact of produced leather. Moreover, the leather treated with tannic acid, is supposed to possess at a certain level the antimicrobial and antifungal properties of the used metal-tannin complexes [1, 2]. The leather treated by titanium (IV) - tannin solution is very soft, flexible and possess good thermal stability [3]. The leather technol-ogy calls for proper data about the Ti-TA complex com-position and stability so that to achieve a fine tuning of the composition of tanning solutions. The ratio between metal and tanning reagent is a key factor for minimizing the unused quantities of tannic acid in wastewaters. On

the one hand, high levels of tannic acid in water provoke an intensive grow of algae, on the other hand, tannin decomposition rapidly increases water COD and causes damages on aqueous ecosystems. A detailed study of the Ti (III) – tannic acid mixture in a combined tanning procedure was reported by Bo Teng et al. [3]. Tannins with different origin and different content of gallic acid were studied. The results presented showed that the quality of the treated leather depends on the structure and origin of the used tannins.

The data about Ti-TA complexes found in the lit-erature are very scarce. Tannic acid was reported as an effective precipitating reagent for titanium determina-tion in chloride solutions [4]. Spectrophotometric de-termination based on titanium complexation by tannin and thioglycolic acid was described [5], however the structure and the origin of studied tannin, as well as the


595

absorption spectra of the formed compound were not presented. The strong complexation of Ti (IV) by tan-nin was used for photosensitization of nanocrystaline TiO2 films [6]. The force of induced photoelectricity depends on the structure and origin of the used tannin. Electrospray mass spectrometry was applied to study copper complexation by ligands derived from tannic acid [7]. Although the proposed method is important for characterization of different classes of ligands found in natural waters, quantitative data about their stability constants are not presented.

The chemistry of Ti (IV) salts tanning is reported to be dominated by the titanyl ion TiO2+ [1, 9]. The com-plexes formed in water-sulphuric acid are described as chains of titanium ions bridged by hydroxy and sulphato ligands [10]. An advantage of tanning with titanium (IV) is the resulting white leather. However, Ti (IV) salts tends to hydrolyse and precipitate in dilute solution [1]. The hydrolysis of such titanium complexes proceeded through the coordination of hydroxo groups and forma-tion of oxo bridges. The higher the concentration of Ti (IV) and sulphuric acid is the more extensive the degree of polymerization is [9]. It was proposed that mixtures of the metal sulphates could be stabilised against hydrolysis at pH 4 by complexing (masking) with gluconate [1].

In the research literature tannic acid (tannin, gal-lotannin, TA) is used to designate a large group of naturally occurring polymers with different structure and molecular mass. This fact illustrates the main dif-ficulty of using the available in the literature data: the lack of specified information about the molecular mass and the origin or structure of studied tannins limits the application of available data and the comparability of the results. This study is focused on tannin (C76H32O46) with molecular mass 1 701.20 g/mol and structure de-scribed in [11], often named as “Chinese tannin” as it is found in a pure form in Chinese gals (Rhus semilata) and Sumac (Rhus typhina) [12]. Our interest to Sumac is determined on the one hand, by the fact that Sumac is a plant largely available in Europe traditionally used for vegetable leather tanning [13], and on the other hand, it could be isolated in enough pure form so that comparable quantitative data to be reported.

The aim of this study is to characterize the TiO2+-TA complex: its structure, stoichiometry and stability constant. The study was performed at formate buffer solution. The chosen experimental conditions are sup-

posed to correspond more closely to the combined tanning conditions than the acetate buffer experiments often found in the literature [5]. Additionally, our recent study reported that tannic acid (Chinese tannin) is a monoprotic acid with pK = 4.2 and could be regarded as a moderately strong acid [11]. The results showed that the proton of the phenolic group in the next-to-last gal-loyl residue in the tannin chain is more likely to govern the protolytic reaction.

EXPERIMENTAL

ReagentsThe stock solution of TiO2+ (2.087x10-2 M) was pre-

pared from TiO2 (p.a.; Merck 99.99 %) fused with K2S2O8 at 780oC (p.a.; Merck). The obtained melt was dissolved in 25 mL 33 % v/v H2SO4 at heating, after cooling the solution was diluted up to 100.0 mL with d. H2O. Work-ing standard solutions were daily prepared from the stock solution by appropriate dilution. The tannin stock solution (1.000 x 10-3 M) was daily prepared from dried at 105oC for 1h tannin reagent (p.a. Fluka, 403040) and dissolving in d.H2O. The formate buffer (0.2 M, pH = 4.00 ± 0.02) was prepared from formic acid and potassium hydroxide (p.a.; Merck). The pH was potentiometrically controlled by Boeco (Germany) combined pH electrode. Distilled water was used throughout the experiments.

ProceduresMolar ratio method. Working solutions at different

tannin/TiO2+ molar ratios were prepared by mixing 25 mL formate buffer (pH 4.00) with calculated volumes of the tannin stock solution (10-3 M) and finally adding the appropriate aliquots of 1.070 x 10-3 M solution of TiO2+. The final volume of 50.00 mL was obtained by dilution with d. H2O. After 20 min reaction time, the absorbance was measured at 322 nm in 1 cm cuvette. A blank containing the same quantity of tannin solution in formate buffer was used as a reference. The measure-ments were made at 25±1oC. The spectra were acquired at Cary 100 Varian spectrophotometer (Agilent).

IR spectrometry. Titanyl-tannin (TiO-TA) complex was obtained by mixing TiO2+ and tannin solution at molar ratio 1:2 at pH 4.00 (formate buffer). The obtained yellow solution was stable during one hour. After 24 h the precipitate was formed. The obtained precipitate was dried at room temperature. The IR spectra were registered on KBr by Alfa Aesar spectrometer.


596


NMR study of the tanninThe NMR spectra of the studied tannin is presented

on Fig. 1. As can be seen, the number of the protons from the galoylic part of the molecule is 10 times higher than the number of the protons in the glucose part. This fact proves the assumed number of galoylic residues in tannin molecule. Additionally, a large signal at 7 ppm showed that some tannin monomers combined to form oligomers. The carbon NMR spectra of the tannin couldn’t be registered due to the low solubility of tannin in denatured solvents. The structure of studied tannin is presented on the Fig. 1B [11].

Spectroscopic study of the reaction between TiO2+ and tannin

Electronic spectra of titanyl, tannin and titanyl-tannin mixture in formate buffer at pH 4 are presented on Fig. 2. The formed complex is clearly manifested by a red shift of wave length of maximum absorption from 275 nm (tannin solution) to 300 nm (solution of TiO-TA complex) (∆λ = 25 nm) and by a new shoulder appearing at 400 nm. The color of the complex aque-ous solution was bright yellow and the spectra profile was independent of the chemical composition of used buffers: acetate and formate. The buffer compositions at pH around 4 were chosen based on literature data [5]. However, the concentration of acetate buffer should be very high (4 M) to maintain a sufficient buffer capacity which was related to severe inconveniences in routine laboratory work. Moreover, a slight increase of the absorbance at 320 nm was noticed in formate buffer (pH 4.00) compared to the absorbance of the complex obtained in acetate buffer (pH 3.40). As regarding the application of titanyl-tannin complex in leather technol-

ogy, additional advantage of formate buffer as a medium for complex study is the fact that the procedure of leather pickling is performed in HCOOH solutions. Thus the study of titanyl-tannin complex was further performed in a formate buffer. The spectra of the formed complex measured against an analytical blank containing tannin and formate buffer are presented on Fig. 3. The absorb-ance of pure tannin was extracted from the spectra of the titanyl-tannin complex to obtain the pure complex spectra. The differential absorbance at 270 nm due to the absorption by C=O groups in tannin molecule [14] had negative values indicating that the C=O groups did not absorb UV- light at this spectral region. The formation of titanyl-tannin complex by coordination via C=O could be supposed. The absorbance of the complex depended on the concentration of titanyl ions in the solution (Fig. 3). The maximum differential absorbance of the obtained complex was measured at 322 nm. It is worth to be

Fig. 1. NMR spectra (A) and proposed structure (B) of the studied tannin.

Fig. 2. UV-Vis spectra of: (1) TiO2+ (C = 8.35x10-6M); (2) tannin (C = 8.54x10-6 M) and (3) TiO2+-tannin mixture at pH 4 (0.1 M formate buffer). Blank: d. H2O.

a) b)


597

mentioned that at this wave length the complex TiO(OH)i, formed in formate buffer solution of TiO2+ at pH 4.00 did not absorb photons (Fig. 2, curve 1).

The complex formation was verified by IR spectros-copy on KBr. For IR study the complex was obtained at molar ratio of TiO2+ to tannin equal to 1:2. The obtained solution of the complex was stable for an hour. The yellow colored complex was obtained by precipitation for 24 h. IR spectra of TiO2, tannin and the complex are presented on Fig. 4. In the region of hydrogen bonded OH groups (3500-3200 cm-1) a strong wide band at 3289 -3228 cm-1 was observed in the tannin spectrum (Fig. 4A). The band was assigned to the stretching vi-brations of hydrogen bonded phenolic hydroxyl groups (ν (OH)) and the formation of associates between tannin molecules and also intramolecular hydrogen bonds were supposed. The corresponding band wasn’t observed in TiO2 spectrum. The moderately strong band at 3424 cm-1

in the IR spectra of the complex could be also assigned to O-H stretching vibration of coordinated OH groups or of lattice water molecules [15].The reduced intensity of the band in the region 3500-3200 cm-1 showed the decrease in the number of H-bonded OH groups.The positive shift of the band (ν (OH)= 3424 cm-1; ∆ν(OH) = 135 cm-1) suggested the coordination of tannin mol-ecules with the metal ions via deprotonation [16]. The band due to C=O stretching vibration that appeared at 1715 cm-1 in the aromatic carbonic acids (Fig. 4A) [17] was almost vanished in the complex. The decrease in the band intensity confirmed the participation of oxygen in the C=O---M bond. The changes in the spectra profile in the region 1300-1000 cm-1 prove that phenolic oxygen

was also involved in the coordination of metal ion. The IR data suggest that the aromatic carboxylic oxygen (Ar-C=O) and probably phenolic oxygen are involved in the coordination of titanyl ion. Hence in the studied complex tannin acts as a polydentate ligand coordinating through the oxygen of C=O and OH groups. The changes of the number and the intensity of the bands in the region 1700-700 cm-1 might be due to chelate rings formation in the complex. The coordination of OH ions or H2O was also supposed based on bands at 3424 cm-1 and 787 cm-1 [18]. As can be seen from Fig. 4A and 4C, the large peak at 598-533 cm-1 in TiO2 spectrum disappeared in the spectrum of the complex and a new peak could be found at 455 cm-1. The absorption band at 455 cm-1 was attributed to Metal---O stretching vibrations. Based on the presented data, the following composition of the complex in a solid form, obtained after precipitation and drying, could be supposed [Ti(OH)2TA]+ [21, 22].

Fig. 3. UV-Vis spectra of titanyl-tanin complex: (A) CTiO2+

= Ctanin = 8 x 10-6 M and (B) CTiO2+ = Ctanin = 2x10-5 M in formate buffer (pH 4.00) acquired against analytical blank containing the same tannin concentrations.

Fig. 4. IR spectra of (A) TiO2, (B) tannin and (C) titanyl-tannin complex.

a)

b)

c)


598

Spectrophotometric study of the titanyl-hydroxo-tannin complex

The molar ratio method was chosen for this study as the exact concentrations of metal and ligand for practically complete complex formation process could be easily obtained from the curves A = f(Ctannin/Ctitanyl). The obtained results are presented on Fig. 5. Each curve was constructed from a large number of experiments in order to obtain reliable results; each point was obtained as a mean of six values of absorbance of two solutions at the same concentrations.The formation of a complex at 1:1 molar ratio between TiO2+ – tannin was proved by the intercept of two linear parts of the curves. The obtained results have showed that the absorbance in-creased linearly with the concentration of titanyl ions at titanyl/tannin ratios more than 2 (Fig. 5B). The fol-lowing equation was obtained: A = 0.002 + 37 453xC (TiO2+) with correlation coefficient of 0.999 (Fig. 5B). As it was described above, the only absorbing species at 322 nm was the titanyl-tannin complex (Fig. 2, curve 3). Hence, the concentration of the formed complex TiO-TA increased linearly with the concentration of TiO2+ and the molar absorptivity of TiO-TA complex could be cal-culated from the slope of the curve presented on Fig. 5B: ε = 3.75x104 L mol-1cm-1. The results showed also that the Beer’s law was obeyed in the studied concentration interval of the complex.

At the conditions of the present study (pH 4, formate buffer), a formation of TiO(OH)+ complex could also proceed in a considerable extent. The calculated values of alpha coefficients, based on the previously reported

stability constant of titanyl-hydroxo complex TiO(OH)+ :

lgβ = 13.7 [19], are presented on Fig. 6. The logarithmic

value of αTiO(OH) at pH 4 was 3.7 or . As

it could be seen from the Fig. 2, curve 1, the species TiO(OH)+ did not absorb photons at the wave length chosen for the absorbance measurement in the mole ratio method (322 nm).

Based on the above mentioned results, the formation of a mixed ligand titanyl-hydroxo-tannin TiO(OH)TA complex was supposed at the studied conditions. The coordination of OH- as a second ligand has been already reported for some Ti (IV) or titanyl complexes [21, 22]. The following mechanism of the complex formation and corresponding equilibrium constants were proposed:

Fig. 5. Molar ratio plots at different concentrations of TiO2+ at pH 4: (1) 6.04x10-6 M; (2) 8.35x10-6 M; (3) 1.15 x10-5 M and (4) 2.00x10-5 M. The absorbance was measured at 322 nm against an analytical blank.

Fig. 6. Logarithmic values of αTiO(OH) at different pH.


599

(1)

(2)

(3)

The overall equilibrium constants could be described as:

(4)

where K’ is defined as

(5)

and the stability constant of the mixed ligand titanyl-hydroxo-tannin complex was denoted as:

(6)

The value of K’ at constant pH was experimen-tally determined from the molar ratio measurements. The results are presented in Table 1. The equilibrium concentrations of titanyl ion, tannin and the complex at 0.9<Ctannin/Ctitanyl>1 were calculated based on the Beer’s law and the mass law equations.The obtained values

of K’ (eq.5) at pH 4 are presented in Table 1 and the obtained mean value was lgK’=6.53 ± 0.05.

The stability constant of the formed complex TiO(OH)TA was calculated according to the theory of mixed ligand complexes [19,20]. The stability constant β of TiO(OH)TA was calculated based on the equation derived from eq. 4:

(7)

The step stability constant β2 of the process de-scribed by eq. 2 was also estimated starting from the equation of the stability constant of mixed ligand titanyl-hydroxo-tannin complex:

(8)

where and lg β2= 2.83.

CONCLUSIONS

The results obtained in this study warrant the con-clusion that at concentrations in the interval 5x10-6 – 2x10-5 M and at pH 4 the main titanium (IV) species in titanyl tannin formate solutions is titanyl-hydroxo-tannin complex [TiO(OH)TA]. It was proved by UV-Vis and IR spectrometry that the aromatic carboxylic oxygen (Ar-C=O) and probably phenolic oxygen of tannin are involved in the coordination of titanyl ion. It was sup-posed that in a solid form the complex composition is [Ti(OH)2TA]+. The stability constant of TiO(OH)TA was estimated: lgβ = 16.53. The obtained molar ratio curves and calculated values of the derived constants could be used by leather technologists for selecting an appropriate composition of tanning solutions.

AcknowledgementsThe financial support of the University of Chemical

Technology and Metallurgy through the Science and Research program (project Nr 11156/2013) is gratefully acknowledged.

REFERENCES

1. A.D. Covington, Modern tanning chemistry, Chem. Soc. Rev., 26, 1997, 111-126.

2. A. Higazy, M. Hashem, A. ElShafei, N. Shaker, M.A. Hady, Development of anti-microbial jute fabrics via in situ formation of cellulose-tannic acid-metal ion complex, Carbohydrate Polymers, 79, 2010, 890-897.

3. B. Teng, X. Jian, W. Chen, Effect of gallic acid content on tannin-titanium (III) combination tanning, Leather Footwear J., 13, 2013, 3-13.

4. W.R. Schoeller, H. Holness, The precipitation of titanium by tannin from chloride solutions, Analyst, 70, 1945, 319-323.


600

5. S. Benerjee, Spectrophotometric determination of titanium with tannin and thyoglycollic acid and its application to titanium-treated steels and ferrous and non-ferous alloys, Talanta, 33, 1986, 360-362.

6. K. Tennakone, G.R.R.A. Kumara, A.R. Kumaras-inghe, P.M. Sirimanne, K.G.U. Wijayantha, Efficient photosensitization of nanocrystalline TiO2 films by tannins and related phenolic substances, J. Photo-chem. Photobiol. A: Chemistry, 94, 1996, 217-220.

7. A.R.S. Ross, M.G. Ikonomou, K.J. Orians, Character-ization of dissolved tannins and their metal-ion com-plexes by electrospray ionization mass spectrometry, Anal. Chim. Acta, 411, 2000, 91-102.

9. V. Sivakumar, K. Jeyaraj, R. Chandrasekar, G. Swaminathan,Tanning Alternatives, Leather Inter., 2008, http://www.leathermag.com/features/feature-tanning-alternatives/.

10. K. Manolov, Short inorganic chemistry, Technika, Sofia, 1979, pp. 156-164, (in Bulgarian).

11. L. Costadinnova, M. Hristova, T. Kolusheva, N. Stoilova, Conductometric study of the acidity properties of tannic acid (chinese tannin), J. Chem. Technol. Metall, 47, 2012, 289-296.

12. J.C. Bickley, Vegetable tannins, Leather Conserva-tion Centre, Northmapton, 1991, 16-23.

13. Silvateam, Vegetable Tanning: tradition, innovation, naturalness and beauty, en.silvateam. com, last ac-cessed 01.07.2014.

14. T. Owen, Fundamentals of UV-visible spectroscopy,

Agilent Technol., 2000, pp. 10.15. A.C. Tella, W.A. Osunniran, S.O. Owalude, O.M.

Olabemiwo, Synthesis and characterization of some transition metal complexes of bis(2-hydroxy-4-me-thoxyacetophenone) ethylenediimine, Experiment, 16, 2013, 1123-1129.

16. R.S. Peres, E. Cassel, D.S. Azambuja, Blackwattle tannin as steel corrosion inhibitor, ISRN Corrosion, 2012, Article ID 937920, doi:10.5402/2012/937920.

17. M.A. Pantoja-Castro, H. González-Rodríguez, Study by infrared spectroscopy and thermogravimetric analysis of tannins and tannic acid, Rev. Latinoamer. Quím., 39, 2011,107-111.

18. S. Verma, S. Shrivastva, R. Shrivastva, Synthesis, characterization, and physicochemical studies of mixed ligand complexes, J. Chemistry, 2013, Article ID 309179, http://dx.doi.org/10.1155/2013/309179.

19. J. Incedy, Analytical applications of complex equi-libria, Mir, Moscow, 1979, pp. 50-51; pp. 156-161; p. 290, (in Russian).

20. A. Ringbom, Complexation in analytical chemistry, Interscience Publ., 1963, pp.29.

21. G.M.H. Van de Velde, The oxalato complexes of titanium (IV). I. Mononuclear Ti(OH)2(C2O4)2

2- in solution, J. Inorg.Nucl. Chem., 39, 1977, 1357-1362.

22. G.M.H. Van de Velde, S. Harkema, P. J. Gelling, The crystal and molecular structure of ammonium titanyloxalate, Inorg. Chim. Acta, 11, 1974, 243-252.

Stanislava Yordanova, Ivan Petkov, Stanimir Stoyanov

601


SOLVATOCHROMISM OF HOMODIMERIC STYRYL PYRIDINIUM SALTS


Faculty of Chemistry and PharmacySofia University “St. Kliment Ohridsky”1 James Baurchier blvd., 1164 Sofia, BulgariaE-mail::[email protected]

ABSTRACT

The solvatochromic properties of homodimeric styryl pyridinium salts are analyzed on the ground of Lippert-Mataga correlation, solvent scales of molar electronic transition energies ET(30) values, multilinear regression with the Kamlet-Taft solvent scales and continuum McRae-Bayliss model. The protic and aprotic solvents refer to two isolated domains in the Lippert-Mataga and Reichardt plots, whereas fitting to Kamlet-Taft scales gives a linear regression valid for both types of solvents studied. Poor linearity is observed in case of McRae equation application. An intermolecular hydrogen bonding mechanism is advanced to explain the results obtained.

Keywords: cyanine dyes, styryl pyridinium tetraphenylborates, linear solvation energy relationship (LSER), solvato-chromism.


INTRODUCTION

The color change of a compound induced by ex-ternal influences, e.g., by solvents (solvatochromism) [1, 2], salts (halochromism) [3], and temperature (ther-mochromism) [4] has been intensively studied over the past decades.

Cyanine dyes which possess an electron pushing (donor) group and an electron withdrawing (acceptor) group at both ends of the molecule are characterized by an extremely large dipole moment in their ground state. They find successful application in the laser technology [5 - 7], the optical data storage [8], in the non-linear optics [9] and molecular electronics [10].

Donor–(π-bridge)–acceptor materials like mero-cyanine dyes have been lately extensively studied for second-harmonic generation and application for Non-Linear Optical (NLO) materials [11, 12]. Such molecules, similar to those we report here, have a large ground-state dipole moment, aggregate in less polar solvents and exhibit negative solvatochromism [13].

Some of the more polar examples are found to adopt an antiparallel arrangement both in a solution [14] and within a monolayer [15] formed. Several literature studies discuss the solvatochromism of such dyes. Al-Ansari and Ball [16] study four styryl pyridinium dyes and find that the absorption maximum position depends on the polarity and hydrogen bonding ability of the solvents, while hypsochromic shifts are observed with increasing solvent’s polarity or acidity. However, no solvatochromism is observed in the emission spectra. Gawinecki [17] studies eighteen styryl pyridinium cya-nine dyes. He reports that there is no linear correlation between absorption energy, Eabs, and Reichardt solvent polarity parameter, ET(30), even after rejection of weakly polar solvents. He finds no correlation between ΔEabs and Kamlet-Taft’s parameters for solvent polarity (π*) and hydrogen bonding ability (α and β). It is evident from the literature survey there is a contradiction in the re-sults concerning the solvent effects on the spectroscopic properties of styryl pyridinium salts. In this aspect, the current study contributes to get extra information on the


602

absorption and fluorescence characteristics of these dyes.In this paper we describe the changes in the absorp-

tion and emission spectra for a series of styryl pyridinium tetraphenylborates in solvents of different polarity. The solvatochromism of these compounds were analyzed on the ground of Lippert-Mataga correlation [18 - 20], solvent scales of molar electronic transition energies, ET(30), values [21], multilinear regression with the Kamlet-Taft solvent scales [22, 23] and continuum McRae-Bayliss model [24, 25].

EXPERIMENTAL

The general synthetic scheme for preparation of the styryl pyridinium tetraphenylborates was previously reported [26, 27]. UV/Vis spectrophotometric investiga-tions were performed using Thermo Spectronic “Unicam UV 500” spectrophotometer. Fluorescence spectra were recorded on a Varian “Cary Eclipse” scpetrofluorom-eter. All spectra were recorded using 1 cm pathlength

synthetic quartz glass cells at room temperature. The fluorescence quantum yield (ΦF) was determined by the gradient method in reference to a standard Acridine yellow in ethanol (ΦF = 0.47 [28]). All solvents used in this study were of spectroscopic grade.

The structures of the homodimeric dyes under study are presented in Table 1.


Absorption and emission properties of the com-pounds studied

The UV/Vis absorption spectra are investigated in various solvents of different dipolarity/polarizability and hydrogen-bonding ability. The dyes studied exhibit several absorption bands in the region from 250 nm to 600 nm of the electromagnetic spectrum. The energies (cm-1) of the longest absorption band are summarized in Table 2.

The influence of the substituent’s nature can be il-lustrated by some representative spectra of compounds

Table 1. Compounds studied.

X CH CH N CH

R

CH2 CH

R

N CH CH Xn

Ph4B Ph4B

№ Compound X R n Abbreviation

1 (E)-1,1’-(butane-1,4-diyl)bis(4-(4-hydroxystyryl)-pyridinium)

tetraphenylborate

OH H 2 H4

2 (E)-1,1’-(pentane-1,5-diyl)bis(4-(4-hydroxystyryl)-pyridinium)

tetraphenylborate

OH H 3 H5

3 (E)-1,1’-(hexane-2,5-diyl)bis(4-(4-hydroxystyryl)-pyridinium)

tetraphenylborate

OH CH3 2 H6

4 (E)-1,1’-(butane1,4-diyl)bis(4-(4-(N,N-dimethylamino)-styryl)-

pyridinium) tetraphenylborate

N(CH3)2 H 2 D4

5 (E)-1,1’-(pentane-1,5-diyl)bis(4-(4-(N,N-dimethylamino)-styryl)-


N(CH3)2 H 3 D5

6 (E)-1,1’-(hexane-2,5-diyl)bis(4-(4-(N,N-dimethylamino)-styryl)-


N(CH3)2 CH3 2 D6

7 (E)-1,1’-(butane1,4-diyl)bis(4-styryl-pyridinium) tetraphenylborate H H 2 S4

8 (E)-1,1’-(pentane-1,5-diyl)bis(4-styryl-pyridinium)

tetraphenylborate

H H 3 S5

9 (E)-1,1’-(hexane-2,5-diyl)bis(4-styryl-pyridinium) tetraphenylborate H CH3 2 S6


603

of identical spacer in the same solvent (Fig. 1). The main

absorption maximum, which appears due to a π→π* transition in the chromophore system of the unsubsti-tuted styryl pyridinium fragment, is ca 350 nm (see series S). As expected, the electron-donor substituents cause a bathochromic shift – to approximately 400 nm for the hydroxyl and ca 490 nm for the dimethylamino group. Moreover, a donor-π-acceptor type conjugated system is formed in the substituted compounds. The earlier work of Huang et al. [29, 30] on a large number of flexible dichromophoric compounds documents that the visible absorption spectra exhibit an intramolecular

charge transfer transitions (S0→CT) affecting the elec-tron pairs in the donor and the cationic nitrogen atom of the pyridinium fragment. The nature of the CT-band is verified by the following indications – it is relatively broad, intense and structureless, and its position depends strongly on the solvent type (see below).

The emission spectra of the dyes studied are char-acterized by broad and structureless bands and large Stokes’ shifts. The latter differences in case of low and high polar solvents are ca 2000 cm-1 which is a further indication for charge transfer transitions. It is worth adding that the large Stokes shift suggests difference between the excited state geometry and that of the ground state. The energies (cm-1) of the fluorescence

emission band are summarized in Table 3. The emission wavelength, fluorescence intensity, fluorescence quan-tum yields and lifetimes are strongly structure-related and solvent-dependent. The low fluorescence quantum yields may be attributed to the ultrafast internal twist-ing process in the excited state of the molecule. This is in good agreement with the properties of hemicyanine dyes described earlier [31, 32].

The steady-state fluorescence spectra of the dyes studied exhibit fluorescence bands in the region from

Table 2. Energies of the absorption maxima (nabs.10-3 cm-1) of the compounds studied.

solvent H4 H5 H6 D4 D5 D6 S4 S5 S6

H2O 25.19 25.38 25.51 19.57 19.92 19.80 27.40 28.33 28.33

DMSO 25.13 25.13 25.06 20.79 20.83 20.79 28.49 28.57 28.57

DMF 25.13 25.25 25.19 20.83 20.92 20.83 28.57 28.57 28.65

MeCN 25.64 25.84 25.71 20.83 20.88 20.88 28.65 28.82 28.82

MeOH 24.88 25.06 25.00 20.58 20.66 20.58 28.41 28.57 28.41

EtOH 24.39 24.57 24.51 20.24 20.28 20.33 28.01 28.09 28.09

Acetone 25.19 25.45 25.00 20.62 20.90 20.66 28.65 28.82 28.57

2-PrOH 24.10 24.04 24.63 20.41 20.45 20.33 28.01 28.25 28.09

THF 24.94 25.06 25.00 20.75 20.83 20.66 28.57 28.57 28.57

EtOAc 25.13 25.19 24.94 20.83 20.83 20.75 28.49 28.65 28.41

Fig. 1. Absorption spectra of H4, S4 and D4 in Methanol, c = 10-5 M.

250 300 350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

Abso

rban

ce, a

.u.

Wavelength, nm

S4 H4 D4


604

420 nm to 440 nm for unsubstituted dyes (λex = 350 nm), between 500 nm and 530 nm for hydroxy substituted derivatives (λex = 400 nm) and between 600 nm and 640 nm for dimethylamino substituted salts (λex = 490 nm).

The compounds studied do not generally show intensive emission. The relaxation processes in the hemicyanines excited state have been studied for sev-eral decades and the display of twisted intramolecular charge transfer (TICT) [33] is one of its most discussed features. The TICT state is considered a non-radiative but favorable state, especially in polar solvents. Its formation is responsible for the low fluorescence quantum yield. There is no consensus concerning the torsion mode of TICT formation of hemicyanine but the most recently published works tend to consider that twisting takes place around the C-C single bond [30].

Linear Solvation Energy RelationshipMcRae-Bayliss equationWe used the continuum McRae-Bayliss model of

solvatochromism. The analysis carried out is based on Equation (1):

(1)

where νmax is the solute transition energy in a solution (cm-1), ν0 is the solute transition energy in the gas phase, n is the refractive index, while ε is the dielectric con-stant. A is the electronic polarization term describing the transient polarization induced by the solute on the solvent molecules during excitation. B is the orientation polarization term describing the electrostatic forces be-tween the solute and polar solvent molecules.

The performed multiple regression analysis of the experimental data did not show a satisfying correlation between the measured wavenumbers and the terms in the McRae-Bayliss equation illustrated by the outlined deviation (Fig. 2). This is due to the theoretical basis of the McRae model which treats the solvent as a con-tinuum. Besides, the polarization changes are the only effects taken into account. It is evident that the Franck-Condon excited state in this type of chromophores is characterized by a dipole moment which is inverse in respect to the ground state. This is due to a charge transfer transition upon irradiation, while the first solva-tion cage remains intact in its timescale. The relaxation processes in such a non-equilibrium environment are of more complex nature. They include full reorientation of the solvent molecules adjacent to the excited particle and

Table 3. Energies of the emission maxima (l em.10-3 cm-1) of the compounds studied.

solvent H4 H5 H6 D4 D5 D6 S4 S5 S6

H2O 19.61 19.61 19.72 15.92 15.95 15.87 23.09 23.31 23.26

DMSO 19.23 19.34 19.30 16.13 16.10 16.10 23.04 23.15 23.15

DMF 19.38 19.34 19.38 16.23 16.21 16.23 23.09 22.94 22.99

MeCN 20.00 20.00 19.96 16.16 16.21 16.21 23.09 22.83 22.88

MeOH 19.61 19.61 19.61 16.34 16.47 16.39 23.36 23.15 23.15

EtOH 19.65 19.65 19.57 16.47 16.47 16.50 23.47 23.20 23.42

Acetone 19.57 19.61 19.72 16.26 16.29 16.23 23.26 23.09 23.09

2-PrOH 19.92 19.84 19.92 16.64 16.29 16.64 23.70 23.81 23.70

THF 19.76 19.80 19.84 16.64 16.67 16.67 24.39 24.51 24.51

EtOAc 19.80 19.80 19.72 16.67 16.72 16.67 23.31 23.53 23.36


605

their further interaction with the outer solvation shells, which makes the McRae model inapplicable.

Lippert-Mataga equationSolvent effects on the dyes photophysical properties

are due to their interaction with the environment (such as the solvation cage) via dipole-dipole interaction, hydrogen bonds formation, charge transfer processes, reorientation of the dipole moments of the solvents upon excitation of the fluorophores, etc. [34]. General and specific solvents effects as well as the structural relaxation of the molecular skeleton at the excited state determine the intrinsic origin of the Stokes’ shift. The simplest consideration of the general solvents effect on the Stokes shift is based on the applicability of the Lippert-Mataga equation (2).

(2)

,

where Δν stands for the Stokes’ shift, h is the Planck’s constant, c is the velocity of light in vacuum, a is the Onsager cavity radius, Δf is the orientation polarizability, µe and µg are the ground-state dipole in the ground-state and the excited dipole in the excited-state geometry, cor-respondingly, while ε0 is the permittivity of the vacuum.

The Lippert-Mataga correlations of the Stokes’ shifts of compounds H1 are illustrated in Fig. 3. Poor linearity is found for the overall regression for all compounds studied. The protic and the aprotic solvents are clearly divided, i.e. two isolated domains appear in the plot. Such isolation in the Lippert-Mataga plots is reported in case of polarity probes like PRODAN and its ana-logue [35]. Besides, the exclusive regression of the two isolated domains gives good linearity. It is worth noting that the Lippert-Mataga model cannot explain a specific solvent–fluorophore interaction because it does not con-sider chemical interactions. Therefore, the non-linearity of the correlations considered above should indicate a specific solvent effect, most probably intermolecular hydrogen bonding between the solute and the solvent molecules [36].

Reichardt modelA solvent scale accounting of the specific solvent

effect contributions was used aiming a better description of solvatochromism observed. Thus the Stokes’ shifts (cm-1) were correlated with the molar electronic transi-tion energies, i.e. the ET(30) values. A representative correlation for H1 is depicted in Fig. 4. Poor linearity is observed – the protic and the aprotic solvents in case of all compounds studied are again separated in two

Fig. 2. Relationship between the experimental and the calculated by the McRae equation transition energies for H4 (А) and D4 (B).

23500 24000 24500 25000 2550023500

24000

24500

25000

25500

n cal

cula

ted

nexperimental

(A)

Fig. 3. Lippert-Mataga relationship between the Stokes shift values (ΔνSt, cm-1) and Δf (see Eq. 2) for H1 (the squares refer to the aprotic solvents, while the triangles – to the protic one): 1 - Ethyl acetate, 2 - Tetrahydrofuran, 3 - Dimethyl sulfoxide, 4 - N,N-Dimethylformamide, 5 - Acetone, 6 - Acetonitrile; 7- 2-propanol, 8 - Ethanol, 9 - Methanol, 10 - water.

0.15 0.20 0.25 0.30 0.353000

4000

5000

6000

7000

8000

0.15 0.20 0.25 0.30 0.353000

4000

5000

6000

7000

8000

∆f

10

1

7

9

8

6543

∆nSt

2


606

isolated domains, each of good linearity but a different slope. This suggests that the Reichardt scale is not ap-plicable to this type of chromophores. In fact this is a common weakness of the experimentally derived scales which can be correctly used only if the compound stud-ied has behavior upon excitation similar to that of the probe molecule.

Multilinear regression of the photophysical prop-erties using the Kamlet-Taft scale

The solvent dependency of the photophysical prop-erties of a dye, such as νabs (absorption energies), νem (emission energies) and the Stokes shifts (ΔνSt, in cm-1) can also be described by multilinear regression with the application of Equation (3). The solvatochromic param-eters α, β, and π* are summarized in Table 4.νmax = ν0 + aα + bβ + s (π* + dδ) (3)

seven nitro aromatic indicators). This electronic transi-tion is connected with an intramolecular charge transfer from the electron-donor part to the acceptor one through the aromatic system. The correlations provide statisti-cally a solid base for understanding the manifold solvent effects on the solvatochromic long wavelength UV/Vis absorption band of the dyes studied. The linear solvation energy relationships (LSERs) show a high quality for a multi-linear regression with 6 degrees of freedom, as indicated by the correlation statistics - coefficients R2 are higher than 0.9 (see Table 5) and F-stat is lower than 10 in all cases.

For the hydroxy-substituted derivatives the coef-ficients s are positive. This suggests that the excited state is more polar than the ground one and positive solvatochromism is expected. It can be concluded that the term α plays no role since the calculated values are close to zero. It is seen that the values of b are larger than those of a. This means that the hydrogen bond ac-cepting (HBA) ability of the solvent affects the transition energy to a greater extent than its hydrogen bond donor (HBD) ability.

An opposite conclusion can be drawn for compounds from series D. The HBD ability of the solvents is more important than HBA as the values of b are smaller than those of a. The negative sign of s indicates that the first singlet excited state of these molecules is better stabi-lized when the solvents’ polarity increases. This is in good agreement with the results reported in ref. [37].

It can be further concluded that the three parameters

Fig. 4. Relationship between the Stokes shift values (ΔνSt, cm-1) and ET(30) for H1 (the squares refer to the aprotic solvents, while the triangles – to the protic one): 1 - Tet-rahydrofuran, 2 - Ethyl acetate, 3 - Acetone, 4 - N,N-Di-methylformamide, 5 - Dimethyl sulfoxide, 6 - Acetonitrile; 7- 2-propanol, 8 - Ethanol, 9 - Methanol, 10 - water.

35 40 45 50 55 60 654000

4500

5000

5500

6000

6500

7000

7500

35 40 45 50 55 60 654000

4500

5000

5500

6000

6500

7000

7500

ET(30)

10

9

8

7

654

32∆n

St

1

where νmax represents the property to be correlated, ν0 stands for the property related to a standard process, α is the hydrogen bond donor acidity, β is the hydrogen bond acceptor basicity, π* is the dipolarity-polarizability, δ stands for the polarizability correction term while a, b, s and d are coefficients which are characteristic for a given compound.

The π* scale is based on the solvent-induced shifts of the longest wavelength (π → π* absorption band of

Table 4. Solvent constants and empirical solvatochromic parameters used in the study.

solvent ε1 n2 ET(30) π* α β

H2O 80 1,34 63,1 1,09 1,71 0,47

DMSO 47,2 1,479 45,1 1 0 0,76

DMF 38,3 1,429 43,2 0,88 0 0,69

MeCN 37,5 1,346 45,6 0,75 0,19 0,40

MeOH 32,7 1,3284 55,4 0,60 0,98 0,66

EtOH 24,55 1,3613 51,9 0,54 0,86 0,75

Acetone 20.7 1,359 42,2 0,71 0,08 0,43

2-PrOH 19,92 1,3772 48,4 0,48 0,76 0,95

THF 7,50 1,407 37.4 0,58 0 0,55

EtOAc 6,02 1,373 38,1 0,55 0 0,45

1 dielectric constant; 2 refractive index


607

considered have no significant effect on the νmax values for the compounds of series S.

The correlation between the absorption maxima (cm-1) predicted by the multilinear regression and the experimental values obtained is presented in Fig. 5. A linear correlation is observed for all compounds studied.

CONCLUSIONSThe solvatochromic properties of homodimeric sty-

ryl pyridinium salts were analyzed with the application

of various Linear Free Energy Relationship methods. McRae-Bayliss, Lippert-Mataga and Reichardt equa-tions were found not applicable for quantitative descrip-tion of the absorption and emission energies as a function of the respective solvent parameters. The Kamlet-Taft model describing hydrogen bond formation gave excel-lent results in terms of statistics - the correlation coef-ficients were found higher than 0.9 for all compounds studied. The values of the constants in the corresponding equations were obtained and the parameter of greatest importance for each series was determined.

REFERENCES

1. C. Reichardt, Solvatochromic dyes as solvent polarity indicators, Chem. Rev., 94, 1994, 2319-2358.

2. M. Panigrahi, S. Dash, S. Patel, P.K. Behera, B.K. Mishra, Reversal in solvatochromism in some novel styrylpyridinium dyes having a hydrophobic cleft, Spectrochim. Acta A Mol. Biomol. Spectrosc., 68, 2007, 757-762.

3. Y.-S. Jung, J.-Y. Jaung, Halochromism of pyridinium azomethine ylides stabilized by dicyanopyrazine group, Dyes Pigm., 65, 2005, 205-209.

4. E. Lambi, D. Gegiou, E. Hadjoudis, Thermochromism and photochromism of N-salicylidenebenzylamines and N-salicylidene-2-aminomethylpyridine, J. Photo-

Table 5. Calculated values for a, b and s coefficients from Kamlet-Taft equation.

compound ν0, cm-

1 a, cm-1 b, cm-1 s, cm-1 R2

H1 26161 172 -2713 1409 0.981

H4 25364 -226 -1627 978 0.964

H5 25739 -176 -2030 945 0.960

H6 25040 5 -1057 918 0.989

D4 20944 -605 117 -270 0.907

D5 21018 -513 -134 -69 0.957

D6 20843 -502 -31 -49 0.948

S4 28961 -585 -295 -261 0.905

S5 28980 -230 -727 130 0.915

S6 28555 -247 -531 464 0.946

Fig. 5. A Linear relationship between the experimental and the predicted absorption (ν, cm-1) of H4 using multilinear regression on the ground of Kamlet-Taft scales.

24000 24500 25000 25500 2600024000

24500

25000

25500

26000

n calcu

late

d

nexperimental


608

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610


ELECTRIC-ARC-CARBOTHERMIC PROCESSING OF AGGLOMERATED WASTE DISPERSAL MATERIALS FROM FERROUS METALLURGY

Daniela Grigorova, Maxim Marinov, Rossitza Paunova


ABSTRACT

The present paper reports results referring to the production of pig iron under laboratory conditions using a low shaft electric furnace. Wastes from ferrous metallurgy subjected to a preliminary agglomeration are used as iron containing raw materials in the sintering charge. Material and heat balance calculations are carried out. It is found that sinters of high waste powders and sludge content can be used to give pig iron of low content of sulphur, phosphorus and non ferrous metals. The chemical composition, the yield and the specific energy consumption are found close to the existing standards.

Keywords: waste materials, balance calculations, agglomeration, pig iron, a low shaft electric furnace.


INTRODUCTION

The practice of some countries verifies the economi-cal effectiveness of pig iron production using electric arc furnace at predetermined technical and economical conditions [1 - 4]. These furnaces have less requirements referring to the chemical and mechanical indexes of the ore when compared to those used in case of blast fur-nace production. Under identical conditions the metal obtained in a low shaft electric furnace is of higher quality compared to that of the pig iron produced in a blast furnace [5 - 7]. Besides iron ores which are not applicable to the blast furnace process can be utilized if low shaft electric furnaces are used. Hence the latter extend the possibilities for iron ores use [8 - 10].

Most of the waste powders from the gas cleaning equipment used in the extraction metallurgy are currently a subject of landfilling. These are low price materials but have a relatively high iron content ranging from 29.89 % (blast furnace powders) to 57.02 % (waste powders from

LD converters) [11, 12]. Their processing аs eventual blast furnace charge is not advisable because of pollution problems [11, 12].

The aim of the present paper is to carry out material and heat balance calculations of agglomerated mixtures of waste materials coming from the extraction metal-lurgy and to produce pig iron in a low shaft electric furnace using the sinters obtained.

EXPERIMENTAL

Waste powders and sludge from the extractive met-allurgy require agglomeration preceding their further processing. That is why those used in the present inves-tigation were sintered. The charge materials comprised of stack mixture, limestone, coke breeze, waste mixture and sludge. The waste mixture contained 33.80 % of sinter dust from an electrostatic precipitator, 35.88 % of blast furnace powder, 28.32 % of oxygen converter (BOF) powders and 2 % of electric arc furnace (EAF)


611

dusts. The mixture composition was selected on the ground of the usual annual generation of these materials under actual processing conditions. Two mixtures were prepared for the investigation carried out. The first one was a reference one as its composition was analogous to that generally used. 2 % of wastes and 10 % of sludge were added to the charge of the second one. The sinters were obtained under laboratory conditions. Their chemi-cal composition is shown in Table 1.

A monophase electric arc furnace of 75 kVA, AC power supply and secondary voltages of 15V, 20V, 25V, 30V, 35V and 40 V was used (Fig. 1). The secondary electric circuit consisted of a vertically suspended mov-able graphite electrode and a bottom water cooled copper electrode. The furnace control was electrical, while the

driving was of a hydraulic type [13]. The gas and the powder emissions from the furnace bath space were exhausted by two independent gas conduits.


The charge used consisted of sinters, limestone and coke breeze as presented in Table 1. The composition of the two systems studied is shown in Table 2. The chemical composition of the slag obtained is outlined in Table 3. The iron content in the second system is lower by 0.35 % when compared to that of the reference one. The content of phosphorus decreases in the second sys-tem as well. The content of all other components varies in relatively narrow limits. The material balance of the process is presented in Table 4.

The output includes sinters, coke breeze, limestone and 0, 6 kg electrodes per 100 kg sinter. The coke breeze is partially replaced by anthracite in the second system. Higher total input of the reducing agent is required in this case.

The heat balance of the process is shown in Table 5.Five melting operations are carried out. The first two

refer to the first system, while operations from 3 to 5 refer to the second one. The latter includes 2 % of wastes, 10 % of sludge and CaO/SiO2 to provide basicity of 1.3. In this case anthracite is used as a partial substitute of the coke breeze. The quantities of the charge materials used and those of metal and slag obtained are shown in Table 6. The furnace in the course of pig iron tapping is shown in Fig. 2.

The chemical composition of the metal obtained

Fe2O3 FeO FeS MnO SiO2 Al2O3 CaO MgO BaO P2O5 ZnO PbO CuO SO3 others

1 70.39 10.00 0.01 3.06 3.48 1.40 6.97 2.32 1.52 0.02 0.02 0.19 0.04 0.38 0.19

2 72.40 10.00 0.01 3.15 3.59 1.36 4.78 2.23 1.46 0.02 0.12 0.30 0.04 0.35 0.18

Table 1. Chemical composition of the sinters obtained, %.

Fig. 1. Scheme (cross section) of a monophase electro arc furnace:1 - furnace roof; 2 - furnace bath; 3 - graphite electrode; 4 - fire clay bricks; 5 - furnace jacket; 6 - tap hole (runner); 7 - foam thermal insulator; 8 - graphite refractory ramming; 9 - graphite electrode; 10 - water cooled copper plate.

Elements Fe Mn Si P Al C S Ca Mg Pb Ba

1 92.129 2.513 0.732 0.022 0.306 3.766 0.016 0.268 0.101 0.037 0.102

2 91.476 3.257 0.779 0.016 0.304 3.621 0.021 0.208 0.120 0.069 0.116

Table 2. Chemical composition of the pig iron obtained, %.


612

Input

Output

1 2 1 2 kg. % kg. % kg. % kg. %

Sinter 100.00 79.65 100.00 77.22 Pig iron 63.931 50.92 67.357 52.01 Coke breeze 23.91 19.04 26.767 20.67 Slag 16.353 13.03 16.681 12.88 Electrodes 0.60 0.48 0.60 0.46 Gases 43.517 34.66 44.781 34.58

Limestone 1.034 0.82 2.140 1.65 Calculated Losses

1.74 1.39 0.69 0.53

Total 125.543 100.0 129.51 100.0 Total 125.54 100.0 129.51 100.0

Table 4. Material balance, kg/100 kg of sinter.

Input

1 2 кJ % кJ %

1. Q- combustion of C-CO 1891.3 0.18 1892.0 0.175 2. Q - slag forming 2092.5 0.19 2227.6 0.205 3. Q - silicates 3331.5 0.31 3640.5 0.336 4. Q - charge materials 250

С 2587.6 0.24 2587.9 0.239

5. Electric power 1067552.3 99.08 1073671.2 99.045 Total 1077455.1 100.00 1084019 100

Output

1 2

кJ % кJ % 1. Q - dissociation of the oxides

355271.3 32.97 349371 32.229

2. Q – dissociation of carbonates

1742.1 0.16 3607.734 0.333

3. Q – pig iron 135000.0 12.53 135000 12.454 4. Q - slag 49879.8 4.63 48290.95 4.455 5. Q - off gas 393396.1 36.51 404824.3 37.345 6. Q – slagging of the S 1628.2 0.15 1531.386 0.141 7. Heat losses 140537.6 13.04 141393.8 13.043 Total 1077455.1 100.00 1084019 100

Table 5. Heat balance, kg/100 kg of sinter.

№ Charge materials

Metal, kg Slag, kg Sinter, kg Pellets, kg

Coke breeze, kg

Anthracite, kg Limestone, kg

1. 16 4 5 - 0.20 9.7 4.5 2. 16 4 5 - 0.20 10.4 3.7 3. 20 - 3 3 0.40 11.8 4.2 4. 20 - 3 3 0.40 12.6 3.9 5. 20 - 3 3 0.40 12.6 4.1

Table 6. Experimental data referring to the metal and slag.

Elements SiO2 Al2O3 CaO MgO MnO PbO BaO S

1 29.008 11.711 37.213 11.650 1.534 0.155 6.990 1.738

2 27.213 12.751 37.213 13.455 1.951 0.300 8.227 1.603

Table 3. Chemical composition of the slag obtained, %.


613

is shown in Table 7. The figures pointed out have to be compared to those referring to high manganese pig iron in accordance with FN27-4-86 – I. The chemical composition of the latter is as follows: 0,5-1,00 % of Si, less than 3,50 % of Mn, less than 0,15 % of P and less than 0,05 % of S. The specific electric consumption in the experiments described is 9,84 kWh/kg. The compara-tive consideration of the values presented shows that the waste poly-dispersed materials can be used for pig iron production in a low shaft electric furnace.

CONCLUSIONS

The possibility to use as sintering charge waste mixtures containing 33.80 % of sinter powders, 35.88 % blast furnace powders, 28.32 % of BOF converter pow-ders, 2 % of electric arc furnace powders and landfilling sludge is theoretically and experimentally investigated.

Melting operations for pig iron production in a low shaft electric furnace are carried out. They refer to a

system using a standard sinter and another one contain-ing waste dispersed powders, sludge and anthracite substituting some of the coke breeze usually introduced.

It is found that sinters of high waste powders and sludge content and cheaper reducing agent can be used in a low shaft electric furnace to produce pig iron of low content of sulphur, phosphorus and non ferrous metals.

The values referring to the chemical composition, the yield and the specific energy consumption are very close to the standard one.

REFERENCES

1. F.Cirkel, Preparation of Pig Iron in Electric Furnace, Stahlu. Eisen, 1906.

2.B. Chatterjea, Low Shaft Furnaces as an Alternative to the Blast Furnace -Their Place in an Integrated Iron and Steel Plant, Iron and Coal Trades Review, 1956.

3. W.E. Krebs and D.J. Ram, Pig Iron Smelting Without Me-tallurgical Coke, Trans. Ind. Inst Metals, 5, 1957, 51-76.

4. M.N. Dastur and R.D. Lalkaka, Future of Low Shaft Fur-naces in India, Trans. Ind. Inst. Metals, 5, 1956, 59-82.

5. A.S. Belkin, M.A. Tsejtlin, G.P. Zuev, L.S. Zagajnov, S.G. Murat, A.G. Sitnov, A.A. Mazun, E.B Tkachev, V.N. Oguenko, Method of Pig Iron Smelting in Arc Furnace, Patent RU2142516, 1999.

6. C. Hart, Production of Pig Iron in the Electric Furnace, AIME Transactions, http://www.onemine.org/, 2008.

7. B. Konstantin, O. Mikhail, B. Aleksandrov, Method for Melting Iron-Carbon Alloys in Hearth Furnaces, Patent RU 2055908, 1996.

8. F. Edneral, A. Filipov, Electrical Metallurgy of Steel and Ferro-alloys Calculations, Moscow, Russia, 1956, (in Russian).

9. V.P. Elutin, U.A. Pavlov, B.V. Levin, Ferroalloys Mo-scow, Metallurgy, 1951, (in Russian).

№ Elements Fe Si Cu S Mn Al Ca 1. Metal 1 91.47 0.71 0.04 0.02 2.3 0.31 0.03 2. Metal 2 91.86 0.68 0.03 0.02 2.4 0.28 0.01 3. Metal 3 91.46 0.68 0.05 0.02 2.87 0.29 0.02 4. Metal 4 91.34 0.78 0.09 0.03 2.26 0.31 0.04 5. Metal 5 91.38 0.76 0.09 0.02 2.31 0.32 0.03

Table 7. Chemical composition of the metal obtained, %.

Fig. 2. Laboratory 75 kVA monophase electric arc furnace in course of metal tapping.


614

10. R. Durrer, Metallurgic processing of iron ore Moscow, Russia, 1960, (in Russian).

11. Cv. Dimitrova, I. Jabinski, J. Jordanov, Cv. Canev, P. Lesidrenski, G. Shopov, Industry and Environment, Sofia, Bulgaria, 1993, (in Bulgarian).

12. P. Ostapenko, N. Miasnikov, Wasteless technology, Moscow, Nedra, 1988, (in Russian).

13. A. Avramov, Metallurgy of ferroalloys Technology, Sofia, Bulgaria, 1988, (in Bulgarian).

Daniela Grigorova, Tsvetan Tsanev, Maxim Marinov

615


UTILIZATION OF WASTE POWDER AND SLUDGE IN IRON ORE SINTERING PROCESS


University of Chemical Technology and Metallurgy8 Kl. Ohridski, 1756 Sofia, BulgariaE-mail:[email protected]

ABSTRACT

Extraction metallurgy is characterized by low rate waste emissions utilization. The greater part of the dust and sludge generated by the gas cleaning facilities does not return to production. Their accumulation leads to eco-logical problems.

The present study treats theoretically calculated and experimentally obtained series of sinters of various basicity and concentrations of sludge and waste mixture. The latter contain sinter powder, blast furnace powder, convertor dust and electric arc furnace powder of a ratio corresponding to that yearly observed under real produc-tion conditions. The main characteristics of the sintering process are established. The results reveal that the main characteristics of sintering process do not significantly change in case of using waste powders and sludge.

Keywords: sintering, simulation model, waste powders, sludge.


INTRODUCTION

Sintering of the iron ore containing materials is required to obtain a product, whose treatment allows regular and intensive stock of the blast furnaces. Iron ores concentrates and waste iron containing powders are subjected to advanced sintering [1-3]. An important and durable trend at advance preparation of the raw materials for the blast furnace process is the sintering process. It is a thermal method for agglomeration. The heat required is additionally provided or comes from fuel combustion of the charged materials. The introduction of sintered iron ore containing charges to the blast furnace results in lower coke consumption and improved pig-iron quality [4 - 6].

The gas-treatment stage of the technological processes in extraction ferrous metallurgy generates considerable amounts of waste materials (powders and slurries). The total dust quantity emitted by sintering is equal to 4.34 t/24 h, while 4.65 t/24 h come from the blast furnace process. The total dust emitted by LD- converters is evaluated to 3.67 t/24 h, while that from

electric arc furnaces to 0.26 t/24 h [7].The average Fe content of the waste powders and

slurries which are deposed is up to 50%. Besides, the storage of these wastes represents an environmental problem [7, 8]. There are applications [9 - 14] that provide the waste powder and sludge utilization in the sintering process. A methodology for the latter simula-tion is presented [15 - 17].

The present study is aimed at the simulation of a sintering process carried out with raw materials contain-ing extraction metallurgy wastes.

EXPERIMENTAL

A mathematical method described in [17] and devel-oped as simulation PC program “Aglprice” was used for sintering modeling. The charge materials corresponded to those used under production conditions. Eight variants were developed. The first one was a base line variant analogous to that generally used under real production conditions. In the rest of the variants secondary materials (powders and sledge) from the extraction metallurgical


616

processes were introduced. In case of variants from 2 to 8 the added waste mixture was 2 %. It contained sinter powder (33.80 %), blast furnace powder (35.88 %), convertor dust (28.32 %) and electric arc furnace powder (2 %). The composition corresponded to that required for its complete utilization as well as the components yearly generation. Sludge was added in amounts refer-ring to 4%, 8% and 10 % in case of variants 2, 3 and 4. The basicity was equal to 2. In case of variants from 5 to 8 the basicity was 1.3 , while the sludge content cor-responded to 4 %, 8 %, 10 % and 20 %, respectively. All variants developed referred to wet content of 10 %, FeO content of 10 %, powder and mechanical losses of 2 % and 35 % of hot sinter for recycling. The chemical composition of the materials used is shown in Table 1.

Certain devices were used for the physical modeling of the sinter process [4]. The sinters were produced on a semi-industrial sinter installation under laboratory conditions. The raw materials were introduced using a scale of 5 g accuracy. A laboratory type mixer was used for the mixing, moistening and granulation of the raw materials. The agglomeration cup was of a square sec-tion (140mmx140 mm). The cup was placed on landing 8 where a grate was installed. The laboratory installation scheme is shown in Fig.1.

The waste gases temperature was measured by a thermocouple. The charge prepared was placed in the ag-glomeration cup. The layer height was determined by the experimental conditions used. Each experiment started with the ignition of the layer over the whole surface.

Upon process finalization the vacuum pump func-tioned for 2 extra min to provide cooling of the sinter obtained. Then the latter was weighted and sieved.


The theoretical chemical composition of the sinters obtained is shown in Table 2. The reference to the base variant shows that the chemical composition in case of added waste materials changes within narrow limits. Thus the amount of iron (III) oxide in variant 4 whose sludge content is equal to 10 % decreases from 70. 39 % to 69.84 %, while that of hematite is more than 2 % higher in the variants whose basicity is 1.3. The content of ZnO and PbO increases with the increase of the waste sludge presence. In analogy with the iron content that of BaO decreases from the initial 1.52 % (in case of the base variant) to 1,41 % in variant 4. The coarse coefficient of the charge materials decreases with the increase of the waste material presence from 0.97 to 0.91.

The material balance in Table 3 illustrates the change in the charge materials at sintering. The input includes

Fig. 1. Sinter installation scheme: 1 - electric motor, 2 – con-nector, 3 – exhauster, 4 - cleaning outlet, 5 - dust capturing, 6 - sliding rule, 7 – cup, 8 – landing, 9 - pressure gauge, 10 - millivolt meter.

Materials Total Fe-content

Total Mn-content

SiO2 Al2O3 CaO MgO C volatile

iron ore bedding

54.13 2.25 2.56 0.99 3.52 2.16 0.16 3.59

limestone 0.07 - 1.99 0.92 53.39 0.59 - 0.79 coke breeze 4.30 0.25 14.50 6.46 2.14 0.31 66.30 2.81 hot sinter for recycling

58.52 1.99 3.48 1.20 6.36 1.12 - -

waste mixture 43.52 2.15 7.85 4.95 7.54 1.00 4.50 4.38 sludge 46.80 1.91 3.95 0.54 11.14 0.91 - 13.53

Table 1. Chemical composition of the materials used , %.


617

iron ore, limestone, aglofuel, air as well as waste materi-als additives. The output includes sinter, hot sinter for recycling and waste gases. The balance is calculated on the ground of the production of 100 kg of sinter.

In case of a constant content of the waste mixture the iron ore consumption decreases from 105.04 % to 87.90 % with the increase of the waste sludge (going from the

base down to the last variant). The change in aglofuel is within narrow limits. The quantity of limestone decreases in variants 5 and 6, while it is not required in the last two variants. This is explained with the lower basicity (1.3) of the mixtures considered. The addition of waste materials affects the waste gases quantities. It is 76.17 % in variant 1 and goes down to 5.17 % in variant 8.

Variant 1 2 3 4 5 6 7 8

basicity 2 basicity 1.3 Fe2O3 70.39 69.96 69.88 69.84 72.58 72.53 72.40 71.68 FeO 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 FeS 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 MnO 3.06 3.06 3.05 3.05 3.16 3.15 3.15 3.12 SiO2 3.48 3.62 3.67 3.69 3.52 3.57 3.59 3.73 Al2O3 1.40 1.45 1.42 1.41 1.39 1.37 1.36 1.32 CaO 6.97 7.24 7.34 7.39 4.57 4.64 4.78 5.55 MgO 2.32 2.25 2.21 2.19 2.29 2.25 2.23 2.11 BaO 1.52 1.49 1.44 1.41 1.54 1.49 1.46 1.32 P2O5 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 ZnO 0.02 0.06 0.10 0.12 0.06 0.10 0.12 0.22 PbO 0.19 0.24 0.28 0.30 0.25 0.28 0.30 0.39 CuO 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 SO3 0.38 0.37 0.35 0.35 0.37 0.36 0.35 0.32 others 0.19 0.18 0.18 0.17 0.19 0.18 0.18 0.16

Table 2. Chemical composition of the sinters obtained, %.

Variant 1 2 3 4 5 6 7 8

basicity 2 basicity 1.3 Material balance INPUT

iron ore mixture 105.04 99.44 95.88 94.11 102.88 99.37 97.47 87.90 limestone 5.94 5.72 5.30 5.09 0.51 0.03 - - aglofuel 4.67 4.47 4.39 4.36 3.86 3.78 3.76 3.70 hot sinter for recycling

35.00 34.96 34.93 34.92 35.00 34.99 35.00 35.01

was

te

mat

eria

l

waste mixture

- 2.00 2.00 2.00 2.00 2.00 2.00 2.00

sludge - 4.00 8.00 10.00 4.00 8.00 10.00 20.00

air 48.95 48.14 47.32 46.89 42.18 41.27 41.08 40.21 wet 11.57 11.56 11.56 11.56 11.33 11.32 11.32 11.36 Total income 211.17 210.19 209.38 208.93 201.76 200.76 200.63 200.18

Material balance OUTPUT sinter 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 hot sinter for recycling

35.00 34.96 34.93 34.92 35.00 34.99 35.00 35.01

waste gases 76.17 75.33 74.45 74.01 66.76 65.77 65.63 65.17 Total production 211.17 210.29 209.38 208.93 201.76 200.76 200.63 200.18

Table 3. Material balance, kg per 100 kg sinter.


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Eight sintering operations are carried out under laboratory condition aiming to evaluate the impact of the different waste materials introduced. The materials ratio in the charge follows the material balance calcu-lations shown in Table 3. The agglomeration is carried out in presence of a constant content of hot sinter for recycling (35 %). The first charge is prepared in absence of waste materials.

The sinter period is 15-16 min. Only in case of sinter 8 containing 2 % of wastes mixture and 20 % of sludge the period is 19 min. The vacuum change below the grate is shown in Fig. 2 for the different variants. It is seen that it is in the range from 520 mm to 530 mm (water column). In case of basicity of 1.3 the vacuum reaches 610 mm (water column) (see variant 8). The difference of 80 mm is probably due to the increased amount of

sludge and the absence of limestone. In addition worse granulation is observed in the last case pointed above.

The changes in fume gases temperature are shown in Fig. 3. It is seen that the value is 310oС in case of the base variant. It increases to 320 oС for variants 2, 3 and 7, while it goes up to 380oС for variants 5 and 6. The temperature increases to 400oС in case of variants 4 and 8 where the percentage of the waste sludge used is higher. It is worth adding that temperature changes are expected in the course of the sintering process.

The physical (macro) structure of the sinter obtained in case of variant 4 is shown in Fig. 4. Corresponding to Miler’s classification [1] it has a porous structure with irregularly distributed large pores. This result indicates that the charge is sintered with increased sintering losses. This is most probably due to the fine composition of the

Fig. 2. Vacuum change below the grate in the course of the sintering process.

Fig. 3. Changes in the fume gases temperature in the course of the sintering process.


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initial material or the low quality of the charge prelimi-nary pelletization.

The structure of the sinters obtained in case of vari-ants 7 and 8 (using 10 % and 20 % of sludge) is similar. All other sinters obtained are of a spongy structure with pores of 2 - 4 mm uniformly distributed in the sinter bulk. Тhe results considered show that the physical struc-ture of the sinters is genetically related to the physical structure of the charge materials used.

The fraction distribution of the sinters (in %) is shown in Fig. 5.

It is seen that the fractions above 30 mm amount to 28 % with the exclusion of variant 1. Fractions of 20 - 12 mm amount to 26.4 %, while those of 5 mm and below are mainly obtained in case of the base variant. They amount to 34.75 %. This is most probably due to the sinter charge preparation. In all other variants the

quantity distribution varies between 6.56 % and 16.38 %. Taking into consideration all variants studied the average fraction is under 12 mm and its amount refers to 32 %. In fact the hot sinter is that to be used for recycling.

CONCLUSIONS

The sinter composition is relatively constant and it can’t be materially distinguished from that obtained in the corresponding industrial process. The addition of sludge to the charge increases the quantity of non-ferrous metals as ZnO goes up to 0.22 %, while PbO is increased to 0.39 %. The total Fe content in the sinter is 57.06%. going from variant 1 to 4 it decreases slowly to 56.68 %. The total Fe content decreases proportionally to the amount of the waste materials used.

The consumption of iron ore mixture decreases in case of all variants with the increase of the percentage of waste powders and sludge added. It is 105.04 kg in the base variant and goes down to 87.90 kg per100 kg of sinter.

The experimental results obtained show that the waste materials introduction does not affect the sinter-ing process. Its parameters remain relatively unchanged. The fraction analysis is carried out without additional breakage. The amount of the sinter obtained by fine iron ore and waste materials (using up to 10% sludge) is ac-ceptable. Besides it can be further used as a charge for the blast furnace process. The ratio of the components of the waste material mixture is chosen to guarantee the full utilization of the latter under real production conditions.

Fig. 4. Macrostructure of the sinter obtained.

Fig. 5. Fraction composition.


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REFERENCES

1. B. Drakaliiski, Tcv.P. Tzanev, Metallurgy of pig iron, Technics and Kremikovtzi JSCo, Sofia, 1998, (in Bulgarian).

2. Е.F. Vegman, Theory and technology of sintering pro-cesses, Moscow, Russia, 1981, (in Russian).

3. V.I. Кorotich, Foundations of the theory and technology for preparation of raw materials for blast furnace melt-ing operation , Metallurgy, Moscow 1978, (in Russian).

4. А.G. Аvramov, I.H. Ivanchev, Tcv.P. Tzanev, Metallurgy of iron, Technology, Sofia, Bulgaria, 1994, (in Bulgarian).

5. Е. F. Vegman, Enrichment of ores and concentrates, Metallurgy, Moscow 1968, (in Russian).

6. A. Ghosh, A. Chatterjee, Iron Making And Steelmaking: Theory And Practice, PHI Learning Pvt. Ltd., 2008.

7. Cv. Dimitrova, I. Jabinski, J. Jordanov, Cv. Canev, P. Lesidrenski, G. Shopov, Industry and Environment, Sofia, Bulgaria, 1993, (in Bulgarian).

8. BAT Reference Document on the production of iron and steel, European Commision, IPPC Bureau, 2000.

9. N.P. Nayak, Characterization and Utilization of Solid Wastes Generated from Bhilai Steel Plant, Dep. Min. Eng., Rourkela, 2008.

10. G.V Korshikov V.V. Grekov, A.K. Semenov, S.L. Zevin, V.N. Grigoriev, I. S. Jarikov, E.G. Korshikova, V.V. Chujkov, A.S. Kuznetsov, V. L. Emeljanov,

Process of Mixing Sinter of Different Basicity from Iron Containing Waste of Metallurgy”, Patent RU2221880, 2004, (in Russian).

11. R.C. Gupta, Utilisation of Solid Waste from Conventional Steel Plants for Preparation of Value Added Products, Environmental Management in Metallurgical Industries, EMMI, 2010, 27-34.

12. F.M. Yamamoto, Recycling process for sludges and dusts from blast furnace and steel works, Patent WO2005017216, 2005.

13. N.V. Fedorenko, V.I. Danilov, A.T. Korotkikh, Use of iron-bearing wastes in sinter and pig iron production, Metallurgist, 29, 12, 1985, 376-378.

14. G.V. Koloshikov, V.V. Grekov, A.K. Semenov, S.L. Zevin, V.N. Grigorev, I.S. Jarikov, Process of Mixing Sinter of Different Basicity from Iron-Containing waste of Metallurgy, Patent RU 2221880, 2004.

15. J. Mitterlehner G. Loeffler, F. Winter, H. Hofbauer, H. Schmid, E. Zwittag, Modeling and simulation of heat front propagation in the iron ore sintering process, ISIJ International, 44, 2004, 11-20.

16. R. Venkataramana, S.S. Gupta, P.C. Kapur and N. Ramachandran, Mathematical Modelling and Simulation of the Iron Ore Sintering Process, Tata Search, 1998, 50-55.

17. I Ivanchev, Tsv. Tsanev, Production of Pig Iron- Guidance, Technology, Sofia, Bulgaria, 1979, (in Bulgarian).

Sergey Lezhnev, Irina Volokitina, Toncho Koinov

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RESEARCH OF INFLUENCE EQUAL CHANNEL ANGULAR PRESSING ON THE MICROSTRUCTURE OF COPPER

Sergey Lezhnev1, Irina Volokitina1, Toncho Koinov2

1 Karaganda State Industrial University, Temirtau, Kazakhstan E-mail: [email protected] Department of Physical Metallurgy and Thermal Equipment University of Chemical Technology and Metallurgy 8 Kl.Ohridski, 1756, Sofia, Bulgaria E-mail: [email protected]

ABSTRACT

The effect of equal-channel angular pressing (ECAP) in step die on the microstructure and properties of preheated copper is studied. It is found that the grain size after ECAP depends slightly on the preliminary thermal treatment, but the quenching applied decreases copper hardness values by 15 %, which in turn provides to decrease the pressing force during the first pass from 620 kN to 510 kN. The minimum average grain diameter reached in case of quenching alloy M1 at 700oС, ECAP in step die at room temperature and 6 cycles of deformation is 0.6 microns.

Keywords: microstructure, properties, copper, pressing force.


INTRODUCTION

A number of methods based on severe plastic defor-mations (SPD) proceeding in the bulk of the correspond-ing body have been developed during the past decade. These processes include multi-axis forging [1], twist extrusion [2], upsetting with torsion [3], etc. However, these methods of deformation are inefficient and time-consuming, because in some cases shear deformations are accompanied by large single reductions, while in others great deformation forces are required.

The ECAP method [4] is the most successful method of severe plastic deformation used today. It has great potential of producing ultrafine-grained structure with a uniform equiaxed structure and grain boundaries of pre-dominant high-angle disorientation. It is worth adding that the workpiece retains its original size. In fact the uniform compression scheme provided excludes the formation of micro-and macro material discontinuities in the course of deformation resulting thus in high-quality pieces.

A number of designs templates to implement equal-channel angular pressing are developed. These are the equal channel angular die [5], the equal channel step die [6], the equal channel angular die with two dimensional metal flow [7], the equal channel step die with three dimensional metal flow [8], etc.

Many scientists study the process of metal deforma-tion by equal-channel angular pressing. This is also done in the group of Prof. A.B. Naizabekov at the Karaganda State Industrial University (Kazakhstan). Good results referring to the production of ferrous and non-ferrous metals (aluminum, copper) with ultra-fine grain structure by equal channel angular pressing in equal channel step die are already obtained (Fig. 1).

This paper investigates the effect of severe plastic deformation in pressing copper workpieces using the equal channel step die. The effects of the preliminary and post thermal treatment on the metal microstructure evolution and strengthening are studied as well. The interest is mainly determined by the metal properties.


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Copper has a low coefficient of friction combined with high ductility and good resistance to corrosion in a number aggressive media as well as good electrical conductivity. Furthermore, it is easily processed by forming in a cold resulting in more than 90 % of total reduction. There is an extra point which needs to be addressed. It refers to the fact that the equal channel angular pressing does not always produce metal of ultrafine grain structure in the course of a small number of cycles, which in turn requires the in-troduction of preliminary and final thermal treatment. The preliminary thermal treatment is applied to avoid brittle fracture products in case of severe plastic deformation. It adapts the alloys structure prior to the process providing to escape from lattice distortions and stock of residual en-ergy. Thus improved mechanical properties are obtained.

EXPERIMENTAL

Technical copper grade M1 was used. The samples were subjected to preliminary thermal treatment which included annealing, quenching and normalizing to the standard mode. Square section (15 mm´15 mm) samples of 70 mm length were subjected to ECAP in an equal channel step die. The channels junction angle was 125°, while the tilting of the workpiece around the longitudinal axis [9] was 90°. The friction between the tool and the workpiece was decreased by using palm oil as a lubricant. The deformation was carried out at temperatures lower than that of the material recrystallization [10] to eliminate the effect of deformational-stimulated growth of grains usually brought about by ECAP temperature increase.

The experiments were carried out in three modes: mode 1 referred to pressing temperature of 25°C; mode 2 - to pressing temperature of 90°C; mode 3 - to pressing temperature of 180°C. In all cases the number of cycles applied was equal to 6.

As pointed above heating at temperatures higher than that of recrystallization resulted in significant grain growth and sharp decrease of copper strength. That is why it was necessary to determine the recrystallization temperature. Its approximate value was estimated on the ground of formulas reported [11] and further verified by a laboratory experiment. Samples after ECAP were cut into thin plates of thickness of 5 mm and heated for 1 hour at a temperature in the range from 100oC - 270oС. The subsequent cooling of the samples was carried out in water. The samples treated were studied using an opti-cal microscope and a transmission electron microscope. They were also tested using the torsion-tensile machine. The testing referred to the median plane of the sample aiming to avoid the effect of peripheral areas. Obtained samples were examined in two sections: the transverse and longitudinal direction.

Thin samples were prepared to carry out the metal-lographic analysis. A standard procedure was applied using Leica optical microscope equipped with a micro hardness tester. Aiming further analysis workpieces of thickness of 250 mm were cut using a high-precision cut-ting machine and subjected to fine polishing to remove the cold-worked layer and electrolytic thinning using Tenupol 5. Then they were examined by JEM2100 JEOL transmission electron microscope.

The chemical composition of the samples was deter-mined using the multifunction device RIKOR. The me-chanical properties of the alloys undergone heat treatment and subsequent ECAP were studied by the torsion-tensile machine MI40KU. Standard cylindrical samples were used. The tensile speed was 0.5 mm/min corresponding to a speed of deformation of 0.56 × 10-3 s-1.

RESULTS AND DISCUSSIONMaterialThe chemical composition of the samples studied

is given in Table 1.

Fig. 1. An equal-channel step die for pressing.

punch die workpiece

Chemical element Cu Fe Zn О Content, % 99,94 0,005 0,004 0,051

Table 1. Chemical composition of copper M1.


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MicrostructureCopper alloys can become either softer or harder

by heat treatment. However, unlike other alloys, they become harder by slow cooling in air, while softer in case of rapid cooling in water.

Fig. 2 shows optical photos of the microstructure of the original sample and that subjected to preliminary thermal treatment. It is seen that the structure of the original sample is characterized by the presence of a large number of twins (Fig. 2a), while that obtained upon deformation by annealing shows equiaxed grains with the presence of twins (Fig. 2b).

The effectiveness of ECAP can be evaluated on the ground of the comparative examination of the microstructure of the copper alloys prior to and after the deformation applied. The alloy M1 microstructure obtained after pressing at different temperature values is illustrated in Figs. 3 - 5.

The changes in copper microstructure brought about by ECAP were investigated by transmission electron microscopy. Aiming a comparison the photos were taken after the 6th ECAP cycle at various temperatures

of pressing. Most of the strongly deformed metals and alloys

lack ductility which leads to significant fragility. The plastic properties of the metal can be improved by reducing the stress. This is usually achieved through annealing, seasoning, and tempering of the metal. But the desired final heat treatment depends also on the recrystallization temperature.

The structure formed during ECAP at room tem-perature determines the behavior of copper during the subsequent heating. Samples subjected to quenching and 6 pressing cycles at 25oC were referred to as it was found that they had the most fine-grained structure.

Mechanical propertiesFig. 8 illustrates the Vickers micro-hardness ob-

served. Results referring to the microhardness and the average grain diameter of the copper alloy M1 after preliminary heat treatment are shown in Table 2.

The ECAP tensile test was followed by compressing and determination of the Vickers microhardness.

The strength properties of alloys are characterized by

d)

a) b) 100 μm 50 μm 100 μm

c) 100 μm 100 μm

Fig. 2. Optical photos of the microstructure of copper prior to and after preliminary thermal treatment, x100a) prior to thermal treatment; b) after annealing; c) after normalization; d) after quenching.


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a) b)

c) d)

20 μm 20 μm

20 μm 20 μm

Fig. 3. Optical photos of copper microstructure of after 6 cycles of ECAP in a step die at 25°C, x1000 a) prior to treatment; b) after annealing; c) after normalization; d) after quenching.

a) b)

c) d)

20 μm

20 μm 20 μm

20 μm

Fig. 4. Optical photos of the microstructure of copper after 6 cycles of ECAP in step die at a temperature of 90°C, x1000. a) prior to treatment; b) after annealing; c) after normalization; d) after quenching.


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Fig. 5. Optical photos of the microstructure of copper after 6 cycles of ECAP in a step die at a temperature of 180°C, x1000. a) prior to treatment; b) after annealing; c) after normalization; d) after quenching.

a) b)

c) d)

20 μm 20 μm

20 μm 20 μm

Fig. 6. Microstructure of copper after 6 cycles of ECAP in a step die observed by a transmis-sion electron microscope at: a) 25oC; b) 90oС; с) 180oС.

a) b)

с)


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the values of the yield strength σт and tensile strength σв, while the plastic characteristics are characterized by the values of contraction ratio and elongation samples prior to the destruction [12]. According to the mechanical tests the tensile strength of alloy M1 depends on the passes number and the temperature (Figs. 9-11).

In case of cold deformation the alloy‘s most intense hardening is observed within 2-3 passes. Then the pro-cess of hardening slows down. The temperature increase

to 180oC leads to a significant decrease of the alloy’s deformation resistance. The most intensive hardening of the alloy occurs at this temperature at a relatively low degree of deformation (2 passes). Then the hard-ening process slows down. Dynamic loss of strength is observed on further deformation. The structure obtained is inequigranular with a large average grain diameter.

The consideration presented above leads to the conclusion that pressing at room temperature is the most

Fig. 7. The microstructure of copper M1, annealing T of 270oС.

Fig. 8. Determination of micro-hardness of copper.

Type of heat treatment Alloy of copper Hardness Н10

Average grain diameter, µm

Initial М1 97,5 200 Annealing М1 104,6 260

Normalizing М1 109,5 120 Quenching М1 96,8 145

Table 2. Results referring to Vickers microhardness and the average grain diameter for the alloys M1 after pre-liminary heat treatment.

Fig. 9. Dependence of the tensile strength of alloy M1 on the passes number at room temperature.

Fig. 10. Dependence of the tensile strength of alloy M1 on the passes number at 90°C.


627

efficient. Therefore, other mechanical characteristics are given only for alloy M1 after 6 ECAP cycles at room temperature (Table 3).

DISCUSSIONEffect of ECAP in step die on copper micro-

structure Metallographic analysis of copper after ECAP in

step die shows that during the initial pressing stage the initial grains are oriented at an angle to the sample axis, while their substructure is badly etched. After the third cycle the sample’s structure becomes partially meshed and polygonized. Formation of the submicrocrystalline structure at ECAP is a mesh structure. It is found that the formation of nanostructures by severe plastic deforma-tion has a pronounced phase character. In order to facili-tate the description of the processes taking place with copper at ECAP the deformation process is conditionally divided into stages. The first stage refers to a small de-gree of deformation for equal-channel angular pressing (1-2 passes). It is characterized by the appearance of a mesh structure: a dislocation is redistributed in the bulk of the grains leading to the formation of plexus blurred walls. The dislocation density of the latter is higher than that of the surrounded regions. The average size of such cells is ca 0.5-2 microns and the wall thickness is one order smaller [14]. The increase of the degree of deformation leads to cell size changes and increase of their blurred boundaries. The formation of tangles and plexus of dislocations starts to fill the entire volume of the initial grains (Fig.12) with the further increase of the degree of deformation.

Due to excess of the same sign dislocations in the wall the adjacent cells are disoriented to angles 1-20o and direction of reversal at the boundaries between differ-ent adjacent cells are different, so at the mesh structure macroscopic changes of crystallite orientations is not obtained [13].

The second stage refers to 4-5 passes at ECAP. This is accompanied by the formation of transitional struc-tures of mesh and nanostructures features accompanied by large misorientation. The increase of the deforma-tion degree leads to a certain decrease of the average size of the cells and disorientation increase at the cell boundaries.

Twinning takes place during the second stage.

Fig. 11. Dependence of the tensile strength of alloy M1 on the passes number at 180°C.

Type of heat treatment

Alloy of copper

Hardness Н10

Yield strength σт, MPa

Tensile strength σв, MPa

ψ, % δ, %

Initial М1 229,4 402 469 25,5 3,6 Annealing М1 215,7 360 374 36,3 8,0

Normalizing М1 236,8 402 502 22,2 5,7 Quenching М1 246,1 460 560 26,0 7,2

Table 3. Mechanical testing results for copper alloy M1 after 6 cycles of ECAP at room temperature.

Fig. 12. Schematic representation of the mesh structure formation in ECAP course: A - distribution of dislocations at the grain boundaries; B - dislocations blocked by sub-grains borders; C - mesh structure formation.


628

This process is activated when the deformation diso-rientation’s fields (cells) become so small in size that the generation of lattice dislocations in them becomes impossible. The twin boundaries represent a different coherent structure and besides they are randomly dis-tributed in the metal. Therefore specific crystallographic shifts leading to their formation do not make a significant contribution to the active deformation of the metal. In this context, twinning is a complementary mechanism of grains grinding.

A homogeneous nanostructure is formed during the third stage (after more than 6 passes). The structure of the grains experiences strong elastic distortions caused by grain boundaries long-range stresses.

The results obtained provide to advance a model of microstructure evolution in the course of severe plastic deformation. It describes the transition in the course of severe plastic deformation process from a mesh to grain structure, which is characterized by angle grain bounda-ries. Mesh structure is formed during the initial stage of deformation. It is then transformed during the SPD. Thus the cell walls become more narrow and streamlined. However, with the further deformation proceeding the dislocation density in the walls becomes higher than the critical one. This leads to the development of a reverse transition consisting in the annihilation of opposite sign dislocations. As a result excessive dislocation of two signs playing different roles appears in the cell walls. The increased misorientations and transformation of the grains cells are due to dislocations of Burgers vector perpendicular to the boundary.

Sliding is another type of dislocation which forms long-range stress fields. It leads to an increase of the elastic microdistortions and atomic displacement of the crystal lattice sites. This results in grain boundary sliding and relative displacement of the grains.

The model advanced is in agreement with many ex-perimental results found by electron microscopy studies of materials subjected to severe plastic deformation, i.e. the equiaxed shape of the grains, the significant distor-tions of the crystal lattice and the presence of a high density of dislocations in the grain boundaries.

The microstructure analysis of alloy M1 after ECAP shows that intensive grain refinement is observed after each cycle of deformation. It is also found that after 4-5 cycles of pressing the grain structure becomes substan-tially uniform in all sample directions.

As pointed above annealing, normalizing, quenching and tempering have to be applied prior to ECAP aim-ing to avoid brittle fracture of the product, to improve the efficiency of grain refinement and to improve the mechanical properties of the material. The present study shows that all preliminary heat treatments considered resulted in grain grinding. But the final grain size after ECAP is found almost identical. Hence it is concluded that the grain size after ECAP depends little on the pre-liminary heat treatment. It is worth adding that increased quenching reduces copper hardness by 15%, which helps to reduce the pressing force during the first pass from 620 to 510 kN.

The increase of the pressing temperature decreases the pressing force required and the deformation resist-ance, but increases the unevenness of the metal flow. The same effect is observed with the increase of the tempera-ture gradient between the workpiece and the container. This results in uneven distribution of the deformation resistance over the cross section of the workpiece. Besides, the peripheral layers cooling leads to a more rapid flow of the inner layers of the workpiece (Fig. 13).

Well outlined anisotropy of properties is observed at pressing in the range of 90oС - 180oС. It consists in increase of the strength characteristics and decrease of the plasticity along the cross direction when compared to those referring to the longitudinal one.

The minimum average grain diameter obtained by pressing alloy M1 in equal channel step die is 0.6 mi-crons. Such grain size is obtained after quenching and ECAP at room temperature and 6 cycles of deformation.

Fig. 13. The microstructure of copper M1 flow.


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All data presented above lead to the conclusion that pressing of copper alloy M1 at room temperature is the most efficient.

The analysis of alloy M1 microstructure after re-crystallization annealing shows that annealing at tem-peratures in the range of 100oС -160oС coincides with the temperature range of the reverse transition, which is characterized by a gradual decrease of the dislocation density and the concentration of excess defects as well as by redistribution of dislocations leading to a decrease in the level of microdistortions. At annealing temperature of 220oС a primary recrystallization process starts, i.e. new recrystallized grains appear from the deformed matrix at the boundaries in the transition bands (Fig. 7). The further increase of the annealing temperature to 270°C leads to rapid collective recrystallization in samples undergone ECAP. This results is in agreement with the observations of S.S. Gorelik [14] for small grains (d ≤ um). The value of the driving force of col-lective recrystallization is of the order of magnitude of the driving forces of primary recrystallization.

Changes in the mechanical properties of the alloy under ECAP effect

The graph presenting the dependence of alloy M1 tensile strength on the number of passes at different pressing temperatures suggests an intensive growth of the strength characteristics within the first 2-3 press cy-cles. The next cycle is also connected with steady growth of strength characteristics but the latter is less intensive. The temperature increase to 180oС leads to a significant decrease of the alloy’s deformation resistance. The most intensive hardening of the alloy at this temperature is obtained at a relatively low degree of deformation (2 passes). Then the hardening process slows down, while during the subsequent deformation dynamic loss of strength starts to outline. This results in less processes structure with grains of a large average diameter.

Hence it can be concluded that pressing copper alloy M1 at room temperature the most efficient.

The maximum values of the mechanical strength characteristics of alloy M1 are obtained after quenching and pressing at room temperature. They refer to σт = 460 MPa and σв = 560 MPa. It is evident that the tensile strength is increased almost twice when compared to that of the initial large grain material.

The values of Vickers microhardness for all alloys

demonstrate a steady almost linear increase with each new cycle of deformation. The maximum value reached is equal to 246.1 units. The mechanical tests carried out verified the value of recrystallization temperature obtained by the metallographic investigations. The tem-perature referring to the sharp decrease in microhardness of 49.3 units (20 %), coincides with that of the appear-ance of new recrystallized grains.

CONCLUSIONS

The present work reports data on the effect of equal channel angular pressing in step die on the microstruc-ture and properties of the heat-treated copper at various pressing temperatures. The main conclusions refer to:

A noticeable decrease of the grains size after each cycle of deformation (after quenching from 145 microns to 0.6 microns for 6 cycles of deformation on the route Вс at room temperature);

A significant strength increase when compared to the initial one. The tensile strength increases almost twice, while the plasticity decreases less than in other metal forming methods;

Almost no effect of the preliminary heat treatment on the grain size. Quenching reduces the hardness of copper by 15 %, which helps to decrease the pressing of the first passes from 620 to 510 kN and to obtain more fine-grained structure;

The increase of the pressing temperature decreases the pressing force and the deformation resistance, but the unevenness of the metal flow increases. The latter effect is also observed on increase of the temperature gradient between the workpiece and the container. The results obtained refer to an uneven distribution of the deformation resistance upon the cross section of the workpiece. The peripheral layers cooling leads to a more rapid flow of the internal layers of the workpiece. Pressing in the range of 90oС -180oС results in strong anisotropy of the properties is observed - the strength characteristics are higher while the plasticity is lower along the cross direction when compared to those along the longitudinal direction. That is why pressing of copper alloy M1 at room temperature is to be chosen.

The recrystallization temperature is that of the microhardness sharp decrease of 49.3 units (20 %) as it coincides with the appearance of new, recrystallized grains in the alloy structure.


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Acknowledgements

The financial support of the project “Getting high-quality materials by a combination of thermal treatment and severe plastic deformation” within the frameworks of “2012-2014 Research Grants” program is gratefully acknowledged.

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13. I.I. Novikov, The theory of heat treatment of metals, Moscow, Metallurgy, 1986, (in Russian).

14. S.S. Gorelik, Recrystallization of Metals and Alloys, Moskow, Metallurgy, 1967, (in Russian).

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Journal of Chemical Technology and Metallurgy is a specialized scientific edition presenting original research results in the field of chemical technology and metallurgy, chemical engineering, biotechnology, indus-trial automation, environmental protection and natural sciences. The articles published in Journal of Chemical Technology and Metallurgy refer to:

Inorganic Chemistry; Organic Chemistry; Analytical Chemistry; Physical Chemistry;

Organic Synthesis and Fuels; Polymer Engineering; Textile and Leather; Cellulose, Paper and Polygraphy; Inorganic and Electrochemical Productions;

Chemical Engineering; Industrial Automation; In-formation Technology; Biotechnology; Economics and Management of Chemical and Metallurgical Industry; Sustainable Development and Environmental Protection;

Physical Metallurgy; Metallurgy of Iron and Cast-ing; Metallurgy of Non-ferrous and Semiconducting Materials; Technology of Silicates; Nanomaterials.

Journal of Chemical Technology and Metallurgy publishes full-length research papers, critical and book reviews. Research papers are expected to be complete and authoritative accounts of work that has significance and interest, presented clearly and concisely. Critical reviews are commissioned by the Editor-in-Chief. Au-thors intending to offer critical reviews are invited first to contact the Editor-in-Chief.

Contributions will only be considered for publica-tion if they are relevant to the topics pointed above. Presentation and discussion should be at the level of the Journal status gained. The language can be a reason for rejection if below an acceptable level of clarity. De-tailed descriptions of the equipment used should only be given if the latter is new. Papers reporting experimental data without adequate interpretation are not acceptable. Contributions will be accepted for publication only on the recommendation of referees.

Submission of ManuscriptsManuscripts should be submitted to the Editor-in-

Chief at the following address:University of Chemical Technology and Metallurgy8 Kliment Ohridski blvd., Sofia 1756, Bulgaria, (for JCTM)Manuscripts may be submitted electronically to

e-mail: [email protected]

GUIDE FOR AUTHORS

Manuscript preparationIt is important that the file be saved in the native

format of the wordprocessor used. All contributions should be typed, double-spaced, 12-pt font. All pages must be numbered in sequence. Most formatting codes will be removed and replaced on processing the article. In particular, do not use the wordprocessor’s options to justify text or to hyphenate words. However, do use bold face, italics, subscripts, superscripts, etc. Do not embed graphically designed equations or tables, but prepare these using the wordprocessor’s facility. When preparing tables, if you are using a table grid, use only one grid for each individual table and not a grid for each row. If no grid is used, use tabs, not spaces, to align columns.

Essential title page informationTitle. It should be concise and informative. Titles

are often used in information-retrieval systems. Avoid abbreviations and formulae where possible.

Author names and affiliations. The names of the authors have to be clearly indicated. Present the authors’ affiliation addresses (where the actual work was done) below the names. Provide the full postal address of each affiliation, including the country name.

Corresponding author. Clearly indicate who will handle correspondence at all stages of refereeing and publication, also post-publication. Ensure that phone numbers (with country and area code) are provided in addition to the e-mail address and the complete postal address. Contact details must be kept up to date by the corresponding author.

Present/permanent address. If the author has moved since the work described in the article was done, or was visiting at the time, a “Present address” (or “Permanent address”) may be indicated as a footnote to that author’s name. The address at which the author actually did the work must be retained as the main, af-filiation address. Superscript Arabic numerals are used for such footnotes.

AbstractA concise and factual abstract is required. The ab-

stract should state briefly the purpose of the research, the principal results and major conclusions. An abstract is often presented separately from the article, so it must be able to stand alone. For this reason, References should be avoided, but if essential, then cite the author(s) and year(s). Also, nonstandard or uncommon abbreviations

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should be avoided, but if essential they must be defined at their first mention in the abstract itself.

Article structureIntroductionState the objectives of the work and provide an

adequate background. Avoid a detailed literature survey or a summary of the results.

ExperimentalProvide sufficient details to allow the work to be

reproduced. Methods already published should be indi-cated by a reference: only relevant modifications should be described.

Results and DiscussionResults and the discussion provided should be clear

and concise.ConclusionsThe main conclusions of the study have to be pre-

sented in a short Conclusions section.AcknowledgementsAcknowledgements are given in a separate section

at the end of the article before the references.References should be cited in the text by an Arabic

numeral in square brackets, listed numerically and typed double-spaced on a separate sheet at the end of the paper. References should be arranged in the order in which they appear in the text. Journal titles should be abbreviated ac-cording to the latest Chemical Abstracts Service Source Index. Only articles that have been published or are in press should be included in the references. Unpublished results should be cited as such in the text. References in non-English language should be translated in English. The original language must be shown in brackets. In the reference list, the styling, punctuation and capitalization should be as follows:

For journals:S.K. Maji, A. Pal, T. Pal, A. Adak, Modeling and

fixed bed column adsorption of As(III) on laterite soil, Sep. Purif. Technol., 56, 2007, 284-290.

For books:A. Krueger, Carbon Materials and Nanotechnology,

Weinheim, WILEY-VCH Verlag GmbH & Co. KGaA, 2010.

For edited books:F.W. Cooke, J.E. Lemons, B.D. Ratner, in: B.D.

Ratner, A.S. Hoffman, F.J. Lemons (Eds.), Biomaterials Science, Academic Press, San Diego, 1996, p. 11.

For conference proceedings, symposia, etc.:E. Motta, D. Rajpathak, Z. Zdrahal, R. Roy, The

Epistemology of Scheduling Problems, F.V. Harmelen (ed.), Proceedings of the 15th European Conference on Artificial Intelligence, Lyon, France, 2002, 215-219.

When you are citing papers published in Journal of Chemical Technology and Metallurgy you should use the abbreviation of the title as follows:

J. Chem. Technol. Metall.

Formulae should be typewritten (Equation Editor) with ample space around them. The meanings of all symbols have to be given immediately after the equa-tion in which they are first used. Equations should be sequentially numbered (on the right side of the equation and in parentheses). Subscripts and superscripts should be clearly indicated. Greek letters and other non-Latin symbols should be typed using Times New Roman Gr or Symbols.

Units should be given in SI system. They should be in the form, e.g. g cm-1 rather than g/cm.

Figures should be numbered consecutively with Arabic numerals. All figures should be supplied on separate sheets. All illustrations must be readable when reduced to a width of 75 mm (single column figure) or 160 mm (double column figure). Photographs, charts and diagrams are all to be referred to as “Figures(s)” and should be numbered consecutively in the order to which they are referred. They should accompany the manuscript, but should not be included within the text. All illustrations should be clearly marked with the figure number. Figure captions should be typed double-spaced on a separate sheet. Figures should not include text. Graphic files should be in TIFF or JPG with resolution not less than 300 dpi.

Tables should be numbered consecutively with Arabic numerals in the order of appearance in the text. Each table should be given on separate sheet with a short descriptive title directly above it, with essential footnotes below.

One copy of the manuscript in English and one in Bulgarian (for Bulgarian authors) with a set of original illustrations and a disk must be submitted. The manu-script should be written with true-type font Times New Roman size 12 with double line spacing. Manuscripts of papers submitted to the journal will not be sent back to the authors.

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