applied catalysis a: general
TRANSCRIPT
Mf
Za
b
c
a
ARRAA
KBHMCZ
1
rBf(fimtc
obNuttfllm
0h
Applied Catalysis A: General 427– 428 (2012) 58– 65
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
jo u r n al hom epage: www.elsev ier .com/ locate /apcata
echanochemical preparation and characterization of CaO·ZnO used as catalystor biodiesel synthesis
ˇeljka Kesic a, Ivana Lukic a, Dragana Brkic a, Jelena Rogana, Miodrag Zdujic b, Hui Liuc, Dejan Skalaa,∗
University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, SerbiaInstitute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, SerbiaUniversity of Geosciences, School of Environmental Studies, Wuhan 430074, PR China
r t i c l e i n f o
rticle history:eceived 19 December 2011eceived in revised form 19 March 2012ccepted 22 March 2012vailable online 1 April 2012
a b s t r a c t
In this study, the synthesis of biodiesel or fatty acid methyl esters (FAME) from sunflower oil and methanolusing CaO·ZnO catalyst was investigated. Catalyst was synthesized by ball milling of Ca(OH)2 and ZnOpowder mixture with the addition of water (BMH), as well as solely by ball milling of mentioned pow-ders (BM) and subsequent calcination at 700 ◦C in air atmosphere. For comparison, the CaO·ZnO mixedoxide was also prepared using usual coprecipitation procedure (CP) followed by calcination at 700 ◦C of
eywords:iodieseleterogeneous catalystethanolysis
aOnO
the formed calcium zinc hydroxide hydrate. The BMH, BM and CP catalysts were characterized by X-raydiffraction (XRD), thermogravimetric analysis (TGA), infrared spectroscopy (FTIR), particle size distribu-tion measurement and scanning electron microscopy (SEM and SEM-EDS). In addition, specific surfacearea (BET), solubility in methanol at 60 ◦C and alkalinity (Hammett indicator method) were also deter-mined. The activity of BMH, BM and CP catalysts for biodiesel synthesis were tested at 60 ◦C and 1 bar,using molar ratio of sunflower oil to methanol of 1:10 and with 2 wt% of catalyst based on oil weight.
. Introduction
Due to the limitation of fossil fuel and its toxicity today’sesearch is directed towards alternative renewable sources.iodiesel is non-toxic and biodegradable renewable fuel derived
rom vegetable oils, animal fats or used cooking oils. BiodieselFAME, Fatty Acid Methyl Esters) is usually produced by transesteri-cation of triglycerides, the main constituent of vegetable oil, withethanol or ethanol. The most important parameters that affect
he rate of transesterification are reaction temperature, type andoncentration of catalyst as well as alcohol to oil molar ratio [1].
In order to improve the rate of transesterification and yieldf FAME the reaction of transesterification could be catalyzedy homogeneous (alkalies and acids) or heterogeneous catalysts.owadays, homogeneous base catalysts are the most frequentlysed in industry, since the process is faster under mild reac-ion conditions compared to acid catalyzed reaction. However,heir utilization in vegetable oils transesterification very oftenorms soaps as undesirable byproducts, which in turn generatesarge amounts of wastewater during the separation of the cata-
yst and formed products. Heterogeneous catalyst could overcomeentioned drawbacks of homogeneous catalysts; they can reduce
∗ Corresponding author. Tel.: +381 11 3303 710; fax: +381 11 3370 473.E-mail address: [email protected] (D. Skala).
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2012.03.032
© 2012 Elsevier B.V. All rights reserved.
production costs, be reused, regenerated [2] and, finally, heteroge-neous catalysts could be operated in continuous processes.
A variety of solid catalysts for biodiesel production has beeninvestigated. Those include alkaline earth base oxides [3–12], zeo-lites and modified zeolites [13], hydrotalcites [14], and alkali oralkaline earth oxides on porous support [15–17]. Some of them areproduced using complex and expensive procedures, which is a bigdisadvantage for their industrial application.
Among the heterogeneous base catalysts, CaO is the moststudied due to its low price and desired activity (Table 1). Thecatalytic activity of CaO strongly depends on calcination temper-ature [3,5] and used precursor [3,6]. Since CaO, known to be activein methanolysis reaction (yield over 90% after 90 min [3]), tendsin smaller extent to be leached by methanol [7] it is important toimprove its properties by fixing it to some support, e.g. silica [15],alumina [16], or ZnO [17]. The support is usually a porous materialproviding higher surface area, with catalytic activity ranging fromvery small to none.
One of the catalysts showing excellent activity under moder-ate reaction conditions (reaction time 3 h, FAME yield 94% andcatalyst can be reused up to 3 times with FAME yield above 90%[18]) is the mixture of CaO and ZnO oxides [17–20]. It might beobtained by calcination of calcium zinc hydroxide hydrate (calcium
zincate dihydrate – CaZn2(OH)6·2H2O) synthesized by coprecipita-tion of ZnO and Ca(OH)2 added to 20% KOH solution [19]. Proposedmethod of CaZn2(OH)6·2H2O synthesis consists of several steps.One of them is long-lasting, as is the washing of formed calcium
Z. Kesic et al. / Applied Catalysis A: General 427– 428 (2012) 58– 65 59
Table 1Literature review of the activity of CaO, ZnO, CaO·ZnO mixed oxides and supported CaO as heterogeneous catalysts for biodiesel production.
Catalyst Oil wt% Reaction conditions Yield, % Reference
T, ◦Catmosphere
Molar ratio Time, h
CaO Sunflower 1 60N2
13:1 1.5 >90 [3]
CaO Rapeseed 0.66 60 7.2:1 3 90 [8]CaO Soybean 1 65
N2
18:1 1 90 [9]
CaO Soybean and waste cooking 1 65 14:1 2 99 [10]CaO Soybean 1 65 12:1 2 99 [11]CaO/SBA15 Sunflower 1 60 12:1 5 95 [15]CaO/Al2O3 Palm 3.5 65 12:1 5 95 [16]CaO·ZnO Sunflower 1.3 60 12:1 2 >90 [17]CaO·ZnOCa/Zn ratio of 0.25
Palm kernel 10 60 30:1 1 >94 [18]
CaZn2(OH)6·2H2O Sunflower 4 60N2
12:1 0.75 >90 [19]
CaO·ZnO Jatropha curcas 4 65 15:1 6 >80 [20]ZnO Soybean 5 100 55:1 7 15 [21]
Table 2Synthesized catalysts – used working condition, basicity and surface area.
Catalyst denotation Molar ratio ofCa(OH)2 and ZnO,medium
Preparationmethod
Calcinationtemperature, ◦C
Basic strength(H−)
Basicity(mmol g−1)
Surface area(m2 g−1)
CP Ca(OH)2:ZnO 1:2,KOH solution
Co-precipitation / / / /
CP700 Ca(OH)2:ZnO 1:2,KOH solution
Co-precipitation 700 9.3–10 1.31 5.7
BMH Ca(OH)2:ZnO1:2 + H2O
Ball-millinga 7 h / / / /
BMH700 Ca(OH)2:ZnO1:2 + H2O
Ball-milling 7 h 700 9.3–10 2.94 8.6
BM Ca(OH) :ZnO 1:2 Ball-milling 7 h / / / /700
ut mil
zttdc
mtMfi
cdaspctt
2
2
Ztv
2
BM700 Ca(OH)2:ZnO 1:2 Ball-milling 7 h
a A vessel was periodically open to atmosphere while other was closed througho
inc hydroxide hydrate with distilled water until neutral condi-ion. Namely, the presence of KOH, even in small traces, influenceshe methanolysis reaction as catalyst, which in return can make itifficult to dissolve true catalytic activity of CaO·ZnO obtained afteralcinations CaZn2(OH)6·2H2O at appropriate temperature.
The synthesis of CaZn2(OH)6·2H2O by mechanochemical treat-ent of Ca(OH)2 and ZnO powders in water medium with the goal
o use it as rechargeable anodic material was recently reported [22].echanochemical treatment of different powders is widely used
or the activation and synthesis of broad class of materials [23,24],ncluding catalysts [25].
The goal of this study is to investigate synthesis of CaO·ZnOatalyst by mechanochemical treatment of Ca(OH)2 and ZnO pow-er mixture with the addition of required water amount, as wells to apply the same procedure but without added water, andubsequent calcinations at 700 ◦C. The classical coprecipitationrocedure of CaZn2(OH)6·2H2O synthesis followed by the samealcination procedure at 700 ◦C was also performed to comparehe catalytic activity with samples obtained by mechanochemicalreatment.
. Experimental
.1. Catalyst preparation
Ca(OH)2 (Centrohem, Belgrade, Serbia) and ZnO (Kemika,agreb, Croatia) were used for catalyst synthesis. Mechanochemicalreatment was carried out in the planetary ball mill Fritsch Pul-erisette 5, in air atmosphere. Two zirconia vials of 500 cm3 volume
9.3–10 2.08 7.2
ling.
each charged with 500 g zirconia 10 mm diameter balls were usedas milling mediums. The balls to powder mass ratio was approx.30. A powder mixture of Ca(OH)2 and ZnO, in the molar ratio of1:2, with, as well as without, stoichiometrically required additionof water (4.66 g) were used as starting materials for mechanochem-ical treatment. Angular velocity of supported (basic) disc, measuredby tachometer, was 250 rpm (26.2 rad s−1).
Calcium zinc hydroxide hydrate (CaZn2(OH)6·2H2O) was pre-pared by coprecipitation according to Ziegler and Johnson’sprocedure [26].
Prepared catalysts are denoted as follows: CP for precipitated,BMH for ball-milled with addition of H2O and BM for ball-milledwithout addition of H2O (subscript number represents the temper-ature of calcination, i.e. 700 ◦C) (all the information related to thepreparation of BMH and BM samples is shown in Table 2).
2.2. Catalyst characterization
XRD patterns were recorded with Ital Structure APD2000 X-raydiffractometer in Bragg–Brentano geometry using CuK� radiation(� = 1.5418 A) and step-scan mode (range: 10–70◦ 2�, step-time:0.50 s, step-width: 0.1◦).
Thermogravimetric analysis (TGA) was carried out on SDT Q600instrument in air atmosphere, with the flow rate of 100 mL min−1
at the 20 ◦C min−1 heating rate ranging from 25 to 800 ◦C.
Fourier-transform infrared (FTIR) spectra were recorded usingBOMEM (Hartmann & Braun) spectrometer. Measurements wereconducted in wave number range of 4000–400 cm−1, with 4 cm−1
resolution.
6 is A: General 427– 428 (2012) 58– 65
cee
aw
ouEs
tc(cwoowtb(
2
omaiEw23adct0tf111dat
2
htFwuttwtws
m
0 Z. Kesic et al. / Applied Catalys
The sample morphology and elemental chemical analysis wereharacterized at room temperature by Quanta 200 SEM systemquipped with EDS detector (FEI company, Netherlands). The accel-rating voltage was 20 kV.
The particle size distribution was measured by laser particle sizenalyzer (PSA) on Mastersizer 2000 (Malvern Instruments Ltd., UK),hich covers the particle size range of 0.02–2000 �m.
BET surface areas of the synthesized catalysts were carriedut according to the multipoint N2 adsorption–desorption methodsing SSA-4200 Surface Area & Pore Size analyzer (Beijing Builderlectronic Technology Co., Ltd., China). Prior to measurements, allamples were outgassed overnight under vacuum at 573 K.
Hammett indicator experiments were conducted to determinehe basic strength of catalysts. The following Hammett indi-ators were used: phenolphthalein (H− = 9.3), thymolphthaleinH− = 10.0) and 4-nitroaniline (H− = 18.4). Typically, 500 mg of theatalyst was mixed with 1 mL of Hammett indicators solution thatas diluted in 20 mL methanol. After 2 h of equilibration the color
f the catalyst was noted. The basic strength of the catalyst wasbserved to be higher than the weakest indicator that under-ent the color change, and lower than the strongest indicator
hat underwent no color change. To measure the basicity of solidases, the method of Hammett indicator-benzene carboxylic acid0.02 mol L−1 anhydrous ethanol solution) titration was used.
.3. Methanolysis reaction
The catalytic activity was evaluated in the transesterificatonf commercial edible sunflower oil (Dijamant, Zrenjanin, Serbia;olecular weight 876.6 g mol−1, acid value of 0.202 mg NaOH g−1)
nd methanol (99.5% purity, Fluka, Switzerland). All the exper-ments were conducted in 300 cm3 batch autoclave (Autoclavengineers) equipped with a heater and a mixer at 60 ◦C and 1 bar,ith the molar ratio of methanol to sunflower oil of 10:1 and with
wt% of catalyst based on oil weight. The agitation speed was00 rpm. The reaction samples were taken out from the reactort different reaction times, and after filtration 1 �L aliquots wereiluted with chloroform and obtained solutions analyzed by gashromatography (Varian 3400) with FID detector, on-column injec-or and fused silica capillary column (5 m × 0.53 mm film thickness.5 �m). Injector temperature was 330 ◦C, FID detector 345 ◦C, andhe temperature program of the chromatographic system was asollows: 2 min isothermal at 50 ◦C, 50–110 ◦C with 50 ◦C min−1,
min isothermal at 110 ◦C, followed by 4 ◦C min−1 ramp to 170 ◦C, min held at 170 ◦C, 170–340 ◦C with 20 ◦C min−1 and at the end5 min maintained at 340 ◦C. Quantitative analysis of FAME wasone using correction factors for FAME, tri-, di- and monoglyceridesnd glycerol. Calculated correction factors were used for calculatinghe mass percentage of FAME.
.4. Catalyst stability
The presence of catalyst in solution might imply to possibleomogeneous contribution to the reaction, which requires addi-ional steps of washing and purification of the biodiesel fraction.or this reason, an experimental procedure to evaluate lixiviationas employed. It consisted of mixing the catalyst with methanolnder the same experimental conditions as used in the transes-erification process but without the oil presence. After 4 h of suchreatment, the catalyst was removed by filtration, and methanolas mixed with the necessary volume of sunflower oil, and main-
ained at 60 ◦C for 4 h. If catalysts were lixiviated, some conversion
ould be observed due to the homogeneous contribution in theystem.The solubility of the catalyst in methanol at 60 ◦C was also deter-
ined by measuring the calcium(II) and zinc(II) concentration with
Fig. 1. XRD pattern of CP (a), BM (b), BMH (c), initial Ca(OH)2 (d) and ZnO (e).
HITACHI Z-2000 polarized zeeman atomic absorption spectropho-tometer.
The possibility to reuse the catalyst was also tested to check itscapacity to provide the same catalytic activity. The recycled useof catalyst BMH700 was carried out. After filtration, the recoveredBMH700 catalyst was reused in the next run. Then the reaction testwas repeated 4 times using the same ratio of the oil to catalyst andmethanol to oil.
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 shows the XRD patterns of coprecipitated calcium zinchydroxide hydrate (a), mechanochemically treated Ca(OH)2 andZnO with/without water addition (b and c), as well as initialCa(OH)2 and ZnO powders (d and e).
XRD of CP shows intense peaks of CaZn2(OH)6·2H2O (JCPDS 25-1449) with few weak peaks assigned to ZnO (JCPDS 36-1451), con-firming that calcium zinc hydroxide hydrate (CaZn2(OH)6·2H2O)was synthesized (Fig. 1a).
XRD analysis of BMH reveals calcium zinchydroxide hydrate and zinc oxide with no detectable Ca(OH)2(Fig. 1c). The presence of ZnO peaks in BMH indicates that theformation of calcium zinc hydroxide hydrate was not completed.Peak at 29.4 2� is assigned to CaCO3 (JCPDS 5-586), which is partlyobtained by the reaction of Ca(OH)2 with CO2 from air and partlyoriginates from the initial Ca(OH)2 (Fig. 1d). It can be concludedthat mechanochemical reaction of Ca(OH)2 and ZnO powderswith added stoichiometrical amount of water yielded a mixture ofCaZn2(OH)6·2H2O, ZnO and CaCO3 phases.
Mechanochemical treatment of Ca(OH)2 and ZnO without theaddition of water obviously does not lead to formation of calciumzinc hydroxide hydrate, as XRD analysis revealed (Fig. 1b). Also, itshould be noted that other calcium compounds could not be iden-tified, probably due to amorphization of CaCO3 and Ca(OH)2 duringball milling, showing similar mechanism to the Cu/ZnO system [27].Thus, XRD pattern of BM sample shows only peaks of ZnO.
XRD analysis of the samples treated at 700 ◦C reveals that cal-cination, regardless of the procedures of precursor preparation
(coprecipitation or ball milled Ca(OH)2 and ZnO powders eitherwith or without the addition of water) leads to the formation ofCaO (JCPDS 37-1497) and ZnO mixture, with other phase(s) beingundetectable (Fig. 2). It is very likely that fine CaO particles are
Z. Kesic et al. / Applied Catalysis A: General 427– 428 (2012) 58– 65 61
et
dbeZbsarw3(aliwtCfC
Fig. 2. XRD pattern of CP700 (a), BM700 (b) and BMH700 (c).
mbedded in ZnO matrix, suggesting that ZnO actually representshe support for CaO.
TGA analysis of CaZn2(OH)6·2H2O is characterized by two-stepecomposition [28]. The first dominant step of weight loss coulde observed from 120 to 180 ◦C, which may be attributed to thelimination of hydrated water and dehydration of Zn(OH)2 to formnO. In respect to the initial composition of used powders, it shoulde approximately 23.3%. The weight loss at about 385 ◦C corre-ponds to the dehydratation of Ca(OH)2 (5.8%) [28]. A weight losst 700 ◦C (Fig. 3) indicates the presence of calcium carbonate.Theesults of TG analysis for CP is close to theoretical calculation (firsteight loss up to 200 ◦C was 18.8%) and weight loss at 700 ◦C being
.0%, corresponding to the presence of CaCO3 in the initial Ca(OH)2Fig. 1). In fact, TG analysis of Ca(OH)2 (not given) revealed that itlready contains some amount of CaCO3.For BMH the first weightoss up to 200 ◦C was 12.9%, which is less than theoretical one andn agreement with XRD results that mechanochemical synthesis
as not completed. Also, the weight loss of 7.0% at 700 ◦C indicates
hat during mechanochemical treatment the reaction of CO2 anda(OH)2 to CaCO3 occurred. Most likely, Ca(OH)2 reacts with CO2rom the vial environment at the very beginning of milling becauseaCO3 could not be formed by the reaction between calcium zinc
Fig. 3. TGA curves of CP, BM and BMH.
Fig. 4. FTIR spectrum of: CP (a); CP700 (b); BMH (c); BMH700 (d), BM (e) and BM700
(f).
hydroxide hydrate and CO2 from air [19]. Such conclusion wasalso proven after analysis of mechanochemically prepared sam-ples using different working procedure – one vessel was openedevery hour during 7 h of milling, thus enabling contact of ZnO andCa(OH)2 powder mixture with air, while other vessel being keptclosed throughout milling. The results of TG analysis revealed thesame amount of CaCO3 in both cases, indicating that formation ofCaCO3 took place at the beginning of the milling process [29]. Thisis also in agreement with the result of Lopez Granados et al. [3] whoshowed that the carbonation of CaO is very rapid (couple of minutesare required to extensively carbonate the sample). Furthermore, itis also known that CaCO3 does not catalyze the methanolysis oftriglycerides, and that its transformation into the active CaO phasecould be only realized at high temperatures.
According to TGA of BM, there were three weight-loss steps. Thefirst and second weight losses are the result of Ca(OH)2 decompo-sition (about 7 wt%) and obviously above 600 ◦C decomposition ofCaCO3 occurred giving 8.0 wt% of weight loss.
FTIR spectra of the CP, CP700, BMH, BMH700 and BM catalysts areshown in Fig. 4.
Typical bands for calcium zinc hydroxide hydrate were detectedfor CP and BMH sample (Fig. 4a and c). The hydroxyl ions are char-acterized by sharp bands appearing between 3700 and 3500 cm−1.Therefore, two sharp bands at 3615 and 3505 cm−1, which can beseen at Fig. 4a and c, are assigned to �(OH) stretching vibrations[30]. The weak band at 3643 cm−1 is normally ascribed to OH groupsof Ca(OH)2. The OH groups also forms a bridge between the twometals. The bridging OH bending mode is visible at 940 cm−1 [31].The wide band that is observed in the region of 3400–3100 cm−1 iscaused by the stretching of water molecules. Also, the HOH bendingmode of lattice water appears at 1600 cm−1, and this band exists inall FTIR spectra, although for BM catalyst prepared without wateris very weak. The stretching bands at 3150, 3034 and 2880 cm−1
are attributed to the O H groups from H2O molecules [30]. Theband at 1070 cm−1 attributed to the Zn O H bending vibration isobservable for CP and BMH catalysts.
The presence of carbonates in all the samples is confirmedby the broad band centered at 1465 cm−1 [19]. This band canbe assigned to stretching vibrations of O C O [3]. The bands at
−1
874 and 712 cm arises from carbonates group as well, and theasymmetrical stretch of CO2 gives a band in the FTIR spectrumat 2350 cm−1 [31]. The increase amount of carbonates was deter-mined for the samples of catalysts obtained by ball-milling.
62 Z. Kesic et al. / Applied Catalysis A: G
c1ubrvhhb
nB
bwibpt
ppagCacfis(
s
ictt1tsTt
which poses desirable catalytic activity was also tested in this study.
Fig. 5. Particle size distribution of CP700, BM700 and BMH700.
FTIR analysis of calcined samples indicates the presence ofalcium carbonate (lower amount of calcium carbonate, band at465 cm−1, in comparison to the amount of CaCO3 in initial Ca(OH)2sed for synthesis), as well as OH groups of Ca(OH)2 and the HOHending mode of lattice water (band at 3615 cm−1) which is theesult of the fact that carbonation and the hydration of CaO in air isery rapid with just a few minutes being required to carbonate andydrate the sample [3]. Bands at frequencies lower than 500 cm−1
ave not been explored yet, but it is supposed they refer to theonds in the oxides of Zn and Ca [30].
The particle size distribution of the samples prepared by calci-ation at 700 ◦C of different precursors, namely CP700, BM700 andMH700 catalysts, is shown in Fig. 5.
Remarkably difference is notable between the catalyst obtainedy coprecipitation and obtained by ball milling with addition ofater. The particle size distribution of the CP700 and BM700 catalyst
s uniform with the size range of 0.2–30 �m, while for the BMH700imodal distribution is obtained: a larger fraction of the powderarticles is within the size range of 0.2–3 �m and the rest is withinhe range of 3–40 �m.
The difference of median particle sizes of the CP700 and BM700owders is obvious, while BMH700 sample has the smallest medianarticle size. The median particle sizes of CP, BMH and BM samplesre 12.0, 6.2 and 3.9 �m, respectively (particle size distribution notiven), and after calcination at 700 ◦C decrease to 8.6 and 3.8 �m forP700 and BMH700 catalyst, respectively, while for BM700 remainedlmost the same (3.8 �m). Such an effect could be expected becausealcination process of CP and BMH causes removal of H2O and CO2rom the CaZn2(OH)6·2H2O and CaCO3, inducing particle crush-ng and diminution. Hence, the particle size distribution shifts tomaller values for CP700 and BMH700 catalysts but not for BMH700Fig. 5).
The bimodal particle size distribution of the BMH700 catalyst,hown in Fig. 5, can be explained by the SEM analysis (Fig. 6).
The major difference in the morphology can be seen from SEMmages of mechanochemically synthesized catalyst with water andatalyst prepared by coprecipitation method. The SEM images ofhe CP700 (Fig. 6a and b) show the existence of large, plate like par-icles of hexagonal shape, with particle sizes ranging from less than
to 20 �m, while BMH700 (Fig. 6e and f) shows the agglomera-ion of small round-shape particles, including nano-particles. The
mall particles were merged together giving large agglomerates.he existence of the very small, nano sized particles along withhe large agglomerates could be the explanation for the resultseneral 427– 428 (2012) 58– 65
obtained from particle size analysis where bimodal distributionwas determined (Fig. 5). In the BM700 sample (Fig. 6c and d) irregu-lar plate like grains are found with small particles on them. Smooth,polygonal plates observed at CP700 and BM700 catalysts appear tobe ZnO crystals [32] with small particles of CaO [33] dispersed onthem. It appears that better dispersion is achieved when ball millingprocedure for catalyst preparation was used, and this is the mostpronounced in the case of BMH700 where small round particles ofCaO entirely covered ZnO crystals.
Several typical points on the surface of each sample wereselected for SEM-EDS analysis to determine the atomic distributionof Ca, Zn and O and an average value for each catalyst is considered.The atomic Zn/Ca ratios for BM700 and BMH700 are 1.92 and 1.9,respectively, which is close to the theoretical values correspondingto the calcium zinc hydroxide hydrate, while determined atomicZn/Ca ratio for CP700 is 3.60, most likely due to the heterogeneousdistribution of ZnO and CaO [19].
Basic strength of all calcined samples, i.e. mechanochemicallyobtained BMH700 and BM700 and the catalyst prepared by coprecip-itation method CP700, was lower than H− = 18.4 and H− = 10 sinceno color change was observed. All catalysts had the basic strength inthe range of 9.3–10. Higher basicity was found for BMH700 catalystand BM700 compared to CP700 (Table 2).
Better dispersion of CaO on the surface of ZnO could remarkablyincrease the basicity of the catalyst. The increased basicity could beattributed to the preparation method. According to the research ofWatanabe et al. the surface basicity of the alkaline earth hydroxidescould be increased by milling [34]. Obviously, basicity of BMH700is the highest, and it could influence the catalytic activity for thebiodiesel synthesis. Namely, the reaction activity depends on thenumber of basic sites present in the catalysts as well as on theirstrength [35].
BET analysis was carried out in order to determine the specificsurface area of the catalysts used in this study (Table 2). The ballmilled and calcined catalysts exhibited a slightly higher surfacearea than the CP700. However, the difference of specific surfacearea between prepared samples of catalyst is not crucial for theircatalytic activity indicating that all prepared samples of CaO·ZnOcan be characterized as catalysts with small surface area and lowporosity.
3.2. Activity of synthesized catalysts
Beforehand, the catalytic behavior of non-calcined sampleswas also investigated. CP catalyst dried at 100 ◦C for 45 min (toremove H2O absorbed in contact with air) did not show activityin methanolysis of sunflower oil. Drying procedure is necessarysince the presence of water is not desirable because it influ-ences the saponification reaction. The removal of water from wetcoprecipitated CaZn2(OH)6·2H2O suggests that crystal water andcorresponding hydroxides could not catalyze reaction betweenmethanol and triglycerides.
BMH also showed poor activity (23% yield of FAME after 4 h),but it was still noticeable better comparing to precipitated cal-cium zinc hydroxide hydrate. Martín Alonso et al. [6] reported thatneither CaCO3 nor Ca(OH)2 were active in transesterification oftriglycerides with methanol, while Kouzu et al. [11] pointed outthat Ca(OH)2 showed some but very low activity (12% yield after1 h of reaction).
The initial idea to obtain CaO·ZnO mixture from ball milledCa(OH)2 and ZnO powders without the addition of water (BM)
However, the sample BM did not show activity in methanolysisreaction, indicating that Ca(OH)2 was not transformed to active CaOduring milling. Such finding is in agreement with the XRD result,
Z. Kesic et al. / Applied Catalysis A: General 427– 428 (2012) 58– 65 63
); BM7
w(
acB
seao
aeawocst
Fig. 6. SEM images of CP700 (a) and (b
hich shows that only ZnO phase was identified on XRD patternFig. 1).
Finally, the FTIR analysis proved that crystal water, hydroxides,s well as formed carbonates could be removed or transformed intoorresponding oxides only after calcination at 700 ◦C (activation ofMH, BM and CP).
It is clear from activity test of prepared catalysts for biodieselynthesis presented in Fig. 7 that all samples were active for trans-sterification reaction of sunflower oil with methanol after theirctivation at 700 ◦C. The calcination step at 700 ◦C is necessary forbtaining desired activity of BMH, BM and CP samples.
The catalysts prepared by different methods showed differentctivity. The best conversion of triglycerides to fatty acid methylsters with 92% yield of FAME after 2 h and over 97.5% after 4 h waschieved with BMH700 catalyst. The high yield was also obtainedith BM700 catalyst, but longer time was required (3 h) to reach
ver 90% the FAME content in an oil phase. The obtained resultslearly indicate that BMH700 catalyst is more active, and thathorter time is necessary to reach desired composition of reac-ion mixture compared to BM700 catalyst. The catalyst prepared
00 (c) and (d); and BMH700 (e) and (f).
by coprecipitation method was less active giving only 84% of FAMEin oil phase after 4 h. The prepared sample BMH was also calcinedat 400 ◦C and BMH400 tested in the methanolysis of sunflower oil;after 4 h of reaction, the obtained yield of FAME was 89.6%. Activ-ity of BMH400 and obtained FAME yield was used to determinethe appropriate temperature for calcination of BMH sample being700 ◦C (BMH700 gave 97.1% FAME yield after 3 h). Such decision wasproven by TG analysis of BMH sample which indicates the presenceof larger amount of calcium carbonate which is the reason whycalcination at higher temperature is necessary.
Although the XRD analysis of CP, BMH and BM catalysts hasshown that for CP and BMH procedure of catalyst synthesis, theCaZn2(OH)6·2H2O was synthesized, which is not in the case forBM catalyst preparation. In all cases CaO·ZnO powder mixture wasobtained after calcination at 700 ◦C. Thus, the difference in theactivity of both mechanochemically synthesized catalysts and cat-
alyst prepared by coprecipitation procedure could be related to thedifferent particle size and particle size distribution, as well as tothe alkalinity of prepared catalysts. The results of biodiesel synthe-sis confirmed such assumption that simple relation between the
64 Z. Kesic et al. / Applied Catalysis A: G
Fct
ptdafaht
3
nshnd[wiTpfooc
fni2TCroceapt
pp
[
ig. 7. FAME yields for catalysts synthesized by coprecipitation or ball milling andalcined at 700 ◦C; experimental conditions: 60 ◦C and 1 bar, molar ratio of methanolo sunflower oil of 10:1 and with 2 wt% of catalyst.
articles size and time necessary to reach the same conversion ofriglycerides exists. Obviously, the rate of reaction increased withecreasing particle sizes, as the effect of larger external surfacerea of catalyst and decreased resistance for triglyceride diffusionrom the bulk liquid phase to that surface. Taking the alkalinitynd the catalytic activity into account, it could be concluded thatigher alkalinity of prepared catalyst increases the conversion ofriglyceride.
.3. Catalyst stability
A key aspect in the development of solid catalysts for heteroge-eous biodiesel synthesis is avoiding undesired lixiviation of activepecies. If lixiviation is high, the active species could also act as aomogeneous catalyst and thus, the advantages of the heteroge-eous catalyst would not really exist. The previous studies haveemonstrated that bulk CaO is partially dissolved in the methanol7]. To evaluate lixiviation, catalyst BMH700 was placed in contactith methanol under the same experimental conditions as used
n the transesterification process of sunflower oil (triglycerides).he catalyst was removed by filtration and then the methanol waslaced in the contact with the vegetable oil, and maintained at 60 ◦Cor 4 h. The analysis of the resulting solution revealed a FAME yieldf 0.51%, indicating that lixiviation was negligible. The FAME yieldf 0.51% in 4 h can be associated to the very small amount of solidatalyst, which is not removed from methanol after filtration.
The data of catalyst solubility in methanol is very important forurther application of solid catalyst. The reaction will be homoge-eous if the catalyst is soluble in reactants. The amount of Ca2+
on that can be present in methanol has been investigated after h catalysts with methanol at 60 ◦C contact and catalysts removal.he result showed that the concentrations of dissolved Ca2+ fromP700, BM700 and BMH700 catalysts were 4.32, 48.7 and 8.90 mg L−1,espectively. López Granados et al. [7] reported that the solubilityf CaO was 134 and 169 mg L−1 CaO in methanol after 1 and 3 hontact time, respectively. Expressed as Ca2+ concentration, it isquivalent to 96 and 121 mg L−1of Ca2+ methanol. It indicates thatll the catalysts sample prepared in this study are more stable thanure CaO, thus suggesting that mixed CaO and ZnO oxides stabilized
he CaO or active phase and decrease the rate of leaching process.To found out whether deactivation of the active sites due to theiroisoning by adsorption of triglycerides or some other moleculeresent in the reaction mixture occurs, used catalyst BMH700 was
[
[
eneral 427– 428 (2012) 58– 65
filtered and reused in several repeated biodiesel synthesis underthe same reaction conditions. Achieved FAME yield after fourth runwas 90%. Therefore, it was concluded that BMH700 have good activ-ity and might be reused several times. The reason for stability ofCaO could be attributed to its strong interaction with less solubleZnO.
4. Conclusion
The synthesis of CaO·ZnO mixtures was carried out and tested inreaction of methanol and sunflower oil. Different samples of CaOprecursor were obtained by: (a) ball milling of Ca(OH)2 and ZnOpowders with or without the addition of water, and (b) coprecip-itation of ZnO and Ca(OH)2 added to 20% KOH solution. Preparedprecursors were calcinated at 700 ◦C leading to the formation ofCaO·ZnO mixtures in all cases. After the calcination at 700 ◦C, cat-alyst obtained by ball milling was more active compared to thecatalyst calcined prepared by coprecipitation at the same temper-ature. The reasons for the different activity of synthesized catalystscould be explained by the difference of their basicity and the differ-ence of their particle sizes. These facts imply the importance of thepreparation procedure, which significantly affect the properties ofthe catalysts, as well as the catalytic activity.
The highest catalytic activity exhibits the catalyst obtained bymechanochemical treatment of Ca(OH)2 and ZnO powders withadded water, and subsequent calcination at 700 ◦C. Such preparedcatalyst gave FAME formation of 97.5% after 4 h reaction of sun-flower oil and methanol (1:10 molar ratio) in a batch reactor at60 ◦C. Also, the results showed that the CaO·ZnO catalysts werepractically insoluble in methanol indicating that CaO properties areimproved by mixing with ZnO. Repeated test of biodiesel synthesiswith same amount of BMH700 catalyst showed that catalyst keepacceptable activity. Namely, decrease of FAME yield from 97.1%after first to 90% after fourth use of same amount of catalyst wasexperimentally determined.
Acknowledgments
This work was financially supported by the Ministry of Edu-cation and Science of the Republic of Serbia (Grant No. 45001).The authors thank M.Sc. Zoran Stojanovic ITS SASA for performingparticle size measurements and to Prof. Nevenka Rajic for valu-able comments related to TG/DTG analysis and characterization ofprepared samples of catalysts.
References
[1] Y.C. Sharma, B. Singh, S.N. Upadhyay, Fuel 87 (2008) 2355–2377.[2] S. Glisic, I. Lukic, D. Skala, Bioresour. Technol. 100 (2009) 6347–6354.[3] M. Lopez Granados, M.D. Zafra Poves, D. Martın Alonso, R. Mariscal, F. Cabello
Galisteo, R. Moreno-Tost, J. Santamaría, J.L.G. Fierro, Appl. Catal. B 73 (2007)327–336.
[4] M. Di Serio, M. Ledda, M. Cozzolino, G. Minutillo, R. Tesser, E. Santacesaria, Ind.Eng. Chem. Res. 45 (2006) 3009–3014.
[5] V. Veljkovic, O. Stamenkovic, Z. Tododrovic, M. Lazic, D. Skala, Fuel 88 (2009)1554–1562.
[6] D. Martín-Alonso, F. Vila, M. Ojeda, M. López Granados, J. Santamaría-González,Catal. Today 158 (2010) 114–120.
[7] M. López Granados, D. Martín-Alonso, I. Sadaba, R. Mariscal, P. Ocon, Appl. Catal.B 89 (2009) 265–272.
[8] A. Kawashima, K. Matsubara, K. Honda, Bioresour. Technol. 100 (2009)696–700.
[9] M. Kouzu, T. Kasuno, M. Tajika, S. Yamanaka, J. Hidaka, Appl. Catal. A 334 (2008)357–365.
10] M. Kouzu, S. Yamanaka, J. Hidaka, M. Tsunomori, Appl. Catal. A 355 (2009)
94–99.11] M. Kouzu, T. Kasuno, M. Tajika, Y. Sugimoto, S. Yamanaka, J. Hidaka, Fuel 87(2008) 2798–2806.
12] G. Arzamendi, E. Arguinarena, I. Campo, S. Zabala, L.M. Gandia, Catal. Today133–135 (2008) 305–313.
s A: G
[
[[
[
[
[
[
[
[
[
[
[
[
[[
[[
[[
[
Z. Kesic et al. / Applied Catalysi
13] G.J. Suppes, M.A. Dasari, E.J. Doskocil, P.J. Mankidy, M.J. Goff, Appl. Catal. A 257(2004) 213–223.
14] W. Xie, H. Peng, L. Chen, J. Mol. Catal. A: Chem. 246 (2006) 24–32.15] M.C.G. Albuquerque, I. Jiménez-Urbistondo, J. Santamaría-Gonzáles, J.M.
Mérida-Robles, R. Moreno-Tost, E. Rodríguez-Castellón, A. Himénez-López,D.C.S. Azevedo, C.L. Cavalcante Jr., P. Maireles-Torres, Appl. Catal. A 334 (2008)35–43.
16] M. Zabeti, W.M.A.W. Daud, M.K. Aroua, Fuel Process. Technol. 91 (2010)243–248.
17] A.C. Alba-Rubio, J. Santamaría-Gonzáles, J.M. Mérida-Robles, R. Moreno-Tost, D.Martín-Alonso, A. Jiménez-López, P. Maireles-Torres, Catal. Today 149 (2010)281–287.
18] C. Ngamcharussrivichai, P. Totarat, K. Bunyakiat, Appl. Catal. A 341 (2008)77–85.
19] J.M. Rubio-Caballero, J. Santamaría-Gonzáles, J.M. Mérida-Robles, R. Moreno-Tost, A. Jiménez-López, P. Maireles-Torres, Appl. Catal. B 91 (2009) 339–346.
20] Y.H. Taufiq-Yap, H.V. Lee, M.Z. Hussein, R. Yunus, Biomass Bioenergy 35 (2011)
827–834.21] W.M. Antunes, C.O. Veloso, C.A. Henriques, Catal. Today 133–135 (2008)548–554.
22] X.-M. Zhu, H.-X. Yang, X.-P. Ai, J.-X. Yu, Y.-L. Cao, J. Appl. Electrochem. 33 (2003)607–612.
[
[[
eneral 427– 428 (2012) 58– 65 65
23] E. Avvakumov, M. Senna, N. Kosova, Soft Mechanochemical Synthesis, KluwerAcademic Publishers, 2001.
24] P. Baláz, Mechanochemistry in Nanoscience and Minerals Engineering,Springer-Verlag, Berlin, Heidelberg, 2008.
25] S. Petrovic, A. Terlecki-Baricevic, Lj. Karanovic, P. Kirilov-Stefanovic, M.Zdujic, V. Dondur, D. Paneva, I. Mitov, V. Rakic, Appl. Catal. B 79 (2008)186–198.
26] F. Ziegler, C.A. Johnson, Cem. Concr. Res. 31 (2001) 1327–1333.27] H.L. Castricum, H. Bakker, B. van der Linden, E.K. Poels, J. Phys. Chem. B 105
(2001) 7928–7937.28] S. Wang, Z. Yang, L. Zeng, Mater. Chem. Phys. 112 (2008) 603–606.29] A.M. Kalinkin, E.V. Kalinkina, O.A. Zalkind, T.I. Makarova, Inorg. Mater. 41 (2005)
1073–1079.30] T. Lin, M.A. Mollah, R. Vempati, D. Cocke, Chem. Mater. 7 (1996) 1974–1978.31] K. Nakamoto, Infrared, Raman Spectra of Inorganic and Coordination Com-
pounds, 5th ed., John Wiley & Sons, New York, 1997.32] F. Liu, Y. Zhang, Ceram. Int. 37 (2011) 3193–3202.
33] Dj. Vujicic, D. Comic, A. Zarubica, R. Micic, G. Boskovic, Fuel 89 (2010)2054–2061.34] T. Watanabe, J. Liao, M. Senna, J. Solid State Chem. 115 (1995) 390–394.35] S. Bancquart, C. Vanhove, Y. Pouilloux, J. Barrault, Appl. Catal. A 218 (2001)
1–11.