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Applied Catalysis A: General 466 (2013) 123– 130

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l h om epage: www.elsev ier .com/ locate /apcata

tructure and composition of hard coke deposited on industrial fluidatalytic cracking catalysts by solid state 13C nuclear magneticesonance

abita Behera, Piyush Gupta, Siddharth S. Ray ∗

SIR-Indian Institute of Petroleum, Dehradun 248005, India

a r t i c l e i n f o

rticle history:eceived 5 April 2013eceived in revised form 19 June 2013ccepted 25 June 2013vailable online 4 July 2013

a b s t r a c t

Carbonaceous deposits (hard coke) were studied by various solid state 13C nuclear magnetic resonance(NMR) techniques after demineralization of the spent and regenerated fluid catalytic cracking (FCC)catalysts obtained from Indian refineries. A number of structural parameters such as aromaticity, H/Cratio, fraction of protonated (f P

a ) and non-protonated (f NPa ) aromatic carbons, number of pericondensed

rings per average molecule (Nperi), and aromatic condensation index (�ar) are derived from the NMR

eywords:CC catalystsard coke

3C NMRericondensed aromatics

data. Findings of thirty-two and thirty-nine aromatic rings in coke from regenerated catalysts and fiveand nineteen rings in coke from spent catalysts are rationalized by various parameters such as feed,temperature and process conditions of FCC reactor on the basis of condensation. The coke are foundmore condensed in regenerated catalysts compared to spent catalysts revealing that the temperaturehas a marked effect in the evolution of coke structure whereas feeds and other process conditions governthe nature and composition of coke.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

In all petroleum refineries, the fluid catalytic cracking unitFCCU) plays an important role for gasoline production whereeavy oil feedstocks are catalytically cracked into gasoline andeating oil at 480–550 ◦C. Following the cracking reaction, the spentatalyst is first exposed to steam (480–550 ◦C) to remove occludedydrocarbons, and then is sent to regenerator where, coke, the car-onaceous deposits, are removed by air at temperature in the rangef 600–700 ◦C. The regenerated FCC catalyst is mixed with someresh catalysts to maintain steady activity in the FCCU reactionone. During the process, the catalyst deactivation via coke deposi-ion is one of the major concerns in petroleum refineries [1,2] sincehere is heavy loss of catalyst after every run. This economical con-traint requires continuous improvement of catalyst to reduce cokeormation in order to make the process relatively cost effective.

It is generally considered that during coking, a complex mixturef condensed polyaromatic compounds is formed on the surfacef catalysts or inside the pores and/or channels of the catalysts as

by-product which further evolve to bulky condensed insolublerganic matter under severe process and regeneration conditions3–5] leading to coke. Formation of some aliphatic coke is also

∗ Corresponding author. Fax: +91 135 2660202.E-mail addresses: ssray@iip.res.in, ssiddharthray@yahoo.com (S.S. Ray).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.06.038

reported [5]. The catalyst acts as a heat carrier between regener-ator and reactor to facilitate the role of heat balancing. Therefore,the nature and amount of the coke on the catalyst affect the tem-perature of the reactor. Since coke on catalyst plays an importantrole in heat balance of the FCCU, its profitability is linked to cokein many ways [1]. Information on the nature and composition ofcoke is needed for designing the coke resistant catalysts, optimiza-tion strategies for best refining process to steer the desired productslate and coke selectivity.

Coke having H/C ratio between 0.3 and 1.0, in general, are classi-fied into two categories; the coke that is soluble in common organicsolvents such as CHCl3 and CH2Cl2 is termed as soluble coke andanother completely insoluble in any solvent is known as insolubleor hard coke (HC). But major part of the coke (more than 90%) isinsoluble in nature. Another group of researchers used to definetypes of coke in to five categories: (i) catalytic coke, (ii) catalyst-to-oil coke, (iii) thermal coke, (iv) additive coke and (v) contaminantcoke in the review [6] and references therein. However, we areusing here the concept of soft and hard coke which is most gen-eral.

The structure and composition of the soluble coke are stud-ied by various analytical techniques like high performance liquid

chromatography (HPLC), gas chromatography–mass spectrome-try (GC–MS), and infra-red (IR) spectroscopy [5,7,8]. Spectroscopictechniques for surface studies like scanning electron microscopy(SEM), tunneling electron microscopy (TEM) and X-ray photo

1 sis A:

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24 B. Behera et al. / Applied Cataly

lectron spectroscopy (XPS) [9,10] used for structural character-zation are not sufficient to provide the information on chemicalature of HC. Characterization of insoluble coke is still remainings a great challenge to scientists due to its complex nature of com-osition and structure along with its insolubility factor. Only aew numbers of studies have been reported for characterizationf insoluble coke due to the problems associated with their forma-ion at low concentration (<1%, w/w) on catalysts and the above

entioned factors. Therefore, detailed studies involving the char-cterization of coke have been of particular interest for refiningndustry. Moreover, greater attention is required to the nature andegree of condensation of the aromatic structure of insoluble coke.uch attention could not be focused mainly due to the fact that

hese insoluble fractions are not amenable to analytical techniquesuch as HPLC, GC–MS. This problem can be addressed well by solidtate 13C NMR.

Among various spectroscopic techniques, solid state 13C NMRas the unique ability to study the local structure in detail. 13CMR distinguishes various types of carbons like aliphatic, aromatic,henolic, etherial, carboxylic carbons present in complex insolublerganic matters [11–15]. All the NMR studies are done with coalamples. Bloch decay NMR, known as single hard pulse excitationSHPE-NMR) has been considered as the best technique for quanti-ative 13C NMR analysis of coals and solid fuels, but it takes longxperimental time. High-resolution 13C NMR using magic anglepinning (MAS) and cross polarization (CP) techniques with highower decoupling has been used for the study of coals, oil shalesnd other carbonaceous materials [15,16]. CP, in combination withAS (CP-MAS), is very popular for characterization of different

ypes of hydrogen bonded carbons, here after referred as proton-ted carbons, in short time and can be used for semi quantitativeurpose. The only carbons that cannot be observed by CP are thearbons near paramagnetic ions or graphitic carbons [17]. Furthery introduction of a short relaxation delay (dephasing time) intohe standard CP method, termed as dipolar dephasing (DD) with CPCP-DD), various protonated and non-protonated carbons [18,19]an be differentiated. However, high concentration of coke is nec-ssary to achieve sufficient sensitivity in standard solid state 13CMR experiments; otherwise it consumes very long experimental

ime.Fundamental deactivation studies of coke on the spent catalysts

rocessed with various model compounds like ethylene, propylene,-hexane, n-heptane, aromatics and naphthenes have been done byMR [20–23]. As the industrial FCC catalysts contain coke in very

ow concentration (approx. <2%, w/w), not much of work has beenone on FCC coke. Due to very low occurrence, the coke is to beoncentrated by demineralization [21] as proposed by Guisnet andagnoux [5]. Only Snape et al. [24–26] characterized the coke on

CC catalysts by 13C NMR. Our present study is different from thems the coke obtained in our case is from real refinery conditions asell as we have used various advanced NMR strategies to under-

tand the details of FCC coke. Also in this work we have attemptedo study the nature of coke in as much detail as possible. We haveemonstrated in our previous work [27–29] that various modernolid state NMR techniques can be the best bet for studying thehemical nature of coke on the spent catalysts formed in differ-nt hydrocarbon conversion processes in pilot plants. Behera et al.30,31] studied the effect of feeds and catalysts on structure of FCCoke obtained from different Indian refineries.

The present paper deals with the study of HC deposited on spentnd regenerated FCC catalysts in refineries by various techniquesncluding different solid state NMR. The quantitative structural

arameters and average number of aromatic rings (NR) of cokeere derived from NMR data. A parameter named as condensa-

ion index describing the average perimeter of coke was derived toupport the condensed coke structure.

General 466 (2013) 123– 130

2. Experimental

Two spent and two regenerated catalysts were obtained fromthe FCCU of Indian refineries. The feedstock used for cracking in onerefinery is the blend of heavy waxy distillates and vacuum gas oil(VGO) and for other refinery it is VGO. The physico chemical prop-erties of these feeds along with detailed composition and structurefrom NMR data are given elsewhere [31]. These data also help inunderstanding coke formation during the process.

2.1. Extraction of hard coke

The HCs were extracted from spent and regenerated catalystsby demineralization [5,21]. The purpose of HCl treatment prior todemineralization is to remove any paramagnetic species or rareearth ions present in the catalysts. The demineralization was car-ried out in closed chambers. The vacuum dried HCl extract wastreated with 40% hydrofluoric acid for 10 h with 20 ml of HF pergram of sample and filtered. The filtration was carried out inTeflon filtration setup with suction obtained from Millipore. Theresidue was again refluxed in chloroform to remove the solu-ble coke present, if any, and then filtered. The coke concentratesare termed as insoluble coke or HC. This extraction process wasrepeated several times to obtain reasonable amount of coke sam-ples for characterization studies. The HC has carbon content varyingbetween 26 and 43% (w/w). The coke were named as HC1, HC2, HC3and HC4 where HC1 and HC4 were obtained from spent catalystsand HC2 and HC3 were from the regenerated catalysts.

2.2. NMR experiments

All 13C NMR spectra were recorded on a Bruker DRX 300spectrometer using 4 mm broad band MAS probe, operating at75.45 MHz for carbon resonance frequency and 300.13 MHz forproton decoupling. Approximately 100 mg of dried and finely pow-dered coke samples were packed in the ZrO2 rotor fitted with Kel-Fcap. For the quantitative estimation, Bloch decay 13C MAS NMRexperiment was performed for all samples using a 5 �s 13C �/2pulse, 50 s recycle delay time and MAS rate of 12.5 kHz. All spec-tra were externally referenced to 132.2 ppm peak of hexamethylbenzene as secondary reference with respect to tetramethylsilane.The 13C CPMAS NMR were carried out for all the coke sam-ples at 12.5 kHz spinning speed with 50 kHz and 65 KHz radiofrequency fields respectively for carbon and proton for optimalHartman–Hahn conditions. The optimized contact time of 1 mswas used for efficient polarization transfer with 4 s recycle delayto acquire the CPMAS spectra. CP time constants, TCH and pro-ton spin lattice relaxation times, T1�(H) in rotating frame weredetermined by performing CP dynamics with variable contact timeexperiments using 20 delays between 50 �s and 10 ms. The TCH andthe T1�(H) were obtained from non-linear least square fitting ofcarbon magnetization values. Besides giving TCH and T1�(H) values,the optimized contact times for each sample obtained from theseexperiments were used in respective CPMAS experiments. The CP-DD experiments were performed for each sample with 17 timedelays between 50 �s and 1 ms and with accumulated 2000 scansfor each delay. Proper combinations of Gaussian and Lorentzianfunctions were used to fit the decay of integral intensity of aromaticpeak as a function of time.

2.3. Elemental, thermogravimetry, ultraviolet and infraredspectroscopy

Other analytical techniques such as elemental, IR and ultravio-let (UV) spectroscopy and thermo-gravimetric analysis (TGA) werecarried out on coke samples to complement the NMR findings. The

B. Behera et al. / Applied Catalysis A: General 466 (2013) 123– 130 125

Table 1Normalized element analysis data of coke.

%C %H %N %S H/C Coke (w/w, %)

HC1 92.97 2.9 3.75 0.36 0.37 1.74HC 97.73 2.22 – – 0.27 0.84

ePmmmtptscrirPsbeastuaaSaam±saoaF

3

iba

3

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The broadening of the signal might be due to inhomogeneousbroadening from many different aromatic carbonaceous speciesincluding polyaromatics. In these spectra, some contribution fromhomogeneous broadening cannot be ruled out. A sharp peak at

2

HC3 93.5 2.59 3.02 0.13 0.33 0.64HC4 91.20 4.18 2.51 1.88 0.54 0.46

lemental analysis of the coke concentrates was performed on aerkin Elmer 2400 Series II CHNS/O analyzer using the combustionethod. Cystine was taken as a standard for K factor measure-ent. The product gases were separated through GC and wereeasured as a function of thermal conductivity and compared

o the standards. All the quantitation was performed on weightercent basis, using accurately weighed samples. IR recordings ofhe coke samples were performed with Perkin Elmer 1760 FTIRpectrometer using KBr pellets of 13 mm diameter (2.0%, w/w, ofoke/spectroscopic grade KBr mixture). All the IR spectra wereecorded in the range of 400–4000 cm−1 with 4 cm−1 resolutionn transmittance mode with background correction. UV diffusedeflectance spectra of all the coke samples were recorded on aerkin Elmer Lambda 19 spectrometer equipped with an integratedphere (B013-9941). Standard BaSO4 plates were clamped overoth the sample and the reference ports to initialize the spectrom-ter. By setting the slit width at 4 nm, the spectra were recordedt room temperature in the range 200–800 nm with a scanningpeed of 240 nm/min. The signal in transmittance mode was plot-ed against the wavelength to show the variation in nature ofnsaturated components of coke. When required, magnificationnd consequently second derivative of spectra were obtained. TGAnalyses were carried out using Setsys TG-18 thermal analyzer ofetaram. Approximately 10 mg of coke concentrates were taken inlumina crucible and heated at a rate of 10 ◦C/min up to 800 ◦C inir atmosphere with air flow rate at 100 ml/min. Accuracy of theaintained temperature and precision of weight measurement are1.0 ◦C and 0.4% respectively. SEM was conducted to analyze micro

tructural changes through morphology on coke samples. Prior tonalysis by SEM the samples were dried using a vacuum dryingven at 45 ◦C after which they were mounted on sample holdersnd coated with gold. SEM images of samples were taken by usingESEM, Quanta 200 F (Netherlands) at a voltage of 10–30 kV.

. Results and discussion

The main objective of this work is to study the nature of differentnsoluble FCC coke by various 13C NMR techniques supplementedy UV, IR and TGA. We have also attempted to determine the vari-tions in the nature and composition of coke.

.1. Elemental analysis

From the coke extraction data it is found that the used FCC cata-ysts contain 1.74, 0.84, 0.64, 0.46% (w/w) of insoluble coke as givenn Table 1 along with the normalized elemental composition of HCs.

The H/C ratio of these coke varies from 0.27 to 0.54. Two coke,C2 and HC3, representing two lower H/C ratios may have moreondensed rings compared to HC1 and HC4. The table also showsresence of nitrogen and sulfur in the coke which are sourced fromeactant feeds.

.2. NMR results

Solid state SHPE with MAS and CPMAS were employed to quan-itatively measure the aromaticity of the insoluble coke deposits.igs. 1 and 2 show respectively the SHPE with MAS and CPMAS

Fig. 1. SHPE/MAS NMR spectra showing different spectral profiles and peaks in thearomatic regions of (a) HC1, (b) HC2, (c) HC3 and (d) HC4.

spectra of coke concentrates HC1, HC2, HC3 and HC4. Though thespectra of coke look similar, they have some finer variations andquantitative difference. The assignment of isotropic chemical shiftto various carbons in the spectra is given in Table 2. Each SHPEspectrum shows an intense peak approximately at 130 ppm due toaromatic carbons accompanied by two side bands at both sides ofisotropic signal. This aromatic peak spreads from 100 to 160 ppmwith spectral width at half height (��1/2) being 2113, 2037, 2264and 2188 Hz for HC1, HC2, HC3 and HC4.

Fig. 2. CP/MAS NMR spectra of aromatic and aliphatic regions for (a) HC1, (b) HC2,(c) HC3 and (d) HC4.

126 B. Behera et al. / Applied Catalysis A: General 466 (2013) 123– 130

Table 2Assignment of chemical shifts of various carbons found in coke.

Chemical shift (ppm) Type of carbons

0–50 Aliphatic carbons50–90 Quaternary Cali, Cali O Cali, Cali O Ca, Cali OH90–110 Pyrrole/pyridine carbons

1mHaiHcfl

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ascc(nsTaspTtsbcirugte5CCtfd

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Table 3Aromaticity (fa) estimated from SHPE and CP NMR data of various coke.

Coke Aromaticity (fa)

SHPE CP

HC1 0.92 0.87

that the optimum contact time is 1–2 ms for aromatic carbons and0.5–1 ms for that of aliphatic carbons. Thus a compromised value of1 ms was taken for acquisition of 13C CP MAS NMR spectra of cokedeposits.

110–165 Aromatic carbons>165 Carbonyl carbons

14 ppm associated with aromatic peak indicates the presence ofore protonated carbons and is markedly prominent in cases ofC1 and HC4. Reduction in intensity of this 114 ppm peak in HC2nd HC3 indicates that the amount of protonated aromatic carbonss reduced in these two coke compared to their spent counterparts.ere reduction is due to oxidization of these two coke to higherondensed polyaromatics during regeneration under steam and airow at high temperature.

Besides the strong aromatic signals, small peaks in 100–110 ppmre assigned to pyrollic type of structure in coke concentrates inC1, HC3 and HC4. This assignment was also supported by the ele-ental analysis data that show the presence of nitrogen in HC1,C3 and HC4. The nitrogenous coke is formed at the earlier stage ofracking while hydrocarbon type coke are formed in the latter stagef cracking. Qian et al. observed the presence of nitrogen contain-ng aromatic structures while characterizing the FCC coke by XPS10].

However, in the aliphatic region, ill-defined signals are observeds a small hump in 10–50 ppm region. Very low signal inten-ity of aliphatic region with respect to that of aromatic regionlearly implies that the coke contain fewer amounts of aliphaticarbons and are dominated by aromatics. Small peaks of aliphatic10–50 ppm) and substituted aromatic carbon (∼154 ppm) reso-ances are observed in HC1 and HC4. Low intensity aliphatic signalsuggest the presence of side chains in aromatics but in low quantity.he substituted aromatic carbons are either bridgehead or proton-ted rather than alkylated which is evident from the presence ofignal at 154 ppm. Besides the aliphatic and aromatic signals, smalleaks around 50–90 ppm are observed in all the coke samples.hese peaks indicate that the aliphatic carbons contain ethereal, orhiol structures. The presence of oxygen atom in the hydrocarbontructure might be due to high temperature oxidation of hydrocar-ons in presence of steam and air during regeneration. Aliphatichains are more prone to be oxidized than the ring carbons lead-ng to aliphatic ethereal or thiol structures. Possibility of selectiveing opening following oxidation cannot be ruled out dependingpon the severity of the condition. These types of structures areenerally formed during high temperature oxidation above 700 ◦C,he set temperature for regeneration in FCCU. The percentage ofthereal carbons as determined from SHPE NMR data is roughly. The intensity of these ethereal carbons is greatly enhanced byP experiments. There is a manipulation of intensity during theP experiment, where the magnetization of protons is transferredo carbons for enhancing their intensity. Therefore, data obtainedrom CP NMR cannot be used for quantification. Here, we used CPata only for confirming the presence of ethereal carbons.

.2.1. Aromaticity of cokeAromaticity is an important characteristic of coke, which gov-

rns the nature of coke. In NMR, aromaticity is determined fromhe intensity ratio of aromatic carbons including the associated sideands to the total carbons. In case of SHPE with slow MAS rate, side

ands of isotropic aromatic signal overlap with the aliphatic car-on resonance and that happens when the spinning frequency is

ess than the magnitude of chemical shift anisotropy. Thus, careas been taken to optimize the spinning speed so that the side

HC2 0.98 0.74HC3 0.99 0.93HC4 0.86 0.86

bands would appear outside the spectral range of aliphatic region.In the present case, the samples were spun at 12.5 kHz speed toobtain “sideband overlap free” spectra for quantification. Aromatic-ity (refer to footnote of Table 5) derived from SHPE NMR spectraof all coke samples are given in Table 3. Aromaticity values liewithin 0.86 to 0.99 (from SHPE NMR) indicating all the coke arehighly aromatic. However, regenerated coke have comparativelyhigher aromaticity than the spent coke thus showing their morecondensed ring structures.

Relaxation time measurements (here called CP dynamics) wereperformed not only to measure TCH and T1�(H) values but also toobtain optimized CP conditions for aromatic and aliphatic carbons.Fig. 3 shows the relaxation curves i.e. the growth and decay ofequilibrium magnetization of aromatic carbons in coke samples.

Here, TCH and T1�(H) values represent the individual polar-ization transfer time and proton relaxation time in the rotatingframe. These two parameters are measured by least square fittingmethod for change in intensity of aromatic carbons with time ineach experiment. The TCH and T1�(H) values determined from thecurves were found to vary between 66–113 �s and 6.6–17 ms foraromatic carbons. Higher value of T1�(H) in HC4 indicates that thecoke molecules in this case have higher degrees of movement thatmay be due to their less bulky nature.

Lower values of T1�(H) in other coke indicate that theyhave a very short proton spin diffusion limit, i.e. strongproton–proton dipolar interactions. More number of condensedrings imposes the rigidity and thus set the short proton diffu-sion limit with shorter relaxation time for protons. Since CP canprovide only semi-quantitative results with optimized contact time(TCH � �CH � T1�(H)), the aromaticity value obtained from CP dif-fers from SHPE. From the analysis of relaxation data it was found

Fig. 3. Built-decay curves of intensities from aromatic carbons with contact time of1 ms for HC1, HC2, HC3 and HC4 coke.

B. Behera et al. / Applied Catalysis A:

autvssTSi

3

otetatcfRofmfaLttdts

TRC

trip

Fig. 4. CP-DD NMR spectra at dephasing time of 40 �s: HC1 (a) and HC4 (b).

The aromaticity value obtained from CP is compared with theromaticity determined from SHPE with MAS NMR experimentssing a pulse recycle delay of 50 s. One of the spectra was recordedhrice, manually integrated five times for the aromaticity. Thealue was found repeatable to an accuracy of 0.5%. However, poorensitivity of SHPE experiment is mostly due to inherent lower sen-itivity of carbon and time constraints for large number of scans.he discrepancy in the aromaticity values obtained from CP andHPE is because of the loss of signal due to mismatching of CP raten case of non-protonated aromatic carbons.

.2.2. Nature of the aromatic carbonsSince aromaticity alone is unable to provide the complete nature

f the coke molecule, more quantitative information from differentypes of aromatic carbons is always desired. Therefore, we havestimated the percentage of protonated, bridgehead and substi-uted aromatic carbons in these coke samples to know the nature ofromatic structures and the reason for their variation. The quanti-ative information about protonated and non-protonated aromaticarbons is achieved by performing DD experiments based on dif-erences in the magnitude of heteronuclear dipolar couplings.epresentative DD spectra for HC1 and HC4 at the dephasing timef 40 �s are shown in Fig. 4 to emphasize that the DD spectra of dif-erent coke are different. The aromatic peak centered on 130 ppm is

ainly due to the quaternary aromatic carbons. The data obtainedrom the analysis of CP-DD and CP dynamics of all coke samplesre given in Table 4. I0L and I0G correspond to the intensity of initialorentzian and Gaussian components while T2L and T2G correspondo the relaxation time of Lorentzian and Gaussian decays. The pro-onated aromatic carbons, called as Gaussian carbons have stronger

ipolar coupling there by dephasing at a faster rate (∼40 �s) thanhe non-protonated aromatic carbon (∼300 �s). The peak inten-ity decreases rapidly during first 40 �s due to decay of protonated

able 4elaxation parameters of aromatic carbons in coke obtained from CP dynamics andP-dipolar dephasing NMR.

CP dynamicsa CP-dipolar dephasinga

TCH (�s) T1�(H) (ms) T2G (�s) T2L (�s) I0G I0L

HC1 80 7.6 30 300 0.25 0.75HC2 95 8.8 27 150 0.27 0.73HC3 66 6.6 23 270 0.29 0.71HC4 113 17 24 140 0.49 0.50

a Abbreviations used for the parameters: Individual polarization transfer time inhe rotating frame (TCH); proton relaxation time in the rotating frame (T1�(H));elaxation time of Gaussian decays (T2G); relaxation time of Lorentzian decays (T2L);ntensity of initial Gaussian components (I0G); intensity of initial Lorentzian com-onents (I0L).

General 466 (2013) 123– 130 127

aromatic carbons that are governed by T2G. After initial decay ofprotonated carbons, the intensity of the remaining non-protonatedcarbons has Lorentzian decay with modulation by MAS rotor fre-quency. The Lorentzian carbons have decay constant that rangewithin 140–300 �s while Gaussian carbons have the range of decayconstant from 23 �s to 30 �s. The fraction of non-protonated car-bons represented by I0L is 0.5 in HC4 and remains around 0.7 forrest of the three coke. HC1, HC2 and HC3 show little variationin non-protonated carbons while HC4 shows a wide deviation innon-protonated aromatic carbon content as shown in Table 4. Thisindicates that the nature of coke in HC4 is very different from otherthree coke. The lower value for I0L in HC4 indicates less condensednature of coke. Since in analysis of CP-DD NMR total carbon isnormalized to one, the protonated carbons represented by I0G arecomplementary to I0L as shown in Table 4.

3.3. IR, UV, TG and SEM results

The IR analysis of coke samples gives information about variousfunctional groups and molecular structure that are used to aug-ment the NMR findings. CH3/CH2 stretching bands around 2851and 2922 cm−1 with corresponding bending vibration at 1457 cm−1

indicates the presence of methyl/methylene groups in all these cokesamples. C Ostr vibration corresponding to the peak at 1115 cm−1

for all coke shows that the presence of etherial carbons may beas secondary or tertiary in nature and is in conformity with ourobservation from NMR data as discussed earlier. The usual cokeband assigned to 1585 cm−1 is due to carbon–carbon bond stretch-ing and is found prominently in samples HC1 and HC2. But broadpeak at 1637 cm−1 which is due to C Cstr indicates the presence ofdifferent types of polyaromatic carbons found in all the coke sam-ples. In samples HC1, HC3 and HC4, a small shoulder to the cokeband appeared at 1697 cm−1 indicating the presence of nitrogenouscompounds in the aromatic ring of coke. The shift of coke band tohigher wavelength indicates more condensed nature of ring systemin HC2 and HC3.

All the UV spectra of solid coke samples are looking similaras they are taken from diffuse reflectance mode. However, withmagnification and second derivative of each spectrum we couldmark the differences in four coke samples. All coke samples showedprominent bands at 450 and 500 nm due to presence of poly alkylaromatics and condensed aromatic compounds. In all cases, a broadabsorption peak centered at 300 nm was observed that might bedue to the traces of undissolved catalysts (in ppm level). A smallpeak at higher wavelength side at 680 nm indicates the presence ofmore condensed or polar compounds.

Though TGA mainly estimates the amount of carbons from thepercentage of weight loss during oxidation, here the amount can-not represent the true percentage of carbonaceous deposits. Thisis because, instead of taking FCC catalyst with coke, we have takenonly coke after removing the matrix. The percentage of high tem-perature coke determined by TGA is found to be 52.5, 32.0, 58.2and 60.6% for HC1, HC2, HC3 and HC4. Therefore, we tried to com-pare the proportion of coke in different temperature regions. Allthe TGA profiles of dw/dT vs. T plots for four samples are shown inFig. 5. In general, all samples exhibit three different regions; regionI for T < 180 ◦C ascribed to the loss of water and volatile species,region II for 180 ◦C < T < 330 ◦C due to desorption of low tempera-ture coke as CO and CO2 and region III for 330 ◦C < T < 700 ◦C due tohigh temperature coke.

The coke in region II contains more mobile carbonaceousresidues or unstripped reactants or products. High temperature

coke in region III are bulky and condensed in nature. On the basisof literature data [32] on oxidation of different forms of carbonssuch as fullerenes, diamonds, graphite and carbon nanotubes, wecan infer from our TGA profiles that the coke from 330 to 700 ◦C

128 B. Behera et al. / Applied Catalysis A: General 466 (2013) 123– 130

f coke

rf

aH4Sba

Fig. 5. Thermograms with dw/dT curves o

epresents more than one form of carbon. A close look at peaks ofour coke samples shows different profiles.

The peaks of HC1 is more broadened compared to the regener-ted coke HC2 with maximum at 460 ◦C and shoulder at 550 ◦C.C2 gives intense peak around 425 ◦C with two shoulders at

60 ◦C and at 550 ◦C. HC3 gives two peaks at 525 ◦C and 595 ◦C.imilarly, HC4 gives two peaks at 525 ◦C and 595 ◦C like HC3ut peak at 595 ◦C is less broadened. Therefore, the regener-ted coke HC3 has more polycondensed aromatics. Two general

for (a) HC1, (b) HC2, (c) HC3 and (d) HC4.

inferences are derived from this analysis. Broadened peaks giveinformation of more heterogeneity in coke structure and multi-ple peaks indicate the presence of different types of condensedring systems. These findings helped to interpret our NMR datato account for different number of rings in coke as discussed in

next section. Furthermore, the SEM results, Fig. 6 of two spentand regenerated coke show the morphological property wherecondensed aromatic ring islands are evidenced from layered pro-files.

B. Behera et al. / Applied Catalysis A: General 466 (2013) 123– 130 129

ing layered structures of condensed aromatic rings.

3

abcdiNaecrtib

mft

TA

(bfan

sf(o(H

Fig. 6. SEM images of (a) HC1 and (b) HC3 show

.4. Average structure of insoluble coke

A number of structural parameters such as fraction of proton-ted (f P

a ) and non-protonated (f NPa ) aromatic carbons, fraction of

ridge-head aromatic carbon (f Bra ), mole fraction of bridgehead

arbons (�b), fraction of aliphatic carbon (fali), number of pericon-ensed rings per average molecule (Nperi), aromatic condensation

ndex (�ar), and H/C ratio are derived from the data of variousMR experiments and are given in Table 5. NMR analysis shows

wide variation in the nature of coke in terms of these param-ters. HC1 contains highest fraction of non-protonated aromaticarbons while HC4 contains less non-protonated carbons. Thus, theegenerated coke have higher non-protonated carbons comparedo their spent counterparts and the number of bridgehead carbonss relatively more in these regenerated coke. The mole fraction ofridgehead carbons varied from 0.44 to 0.73.

From the fraction of non-protonated aromatic carbon, it is esti-

ated that the fraction of bridgehead carbons (f Br

a = CBr/Car) variesrom 30 to 71% of the total aromatic carbons. In this calcula-ion we assumed one to one correspondence between number

able 5verage structural parameters obtained from NMR analysis.

Average structural parameters of cokea HC1 HC2 HC3 HC4

f Pa 0.22 0.26 0.29 0.44

f NPa 0.69 0.72 0.69 0.44

f Bra 0.57 0.71 0.68 0.30

fali 0.08 0.01 0.01 0.12Mol fraction of bridgehead carbons (�b) 0.73 0.72 0.69 0.44Number of pericondensed rings per

average molecule (Nperi)10 23 19 5

�ar 1.9 2.62 2.26 0.53

f CHnO−

ali0.46 0.97 1 0.75

f CH3ali

0.54 0.03 – 0.25H/CNMR 0.23 0.22 0.28 0.47H/CEA 0.37 0.27 0.33 0.54

a Formulae and abbreviations used: Fraction of aliphatic carbons (fali); aromaticityfa) = fraction of aromatic carbons to total carbons = intensity of aromatic car-ons/intensity of total carbons; fraction of protonated aromatic carbons (f P

a );raction of nonprotonated aromatic carbons (f NP

a ) (includes fraction of bridgeheadnd substituted aromatic carbons); fraction of bridgehead aromatic carbons (f Br

a );umber of pericondensed ring per average molecule (Nperi); aromatic conden-

ation index (�ar); fraction of oxygenated protonated aliphatic carbons (f CHnO−

ali);

raction of aliphatic methyl carbons (f CH3ali

); hydrogen to carbon ratio from NMRH/CNMR); hydrogen to carbon ratio from elemental analysis (H/CEA); fractionf peripheral carbons: aromatic carbons that are protonated and substitutedf P+Sa ); f Br

a = fa − (f NPa − fali); Nperi = 1 + (Car,unit − 6)/3 where Car/unit = 6/(1 − f Br

a )2;/C = H/(Carfa) + (1 − fa)H/Cali; �ar = f Br

a /f P+Sa .

Fig. 7. Condensed aromatic systems derived from calculated �ar value for (a) HC1,(b) HC2, (c) HC3 and (d) HC4.

1 sis A:

octticp

Mvbodtdptoa�rptemge�tiwtr

4

sovobFcrmctvreamd

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30 B. Behera et al. / Applied Cataly

f substituted aromatic carbons and aliphatic carbons. If peri-ondensed aromatic rings are drawn to fit f Br

a values, five towenty-three pericondensed rings are required to fit the data forhese coke. Here the average perimeter of condensed aromatic ringss calculated from the aromatic condensation index, �ar. This �ar isomputed from the ratio of fraction of bridgehead to fraction oferipheral carbons (�ar = f Br

a /f P+Sa ).

In the present study, the �ar value varies from 0.53 to 2.62.ore condensation in ring system occurs with increase in the �ar

alue for a particular coke. Lower �ar value of HC4 suggests lessulky nature of coke as a consequence of less condensation. It isbserved that the coke of regenerated catalysts are found more con-ensed in nature compared to the coke from spent catalysts. Fromhe calculated �ar, we tried to simulate the most probable con-ensed aromatic structure by using “chem window” software. Therobable structures for all coke samples are given in Fig. 7. Thirtywo and thirty nine aromatic rings are possibly present in cokebtained from regenerated catalysts while five and nineteen ringsre present in coke from spent catalysts to have the closest value ofar. In this Fig. 7 we have also shown the number of pericondensedings (NPCR) and number of catacondensed rings (NCCR) for therobable coke structures with total number of rings. Along withhese condensed aromatic structures, there is possibility of pres-nce of other lower aromatic structures as coke are thought to beixture of aromatic compounds. This structural investigation sug-

ests that high temperature regeneration has a marked effect on thevolution of coke molecules to condensed structures. The differentar value of coke samples implies differences in their structures

hat are considered to arise primarily from the major differencen the feedstock and type of catalyst used in the reaction, along

ith some other factors like stripping condition and concentra-ion of Ni and V in feeds that affect the combustion behavior in theegenerator.

. Conclusions

Various solid state NMR techniques are found to be promising totudy average structure of carbonaceous residues. We found fromur investigation that coke has different aromatic structures witharieties of aliphatic substitutions. With careful measurements andbservations, the protonated and non-protonated aromatic car-ons can be quantified from dipolar dephasing experiments. TheCC coke of Indian refineries are different from each other andomprise five to thirty nine aromatic rings. The insoluble coke inegenerated catalysts are found to contain more condensed aro-atic rings than the coke from spent catalysts. Factors such as

omposition of catalysts, nature of feeds, parameters of FCC reac-ors, regeneration conditions, etc. are responsible for the wideariation in coke structure. Depending upon the nature of feed andegeneration parameters, the coke from spent catalysts can have

therial coke. All these structural information including ring sizend aliphatic substitutions will be helpful to predict the probableechanism of coke formation in a refinery process and can lead to

evelopment of better FCC catalyst with lesser deactivation. This

[

[[[

General 466 (2013) 123– 130

structural information can also lead to development of high valuedcarbon compounds in the refinery.

Acknowledgements

We are thankful to Director, IIP for permitting to publish thiswork. We are also thankful to Prof. M. Hunger, Univ. of Stuttgart,Germany for thermal analysis and valuable discussions. BB wouldlike to acknowledge CSIR, India for providing senior research fel-lowship during this work. This work is partially supported throughfunding by CSIR vide Project No. OLP 521219.

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