# Multitechnique Characterization of Coke Produced during Commercial Resid FCC Operation

Henrique Cerqueira, Carsten Sievers, Guy Joly, Patrick Magnoux, Johannes Lercher

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Henrique Cerqueira, Carsten Sievers, Guy Joly, Patrick Magnoux, Johannes Lercher. Multitechnique Characterization of Coke Produced during Commercial Resid FCC Operation. Industrial and engineering chemistry research, 2005, 44 (44), pp.2069. 10.1021/ie048963k. hal-00288627

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# Multitechnique Characterization of Coke Produced during Commercial Resid FCC Operation

Henrique S. Cerqueira,*,† Carsten Sievers,‡ Guy Joly,§ Patrick Magnoux,§ and Johannes A. Lercher‡

Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo A. Miguez de Mello (Cenpes), Pesquisa e Desenvolvimento do Abastecimento, Tecnologia em FCC. Ilha do Fundão, Av. Jequitibá 950, Rio de Janeiro, 21941-598 RJ, Brazil, Institut für Technische Chemie, Technische Universität München. Lichtenbergstrasse 4, D-85748 Garching, Germany, and Université de Poitiers, UMR CNRS 6503. 40, Avenue du Recteur Pineau, 86022 Poitiers cedex, France.

The characterization of industrial coked resid fluid catalytic cracking (RFCC) catalysts is reported. The aim is to provide insight into the coke deposition on commercial resid fluid catalytic cracking catalysts sampled after the stripper of a commercial RFCC unit and to relate it to the potential process chemistry. Physicochemical techniques were used to characterize the used catalysts and the deposited coke. 95% of the coke was insoluble in CH₂Cl₂. This coke was located in the mesopores of the catalyst matrix. The results suggest the existence of domains of polyaromatic and heterogeneously distributed coke, in which saturated hydrocarbons are trapped. IR spectroscopy of adsorbed pyridine shows that the largest fraction of strong Brønsted acid sites is free after the catalyst has passed the stripper. The results indicate that for RFCC not the local deactivation of the acid sites but rather blocking of domains of the catalyst is the most important mode of deactivation after passing through the riser reactor.

## Introduction

Coke formation is the main cause of temporary catalyst deactivation during the transformation of organic compounds. In the refining process, coke can be defined as the byproducts that are retained inside the catalyst particles after the reaction. The retention of these molecules is due to their low volatility, trapping, and adsorption on acid sites.¹ Depending on the operating conditions, the coke molecules may react further even in the absence of other reactants.²

The deactivation of zeolite-based catalysts by coke is caused by poisoning of acid sites or pore blockage.³ In the former case, one coke molecule blocks one active site affecting the activity linearly, but mainly the reaction selectivity.⁴⁻⁸ The deactivating effect is much more pronounced in the case of pore blockage, with one coke molecule blocking the access of reactants to more than one active site.⁹⁻¹³ Prior to pore blockage, coke molecules can be heterogeneously distributed over the zeolite crystallites, being more concentrated at the external surface (pore mouth). If this is the case, the effective pore diameter is reduced, and the diffusion resistance of reactants in the crystallites is enhanced.¹⁴⁻¹⁶ For higher coke contents, the heavier coke molecules can accumulate on the outer surface and locally block the access of reactant molecules to the pore openings.

In the conventional fluid catalytic cracking (FCC) process with zeolite Y-based catalysts, a significant portion of the feedstock is converted into coke. Those conventional FCC units operate in a heat balance. The

heat produced by the combustion of coke is used in various ways: (i) to heat the feed to the reaction temperature, (ii) to provide energy to the endothermic cracking reactions, (iii) to heat the combustion air and (iv) the coke on the spent catalyst to the regenerator temperature, (v) to supply the heat lost from the reactor/regenerator, and (vi) to heat the steam to exit temperature.¹⁷

Because of the new discoveries of heavy oil deposits, many FCC units are beginning to process feedstocks with a higher tendency to form coke. This is particularly true for resid FCC units, which are designed to convert 100% residue from the atmospheric distillation tower. To maximize the profitability of those resid FCC units, part of the heat produced through the combustion of coke is recovered by means of catalyst coolers that control the regenerator dense-phase temperature and produce steam.

Although the understanding of catalyst deactivation by coke has seen substantial progress,¹⁸⁻²³ detailed characterization of the deposits is rare and even more so for commercially operated (FCC) catalyst samples. The chemical identity of such coke components can be determined through various spectroscopic techniques such as electron paramagnetic resonance (EPR),²⁴ magic angle spinning ¹³C nuclear magnetic resonance (¹³C MAS-NMR),²¹,²³⁻²⁷ UV,²⁸,²⁹ and IR spectroscopy,¹,²⁸ X-ray diffraction (XRD),³⁰ and Raman spectroscopy.³¹ Most of those techniques are carried out under static conditions, but the deposition of carbonaceous materials can also be followed by in situ IR spectroscopy followed by on-line gas chromatography (GC).³²⁻³⁶ Complementary information about coke location can be obtained by ¹²⁹Xe NMR³⁷,³⁸ and X-ray photoelectron spectroscopy.³⁹,⁴⁰ However, in most instances, these approaches require total separation of coke from the catalyst, involving the destruction of the catalyst by HF followed by the dissolution of the soluble coke compounds in an organic

<footnote>* To whom correspondence should be addressed. E-mail: henriquecerqueira@petrobras.com.br. Fax: +55 21 3865-6626. Tel.: +55 21 3865-6635.</footnote>

<footnote>† Petrobras, Cenpes.</footnote>

<footnote>‡ Institut für Technische Chemie.</footnote>

<footnote>§ Université de Poitiers.</footnote>
solvent.⁴¹,⁴² The heavy fraction corresponding to the insoluble coke is often only characterized by the average H/C ratio. As a consequence, little information on structure–property relationships exists despite the great number of physicochemical characterization tools applied.

Contrary to many previous studies with model com-
pounds that have studied the soluble coke composition
in detail, in the present paper, a multitechnique ap-
proach was used in order to obtain information about
the chemical identity and composition of insoluble coke
compounds as well as their location on commercial resid
FCC catalyst samples.

**Experimental Section**

**Spent Catalyst and Feedstock Specification.** The fresh catalyst used in this unit consists of a blend of fresh catalyst, specially developed for resid applications, and a low-metal “flushing” equilibrium catalyst. Details about the fresh catalyst formulation⁴⁹ and the flushing⁵⁰ can be found elsewhere. Two spent catalyst samples were collected from a commercial resid FCC unit after the stripper. The main difference between the two samples was the excess of oxygen in the resid FCC unit regenerator: 1.5 and 0.13 mol % for samples A and B, respectively. The feedstock processed was a residue from the crude distillation tower with 16.3 °API, 4800 ppm of nitrogen (1800 ppm of basic nitrogen), 0.65 wt % of sulfur, 19 ppm of vanadium, 11 ppm of Ni, and a Conradson carbon residue of 8.2 wt %. The feedstock consists of 1.5 wt % of compounds insoluble in n-heptane, 36 wt % of saturated compounds, and 10, 11, 4, and 2 wt % of mono-, di-, tri-, and polyaromatics, respectively.

**Physicochemical Characterization of Spent Catalyst Samples.** The spent catalysts were characterized with various physicochemical techniques. The carbon content was measured by total combustion in air at 850 °C in a LECO 244 unit. The average hydrogen to carbon (H/C) ratio was determined on the basis of the carbon content of the coke and the amount of water produced during the combustion of 12 g of spent catalyst sample in a fixed-bed microactivity test unit. The bottom of the reactor was filled with inert silica in order to retain stripped coke molecules that might be eventually stripped from the catalysts, allowing a more reliable estimate of the H/C ratio. The samples were pretreated under nitrogen flow (30 mL min⁻¹) for 1 h at temperatures between 100 and 250 °C, followed by a plateau of 30 min at 250 °C. The combustion was performed with pure oxygen (30 mL min⁻¹) at 580 °C for 4 h. The humidity of oxygen was taken into account.

IR analysis of the spent catalysts was performed with thin wafers (10–15 mg) of previously sieved (<325 mesh) samples. The wafers were pretreated at 450 °C under vacuum (1.3 × 10⁻⁴ Pa) for 4 h. The IR spectra were recorded at room temperature. The spent catalyst samples were exposed to 7.1 × 10⁻⁴ Pa pyridine for 15 min at room temperature, followed by outgassing for 2 h at 150 °C in a vacuum to remove physisorbed pyridine. Finally, the samples were heated to 300 °C, to determine the concentration of strong acid sites. The analysis was conducted with a NICOLET Magna 650 FT-IR spectrometer.

To characterize the nature of carbonaceous deposits
in the coke, ¹³C NMR was performed without pretreat-
ing the samples. A Bruker DSX 300 spectrometer was

used. Cross-polarization (CP) was applied to improve
the signal-to-noise ratio of the spectrum.

Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy, MALDI-TOF and LDI-TOF-MS, have been successfully applied for the determination of large (bio)organic molecules.⁴³,⁴⁴ The methods utilize the energy of a short laser pulse for desorbing and ionizing sample molecules, which are subsequently analyzed in a time-of-flight mass spectrometer. In LDI-TOF, the sample itself is desorbed and ionized by the laser beam or postionized either by another laser or with an electron beam.⁴⁵ In MALDI-TOF, on the other hand, a matrix, typically an aromatic acid, is mixed with the sample. The matrix absorbs the laser power, is vaporized, and carries with it the analyte molecules into the gas phase, where they can be ionized by cation addition or by charge transfer from the formed matrix ions. The different ionization mechanisms have been discussed elsewhere.⁴⁶ The primary advantage of the laser desorption/ionization is the relatively low energy input in the desorption process, which minimizes fragmentation. Thus, the measured mass spectrum directly represents the molecular weight distribution of the analyzed mixture. In turn, however, this eliminates all structural information.

Recently, both techniques were successfully adapted
to studying deactivated zeolite catalysts with and
without destruction of the matrix. The detailed attribu-
tions of the coke species in these two papers were the
basis for the attributions in the present contribution.⁴⁷,⁴⁸

For LDIMALDI-TOF MS, 0.15 g of the coked catalyst was suspended in 1 mL of distilled water and sonicated for 15 min. For MALDI experiments, 0.1 g of 2,5-dihydroxybenzoic acid (DHB) was added. From each solution, 0.5 µL was deposited on a position of the sample holder and dried overnight at room temperature. The measurements were performed in reflectron mode using a Bruker Biflex III MALDI-TOF mass spectrometer equipped with an N₂ laser (ν = 337 nm) operating at a pulse rate of 3 kHz. The ions were accelerated with pulsed ion extraction after a delay of 50 ns by a voltage of 28.5 kV. A microchannel plate was used for detection.

**Characterization of Soluble Coke.** The coked samples were submitted to a direct Soxhlet procedure with CH₂Cl₂ in order to recover the soluble coke compounds, which are not sterically blocked in the catalyst structure. The analysis of the coke molecules directly soluble in CH₂Cl₂ was made by gas chromatography and mass spectroscopy (GC-MS).

**Characterization of Insoluble Coke.** After the extraction, the samples were treated with an HF solution in order to remove silica from the structure, following a procedure previously described.⁴¹ The insoluble coke, which was recovered after destruction of the catalyst structure, was analyzed by various characterization techniques. Carbon and hydrogen content were measured after drying in air or Ar by total combustion at 1020 °C in He and O₂ with a Thermoquest NA2100 analyzer. For IR studies, the insoluble coke was pressed into thin wafers and pretreated at 200 °C in a vacuum (1.3 × 10⁻⁴ Pa) for 1 h. All IR spectra were recorded at room temperature. Oxidation in an air flow and degradation of insoluble coke in argon were carried out with a thermal desorption analysis and thermal gravimetric analysis (TDA-TGA), heating a 20-mg insoluble coke sample from 20 to 750 °C at 2 °C/min.

Transmission electron microscopy (TEM) pictures of
insoluble coke were obtained at both low and high
<figure><img src="image_2.png" /><figcaption>**Figure 1.** IR spectra of coke deposits over sample A. Regions (a) 1700–1300 cm⁻¹ and (b) 3200–2700 cm⁻¹.</figcaption></figure>

resolution using a Philips CM120. LDI- and MALDI-TOF-MS measurements were also performed for the insoluble coke following the same procedure for the spent catalyst samples.

Degradation experiments were carried out by introducing 10 mg of insoluble coke directly into the MS source at 350 °C and increasing the temperature rapidly (5 °C ms⁻¹) to 650 °C; the products formed were also analyzed by GC.

Oxidative degradation was performed with sodium dichromate, an oxidizing agent reasonably selective for coke oxidative degradation (i.e., being able to oxidize substituted polycyclic aromatic compounds with a minimum of ring degradation).⁵¹ In this experiment, insoluble coke diluted with water was separated and introduced into 0.4 M K₂Cr₂O₇ solution. Some quantity of 18 M sulfuric acid (1 or 2 mL) was added. This solution was heated under reflux and agitation for 3, 5, or 19 h. After cooling to ambient temperature, CH₂Cl₂ was added, and the organic phase was separated and analyzed by GC-MS. It was confirmed that this treatment (3 h reflux with 1 mL H₂SO₄) was able to oxidize anthracene without any paraffin production.

## Results and Discussion

**Characterization of Spent Catalysts.** The carbon content of samples A (1.5 mol % excess O₂ in regenerator) and B (0.13 mol % excess O₂ in regenerator), obtained at different days from the same commercial resid FCC unit, were 1.05 and 1.15 wt %, respectively. The H/C ratio of the coke in these samples was 0.79 and 1.11, as derived from the carbon content and the amount of water produced during the combustion of 12 g of spent catalyst sample in a fixed-bed reactor. Sulfur and nitrogen levels were below detection limits.

The IR spectra of spent catalysts showed bands at 1600 cm⁻¹, characteristic of unsaturated hydrocarbons and aromatic compounds,¹,²⁸ as well as bands corresponding to CH vibrations in unsaturated hydrocarbons/aromatic compounds (3030–3070 cm⁻¹) and CH vibrations of saturated groups (2920 cm⁻¹) (Figure 1).

IR spectra of adsorbed pyridine, allowing one to estimate the concentration of accessible Brønsted and Lewis acid sites (see Table 1), suggest that, despite the low coke content, both kinds of acid sites are affected. It is however striking that sample A (slightly lower coke content) has retained all, while sample B has only lost a small fraction (20%) of the strong Brønsted acid sites. Because the strong Brønsted acid sites are related with

<table><caption>**Table 1.** Acid Sites Determined by IR Spectra after Pyridine Ad/Desorption</caption><thead><tr><th>sample</th><th>Lewis total<sup>a</sup><br>(a.u.)</th><th>Brønsted total<sup>a</sup><br>(a.u.)</th><th>Lewis strong<sup>b</sup><br>(%)</th><th>Brønsted strong<sup>b</sup><br>(%)</th></tr></thead><tbody><tr><td>E-catalyst</td><td>0.40</td><td>0.21</td><td>32.5</td><td>47.6</td></tr><tr><td>A</td><td>0.18</td><td>0.11</td><td>16.7</td><td>100</td></tr><tr><td>B</td><td>0.12</td><td>0.08</td><td>25.0</td><td>100</td></tr></tbody></table>

the zeolite component, the results suggest that the zeolite component was not affected after the stripper and that the majority of coke is located in the matrix (i.e., on weak Brønsted and a significant fraction of Lewis acid sites).

¹³C NMR spectra of samples A and B show a large signal attributed to aromatic compounds (130 ppm) (Figure 2) and a less intense signal around 18–20 ppm attributed to aliphatic hydrocarbon groups. Although CP-NMR does not provide quantitative results, it allows the rough estimation that 80–90% of the carbon is in aromatic rings, while 10–20% is part of saturated groups/molecules. With the H/C ratios being approximately 0.79 and 1.11, respectively, (whereas saturated species have an H/C ratio of approximately 2), the aromatic molecules in the mesopores must be rather large (i.e., present a kinetic diameter greater than 12 Å).¹³ The difference in the H/C ratio indicates that the size of these condensed aromatic molecules is smaller for sample B. This is in agreement with the fact that a part of these molecules directly blocks Brønsted acid sites in the pores of the zeolite component.

LDI and MALDI-MS analyze only the aromatic fraction of the coke. The desorbed molecules are usually observed in a cationic form (protonated species and complexes with Na⁺ and K⁺ ions). The most obvious differences between the LDI and MALDI spectra are explained with species directly derived from the matrix (DHB). Four strong peaks are important: 155 m/z (DHB + H⁺), 137 m/z (DHB + H⁺ – H₂O), 177 m/z (DHB + Na⁺), and 273 m/z (DHB dimer formed by esterification + H⁺). Apart from these matrix-associated peaks, only minor differences were observed between the LDI- and MALDI-TOF mass spectra, suggesting that the aromatic molecules absorb the energy of the laser (see Figure 3).⁴⁷,⁵⁴,⁵⁵ The MS peaks of the untreated spent catalyst samples correspond to coke formed on the outer surface of the catalyst particles. Even masses dominate the spectrum of both catalysts. The spectrum of catalyst B

<footnote><sup>a</sup> Concentration of sites able to retain pyridine at 150 °C.</footnote>

<footnote><sup>b</sup> Fraction of sites able to retain pyridine at 300 °C.</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** $^{13}$C NMR of coked samples. (a) Sample A. (b) Sample B.</figcaption></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) LDI- and (b) MALDI-TOF spectra of the spent catalysts.</figcaption></figure>

also shows a series of odd masses between 215 and 285 *m/z* in addition to mostly even masses.

Because the detected species are being charged by the addition of a proton, the masses observed are increased by 1. However, we will refer to the masses of the molecules without the addition of a proton. Therefore, even masses are assigned to nitrogen-free compounds, whereas odd masses are assigned to compounds containing a nitrogen atom. It should be mentioned that even masses can also result from hydrocarbons with an even number of nitrogen atoms or radical ions.⁴⁸ Both possibilities were excluded, however, as molecules with more than one nitrogen atom are unlikely because of the low nitrogen content of the coke, and radical cations are not likely to be formed in the environment present.

The mass spectra of the spent catalysts showed a distribution of species with masses up to approximately 400 *m/z*. Within this distribution, the difference of two adjacent peaks was 1 *m/z*, indicating a very heterogeneous character of the coke. The intensity of most of

<figure><img src="image_6.png" /><figcaption>**Table 2.** Elemental Analysis of Insoluble Coke</figcaption></figure>

<table><thead><tr><th></th><th>C (wt %)</th><th>H (wt %)</th><th>N (wt %)</th><th>S (wt %)</th><th>N/C</th><th>H/C</th></tr></thead><tbody><tr><td colspan="7">Sample A</td></tr><tr><td>spent catalyst</td><td>12.71</td><td>0.76</td><td>0.38</td><td>0.11</td><td>0.026</td><td>0.72</td></tr><tr><td>air 100 °C</td><td>13.60</td><td>1.38</td><td>0.44</td><td>0.06</td><td>0.028</td><td>0.78</td></tr><tr><td>argon 250 °C</td><td>12.30</td><td>0.76</td><td>0.26</td><td>0.07</td><td>0.018</td><td>0.77</td></tr><tr><td>argon 350 °C</td><td>12.07</td><td>0.57</td><td>0.27</td><td>0.07</td><td>0.019</td><td>0.57</td></tr><tr><td colspan="7">Sample B</td></tr><tr><td>spent catalyst</td><td>14.98</td><td>0.83</td><td>0.56</td><td>0.11</td><td>0.032</td><td>0.66</td></tr><tr><td>argon 350 °C</td><td>14.15</td><td>0.71</td><td>0.38</td><td>0.09</td><td>0.023</td><td>0.60</td></tr></tbody></table>

the peaks is relatively low compared to the most prominent ones in the respective spectra.

Various series of masses with differences of 14 *m/z* were observed in all spectra. A typical example in the LDI spectrum of sample A is the series starting at 216 *m/z*. The next six peaks (up to 300 *m/z*) were clearly observed with a distribution maximum at 258 *m/z*. These series are assigned to related compounds, which differ by one additional CH₂ group, so that the second peak with a mass of 230 *m/z* would contain one additional methyl group. Generally, these series follow a Gaussian distribution. In the spectrum of sample B, two significant series are observed starting at 208 and 215 *m/z* with the most intensive peaks at 222 and 243 *m/z*, respectively.

In addition to these series, several isolated peaks with high intensities were observed. The most prominent of these peaks had masses of 380 and 412 *m/z*, respectively. The peak at 380 *m/z* can be explained with molecular formulas of C₃₀H₂₀, while the molecular formula C₃₃H₁₆ results in a mass of 412 *m/z*. To provide a general idea of molecules with these masses, possible structures of coke molecules are shown in Table 3.

**Analysis of Coke Reclaimed by Soxhlet Treatment.** The two spent catalyst samples were Soxhlet extracted.²¹ This method removes external soluble coke by dichloromethane under reflux (6 h). After this, the extract was analyzed by GC-MS. For both catalysts, only 5% of the coke can be extracted by Soxhlet.

Despite the low extraction yield (5%), extracts were analyzed by GC-MS and show the presence of various compounds such as xylenes, trimethylbenzenes, methylnaphthalenes, methylanthracenes (phenanthrenes), and alkylnaphthenes (from C₁₄H₂₈ to C₂₀H₄₀).

**Characterization of Insoluble Coke.** After the Soxhlet, the catalyst was reclaimed and treated with HF solution in order to dissolve the silica fraction and
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Insoluble coke oxidation in air. (a) Sample A. (b) Sample B.</figcaption></figure>

<table><caption>**Table 3.** Main Components Observed in the LDI- and MALDI-TOF Mass Spectra</caption><thead><tr><th>Mass (m/z)</th><th>Possible assignment</th><th>Formula</th><th>H/C</th></tr></thead><tbody><tr><td>167</td><td><img src="image_0_1.png" alt="Chemical structure of 1,3-diphenylpropan-2-one"></td><td>C<sub>12</sub>NH<sub>9</sub></td><td>0.75</td></tr><tr><td>216</td><td><img src="image_0_2.png" alt="Chemical structure of 1,3-diphenylpropan-2-one"></td><td>C<sub>17</sub>H<sub>12</sub></td><td>0.71</td></tr><tr><td>217</td><td><img src="image_0_3.png" alt="Chemical structure of 1,3-diphenylpropan-2-one"></td><td>C<sub>16</sub>NH<sub>11</sub></td><td>0.69</td></tr><tr><td>267</td><td><img src="image_0_4.png" alt="Chemical structure of 1,3-diphenylpropan-2-one"></td><td>C<sub>20</sub>NH<sub>13</sub></td><td>0.65</td></tr><tr><td>380</td><td><img src="image_0_5.png" alt="Chemical structure of 1,3-diphenylpropan-2-one"></td><td>C<sub>30</sub>H<sub>20</sub></td><td>0.67</td></tr><tr><td>412</td><td><img src="image_0_6.png" alt="Chemical structure of 1,3-diphenylpropan-2-one"></td><td>C<sub>33</sub>H<sub>12</sub></td><td>0.36</td></tr></tbody></table>

to liberate the coke compounds. All the coke liberated after HF treatment was insoluble in dichloromethane. The insoluble coke was separated, dried (after eliminating the HF-containing solution), and characterized by elemental analysis, IR spectroscopy, electron microscopy, pyrolysis followed by MS, and chemical degradations.

**A. Elemental Analysis.** Insoluble coke from samples A and B was analyzed after three different treatments: (i) after extraction with HF and drying at room temperature, (ii) after extraction with HF and drying in air at 100 °C (24 h), and (iii) after extraction with HF and treatment in Ar at 250 or 350 °C. Table 2 compiles the elemental analyses of insoluble cokes after these treatments. In each sample, the H/C/N elemental analysis showed that the remainder contained a large fraction of alumina, which retained the original shape of the crystal (see TEM results).

Elemental analysis suggests that insoluble coke does not only contain large structures with more than ten condensed aromatic rings. Whatever the treatment of the spent catalyst, the H/C ratio was in the range 0.6–

0.8, which is in qualitative agreement with values reported by Moljord et al. found in laboratory experiments with catalysts that contained similar concentrations of coke deposits.⁵² Only for sample A, pretreatment in Ar at 350 °C drastically decreased the H/C ratio of insoluble coke. The H/C ratio of insoluble coke in sample A was between 0.72 and 0.78, and hence, the coke must, therefore, have a lower degree of condensation compared to the coke deposited on sample B. Nitrogen and sulfur that were below detection limits for the spent catalysts were now detected on both samples. The N/C ratio was 0.026 and 0.032 for samples A and B, respectively. These values correspond well with the results by Qian et al., which were obtained in a laboratory-scale FCC unit.⁴⁰ A comparison with the feed composition shows that nitrogen was somewhat enriched in coke. In line with Barth et al., this indicates that molecules containing nitrogen are important coke precursors and are preferentially retained.⁵³ The weaker preferential interaction of the nitrogen-containing molecules in the resid FCC catalysts compared to the conventional FCC indicates that the nitrogen-containing molecules are larger in resid FCC operations and therefore do not interact with the strong acid sites in the zeolite fraction. This is in agreement with the large fraction of free Brønsted acid sites noted in the two samples. In contrast to the results observed with conventional FCC catalysts, for both samples the N/C ratio decreased during the Ar treatment at 350 °C supporting, a relatively weak and nonlocalized adsorption of the nitrogen-containing molecules.

**B. Temperature-Programmed Oxidation (TPO).** TPO of insoluble coke shows maximum coke combustion at 460–470 °C (Figure 4). The losses of weight were equal to 17% and 20% for insoluble coke from samples A and B, respectively. In argon, the removal of coke was more difficult, resulting in maximum coke removal close to 580 °C, which is 100 °C higher than in air (Figure 5). Upon heating to 650 °C under argon, the losses in weight were equal to 18% and 20% for insoluble coke from samples A and B, respectively, showing that all the coke can be removed.

**C. IR Spectroscopy.** The IR spectra of insoluble coke (Figure 6) showed aromatic bands at 1617–1623 cm⁻¹, and for insoluble coke from sample A, the bands of aliphatic groups at 2920, 2846, and 1437–1443 cm⁻¹ confirm that coke deposited on sample A was slightly less aromatic than that formed on sample B.

**D. LDI- and MALDI-TOF Analysis.** In contrast to the spent catalyst (see Figure 3, masses above 450 *m/z* were not observed), much larger masses were observed in the HF-treated sample (see Figure 7). The maxima of the mass distribution (following a Gaussian profile)
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Insoluble coke thermodesorption in argon. (a) Sample A. (b) Sample B.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 6.** IR spectroscopy of insoluble coke from samples A and B.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 8.** TEM photographs of insoluble coke from sample B. (a) Low magnification and (b) high magnification.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** (a) LDI- and (b) MALDI-TOF spectra of the insoluble coke.</figcaption></figure>

were around 740 and 560 for samples A and B, respectively. Prominent masses for the isolated coke were 486 and 560 *m/z*. These peaks are assigned to polyaromatic hydrocarbons. Because of their molecular diameters, it is concluded that these molecules are located in the mesopores of the catalyst.⁴⁷

The very broad distribution of molecular masses and the higher distributions observed in the LDI- and

MALDI-TOF experiments suggests that in the mesopores a wide variety of species are formed that are hardly stripped from the catalyst before regeneration. Odd and even masses were observed. The deposits show a common tendency of peaks with odd masses up to 400 *m/z*, while at higher masses, even masses dominate. This observation is in line with the results of the elemental analysis under argon, indicating that nitrogen is found mainly in smaller coke molecules. It is worth mentioning that many of the prominent peaks above 400 *m/z* have a corresponding peak with a mass difference of 50 *m/z*. Some examples from the LDI-TOF spectrum of sample A are the pairs at 412, 462 and 426, 476, as well as 486, 436 *m/z*. These are assigned to the enlargement of the condensed rings by one (four carbon atoms).⁴⁷ However, long series such as those with steps of 14 *m/z* (substitution of H by CH₃ group) for the spent catalyst were not observed.

**E. TEM and Electron Diffraction X-ray (EDX) Analysis.** TEM photographs were taken of the insoluble coke from sample B obtained after HF and CH₂Cl₂ treatments. Figure 8a shows a heterogeneous distribution of coke and alumina, represented as black particles. Some less-colored parts were attributed to the presence of remaining zeolite crystallite and/or matrix. At high magnification (Figure 8b), it was possible to show that the remaining alumina forms rectangular platelets retaining the shape of zeolite crystallites. The average size of these rectangular platelets was about 400 × 200 nm² (0.4 × 0.2 µm²), which correlates to the size of zeolite crystallites. From these results, we suggest that, after the HF treatment, the remaining alumina assumes the form of zeolite crystal.

This hypothesis was confirmed by analysis of the composition of some platelets by EDX. A great fraction of the material was amorphous alumina showing very low Si/Al ratios, which suggests that silica, as expected, was preferentially destroyed by the HF treatment.
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Potential aromatic compounds detected by mass spectrometry analysis after pyrolysis of insoluble coke from (a) sample A and (b) sample B.</figcaption></figure>

<table><caption>**Table 4.** Elemental Analysis of Insoluble Coke after Pyrolysis</caption><thead><tr><th></th><th>C (wt %)</th><th>H (wt %)</th><th>N (wt %)</th><th>S (wt %)</th><th>N/C</th><th>H/C</th></tr></thead><tbody><tr><td colspan="7">Sample A</td></tr><tr><td>before pyrolysis</td><td>17.2</td><td>1.1</td><td>0.56</td><td>0.21</td><td>0.028</td><td>0.76</td></tr><tr><td>after pyrolysis</td><td>10.4</td><td>0.46</td><td>0.31</td><td>0.10</td><td>0.026</td><td>0.53</td></tr><tr><td>loss (%)</td><td>40</td><td>58</td><td>45</td><td>52</td><td></td><td></td></tr><tr><td colspan="7">Sample B</td></tr><tr><td>before pyrolysis</td><td>19.8</td><td>1.1</td><td>0.68</td><td>0.27</td><td>0.029</td><td>0.67</td></tr><tr><td>after pyrolysis</td><td>12.9</td><td>0.56</td><td>0.45</td><td>0.19</td><td>0.030</td><td>0.52</td></tr><tr><td>loss (%)</td><td>35</td><td>49</td><td>34</td><td>30</td><td></td><td></td></tr></tbody></table>

Additionally, significant concentrations of carbon and nickel were observed. The data show clearly that the insoluble coke is heterogeneously distributed in the remainder reclaimed after HF treatment.

**F. Pyrolysis Analysis.** To functionalize the insoluble coke, both samples were pyrolyzed at 650 °C, and the products liberated by this treatment were analyzed by GC-MS. From the insoluble coke of sample A, pyrene and/or fluoroanthene compounds were the main products liberated (Figure 9a). Additionally, aromatic compounds containing two (naphthalene) to five (benzopyrene) aromatic rings were found. Alkanes ($C_{12}-C_{18}$) and benzoquinoline were also detected confirming the presence of nitrogen compounds. The same compounds were found by pyrolysis of insoluble coke from sample B (Figure 9b) with a higher fraction of nitrogen-containing molecules (isoquinoline and benzoquinoline). However, because the increase in temperature was extremely rapid, not all the coke was functionalized. In fact, after pyrolysis and subsequent extraction, elemental analysis (Table 4) indicated that only 35% to 40% of the coke had been removed. The atomic H/C ratio of the two samples decreased on average from 0.72 to 0.52, indicating that the residue had an even higher degree of aromaticity. Note that similar aromatic hydrocarbon molecules were found trapped in HY cavities after the transformations of various hydrocarbons.<sup>11-13,56</sup>

**G. Oxidative Degradation.** In another attempt to functionalize the insoluble coke, both samples were treated in a mixture of $K_2Cr_2O_7$ and $H_2SO_4$. It was verified that this treatment was able to oxidize anthracene without producing paraffins. After 3 h, the degradation of both samples was considered complete, and the yield was between 30 and 40 wt %. Analysis by GC/MS of the extracts obtained from the 3 and 5 h treatments show a large fraction of $C_{22}-C_{31}$ alkanes. Alkanes from $C_{12}$ to $C_{32}$ were obtained after a treatment for 19 h in $K_2Cr_2O_7 + 2 mL H_2SO_4$.

<figure><img src="image_13.png" /><figcaption>**Table 5.** Atomic H/C Ratio of Different Hydrocarbons</figcaption></figure>

<table><thead><tr><th>Compound</th><th>Formula</th><th>H/C</th></tr></thead><tbody><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>8</sub>H<sub>18</sub></td><td>2.25</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>8</sub>H<sub>16</sub></td><td>2.00</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>6</sub>H<sub>6</sub></td><td>1.00</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>12</sub>NH<sub>9</sub></td><td>0.75</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>14</sub>H<sub>10</sub></td><td>0.71</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>17</sub>H<sub>12</sub></td><td>0.71</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>20</sub>H<sub>18</sub></td><td>0.90</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>33</sub>H<sub>12</sub></td><td>0.36</td></tr><tr><td>~<br>~<br>~<br>~<br>~<br>~<br>~</td><td>C<sub>45</sub>H<sub>22</sub></td><td>0.49</td></tr></tbody></table>

It could be speculated that these alkanes either stem from (i) the oxidation of long-chain alkyl aromatics with formation of acid components followed by decarboxylation of these acids or (ii) from the liberation of alkanes trapped in zeolite cavities after degradation of polyaromatic compounds. The second hypothesis seems to be more probable, considering the overall chemistry of hydride transfer and redistribution during cracking.

The relative amounts of alkanes are estimated from the H/C ratio. On the basis of the value of 0.72 for the H/C ratio of the insoluble coke (vide supra) and assum-
ing that the average H/C of only the polyaromatic coke compounds is equal to 0.55 (Table 5), it can be estimated that a maximum of 10 wt % of the total coke is constituted of alkanes.

## Conclusions

Coking of resid FCC catalysts in a commercial unit seems to follow a very peculiar mode of carbon deposition, which drastically differs from the conversion of light model compounds used in laboratory cracking. The results show consistently that the coke consists mainly of heavy condensed aromatic compounds, which are hardly soluble in CH₂Cl₂. These molecules are located in the mesopores of the catalyst matrix and binder. This is concluded from the fact that both used catalyst samples showed a large fraction of free strong Brønsted acidic hydroxyl groups. Note that strong Brønsted acid sites in sample A were not affected by coking, while a decrease of only 20% was observed for sample B. Thus, the bulky molecules present in the feed and formed during the cracking process do not enter the pores of the zeolite component, which (the micropores) in turn would account for the majority of the conversion of small molecules such as light alkanes. From the two samples investigated, sample A (higher oxygen concentration in the regenerator) shows a lower coke content than sample B and a higher H/C ratio of the deposits.

Four kinds of coke molecules were identified: (i) a small amount of soluble coke, recovered by a direct Soxhlet procedure via solution in CH₂Cl₂, (ii) very polyaromatic coke deposited on the zeolite crystallite and/or on alumina, (iii) aromatic molecules trapped and liberated by pyrolysis through degradation of insoluble coke, and (iv) alkanes trapped inside the domains of aromatic molecules. These latter molecules were only detected after an oxidative degradation with a K₂Cr₂O₇/ H₂SO₄ mixture and account for less than 10 wt % of the coke. It is important to note that the condensed aromatic ring systems have partly aliphatic chains evidenced by the detailed analysis of LD-MS spectra.

The nitrogen-containing compounds were mainly found in the somewhat smaller molecules and seem to deplete with oxidative and thermal treatment, in contrast to the results observed with gasoil FCC operation. The results obtained with the commercial FCC catalyst samples showed a good agreement with the results of several previous coke characterization studies on laboratory-scale reactors at similar conditions.

## Acknowledgment

We would like to thank Prof. M. Guisnet (Université de Poitiers) for many scientific discussions and Herr Krause (Technische Universität München) for his support with the LDI- and MALDI-TOF measurements.

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