Coke Formation during Catalytic Cracking of C8 Aliphatic

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Ind. Eng. Chem. Res. 2003, 42, 2292-2298

Coke Formation during Catalytic Cracking of C8 Aliphatic Hydrocarbons over Ultrastable Y Zeolite Aristidis A. Brillis and George Manos* Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.

The coking of an ultrastable Y zeolite was investigated during cracking of n-octane, isooctane, and 1-octene in a fixed-bed reactor over a temperature range of 523-623 K, various residence times, and various reactant compositions. The coking tendencies were found to be in the following descending order: 1-octene > isooctane > n-octane. In all cases, the coke content increased with increasing reactant feed composition or residence time. However, the coke amount formed was not proportional to the reactant feed composition, because of a strong pseudo-zeroth-order initial coking on strong acidic sites. Coke formation during the first minute of TOS was significantly faster than at later stages, because of the very strong zeolitic acid sites, which deactivated rapidly. Differences in coke formation at different compositions started to build only later at much lower rates. Similarly, coke content increased with increasing reaction temperature at high compositions. However, at low reactant compositions, the coke content decreased with increasing temperature for isooctane and 1-octene. This was due to an increase in desorption of the olefin intermediates responsible for coke formation into the gas phase with increasing temperature. 1. Introduction Hydrocarbon reactions over acidic zeolite catalysts are accompanied by the formation of heavy, low-boilingpoint, high-molar-mass byproducts that deposit on the surface of the catalyst and cause deactivation.1-3 These secondary byproducts remain trapped either in the zeolite pores or on the external surface of the crystallites. The term coke is used for the mixture of these numerous components that remain at ambient conditions after the removal of the reaction mixture and the term coke precursors for components intermediate in the complex reaction network that forms coke. The catalyst deactivation rate obviously depends on the relative rate of formation of these byproducts, a rate that can be very different from one catalytic system to another, depending on both the catalyst and the reaction network. Thus, in reforming, only 1 carbon atom out of 200 000 activated by the catalyst is transformed into coke, whereas in the cracking of heavy petroleum fractions, this can be more than 1 out of 20.4 The rate of coke formation and the composition of the coke furthermore depend on the number and strength of active sites, the pore structure, and the temperature. The effect of the zeolite pore structure is not limited to steric constraints on the formation of coke precursors. The contact time of an organic molecule with the active sites depends on the rate of diffusion of these molecules and, hence, on the characteristics of the diffusion path inside the zeolite crystallites: the length compared to the crystallite size, the size of the pore apertures, the size of the channel intersections, and the density of the acid sites.2 Coke is formed preferentially on the strongest acid sites and causes their deactivation. As these sites are the most active, the initial deactivating effect of coke is more pronounced than it would be if all of the active * To whom correspondence should be addressed. Fax: +4420 73832348. E-mail: [email protected].

sites were of the same strength. Therefore, a higher density of strong acid sites leads to a higher coke content.5-6 As the Arrhenius equation describes, increasing the temperature increases the rate of reaction. Therefore, coke formation is also expected to increase with increasing temperature. Higher temperatures on the other side increase the desorption rate of coke precursors. At high temperatures, the retention of coke is mainly due to trapping in the blocked pores, whereas at low temperatures, coke retention is due to a stronger adsorption that lowers the volatility of the formed molecules.2,3,7 The scope of the present work was to study the coking of an acidic ultrastable Y zeolite (USHY) by similar reactants. Components with the same number of carbons but different structures were chosen, namely, a normal octane, a branched paraffin isooctane, and the olefin 1-octene. 2,2,4-Trimethylpentane was chosen as the isooctane. In addition to the effect of the nature of the reactant, the effects of residence time and reactant composition on the coking process and catalyst deactivation at short times on stream were investigated, as well as temperature effects. 2. Experimental Section 2.1. Materials. The USHY zeolite catalyst was provided in powder form with an average particle size of 1 µm, an original Si/Al ratio of 2.5, and a framework Si/ Al ratio of 5.7. Its micropore area was 532.4 m2/g, and its micropore volume 0.26 cm3/g. The measured BET surface area was 590 ( 23.5 m2/g. The catalyst was pressed into pellets, crushed, and sieved, producing particles with sizes in the range of 1.0-1.7 mm. Before each experiment, the catalyst was dried in an oven at 473 K for 2 h. All hydrocarbon reactants, normal octane, isooctane (2,2,4-trimethylpentane), and 1-octene, were supplied by Fluka Chemicals (99% purity). Nitrogen (99% purity) was used as the carrier gas.

10.1021/ie020460w CCC: $25.00 © 2003 American Chemical Society Published on Web 05/06/2003

Ind. Eng. Chem. Res., Vol. 42, No. 11, 2003 2293 Table 1. Residence Times (min) under All Experimental Conditions flow rate (mL/min) 100 190

temp (K)

25

reactant feed composition

523 523 523 573 573 573

0.0168 0.0149 0.0153 0.0136

0.0044 0.0042 0.0037 0.0040 0.0038 0.0034

0.0023 0.0022 0.0019 0.0021 0.0020 0.0017

0.05 0.1 0.2 0.05 0.1 0.2

623 623 623

0.0141 0.0125

0.0037 0.0035 0.0031

0.0019 0.0018 0.0016

0.05 0.1 0.2

2.2. Experimental Procedure. The experiments were performed at a temperatures of 523-623 K and atmospheric pressure in a stainless steel isothermal tubular fixed-bed reactor. The amount of catalyst used in each experiment was 0.65 g. The carrier gas passed through a saturator placed in a heated water bath at three different flows, namely, 25, 100, and 190 mLN/ min, whereas the reactant composition in the feed was varied between 5, 10, and 20% by adjusting the temperature of the water bath. An overview of the experimental conditions during all runs with the corresponding residence times is presented in Table 1. Upon completion of the experiment, nitrogen was flowed directly through the reactor for 1 h to ensure that any residual vapors throughout the system were discharged. Once the reactor had cooled to 373 K, it was disconnected from the rig and opened, to allow the catalyst to be removed. The entire zeolite amount was then placed into a TGA (thermal gravimetric analysis) apparatus. The temperature of the TGA instrument was raised first to 473 K for 30 min under flowing nitrogen to remove adsorbed reaction-mixture components. A period of 30 min was found to be enough for this task, as the TGA weight indication stabilized during this time. Then, the temperature was increased to 1123 K at a rate of 10 K/min under flowing air. The amount of coke present in the zeolite was calculated by the difference between initial and final masses of the catalyst sample, with the initial mass taken as the mass of the sample at 473 K

% coke )

Figure 1. Coke content after 20 min of n-octane cracking over USHY at different reactant compositions and temperatures.

Figure 2. Coke content after 20 min of isooctane cracking over USHY at 20% reactant composition, different residence times, and different temperatures.

(mass before coke burning) (mass after coke burning) (mass after coke burning)

3. Results and Discussion 3.1. n-Octane Results. The amounts of coke produced during n-octane reactions are presented in Figure 1. One can clearly observe that, although the two reactant compositions were in a ratio of 2:1, the coke content was only slightly higher for the high-composition experiments. At all reaction temperatures, the difference in coke yield between the higher and lower reactant compositions was less than 0.5%. The reason for this low difference in coke concentration is the preferential initial coking of the strong acid sites, which shows pseudo-zeroth-order behavior with regard to the reactant. Coking occurs on acidic catalytic sites, and the coke formation rate increases with the strength of these acid sites.8 Zeolites contain strong acidic sites that promote coking tremendously. Coking on strong zeolitic sites is rapid and independent of the hydrocarbon content of the reaction mixture, showing pseudo-zerothorder behavior. It occurs in the first minute of time on


stream (TOS). Experiments with other reactants showed that the majority of coke was formed during the first minute, confirming the above explanation. 3.2. Isooctane Results. With isooctane as the reactant, the amounts of coke produced were larger than those produced from n-octane and smaller than those produced from 1-octene. This was expected, as branched paraffins are more reactive than normal paraffins but less reactive than olefins. The isooctane results are presented in Figures 2-4. In each of these graphs, isooctane has the same feed composition but different residence times. The coke content with isooctane decreased with decreasing residence time. Higher flow rates of inert gas resulted in higher amounts of olefins being desorbed into the gas phase and, therefore, fewer coke precursors

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Ind. Eng. Chem. Res., Vol. 42, No. 11, 2003 Table 2. Isobutene Mole Fractions during Isooctane Cracking over USY at TOS of 5 and 15 min at Various Temperatures, Inert Gas Flow Rates, and Reactant Feed Compositions flow rate (mL/min) 25

100

190

temp TOS ) TOS ) TOS ) TOS ) TOS ) TOS ) reactant feed (K) 5 min 15 min 5 min 15 min 5 min 15 min composition


being left on the catalyst surface, which formed less coke. In addition, at high reactant composition, the amount of coke increased with increasing reaction temperature. This was expected, since reaction rates increase with temperature. However, the trend of increasing amount of coke with increasing reaction temperature was surprisingly reversed at the other two lower-composition values. The coke amounts decreased with temperature for both of these sets of experimental runs. A possible explanation for lower coke contents with increasing temperatures could be the fact that the activation energies of the reactions responsible for coke formation could be lower than the activation energies of the secondary reactions of the olefins responsible for coke production. In that case, the temperature would increase both reaction rates, but the reactions forming secondary gaseous products would be enhanced to a greater extent, leaving much less of the adsorbed olefinic species to form coke. The end result would be less coke formation, despite the increase in coking kinetic constant. The concentration of coke precursors would be much lower, resulting in a net decrease of the coking rate. To confirm this statement, a simple model was set up, as described in a following section. The simulation results confirmed that this is the cause of the unusual temperature behavior. A similar explanation, namely, that this behavior is the result of easier retention of coke precursors in the zeolite micropores at lower temperatures, was given by Cerqueira et al.,9 who observed the same coke reversal with temperature during m-xylene transformation over USHY zeolite at 520 and 720 K. We assumed that, with isooctane as the reactant, isobutene was responsible for coke formation. In all cases, the yield of gaseous isobutene increased with increasing reaction temperature and with decreasing residence time (see Table 2). This behavior would explain the smaller amounts of coke produced with increasing reaction temperature at low reactant compositions and the smaller amounts of coke with decreasing residence time for the same range of reactant compositions. Because greater amounts of isobutene desorbed into the gas phase with increasing reaction temperature, lower amounts of adsorbed isobutene would undergo secondary reactions, and therefore, less coke would be produced with increasing reaction temperature. Therefore, it is not a gross generalization to say that the higher the yield of isobutene exiting the reactor, the lower the amount of coke produced, regardless of the effect of temperature or residence time.

523 573 623 523 573 623 523 573 623

0.007 0.011 0.020 0.004 0.005 0.008

0.012 0.019 0.028 0.012 0.014 0.020

0.020 0.024 0.047 0.020 0.038 0.052 0.013 0.023 0.025

0.036 0.040 0.057 0.032 0.062 0.069 0.02 0.034 0.025

0.028 0.055 0.083 0.030 0.051 0.076 0.021 0.043 0.048

0.036 0.062 0.098 0.037 0.069 0.100 0.028 0.048 0.120

0.05 0.05 0.05 0.1 0.1 0.1 0.2 0.2 0.2

Figure 5. Coke weight loss percentage not as the total weight difference from the beginning of the TGA experiment, but as the weight difference between different initial temperatures (coke treatment temperatures) and the final temperature of 1073 K (isooctane cracking over USHY, TOS ) 20 min).

However, this decrease in coke amount with increasing reaction temperature was not observed in the experiments at high reactant composition, even though the isobutene yield followed exactly the same trends as for the other two lower compositions. Obviously, at the higher composition, the resulting decrease in adsorbed coke precursors was not enough to compensate for the increased coking kinetic constant. To verify that the decrease in coke production with increasing temperature was not artificial, an alternative method was used to calculate the coke content. The earlier results might have been skewed by higher amounts of volatile coke components formed at higher reaction temperatures. In this new method, the coked catalyst sample in the TGA apparatus was first treated at various temperatures, 473 K being the usual temperature, to remove any coke components volatile at that temperature. The coke amount was then calculated as the weight decrease between the specified TGA treatment temperature and the final temperature of 1073 K, where no coke is left on the catalyst. The coke amounts calculated in this way excludes coke components that are volatile at the specified TGA treatment temperature. The results are presented in Figure 5 as coke content percentages against treatment temperature. They show that, independently of the chosen initial treatment temperature, the coke content decreased with increasing reaction temperature. 3.3. 1-Octene Results. Using 1-octene as the reactant, we confirmed the pattern that olefins produce more coke than alkanes, straight or branched. In all experiments performed with 1-octene as the reactant, the coke

Ind. Eng. Chem. Res., Vol. 42, No. 11, 2003 2295

Figure 6. Coke content after 20 min of 1-octene cracking over USHY at high residence time, different reactant compositions, and different temperatures.

Figure 7. Coke content after 20 min of 1-octene cracking over USHY at 10% reactant compositions, different residence times, and different temperatures.

amounts far surpassed those from the other two reactants. The 1-octene coke results are presented in Figures 6 and 7. The highest coke amount was observed in the experiment with the highest residence time, highest reactant composition, and highest temperature of 623 K. This result confirms the literature trend that higher temperatures and reactant compositions deposit more coke on the catalyst. However, this trend was reversed in all cases with low reactant composition, similarly to the isooctane coke results. The coke amounts decreased with increasing temperature. The explanation of higher coke amounts with decreasing temperature is the same as that presented for isooctane. In this case, with 1-octene as the reactant, more olefins were present in the product spectrum, namely, 1-octene isomers, which should contribute considerably to coke formation. For the highcomposition case (20%), the C8 isomers exhibited significant differences in their yields. In particular, at the end of TOS, we found that the lower the temperature, the larger the yield of C8 isomers. This means that, because more olefins exited the reactor at lower temperatures, fewer secondary reactions took place, resulting in less coke. This could explain why the above temperature reversal did not take place at reactant feed compositions of 20%. Similar decreases in the coke amount with increasing temperature were observed for both low-residence-time experiments (Figure 7). As in the case of isooctane, the activation energy of the reaction responsible for coke formation plays a significant role in the coke trend. When the coking activation energy is lower than the activation energies of the secondary olefinic reactions to form gaseous products, coke production decreases with increasing temperature.

Figure 8. Coke content, after 30 min in N2 flow at 673 K, during 1-octene cracking over USHY at 10% reactant composition, different residence times, and different temperatures.

This statement is discussed in the next section, where a simple model is developed. Because of the opposite temperature dependencies of the coke amounts at different feed compositions, a reversal in the coke trend with feed composition was also observed (Figure 6). With n-octane and isooctane, lower reactant compositions resulted in the production of lower amounts of coke. However, for 1-octene, this was not the case. At 523 K, the amount of coke produced for the highest reactant composition was a little more than 12%, whereas for the lower reactant composition, the amount of coke produced was nearly 15%. Similarly, for 573 K, the lower reactant composition produced more coke. This behavior can again be explained by the selectivities to 1-octene isomers. The C8 isomer yield for the low composition was around 0.15, whereas for the high composition, it was around 0.35. Therefore, as more C8 isomers exited the reactor with TOS, lower amounts of coke would be produced. Similarly, for the 573 K experiments, the C8 isomers had a yield difference of around 0.05, resulting in a coke difference of 0.05%. Comparing the coke results at different residence times, the conclusions drawn are in accordance with the selectivities obtained for the products. The shorter the residence time, the lower the amount of coke produced, because the conversion of 1-octene isomers into products was lower. To verify the above conclusions, a different method was used for coke analysis. It involved the passage of high nitrogen flows at 673 K immediately after the end of the experiment for 30 min. The results showed a much smaller decrease in coke amounts with reaction temperature, but a decrease nevertheless. The coke amounts found with this method were less than those obtained earlier, which is in accordance with the theory that significant amounts of volatile compounds have evaporated, leaving only the heavy byproducts of the coking reactions in the catalyst. These results are presented in Figure 8. Three additional experiments were performed at 573 K using the middle residence time of 0.228 s, a N2 flow of 100 mLN/min, and the low reactant composition of 10% at different reaction times, to better understand how rapidly coke was formed. The results are presented in Figure 9. It is fairly obvious that coke formation is an extremely rapid process at the beginning of catalyst exposure to the reaction mixture. The gradient of the coke content curve is extremely high during the first minute, whereas it becomes much flatter after that. After 1 min, the coke content shows a linear dependence

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This reaction scheme is represented by

RfP+O

(R1)

OfP

(R2)

OfC

(R3)

For a similar catalytic system, Corma et al.10 assumed pseudo-first-order kinetics. Therefore, for our reaction scheme, we also assumed pseudo-first-order kinetics

ri ) kiCK Figure 9. Coke content after different reaction times of 1-octene cracking over USHY at 10% reactant compositions, medium residence times, and 573 K.

(1)

where ri is the kinetic rate for reaction i (mol gcat-1 min-1); ki is the kinetic constant for reaction i (cm3 gcat-1 min-1); and CK is either the reactant concentration CR for reaction 1 or the olefin concentration CO for reactions 2 and 3, i.e., the concentration of the reactant of the corresponding reaction (mol /cm3). Mass balances for reactant R, paraffin P, olefin O, and coke C were formulated as follows

mole balance for reactant R V°

Figure 10. Coke content after different reaction times of 1-octene cracking over USHY at 10% reactant composition, different residence times, and different temperatures.

dCR ) - k1φ1CR dW

(2)

where V° is the volumetric flow rate (cm3 min-1); W is the weight of the catalyst (gcat); and φ1 is the activity factor, which is equal to 1 at TOS ) 0 and decreases with increasing concentration of coke on the catalyst (CC), following the equation10

φi ) exp(-aiCC)

(3)

mole balance for product paraffin P on TOS. More than two-thirds of the coke formed during the first 20 min was actually produced in the first minute of TOS. As the reaction time increased, the coke formation rate decreased, indicating a deactivation of the coking itself. At 4 times the reaction time, i.e., at 80 min, coke was still formed, but its rate of production was more than halved, reaching a value of 23%. This is an indication of the high activity of the USHY catalyst used in these experiments. A more complete picture of the coking behavior at short TOS is presented in Figure 10. 3.4. Simple Model of Coking. In this section, a simple model of our catalytic system is developed, to elucidate the trend of lower coke contents with increasing temperature occurring at low reactant compositions. The explanation put forward in the previous section was that, because olefins were responsible for coke production and because greater amounts of olefins exited the reactor at higher reaction temperatures, fewer olefinic secondary reactions to coke would take place and, therefore, less coke would be produced at higher reaction temperatures. The reason for this was believed to be the lower activation energy values for coking than for secondary olefinic reactions. A simplified reaction scheme for the cracking of isooctane (R) is adopted from Corma et al.10 Via cracking, reactant (R) produces paraffins (P) and olefins (O). However, some of the olefins undergo secondary reactions, giving paraffins (P) and coke (C) on the catalyst.

V°

dCP ) k1φ1CR + k2φ2CO dW

(4)

mole balance for olefin O V°

dCO ) k1φ1CR - (k2φ2 + γ3k3φ3)CO dW

(5)

The rate of coke formation is given by

dCC ) γk3φ3CO dt In the above equations, γ3 is the stoichiometric coefficient of olefin in reaction 3, and γ is the molecular weight of the olefin involved in the formation of coke (reaction 3). From the above model, the initial coking rate over fresh catalyst was calculated, assuming that the activity factors for all three reactions were equal to 1, i.e., φ1 ) φ2 ) φ3 ) 1. In the previous section, it was assumed that the coke content decreased with increasing temperature when the activation energies of the coking reactions were greater than the activation energies of the olefin hydrogenation into paraffins (responsible for coke production). The simulated results shown in Figure 11, with the activation energy of reaction 2 smaller than the activation energy of reaction 3 (coking), indeed demonstrate that the coke content increases with temperature, as usual.

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Figure 11. Simulated average initial coking rate during isooctane cracking over USHY, with kinetic constant values at 523 K of k1 ) 500 cm3 gcat-1 min-1, k2 ) 1000 cm3 gcat-1 min-1, and k3 ) 10 cm3 gcat-1 min-1 and activation energies of Ea1 ) 65 kJ mol-1, Ea2 ) 80 kJ mol-1, and Ea3 ) 100 kJ mol-1.

Figure 12. Simulated average initial coking rate during isooctane cracking over USHY, with kinetic constant values at 523 K of k1 ) 500 cm3 gcat-1 min-1, k2 ) 1000 cm3 gcat-1 min-1, and k3 ) 10 cm3 gcat-1 min-1 and activation energies of Ea1 ) 65 kJ mol-1, Ea2 ) 100 kJ mol-1, and Ea3 ) 80 kJ mol-1.

In all simulated results, the average initial coking rate is presented in units of gcoke gcat-1 min-1, as we intended to show the reverse temperature effect on coke observed in various experimental runs. Another assumption made was that reaction 2 was much faster than reaction 1, which was faster than reaction 3, similarly to the assumptions made by Corma et al.10 These assumptions are manifested in the values of the preexponential factors of the kinetic constants. On the other hand, in our simulated system, when the activation energy of reaction 2 was higher than the activation energy of reaction 3, the average initial coking rate decreased with increasing reaction temperature (Figure 12). This means that reaction 2 was more enhanced than reaction 3 (coking), i.e., most of the olefin produced by reaction 1 would be consumed to form more paraffin through reaction 2, leaving less olefin to produce coke by reaction 3. Therefore, less coke would be produced with increasing reaction temperature. Another interesting point from the simulated results can be seen in Figure 13. For a carefully chosen set of parameters, the coke content seemed to exhibit a maximum at the intermediate temperature of 573 K. In case A, when the activation energy of reaction 2 was slightly smaller than the activation energy of reaction 3, the average initial coking rate showed an expected increase between 523 and 573 K, followed by a decrease between 573 and 623 K. With reversed activation energies for reactions 2 and 3 (case B), the average initial coking rate increased between 523 and 573 K, but then decreased to an even smaller value for the 623

Figure 13. Simulated average initial coking rate during isooctane cracking over USHY at 523 K with kinetic constant and activation energy values of (A) k1 ) 500 cm3 gcat-1 min-1, k2 ) 1000 cm3 gcat-1 min-1, k3 ) 10 cm3 gcat-1 min-1; Ea1 ) 65 kJ mol-1, Ea2 ) 80 kJ mol-1, Ea3 ) 85 kJ mol-1 and (B) k1 ) 500 cm3 gcat-1 min-1, k2 ) 1000 cm3 gcat-1 min-1, k3 ) 10 cm3 gcat-1 min-1; Ea1 ) 65 kJ mol-1, Ea2 ) 85 kJ mol-1, Ea3 ) 80 kJ mol-1.

K reaction temperature. Because both reactions 2 and 3 became much faster than reaction 1, there was not enough raw material, i.e., olefin, to form coke at the highest reaction temperature, resulting in this maximum in the average initial coking rate. Note that, in the simulated results, we selected sets of activation energies in a reasonable range that were able to reproduce the three different tendencies observed for the coke dependence on temperature. In conclusion, it was possible to simulate a decreasing coke content with increasing temperature, as experimentally found, if the activation energies of the reactions involved in our catalytic system were reversed, i.e., favoring the secondary reactions of the olefins to produce more products instead of coke. 4. Conclusions The coking tendencies during cracking of C8 aliphatic hydrocarbons over USHY zeolite catalyst were found to be in the following descending order: 1-octene > isooctane > n-octane. In all cases, the coke content increased with increasing reactant composition or residence time. Coke formation during the first minute of TOS was significantly higher than that at later stages, as a result of very strong zeolitic acid sites, which enhanced coking and deactivated rapidly, causing a steep decrease of coking rate. Similarly, the coke content increased with increasing reaction temperature at high reactant compositions. However, at low reactant feed compositions (5 and 10%), the coke content decreased with increasing temperature for both isooctane and 1-octene, because of higher desorption of the olefinic coke precursors responsible for coke formation into the gas phase with increasing reaction temperature. Simulation results confirmed the above findings. When the activation energy of the coking reaction was lower than the activation energies of the secondary reactions forming paraffins from olefin intermediates, the coke content decreased with increasing temperature. As the strongest deactivation was observed in the first minute of time on stream, future work should focus on experiments at very short TOS and analysis of the reaction mixture at shorter intervals of TOS. Novel sampling methods should be developed that enable accurate multiple sampling during the first minute of time on stream.

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Literature Cited (1) Guisnet, M.; Magnoux, P. Coking and deactivation of zeolites. Influence of pore structure. Appl. Catal. 1989, 54, 1. (2) Guisnet, M.; Magnoux, P. Composition of the carbonaceous compounds responsible for zeolite deactivation. Modes of formation. In Zeolite Microporous Solids: Synthesis, Structure and Reactivity; Derouane, E. G., Lemos, F., Naccache, C., Ramoˆa Ribeiro, F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; p 437. (3) Guisnet, M.; Magnoux, P. Deactivation of zeolites by coking. Prevention of deactivation and regeneration. In Zeolite Microporous Solids: Synthesis, Structure and Reactivity; Derouane, E. G., Lemos, F., Naccache, C., Ramoˆa Ribeiro, F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; p 457. (4) Barbier, J. Deactivation of reforming catalysts by cokings A review. Appl. Catal. 1986, 23, 225. (5) Magnoux, P.; Roger, P.; Canaff, C.; Fouche, V.; Gnep, N. S.; Guisnet, M. New technique for the characterization of carbonaceous compounds responsible for zeolite deactivation. Stud. Surf. Sci. Catal. 1987, 34, 317.

(6) Babitz, S. C.; Kuehne, M. A.; Kung, H. H.; Miller, J. T. Role of Lewis acidity in the deactivation of USHY zeolites during 2-methylpentane cracking. Ind. Eng. Chem. Res. 1997, 36, 3027. (7) Magnoux, P.; Boucheffa, Y.; Joly, G.; Guisnet, M.; Jullian, S. Formation of carbonaceous compounds from propene and isobutene over 5A zeolite adsorbents. Stud. Surf. Sci. Catal. 1987, 34, 427. (8) Manos, G.; Hofmann, H. Disproportionation of ethylbenzene on an ultrastable Y-zeolite: Investigation of the coking mechanism in an integral reactor. Chem. Ztg. 1990, 114, 183. (9) Cerqueira, H. S.; Ayrault, P.; Datka, J.; Guisnet, M. Influence of coke on the acid properties of a USHY zeolite. Microporous Mesoporous Mater. 2000, 38, 197. (10) Corma, A.; Miguel, P. J.; Orchilles, A. V. Product selectivity effects during cracking of alkanes at very short and longer times on stream. Appl. Catal. A: Gen. 1996, 138, 57.

Resubmitted for review November 18, 2002 Revised manuscript received March 10, 2003 Accepted March 24, 2003 IE020460W

Coke Formation during Catalytic Cracking of C8 Aliphatic

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