Catalytic Conversion of Thiophene under Mild Conditions over a ZSM

Ind. Eng. Chem. Res. 2009, 48, 7505–7516

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Catalytic Conversion of Thiophene under Mild Conditions over a ZSM-5 Catalyst. A Kinetic Model Lisette Jaimes and Hugo de Lasa* Chemical Reactor Engineering Centre (CREC), Department of Chemical and Biochemical Engineering, Faculty of Engineering, UniVersity of Western Ontario, London, Ontario, Canada N6A 5B9

Currently, refiners consider the post-treatment of FCC gasoline processes to be a viable and likely less costly path for meeting environmental regulations on sulfur. Several promising catalytic desulfurization post-treatment processes do not require hydrogen addition and use zeolites. This type of desulfurization leads to significant levels of coke, and as a result, it is being considered for implementation in twin fluid beds (reactor and regenerator) to maintain the catalyst activity. This study evaluates the conversion of thiophene on H-ZSM-5 zeolite in a silica matrix. Experiments were carried out in the CREC fluidized riser simulator under mild conditions using thiophene/n-octane and thiophene/1-octene mixtures. The results show a high and selective thiophene conversion. It is speculated that thiophene conversion takes place via ring opening and alkylation to form H2S, aromatics, alkylthiophenes, benzothiophene, and coke. These observations are in agreement with previous thermodynamic analyses. On this basis, a reaction network and kinetic model are proposed. Numerical regression leads to kinetic parameters with narrow spans, suggesting that the proposed model satisfactorily simulate thiophene removal under the suggested gasoline post-treatment conditions. 1. Introduction Environmental regulations imposing very low concentrations of sulfur in fuels, with limits of 30 ppm in gasoline and 15 ppm in diesel, have been in effect since 2004.1 Fluid catalytic cracking (FCC) gasoline is, by far, the most important source of sulfur emissions, contributing up to 85-95%. An updated review of FCC gasoline desulfurization alternatives can be found in ref 2. There are three main available options for producing FCC gasoline with very low sulfur content: hydroprocessing of the FCC feedstock, reduction of the sulfur content during the FCC process, and desulfurization of FCC gasoline. The hydroprocessing of FCC feedstock entails high investment and operating costs because of the high process severity and H2 consumption, whereas gasoline and diesel sulfur specifications cannot be met by varying the FCC operating conditions. This explains why most refiners are turning toward post-treatment technologies for FCC gasoline as the most viable and least costly path for meeting sulfur environmental regulations. Conventional hydrotreating decreases the quality of FCC gasoline as a result of octane number losses. Moreover, the use of hydrogen adds significant processing costs. As a result, the most recent gasoline desulfurization studies have considered desulfurization circumventing the need of the use of hydrogen.2 Among the most promising catalytic desulfurization options not requiring the use of hydrogen, zeolites as either adsorbents or catalysts appear to have great potential. The selective adsorption and reaction of thiophene in model hydrocarbon mixtures using zeolites has been widely investigated.2,3 This type of desulfurization leads to significant levels of coke. As a result, to maintain catalyst activity, it has been proposed that this approach be implemented in a fluidized-bed reactor and fluidized-bed regenerator system. In this sense, the SO2 produced during catalyst regeneration can be removed from the flue gases by a variety of methods, with the following being the most common: (a) wet scrubbing using a slurry of alkaline sorbent, usually limestone or lime, or seawater to scrub the gases; (b) spray-dry scrubbing using similar sorbent slurries; and (c) dry

sorbent injection systems. These desulfurization processes are able to remove 95% or more of the SO2 in flue gases. Regarding the use of zeolites as adsorbents and/or catalysts, USY, Y, and ZSM-5 are the preferred alternatives given that they have pore sizes in the 7-13 Å range. Desulfurization with Y and USY appears not to be favored, however, considering the significant gasoline losses (up to 60%) that result.4 In contrast, at the appropriate conditions, ZSM-5, with a pore size of 5.4 Å, provides good access, evacuation, and selective adsorption of sulfur-containing molecules.2,3 It is with these facts in mind that ZSM-5 zeolite was selected as the catalyst of choice for the desulfurization of gasoline-range molecules in the present study. Considering the facts described above, along with the fact that thiophenic species represent more than 60% of the total sulfur content in FCC gasoline,5 the current research evaluates the conversion of thiophene on H-ZSM-5 zeolite crystallites dispersed in a silica matrix. Experiments were carried out in the CREC fluidized riser simulator6 under mild conditions using thiophene/n-octane and thiophene/1-octene mixtures as representatives of gasoline. Experimental results along with previous thermodynamic evaluations and mechanistic interpretations allow the most probable reaction network for thiophene over zeolite H-ZSM-5 to be identified. On this basis, a kinetic model that describes the cracking behavior of thiophene under gasoline desulfurization conditions is proposed, and the associated reaction rate kinetic parameters are estimated. 2. Experimental Section The model compound thiophene, C4H4S (Aldrich, 99% purity), was selected as the key sulfur species in gasoline, with gasoline being simulated by n-octane (Aldrich, 98% purity) and 1-octene (Aldrich, 98% purity), as both hydrocarbons have boiling points that fall in the middle of the gasoline boiling range. A commercial catalyst designated as Z-2, supplied by Albemarle, was used for the catalytic cracking tests. This catalyst was selected for this study because of its high content of

10.1021/ie8020126 CCC: $40.75  2009 American Chemical Society Published on Web 07/20/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009

H-ZSM-5 supported in a matrix (49.7%), as confirmed by X-ray diffraction (XRD), temperature-programmed desorption (TPD), and surface area analysis. The Z-2 catalyst, already thermally treated by Albermarle, was further calcined in the CREC riser simulator at 550 °C under flowing air for 25 min before catalytic tests were performed. Thermal and catalytic runs were performed in the CREC fluidized riser simulator.6 The riser simulator consists of two outer shells, a lower section and an upper section, that permit the catalyst to be loaded and unloaded easily. This reactor was designed in such way that an annular space is created between the outer portion of the basket and the inner part of the reactor shell. Upon rotation of the impeller located in the upper section, gas is forced outward from the center of the impeller toward the walls. This creates a lower pressure in the center region of the impeller, inducing an upward flow of gas through the catalyst chamber from the bottom of the reactor annular region where the pressure is slightly higher. The reactor flow rate is a function of the set impeller velocity and the set reactor pressure. The impeller provides a fluidized bed of catalyst particles, as well as intense gas mixing inside the reactor. The riser simulator operates in conjunction with a series of sampling valves that, following a predetermined sequence, allow hydrocarbons to be injected and products to be withdrawn in short periods of time. The sampling system also allows the reaction product sample to be sent to the analytical system. A four-port chromatographic valve (4PV) connects the reactor with the air/argon supply at one end and with the vacuum system at the other end. With the valves in the open position, the gases pass through the 4PV and the reactor and finally go to the vacuum box. When the valves are in the closed position, the reactor is completely isolated from the rest of the system. The reaction time is set with a timer connected to the actuator of the 4PV. This timer is linked to a microswitch located in the manual injector. When the plunger of the syringe is pushed all the way forward to deliver its contents to the reactor, the injector switch is pressed, and the timer is started. Once the required reaction time is reached, the actuator opens the 4PV, and the reactor is emptied as a result of the pressure difference between the reactor and the vacuum box. Finally, the reaction product sample is sent to the analytical system. For the kinetic study, thiophene/n-octane and thiophene/1octene mixtures were reacted in an argon environment (2 atm), at mild temperatures (350-450 °C), for contact times from 5 to 20 s, concentrations of thiophene between 0 and 5 wt %, a catalyst-to-feedstock ratio of 5, and an impeller velocity of 5000 rpm. All thermal and catalytic runs were repeated at least three times to ensure the reproducibility of the results. An important observation from these runs was that the mass balance closures, which included all chemical species being fed to and removed from the reactor, were in the range of (7%, with most of the balances in the (3% range. The gaseous reaction products were analyzed in an Agilent GC/MSD system. An Agilent 5973N mass-selective detector (MSD) was used to identify the reaction products, whereas an Agilent 6890N gas chromatograph (GC) with a flame photometric detector (FPD) and a flame ionization detector (FID) allowed their quantification. The FPD was utilized for the selective detection of sulfur compounds at parts-per-million levels in hydrocarbon mixtures, with calibration curves to correlate calculated areas and concentrations of sulfur in hydrocarbon mixtures obtained prior to the experimental runs. In addition, the FID was employed to quantify the hydrocarbon species on a sulfur-free basis. Two HP-1 dimethylpolysiloxane

Figure 1. X-ray diffraction of Z-2 catalyst.

capillary columns with a length of 50 m, a nominal diameter of 0.50 mm, and a nominal film thickness of 0.5 µm permitted the separation of the components present in the samples. The coke deposited on the catalysts after the tests was measured in a total organic carbon analyzer (TOC-V) with a solid sample module (SSM-5000) from Mandel, and the sulfur amount in the coke was determined by difference assuming 100% sulfur balance. Some experimental measurements of sulfur on the catalyst were performed with an elemental analyzer (CNS LECO-932) for comparison. It was observed that deviations between the calculated and experimental values were lower than 6%. 3. Catalyst Characterization Physicochemical characterization of the Z-2 catalyst was performed, including particle size distribution (PSD), average bulk density (ABD), X-ray diffraction (XRD), electron probe microanalysis (EPMA), temperature-programmed desorption (TPD), and surface area (BET and T-plot). Additionally, H-ZSM-5 synthesized in the CREC laboratories7 was used as a reference for assessing zeolite properties including crytallinity. The average particle size was assessed at 84 µm, with the 0-20- and 0-40-µm catalyst fractions being limited to 10 and 19 vol %, respectively. This limited fraction of fines prevented plugging of the reactor porous plates. The apparent density of the catalyst was measured at 1535 kg/m3 by a method established at CREC. Using these physical parameters and considering the gas density for pure n-octane at 200 kPa and 400 °C to be equal to 0.245 kg/m3, it was concluded that the Z-2 catalyst particles belong to group A in Geldart’s powder classification chart,8 a particle group considered to display good fluidization. XRD was used to confirm the presence of H-ZSM-5 zeolite in the Z-2 catalyst sample. The XRD analysis was performed using Cu KR radiation in a rotating anode X-ray diffractometer over a 2θ range from 2° to 82°. The XRD patterns for both Z-2 catalyst and H-ZSM-5 zeolite (Figures 1 and 2, respectively) display peaks at similar 2θ values, confirming that the crystals in the Z-2 catalyst are indeed those of H-ZSM-5 zeolite. Comparison of the characteristic peak at 23.6° shows an XRD peak relationship close to the zeolite/matrix ratio reported by Albermarle, with 49.7% crystallinity as assessed with XRD. The chemical composition of the Z-2 catalyst, measured using EPMA, showed that the Si/Al ratio for this catalyst was 3.46, with a low sodium content of 0.08 wt %. TPD experiments were performed using an AutoChem II analyzer from Micromeritics. A 0.172-g sample was pretreated by a helium purge for 2 h at 500 °C. Ammonia was adsorbed


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Figure 4. Conversion of thiophene versus time. Reaction conditions: T ) 350, 375, 400, 425, and 450 °C; catalyst/feedstock ratio ) 5; feed composition ) 5 wt % thiophene/95 wt % n-octane. Error bars correspond to standard deviations of three repeated trials. Figure 2. X-ray diffraction of H-ZSM-5 reference zeolite.

Figure 3. TPD spectra of Z-2 catalyst and H-ZSM-5 reference zeolite.

for 1 h at 100 °C using a NH3/He gas mixture (4.52% ammonia, 95.58% helium). After dosing, the sample was purged in He for 1 h at the adsorption temperature. During the TPD experiments, the temperature of the sample was increased linearly by 15 °C min-1 up to 500 °C in flowing helium. Ammonia TPD for both the Z-2 catalyst and H-ZSM-5 zeolite are reported in Figure 3. Deviations of the Z-2 catalyst acidity under pretreatment conditions, as described in section 2, were smaller than 8%. The TPD spectra display two consistent desorption peaks centered at 220 and 430 °C, confirming that both Z-2 catalyst and H-ZSM-5 zeolite contain weak acidic sites and strong acidic sites. Furthermore, the total acidities for both Z-2 catalyst and H-ZSM-5 zeolite were determined by integrating the TPD spectra (Figure 3). The Z-2 catalyst displayed a total acidity of 0.048 mmol of NH3/g, whereras the H-ZSM-5 reference zeolite showed a higher 0.063 mmol of NH3/g value, a valuable result showing a higher proportional contribution of weaker sites in the Z-2 catalyst. The specific surface area of the Z-2 catalyst was measured using an ASAP 2010 analyzer (from Micromeritics). Before the measurements, samples weighing between 0.15 and 0.2 g were degassed at 643 K for 4 h. Adsorption isotherms were measured under the relative pressure range from ∼10-6 to 1. The total Z-2 catalyst surface area (BET) observed was 92 m2/g, with the zeolite (micropore) after the T-plot calculations contributing 71 m2/g and the matrix contributing 21 m2/g. On the basis of the Z-2 catalyst characterization, it is possible to conclude that (a) the physical properties of the catalyst particles are adequate for good fluidization; (b) the catalyst presents both weak and strong acid sites; and (c) XRD, TPD, and surface area data for the catalyst confirm a high concentra-

Figure 5. Conversion of thiophene versus time. Reaction conditions: T ) 350, 375, 400, 425, and 450 °C; catalyst/feedstock ratio ) 5; feed composition ) 5 wt % thiophene/95 wt % 1-octene. Error bars correspond to standard deviations of three repeated trials.

tion of H-ZSM-5 zeolite, the main component accounting for its catalytic activity. 4. Experimental Results and Discussion 4.1. Thermal Cracking. Thermal cracking runs, without catalyst loaded in the CREC riser simulator reactor, were initially performed to determine possible thermal cracking of thiophene, n-octane, and 1-octene under the most severe reaction conditions. Whereas no thiophene conversion was observed at 450 °C and 20 s, n-octane and 1-octene exhibited small conversions of 0.44% and 1.89%, respectively. Therefore, thermal cracking was neglected under the conditions studied, and the conversion observed during the catalytic runs (catalyst-loaded) truly represents the H-ZSM-5 catalytic effects. 4.2. Catalytic Runs: Conversion of Thiophene. The catalytic cracking behavior of thiophene was evaluated using thiophene/n-octane and thiophene/1-octene mixtures as feeds to the CREC riser simulator. In addition, catalytic experiments were conducted using pure n-octane and 1-octene. It was observed that up to four consecutive catalytic runs could be performed without significant catalyst deactivation. As a result, the catalyst was regenerated after every four consecutive runs. This also allowed the kinetic model to be established neglecting the influence of catalyst deactivation. Figures 4 and 5 report thiophene conversions for thiophene/ n-octane and thiophene/1-octene mixtures, respectively, with concentrations of 5 wt % of thiophene. As expected, the thiophene conversion increases progressively with reaction time in the 5-20 s range, reaching a maximum value of 36% at 450 °C and 20 s. A comparison of Figures 4 and 5 shows a higher thiophene conversion for the thiophene/n-octane mixture than for the

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Figure 6. Thiophene adsorption/conversion selectivity in hydrocarbon mixtures. Reaction conditions: T ) 350, 375, 400, 425, and 450 °C; catalyst/ feedstock ratio ) 5; reaction time ) 5, 10, 15, and 20 s; feed composition ) (() 5 wt % thiophene/95 wt % 1-octene and (∆) 5 wt % thiophene/95 wt % n-octane.

thiophene/1-octene mixture, for reaction times and temperatures above 15 s and 400 °C. This behavior can be explained by taking into consideration the facts that n-octane is a better hydrogen donor than 1-octene and that reactions involving thiophene ring opening and hydrogen transfer are endothermic and, therefore, are thermodynamically favored at higher temperatures. On the other hand, the thiophene conversion displays higher values in the presence of 1-octene than in the presence of n-octane for the lower reaction times (t < 15 s) and temperatures (T < 400 °C). This result can be justified given that olefins are more reactive over H-ZSM-5 zeolites than paraffins. In addition, olefins are a better source of light olefins for thiophene alkylation reactions, as these reactions are exothermic and therefore thermodynamically favored at lower temperatures. In addition, to assess the competitive conversion of thiophene and hydrocarbons (n-octane or 1-octene), a selectivity parameter can be defined as STh )

XTh /WTh XHC /WHC

Figure 7. Conversion of thiophene versus initial thiophene concentration. Reaction conditions: T ) 450 °C, catalyst/feedstock ratio ) 5, time ) 20 s. Error bars correspond to standard deviations.

Figure 8. Conversion of n-octane versus time. Reaction conditions: T ) 350, 400, and 450 °C; catalyst/feedstock ratio ) 5; feed composition ) 100 wt % n-octane. Error bars correspond to standard deviations.

(1)

where XTh and XHC are the thiophene and hydrocarbon conversions, respectively, based on WTh and WHC, the corresponding mass fractions of thiophene and hydrocarbon (n-octane or 1-octene) injected as reactants. At close initial reactant concentrations, a selectivity parameter of more than 1 means that the thiophene conversion is higher than the hydrocarbon conversion. On the other hand, at close thiophene and hydrocarbon conversions, variations in the selectivity parameter are directly related to differences in the initial thiophene and hydrocarbon concentrations. This selectivity parameter lumps the combined effects of adsorption and reaction, exceeding a value of 5 in all cases (see Figure 6). Given the significant differences in gas-phase concentrations between hydrocarbon and thiophene, this result confirms that there is either a greater adsorption affinity of thiophene versus both n-octane and 1-octene or alternatively a much higher intrinsic rate of thiophene dehydrosulfidation compared to the hydrocarbon cracking rate. Furthermore, to explore the reaction order of the thiophene reaction, experiments at different thiophene contents in n-octane were performed with the remaining reaction conditions held constant: catalyst/feedstock ratio ) 5, temperature ) 450 °C, and time ) 20 s. Results of these experiments are reported in Figure 7. From this figure, it can be observed that the thiophene conversion increases with increasing initial concentration at low thiophene content in the reactant mixture and that the conversion of this sulfur species remains fairly constant for initial concen-

Figure 9. Conversion of n-octane versus time. Reaction conditions: T ) 350, 375, 400, 425, and 450 °C; catalyst/feedstock ratio ) 5; feed composition ) 5 wt % thiophene/95 wt % n-octane. Error bars correspond to standard deviations.

trations higher than 5 wt %. The recorded dependence of the thiophene conversion on the initial concentration suggests a second-order thiophene reaction for the thiophene conversion at low concentration and a first-order reaction at higher concentrations. 4.3. Catalytic Runs: Conversions of n-Octane and 1-Octene. n-Octane and 1-octene conversions are reported in Figures 8-11. These figures report the results of experiments performed using 100 and 95 wt % hydrocarbons (with the balance thiophene). The runs show an expected and progressive increase of the hydrocarbon conversion with reaction time and temperature. In addition, the n-octane conversions (Figures 8 and 9) are lower than those for 1-octene (Figures 10 and 11), with these results being consistent with the expected reactivity differences of paraffins and olefins over H-ZSM-5 zeolites.9 Moreover, to specifically establish the reaction orders of the n-octane and 1-octene reactions, catalytic runs were performed with different gas-phase initial hydrocarbon concentrations. In


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Table 1. Hydrocarbon and Sulfur Product Distributions from the Catalytic Conversion of Pure Hydrocarbon and Thiophene/ Hydrocarbon Mixtures at 450 °Ca reactant mixture n-C8

n-C8/5 wt % Th

C8d

C8d/5 wt % Th

Hydrocarbon Products (wt %)

Figure 10. Conversion of 1-octene versus time. Reaction conditions: T ) 350, 400, and 450 °C; catalyst/feedstock ratio ) 5; feed composition ) 100 wt % 1-octene. Error bars correspond to standard deviations.

C2s C3s iso-C4 n-C4 C4d C5 C5d unreacted n-C8 or C8d aromaticsb

1.47 11.68 2.99 4.49 3.14 3.20 1.36 69.69 1.98

1.57 10.48 2.58 4.36 2.35 2.35 1.16 68.59 2.92

1.52 12.24 5.38 3.05 9.42 3.01 4.99 52.69 7.71

1.49 12.79 4.95 3.22 7.08 2.95 4.46 49.27 9.68

Sulfur Species (ppm) H2S unreacted thiophene (wt %) methylthiophene ethylthiophene 2-(1-methylethyl)thiophene propylthiophene benzothiophene methylbenzothiophene H2S/(alkylthiophene + benzothiophene)c Figure 11. Conversion of 1-octene versus time. Reaction conditions: T ) 350, 375, 400, 425, and 450 °C; catalyst/feedstock ratio ) 5; feed composition ) 5 wt % thiophene/95 wt % 1-octene. Error bars correspond to standard deviations.

coke (wt %)d sulfur on catalyst (wt %)e hydrocarbon conversion (%) thiophene conversion (%)

333 3.38 994 686 315 177 31 0

85 3.82 304 172 2125 319 35 10

0.48

0.10

0.04

0.07 0.13

0.05

0.29 0.13

30.46

27.85 36.03

47.44

48.57 33.09

a Reaction time ) 20 s, catalyst/feedstock ratio ) 5. b Including benzene, toluene, p-xylene, and ethylbenzene. c Mole ratio. d Weight of coke/weight of catalyst. e Weight of sulfur/weight of catalyst.

Figure 12. Conversions of n-octane and 1-octene versus hydrocarbon injected into the reactor. Reaction conditions: T ) 450 °C, catalyst/feedstock ratio ) 5, reaction time ) 20 s, feed composition ) 100 wt % hydrocarbon. Error bars correspond to standard deviations.

these experiments, the following conditions were kept constant: catalyst/feedstock ratio ) 5, temperature ) 450 °C, and time ) 20 s. The results of these experiments are reported in Figure 12. A lack of dependence of the hydrocarbon conversion on the initial hydrocarbon concentration is observed, with this finding suggesting a first-order reaction for both the cracking of n-octane and the cracking of 1-octene. 4.4. Catalytic Runs: Product Distribution. Table 1 reports hydrocarbon and sulfur product distributions for the catalytic runs performed using pure n-octane, pure 1-octene, and thiophene/ n-octane and thiophene/1-octene mixtures at 450 °C and 20 s. One can observe in Table 1 that the conversions of n-octane and 1-octene produce mainly light hydrocarbons (C2-C4), as well as aromatics, with these chemical species representing a typical product distribution for n-octane and 1-octene catalytic cracking over H-ZSM-5 zeolites.10

Concerning the catalytic conversion of thiophene in the presence of n-octane or 1-octene, the main sulfur products were H2S, alkylthiophenes, benzothiophene, and coke. The production of H2S confirms the contribution of thiophene conversion via reactions involving thiophene ring opening and hydrogen transfer. In this sense, given that n-octane is a better hydrogen donor than 1-octene, a higher H2S production level from thiophene conversion in thiophene/n-octane mixtures is considered an expected result. It was also noticed that there is a higher production of methylthiophene in the conversion of thiophene/n-octane mixtures. This result suggests that methylthiophene production from thiophene ring-opening and hydrogen-transfer reactions is a preferred pathway over alkylation reactions. The influences of thiophene alkylation reactions were also apparent given the following observations: (a) reduction of light olefins with progressive alkylthiophene formation, (b) observed H2S/(alkylthiophenes + benzothiophene) mole ratio lower than unity, and (c) production of methylbenzothiophene and formation of thiophenes with multiple alkylated groups such as 2-(1methylethyl)thiophene. Moreover, thiophene alkylation reactions are a favored reaction path over hydrogen transfer if thiophene is converted in the presence of 1-octene instead of n-octane. 1-Octene is a better source of light olefins than n-octane, which leads to the following results: (a) a lower H2S/(alkylthiophenes + benzothiophene) molar ratio, namely, 0.10 versus 0.48; (b) a higher level of thiophenes with multiple alkylated groups such as 2-(1methylethyl)thiophene; and (c) a similar sulfur content with a much higher coke yield, given the increased influence of thiophene-olefin oligomerization reactions.

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Figure 13. Thiophene reaction scheme: (a) thiophene ring opening with formation of adsorbed butenylthiophene intermediates, (b) thiophene protonation and alkylation.

On the other hand, as reported in a previous contribution,3 the production of aromatics consistently increases in the presence of thiophene in the two model compound mixtures (cf. Table 1). This result is attributed to the influence of thiophene conversion via bimolecular reactions involving two neighboring acid sites, followed by the quick alkylation of benzene. Conversion of thiophene to aromatic products was also reported by Li et al.11 They performed isotopic tracer studies of thiophene desulfurization reactions, measuring the chemical and isotopic composition of products formed from 13C-labeled C3H8 and unlabeled C4H4S mixtures on H-ZSM-5. They concluded that propane-thiophene reactions on H-ZSM-5 lead to the formation of aromatic products (benzene, toluene, and xylenes) containing carbon atoms from both propane and thiophene. 5. Reaction Mechanism of Thiophene over H-ZSM-5 Based on the experimental results of the present study, molecular modeling, and thermodynamic analysis, a detailed mechanistic interpretation of thiophene removal over H-ZSM-5 was proposed by Jaimes et al.3 This mechanism involves interactions between thiophene and surface acidic OH groups followed by (a) thiophene ring opening with formation of adsorbed sulfur butadienyl groups on the catalyst surface or (b) thiophene protonation with formation of ionic species, as shown in Figure 13. After thiophene adsorption and ring opening, the key adsorbed sulfur butadienyl intermediate can react with another thiophene molecule (Figure 13a.1), yielding hydrogen sulfide and an adsorbed butenylthiophene species. These adsorbed thiophene

fragments on H-ZSM-5 are highly unsaturated and unable to desorb as stable molecules without a separate source of hydrogen. In this sense, the adsorbed butenylthiophene intermediate can undergo further reactions such as (1) formation of ethylene and benzene via reaction of two adsorbed neighboring thiophene-derived species, with hydrogenation of thiophene fragments remaining after ring opening and H2S evolution (Figure 13a.2) and benzene produced in two neighbor Brønsted sites; (2) propylene and methylthiophene formation via hydrogenation and cracking of adsorbed butenylthiophene intermediary species (Figure 13a.3); or (3) formation of benzothiophene via cyclization of the adsorbed butenylthiophene intermediary species (Figure 13a.4), with benzothiophene produced at an isolated Brønsted site. On the other hand, not only thiophene ring opening but also thiophene alkylation reactions occur. Alkylation of thiophene over H-ZSM-5 can proceed via thiophene protonation on a Brønsted acid site and reaction of the carbenium ion formed with an olefin to produce an alkylthiophene (Figure 13b). Even more, long-chain alkylated thiophenes can either be the final product or react further (a) via protolytic cracking to give an olefin and a short alkylthiophene (Figure 13b.1) or (b) via cyclization of long-chain alkylthiophenes to produce bicyclic compounds that can then undergo dehydrogenation to benzothiophene or alkylbenzothiophenes (Figure 13b.2). Cyclization of long-chain alkylated thiophenes is thermodynamically favored over cracking.3 On the basis of the mechanisms described above, n-octane and 1-octene can be considered as hydrogen donors in thiophene


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(3) The thiophene adsorption takes place at equilibrium on one acid site, likely a Brønsted acid site (Figure 13a) KTh

Th + BS 798 ThBS

Figure 14. Proposed reaction network for thiophene.

ring-opening reactions, as well as co-reactants in thiophene alkylation reactions, with n-octane being a better hydrogen donor and 1-octene being a better source of light olefins. 6. Thiophene Kinetic Model 6.1. Model Development. The first step in thiophene kinetic modeling was to define the components that would be used in the model. For the purposes of the present study, the system was simplified by lumping the compounds into groups, as a function of their molecular characteristics (aromatics) or boiling points (sulfur product species that fall into the gasoline boilingpoint range). The lumps defined are as follows: H2S, SG (alkylthiophenes, benzothiophene, and methylbenzothiophene), aromatics (benzene and alkylbenzenes), and SC (sulfur on catalyst). This lumped kinetic scheme was used to simplify the calculations because of the large number of compounds involved, the majority of which are present in small concentrations (cf. Table 1). An interesting previous work by Valla et al.12 showed that the conversion of sulfur species in gas oils can be satisfactorily represented using three lumps. In the present study, the proposed network for thiophene cracking is expanded as shown in Figure 14. The proposed kinetic model also involves the hydrocarbon used to represent the gasoline (n-octane or 1-octene). The inclusion of this component is considered necessary to stand for the gasoline lost and to account for the fact that these hydrocarbons also convert to aromatics, one of the products resulting from thiophene conversion. On this basis, two lumps were used to define the conversions of n-octane and 1-octene: aromatics (benzene and alkylbenzenes) and nonaromatics (including the rest of the hydrocarbon products). The selected reaction network for thiophene and hydrocarbons, along with the thiophene reaction mechanism over H-ZSM-5 detailed in section 4, were used to propose a kinetic model that includes all reactants (thiophene, n-octane, and 1-octene) and products (H2S, SG, SC, and aromatics). The main modeling assumptions and reaction steps considered are as follows: (1) The selective thiophene adsorption over n-octane or 1-octene is based on the experimental observations (Figure 6). (2) A first-order reaction rate for both n-octane and 1-octene is considered. This assumption is adopted given the observed lack of dependence of the hydrocarbon conversion on the initial concentration (Figure 12). Thus, the proposed hydrocarbon rate expression is given by -rHC ) (kHC + kAr2)PHC

(2)

where kHC and kAr2 are the rate constants of nonaromatic and aromatic products, respectively, and PHC is the partial pressure of the reactant hydrocarbon (n-octane or 1-octene).

(3)

where KTh represents the adsorption equilibrium constant of thiophene and BS represents the Brønsted acid site. (4) The surface reactions are rate-limiting steps, as in a previous thermodynamic analysis.3 (5) The H2S formation reaction is the result of an adsorbed thiophene molecule reacting with a second gas-phase thiophene molecule (Figure 13a.1). This suggested mechanism leads to the reaction rate expression rH2S )

kH2SKThPTh2

(4)

1 + KThPTh

where kH2S is the H2S rate constant and PTh is the thiophene partial pressure. (6) The production of nonadsorbed aromatics is the outcome of the reaction of two close surface-adsorbed thiophene-derived species (Figure 13a.2) and the cracking of n-octane or 1-octane rAr )

kAr1KThPTh2 (1 + KThPTh)2

+ kAr2PHC

(5)

where kAr1 is the rate constant of aromatics produced from thiophene conversion. (7) The reaction of adsorbed thiophene with an olefin or another thiophene molecule in the gas phase (Figure 13a3,a4,b) leads to nonadsorbed sulfur species with boiling points falling into the gasoline range (defined as SG). An overall first-order SG reaction rate is suggested to describe this rate rSG )

kSGKThPTh 1 + KThPTh

(6)

where kSG is the SG rate constant. (8) The thiophene oligomerization reactions involving adsorbed thiophene and olefins are the precursor steps leading to coke formation, with a sulfur-contained-in-coke rate given by rSC )

kSCKThPTh 1 + KThPTh

(7)

where kSC is the SC rate constant. Taking into account all of the above-mentioned steps and assumptions, the thiophene rate expression can be summarized as -rTh )

kH2SKThPTh2 1 + KThPTh

+

kAr1KThPTh2 (1 + KThPTh)

2

+

(kSG + kSC)KThPTh 1 + KThPTh (8)

This thiophene rate expression is in agreement with the experimentally observed thiophene conversion rates, where the overall rate suggests a second-order reaction at low thiophene concentration and a first-order reaction at higher thiophene concentration (see section 4.3). Regarding the reaction rate, the CREC riser simulator equation, which is applicable to a bench-scale isothermal wellmixed batch reactor unit, can be used to express the change in the number of moles of species i ri )

VR dCi Wc dt

(9)

7512


Table 2. Values of Guesses for Kinetics Constants at 450 °Ca

a

pure n-C8

thiophene/n-C8

pure 1-C8d

thiophene/1-C8d

app kHC ) 2.32 × 10-5 R2 ) 0.9817

app kHC ) 2.07 × 10-5 R2 ) 0.9876 KTh ) 19.40 ki ) 2.07 × 10-6 R2 ) 0.5998

app kHC ) 3.96 × 10-5 R2 ) 0.9977

app kHC ) 4.37 × 10-5 R2 ) 0.9728 KTh ) 3.05 ki ) 2.39 × 10-5 R2 ) 0.6385

app and ki are in units of mol gcat-1 s-1 atm-1; KTh is in units of atm-1. ki ) kH2S ) kAr1 ) kSG ) kSC; kHC

where Ci is the molar concentration of species i, VR is the reactor volume, and Wc is the catalyst weight. Finally, under the conditions of the present study (reaction pressures lower than 4 atm), the ideal gas law was assumed to correlate the reactant molar concentration (Ci) with its partial pressure (Pi), as needed to solve the differential equation system (eqs 2 and 4-8). 6.2. Kinetic Parameter Estimation. The postulated rate expressions lead to mathematical models that are highly nonlinear with respect to their parameters, particularly those in which the adsorption constant (KTh) appears in both the numerator and the denominator of the expression. Therefore, parameter estimation was performed using the nonlinear leastsquares regression routine nlinfit.m, available in the optimization toolbox of MATLAB, version 5.6. This routine uses the Gauss-Newton algorithm with the Levenberg-Marquardt modifications for global convergence. The integration of the differential system (eqs 2 and 4-8) required in the parameter estimation was performed numerically using the function ode113, and the function nlparci was used to produce the 95% confidence interval for each estimated parameter. The reaction data used to estimate the kinetic parameters corresponding to the thiophene adsorption (KTh) and the ratelimiting surface reaction rates (ki) were obtained from catalytic runs with pure hydrocarbon (n-octane and 1-octene) and mixtures of thiophene/n-octane and thiophene/1-octene. The conditions of the catalytic experiments were set as follows: 2.5 and 5 wt % of thiophene in thiophene/hydrocarbon mixtures; 2 atm of argon; catalyst/feedstock ratio of 5; residences times of 5, 10, 15, and 20 s; and reaction temperatures of 350, 375, 400, 425, and 450 °C. As mentioned in the previous sections, the catalytic runs were repeated at least three times, and the mass balance closures were below 7%, which is well within the range of typical closures achieved in the CREC riser simulator.7 6.2.1. Kinetic Guess Values. The nlinfit function requires initial guesses for the unknown parameters. First-guess values for kinetic constants and the thiophene equilibrium constant were obtained by simplifications of eqs 2 and 8. In this sense, an apparent kinetics constant (kapp HC ), that includes the rate constants of both nonaromatic (kHC) and aromatic (kAr2) products, is defined for the hydrocarbon reaction rate app PHC -rHC ) kHC

(10)

After substituting eq 9 into eq 10 and integrating, eq 11 is obtained.

( )

ln

PHC0 W RT app c ) kHC t PHC VR

(11)

Linear regression of eq 11 on a semilogarithmic plot gives the app quantity kHC Wc/VR. Considering that Wc and VR are known, the app kHC parameter can be calculated. Table 2 reports the values of app obtained at 450 °C, along with their coefficients of kHC determination (R2). Regarding the thiophene rate, a second-order reaction and thiophene adsorption at one acid site is assumed. Also, an

Table 3. Kinetic Parameters at 450 °C for Thiophene/n-Octane and Thiophene/1-Octene Mixtures thiophene/n-C8 mixture parameter

value 2.64 × 4.28 × 3.91 × 11.12 3.16 × 1.88 × 1.93 ×

a H2S b SG b SC c Th a Ar1 a Ar2 a n-C8

k k k K k k k k1-C8da

thiophene/1-C8d mixture

error -6

10 10-7 10-6 10-8 10-6 10-5

(1.04 (1.32 (1.20 (6.41 (1.02 (2.68 (1.31

value -6

× 10 × 10-7 × 10-6 × 10-8 × 10-9 × 10-8

a In units of mol gcat-1 s-1 atm-1. units of atm-1.

b

4.05 × 4.52 × 4.12 × 10.82 2.34 × 8.39 ×

error -7

10 10-7 10-6 10-6 10-4

3.26 × 10-5

(2.43 (1.69 (1.37 (6.74 (1.56 (1.54

× 10-7 × 10-7 × 10-6 × 10-6 × 10-6

(1.42 × 10-8

In units of mol gcat-1 s-1.

c

In

Table 4. Guessed Values for Activation Energies and Heats of Adsorption thiophene/n-C8 mixture a


parameter

value

R2

value

R2

EH2S ESG ESC Th ∆Hads EAr1 En-C8 E1-C8 EAr2

124.4 138.1 103.7 -19.6 483.0 43.7

0.8982 0.6182 0.5272 0.9991 0.8128 0.8509

97.1 130.4 23.7 -20.8 141.8

0.7660 0.8267 0.9145 0.9957 0.7147

322.2

0.5618

27.4 201.6

0.9150 0.5969

a

units [kJ/mol].

app apparent thiophene kinetic constant (kTh ) is defined as the product of the thiophene adsorption equilibrium constant (KTh) and a global rate constant, where the latter is the sum of the individual rate constants (kH2S, kAr1, kSG, kSC). Equation 12 gives the simplified thiophene rate

-rTh )

2 kapp Th PTh 1 + KThPTh

(12)

Equation 13 is obtained after substituting eq 9 into eq 12 and reparametrizing to provide a linear model -

KTh PTh 1 1 ) app + app dPTh /dt k′Th PTh k′Th

(13)

with the primed apparent constant representing app k′app Th ) kTh

WcRT WcRT ) (kH2S + kAr1 + kSG + kSC)KTh VR VR (14)

Linear regression of eq 13 allows for the determination of the ′app and KTh. parameters kTh ′app Once kTh is calculated, one can assume that the individual rate constants kH2S, kAr1, kSG, and kSC are equal, allowing firstguess values for them to be obtained. The kinetic values obtained at 450 °C are reported in Table 2. The R2 values obtained from the linear regression of eq 11 are close to 1. This result indicates that the proposed hydro-

Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 Table 5. Intrinsic Kinetic Parameters of the Proposed Thiophene Kinetic Model, along with Their 95% Confidence Intervals thiophene/n-C8 mixture


parameter

value

span for 95% confidence

kH0 2Sa EH2Sb 0 c kSG ESGb 0 c kSC ESCb K0Thd Th b ∆H ads 0 a kAr1 EAr1b 0 a kn-C 8 En-C8b 0 da k1-C8 E1-C8b 0 a kAr2 EAr2b σe m

8.74 × 10-7 86.8 2.5 × 10-7 49.8 2.77 × 10-6 26.9 14.02 -19.3 9.6 × 10-9 305.2 1.41 × 10-5 28.2

(3.46 × 10-7 (7.2 (7.68 × 10-8 (6.5 (8.45 × 10-7 (6.2 (8.08 (12.1 (5.12 × 10-9 (82.9 (9.6 × 10-9 (5.6

6.73 × 10-7 95.1 2.44 × 10-5 264

(9.6 × 10-10 (4.7

a

-1

-1

-1

b

value 2.41 × 58.1 7.72 × 160.4 3.15 × 23.2 13.94 -21.0 1.74 × 193.3

10-7 10-8 10-6

10-7

2.22 × 10-5 31.0 5.25 × 10-6 35.5 1.26 × 10-3 192

span for 95% confidence (1.45 (32.1 (2.90 (11.3 (1.04 (10.7 (8.68 (14.0 (1.16 (31.5

× 10-7 × 10-8 × 10-6

× 10-7

(9.66 × 10-9 (0.1 (9.66 × 10-9 (4.7

calculated from this simplified model were used as first guesses for the numerical solution of the more complex thiophene kinetic model (eqs 4-8). 6.2.2. Kinetic Parameters as a Function of Temperature. Once a first set of kinetic parameters had been obtained, a new search for the kinetic parameters was initiated using the parameter guesses in section 6.2.1; eqs 2 and 4-8; and the data obtained at 350, 375, 400, 425, and 450 °C. The MATLAB calculation procedure is explained in section 6.2. Table 3 reports the kinetic parameters obtained at this second adjustment for mixtures of thiophene in n-octane and 1-octene at 450 °C, along with their 95% confidence limits. 6.3. Intrinsic Kinetic Parameter Estimation. To obtain the intrinsic kinetic parameters (activation energies and preexponential factors), the kinetic parameters ki and KTh (eqs 2 and 4-8) were allowed to vary with temperature using an Arrhenius relationship centered on an average temperature of 400 °C

[ (

ki ) k0i exp c

In units of mol gcat s atm . In units of kJ/mol. In units of mol gcat-1 s-1. d In units of atm-1. e σ ) [∑(Xexperimental - Xestimated)2]1/2/ (m - p), where m is the number of data points without considering repetitions and p is the number of model parameters (p ) 14).

Figure 15. Predicted and experimental thiophene yields from the catalytic conversion of thiophene/n-octane mixtures over ZSM-5 at various temperatures, residence times, and thiophene initial concentrations (2.5 and 5 wt %).

Figure 16. Predicted and experimental n-octane yields from catalytic conversion over ZSM-5 at various temperatures, residence times, and thiophene initial concentrations (0, 2.5, and 5 wt %).

carbon kinetic model (eq 10) and the estimated parameters, kapp HC, adequately represent the experimental data. On the other hand, the simplified kinetic model for thiophene (eq 12) does not correlate the experimental data as well, as is apparent from the 0.5998 and 0.6385 R2 values obtained for the fitting of eq 13. Despite this result, the kinetic parameters

7513

[

KTh ) K0Th exp -

Ei 1 1 R T Tavg

(

)]

Th ∆Hads 1 1 R T Tavg

(15)

)]

(16)

where ki is the reaction rate constant of component i, k0i is the pre-exponential factor or reaction rate constant at 400 °C, Ei is 0 is the thiophene adsorption equilibthe activation energy, KTh Th is the thiophene heat of rium constant at 400 °C, ∆Hads adsorption, R is the gas constant, and Tavg is the average temperature. Regarding eqs 15 and 16, one should notice that the Arrhenius centered form reduces the correlation between the preexponential factor and the activation energy, thereby improving the statistical properties of the estimates for the pre-exponential factor. Substitution of eqs 15 and 16 into the proposed rate expressions (eqs 2 and 4-8) provides a new differential equation system to be solved, with the intrinsic kinetic parameters 0 Th and ∆Hads , and corresponding to the thiophene adsorption, KTh 0 to the rate-limiting surface reaction rates, ki and Ei, as the parameters to be estimated. The initial values used for the kinetic parameters to solve the new differential equation system were those derived in section 6.2.2. In this sense, the kinetic parameters at 400 °C 0 and k0i ). The were used as pre-exponential guess values (KTh Th ) initial activation energy (Ei) and heat of adsorption (∆Hads values were obtained from linear regression of the Arrhenius expressions (eqs 15 and 16) on a semilogarithmic plot. Table 4 Th and their linear shows the results obtained for Ei and ∆Hads regression errors (R2). Finally, a total of 14 parameters were adjusted simultaneously by nonlinear multivariable regression of the experimental data using the MATLAB calculation procedure explained in section 6.2 and the above guess values. Table 5 summarizes the intrinsic kinetic parameters estimated, along with their 95% confidence intervals and the standard deviations of the residuals (σ), showing the quality of the fits. Inspecting the results of the parameter estimation, it can be noticed that the standard deviation (σ), calculated from the sum of the squares of the residuals, shows the quality of the fit. This result is particularly relevant given the large number of parameters adjusted, and it was reached because of the representative number of data points obtained (m). Moreover, the intrinsic kinetic parameters corresponding to the rate-limiting surface reaction rates, k0i and Ei, are significant

7514


Figure 17. Predicted and experimental sulfur products and aromatics yields from the catalytic conversion of thiophene/n-octane mixtures over ZSM-5 at various temperatures, residence times, and thiophene initial concentrations (0, 2.5, and 5 wt %).

Figure 18. Predicted and experimental thiophene yields from the catalytic conversion of thiophene/n-octane mixtures over ZSM-5 at various temperatures and residence times and a 5 wt % thiophene initial concentration.

Figure 19. Predicted and experimental 1-octene yields from catalytic conversion over ZSM-5 at various temperatures, residence times, and thiophene initial concentrations (0 and 5 wt %).

at the 95% confidence level, with this result showing that reparameterization and temperature centering were successful in reducing the overall correlation between the parameters. The 0 Th and ∆Hads , showed a thiophene adsorption parameters, KTh

higher degree of correlation, which leads to their large 95% confidence intervals, an expected result considering that the adsorption constant (KTh) appears both in the numerator and in the denominator of the rate expressions. This result is consistent with the observations of others.13 Regarding the energies of activation (Ei), it is important to emphasize that their magnitudes are in accord with very limited or a total lack of mass- and heat-transport controls. In fact, smaller energies of activation are normally indicators of potential mass- and heat-transport limitations. These results agree with the proposed kinetic model, where the surface reaction is the limiting step. Higher activation energy values were observed for the production of aromatics and H2S from thiophene conversion (EAR1 and EH2S, respectively). This result can be explained by the fact that the formation of these compound involves multistep reactions with significant numbers of transition states in which bond breaking is substantial, as detailed in Figure 13a. The signs assigned to the activation energies are consistent with the expected dependence of these constants on temperature. A positive value of Ei indicates a thiophene conversion intrinsic constant favored by higher temperatures, whereas negative ∆H Th ads shows an adsorption process that is negatively affected by increased temperature. Finally, the intrinsic kinetic parameters obtained in the present work are the first attempt to simulate thiophene catalytic conversion over a ZSM-5 catalyst under gasoline desulfurization conditions, so the actual results could not be compared with previous works. 6.4. Thiophene Kinetic Modeling Results. The experimental yields and the model-predicted values for thiophene/n-octane mixtures obtained using the intrinsic kinetic parameters reported in Table 5 are shown in Figures 15-17. It can be observed that the proposed kinetic model gives accurate predictions of the sulfur and hydrocarbon conversions, as the calculated thiophene and n-octane concentrations are comparable to the experimental data (Figures 15 and 16).


7515

Figure 20. Predicted and experimental sulfur products and aromatics yields from the catalytic conversion of thiophene/1-octene mixtures over ZSM-5 at various temperatures, residence times, and thiophene initial concentrations (0 and 5 wt %).

Additionally, Figure 17 reports a reasonable random distribution of sulfur product yields and aromatics with respect to the 45° perfect-agreement case, with this result indicating that all of the individual lumps are predicted satisfactorily. Similarly, the predicted and experimental lump yields from the conversion of thiophene/1-octene mixtures are compared in Figures 18-20. It can be observed that the proposed kinetic model with the parameters reported in Table 5 also adequately describes the experimental results for the various lumps. Furthermore, according to the kinetic modeling results, the proposed second-order reaction kinetics for thiophene cracking provides a good description of the system. 7. Conclusions The following are the most relevant conclusions of this research: (1) Catalytic thiophene dehydrosulfidation takes place without hydrogen addition. The conversions of all reactants (n-octane, 1-octene, and thiophene) progressively increase with reaction time and temperature. The thiophene adsorption/conversion selectivity is consistently greater than 1, indicating a distinctive and favorable adsorption affinity of thiophene versus both n-octane and 1-octene or, alternatively, a much higher intrinsic rate of thiophene dehydrosulfidation compared to the hydrocarbon cracking rate. (2) Thiophene conversion over H-ZSM-5 under the proposed gasoline desulfurization conditions leads mainly to coke, H2S, aromatics, alkylthiophenes, and benzothiophene. (3) The derived reaction mechanisms suggest that the adsorption and cracking of sulfur compounds on H-ZSM-5 zeolites are very important steps. On this basis, it is postulated that thiophene conversion occurs according to the selective adsorption of thiophene with the formation of carbenium ions as transition states followed by (a) thiophene ring opening and thiophene bimolecular reactions with or without hydrogen transfer or (b) alkylation reactions.

(4) It was also verified that gasoline components can play a key role as hydrogen donors in thiophene ring-opening reactions, as well as co-reactants in thiophene alkylation reactions. (5) A first kinetic model of thiophene conversion over ZSM-5 catalyst under gasoline desulfurization conditions is proposed that considers a four-lump thiophene reaction network. This reaction network accounts for all reactants (thiophene, n-octane, and 1-octene) and products (H2S, SG, SC, and aromatics). The associated reaction rate kinetic parameters were estimated using a two-level parameter selection. The resulting kinetic parameters and the proposed kinetic model were found to satisfactorily describe the catalytic conversion of thiophene under gasoline desulfurization conditions. Acknowledgment We acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada (NSERCCanada) for the development of this work. Notation ABD ) average bulk density BET ) Brunauer-Emmett-Teller C8d ) 1-octene Ci ) molar concentration of species i (mol/cm3) CREC ) Chemical Reaction Engineering Centre Ei ) activation energy of lump i (kJ/mol) EPMA ) electron probe microanalysis FCC ) fluid catalytic cracking FID ) flame ionization detector FPD ) flame photometric detector GC ) gas chromatography KTh ) thiophene adsorption equilibrium constant (atm-1) kH2S ) H2S rate constant (mol gcat-1 s-1 atm-1) kHC ) rate constant of nonaromatic products from n-octane or 1-octene conversion (mol gcat-1 s-1 atm-1) kn-C8 ) rate constant of n-octane (mol gcat-1 s-1 atm-1) k1-C8d ) rate constant of 1-octene (mol gcat-1 s-1 atm-1)

7516


kAr2 ) rate constant of aromatics from HC cracking (mol gcat-1 s-1 atm-1) kAr1 ) rate constant of aromatics produced from thiophene conversion (mol gcat-1 s-1 atm-1) kSG ) SG rate constant (mol gcat-1 s-1) kSC ) SC rate constant (mol gcat-1 s-1) ki0 ) pre-exponential factor or reaction rate constant at 673 K 0 ) thiophene adsorption equilibrium constant at 673 K (atm-1) KTh MSD ) mass-selective detector n-C8 ) n-octane PHC ) n-octane or 1-octene partial pressure (atm) PTh ) thiophene partial pressure (atm) PSD ) particle size distribution R ) gas constant (82.05 atm cm3 mol-1 K-1) SC ) sulfur-on-catalyst lump SG ) alkylthiophenes, benzothiophene, and methylbenzothiophene lump SSM ) solid sample module Sth ) thiophene adsorption/conversion selectivity T ) temperature (K) Tavg ) average temperature (673 K) Th ) thiophene TOC ) total organic carbon analyzer TPD ) temperature-programmed desorption VR ) reactor volume (59.47 cm3) Wc ) catalyst weight (0.8 g) WTh ) mass fraction of thiophene WHC ) mass fraction of n-octane or 1-octene XRD ) X-ray diffraction XTh ) conversion of thiophene (%) XHC ) conversion of n-octane or 1-octene (%) Th ) thiophene heat of adsorption (kJ/mol) ∆Hads σ ) standard deviation

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ReceiVed for reView December 30, 2008 ReVised manuscript receiVed March 16, 2009 Accepted March 20, 2009 IE8020126

Catalytic Conversion of Thiophene under Mild Conditions over a ZSM

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