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Effect of Reaction Pressure and Carrier Gas on Toluene Disproportionation over Molybdenum-ZSM-5 Catalyst S. Al-Khattaf,*,† M. A. Ali,† and A. Al-Amer‡ Research Institute and Chemical Engineering Department, King Fahd UniVersity of Petroleum and Minerals, Dhahran 31261, Saudi Arabia ReceiVed September 29, 2007. ReVised Manuscript ReceiVed NoVember 8, 2007
This study was aimed at evaluating the effects of reaction pressure and temperature on toluene disproportionation using a ZSM-5-based catalyst impregnated with molybdenum. The results of the study showed that both toluene conversion and xylene selectivity were strongly dependent on temperature, pressure, and the kind of carrier gas used. Toluene conversion compared at the same temperature and pressure was found to be higher when nitrogen instead of hydrogen was used as the carrier gas. For both carrier gases, it was found that toluene conversion increased with both pressure and temperature. Furthermore, it was observed that catalyst deactivation was more rapid and more severe with nitrogen as the carrier gas than with hydrogen.
1. Introduction Toluene disproportionation (TDP) is a very useful reaction for converting surplus toluene into the more valuable xylene, which is a key raw material in the petrochemical industry for the production of a wide range of materials such as polyester.1 Over the past few decades numerous studies have been conducted to develop more efficient catalysts as well as to determine optimized reaction conditions for this very important reaction. The main reaction products of toluene disproportionation are benzene and mixed xylene comprised of the three isomers, namely p-xylene, o-xylene, and m-xylene. Both oxylene and m-xylene are usually processed further by isomerization reaction to produce the more valuable p-xylene, which is mainly oxidized to form terephthalic acid used for the production of the world’s most important polymer, poly(ethylene teraphthalate) (PET). Because of the growth in demand for p-xylene, new technologies are needed to convert low-valued aromatics into mixed xylenes and specifically into p-xylene. Many researchers have investigated different types of catalysts based on acidic zeolites such as Y, ZSM-5, mordenite, and MCM-22 and have reported values of toluene conversion and xylene selectivity.2–14 The effects of crystal size and siliconto-aluminum ratio of ZSM-5 have also been studied using the * Corresponding author: Fax 966-3-8604509, e-mail skhattaf@ kfupm.edu.sa. † Research Institute. ‡ Chemical Engineering Department. (1) Tsai, T.-C.; Liu, S. B.; Wang, I. Appl. Catal. 1999, 181, 355–398. (2) Uguina, M. A.; Sotelo, J. L.; Serrano, D. P. Appl. Catal. 1991, 76, 183–198. (3) Chaudhari, P. K.; Saini, P. K.; Chand, S. J. Sci. Ind. Res. 2002, 61, 810–816. (4) Kareem, A.; Chand, S.; Mishra, I. M. J. Sci. Ind. Res. 2001, 60, 319–327. (5) Bhavani, A.; Karthekayen, D.; Rao, A.; Lingappan, N. Catal. Lett. 2005, 103, 89–100. (6) Tsai, T.-C.; Chen, W. H.; Liu, S.-B.; Tsai, C.-H.; Wang, I. Catal. Today 2002, 73, 39–47. (7) Qingling, C.; Xin, C.; Liansheng, M.; Wencai, C. Catal. Today 1999, 51, 141–150. (8) Li, Y.-G.; Jun, H. Appl. Catal. 1996, 142, 123–137. (9) Fujimoto, K.; Yang, M.-G.; Nakamura, I. Appl. Catal. 1995, 127, 115–124.
activation method and pelletization of zeolite.2 Catalytic activity was enhanced using lower silicon-to-aluminum ratio zeolite. IR spectroscopic measurements using pyridine revealed that during the activation step a high H+ concentration in the cation exchange increased the strength of the Bronsted sites, thus increasing the number of acid sites active in toluene disproportionation. Chaudhari and co-workers3,4 have investigated TDP reaction using ZSM-5 as well as Y-zeolite catalysts ionexchanged with active metals: Ni, Pb, and Cr. The reactions were carried out under atmospheric pressure, and the effects of the metal type loaded, silica–alumina ratio of the zeolite, type of zeolite, and particle size were studied. The percentage yield of pxylene was found to increase with the ion-exchanged zeolites. A yield of 2.3% p-xylene was obtained with Cr-loaded HZSM-5 at 600 °C and 1 atm. Mavrodinova et al.11–14 enhanced p-xylene selectivity during TDP over several zeolite types by replacing the zeolite proton by InO+ cations. Tsai15 reported a novel method of TDP using hydrogen to reactivate the active sites on coked mordenite. Recently, several researchers have focused their work on the effect of catalyst acidity on toluene conversion and p-xylene selectivity.16–19 Al-Khattaf20 studied TDP in a fluidized bed using Y-zeolite and reported that toluene conversion was very low when the concentration of acid sites of the zeolite is low. In the case of Y-zeolite having high concentration (10) Uguina, M. A.; Sotelo, J. L.; Serrano, D. P. Ind. Eng. Chem. Res. 1993, 32, 49–55. (11) Mavrodinova, V. P.; Papova, M. D.; Neinska, Y. G.; Minchev, C. I. Appl. Catal. 2001, 210, 397–408. (12) Mavrodinova, V. P.; Papova, M. D.; Mihalyi, R. M.; Pal-Borbely, G.; Minchev, C. I. Appl. Catal. 2003, 248, 197–209. (13) Mavrodinova, V. P.; Papova, M. D.; Mihalyi, R. M.; Pal-Borbely, G.; Minchev, C. I. Appl. Catal. 2004, 262, 78–83. (14) Mavrodinova, V. P.; Papova, M. D. Catal. Commun. 2005, 6, 247– 252. (15) Tsai, T.-C. Appl. Catal. 2006, 301, 292–298. (16) Zhu, Z.; Xie, Z.; Chen, Q.; Kong, D.; Ki, W.; Yang, W.; Li, C. Microporous Mesoporous Mater. 2007, 101, 169–175. (17) Wang, K.; Wang, X.; Li, G. Catal. Commun. 2007, 8, 324–328. (18) Liu, L.; Cheng, M.; Ma, D.; Hu, G.; Pan, X.; Bao, X. Microporous Mesoporous Mater. 2006, 94, 304–312. (19) Wang, K.; Wang, X.; Li, G. Microporous Mesoporous Mater. 2006, 94, 325–329. (20) Al-Khattaf, S. Energy Fuels 2006, 20, 946–954.
10.1021/ef700582b CCC: $40.75 2008 American Chemical Society Published on Web 12/06/2007
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of acid sites, toluene conversion occurred mainly through dealkylation and pairing reactions producing more gases. It is well documented that zeolite-based catalysts can be used to increase p-xylene selectivity via toluene disproportionation.21 Olson and Haag22 have investigated the role of external active sites and diffusional path length on p-xylene selectivity by studying the effect of zeolite crystal. Cejka and Wichterlova23 have published a detailed review of the production of monoand dialkylbenzenes using zeolites explaining the effects of acidity and structure of zeolite and the reaction conditions on the activity and selectivity. This paper presents a systematic study of toluene disproportionation using a ZSM-5-based catalyst modified with molybdenum metal using the impregnation method. Reactions were carried out in the presence of nitrogen and hydrogen carrier gases and at different pressures to observe the effect of these variables. The main purpose of loading molybdenum on the zeolite was to observe its effect on toluene conversion, xylene selectivity, and coke formation in the presence of hydrogen and nitrogen as carrier gases. Most commercial processes utilize 30 bar of hydrogen pressure in the presence of zeolite-based catalysts.6 2. Experimental Section 2.1. Catalyst Preparation. The catalyst sample used in this study is based on a ZSM-5 zeolite loaded with molybdenum metal through the impregnation method. The ZSM-5 zeolite used was acquired from CATAL International, UK, in the H-form and has a SiO2/Al2O3 ratio of 30 and a surface area of 400 m2/g. The alumina binder used in preparing the extrudates was Cataloid AP-3 obtained from CCIC, Japan. Cataloid AP-3 contains 75.4 wt % alumina, 3.4 wt % acetic acid, and water as a balance. The procedure used for preparation the catalyst is described below. First, the alumina binder was dispersed in water and stirred for 30 min to produce a thick slurry. The ZSM-5 zeolite was then mixed with the alumina slurry to produce a thick paste. Following this, the paste was subjected to an extruder to produce extrudates with thickness of 1 mm which were initially dried at room temperature for 4 h and then at 120 °C for 2 h. After the drying, the extrudates were calcined at 500 °C for 2 h in a temperature-programmed furnace. An aqueous solution containing hexaammonium heptamolybdate tetrahydrate was then used to impregnate the extrudates with 3% molybdenum. The metal-loaded extrudates were then dried again at room temperature and in an oven at 120 °C and finally calcined at 500 °C. The prepared catalyst was designated as Mo/Z. In order to investigate the effect of the alumina binder on toluene conversion and product composition, extrudates of 1.5 mm size were prepared using the alumina binder (AP-3) in water. These extrudates were dried and calcined at conditions similar to those used for the Mo/Z catalyst. 2.2. Catalyst Characterization. The Mo/Z catalyst was characterized for surface area, porosity characteristics, and acidity using the Quantachrome Autosorb-1C chemisorption-physisorption analyzer equipped with thermal conductivity detector. The BET surface area, pore volume, and pore size distribution were obtained from nitrogen adsorption–desorption isotherms measured at 77 K. The Autosorb-1C system is equipped with a high-sensitivity, thermal conductivity detector, and a precise vacuum volumetric method was used for the analysis of BET surface area and mesopore and micropore size distributions. The sample was outgassed and evacuated for 2 h at a temperature of 100 °C and then kept under the flow of nitrogen at 350 °C for 1 h to allow for the adsorption of nitrogen gas. When the adsorption reached equilibrium, the process was completed by desorbing the adsorbed nitrogen. At the (21) Chen, N. Y.; Kaeding, W. W.; Dwyer, F. G. J. Am. Chem. Soc., 1979, 101, 6783–6784. (22) Olson, D. H.; Haag, W. O. ACS Symp. Ser. 1984, 248, 275–307. (23) Cejka, J.; Wichterlova, B. Catal. ReV. 2002, 44, 375–421.
Al-Khattaf et al. end of the run, the system performed the necessary calculations and correlations, and the complete results were produced. The measurement of the acidity of the Mo/Z catalyst was carried out by the temperature-programmed adsorption and desorption of ammonia in the temperature range 100-800 °C. The equipment employed was the same Autosorb-1C used for surface area measurement. 0.19 g of the catalyst sample was heated from room temperature to 500 °C at the rate of 20 °C/min and evacuated for 2 h under the flow of helium. The sample was then cooled to 100 °C still under the flow of helium. At this temperature, the flow was switched to ammonia, allowing for the adsorption of ammonia for 10 min with the temperature held constant. Afterward, the flow of ammonia was then stopped, and the sample was purged at 100 °C under helium to remove physisorbed ammonia. The removal of the physisorbed ammonia was monitored by the TCD response. It was made sure that all the physisorbed ammonia at 100 °C was removed. Once the TCD at 100 °C shows no more signal, indicating that all the physisorbed ammonia has been removed, the actual measurement of the acidity started by temperature-programmed desorption. The desorption of ammonia was carried out under helium, and the sample was heated up to 800 °C at the rate of 10 °C/min. The response of the TCD was recorded as a function of desorption temperature, and the acidity was calculated. An ammonia TPD chromatogram for the Mo/Z catalyst showing the weak and strong acidities is shown in Figure 1. Coke deposited on the spent catalysts was determined by the CHNS elemental analyzer Model Vario EL from Elementar, Germany. The method is based on combustion of the coke to produce carbon dioxide, which is then quantified. The surface area, porosity, and ammonia-based acidity characteristics of the catalyst are given in Table 1. 2.3. Catalyst Evaluation. The Mo/Z catalyst was evaluated in a fixed-bed reactor system at 1.5 LHSV, using nitrogen and hydrogen as carrier gases with pure toluene as the feed. To determine the effects of carrier gas, reaction pressure, and temperature on toluene conversion and xylene selectivity, two sets of experimental runs were made. The first was carried out using nitrogen and then hydrogen as the carrier gas at a fixed temperature of 400 °C under pressures of 1, 5, 10, 30, and 50 bar. The second was carried out also using both nitrogen and hydrogen as carrier gases at a fixed pressure of 30 bar at temperatures of 375, 400, and 425 °C. Furthermore, a run at a pressure of 30 bar and temperature of 400 °C was made to determine the effect of carrier gas on toluene conversion and catalyst deactivation with time on stream. Seperate runs were also made using the alumina binder to investigate its effect on toluene conversion and product distribution. As regards the experimental procedure, 15 mL of the catalyst was charged into the middle portion of the reactor with total volume of 50 mL provided with inert in the upper and lower portions of the reactor. The catalyst was then calcined and activated in situ in the presence of nitrogen prior to the reaction. At 300 °C, toluene was pumped into the reactor at a predetermined rate, and the temperature was raised to the desired reaction temperature. The reaction product was separated in a gas–liquid separator. Both gas and liquid samples were collected after about 6 h on stream, the time at which the reaction conditions were stabilized and the products collected were representative. Each batch of catalyst was used for a particular temperature and pressure and then replaced with a fresh one at the end of the run. These runs were carried out to observe the effects of pressure, type of carrier gas, reaction temperature, and time on stream on the activity and selectivity of the Mo/Z catalyst. Extrudates of alumina binder (AP-3) were also tested in the reactor at 400 °C, 1.5 LHSV, and a pressure of 30 bar in the presence of hydrogen and nitrogen gases to observe the effect of the alumina binder on the toluene conversion. The liquid reaction products were analyzed using a PIONA analyzer based on a Shimadzu gas chromatograph GC-2010 and a Shimadzu autoinjector AOC-20i. The PIONA analyzer was equipped with FID detector and an autosampler. The hydrocarbon separation was carried out on 50 m long and 0.15 mm diameter BP1PONAM50-050 column under temperature-programmed conditions. The
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Figure 1. Ammonia TPD chromatogram of the Mo/Z catalyst showing the weak and strong acidities. Table 1. Surface Area, Porosity, and Acidity of Mo/Z Catalyst no.
property
value
1 2 3 4 5 6 7 8 9
surface area, m2/g external surface area, m2/g micropore surface area, m2/g total pore volume, cm3/g micropore volume, cm3/g average pore diameter, nm total ammonia acidity, mmol/g % weak acidity (100–380 °C) % strong acidity (380–800 °C)
333.2 110.4 223.3 0.391 0.094 4.69 0.625 80.6 19.4
components were separated according to their boiling points. The components were identified using a calibration that was accomplished using a standard hydrocarbon mixture sample having the components of known composition. The composition of the gaseous product was analyzed by a Shimadzu gas chromatograph model 14B equipped with a flame ionization detector and thermal conductivity detector. The hydrocarbons C1 to C5, nitrogen, and hydrogen were separated on alumina PLOT, 50 m capillary column.
3. Results and Discussion 3.1. Toluene Conversion. Figure 2a shows the plot of toluene conversion at 400 °C as a function of pressure using nitrogen and hydrogen as carrier gases. From the plot, it can be observed that toluene conversion increased with pressure for the two cases up to 30 bar, beyond which it remains fairly constant up to 50 bar. It is also clear that within the pressure range investigated (1–50 bar) toluene conversion was always higher when nitrogen was used as the carrier gas instead of hydrogen. The initial increase in toluene conversion between 1 and 30 bar can be attributed to the fact that within this pressure range increasing the pressure enhanced the adsorption of toluene on the active sites, leading to increased toluene conversion. Further increase in pressure beyond 30 bar only resulted in a slight increase in conversion probably because the adsorption of toluene had already reached saturation with the occupation of most of the active sites. Figure 2b shows the plot of toluene conversion at a pressure of 30 bar as a function of reaction temperature using nitrogen and hydrogen as carrier gases. As expected for equilibriumlimited endothermic reactions, toluene conversion increased with temperature in both cases. Within the temperature range of
Figure 2. Conversion of toluene using hydrogen (9) and nitrogen (2) carrier gases as a function of (a) reaction pressure at 400 °C and (b) reaction temperature at 30 bar.
investigation (375–425 °C), it can be seen that toluene conversion was higher with nitrogen as the carrier gas than with hydrogen. However, the difference in toluene conversion for the two cases decreased as temperature was increased, approaching zero at 425 °C. This may be due to the fact that at high temperatures in the region of 425 °C coke formation and hence catalyst deactivation are more severe in the presence of nitrogen than hydrogen. 3.2. Xylenes Selectivity. Figure 3 depicts the plot of xylene selectivity at a temperature of 400 °C as a function of pressure in the range of 1–50 bar using nitrogen and hydrogen as the carrier gas. In both cases, the plot shows that xylene selectivity decreased as pressure was increased. Initially (between 1 and 8 bar), the selectivity of xylene when nitrogen was used as the carrier gas was higher than when hydrogen was used. However, at pressures above 8 bar, the xylene selectivity with nitrogen as the carrier gas fell drastically and goes below that when hydrogen was used as the carrier gas. In the case where
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Figure 4. Xylenes selectivity using hydrogen (9) and nitrogen (2) carrier gases as a function of reaction temperature at 30 bar pressure. Figure 3. Xylenes selectivity using hydrogen (9) and nitrogen (2) carrier gases as a function of reaction pressure at 400 °C.
hydrogen was used as the carrier gas, xylene selectivity showed only a slight decrease between 1 and 30 bar, after which it remained approximately constant up to 50 bar. Individual m-xylene, p-xylene, and o-xylene showed a similar trend. In the presence of both nitrogen and hydrogen as carrier gas, secondary and tertiary reactions occurred in which some of the xylene formed reacted further with toluene to produce trimethybenzenes and benzene. This resulted in an increase of benzene concentration and a decrease in the concentration of toluene. Furthermore, the trimenthylbenzene formed reacted with toluene to produce higher aromatics that serve as coke precursors. The latter reaction takes place on the external active sites of the zeolite. Generally, the amount of coke deposit when nitrogen was used as the carrier gas was higher than when hydrogen was used. This observation is supported by the fact that when nitrogen was used as the carrier gas, no trimethylbenzene was detected in the reaction product owing to its conversion to coke precursors. At a pressure of 50 bar, the amount of coke deposited on the catalyst resulted in low values of toluene conversion. On the other hand, when hydrogen was used as the carrier gas, the reaction products were stabilized after the primary reaction, thereby reducing the extent of the secondary and tertiary reactions. In this case, some amount of trimethylbenzene was detected in the reaction product while some went into coke formation. Figure 4 shows the plots of xylene selectivity at a pressure of 30 bar as a function of temperature. It can be seen from the plots that when hydrogen was used as the carrier gas, xylene selectivity decreased as temperature was increased from 375 to 425 °C. This is due to the fact that with hydrogen as the carrier gas, toluene conversion was relatively low, leading to higher xylene selectivity. Although increasing the temperature from 375 to 425 °C resulted in increased toluene conversion, other secondary reactions also become more severe with temperature,
resulting in an appreciable decrease in the selectivity of xylene. Similarly, when nitrogen was used as the carrier gas, xylene selectivity was found to be affected by toluene conversion. In this case, xylene selectivity was found to decrease with temperature up to 400 °C, beyond which it was approximately constant up to 425 °C. In addition, it was observed that the difference in xylene selectivity when nitrogen and hydrogen were used as carrier gases decreased as temperature was increased from 375 to 425 °C. From the plots in Figure 4, it can be seen that this difference was approximately zero at 425 °C. This observation can be attributed to the increased severity of other secondary reactions, leading to the formation of benzene and C9+ hydrocarbons. However, thermodynamic equilibrium was found to be maintained between the xylene isomers for all reaction temperatures and pressures. Light hydrocarbons such as methane, ethane, and propane were not detected in the analysis of the gaseous products of the reaction. Only the carrier gases—hydrogen or nitrogen—were detected. This suggests that dealkylation did not occur and that toluene was selectively converted to xylene and benzene. 3.3. Benzene Selectivity. The selectivity of benzene as a function of reaction pressure at 400 °C is plotted for both nitrogen and hydrogen as carrier gases, as shown in Figure 5a. When nitrogen was used as the carrier gas, it was observed that benzene selectivity increased over the investigated pressure range, having a value of approximately 55 and 68 mol % at 1 and 50 bar, respectively. This shows that with increased pressure a higher amount of benzene was produced as a result of secondary and tertiary reactions, leading to the formation of coke. On the other hand, when hydrogen was used as the carrier gas, benzene selectivity initially decreased as pressure was increased from 1 bar to about 10 bar and then remained fairly constant up to 50 bar. This observation points to the fact that the presence of hydrogen alongside the molybdenum prevents undesirable secondary and tertiary reactions that consume xylene to produce higher or lower aromatics.
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Figure 5. Selectivity of benzene using hydrogen (9) and nitrogen (2) carrier gases as a function of (a) reaction pressure at 400 °C and (b) reaction temperature at 30 bar.
Figure 6. C9+ aromatics produced using hydrogen (9) and nitrogen (2) carrier gases as a function of (a) reaction pressure at 400 °C and (b) reaction temperature at 30 bar.
Furthermore, the selectivity of benzene at a reaction pressure of 30 bar was plotted as a function of reaction temperature with nitrogen and hydrogen as the carrier gas, as shown in Figure 5b. It is evident from the plot that in both cases benzene selectivity increased as temperature was raised from 375 to 400 °C. However, it was observed that the rate at which benzene selectivity increased with temperature was higher when nitrogen was used instead of hydrogen as the carrier gas. This is indicative of a more rapid and a more severe catalyst deactivation under the nitrogen environment as compared to hydrogen, especially at higher temperatures. 3.4. C9+ Aromatics Produced. Figure 6a shows a plot of the amount of C9+ aromatics produced at 400 °C as a function of pressure for the two cases where nitrogen and hydrogen were used as carrier gases. In the case in which nitrogen was used as the carrier gas, it can be observed from the plot that the amount of C9+ aromatics produced increased with pressure over the range of 1 bar to about 10 bar and then remained constant up to 50 bar. On the other hand, when hydrogen was used as the carrier gas, the amount of C9+ aromatics produced increased
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with pressure up to about 30 bar, after which it remained almost constant up to 50 bar. Over the pressure range investigated it was found that more C9+ aromatics were produced using hydrogen rather than nitrogen as the carrier gas and, of the C9+ aromatics produced, 1,2,4-trimethylbenzene was found to have the highest concentration. The other C9+ compounds detected in the product included 1,3,5-trimethylbenzene, 1,2,3-trimethylbenzene, 1-methyl-3-ethylbenzene, 1-methyl-4-ethylbenzene, and 1-methyl-2-ethylbenzene. The increase in trimethylbenzene with pressure probably has to do with adsorption. It is well-known that the adsorption of hydrocarbons increases with pressure. Hence, as pressure was increased, the rate of adsorption of xylene molecules on the external sites of the zeolite increased, leading to an increase in the rate of transalkylation reaction between toluene from the feedstock and the adsorbed xylene. The latter reaction produces trimethylbenzene. Figure 6b shows a plot of the amount of C9+ aromatics produced at a reaction pressure of 30 bar as a function of reaction temperature for both nitrogen and hydrogen as carrier gas. From the plot, it can be seen that in both cases the amount of C9+ aromatics produced increased as temperature was increased from 375 to 425 °C. The plot also shows that more C9+ aromatics were produced when hydrogen was used as the carrier gas instead of nitrogen. This is because in the presence of reactive hydrogen further reactions leading to coke formation were reduced as compared to the reactions in the presence of inert nitrogen. 3.5. Effect of Carrier Gas Type and Pressure. The conversion of toluene under nitrogen carrier gas was found to be higher than hydrogen as the carrier gas. A similar observation has been reported in the literature,24 and the results of toluene disproportionation under nitrogen were found to be in agreement with our study. The dependence of toluene conversion on the type of carrier gas can be explained on the basis of the atomic and molecular sizes of the carrier gas and its behavior during the reaction process. Nitrogen is a bigger molecule compared to hydrogen and it is relatively inert, whereas hydrogen is the smallest molecule and it participates in the reaction network responsible for the production of BTX and higher aromatics. In the presence of nitrogen, toluene adsorption, diffusion, and reaction mainly occur on the active sites of the catalyst, and this leads to higher toluene conversion. Although some nitrogen molecules may be physically adsorbed on the active sites, however due to its inertness, nitrogen does not participate in the reaction stage. Unlike nitrogen, when hydrogen is used, it is adsorbed along with toluene and takes part in the reaction. It should be noted that, because of its smaller size, the diffusion of hydrogen is faster than that of toluene. Thus, there are competing reactions where hydrogen occupies some of the active sites and dissociates into atomic hydrogen which participates in the reactions. This competing adsorption, diffusion, and activation of hydrogen molecule on the active sites may lower toluene conversion. This phenomenon can also be explained on the basis of Bronsted and Lewis acidity. The presence of hydrogen may lead to a decrease in Bronsted acidity and thus decrease in the concentration of intermediates benzylic carbocations, leading to lower toluene conversion.25,26 The conversion of toluene was found to increase with pressure in the presence of nitrogen and hydrogen carrier gases within the range of 1-30 bar. From 30 to 50 bar, toluene conversion (24) Schult-Ekloff, G.; Jaeger, N. I. Appl. Catal. 1987, 33, 73–81. (25) Gnep, N. S.; Guisnet, M. Appl. Catal 1981, 1, 329–340. (26) Guisnet, M. J. Catal. 1984, 88, 249–259.
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Figure 7. Toluene conversion at 400 °C, 30 bar, and 1.5 LHSV as a function of time on stream in the presence of hydrogen (9) and nitrogen (2) carrier gases.
remained almost insensitive to pressure in both cases. This shows the limits of adsorption, diffusion, and reactivity as a function of reaction pressure. Singer et al.27 have reported a very recent study on the effect of pressure on the alkylation of toluene with ethane. They had shown that toluene conversion during the alkylation reaction increased from 17 to 28% at 350 °C by increasing the reaction pressure from 30 to 50 bar. This is probably due to the fact that catalyst deactivation is reduced at higher pressures because of the stronger adsorption and reaction of ethane molecules on the catalysts at these high pressures. This can also be due to the increase in adsorption, diffusion, and reactivity of ethane molecules on the catalyst at higher pressures because of its relatively smaller size compared to the bigger molecules such as toluene. 3.6. Effect of Alumina Binder. The results of the catalytic reactions carried out at 400 °C, 1.5 LHSV, and pressure of 30 bar in the presence of hydrogen and nitrogen gases using extrudates of alumina binder showed very little toluene conversion. Similar conversions and product distributions were observed with both nitrogen and hydrogen as carrier gases. This shows that the alumina binder plays no significant role in the production of the aromatics as well as the production of gaseous hydrocarbons. 3.7. Time on Stream Data Showing Deactivation. Additional catalytic reaction was carried out at 400 C, 1.5 LHSV, and a pressure of 30 bar with both nitrogen and hydrogen as carrier gases. Product samples were collected at 06, 16, 22, 30, and 40 h on stream. The results were used to produce deactivation curves of the Mo/Z catalyst under both hydrogen and nitrogen carrier gases, as shown in Figure 7. From this figure, it can be observed that with nitrogen as the carrier gas toluene conversion decreased faster and reached less than half of its initial value within 40 h, while toluene conversion in the presence of hydrogen decreased only slightly during the same time on stream. Consequently, the amount of benzene and xylenes also decreased drastically using nitrogen as compared to hydrogen as the carrier gas. 3.8. Coke on Spent Catalysts. The amount of coke deposited on the spent catalysts obtained from catalytic runs made under nitrogen and hydrogen carrier gases is plotted in Figure 8. Coke is basically formed as a result of secondary and further reactions of toluene with xylenes and the higher aromatics produced, and the severity of these reactions is determined by the type of carrier gas. The data shows that the spent catalysts obtained under hydrogen carrier gas at pressures in the range of 1-50 bar possess lower coke contents as compared to the spent catalysts obtained under nitrogen carrier gas. This could be because of contribution from several factors. One of these could be due to the fact that hydrogen takes part in these reactions occupying (27) Singer, D.; Rezai, S. A. S.; Sealy, S.; Traa, Y. Ind. Eng. Chem. Res. 2007, 46, 395–399.
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Figure 8. Effect of reaction pressure on coke deposited on the spent catalysts at 400 °C in the presence of hydrogen (9) and nitrogen (2) carrier gases.
some active sites, reducing the severity of the secondary, tertiary, and further reactions which produces multialkyl-substituted benzene and polyaromatics which are major coke precusors. On the other hand, nitrogen being an inert gas does not take part in the reaction, and thus a larger amount of active sites are available for secondary and tertiary reactions that eventually lead to higher coke formation. Another reason may be that in the presence of reactive hydrogen the coke precursors are desorbed easily from the catalyst surface as compared to inert nitrogen carrier gas. The amount of coke on the spent catalyst under nitrogen carrier gas decreased from 30 to 50 bar pressure. This effect may be due to extraction and desorption of coke precursors at high pressure.27 4. Conclusions The following conclusions can be drawn from this study. 1. Under similar reaction conditions, toluene conversion was found to be higher when nitrogen instead of hydrogen was used as the carrier gas. In both cases, toluene conversion measured at 400 °C was found to increase with pressure over 1-30 bar, after which it remained practically constant up to 50 bar. 2. Toluene conversion measured at 30 bar and temperature of 375 °C was lower when hydrogen rather than nitrogen was used as the carrier gas. However, this difference reduced significantly as temperature was increased approaching a value of zero at 425 °C. 3. Xylene selectivity was found to be higher and more stable when hydrogen instead of nitrogen was used as the carrier gas. Measurements at 400 °C showed that xylene selectivity decreased with pressure in both cases. However, the decrease was found to be more severe when nitrogen was used as the carrier gas. 4. At a reaction pressure of 30 bar, xylene selectivity was found to deacrease with reaction temperature when both nitrogen and hydrogen were used as the carrier gas. The decrease was sudden and more evident when hydrogen was used as the carrier gas. At 425 °C, the xylene selectivity was approximately the same for both cases. 5. Under similar reaction conditions, benzene selectivity was found to be higher when nitrogen instead of hydrogen was used as the carrier gas. Selectivity measurements at 400 °C showed that benzene selectivity increased with reaction pressure when nitrogen carrier gas was used but remained almost unchanged when hydrogen carrier gas was used. 6. With nitrogen as the carrier gas, the rate at which xylene is consumed through transalkylation of toluene with xylene competes with the rate at which xylene is formed through the disproportionation of toluene. This is due to the unavailability of reaction hydrogen. The effect of these competing
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reactions is the production of higher amount of benzene, lower amount of C9+ aromatics, and higher coke formation, resulting in faster catalyst deactivation compared to the use of hydrogen as the carrier gas. This was confirmed by the higher amount of coke measured on the spent catalyst from catalytic reactions where nitrogen carrier gas was used as compared to when hydrogen gas was used. This was further confirmed by the time-on-stream study of the catalyst which shows that toluene conversion decreased much faster when nitrogen was used as the carrier gas instead of hydrogen. It was observed that within 40 h time-on-stream, toluene conversion decreased by more than half of its initial value while in the presence of hydrogen carrier gas the decrease in toluene conversion was only slight.
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7. The alumina binder played no significant role both in toluene conversion and in the production of aromatics. Only negligible amounts of toluene conversion were observed with both nitrogen and hydrogen carrier gases. 8. Light hydrocarbons such as methane, ethane, and propane were not detected in the gas samples collected from the reaction product. The analysis of the gas showed only the presence of the carrier gas used. Acknowledgment. This project is supported by the King AbdulAziz City for Science and Technology (KACST) under project # AR-22-14. The support of the King Fahd University of Petroleum and Minerals is highly appreciated. The authors thank Mr. Khalid Al-Nawad for his help during the experimental work. EF700582B
