Energy & Fuels 2006, 20, 919-922
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Propane Conversion to Aromatics on Highly Active H-GaAlMFI: Effect of Thermal Pretreatment T. V. Choudhary,*,† A. Kinage,‡ S. Banerjee,‡ and V. R. Choudhary‡ ConocoPhillips Company, BartlesVille Technology Center, BartlesVille, Oklahoma 74004, and Chemical Engineering DiVision, National Chemical Laboratory, Pune 411008, India ReceiVed October 5, 2005. ReVised Manuscript ReceiVed March 24, 2006
The propane aromatization activity over H-GaAlMFI has been investigated after different thermal pretreatments (calcination temperature range: 500-800 °C) at a reaction temperature of 500 °C and in the space velocity range of 1500-12000 cm3/g/h. The calcination temperature had a strong influence on the strong acidity of the zeolite, propane conversion, and aromatic distribution. The decrease in acidity with increased thermal treatment severity correlated well with the decrease in the framework Ga. There was a considerable decrease in propane aromatization activity at calcination temperatures above 600 °C. The effect of calcination temperature on the propane aromatization activity can be explained in terms of change in the strong acidity of the zeolite and the extraframework Ga content. The selectivity for aromatics was found to decrease with increasing thermal pretreatment severity, while that for methane, ethane, and propylene was found to increase. The aromatic distribution was also influenced by the thermal pretreatment; the selectivity for benzene increased with increasing pretreatment temperature, while the selectivity for toluene, C8 aromatics, and C9+ aromatics decreased.
Introduction Conversion of propane to aromatics, a commercially important reaction, has been investigated over a variety of Ga-based ZSM-5 type zeolites, physically mixed Ga2O3 and H-ZSM-5, Ga-exchanged or Ga-impregnated H-ZSM-5 (Ga/H-ZSM-5),1-3 H-gallosilicates (GaMFI),4-10 and H-galloaluminosilicate (HGaAlMFI).11 The high aromatization activity arises due to the bifunctional nature of these zeolites, high dehydrogenation function due to the presence of extraframework Ga species (in combination with zeolitic protons), and high acidity due to framework (FW) Al and/or Ga.2 Among the various Ga-modified zeolites, GaAlMFI shows the highest activity/selectivity for the propane aromatization process.11 The superior performance of these zeolites has been attributed to high zeolitic acidity in combination with well* Corresponding author. E-mail:
[email protected]. † ConocoPhillips Co. ‡ National Chemical Laboratory. (1) Ono, Y. Catal. ReV.-Sci. Eng. 1992, 34, 179. (2) Choudhary, V. R.; Mantri, K.; Sivadinarayana, C. Microporous Mesoporous Mater. 2000, 37, 1. (3) Giannetto, G.; Monque, R.; Galliasso, R. Catal. ReV.-Sci. Eng. 1994, 36, 271. (4) Bayense, C. R.; vanderPol, A. J. H. P.; vanHoof, J. H. C. Appl. Catal. 1991, 72, 81. (5) Bayense, C. R.; vanHoof, J. H. C. Appl. Catal., A 1991, 79, 127. (6) Giannnetto, G.; Montes, A.; Gnep, N. S.; Florentino, A.; Cartraud, P.; Guisnet, M. J. Catal. 1993, 145, 86. (7) Choudhary, V. R.; Kinage, A. K.; Sivadinarayana, C.; Sansare, S. D.; Guisnet, M. Catal. Lett. 1995, 33, 401. (8) Choudhary, V. R.; Kinage, A. K.; Sivadinarayana, C.; Devdas, P.; Sansare, S. D.; Guisnet, M. J. Catal. 1996, 158, 34. (9) Choudhary, V. R.; Devdas, P.; Kinage, A. K.; Sivadinarayana, C.; Guisnet, M. J. Catal. 1996, 158, 537. (10) Choudhary, V. R.; Sivadinarayana, C.; Kinage, A. K.; Devdas, P.; Guisnet, M. Appl. Catal., A 1996, 136, 125. (11) Choudhary, V. R.; Kinage, A. K.; Sivadinarayana, C.; Guisnet, M. J. Catal. 1996, 158, 23.
dispersed extraframework Ga species.12,13 Our previous studies have shown that the zeolitic acid sites and extraframework Ga species in GaMFI are strongly influenced by different thermal treatments.10 The different thermal pretreatments are also expected to affect the zeolitic acid sites and extraframework Ga species in case of the highly active GaAlMFI zeolite. Despite its superior performance as compared to other Ga-based zeolites, the effect of thermal treatment (calcination temperature) on the propane aromatization activity of H-GaAlMFI has not yet been reported. The present work was undertaken to understand the influence of thermal treatment on the acidity/propane aromatization activity of H-GaAlMFI zeolite. Experimental Section Catalyst Synthesis. H-GaAlMFI zeolite was synthesized by the hydrothermal crystallization from a gel consisting of Na-trisilicate (Fluka), gallium nitrate (Aldrich), aluminum nitrate (BDH), TPABr (Aldrich), sulfuric acid, and deionized water in a stainless steel autoclave at 180 °C for 96 h. The zeolite crystals were washed thoroughly with deionized water and dried at 120 °C for 10 h. After calcination at 500 °C for 15 h under static air, it was converted into its NH4 form by repeated exchanging with 1 M ammonium nitrate solution at 80 °C. The different zeolite samples were obtained by deammoniation of their NH4 form by calcination at different temperatures (500-800 °C) for 1 h in a flow of moisture-free air (1800 cm3/g/h). Catalyst Characterization. The strong protonic acidity was measured in terms of the amount of pyridine chemisorbed on the zeolite at 400 °C. The pyridine chemisorption (at 4000 °C) on ZSM-5 type zeolite essentially measures the protonic acid sites because the Lewis sites are not accessible to pyridine due to (12) Choudhary, V. R.; Kinage, A. K.; Choudhary, T. V. Appl. Catal., A 1997, 162, 239. (13) Choudhary, T. V.; Kinage, A. K.; Banerjee, S.; Choudhary, V. R. Microporous Mesoporous Mater. 2004, 70, 37.
10.1021/ef050328j CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006
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Figure 1. Influence of calcination temperature on strong acidity of GaAlMFI zeolite (mmol/g).
structural constraints.14 The error in reproducibility for strong acidity measurement was within 5%. Details of the procedure to determine acidity by this method have been described earlier.8 The FW Si/ (Ga +Al) ratios were determined from 29Si MAS NMR, and the individual FW Si/Al and Si/Ga ratios were estimated from the FW Si/(Ga+Al), octahedral Al (determined from 27Al MAS NMR), and bulk concentration of Al in the zeolite. The T-sites of ZSM-5 are not crystallographically identical, and hence the determination of FW Si/(Al+Ga) is somewhat complicated.15 The % repeatability error is expected to be between 5% and 10%. The MFI structure of the zeolites was confirmed by XRD. Activity Tests. The propane aromatization reaction was carried out in an atmospheric pressure continuous flow quartz reactor equipped with a Chromel-Alumel thermocouple in the center of the catalyst bed (1 g of catalyst and 1:1 propane-N2 mix as feed). The conversion and selectivity data at different space velocities (1500-12000 cm3/g/h) were obtained at 500 °C in the absence of catalyst deactivation (initial activity and selectivity). This was accomplished by employing the square pulse technique by passing the reaction mixture over a fresh catalyst for a short period (2-5 min) under steady state and then replacing the reaction mixture with pure N2 during the period of product analysis. The reaction products were analyzed via an on-line GC with FID, using Poropak-Q (3 mm × 3m) and Benton-34 (5%) and dinonylthalate (5%) on Chromosorb-W (3 mm × 5m) columns. The error for reproducibility in conversion was (4%, and selectivity was less than (5%.
Results and Discussion Figure 1 shows the influence of calcination temperature on the protonic strong acidity of the GaAlMFI zeolites. The strong acidity decreases linearly (from 0.59 to 0.06) with increasing calcination temperature. The acidity of the GaAlMFI zeolite may be attributed to the tetrahedral Ga and Al in the framework.8,12,16 It is, therefore, apparent that the thermal treatments influence the content of the tetrahedral Ga and/or Al of the GaAlMFI zeolite. Figure 2 shows the effect of calcination temperature on the GaAlMFI framework (FW) composition. There is a considerable increase (from 43 to 76) in the FW Si/ Ga ratio with the increase in the calcination temperature; this suggests that there is extensive degalliation during the different thermal treatments. It is noteworthy that the Si/Al ratio does not change as a result of the thermal treatment. This clearly illustrates that the FW Al is far more stable than the framework (14) Nayak, V. S.; Choudhary, V. R. J. Catal. 1983, 81, 26. (15) Thomas, J. M.; Lin, X.-S. J. Phys. Chem. 1986, 90, 4843. (16) Choudhary, T. V.; Kinage, A. K.; Banerjee, S.; Choudhary, V. R. Microporous Mesoporous Mater. 2005, 87, 23.
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Figure 2. Influence of calcination temperature on framework (Si/Ga and Si/Al) ratio.
Figure 3. Plot of strong acidity (mmol/g) of the different thermally treated zeolites as a function of their FW Si/Ga ratio.
Ga. This is in line with a previous study wherein a comparison of the H-GaMFI and H-ZSM-5 zeolites (having nearly the same Si/Ga or Al ratio) for their hydrothermal stability revealed a considerably lower stability for the H-GaMFI zeolite.9 Because the ionic radius of Ga3+ (0.62 Å) is larger than that of Al3+ (0.51 Å), the isomorphous substitution of Al by Ga is expected to result in a relatively less stable MFI structure, more susceptible to degalliation due to hydrothermal/thermal pretreatment. Figure 3 shows a plot of the Si/Ga ratio and strong acidity for the different zeolites investigated in this study. A linear correlation is observed with the Si/Ga ratio and strong zeolitic acidity. Because the Si/Al ratio does not change with calcination temperature, the FW Al is not expected to influence the change in the strong acidity. This shows that FW Ga content determines the difference in the strong acidity for the different thermally treated zeolite samples. Increase in thermal treatment severity of the H-GaAlMFI results in an increase in the degalliation, which in turn decreases the strong acidity of the zeolite. It, however, increases the content of the extra-FW Ga species. A decrease in the propane conversion and aromatic yield was observed with increasing calcination temperature (Figure 4) at a reaction temperature of 500 °C (GHSV ) 1500 and 3000 cm3/g/h); the decrease was particularly severe at higher (above 600 °C) calcination temperatures. The decrease in the propane conversion and aromatic yield with calcination temperature can be attributed to the decrease in the strong acidity of the zeolite. It is noteworthy that the strong acidity decreases linearly with increase in the calcination temperature. If strong acidity was the only factor controlling the propane aromatization activity,
Propane ConVersion to Aromatics
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Figure 4. Influence of calcination temperature on total propane conversion and aromatic yield (%) at space velocity of 1500 and 3000 cm3/g/h; reaction temperature ) 500 °C.
a linear decrease in the aromatic yield should have been observed. However, this is not the case. As mentioned earlier, there is extensive degalliation of the framework Ga with increasing thermal treatment severity; this increases the content of the extra FW Ga of the zeolite. From previous studies, it is known that the extra-FW Ga species also plays a role in defining the propane/lower alkane aromatization activity.11,17,18 The simplistic mechanism for propane aromatization over GaAlMFI is provided below: extra FW Ga
H+
C3H8 98 C3H6 98 H+ and extra FW Ga
C6-C9 oligomer 98 aromatics The Ga species are mainly involved in the dehydrogenation of propane and dehydrocyclization reactions, while the protonic acidity is required for oligomerization and dehydrocyclization reactions. The overall propane aromatization reaction is therefore controlled by both the Ga species and the protonic acidity. The loss of protonic acidity affects the oligomerization and cyclization steps and thereby reduces the aromatization activity. The present work indicates that the increasing extra FW Ga content with increasing calcination temperature counters (to a certain extent) the activity decrease occurring from the loss in strong acidity. However, after a certain optimum content of extra FW Ga is reached (probably occurring between calcination temperatures of 600-700 °C), there is no further contribution from the increase in extra FW Ga content; after this point, the change in aromatization activity with increasing calcination temperature is controlled only by change in strong acidity of the zeolite. This discussion is in good agreement with earlier studies,19,20 wherein we had undertaken an in-depth study of the influence of zeolitic acidity and extra-FW Ga (over different Ga-modified ZSM-5 catalyst) on the ethylene and heptane aromatization reactions. Based on these studies, the optimum extra-FW Ga/ strong protonic acid sites ratio for the aromatization of n-heptane and ethylene on Ga-modified ZSM-5 catalysts passed through a maximum (at ∼1.0). (17) Choudhary, V. R.; Mondal, K. C.; Mulla, S. A. R. Angew. Chem., Int. Ed. 2005, 44, 4381. (18) Choudhary, T. V.; Kinage, A. K.; Banerjee, S.; Choudhary, V. R. Catal. Commun. 2006, 7, 166. (19) Choudhary, V. R.; Devadas, P.; Banerjee, S.; Kinage, A. K. Microporous Mesoporous Mater. 2001, 47, 253. (20) Choudhary, V. R.; Mulla, S. A. R.; Banerjee, S. Microporous Mesoporous Mater. 2003, 57, 317.
Figure 5. Influence of calcination temperature on the selectivity of the products formed during propane aromatization reaction; reaction temperature ) 500 °C and propane conversion ∼48%.
Previous studies have shown that the selectivity of the products formed in the propane aromatization reaction is strongly influenced by total propane conversion.13 Therefore, in this study, the selectivity for the products in the propane aromatization reaction for the different thermally treated samples is compared at a near constant conversion of 48% (Figure 5); this was possible because data for each zeolite were collected at different space velocities (1500-12 000 cm3/g/h). The selectivity for the 800 °C calcined zeolite is not shown in the comparison because there was no overlapping conversion for this sample with the 500 °C calcined zeolite at any space velocity. The following trends in product selectivity are observed with increasing thermal treatment severity: (a) selectivity for aromatics decreases; (b) selectivity for methane, ethane/ethylene, and propylene increases; and (c) selectivity for C4 species remains unaffected. With increasing thermal severity, the protonic strong acidity (which catalyzes the oligomerization and cyclization reactions) is decreased, resulting in a decrease in the aromatics selectivity. Correspondingly, the formation of extra-FW Ga species (which catalyze the propane dehydrogenation) is increased, resulting in the increase of propylene selectivity. Further, with decreasing strong acidity, other acid-catalyzed reactions, such as dimerization, cracking/dealkylation, and hydrogen transfer reactions, become relatively more predominant, causing the observed increase in the selectivity for methane and C2 hydrocarbons. However, because C4 hydrocarbons are more reactive than C2 hydrocarbons, their selectivity is not affected. The aromatic product distribution (Figure 6) is also influenced by the calcination temperature (500-700 °C). The effect is, however, more pronounced, when the calcination temperature is increased from 500 to 600 °C. The following trends in selectivity are observed with increasing thermal treatment severity: The selectivity for benzene increased with increasing pretreatment temperature, while the selectivity for toluene, C8 aromatics, and C9+ aromatics decreased. With decreasing strong acidity (increasing calcination temperature), the propene dimerization (as opposed to trimerization) is expected to become more predominant, resulting in the formation of more benzene as compared to other aromatics. It may also be noted that the aromatics distribution is controlled by different aromatics transformation reactions (alkylation/dealkylation and disproportionation) after the formation of aromatics from propane.19,21 (21) Bhan, A.; Hsu, S. H.; Blau, G.; Caruthers, J. M.; Venkatasubramanian, J. M.; Delgass, W. N. J. Catal. 2005, 235, 35.
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Figure 6. Influence of calcination temperature on aromatic product distribution (wt %); reaction temperature ) 500 °C and propane conversion ∼48%.
The dealkylation reactions may become relatively more predominant with decreasing strong acidity, and this may also be the cause for the increased benzene selectivity with increasing thermal treatment severity. Conclusions The salient features of the study are summarized below: (a) The thermal pretreatments had a strong influence on the
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FW Si/Ga ratio; the Si/Ga ratio increased linearly with increasing calcination temperature. (b) In contrast to the unstable FW Ga, the FW Al was exceedingly stable to the thermal pretreatments; no change in the Si/Al ratio was observed even after the most severe thermal pretreatment undertaken in this study. (c) The strong zeolitic acidity decreased linearly with increasing thermal severity; it was found to correlate very well with the FW Si/Ga ratio. (d) The thermal pretreatments had a profound influence on the propane aromatization activity. There was a significant decrease in propane conversion at temperatures above 600 °C. The effect of calcination temperature on the propane aromatization activity can be explained on the basis of the change in the strong acidity of the zeolite and extraframework Ga content. (e) The selectivity for aromatics was found to decrease with increasing thermal pretreatment severity, while that for methane, ethane, and propylene was found to increase. (f) The aromatic distribution was also influenced by the thermal pretreatment; the selectivity for benzene increased with increasing pretreatment temperature, while the selectivity for toluene, C8 aromatics, and C9+ aromatics decreased. Acknowledgment. S.B. is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, for awarding him a Senior Research Fellowship. EF050328J
