Influence of various catalyst factors on time-on-stream activity of Ga/H-ZSM-5 in propane aromatization

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1 Indian Journal of Chemical Technology Vo\. 6, May 1999, pp Influence of various catalyst factors on time-on-stream activity of Ga/H-ZSM-5 in propane aromatization Vas ant R Choudhary*, Kshudiram Mantri & Chinta Sivadinarayana Ch.e:mical Engineering Division, National Chemical Laboratory, Pune 411 8, India Received 11 February 1999; accepted 2 April 1999 Influence of various zeolite catalyst factors such as, Si/AI ratio (35-132), degree of W exchange (2-98%), calcination temperature (6-8 C) and Ga-loading on time-on-stream activity/selectivity of GaIH-ZSM-5 in the propane aromatization has been investigated. lbe time-on-stream activity in the propane-to-aromatics conversion is found to be influenced strongly by the above zeolitic factors. The GaIH-ZSM-5 zeolite catalysts undergo quite rapid deactivation due to coking. Aromatization of lower alkanes (C 3 -C.j alkanes) is a process of great practical importance. This process has been extensively investigated during the last decade and covered In critical reviews ' - 3. Gaimpregnated H-ZSM-5 (Ga/H-ZSM-5) shows high activity in the aromatization of lower alkanes 4-7. This zeolite catalyst, however, is deactivated in the aromatization process due to coking quite fast 8-1O The deactivation of the zeolite catalyst in the aromatization process is expected to be strongly influenced by the zeolite factors affecting the acidity and/or catalytic activity of the zeolite. However, infonnation on the zeolite catalyst factors, such as Sil Al ratio, degree of H+ exchange, calcination temperature and Galoading of Ga/H-ZSM-5 zeolite, on its time-onstream activity/selectivity in the lower alkane aromatization process is scarce. This investigation was therefore undertaken with the objective of investigating the influence of the zeolite factors on the timeon-stream activity/selectivity of Ga/H-ZSM-5 zeolite in the aromatization of propane. Experimental Procedure Ga/H-ZSM-5 zeolites (Ga-Ioading = 1. wt% and 3. wt%) were prepared by impregnating Ga-nitrate (Aldrich) on NH 4 -ZSM-5 and H.Na. ZSM-5 zeolites using the incipient wetness impregnation technique, drying at 12 C for 1 h and calcining the impregnated zeolite under static air for 4 h (at 6 or 8 C). The NH 4 -ZSM-5 zeolites were prepared by exchanging calcined ZSM-5 zeolites with aqueous I M ammonium nitrate solution at 8 C for Ih; the exchange was repeated five times. H.Na-ZSM-5 (Si/AI = 35) with different degrees of H+ exchange were prepared by exchanging H-ZSM-5 (obtained by deammoniation of the NH 4 -ZSM-5 at 6 C for 1h under static air) with an aqueous solution of sodium nitrate (.5 or.1 M) at 3 C. The ZSM-5 zeolites were synthesized and characterized by the procedures described elsewhere " '2. Before use, the zeolite catalysts were pretreated in a flow of moisture-free N2 at 6DO C for a period of 1 h and then in H2 flow at 6 C for 1h. The MFI structure of the zeolite was con finned by XRD (using Philips Defractometer 173 series and CuKa source of radiation). The 27 AI MAS NMR of all the zeolites indicated that all the AI in the zeolites are tetrahedral 13 The number of strong acid sites on the zeolite catalysts was measured in tenns of the pyridine chemisorbed at 4 C. The acidity was detennined by using the GC adsorption/desorption technique 14. Propane aromatization reaction (at 55 C) was carried out at atmospheric pressure in a continuous flow quartz reactor (i.d. 13 mm) provided with a chrome1- alumel thennocouple in the center of catalyst bed (containing 1-2 g catalyst), using a propane-nitrogen mixture (33 mol% propane) as a feed with a space velocity of 31 cm 3 g-'h- '. The reaction products were analyzed by an on-line GC with FID, using Poropak-Q (3 mm x 3 m) and Benton-34 (5%) and dinonylphthalate (5%) on Chromosorb-W (3 mm x 5 m) columns.

2 CHODHRY el al.: INFLENCE OF CATALYST FACTOR ON GAIH-ZSM Results and Discussion Influence of Si/AI ratio of Ga/H-ZSM-5 on its time-on-stream activity In ortier to study the influence of Sil Al ratio on the perfoance of GalH-ZSM-S. zeolite in the propane aromatization, the zeolite catalysts having different Sil AI ratios (Sil AI = ) but having the same Galoading (1 wt % Ga), degree of H+ exchange (97-98%) and calcination temperature (6 C) were used. Results in Fig. I show a strong influence of the Si/AI ratio on the strong acidity (measured in terms of the pyridine Ghemisorbed at 4 C) of the GaIH-ZSM-5 zeolite. Strong acidity is decreased sharply with increasing the Sil Al ratio of the zeolite catalyst. Amount of coke deposited on the zeolite catalyst is also decreased sharply with increasing the Sil Al ratio of the zeolite (Fig. 1). This is consistent with the fact that the strong acidity of H-ZSM-5 is attributed to the tetrahedral (or framework) AIlS. The time-on-stream activity of GaIH-ZSM-5 zeolite with different Sil AI ratios in the propane aromatization (at 55 C and GHSV 31 cm 3.g- l.h-.,. is 1.5 shown in Fig. 2. The propane aromatization activity is continuously decreased for the zeolites with Sil Al ratios 35 and 54, but it is decreased only to a small extent for the zeolite with igher Sil Al ratio (Sil Al = 132) ' with increasing the time-on-stream. The influence of time-on-stream on the product selectivity in the propane aromatization is shown in Fig. 3. With the increase in the time-on-stream, the selectivity for aromatics is decreased, the selectivtty for ethylene and ethane is increased, whereas the selectivity for methane, ethane and C 4 + aliphatics is effected only to a small extent. The observed influence of the timeon-stream on the product selectivity could also be due to the decrease in the propane conversion with increase in the time-on-stream. The product selectivity.. 16 is strongly dependent upon the propane conversion. Influence of degree of W exchange of Ga/H.Na-ZSM-5 on its time-on-stream activity In order to study the effect of H+ exchange or Na/Ga ratio on the performance of GaIH-Na-ZSM-5 zeolite in the propane aromatization, the zeolite , (3) Conversion to... ornatics , 15 1 S Al- 35 5rAl Al J-...L L-_--L.....L-_---lL--_-;.3 c. c (b) Total conversion "??-.2 '6. g til.1 SO/AI- 3S 1 SO/AI - 54 SO/AI SiiAI ratio Fig. I-Effect of Si/AI ratio of Ga (I wt%)/h-zsm-5 on its strong acidity and coke deposition on the catalyst in the propane aromatization (at 55 C and GHSV = 31 cm) g-i h- ' ).. o 2 3 " 5 6 Time-<>n-stream, h Fig. 2-Variation with time-on-stream of the propane conversion - (a) to aromatics and (b) total conversion, in the propane aromatization (at 55 C and GHSV = 31 cm) g-i h- ' ) over Ga (I wt%)/h-zsm-5 with different Si/AI ratios.

3 INDIAN 1. CHEM. TECHNOL., MAY '*' 3 CH. C1H. C1H, C)H, i- y c.. : lor II t) V) 2 '#. i ]".;;! o.5 be. t-...l.-.l.-.---jl-_-l--l.3 1 Fig. 3-lnfluence of time-on-stream on the product selectivity in the pro pane aromatization (at 55 C and GHSV = 31 cm 3 g- I h- I ) over Ga( I wt%)/h-zsm-5 (Si/AI = 35). catalysts having different degrees of H+ exchange (2-98%) but same SiiAl ratio (SiiAI = 35), Ga loading (1 wt % Ga) and calcination temperature (6 C) were used. Results in Fig. 4 show that the strong acidity and amount of coke deposited on the zeolite catalyst are almost linearly increased with increasing the degree of fr exchange. The propane conversion or aromatization activity of the zeolite with 66%,88% and 98% ofh+exchange are decreased, but it is not affected for the zeolite with the H+ exchange of 2% with increasing the timeon-stream (Fig. 5). The influence of strong acidity on the time-on-stream activity and extent of coke deposition are consistent with that obsenred for the zeolite with different Si/Al ratios. Influ ence of calcination temp-erature of Ga/H-ZSM-5 on its time-on-stream activity In order to study the influence of calcination temperature of Ga/H-ZSM-5 on its perfonnance in the propane aromatization, the zeolite catalysts having the same Sil Al ratio (Sil AI=35); degree of H+ ex- o 2 Degree ofh+ exchange, 'Yo Fig. 4--Effect of degree of W exchange of Ga( I wt%)/h-zsm-5 on its strong acidity and coke deposition on the catalyst in the propane arornati zati on (at 55 C and GHSV = 3 1 cm 3 g-i h- I ). change (98%) and Ga-Ioading (1 wt% Ga) but calcined at two different temperatures (6 and 8 C) were used. The strong acidity of the zeolite catalyst is decreased from.28 to.9 mmol g- I with the increase in its calcination temperature from 6 to 8 C. Coke deposited on the zeolite catalyst for the timeon-stream of 6.7 ±.3 h was also decreased from l.46 wt% to.85 wt% with the increase in the calcination temperature from 6 to 8 C. The time-on-stream propane conversionlaromatization activity data for the zeolite catalvst calcined at 6 and 8 C are presented in Fig. 6. With the increase in the time-on-str eam, total conversion and conversion to aromatics are decreased significantly for the zeolite calcined at 6 C, but these are almost unaffected for the zeolite calcined at 8 C. The effect of time-on-stream is consistent with that observed for the zeolite catalyst with different SiiAI ratios and dgrees of H+ exchange, when the acidity is taken in consideration. -

4 CHODHRY et al.: INFLENCE OF CATALYST FACTOR ON GAlH-ZSM '<f? r: 'j!! u > c: '" u'" A 66'"... '" u'" A 66'" 2'" 6 7 Fig. 5-Yariation with time-on-stream of the conversion of propane (total and to aromatics) in the propane aromatization (at 55 C and GHSY = 31 cm} g-' h-') over Ga(l wt%)/h.na ZSM-5 with different degrees of W exchange. 2r ,.. 15 (8) Convmion 1 aromatics 1 S c. > c u (b) To conversion Fig. 6--Yariation with lime-on-stream of Ihe - (a) conversion to aromatics and (b) total conversion of propane in the propane aromatization (at 55 C and GHSV = 31 cm} g-' h-') over Ga(lwt%)/H-ZSM-5 calcined at different temperatures. '$ iii fr " 3r ,.3 bo.28 E..26 " u." til) g.24 CI).22 2 Ga-Ioading, wt % Fig. 7-Effect of Ga-Ioading of GalH-ZSM-5 on its strong acidity and coke deposition on the catalyst in the propane aromatization (at 55 C and GHSV = 31 cm} g-' h-'). Influence of Ga-Ioadiog of Ga/H-ZSM-5 on its time-oo-stream activity In order to study the influence of Ga loading of GaIH-ZSM-5 zeolite on its performance in the propane aromatization, gallium at different loadings (, 1 and 3 wt% Ga) was impregnated on the NH4 ZSM-5 (Si/Al = 35, and NH 4 + exchange = 98 %) and the resulting zeolite catalyst was calcined at 6 C. The influence of Ga-Ioading of GaIH-ZSM-5 zeolite on its strong acidity in the propane aromatization is shown in Fig. 7. The strong acidity of the zeolite arc decreased with increasing the Ga-loading from to 3 wt % Ga. The decrease in the acidity is mostly due to the replacement of some of the zeoiitic protons by gallium ionic species. The nature of the gallium species (Ga}' or GaO+) in the zeolite channels is not yet fully understood 1.3. The time-on-stream activity and selectivity data for the Ga/H-ZSM-5 zeolite with different Ga loadings in the propane aromatization at the space velocity of 31 cm)g- 'h- ' are presented in Fig. 8. With the 3

5 17 INDIAN J. CHEM. TECHNOL., MAY Fig. 8--Variation with time-on-stream of propane conversion (a) to aromatics and (b) total conversion in the propane aromatization over GaIH-ZSM-5 with different Ga-Ioadings (at 55 C and GHSV = 31 cm J g-i h- I ). increase in time-on-stream, the total conversion and conversion to aromatics an! decreased for the' zeolite catalyst containing extra framework Ga; the decrease is, however, more pronounced for the catalyst with 3 wt% Ga. For the H-ZSM-5 zeolite, the propane conversion is very small and there is a little or no influence of time-on-stream on the propane conversion activity of the zeolite. The coke deposited on the zeolite catalyst in the propane aromatizati<;m for a time-on-stream of 6.7±.3 h is increased with increasing the Ga-Ioading (Fig. 7). This is expected mostly because of the increase in the dehydrogenation activity of the catalyst due to the presence of more and more Ga-oxide species in the zeolite channels. Fig. 9 shows that the p-xlm-x product ratio is very significantly higher for the zeolite catalyst with the higher Ga-Ioading. The p-xlm-x ratio is changed only to a small extent due to the deactivation due to coking of the zeolite catalyst with 1 wt % Ga, but it is strongly influenced (it is increased markedly with increasing the time-on-stream) for the catalyst with 3 wt% Ga. o Fig. 9--Variation with time-on-stream of xylene ratios in the propane aromatization (at 5.5 C and GHSV = 31 cm J g-i h- I ) over GaIH-ZSM-5 with different Ga loadings. The observed increase in the p-xjm-x ratio para selectivity of the zeolite is due to the increase in the diffusional resistance in the zeolite channels, resulting from the reduction in the effective zeolite channel diameter due to the presence of Ga-oxide species in the zeolite channels. This could also be due to the blockage and/or the reductkm in the size of some of the channel openings due to presence of Ga-oxide species at the external surface of the zeolite crystallites. The observed large increase in the p-xlm-x ratio with the increase in the time-on-stream (or with increasing the extent of deactivation due to coking) is attributed to reduction in the effective channel size, and also to a partial or complete blockage of some of the channel openings by the coke molecules. A combined effect produced by the coking and the Ga-oxide species is thus expected to produce the observed large increase in the para-electivity with increasing the Ga loading and/or deactivation of the z( olite catalyst in the propane aromatization. Conclusion The time-on-stream activity in the propane-toaromatics conversion. is strongly influenced by the

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