Pore opening size alteration of zeolite and its alkylation activity stabilization by coking

Transcription

1 Indian Journal of Chemical Technology Y0l. 3, November 1996, pp Pore opening size alteration of zeolite and its alkylation activity stabilization by coking Y S Bhat, J Das & A B Halgeri* Research Centre, Indian Petrochemicals Corporation Limited, Baroda , India Received 15 December 1995; accepted 1 July 1996 Medium pore, ZSM-5 zeolite as subjected to chemical vapour deposition to alter its pore opening size. The extent of alteration and its effect on shape selectivity during toluene ethylation has been investigated. The time on stream behaviour of the' pore size altered zeolite shoed a deactivating trend ith enhancement in hours on stream. The stabilization of the alkylation activity of the catalyst has been achieved by controlled coking and the details m:ediscussed. The unique feature of ZSM-5 zeolite is its shape selectivity hich is attribut~d to the pore structure of the zeolite. This effect is more hen the dimensions of reactant and product molecules are close to the pore opening size of the zeolite. The shape selectivity aspect of the zeolite has been made use in many hydrocarbon conversion reactions 1 There has been continuous efforts to modify the shape selectivity feature of the zeolite by altering the pore opening dimension and to make use of it in synthesizing high purity dialkylbenzenes2-4 Chemical vapour deposition of tetra alkoxy silane compound on zeolite is. found to be a u~eful technique for altering the pore opening sizes,6, here the silica deposits on the external surface as a thin layer hich narros the pore openings of the zeolites. Simultaneously silica layer deactivates acid sites on the external surface as the coated silica is inert in nature. This method has been applied to H-mordenite and H-ZSM-5 and its effect during cracking of octane isomers and methanol conversion is reported. The structural analysis of the deposited material as carried out ith various spectroscopic techniques 7 The EXAFS measurement shoed that the material as present not as particles but as a thin layer and the XPS and TEM measurements shoed that the material covered uniformly the external surface. The' pore opening alteration after silica deposition maybe explained by the fact that the deposited silica layer consists of siloxane and Si-O-Al bonds. This results in the variation in bond angles beteen silica layer and the zeolite frameorks. The' siloxane bond of the *Author to hom correspondence should be addressed. silica layer protrudes into the pore, due to this pore opening is narroed. The aim of the present study is to correlate the extent of pore size alteration of ZSM-5 zeolite and its effect on the shape selectivity during toluene alkylation ith ethanol. The aspect of catalyst stability during long hours of on stream is also discussed. Experimental Procedure The zeolite used in this ork as synthesized' according to the patented literatures. It as characterised for structural, morphological and acidity features by physico-chemical techniques such as XRD, SEM, MAS-NMR, FT-IR and TPD of ammonia. The Si-Al ratio as determined by a combinatioin of et chemical analysis and atonuc absorption spectrophotometry. Before silica deposition the as synthesized zeolite as converted to protonic form and given the shape of cylindrical extrudates. The protonic form of the zeolite as admixed ith inert alumina binder in the proportion (t%) 70:30 and extruded in a laboratory extruder. The extrudates of 2 mm diameter ere cut into the size of 5 mm length. The extrudates ere calcined at 500 C for 6 h. The alkylation reaction as ell as silica deposition as carried out in a tubular fixed-bed, ~on flo glass reactor at atmospheric pressure. To grams of zeolite catalyst as loaded in the reactor for each run. The silica deposition as carried out by chemical vapour deposition of bulky silicon compound tetraethyl orthosilicate9 A 7.5% Si(OC2Hs)4 solution in 60:40. toluene and methanol as vaporized and passed through the catalyst bed maintained at 240 C. The silicate solution as fed at a, rate of 6

2 3 INDIAN J. CHEM. TECHNOL., NOVEMBER 1996 eolite* Table I-Performance comparison of silynated and parent zeolite for toluene ethylation zeolite Parent Silynated Ethyl -para toluene z fz 40 Q. Others = benzene, ethylbenzene, xylenes, C9 aromatics other than ethyl toluenes and CIO aromatics. Temperature = 325 C, WHSV = 3 h -I, Toluene/Ethanol mol ratio =4. *4.5 t % SiOz deposition. TOLUENE CONV SILICA DEPOSITED, Wt.% 5 6 mvh and a flo of 60 mvmin hydrogen as maintained so that there as uniform contact of the vapour ith the catalyst. After the catalyst as in contact ith the vapour for the desired period, hydrogen flo as changed to nitrogen. The nitrogen gas as further replaced by air and the reactor temperature as raised to 540 C and kept for 10 h to decompose tetraethyl orthosilicate to silica. This procedure resulted in a uniform deposition of silica throughout the catalyst bed. The mixture of reactants, toluene plus ethanol in different proportions as fed by a Sage syringe pump, vaporized in a preheater assembly and passed through the catalyst bed maintained at the desired reaction temperature. The products of the reaction ere condensed in a chilled ater cooled condenser at the reactor outlet and analysed in a Varian Vista 6000 gas chromatograph equipped ith 0.5 mm x 50 m LB-550 capillary column. Results and Discussion The zeolite prepared is of high Si/ AI ratio of 290 as estimated by a combination of et chemical analysis and atomic absorption spectrophotometry. The XRD pattern indicated the material contains only ZSM-5 phase. The morphology of the crystallite is pseudocubic ith to flat surfaces and rest of the four faces being joined by curved edges. The average size is about 5.5 x 5.5 x 4.4 cubic microns. The total acidity as estimated by temperature programmed description in terms of milli mole equivalent of ammonia as Al NMR spectrum indicated the absence of any extra' frameork aluminium. Table 1 compares the performance of silynateq Fig. I-Alkylation activity of ZSM-5 zeolite silylated to different extent. Temperature = 350 C, Toluene/Ethanol mol ratio=4 WHSV=3 h-i and the parent zeolite for toluene ethylation. The silylated catalyst exhibited a loer toluene conversion and higher para-ethyltoluene selectivity. Fig. 1 presents the results of the run in hich alkylation as carried out over the zeolite siiylated to different extent. With progressive silylation para-ethyltoluene selectivity as indicated by the fraction of para-isomer among ethyltoluenes gradually enhanced from 60 to 95%. Simultaneously toluene conversion came don from to 14%. This is due to decrease in pore opening size of the zeolite. The alteration in the pore opening size can easily be monitored by a standard test involving probe molecules of different kinetic diameter1o. It has been reported that the improvement in para-selectivity by the modifications of HZSM-5 ith oxides of P, Mg and B as due to the suppression of the isomerization of the primarily produced para-isomer1l The role of the external surface sites in reactions ith ZSM-5 catalysts is also reported 12. It is found that the para-isomer formed selectively inside ZSM-5 channels, hile isomerization proceeded just on the external surfaces and that the improvement in para-selectivity by the modification as due to the inactivation of the acid sites on the external surfaces. The paraselective alkylation of toluene ith methanol over ZSM-5 zeolites is also studied 13. The reactions of toluene alkylation and xylene isomerization are considered on both the external and internal surface catalytic sites of the zeolite crystals. CVD zeolites exhibits super shape selectivity 14. The study reports the deposition of ultrathin layer of Si02 to

3 BHAT et al: PORE OPENING SIZE ALTERATION OF ZEOUTE f- p-et SELECT +~+~ ~ I Ẓ... ~ I- 60 z S TEMPERAHJRE, c 375.fOl.UEN E CONV Fig. 2-Effect of reaction temperature on the activity selectivity of silylated ZSM-5 zeolite. WHSV=3 h-i, Toluene/Ethanol mol ratio = 4, t% Si02 deposited = 4.5 cover the external surface of zeolites and the thin layer of silica is effective in controlling the pore opening size ithout changing the internal structure. It ill be interesting to see the effect of temperature on the activity and selectivity of the pore size regulated ieolite. Fig. 2 depicts the results of the runs in hich reaction temperature as varied from 275 to 375 C. This rise in reaction temperature resulted increase in ethyltoluene yield, hile the trends got reversed in the temperature range C. Only a slight decrease in paraethyltoluene selectivity as observed in this temperature range. This may be due to the fact that initially ith.enhancement in temperature alkylation activity increased, hile the dealkylation of the alkylated product back to toluene and ethylene favoured at temperatures above 325 C. When the alkylation as carried out over a long period it as found that the catalyst stability as affected due to rapid deactivation (Fig. 3). The run as carried out for 15 h. The zeolite shoed signs of deactivation, toluene conversion decreased from 15 to 3% and para-ethyltoluene selectivity enhanced from 95 to 100%. This behaviour can be ascribed to steady coke formation inside the zeolite channels, hi<;h affects the conversion and selectivity. The coke formation seems to be initiated by the ethanol, hich, being a smaller molecule than toluene, diffuses faster inside the channels of ZSM-5 and gets converted to ethylene. Further, this ethylene oligomerizes to coke precursor. This is in line ith an earlier study hich reveals that internal coke forms by the polymerization of small 0lefinsl5 This phenomenon as observed for all the temperatures in the range C. This can be attributed to the narroing of the pore opening entrance. o o TIME ON STREAM, h Fig. 3- Time on stream behaviour of silylated ZSM-5 zeolite. Temperature = 350'C, Toluene/Etha.nol mol ratio = 4, WHSV= 3 h-i, Wi % Si02 deposited = 4.5 Because of pore opening alteration the reactant molecules are retained for a longer time inside the channel and coke is formed on the stronger acidic sites. This coke gros further and covers the active sites and catalytic activity drastically reduces. This argument seems to be more logical in the context of finding that higher the extent of pore opening size alteration, faster the rate of'deactivation during alkylation.,several attempts ere made to stabilize the catalytic activity of the silynated zeolite for long hours of on stream. Among the different techniques tried for this purpose, only controlled coking of the catalyst orked very effectively. The controlled coking is carried out by disproportionating ethylbenzene hich goes through dealkylation/realkylation mechanism. Some of the dealkylated ethyl group produces ethylene hich goes to coke forming reactions. This coke is deposited on the higher strength acid sites inside the zeolite channel and the acidity of the zeolite is modulated. An XPS study of coke on ZSM-5 provides a strong evidence that coke fills ZSM-5 channel system initially before any significant amount of coke is formed on the external surfacel6 It has been reported that hen the pore system is constituted of interconnecting channels ithout cavities, like in ZSM-5, deactivation occurs initially through limitation of the access to sites then through blockage of the access to the sites of the channel intersection in hich the coke molecules are situated 17. Lastly, at high coke content, coke molecules located on the outer surface of crystallites can block access to the sites completely. It may be noted here that the main difference

4 322 INDIAN 1. CHEM. TECHNOL., NOVEMBER 1996 ell Z ~ ell o... I. S1LYNATEO lsh-s 2. COKED SH.YNATED lsh-5 15 TOLUENE CONY. 100 u... C z ~10 ll P-[T SELECT ~ u 96 ll TEMPERATURE,oC Fig. 4- TPD patterns of silylated ZSM-5 zeolite ith and ithout coking. 5 beteen ethylene produced from ethylbenzene and, oletins and ethylene formed from methanol is the quantity. During ethylbenzene conversion over ZSM-5 zeolite, the major reaction is disproportionation via dealkylationlrealkylation mechanism, only a very small quantity of ethylene is produced from the - C2Hs group detached from the aromatic ring, hile the reactions of methanol over ZSM-5 zeolite formed considerable quantities of ethylene and other oletins. Accordingly, the pattern of the acid sites coking is different in the above to cases. The change in acidity of the zeolite after coking is established from the ammonia TPD patterns (Fig. 4). The TPD profile consisted of a high and a lo temperature peak correspnding to strong and eak acid sites. Coking changes the distribution of lo and high temperature peaks. The total acidity of the zeolite before and after coking, in terms of millimoles of ammonia per gram of zeolite as and , respectively. In the present case, the catalyst as coked to achieve 98.5% selectivity for para-diethylbenzene. Soon after this, toluene and ethanol vapour mixture as passed over the catalyst to carry out the alkylation. The pre-coked catalyst shoed a steady acitivity in terms of consistent toluene conversion, 13.8 and 96.1% selectivity for paraethyltoluene. The selectivity for para-ethyltolulene as less than para-diethylbenzene hich is due to the difference in kinetic diameters of these molecules]. Hence, coking suppresses some of the active sites leading to the decrease in total acidity and stabilizes the catalytic activity of the zeolite. The results of the alkylation run over the zeolite catalyst hich is modified by controlled coking is reported in Fig. 5. Unlike the unmodified zeolite the coked catalyst shoed a steady activity throughout stream. the run carried out for 40 h on TIME" ON STREAM, h Fig. 5- Time on stream behaviour of coked silylated.zsm-5 zeolite. Temperature = 350 C, ToluenelEthanol mol ratio = 4, WHSV = 3 h - I, t% Si02 deposited = 4.5, The zeolite, having more strong acid sites, favour cracking! coking reactions, the zeolite possessing more of medium strength acid sites is desired for alkylation, hile the zeolite containing more acid sites of loer strength is preferred for isomerization reaction. Controlled coke formation during ethylbenzene conversion covers the sites of higher strength. Due to this, the coked zeolite contains more acid sites of the strength required for alkylation and exhibits enhanced stability. Conclusions Silynation of medium pore ZSM-5 zeolite by chemical vapour deposition technique alters the pore opening size. The modification enhanced shape selectivity during toluene ethylation. Due to the narroing of pore opening size the catalyst stability got affected ith the result a deactivating trend as observed. Controlled coking of the zeolite after silynation and before alkylation stabilized the catalyst activity. A combination of pore size alteration and controlled coking is a good technique to enhance ZSM-5 zeolite. the shape selectivity of References I Chen N Y, Garood W E & Dyer F G, in Shape Selective Catalysts, if!. Industrial Applications (Marcel Decker Inc.. NeYork), Bhat Y S & Halgeri A B, Appl Cat, 101 (1993) Bhat Y S & Halgeri A B, Stud Surf Sci Cata/, 83 (1994) Bhat Y S, Das J & Halgeri A B, J Catal, 155 (1995) 154., I ~I

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