Bielanski Adam

Małecka A.

Hungarian

University of Szeged

CATALYTIC CUMENE CRACKING ON H-Z3M-5 TYPE ZEOLITES A. BIELANSKI a'b, A. MAI.ECKA b Department of Chemistry, Jagiellonian University, Cracow, Poland (a) Institute of Catalysis and Surface Chemistry of the Polish Academy of Sciences, Cracow, Poland (b) ABSTRACT The effect of cation exchange degree of H-ZSM-5 type zeolite the mode of its decationation and its granulation on the zeolite catalytic activity in the case of cumene cracking was investigated. Besides ben-zene and propylene the main products of this reaction butenes and pen-tenes as well as n-propylbenzene appeared as the products of side reactions. Owing to the shape selectivity no saturated hydrocarbons (except traces of butane) nor ethylbenzene and toluene were forming which are typical products of side reactions on HNaY zeolites. Gene-rally the H-ZSM-5 zeolites decationated by the substitution of Na+ by NH^ are more active than those decationated by the treatment with hydro-chloric acid solution exhibiting comparable content of Brttnsted acid sites.However this effect was definitely more distinct in the case of highly exchanged zeolites (cation exchange degree about 90 %) . x INTRODUCTION Despite the fact that cumene cracking belongs to the frequently used catalytic test reactions it has not been more thouroughly studied on the ZSM-5 type zeolites. Nayak and coll. [1] described a series of experiments in which cumene conversion was determined at 673 K on a series of H-ZSM-5 zeolites differing by the cation exchange degree. The present authors [2] compared the behaviour of HNaY and H-ZSM-5 type zeolites as the catalysts of cumene cracking and stated distinct diffe-rences in the yield and the composition of the side reactions products. The aim of the present investigation was to study the catalytic cracking of cumene on H-ZSM-5 type zeolites of different cation exchange degree the acid properties of which were well characterized. Simultaneously the effects of the decationation mode and the granulation of the samples were investigated. EXPERIMENTAL The samples of ZSM-5 type zeolite (Si/Al molar ratio equal to 47) were obtained at the Institute of Industrial Chemistry, Warsaw, as described in [3]. The decationation was carried out by J. Datka either via substitution of Na+ ions with NH^ ( symbol NZ} or by the treatment with hydrochloric acid solution ( symbol HZ), The samples with the highest cation exchange degree were also obtained in the case of which both methods of decationation were applied one after another but in different sequences ( symbols HNZ and NHZ). The acid properties of the same samples were investigated in this laboratory by J. Datka and Tufc-nik and are described by them in full detail in their paper presented at this conference [4] and also in ref. [5]and[6], The main data charac terizing the samples are given in the Table. The numbers in the symbols: of samples give the percentage of cation exchange. Strong BrBnsted acid centres are formed by OH groups with the stretching frequency 3609 cm-^ Catalytic tests were carried out using either the samples not granu-lated containing crystallites of ~ l^n or the samples obtained by pressing the same fine crystalline powder, crushing the pellets and selecting the sieve fraction of grains 90-190/tm. Table Sample Apparent Concentration of acid Rate constant of cumene and ca- activation center per unit cell* cracking min-^ tion ex-change CilCig J k3 mol"1 Brönsted Lewis K670 K630 degree % t S cn strong weak fc 1 rj c ß o> powder granul. NZ-31 62 39 0 .29 0 .30 0 .59 0 .10 70 100 31 59 NZ-43 54 44 0 .47 0 .36 0 .83 0 .15 • 144 145 85 88 NZ-83 52 30 0 .85 0 .39 1.24 0 .18 238 218 ' 135***' 152 HZ-42 58 49 0 .38 0 .35 0 .73 0.14 143 179 70xX 115 HZ-81 35 38 0 .82 0 .26 1.08 0 .18 129 205 77 136 HZ-88 30 37 1.03 0 .30 1.33 0 .17 168 242 126 167XX HNZ-90 38 35 0 .96 0 .22 1.18 0 .10 115 209 72 140 HNZ-88 55 36 1.04 0 .21 1.25 0 .13 183 278 115 196 x - Determined by J. Datka and E. Tufcnik £"5,6] ; xx - Extrapolated j xxx - Interpolated. Catalytic tests were carried out in a pulse microreactor connected on line with a gas chromatograph. Helium was used as a carrier gas into the flow of which (1200 cnrh ) the doses of 3 /tl of cumene were intro-duced. The samples 0.02 g of catalyst diluted with 0.1 g of powdered quartz were activated "in situ" for 2 h at 720 K in the stream of helium. Analytically pure cumene (POCh Gliwice, Poland) was kept over metallic sodium and distilled before experiments. A Giede 18.6 Gas Chromatograph with programmed heating was used for the analysis of the reaction pro-ducts applying a chromatographic column (length 2.5 m, diameter 4 mm) filled with Chromosorb W covered with 14 % of silicon oil DC 200. Katha-rometric detection was applied. The reaction products were identified using Mass Spectrometer LKB 9000 s. Two se'ries of catalytic tests were carried out. In constant tem-perature tests the dependence of activity and selectivity on the number of cumene pulses was investigated. In polythermic tests executed usually between 570 and 670 K at each temperature only one pulse was introduced in order to avoid the effects of ageing. The rate constant of cumene conversion was calculated assuming 1st order of the reaction, RESULTS AMD THEIR DISCUSSION Reaction products. Besides the main reaction products benzene and propylene some amounts of side reactions products were also detec-ted : butenes, pentenes and n-propylbenzene but only traces of butane. The olefins were the products of the transformations of primary pro-pylene as verified by the separate experiments in which pure propylene was introduced on the catalyst. The fact that olefins were the predomi-nant product of propylene secondary reactions and only traces of alka-nes (butane) were present which on the other hand are the main products of propylene secondary reactions on NaHY zeolites [2] is a typical shape selective effect. Presumably it is connected with different pro-perties of propylene oligomer forming in both cases. In ZSM-5 zeolites? the narrow pores allow only the formation of linear oligomer which, as it iŝ  supposed, decomposes into olefins of various chain length while the tireedimensional oligomer forming in large cavities of faujasite type zeolites: disproportionates into the hydrogen rich (alkanes) and hydrogen deficient precursor of the coke [7]. Only one aromatic product of side reactions was detected i.e. the n-propylbenzene which is the product of cumene isomerization. According to [8] the latter reaction occurs with the formation of monomolecular 391 transition complex. No ethylbenzene nor toluene were formed which were observed in the case of NaHY zeolites. They are considered to be formed via bimolecular transition complexes [8] which obviously can not be formed in the narrow pores of H-ZSM-5 zeolite. Ageing of the catalysts. The ageing of the catalysts was follo-wed at 600 and 670 K. Pig. 1 (full line) shows that the deactivation of the catalyst with the increasing number of cumene pulses was slow being more distinct at lower temperature and in the case of the catalyst ex-hibiting smaller cation exchange degree. The fact that deactivation was slower at higher temperatures can be explained by a more rapid de-sorption of precursors of the coke deactivating the catalyst. No influ-ence of the granulation on the ageing of the catalyst (dashed lines on Pig. 1) was observed. Fig. 1. Conversion of cumene as the function of the number of cumene pulses in the case of HNZ-90 and HZ-42 samples non-granulated (full line) and granulated (dashed line) . Fig. 2 shows on the example of HZ-1 sample the effect of the de-activation on the composition of reaction products. It is seen that the mole % of benzene in the initial pulses exceeded the sum of mole % of propylene unchanged and propylene transformed into butenes. The deficit of propylene must be due at least partially to its irreversible ad-sorption resulting in the slow catalyst deactivation but also some amounts of it may be only slowly desorbed out of the catalyst and not be included in the pulse of products introduced on the chromatographic column. Similarily as it was observed in [2] catalyst deactivation results in the diminishing of the amounts of side reactions products s increasing the content of propylene, decreasing the content of other olefins. Simultaneously the relative content of benzene with respect to 392 the total content of propylene is decreasing and slowly approaching the molar ratio 1s1 thus indicating that the deficit of propylene is growing less and the catalyst is becoming more selective in respect to the main cracking reaction. This indicates that the secondary transfor-mations of propylene occur on the acid centres - presumably the stron-gest ones - which are poisoned at first. Pig. 2. Composition of the reaction products on the HZ-42 non-granulated at 670 and 600 K as the function of the number of cu-mene pulses (mol %), a benzene • propylene o butenes (expressed as the number of moles of converted propylene') x n-propylbenzene The effect of cation exchange degree and the mode of decate-nation. Pig» 3 shows the dependence of rate constant on the temperature as obtained in polythermic tests: carried out with not granulated samples. When comparing the activity of various samples the following statements can be done : 1. Within both series of zeolites NZ and HZ the catalytic activity is increasing with the increasing cation exchange degree and also with the acidity as given by the concentration BrBnsted acid centres. 2. The effect of the decationation mode can be followed by comparing samples HZ-42 and NZ-43 with cation exchange degree 42 resp. 43 % and also on the samples HZ-88, HNZ-90 and NHZ-88 with 88-90 % of cation ex-change. In the further case the activities of both samples are similar within the whole range of temperature but the sample NZ-43 with higher number of pulses number of pulses 393 200-c 'e c o o o 8. 100 NZ-83 NHZ-88 HNZ-90 NZ-31 sfeo ' sfeo ' sbo ' 6*20 ' ¿ 0 ' 6^0 ' Temperature, K Fig. 3. Rate constant of the non-granulated catalysts as the function of temperature content of BrOnsted acid centres was always more active. In the latter case the discrepancies) in the activity are much more pronounced and the sequence of activity HNZ-90 NHZ-88 HZ-88 corresponds well to the sequence of the total concentration of BrBnsted acid centres. However it should be observed that the sample NZ-83 with 1.24 BrBnsted acid centres per unit cell practically identical with that of the sample NHZ-88 is more active than all the other samples. The apparent activation energies of catalytic reaction of NZ-83 and NHZ-88 are not very much differing but the activation energy in the case of the latter sample is somewhat lower and the observed differences may be attributed to the higher acti-vity of catalytically active BrBnsted acid centres. However the second possibility is that in NHZ-88 sample the acid centres although accessible to pyridine molecules in the course of determination of their concen-tration, are only partially accessible to the cumene molecules. This 394 might he due to some structural changes in NHZ-88 sample. 3. The apparent activation energy of cumene conversion decreased within both series of samples NZ and HZ with increasing cation exchange degree. With the exception of NHZ-88 sample this rule seems to be also approximately valid for the whole set of the results. The decrease of activation energy with the decreasing cation exchange degree was also observed in some other cases as e.g. in the case of butene isomer!-sation on NaHY zeolites [9]. There are several possible reasons of such effect. First of all the increasing number of acid centres playing the role of catalytically active sites results in much faster reaction in the zeolite pores and in such situation diffusion becomes a factor influencing the overall rate of the process. Secondly the in-creased acid strength of the BrBnsted acid centres may lower the ener-gy needed for the formation of transition complex. Finally the possible increase of the adsorption heat of the substrate may cause the decrease in the apparent energy of activation even if the true activation energy remains unchanged. The effect of sample granulation, is shown in Fig. 1 in which full lines correspond to the not granulated samples and the dashed lines to the granulated ones. It is seen that in the case of HZ-42 sample exhibiting relatively low activity the effect of granulation is not pronounced. On the other hand In the case of highly active HNZ-90 sample the conversion on the granulated catalyst is definitely higher. This can be attributed to the fact that the reagents penetrating into the pores between particular crystallites remain for a longer period of time in the contact with the catalyst and the effect is similar to that obtained by the prolongation of contact time. The effect''diffusion in the intercrystalline pores of the granules is the obvious reason ahy granulating the samples lowers the apparent activation energy as it is seen from the data presented in the Table. REFERENCES 1. Nayak V.S., Choudry V.R., J. Catal., 81,, 26 (1983). 2. Bielariski A., Maifecka A., to be published. 3. PRL Patent 82967 (19731. 4. Datka J., Tuinik E., paper presented at this conference. 5. Datka J., Tuznlk E., in print. 6. Datka, J., J.C.S. Faraday I, 2§, 2437 (1980). 7. Datka, J., in print. 8. Best, B., Wojciechowski, B.W., J. Catal., 42 (1977K 9. Bielariski, A., Datka, J., Drelinkiewicz, A., Kaïecka, A., Acta Phys. et Chem. Szeged., 89 (1978), Proc. Symposium on Zeolites, Szeged 1978. 3 9 6 

Catalytic cymene cracking on H-ZSM-5 type zeolites

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Catalytic cymene cracking on H-ZSM-5 type zeolites

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