Beltrame, P.

Carniti, P.

Forni, L.

Zuretti, G.

English

Beltrame P.

Carniti P.

Forni L.

Zuretti G.

University of Szeged

TOLUENE AND XYLENE DISPROPORTIONATION OVER HZSM-5 CRYSTALS OF DIFFERENT SIZE P.BELTRAME, P.L.BELTRAME, P.CARNITI, L.FORNI, G.ZURETTI Dipartimento di Chimica fisica ed Elettrochimica, Universita di Milano, Via Golgi 19, 20133 Milano, Italy ABSTRACT Toluene, xylene (isomer equilibrium mixture) or 1,2,4-trimethyl-benzene (1,2,4-TMB) were fed to a fixed-bed reactor in a flow of H2 or N2, at 300 to 360°C. The catalyst was HZSM-5, employed as mixed crystals or one of three separate fractions from 2.5 to 7.5 pm average size. A few runs were also carried out after; poisoning the catalyst outer sur-face with 4-methylquinoline. The catalyst crystal size showed no effect on the disproportion-ation of toluene and a slight effect on that of xylene. Xylene gave also demethylation, which was the main reaction of 1,2,4-TMB. Poison-ing showed a marked effect on selectivity in the case of toluene and on fractional conversion for xylene disproportionation, and deeply af-fected the reactions of 1,2,4-TMB. INTRODUCTION Toluene disproportionation on unmodified HZSM-5 catalysts pro-ceeds usually to an equilibrium mixture of xylene isomers [1]. The para--selectivity could be increased up to 35% by reaction over "large cry-stals" of catalyst (3 ym size) at 550°C [2]. More impressive results were achieved by using ZSM-5 catalysts modified with Mg, P or B [1,3], or partially deactivated by coking [4]. Xylene reactions over HZSM-5 were studied at 250-300°C, finding a strong selectivity for 1,2,4-trimethylbenzene in the disproportion-ation, which was accompanied by isomerization [5]. Dealkylation was al-so found at 370-400°C [6J. We undertook a study of toluene and xylene disproportionation on HZSM-5 crystals of different size, looking for shape-selectivity ef-fects and aiming at discriminating between channel diffusion resistance and transition state steric inhibition as their cause [7]. In order to get further information on catalyst behaviour, a few runs were also carried out on 1,2,4-trimethylbenzene. The catalyst employed was the same zeolite, of high and stable activity, previously used for kinetic measurements on toluene dispro-portionation [8]. However, for the present work, besides using "mixed crystals" (M), three samples with average crystal size 7.5 pm (A), 4 vim (B) and 2.5 ym (C) were separated and tested. The following reactions were carried out on each catalyst sample in the order given: a) toluene disproportionation (carrier H2) in stan-dard kinetic conditions; b) toluene disproportionation (carrier H2) at higher conversion; c) reactions of xylenes, fed as an equilibrium mix-ture of isomers (carrier H2); d) (only on catalysts B and M) reactions of 1,2,4-trimethylbetfizene (carrier H2); e) toluene disproportionation (prolonged runs with carrier N2). For some reactions over catalyst M, poisoning experiments were also effected, by using 4-methylquinoline, which should inhibit only the acid sites on the outer surface of HZSM-5 [9,10], because of its molecular size. EXPERIMENTAL Materials. All reactants and standards were commercial "pure re-agents", which gave single peaks by GC analysis, apart from 1,2,4-tri-methylbenzene, that contained ca.1% of 1,3,5-isomer. Catalyst. HZSM-5 was prepared by a standard procedure [11]; it was the same binderless catalyst (SiC^/AljO^ wt.ratio 15.0) described elsewhere [8] as HZSM-5(HA). Samples A, B and C were separated from it by elutriation in deionized water, leaving behind a small amount of smaller particles. Crystal sizes were evaluated on SÊ I micrographs. All samples were pressed, crushed and sieved to 40-80 mesh before use. Elemental analysis by A.A. revealed differences between the samples, the content of aluminium (as AljOj) being 4.63% for A and for B, 5.91% for C and 5.06% for the mixed crystals. However, surface acidity, as measured by titration with n-butylamine in anhydrous solvent [12] (pK ¿1.5), was found scarcely dependent on A1 content: A, 1.0; B, 1.1; C, 1.0 meq/g. When using the bulky tributylamine, the corresponding acidity of the mixed crystals was titrated as 0.075 meq/g. Apparatus and procedure. A tubular flow reactor with a fixed bed of catalyst (0.5 g), immersed in a thermostatted bath, was employed [8]. Reactants were fed by a glass syringe with PTFE piston and with a car-rier to hydrocarbon ratio of 5, at atmospheric pressure (partial pres-1 cr\ sure of the hydrocarbon in the feed =17 kPa). The reactor effluent was passed through a condensing trap, kept at ca.-17°C> and liquid products analyzed by GC, using a standard 1:1 Didecylphthalate-Bentone 34 coliimn 4.5m long. Selective poisoning experiments were performed following a repor-ted procedure [9], applied to a series of runs on the same catalyst sample. Before each reaction a single dose of 4-methylquinoline was in-jected (first run, 15 ill; successive runs, 5 yl, to compensate for a possible partial desorption of the poison). Reactions were carried out in the order a, c, d, a. RESULTS Products were usually analyzed by gas-chromatography. A more det-ailed identification, based on GC-MS analyses, was carried out only for runs on the "mixed crystals" catalyst. The products from a feed of toluene (T) were mainly benzene (B) and the three xylenes (DMB). Small amounts of C9 hydrocarbons were also found, i.e. ethyltoluenes (EMB), not resolved by the standard GC anal-ysis, and trimethylbenzenes (TMB), which were resolved and found to be predominantly represented by the 1,2,4-isomer. The reaction of xylenes, effected by feeding a mixture close to the isomeric equilibrium, gave toluene and trimethylbenzenes (mostly 1,2,4-TMB) as main products, but also benzene, ethyltoluenes and C10 hydrocarbons in minor amounts. A GC-MS analysis, with a column longer than the standard one, showed that the three isomers of EMB were all present, in the abundance order characteristic of equilibrium (m>p>o). As to the C10 hydrocarbons, they were mainly recognized by GC-MS in two peaks, with M.W. 134 and 132, respectively. The former peak correspon-ded neither to a tetramethylbenzene nor to a diethylbenzene nor to a propyltoluene; it could reasonably be an ethyldimethylbenzene. The lat-ter peak had a retention time just in the region of the tetramethyl-benzenes (TTMB), but it was ascertained that TTMB's were absent or negligible; the relevant compound could be a vinyldimethylbenzene. When feeding 1,2,4-TMB, the reaction products were mainly xylenes and toluene, with lesser amounts of benzene and C10 hydrocarbons, and a partial isomerization to 1,2,3-TMB. The latter was detected as a shoulder on \the peak of the reactant, but sometimes it was not enough for a quantitative integration; in these cases it was only approxim-ately estimated. As to the C10 hydrocarbons, the GC-MS analysis showed that they were a mixture of isomeric tetramethylbenzenes. On the basis of the products identified, the chemical behaviour of the system was summarized as follows: disproportlonation 2 T — * B + DMB (1) 2 DMB — • T + C9 (2) 2 C9 — • DMB + C10 (3) demethylation isomerization DMB —p- T + gas (4) 1,2,4-TMB —> DMB + gas (5) 1,2,4-TMB 1,2,3-TMB (6) Demethylation reactions, when xylenes or 1,2,4-TMB were fed to the reactor, were evidenced by the sum of the side-chain carbons in the products falling short of the theoretical. Fractional conversions for disproportlonation (y^g) > demethyl-ation (Yjjgj,,) » and isomerization (y^so) were therefore defined as fol-lows, for different feed compositions (x̂  = molar fraction of compon-ent i) : Feed: toluene Feed: xylenes Feed: 1,2,4-TMB ^dis = *B + ZXDMB + 2 L X C 9 ( 7 ) Ydis = 2 ( . E X C 9 + 2 E X C 1 0 ) ( 8 ) ^dem " 2 " XT -2IXDMB " 3 Z X C 9 " 4 E X C 1 0 ( 9 ) ^iso = X123-TMB <10) ydis = 2 L X C 1 0 < 1 1 > ^dem = 3 ~ XT ~ 2IXDMB - 3 E X C 9 " 4 Z X C 1 0 ( 1 2 ) Reaction selectivities were also evaluated. The isomer distrib-ution within the xylene fraction of products was expressed as SQ, Sm and S , defined as S =x„ /Ex.^. and analogously for and S . For p O O—LirLD L/PU) «1 p the reaction of toluene, a further selectivity S D M B was given by the ratio Zx,,^/ (Zx„irD+2Ex_Q) , which represents the fraction of DMB produced DMB DMB L y by reaction (1) and not entered into further disproportionation. Anal-ogously, for a xylene feed, the ratio Sc9 [=Zxcg/ (Zxc9+2Zxc^0)] represents the fraction of C9 hydrocarbons produced by reaction (2) and not fur-ther reacted. For the reaction of xylenes, the isomer distribution within the C9 fraction was firstly evaluated as the ratio ZxTMB/Zxc9 (the differ-352 ence to 1 gives the relative weight of the EMB product), and then as the isomer fractions within the TMB product, essentially as s^24 (the ratio x124_TMB/ExTMB). For comparison, it can be mentioned that the equilibrium frac-tions of isomeric xylenes at 300-360°C are calculated as: ortho, 0.22; meta, 0.54; para, 0.24. The equilibrium fractions of TMB1s at 350°C are similarly calculated as: 1,2,3-TMB, 0.07; 1,2,4-TMB, 0.66; 1,3,5--TMB, 0.27 [13]. Experimental results, worked out as described above, are presen-ted in Tables 1-4 and in Figure 1. Table 1 Disproportionation of toluene in standard runs (carrier H2; T=300°C; W/F°=164 g h/mol). Average values of measurements.3 Catalyst (size) Ydis SDMB So Sm SP A (7.5 pm) 0.056 0.95 0.22 0.54 0.24 B (4 pm) 0.064 0.9)5 0.21 0.53 0.26 C (2.5 pm) 0.050 0.96 0.20 0.55 0.25 M (mixed) 0.052 0.95 0.21 0.53 0.26 Mb 0.038 0.95 0.09 0.20 0.71 a C9 hydrocarbons were exclusively EMB, within the detection limits; k sampling 6 h after poisoning by 4-Me-quinoline(15 pi). Table 2 Disproportionation of toluene at higher conversion (carrier H2; T=360°C; W/F°=504 g h/mol). Average values of measurements.3 Catalyst Ydis SDMB So Sm SP A 0.219 0.98 0.22 0.51 0.27 B 0.258 0.97 0.22 0.53 0.25 c 0.213 0.98 0.22 0.54 0.24 M 0.205 0.98 0.22 0.53 0.25 a C9 hydrocarbons were EMB and TMB in an approximate ratio of 2:3. Table 3 Reaction of xylenes3 (carrier H2; T=350°C; W/F°Mfi=94 g h/mol). Measurements after ca.24 h on-stream. Catalyst y d ± s y d e m Sc9 Z x ^ / E x ^ S 1 2 4 A 0.099 0.075. .,-0.94 0.98 1.00 B 0.108 0.082 0.92 0.98 1.00 c 0.143 0.063 0.94 0.98 0.97' M 0.134 0.051 0.94 ' 0.99 1.00 Mc 0.023 0.049 0.92 0.89 1.00 isomers in the feed: o, 0.22; m, 0.54; £, 0.24; unreacted xyl-enes: o, 0.22-0.24; m, 0.52-0.54; £, 0.24; b 1,2,3- and 1,3,5--TMB were both present; C after poisoning by 4-Me-quinoline (15+ 5 pi injected'30 and 3 h, respectively, before sampling). Table 4 Reaction of 1,2,4-trimethylbenzene (carrier H2; T=350°C; w/ftmb = 105 g h/mol). Average values of measurements. Catalyst ydis yiso ydem m B M M* 0.0030 0.0264 0.0753 0.20 0.52 0.28 0.0039 0.01-0.02 0.0721 0.21 0.51 0.28 0.0018 nil 0.0253 0.16 0.45 0.39 after poisoning by 4-Me-quinoline (15+5 pi injected 34 and 7h, respectively, before sampling. 0.10 Figure 1. Disproportionation of toluene in prolonged runs with carrier N2 ( T = 300°C; W/F°=164 g h/mol): y ^ (up-per plot) and Sp (lower plot) vs. time-on-stream. Catalyst sanples: *,A; o,B; A,C; X,M. Average values of Sj^ were in the range 0.97-0.98. 354 DISCUSSION According to the measured kinetics [8], for standard runs (a) on toluene a value 0.052 for and an equilibrium xylene isomer dis-tribution were expected. Results in Table 1 show that this was actual-ly found, within experimental error, not only for the mixed crystals of HZSM-5, but even for the largest particles (catalyst A). This evi-dences that channel diffusion resistance is negligible in this case and that the measured kinetics refers to the reaction under chemical control. Values of ydis/ Sq/ Sm and Sp appear independent of catalyst crystal size also for runs (b) to higher conversion (Table 2). Poisoning was effected on runs (a) by injecting a quantity of 4-methylquinoline exceeding by an order of magnitude the amount re-quired to saturate an HZSM-5 catalyst [9], although corresponding to only 1/5 of the theoretical to neutralize all medium-to-high strength (pK ¿1.5) acid sites, should the base penetrate the pores. The poison a effects on the toluene standard run over M catalyst (Table 1) were a slight decrease of y^g and a substantial change in the xylene selec-tivities, favouring p-xylene. The former effect proves that toluene disproportionation occurs mainly in the zeolite pores; the latter is very likely due to the fact that the usual isomerization of the prefer^ red primary product on the acid sites at the pore mouth is prevented by the poison. As to the C9 fraction, EMB was its only component for runs (a) and it was largely present also in runs (b). A side-chain attack is known to occUr on X and Y zeolites exchanged with large or medium size alkali cations, from Cs+ to Na+, for alkylations of toluene [14], xyl-ene [15] and pyridine [16] by methanol. Perhaps a similar mechanism operates in the present case, due to residual Na+ ions and favoured also by the smaller pore diameter of the zeolite. TMB, when present, was predominantly the 1,2,4-isomer. It can be noticed that SDMfi was close to unity, that is the further reaction of xylenes was a very restricted phenomenon. Xylene reactions (runs c; Table 3) did not include isomerization, because isomeric equilibrium was present in the feed and was preserved in the unreacted xylene output. The various catalyst samples, while showing slight differences in overall activity (ydis + Y^^, = 0.174, 0.190, 0.206 and 0.186 for A, B, C and M, respectively), presented more marked differences in the relative weight of the two reactions. De^ methylation gained more weight when the catalyst was in larger crystals 355 (A and B); on the contrary, y d i g increased in the order A<B<C. The ef-fect of 4-Me-quinoline was remarkable, the poison scarcely affecting demethylation but heavily depressing disproportionation. It can be suggested that demethylation occurs mainly within the catalyst pores and without intervention of significant diffusive resistance, while disproportionation, in the absence of poison, occurs partly on the outer surface and partly within the zeolite pores, in this case with significant channel diffusion resistance effects (see values of y ^ s ) • Disproportionation gave almost always 1,2,4-TMB as the only tri-methylbenzene, independently of the site of reaction; it seems there-fore that the predominance of this isomer, which is favoured by elec-tronic as well as by statistical factors, is not clear evidence of shape-selectivity. The fraction of EMB within C9's is a more interes-ting- parameter, since the figures in Table 3 (1-2% EMB on unpoisoned catalysts, 11% after poisoning) show that the rate of formation of EMB was little affected by poisoning, which instead markedly reduced the rate of TMB formation, indicating that the former reaction does not require those acid centres that catalyze the latter one. Surprisingly, TTMB was practically absent from the C10 fraction of runs (c). Apparently, reaction (3) occurs mainly to C9 molecules just formed within the pores, thus favouring side-chain attack, with the possible formation of ethyldimethylbenzene and vinyldimethylben-zene. In the mentioned cases of side-chain methylation [14-16], for-mation of both ethyl and vinyl side groups was common. The main reaction of 1,2,4-TMB (runs d; Table 4) was demethyl-ation. The small amount of disproportionation gave TTMB, as expected, and no EMB was found among the products. Isomerization resulted rather slow, so justifying the very high values of S^24 of runs (c) at the same temperature. Poisoning depressed demethylation and disproportion-ation, while preventing isomerization, to the point that the small im-purity of 1,3,5-TMB, present in the feed, was found (only in this case) as unreacted. Poisoning slightly favoured p-xylene. Results of selectively poisoned reactions have been reported up to an overall addition of 20 pi of 4-Me-quinoline and for samples taken up to 34 h after the first poison injection. Results obtained from runs (d) and (a) after further poison additions showed an abrupt de-crease of the catalytic activity, so comprimising the reliability of GC analyses. Prolonged reaction of toluene in N9 (runs e) gave similar re-356 suits for catalysts A, B and C (Figure 1). A "coking" effect was de-tected, giving decreasing values of y d i s and increasing values of S^ (up to 0.32) vs. time-on-stream. Interestingly, the activity of the mixed-crystal sample M was more constant than for samples A-C. This might be connected with slight structural alterations introduced by the elutriation procedure. REFERENCES 1. Kaeding, W.W., Chu, C., Young, L.B., Butter, S.A., J.Catal. £9, 392 (1981) . 2. Chen, N.Y., Kaeding, W.W., Dwyer, F.G., J.Amer.Chem.Soc. 101, 6783 (1979). 3. Young, L.B., Butter, S.A., Kaeding, W.W., J.Catal. 2§» 418 (1982). 4. Beltrame, P., Beltrame, P.L., Carniti, P., Forni, L., React.Kinet. Catal.Letters V), 213 (1982). 5. Collins, D.J., Medina, R.J., Davis, B.H., Can.J.Chem.Eng. 6M, 29 (1983) . 6. Nayak, V.S., Choudhary, V.R., Appl.Catal. 4, 333 (1982). 7. Haag, W.O., Lago, R.M., Weisz, P.B., Discuss.Farad.Soc. 72, 317 (1981). 8. Beltrame, P., Beltrame, P.L., Carniti, P., Forni, L., Zuretti, G., submitted to Zeolites. 9. Anderson, J.R., Foger, K., Mole, T., Rajadhyaksha, R.A., Sanders, J.V., J.Catal. 58, 114 (1979). 10. Yashima, T., Sakaguchi, Y., Namba, S., in "New Horizons in Catal-ysis" (Eds.T.Seiyama, K.Tanabe), Elsevier Sci.Publ.Co., Amsterdam, 1981, p.739. 11. Argauer, R.J., Landolt, G.R., U.S.Pat. 3 702 886 (1972). -12. Bertolacini, R.J., Anal.Chem. 35.» 5 9 9 (1963). / 13. Stull, D.R., Westrum, E.F.Jr, Sinke, G.C., "The Chemical Thermo-dynamics of Organic Compounds", Wiley Inc., New York, 1969. 14. Yashima, T., Sato, K., Hayasaka, T., Hara, N., J.Catal. £6, 303 (1972). 15. Itoh, H., Hattori, T., Suzuki, K., Murakami, Y., J.Catal. 79» 21 (1983) . 16. Kashiwagi, H., Enomoto, S., Chem.Pharm.Bull. 30, 404 (1982). 357 

Toulene and xylene disproportination over HZSM-5 crystals of different size

http://acta.bibl.u-szeged.hu/24287/1/phys_chem_031_fasc_001_002_349-357.pdf

Toulene and xylene disproportination over HZSM-5 crystals of different size

Abstract

Similar works

Full text

Available Versions

University of Szeged