Alkylation of toluene with methanol on zeolites. The role of electronegativity on the chain or ring alkylation N. Giordano and L. P i n o
Istituto CNR-TAE, Via Salita S. Lucia Sopra Contesse, 39, 98013 Pistunina-Messina, Italy and S. Cavallaro and P. V i t a r e l l i
Istituto di Chimica lndustriale dell' Universita di Messina, Via Dei Verdi, 98100 Messina, Italy and B.S. Rao
Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India (Received 11 November 1985)
Catalysed alkylation of toluene with methanol on zeolites proceeds in two different ways, depending upon the nature of the zeolite, and gives rise to ethylbenzene--styrene and/or xylenes. The results indicate that the activity and selectivities are dictated by the acidity and electronegativity of the zeolites with the most electropositive ones producing ethylbenzene and styrene and the most electronegative catalysing formation of xylenes. Keywords: Catalysis; acidity; toluene alkylation
INTRODUCTION Alkylation of toluene and benzene with methanol using zeolites has been reported in numerous papers I-2°. Formation of xylenes is found mainly on a'ci~lic zeolites I-:¢'6'9"1:~'17'1~'='°, and eth l b e n z e n e a n d " " i -Y'3J48 " styrene on alkah-exchanged faujas tes ' ' . As these products are industrially important chemicals, it is not surprising that extensive work has been carried out, with particular emphasis on p-xylene selectivity. In this study, we propose a correlation between the alkylation activity and intermediate electronegativity, using experimental data obtained tinder uniform conditions. Experiments were carried out using zeolites of various electronegativities and structural type, ranging from Y to ZSM-5. A survey of the literature shows a variety of other resuhs in terms of conversions and selectivities, under varying experimental conditions. Zeolitic properties have been related to Sanderson's electronegativity21 on several occasions 22-26. As a unifying principle in zeolite chemistry one of the most important attributes of intermediate electronegativity is its structural insensitivity which allows comparison of structurally dlssmular zeohtes- . The validity of the treatment can be seen in the correlation of electronegativity and catalytic activity of zeolites in a complex reaction, such as the conversion of methanol to hydrocarbons 2s. The present work is an extension of these concepts to another zeolitecatalysed reaction, namely alkylation of toluene with methanol. .
EXPERIMENTAL NaY, NaX, NaM, CsX, CsM, KX and HZSM-5 (AI), 0144-2449/87/020131-04 $03.00 © 1987 Butterworth & Co. (Publishers) Ltd
HZSM-5 (Fe), HZSM-5 (B) binderless zeolites were used throughout in this study in 10-22 mesh powder form. NaY, NaX, and NaM were commercially available, whereas Cs and K ion-exchanged zeolites were prepared according to standard procedures. HZSM-5 and Fe- and B-silicates of the HZSM-5 type (denoted as (AI), (Fe) and (B), respectively), were prepared according to the procedure outlined in Ref. 29. Methanol and toluene were C. Erba reagent grades (99% purity).
Apparatus and procedure A catalytic reactor (stainless steel, 15 cm long, 0.5 cm inner diameter) was heated by a wire-wound furnace of 10 cm constant zone (500 ° + 3°C). Reactants in the correct mole ratios were fed by vaporizing at two different controlled temperatures (__ 0.1°C) and by carrying the vapours to a pre-heater (at 120°_+2°C) by means of a carrier gas (helium). Binderless, 10-22 mesh, catalyst pellets were placed in the microreactor and a thermocouple was kept at the centre of the catalyst bed. Catalyst pretreatment was carried out by heating at 500°C in air and helium (1 h each). Experiments were carried out at 400°_+2°C with a toluene to methanol mole ratio of 3.16, WHSV = 10.5 h -I and at a He flow of 85 cm 3 rain -~. Reactants and products were analysed on a gas chromatographic column (6 ft long, I/8 inch external diameter) loaded with 1.75% SP 200, 1.75% Bentone-34 supported on 100-120 Supelcoport using a flame ionization detector. Yields of the various products have been calculated on the basis of the product analysis, in relation to the toluene/methanol ratio, on the assumption that toluene does not undergo cracking but just de-
ZEOLITES, 1987, Vol 7, March
Alkylation of toluene with methanol: N. Giordano et al.
alkylates and that methanol is the only 'coked' product. The sum of the moles of aromatic products corresponds to a theoretical CHsOH concentration, as calculated from the initial toluene/CH~OH ratio. Methanol conversion reported in Tables I and 2 represents the conversion of methanol to products and coke, and is calculated as: CMcOH = 1 --
where ICHsOHI.... is the molar methanol fi'action (g M~OH) analysed on exit and ICH3OHt~,I,.' is the molar methanol fraction calculated as a function of the initial toluene/methanol ratio (3.16). Conversion of toluene has been calculated on the basis of the product analysis as: CTol. = 1 -- ~.rol. XBe,z. where XB..... is the sum of the molar fractions of the aromatic compounds. The yields of various (poly)alkylation products are expressed as: }"i
This procedure accounts approximately fi)r the coking of methanol. Oil the basis of the above reasonable assumptions, the difference between ICHsOHI,-,m. and the total number of methyl groups in alkylation products represents the moles of CHsOH transTable 1
formed to coke which are not detectable by gas chromatography. Zeolites were classified in terms of product yields and intermediate Sanderson's electronegativity (Sire.) and hydrogen charge (OH+); the latter values were calculated according to Ho(:evar'~5 (see Equations 1 and 13 in Ref. 25) on the basis of the reported stoichiometric composition.
Figure I shows the yields of alkylation of toluene with methanol as a function of the intermediate Sanderson electronegativity for data taken from literature, obtained under a variety of experimental reaction conditions (see Table I). From Figure 1 it can be seen that the reaction proceeds to either chain alkylation products (ethylbenzene and styrene) or ring alkylation products (xylenes and polyalkylates). Alkylation of the toluene chain takes place only on highly basic catalysts (e.g. alkaline metal exchanged zeolite X and Y), whereas acid sites favour ring alkylation8. As shown in Figure 1, the fi)rmation of ethylbenzene and/or styrene over zeolites X and Y modified by alkaline metals has been noticed by various authors sA'7'~, hoh et al. 7 stress the importance of the acidity of alkali and alkaline metal exchanged zeolite X catalysts on alkylation of p-xylene with methanol.
Experimental values taken from the literature
1 2 3 4 6 8 9 13 17 18 20
225 225 425 410 700 425 400-550 450 250 225-350 400
2 2 6 5 9 6 15-1.5 2 2 2 2
WHSV or W/F 120 120 25 950 1.91 40 5-17 1 120 120 0.4
(W/F) (W/F) (W/F) (W/F) (WHSV) (W/F) (WHSV) (LHSV) (W/F) (W/F) (WHSV)
Sint Figure 1 Alkylation of toluene with methanol, results taken from literature sources. (O) Alkylation to xylenes; (D) alkylation to ethylbenzene and styrene. Numbers on graph refer to the reference used; for data see Table 1
Table 2 Experimental results of this study. Time on stream, 5 min; helium flow rate, 85.0 ml min-1; temperature, 400°+_2°C;toluene/ MeOH ratio, 3.16 (mole/mole); MeOH + toluene total flow, 12 ml h 1
no. 1 2 3 4 5 6 7 8
NaX KX CsX NaY CsM ZSM-(AI) ZSM-(Fe) ZSM-(B)
NaesAIseSiloeO3s4 1.23 3.260 KesAlesSiloeO3s4 1.23 3.051 CsasAlesSilosO3~ 1.23 2.894 Nasc,AlseSi13603~ 2.43 3.549 CseAleSi4oOs6 5.00 3.641 Hs.3AIs.3Siso.7Ols2 17.11 4.223 Hs.3Fes.3Siso.70192 17.11 b 4.224 Hs.3Bs.3Sise.7Ols2 17.115 4.244
=On toluene basis (mole %) ~3i/Fe or Si/B ratio =Traces
ZEOLITES, 1987, Vol 7, March
C C Yields a MeOH Toluene (%) (%) Ethylbenzene p-Xylene m-Xylene o-Xylene StyreneTMB
-0.074 57.8 -0.127 36.6 -0.167 95.0 - 0 . 0 0 0 40.0 0.023 44.6 0.172 94.0 0.172 100.0 0.177 99.4
0.8 5.5 9.0 1.5 2.4 20.6 14.1 27.8
0.2 2.0 7.3 c 0.4 0.0 0.0 0.0
0.0 0.1 0.0 c 0.3 12.6 6.9 17.3
0.0 0.6 0.2 c 0.8 4.4 3.2 3.7
0.0 0.1 0.2 c 0.4 2.4 3.0 3.8
0.5 1.5 0.6 c 0.1 0.0 0.0 0.0
0.0 0.4 0.4 c 0.1 0.9 0.9 2.9
Alkylation o f toluene with m e t h a n o l : N. Giordano et al.
Unland et al. 4 show, spectroscopically, that the electrostatic potential and geometry of the structural supercages determine the selectivity of zeolite catalysts e x c h a n g e d with alkaline metals and partially substituted with boron. However, other u t h o r s 1'2"5"6'9'I~;'12'1"~'18-20 indicate the selective formation of ring alkylation products over acidic zeolites (see Figure I). The reaction occurs on relatively acidic ZSM-5 zeolites 5'6'9'1°'12"13 or X and Y zeolites which have been rendered even more acidic by cation exchange 16'I° or dealumination 19. According to Yashima et al.l catalytic methylation of toluene over exchanged zeolite Y inc~-eases in the order: monovalent < HY < REY. Moreover, the selectivity highly favours p-xylene formation, which differs greatly from thermodynamic equilibrium of the three xylene isomers (para ~ ortho ~ 25%, meta ~ 50%). Other authors2'3 indicate, qualitatively, a dependence of the conversion and selectivity on the Br6nsted acidity of the catalyst. Upon high temperature calcination of zeolites, the p-xylene selectivity increases in the H-form catalysts exchanged with Mn 2; however, addition of small amounts of HCI strongly inhibits para-selectivity 2. Alkylation of toluene over zeolite HY and HZSM-5, impreKnated with MgO, has been studied by Yashima et al." between 300 ° and 700°C. Due to neutralization of the Br6nsted acid sites of the non-pretreated catalysts, HZSM-5 calcined at 950°C, or charged with MgO, exhibits strong para-selectivity (>80%). Kaeding et al. "5 attribute the para-selectivity in B and P impregnated ZSM-5 catalysts to the zeolite porosity. Similarly, Young et al. ° attribute paraselectivity of B, P and Mg modified HZSM-5 zeolites to reduction of the pore dimensions of the original zeolites. This is in accordance with Wei l°, who concludes that the diffusivity of the p-xylene isomer in ZSM-5 is several orders of magnitude higher than that of other isomers. Barisano et al. 12 report a high para-selectivity (>90%) over ZSM-5 modified by K and Mg. Methylation of toluene over zeolite Fu- 1 takes place exclusively on the external catalyst surface 13, while Zielinski and Sarbat Is reveal a difference between the orthoand para-selectivity of zeolites X and Y exchanged with rare-earths. Whereas faujasite X prefers orthosubstitution, Y leads to higher para-selectivity. Also, in 12
[ ]~ \
Figure 3 Conversion of methanol (©) and toluene (17) versus intermediate electronegativity, S,,t.. Numbers on graph indicate the run number; for data see Table 2
this case, Br6nsted acidity is responsible for the high catalytic activity. The experimental results mentioned above, from the literature, as shown in Figure 1, have been extended by our additional data obtained using the zeolites in Table 2. For the catalytic performance of these zeolites, see Figures 2--4.
DISCUSSION As seen in Figure 1, the yields of the methylation of toluene over a wide variety of zeolites obtained in non-uniform experimental conditions (Table I), vary with the intermediate Sanderson electronegativity in a parabolic mode. Chain alkylation (with formation of ethylbenzene and styrene) occurs for Si,~. < 3.6, whereas ring alkylation takes place for higher values. This is in agreement with previous observations ~'s that basic sites are a prerequisite for the side chain alkylation of toluene, whereas benzene ring alkylation takes place on acidic sites. Ethylbenzene and styrene are only formed over alkali metal cation zeolites for which a m p h o t e r i c oxide b e h a v i o u r has been predicted 24 . The fact that no catalytic reaction is observed for basic A-type zeolites 4 is due to the inability of toluene to reach the active sites located inside zeolite cavities with 4/~ pore openings (kinetic diameter 6.1 /~). Some scattered data may be due to variations in experimental conditions or badlydefined zeolite composition.
Figure 2 Alkylation of toluene with methanol, this study. (C)) Alkylation to xylenes; (I-1) alkylation to ethylbenzene and styrene. NUmbers on graph indicate the run number; for data see Table 2
• Sint Figure 4 Trimethylbenzene (TMB) yield in the polyallo/lation of toluene versus intermediate electronegativity, Sint.. Numbers on graph indicate the run number; for data see Table 2
ZEOLITES. 1987. Vol 7. March
Alkylation of toluene with methanol: N. Giordano et al.
T h e observed correlation between electronegativity and total catalytic yields is confirmed by our results (Figure 2) using the zeolites listed in Table 2. It should be noted that methanol and toluene conversions vary with Sire. in a similar fashion (Figure 3), irrespective of the particular experimental conditions. Highest conversions are found at extreme values of Sint. (2.8 and 4.2). Alkali metal ion faujasites were used as catalysts for low Sint. values, and pentasil zeolites were used for the higher electronegativity values (see Table 2). T h e transition from alkali-exchanged zeolites to acidic ones occurred at Sint. ~ 3.6 (Figure 3). This point marks the change from chain to ring alkylation. It is of interest to note that the mean electronegativity of the toluene molecule has the same value. This can be explained by the fact that both ring and chain aikylation occur by initial adsorption of toluene on the catalytic substrate. According to Mortier'-'', the formation of a chemical bond is accompanied by a shift in electric charge from the more basic to the more acidic centre. T h e chemical bond is weakest Ibr equal electronegativity of substrate and adsorbent. Also, the yields in polyalkylates (trimethylbenzene, TMB), which never exceeded ~ 3% in our experimental conditions, vary with Si,,,. (see Figure 4) with enhanced production of TMB at high Si~. values, in accordance with the formation of the monoalkylates. T h e low TMB concentration and increase in yield ['or the more acidic zeolites, show that polyalkylation follows a consecutive route with respect to primary alkylation and that T M B is more easily formed from xylenes than from ethlybenzene or styrene 5. Using an excess of toluene (toluene/ methanol = 3.16), formation of polyalkylates is minimized 5.
CONCLUSIONS Our findings show that the behaviour of zeolites with different structure, Si/AI ratios, nature of the exchanged cation etc., in the methylation reaction of toluene can be attributed to the electronegativity concept. On more acidic zeolites (as expressed by Sanderson's intermediate electronegativity, Si,t.), benzene ring alkylation with xylene formation occurs whereas more basic catalysts are selective towards chain alkylation products. Minimum catalytic activity is noticed when Si,,t. is equal to the value of the
ZEOLITES, 1987, Vol 7, March
reactant (toluene). T h e resuhs strongly support the relevance of electronegativity in catalytic reactions. REFERENCES 1 Yashima, T., Ahmad, H., Yamazaki, K., Katsuta, M. and Hara, N. J. Catal. 1970, 16, 273 2 Yashima, T., Katsuta, M., Ahmad, H., Yamazaki, K. and Hara, N. J. Catal. 1970, 17, 151 3 Yashima, T., Sato, K., Hayasaka, T. and Hara, N. J. Cata/. 1972, 26, 303 4 Unland, M.L. and Barker, G.E., 'Catalysis of organic reactions', (Ed. W.R. Moser) Marcel Dekker, New York, USA, 1980, p. 51 5 Kaeding, W.W., Chu, C., Young, L.B., Weinstein, B. and Butter, S.A.J. Catal. 1981, 67, 159 6 Yashima, T., Sakaguchi, Y. and Namba, S. Stud. Surf. Sci. Catal. Pt. A, New Horiz. Catal. 1981, 7, 739 7 Itoh, H., Hattori, T., Suzuki, K., Miyamoto, A. and Murakami, Y. J. CataL 1981, 72, 170 8 Itoh, H., Miyamoto, A. and Murakami, Y. J. Catal. 1980, 64, 284 9 Young, L.B., Butter, S.A. and Kaeding, W.W.J. CataL 1982, 76, 418 10 Wei, J. J. CataL 1982, 76, 433 11 Beltrame, P., Beltrame, P.L., Carniti, P. and Forni, L. in 'Proceedings of the Fourth Italo-Czechoslovak Symposium on Catalysis', Univercitt&, Torino, Italy, 1983, p. 83 12 Barisano, E., Ciambelli, P., De Simone, V., Bagnasco, G., Bianchi, R. and Russo, G. in 'Proceedings of the Fourth Italo-Czechoslovak Symposium on Catalysis', Torino, Italy, 1983, p. 91 13 Parker, D.G. Appl. Catal. 1984, 9, 53 14 Satio, Y. and Tsuchiya, S. J. Catal. 1976, 42, 288 15 Tanabe, K., Ichikawa, I., Ikeda, H. and Hattori, H. J. Res. Inst. Catal. Hokkaido Univ. 1972, 19, 185 16 Coughlan, B., Carrol, W.W. and Nunan, J. Chem. Ind. 1981, 10, 363 17 Itoh, H., Hattori, T., Suzuki, K. and Murakami, Y. J. Catal. 1983, 79, 21 18 Zielinski, S. and Sarbak, Z. React. Kinet. Catal. Lett. 1981, 16, 119 19 Naum, N., Ababi, V., Mihaila, G. and Popovici, E. Chim. Ing. Chim. 1980, 26, 61 20 Bhat, S.G.T.J. Catal. 1982, 75, 196 21 Sanderson, R.T. 'Chemical Bonds and Bond Energy', 2nd Edn., Academic Press, N.Y., USA, 1976 22 Mortier, J.W.J. Catal. 1978, 55, 138 23 Jacobs, P.A., Mortier, W.J. and Uytterhoeven, J.B.J. Inorg. Nucl. Chem. 1978, 40, 1919 24 Barthomeuf, D. J. Phys. Chem. 1984, 88(1), 42 25 Ho6evar, S. and Drzaj, B. J. Catal. 1982, 73, 205 26 Ghosh, A.K. and Curthoys, G. J. CataL 1984, 86, 454 27 Dwyer, J., Fitch, F.R. and Nkang, E.E.J. Phys. Chem. 1983, 87, 5402 28 Giordano, N., Vitarelli, P., Cavallaro, S., Ottan~, R. and Rao, B.S.J. Catal., submitted for publication 29 Kulkarni, S.B., Shiralkar, V.P., Kotasthane, A.N., Borade, R.B. and Ratnasamy, P. Zeolites 1982, 2, 313