Alkylation of toluene with methanol IIl:Para-selectivity on modified ZSM-5 zeolites S. Cavallaro Istiluto di Chimica Indush~ale - U~dz,ersita' di Messina, Via Dei Verdi, 98100 Messina, Italy and L. Pino, P. Tsiakaras and N. Giordano lstilut9 CNR-TAE, Via Salita S. Lucia Sopra Contesse, 39, 98013 Pistunina-Messi,a, Italy, and B.S. Rao On leave from Physical Chemish 3, Divi.~io,, National Chemical Laboratory, Pune 411008, India (Received 17 March 1986; revised I September 1986) Selectivity factors S have been derived for three xylene isomers obtained by methylation of toluene over ZSM-5 zeolites modified by the introduction of Fe, B and P ions. In the experimental conditions the reaction proceeds with formation of o+p xylene up to onset of isomerization. Our results and previously published independent data show a close correlation of S and the guest electronegativity, indicating a role of the acidity in the reaction mechanism. The para-selectivity is attributed to the selective adsorption of o+m xylenes on the acidic reaction site; the less polar p-isomer is rapidly desorbed. Keywords: Alkylation; methylation; para-selectivity; ZSM-5; toluene; methanol; xylene INTRODUCTION Alkylation of toluene with methanol over zeolites may lead either to ethylbenzene/styrene or xylenes as a result of insertion of the methyl-group in the chain or in the ring I. The selectivity varies from basic zeolites (e.g. potassium-exchanged A and Y zeolites'-') to acidic zeolites (H-zeolites, ZSM-5, etc.:~-~), as discussed in previous work'. In particular, methyhlti,)n of t,,luene over acidic catalysts with xylene formation has exten- sively been investigated :~-'i. The reaction pr,,ceeds in three consecutive steps according t,,: Tol + MeOH ~3, 0 + p-xylene ~-~ 0 + m + p-xylene-~ tri- + tetramethylbenzene At high MeOH concentrations it is possible that isomerization of 0- and p-xylenes formed in the first reaction step proceeds with a bimolecuhlr mechanism accordin[j to the following scheme of parallel reactions':: p-xylene 1,2,4-Me3CoH:~ m-xylene o-xylene 1,3,5-Me3C6H3 1,2,3- M e3Ct~H:~ 0144-2449/87/050408-04 $03.00 © 1987 Butterworth & Co. (Publishers) Ltd 409 In this way formation of 1,2,4- and 1,2,3,- trimethylbenzene (TMB) is rationalized. In turn, m-xylene is fornled by dealkylation of TMB up to equilibrium conditions. At low ('H:~OH concentra- tions (with respect to toluene) and at relatively low contact times, both the bimolecular mechanism and the last step of the consecutive mechanism are blocked, and methylation over ZSM-5 catalysts no longer forms polyalkylates (TMB). According to Kaeding et al. :~ the reaction proceeds selectively to the commercially more interesting p-xylene isomer using ZSM-5 catalysts modified by B, P, etc. In this case the tlaermodynamic equilibrium conditions :j (meta ~ 50%, ortho- ~ para- "~ 25%) are overcome, and leading to an excess of the para- isomer. The resuh has been explained hy the different steric encumbrance of the three isomers (m > 0 > p) and the microporosity of the modified catalysts :~. However, diffusion in the micropores may not be the only reason for the high para-selectivity of shape-selective zeolites. Bhat l° suggests that claemisorption of va- rious isomers and/or toluene can also play a signifi- cant role in the performance of the catalyst. It is reasoned that ions such as P and B in the micropores of the ZSM-5 decrease the free diameter of" the catalyst and favour diffusion of the sterically less requiring p-isomer. The rapid release of p-xylene shifts the thermodynamic equilibrium towards this isomer and enhances the para-selectivity. In order to evaluate the effect of chemical adsorp- tion of the reaction products on toluene methylation, we have investigated the correlation between selectiv- ity and electronegativity of the ions by modifying the ZSM-5 lattice. In fact, as the catalytic reaction occurs