Liquid-phase regioselective benzylation of bromobenzene and other aromatics over microporous zeolites

Abstract

Various solid acids were investigated for the liquid-phase benzylation of aromatics to industrially important substituted diphenylmethanes. Large-pore zeolites, particularly H-beta (BEA), were active and regioselective for the parasubstituted product in the benzylation of arenes. With deactivated aromatics such as bromobenzene, the selectivity towards the para-substituted product decreased with reaction time. The decline in selectivity is attributed to the partial blockage of the zeolite channels by high molecular weight deposits, leaving only non-selective catalytic sites at the outside of the zeolite crystallites. These deposits result from self-condensation of benzyl chloride, and their formation can be greatly suppressed when the concentration of benzyl chloride in the reaction mixture is kept low. By adding benzyl chloride continuously to the reaction mixture, a selectivity towards the para-substituted product of more than 80% was achieved. Mesoporous materials such as montmorillonite K10 and H-Al-MCM-41 were active but not regioselective for the reaction. The pores of these materials are too large to impose restraints on the geometry of the products or the transition state. H-ZSM-5 with smaller pores was less active and showed no regioselectivity, because the active sites inside the channel system of this medium-pore zeolite are less accessible to the reactants, and most of the reaction is catalyzed at the outer surface. © 2002 Elsevier Science Inc. All rights reserved.