Activity and selectivity of zeolites MCM-22 and MCM-58 in the alkylation of toluene with propylene

The influence of the pore architecture of zeolites MCM-22 and MCM-58 on their catalytic activity and selectivity in the gas-phase alkylation of toluene with propylene has been investigated. The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-membered-ring pores in the structure of this zeolite, no enhanced selectivity to p-cymene is observed. Moreover, also n-propyltoluenes, which are usually formed over three-dimensional 10-membered-ring zeolites (e.g., ZSM-5 and ZSM-11) via bimolecular transalkylation/isomerization reactions of cymenes with toluene, are only formed to a negligible extent. This has been rationalized by the assumption that most of the reactions occurring in this system take place on acid sites at or close to the external surface. This is supported by the FTIR-spectroscopic observation that p-cymene can reach only about one half of the bridging OH groups, while the smaller d(3)-acetonitrile reaches/covers virtually all remaining Bronsted and Lewis acid sites. Over zeolite MCM-58, relatively high selectivities to n-propyltoluenes were observed, which has been ascribed to the peculiar undulating 12-membered-ring channel system of this zeolite. It is proposed that the 1,2-p-ditolylpropane-type transition state required for the formation of n-propyltoluenes is preferentially located in positions similar to those which are occupied by the N-benzyl-1,4-diazabicyclo[2.2.2]octane cations used as templates for the synthesis of zeolite MCM-58. (C) 2002 Elsevier Science Inc. All rights reserved.