ON THE MECHANISM OF TOLUENE DISPROPORTIONATION IN A ZEOLITE ENVIRONMENT

A study of the mechanism of toluene disproportionation catalyzed by th e zeolite ZSM-5 has been conducted using toluene-alpha,alpha,alpha-d(3 ). The primary mechanism is a benzylic cation chain reaction. This con clusion is deduced from the fate of the deuterium label, observation o f a kinetic induction period as well as a primary kinetic isotope effe ct, and rate acceleration upon addition of small amounts of diphenylme thane during reaction. The magnitude of the kinetic isotope effect, 1. 88 at 250 degrees C, implies the existence of a nearly symmetrical, li near configuration of C-H-C in the transition state of the hydride tra nsfer step between the toluene methyl and the benzylic cation. Such a model of the transition state fits well inside the straight channel of ZSM-5, based on steric considerations alone.