On the mechanism of toluene disproportionation in a zeolite environment

Article abstract of DOI:10.1021/ja00142a007

A study of the mechanism of toluene disproportionation catalyzed by the zeolite ZSM-5 has been conducted using toluene-α,α,α-d₃. The primary mechanism is a benzylic cation chain reaction. This conclusion is deduced from the fate of the deuterium label, observation of a kinetic induction period as well as a primary kinetic isotope effect, and rate acceleration upon addition of small amounts of diphenylmethane during reaction. The magnitude of the kinetic isotope effect, 1.88 at 250 °C, implies the existence of a nearly symmetrical, linear configuration of C-H-C in the transition state of the hydride transfer 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.