HZSM-5-catalyzed isomerization of [1- 13C]toluene

Abstract

There is ample evidence to support the view that xylene isomerization follows a simple 1,2-methyl migration scheme with an aluminum chloride catalyst and that the contribution of 1,3 migrations is immeasurably small. When an even smaller-pore heterogeneous catalyst, a LaY zeolite, is used for xylene isomerization, methyl shifts other than 1,2 migrations appear to make a significantly larger contribution to the overall conversion than for the amorphous, larger-pore silica-alumina catalyst. When the even smaller-pore zeolite ZSM-5 was used, the products had essentially the equilibrium composition. Thus, the results with heterogeneous catalysts could be viewed as supporting, on the one hand, a diffusion disguise in which a series reaction mechanism of 1,2-methyl migrations appear experimentally as a coupled three-component reaction network or, on the other hand, to involve an isomerization mechanism that proceeds by a transalkylation mechanism. It was therefore of interest to learn whether ZSM-5 catalyzed isomerizations followed the typical mechanism of a series of 1,2-methyl shifts. Toluene, since it has a considerably smaller kinetic diameter than o- or m-xylene, should have the least diffusional resistance of any aromatic compound suitable for isomerization studies. Consequently, toluene in which the ring was labeled with /sup 13/C was converted with a portionmore » of the same ZSM-5 used by Collins et al. in their xylene isomerization studies in which the products were near their equilibrium value. 13 references.« less