Does hierarchical structure affect the shape selectivity of zeolites? Example of transformation of n-hexane in hydroisomerization

Abstract

The large mesoporous surfaces of hierarchical zeolites without confinement of the active sites leads to the question of whether the shape selectivity of these zeolites is essentially altered. The hydroisomerization of n-hexane to mono- and dibranched C6 isomers was carried out as a model reaction governed by the shape selectivity to obtain insight into the effects of the secondary mesoporous structure. A series of micro- and micromesoporous ZSM-5, MOR, ∗BEA, and Y zeolites with regular microporosity or well-developed mesoporosity were prepared by synthesis and postsynthesis treatment, and their selectivity was investigated. We demonstrate that isomerization is exclusively controlled by concurrent interaction of the reactant with the Brønsted site and the inner walls of the zeolite channels, irrespective of the mesoporosity. The reaction is facilitated only by a combination of dispersive forces in the inert void and acid sites in the confined environment. The shape selectivity of micromesoporous zeolites is thus fully preserved.