Hydrolysis of zeolite framework aluminum and its impact on acid catalyzed alkane reactions

Abstract

Short exposure of HZSM-5 zeolites to water vapor at high temperatures (mild steaming) enhances the rates of alkane cracking and dehydrogenation as well as the rates of exchange between H2 and D2. Solid state 27Al MAS NMR results show that mild steaming leads to metastable aluminum oxide species, hypothesized to be partially framework-bound. Combining double quantum magic-angle spinning nuclear magnetic resonance experiments and kinetic analysis of cracking reactions allows us to conclude that the presence of these aluminum oxide clusters in the vicinity of Brønsted acid sites leads to an increase in the activation entropies during alkane cracking, induced by increasing steric constraints. Prolonged steaming results, in contrast, in the extraction of framework Al, which subsequently forms aggregated extra-framework Al oxide species partly blocking Brønsted acid sites and partly deposited at the outer surface of the crystallites.