Promotion of adsorptive and catalytic properties of zeolitic Brønsted acid sites by proximal extra-framework Si(OH)x groups

Steric confinement in zeolites influences the catalytic conversion of alkanes. For zeolitic Brønsted acid sites, proximate extra-framework species introduce additional confinement to the pore constraints, enhancing the catalytic activity of alkane cracking. Although extra-framework alumina has been the most studied, here we report the element-specific impact of silica species. By grafting extra-framework silica species close to Brønsted acid sites in H-ZSM-5 zeolite, the binding of bases like pyridine and amines is strengthened via van der Waals interactions with their aryl or alkyl chains. Brønsted acid sites close to extra-framework silica achieve a higher reaction rate of protolytic cracking of n-pentane via enthalpic (unlike entropic, as with extra-framework alumina) stabilization of the transition state by 24–51 kJ mol−1. The lower activation energy points to an earlier transition state than in the presence of extra-framework alumina, with a better stabilization of the carbonium ions in the transition state compared to the parent zeolite. Placing extra-framework aluminium species in the proximity of Brønsted acid sites is one of the most effective ways of tuning the energetics of zeolite-catalysed reactions. Here, using pentane cracking as an example, the authors show that grafting extra-framework silica species instead represents a valuable alternative way to modulate zeolite acitivty.