Are the Brønsted acid sites in amorphous silica–alumina bridging?

Abstract

Competing models exist to explain the differences in the activity of zeolites and amorphous silica–aluminas. Some postulate that silica–alumina contains dilute zeolitic bridging acid sites, while others favor a pseudo-bridging silanol model. We employed a selective isotope labeling strategy to assess the existence of Si–O(H)–Al bonds using NMR-based distance measurements.