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

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. The identity of the active sites in amorphous silica-alumina has been a hotly debated topic. Using selective isotope labeling and quantitative NMR distance measurements we confirmed that the active sites are indeed zeolitic bridging Bronsted sites.

Automated summary

The study confirmed that the active sites in amorphous silica-alumina are zeolitic bridging Bronsted sites, using selective isotope labeling and quantitative NMR distance measurements.