An FTIR study of the surface acidity of USY zeolites:: Comparison of CO, CD3CN, and C5H5N probe molecules

A comparison of three basic probe molecules of varying strength, namely pyridine, acetonitrile, and carbon monoxide, was made, and their suitability toward the characterization of surface acidity in zeolitic materials was assessed. Two test samples were employed: both ultrastable Y (USY) zeolites but with differing silica:alumina ratios and degree of extraframework material. Carbon monoxide proved to be the most selective probe, not only being able to differentiate Bronsted (BAS) from Lewis acid sites (LAS), but BAS with varying acid strength as well. However, its weak basicity did not allow interaction with OH groups situated within the sodalite cages, this only being achieved with the two more basic probes. OH groups interacting with extraframework material also failed to form H-bonds with CO. Both pyridine and acetonitrile interacted with all OH groups, and pyridine was protonated by the most acidic bridged species. Although LAS could be told apart from BAS, differentiation of different types of LAS and BAS among themselves was not directly possible with these two molecules.