Acid sites in mesoporous Al-SBA-15 material as revealed by solid-state NMR spectroscopy

Abstract

The nature of acid sites in Al-SBA-15 materials with various Si/Al ratios was studied by various NMR techniques including 27Al, 29Si, 1H, 31P MAS (magic-angle spinning), and some double-resonance methods such as 1H/ 27Al transfer of population in double resonance (TRAPDOR). XRD and 27Al NMR results indicate that Al has been successfully incorporated into the framework of SBA-15 materials by post-synthesis method. The bridging hydroxyl groups (SiOHAl) associated with Brönsted acid sites are invisible either in 1H MAS or in 1H/ 27Al TRAPDOR spectra of the dehydrated Al-SBA-15 materials. However, after the adsorption of basic probe molecules, such as trimethylphosphine oxide (TMPO), trimethylphosphine (TMP) and acetone, 31P MAS and 13C CP MAS NMR experiments strongly support the presence of Brönsted acid sites. NMR results suggest that Brönsted acid sites in dehydrated Al-SBA-15 materials are in the form of terminal silanol groups in the vicinity of aluminum atoms that can be induced to generate the bridging hydroxyl groups by the adsorbed basic probe molecules. Although the acid strength of Al-SBA-15 materials detected by different probe molecules is different, it is comparable to that of microporous zeolites. It is noteworthy that no Lewis acid site can be found in the Al-SBA-15 materials.