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

The nature of acid sites in Al-SBA-15 materials with various Si/Al ratios was studied by various NMR techniques including Al-27, Si-29, H-1, P-31 MAS (magic-angle spinning), and some double-resonance methods such as H-1/Al-27 transfer of population in double resonance (TRAPDOR). XRD and Al-27 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 Bronsted acid sites are invisible either in 1H MAS or in H-1/Al-27 TRAPDOR spectra of the dehydrated Al-SBA-15 materials. However, after the adsorption of basic probe molecules, such as trimethylphosphine oxide (TWO), trimethylphosphine (TMP) and acetone, P-31 MAS and C-13 CP MAS NMR experiments strongly support the presence of Bronsted acid sites. NMR results suggest that Bronsted 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. (c) 2006 Elsevier Inc. All rights reserved.