IR and NMR studies of mesoporous alumina and related aluminosilicates

The synthesis of mesoporous alumina and alummosilicates of Si/Al from 0.2 to 10 was done, and the status of Si and Al was followed by Si-29 and Al-27 MAS NMR. The properties of acid sites were studied by following the IR spectra of CO, N-2, pyridine and ammonia adsorbed. The mesoporous alumina contained only surface weak Lewis acid sites, the insertion of Si resulted in loss on such Lewis sites and in the formation of Bronsted sites. The Al-OH bands decreased (the Al-IV-OH band related to tetrahedral Al species diminished the first) and Si-OH-Al one (IR band at 3605 cm(-1))-similar as in the case of zeolites appeared as well as Si-OH groups (IR band at 3740 cm(-1)). The alummosilicates of higher Si content contained also strong Lewis acid sites formed by the dehydroxylation of Si-OH-Al groups (process similar as in zeolites), which occurred at the activation. The concentration of both Lewis and Bronsted sites was determined by quantitative IR studies of ammonia and pyridine adsorption. The concentration of Bronsted acid sites increased with Si content due to the formation of acidic Si-OH-Al, and at high Si/Al, it increases also due to the growing surface area. The concentration of Lewis sites decreased with Si content due to the transformation of surface Al species, which are potential Lewis sites into Si-OH-Al and also to dilution of the material with Si species. The acid strength of Lewis sites increased with Si content due to increasing contribution of strongly acidic sites produced by dehydroxylation. The mesoporous aluminosilicate (Si/Al = 10) studied in this research showed higher concentration and higher acid strength of Bronsted sites than HMCM-41 of the same Si/Al-it may be an potential catalyst in fine chemistry reactions. (c) 2005 Elsevier B.V. All rights reserved.