IR and NMR studies of mesoporous alumina and related aluminosilicates

Abstract

The synthesis of mesoporous alumina and aluminosilicates of Si/Al from 0.2 to 10 was done, and the status of Si and Al was followed by 29Si and 27Al 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 Brønsted 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 aluminosilicates 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 Brønsted sites was determined by quantitative IR studies of ammonia and pyridine adsorption. The concentration of Brønsted 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 Brønsted sites than HMCM-41 of the same Si/Al—it may be an potential catalyst in fine chemistry reactions.