Mesoporous MCM-41 aluminosilicates as model silica-alumina catalysts: spectroscopic characterization of the acidity

摘要

The synthesis procedure brings about a low threshold value for the surface concentration of Al species and renders MCM-41 a model system for the study of silicaalumina phases. On severely outgassed samples, two types of Lewis sites are present, differing in their protrusion from the surface, as well as Bronsted sites engaged in lateral H-bonding, evidenced by the interaction with NH_3 and CO. The conversion of Lewis sites into Bronsted ones by water adsorption shows that the Bronsted species are Si(OH)Al, typical of zeolites, and Si-Al(OH)-Si, found in dealuminated zeolites. The flexibility of the amorphous framework accounts for the differences with zeolitic aluminosilicates.