Acidity Characterization of Amorphous Silica–Alumina

Abstract

Surface characterization of amorphous silica–alumina (ASA) by COads IR, pyridineads IR, alkylamine temperature-programmed desorption (TPD), Cs+ and Cu(EDA)22+ exchange, 1H NMR, and m-xylene isomerization points to the presence of a broad range of Bronsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Bronsted acid sites (BAS), typically at concentrations lower than 10 μmol/g. The general procedure for alkylamine TPD, which probes both Bronsted and Lewis acidity, is modified to increase the selectivity to strong Bronsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 μmol/g, which can be quantified by COads IR. Cu(EDA)22+ exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with...