The application of paramagnetic complexes of probe molecules for the investigation of the Lewis acidity of aluminas

The Lewis acidity of aluminas formed by dehydration of aluminium hydroxides is studied by means of the EPR method, using paramagnetic complexes of indicators (nitrogen oxide, 2,2,6,6-tetramethylpiperidine-N-oxyl, anthraquinone) with coordinatively unsaturated aluminum cations at the solid surface. The nature, concentration and mutual disposition of Lewis acid sites on the oxide surfaces are determined. A correlation is shown to exist between the EPR data and the IR spectroscopy data of adsorbed carbon monoxide. The role of the alumina crystal structure in the formation of Lewis acid sites is discussed.