A quantum-chemical approach to the generation of solid acidity in composite metal oxides

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Various microscopic surface structures of silica–alumina and silica–magnesia which can generate Brønsted acidity are discussed via molecular-orbital calculations using cluster models, together with indices for Brønsted acidity. Mechanisms explaining the origin of the Brønsted acidity of the solid surface are proposed in which a Lewis-acid site on the surface is responsible for Brønsted acidity by interacting with a residual OH group nearby. A special structure consisting of a surface 3-coordinated Si and an adjacent 4-coordinated Al under the surface is shown to be capable of generating particularly strong acidity