Preparation, Characterization, and Acidity of a Silica Gel/Tungsten Oxide Solid Acid

Solid acidity is measured by simultaneously solving data from a calorimetric titration and an adsorption isotherm measurement to provide the amount, equilibrium constant, and enthalpy for each acid site. The utility of this procedure, referred to as cal-ad, in the synthesis of solid acids is illustrared with the preparation and characterization of a new hydrated tungsten oxide−silica-supported acid catalyst, referred to as SG-W. Cal-ad shows that SG-W, prepared from WCl 6, is a stronger acid with more Bronsted sites per gram than tungsten oxide, tungstic acid, or silica-supported WO 3 prepared by thermal decomposition of ammonium tungstate (SG-WO 3 ). Surface studies show a much better dispersion of tungsten in SG-W than in SG-WO 3 . SG-W adds to a growing series of reproducible, characterized solid acids of varying strengths. A quantitative cal-ad series of solid acid strengths is given to provide a new scale for correlating acidity with catalytic activity and to establish acidity thresholds for different acid-catalyzed transformations. Solid acids with varying acidities are compared as catalysts for alcohol dehydration, light paraffin isomerization, and the alkylation of phenol with methanol to illustrate this approach.