Acid and catalytic properties of nonstoichiometric aluminum borates

Abstract

This paper details an investigation on the structure-acidity-catalytic activity relationship in aluminum borate mixed oxides. This aluminum borate mixed oxide is amorphous, is refractory, and exhibits properties which are dependent on the exact stoichiometry of the material. A series of catalysts with B Al ratios ranging from 0 to 1 was prepared and characterized with respect to surface area, pore volume, and thermal stability. Nuclear magnetic resonance spectroscopy of 11B and 27Al was used to determine the local structure. Two boron species were identified: a tetrahedral BO 4 and a trigonal BO 3. The relative amounts of the two boron species were dependent on the B Al stoichiometry. Three aluminum species were identified: a tetrahedral AlO 4, an octahedral AlO 6, and an AlO 4 with four B next-nearest neighbors. The strength and concentration of acid sites present on the surface of the aluminum borates were determined using adsorbed indicators. The presence and coordination of boron were seen to have a dramatic effect on the acidity of these materials with the acidity being closely related to the relative concentration of BO 4. 2-Propanol dehydration was used as a probe reaction to measure the acidity. Results from this reaction have shown these materials to indeed be highly acidic and catalytically active.