Hydrothermal synthesis of a layered-type W–Ti–O mixed metal oxide and its solid acid activity

Abstract

A layered-type W–Ti–O mixed oxide was synthesized by hydrothermal synthesis from an aqueous solution of ammonium metatungstate and titanium sulfate. To avoid the formation of titania, oxalic acid was used as a reductant. Optimized synthesis led to rod-like particles comprised of MO₆ (M = W, Ti) octahedra connected in a corner-sharing fashion in the c-direction and in the form of micropore-containing {W₆O₂₁} units in the a–b plane. The surface area, acidity and acid catalytic activity (alkylation) increased with the amount of the layered-type W–Ti–O phase. Strong Brønsted acid sites formed due to the thermal release of ammonia from the uncalcined precursor. Calcination at 400 °C led to the highest acidity and alkylation activity. Alkylation of benzyl alcohol and toluene led to heavy product formation due to over-alkylation of the product. The selectivity to the mono-alkylated product was improved by the addition of water, which competes with the selectively formed products for adsorption on the acid sites. FT-IR measurements showed that the layered-type W–Ti–O possesses Brønsted acid sites and at least two different Lewis acid sites. The stronger Lewis acid sites can be converted into Brønsted acid sites in the presence of water, and the weaker Lewis acid sites functioned in the presence of water. This water tolerance of Lewis acid sites is an important characteristic of layered-type W–Ti–O, as it allows the bifunctional catalyst to convert 1,3-DHA into lactic acid in water.