Acid catalytic properties of reduced tungsten and niobium-tungsten oxides

Abstract

Tungsten oxide and mixed niobium–tungsten oxides were prepared by calcination of re-precipitated tungsten and niobium precursors. These materials were characterized by N2 physisorption, XRD, XPS, UV–vis and UV Raman spectroscopy and H2-TPR. With increasing calcination temperature the oxides were seen to transform from a mixture of h-WO3 and HATB (T<400°C) to predominantly h-WO3 (400°C<T<470°C) and m-WO3 (T>470°C). Ammonium ion and ammonia are essential to retain the hexagonal structure of the intermediate phase. The reducibility of the various samples was linked to their structure. In-situ XRD point to formation of β-W in the presence of Pd when the precursor was deeply reduced. When the precursor predominantly contained h-WO3, (NH4)0.33−x(H3O)yWO3−z was obtained upon reduction of tungsten and mixed niobium–tungsten oxide materials. These exhibited good performance in the bifunctional hydroisomerization of n-alkanes, outperforming amorphous silica–alumina and tungstated zirconia catalysts.