Acid catalytic properties of reduced tungsten and niobium-tungsten oxides

Tungsten oxide and mixed niobium-tungsten oxides were prepared by calcination of re-precipitated tungsten and niobium precursors. These materials were characterized by N-2 physisorption, XRD, XPS, UV-vis and UV Raman spectroscopy and H-2-TPR. With increasing calcination temperature the oxides were seen to transform from a mixture of h-WO3 and HATB (T < 400 degrees C) to predominantly h-WO3 (400 degrees C < T < 470 degrees C) and m-WO3 (T > 470 degrees 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 beta-W in the presence of Pd when the precursor was deeply reduced. When the precursor predominantly contained h-WO3, (NH4)(0.33-x)(H3O)(y)WO(3-)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. (C) 2014 Elsevier B.V. All rights reserved.