Acid catalytic properties of reduced tungsten and niobium-tungsten oxides

• Structure of (Nb)WO3 modified by varying the calcination temperature.
• NH3 and NH4+ stabilize hexagonal WO3.
• Intermediate calcination temperature and Pd reduction: (NH4)0.33−x(H3O)yWO3−z.
• (NH4)0.33−x(H3O)yWO3−z strong acid component in reduced Pd/(Nb)WOx.
• Pd-loaded oxides are highly acidic and active hydroisomerization catalysts.

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.

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