Characterization of WOx/CeO2 catalysts and their reactivity in the isomerization of hexane

Two series of WOx–CeO2 catalysts with a W content between 4.0 and 12 wt% prepared by coprecipitation (CW-PP) and impregnation (CW-IMP) of CeO2 were investigated with the aim of examining the role of tungsten and the support in acid-catalyzed reactions. The catalysts were characterized with several in situ techniques (XRD, Raman, XPS, and NH3-DRIFT), and nitrogen adsorption at 77 K. XRD results pointed to the preferential formation of crystalline Ce2(WO4)3 which was completed at 1173 K. Crystalline WO3 was absent in both series of CW-IMP and -PP catalysts. Raman spectra of CW-PP catalysts suggested the possible formation of WO3 only in the catalyst with 11.9 wt% W calcined at 1173 K. For the CW-IMP catalysts, the bands assigned to amorphous WO3 were clearly seen in samples calcined at 1173 K, with their intensity depending on the W content. The treatment of the catalysts under hydrogen resulted in a decrease of the characteristic Raman band of ceria, and changes of the Ce2(WO4)3 phase. NH3-DRIFT spectra showed the presence of both Lewis and Brønsted acid sites, the strength of which increased with calcination temperature. The effect of hydrogen in improving the acidity consists either of a degradation of the Ce2(WO4)3 phase with the formation of small WO3 domains or of some new arrangements like Lindqvist polyanions, both species being susceptible to an easy transformation of Lewis to Brønsted acid sites. Isomerization of hexane over WOx/CeO2 catalysts produced mono- and di-alkylated hydrocarbons and methylcyclopentane. Increasing tungsten contents enhanced the activity and selectivity to methylcyclopentane. The presence of hydrogen improved the stability of the catalysts but decreased their activity, and resulted in important changes of the selectivity: the mono- and di-alkylated hydrocarbons predominated.