Catalysis on Pd/WO3 and Pd/WO2: II. Effect of Redox Treatments in Hexanes and Hexenes Re-forming Reactions

Abstract

Skeletal rearrangements of alkanes and alkenes were investigated on Pd/WO3 and Pd/WO2 catalysts following various activation treatments. Catalysts reduced at low temperature (350°C) are active and selective for isomerization of hydrocarbons. Palladium loses its intrinsic catalytic properties for isomerization but keeps partly its (de)hydrogenation properties. Hence we postulated that an interaction between palladium and tungsten oxide takes place under an H2 stream. The catalysts exhibit high selectivity in isomerization which is linked to the presence of acidic active centers, mainly Brønsted sites, on WO3 or W20O58 phases. Catalytic properties were interpreted by the traditional bifunctional mechanism, including the formation of carbocation species. Palladium metallic function, required for alkane dehydrogenation, deactivates with time under reactants but is easily regenerated by air exposure at 400°C. Striking results were obtained after exposures under oxygen and traces of water, providing an increase in the number of Brønsted acidic sites and in the acidity strength leading to an improvement of activity and isomerization selectivity. Catalysts reduced at higher temperature (600°C) are very active and selective for isomerization. Referring to our previous paper (35), devoted to catalyst characterizations (BET, TPR, XRD, XAS, XPS), we suggested that catalytic properties are then due to the W3O phase. The catalytic behaviour of such a phase has been interpreted with a monofunctional mechanism including tungstenacyclobutane species as intermediates.