Catalysis on Pd/WO3 and Pd/WO2 - II. Effect of redox treatments in hexanesand hexenes re-forming reactions

Skeletal rearrangements of alkanes and alkenes were investigated on Pd/WO3 and Pd/WO2 catalysts following various activation treatments. Catalysts red uced at low temperature (350 degreesC) are active and selective for isomeri zation 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 take s place under an H-2 stream. The catalysts exhibit high selectivity in isom erization which is linked to the presence of acidic active centers, mainly Bronsted sites, on WO3 or W20O58 phases. Catalytic properties were interpre ted by the traditional bifunctional mechanism, including the formation of c arbocation species. Palladium metallic function, required for alkane dehydr ogenation, deactivates with time under reactants but is easily regenerated by air exposure at 400 degreesC. Striking results were obtained after expos ures under oxygen and traces of water, providing an increase in the number of Bronsted acidic sites and in the acidity strength leading to an improvem ent of activity and isomerization selectivity. Catalysts reduced at higher temperature (600 degreesC) are very active and selective for isomerization. Referring to our previous paper (35), devoted to catalyst characterization s (BET, TPR, XRD, XAS, XPS), we suggested that catalytic properties are the n due to the W3O phase. The catalytic behaviour of such a phase has been in terpreted with a monofunctional mechanism including tungstenacyclobutane sp ecies as intermediates. (C) 2000 Academic Press.