Alkane dehydrogenation with silica supported platinum and platinum-gold catalysts derived from phosphine ligated precursors

In this paper we describe the preparation, catalysis of hexane and propane conversion, and characterization results of several phosphorus containing Pt and Pt-Au catalysts on silica supports. The catalysts were prepared by adsorption from solution of mono- and bi-metallic molecular precursors that are ligated by triphenylphosphine. The effects of phosphorus in the calcined and activated catalysts are much greater than that of gold and lead to remarkable changes in selectivity and stability compared with conventional, non-phosphorus containing Pt/SiO₂ and co-deposited Pt-Au/SiO₂ catalysts. All catalysts derived from triphenylphosphine precursors showed high selectivity and activity towards dehydrogenation products. Other reactions, namely cyclization, isomerization and cracking, were severely inhibited. Preliminary results for propane conversion show that phosphorus containing catalysts are excellent catalysts for propane dehydrogenation (90% selectivity to propylene at 35% conversion, 550°C, SV = 0.5 h^-1). The phosphorus containing catalysts also have much greater stability on stream than the non-phosphorus containing catalysts. DRIFTS data on adsorbed CO show that the presence of phosphorus in the catalysts has a significant effect on the CO stretching frequency and causes an upward (blue) shift of about 10 cm^-1. TPD of adsorbed CO showed that desorption of CO from a phosphorus containing Pt catalyst had a maximum at ca. 140°C while CO desorption from a conventional Pt/SiO₂ catalyst peaked between 240-270°C. These results are consistent with a phosphorus ligand effect on Pt. Preliminary TEM data with a phosphorus containing Pt catalyst showed very small Pt particles estimated to be less than 1 nm.