The epoxidation of propene over gold nanoparticle catalysts

Different gold nanoparticle catalysts on titania, silica, and titanosilicate supports are compared in the hydro-epoxidation of propene. All catalysts tested were active in the propene epoxidation, with Au/TiO2 showing the highest activity at low temperature, but also a high rate of deactivation. It is shown that the deactivation of the catalysts is directly related to a side reaction of a bidentate propoxy reaction intermediate. This species can react to produce propene oxide, but as a side reaction it can also produce very strongly adsorbed species, most likely carbonates, which cause a reversible deactivation. There are no indications that the catalyst deactivation is caused by changes in the size or the state of the gold nanoparticles. Catalysts containing a lower amount of titania dispersed in or on a silica support are more stable, but require a higher reaction temperature for a similar activity. Ti-SBA-15 is the most promising support material, but the gold deposition inside the structure requires further optimization. A key factor, that needs to be solved for all gold catalysts, is the efficiency in which hydrogen is used as a coreactant. The currently obtained hydrogen efficiencies of up to 10% are insufficient to run a process profitably.