Direct propylene epoxidation over barium-promoted Au/Ti-TUD catalysts with H2 and O2: Effect of Au particle size

A catalyst consisting of gold supported on a Ti-containing silicate mesoporous material (TUD) with 13-nm pores was used for the epoxidation of propylene with mixtures of H2 and O2. The catalyst activity was enhanced by the addition of a Ba promoter. The pH of deposition was important in controlling the gold loading and particle size. A pH of 7 gave a Au loading of 2.7 wt% and an average particle size of 2.0 nm, as determined by transmission electron microscopy, whereas a pH of 9 produced a much lower Au loading of 0.11 wt% and smaller particles of size about 0.9 nm, as estimated by X-ray absorption fine structure measurements. At 423 K and 0.1 MPa total pressure, the catalyst prepared at pH 7 gave a steady-state propylene conversion of 2.1%, a propylene oxide (PO) selectivity of 79%, and a H2 efficiency of 3.8%, whereas that prepared at pH 9 gave a conversion of 1.4%, a PO selectivity of 99%, and a H2 efficiency of 17%. The turnover frequency for PO production based on total gold increased on going to the sample prepared at pH 9. Thus, it is concluded that very small Au particles (about 1 nm) are the most active for epoxidation, whereas larger Au particles (about 2 nm) are less active because they promote direct H2 oxidation to H2O. X-ray absorption near-edge spectroscopy results indicated that at reaction conditions, the small particles had partially oxidized gold but the larger particles had metallic gold, suggesting that the smaller particles had high coverage of oxygen or oxygen-derived species.