Effect of gold oxidation state on the epoxidation and hydrogenation of propylene on Au/TS-1

Gold nanoparticles on titanium oxide and titania-silica supports are active for the formation of propylene oxide by the oxidation of propylene with hydrogen and oxygen mixtures. Cyanide treatment of gold supported on titanosilicate zeolite supports (Au/TS-1) yielded unexpected results. Catalysts treated with weak solutions of sodium cyanide resulted in preferential removal of small gold particles, while catalysts treated with strong solutions resulted in dissolution of the gold and re-precipitation as gold (+1) cyanide. X-ray absorption spectroscopy demonstrated that catalysts that produce propylene oxide in the presence of hydrogen and oxygen mixtures had supported gold (+3) oxide nanoparticles of 3 nm size after synthesis, which were reduced to gold metal at reaction conditions. Samples treated with strong solutions of sodium cyanide resulted in supported gold (+1) cyanide particles of large size, 9–11 nm. These particles did not produce propylene oxide but, surprisingly, showed high selectivity toward propylene hydrogenation. Increasing gold (+1) cyanide particle size resulted in a decrease in hydrogenation activity.

Treatment of gold samples supported on TS-1 with sodium cyanide solutions resulted in the precipitation of gold (I) cyanide. When this occurred, the selectivity of the reaction of propylene with hydrogen and oxygen mixtures shifted from propylene oxide to propylene, a selectivity characteristic of Pt.Figure optionsDownload high-quality image (47 K)Download as PowerPoint slideHighlights
► Treatment of Au nanoparticles on TS-1 with NaCN solutions precipitated gold (I) cyanide.
► Metallic gold Au(0) produced propylene oxide (PO) in the reaction of C3H6 with H2/O2.
► Gold (I) cyanide produced propane in the same reaction, a product characteristic of Pt.
► In situ X-ray absorption showed that Au(0) formed PO but Au(+1) formed propane.
► Au(+1) has Pt-like properties.