Enhancement of Catalyst Performance in the Direct Propene Epoxidation: A Study into Gold–Titanium Synergy

Abstract

Enhanced productivity toward propene oxide in the direct propene epoxidation with hydrogen and oxygen over gold nanoparticles supported on titanium‐grafted silica was achieved by adjusting the gold–titanium synergy. Highly isolated titanium sites were obtained by lowering the titanium loading grafted on silica. The tetrahedrally coordinated titanium sites were found to be favorable for attaining small gold nanoparticles and thus a high dispersion of gold. The improved productivity of propene oxide can be attributed to the increased amount of the interfacial AuTi sites. The active hydroperoxy intermediate is competitively consumed by epoxidation and hydrogenation at the AuTi interface. A higher propene concentration is favorable for a lower water formation rate and a higher formation rate of propene oxide. Propene hydrogenation, if occurring, can be switched off by a small amount of carbon monoxide.

Effect of Ti incorporated MWW supports on Au loading and catalytic performance for direct propylene epoxidation

“…The average gold NPs sizes are about 2 nm in 33-Ti-YNU-1, and 1.5 nm in 100-Ti-YNU-1 and A-Ti-YNU-1. These results are consistent with the literature reports in [3,5]. With the comparison of Fig.…”

“…One is the specific rate or yield per hour based on per kg of catalyst and are called PO formation rate. The other PO rate is turnover Rate (TOR) based on per gram of gold, and that can demonstrate the Au efficiency [3].…”

“…Gold is believed to firstly catalyze the reaction of O 2 /H 2 to form hydrogen peroxide, which then migrates to Ti(IV) sites on the supports to form titanium hydroperoxide species, and the latter react with propylene to form epoxide [1,3,45,46,47]. For 4.5Au/xTi-YNU-1 catalysts, the actual Au loading and the corresponding Au NPs size decrease with the Ti content in the supports.…”

“…Oyama et al [4] reported that very small Au particles (about 1 nm) supported on mesoporous Ti-TUD synthesized by hydrothermal reaction are the most active for epoxidation. On the other hand, Nijhuis's group [3,5] reported that Au NPs supported on different supports had different optimum sizes to give best activity in direct epoxidation of propylene. The Au average NPs in the range of 1.5-2.8 nm supported on Ti-SBA-15 prepared by Ti grafting method [5] and Au NPs of sub-nanometer to 6 nm supported on the Ti-grafted silica [3] are most active.…”

“…For the direct propylene epoxidation over Au-Ti catalysts, it is generally agreed that the size of Au particles and the nature of the titaniumcontaining supports are important factors to the activity of the catalysts. Various titanosilicates, including TiO 2 /SiO 2 [3], Ti-TUD [4], Ti-SBA-15 [5], Ti-MCM-41 [6], Ti-MCM-48 [7], Ti-MWW [8], Ti-␤ [9], TS-2, TS-1 [10,11], and alkaline treated TS-1 [12] have been investigated for propylene epoxidation. Chowdhury et al [13] declared that Au particles supported on anatase TiO 2 of 2-5 nm in size were responsible for epoxidation, whereas particles smaller than 2 nm were responsible for hydrogenation to form propane and those larger than 5 nm were responsible for propylene combustion.…”