Synthesis and characterization of Ti-MCM-41 and vapor-phase epoxidation of propylene using H2 and O2 over Au/Ti-MCM-41 Original Research Article

Vapor-phase epoxidation of propylene to propylene oxide (PO) using H2 and O2 has been carried out over Au deposited on Ti-MCM-41 and TiO2–SiO2 mixed oxides having various Ti/Si molar ratios ranging from 0.25/100 to 6.0/100. The Ti-MCM-41 supports were prepared by using two different structure-directing organic templates: dodecyltrimethylammonium hydroxide (C12TMACl) and cetyltrimethylammonium hydroxide (C16TMACl). TiO2–SiO2 mixed oxides were prepared by a sol–gel process. Gold nano-particles were loaded by deposition–precipitation (DP) using various precipitating agents: LiOH, NaOH, KOH, RbOH, and CsOH. Au/Ti-MCM-41 gives increased propylene conversion with an increase in Ti/Si ratio and the PO yield passes through a maximum (conversion=3.2%, PO selectivity=93.5%) at 100°C at a Ti/Si molar ratio of 3/100. However, at higher reaction temperatures of ≥120°C, PO selectivity decreases with an increase in the Ti/Si ratio. At the reaction temperatures of 150 and 200°C, to get better performance, the ideal catalysts should have Ti/Si ratios of 2/100 and 1.5/100, respectively. The critical balance among Ti-content, Au loading and the reaction temperature is important to achieve the higher propylene conversion, PO selectivity above 90% and lower H2 (and O2) consumption. Gold supported on TiO2–SiO2 mixed oxides showed very poor performance, probably due to higher concentrations of extra framework titanium and lower dispersion of gold particles.