Vapor-Phase Epoxidation of Propene Using H2 and O2 over Au/Ti–MCM-48

Vapor-phase epoxidation of propene using H2 and O2 over homogeneously dispersed gold particles deposited by a deposition–precipitation (DP) method on the supports Ti–MCM-41 (hexagonal) and Ti–MCM-48 (cubic) is studied at a space velocity of 4000 h−1·cm−3/g(cat.) and at reaction temperature of 100 or 150°C. Influences of the various parameters investigated in the case of Ti–MCM-48 support are Si/Ti (30, 50, 75, and 100) ratios, precipitating agents (LiOH, NaOH, KOH, RbOH, or CsOH) for Au deposition, pH (6–8), Au loading (2–25 wt% in solution), and calcination temperature (150–500°C). The supports were characterized by XRD, UV–vis, FT-IR, and surface area measurements, whereas supported gold catalysts were characterized by TEM, ICP, and XPS techniques. Better performance was observed with Ti–MCM-48 (initial conversion = 5.6%, PO selectivity = 92%), due to its three-dimensional pore system, than with Ti–MCM-41 (initial conversion = 5.1%, PO selectivity = 88%) in the propene epoxidation at 150°C. GC–MS investigation of the extracted species using organic solvent from the used catalyst revealed that acidic as well as oligomeric species accumulated on the catalyst surfaces. These species are assumed to cause the catalyst deactivation. Silylation of the catalyst Au/Ti–MCM-48 prohibits it from getting deactivated faster and also helps to improve PO selectivity and decrease H2 consumption. Based on the above experimental results, a probable reaction mechanism is explained.