Impact of Titanium in Controlling Silver Particle Size on Enhancement of Catalytic Performance of AgMoO₃/Ti-HMS for...

In this work, direct gas-phase epoxidation of propylene (DPO) to propylene oxide by molecular oxygen has been studied by using Ag-MoO₃ supported on titanium-containing hexagonal mesoporous silica (Ti-HMS ) of different Si/Ti molar ratios. The promotion effect of NaCl on the synthesized catalysts has also been investigated. Among the studied supports, the hexagonal mesoporous silica (HMS) with a Si/Ti ratio of 10 was the most suitable one for production of propylene oxide (PO). The optimal performance of the AgMo/Ti-HMS₁₀ catalyst in DPO exhibited a selectivity to PO of 43.2% with a propylene conversion of 14.1%, at 400 °C, 0.1 MPa, and a space velocity of 12,000 h^-1. The catalyst verified good stability over at least 20 h on stream. Only 2.7% PO selectivity with a propylene conversion of 10.1% was achieved over the AgMo/HMS sample. The incorporation of Ti into the HMS frame could optimize the particle size distribution of Ag, producing Ag nanoparticles with an average size of 6.8 nm compared with that of Ag/HMS (24.3 nm). The in situ Raman spectrum of AgMo/Ti-HMS₁₀ heated in a stream of C₃H₆/He at 400 °C showed new bands at 616, 390, and 210 cm^-1, characteristic of the Ag Mo O intermediate phase. The obtained results suggested that this formed AgMo/Ti-HMS₁₀ phase could most likely be relevant for selective epoxidation of propylene. However, during the reaction of C₃H₆ with AgMo/HMS, the formation of this intermediate was hardly detected. On the other hand, the hydrogen temperature-programmed reduction measurements indicated improved reducibility of MoO₃ in the AgMo/Ti-HMS₁₀ catalyst, which acknowledged the role of Mo⁶⁺ in gaining electrons from silver to form positively charged Ag. This could reduce the effective charge of the adsorbed oxygen on silver sites and in turn favor the epoxidation path of propylene rather than the combustion route. Also, during the reaction of C₃H₆ with NaAgMo/Ti-HMS₁₀ at 400 °C, two new Raman bands were detected at 277 and 350 cm^-1, characteristic of Ag₂MoO₄.