Direct epoxidation of propylene with molecular oxygen over Ag–MoO 3/ZrO 2 catalyst

Abstract

Direct gas-phase epoxidation of propylene to propylene oxide (PO) by molecular oxygen was studied over the MoO 3-modified supported-silver catalyst. The effect of promoter and support on the performance of the Ag–MoO 3 catalyst and their role were investigated by XPS, XRD, SEM, BET surface area, NH 3-TPD, CO 2-TPD techniques and so on. As the promoter, MoO 3 plays a great role of electron- and structure-type bi-functional promoter. Of all the supports studied, ZrO 2 is the most suitable one for the Ag–MoO 3 catalyst. The presence of support can regulate the size of the Ag–MoO 3 particles and the pore radii of catalyst, and decrease the loading of MoO 3 from ∼50 wt.% in the unsupported Ag–MoO 3 catalyst to ∼4 wt.% in the Ag–MoO 3/ZrO 2 catalyst. The properly weak Lewis acidic sites, the larger size of the Ag–MoO 3 particles and pore channel on the Ag–MoO 3/ZrO 2 catalyst are beneficial to improve epoxidation performance of the catalyst. The effect of the reaction temperature and space velocity on the catalytic epoxidation of propylene was also investigated. The low temperature or high space velocity can decrease the deep oxidation of propylene oxide to improve the selectivity to PO. Over the 20%Ag–4%MoO 3/ZrO 2 catalyst at 400 °C, 0.1 MPa and space velocity of 7500 h −1, the selectivity to PO of 60.3% was achieved with the O 2 conversion of 4.8%; under the space velocity of 12 000 h −1, the selectivity to PO was 71.5% with 2.5% O 2 conversion.