(668e) Effect of K Promotion and Gas Pretreatment of Highly Dispersed Cu/SiO2 on Direct Propylene Epoxidation

This work investigates the effect of K promotion and pretreatment on the gas-phase epoxidation of propylene to propylene oxide (PO) with molecular oxygen on Cu/SiO₂. Systematic fixed bed reactor, H₂-TPR, and XPS studies were carried out to elucidate the effect of K on PO selectivity. Cu catalysts were prepared by charged enhanced deposition precipitation of Cu(NO₃)₂ on amorphous SiO₂ (320 m²/g). Potassium acetate was added to the Cu/SiO₂ by incipient witness impregnation to obtain K-Cu/SiO₂ with different K/Cu ratios. FBR studies were conducted at 1 atm and 250 ℃ on Cu/SiO₂ and K-Cu/SiO₂ catalysts pretreated in O₂ or H₂. The unpromoted catalysts showed high selectivity to acrolein and other products (allylic alcohol, acetaldehyde and acetone) with low PO selectivity (5-10%) regardless of the pretreatment. K promotion of O₂ pretreated catalysts increased PO selectivity, becoming the product with the second highest selectivity after acrolein. The O₂ pretreated K-Cu catalysts (K/Cu =1) gave the highest PO selectivity (~20%). H₂ pretreatment of K-Cu catalysts increased PO selectivity up to 50%, making it the main reaction product. H₂-TPR of spent H₂ pretreated K-Cu catalyst showed a primary reduction peak that was between that for unpromoted and O₂ pretreated K-Cu catalysts. XPS analysis of spent catalysts showed that H₂ pretreated K-Cu one exhibited the most significant Cu⁰/Cu⁺ shift to higher binding energies with respect to fresh and unpromoted catalysts. The K2p peak in the H₂ pretreated K-Cu catalyst also shifted to higher binding energies relative to the O₂ pretreated one. These results indicate that the role of the H₂ pretreatment is to enhance the K-Cu interaction and stabilize active Cu sites (Cu^m+) for propylene epoxidation. Ongoing work is focused on monitoring charge transfer and identifying reactive intermediates involved in acrolein and PO formation on the catalyst surface using UV-Vis and IR modulation excitation spectroscopy techniques.