Propylene oxide (PO) is a crucial organic chemical intermediate. The direct epoxidation of propylene with molecular oxygen offers 100% theoretical atomic economy, representing a most desired route for PO production. However, this aerobic epoxidation reaction suffers from the apparent trade-off between propylene conversion and PO selectivity, which remains a key challenge in catalysis.

A recent research by Nankai University research team, titled “Direct Propylene Epoxidation with Molecular Oxygen over Titanosilicate Zeolites”, has been published in the journal National Science Review. Dr. Li Weijie and Dr. Qin Bin from College of Chemistry, Nankai University are the joint first authors. Professor Li Landong acts as the corresponding author and Professor Ma Ding from Peking University as a co-corresponding author. The team developed a Ti-Beta zeolite catalyst containing framework five-coordinated titanium sites for the direct epoxidation of propylene with molecular oxygen. Typically, 17% propylene conversion and 85-90% propylene oxide selectivity can be achieved simutaneously, matching the level of industrial ethylene epoxidation process and demonstrating a key breakthrough in the reaction.

Through the combination of transient kinetic analysis and theoretical calculations, the reaction pathway and mechanism for the direct epoxidation of propylene with molecular oxygen could be revealed. The Ti defective sites in Beta zeolite framework, namely the H-terminated pentacoordinated Ti species, are identified as the preferred active sites for propylene aerobic epoxidation. The epoxidation begins with the reaction between lattice oxygen from Ti-OH and the propylene molecule, following the Mars-van Krevelen mechanism to generate the first propylene oxide molecule. Subsequently, molecular oxygen chemisorbs on the oxygen-deficient titanium center, forming a Ti-OO species that will react with another propylene molecule to produce the second propylene oxide molecule and restore to its initial state. This novel reaction mechanism offers new insights into the selective catalytic oxidation process and paves the way for the rational design of catalyst system. This breakthrough is expected to spark new technologies for the industrial production of propylene oxide.

Reaction Pathway and Energy Profiles of Direct Epoxidation of Propylene with Molecular Oxygen Catalyzed by Ti-Beta

Paper link: https://doi.org/10.1093/nsr/nwae305

（Edited and translated by Nankai News Team.)