Efficient epoxidation of propylene over non-noble nickel-based catalyst promoted by alkali metals

The application of non-noble metal catalysts in the catalytic direct gas-phase epoxidation of propylene with H₂ and O₂ to produce propylene oxide is valuable and challenging. The introduction of alkali metal promoters is one of the effective methods to improve the catalytic activity of catalysts. Herein, a series of alkali metal (Li, Na, K, Rb, and Cs)-promoted Ni/TS-1 catalysts were prepared to deeply understand the effect of alkali metals on the structure-activity relationship for gas-phase epoxidation of propylene. Among them, the Na-Ni/TS-1 catalyst exhibits the highest catalytic activity (propylene conversion of 7.35% and PO formation rate of 157.9 g h⁻¹ kg_cat⁻¹) and the best stability (long-term stability exceeding 140 h at 200 °C). X-ray absorption and photoelectron spectroscopy revealed that the electronic structure of Ni can be tuned by the addition of alkali metal promoters. NH₃-TPD-MS, CO₂-TPD-MS, and C₃H₆-TPD-MS results indicate that the acidity of the catalyst can also be adjusted by the introduction of alkali metal, whereas the Na-Ni/TS-1 catalyst exhibits the strongest C₃H₆ adsorption capacity. Thus, the suitable acid-base properties, unique electronic properties of Ni species, and the strongest propylene adsorption capacity resulted in improved propylene gas-phase epoxidation activity of Na-Ni/TS-1 catalyst. This study not only provides a new strategy for the practical application of nickel-based catalysts in the gas-phase epoxidation of propylene but also provides insights into the promoting effect of alkali metals.