Regulation of Co²⁺ cations on the content of Brönsted acid site and oxygen vacancy of WO_x to improve the epoxidation performance of 1-hexene

In this study, the Co-WO_x catalyst was successfully prepared by directly introducing Co²⁺ dopant in a dynamic solvothermal synthesis process, and the obtained Co-WO_x was used for the catalytic epoxidation of 1-hexene. The structures of WO_x before and after the doping were analyzed by XRD, SEM, TEM, Raman, XPS as well as in-situ NH₃-FTIR. The results show that the doping of Co²⁺ has not obvious effect on the crystal phase and main growth direction of WO_x, but can effectively reduce the content of Brönsted acid (B acid) site on the surface of WO_x catalyst and increase the content of oxygen vacancy at the same time. In the epoxidation reaction, the obtained Co-WO_x catalyst (Co/W = 0.1) can increase the selectivity of 1,2-epoxyhexane from 26.9% of pure WO_x to 55.7% with a 5.3% decrease in 1-hexene conversion. The improvement of Co-WO_x performance is mainly attributed to two aspects: (1) the reduction of B acid site on the surface of WO_x inhibits the ring opening hydrolysis of 1,2-epoxyhexane; (2) The increase of oxygen vacancies on the surface of WO_x promotes the activation of H₂O₂, ensuring that the conversion rate of 1-hexene does not decrease significantly, and an increase in the utilization of oxidant H₂O₂ by 13.5%. Combined with the characterization results and reaction data, the epoxidation mechanism of 1-hexene with W−O−OH as active intermediate is proposed.