Co^(2+)调控WO_(x)表面Brönsted酸和氧空位含量用于提高1-己烯环氧化性能

Regulation of Co^(2+) cations on the content of Brönsted acid site and oxygen vacancy of WO_(x) to improve the epoxidation performance of 1-hexene

本研究采用动态溶剂热合成法,在WO_(x)的制备过程中直接引入Co^(2+)得到了Co-WO_(x)催化剂,并将其用于1-己烯的催化环氧化。通过XRD、SEM、TEM、Raman、XPS等多种表征手段以及原位NH_(3)-FTIR对Co^(2+)引入前后WO_(x)的结构进行了系统分析。结果表明,Co^(2+)的引入对WO_(x)的晶型和晶体主生长方向无明显影响,但有效减少了其表面Brönsted酸(B酸)含量,同时增加了其表面氧空位含量。在环氧化反应中,所得Co-WO_(x)催化剂(Co/W=0.1)在1-己烯转化率降低5.3%的情况下,可以将1,2-环氧己烷的选择性从纯WO_(x)的26.9%提高至55.7%。Co-WO_(x)催化剂环氧化性能的提高主要归因于两个方面:一是,WO_(x)表面B酸位点减少抑制了1,2-环氧己烷的开环水解;二是,WO_(x)表面氧空位增多促进了H_(2)O_(2)的活化,保证了1-己烯转化率降幅不大,而且使氧化剂H_(2)O_(2)的利用率提高了13.5%。结合表征结果和反应数据,提出了以W−O−OH为活性中间体的1-己烯环氧化反应机理。

In this study,the Co-WO_(x) catalyst was successfully prepared by directly introducing Co^(2+)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 NH_(3)-FTIR.The results show that the doping of Co^(2+)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 H_(2)O_(2),ensuring that the conversion rate of 1-hexene does not decrease significantly,and an increase in the utilization of oxidant H_(2)O_(2) 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.