Co2+调控WOx表面Brönsted酸和氧空位含量用于提高1-己烯环氧化性能

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

  • 摘要: 本研究采用动态溶剂热合成法,在WOx的制备过程中直接引入Co2+得到了Co-WOx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NH3-FTIR对Co2+引入前后WOx的结构进行了系统分析。结果表明,Co2+的引入对WOx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-WOx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WOx的26.9%提高至55.7%。Co-WOx催化剂环氧化性能的提高主要归因于两个方面:一是,WOx表面B酸位点减少抑制了1,2-环氧己烷的开环水解;二是,WOx表面氧空位增多促进了H2O2的活化,保证了1-己烯转化率降幅不大,而且使氧化剂H2O2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。

     

    Abstract: In this study, the Co-WOx catalyst was successfully prepared by directly introducing Co2+ dopant in a dynamic solvothermal synthesis process, and the obtained Co-WOx was used for the catalytic epoxidation of 1-hexene. The structures of WOx 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 WOx, but can effectively reduce the content of Brönsted acid (B acid) site on the surface of WOx catalyst and increase the content of oxygen vacancy at the same time. In the epoxidation reaction, the obtained Co-WOx catalyst (Co/W = 0.1) can increase the selectivity of 1,2-epoxyhexane from 26.9% of pure WOx to 55.7% with a 5.3% decrease in 1-hexene conversion. The improvement of Co-WOx performance is mainly attributed to two aspects: (1) the reduction of B acid site on the surface of WOx inhibits the ring opening hydrolysis of 1,2-epoxyhexane; (2) The increase of oxygen vacancies on the surface of WOx 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.

     

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