CHEMICAL SCIENCE

New progress has been made in the preparation of epoxy alkanes by selective oxidation


Recently, the team of Tang Zhiyong and Li Guodong, researchers of the National Center for Nanoscience, has made new progress in regulating the selective oxidation of olefins under the air atmosphere of bimetallic sites in the regulation of multimetallic organic frameworks (MOFs). The study has been published online in the Journal of the American Chemical Society.

Synergistic catalytic epoxidation of olefins under two-site air atmosphere. Photo courtesy of the National Nano Center

Epoxy compounds are important intermediates in the industrial production of flavors, pharmaceuticals and organic synthesis. Direct epoxidation of olefins is considered to be a promising preparation method due to its simple technical route and low pollution. To date, a variety of oxidants have been reported for epoxidation of olefins, including supplementing the epoxidation of olefins in an oxygen atmosphere with additives such as aldehydes.

Due to the high bonding energy of O2 molecules, it is difficult to directly activate under mild conditions, and competitive side reactions such as “carbon-carbon” bond cleavage and isomerization also limit the selectivity of epoxy compounds. The maximum yield of phenylethane in O2 atmosphere and without additives is below 80%, and achieving both high conversion and high selectivity of epoxy compounds remains challenging. Catalysts such as cobalt oxide and cobalt-doped zeolite can effectively activate O2 molecules and obtain high catalytic activity, but the selectivity of epoxy compounds is low.

MOF contains different metal nodes, especially the presence of variable valence sites, which provides more possibilities for the preparation of multifunctional catalysts and selective catalysis. Based on this, the research team synthesized CuCo-MOF-74 containing different proportions of copper-cobalt nodes by solvothermal method. The framework not only contains monometallic sites of divalent copper and divalent cobalt ions, but also generates monovalent copper ions with lower coordination sites. Under the conditions of 95oC, air atmosphere and no additives, when catalyzing the selective oxidation of olefins to prepare epoxy alkanes, the catalytic activity is significantly higher than that of monometals and their physical mixed materials, which can achieve a conversion rate of 97.6% of styrene and a selectivity of 87.6% of epoxyalkanes, and an increase of about 25.8%, which is better than the performance of heterogeneous catalysts under the currently reported oxygen atmosphere, and can also maintain good stability in cyclic reactions.

In situ synchrotron radiation characterization and near-normal pressure X-ray photoelectron spectroscopy showed that both monovalent copper ions and divalent cobalt ions could activate O2 molecules. Based on experimental results and theoretical calculations, the researchers proposed a two-site reaction path for adsorbing oxygen and co-activating styrene, which improved the activation of O2, and the π bonds in oxygen and styrene in the formed oxygen and styrene co-adsorption intermediate states were more likely to break to form epoxy alkanes. In addition, CuCo-MOF-74 also exhibits good catalytic activity and epoxy product selectivity when other types of olefins are used as substrates. A similar method can also be used to construct bimetals containing monometallic sites, monovalent copper ions and other divalent metal nodes. This work opens up new ideas for constructing different isolated sites for achieving challenging selective oxidation reactions. (Source: Zhang Shuanghu, China Science News)

Related paper information:https://doi.org/10.1021/jacs.3c00460



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