Aerobic Baeyer-Villiger oxidation of ketones over mesoporous Mn-Ce and Mn-Co composite oxides in the presence of benzaldehyde: The effect of valence state

Aerobic Baeyer-Villiger oxidation of ketones into corresponding esters were performed over mesoporous Mn-Ce and Mn-Co composite oxides prepared by co-impregnation method and benzaldehyde was used as the sacrificing agent. It was found that the best conversion and selectivity were obtained when the Mn/Ce mole ratio was 1:2 (denoted as Mn0.33Ce0.67Os) through investigating the effect of Mn/Ce mole ratio on the catalytic activity. Interestingly, Mn0.33Ce0.67Os (Mn/Co mole ratio was 1:2) catalyst had no catalytic activity for this reaction. Xray diffraction (XRD) results showed that MnOx was completely dissolved into the CeO2 lattice, and Mn0.33Ce0.67Os sample was existed in the form of spinel structure (Co, Mn) (Mn, Co)(2)O-4 and obvious peaks of Co3O4. Nitrogen adsorption-desorption measurements revealed that Mn0.5Ce0.5Os, Mn0.33Ce0.67Os, Mn0.33Ce0.67Os and Mn0.33Ce0.67Os materials possessed mesoporous structure. X-ray photoelectron spectroscopy (XPS) analysis showed that the highest concentration of Ce3+ on the surface of Mn0.33Ce0.67Os catalyst, which could create unsaturated chemical bonds, charge imbalance, and oxygen vacancies on the surface of samples, it is beneficial to the oxidation reaction. Additionally, XPS data revealed that Mn0.33Ce0.67Os catalyst has great potential in catalytic oxidation due to the large amount of active oxygen species on the surface. In addition, manganese and cobalt species on the surface of Mn0.33Ce0.67Os are almost present in the form of Mn2+ and Co3+, it is known that Mn2+ can serve as a potent antioxidant and scavenge the activity of active oxygen species, while Co3+ ions can prevent the formation of oxygen vacancies and inhibit the release of reactive oxygen species, thus the presence of Mn2+ and Co3+ inhibited the catalytic activity of Mn0.33Ce0.67Os catalyst. The diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-vis) and hydrogen temperature-programmed reduction (H-2-TPR) results are in accordance XPS data.