Gold nanoparticles supported on Ce-Zr oxides for the oxidative esterification of aldehydes to esters

摘要

Au nanoparticles supported on Ce-Zr oxides were prepared and characterized in order to study the role of the support in the oxidative esterification of aldehydes in the presence of molecular oxygen. Ce-Zr solid solutions were synthesized by using (NH4)(2)Ce(NO3)(6) as a precursor, while the mixed oxides were obtained by using a Ce(NO3)(3) precursor. The solid solutions exhibited a smaller crystallite size, higher BET surface area, larger amount of H-2 consumption, and higher acidity and basicity than the mixed oxides at the same Ce/Zr molar ratio due to the incorporation of Zr4+ into the ceria lattice. The effect of the support was investigated because all the samples presented similar Au particle sizes, as confirmed by the HAADF-STEM study. Supports with higher reducibility showed better performance by activating methanol to methoxy and facilitating the beta-H elimination of hemiacetal. We also found that the formation of hemiacetal was enhanced by the acidic sites and basic sites of Au catalysts supported on solid solutions possessing similar reducibility. A plausible reaction mechanism for the oxidative esterification of aldehydes on Ce-Zr solid solution-supported Au nanoparticles was proposed. The screened catalyst was also applicable to the oxidative esterification of different benzylic aldehydes, producing high yields. This catalyst could be reused after a simple separation eight times, keeping a high selectivity of above 99%.