Controlled Metal-Support Interactions in Au/CeO2-Mg(OH)2 Catalysts Activating the Direct Oxidative Esterification of Methacrolein with Methanol to Methyl Methacrylate.

The strong metal-support interaction (SMSI) between the three components in Au/CeO2-Mg(OH)2 can be controlled by the relative composition of CeO2 and Mg(OH)2 and by the calcination temperature for the direct oxidative esterification of methacrolein (MACR) with methanol to methyl methacrylate (MMA). The composition ratio of CeO2 and Mg(OH)2 in the catalyst affects the catalytic performance dramatically. An Au/CeO2 catalyst without Mg(OH)2 esterified MACR to a hemiacetal species without MMA production, which confirmed that Mg(OH)2 is a prerequisite for successful oxidative esterification. When Au/Mg(OH)2 was used without CeO2, the direct oxidative esterification of MACR was successful and produced MMA, the desired product. However, the MMA selectivity was much lower (72.5%) than that with Au/CeO2-Mg(OH)2 catalysts, which have an MMA selectivity of 93.9-99.8%, depending on the relative composition of CeO2 and Mg(OH)2. In addition, depending on the calcination temperature, the crystallinity of the CeO2-Mg(OH)2 and the surface acidity/basicity can be remarkably changed. Consequently, the Au-nanoparticle-supported catalysts exhibited different MACR conversions and MMA selectivities. The catalytic behavior can be explained by the different metal-support interactions between the three components depending on the composition ratio of CeO2 and Mg(OH)2 and the calcination temperature. These differences were evidenced by X-ray diffraction, X-ray photoelectron spectroscopy, and CO2 temperature-programmed desorption. The present study provides new insights into the design of SMSI-induced supported metal catalysts for the development of multifunctional heterogeneous catalysts.

A Multifunctional Au/CeO2-Mg(OH)2 Catalyst for One-Pot Aerobic Oxidative Esterification of Aldehydes with Alcohols to Alkyl Esters.

Au nanoparticles bound to crystalline CeO2 nanograins that were dispersed on the nanoplate-like Mg(OH)2, denoted as Au/CeO2-Mg(OH)2, were developed as the highly active and selective multifunctional heterogeneous catalyst for direct oxidative esterification of aldehydes with alcohols to produce alkyl esters under base-free aerobic conditions using oxygen or air as the green oxidants. Au/CeO2-Mg(OH)2 converted 93.3% of methacrylaldehyde (MACR) to methyl methacrylate (MMA, monomer of poly(methyl methacrylate)) with 98.2% selectivity within 1 h, and was repeatedly used over eight recycle runs without regeneration. The catalyst was extensively applied to other aldehydes and alcohols to produce desirable alkyl esters. Comprehensive characterization analyses revealed that the strong metal-support interaction (SMSI) among the three catalytic components (Au, CeO2, and Mg(OH)2), and the proximity and strong contact between Au/CeO2 and the Mg(OH)2 surface were prominent factors that accelerated the reaction toward a desirable oxidative esterification pathway. During the reaction, MACR was adsorbed on the surface of CeO2-Mg(OH)2, upon which methanol was simultaneously activated for esterifying the adsorbed MACR. Hemiacetal-form intermediate species were subsequently produced and oxidized to MMA on the surface of the electron-rich Au nanoparticles bound to partially reduced CeO2-x with electron-donating properties. The present study provides new insights into the design of SMSI-induced supported-metal-nanoparticles for the development of novel, multifunctional, and heterogeneous catalysts.