A Facile Route to Prepare Eggshell Gold Pellet Catalysts for Oxidative Esterification of Methacrolein

Using 40–100 mesh MgAl mixed metal oxide (MA) pellets as supports, Au/MA pellet catalysts are prepared for the oxidative esterification of methacrylaldehyde (MAL) by the urea deposition–precipitation method. By XRD, SEM, N2 adsorption, ICP, EDS, STEM, CO2-TPD, and the CO oxidation model reaction, the effects of support calcination temperature on the structure and surface properties of MA, the structure of the gold catalyst, and its catalytic performance in the MAL oxidative esterification are studied. By examining the influences of the addition timing of urea and Mg/Al ratio on the structure and catalytic performance of gold catalysts, the formation mechanism of eggshell-type gold catalysts is explored. It is found that the supports calcined at ≥700 °C have larger pore sizes, pore volumes, and lower surface hydroxyl groups but higher spinel contents. It is shown that the pore structure and Mg/Al ratio of the MA support have a significant impact on the loading of gold. Au/MA-700 has an initial MAL conversion of 99.8% and an MMA selectivity >94% at 80 °C for 1 h, which exceeds most of the reported catalysts. For the first time, it is found that eggshell-type gold catalysts can be easily formed by premixing urea with chloroauric acid before adding 40–100 mesh support pellets, while homogeneous gold catalysts can be obtained by premixing a chloroauric acid aqueous solution with the support pellets before adding urea. The urea deposition–precipitation method of premixing urea with chloroauric acid before adding the support pellets can open up a new idea for the preparation of eggshell-type gold catalysts.