One-pot synthesis of finely-dispersed Au nanoparticles on ZnO hexagonal sheets for base-free aerobic oxidation of vanillyl alcohol

Abstract

Advanced synthesis of highly-dispersed Au nanoparticles is vital to catalysis and green chemistry. In this work, a facile one-pot coordination–calcination method was developed to prepare uniform Au nanoparticles loaded on ZnO using metal ions and the 2-methylimidazole ligand. This Au/ZnO–MZ catalyst was efficient and stable towards sustainable synthesis of vanillin via the base-free aerobic oxidation of vanillyl alcohol. A turnover number of 335 and 100% yield of vanillin were obtained at 120 °C and 5 bar of O₂, the best result to date among all the supported noble-metal catalysts. Meanwhile other Au/ZnO catalysts were prepared for comparison by a one-pot precipitation method using ammonia, triethylamine and sodium hydroxide, respectively. The Au/ZnO–MZ catalyst demonstrated unique properties as shown by PXRD, N₂ physisorption, SEM, TEM, H₂-TPR, O₂-TPD, XPS, PL spectroscopy and CO adsorbed DRIFT-IR. Small and metallic-state Au nanoparticles (about 2.2 nm) as active sites were highly dispersed on ZnO hexagonal nanosheets. Sufficient surface oxygen species bearing enhanced oxygen mobility were observed for ZnO carriers. The strong metal–support interaction due to the interfacial electron transfer from Au to ZnO was elucidated. These catalytic features allowed excellent base-free oxidation catalysis by lowering the activation energy. Moreover, the Au/ZnO–MZ catalyst displayed a wide substrate scope of alcohols to produce the corresponding aldehydes. The present findings may probably inspire new preparation methods and applications for Au heterogeneous catalysts.