Confined Gold Nanoparticles in Sn-Beta Zeolite via Molecule Linkage Strategy: An Efficient and Stable Catalyst for Conversion of Glycerol to Methyl Lactate

Glycerol, as the main byproduct in biodiesel production, is attractive for the synthesis of value-added chemicals with the rapid increase of the biodiesel market in recent years. In this study, we developed a bifunctional Au/Sn-Beta catalyst via a molecule linkage strategy for the one-pot catalytic conversion of glycerol to methyl lactate (MLA) with molecular oxygen as the oxidant. Bifunctional Au/Sn-Beta was constructed using β-mercaptoethylamine as a linkage molecule to anchor negative Au species on the negative surface of Sn-Beta zeolites. β-Mercaptoethylamine with a well-defined structure of head and tail functional groups was anchored on Sn-Beta zeolite through the interaction of amine groups with the framework Sn sites and weakly acidic silanols. Gold species with negative charge were captured by thiol groups of β-mercaptoethylamine anchored on Sn-Beta. Upon calcination, highly dispersed Au nanoparticles (∼2 nm) were confined in the crystal of Sn-Beta zeolite, which was proved by Tomogram-section TEM images. Linkage molecules with different structures were employed. The role of the linkage molecule was revealed by XPS, FTIR, and Raman spectroscopy. It was observed that electrons were transferred from Au sites to Sn sites, which facilitated the dispersion and stabilization of Au sites. A yield of 79.3% to MLA with 100% of glycerol conversion was achieved over the prepared Au/Sn-Beta at 160 °C for 1.5 h, which was much higher than that over Au/Sn-Beta prepared by the wet impregnation method. The positive charge on Au sites decreased the catalytic oxidation ability of the catalyst. The catalyst showed good catalytic activity and stability in ten batch reaction runs.