Gold-mediated selective transformation of lignin models to aromatic esters in the presence of molecular oxygen

A highly efficient and selective transformation of lignin-derived vanillin and vanillyl alcohol to prepare methyl vanillate has been developed in the presence of molecular oxygen. A series of supported Au, Pd and Pt catalysts were prepared and employed for the selective oxidation of vanillin. It is found that, in the presence of Au-h/Al₂O₃ catalyst, a 98.1% conversion of vanillin and 99.9% selectivity of methyl vanillate was obtained in methanol under 0.3 MPa of O₂ at 140 °C for 4 h. Moreover, the oxidative esterifications of isovanillin, syringaldehyde, p-anisaldehyde and 3, 4, 5- trimethoxybenzaldehyde were successfully performed, and 70.2–98.0% yields of methyl aromatic esters are obtained. The recycling experiment showed that the Au-h/Al₂O₃ still keep a high catalytic activity after being reused five times. Next, the oxidation of vanilly alcohol was studied, and 99.9% conversion and 99.6% selectivity of methyl vanillate was attained at 140 °C for 12 h with the Au-h/Al₂O₃ as catalyst. Finally, based on the reaction phenomena and experimental results, a possible reaction mechanism for the oxidation of vanillyl alcohol is proposed.