Efficient aerobic oxidative esterification of furfural to methylfuroate over Au/Al2O3 catalysts in base-free medium†

Oxidative esterification of furfural to methylfuroate was conducted in base-free medium over Au catalysts supported on ZrO₂, CeO₂, TiO₂, ZnO and Al₂O₃, and the Au/Al₂O₃ catalyst exhibited a higher activity than the other prepared catalysts. The characterization results of these Au catalysts showed that the Au/Al₂O₃ catalyst possessed good dispersion of Au nanoparticles and suitable surface acidity, leading to better performance of the catalyst. The effects of reaction time, reaction temperature, initial O₂ pressure and catalyst dosage on the oxidative esterification of furfural over Au/Al₂O₃ were investigated, and furfural conversion and methylfuroate selectivity reached 99.8% and ∼100%, respectively, under the conditions of 140 °C, 1 MPa O₂, 1 h and 0.2 g catalyst. Meanwhile, the furfural/methanol mole ratio in this work was as high as 1/27, which is higher than that reported in the literature and is more promising for commercial application. The catalytic performance of Au/Al₂O₃ was found to decrease slightly with the use time, and the aggregation of Au particles in the reaction was suggested to account for the decrease in activity. The functional mechanism of the catalyst was investigated via contrast experiments, and it was speculated that the acid sites of Al₂O₃ might promote the adsorption of furfural on the surface of catalyst, and Au catalyzed both steps of the oxidative esterification of furfural to methylfuroate.