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 O-2 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 similar to 100%, respectively, under the conditions of 140 degrees C, 1 MPa O-2, 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.