On the nature of the active sites in the selective oxidative esterification of furfural on Au/ZrO2 catalysts

Two catalysts,based ongold on zirconia and prepared in the same way, except for the different thermal treatment in air after the addition of gold (0.4% wt), show a quite different behavior in furfural oxidative esterification, particularly in the selectivity to 2-MF (methyl 2-furoate). The analysis of their catalytic behavior, in comparison with the ZrO2 support and a reference Au/TiO2 catalyst (AUROliteTM), provides a series of indications on the reaction mechanism and the nature of the sites. It is evidenced in particular that i) the addition of gold enhances the activity of the support in furfural conversion by a factor of about ten, ii) the creation of active sites for the further conversion of the acetal intermediate to 2-MF increases the reaction rate of furfural conversion, mainly by shifting the equilibrium of the furfural to acetal reaction to right, iii) the precursor sites for the acetal to 2-MF oxidative conversion form during the thermal treatment in air from 200°C to 400°C, due to a reduction of gold occurring during this treatment with partial redispersion of gold and formation of very small Au NPs (nanoparticles) with average size of 0.7 nm, iv) the active sites for the acetal to 2-MF oxidative conversion are associated likely to these small Au NPs, but they form in-situ at the initial stage of reaction.