New insights on the role of perimeter sites in oxygen and methanol activation during furfural oxidative esterification over gold supported catalysts

Au/ZrO2 catalysts (1.5 wt% Au loading) undergone to a final calcination at different temperature (423, 573, 773 and 923 K) to opportunely modulate the Au size were proven to be highly active and selective in furfural oxidative esterification to methyl-2-furoate. The catalysts were tested according to previously optimized reaction conditions, employing O2 as benign oxidant and without any base co-catalyst. The strong dependence of the catalytic activity on the Au size was previously ascribed to the presence of Au clusters able to dissociate O2 and producing atomic O that can activate methanol. However, despite the absence of clusters, also the catalysts containing larger Au nanoparticles were active, even if conversion and selectivity were remarkably lower if compared to the other samples. The nature and the role of Au active sites exposed at the surface of clusters and nanoparticles were then investigated by HR-TEM, CO chemisorption and FTIR spectroscopy of adsorbed CO, molecular oxygen, methanol and furfural to establish structure-activity relationships. Metal nanoparticles can activate oxygen and form superoxo species and atomic O, whereas experiments performed on the pre-hydrated samples revealed that Au perimeter sites at the metal-support interface play a key role in methanol activation during furfural oxidative esterification.