Aerobic oxidative coupling of 2-naphthols catalyzed by flame-made VOx/t-ZrO2: Effect of metal oxide support and VOx content on catalytic activity

Oxidative coupling of 2-naphthols to 1,1'-bi-2-naphthols in the air at 50 °C was conducted by using VOx supported on metal oxides prepared by flame spray pyrolysis. Among the tested catalysts, the highest yield (91%) was achieved using VOx/t-ZrO2 (containing 7.8wt% VOx as V2O5). The catalytic activity decreased as the VOx content increased. At lower VOx content, the catalyst primarily consisted of highly dispersed vanadate species with square pyramidal coordination. As the VOx content increased, the fraction of highly dispersed vanadate decreased, whereas that of V2O5 particles increased. H2 temperature-programmed reduction measurements revealed that highly dispersed vanadate species exhibited lower reduction temperatures than V2O5 particles. Therefore, our results suggest that the small vanadate clusters supported on t-ZrO2 are the active sites for the coupling reaction.