Efficient Esterification and Transesterification of Used Cooking Oil Using 12-Tungstophosphoric Acid (TPA)/Nb2O5 Catalyst

12-Tungstophosphoric acid (TPA, 5−30 wt %) supported on niobia (Nb2O5) catalysts were prepared, and their efficacy as solid acid catalysts for the transesterification of used cooking oil (with high free fatty acids) with methanol was investigated. The catalysts prepared were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), NH3−temperature-programmed desorption (TPD), laser Raman, Brunauer−Emmett−Teller (BET) surface area, and scanning electron microscopy (SEM) techniques. Among the catalysts, 25 wt % TPA/Nb2O5 was the most promising one with the highest ester yield and having the ability to catalyze transesterification and esterification reactions simultaneously. Further, reaction parameters, such as catalyst weight, methanol/oil mole ratio, and reaction temperature, were also optimized with the most active catalyst, i.e., 25 wt % TPA/Nb2O5. It was found that, at 200 °C, 18:1 alcohol/oil molar ratio, and 3 wt % catalyst loading, a maximum ester yield of 92 wt % could be obtained. The catalyst was recycled and reused with negligible loss in activity. The kinetic studies suggested that the reaction follows the first-order kinetics.