Efficient solid acid catalysts for esterification of free fatty acids with methanol for the production of biodiesel

12-Tungstophosphoric acid (TPA) with varying contents supported on SnO2 catalysts was prepared and their efficacy as solid acid catalysts for the esterification of palmitic acid with methanol was investigated. The catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), temperature programmed desorption (TPD) of NH3, Laser Raman, Brunner–Emmett–Teller (BET) surface area and scanning electron microscopy (SEM) techniques. The esterification activity depends on the amount and dispersion of TPA on SnO2 which in turn relates to the amount of acidity. Among the catalysts, 15 wt% TPA/SnO2 was the most promising one with the highest fatty acid conversion. Further, reaction parameters such as catalyst concentration, methanol to palmitic acid mole ratio and reaction temperature were also optimized. The catalyst was recycled and reused with consistent activity. A pseudo first order model was applied to correlate the experimental kinetic data and kinetic parameters were evaluated. The activation energy of the catalysts is comparable to that of mineral acids and lower than usual solid acid catalysts.