Sulfated metal oxides: eco-friendly green catalysts for esterification of nonanoic acid with methanol

Considering the need of applicability of green chemistry in research, a series of heterogeneous catalysts, viz., sulfated iron oxide, zirconia supported tungstophosphoric acid and sulfated zirconia have been synthesized by a solvent-free method. The prepared catalysts were used in the esterification of nonanoic acid with methanol and were compared with ion exchange resins for the assessment of their catalytic performance. Sulfated iron oxide was found to be best with an acid conversion of 83%, which is quite comparable with Amberlyst 15 and Dowex50Wx2. The high catalyst loading, cost, low thermal stability, and long reaction time make ion exchange resins uneconomical to use over other alternatives that result in same efficiency with low cost. Sulfated iron oxide was further optimized for its preparation conditions for high catalytic performance in the esterification reaction. The catalysts were characterized for their crystallinity, surface morphology, composition, weight loss, and structure by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The evaluated catalysts were compared on the basis of their preparation time, catalytic performance, catalyst loading, reaction time, and overall cost.