Esterification of free fatty acids using acidic metal oxides and supported polyoxometalate (POM) catalysts

The valorization of various types of low-quality vegetable oils with relatively high content of free fatty acids (FFA) is a research field of increased interest. The esterification of oleic acid, a highly abundant FFA in waste oils, with trimethylolpropane (TMP) towards the production of the corresponding triesters, which can be used in lubricants or hydraulic fluids, was studied in this work. The catalysts tested were various meso/macroporous oxides, such as ZrO₂, 3.5%SiO₂-ZrO₂, SiO₂-Al₂O₃, SBA-15 and Al-SBA-15 with varying textural and acidic properties and the corresponding supported heteropoly acid catalysts, prepared by the use of phosphotungstic (H₃PW₁₂O₄₀) and tungstosilicic (H₄SiW₁₂O₄₀) acids. The catalysts were characterized by ICP-AES, XRD, FT-IR, N₂ porosimetry and FT-IR coupled with pyridine adsorption. The main product at 200 °C and after 6 h of reaction was the triester (TE) of oleic acid with selectivity values ranging from 85% to 99% at conversion degrees of 82% to 96%. The two aluminosilicate catalysts that combine Brønsted and Lewis acidity (i.e. SiO₂-Al₂O₃ and Al-SBA-15) showed slightly higher yields of triester (∼83%) compared to the rest oxides (∼78%), while the Brønsted acidity induced by the two heteropoly acids, especially by PW, provided very high reactivity and selectivity towards triesters, irrespective of the type of support used, reaching triester yield values of 95% (96.2% conversion, 98.7% selectivity) with the 20%PW/ZrO₂ catalyst. The most stable catalysts against leaching of heteropoly acids were those supported on 3.5%SiO₂-ZrO₂ and SiO₂-Al₂O₃.