Synthesis of a modified heteropolyacid and evaluation as a phase-transfer catalyst for soybean oil epoxidation

Abstract

Novel modified heteropolyacid (HPA) catalysts have been developed for the epoxidation of unsaturated vegetable oils. These were synthesized through hybridization of phosphotungstic acid (PTA) with cetyltrimethylammonium bromide (CTAB), tetrabutylammonium bromide (TBAB), and 1-dodecyl-3-methylimidazolium bromide (C_12mimBr). Characterization of the modified HPAs was conducted using FT-IR, XRD, and TGA, and they were assessed as phase-transfer catalysts in the epoxidation of soybean oil (SO) with H₂O_2, in a two-phase system, without a percarboxylic acid intermediate. The performance of the epoxidation reaction was evaluated based on conversion (change of iodine value), selectivity (oxirane value), and productivity. The catalyst that exhibited superior performance was the phosphotungstic acid modified with CTAB corresponding to [C₁₉H₄₂N]₃PW₁₂O₄₀ (CTA₃PTA). Subsequently, it was found that the introduction of acidity to the reaction medium significantly enhanced the catalytic activity of CTA₃PTA by promoting active peroxo forms. Different ion exchange resins (Dowex 50WX2, Amberlyst 15, and Amberlite IR-120) were explored as co-catalysts for reaction. Effective epoxidation conditions were achieved at a temperature of 60 ºC, a molar ratio of double bond to H₂O₂ of 1:2, and 2 g of Amberlite IR-120, resulting in a 4.6 % wt. oxirane value. Finally, the structure-performance relationship was discussed focusing on the influence of acidity in the epoxidation reaction.