Optimization of Methyl Oleate Synthesis with Sulfonated Pyrrolidonium Ionic Liquids as Catalysts Using the Response Surface Methodology

In order to improve the efficiency of biodiesel production from esterification of free fatty acids, an alternative to sulfuric acid has been explored in this study. These catalysts including three pyrrolidonium ionic liquids (ILs), 1-methyl-2-pyrrolidonium hydrogen sulfate ([Hnmp]HSO4), 2-pyrrolidonium hydrogen sulfate ([Hnhp]HSO4), and 1-(3-sulfonic acid) propyl-2-pyrrolidonium hydrogen sulfate ([C3SO3Hnhp]HSO4), were applied as catalysts to produce biodiesel through the esterification reaction of oleic acid with methanol. The catalytic performances of the synthesized ILs for the esterification of oleic acid were evaluated, and [C3SO3Hnhp]HSO4 exhibited the best catalytic activity among all tested acidic ILs. Moreover, the esterification of oleic acid with methanol by the [C3SO3Hnhp]HSO4-catalyzed was systematically explored, and the reaction conditions were further optimized through a single-factor experiment, the Plackett-Burman design, and a response surface methodology. It was found that optimum response for oleic acid conversion was 97.4% under reaction condition of using catalyst dosage of 12.5%, methanol/oleic acid molar ratio of 9:1, reaction time at 4 h and reaction temperature at 70 °C. In addition, the catalytic activity of [C3SO3Hnhp]HSO4 still remained high level after 5 cycles. In a conclusion, the IL [C3SO3Hnhp]HSO4 has great potential as a catalyst for producing fatty acid methyl esters via the esterification reaction.