Size Selectivity in the Hydroxylation of Esters of Unsaturated Fatty Acids

The one-pot two-steps hydroxylation of fatty acid esters with H2O2 and acetic acid catalyzed by acidic ion exchange resins is reinvestigated aiming to shed light on the relationship between the structure of different substrates and the efficiency of different resins in promoting the hydrolysis of the intermediate epoxides to the corresponding diols. 1H-NMR analysis of crude reaction mixtures highlight a correlation between the drying process of the resins, their cross-linking degree, and their effectiveness in promoting such a hydrolysis. Employing dry Dowex 50WX2 as a catalyst, a close relationship is found between the steric hindrance of the substrates and the recovered diol/epoxide ratio, with less hindered substrates preferentially leading to the formation of high diol/epoxide ratios. Accordingly, this procedure is successfully applied to the valorization of the methyl ester of a waste cooking oil. Conversions and selectivity kinetics increase significantly when the amount of the catalyst is increased leading to successfully apply this one-pot two-steps hydroxylation reaction to esters of unsaturated fatty acids sterically bulkier than methyl oleate. Spectroscopic and morphological characterization of Dowex 50WX2 shows that the drying process and the interaction with the reagents affect the morphology of the catalyst and promotes the hydrolysis of the intermediate epoxides. Practical applications: Hydroxylated fatty acids are valuable intermediates in the synthesis of a variety of useful biochemicals, including mono- and di-carboxylic acids, biolubricants, and cosmetics. Their work describes the successful application to a variety of esters of unsaturated fatty acids and to the methyl esters of a vegetable oil and a waste cooking oil, of a straightforward hydroxylation procedure, catalysed by commercially available acidic ion exchange resins and occurring under mild and easy to set-up conditions. The evidence reported on the different catalytic activities and on the structural characterization of the as-received and dried acidic ion exchange resins are of practical importance in their employment as acidic catalysts.