Title: Heterogeneous catalytic esterification of omega-sulfhydryl fatty acids: Avoidance of thioethers, thioesters, and disulfides Authors
Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 22, 2011
Publication Date: November 15, 2011
Citation: Compton, D.L., Jackson, M.A. 2011. Heterogeneous catalytic esterification of omega-sulfhydryl fatty acids: Avoidance of thioethers, thioesters, and disulfides. Journal of the American Oil Chemists' Society. 88:1799-1805. Interpretive Summary: This research demonstrated the superiority of solid acid catalysts over stronger acid catalysts for the formation of sulfur containing fatty acid ethyl esters. Our mission is to develop new, value added uses for commodity crops and crop oils in an effort to mitigate the effects of climate change through the development of domestically derived, bio-based chemicals and products. Sulfur containing fatty acids derived from plant oils are used in many applications such as making electrodes for vitamin C detection, glucose detection, and analytical studies relevant to pacemakers and interstitial biosensors. Many of these sulfur containing fatty acids and their esters (similar in chemical structure to biodiesel) which are used in this type of research are not commercially available, and the current method for making the esters results in many unwanted side products. Our results are important because we have discovered a route to these sulfur containing fatty acid esters using solid acid catalysts which is somewhat slower but more efficient because it avoids the formation of unwanted side products. Thus, the purification and isolation of the sulfur containing fatty acid esters are greatly simplifying.
Technical Abstract: Two mesoporous silicas functionalized with propylsulfonic (SBA-15-PSA) and arenesulfonic (SBA-15-ASA) acid groups, and a highly acidic, functionalized styrene divinylbenzene copolymer ion exchange resin (Amberlyst-15) were examined for their ability to catalyze the ethanolic esterification of the N-terminal sulfydryl fatty acid, 11-mercaptoundecanoic acid (MUA), without catalyzing unwanted side reactions at the sulfhydryl group. All three solid acid catalysts catalyzed the MUA esterification in excess ethanol. The activation energy for the catalytic esterifications were determined from 50 to 75°C, resulting in apparent Ea of 54, 71, and 59 kJ/mol for SBA-15-PSA, SBA-15-ASA, and Amberlyst-15, respectively. GC-MS analysis determined that all three catalysts produced near quantitative conversion of MUA to its ethyl ester with very little reactivity towards the sulfhydryl group. This was a marked improvement over the esterifications catalyzed by sulfuric and p-toluenesulfonic acids which produced thiothers and disulfide side products. The MUA ethyl ester synthesis was demonstrated on a gram scale at 70°C catalyzed by Amberlyst-15, and the desired product was isolated in 80% yield at > 95% purity with a minimum of purification.