Synthesis of Sugar Fatty Acid Esters using Lipase Immobilized in Supported Sol-Gels
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Sugar fatty acid esters are of practical importance and have a variety of applications that include biodegradable detergents and emulsifiers in resin polymerization. Traditionally, they are synthesized chemically with low selectivity and different degrees of esterification. In this study, different sugar esters were enzymatically synthesized from a variety of sugars/sugar alcohols and fatty acids. The removal of water in an esterification reaction is critical in determining the reaction direction due to the reversibility of the reaction. Furthermore, if the water generated in the reaction is not removed continuously lower ester yields and a significant reduction in the activity and stability of immobilized enzymes can occur due to partial inhibition. The approach taken here was to investigate the feasibility of using Celite® supported sol-gel immobilized enzymes (CSSIE) as biocatalysts. The sol-gel consisted of PTMS (trimethoxypropylsilane) and TMOS (tetramethylorthosilicate) and was supported by three different types of Celite® – R632, R633 and R647. CSSIE was dried and reused a number of times without a significant loss of activity. The CSSIE were found to behave as highly porous adsorbents with a high capacity to absorb water by selective uptake of polar substances. Their strong affinity for water not only prevented significant co-adsorption problems but also offered the consistent performance required for industrial applications. Maximum yield of product was obtained using CSSIE and results were compared to literature data using a variety of biocatalysts and moisture adsorbing media including unsupported sol-gels, Novozym, Celite® and molecular sieves. Although the moisture adsorbing capacity of the CSSIE decreased with reuse due to saturation, sol-gel aging marginally increased enzyme activity with a half life of 8 repeated uses with the highest conversion obtained when using the supported sol-gel 8-days after initial preparation. No appreciable side products were observed for any of the reaction combinations examined and the highest product yield (96.7%) was obtained for the product sorbitan monooleate. Experiments were also conducted to determine conversion and moisture content as a function of substrate molar ratio, quantity of enzyme, type of solvent, nature of the fatty acid and reaction temperature. The highest conversion was obtained by the use of acetone as the reaction solvent, longer chain fatty acids and a reaction temperature of 40°C. The maximum protein immobilization of 85% of the supplied protein was obtained using sol-gel supported by Celite®-R633. Sol-gel clusters on the surface of the Celite® were observed following repeated reuse of the CSSIE which may partially explain the decrease in conversion observed with reuse.