The Catalytic Properties Of Microemulsions For Esterification And Transesterification

It is well known that direct esterification of carboxylic acid/alcohol and transesterification of ester/alcohol play important roles in the production of organic esters, especially for some important products and intermediates in the organic, bioorganic, pharmaceutical industries and related fine-chemical synthesis. However either of the reactants in excess and/or the removal of the formed alcohol (or water) from products are generally required to promote the equilibrium process in favor of the products side. At present, most of the works reported in the literatures have been based on esterification reactions using longer-chain length fatty acids and alcohols or short-chain fatty acids and alcohols. Catalysts that can be used to catalyze the esterification and transesterification have two main kinds of catalyst, one is chemical catalyst and the other is biocatalyst. Enzymes are a kind of biocatalyst and are efficient catalyst in synthetic chemistry, their catalytic activity with unnatural substrates attracts much attention because of their much milder reaction conditions and friendly environments. In recent years, one of the most intensively studied areas has been the technique of entrapping enzymes in reverse micelles or microemulsion. A microemulsion is a thermodynamically stable, isotropic, optically transparent solution consisting of water, oil and a surfactant. Often,the formation of microemulsion requires the presense of a cosurfactant. Depending on the microstructure, microemulsion can be divided into three main cases: an oil-in-water(o/w), a bicontinuous structures, an water-in-oil(w/o) that is aqueous droplets(stabilized by surfactant) is dispersed in a continuous organic medium(also called as reverse micelles). For enzyme-catalysed reactions a w/o type is usually used. The enzyme is solubilized in the water droplets of the microemulsion while the oil-soluble substrates are dissolved in the continuous, organic phase. The reactions take place at the oil-water interface and the products are distributed between the oil phase or the water phase. This kind of system mainly has two advantages: (1) Lipase molecules can be entrapped in water-containing micro-drops, avoiding direct-contract with unfavorable organic medium and retaining their catalytic activity. (2) Larger polar/apolar interfacial area improves the interaction between the enzyme and substrates. Surfactant is a very important factor in formation of microemulsion system, most of the enzymatic reactions were performed in microemulsion or reverse micelles stabilized by AOT and CTAB surfactant, few attention has been paid to the other surfactants. So we initiated our experiment to use DBSA, a relatively cheap and widely used commercial surfactant to form reverse microemulsion. Based on above mentioned, a model esterification reaction of hexanol and hexanoic acid, which was catalyzed by Candida cylindracea lipase in DBSA/cyclohexane microemulsion system, was undertaken to compared lipase activity in this system with AOT/cyclohexane microemulsion system and cyclohexane system. To us surprised, the results of our experiments showed that even if at mild conditions, esterification reaction in the DBSA system can perform very rapidly and obtain relatively considerable conversion whether catalyzed by Candida cylindracea (Ccl) or not as compared with AOT microemulsion system and organic medium system, this result encouraged us to further explore the reaction mechanism. According to our investigation, we concluded that relatively high conversion in DBSA system is ascribed to the two-fold functions of DBSA, which are surfactant as well as acid catalyst. So this proved that DBSA microemulsion should be a novel and promising esterification system in improving reaction rate and conversion under relatively mild reaction conditions.