Microwave irradiated immobilized lipase catalyzed synthesis of alkyl benzoate esters by transesterification: mechanism and kinetic modeling

Alkyl benzoate esters have gained importance due to their application in a variety of industries such as flavor, cosmetics, and pharmaceuticals. Effect of microwave irradiation in the enzymatic transesterification of methyl benzoate with different alcohols, viz., n-butanol, n-pentanol, n-hexanol, n-octanol, benzyl alcohol, isoamyl alcohol, and 2-ethyl-1-hexanol, was investigated. Synthesis of n-hexyl benzoate was chosen as the model reaction. Different enzymes such as Novozym 435, Lipozyme TL IM, Lipozyme RM IM, and Lipase AYS Amano were screened under microwave irradiation. Novozym 435 was the most active catalyst. To establish the kinetics and mechanism for Novozym 435 catalyzed transesterification of methyl benzoate with n-hexanol, the effects of various parameters affecting the conversion and rate of reaction were studied. Under microwave synergism, an increase in initial rates up to 6.5-fold was observed. Twenty millimoles of methyl benzoate and 10 mmol of n-hexanol in n-heptane with immobilized Candida antarctica lipase B, i.e., Novozym 435, as biocatalyst showed an optimal conversion of 97% at 60 °C in 6 h. Based on initial rate and progress curve data, the reaction was found to follow the ternary complex ordered bi–bi mechanism with inhibition by n-hexanol