Enzymatic transesterification of castor oil for the synthesis of methyl ricinoleate

Abstract

Transesterification of vegetable oils has been investigated for the synthesis of methyl esters as diesel substitutes. Chemical methanolysis of natural oils and fats has been the most effective process for the production of fatty acid esters, but the conventional methanolysis has several drawbacks such as the difficulty of glycerol recovery, the need of catalyst removal, and the excessive energy consumption. Several studies investigated the enzymatic transesterification of vegetable oils to solve the problems of chemical methanolysis. However, because of the methanol inhibition, the efficiency of enzymatic transesterification was too low. In this study, among the thirteen tested lipases, Novozym 435, a lipase from Candida antarctica, showed the highest activity on the methanolysis of castor oil. The maximum yield of the methyl ricinoleate using 2 wt% of Novozym 435 and six molar ratio of methanol to oil was 95% at 50℃ during 24hr. However, the conversion of castor oil to the methyl esters decreased considerably as the lipase was used repeatedly. We found that the main reasons were the irreversible inactivation of the lipase by contact with methanol and the mass transfer limitation by the castor oil and glycerol. To solve these problems, we developed the treatment methods: stepwise addition of methanol, addition of organic solvent, and washing with methanol-water solution. Stepwise methanol addition was able to reduce the relative amount of methanol contacting with the lipase. The first-step reaction was conducted in the presence of 3g of castor oil, 60mg of Novozym 435, and 1/3 molar equivalent of methanol for the stoichiometric amount; the second-step reaction in the presence of the first step mixture and 1/3 molar equivalent of methanol; the third-step reaction in the presence of the second step mixture and 1/3 molar equivalent of methanol. But, the yield of methyl ricinoleate was not higher than 80% because of reverse reactions from diricinoleate and monorici...