Combining solvent engineering and thermodynamic modeling to enhance selectivity during monoglyceride synthesis by lipase-catalyzed esterification

Monoglyceride synthesis by Rhyzomucor or miehei lipase was investigated via direct esterification between glycerol (adsorbed onto silica gel) and olei c acid in organic solvents. The main difficulty is to avoid the unwanted pr oduction of di- and tri-glycerides. It was demonstrated that an increase in solvent polarity, using mixtures of n-hexane and 2-methyl-2-butanol (2M2B) , improves drastically the selectivity toward monoglyceride formation. In p ure n-hexane, the monoglyceride represents only 6 molar % of the total prod ucts at the thermodynamic equilibrium (34 and 60% for di- and tri-glyceride respectively). Use of an equivolume mixture of n-hexane/2M2B enables a pro duct mixture to be obtained containing 94% of monoglyceride at equilibrium (2.3 and 0% for di- and tri-glyceride respectively). This positive effect i s counterbalanced by a decrease both in initial velocities and in substrate conversion at thermodynamic equilibrium. A modeling, able to predict the three thermodynamic equilibria governing th e 3 consecutive reactions, hosed on activity coefficient calculations using the UNIFAC model, is proposed. It takes into account both the partition of water between solvent and immobilized catalyst, and the partition of glyce rol between solvent and silica gel. A good correlation with experimental da ta obtained in n-hexane/2M2B mixtures was observed. (C) 2001 Elsevier Scien ce Inc. All rights reserved.