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

Monoglyceride synthesis by Rhyzomucor miehei lipase was investigated via direct esterification between glycerol (adsorbed onto silica gel) and oleic acid in organic solvents. The main difficulty is to avoid the unwanted production 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 pure n-hexane, the monoglyceride represents only 6 molar % of the total products at the thermodynamic equilibrium (34 and 60% for di- and tri-glyceride respectively). Use of an equivolume mixture of n-hexane/2M2B enables a product mixture to be obtained containing 94% of monoglyceride at equilibrium (2.4 and 0% for di- and tri-glyceride respectively). This positive effect is 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 the 3 consecutive reactions, based 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 glycerol between solvent and silica gel. A good correlation with experimental data obtained in n-hexane/2M2B mixtures was observed.