Enzymatic synthesis of polyol seters in aqueous - organic two-phase systems

The last decade increasingly attention is paid to lipases as catalysts for synthesis of components, such as fatty acid-based surfactants, flavors, edible oil equivalents, monomers and polymers, and amides. In this thesis, the lipase-catalyzed esterification of polyols and fatty acids is described. These esters consist of a nonpolar part (fatty acid) and a polar part (polyol). Therefore, polyol esters have surface-active properties and are used as emulsifier in food, pharmaceutics; and cosmetics. One of the aims of this thesis is to develop a reaction system for the esterification of polyols (carbohydrates) and fatty acids, without any modification of the substrates. Also, high reaction rates are desired.Enzymatic esterification is often performed in the presence of organic solvents. Besides activity and stability of the enzymes, the solvents will affect the equilibrium position of reactions. In literature, models were described for the prediction of the equilibrium position in dilute two-phase systems. However, for industrial applications, high product concentrations are desired, which implicate the use of nondilute reaction systems. Another aim of this thesis is to gain a better insight in factors that affect the equilibrium position of a reaction and to predict the product concentrations at equilibrium in non-dilute two-phase systems.In chapter 2 and 3, the lipase-catalyzed esterification of sorbitol and fatty acid is studied in two different two-phase reaction systems. In chapter 2, 2-pyrrolidone is used as a cosolvent for sorbitol. In this study, the lipase from Chromobacterium viscosum is used and the initial esterification rate is high as compared to literature data. The water activity is found to be important for the ester concentrations at equilibrium. High concentrations of the cosolvent 2-pyrrolidone should be avoided, because these will inactivate the lipase. In the reaction system that is described in chapter 3, water is used to dissolve sorbitol. Candida rugosa lipase is used in this study and initial esterification rates are slightly higher than in chapter 2. The water activity is dependent on the sorbitol mole fraction in the aqueous phase and lowering of the water activity is limited by the solubility of sorbitol. A two-phase membrane reactor is a suitable type of reactor, since the water activity of the aqueous phase can be kept constant during the experiment and lipase possesses a good stability. In both reaction systems, besides sorbitol also glucose and fructose can be used as a substrate, while disaccharides, such as sucrose, are not reactive at all.In chapter 4, the lipase-catalyzed esterification of glycerol and decanoic acid has been studied in aqueous-organic two-phase systems. The addition of an organic solvent is found to influence the ester mole fractions at equilibrium. For the synthesis of polar products (monoesters), a polar solvent (low log P) is favorable, while for the synthesis of nonpolar products (triesters), it is better to choose a nonpolar solvent (high log P). The computer program 'Two-phase Reaction Equilibrium Prediction' (TREP) has been developed for the prediction of the ester concentrations in nondilute two-phase systems, in case both the reaction equilibrium as well as the phase equilibrium are achieved. This program is based on mass balances and the UNIFAC group contribution method. Deviations in the prediction with TREP are generally less then a factor of 2 and are due to inaccuracies of the UNIFAC group contribution method.The lipase-catalyzed acylglycerol synthesis with fatty acids of different chain length is studied in chapter 5. For predictions with TREP, one set of equilibrium constants is used for monoester, diester, and triester synthesis. It is shown that with this set the equilibrium position of the reaction between glycerol and all saturated fatty acids with a chain length from 6 to 18 and oleic acid can be calculated within some margins. For fatty acids with different chain length, the ester mole fractions at equilibrium are clearly different. With the short-chain hexanoic acid, the monoester mole fraction is highest, while for the long-chain oleic acid, the diester mole fraction is the highest one. Besides the equilibrium position, also the reaction rates are affected by the solvent that is added. In polar solvents, the monoester production rate is enhanced. This is caused by the shift in the equilibrium mole fractions.In chapter 6, the effect of solvents on the esterification of decanoic acid and several alcohols, such as 1-dodecanol, 1-butanol, 1,3-propanediol, and sorbitol is studied. In agreement with the previous results, the ester mole fractions at the reaction equilibrium are dependent on the solvability of the ester in the organic phase. This effect is most striking for the polar sorbitol esters. Almost no esters are present at equilibrium in systems with nonpolar solvents, while reasonable high ester mole fractions can be obtained in systems with polar solvents. In contrast with the results of chapter 5, the equilibrium constants are clearly affected by the type of alcohol that is chosen as a substrate. Calculations with TREP showed that the calculated ester mole fractions did not deviate more than a factor of 1.5 from the measured ones. However, it appears that the calculated water mole fractions deviate systematically in the downwards direction.Chapter 7 shows a comparison between models in literature for the prediction of the equilibrium position in dilute two-phase reaction systems and calculations with TREP. It is shown that the models from literature are limited to reaction systems in which partition coefficients are constant. The program TREP can be used for nondilute as well as dilute reaction systems.Furthermore, this chapter shows that the ester mole fractions at equilibrium can be increased with increasing temperature. This is due to the increase of the solubility of sorbitol with increasing temperature. Most pronounced is the effect of temperature on the reaction rate, which is increased enormously. However, for long-term processes at high temperatures it is important that heat-stable lipases will be used.