Application Of Phenylboronic Acid To The Enzymatic Synthesis Of Sugar Ester

Sugar esters are non-ionic, nontoxic and biodegradable surfactants that can be synthesized from renewable resources. Their hydrophilic-lipophilic balance (HLB) may be modulated by varying the fatty acid, the sugar and the degree of substitution. Now they have been widely used in food, pharmaceutical and cosmetic industries. Enzymatic synthesis of sugar esters in non-aqueous media faces with two difficulties: the one is the low solubility of sugar in organic solvent, the other is the impurity of ester obtained. This paper deals with the improvement of the synthesis of sugar ester by the addition of phenylboronic acid. The effect of phenylboronic acid on the synthesis of sugar ester were at first studied.The introduction of phenylboronic acid increased the solubility of glucose and improved the specificity of the reaction. At the optimal molar ratio of glucose to phenylboronic acid, that is 1:2, the solubility of glucose was increased two times and the production only monolauroyl glucose ester was produced. The initial reaction rate was doubled when phenylboronic acid was added. The introduction of phenylboronic acid increased remarkably the conversion of monosaccharide and that of fatty acids with medium carbon chains, but had little effect on disaccharide and fatty acids with long carbon chains.The batch synthesis of lauroyl glucose esters in the presence of phenylboronic acid was then optimized. The optimal conditions of synthesis were obtained as follows:glucose 50 mmol/L, molar ratio of glucose to phenylboronic acid 1:2, molar ratio of lauric acid to suagr 3:1, dosage of lipase 20 g/L and that of molecular sieves 100 g/L, solvent tert-butyl alcohol, reaction temperature 50℃, reaction time 24 h. A conversion ratio up to 90.1% was obtained.The effect of phenylboronic acid on the continuous synthesis of sugar ester was also investigated. Optimal conditions for continuous production in tert-bytul alcohol were as follows: 1,2:3,5-glucose phenylboric ester 50 mmol/L, lauric acid 150 mmol/L, dosage of lipase 10 g and that of molecular sieve 5 g per reator, 5 g molecular sieve were dayly replaced, flow rate 0.2 mL/min. The productivity of this CSTR reactor has reached to 12.4 g/(L.d), which was much more than that of batch reaction. The conversion of maltose is 59.2%. The separation and purification of lauroyl glucose ester was also tested. After washing the reaction mixture by hexane and then centrifugated for two times, the purity of the final product was 89.46%. A flow sheet was proposed for the industrial production.The equilibrium of water at different reaction steps was determined by the Karl Fischer method. The adsorption isotherm of water on 3A molecular sieve in tert-butyl alcohol was determined by the same method. A mathematic model was proposed on the base of the mass balance of water and the law of mass action for predicting the equilibrium conversion ratio for the synthesis of lauroyl glucose ester. The validity of the model was confirmed. The predicted conversions for monolauroyl glucose ester agreed well with the experimental data except for the cases where the molar ratio of the lauric acid to the alcohol was high and the excess amount of molecular sieve.