| Epoxidized fatty acid methyl ester is a kind of PVC-plasticizers and stabilizers which can be used as reactive diluents for paints and biodegradable biolubricants because of its compatibility, friction reducing and dispersity. In addition epoxidized fatty acid methyl ester can be used as intermediates for production of polyurethane, polyester and epoxy resins by ring opening or polymerization because of the highly reactive oxirane ring due to polarity and ring strain. Epoxidized fatty acid methyl esters are widely used in plastics industry, polymer science and so on.Replaced the aqueous hydrogen peroxide (aq.H2O2) with the complex urea-hydrogen peroxide (UHP) as oxidant, soybean oil methyl ester (SOME) was epoxidized in solvent-free system catalyzed by immobilized lipase Novozym435. Hydrogen peroxide concentration in organic phase during epoxidation was determined by ultraviolet spectrophotometer(UV), which confirmed that UHP could release hydrogen peroxide in a controlled manner. Reaction parameters including enzyme loading, rotate speed, temperature and the amount of UHP were optimized. The best conditions for the epoxidation had been found to be:40℃,150rpm, molar ratio of hydrogen peroxide contained in UHP to carbon double bonds1.1:1, enzyme loading of40mg/g. Under the optimized conditions the oxirane value achieved4.94%(wt) after11h. Scale-up of the epoxidation in a reactor with volume of200mL was investigated, where a stainless steel screen was used to separate UHP from enzyme. The oxirane value achieved4.24%(wt) and could maintain at3.3%(wt) after three cycles, which indicating the improvement of lipase stability, while the residual enzyme activity was4%when aq.H2O2was used as oxidant in solvent-free system.In the chemo-enzymatic epoxidation, when using UHP as oxidant without other oxygen carrier, SOME could be as oxygen carrier by reacting with hydrogen peroxide to give peroxide acids catalyzed by lipase. However, TOME was not able to be epoxidized in the system only when stearic acid was added as oxygen carrier and the oxirane value could achieve6.14%(wt) after18h. It indicated that TOME can not react with hydrogen peroxide to give peroxide acids catalyzed by lipase because of the conjugate double bonds in TOME.The epoxidation of TOME in the formic acid self catalysis system was also investigated. Reaction parameters including temperature, the amount of formic acid and hydrogen peroxide were optimized. The best conditions for the epoxidation had been found to be:60℃,450rpm, molar ratio of hydrogen peroxide and formic acid to carbon double bonds1.5:0.5:1. Under the optimized conditions the oxirane value could achieve5.91%(wt) after3h. The rate constants obtained at60℃was13.5×10-6L-mol-1·s-1, enthalpy of activation was27.3KJ·mol-1, the entropy of activation and free energy of activation were-256.98J·mol-1·K-1,58.10KJ·mol-1respectively. These value of constants indicated that the reaction was non-spontaneous under the present experimental conditions because the free energy of activation was positive and the present reaction was endothermic in nature because the enthalpy of activation was positive, so an increase in the reaction temperature may lead to an increase of oxirane value. Activation energy of the epoxidation of TOME was30.1KJ·mol-1, lower than that of other oil. Due to the electron provided by conjugated carbon-carbon double bonds, the cyclic oxygen ion intermediate could be stabilized and the activation energy was lower. Therefor, TOME can be easierly epoxidized... |
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