Enzymatic Synthesis Of Fatty Acid Sugar Esters In Ionic Liquid Systems

Lipases can be used to catalyze both esterification and transesterification reactions to synthesis sugar fatty acid esters(SFAEs). Enzymatic synthesis of sugar esters needs to be carried out in a non-aqueous medium, but the major problems that restrain the extensive use of enzyme-catalyzed sugar esters production include the cost of the enzyme as the catalyst, the choice of solvent as the reaction medium, and the selection of substrate. In view of the above problems, we have screened 13 ionic liquids(IL) and 14 deep eutectic solvents(DES) as reaction medium for sugar ester synthesis; we have investigated the use of IL/2-methyl-2-butanol(2M2B) bisolvent system for both esterification and transesterification reaction with different combinations of substates to synthesize sugar fatty acid esters; and we have carried out activity tests for 6 commercial lipases as catalysts used in sugar ester synthesis and their immobilization(by forming cross-linked enzyme aggregates(CLEA) and adsorbing enzymes on hydrophobic/ion exchange functionalized diatomite), respectively.The major contents are given below:1. Enzyme screeningAmong the six commercial lipases that have been screened, Novozym 435 and Lipozyme TLIM(both products of Novozymes, Denmark) can provide superior conversions when being used as catalysts for SFAE synthesis.2. Enzymatic synthesis of sugar fatty acid estersAmong the 13 ILs that have been screened as solvents for enzymatic SFAE synthesis, [HMIm][TfO] was found to be the one to yield the highest conversion. The bisolvent system constituted by this IL and 2M2 B has been utilized as reaction medium for SFAE synthesis with two reactions:(1) transesterification of glucose and fatty acid vinyl esters, catalyzed by Lipozyme TLIM; and(2) esterification of methyl glucoside and fatty acids, catalyzed by Novozym 435. The former was optimized by using Box-Behnken Design(BBD) of Response Surface Methodology(RSM), and a superior conversion of 94.0% was obtained. Deep eutectic solvents can also be used as reaction medium for enzymatic SFAE synthesis, but with a poor conversion. For both reactions, the conversions increased with an increase in the length of the fatty acid chain. A high conversion of 99.6% was obtained within 7 h for the synthesis of glucose stearate by using the first reaction(transesterification), whereas the second reaction(esterification) yielded a conversion of 63.0% within 24 h when stearic acid was used as the acyl donor.3. Lipase immobilizationImmobilization of Aspergillus niger lipase(ANL) was conducted by using the CLEA technology, and the process was optimized by adopting Response Surface Methodology(RSM) with Central Composite Rotatable Design(CCRD). The catalytic properties of ANL-CLEA were investigated, and its surface morphology was examined by using scanning electron microscope(SEM). The results showed that the surface of ANL-CLEA was rich in pore structure. In aqueous solution, ANL-CLEA was significantly more stable than the free enzyme. ANL-CLEA was also catalytically active in non-aqueous medium such as organic solvents(e.g., 2M2B) and ionic liquids(e.g., [HMIm][TfO]), and a conversion of 44.5% was obtained in 24 h when it was used to catalyze the transesterification of glucose and vinyl laurate for synthesis of glucose laurate in 2M2 B.Thermomyces lanuginosus Lipase(TLL) was immobilized on the octyl and sulfonic acid co-bonded celite(OSC) via hydrophobic and strong cation-exchange interactions. The immobilized enzyme showed great catalytic activity in the [HMIm][TfO]/2M2B(3/7, v/v) bisolvent system. Under the optimal conditions, the conversion of the transesterification reaction to produce glucose laurate was 95.0% in 24 h, which was similar to that of the commercial Lipozyme TLIM.