Study On Synthesis And Resolution By Lipase-catalyzed Transesterification In Ionic Liquids

Possessing the characters of non-volatile, thermal stability and environment friendly, ionic liquids were labeled to "green" solvent. Its character of high polarity also extends the range of reaction media for enzymatic catalysis. In our study, the synthesis of spice (geraniol acetate) and the resolution of racemates (menthol, (R, 5)-4-hydroxy-3-methyl-2-(2-propenyl)-2-cyclopenten-1-one ((R, S)-HMPC)) were achieved in ionic liquids by three lipase that are usually used in biocatalysis. Meanwhile, some problems associated with enzyme catalysis in ionic liquids were also addressed.With a water-soluble ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) as the target product, the synthesis methods of ionic liquids by ultra sound were studied. It was found that the intermediate (1-butyl-3-methylimidazolium bromide, [BMIM][Br]) could be synthesized in short period of time (about 1 h) if the ultra sound treatment conditions (740 W～760 W, work 4 s, pause 5 s) were controlled and appropriate starting materials (1-bromobutane, 1-methylimidazoium) were selected. A one pot synthesis of ionic liquid [BMIM][BF₄] under the treatment of ultra sound was also carried out in this study, setting water bath temperature at 80 °C during ultra sound treatment about one hour may result in high yield of product (>50 %).Transesterification of geranol and vinyl acetate in ionic liquids were investigated to synthesize geranyl acetate using immobilized Candida antarctica lipase B as biocatalyst. The reuse of ionic liquids and lipase were also studied in this reaction. The optimum reaction conditions were as followings: at molar ratio of 9.42 (vinyl acetate/geranol), a_w=0.28, 30 °C in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]). It was difficult to separate immobilized lipase from ionic liquids through filtration, but successive batch reaction may be a good way for reuse of ionic liquids and lipase. The volume of ionic liquid and catalytic ability of enzyme did not reduce significantly after being reused twelve times at the optimum conditions.The resolution of menthol in ionic liquids was investigated using Candida cylindracea lipase as biocatalyst. In the meantime, some methods that can increase selectivity of lipase in ionic liquids were also explored. The results showed that adequate propionic anhydrate concentration (propionic anhydrate to menthol at molarratio of 1:1), proper enzyme amount and low water activity (water activity below 0.63) can increase selectivity of enzymatic reaction and improve resolution of menthol in ionic liquids.Resolution of (R, S)-HMPC was achieved in ionic liquids using Pseudomonas cepacia lipase as biocatalyst. Meanwhile, the catalytic efficiency of different enzyme formulations which were prepared by different methods (coprecipitation, lyophilization, preparation of enzyme coated micro crystals) were evaluated in ionic liquids. The resolution of (R, S)-HMPC in ionic liquids by commercial lipase was studied in the first place and the resolution of (R, S)-HMPC in vinyl acetate was chosen as control reaction. The results showed that lipase exhibited higher initial rate and thermostability in ionic liquid [BMIM][PF₆]. The optimum resolution conditions were as followings: a_w 0.17,40 °C, pH 7. Further study on enzyme formulation showed that lipase co-precipitated with Sorbitan monostearate (Span 60) or stearic acid can drastic improve conversion in [BMIM][PF₆]. Preparation of lipase with β-cyclodextrin using two methods (lyophilization, preparation of enzyme coated micro crystals) both can improve conversion in the ionic liquids. Additional experiments were performed to investigate catalytic efficiency and some properties of lipase lyophilized with different cyclodextrins. The results showed that E value and conversion of the reaction were determined by cyclodextrin types and reaction media, lipase lyophilized with hydroxypropyl-β-cyclodextrin can achieve better catalytic result when the reactions were performed in [BMIM][PF₆]. The initial rate of the reaction markedly increased as the amount of cyclodextrins used for lyophilization gradually increased.