The Mechanism And Application Of Ionic Liquids Mediating Enzymatic Alcoholysis Reaction

Glyceride alcoholysis catalyzed by lipase is an important reaction type for preparing diacylglycerol,monoglyceride and fatty acid esters,which has the advantages of mild condition and high catalytic efficiency.At present,organic solvents are generally used as medium,but have many disadvantages.As a new type of reaction medium,ionic liquids have some good characteristics,such as low vapor pressure,good thermal stability and chemical stability and so on,which can make up the drawbacks of organic solvents.Therefore,this paper intended to use ionic liquids instead of organic solvents as medium for enzymatic glycerol ester alcoholysis reaction.In this paper,the enzymatic reaction of monoglyceride with methanol was studied as a model and initial activities and stability of lipase were taken as the main indexes to explore the effect of ionic liquids on enzymatic alcoholysis.In addition,oils consisted of different fatty acids were used as acyl donors to verify the effect of ionic liquids on the preparation of fatty acid esters by enzymatic alcoholysis.Finally,the feasibility of practical application of ionic liquids was investigated by the preparation of fatty acid methyl esters.The main research contents were as follows:Firstly,the effect of ionic liquid on initial activities of three lipases was studied by the alcoholysis reaction of monoglyceride and methanol,the initial activity of lipase was detected by gas chromatography,and analyzed the correlation between the main properties of ionic liquids and initial lipase activities.The results indicated that the anion of ionic liquids based on [Tf2N]-had a significant effect on initial lipase activities,and the initial lipase activities higher in ionic liquid based on low nucleophilic anion were higher than those in others.In addition,the cation of ionic liquids had a certain correlation with the initial lipase activities.As the cationic chain lengthened,the initial lipase activities were gradually increased.The hydrophobicity of ionic liquids were positively correlated with the initial lipase activities(p<0.01),and the correlation coefficient was about 0.9.Hydrogen bond basicity and polarity were negatively correlated with the initial lipase activities,and the correlation coefficient was about 0.5~0.7,while the viscosity was almost independent of the initial lipase activities.Taking an example of Novozym 435,the stability and secondary structure of lipase were investigated in ionic liquids which lipases owned activities.The results showed that ionic liquids based on low nucleophilic anions could maintain the stability of lipase,and the secondary structure of lipase immersed in these ionic liquids was more similar with the original lipase than those in other ionic liquids.Secondly,the effects of ionic liquids on the preparation of fatty acid esters by enzymatic alcoholysis were studied by some oils consisted of different fatty acid compositions as acyl donor.The production of fatty acid ester was measured by gas chromatography.The result indicated that short chain fatty acids were easier to alcoholysis than long chain fatty acids.The production of fatty acid ester was higher in ionic liquids based on [Tf2N]-and [PF6]-,and the influence of anions was more obvious.The properties of ionic liquids had correlations with the production of fatty acid ester.With the increase of ET N and β,the production of fatty acid ester generally showed a decreasing trend.With the increase of log P,the production of fatty acid ester increased(p<0.05).Viscosity had no obvious correlation with the production of fatty acid ester.Among these properties,there was higher correlation coefficient between log P,β and fatty acid ester production,which reached approximately 0.9.These similar results were observed in the preparation of fatty acid ester based on different fatty acid compositions of oils.Finally,the effect of ionic liquids on the positional selectivity of lipase with positional specific was studied.The results showed that the larger the polarity of ionic liquid,the weaker the positional selectivity of lipase.Therefore,when ionic liquids were selected as the medium for the preparation of the fatty acid ester by the alcoholysis reaction,both of lipase activities and the positional selectivity should be considered.The preparation of fatty acid methyl esters with high oleic sunflower oil and methanol was used to verify the applicability of ionic liquids as reaction medium.The methanol was added for one time,and the reaction conditions were optimized respectively: Lipozyme TL IM as catalyst,[BMIM][PF6] as reaction medium,substrate molar ratio(oil and methanol)of 1:3,immobilized enzyme amount(percent of substrate quality)of 15%,ionic liquid amount(percent of substrate quality)of 75%,temperature of 40oC,reaction time of 12 h.The yield of fatty acid methyl ester reached 90.7%.After the ionic liquid was reused 6 times,the yield of fatty acid methyl ester was still up to 86.4%.