Application Of Free Or Immobilized Lipase In Transesterification Of Carbonate

With the growing demand of human life and scientific advancements, catalyst application is becoming more and more important. Meanwhile, an increasing diversity of catalysts were designed to fulfill specific needs. The wide application of catalysts have significantly improved the efficiency of production, yet it brings a variety of new problems, especially the issue of environmental pollution encountered when heavy metal elements get involved. Nowadays the concept of green chemistry is becoming widely acknowledged and considerable amount of scientific research are devoted to eliminate the adverse effect of catalysts.How to resolve this issue? A promising approach is the development of enzyme catalysts. Enzyme, as a biological catalyst, shares a lot of common features with the ordinary molecular catalysts, but also has many distinctive properties per se. Compared to the general catalytic chemical reactions, enzyme-catalyzed reactions are mostly going on smoothly under mild conditions and the corresponding catalytic efficiency is severely enchanced by, usually, about 7 folds and more.With the prosperity of enzyme engineering, extending the application of the enzyme in the reaction regarding organic compounds (e.g., some esterification reactions in water that are very difficult to set off) attracts increasingly amount of attention. In this thesis, enzyme Novozym435 is used for the first time to catalyze the esterification reaction of dimethyl carbonate (DMC) with isoamyl alcohol to yield an asymmetric carbonate, which extends the range of application of enzyme Novozym435. Since biological enzymes are very expensive, the recycling of enzymes after production is a very hot topic in literature. Moreover, immobilization of enzyme is an effective and feasible method to improve recycling rates. Therefore, exploring the fixation method of free enzyme becomes very valuable. As industrial production applications that require immobilization process should be non-toxic to the greatest extent possible, energy-conservative, and should be simple and convenient to improve the activity of biological enzyme. Further more, immobilized carrier of its use should be cheap and easy to get and the obtained immobilized enzyme could be easily separated from the reaction system. The main focus of our research is to explore enzyme-catalyzed transesterification reaction of DMC with alcohol and to look for cheap enzyme with its operability of immobilization and higher catalytic activity for the purpose of replacing expensive commercial enzyme, and to extend its range of applications. This thesis can be divided into five parts:Chapter 1:The free enzyme and immobilized enzyme were reviewed from the aspects of concepts, principles, application and the current status of development. Besides, the catalytic applications of enzyme, lipase in particular were systematically introduced.Chapter 2:Commercialized enzyme Novozym435 was firstly introduced into the transesterification reaction between dimethyl carbonate (DMC) and iso-amyl alcohol for producing an asymmetric carbonate. Even at mild temperature and pressure conditions, Novozym435 catalyst showed good catalytic activity. At optimized reaction conditions (reaction temperature:60?, reaction time:48h, (to DMC) of Novozymes4350.98 wt%, isoamyl alcohol/DMC molar ratio:1:12), the conversion rate of isoamyl alcohol is 93.40%. After 9 cycles, the recycled Novozym435 can still achieve 91% of the catalytic activity in the unrecycled case.Chapter 3:Through a series of screening of biological free enzyme, Aspergillus niger lipase and porcine pancreatic lipase were selected for the transesterification of DMC with iso-amyl alcohol. At optimized conditions, i.e., the amount of Aspergillus niger lipase was 8.17wt%(to DMC) and alcohol ester molar ratio is 1:12, the conversion effeciency of isoamyl alcohol is 83.56%.Chapter 4:After screening the different fixation materials, YWD09A5 resin and X-5 resin were selected as the immobilized materialfor Aspergillus niger. After condition optimization (i.e., free enzyme and the carrier mass ratio is 5:4, pH= 7, the concentration of enzyme solution is 80mg/mL), the optimum catalytic activity was obtained after immobilization. At optimized reaction conditions (the temperature is 60?, the molar ratio of alcohol esters 1:12, immobilized lipase Aspergillus niger 5.23% (to DMC quality ratio)), the conversion rate of isoamyl alcohol is 83.7%. Moreover, after immobilization, the dosage of enzyme is reduced to 73.2% of the original amount, and after 9 cycles, the recycled immobilized enzyme still achieved 70% of the catalytic activity of the initial efficiency.Chapter 5:In order to investigate the using scope and limitation of the Immobilized Aspergillus niger lipase, different diol ester were used to set off transesterification with DMC. GC-MS was applied to characterize the product yield. Most of the diols could react with DMC and the corresponding products were mainly monomolecular substituents. Sterical hindrance is one of the main factors determining the yield of reaction. Moreover, highest yield was achieved for those reactions whose products possess a five-member ring structure, the yield is lesser with products being a six-membered ring, followed by the seven-membered ring structure. Furthermore, the functional group in diol also had an effect on the conversion rate. When the substituent was an electron donating group (e.g. unsaturated bond, aromatic ring, etc.), the conversion rate of the diol could be improved; when the substituent was an electron withdrawing group (such as chlorine atom), the conversion rate of diol sharply declined; what's worse, the transesterification would be inhibited completely.