Study On The Synthesis Of L-lactide From Cellulosic L-lactic Acid

After long-term research,the dry biorefinery technology of lignocellulosic biomass has basically overcome the main technical obstacles in the process of pretreatment,detoxification,saccharification and fermentation,and has realized the production of cellulosic L-lactic acid fermentation with high concentration,high chirality and low residual sugar.However,the synthesis of poly(lactic acid)precursor L-lactide from cellulosic L-lactic acid based on dry biorefinery technology has not been reported.In this study,the synthesis of L-lactide from L-lactic acid produced by lignocellulosic feedstock was studied.Firstly,in this thesis,the cellulosic L-lactic acid obtained by dry biorefinery technology was separated and purified by conventional and easy-to-operate separation and purification operations(centrifugation,decolorization,crystallization,acidification,and refinement)to obtain a recovery rate of 69.30%and a purity of 98.19%(w/w)cellulosic L-lactic acid.Among them,the decolorization rate of L-lactic acid is 98.37%,which completely removed the residual phenolic substances derived from lignin degradation after biological detoxification.Then,L-lactide with an optical purity of 98.95%,a chiral purity of 98.99%and a chromatographic purity of 94.66%was successfully synthesized by using the refined cellulosic L-lactic acid.Finally,the structure analysis(NMR,FT-IR),melting point determination,molecular weight determination and element content determination of L-lactide from lignocellulose were carried out.Compared with the L-lactide standards,it is found that the characterization results of L-lactide derived from lignocellulose were completely consistent with the standards.In this thesis,a conventional and easy-to-operate purification method was used to obtain polymer-grade cellulosic L-lactic acid;the goal of synthesizing poly(lactic acid)precursor L-lactide from L-lactic acid based on dry biorefinery technology was achieved,which provides a very important basic research basis for the synthesis of non-grain-based polymers from lignocellulosic biomass.In order to further simplify the purification steps of cellulosic L-lactic acid,the long-term adaptive evolution of L-lactic acid engineering bacteria Pediococcus acidilactici ZY271 was studied in low pH environment.In order to enhance the adaptability of P.acidilactici ZY271 in low pH environment,a long period of continuous transfer culture was carried out under the condition without using pH neutralizer.With the decrease of pH,the cell viability of adaptive evolutionary strains was significantly higher than that of their parents,and the production of L-lactic acid was also increased to a certain extent.Under the condition of pH 4.0(close to the pKa value of L-lactic acid),the cell viability of the adaptive evolutionary strain was 4.21 times higher than that of the parent strain,and the yield of L-lactic acid increased by 34.67%.