Study On Efficient Synthesis Of L-Lactide

Among many bio-based materials,polylactic acid（PLA）is the most promising and popular material because of its ideal properties:no pollution to the environment,natural degradation,good biocompatibility and various uses.High molecular weight PLA is usually prepared by lactide ring opening polymerization.In industrial production,tin-based catalysts（such as Sn（Oct）₂,Sn Cl₂,Sn O）are widely used in the production of lactide.However,there are some problems in the synthesis of lactide,such as high energy consumption,low product yield and low chemical purity.The residual tin-based catalyst has strong neurotoxicity,and the PLA prepared from the lactide can’t be used in the fields of food,health and medicine.Therefore,it is of great significance to develop efficient and green catalysts to replace tin-based catalysts.Therefore,this subject mainly study the preparation process and mechanism of L-lactide by two-step method with zinc lactate（ZnO）and ZnO-creatinine（CR）composite as catalysts.Firstly,the process parameters of free water removed from L-lactic acid in the early stage were explored.It was found that the free water removal rate of L-lactic acid reached90%at 85℃and under vacuum of 50～55 KPa for 2.5 h.On this basis,the catalytic effect of zinc lactate,ZnO,cerium chloride and zinc acetate on the catalytic synthesis of L-LA was studied.The results showed that the crude yield of L-LA was 67.72%and the precision yield was 20.04%under the catalysis of ZnO nanoparticles.In addition,the purified L-LA was characterized by nuclear magnetic resonance hydrogen spectrum（¹H NMR）and Fourier transform infrared spectroscopy（FTIR）.The shift of group characteristic peak shows that the synthetic product was determined as L-LA.At the same time,the melting point of purified L-LA is characterized by differential scanning calorimetry,and the melting point reached99.15℃.Secondly,the effects of catalyst content,debonding water reaction temperature,debonding water reaction time and distillation temperature on the yield of L-LA are investigated.The results show that the optimum preparation process of L-LA is polycondensation stage,the content of ZnO is 0.2 wt.%,the reaction is at 120℃and under vacuum of 50～55 KPa for 5 h to form LA oligomer,the crude L-LA is evaporated at 240℃and under vacuum of 80～90 a,and finally the refined L-LA is purified by absolute ethanol.The yield of L-LA is 40.98%.The distillation temperature has a great influence on the yield of L-LA.The ¹H NMR analysis of crude lactide and depolymerization substrate at different stages under better conditions showed that meso-LA appeared in the cyclization process,and the content of meso-LA gradually increased with the increase of time at high temperature.After reacting at 240℃for 30 min,the content of meso-LA reached 15.31%.The results are consistent with those of crude lactide.Finally,co-catalyst CR is added to further optimize the preparation process of L-LA.The contents of ZnO and CR are both 0.2wt.%,and ZnO is added in the dehydration stage and CR is added in the cracking stage.The refined yield of L-LA calculated by mass method is 45.58%,which is about 5%higher than that using ZnO as catalyst alone.The ¹H NMR results and HPLC analysis show that the appearance time of meso-LA in the cyclization reaction is longer than that in the later stage.The addition of CR reduced the formation rate of meso-LA,and the content of meso-LA is lower 10%than using ZnO as catalyst after 30min reaction at 240℃,which was also the reason for the increase of yield after the addition of CR.