Chemical Depolymerization Of Polyester Materials Catalyzed By [Bmim]Cl·FeCl₃ And DBU-based Ionic Liquids

Polyester materials including poly?bisphenol A?carbonate?PC?,poly?lactic acid??PLA?and poly?ethylene terephthalate??PET?derived from fossil energy have been widely used in automobiles,packaging,and electronic appliances due to their excellent properties.However,the waste polyesters have brought great waste of resources and environmental pollution.From the perspective of“green chemistry”and“sustainable development”,the recycling of waste plastics has become more and more important.For the waste polyester materials,the traditional disposal methods include landfill,incineration and physical recycling.These cause the environmental pollution and the waste of resources.Moreover,the mechanical and physical properties of the recycled polyester materials are greatly reduced.Therefore,chemical depolymerization of waste polyester materials has attracted great attention,as it could convert waste polyester into their starting monomers or other valuable chemicals for virgin plastic reproduction.Ionic liquid refers to a salt ionic liquid composed entirely of organic cations and inorganic or organic anions.It is a kind of solvent and catalyst with excellent properties,which has been widely used in polymer degradation in recent years.In this project,ionic liquid with simple synthesis and excellent effect was selected as the catalyst to achieve the chemical depolymerization of PC under mild conditions.A series of Lewis acid ionic liquids[Bmim]Cl·FeCl₃ with different acid strengths were first synthesized,they were used to catalyze the alcoholysis of PC.The effects of reaction temperature,catalyst concentration,alcohol dosage and reaction time on the alcoholysis reaction were investigated.A 100%PC conversion with 97.2%bisphenol A?BPA?yield was obtained under the conditions of 5.0 mol%[Bmim]Cl·2.0FeCl₃loading,120 ^oC,n?CH₃OH?/n?PC?=6:1 for 3.0 h.The influence of different alohol structures on the PC alcoholysis reaction was evaluated.It showed that methanol as an alcoholysis reagent could afford a highest BPA yield.Moreover,the catalyst could be reused six times without a significant decrease of catalytic activity.The mechanism and kinetics for the PC methanolysis were studied,the reaction followed a first-order kinetic model and the activation energy was 98.9 kJ/mol^-1.Several DBU-based ionic liquids were synthesized by simple one-step neutralization method,and their catalytic performance in PC alcoholysis reaction was studied.The effects of different catalyst structures and catalytic reaction conditions on PC alcoholysis reaction were investigated.On the basis of the experiment results,[HDBU][LAc],[HDBU][OAc]and[HDBU][PAc]showed slight difference in activity,mainly due to the similar acidity-alkalinity of the synthesized DBU-based ionic liquid.The[HDBU][LAc]with super thermostability was selected as the catalyst to study the influence rules of different reaction parameters.The optimum reaction conditions were obtained:T=120 ^oC,t=1 h,n?CH₃OH?/n?PC?=5:1,and the[HDBU][LAc]loading was 0.80 mol%of PC.Under the above conditions,a 100%PC conversion with 95.0%yield was obtained.The effects of different alcohol structures on the catalytic activities were investigated.The reactivity decreased with the increase of the steric hindrance of the alcohol structure.After 6 cycles of catalyst recycling,there was no significant decrease for the PC degradation rate and BPA yield,indicating a good structural stability.Combined with the experimental results,the mechanism of[HDBU][LAc]catalyzed PC methanol alcoholysis reaction was discussed in depth.On the basis of the above studies,a series of basic ionic liquids with DBU as cations and different substituted imidazoles as anions were synthesized by one-step method in order to further reduce the energy consumption of PC alcoholysis reaction and to show the catalytic universality of other waste polyester materials for alcoholysis.The effects of different imidazolium anions on PC alcoholysis was compared,and[HDBU][Im]showed excellent catalytic activity.Thereafter,the reaction conditions such as reaction time,reaction temperature,catalyst amount and methanol dosage were optimized.The PC could be completely alcoholized,and the BPA yield was over 96%under the optimal conditions of 70 ^oC,n?CH₃OH?/n?PC?of 5:1 and catalyst concentration of 1.60 mol%for 2.0 h.Different substituted alcohols were examined for PC depolymerization over[HDBU][Im]catalyst.Given the potential of[HDBU][Im]as catalyst,the versatility to methanolysis of other polyester wastes,such as PLA,PET and PHB were also examined and compared.The catalysts were easy to reuse for at least six cycles without deactivation.Moreover,combining the obtained results with activation analysis,a synergistic catalysis mechanism resulting from cation-anion ions was proposed.