Construction And Synergic Mechanism Of PET Degradation Using Organic Base Complex Catalyst

Polyethylene terephthalate（PET）is one of the most widely used thermoplastic polymer materials in daily life due to its excellent physical and chemical properties,easy processing and high economy.This has led to a series of problems such as environmental pollution,waste of resources and human health hazards.According to statistics,the annual waste of PET polyester reaches tens of millions of tons,the recycling of waste PET can reduce the waste of oil resources and reduce carbon dioxide emissions,which is of great significance to the recycling of resources and the realisation of carbon neutrality and carbon emissions.The glycolysis of PET is an effective way to recycle PET into bis（hydroxyethyl）terephthalate（BHET）,which can be purified to achieve PET recycling or other functional polyesters.One of the key factors determining the speed of PET glycolysis is the choice of catalyst.With the in-depth research on alcoholysis catalysts,the advantages of organic catalysts in degrading PET are gradually explored,and metal ions can promote the breakage of PET molecular chains.In this thesis,different types of catalysts are designed to degrade PET,and the correlation between different structural catalysts and PET degradation efficiency is investigated,and the possible catalytic reaction mechanisms involved in different catalysts.This provides an important theoretical basis for the efficient degradation of PET and the separation and purification of products.The main studies are as follows:In this paper,a series of metal salt catalysts and 1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）were selected for the complex degradation of waste PET to investigate the effect of the composite catalysts on PET alcoholysis,and the effect of different reaction parameters on the alcoholysis reaction of PET glycols was investigated.Compared with the conventional metal salt catalysts,the combination of metal salt and organocatalyst improved the catalytic activity,and Zn（OAc）₂/DBU promoted the molecular chain breakage of PET through the interaction between zinc ions and DBU.n（Zn（OAc）₂/DBU=1:2）,the PET was completely degraded by the reaction at 180°C for77 min,and the yield of monomeric BHET reached 78.2%by SEM showed that the PET was mainly undergoing surface fracture and the glycol kinetic reaction of PET was considered as a first order reaction and suggested a possible acid-base synergistic mechanism.To further achieve green,mild and efficient catalytic recovery of PET,amino（-NH₂）was introduced as a basic site to enhance the attack on PET ester groups.A series of DBU-based non-metallic deep eutectic solvents,[DBUH][DMU],were synthesized with good catalytic activity in PET glycolysis.The catalyst structures were analysed by nuclear magnetic hydrogen spectroscopy（¹H-NMR）and Fourier infrared spectroscopy（FT-IR）,and the results showed that the deep eutectic solvents were successfully synthesized,and the reaction conditions were optimised at DESs additions of 3.2 wt%,m（PET:EG）=1:3,70 min at 180°C,PET conversion up to 100%,BHET yield up to82.62%.The apparent reaction activation energy of low eutectic solvent-catalyzed PET glycolysis was determined to be 160.75 k J/mol.A mechanistic speculation study of[DBUH][DMU]was carried out by NMR hydrogen and carbon spectroscopy,and DFT calculations.It was further verified that a hydrogen bonding interaction between DBU and ethylene glycol was proposed,with the nitrogen atom in DBU forming a hydrogen bond with the hydrogen atom on the hydroxyl group in EG,thus enhancing the electronegativity of the hydroxyl group on EG and facilitating its nucleophilic attack on the PET carbonyl carbon.In addition,in ethylene glycol,the hydroxyl hydrogen atom interacts with the amino nitrogen atom in 1,3-dimethylurea by hydrogen bonding,which helps to enhance the electronegativity of the hydroxyl oxygen atom in ethylene glycol,and the synergistic interaction between[DBUH][DMU]promotes the breakage of the PET chain.In this paper,we have studied different complex catalytic systems for the alcoholysis of PET with different organic bases.A series of experimental preparations and data characterization have shown the efficient application of organic bases in the alcoholysis of PET,which provides a theoretical reference for organocatalysts in the degradation of waste polyester.