Study On The Kinetics Of Polyethylene Terephthalate (PET) Alcoholysis Reaction

Polyethylene terephthalate?PET?is a kind of high performance thermoplastic polymer materials.Its consumption in industrial production is huge.Due to its chemical inertness,it is difficult to be degraded by microorganisms in the natural environment.With great impact,the recycling of PET can not only reduce environmental pollution,but also turn waste PET into treasure.Ionic liquids have the advantages of good thermal stability,acid strength and structure that can be adjusted and reused,etc.In recent years,they have been widely used as catalysts or solvents for organic reactions.Ionic liquids have been used to catalyze the degradation reaction of PET.Our laboratory had synthesized double acid type ionic liquids that can efficiently catalyze PET degradation.However,the reaction kinetics and mechanism of PET degradation have not been studied.The study of kinetics and reaction mechanism is conducive to provide a theoretical basis for the industrial recycling of waste PET,and promote the recycling of waste PET.Therefore,the kinetics and mechanism of PET alcoholysis reaction are studied in this paper.An ionic liquid was synthesized[HO₃S-?CH₂?₃-NEt₃]Cl-ZnCl₂?x=0.67?and the structure and acidity were characterized with the fourier infrared spectroscopy and Hammett's indicator.The results showed that the ionic liquid had double acid and the acidity of 2.01.The[HO₃S-?CH₂?₃-NEt₃]Cl-ZnCl₂?x=0.67?and ZnCl₂ as catalysts,respectively,m?PET?:m?catalyst?5:1,m?PET?:m?CH₃OH?1:4,reaction temperature180¹95 ^o C,compared the kinetics of methanol alcoholysis of PET,the results showed that the double acid ionic liquid because of the synergistic catalytic effect of anion and cation,the activation energy of the reaction was lower.The reaction activation energy of the double acid ionic liquid catalytic system was 16.08 kJ/mol,and the reaction activation energy of the traditional catalyst ZnCl₂ catalytic system was 24.28 kJ/mol.Diacid ionic liquid[HO₃S-?CH₂?₃-NEt₃]Cl-ZnCl₂?x=0.67?and zinc acetate as catalysts,respectively,m?PET?:m?catalyst?5:2,n?PET repeat unit?:n?C₄H₉OH?1:3,reaction temperature was 190²05^oC,the kinetics of the n-butanol alcoholysis of PET was investigated.The results showed that the double acid ionic liquid has good catalytic effect and low activation energy.The reaction activation energy of the ionic liquid catalytic system was 39.94 kJ/mol.The reaction activation energy of the traditional catalyst Zn?CH₃COO?₂ system was 48.24 kJ/mol.The double acid ionic liquid[HO₃S-?CH₂?₃-NEt₃]Cl-ZnCl₂?x=0.67?was used as the catalyst.The kinetics of the isooctyl alcoholysis of PET at m?PET?:m?catalyst?5:2,n?PET repeat unit?:n(C₈H₁₇OH)1:5,180²10 ^oC was investigated.The activation energy of the reaction was 46.72 kJ/mol.The mechanism of the reaction was studied.It was found that the alcoholysis process started from the breakage of the PET chain lactone bond and the chain end ester bond.When ZnCl₂ was used as the catalyst,the first was the interaction between the cation Zn²⁺and the carbonyl oxygen on the PET chain.The carbonyl carbon was protonated and then the hydroxyl oxygen in the alcohol molecule attacks the carbonyl carbon positive ion to form a tetrahedral intermediate transition state.Subsequently,the carbon-oxygen bond interacting with Zn²⁺was broken,and the electrons on the oxygen atom in the PET chain proceed.The Zn²⁺was leaved form carbon-oxygen double bond,at the same time,the hydroxyl hydroxyl bond breaked in the alcohol molecule,the hydrogen leaving and the broken carbonyl oxygen group combined,repeat the above reaction process to generate monomer.With Zn?CH₃COO?₂ as a catalyst,the first was that zinc acetate reacts with the alcohol molecule to form two metal alkoxides.The metals in both metal alkoxides have empty orbits,and the lone pair electrons of the ester carbonyl oxygen coordinate with the metal empty orbits.The diacid ionic liquid catalyst was a cation in the ionic liquid[HO₃S-?CH₂?₃-NEt₃]⁺that interacted with the carbonyl oxygen on the PET chain,allowing the carbonyl carbon to be rapidly protonated while the anionic Zn₂Cl₅^-interacted with alcohol hydroxyl oxygen,maked hydroxyl oxygen easier to attack carbonyl carbon.From the above results,it can be seen that the double acid ionic liquid has a good catalytic effect as a catalyst for the degradation reaction of PET,the activation energy of the reaction was lower,and the reaction rate was also faster.