Theoretical Study On Thermal Degradation Mechanism Of Polyethylene Terephthalate Model Compound

Poly(ethylene terephthalate（PET）are widely used in all walks of life because of the excellent physical properties and chemical properties,but it is difficult to spontaneously degrade in the natural condition.The usage of PET plastic leads to serious pollution for the environment.Pyrolysis is considered as a feasible solution for the treatment and recycling of PET waste.And the evolution and formation mechanisms of PET products by pyrolysis through research theoretically can offer a certain theory basis for the heat recovery and application of PET waste.Therefore,the computation of the bond dissociation energy of PET dimer model compounds,as well as the formation mechanism of pure thermal degradation and catalytic pyrolysis products of PET were studied from the microscopic level via the method density functional theory（DFT）in this paper.And the effect of temperature on the main elementary reactions of the initial pure pyrolysis of PET was considered.The optimization of the molecular geometry of all the molecules of substances involved in the formation process of pyrolysis products of PET dimer model compounds was carried out by Gaussian 09,and the calculation of frequency was performed,as well as the thermodynamic and kinetic parameters of all possible pyrolysis reaction paths were acquired.The analysis of products evolution mechanisms during the pure pyrolysis and catalysis of PET were proceeded in order to study the reaction mechanisms of thermal degradation and catalytic pyrolysis of PET.The main results are as follows:1.In the theoretical calculation process of PET dimer bond dissociation energy（BDE）,the conclusions indicate that the calculation result by using B3P86 method is closer to the experimental reference values than the result calculated by M06-2X method,and it is more suitable for the calculation of PET BDE.The BDE of C-C_aromaticbond of the main chain of the PET dimer is the largest,and the BDE of C-C bond is the smallest,followed by the alkoxy C-O bond.2.In the process of PET pure pyrolysis,the DFT method of B3P86/6-31++G（d,p）was used to carry out related theoretical calculation of all possible reaction paths.It found that PET compound mainly undergoes concerted reactions on the middle and the end of the main chain to generate carboxylic acid and vinyl terminal group products through the six-membered cyclic transition states,and then degraded into other products,and the concerted reactions occurred through the four-membered cyclic transition states may be competitive reactions.The radical reaction,through the cleavage of the main chain C-C bond and C-O bonds,may not occur during the initial pyrolysis reaction of PET.The analysis of kinetic and thermodynamic shows that terephthalic acid,vinyl terephthalate,acetaldehyde,CO₂and divinyl terephthalate are the main products of PET initial pyrolysis.And the degradation reactions of terephthalic acid,vinyl terephthalate and divinyl terephthalate were analyzed and discussed.In order to understand the influence of temperature on PET pyrolysis process comprehensively,the same DFT method was adopted to calculate the thermodynamic parameters of main elementary reactions in the initial pure pyrolysis paths of PET（Path 1,Path 2,Path 3 and Path 4）at different temperatures（298 K,450 K,650 K,750 K,850 K,1000 K,1200 K）.The results show that the change of temperature may lead to the values ofΔH,ΔG,andΔS change,and the more the increase of temperature,the more theΔG values of each reaction reduce.It can be indicated that the possibility of the occurrance of reactions naturally may be improved by increasing temperature.TheΔG values of main elementary reaction of Path 1 are all less than zero at different temperatures,theΔG values of main elementary reactions of Path 2 are more than zero in the range of 298 K to 450 K,butΔG values are less than zero at 450 K and above.It can be indicated that the minimum temperature at which the concerted reactions can spontaneously occur in the initial pyrolysis process of PET is 450 K.TheΔG values of main elementary reactions for radical reactions are all larger than zero at the set temperatures,it indicates that the possibility of occurrence of radical reactions in the initial pyrolysis process of PET is small or the radical reactions need to be carried out spontaneously at higher temperature,which is consistent with the conclusion of pure pyrolysis reaction mechanism of PET.3.In the process of catalytic pyrolysis of PET,the possible reaction paths were designed.The molecular structures of all reactions were optimized geometrically and the frequencies of them were reckoned to acquire the thermodynamic as well as kinetic parameters of each possible catalytic reaction path.The calculated results show that the energy barrier of capturing protons from carboxyl group of main chain is lower than that of capturing protons from alkyl group of main chain by Ca O under alkaline conditions,which is more favorable for decarboxylation reaction.Under acidic conditions,the protons provided from acid catalyst may attack the oxygen atoms of the C=O bond in PET main chain to form a cationic intermediate,which then cracks on the alkyl bond with the lowest bond energy.Compared with pure pyrolysis,the little influence of catalysts on the component of the products of initial pyrolysis,but they can change the component of the secondary pyrolysis products of PET.Adding catalysts can change the composition of the secondary pyrolysis products by changing the optimal reaction path of degradation.During the reaction processes,both alkaline and acidic catalysts can reduce the initial reaction energy barrier to a certain extent,making the reaction easier to proceed.To sum up,the initial reaction of PET pure pyrolysis may mainly performed concerted reactions through six-membered cyclic transition states to generate the main products of terephthalic acid,vinyl terephthalate and divinyl terephthalate.And the increase in temperature can increase the possiblity of the occurrence of reactions spontaneously.Alkaline and acidic catalysts can change the composition of secondary pyrolysis products by changing the optimal reaction paths of degradation.And they can reduce the initial reaction energy barrier of the reaction to make the reaction easy to carry out to a certain extent.