聚对苯二甲酸乙二醇酯二聚体模化物键离能的理论研究

Theoretical study on bond dissociation energy of polyethylene terephthalate dimer compound

采用密度泛函理论方法计算了聚对苯二甲酸乙二醇酯(PET)二聚体模化物的键离能,并设计PET热解的3条可能路径,分析PET热解机理.由于乙酸甲酯与PET具有相同的酯基官能团,因此以乙酸甲酯为简单模型参照物,采用M06-2X,B3P86,B3LYP以及BHandHLYP方法分别在基组LanL2DZ,6-31G(d),3-21G和6-31++G(d,p)水平下对乙酸甲酯的键离能进行计算.通过计算可知,B3P86与M06-2X方法的计算结果与iBonD数据库的乙酸甲酯实验测定值最接近.因此本研究采用B3P86与M06-2X方法对PET的键离能进行计算.计算结果表明:PET的各键中C—C(aromatic)键的键离能最大,主链上的C—C键离能最小,其次是C—O键.在PET的可能热解路径中,PET可能主要通过主链进行协同反应,生成苯甲酸、对苯二甲酸等有机酸以及CH3CHO和CO2等气体产物.

Polyethylene terephthalate(PET) as one of the most commonly used plastic raw materials, has been widely applied in all walks of life. Due to the excellent physical and chemical properties of PET polymer, it is difficult to spontaneously degrade under natural conditions, so the use of PET plastic products has caused serious staining problems for environment. Pyrolysis, as one of the most effective methods to treat plastic waste, has attracted extensive attention in recent years, and a mass of experimental studies have been conducted on PET pyrolysis. The bond dissociation energy is the energy required for substances to break chemical bonds. The smaller the bond dissociation energy, the more likely it is to break the chemical bond. In order to further study the theoretical mechanism of PET pyrolysis, it is necessary to understand the energy required to break the bonds on the main chain of PET.Therefore, the bond dissociation energy of polyethylene terephthalate dimer was calculated by density functional theory(DFT) methods, and three possible routes of PET pyrolysis were designed to analyze PET pyrolysis mechanism in this study. In order to figure out the energy required for bond fracture of PET backbone chain, the B3 P86 and M06-2 X methods were used to compute the bond dissociation energy of PET dimer model compound with various basis sets in this study, respectively. The calculation results show that the bond dissociation energy of C—C(aromatic) bonds is the highest in various bonds of PET, and the C—C bonds of the main chain have the lowest dissociation energy, followed by the C—O bonds, as well as the bond dissociation energy values of the C—O bonds on the main chain ester group is higher than that on the alkyl group. In addition, the calculated results also indicate that the values calculated by M06-2 X are higher than those computed by B3 P86 method. Moreover, in the possible pyrolysis pathway of PET, PET may mainly carry out synergistic reaction on the backbone chain rather than radical reactions to generate organic acids such as benzoic acid, terephthalic acid and gas products such as CH3CHO and CO2.