Theoretical Study On The Degradation Mechanism Of Polystyrene

Polystyrene,abbreviated as PS,is widely used because of its advantages of good transparency,easy secondary processing,thermal conductivity does not change with temperature,radiation resistance,and high pyrolysis oil yield.The world output of PS accounts for the fourth place in general-purpose plastics,which results in a large amount of PS waste plastics that still need to be processed reasonably.The pure thermal degradation and catalytic cracking of PS waste plastics is currently a feasible solution for efficient recycling.In this paper,the density functional theory B3LYP/6-311G(d)method was used to study the pure thermal degradation mechanism and catalytic cracking(acidic catalytic cracking and alkaline catalytic cracking)of PS model compounds,and considered the effect of temperature on pure thermal degradation and catalytic cracking.The main products of PS pure thermal degradation and alkaline catalytic cracking are styrene,followed by aromatic compounds such as toluene,?-methylstyrene,ethylbenzene and dimer.The main product of acidic catalytic cracking is styrene,followed by aromatic compounds such as benzene,benzene derivatives,toluene,?-methylstyrene,ethylbenzene and dimer.In this paper,the path design of the products of pure thermal degradation and catalytic cracking of PS was carried out.The geometrical structure optimization and frequency calculation of all reaction molecules in the reaction process were carried out.The standard kinetic parameters and thermodynamic parameters of each pyrolysis path were obtained.The research results obtained are as follows:1.PS pure thermal degradation reactions mainly include main chain C-C bond homolysis reaction,?-fragmentation reaction,hydrogen transfer reaction,and free radical termination reaction.The path design and theoretical calculation and analysis were carried out for the above various reactions.The calculation results show that the main formation mechanism of styrene is the chain end ?-fragmentation reaction of free radicals;the dimer is mainly formed by the reaction mechanism of 1,3 hydrogen transfer within the molecule.?-methyl styrene first undergoes intra-molecular 1,2 hydrogen transfer,and then undergoes beta-fragmentation formation;the hydrogen atoms in the main chain are captured by benzyl radicals to form toluene;ethylbenzene is obtained by phenethyl radicals Hydrogen atoms are formed.Kinetic analysis shows that the energy barrier required for the formation mechanism of styrene is lower than the energy barrier required for the formation of other products,so styrene is the main thermal degradation product.2.For two types of catalytic cracking(acidic catalytic cracking and basic catalytic cracking)reactions,path design and theoretical calculation and analysis are performed,and geometrical structure optimization and frequency calculation are performed on the molecular geometry of the participating reactions to obtain standard kinetic and thermodynamic parameters of the catalytic cracking path.The calculation results show that in the alkaline catalytic cracking reaction,the energy barrier of the rate control step to form the optimal reaction path of the product styrene monomer is 68.2 kJ/mol;in the acid catalytic cracking,the optimal reaction path to form the styrene monomer The energy barrier of the rate control step is 151.9 kJ/mol;the optimal reaction rate control step of the two types of catalytic cracking to form the product styrene monomer is lower than the energy barrier of the other products,so it is formed in the two types of catalytic cracking The main products are styrene monomer.The main products of PS pure thermal degradation and PS catalytic cracking are both styrene,but there is a big difference in the distribution of aromatic products.Compared with pure thermal degradation,an obvious feature of acid catalysis is the formation of the product benzene as well as indene and indene derivatives.The removal of benzene reduces the content of phenyl,which reduces the yield of the main product styrene monomer.Therefore,acid catalysis shows a negative catalytic effect,which reduces the yield of styrene monomer,the main product of PS cracking;alkaline catalysis shows a positive catalytic effect,which increases the yield of styrene monomer,the main product of PS cracking.3.On the basis of pure thermal degradation and catalytic cracking,the PS is further studied,and the thermodynamic quantity change value ? of each reaction path in pure thermal degradation and catalytic cracking at different temperatures(298K,450 K,600K,750 K,900K,1050K)is obtained ?S,?E,?H,?G.calculated various standard thermodynamic parameters of pure thermal cracking and catalytic cracking reactions at different temperatures.The calculation results show that in pure thermal degradation,when the reaction temperature reaches 750 K,all reaction paths can spontaneously react;in acid-catalyzed cracking reaction,when the reaction temperature reaches 450 K,all reaction paths can spontaneously react;In the catalytic cracking reaction,all reactions are endothermic reactions,and under standard conditions,all reaction paths can spontaneously react.When the temperature is increased,the ?G of each reaction of pure thermal degradation and catalytic cracking becomes smaller,indicating that high temperature is conducive to the occurrence of each reaction,and the increase of the reaction temperature can increase its spontaneity,and the ?H of the endothermic reaction changes The value is greater than the ?H change value of the exothermic reaction,indicating that the increase in temperature is more conducive to the cracking of the endothermic reaction,and because the ?G of the exothermic reaction is smaller,increasing the temperature is more beneficial to increase the yield of styrene monomer.In summary,the main products of pure thermal degradation and catalytic cracking of PS are styrene.For the formation of the main products,basic catalytic cracking shows positive catalysis and acidic catalytic cracking shows negative catalysis;for pure thermal degradation of PS and catalytic cracking,increasing the temperature is conducive to the formation of the main product.This is consistent with the relevant experimental results.