Density functional theory study on the catalytic degradation mechanism of polystyrene

Abstract

The density functional theory method of B3LYP/6-311G(d) was used to study two catalytic degradation (acid-catalyzed and alkali-catalyzed) reaction mechanisms of polystyrene (PS). The geometric structure optimization and frequency calculations of all the molecules involved in the catalytic degradation were performed, and the standard thermodynamic parameters of each catalytic cracking path were obtained. The calculation results show that the energy barrier of the optimal reaction path’s rate control step to form styrene monomer is 68.2 kJ/mol in the alkali-catalyzed degradation reaction paths. In the acid-catalyzed cracking paths, the energy barrier of the optimal path’s rate control step to form styrene is 151.9 kJ/mol. The energy barriers of rate control steps for the formation of styrene monomer in both types of these catalytic cracking reactions are lower than those for other products, so the main degradation product in the two types of catalytic degradation is styrene monomer. Compared with pure thermal degradation, an obvious feature of acid-catalyzed degradation is the formation of benzene, indene, and derivatives of indene. The formation of benzene reduces the phenyl content of the PS main chain, which results in a reduction in the yield of styrene monomer. However, an alkali catalyst shows a positive catalytic effect, which increases the yield of styrene monomer.

Current Prospects for Plastic Waste Treatment

“…Furthermore, adding a catalyst reduces the residence time of polymer degradation in the reactor and decreases the process temperature, by lowering the activation energy by breaking the chain of C–C bonds. PS catalytic depolymerization can be split into acid and alkaline [ 113 ]. Numerous research works on the catalytic pyrolysis of PS have been performed, including metallic oxides (alumina, alumina-silica, CuO/Al 2 O 3 , BaO, Al 2 O 3 , SiO 2 , K 2 O, CaO, or silica), assisted transition metals, mesoporous materials (K 2 O−BaO/MCM−4, K 2 O/Si−MCM−41, MCM−41 sepiolite derived from nature), as well as clay (pyrophyllite, albite, halloysite, montmorillonite) [ 112 , 114 ].…”

“…PS thermal depolymerization can be applied to produce a styrene monomer. Nevertheless, this method has certain drawbacks, including equipment blockages and high-temperature requirements [ 113 ].…”