Investigation on the role of hydrogen in polyethylene hydropyrolysis process via ReaxFF molecular dynamic simulation

Abstract

Pyrolysis is one of the promising ways to valorize waste plastics into valuable products, and the introduction of hydrogen can further enhance the quality of products. In this study, the role of hydrogen in the catalyst-free hydropyrolysis of polyethylene (PE) at different temperatures was systematically investigated via ReaxFF molecular dynamic simulation. The accuracy of the model was validated by comparing with the thermogravimetric experimental results and the corresponding kinetics, as well as an appropriate degree of polymerization of PE was screened. The results indicate that temperature played a decisive role in the hydropyrolysis of PE. An increase in temperature significantly accelerated the rate of breakage of C-C and C-H bonds in PE, thus improving the yield and shifting the product distribution towards low carbon numbers. The initial hydrogen in the system had an almost negligible effect on the yield, while its effect on the product distribution was opposite to the temperature. The presence of hydrogen slowed down the stretching process of the C-C bond through intermolecular forces, thus inhibiting the cleavage of different types of C-C bonds. As a result, the product distribution shifted to the high carbon numbers and the average unsaturation of products decreased. This effect of hydrogen gradually diminished when the temperature increased. In addition, high temperature also promoted the dehydrogenation reaction rate of PE and its depolymerization intermediates, which increased the average unsaturation of products; however, the involvement of hydrogen in the chemical reaction during the middle and late stages of the reaction at high temperatures in turn led to a certain decrease in the average unsaturation of the products. This study provides a theoretical basis for the design and optimization of PE hydropyrolysis process.