Microwave-assisted catalytic pyrolysis of polyethylene for hydrogen Production: Mechanistic investigation by ReaxFF MD simulation and experiment validation

Hydrogen-rich polyolefins are ideal candidates for producing hydrogen and value-added carbon materials. Challenges with conventional pyrolysis of plastics include slow heating, incomplete decomposition, and high energy consumption. Microwave-assisted catalysis is a promising technology to tackle aforementioned problems, thus promoting effective and selective plastic decomposition. However, the underlying mechanisms of positive effects with microwave heating are not fully understood and cannot be easily revealed by experiments. Given such condition, the current study adopted ReaxFF MD method to simulate PE decomposition in conventional, conventional catalytic, and microwave-assisted catalytic pyrolysis. Shifting the strategy from conventional to microwave heating changed the decomposition pathways of PE from two-step pyrolysis-reforming to one-step C-H bond dissociation. The reaction mechanism of microwave-catalyzed pyrolysis to produce hydrogen was revealed by identifying the key intermediate as C-Fe-H covalent bond. Kinetic analysis showed that activation energies with C-H bond dissociation decreased by 27 kJ/mol with microwave heating, and the increase of catalyst loading could facilitate carbon deposition and hydrogen production. The simulation results were verified by microwave and conventional pyrolysis experiments. It was found both the yields and selectivity of hydrogen with microwave pyrolysis were significantly higher. The results provide insightful understanding of reaction mechanism with microwave-assisted catalytic pyrolysis.