The catalytic hydrocracking of polypropylene (PP) into light alkanes (C₁–C₅) was carried out over metal-free HMFI zeolite at 280 °C under 1 MPa H₂. In situ DRIFTS and UV–vis spectroscopy revealed the roles of external Brønsted acid sites and the processes involving the formation and consumption of oligomeric species as reaction intermediates. DFT calculations supported a low-barrier pathway involving carbocation and alkoxide intermediates, allowing for the selective formation of C₃–C₄ hydrocarbons.

Abstract

Chemical recycling of polyolefins represented by polyethylene (PE) and polypropylene (PP) via catalytic cracking has emerged as a promising strategy for converting waste plastics into valuable hydrocarbons. In this study, we investigated the selective hydrocracking of PP into light alkanes (C₁–C₅) using zeolite catalysts at 280 °C under 1 MPa H₂. An HMFI zeolite with high Al content exhibited the best catalytic performance among various zeolite catalysts tested. In situ DRIFTS comparing bare HMFI and externally-silylated HMFI suggested that the external surface Brønsted acid sites serve as the active sites for the cracking of PP. Combination of in situ DRIFTS and UV–vis spectroscopy analyses identified the formation and consumption of oligomeric species as a reaction intermediate during reaction. Density functional theory (DFT) calculations suggested that a route in which the carbocation and alkoxide intermediates generated by hydrocracking of PP undergo low-energy barrier transformations into gaseous products such as C₃ and C₄ hydrocarbons. This study advances the development of sustainable polyolefin recycling technologies.