Selective Polyethylene Hydrocracking to Liquid and Gaseous Hydrocarbons over Co–Ni Catalysts Supported on H-BEA Zeolite

Abstract

The global plastic waste crisis demands efficient catalytic solutions to convert polyolefins into valuable fuels and chemicals. While noble metal catalysts have shown high activity for hydrogenolysis, their high cost and scarcity hinder large-scale implementation. Hydrocracking over earth abundant acid catalysts such as zeolites remains limited by catalyst deactivation, poor selectivity, and sensitivity to moisture and contaminants. In this study, a series of cobalt–nickel bimetallic catalysts supported on H-BEA zeolite (Co–Ni/BEA) were designed to address these challenges. A Co–Ni/BEA catalyst with approximately a 1:1 ratio of Co:Ni, prepared via co-impregnation, outperformed monometallic and physical mixture analogues during hydrocracking at 265°C and 20 bar H2, achieving high liquid alkane yields while suppressing methane formation. Temperature- and pressure-dependent studies of model polyethylene (tetracosane) hydrocracking, in combination with a suite of catalyst characterization, suggest that Co improves activity, while Ni incorporation modulates the hydrogenolysis pathway, reducing terminal C–C cleavage and improving hydrogen utilization via spillover. In the presence of water, Co–Ni/BEA retained high activity and low coke formation, while metal-free H-BEA produced carbonaceous deposits which clog pore sites, emphasizing the necessity of metal sites for water-tolerant upcycling. The Co–Ni/BEA catalyst selectively formed C3–C7 hydrocarbons from LDPE, demonstrating the use of earth-abundant, inexpensive transition metals as a sustainable alternative to noble metals. These findings establish a scalable, selective, and water-tolerant catalyst platform for polyethylene waste valorization.