Bridging plastic recycling and clean energy production: hydrogen-free catalytic pyrolysis of polyethylene over CuMgAlOx for high-yield diesel fuel generation†

Abstract

The increasing annual production of plastic waste, particularly polyolefins, underscores the critical need to valorize these waste polyolefin plastics by converting them into high-value products. Herein, we report a catalytic pyrolysis technology for converting high-density polyethylene (HDPE) into diesel fuel using a CuMgAlO_x mixed metal oxide catalyst, which exhibits rapid reaction kinetics without requiring additional solvents, hydrogen, or precious metals. Experimental results reveal nearly complete conversion of polyethylene under optimized conditions at 500 °C with 60 s residence time, achieving an exceptional pyrolysis oil yield of 85.7 wt% – significantly surpassing current literature values. The pyrolysis oil primarily comprises alkanes (39.6%), alkenes (42.4%), and alkylaromatics (18.0%) as determined by chromatographic analysis. Comprehensive physicochemical characterization shows that the liquid product exhibits favorable diesel fuel properties: low kinematic viscosity (1.8 mm² s⁻¹), appropriate density (823.8 kg m⁻³), and a high calorific value (49.3 MJ kg⁻¹), all conforming to standard diesel specifications. Leveraging the structural similarity between polyethylene's long-chain alkane backbone and hexadecane, we employed the latter as a probe molecule to elucidate the catalytic pathway. This approach confirmed that the CuMgAlO_x catalyst facilitates hydrogen transfer and aromatization reactions. Mechanistic investigations through radical quenching experiments established the dominance of a free radical-mediated decomposition pathway in the catalytic pyrolysis process. Furthermore, the developed kinetic model accurately predicts product distributions across operational parameters (450–600 °C, 10–60 s residence time), demonstrating robust agreement with experimental data. This study presents an efficient catalytic strategy for polyolefin valorization, offering a technically viable solution to address plastic waste management challenges while producing high-value transportation fuels. The methodology advances plastic recycling paradigms by enabling direct transformation of waste polyethylene into energy-dense hydrocarbon fuels through an optimized heterogeneous catalytic system.