Evaluation of two approaches for the synthesis of hierarchical micro-/mesoporous catalysts for HDPE hydrocracking

Abstract

Plastic waste management has become a pressing global issue. A viable and sustainable alternative to incineration is the conversion of polyethylene into chemicals or fuels by through hydrocracking. To improve the catalytic performance during hydrocracking, bifunctional catalysts are required, in which the zeolite imparts the acid function, and the metallic function is provided by a noble or transition metal, such as nickel. In this study, acid supports were synthesized using two strategies, namely zeolitisation and desilication, for comparison. The synthesized materials exhibited Si/Al molar ratios of approximately 10, hierarchical micro-/mesoporosities, and a bifunctional character after incorporation of nickel up to 5 wt%. The materials were extensively characterized by various techniques, including powder X-Ray diffraction, N2 sorption, acidity measurement, and scanning electron microscopy. The characterization results showed that the desilicated HZSM5 zeolite was the most effective support for nickel impregnation, leading to a quantitative conversion of High-Density Polyethylene (HDPE) by hydrocracking and the formation of predominantly hydrocarbons with 5 carbon atoms. A clear disparity in composition, with a prevalence of a gasoline-type fraction, was observable in the liquid phase from HZSM5 to Ni particles supported on hierarchical HZSM-5 (Ni@m-HZSM-5w). The hierarchy factor (HF), the molar ratio between Lewis and Brønsted acid sites, and the accessibility factor (ACI) were combined to form the interplay factor (IF). The investigation resulted in materials with IF values between 0.35 and 7, and a positive correlation between HDPE conversion and IF values is observed. In conclusion, this study suggests that the desilication of HZSM5 zeolite is a promising route for the development of efficient catalysts for the hydrocracking of plastic waste.