Analyzing HDPE Thermal and Catalytic Degradation in Hydrogen Atmosphere: A Model-Free Approach to the Activation Energy

Despite the great interest in thermochemical processes for converting plastic waste into chemical feedstocks or fuels, their kinetics are still a less studied topic, especially under reductive conditions. In the present work, non-isothermal thermogravimetric analysis is used to study the thermal and catalytic conversion of HDPE promoted by parent and metal-based H-USY (15) and H-ZSM-5 (11.5) zeolites under a reducing hydrogen atmosphere. Additionally, the respective kinetic parameters (apparent activation energy, Ea, and frequency factor, A) were determined by applying two distinct model-free methods: Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). The results showed that Ea of the thermal degradation of HDPE has an average value of 227 kJ/mol for both methods. In the presence of H-USY (15) and H-ZSM-5 (11.5) zeolites, Ea is strongly reduced and is highly dependent on conversion. In the case of H-USY (15), Ea varies from 78 to 157 kJ/mol for the KAS method and from 83 to 172 kJ/mol for the FWO method. Slightly lower values are reported for H-ZSM-5, with Ea values in the range of 53–122 kJ/mol for KAS and 61–107 kJ/mol for FWO. The presence and type of the metal source (Ni, Pt, or Pd) also affect the kinetic parameters of the reaction. The mean Ea values follow the order: Ni > Pt ≈ Pd for H-USY (15) or H-ZSM-5 zeolites. Accordingly, both parent and metal-based H-USY (15) and H-ZSM-5 zeolites can significantly reduce energy consumption in HDPE hydrocracking, thus promoting a more sustainable conversion of plastic waste.