Analyzing HDPE Thermal and Catalytic Degradation in Hydrogen Atmosphere: A Model-Free Approach to the Activation Energy
Cátia S. Costa, Auguste Fernandes, Marta Muñoz, M. Rosário Ribeiro, João M. Silva
Abstract
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.