Investigation of Catalyst Activity in Iron Oxide-Based Catalytic Pyrolysis of High Density Polyethylene and Polypropylene Plastic Wastes: Kinetic and Thermodynamic Analysis

In the chemical recycling of plastic wastes, many catalysts based on transition metals (Fe, Co, Mo, Ni, etc.) with different physicochemical activities are used. Impregnating metal salts (such as nitrates) prepare these metal-containing catalysts with high water solubility onto acidic and mostly spherical (1-3 mm) supports such as SiO2, Al2O3. These catalysts are used in the catalytic pyrolysis of many thermoplastic polymers such as polyethylene(s), polypropylene, polystyrene, polyvinyl chloride, polyethylene terephthalate in inert atmospheres. In this study, SiO2-supported iron oxide-based catalysts with spherical structures of 1-3 mm in diameter were prepared by applying the impregnation method. The recycling of disposable waste plastic injectors produced from high density polyethylene (HDPE) and polypropylene (PP) plastics into chemical products by catalytic pyrolysis was investigated. Total product conversions (a) of both PP and HDPE plastic wastes increased at different rates depending on the temperature. In the catalytic pyrolysis of PP plastic wastes, the activation energy was calculated as 133.81 kJ/mol and in the catalytic pyrolysis of HDPE plastic wastes, it was calculated as 171.98 kJ/mol. This result shows that the pyrolysis conditions of HDPE plastic wastes are more endothermic than PP and therefore more energy is required. This situation showed similar results in thermodynamic variables and the enthalpy values were positive at all temperatures in the plastic waste pyrolysis, which is an endothermic reaction. It was observed that there were significant decreases in the enthalpy values for both waste plastics with the use of catalyst. The calculated entropy and free energy values also showed that these reactions did not tend to spontaneously.