Catalytic valorization of oriented polystyrene waste into high-grade liquid fuel: Kinetic and thermodynamic insights into an energy-efficient reaction pathway

The growing accumulation of Oriented Polystyrene (OPS) waste necessitates efficient catalytic technologies that support resource recovery within a circular economy. This study investigates the thermal-catalytic conversion of OPS into high-grade liquid fuel using a defect spinel γ-Al 2 O 3 solid acid catalyst. Isothermal cracking experiments were conducted at 350–500 °C with catalyst loadings ranging from 0 to 20 wt%, and the reaction mechanism was elucidated through solid-state kinetic modeling coupled with activated complex thermodynamic analysis. The optimum condition (450 °C, 5 wt% γ-Al 2 O 3 ) produced a high-quality liquid fuel with a gasoline-equivalent fraction of 73.9–75.6% and a high heating value of approximately 40.3 MJ/kg. Kinetic analysis revealed that this optimum catalyst loading fundamentally altered the rate-determining step, shifting the reaction from diffusion-controlled degradation to first-order kinetics dominated by catalytic C C bond cleavage. This transition resulted in a massive surge in the apparent pre-exponential factor ( A = 146.49 min −1 ), which effectively offset the apparent activation energy barrier. This interplay reflects a significantly enhanced apparent molecular collision frequency, which thermodynamically yields a lower Gibbs free energy of activation (Δ G ‡ ). In contrast, excessive catalyst loadings (15–20 wt%) promoted steric hindrance and pore blockage arising from the bulky phenyl structure of OPS, restricting reactant accessibility and shifting the reaction back toward a diffusion-controlled regime. These findings demonstrate that optimizing γ-Al 2 O 3 loading not only maximizes liquid fuel production but also improves the energetic efficiency of catalytic depolymerization through favorable kinetic and thermodynamic pathways. The mechanistic insights provide a scientific basis for designing energy-efficient catalytic processes for plastic waste valorization and advancing the fundamental development of low-carbon waste-to-fuel technologies.