Calophyllum inophyllum oil as dual function microwave absorber and reactive co-feedstock in microwave-assisted plastic pyrolysis: An experimental case study

Plastic waste exhibits inherently low dielectric loss, limiting its direct applicability in microwave-assisted pyrolysis due to inefficient microwave absorption. Conventional approaches overcome this limitation by adding inert microwave susceptors (e.g., activated carbon or silicon carbide), which increase system complexity and require post-reaction separation without contributing to fuel formation. This study addresses this critical limitation by proposing a dual-function strategy using Calophyllum inophyllum oil (CIO) as both an intrinsic microwave absorber and a reactive co-feedstock in catalytic microwave co-pyrolysis of polystyrene (PS). The research hypothesis is that CIO, due to its polar functional groups and liquid-phase characteristics, can simultaneously enhance microwave energy coupling and participate in hydrogen-transfer reactions, thereby improving liquid fuel yield and quality while eliminating the need for external absorbers. Experiments were conducted in a laboratory-scale microwave reactor integrated with HZSM-5 catalyst and in situ fractionation. Under optimal conditions (600 W, 10 wt% HZSM-5, PS:CIO = 1:1), the system produced a maximum liquid yield of 74.65 wt%, which is significantly higher than individual pyrolysis of PS (25.42 wt%) and CIO (23.24 wt%) under comparable conditions, confirming a strong synergistic effect. The resulting liquid exhibited a high heating value of 43.8 MJ kg −1 , comparable to conventional fossil fuels. GC–MS analysis revealed dominance of gasoline-range aromatic hydrocarbons, particularly toluene (56.61 wt%) and styrene (22.36 wt%), indicating effective depolymerization, hydrogen transfer, and catalytic upgrading pathways. Beyond yield enhancement, the use of CIO eliminates the need for inert microwave absorbers, simplifying reactor design and improving process integration. This dual-function concept represents a process-intensified approach that couples electromagnetic heating, radical chemistry, and catalytic upgrading within a single system. From an industrial perspective, the proposed strategy offers potential advantages in reducing operational complexity and enabling more efficient waste-to-fuel conversion. This experimental case study provides a laboratory-scale proof of concept for replacing inert microwave absorbers with renewable reactive oils in plastic pyrolysis systems. However, further work involving continuous reactor validation, catalyst reuse, product upgrading, complete fuel-property testing, techno-economic analysis, and life-cycle assessment is required before industrial application can be established. The findings highlight a promising pathway toward scalable and economically viable plastic waste valorization, while identifying future challenges related to energy efficiency, reactor scale-up, and techno-economic performance.