Hydrothermal liquefaction of polyethylene terephthalete and nitrile butadiene rubber plastic waste blend into fuels using Pd/Fe3O4/zeolite composite catalyst

Abstract

This study investigated the catalytic hydrothermal liquefaction (HTL) of mixed plastic waste feedstock into fuel using a Pd/Fe₃O₄/zeolite composite catalyst. The plastic waste feedstock was composed of polyethylene terephthalate (PET) and nitrile butadiene rubber (NBR) from nitrile gloves, both of which are widely used but pose significant environmental challenges due to their poor biodegradability. The work addressed the problem of high sulfur and oxygen heteroatom content in HTL-derived fuels, which lowers calorific value, increases viscosity, and contributes to engine wear and environmental pollution. The main objective was to develop and evaluate the Pd/Fe₃O₄/zeolite composite catalyst for producing high-quality fuels with reduced sulfur and oxygen content. The catalyst was synthesized from natural zeolite and magnetite via co-precipitation and palladium impregnation, then characterized using SEM-EDX, FTIR, XRD, and XRF. HTL experiments were conducted in a batch reactor with varying PET–NBR blend ratios, catalyst dosages, reaction times, and temperatures, optimized using response surface methodology (RSM). Results showed that the Pd/Fe₃O₄/zeolite catalyst achieved a maximum fuel yield of 55.4 wt.% under optimal conditions (350 °C, 180 min, PET–NBR ratio of 4:1, catalyst dose 0.2 g). Catalytic modification significantly reduced sulfur content from 9.41 wt.% to 1.85 wt.% and oxygen content from 9.10 wt.% to 3.00 wt.%. Improvements were also observed in fuel properties, including flash point (92.5 °C), kinematic viscosity at 40 °C (5.601 cSt), and higher heating value (40.806 MJ/kg). The study concludes that Pd/Fe₃O₄/zeolite is an effective multifunctional catalyst for HTL of PET–NBR waste, enabling simultaneous deoxygenation, desulfurization, and fuel quality enhancement. Interestingly, the catalyst can be recycled and reused a number of times without significant loss in activity. This catalytic approach offers a sustainable waste-to-energy pathway that mitigates plastic pollution while producing cleaner, high-energy liquid fuels.