Sustainable aromatic chemicals from catalytic hydrothermal co-liquefaction of lignin and polystyrene using transition metal carbides

Abstract

Valorisation of lignin and plastic waste is critical for advancing sustainability, circular bioeconomy and environmental pollution control. Effective production of high value chemicals from such waste feedstocks remains and attractive option. This study investigates the catalytic hydrothermal co-liquefaction of alkaline lignin and polystyrene in water to produce benzene, toluene, ethylbenzene, and xylenes (BTEX), using inexpensive transition metal carbides (TMCs) as alternatives to costly noble metal catalysts. The process eliminates the need for external hydrogen or organic solvents, relying instead on in-situ hydrogen generation via steam reforming and polymer decomposition. Among the tested catalysts, molybdenum carbide (Mo2C), titanium carbide (TiC), silicon carbide (SiC), and vanadium carbide (VC) demonstrated promising catalytic activity, with VC achieving the highest liquid yield (76.3 wt%) and lowest char yield under nitrogen. Additionally, the TMCs significantly enhanced monoaromatic hydrocarbon (MAH) production, surpassing noble metals in performance and selectivity. Mechanistic insights revealed positive synergistic effects between the lignin and polystyrene feedstock, promoting deoxygenation and enhancing BTEX formation while suppressing char formation. Reusability assessments confirmed the stable catalytic performance of Mo2C over multiple cycles with minimal performance loss. This work indicates that TMCs are viable, cost-effective, and potentially scalable catalysts for sustainable BTEX production from mixed biogenic and plastic waste feedstocks, aligning with global sustainability goals.