Coproduction of High-Value Oils and Carbon Nanotubes via Polystyrene Pyrolysis with Biomass-Derived Iron Catalysts

The coproduction of high-value oils and carbon nanotubes (CNTs) from waste polystyrene (PS) represents a promising approach for plastic waste valorization and circular economy development. In this study, three biomass-derived iron catalysts (W-Fe-MEC, Y-Fe-MEC, and H-Fe-MEC) were synthesized from metasequoia sawdust through one-step carbonization, two-step carbonization, and hydrothermal carbonization methods, respectively. These catalysts were evaluated for catalytic pyrolysis of PS to achieve the simultaneous production of oils and CNTs. W-Fe-MEC achieved the highest liquid product yield of 82.7% at 500 °C, with enhanced yields of valuable small-molecule oils, including toluene (7.4%) and ethylbenzene (10.9%). The two-step carbonization method used for Y-Fe-MEC led to a more uniform iron distribution. This catalyst was effective in promoting solid product formation at higher temperatures, reaching a maximum solid yield of 51.6% at 800 °C. Iron nanoparticles catalyzed C–C and C–H bond scission, facilitating the formation of small-molecule aromatic compounds and ordered carbon structures, with the metal particles functioning as active catalytic sites for hydrocarbon cracking and nucleation centers for CNT growth via chemical vapor deposition, exhibiting both conventional tip-growth and bamboo node growth mechanisms. This waste-to-value approach demonstrates effective plastic waste management while producing high-value chemicals and nanomaterials, contributing to the circular economy and sustainable chemical engineering.