Enhancing the activity of Zn, Fe, and Ni-embedded microporous biocarbon: Towards efficiently catalytic fast co-pyrolysis/gasification of lignocellulosic and plastic wastes

This study explored an energy-efficient and cost-effective method to synthesize three-dimensional metal-embedded microporous carbocatalysts. Pellet biochar manufactured with compressed and porous structure was used as the carbonaceous precursor, which was modulated by diverse metal chlorides in the single-step thermal process, fulfilling the synchronous pore-forming, metal-doping, and graphitization. The as-synthesized carbocatalysts were characterized in detail by using N₂ physisorption, SEM, TEM, EDX, XRD, TPO, TGA, FTIR, XPS, Raman, CHNS elemental analysis, etc. It was found that the metal-embedded carbocatalysts possessed well-developed 3D microporous structures with the highest specific surface area of 964 m²/g. The catalytic activities of these catalysts were investigated during on-line and ex-situ catalytic fast co-pyrolysis of wheat straw and plastic waste. It was observed that the carbon yield of bio-oils could reach over 60 C% by using Zn@C as the catalyst at 500 °C, and the HHV of bio-oils peaked at 38.52 MJ/Kg in the presence of Ni@C at 500 °C. Moreover, these carbcatalysts at 500 °C favored production of hydrocarbons with a relative content up to 98%; in particular, monocyclic aromatics presented the highest selectivity (nearly 60%). Among metal-embedded carbcatalysts, Ni@C at 800 °C was in favor of H₂ (157 NmL/g_feedstock) and syngas (273 NmL/g_feedstock) production; importantly, Ni@C also promoted the generation of carbon nanotubes. Additionally, the thermal degradation behaviors and kinetics of non-catalytic and catalytic co-pyrolysis of biomass and plastic waste over the as-synthesized catalysts were also tested by thermogravimetric analysis. Finally, a rational reaction mechanism regarding ex-situ catalyst fast co-pyrolysis of biomass and plastic waste over catalytically active sites on the as-synthesized catalysts was elucidated. Accordingly, this work provides a great potential of using the promising carbocatalysts to co-valorize biomass and plastic waste into the integrated harvests of monocyclic aromatics, syngas, and valuable carbons.