Catalytic Upgrading of Wheat Straw-Derived Hydrothermal Liquefaction Biocrude over Fe-Impregnated Hydrochar

Abstract

Hydrothermal liquefaction (HTL) of lignocellulosic biomass produces biocrude oil with inherently high oxygen content, which limits its direct utilization as a transportation fuel. In this study, Fe-based catalysts supported on hydrochar derived from wheat straw were developed and evaluated for catalytic upgrading of HTL biocrude. The hydrochar support was thermally pretreated at 400–600 °C to tailor its textural properties prior to metal impregnation. Increasing the calcination temperature significantly improved the surface area and pore volume of the hydrochar, with BET surface area increasing from 49 m2 g–1 at 400 °C to 228 m2 g–1 at 600 °C. Iron (10 wt %) was subsequently impregnated onto the pretreated hydrochars, followed by incorporation of secondary metals (Ni, Co, and Cu at 3.5 wt %) to prepare bimetallic catalysts. Structural characterization using BET, XRD, and FTIR confirmed the development of mesoporous carbon supports and the successful dispersion of metal species on the hydrochar matrix. Catalytic upgrading of HTL biocrude was carried out under hydrotreating conditions to reduce oxygen-containing functional groups and improve fuel-related properties. FTIR analysis of the upgraded bio-oils revealed a substantial reduction in oxygenated functionalities, including hydroxyl, carbonyl, and ether groups, along with an increase in aliphatic hydrocarbon structures. Among the catalysts investigated, CuFe/HC demonstrated more effective deoxygenation compared to NiFe/HC and CoFe/HC, indicating the importance of metal composition in enhancing catalytic activity. These findings demonstrate that hydrochar-derived carbon materials can serve as effective catalyst support for biocrude upgrading, offering a sustainable approach for improving the quality of HTL-derived biofuels.