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In-situ catalytic hydrodeoxygenation (HDO) of bio-tar in supercritical ethanol for upgraded biofuel was investigated. Highly mesoporous KOH-treated AC (KOH-AC) was synthesized by chemical activation and used as the support for Ni-based catalysts. Among the tested catalysts (AC, KOH-AC, Mg-Ni-Mo/AC, and Mg-Ni-Mo/KOH-AC), Mg-Ni-Mo/KOH-AC with a high surface area of 1310.1 m2/g and a well-developed mesoporous structure exhibited much higher catalytic performance for the HDO of bio-tar. The effects of different reaction temperatures (300–400 °C) and residence time (0–120 min) on the HDO of bio-tar over Mg-Ni-Mo/KOH-AC were also examined. Enhanced properties of liquid fuel with a higher heating value (HHV) of 36.2 MJkg−1, an oxygen content of 11.7 wt%, and a total acid number (TAN) value of 8.6 mgKOHg−1 were obtained from bio-tar over a Mg-Ni-Mo/KOH-AC at 400 °C for 120 min. In these conditions, acids, aldehydes, and oxygenated phenols present in bio-tar (>67 area%) were effectively converted to high value-added species including aromatics, hydrocarbons, and alkyl phenols in upgraded bio-tar (>77 area%) via esterification, hydrogenation, deoxygenation, and ring-alkylation reactions. A series of Mg-Ni-Mo/KOH-AC catalyst recycle test showed the deposition of coke on the catalyst, which became a major reason for the catalyst deactivation.

An efficient method to upgrade bio-oil derived from fast pyrolysis of woody biomass was investigated in supercritical ethanol without external hydrogen supply. A set of Ni-based catalysts (Ni, Ni-Mo, and Mg-Ni-Mo) supported on activated charcoal (AC) was tested to understand their catalytic effect on the yield and the quality of upgraded liquid fuel in supercritical ethanol. Mg-Ni-Mo/AC was found to be the most effective catalyst among the tested Ni-based catalysts; the liquid product obtained at 350 °C showed the highest yield (70.5 wt%) and the lowest oxygen-to-carbon (O/C) molar ratio (0.19). The catalyst also formed the lowest amount of solid product (9.8 wt%) at 350 °C by suppressing coke formation. At 350 °C, a higher quality liquid product with a total acid number (TAN) value of 6.2 mgKOHg−1 and a higher heating value (HHV) of 33.4 MJkg−1 was obtained compared with bio-oil feed (TAN: 48.0 mgKOHg−1 and HHV: 19.5 MJkg−1). A gas chromatography–mass spectrometer (GC–MS) analysis of the liquid product demonstrated that major oxygenated components in the bio-oil feed such as acids, aldehydes, and levoglucosan were effectively destroyed through hydrogenation, esterification, and deoxygenation reactions in the catalytic upgrading process.