Efficient upgrading of pyrolysis bio-oil over Ni-based catalysts in supercritical ethanol

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.