Catalytic hydrotreatment of bio-crude produced from the hydrothermal liquefaction of aspen wood: a catalyst screening and parameter optimization study

Abstract

Lignocellulosic plant matter, as a second generation biomass, has potential as a feedstock for the production of liquid bio-fuels providing an alternative to fossil fuels. Herein, we report detailed catalyst screening and parameter optimization for catalytic hydrotreatment of bio-crude produced from the continuous hydrothermal liquefaction (HTL) of aspen wood. Three different commercial metal oxide catalysts, NiW/Al₂O₃ and NiMo/Al₂O₃ with a high and low NiMo loading, were examined. Elemental analysis showed significant oxygen expulsion from 10.7 wt% in the bio-crude down to a minimum of 0.7 wt% in the hydrotreated bio-oil. Considering the degrees of hydrodeoxygenation (HDO) along with yields, NiMo/Al₂O₃ with a high NiMo loading showed the best performance with a yield of 71.9 wt% and an oxygen content of 2.4 wt% for the final bio-oil, followed by low loaded NiMo/Al₂O₃ (yield 67.4 wt%, oxygen content 3.8 wt%) and NiW/Al₂O₃ (yield 58.7 wt%, oxygen content 6.9 wt%) under relatively mild conditions. Characterization by gas chromatography coupled with mass spectrometry pointed towards possible conversion pathways of the main bio-crude components during hydrotreatment. These included the conversion of substituted cyclopentenones to cycloalkanes, and oxygen-containing substituted polycyclic aromatic hydrocarbons (PAHs) to polycyclic aromatic hydrocarbons (e.g. anthracene, phenanthrene and naphthalene) and cracking of fatty acids to aliphatic hydrocarbons. The effects of initial hydrogen pressure and reaction time were investigated. The results demonstrate the potential of upgrading the bio-crude produced from the HTL of aspen wood to a hydrocarbon product with properties similar to petroleum derived transportation fuels.