Catalytic Upgrading of Water-Soluble Biocrude from Hydrothermal Liquefaction of Chlorella

Hydrothermal liquefaction of microalgae produces water-insoluble biocrude that spontaneously separates from aqueous phase by gravity. A small proportion of water-soluble biocrude can be obtained via organic solvent extraction from the aqueous phase. This work explored catalytic hydrothermal upgrading of the water-soluble biocrude with five varieties of catalysts (i.e., Pt/C, Pd/C, Ru/C, Pt/C + Pd/C, and newly synthesized CoNiMoW/γ-Al₂O₃) for the first time. The results show that the upgraded oil by Pt/C had the highest yield and energy recovery but the second lowest quality with respect to elemental composition and heating value. Pd/C led to the highest heating value and the lowest yield and energy recovery of upgraded oil, as well as the largest yields of CH₄ and C₂H₆ simultaneously. Both Pt/C and CoNiMoW/γ-Al₂O₃ performed well in converting high-boiling-point macromolecules into smaller molecular compounds in water-soluble biocrude upgrading. More than 70% of components in upgraded oils were in the distillation range of 150-350 °C, in accord with that of petroleum diesel. A trade-off between bio-oil yield and quality is required to be made for catalyst selection in water-soluble biocrude upgrading. From the perspective of relatively high yield and quality of upgraded oil, CoNiMoW/γ-Al₂O₃ was a good option in catalytic hydrothermal upgrading of water-soluble biocrude.