Hydrothermal catalytic processing of pretreated algal oil: A catalyst screening study

• A two-step process for hydrothermally treating an algal biocrude was developed.
• Pretreatment (step-1) reduced the N and increased the flowability of algal biocrude.
• Catalytic upgrading (step 2) effected greater desirable changes as did step 1.
• Combination of Ru/C and Raney Ni performed better than either catalyst alone.
• Upgrading with two-component catalyst could be a productive avenue for additional research.

We herein report on the catalytic processing of pretreated algal oil that was produced from the hydrothermal liquefaction of Chlorella pyrenoidosa. The activities of 5% Pt/C, 5% Pd/C, 5% Ru/C, 5% Pt/C (sulfided), Mo2C, MoS2, alumina, CoMo/γ-Al2O3 (sulfided), Ni/SiO2–Al2O3, HZSM-5, activated carbon, and Raney-Ni for hydrothermal hydrodeoxygenation and hydrodenitrogenation of the pretreated algal oil at 400 °C were determined. Ru/C showed the best performance for deoxygenation and Raney-Ni was the most suitable catalyst for denitrogenation. The upgraded oil from the Ru/C catalyzed reaction contained the highest hydrocarbon content, the highest fraction of material boiling below 400 °C, and the highest higher heating value (45.1 MJ/kg). All of the metal catalysts produced freely flowing upgraded oil. The combination of Ru/C and Raney Ni, which showed very good deoxygenation and denitrogenation of the oil, produced upgraded oil that retained 86% of the heating value in the original pretreated oil. This upgraded oil was also produced in a higher yield (77.2 wt.%) and with lower gas (9.4 wt.%), coke (15.6 wt.%) and water-soluble products yields (2.1 wt.%) than from either catalyst alone. These results indicate that a two-step biocrude treatment strategy (noncatalytic treatment followed by catalytic upgrading) and a two-component catalyst bed in the second step represents an intriguing option for the hydrothermal catalytic upgrading of algal biocrude.

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