Savage et al. demonstrated hydrothermal liquefaction to produce a crude bio-oil from wet algae paste and then hydrothermal catalytic upgrading of the biocrude to produce hydro­carbon product in high yield. This work provides new results on the liquefaction pathways and kinetics and on the roles and effectiveness of different upgrading catalysts for removing heteroatoms from algae and reducing the viscosity of the biocrude (Savage et al., 2012b).

Duan et al. reported the catalytic hydrotreatment of crude bio-oil produced from the hy­drothermal liquefaction of microalgae (Nannochloropsis sp.) over Pd on C (5% Pd/C) in super­critical H2O (SCW) at 400°C and 3.4 MPa high-pressure H2. Longer reaction times and higher catalyst loadings did not favor the treated oil yield due to the increasing amount of gas and coke products formation but did lead to treated bio-oil with higher HHV (41-44 MJ kg-1) than that of the crude feed. Highest HHV of treated oil (ca.44 MJ kg-1) was obtained after 4 h using an 80% intake of catalyst on crude bio-oil. The product oil produced at longer reaction times and higher catalyst loadings, which was a freely flowing liquid as opposed to being the vis­cous, sticky, tar-like crude bio-oil material, was higher in H and lower in O and N than the crude feed, and it was essentially free of S (below detection limits). Typical H/C and O/C molar ratio ranges for the bio-oils treated at different reaction times and catalyst loadings were 1.65-1.79 and 0.028-0.067, respectively. The main gas-phase products were unreacted H2, CH4, CO2, C2H6, C3H8, and C4H10. Overall, many of the properties of the treated oil obtained from catalytic hydrotreatment in SCW in the presence of Pd/C are very similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011a).

Duan and Savage determined the influence of a Pt/C catalyst, high-pressure H2, and pH on the upgrading of a crude algal bio-oil in supercritical water (SCW). The SCW treatment led to product oil with a higher heating value (ca.42 MJ kg-1) and lower acid number than the crude bio-oil. The product oil was also lower in O and N and essentially free of sulfur. Including the Pt/C catalyst in the reactor led to freely flowing liquid product oil with a high abundance of hydrocarbons. Overall, many of the properties of the upgraded oil obtained from catalytic treatment in SCW are similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011b).