Hydrothermal liquefaction of a microalga with heterogeneous catalysts

We produced crude bio-oils from the microalga Nannochloropsis sp. via reactions in liquid water at 350 °C in the presence of six different heterogeneous catalysts (Pd/C, Pt/C, Ru/C, Ni/SiO₂-Al ₂O₃, CoMo/γ-Al₂O₃ (sulfided), and zeolite) under inert (helium) and high-pressure reducing (hydrogen) conditions. To our knowledge, this is the first application of common hydrocarbon processing catalysts to microalgae liquefaction in water. In the absence of added H₂, all of the catalysts tested produced higher yields of crude bio-oil from the liquefaction of Nannochloropsis sp., but the elemental compositions and heating values of the crude oil (about 38 MJ/kg) were largely insensitive to the catalyst used. The gaseous products were mainly H₂, CO₂, and CH₄, with lesser amounts of C ₂H₄ and C₂H₆. The Ru and Ni catalysts produced the highest methane yields. Only the zeolite catalyst produced significant amounts of N₂. Typical H/C and O/C atomic ratios for the crude bio-oil are 1.7 and 0.09, respectively. In the presence of high-pressure H₂, the crude bio-oil yield and heating value were largely insensitive to the presence or identity of the catalyst. The presence of either the hydrogen or the higher pressure in the reaction system did suppress the formation of gas, however. The total gas yield was always lower in H ₂ than it was in analogous experiments without H₂ and at lower pressure. In both the presence and absence of H₂, the supported Ni catalyst produced a crude bio-oil with a sulfur content below the detection limits. This apparent desulfurization activity for the Ni catalyst was unique to this material.