Figure 11. Experimental rate constant for MTBE hydrolysis in HTW. (Reprinted with permission from ref 137.

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We converted the marine microalga Nannochloropsis sp. into a crude bio-oil product and a gaseous product via hydrothermal processing from 200 to 500 °C and a batch holding time of 60 min. A moderate temperature of 350 °C led to the highest bio-oil yield of 43 wt %. We estimate the heating value of the bio-oil to be about 39 MJ kg−1, which is comparable to that of a petroleum crude oil. The H/C and O/C ratios for the bio-oil decreased from 1.73 and 0.12, respectively, for the 200 °C product to 1.04 and 0.05, respectively, for the 500 °C product. Major bio-oil constituents include phenol and its alkylated derivatives, heterocyclic N-containing compounds, long-chain fatty acids, alkanes and alkenes, and derivatives of phytol and cholesterol. CO2 was always the most abundant gas product. H2 was the second most abundant gas at all temperatures other than 500 °C, where its yield was surpassed by that of CH4. The activation energies for gas formation suggest the presence of gas-forming reactions other than steam reforming. Nearly 80% of the carbon and up to 90% of the chemical energy originally present in the microalga can be recovered as either bio-oil or gas products.

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−Al2O3, 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 C2H4 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 H2 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.