Catalytic hydrothermal liquefaction of microalgae to bio-oil with in-situ hydrogen donor formic acid

Hydrothermal liquefaction (HTL) can directly convert high water content microalgae into bio-oil, but the bio-oil quality needs to be improved before the actual utilization. In this paper, the influences of hydrogen donor formic acid and catalysts on bio-oil properties via HTL of Chlorella were investigated. The results showed that the addition of formic acid and catalysts decreased the bio-oil yield slightly but increased the carbon, hydrogen content and higher heating values of the bio-oil. The carbon content and heating value of bio-oil derived from Au/C3N4 catalyst with formic acid were 74.66% and 38.34MJ/kg, respectively. Nitrogen heterocyclic and amine compounds were the main components of the bio-oil in the blank experiment, and the peak area percentages of them were 31% and 28% respectively. When formic acid was added as hydrogen donor, the content of nitrogen heterocyclic compounds decreased while the content of amine compounds increased. Furthermore, Ru/C or Au/C3N4 increased the content of hydrocarbons in the bio-oil, and the highest peak area percentage of hydrocarbons in Au/C3N4 bio-oil was 42%. Besides, thermogravimetric analysis of the bio-oil indicated that the distillates below 200 °C enhanced 10% compared with the blank bio-oil, and the fraction below 400 °C reached 84% after addition of formic acid and Au/C3N4. The results of this study showed that the introduction of formic acid and catalyst to HTL could improve the quality of bio-oil obviously. •The heating value of bio-oil from Au/C3N4 catalyst with formic acid was 38.34MJ/kg.•Formic acid decreased the content of nitrogen heterocyclic compounds in the bio-oil.•Ru/C or Au/C3N4 increased the content of hydrocarbons in the bio-oil.•Fraction below 400 °C in the bio-oil from Au/C3N4 with formic acid accounted for 84%.