Hydrothermal liquefaction (HTL) of microalgae, a process that uses water at high temperature and high pressure to make a renewable crude bio-oil, is receiving increased attention. Understanding the governing reaction pathways for the biomolecules in the microalgae cell could lead to improved conversion processes. This review collects information pertinent to the behavior of microalgae biomolecules (e.g., proteins, polysaccharides, lipids, chlorophyll) and their hydrothermal decomposition products (e.g., amino acids, sugars, fatty acids) in high temperature water (HTW). We report on studies involving individual compounds and their mixtures. The mixture systems are particularly important as they move closer to mimicking the true chemistry of HTL of microalgae by providing opportunities for interactions between different molecules that would be present during HTL. Throughout this review, we highlight gaps in the understanding of different chemical reactions that may take place during HTL of microalgae.

We explored the hydrothermal cracking of palmitic acid by HZSM-5 zeolite in a batch reactor. The major liquid products were aromatic hydrocarbons (e.g., xylenes, toluene) and alkanes (e.g., 2-methyl-pentane, heptane). The major gaseous products were CO and CO2, but appreciable yields of propane and butane were also obtained. The effects of batch holding time, temperature, hydrogen pressure, and water density on product yields were elucidated. Total yields of gas and liquid products exceeding 90 wt % are available at reaction conditions of 400 °C, 180 min, and either with no added H2 and a water density of 0.1 g/mL or less or with added H2 and a water density of 0.15 g/mL. The activation energy for palmitic acid disappearance is 31 ± 1 kJ/mol. The zeolite catalyst undergoes some modest structural changes under the hydrothermal reaction conditions employed, but the catalyst can be regenerated by controlled oxidation to remove coke and calcination to restore the structure. These results demonstrate the technical feasibility of using HZSM-5 under hydrothermal conditions to produce valuable chemicals from renewable fatty acid feedstocks.

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