To understand the reactivity of wet algal biomass, it is necessary to understand the reac­tivity of model compounds (components of algal biomass). Experiments with wet algal bio­mass are very useful for understanding how the yields and comparison of different product fractions (e. g., crude bio-oil, aqueous phase products, gaseous products, and solid products) vary with hydrothermal processing conditions. Such data can be used to develop phenome­nal kinetics models that have utility for process design and optimization. Such data provide little insight into the details of the chemistry that occurs. However, to elucidate some of these details, several studies have been carried out with simpler organic molecules (phytol, ethyloleate, phenylalanine, and a model phospholipid) that mimic the structural features and functional groups present in microalgae and/or crude algal bio-oil from hydrothermal liquefaction (Savage et al., 2012a).

Changi et al. examined the behavior of phytol, an acyclic diterpene C20-alcohol and a model compound for algal biomass, in high-temperature water (HTW) at 240°C, 270°C, 300°C, and 350°C. Under these conditions, the major products include neophytadiene, isophytol, and phytone. The minor products include pristene, phytene, phytane, and dihydrophytol. Neophytadiene is likely formed via dehydration of phytol, whereas isophytol can be obtained via an allylic rearrangement. Phytol disappearance follows first-order kinetics with activation energy of 145 ± 20 kJ mol-1 and a pre-exponential factor of 109.94 ±0.12 s-1. Delplot analysis discriminated between primary and nonprimary products and led to a potential set of reaction pathways. A kinetics model based on the proposed path­ways was consistent with the experimental data (Changi et al., 2012).

Formic acid, acetic acid, lactic acid, glycolic acid, 2-hydroxybutyric acid, succinic acid, malic acid, mannuronic acid, and guluronic acid were obtained by the hydrothermal treat­ment of alginate. The total yield of the organic acids was 46% at maximum yield 350°C, 40 MPa, and 0.7 s reaction time (Aida et al., 2012). The formation of organic acids suggests that the carboxyl group structure of the alginate is preserved during the hydrothermal de­composition of the alginate. The formation of dicarboxylic acids is evidence that oxidation reactions occur during the hydrothermal treatment, introducing carboxyl groups into the de­composition products. The product distribution indicates that both acid and base catalyzed reactions occur during the hydrothermal treatment of alginate. Hydrothermal treatment of uronic acid, glucuronic acid, gave the same organic acids as those obtained from hydrother­mal treatment of alginate (Aida et al., 2012).