Hydrothermal liquefaction coupled with in-situ hydrogenation: A route to 5-Methylfurfural from algal biomass models

HTL of rhamnose and glucuronic acid enables 5-methylfurfural production • Formic acid enhances MF via synergistic acid catalysis and hydrogen donation • Alcohol-water co-solvents and aqueous phase recycling further promote MF formation • Aqueous phase recycling achieves the highest MF relative content of 68.9% Hydrothermal liquefaction (HTL) provides a promising route for converting algal carbohydrates into value-added furan-based chemicals; however, achieving selective production of 5-methylfurfural (MF) remains challenging due to the complex competition among dehydration, hydrogenation, condensation, and polymerization reactions. In this study, rhamnose and glucuronic acid serve as representative model compounds of algal sulfated polysaccharides to investigate the regulation of MF formation through three in-situ hydrogen donation strategies, including formic acid addition, alcohol-water co-solvents, and aqueous phase recycling. The results demonstrate that hydrogen donation significantly alters the HTL reaction network by enhancing hydrogen transfer and stabilizing reactive intermediates, thereby promoting the selective formation of MF. Formic acid addition improves MF selectivity through the synergistic effects of acid catalysis and hydrogen donation, while alcohol-water co-solvents optimize the reaction microenvironment and suppress secondary polymerization. Among the investigated strategies, aqueous phase recycling exhibits the best performance by establishing an autocatalytic reaction environment, achieving a maximum MF relative content of 68.9% after the second recycling cycle.Furthermore, response surface methodology further identifies formic acid concentration and reaction temperature as the dominant factors affecting MF formation, while reaction time mainly exhibits a nonlinear effect. The systematic comparison of in-situ hydrogen-transfer strategies and the identification of aqueous-phase recycling as an effective approach for enhancing MF selectivity highlight the main contribution of this work. These findings provide insights into selective MF production from algal carbohydrate resources. Download: Download high-res image (205KB) Download: Download full-size image