Integrated Valorization of Macroalgae in a Recyclable Ionic Liquid: Hydrothermal Production of Formic and Levulinic Acids with Product Recovery and Biohydrogen Generation

Abstract

Abstract Marine macroalgae are drawing growing interest as underutilized feedstocks for chemicals and fuels and as high-productivity, carbon-assimilating biomass that could contribute to carbon capture and sequestration strategies. Here, we investigate an ionic-liquid-assisted hydrothermal conversion of Gracilaria verrucosa (Rhodophyta; red alga) to formic acid (FA) and levulinic acid (LA) using 1-methyl-3-(4-sulfobutyl)imidazolium hydrogen sulfate ([SBMim][HSO4]). Under optimized conditions (210 °C, 5 h), FA and LA were produced in 18% (3.0 g/L) and 38% (6.36 g/L) yields, respectively. In a 10 g scale experiment, FA was recovered as an aqueous solution in 4% isolated yield via vacuum distillation, while LA was obtained as a solid in 29% isolated yield via ethyl acetate extraction. The ionic liquid was reconditioned using an activated carbon–Celite–S108H protocol and reused over ten consecutive cycles while maintaining FA/LA yields. Preliminary PMI/E-factor and break-even analyses further clarified the sustainability advantages and lab-scale economic limitations of the process. Importantly, the recovered FA was evaluated as a substrate for biohydrogen production using the hyperthermophilic archaeon Thermococcus onnurineus NA1, which couples formate oxidation to growth. Macroalgae-derived FA supported a hydrogen production rate comparable to that obtained with chemical-grade FA in batch culture. Overall, this work demonstrates a recyclable, ionic-liquid-assisted hydrothermal platform for converting marine macroalgae into value-added organic acids, integrating product recovery with downstream biohydrogen generation and underscoring the potential of macroalgal biorefineries for sustainable chemicals and energy.