Structure-dependent nitrogen transformation in hydrothermal liquefaction of amino acids

Hydrothermal liquefaction (HTL) offers a promising route for converting wet biomass into biocrude, yet nitrogen-rich feedstocks like microalgae pose challenges due to nitrogen contamination. This study investigates how the structure of model amino acids—leucine (neutral), arginine (basic), and aspartic acid (acidic)—affects nitrogen migration during HTL. Nitrogen partitioning across product phases was quantified via orthogonal experiments and GC–MS, when degradation mechanisms were elucidated via Density Functional Theory (DFT) simulations. As shown by the experimental results, leucine achieved the highest biocrude yield (31.61%) with nitrogen retained as amides and diketopiperazines. Arginine yielded minimal oil (7.94%) and favored aqueous nitrogen-heterocycles, while aspartic acid (29.53%) released nitrogen mainly into aqueous and gaseous phases via decarboxylation. DFT confirmed leucine follows two competing routes: DKP formation and oxidative cleavage yielding 2-pyrrolidone. These findings reveal a structure-reactivity-distribution relationship linking amino acid functionality to nitrogen fate, offering guidance for feedstock selection and HTL optimization to enhance biocrude quality.