5-Hydroxymethylfurfural (HMF) generation from hydrothermal liquefaction of algal modeling compound: A comprehensive investigation on the experimental exploration and formation mechanism

The reaction mechanism for the selective conversion of algal polysaccharides into platform chemicals such as 5-hydroxymethylfurfural (HMF) during hydrothermal liquefaction (HTL) remains unclear, which hinders process optimization. To address this issue, this study selected L -rhamnose, D -glucuronic acid, and D -glucose as model compounds. An integrated approach combining microwave-assisted HTL experiments, deuterium isotope tracing, and quantum chemical calculations was employed to investigate the reaction mechanism. In the microwave-assisted glucose HTL experiment, a relative HMF yield exceeding 90 % was achieved. Under optimal conditions of 190 °C for 30 min, the relative content of HMF reached 96.3 %. When the reaction temperature was further increased or the reaction time prolonged, secondary conversion of HMF to furfural (FF) was initiated. Deuterium tracing experiments using GC-MS and 1 H NMR confirmed that hydrogen atoms on hydroxyl-bearing carbons in HMF preferentially undergo deuterium exchange, indicating site-specific dehydration in carbohydrate conversion. Based on density functional theory calculations, four formation pathways for 5-hydroxymethylfurfural and two formation pathways for furfural were proposed, and their thermodynamic parameters were systematically evaluated. Pathway 4 for HMF and Pathway 6 for FF were identified as the most favorable due to their lower energy barriers and advantageous thermodynamic properties. This study clarifies the bond cleavage preferences and reaction network mechanisms of algal-specific carbohydrates under HTL conditions. The findings provide important theoretical insights for optimizing algal HTL parameters and establish a basis for understanding selective regulation in carbohydrate thermochemical conversion. • Microwave-assisted HTL achieves 96.3 % HMF yield at 190°C. • Deuterium tracing reveals C1 hydrogen exchange specificity (36.9 % substitution). • DFT identifies optimal HMF (Pathway 4) and FF (Pathway 6) formation routes. • Frontier orbital theory deciphers carbohydrate dehydration selectivity.