Breaking the Concentration Barrier: Triethylbenzylammonium Chloride-Based Acidic Deep Eutectic Solvents for Sustainable 5-Hydroxymethylfurfural Production via Self-Regulated Dynamic Equilibrium Mechanism

5-Hydroxymethylfurfural (5-HMF), a key platform chemical derived from biomass, continues to face significant challenges in achieving efficient production under industrially viable conditions, particularly due to a prolonged reaction time and low yields at high substrate concentrations. This study designed a ternary acidic deep eutectic solvent (DES) system using triethylbenzylammonium chloride (TEBAC), dicarboxylic acids, and cosolvents. The optimized system (TEBAC/oxalic acid dihydrate (OAD)/H 2 O = 3:1.2:4.5) enabled high 5-HMF yields of 87.87 ± 0.10% from 10 wt % fructose at 90 °C in just 1.5 h while maintaining strong performance at higher substrate loadings, achieving 81.06 ± 1.29% at 20 wt % and 63.93 ± 0.93% at 50 wt %, significantly surpassing conventional methods. A central innovation of this work is the identification of a self-regulated dynamic equilibrium mechanism involving a difructose anhydride (DFA) intermediate. DFA dynamically suppresses humin formation while decomposing gradually to replenish reactive species, thereby sustaining high 5-HMF productivity. This mechanistic pathway was supported by detailed spectroscopic analyses, including FT-IR, 1 H/ 13 C NMR, and LC-MS. Moreover, the TEBAC-based acidic DES exhibited tunable hydrogen-bond networks and synergistic acid–base behavior, overcoming viscosity and stability limitations commonly associated with traditional solvent systems. This work offers a scalable and sustainable strategy for biomass valorization.