Direct conversion of underutilized tropical fruit wastes to 5-hydroxymethylfurfural using a strongly acidic deep eutectic solvent: mechanistic study, renewable extraction, and life cycle assessment

A highly efficient and sustainable route for the direct production of 5-hydroxymethylfurfural (5-HMF) from untreated lignocellulosic biomass is reported using a novel Brønsted-Lewis deep eutectic solvent (DES) based on betaine hydrochloride and AlCl₃ (HBetCl:AlCl₃), which functions simultaneously as a solvent and catalyst. This research demonstrates the inaugural utilization of a betaine-derived chloroaluminate deep eutectic solvent (DES) for the synthesis of 5-HMF, offering remarkably robust and adjustable dual acidity without necessitating biomass pretreatment, supplementary mineral acids, or other organic solvents. Under mild conditions of 135 °C, 150 min, 10 vol% water, HBetCl : AlCl₃ = 1 : 0.5, jackfruit rind produced an excellent 5-HMF yield of 66.42 ± 0.15 mol%. Density functional theory (DFT) simulations show a strong synergistic effect between Brønsted and Lewis acidic sites. Chloroaluminate species lower the glucose-to-fructose isomerization barrier from more than 110 kJ mol^-1 to 46 kJ mol^-1, which is also supported by kinetic modeling. The process also showed that bio-based 2-methyltetrahydrofuran (2-MeTHF) could be used to recycle DES effectively for 5-HMF extraction. The DES kept its high catalytic performance over several cycles, with little AlCl₃ leaching and good phase separation. It retained 65.23% yield after the first recycle and stayed active for five runs in a row. Furthermore, a cautious cradle-to-gate life cycle evaluation also shows that 5-HMF has a global warming potential of 15.6 kg CO₂ eq. per kg, which is a good starting point for sustainable biomass valorization.