Ultra-Dilute SnCl4-Catalyzed Conversion of Concentrated Glucose to 5-Hydroxymethylfurfural in Aqueous Deep Eutectic Solvent.

5-Hydroxymethylfurfural(HMF) is a versatile chemical synthesized from glucose dehydration catalyzed by metal chloride (MCl_x) in deep eutectic solvents (DESs). However, the low glucose concentration and high catalyst dosage hinder large-scale HMF production. Herein, we report an aqueous DES of tetraethylammonium bromide(TEAB)-glucose for converting concentrated glucose (40 wt %, relative to TEAB) using ultra-dilute SnCl₄ (0.25 mol %), achieving a 62 % yield of HMF. Ultra-dilute MCl_x-catalyzed selective conversion of glucose is feasible only when combining SnCl₄ with Br-based DES, which is elucidated by density functional theory and molecular dynamic calculations. Using SnCl₄ is essential due to its higher glucose isomerization activity than AlCl₃ and CrCl₃, which can be attributed to its low-barrier coordination with glucose and its barrier-free separation from fructose. Halide anions in DESs strongly interact with glucose, hindering the MCl_x-glucose coordination and thereby reducing MCl_x's activity for glucose isomerization. Consequently, Br-based DESs facilitate higher activity of MCl_x than Cl-based DESs, due to the weaker interaction between halide anion and glucose. In addition, we elucidated the side reactions including condensation, polymerization, and isomerization, and proposed a reaction network. Our findings clarify the differential activity of MCl_x and the impact of halide anions in DESs on MCl_x's activity.