Unveiling the catalytic effects of Brønsted acidic ionic liquid on quantitative α-glucose conversion to 5-HMF: Experimental and computational studies

Effective biomass conversion to 5-HMF(5-hydroxymethylfurfural) is still a challenge and needs to be improved because of the 5-HMF importance as building blocks for valuable monomers and fuel precursors. We suggest for the first time utilization of low-cost metal-free Brønsted acidic Ionic liquid (IL) N,N-Diethyl-1,4-phenylenediamine hydrogen sulfate, [DPhDA]HSO₄ as a catalyst for the α-glucose dehydration to 5-HMF. Quantitative α-glucose conversion is achieved via optimizing the reaction condition: 91.4% 5-HMF yield with 30 mol% [DPhDA]HSO₄ in the presence of DMSO as a solvent at 160 °C in 30 min (TOF 6.1 h⁻¹). 3-fold increase in reaction time leads to higher (94%) 5-HMF yield at 160 °C with lower TOF 1.6 h⁻¹. The IL surprising catalytic performance is scrutinized with its amphiprotic nature (availability of basic and acidic spots in its structure (Fig. 1)). Further computational mechanistic studies revealed the function of the catalytic sites in the α-glucose dehydration. We calculated five-membered ring formation and water extrusion in one concerted transition state (TS2A, ΔG^‡ = 38.8 kcal/mol) following the α-glucose ring-opening (TS1, ΔG^‡ = 20.4 kcal/mol). Further four steps (sp³-s, C–H bond cleavages (TS3, TS6) and dehydroxylation (TS4A, TS5)) are quite straightforward to reach the product (5-HMF).