Recyclable Brønsted-Lewis acidic ionic liquids enable high-yield biomass valorization to platform chemicals in aqueous biphasic systems

Abstract

Multiple product formation in biorefineries maximizes biomass valorization, resource efficiency, process integration, and flexibility in adapting to fuels, chemicals, and materials demand. We report a Brønsted acidic (BAIL) and Brønsted-Lewis acidic ionic liquids (BLAILs) that promote tandem biphasic extraction-conversion-separation of levulinic acid (LA), 5-hydroxymethylfurfural (HMF) and furfural (FFR) from (hemi)cellulose in corn cobs and giant cane biomass. Reacting 1-benzyl-1H-imidazole and 1,4-butane sultone, afforded 1-benzyl-3-(4-sulfonatobutyl)imidazolium (zwitterion 1). This was followed by protonation of zwitterion 1, leading to 1-benzyl-3-(4-sulfobutyl)-1H-imidazole-3-ium (BAIL 2), which was treated, separately, with FeCl3, ZnCl2, SnCl2, and NiCl2 to give BLAILs (3a-d) with larger anions (FeCl4−, ZnCl3−, SnCl3−, and NiCl3−). These IL catalysts mediated raw biomass conversion via extraction-hydrolysis-dehydration and separation of FFR, LA and HMF. Under optimized conditions, BAIL (2) achieved 91 % FFR yield, while the BLAIL, incorporating FeCl4−, yielded 95 % FFR. The sequence of the BLAILs’ catalytic activity, which corresponded to their Lewis acidities, was FeCl4− > SnCl3− > ZnCl3− > NiCl3. Post-reaction solid residues characterized using SEM, PXRD, and FT-IR, revealed significant structural changes in biomass, including increased crystallinity, attributed to type I microcrystalline cellulose. This work establishes an efficient, high-yielding, and selective method for converting and separating FFR, HMF, LA, and pure microcrystalline cellulose from biomass using recyclable, earth-abundant metal-based ILs.