Catalytic conversion of herbal residue carbohydrates to furanic derivatives in a deep eutectic solvent accompanied by dissolution and recrystallisation of choline chloride

Catalytic conversion of lignocellulosic biomass into furfural (FF) and 5-hydroxymethylfurfural (5-HMF) requires a high reaction temperature (150–230 °C) in aqueous/organic systems. In this study, a novel choline chloride–formic acid-SnCl₄·5H₂O (ChCl–Fa–SnCl₄) catalytic system was designed to transform herbal residues (HRs) carbohydrates into furanic derivatives at temperatures below 140 °C. The total yield of FF and xylose from HR of Heteropogon contortus (L.) Beauv (HRH) was 67.5% at 140 °C for 10 min, while the total yield of 5-HMF and glucose from HR of Anemarrhena asphodeloides Bunge was 93.8% at 120 °C. Characteristics of substrates such as molecular weight had a great influence on formation of furanic derivatives and sugar release. Surprisingly, the dissolution and recrystallisation characteristics of ChCl make this catalytic system sustainable and economically attractive. Recovery of ChCl and SnCl₄ was 73.3% following simple natural recrystallisation, and recovered material retained a high chemical reactivity for FF and 5-HMF production. Moreover, 100% lignin dissolution was achieved in this system at 140 °C, and ~ 77% could be precipitated and recovered by adding water.