Effective transformation of cellulose to 5-hydroxymethylfurfural catalyzed by fluorine anion-containing ionic liquid modified biochar sulfonic acids in water

This article first discloses that the fluorine anion-containing ionic liquids-functionalized biochar sulfonic acids (BCSA-IL-F_1–3s), which were simply synthesized by an ionic exchange of 1-trimethoxysilylpropyl-3-methylimidazolium chloride (IL-Cl) grafted on the BCSA with CF₃SO₃H (HF₁), HBF₄ (HF₂), HPF₆ (HF₃), respectively, can efficiently catalyze cellulose hydrolysis into reducing sugars (RSs) and 5-hydroxymethyl furfural (HMF) in water under microwave irradiation. This process provides a very high catalysis efficiency (turnover numbers, 4.03–4.89) at mild temperature (80 °C) for 3 h, but also possesses an excellent repeatability. More outstandingly, they can achieve much higher HMF yields (12.70–27.94%) compared to the IL-Cl-functionalized BCSA catalyst (HMF yields are lower than 0.1%) under the same reaction conditions. This is likely because the introduction of IL-F_1–3s groups can significantly improve the accessibility, acidity and thermal stability of BCSA’s SO₃H sites, as supported by evidence from a solid ³¹P NMR spectrum and thermogravimetric analysis. It is proposed that the good selectivity for HMF perhaps originates from a co-catalysis action of the IL-F_1–3s and SO₃H groups on BCSA-IL-F_1–3s in the further conversion of RSs to HMF.