Enhanced catalytic conversion of cellobiose/cellulose to 5-hydroxymethylfurfural using dual catalysts: Sulfonated activated carbon and Lewis acid catalyst

An activated carbon (SX+) was functionalized with sulfonic groups via the diazonium coupling method to impart Brӧnsted (B) acidity to catalyze cellobiose and cellulose upgrading into 5-HMF. X-ray photoelectron spectroscopy (XPS) confirmed SO₃H groups grafting, which significantly enhanced the total acidity to 1.33 mmol/g cat., as confirmed by Boehm titration. The B nature of acidity was verified by ³¹P ssNMR. The catalytic activity of SO₃H/SX+ alone and with AlCl₃ as Lewis (L) acid was evaluated by optimizing reaction conditions such as temperature, solvent, and B/L ratio. A ∼47 % 5-HMF yield was obtained with 100 % cellobiose conversion at 150 °C, 4 h with SO₃H/Al ratio of 1:10 in Milli-Q water (MQ)/tetrahydrofuran (THF) solvent. When AlCl₃ was replaced with heterogeneous Lewis acidic catalysts (γ-Al₂O₃, AlOOH, and TiO₂), TiO₂ combined with SO₃H/SX+ showed similar catalytic activity, achieving ∼50 % yield of 5-HMF with 100 % cellobiose conversion, attributed to its high total acidity (2.4 × 10⁻² mol/g cat.). 5-HMF was isolated from the reaction mixture using liquid-liquid extraction, resulting in ∼98 % pure 5-HMF. Moreover, SO₃H/SX+ efficiently cleaved the glycosidic bonds of microcrystalline cellulose by giving a ∼24 % 5-HMF yield when combined with Lewis acid TiO₂. This study presents a novel dual catalytic system, utilizing an optimized B/L acidity ratio to achieve efficient cellulose conversion into 5-HMF.