Efficient Alcoholysis Reaction from Furfural Alcohol to Ethyl Levulinate by Sulfonic Acid-Functionalized Furfural Residue

Five biomass-based materials: sesame straw (SS), corn cobs (CC), bamboo powder (BP), wheat straw (WS), and furfural residue (FR), were sulfonated using sulfuric acid, as well as some commercial sulfonated catalysts, like Amberlyst, Nafion were compared for the production of ethyl levulinate (EL) from furfuryl alcohol (FAL). Remarkably, under optimum conditions (50 mg of 8% SO3H-FR at 170 °C for 5 h), 100% FAL conversion and 90% EL yield were achieved, along with stable recyclability in continuous flow. Dynamic evolution of GC in the function of reaction time under different temperatures was performed, indicating that FAL conversion to ethoxymethylfuran (EMF) occurs under a lower temperature (150 °C) and short time (45 min). On the contrary, the intermediates 5, 6, and 7 required a higher temperature for EL formation with the concomitant release of diethyl ether (DEE). Comprehensive characterizations demonstrated significant changes in the chemical composition of sulfur- or nitrogen-containing functional groups after sulfonation. In particular, the −S–O types (especially −SO3H) play a more critical role in EL production than the sulfur content alone. The adsorption energies and charge density differences of FAL (Eads) on pristine FR and the sites of −SO3H, −SO4, pyrrolic N, pyridinic N, and their complexes were compared. The FAL adsorption energy followed the order: Eads (complex) > Eads (−SO4) > Eads (−SO3H) > Eads (pyrrolic N) > Eads (pyridinic N)>Eads (pristine), indicating a synergistic adsorption effect among these functional groups in FR. Additionally, the electron transfer capacity was significantly enhanced after −SO3H doping.