High-Efficient Conversion of Cellulose to Levulinic Acid Catalyzed via Functional Brønsted–Lewis Acidic Ionic Liquids

Levulinic acid (LA) is considered as an important intermediate for production of chemicals, resins, polymers, and fuel additives. Herein, five Bronsted–Lewis acidic ionic liquids (ILs) were synthesized and their acidities of ILs were characterized using FT-IR spectroscopy. The order of their acidic strength was similar to that of the used metal chlorides (ZnCl2 > FeCl3 > CuCl > CrCl3). Cellulose was effectively converted to LA in pure water at 180 °C in 10 h without additives, and the maximum yield of LA was more than 49% over the Bronsted–Lewis IL [HO3S-(CH2)3-py]Cl-FeCl3. The synergistic catalytic effect of the Lewis and Bronsted acidic centers of the catalyst played a significant role for the conversion of cellulose to LA. Some solid residues were formed along with the synthesis of LA, and the result of its characterization suggested that they had good thermal stability and their structures were benzene or amorphous (partially hydrogenated) fused benzene ring. Otherwise, the IL reusability was good, and when it was reused for five times, its catalytic performance was not changed. Therefore, the conversion route for cellulose to LA in pure water with the Bronsted–Lewis acidic ILs provides an environmentally friendly method for biobased-platform from biomass.