Direct conversion of cellulose to levulinic acid using SO3H-functionalized ionic liquids containing halogen-anions

Abstract

The conversion of cellulose to levulinic acid (LA) and other platform chemicals is a promising route to solve the energy crisis. For this purpose, a series of new SO3H-functionalized ionic liquids (ILs), containing different halogen-anions were synthesized and applied for catalytic conversion of cellulose to LA. The structure, thermal stability, and acidity of SO3H-functionalized ILs were characterized by Nuclear magnetic resonance (NMR) spectra, Fourier-transform infrared (FT-IR), High resolution mass spectrometry (HRMS), thermogravimetric analysis (TGA), and Hammet acidity method. The results revealed that as the size of halogen anions increased, the Hammett acidity gradually increased from SO3H-functionalized ILs containing fluorine- to chlorine- and then to bromine-anions. When SO3H-functionalized ILs were used as a catalyst for the production of LA from cellulose, the yield of LA gradually increased as the size of halogen-anions increased from fluorine- to chlorine- and then to bromine-anions. Among the SO3H-functionalized ILs, 1,1-bis(3-(4-sulfobutyl)imidazolium-1-yl)butylene bromine ([BSim]Br) exhibited the best catalytic activity with 76.5% yield of LA in cellulose/[BSim]Br/H2O (m/m/m = 1:9:8) reaction system at 180 °C for 20 min, which was the highest yield compared to previous literature reports using ILs as catalyst. Besides, the [BSim]Br was easily recovered and showed high catalytic activity after five cycles. The efficient catalyst, [BSim]Br is then proposed to have a great potential for mass production of LA from biomass.