Metal-Free and Selective Oxidation of Furfural to Furoic Acid with an N-Heterocyclic Carbene Catalyst

Abstract

Aerobic oxidation of biomass-derived furfural to furoic acid was studied with an N-heterocyclic carbene as a homogeneous catalyst. Carbene species generated in situ on 1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride with a strong organic base (1,8-diazabicyclo[5.4.0]undec-7-ene) was highly active and selective for the formation of furoic acid in dimethyl sulfoxide at 40 °C. This reaction initiates the formation of a Breslow intermediate between an N-heterocyclic carbene and a furfural molecule and the subsequent activation of molecular O2. While the active carbene catalyst promoted furfural dimerization to afford furoin as a side reaction, furoin was decomposed into the Breslow intermediate and furfural through a reverse reaction, which were then converted quantitatively to furoic acid. Kinetic studies revealed that the apparent activation energy for this furfural oxidation was only 20 kJ mol–1, which is significantly lower than that with a supported Au catalyst (30.4 kJ mol–1). The N-heterocyclic carbene catalyst can oxidize various furan-based aldehydes with high selectivity; however, the electron-withdrawing group bonded to the furan ring has a negative effect on the reaction rate. Furfural can also be oxidized selectively to furoic acid, even in the presence of byproducts that are formed by acid-catalyzed dehydration of xylose with Amberlyst-70. As a result, a sequential reaction system based on initial dehydration and subsequent aerobic oxidation was developed for the production of furoic acid from xylose, without the need for furfural purification, using Amberlyst-70 (a solid acid) and an N-heterocyclic carbene catalyst.