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

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 degrees 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 O-2. 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.