Efficient Formation of 2,5-Diformylfuran in Ionic Liquids at High Substrate Loadings and Low Oxygen Pressure with Separation through Sublimation

The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) using oxygen (1 atm) with a TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) and CuCl catalyst system is investigated using a range of imidazolium-based ionic liquids (ILs) and various bases at different HMF substrate loadings (10-50%). This represents the first example of HMF to DFF conversion in ionic liquid media under homogeneous catalysis conditions, revealing dramatic differences in performance between the ILs. In the noncoordinating, hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][NTf2]), 90% DFF yield is obtained at 5 mol % catalyst loading after 6 h at 80 degrees C at a very high 40% HMF loading. Increasing the temperature to 100 degrees C leads to a lower yield, attributed to the loss of volatile TEMPO from the reaction medium. A system using TEMPO and pyridine immobilized within the ionic liquid [bmim][NTf2] results in selective conversion of HMF to high purity DFF. It also allows the DFF formed to be isolated by sublimation in 81% yield before a further cycle is performed. Subsequent catalyst deactivation is probed by X-ray photoelectron spectroscopy (XPS). Synthesis from fructose in a two-step process achieves a 55% isolated DFF yield. This approach overcomes significant drawbacks previously reported for this transformation, such as solvent toxicity, separation and purification problems, and the need for high oxygen pressures. Further oxidation of HMF with this system leads to a 62% yield of 5-formyl-2-furancarboxylic acid (FFCA). The separation of this compound can be achieved by sublimation of DFF followed by solvent extraction.