Selectivity enhancement in the aqueous acid-catalyzed conversion of glucose to 5-hydroxymethylfurfural induced by choline chloride

In this work we wish to show that choline chloride (ChCl), a cheap and safe quaternary ammonium salt industrially produced at a few thousand tons per year through a 100% atom economy process, is capable of enhancing the selectivity of metal chlorides such as AlCl3, FeCl3 and CuCl2 in the aqueous tandem isomerization/dehydration of glucose to HMF. Under optimized conditions, 70% yield of HMF was obtained in a water/methylisobutylketone (MIBK) biphasic system which is a competitive yield to those traditionally obtained in imidazolium-based ionic liquids in the presence of hazardous chromium salts. By means of counter experiments, we show that the selectivity enhancement is optimal for a ChCl content of 50 wt% in water. At higher loading of ChCl, strong molecular interaction occurs between ChCl and HMF making the extraction of HMF from the aqueous phase difficult with MIBK, thus enhancing side reactions of HMF with water and hexoses to unwanted products. Interestingly, this process can be transposed to the direct conversion of cellulose to HMF which is an even more challenging reaction. In this case, we show that combination of FeCl3 with AlCl3 allowed cellulose to be converted to HMF with 49% yield in a one pot reaction. From the viewpoint of sustainable chemistry, this work shows noticeable advantages such as the use of (1) water as a solvent, (2) ChCl as a cheap and safe additive, (3) cheap and naturally abundant metals (Al, Fe, and Cu) and (4) renewable raw materials.