Turning Point toward the Sustainable Production of 5-Hydroxymethyl-2-furaldehyde in Water: Metal Salts for Its Synthesis from Fructose and Inulin

5-Hydroxymethyl-2-furaldehyde (HMF) is one of the top 12 biobased chemicals, precursor of renewable monomers and biofuels. This key molecule can be obtained from the dehydration of monosaccharides and polysaccharides. In this work, the synthesis of HMF from fructose and inulin was investigated in the presence of a wide range of metal salts adopting a truly sustainable catalytic approach: water as reaction medium, appreciable substrate concentration (10 wt %), very low amounts of catalyst (1-3 mM) and microwave heating. Copper salts, in particular copper(II) nitrate, afforded the best performances in terms of both HMF yield and selectivity. The influence of the most important parameters (temperature, time and Cu(NO 3 ) 2 concentration) on the reaction performance was also investigated through a statistical modeling, which showed that the remarkable HMF yield of 54 mol % was reached at 188 °C after 9 min. Under these optimized reaction conditions, an analogous outstanding HMF yield was also achieved starting from the polysaccharide inulin.