Copper phosphotungstate-catalyzed microwave-assisted synthesis of 5-hydroxymethylfurfural in a biphasic system

In this paper, we have described a novel route to produce 5-hydroxymethylfurfural (HMF), a valuable platform molecule obtained from biomass. Metal-exchanged Keggin heteropolyacid salts were used as catalysts, in microwave-assisted reactions carried out in a water-ethyl acetate biphasic system. To avoid the use of homogenous Brønsted acid catalysts, which are corrosive and difficult to be reused, we have exchanged the protons of the Keggin heteropolyacids with transition metal cations. These salts were evaluated in the fructose dehydration, being the Cu_3/2PW₁₂O₄₀ the most active and selective catalyst, achieving 81% of HMF yield, after 15 min reaction at 413 K under microwave irradiation. The effects of metal cation, anion or heteropolyanion present in the catalyst were evaluated. The greatest efficiency of the Cu_3/2PW₁₂O₄₀ was attributed to its high Lewis acidic strength which allows its coordinates with the water molecules, consequently generating H₃O⁺ ions in the reaction medium. In addition, after assessing reactions of fructose dehydration in the presence of other Copper salts i.e., CuCl₂ or Cu(NO₃)₂], we conclude the anion plays too a key role. The higher softness of phosphotungstic anion should stabilize protonate intermediates better than chloride or nitrate anions, favouring this way the reaction. Finally, although the catalyst has been soluble, it was easily reused by removing the aqueous phase and adding a new load of the substrate dissolved in ethyl acetate. The runs were successfully repeated without the loss of activity of the catalyst.

Graphical abstract