Synergy of boric acid and added salts in the catalytic dehydration of hexoses to 5-hydroxymethylfurfural in water

Abstract

In combination with various salts boric acid, B(OH)₃, was shown to be an efficient, weak Lewis acid catalyst in the aqueous dehydration of fructose to 5-hydroxymethylfurfural (HMF) due to strong complexation between the boron atom and the hexoses. In the dehydration of a highly concentrated aqueous fructose solution (30 wt%), a HMF yield of 60% was achieved at 92% fructose conversion and a HMF selectivity of 65% with 100 g L⁻¹B(OH)₃ and 50 g L⁻¹sodium chloride in the aqueous phase and methyl-isobutylketone (MIBK) as extracting solvent. Furthermore, the dehydration of glucose resulted in a yield of 14% HMF at 41% glucose conversion after 5 h at similar conditions. These results are highly competitive with currently reported aqueous HMF dehydration systems. In combination with the non-corrosive and non-toxic nature of the boric acid, compared to other well known homogeneous catalysts applicable for the dehydration process (e.g., H₂SO₄ and HCl), clearly, the boric acid-salt mixture is a very attractive catalyst system.