Catalytic Activity of Lanthanide(III) Ions for the Dehydration of Hexose to 5-Hydroxymethyl-2-furaldehyde in Water

Abstract All of the lanthanide(III) (La3+–Lu3+) efficiently catalyzed the dehydration of hexose in water at 140 °C to produce 5-hydroxymethyl-2-furaldehyde (HMF) without accompanying further hydrolysis to levulinic acid, which is routinely obtained as a by-product in the conventional Brønsted acid-catalyzed reactions. The relationships between the catalytic activity and the ionic radius (ri) of lanthanides(III) for the dehydration of d-fructose and d-glucose were found to have a double-peak profile maximizing at Pr or Nd (ri 1.00 Å) and Er or Yb (ri 0.87 Å) with a dip at Sm (ri 0.96 Å). Kinetic analyses revealed that the rate-determining step is not the complex formation of lanthanide(III) ion with the saccharide molecule, but a subsequent reaction from the substrate-catalyst complex. Possible reasons why the catalytic activity–ionic radius relationships show a two-peak profile are discussed.