Catalytic activity of Lanthanide(III) ions for the dehydration of hexose to 5-hydroxymethyl-2-furaldehyde in water

All of the lanthanide(III) (La3+-Lu3+) efficiently catalyzed the dehydration of hexose in water at 140 degreesC 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 Bronsted acid-catalyzed reactions. The relationships between the catalytic activity and the ionic radius (Ti) 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 (r(i) 1.00 Angstrom) and Er or Yb (r(i) 0.87 Angstrom) with a dip at Sm (r(i) 0.96 Angstrom). 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.