The insights into the catalytic performance of rare earth metal ions on lactic acid formation from biomass via microwave heating

The unique properties of rare earth metal offer attractive potentials for the application as catalyst in biomass valorization. In present work, we found that rare earth metal ions exhibited specific catalytic activity for lactic acid production from biomass, giving 75.9% and 71.0% carbon molar yield of lactic acid from glucose and furfural residue, respectively. The activity of lanthanide(III) ions increased monotonically with the atomic number, as well as the decreasing ionic radius, from La3+ to Lu3+. The increasing oxidation potential with atomic number might facilitate the disproportionation reaction in glucose conversion via an intramolecular H-shift. DFT calculation further revealed that smaller ionic radius of lanthanide(III) ions led to stronger interaction between lanthanide(III) ions and glucose/fructose, as well as the less composition of 4f in LUMO that enabled more activation of glucose. More importantly, lanthanide(III) with smaller ionic radius exhibited higher electron-withdrawing ability from the oxygen atom in -C = O. This enhanced the electrophilicity of C2, C3, and C4 atoms, and led to the more exposure especially for C4 atom, thereby significantly weakening C3-C4 bond. In addition to the increasing orbital-stabilization ability at the bond-breaking site in the corresponding transition state with atomic number, the energy barrier for C3-C4 cleavage in fructose was greatly reduced by heavy lanthanide(III) ions. These factors contributed to the special activity for lactic acid formation from biomass. The insights in this work might give some useful clues for the widespread application of rare earth metal in biomass valorization.