Lewis Acid and Base Catalysis of YNbO₄ Toward Aqueous-Phase Conversion of Hexose and Triose Sugars to Lactic Acid in Water

Amphoteric YNbO₄ was synthesized by the simple coprecipitation using (NH₄)₃[Nb(O₂)₄] and Y(NO₃)₃, and examined as a new solid acid-base bifunctional catalyst for various reactions including aqueous-phase conversion of glucose to lactic acid. After drying the white precipitate at 353 K for 3 h, the resultant oxide is an amorphous YNbO₄ with high densities of Lewis acid sites (0.18 mmol g⁻¹) and base sites (0.38 mmol g⁻¹). Negatively-charged lattice oxygen of amorphous YNbO₄ functioned as Lewis base sites that promote a Claisen-Schmidt-type condensation reaction with acetylacetone and benzaldehyde with comparable activity to reference catalysts. Amorphous YNbO₄ can also be applicable to the production of lactic acid from glucose in water, which gives relatively high yields (19.6 %) compared with other reference catalysts. Mechanistic studies using glucose-1-d and ²H nuclear magnetic resonance spectroscopy (NMR) revealed that YNbO₄ first converts glucose to two carbohydrates (glyceraldehyde and pyruvaldehyde) through dehydration via the formation of 3-deoxyglucosone and subsequent retro-aldolization, and these intermediates are then converted to lactic acid by both dehydration and isomerization through hydride transfer.