Efficient Epimerization of Aldoses Using Layered Niobium Molybdates.

Area of Science:

• Catalysis
• Materials Science
• Carbohydrate Chemistry

Background:

• Sugar epimerization is crucial for carbohydrate modification.
• Developing efficient and reusable catalysts for epimerization in aqueous media remains a challenge.
• Traditional catalysts like bulk molybdenum oxide can dissolve, leading to loss of activity and product contamination.

Purpose of the Study:

• To investigate the catalytic activity of layered mixed oxides, specifically LiNbMoO6 and HNbMoO6, for sugar epimerization.
• To compare the performance of these heterogeneous catalysts with bulk molybdenum oxide (MoO3).
• To elucidate the reaction mechanism and identify the active sites responsible for catalysis.

Main Methods:

• Synthesis and characterization of LiNbMoO6 and HNbMoO6.
• Catalytic epimerization reactions of glucose, mannose, xylose, and arabinose in water.
• Analysis of reaction products using chromatography and spectroscopy.
• 13C isotopic labeling experiments to study the reaction mechanism.
• Reusability tests of the heterogeneous catalysts.

Main Results:

• Both LiNbMoO6 and HNbMoO6 efficiently catalyzed the epimerization of glucose, mannose, xylose, and arabinose in water, reaching near-equilibrium yields (24-29%) within hours.
• The layered mixed oxides demonstrated excellent reusability as heterogeneous catalysts without loss of activity.
• In contrast, bulk MoO3 dissolved completely during the reaction, indicating its instability.
• 13C substitution experiments confirmed a 1,2-rearrangement mechanism, with surface Mo octahedra identified as the active sites.
• HNbMoO6 also facilitated the conversion of cellobiose to mannose via hydrolysis and subsequent epimerization.

Conclusions:

• Layered LiNbMoO6 and HNbMoO6 are effective, reusable heterogeneous catalysts for sugar epimerization in water.
• These catalysts offer a stable and efficient alternative to homogeneous or dissolving catalysts.
• The study identified surface Mo octahedra as key active sites and elucidated the 1,2-rearrangement mechanism.
• HNbMoO6 shows potential for converting disaccharides into valuable monosaccharides.