Efficient Epimerization of Aldoses Using Layered Niobium Molybdates

Both non-acidic LiNbMoO₆ and strongly acidic HNbMoO₆ efficiently catalyze the epimerization of sugars including glucose, mannose, xylose, and arabinose in water. The reactions over these oxides reached almost equilibrium within a few hours where yields of corresponding epimers from glucose, xylose, and arabinose were 24-29 %. The layered mixed oxides functioned as heterogeneous catalysts and could be reused without loss of activity, whereas bulk molybdenum oxide MoO₃ was completely dissolved during the reaction. A ¹³C substitution experiment showed that the reaction proceeds through a 1,2-rearrangement mechanism. The surface Mo octahedra were responsible for the activity. The layered HNbMoO₆ could also afford mannose from cellobiose through hydrolysis and successive epimerization.