The catalytic epimerization of glucose to mannose over an organic-inorganic hybrid based on Keggin-type phosphomolybdate

The epimerization of glucose into mannose is of great significance for high-value utilization of monosaccharides. Extensive studies on the epimerization of glucose into mannose have been conducted using molybdenum-based catalysts, owing to their good activities for the epimerization reaction. Nonetheless, a great challenge for this reaction is that molybdenum species are prone to leaching into solution, which is a primary reason for catalyst deactivation. Additionally, the reaction usually gives a very low mannose yield and poor selectivity because it is a thermodynamic equilibrium reaction with sugar prone to side reactions at high temperatures. Herein, a metal-organic framework (MOF) material incorporating Keggin-type phosphomolybdate was explored as the catalyst for the epimerization of glucose. Under optimal conditions, an excellent mannose yield of up to 32.5% with an impressive selectivity of 94% was achieved. When other aldoses such as arabinose were used as starting materials, the catalyst also afforded high yield and product selectivity in the epimerization reaction. This novel hybrid material, a polyoxometalate-based metal-organic framework (POMOF), has been introduced to catalyze the conversion of sugars for the first time. Notably, in contrast to most molybdenum-based catalysts, the target hybrid showed good stability after five successive cycles. Furthermore, the reaction mechanism of the glucose-to-mannose epimerization was proposed, confirming the intramolecular carbon-skeleton rearrangement pathway (involving C-1 and C-2 positions) by isotope-labeling trace experiments. The epimerization of glucose into mannose is of great significance for high-value utilization of monosaccharides.