Glucose Isomerization and Epimerization over Metal‐Organic Frameworks with Single‐Site Active Centers

Regulating reaction pathway of glucose isomerization and epimerization is essential for production of high-fructose corn syrup and rare sugars. Herein, we show a molecular-level observation on glucose transformation using metal-organic frameworks (MOFs) with single-site active centers (SSACs) as heterogeneous catalysts. It is found that Lewis acidic Cr (III) sites of MIL-101 prefer to catalyze glucose-to-fructose isomerization in either water or methanol solvent. Elevating reaction temperature can promote glucose-to-mannose epimerization but without intramolecular carbon shift. The Lewis acidic Zr (IV) sites of UiO-66 can catalyze glucose isomerization and epimerization in aqueous media simultaneously, where glucose-to-mannose epimerization proceeds by way of intramolecular carbon-skeleton rearrangement (C-1 and C-2), following the cleavage of the bond between C-2 and C-3 and formation of new bond between C-1 and C-3. It is proposed that the difference in catalytic behavior might be related to the polarizability of SSACs and particularly to their local coordination environment.