Nature of Glucose Epimerization Catalyzed by Mo-Containing Bulk Catalysts in Aqueous Phase

Abstract

The nature of Mo-catalyzed glucose epimerization in the aqueous phase was elaborately studied. We herein formulate the thermodynamic properties (e.g., Δ_rH_T, Δ_rG_T, and K_eq.T) of the reversible epimerization by collecting the equilibrium composition. The isotopic tracing and NMR spectra show that the overall tautomerization network encompasses the reversible epimerization and isomerization and the irreversible degradation of all hexoses. The leaching tests and kinetic and spectroscopic studies reveal that glucose epimerization catalyzed by Mo-containing solid catalysts in the aqueous phase resembles homogeneous catalysis. All catalysts enable a near-equilibrium yield of mannose (28%) at 373 K except MoP but undergo a different kinetic course of which MoN is the best catalyst according to the apparent kinetic parameters. The molybdenum species dissolved in an aqueous solution evolves into the truly active centers of the Mo^VI–O–Mo^VI bridged polymolybdates. Moreover, we propose that a single Mo center as Lewis acidic site coordinates with the aldoses to form a bidentate complex, which thereby contributes two different mechanisms to generate the epimers, viz., the intramolecular 1, 2-carbon exchange and two-step isomerization. The former proceeds through a three-membered cyclic transition state (TS_C) that mediates the simultaneous cleavage of the bond between C-2 and C-3 and formation of the bond between C-1 and C-3, whereas the latter undergoes two hydride transition states (TS_H-1 and TS_H-2) via the hydride transfer twice, leading to the chiral inversion of the configuration at C-2. Last but not least, the presence of phosphates in an aqueous solution leads to the deactivation of Mo-based catalysts because of the interplay between glucose and phosphates.