(134f) Modeling the Aldose- Ketose Isomerization by Lewis Acids in the Gas Phase and Aqueous Media. A Detailed Mechanistic Study
Conversion of biomass-derived carbohydrates into furan derivatives is a crucial step for the manufacture of value-added chemicals and fuels from abundant biomass. Although considerable success has been achieved in the dehydration of fructose to 5-hydroxylmethylfurfural (HMF), the economics of the process depend heavily on the cost of fructose. Thereby it is extremely important to selectively convert glucose, a cheap hexose, to fructose. Researchers at the Catalysis Center for Energy Innovation have recently discovered homogeneously and heterogeneously catalyzed processes for this isomerization chemistry in aqueous phase using CrCl3or the zeolite catalyst Sn-beta, respectively.
The aldose-ketose isomerization is a simple but mechanistically ambiguous reaction involving the formal transfer of two hydrogen atoms, from C2 and O2 to C1 and O1 of an α-hydroxy aldehyde to form the corresponding α-hydroxy ketone. In the present paper we employ electronic structure calculations and first-principles Molecular Dynamics simulations (Car-Parrinello MD with Metadynamics acceleration) to investigate the mechanism and energetics of the Lewis-acid catalyzed glucose-fructose isomerization reaction, first in the gas phase and then in solution with explicit water molecules. Some of the findings that we shall present include:
(1) Formally, the Lewis acid catalysis favors the so-called hydride transfer mechanism from the C2 to the C1 carbon, which is activated by initial deprotonation of the hydroxyl group at the C2 carbon.
(2) By a thorough analysis of the electron charge distribution during the hydride transfer, by means of Natural Bond Orbital analysis and Electron Localization Function analysis, we find that the H transfer from C2 to C1 is more consistent with an electron-proton-electron transfer, both in the gas phase and in solution.
(3) In solution, the solvent plays a very active role in the initial deprotonation of the C2-OH and in the final proton back-donation to the C1-O, the latter being water-mediated.
(4) We present detailed free energy surfaces for all elementary steps of the reaction in solution and compare the MD-predicted equilibrium structures with those revealed by Extended X-ray Absorption Fine Structure Spectroscopy.