(530c) On the Role of Na Cations and Solvent in Glucose Isomerization and Epimerization in Sn-BEA

The efficient conversion of glucose to platform chemicals such as 5-hydroxymethylfurfural (HMF) is crucial in establishing new green chemistry routes to useful products ranging from transportation fuels to chemical intermediates. Since the direct conversion of glucose to HMF is challenging, isomerization of glucose to fructose provides an alternative path that leverages the more favorable dehydration of fructose to HMF. Davis and coworkers^{1, 2} have recently demonstrated that the hydrolyzed metal site of the Lewis-acidic zeolite Sn-BEA is particularly selective to glucose-fructose isomerization. However, the selectivity tilts in favor of the epimerization reaction to mannose in Na-exchanged Sn-BEA (Na-Sn-BEA). The reasons behind the selectivity switch have been the source of speculation and debate. Using DFT calculations, we have been able to uncover for the first time the reasons underlying the change in selectivity. Specifically, we shall show that selectivity in Na-Sn-BEA is effectively determined by H₂O molecules vicinal to the active site.

The glucose-fructose isomerization proceeds via a 1,2-hydride shift (HS) from the C2 to the C1 carbon atom, while there are two pathways for mannose formation: a direct pathway via an intramolecular 1,2-carbon shift (CS) from C2 to C1 and an indirect pathway via a second 1,2-HS from C1 to C2 through fructose. We have compared all three reaction pathways both in the absence and presence of water molecules in the vicinity of the active site. In Sn-BEA, the isomerization to fructose is the kinetically dominant pathway, while mannose is formed via the indirect epimerization pathway. In Sn-BEA, water does not influence the selectivity, but it does open the energy gap between the isomerization and epimerization pathways, making the latter even less favorable on account of the extensive solvent re-organization that it entails. In Na-Sn-BEA the lack of water molecules at the active leads to an incorrect prediction that favors isomerization. However, calculations with water at the active site clearly demonstrate that epimerization via the direct 1,2-CS pathway is favored over isomerization, in complete agreement with experiment. Upon coordination to Na⁺, the polar H₂O molecules screen the strong electrostatic interaction between the C3 carbon of GLU and Na⁺ and thus facilitate the 1,2-CS, which is now the preferred pathway. Our calculations resolve for the first time the role of water and Na cations in the chemistry while rationalizing the experimental data.

References

[1] Bermejo-Deval, R.; Orazov, M.; Gounder, R.; Hwang, S. J.; Davis, M. E., ACS Catalysis 2014, 4 (7), 2288-2297.

[2] Hwang, S. J.; Gounder, R.; Bhawe, Y.; Orazov, M.; Bermejo-Deval, R.; Davis, M. E., Topics in Catalysis 2015, 58 (7-9), 435-440.