(116f) Active Sites in Sn-Beta for Glucose Isomerization to Fructose and Epimerization to Mannose

Framework tin sites located in a hydrophobic, pure-silica molecular sieve with the zeolite beta framework topology (Sn-Beta) are known to predominantly catalyze the isomerization of glucose to fructose via a Lewis acid-mediated 1, 2 intramolecular hydride shift in water [1]. Additionally, a Lewis acid-mediated 1, 2 intramolecular carbon shift that leads to the direct epimerization of glucose to mannose has been reported with Sn-Beta in methanol [2].

In this work, we show that, in both water and methanol solvents, alkali-free Sn-Beta predominantly isomerizes glucose to fructose via the 1, 2 intramolecular hydride shift. However, increasing degree of post-synthetic Na⁺ exchange of Sn-Beta is shown to progressively alter the reaction pathway towards the 1, 2 intramolecular carbon shift that forms mannose from glucose. Na⁺remains exchanged onto these silanol groups during reaction in methanol solvent, leading to a nearly complete shift in selectivity towards glucose epimerization to mannose. In contrast, decationation occurs during reaction in aqueous solutions and gradually increases the reaction selectivity to isomerization at the expense of epimerization. The decationation and the concomitant changes in selectivity can be eliminated by addition of NaCl to the aqueous reaction solution.

Pre-adsorption of ammonia onto Sn-Beta leads to nearly complete suppression of infrared (after deuterated acetonitrile adsorption) and ¹¹⁹Sn NMR spectroscopic signatures attributed to open Sn sites, and of fructose yields from the reaction of glucose in water and methanol. These data provide evidence that the open Sn site with a proximal silanol group is the active site for the 1, 2 intramolecular hydride shift in glucose to fructose isomerization, while the open site with Na-exchanged silanol groups is the active site for the 1, 2 intramolecular carbon shift in glucose to mannose epimerization.

[1] Bermejo-Deval, R.; Assary, R. S.; Nikolla, E.; Moliner, M.; Román-Leshkov, Y.; Hwang, S.-J.; Palsdottir, A.; Silverman, D.; Lobo, R. F.; Curtiss, L. A.; Davis, M. E. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 9727–9732.

[2] Bermejo-Deval, R.; Gounder, R.; Davis, M. E. ACS Catal. 2012, 2, 2705–2713.

Acknowledgements

This work was financially supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004.  M.O. wishes to acknowledge funding from the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1144469. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

R.G. current address: School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA.

M.O. and R.B.D. contributed equally to this work.