We investigate the isomerization and epimerization of glucose to fructose and mannose in Sn-BEA and Na-exchanged Sn-BEA using density-functional theory calculations on periodic BEA crystals. We compare reaction pathways both in the absence and presence of water molecules in the vicinity of the active site and find that water effectively determines the selectivity in Na-Sn-BEA. We identify two competing epimerization pathways, one involving direct 1,2-carbon shift and the other involving 1,2-hydride shift via fructose. In Sn-BEA, the isomerization to fructose is the kinetically dominant pathway, while mannose is formed via the indirect 1,2-hydride shift epimerization pathway. In Na-Sn-BEA, the kinetically dominant pathway is epimerization to mannose via the direct 1,2-carbon shift pathway (Bilik mechanism) only in the presence of water solvent in the vicinity of the active site, whereas isomerization is preferred in the absence of water. We argue that polar water molecules that coordinate around the Na cation screen strong electrostatic interactions between Na+ and the glucose backbone that are responsible for the strong inhibition of the 1,2-carbon shift mechanism in the absence of water. In Sn-BEA, the presence of water does not influence the selectivity. Our calculations resolve for the first time the role of water and Na cations in the catalytic activity of Sn-BEA, and rationalize the experimental data.
Bibliographical noteFunding Information:
This work was 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, Basic Energy Sciences under Award Number DE-SC0001004. The authors also acknowledge the resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy under Contract DE-AC02-05CH11231. Additional support was provided through the use of Information Technologies (IT) resources at the University of Delaware, specifically the high-performance computing resources.
- Glucose epimerization
- Glucose isomerization
- Na-exchanged Sn-BEA zeolite
- Periodic density-functional theory
- Sn-BEA zeolite