TY - JOUR
T1 - The origin of selectivity in the conversion of glucose to fructose and mannose in Sn-BEA and Na-exchanged Sn-BEA zeolites
AU - Li, Sha
AU - Josephson, Tyler
AU - Vlachos, Dionisios G.
AU - Caratzoulas, Stavros
N1 - Publisher Copyright:
© 2017
PY - 2017
Y1 - 2017
N2 - 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.
AB - 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.
KW - Glucose epimerization
KW - Glucose isomerization
KW - Na-exchanged Sn-BEA zeolite
KW - Periodic density-functional theory
KW - Sn-BEA zeolite
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U2 - 10.1016/j.jcat.2017.09.001
DO - 10.1016/j.jcat.2017.09.001
M3 - Article
AN - SCOPUS:85029575991
SN - 0021-9517
VL - 355
SP - 11
EP - 16
JO - Journal of Catalysis
JF - Journal of Catalysis
ER -