(508b) Tin-Containing Beta Zeolite Catalyst for the Isomerization of Glucose: Insights On the Mechanism and the Active Site

Nikolla, E., California Institute of Technology
Moliner, M., Instituto de Tecnologia Quimica (UPV-CSIC)
Hwang, S., California Institute of Technology
Davis, M. E., California Institute of Technology

Biomass is being investigated as feedstock for the production of chemicals and transportation fuels of the future. Carbohydrates represent 75% of the annual renewable biomass. An approach to convert biomass-based carbohydrates to fuels and chemicals involves the production of chemical intermediates such as 5-hydroxymethylfurfural (HMF) that can serve as a gateway chemical for the production of high value products. We have recently reported that a tin containing zeolite beta catalyst (Sn-Beta) combined with a Brønsted acid catalyst such as HCl can efficiently convert sugars such as glucose to HMF under aqueous conditions with high selectivities (>70%). In this system, Sn-Beta zeolite is responsible for the isomerization of glucose to fructose, a process that has been a challenge to catalyze using heterogeneous catalysts under aqueous conditions. We have reported that Sn-Beta zeolite can efficiently catalyze the isomerization of glucose to fructose in aqueous media at low pH in the presence of HCl and an inorganic salt (Proc. Natl. Acad. Sci., 107 (2010) 6164, ACS Catal, 1 (4), (2011) 408).

Here, we will discuss efforts focused on obtaining a fundamental understanding of the nature of the active site in the Sn-Beta zeolite and the mechanism of the isomerization reaction of glucose to fructose. We have utilized solid-state NMR experiments to probe the environment of tin in the zeolite.  Sn-Beta zeolite is synthesized with labeled Sn-119 in order to provide a solid that can be effectively analyzed by solid-state NMR. We have investigated the state of the tin when the zeolite is exposed to various conditions including glucose isomerization conditions, and the results will be discussed. We have also performed NMR experiments with deuterated and C-13 labeled glucose to study the interactions of the reactant with the tin active site in the zeolite. Detailed kinetic studies including isotope-labeling experiments have been conducted in order to obtain kinetic parameters such as activation energies and rate limiting steps for the isomerization of glucose to fructose. An overall picture of the state of the tin active center and the reaction pathway is emerging and will be presented.