Site and Structural Changes to Sn-Beta Zeolites in Aqueous Media and Their Consequences for Glucose Isomerization Catalysis

As the pressure to reduce greenhouse gas emissions increases, biofuels provide a promising alternative to fossil-derived fuels due to their overall lower net CO₂ production. One biofuel production pathway involves the conversion of sugars from cellulosic biomass sources, and the isomerization of glucose to fructose is an important reaction step in this pathway. This reaction can be catalyzed by isolated tin atoms within the framework of Beta zeolites (Sn-Beta) [1], in which the proposed active site is a hydrolyzed Sn site in open coordination within the framework ((SiO)₃-Sn-(OH)) [2, 3]. Here, we report the effects of exposing Sn-Beta catalysts to aqueous media at high temperatures (373 K), prior to measuring glucose isomerization rates. Catalysts exposed to water at 373 K for 0-60 minutes prior to reaction showed rates (per total Sn) that were up to 2.6 times higher than on catalysts that were not exposed to water prior to reaction. Longer exposure times to water (>60 minutes) prior to reaction resulted in rates (per total Sn) that decreased with increasing exposure time, indicating catalyst deactivation. In order to determine how the catalyst structure changed during these activation and deactivation periods, pyridine and deuterated acetonitrile infrared spectroscopy were used to quantify the types of Sn sites present on the catalyst before and after hot water treatment. Among the different samples studied, the magnitude of the rate enhancement (per total Sn) observed after hot water treatment increased systematically as the fraction of open Sn sites (per total Sn) present initially on the catalyst decreased. This suggests that the activation period forms new hydrolyzed open Sn sites in the presence of water that become available as active sites for the reaction. Hydrolysis of framework bonds caused long-term catalyst deactivation, however, resulting from both tin leaching and defect silanol group formation. These findings are consistent with open Sn sites behaving as the dominant active site for glucose isomerization in Sn-Beta, and provide new insights into the effects of water exposure on site and structural changes to Sn-Beta zeolites.

[1] M. Moliner, Y. Román-Leshkov, and M.E. Davis, Proc. Natl. Acad. Sci. USA 107 (2010) 6164â??6168.

[2] R. Bermejo-Deval, et al., ACS Catal. 4 (2014) 2288-2297.

[3] J.W. Harris, et al., J. Catal. 335 (2016) 141-154.