(458f) Methods for Quantifying Open and Closed Lewis Acid Sites in Hydrophobic and Hydrophilic Sn-Beta Zeolites That Catalyze Glucose Isomerization

Tetravalent heteroatoms (M⁴⁺) in pure-silica zeolites can behave as Lewis acid centers that catalyze glucose-fructose isomerization via intramolecular 1,2-hydride shift. These heteroatoms can either be fully-coordinated (closed sites, M-(OSi)₄) or partially-coordinated (open sites, (HO)-M-(OSi)₃) in silica frameworks, each structure with its own reactivity and each formed with a density that depends on the synthetic and post-synthetic protocols used. We have prepared Sn-BEA zeolites using different routes reported in the literature, including by direct hydrothermal synthesis in fluoride media using dealuminated BEA seeds and by post-synthetic grafting of Sn atoms in vacancy defects of dealuminated BEA zeolites. Infrared spectra of Sn-BEA zeolites exposed to controlled amounts of various base titrants (pyridine, deuterated acetonitrile) were used to determine integrated molar extinction coefficients (IMEC) and to quantify of open and closed Lewis acid sites. The number of total Lewis acid sites on each Sn-containing sample (zeolites, amorphous silicas) was quantified using IMEC values and IR band areas of samples saturated with pyridine or CD₃CN, and by temperature-programmed desorption of samples saturated with NH₃ and n-propylamine. These four base titrants gave consistent counts of the fraction of total Sn present as Lewis acidic sites on each material, which ranged from 0.51 ± 0.02 to 1.07 ± 0.03 depending on the synthesis procedure. IR spectra of samples saturated with CD₃CN showed that the fraction of total Sn present as open Lewis acid Sn sites varied from 0.16 ± 0.02 to 0.76 ± 0.11, and the ratio of open to closed sites varied from 0.4 to 2.2, depending on the synthesis procedure. Effective first-order rate constants for glucose isomerization in water (per open Sn; 373 K), normalized by the number of open Sn sites, which experiment [1] and theory [2] suggest are the active sites for glucose-fructose isomerization in Sn-BEA, were within 3x on hydrophobic Sn-BEA samples of varying Si/Sn ratio (100-170) that were synthesized under equivalent conditions (time, temperature, seed used). Effective first-order rate constants (373 K) on hydrophilic Sn-BEA samples prepared by grafting Sn sites into vacancy defects were lower (by 60x), extending the previous reports of lower effective first-order isomerization rate constants (by 10-30x, 373 K) on hydrophilic than on hydrophobic Ti-BEA zeolites [3]. These findings establish straightforward methods to quantify open and closed Lewis acid sites in zeolites, and show that the synthesis methods used influence the ratio of open and closed sites and the hydrophobicity of the final material, both of which are structural features with dramatic consequences for glucose-fructose isomerization reactivity.

[1] R. Bermejo-Deval, M. Orazov, R. Gounder, S.J. Hwang, M.E. Davis, ACS Catalysis, 4 (2014) 2288-2297.

[2]  a) R. Bermejo-Deval et al., PNAS, 109 (2012) 9727-9732; b)  N. Rai, S. Caratzoulas, D.G. Vlachos, ACS Catalysis, 3 (2013) 2294-2298; c) Y.P. Li, M. Head-Gordon, A.T. Bell, ACS Catalysis, 4 (2014) 1537-1545.

[3] R. Gounder, M.E. Davis, Journal of Catalysis, 308 (2013) 176-188.