(764d) Controlled Tin Insertion into Zeolite Framework Vacancy Defects and Catalytic Consequences for Sugar Isomerization

Alternative synthesis routes to hydrothermal crystallization of Sn-Beta zeolites have focused on the post-synthetic incorporation of stannic precursors within framework vacancy defects in dealuminated supports via vapor-phase deposition [1], solid-state ion exchange [2], and liquid-phase grafting under isopropanol reflux [3], each with different consequences for the density and coordination of framework Sn atoms and density of residual silanol defects. We report that grafting Sn heteroatoms in dichloromethane solvent (333 K) enables preparing Sn-Beta zeolites comprising a wide range of Sn content (Si/Sn = 30-144; 1.4-6.1 wt% Sn) [4] and higher framework Sn densities than reported for isopropanol reflux (Si/Sn > 120) [3], because Sn incorporation is limited during isopropanol reflux (383 K) by competitive adsorption of isopropanol at vacancy defects, consistent with IR and TPD evidence [4]. As a result, dichloromethane-assisted grafting of stannic chloride into vacancy defects provides a route to prepare Sn-Beta zeolites with controlled densities of framework Sn and residual vacancy defects. IR spectra of Sn-Beta samples titrated with CD₃CN (303 K) was used to quantify the distribution of Sn heteroatoms in an open ((HO)-Sn-(OSi)₃, 2316 cm^-1) and closed (Sn(OSi)₄, 2308 cm^-1) configuration [5], providing evidence that open Sn sites are formed preferentially at low Sn framework densities. Thus, first-order glucose-fructose isomerization rate constants (373 K) decreased systematically with increasing Sn content when normalized to the total number of Sn atoms. Isomerization rate constants (373 K) normalized to open Sn sites, however, were invariant (within 2x) among twelve Sn-Beta samples prepared via liquid-phase grafting under dichloromethane reflux (333 K), highlighting the single-site behavior of open framework Sn sites in post-synthetically prepared Sn-Beta for glucose isomerization [4]. Vapor-phase CH₃OH adsorption isotherms (293 K) of Sn-Beta zeolites of varied Sn content and residual density of vacancy defects suggest that removing internal hydrophilic binding sites may increase glucose-to-fructose isomerization rate constants (per open Sn, 373 K) for post-synthetically prepared solid Lewis acids. These findings provide insight into how reflux solvents participate in grafting procedures used to incorporate heteroatoms within vacancy defects, and highlight the importance of quantifying active sites to normalize rate data prior to kinetic interpretation.

[1] P. Li et al., J. Phys. Chem. C 115 (2011) 3663-3670.

[2] C. Hammond et al., Angew. Chem. Int. Ed. 51 (2012) 11736-11739.

[3] J. Dijkmans et al., J. Catal. 330 (2015) 545-557.

[4] J.C. Vega-Vila et al., J. Catal. 344 (2016) 108-120.

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