Molecular Structure and Confining Environment of Sn Sites in Single-Site Chabazite Zeolites

Chabazite (CHA) molecular sieves, which are industrial catalysts for the selective reduction of nitrogen oxides and the conversion of methanol into olefins, are also ideal materials in catalysis research because their crystalline frameworks contain one unique tetrahedral site. The presence of a single lattice site allows for more accurate descriptions of experimental data using theoretical models and consequently for precise structure-function relationships of active sites incorporated into framework positions. A direct hydrothermal synthesis route to prepare pure-silica chabazite molecular sieves substituted with framework Sn atoms (Sn-CHA) was developed, which is required to predominantly incorporate Sn within the crystalline lattice. Quantitative titration with Lewis bases (NH₃, CD₃CN, and pyridine) demonstrates that framework Sn atoms behave as Lewis acid sites which catalyze intermolecular propionaldehyde reduction and ethanol oxidation as well as glucose-fructose isomerization. Aqueous-phase glucose isomerization turnover rates (per accessible Sn, 398 K) on Sn-CHA are four orders of magnitude lower than on Sn-Beta zeolites, but similar to those on amorphous Sn-silicates. Further analysis of Sn-CHA by dynamic nuclear polarization enhanced solid-state nuclear magnetic resonance (DNP NMR) spectroscopy enables measurement of ¹¹⁹Sn NMR chemical shift anisotropy (CSA) of Sn sites. Comparison of experimentally determined CSA parameters to those computed on cluster models using density functional theory supports the presence of closed sites (Sn-(OSi≡)₄) and defect sites ((HO)-Sn-(OSi≡)₃) adjacent to a framework Si vacancy), which respectively become hydrated hydrolyzed-open sites and hydrated defect sites when Sn-CHA is exposed to ambient conditions or aqueous solution. Kinetic and spectroscopic data show that large substrates (e.g., glucose) are converted only on Sn sites located within disordered mesoporous voids of Sn-CHA, which are selectively detected and quantified in IR and ¹⁵N and ¹¹⁹Sn DNP NMR spectra using pyridine titrants. This integrated experimental and theoretical approach allows precise description of the primary coordination and secondary confining environments of Sn active sites isolated in crystalline silica frameworks and establishes the role of confinement within microporous voids of Beta zeolites for aqueous-phase glucose isomerization catalysis.