(582bj) Spectroscopic and Kinetic Assessment of Sn Sites Incorporated into Chabazite Frameworks at Intracrystalline and Extracrystalline Locations

Chabazite molecular sieves with isomorphously substituted framework tin heteroatoms (Sn-CHA) were synthesized via fluoride-mediated hydrothermal routes, and constitute model stannosilicate zeolites containing a single crystallographically unique tetrahedral-site (T-site). Lewis acid sites in Sn-CHA were titrated by ammonia and deuterated acetonitrile and quantified from temperature programmed desorption profiles and infrared (IR) spectra, respectively, giving values identical to the total Sn content. Thus, integrated molar extinction coefficients for CD₃CN IR vibrations measured on Sn-Beta [1] can be used for Sn-CHA. The predominance of framework Sn centers is also consistent with UV-visible and X-ray absorption spectra that detect tetracoordinate Sn centers under dehydrated conditions. IR spectra of Sn-CHA zeolites titrated by pyridine indicate that ~20% of the framework Sn sites are located at extracrystalline surfaces, consistent with ¹⁵N DNP-NMR spectra of ¹⁵N-pyridine saturated Sn-CHA that detect pyridine bound to Lewis acidic Sn sites. Despite the presence of a single T-site in CHA, ¹¹⁹Sn dynamic nuclear polarization NMR (DNP-NMR) detected two Sn sites with the same isotropic chemical shift but with different chemical shift anisotropy (CSA) parameters in hydrated Sn-CHA zeolites, reflecting two Sn sites of the same bond-connectivity but with slight differences in the symmetry of bound oxygen atoms. Comparison of experimentally measured ¹¹⁹Sn NMR chemical shifts and spans with those estimated using density functional theory (DFT) supports the presence of framework Sn sites with (“defect open”) and without (“closed”) neighboring framework Si vacancy defects. Aqueous-phase glucose isomerization turnover rates (1% (w/w) glucose, 398 K) on Sn-CHA zeolites, normalized by the number of sites titrated by pyridine, were similar to rates on an amorphous stannosilicate xerogel (Sn-xerogel), consistent with the presence of framework Sn sites at extracrystalline surfaces. Intermolecular reactions involving Meerwein-Pondorff-Verley reduction of propionaldehyde with Oppenauer oxidation (MPVO) of ethanol confirm that Lewis acidic Sn sites within microporous voids of Sn-CHA are capable of catalyzing intermolecular hydride shift steps. The approach used here, which combines the synthesis of model catalysts with theoretical and experimental probes sensitive to local structure, enables more precise descriptions of the structures and locations of metal heteroatoms substituted within zeolite frameworks.

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