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Time-resolved C, Na, Al, and Si MAS NMR has been applied in situ for monitoring the hydrothermal synthesis of zeolite BEA. Isotopic labelling with Si and C isotopes has been used to follow the fate of siliceous species and structure directing agent (( CH -CH ) NOH). Two mechanistic pathways, namely solution-mediated and solid-solid hydrogel rearrangement have been distinguished for two synthesis procedures studied. The mechanisms of structure-directing behavior of TEA cations in two reaction pathways have been elucidated. The results show that multinuclear MAS NMR can serve as a superior tool for monitoring hydrothermal synthesis of various solids including zeolites, zeotypes, mesoporous materials, metal-organic frameworks and so on and for the design of novel outstanding materials for different applications.

(119)Sn CPMG MAS NMR is demonstrated to be a fast and efficient method for characterization of Sn-sites in Sn-containing zeolites. Tuning of the CPMG echo-train sequence decreases the experimental time by a factor of 5-40 in the case of as-synthesized and hydrated Sn-BEA samples and by 3 orders of magnitude in the case of dehydrated Sn-BEA samples as compared to conventional methods. In the latter case, the reconstruction of the quantitative spectrum without the loss of sensitivity is shown to be possible. The method proposed allows obtaining (119)Sn MAS NMR spectra with improved resolution for Sn-BEA zeolites with natural (119)Sn isotope abundance using conventional MAS NMR equipment.

Zeolites, although key materials used in industrial processes, remain poorly understood on a molecular level despite their well-defined crystal lattices. In fact, obtaining a direct spectroscopic signatures and resolving the structure of Lewis acid sites (LAS) has remained a challenge. In this work, thanks to 1D and 2D 1H, 15N, and 27Al MAS NMR spectroscopy, carried out at different temperatures (from 298 down to 107 K), we were able to obtain the NMR spectroscopic signatures of LAS and Brønsted acid sites (BAS) in mordenite zeolite in the presence and the absence of adsorbed pyridine (Py). Combined with DFT modeling, this information enabled the structure of LAS to be revealed, namely (SiO)3Al sites interacting with pyridine, thus indicating that the corresponding base-free framework-associated sites are pseudo tricoordinated Al sites, namely tricoordinated Al sites interacting with an additional coordinated adjacent siloxane bridge. With this information in hand, we propose a molecular-level understanding on how the AlIV and AlVI framework and framework-associated sites evolve upon dehydration and exposure to Py into BAS and LAS, and their associated Py adducts. By measuring and analyzing the changes in quadrupolar coupling constants (CQ) that reflect electrical charge distribution around the nuclei, we further show that the lower C Q values observed at 298 K are due to residual dynamics that makes the electric field around aluminum nuclei more symmetric. Thus, NMR spectroscopic signatures of 27Al greatly vary with temperature; this information illustrates the importance of accounting for the temperature effect when confronting experimental and calculated C Q values of the corresponding aluminum sites in zeolites in order to obtain accurate structural assessment.

The Lewis and Brønsted acid properties of SnBEA zeolites as well as SnO2 supported on silica BEA have been examined by means of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile. Three types of surface sites have been detected and assigned to (i) framework Sn centers possessing Lewis acid properties; (ii) weak Brønsted acid sites associated with framework tin atoms; and (iii) nonframework SnO2 particles which possess Lewis acidity. The total amount of Lewis sites can be determined using pyridine as a probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm–1 is attributed to strong framework sites, whereas the band at 2287 cm–1 is attributed to nonframework sites. Brønsted acid sites can be characterized using 2,6-ditertbutylpyridine (bands at 3363, 1613, and 1530 cm–1) and deuterated acetonitrile (band at 2308 cm–1). The relative amount of Lewis and Brønsted acid sites on SnBEA can be varied by treatment in hydrogen at different temperatures.