Direct Observation of Tin in Different T-Sites of Sn-BEA by One- and Two-Dimensional Sn MAS NMR Spectroscopy.

The direct and quantitative identification of active sites is crucial for the development of zeolite catalysts and their implementation in industry. Herein we report on the application of one-dimensional Sn direct polarization (DP) and rotational echo double-resonance (REDOR) and two-dimensional Sn magic-angle tuning (MAT) NMR spectroscopy for the identification of different Sn sites in fully dehydrated Sn-BEA zeolite. It is demonstrated that Sn magic-angle spinning (MAS) NMR techniques, modified by Carr-Purcell-Meiboom-Gill (CPMG) echo-train acquisition allow to resolve three groups of NMR signals, which can be attributed to three groups of nonequivalent T-sites based on the existing theoretical predictions: (I) T9, T4, and T3; (II) T2, T1, and T8; and (III) T7, T5, and T6. Results suggest that the sites attributed to group III are the most populated in Sn-BEA samples obtained via the fluoride route. The attribution of NMR lines to different T-sites in the structure of BEA allows for the establishment of structure-reactivity relationship and therefore for further improvement of Sn-BEA catalysts.