Time-Resolved In Situ MAS NMR Monitoring of the Nucleation and Growth of Zeolite BEA Catalysts under Hydrothermal Conditions

Abstract

Time-resolved ¹³ C, ²³ Na, ²⁷ Al, and ²⁹ Si MAS NMR has been applied in situ for monitoring the hydrothermal synthesis of zeolite BEA. Isotopic labelling with ²⁹ Si and ¹³ C isotopes has been used to follow the fate of siliceous species and structure directing agent ((¹³ CH₃ -CH₂ )₄ 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.

Keywords: NMR spectroscopy; hydrothermal synthesis; in situ mechanistic studies; time-resolved spectroscopy; zeolite BEA.