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.

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 followthe fate of siliceous species and structure directing agent (( 13 CH 3 À CH 2 ) 4 NOH). Tw om echanistic pathways, namely solution-mediated and solid-solid hydrogel rearrangement have been distinguished for two synthesis procedures studied. The mechanisms of structuredirecting behavior of TEA + cations in two reaction pathways have been elucidated. The results show that multinuclear MAS NMR can serve as asuperior tool for monitoring hydrothermal synthesis of various solids including zeolites,z eotypes,m esoporous materials,metal-organic frameworks and so on and for the design of novel outstanding materials for different applications.Zeolites and zeolite-type materials are of great industrial importance for catalysis,a dsorption, separation, and ion exchange. [1] Understanding the mechanism of their synthesis is crucial for further improvement of these materials and rational design of novel ones. [2] In this respect, the development of the techniques for direct monitoring of hydrothermal synthesis in situ is highly desirable. [3] Among the in situ spectroscopic techniques available,MAS NMR spectroscopy is considered to be one of the most informative, [3, 4] since it is based on the direct observation of atomic nuclei that compose the frameworks of reagents,products,and intermediates and therefore provides au nique advantage of probing the local structure at the molecular level and to access the crystallization process from the very beginning. However,t he application of this technique is strongly impeded by many experimental difficulties imposed by severe conditions of hydrothermal synthesis (temperatures up to 473 K, pressure up to 20 bars,a ggressive alkaline or acidic media). Therefore the application of this technique for long time was restricted to experiments under static conditions or at relatively low temperature (up to 373 K) and was limited to the studies of few zeotypes. [3, 4] Ther ecent development of specially constructed autoclave-type MAS NMR cells for standard Va rian [5] and Bruker [6] MAS NMR equipment opens completely new perspectives for the investigation of hydrothermal synthesis in situ. However,further progress in this field is limited by low natural abundance of 29 Si and 13 Ci sotopes,p reventing timeresolved monitoring of silicon and carbon nuclei, which are of the major importance for zeolite synthesis:t he former being the main component of zeolites,t he latter being the main component of organic structure directing agent (OSDA) responsible for zeolite structure formation. Therefore isotopic labeling of silicon and carbon nuclei is crucial for further progress in this field.Herein we report the first attempt for the time-resolved monitoring of zeolite BEA synthesis by means of in situ 13 C and 29 Si MAS NMR along with 23 Na and 27 Al MAS NMR. Isotopically labeled silica gel and TEAOH (( 13 CH 3...