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

Abstract

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...

A Strategy for Probing the Evolution of Crystallization Processes by Low-Temperature Solid-State NMR and Dynamic Nuclear Polarization

“…5,6 In recent years, there has been increasing interest in the development and application of solid-state NMR strategies for in-situ monitoring of crystallization, particularly to establish the sequence of solid phases present as a function of time. [7][8][9][10][11][12][13][14] The time-resolution that can be achieved in in-situ solid-state NMR studies of crystallization depends on the time to record an individual spectrum of adequate quality to distinguish the different solid forms present at each stage of the process. However, even to record a simple one-dimensional solid-state NMR spectrum may require several tens of minutes (depending on isotopic abundance, the type of NMR measurement, relaxation times, magnetic field strength, etc), and the use of more sophisticated measurement techniques (e.g., two-dimensional correlation spectra, which yield more detailed structural insights) is generally not viable within the context of in-situ solid-state NMR studies of crystallization.…”

“…37,38 According to literature, crystallization of glycine from water at neutral pH produces the meta-stable  polymorph, while crystallization of glycine from deuterated water has been reported 35,4,8,39 to increase the probability of forming the  polymorph. The isotropic 13 C chemical shifts for the carboxylate carbon in the , …”

“…We note that the increased DNP sensitivity allows a residual amount of This DNP sensitivity enhancement has been exploited to investigate the intermolecular interactions in the glycine/water glass phase in more detail by measuring the 2D DNP-enhanced 13 C-13 C doublequantum (DQ) dipolar correlation spectrum (Figure 4) for the water/glycerol crystallization solution quenched at 5 min. As 1-13 C-glycine was used, correlation peaks in the 2D 13 C-13 C DQ dipolar correlation spectrum arise only for 13 C ... 13 C intermolecular distances between 13 C nuclei in the carboxylate groups of different glycine molecules. Furthermore, as this spectrum was recorded with a relatively short recoupling time (2 ms), we estimate that a correlation signal is observed when the distance between two 13 C nuclei is less than 5 ̊.…”

“…DNP-enhanced13 C-13 C DQ dipolar correlation spectrum recorded (recoupling time, 2 ms) for a rapidly quenched crystallization solution containing 1-13 C-glycine and AMUPol in water/glycerol. The crystallization time before quenching was 5 min.…”

In situ imaging of two-dimensional surface growth reveals the prevalence and role of defects in zeolite crystallization

“…For instance, zeolite growth occurs by nonclassical pathways (Fig. 1A) involving the attachment of precursors ranging from oligomers to amorphous particles and small crystallites (20)(21)(22)(23)(24). These processes involve disorder-to-order transitions governed by dissolution and reprecipitation of amorphous precursors, which transpires in tandem with a classical pathway involving monomer addition.…”