Stacking disorder and polymorphism in zeolite and zeolite-like materials hinder their structural characterization. In this work, we propose an advanced approach based on applying "pure shift" solid-state 29 Si nuclear magnetic resonance (NMR) spectroscopy for the structural investigation of zeolitic materials containing intergrown polymorphs. The approach developed in the case study of zeolite beta allows for the resolution of 21 29 Si signals, attributing them to non-equivalent T sites in polymorphs A, B, and C, reconstruction of individual 29 Si magic angle spinning NMR spectra for each polymorph, and determination of the polymorph composition with higher accuracy than Xray diffraction. The results reveal that two widely used synthetic routes for zeolite beta, alkaline and fluoride synthesis, lead to different polymorph compositions. These findings indicate that "pure shift" solid-state 29 Si NMR can serve as a superior tool for the elucidation of polymorphism in zeolites.

31 P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy of adsorbed alkyl-substituted phosphine oxides has witnessed tremendous progress during the last years and has become one of the most informative and sensitive methods of zeolite acidity investigation. However, quantitative evaluation of the number of sites is still a challenge. This study clarifies the main origin of errors occurring during NMR experiments, introduces the appropriate standards (both internal and external), and determines the relaxation parameters and the conditions for the acquisition and integration of spectra. As a result, a methodology for the quantitative measurement of the content of Brønsted and Lewis sites and the amount of internal and external silanol groups is established. The application of probe molecules of different sizes (namely, trimethylphosphine oxide (TMPO), tri- n -butylphosphine oxide (TBPO), and tri- n -octylphosphine oxide (TOPO)) is shown to be a good tool for distinguishing between the active sites inside the zeolite pores, mesopores, and on the outer crystal surface. The methodology proposed is verified on BEA zeolites different in composition, texture, and morphology.

The review analyzes the potential of 31Р MAS NMR spectroscopy of adsorbed trisubstituted phosphines and phosphine oxides for the investigation of the acidic properties of molecular sieve catalysts. The main methodological approaches to identifying sites of different types are considered. The assignment of signals in the 31Р MAS NMR spectra of methyl-, ethyl-, n-butyl-, and phenyl-substituted phosphines and their oxides adsorbed on zeolites of different structural types and other molecular sieves to Lewis and Brønsted acid sites is reported, and the potential of 31Р MAS NMR spectroscopy as a tool for the determination of the strength of these sites and their amount is analyzed. It is shown that varying the size of the substituent allows assessment of the localization and accessibility of acid sites in zeolites.

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Time-resolved in situ 13C, 27Al, and 29Si MAS NMR and ex situ XRD, X-ray fluorescence, and TG studies have been applied for the elucidation of the mechanisms of hydrothermal synthesis of zeolite BEA from a clear solution and a dense hydrogel. Isotopic labeling with 29Si and 13C isotopes has been used to follow the fate of siliceous species and structure-directing agent ((13CH3-CH3)4NOH)). The results point to different mechanistic pathways depending on silicate speciation in the initial reaction mixture: (i) an aggregation/nucleation process for the reaction mixture prepared from a colloidal silica source and (ii) solution-mediated crystallization for the reaction mixture prepared from a dense silica hydrogel. The products obtained have been characterized using a set of physicochemical methods including XRD, X-ray fluorescence analysis, TEM, low-temperature nitrogen adsorption–desorption, and NH3-TPD. The results demonstrate that the crystallization mechanism affects the crystal size and morphology: the aggregation/nucleation process leads to very tiny crystals with a size of 5–15 nm, whereas solution-mediated crystallization results in dense polycrystals with a size of 300–700 nm with the core enriched with Al. The evaluation of the catalysts in liquid-phase alkylation of benzene with propylene points to the higher activity of zeolite BEA obtained from a colloidal silica source.