Direct Identification of Zeolite Beta Polymorphs by Solid-State ²⁹Si Nuclear Magnetic Resonance Spectroscopy

Abstract

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.

Comparing Cosine and Sine Functions for Calculating the Magnetic Shielding of 29Si Nuclei across Siliceous Zeolites

“…). Extreme experimental CSs 6,15 (upper and lower values) used for scaling of the calculated shielding values for BEA/A are given in bold. b CS of ±0.05 ppm accuracy.…”

“…According to them, we conclude that MAD ε = 0.63 ppm calculated by us for FER (Table S1) S2. The application of two different XRD models allowed avoiding the contradiction between NMR spectra of BEA/A (opposite positions of the 29 Si NMR peaks related to T1 and T2 sites) obtained either by Kolyagin et al 6 and Fyfe et al, 7 on the one hand, or by Marti ́nez-Inẽsta et al, 15 on the other. It also characterizes the sufficient accuracy of the method applied herein.…”

“…It allowed us to check the correlation between the measured shifts 6 (Table 1) and the ones calculated here using the geometry of polymorph BEA/B 10 (triangles in Figure 2a, r = 0.431). In parallel, the measured shifts 6 based on the XRD (the X-ray diffraction (XRD) abbreviation is not totally justified for ref 15 because an iterative fitting of BEA/A parameters including data of synchrotron and neutron scattering together with the XRD data 15 was considered) geometry 15 were compared with the shifts evaluated via the equation 15 (Si) 255. 23 3.8459…”

“…Their assignments, in close matching with the data of Fyfe et al , provided clarification regarding the B and C polymorphs. However, without computational analyses of the CSs, the question regarding which BEA geometry provides the best agreement with the accurate sets of NMR data remains unanswered . In this regard, computational analyses of zeolite polymorphs can be a challenging and expensive task.…”

“…The capping H or OH groups joined to the different types of O/Si atoms of the clusters around the T N sites for comparable sizes thus result in variations of the total energies. Therefore, on the other hand, finding periodic models of BEA polymorphs that can facilitate estimations of the δ­( 29 Si) CSs comparable to the recent accurate experimental data is highly desirable . With a sufficiently accurate BEA geometry, one could seek a strong correlation between CSs and the spatial or electronic characteristics of the BEA system as well as to verify the assignments theoretically.…”