On Layered Silicates and Zeolitic Nanosheets

Abstract

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Non‐Topotactic Transformation of Silicate Nanolayers into Mesostructured MFI Zeolite Frameworks During Crystallization

“…[4,5] These displacements in the 29 Si signals establish that the local bonding environments of the nanolayered silicates are transforming and, as they do so,a re correlated with Q 42 9 Si signals (pink band in Figure 1a)t hat are in the spectral region À110 to À114 ppm of MFI zeolite frameworks that is distinct from any of the Q 4 nanolayered silicate signals (specifically sites 4and 5 at À108.8 and À114.9 ppm). Thed isplaced signals are associated with regions of the nanolayered silicate frameworks that are rearranging to form mesostructured MFI zeolite frameworks,consistent with the 2D J-mediated NMR and electron microscopy results discussed below.Dissolution and recrystallization would produce physical mixtures of macroscopically separated nanolayered silicate and MFI zeolite species that would generally be distant from each other and result in negligible correlated intensity in the 2D 29 Si{ 29 Si}NMR spectra, contrary to the assertion by Goesten, et al [26] that such commingled phases would be "apt to magnetization transfer in the 2D NMR experiment. "…”

“…If the MFI zeolite frameworks formed only through dissolution and reprecipitation, as suggested by Goesten, et al, [26] the nanolayered silicate and MFI zeolite moieties would not be covalently bonded to any significant extent. The presence of the J-mediated 29 Si signals in Figure 1b is instead consistent with the non-topotactic rearrangement of nanolayered silicates to form the MFI zeolite frameworks.T he regions over which such transformations occur are sufficiently numerous and large that they can be resolved in the representative TEM image shown in Figure 4b of Messinger, et al, [1] which we have modified in Figure 1c to emphasize aregion in which nanolayered silicate species are rearranging into mesostructured MFI zeolite nanosheets (red box in Figure 1c).…”

“…[1] are dismissed as showing "two different phases on top of each other rather than 'sheets in transformation'." Goesten, et al provide 1D 29 Si{ 1 H} CP-MAS NMR spectra to support their contentions, [26] though such low-resolution 1D NMR measurements are ambiguous. and saw no evidence for the nanolayered silicate intermediate under their synthetic and characterization conditions. Herein, we respond to each of the points raised by Goesten, et al [26] and provide additional evidence that corroborates the analyses and strengthens the conclusions that, under the synthetic conditions used in Messinger, et al, [1] the crystallization of MFI zeolite nanosheets proceeded predominantly via an anolayered silicate intermediate.W hile Goesten, et al observe that such non-topotactic transformations have been "rare" in the past, there has until now been alack of experimental probes capable of detecting such subtle molecular rearrangements.H owever,s uch challenges are overcome by 2D 29 Si{ 29 Si}N MR techniques which provide information on the atomic-level interactions of dipolar-and J-coupled pairs of 29 Si nuclear spins,whose local (< 1nm) bonding environments are manifested by their respective isotropic chemical shifts.I no ne-dimensional (1D) NMR spectra, such as 1D 29 Si{ 1 H} cross-polarization (CP) magic-angle spinning (MAS) spectra, the 29 Si isotropic shifts often overlap,a si st he case for the heterogeneous intermediate and final mesostructured MFI zeolite products.…”

“…crystallization ·d ynamic nuclear polarization · layered silicates ·solid-state NMR spectroscopy · zeolites Inarecent Communication in this journal, [1] we reported on the co-development of crystalline and mesoscopic order during the hydrothermal synthesis of mesostructured MFI zeolite nanosheets.C ombined X-ray diffraction (XRD), transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (NMR) spectroscopy analyses provided evidence for the transformation of an anolayered silicate intermediate into mesostructured MFI zeolite frameworks.G oesten, et al [26] have raised questions about the interpretations of the experimental results and their bearing on the overall conclusions with respect to the crystallization of the MFI nanosheets.Aprimary issue is whether zeolite crystallization can proceed through non-topotactic rearrangement of framework bonds.They suggest that the nanolayered silicate species present during the intermediate stages of the MFI nanosheet synthesis are an "artifact" and play no role in the subsequent crystallization of the MFI zeolite product. They assert that the MFI zeolite nanosheets crystallize through dissolution of the initially formed mesostructured (non-crystalline) silica and the nanolayered silicate,followed by subsequent reprecipitation of the MFI zeolite nanosheets.…”

Unraveling Direct Formation of Hierarchical Zeolite Beta by Dynamic Light Scattering, Small Angle X-ray Scattering, and Liquid and Solid-State NMR: Insights at the Supramolecular Level

“…The investigation of zeolite self-assembly and crystallization presents a persisting challenge. 26 It is generally accepted that weak and strong interactions between template and building units lead to the formation of aluminosilicate framework around structuredirecting agents (SDAs). The mechanism of zeolite formation is commonly studied in colloidal precursor suspensions permitting the growth process to be followed with a number of noninvasive techniques.…”