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Keenan, Caroline D. ; Herling, Markus M. ; Siegel, Renée ; Petzold, Nikolaus ; Bowers, Clifford R. ; Rößler, Ernst ; Breu, Josef ; Senker, Jürgen:
Porosity of Pillared Clays Studied by Hyperpolarized ¹²⁹Xe NMR Spectroscopy and Xe Adsorption Isotherms.
In: Langmuir.
Bd. 29
(2013)
Heft 2
.
- S. 643-652.
ISSN 1520-5827
DOI: https://doi.org/10.1021/la304502r
Abstract
The influence of the layer charge on the microstructure was studied for a series of three hybrid pillared interlayered clays based on the organic dication Me2DABCO2+ and charge reduced synthetic fluorohectorites. To get a detailed picture of the local arrangements within the interlayer space, multinuclear solid-state NMR spectroscopy was performed in conjunction with high-resolution 129Xe MAS NMR, temperature-dependent wide-line 1D and 2D 129Xe NMR, and Ar/Ar(l) and Xe/Xe(l) physisorption measurements. The resulting layer charge (x) for the three samples are 0.48, 0.44, and 0.39 per formula unit (pfu). The samples exhibit BET equivalent surfaces between 150 and 220 m2/g and pore volumes which increase from 0.06 to 0.11 cm3/g while the layer charge reduces. 1D and 2D 1H, 13C, 19F, and 29Si MAS data reveal that the postsynthetic charge reduction induces regions with higher defect concentrations within the silicate layers. Although the pillars tend to avoid these defect-rich regions, a homogeneous and regular spacing of the Me2DABCO2+ pillars is established. Both the Ar/Ar(l) physisorption and 129Xe NMR measurements reveal comparable pore dimensions. The trend of the temperature-dependent wide-line 129Xe spectra as well as the exchange in the EXSY spectra is typical for a narrow 2D pore system. 129Xe high-resolution experiments allow for a detailed description of the microstructure. For x = 0.48 a bimodal distribution with pore diameters between 5.9 and 6.4 Å is observed. Reducing the layer charge leads to a more homogeneous pore structure with a mean diameter of 6.6 Å (x = 0.39). The adsorption enthalpies ΔHads determined from the temperature-dependent 129Xe chemical shift data fit well to the ones derived from the Xe/Xe(l) physisorption measurements in the high-pressure limit while the magnitude of ΔHads in the low-pressure limit is significantly larger. Thus, the 129Xe data are influenced by adsorbate–adsorbent as well as adsorbate–adsorbate interactions.