Porosity of Pillared Clays Studied by Hyperpolarized 129Xe NMR Spectroscopy and Xe Adsorption Isotherms

详细信息    查看全文

• 作者：Caroline D. Keenan ; Markus M. Herling ; Ren茅e Siegel ; Nikolaus Petzold ; Clifford R. Bowers ; Ernst A. R枚ssler ; Josef Breu ; J眉rgen Senker
• 刊名：Langmuir
• 出版年：2013
• 出版时间：January 15, 2013
• 年：2013
• 卷：29
• 期：2
• 页码：643-652
• 全文大小：477K
• 年卷期：v.29,no.2(January 15, 2013)
• ISSN：1520-5827

文摘

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 脜 is observed. Reducing the layer charge leads to a more homogeneous pore structure with a mean diameter of 6.6 脜 (x = 0.39). The adsorption enthalpies 螖H_ads 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 螖H_ads in the low-pressure limit is significantly larger. Thus, the ¹²⁹Xe data are influenced by adsorbate鈥揳dsorbent as well as adsorbate鈥揳dsorbate interactions.