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

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 Me(2)DABCO(2+) 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 Xe-129 MAS NMR, temperature-dependent wide-line ID and 2D Xe-129 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 m(2)/g and pore volumes which increase from 0.06 to 0.11 cm(3)/g while the layer charge reduces. 1D and 2D H-1, C-13, F-19, and Si-29 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 Me(2)DABCO(2+) pillars is established. Both the Ar/Ar(l) physisorption and Xe-129 NMR measurements reveal comparable pore dimensions. The trend of the temperature-dependent wide-line Xe-129 spectra as well as the exchange in the EXSY spectra is typical for a narrow 2D pore system. Xe-129 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 angstrom is observed. Reducing the layer charge leads to a more homogeneous pore structure with a mean diameter of 6.6 angstrom (x = 0.39). The adsorption enthalpies Delta H-ads determined from the temperature-dependent Xe-129 chemical shift data fit well to the ones derived from the Xe/Xe(1) physisorption measurements in the high-pressure limit while the magnitude of Delta H-ads in the low-pressure limit is significantly larger. Thus, the Xe-129 data are influenced by adsorbate-adsorbent as well as adsorbate-adsorbate interactions.