Structural Characterization of Pristine and Modified Fluoromica Using Multinuclear Solid-State NMR

The structure of a synthetic fluoromica has been investigated and elucidated by complementary Si-29, H-1, F-19, Na-23, and Al-27 single and double resonance experiments. The phyllosilicate possesses a charge heterogeneity arising from the presence of different charged sites in the octahedral sheets of the T-O-T layers. Part of the magnesium atoms in the octahedral sheets is missing, creating vacancies. Charge balancing proceeds mostly by incorporation of sodium ions into the interlayer space and substitution of Mg by sodium in the lattice. The latter process produces a nonexchangeable fraction of sodium ions (similar to 20% of the total inventory) and creates multiple sites for the fluoride ions, as evidenced from F-19 NMR spectroscopy. Detailed quantitative information about the sodium fluoride distance geometries were obtained from F-19/Na-23 double rotational echo double resonance (REDOR) experiments; furthermore, different types of fluoride sites could be discriminated on the basis of F-19/Na-23 double resonance experiments. The chemical reactivity of this system was modified by ball milling, cation exchange with octadecylammonium ions, and grafting with an organosilane (gamma-aminopropyltriethoxysilane), and the corresponding structural consequences were probed by multinuclear single and double resonance NMR techniques.