Fluorine Route Synthesis of Montmorillonites Containing Mg or Zn and Characterization by XRD, Thermal Analysis, MAS NMR, and EXAFS Spectroscopy

EXAFS spectroscopy, combined with X-ray powder diffraction, chemical and thermal analysis, scanning electronic microscopy, 29Si, 27Al, and 19F MAS NMR spectroscopy, are used in the structural analysis of montmorillonites, synthesised in an acidic fluoride medium. Hydrothermal synthesis performed under mild conditions (493 K, autogenous pressure) enables the formation of montmorillonite clays, containing Al and Zn or Mg in the octahedral sheet. It is shown that montmorillonites can only be synthesized in a narrow range of compositions. An accurate value of the Al for Si substitution rate in the tetrahedral sheet is determined by using 27Al MAS NMR spectroscopy performed under definite conditions. Some interatomic distances are determined by EXAFS spectroscopy (RMg−O = 2.11 A, RZn−O = 2.08 A, RZn−Al = 2.98 A, RZn−Zn = 3.11 A) which reveals strong local distortions in the octahedral sheet with respect to the ideal montmorillonite structure. Lengthening of the out-of-plane Zn−Si(Al) distances also shows the swelling of the whole layer in the neighbourhood of Zn. Moreover, the combination of EXAFS and 19F MAS NMR spectroscopic data reveals a clustering of the divalent elements of the octahedral sheet, which is related to a possible local trioctahedral character of these materials.