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 th ermal analysis, scanning electronic microscopy, Si-29, Al-27 and F-19 MAS N MR spectroscopy, are used in the structural analysis of montmorillonites, s ynthesised in an acidic fluoride medium. Hydrothermal synthesis performed u nder 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 t he tetrahedral sheet is determined by using Al-27 MAS NMR spectroscopy perf ormed under definite conditions. Some interatomic distances are determined by EXAFS spectroscopy (RMg-O = 2.11 Angstrom, RZn-O = 2.08 Angstrom, RZn-Al = 2.98 Angstrom, RZn-z = 3.11 Angstrom) which reveals strong local distort ions in the octahedral sheet with respect to the ideal montmorillonite stru cture. Lengthening of the out-of-plane Zn-Si(Al) distances also shows the s welling of the whole layer in the neighbourhood of Zn. Moreover, the combin ation of EXAFS and F-19 MAS NMR spectroscopic data reveals a clustering of the divalent elements of the octahedral sheet, which is related to a possib le local trioctahedral character of these materials.