FULL PAPER Fluorine Route Synthesis of Montmorillonites Containing Mg or Zn and Characterization by XRD, Thermal Analysis, MAS NMR, and EXAFS Spectroscopy Marc Reinholdt,* [a,b] Jocelyne Miehe ´-Brendle ´, [a] Luc Delmotte, [a] Marie-He ´le `ne Tuilier, [b] Ronan le Dred, [a] Robert Corte `s, [c] and Anne-Marie Flank [c] Keywords: Montmorillonite / Clays / Synthesis / Fluorine / Solid-state NMR / EXAFS spectroscopy EXAFS spectroscopy, combined with X-ray powder diffrac- tion, chemical and thermal analysis, scanning electronic mi- croscopy, 29 Si, 27 Al, and 19 F MAS NMR spectroscopy, are used in the structural analysis of montmorillonites, syn- thesised in an acidic fluoride medium. Hydrothermal syn- thesis 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 27 Al MAS NMR spectroscopy performed under def- Introduction Smectites present in soils and sediments are one of the largest and most important classes of the phyllosilicate clay minerals group. Their ability to swell upon contact with water and other solvents is extensively used in two fields of application. The first one is related to the properties of adsorption (use in the selective adsorption of toxic com- pounds from water, detergents, organic chemistry, etc.). [1] The other one concerns pillared clays which are commonly used in the field of catalysis. [2,3] The 2:1 structure of smec- tites consists of two sheets of distorted SiO 4 tetrahedra con- nected by a sheet of Al(M)(OH) 2 O 4 octahedra, M being a divalent element (Figure 1a). The silicon and metal atoms occupy the centre of the tetrahedra or the octahedra, with the oxygen atoms and hydroxy groups (OH) located at the corners. Smectites are divided into two groups: the diocta- hedral smectites for which the octahedral sheet contains two Al(M) elements in the centre of two out of three oc- tahedra, the third one being vacant; and the trioctahedral smectites which contain only divalent elements in the octa- hedral sheet without vacancies. Montmorillonite, which be- longs to the former group, has been extensively studied. Among the dioctahedral smectites, the distinctive feature of montmorillonite is that transition elements are included in [a] Laboratoire des Mate ´riaux Mine ´raux, UPRES-A 7016, 3 rue A. Werner, 68093 Mulhouse Cedex, France [b] Laboratoire de Physique et Spectroscopie Electronique, UPRES-A 7014, 4 rue des Fre `res Lumie `re, 68093 Mulhouse Cedex, France [c] Laboratoire pour l’Utilisation du Rayonnement Electromagne ´tique (LURE), Ba ˆt 209D, Centre Universitaire, B. P. 34, 91898 Orsay Cedex, France Eur. J. Inorg. Chem. 2001, 2831-2841  WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001 1434-1948/01/1111-2831 $ 17.50+.50/0 2831 inite conditions. Some interatomic distances are determined by EXAFS spectroscopy (R Mg-O = 2.11 A ˚ , R Zn-O = 2.08 A ˚ , R Zn-Al = 2.98 A ˚ , R Zn-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 19 F 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. the octahedral sheet whereas Al can be split into the tetra- hedral (Al IV ) and octahedral (Al VI ) sheets. The Al atoms are partially replaced by either Mg or Fe atoms, thereby creating a charge deficiency within the unit structure. This results in a small negative charge on the basal plane of the silica tetrahedra which is balanced by exchangeable cations present in the interlayer spacing. In natural montmorillon- ites these cations are usually calcium or sodium according to the weather agent associated with the formation of the mineral. The crystal structure of montmorillonite has a C2mm symmetry. The ideal formula per half a unit cell, without tetrahedral substitution, is R 2x (Al 2(1-x) M 2x )Si 4 O 10 (OH) 2 , where R represents the interlayer cations and x the layer charge ran- ging from 0.05 to 0.54. Natural montmorillonites usually present isomorphic substitutions in both tetrahedral and octahedral layers. Moreover, depending on the deposits, various structural compositions and particle sizes can be found. These drawbacks, combined with the presence of im- purities, limit their use in some applications. One way to avoid these disadvantages is to synthesize such compounds; this enables the control of the chemical composition by fine tuning the preparative methods. Many studies have been performed in this field, as recently mentioned by Kloprogge et al., but only four papers mentioned the synthesis of pure montmorillonite. [4] The first one, by Otsubo and Kato, de- scribed Zn for Al partially substituted montmorillonite in the ZnO-Al 2 O 3 -SiO 2 system, [5] the Zn and Al atoms be- ing located in the octahedral sheet and the Si atoms in the tetrahedral sheet. The clay has been prepared under hydro- thermal conditions in an alkali medium at temperatures