This paper reviews the synthesis of smectites and porous pillared clay catalysts. Synthetic as well as natural smectites serve as precursors for the synthesis of Al, Zr, Ti, Fe, Cr, Ga, V, Si, and other pillared clays as well as mixed Fe/Al, Ga/Al, Si/Al, Zr/Al and other mixed metal/Al pillared clays. The use of these pillared clays in some catalytic reactions is also briefly reviewed.

Synthesis of Smectites and Porous Pillared Clay Catalysts: A Review

Smectites are one of the most important groups of phyllositicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature ( 1000~ or pressures >10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrother- mal conditions are the most numerous and the most successful in terms of smectite yield and phase- purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The pres- ence of Mg 2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temper- atures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under- and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermody- namically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.

Synthesis of smectite clay minerals; a critical review

For two series of aluminosilicate glasses on the SiO 2 -NaAlO 2 and SiO 2 -CaAl 2 O 4 joins, 29 Si magic-angle-spinning (MAS) NMR spectra were measured. Systematic variations in peak positions and widths with composition are closely related to the extent of ordering of Si and Al cations. A statistical thermodynamic model based on the quasi-chemical approximation was formulated to calculate the proportions of SiO 4 groups with varying numbers of Al neighbors and thus to quantify the extent of ordering. Multiple spectra in each compositional series were fitted simultaneously with several peaks representing each of these structural species and with area constraints generated by the model. The extent of aluminum avoidance (Q), which was defined using the relative lattice energy differences among the linkages Si-O-Si, Si-O-Al, and Al-O-Al, was optimized for each series. For the calcium aluminosilicates, the best fit is with 0.8< or =Q< or =0.875, where Q = 1 represents perfect Al-avoidance. For the sodium series, Q was found to be larger (0.93< or =Q< or =0.99), as expected from energetic considerations and from known variations in ordering in minerals. The contributions to the overall configurational entropy and heat capacity from Si-Al disorder can be calculated, and are significant fractions of experimentally estimated values. However, major contributions must also come from other sources of disorder, such as "topological" disorder of bond angles and length.

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The degree of aluminum avoidance in aluminosilicate glasses

We use molecular dynamics as a tool to understand the structure and phase transitions [Osman, M A; et al J Phys Chem B 2000, 104, 4433−4439 Osman, M A; et al J Phys Chem B 2002, 106, 653−662] in alkylammonium micas The consistent force field 91 is extended for accurate simulation of mica and related minerals We investigate mica sheets with 12 octadecyltrimethylammonium (C18) ions or 12 dioctadecyldimethylammonium (2C18) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary The nitrogen atoms are 380−390 pm away from the superficial silicon−aluminum plane With increasing temperature, rearrangements of C18 ions on the mica surface are found, while 2C18 ions remain tethered due to geometric restraints We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche−tra

Structure and phase transitions of alkyl chains on mica.

Sorption is one of the most important chemical processes in soil. It affects the fate and mobility of nutrients and contaminants in soils and waters greatly. This per critically reviews the mechanisms of sorption/desorption phenomena at the mineral/water interface. A brief discussion of macroscopic, equilibrium approaches for describing sorption processes is provided. However, emphasis will be placed on the importance of understanding the rates of sorption/desorption processes and coupling the kinetic investigations with in-situ atomic/molecular resolution surface techniques. The use of X-ray absorption fine structure (XAFS) spectroscopy and scanning force microscopy (SFM) will be emphasized.

A critical assessment of sorption-desorption mechanisms at the soil mineral/water interface

29Si MAS-NMR spectra were obtained for vermiculite with tetrahedral composition x=Al/(Al+Si)=0.28, a synthetic mica with x=0.43, and margarite with x=0.5. Comparison between the observed and Monte-Carlo simulated spectra was used to test different Al-Si distribution schemes in the tetrahedral sheet. The results of this analysis are interpreted including earlier data corresponding to lower Al for Si substitutions (0.12≤x≤0.28), and it is shown that for all samples (8 compositions in the range 0.1≤x≤0.5) the Al-Si distribution complies with the principle of homogeneous dispersion of charges (HDC). Thus, Al-Si distribution models for any particular x can be predicted on the basis of a simple crystallochemical criterion. HDC is a short range order concept and implies that, within the restrictions imposed by the principle, charges be distributed at random. For increasing values of x, however, the observance of HDC implies a coupling between adjacent hexamer rings which restricts the degrees of freedom for random distribution and is equivalent to a progressive extension of order. This is reflected by the frequencies of meta and para substitutions in the hexamer rings, which vary in a characteristical, continuous way with x, until the fully ordered distribution scheme of margarite (x=0.5) is reached. Partial long-range order for x<0.5, with alternating Si and (Al+Si) tetrahedra and all substitutions in meta disposition, could not be confirmed.

29 Si MAS-NMR spectroscopy of mica-type silicates: Observed and predicted distribution of tetrahedral Al-Si

Tetrahedral cation ordering in layer silicates by 29Si NMR spectroscopy

Dispersion of charge deficits in a tetrahedral sheet of phyllosilicates: analysis from silicon-29 NMR spectra

The analysis of 29Si MAR-NMR spectra of micas and vermiculite shows that the two main factors that govern the Si, Al distribution in the tetrahedral sheet of these minerals are: (i) Loewenstein's rule (avoidance of Al-O-Al linkages), and (ii) local balance of charges. In muscovite, phlogopite and vermiculite, Al is randomly distributed in hexagonal ring sites (meta and para dispositions equally favoured), but complying with the two above restrictions. In margarite Si and Al are observed to alternate strictly, in agreement with previous X-ray diffraction analysis.

http://iopscience.iop.org/article/10.1088/0022-3719/18/1/009/pdf

Si, Al distribution in micas; analysis by high-resolution 29 Si NMR spectroscopy

The arrangement of water molecules in one- and two-layer hydrates of high-charged vermiculites, saturated with alkaline (Li+, Na+) and alkali-earth (Mg2+, Ca2+, Ba2+) cations, has been analyzed with 1H NMR spectroscopy. Two different orientations for water molecules have been found, depending on the hydration state and the sites occupied by interlayer cations. As the amount of water increases, hydrogen bond interactions between water molecules increase at expenses of water−silicate interactions. This interaction favors water mobility in vermiculites. A comparison of the temperature dependence of relaxation times T1 and T2 for one and two-layer hydrates of Na-vermiculite shows that the rotations of water molecules around C2-axes and that of cation hydration shells around the c*-axis is favored in the two-layer hydrate. In both hydrates, the anisotropic diffusion of water takes place at room temperature, preserving the orientation of water molecules relative to the silicate layers. Information obtained by N...

J. Sanz

Papers

29 Si MAS-NMR spectroscopy of mica-type silicates: Observed and predicted distribution of tetrahedral Al-Si

Tetrahedral cation ordering in layer silicates by 29Si NMR spectroscopy

Dispersion of charge deficits in a tetrahedral sheet of phyllosilicates: analysis from silicon-29 NMR spectra

Si, Al distribution in micas; analysis by high-resolution 29 Si NMR spectroscopy

Arrangement and mobility of water in vermiculite hydrates followed by 1H NMR spectroscopy.