Smectite types

The Smectite Clays. The smectite-type clays are distinctive in that they expand and cause significant destruction to synthetic (human-made) structures. In this type of 2 1 clay, isomorphous substitution occurs in the aluminum sheet. If there is substitution of lower-oxidation-state metal such as magnesium, there will be an unsatisfied pair of bonding electrons in the interior of the crystal and there will be no noticeable change in the surface. Because the charge is in the interior of the crystal, its attraction for cations is diminished by distance. Thus, smectite crystals are not held together strongly by cations and are able to incorporate more water and ions between sheets when the environment is wet and less when it is dry. 

Clay minerals with their own surface properties affect the near surface water in different ways. The adsorbed water in the case of kaolinite consists only of water molecules ( pure water), whereas water adsorbed on a smectite-type mineral is an aqueous solution, due to the presence of exchangeable cations on the 2 1 layer sihcate. Sposito (1989) noted the generally accepted description that the spatial extent of adsorbed water on a phyUosilicate surface is about 1.0 nm (two to three layers of water molecules) from the basal plane of the clay mineral. 

Porous Smectite-type Materials Containing Catalytically Active Divalent Cations in Octahedral Sheets... 

Smectite-type materials containing transition metal divalent cations (Ni2+, Co2, and Zn2+) in octahedral sheets were synthesized. The synthetic smectites were thermally stable and had large surface areas and high pore volumes after evacuation at 873 K. Catalytic activities of synthetic smectites were investigated. The Ni2 -containing smectites were active for the isomerization of 1-butene and the oligomerization of ethylene. The Co2+-containing smectites were active for the hydrodesulfurization of thiophene. 

Smectite-type materials were synthesized with a hydrothermal method [5]. The aqueous solution of sodium silicate (Si02 / NajO= 3.22) and sodium hydroxide was mixed with the aqueous solution of metal chloride to precipitate Si-M (M divalent metal cation, Si M = 8 6) hydroxides. The precipitation pH of Si-M hydroxide was controlled by changing the molar ratio of sodium hydroxide to sodium silicate. After separating and washing of Si-M hydroxide, slurries were prepared from Si-M hydroxide and water. The Si-M slurries were treated hydrothermally in an autoclave at 473 K under autogaseous water vapor pressure for 2 h. The resultant samples were dried at 353 K then we obtained smectite samples. The smectite-type materials are denoted by the divalent species in octahedral sheets and BET surface area, e.g., Ni-481 for the Ni2+ substituted smectite-type material with a surface area of 481 m2g. ... 

Table 1 shows the properties of smectite-type materials prepared. Smectite materials prepared at lower pH had fewer sodium ions, higher surface areas, and larger pore volumes for a series of samples containing the same divalent cation species (nickel and cobalt) in the octahedral sheet. The adsorption of methylene blue on all the synthetic smectites shows that the smectite fragments are negatively charged. The Si M ratios of synthetic smectites were about 8 6, indicating that most of divalent cations exist in octahedral layers and small amount of divalent cations would exist as hydroxide or oxide cluster in smectite materials. However, the amounts of the hydroxide or oxide cluster were small, because only smectite structures were observed in XRD patterns and EXAFS Fourier transforms of synthetic smectites calcined at 873 K. 

The nitrogen adsorption-desorption isotherms at 77 K showed that the pore structures of smectite-type materials are of a bottle-neck type [3]. The surface areas of Ni-481 and Ni-359 treated at 873 K were 381 and 184 m2 g 1, respectively (Figure 2). The synthetic smectites have large surface areas because many small fragments with the same smectite structure are intercalated in the interlayer region [4]. 

Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering horizons of podzols. Clay Miner., 5 183-193. 

Upon addition of stoichiometric amount of bpy to Cu11 exchanged smectite-type montmorillonite, the formation of a Cun(bpy)2 complex was claimed, which was very active and selective for the oxidation of alcohols into the corresponding ketones.[96] Unfortunately, neither the nature of the secondary porosity, the complex geometry nor its location was investigated in further detail. 

Hectorite is an aluminum-free mineral of the smectite type. Isomorphous substitution could occur at tetrahedral silicon sites as well as at the octahedral sites originally occupied by lithium and magnesium. Monitoring the x-ray powder diffraction patterns as a frmction of crystallization time, it was found that the hydrothermal crystallization was complete after 12h at 200°C, independent of the alumina content of the reaction mixture. However, NMR spectroscopy proves that some structural change still occurs after this time period. 

There is considerable current interest in the design of new catalysts by interchelating clay minerals of the smectite type with redox metal ions, leading to the formation of oxidation catalysts with interesting (shape-selective) properties [38]. For example, vanadium-pillared montmorillonite (V-PILC) proved to be an... 

Hitherto the most successful pillaring has been carried out on smectite type clays. The original research on pillaring included several series of unsuccessful experiments on mica, vermiculite and the sheet silicic acid minerals magadiite, silhydrite and kenyaite. Recently renewed attention has focused on these and other alternative sheet structures. They include rectorite, zirconium phosphates, tetrasilicic micas, hydrotalcites and silicic acids, the general structural features of which are shown in Figure 4. 

Mostly focused on cationic clays, and particularly on montmorillonite and hectorite, smectite-type layered silicates and clay-based nanofillers have recently been extended to the family of LDH. Hydrotalcite-like LDH materials are described according to the ideal formula, [M1/ xM"l(OH)2]frl(lra [A H20]inter, where Mn and Mm are metallic cations, A the anions, and intra and inter denote the intralayer and interlayer domain, respectively. The structure consists of brucite-like layers constituted of edge-sharing octahedra. The presence of trivalent cations induces positive charges in the layers that are counterbalanced by interlamellar anions (Scheme 15.16). 

The clay mineral spectrum is made up essentially of illite, kaolinite, chlorite and mixed-layer swelling clays of the illite-smectite type. The qualitative and quantitative composition of the clay minerals depends on their depositional environment. In littoral zones where the influence of the continents is pronounced, the kaolinite content may reach 30% whereas in the deeper parts of the sea it will be only 10%. For the mixed-layer clays the opposite trend may be observed. The widespread presence of these minerals, the genetic conditions of which are similar to those of smectite, suggests an arid climate over the East Saharan synclinorium during the Visd. This also explains the formation of evaporites in isolated highly saline basins in the western Saharan region. 

Fig. 4.1. Transformation of mixed-layer days of illite-smectite types (unordered) into ordered phase of vermiculite-chlorite type vs. transition of Fe into Fe. Paleozoic sediments of the Illizi Basin...

The self-assembly of the 2 1 smectite-type clay containing thiol groups on Au was performed in a chloroform suspension of clay [91]. The formation of a monolayer or a partial double layer was demonstrated from the frequency change. In other studies on the clay-modified electrodes, the clay-precoated QCM was used to examine the swelling of the clay films, and the adsorption and redox of the metal complexes. 

Fig. 7.7 Square wave voltammetric traces recorded on 2 x 10 M mesotrione, 0.1 M acetate buffer solution (pH = 6) at a (a) bare glassy carbon electrode, (i>) natural Cameroonian smectite-type clay-modified electrode, (c) cetyltrimethylammonium functionalized smectite-type clay-modified electrode, and (d) didodecyldimethyl ammonium functionalized smectite-type clay-modified electrode. Inset reports the chemical structure of mesotrione (Reproduced from Ref. [75] with the permission of Elsevier)...

Synthesis of smectite-type clay minerals, such as saponites, may ciieumvcnt the above mentioned difficulties. The usually demanding hydiolfaermal treatment [3>4, the lo duration of the synthesis [5], and problems with the scale-up of the jxq>aratk procecterc. however, have thus far severely limited die use of synthetic clays as solid add catalysts. 

The "Transition Stretch " (Suarez Vega, 1974), also known as "brown clay and black shale with gypsum and anhydrite" (Pier et al., 1995), is located between the two previously described lithologic sets. Comprises a series of black and brown shales (smectites type), with frequent levels of sulfates (gypsum and anhydrite). Its thickness is highly variable, fiom absence of up to 20 or even 60 meters. 

Tonle IK, Ngameni E, Walcarius A (2005) Preconcentration and voltammetric analysis of mercury(II) at a carbon paste electrode modified with natural smectite-type clays grafted with organic chelating groups. Sensor Actuat B 110 195-203... 

A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering... 

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