Smectite clays charge

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

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. 

Stucki JW, Golden DC, Roth CB. 1984. Effect of reduction and reoxidation of strucmral iron on the surface charge and dissolution of dioctahedral smectites. Clays and Clay Minerals 32 350-356. 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

Montmorillonite is a layered smectite clay. It has a central gibbsitic octahedral layer of alumina sandwiched between two tetrahedral layers of sUica. But usually isomorphous substitution occurs and natural clay has Al substituted by Mg and Fe leading to negative charge on the layers. The interlamellar space between the layers is occupied by hydrated cations usually Na", K Ca" etc. balancing the negative charge on the layCTS. 

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. 

The smectite clays, for example, montmorillonite, hectorite, beidellite, and the mica minerals, for example, talc and pyrophillite possess a structure which is exemplified in Figure 67. The host lattice is formally composed of three sublayers two tetrahedral Si/O and one octahedral M/O, OH (M = Al, Mg) central layer. These layers bear an excess negative charge and compensation of these charges is achieved by the presence of interlayer cations. 

Clays are colloidal layered hydrous aluminosilicates. There are relatively few examples of porphyrin intercalation into clays reported, mostly with either smectite clays or layered double hydroxides (LDH). Smectite clays consist of negatively charged layered aluminosilicate sheets. These sheets are separated by cations and water molecules. The... 

Clays are classified on the basis of their crystal structure and the amount and locations of elelectric charge (defidt or excess) per unit cell. Crystalline days range from kaolins, which are relatively uniform in chemical composition, to smectites, which vary in their composition, cation exchange p>rop>erties, and ability to expand. The most commonly employed smectite clay for the preparation of polymeric nanocomposites is bentonite, whose main mineral component is montmorillonite (Utracki, 2004). 

Another very important property of smectite clays for pillaring is their ability to be swollen by water. Swelling provides access to the exchange ions on the basal surfaces, and allows facile replacement by pillaring cations. The charge... 

Layered double hydroxides (LDHs) are complementary to smectite clays insofar as the charge on the layers and the gallery ions is reversed that is, the host layers of an LDH are 2D polyhydroxy cations and the gallery species hydrated anions. The compositions of LDHs are represented by the general formula [M j.jM j(OH)2][A"]jjy, z H2O, where A" is the gallery anion, and... 

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