Smectite clay structure

The discussion of clay surface compatibility with polymers in this section will focus primarily on montmorillonite as the example clay. The characteristics discussed will only vary by degree for other smectic clays. 

It is important to understand the structure and general properties of smectic clays. The primary smectite phyllosilicates chosen are analogs 

Two types of crystallographic filling of the octahedral metal site characterize the structural filling. In the case of talc, hectorite, some vermicu-lite, and some micas, all three crystallographic sites are occupied by divalent ions such as magnesium. These are normally designated by the term trioctahedral. The other minerals only have cations at two out of the three sites and are referred to as dioctahedral. The predominate cation in these cases is Al or, in some cases, Fe .  

The other key difference is the level and type of isomorphous substitution in each of these minerals. Among the trioctahedral clays, talc has no isomorphous substitution, hectorite has moderate substitution of Li for Mg and vermiculite and mica have high levels of substitution. 

In the dioctahedral series, pyrophyllite has no substitutions, montmor-illonite has medium amounts of substitution, and vermiculites and mica have high amounts of substitution. 

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FIGURE 2.2 The smectite clay structure. (From http //www.pslc.ws/macrog/mpm/composit/nano/stmct3 l. htm, access date 4.11.06.)... 

Many varieties of clay are aluminosilicates with a layered structure which consists of silica (SiOa" ) tetrahedral sheets bonded to alumina (AlOg ) octahedral ones. These sheets can be arranged in a variety of ways in smectite clays, a 2 1 ratio of the tetrahedral to the octahedral is observed. MMT and hectorite are the most common of smectite clays. 

Alvarez-Cohen et al. [91] explicitly showed that microbial transformation rates of trichloroethylene (TCE) were proportional to the aqueous TCE concentrations and independent from zeolite-sorbed TCE concentrations. Apparently in contrast to these findings, Crocker et al. [92] reported on the direct bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay to Pseudomonas putida 17848, but not to Alcaligenes sp. strain NP-Alk. It should be noted that sorption to the hexadecyl chains of HDTMA resembles more the solubilisation by a surfactant than adsorption to a solid surface. Possibly, hydrophobic surface structures of strain 17848 allowed the close contact with HDTMA, thereby facilitating the uptake of naphthalene by a lipophilic pathway. 

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. 

Kostka JE, Wu J, Nealson KH, Stucki JW. 1999. The impact of structural Fe(III) reduction by bacteria on the surface chemistry of smectite clay minerals. Geochimica et Cosmochimica Acta 63 3705-3713. 

FIGURE 7.26 Idealized structure of the layers in a smectite clay (a) showing only the oxygen/hydroxyl framework (b) also depicting aluminium and silicon positions. 

SCHULTZ (L.G.), 1969. Lithium and potassium absorption, dehydroxylation temperature, and structural water content of aluminous smectites. Clays and Clay Min. 1 7, 115-49. 

Many of the same ionic surfactants used for the assembly of mesostructured molecular sieve catalysts [1-4] and related bulk phases [5] can be intercalated in a variety of layered host structures [6]. We have recently demonstrated that some of these mesostructure - forming surfactants retain their structure directing properties when intercalated in the galleries of smectite clays. In a manner quite analogous to bulk mesostructure formation, the intercalated surfactants direct the assembly of an open framework metal oxide (silica) structure within the constrained gallery regions of the layered host (7). The resulting porous intercalates are referred to as porous clay heterostructures (PCH). 

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. 

In this work, and based on the synthesis procedure previously described [4], we develop a new PCH structure based on a synthetic smectite clay host. The synthesis involves a two-step mechanism in a first step, cationic surfactants are ion-exchanged on the clay. Then, in... 

The key structural features of smectite clays of relevance to discussions of their catalytic activity and selectivity are the following ... 

Montmorillonites (smectite clays) have structures resembling that of pyrophyllite but the structure is not electrically neutral. Exchangeable cations are located in interlamellar regions of the clay and, furthermore, the clay can be flocculated such that the plate-like crystals compact with parallel c-axes to give coherent layers. The smectites are then attractive materials with which to modify electrodes. 

In some pipe deposits in geothermal power plants, arsenic is associated with clays or other silicate minerals rather than sulfides or (oxy)(hydr)oxides. Pascua et al. (2005) found that about 80 % of the arsenic in pipe scales from a Japanese geothermal power plant was associated with Mg-rich smectite clays. The arsenic (mostly III) was probably located in the crystalline structures of the clays and/or present as submicron inclusions. 

Figure 1. Generalized structure of a 2 1 phyllosilicate (e.g., smectite clay). There are two different octahedral sites corresponding to cis-M04(0H)2 and trans-MO OH. (Modified from Grimm, R.E. "Clay Mineralogy" McGraw-Hill, 1968).

Variable Oxidation States of Iron in the Crystal Structure of Smectite Clay Minerals... 

One can only speculate as to the exact identity of the Z q or Zl species, but the presence or formation of (0 )x (e )x centers in the clay structure should be investigated. Ideas advanced by Coyne (43) and Freund and Batllo (44) regarding (0 ) and (02 ) centers in clay minerals could, therefore, play an important part in the development of a complete understanding of the Fe reduction mechanisms in smectites ... 

M. Dijkstra, J.-P. Hansen, and P. Madden, Statistical model for the structure and gelation of smectite clay suspensions, Phys. Rev. E55, 3044—3053 (1997). 

Teppen et al. [89] have used a flexible model for clay minerals that allows full movement of the M-O-M bonds in the clay structure, where M represents Si, Al, or other cations in the octahedral sheet. This model was used in MD simulations of interactions of hydrated clay minerals with trichloroethene [90, 91]. The simulations suggest that at least three distinct mechanisms coexist for trichloroethene sorption on clay minerals [90], The most stable interactions of trichloroethene with clay surfaces are by full molecular contact, coplanar with the basal surface. The second type more reversible, less stable is adsorption through single-atom contact between one chlorine atom and the surface. In a third mechanism, trichloroethene interacts with the first water layer and does not interact with clay surface directly. Using MC and MD simulation the structure and dynamics of methane in hydrated Na-smectite were studied [92], Methane particles are solvated by approximately 12-13 water molecules, with six oxygen atoms from the clay surface completing the coordination shell. 

Layered silicate clays intercalated by pillaring poly-oxocations are precursors to an important class of mi-croporous catalysts. Smectite clay was the only host structure known to be pillarable by purely inorganic oxo ions. Recently, layered double hydroxides (LDH) pillaring oxo ions were reported by Pinnavaia and coworkers [79, 80]. 

More recently, various attempts have been made to develop cracking catalysts from pillared smectite clays, in which the layers are separated and held apart by the intercalation of large cations. Pillared clays (PILCs) have large surface areas within fairly well-ordered micropore structures (pore widths in die approximate range 0.6-1.2 nm). It is not surprising that these materials have attracted considerable interest with the prospect of an alternative type of catalytic shape selectivity (Thomas, 1994 Thomas etal., 1997 Fripiat, 1997). 

Figure 3 The layered structure of smectite clays such as montmorillonite or hectorite is represented. The aliuniniun sites are octahedral and the silicon sites are tetrahedral. The interlayer space is filled with water and metal cations...

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

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