Smectites tetrahedrally charged

It is believed that CuO O) " - orients in vermiculite interlayers (2 water layers thick) as shown in Figure 18B (6). Yet, the smectites with more vermiculite-like properties (high tetrahedral charge, high total charge) showed no evidence of this orientation, even in cases where two layers of water were situated between the plates. It is necessary to conclude that Cud O) " or Cu(H20)5 + ions are found in the two-layer hydrates of the smectites, with the orientation shown in Figure 18C. 

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. 

Arene complexes are also formed with other Cu +- or Ag +-saturated smectites. Charge location (tetrahedral or octahedral) has an influence on the formation of these compounds, with complexation easier in octahedrally charged smectites. This is because water is more firmly bound and more difficult to eliminate in tetrahedrally charged smectites thus, aromatic molecules will have greater difficulty entering into direct coordination with the cations (56). Also, arene com-... 

At that time, Plee et al. stated that the layer reactivity is solely related to the origin of the layer charge, and only occurs in case of tetrahedrally charged smectites [58]. Later, however, Pinnavaia et al. proved that the mechanism of cross-linking was also possible for the octahedrally substituted fluorohectorite clay [57]. present in the former clay, was found to be responsible for the labiliza-tion of the Si-0 bonds of the tetrahedral layer (40). This promotes the linking between the host layers and the pillars to form Siday-O-Alpmar covalent bonds by an inversion of the Si04-tetrahedrons. This mechanism of cross-linking for fluorohectorite is represented in Fig. 14. 

Figure 1. Pyrophyllite, reference model of dioctahedral smectites centrosymmetrical layer, (a) Projection on the biperiodic plane of the lattice (ab). (b) Projection on a plane perpendicular to the b axis. Only those atoms situated between the planes x and x of the projection (a) are represented. Arrows A indicate the centers of the hexagonal cavities of the surface of the layer. Arrows M and B indicate eventual localization of negative charges created by isomorphous replacements. Af—octahedral charges (montmorillonite) tetrahedral charges (beidellite).

Figure 3. Talc, reference model of trioctahedral smectites. A— hexagonal cavities H and S— eventual localization of the negative charges created by isomorphous replacements H—octahedral charges (hectorite) S— tetrahedral charges (saponite).

The ordered and semi-ordered forms are observed only on smectites having abundant tetrahedral charges (saponite, beidellite). It can, therefore, be concluded that the tetrahedral charges tend to impose the configuration conforming to the sketch of Figure 9. 

Smectites are stmcturaUy similar to pyrophylUte [12269-78-2] or talc [14807-96-6], but differ by substitutions mainly in the octahedral layers. Some substitution may occur for Si in the tetrahedral layer, and by F for OH in the stmcture. Deficit charges in smectite are compensated by cations (usually Na, Ca, K) sorbed between the three-layer (two tetrahedral and one octahedral, hence 2 1) clay mineral sandwiches. These are held relatively loosely, although stoichiometricaUy, and give rise to the significant cation exchange properties of the smectite. Representative analyses of smectite minerals are given in Table 3. The deterrnination of a complete set of optical constants of the smectite group is usually not possible because the individual crystals are too small. Representative optical measurements may, however, be found in the Uterature (42,107). 

Vermiculites have a 2 1 layer structure similar to smectites, but expand less freely in water, presumably because of the higher layer charge in the former minerals. Most of this structural charge resides in the tetrahedral layers of the vermiculite platelets. Even when fully wetted, vermiculites do not expand beyond the two water-layer stage ( " 1.5 nm c-spacing). 

The layer silicates comprise tetrahedral sheets of silica and octahedral sheets of aluminium and magnesium hydroxide, with varying amounts of the Si, Al and Mg replaced by cations of lower valence giving the lattice a net negative charge. Two basic combinations occur 1 tetrahedral sheet with 1 octahedral (e.g. kaoUnite, halloysite), and 2 tetrahedral with 1 octahedral (e.g. smectite, vermiculite, illite). 

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