Interlayer substitution

In the conventional precipitation and sol-gel routes, the resultant bimessite or buserite materials are usually Na type or K type, in which the cations are located in the interlayers, and MnOe octahedral units form the Cdl2-type structure in the layers. The stmcture and propaties of bimessite and buserite can be modified to load other cations, including alkali metals, alkaline earths, and transition metals, via two pathways. One pathway is interlayer substitution via ion exchange method at room temperature [14,22]. In this process, Na- or K-bimessite are stirred in a solution of dopants. Divalent metal cations, such as Mg ", Ni ", Co ", have been successfully ion exchanged with Na-birnessite mataials and used to prepare todorokite-type materials with tunnel stractures [22],... 

Talc and Pyrophyllite. Talc (qv) and pyrophjlhte are 2 1 layer clay minerals having no substitution in either the tetrahedral or octahedral layer. These are electrostatically neutral particles (x = 0) and may be considered ideal 2 1 layer hydrous phyUosiHcates. The stmctural formula of talc, the trioctahedral form, is Mg3Si402Q(0H)2 and the stmctural formula of pyrophylUte, the dioctahedral form, is Al2Si402Q (OH)2 (106). Ferripyrophyllite has the same stmcture as pyrophylUte, but has ferric iron instead of aluminum in the octahedral layer. Because these are electrostatically neutral they do not contain interlayer materials. These minerals are important in clay mineralogy because they can be thought of as pure 2 1 layer minerals (106). 

We have done our experiments with hectorite, which is a 2 1 smectite that develops negative layer charge by substitution of Li for Mg in the octahedral sheet.Samples were prepared by multiple exchange in 1.0 and 0.1 M CsCl solutions until essentially complete Cs-exchange was reached (97% of the interlayer cations). Temperature dependent data are essential to interpret the results, because there is rapid exchange of Cs among different interlayer sites at room temperature (RT). 

In the pyroaurite structure the brucite layers are cationic. However, on oxidation the resultant brucite layers in y - NiOOH are anionic. To preserve electroneutrality, cations and anions are exchanged in the intercalated layer during the oxidation-reduction process. This is illustrated in Fig. 4. In the case of Mn-substituted materials, some Mn can be reduced to Mn(II). This neutralizes the charge in the brucite layer this part of the structure reverts to the P - Ni(OH)2 structure and the intercalated water and anions are expelled from the lattice. With this there is a concomitant irreversible contraction of the interlayer spacing from 7.80 to 4.65A [72]. 

Bronsted acid sites) or metal atoms with unsatisfied coordination (Lewis acid sites) react with water to form surface charge (13). Isomorphic substitution in the interlayer region of layered silicates results in a negative surface charge. In each case chemical "exchange" of ions between phases results in the formation of surface charge and the development of an electrical potential. 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

The basic structure of an LDH may be derived by substitution of a fraction of the divalent cations in a brucite lattice by trivalent cations such that the layers acquire a positive charge, which is balanced by intercalation of anions (and, usually, water) between the layers. It is the possibility of varying the identity and relative proportions of the di- and trivalent cations as well as the identity of the interlayer ions that gives rise to the large variety of materials having the general formula [M S xM x(OH)2] [A" ] c/ yH20, which... 

Phenols, particularly the highly chloro-or nitro-substituted variety, are an important group of organic contaminants which, at typical ambient pH, can be present in groundwater predominantly as phenolate anions. Ulibarri et al. [154] studied the adsorption capacity of 2,4,6-trinitrophenol (TNP) on Mg/Al LDHs and their calcined products. The adsorption of TNP on LDHs by anionic exchange is dramatically affected by the identity of the interlayer anion and LDH chlorides have an adsorption capacity of more than 4 times that of LDH carbonates. However, calcined LDH carbonates are more effective adsorbents than those derived by calcination of LDH chloride samples. This possibly reflects the higher surface area of the former species. 

Montmorillonite, one of the most commonly encountered smectites, is similar to pyrophyllite (2 1) but has some interlayer cations and extra water. In pyrophyllite the layers are neutral because Si " in the tetrahedral sheet is not replaced by Al. In the smectites there is substitution of Al for Si " in the tetrahedral sheets, and occasionally Al appears in octahedral locations as well (for the names assigned to the end members, see Brindley and Brown, 1980, pp. 169-170.) The charge imbalances of the substitutions are compensated by interlayer cations, usually Na or Ca. These cations are easily exchangeable. The hydration level of the smectites is also variable. These minerals are very responsive to changes in water content as well as to the salt contents of the water. Other liquids that might be associated with the minerals and temperature can also effect changes in the chemical and crystal structure. 

Anticorrosive paints containing pigments with either chemical or electrochemical action may induce formation of protective coatings at the metal-paint interlayer (Etz-rodt, 1993). These protective coating may be metal-substituted iron oxides iron phosphate precipitates or even a green rust - Fe Fe" 0HigC03 4H2O (Chemical Week, 1988). 

Thus, for example, for a system of benzyl chloride, butyl bromide, and the hydrotalcite-like material, we expect the following reactions to occur. Butyl bromide would undergo a halide substitution by the interlayer Cl anions, leaving Br anions in the interlayer space the intercalated Br anions would, in turn, attack benzyl chloride to yield benzyl bromide leaving Cl anions in the interlayer space, the interlayer Cl anions being cycled as follows ... 

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