Cationic clay

Two classes of clays are known [3] (i) cationic clays (or clay minerals) that have negatively charged alumino-silicate layers balanced by small cations in the interlayer space (e.g. K-10 montmorillonite) and (ii) anionic clays which have positively charged brucite-type metal hydroxide layers balanced by anions and water molecules located interstitially (e.g. hydrotalcite, Mg6Al2(0H)igC034H20. 

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

Jaynes WF, Vance GF (1999) Sorption of benzene, toluene, ethylbenzene and xylene (BTEX) compounds by hectorite clays exchanged with aromatic organic cations. Clays Clay Miner 47 358-365 Johnston CT, De Oliveira MF, Teppen BJ, Sheng G, Boyd SA (2001) Spectroscopic study of nitroaromatic-smectite sorption mechanisms. Environ Sci Technol 35 4767-4772... 

Clay-polymer nanocomposites have proven to be interesting candidates as gas barrier materials preventing permeation of volatile gases by creating a long path for diffusion and as flame-retardant materials. Previous work mainly involves the utihzation of cationic clays, although LDH materials... 

Summarizing, the efficiency of the oxidant use can substantially be improved with a hydrophobic LDH. There seems to be an optimum surface polarity, which in the present set of catalysts is most closely approached by the pTos-exchanged material. Similar effects of surface polarity on catalytic performance have been observed for cationic clays [16]. 

Calvet, R. (1975) Dielectric properties of montmorillonites saturated by bivalent cations, Clays and Clay Minerals 23, 257-265... 

MePOR species and other complexes in cationic clays can be located at the edges of packed platelets, in the interlamellar space or in the mesopores present (Scheme 10.9). A review of the early data in this area is available.[86] The flat metallo macrocycles under clay synthesis conditions help to induce layer silicate formation, the complexes being intercalated between the layers. Whereas with monooxygen atom donors, alkanes can be oxygenated with significantly enhanced activities compared with the homogeneous case, in every case the expected products (ol/on) were obtained. Competitive oxygenation of adamantane and pentane shows lower... 

Cationic clays have also been used as supports for Cu. Cu-doped alumina-pillared montmorillonites have been employed in the oxidation of toluene and of xylenes with H2C>2. The pillaring and the Cu exchange are performed under acidic conditions at pH 2 and 3.5, respectively. It is unclear whether the Cu2+ remains fully associated with the clay in the presence of H2O2, which is itself acidic. Moreover, the reactions are unselective mixtures of ring-hydroxylated and side chain-oxidized products are obtained (180). 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

It should be noted that although the quaternary ammonium is nominally chosen as the modifier to compatibilize the cationic clays with the polymer matrix, this does not refer to the processing aids or compatibilizers that help disperse the clay particle into the polymer matrix and set up the PN structure the compatibilizers may not necessarily be part of the interface between the polymer and the clay. For instance, the graft copolymer of ethylene or propylene with maleic anhydride (PE-g-MA or PP-g-MA) has proven to be an excellent compatibilizer/disperser for the PE/ or PP/ clay nanocomposite,47 but the graft copolymer is not part of the interface of the modified clay. 

The interest in LDHs is much more recent than their cationic equivalents (cationic clays such as montmorillonite and saponite). This is, in part, because cationic clays are much more abundant in nature and have an important role in many soil processes, for example. It is clear, however, that the potential for creating novel supramolecular structures, either directly as intercalates or for orienting organic molecules at reactive surfaces, is likely to increase. 

Figure 25. Schematic representation of 2 1 (trioctahedral and dioctahedral) cationic clays.

Dimer acids. Dimer acids are produced by heating monoene or diene fatty acids (e.g., tall oil acids, a byproduct of wood pulping) with a cationic clay catalyst (92). Typical conditions are 4% montmoriUonite at 230°C for 4—8 hours. After distillation, the product is a complex mixture of acyclic, cyclic, and bicyclic dimers along with some trimer. Dimer acids are dibasic and react with diamines and tria-mines to give polyamides. Imidazole derivatives are used as corrosion inhibitors and esters as lubricants. 

In combination with clay minerals, such as through bridging by polyvalent cations (clay-metal-humus), hydrogen bonding, van der Waal s forces, and in other ways as discussed by Greenland (1971) and Theng (1979). 

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). 

Sorption capacity manifested by binding and exchanging univalent and bivalent cations. Clay soils are characterized by a high sorption capacity. 

We report here preliminary results of the physicochemical characterization of a composite material obtained by combining the cethyltrimethylammonium cations clay insertion procedure with the room temperature synthesis of mesoporous materials inside of clay layers. The Romanian bentonite, containing 64% montmorillonite was used. The organic cations are incorporated within the interlayer region of the clay, serving to prop of>en the layers and to allow incorporation of the silicon source for MCM-4I synthesis. The obtained materials display a high thermal stability and molecular sieve properties. 

D. G. Edwards, A. M. Posner, and J. P. Quirk, Repulsion of chloride ions by negatively charged clay surfaces. Part 2. Monovalent cation montmorillonites, Trans. Faraday Soc. 61 2816 (1966). Part 3. Di- and trivalent cation clays, Trans. Faraday Soc. 61 2820 (1966). L. L, Schramm and J.C.T. Kwak, Interactions in clay suspensions The distribution of ions in suspension and the influence of tactoid formation, Colloids Surfaces 3 43 (1982). 

G.M. Do Nascimento, P.S.M. Barbosa, V.R.L. Constantino, and M.L.A. Temperini, Benzidine oxidation on cationic clay surfaces in aqueous suspension monitored by in situ resonance Raman spectroscopy, Coll. Surf. A Physicochem. Eng. Aspects, 289, 39-46 (2006). 

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