Montmorillonites ion exchange

Laszlo and co-workers have reported that K10 montmorillonites ion-exchanged with various transition metal cations are effident catalysts for Friedel-Crafts alkylation of aromatics with a wide range of alkylating agents such as alcohols, alkenes and alkyl halides.25... 

Montmorillonite ion-exchanged with ammonium ions was used as the organophihc clay. This type of montmorillonite is called C18-Mt . 

Hwu and Jiang used a montmorillonite ion exchanged with stearyltrimethylammonium to support MAO, followed by catalyst Propylene polymerization was conducted in... 

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

Bentonite is an impure clay that is formed by weathering of volcanic tuffs. It contains a high content of montmorillonite. Bentonites exhibit properties such as ability to swell, ion exchange, and thixotropy. Properties can be modified by ion exchange, for example, exchange of earth alkali metals to alkali metals. The specific surface can be modified with acid treatment. Organophilic properties can be increased by treatment with quaternary ammonia compounds. 

As noted above, adsorption isotherms are largely derived empirically and give no information on the types of adsorption that may be involved. Scrivner and colleagues39 have developed an adsorption model for montmorillonite clay that can predict the exchange of binary and ternary ions in solution (two and three ions in the chemical system). This model would be more relevant for modeling the behavior of heavy metals that actively participate in ion-exchange reactions than for organics, in which physical adsorption is more important. 

The catalytic application of clays is related closely to their swelling properties. Appropriate swelling enables the reactant to enter the interlamellar region. The ion exchange is usually performed in aquatic media because the swelling of clays in organic solvents, and thus the expansion of the interlayer space, is limited and it makes it difficult for a bulky metal complex to penetrate between the layers. Nonaqueous intercalation of montmorillonite with a water-sensitive multinuclear manganese complex was achieved, however, with the use of nitromethane as solvent.139 The complex cation is intercalated parallel to the sheets. 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

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] ... 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

Ion exchange of various radionuclides on Na+-montmorillonites (from Shiao et al., 1979). 

The data are "normalized" with regard to the ion exchange capacity C of the sorbents. The sorption curves of the illite and of the < 40-pm chlorite are strongly non-linear, whereas that of the montmorillonite approaches linearity. 

As the figure shows the exchange of Sr2+ on Na+-montmorillonite fits the ion exchange theory very well. But the adsorption of heavy metals cannot be accounted for by this theory. Co(II) behaves as if it were monovalent Kd for americium is independent of [Na+] (americium occurs at pH = 6.5 as a hydroxo complex). 

Shiao, S. Y. (1979), "Ion Exchange Equilibria Between Montmorillonite and Solutions of Moderate to High Ionic Strength", in Radioactive Water in Geological Storage, ACS Symposium 100, Amer. Chem. Soc. 

Extremely high ion exchange affinities are however sometimes observed for alkali metals (e.g. Cs) and transition metal ion complexes in clay minerals and zeolites. The objective of this paper is to give an account of the factors which are involved in these high selectivity phenomena. The discussion will be focussed mostly on montmorillonites and faujasites as representatives of the phyllosilicate and tectosilicate groups. 

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. 

Ion exchange reactions between cations present in groundwater and in the interlayer space of the montmorillonite structure. These are mainly the replacement of 2Na+ by Ca2+ and Mg2+ present in ground-waters. This triggers the dissolution of carbonates according to equilibria like the one exemplified by equation (1). The ion-exchange reactions can be described by ... 

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