Montmorillonite cation-exchanged montmorillonites

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

The first examples of cationic exchange of bis(oxazoline)-metal complexes used clays as supports [49,50]. Cu(II) complexes of ligands ent-6a, 6b, and 6c (Fig. 15) were supported on three different clays laponite (a synthetic clay), bentonite, and montmorillonite KIO. The influence of the copper salt from which the initial complexes were prepared, as well as that of the solvent used in the cationic exchange, was analyzed. 

FIG. 4 Experimental (vertical bars) and simulated (symbols) values of the d-spacings for aUcy-lammonium-exchanged clay at three different cation exchange capacities (CECs) (a) SWy2 mont-morillonite, CEC = 0.8 meq/g (b) AMS montmorillonite (Nanocor), CEC = 1.0 meq/g (c) fluoro-hectorite (Dow-Corning), CEC = 1.5 meq/g. (Erom Ref. 30.)... 

Clay materials show a different behavior. They are either cation-poor or cation-rich sheet silicates. They can swell by taking up varying amounts of water between the sheets. If the intercalated cations are hydrated as in montmorillonite, they act as cation exchangers. Montmorillonite, especially when it has intercalated Ca2+ ions, has thixotropic properties and is used to seal up drill holes. The effect is due to the charge distribution on... 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

Exchange of organic ammonium cations. Exchange selectivity of monovalent alkyl ammonium cations in montmorillonites (40-41) and octahedrally substituted synthetic clay minerals (laponite) increases with their chain length (42) and along the series... 

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

However, when protonated TEMPAMINE adsorbs by cation exchange on fully hydrated layer silicate clays (10, 11), the spectrum becomes less symmetrical as shown in Figure 5. The beidellite and montmorillonite spectra have line shapes typical for nitroxide molecules with rotational frequencies on the order of 10 Hz (17). 

Montmorillonite An iron-rich clay mineral that has a very high cation exchange capacity. Unlike the other clay minerals, a significant amount of sedimentary montmorillonite is hydrothermal in origin. 

Montmorillonite KIO clay and its various cation-exchanged forms have been found to promote the formation of an unexpected product, p-nitrosodiphenylamine, from iV-phenylhydroxylamine, rather than the typical Bamberger products. A Bamberger rearrangement has been shown to occur during the metabolism of 2,4,6-trinitrotoluene... 

Activated charcoal Anion-exchange resin Cation-exchange resin Cellulose powder Cellulose triacetate Kaolinite Melfort loam Montmorillonite Peat... 

The smectite group of clay minerals is also poorly crystalline but perhaps better known because of their cation exchange capacity and their occurrence in the bentonite clays. A general formula for montmorillonite, which is one of the dioctahedral smectites is... 

Interestingly, the use of Sc(OTf)3 as the promoter gave hydroxyalkyl-6,7-dihydrobenzofuran-4(57/)-one derivatives 17 from D-ribose (9), whereas scandium cation-exchanged montmorillonite (Sc -mont) afforded hydroxyalkyl-3,3,6,6,-tetramethyl-3,4,5,6,7,9-hexahydro-l//-xanthene-l,8(27/)-dione (18) in good yield [97] (Scheme 3). 

The dichroic properties of the Li modiEed montmorillonite were followed by orientation of thin films (0-45°) in the IR beam (FTIR Brucker IFS 88). The cationic exchange capacity (CEC) of the samples calcined at 400°C was evaluated. 

The Pechmann and Knoevenagel reactions have been widely used to synthesise coumarins and developments in both have been reported. Activated phenols react rapidly with ethyl acetoacetate, propenoic acid and propynoic acid under microwave irradiation using cation-exchange resins as catalyst <99SL608>. Similarly, salicylaldehydes are converted into coumarin-3-carboxylic acids when the reaction with malonic acid is catalysed by the montmorillonite KSF <99JOC1033>. In both cases the use of a solid catalyst has environmentally friendly benefits. Methyl 3-(3-coumarinyl)propenoate 44, prepared from dimethyl glutaconate and salicylaldehyde, is a stable electron deficient diene which reacts with enamines to form benzo[c]coumarins. An inverse electron demand Diels-Alder reaction is followed by elimination of a secondary amine and aromatisation (Scheme 26) <99SL477>. 

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

Cation exchange studies on montmorillonites have shown a number of interesting relations regarding ionic distribution between aqueous solutions and the silicate (Deist and Tailburdeen, 1967 Hutcheon, 1966 ... 

The apparent discrepancy could reside in the fact that if potassium ions are available at all, they will form a mica at temperatures near 100°C. Montmorillonite structures below these conditions (pressure and temperature) need not contain potassium at all. However, at the correct physical conditions the 2 1 portion of the montmorillonite must change greatly (increase of total charge on the 2 1 unit) in order to form a mica unit in a mixed layered mineral phase. Since neither Na nor Ca ions will form mica at this temperature, potassium will be selectively taken from solution. Obviously this does not occur below 100°C since cation exchange on montmorillonites shows the reverse effect, i.e., concentration of calcium ions in the interlayer sites. If potassium is not available either In coexisting solids or in solutions, the sodi-calcic montmorillonite will undoubtedly persist well above 100°C. 

FOSTER (M.D.), 1951. The importance of the exchangeable magnesium and cation exchange capacity in the study of montmorillonitic clays. 

HUTCHEON (A.T.), 1966. Thermodynamics of cation exchange on clay Ca-K montmorillonite. Journ. Soil Sci. 17, 339-55. 

Cast, R. G., "Alkali Metal Cation Exchange on Chambers Montmorillonite," Soil Sci. Soc. Amer. Proc., (1972), 36, 14. 

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