Structural montmorillonite

Montmorillonite is a clay similar to kaolinite (Section 1.4.8) but differs in its detailed structure. Montmorillonite clay is a hydrous alumina silicate mineral whose lamellae are constructed from an octahedral alumina sheet sandwiched between two tetrahedral silica sheets (2 1 layer silicate), while kaolinite has alternating an octahedral alumina sheet and a tetrahedral silica sheet (1 1 layer silicate). The crystal structure of montmorillonite was worked out by Hoffmann et al [132] and is shown in Fig. 1.22. It is similar but different from kaolinite... 

Smectite clay - Like mica, smectite cl (commonly called bentonite) has either a pyrophyllite or talc structure. Montmorillonite, a common high-aliunimun smectite, can be characterized by die pyrophyllite crystal structure with a small amount of octahedral Al replaced by Mg. The resulting charge imbalance is compensated by exchangeable cations, usually Na or Ca, between the laminae. In addition to diese counterions, oriented water, similar to that in vermiculite, occupies the interlaminar space. When Ca is the exchangeable cation, there are two water l ers, as in vermiculite when Na is the counterion, there is usually just one water layer. Figure 18 shows the montmorillonite structure. 

Silicates with layer. structures include some of the most familiar and important minerals known to man, partieularly the clay minerals [such as kaolinite (china clay), montmorillonite (bentonite, fuller s earth), and vermiculite], the micas (e.g. muscovite, phlogopite, and biotite), and others such as chrysotile (white asbestos). 

Figure 9.12 Schematic representation of the structures of muscovite mica, (K2Al4(Si6Ali)02o(OH)4], hydrated montmorillonite, [Al4Sig02o(OH)4].xH20 and chlorite, (MgioAl2(Si6Al2)02o(6H)i6], see text.

Fig. 8-2 Structure of a 1 1 (kaolinite) and a (montmorillonite) layer-silicate clay mineral.

FIG. 1 Idealized structure of a montmorillonite layer showing two tetrahedral-site sheets fused to an octahedral-site sheet (2 1 type). (From Ref. 8.)... 

We alluded earlier to the variety of structural modifications which may he observed in sheet silicates. Clearly it is a matter of considerable in jortance to he able to determine if, for example, the aluminium content within a clay arises p a ely from octahedral substitution (as in montmorillonite) or whether there is some tetrahedral component (as in heidellite). a1 MASNMR readily provides the necessary answers. Figvire 1 illustrates the a1 spectrum for a synthetic heidellite material with Na as charge balancing cation. Aluminium in two distinct chemical environments is observed, with chemical shifts corresponding to octahedrally and tetrahedrally co-ordinated aluminium. 

Fewer controlled experiments have been carried out for purely aquatic systems. Montmorillonite complexes with benzylamine at concentrations below 200 pg/L decreased the extent of mineralization in lake-water samples, although a similar effect was not noted with benzoate (Snbba-Rao and Alexander 1982). Even in apparently simple systems, general conclusions cannot therefore be drawn even for two structurally similar aromatic compounds, both of which are readily degradable nnder normal circumstances in the dissolved state. 

Bakke et al. (1982) have shown how montmorillonite catalyses chlorination and nitration of toluene nitration leads to 56 % para and 41 % ortho derivative compared to approximately 40 % para and 60 % ortho derivatives in the absence of the catalyst. Montmorillonite clays have an acidity comparable to nitric acid / sulphuric acid mixtures and the use of iron-exchanged material (Clayfen) gives a remarkable improvement in the para, ortho ratio in the nitration of phenols. The nitration of estrones, which is relevant in making various estrogenic drugs, can be improved in a remarkable way by using molecular engineered layer structures (MELS), while a reduction in the cost by a factor of six has been indicated. With a Clayfen type catalyst, it seems possible to manipulate the para, ortho ratio drastically for a variety of substrates and this should be useful in the manufacture of fine chemicals. In principle, such catalysts may approach biomimetic chemistry our ability to predict selectivity is very limited. 

Montmorillonite K10 was also used for aldol the reaction in water.280 Hydrates of aldehydes such as glyoxylic acid can be used directly. Thermal treatment of K10 increased the catalytic activity. The catalytic activity is attributed to the structural features of K10 and its inherent Bronsted acidity. The aldol reactions of more reactive ketene silyl acetals with reactive aldehydes proceed smoothly in water to afford the corresponding aldol products in good yields (Eq. 8.104).281... 

As has been confirmed by XRD, the framework of montmorillonite has been partly destroyed due to the calcination under high temperature. Most diffraction peaks of montmorillonite are faint. After hydrothermal crystallization the characteristic Bragg reflections for zeolite Beta structure at 7.7° and 22.42° 20 are detected in the composite, indicating the presence of the Beta phase. 

Figure 2 shows us the N2 adsorption-desorption isotherm of Beta/montmorillonite composite. At low relative pressure a sharp adsorption of nitrogen indicates the existence of large amount of micropore. The hysteresis shown in figure 2 is ascribed to type H4 which usually can be observed on layered clay and other materials [2], It is obvious that part of the pore structure in montmorillonite is still preserved after calcination under high temperature and the following hydrothermal crystallization. 

Calcium magnesium acetate, 1 127 Calcium magnesium carbonate, health and safety factors related to, 15 74 Calcium monosulfoaluminate, 5 477t Calcium montmorillonite, 6 686, 696 structure and composition, 6 668-669 Calcium nitrate, in nitrogen fertilizers,... 

Figure 4.7 Structure of one of the smectite group of clay minerals montmorillonite, Al2Si40io(OH)2.nH20. (After Evans, 1966 Figure 11.08b, by permission of Cambridge University Press.)...

Chatteijee, A., Ebina, T., and Mizukami, F. 2005. Effects of water on the structure and bonding of resorcinol in the interlayer of montmorillonite nanocomposite—a periodic first principle study. J. Phys. Chem. B 109 7306-7313. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

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