Natural montmorillonite clays

The natural montmorillonite clays consist of several hundred individual platelike particles of dimensions 1 jum x 1 jum x 1 nm, held together by electrostatic forces with a gap of approximately 0.3 nm between two adjacent particles. The structure at the atomic level is shown in Figure 13.28 (77). The sodium montmorillonite layer is a crystalline 2 1 layered clay mineral in which a central alumina octahedral sheet is sandwiched between two silica tetrahedral sheets. These structures are sometimes called smectite clays, because of their layered structure see Figure 7.1. Note that this clay mineral comprises silicate layers in which the fundamental unit is planar. In the gap between the silicate layers are sodium ions. The gap is widely known as a gallery or an interlayer. The density of montmorillonite clays vary slightly with composition, but is generally near 2.5 g/crn (78). 

Table 5.1 Material compositions used in chemical modification of natural montmorillonite clay (Na-MMT) [46]...

Figure 5.2 XRD patterns of modified clay particles and natural montmorillonite clay particles (Na-MMT). See text for sample nomenclature. Reproduced with permission from [46].

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

Acidic clay catalysts can also be used in alkylation with alcohols 98 The main advantages of these catalysts are the reduced amount necessary to carry out alkylation compared with conventional Friedel-Crafts halides, possible regeneration, and good yields. Natural montmorillonite (K10 clay) doped with transition metal cations was shown to be an effective catalyst 200... 

Bentonite occurs commercially as powders ranging in colors and tints from off white to pale brown to gray depending on the cations present in natural deposits. It comprises natural smectite clays consisting primarily of colloidal hydrated aluminum silicates of the montmorillonite or hectorite type of minerals with varying quantities of alkalies, alkaline earths, and iron. It is insoluble in water, in alcohol, in dilute acids, and in alkalies. 

Bishop J. L., C. M. Pieters, and J. O. Edwards. 1994. Infrared spectroscopic analyses on the nature of water in montmorillonite. Clays Clay Miner. 42 702-716. 

There are historic reports of floods in the area (32 J. Rutherford, personal communication) and of an earthquake in 18 B.C. (33). The movement of the expansive Esna shale bed formation that underlies the whole area is also a source of disruption (32). The Esna shale bed formation, which contains montmorillonite clay, swells upon hydration by about 12.5% (32). Therefore, if an adequate amount of water is available, for example from increased irrigation, humidification, or flooding, the movement of this shale formation accelerates. Collectively, these dramatic natural events are the most probable causes of the loss of plaster and painted murals in the lower chamber of the tomb prior to its discovery, and they increasingly (32) threaten tombs in the Thebes area. 

Natural bleaching clays are aluminum silicates (bentonite, atta-pulgite, and montmorillonite), containing relatively high amounts of Mg, Ca, or Fe. The clays are generally activated by heat treatment. The high metal content, however, limits the adsorptive activity of these... 

Most of the ways in which natural water/solid interfaces influence the rates of transformation of pesticide compounds, however, are consistent with the various effects of related chemical species on these reactions in homogeneous solution, discussed in preceding sections. Russell et al. (1968), for example, observed that the hydrolysis of atrazine at low pH was greatly accelerated in suspensions of montmorillonite clay, relative to the rate ofreaction in homogeneous solution at the same pH. The authors attributed this effect to the acid-catalysis of the reaction by the clay surface, which exhibited a pH that was considerably lower (by as much as 3-4 pH units) than that of the bulk solution. As noted previously, the sorption of pesticide... 

Hormite clay is naturally more susceptible to acid-activation than montmorillo-nite lower levels of acid are required and can be directly applied (i.e., by simple spraying or pugging). Consequently, the slurry technique described above is not required, and the cooking, filtration, and washing steps can be eliminated (31). Data in Table 1 compare the response of hormite and montmorillonite clays with acid-activation. 

Reynolds, R.C. and Hower, J., 1970. The nature of interlayering in mixed-layer illite— montmorillonite. Clays Clay Miner., 18 25—36. 

---