Aluminosilicates montmorillonite

FIGURE 14.1 Crystal structure of 2 1 type aluminosilicate (montmorillonite). 

The mineral clays are hydrous aluminosilicates (montmorillonites). Their ability to acconmiodate a broad range of guest molecules is brought about by the extensive expansion of its lamellar structures as indicated in Scheme 15. The lamellar system consists of aluminosilicate sheets incorporating alternatively [Si04l " tetrahedra and [A104(0H)2] " octahedra [60]. 

H. Liu, D.W. Kim, A. Blumstein, J. Kumar, and S.K. Tripathy, Nanocomposite derived fi"om intercalative spontaneous pol3fmerization of 2-eth3fnylpyridine within layered aluminosilicate montmorillonite, Chem. Mater., 13, 2756-2758 (2001). 

In this chapter, we demonstrate the potential of such agents as catalysts/promoters in key steps for the derivatization of sugars. The most significant catalytic approaches in carbohydrate chemistry that use aluminosilicate porous materials, namely zeolites and montmorillonite clays, are reviewed and discussed. Silica gel is a porous solid silicate that has also been used for heterogeneous catalysis of organic reactions in general. We include here its usefulness as promoter and reagent support for the reactions under consideration. 

Taking this one step further, perhaps even an inorganic gene may have been provided by clay mineral sources. Earliest clay samples are of a mineral called montmorillonite that consists of sheets of aluminosilicates in which Fe2+, Fe3+ and Mg2+ are substituted for some of the Al3+, and Al3+ is substituted for Si4+. The oxygen content of the layers does not change and the alternative valencies allow the production of positive and negatively charged layers. Dramatically, Paecht-Horowitz and co-workers showed that the amino acid adenylate could be polymerised with up to 50 units on the montmorillonite surface in aqueous solution. Similar condensation reactions for carbohydrates on hydrotalcite surfaces have... 

In addition to stabilizing organic products by reaction with metal-exchanged clays, as indicated above, aluminosilicate minerals may enable the preparation of metal organic complexes that cannot be formed in solution. Thus a complex of Cu(II) with rubeanic acid (dithiooxamide) could be prepared by soaking Cu montmorillonite in an acetone solution of rubeanic acid (93). The intercalated complex was monomeric, aligned with Its molecular plane parallel to the interlamellar surfaces, and had a metal ligand ratio of 1 2 despite the tetradentate nature of the rubeanic acid. 

Clays - Kaolin, Illite, M. L. Clays, Montmorillonite, Misc. Aluminosilicates... 

Clays have a loose layer structure (Figure 4.6). Characteristic minerals are montmoril-lonite and beidellite. Aluminosilicates such as montmorillonite, kaolinite, and feldspar can act as cation and anion exchangers. 

Advantage has also been taken of dispersed clays to host small particles [478]. Montmorillonite clays are colloidal, layered aluminosilicates with ex-... 

Aluminum is present in many primary minerals. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites (Bodek et al. 1988). Aluminum is found in the soil complexed with other electron rich species such as fluoride, sulfate, and phosphate. 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

The formation of edge charges of minerals have been discussed in Chapter 1, Section 1.3.21. It has been shown that aluminosilicates (including montmorillonite) have two types of surface (aluminol and silanol) sites, and their protolytic processes have been expressed by Chapter 1, Equations 1.54-1.56. For simplicity, the reaction equations are repeated here. For aluminol sites,... 

The acidic destruction of montmorillonite results in the release of silicon and aluminum. The initial fast exchange of surface cations by hydrogen ions is followed by the release of aluminum and silicon. The dissolution rate of Si is higher than that of A1 and is influenced by the relative ratios of basal siloxane and edge surfaces. The shift of pH to more basic values by the ion-exchange processes and the hydrolysis of dissolved species induce the formation of secondary amorphous solids, initiating the formation of amorphous aluminosilicates (Sondi et al. 2008). 

The Mossbauer spectrum of the original sodium-bentonite shows the isomer shift and quadrupole splitting values as usual for montmorillonite (Stevens et al. 1983). The values 6 and d, exhibit Fe3+, while the values of S2 andd2reveal Fe2+ microenvironments. These values are typical for Fe3+ and Fe2+ ions that are in the central positions of octahedrons of aluminosilicates (Kuzmann et al. 1998 Stevens and Stevens 1978). The relative areas (A s) show that the Fe3+ species is dominant as usual for Na-bentonites (Stevens et al. 1983). The spectra at the temperature of liquid nitrogen show no magnetically split components. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

Figures 2.21 and 2.22 refer to the adsorption of low molecular weight aliphatic alcohols from alcohol + benzene mixtures on montmorillonite. This adsorbent Is a so-called swelling clay mineral, meaning that it consists of packages of thin (aluminosilicate) layers that, under certain conditions, swell to give ultimately a dispersion of the individual sheets. Upon this swelling the specific surface area increases dramatically, it can readily reach several hundreds of m g" On adsorption from solution the swelling is determined by the extent to which one or both of the component(s) penetrate(s) between these sheets. In other words, we are dealing here with a non-inert adsorbent. The gas adsorption equivalent has been illustrated in fig. 1.30.

Palygorskite and sepiolite are magnesium-rich fibrous aluminosilicates that have been identified in basal deep-sea sediments (Table 4 e.g., Hathaway and Sachs, 1965 Bowles et al., 1971 Bonatti and Joensuu, 1968 Church and Velde, 1979 Jones and Galan, 1988 Velde, 1985). These phases are commonly associated with smectite, and it has been suggested that they originate by alteration of montmorillonite by low-temperature, magnesium-rich, hydrothermal solutions (Bonatti and Joensuu, 1968), e.g.. 

Studies of the sorption of plutonium are complicated by the high redox reactivity of plutonium. Sorption of Pu(V) by pure aluminosilicates and oxyhydroxide phases is usually characterized by initial rapid uptake followed by slow irreversible sorption and may represent a reductive uptake mechanism catalyzed by the electrical double layer of the mineral surface (Turner et al., 1998 Runde et al., 2002a). In Yucca Mountain waters, the ranges for Pu(V) uptake by hematite, montmorillonite, and silica colloids were 4.9xl0 mLg to 1.8 X 10 mL g 5.8 X 10 mL and 8.1 X 10 mL g, respectively. These are much higher than those observed for Np(V) in the same waters as described previously. High surface redox reactivity for... 

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