Synthetic minerals smectites

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

Synthesis of smectite-type clay minerals, such as saponites, may ciieumvcnt the above mentioned difficulties. The usually demanding hydiolfaermal treatment [3>4, the lo duration of the synthesis [5], and problems with the scale-up of the jxq>aratk procecterc. however, have thus far severely limited die use of synthetic clays as solid add catalysts. 

The natural smectite hectorite, when obtained from the Clay Repository of the Clay Minerals Society, still contains a lot of carbonate impurities. In order to eliminate these impurities, hectorite has to be pretreated with a sodium acetate/acetic acid buffer of pH 4 [53]. In this way, all carbonates are transformed into H2CO3, whereupon H2O and CO2 (liberated from the solution) are formed. Subsequent exchange in a NaCl solution and removal of the excess chloride give Na+-hectorite. The synthetic laponite, supplied by Laporte Inorganics, is already in the Na -form and free of any impurity. 

The most intense research devoted to clay-polymer nanocomposites concerns natural (montmorillonite, hectorite, saponite) or synthetic (laponite, fluorohec-torites) smectites because these ID nanofiUers have a layer thickness of the order of one nanometer. The other dimensions of the clay mineral are about 1000 times larger than the thickness, i.e., they are at the micrometer scale. Figure 3 shows the different structures that can be formed by the association of these clay minerals with polymers. 

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