Sepiolites

The sheets formed by the apices of the tetrahedra are completed by hydroxyls and magnesium ions in octahedral coordination link the sheets. The one structural arrangement has nine octahedral sites and the other only eight. Both structures have channels on both sides and top and bottom of each ribbon which contains water molecules (zeolitic water). Additional water is bound to the edge of the ribbons and hydroxyls occur in the structure proper. 

Martin-Vivaldi and Cano-Ruis (1955) found from 50 sepiolite analyses a mean value of 53.9% Si02 with a standard error of 1.9. 

24% MgO. From Nagy-Bradley s (1955) structural formula, 6Si02 MgO 4H20, adding four extra molecules of water to make it comparable with the analytical data, we obtain for the silica 54.2% and for the MgO 24.2%.  

Their data indicate that the moles of MgO per 100 g range from approximately 0.4 to 0.7 with a modal value near 0.6. The moles of (Al2 03 + Fe2 O + FeO + Mn) range from approximately 0.0 to 0.15. These data also indicate that eight octahedral sites are filled and in only a few samples is substitution of Mg by trivalent cations enough to account for the filling of only seven octahedral sites. 

Hathaway and Sachs (1965) sepiolite, Mid-Atlantic Ridge analysts Paul Elmore, Sam Botts, Gillison Chloe, Lowell Artis and H. Smith. 

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Moisture. The presence of water in a filler is not usually beneficial. Most fillers added to adhesives have a moisture content lower than 1 wt%. Only precipitated silicas and sepiolite contain about 5-10 wt% moisture. For some applications, fillers must be completely dried to exhibit adequate performance. Moisture absorbed on the surface of fillers impacts the rate and extent of curing of rubber base adhesives. 

IR spectroscopy of two supports was used for the determination of their surface acidity. The presence of Lewis acid sites on the surface of sepiolite allowed the preparation of a catalyst able to transform citral into menthol in fairly good yield under veiy mild conditions (90°C, 1 barH2). 

A very different picture comes from the IR spectra of sepiolite. The different spectra of adsorbed CO over sepiolite are represented in Figure 9.2. Introduction of CO resulted in the appearance of a series of absorption bands with v... 

The intense absorption band at v (CO) 2154 cm" that appears at high CO pressure can be attributed to the hydrogen-bonded complex of CO with silanol OH-groups. Its appearance corresponds to a slight shift to lower frequencies of the absorption band at v (OH) 3742 cm corresponding to the stretching of Si-OH groups. The absorption band at v (CO) 2136 cm can be ascribed to physical-adsorbed CO over sepiolite. From these results we can conclude that on this particular kind of silica no acid sites are present, whereas sepiolite shows both weak and medium Lewis acid sites. 

On the contrary, the Lewis acid sites present on the snrface of sepiolite make the Cn/sepiolite catalyst extremely active in promoting the ene reaction of citronellal. Thns, citronellal never accnmnlates in the reaction mixtnre bnt it is com erted into isopulegol as soon as it forms. Hydrogenation of isopnlegol is very slow nnder these reaction conditions, bnt this simple catalyst is able to produce menthol in a one-pot-one-step reaction under very mild experimental conditions. Notably dehydration products, which give account of 40% of the reaction mixture obtained over Ni-H-MCM-41 [4], are kept under 20% over both Cu catalysts. 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

Phyllosilicates, in addition to talc and silica, have recently been evaluated for their use as tableting excipients. These compounds include the smectites, pa-lygorskites, and sepiolites [85a]. Although they show some promise, current levels of metallic impurities are currently too high for use in pharmaceutical preparations. 

Sepiolite (or meerschaum) White or off-white, soft, fibrous earthy masses, easily carved and polished... 

Sepiolite, also known as meerschaum or sea foam, is a white or off-white soft clay of low density. It is composed of hydrated magnesium silicate and occurs in consolidated masses that are easily carved and whose surfaces can be smoothed and endowed with a durable polish. It is for this reason that meerschaum has been used, since antiquity, for making ornaments and decorative carvings. For many centuries meerschaum has been intensively mined in Greece and Turkey. 

Layered inorganic solids have been used for site isolation, for example, nickel phosphine complexes confined within the interlayer spaces of sepiolite have been used as olefin hydrogenation catalysts [63], and similarly there has been the encapsulation of metal complexes into zirconium phosphates [64], The principal idea is illustrated in Figure 5.8. The metal complex can be encapsulated by covalent means (a) or by non-covalent interactions (b). 

The pyrophoricity of nickel-on-sepiolite catalysts after use in petroleum processing operations may be caused by the presence of finely divided nickel and/or carbon. See other pyrophoric catalysts... 

Sandi, G., Joachin, H., Lu, W., Prakash, J., and Tassara, G., Comparison of the electrochemical performance of carbon produced from sepiolite with difference surface characteristics, J. of New Materials for Electrochemical Systems, (2003) 6, 75-80. 

Fig. 1.10 SEM micrographs of chitosan-based bio-nanocomposites involving (A) sepiolite and (B) montmorillonite as the inorganic moiety.

In the calculation results (Fig. 24.1), amorphous silica, calcite (CaCCF), and sepiolite precipitate as water is removed from the system. The fluid s pH and ionic strength increase with evaporation as the water evolves toward an Na-C03 brine (Fig. 24.2). The concentrations of the components Na+, K+, Cl-, and SO4- rise monotonically (Fig. 24.2), since they are not consumed by mineral precipitation. The HCO3 and Si02(aq) concentrations increase sharply but less regularly, since they are taken up in forming the minerals. The components Ca++ and Mg++ are largely consumed by the precipitation of calcite and sepiolite. Their concentrations, after a small initial rise, decrease with evaporation. 

Fig. 24.1. Volumes of minerals (amorphous silica, calcite, and sepiolite) precipitated during a reaction model simulating at 25 °C the evaporation of Sierra Nevada spring water in equilibrium with atmospheric C02, plotted against the concentration factor. For example, a concentration factor of x 100 means that of the original 1 kg of water, 10 grams remain.

Calcite and sepiolite precipitate in large part because of the effects of the escaping C02. The corresponding reactions are,... 

Figure 24.3 compares the calculated composition of the evaporated water, concentrated 100-fold and 1000-fold, with analyses of waters from six saline alkaline lakes (compiled by Garrels and Mackenzie, 1967). The field for the modeled water overlaps that for the analyzed waters, except that Ca++ and Mg++ are more depleted in the model than in the lake waters. This discrepancy might be explained if in nature the calcite and sepiolite begin to precipitate but remain supersaturated in the fluid. 

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