Structural sepiolite

Sepiolite clay (<100 mesh) was heated in air at 120°C in order to remove the zeolitic and surface bound water molecules. The partially dehydrated clay mineral was subsequently exposed to acetone vapor at room temperature for a period of four days. H and 29Si CP MAS-NMR experiments revealed that the acetone molecules penetrated into the microporous channels of the sepiolite structure. Broad line 2H NMR studies using acetone-d6 revealed that, in addition to fast methyl group rotations, the guest acetone-d6 molecules were also undergoing 2-fold re-orientations about the carbonyl bond. The presence of acetone-d6 molecules adsorbed on the exterior surfaces of the sepiolite crystals was also detected at room temperature. 

From the results of the AAS experiments, we can conclude that most sepiolite structure is destroyed during the catalyst synthesis due to the loss of constitutional water. Thus, we may assume that the support in Ni/Sep catalysts is basically made up mostly of a 3Si02.2Mg0 amorphous mixture. However, X-Ray and TEM experiments indicate that a smaller portion of sepiolite structure remains undamaged. Thus, in Fig. 1 we can see metal particles supported on the typical noddies constituting the sepiolite structure. 

Bearing in mind the difficulties of interpretation because of the lack of precise crystal structural detail characteristic of the diffraction data obtained from fibrous materials, chemical analyses indicate that the composition of sepiolite can be expressed as... 

Substitution and variations in the tetrahedral sites change the manner of side linkages for the ribbons, effecting the octahedral cation and water associations. In addition, different ribbon widths can lead to different numbers of octahedral cations. Variation in the width of chains and substitution of cations and water are easily accomplished, which means that accurate and consistent chemical and crystal structural data on these minerals are difficult or, at best, approximate. However, the minerals do form fibers with a consistent fiber axis repeat of about 0.512 nm (Preisinger, 1959 Rautureau et al., 1972). Sepiolite and palygorskite represent the widest possible structural and chemical diversity among fibrous silicate minerals. 

Fig. 2.16 Schematic representations of the structures suggested for sepiolite and palygorskite. The ribbonlike arrangement of silicate chains alternates with hydroxyl and water areas. (A) Sepiolite, the (001) projection, showing the cross section of three 2 1 silicate chains and associated water and hydroxyl groups. (B) Palygorskite, the (100) projection, showing the cross section of two silicate chains and associated water and hydroxyl groups.

There are several other types of minerals commonly found in clay particle size mineral assemblages (i.e.,< 2 microns diameter, Krumbein and Pettijhon, 1938). Aside from quartz and amorphous materials, the two most important mineral groups are sepiolite-palygorskite and zeolites. These two groups are similar in that they both contain free 1 0 molecules in their structure. However the Si-0 linkage is quite different in each case. 

Sepiolite and palygorskite are frequently associated In natural deposits. They are both fibrous in form, a characteristic dictated by their chain-type (linear) structure. They contain hydroxyls, zeolitic... 

High pressure studies using natural sepiolite and palygorskite (Frank-Kameneckiji and Klockova, 1969) indicate that these minerals can contain variable quantities of silica because they exsolve quartz while retaining their basic structural and mineral identity. These experiments also demonstrate that the natural minerals are compositionally intermediate between talc or montmorillonite and quartz. These latter phases are formed upon the thermal breakdown of sepiolite and palygorskite under conditions of 1 and 2Kb total pressure. Both sepiolite and palygorskite appear to remain stable in sequences of buried rocks, at least up to the depth where fully expandable dioctahedral montmorillonite disappears (Millot, 1964). 

Currently, a wide range of technological applications is based on the sorptive and catalytic properties of sepiolite [2], Sepiolite is increasingly being used as a decolorizing agent [3], as a catalyst or catalyst carrier [4-6], and as odorant adsorbents in environmental applications [7-9], Several papers have appeared recently that examine the structural, textural and sorptive properties of untreated sepiolite [10-12] and of sepiolite subjected previously to acid and/or thermal treatment [13-16], Sepiolite has also been used recently as... 

Fig. 1 Structure of sepiolite projected on the (001) plane a) at room temperature b) after heating in air to 120°C and c) after heating in air to 500°C (Adapted from Ref 1).

Finally, the 29Si CP/MAS-NMR spectrum of a partially dehydrated sepiolite that was subsequently exposed to acetone vapor is presented in Fig. 2c, and is strikingly similar to the spectrum of the original, untreated sepiolite (Fig. 2a). Since zeolitic water molecules are not present in this sample, and in light of the discussion of the partially dehydrated sepiolite sample, it appears that the acetone molecules have penetrated inside the microporous channels and reversed the structural changes that were caused by partial dehydration. Thus Fig. 2c confirms that acetone molecules enter the microporous channels of sepiolite, and are not simply adsorbed on the crystallite exterior surfaces. 

Palygorskile and Sepiolite. Palygorskite (anapalgite) and sepiolitc arc day minerals in which the 2 1 layers are linked together in chain-likc or a combination of chain-sheet structures. 

Palygorskite and sepiolite are different from other clay minerals in the manner in which the 2 1 layers arc joined Ralher than being joined in a ennlinuous manner, the tetrahedral sheets are joined to an adjacent inverted tetrahedral layer, making the octahedral layers noncontinuous and leaving an open channel in the mineral structure. 

Attapulgite and sepiolite are clay minerals with a chain structure. The former has five octahedral positions and the latter either eight or nine. Both have relatively little tetrahedral substitution. The octahedral positions in sepiolite are filled largely with Mg and those in attapulgite with approximately half Mg and half Al. 

Fig.28. The relation of percent octahedral occupancy to RJ+/(R3+ +R2 +) for layer structure and chain structure clays, = saponite = attapulgite x = sepiolite (nine octahedral positions) o = sepiolite (eight octahedral positions).

---