Structural saponite

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

A brief comparison between fluorohectorite PCH and saponite PCH is made in Table 2 showing the structural similarities of the two mesostructured clays. 

Table XXXVIII). Brindley (1955) has suggested that stevensite is a mixed-layer talc-saponite however, Faust et al. (1959) considered it to be a defect structure with a random distribution of vacant sites in the octahedral sheets. A small proportion of domains with few or no vacancies would then be present having characteristics of talc. The layer charge in stevensite is due to an incompletely filled octahedral sheet (Faust and Murata, 1953). This deficiency is minor (0.05—0.10) and the resulting cation exchange capacity is only about one-third that of the dioctahedral montmorillonites (100 mequiv./lOO g.).

Chemical analyses and structural formulas of some saponites... 

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).

The interest in LDHs is much more recent than their cationic equivalents (cationic clays such as montmorillonite and saponite). This is, in part, because cationic clays are much more abundant in nature and have an important role in many soil processes, for example. It is clear, however, that the potential for creating novel supramolecular structures, either directly as intercalates or for orienting organic molecules at reactive surfaces, is likely to increase. 

Effect on clay dispersion, 414 Hazard, 423-426 Saponite, 123-124 Structure, 124 Composition, 124... 

The 2 1 layer clays (i.e. with three-sheet layers) include a group of expanding or swelling clays, which comprise the smectites (e.g. montmorillonite, saponite and hectorite) and the vermiculites. The basic structure of a smectite is shown in Figure... 

Both natural clays and their alnminium oxide pillared analogues have also been tested for the catalytic cracking of polyethylene [49-51]. The clays investigated include mont-morillonite and saponite. They possess a layered structure which can be converted into a two-dimensional network of interconnected micropores by intercalation of molecular moieties. In the case of alnmininm pillared clays, these materials show a mild acidity... 

Although the possible catalytic applications of PILCs is the subject more extensively studied, large efforts have also been devoted to the characterisation of the microporous structure of these materials. To this end, the application of both classical and novel models based on gas adsorption has been reported in the literature. The adsorption data depend on the internal physical and chemical structure of the solid and on the nature of the adsorbate molecule. Thus, the adsorption results contain information about structural and energetic properties of the materials surface [8]. In the following, the methods considered in this work to investigate the microporous properties of several alumina-pillared saponites are briefly presented. 

Figure 8.13. Effect of relative humidity (P/P ) on the c-axis spacing of Na+ and Ca + exchange forms of smectite. (Adapted from H. Suquet et al. 1975. Swelling and structural organization of saponite. Clays Clay Min. 23 1-9.)...

The X-ray diffraction pattern of saponite exhibits a relatively weak and broad (001) reflection compared with montmorillonite, indicating a lack of long-range layer ordering. This characteristic of saponite can be ascribed to the three-dimensional voluminous house-of-cards structure. The house-of-cards structure also contributes to the high catalytic activity in comparison to montmorillo-nites, which do not form the house-of-cards structure. Alkylation activity... 

M. M. Mortland and K. V. Raman, Surface acidity of smectites in relation to hydration, exchangeable cation, and structure, Clays and Clay Minerals 16 393 (1968). See also J. D. Russell, Infrared study of the reactions of ammonia with montmorillonite and saponite, Trans. Faraday Soc. 61 2284 (1965), andM. M. Mortland, Protonation of compounds at clay mineral surfaces, Trans. 9th Int. Cong. Soil Sci. (Adelaide) 1 691 (1968). 

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