Synthetic smectite

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... 

Smectites are 2 1 charged layered silicates from natural (montmorillonite, hectorite, beidellite, saponite etc.) or synthetic (synthetic fluorohectorites, such as... 

The Smectite Clays. The smectite-type clays are distinctive in that they expand and cause significant destruction to synthetic (human-made) structures. In this type of 2 1 clay, isomorphous substitution occurs in the aluminum sheet. If there is substitution of lower-oxidation-state metal such as magnesium, there will be an unsatisfied pair of bonding electrons in the interior of the crystal and there will be no noticeable change in the surface. Because the charge is in the interior of the crystal, its attraction for cations is diminished by distance. Thus, smectite crystals are not held together strongly by cations and are able to incorporate more water and ions between sheets when the environment is wet and less when it is dry. 

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. 

Fast deactivation rates due to coking and the limited hydrothermal stability of pillared clays have probably retarded the commercial development of these new type of catalysts and prevented (to date) their acceptance by chemical producers and refiners. However, there is a large economic incentive justifying efforts to convert inexpensive (i.e. 40-100/ton) smectites into commercially viable (pillared clay) catalysts (56). Therefore, it is believed that work on the chemical modification of natural (and synthetic) clays, and work on the preparation and characterization of new pillared clays with improved hydrothermal stability are, and will remain, areas of interest to the academic community, as well as to researchers in industrial laboratories (56). 

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. 

In this work, and based on the synthesis procedure previously described [4], we develop a new PCH structure based on a synthetic smectite clay host. The synthesis involves a two-step mechanism in a first step, cationic surfactants are ion-exchanged on the clay. Then, in... 

Smectite-type materials containing transition metal divalent cations (Ni2+, Co2, and Zn2+) in octahedral sheets were synthesized. The synthetic smectites were thermally stable and had large surface areas and high pore volumes after evacuation at 873 K. Catalytic activities of synthetic smectites were investigated. The Ni2 -containing smectites were active for the isomerization of 1-butene and the oligomerization of ethylene. The Co2+-containing smectites were active for the hydrodesulfurization of thiophene. 

Table 1 shows the properties of smectite-type materials prepared. Smectite materials prepared at lower pH had fewer sodium ions, higher surface areas, and larger pore volumes for a series of samples containing the same divalent cation species (nickel and cobalt) in the octahedral sheet. The adsorption of methylene blue on all the synthetic smectites shows that the smectite fragments are negatively charged. The Si M ratios of synthetic smectites were about 8 6, indicating that most of divalent cations exist in octahedral layers and small amount of divalent cations would exist as hydroxide or oxide cluster in smectite materials. However, the amounts of the hydroxide or oxide cluster were small, because only smectite structures were observed in XRD patterns and EXAFS Fourier transforms of synthetic smectites calcined at 873 K. 

The nitrogen adsorption-desorption isotherms at 77 K showed that the pore structures of smectite-type materials are of a bottle-neck type [3]. The surface areas of Ni-481 and Ni-359 treated at 873 K were 381 and 184 m2 g 1, respectively (Figure 2). The synthetic smectites have large surface areas because many small fragments with the same smectite structure are intercalated in the interlayer region [4]. 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

Gougeon, R.D., Reinholdt, M., Delmotte, L., Miehe-Brendle, J., Chezeau, J.M., Le Dred, R., Mar-chal, R., Jeandet, P. (2002). Direct observation of polylysine side-chain interaction with smectites interlayer surfaces through H- A1 heteronuclear correlation NMR spectroscopy. Langmuir, the American Chemical Society Journal of Surfaces and Colloids, 18, 3396-3398 Gougeon, R.D., Soulard, M., Reinholdt, M., Miehe-Brendle, J., Chezeau, J.M., Le Dred, R., Mar-chal, R., Jeandet, P. (2003). Polypeptide adsorption onto a synthetic montmoriUonite A combined solid-state NMR, X-ray diffraction, thermal analysis and N2 adsorption study. Eur. J. Inorg. Chem., 2003, 1366-1372... 

There has been renewed interest in catalytically active clays since the report by Swift and Black ( 1) to the effect that replacement of octahedrally coordinated aluminium ions by nickel or cobalt in synthetic smectite clays, as done by Granquist ( ), results in a new type of catalyst, called nickel- (or cobalt-) substituted mica montmorillonite (Ni(Co)SMM), which is very active in the isomerization and cracking of hydrocarbons. 

The necessity to develop hydrotreating catalysts with enhanced activity stimulates the search for alternative catalyst supports. It was shown that clay-supported transition metal sulfides can efficiently catalyze hydrodesulfurization (HDS) of thiophene [1-3]. However, the large scale application of the catalysts based on natural clays is still hampered, mainly due to the difficulties in controlling the chemical composition and textural properties. Synthetic clays do not suffer from these drawbacks. Recently, a novel non-hydrothermal approach was proposed for the synthesis of some trioctahedral smectites, namely saponite... 

The XRD patterns of the samples obtained (Fig. 1) are consistent with those of synthetic and natural stevensites [7,8J, evidencing formation of the trioctahedral smectite structure, as indicated by the (060) reflection at about 1.52 A. However, intensities of the X-ray reflections vary for the particular solids, which is caused probably by differences in the crystallinity and/or in size and stacking of particles. All the samples show a typical smectite ability to swell in ethylene glycol and exhibit ion-exchange capacity comparable with that of the natural stevensite (of about 50 mequiv/g). 

Rectorite is one of an almost infinite number of randomly mixed layer clays (411. collectively called illites, which include several other ordered interstratified varieties, including chlorite, corrensite and allevardite. Rectorite has the advantages of a mica and a smectite, in that alternate interlayers are expanding and non-expanding. It may be viewed as an ordered synthetic mica-montmorillonite (SMM) in the nomenclature familiar to catalytic... 

Polyvinyl butyral Potassium polyacrylate Potassium silicate Potato (Solanum tuberosum) starch, Quaternium-18 bentonite Quaternium- 18/benzalkonium bentonite Quaternium-18 hectorite Rhodapon CAV Shellac, Silica, amorphous hydrated. Silica dimethyl sllylate Silica, hydrated Smectite Sodium chloride Sodium hydrosulfite Sodium magnesium fluorosilicate Sodium polyacrylate Sodium polymelhacrylate Sodium polystyrene sulfonate Sodium slllcoaluminate Sodium stearate Sodium sulfate Stannic oxide Stearalkonlum bentonite Stearalkonium hectorite Stearamide, Stearamide MEA-stearate Stearyl alcohol, Stearyl stearate, Synthetic wax, Tallamide DEA, Tetradecylelcosanol Tetrasodium etidronate Tridecyl alcohol Tridecyl stearate Trihydroxystearin, Trilinoleicacid, Tristearin, Urea-formaldehyde resin. Wheat (Triticum vulgare) starch Xanthan gum. Zinc laurate... 

PEG-5M PEG-7M PEG-9M PEG-23M PEG-45M PEG-12 ricinoleate PEG-2 stearate PVM/MA copolymer Smectite thickener, paper coatings Hydroxyethylcellulose thickener, paper finishes Beeswax, synthetic thickener, paper sizing Xanthan gum thickener, papermaking PEG-14M... 

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