Hectorite pillaring

ESR has also been used in the characterization of species adsorbed on pillared clays, i.e. smectites with hydroxy-aluminium interlayers. Adsorption of Cu(II) on hydroxy-aluminium hectorite produced mobile hexaaquacopper(II) and Cu(II) chemisorbed to... 

Mesoporous materials in which rearranged interlayer anisotropic silicates act as long pillars were produced as the heat-treated products of organophilic hectorites. 

The mesoporous materials produced from a precursory hectorite synthesized at 150°C had a total specific surface area of 848 m g a pore volume of 0.98 cm g and an average pore diameter of 46 5, values which are significantly higher than those of conventional pillared clays. 

Al3+-exchanged synthetic hectorite is a good catalyst for these conversions, and the 13C NMR spectrum obtained in the interlamellar, proton-catalyzed addition of water to 2-methylpropene is indistinguishable (Fig. 79) from that of f-butanol. Doubtless studies of this kind, where natural-abundance, 3C NMR signals are used to probe the chemical identity and motional freedom of reactant and product species situated in the interlamellar spaces of clays or pillared clays (see below), will become increasingly popular. Using l3C NMR linewidths and spin-lattice relaxation studies, Matsumoto et al. (466) have succeeded in discriminating between the internal and external surfaces of pillared montmorillonites. 

According to Cool and Vansant (1996), pores between 0.7 and 1.1 nm are probably present in all pillared clays, whereas the narrow and wider pores are particular features of the Zr-laponite and Zr-hectorite. A relatively high adsorption affinity (i.e. the low pressure capacity) of Zr-laponite for cyclohexane was attributed to the presence of a large number of narrow pores, giving rise to enhanced adsorbate-adsorbent interactions. 

The preparation and properties of smectites pillared with A1 hydroxy oligomers have been extensively investigated. Most of the work has been performed on montmoriUonite and hectorite, which have been intercalated with a great variety of pillaring agents and oUgocations. 

Many other materials, including synthetic aluminas, aluminum carbonates, aluminum silicates, magnesium silicates, various forms of attapulgite and sepiolite (81-83), alumina-pillared acid-activated montmorillonite (84), synthetic mica mont-morillonite, HY-zeolite, zirconium phosphate (85), mica, kaolin, and synthetic hectorite (86), have been evaluated for their ability to purify virgin fats and oils, but none were as good as acid-activated bentonite. 

The acidities of cliys and pillared clays are between those of amorphous aluminosilicates and zeolites. Pillared clays can provide large-pore two-dimensional networks. Hectorite, montmorillonite, saponite and beidellite are the clays most often used to make pillared clays. Most pillared clays coke and deactivate st. This, and low thermal and hydrothermal stabilities have so far limited catalytic applications. Al, Ti, Zr, Cr, Si, and Fe and their mixtures give more stable pillars than those tried in the past. Occelli and Robson reviewed pillrued clays [52]. 

Exploration of the pillar-clay sheet reactivity and connectivity also indicate the important role of the specific clay type. 27 1 and 29si-MASNMR experiments have shown distinctive differences between pillaring mechanisms in trioctahedral hectorite and dioctahedral montmorillonite. Whereas Plee et al. (22) concluded that chemical crosslinking may occur between the pillar and tetrahedral layer in a beidellite montmorillonite, Pinnavaia et al. (23) showed that it did not occur in a hectorite. These are the first observations of a complex process that may depend upon several structural and chemical factors, such as substitution of Al in the tetrahedral layer, or the need for vacancies in the octahedral layer to allow rotation of structural units or migration of reactant species to facilitate crosslinking. Ongoing research should further elucidate refinements on these mechanisms, and direct the technology towards more optimized catalysts - presumably those which form chemical bonds between the pillar and clay layer. 

Since all conditions for the Hofmann-Klemen effect are fulfilled, the liberated protons will migrate into the empty octahedral positions of dioctahedral clays. Some problems are encountered for this type of clays, since a large part of the CEC is now lost. When the trioctahedral hectorite and laponite serve as host for pillaring, the problem of proton migration can be avoided. The protons remain present in the interlayer region of the PILCs and are still available for further ion exchange. 

Pinnavaia et al. described the bonding of the pillars to the hectorite clay sheets, upon calcination, as a layer cross-linking mechanism [57]. Si and Al MAS-NMR data on Al-pillared smectite clays indicated the existence of two mechanisms for the linking of the pillars to the clay sheets. It was shown that the layer composition of the host clay plays a very important role in determining the layer reactivity upon heating. 

Zielke and Pinnavaia [106] compared the adsorption of several chlorinated phenols by pillared montmorillonite and Laponite (Laponites are synthetic hectorite-like materials). Pentachlorophenol, which is the strongest Brensted acid, is best adsorbed by the polyoxoaluminum derivative and less by the polyhydroxo form. The adsorption capacity decreases strongly with increasing solution pH (from pH = 4.7 to 7.4), showing that this pollutant is adsorbed in undissociated form. The effectivity of the smectites (at pH = 4.7) is polyoxoaluminum laponite > polyhydroxoaluminum laponite > polyoxoduminum montmorillonite > polyoxochromium montmorillonite. Sodium laponite and montmorillonite show no tendency to adsorb the pollutant from aqueous solution. 

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