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Copper metal clusters of 4–5 Å have been intercalated in the interlayers of montmorillonite by in situ reduction of bulky copper acetate hydroxide species with ethylene glycol (polyol process) at 195 °C. Various amounts of CuII as copper acetate hydroxide hydrate in excess of ion-exchange capacity were introduced into the interlayers by titrating copper acetate solution with sodium hydroxide in the presence of montmorillonite. The exchange of fully developed copper acetate hydroxide hydrate interlayers in the montmorillonite gave a basal spacing of 19.6 Å. The reduced samples exhibited a basal spacing of 14.4 Å indicating the presence of 4.8 Å copper metal clusters after subtracting the thickness of the silicate layer (9.6 Å). Further evidence for formation of copper metal clusters in the interlayers was obtained from transmission electron microscopy (TEM) and UV-VIS-NIR spectroscopy. Both water sorption isotherms and nitrogen B.E.T. surface-area measurements indicated the porous nature of Cu metal intercalated phases similar to pillared clays. During the process of reduction, large amounts of copper metal particles were expelled from the interlayers which grew to ca. 0.1-0.5 µm on the external surfaces of montmorillonite.
Truly nanocomposite materials that are stable to about 400 to 700°C can be prepared by intercalating oxides or metal clusters of about 0.4 to 2.0 nm in between ~1.0 nm layers of smectite clays. Both the chemistry and size of intercalates (pillars) can be varied to introduce unique catalytic, molecular sieving, dehumidifying and adsorption properties in these materials. The intercalated clays also provide opportunities to prepare compositionally and stoichiometrically diverse nanocomposite precursors to high temperature structural and electronic ceramics. Although montmorillonite is the most widely used host, further designing in properties can be achieved by using other members of smectite family having subtle crystal chemical and compositional variations, such as beidellite, nontronite, saponite or hectorite. The sol-gel chemistry involving the preparation of positively charged mono- or multiphasic solution-sol or colloidal-sol particles is a viable approach to introduce chemically diverse oxide particles in the interlayers of smectite. Reduction of transition metal ions or complexes in the interlayers of smectite to zerovalent metal clusters/particles using polar liquids is another novel approach to develop catalytically active, high surface area materials.
