1654
Ind. Eng. Chem. Res. 1992,31, 1654-1659
MATERIALS AND INTERFACES Preparation of Hectorite Clays Utilizing Organic and Organometallic Complexes during Hydrothermal Crystallization Kathleen A. Carrado Chemistry Division, Argonne National Laboratory, 9700 S. Cuss Ave., Argonne, Illinois 60439
A method for incorporating a remarkably diverse variety of intercalants directly during hydrothermal synthesis of hedorite layer-silicate clays has been developed. Refluxing a gel of silica sol, magnesium hydroxide sol, and lithium fluoride for just 2 days in the presence of an organic or organometallic intercalant results in crystalline products containing either (a) organic dye molecules such as ethyl violet and methyl green, (b) dye molecules such as alcian blue that are based on a Cu(I1) phthalocyanine complex, or (c) transition metal complexes such as Ru(I1) phenanthroline and Co(II1) sepulchrate. The following properties of any intercalant are found to be necessary (i) water solubility, (ii) positive charge, and (iii) thermal stability under moderately basic (pH 9-10) aqueous reflux conditions. The materials, and ion-exchanged natural hectorites for comparison, are characterized by X-ray powder diffraction, N2 BET surface area measurements, microanalysis, and thermal gravimetric analysis. Introduction The synthesis of new materials with potential as acidic catalysts or supports is of interest to many concerns that employ zeolites and clays in these traditional uses, e.g., the petrochemical industry. These minerals are crystalline, open-framework silicates that have pores and channels (in the case of three-dimensional zeolites) or galleries (twodimensional clays and pillared clays) available as microenvironments for chemical reaction. The smectite clay minerals of specific interest in this study have two tetrahedral silicate layers sandwichihg a central octahedral layer in a so-called 2:l arrangement (Grim, 1968); see Figure 1. Isomorphous substitutions in the lattice such as Mg(I1) for Al(II1)in the octahedral layers of montmorillonite or Li(1) for Mg(I1) in the octahedral layers of hectorite cause an overall negative charge that is compensated by the presence of interlayer, or gallery, cations. A significant amount of interlayer water is also present and the cations are easily exchangeable. The d(001) spacing indicated in Figure 1 is the crystallographic unit cell dimension along the c-axis, and includes the fmed measurement of the clay layer (-9.6 A) along with the variable interlayer or gallery spacing. The usual method for modifying such silicate minerals is by ion exchange, where the alkali metal or alkaline earth cation is replaced by, for example, a catalytically active species (Pinnavaia, 1983). However, a more direct method that offers synthetic potential is to incorporate intercalants, espeically more complex molecules, during hydrothermal crystallization of the silicate. This has recently been demonstrated for hectorite clays that were made in the presence of water-soluble porphyrins and metalloporphyrins (Carrado et al., 1991). This method has now proven to be amenable to a wide variety of organics and organometallics; see Figure 2. The adsorption characteristics of dye-clay mineral systems have been extensively studied in terms of staining tests (Grim, 1968) and metachromasy (Yariv et al., 1991; Grauer et al., 1987; Cohen and Yariv, 1984) (appearance
of a blue-shifted band), which has implications in geochemistry and agriculture (Yariv et al., 1991), and fluorescence to probe photoprocesses at solid surfaces (Middleton and Jennings, 1991; Endo et al., 1989; Grauer et al., 1984) and photoredox reactions in organized media (DellaGuardiaand Thomas, 1983; Nijs et al., 1982). The dyes methyl green (MG) and alcian blue (AB) are biological stains, and ethyl violet (EV)is known to display metachromasy with clays (Yariv et al., 1991). The other intercalants used are bulky transition metal complexes, such as Ru(I1) (o-phenanthroline) and Co(II1) sepulchrate. Metal chelates such as Ru(I1) and Fe(II1) phenanthrolines adsorbed onto clays have been studied by electric dichroism (Taniguchi et al., 1990,1991),in terms of ligation (Berry et al., 1990; Berkheiser and Mortland, 1977), and make interesting props of the gallery space as pillars. All synthesized materials were characterized by X-ray powder diffraction (XRD), microanalysis, N2 BET surface area measurements, and thermal grayimetric analysis.
Experimental Section Materials. Synthetic hectorites were prepared as described previously (Carrado et al., 1991). Aqueous gels consisting of 0.06:0.20:1.00:1.52 organic comp1ex:LiF: MgO:Si02 molar ratios were allowed to reflux for 2 days, then centrifuged, washed until the decant was clear, and air-dried. Only freshly prepared Mg(OH), sols were used as the source of MgO, which were thoroughly washed after precipitation from MgClz in dilute "*OH. The purely inorganic clay synthetic Li-hectorite used gel molar ratios of 0.262.00:1.52 LiF:MgO:SiOz. All organic complexes and other gel components were used as received from Aldrich except for the silica source, which was Ludox HS-30, a Na+-stabilized 30% silica sol obtained from Du Pont. For ion-exchange experiments the natural hectorite SHCa-1 (from near Hector, CA) was employed. As received from the Source Clays Repository, Columbia, MO, this hectorite contains nearly 50% by weight calcium carbonate im-
0888-5885f 92f 263I-1654$03.0O/O 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1655 100
Exchangeable Cations, H,O
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Figure 2. Structures of pillaring cations used in this study; the R in the 8GX dye group in alcian blue is CHPSC[N(CH3)P]N(CH3)2+C1and CH2(C5H4N)+C1- in the pyridine variant.
purity; this was removed prior to use, along with some iron, by conventional sedimentation techniques. By X-ray powder diffraction, it is estimated that
