A montmorillonite exchanged with large hydroxy-Al cations was thermally treated to convert the hydroxy cations to oxide pillars and to generate permanent microporosity in the interlayers. The pillared clay products were characterized by X-ray powder diffraction (XRD) and water-sorption measurements to delineate the effect of aging of the pillaring solutions, the methods of drying and concentration of clay-water suspensions, and the temperatures of calcination on micropore volumes. Although samples prepared at different conditions gave d(001) reflections of 17–17.5 Å, at least to 500°C, the concentrations of clay-water suspensions and the temperatures of calcination significantly changed the pore-size distribution and the volume of micropores present in the sample, as calculated from the water-sorption isotherms. A narrow pore-size distribution having little or no macroporosity or external surface condensation was observed in samples prepared by the addition of the pillaring solution directly to the clay (solid) without first making a clay-water suspension and calcining the sample at ≥400°C. About 87% of the total volume was found to be micropores of ≤14–17 Å in the sample calcined at 600°C. Aging the pillaring solution, however, did not influence either the water-sorption isotherms or the XRD patterns significantly under the conditions specified. Essentially the same results were obtained for samples prepared from both hydroxy-Al polymer (OH/Al = 2) and aluminum chlorohydrol (ACH) solutions.

Although alumina-pillared clays exhibited an extreme type-I isotherm (Langmuir type) for nitrogen adsorption, the sorption of water vapor gave an isotherm of unusual shape that fit neither a BET nor a Langmuir equation. This latter behavior has previously been attributed to a unique pore size and hydrophobicity and is apparently common to all pillared montmorillonite materials. The hydrophobicity of the pillared clay apparently developed on calcination by the migration of protons from the interlayers to the octahedral sheets, in which sites of cationic substitution (negative charge) were located. Protons migrated back to the interlayers by treatment with NH3, which subsequently converted to NH4+ in the interlayers. The exchange of the NH4+ by Ca2+ introduced hydrophilicity to the alumina-pillared clay, which was reflected both in the shape of the water isotherm (close to moderate Brunauer type I), the heat of sorption, and the total sorption capacity.

Polynuclear hydroxy-Al cations were prepared by partially neutralizing dilute solutions of aluminum chloride. These cations were introduced in the interlayer space of montmorillonite by cation exchange, which formed heat-stable pillars between the silicate layers. Polynuclear hydroxy-Al was preferentially adsorbed on montmorillonite compared with monomer-Al; the maximum amount adsorbed was ∼400 meq/100 g of montmorillonite. Of this amount 320 meq was non-exchangeable. The 001 X-ray powder diffraction reflection of the polynuclear hydroxy-Al-montmorillonite complex was at 27 Å, with four additional higher-order basal reflections, giving an average d(001) value of 28.4 Å. This complex was thermally stable to 700°C. An analysis of the basal reflections by the Fourier transform method indicated that the 28-Å complex had a relatively regular interstratified structure of 9.6- and 18.9-Å component layers with a mixing ratio of 0.46:0.54. This ratio implies that the hydroxy-Al pillars occupied every second layer. Considering the relatively small amount of Al adsorbed and the thermally stable nature of the structure, the hydroxy-Al pillars must have been sparsely but homogeneously distributed in the interlayer space.

The reactions of the tris(acetylacetonato)silicone(IV) cation (Si(acac)3+) with Na+-, Mg2+-, and Co2+-exchange forms of hectorite and montmorillonite have been investigated to understand better the formation process of clays pillared by silicic acid. In acetone as the solvating medium, Si(acac)3+ binds to the Na+- and Mg2+-clays with the desorption of only a small fraction (~5%) of the initial exchange cation, suggesting that the complex binds as the ion pair [Si(acac)3+][Cl−]. With the Co2+-clays, however, the exchange cation is desorbed quantitatively, and Si(acac)3+ binding is accompanied by the formation of an acetone-solvated CoCl2 solution complex which helps to drive the ion-exchange reaction. Thus, Co2+-smectites react with Si(acac)3+ in acetone to produce homoionic Si(acac)3+ intercalates, whereas Na+-and Mg2+-smectites produce mixed-ion intercalates. The interlayer hydrolysis of Si(acac)3+ to silicic acid in the homoionic Si(acac)3+- and mixed-ion Na+/Si(acac)3+- and Mg2+/Si(acac)3+-exchange forms of montmorillonite films is diffusion controlled. In water as the solvating medium, the reaction of Si(acac)3+ with Mg2+- or Co2+-montmorillonite results in the desorption of the exchange cations on a time scale which is comparable to that observed for the solution hydrolysis of Si(acac)3+. Thus, the precipitation of silicic acid from aqueous solution competes strongly with the formation of interlayer silicic acid. With aqueous Na+-montmorillonite dispersions, however, a significant fraction of the exchange cations desorbed rapidly upon Si(acac)3+ binding, and the formation of interlayer silicic acid is favored over the precipitation of Si(OH)4.

Titanium was introduced into the montmorillonite structure by cation exchange with polymeric Ti cations, formed by partial hydrolysis of TiCl4 in HCl. On further hydrolysis and heating, TiO2 pillars in the form of anastase were formed between the montmorillonite layers. The resulting TiO2-cross-linked montmorillonites possessed surface areas in the range 200–350 m2/g and pore volumes of about 0.2 cm3/g and were thermally and hydrothermally stable to 700°C. The basal spacing of products heated at temperatures > 200°C was about 28Å, as determined by X-ray powder diffraction and by N2-desorption pore-size analysis. The surface area increased and the pore volume decreased with increasing HCl-concentration in the Ti-solution. The uptake of TiO2 by the montmorillonite, the surface area, and the pore volume increased with increasing amount of Ti added in the preparation, to about 10 mmoles of Ti/g of montmorillonite. A further increase in the amount of Ti added resulted in a decrease in surface area, but the pore volume and the uptake of TiO2 remained almost constant. The high porosity and the interlayer spacing of the product are consistent with a structure similar to that previously proposed for smectites, cross-linked with hydroxy-Al oligocations.

The sintering of montmorillonites pillared by hydroxy-Al species was investigated by several techniques. The change of the microporosity of the pillared interlayer clay with temperature shows that sintering was mainly due to the loss of microporosity. On calcination of the parent clay at 760°C, the X-ray powder diffraction pattern did not change; the 060 line shifted only slightly from 8.988 to 9.017 Å. The infrared (IR) spectrum suggested some dehydroxylation, but the structure appears to have been preserved. On calcination of the pillared clay at >700°C, the intensity of the 001 line decreased steadily, the 060 line broadened, and the b parameter decreased slightly from 8.988 to 8.928 Å. Fe3+ ions apparently occupied only one site after calcination at 300°C and two sites after calcination at 700°C. Smaller isomer shifts and higher quadrupole splittings in the Mössbauer spectra suggest that calcination produced more covalent bonding of Fe3+ and a highly distorted structure. The IR spectrum was significantly modified by calcination of the sample, and the 935- and 1125-cm−1 bands disappeared and the 1035-cm−1 band broadened. These results suggest that at <700°C the pillars sintered and that the clay progressively decomposed at >750°C. The pillaring-induced decrease of thermal stability of the clay was likely due to the introduction of protons during the first step of the preparation. The thermal stability of these Al-pillared interlayer complexes is comparable to that reported for hydroxy-silicoaluminum montmorillonites and fluorhectorites.