Rasik H. Raythatha

On the pillaring and delamination of smectite clay catalysts by polyoxo cations of aluminum

Abstract Two types of polyoxoaluminum solutions, base-hydrolyzed A1Cl 3 and aluminum chlorohydrate (ACH) with OH/A1 ratios of 2.42 and 2.50, respectively, have been used as reagents for the pillaring of the smectite clays montmorillonite and nontronite. Selective adsorption studies demonstrate that the method used to dry the flocculated clay layers is far more important than the choice of pillaring reagent or clay layer charge in determining the apparent pore size of pillared products. Air-drying leads to zeolite-like products with a pore opening ⪢6.2 A and 27 Al NMR spectroscopy indicates the polyoxo cation nuclearity to be larger in ACH than in base-hydrolyzed AlCl 3 , both reagents give pillared products with similar aluminum contents, pore sizes, thermal stabilities and catalytic properties. These similarities suggest the same type of oxo cations, probably Al 3 Keggin ions, are formed on the intracrystal surfaces. Also, the amount of Al bound per unit cell of pillared clay varies only over a small range (2.78 – 3.07), and the variation is not correlated with layer charge. This latter result suggests that a more or less uniform monolayer of hydrated polyoxo cations is formed in the interlayers, and that electrical neutrality is achieved through hydrolysis of the pillaring cations.

Pillared-clay catalysts containing mixed-metal complexes I. Preparation and characterization

Abstract High-surface-area pillared clays were prepared from naturally occurring montmorillonites by exchanging interlayer ions with polyoxocations containing (i) iron, (ii) aluminum, (iii) discrete mixtures of (i) and (ii), or (iv) iron and aluminum located within the same complex. The valence state, solid-state properties, and stability of these pillars were determined following reduction and oxidation using Mossbauer spectroscopy, X-ray diffraction, and BET surface area measurements. Controlled atmosphere electron microscopy and transmission electron microscopy were also used to follow the nucleation and sintering behavior of the pillars during reduction. Mossbauer data suggested interlayer formation of metallic iron domains following reduction of types (i) and (iii) pillared systems. The magnetic properties and the oxidation behavior deduced from Mossbauer analysis and the complementary insights provided by XRD strongly indicated that these crystallites were in the form of thin-film/pancake-shape islands most likely conforming to the geometry of the interlayer region. Reduced domains remained accessible to the gas phase and in some cases resisted sintering during reduction/oxidation cycles. Reduction of the iron phase could be enhanced by addition of platinum to the sample. The absence of Mossbauer features attributable to FePt alloys and the onset of iron reduction, from Fe3− to Fe2+, at room temperature suggested that reduction was facilitated by hydrogen spillover from platinum. The expanded structures of types (ii) and (iii) pillared systems were found to be relatively stable following reduction up to 723 K due to the irreducible nature of discrete aluminum pillars under these conditions. At appropriate iron pillar to aluminum pillar ratios, results obtained from type (iii) pillared systems also indicated that at least one monolayer of Fe2+ was preferentially decorated/accommodated at the surfaces of the aluminum oxide pillars. This behavior was attributed to the relatively stronger interaction of iron with alumina than with silica and was triggered at temperatures ≤673 K by introducing platinum, and presumably hydrogen atoms, to the specimen. On the basis of the findings noted above, intercalation of clays with mixtures of chemically distinct pillars appears to provide a unique method for preparing highly dispersed metallic or even bimetallic catalysts possessing two-dimensional sieve-like behavior with high overall surface areas and high loadings of the active metal.

Clay intercalation catalysts interlayered with rhodium phosphine complexes. Surface effects on the hydrogenation and isomerization of 1-hexene

Clay intercalation catalysts formed by interlayering of Na/sup +/-hectorite with rhodium phosphine complexes of the type Rh(NBD)(PPh/sub 3/)/sub 2//sup +/ and Rh(NBD)(dppe)/sup +/, where NBD = norbornadiene and dppe = 1,2-bis(diphenylphosphino)ethane, were examined as catalyst precursors for the hydrogenation-isomerization of 1-hexene in methanol. Relative to reaction under homogeneous solution conditions, the intercalated catalysts exhibit a much lower tendency to isomerize the substrate to the less reactive internal olefin 2-hexene. In the case of Rh(NBD)(PPh/sub 3/)/sub 2//sup +/-hectorite, the dramatic dependence of the hexane: 2-hexene product ratio on substrate concentration and water content indicates that the intrinsic Bronsted acidity of partially hydrated Na/sup +/ ions in the clay interlayers causes the protonic equilibrium between surface intermediates responsible for the isomerization and hydrogenation pathways to be shifted in favvor of hydrogenation. Rh(NBD)(dppe)/sup +/-hectorite also favors hydrogenation over isomerization relative to homogeneous solution. In this case, however, a catalytically important protonic equilibrium is not involved in the reaction mechanism, and the reaction is insensitive to factors which influence surface acidity. The results demonstrate that surface chemical effects can dramatically alter the catalytic properties of metal complexes immobilized in clay interlayers.

Substrate rigidity effects in mixed layered solids

Abstract The composition dependence of the d-spacing of Cs1−xRbx vermiculite has been determined for 0 ≤ × ≤ 1. It exhibits a step-like drop with increasing x at a composition that is unexpectedly lower than that at which Cs1−xRbxC8 shows a more gradual drop. This results attributed to in-plane substrate distortions which preserve the rigidity of the vermiculite layer. Measurements of the torsional Raman mode of Cs1−xRbx vermiculite support this analysis.

Mechanism of synthesis of 10-Aa hydrated kaolinite

The synthesis of 10-Å hydrated kaolinite was accomplished by: (1) direct reaction of HF with a dimethyl sulfoxide (DMSO)-kaolinite intercalate and water washing; (2) methanol washing of a DMSO-kaolinite intercalate followed by reaction with any alkali fluoride salt and water washing; and (3) room-temperature (RT) water-washing of a methanol-washed DMSO-kaolinite intercalate. In all syntheses the optimum yield required a kaolinite in which DMSO was bound strongly to the interlayer surface. In the first synthesis, water inclusion between clay layers appeared to be facilitated by the reduction of cohesive interlayer forces brought about by replacement of surface and edge OH− by F−. The fluorination reaction was accomplished either by direct reaction of HF or by HF produced through the hydrolysis of NH4F at 60°C. In the second synthesis, intercalated DMSO was replaced by methanol. F− solvated readily in methanol but not in DMSO. Consequently, F− produced through hydrolysis of the alkali fluoride salt entered the interlayer space and contributed to the fluorination reaction. Furthermore, the diffusion of methanol out of the interlayer space during the RT-washing step was slowed by F≈ solvation which aided the exchange of methanol for water. High yields of 10-Å kaolinite hydrate were obtained irrespective of choice of alkali fluoride salt. The third synthesis was dependent on matching the diffusion of methanol out of the interlayer space with diffusion of water into this space. At room temperature the diffusion rates were close enough to maintain the clay in the expanded state throughout the hydration process, and high yields of 10-Å kaolinite hydrate were obtained. At 60°C the diffusion rates were too dissimilar, and very low yields of hydrate were obtained.

Hydrogenation of 1,3-butadienes with a rhodium complex-layered silicate intercalation catalyst

Abstract Rh(NBD)(dppe) + (NBD = norbornadiene, dppe = 1,2-bis(diphenylphosphino ethane) intercalated in hectorite, a swelling layered silicate, catalyzes t the overall 1,2 and 1,4 addition of hydrogen to 1,3-butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene at rates which range from −5 to 0.83, relative to the homogeneous catalyst. The yields of 1,2 addition products are 1.5 to 2.3 times higher than those obtained under homogeneous reaction conditions. The intercalation effects depend in part on the extent to which the silicate interlayers are swelled by solvent.

Pressure effects on water-intercalated kaolinite

Abstract Water-intercalated kaolinite was studied in various fluids under pressure up to 30 kbar by x-ray diffraction. With the application of pressure, pressure-induced intercalation and an acceleration of diffusion-limited intercalation were observed. The intercalated compounds were stable at ambient pressure in the presence of solvent, but they quickly deintercalated on drying.