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

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 Mössbauer 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. Mössbauer 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 Mössbauer 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 Mössbauer features attributable to FePt alloys and the onset of iron reduction, from Fe^3- 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.