Metal oxide pillars

In an effort to more fully elucidate the structure and reactivity of metal oxide pillared clays, we have been investigating the structure-reactivity properties of chromia-pillared derivatives (17). In the following sections, we provide an example of the structure-catalytic reactivity properties of chromia-pillared montmorillonites. Also, we report our initial efforts to structurally characterize the intercalated chromia aggregates by Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong K-edge absorption and fine structure suitable for determination of metal-oxygen bond distances in the pillars. 

Many cation-exchanged clays are suitable for the production of metal-oxide-pillared clays. The hydrolysis of the cation helps the pillaring step, so, at first, the aluminum ion was applied as a pillaring agent. Later, other elements were also used, for example, zirconia chromium iron transition metal elements and some lantanoids, organometallic complexes, surfactants, and polymers. 

These clays have been hybridized with diverse structural types of components such as nanoparticles, clusters, complex compounds, polymers, molecules, and ions. Their potential apphcations are found in many fields as inorganic catalysts, adsorbents, ceramics, coatings, and even drug delivery carriers. Various preparation methods have been developed such as pillaring, intercalation, and delamination techniques. The representative examples include organic-clay hybrids," metal oxide-pillared clays, " and bioclay hybrids. ... 

Metal oxide pillared days typically are chemically and thermally more robust than their microporous organo day coimterparts. These derivatives are... 

Protons are released upon heating which in part balance the negative charge of the host clay layers. A number of review articles have recently appeared which summarize the synthesis and physical properties of metal oxide pillared days derived fix>m the intercalation of polyoxocations of aluminum, zirconium, chromium and many other metals [10-12]. The Lewis acid sites provided by coordinatively unsaturated metal ion sites on the pillar and the Bronsted addity formed upon thermolysis imparts novel chemical catalytic properties [13,14]. Since the pores between pillars often are larger than those foimd in conventional zeolites, there is considerable interest in the use of metal oxide pillared clays for the processing of large organic molecules, espedally petroleum [14-17]. 

The concept of pillaring is very straightforward and consists of two main steps first, the interlamellar small cations are exchanged for other, bulky ions. A second or calcination step converts the inorganic polyoxycation precursors into rigid, stable metal oxide pillars, tightly bonded to the clay layers (Fig. 2). 

Yoda, S., Nagashima, Y., Endo, A., Miyata, T., Otake, K., and Tsuchiya, T. (2005) Nanoscale architectiu e of metal-oxide-pillared clays using supercritical CO2. Adv. Mater., 17, 367-369. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

General procedures for the preparation of pillared clays are schematically illustrated in Fig. 1. The first and most important reaction for the introduction of pillars is ion-exchange the hydrated interlayer cations of montmorillo-nite are exchanged with precursory polynuclear metal hydroxy cations. After the ion-exchange, the montmorilIonite is separated by centrifugation and washed with water several times to remove excess hydroxy ions. The interlayered hydroxy cations are then converted into the respective oxide pillars by calcination. The precursors developed so far and the interlayer spacings of their... 

Reduction of metal oxides, intercalated between the 72 clay layers (pillared clays), led to metal-intercalated clay nanocomposites... 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Catalyst discovery research—metal oxides and supports, shape selective and hetero metal substituted molecular sieves, pillared clays, biomimetic, methan-otropic and other bio systems and combinatorial catalytic screening techniques, liquid phase homogeneous systems. 

The other type of transformation process of the interlayer cation is pillaring (Chapter 1, Section 1.3.5). In this process, metal oxide chains are formed in the interlayer space upon thermal treatment of different cation-exchanged mont-morillonites. As a result, the basal spacing, that is, the size of the interlayer space, increases. The pillared montmorillonites are widely applied in catalytic reactions, as evidenced by over 600 scientific publications in the last 10 years (e.g., Fetter et al. 2000 Johnson and Brody 1988 Perez Zurita et al. 1996). 

An equivalent surface area of 460 m g was determined from the monolayer volume, Vj. The value obtained for the dimensionless energetic constant, C=260, was characteristic of a microporous material. Although the BET surface area may not be a physically precise quantity due to the fact that the nitrogen molecule does not exhibit the same cross-sectional area in a microporous environment as on a flat surface, the BET value is useful for comparisons of relative porosities among a related class of adsorbents. For instance, smectite clays pillared by metal oxide aggregates typically exhibit BET surface areas in the range 150 - 400 m /g. Thus, the TSLS complex is among the more porous intercalated nanocomposites derived from smectite clays. 

The anionic polyoxometalates comprise a much larger class of metal oxide-based pillaring agents (18). It was proposed that hydrotaldtes (also referred to as layered double hydroxides, or LDHs) would be suitable host materials for pillaring with polyoxometalates. Subsequently, a route for synthesizing isopol3rmetalate-pillared hydrotalcites via organic-anion-pillared precursors was developed (19-20). 

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