Clays metal catalyst supports

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. 

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],... 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Over the past 15-20 years, there has been a renewed and growing interest in the use of clay minerals as catalysts or catalyst supports. Most of this interest has focused on the pillaring of smectite clays, such as montmorillonite, with various types of cations, such as hydrated metal cations, alkylammonium cations and polycations, and polynuclear hydroxy metal cations (1-17). By changing the size of the cation used to separate the anionic sheets in the clay structure, molecular sieve-like materials can be made with pore sizes much larger than those of conventional zeolites. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

In order to improve the textural properties of particle-clay nanohybrids, bulky organic cations are intercalated as a kind of template into particle-intercalated clays before stabilization procedures. Intercalation of the organic cations results in the removal of some of the intercalated nanoparticles and/or in their rearrangement. Subsequent calcination leads to formation of additional pore space that is highly correlated to the geometry and size of the templates. This technique allows fine tuning of textural properties in the preparation of particle-clay nanohybrids. The clay nanohybrids intercalated with metals, oxides, and complexes have a broad range of applications. In particular, metal oxide particle-pillared clays have excellent potentials as catalysts, catalyst supports, selective adsorbents, etc. " ... 

The use of pillared clays as metal supports has also been reported. The more defined interlamelar spacing available with these supports should give a more predictable shape selectivity to the resulting supported metal catalysts. Further, since the pillars prevent the collapse of the layers on drying and further heating, the pillared clay supported metals salts can be calcined and reduced under conditions that can give the best metal dispersion without any concern for a change in the structure of the support. ... 

The necessity to develop hydrotreating catalysts with enhanced activity stimulates the search for alternative catalyst supports. It was shown that clay-supported transition metal sulfides can efficiently catalyze hydrodesulfurization (HDS) of thiophene [1-3]. However, the large scale application of the catalysts based on natural clays is still hampered, mainly due to the difficulties in controlling the chemical composition and textural properties. Synthetic clays do not suffer from these drawbacks. Recently, a novel non-hydrothermal approach was proposed for the synthesis of some trioctahedral smectites, namely saponite... 

The PIL-clay and zeolitic materials synthesized are suitable as support for preparing heterogeneous catalysts by anchoring an organometallic complex by ion exchange. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with lamellar structures, while better dispersion was shown by the catalysts supported on zeolitic structures. 

The high surface area of clays also makes them particularly attractive as catalyst and reagent supports. Discussion in this chapter will be largely confined to clay-based catalysts in which metal complexes or other ions are specifically incorporated in the clay matrix. Other clay-based catalytic materials will be discussed in Chapter 4. 

The earliest material to resemble a supported metal catalyst was made by Dobereiner, who mixed platinum black with clay in order to dilute its catalytic action. This remains a significant objective, since for most purposes it would be quite impractical to employ undiluted metal. The use of supported metals facilitates handling and minimises metal loss, a particularly important consideration with the noble metals by appropriate choice of the physical form of the support, they can be used in various types of catalytic reactor, such as fixed-bed or fluidised-bed configurations. The support has often been regarded as catalytically inert, but in addition to those cases such as bifunctional catalysts, where the acidic support has long been known to play a vital role, there is a growing number of examples of participation by the support in catalytic processes. The support surface also facilitates the incorporation of modifiers, such as promoters or selective poisons. 

Torok, B., Balazsik, K., Kun, 1., Szollosi, G., Szakon5d, G., Bartok, M. (1999) Homogeneous and Heterogeneous Asymmetric Reactions. Part 13. Clay-Supported Noble Metal Catalysts in Enantioseleetive Hydrogenations, Stud. Surf. Sci. Catal. 125, 515-522. 

Ethylene polymerization catalysts based on late transition metals may be intrinsically more compatible with clay supports than early transition metal catalysts, because of their higher tolerance for water and other polar impurities. Two families of late transition metal catalysts have been particularly successful in producing high molecular weight polyethylene those based on bis(imino)pyridine ligands and those containing a-diimine or related ligands. 

Ray et al. [24] treated Cloisite 20A (montmorillonite modified with dimethyl-ditallow-containing approximately 65% Cig, 30% Cis, and 5% Ci4-ammonium cation chains) with a MAO solution, after vacuo-drying at 100 °C. The resulting MAO-treated clay was subsequently used for ethylene polymerizahon in the presence of a late transition metal catalyst (2,6-bis[l-(2,6-diisopropylphenylimino)ethyl] pyridine iron(ll) dichloride) and additional MAO in a glass reactor. They compared the result with homogeneous polymerization with the same catalyst in the presence of Cloisite 20A and observed that the supported catalyst was more efficiently exfoliated than when only a mixture of catalyst and clay was used. This comparison led them to conclude that at least some of the active centers resided within the clay galleries. Inductively coupled plasma (ICP) measurements showed that all MAO and catalyst remained in the solid catalyst after drying. 

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