Solids as Heterogeneous Catalysts

A catalyst is a substance that increases the rate at which a chemical reaction approaches equilibrium, while not being consumed in the process. Thus, a catalyst affects the kinetics of a reaction, through provision of an alternative reaction mechanism of lower activation energy, but cannot influence the thermodynamic constraints governing its equilibrium. 

Although the general definition of a catalyst given above emphasizes the acceleration of the approach to equilibrium, the selectivity of a catalyst is often of more importance than its overall catalytic activity. An unselec-tive catalyst may accelerate undesirable reaction pathways as well as, or more than, the desired one. A commercially important example of selective 

The presence of unbalanced attractions at the surface of a solid—say, a metal such as nickel—means that small molecules will tend to become rather loosely attached to the surface in one or (more likely) several molecular layers with an exothermic adsorption energy ranging to about —20 kJ mol-1 for nonpolar molecules. (The term adsorption is used to denote surface sorption without penetration of the bulk solid, which would be called absorption.) No chemical bonds are formed or broken. This state is usually called physical adsorption or physisorption. If, however, the adsorbate forms chemical bonds with the surface atoms, the adsorption process is called chemisorption. Chemisorption can be quite strongly exothermic (—40 to —800 kJ mol-1) but involves only the first monomolecular layer of adsorbate. 

As Fig. 6.1 shows, with reference to the adsorption of hydrogen on nickel, 

In fact, the most catalytically active transient oxygen species on surfaces is often the singly charged atom 0 . 

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The mesoporous solids have great potential for use as heterogeneous catalysts, which have the advantage of being simpler to use than homogeneous catalysts as they can be separated by filtration at the end of a reaction. A great deal of research has gone into various modifications of the structure to make them suitable for particular reactions. The aluminosilicate walls of mesoporous materials can be modified in various ways ... 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

In addition to this, related areas such as liquid CO2 and C02-expanded solvents should not be overlooked. Many additives and complex modifiers are being used to facilitate reactions in SCCO2 and perhaps the use of a small amount of organic solvent (perhaps from a bio-feedstock) could be justified in order to reduce the cost of a process and therefore lead to its uptake by industry. In addition to this, continued research into biphasic systems C02-water, C02-ionic liquids, CO2-PEG/surfactants and CC -solids (including heterogeneous catalysts) is needed to deliver pure products and reduced cost to future end-users of this technology. 

Hino, T., Anzai, T. and Kuramoto, N. (2006) Visible-light induced solvent-free photooxygenations of organic substrates by using [60]fullerene-linked silica gels as heterogeneous catalysts and as solid-phase reaction fields. Tetrahedron Letters, 47 (9), 1429-1432. 

A variety of related structures can be identified with 6,8, and 12-fold coordination of the A cation and four or sixfold coordination of the anion. In fact, the chemistry of ABO4 temarys is extremely complicated with solid solutions and phase transitions being common. Lattice defects may be introduced easily by appropriate dopings. Scheelites and its relatives have been studied intensively for their properties as heterogeneous catalysts, as host materials for impurity activated luminescent materials, and for specialized optical uses see Oxide Catalysts in Solid-state Chemistry and Section 4.4). 

Heterogeneous catalysts are defined as solids or mixture of solids that are used to accelerate a chemical reaction without undergoing change themselves. The types of solids used in industry as heterogeneous catalysts include simple oxides, mixed oxides, metal salts, solid acids and bases, metals, and dispersed metals. Catalysts are used in a wide variety of chemical and environmental processes worldwide. The global value of fuels and chemicals produced by catalytic routes is about US 2.4-3 trillion per year. About 20% of all products produced in the United States are derived from a catalytic process of some form. As important as catalysis is to the world economy, the number of various chemicals used as a catalyst as well as the form and shape of the material vary as much as the number of processes that use catalysts. Fig. 1 is a picture of a number of various types of catalysts and illustrates the numerous possibilities of shapes and sizes. Naturally, the preparation processes of such a wide variety of products is also numerous. 

The oxidation of methane has also been studied with heteropoly oxometalates as heterogeneous catalysts (ref. 17). While a number of these solids was examined the highest activity and selectivity were found with 12-molybdophosphoric acid supported on silica with N O as oxidant. 

The characterization of complex solids, such as heterogeneous catalysts, composites or ceramics, involves the determination of many parameters. Several techniques are always needed to obtain all the necessary data to yield these parameters. For example, to characterize a catalyst surface, one might apply a combination of XPS (X-ray Photoelectron Spectroscopy), various forms of electron microscopy and LEIS (Low Energy Ion-scattering Spectroscopy) to produce a correct model. These techniques involve separate areas of expertise, the coordination of which, essential to the success of the project, requires a dedicated effort. 

Zeolites are microporous crystalline solids which find a wide variety of industrial applications in the fields of ion exchange and separation, purification and catalytic transformation of organic compounds. As heterogeneous catalysts, most of their uses have been as acid catalysts where the combination of high acidity, high specific surface area and the shape selectivity derived from the size and shape of their... 

The neutral SO4 species is a reactive intermediate.) In another reaction sequence, solid particles in the atmosphere can act as heterogeneous catalysts for the reaction... 

Maximum effort has been directed toward the use of solid acid catalysts. In fact, heterogeneous catalysts can be easily separated from the reaction mixture and reused they are generally not corrosive and do not produce problematic side products. Different classes of materials have been studied and utilized as heterogeneous catalysts for Friedel-Crafts acylations these include zeolites (acid treated), metal oxides, and heteropoly acids already utilized in hydrocarbon reactions. Moreover, the application of clays, perfluorinated resinsulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, are the subject of intensive studies in this area. 

In this chapter, the use of solid acids as heterogeneous catalysts for the Friedel-Crafts acylahon reaction is described. Our review is split up into seven sechons, describing the application of zeolites, clays, metal oxides, sulfated zirconia, heteropoly acids. Nation, and other less-utilized solid catalysts (i.e., graphite). When possible, the relationship between the acid properhes of the solids (namely, Bronsted and Lewis types) and the catalytic efficiency is shown, as well as the role of the active site location on the catalyst surface. ... 

The success of the fuel and chemical industries as we know them is a direct result of the discovery of catalysts for carrying out specific reactions or sets of reactions at high rates and with high selectivity for the desired products. It is, therefore, not surprising that over 90% of the chemical processes in use require one or more catalysts. The preponderance of solid, or heterogeneous, catalysts... 

In the course of this chapter we will treat a number of other novel materials as well. Solids such as zirconium phosphates also possess layered structures and represent some of the newest materials being explored as heterogeneous catalysts. Although the zirconium phosphates have not yet been as widely studied by NMR as the clays, we review here some of the salient work done so far. This subject promises to be a thriving area of research in the near future. 

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