Porous solids as catalysts

Many essential transformations of carbohydrates regularly employ toxic and corrosive reagents, such as Lewis acids and strong mineral acids. Hence, the use of zeolites and related porous solids as catalysts in such reactions affords a practical and environmentally compatible alternative to the standard protocols. 

The diffusion rates of fluids, particularly gases, throagh porous solid materials are of particular interest because of the common practice of using porous solids as catalysts and adsorbents. Diffusions transport rates within the pores may limit the rates by which such procesms occur. 

The intramolecular 5,e-epoxycarbonyl rearrangement of 8a,17-epoxy-14,15-dinorlabdan-13-one I to give stereoselectively two diastereoisomeric ketals II or III was promoted in heterogeneous media using silicic porous solids as catalysts, making possible to choose a convenient selective system through an adequate combination of solvent, catalyst and temperature. 

Nanocatalysis is a rapidly growing field which involves the use of nanostruc-tured materials such as NPs, nanofilms, and nano porous solids, as catalysts. In comparison with their bulk counterparts, the nano-structured catalytic materials possess high surface-to-volume ratio, high surface energy, and different electronic state, which result in unique catalytic activity in many reactions. Conventionally, soHd-supported nano-metal particles (NMPs) have been widely studied in various reactions. Besides the specific function of a soHd base to build the catalytically... 

When porous solids are being used as catalysts or as reactants, the rate constant k in the global equation is replaced by rjk. Consequently, this equation applies to porous solids as well as nonporous solids. 

In reactions using amorphous solids as catalysts, where reaction occurs at the surface of the solid, the mobility of the reactants is reduced from three dimensions to two. In effect this concentrates the reactants in small, localised areas and thus raises the rate of reaction. In lamella or porous solids the limitations may be even greater. One type of catalysis common with porous solids when two species are in competition in a reaction involves biasing the process in favour of one species by use of a solid that allows only one of the competitors to enter the pores. This essentially separates the two species, allowing a selective reaction to occur. It is also possible to control the outcome of a reaction by selecting one of a set of possible products on the basis of its shape. Quite commonly reactions produce more than one product, or isomers of the same material, that have different physical shapes. By restricting the space in which the reaction occurs it is sometimes possible to control which product is formed or which can escape from the pores of the solid. 

Another way in which catalysis can be achieved using solid materials is to use them as supports for other catalysts such as metal salts. In this way a catalytic species is held immobile on the solid phase. This means that with amorphous solids a highly dispersed layer of catalyst is created and with lamella or porous solids the catalyst is held in a restricted space. The combination of the catalyst and the restriction of molecular movement brought about by the solid can give powerful control over reactive species. 

Porous surfaces of inorganic solids such as clays, silica gel, alumina and zeolites are the commonest systems used as catalysts in Diels-Alder reactions. 

As an example, consider the use of PVPy as a solid poison in the study of poly(noibomene)-supported Pd-NHC complexes in Suzuki reactions of aryl chlorides and phenylboroiuc acid in DMF (23). This polymeric piecatalyst is soluble under some of the reaction conditions employed and thus it presents a different situation from the work using porous, insoluble oxide catalysts (12-13). Like past studies, addition of PVPy resulted in a reduction in reaction yield. However, the reaction solution was observed to become noticeably more viscous, and the cause of the reduced yield - catalyst poisoning vs. transport limitations on reaction kinetics - was not immediately obvious. The authors thus added a non-functionalized poly(styrene), which should only affect the reaction via non-specific physical means (e.g., increase in solution viscosity, etc.), and also observed a decrease in reaction yield. They thus demonstrated a drawback in the use of the potentially swellable PVPy with soluble (23) or swellable (20) catalysts in certain solvents. 

Improved characterization of the morphological/microstructural properties of porous solids, and the associated transport properties of fluids imbibed into these materials, is crucial to the development of new porous materials, such as ceramics. Of particular interest is the fabrication of so-called functionalized ceramics, which contain a pore structure tailored to a specific biomedical or industrial application (e.g., molecular filters, catalysts, gas storage cells, drug delivery devices, tissue scaffolds) [1-3]. Functionalization of ceramics can involve the use of graded or layered pore microstructure, morphology or chemical composition. 

In this chapter, we demonstrate the potential of such agents as catalysts/promoters in key steps for the derivatization of sugars. The most significant catalytic approaches in carbohydrate chemistry that use aluminosilicate porous materials, namely zeolites and montmorillonite clays, are reviewed and discussed. Silica gel is a porous solid silicate that has also been used for heterogeneous catalysis of organic reactions in general. We include here its usefulness as promoter and reagent support for the reactions under consideration. 

Unsupported, or bulk cobalt catalysts are commonly used as model systems to avoid the influence of support interactions. Bulk cobalt (also known as cobalt black) is typically produced by reduction of Co304, leading to a porous solid with a low... 

The illustrations shown are just a portion of a variety of textures of real porous solids, also used as adsorbents and catalysts. It is obvious that when one goes from descriptions to quantitative... 

Wheeler [16] proposed that the mean radius, r, and length, L, of pores in a catalyst pellet (of, for that matter, a porous solid reactant) are determined in such a way that the sum of the surface areas of all the pores constituting the honeycomb of pores is equal to the BET (Brunauer, Emmett and Teller [17]) surface area and that the sum of the pore volume is equed to the experimental pore volume. If represents the external surface area of the porous particle (e.g. as determined for cracking catalysts be sedimentation [18]) and there are n pores per unit external area, the pore volume contained by nSx cylindrically shaped pores is nSx nr L. The total extent of the experimentally measured pore volume will be equal to the product of the pellet volume, Vp, the pellet density, Pp, and the specific pore volume, v. Equating the experimental pore volume to the pore volume of the model... 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

As with thermal conductivity, we see in this section that disorder can greatly affect the mechanism of diffusion and the magnitude of diffusivities, so that crystalline ceramics and oxide glasses will be treated separately. Finally, we will briefly describe an important topic relevant to all material classes, but especially appropriate for ceramics such as catalyst supports—namely, diffusion in porous solids. 

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