Solid catalysts zeolite heterogeneous catalysis

Adsorption microcalorimetry is the measure of the heat of adsorption evolved when dosing measured small amounts of a vapor probe on a surface. Cardona-Martinez and Dumesic [36] summarized the results obtained for oxides, zeolite, and metal catalysts before 1992. Summaries of the application of these techniques to gas-solid interactions and heterogeneous catalysis have been published recently [37-39]. As done by Auroux and Gervasini [40] for a number of binary metal oxides, calorimetric studies of the acidity and basicity are mostly performed using ammonia as an acidity probe and carbon dioxide as a basicity probe [41]. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

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. 

Application of transmission electron microscopy (TEM) techniques on heterogeneous catalysis covers a wide range of solid catalysts, including supported metal particles, transition metal oxides, zeolites and carbon nanotubes and nanofibers etc. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

Obviously, it is very desirable to substitute these modes of benzylic ether preparation by an heterogeneous catalysis process. Clays (50) and resins (51, 52) which were the first solid acid catalysts used have given low or moderate yields. The first experiments with zeolites were carried out by Rhodia (53, 54) on the etherification of vanillic alcohol (A) in a batch reactor over a HBEA zeolite with a Si/Al ratio of 12.5 ... 

Owing to the great interest in the argument, minireviews have been published on the use of solid catalysts in Friedel-Crafts acylation. Kouwen-hoven and van Bekkum, in a chapter of the Handbook of Heterogeneous Catalysis, faced the basic problem of the use of zeolites in the reaction. A further essential overview of the same argument was reported by Metivier in Fine Chemicals through Heterogeneous Catalysis Furthermore, Bezouhanova described the synthetic aspects of the zeolite-catalyzed preparation of aromatic ketones. ... 

Reaction on the surface of a solid on the one side and in the lattice of a crystalline solid on the other side may be regarded as two extreme cases of heterogeneous catalysis, the majority of porous commercial catalysts lying in between. A more detailed understanding of the kinetics of reactions in zeolites could be important, therefore, from a very general viewpoint. 

Reactions catalyzed by zeolites and other solid catalysts proceed in highly heterogeneous systems. The determination of local diffusivities in such systems by conventional gravimetric or flow methods [3-5] is very complicated and incurs the risk of misinterpretation, since all these methods are sensitive to the response of the whole system rather than to the mobility of individual species within well-defined regions. With reference primarily to diffusion studies in zeolitic adsorbent-adsorbate systems, the present chapter will show how NMR spectroscopy is a most versatile tool for investigating molecular mass transfer phenomena in heterogeneous catalysis. Information may be provided with respect to both microscopic and macroscopic dimensions, involving the observation of molecular distributions as well as the diffusion paths of individual molecules. 

Here, we will first present briefly the general chemistry at work in deNOx catalysis. Then we wiU focus on the various specific nanometric issues in the domain. NOx decomposition mainly takes place on a metal center, as do most redox reactions in heterogeneous catalysis. We will therefore focus on how nanometric size influences the role of metal particles in this domain. It is very much linked to interaction with the oxide support, which wiU be the subject of the following part Zeolites are very specific oxide supports, controlling the distance between reactants and imposing chemical reactivity within the nanometer scale. Metal particles formed in zeoHtes have their size controlled between 0.5 and several dozens of nm, sometimes with Angstrom accuracy. We wiU next deal with three-way catalysts, mainly used for automotive deNOx. They involve very complex solids, where particle size is a key issue for activity. Finally, we wiU mention quickly new nanometric sohds like carbon nanotubes, which have appeared recently in deNOx catalysis. 

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