Zeolite particles, typical shapes

Particle Morphology Typical shapes of zeolite particles from these preparations are shown in Figures 3. 4, and 5. 

In general, zeolites are crystalline aluminosilicates with microporous channels and/or cages in their structures. The first zeolitic minerals were discovered in 1756 by the Swedish mineralogist Cronstedt [3], Upon heating of the minerals, he observed the release of steam from the crystals and called this new class of minerals zeolites (Greek zeos = to boil, lithos = stone). Currently, about 160 different zeolite structure topologies are known [4] and many of them are found in natural zeolites. However, for catalytic applications only a small number of synthetic zeolites are used. Natural zeolites typically have many impurities and are therefore of limited use for catalytic applications. Synthetic zeolites can be obtained with exactly defined compositions, and desired particle sizes and shapes can be obtained by controlling the crystallization process. 

A typical value for m is between 0.54 and 0.72. For dynamic holdup, the value of 0.72 can be used for irregular-shaped particles similar to activated carbon and zeolites. 

Catalyst particles generally consist of a metal deposited onto the surface of a support and are denoted by metal/support, e.g. Pd/C indicates palladium metal on a carbon support. Among the metals used for catalysis, Pd is often found to be the most active metal. (Augustine 1965) For example, in the aqueous hydrodechlorination of 1,1,2-trichloroethane, Pd catalysts achieved significantly more conversion than Pt or Rh catalysts. (Kovenklioglu et al. 1992) Catalyst supports can vary in shape, size, porosity and surface area typical materials include carbon, alumina, silica and zeolites. 

Crystals of the synthetic zeolites are usually quite small and often exhibit various forms of twinning and intergrowth. With some zeolites, individual crystallites (e.g. cube-shaped NaA) of size < SO nm have been identified by electron microscopy, but the agglomerate sizes are generally in the approximate range 1-10 pm. For example, the particle size distribution over this range of a typical NaA powder was reported to be of a broad log-normal character (Breck, 1974, p. 388). 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

Zeolites have been used for years as supports for metal catalysts [1-5]. Such catalysts are typically made by impregnation of the zeolite with an aqueous solution of a metal salt, followed by calcination and reduction in hydrogen. Because the metal particles in such catalysts are typically extremely small and nonuniform in size and shape, often being present both inside and outside the zeolite pore structure, their structures are not well understood. This structural complexity provides a fundamental motivation for preparing and investigating structurally simple zeolite-supported metals, those that are so small and uniform as to be nearly molecular in character and located almost entirely within the zeolite pores investigations of well-defined... 

We now turn to the question of texture within the particle, i.e., surface area, pore shape, and size distribution. Particles are formulated by agglomerating microparticles produced during a precipitation phase, as shown in Fig. 1.4. Approximately 100 m in size, these microparticles consist of a complex porous solid. Pores typically range from l.S to 15 nm in radius. Formerly termed micropores, these channels are now called mesopores. The name micropores is reserved for those less than 1.5 nm in radius, usually found in zeolites. 

The importance of XAS spectroscopy in oxidation catalysis research results from its abiUty to provide information on the local structure of active redox sites under reaction-Uke conditions, which can often hardly be obtained by other methods, such as XRD which is restricted to crystaUine material, EPR spectroscopy which only detects species with unpaired electrons, or UV-vis diffuse reflectance and Raman spectroscopy which are less sensitive to strongly absorbing TMI in reduced valence states. Thus, in situ EXAFS has been widely used to determine the local environment and redox behaviour of TMI in nanoporous oxides. A typical example is discussed in 19.3.2.2a for Fe-ZSM-5 zeolites. Another major application is the determination of size and shape of supported metal clusters, especially for particles smaller than... 

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