Zeolites compounds

Another way to work in transient conditions is to stop suddenly (or conversely to instantaneously introduce) one of the reactants, in order to destabilize the system and to enhance the concentration of labile species. With this method, for example, Poignant et al. studied the DeNO. reaction mechanism on a H—Cu-ZSM-5 catalyst, using propane or propene as reducing agents. The introduction of 2000 ppm of hydrocarbon in a flow of NO (2000 ppm) + 5% 02 allowed to evidence the formation of acrylonitrile, which behaved as an intermediate. Its reactivity with NO+ species constituted a fundamental point to describe a detailed SCR mechanism for NO removal on zeolitic compounds [137],... 

Molecular sieves are used in a variety of fuel processing applications. Uses include drying and water removal from fuel, product purification, hydrocarbon separation and catalysis. Molecular sieves are composed of sodium and calcium aluminosilicate crystals which have been produced from natural or synthetic zeolite compounds. The crystals are dehydrated through heating and are processed to ensure that pore sizes are tightly controlled. 

Zeolite Compound Melting point (°C) Heat treatment ... 

Effects of crystalline structure and acidity differentiate the catalytic behavior of ZSM5, USY and mordenite zeolites. Compounded with the nature of the support, the location of nickel particles leads to very peculiar behaviors in the formation of low-temperature coke during the hydrogenation of phenylacetylene. The principal differences in the high temperature deactivation are determined by the size specificity of the zeolitic supports, and by the high acidity available to the reactant molecules, especially for the USY support. The contribution of nickel to coke formation at low temperatures occurs mainly at the internal surface of Ni/mordenite and Ni/USY and at the external surface of Ni/ZSM-5. This conclusion is supported by the TPR patterns as well as by the relative values of low, intermediate and high temperamre coke for each individual support. 

The calcium chloride and residual sodium chloride are washed away and the regenerated sodium zeolite compound can be used again. The process can be repeated almost indefinitely, subject only to the very gradual breaking down of the granules of the compound into powder. 

Soluble potassium may be made available to plants by the action of biological end products upon. the zeolite compounds of the soil. Maclntire has shown, however, that the economic additions of calcic and inorganic materials function to protect native soil potassium and give a repressive rather than a liberative chemical effect. 

Lastly, a wide selection of papers, focus the most recent achievements in the application of zeolitic compounds, in the use of oxide materials in the fuel cells technology and in the understanding of the complex mechanisms of oxide-based materials in the soil system. 

Here reactions (10) and (11) highlight the comparison that can be drawn between an electron trapped in a cavity defined by a tetrahedron of sodium cations in the sodalite cage, and an electron trapped within solvent cavities in liquid ammonia. Furthermore, the ability to prepare trapped electrons in zeolites, in stoichiometric or near stoichiometric amounts, has led to the description inorganic electrides [88, 90]. Together these ideas have been found to constitute a remarkably robust conceptual framework for rationalizing the properties of a wide range of metal-zeolite compounds [88], whose main strength lies in its ability to do so without recourse to detailed structural information, which is available in only a few cases. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Schunk S A and Schuth F 1998 Synthesis of zeolite-like inorganic compounds Molecular Sieves Science and Technology vo 1, ed H G Karge and J Weitkamp (Berlin Springer) pp 229-63... 

The most common compound is sodium chloride, but it occurs in many other minerals, such as soda niter, cryolite, amphibole, zeolite, etc. 

Over the years, thousands of compounds have been tried as cracking catalysts. These compounds fall into two general categories natural and synthetic. Natural catalyst, as the name denotes, is a naturally occurring clay that is given relatively mild treating and screening before use. The synthetic catalysts are of more importance because of their widespread use. Of the synthetic catalysts, two main types are amorphous and zeolitic. 

Molecular sieves (dehydrated zeolite) purify petroleum products with their strong affinity for polar compounds such as water, carbon dioxide, hydrogen sulfide, and mercaptans. The petroleum product is passed through the sieve until the impurity is sufficiently removed after which the sieve may be regenerated by heating to 400 - bOO F. 

Zeolites, 321, 322 Ziegler-Natta catalysts, 174 Zirconium chlorides, 194 Zirconium compounds as catalysts, 188... 

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

While Cordfuncke [997] believes that there are only four stable compounds in the U03—NH3—H20 system, the results of Stuart et al. [998, 999] indicate the existence of a continuous non-stoichiometric phase containing the NH4 ion and possessing zeolitic properties U02(0H)2 x (ONH4 )x yH20. 

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

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