Molecular sieve zeolite

The structures of the bronzes are built up of (Cr02) sheets with K+ ions inserted in distorted octahedral sites for 0.70 0.77 and trigonal prismatic 

Zeolites are three-dimensional, crystalline networks of A104 and Si04 tetrahedra, a unit negative charge being associated with each A104 tetrahedron in the framework. Their crystallization is often a nucleation-controlled process occurring 

Synthesis methods are described below for four very different but important zeolite structures, A,2 Y,3 tetramethylammonium (TMA) offretite4 and tetra-propylammonium (TPA) ZSM-5.5,6 These four were selected because they span the composition range from 1 1 Si Al to a potentially aluminum-free zeolite structure (A to ZSM-5). In addition, these syntheses provide examples of fundamental concepts in crystallization such as templating (TMA offretite and ZSM-5), low-temperature nucleation (Y), and variable reactant (silica) sources. 

The chemical description of a zeolite synthesis mixture requires special comment. It is conventional to present reaction mixtures as mole ratios of added ingredients  

Determination of purity in zeolites is a second area of concern. Elemental analysis is generally not a satisfactory criterion since almost all zeolite structures can exist in a range of compositions (i.e., of Si02 A1203 ratio). For example, the A structure has been crystallized with a Si02 Al203 ratio from 2 to 6 7 at the other extreme, ZSM-5 has even been synthesized with essentially no aluminum.6 

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D. W. Breck, Zeolite Molecular Sieves, Wiley-Interscience, New York, 1974. 

A. Dyei, Al Introduction to Zeolite Molecular Sieves, ]oha Wiley Sons, Inc., New York, 1988, 102—105. 

The more permeable component is called the. st ga.s, so it is the one enriched in the permeate stream. Permeability through polymers is the product of solubihty and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 22-23 after Breck, Zeolite Molecular Sieves, Wiley, NY, 1974, p. 636). 

Noble gas hydrates are formed similarly when water is frozen under a high pressure of gas (p. 626). They have the ideal composition, [Gg(H20)46], and again are formed by Ar, Kr and Xe but not by He or Ne. A comparable phenomenon occurs when synthetic zeolites (molecular sieves) are cooled under a high pressure of gas, and Ar and Kr have been encapsulated in this way (p. 358). Samples containing up to 20% by weight of Ar have been obtained. 

Brec D. W., Zeolite Molecular Sieves Structure, Chemistry, and " New York Wiley Interscience, 1974. 

D. M. Ruthven, M. F. M. Post 2001, (Diffusion in zeolite molecular sieves), in Introduction to Zeolite Science and Practice, eds. H. van Bekkum, E. M. Fla-nigen, J.C. Jansen, Elsevier, Amsterdam. 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

In the above discussion, we have presumed that the tortuosity factor t is characteristic of the pore structure but not of the diffusing molecules. However, when the size of the diffusing molecule begins to approach the dimensions of the pore, one expects the solid to exert a retarding influence on the flux and this effect may also be incorporated in the tortuosity factor. This situation is likely to be significant in dealing with catalysis by zeolites (molecular sieves). 

ENSORB [ExxoN adSORBtion] A process for separating linear from branched hydrocarbons, using a zeolite molecular sieve. The adsorbed gases are desorbed using ammonia. It operates in a cyclic, not a continuous, mode. Developed by Exxon Research Engineering Company, and used by that company on a large scale at the Exxon refinery in Baytown, TX. Asher, W. J., Campbell, M. L., Epperly, W. R., and Robertson, J. LHydrocarbon Process., 1969, 48(1), 134. 

IsoSiv [Isomer separation by molecular sieves] A process for separating linear hydrocarbons from naphtha and kerosene petroleum fractions. It operates in the vapor phase and uses a modified 5A zeolite molecular sieve, which selectively adsorbs linear hydrocarbons, excluding branched ones. Developed by Union Carbide Corporation and widely licensed, now by UOP. The first plant was operated in Texas in 1961. By 1990, more than 30 units had been licensed worldwide. See also Total Isomerization. 

OlefinSiv A process for isolating isobutene from a mixture of C4-hydrocarbons by chromatography over a zeolite molecular sieve. Developed by the Linde Division of the Union Carbide Corporation, as one of its IsoSiv family of processes. 

PuraSiv Hg An adsorptive process for removing mercury vapor from gaseous effluents from the Castner-Kellner process by TSA. The adsorbent is a zeolite molecular sieve containing silver. Developed by UOP... 

PuraSiv N A process for removing nitrogen oxides from the tail gases from nitric acid plants, using an acid-resistant zeolite molecular sieve. Developed by the Union Carbide Corporation in 1971. Not to be confused with PuraSiv HR, Type N (see previous entry). 

Dyer, A. (1988). An Introduction to Zeolite Molecular Sieves, p. 149, John Wiley Sons, New York... 

Zeolite molecular sieves are widely used as solid acid catalysts or catalyst components in areas ranging from petroleum refining to the synthesis of intermediates and fine chemicals (112,113). An important reason for their widespread use is the flexibility they oflFer regarding the tailoring of the concentration and nature of catalytically active sites and their immediate environments. We note that discrimination between chemical and structural aspects works well at a conceptual level, but one faces quite severe limitations as soon as one tries to separate the contributions of the two effects. The complexity arises because the chemical properties of a particular molecular sieve are connected with its framework density. 

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