Zeolites microcrystalline

Furthermore, aluminosilicates are available in a variety of different structural types including lamellar clays and three-dimensional microcrystalline zeolites. Such solids can be useful... 

Results from diverse experimental methods were combined to arrive at structure proposals for ECR-1, a typical example of a microcrystalline zeolite. The zeolite features a 12-ring single channel, formed by a regular alternation of connected sheets of mordenite and mazzite. Crucial clues to the structure came from the HREM observation of a mazzite epitaxial overgrowth on an ECR-1 crystal. Electron and x-ray diffraction, infra-red spectroscopy and synthesis phase relationships were essential additional data sources. 

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

The sodium forms of zeolites were obtained mostly by courtesy of WOLFEN A-zeolite was provided by BAYER and H-ZSM-5 by DEGUSSA fi om Germany. ICI U.K pro"vided EU-1 samples with different Si/Al ratio (29, 121 and 712). Clinoptilolite (C,HEU) was microcrystalline zeolitic tuff fiom sedimentary deposit (Horseshoe Dam, Arizona, U.S.A.) and proved to be quite pure (about 96%). 

For about two hundred years, zeolites were considered minerals of no practical interest, because of their exiguity in known occurrences in vugs and cavities of basalts (rocks with silica content ranging between 45% and 52%) and other trap-rock formations. However, the discovery of their unique and attractive properties useful for many applications of environmental and industrial relevance, promoted search campaigns to find more significant natural zeolite formations. Evidence for the existence of microcrystalline zeolite occurrences of volcanic origin had sporadically been reported in the literature (see, e.g., [17]), but nobody would have expected that so huge zeolite deposits would have been found in a few decades all over the world (see Sec. 4). 

The preparation was performed on a commercial microcrystalline beta zeolite. The zeolite was treated with the Fenton s reagent and less than 0.3 wt% of carbon remained after the treatment. The porosity was fully developed as revealed by the pore-size distribution. Elemental analysis combined with TPR did confirm the high degree of Fe-exchange (98%) on the Bronsted sites. 

Figure 1. High-resolution electron micrograph and corresponding optical transform (inset) of an x-ray amorphous zeolite-Y specimen that has undergone ion-exchange with a solution containing U022+ ions. The microcrystalline regions are rendered visible by the locally ordered U022+ ions. ( See text.)...

Maher, P.K. and Scherzer, J. (1970) Method of preparing microcrystalline faujasite-type zeolite. US Patent 3,516,786. 

While single crystals of natural minerals readily lend themselves to conventional methods of structural investigation, synthetic zeolites, which are almost always microcrystalline, must of necessity be studied by the less powerful powder X-ray diffractometry. The development of high-resolution solid-state NMR techniques was therefore very timely. 

Microcrystalline solids such as zeolites and zeolite like structures have shown the utility of those properties in the domain of acid catalysis. However, little is known on their possibilities as base catalysts. It has been shown [ref. 1,2] that zeolites have basic sites which are able to catalyze reactions needing weak and medium basic strengths. Moreover, a correlation between the basicity and the Sanderson s average electronegativity Df the framework has been observed [ref. 3], Then, their activity as base catalysts can be modified by changing the countercation [ref. 4], the framework Si/Al ratio, or by introducing atoms other than Si and Al in the framework [ref. 5],... 

Pyroclastic rocks include scoria and pumices, which show variably por-phyritic textures with dominant leucite phenociysts and minor clinopyrox-ene, some mica, and rare olivine, set in a microcrystalline to glassy matrix that has been generally affected by strong secondaiy processes, with abundant zeolites and clay minerals (Fomaseri et al. 1963 Trigila et al. 1995). Accessory phases include Fe-Ti oxides, apatite and rare garnet. 

In this paper we shall describe how high resolution electron microscopy (HREM) can be used in conjunction with selected area electron diffraction (SAED) to probe the local structure of zeolitic solids (2, 5 8) which are often microcrystalline, multi-phasic or twinned. We shall also refer to the application of optical diffractometry (4, 9-11) as a supplemental procedure either for interpretation of electron micrographs, or for analogue diffraction studies of model systems. 

Many inorganic ion exchangers have been studied for the removal of strontium and cesium ions, due to their high selectivity for specific cations, thermal and radiation stabilities [1 S], In spite of those advantages, they have been limted in their extensive applications because of the pressure drop problem in column operation when they are used in microcrystalline or powdered form. To solve this problem, in the case of synthetic zeolites, clay minerals are often used for pelletization, but it still causes dissolution of clay minerals in aqueous solution. 

The.se will act as Lewis aads. Hie catalytic silc.s occur at high Jensiiy and are uniform in their activity (as opposed to amorphous solids bectiuse of the microcrystalline nature of the zeolites. 

The broad lines obtained for solid-state NMR spectra without applying any line-narrowing improvements are due to the different behaviour of nuclear spin interactions in solids compared to liquids. These interactions are averaged to zero or reduced to the isotropic values in liquids by the fast molecular motions, whereas the fixed (and different) orientations (with respect to the external magnetic field Bq) of the local environments of NMR active isotops in the rigid lattice of a solid cause line broadenings. The recorded broad NMR line patterns are superpositions of resonances from randomly oriented individual nuclei due to a random distribution of different orientations, since zeolitic materials usually are microcrystalline powders. Table 1 summarizes the nuclear spin interactions and their behaviour in liquids versus solids (17). 

Whilst layer lattice clay minerals are finely divided microcrystalline minerals, which together with their variable composition limits their choice as ion exchangers in industrial processes, the naturally occurring zeolites are largely macrocrystalline. As industrial ion exchangers they are preferable and furthermore they are readily synthesized and in this form may possess even better ion exchange capacities than their naturally occurring counterparts. Artificial zeolites, or... 

Microcrystalline feldspathoids and zeolites, both frame-work silicate structures, can precipitate from very alkaline solutions near 20 C at the expense of layer silicates, but true crystalline feldspars do not form at measurable rates in this way. 

Measuring the acidity of the Bronsted and Lewis acid sites is problematic. The adsorption and desorption of various amines has been used, but there is some disagreement about what it means.160 Some workers prefer to use isopropylamine, which desorbs only from Bronsted sites. Solid-state nuclear magnetic resonance (NMR) has also been used in the study of reactions on zeolites.161 Many zeolites crystallize into crystals that are too fine for conventional X-ray analysis. A new method that uses synchrotron X rays on microcrystalline powders promises to make it much easier to determine the structures of zeolites and related materials.162... 

Figure 2 (Left) Commercial (TOSOH) microcrystalline beta zeolite, comprising crystals smaller than 1 pm. (Right) Combined detemplation and Fe-ion exchange.

Sample Preparation Slurries, Solvent Evaporated Samples and Equilibrium Samples. The adsorption of probes on microcrystalline cellulose or native cellulose, on the surface of different pore size silicas or alumina, and on silicalite surfaces (or other zeolites) has to be made in a differentiated manner, according to each s adsorbent characteristics. Adsorption of probes onto these powdered solids can be performed from a solution containing the probe or from a gas phase. 

By the 1980s most of the aluminosilicate zeolites currently used industrially were known, and the emphasis shifted to the study of these materials using a range of powerful new techniques that came of age at this time. These included, in particular, solid state NMR, X-ray and neutron powder diffraction analysis, high resolution electron microscopy and computational methods. All were ideal for the study of structural details of solids that were rarely available, and never used in industrial applications, other than as microcrystalline powders. All these techniques are applicable to the bulk of the solid - this in turn makes up the (internal) surface, which is accessible to adsorbed molecules. Since the techniques are able to operate under any conditions of gas pressure, they may be used to extract structural details in situ under the operating conditions of ion exchange, adsorption and catalysis. In particular, zeolitic systems have proved ideal for the study, understanding and subsequent improvement of solid acid catalysts. 

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