Morphology, zeolites

Zeolite crystals can be grown in sizes ranging from 0.5 pm to several hundreds pm and often have a characteristic morphology. Thus type A zeolites are cubes,... 

This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported. 

A novel zeolite material possessing an inherent hierarchical structure with good mechanical and chemical strength has been prepared by the LbL assembly of zeolite nanocrystals and PDDA on the diatomite substrates [129]. The diatomite used has a disk-like morphology (Figure 7.12A) and exhibits abundant and uniform macropores (about 300-500 nm) in the diatomite plates (Figure 7.12B). The zeolite-diatomite (ZD)... 

The synthesis procedure led to a highly crystalline MCM-22 zeolite, as indicated by XRD data (Fig. 1A, a). The introduction of platinum by ion exchange procedure does not modify the crystallinity of the zeolite (Fig. 1A, b). SEM micrographs show that the sample obtained in static hydrothermal conditions is characterized by the presence of homogeneous round-shaped particles of ca. 10 pm in diameter formed by aggregation of lamellar particles [7,8], The morphology of the zeolite is not modified after platinum introduction (Fig. IB). 

Phase purity and crystallinity of the obtained zeolites were characterized using XRD (Siemens 5005). The morphology and crystal size were determined using SEM (Philips... 

Irrespectively of the iron content, the applied synthesis procedure yielded highly crystalline microporous products i.e. the Fe-ZSM-22 zeolite. No contamination with other microporous phases or unreacted amorphous material was detected. The SEM analysis revealed that size and morphology of the crystals depended on the Si/Fe ratio. The ZSM-22 samples poor in Fe (Si/Fe=150) consisted of rice-like isolated crystals up to 5 p. On the other hand the preparation with a high iron content (Fe=27, 36) consisted of agglomerates of very small (<0.5 p) poorly defined crystals. The incorporation of Fe3+ into the framework positions was confirmed by XRD - an increase of the unit cell parameters with the increase in the number of the Fe atoms introduced into the framework was observed, and by IR - the Si-OH-Fe band at 3620 cm 1 appeared in the spectra of activated Fe-TON samples. 

Crystal habit (or morphology) is an important property for the industrial use of zeolites. Manipulating habit can be used to tailor the number of pores exposed to the surface of a crystal, as well as the length of pore channels. The shape of a crystal can vary because the different faces grow at different rates relative to one another. 

During the last decade large progresses have been performed in the so much difficult art of zeolites synthesis. As a consequence, the amounts of structural defects and chemical impurities have been reduced in zeolite samples (crystallites of larger sizes and well-defined morphology have been synthesized ). At the same time, the zeolite sorption capacities increase. Such an observation is well illustrated by the sorption... 

Scanning electron microscopy indicated that the zeolites crystals are homogeneously dispersed in the surface and the core of the composites. Figure 2 presents micrographs of cross-sections of the chitosan-zeolite spheres and shows that the morphology of the zeolite crystals has not been affected by the gelling of chitosan. 

The calcination of the composites eliminates the totality of the organic fraction but let unaltered the overall morphology of the material (Fig. 4 (b)) confirming that natural hydrocolloids are effective binders for the shaping of zeolites. 

One difficulty with many synthetic preparations of semiconductor NCs that complicates any interpretation of NMR results is the inevitable distribution of sizes (and exact shapes or surface morphologies). Therefore attempts to make semiconductors as a sort of molecular cluster having a well-defined stoichiometry are of interest to learn potentially about size-dependent NMR parameters and other properties. One approach is to confine the semiconductor inside a template, for instance the cuboctahedral cages of the sodalite framework or other zeolite structures, which have been characterized by multinuclear NMR methods [345-347], including the mesoporous channel material MCM-41 [341, 348]. 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

The water and A1 contents decrease while the amount of M cations per unit cell of zeolite increases. For Li and Na,the morphology consists of clusters of polycrystalline aggregates. Better outlined single crystals are observed for K and Rb and additional pronounced twinning appears for (Cs)ZSM-5 (Figure 10). 

Figure 10. SEM micrographs comparing sizes and morphologies of the various (M)ZSM-5 zeolites (Reproduced with permission, from ref (26) Elsevier, Sci. Publ. Co.).

Dispersion of POMs onto inert solid supports with high surface areas is very important for catalytic application because the surface areas of unsupported POMs are usually very low (—10 m2g). Another advantage of dispersion of POMs onto inert supports is improvement of the stability. Therefore, immobilization of POMs on a number of supports has been extensively studied. Silica and active carbon are the representative supports [25], Basic supports such as MgO tend to decompose POMs [101-104], Certain kinds of active carbons firmly entrap POMs [105,106], The maximum loading level of POMs on active carbons is 14 wt% [107], Dispersion of POMs onto other supports such as zeolites, mesoporous molecular sieves, and apatites, is of considerable interest because of their high surface areas, unique pore systems, and possibility to modify their compositions, morphologies, and sorption properties. However, a simple impregnation of POM compounds on inert supports often results in leaching of POMs. 

Figure 1.5. Schematic view of some channels in a hexagonal zeolite crystal with cylinder morphology.

Figure 1.11. Schematic view of a zeolite L crystal loaded with type 1 (Fig. 1.8) dyes with electronic transition moments aligned along the axis of the channels. Left Side view of the morphology, size, and optical anisotropy of the material. Right Front view of a few individual dye-filled channels. The polarization of absorbed and emitted light is indicated.

We reported the preparation of sophisticated bipolar three-dye photonic antenna materials for light harvesting and transport [22]. The principle is illustrated in Figure 1.12. Zeolite L microcrystals of cylinder morphology are used as host for organizing several thousand dyes as monomers into well-defined zones. 

Given the complex nature of the crystal structure and small crystal size with an anisotropic morphology of UZM-5, the normal X-ray diffraction patterns were not sufficient to deduce an unambiguous structure. Thus a multi-technique approach was required to successfully solve the structure, to explain the adsorption properties and by analogy to the structure of other zeolites in order to assess potential applications. 

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