Zeolites X-ray diffraction

For calcium zeolites, x-ray diffraction and infrared studies together have shown that the first 16 or 18 calcium ions introduced into the unit cell of the sodium form are located in the hexagonal prisms and sodalite portions of the structure 1, 2, 23). Addition of further calcium ions results in the occupation of sites in the supercages. The changes in intensity of the hydroxyl infrared bands and the onset of the cation-pyridine interactions indicate that the same preferential location of calcium ions in calcium hydrogen Y zeolite occurs (23). 

The first analytical tool to assess tire quality of a zeolite is powder x-ray diffraction. A collection of simulated powder XRD patterns of zeolites and some disordered intergrowths togetlier witli crystallographic data is available from tlie IZA [4o]. Phase purity and x-ray crystallinity, which is arbitrarily defined as tlie ratio of tlie intensity of... 

Karge FI G, Flunger M and Beyer FI K 1999 Characterization of zeolites—infrared and nuclear magnetic resonance spectroscopy and x-ray diffraction Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L... 

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

On the other hand, in.the case of the nonionic surfactants C-15, NP-15 and 0-15 (the nonionic surfactant/cyclohexane system), mono-dispersed silicalite nanocrystals were obtained as shown in Fig. 1(c), 1(d) and 1(e), respectively. The X-ray diffraction patterns of the samples showed peaks corresponding to pentasile-type zeolite. The average size of the silicalite nanocrystals was approximately 120 nm. These results indicated that the ionicity of the hydrophilic groups in the surfactant molecules played an important role in the formation and crystallization processes of the silicalite nanocrystals. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

These results are consistent with XRD (X-ray diffraction) results. The amounts of K-feldspar, K-mica and chlorite are higher in the altered rocks closer to the veins and Ca-zeolites and smectite decrease in amounts towards periphery of the alteration zones. 

For the crystalline materials, high resolution X-ray diffraction experiment is a powerful tool to derive accurate electron density even for large systems like zeolites. In this study, we are interested in the experimental electron density distribution in the scolecite CaAl2Si3O10 3H20 in order to make comparison with its sodium analogue natrolite Na2Al2Si3Oi0 2H20 for which the electron density has been reported recently [1,2],... 

Ghermani, N.E., Lecomte, C. and Dusausoy, Y. (1996) Electrostatic properties in zeolite-type materials from high-resolution X-ray diffraction the case of natrolite, Phys. Rev. B, 53, 5231-5239. 

Zeolite structures sometimes remain unsolved for a long time, because of either their complexity, the minute size of the crystallites or the presence of defects or impurities. One extreme example of stacking disorder is provided by zeolite beta [1,2], Different stacking sequences give rise to two polymorphs (A and B) in zeolite beta that always coexist in very small domains in the same crystal. Not only do the small domains make the peaks in the powder X-ray diffraction pattern broad and thereby exacerbate the reflection overlap problem, but the presence of stacking faults also gives rise to other features in the diffraction pattern that further complicate structure solution. 

Although its main use is still the identification of crystalline phases, X-ray diffraction is also the most used technique for the determination of the location of extraframework cations. XRD is well suited to perform structural characterisation of dehydrated zeolites since the framework is highly crystallised and the extraframework cations are often heavy elements. 

Keywords solid-state ion exchange, high-temperature X-ray diffraction, zeolite Cd-Y. 

The existence of small quantities of zeolite in Y-AI2O3 was verified by X-Ray diffraction with the K ray of cobalt, especially for the samples with the smallest contents of zeolite. 

The chemical compositions of the samples were obtained by ICP in a Varian 715-ES ICP-Optical Emission Spectrometer. Powder X-ray diffraction was performed in a Philips X pert diffractometer using monochromatized CuKa. The crystallinity of the zeolites was obtained from the intensity of the most intense reflection at 23° 20 considering the parent HZ5 sample as 100% crystalline. Textural properties were obtained by nitrogen physisorption at -196°C in a Micromeritics ASAP 2000 equipment. Surface areas were calculated by the B.E.T. approach and the micropore volumes were derived from the corresponding /-plots. Prior to the adsorption measurements the samples were degassed at 400°C and vacuum overnight. 

Spackman, M.A. and Weber, H.-P. (1988). Electrostatic potential in dehydrated sodium zeolite A from low resolution X-ray diffraction data. J. Phys. Chem. 92, 794-796... 

X-ray diffraction measurements indicated that the zeolite rock consisted primarily of clinoptilolite (60-70%), volcanic glass (10%), feldspar (10%) and minor quantities of cristobalite, quartz and plagioclase (20%). Fig. 1 represents the XRD pattern of ODA-Clinoptilolite-rich tuff used for arsenate or chromate removal from aqueous solutions. 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

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