Zeolite chabazite

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

Regli L, Zecchina A, Vitillo J G, Cocina D, Spoto G, Lamberti C, Lillerud K P, Olsbye U and Bordiga S (2005), Hydrogen storage in chabazite zeolite frameworks , Phys Chem Chem Phys, 7, 3197. 

D. T. Bostick, W. D. Arnold, Jr., P. A. Taylor, D. R. McTaggart, M. W. Burgess, and B. Guo, Evaluation of Improved Techniques for the Removal of Sr and Cs from Process Wastewater and Groundwater Chabazite Zeolite Baseline Study, Report ORNL/TM-12903, Oak Ridge National Laboratory, Oak Ridge, TN, April 1995. 

Zeolites occur widely in nature and have been known for more than 250 years as aluminosilicate minerals, i.e., faujasite, mordenite, offretite, ferri-erite, chemierionite, and chabazite. Zeolites are inorganic crystalline solids with small pores (1-20 A diameter) running throughout the solid. Zeolites... 

Xu S, Zheng A, Wei Y, et ah Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites, Angew Chem Int Ed 52 11564-11568, 2013. 

M. Calligaris, A. Mezzetti, G. Nardin and L. Randaccio. Cation sites and framework deformations in dehydrated chabazites. Crystal structure of a fully silver-exchanged chabazite. Zeolites 4, 1984, 323-328. 

Zeolites are tire product of a hydrotliennal conversion process [28]. As such tliey can be found in sedimentary deposits especially in areas tliat show signs of fonner volcanic activity. There are about 40 naturally occurring zeolite types. Types such as chabazite, clinoptilolite, mordenite and phillipsite occur witli up to 80% phase purity in quite large... 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Size limit for Ca-rich chabazite, Linde sieve 5 A, Ba-zeolite and gmelinite about here ( 0.49 nm)... 

The low silica zeolites represented by zeolites A and X are aluminum-saturated, have the highest cation concentration and give optimum adsorption properties in terms of capacity, pore size and three-dimensional channel systems. They represent highly heterogeneous surfaces with a strongly hydrophilic surface selectivity. The intermediate Si/Al zeolites (Si/Al of 2-5) consist of the natural zeohtes eri-onite, chabazite, clinoptilolite and mordenite, and the synthetic zeolites Y, mordenite, omega and L. These materials are still hydrophilic in this Si/Al range. 

Barter, R.M. (1948) Synthesis of a zeolitic mineral with chabazite-like sorptive properties. /. Chem. Soc., 127 Barter, R.M. and Riley, D.W. (1948) Sorptive and molecular sieve properties of a new zeolitic mineral. /. Chem. Soc., 133. 

Commercially significant zeolites include the synthetic zeolites type A (LTA), X (FAU), Y (FAU), L (LTL), mordenite (MOR), ZSM-5 (MFI), beta ( BEA/BEC), MCM-22 (MTW), zeolites E (EDI) andW (MER) and the natural zeolites mordenite (MOR), chabazite (CHA), erionite (ERl) and clinoptiloUte (HEU). Details of the structures of some of these are given in this section. Tables in each section lists the type material (the common name for the material for which the three letter code was established), the chemical formula representative of the unit cell contents for the type material, the space group and lattice parameters, the pore structure and known mineral and synthetic forms. 

Barrer, R.M. (1948) Synthesis of a zeolitic mineral with chabazite-like sorptive properties. J. Chem. Soc., 127-132. 

Woodcock, D.A. and Lightfoot, P. (1999) Negative thermal expansion in the siliceous zeolites chabazite and ITQ-4 a neutron power diffraction smdy. Chem. Mater., 11, 2508-2514. 

A new transition-state-searching algorithm was used to determine the mechanism for methanol condensation to form dimethyl ether within the microporous environment of the zeolite, chabazite, using periodic boundary conditions and density functional theory. An acid site in the zeolite produces MeOH2+ for nucleophilic attack by a second adsorbed MeOH molecule. 

This hypothesis does not agree with the HRMAS Si-NMR results of Bodart et al. [3], who found that the Al atoms in chabazite are distributed statistically. However, the low resolution of the Si-NMR spectra and the superimposition of different Si(nAl) lines for zeolites of the chabasite group with two independent T sites make these results uncertain. 

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