Structures and Cation Sites

Smyth J. R. and Bish D. L. (1988). Crystal Structures and Cation Sites of the Rock-Forming Minerals Allen and Unwin, Boston. 

Figure 7.12. Structure and cation sites of chabazite (Caiiigaris and Nordin, 1982).

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

This section is devoted to structural considerations and characterization of the more important zeolites with SiOa/AliOa ratios greater than 5, excluding Y-type zeolite covered already by Rudham and Stockwell. In order to consider catalytic properties in depth it is necessary to obtain as much information as possible about the zeolite framework, port sizes, and cation sites. It will be noted that the information available gives a picture that is far from complete. Even for mordenite there are many gaps in our knowledge. 

Figure 3. Schematic representation of H20 and cation sites in the analcite structure...

For the a-Ba2P207 composition, the crystal structure and cation coordination differs from the others. This, plus the larger size of the Ba2+ cation, causes a high crystal field intensity to appear at the activator site, cind results in a major shift in emission wavelengths. 

The Kang-Eyring system interpreting fluorite related anion deficient structures has recently been extended to a wide range of fluorite related structures with vacancies in both anion and cation sites [28, 29]. 

Titanium(ll) oxide is manufactured by heating Ti02 and Ti in vacuo. It is a black solid and a metallic conductor which adopts an NaCl-type structure with one-sixth of both anion and cation sites unoccupied. The oxide is a non-stoichiometric compound with a composition typically in the range TiOo 82 TiOi 23. A commercial use of TiO is in electrochromic systems (see Box 23.4). Conducting properties of the first row metal(II) oxides are compared in Section 28.3. 

Air Separation by AgULSX. Mixed Li, Ag-LSX with different contents of Ag were prepared and characterized by Hutson and Yang (2000a). These samples were heat-treated in vacuo at various temperatures and their structures (including cation sites) were determined with powder neutron diffraction. The cation sites are summarized in Table 10.3. Structural characterization revealed the presence of cations at site II in mixed Li,Ag-LSX zeolites that were vacuum-dehydrated at 450 °C. Cations in this site II are more interactive with the atmospheric sorbates of interest than silver at the conventional site II location. Vacuum dehydration at 450 °C induced thermal migration of Ag+ from site II to site II. This is clearly seen in Table 10.3. Furthermore, the number of Ag cations at site II per unit... 

Bemuy-Lopez C, Allix M, Bridges CA, Claridge JB, Rosseinsky MJ (2007) SroMgMoOe d structure, phase stability, and cation site order control of reduction. Chem Mater 19 1035-1043... 

There are several points to note about the GaAs (100) surface. First, the surface reconstructs into dimer rows as does silicon (100) 2x1. Flowever the arrangement of these dimers is more complex to account for the difference in chemistry of the anion and cation sites and to get the electron pairing that drives reconstruction correct. The structure removes half of the top layer of atoms and some of the second layer atoms to expose a third layer of the same chemistry as the top layer. These exposed atoms... 

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