Zeolite anions

From Table V, the zeolite anion is more electronegative than F. Also shown in Table V are the electron occupancies in the 5s orbital of the Ag+ that is bonded to the anion. For a perfect anion, the Ag+ to which it is bonded should have an empty 5s orbital. Again, it is seen that Ag+ in AgZ has the lowest occupancy in its 5s orbital, indicating that the Z (i.e., zeolite framework anion) is the most electronegative anion. 

Relative Electronegativities of Zeolite Anion and Halides—Comparison of Anion Net Charges Calculated by Molecular Orbital Theory using B3LYP/LANL2DZ with NBO Method ... 

N2 and O2 adsorption capacities for Li faujasite, 103 naturally occurring, 97-98 pore size distribution, 89 relative electronegativities of zeolite anion and halides, 101... 

Table 2. Summary of energies of adsorption for thiophene and benzene in kcal/mol calculated from molecular orbital theory (Z denotes Zeolite anion)... 

Table 7.2. Relative electronegativities of zeolite anion and haiides. Comparison of anion net charges caicuiated by molecuiar Orbitai Theory...

Z denotes zeolite anion using the cluster model shown in Figure 8.3 From Yang et al., 2002.)... 

It is well suited for the preparation of metal clusters which are difficult to prepare by ion exchange and reduction, e.g., when cations are difficult to reduce because of their low redox potential or because they tend to be highly stabilized by zeolite anionic frameworks (e.g., in SI or SI sites of faujasites). There are many reports on the preparation of iron clusters in Y zeolites from Fe(CO)5 [42,45-51], Fe2(CO)g and Fe3(CO)i2 [52] precursors. Cobalt clusters were prepared by adsorption and thermal decomposition of CofCOljNO [42, 53, 54] and Co2(CO)g [55-57]. Co2(CO)g was decomposed by microwave discharges allowing better control of the decomposition than is possible with thermal treatments [55]. 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

The four oxygen anions in the tetrahedron are balanced by the -i-4 oxidation state of the silicon cation, while the four oxygen anions connecting the aluminum cation are not balanced. This results in -1 net charge, which should be balanced. Metal cations such as Na", Mg ", or protons (H" ) balance the charge of the alumina tetrahedra. A two-dimensional representation of an H-zeolite tetrahedra is shown ... 

Among the three-dimensional silicates are the zeolites, which contain cavities or tunnels in which Na+ or Caz+ ions may be trapped. Synthetic zeolites with made-to-order holes are used in home water softeners. When hard water containing Ca2+ ions flows through a zeolite column, an exchange reaction occurs. If we represent the formula of die zeolite as NaZ, where Z represents a complex, three-dimensional anion, the water-softening reaction can be represented by the equation... 

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

The Tg-based octa-anion TglO lg can be seen as a model for the well-known D4R found in inorganic structures such as zeolite A. Synthesis of TglO Jg can be achieved more readily than many other POSS species and it can be obtained in quantitative yield from the reaction of a tetra-alkoxysilane with H2O (10 equiv./Si) in the presence of Me4NOH (1 equiv./Si) in methanol at room temperature for 1 day (Figure Alcoholysis of T8[OSiMe2H]g causes terminal Si-O bond... 

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

Pillared clays (MELS) are aLso covered in this review. MELS have three-dimensional network structure like zeolites, and, unlike clays, which have two-dimensional layered structures, pillared cationic and anionic clays have been studied. Phthalocyanins intercalated in anionic clays have given interesting results for wastewater purification (Vaccari, 1998). 

Zeolites are structurally related to colorless sodalite, Na4Cl[Al3Si3012], and to deeply colored ultramarines. These have aluminosilicate frameworks that enclose cations but no water molecules (Fig. 16.25). Their special feature is the additional presence of anions in the hollows, e.g. Cl-, S()4, S2, or S. The two last-mentioned species are colored radical ions (green and blue, respectively) that are responsible for the brilliant colors. The best-known representative is the blue mineral lapis lazuli, Na4S (.[Al3Si3012], which is also produced industrially and serves as color pigment. 

Due to blockage of the main zeolite channels by anions present in the structure, few applications have been reported for these tectosilicates. However, the presence of carbonate anions could make them suitable as antacids. Our group has worked with a series of natural minerals as antacids, such as clays [4], modified carbons [5] and zeolites[6]. In this way, a carbonated cancrinite and its intermediate phase were tested as an antacid with a synthetic gastric juice. 

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