Zeolite 52 state

POLLUTION ABATEMENT USING ZEOLITES STATE OF THE ART AND FURTHER NEEDS... 

From the comparison of Table 1 and Table 3 it can be seen that the chemical shifts of the end-methyl groups are greatly influenced by the occlusion of the ions in the zeolitic channels. Indeed, a variation of 2-3 ppm to high field occurs for the -CH3 groups while going from the gel to the zeolitic state. In addition, the line width of the carbon NMR lines of the ions in the gel phase are narrower, showing a higher rotational mobility of these tetraalkylammonium ions. 

Engelhardt G and Gunter 1987 High-Resolution Solid-State NMR of Silicates and Zeolites (Chichester Wiley)... 

Since zeolites are metastable crystallization products tliey are subject to Ostwald s mle which states tliat metastable phases are initially foniied and gradually transfonii into tlie tlieniiodynaniically most stable product. The least stable zeolitic phase (tliat witli tlie lowest framework density) is tlierefore foniied first and consumed with furtlier syntliesis time at tlie expense of a more stable phase due to a continuous crystallization/redissolution equilibrium. 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

J. S. Falcone, Jr., andJ. G. Blumberg, in O. Hutzinger, ed.. Handbook of Environmental Chemisty, Vol. 3, Pt. F, Springer-Vedag, Berlin, 1992. G. EngeUiardt and D. Michel, High Resolution Solid State NMR of Silicates and Zeolites, Chichester, John Wiley Sons, Ltd., 1987. 

J. Weitkamp and co-woikeis, eds., Zeolite and Telated Microprous Materials, State of the Art 1994, Studies in Suface Science and Catalysis, Vol. 84, Elseviei Science Publishing Co., Inc., New Yoik, 1994, pp. 653—659. 

Volume 84 Zeolites and Related Microporous Materials State of the Art 1994. 

Examples of the unique insights obtained by solid state NMR applications to materials science include the Si/Al distribution in zeolites, the hydrogen microstructure in amorphous films of hydrogenated silicon, and the mechanism for the zeolite-catalyzed oligomerization of olefins. ... 

In a recent paper Pijpers et al. [2.42] have reviewed the application of XPS in the field of catalysis and polymers. Other recent applications of XPS to catalytic problems deal with the selective catalytic reduction of using Pt- and Co-loaded zeolites. Although the Al 2p line (Al from zeolite) and Pt 4/ line interfere strongly, the two oxidation states Pt and Pt " can be distinguished after careful curve-fitting [2.43]. 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Zeolite-supported Co catalyst was synthesized by solid-state ion exchange using the procedure described by Kucherov and Slinkin[16, 17], CoO... 

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 ... 

As stated above, a typical zeolite consists of silicon and aluminum atoms that are tetrahedrally joined by four oxygen atoms. Silicon is in a +4 oxidation state therefore, a tetrahedron containing silicon is neutral in charge. In contrast, aluminum is in a +3 oxidation state. This indicates that each tetrahedron containing aluminum has a net charge of -1, which must be balanced by a positive ion. 

At this state of the catalyst synthesis there are two approaches for further treamient of NaY. Depending on the particular catalyst and the catalyst supplier, further treatment (rare earth exchanged) of NaY can be accomplished either before or after its incorporation into the matrix. Post-treatment of the NaY zeolite is simpler, but may reduce ion exchange efficiency. 

Zeolites are crystalline alumina-silicates having a regular pore structure. Their basic building blocks are silica and alumina tetrahedra. Each tetrahedron consists of silicon or aluminum atoms at the center of the tetrahedron with oxygen atoms at the comers. Because silicon and aluminum are in a +4 and +3 oxidation state, respectively, a net charge of -1 must be balanced by a cation to maintain electrical neutrality. 

Vanadium in the feed poisons the FCC catalyst when it is deposited on the catalyst as coke by vanadyl porphydrine in the feed. During regeneration, this coke is burned off and vanadium is oxidized to a oxidation state. The vanadium oxide (V O ) reacts with water vapor in the regenerator to vanadic acid, HjVO. Vanadic acid is mobile and it destroys zeolite crystal through acid-catalyzed hydrolysis. Vanadic acid formation is related to the steam and oxygen concentration in the regenerator. 

As with 16-38, these reactions have sometimes been performed with acid catalysts. Ultrasound has been used to promote the reaction, and it has also been done without solvent using microwave irradiation. Another solid-state variation is done on moist LiBr. Zeolites have been used to promote the... 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

The presence of the zeolite cavity dramatically lowers the activation energy for the protonation of toluene. It is mainly due to screening of the charges in the transition state due to the polarizable lattice oxygen atoms. In the transition state, a positive charge develops on protonated toluene. 

As long as there are no important steric contributions to the transition-state energies, the elementary rate constant of Eq. (1.22) does not sensitively depend on the detailed shape of the zeolite cavity. Then the dominant contribution is due to the coverage dependent term 9. 

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