Zeolite X cations

NH3 is similar to H2O in that they both possess large dipole moments and are both small molecules. The presence of NH3 in a zeolite is chemically similar to the presence of H2O in a zeolite. Therefore, the hydrated cation distribution in zeolites is probably more typical of NH3 adsorption in zeolites than the dehydrated cation distribution. According to Breck (18), for hydrated zeolite X, cations are found in sites SI, SI, SII, and SIV. Of these sites, SI, SII, and SIV would all be adsorption lattice solution sites. The cationic and anionic lattice solution sites (in the supercavity of NaX) are illustrated in Figure 8. For NH3, the subscript J1 will refer to SII sites, the subscript J2 will refer to SI sites, and J3 will refer to SIV sites. The anionic sites are two and are (l) in the center U-membered ring of the connecting frame and (2) near the center of the 0(2)—0(1)—0(l) triad of oxygen atoms. For NH3, the subscript il will refer to the first anionic site the subscript i2 will refer to the second anionic site. 

Fig. 4. Model of the ciystal structure of zeolites X, Y, and the mineral faujasite. At the tight is shown the tetrahedral arrangement of tmncated octahedra surrounding one large cavity. On the left the packing model of zeohte X is shown, containing three types of Na cations.

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm 1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu+ and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex. 

Panov, A.G., Larsen, R.G., Totah, N.I., Larsen, S.C. and Grassian, V.H. (2000). Photooxidation of toluene and p-xylene in cation-exchanged zeolites X, Y, ZSM-5, and beta the role of zeolite physicochemical properties in product yield and selectivity. J. Phys. Chem. B 104, 5706-5714... 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

FAU type zeolites exchanged with many different cations (Na, K, Ba, Cu, Ni, Li, Rb, Sr, Cs, etc.) have been extensively studied. The unit cell contents of hydrated FAU type zeolite can be represented as M,j(H20)y [A Sii92 0384] -FAU, where x is the number of A1 atoms per unit cell and M is a monovalent cation (or one-half of a divalent cation, etc.). The number of A1 atoms per cell can vary from 96 to less than 4 (Si/Al ratios of 1 to more than 50). Zeolite X refers to zeolites with between 96 and 77 A1 atoms per cell (Si/Al ratios between 1 and 1.5) and Zeolite Y refers to zeolites with less than 76 A1 atoms per cell (Si/Al ratios higher than 1.5). 

The shape selectivity of zeolites is influenced by the location and distribution of charge-compensating cations. The charge-compensating ions other than protons are all quadrupolar. and Li NMR spectra of dehydrated LiX-1.0 identified three crystallographically distinct sites [221]. In the case NaX with Si/Al ratio of 1.23, six distinct sodium sites were identified using fast Na NMR, DOR and nutation techniques [222]. Na MQMAS has been extensively studied for zeolites X and Y [155]. Other cations like Cs and La in zeolites have also been investigated [155,... 

Table 6.6 Durene/isodurene selectivity as a function of X-zeolite exchanged cation.

Fig. 5 (a) shows the nitrogen adsorption isotherms of aluminum hydroxy pillared clays after heat-treatment at 300-500°C. These are of the typical Langmuir type isotherm for microporous crystals. Fig, 5 (b) shows the water adsorption isotherms on the same Al-hydroxy pillared clays [27]. Unlike the water adsorption isotherms for hydrophilic zeolites, such as zeolites X and A, apparently these isotherms cannot be explained by Langmuir nor BET adsorption equations the water adsorption in the early stages is greatly suppressed, and shows hydrophobicity. Water adsorption isotherms for several microporous crystals [20] are compared with that of the alumina pillared clay in Fig. 6. Zeolites NaX and 4A have very steep Langmuir type adsorption isotherms, while new microporous crystals such as silicalite and AlPO -S having no cations in the...

Zeolites are crystalline aluminosilicates whose primary structure is formed by Si04 and A104 tetrahedra sharing the edges . Their tertiary structure forms uniform channels and cavities of molecular dimensions that are repeated along the zeolite lattice. Due to the lower valence of the aluminium relative to silicon, the excess negative charge (one per A1 atom) is balanced by alkali metal cations, mainly Na". An important class of the zeolite family are the faujasites, known as zeolites X and Y, which have the typical composition for the unit cell as follows ... 

Fig. 18. The different cation sites in the faujasite structure (zeolites X and Y) (25S). The figure has been simplified, but the oxygen ions of the windows are not equivalent (see text).

Barrer showed these hydrogen zeolites, mordenite and chabazite, to be crystalline using x-ray diffraction, and stated, Hydrogen zeolites are effectively crystalline aluminosilicic acids, the salts of which are their diverse cation exchange products." Szymanski, Stamires, and Lynch (13) used simple thermal decomposition of an ammonium zeolite X in an attempt to prepare the hydrogen zeolite... 

One of the most promising methods for controlling the intensity and selectivity of processes is the introduction of various substances into the reaction mixture. Venuto et al. (58) attained a highly selective dehydrogenation of hydrocarbons over cation exchanged zeolite X by conducting the reaction in the presence of NH3. It is also well known that the addition of small amounts of water increases the activity of zeolites for carbo-nium-ion type reactions cracking (59), alkylation (58), isomerization (56,60), disproportionation (60,61,62) and others (56). 

The model has been tested for a wide variety of gas-zeolite combinations. Gases of increasing complexity were considered Ar(non-polar), (quadrupole moment, no dipole moment), NgO (quad-rupole moment, small dipole moment), and NHg (large dipole moment, small quadrupole moment). The zeolites tested were all in the synthetic faujasite family however, they ranged from the cation-rich zeolite X to the cation-poor zeolite Y. Cation geometries considered in the tests were those typical of the dehydrated zeolite form and those typical of the hydrated geometry (associated with NHg adsorption). Two forms of representative cations were considered, Li and Na+. 

Zeolites can be hydrophilic or hydrophobic due to the different Si/Al ratios within the zeolite framework. Organic molecules rely on H-bonding, electrostatic, and 77-cation interactions for effective zeolite absorption, and these interactions will clearly be influenced by the number of cation sites present. As expected, the more Si present, the more hydrophobic the zeolite and, therefore, the greater the ability of these materials to interact with hydrophobic organic molecules or to exclude hydrophilic molecules, such as water. Zeolites X/Y have a Si/Al content at or close to 1 and are highly hydrophilic absorbants. Pentasil zeolite ZSM-5, which... 

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