Cations Chabazite

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. 

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... 

This facile loss of water was shown to be reversible by Damonr (1840) and, in 1858, Eichom showed that the zeohte chabazite contained aUcah and alkahne earth metals, which were capable of being reversibly replaced, that is, the zeolite exhibited cation-exchange properties. Analysis of zeohte minerals showed them to be aluminosilicates and their easy loss of water and cation exchange was evidence for the open nature of their structures, often likened to a sponge. The description zeohtic water has been widely used to describe loosely held water in any sohd. 

Structure but with chains joined together in a different way, as shown in Fignre 9. The differences can be seen to be variations in the relative spatial orientations of the linked tetrahedra described as np (U) and down (D), and adjacent tetrahedra are arranged as two pointing up and two down (UUDD). Note that, again, restricting S8R units have been created (Table 5), but of different diameters to those seen in chabazite. The cations reside close to the S8R positions. 

The conversion of P to chabazite proceeds at a superior rate compared with the conventional synthesis from an alumia-silicate hydrogel. Our ESCA studies have shown that a superficial cation exchange can take place even in the eibsence of subsequent conversion. Shielded organocations like tetrapropyl ammonium give poorer surface ion-exchange. Ttiis may be related to the slower rates of organozeolite synthesis observed in this study. Tlie ion-exchange which occurs on the surface may be a preliminary step in the conversion. 

Increasing attention has been given to the structures of zeolitic crystals, not only on account of their practical value as selective sorbents, but also because of the remarkable pore systems which have been revealed. As a result considerable new information exists about the anionic frameworks, although the disposition of the relatively mobile intracrystalline water and cations is intrinsically more difficult to determine. Four structures which have certain related features, and which are of interest as molecular sieves, are those of chabazite, gmelinite, levynite and erionite, for which hexagonal unit cells may be given as follows ... 

Two additional conditions must be satisfied for these dimensions rigorously to determine molecular sieve action. First, when water is removed by heat and evacuation, the rings must keep their stereochemical configuration and secondly, cations must not be so located as to block these rings. The first condition is approximately fulfilled the latter is not in all cases. Thus, in aluminous, and therefore cation-rich, synthetic near-chabazites, sodium ions are so numerous, and so placed, as to prevent molecule diffusion, except of small polar molecules like water. 3... 

E for tracer diffusion of monovalent cations in chabazite (3) does not seem to be very different from E for water in the same zeolite ... 

Heats of immersion in water have been determined for a number of outgassed porous crystals enriched by ion exchange in various cations (zeolites X, Y, A, chabazite, and synthetic ferrierite), and for clinoptilolite and mordenite in their Na-forms, decationated, and in various stages of de-alumination. Finally, heats of immersion were determined in NaX, NaY, NaA, and (Ca,Na) chabazite in which the crystals initially contained various known loadings of zeolitic water. From the results, the influence of the exchange cations upon integal heats of sorption of water, AH, and other derived heats have been evaluated and discussed. 

Cation Form Zeolite X Zeolite Y Zeolite A Chabazite Ferrierite... 

Of the cation forms of chabazite, Cs-chabazite contains about 20% of residual Ca while the other modifications should contain less than 10% of this ion (8). In zeolite X, the Rb, Cs, Mg, and Ba forms contain about 30, 30, 27, and 23 residual Na" ions per unit cell (14) out of a total in our... 

Relations between Heats and Cation Radii. For any cation—water pair, the ion-dipole energy should vary inversely as the square of the center-to-center distance between ion and water molecule—i.e., as (+ 1.40)", where Vc is the cation radius in A and 1.40A is taken as the radius of water. Accordingly, correlations might be sought between qn, Qi, or Quo and (r + 1.40)" or Tc. Such plots proved qualitatively the same whether (fc + 1.40)" or was the abcissa. Figures 4 and 5 show several of the observed relations with for zeolites X and Y and for chabazite. 

As an illustration of the current state of the art for electronic spectroscopy of transition metal ions in zeolites, refer to the recent review by Schoonheydt of Cu2+ in different zeolites [56]. Schoonheydt shows that experimental measurement of diffuse reflectance spectra (and in the case of Cu2 + EPR spectra) must be combined with theoretical calculations if a complete interpretation is to be made. The exact frequencies of the d-d transitions in the electronic spectrum of Cu2+ are independent of the zeolite structure type, the Si Al ratio, and the co-exchanged cations, but depend solely on the local coordination environment. Figure 20 shows the diffuse reflectance spectrum of dehydrated Cu-chabazite the expanded portion reveals the three d-d transitions in the region around 15000 cm l. 

A computational exploration of cation locations in high- silica Ca-Chabazite... 

The location of extra framework cations is a major problem in characterising zeolites. Simulation is becoming an increasingly powerful tool for the exploration and rationalisation of cation positions, since it not only allows atomic level models to be compared to bulk experimental behaviour, but can also make predictions about the behaviour of systems not readily accessible to experimental probing. In the first part of this work we use the Mott-Littleton method in conjunction with empirical potential energy functions to predict and explore the locations of calcium cations in chabazite. Subsequently, we have used periodic non-local density functional calculations to validate these results for some cases. 

The primary objective of this study was to look at the locations of calcium cations in high silica chabazite. In the limiting case, a calcium ion associated with a pair of aluminium atoms in the framework will not be influenced by any other aluminium. In a slightly more realistic case, the influence of the other aluminium atoms can be regarded, to a first approximation, as a uniform isotropic background that will not affect the relative energies of the different calcium sites. A natural approach to this situation, is to model the chabazite initially as being purely siliceous, then introduce just two aluminium atoms and a calcium cation as a defect. 

For the high-silica chabazite, the barriers to migration were studied and, as expected, diffusion of cations can be ruled out for anhydrous Ca-chabazite at reasonable temperatures. 

Cation Exchange in Mordenite. Until 1974 there had been few systematic studies of ion exchange in mordenite and only Rees had determined isotherms. Peculiarities were found which had not been observed in A, X, and Y zeolites or in chabazite, in that only limited exchange of divalent ions could be achieved. Using a natural mordenite from Harbourville, Nova Scotia, Rees found for... 

The first natural microporous aluminosilicate, i.e., natural zeolite, was discovered more than 200 years ago, and after long-term practical applications, the intrinsic properties of natural zeolites such as reversible water-adsorption capacity were fully recognized.13 41 By the end of the 19th century, during exploitation of ion-exchange capacity of some soils, it was found that natural zeolites exhibited similar properties some cations in natural zeolites could be ion-exchanged by other metal cations. Meanwhile, natural chabazite could adsorb water, methanol, ethanol, and formic acid vapor, but could hardly adsorb acetone, diethyl ether, or benzene. Soon afterwards, scientists began to realize the importance of such features, and use these materials as adsorbents and desiccants. Later, natural zeolites were also used widely in the field of separation and purification of air. 

---