Zeolite cations

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

In a later study, Wang and Lunsford (263) used the alkaline-earth zeolites to determine whether any systematic change in the crystal field effect on the adsorbed Oj species could be detected as the cation is varied from Mg2 + to Ba2 +. The results show that three or more different adsorption sites are present on each of the cationic zeolites and that there is no significant trend in the energy splitting of the nt levels of the Oj ion as one goes from Mg2 + to Ba2+. 

The templating theory is based on a stereospecificity which cannot be separated from the chemistry of the cation. Zeolites are crystallized in alkaline solutions, most readily at a pH greater than 11, limiting the cations used in zeolite synthesis to alkali, some alkaline earths, and organic cations... 

The ability of water molecules to promote a reaction depends on many factors. In most cases, zeolites with monovalent cations have low activity. However, the addition of water molecules to X and Y zeolites with monovalent ions increased the isomerization of cyclopropane (63). De-cationized zeolites can be promoted readily with water, and the process is reversible (2, 60, 64). It was shown (2) that the promoting ability of water molecules in faujasites is less when the Si02/Al203 increases. Dealu-minated faujasites are even more difficult to promote. For erionite and mordenite the maximum effect of water was observed only after treatment with liquid water and subsequent heating (2). The effect of water on zeolites saturated with polyvalent cations is less pronounced (65, 66, 67). However, the presence of multivalent cations stabilizes the catalytic activity. Water and alcohols were reported to promote ion exchanged zeolites for n-pentane isomerization (68) and n-hexadecane hydrocracking (69). 

Introducing 3.7 to 13.2 La3+ ions/unit cell in the Na-8.7 zeolites or 4 La3+ ions/unit cell in the D.Na-5.4 zeolite raises the thermal stability substantially. The lanthanum samples have the same activity after a 900°C pretreatment than after a 550°C pretreatment whereas the Na-8.7 material starts to lose its activity at 700° C. This high stability of polyvalent cationic zeolites has been reported previously in few studies but at temperatures lower than in our work (29, 80,31). 

Specific adsorbents with positive surface charges. Acidic hydroxyl groups (hydroxylated acid oxides such as silica), aprotic acid centers, or small radius cations (zeolites) on the surface. Adsorbents of this type will interact with molecules which have locally concentrated electron densities, that is. Group B and Group D molecules. 

A number of simple and inexpensive materials catalytically promote the cobalt-carbonylation (Reaction 2) in aqueous solution. These include ion-exchange resins, zeolites, or special types of activated carbon. Formation of the active catalyst in a separate reactor is thus economically feasible. The mechanism of this catalysis has not yet been elucidated and seems to differ for each promoter mentioned. After an induction period during which the cobalt fed to the reactor is partially retained by the promoter, fully active materials have absorbed cobalt carbonyl anion Co(CO)4 (ion exchange resins), Co2+ cation (zeolites), or a mixture of Co2+, cobalt carbonyl hydride, and cluster-type cobalt carbonyls (activated carbon). This can be shown by analytical studies (extraction, titration, and IR studies) of active material withdrawn from the reactor. 

Classifi- cation Zeolite N Free aperture of channels (A) Classifi- cation Zeolite N" Free aperture of channels (A)... 

The Al-rich (cationic) zeolites have highly polar internal surfaces. The polarity increases with increasing cation charge and decreasing cation size. However, the relationship between the nature of the cation and the surface properties is complex because the differences in cation location (sites) must also be considered. 

Sherry, in the last few years, has been generalizing all knowledge about selectivity during ion exchange in zeolites this author has summarized the selectivity rules as follows [21] every zeolite has preference for Na+ instead of Li+ and NH4 instead of Na+ zeolites with a low Si/Al ratio have preference for Ca2+ and zeolites with a high Si/Al ratio have preference for alkaline cations zeolites are selective for polarizable cations and the electroselectivity, molecular sieving, and space limitations rules are valid. 

Table 2.—Adsorption of carbon monoxide on several bivalent-cation zeolites, SAMPLE compositions, LANGMUIR COMBINING CONSTANTS K AND NUMBERS o OF EFFECTIVE SITES, NON-SPECIFIC ADSORPTION COEFFICIENTS C, AND RELATIVE MOLECULAR OPTICAL DENSITIES Dq...

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. 

An increase in the values of retention volumes of compounds on de-cationized zeolites may be explained both by a stronger dehydration of the remaining sodium cations and by a possible formation of active three-coordinate aluminum groups. 

Alfred E. Hirschler (Sun Oil Co., Marcus Hook, Pa. 19061) I would like to point out two significant omissions in Rabo s review dealing with Bronsted sites in zeolites. The first substantive evidence for the presence of Bronsted sites in cationic zeolites was our 1963 paper (/. Catalysis 1963, 2, 428), which showed that arylmethanol indicators generate car-bonium ions on contact with CaX and other cationic zeolites. The cited work of Norton did not establish the presence of Bronsted acidity for two reasons. He used Ho indicators which give acid colors with Lewis as... 

It should also be noted that the decrease in the micropore volume induced by the grafting of the cationic zeolites is half that observed for the corresponding protonic zeolites. This result provides argument for the involvement of framework hydroxyl groups in the anchoring reaction. 

Because cation zeolites such as Na-X can adsorb a large amount of alkali metals to form metal clusters, a large number of extra electrons in unit volume can be produced in the compounds. When the concentration of these extra electrons reaches a certain value, it is possible for the metal-cluster-containing host-guest material to undergo transition from an insulator to a conductor. In particular, when there are crystallographically onedimensional channels, the formed one-dimensional metal clusters may become quantum wires (Figure 9.7). lx 211... 

---