Ion exchange, zeolites for

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). 

As described above, the catalytic activity of metal ion-exchanged zeolites for aniline formation has a good correlation with electronegativity and with the formation constant of ammine complexes of metal cations. The order of the activity agrees with the Irving-Williams order. These facts give irrefutable evidence that the transition metal cations are the active centers of the reaction. 

Ion exchange zeolites are builders in washing powder, where they have gradually replaced phosphates to bind calcium. Calcium and, to a lesser extent, magnesium in water are exchanged for sodium in zeolite A. This is the largest application of zeolites today. Zeolites are essentially nontoxic, and pose no... 

Alkali metal ion-exchanged zeolites and occluded alkali metal oxide zeolites have been investigated extensively and applied as basic catalysts for a variety of organic transformations (1,41,221,222). Zeolites modified with alkaline earth compounds have been applied much less frequently as base catalysts for organic reactions. 

Copper Ion-exchanged Zeolites as Active Catalysts for Direct Decomposition of Nitrogen Monoxide... 

K. The results indicate that Cu-ZSM5 is the most active catalyst at 773 K for the decomposition of dilute NO gas. The order of activity is Cu-ZSM5 > Ag-Co304 > La-Sr-Co(Cu)-0 > Pt/Al203 > Y-Ba-Cu-O/MgO. Regarding transition metal ion-exchanged zeolites, catalytic activities in the reduction of NO with or and the adsorption state of NO were extensively... 

UOP molecular sieves (UOP) has developed the lonsiv family of ion exchange resins for the extraction of radionuclides from wastewater. lonsiv TIE-96 is composed of a titanium-coated zeolite (Ti-zeolite) and is used to separate plutonium, strontium, and cesium from alkaline supernatant and sludge wash solutions. The technology was developed by Pacific Northwest Laboratory (PNL) for use at the West Valley, New York, nuclear waste facility. The technology is commercially available. 

A further possibility for side-chain alkylation of toluene is oxidative methylation with methane. Catalysts with occluded alkali metal oxides, prepared by impregnating zeolites with alkali metal hydroxides followed by calcination, usually exhibit better performance.441 Further enhancement was achieved by impregnating ion-exchanged zeolites 442 Significant improvements in stability and the yields of Cg hydrocarbons were also observed when NaX was impregnated with 13% MgO which was found to increase the amounts of active sites.443... 

Figure 2. Frequency of the D6-ring band vs. degree of exchange for ion-exchanged zeolites...

In this work the reactions of ammonia with chlorobenzene and benz-aldehyde over a series of metal ion-exchanged zeolites were investigated by the microreactor method, and attempts were made to relate the catalytic activity of the zeolites to properties of metal cations. Ammonia was a reactant and a poison for acidic sites. 

This is the same order of the catalytic activity of transition metal in exchanged zeolites for aniline formation. Irving and Williams (15, 16) pointed out also that there is a clear correlation between complex stability and the second ionization potential. As a matter of fact, a good correlation was found between the catalytic activity and the second ionization potential of divalent ions. (We thank the reviewer for pointing out this correlation.)... 

Heterogeneous catalysts for liquid phase oxidations can be divided into three different categories (a) supported metals (e.g. Pd/C), (b) supported metal ions (e.g. ion exchange resins, metal ion exchanged zeolites) and (c) supported oxometal (oxidic) catalysts (e.g. Ti1v/SiOg, redox zeolites, redox pillared clays). This division of the various catalyst types will be used as a framework for the ensuing discussion. 

Taking into account the nature of the Wacker concept (305), it is evident that early work is available with Pd2+ and Cu2+ ion-exchanged zeolites (306-308). Because ion exchange was done with Pd(NH3)42+, it was not clear whether residual ammonia was an essential requirement for good catalyst selectivity and stability (Scheme 3). 

In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening by replacement of Ca2-with Na+ or another ion. When dehydrated they have important catalytic applications, promoted by the Brensted acid sites, and by the large area of internal surface. They are used for the cracking of petroleum and for the isomerisation of hydrocarbons, where limited pore size exerts a shape selectivity, which allows one desirable product to be formed in high yield. 

Impregnation and ion-exchange methods for preparation of supported catalysts are discussed in detail in Section 2.2.1.1. Only ion exchange in the solid state as a novel method to prepare zeolite catalysts is described here. 

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