Zeolites as ion-exchangers

A tremendous variety of structures is known, and some of the three-dimensional network ones are porous enough to show the same type of swelling phenomena as the layer structures—and also ion exchange behavior. The zeolites fall in this last category and have been studied extensively, both as ion exchangers and as gas adsorbents (e.g.. Refs. 185 and 186). As an example, Goulding and Talibudeen have reported on isotherms and calorimetric heats of Ca -K exchange for several aluminosilicates [187]. 

Some studies and patents related to the etherification reaction between glycerol and isobutene are available in the literature, which report the production of the ethers with various catalysts, such as zeolites [19], ion-exchange resins [20, 21] and some homogeneous catalysts, e.g., p-toluene sulfonic acid [21, 22] and methane sulfonic acid [22], Recent studies, however, have concentrated on the etherification of glycerol with isobutene on ion-exchange resins [8, 23]. 

In 1962 Mobil Oil introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst In 1969 Grace described the first modification chemistry based on steaming zeolite Y to form an ultrastable Y. In 1967-1969 Mobil Oil reported the synthesis of the high silica zeolites beta and ZSM-5. In 1974 Henkel introduced zeolite A in detergents as a replacement for the environmentally suspect phosphates. By 2008 industry-wide approximately 367 0001 of zeolite Y were in use in catalytic cracking [22]. In 1977 Union Carbide introduced zeolites for ion-exchange separations. 

Synthetic zeolites and other molecular sieves are important products to a number of companies in the catalysis and adsorption areas and numerous applications, both emerging and well-established, are encouraging the industrial synthesis of the materials. There are currently no more than a few dozen crystalline microporous structures that are widely manufactured for commercial use, in comparison to the hundreds of structures that have been made in the laboratory. See Chapter 2 for details on zeolite structures. The highest volume zeolites manufactured are two of the earliest-discovered materials zeolite A (used extensively as ion exchangers in powdered laundry detergents) and zeolite Y (used in catalytic cracking of gas oil). 

Ultramarines are three-dimensional cage-like structures. They differ from feldspars and zeolites because of the large spaces within the structures that can contain cations and anions but not water, illustrating a natural buckeyball-like structure and cavity, and a diversity of environment between the internal and external cages. Ultramarines can act as ion-exchangers for both anions and cations. The blue color of ultramarines is due to the presence of the ion although a yellow ion S2 also exists in the same structure. 

After soil and clays, natural and synthetic aluminum silicates and synthetic zeolites were tested as ion-exchange materials by other scientists. However, the first practical applications of ion exchange took place hi the early 20th century. 

Unsupported particulates, like their powder counterparts, contain active sites without the addition of other catalytic species. Synthetic zeolites and Si02-Al203 catalysts used for cracking heavy oils to gasolines are catalytic due to their acid sites. They are produced by chemical reactions between the various components but can be found in nature. These materials are often modified by chemical techniques such as ion exchange however, the impregnation techniques typical of dispersed catalysts are not used. Promoters can be added to enhance performance. 

We should remember that low temperature zeolite synthesis started with Milton s gel synthesis of A and X in the early 1950 s(l). Large pore zeolites were substantially unavailable at that time. Natural faujasite was rare as it still is. Large port mordenite was still ten years in the future as were zeolites L and Omega. Zeolites were regarded as ion exchangers or as selective sorbents, not as catalysts. 

As ion exchanger, zeolites play an important role in volcanic rocks and marine... 

For instance, nafion composites give promising results. In the Pechmann reaction as well as in the esterification of dicyclopentadiene, the Nafion/silica composites are superior to pure Nafion, zeolites and ion exchange resins. We find high conversions at low temperatures, which can be attributed to accessible acidic sites with high acidic strength. 

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