Zeolite chemistry cation exchange

Cation binding, in supramolecular chemistry, 24 40-43 Cation bridging, 11 634-635 Cation complexation, routes to, 24 41 Cation exchange, in zeolites, 16 826 Cation-exchange catalysts, 10 477-478 12 191... 

Zeolite surface chemistry resembles that of smectite clays. In contrast to clays, however, natural zeolites can occur as millimeter- or greater-sized particles and are free of shrink-swell behavior. As a result, zeolites exhibit superior hydraulic characteristics and are suitable for use in filtration systems (Breck 1974) and as permeable barriers to dissolved chemical migration. Internal and external surface areas up to 800 m2 g have been measured. Total cation exchange capacities in natural zeolites vary from 250 to 3000 meq kg 1 (Ming and Mumpton 1989). External cation exchange capacities have been determined for a few natural zeolites and typically range from 10 to 50 percent of the total cation exchange capacity (Bowman et al. 1995). 

It is clear that the Wacker cycle in a CuPdY zeolite incorporates the traditional features of the homogeneous catalysis combined with typical effects of a zeolite (303, 310). It also follows that whereas other cation exchangers in principle will show Wacker activity after cation exchange with Cu/Pd ions, the cage and pore architecture will probably be less suitable for Wacker chemistry than those of the faujasite structure. This is the case for fluoro-tetrasilicic mica, a synthetic layer silicate that swells under reaction conditions and allows access to the interlayer space (311). 

To deepen into the effect of the surface chemistry of the zeolite on CO2 adsorption, zeolite NaY was exchanged with alkaline cations (Li and K ) and different divalent cations (Ca, Sr and Ba ). The exchange of Na cations by alkaline species will aid in analysing the effect of the polarising power of the cation. Moreover, the ion exchange of Na cations by a divalent cation results in a decrease of the concentration of cations existing in the zeolite framework. Figures 6 and 8 contain the characteristic curves obtained for the above samples. 

In an effort to probe catalytic sites and their stability, the zeolite catalyst, ZSM-5 was Investigated by impedance and fourier transform infrared spectroscopies as a function of aluminium substitution and cation exchange. Samples were provided by Chemistry Division, DSIR, with (Si + A1)/A1 ratios of ao, 1000, 500, 200, 136 and 40. Crystallite size and morphology varied somewhat with aluminium content but typically the samples had crystal size distributions in the range 0.2 pus to 2 (im. 

Thirdly, chemical properties must be elucidated. For example, ion exchange was recognized early as a characteristic property, yet we know little about the ion exchange behavior of most of the zeolites. The ion exchange character of some zeolites, reviewed above, has been studied in detail and in depth. These in-depth studies have revealed unexpected cation selectivities and have shown that zeolites may make interesting ion separations. Optimization of these properties may lead to important applications in solving environmental problems. The novel application of the zeolite properties in nuclear chemistry has been demonstrated. The Szilard-Chalmers reaction to form actinides within the structure of zeolite X by the usual (n/y) reaction is known. Recoil of the nuclide after neutron capture permits easy elution (24). 

Since the development of zeolite, chemistry transalkylation has been studied mainly using zeolite catalysts. Frilette used natural mordenite treated by acid. The activity was much hi er than amorophous Si02 —AI2O3, but the activity could not be maintained. Benesi reported that mordenite was about 8 times more active than Y-type zeolites and that the active centers were Brensted acid sites. Various efforts including dealumination and cation exchange have been made to improve the aging. 

There are only a few Mossbauer nuclei which are interesting in zeolite chemistry and, thus, candidates for application of Mossbauer spectroscopy in soUd-state ion exchange. However, among them is one of the most important elements, viz., iron, which has also attracted much attention in zeoUte chemistry as a key component of possible catalyst formulations. Mossbauer spectroscopy proved to be exceptionally successful in discriminating Fe + and Fe + cations residing on extra-framework sites after introduction of iron via soHd-state ion exchange. Moreover, Mossbauer spectroscopy provides information about the various coordinations of Fe + and Fe in zeoUte lattices (cf. Sect 5.3.4). 

We discuss here a combined process including detemplation and Fe incorporation by ion-exchange in the zeolite framework [147]. To achieve this, oxidants to decompose the organic template and Fe-cations for exchange are needed. Both requirements are in harmony with Fenton chemistry. The OH radicals can oxidize the template and the Fe-cations be exchanged simultaneously. 

Whereas the synthesis of zeolite occurs in nature and in the laboratory under strongly basic conditions (pH 9-11), they are widely used as catalysts in hydrocarbon chemistry under their acidic form. In order to obtain acidic zeolites, the alkali cations (K, Li, Na, Ca, etc.) are first exchanged by NH4+C1 followed by heating which, after release of ammonia, leaves the proton loosely attached to the framework on the Si-O-Al bridging group (Figure 2.19). 

The author of this book has been permanently active during his career in the held of materials science, studying diffusion, adsorption, ion exchange, cationic conduction, catalysis and permeation in metals, zeolites, silica, and perovskites. From his experience, the author considers that during the last years, a new held in materials science, that he calls the physical chemistry of materials, which emphasizes the study of materials for chemical, sustainable energy, and pollution abatement applications, has been developed. With regard to this development, the aim of this book is to teach the methods of syntheses and characterization of adsorbents, ion exchangers, cationic conductors, catalysts, and permeable porous and dense materials and their properties and applications. 

Supercages of X and Y zeolites contain a large number of exchangeable cations (Fig. 6). Of the various cations, those of type II and III, present within the supercages, can interact with the included guest molecules. By careful choice of the cations one can control the chemistry that takes place inside the supercage. 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

The chemistry of natural zeolites may have important effects on their ion exchange properties, mainly in terms of selectivity. It is well known that selectivity is a function of various parameters, depending on (1) framework topology, (2) ion size and shape, (3) charge density on the anionic framework, (4) ion valence and (5) electrolyte concentration in the aqueous phase [51]. Within the same zeolite type, the variation of the framework composition (in practice, Si/Al ratio) and therefore of the framework charge density, affects the cation selectivity [52], as it has experimentally been proven for phillipsitc [53]. It is improper, stricto sensu, to compare with each other, in terms of selectivity behaviour, different zeolites having... 

Last but not least, the versatility of zeolites is demonstrated by exchanging the acidic proton with deuteron which enables investigation of interesting mechanisms related to catalysis and by exchanging the proton with transition metal cations, such as Cu(I), and opens new areas of enviromnentaUy friendly organic chemistry. For these reasons, we are including in this chapter acidic-zeolite catalysed reactions from our own work which can be best understood as examples of confinement effects superelectrophilic, Cu(I) catalysed Click chemistry, and specific H/D exchange reactions. 

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