Ions in Ion-Exchange Positions

Thus, the introduction of palladium does not results in any significant alteration in the relative distribution of Co2+ in the exchange positions, but it is probably responsible by a re-definition of cobalt oxide species, that leads to a decrease of C03O4 and the appearing of Co-oxo ions. 

However, the author believes that the metals in the weathered samples are held in ion exchange positions or as simple salts of the organic matter rather than by chelation, and that the accumulation of these abnormally large amounts of most of the elements was synchronous with the weathing process. Data from these samples suggest that basing minor element distribution in carbonaceous rocks on data from weathering samples could lead to erroneous interpretation. 

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

Co-containing zeolites of MFI structure were synthesized using alkaline media. The orthorhombic-monoclinic symmetry transition suggests that at least the Co(II) ions also occupy tetrahedral framework positions. The XPS data clearly show that the samples contain both framework tetrahedral and extraframework octahedral Co(II) ions at ion exchange positions. The diffuse reflectance UV-visible spectra show unambiguously the presence of tetrahedral Co(II) ions in the structure. 

In contrast to the stabilization of monovalent exchange cations in zeolites the structural aspects of stabilization of multivalent metal exchange cations are not quite clear. For example, the ion exchange position for bivalent metal cations should be formed by two lattice Al atoms. According to the traditional point of view the aluminum distribution over zeolite lattice is predominantly stochastic. It creates a variety of mutual localization of two nearest lattice Al atoms and results in a number of possible ion exchange structures for the bivalent cation stabilization. On the other hand, it is evident that structural peculiarities of bivalent metal cations stabilization influence the adsorption ability and catalytic reactivity of the cation. 

The formation of oxide-hydroxide metal clusters is considered in Sect. 20.4. Various places of their localization are discussed. One of them is associated with accommodation of metal oxide species in the cationic position of zeolites. The (Zn302) cluster formation was smdied and its activity in the dehydrogenation of ethane was calculated. Besides the condensation of polynuclear oxide species in ion-exchange positions a possibility of the grafting of small metal oxide clusters to zeolite or to pure silica lattice was considered on the example of immobilization of ZnO, (ZnO)2 and (ZnO)3 species. 

Structures of styrene, divinylbenzene, and a styrene-divinylbenzene co-polymer modified for use as an ion-exchange resin. The ion-exchange sites, indicated by R, are mostly in the para position and are not necessarily bound to all styrene units. 

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

Substituted Phenols. Phenol itself is used in the largest volume, but substituted phenols are used for specialty resins (Table 2). Substituted phenols are typically alkylated phenols made from phenol and a corresponding a-olefin with acid catalysts (13). Acidic catalysis is frequendy in the form of an ion-exchange resin (lER) and the reaction proceeds preferentially in the para position. For example, in the production of /-butylphenol using isobutylene, the product is >95% para-substituted. The incorporation of alkyl phenols into the resin reduces reactivity, hardness, cross-link density, and color formation, but increases solubiHty in nonpolar solvents, dexibiHty, and compatibiHty with natural oils. 

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