External cationic sites

The external versus internal surface has also been investigated in the case of Co-H-zeolites active in the selective catalytic reduction of NO by methane [20]. The highly hindered nitrile, ortho-toluonitrile (oTN) has been used, co-adsorbed with CO and NO. The pre-adsorption of oTN poisons the external cationic sites of Co-H-ZSM5. Successive adsorption of CO shows that some of the Co ions are on the inner surface and some on the outer surface. The coadsorption of oTN and NO shows that trivalent Co ions are actually located in the internal cavity surface of Co-H-ZSM-5 while divalent Co ions distribute almost equally in the internal and external surface. 

The crystal structure analysis of palladium-exchanged zeolite allows the determination of initial cation positions in the dehydrated porous framework. Similar studies after reduction by hydrogen at various temperatures should permit the observation of palladium removal from the cation sites and thus the estimation of the reduction level. Moreover, the presence of metal on the external surface is easily detected. Hence, x-ray diffraction techniques should give a good picture of hydrogen reduction of palladium in Y zeolites. 

Unlike kaolinite and montmorillonite, there are several sites for ion exchange reactions to occur on mica and vermiculite (Bolt et al., 1963 Sparks and Jardine, 1984). Bolt et al. (1963) studied potassium exchange on mica and proposed three sites for reactivity. Slow reactions were ascribed to interlattice exchange sites, rapid reactions to external planar sites, and intermediate reactions to readily exposed edge sites. Sawhney (1966) found two distinct reaction rates for cesium exchange on a Ca-vermiculite. The first reaction was ascribed to a rapid exchange of cesium with cations on external planar surface sites and interlattice edges, followed by a second, slow reaction in which cesium diffuses into the interlayers. 

A modified pentasil was prepared in which protonated sites reside externally and cationic sites reside internally. Coked samples of this zeolite were characterized by 129Xe NMR. Even at high coke levels, only slight blockage of channels was observed. In contrast the fully protonated H-ZSM-5 was shown to undergo coke deposition resulting in channel blockage. These observations can be rationalized by a model in which deposition of coke in the channels or at... 

Bentonite rocks have many uses in the chemical and oil industries and also in agriculture and environmental protection. The usefulness of bentonite for each of these applications is based on its interfacial properties. These properties are determined by geological origin, chemical and mineral composition (especially montmorillonite content), and particle size distribution, and they include the specific surface area (internal and external), cation-exchange capacity (CEC), acid-base properties of the edge sites, viscosity, swelling, water permeability, adsorption of different substances, and migration rate of soluble substances in bentonite clay. 

External versus Internal Cationic Sites and Complex Interactions in Cation Exchanged Zeolites... 

However, given the properties of the films, other pathways may exist for charge transfer where the films play a more active role. As an example, the polymers of concern here contain fixed cationic sites and, as noted before, can function as anion exchange membranes. At sufficiently thick films the usual electrode response for a cationic couple in the external solution can be quenched by the film. However, anionic couples like Fe(CN)g -/3-can enter the films, arid within the films their response becomes that of an electrostatically-bound couple as noted above. In fact, the concentrating effect of the ion exchange membrane can lead to a very sensitive electroanalytical technique for anions. 

Figure 3.11. Depiction of Na /Ca cation segregation between internal and external exchange sites of layer silicate clay particles.

The way this structure reacts to heat, pressure, and cation substitution, reflects the basic linkage between tetrahedral chains and the linking poly-hedra. Tetrahedra are more rigid than the cation polyhedra. The cation sites are the ones which respond the most to external conditions, while the... 

A further anticipated consequence of the above mechanism would be an appreciable reduction in polycation sorption in the presence of a cationic surfactant. Not only would the latter species compete for the external anionic sites of the keratin, but it is known to penetrate into the body of the hair (11). Measurements in fact showed that the surfactant... 

The nature and location of protonic and cationic sites of Co-H zeolites (FER, MOR, MFl) were studied by UV-vis and IR spectroscopy of adsorbed hindered nitrides [04M3]. They showed, in exchanged Co-H-FER, that Co ions are distributed between the internal and external surface of zeolite. The UV-vis speetia of eobalt were similar in partially exchanged zeolites and on rrricroporous silica-altrrttina (srrrface Co species). The UV-vis spectra of Co-species in exchanged zeolites resrrlt to be quite irrserrsitive to the nature of the errvironmerrt where the ions are located. 

Recently, Robinson (1969) has made a kinetic study of p-nitrophenyl-phosphatase in a partially purified ATPase preparation from rat brain from which he proposes that for ATPase there is a concerted process with interdependent coupled cation activators and not two sequential mechanisms each with its own cation activator. Thus, instead of a Na site being translocated to become an external K site in the enzyme molecule, Na+ andK sites would coexist. 

No external protection agent is needed for St-Cl in rac-Me2Si[2-Me-4-Ph (Ind)]2ZrCl2/MAO-catalyzed polymerization of propylene. However, both the OH and NH2 groups are highly sensitive to the metallocene cationic site. The silane groups not only provide effective protection for both the OH and NH2 functional... 

Zeolites and molecular sieves are crystalline porous metal oxides possessing enormous internal surface area (—1000 mVg), where every metal cation is either an internal or external surface site [3]. Zeolites are composed of an aluminosilicate framework of the type M ,/ [(A102)jc(Si02) ] zH20 (where n is the charge on the metal cation, which is typically Na, K, Ca ). Molecular sieves differ from the aluminosilicate zeolites because they possess framework atoms other than aluminum or silicon (P, Ga, Ti, V, etc.). Zeolites and molecular sieves possess a wide variety of properties pore size, pore shape, dimen-... 

Carbohydrate derivatives bearing charged sites (as ammonium or phosphonium components) clearly held significant potential not only with the cyclodextrins, but also for other carbohydrate species owing to the wealth of functionality present within the structure and the possibilities for interaction with solvents and other external anionic components. Owing to the size of the cationic pwrtion and the wealth of associated anionic species in a multitude of possible locations relative to the individual cationic sites, the possibilities of cationic carbohydrate derivatives for serving in ionic liquids are quite sizable. 

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