Internal cationic sites

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

The interactions of (212) and related species with monovalent and divalent metal cations have been studied by nmr spectroscopy (Lehn Simon, 1977). The study indicated that sequential formation of mono-and di-cation complexes occurs (see [4.7]). These studies, and especially a 13C nmr study of the 1 1 complex of (212) with barium nitrate, suggest that the 1 1 species are unsymmetrical with the metal ion being contained in one of the azacrown cavities. Nevertheless, the nmr data also indicate that these 1 1 species undergo internal cation exchange between the respective azacrown sites. This intramolecular dynamic behaviour serves... 

Li+ may bind at the Na+ site with activation42 while K+ binds with inhibition. For these internal Na+ sites, Km (Na+) = 1.23 mmol dm 3 and KT (K+) = 4.5 mmol dm-3. Both high and low affinity sites exist for K+ for the high affinity sites Km (K+) = 0.08 mmol dm 3 and (Na+) = 14.3 mmol dm 3, thus showing good discrimination between K+ and Na+. The low affinity sites have Km (K+) = 1.2 mmol dm-3 and appear to be involved in the phosphatase reaction Na+ binds with inhibition [X, (Na+) = 4.5 mmol dm-3]. Other Group IA cations activate at the K+ sites thus there is evidence for two sites for Rb+ per ouabain site. 

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... 

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. 

Zeolites are crystalline aluminosilicates found in nature and have been prepared synthetically since the 1800s. Their robust structure, porous architecture and internal acidic sites make them excellent sorption materials, ionic exchange materials and catalysts for industrial processes. There are numerous known zeolite structures that have been obtained naturally or prepared synthetically through various routes. In the simplest cases the porous architecture can be altered by exchanging different sized cations into the pores to vary pore diameter or volume. 

In the last decade, xenon has proven to be an efficient sorbate for probing the pore structure and the internal surface of adsorbents by NMR spectroscopy [28-30]. The advantage of xenon in comparison with other adsorbates is brought about by the large chemical shifts of Xe NMR as a consequence of the large electron shell and by the fact that xenon as a noble gas leaves the adsorbent structure essentially unaffected. In particular, in zeolite research Xe NMR has been successfully applied to probe pore and channel dimensions [31], cation distributions [32], cation sites [33], and cation mobilities [34,35] as well as matter depositions and lattice defects [36-38]. 

Membrane currents with lipid-soIuble anions show saturation at high ionic concentrations, and for the anions, at least, the major obstacle to movement is diffusion through the aqueous stationary layers on either side of the lipid bilayer. The large hydrophobic groupings associated with the lipid-soIuble ions are able to mask the charge in ways which make it possible for them to penetrate the hydrophobic interior of the bilayers. This immediately suggests a way in which carriers for small ions may operate. What is required are bulky molecules with hydrophobic outer surfaces which may easily partition into cell membranes, yet at the same time contain internal hydrophilic sites which can accommodate a dehydrated cation. 

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. 

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