Electrostatic field within zeolites

The contents of zeolites with neutral frameworks must themselves be neutral. Accordingly, only molecules and/or salts with no excess ions to unbalance charge may be present. Because of the weak electrostatic fields within these zeolites, there is little reason for ionization to occur in other words, they are poor solid electrolytes. For the same reason, sorption energies are small and these zeolites are relatively easy to empty. 

To make further progress it will be necessary to employ well defined micro-porous and small pore mesoporous adsorbents and have available non-porous reference solids of the same surface structure. This will be possible with the current progress in material synthesis procedures (e.g. organised amorphous structures obtained in the presence of templating agents [62,63]. We have to note that zeolites are often not suitable reference materials because the strong electrostatic fields within the crystalline cavities can polarise the gas molecules. Therefore the adsorption equilibrium in these materials is not solely a function of the size of the adsorbate. 

The dealumination process is associated with a change in the porosity within the crystals and may sometimes cause a drastic loss of crystallinity. The microporous adsorbents of the faujasite type are so arranged that the Si/Al ratio increases as the munber of cations and the average electrostatic field within the framework decrease. To assess the effect of the Si/Al ratio on the activity and acidity of Y zeolites, it is desirable to compare samples with similar extents of exchange, since the degree of exchange has a significant influence on the catalytic and acidic properties of faujasites. 

The solution is assumed ideal. It is incompressible all lattice sites are filled with some species of molecule. All species of molecules at the lattice sites are of equal (or nearly equal) size. For physical reasons, only one molecule can occupy each lattice site. Since there is a distribution of adsorptive energies within the zeolite, corresponding to the locally varying electrostatic field, the adsorption problem is approached from the standpoint of a superposition of several solutions - all the sites in each being identical. The number of solutions that must be considered equals the number of different adsorptive energy sites that are found within the zeolite. 

Nucleophilic photosubstitution reactions of benzylic chlorides have also been observed to occur with nucleophiles other than the alcohol solvents. n-Nucleophiles such as amine solvents147 and halide ions and acetate ions148, as well as 7r-nucleophiles such as toluene149 have been used. The latter, a photoalkylation reaction, was achieved by irradiation of benzyl chloride absorbed within zeolite micropores in a slurry in cyclohexane. In cyclohexane itself only products of PhCH2 are formed. This large medium effect is due to the strong electrostatic fields experienced in the zeolite cavities149. 

Although the reduction step has commonly been effected with H2, the use of alkali metal (Na°) vapors has been employed by Rabo (48) for reduction of Ni++Y to Ni+Y. Generally, the stronger the electrostatic field of the intrazeolitic cation, the greater the tendency toward reduction (48). Metals such as Hg, Cd, and Zn, formed within the faujasite structure by H2-reduction of the corresponding bivalent cations, can also be removed from the zeolite by heating at high temperatures in a stream of H2 (170). 

The final methods which will be discussed are those which consider the electrostatic potential or field within a zeolite cage or skeleton, and the effect that this may have upon possible catalytic sites and absorbed molecules. A number of these calculations have been carried out, and they can be divided into two categories those which use the monopole approximation and simply calculate the field or potential due to point charges placed at atom centres , or those which use a more rigorous approach and calculate the potential directly from the molecular wavefunction . ... 

A slightly different empirical scheme has also been used to look at cation locations. The work of No and co-workers has been concerned with setting up a force field based upon electrostatic, repulsion, dispersion, polarisation and harmonic bond stretching energy contributions , which can be used to assess interactions within zeolites. In setting up this force field, the parameters have been optimised for Na-zeolite A, however, this has been extended to in-... 

Xenon-129 NMR has also been used as a suitable probe to investigate the electrostatic field that adsorbed xenon adatoms experience within zeolitic pores. The chemical shift of adsorbed Xe is a sensitive function of the electrostatic field that it experiences, as well as depending on collisions between... 

In all cases the zeolite structure will impose environmental conditions differing from those encountered on a "flat surface. The zeolite will behave as a solid ionic solvent [7] and embed its guests within a high electrostatic field (10 V.m" ). Its anionic framework will be able to act as a macroanion or a polydentate ligand. Further, the finite size of the channels and intersections will limit the access to the internal void volume of the entities used in the preparation of... 

The zeolite framework was described by a specific force field developed by van Santen et al. [11] while the hydrocarbon molecules and their interaction among themselves and with the zeolite lattice were described by the generic force field Drdding n [12]. All the internal coordinates of the alkane molecules were allowed to fully relax. The nonbonded interactions (electrostatic and van der Waals) were computed for aU atoms within a cutoff-radius of 12A. Periodic boundary conditions were imposed along the three axes of the zeolite model to simulate an infinite crystal. 

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