Zeolites interaction within

Due to the confined environment and cation-jr interactions within the zeolite cavities as well, it is expected that the regioselectivity in the photooxygenation of trisubstituted... 

Figure 28 Cation-alkene interaction within a supercage of cation-exchanged Y-zeolite may control the conformations of allylic hydrogens. For allylic hydrogen abstraction, Ha and Hb should be parallel to the tt orbital (see top).

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

The zeolite interaction with this compound was evidenced by the low frequency shift experienced by the CO vibration of the face bridged carbonyls. Competition between residual or added water was witnessed suggesting that the solvating properties of the zeolite and water were similar and rather weak. Therefore the stabilization of these zerovalent carbonyls within the zeolite porous structure should be attributed to a cage rather than to a chemical effect. 

NaY, KY, RbY, and CsY zeolites efficiently produced the corresponding trans isomers 21b-d. In sharp contrast to the very low des (0-5%) obtained upon irradiation of 20b-d in isotropic dichloromethane/hexane solutions, photolyses of the same substrates in zeolites gave much improved des of up to 55% for 21b in NaY. The de of 21b critically depends on the countercation of Y zeolite, affording 50% for LiY, 55% for NaY, 30% for KY, 22% for RbY, and 5% for CsY. In LiY 1-phenylethyl amide 20d afforded 21b in de as high as 80%. Ramamurthy emphasized the importance of the cation-substrate interaction within the supercage for controlling the diastereodifferentiating photoreaction [69,70]. 

The Co-exchanged zeolites were not effective catalysts for the oxidation of cyclohexane. The cobalt exchanged ions were not stabilized enough by the zeolite interactions and part of these cations were released in the oxidation medium. Thus, we decided to explore the activity of P-zeolites in which cobalt ions were incorporated into the framework. We hoped that the incorporation would increase the stability of the cation within the solid. We studied the catalytic activities of cobalt substituted P-zeolites containing aluminium (Co-Al-BEA) and boron (Co-B-BEA) towards the oxidation of cyclohexane into adipic acid. 

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-... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Lowering the temperature has a similar effect on the deuterium spectra as does increased loadings. In Figure 3, spectra for benzene-d6/(Na)X at 0.7 molecules/supercage over the temperature range 298 to 133 K are shown. It is observed that both benzene species are detected simultaneously between 228 and 188 K. Below this temperature the oriented benzene species becomes the predominant form. A similar situation occurs for polycrystalline benzene-dg in which two quadrupole patterns, one static and the other motionally narrowed due to C rotation, are observed to coexist at temperatures between 110 and 130 K (7). This behavior has been attributed to sample imperfections (8) which give rise to a narrow distribution in correlation times for reorientation about the hexad axis. For benzene in (Na)X and (Cs,Na)X such imperfections may result from the ion/benzene interaction, and a nonuniform distribution of benzene molecules and ions within the zeolite. These factors may also be responsible for producing the individual species. However, from the NMR spectra it is not possible to... 

Additional information can be obtained by employing double resonance methods which probe the dipolar interaction of 29Si and 27A1 within the zeolite framework [93, 94]. The underlying NMR methods will be considered in more detail in Sects. 3 and 4 of this chapter. 

Stratakis, M., Rabalakos, C., Mpourmpakis, G. and Froudakis, G.E. (2003). Ene hydroperoxidation of isobutenylarenes within dye-exchanged zeolite Na-Y control of site selectivity by cation-arene interactions. J. Org. Chem. 68, 2839-2843... 

Pr4N+ cations were recognized to form complexes with silica-te (46) or aluminosilicate (23) species and subsequently to cause replication of the so formed framework structure via stereo-specific interactions (template effect). During this process, they are progressively incorporated and stabilized within the zeolite framework (1 1,47,48). Rollmann (37) has shown that initial Pr +/... 

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