Zeolites electrostatic effect

Moreover, recently the application of zeolites in organic synthesis demonstrated their ability to achieve reactions necessitating usually superelectrophilic intermediates as in superacid media.144 These results cannot be rationalized on the basis of the acidic character alone and needs the understanding of confinement effects145 in which case the zeolite cage behaves like a nanosized reactor favoring the contact between reactant and specific acidic sites. The electrostatic effects may also be important. 

One of the important differences between molecules in zeolite cages and species isolated in other matrices such as cyclodextrins [22], micelles [23], vesicles [24] and polymers [25] arises from the fact that the zeolite can be an active host and influence the structure and reactivity of the encapsulated molecules. The nature of the intra-zeolitic environment has been extensively studied and the influence of polarity, steric and electrostatic effects on encapsulated molecules is beginning to be well understood. 

In summary, it is dear that the zeolite is a novel host for the entrapment of molecules and the rigidity and the charged nature of the framework allow for steric and electrostatic effects on the encapsulated molecules. Isolation of entrapped molecules can also influence their reactivity. The interest in zeolites as hosts for electron-transfer reactions stems from a combination of properties, including... 

It seems that practical implementation of this type of selective catalysts will require a medium in which (very) polar products can be removed from the zeolite phase. Unfortunately, no attention has been paid in literature to such issues. On the contrary, some attention has been devoted to host modification after exchange of NaY with other alkali metal cations [37]. The cyclohexene epoxidation activity increases with decreasing size of the charge compensating cation pointing to the influence of steric effects or of electrostatic effects on the activity. In competitive experiments using cyclohexene and 1-octene as feed, the reactivity of the smaller substrate is suppressed, indicating that competitive sorption is involved as well [37],... 

A DFT study of the isomerization reactions of toluene catalyzed by acidic zeolite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed in detail. The uses of periodical structure calculations as well as small cluster approach method calculations allow to analyze the effect of zeolite electrostatic contributions and steric constraints on reactivity. 

Figure 7.11 The wavenumbers of (left) the C=0 stretch of carbon monoxide and (right) the N = N stretch of dinitrogen adsorbed on a series of cation-exchanged zeolites, plotted against the inverse square of the cation- adsorbate distance. A linear relationship suggests that electrostatic effects are dominant in determining the frequency shift. [Reproduced from reference 75 with permission. Copyright 1995 American Chemical Society.]...

Early studies by Scurrell and coll, demonstrated the use of rhodium zeolites as catalysts for the carbonylation of methanol into methyl acetate in the presence of methyl iodide (65). It was hoped that due to their electrostatic field zeolites would effect the direct carbonylation of methanol without the help of the iodide promoter. In fact, as the CH3OH/CH3I ratio increased, increasing amounts of CH4 and CO2 were produced indicating that the reaction... 

The efiect is thus not related to geometrical constraints induced on complexes anchored in mesoporous charmels (sometimes also called as confinement efiect, even if this definition is not properly correct), neither to shape-selectivity effects as possible in zeolites, since the size of mesoporous charmels is much larger than those of micro-porous materials. Instead, an effective modification on the characteristics of the fluids is observed due to the electrostatic field generated by the charmel walls. This is an enthalpic effect versus an entropic effect as observed when the modification is instead related to limitations in the translation modes of molecules. Recently, it was also demonstrated that wall curvature influence the molecular orientation of the... 

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

Zeolites can be hydrophilic or hydrophobic due to the different Si/Al ratios within the zeolite framework. Organic molecules rely on H-bonding, electrostatic, and 77-cation interactions for effective zeolite absorption, and these interactions will clearly be influenced by the number of cation sites present. As expected, the more Si present, the more hydrophobic the zeolite and, therefore, the greater the ability of these materials to interact with hydrophobic organic molecules or to exclude hydrophilic molecules, such as water. Zeolites X/Y have a Si/Al content at or close to 1 and are highly hydrophilic absorbants. Pentasil zeolite ZSM-5, which... 

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. 

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