Zeolite framework stabilization

Zeolite framework stabilization effects uniformly all transition states and charged transient intermediates, and does not effect the neutral intermediates. Similar effect of the zeolite framework has been described by Corma et al. for another reaction. 

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

Figure 2 clearly indicates a large variability in both the sites occupied by Al atoms and the concentration of the Al atoms in these sites for the ZSM-5 samples studied. Even samples with similar framework aluminum concentration exhibit very different Al sitings as well as Al concentrations in the individual occupied T sites. This fact clearly rules out that the Al siting in the zeolite framework is random or is controlled by thermodynamic stability. It rather shows that the conditions of the synthesis are responsible for the Al siting in the framework. 

Ab initio MO studies on the ordering of Si and A1 atoms in zeolite frameworks and on the stabilizing effects of counter ions... 

Several reaction mechanisms have been suggested to explain the framework stabilization of zeolites upon dea-lumination ... 

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 +/... 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Easily ionizable anthracene forms the cation-radical as a result of sorption within Li-ZSM-5. In case of other alkali cations, anthracene was sorbed within M-ZSM-5 as an intact molecule without ionization (Marquis et al. 2005). Among the counterbalancing alkali cations, only Li+ can induce sufficient polarization energy to initiate spontaneous ionization during the anthracene sorption. The lithium cation has the smallest ion radius and its distance to the oxygen net is the shortest. The ejected electron appears to be delocalized in a restricted space around Li+ ion and Al and Si atoms in the zeolite framework. The anthracene cation-radical appears to be in proximity to the space where the electron is delocalized. This opens a possibility for the anthracene cation-radical to be stabilized by the electron s negative field. In other words, a special driving force for one-electron transfer is formed, in case of Li-ZSM-5. 

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