Zeolites molecule movement

Although it is not possible for the chemist to absolutely control the movement of individual atoms or molecules in zeolite structures, the nature of the structure itself results in channels that direct the molecular motions (Fig. 13). Furthermore, the sizes and shapes or the channels determine which molecules can form most readily, and which can leave readily. A molecule that cannot leave (Fig. 1.4) is apt to react further. This may have important consequences A catalyst (ZSM-5) that is structurally related to boggsite is used in the alkylation or toluene by methanol to form pui u-xylene. The methanol can provide methyl groups to make all three (ortho, meta. and para)... 

Note that the size of the terf-butyl alcohol molecule (5.4 X 6.8 X 6.8 A) distinctly exceeds the aperture of ZSM-5 channel, so that a serious mutual perturbation of the geometry of the ferf-butyl alcohol molecule by the zeolite, and vice versa, operates during diffusion. (The translational movement of the tert-butyl alcohol molecule inside HZSM-5 channel must resemble that of a rabbit inside a snake.) For most of the time, adsorbed alcohol molecules reside... 

The delocalized state can be considered to be a transition state, and transition state theory [105], a well-known methodology for the calculation of the kinetics of events, [12,88,106-108] can be applied. In the present model description of diffusion in a zeolite, the transition state methodology for the calculation of the self-diffusion coefficient of molecules in zeolites with linear channels and different dimensionalities of the channel system is applied [88], The transition state, defined by the delocalized state of movement of molecules adsorbed in zeolites, is established during the solution of the equation of motion of molecules whose adsorption is described by a model Hamiltonian, which describes the zeolite as a three-dimensional array of N identical cells, each containing N0 identical sites [104], This result is very interesting, since adsorption and diffusion states in zeolites have been noticed [88],... 

L. V. C. Rees (Imperial College, London) When zeolites have increasing amounts of polar molecules sorbed in them, the cations show a great increase in mobility when certain low fillings are obtained. Is it not possible that the decrease in the heat of sorption of NH at low isotherm temperatures represents the point where the cations loosen their attachment to the framework This would be an endothermic movement of a positive charge from the negative framework. At 300 °C, the cations would have sufficient thermal energy to mask this effect. 

Each zeolite framework has a system of channels through which ions and guest molecules may enter and leave. The movements of ions and molecules are limited by the diameters of these charmels.l> These chatmels may extend in one dimension, like many... 

Other nomenclature is associated with these channels. Narrow points in a channel are called windows or, less definitively, rings. These further limit the movements of ions and molecules. Wider areas between windows and/or at channel intersections are called cavities or cages, and these may be the primary source of the intrazeolitic volume which is available for ions and molecules. This volume can be as large as half of the total volume of a zeolite crystal. Cavities accessible from only one side may be called side-pockets. A zeolite whose channels are of relatively uniform diameter may be viewed as having neither windows nor cages. 

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