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Black plate (269,1) hydrocarbons which eventually deactivate (‘‘poison’’) the catalyst. Each cage or channel with its occluded organic and inorganic species in its entirety may be considered as a supramolecule which acts as a reactant to form various product species, leading to structure-activity and structure-selectivity relationships which permit the modification of the catalyst with degrees of freedom that appear closer to homogeneous catalysts than heterogeneous catalysts168. However, closure over this matter is not unequivocal and the issue of forming the very first C2C bond remains the subject of investigation and speculation, although this may be more or less a matter of ‘‘fine-tuning’’, if the most important elements really are comprehended. The hydrocarbon pool model implies that reactions occurring during an initial kinetic induction phase need not necessarily be similar to those on a working catalyst. It remains unresolved however, which induction reactions – if any – might contribute to the hydrocarbon pool. The results of a theoretical study169 have been published which aims to integrate a large range and variety of possible direct reactions and so to deduce whether ethylene can be produced solely from methanol, and which stable intermediates might thereby be formed directly, to provide an understanding of how that vexed first C2C bond is indeed formed. Over 80 stationary points in the MTO process were calculated which provided both rate coefficients at 720K and reaction barriers at 0 K. However, no direct pathway could be identified that led from from methanol to ethylene (or for that matter to any intermediate containing a C2C bond), in accord with the observation that methanolydimethylether are relatively unreactive on H-ZSM-5 when there are no organic impurities present that can provide a hydrocarbon pool. Two bottlenecks were identified from the calculations: one being the instability of ylide intermediates, and also that the frequently invoked concerted C2C bond formation reactions actually have very high energy barriers, which is due to the weakly basic character of the zeolite oxygen bridge. The calculations allow a clear definition of those intermediate species which are likely to interact with the hydrocarbon pool, which are not only the mixture of methanol and its condensation products dimethyl ether and water, but also the trimethyloxonium cation, which might act as a source of methyl groups, and also of methane and formaldehyde. In summary, the authors note that neither a single mechanism nor a combination of steps among the calculated scheme of reactions can link methanol to ethylene. They conclude that the theoretical work provides hence strong support for the www.scienceprogress.co.uk Properties and applications of zeolites 269PDF Image | Properties and applications of zeolites
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