Adsorption Equilibria and Catalytic Selectivity

The observed optimum in selectivity for the branched product relates strongly to the change in shape of the linear molecule and its larger effective volume at intermediate micropore sizes. In an analogous fashion, the different responses observed between the MFI 

At 1 kPa, for strongly adsorbing Cio alkanes in MEL-type zeolite, there is a large preference for adsorption of the linear alkane. This preference is much less than for the MFI zeolite. Differences appear at high micropore occupation. Competitive adsorption suppresses the formation of i-Cio in MEL owing to the difference in the channel cross-section geometry, where branched alkanes prefer to adsorb. As a consequence, the rate of n-Cio conversion is low towards i-Cio. The MFI zeolite, therefore has the superior rate since the rate of iCio formation is higher. The reaction products are the result of consecutive reactions of i-Cio. In contrast, as one notes from Fig. 4.39, in MEL at 10 kPa for C7 there is no such preference in adsorption for the n-C7 versus i-C7 molecule since under these conditions the adsorption concentration is still too low. 

An approximate expression to compute the rate of productP, formation is 

When desorption is rate limiting, this expression reduces to  

At the high pore fillings conventionally used, product molecules can be assumed to be equilibrated in the micropores. This implies, as we have seen, that their relative concentration is determined by the molecular chemical potential in the micropores. Since the diffusion constants can also be a strong function of concentration, one has to apply these expressions with care, or even better one should use expressions that include the concentration dependence of diffusion constants such as the Maxwell-Stefan expression. 

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Shape-Selective Microporous Catalysts, the Zeolites 205 4.5 Adsorption Equilibria and Catalytic Selectivity... 

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