Several commercial aluminas, silica-aluminas and clays are investigated from a standpoint of catalyst characterization and the influence of partial deactivation on their activities for dehydration of primary alcohols. The states of the various catalysts are characterized by calorimetric titrations with n-butylamine and trichloroacetic acid and the resulting heat of adsorption curves are utilized to obtain acidity and basicity distributions for each catalyst state. A division of these distributions into groups of suitable acidic and basic site pairs leads to the development of a correlation between the acid-base distributions and the catalyst activities. The postulates of the correlation are in agreement with the reaction mechanism previously proposed in the literature.
Several of these catalysts are subjected to poisoning by ammonia and organic bases of different strengths. Subsequent evaluation of the acid-base distributions of the deactivated catalysts show subtle changes in the basicity distributions depending upon the strength of the poison. The correlation developed earlier is used to predict the activities and selectivities of the deactivated catalysts. The good agreement between the predictions and the experimental results substantiate the usefulness of the correlation.Subtle changes in selectivity caused by poisoning have been explained by the corresponding changes in the acid-base distributions, thus proving the importance of such characterization.
Kinetics of methanol and ethanol dehydration over some of these catalysts have been studied to ascertain effects of changes in the catalyst state.The rate expression
r = kKAcA1/2/(1 + KAcA1/2 + KWcW)
describes the experimental data for all the catalysts in their fresh as well as poisoned states. Significant variations in k, KA and KW are observed depending upon the catalyst states. Comparison of kinetics on fresh and poisoned catalyst states shows that poisoning increases the KA and KW for ether formation in contrast to a decrease in these constants for olefinformation. These variations are attributed to interactions among poisonmolecules and acid-base site pairs, thereby lending support to the reaction mechanism. Certain implications of nonseparable kinetics are investigatedto show significant changes in total conversion and product distribution upon reversal of flow direction through a graded reactor.
