Investigation o f the active surface of aluminum oxide by the method of selective etching
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It was shown on the basis of data on the reversible and irreversible poisoning of aluminum oxide catalysts (by dioxane, pyridine, compounds of heavy metals, etc.) in the dehydration of isopropanol in the adsorption layer and under flowing conditions that the number of active portions of the surface comprises 1015–1016/m2, i.e., approximately an order of magnitude less than has been indicated in the literature.
The total number of alcohol molecules decomposing in the adsorption layer significantly exceeds the number of active portions on the corresponding catalysts, which is evidence of the occurrence of surface regeneration of the centers on account of migration of adsorbed molecules.
In the case of high degrees of coverage in the adsorption layer, the low-temperature reaction of dehydration of alcohol does not take place and acquires an appreciable rate only in the case of removal of the excess alcohol molecules (without changing the temperature).
When the process is conducted under flowing conditions at atmospheric pressure, the basic role is played by the same relatively scanty group of active sites, which can be determined according to the data of the reaction in an adsorbed layer and, consequently, increasing the temperature leads not so much to an increase in the number of active sites as to an increase in the frequency of alternation of the elementary events of the reaction on each site.
KeywordsPyridine Dehydration Isopropanol Flowing Condition Dioxane
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