Specific Poisoning on Alumina Surfaces

Two reactions for which specific poisoning experiments have contributed to the elucidation of the reaction mechanisms and permit evaluation of the possibilities and pitfalls of the technique are discussed as examples in this section. The first example is the dehydration of alcohols on alumina catalysts, and the second, the isomerization of olefins on the same type of catalyst. 

Various reviews have appeared in the past dealing with the dehydration reaction of alcohols (27, 28, 337-339). The elimination of water from aliphatic alcohols on alumina is known to proceed through two possible routes, namely, monomolecular olefin formation, 

Early poisoning experiments using nitrogen bases such as ammonia, pyridine, and piperdine have shown that the secondary isomerization of the primary ole-finic products can be completely suppressed, whereas the dehydration activity of the alumina catalyst was only slightly influenced by these poisons (30, 31,341-344). This is a typical example of selective poisoning, where a consecutive reac- 

Poisoning experiments with varying amounts of preadsorbed pyridine have recently been carried out by KnOzinger and Stolz (47). Pyridine is solely held by Lewis acid sites under the experimental conditions as shown by infrared spectroscopy. The rate of isobutylene formation from f-butanol was essentially independent of the degree of poisoning, and the true activation energy of the reaction remained constant at 25 kcal/mole, when the number of preadsorbed pyridine molecules varied between 3 and 9X 10n/m2. It thus, appears that Lewis sites which retain pyridine at temperatures between 550° and 150°C, respectively, do not interfere in this reaction. 

In the case of isobutanol dehydration, a promotional effect is observed (47). Isobutanol forms a surface carboxylate under reaction conditions (340), and this surface species gives rise to a typical symmetric COO -stretching vibration at 1567 cm 1. The CH-stretching vibration of the methylene group of isobutanol at 2870 cm-1 disappears on formation of the oxidized species. Consequently, the intensity of the 1567-cm 1 band can be taken as a measure of the surface concentration of the carboxylate species, whereas the intensity of the 2870-cm 1 band represents the surface concentration of molecular alcohol. The concentra- 

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