Active sites in zeolites. Part 4.—n-Butene isomerization over deammoniated and partly hydrolysed NH4Y zeolites

Abstract

The isomerization of n-butenes was carried out on a series of NH₄Y catalysts pretreated at between 823 and 1 073 K under hydrolysing conditions or under conditions which produce hydrogen or dehydroxylated zeolites. Reaction parameters were determined in a continuous flow and in a circulation system. Poisoning experiments with pyridine and 2,6-dimethylpyridine in a pulse microcatalytic system combined with i.r. spectroscopy were applied to determine the number of active sites. The initial activity for but-1-ene disappearance changes in parallel with the supercage acidity. Poisoning experiments reveal that the supercage hydroxyl groups act as active centres. Turnover numbers calculated for these sites indicate that these hydroxyl groups are more heterogeneous in hydrolysed zeolites. On a prepoisoned catalyst the n-butene isomers are converted via a s-butyl carbonium ion. It is concluded that the self-poisoning of the catalyst is due to irreversible consumption of the supercage hydroxyl groups.

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