NH4-exchange: a sensitive method for the characterization of structurally modified zeolites

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Abstract
The ammonium-exchange behaviour of the zeolite mordenite has been examined after structural modification with silicon hydrides. The changes in capacity and selectivity as a function of the degree of modification, the pretreatment conditions and the type of applied mordenite reveal features related to the nature and location of the incorporated groups. New hydroxyl groups, created after hydrolysis of Si—H bonds, have been shown to take part in ion-exchange processes. Silicon hydride modification occurs mainly in the large channels of mordenite, as has also been observed during gas-adsorption experiments. The new hydroxyl groups are only weakly polarized and thus the exchange capacity drops rapidly. On the other hand, the presence of extra-framework entities in the large channels promotes the reaction at the normally less accessible side-pockets. The new OH groups on these sites have a higher polarity and the loss in exchange capacity due to chemisorption becomes minimal.

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