Abstract
The H/D exchange reaction between isobutane and acidic USY zeolite was studied using mass spectrometry and in situ1H NMR. An inhibiting effect of hydrogen on this reaction was observed. The H/D exchange is enhanced by both in situ and ex situ addition of small amounts of isobutene. A kinetic isotope effect of 1.5 was found for the H/D exchange of isobutane and 2-deuterium-labeled isobutane over D2O-exchanged HUSY (DUSY), as well as nondeuterated and perdeuterated isobutane over HUSY. The occurrence of a primary kinetic isotope effect (KIE) indicates that the slow step of the H/D exchange reaction involves a cleavage of the tertiary CH (or CD) bond in isobutane. After addition of small amounts of isobutene to the feed or pretreating the catalyst with small amounts of isobutene before starting the H/D exchange reaction, the KIE still exists, suggesting that the limiting step for the H/D exchange reaction between isobutane and acidic zeolite is the hydride transfer from an alkane to the exchanged cation.
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