Mechanism of 2-propanol dehydrogenation with suspended nikel fine-particle catalyst.

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Selective dehydrogenation of 2-propanol was performed with a suspended nickel fine-particle catalyst at high activity under refluxing conditions. The isotope effects, kH⁄kD for (CH3)2CDOH and (CD3)2CDOD were found to be 2.56 and 2.69, respectively, in contrast to 1.68 for (CH3)2CHOD, suggesting that the C–H bond dissociation at the methine position is rate-determining. With regard to (CH3)2CHOD and (CH3)2CDOH, a distinct difference was observed for the deuterium distributions among the liquid-phase components after the reaction, since deuterium transfer from the hydroxyl to the methyl and other groups proceeded tremendously for (CH3)2CHOD, whereas the methine group of (CH3)2CDOH changed little, even after a 5-h reaction under refluxing conditions. This contrast was well accounted for by the difficult splitting of the methine C–H bond and the facile dissociation of the hydroxyl group, in addition to a gradual H–D exchange due to the keto–enol equilibriation of the adsorbed acetone and the hydrogen transfer from 2-propanol to acetone. Distributions of H2, HD, and D2 were interpreted as well, which varied with the reaction periods and the kinds of substrates. Based on the proposed mechanism, strategies for a catalyst improvement suitable for a newly-proposed chemical heat pump system are discussed.
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