| Attributs | Valeurs |
|---|
| type
| |
| Is Part Of
| |
| Subject
| |
| Title
| - Hydrogen/Deuterium Exchange Kinetics by aSilica-Supported Zirconium Hydride Catalyst: Evidencefor a σ-Bond Metathesis Mechanism
|
| has manifestation of work
| |
| related by
| |
| Author
| |
| Abstract
| - Silica-supported molecular zirconium hydride species catalyze the hydrogenolysis of paraffins by initially activating carbon−hydrogen bonds under relatively mild conditions. Uncertainties regarding the nature of the hydrocarbon activation mechanism led us to study several H/D exchange processes. Measured H/D exchange rates between methane and hydrogen support the view that these reactions proceed via σ-bond metathesis. Evidence for this mechanism is based on the low energy of activation and the high negative entropy of activation.
- Silica-supported molecular zirconium hydride species have been the subject of numerousinvestigations and have been identified as highly active olefin polymerization catalysts.Recently it has been shown that these materials catalyze hydrogenolysis of paraffins byactivating carbon−hydrogen bonds under relatively mild conditions. Uncertainties regardingthe nature of the hydrocarbon activation mechanism led us to study several H/D exchangeprocesses. The rate and temperature dependence of H/D exchange between H2 and D2 andbetween CH4 and D2 was determined utilizing a glass recycle reactor in which reactantsand products were repeatedly passed over the catalyst. A sequential, single-atom exchangeprocess was observed for both exchange processes. More specifically, H2/D2 exchange to anequilibrium distribution was instantaneous even at liquid nitrogen temperatures. Weestimate the upper bound for the activation energy to be about 2 kcal/mol. Moreover, theexchange kinetics between CH4 and D2 are characterized by a large negative entropy ofactivation (−27 ± 3 eu) and relatively low energy of activation (∼7 ± 1 kcal/mol), consistentwith a σ-bond metathesis pathway.
|
| article type
| |
| is part of this journal
| |
