Hydrogen/deuterium exchange kinetics by a silica-supported zirconium hydride catalyst:: Evidence for a σ-bond metathesis mechanism

Silica-supported molecular zirconium hydride species have been the subject of numerous investigations and have been identified as highly active olefin polymerization catalysts. Recently it has been shown that these materials catalyze hydrogenolysis of paraffins by 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. The rate and temperature dependence of H/D exchange between H-2 and D-2 and between CH4 and D-2 was determined utilizing a glass recycle reactor in which reactants and products were repeatedly passed over the catalyst. A sequential, single-atom exchange process was observed for both exchange processes. More specifically, H-2/D-2 exchange to an equilibrium distribution was instantaneous even at liquid nitrogen temperatures. We estimate the upper bound for the activation energy to be about 2 kcal/mol. Moreover, the exchange kinetics between CH4 and D-2 are characterized by a large negative entropy of activation (-27 +/- 3 eu) and relatively low energy of activation (similar to7 +/- 1 kcal/mol), consistent with a a-bond metathesis pathway.