Alkane metathesis by tantalum metal hydride on ferrierite: A computational study

The full catalytic cycle for the self-metathesis of ethane was studied by density functional theory (DFT). The active site was a Tadihydride grafted on a Brønsted acid site [(tAlO)₂Ta(H₂)] of the internal pore surface of the FER zeolite. The transition state geometries and activation energies were determined through the nudged elastic band (NEB) method for each elementary step, and the complete cycle was found to be thermodynamically consistent. Investigated elementary steps include ethane C-H s-bond activation, ethylene desorption through α and β hydrogen elimination mechanisms, Ta-ethylcarbene formation, olefin metathesis, and hydrogenation of olefin metathesis products. For the activation of ethane, as compared to catalytic systems involving zeolitesupported Ga and Zn, a low barrier (̃64 kJ mol^-1) was observed. In the olefin metathesis step, where Ta-ethylcarbene reacts with ethylene, it was found that the Ta- metallacyclobutane has a relatively high stability (̃143 kJ mol ^-1) as compared to similar metallacyclobutane species and that the forward decomposition of the Ta-metallacyclobutane is the most energetically demanding step.