Ethene Dimerization on Zeolite-Hosted Ni Ions: Reversible Mobilization of the Active Site

The
active site in ethene oligomerization catalyzed by Ni-zeolites
is proposed to be a mobile Ni­(II) complex, based on density functional
theory-based molecular dynamics (DFT-MD) simulations corroborated
by continuous-flow experiments on Ni-SSZ-24 zeolite. The results of
the simulations at operating conditions show that ethene molecules
reversibly mobilize the active site as they exchange with the zeolite
as ligands on Ni during reaction. Microkinetic modeling was conducted
on the basis of free-energy profiles derived with DFT-MD for oligomerization
on these mobile [(ethene)2-Ni-alkyl]+ species.
The model reproduces the experimentally observed high selectivity
to dimerization and indicates that the mechanism is consistent with
the observed second-order rate dependence on ethene pressure.