(314f) Ethylene Oligomerization on Ni/Uio-66: Rates, Mechanism, and Site Densities

UiO-66 metal organic frameworks (MOFs) with nickel supported on the zirconium oxide framework (Ni-UiO-66) omit the use of cocatalysts and activators for ethylene oligomerization typical in homogeneous catalysts, but rates on MOF-supported catalysts are characterized with induction periods dependent on process parameters. We show that higher ethylene pressures result in shorter transients and higher intrinsic rates for ethylene oligomerization on Ni-UiO-66, while maintaining unprecedented stability of the catalyst (>15 days on-stream) by creating relevant active sites on-stream. Increased intrinsic rates (~2-10x) but invariant selectivity-conversion characteristics are observed at the same ethylene reference pressure (106 kPa) after exposing the catalyst to higher pressures (657 kPa and 1000 kPa) of ethylene. These observations suggest that only one type of active site is generated during the reaction and the enhancement in rate can be attributed to the abundance of active sites. Chemical titration experiments with NO (0.02 – 0.15 μmol s^-1) in situ during ethylene oligomerization enumerate relevant nickel active sites to affirm nickel active sites are engendered by higher ethylene pressures. Intrinsic ethylene oligomerization rates exhibit first order dependence in ethylene pressure and activation energies of 81 kJ mol^-1. Density functional theory (DFT) on cluster models of Ni-UiO-66 is employed to compare computed and experimental activation enthalpies to reveal that a Phillips-type mechanism is consistent with the induction period and steady state kinetics observed for ethylene oligomerization.