Ethene Oligomerization in Ni-Containing Zeolites: Theoretical Discrimination of Reaction Mechanisms

Ni-containing porous aluminosilicates
are promising heterogeneous
catalysts for oligomerization of ethene, but little is known about
the catalytic cycle. In addition, it remains unclear why the aluminosilicates
work without the alkyl aluminum cocatalyst needed in homogeneous catalysis.
As the first of its kind, this work uses density functional theory
(DFT) to identify the most probable mechanism of oligomerization and
active site formation. The periodic DFT calculations employed the
BEEF-vdW functional to consider both short-range interactions involved
in bond formation and long-range interactions with the zeolite framework.
The calculations targeted Ni-containing SSZ-24 zeolite as a representative
catalyst and considered Ni+, Ni2+ ions, and
neutral nickel atoms as active sites. We investigated the catalytic
cycles of the metallacycle and Cossee–Arlman mechanisms that
have been proposed in the literature, in addition to a new proton-transfer
mechanism. Free energy profiles were derived at a typical experimental
reaction temperature of 393 K and used to kinetically discriminate
the mechanisms with the energetic span model. On the basis of the
results, we predict the Cossee–Arlman mechanism known from
homogeneous catalysts to prevail also in the zeolite catalyst. The
calculated intrinsic enthalpy of activation of 77 kJ/mol for ethene
dimerization agrees well with available experimental data. We further
propose a mechanism for formation of the active nickel–alkyl
species by reaction between ethene and isolated Ni2+ ions.
The results hence provide a solid starting point for experimental
investigations of the catalytic cycle, to validate our predictions
and ultimately determine the atom-scale properties that control catalytic
activity.