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.