Evidence for the Coordination–Insertion Mechanism of Ethene Dimerization at Nickel Cations Exchanged onto Beta Molecular Sieves

The mechanistic origin of alkene dimerization on Ni sites supported on aluminosilicates has been ascribed to both coordination–insertion (i.e., Cossee-Arlman) and metallacycle-based cycles; the latter is often invoked in the absence of externally supplied cocatalysts or activators that generate Ni-hydride or Ni-alkyl species to initiate coordination–insertion cycles. Determining the prevalent reaction mechanism at Ni sites is often complicated by the formation and consumption of alkene dimer products via oligomerization and other parallel reactions (e.g., cracking, isomerization) that occur at Bronsted acid sites on supports. Here, ethene dimerization (453 K) was studied on Beta zeolites synthesized to contain predominantly exchanged Ni2+ sites according to site balances determined by cation exchange, and to Ni structure determined by CO infrared, UV–visible, and Ni K-edge X-ray absorption spectroscopies. The catalytic behavior of Ni2+ sites was isolated by suppressing contributions from residual H+ sites...