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 on support structures, either by selectively poisoning them with Li+ cations or NH4+ species, or by weakening them using a zincosilicate support. Bronsted acid sites form linear dimers (1-butene, cis-2-butene, trans-2-butene) in thermodynamically equilibrated ratios, in addition to their skeletal isomers (isobutene) and products of subsequent oligomerization-cracking cycles; hence, isobutene formation rates serve as a kinetic marker for the presence of H+ sites. After residual H+ sites deactivate during initial reaction times or when they are suppressed prior to reaction, linear butene isomers form in nonequilibrated ratios that are invariant with ethene site-time, reflecting primary butene double-bond isomerization events catalyzed at Ni2+-derived active sites. Ni-zeolites pretreated in oxidative environments (5 kPa O-2, 773 K) show transient activation periods during initial reaction times at dilute ethene pressures (<0.4 kPa) but not at higher ethene pressures (>0.4 kPa) or in the presence of co-fed hydrogen (5 kPa). This behavior is consistent with in situ ethene-assisted formation of [Ni(II)-H](+) intermediates, which isotopically scramble H-2-D-2 mixtures (453 K) and are quantified from surface H/D exchange reactions (453 K). Taken together, these findings provide unambiguous evidence for the coordination insertion mechanism as the dominant route for alkene dimerization at Ni2+ cations exchanged onto molecular sieves.