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

Abstract

The mechanistic origin of alkene dimerization on Ni sites supported on aluminosilicates has been ascribed to both coordination−insertion (i.e., Cossee-Arlman) and metallacyclebased cycles; the latter is often invoked in the absence of externally supplied cocatalysts or activators that generate Ni-hydride or Nialkyl 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 Brønsted acid sites on supports. Here, ethene dimerization (453 K) was studied on Beta zeolites synthesized to contain predominantly exchanged Ni 2+ 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 Ni 2+ sites was isolated by suppressing contributions from residual H + sites on support structures, either by selectively poisoning them with Li + cations or NH 4 + species, or by weakening them using a zincosilicate support. Brønsted 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 Ni 2+ -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 Ni 2+ cations exchanged onto molecular sieves.

Olefin Oligomerization by Main Group Ga3+ and Zn2+ Single Site Catalysts

“…Ethylene insertion to the metal hydride regenerates the metal alkyl intermediate completing the catalytic cycle. 12,13 It is generally accepted that empty d orbitals of the transition metal catalysts are required for ole n coordination-insertion and β-hydride elimination and the formation of metal alkyl and metal hydride reaction intermediates. 20 Recently, silica-supported, single site Ga 3+ and Zn 2+ were reported for alkane dehydrogenation and ole n hydrogenation where metal hydride and alkyl intermediates and ole n insertion and β-hydride elimination elementary steps were proposed.…”

Catalytic activation of ethylene C-H bonds on uniform d8 Ir(I) and Ni(II) cations in zeolites: toward molecular level understanding of ethylene polymerization on heterogeneous catalysts

“…12 This attribution was subsequently supported for ethylene dimerization on Ni/BEA, although Ni-H species were not observed directly. 19 Until now, it remained unclear how ethylene, in the absence of M-hydride species, can polymerize considering the importance of M-H intermediates in the Cossee-Arlman mechanism. Theoretical studies have identified potential mechanisms for ethylene dimerization on Ni/BEA where the metallocycle, protontransfer, and Cossee-Arlman mechanisms were compared.…”

“…Though reported for other d 8 metals, it is not straightforward to generate uniform Ni(II) species since they may graft to both silanol nests and various extra-framework zeolite positions, evidenced by IR spectroscopy of CO adsorption. 19 This brought into question the true active center for ethylene oligomerization activ-ity. 21 To better understand the active centers for ethylene dimerization, well-defined supported complexes of Ir(I) and Ni(II) were generated, characterized, and tested in this study.…”

Activation of Ethylene C-H Bonds on Uniform D8 Ir(I) and Ni(II) Cations in Zeolites: Catalytic Butadiene and Butenes Production Under Mild Conditions

“…12 This attribution was subsequently supported for ethylene dimerization on Ni/BEA, although Ni-H species were not observed directly. 19 Until now, it remained unclear how ethylene, in the absence of M-hydride species, can polymerize considering the importance of M-H intermediates in the Cossee-Arlman mechanism. Theoretical studies have identified potential mechanisms for ethylene dimerization on Ni/BEA where the metallocycle, proton-transfer, and Cossee-Arlman mechanisms were compared.…”

“…Though reported for other d 8 metals, it is not straightforward to generate uniform Ni(II) species since they may graft to both silanol nests and various extra-framework zeolite positions, evidenced by IR spectroscopy of CO adsorption. 19 This brought into question the true active center for ethylene oligomerization activity. 21 To better understand the active centers for ethylene dimerization, well-defined supported complexes of Ir(I) and Ni(II) were generated, characterized, and tested in this study.…”