Isolated Ni²⁺ Cations as the Active Centers for 1-Butene Dimerization in Zeolites

We have identified isolated Ni²⁺ cations, ion-exchanged at the Al-pair sites, as the active centers for 1-butene dimerization under supercritical reaction conditions (<i>T</i> ≈ 433 K and <i>p</i> _butene ≈ 42.5 bar) on three different zeolite frameworks, viz., small-pore CHA, medium-pore MFI, and large-pore FAU. The linear octene selectivity, at low 1-butene conversions, decreased systematically with the size of the pore openings of the zeolites: CHA (∼50%) ≈ MFI (∼46%) > FAU (∼27%). The turnover frequency for 1-butene conversion, on the other hand, followed the order: FAU (∼19.2 mol_butene mol_Ni ^-1 s^-1) ≫ MFI (∼0.36 mol_butene mol_Ni ^-1 s^-1) ≈ CHA (∼0.33 mol_butene mol_Ni ^-1 s^-1). Butene dimerization proceeds via a Cossee-Arlman-type coordination-insertion mechanism on in situ generated Ni-butyl complexes as the active reaction centers. The differences in reactivity stem from a lower intrinsic activation energy for C-C coupling in FAU due to the more spacious environment of its supercages. The larger pores of FAU stabilize adsorbed 1-butene less than the pores of CHA and MFI frameworks but stabilize the bulkier C-C coupling transition state better than the latter two. These varying degrees of stabilization of reactant and transition states result in the almost two orders of magnitude higher dimerization activity in Ni-exchanged FAU zeolites compared to CHA or MFI.