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

Area of Science:

• Heterogeneous catalysis
• Zeolite chemistry
• Organometallic chemistry

Background:

• Zeolites are widely used as catalysts in petrochemical processes.
• Nickel-based catalysts are effective for olefin transformations.
• Understanding active sites and reaction mechanisms is crucial for catalyst design.

Purpose of the Study:

• To identify the active sites for 1-butene dimerization in Ni-exchanged zeolites.
• To investigate the influence of zeolite framework topology on catalytic performance.
• To elucidate the reaction mechanism and factors controlling activity and selectivity.

Main Methods:

• Ion-exchange of Ni2+ into CHA, MFI, and FAU zeolite frameworks.
• 1-butene dimerization reactions under supercritical conditions.
• Analysis of reaction products to determine selectivity and conversion.
• Kinetic studies to determine turnover frequencies and activation energies.

Main Results:

• Isolated Ni2+ cations at Al-pair sites are the active centers.
• Linear octene selectivity decreases with increasing zeolite pore size (CHA ≈ MFI > FAU).
• Turnover frequency increases significantly with pore size, with FAU showing the highest activity (approx. 2 orders of magnitude higher than CHA/MFI).

Conclusions:

• 1-butene dimerization proceeds via a Cossee-Arlman-type mechanism involving Ni-butyl complexes.
• Zeolite pore size influences reactivity by differential stabilization of reactants and transition states.
• FAU's larger pores enhance C-C coupling, leading to superior dimerization activity.