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

## TL;DR

This paper shows that isolated Ni2+ cations in zeolites are responsible for 1-butene dimerization, with activity and selectivity influenced by the size of the zeolite pores.

## Contribution

The study identifies isolated Ni2+ cations as active centers and explains how pore size affects dimerization activity and selectivity.

## Key findings

- Linear octene selectivity decreases with increasing zeolite pore size.
- FAU zeolites show much higher dimerization activity due to lower activation energy and better transition state stabilization.
- Butene dimerization occurs via a Cossee–Arlman-type coordination-insertion mechanism.

## Abstract

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 (T ≈
433 K and p
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 molbutene molNi
–1 s–1) ≫
MFI (∼0.36 molbutene molNi
–1 s–1) ≈ CHA (∼0.33 molbutene molNi
–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.