Metal–organic frameworks (MOFs) hold great potential
in
heterogeneous catalysis due to their unique properties and tunable
structures. Zeolitic imidazolate frameworks (ZIFs) are series of classical
MOFs that have been widely studied, where Zn2+ is coordinated
with four imidazoles to form a tetrahedral structure. ZIF-L, one type
of ZIF, is known for its unique two-dimensional (2D) leaf-like morphology
and large external specific surface area. In this work, we reported
a facile and green synthesis of the Ni-loaded ZIF-L (Ni-ZIF-L) catalysts,
where water as solvent and product collection by filtration make the
preparation easy for large-scale industrial production. Considering
the incompatibility of the d8 electron configuration of
Ni with the three-dimensional (3D) framework of ZIF-L, Ni can only
be selectively anchored to the surface of ZIF-L. The Ni-ZIF-L has
shown high crystallinity and 2D leaf-like microflakes similar to those
of pure ZIF-L. After systematic analysis, we speculate that the Ni
sites in Ni-ZIF-L samples have a square planar configuration, which
should be coordinated by two imidazole groups from the framework,
one NO3
– group, and one free imidazole
group. Ni-ZIF-L possesses fully exposed Ni active sites and a large
specific surface area, accounting for the good performance for ethylene
dimerization. In addition, the large particle size of the Ni-ZIF-L
catalyst is beneficial for separation. With the assistance of cocatalyst,
Ni-ZIF-L achieves an average ethylene turnover frequency of 330 320
moles of ethylene per mole of Ni per hour (1-butene selectivity >90%))
under 40 °C and 30 bar, comparable to the activity of the benchmark
heterogeneous catalyst. The isotope experiments are used to illustrate
that the ethylene dimerization catalyzed by Ni-ZIF-L follows the Cossee-Arlman
mechanism, which also rationalizes the high catalytic activity and
the small amount of isomerization and trimerization product formation.