Ni-Loaded 2D Zeolitic Imidazolate Framework as a Heterogeneous Catalyst with Highly Activity for Ethylene Dimerization

Abstract

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.