Ni-Loaded 2D Zeolitic\nImidazolate Framework as a Heterogeneous\nCatalyst with Highly Activity for Ethylene Dimerization

Abstract

Metal–organic frameworks (MOFs) hold great potential\nin\nheterogeneous catalysis due to their unique properties and tunable\nstructures. Zeolitic imidazolate frameworks (ZIFs) are series of classical\nMOFs that have been widely studied, where Zn²⁺ is coordinated\nwith four imidazoles to form a tetrahedral structure. ZIF-L, one type\nof ZIF, is known for its unique two-dimensional (2D) leaf-like morphology\nand large external specific surface area. In this work, we reported\na facile and green synthesis of the Ni-loaded ZIF-L (Ni-ZIF-L) catalysts,\nwhere water as solvent and product collection by filtration make the\npreparation easy for large-scale industrial production. Considering\nthe incompatibility of the d⁸ electron configuration of\nNi with the three-dimensional (3D) framework of ZIF-L, Ni can only\nbe selectively anchored to the surface of ZIF-L. The Ni-ZIF-L has\nshown high crystallinity and 2D leaf-like microflakes similar to those\nof pure ZIF-L. After systematic analysis, we speculate that the Ni\nsites in Ni-ZIF-L samples have a square planar configuration, which\nshould be coordinated by two imidazole groups from the framework,\none NO₃^– group, and one free imidazole\ngroup. Ni-ZIF-L possesses fully exposed Ni active sites and a large\nspecific surface area, accounting for the good performance for ethylene\ndimerization. In addition, the large particle size of the Ni-ZIF-L\ncatalyst is beneficial for separation. With the assistance of cocatalyst,\nNi-ZIF-L achieves an average ethylene turnover frequency of 330 320\nmoles of ethylene per mole of Ni per hour (1-butene selectivity >90%))\nunder 40 °C and 30 bar, comparable to the activity of the benchmark\nheterogeneous catalyst. The isotope experiments are used to illustrate\nthat the ethylene dimerization catalyzed by Ni-ZIF-L follows the Cossee-Arlman\nmechanism, which also rationalizes the high catalytic activity and\nthe small amount of isomerization and trimerization product formation.