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.

Ultra‐Highly Active Ni‐Doped MOF‐5 Heterogeneous Catalysts for Ethylene Dimerization

“…The the unique d 8 electronic configuration of Ni 2+ . [22,23] The activity for catalyzing ethylene dimerization on Ni-ZIF particles is the highest among all heterogeneous catalysts and exceeds most homogeneous catalysts reported so far. At 30 bar and 35 °C, Ni-ZIF with 0.7 wt% Ni loading showed a TOF of 297 000 h −1 , corresponding to 992 g of butene per gram of Ni-ZIF per hour.…”

“…To directly illustrate the superiority of this catalyst for catalyzing ethylene dimerization, we compared its activity with that of the reported heterogeneous catalysts. [11,15,[19][20][21][22][23][29][30][31][32][33][34][35][36][37][38] As shown in Figure S12 (Supporting Information), the average TOF of Ni-MOF-5 is higher than most of the reported heterogeneous catalysts, except Ni-ZIF-8 and Ni-ZIF-L. We speculate that compared with the reported catalysts with small pore sizes and catalysts obtained by post-synthesis that caused structural breakage and pore blockage, our catalyst synthesized by one-step synthesis have relatively large straight channels (≈1.2 nm) and regular structure to achieve a high TOF. Meanwhile, the TOF of Ni-MOF-5 is lower than those of ZIF-8 and ZIF-L with Ni active centers anchored on the surface of the catalysts, which also supports the speculation.…”

“…[ 20,21 ] Very recently, we designed one kind of striking MOF Ni‐ZIF by a simple one‐pot synthesis method, where Ni can only be selectively anchored on the surface of the particles because of the unique d 8 electronic configuration of Ni 2+ . [ 22,23 ] The activity for catalyzing ethylene dimerization on Ni‐ZIF particles is the highest among all heterogeneous catalysts and exceeds most homogeneous catalysts reported so far. At 30 bar and 35 °C, Ni‐ZIF with 0.7 wt% Ni loading showed a TOF of 297 000 h −1 , corresponding to 992 g of butene per gram of Ni‐ZIF per hour.…”

Facile preparation of a Ni–imidazole compound with high activity for ethylene dimerization

“…16 Subsequently, another Ni-ZIF-L catalyst with 0.49 wt% Ni loading was also developed in our group, which showed an even higher activity with a TOF of 330 320 h −1 . 17 However, we realized that the Ni loading amount correlated with the external surface area of the support determined the overall performance of these catalysts.…”

Heterogenization of molecular catalysts within porous solids: the case of Ni-catalyzed ethylene oligomerization from zeolites to metal–organic frameworks

“…As an alternative to ZIF-8, Dong and coworkers recently reported Ni-ZIF-L as highly active ethylene dimerization catalyst. 141 ZIF-L present similar composition and chemical environment than ZIF-8 but with different topology, with quite lower surface area of ca. 160 m 2 g À1 , which allows the insertion of nickel atoms only at the surface of the MOF's particles.…”