Ning Yang#, Hong-Xin Li#, Logan Ritter, Guo-Tong Du, Xin-Ai Guo, Brian Space, Dong-Xu Xue*. Inorg. Chem. 2024, DOI: 10.1021/acs.inorgchem.4c02473

Ethylene (C₂H₄), a cornerstone in the pantheon of petrochemical products, is instrumental in the synthesis of a plethora of valuable organic compounds and polymers. The impurities of ethane and acetylene in ethylene products can´t meet the requirements of industrial use. The traditional method of obtaining high purity ethylene has high energy consumption and high cost, which hinders its practical application. Therefore, it is of great significance to find a low cost and high efficiency ethylene separation technology.

Figure 1. Schematic representation showing the assembly of Ce-NTB-rtk. Top: the frameworks of kgd and rtl are split and then merged to form Ce-NTB-rtk; bottom: kgd and rtl nets are merged to form rtk network. Ce = green, C = gray, N = blue/pink, O = red and F = cyan. Hydrogen atoms and the terminally coordinated methanol molecules are omitted for clarity.

Metal-organic frameworks (MOFs), a new class of crystalline porous materials renowned for their tunable pore sizes, diverse chemical functionalities, and high surface areas. Adsorptive purification technology based MOFs, with its potential for energy efficiency and operational cost reduction, has been hailed as a promising avenue for the future of gas separation. While MOFs have been explored for the separation of binary mixtures such as C₂H₂/C₂H₄ or C₂H₆/C₂H₄, respectively, the simultaneous removal of C₂H₂ and C₂H₆ from a ternary mixture is a more complex and challenging task. This is largely due to the intermediate quadrupole moment and polarizability of C₂H₄, which fall between those of C₂H₂ and C₂H₆, making it difficult to simultaneously remove C₂H₂ and C₂H₆ in a single step. Although a few MOF materials have been reported to enable single-step purification of ethylene from a ternary C₂ mixture, the design and synthesis of such materials remains a pressing need and a formidable challenge.

Figure 2. Experimental column breakthrough curves for (a) C₂H₂/C₂H₄/He (25/25/50) mixture under a flow of 2 mL min^-1, (b) C₂H₆/C₂H₄/He (25/25/50) mixtures under a flow of 2 mL min^-1 and (c) C₂H₂/C₂H₄/C₂H₆/He (16.5/16.5/16.5/50) mixture under a flow of 1.2 mL min^-1 in an absorber bed packed with Ce-NTB-rtk at 298 K and 1.0 bar. (d), (e) and (f) are their regenerative tests, respectively.

In our recent work, we have focused on the development of innovative MOFs through a ligand-conformer strategy. By utilizing amide or imine inserted tritopic carboxylate ligands, novel three-way rod and multi-cage zirconium-based MOFs have been successfully constructed. Additionally, cerium, a non-toxic, cost-effective, and abundant metal element, has been less explored in the construction of new MOFs compared to other rare-earth metals. Most cerium-based MOFs reported in the literature are structured from rod inorganic building units, while cluster-based cerium MOFs are relatively scarce. This presents an opportunity to expand the scope of cerium-based materials and to investigate their potential in various applications.

Figure 3. Preferential (a) C₂H₂, (b) C₂H₄, and (c) C₂H₆ molecule binding sites within Ce-NTB-rtk simulated with GCMC.

Building on above considerations, we have introduced a propeller-like ligand as the connecting node and utilized cerium as the metal source to construct a new cerium-based MOF material, i.e., Ce-NTB-rtk. In this structure, in situ generate a rare tetranuclear cerium cluster of [Ce₄F₂(O₂C-)₁₂], such cluster is observed for cerium ion for the first time, which is further connected via NTB linkers in a mode of μ₅-η²:η¹:η¹:η²:η¹. Topological analysis reveal that it is a new (3,3,12)-c net. Considering the occurrence of two crystallographically independent NTB linkers (i.e., ligand conformer) within Ce-NTB-rtk, such (3,3,12)-c framework can be split into a 2D (3,6)-c kgd net and a 3D (3,6)-c rtl one, consequently resulting in a merged (3,3,12)-c rtk network. The results of low-pressure gas adsorption isotherms, IAST selectivity and adsorption enthalpy data indicate that Ce-NTB-rtk has the potential to purify ethylene from ternary C₂ gas in one step, which is further verified by breakthrough test and GCMC theoretical calculation. The good separation performance of Ce-NTB-rtk is mainly due to its suitable microporous cavity and nonpolar microenvironment. This discovery further illuminates the efficacy of the ligand-conformer approach in the synthesis of novel metal clusters and the subsequent creation of porous frameworks, paving the way for their application in cutting-edge technologies.

First Authors: Ying Ning, master’s candidate, Li Hongxin, doctoral candidate, Shaanxi Normal University

Correspondence Author: Prof. Xue Dongxu, Shaanxi Normal University

Full Text Link: https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c02473