CHEMICAL SCIENCE

A novel pseudo-three-dimensional MOFs channel regulation strategy improves the separation performance of structurally similar gases


Recently, ren Qilong and Yang Qiwei teams of Zhejiang University collaborated with Dr. Xin He of the Massachusetts Institute of Technology and Professor Norbert Stock of the University of Kiel in Germany to propose a strategy for using three-dimensional bridge alkane ligands to achieve “quasi-three-dimensional” pore regulation. On July 18, 2022, the study, titled “Tunable Confined Aliphatic Pore Environment in Robust Metal–Organic Frameworks for Efficient Separation of Gases with a Similar Structure,” was published in J. A. Thompson. Am. Chem. Soc. Journal.

Metal-organic frameworks (MOFs) have periodic crystal structures formed by the self-assembly of inorganic metal nodes or metal clusters combined with organic ligands, and their modular construction methods and highly adjustable structural characteristics create a rich variety of ordered pores, providing conditions for meeting different application needs. Especially for isomorphic MOFs, it is easier to achieve fine adjustment of pore size and pore environment by changing the type of metal atom, the length of ligand molecules, or the derivatization of ligands without changing the topological type. The first strategy can impose subaeration-level changes in the node positions in the framework to achieve near-zero-dimensional fine adjustment in the coordination network, but due to the limitations of coordination and coordination capabilities, the available metal types are often limited; The latter two strategies have been widely used in various MOFs, but subject to the planar structure and chemical properties of aromatic rings, they can usually only extend or contract in approximately one dimension, which restricts the diversity of pore channels to a certain extent.

Figure 1: Schematic diagram of the channel regulation strategy

Recently, ren Qilong and Yang Qiwei teams of Zhejiang University collaborated with Dr. Xin He of the Massachusetts Institute of Technology and Professor Norbert Stock of the University of Kiel in Germany to propose a strategy for using three-dimensional bridge alkane ligands to achieve “quasi-three-dimensional” pore regulation. By adjusting the ring size and number of rings of the bridge ligand, the one-dimensional length of the ligand and its three-dimensional filling degree to the pore space can be changed at the same time, and it is possible to achieve richer and more diverse pore size and fine regulation of the pore environment than the regulation method based on the aromatic ring plane structure. At the same time, the structural rigidity of the bridge alkyl ligand is conducive to the formation of a stable frame structure with permanent porosity, and the non-aromatic nature of the bridge alkyl group creates an atypical near-fat pore environment, which in turn provides the possibility of enhancing the adsorption and separation of specific gases. Based on this strategy, two isomeric, water-stable new aluminum MOFs, ZUL-C1 and ZUL-C2 were successfully synthesized and applied to ch4/C2H6/C3H8 and Xe/Kr with similar molecular structures, and excellent separation results were obtained.

Figure 2: Schematic diagram of the structure of ZUL-C1 and ZUL-C2

The adsorption and separation properties of the materials were investigated by one-component adsorption equilibrium experiment and multi-component dynamic penetration experiment. From ZUL-C1 to ZUL-C2, the gas adsorption amount and IEST separation selectivity at low pressures have been significantly improved. For CH4/C2H6/C3H8 systems, ZUL-C2 exhibited the highest C2H6 adsorption capacity (1.16 mmol/g, 1 kPa) and the highest C2H6/CH4 (10:85) and C3H8/CH4 (5:85) selectivity (82, 741) at low pressures. For the Xe/Kr system, ZUL-C2 also combines high adsorption and high selectivity, not only obtaining excellent high purity Kr yield (5.54 mmol/g) in the dynamic separation of Xe/Kr (20:80) binary mixtures (model matter of air separation process by-products), but also in ultra-low concentration mixtures (model of nuclear posttreatment exhaust gases: 400 ppm Xe, 40 ppm Kr, 21% O2, 0.91% Ar, Excellent Xe adsorption (27.4 mmol/kg) was obtained in the dynamic separation of the remaining N2).

Figure 3: Separation performance of ZUL-C1 and ZUL-C2 on CH4/C2H6/C3H8 system and Xe/Kr system

Through the simulation and calculation of dispersion correction density functional theory (DFT-D), it is found that the increase in the number of ligand fat rings and the group product of the bridge ring group in the ZUL-C2 material effectively reduces the pore size in a specific direction, strengthens the pore limiting effect, and forms a concave and uneven pore surface with more alkyl sites, enhances the interaction between the gas molecules and the hydrogen bond, dipole and van der Waals force of the ZUL-C2 framework, and promotes the efficient capture of C2H6, C3H8 and Xe molecules by the frame pores.

Figure 4: Adsorption sites of C2H6, C3H8, and Xe in ZUL-C1 (a-c) and ZUL-C2 (d-f).

The successful application of the “quasi-three-dimensional” pore regulation strategy proposed in this study in CH4/C2H6/C3H8 separation and Xe/Kr separation shows its effectiveness and development potential, and is expected to provide useful guidance for material design and precise regulation of other challenging structurally similar substance separation processes. (Source: Science Network)

Related paper information:https://doi.org/10.1021/jacs.2c05448



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