Identical Composition and Distinct Performance: How ZIF-8 Polymorphs’ Structures Affect the Adsorption/Separation of Ethane and Ethene

Abstract

Zeolitic imidazolate frameworks (ZIFs) can separate olefins from paraffin with very little energy consumption, which is critical for the chemical industry. They can form a large diversity of polymorphs, which show identical chemical formulas but distinct olefin/paraffin separation performance. The adsorption and separation of ethane/ethene on several ZIF-8 polymorphs, some recently synthesized in experiments, were studied by grand canonical Monte Carlo and ideal adsorbed solution theory simulations in this work. These two methods yielded consistent results, which also agree with experiments. The adsorption mechanism, especially the relationship between the spatial arrangement of the metal and ligand in these polymorphs and the adsorption of olefins/paraffin, was revealed via in-depth investigation of the simulation data. The adsorption of ethane/ethene and propane/propene was compared to further elucidate that the low pressure uptake well correlates with ZIF–gas interaction, for example, the heat of adsorption and Henry coefficient, while high pressure uptake correlates with the ZIFs’ geometrical structures, such as porosity or accessible surface area. The adsorption competition between ethane and ethene in the mixture and the separation performance of the ZIF polymorphs were also investigated. Our work could help understand how ZIFs’ structures affect their olefin/paraffin adsorption behavior and develop nanoporous materials with improved separation performance.

Effects of Ligand Substituents on the Character of Zn-Coordination in Zeolitic Imidazolate Frameworks

“…All of these crystals have the same network topology (SOD). 46 The rhombohedral unit cell adopted in the calculations of all the considered ZIF-8-X (with X = CH 3 , H, Br, Cl), see Figure 1a, is retrieved from the initial cubic cell using the code seekpath. 47 To preserve the symmetry of the initial structures, the input lattice parameters of all of the considered ZIF-8 variants (see Table S1 of the Supporting Information) are kept constant during optimization, which thus entails only the minimization of the interatomic forces.…”

“…In our bulk models, we took the molecular orientation giving rise to a bulk with I̅ 43 m symmetry. All of these crystals have the same network topology (SOD) . The rhombohedral unit cell adopted in the calculations of all the considered ZIF-8-X (with X = CH 3 , H, Br, Cl), see Figure a, is retrieved from the initial cubic cell using the code seek-path .…”

Understanding and controlling the nucleation and growth of metal–organic frameworks

“…61 Since polymorphs can display different crystal structures while retaining identical molecular composition, the structure-function relationship based on structural parameters can be investigated. 62,63 Polymorph transition can be induced post-synthesis through physical steps like additional heating, 64 pressure, or template mediation. 65 Typically, post-synthetic phase transitions result in the formation of more dense, thermodynamic crystalline phases through reconstruction or displacement of internal structures.…”

Understanding the Nucleation and Growth of ZIF-8 Polymorphs

“…4−6 Polymorphism in MOFs can be a determining factor in function, as demonstrated by the ZIF-8 system where enhanced separation of ethane and ethene is observed for more dense ZIF-8 structures such as ZIF-8_I4̅ 3m, ZIF-8_R3m, and ZIF-8_Cm. 7 While much work has focused on the characterization of MOF polymorphs 8 and the synthetic factors affecting polymorph selection, the mechanisms by which different polymorphs form have largely been unexplored. This leaves a substantial knowledge gap in our understanding of MOF nucleation and growth.…”

“…Metal-organic frameworks (MOFs) are a class of nanoporous materials consisting of inorganic metal nodes that are bridged by organic linkers. , MOFs have a wide range of applications due to their ultrahigh surface area, fixed porosity, and high thermal stability . MOFs can exhibit polymorphism, where the MOF structure contains identical chemical composition, but varies in network topology. − Polymorphism in MOFs can be a determining factor in function, as demonstrated by the ZIF-8 system where enhanced separation of ethane and ethene is observed for more dense ZIF-8 structures such as ZIF-8_ I 4̅3 m , ZIF-8_ R 3 m , and ZIF-8_ Cm …”