Defining New Limits in Gas Separations Using Modified ZIF Systems

Zeolitic-imidazolate frameworks (ZIFs) are candidate materials for the next generation of membranes for cheaper, "greener" separations. More than a decade after ZIF introduction, the high propylene/propane selectivity of ZIF-8 and ZIF-67 is the only example of ZIF membranes with remarkable selectivity efficiency despite their numerous advantages over other families of materials. Herein, we demonstrate the effectiveness of molecular-scale modification in the design of new ZIF materials useful for the separation of important and highly challenging mixtures such as He/CH₄, H₂/CH₄, O₂/N₂, CO₂/CH₄, and CO₂/N₂. Via computational methods, metal and linker substitutions are employed to produce new ZIF-8 variants with a finely discretized range of aperture sizes, as these govern the kinetic-based selectivity of the material. The permeability and selectivity through the ZIF-8 variants of the gases under study are estimated, and their performance is compared with an extensive number of polymeric, metal-organic framework, covalent-organic framework, and mixed-matrix membranes. The comparison shows that some of the ZIF-8 analogues can be used as membranes of unprecedented high separation performance. The scope of this work is to highlight the effectiveness of the molecular level design as means of membrane development to address the global need for cheaper separation methods and CO₂ emission reduction.