(673f) Adsorption-Enhanced, Plasticization Resistant Composite Membranes Using Metal–Organic Framework Nanocrystals

Here, we have developed a method for improving the gas separation performance for membranes in a broadly applicable and synergistic way. Specifically, we address two fundamental problems with conducting gas separations in neat polymer materials, namely selectivity/permeability tradeoffs and membrane plasticization. These two phenomena limit the application of membrane materials in numerous processes, and one severe case is the separation of ethylene from ethane. As the condensability and kinetic diameter of these gases are similar, there is no good chemical handle to conduct a separation. Introducing adsorption-based selectivity by compositing nanocrystals of the metal–organic frameworks M₂(dobdc) (M = Mg, Mn, Co, Ni; dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate) with the polyimide 6FDA-DAM (6FDA = 4,4¢-(hexaflouroisopropylidene)diphthalic anhydride; DAM = 2,4,6-trimethyl-1,3-phenylenediamine), a unique chemical handle is used to improve the permeability of ethylene and the selectivity for ethylene over ethane. Additionally, we found that the metal–organic frameworks physically crosslink the polymer, which addresses plasticization, the second major issue with polymers for this gas separation.

We then expanded on this work by addressing another important gas separation that is limited by plasticization - natural gas purification - by again applying the crosslinking effects of the metal–organic framework nanocrystals. In this case, nanocrystals of the metal–organic framework Ni₂(dobdc) were incorporated into six different polyimides that can be used for this gas separation. This resulted in significantly improved plasticization response in the majority of materials, as measured by CO₂ permeation hysteresis and binary gas permeation measurements. Notably, high selectivity is retained under pressures of CO₂ in excess of 20 bar. These pressures represent some of the most aggressive feed environments that might be encountered in the field. Together, these works introduce a novel handle to improve permselectivity in addition to a new crosslinking mechanism to improve membrane stability.