(459d) Synthesis of MOF-Based Membranes through Chemical Vapor Growth

Metal organic framework (MOF) membranes possess unique properties for gas separation. To make MOF membranes a viable separation technology requires development of cost-effective synthesis of thin and high quality MOF membranes on scalable polymer support, or embedding nano-sized MOF crystals into polymer matrix to form defect-free mixed matrix membranes (MMMs). It was recently reported that atomic layer deposited ZnO can be transformed to ZIF-8 by exposure to 2-methylimidazole vapor [Nat. Mater.2016, 15:304–310]. Here, we developed a modified chemical vapor growth method as novel techniques for the synthesis of MOF-based supported membranes and MMMs. In this method, ZnO nanocrystals of ~5 nm in diameter were synthesized and used as metal source to react with vapor of organic ligands forming microporous ZIF structures.

For supported MOF membranes, this chemical vapor growth eliminates the homogeneous nucleation/growth of MOFs and the re-dissolution of membrane materials during membrane synthesis, enabling the formation of conformal ZIF-8 membranes as thin as 300 nm on porous polyethersulfone substrate through vapor treatment of a thin coating of ZnO nanocrystals. The membrane exhibits a molecular sieving effect towards gas molecules with the He/N₂ and CO₂/N₂ ideal selectivities close to those of ZIF-8 membranes made by the conventional liquid-phase synthesis. For MOF-based MMMs, the non-selective MOF-polymer interface remains a major challenge to overcome. In this work, a novel approach was developed to in situ form MOFs inside the polymer matrix by the chemical vapor growth method. Polymer/ZnO nanocomposite membranes were first prepared, followed with a ligand vapor treatment process to convert the embedded ZnO nanocrystals to MOFs inside the polymer matrix. The volume expansion of ZnO after being converted to MOF (~17 times for complete conversion to ZIF-8) is expected to occupy the ZnO-polymer interface, producing defect-free MOF-based MMMs. SEM and XRD confirmed the successful in situ phase transformation of ZnO to ZIF-8 inside the polymer matrix. The mechanical properties, gas permeation and separation properties of these interesting MMMs will also be discussed.