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