Vapor-Phase Synthesis and Modification of Metal-Organic Framework Membranes

Metal-organic frameworks (MOFs) hold promise as separation membranes due to their structural diversity stemming from plethora of metal/organic linker combinations enabled by direct synthesis and by post-synthetic modifications. Although relatively rapid progress for highly selective MOF membranes has been achieved, commercialization is still limited by the high processing costs and limited scalability associated with solvothermal processing. The recently introduced solvent-free synthesis method for ZIF membranes, called ligand induced permselectivation (LIPS), holds promise for overcoming this obstacle (Science 361, 1008-1011 (2018)). The LIPS method consists of a combination of atomic layer deposition of a dense oxide inside the mesopores of a porous support followed by transformation of the dense oxide deposit to nanoporous ZIF by exposure to sublimated vapors of an imidazolate ligand. It enables the formation of thin nanocomposite films consisting of ZIF deposited inside mesopores. Vapor-phase conversion of the impermeable ZnO to ZIF increases flux of certain gases more than others. For example, propylene flux is nearly 100 times faster than propane flux resulting in a commercially attractive combination of flux and selectivity. Systematic characterization of the microstructure of LIPS-ZIF-8 membranes and their permeation properties under industrially relevant conditions will be discussed. Moreover, the facile modification of ZIF-8 membranes made by LIPS (LIPS-ZIF-8) through a vapor phase ligand treatment (VPLT) method to tune their separation performance towards smaller molecules will be presented (Angew. Chem. Int. Ed. 58, 16390 –16394 (2019)). VPLT of a MOF membrane is shown to allow for systematic regulation of permeation properties. Of particular interest is the observed stable O₂/N₂ selective performance, which compares favorably with that of optimized gas separation polymeric membranes. It will be argued that the combination of LIPS and VPLT opens unprecedented levels of tunability for scalable thin film MOF processing for membrane and other thin film applications.