Journal Article oxygen permeation 2D channels MOF membrane MOFs Research Support, Non-U.S. Gov't angstrom‐scale defect cation stoichiometry cluster defect confined metals gas separation hollow fibers linker defect low temperature membranes mixed ionic and electronic conductors mixed-matrix membranes molecular recognition organic separation perovskite oxyfluoride membrane perovskites propylene/propane separation size-sieving