Functional MOF-guided mixed matrix membranes for CO2 separation: Current breakthroughs and future horizon

Global warming due to an unbridled increment in CO 2 levels in the atmosphere is the greatest threat to global sustainability. Therefore, gas separation/purification is an urgent need. The reduced cost, simplicity, energy efficiency, and green process have made the polymeric membrane technology as the most interest candidate for carbon capture during the last few decades. However, the “trade-off” effect between permeability-selectivity has restricted large-scale applications of single/pristine membranes. Mixed matrix membranes (MMMs) formed by inserting inorganic nanomaterials into polymers have improved membranes' effectiveness to reach Robeson's bounds, however, the defective interface structure has reduced their performance. Accordingly, metal-organic frameworks (MOFs) have been introduced as alternative compounds due to their structural diversity, flexibility, ultra-high surface areas, and open metal sites, not presented by conventional nanofillers. Interfacial design strategies (surface functionalization) can further advance polymer/filler compatibility so that an almost defect-free interface with improved thermomechanical stability is envisaged. First, a comparison is made between membrane and other methods for CO 2 /CH 4 and CO 2 /N 2 separation. MMMs' basic principles and diverse kinds of nanofillers are then reviewed. MOFs' basic principles and their advantages over other fillers are explained. Functional groups, functionalization approaches (pre- and post-synthetic functionalization), and surface functionalities on tuning permeability are investigated. To specialize, the most used and more permeable polymers (i.e., a family of polymers like polyimides (PIs)) modified with functionalized MOFs are introduced. The synergetic effects and mutual interactions between polymers/nanofillers are scrutinized. Lastly, the biggest technical issues of current MMMs and suggestions for future studies are highlighted.