Climate change, attributed largely to atmospheric CO2, continues to threaten the global environment and its inhabitants. Numerous efforts have endeavored to design membranes to remove CO2 from both industrial processes involving flue gas and natural gas, and have recently emphasized permeability rather than selectivity primarily through the development of such materials as polymers of intrinsic microporosity. What if, however, we instead focused on selectivity? In this work, we explore two approaches starting from different ends of the permeation spectrum that are used to fabricate organic membranes capable of CO2 ultraselectivity (i.e., CO2/N2 selectivity &gt; 100). In one instance, the starting point is a sustainable bionanoparticle, micro/nanofibrillated cellulose (MNFC), which by itself acts as a barrier to gas permeation. Upon addition of a hydrophilic ionic liquid (IL) to coat and separate MNFC fibrils followed by subsequent use of humidified feed gas, a “gate-opening” mechanism can be activated at intermediate relative humidity levels so that CO2 is permitted to selectively permeate through the hydrated IL. The selectivity levels measured here for such hybrid membranes exceed 300. In the second case considered, the starting point is a low-selectivity, ultrapermeable (CO2 permeability &gt; 1000 Barrer) membrane that is surface-functionalized to introduce CO2-philic groups. By integrating these components and two different transport mechanisms together, CO2 concentrates on the CO2-philic membrane surface and then quickly permeates through the high-permeability substrate polymer, yielding ultraselective and ultrapermeable membranes that not only defy the so-called Robeson upper bound but also remain economically viable and process compatible. The performance of these membranes, as discerned from selectivity-permeability trade-off plots, lies far above a new upper bound proposed on the basis of recently published results.

An integrated materials approach to ultraselective polymer membranes for carbon capture: Breaking through the upper bound

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