Enhanced natural gas purification by a mixed-matrix membrane with a soft MOF that has a CO₂-adapted nanochannel

Mixed-matrix membranes, which are fabricated by incorporating dispersed fillers into continuous polymers, have been considered as a promising and feasible platform for highly efficient gas separation. However, few have impurity-adapted nanochannels. Herein we present a group of mixed-matrix membranes (MMMs) incorporating a soft metal-organic framework (NTU-88c, activated NTU-88) into 6FDA-DAM. The rotation of the pyridyl rings of the ligands in NTU-88c creates a nanochannel with an aperture size of 3.4 Å, which falls between the kinetic parameters of CO₂ (3.3 Å) and CH₄ (3.8 Å). The membrane shows enhanced CO₂ permeability (1140 Barrer) at 60 °C, accompanied by a good separation factor of 45. In addition, an increase in feed gas pressure 1.0 MPa (25 °C) resulted in a notable enhancement in CO₂ permeability (645 Barrer), accompanied by a high CO₂/CH₄ separation factor (41). More importantly, the MMM has demonstrated durable and stable CO₂/CH₄ separation performance in long-term test, exceeding the Robeson upper-bound, suggesting a promising potential for natural gas purification. Moving forward, the findings of this work not only demonstrate the importance of porous fillers that have impurity-adapted nanochannels for membrane separation, but also offer a route to design and construct high-performance MMMs for feasible applications.