Effect of a Fe-Doped g-C ₃ N ₄ Filler on Biogas Separation Performance in Polymers of Intrinsic Microporosity (PIM-1)

Integrating nanomaterials with tunable surface functionalities into polymer matrices offers a promising route for developing high-performance mixed matrix membranes (MMMs). Among these, graphitic carbon nitride (g-C 3 N 4 ) facilitates gas transport and enhances membrane permeability due to its intrinsic nanopores. However, while improving the CO 2 permeability, pristine g-C 3 N 4 often compromises the CO 2 /CH 4 selectivity. To address this trade-off, iron-doped g-C 3 N 4 (Fe-g-C 3 N 4 ) was incorporated into a polymer of intrinsic microporosity (PIM-1), possessing a high free volume and excellent gas diffusion characteristics. Iron doping served to increase the CO 2 -philicity of g-C 3 N 4 and strengthen its interaction with the CO 2 molecules, thereby improving both permeability and selectivity. The resulting Fe-g-C 3 N 4 /PIM-1 MMMs demonstrated enhanced CO 2 permeability and CO 2 /CH 4 selectivity, indicating their potential for use in CO 2 separation applications. Incorporating 0.5 wt % Fe-doped g-C 3 N 4 into the PIM-1 matrix led to a 38.5% increase in CO 2 permeability compared to pristine PIM-1 and a 12.7% improvement over the undoped g-C 3 N 4 -based MMM. While undoped g-C 3 N 4 reduced the CO 2 /CH 4 selectivity, Fe doping had the reverse effect, enhancing the selectivity by 7.2% relative to the undoped system. Notably, the Fe-g-C 3 N 4 /PIM-1 MMMs approach the 2008 Robeson upper bound for the CO 2 /CH 4 separation. In addition, the membrane exhibited improved mechanical strength and antiaging and antiplasticization characteristics, collectively outperforming pristine PIM-1.