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-C3N4) facilitates gas transport and enhances membrane permeability due to its intrinsic nanopores. However, while improving the CO2 permeability, pristine g-C3N4 often compromises the CO2/CH4 selectivity. To address this trade-off, iron-doped g-C3N4 (Fe-g-C3N4) 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 CO2-philicity of g-C3N4 and strengthen its interaction with the CO2 molecules, thereby improving both permeability and selectivity. The resulting Fe-g-C3N4/PIM-1 MMMs demonstrated enhanced CO2 permeability and CO2/CH4 selectivity, indicating their potential for use in CO2 separation applications. Incorporating 0.5 wt % Fe-doped g-C3N4 into the PIM-1 matrix led to a 38.5% increase in CO2 permeability compared to pristine PIM-1 and a 12.7% improvement over the undoped g-C3N4-based MMM. While undoped g-C3N4 reduced the CO2/CH4 selectivity, Fe doping had the reverse effect, enhancing the selectivity by 7.2% relative to the undoped system. Notably, the Fe-g-C3N4/PIM-1 MMMs approach the 2008 Robeson upper bound for the CO2/CH4 separation. In addition, the membrane exhibited improved mechanical strength and antiaging and antiplasticization characteristics, collectively outperforming pristine PIM-1.