Improving the CO2 permeability and selectivity of Pebax mixed-matrix membranes by constructing an "expressway" using pyrazine-based nitrogen-doped porous carbon with nitrogen-containing groups and meso-microporous structure
To overcome the "trade-off" effect and simultaneously increase the CO 2 permeability and selectivity of the traditional Pebax membranes. The pyrazine-based nitrogen-doped porous carbon (NPC) was incorporated into Pebax-2533 to fabricate mixed-matrix membranes (MMMs). Firstly, a novel porous organic polymer (POP-1) was prepared using melamine and 2,6-dichloropyrazine as carbon and nitrogen sources. Five NPCs of different nitrogen content (NPC-1, NPC-2, NPC-3, NPC-4, and NPC-5) were obtained by adjusting the carbonization temperature and activator type. The NPCs had a large specific surface area (S BET , 976–2500 m 2 g -1 ) and nitrogen content (up to 7.6–12.9%). The CO 2 adsorption capacity (2.90–3.49 mmol/g), and CO 2 /N 2 selectivity (23.2–37.8) of the NPCs were also high, and their cycle regeneration performance was good. Using Materials Studio 7.0 software to calculate the CO 2 adsorption sites and binding energies of NPCs’ precursors, it was confirmed that the NPCs had a strong adsorption capacity for CO 2 . Then, the synthesized NPC-1 with meso-microporous structure and nitrogen-enriched units were used to prepare NPC-1/Pebax MMMs by the solution casting method. By introducing alkaline nitrogen-containing groups, a continuous meso-microporous structure was constructed within the membrane, forming an "expressway" channel that can promote rapid CO 2 transport. It could significantly improve the CO 2 permeability and CO 2 /N 2 selectivity of the MMMs. When the CO 2 permeability of the MMMs with an NPC-1 content of 3 wt% was 457 Barrer, the CO 2 /N 2 selectivity reached 64.7, much higher than that of the pristine Pebax membrane (approaching the revised Robertson upper bound in 2019). The CO 2 permeability, solubility, and diffusivity coefficient of the MMMs with NPC-1 content of 7 wt% were 491 Barrer, 61.36 cm 3 (STP)/(cm 3 ·cmHg), and 7.05 cm 3 (STP)/(cm 3 ·cmHg), respectively, which were 211%, 101%, and 62% higher than the pristine Pebax membrane. In addition, the 3NPC/Pebax MMMs exhibit excellent mechanical properties (up to 34 MPa), and the gas separation performance does not rapidly decrease after long-term stability for 360 h (15%CO 2 /N 2 ). The above results indicated that pyrazine-based NPC has good CO 2 adsorption ability and can improve the CO 2 permeability and selectivity of Pebax membranes, which provides a further research foundation for the application of NPC in polymer membranes.
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