Crystallinity-Enhanced CO2 Adsorption by Sodium Poly(Heptazine Imide) Frameworks.

This work presents sodium poly(heptazine imide) (NaPHI)-based materials, synthesized in a NaCl medium, as highly effective platforms for CO2 capture. High crystallinity-an often-overlooked aspect in PHI frameworks-is identified as a key factor governing CO2 adsorption capacity in microporous structures. Thermogravimetric analysis and manometric studies reveal a CO2 uptake of ≈3.8 mmol g-1, at 1 bar and 25 °C, surpassing most reported PHI-based adsorbents under similar conditions. Exchanging Na+ with K+ or Rb+ preserves CO2 adsorption performance, whereas Cs+ incorporation induces structural distortion, greatly reducing CO2 adsorption capacity in PHI. These materials exhibit excellent cyclic stability (20 cycles) without degradation and CO2 adsorption capacity loss. Notably, at flue gas-relevant temperature (100 °C), NaPHI attains a CO2 capacity of 2.1 mmol g-1, doubling the performance of benchmark Zeolite 13X (1.1 mmol g-1). Ideal Adsorbed Solution Theory confirms remarkable CO2/N2 selectivity (≈3.8 mmol g-1 vs typical N2 adsorption of 0.3 mmol g-1), a critical property for postcombustion CO2 capture. These findings position PHI-based materials as a disruptive platform for CO2 adsorption, offering 1) straightforward synthesis from readily available precursors, 2) promising scalability, and 3) outstanding performance.