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