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.