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 (TGA) and manometric studies revealed a CO₂ 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+ preserved CO2 adsorption performance, whereas Cs+ incorporation induced structural distortion, greatly reducing CO2 adsorption capacity in PHI. These materials exhibited excellent cyclic stability (20 cycles) without degradation and CO2 adsorption capacity loss. Notably, at flue gas-relevant temperature (100 °C), NaPHI attained a CO₂ capacity of 2.1 mmol/g, doubling the performance of benchmark Zeolite 13X (1.1 mmol/g). Ideal Adsorbed Solution Theory (IAST) confirmed remarkable CO₂/N₂ selectivity (~3.8 mmol/g vs. typical N₂ adsorption of 0.32 mmol/g), a critical property for post-combustion CO2 capture. These findings position PHI-based materials as a disruptive platform for CO₂ adsorption, offering (i) straightforward synthesis from readily available precursors, (ii) promising scalability, and (iii) outstanding performance.