Calcium Poly(Heptazine Imide): A Covalent Heptazine Framework for Selective CO₂ Adsorption

Potassium poly(heptazine imide) (KPHI) has recently garnered attention as a crystalline carbon nitride framework with considerable photoelectrochemical activity. Here, we report a Ca²⁺-complexed analogue of PHI: calcium poly(heptazine imide) (CaPHI). Despite similar polymer backbone, CaPHI and KPHI exhibit markedly different crystal structures. Spectroscopic, crystallographic, and physisorptive characterization reveal that Ca²⁺ acts as a structure-directing agent to transform melon-based carbon nitride to crystalline CaPHI with ordered pore channels, extended visible light absorption, and altered band structure as compared to KPHI. Upon acid washing, protons replace Ca²⁺ atoms in CaPHI to yield H⁺/CaPHI and enhance porosity without disrupting crystal structure. Further, these proton-exchanged PHI frameworks exhibit large adsorption affinity for CO₂ and exceptional performance for selective carbon capture from dilute streams. Compared to a state-of-the-art metal organic framework, UTSA-16, H⁺/CaPHI exhibits more than twice the selectivity (∼300 vs ∼120) and working capacity (∼1.2 mmol g^-1 vs ∼0.5 mmol g^-1) for a feed of 4% CO₂ (1 bar, 30 °C).