Calcium Poly(Heptazine Imide): A Covalent Heptazine Framework for Selective CO<sub>2</sub> Adsorption
James N. Burrow, Ryan A. Ciufo, Lettie A. Smith, Yu Wang, David C. Calabro, Graeme Henkelman, C. Buddie Mullins
Abstract
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).