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

Area of Science:

• Materials Science
• Chemistry
• Environmental Science

Background:

• Potassium poly(heptazine imide) (KPHI) is a carbon nitride framework with photoelectrochemical applications.
• Crystalline carbon nitride materials offer potential for various chemical processes.

Purpose of the Study:

• To synthesize and characterize a Ca2+-complexed analogue of KPHI, termed calcium poly(heptazine imide) (CaPHI).
• To investigate the structural and electronic properties of CaPHI and its proton-exchanged derivative (H+/CaPHI).
• To evaluate the performance of H+/CaPHI for selective carbon dioxide capture.

Main Methods:

• Spectroscopic and crystallographic characterization to determine crystal structure and properties.
• Physisorption analysis to assess porosity.
• Gas adsorption experiments to evaluate CO2 capture performance.

Main Results:

• CaPHI exhibits a different crystal structure than KPHI, with Ca2+ acting as a structure-directing agent.
• Proton exchange in CaPHI yields H+/CaPHI, enhancing porosity without altering crystal structure.
• H+/CaPHI demonstrates significantly higher CO2 selectivity and working capacity compared to the metal-organic framework UTSA-16.

Conclusions:

• Ca2+ templating provides a route to engineer crystalline carbon nitride frameworks with tailored properties.
• Proton-exchanged poly(heptazine imide) materials are highly effective for selective CO2 capture from dilute streams.
• H+/CaPHI presents a promising alternative to existing materials for carbon capture technologies.