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).
A new concentration‐swing frequency response system was developed to evaluate water kinetics in non‐binder containing zeolite Linde Type A (LTA) crystals with frequencies up to 1 Hz. For commercial micron‐sized 3A crystals, the mass transfer rate is too fast to be determined accurately due to reduced sensitivity near the system limits. Larger LTA crystals (~15 μm) were synthesized in house and ion exchanged to three potassium concentrations (0, 48, and 77 wt%) to aid in the accurate determination of mass transfer mechanism and rates. Micropore diffusion with parallel sites, not a surface barrier, described all the data well, consistent with the bimodal distribution of crystal sizes from scanning electron microscope images. The transport diffusivity for water in zeolite 3A crystals near saturated loadings at 25°C is about 1 × 10−12 m2/s, 10 times slower than 4A crystals around 10−11 m2/s, both supporting fast micropore diffusion time constants >1 s−1 for micron‐sized crystals.
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