Ligand-Mediated Defects Unlock Fast and Regenerable CO₂ Capture in NICS-24 Metal-Organic Framework

Indoor CO₂ capture is an emerging strategy to improve air quality while reducing the energy cost of ventilation. Practical adsorbents must exhibit high affinity at low partial pressures, fast sorption kinetics, and energy-efficient regeneration. Here, we show that the Zn-oxalate guanazolate framework NICS-24, a promising platform for CO₂ capture at 400-1000 ppm, can be transformed into a high-performance indoor sorbent via ligand-mediated defect engineering (LMDE). Postsynthetic treatment with CF₃- and SCH₃-functionalized azolate linkers (denoted as FMeTz and SMeTz) induces an equilibrium-limited partial Zn-leaching process that generates vacancy-type defects while preserving the long-range crystallinity and Zn-linker connectivity. Comprehensive characterization (PXRD, EDS, ICP, XPS, PALS, FTIR, and multinuclear SSNMR) reveals subtle unit-cell contraction, increased local disorder, and modest expansion of accessible ultramicropore free volume. These defect-related changes markedly enhance CO₂ adsorption and transport. At 25 °C and 1000 ppm, the NICS-24-modified materials show more than a 2-fold increase in uptake (0.36 to 0.77 mmol/g), along with rapid kinetics and improved thermal-swing regeneration. The SMeTz-treated material retains high working capacity after regeneration at 70 °C and exhibits stable cycling. In situ DRIFTS and ex situ SSNMR confirm reversible physisorption dominated by interaction with defect-associated O-H/N-H environments, directly linking vacancy-type defect domains to enhanced low-pressure uptake, accelerated transport kinetics, and regenerability. Although water adsorption remains competitive, the frameworks are hydrolytically robust, highlighting LMDE-modified NICS-24 as a promising platform for indoor CO₂ capture under dry conditions.