Diaminopropane-Functionalized MOF-PVDF Composite Beads for Indoor CO₂ Adsorption with Improved Humidity Tolerance

Indoor carbon dioxide (CO₂) levels in enclosed spaces frequently exceed the recommended limit of 1000 ppm, creating a strong demand for sorbents that can efficiently capture dilute CO₂. Amine-functionalized metal-organic frameworks (MOFs) are promising in this regard due to their strong CO₂ affinity, but their long-term stability is compromised by competitive water binding and amine loss. In this work, Mg₂(hob) [hob^4- = 5,5'-(hydrazine-1,2-diylidenebis(methanylylidene))bis(2-oxidobenzoate)] was employed as a platform and functionalized with propylene-linked diamines, which offer higher thermal stability and reduced volatility under humid conditions compared to ethylene-linked analogues. Among the series, Mg₂(hob) functionalized with N-methyl-1,3-propanediamine (mpn-MOF) showed the highest CO₂ uptake (10.3 wt % at 1000 ppm and 25 °C), together with a pronounced low-pressure cooperative adsorption step. To further improve water resistance and enable practical shaping, Mg₂(hob) was blended with poly(vinylidene fluoride) (PVDF) to fabricate bead-shaped composites containing 15, 25, and 35 wt % PVDF, followed by postsynthetic mpn functionalization. Notably, mpn-MOF@PVDF25 retained most of its CO₂ uptake after 15 days at 40% relative humidity. This demonstrates a practical approach that combines propylene-linked diamine functionalization with hydrophobic polymer shaping to achieve long-term indoor CO₂ capture.