Revealing an unusual temperature-dependent CO₂ adsorption trend and selective CO₂ uptake over water vapors in a polyamine-appended metal-organic framework

We report the development of a tetraethylenepentamine (tepa)-functionalized Mg₂(dobpdc) (T02, T2, T20, and T200; the number denotes equivalents of tepa used with respect to the framework), which uncommonly exhibited a gradual increase in CO₂ uptake with increasing temperature when polyamine groups were heavily loaded into the framework (T20 and T200). For the moderately loaded sample (T2), a significant working capacity (11.2 wt%) and a cyclable CO₂ performance at 40 °C and 15% CO₂ pressure were obtained. The abrupt increase observed in the CO₂ isotherms indicates that the tepa-loaded samples involve a CO₂ insertion mechanism. Interestingly, all amine groups in tepa can partly participate in the formation of ammonium carbamate ion pairs, which is the first observation of this in amine-functionalized metal-organic frameworks. Remarkably, we demonstrate that high loading of tepa allowed for selective CO₂ uptake over H₂O vapors, together with enhanced framework stability under humid conditions.