Phase Change-Mediated Capture of Carbon Dioxide from Air with a Molecular Triamine Network Solid

The efficient removal of CO₂ from exhaust streams and even directly from air is necessary to forestall climate change, lending urgency to the search for new materials that can rapidly capture CO₂ at high capacity. The recent discovery that diamine-appended metal-organic frameworks can exhibit cooperative CO₂ uptake via the formation of ammonium carbamate chains begs the question of whether simple organic polyamine molecules could be designed to achieve a similar switch-like behavior with even higher separation capacities. Here, we present a solid molecular triamine, 1,3,5-tris(aminomethyl)benzene (TriH), that rapidly captures large quantities of CO₂ upon exposure to humid air to form the porous, crystalline, ammonium carbamate network solid TriH(CO₂)_1.5·<i>x</i>H₂O (TriHCO₂). The phase transition behavior of TriH converting to TriHCO₂ was studied through powder and single-crystal X-ray diffraction analysis, and additional spectroscopic techniques further verified the formation of ammonium carbamate species upon exposing TriH to humid air. Detailed breakthrough analyses conducted under varying temperatures, relative humidities, and flow rates reveal record CO₂ absorption capacities as high as 8.9 mmol/g. Computational analyses reveal an activation barrier associated with TriH absorbing CO₂ under dry conditions that is lowered under humid conditions through hydrogen bonding with a water molecule in the transition state associated with N-C bond formation. These results highlight the prospect of tunable molecular polyamines as a new class of candidate absorbents for high-capacity CO₂ capture.