Dynamic guanidinium sulfate salt for selective carbon dioxide adsorption with negative pressure inflexion.

Hydrogen bonds' flexible distances and moderate strength entitle compounds to dynamic properties under external stimuli. Here we report multiple phase transitions and counter-intuitive CO2 adsorption behavior of dynamic guanidinium sulfate (GS) salt assembled via hydrogen-bonds. Exploration based on the energy landscape generated by crystal structure prediction (CSP) reveals three porous GS phases with stability of α > β > γ and the inverse order of porosity, agreeing with experimental results. Transformations among polymorphs via heating or compressing involve ion rearrangement. Adsorption isotherms of β-GS indicate that CO2 firstly enters the isolated cavities at a low gating pressure, and further increasing CO2 pressure leads to the continuous gas uptake but reduced pressure at a critical point and thus an unexpected negative pressure inflexion (NPI), followed by the final adsorption saturation. Theoretical calculations demonstrate that the NPI behavior stemmed from the GS structural transition from β to more porous γ-phase, with the γ-GS phase becoming more energy-favorable as CO2 uptake increases. Specific supramolecular interactions ensure CO2 selectivity and easy regeneration. With a CO2 uptake of 4.2 mmol g-1 (273 K, 100 kPa), GS salt exhibits great promise for CO2 capture and transport, demonstrating the potential of simple hydrogen-bonded salts as adaptive materials.