Hydration Energetics of a Diamine-Appended Metal-Organic Framework Carbon Capture Sorbent

mmen-Mg-2(dobpdc) (mmen = N,N'-dimethylethylenediamine, dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) is a diamine-appended metal-organic framework (MOF) material with promising future as an efficient CO2 capture sorbent for industry applications. Here, using adsorption calorimetry, the energetic landscape of mmen-Mg-2(dobpdc) hydration has been revealed. Specifically, at near-zero water coverage, hydration results in the most exothermic differential enthalpy of adsorption of -110.9 +/- 3.1 kJ/mol water. A differential enthalpy plateau at -65.8 +/- 4.7 kJ/mol water is observed at the intermediate degree of hydration, which corresponds to water-diamine chemisorption through hydrogen bonding. Eventually, mmen-Mg-2(dobpdc) hydration is concluded at the second differential enthalpy plateau at -44.2 +/- 1.8 kJ/mol water suggesting condensation of water within the internal channel space of mmen-Mg-2(dobpdc). The position of the second plateau at about -44.0 kJ/mol water strongly suggests formation of liquid-like water clusters within the hydrophilic nanoconfinement environment. This comprehensive study energetically distinguishes the guest-host interfacial bonding and guest-guest intermolecular interactions for the water-mmen-Mg-2(dobpdc) system, which provides fundamental thermodynamic data to enhance our understanding of the behavior of CO2 capture sorbents in the presence of moisture.