A Diaminopropane-Appended Metal–Organic Framework Enabling Efficient CO₂ Capture from Coal Flue Gas via a Mixed Adsorption Mechanism

A new diamine-functionalized metal-organic framework comprised of 2,2-dimethyl-1,3-diaminopropane (dmpn) appended to the Mg²⁺ sites lining the channels of Mg₂(dobpdc) (dobpdc⁴⁻ = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) is characterized for the removal of CO₂ from the flue gas emissions of coal-fired power plants. Unique to members of this promising class of adsorbents, dmpn-Mg₂(dobpdc) displays facile step-shaped adsorption of CO₂ from coal flue gas at 40 °C and near complete CO₂ desorption upon heating to 100 °C, enabling a high CO₂ working capacity (2.42 mmol/g, 9.1 wt %) with a modest 60 °C temperature swing. Evaluation of the thermodynamic parameters of adsorption for dmpn-Mg₂(dobpdc) suggests that the narrow temperature swing of its CO₂ adsorption steps is due to the high magnitude of its differential enthalpy of adsorption (Δhads = -73 ± 1 kJ/mol), with a larger than expected entropic penalty for CO₂ adsorption (Δsads = -204 ± 4 J/mol·K) positioning the step in the optimal range for carbon capture from coal flue gas. In addition, thermogravimetric analysis and breakthrough experiments indicate that, in contrast to many adsorbents, dmpn-Mg₂(dobpdc) captures CO₂ effectively in the presence of water and can be subjected to 1000 humid adsorption/desorption cycles with minimal degradation. Solid-state ¹³C NMR spectra and single-crystal X-ray diffraction structures of the Zn analogue reveal that this material adsorbs CO₂ via formation of both ammonium carbamates and carbamic acid pairs, the latter of which are crystallographically verified for the first time in a porous material. Taken together, these properties render dmpn-Mg₂(dobpdc) one of the most promising adsorbents for carbon capture applications.