A Diaminopropane-Appended Metal-Organic Framework Enabling Efficient CO 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 Mg sites lining the channels of Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) is characterized for the removal of CO from the flue gas emissions of coal-fired power plants. Unique to members of this promising class of adsorbents, dmpn-Mg(dobpdc) displays facile step-shaped adsorption of CO from coal flue gas at 40 °C and near complete CO desorption upon heating to 100 °C, enabling a high CO 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-Mg(dobpdc) suggests that the narrow temperature swing of its CO adsorption steps is due to the high magnitude of its differential enthalpy of adsorption (Δh = -73 ± 1 kJ/mol), with a larger than expected entropic penalty for CO adsorption (Δs = -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-Mg(dobpdc) captures CO effectively in the presence of water and can be subjected to 1000 humid adsorption/desorption cycles with minimal degradation. Solid-state C NMR spectra and single-crystal X-ray diffraction structures of the Zn analogue reveal that this material adsorbs CO 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-Mg(dobpdc) one of the most promising adsorbents for carbon capture applications.