Phillip J. Milner

Steaming out captured CO 2 Although natural gas is less carbon dioxide (CO 2 )–intensive than coal, capturing its emitted CO 2 can be more challenging because combined-cycle natural gas combustion has a CO 2 concentration that is only one-third of that of coal combustion and contains high concentrations of oxygen and water. Kim et al. report on a tetraamine-functionalized magnesium metal–organic framework that displays two-step cooperative CO 2 adsorption that leads to a high CO 2 capacity and adsorption enthalpy (see the Perspective by Peh and Zhao). This material could capture CO 2 from humid air and could be regenerated with steam, a method that is more economical than temperature or pressure swing methods. Science , this issue p. 392 ; see also p. 372

In the transition to a clean-energy future, CO2 separations will play a critical role in mitigating current greenhouse gas emissions and facilitating conversion to cleaner-burning and renewable fuels. New materials with high selectivities for CO2 adsorption, large CO2 removal capacities, and low regeneration energies are needed to achieve these separations efficiently at scale. Here, we present a detailed investigation of nine diamine-appended variants of the metal–organic framework Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) that feature step-shaped CO2 adsorption isotherms resulting from cooperative and reversible insertion of CO2 into metal–amine bonds to form ammonium carbamate chains. Small modifications to the diamine structure are found to shift the threshold pressure for cooperative CO2 adsorption by over 4 orders of magnitude at a given temperature, and the observed trends are rationalized on the basis of crystal structures of the isostructural zinc frameworks obtained from in situ single-crystal X-ray diffraction experiments. The structure–activity relationships derived from these results can be leveraged to tailor adsorbents to the conditions of a given CO2 separation process. The unparalleled versatility of these materials, coupled with their high CO2 capacities and low projected energy costs, highlights their potential as next-generation adsorbents for a wide array of CO2 separations.

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.

(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.

capture materials for this separation. This broadening in the scope of current carbon capture research is urgently needed to accelerate the deployment of transformational carbon capture technologies.