High-Capacity, Cooperative CO 2 Capture in a Diamine-Appended Metal-Organic Framework through a Combined Chemisorptive and Physisorptive Mechanism.

Diamine-appended Mg 2 (dobpdc) (dobpdc 4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are promising candidates for carbon capture that exhibit exceptional selectivities and high capacities for CO 2 . To date, CO 2 uptake in these materials has been shown to occur predominantly via a chemisorption mechanism involving CO 2 insertion at the amine-appended metal sites, a mechanism that limits the capacity of the material to ∼1 equiv of CO 2 per diamine. Herein, we report a new framework, pip2-Mg 2 (dobpdc) (pip2 = 1-(2-aminoethyl)piperidine), that exhibits two-step CO 2 uptake and achieves an unusually high CO 2 capacity approaching 1.5 CO 2 per diamine at saturation. Analysis of variable-pressure CO 2 uptake in the material using solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that pip2-Mg 2 (dobpdc) captures CO 2 via an unprecedented mechanism involving the initial insertion of CO 2 to form ammonium carbamate chains at half of the sites in the material, followed by tandem cooperative chemisorption and physisorption. Powder X-ray diffraction analysis, supported by van der Waals-corrected density functional theory, reveals that physisorbed CO 2 occupies a pocket formed by adjacent ammonium carbamate chains and the linker. Based on breakthrough and extended cycling experiments, pip2-Mg 2 (dobpdc) exhibits exceptional performance for CO 2 capture under conditions relevant to the separation of CO 2 from landfill gas. More broadly, these results highlight new opportunities for the fundamental design of diamine-Mg 2 (dobpdc) materials with even higher capacities than those predicted based on CO 2 chemisorption alone.