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

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