The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal-organic frameworks of the type diamine-M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) have shown promise for carbon-capture applications, although questions remain regarding the molecular mechanisms of CO₂ uptake in these materials. Here we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO₂ chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and van-der-Waals-corrected density functional theory (DFT) calculations for 13 diamine-M₂(dobpdc) variants, we demonstrate that the canonical CO₂ chemisorption products, ammonium carbamate chains and carbamic acid pairs, can be readily distinguished and that ammonium carbamate chain formation dominates for diamine-Mg₂(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves the formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO₂ uptake in diamine-M₂(dobpdc) materials. Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO₂ adsorption within diamine-M₂(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.

Carbon capture at fossil fuel-fired power plants is a critical strategy to mitigate anthropogenic contributions to global warming, but widespread deployment of this technology is hindered by a lack of energy-efficient materials that can be optimized for CO₂ capture from a specific flue gas. As a result of their tunable, step-shaped CO₂ adsorption profiles, diamine-functionalized metal-organic frameworks (MOFs) of the form diamine-Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are among the most promising materials for carbon capture applications. Here, we present a detailed investigation of dmen-Mg₂(dobpdc) (dmen = 1,2-diamino-2-methylpropane), one of only two MOFs with an adsorption step near the optimal pressure for CO₂ capture from coal flue gas. While prior characterization suggested that this material only adsorbs CO₂ to half capacity (0.5 CO₂ per diamine) at 1 bar, we show that the half-capacity state is actually a metastable intermediate. Under appropriate conditions, the MOF adsorbs CO₂ to full capacity, but conversion from the half-capacity structure happens on a very slow time scale, rendering it inaccessible in traditional adsorption measurements. Data from solid-state magic angle spinning nuclear magnetic resonance spectroscopy, coupled with van der Waals-corrected density functional theory, indicate that ammonium carbamate chains formed at half capacity and full capacity adopt opposing configurations, and the need to convert between these states likely dictates the sluggish post-half-capacity uptake. By use of the more symmetric parent framework Mg₂(pc-dobpdc) (pc-dobpdc^4- = 3,3'-dioxidobiphenyl-4,4'-dicarboxylate), the metastable trap can be avoided and the full CO₂ capacity of dmen-Mg₂(pc-dobpdc) accessed under conditions relevant for carbon capture from coal-fired power plants.

Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4-dioxidobiphenyl-3,3-dicarboxylate) metal-organic frameworks have emerged as promising candidates for carbon capture owing to their exceptional CO2 selectivities, high separation capacities, and step-shaped adsorption profiles, which arise from a unique cooperative adsorption mechanism resulting in the formation of ammonium carbamate chains. Materials appended with primary,secondary-diamines featuring bulky substituents, in particular, exhibit excellent stabilities and CO2 adsorption properties. However, these frameworks display double-step adsorption behavior arising from steric repulsion between ammonium carbamates, which ultimately results in increased regeneration energies. Herein, we report frameworks of the type diamine-Mg2(olz) (olz4- = (E)-5,5-(diazene-1,2-diyl)bis(2-oxidobenzoate)) that feature diverse diamines with bulky substituents and display desirable single-step CO2 adsorption across a wide range of pressures and temperatures. Analysis of CO2 adsorption data reveals that the basicity of the pore-dwelling amine─in addition to its steric bulk─is an important factor influencing adsorption step pressure; furthermore, the amine steric bulk is found to be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2-Mg2(olz) (ee-2 = N,N-diethylethylenediamine), adsorbs >90% of the CO2 from a simulated coal flue stream and exhibits exceptional thermal and oxidative stability over the course of extensive adsorption/desorption cycling, placing it among top-performing adsorbents to date for CO2 capture from a coal flue gas. Spectroscopic characterization and van der Waals-corrected density functional theory calculations indicate that diamine-Mg2(olz) materials capture CO2 via the formation of ammonium carbamate chains. These results point more broadly to the opportunity for fundamentally advancing materials in this class through judicious design.

Diamine-appended Mg₂(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 CO₂. To date, CO₂ uptake in these materials has been shown to occur predominantly via a chemisorption mechanism involving CO₂ insertion at the amine-appended metal sites, a mechanism that limits the capacity of the material to ∼1 equiv of CO₂ per diamine. Herein, we report a new framework, pip2-Mg₂(dobpdc) (pip2 = 1-(2-aminoethyl)piperidine), that exhibits two-step CO₂ uptake and achieves an unusually high CO₂ capacity approaching 1.5 CO₂ per diamine at saturation. Analysis of variable-pressure CO₂ 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-Mg₂(dobpdc) captures CO₂ via an unprecedented mechanism involving the initial insertion of CO₂ 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 CO₂ occupies a pocket formed by adjacent ammonium carbamate chains and the linker. Based on breakthrough and extended cycling experiments, pip2-Mg₂(dobpdc) exhibits exceptional performance for CO₂ capture under conditions relevant to the separation of CO₂ from landfill gas. More broadly, these results highlight new opportunities for the fundamental design of diamine-Mg₂(dobpdc) materials with even higher capacities than those predicted based on CO₂ chemisorption alone.