Elucidating CO₂ Chemisorption in Diamine-Appended Metal–Organic Frameworks

Abstract

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 thirteen 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 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. Structure files generated by DFT calculations with the PBE functional and the D3 vdW correction (CIF).

Kinetics of cooperative CO2 adsorption in diamine-appended variants of the metal–organic framework Mg2(dobpdc)

“…9, consistent with our previous ndings that these materials capture CO 2 by the same mechanism. 26,52 In each case, k A increases with decreasing temperature, indicative of inverse Arrhenius behavior, as observed for m-2-m-Mg 2 (dobpdc) ( Fig. S13-S18 †).…”

“…10a). 27,52 Similarly, the kinetics prole of dmpn-Mg 2 (dobpdc) is distinct from that of the other diamine-appended variants (Fig. 10b and c), with no induction period observed even at temperatures close to the adsorption step.…”

“…27 Notably, the complex CO 2 chemisorption mechanism in dmpn-Mg 2 (dobpdc) ultimately leads to the formation of carbamic acids interacting with ammonium carbamates, and this material exhibits a mixture of $4 chemisorbed species in the early stages of CO 2 adsorption. 52 Given this complexity and the promising attributes of dmpn-Mg 2 (dobpdc) for implementation in practical processes, we sought to investigate its adsorption kinetics from a gas stream of 15% CO 2 in N 2 . Consistent with previous ndings, the 15% CO 2 adsorption isobar for this material exhibits a step around 60 C that is more broad than those of other diamine-appended Mg 2 (dobpdc) variants, reecting its complex adsorption mechanism (Fig.…”

“…These dissimilar kinetics proles may result from dmpn-Mg 2 (dobpdc) exhibiting a mixture of approximately four chemisorbed species in the early stages of CO 2 adsorption, while a 1 : 1 mixture of ammonium carbamates:carbamic acids is formed following saturation with CO 2 and long equilibration times. 52…”

Solid‐state NMR studies of the acidity of functionalized metal–organic framework UiO‐66 materials

“…[35] Solid-state NMR spectroscopy was frequently utilized for the exploration of the chemical structures, dynamic behavior, and host-guest interaction of MOFs. [36][37][38][39][40][41][42][43][44] To investigate the chemical composition of these activated UiO-66-X (X = -H, -2COOH, -SO 3 H) and multivariate functionalized UiO-66-X samples, 1 H and 13 C MAS NMR experiments were conducted. Figure 2 shows the 1 H MAS and 13 C CP/MAS NMR spectra of dehydrated UiO-66, UiO-66-SO 3 H, UiO-66-2COOH and multivariate functionalized UiO-66-X samples.…”