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.

Recent Progress on Microfine Design of Metal–Organic Frameworks: Structure Regulation and Gas Sorption and Separation

“…In 2017 and 2018, they reported a series of diamineappended variants of MOF Mg 2 (dobpdc) (dobpdc 4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) and found the different appended diamine could cause new adsorption mechanisms. [93,94] Later, in 2020, Long and co-workers reported a series of alcoholamine-and alkoxyalkylamine-functionalized MOF Mg 2 (dobpdc). [95] The modified materials were confirmed isoreticular to previous analogs by powder X-ray diffraction (PXRD).…”

Nuclear Magnetic Resonance Studies of Carbon Dioxide Capture

“…Figure 2a shows a common ex situ approach for studying solid-adsorbents for CO2 capture, which we term "ex situ MAS". 13,17,21 In this approach, a MAS rotor is packed with the solid adsorbent and is transferred to a gas-dosing manifold, with the rotor left open. The sample is then evacuated before dosing CO2 gas.…”

“…As discussed previously, 13 C NMR experiments can be implemented easily and provide a wealth of information about the number and type of carbon environments present in sorbent materials. More detailed information about the binding mechanisms can be gained by exploring the local connectivity of the carbon atoms through correlation experiments.…”

“…We then show three case studies of how NMR methods have been developed to understand CO2 adsorption and diffusion mechanisms in MOFs. [13][14][15]…”

Insights into the Interaction between Immobilized Biocatalysts and Metal–Organic Frameworks: A Case Study of PCN-333

“…28−30 For example, Fragai et al 31 showed that the three-dimensional structures of the enzymes were retained after immobilization in a peptide-templated silica gel. Indeed, ssNMR spectroscopy has been applied as a general approach to study MOFs and their interactions with small guest species 32 (e.g., H 2 , 33 xenon, 34 CO, 35 CO 2 , 36,37 nitric oxide, 38 H 2 O, 39 hydrocarbon, 40 aromatics, 41,42 etc. ), providing complementary information for host−guest interaction.…”