Cooperative Carbon Dioxide Adsorption in Alcoholamine- and Alkoxyalkylamine-Functionalized Metal-Organic Frameworks.

A series of structurally diverse alcoholamine- and alkoxyalkylamine-functionalized variants of the metal-organic framework Mg2 (dobpdc) are shown to adsorb CO2 selectively via cooperative chain-forming mechanisms. Solid-state NMR spectra and optimized structures obtained from van der Waals-corrected density functional theory calculations indicate that the adsorption profiles can be attributed to the formation of carbamic acid or ammonium carbamate chains that are stabilized by hydrogen bonding interactions within the framework pores. These findings significantly expand the scope of chemical functionalities that can be utilized to design cooperative CO2 adsorbents, providing further means of optimizing these powerful materials for energy-efficient CO2 separations

Elucidating CO2 Chemisorption in Diamine-Appended Metal–Organic Frameworks

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; dobpdc4− = 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

Enantioselective Recognition of Ammonium Carbamates in a Chiral Metal–Organic Framework

Chiral metal–organic frameworks have attracted interest for enantioselective separations and catalysis because of their high crystallinity and pores with tunable shapes, sizes, and chemical environments. Chiral frameworks of the type M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) seem particularly promising for potential applications because of their excellent stability, high internal surface areas, and strongly polarizing open metal coordination sites within the channels, but to date these materials have been isolated only in racemic form. Here, we demonstrate that when appended with the chiral diamine <i>trans</i>-1,2-diaminocyclohexane (dach), Mg₂(dobpdc) adsorbs carbon dioxide cooperatively to form ammonium carbamate chains, and the thermodynamics of CO₂ capture are strongly influenced by enantioselective interactions within the chiral pores of the framework. We further show that it is possible to access both enantiomers of Mg₂(dobpdc) with high enantiopurity (≥90%) via framework synthesis in the presence of varying quantities of d-panthenol, an inexpensive chiral induction agent. Investigation of dach–M₂(dobpdc) samples following CO₂ adsorptionusing single-crystal and powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and density functional theory calculationsrevealed that the ammonium carbamate chains interact extensively with each other and with the chiral M₂(dobpdc) pore walls. Subtle differences in the non-covalent interactions accessible in each diastereomeric phase dramatically impact the thermodynamics of CO₂ adsorption

Enantioselective Recognition of Ammonium Carbamates in a Chiral Metal–Organic Framework

Chiral metal–organic frameworks have attracted interest for enantioselective separations and catalysis because of their high crystallinity and pores with tunable shapes, sizes, and chemical environments. Chiral frameworks of the type M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) seem particularly promising for potential applications because of their excellent stability, high internal surface areas, and strongly polarizing open metal coordination sites within the channels, but to date these materials have been isolated only in racemic form. Here, we demonstrate that when appended with the chiral diamine <i>trans</i>-1,2-diaminocyclohexane (dach), Mg₂(dobpdc) adsorbs carbon dioxide cooperatively to form ammonium carbamate chains, and the thermodynamics of CO₂ capture are strongly influenced by enantioselective interactions within the chiral pores of the framework. We further show that it is possible to access both enantiomers of Mg₂(dobpdc) with high enantiopurity (≥90%) via framework synthesis in the presence of varying quantities of d-panthenol, an inexpensive chiral induction agent. Investigation of dach–M₂(dobpdc) samples following CO₂ adsorptionusing single-crystal and powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and density functional theory calculationsrevealed that the ammonium carbamate chains interact extensively with each other and with the chiral M₂(dobpdc) pore walls. Subtle differences in the non-covalent interactions accessible in each diastereomeric phase dramatically impact the thermodynamics of CO₂ adsorption