Cooperative Carbon Dioxide Capture in Diamine-Appended Magnesium-Olsalazine Frameworks.

Diamine-appended Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks have emerged as promising candidates for carbon capture owing to their exceptional CO 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 ,-diamines featuring bulky substituents, in particular, exhibit excellent stabilities and CO adsorption properties. However, these frameworks display double-step adsorption behavior arising from steric repulsion between ammonium carbamates, which ultimately results in increased regeneration energies.

Porous materials for carbon dioxide separations.

Global investment in counteracting climate change has galvanized increasing interest in carbon capture and sequestration (CCS) as a versatile emissions mitigation technology. As decarbonization efforts accelerate, CCS can target the emissions of large point-source emitters, such as coal- or natural gas-fired power plants, while also supporting the production of renewable or low-carbon fuels. Furthermore, CCS can enable decarbonization of difficult-to-abate industrial processes and can support net CO removal from the atmosphere through bioenergy coupled with CCS or direct air capture.

Hysteresis curves reveal the microscopic origin of cooperative CO adsorption in diamine-appended metal-organic frameworks.

Diamine-appended metal-organic frameworks (MOFs) of the form Mg(dobpdc)(diamine) adsorb CO in a cooperative fashion, exhibiting an abrupt change in CO occupancy with pressure or temperature. This change is accompanied by hysteresis. While hysteresis is suggestive of a first-order phase transition, we show that hysteretic temperature-occupancy curves associated with this material are qualitatively unlike the curves seen in the presence of a phase transition; they are instead consistent with CO chain polymerization, within one-dimensional channels in the MOF, in the absence of a phase transition.

Cooperative carbon capture and steam regeneration with tetraamine-appended metal-organic frameworks.

Natural gas has become the dominant source of electricity in the United States, and technologies capable of efficiently removing carbon dioxide (CO) from the flue emissions of natural gas-fired power plants could reduce their carbon intensity. However, given the low partial pressure of CO in the flue stream, separation of CO is particularly challenging. Taking inspiration from the crystal structures of diamine-appended metal-organic frameworks exhibiting two-step cooperative CO adsorption, we report a family of robust tetraamine-functionalized frameworks that retain cooperativity, leading to the potential for exceptional efficiency in capturing CO under the extreme conditions relevant to natural gas flue emissions.

Kinetics of cooperative CO adsorption in diamine-appended variants of the metal-organic framework Mg(dobpdc).

Carbon capture and sequestration is a key element of global initiatives to minimize anthropogenic greenhouse gas emissions. Although many investigations of new candidate CO capture materials focus on equilibrium adsorption properties, it is also critical to consider adsorption/desorption kinetics when evaluating adsorbent performance. Diamine-appended variants of the metal-organic framework Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are promising materials for CO capture because of their cooperative chemisorption mechanism and associated step-shaped equilibrium isotherms, which enable large working capacities to be accessed with small temperature swings.

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 Mg (dobpdc) are shown to adsorb CO 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 CO adsorbents, providing further means of optimizing these powerful materials for energy-efficient CO separations.

Amine Dynamics in Diamine-Appended Mg(dobpdc) Metal-Organic Frameworks.

Variable-temperature N solid-state NMR spectroscopy is used to uncover the dynamics of three diamines appended to the metal-organic framework Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), an important family of CO capture materials. The results imply both bound and free amine nitrogen environments exist when diamines are coordinated to the framework open Mg sites. There are rapid exchanges between two nitrogen environments for all three diamines, the rates and energetics of which are quantified by N solid-state NMR data and corroborated by density functional theory calculations and molecular dynamics simulations.

Correction: Enhancement of CO binding and mechanical properties upon diamine functionalization of M(dobpdc) metal-organic frameworks.

[This corrects the article DOI: 10.1039/C7SC05217K.].

Elucidating CO 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-M(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) have shown promise for carbon-capture applications, although questions remain regarding the molecular mechanisms of CO uptake in these materials. Here we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO chemisorption.

Cooperative adsorption of carbon disulfide in diamine-appended metal-organic frameworks.

Over one million tons of CS are produced annually, and emissions of this volatile and toxic liquid, known to generate acid rain, remain poorly controlled. As such, materials capable of reversibly capturing this commodity chemical in an energy-efficient manner are of interest. Recently, we detailed diamine-appended metal-organic frameworks capable of selectively capturing CO through a cooperative insertion mechanism that promotes efficient adsorption-desorption cycling.

Near-Perfect CO/CH Selectivity Achieved through Reversible Guest Templating in the Flexible Metal-Organic Framework Co(bdp).

Metal-organic frameworks are among the most promising materials for industrial gas separations, including the removal of carbon dioxide from natural gas, although substantial improvements in adsorption selectivity are still sought. Herein, we use equilibrium adsorption experiments to demonstrate that the flexible metal-organic framework Co(bdp) (bdp = 1,4-benzenedipyrazolate) exhibits a large CO adsorption capacity and approaches complete exclusion of CH under 50:50 mixtures of the two gases, leading to outstanding CO/CH selectivity under these conditions. In situ powder X-ray diffraction data indicate that this selectivity arises from reversible guest templating, in which the framework expands to form a CO clathrate and then collapses to the nontemplated phase upon desorption.

Enhancement of CO binding and mechanical properties upon diamine functionalization of M(dobpdc) metal-organic frameworks.

The family of diamine-appended metal-organic frameworks exemplified by compounds of the type mmen-M(dobpdc) (mmen = ,'-dimethylethylenediamine; M = Mg, Mn, Fe, Co, Zn; dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are adsorbents with significant potential for carbon capture, due to their high working capacities and strong selectivity for CO that stem from a cooperative adsorption mechanism. Herein, we use first-principles density functional theory (DFT) calculations to quantitatively investigate the role of mmen ligands in dictating the framework properties. Our van der Waals-corrected DFT calculations indicate that electrostatic interactions between ammonium carbamate units significantly enhance the CO binding strength relative to the unfunctionalized frameworks.

Overcoming double-step CO adsorption and minimizing water co-adsorption in bulky diamine-appended variants of Mg(dobpdc).

Alkyldiamine-functionalized variants of the metal-organic framework Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are promising for CO capture applications owing to their unique step-shaped CO adsorption profiles resulting from the cooperative formation of ammonium carbamate chains. , (1°,2°) alkylethylenediamine-appended variants are of particular interest because of their low CO step pressures (≤1 mbar at 40 °C), minimal adsorption/desorption hysteresis, and high thermal stability. Herein, we demonstrate that further increasing the size of the alkyl group on the secondary amine affords enhanced stability against diamine volatilization, but also leads to surprising two-step CO adsorption/desorption profiles.

Unexpected Diffusion Anisotropy of Carbon Dioxide in the Metal-Organic Framework Zn(dobpdc).

Metal-organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is crucial for the practical application of these materials as adsorbents or in mixed-matrix membranes, since the rate of gas diffusion within the adsorbent pores impacts the required size (and therefore cost) of the adsorbent column or membrane. Here, we investigate the diffusion of CO within the pores of Zn(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations.

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 M(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc = 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 trans-1,2-diaminocyclohexane (dach), Mg(dobpdc) adsorbs carbon dioxide cooperatively to form ammonium carbamate chains, and the thermodynamics of CO capture are strongly influenced by enantioselective interactions within the chiral pores of the framework.

A Diaminopropane-Appended Metal-Organic Framework Enabling Efficient CO Capture from Coal Flue Gas via a Mixed Adsorption Mechanism.

A new diamine-functionalized metal-organic framework comprised of 2,2-dimethyl-1,3-diaminopropane (dmpn) appended to the Mg sites lining the channels of Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) is characterized for the removal of CO from the flue gas emissions of coal-fired power plants. Unique to members of this promising class of adsorbents, dmpn-Mg(dobpdc) displays facile step-shaped adsorption of CO from coal flue gas at 40 °C and near complete CO desorption upon heating to 100 °C, enabling a high CO working capacity (2.42 mmol/g, 9.

Controlling Cooperative CO Adsorption in Diamine-Appended Mg(dobpdc) Metal-Organic Frameworks.

In the transition to a clean-energy future, CO separations will play a critical role in mitigating current greenhouse gas emissions and facilitating conversion to cleaner-burning and renewable fuels. New materials with high selectivities for CO adsorption, large CO removal capacities, and low regeneration energies are needed to achieve these separations efficiently at scale. Here, we present a detailed investigation of nine diamine-appended variants of the metal-organic framework Mg(dobpdc) (dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) that feature step-shaped CO adsorption isotherms resulting from cooperative and reversible insertion of CO into metal-amine bonds to form ammonium carbamate chains.

Tuning the Adsorption-Induced Phase Change in the Flexible Metal-Organic Framework Co(bdp).

Metal-organic frameworks that flex to undergo structural phase changes upon gas adsorption are promising materials for gas storage and separations, and achieving synthetic control over the pressure at which these changes occur is crucial to the design of such materials for specific applications. To this end, a new family of materials based on the flexible metal-organic framework Co(bdp) (bdp = 1,4-benzenedipyrazolate) has been prepared via the introduction of fluorine, deuterium, and methyl functional groups on the bdp ligand, namely, Co(F-bdp), Co(p-F-bdp), Co(o-F-bdp), Co(D-bdp), and Co(p-Me-bdp). These frameworks are isoreticular to the parent framework and exhibit similar structural flexibility, transitioning from a low-porosity, collapsed phase to high-porosity, expanded phases with increasing gas pressure.