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 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.
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Materials capable of selectively adsorbing or releasing water can enable valuable applications ranging from efficient humidity and temperature control to the direct atmospheric capture of potable ...water. Despite recent progress in employing metal-organic frameworks (MOFs) as privileged water sorbents, developing a readily accessible, water-stable MOF platform that can be systematically modified for high water uptake at low relative humidity remains a significant challenge. We herein report the development of a tunable MOF that efficiently captures atmospheric water (up to 0.78 g water/g MOF) across a range of uptake humidity (27-45%) employing a readily accessible Zn bibenzotriazolate MOF, CFA-1 (Zn5(OAc)4(bibta)3, H2bibta = 1H,1H'-5,5'-bibenzod1,2,3triazole), as a base for subsequent diversification. Controlling the metal identity (zinc, nickel) and coordinating nonstructural anion (acetate, chloride) via postsynthetic exchange modulates the relative humidity of uptake, facilitating the use of a single MOF scaffold for a diverse range of potential water sorption applications. We further present a fundamental theory dictating how continuous variation of the pore environment affects the relative humidity of uptake. Exchange of substituents preserves capacity for water sorption, increases hydrolytic stability (with 5.7% loss in working capacity over 450 water adsorption-desorption cycles for the nickel-chloride-rich framework), and enables continuous modulation for the relative humidity of pore condensation. This combination of stability and tunability within a synthetically accessible framework renders Ni-incorporated M5X4bibta3 promising materials for practical water sorption applications.
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Devices that utilize the reversible capture of water vapor provide solutions to water insecurity, increasing energy demand, and sustainability. In all of these applications, it is important to ...minimize water adsorption–desorption hysteresis. Hysteresis is particularly difficult to avoid for sorbents that bind water strongly, such as those that take up below 10% relative humidity (RH). Even though the theoretical factors that affect hysteresis are understood, understanding the structure–function correlations that dictate the hysteretic behavior in water sorbents remains a challenge. Herein, we synthesize a new hexagonal microporous framework, Ni2Cl2BBTQ (H2BBTQ = 2H,6H-benzo1,2-d4,5-d′bistriazolequinone), to elucidate these principles. Uniquely among its known isoreticular analogues, Ni2Cl2BBTQ presents unusually high hysteresis caused by strong wetting seeded by a particularly strong zero-coverage interaction with water. A combination of vibrational spectroscopies and detailed molecular dynamics simulations reveals that this hysteretic behavior is the result of an intricate hydrogen-bonding network, in which the monolayer consists of water simultaneously binding to open nickel sites and hydrogen bonding to quinone sites. This latter hydrogen-bonding interaction does not exist in other isoreticular analogues: it prevents facile water dynamics and drives hysteresis. Our results highlight an important design criterion for water sorbents: in order to drive water uptake in progressively dry conditions, the common strategy of increasing hydrophilicity can cause strong wetting and the formation of superclusters, which lead to undesirable hysteresis. Instead, hysteresis-free water uptake at extremely low humidity is best promoted by decreasing the pore size, rather than increasing hydrophilicity.
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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. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and van-der-Waals-corrected density functional theory (DFT) calculations for 13 diamine-M
(dobpdc) variants, we demonstrate that the canonical CO
chemisorption products, ammonium carbamate chains and carbamic acid pairs, can be readily distinguished and that ammonium carbamate chain formation dominates for diamine-Mg
(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn-Mg
(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 CO
uptake in diamine-M
(dobpdc) materials. Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO
adsorption within diamine-M
(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.
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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) (dobpdc4– = 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-dobpdc4– = 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.
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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 aminein addition to its steric bulkis 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.
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Diamine-appended Mg2(dobpdc) (dobpdc4– = 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.
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Diamine-appended Mg
(dobpdc) (dobpdc
= 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are promising candidates for carbon capture that exhibit exceptional selectivities and high ...capacities for CO
. To date, CO
uptake in these materials has been shown to occur predominantly via a chemisorption mechanism involving CO
insertion at the amine-appended metal sites, a mechanism that limits the capacity of the material to ∼1 equiv of CO
per diamine. Herein, we report a new framework, pip2-Mg
(dobpdc) (pip2 = 1-(2-aminoethyl)piperidine), that exhibits two-step CO
uptake and achieves an unusually high CO
capacity approaching 1.5 CO
per diamine at saturation. Analysis of variable-pressure CO
uptake in the material using solid-state nuclear magnetic resonance (NMR) spectroscopy and
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that pip2-Mg
(dobpdc) captures CO
via an unprecedented mechanism involving the initial insertion of CO
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 CO
occupies a pocket formed by adjacent ammonium carbamate chains and the linker. Based on breakthrough and extended cycling experiments, pip2-Mg
(dobpdc) exhibits exceptional performance for CO
capture under conditions relevant to the separation of CO
from landfill gas. More broadly, these results highlight new opportunities for the fundamental design of diamine-Mg
(dobpdc) materials with even higher capacities than those predicted based on CO
chemisorption alone.
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Functional organic–inorganic hybrid materials with tunable properties are useful across many application areas, ranging from gas storage to electronics, flame retardants, separations, and catalysis. ...Combining polymers, with a suite of functional groups and conformational flexibility, and inorganic nanoparticles, with tunable surface chemistry and composition, yields hybrids with novel functional properties. Specifically, in catalysis, control of the electronic environment at a metal interface is paramount in determining the catalytic properties. In this contribution, we describe a modular process to prepare porous polymer–nanocrystal (NC) composites in a hierarchical, multilayered synthesis, in which multiple parameters can be accurately tuned: polymer functional groups and the corresponding pore structure, the polymer layer thickness, and the NC size, shape, and composition. This process provides for a variety of controlled materials with high surface area, tunable chemistry, and thermal and chemical stabilities. Furthermore, we demonstrate their utility for shape- and size-selective catalytic conversions both in oxidation and hydrogenation reactions, where they show increased selectivity by orders of magnitude compared to conventional polymer-supported metal catalysts. In light of the high degree of control in the composite structure, this method allows for the design and realization of catalysts for several reactions and reaction environments and for nanomaterials with other applications.
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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 CO
capture from a specific flue gas. As a result of their tunable, step-shaped CO
adsorption profiles, diamine-functionalized metal-organic frameworks (MOFs) of the form diamine-Mg
(dobpdc) (dobpdc
= 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are among the most promising materials for carbon capture applications. Here, we present a detailed investigation of dmen-Mg
(dobpdc) (dmen = 1,2-diamino-2-methylpropane), one of only two MOFs with an adsorption step near the optimal pressure for CO
capture from coal flue gas. While prior characterization suggested that this material only adsorbs CO
to half capacity (0.5 CO
per diamine) at 1 bar, we show that the half-capacity state is actually a metastable intermediate. Under appropriate conditions, the MOF adsorbs CO
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 Mg
(pc-dobpdc) (pc-dobpdc
= 3,3'-dioxidobiphenyl-4,4'-dicarboxylate), the metastable trap can be avoided and the full CO
capacity of dmen-Mg
(pc-dobpdc) accessed under conditions relevant for carbon capture from coal-fired power plants.
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