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. Small modifications to the diamine structure are found to shift the threshold pressure for cooperative CO adsorption by over 4 orders of magnitude at a given temperature, and the observed trends are rationalized on the basis of crystal structures of the isostructural zinc frameworks obtained from in situ single-crystal X-ray diffraction experiments. The structure-activity relationships derived from these results can be leveraged to tailor adsorbents to the conditions of a given CO separation process. The unparalleled versatility of these materials, coupled with their high CO capacities and low projected energy costs, highlights their potential as next-generation adsorbents for a wide array of CO separations.

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, 9.1 wt %) with a modest 60 °C temperature swing. Evaluation of the thermodynamic parameters of adsorption for dmpn-Mg(dobpdc) suggests that the narrow temperature swing of its CO adsorption steps is due to the high magnitude of its differential enthalpy of adsorption (Δh = -73 ± 1 kJ/mol), with a larger than expected entropic penalty for CO adsorption (Δs = -204 ± 4 J/mol·K) positioning the step in the optimal range for carbon capture from coal flue gas. In addition, thermogravimetric analysis and breakthrough experiments indicate that, in contrast to many adsorbents, dmpn-Mg(dobpdc) captures CO effectively in the presence of water and can be subjected to 1000 humid adsorption/desorption cycles with minimal degradation. Solid-state C NMR spectra and single-crystal X-ray diffraction structures of the Zn analogue reveal that this material adsorbs CO via formation of both ammonium carbamates and carbamic acid pairs, the latter of which are crystallographically verified for the first time in a porous material. Taken together, these properties render dmpn-Mg(dobpdc) one of the most promising adsorbents for carbon capture applications.

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).

Natural gas has become the dominant source of electricity in the United States, and technologies capable of efficiently removing carbon dioxide (CO2) from the flue emissions of natural gas–fired power plants could reduce their carbon intensity. However, given the low partial pressure of CO2 in the flue stream, separation of CO2 is particularly challenging. Taking inspiration from the crystal structures of diamine-appended metal–organic frameworks exhibiting two-step cooperative CO2 adsorption, we report a family of robust tetraamine-functionalized frameworks that retain cooperativity, leading to the potential for exceptional efficiency in capturing CO2 under the extreme conditions relevant to natural gas flue emissions. The ordered, multimetal coordination of the tetraamines imparts the materials with extraordinary stability to adsorption-desorption cycling with simulated humid flue gas and enables regeneration using low-temperature steam in lieu of costly pressure or temperature swings.

Judicious choice of framework structure allows for single CO2 adsorption steps with bulky primary,secondary diamines appended to metal–organic frameworks.

Supported by increasingly available reserves, natural gas is achieving greater adoption as a cleaner-burning alternative to coal in the power sector. As a result, carbon capture and sequestration from natural gas-fired power plants is an attractive strategy to mitigate global anthropogenic CO 2 emissions. However, the separation of CO 2 from other components in the flue streams of gas-fired power plants is particularly challenging due to the low CO 2 partial pressure (∼40 mbar), which necessitates that candidate separation materials bind CO 2 strongly at low partial pressures (≤4 mbar) to capture ≥90% of the emitted CO 2. High partial pressures of O 2 (120 mbar) and water (80 mbar) in these flue streams have also presented significant barriers to the deployment of new technologies for CO 2 capture from gas-fired power plants. Here, we demonstrate that functionalization of the metal−organic framework Mg 2 (dobpdc) (dobpdc 4− = 4,4′-dioxidobiphenyl-3,3′dicarboxylate) with the cyclic diamine 2-(aminomethyl)piperidine (2-ampd) produces an adsorbent that is capable of ≥90% CO 2 capture from a humid natural gas flue emission stream, as confirmed by breakthrough measurements. This material captures CO 2 by a cooperative mechanism that enables access to a large CO 2 cycling capacity with a small temperature swing (2.4 mmol CO 2 /g with ΔT ≥ 100°C). Significantly, multicomponent adsorption experiments, infrared spectroscopy, magic angle spinning solid-state NMR spectroscopy, and van der Waals-corrected density functional theory studies suggest that water enhances CO 2 capture in 2-ampd−Mg 2 (dobpdc) through hydrogen-bonding interactions with the carbamate groups of the ammonium carbamate chains formed upon CO 2 adsorption, thereby increasing the thermodynamic driving force for CO 2 binding. In light of the exceptional thermal and oxidative stability of 2-ampd−Mg 2 (dobpdc), its high CO 2 adsorption capacity, and its high CO 2 capture rate from a simulated natural gas flue emission stream, this material is one of the most promising adsorbents to date for this important separation. ■ INTRODUCTION 40 The combustion of fossil fuels in the energy sector is currently 41 responsible for the release of 32 Gt/year of CO 2 into the 42 atmosphere, or approximately 65% of annual anthropogenic 43 greenhouse gas emissions. 1,2 To limit the contribution of these 44 emissions to global climate change, mitigation strategies are 45 needed during the transition to cleaner fuel sources. 2 One of 46 the most widely studied emission mitigation strategies is 47 postcombustion carbon capture and sequestration (CCS), in 48 which CO 2 is selectively removed from the flue gas streams of 49 fossil fuel-or biomass-fired power plants and sequestered 50 underground. 1−4 To date, the large majority of efforts toward 51 implementing CCS have focused on coal-fired power plants, 52 which are currently responsible for approximately 45% of 53 energy-related CO 2 emissions. 4,5 However, global consump-54 tion of natural gas has been increasing steadily, and i...

The stable Pd(0) species [(1,5-cyclooctadiene)(L•Pd) 2 ] (L = AdBrettPhos) has been prepared and successfully evaluated as a precatalyst for the fluorination of aryl triflates derived from biologically active and heteroaryl phenols, challenging substrates for our previously reported catalyst system. Additionally, this precatalyst activates at room temperature under neutral conditions, generates 1,5-cyclooctadiene as the only byproduct, and leads to overall cleaner reaction profiles.Fluorination of aromatic rings is a widely used strategy for modifying the biological activities of potential pharmaceutical and agrochemical agents. 1 In addition, 18 F-substituted compounds are important radiotracers for positron emission tomography (PET). 2 Aryl fluorides are typically installed early in a target molecule's synthesis using the harsh BalzSchiemann reaction, making the synthesis of 18 F-radiotracers and highly functionalized fluorinated materials difficult. Although a number of methods for electrophilic aryl fluorination with Ag, 3 Pd, 4 and Cu 5 catalysts, and without added transition metals, 6 have been developed to address this need, these reactions typically do not tolerate easily oxidizable functional groups such as tertiary amines and electron-rich heterocycles, result in 5-50% reduction of the starting material, and/or require the synthesis of unstable or toxic organometallic reagents. The direct transformation of aryl (pseudo)halides to aryl fluorides using a metal fluoride salt is a promising alternative to electrophilic fluorination in terms of generality and practicality 7 that has received less attention than electrophilic fluorination methods. 8 To this end, we reported the successful coupling of aryl triflates with CsF using a Pd catalyst based on the bulkyl biaryl phosphine ligand tBuBrettPhos (1) (Figure 1). 9,10 However, there remains a strong need for the further development of simple methods for aryl fluorination that demonstrate broad substrate scope and clean reaction profiles.Our original catalyst system of [(cinnamyl)PdCl] 2 /1 facilitates the catalytic fluorination of a variety of aryl triflates with minimal formation (<5%) of the corresponding reduction product. 9b However, this method suffers from poor reactivity with highly electron-rich and * sbuchwal@mit.edu. Supporting Information AvailableProcedural and spectroscopic data for all compounds are provided. This data is provided free of charge at http://pubs.acs.org. (Figure 1) was found to be more capable in the fluorination of these substrates (Table 1, Entry 2), though formation of two regioisomeric aryl fluorides was observed in the case of estrone triflate. 9b The effectiveness of a catalyst based on 2 is likely due to the faster rate of reductive elimination from Pd-F intermediates bearing 2 compared to those bearing 1. 11 However, the use of [(cinnamyl)PdCl] 2 as the source of active Pd requires 1.5 equiv of 2 relative to Pd to be added and results in generation of one equivalent of "Cl − ", which participates in a compe...

Purification of the C alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene remains among the most challenging industrial separations, due to the similar shapes, boiling points, and polarities of these molecules. Herein, we report the evaluation of the metal-organic frameworks Co(dobdc) (dobdc = 2,5-dioxido-1,4-benzenedicarboxylate) and Co( m-dobdc) ( m-dobdc = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multicomponent breakthrough measurements. Remarkably, Co(dobdc) distinguishes among all four molecules, with binding affinities that follow the trend o-xylene > ethylbenzene > m-xylene > p-xylene. Multicomponent liquid-phase adsorption measurements further demonstrate that Co(dobdc) maintains this selectivity over a wide range of concentrations. Structural characterization by single-crystal X-ray diffraction reveals that both frameworks facilitate the separation through the extent of interaction between each C guest molecule with two adjacent cobalt(II) centers, as well as the ability of each isomer to pack within the framework pores. Moreover, counter to the presumed rigidity of the M(dobdc) structure, Co(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.