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.

We have found that the 3D zeolitic imidazolate framework ZIF-7 exhibits far more complex behavior in response to the adsorption of guest molecules and changes in temperature than previously thought. We believe that this arises from the existence of different polymorphs and different types of adsorption sites. We report that ZIF-7 undergoes a displacive, nondestructive phase change upon heating to above ∼700 °C in vacuum, or to ∼500 °C in CO2 or N2. This is the first example of a temperature-driven phase change in 3D ZIF frameworks. We predicted the occurrence of the high-temperature transition on the basis of thermodynamic arguments and analyses of the solid free-energy differences obtained from CO2 and n-butane adsorption isotherms. In addition, we found that ZIF-7 exhibits complex behavior in response to the adsorption of CO2 manifesting in double transitions on adsorption isotherms and a doubling of the adsorption capacity. We report adsorption microcalorimetry, molecular simulations, and detailed XRD investigations of the changes in the crystal structure of ZIF-7. Our results highlight mechanistic details of the phase transitions in ZIF-7 that are driven by adsorption of guest molecules at low temperature and by entropic effects at high temperature. We derived a phase diagram of CO2 in ZIF-7, which exhibits surprisingly complex re-entrant behavior and agrees with our CO2 adsorption measurements over a wide range of temperatures and pressures. We predicted phase diagrams of CH4, C3H6, and C4H10. Finally, we modeled the temperature-induced transition in ZIF-7 using molecular dynamics simulations in the isobaric-isothermal ensemble, confirming our thermodynamic arguments.

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 crystal structure of the aluminosilicate MCM-68 was solved from synchrotron powder diffraction data by the program FOCUS. The unit cell framework contains Si100.6Al11.4O224. This material crystallizes in space group P42/mnm, where, after Rietveld refinement, a=18.286(1) A and c=20.208(2) A. A three-dimensional framework is found that contains continuous 12-ring channels and two orthogonal, intersecting, undulating 10-ring channels. Rietveld refinement of the model coordinates optimizes the framework geometry, to match the observed intensity profile by Rwp=0.1371, R(F2)=0.1411. It is not possible to determine the location of approximately 0.84 K+ cations remaining in the unit cell after the material is steamed and then dehydrated. The framework model also successfully predicts observed electron diffraction data in two projections, and the tetragonal projection can be determined independently from these data by direct methods. The calculated density of the framework structure is 1.66 g/cm3, and the T-site framework density is 16.6 T/1000 A3.

As natural gas supplies a growing share of global primary energy, new research efforts are needed to develop adsorbents for carbon capture from gas-fired power plants alongside efforts targeting emissions from coal-fired plants.

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