RESEARCH ARTICLE◥

GAS SEPARATIONS

Cooperative carbon capture and steam regenerationwith tetraamine-appended metal–organic frameworksEugene J. Kim1, Rebecca L. Siegelman1,2*, Henry Z. H. Jiang1, Alexander C. Forse1,3,4†,Jung-Hoon Lee5,6,7, Jeffrey D. Martell1‡, Phillip J. Milner1§, Joseph M. Falkowski8,Jeffrey B. Neaton5,6,9, Jeffrey A. Reimer2,3, Simon C. Weston8, Jeffrey R. Long1,2,3¶

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

Carbon dioxide (CO2) emissions from fos-sil fuel combustion and industrial pro-cesses account for as much as 65% ofanthropogenic greenhouse gas emissions(1–3). Carbon capture and sequestration

(CCS), wherein CO2 is separated from the flueemissions of large point sources and perma-nently sequestered underground, is widelyrecognized as an essential component of strat-egies for meeting the ambitious climate targetsestablished at the Paris Climate Conference(4). The development of capture technologyhas largely focused on coal flue emissions (5),but worldwide use of natural gas is projectedto exceed that of coal by ~2032, necessitatingthe rapid development of CCS technology fornatural gas emissions (6). The U.S. Departmentof Energy (DOE) has set an ambitious targetof 90% capture of the CO2 from natural gasflue streams (7), which is particularly chal-lenging given that the CO2 concentration in

natural gas combined-cycle (NGCC) emissions istypically only ~4%, comparedwith~12 to 15% forcoal emissions (7, 8). Additionally, NGCC emis-sions contain high concentrations of O2 (12.4%)andwater (8.4%), and thus effective technologiesmust be stable and maintain CO2 capture per-formance in the presence of these species (7, 9).Aqueous amine solutions, the most mature

carbon-capture technology as of now (10), aresusceptible to oxidative and thermal degra-dation and have low CO2 cycling capacities(11). Porous solid adsorbents such as zeolites,silicas, andmetal–organic frameworksareemerg-ing as promising CO2-capture materials be-cause of their high surface areas, lower intrinsicregeneration energies, higher stabilities, andtunable surface chemistries (12, 13). Takinginspiration from amine-functionalized silicas(14), we and others have shown that amine-functionalized metal–organic frameworks cancapture CO2 in the presence of water (15, 16). Amajor consideration for adsorptive CO2 cap-ture from natural gas flue emissions is that thelow partial pressure of CO2 requires materialswith high adsorption enthalpies. In turn, highertemperatures or lower pressures are neededto desorb capturedCO2, and these regenerationconditions are costly and can substantially af-fect performance (12). The use of steam for CO2

recovery has been proposed as a cost-effectivestrategy for amine-functionalized adsorbents(17–22), particularly because low-grade steamis inexpensive and readily available in mostindustrial processes. Unfortunately, engineer-ing adsorbents with stability in the presence ofsteam is an ongoing issue (19, 23–25).We recently reported the promising CO2-

capture properties of the diamine-appendedframework N,N′-dimethylethylenediamine–

Mg2[dobpdc (4,4′-dioxidobiphenyl-3,3′-dicarboxylate)](Fig. 1A) (26). In this material, CO2 adsorptionproceeds through a cooperative mechanismin which the gas inserts into the Mg–N bondsto form chains of oxygen-bound carbamatespecies charge-balanced by neighboring am-monium groups running along the pores (27).This mechanism gives rise to unusual step-shaped CO2 adsorption profiles and improvedmaterial CO2 cycling capacities that can beachieved with smaller pressure or tempera-ture swings than are required for traditionaladsorbents (27). The CO2-capture propertiescan further be tuned by varying the appendeddiamine, leading to adsorbents with potentialfor CO2 capture from coal flue gas emissions(Fig. 1B) (28–31) and even NGCC emissions (32).Nonetheless, these materials are susceptible

to diamine volatilization upon regeneration,which has limited their applicability in prac-tical capture processes (30). We now reportthat tetraamine-functionalizedMg2(dobpdc)materials (Fig. 1C) exhibited high thermalstability and cooperatively captured CO2 atconcentrations as low as parts per million(ppm), enabled by a two-step ammonium car-bamate chain-formation mechanism. The or-dered, multimetal coordination mode achievedwith the tetraamines markedly increased thestability of these adsorbents and enables CO2

desorptionwith inexpensive steam in tetraamine-functionalized Mg2(dobpdc).

Synthesis and structure oftetraamine-functionalized Mg2(dobpdc)

Our pursuit of tetraamine-functionalizedMg2(dobpdc) was motivated by structuralanalysis of diamine-appended analogs, whichsuggested that tetraamines with chain lengthson the order of two N-alkylethylenediaminescould bridge nearest-neighbor metals acrossthe pore (Fig. 1, B and C) (28). Such multiple-metal coordination could yield adsorbentsexhibiting cooperative CO2 capture and great-ly enhanced thermal and hydrolytic aminestability. Soaking Mg2(dobpdc) with varioustetraamines (Fig. 1D) in toluene (materialsand methods) produced loadings of ~1 tetra-amine per Mg2+ site, or twice the desired 1:2tetraamine:metal ratio (Fig. 1C), as determinedby 1H nuclear magnetic resonance (NMR) ofacid-digested samples. The desired loadingcould be achieved through subsequent ther-mal activation (materials and methods, figs. S3and S4, and tables S1 and S2). Thermogravi-metric analysis indicated that the tetraamine-grafted Mg2(dobpdc) materials were resistantto further diamine loss up to ~250° to 290°C(figs. S3 and S4). Here, we refer to the tetra-amines using shorthand based on the numberof carbon atoms in the alkyl groups bridgingthe amine moieties; for example, N,N′-bis(3-aminopropyl)-1,3-diaminopropane and N,N′-bis(3-aminopropyl)-1,4-diaminobutane are

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1Department of Chemistry, University of California, Berkeley,CA 94720, USA. 2Materials Sciences Division, LawrenceBerkeley National Laboratory, Berkeley, CA 94720, USA.3Department of Chemical and Biomolecular Engineering,University of California, Berkeley, CA 94720, USA. 4BerkeleyEnergy and Climate Institute, University of California,Berkeley, CA 94720, USA. 5Department of Physics,University of California, Berkeley, CA 94720, USA.6Molecular Foundry, Lawrence Berkeley National Laboratory,Berkeley, CA 94720, USA. 7Computational Science ResearchCenter, Korea Institute of Science and Technology, Seoul02792, Republic of Korea. 8Corporate Strategic Research,ExxonMobil Research and Engineering Company, Annandale,NJ 08801, USA. 9Kavli Energy Nanosciences Institute,University of California, Berkeley, CA 94720, USA.*Present address: DuPont de Nemours, Wilmington, DE 19803, USA.†Present address: Department of Chemistry, University of Cambridge,Cambridge CB2 1EW, UK. ‡Present address: Department ofChemistry, University of Wisconsin–Madison, Madison, WI 53706,USA. §Present address: Department of Chemistry and ChemicalBiology, Cornell University, Ithaca, NY 14853, USA.¶Corresponding author. Email: jrlong@berkeley.edu

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referred to as 3-3-3 and 3-4-3, respectively(Fig. 1D).Single-crystal x-ray diffraction (XRD) data

obtained for 3-3-3– and 3-4-3–appended variantsof the isostructural framework Zn2(dobpdc)revealed that the tetraamines coordinate in ahighly ordered fashion (Fig. 1, E and F). Tetra-amine 3-3-3 bound two metal centers sepa-rated by a distance of 10.4637(11) Å (Fig. 1E),whereas 3-4-3 bridged metal sites separatedby 16.8312(19) Å (Fig. 1F). Longer tetraaminescould ostensibly also be accommodated inthe framework and bridge metal atoms ateven greater distances. In 3-3-3–functionalizedZn2(dobpdc), extensive intramolecular hydrogenbondingwas observed,whereas 3-4-3–Zn2(dobpdc)primarily exhibited intermolecular hydrogenbonding between adjacent tetraamines along thepore direction. These experimental structuresconfirmed the tetraamine coordination modein M2(dobpdc) and accounted for the extremelyhigh thermal stability of these materials.

Adsorption properties and optimizationfor NGCC postcombustion capture

All of the Mg2(dobpdc)(tetraamine) variantsexhibited sharp step-shaped CO2 adsorptionprofiles consistent with cooperative adsorp-tion and ammonium carbamate chain forma-tion (Fig. 2 and figs. S14 to S16) (27, 28, 30, 32).In each case, the CO2 adsorption capacity at

30°C approached the theoretical capacity oftwo CO2 molecules per tetraamine. With theexception of Mg2(dobpdc)(3-3-3), the frame-works exhibited two-step adsorption profiles,with each step corresponding to half of thetheoretical capacity. Analogous two-step adsorp-tion profiles were previously observed in bulkydiamine-appended variants of Mg2(dobpdc)and were attributed to steric conflict betweenneighboring ammonium carbamate chainsacross the pore (30). In the case of tetraamine-appended Mg2(dobpdc), we hypothesize thatinitial chemisorption of CO2 occurred at oneamine end, generating ammonium carbamatesrunning along one vertex of the hexagonalpore. The formation of the second set of am-monium carbamate chains likely required re-orientation of the unreacted, bound amines.Consistent with this proposed mechanism,

increasing tetraamine length coincides with adecrease in the material step separations up to3-3-3, for which the functionalized frameworkdisplayed only a single adsorption step. Two-stepped adsorption returned in the case ofMg2(dobpdc) appended with the slightly larger3-4-3, which we attribute to this tetraaminelikely bridging two metals at a longer distancethan 3-3-3 and the other smaller tetraamines(Fig. 1, E and F). Furthermore, when appendedwith triamines 3-3 [bis(3-aminopropyl)amine]and 3-4 [N-(3-aminopropyl)-1,4-diaminobutane],

which can only form one set of ammoniumcarbamate chains, the Mg2(dobpdc) frameworksexhibited CO2 capacities that correspond toadsorption of one molecule of CO2 for everytwo Mg2+ sites (figs. S12 and S13). For practicalapplications, a single-step adsorption profile,or a two-step adsorption profile with closelyspaced adsorption steps, is desirable to max-imize the operating range over which the fullmaterial adsorption capacity can be accessed (32).The temperatures of the first adsorption step

for Mg2(dobpdc)(3-3-3) and Mg2(dobpdc)(3-4-3)are 163° and 150°C, respectively. These tempe-ratures are among the highest reported for anyamine-appended variant of Mg2(dobpdc)(26, 28–30, 32). Traditional adsorbents withLangmuir-type adsorption profiles typicallyshow optimal performance at the lowest pos-sible adsorption temperature, where the ad-sorption capacity is maximized. The highadsorption step temperatures for the tetraamine-appendedmaterials directly correlated withadsorption steps at low pressures sufficientto enable 90% CO2 capture from an NGCC fluestream (Fig. 2B and figs. S14 to S16) (7). Forexample, the CO2 adsorption isotherm forMg2(dobpdc)(3-2-3) at 75°C indicated that thematerial should be able to reduce the CO2

content in an NGCC flue stream to 0.4% (fig.S16). Additionally, our preliminary results (figs.S28 and S39) indicate that these materials may

Kim et al., Science 369, 392–396 (2020) 24 July 2020 2 of 5

Fig. 1. Diamine versus tetraaminecoordination in M2(dobpdc). (A) Illus-tration of a hexagonal channel ofMg2(dobpdc) viewed in the ab-plane,using single-crystal XRD data for theisostructural framework Zn2(dobpdc).(B and C) Diamine-functionalizedmaterial features coordination of onediamine to each Mg2+ site (28) (B),whereas tetraamines can coordinate totwo Mg2+ sites (C). (D) Tetraaminesexplored in this work and their abbre-viations. (E and F) Single-crystalXRD structures (100 K) of Zn2(dobpdc)functionalized with 3-3-3 and 3-4-3tetraamines, respectively. The tetra-amines span metal centers across thepore that are 10.4637(11) Å apart(3-3-3) and 16.8312(19) Å apart(3-4-3). Green, light blue, gray, red,blue, and white spheres representMg, Zn, C, O, N, and H, respectively.

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also be suitable for direct capture of CO2 fromair. Whereas most tetraamine-appended ma-terials exhibit stable CO2 cycling capacities,Mg2(dobpdc)(3-3-3) shows a gradual decreasein CO2 cycling capacity over time (fig. S10);thus, Mg(dobpdc)(3-4-3) was chosen for fur-ther study because of its practical step posi-tion and fundamentally interesting two-stepadsorption behavior.The adsorption data for Mg2(dobpdc)(3-4-3)

suggested that in a postcombustion-captureprocess fromNGCC flue emissions, this frame-

work could achieve a 90% CO2 capture rate,referring to the removal of CO2 to a residualconcentration of 10% that of the feed (reduc-tion from 40 to 4 mbar at atmospheric pres-sure). Single-component isotherms collectedbetween 90° and 120°C reveal that step-shapedCO2 capture at 4 mbar (corresponding to 90%capture) was retained up to 90°C under dry con-ditions (Fig. 2B). Using the Clausius–Clapeyronrelationship, we calculated differential adsorp-tion enthalpy (Dhads) and entropy (Dsads) val-ues of 99 ± 3 kJ/mol and 223 ± 8 J/mol·K,

respectively, at a loading of 1 mmol CO2/g(figs. S19 and S20). These values followed acorrelation previously observed between Dhadsand Dsads values determined for diamine-appended Mg2(dobpdc) (fig. S21) (28). TheDhads value for Mg2(dobpdc)(3-4-3) is amongthe highest reported for amine-functionalizedmaterials. Such a high enthalpy of adsorptionshould enable CO2 capture at high tempera-tures and result in lower energy consumptionin a temperature-swing adsorption process byminimizing the required temperature swing

Kim et al., Science 369, 392–396 (2020) 24 July 2020 3 of 5

Fig. 2. CO2 uptake in tetraamine-appended Mg2(dobpdc). (A) Adsorptionisobars obtained through thermogravimetricanalysis of Mg2(dobpdc)(tetraamine)under pure CO2 at atmospheric pressure.Dashed lines indicate the theoreticalcapacities for binding of one and two CO2molecules per tetraamine. (B) Adsorption(filled circles) and desorption (opencircles) isotherms for CO2 uptakein Mg2(dobpdc)(3-4-3) at 90°, 100°, 110°,and 120°C. The CO2 pressures in anuntreated NGCC flue emission stream(40 mbar) and after 90% capture(4 mbar) are indicated with light grayand dark gray lines, respectively.

Fig. 3. Spectroscopic investi-gation of CO2 adsorption inMg2(dobpdc)(3-4-3). (A) Rawin situ DRIFTS spectra of acti-vated 3-4-3–Mg2(dobpdc) (graycurve), 3-4-3–Mg2(dobpdc)dosed with 400 mbar of 12CO2

and 13CO2 at 120°C (dark blueand yellow curves, respec-tively), and the isotopic differencespectrum (teal curve). The13CO2 spectrum was used as abaseline to isolate vibrationscaused by inserted CO2. Vibra-tions corresponding to diagnosticcarbamate bands are labeled.(B) Room-temperature 13C solid-state magic angle spinningNMR spectrum (16.4 T) ofMg2(dobpdc)(3-4-3) dosedwith 1038 mbar of 13CO2. Theresonance at 162.6 ppmwas assigned as carbamate.Asterisks mark spinning sidebands. (C) 1H13C hetero-nuclear correlation (contact time100 ms) spectrum and correlationassignments. (D) Predictedstructures of Mg2(dobpdc)(3-4-3),Mg2(dobpdc)(3-4-3)(CO2), andMg2(dobpdc)(3-4-3)(CO2)2obtained from structural relaxations with vdW-corrected DFT. Green, gray, red, blue, and white spheres represent Mg, C, O, N, and H atoms, respectively.

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(33–37). Additionally, Mg2(dobpdc)(3-4-3) ex-hibited minimal adsorption-desorption hys-teresis at atmospheric pressure (fig. S11).

Cooperative adsorption mechanism

Weused in situ infrared (IR) and solid-stateNMRspectroscopies to characterize Mg2(dobpdc)(3-4-3)before and after CO2 adsorption to investigatethe two-step cooperative adsorption mecha-nism. In situ diffuse reflectance IR spectroscopy(DRIFTS) characterization of the frameworkin the presence of CO2 revealed a characteristiccarbamate C–N stretch at 1339 cm−1 and a C–Ostretch at 1689 cm−1 (isotopic difference spec-trum, Fig. 3A) (27–30). In addition, spectracollected simultaneously with a volumetric,equilibrium CO2 isotherm at 120°C indicatedthat the ammonium carbamate species wasresponsible for cooperative adsorption (fig. S25).Distinctive features corresponding to hydrogenbonding between neighboring ammoniumcarbamate chains were observed in the N–Hregions of the difference spectrum (3300 to3000 and 3500 to 3400 cm−1), and distincthydrogen bonding environments were char-acterized for each of the two adsorption steps(fig. S26).Corroborating evidence for ammonium car-

bamate formation was obtained from magicangle spinning solid-state NMR experiments.A single carbamate resonance was observedat 162.6 ppm in the 13C NMR spectrum ofMg2(dobpdc)(3-4-3) dosed with 1.04 bar of 13CO2

(Fig. 3B), which coincides with resonances re-ported previously for ammonium carbamateformed upon 13CO2 adsorption in diamine-appended Mg2(dobpdc) (38). The presenceof a single carbamate feature suggested thatall of the chemisorbed CO2 was in the samechemical environment after adsorption at1 bar. We also observed five resonances for thetetraamine alkyl carbons, supporting a sym-metric reaction between the tetraamine andCO2 at full capacity (Fig. 3B). Two-dimensionalheteronuclear correlation data further revealedthat CO2 reacted with the primary amine andthat a secondary ammonium group formednearby the carbamate (Fig. 3C) (38), and solid-state 15N NMR data confirmed the symmetricformation of ammonium and carbamate species(fig. S27). The NMR and IR results confirmedthat our tetraamine-functionalized Mg2(dobpdc)variants operated through the envisaged two-step cooperative adsorption mechanism.The structures of Mg2(dobpdc)(3-4-3) after

adsorption of CO2 to half and full capacitywere further investigated using van der Waals(vdW)–corrected density functional theory(DFT) calculations (Fig. 3D). Lattice param-eters obtained from high-resolution powderXRD (PXRD) experiments under vacuumand after exposure to 1-bar CO2 were usedas starting points for structural relaxations(fig. S29 and table S4). The computed binding

energies (DEads) for the structures are within±10 kJ/mol of the corresponding experimentalDhads values (table S7) (38). At full CO2 capacity(i.e., two molecules of CO2 per tetraamine),our calculations support two-step coopera-tive insertion of CO2, and the calculatedNMR chemical shifts for this structure are ingood agreement with the experimental values(table S8) (38). The computed DEads valuesindicate that the half-capacity structure wasmore stable than the full-capacity structure,which further supports our proposed two-step cooperative adsorption mechanism. Inaddition, the tetraamines in our optimizedstructure of activatedMg2(dobpdc)(3-4-3) bridgedthe set of metals spanning ~16.7 Å in the ab-plane of the framework, consistent with thesingle-crystal structure of Zn2(dobpdc)(3-4-3).Comparing the activated and full CO2 capacitystructures with synchrotron PXRDpatterns byRietveld analysis shows reasonable fits (figs.S32 and S33 and table S9). Taken together,our results from XRD, NMR, IR, and DFT in-

dicate that tetraamine-functionalized frame-works indeed enhance cooperative CO2 ad-sorption relative to diamine-appendedmaterials.

Carbon capture from flue gaswith steam desorption

Because flue gas streams are saturated withwater, any candidate carbon-capture mate-rial must be able to adsorb CO2 under humidconditions. When exposed to CO2 streamswith ~2.6%H2O (materials andmethods) (32),Mg2(dobpdc)(3-4-3) again exhibited step-shapedisobars, with steps shifted to higher temper-atures relative to those under dry CO2 (fig. S40),as has been reported previously for diamine-appended Mg2(dobpdc) (29, 30, 32). Notably,this temperature shift indicated that waterenhanced CO2 binding in the material andshould promote CO2 capture from a flue gasstream (29, 30, 32). Furthermore, essentiallyno changes in the CO2 adsorption profile andcapacity were observed after the material washeld under flowing air at 100°C for 12 hours,

Kim et al., Science 369, 392–396 (2020) 24 July 2020 4 of 5

Fig. 4. CO2 adsorption in Mg2(dobpdc)(3-4-3) in the presence of water. (A) CO2 breakthrough profile forMg2(dobpdc)(3-4-3) under simulated humid (~2.6% H2O) natural gas flue gas (4% CO2 in N2) at 100°C andatmospheric pressure. (B) Extended temperature-swing cycling of Mg2(dobpdc)(3-4-3) carried out using athermogravimetric analyzer at atmospheric pressure. Adsorption conditions: humid (~2.6% H2O) 4% CO2 inN2 for 5 min at 100°C; desorption conditions: humid (~2.6% H2O) CO2 stream at 180°C for 1 min. (C) IRspectra showing 15 cycles of adsorption under humid CO2 (bright green spectra) and desorption undersimulated steam (light gray spectra, estimated 51 to 65% water content, balance N2). Increasingly darkercolored curves indicate progressive cycles. Spectra of the activated framework under flowing steam andN2 were used as a baseline. (D) Plot of the integrated area for the carbamate band in (C) (~1290 to 1360 cm−1)versus adsorption cycle number, illustrating stable formation of the carbamate species across 15 cycles.

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indicating the stability of the framework tooxygen in the flue gas stream (fig. S43).Wealsoperformedbreakthroughexperiments

at 100°C under the same humid conditions tosimulate a real fixed-bed adsorption process.Under these conditions, Mg2(dobpdc)(3-4-3)exhibited a high CO2 capture rate of 90%witha breakthrough capacity of 2.0 ± 0.2 mmol/g(Fig. 4A and figs. S45 to S48). It also exhibitedminimal tetraamine volatilization during thecourse of 1000 CO2 adsorption-desorption cy-cles performed under humid conditions usinga thermogravimetric analyzer, and the mate-rial remained crystalline with a stable cyclingcapacity of 13.6 g/100 g (Fig. 4B and fig. S50and S51). Thus,Mg2(dobpdc)(3-4-3) could cyclea large quantity of CO2 while meeting the U.S.DOE target for capturing 90% of the CO2 fromnatural gas flue emissions (7).The high breakthrough capacity and excep-

tional stability of Mg2(dobpdc)(3-4-3) moti-vated us to explore the use of steam in place ofheating at 180°C to desorb CO2 from the ma-terial. Low-grade steam could offer advantagesover both temperature-swing and pressure-swing adsorption processes for postcombus-tion CO2 capture (17–22). The viability of usingsteam for CO2 desorption was tested in acustom-built flow-through cell for in situDRIFTSequipped to cycle between flowing humid CO2

and simulated steam (estimated 51 to 65%water content, balance N2; table S10) at 120°Cand ~1 atm (Fig. 4, C and D). The IR spectracollected for Mg2(dobpdc)(3-4-3) over thecourse of 15 cycles showed repeated growthand disappearance of both the ammonium(2800 to 1800 cm−1) andC–N (1339 cm−1) bandsassociated with carbamate chain formation,indicating the stability of the cooperative in-sertion mechanism to steam-regeneration con-ditions. Postcycling isobaric analysis of thematerial revealed that step-shaped adsorptionwas retained to a high capacity (~16.6 g/100;figs. S52 to S55 and table S11), and 1H NMRanalysis revealed that the tetraamine loadingremained unchanged (table S12). By contrast,the representative diamine-appended mate-rial Mg2(dobpdc)(N-ethylethylenediamine)2(28, 30) exhibited substantial amine volatil-ization after only a single steam desorptioncycle (table S13 and fig. S56). The impressiveperformance of Mg2(dobpdc)(3-4-3) under-scores the stabilization afforded by the multi-metal coordination modes.

Outlook

We have developed a class of tetraamine-functionalized metal–organic frameworks thatexhibit cooperative CO2 adsorption and greatlyenhanced stability compared with previousdiamine-functionalized materials as a resultof multiple, orderedmetal-amine interactions.The nature of amine coordination in thesematerials gives rise to two-step adsorption of

CO2 and large adsorption enthalpies suitablefor CO2 capture from simulated natural gasflue streams. The top-performing material,Mg2(dobpdc)(3-4-3), achieved a large CO2 ad-sorption capacity of 2.0 ± 0.2 mmol/g in thepresence of water while meeting the DOE tar-get for 90% CO2 capture from natural gas flueemissions.Most notably, however, the enhancedstability of these tetraamine-functionalizedframeworks enables regeneration using directsteam contact, a pathway that could afford sub-stantial energy savings compared with tradi-tional processes. Overall, the molecular-leveldesign strategy implemented in this work rep-resents a step-change in the industrial viabilityof adsorption-based CO2 capture from naturalgas flue emissions, a separation of growingimportance in the global energy landscape.

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ACKNOWLEDGMENTS

We thank H. N. Redfearn for carrying out initial NMR measurementson tetraamine-functionalized metal–organic frameworks, H. Furukawaand J. Oktawiec for helpful discussions, and K. R. Meihaus foreditorial assistance. Funding: We acknowledge ExxonMobilResearch and Engineering Company for financial support of thiswork. Single-crystal XRD data were collected at beamlines 11.3.1and 12.2.1 of the Advanced Light Source at Lawrence BerkeleyNational Laboratory, a user facility supported by the director, Officeof Science, Office of Basic Energy Sciences, of the DOE (contractno. DE-AC02-05CH11231). Synchrotron PXRD data were collectedon the 17-BM-B Beamline at the Advanced Photon Source, a U.S. DOEOffice of Science User Facility operated by Argonne NationalLaboratory. Use of the Advanced Photon Source at Argonne NationalLaboratory was supported by the U.S. DOE, Office of Science,Office of Basic Energy Sciences (contract no. DE-AC02-06CH11357).Work at the Molecular Foundry was supported by the Office ofScience, Office of Basic Energy Sciences, U.S. DOE (contract no.DE-AC02-05CH11231). Additional computational resources wereprovided by the DOE (NERSC) and the KISTI Supercomputing Center(project no. KSC-2019-CRE-0149). This research also used the Saviocomputational cluster resource provided by the Berkeley ResearchComputing program at the University of California, Berkeley. Wethank the Philomathia Foundation and Berkeley Energy andClimate Institute for support of A.C.F. through a postdoctoralfellowship. J.-H.L.’s work was supported by the KIST InstitutionalProgram (project no. 2E30460). We thank the Miller Institutefor Basic Research in Science for postdoctoral fellowship supportof J.D.M. We thank the National Institute of General MedicalScience of the National Institutes of Health for a postdoctoralfellowship for P.J.M. (F32GM120799). The content is solely theresponsibility of the authors and does not necessarily representthe official views of the National Institutes of Health. Authorcontributions: E.J.K., R.L.S., J.D.M., P.J.M., J.M.F., S.C.W., andJ.R.L. formulated the project. E.J.K. synthesized the materials.E.J.K. collected and analyzed the gas adsorption data. R.L.S.and E.J.K. collected and analyzed the single-crystal XRD data.H.Z.H.J. and E.J.K. collected and analyzed the IR spectra. A.C.F.and E.J.K. collected and analyzed the NMR spectra. J.-H.L.collected and analyzed the computational data. H.Z.H.J. collectedand analyzed the PXRD data. R.L.S. collected and analyzed thebreakthrough data. H.Z.H.J. and E.J.K. collected and analyzedthe steam desorption cycling data. E.J.K. and J.R.L. wrotethe manuscript, and all authors contributed to revising themanuscript. Competing interests: The authors declare thefollowing competing financial interest(s): J.R.L. has a financialinterest in and serves on the board of directors of MosaicMaterials, a start-up company working to commercialize metal–organic frameworks for gas separations. S.C.W., J.M.F., J.R.L.,E.J.K., R.L.S., J.D.M., and P.J.M. are inventors on patentapplication US16/175,708 (international: PCT/US2018/058287)held and submitted by the University of California, Berkeley, andExxonMobil Research and Engineering Co. that covers polyamine-appended metal–organic frameworks for carbon dioxideseparations. Data and materials availability: The supplementarymaterials contain complete experimental and spectral detailsfor all new compounds reported herein. Crystallographicdata have been made available free of charge from the CambridgeCrystallographic Data Centre under reference numbers CCDC1996313 and 1996314.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/369/6502/392/suppl/DC1Materials and MethodsFigs. S1 to S56Tables S1 to S14References (39–61)Data S1 and S2

20 February 2020; accepted 9 June 202010.1126/science.abb3976

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frameworksorganic−Cooperative carbon capture and steam regeneration with tetraamine-appended metal

Milner, Joseph M. Falkowski, Jeffrey B. Neaton, Jeffrey A. Reimer, Simon C. Weston and Jeffrey R. LongEugene J. Kim, Rebecca L. Siegelman, Henry Z. H. Jiang, Alexander C. Forse, Jung-Hoon Lee, Jeffrey D. Martell, Phillip J.

DOI: 10.1126/science.abb3976 (6502), 392-396.369Science 

, this issue p. 392; see also p. 372Sciencemethods.air and could be regenerated with steam, a method that is more economical than temperature or pressure swing

from humid2capacity and adsorption enthalpy (see the Perspective by Peh and Zhao). This material could capture CO2 adsorption that leads to a high CO2organic framework that displays two-step cooperative CO−magnesium metal

report on a tetraamine-functionalized et al.coal combustion and contains high concentrations of oxygen and water. Kim concentration that is only one-third of that of2challenging because combined-cycle natural gas combustion has a CO

can be more2intensive than coal, capturing its emitted CO−)2Although natural gas is less carbon dioxide (CO2Steaming out captured CO

ARTICLE TOOLS http://science.sciencemag.org/content/369/6502/392

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2020/07/22/369.6502.392.DC1

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REFERENCES

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