COMMUNICATION
/JACS Strong and Reversible Binding of Carbon Dioxide in a Green
MetalÀOrganic Framework
Jeremiah J.Gassensmith,†Hiroyasu Furukawa,‡Ronald A.Smaldone,†Ross S.Forgan,†
Youssry Y.Botros,†,§,||,^Omar M.Yaghi,‡,#and J.Fraser Stoddart*,†,#
†Department of Chemistry and§Department of Materials Science,Northwestern University,2145Sheridan Road,Evanston, Illinois60208-3113,United States
‡Department of Chemistry and Biochemistry,University of California,Los Angeles,607Charles E.Young Drive East,Los Angeles, California90095-1569,United States
)Intel Labs,Building RNB-6-61,2200Mission College Boulevard,Santa Clara,California95054-1549,United States
^National Center for Nano Technology Research,King Abdulaziz City for Science and Technology,P.O.Box6086,Riyadh11442, Kingdom of Saudi Arabia
#NanoCentury KAIST Institute and Graduate School of EEWS(WCU),Korea Advanced Institute of Science and Technology(KAIST), 373-1Guseong Dong,Yuseong Gu,Daejeon305-701,Republic of Korea
b Supporting Information
ABSTRACT:The efficient capture and storage of gaseous
CO2is a pressing environmental problem.Although porous
metalÀorganic frameworks(MOFs)have been shown to be
very effective at adsorbing CO2selectively by dint of dipoleÀ
quadruple interactions and/or ligation to open metal sites,the
gas is not usually trapped covalently.Furthermore,the vast
majority of these MOFs are fabricated from nonrenewable
materials,often in the presence of harmful solvents,most of
which are derived from petrochemical sources.Herein we
report the highly selective adsorption of CO2by CD-MOF-2,
a recently described green MOF consisting of the renewable
cyclic oligosaccharideγ-cyclodextrin and RbOH,by what isreactive metal
believed to be reversible carbonfixation involving carbonate
formation and decomposition at room temperature.The
process was monitored by solid-state13C NMR spectroscopy
as well as colorimetrically after a pH indicator was incorpo-
rated into CD-MOF-2to signal the formation of carbonic
acid functions within the nanoporous extended framework.
I n view of the predicted detrimental effects of CO2emission, capture of CO2from gaseous waste streams has become an urgent scientific objective.1Recently,several approaches toward the capture of CO2have employed porous metalÀorganic frameworks(MOFs)2assembled by linking organic and inor-ganic building blocks.While these advances in the technology of CO2capture are noteworthy3for their high storage capacity,the vast majority of these MOFs are fabricated from nonrenewable materials in harmful solvents,many of which are derived from petrochemical sources.4We recently reported5the discovery of a series of MOFs composed ofγ-cyclodextrin(γ-CD)(Figure1a), a product prepared6microbiologically from starch(amylose)and thus obtained from atmospheric carbon andfixed by photosynthesis. Theγ-CD tori are coordinated to each other by alkali metal cations in units of ,(γ-CD)6(Figure1b),forming three-dimensional (3D)body-centered-cubic(bcc)extended structures.These CD-MOFs,which are crystallized from water and either methanol or ethanol,are inexpensive and,importantly,“green”in the sense that they can be synthesized from renewable sources that are themselves derived from water,CO2,and nontoxic metal salts.We seek to apply this green material in the international green initiative tofind methodologies for trapping CO2in exhaust gases produced by combustion of organic matter.MOFs have been considered for this task,and there are bro
adly two distinct mechanisms by which reversible CO2capture occurs within these frameworks.One method is binding of CO2to vacant coordination sites on metal atoms.7 While this approach has led to materials with high selectivity for CO2,the effect of water(a combustion product)on binding is yet to be determined.Another method uses weakly nucleophilic or polar functional groups that bind CO2in a physisorptive manner by means of dipole interactions.8While this method is likely to be far
less Figure1.(a)Structural formula ofγ-cyclodextrin(γ-CD)with the primary hydroxyl groups colored red.(b)Stick representation of a single cubic(γ-CD)6unit of the extended framework of activated CD-MOF-2. The primary faces of the sixγ-CD tori point inward,while the secondary faces are oriented outward and are coordinated by24Rb+cations to another six(γ-CD)6units,forming a3D extended bcc structure w
herein gases may pass through portals defined by(i)cylindrical channels of aligned CD tori with diameters of∼0.9nm and(ii)smaller aligned triangular-shaped windows.The spherical inner cavities of the (γ-CD)6cubes have a diameter of∼1.7nm and are lined with24primary hydroxyl groups,whose O atoms are shown as red spheres.(c)Space-filling representation of the(γ-CD)6unit in which the sixγ-CD rings forming the sides of the cube are shown in different colors. Received:July13,2011
Published:August30,2011
a ffected by water,the constituent materials are typically toxic and the selectivity for CO 2over other gases is smaller than for methods using open metal sites.Fixing CO 2as carbamates by using pools of amines 9has been explored extensively,but the carbamate end product is thermodynamically very stable,making recycling impractical because heat (and thus more energy)is needed to liberate the CO 2and regenerate the free amine.Nevertheless,inspired by the notion of using weakly nucleophilic functional groups to fix CO 2chemically and reversibly,we found in CD-MOF-2a surfeit of free alcohol groups (Figure 1a)and anions 10to help sustain carbonic acid formation.Initial CO 2gas-uptake experiments with CD-MOF-2revealed an atypically strong a ffinity between CO 2and the MOF at low pressures,an observation that is indicative of a chemisorptive process.11To determine the role a chemisorptive process might have on gas adsorption,isotherms were measured for both CO 2and CH
4with CD-MOF-2at incremental temperatures (Figure 2).The total uptake of CO 2in the low-pressure region (<1Torr)was clearly una ffected by temperature over the range 273À298K,with a notable in flection point at ∼23cm 3/g regardless of the sample temperature.At these low pressures,the selectivity (as the ratio of the initial slopes)for CO 2over CH 4was nearly 3000-fold,a considerable di fference that is unmatched in the literature as far as we can tell.12
Additionally,the steep slopes of the isotherms in this region (Figure 2inset)suggest a strong binding event that would equate with the formation of a covalent bond.Notably,the abrupt transition in higher-pressure regimes (>1Torr)becomes much more dependent upon temperature,as indicated by the 30%greater uptake of CO 2at 273K than at 298K.These observations are consistent with covalent bond formation occurring preferentially atlow pressures and giving way to physisorption at elevated pressures,with the change in uptake mechanism occurring when the CO 2content of the MOF is ∼23cm 3/g.We were able to obtain these isotherms repeatedly on the same sample,showing the process to be fully reversible at room temperature.
Alkylcarbonic acids are known 13to form as a result of the reaction between CO 2and free primary alcohol groups.Although it has been pointed out 14that the addition of nucleo-philic groups,15speci fically primary amines,to MOFs through the rational design of struts 16or by way of postsynthetic modi
fi-cation improves CO 2capture,to our knowledge no spectro-scopic evidence has been provided for the formation of the
resultant organic acids.The free hydroxyl groups 17located on each individual γ-CD torus seem to be capable of serving as reactive functional groups for reversible carbonic acid formation (Figure 3a).Spectroscopic evidence showing the solid-state reactivity of γ-CD with CO 2was obtained by cross-polarization magic-angle-spinning (CP/MAS)13C NMR spectroscopy.For the solid-state NMR spectroscopic experiments,crystalline sam-ples were activated by exchanging the aqueous methanolic solution with dichloromethane before being evacuated and dried at low pressure (<2.0Â10À3Torr)for 2days to remove all of the remaining water.The activated CD-MOF was exposed to an atmosphere of dry CO 2for 10min and transferred into an airtight zirconium solid-state NMR rotor.The 13C NMR spec-trum of a crystalline sample of activated CD-MOF-2(Figure 3b,bottom)shows separate peaks for C1and C10as well as for C4and C40as a result of the commuted symmetry induced by the alternating Rb +cations on the primary and secondary faces of the γ-CD tori.Upon exposure to CO 2,a new peak 18centered at 158ppm emerged (Figure 3b,top),consistent with formation of a carbonate.19To rule out the possibility that OH Àcounterions of CD-MOF-2are the reactive agents,forming carbonate anions 20by reaction with CO 2,we examined CD-MOFs prepared from RbF or potassium benzoate that are is
ostructural with CD-MOF-2but contain non-nucleophilic and weakly basic counterions.The CP/MAS spectra of both MOFs showed identical reso-nances at 158ppm,precisely the chemical shift found in CD-
MOF-2upon exposure to CO 2.
Figure 2.Gas adsorption isotherms for activated CD-MOF-2,illustrating the uptake of CO 2measured consecutively at 273K (blue squares),283K (green circles),and 298K (black triangles)to be contrasted with the uptake of CH 4at 298K (red diamonds).Solid symbols indicate gas sorption and open symbols gas desorption.The initial steep rises observed at very low CO 2pressures reach the same value of ∼23cc/g regardless of tem-perature and are believed to
be characteristic of a chemisorption process.11
Figure 3.(a)Schematic diagram illustrating the equilibrium proposed to exist during the chemisorption of CO 2by CD-MOF-2,expressed in the context of the structural formula of one of the four repeating maltosyl units present in a single γ-CD torus.(b)CP/MAS 13C NMR spectra (400MHz,room temperature)of activated CD-MOF-2before (bottom)and after (top)exposure to CO 2.Upon exposure to CO 2,CD-MOF-2shows a new peak at 158ppm due to the formation of carbonic acid functions.This additional resonance is accompanied by changes in the chemical shifts of other peaks in the spectrum,supporting the observation that a chemical reaction between the gaseous CO 2and the framework of activated CD-MOF-2has occurred.Gaseous CO 2is not detected by this CP/MAS NMR method but is usually observed as a very sharp singlet at 126ppm in direct-polarization experiments.
We speculate that this favorable reactivity arises because the γ-CD units of CD-MOF-2possess a signi ficant number of acces-sible hydroxyl groups that de fine the circumference of a large (1.7nm diameter)pore and have a high capacity for CO 2relative to ambient pressures.These large local reactant concentrations facilitate the formation of bound CO 2molecules in ways reminiscent 21of the enhanced reactivity observed within supra-molecular host/guest complexes that arises from the greater local concentration within a supramolecular ensemble,which can increase the reactivity by orders of magnitude in some cases.If we are correct in assuming that formation of a carbonate ester by reaction with a hydroxyl group of a γ-CD occurs as the primary mechanism,it should be possible to detect the CO 2uptake using an inexpensive and readily available pH indicator.Thus,methyl red,a zwitterionic azobenzene-based pH indicator,was di ffused into the pores of CD-MOF-2by suspension in a CH 2Cl 2solution of the dye.The red solution was decanted,and brilliant yellow crystals were obtained after washing and drying in vacuo (Figure 4a).The yellow color arises from the incorporated methyl red,which undergoes partial anion metathesis (and thus deprotonation)with the counterions in the pore structure,an observation consistent with previous findings.5a The dried crystals were transferred to a scintillation vial,which was then exposed to both dry (from a tank)and humid (from sublimed dry ice)CO 2.The initial color change from yellow to orange/red occurred quickly (Figure 4b),regardless of the CO 2source,and after 5min,no further color change could be discerned by the na
ked eye.When the source of CO 2was removed,the crystals reverted to a yellow color (Figure 4c),indicating that the transient carbonic acid function had returned to the alcohol,liberating CO 2.This Le Ch ^a telier-like process was repeated more than 10times with no apparent fatigue observed on the material.
This strong uptake of CO 2occurs only when the γ-CD tori are arranged in the solid state as CD-MOF.22Pulverizing the crystals of this MOF turned it into an amorphous powder,as determined by powder X-ray di ffraction (PXRD)measurements (Figure 5a),23and quelled the carbonate formation,as deter-mined by solid-state NMR spectroscopy.The surface accessi-bility of these hydroxyl groups in the crystalline state far exceeds
that in the amorphous state,as determined (Figure 5b)by Langmuir and BET analysis [see Figure S3in the Supporting Informa-tion (SI)].Additionally,no evidence of carbonate formation was observed by CP/MAS NMR spectroscopy of pure γ-CD after exposure to CO 2.We repeated the CO 2uptake color-change experiments (Figure 5c)on pulverized CD-MOF-2and found that the amorphous material did not change color upon exposure to CO 2,in contrast to a pristine crystalline sample.
In conclusion,we have established that CD-MOF-2is highly selective for the absorption of CO 2at low pressures.24This selectivity is believed to be the result of chemisorption and to rely upon the free hydroxyl groups present in CD-MOF-2,which act as reactive hotspots for the formation of carbonic acid groups when CO 2di ffuses into the framework.Furthermore,the formation of an inherently acidic product as a result of this chemisorption allowed for the detection of CO 2colorimetrically by swapping anions with a small-molecule pH indicator within the MOF structure.We have also shown that the strong and selective uptake of CO 2is a property unique to nanoporous crystalline CD-MOF-2and is not observed for other CD-based mixtures with amorphous morphologies.The fact that CD-MOFs can be made from environmentally benign materials whose synthesis is essentially carbon-neutral and have the demon-strated ability to absorb CO 2from the atmosphere makes
them promising materials for carbon fixation.
Figure 4.Photographs of crystalline activated CD-MOF-2samples with incorporated methyl red at selected time intervals during CO 2sorption and desorption processes.(a)Yellow crystals of activated CD-MOF-2prior to CO 2exposure.It is postulated that interstitial OH Àcounterions (CD-MOF-2is prepared from γ-CD and RbOH)maintain a basic environment within the framework,leading to deprotonation of the methyl red dye.(b)Red crystals obtained after exposure of the yellow crystals from (a)to CO 2over the course of 5min.It is proposed that chemisorption of CO 2at the many OH groups facing into the cavities of the (γ-CD)6units results in the formation of carbonic acid functions throughout the extended framework,producing an acidic environment that protonates the dye,turning it red.(c)Crystals that reverted back to yellow following removal of the CO 2atmosphere,allowing ambient air to enter the vial over the course of 5min.It is hypothesized that the metastable carbonic acid species dissociate and release CO 2,returning the interior
of the framework to a basic environment.
Figure 5.(a)PXRD analysis of pristine CD-MOF-2(red,top)and a sample ground in a mortar and pestle for 7min (blue,bottom).The grinding process reduced CD-MOF-2into an amorphous powder,as re flected in the di ffraction pattern.(b)CO 2uptake isotherms (298K)for crystalline CD-MOF-2(red circles)and a sample ground into an amorphous powder (blue squares).The powdered sample exhibite
d very little CO 2sorption and no steep rise in the low-pressure region.(c)Photograph of activated CD-MOF-2with incorporated methyl red that was (left)ground to an amorphous powder or (right)allowed to remain pristine and then exposed to CO 2for 5min.The insets show magni fied images.No color change was evident in the amorphous sample,while the crystalline material underwent the expected change to a red color.
’ASSOCIATED CONTENT
b Supporting Information.Synthesis and characterization of all compounds and additional experimental results.This material is available free of charge via the Internet at ’AUTHOR INFORMATION
Corresponding Author
stoddart@northwestern.edu
’ACKNOWLEDGMENT
This research was supported in the U.S.by the National Science Foundation under Grant CHE-0924620and in the U.K.by the Engineering and Physical Research Council under Grant EP/ H003517/
1.In addition,the authors thank our joint collaborators Dr.Turki S.Al-Saud and Dr.Mohamed B.Alfageeh from KACST in Saudi Arabia for funding and support and the Non-Equilibrium Energy Research Center(NERC),which is an Energy Frontier Research Center(EFRC)funded by the U.S.Department of Energy, Office of Basic Energy Sciences(DOE-BES)under Award DE-SC0000989.O.M.Y.was supported by the Center for Gas Separations Relevant to Clean Energy Technologies,an EFRC funded by DOE-BES under Award DE-SC0001015.O.M.Y.and J.F.S.were supported by the WCU Program(NRF R-31-2008-000-10055-0)funded by the Ministry of Education,Science and Technology,Korea.
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