Science-2010-Vaidhyanathan-650-3

DOI: 10.1126/science.1194237
, 650 (2010);
330 Science , et al.Ramanathan Vaidhyanathan Amine-Functionalized Nanoporous Solid Binding Within an 2Direct Observation and Quantification of CO
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w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m
Direct Observation and Quantification of CO 2Binding Within an
Amine-Functionalized Nanoporous Solid
Ramanathan Vaidhyanathan,1*Simon S.Iremonger,1George K.H.Shimizu,1*Peter G.Boyd,2Saman Alavi,2Tom K.Woo 2*
Understanding the molecular details of CO 2-sorbent interactions is critical for the design of better carbon-capture systems.Here we report crystallographic resolution of CO 2molecules and their binding domains in a metal-organic framework functionalized with amine groups.Accompanying computational studies that modeled the gas sorption isotherms,high heat of adsorption,and CO 2
lattice positions showed high agreement on all three fronts.The modeling apportioned specific binding interactions for each CO 2molecule,including substantial cooperative binding effects among the
guest molecules.The validation of the capacity of such simulations to accurately model molecular-scale binding bodes well for the theory-aided development of amine-based CO 2sorbents.The analysis shows that the combination of appropriate pore size,strongly interacting amine functional groups,and the cooperative binding of CO 2guest molecules is responsible for the low-pressure binding and large uptake of CO 2in this sorbent material.T
he capture and storage of CO 2emitted from industrial processes are global chal-lenges.In many industrial processes,CO 2
is present at low partial pressures among other gases that ideally should be recycled.The currently employed capture method involves alkanolamine-based solvents that act as CO 2scrubbers (1,2)by chemisorptive formation of N-C bonded carbamate species (bonding energies are typically 100kJ mol −1).Regeneration of the amine requires cleavage of this covalent bond by heating (at 100°to 150°C)to release CO 2.Major drawbacks of this process include the corrosive nature and volatility of the amines,their occasional decomposition,and most prominently,the high energy cost of their regen-eration (1,2).The challenge is thus to couple ef-ficient CO 2capture with facile release in a sorbent material.
Porous systems,including zeolitic/zeotypic ma-terials (3,4),mesoporous silica (5–8),porous carbon (9),and,more recently,metal-organic frameworks (MOFs)(10–19),have been inves-tigated for CO 2storage.Merging the inherent sorptive behavior of porous solids with less-basic amines offers a route to the sort of easy-on/easy-off materials described above.Less-basic amines would favor physisorption over chemisorption of CO 2,thus greatly reducing the energy of regeneration.This prospect has prompted research on many amine-functionalized solid materials that on the whole demonstrates that amines can enhance CO 2uptake (5–8,17–19).Despite this conclusion,experimental insights at a molecular level on the nature of the NH 2···CO 2interaction are lacking (20).For materials such as silica and carbon,a
combination of factors (lack of order,large voids,flexible amine groups,and random adsorption sites)makes the study of individual sorptive interactions virtually unfeasible.In contrast,the crystallinity of
MOFs enables diffraction experiments to study structure at a molecular level.Beyond character-ization of the framework,in exceptional cases,x-ray or neutron diffraction can allow direct visu-alization of gases within pores (21–27)to eluci-date specific binding interactions and enable better sorbent design.Locating gas molecules in a MOF is challenging,but the systems typically display strong confinement effects on the guest molecules and/or specific sites of strong interaction (such as bare metal sites)that can serve as excellent models for understanding the interactions of gases in all porous systems.
We previously noted (28)that the MOF Zn 2(Atz)2(ox)(1)(Atz,3-amino-1,2,4-triazole;ox,oxalate)showed CO 2uptake at low pressures with an initial heat of adsorption (D H ads )of ~40kJ mol −1(Fig.1and fig.S4).After manifesting the expected trend of decreased D H ads with in-creased coverage,the material showed a subse-quent increase in D H ads ,which remained over 35kJ mol −1during continuous gas exposure,sug-gesting a cooperativity effect in the binding mech-anism.Here we describe a detailed study,via a combination of crystallographic and computa-tional methods,of the nature of CO 2binding in 1.In a crystal structure of 1loaded with CO 2mol-
1
Department of Chemistry,University of Calgary,Calgary T2N 1N4,Canada.2Department of Chemistry,Centre for Catalysis Research and Innovation,University of Ottawa,Ottawa,K1N 6N5,Canada.
*To whom correspondence should be addressed.E-mail:vvramana@ucalgary.ca (R.V.);gshimizu@ucalgary.ca (G.K.H.S.);twoo@uottawa.ca
(T.K.W.)
Fig.1.(A )Structure of the Zn-Atz layer in 1(Zn,cyan;C,black;N,blue;H,not shown).(B )Three-dimensional structure of 1,wherein the Zn-Atz layers are pillared by oxalate moieties (O,red)to form a six-connected cubic network shown as green struts.(C )Adsorption isotherms for different gases carried out using 1.The inset shows heat-of-adsorption data calculated with the CO 2isotherms measured at 273and 293K (fig.S5).The zero-loading heat of adsorption was calculated,using a model based on the virial equation (figs.S6and S7and table S2),to be 40.8T 0.8kJ mol −1.
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ecules,the CO 2binding sites are readily identi-fied,even from room temperature diffraction data.The characteristics of CO 2uptake in 1,including isotherm,heat of adsorption,and location of CO 2molecules,are modeled with high accuracy via a combination of classical grand canonical Monte Carlo (GCMC)simulations,molecular dynamics (MD)simulations,and periodic density functional theory (DFT)calculations.Thus,1·(CO 2)1.3serves to calibrate these methods for modeling gas sorp-tion in MOFs.The modeling enables the par-titioning of CO 2binding in 1into components based both on neighboring groups and the nature of interaction (electrostatics/dispersion).
Single crystals of 1were prepared via a pro-cedure modified from that previously reported (28),for the phases with hydrated and evacuated pores (29).X-ray crystallography of evacuated 1showed no electron density in the voids,thus confirming the effectiveness of the activation pro-cedure and the stability of the crystal.CO 2was then loaded into evacuated crystals of 1,and x-ray dif-fraction experiments conducted at 123,173,195,and 293K all yielded refinable data.The CO 2molecules could be located within the pores in all cases and were ordered except at 293K,where the disorder could be modeled.The 173-K data set yielded the best refinement parameters [refinement factor (R )=2.7%,weighted R (R w )=6.5%]and will be used for structural discussions.
The 173-K structure refinement gave a for-mula of 1·(CO 2)1.30,which agrees well with the calculated loading of 1.35CO 2from the adsorp-tion isotherms at 840mbar and 293K (which are comparable conditions to those of single-crystal experiments).In the lattice Zn,aminotriazolate layers are pillared by oxalate ions,with free amine groups lining the pores (Fig.1).
Within each pore,two independent CO 2bind-ing sites were located:CO 2-I [O(100)-C(100)-O(101)]and CO 2-II [O(200)-C(200)-O(201)](Fig.2and fig.S1).CO 2-I,near the free amine group,was 80%occupied,and CO 2-II,closer to the ox-alates,was 50%occupied;in filled pores,neither CO 2molecule showed positional disorder.CO 2could interact with an amine via N-H···O hydro-gen bonding or via an interaction between the N lone pair and the C atom of CO 2.The H atoms of the amine groups were readily located in the x-ray structure of 1·(CO 2)1.30,enabling direct vi-sualization of the H-bonding.CO 2-I was adjacent to the amine,with its electropositive C atom oriented toward the electronegative N atom [C(d +)···N(d –)=3.151(8)Å;the C-N of mono-ethanolamine carbamate was modeled as 1.45Å(30)and factoring C and N van der Waal radii =3.25Å].The bowing of the protons on the N atoms (~21°from the mean triazole plane)confirms that the lone pair is not delocalized into the triazole ring (Fig.2).Both O atoms of CO 2-I are within the range of longer H-bonds to the amine [N4-H···O100=3.039(4),O101=3.226(9)Å],with angles also corroborating very weak interactions [∠N-H···O:O100=97.486(1)°;O101=95.822(1)°].The C atom of CO 2-I also interacts with an oxalate O atom lining the pore [C(d +)…O(d –)=3.155(8)Å].The amine under-goes stronger H-bonding with oxalate O atoms [N4-H···O2=2.888(7),O4=2.122(2)Å,∠N-H···O =154.718(1)°and 137.843(1)°,respectively].
CO 2-II was located between oxalate groups along the b axis.The O(d –)of CO 2-II interacts with the C(d +)of the oxalate [O(d –)(CO2)…C(d +)(Ox)=2.961(5)Å].In contrast to CO 2-I,CO 2-II interacts with a proximal amine via only a single H-bond [(N8-H···O200=2.783(8),∠N-H···O =101.953(1)°].There is an interaction between CO 2molecules as one O atom of CO 2-I forms a contact with the C of CO 2-II [(C(d +)…O(d –)=3.023(7)Å,C200···O100].This CO 2-CO 2interaction is high-ly relevant because it is most likely the origin of the observed increasing D H ads with loading.Regarding the geometries of the CO 2mole-cules,both appear,with sizeable uncertainties,slightly bent [∠O-C-O:CO 2-I,175.72(1.01)°;CO 2-II,177.15(1.66)°].Given that a 3s range of angles approaches or includes linearity and the fact that neither chemical intuition nor the modeling studies support a nonlinear structure,the apparent bend of the CO 2molecules probably arises from their positional distributions rather than any true distortion in bonding.The C-O bond lengths [CO 2-I:1.137(8),1.079(9)Å;CO 2-II:1.141(13),1.125(13)Å]were slightly shorter than reported [1.155(1)Å]in a 150-K/ambient pressure structure of pure CO 2(31).V ariable-temperature x-ray crys-tallography showed that although the CO 2bond lengths and angles varied slightly with temperature,their occupancies and orientations did not appre-ciably change (table S1).A decrease in total gas uptake would be expected with increasing tem-perature;however,for 1,this did not change be-tween 195and 273K.This observation,coupled with the order of the gas molecules,reinforces the fact that the collective interactions are highly favorable for CO 2binding.Insight regarding the specific interactions was gained through compu-tational modeling.
To investigate the nature of the CO 2interactions with 1and the cooperative guest-guest binding effects,we used a combination of dispersion-corrected (32)periodic DFT calculations and clas-sical GCMC simulations and MD simulations (33),in which the partial charge parameters were derived from the periodic DFTcalculations by means of the REPEA T method (34).Figure 3A displays the ex-cellent agreement between the experimental and GCMC-simulated CO 2adsorption isotherms of 1at 273K.The parameters associated with the CO 2-1intermolecular potential were not adjusted to obtain this quality of fit.Figure 3B shows a center-of-mass probability density plot from a GCMC simulation of 1(at 850mbar and 273K),where darker regions reveal a greater probability of finding CO 2.Even at 273K,binding sites are well localized,and the symmetry of the probability clouds sug-gests two distinct binding sites,corroborating the crystallography.To compare the CO 2binding sites determined computationally and crystallographically,the experimental CO 2positions are superimposed on three-dimensional isosurfaces of the probabilities for C and O in Fig.3C.The remarkable agreement between the simulated and experimental CO 2po-sitions suggests that GCMC simulations that are often used to study gas adsorption in MOFs (33,35)
Fig. 2.X-ray structure of CO 2binding in 1·(CO 2)1.3at 173K.(A )The role of the amine group of Atz in binding CO 2-I is depicted.The H atoms of the amine group (located crystallographically)H-bond to oxalate O atoms,directing the N lone pair toward the C(d +)atom of the CO 2mole-cule.H-bond distances shown are for H-acceptor interactions.(B )Both crystallographically independent CO 2molecules are shown trapped in a pore,showing the cooperative in-teraction between CO 2-I and CO 2-II molecules.The CO 2…NH 2interaction is represented as a dotted purple bond,and the CO 2…CO 2interaction is indicated as a dotted yellow bond.(C )This panel shows the other interactions present.The CO 2-I …Ox interactions are shown in orange,and the CO 2…NH 2hydrogen bond in-teractions are shown in green.For clarity,H atoms are shown in
purple.
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not only reproduce adsorption isotherms but can also accurately reproduce specific binding sites.The nature of the CO 2binding was further studied with dispersion-corrected periodic DFT calculations.With all 16binding sites in the unit cell occupied,the resulting fully optimized ge-ometry is in excellent agreement with the x-ray structure,including the relevant CO 2-amine and CO 2-oxalate distances (fig.S8).The exception to this was the geometries of both CO 2molecules,which,unlike their geometry in the x-ray anal-ysis,were optimized to linear configurations,re-vealing that there is minimal geometric distortion from interaction with the framework.Radial dis-tribution plots extracted from the GCMC sim-ulations are also in agreement with the geometric parameters determined by crystallography (fig.
S10).Although specific binding sites were located experimentally,the CO 2binding is expected to be dynamic in nature.This prospect has been in-vestigated via classical MD simulations,which show that,in a 1.4-ns time span,the CO 2mole-cule can hop between several of the binding sites I and II (Fig.3D).
The CO 2binding energies were calculated at various occupancies at the DFT level (29).When 1is empty,CO 2-I has a binding energy of 39.6kJ mol −1,which is in good agreement with the experimental zero-loading D H ads (40.8T 0.8kJ mol −1).We see a strong cooperative enhance-ment of CO 2binding that increases with loading.Specifically,the binding energy of CO 2-II in-creases by 4.6kJ mol −1to 37.0kJ mol −1when an adjacent site I is occupied.When 1is fully oc-cupied less one binding site,the binding energy for CO 2-II increases to 38.1kJ mol −1,whereas that of CO 2-I is 44.2kJ mol −1.Further insight into the nature of the CO 2binding can be gained by partitioning the total binding energy.This analysis reveals that 82%of the binding energy of site I is due to dispersion,whereas 18%results from electrostatics (29).This contrasts with site II,where the binding energy is almost entirely (99%)due to dispersion interactions.
The cooperative binding in 1can be attributed to a combination of dispersion and electrostatic interactions between CO 2molecules,which can be quantified by using DFT to evaluate the interaction energy between two CO 2molecules at sites I and II in a vacuum (29).This calculation gives a CO 2-CO 2interaction energy of 3.9kJ mol −1,which accounts for most of the observed 4.6–kJ mol −1binding enhancement.Of the 3.9–kJ mol −1interaction between CO 2molecules,66%can be attributed to dispersion and 34%can be assigned to electrostatics.The same analysis,with the MOF fully loaded,attributes 61%of the interaction to dispersion and 39%to electrostatics,suggesting similar cooperativity at higher loadings.Several key insights obtained from the de-tailed analysis of the binding interactions in 1have implications for the design of future materials used for the physisorption of CO 2.The relatively high binding energies observed in the material are dom-inated by dispersion interactions.Because CO 2has a substantial quadrupole moment,there is op-portunity to further increase the binding energies through appropriately designed binding sites that maximize the electrostatic interactions with CO 2.The importance of the cooperative guest binding to the uptake of CO 2in 1is another key insight.In particular,the proper mutual orientation and high density of binding sites can be used as a strategy to increase CO 2binding energies.Further,we believe that these strategies can be incorporated into mate-rials with larger pores,in order to increase the overall CO 2uptake capacity and binding energies,thereby improving the uptake properties in the im-portant low –partial-pressure regime.In the broadest sense,via detailed modeling,this study lays the groundwork for the design of easy-on/easy-off physisorptive materials for CO 2capture,designed from the characteristics of the guest molecules out-ward rather than from the host framework inward.
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Fig.3.(A )Comparison of the simulated (red)and experimental (blue)CO 2gas adsorption isotherm of 1at 273K.The inset shows a comparison of experimental and simulated heats of adsorption as a function of guest loading.(B )Center-of-mass probability density plots of CO 2molecules in 1from a GCMC simulation at 850mbar and 273K.Probabilities along the b axis are summed and projected onto the ac plane.(C )Comparison of the location of the CO 2binding sites in 1obtained from x-ray analysis and those predicted by simulation.Probability isosurface plots of the CO 2O atoms are shown as transparent red surfaces and those of the C atoms as transparent cyan surfaces.The 16experimentally determined CO 2molecule positions are shown as tubes.An isosurface value of 0.15Bohr −3is used.(D )Trace of a single CO 2molecule during a MD simulation of 1with five CO 2molecules per unit cell or a loading of 1.6mmol g –1.Shown are 23successive snapshots,separated by 62.5ps.Other CO 2molecules present in the simulation are not shown.In (B)and (D),the approximate locations of binding sites I and II in a single channel are shown with dotted ellipses (site I,black;site II,red).In (B)and (C),a single unit cell of MOF 1is shown,whereas in (D)a 2×1×1cell is depicted.In (B)to (D),the MOF framework is shown as lines and the CO 2molecules are shown as tubes (C,light blue;O,red;N,dark blue;Zn,not shown).
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Alberta Energy Research Institute for partial financial support of this work.T.K.W.and P.G.B.thank the Canada Research Chairs Program and the High Performance Computing Virtual Laboratory for financial support.Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC)under reference numbers CCDC 782638to 782642.These data can be obtained free of charge via /conts/retrieving.html (or from the CCDC,12Union Road,Cambridge CB21EZ,UK).
Supporting Online Material
/cgi/content/full/330/6004/650/DC1Materials and Methods SOM Text
Figs.S1to S12Tables S1to S5References Movie S1
25June 2010;accepted 30September 201010.1126/science.1194237
The Occurrence and Mass
Distribution of Close-in Super-Earths,Neptunes,and Jupiters
Andrew W.Howard,1,2*Geoffrey W.Marcy,1John Asher Johnson,3
Debra A.Fischer,4Jason T.Wright,5Howard Isaacson,1Jeff A.Valenti,6Jay Anderson,6Doug N.C.Lin,7,8Shigeru Ida 9
The questions of how planets form and how common Earth-like planets are can be addressed by measuring the distribution of exoplanet masses and orbital periods.We report the occurrence rate of close-in planets (with orbital periods less than 50days),based on precise Doppler
measurements of 166Sun-like stars.We measured increasing planet occurrence with decreasing planet mass (M ).Extrapolation of a power-law mass distribution fitted to our measurements,d f /dlog M =0.39M −0.48,predicts that 23%of stars harbor a close-in Earth-mass planet (ranging from 0.5to 2.0Earth masses).Theoretical models of planet formation predict a deficit of planets in the domain from 5to 30Earth masses and with orbital periods less than 50days.This region of parameter space is in fact well populated,implying that such models need substantial revision.T
he architecture of our solar system,with small rocky planets orbiting close to the Sun and gas-liquid giant planets farther out,provides key properties that inform theories of planet formation and evolution.As more plan-etary systems are discovered,the planet occur-rence fractions and distributions of mass and orbital distance similarly shape our understanding of how planets form,interact,and evolve.Such properties can be measured using precise Doppler measurements of the host stars that interact gravi-tationally with their planets.These measurements reveal the planetary orbits and minimum masses (M sin i ,due to unknown orbital inclinations i ).In the core-accretion theory of planet forma-tion,planets are built from the collisions and sticking together of rock-ice planetesimals,grow-ing to Earth size and beyond,followed by the gravitational accretion of hydrogen and helium gas.This process has been simulated numeri-cally (1–4),predicting the occurrence of planets in a two-parameter space defined by their masses and orbital periods (P ).These simulations pre-dict that there should be a paucity of planets,a “planet desert ”(3),in the mass range from ~1
to 30Earth masses (M Earth )orbiting inside of ~1astronomical unit (AU),depending on the exact treatment of inward planet migration.We used precise Doppler measurements of a well-defined sample of nearby stars to detect planets having masses of 3to 1000M Earth or-biting within the inner 0.25AU.The 235main-sequence G-,K-,and M-type dwarf stars in our NASA –University of California Eta-Earth Survey were selected from the Hipparcos cat-alog on the basis of brightness (V <11),distance (<25pc),luminosity (M V >3.0),low chromo-spheric activity (log R ′HK <−4.7),lack of stellar companions,and observability from Keck Obser-vatory.The resulting set of stars is nearly free of selection bias;in particular,stars were neither included nor excluded based on their likelihood to harbor a planet.[The stars and planets are listed in the supporting online material (SOM).]Here we focus on the 166G-and K-type stars,with masses of 0.54to 1.28solar masses and B −V <1.4.We analyzed previously announced planets,new candidate planets,and nondetections on a star-by-star basis to measure close-in planet oc-currence as a function of planet mass.
We measured at least 20radial velocities (RVs)for each star,achieving 1m s −1precision (5)with the HIRES echelle spectrometer (6)at Keck Observatory.To achieve sensitivity on time scales ranging from years to days,the observa-tions of each star were spread over 5years,with at least one cluster of 6to 12observations in a 12-night span.Stars with candidate planets were observed intensively,leading to several discov-eries (5,7,8).In total,33planets (Fig.1)have been detected around 22stars in our sample (5,7,9–22),some of which were discovered by other groups.Sixteen of these planets have P <50days.Our analysis also includes five can-didate low-mass planets from the Eta-Earth Survey with P <50days and false alarm prob-abilities (5)of <5%.
1
Department of Astronomy,University of California,Berkeley,CA 94720,USA.2Space Sciences Laboratory,University of California,Berkeley,CA 94720,USA.3Department of Astro-physics,California Institute of Technology,Pasadena,CA 91125,USA.4Department of Astronomy,Yale University,New Haven,CT 06511,USA.5Department of Astronomy and As-trophysics,The Pennsylvania State University,University Park,PA 16802,USA.6Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218,USA.7University of Cal-ifornia Observatories/Lick Observatory,University of California,Santa Cruz,CA 95064,USA.8Kavli Institute for Astronomy and Astrophysics,Peking University,Beijing,China.9Tokyo Institute of Technology,Ookayama,Meguro-ku,Tokyo 152-8551,Japan.
*To whom correspondence should be addressed.E-mail:howard@
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