Microporous and Mesoporous Materials 130 (2010) 76–82

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Computational study of the effect of organic linkers on natural gas upgradingin metal–organic frameworks

Wei Mu, Dahuan Liu, Qingyuan Yang *, Chongli ZhongLab of Computational Chemistry, Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 April 2009Received in revised form 2 September 2009Accepted 21 October 2009Available online 23 October 2009

Keywords:SeparationMolecular simulationDensity functional theoryMetal–organic frameworks

1387-1811/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.micromeso.2009.10.015

* Corresponding author. Tel.: +86 10 64418935.E-mail address: qyyang@mail.buct.edu.cn (Q. Yang

In this work a hierarchical multiscale approach combining grand canonical Monte Carlo simulation anddensity functional theory calculation was performed to study the effect of the chemical properties of nineorganic linkers on CO2/CH4 mixture separation in metal–organic frameworks (MOFs). The computationalresults show that the organic linkers decorated with the electron-donating groups can strengthen the dis-tribution of the electrostatic field in the pores of MOFs, and greatly enhance the adsorption selectivity ofCO2/CH4 mixture in MOFs. This enhancement becomes stronger with the increase of the electron-donat-ing ability of groups. In addition, this work also demonstrates that the negative steric hindrance effects onthe separation behavior should be considered when the organic linkers are modified with multiple sub-stitutions in designing new materials. The knowledge obtained is expected to provide useful informationfor tailoring the electrostatic properties of MOFs for separation of various gas mixture systems of practi-cal importance.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

The combustion of vast amounts of carbon-based fossil fuels hasled to one of the most serious global environmental problems. Nat-ural gas mainly composed of methane (CH4) is thought as one ofthe cleanest carbon fuels due to the low emission of carbonaceousgreenhouse gas. However, the presence of carbon dioxide (CO2) innatural gas reduces its calorie content and also induces pipelinecorrosion. A widely recognized strategy for the feasible CO2 cap-ture technology is that minimal environmental impact and lowcosts should be achieved. The currently commercial amine-basedsystems used for CO2 removal bear several drawbacks such as cor-rosion control and considerable energy needed for solvent regener-ation [1]. Pressure swing adsorption (PSA) technology based on theporous adsorbents is known to be one of the most efficient andaffordable processes for CO2 removal from natural gas [2]. Manyresearch efforts have been carried out to identify and to developa suitable porous material with high CO2 affinity and capacity.Although the conventionally involved zeolite materials like zeo-lites 13X and NaY have been claimed to have high performancefor separation of CO2 from mixtures [3,4], it is difficult to regener-ate them without significant heating which leads to low productiv-ity and great expense [5].

Owing to their flexibility to design through control of the archi-tecture and chemical functionality of the pores, metal–organic

ll rights reserved.

).

frameworks (MOFs) have emerged as a new family of nanoporousmaterials that offer promising applications for gas storage and sep-aration [6,7]. Up to date, many studies have been carried out toinvestigate the removal of CO2 from natural gas in MOFs. For exam-ple, experiments have been carried out to study the ‘‘breathing” ef-fect of MIL-53 framework on CO2/CH4 mixture separation [8].Bastin et al. investigated the adsorption of CO2/CH4 mixture inMOF-508b by fixed-bed adsorption [9]. In addition, Babarao et al.performed molecular simulation to study the separation behaviorsof CO2/CH4 mixture in MOFs [10] and compared with other nano-porous materials [11]. Ideal adsorbed solution theory (IAST) calcu-lations have been used to study the adsorption of CO2/CH4 mixturein mixed ligand and carborane-based MOFs by Snurr and co-work-ers [12,13]. Keskin and Sholl conducted the atomically detailed cal-culations to predict the performance of MOF membranes for CO2/CH4 mixture separation [14,15]. Yang and Zhong investigated theenhancement of the electrostatic characteristic of MOFs on theselective adsorption of CO2 from its mixture with CH4 [16,17]. Mar-tín-Calvo et al. assessed the adsorption and separation of naturalgas in IRMOF-1 and Cu-BTC by molecular simulations [18]. All ofthese studies have demonstrated that MOFs could act as the poten-tial nanoporous materials for natural gas upgrading. However,none of them have been performed to examine the electron-com-bining ability of the organic linker on the separation of natural gasin MOFs. The different organic linkers may induce the great differ-ence in the electrostatic fields in MOFs, which is particularly usefulfor the separation of mixture consisting of components with largedifference in the quadrupole moments [17]. Thus, based on the

W. Mu et al. / Microporous and Mesoporous Materials 130 (2010) 76–82 77

combination of the grand canonical Monte Carlo simulations(GCMC) and density functional theory (DFT) calculations, a system-atic computational study was performed in this work to study theseparation of CO2/CH4 mixture in MOFs by introducing four typesof functional groups (–CH3, –F, –OH and –NH2) into the organic lin-ker in IRMOF-1. The knowledge obtained is expected to provideuseful information for tailoring the electrostatic properties ofnew MOFs for separation of various gas mixture systems of practi-cal importance.

2. Models and computational details

2.1. MOF structures

In the present work, IRMOF-1, a representative of the high por-ous MOFs, was selected as the host material, which has beenwidely studied both experimentally [19,20] and theoretically[14–16,18,21]. The guest-free framework structure of IRMOF-1was constructed from its experimental single-crystal X-ray diffrac-tion (XRD) data [22], and the unit cell crystal structure is shown inFig. 1. As can be seen from this figure, the crystal structure of IR-MOF-1 is very simple that consists of each oxide-centered Zn4Otetrahedron connected by six 1,4-benzenedicarboxylate (BDC)linkers to form a three-dimensional porous cubic framework.

In order to understand how the electron-combining abilities ofthe organic affect the separation of CO2/CH4 mixture in MOFs, fourtypes of functional groups were adopted in this work to substitutethe H atoms in the organic linkers of IRMOF-1, resulting in eightdifferent organic linkers as shown in Fig. 2. The eight designedMOF materials with these organic linkers were denoted as a gen-eral formula, IRMOF-(XX)n, where XX and n denote the type andnumber of the functional groups used to substitute the H atomsin the BDC linkers of IRMOF-1, respectively. It should be pointedout that the designed IRMOF-NH2 is identical to the available IR-MOF-3 [22].

2.2. Computational methods

2.2.1. Geometry optimization of the full periodic MOF structuresIn order to study the structural stabilities of the designed MOFs,

DFT calculations with periodic boundary conditions were per-formed to optimize the structures of their unit cells. In these calcu-lations, the gradient corrected (GGA) correlation functional ofPerdew and Wang (PW91) was used with the precise numericalbasis set DNP (double numerical plus polarization). All of the calcu-lations were carried out using the DMol3 code as implemented inthe Materials Studio package [23]. The above DFT methodology

Fig. 1. Unit cell crystal structure of IRMOF-1 viewed along the [1 0 0] direction (Zn,blue; O, red; C, gray; H, white). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

has been successfully employed to study the structures of MOFs[24,25].

2.2.2. Atomic partial charge calculationsIn all simulations, atomic partial charges for the MOFs are re-

quired as input parameters. Similar to the method in the early pio-neering works by Ganz et al. [21] and others [26], cluster modelswere cut from the optimized structures of the eight designed MOFsand the experimental one of IRMOF-1 as shown in Fig. 3, where thecluster models of IRMOF-1 and IRMOF-(NH2)3 together with thespecified atomic types are illustrated as the examples. For all ofthe cleaved clusters, the terminations of them were saturated withmethyl (–CH3) groups. Prior to calculating the atomic partialcharges, these cluster models were firstly optimized by DFT meth-od. The Perdew–Burke–Ernzerhof (PBE) exchange-correlation func-tional along with the polarized diploid-f valence basis set (DZVPP)is applied in these calculations [27]. After the optimizations, thesecluster models were used to calculate the atomic partial charges inthe frameworks of the MOFs. As done in previous works[16,17,21,26], DFT calculations with B3LYP functional were carriedout to compute the charge distributions, and the basis set LANL2DZwas used for metal atom Zn, while 6 � 31 + G� was used for theremaining atoms. The electrostatic charges were used as the atom-ic partial charges, and the ChelpG method was adopted, which hasbeen recognized as the most popular and reliable electrostaticcharge calculation method [28]. Details of the calculations can befound elsewhere [16,17,21]. All the calculations were performedusing the GAUSSIAN 03 suite of programs [29], and the calculatedresults are given in Tables 1 and 2.

2.2.3. Force fieldsIn this study, a single site model with the Lennard–Jones (LJ)

interaction was used to describe a CH4 molecule, and the potentialparameters (rCH4 = 3.73 Å and eCH4 /kB = 148.0 K) were taken fromthe TraPPE force field [30]. CO2 was modeled as a rigid linear mol-ecule with three charged LJ sites located on each atom. A combina-tion of the site–site LJ and coulombic potentials was used tocalculate the CO2–CO2 intermolecular interaction. The LJ potentialparameters for atom O (rO = 3.011 Å and eO/kB = 82.96 K) and atomC (rC = 2.789 Å and eC/kB = 29.66 K) in CO2 molecule were takenfrom the work of Babarao and Jiang [11]. The C–O bond length is1.18 Å. In this model, the intrinsic quadrupole moment is approx-imately described by the partial point charges centered at each LJsite (qO = �0.288 e and qC = 0.576 e).

For the MOFs studied in this work, an atomistic representationwas used to model all of them. Similarly, the CO2-adsorbent inter-actions were described by a combination of the site–site LJ andCoulombic potential, while the CH4-adsorbent interactions wereonly considered by the site–site LJ potential. The LJ potentialparameters for the framework atoms in MOFs were taken fromthe universal force field (UFF) [31], as listed in Table 3. In our sim-ulations, all the LJ cross-interaction parameters were determinedby the Lorentz–Berthelot mixing rules.

2.2.4. Simulation techniquesGrand canonical Monte Carlo (GCMC) simulations were em-

ployed to calculate the adsorption of pure components and theirmixtures with equimolar composition in MOFs at 298 K. Detailson the method can be found in our previous work [16,17]. Similarto previous works [10–18], all the MOFs were treated as rigidframeworks, since the effects of the dynamics of MOFs become sig-nificant only when the guests are large and/or strong guest–hostinteractions exist in the system at room temperature. The numberof the unit cells of MOFs adopted in the simulation cell is 2 � 2 � 2.At very low pressures, the molecules adsorbed in MOFs are verysmall, thus, simulation cells with 4 � 4 � 4 size were used at these

Fig. 2. Eight different organic linkers adopted in this work.

Fig. 3. Model clusters used for the calculations of atomic partial charges: (a)IRMOF-1 and (b) IRMOF-(NH2)3.

Table 1Atomic partial charges of IRMOF-1 and IRMOF-XX with single substitution (unit of e).

Atom types IRMOF-1 IRMOF-NH2 IRMOF-OH IRMOF-F IRMOF-CH3

O1C �1.996 �1.861 �1.872 �1.868 �1.847Zn 1.637 1.501 1.503 1.502 1.487O1 �0.757 �0.707 �0.688 �0.688 �0.684C1 0.671 0.623 0.598 0.605 0.594C2 0.079 �0.022 0.043 0.034 0.027C3 �0.122C3X 0.358 0.333 0.287 0.162C3Y �0.128 �0.138 �0.130 �0.121C4X �0.306 �0.342 �0.270 0.273C4Y �0.214 �0.215 �0.189 �0.192C5 0.157 0.193 0.167 0.161H3 0.125 0.150 0.149 0.155 0.144O3C �0.564H3O 0.419C3C �0.201H3C 0.062F �0.202N3 �0.800H3N 0.353

Table 2Atomic partial charges of IRMOF-XX with multiple substitutions (unit of e).

Atomtypes

IRMOF-(OH)2

IRMOF-(NH2)2

IRMOF-(NH2)3

IRMOF-(NH2)4

O1C �1.872 �1.876 �1.871 �1.878Zn 1.500 1.513 1.532 1.545O1 �0.690 �0.800 �0.800 �0.916C1 0.633 0.832 0.837 1.076C2 0.083 0.166 �0.302 �0.421C3 0.265 0.531 0.328C3X 0.354C3Y 0.364C4 �0.327 �0.325C4X 0.264C4Y �0.394C5 �0.097H3 0.181 0.211 0.218O3C �0.545H3O 0.402N3 �1.12 �0.906 �0.994H3N 0.473 0.403 0.448

78 W. Mu et al. / Microporous and Mesoporous Materials 130 (2010) 76–82

state points so that enough molecules are accommodated to guar-antee the simulation accuracy. A cutoff radius of 12.8 Å was ap-plied to the LJ interactions, and the long-range electrostaticinteractions were handled using the Ewald summation technique.Periodic boundary conditions were applied in all three dimensions.Gas-phase fugacities used to perform GCMC simulations were cal-culated with Peng–Robinson equation of state. For each state point,GCMC simulation consisted of 10,000,000 steps to guarantee equil-ibration followed by 10,000,000 steps to sample the desired ther-modynamic properties.

3. Results and discussion

3.1. Stability and atomic partial charges of MOFs

Periodic DFT calculations were performed to validate whetherthe structures of the eight designed MOFs are stable. From the cal-culation results, we found that the unit cell length of IRMOF-NH2

(i.e., IRMOF-3) is 25.703 Å, which is in good agreement with theexperimental value (25.7465 Å) [22]. In addition, the DFT calcula-tion results also show that the unit cell lengths of other seven

Table 3LJ potential parameters for the framework atoms in MOFs studied in this work.

LJ parameters MOF_Zn MOF_C MOF_O MOF_H MOF_N MOF_F

r (ÅA0

) 2.46 3.43 3.12 2.57 3.26 3.00

e=kB 62.40 52.84 30.19 22.14 34.69 25.14

Fig. 4. Optimized unit cell crystal structures of: (a) IRMOF-(OH)2 and (b) IRMOF-(NH2)4.

W. Mu et al. / Microporous and Mesoporous Materials 130 (2010) 76–82 79

new designed MOF materials lie in the range from 25.61 to 25.68 Å.Thus, the unit cell lengths of the eight designed MOFs are veryclose to that of IRMOF-1 (25.832 Å). Fig. 4 shows the optimizedunit cell crystal structures of IRMOF-(OH)2 and IRMOF-(NH2)4 asexamples, where the unit cell lengths are 25.673 and 25.612 Å,respectively. As can be seen from this figure, the optimized crystalstructures do not collapse, and keep the similar three-dimensionalporous cubic framework as that of IRMOF-1. These results indicatethat the structures of these eight MOFs are stable, and can be fur-ther used for the following investigations in this work.

Based on the DFT calculations, the atomic partial charges ineach MOF studied in this work were calculated, and the resultsare presented in Tables 1 and 2, where Table 1 shows the resultsfor IRMOF-1 and IRMOF-XX (XX = –NH2, –OH, –F and –CH3) withsingle substitution in the organic linker, while Table 2 shows theresults for IRMOF-(XX)n (XX = –OH and –NH2) with multiple substi-tutions. As shown in Tables 1 and 2, for each IRMOF-(NH2)n withdifferent number of substituent group –NH2, although the chemi-cal environments for the atoms N are somewhat different, theiratomic partial charges are very close. Thus, the atoms N in each IR-MOF-(NH2)n material are classified as the same atomic types withequal atomic partial charges. In addition, Tables 1 and 2 also showthat, the atomic partial charges in the eight designed MOFs aremuch different from those in the host material IRMOF-1, especiallyin the MOFs modified by –NH2 and –OH groups. Since the organiclinkers have been identified as the secondary adsorption sites for

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(a)

Fig. 5. Comparison of the simulated and experimental [32–34] adsorption isotherms of: (77 K.

the adsorption of adsorbate molecules in MOFs [10–18], variationsof the chemical properties of the organic linkers by modificationshould have great impacts on the separation of the mixture consid-ered in this work.

3.2. Validation of the force field

To confirm the reliability of the force field adopted in this work,the adsorption isotherms of pure CO2 and CH4 in IRMOF-1 weresimulated at 298 K and compared with their experimental data,as shown in Fig. 5a. It can be found in this figure that the simulatedisotherms are both in good agreement with the correspondingexperimental data of pure CO2 and CH4 in IRMOF-1 [32,33]. Forthe other eight MOF materials, to the best of our knowledge, thereare no experimental data for the adsorption isotherms of CO2 andCH4 in them. In order to further validate whether the above forcefield also works for them, the adsorption of H2 at 77 K in IRMOF-NH2 (i.e., IRMOF-3) was simulated and compared with the avail-able experimental data [34]. The potential model for H2 moleculeused here was the same as that in our previous work [35]. The re-sults shown in Fig. 5b indicate that the simulations enable goodreproduction of the experimental isotherm. Therefore, it could bereasonably thought that the force fields adopted in this work arereliable to investigate the adsorption behaviors of CO2/CH4 mixturein IRMOF-1 and other eight derivative MOF materials.

3.3. Adsorption selectivity in MOFs

In separation processes, a good indication of the separation abil-ity is the selectivity of a porous material for different componentsin mixtures. The selectivity for component A relative to componentB is defined by S = (xA/xB)(yB/yA), where xA and xB are the mole frac-tions of components A and B in the adsorbed phase, respectively,while yA and yB are the mole fractions of components A and B inthe bulk phase, respectively.

Fig. 6a compares the simulated selectivities of CO2 from equi-molar CO2/CH4 mixture in IRMOF-1 and IRMOF-XX with singlesubstitutions (XX = –NH2, –OH, –F and –CH3) at 298 K, where theerror bars shown on the simulated data are indicative of the statis-tical uncertainties, and the heights of them are comparable to thesizes of the data symbols. In these MOFs, functional groups –NH2, –OH and –CH3 are the electron-donating groups, while –F is theelectron-withdrawing group. Among them, the electron-donatingability follows the order –NH2 > –OH > –CH3, and this ability of –CH3 group is very weak. Compared with IRMOF-1, the resultsshown in Fig. 6a clearly show that the selectivities in IRMOF-NH2

and IRMOF-OH are the highest, and the selectivities in IRMOF-Fand IRMOF-CH3 are comparative to those in IRMOF-1 with onlyslightly larger values. These observations indicate that the intro-

0.00 0.02 0.04 0.06 0.08 0.100.0

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a) pure CO2 and CH4 in IRMOF-1 at 298 K and (b) H2 in IRMOF-NH2 (i.e., IRMOF-3) at

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IRMOF-(NH2)

2 IRMOF-(NH

2)

3

IRMOF-(NH2)

4

Fig. 6. Selectivities of CO2 from equimolar CO2/CH4 mixture at 298 K in: (a) IRMOF-1 and IRMOF-XX with single substitutions (XX = NH2, OH, F and CH3), (b) IRMOF-(XX)2

with double substitutions (XX = –NH2 and –OH), (c) IRMOF-(NH2)n with different substituent number of –NH2 group (n = 1–4). The error bars shown on the simulation dataare indicative of the statistical uncertainties.

80 W. Mu et al. / Microporous and Mesoporous Materials 130 (2010) 76–82

duction of functional groups with stronger electron-donating abil-ity into the organic linkers can enhance the adsorption selectivityof CO2/CH4 mixture in MOFs, while a very weak effect occurs forthe introduction of the electron-withdrawing group.

To compare the effects of the electron-donating abilities of thefunctional groups –NH2 and –OH on the selectivities of CO2 fromCO2/CH4 mixture, Fig. 6b shows the selectivities in IRMOF-(XX)2

with double substitutions (XX = –NH2 and –OH), and the error barsare also presented to indicate the statistical uncertainties. Obvi-ously, the selectivities in IRMOF-(NH2)2 are higher than those in IR-MOF-(OH)2 in the pressure range examined in this work. Theseresults are caused by the fact that the electron-donating abilityof –NH2 group is stronger than that of –OH group, which leadsthe more distinct changes in atomic partial changes (see Table 2),and thus increases the electrostatic interactions between CO2 andMOFs.

To investigate the effects of the electron-donating groups on theselectivity in more detail, GCMC simulations were further per-formed to study the separation behaviors of the equimolar CO2/CH4 mixture in IRMOF-(NH2)n (n = 1–4) at 298 K. The simulationresults are shown in Fig. 6c, where those in IRMOF-1 are alsoshown for comparison. It can be found in this figure that the selec-tivities become larger with the increasing of the substituent num-ber of –NH2 group, which indicates the changes of the electrostaticfield in MOFs are more in favor of CO2 adsorption with more elec-tron-donating groups presented in the organic linkers. In addition,Fig. 6c also shows that the selectivities in IRMOF-(NH2)3 and IR-MOF-(NH2)4 almost have the same values, and this phenomenamay be attributed to the steric hindrance effects of the additionalpendant –NH2 group presented in the phenylene space in IRMOF-(NH2)4, which essentially offsets the electrostatic enhancement ef-fects of –NH2 groups on the selectivities. Thus, it should balancethese two factors when incorporating the electron-donatinggroups into the organic linkers in designing new materials forthe practical applications.

3.4. Impact of HOMOs on the adsorption selectivity

It has been found that the occupying size of the conjugationspace of the highest occupied molecular orbitals (HOMOs) canqualitatively indicate the adsorption strength for polar or polariz-able molecules [34]. Due to the perfect symmetry of the –NH2

groups in the organic linkers of IRMOF-(NH2)4, the HOMOs calcula-tion was performed in this material as well as in IRMOF-1 for com-parison. Both the calculations were performed at DFT/B3LYP levelusing the GAUSSIAN 03 suite of programs [29], and the basis setLANL2DZ was used for atoms Zn, while 6 � 31 + G� for the rest ofthe atoms. Compared with the calculated results for IRMOF-1shown in Fig. 7a, due to the conjugation effects of the lone pairelectrons in –NH2 groups, Fig. 7b shows that the delocalization ofthe HOMOs around the organic linker in IRMOF-(NH2)4 was moreenhanced, and the electronic cloud of the HOMOs occupies muchlarger conjugated space, which can provide more polarized elec-tron densities of the framework that are advantageous to the inter-actions with the orbitals of CO2 molecules. Thus, the incorporationof electron-donating group like –NH2 into the organic linkers ismore favorable for the electrostatic interactions of CO2 moleculeswith the framework of MOFs, and may consequently enhancesthe adsorption selectivity of CO2/CH4 mixture in MOFs.

3.5. Relationship between the electrostatic fields in MOFs and theselectivity

To understand the enhanced electrostatic interactions of CO2

molecules with the framework of IRMOF-(NH2)4, the contour mapsof the electrostatic potentials for IRMOF-1 and IRMOF-(NH2)4 werefurther calculated, as shown in Fig. 8. The electrostatic potentials inMOFs were calculated using Dmol3 as implemented in the Materi-als Studio package [23], based on BLYP exchange-correlationfunctional of GGA level coupled with DNP basis set. Comparedwith the results shown in Fig. 8a for IRMOF-1, Fig. 8b shows that

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4

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Case 1 Case 2 Case 3 Case 4

Fig. 10. Effects of CO2–MOF electrostatic interactions on the selectivities of CO2

from the equimolar CO2/CH4 mixture at 298 K: case 1, in IRMOF-1 by switching offthe CO2–MOF electrostatic interactions; case 2, in IRMOF-(NH2)4 by switching offthe CO2–MOF electrostatic interactions; case 3, in IRMOF-1 with the CO2–MOFelectrostatic interactions considered; case 4, in IRMOF-(NH2)4 with the CO2–MOFelectrostatic interactions considered. The error bars used to indicate the statisticaluncertainties are within the data symbols.

Fig. 7. The highest occupied molecular orbitals of the organic linker in: (a) IRMOF-1 and (b) IRMOF-(NH2)4.

Fig. 8. The contour maps of electrostatic potentials (ESP) for: (a) IRMOF-1 and (b)IRMOF-(NH2)4.

Fig. 9. Contour plots of adsorption selectivities of CO2 from equimolar CO2/CH4

mixture at 1.0 MPa and 298 K in planes through the organic linkers and the metalclusters area in: (a) IRMOF-1 and (b) IRMOF-(NH2)4.

W. Mu et al. / Microporous and Mesoporous Materials 130 (2010) 76–82 81

IRMOF-(NH2)4 has an electrostatic potential with a larger gradientand higher absolute values in the two sides of the organic linker,which reflects a stronger electrostatic field in the pores of IR-MOF-(NH2)4 than that in IRMOF-1.

To illustrate the relationship between the distribution of elec-trostatic field in MOF and the adsorption selectivity, the selectivitydistributions of CO2 from the equimolar CO2/CH4 mixture in IR-MOF-1 and IRMOF-(NH2)4 at 1.0 MPa and 298 K were calculatedand are shown in Fig. 9, where the contour plots of the selectivitiesare in the planes through the organic linkers and the metal clustersarea. Obviously, the selectivity distributions are heterogeneous inboth materials, and Fig. 9b also shows that the local selectivitiesaround the organic linkers in IRMOF-(NH2)4 are much larger thanthose around the linkers in IRMOF-1. These observations are ingood agreements with the results shown in Fig. 8 for the distribu-tions of electrostatic fields in two MOFs.

In order to more deeply investigate the effects of the electro-static fields resulted from the above electrostatic potentials onthe adsorption selectivity, two additional simulations were per-formed at 298 K for the equimolar CO2/CH4 mixture in IRMOF-1and IRMOF-(NH2)4: case 1, switching off the CO2-IRMOF-1 electro-static interactions, and case 2, switching off the CO2-IRMOF-(NH2)4

electrostatic interactions. The simulation results are presented inFig. 10, where the results shown in Fig. 6 for IRMOF-1 (case 3)and IRMOF-(NH2)4 (case 4) with these electrostatic interactions

considered are also included for comparison. Evidently, in cases 1and 2, where only the dispersive interactions exist between adsor-bates–MOFs, the selectivities in two materials are comparativewith somewhat larger values in IRMOF-(NH2)4. These observations

Fig. 11. Contour plots of adsorption selectivities of CO2 from equimolar CO2/CH4

mixture at 1.0 MPa and 298 K in planes around the linkers and the metal clustersarea in: (a) IRMOF-1 by switching off the electrostatic interactions between CO2 andMOF and (b) IRMOF-(NH2)4 by switching off the electrostatic interactions betweenCO2 and MOF.

82 W. Mu et al. / Microporous and Mesoporous Materials 130 (2010) 76–82

are in good agreements with the selectivity distributions shown inFig. 11 for both cases at 1.0 MPa. When the electrostatic interac-tions between the CO2 molecules and MOFs are taken into account,the increasing of the selectivities in IRMOF-(NH2)4 is much moresignificant than that in IRMOF-1 in the whole pressure rangeexamined, which can be attributed to the fact that the electrostaticfield in the former is stronger as shown in Fig. 8. Therefore, it isclear that the strong electrostatic interactions produced by theelectrostatic fields in MOFs can largely enhance the adsorptionselectivity of CO2/CH4 mixture in them.

In addition, a combination of site–site LJ and Coulombic poten-tials was used for a CO2 molecule due to its large quadrupole mo-ment represented by the atomic partial charges, while only the LJpotential was used for a CH4 molecule because of its zero quadru-pole moment. As can be seen from Fig. 10, if this difference in thequadrupole moments of CO2 and CH4 molecules is considered inthe simulations, the electrostatic interactions of CO2–MOFs re-sulted from the electrostatic fields in both MOFs can largely en-hance the adsorption selectivities of CO2 from CO2/CH4 mixture,especially in IRMOF-(NH2)4 because of its stronger electrostaticfield. Thus, it may be concluded that, the larger difference in thequadrupole moments of the components, the greater the separa-tion ability of the MOFs in favor of the component with largerquadrupole moment.

4. Conclusions

In this work, a computational study was performed to examinethe effects of organic linkers with chemical modifications on theadsorption separation of CO2/CH4 mixture in MOFs. The resultsfrom GCMC simulations show that the incorporation of the elec-tron-donating groups into the organic linkers can largely enhancethe adsorption selectivity of MOFs for CO2/CH4 mixture separation,while almost no influence by decorating with electron-withdraw-ing groups. This enhancement becomes more evident with the

increase of the electron-donating ability of the decorated group.The underlying mechanisms were further studied: the organiclinkers modified with the electron-donating groups can greatlystrengthen the distributions of the electrostatic field in the poresof the MOFs, and thus are beneficial for the separation of CO2/CH4 mixture. Furthermore, this work shows that it should balancethe separation enhancement of the electron-donating groups andtheir steric hindrance effects in designing new materials. On thebasis of the results obtained, it may be concluded that the incorpo-ration of the electron-donating groups into the organic linkers is apromising strategy to develop MOFs with high separation perfor-mance for gas mixtures with components having large differencein quadrupole moments.

Acknowledgments

The financial supports of the NSFC (Nos. 20725622, 20706002,and 20876006) and Beijing Nova Program (No. 2008B15) aregreatly appreciated.

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