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Theoretical studies of CO2 adsorption mechanism on linkers of metal–organic frameworks
Fuel 95 (2012) 521–527
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Theoretical studies of CO2 adsorption mechanism on linkers of metal–organic frameworks Yang Liu a, Jing Liu a,⇑, Ming Chang b, Chuguang Zheng a a b
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 2 September 2011 Received in revised form 23 September 2011 Accepted 29 September 2011 Available online 20 October 2011 Keywords: CO2 adsorption MOFs Chemical modification Flue gas
a b s t r a c t Capturing CO2 from the flue gases for sequestration is currently a key issue in environmental protection. Metal–organic frameworks (MOFs) are a new class of porous materials and have shown great promise for CO2 adsorption and separation applications. While the linkers of MOFs influence the CO2 adsorption performance greatly, the mechanism about it is still not clear. In this work, density functional theory calculations were performed to study CO2 adsorption mechanism on linker of isoreticular metal–organic framework-1 (IRMOF-1). The effect of model sizes was investigated by comparing the adsorption energies of different models with CO2 located on the same adsorption sites. The results indicate that model (HCOO)5Zn4O(BDC)Zn4O(HCOO)5 is sufficient to calculate CO2 adsorption on linker of IRMOF-1. Eight different positions on linker of IRMOF-1 with three orientations of CO2 were studied in detail to understand the mechanism of CO2 adsorption. The side position with CO2 parallel attack at hydrogen side of linker edge is the most favorite adsorption site. The effects of chemical modifications on CO2 adsorption were studied, and the adsorption energies were found to be significantly increased. This work will be helpful to design and synthesis new materials that have higher CO2 adsorption abilities. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The combustion of fossil fuels in power plants has generated a huge amount of CO2, which is the major component of greenhouse gases (GHGs) and plays a significant role in global warming. An appealing approach for solving this problem is the development of materials that are capable of recovering CO2 from the flue gas [1]. In recent years, new synthetic and natural organic porous materials have continued to be developed for this purpose, particularly the metal–organic frameworks (MOFs) [2]. MOFs are crystalline compounds formed by using organic ligands to connect small metal-based clusters of atoms and have been considered as potential materials used in hydrogen storage [3], ion exchange [4], as well as CO2 capture [5], due to their tunable properties. Desirable characteristics of MOFs are low energy requirement for regeneration, good thermal stability, tolerance to contaminants, attrition resistance, and low cost [1]. To date, many experiment studies have been performed on CO2 adsorption in MOFs. For example, Millward and Yaghi [6] tested the CO2 adsorption abilities of nine MOFs with different topologies and compared them with Zeolite 13X and MAXSORB which two were considered as benchmark materials. The results showed that MOF-5 (or IRMOF-1) and MOF-177 had a volumetric CO2 capacity ⇑ Corresponding author. Tel./fax: +86 27 87545526. E-mail address: [email protected] (J. Liu). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.09.057
of 22 and 33.5 m mol g1 at room temperature and 35 bar, which was much higher than the benchmark materials and any other porous materials reported before. In addition, the ability to synthesize MOFs with various organic linkers and metal joints provides tremendous flexibility in tailoring this porous material to have specific physical characteristics and chemical functionalities [7] for the application of special gas sorption, e.g. CO2 capture. For design and operation of the new class of MOFs materials with higher CO2 adsorption abilities, understanding the adsorption mechanism of CO2 molecules in MOFs are of great importance. While the nature of the MOF–CO2 interaction and the manner in which CO2 is adsorbed onto the structure is still unclear. Answering these questions is expected to hold the key to optimize these materials for practical CO2 capture applications. Although experimental methods, such as inelastic neutron scattering [8] and Raman spectroscopic investigation [9], can provide useful information for the interaction between MOFs and gas, they are limited to only a few MOF materials. Theoretical calculations are useful ways to provide general and specific clues to enhance the adsorption properties and the underlying adsorption mechanism of MOFs [10]. Quantum chemical methods, in the form of density functional theory (DFT), are increasingly used to understand mechanisms in both homogeneous [11–13] and heterogeneous [14,15] systems, which prove to be powerful tools, and can solve problems beyond the ability of experiments. In the previous studies, DFT calculations have been successfully applied to
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carboxylate (BDC) to link together Zn4O clusters. The resulting framework with formula unit Zn4O(BDC)3 consists of cubic pores, where BDC forms the edges of the cubes and the Zn4O clusters form the vertexes (see Fig. 1a). The width of each pore is approximately 13 Å. The initial atomic coordinates of this material were taken directly from the refined structure obtained by X-ray diffraction [22].
study H2 adsorption mechanism in MOFs [16,17], but little has been done on CO2 adsorption. In addition, although it had been widely considered that the linkers of MOFs played a significant role during the adsorption procedure [18,19], few theoretical calculations have been performed to explore the mechanism of CO2 adsorption on the linkers of MOFs. Torrisi et al. [20,21] investigated the impacts of ligands on CO2 adsorption in MOFs and they focused mainly on the interaction of CO2 with functionalized benzenes. However, the model considered in their works is simple and may be not adequate to describe the influences of the zinc oxide cluster fragments. Thus the mechanism of CO2 adsorption on linkers of MOFs should be further studied. Finally, a relatively low adsorption energy means a weak interaction strength between the guest molecule and adsorbent, and then make adsorption behavior difficult to occur especially at low-pressure stage. Thus how to obtain relatively higher adsorption energy is one of the key points to enhance CO2 uptake capacity of MOFs. The objective of the current study is to apply density functional theory to find out the nature of the interaction force between CO2 molecule and MOF structure, and then do some chemical modifications on the linker, such as increasing the size of linker, adding functional groups as well as Li-doping, to enhance these interactions. To the authors’ knowledge, this is the first theoretical study involved CO2 adsorption in Li-doped MOFs. We aimed to give guides to synthesizing new metal organic frameworks with higher CO2 adsorption abilities.
2.2. The models and CO2 adsorption sites
2. Computational methods
As shown in Fig. 1a, the unit cell of IRMOF-1 is too large to calculate every possible configuration of the adsorbed CO2 molecule. In order to evaluate possible model systems and to save computation time, we constructed two cluster models from the periodic structure, named model 1 and model 2 as shown in Fig. 1b and c. For model 1, only a single linker was considered. Model 2 was a side of the crystal structure of IRMOF-1 with 10 BDCs replaced by H atom, namely (HCOO)5Zn4O(BDC)Zn4O(HCOO)5. And also, the comparison of these two different models will be helpful to explain the mechanism of CO2 adsorption on linker of IRMOF-1. The knowledge of adsorption sites of adsorbate molecules in a porous material is very important in understanding the adsorption mechanism. Eight different positions were specified to investigate CO2 interaction with the linker as presented in Fig. 1c. Three possible orientations of molecules were also taken into account: parallel (P), cross (C) and vertical (V) attack to the model clusters. Thus, 24 different CO2-model cluster configurations in total were studied in this work. More detailed descriptions are shown in Fig. 2.
2.1. Initial structure of IRMOF-1
2.3. Computational details
This work was mainly focused on IRMOF-1, one of the most widely studied MOFs. IRMOF-1 is formed by using 1,4-benzenedi-
All of the theoretical calculations were performed by DFT method using the DMol3 code [23]. The DFT method in this code uses fast
(a)
(b) Zn O C H
C
C1
B2 B1 C2 H E
(c)
Zn
O
S
H
C
Fig. 1. (a) The unit cell of IRMOF-1; (b) Model 1, organic-linker-only model; (c) Model 2, (HCOO)5Zn4O(BDC)Zn4O(HCOO)5. C, central of the linker; E, edge of the linker; S, hydrogen side of the linker edge; B1, bond of type 1 in the linker; B2, bond of type 2 in the linker; C1, carbon atom of type 1 in the linker; C2, carbon atom of type 2 in the linker; H, hydrogen atom in the linker (Zn, light blue spheres; O, red spheres; C, gray spheres and H, white spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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convergent three-dimensional numerical integration to calculate the necessary matrix elements and is also an efficient and exact method for calculating the electrostatic potential. Perdew–Wang 1991 (PW91) [24] is generalized gradient approximation (GGA) level functional, and is considered adequate with quantitative precision for the investigation of weak Vander Waals force between MOFs and CO2 [25]. Thus, we used DFT with the PW91 exchange–
PS
CS
correlation functional for all our calculations. The double numeric polarization (DNP) [23] basis set, which is comparable to 6-31G(d, p), was used to describe the atomic orbital. DFT Semi-core Pseudopots (DSPPs) [26] which was developed specifically for DMol3 calculations was used to set the type of core treatment. A real-space orbital global cutoff of 3.7 Å is applied and the convergence threshold parameters for the optimization were 105 (energy), 2 103
VS
C O O PE
CE
PC
CC
VE
VC
PB1
CB1
VB1
PB2
CB2
VB2
PC1
CC1
VC1
PC2
CC2
VC2
PH
CH
VH
Fig. 2. Various configurations of CO2 to linker model: the first letter means the orientations of CO2 molecule, the rest means positions located in the linker. For example, PC means CO2 in parallel orientation in C position. Detail descriptions of the position can be seen in Fig. 1 (Zn, light blue spheres; O, red spheres; C, gray spheres and H, white spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(gradient), and 5 103 (displacement), respectively. The DMol3 code uses exact DFT spherical atomic orbitals. Therefore, basis set superposition error effects (BSSEs) are not considered in calculating the energies because of the minimization of BSSE effects when compared to less complete Gaussian basis functions [25,27]. Atomic charges were obtained by Mulliken analysis method. The calculation process includes geometry optimizations and energy calculation. Full geometry optimizations, i.e. optimization over all atoms, were performed to optimize the geometries of model 1 and model 2 and their corresponding CO2 adsorbed intermediates to a minimal point on the potential surface using the methods described above. Once the geometry optimization was completed, the energies were calculated using the methods consistent with geometry optimization. The adsorption energy (Eads) of an adsorbate ‘‘A’’ on a substrate ‘‘B’’ can be calculated as:
3. Results and discussion
CO2 adsorbing on model 1 is found with an adsorption energy of 5.68 kJ/mol, which is close to the MP2/6-311 + G(2d,2p) value [20]. Furthermore, for CO2 molecule parallel attack on the linker (P site), the adsorption energy with a value of 5.27 kJ/mol is also in good agreement with the previous MP2 level study (5.32 kJ/mol, calculated with similar model as model 2) [7], but which requires large computational costs compared to our work. Thus it can be concluded that the method used in this work (PW91/DNP) is adequate to describe the intermolecular interactions between CO2 molecules and MOF linkers and provide a reasonable balance between accuracy and computational expense. It is obvious that the adsorption energies of CO2 on model 2 ((HCOO)5Zn4O(BDC)Zn4O(HCOO)5) are different from that on model 1 (organic-linker-only model) to a certain extent. For instance, regarding the side positions (PS, CS and VS), it is remarkable that the adsorption energies of CO2 on model 2 are significantly stronger than that on model 1. In addition, as described in above, the most stable configuration of model 1 is found at PC site with adsorption energy of 5.68 kJ/mol, while the most stable configuration of model 2 is found at PS site with a higher adsorption energy of 7.01 kJ/mol. These differences are due to the change of electronic environment from the isolated linker to the linker with two Zn4OCOO(COOH)5 fragments which is similar with the trend of H2 adsorption in IRMOF-1 [16]. That is to say, the existing of Zn4OCOO(COOH)5 causes the charge redistribution and this change is critical to the adsorption capabilities of organic linkers [17]. Thus it can be concluded that compared to model 1, the model 2 ((HCOO)5Zn4O(BDC)Zn4O(HCOO)5) is sufficient to calculate CO2 adsorption on linker of IRMOF-1 and is used in the following calculations.
3.1. Model selection
3.2. CO2 adsorption on linker of IRMOF-1
Nine different adsorption sites were selected to evaluate the two possible model systems. The adsorption energies calculated are shown in Table 1. Firstly, the most stable conformation of
The adsorption energies of all the 24 configurations were calculated separately and the results were shown in Table 1. In general, when CO2 interacts with the linker of IRMOF-1, the CO2 molecule significantly prefers parallel (P) and cross (C) orientations to vertical (V). Taking C1 position as an example, the adsorption energies are 4.62 and 4.88 kJ/mol respectively for P and C orientations, but the value is lower to 0.91 kJ/mol when the CO2 molecule vertical attack at the same position. This phenomenon can be simply due to the fact that the main interaction force between CO2 molecule and IRMOF-1 linker is van der Waals force. Based on the calculation results, the side position with CO2 parallel attack at hydrogen side of linker edge (PS) is the most favorite adsorption site and the adsorption energy is 7.01 kJ/mol, as shown in Table 1. Regarding the interactions of CO2 molecule at this site, the interatomic distances (see Fig. 3) suggest two scenarios: (i) interaction between the oxygen atom of the CO2 molecule with the hydrogen atom of the C6H4 group (dH–O = 2.63 Å); (ii) interactions between the CO2 molecule and the carboxyl group of the framework (the nearest distances between the oxygen atoms of CO2 molecule and the oxygen atoms of carboxyl group are 3.53, 4.30, 4.38 Å respectively). The former one can be mainly attributed to the dispersive forces, while the latter one is the electrostatic forces caused by the Zn4OCOO(COOH)5 unit. Thus one can expect that the increase of the adsorption energy compared with the organic-linker-only model can mainly be attributed to the influence of the electrostatic forces caused by the Zn4OCOO (COOH)5 unit, especially the O atoms of the –COO parts. In addition, for the other two side positions (CS and VS), the adsorption energies are also relatively higher than other positions with the same CO2 orientation which can also be due to the influence of the existing of Zn4OCOO(COOH)5 unit. Besides the side positions, for the positions above the face of the linker, the C2 position with CO2 molecule parallel attack (PC2) is
Eads ¼ EðABÞ ðEðAÞ þ EðEÞÞ
ð1Þ
where E(A) is the total energy of the adsorbate, and E(B) is the total energy of the substrate, while E(AB) is the total energy of adsorbate/ substrate system in equilibrium state. Adsorption of the adsorbate is exothermic if Eads is negative. A higher negative value of Eads corresponds to a stronger adsorption. The adsorption energy of CO2 on MOFs is an important parameter related to the CO2 adsorption capacity of MOFs. An enhancement in the adsorption energy would be highly beneficial to the CO2 capture applications in the MOF systems.
Table 1 Adsorption energies (kJ/mol) between CO2 and various linker models. Adsorption sitea
Orientation
Position S
E
C
B1
B2
C1
C2
H
P C V P C V P C V P C V P C V P C V P C V P C V
Adsorption energy Model 1b
Model 2c
2.57 1.28 0.37 3.03 2.99 1.22 5.68 5.54 0.42
7.01 6.54 3.85 2.64 4.23 1.87 5.27 4.75 3.41 4.74 4.07 2.44 4.73 4.07 2.44 4.62 4.88 0.91 5.88 2.88 3.26 3.84 5.00 2.50
a Adsorption sites were named by the position in IRMOF-1 and orientation of CO2 molecule as shown in Fig. 2. b Model 1 represents organic-linker-only model. c Model 2 represents (HCOO)5Zn4O(BDC)Zn4O(HCOO)5.
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linker, as shown in Fig. 4. For comparison, the CO2 molecule was located at the same site: CC site (CO2 cross attack on face of linker). 3.4. Increasing the size of the linker
O
O 3.53Å 2.63Å
O
4.30Å
O 4.38Å
C O
Fig. 3. Interaction of CO2 molecule with IRMOF-1 at PS site (Zn, light blue spheres; O, red spheres; C, gray spheres and H, white spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
more favorable than other sites. The adsorption energy is 5.88 kJ/ mol. The relatively higher adsorption capacity at this site can be explained as follows: the C2 position is close to the Zn4OCOO (COOH)5 unit and CO2 molecule with parallel orientation is more likely to be influenced by the electrostatic forces than the cross and vertical orientation. Moreover, the PC and CH sites are also the possible adsorption sites because the adsorption energies at these two sites are very close to that of PC2 site.
The linker was firstly increased in size from a single benzene ring to double benzene ring, formed the IRMOF-8 linker, as shown in Fig. 4a. The CO2 molecule was placed on the face of one of the benzene rings. The adsorption energy increases from 4.75 to 5.70 kJ/mol, an increase of 20% compared to that of model 2. This may be attributed to the fact that larger charge transfer takes place in IRMOF-8 and this charge separation induces the large charge gradient of the electrostatic potential locally [28]. Calculations were also performed with two CO2 molecules placed on the modified linker and the results showed that the new linker is large enough to provide another site for CO2 to bind. The distance between these two CO2 molecules is 3.40 Å which is much shorter than the length of the linker, as shown in Fig. 5a. Pairing energy (Epairing) was also calculated using the following equation:
Epairing ¼ E12 ðE1 þ E2 Þ
ð2Þ
Detail descriptions of the equation can be found in previous article [28]. The pairing energy is 3.01 kJ/mol, which means the first adsorbed CO2 molecule is helpful to the following CO2 adsorption through van der Waals interactions. Thus besides the MOF– CO2 interactions, CO2–CO2 interactions may be another key point during CO2 adsorption in MOFs.
3.3. Chemical modifications on linker
3.5. Adding functional groups
According to the above calculations, the adsorption energies between CO2 and IRMOF-1 are typically small (lower than 7.01 kJ/mol). To improve the CO2 adsorption performance of MOFs at low pressure stage, it is a useful way to increase the adsorption energy and several methods are taken to modify the structure of
By adding a –NH2 group, as shown in Fig. 4b, one produces the IRMOF-3 linker, the adsorption energy at CC site increases to 5.36 kJ/mol, an increase of 13%. Additional configuration, with CO2 parallel attack at the NH2 side, was also evaluated and the adsorption energy is 9.80 kJ/mol, which is even slightly
(a)
(b)
H
(c)
(d)
N
C H
O
(e)
N H
(f)
Li
Fig. 4. Model clusters induced by chemical modifications. (a) Linker with double benzene ring; (b) linker adding a –NH2 group; (c) linker by adding a –NO2 group; (d) linker by adding a –CH3 group; (e) linker by adding four –CH3 group; (f) Li-doped BDC linker (Zn, light blue spheres; O, red spheres; C, gray spheres; H, white spheres; N, dark blue spheres and Li, pink spheres) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(a)
O C O
3.40Å
4.90Å
(b)
H
H
2.36 Å N
(c)
-0.28 |e|
O N
3.19Å -0.28 |e| +0.56 |e|
+0.35 |e| O -0.28 |e| 3.37Å -0.29 |e|
(d) 3.20 Å
3.11 Å
116o 123
o
-0.21 |e| +0.59 |e| -0.313 |e|
(e) 2.64Å Li
+0.46 |e|
Fig. 5. (a) Configuration with two CO2 molecule placed on double benzene ring linker; (b) configuration with CO2 parallel attack at the –NH2 side; (c) configuration with CO2 cross attack at–NO2 side; (d) CO2 adsorption on –CH3 substituted linker; (e) CO2 adsorption on Li-doping linker (Zn, light blue spheres; O, red spheres; C, gray spheres; H, white spheres; N, dark blue spheres and Li, pink spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
higher than the value (9.27 kJ/mol) at metal site [19]. It means that the adsorption site in IRMOF-3 locates near the functional group is favorable to the metal sites. That is to say, the CO2 molecule will first adsorb on the linker during the adsorption process, which is different from IRMOF-1. In addition, higher adsorption energies of IRMOF-3 compared with IRMOF-1 means a sharp increase at the initial stage of adsorption isotherm, which agrees
well with the experimental results [6]. The distance between oxygen and hydrogen atom is 2.36 Å, as shown in Fig. 5b. It can be concluded that the hydrogen atom of –NH2 interacts with the lone pair electron of oxygen atom of CO2 molecule and forms hydrogen-bond, which is significantly stronger than van der Waals force. Another new linker by adding a –NO2 group was also produced, as shown in Fig. 4c. Firstly, after the geometry optimization, the plane of –NO2 group is vertical to the plane of benzene linker, which is different to the optimized structure obtained in the previous work [21] in which the substituted nitro group is co-planar with the benzene ring. The difference is due to the effects of steric configuration induced by the existing of the nearby Zn4OCOO(COOH)5 fragment and may influence the adsorption behavior of CO2 on linker to some extent. The adsorption energy increases to 7.05 kJ/mol, an increase of 48%. The additional adsorption site, with CO2 cross attack at –NO2 side, was also evaluated. The adsorption energy is 10.39 kJ/mol, an increase of 59% compared to that of the CS site of model 2. The enhancement of CO2 uptake capacity at this site is mainly due to the electronic interaction between oxygen atoms of –NO2 group and carbon atom of CO2, at distances about 3.19 and 3.37 Å, as shown in Fig. 5c. Some other chemical modifications were also investigated. By adding a –CH3 group, as shown in Fig. 4d, the adsorption energy increases to 6.02 kJ/mol (an increase of 27%). These enhancement can be attributed to the substitution of the –CH3 group which donates electronic charges (0.06 electron, obtained by Mulliken analysis) into the aromatic ring and improves the quadrupole–p electron interaction. And then we produced the IRMOF-18 linker by adding four –CH3 groups, as shown in Fig. 4e, and the adsorption energy is 10.86 kJ/mol with an increase of 129%. The Mulliken analysis results show that 0.14 electrons are injected into the aromatic ring, which is responsible to the improvement of CO2 adsorption energy. In addition, as shown in Fig. 5d, weak hydrogen bond interactions are found between the oxygen atom of CO2 and the neighboring hydrogen atoms of –CH3 groups, with distances of 3.11 Å and 3.20 Å, and C@H O angle of 122° and 116° respectively. While the shortcoming for adding four –CH3 groups is that these groups may partially block the metal sites because they are located near the zinc oxide corners and result in reduced accessible surface area. 3.6. Li-doping on the linker Doping MOFs with Li or Li+ has been a useful way to increase H2 adsorption energy to MOFs [29,30]. However, researches have not been performed in the area of CO2 adsorption in Li-doping MOFs. Thus the adsorption energy of CO2 was firstly calculated with the presence of a Li dopant. The Li atom with no charge was placed on the center of the linker and the Li-linker system therefore represents a model for CO2 adsorption on Li-doped linker of IRMOF-1, as shown in Fig. 4f. Atomic charge was calculated through Mulliken analysis and it has been found that Li atom contributed 0.46 electron to the linker, compared with 0.6 electron from the zinc oxide cluster in the MOF crystal [31], which means a significant change of electronic environment. Interestingly, CO2 was initially located at the CC site, but it finally moved to VC site after geometry optimization. This phenomenon indicates that there is a strong interaction between Li atom and O atom of CO2 molecule via electronic force, and the doped Li atom is an unsaturated metal center to which CO2 can bind. The distance between Li atom and O atom of CO2 molecule is 2.638 Å, as shown in Fig. 5e. Finally, the calculated adsorption energy is 24.76 kJ/mol, which indicates that Li doping on linker greatly increases the CO2 adsorption capacity.
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4. Conclusions Density functional theory studies were performed to calculate the adsorption energies of CO2 on various IRMOF-1 linker sites. CO2 was assigned to lie in the configurations parallel (P), cross (C) and vertical (V) to linker domain. The model of (HCOO)5 Zn4O(BDC)Zn4O(HCOO)5 is sufficient to represent interactions on linker domains of the IRMOF-1 with CO2. The optimal adsorption site on linker for the CO2 is the side position with CO2 parallel attack at hydrogen side of linker edge. While the main interaction force between CO2 and the linker of IRMOF-1 is van der Waals force, electrostatic force induced by Zn4O(HCOO)5 cluster can also significantly influence the adsorption behavior. The effects of chemical modifications of the linker, such as increasing the size of linker, adding functional groups and Li-doping, were studied and the adsorption energies were found to be significantly increased, which indicates that it is an effective way to enhance CO2 adsorption capacity by chemical modifications to the linker of MOFs. The density functional method is an effective method to reveal the mechanism of CO2 adsorption on the linkers of MOFs. This work will be helpful to design and synthesis new materials that have higher CO2 adsorption abilities. Acknowledgements This work was supported by Natural Science Foundation of China (50936001), 973 Program of China (2011CB707301, 2010CB227003) and Program for New Century Excellent Talents in University (NCET-10-0412). References [1] Figueroa JD, Fout T, Plasynski S, McIlvried H, Srivastava RD. Advances in CO2 capture technology–the US department of energy’s carbon sequestration program. Int J Greenh Gas Con 2008;2:9–20. [2] Banerjee R, Phan A, Wang B, Knobler C, Furukawa H, O’Keeffe M, et al. Highthroughput synthesis of zeolitic imidazolate frameworks and application to CO2 capture. Science 2008;319:939–43. [3] Wong-Foy AG, Matzger AJ, Yaghi OM. Exceptional H2 saturation uptake in microporous metalorganic frameworks. J Am Chem Soc 2006;128:3494–5. [4] Snurr RQ, Hupp JT, Nguyen ST. Prospects for nanoporous metal-organic materials in advanced separations processes. AIChE J 2004;50:1090–5. [5] Finsy V, Ma L, Alaerts L, De Vos DE, Baron GV, Denayer JFM. Separation of CO2/ CH4 mixtures with the MIL-53(Al) metal–organic framework. Micropor Mesopor Mater 2009;120:221–7. [6] Millward AR, Yaghi OM. Metalorganic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J Am Chem Soc 2005;127:17998–9. [7] Walton KS, Millward AR, Dubbeldam D, Frost H, Low JJ, Yaghi OM, et al. Understanding inflections and steps in carbon dioxide adsorption isotherms in metal–organic frameworks. J Am Chem Soc 2008;130:406–7. [8] Rowsell JLC, Eckert J, Yaghi OM. Characterization of H2 binding sites in prototypical metalorganic frameworks by inelastic neutron scattering. J Am Chem Soc 2005;127:14904–10.
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[9] Siberio-Pérez DY, Wong-Foy AG, Yaghi OM, Matzger AJ. Raman spectroscopic investigation of CH4 and N2 adsorption in metal–organic frameworks. Chem Mater 2007;19:3681–5. [10] Kim D, Lee TB, Choi SB, Yoon JH, Kim J, Choi SH. A density functional theory study of a series of functionalized metal–organic frameworks. Chm Phys Lett 2006;420:256–60. [11] Liu J, Qu WQ, Yuan JZ, Wang SC, Qiu JR, Zheng CG. Theoretical studies of properties and reactions involving mercury species present in combustion flue gases. Energy Fuels 2010;24:117–22. [12] Zheng CG, Liu J, Liu ZH, Xu MH, Liu YH. Kinetic mechanism studies on reactions of mercury and oxidizing species in coal combustion. Fuel 2005;84:1215–20. [13] Xu MH, Qiao Y, Liu J, Zheng CG. Kinetic calculation and modeling of trace element reactions during combustion. Powder Technol 2008;180:157–63. [14] Liu J, Cheney MA, Wu F, Li M. Effects of chemical functional groups on elemental mercury adsorption on carbonaceous surfaces. J Hazard Mater 2011;186(1):108–13. [15] Liu J, He MF, Zheng CG, Chang M. Density functional theory study of mercury adsorption on V2O5 (0 0 1) surface with implications for oxidation. Proc Combust Inst 2011;33(2):2771–7. [16] Lee TB, Kim D, Jung DH, Choi SB, Yoon JH, Kim J, et al. Understanding the mechanism of hydrogen adsorption into metal organic frameworks. Catal Today 2007;120:330–5. [17] Mueller T, Ceder G. A density functional theory study of hydrogen adsorption in MOF-5. J Phys Chem B 2005;109:17974–83. [18] Zhang L, Wang Q, Liu YC, Wu T, Chen D, Wang XP. Interactions of hydrogen molecules with metal–organic frameworks at adsorption sites. Chem Phys Lett 2009;469:261–5. [19] Pianwanit A, Kritayakornupong C, Vongachariya A, Selphusit N, Ploymeerusmee T, Remsungnen T, et al. The optimal binding sites of CH4 and CO2 molecules on the metal–organic framework MOF-5: ONIOM calculations. Chem Phys 2008;349:77–82. [20] Torrisi A, Mellot-Draznieks C, Bell RG. Impact of ligands on CO2 adsorption in metal–organic frameworks: first principles study of the interaction of CO2 with functionalized benzenes. I. Inductive effects on the aromatic ring. J Chem Phys 2009;130:194703. [21] Torrisi A, Mellot-Draznieks C, Bell RG. Impact of ligands on CO2 adsorption in metal–organic frameworks: first principles study of the interaction of CO2 with functionalized benzenes. I. Effect of polar and acidic substituents. J Chem Phys 2010;132:044705. [22] Li HL, Eddaoudi M, O’Keeffe M, Yaghi OM. Design and synthesis of an exceptionally stable and highly porous metal–organic framework. Nature 1999;402:276–9. [23] Delley B. From molecules to solids with the DMol3 approach. J Chem Phys 2000;113:7756–64. [24] Perdew JP, Wang Y. Accurate and simple analytic representation of the electron–gas correlation energy. Phys Rev B 1992;45:13244–9. [25] Ramsahye NA, Maurin G, Bourrelly S, Llewellyn PL, Serre C, Probing Loiseau T, et al. J Phys Chem C 2008;112:514. [26] Delley B. Hardness conserving semilocal pseudopotentials. Phys Rev B 2002;66:155125–34. [27] Delly B. An all-electron numerical method for solving the local density functional for polyatomic molecules. J Chem Phys 1990;92:508–17. [28] Kim D, Junga DH, Choib SB, Yoonb JH, Kimb J, Choic K, et al. A density functional theory study on the interaction of hydrogen molecules with aromatic linkers in metal–organic frameworks. J Phys Chem Solids 2008;69:1428–31. [29] Kolman SJ, Chan B, Jordan MJT. Modelling the interaction of molecular hydrogen with lithium-doped hydrogen storage materials. Chem Phys Lett 2008;467:126–30. [30] Mavrandonakis A, Tylianakis T, Studos AK, Froudakis GE. Why Li doping in MOFs enhances H2 storage capacity? a multi-scale theoretical study. J Phys Chem C 2008;112:7290–4. [31] Sagara T, Klassen J, Ganz E. Computational study of hydrogen binding by metal–organic framework-5. J Chem Phys 2004;121:12543.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Theoretical studies of CO2 adsorption mechanism on linkers of metal–organic frameworks Yang Liu a, Jing Liu a,⇑, Ming Chang b, Chuguang Zheng a a b
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e
i n f o
Article history: Received 2 September 2011 Received in revised form 23 September 2011 Accepted 29 September 2011 Available online 20 October 2011 Keywords: CO2 adsorption MOFs Chemical modification Flue gas
a b s t r a c t Capturing CO2 from the flue gases for sequestration is currently a key issue in environmental protection. Metal–organic frameworks (MOFs) are a new class of porous materials and have shown great promise for CO2 adsorption and separation applications. While the linkers of MOFs influence the CO2 adsorption performance greatly, the mechanism about it is still not clear. In this work, density functional theory calculations were performed to study CO2 adsorption mechanism on linker of isoreticular metal–organic framework-1 (IRMOF-1). The effect of model sizes was investigated by comparing the adsorption energies of different models with CO2 located on the same adsorption sites. The results indicate that model (HCOO)5Zn4O(BDC)Zn4O(HCOO)5 is sufficient to calculate CO2 adsorption on linker of IRMOF-1. Eight different positions on linker of IRMOF-1 with three orientations of CO2 were studied in detail to understand the mechanism of CO2 adsorption. The side position with CO2 parallel attack at hydrogen side of linker edge is the most favorite adsorption site. The effects of chemical modifications on CO2 adsorption were studied, and the adsorption energies were found to be significantly increased. This work will be helpful to design and synthesis new materials that have higher CO2 adsorption abilities. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The combustion of fossil fuels in power plants has generated a huge amount of CO2, which is the major component of greenhouse gases (GHGs) and plays a significant role in global warming. An appealing approach for solving this problem is the development of materials that are capable of recovering CO2 from the flue gas [1]. In recent years, new synthetic and natural organic porous materials have continued to be developed for this purpose, particularly the metal–organic frameworks (MOFs) [2]. MOFs are crystalline compounds formed by using organic ligands to connect small metal-based clusters of atoms and have been considered as potential materials used in hydrogen storage [3], ion exchange [4], as well as CO2 capture [5], due to their tunable properties. Desirable characteristics of MOFs are low energy requirement for regeneration, good thermal stability, tolerance to contaminants, attrition resistance, and low cost [1]. To date, many experiment studies have been performed on CO2 adsorption in MOFs. For example, Millward and Yaghi [6] tested the CO2 adsorption abilities of nine MOFs with different topologies and compared them with Zeolite 13X and MAXSORB which two were considered as benchmark materials. The results showed that MOF-5 (or IRMOF-1) and MOF-177 had a volumetric CO2 capacity ⇑ Corresponding author. Tel./fax: +86 27 87545526. E-mail address: [email protected] (J. Liu). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.09.057
of 22 and 33.5 m mol g1 at room temperature and 35 bar, which was much higher than the benchmark materials and any other porous materials reported before. In addition, the ability to synthesize MOFs with various organic linkers and metal joints provides tremendous flexibility in tailoring this porous material to have specific physical characteristics and chemical functionalities [7] for the application of special gas sorption, e.g. CO2 capture. For design and operation of the new class of MOFs materials with higher CO2 adsorption abilities, understanding the adsorption mechanism of CO2 molecules in MOFs are of great importance. While the nature of the MOF–CO2 interaction and the manner in which CO2 is adsorbed onto the structure is still unclear. Answering these questions is expected to hold the key to optimize these materials for practical CO2 capture applications. Although experimental methods, such as inelastic neutron scattering [8] and Raman spectroscopic investigation [9], can provide useful information for the interaction between MOFs and gas, they are limited to only a few MOF materials. Theoretical calculations are useful ways to provide general and specific clues to enhance the adsorption properties and the underlying adsorption mechanism of MOFs [10]. Quantum chemical methods, in the form of density functional theory (DFT), are increasingly used to understand mechanisms in both homogeneous [11–13] and heterogeneous [14,15] systems, which prove to be powerful tools, and can solve problems beyond the ability of experiments. In the previous studies, DFT calculations have been successfully applied to
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carboxylate (BDC) to link together Zn4O clusters. The resulting framework with formula unit Zn4O(BDC)3 consists of cubic pores, where BDC forms the edges of the cubes and the Zn4O clusters form the vertexes (see Fig. 1a). The width of each pore is approximately 13 Å. The initial atomic coordinates of this material were taken directly from the refined structure obtained by X-ray diffraction [22].
study H2 adsorption mechanism in MOFs [16,17], but little has been done on CO2 adsorption. In addition, although it had been widely considered that the linkers of MOFs played a significant role during the adsorption procedure [18,19], few theoretical calculations have been performed to explore the mechanism of CO2 adsorption on the linkers of MOFs. Torrisi et al. [20,21] investigated the impacts of ligands on CO2 adsorption in MOFs and they focused mainly on the interaction of CO2 with functionalized benzenes. However, the model considered in their works is simple and may be not adequate to describe the influences of the zinc oxide cluster fragments. Thus the mechanism of CO2 adsorption on linkers of MOFs should be further studied. Finally, a relatively low adsorption energy means a weak interaction strength between the guest molecule and adsorbent, and then make adsorption behavior difficult to occur especially at low-pressure stage. Thus how to obtain relatively higher adsorption energy is one of the key points to enhance CO2 uptake capacity of MOFs. The objective of the current study is to apply density functional theory to find out the nature of the interaction force between CO2 molecule and MOF structure, and then do some chemical modifications on the linker, such as increasing the size of linker, adding functional groups as well as Li-doping, to enhance these interactions. To the authors’ knowledge, this is the first theoretical study involved CO2 adsorption in Li-doped MOFs. We aimed to give guides to synthesizing new metal organic frameworks with higher CO2 adsorption abilities.
2.2. The models and CO2 adsorption sites
2. Computational methods
As shown in Fig. 1a, the unit cell of IRMOF-1 is too large to calculate every possible configuration of the adsorbed CO2 molecule. In order to evaluate possible model systems and to save computation time, we constructed two cluster models from the periodic structure, named model 1 and model 2 as shown in Fig. 1b and c. For model 1, only a single linker was considered. Model 2 was a side of the crystal structure of IRMOF-1 with 10 BDCs replaced by H atom, namely (HCOO)5Zn4O(BDC)Zn4O(HCOO)5. And also, the comparison of these two different models will be helpful to explain the mechanism of CO2 adsorption on linker of IRMOF-1. The knowledge of adsorption sites of adsorbate molecules in a porous material is very important in understanding the adsorption mechanism. Eight different positions were specified to investigate CO2 interaction with the linker as presented in Fig. 1c. Three possible orientations of molecules were also taken into account: parallel (P), cross (C) and vertical (V) attack to the model clusters. Thus, 24 different CO2-model cluster configurations in total were studied in this work. More detailed descriptions are shown in Fig. 2.
2.1. Initial structure of IRMOF-1
2.3. Computational details
This work was mainly focused on IRMOF-1, one of the most widely studied MOFs. IRMOF-1 is formed by using 1,4-benzenedi-
All of the theoretical calculations were performed by DFT method using the DMol3 code [23]. The DFT method in this code uses fast
(a)
(b) Zn O C H
C
C1
B2 B1 C2 H E
(c)
Zn
O
S
H
C
Fig. 1. (a) The unit cell of IRMOF-1; (b) Model 1, organic-linker-only model; (c) Model 2, (HCOO)5Zn4O(BDC)Zn4O(HCOO)5. C, central of the linker; E, edge of the linker; S, hydrogen side of the linker edge; B1, bond of type 1 in the linker; B2, bond of type 2 in the linker; C1, carbon atom of type 1 in the linker; C2, carbon atom of type 2 in the linker; H, hydrogen atom in the linker (Zn, light blue spheres; O, red spheres; C, gray spheres and H, white spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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convergent three-dimensional numerical integration to calculate the necessary matrix elements and is also an efficient and exact method for calculating the electrostatic potential. Perdew–Wang 1991 (PW91) [24] is generalized gradient approximation (GGA) level functional, and is considered adequate with quantitative precision for the investigation of weak Vander Waals force between MOFs and CO2 [25]. Thus, we used DFT with the PW91 exchange–
PS
CS
correlation functional for all our calculations. The double numeric polarization (DNP) [23] basis set, which is comparable to 6-31G(d, p), was used to describe the atomic orbital. DFT Semi-core Pseudopots (DSPPs) [26] which was developed specifically for DMol3 calculations was used to set the type of core treatment. A real-space orbital global cutoff of 3.7 Å is applied and the convergence threshold parameters for the optimization were 105 (energy), 2 103
VS
C O O PE
CE
PC
CC
VE
VC
PB1
CB1
VB1
PB2
CB2
VB2
PC1
CC1
VC1
PC2
CC2
VC2
PH
CH
VH
Fig. 2. Various configurations of CO2 to linker model: the first letter means the orientations of CO2 molecule, the rest means positions located in the linker. For example, PC means CO2 in parallel orientation in C position. Detail descriptions of the position can be seen in Fig. 1 (Zn, light blue spheres; O, red spheres; C, gray spheres and H, white spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(gradient), and 5 103 (displacement), respectively. The DMol3 code uses exact DFT spherical atomic orbitals. Therefore, basis set superposition error effects (BSSEs) are not considered in calculating the energies because of the minimization of BSSE effects when compared to less complete Gaussian basis functions [25,27]. Atomic charges were obtained by Mulliken analysis method. The calculation process includes geometry optimizations and energy calculation. Full geometry optimizations, i.e. optimization over all atoms, were performed to optimize the geometries of model 1 and model 2 and their corresponding CO2 adsorbed intermediates to a minimal point on the potential surface using the methods described above. Once the geometry optimization was completed, the energies were calculated using the methods consistent with geometry optimization. The adsorption energy (Eads) of an adsorbate ‘‘A’’ on a substrate ‘‘B’’ can be calculated as:
3. Results and discussion
CO2 adsorbing on model 1 is found with an adsorption energy of 5.68 kJ/mol, which is close to the MP2/6-311 + G(2d,2p) value [20]. Furthermore, for CO2 molecule parallel attack on the linker (P site), the adsorption energy with a value of 5.27 kJ/mol is also in good agreement with the previous MP2 level study (5.32 kJ/mol, calculated with similar model as model 2) [7], but which requires large computational costs compared to our work. Thus it can be concluded that the method used in this work (PW91/DNP) is adequate to describe the intermolecular interactions between CO2 molecules and MOF linkers and provide a reasonable balance between accuracy and computational expense. It is obvious that the adsorption energies of CO2 on model 2 ((HCOO)5Zn4O(BDC)Zn4O(HCOO)5) are different from that on model 1 (organic-linker-only model) to a certain extent. For instance, regarding the side positions (PS, CS and VS), it is remarkable that the adsorption energies of CO2 on model 2 are significantly stronger than that on model 1. In addition, as described in above, the most stable configuration of model 1 is found at PC site with adsorption energy of 5.68 kJ/mol, while the most stable configuration of model 2 is found at PS site with a higher adsorption energy of 7.01 kJ/mol. These differences are due to the change of electronic environment from the isolated linker to the linker with two Zn4OCOO(COOH)5 fragments which is similar with the trend of H2 adsorption in IRMOF-1 [16]. That is to say, the existing of Zn4OCOO(COOH)5 causes the charge redistribution and this change is critical to the adsorption capabilities of organic linkers [17]. Thus it can be concluded that compared to model 1, the model 2 ((HCOO)5Zn4O(BDC)Zn4O(HCOO)5) is sufficient to calculate CO2 adsorption on linker of IRMOF-1 and is used in the following calculations.
3.1. Model selection
3.2. CO2 adsorption on linker of IRMOF-1
Nine different adsorption sites were selected to evaluate the two possible model systems. The adsorption energies calculated are shown in Table 1. Firstly, the most stable conformation of
The adsorption energies of all the 24 configurations were calculated separately and the results were shown in Table 1. In general, when CO2 interacts with the linker of IRMOF-1, the CO2 molecule significantly prefers parallel (P) and cross (C) orientations to vertical (V). Taking C1 position as an example, the adsorption energies are 4.62 and 4.88 kJ/mol respectively for P and C orientations, but the value is lower to 0.91 kJ/mol when the CO2 molecule vertical attack at the same position. This phenomenon can be simply due to the fact that the main interaction force between CO2 molecule and IRMOF-1 linker is van der Waals force. Based on the calculation results, the side position with CO2 parallel attack at hydrogen side of linker edge (PS) is the most favorite adsorption site and the adsorption energy is 7.01 kJ/mol, as shown in Table 1. Regarding the interactions of CO2 molecule at this site, the interatomic distances (see Fig. 3) suggest two scenarios: (i) interaction between the oxygen atom of the CO2 molecule with the hydrogen atom of the C6H4 group (dH–O = 2.63 Å); (ii) interactions between the CO2 molecule and the carboxyl group of the framework (the nearest distances between the oxygen atoms of CO2 molecule and the oxygen atoms of carboxyl group are 3.53, 4.30, 4.38 Å respectively). The former one can be mainly attributed to the dispersive forces, while the latter one is the electrostatic forces caused by the Zn4OCOO(COOH)5 unit. Thus one can expect that the increase of the adsorption energy compared with the organic-linker-only model can mainly be attributed to the influence of the electrostatic forces caused by the Zn4OCOO (COOH)5 unit, especially the O atoms of the –COO parts. In addition, for the other two side positions (CS and VS), the adsorption energies are also relatively higher than other positions with the same CO2 orientation which can also be due to the influence of the existing of Zn4OCOO(COOH)5 unit. Besides the side positions, for the positions above the face of the linker, the C2 position with CO2 molecule parallel attack (PC2) is
Eads ¼ EðABÞ ðEðAÞ þ EðEÞÞ
ð1Þ
where E(A) is the total energy of the adsorbate, and E(B) is the total energy of the substrate, while E(AB) is the total energy of adsorbate/ substrate system in equilibrium state. Adsorption of the adsorbate is exothermic if Eads is negative. A higher negative value of Eads corresponds to a stronger adsorption. The adsorption energy of CO2 on MOFs is an important parameter related to the CO2 adsorption capacity of MOFs. An enhancement in the adsorption energy would be highly beneficial to the CO2 capture applications in the MOF systems.
Table 1 Adsorption energies (kJ/mol) between CO2 and various linker models. Adsorption sitea
Orientation
Position S
E
C
B1
B2
C1
C2
H
P C V P C V P C V P C V P C V P C V P C V P C V
Adsorption energy Model 1b
Model 2c
2.57 1.28 0.37 3.03 2.99 1.22 5.68 5.54 0.42
7.01 6.54 3.85 2.64 4.23 1.87 5.27 4.75 3.41 4.74 4.07 2.44 4.73 4.07 2.44 4.62 4.88 0.91 5.88 2.88 3.26 3.84 5.00 2.50
a Adsorption sites were named by the position in IRMOF-1 and orientation of CO2 molecule as shown in Fig. 2. b Model 1 represents organic-linker-only model. c Model 2 represents (HCOO)5Zn4O(BDC)Zn4O(HCOO)5.
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linker, as shown in Fig. 4. For comparison, the CO2 molecule was located at the same site: CC site (CO2 cross attack on face of linker). 3.4. Increasing the size of the linker
O
O 3.53Å 2.63Å
O
4.30Å
O 4.38Å
C O
Fig. 3. Interaction of CO2 molecule with IRMOF-1 at PS site (Zn, light blue spheres; O, red spheres; C, gray spheres and H, white spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
more favorable than other sites. The adsorption energy is 5.88 kJ/ mol. The relatively higher adsorption capacity at this site can be explained as follows: the C2 position is close to the Zn4OCOO (COOH)5 unit and CO2 molecule with parallel orientation is more likely to be influenced by the electrostatic forces than the cross and vertical orientation. Moreover, the PC and CH sites are also the possible adsorption sites because the adsorption energies at these two sites are very close to that of PC2 site.
The linker was firstly increased in size from a single benzene ring to double benzene ring, formed the IRMOF-8 linker, as shown in Fig. 4a. The CO2 molecule was placed on the face of one of the benzene rings. The adsorption energy increases from 4.75 to 5.70 kJ/mol, an increase of 20% compared to that of model 2. This may be attributed to the fact that larger charge transfer takes place in IRMOF-8 and this charge separation induces the large charge gradient of the electrostatic potential locally [28]. Calculations were also performed with two CO2 molecules placed on the modified linker and the results showed that the new linker is large enough to provide another site for CO2 to bind. The distance between these two CO2 molecules is 3.40 Å which is much shorter than the length of the linker, as shown in Fig. 5a. Pairing energy (Epairing) was also calculated using the following equation:
Epairing ¼ E12 ðE1 þ E2 Þ
ð2Þ
Detail descriptions of the equation can be found in previous article [28]. The pairing energy is 3.01 kJ/mol, which means the first adsorbed CO2 molecule is helpful to the following CO2 adsorption through van der Waals interactions. Thus besides the MOF– CO2 interactions, CO2–CO2 interactions may be another key point during CO2 adsorption in MOFs.
3.3. Chemical modifications on linker
3.5. Adding functional groups
According to the above calculations, the adsorption energies between CO2 and IRMOF-1 are typically small (lower than 7.01 kJ/mol). To improve the CO2 adsorption performance of MOFs at low pressure stage, it is a useful way to increase the adsorption energy and several methods are taken to modify the structure of
By adding a –NH2 group, as shown in Fig. 4b, one produces the IRMOF-3 linker, the adsorption energy at CC site increases to 5.36 kJ/mol, an increase of 13%. Additional configuration, with CO2 parallel attack at the NH2 side, was also evaluated and the adsorption energy is 9.80 kJ/mol, which is even slightly
(a)
(b)
H
(c)
(d)
N
C H
O
(e)
N H
(f)
Li
Fig. 4. Model clusters induced by chemical modifications. (a) Linker with double benzene ring; (b) linker adding a –NH2 group; (c) linker by adding a –NO2 group; (d) linker by adding a –CH3 group; (e) linker by adding four –CH3 group; (f) Li-doped BDC linker (Zn, light blue spheres; O, red spheres; C, gray spheres; H, white spheres; N, dark blue spheres and Li, pink spheres) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(a)
O C O
3.40Å
4.90Å
(b)
H
H
2.36 Å N
(c)
-0.28 |e|
O N
3.19Å -0.28 |e| +0.56 |e|
+0.35 |e| O -0.28 |e| 3.37Å -0.29 |e|
(d) 3.20 Å
3.11 Å
116o 123
o
-0.21 |e| +0.59 |e| -0.313 |e|
(e) 2.64Å Li
+0.46 |e|
Fig. 5. (a) Configuration with two CO2 molecule placed on double benzene ring linker; (b) configuration with CO2 parallel attack at the –NH2 side; (c) configuration with CO2 cross attack at–NO2 side; (d) CO2 adsorption on –CH3 substituted linker; (e) CO2 adsorption on Li-doping linker (Zn, light blue spheres; O, red spheres; C, gray spheres; H, white spheres; N, dark blue spheres and Li, pink spheres). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
higher than the value (9.27 kJ/mol) at metal site [19]. It means that the adsorption site in IRMOF-3 locates near the functional group is favorable to the metal sites. That is to say, the CO2 molecule will first adsorb on the linker during the adsorption process, which is different from IRMOF-1. In addition, higher adsorption energies of IRMOF-3 compared with IRMOF-1 means a sharp increase at the initial stage of adsorption isotherm, which agrees
well with the experimental results [6]. The distance between oxygen and hydrogen atom is 2.36 Å, as shown in Fig. 5b. It can be concluded that the hydrogen atom of –NH2 interacts with the lone pair electron of oxygen atom of CO2 molecule and forms hydrogen-bond, which is significantly stronger than van der Waals force. Another new linker by adding a –NO2 group was also produced, as shown in Fig. 4c. Firstly, after the geometry optimization, the plane of –NO2 group is vertical to the plane of benzene linker, which is different to the optimized structure obtained in the previous work [21] in which the substituted nitro group is co-planar with the benzene ring. The difference is due to the effects of steric configuration induced by the existing of the nearby Zn4OCOO(COOH)5 fragment and may influence the adsorption behavior of CO2 on linker to some extent. The adsorption energy increases to 7.05 kJ/mol, an increase of 48%. The additional adsorption site, with CO2 cross attack at –NO2 side, was also evaluated. The adsorption energy is 10.39 kJ/mol, an increase of 59% compared to that of the CS site of model 2. The enhancement of CO2 uptake capacity at this site is mainly due to the electronic interaction between oxygen atoms of –NO2 group and carbon atom of CO2, at distances about 3.19 and 3.37 Å, as shown in Fig. 5c. Some other chemical modifications were also investigated. By adding a –CH3 group, as shown in Fig. 4d, the adsorption energy increases to 6.02 kJ/mol (an increase of 27%). These enhancement can be attributed to the substitution of the –CH3 group which donates electronic charges (0.06 electron, obtained by Mulliken analysis) into the aromatic ring and improves the quadrupole–p electron interaction. And then we produced the IRMOF-18 linker by adding four –CH3 groups, as shown in Fig. 4e, and the adsorption energy is 10.86 kJ/mol with an increase of 129%. The Mulliken analysis results show that 0.14 electrons are injected into the aromatic ring, which is responsible to the improvement of CO2 adsorption energy. In addition, as shown in Fig. 5d, weak hydrogen bond interactions are found between the oxygen atom of CO2 and the neighboring hydrogen atoms of –CH3 groups, with distances of 3.11 Å and 3.20 Å, and C@H O angle of 122° and 116° respectively. While the shortcoming for adding four –CH3 groups is that these groups may partially block the metal sites because they are located near the zinc oxide corners and result in reduced accessible surface area. 3.6. Li-doping on the linker Doping MOFs with Li or Li+ has been a useful way to increase H2 adsorption energy to MOFs [29,30]. However, researches have not been performed in the area of CO2 adsorption in Li-doping MOFs. Thus the adsorption energy of CO2 was firstly calculated with the presence of a Li dopant. The Li atom with no charge was placed on the center of the linker and the Li-linker system therefore represents a model for CO2 adsorption on Li-doped linker of IRMOF-1, as shown in Fig. 4f. Atomic charge was calculated through Mulliken analysis and it has been found that Li atom contributed 0.46 electron to the linker, compared with 0.6 electron from the zinc oxide cluster in the MOF crystal [31], which means a significant change of electronic environment. Interestingly, CO2 was initially located at the CC site, but it finally moved to VC site after geometry optimization. This phenomenon indicates that there is a strong interaction between Li atom and O atom of CO2 molecule via electronic force, and the doped Li atom is an unsaturated metal center to which CO2 can bind. The distance between Li atom and O atom of CO2 molecule is 2.638 Å, as shown in Fig. 5e. Finally, the calculated adsorption energy is 24.76 kJ/mol, which indicates that Li doping on linker greatly increases the CO2 adsorption capacity.
Y. Liu et al. / Fuel 95 (2012) 521–527
4. Conclusions Density functional theory studies were performed to calculate the adsorption energies of CO2 on various IRMOF-1 linker sites. CO2 was assigned to lie in the configurations parallel (P), cross (C) and vertical (V) to linker domain. The model of (HCOO)5 Zn4O(BDC)Zn4O(HCOO)5 is sufficient to represent interactions on linker domains of the IRMOF-1 with CO2. The optimal adsorption site on linker for the CO2 is the side position with CO2 parallel attack at hydrogen side of linker edge. While the main interaction force between CO2 and the linker of IRMOF-1 is van der Waals force, electrostatic force induced by Zn4O(HCOO)5 cluster can also significantly influence the adsorption behavior. The effects of chemical modifications of the linker, such as increasing the size of linker, adding functional groups and Li-doping, were studied and the adsorption energies were found to be significantly increased, which indicates that it is an effective way to enhance CO2 adsorption capacity by chemical modifications to the linker of MOFs. The density functional method is an effective method to reveal the mechanism of CO2 adsorption on the linkers of MOFs. This work will be helpful to design and synthesis new materials that have higher CO2 adsorption abilities. Acknowledgements This work was supported by Natural Science Foundation of China (50936001), 973 Program of China (2011CB707301, 2010CB227003) and Program for New Century Excellent Talents in University (NCET-10-0412). References [1] Figueroa JD, Fout T, Plasynski S, McIlvried H, Srivastava RD. Advances in CO2 capture technology–the US department of energy’s carbon sequestration program. Int J Greenh Gas Con 2008;2:9–20. [2] Banerjee R, Phan A, Wang B, Knobler C, Furukawa H, O’Keeffe M, et al. Highthroughput synthesis of zeolitic imidazolate frameworks and application to CO2 capture. Science 2008;319:939–43. [3] Wong-Foy AG, Matzger AJ, Yaghi OM. Exceptional H2 saturation uptake in microporous metalorganic frameworks. J Am Chem Soc 2006;128:3494–5. [4] Snurr RQ, Hupp JT, Nguyen ST. Prospects for nanoporous metal-organic materials in advanced separations processes. AIChE J 2004;50:1090–5. [5] Finsy V, Ma L, Alaerts L, De Vos DE, Baron GV, Denayer JFM. Separation of CO2/ CH4 mixtures with the MIL-53(Al) metal–organic framework. Micropor Mesopor Mater 2009;120:221–7. [6] Millward AR, Yaghi OM. Metalorganic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J Am Chem Soc 2005;127:17998–9. [7] Walton KS, Millward AR, Dubbeldam D, Frost H, Low JJ, Yaghi OM, et al. Understanding inflections and steps in carbon dioxide adsorption isotherms in metal–organic frameworks. J Am Chem Soc 2008;130:406–7. [8] Rowsell JLC, Eckert J, Yaghi OM. Characterization of H2 binding sites in prototypical metalorganic frameworks by inelastic neutron scattering. J Am Chem Soc 2005;127:14904–10.
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