Functional Group Modification of Metal–Organic Frameworks for CO2

Apr 24, 2012 - Reducing the anthropogenic emission of CO2is currently a top priority due to its global warming effect. Capturing CO2 by porous ... A T...
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Functional Group Modification of Metal−Organic Frameworks for CO2 Capture Zhonghua Xiang, Sanhua Leng, and Dapeng Cao* State Key Lab of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China S Supporting Information *

ABSTRACT: Reducing the anthropogenic emission of CO2 is currently a top priority due to its global warming effect. Capturing CO2 by porous materials is a promising approach due to its energetic efficiency and technical feasibility. A promising adsorbent for capturing CO2 should possess not only large BET specific surface areas (SSAs) but also high heat of adsorption. Since the intrinsic quadrupole moment of the CO2 molecule exists, introduction of a polar functional group in the framework of porous materials could enhance CO2 uptake. In this work, we adopt the postsynthetic modification approach to synthesize UMCM-1-NH2-MA (MA = maleic anhydride) material on the basis of UMCM-1-NH2 with an extremely high BET SSA of 4064 m2 g−1 and further explore the effects of free acid functionalities and aromatic amino groups on CO2 capture. The experimental and theoretical results show that, besides amino groups, the polar acidic functionalities also exhibit excellent capability for CO2 capture. Moreover, our firstprinciples calculations indicate that the aromatic imino group loses affinity toward CO2 significantly, compared with the aromatic amino group. In short, we believe that incorporating polar acidic functionalities into the porous materials could be an alternatively suitable approach for enhancing CO2 capture. respectively.40 The NU-100 reported by the Hupp group also exhibits a 2043 mg g−1 CO2 uptake at 40 bar and 298 K.26 The BET SSAs of both MOF-21040 and NU-10026 are larger than 6000 m2 g−1. As for COF materials, COF-102 and COF-103 show a CO2 uptake of 1200 and 1190 mg g−1 at 55 bar and 298 K.41 Ben et al. reported an ultrahigh hydrothermal stability PAF-1, which exhibits a 1300 mg g−1 CO2 uptake at 40 bar and 298 K.42 Most recently, Yuan et al. synthesized a PPN-4 material with a BET SSA of 6400 m2 g−1,43 which has the similar structure with PAF-302,44 and the PPN-4 material shows a CO2 uptake of 1710 mg g−1 at 50 bar and 295 K. In addition, Dawson et al. reported a series of microporous organic polymers (MOPs) with high SSAs (e.g., BET SSA of MOP-A is 4077 m2 g−1) for CO2 uptake.6At 1 bar and 298 K, MOP-E and MOP-B show 78 and 72 mg g−1 CO2 uptakes, respectively. In general, a low pressure (about 1 bar) is energetically advantageous for postcombustion CO2 capture. To obtain a high CO2 capacity at low pressure, the materials should possess a high heat of adsorption. Since the intrinsic quadrupole moment of CO2 molecule exists, introduction of a polar functional group in the framework of porous materials could enhance CO2 uptake. Actually, most of MOFs with open metal

1. INTRODUCTION As is well-known, greenhouse gases mainly include carbon dioxide, methane, ozone, nitrous oxide, fluorinated gases, and so forth. Since the industrial revolution, the fossil-fuel-dependent development results in the fast increase of CO2 concentration in the atmosphere, which has caused the climate change of our planet. As a result, the concentration of CO2 is increased from approximate 260 to 355 ppm with a 36% increment, compared with the one of a thousand years ago.1 Therefore, reducing the anthropogenic emission of CO2 is currently a top priority.2,3 CO2 capture and storage (CCS) have been proposed to reduce its concentration in the atmosphere.4 The pre- and postcombustion capture of carbon dioxide from syngas and flue gas streams is a crucial process of the CCS technology. Among many CCS technologies, capturing CO2 by porous materials is an alternative approach due to its energetic efficiency and technical feasibility.5,6 A lot of porous solids, such as different carbon-based adsorbents (e.g., activated carbons7 and carbon nanotubes8) and zeolites,9,10 have been proposed and widely applied to CO2 capture. Recently, metal−organic frameworks (MOFs)11−32 and covalent-organic frameworks (COFs)33−39 have shown great potential in gas storage due to their high specific surface areas (SSAs), pore volume, and tunable pore size. Many MOFs and COFs exhibit encouragingly high CO2 uptake. The CO2 uptakes of MOF-200 and MOF-210 reported by the Yaghi group reach 2400 and 2396 mg g−1 at 50 bar and 298 K, © XXXX American Chemical Society

Received: December 13, 2011 Revised: April 23, 2012

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Figure 1. Schematic representation of synthesis and postsynthetic modification of three materials in this work: A = UMCM-1, B = UMCM-1-NH2, C = UMCM-1-NH2-MA.

first-principles calculations to confirm our experimental results about the effects of aromatic amino and carboxylic groups on CO2 capture.

sites show large CO2 uptake in the low pressure. At 298 K and 1 bar, the CO2 uptake of Mg/DOBDC (DOBDC = dioxybenzene dicarboxylate) is 352 mg g−1,45 306 mg g−1 for MOF-74 (Co),45 and 256 mg g−1 for MOF-74 (Ni).45 Owing to the presence of pyridine nitrogen, CO2 uptake of the bioMOF-11 reaches 264 mg g−1 at 1 bar and 273 K.46 At similar conditions, all these results overpass the capacities of traditional materials (for instance, zeolite 13-X (207 mg g−1),9 Norbit RB2 (110 mg g−1),9 and active carbon (97 mg g−1)).7 To enhance CO2 uptake, the promising adsorbent should have a reasonably high SSA, relatively large pore volume, and appropriate heat of adsorption.47 Exactly, increasing the heat of adsorption through introducing the tailored binding functionalities is a suitable approach to improve the amount of gas adsorbed. Torrisi et al. studied the functionalized benzenes containing −NO2, −NH2, −OH, −SO3H, and −COOH substituents by using the first-principles calculation and found that all of these substituents could result in the enhancement of the binding energy (B.E.). In particular, the strongest binding appears when lone pair donation and H-bonding interaction occurs simultaneously, as in C6H5SO3H and C6H5COOH,48 which challenges the current research opinion that amine groups are the best for enhancing CO2 capture.3,49−51 Moreover, the Cooper group also experimentally confirms the superiority of carboxyl groups compared with aromatic amino groups by studying the isosteric heats of adsorption for CO2 in a number of functionalized conjugated microporous polymer (CMP) networks (CMP-1, CMP-1-(CH3)2, CMP-1(OH)2, CMP-1-NH2, and CMP-1-COOH).52 There are rare experimental studies on comparisons of amino and carboxyl groups effects for CO2 capture in MOF cases, since it is very difficult to incorporate carboxyl groups into the MOF framework by the traditional one-pot reaction. The facile coordination of carboxyl groups with metals hinders synthesis of the MOF with carboxyl groups. To further study the effects of amino and carboxyl groups in MOFs on CO2 capture, in this work, we employed a postsynthetic modification (PSM) approach53−56 to change UMCM-1-NH2 into the MOF with carboxyl groups (UMCM1-NH2-MA; MA = maleic anhydride). For comparison, we also prepared UMCM-1 without any functional groups. For simplification, we marked UMCM-1, UMCM-1-NH2, and UMCM-1NH2-MA as A, B, and C, respectively. Finally, we also used the

2. EXPERIMENTS AND METHODS 2.1. Experimental Synthesis. All reagents, unless otherwise stated, were obtained from commercial sources (Alfa Aesar, Sigma Aldrich) and were used without further purification. Synthesis of UMCM-1 (A). UMCM-1 was synthesized according to previous published work28 with slight modifications. Zn(NO3)2·6H2O (0.656 g, 2.205 mmol), H2BDC (terephthalic acid, 0.104 g, 0.625 mmol), and H3BTB (4,4′,4″-benzene-1,3,5triyl-tribenzoic acid, 0.233 g, 0.482 mmol, Aldrich) were dissolved in 20 mL of N,N-diethylformamide (DEF) in a 100 mL wide-mouth glass jar. The mixture was ultrasonicated for 15 min. The vial was placed in an oven at 85 °C for 3 days and subsequently transferred into an argon-filled glovebox. The product was isolated by filtration and rinsed with 3 × 10 mL of DMF and then immersed in CHCl3 for 3 days, during which the activation solvent was decanted and freshly replenished three times. It was then dried at 200 °C for about 10 h under an argon atmosphere to yield the product. Elemental Anal. Calcd

Figure 2. Powder X-ray diffraction patterns using Cu Kα radiation for A, B, and C along with a simulated pattern for A. B

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(%) for C44H24O13Zn4: C, 51.7; H, 2.4; O, 20.5. Found: C, 51.6; H, 2.9; O, 20.3. Synthesis of UMCM-1-NH2 (B). UMCM-1-NH2 was synthesized according to previous published work57 with slight modifications. Zn(NO3)2·6H2O (3.213 g, 0.011 mmol), 2aminoterephthalic acid (H2BDC-NH2, 0.400 g, 0.625 mmol), and H3BTB (0.424 g, 0.001 mmol, Aldrich) were dissolved in 10 mL of N,N-diethylformamide (DEF) in a 25 mL glass jar. The following procedures are the same as those of UMCM-1. Elemental Anal. Calcd (%) for C44H25NO13Zn4: C, 51.2; H, 2.4; N, 1.4; O, 20.2. Found: C, 51.1; H, 2.8; N, 1.5; O, 20.7. Synthesis of UMCM-1-NH2-MA (C). UMCM-1-NH2 was activated under vacuum at 200 °C prior to use. A 100 mg portion of activated UMCM-1-NH2 was immersed in 10 mL of anhydrous CHCl3 in a 20 mL vial. Maleic anhydride (9.42 mg) was subsequently added to the mixture. The reaction solution was allowed to stand unperturbed for 3 days at 50 °C. After that, the mixture was isolated by filtration and rinsed with 3 × 10 mL of CHCl3 and then immersed in anhydrous CHCl3 for 1 day again. Finally, the product was dried under vacuum at 200 °C to yield sample C. Elemental Anal. Calcd (%) for C48H28NO16Zn4: C, 51.0; H, 2.5; N, 1.2; O, 22.7. Found: C, 51.3; H, 2.9; N, 1.4; O, 23.1. The percent conversion of sample C is 92 ± 1%, as determined by 1H NMR,57 which is an average (with standard deviations) of three independent experiments. 2.2. Experimental Characterizations. Powder X-ray diffraction (PXRD) measurements were performed with a D/MAX 2000 X-ray diffractometer with a Cu Kα line (λ = 1.54178 Å) as the incident beam. Thermogravimetric analysis (TGA) data were obtained on a STA449C (NETZSCH) instrument, with a heating rate of 2 °C min−1 under flowing Ar. Elemental analysis (C, H, O, and N) was performed on a Thermo Fisher scientific Elemental Analyzer (Ea1112, Beijing Research Institute of Chemical Industry, SINOPEC). Electrospray ionization mass spectrometry (ESI-MS) was performed using a PFEIFF ERGSD301 mass spectrometer, and the data were analyzed using the Xcalibur software suite. For measuring ESI-MS, crystals of UMCM-1-NH2-MA (0.1−1 mg) were digested in 1 mL of MeOH with sonication. 1H NMR spectra were recorded on a Bruker AV600 NMR spectrometer (400 MHz). During measurement, approximately 5 mg of MOF (B or C) was dried under vacuum at 100 °C overnight and digested with sonication in 500 μL of DMSO-d6 and 2.3 μL of 20% DCl. 2.3. Adsorption Measurements. N2 Adsorption. N2 adsorption/desorption isotherms at 77 K were measured by a Micromeritics ASAP 2020. The samples of 150 mg were degassed at 200 °C for 24 h. The pore size distribution data were calculated from the N2 sorption isotherms based on the nonlocal density functional theory (NLDFT) model in the Micromeritics ASAP 2020 software package (the slit pore geometry is assumed). IGA-003 Gravimetric CO2 Adsorption Measurements. The CO2 isotherms at 298 K were measured by using a Hiden Isochema Intelligent Gravimetric Analyzer (IGA-003). In this instrument, an ultrasensitive microbalance of 0.2 μg resolution is mounted in a thermostatted heatsink with high-precision temperature control. The IGA-003 system can operate up to the pressure of 20 bar, and it is equipped with a high-vacuum turbomolecular pump with a dry (membrane) backing pump to ensure minimal contamination of the sample and a microbalance chamber. Prior to the measurement, ∼30 mg of samples was loaded into the IGA-003 and degassed at approximately 10−3 Pa at 350 °C for 24 h. The measurements were then

Figure 3. (a) 1H NMR spectra of B (black) and C (red). Red triangles (five points) and black squares (three points) represent signals of modified and unmodified BDC-NH2, respectively. (b) ESI-MS (negative mode) spectra of the digested C.

Figure 4. N2 adsorption (solid)/desorption (open) isotherms at 77 K. Inset graph refers to pore size distributions of products by incremental pore volume.

carried out under a water bath. The buoyancy corrections were carried out following our previous reports.17,20 The CO2 used for the experiment was of 99.994% purity (Beijing AP Beifen Gases Industry Co., LTD) and were delivered via a T-purge regulator to minimize the risk of gas supply contamination. 2.4. Isosteric Heat of Adsorption (Qst) Calculations. The virial equation of the form given in eq 1 was employed to calculate the enthalpies of adsorption for CO2 on three samples58 m

ln P = ln N + 1/T ∑ aiN i + i=0

C

n

∑ biN i i=0

(1)

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Table 1. Summary of Representative High-Capacity MOF and COF Materials for CO2 Storagea

a

materials

BET SSA [m2 g−1]

B A C COF-103 MOF-205 MOF-177 MIL-101(Cr) COF-102 PPN-4 PCN-68 PAF-1 NU-100 IRMOF-6 MOF-5 Li@CNT@Cu3(BTC)2 MOF-200 PPN-3 MOF-210 CNT@Cu3(BTC)2 COF-8 PPN-2 COF-5 COF-10 PPN-1 MIL-53(Cr) 6 ZIF-8

4064 4092 3887 3530 4460 4526 5500b 3620 6461 5109 5600 6143 2800 3800 857 4530 2840 6240 1458 1350 1764 1670 1760 1249 1463 750 ± 60 1311

pore size [Å]

pore volume [cm3 g−1]

CO2 uptake [mg g−1]

condition

ref

2.11 2.15 1.82 1.54 2.16 1.59 1.90 1.55 3.04 2.13

869 696 651 1038 1029 1008 980 975 898 898 876 838 723 703 660 649 627 598 595 502 486 441 412 393 330 316 238

18 bar/298 K 18 bar/298 K 18 bar/298 K 18.7 bar/298 K 18.7 bar/298 K 18 bar/298 K 20 bar/303 K 18.7 bar/298 K 18 bar/295 K 18 bar/298 K 17 bar/298 K 18 bar/298 K 18 bar/298 K 18 bar/298 K 18 bar/298 K 20 bar/298 K 18 bar/295 K 18 bar/298 K 18 bar/298 K 18.7 bar/298 K 18 bar/295 K 18.7 bar/298 K 18.7 bar/298 K 18 bar/295 K 18 bar/295 K 17 bar/298 K 18 bar/298 K

this work this work this work 41 40 63 66, 67 41 43 68 42 26 64 64 18 40 69 40 18 41 69 41 41 69 21 70 21

13.6, 27.3 13.6, 27.3 13.6, 25.2 12.533 17 8.6/29/34 11.533 15.0−21.8 14−24 17/24/30 10/15 11.2 3.5, 5, 9, 5−100

2.82 0.6 1.19 0.69 3.59 1.70 3.6 0.87 0.69 1.26 1.07 0.69 0.45 0.60

5.8−8.6, 10.1−46.7 3.5, 5, 9, 20−100 16 5.8−8.1, 10.9−46.7 27 16 11.7−46.7 8.7 3.5, 5.2, 8.2 11.8

0.54

It should be noted that the gas adsorption capacities in this table are all in the terms of experimental excess uptake. bLangmuir SSA.

To determine the accuracy of fitting, we define the relative squared 2-norm of the resiual Res as the criterion x

R es =

where P is the pressure expressed in Torr, N is the amount adsorbed in mmol/g, T is the temperature in K, ai and bi are virial coefficients, and m and n represent the number of coefficients required to adequately describe the isotherms. The equation was fitted by using the least-squares method; m and n were gradually increased until the contribution of extra added a and b coefficients was deemed to be statistically insignificant toward the overall fitting, as determined using the t test. The results of t tests are shown in Table S1 (Supporting Information). The values of the virial coefficients a0 through am were then used to calculate the isosteric heat of adsorption using the following expression:

2 1/2

B.E. = E(CO2 /cluster) − E(cluster) − E(CO2 )

(3)

(4)

3. RESULTS AND DISCUSSION To prepare the carboxyl-group-decorated MOF, we here select UMCM-1-NH257 due to its large open channels that allow for

m i=0

x

(∑i = 1 Ei )

where f is the fitted pressure, E is the measured pressure, and x is the number of the measured pressure. 2.5. First-Principles Calculations. All the first-principles calculations were implemented by the Gaussian 03 program package.59 The geometry optimization was performed at the theoretical level of B3LYP/6-31G*. The binding energies (B.E.'s) between CO2 and the studied cluster models were calculated in the framework of Møller−Plesset second-order perturbation theory (MP2) with the 6-31G* basis set, based on the optimized adsorption geometries obtained above. To save calculating time, all the binding energies in the Supporting Information were obtained without performing basis set superposition error (BSSE) calculations, but it is valid to investigate the adsorption site at the same theory level, aiming at selecting the favorite adsorption site. Once we gained the favorite adsorption site, then we performed BSSE calculations at the theoretical level of MP2/6-31G* to obtain the final results in the main text. The B.E. of the molecular complexes were computed by this equation:

Figure 5. Thermogravimetric analysis trace of A, B, and C.

Q st = −R ∑ aiN i

∑i = 1 |fi − Ei|2

(2) D

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facile diffusion of reagents.60 Maleic anhydride was used as the reagent for incorporating the carboxyl groups within the UMCM-1-NH2 to yield the UMCM-1-NH2-MA sample, as shown in Figure 1. Moreover, to study the effect of functional groups, we also prepared UMCM-1 according to our previous synthesis process with a slight modification.20 The structures of A and B were characterized by powder X-ray diffraction (PXRD) patterns, as shown in Figure 2. Note that the two crystals show a very similar structure to the one simulated. The success of incorporating carboxyl groups can be confirmed by the 1H NMR (dissolved in DMSO/DCl) and mass spectrometry (MS) data (see Figure 3). The signal of 1H NMR spectra of digested samples at δ = 6.3 and 6.6 ppm indicates the existence of −CHCH− (Figure 3a and Figure S1 in the Supporting Information). The expected peak for the modified BDC-NH2 ligand (BDC-NH2-MA) at m/z = 278.1 under negative mode was observed in the ESI-MS spectra (Figure 3b). The PXRD of C (Figure 2) indicates that it retains a crystal structure and possesses a framework topology identical to A and B, and introduction of a maleic anhydride group does not disrupt the framework. The porosities of A, B, and C were studied by N2 adsorption isotherms at 77 K (Figure 4). The isotherms exhibit an abrupt secondary jump near P/P0 = 0.2, indicating the presence of mesopores in the three samples. The porous properties of A, B, and C are shown in Table 1. It should be mentioned that the BET SSA of A (4092 m2 g−1) is larger than our previous one (2932 m2 g−1),20 which can be attributed to our modified synthetic method. We prepared sample A in the argon-filled glovebox to avoid the porous property change caused by the crystal surface exposure to the air.61 The BET SSA of B is 4064 m2 g−1, which is larger than the previous reported results (397357and 3200 m2 g−162). The pore volume and pore size distribution of A and B are almost at the same level (see Table 1). After introducing maleic anhydride into B, the BET SSA and pore volume of C decrease from 4064 to 3887 m2 g−1 and 2.11 to 1.82 cm3 g−1, respectively, compared to those of B. Moreover, the pore size of C in the mesopore region also decreases from 27.3 to 25.2 Å due to the formation of longer ligands in the frameworks, which reconfirms the success of incorporating carboxyl groups. The Langmuir SSAs of A, B, and C are summarized in Table S2 (Supporting Information), and the incremental and cumulative pore size distributions of A, B, and C are shown in the inset of Figure 4 and Figure S2 in the Supporting Information. Thermal gravimetric (TG) analysis indicates that the modified MOFs have comparable thermal stability to the parent B and all the three samples can keep thermal stability up to 450 °C (see Figure 5). The CO2 adsorption isotherms of A, B, and C at 298 K are shown in Figure 6a. In the high-pressure region (2−18 bar), sample B exhibits the highest CO2 uptake among the three samples. At 18 bar and 298 K, the CO2 uptake of B reaches 869 mg g−1, while the uptakes of A and C are 696 and 651 mg g−1, which indicate that the amino groups definitely enhance the CO2 adsorption, as proved in the previous reports.49−51 The CO2 uptake of B (869 mg g−1) is comparable to those of the MOFs with ultrahigh SSA to date (1038 mg g−1 for COF103,41 1029 mg g−1 for MOF-205,40 1008 mg g−1 for MOF177,63 and so on), and larger than many other representative MOFs (703 mg g−1 for MOF-5,64 649 mg g−1 for MOF-200,40 502 mg g−1 for COF-8,41 and so on) at the similar condition. More representative high-capacity MOFs and COFs for CO2

Figure 6. (a) Excess CO2 adsorption isotherms of A, B, and C at T = 298 K and P < 18 bar. Solid and open symbols represent adsorption and desorption, respectively. Inset graph refers to the enlargement of the CO2 adsorption isotherms at the P < 0.3 bar region. (b) Calculated isosteric heats of adsorption for CO2 in A, B, and C at T = 298 K.

capture at about 18 bar and 298 K in terms of experimental uptake are summarized in Table 1. The CO2 adsorption behavior in the low-pressure region (0− 0.25 bar) is a little different, as shown in the inset of Figure 6a. The CO2 uptake in C is the highest among the three samples. To better understand the difference between aromatic amino and carboxyl groups on CO2 capture at low pressure, we calculated the isosteric heat of CO2 adsorption (Qst) for A, B, and C by using the virial equation and the adsorption data (Figures S3−S5 in the Supporting Information) collected at T = 298, 303, 308, and 313 K and P < 1 bar. As shown in Figure 6b, the isosteric heat of adsorption of C is the highest, following by that of B, and the isosteric heat of adsorption of A is the lowest. At close to zero loading, the Qst for B is 16.7 kJ mol−1, over 2 times that of A (7.1 kJ mol−1), suggesting that the incorporating amino group extremely enhances the CO2 uptake, compared with A without any polar functionalities. Compared with B, the heat of adsorption of C also increases from 16.7 to 18.6 kJ mol−1 after modifying the amino group into the polar acid group, which suggests that the polar carboxyl group might outperform aromatic amine functionalities for CO2 sorption at low pressure, because the heat of adsorption can reflect the adsorption behavior at low pressure efficiently. To confirm the observation above theoretically, we used the first-principles calculation based on the cluster model to study the adsorption sites of the three samples.39,48 Here, the three distinguishable building blocks BDC, BDC-NH2, and BDC-NH2MA were used, as shown in Figure S6 (Supporting Information), E

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4. CONCLUSIONS In summary, we successfully incorporated carboxyl groups into the MOF framework by the postsynthetic modification approach. All the three MOF materials exhibit high CO2 uptake at T = 298 K and P < 18 bar. At 18 bar and 298 K, the CO2 uptake in B is about 869 mg g−1. Furthermore, the effects of free acid functionalities and aromatic amino groups on CO2 capture were investigated by the combination of experiment and the first-principles calculations. At close to zero loading, the isosteric heat of adsorption of C (the MOF with free acid functionalities; 18.6 kJ mol−1) is the largest, about 2 times that of A (the MOF without any polar functionalities; 7.1 kJ mol−1). Moreover, the heat of adsorption of the MOF with the carboxyl group is larger than that of B (the MOF with the aromatic amino group; 16.7 kJ mol−1). In addition, our first-principles calculations validate the experimental observation, suggesting that the polar carboxyl group might outperform aromatic amine functionalities for CO2 sorption at low pressure. Our firstprinciples calculations also indicate that the aromatic imino group loses affinity toward CO2 capture significantly, compared with the aromatic amino group. In short, we believe that incorporating polar acidic functionalities into the porous materials provides an alternative approach for enhancing CO2 capture, besides the amino groups emphasized in many previous studies.

to represent the three different materials (the residual building blocks in the three structures stay the same, so we only compare the CO2 adsorption in the three linkers). There are mainly five possible adsorption sites for CO2. Among the five sites, sites α and β exist in all the three MOF materials and site γ only exists in B, whereas sites δ and ε only exist in C. Figures S7−S9 in the Supporting Information give all the optimized adsorption geometries of CO2 in the MOFs derived from our first-principles calculations. The most stable one (B.E. = −10.651 kJ mol−1) is on top of the benzene ring in A, while CO2 prefers to locate “on top” of the amino group with an angle of 88.8° with B.E. = −12.728 kJ mol−1 in B. In the C case, the calculated binding energies are −15.930, −18.818, −14.095, −20.917, and −23.801 kJ mol−1 at the five sites, respectively, suggesting that a single CO2 molecule prefers first to locate at the site of a free acid group. It should be mentioned that the binding energy at site γ (“on top” of imino group) is the smallest among the five sites, which implies that CO2 does not prefer to locate around the imino group, not like in the amino group case. The binding energy of “on top” of the imino group is even smaller than that of “on top” of the benzene ring. After the amino group was changed into the imino group, it loses affinity to CO2 molecules significantly, suggesting that the imino group is not as good as the amino group for CO2 capture. By performing BSSE calculations at the theoretical level of MP2/6-31G*, the resulting CO2 favorite adsorption sites in all three materials are shown in Figure 7. Accordingly,

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ASSOCIATED CONTENT

* Supporting Information S

Adsorption isotherms of A, B, and C at different temperatures for calculation of isosteric heat, and partial results from the firstprinciples calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-10-64427616. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National 973 Program (2011CB706900), the Huo Yingdong Foundation (121070), NSF of China (21121064), Doctoral Programs of Ministry of Education (20100010110001), and the Chemical Grid Program from BUCT.

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REFERENCES

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Figure 7. Optimized favorite adsorption sites of the CO2 molecule in the three different distinguishable building blocks from A, B, and C. More information can be found in the Supporting Information. The calculated binding energies (B.E.'s) are also shown. The measured distances are presented in angstroms, and the measured angle is presented in degrees. C, O, H, and N are shown as dark cyan, red, white, and blue spheres, respectively.

all the experimental and theoretical results mentioned above indicate that the polar acidic functionalities might outperform amino functionalities emphasized in many previous studies for CO2 capture at low pressure, which agrees with the most recent results reported by Torrisi and Cooper.48,52,65 F

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3018875 | J. Phys. Chem. C XXXX, XXX, XXX−XXX