High Uptakes of Methane in Li-Doped 3D Covalent Organic

Sep 24, 2009 - To further enhance the methane capacity, we investigated the impact of Li-doping on the methane storage performance of the COFs. Our fi...
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High Uptakes of Methane in Li-Doped 3D Covalent Organic Frameworks Jianhui Lan, Dapeng Cao,* and Wenchuan Wang Division of Molecular and Materials Simulation, Key Laboratory for Nanomaterials, Ministry of Education of China, Beijing University of Chemical Technology, Beijing 100029, China Received June 6, 2009. Revised Manuscript Received August 4, 2009 By using a multiscale theoretical method, which combines the first-principles calculation and grand canonical Monte Carlo (GCMC) simulation, we studied storage capacities of methane in 3D covalent organic frameworks (COFs) and their Li-doped compounds at T = 243 and 298 K. Our results predicted that, at T=298 K and 35 bar, the excess gravimetric capacities of COF-102 and COF-103 reach 17.72 and 16.61 wt % (corresponding to 302 and 285 cm3 (STP)/ g)), which are in good agreement with experimental data, while the excess volumetric capacities of COF-102 and COF-103 reach 127 and 108 v (STP)/v, respectively. The high methane storage capacity of the COFs can be attributed to their ultrahigh surface areas and low densities. To further enhance the methane capacity, we investigated the impact of Li-doping on the methane storage performance of the COFs. Our first-principles calculations show that the Li cation doped in the COFs can enhance the binding of methane to the substrate significantly because of the London dispersion and the induced dipole interaction, due to the strong affinity of Li cation to methane molecules. At T=298 K and relatively low pressures (p < 50 bar), the Li-doping method nearly doubles the methane uptakes of the COFs, compared to the nondoped materials. In particular, at T = 298 K and p=35 bar, the methane volumetric uptakes of Li-doped COF-102 and COF-103 reach 303 and 290 v (STP)/v, respectively, which is more than 2 times those in the nondoped (127 and 108 v (STP)/v). To the best of our knowledge, the Li-doped 3D COFs show the largest methane storage uptakes at room temperature to date.

1. Introduction Nowadays, the coal, petroleum, and other fossil fuels are consumed drastically with the development and progress of human society. Therefore, natural gas as a potential vehicular fuel has attracted more and more research interest due to the urgent demand of exploiting new and effective alternative energy sources. Though natural gas is cheaper than conventional petroleum-based gasoline and diesel fuel, how to efficiently store natural gas, which consists mainly of methane, is still an important subject. Compared to compressed natural gas (CNG) technology, adsorption natural gas (ANG) technology1 is very promising and efficient because ANG just requires relatively low pressures, about 40 bar, to store natural gas in lightweight carriers, whereas CNG often store natural gas in heavy steel cylinders at high pressures, 200-300 bar. To improve the vehicular application of methane, the U.S. Department of Energy (DOE) has set the target of 180 v (STP)/v (standard temperature and pressure equivalent volume of methane per volume of the adsorbent material) for methane storage at 35 bar and around ambient temperatures, so that the energy density of ANG is comparable to that of CNG used in current applications.2,3 *Corresponding author. E-mail: [email protected]. (1) Matranga, K. R.; Myers, A. L.; Glandt, E. D. Chem. Eng. Sci. 1992, 47, 1569. (2) Burchell, T.; Rogers, M. SAETech. Pap. Ser. 2000, 2000-01-2205. (3) Duren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Langmuir 2004, 20, 2683. (4) Zhou, L.; Zhou, Y.; Li, M.; Chen, P.; Wang, Y. Langmuir 2000, 16, 5955. (5) Zhou, L.; Liu, X.; Sun, Y.; Li, J.; Zhou, Y. J. Phys. Chem. B 2005, 109, 22710. (6) Miyawaki, J.; Kaneko, K. Chem. Phys. Lett. 2001, 337, 243. (7) Cao, D.; Zhang, X.; Chen, J.; Wang, W.; Yun, J. J. Phys. Chem. B 2003, 107, 13286. (8) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; Michael, O. K.; Yaghi, O. M. Science 2002, 295, 469. (9) Bekyarova, E.; Murata, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Tanaka, H.; Kahoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 4681. (10) Kowalczyk, P.; Tanaka, H.; Kaneko, K.; Terzyk, A. P.; Do, D. D. Langmuir 2005, 21, 5639. (11) Morales-Cas, A. M.; Moya, C.; Coto, B.; Vega, L. F.; Calleja, G. J. Phys. Chem. C 2007, 111, 6473.

220 DOI: 10.1021/la9020383

Over the past decades, a lot of research interest has been focused on methane adsorption in several types of porous materials,4-19 including carbon nanotubes, molecular sieves, activated carbon, and metal organic frameworks. Zhou et al.4 measured the adsorption of methane in activated carbon at supercritical temperatures experimentally and obtained a storage capacity of about 176 g CH4/kg C at T = 293 K and 35 bar. Miyawaki and Kaneko6 investigated the storage capacity of methane in activated carbon fibers and found that the gravimetric capacity of methane exhibits a maximum of 140 g CH4/kg C at T=303 K and about 100 bar. Bekyarova et al.9 reported that a single wall nanohorn (SWNH) exhibits a high methane storage capacity, reaching 160 v (STP)/v at 35 bar and 303 K. In addition, Cao et al.7 also studied the adsorption storage of methane in single-walled carbon nanotubes (SWNT) arrays by the grand canonical Monte Carlo (GCMC) method and found that at room temperature and 41 bar the total volumetric and gravimetric capacities of methane in the SWNT arrays reach 216 v (STP)/v and 215 g CH4/kg C, respectively. Among the traditional carbon materials, activated carbons are found to possess the highest methane uptake of about 200 v (STP)/v. A new advance in porous materials in the past decade is the successful synthesis of metal organic frameworks (MOFs) due to their interesting structures and various potential applications. Eddaoudi et al.8 reported that IRMOF-6 has high methane (12) Wang, S. Energy Fuels 2007, 21, 953. (13) Lee, J.-W.; Balathanigaimani, M. S.; Kang, H.-C.; Shim, W.-G.; Kim, C.; Moon, H. J. Chem. Eng. Data 2007, 52, 66. (14) Kowalczyk, P.; Brualla, L.; Zywocinski, A.; Bhatia, S. K. J. Phys. Chem. C 2007, 111, 5250. (15) Jhon, Y. H.; Cho, M.; Jeon, H. R.; Park, I.; Chang, R.; Rowsell, J. L. C.; Kim, J. J. Phys. Chem. C 2007, 111, 16618. (16) Wu, H.; Zhou, W.; Yildirim, T. J. Am. Chem. Soc. 2009, 131, 4995. (17) Ma, S.; Sun, D.; Simmons, J. M.; Collier, C. D.; Yuan, D.; Zhou, H.-C. J. Am. Chem. Soc. 2008, 130, 1012. (18) Gallo, M.; Glossman-Mitnik, D. J. Phys. Chem. B 2009, 113, 6634. (19) Wu, H.; Zhou, W.; Yildirim, T. J. Phys. Chem. C 2009, 113, 3029.

Published on Web 09/24/2009

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adsorption performance and can store 155 v (STP)/v at 298 K and 35 bar. Wang12 compared the adsorption storage of methane in MOFs by GCMC simulation and concluded that the accessible surface area and free volume play a main role in determining methane uptake at 298 K. Adsorption of methane in MOF compounds M2(dhtp) (open metal M = Mg, Mn, Co, Ni, Zn; dhtp=2,5-dihydroxy terephthalate) were also investigated by Wu et al.16 It is found that these materials possess exceptionally large densities of open metal sites, and the five M2(dhtp) compounds yield excess methane adsorption capacities ranging from 149 to 190 v (STP)/v at 298 K and 35 bar. Recently, Ma et al.17 synthesized a microporous MOF, PCN-14, and reported that at 290 K and 35 bar PCN-14 exhibits a total methane adsorption capacity of 230 v (STP)/v, corresponding to an excess adsorption capacity of 220 v (STP)/v. To our knowledge, PCN-14 is the first and only MOF material whose methane storage uptake exceeds the DOE target to date. Most recently, a novel family of three-dimensional (3D) covalent organic frameworks (COFs) have been synthesized,20 by self-condensation and co-condensation reactions of the rigid molecular building blocks, tetrahedral tetra(4-dihydroxyborylphenyl)methane (TBPM), and its silane analogue (TBPS), and triangular hexahydroxytriphenylene (HHTP). The reported 3D COFs possess not only extremely high surface areas but also extraordinarily low densities, as presented in Table 1. These characteristics make them promising adsorbents for light gases. So far, a lot of works have been performed to study the hydrogen storage performance of these 3D COFs, and these materials show very high H2 storage capacities.21-23 Furthermore, recent investigations indicate that the Li-doping method can promote the hydrogen storage capacity of porous materials significantly due to the strong affinity of the doped cations to H2, originating from the formation of a dative bond between the electrons of the H2 σ-bond and the empty Li 2s orbital.23-26 Impressively, our previous research indicates that the hydrogen uptakes of Li-doped COFs exceed the DOE target of 6 wt % for hydrogen fuel cell vehicles at room temperature.23 Therefore, in this work, we intend to investigate the methane storage performance of the 3D COFs at both T = 243 and 298 K and further explore whether the Li-doping method can also enhance the methane adsorption in the Li-doped COFs significantly.

2. Computational Details We adopted a multiscale theoretical method to predict the methane storage capacities of the nondoped and Li-doped COFs. (20) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; Cote, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268. (21) Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard, W. A. J. Am. Chem. Soc. 2008, 130, 11580. (22) Klontzas, E.; Tylianakis, E.; Froudakis, G. E. J. Phys. Chem. C 2008, 112, 9095. (23) Cao, D.; Lan, J.; Wang, W.; Smit, B. Angew. Chem., Int. Ed. 2009, 48, 4730. (24) Mavrandonakis, A.; Tylianakis, E.; Stubos, A. K.; Froudakis, G. E. J. Phys. Chem. C 2008, 112, 7290. (25) Mulfort, K. L.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 9604. (26) Han, S. S.; Goddard, W. A., III. J. Am. Chem. Soc. 2007, 129, 8422. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.02 ed.; Gaussian, Inc.: Wallingford, CT.

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Table 1. Unit Cell Parameters, Mass, Density, and Free Volumes of 3D COFs20 materials

a = b = c (A˚)

mass (g/mol)

density (g/cm3)

free volumea (%)

COF-102 27.1771 5083.69 0.42 71.12 COF-103 28.2477 5276.59 0.38 73.25 COF-105 44.8860 9600.52 0.18 88.22 COF-108 28.4010 2351.91 0.17 88.84 a The free volume is defined as the accessible volume of H2 within one unit cell. It is accessible if the potential energy of the interaction between H2 and the solid framework is less than 104 K.

In the multiscale theoretical method, we first calculate the interaction between CH4 and the COFs using the first-principles calculations. Then, the force field (FF) parameters for the interaction between CH4 and the COFs are achieved by fitting the calculated potential energies to the Morse function. Using the force fields, the GCMC simulations are subsequently implemented to predict the adsorption isotherms of CH4 in the COFs and their Li-doped compounds. All the first-principles calculations were implemented by the Gaussian 03 program package.27 2.1. First-Principles Calculations. To investigate methane storage, we first performed high-quality first-principles calculations to obtain the interaction between CH4 and the COFs. Because of the large size of the unit cells in the COFs, the cluster model method was adopted to reduce the huge cost of the firstprinciples calculations. Figure 1a-c shows the four clusters representing the atom types in the COFs, where the C6H6 and B3O3H3 were adopted to represent the three-coordinated C and B atoms as well as the two-coordinated O atom, while the C9H12 and Si3C6H12 were adopted to represent the four-coordinated C and Si atom types, respectively. The potential energies between CH4 and the four clusters were calculated by the MP2/cc-PVTZ method with the basis set superposition error (BSSE) correction. On the basis of these results, the force field parameters for the interactions between CH4 and the COFs can be achieved by fitting the calculated results to the Morse function. The details can be found in section 2.2. To explore the effect of Li-doping on methane adsorption, we must first determine the distribution scheme of Li in the COFs using the first-principles calculations, as reported in our previous work.23 In this Li-doping scheme, each TBPM or TBPS building block supports eight Li atoms, while the HHTP building block just support one Li atom. This scheme guarantees that every Li atom doped in the substrate is positively charged because previous research has indicated that only the positively charged Li atoms exhibit the capability to enhance H2 adsorption, while the negatively charged and neutral Li atoms do not. Therefore, the weight percent of the doped Li atoms was determined as follows: 13.11% (COF-102), 12.63% (COF-103), 8.10% (COF-105), and 8.26% (COF-108). The interaction between methane and the doped Li atom was calculated at the theoretical level of PW91/ 6-311g(d,p) with the BSSE correction (see section 3.2 for details). 2.2. Fitting of Force Fields. On the basis of our calculations, the force field parameters for the interaction between CH4 and the COFs were obtained by fitting to the following Morse potential Uij ðrij Þ ¼ D½x -2x, 2

 ! γ rij x ¼ exp -1 2 re

ð1Þ

where rij is the interaction distance in angstroms. D, γ, and re denote the well depth, the stiffness (force constant), and the equilibrium bond distance, respectively. Figure 2 shows the potential energies obtained from our first-principles calculations DOI: 10.1021/la9020383

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Lan et al. Table 2. van der Waals Force Field Parameters for Nonbond Interaction between CH4 and COFs Derived from Our First-Principles Calculationsa atom typesb

H_

C_R

B_2

O_2

C_3

Si3

Li

D (kcal/mol) 0.052 0.209 0.126 0.255 0.269 0.329 4.700 γ 12.003 12.011 12.020 12.020 12.000 12.000 7.651 3.451 4.050 4.141 4.039 4.052 4.121 2.320 re (A˚) a H_ denotes the H atom bound to the hydrocarbon rings. b The first two characters correspond to the chemical symbol; an underscore appears in the second column if the symbol has one letter. The third column describes the hybridization or geometry: 1=linear, 2=trigonal, R=resonant, 3=tetrahedral.

Figure 1. Cluster models used to represent the 3D COFs. The O atoms in the building blocks are terminated by H, which are not shown here for clarity. Violet, white, pink, green, red, and yellow atoms represent Li, H, B, C, O, and Si, respectively.

Figure 3. Computed CH4 adsorption isotherms in COFs at T = 243 K: (a) total gravimetric isotherms; (b) excess gravimetric isotherms; (c) total volumetric isotherms; (d) excess volumetric isotherms. Figure 2. Potential energies of CH4 on (a) C6H6, (b) B3O3H3, (c) C9H12, and (d) C6Si3H12 derived from first-principles calculations and our force fields, respectively. In (a), (c), and (d), D denotes the vertical distance between the C atom of methane and the surface of the cluster models. In (b), D denotes the distance between the C atom of methane and one O in B3O3H3. Violet, white, pink, green, red, and yellow atoms represent Li, H, B, C, O, and Si, respectively.

and the fitted force fields for the interactions between CH4 and the four clusters. We can see that the potential energies from our force fields are in good agreement with those from the first-principles calculations. The fitted force field parameters originated from our first-principles calculations are listed in Table 2. The interaction between methane molecules is represented by the widely used Lennard-Jones parameters (ε/kB =148.0 K, σ=3.73 A˚).28 2.3. GCMC Simulation. Using the quantum-mechanicsbased force field parameters as input, the GCMC simulations were performed to obtain the adsorption isotherms of methane in the COFs. In GCMC simulations, the temperature, volume, and chemical potential were specified in advance. Moreover, the Widom’s test particle insertion method in a NVT ensemble was used to bridge the relationship between chemical potential and (28) Goodboy, S.; Watanabe, K.; MacGowan, D.; Walton, J.; Quirke, N. J. Chem. Soc., Faraday Trans. 1991, 87, 1952.

222 DOI: 10.1021/la9020383

pressure, as described in our previous work.29 To eliminate the boundary effect, the periodic boundary conditions were applied in all three directions. In addition, a 2  2  2 supercell was used for COF-102, COF-103, and COF-108, while a 1  1  2 supercell was used for COF-105 due to its much larger unit cell.

3. Results and Discussion 3.1. Adsorption of Methane in COFs at 243 and 298 K. Figure 3a-d shows the simulated gravimetric and volumetric adsorption isotherms of CH4 in the COFs at T=243 K. Clearly, the total gravimetric uptakes of CH4 in COF-105 and COF-108 are significantly higher than those for COF-102 and COF-103 at high pressures p > 30 bar due to the larger pore volume and smaller density of the first two materials.20 As is well-known, a smaller pore size in a material can lead to a relatively stronger overlap of the potential fields, originated from the neighboring skeleton atoms, and the material can therefore produce a stronger attraction to adsorbates at low pressures. However, a larger pore volume can lead to a higher capacity of adsorbates at high pressures. At 100 bar, the total gravimetric uptakes of COF-105 and COF-108 are 54.39 and 54.68 wt %, whereas those for COF102 and COF-103 are 34.63 and 35.91 wt %, respectively. (29) Lan, J.; Cheng, D.; Cao, D.; Wang, W. J. Phys. Chem. C 2008, 112, 5598.

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Figure 3b reveals that the COFs show a maximum in the excess uptakes, where the excess amount is defined as the total adsorption amount minus the one of methane in the free volume in the bulk phase. The maxima for COF-102 and COF-103 appear at about 40 bar (28.98 and 29.25 wt % for COF-102 and COF-103, respectively), while those for COF-105 and COF-108 appear within the range from 70 to 80 bar (39.46 and 38.51 wt % for COF-105 and COF-108, respectively). This can be attributed to the larger pore size of the latter, which leads to the lower material density and also generates relatively weaker affinity to CH4. The predicted total volumetric isotherms are presented in Figure 3c. We can see that COF-105 and COF-108 show slightly lower volumetric uptakes of CH4 than COF-102 and COF-103. At 30 bar, the total volumetric uptakes of the COFs have reached 257 (COF-102), 238 (COF-103), 228 (COF-105), and 225 (COF-108) v (STP)/v, respectively. At 100 bar, COF-102 even gives the highest total volumetric uptake of 311 v (STP)/v. Figure 3d presents the predicted excess volumetric isotherms. Compared to COF-105 and COF-108, COF-102 and COF-103 exhibit significantly higher excess volumetric uptakes because of their stronger affinity to CH4 originated from the smaller pore size. As a result, the optimal pressure, corresponding to the maximal excess uptake, is 40 bar for COF-102 and COF-103, while it is 80 bar for COF-105 and COF-108. That is to say, the materials with the stronger affinity to methane present the optimal adsorption at the lower pressure. According to the above results, it can be found that the COFs show extraordinarily high methane storage capacities at 243 K. Figure 4 presents the simulated gravimetric and volumetric adsorption isotherms of CH4 in the COFs at T=298 K. As shown in Figure 4a, at p = 35 bar, the total gravimetric uptakes of COF102, COF-103, COF-105, and COF-108 are 20.39, 19.64, 23.07, and 24.13 wt % (corresponding to 359, 350, 412, and 447 cm3 (STP)/g), respectively. During the submission of this paper, Yaghi et al.30 reported their experimental results about methane adsorption isotherms of the COF-102 and COF-103 at T=298 K, which provides a good evidence to validate our simulation results. Therefore, the total and excess experimental isotherms of methane in COF-102 are also presented in Figure 4a,b for comparison to our simulation results. Impressively, our simulation results are in satisfactory agreement with the experiment data, especially at low pressures of p < 50 bar. As shown in Figure 4a, the simulated total uptake of CH4 in COF-102 is about 27.07 wt % at T = 298 K and p = 70 bar, slightly higher than the experimentally measured value of 25.12 wt %. The slight difference of the methane uptake is reasonable because in the theoretical simulation we used a perfect structure to represent the COFs, while in the practical experiments, some of the porous channels may be blocked, in particular at high pressures, leading to the underestimation of methane capacity. Figure 4b shows the predicted excess gravimetric isotherms of methane in the COFs, from which we can find that COF-105 and COF-108 have nearly the same adsorption capacities to COF-102 and COF-103 at p 40 bar the slight deference between experimental data and simulation results still exists. As mentioned above, at high pressures, the slight overestimation of the simulated excess uptakes of methane in COF-102 is originated from the perfect structures of host materials used in our simulations, while a small part of porous channels in host material may be blocked in practice. It should be noted that COF-102 and COF-103 exhibit almost the same capacities for methane storage both in experiments and in simulations. Therefore, we do not intend to further compare the experimental and simulation results of COF-103 in methane storage. From Figure 4c,d, it can be seen that the both the total and excess volumetric uptakes of CH4 in COF-102 and COF-103 are higher than those in COF-105 and COF-108. Furthermore, the volumetric uptake of methane in COF-102 is the highest among four COF frameworks. At 35 bar, the total and excess volumetric uptakes of CH4 in COF-102 are 151 and 127 v (STP)/v, respectively. In contrast, the excess volumetric capacities of methane in COF-102 and COF-103 are comparable to 135 v (STP)/v of IRMOF-6 and 100 v (STP)/v of IRMOF-14 at T = 298 K and 35 bar.3,8 However, at p = 100 bar, these values of COF-102 increase to 244 and 169 v (STP)/v, respectively. From Figure 4d, it can be found that as the loading increases, the smaller pore size of COF-102 and COF-103 results in a saturation of the excess adsorption at p > 80 bar, while the excess adsorption of COF-105 and COF-108 keeps a nearly linear increase in the case studied. The poor volumetric performance of COF-105 and COF-108 stems from their large pore size, and the solid-fluid interaction cannot result in a sizable excess adsorption. On the basis of the above analysis, it is found that COF-102 and COF103 display not only ultrahigh methane gravimetric storage capacity but also high volumetric storage capacity at room temperature. DOI: 10.1021/la9020383

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Figure 6. Highest occupied molecular orbital (HOMO) for CH4 adsorption at the Li cation in Li-doped COFs. Violet, white, pink, green, red, and yellow atoms represent Li, H, B, C, O, and Si, respectively.

Figure 5. Isosteric heats of CH4 adsorption in 3D COFs at T = 298 K. The experimental data for COF-102 are also shown for comparison.30

For better comparison to experimental data, we also calculated the isosteric heats of CH4 adsorption in 3D COFs by using the fluctuation theory, given by qiso ¼

ÆUæÆNæ -ÆUNæ þ kB T ÆN 2 æ -ÆNæÆNæ

ð2Þ

where Æ...æ denotes the ensemble average, N is the number of particles, and U is the configuration energy of the system. Figure 5 presents the calculated isosteric heats of CH4 in COFs at room temperature, where the experimental isosteric heat of CH4 in COF-102 is also presented for comparison. It is found that for nondoped materials the isosteric heats of CH4 at low loadings are within the range of 6-8 kJ/mol (1-2 kcal/mol) at T= 298 K, which are comparable to the experimental data. In addition, compared to COF-102 and COF-103, the isosteric heats of CH4 in COF-105 and COF-108 are relatively lower because of their relatively larger pore sizes. 3.2. Adsorption of Methane in Li-Doped COFs at 243 and 298 K. As mentioned above, recent studies have indicated that the Li cation doped in COFs and MOFs has strong affinity to H2 and can therefore enhance hydrogen storage. Motivated by these results, in this work, we intend to explore this issue whether this strong affinity can also be observed in the interaction between the doped Li cation and methane. To reduce the computational cost, we still used the cluster model method in our first-principles calculations. As displayed in Figures 6 and 7, a large fragment from COF-105 was selected to represent the COFs, and its three four-coordinated Si atoms were terminated with H atoms. As illustrated in our previous work,23 when a Li atom is placed on the HHTP building block, it prefers to be adsorbed at the openhollow site with the positive charge of 0.42|e| according to Millikan charge analysis (see Figures 6 and 7). Therefore, we adopted this adsorption mode of Li as a representative to study the interaction nature between Li cation and methane. The adsorption geometry of methane on the Li-doped COFs was optimized by using the B3LYP/6-311G(d,p) method (see Figure 6). The binding energy was calculated at the theoretical level of the PW91/6-311g(d,p) method. Our calculations prove that the doped Li cation can also enhance the binding of methane on the surface of substrates, though one Li cation can only adsorb 224 DOI: 10.1021/la9020383

Figure 7. Potential energies calculated from the first-principles calculation as a function of the distance between Li cation and the C atom of methane.

one CH4 molecule due to the steric effect. The calculated binding energy between a methane molecule and the doped Li cation without BSSE correction is about -5.71 kcal/mol. In addition, the distance between Li and the C atom of methane is determined to be about 2.36 A˚. Our results show that there is no bonding interaction between the doped Li cation and the adsorbed CH4 molecule. The Millikan charge of the CH4 molecule induced by adsorbing at the Li cation is about 0.11|e|, indicating an evident charge transfer from CH4 to the Li-doped substrate. Although the CH4 molecule is highly symmetric and nonpolar, the Li-doping method can still enhance the binding strength of methane in the COFs. This is because methane adsorption on the Li cation perturbs the charge distribution of the CH4 molecule and makes it polarized and thus reduces the molecular symmetry and induces dipole moments. The above analysis reveals that the enhanced affinity of the Li cation to CH4 results from not only the London dispersion but also the induced dipole interaction. We have also tried to place the second CH4 molecule to the Li cation. However, it is found that the Li cation does not show evident attraction to the second CH4 molecule, and the Li-C (in CH4) distance is about 4.21 A˚. The Millikan charge of the second CH4 molecule is nearly zero. To further explore the interaction nature of CH4 with the doped Li cation, we also calculated the highest occupied molecular orbital (HOMO) for CH4 adsorption at the Li cation. Langmuir 2010, 26(1), 220–226

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Figure 6 shows the HOMO calculated by using the B3LYP/6311g(d,p) method. When a Li atom is deposited at the openhollow site (see Figures 6 and 7), the side-to-side overlap of Li 2p and C 2p orbitals forms conjugated π bond among Li cation and its nearest two C atoms, and no hybridization exists between Li and the CH4 molecule. To obtain the interaction energies between the CH4 molecule and Li cation, we performed high-quality first-principles calculations by the PW91/6-311g(d,p) method, including the basis set superposition error (BSSE) correction. Figure 7 shows the potential energies of Li-CH4 interaction derived from our firstprinciples calculations and force fields, respectively. It can be found that our force fields give convincing agreement with the first-principles calculations. Using the obtained force field parameters for interaction between CH4 and Li-doped COFs, the adsorption uptakes of CH4 in the Li-doped COFs can be therefore calculated by GCMC simulation. Figure 8 displays the gravimetric and volumetric adsorption isotherms of CH4 in the Li-doped COFs at T = 243 K. Compared to the nondoped COFs, the Li-doping enhances the adsorption of methane significantly. As presented in Figure 8a, at 35 bar, the total gravimetric uptakes of CH4 in Li-doped COF-105 and COF108 show an increase of about 33% compared to the nondoped materials, while those for Li-doped COF-102 and COF-103 show an increase of about 22% compared to the nondoped materials. Most important is that the Li-doping makes methane adsorption of the COFs readily saturated at low pressures. For example, the total gravimetric uptakes of methane in Li-doped COF-102 and COF-103 nearly reach the saturation at about 20 bar at T = 243 K due to their smaller pore size compared to the other two materials. At 100 bar, COF-105 and COF-108 present the highest gravimetric uptakes of 59.09 and 59.41 wt %, respectively. From Figure 8b, it can be seen that the excess gravimetric uptakes of Lidoped COF-102 and COF-103 show maxima at about 20 bar, and they are 37.45 and 38.49 wt %, respectively. In contrast, the maxima for Li-doped COF-105 and COF-108 reach 48.59 and 47.83 wt % at about 60 bar. The predicted total volumetric isotherms are presented in Figure 8c, from which we can see that the volumetric capacities of CH4 in COF-105 and COF-108 are slightly lower than the other two materials. At 35 bar, the total volumetric uptake of methane in Li-doped COF-102 reaches 383 v (STP)/v, which is the highest one among all the four COFs. Figure 8d gives the predicted excess volumetric uptakes, which are enhanced significantly after Li-doping. The excess volumetric uptakes of COF-102 and COF-103 show maxima of 352 and 340 v (STP)/v at about 20 bar. Because of the special importance of room temperature, the methane capacity of Li-doped COFs at T = 298 K was investigated in detail. Figure 9 presents the methane adsorption isotherms of Li-doped COFs at T = 298 K. The snapshots for methane adsorption in Li-doped COF-105 and COF-108 are also presented in Figure 10. After Li-doping, it is found that at T= 298 K the COFs exhibit extraordinarily high methane uptakes, especially at low pressures. As shown in Figure 9a, due to the strong affinity of Li cation to CH4, the total gravimetric capacities of Li-doped COF-102 and COF-103 nearly reach saturation at low pressures, while those of Li-doped COF-105 and COF-108 still exhibit a monotonic increase as the pressure rises. At 35 bar, the total gravimetric uptake of CH4 in Li-doped COF-105 reaches 36.88 wt % (corresponding to 817 cm3 (STP)/g), nearly doubled compared to the nondoped material (23.07 wt % for nondoped COF-105). At 100 bar, the Li-doped COF-105 even exhibit ultrahigh gravimetric uptake of 49.2 wt %. Clearly, the strong binding energy between the doped Li cation and CH4 is responLangmuir 2010, 26(1), 220–226

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Figure 8. Computed CH4 adsorption isotherms in Li-doped COFs at T = 243 K: (a) total gravimetric isotherms; (b) excess gravimetric isotherms; (c) total volumetric isotherms; (d) excess volumetric isotherms.

Figure 9. Computed CH4 adsorption isotherms in Li-doped COFs at T = 298 K: (a) total gravimetric isotherms; (b) excess gravimetric isotherms; (c) total volumetric isotherms; (d) excess volumetric isotherms.

sible to the ultrahigh methane gravimetric capacities of Li-doped COFs, making them the most promising candidates for methane storage. Figure 9b shows that the adsorption ability of Li-doped COFs are improved effectively after Li-doping. The excess uptakes of methane in the four Li-doped COFs reach about 32 wt % at T = 298 K and 35 bar, while these in nondoped COFs are only around 15 wt %, which further confirms the fact that the Li-doping method can nearly double the methane uptake. From Figure 10, we can see that, at low pressures, the adsorbed methane molecules mainly accumulate on the surface of substrates, espeDOI: 10.1021/la9020383

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reach 32.03 and 32.31 wt % (nearly 709 cm3 (STP)/g), respectively, which are the largest among all materials reported.

4. Conclusions

Figure 10. Snapshots and total volumetric adsorption capacities of methane in COF-105 and COF-108 at T=298 K and p=1, 10 MPa, respectively. The simulation boxes for COF-105 and COF108 in the GCMC simulations are 2  2  2 and 1  1  2, respectively.

cially nearby the Li cations. As the pressure increases, the relatively large pores, in which the adsorbate-substrate interaction is very weak, are also filled by methane molecules. Figure 9c displays the predicted total volumetric isotherms of methane in the COFs. At 35 bar, the methane volumetric capacities of Li-doped COF-102 and COF-103 reach 327 and 315 v (STP)/v, which is the largest among all materials reported previously, to our best knowledge. However, the total volumetric uptakes of Li-doped COF-105 and COF-108 are comparatively smaller, which is nearly half of those of the other two Li-doped compounds at p < 50 bar. At 100 bar, the total volumetric uptakes of methane in Li-doped COF-102, -103, -105, and -108 are 363, 357, 256, and 250 v (STP)/v, respectively. Figure 9d gives the predicted excess volumetric isotherms of methane. Obviously, the Li-doped COF-102 and COF-103 show significantly higher excess adsorption capacities than the other two materials due to the smaller pore size of the formers which can display stronger affinity to methane molecules. The excess volumetric uptake of methane in Li-doped COF-102 gives the maximum of 302 v (STP)/v at about 30 bar. On the basis of the above analysis, the Li-doping modification makes the COFs the most promising candidates for methane storage to date. An ideal material for CH4 adsorption should have not only a large accessible surface area but also a high free volume, a low framework density, and strong energetic interactions between the adsorbent and the methane molecules. In particular, at T = 298 K and 35 bar, the Li-doped COF-102 and COF-103 exhibit the largest methane volumetric uptakes and their excess volumetric uptakes reach 302 and 290 v (STP)/v, while the Li-doped COF-105 and COF-108 show the largest methane gravimetric uptakes and their excess gravimetric uptakes

226 DOI: 10.1021/la9020383

In this work, we predicted the methane storage capacities of the recently reported 3D COFs and their Li-doped compounds at T = 243 and 298 K, using our quantum-mechanics-based force fields. At T=243 K and 35 bar, COF-102 and COF-103 display high excess gravimetric uptakes of 28.56 and 28.54 wt %, and their excess volumetric uptakes are 235 and 217 v (STP)/v, respectively. At T = 298 K, the COFs still show high methane adsorption capacities due to their large surface areas and free volumes. At T =298 K and 35 bar, the excess gravimetric uptakes of COF-102 and COF-103 are 17.72 and 16.61 wt %, which are in good agreement with experimental data, whereas their excess volumetric uptakes are 127 and 108 v (STP)/v, respectively. These results indicate that the gravimetric uptakes of methane in COF102 and COF-103 have exceeded the one in the newly reported PCN-14,17 while their volumetric uptakes are comparable to IRMOF-6.8 On the contrary, COF-105 and COF-108 show much higher gravimetric uptakes and evidently lower volumetric uptakes because of their smaller pore sizes and larger pore volumes compared to the other two materials. It is found, for the first time, by our first-principles calculations that the doped Li cation in the COFs can enhance the binding strength of methane to the substrate significantly due to the London dispersion and the induced dipole interaction. The calculated binding energy between CH4 and the doped Li cation is about -5.71 kcal/mol. At both T=243 and 298 K, the Li-doped COFs show exceptionally high methane storage capacities. In particular, at T=298 K and low pressures (p < 50 bar), the methane storage capacities of Li-doped COFs are nearly doubled compared to the nondoped materials. At T=298 K and 35 bar, the total gravimetric uptakes of methane in Li-doped COF-102 and COF-103 reach 33.0 and 32.75 wt %, and their total volumetric uptakes reach 327 and 315 v (STP)/v, respectively. In addition, the Li-doped COFs exhibit also ultrahigh methane excess uptakes, compared to previously reported materials, originating from the strong affinity of Li-doped substrate to methane molecules. At T=298 K and 35 bar, the excess gravimetric uptakes of Li-doped COF-102 and COF-103 reach 31.35 and 30.98 wt %, and the corresponding excess volumetric uptakes are 303 and 290 v (STP)/v, respectively. By comparison, it is found that functionalizing the building blocks of COFs with Li atoms, as introduced in this work, can result in evidently stronger affinity to adsorbates such as CH4 and H2 and therefore improve the storage capabilities of the COFs. To the best of our knowledge, the Li-doped COFs are the most promising adsorbents for methane storage at room temperatures to date. These results provide useful information on modification of COFs for further improving methane storage. Acknowledgment. This work is supported by NSF of China (20776005, 20736002), National Basic Research Program of China (2007CB209706), Beijing Novel Program (2006B17), NCET Program (NCET-06-0095), ROCS Foundation (LX200702), and Novel Team (IRT0807) from the MOE of China, Chemical Grid Program and Excellent Talent of BUCT.

Langmuir 2010, 26(1), 220–226