Comparative Study of Separation Performance of COFs and MOFs for

In this work, grand canonical Monte Carlo (GCMC) simulations were performed to evaluate the separation performance of covalent organic frameworks (COF...
0 downloads 0 Views 2MB Size
Download PDF
2902

Ind. Eng. Chem. Res. 2010, 49, 2902–2906

Comparative Study of Separation Performance of COFs and MOFs for CH4/CO2/ H2 Mixtures Yunhua Liu, Dahuan Liu, Qingyuan Yang, Chongli Zhong,* and Jianguo Mi Laboratory of Computational Chemistry, Department of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China

In this work, grand canonical Monte Carlo (GCMC) simulations were performed to evaluate the separation performance of covalent organic frameworks (COFs) compared with that of metal-organic frameworks (MOFs) for CH4/CO2/H2 mixtures. The simulation results show that the adsorption selectivities of COFs and MOFs are similar. The electrostatic contribution of framework charges in COFs should be taken into account, although it is smaller than that in MOFs. In addition, the present work shows that the ideal adsorbed solution theory (IAST) is applicable to most COFs. 1. Introduction Metal-organic frameworks (MOFs) are a family of hybrid nanoporous materials that are formed by the coordination of metal ions with organic linkers, and they have shown various promising applications in gas storage, separation, and catalysis.1-3 More recently, another new class of porous materials, covalent organic frameworks (COFs), has emerged.4-6 COFs are made of organic linkers held together by boron oxide clusters by means of covalent bonds; these materials have lower densities than MOFs while retaining the unique characteristics of MOFs, such as large surface areas and pore volumes, and are receiving increased attention. Currently, studies on COFs mainly focus on the adsorption behavior of single gases, both experimentally and theoretically,5-10 whereas investigations on the separation of gas mixtures by COFs, which is important in many industrial processes, are scarce. To achieve an understanding of the separation performance of COFs, as well as to have a comparison with MOFs, this work performed a comparative study on the two families of materials. For this purpose, CH4/CO2/H2 mixtures were selected as sample systems for separation because they are important practical systems that are involved in the process of purification of synthetic gas obtained from steam reforming of natural gas. On the other hand, the systems contain components with different characteristics: CH4 is a spherical nonpolar fluid with no quadrupole moment, H2 is a linear nonpolar fluid with a weak quadrupole moment, and CO2 is a linear nonpolar fluid with a strong quadrupole moment. Thus, the study of these mixtures allows us to investigate the behavior of fluids with much different characteristics in COFs and MOFs and to obtain a better understanding of the separation performance of the two families of materials. In addition, the present work also studied the applicability to COFs of the ideal adsorbed solution theory (IAST), which has been recognized as providing good predictions of gas-mixture adsorption in porous materials, such as in zeolites11,12 and MOFs.13,14 2. Models and Simulation Method 2.1. COF and MOF Structures. In this work, 12 materials were investigated, including six COFs (COF-6, -8, -10, -102, -103, and -105)4-6 and six MOFs (Cu-BTC and IRMOF-1, -8, * To whom correspondence should be addressed. Tel.: +86-1064419862. E-mail: [email protected].

-10, -14, and -16).15,16 Only the structures of the COFs are shown in Figure 1, because the IRMOFs and Cu-BTC are wellstudied materials.17 These COFs and MOFs have different topologies, pore sizes, and chemical characteristics to ensure the reliability of the results. The structural properties of all of the materials are summarized in the Supporting Information. 2.2. Force Fields. In the present work, CO2 was modeled as a rigid linear triatomic molecule with three charged LennardJones (LJ) interaction sites, and CH4 was modeled as a single LJ interaction site. The potential parameters were all taken from the TraPPE force field.14 H2 was treated as diatomic molecule modeled by an LJ core located at its center of mass. The quadratic Feynman-Hibbs (FH) effective potential18,19 was employed to calculate all of the LJ interactions to take into account quantum effects on H2 adsorption. The potential parameters for the framework atoms in the MOFs and COFs were taken from the DREIDING force field.20 Detailed parameters of the force fields are given in the Supporting Information. 2.3. Simulation Method. Grand canonical Monte Carlo (GCMC) simulations were employed to calculate the adsorption of pure components and their mixtures in the COFs and MOFs. Similarly to previous works,9,13,14,17 all of the COFs and MOFs were treated as rigid frameworks, with atoms frozen at their crystallographic positions during simulations. A cutoff radius of 12.8 Å was applied to the LJ interactions, and Ewald summations were used to calculate the electrostatic interactions.21 To convert the experimental pressures to fugacities, which are required in the GCMC simulations, we used the Peng-Robinson equation of state. Each GCMC simulation consisted of 1 × 107 steps to guarantee equilibration, followed by 1 × 107steps to sample the desired thermodynamic properties. To estimate the statistical uncertainty, the production phase of each state point was divided into 10 blocks, and the standard deviation of the block average was calculated. A detailed description of the simulation methods can be found in the literature.22 Isosteric heats of adsorption, qst, were also calculated for each component. This quantity can be calculated from simulations as the difference between the partial molar enthalpy of the sorbate in the bulk phase and the partial molar internal energy in the adsorbed phase, which is given by the equation qst ) RT -

10.1021/ie901488f  2010 American Chemical Society Published on Web 02/15/2010

( ) ∂U ∂Nads

T,V

Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

2903

Figure 1. Crystal structures of the COFs used in the simulations: (a) COF-6, (b) COF-8, (c) COF-10, (d) COF-102, (e) COF-103, (f) COF-105 (B, green; Si, blue; O, red; C, gray; and H, white).

Figure 2. Comparison of simulated and experimental excess adsorption isotherms of (a) H2 at 77 K, (b) CH4 at 298 K, and (c) CO2 at 298 K in COF-6 and COF-102.

whereNads is the adsorption loading and U is the internal energy of the sorbate in the adsorbed phase, which includes contributions from both adsorbate-adsorbent and adsorbate-adsorbate interactions.23 In addition, in separation processes, a good indication of the potential for successful separation is the selectivity of a porous material for different components in mixtures. The selectivity for component A relative to component B is defined as S)

( )( ) xA yB xB yA

where xA and xB are the mole fractions of components A and B, respectively, in the absorbed phase and yA and yB are the corresponding mole fractions in the bulk phase. 3. Results and Discussion 3.1. Calculation of Atomic Partial Charges in COFs. In all of the simulations, atomic partial charges in the frameworks were required as input parameters. Those for Cu-BTC and the IRMOFs were taken from the literature,17 whereas for the COFs (COF-5, -6, -8, -10, -102, -103, -105), the atomic partial charges were calculated in this work using density functional theory (DFT) on the basis of the fragmental clusters using the Gaussian 03 package.24 Based on the ChelpG method,25 DFT calculations using the unrestricted B3LYP functional were carried out to compute the atomic partial charges, and the 6-31+G* basis set was used for all atoms. For the cleaved clusters of the COFs,

the terminations are all connected with organic linkers, and therefore, they were saturated with -CH3 groups. This method of computing the atomic partial charges has previously been successfully used in both COFs26 and MOFs.17 The atomic partial charges and the corresponding model clusters are given in the Supporting Information. 3.2. Validation of the Method. The reliability of the force fields employed in this work for gas adsorption in MOFs has been well-validated.27,28 Therefore, here, we validate their reliability and the atomic partial charges obtained only for the COFs. In this work, the excess adsorption isotherms of H2, CH4, and CO2 in the COFs were further simulated and compared with available experimental data.10 To investigate whether the introduction of the partial charges on the frameworks of the COFs improves the reliability of the force field, the excess adsorption isotherms of H2 and CO2 were calculated for three cases: (1) all electrostatic interactions are included, (2) the electrostatic interactions between the fluid molecules and the frameworks are switched off, and (3) all electrostatic interactions involving the fluid molecules are switched off. As can be seen from Figure 2, the isotherms simulated with the inclusion of the partial charges are in better agreement with the corresponding experimental results, indicating that the inclusion of the partial charges is essential and the force fields combined with the atomic partial charges are reliable. In addition, the results in Figure 2a,c demonstrate that the extent of the effect of framework charges is adsorbate-/adsorbent-dependent; it is more evident for H2 in COF-6 than for CO2 in the same COF. On

2904

Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

Figure 3. Selectivities in the COFs and MOFs for (a) CH4 from an equimolar CH4/H2 mixture, (b) CO2 from an equimolar CO2/H2 mixture, and (c) CO2 from an equimolar CO2/CH4 mixture.

Figure 4. Differences of isosteric heats of adsorption in the COFs and MOFs for (a) CH4/H2, (b) CO2/H2, and (c) CO2/CH4.

Figure 5. Effects of electrostatic interactions on a binary CO2/CH4 mixture at 298 K: (a) in the six COFs and (b) in the six MOFs.

the other hand, the effect of the electrostatic interactions between CO2 molecules is more obvious than that for H2 molecules. 3.3. Comparison of the Separation Performances of Gas Mixtures in COFs and MOFs. In this work, we first compared the adsorption selectivities of the six COFs (COF-6, -8, -10, -102, -103, -105) with those of the six MOFs, as reported in Figure 3. It can be seen that, in the studied pressure range, the selectivities of the COFs and MOFs are similar, which indicates that the type of material is not the main factor influencing the possibility of separation. Interestingly, for all of the mixtures considered, COF-6 and Cu-BTC showed the best separation performance. These results can be explained as follows: Cu-BTC is a complex framework with side pockets and channels. Because of the existence of small side pockets, stronger electrostatic interactions and confinement effects apply and play an important role in improving gas separation in the low-pressure range, because the pockets are the preferential adsorption sites for the gas molecules. On the other hand, CuBTC has some open metal sites, which can enhance the adsorption separation of molecules with different quadrupole moments, because a component with a larger quadrupole moment will exhibit stronger electrostatic interactions with open metal sites.23 COF-6 is a two-dimensional framework similar to carbon nanotubes with small pores (see Table S1, Supporting Information), leading to stronger electrostatic interactions and confinement effects. To further understand the good performance of Cu-BTC and COF-6, we analyzed the loading dependency

of the isosteric heats of adsorption for equimolar binary mixtures of CH4, CO2, and H2. Figure 4 shows the differences in the isosteric heats of adsorption. Obviously, the differences in the isosteric heats of adsorption in Cu-BTC and COF-6 are much higher than those in the other materials for all of the binary mixtures considered; this explains why these materials exhibit better performance than the others in terms of separation selectivity. 3.4. Influences of Framework Charges. The effects of framework charges on separations in MOFs were widely studied in our previous work.17 In this work, we further investigated the influences of framework charges on adsorption selectivity in COFs, by performing additional simulations in which the electrostatic interactions between the fluid molecules and the framework atoms wre switched off. The contribution of the framework charges to the adsorption selectivity is defined as the percentage ratio electrostatic contribution )

Swith - Swithout × 100% Swith

where Swith and Swithout denote the adsorption selectivity with and without the fluid-COF electrostatic interactions, respectively. The simulation results are shown in Figures 5 and 6. For comparison, those for the MOFs are also included in the two figures. Although, generally, the electrostatic contributions from the COFs are smaller than those from the MOFs, the effects of framework charges still need to be considered, particularly

Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

2905

Figure 6. Effects of electrostatic interactions on a binary CO2/H2 mixture at 298 K: (a) in the six COFs and (b) in the six MOFs.

Figure 7. Comparison of IAST (lines) and GCMC (symbols) for the selectivities of (a) CH4/H2, (b) CO2/H2, and (c) CO2/CH4 in the COFs at 298 K.

at low pressures. In addition, the effects are more pronounced in the two-dimensional COFs as compared with the threedimensional COFs. 3.5. IAST Predictions. It has been commonly recognized that IAST29 can provide good predictions of the adsorption of gas mixtures in many zeolites,11,12 and in our previous work,13,14 we demonstrated that it is applicable for depicting the adsorption of CH4/CO2/H2 mixtures in noncatenated MOFs and of CH4/ H2 mixtures in catenated MOFs. In this work, IAST calculations were further performed to check whether this is also case for COFs. The adsorption selectivities of binary mixtures of CH4, CO2, and H2 in the COFs, as calculated using GCMC and IAST, are shown in Figure 7. The agreement between IAST and GCMC is good for all of the systems, except for the CO2/H2 mixture in COF-6; a similar observation was also found for Cu-BTC.14 These results can be explained as follows: COF-6 is a twodimensional material with small pores, leading to strong confinement effects on the CO2 molecules but much smaller ones on the H2 molecules. On the other hand, H2 has a weak quadrupole moment, whereas CO2 has a much stronger one, and thus the electrostatic interaction between CO2 and COF-6 is also much stronger than that between H2 and COF-6. These two factors make the system highly nonideal, resulting in large deviations between the IAST and GCMC simulation results. From the results shown in Figure 7, one can conclude that IAST is applicable to most COFs; however, its applicability depends largely on the ideality of the system considered. For

systems in which the confinement effects are highly nonideal, IAST can deviate significantly from the GCMC simulation results. 4. Conclusions This work shows that, for the binary systems considered, the selectivities of COFs and MOFs are similar. The electrostatic contribution of framework charges in COFs is smaller than that in MOFs; nevertheless, this contribution should still be considered, particularly at low pressures. In addition, this work demonstrates that the effects of framework charges are more pronounced in two-dimensional COFs than in three-dimensional COFs, so that IAST is likely to be applicable to most COFs. Acknowledgment The financial support of the NSFC (Nos. 20725622, 20876006, 20821004) is greatly appreciated. Supporting Information Available: Structural properties of the COFs and MOFs studied in this work, description of the force fields employed, and atomic partial charges and the corresponding model clusters of the COFs. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre´, J. Metal-organic frameworkssProspective industrial applications. J. Mater. Chem. 2006, 16, 626.

2906

Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

(2) Rowsell, J. L. C.; Yaghi, O. M. Strategies for Hydrogen Storage in Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2005, 44, 4670. (3) Fe´rey, G. Hybrid porous solids: Past, present, future. Chem. Soc. ReV. 2008, 37, 191. (4) Coˆte´, A. P.; EI-Kaderi, H. M.; Furukawa, H.; Hunt, J. R.; Yaghi, O. M. Reticular Synthesis of Microporous and Mesoporous 2D Covalent Organic Frameworks. J. Am. Chem. Soc. 2007, 129, 12914. (5) Coˆte´, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks. Science 2005, 310, 1166. (6) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Corte´s, J. L.; Coˆte´, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Designed Synthesis of 3D Covalent Organic Frameworks. Science 2007, 316, 268. (7) Garberoglio, G. Computer Simulation of the Adsorption of Light Gases in Covaent Organic Frameworks. Langmuir 2007, 23, 12154. (8) Garberoglio, G.; Vallauri, R. Adsorption and diffusion of hydrogen and methane in 2D covalent organic frameworks. Microporous Mesoporous Mater. 2008, 116, 540. (9) Yang, Q.; Zhong, C. Molecular Simulation Study of the Stepped Behaviors of Gas Adsorption in Two-Dimensional Covalent Organic Frameworks. Langmuir 2009, 25, 2302. (10) Furukawa, H.; Yaghi, O. M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875. (11) Goj, A.; Sholl, D. S.; Akten, E. D.; Kohen, D. Atomistic simulations of CO2 and N2 adsorption in silica zeolites: The impact of pore size and shape. J. Phys. Chem. B 2002, 106, 8367. (12) Challa, S. R.; Sholl, D. S.; Johnson, J. K. Adsorption and separation of hydrogen isotopes in carbon nanotubes: Multicomponent grand canonical Monte Carlo simulations. J. Chem. Phys. 2002, 116, 814. (13) Liu, B.; Yang, Q.; Xue, C.; Zhong, C.; Chen, B.; Smit, B. Enhanced Adsorption Selectivity of Hydrogen/Methane Mixtures in Metal-Organic Frameworks with Interpenetration: A Molecular Simulation Study. J. Phys. Chem. C 2008, 112, 9854. (14) Yang, Q.; Zhong, C. Molecular Simulation of Carbon Dioxide/ Methane/Hydrogen Mixture Adsorption in Metal-Organic Frameworks. J. Phys. Chem. B 2006, 110, 17776. (15) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469. (16) Chui, S. S-Y.; Lo, S. M-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n. Science 1999, 283, 1148. (17) Yang, Q.; Zhong, C.; Chen, J.-F. Computational Study of CO2 Storage in Metal-Organic Frameworks. J. Phys. Chem. C 2008, 112, 1562. (18) Sese´, L. M. Study of the Feynman-Hibbs effective potential against the path-integral formalism for Monte Carlo simulations of quantum manybody Lennard-Jones systems. Mol. Phys. 1994, 81, 1297.

(19) Sese´, L. M. Feynman-Hibbs potentials and path integrals for quantum Lennard-Jones systems: Theory and Monte Carlo simulations. Mol. Phys. 1995, 85, 931. (20) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. DREIDING: A Generic Force Field for Molecular Simulations. J. Phys. Chem. 1990, 94, 8897. (21) Heyes, D. M. Pressure tensor of partial-charge and point-dipole lattices with bulk and surface geometries. Phys. ReV. B 1994, 49, 755. (22) Frenkel, D.; Smit, B. Understanding Molecular Simulations: From Algorithms to Applications, 2nd ed.; Academic Press: San Diego, CA, 2002. (23) Karra, J. R.; Walton, K. S. Effect of Open Metal Sites on Adsorption of Polar and Nonpolar Molecules in Metal-Organic Framework Cu-BTC. Langmuir 2008, 24, 8620. (24) 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.; Bakken, V.; 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 D.01; Gaussian, Inc.: Wallingford, CT, 2004. (25) Francl, M. M.; Carey, C.; Chirlian, L. E.; Gange, D. M. Charges Fit to Electrostatic Potentials. II. Can Atomic Charges Be Unambiguously Fit to Electrostatic Potentials. J. Comput. Chem. 1996, 17, 367. (26) Babarao, R.; Jiang, J. Exceptionally high CO2 storage in covalentorganic frameworks: Atomistic simulation study. Energy EnViron. Sci. 2008, 1, 139. (27) Garberoglio, G.; Skoulidas, A. I.; Johnson, J. K. Adsorption of Gases in Metal Organic Materials: Comparison of Simulations and Experiments. J. Phys. Chem. B 2005, 109, 13094. (28) Du¨ren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Design of New Materials for Methane Storage. Langmuir 2004, 20, 2683. (29) Myers, A. L.; Prausnitz, J. M. Thermodynamics of Mixed-Gas Adsorption. AIChE J. 1965, 11, 121.

ReceiVed for reView September 22, 2009 ReVised manuscript receiVed January 29, 2010 Accepted February 03, 2010 IE901488F