Article pubs.acs.org/JPCC
Water Effects on Postcombustion CO2 Capture in Mg-MOF-74 Jiamei Yu and Perla B. Balbuena* Artie McFerrin Department of Chemical Engineering and Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843-3122, United States ABSTRACT: Mg-MOF-74, one of the best metal−organic framework (MOF) materials for CO2 adsorption and separation, has been widely explored in CO2 capture. In this work, water effects on postcombustion CO2 capture in MgMOF-74 are evaluated by using a combination of Grand Canonical Monte Carlo and density functional theory (DFT) simulations. Our results show that in this MOF the CO2 adsorption capacity is decreased by the presence of water molecules linked to coordinatively unsaturated metal sites (CUMs). This effect is a consequence of the reduction of binding energy between CO2 and the water-coordinated MgMOF-74 framework. DFT calculations indicate that such binding energy is much lower than that without coordinated water. More importantly, we find that the difference of the binding energy between CO2 and CUM in a MOF and that between CO2 and CUM-coordinated water are good descriptors to evaluate water effects on CO2 capture in MOFs with CUMs. At the same time, these investigations further demonstrate that the CUMs in MOFs play an important role in their performance for CO2 capture from a practical stream containing water and other impurities. CO2 separations by these materials is scarce.15−17 On the basis of the limited data available, it is generally accepted that the CO2 adsorption and selectivity decrease with water presence in MOFs since water molecules compete with CO2 for the adsorption sites. However, this is not always the case. For example, it has been found that the presence of 4 wt % water coordinated to the coordinatively unsaturated metal (CUM) sites in the HKUST-1 framework can enhance CO2 capture capacity and selectivity over N2 and CH4.16 In addition, recently we further investigated how water (both coordinated water and water vapor in flue gas) affects the CO2 adsorption and CO2/N2 separation properties in HKUST-1.18 Interestingly, the increase of CO2 adsorption uptake was observed with the increase of hydration level in HKUST-1. Therefore, it might be other important factors that determine the effects of the presence of water during CO2 capture in MOFs. In this work, molecular modeling methods are used to estimate the influence of coordinated water on the CO2 capture in Mg-MOF-74 (also labeled Mg/DOBDC, where DOBDC = 2,5-dioxidobenzene-1,4-dicarboxylate),19 another typical MOF with CUMs, showing almost the highest CO2 adsorption capacity relative to other MOFs reported in the literature to date at room and higher temperatures. The CO2 adsorption and capture in this MOF has been widely and deeply investigated, including the water effects experimentally explored.12,20−26 It has been reported that Mg-MOF-74 can
1. INTRODUCTION How to reduce anthropogenic CO2 emission is one of the most intense concerns nowadays.1−3 As one of the options, carbon capture and storage (CCS) has been proposed and implemented in some cases.1 In CCS, the separation of CO2 from other components of a practical stream is the central task. For postcombustion capture, the separation of CO2 from flue gas (containing N2, water, SO2, and other minimal impurities) is required. It should be pointed out that in these separationrelated issues the separation medium plays a very important role, apart from separation methods and processes.4,5 Metal−organic frameworks (MOFs), as a new class of porous sorbent solids, are attracting wide attention due to their various potential applications, including gas separations.6−9 MOFs have indeed been found in recent years as promising candidates in the adsorption and separation of CO2, as revealed by numerous studies using experimental techniques and computational simulation methods.8−13 Particularly, MOFs allow facile optimization of the pore structure, surface functionalization, and other properties for enhanced CO2 separation performance from gas mixtures, such as flue gas.4,10 In addition to the main components CO2 and N2, many other impurities exist in the flue gas mixture. One important impurity is water. It has been found that the concentration of water in flue gas by volume is 5−7%.14 Although the amount of water is small, it may significantly influence the performance of CO2 capture in MOFs and other porous materials. Barbarao et al.15 showed that even with 0.1% water in the CO2/CH4 mixture the CO2/ CH4 selectivity in rho-ZMOF decreases by 1 order of magnitude. However, the exploration of the water effect on © 2013 American Chemical Society
Received: November 9, 2012 Revised: January 18, 2013 Published: January 23, 2013 3383
dx.doi.org/10.1021/jp311118x | J. Phys. Chem. C 2013, 117, 3383−3388
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provide exceptional CO2 uptake of 23.6 wt % at 0.1 atm and 35.2 wt % at 1 atm.26 In addition, Mason et al. demonstrated the superior performance of Mg-MOF-74 over MOF-177 and zeolites.12 These results indicated the potential application of Mg-MOF-74 as efficient CO2 capture adsorbents. It is indeed well-known that the presence of CUM sites in MOFs can enhance separation of CO2/N2 or CO2/CH4 since CO2 can form a strong interaction with metal centers.27,28 However, as pointed out by Keskin et al. for the adsorption-based separation with MOFs, the coadsorption of CO2 with water in MOFs including Mg-MOF-74 is urgent to be resolved for large-scale application of MOFs for CO2 capture.29 Due to the strong affinity of water molecules to the CUMs, water molecules in a flue gas mixture will be first coordinated with the CUM positions in Mg-MOF-74 when the gas stream passes through it. Therefore, it is very important to investigate the effects of coordinated water in Mg-MOF-74 on CO2 capture performance.
Figure 1. Dry Mg-MOF-74 structure viewed along the Z direction (left); a cluster model resembling the local environment of the unsaturated sites in Mg-MOF-74 (right). Color scheme: Mg atoms, green; O atoms, red; C atoms, gray; and H atoms, lavender.
⇒BE
2. COMPUTATIONAL METHODS Density functional theory (DFT) calculations were performed using the GAUSSIAN 09 suite of programs.30 DFT calculations were performed for three purposes. First we optimized the position of the coordinated water molecules in the frameworks. Second, we calculated the atomic partial charges of the MgMOF-74 framework with or without water coordination for the effective force fields used in Grand Canonical Monte Carlo (GCMC) simulations. Partial charges were calculated using the CHELPG methods.31 Third, we optimized the geometries of CO2 interacting clusters isolated from the unit cells of MgMOF-74 with or without water coordination and estimated the corresponding binding energies (BEs). The exchange and correlation functionals used were B3LYP32 with a 6-31+G* basis set, except for the third purpose, where B3LYP with a 6311++G(d, p) basis set was used for geometrical optimizations and M06 and B97D were used for BE calculations. Compared to the popular functional B3LYP, M06 and B97D show superiority in describing long-range dispersion interactions.33,34 BE calculations were corrected for basis set superposition error (BSSE) using the counterpoised method applied to the optimized structures. The BE is used to evaluate effects of water on CO2 binding strength with CUMs, the primary binding sites for CO2 adsorption in Mg-MOF-74. A cluster model shown in Figure 1 closely resembles the local environment of the unsaturated sites in Mg-MOF-74. The BE is defined as the overall energies of the following reactions. For:
= −(EMg (C7H5O2)5(C6H5O)(H2O)(CO2) 3
− EMg (C7H5O2)5(C6H5O)(H 2O) − ECO2) 3
GCMC simulations35 using the MUSIC code36 were employed to calculate the adsorption of single components and their mixtures in the Mg-MOF-74. The structure of MgMOF-74 used in our study was reported by Caskey et al.26 In our simulations, we have modeled the Mg-MOF-74 as a rigid framework. This is a reasonable assumption for the investigation of coordinated water effects since it has been found that solvent molecules in as-synthesized Mg-MOF-74 materials can be removed upon evacuation to generate CUMs without structural collapse.26 A cutoff radius of 12.8 Å was applied to the LJ potential, and Ewald summations were used to evaluate long-range effects of the electrostatic interactions.37 Each GCMC simulation consisted of 1 × 107 steps to guarantee equilibrium and 1 × 107 production steps. The potential parameters for CO2 and N2 were taken from the TraPPE potentials.38 CO2 was represented as a rigid linear triatomic molecule with one charged Lennard-Jones (LJ) interaction site located at each atom. The LJ potential parameters are σOO = 0.305 nm and εOO/kB = 79.0 K for O···O interactions and σCC = 0.280 nm and εCC/kB = 27.0 K for C···C interactions between CO2 molecules with a C−O bond length of 0.116 nm. Partial point charges are centered at each LJ site with qO = −0.35 e and qC = 0.70 e. The N2 molecule was modeled as a rigid three-site model with two sites located at each of the N atoms and the third one located at the molecule center of mass (COM). The site at each N atom was modeled by a LJ interaction potential with σNN = 0.331 nm and εNN/kB = 36.0 K. The bond length between two N atoms is 0.110 nm. Each N2 molecule was assigned a negative charge on each N atom with qN = −0.482 e and a positive charge at the COM site with qCOM = 0.964 e. The LJ potential parameters for the framework atoms in Mg-MOF-74 were taken from the DREIDING39 or UFF40 force field, as shown in Table 1. Lorentz−Berthelot mixing rules were employed to calculate the pair site−site interactions among the MOF framework and each gas species.
CO2 + Mg 3(C7H5O2 )5 (C6H5O) → Mg 3
(C7H5O2 )5 (C6H5O)(CO2 ) ⇒BE = −(EMg (C7H5O2)5(C6H5O)(CO2) − EMg (C7H5O2)5(C6H5O) 3
3
− ECO2) For:
CO2 + Mg 3(C7H5O2 )5 (C6H5O)(H 2O) → Mg 3
(C7H5O2 )5 (C6H5O)(H 2O)(CO2 ) 3384
dx.doi.org/10.1021/jp311118x | J. Phys. Chem. C 2013, 117, 3383−3388
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compositions of 15:85 and 50:50 for CO2/N2 mixtures in dry and hydrated Mg-MOF-74 frameworks with two different hydration levels. The results shown in Figure 4 indicate that
Table 1. Potential Parameters for the Atoms in the Framework of Mg-MOF-74 atoms
σ (nm)
ε/νB (K)
O C H Mg
0.303 0.347 0.285 0.269
48.16 47.86 7.65 55.86
3. RESULTS AND DISCUSSION To study the coordinated water effects on CO2 capture in MgMOF-74, the positions of hydrogen atoms of the water molecules in the MOF are first optimized at the B3LYP/631+G* level. The position of oxygen atoms of the water molecules was reported in the crystal structure of Mg-MOF-74 by Caskey et al.26 Only the hydrogen atoms of the water molecules are allowed to relax during optimization calculations. After optimization, the hydrogen atoms are observed to lean toward carboxylate oxygen atoms. Figure 2 shows the optimized orientations of water molecules in fully hydrated Mg-MOF-74.
Figure 4. GCMC simulated adsorption isotherms for CO2 in dry and hydrated Mg-MOF-74 at 298 K.
compared with the coordinated water effects on CO 2 adsorption in HKUST-1, where adsorption capacity of CO2 increases with the increase of HKUST-1 hydration level,16 the reverse behavior is observed in Mg-MOF-74. The adsorption capacity of CO2 is decreased by the presence of water molecules coordinated to the CUMs in Mg-MOF-74. The decrease of adsorption capacity in MOFs is undesirable for practical applications of CO2 capture. Therefore, removal of water from both flue gas and/or this MOF material is essential for such applications. Moreover, the adsorption capacity of CO2 decreases with the increase of hydration level in Mg-MOF-74. It should be pointed out that UFF and DREIDING force fields understimate CO2 adsorption in MOF-74 frameworks at low pressures, e.g.,
