Understanding the Effect of Trace Amount of Water on CO2 Capture in

Article pubs.acs.org/IECR

Understanding the Effect of Trace Amount of Water on CO2 Capture in Natural Gas Upgrading in Metal−Organic Frameworks: A Molecular Simulation Study Hongliang Huang, Wenjuan Zhang, Dahuan Liu,* and Chongli Zhong* State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: In this work, molecular simulations were performed to investigate the effect of trace amount of water on CO2 capture in natural gas upgrading process in a diverse collection of 25 metal−organic frameworks (MOFs). The results show that the interaction between water molecules and MOFs plays a crucial role: at the condition of weak interaction, water molecules move freely in the materials and show a negligible effect on the adsorption selectivity of CO2/CH4; while when the interaction is strong enough that water molecules are adsorbed to the preferential adsorption sites in MOFs, the effect can be significant, depending on the strength of water adsorption. In this case, the electrostatic interaction produced by the MOF framework is the dominant factor. This work provides a better understanding of the different behaviors of water effect on CO2 capture observed previously that may guide the future application of MOFs in industrial separations. DHF-PAF-1.17 In the meantime, in some MOFs the small amount of water has a negligible effect on the separation of gas mixtures, for example, the CO2/H2 and CO2/N2 mixtures in bio-MOF-11,18 CO2/CH4 mixture in Zn(BDC) (TED)0.5,19 as well as CO2/CH4 and CO2/N2 mixtures in ZIF-68 and ZIF-69.20 Up to now, the mechanism of the different behaviors caused by the presence of water is not quite clear. Thus, in this work CO2/ CH4, a system that involves in the natural gas upgrading process, was selected as a model mixture to separate, and the effect of trace amount of water was studied systematically using molecular simulations. For this purpose, a diverse collection of 25 MOF materials with typical structures were adopted to help to understand the microscopic mechanisms.

1. INTRODUCTION The escalating level of atmospheric carbon dioxide, the predominant greenhouse gas, is one of the most pressing environmental concerns.1 Carbon capture and storage (CCS) from large-scale sources is the promising option for reducing anthropogenic CO2 emissions, and natural gas sweetening (CO2/CH4) is considered as one of the technologies with the greatest possibility.2 Among the proposed CCS schemes, including amine absorption, cryogenic distillation, adsorption and enzymatic conversion, adsorption in porous materials is energetically efficient and economically competitive, where the performance of these materials is the key factor. Recent investigations have shown that metal−organic frameworks (MOFs), a new family of nanoporous materials,3−6 exhibit higher selective adsorption performance for CO2-containing gas mixtures than traditional porous materials, making them promising candidate in CO2 capture from natural gas.7−9 Therefore, it is worthwhile to investigate the various influencing factors on the separation performance of MOFs, including the structure of materials as well as the operation conditions. Unfortunately, compared to the study on the effect of structure, the investigations on the effect of the operation conditions are scarce, hampering the industrial applications of MOFs.10−12 In practical conditions, gas mixtures, such as natural gas sources, usually contain a small amount of moisture. Thus, it is valuable to investigate the separation performance of MOFs in the presence of water molecules. Very recent studies showed that the presence of water may be beneficial to the separation of a gas mixture. For instance, upon the addition of a trace amount of water into a CO2/H2 mixture, the selectivity in soc-MOF increases at low pressures.13 Similarly, the selectivity of CO2/ CH4 in MIL-101 is enhanced by terminal water molecules.14 In other cases, the addition of water gives negative contributions that can reduce the separation performance substantially, such as CO2/CH4 mixture in rho-ZMOF,15 CO2/H2 mixture in rhtZMOF,16 and CO2/CH4, CO2/H2, and CO2/N2 mixtures in © 2012 American Chemical Society

2. MODELS AND SIMULATION METHOD 2.1. MOF Structures. Twenty-five typical MOFs were selected, including ZIFs (ZIF-8, ZIF-68, ZIF-69, ZIF-70, ZIF-78, ZIF-79, ZIF-80, ZIF-81, ZIF-82),22,23 MIL-47-V,24 Cu-BTC,25 MOF-74-Mg,26 MOF-14,27 MOF-505,28 Cu2L2,29 Cu2L3,29 mixed ligand MOFs (Zn2L2L′-2, Zn2L2L′-3, Zn2L2L′-4, Zn2L2L′-5),30 CUKs (CUK-1, CUK-2),31 edible-MOF,32 NOTT-103,33 and an ionic MOF (Na+-usf-ZMOF).34 These MOFs have different topologies, pore sizes, and chemical characteristics to ensure the sample diversity. The guest-free framework structures were constructed from their corresponding experimental single-crystal X-ray diffraction (XRD) data using Materials Studio Visualizer.35 The structural parameters of the MOFs studied are shown in Table 1. Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 10031

November 22, 2011 April 9, 2012 April 13, 2012 April 13, 2012 dx.doi.org/10.1021/ie202699r | Ind. Eng. Chem. Res. 2012, 51, 10031−10038

Industrial & Engineering Chemistry Research

Article

Table 1. Structural Parameters of the MOFs Studied in This Work MOF ID

name of MOF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

ZIF-8 ZIF-68 ZIF-69 ZIF-70 ZIF-78 ZIF-79 ZIF-80 ZIF-81 ZIF-82 MIL-47-V Cu-BTC MOF-74-Mg MOF-14 MOF-505 a Cu2L2 b Cu2L3 ZnL2L′-2c Zn2L2L′-3d Zn2L2L′-4e Zn2L2L′-5f CUK-1 CUK-2 edible-MOF NOTT-103 Na+-usf-ZMOF

unit cell (Å) a a a a a a a a a a a a a a a a a a a a a a a a a

= = = = = = = = = = = = = = = = = = = = = = = = =

b = c = 16.9910 b = 26.6407, c = 18.4882 b = 26.0840, c = 19.4082 b = 27.0111, c = 18.0208 b = 26.1174, c = 19.4910 b = 25.9263, c = 19.6532 b = 26.3070, c = 1.93610 b = 25.9929, c = 19.6997 b = 26.4422, c = 18.9703 6.8180, b = 16.1430, c = 13.9390 b = c = 26.3430 b = 25.7856, c = 6.77010 b = c = 26.9190 b = 18.4826, c = 24.7130 b = 18.6297, c = 38.4920 b =18.4335, c = 52.3640 10.8860, b = 10.9190, c = 14.0910 12.9758, b = 13.1095, c = 13.9258 12.9938, b = 22.3180, c = 13.0908 15.1644, b = 15.1729, c = 22.2730 18.1020, b = 12.7694, c = 10.9701 11.1223, b = 13.5606, c = 12.1518 b = 42.6517, c = 28.4636 b = 18.5130, c = 45.3540 b = 29.572, c = 51.739

cell angle (degree) α α α α α α α α α α α α α α α α α α α α α α α α α

= = = = = = = = = = = = = = = = = = = = = = = = =

β = γ = 90 β =90, γ = 120 β =90, γ = 120 β =90, γ = 120 β = 90,γ = 120 β = 90,γ = 120 β =90, γ = 120 β = 90, γ = 120 β = 90, γ = 120\ β = γ = 90 β = γ =90 β = 90,γ = 120 β = γ = 90 β = 90,γ = 120 β = 90, γ = 121 β = 90,γ = 122 89.2904, β = 89.0810, γ = 79.6910 85.310, β = 70.710, γ = 84.1930 90, β = 102.582, γ = 90 89.2904, β = 89.0810, γ = 79.6910 γ = 90, β = 103.4 β = γ =90 β = 90, γ = 120 β = 90, γ = 120 β = γ = 120

pore shape

density (cm3/g)

cage/window pore/channel pore/channel pore/channel pore/channel pore/channel pore/channel pore/channel pore/channel channel pocket/channel channel pocket/channel pocket/channel pocket/channel pocket/channel cubic/catenation cubic/catenation cubic/catenation cubic/catenation channel pocket/channel pocket/channel pocket/channel pocket/channel

0.91 1.03 1.00 0.74 1.01 1.05 1.07 1.15 0.80 1.00 0.88 0.93 0.72 0.93 0.65 0.54 1.15 0.97 0.88 0.72 1.46 1.54 0.90 0.58 1.24

a 2

L = terphenyl-3,3″,5,5″-tetracarboxylic acid. bL3 = quaterphenyl-3,3‴,5,5‴-tetracarboxylic acid. cL = 1,4-benzenedicarboxylic acid (BDC); L′ = 4,4′bipyridine (BIPY). dL = 2,6-naphthalenedicarboxylic acid (NDC); L′ = 4,4′-bipyridine (BIPY). eL = 2,6-naphthalenedicarboxylic acid (NDC); L′ = N,N′-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNI). fL = 1,4-biphenyldicarboxylic acid (BPDC); L′ = N,N′-di(4-pyridyl)-1,4,5,8naphthalenetetracarboxydiimide (DPNI).

2.2. Atomic Partial Charges for MOFs. In molecular simulations, atomic partial charges for the frameworks in MOFs are required as input parameters. In this work, these values were estimated using the connectivity-based atom contribution (CBAC) method developed by our group,36,37 as shown in Figure 1. The CBAC method has been proved to be an efficient way to estimate the atomic partial charges in MOFs38−40 and successfully used in the simulations of adsorption and separation in MOFs.41−43 2.3. Force Field. In this work, CO2 was modeled as a rigid linear triatomic molecule with three charged Lennard-Jones (LJ) interaction sites located at each atom. The LJ potential parameters for atom O (σO = 0.305 nm and ε/kB = 79.0 K) and atom C (σC = 0.280 nm and ε/kB = 27.0 K) in CO2 molecule with C−O bond length l = 0.116 nm were taken from the TraPPE force field developed by Potoff and Siepmann.44 Partial point charges centered at each LJ site are qO = −0.35 e and qC = 0.70 e. CH4 was modeled by the united-atom model with LJ interaction, and the potential parameters (σ = 0.373 nm and ε/kB = 148.0 K) were also taken from TraPPE force field.45 Water was modeled by the three-point transferable interaction potentials (TIP3P) model,46 in which the O−H bond length is 0.9572 Å and the ∠HOH angle is 104.52°. More background information of TIP3P water model is provided in the Supporting Information. Both LJ interaction and partial point charges for O atom (σC = 0.3151 nm, ε/kB = 76.42 K and qO = −0.834 e) were considered while only partial point charges were assigned for H atom (qH = 0.417 e). It has been shown that the TIP3P model gives a reasonably good interaction potential compared to the experimental value.47

A combination of the site−site LJ and Coulombic potential was used to calculate the interactions between adsorbate molecules and adsorbents, and the potential parameters for the framework atoms in MOFs were taken from the UFF force field,48 as shown in Table 2. All the LJ cross interaction parameters were determined by the Lorentz−Berthelot mixing rules. The UFF force field combined with the potential models of the gases and water has been successfully used to describe the adsorption and separation in MOFs.13−16,18,49−52 2.4. Simulation Details. In this work, Grand Canonical Monte Carlo (GCMC) simulation was performed to calculate the adsorption of CO2/CH4 and CO2/CH4/H2O mixtures in the MOFs. The bulk composition is 50:50 (equimolar) for CO2/CH4 mixture, and 50:50:0.1 for CO2/CH4/H2O mixture. All the MOFs were treated as rigid framework with atoms frozen at their crystallographic positions. The cutoff radius was set to 12.8 Å for the LJ interactions, and the long-range electrostatic interactions were handled using the Ewald summation technique with tinfoil boundary condition. For each state point, GCMC simulation consisted of 1 × 107 steps to guarantee the equilibration, followed by 1 × 107 steps to sample the desired thermodynamics properties. The isosteric heat of adsorption, Qst, was calculated from53 Q st = RT −

⟨Uff N ⟩ − ⟨Uff ⟩⟨N ⟩ 2

⟨N ⟩ − ⟨N ⟩⟨N ⟩

−

⟨Usf N ⟩ − ⟨Usf ⟩⟨N ⟩ ⟨N 2⟩ − ⟨N ⟩⟨N ⟩ (1)

where R is the gas constant, N is the number of molecules adsorbed, and the bracket ⟨⟩ indicates the ensemble average. 10032

dx.doi.org/10.1021/ie202699r | Ind. Eng. Chem. Res. 2012, 51, 10031−10038


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Figure 1. Atomic partial charges for the MOFs studied in this work. (color code: Zn, yellow; Cu, pink; Co, light blue; V, silver; K, lavender; O, red; C, gray; H, white; N, blue, S, yellow; Cl, light green; Br, brown; Mg, grass green).

The first and second terms on the right-hand side are the contributions from the molecular thermal energy and adsorbate−adsorbate interaction energy, UFF, respectively. The third term is the contribution from the adsorbent−

adsorbate interaction energy, USF. The length of the simulation boxes is larger than 30 Å in each dimension. Equilibrium molecular dynamics (MD) simulation was used to investigate the dynamical behavior of water in MOFs at 10033



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water can be categorized into two groups: one is with negligible effect (for most MOFs, as illustrated in Figures 2a−2d), and the other is with substantial effect (such as ZIF-70, ZIF-78, ZIF-79, ZIF-81, MOF-74-Mg, edible-MOF, and Na+-usf-ZMOF, as illustrated in Figures 2e−2h). To understand the different behaviors of the effect of the trace amount of water on the selectivity, the interactions between water and the materials were studied by calculating the isosteric heat of adsorption of water at infinite dilution (Qst0), as shown in Table 3. The results demonstrate that the MOFs with large Qst0 show a large effect of water, while with the decrease of Qst0, the presence of water has little effect with negligible contributions. That is, only when the interaction between water and MOFs is large enough, then the separation performance may be affected by the presence of trace amount of water. It should be noted that for MOFs with the coordinately unsaturated metal sites (CUMS), such as Cu-BTC, the strong chemical-like interactions between the guest molecules and these adsorption sites cannot be fully captured by molecular simulation with classical force fields, which causes the much lower value of Qst in our calculations compared to the experiment58 or quantum calculation.59 The RDF result also showed that Cu−O(water) distance is about 6 Å, which is larger than the quantum (2.19 Å) and experimental (2.16 Å) values. Such phenomena have been widely observed in many MOFs with CUMS.60 On the basis of the structure of Cu-BTC with all the CUMS (Cu atoms) coordinated with the water molecules, we conducted additional GCMC simulations to examine the effect of water on the separation of CO2/CH4 mixture, as shown in the Supporting Information, Figure S2. Obviously, there are no evident differences between the selectivities for the dry gas mixture and the one with a trace amount of water (0.1% considered here). Thus, the above discrepancy does not influence the conclusions obtained in this work. Furthermore, it is interesting that Qst in ZIF-79 is higher than other ZIFs with similar topology, such as ZIF-78 and ZIF-81. The reason may be the contribution of −CH3 in the frameworks. Previous works have shown that MOFs modified using −CH3 exhibit large affinity to guest molecules including CH4,61 H2,62 and CO2.63 In addition, the bulky −CH3 groups can also reduce the pore size of the large channels in ZIF-79, in which the H2O molecules are also preferentially adsorbed at low pressures. This is different from the situations in ZIFs with other polarity groups, such as -NO2 and -Br in ZIF-78 and ZIF-81, respectively. Previous works have shown that the atomic charges of frameworks in MOFs play an important, and sometimes even dominant role on the adsorption and separation of gases.64,65 Therefore, we further investigated the role of framework charges for the condition of large Qst0 by switching them off in

Table 2. LJ Potential Parameters for the Atoms in the MOFs atom

ε/kB [K]

σ [nm]

MOF-H MOF-C MOF-N MOF-O MOF-Na MOF-Mg MOF-K MOF-S MOF-Cl MOF-Sc MOF-V MOF-Co MOF-Cu MOF-Zn MOF-Br MOF-In

22.14 52.84 34.72 30.19 15.10 55.86 17.61 137.89 114.24 9.56 8.05 7.05 2.52 62.40 126.32 301.45

0.257 0.343 0.326 0.312 0.266 0.269 0.340 0.359 0.352 0.294 0.280 0.256 0.311 0.246 0.373 0.398

298 K. The temperature was held constant with a Nosé-Hoover chain (NHC) thermostat as formulated by Martyna et al.54 The starting configuration for MD simulaion was generated directly from the MC simulation mentioned above. The simulation boxes used in the MD simulations are the same as those in the GCMC simulations, and periodic boundary conditions were also applied in all the three dimensions. The long-range electrostatic interactions were evaluated using the Ewald summation method, and LJ interactions were calculated with a 12.8 Å cutoff radius. The velocity Verlet algorithm was used to integrate Newton’s equations of motion. The time step used in the MD simulations was taken as 0.5 fs. For each MD run, simulation consisted of 2 × 107 steps to guarantee the equilibration, followed by 1 × 107 MD steps to sample the dynamics properties of interest. At least 10 independent simulations were performed for each loading to estimate the statistical error. A detailed description of the simulation methods can be found in ref 55 and our previous work.56,57 The selectivity for component A relative to component B is defined by S = (xA/xB)/(yB/yA), where xA and xB are the mole fractions of components A and B in the absorbed phase and yA and yB are the mole fractions of components A and B in the bulk phase, respectively.

3. RESULTS AND DISCUSSION The simulated selectivities for CO2 in the CO2/CH4 as well as CO2/CH4/H2O mixtures in the 25 MOFs at 298 K are shown in Figure 2, as a function of the bulk pressure up to 3.0 MPa. It can be seen that the effect of the presence of a small amount of

Table 3. Isosteric Heat of Adsorption of Water at Infinite Dilution Cu2L2

Cu2L3

MOFs

Cu-BTC

MOF-14

MOF-505

Qst0 [KJ/mol] MOFs

−25.01 CUK-1

−28.40 CUK-2

−24.36 ZIF-8

Qst0 [KJ/mol] MOFs

−28.63 Zn2L2L′-2

−22.33 Zn2L2L′-3

−13.57 Zn2L2L′-4

−12.81 Zn2L2L′-5

−27.77 ZIF-68

Qst0 [KJ/mol] MOFs

−20.71 ZIF-69

−17.96 ZIF-80

−37.39 ZIF-82

−37.86 ZIF-70

−37.99 ZIF-78

Qst0 [KJ/mol] MOFs

−45.10 ZIF-79

−39.91 ZIF-81

−30.73 edible-MOF

−57.84 MOF-74-Mg

−72.40

−53.93

Qst0 [KJ/mol]

−69.51 10034

−25.45 MIL-47-V

−52.43

−27.11 NOTT-103

−54.21 Na+-usf-ZMOF −89.98



Article

Figure 2. Selectivities of CO2 in equimolar mixtures of CO2/CH4 with and without water as a function of pressure at 298 K.

simulations. As can be seen in Figure 3, the selectivity of CO2 nearly remains unchanged by the presence of water in this case, which is different from the behaviors shown in Figure 2 (2e−2h). This illustrates that the substantial effect of water is largely attributed to the electrostatic interactions produced by the frameworks, since water molecules are highly polar molecules. In fact, the selectivity is more significantly affected by water in MOFs with extra-framework ions, such as socZMOF,13 rho-ZMOF,15 and rht-ZMOF.16

To further understand the microscopic mechanism of the different behaviors of the effect of water on CO 2 selectivity, the location and movement of water in MOFs were further investigated by calculating the mean-squared displacement (MSD) of water in a MOF using MD simulation at 298 K 1 MSD(t ) = N 10035

N

∑ |ri(t ) − ri(0)|2 i

(2)



Article

are comparative, the adsorbed water molecules show little effect on the selectivity, such as in ZIF-69, ZIF-80, and ZIF-82 as shown in Figure 2d. However, the mechanism of the negligible effect is different with that in the MOFs with small Qst0 as shown in Figures 2a−2c.

4. CONCLUSIONS This work shows that the effect of trace amount of water on the adsorption selectivity of CO2 in the CO2/CH4 mixture is MOFdependent, and the effect becomes significant only when the interaction between water molecules and MOF is large enough, in which the electrostatic interactions between the framework and fluids play a crucial role. It is clear that when the interaction between water molecules and MOF is weak enough to allow the water molecules move freely in the materials, the effect on selectivity is negligible. However, at moderate interaction strength, the effect can be either negligible or significant, depending on the competition of the two opposite contributions of water molecules to the adsorption of CO2. The knowledge obtained may provide useful information to guide the application of MOFs in industrial separations.

Figure 3. Selectivities of CO2 in equimolar mixtures of CO2/CH4 with and without water as a function of pressure at 298 K by switching off the framework charges.

where t is the time, N is the number of water molecules, and ri(t) is the center-of-mass position of molecule i at time t. Some typical results are shown in Figure 4, by taking Cu-BTC, MOF505, Cu2L2, Zn2L2L′-4 as the examples of those in which water has little effect, and ZIF-70, ZIF-79, ZIF-81 as the examples of those in which water has substantial effect. As can be seen in Figure 4a, the MSD is very large, which increases gradually with increasing simulation time, implying that the water molecules move freely in these MOFs. This is due to the relatively weak interaction between the MOF material and water molecules as illustrated in Table 3. In this case, the free water behaves like a component in the gas mixtures and has little effect on the separation of CO2/CH4 because of the tiny amount, as shown in Figures 2a−2c. On the contrary, Figures 4b and 4c demonstrate that the MSD of water molecule is small and fluctuates marginally around a certain value with the increase of simulation time. This indicates that water molecules are nearly fixed in these MOFs because the strong interaction between water molecules and the framework makes the water molecules adsorbed on the preferential adsorption sites. In this case, the water molecules may act as additional adsorption sites. Here, a cooperative effect exists for the role of water molecules: on one hand, there is a competitive adsorption of water with CO2, showing negative impact on the selectivity of CO2 in the CO2/ CH4 mixture; on the other hand, the adsorbed water molecules may provide additional adsorption sites for CO2, showing positive impact. This means that the adsorbed water may either enhance or reduce the selectivity of CO2, depending on the competition of the two opposite impacts. Moreover, when they

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ASSOCIATED CONTENT

S Supporting Information *

Background information on the TIP3P water model, and further details on the effect of H2O loading (Figure S1) and of 0.1% H2O on CO2/CH4 selectivity in hydrated Cu-BTC (Figure S2). 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] (D.L.), [email protected]. edu.cn (C.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (Nos. 21136001, 21121064, 20906002), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Contract 20110010130001), and the Fundamental Research Funds for the Central Universities (No. ZZ1102).

Figure 4. Mean-squared displacement of water in the CO2/CH4/H2O mixture in the typical MOFs as a function of simulation time at 298 K and 0.1 MPa. 10036



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