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ReaxFF Molecular Dynamics Simulations of Water Stability of Interpenetrated Metal−Organic Frameworks Xiu Ying Liu,†,‡ Sung Jin Pai,‡ and Sang Soo Han*,‡ †

College of Science, Henan University of Technology, Lianhua Street No. 100, High-tech Zone, Zhengzhou Henan 450001, People’s Republic of China ‡ Computational Science Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea S Supporting Information *

ABSTRACT: Molecular dynamics (MD) simulations using the reactive force field (ReaxFF) have been performed to elucidate the underlying water-induced disruption mechanism of several prototypical interpenetrated MOFs (IRMOF-9, IRMOF-13, and SUMOF4). Through the comparison to the corresponding noninterpenetrated MOFs (IRMOF-10 and IRMOF-14), for both the interpenetrated and noninterpenetrated MOFs, structural collapse was always accompanied by the dissociation of the water molecules, with the produced OH− and H+ forming chemical bonds with the Zn2+ ion and O atom of the ligand, respectively. However, the water stability of the interpenetrated MOFs is less than that of the corresponding noninterpenetrated structures. The reasons for the differences between the MOFs in the resistance to water attack are clarified. The water resistance of the noninterpenetrated MOFs is mainly attributed to the strength of the Zn−Oligand, but, the hydrogen bond has little effect. However, a trade-off between the strength of the Zn−Oligand bond and the hydrogen bond determines the water stability of the interpenetrated MOFs. We expect that our understanding of the water-disruption mechanisms of MOFs will provide helpful guidance for the design of MOFs with a high water-resistance. Additionally, this work shows that ReaxFF simulations could be a useful technique for predicting the hydrothermal stability of MOFs. usually show high water stability;25 ③ installation of protective groups to protect the MOFs from hydrolysis, such as by using hydrophobic surfaces or water-repellant groups.26−28 Theoretically, the first molecular dynamics (MD) simulations18 based on empirical consistent valence force field (CVFF) showed that the replacement of one of the coordinating MOF O atoms by the oxygen of the water leads to the disruption of the IRMOF-1 framework. Using firstprinciples calculations, De Toni et al.19 also reported the liganddisplacement mechanism. However, due to the limitation of the CVFF, the associated hydrolysis reactions were ignored. Using a reactive force field (ReaxFF) to investigate the hydrolysis reactions of several MOFs, we revealed that the structural collapse of MOFs is mainly caused by the breakage of the Zn− ligand bond and water dissociation to further form a hydroxylated Zn cation and a protonated ligand.20 Likewise, Thonauser and co-workers29,30 elucidated that the OH and H products of water dissociation reaction significantly weakened the MOF-74 framework and led to the breakdown of crystal structure. They predicted the dissociation barrier of water and

1. INTRODUCTION Metal−organic frameworks (MOFs) are a new class of crystalline nanoporous materials formed by the self-assembly of metal cations and organic ligands. MOFs exhibit outstanding advantages of high surface area, controlled pore size and adjustable chemical environment, making them promising candidates for use in air separation and purification,1 chemical sensing,2 gas storage,3−6 catalysis,7−9 CO2 capture,10 and other applications. However, water stability remains a bottleneck for their industrial applications due to the ubiquitous presence of moisture in the air and the great difficulty of complete removal of H2O from the gas sources. For example, water vapor with a total volume of 5−7% is always present in the flue gas resulting from fossil fuel combustion.11,12 Therefore, it is highly important to investigate the interaction of water with MOFs and the hydrolysis mechanism of MOFs. Several experimental13−17 and theoretical16,18−23 studies have been conducted to investigate the interactions of MOFs with water molecules. Experimentally, the water stability of MOFs is mainly improved in the following three ways: ① modification of the linkers to exhibit a hydrophobic character, as found for example, in the case of FMOF-1;24 ② use of tri- or tetra-valent metal cations to increase the strength of the metal-linker bond, as seen in the MOFs containing Cr3+, Fe3+ or Zr4+, which © 2017 American Chemical Society

Received: January 21, 2017 Revised: March 14, 2017 Published: March 17, 2017 7312

DOI: 10.1021/acs.jpcc.7b00676 J. Phys. Chem. C 2017, 121, 7312−7318

Article

The Journal of Physical Chemistry C analyzed the effect of water clusters on the stability of framework. And several viable strategies for improving the stability of MOFs were proposed, such as removing the proton adsorption site29 or addition of helium gas.30 Bellarosa and coworkers21−23 used ab initio MD simulations to investigate the water disruption mechanism in various IRMOF-1 series. In particular, they reported that the fully hydrated Be variant of IRMOF-1 was more stable than the Zn or Mg variant; this finding was attributed to the differences in the flexibility of the M4O core and the strength of the M-O bond.21 The ability of Zn to form 5-fold coordination spheres and the increasing basicity of water when forming clusters are the main reasons for the displacement of the organic linker of IRMOF-1.22 By comparing the influence of the ligands on the disruption mechanism of IRMOF homologues, they showed that Brcontaining IRMOF-1 was more stable than its parent compound due to the diminishing cluster-IRMOF interactions and reduced the mobility of the water.23 The aforementioned MOFs have noninterpenetrated structures. However, in many MOF syntheses, long or narrow organic linkers usually lead to interpenetrated frameworks in which two separate frameworks self-assemble with each other.31,32 The interpenetration network structure has proven to be advantageous in selective guest capture,33 stepwise gas adsorption,34 photoluminescence control,35 guest-responsive porosity,36 and other applications. However, to date, the effect of interpenetration on the stability of MOFs in the presence of the water molecules has received little attention. Experimentally, Jasuja and Walton37 evaluated the effect of interpenetration in the framework and basicity of the pillar ligand on the water stability of MOFs. They showed that interpenetration in combination with ligands of relatively high basicity can lead to the MOFs showing good hydrothermal stability, and MOF-508 is one example. However, not all of interpenetrated MOFs have high framework stability, for example, the 2-fold interpenetrated DUT-30(Zn) does not lead to a significant level of water stability.38 To the best of our knowledge, theoretical studies on the hydrothermal stability of interpenetrated MOFs have not yet been reported. In this work, using MD simulations employing ReaxFF, we investigated the effect of interpenetration on the water stability of MOFs. The water-induced disruption mechanism for the interpenetrated MOFs was revealed and compared to that for the corresponding noninterpenetrated MOFs. The reasons underlying the differences in behavior between interpenetrated and noninterpenetrated MOFs were analyzed using the strengths of the metal−ligand bond and the hydrogen bond between the water molecule and Zn4O strut.

Figure 1. Structural units consisting of IRMOF-9, IRMOF-10, IRMOF-13, IRMOF-14, and SUMOF-4. Gray, white, red, and purple balls represent carbon, hydrogen, oxygen, and zinc atoms, respectively.

reactions in large nanoscale systems that cannot be adequately treated by classical force field.39−41 In contrast to other classical force fields, ReaxFF calculates the bond order directly from the instantaneous interatomic distances that are updated in every MD iteration and does not need any connectivity input for the chemical bonds. The ReaxFF parameters describing the systems of MOFs and water molecules used in this work were taken from ref 42 and reproduced the dissociation profiles for a water monolayer on ZnO surface obtained from DFT calculations. In order to validate the accuracy of the ReaxFF for simulations of the dynamics of the MOFs/H2O system, we considered a water dissociation reaction in Zn−MOF-74 and found that the reaction is exothermic by 21.0 kcal/mol, with an energy barrier of 22.5 kcal/mol, which is shown in Figure S1 of Supporting Information. These values are similar to the reported DFT values (energy difference: 23.1 kcal/mol and energy barrier: 25.1 kcal/mol).29 Using the DFT method, Thonhauser et al.30 reported that the energy barrier of water dissociation can be lowered by water clustering (one H2O, 25.1 kcal/mol, and two H2O, 22.1 kcal/mol). Our ReaxFF can also simulate this behavior, as confirmed by the result that the energy barrier for one water molecule is 22.5 kcal/mol; however, the barrier decreases to 15.2 kcal/mol by considering two water molecules. MD simulations were carried out under the isothermal− isobaric (NPT) ensemble to simulate the volume change of MOFs as a function of water content at 300 K and 1 atm. The MD simulations were performed with a parallelized ReaxFF in LAMMPS,43 where a time step of 0.001 ps was used and the simulations were performed up to 2,000 ps. We also tested an effect of the MD time on water stability of the MOFs and found that collapse of the MOF structures induced by hydrolysis reactions were observed within 500 ps, which indicates that the MD time of 2000 ps considered in this work is sufficient for our conclusion. During the simulations, lattice parameters (a, b, c) were changed independently, while, the lattice angles (α, β, γ) were fixed. To eliminate the boundary effects, an infinite threedimensional periodic simulation box was used with 1 × 1 × 1 unit cell for IRMOF-10, IRMOF-13 and IRMOF-14, 2 × 2 × 1 and 2 × 1 × 1 supercells for SUMOF-4 and IRMOF-9, respectively. The lattice parameters of the MOFs are summarized in Supporting Information. The initial structures including MOFs and H2O molecules were constructed using grand canonical Monte Carlo simulation with the universal force field (UFF)44 where the H2O molecules were randomly distributed in free volume of the MOFs.

2. MODELING AND COMPUTATIONAL METHODS The structural units consisting of IRMOF-9, IRMOF-10, IRMOF-13, IRMOF-14, and SUMOF-4 investigated in this work are shown in Figure 1. All these MOFs have the same inorganic secondary building unit [Zn4O(CO2)6] with different organic ligands, where IRMOF-9 and IRMOF-13 are the interpenetrated structures of IRMOF-10 and IRMOF-14 with biphenyl-4,4′-dicarboxylic (BPDC) and pyrene-2,7-dicarboxylate (PDC) linkers, respectively. SUMOF-4 is also an interpenetrated structure with mixed linkers of BPDC and benzene-1,4-dicarboxylic (BDC). ReaxFF was used to describe the dynamics of MOFs including the H2O molecules; this approach, has been demonstrated to be capable of accurately simulating chemical 7313


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The Journal of Physical Chemistry C

3. RESULTS AND DISCUSSION Using five different initial configurations for each MOF at a given water content, we simulated the volume change of MOFs as a function of water content at 300 K and 1 atm. For all the MOFs investigated in this work, the same trends were obtained regardless of the different initial configurations. The average volume changes of MOFs as a function of water content were shown in Figure 2. The frameworks of the interpenetrated

1.93 wt % H2O, respectively. Compared to IRMOF-9 and IRMOF-13, SUMOF-4 is relatively less sensitive to water based on the fact that the structure is maintained with up to 3.09 wt % H2O. However, it still has a lower resistance against the water molecules than the noninterpenetrated IRMOF-10. Accordingly, for MOFs with Zn4O struts considered in this work, interpenetration in the framework does not enhance the water stability of MOFs. To elucidate the disruption mechanism of the interpenetrated MOFs upon exposure to water, MD snapshots for the interpenetrated MOFs were thoroughly investigated and then compared with the noninterpenetrated structures. Figure 3 shows a water-disruption mechanism for IRMOF-9. Here, the water molecules are randomly distributed in the free volume of the MOF at the initial stage (Figure 3a). During the MD simulation, the two interpenetrated frameworks move close to each other due to the hydrogen bond interaction with the water molecules between each framework (Figure 3b), as will be discussed later. Then, the dissociation of a water molecule into OH− and H+ is observed (Figure 3c). The generated OH− forms a chemical bond to the Zn ion in the Zn4O strut of the MOF, and the H+ is bonded to the O atom of the organic ligand (BPDC). Subsequently, more water molecules are further dissociated, and the corresponding chemical bonds are formed. Eventually, the framework of IRMOF-9 collapses (Figure 3d). Similar phenomena were also observed in IRMOF13 and SUMOF-4. For the case of noninterpenetrated MOFs (Figure 4), the following phenomena are observed: water molecules are initially randomly distributed inside the IRMOF-14 (Figure 4a); after interacting with the Zn4O struts, the water molecules are dissociated into OH− and H+ and the produced OH− and

Figure 2. Simulated average volume change of IRMOF-9, IRMOF-10, IRMOF-13, IRMOF-14, and SUMOF-4 as a function of water content (wt %) at 300 K and 1 atm.

IRMOF-9 and IRMOF-13 start to break down at 2.64 and 1.55 wt % H 2O, showing lower water stability than their corresponding noninterpenetrated structures (IRMOF-10 and IRMOF-14) for which the lattice disruptions occur at 3.48 and

Figure 3. MD snapshots of IRMOF-9 with 12 H2O molecules (2.64 wt %) at (a) 0, (b) 25, (c) 200, and (d) 2000 ps. In parts a−c, blue and orange colors are used to highlight the interpenetration structure of the MOF. 7314


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Figure 4. MD snapshots of IRMOF-14 with 10 H2O molecules (1.93 wt %) at (a) 0, (b) 30, and (c) 2000 ps. Color codes of atoms are purple = Zn, red = O, gray = C, and white = H.

H+ are subsequently bonded to the Zn2+ and O atoms, respectively of PDC (Figure 4b); the framework is eventually disrupted (Figure 4c). A similar process is also observed for IRMOF-10. On the basis of the analysis above, it can be found that several common characteristics are observed in the interpenetrated and noninterpenetrated MOFs, that is, the destruction of the framework is accompanied by the dissociation of water molecules, with the OH− forming a chemical bond with Zn ion and the H+ protonating the O atom of the organic ligand. Therefore, the above processes can be described by the following equation:

Table 1. Density, Pore Volume, the Shortest Zn−Zn Distance, and the Average Hydrogen Bond Energy (EHB) between Water Molecule and ZnO4 Cluster in IRMOF-9, IRMOF-10, IRMOF-13, IRMOF-14, and SUMOF-4

Zn4O−ligand n + 2nH 2O

structure

density (g/cm3) 0.660.75

pore volume (cm3/g)

dZn−Zn (Å)

EHB (kcal/mol)

H2O content (wt %)

IRMOF-9 IRMOF-13 SUMOF-4 IRMOF-10 IRMOF-14

0.66 0.75 0.98 0.33 0.37

1.12 0.92 0.64 2.63 2.28

9.05 6.62 10.49 15.08 14.94

−2.75 −3.58 −3.53 −0.86 −1.75

2.64 1.55 3.09 3.48 1.93

distance) between the two adjacent Zn4O struts in the interpenetrated MOFs is much shorter than that in the noninterpenetrated MOFs. For example, the shortest Zn−Zn distances in IRMOF-9, IRMOF-13, and SUMOF-4 are 9.05, 6.62, and 10.49 Å, respectively, whereas the distances in IRMOF-10 and IRMOF-14 are 15.08 and 14.94 Å, respectively. Accordingly, the hydrogen bond interaction between the water molecule and O atoms of the Zn4 O clusters in the interpenetrated MOFs is stronger than in the noninterpenetrated MOFs, leading to a greater attack of the water molecules on the inorganic clusters in the interpenetrated MOFs. Indeed, our ReaxFF simulation reveals that the average hydrogen bond energies per water molecule are −2.75, −3.58, and −3.53 kcal/ mol for IRMOF-9, IRMOF-13, and SUMOF-4, respectively, much larger than the −0.86 and −1.75 kcal/mol for IRMOF-10 and IRMOF-14, respectively (Table 1). The stronger hydrogen

→ Zn4O(OH)2n + n(H 2−ligand)

This equation presents the main disruption mechanism for all MOFs investigated in this paper upon exposure to the water environment. However, it is worth mentioning that a space between two frameworks in the interpenetrated MOFs provides a much stronger water adsorption site than in the noninterpenetrated MOFs; this is mainly caused by the stronger hydrogen bond interaction between the water molecules and the frameworks. This behavior is the origin of the greater susceptibility of the interpenetrated MOFs to water attack relative to their corresponding noninterpenetrated MOFs. Compared to the noninterpenetrated MOFs, the interpenetrated MOFs show larger densities and smaller pore volumes, as seen in Table 1. Thus, the distance (Zn−Zn 7315


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Table 2. Average Hydrogen Binding Energies (kcal/mol) between H2O Molecules and ZnO4 Tetrahedrons and the Critical H2O Content (wt %) Leading to Structural Collapse in each MOFs as a Function of Zn−Oligand Bond Strengtha EZn−Oligand = 160 kcal/mol IRMOF-9 IRMOF-13 SUMOF-4 IRMOF-10 IRMOF-14

EZn−Oligand = 176 kcal/mol



EHB

H2O content

EHB

H2O content

EHB

H2O content

EHB

H2O content

−2.75 −3.58 −3.53 −0.86 −1.75

2.64 1.55 3.09 3.48 1.93

−8.21 −8.94 −5.17 −1.10 −1.39

1.34 0.78 1.05 3.48 2.31

−3.41 −4.69 −4.26 −1.03 −1.88

2.64 2.56 3.10 3.48 2.31

−3.55 −4.78 −4.70 −0.96 −1.61

3.48 3.30 3.34 3.90 3.05

a Here, the hydrogen binding energy corresponds to the total hydrogen binding energy of the system (interaction between H2O and Zn4O tetrahedron plus interaction between H2O molecules) divided by the number of water molecules, which is also time-averaged from 0 to 2000 ps. b Note that the original ReaxFF parameter for the EZn−Oligand optimized from first-principles calculations is 160 kcal/mol.42

disruption of the interpenetrated frameworks occurs at higher water contents, indicating that the metal−ligand bond can also affect the water stability of the interpenetrated MOFs to a certain extent. To summarize, the water stability of the noninterpenetrated MOFs is mainly determined by the strength of the metal−ligand bond, while the hydrogen bond has little effects on it. However, the water resistance of the interpenetrated frameworks is attributed to a delicate balance between the strength of the metal−ligand bond and the energy of hydrogen bond between water and the Zn4O cluster.

bond brings the water molecule closer to the Zn4O, leading to an easier opening up of the Zn−ligand bonds in the interpenetrated frameworks. Once the water molecule dissociates, the dissociated OH− and H+ ions quickly form chemical bonds to the Zn atoms in the Zn4O clusters and the O atoms in the organic ligand, respectively. Therefore, the stronger hydrogen bond can accelerate the breakage of the Zn−O bonds between the Zn4O struts and organic linkers. Our ReaxFF simulations show that all MOFs investigated in this work exhibit poor water resistance due to the relatively weak strength of the Zn−ligand (Zn−O) bond, which is a common weakness of the zinc-carboxylate based MOFs.14,45 ReaxFF-MD simulations of IRMOF-9, IRMOF-10, IRMOF-13, IRMOF-14 and SUMOF-4 including water molecules with different strengths of Zn−Oligand bond were performed to investigate the effect of the metal−Oligand bond strength on the hydrothermal stability of MOFs. Although the strength of the Zn−Oligand bond was arbitrarily increased by up to 30% of the default value of the Zn−Oligand bond (EZn−Oligand = 160 kcal/mol), the same bond-breaking and forming behaviors were observed. The volume changes of the MOFs as a function of water content at 300 K and 1 atm, simulated with different Zn−Oligand bond strengths (Figure S2 of the Supporting Information). Overall, the water stability of the noninterpenetrated IRMOF10 and IRMOF-14 is improved with increasing EZn−Oligand (Table 2). However, for the interpenetrated MOFs, their stability first decreases at EZn−Oligand = 176 kcal/mol (a 10% increase of the default value) and then increases with increasing EZn−Oligand. Table 2 also presents the average energies (EHB) of the hydrogen bond between the water molecule and the Zn4O strut. The change of the hydrogen bond energy as a function of the Zn−Oligand bond strength is much smaller in the noninterpenetrated MOFs than in the interpenetrated MOFs, indicating that the water resistance of the noninterpenetrated MOFs is mostly affected by the strength of the Zn−Oligand bond; however, the effect of the hydrogen bond energy is trivial. On the other hand, the hydrogen bond becomes a significant contributor to the water stability of the interpenetrated MOFs. When EZn−Oligand is 176 kcal/mol, the hydrogen bond energy of the interpenetrated MOFs is maximized and their stability is simultaneously weakest, even in comparison to the simulations with a higher EZn−Oligand. This indicates that during the structural collapse of the interpenetrated MOFs, the hydrogen bond energy plays a more important role than the metal-Oligand bond strength (EZn−Oligand). However, when the EZn−Oligand increases to 192 (20% increase) and 208 kcal/mol (30% increase), the

4. SUMMARY Using MD simulations employing ReaxFF, the water stability of several interpenetrated MOFs (IRMOF-9, IRMOF-13 and SUMOF-4) and noninterpenetrated structures (IRMOF-10 and IRMOF-14) has been investigated to elucidate the effect of interpenetration on the disruption mechanism of the MOFs exposed to water. The volume changes of the MOFs as a function of water content show that the water stability of the interpenetrated MOFs is less than that of the corresponding noninterpenetrated structures. Here, the interpenetrated and noninterpenetrated MOFs exhibit similar hydrolysis mechanisms: structural disruptions are accompanied by the dissociation of the water molecules into OH− and H+, and then the dissociated OH− and H+ form chemical bonds with the Zn2+ ion of the Zn4O struts and the O atom of the organic linker, respectively. However, a space between two frameworks in the interpenetrated MOFs provides a much stronger water adsorption site than in the noninterpenetrated MOFs, which is mainly caused by the stronger hydrogen bond interaction between the water molecules and the frameworks. This behavior is the main reason that the interpenetrated MOFs are more susceptible to water attack than their corresponding noninterpenetrated MOFs. In addition, the effect of the Zn− ligand bond strength on the water stability of the MOFs demonstrates that the water resistance of the noninterpenetrated MOFs is mainly due to the strength of the Zn−Oligand bond. However, a trade-off between the Zn−Oligand bond strength and the hydrogen bond determines the stability of the interpenetrated MOFs toward water molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00676. Additional results on effects of the metal−ligand bond strength on the water stability of MOFs and validations of ReaxFF parameters (PDF) 7316


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AUTHOR INFORMATION

Corresponding Author

*(S.S.H.) Telephone: +82 2 958 5441. Fax: +82 2 958 5451. Email: [email protected]. ORCID

Sang Soo Han: 0000-0002-7925-8105 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.Y.L. is grateful to the National Natural Science Foundation of China (Grant No. 11304079), the China National Scholarship Foundation (Grant No. 201508410255), the Fundamental Research Funds for the Henan Provincial Colleges and Universities in Henan University of Technology (Grant No. 2014YWQQ21), and the Foundation for Young Core Teachers of Higher Education Institutions of Henan Province of China (Grant No. HNQG001316). S.S.H. is grateful to the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF-2016M3D1A1021140) and the KIST institutional project (No. 2E26130).

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ReaxFF Molecular Dynamics Simulations of Water ... - ACS Publications

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