with Different Anions

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Investigation of the Water Adsorption Properties and Structural Stability of MIL-100(Fe) with Different Anions Yen-Ru Chen, Kai-Hsin Liou, Dun-Yen Kang, Jiun-Jen Chen, and Li-Chiang Lin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04399 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Investigation of the Water Adsorption Properties and Structural Stability of MIL-100(Fe) with Different Anions Yen-Ru Chen,1 Kai-Hsin Liou,1 Dun-Yen Kang,1 Jiun-Jen Chen,2,* and Li-Chiang Lin3,* 1

Department of Chemical Engineering, National Taiwan University

No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan

2

Green Energy and Environment Research Laboratories, Industrial Technology Research

Institute No. 195, Sec. 4, Chung Hsing Rd., Chutung, Hsinchu 31040, Taiwan

3

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State

University 151 W. Woodruff Avenue, Columbus, OH 43210, United States

*

Corresponding

authors:

J.-J.

Chen

([email protected]);

L.-C.

Lin

([email protected])

ACS Paragon Plus Environment

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Abstract Investigating metal-organic frameworks (MOFs) as water adsorbents has drawn increasing attention for their potential in energy-related applications such as water production and heat transformation. A specific MOF, MIL-100(Fe), is of particular interest for its large adsorption capacity with the occurrence of water condensation at a relatively low partial pressure. In the synthesis of MIL-100(Fe), depending on the reactants, structures with varying anion terminals (e.g., F-, Cl-, or OH-) on the metal trimer have been reported. In this study, we employed molecular simulations and density functional theory calculations for investigating the water adsorption behaviors and the relative structural stability of MIL-100(Fe) with different anions. We also proposed a possible defective structure and explored its water adsorption properties. The results of this study are in good agreement with the experimental measurements and are in support of the observations reported in the literature. Understandings toward the spatial configurations and energetics of water molecules in these materials have also shed light on their adsorption mechanism at the atomic level.

Keywords: Water vapor adsorption, Metal-organic frameworks, Structural stability, Monte Carlo simulations, Density functional theory calculations


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Introduction Metal-organic

frameworks

(MOFs)

are

an

emerging

class

of

crystalline

microporous/mesoporous materials, composed of metal clusters and organic ligands. MOFs possess a variety of properties such as high porosity, large surface area, and diverse functionality, thus providing opportunities in fields including, but not limited to, gas separations,1-5 catalysis,6-9 drug delivery,10-14 gas storage,15-16 and adsorption refrigeration.1721

In recent years, using MOFs as water adsorbents has drawn increasing attention. MOFs

have been demonstrated for their promise in dehumidification22 and fresh water production.23 As examples, Seo et al. showed that, in terms of adsorption capacity and kinetics, MIL100(Fe) and MIL-101(Cr) can outperform conventional water sorbents such as SAPO-34, NaX, and silica gel.22 Furukawa et al. demonstrated that MOF-801 possesses a high water uptake at a low humidity of P/P0=0.1 and can be regenerated under mild conditions.24 Such properties make MOF-801 a promising material for the production of fresh water from ambient air. The large adsorption capacity at a low relative pressure of MOFs also makes them potential adsorbents in heat pumps for more efficient heat transformation.20-21

Among MOFs reported to date as water adsorbents, MIL-100(Fe) is of particular interest.17,2526

The water adsorption capacity of as-synthesized MIL-100(Fe) can reach as much as 0.75 g-

water/g-MOF at 298 K,17 which transcends many other existing MOF materials (e.g., ZIF-8 of 0.01 g/g at 308 K and HKUST-1 of 0.60 g/g at 298 K).27 MIL-100(Fe) consists of iron(III) and 1,3,5-bezenetricarboxylic (1,3,5-BTC). Its atomic structure can be seen in Figure 1, which has two distinct types of cages with diameters of 2.5 nm and 2.9 nm.28-29 It has a structure formula of FeIII3O(H2O)2X(C6H3(CO2)3)2•nH2O, where X can be F-, or OH-,30 depending on the synthesis conditions. The high water adsorption capacity of MIL-100(Fe) may be attributed to its large surface area (BET surface area is as high as 2300 m2 g-1)31 and


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open-metal sites that strongly coordinate with water molecules.31 In addition to the large capacity, the capillary condensation of water in the framework (i.e., a sharp increase in the adsorption isotherm due to the clustering of water molecules) occurs at a low relative pressure P/P0 of 0.25-0.4, a range suitable for heat transformation applications.17,20

The first experimental synthesis of MIL-100(Fe) reported in the literature involves the use of hydrofluoric acid (HF),29 a highly toxic chemical.30 Several modified synthesis approaches have been therefore developed to avoid using hydrofluoric acid.9,30,32-34 Using the so-called HF-free syntheses, no evident change in the structure of MIL-100(Fe) was found, except that fluorine at the terminal position of the metal cluster (or so-called metal trimer) is replaced by another anion.30 As noted above, hydroxide may replace the terminal fluoride in MIL-100(Fe) obtained from an HF-free synthesis.35 Although MIL-100(Fe) structures with hydroxide have been observed using HF-free conditions, we expect that MIL-100(Fe) with Cl- might be possibly obtained as chloride ions are also commonly available in HF-free syntheses. Accordingly, in this study, we considered MIL-100(Fe) structures with three different anions (F-, Cl-, and OH-). For convenience, these structures were denoted hereafter as MIL100(Fe)_F, MIL-100(Fe)_Cl, and MIL-100(Fe)_OH, respectively.

Although no apparent structural change has been identified, the anions in MIL-100(Fe) play an important role in its stability and crystallinity. The fluoride-functionalized terminal appears to help stabilize the formation of trimeric inorganic building units in MIL-100(Fe), which leads to more effective crystallization compared to other anions.25 The better crystallinity of MIL-100(Fe)_F also results in a high cyclic stability for water adsorption.17,36 Compared to MIL-100(Fe)_F, MIL-100(Fe)_OH has been found to possess a smaller particle


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size and relatively poorly-defined crystal morphology.30 It has also been reported that this structure suffers from a low structural stability in aqueous phase.37

Although MIL-100(Fe)_F has been widely explored both experimentally and computationally for its potential as water adsorbents,17,38 the atomic-level understandings toward the influence of alternative anions in MIL-100(Fe) on their water adsorption properties and structural stability have yet been achieved. In this work, molecular simulations and density functional theory (DFT) calculations were employed to systematically investigate MIL-100(Fe)_F, MIL-100(Fe)_Cl, and MIL-100(Fe)_OH. Monte Carlo simulations in a grand canonical ensemble (GCMC) or canonical ensemble (NVT) were carried out to simulate their water adsorption isotherms as well as to understand the spatial distribution and energetics of water molecules adsorbed in the frameworks. The ground state energy of each structure was also computed using DFT to reveal the relative structural stability. Furthermore, we investigated the water adsorption properties of a possible MIL-100(Fe) with defects.


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Computational Details Structures. The structures used in this work (see Figure 1) were adopted from the Cambridge Crystallographic Data Centre (CCDC: 640536)29 with some modifications. First, we removed residual water molecules inside the pores. Next, the unit cell was converted into a primitive cell in order to reduce the computational cost. We then attached F-, Cl-, or OH- ions onto the metal trimer, leading to the so-called MIL-100(Fe)_F, MIL-100(Fe)_Cl, or MIL-100(Fe)_OH structures, respectively. Note that each trimer only has one anion for the overall charge neutrality of the structures. These three structures (containing approximately 2800 atoms per primitive unit cell) were subsequently relaxed by a cascade of the steepest descent, adjusted basis set Newton-Raphson, and quasi-Newton methods. The structural relaxation was performed using the Forcite module in the commercial Materials Studio® with the DREIDING39 force field. The convergence criteria for energy, force, and displacement were set to 2×10-5 kcal mol-1, 0.001 kcal mol-1 Å-1, and 0.001 Å, respectively. The atomic coordinates of these three structures can be found in the Supporting Information.

Adsorption Isotherms. Monte Carlo simulations in a grand canonical ensemble (GCMC), implemented in an open-source package RASPA 2.0,40 were carried out to compute the adsorption isotherms of water in these three MIL-100(Fe) structures. The temperature of adsorption was set to 300 K. Intermolecular potentials were described using both van der Waals interactions (6-12 Lennard-Jones potential) and long-range Coulombic interactions. The Lennard-Jones parameters of the framework atoms were adopted from the DREIDING39 force field, while their partial charges were assigned by the extended charge equilibrium method (EQeq).41 Water molecules were modeled using the widely used 4-site TIP4P-Ew42 model. The Lorenz-Berthelot mixing rule was applied to determine the Lennard-Jones parameters between two dissimilar atoms. The pair-wise Lennard-Jones potential was shifted


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and truncated at a cut-off radius of 12 Å without tail corrections, while the long-range electrostatic interactions were calculated by the Ewald summation technique with a precision of 1×10-6. A total of approximately 110 million Monte Carlo attempts including random insertion, deletion, reinsertion, rotation, and translation of water molecules were carried out in each of these calculations. The relative ratio of these moves was set respectively to 4: 4: 4: 1: 1. 100 million moves were conducted firstly to ensure that equilibrium had been achieved, and the loading reached approximately a constant after a certain number of Monte Carlo attempts during the system initialization. Subsequently, additional 10 million Monte Carlo steps were conducted in each calculation to calculate the average loading.

Adsorption Distribution and Energetics. To investigate the spatial distribution of adsorbed water molecules within the framework as well as the corresponding adsorption energetics at a specific loading, Monte Carlo simulations in a canonical ensemble (NVT) at 300 K were conducted. The detailed settings of the NVT simulations were similar to that of GCMC calculations but in the absence of random insertion and deletion moves. NVT simulations were also used to compute the binding geometry of a single water molecule in MIL-100(Fe). In these calculations, we performed simulations at a low temperature of 5 K, ensuring the adsorbed phase closes to the lowest energy state. Five replicas of the NVT calculations for each structure were carried out with different initial configurations, and tens of millions Monte Carlo moves were opted in each of the replica. The configuration with the lowest energy was identified as the binding geometry.

Structural Stability. We used the ground state energies of MIL-100(Fe) structures with varying anions to probe their relative structural stability. Considering the size of MIL100(Fe) structures (i.e., ~2800 atoms in the primitive unit cell), it is computationally


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prohibited to compute the ground state energy of these structures using DFT. We therefore constructed small clusters, comprising of one metal trimer and six ligands, as shown in Figure 2 to represent the corresponding full periodic structures. To maintain the neutrality of the cluster and to reduce computational costs, two carboxyl groups in the 1,3,5-BTC linker were replaced by hydrogen atoms. The ground state energy of these clusters was calculated using DFT implemented in the CASTEP43 module of Materials Studio®. Plane-wave periodic DFT that involves few convergence parameters (e.g., cutoff energy and number of k-points) was adopted, and many pioneering efforts have been made for the program optimization.44-45 With a reasonable vacuum space, periodic DFT could be an effective way to study non-periodic cluster systems.46 In these calculations, the generalized gradient approximation (GGA) in the scheme of Perdew–Burke–Ernzerhof (PBE)47 was opted as the exchange-correlation functional without applying additional dispersion corrections. A plane wave energy cutoff of 340.0 eV and the Monkhorst–Pack k-point 1×1×1 mesh48 were used. At least 7 Å of vacuum space was included to minimize the interaction between periodic images. The convergence criteria for energy, force, and displacement were 1×10-5 eV/atom, 0.03 eV Å-1, and 0.001 Å, respectively. In this study, in order to make a meaningful comparison of the computed ground state energies of these clusters that possess different chemical compositions, we further assumed the following two reactions:

MIL-100(Fe)_F + HCl ⇌ MIL-100(Fe)_Cl + HF

Eq. (1)

MIL-100(Fe)_F + H2O ⇌ MIL-100(Fe)_OH + HF

Eq. (2)

From Eq. (1), we defined the relative energy difference between MIL-100(Fe)_F and MIL100(Fe)_Cl to be represented by the difference between E(MIL-100(Fe)_F) – E(HF) and E(MIL-100(Fe)_Cl) – E(HCl). Similarly, from Eq. (2), the energy difference between MIL-


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100(Fe)_F and MIL-100(Fe)_OH can be quantified by the difference between E(MIL100(Fe)_F) – E(HF) and E(MIL-100(Fe)_OH) – E(H2O). E(HF), E(HCl), and E(H2O) are respectively the ground state energies of gas phase HF, HCl, and H2O molecules. These energies were computed using exactly the same level of theory and the same simulation dimension as that adopted in the cluster calculations. All of the above calculations were carried out in the gas phase instead of in a solvent-mediated environment such as under the synthesis condition and/or the condition when the material is filled (or partially filled) with water molecules. Nonetheless, given all of the studied structures are very similar, the gas phase assumption should serve as a decent first-order approximation. Further, as discussed below, our calculation results are in good agreement with those reported experimentally.


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Results and Discussion Water Adsorption. Water adsorption isotherms in MIL-100(Fe)_F, MIL-100(Fe)_Cl, and MIL-100(Fe)_OH at 300 K computed by GCMC simulations are summarized in Figure 3. Isotherms are presented as a function of the relative pressure (P/P0) with P0 to be 3750 Pa at a room temperature according to TIP4P-Ew water model.49 The calculated water isotherm of MIL-100(Fe)_F is in agreement with the experimental ones.17,25 Compared to a previous computational study,38 our isotherm resembles the experimental data more accurately, which may be attributed to the adopted water models. In this work, the 4-site TIP4P-Ew model was adopted, whereas the 3-site SPC/E water model was opted in the previous work. It has been shown in our previous study that the TIP4P-Ew model may better describe water adsorption properties,50 and it has also been widely adopted in the calculations of adsorption in nanoporous materials.51-53 Additionally, Figure 3 shows that the isotherm of MIL-100(Fe)_F possesses a plateau at an uptake of approximately 0.4 g-water/g-MOF (i.e., corresponding to P/P0 of 0.40-0.53 in the simulated isotherm), and this interesting phenomenon can be found in both experimental and simulation isotherms. It has been hypothesized that this may be caused by the difference in water adsorption characteristics between two distinct cages of MIL-100(Fe).17,25 As illustrated in Figure 1, two sizes of cages (2.5 and 2.9 nm) are present in MIL-100(Fe). At a low pressure, water molecules may preferentially adsorb in the small cages (2.5 nm) due to the more favorable interactions between water and the framework until saturation. Thereafter the adsorption in the large cages (2.9 nm) occurs, thus leading to the observed plateau. To validate this hypothesis, snapshots of the water configurations adsorbed in MIL-100(Fe)_F at various pressures are taken from our simulations and shown in Figure 4. For a clear presentation, the MIL-100(Fe) framework is shown as sticks with green and orange colors (represent small and large cage structures, respectively), while the space-filling style was used for water molecules. These atomistic configurations clearly indicate that at a


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low pressure (P/P0 < 0.27), water molecules indeed first occupied the small cages. The condensation of water in the small cages occurred at a P/P0 between 0.27 and 0.40. At a P/P0 of 0.40, Figure 4 shows that the small cages became completely filled with a density of water determined to be around 0.9 g/cm3, whereas nearly no water molecule was adsorbed in the large cages. This preferential adsorption in different types of cages observed in our simulations also agrees with previous findings in related structures.23,25-26,54

The water adsorption capability in MIL-100(Fe) with different anions (Figure 3) demonstrates that the water uptake in both MIL-100(Fe)_Cl and MIL-100(Fe)_OH at 300 K is substantially lower than that of MIL-100(Fe)_F although these three structures possess a nearly identical saturation uptake due to their similar pore structures. Specifically, the sharp increase in the adsorption isotherm of both MIL-100(Fe)_Cl and MIL-100(Fe)_OH occurs at a notably higher pressure, thus suggesting a weaker water-framework interaction was involved in these two structures. As a consequence, different from MIL-100(Fe)_F, the adsorption isotherms of both MIL-100(Fe)_Cl and MIL-100(Fe)_OH do not possess a plateau. Because of their relatively weaker water-framework interactions, the difference in the adsorption characteristics between two types of cages becomes marginal. Additionally, this also indicates that both MIL-100(Fe)_Cl and MIL-100(Fe)_OH may be relatively less promising adsorbent candidates for applications such as water harvesting and heat transformation.

Adsorption Energetics and Binding Geometries. To gain insights into the adsorption energetics of water in MIL-100(Fe) with different terminal anions, the guest-host and guestguest (i.e., guest: water molecules; host: the framework) interaction energies were calculated using NVT simulations (see Figure 5). As discussed above, MIL-100(Fe)_F was indeed


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identified to possess the strongest host-guest interaction among the three MOF structures (Figure 5a) in the low-loading region (below 0.04 g-water/g-MOF). Compared to MIL100(Fe)_Cl and MIL-100(Fe)_OH, the host-guest interaction energy for MIL-100(Fe)_F is lower by as much as 10 kJ/mol. The strong host-guest interaction in MIL-100(Fe)_F explains its significantly higher water uptake in the low-pressure region. This is also reflected on the absence of the sharp uptake increase in both MIL-100(Fe)_Cl and MIL-100(Fe)_OH at a low relative pressure, as the guest-host interaction has been known to facilitate the occurrence of water condensation in confined environments. At a higher loading (> 0.2 g-water/g-MOF), the guest-host interaction energies among the three MOF structures become comparable. Under this condition, most water molecules are anticipated to be at a distance from the framework surface, thus making the overall difference in host-guest interaction become relatively insignificant. For this, we have computed the accumulative number of water adsorbed in the two distinct cage types of the MIL-100(Fe)_F structure as a function of the distance from the center of the cages (see the Supporting Information Figures S1 and S2 for the adsorption in the large and small cages, respectively). For water adsorption in the small cages as an example, at a low loading (0.043 g/g) water molecules are preferentially adsorbed at a distance of 14-17 Å away from the center of the cages (i.e., near the cage surface), as suggested in Figure S1. As noted above, this confirms that the host-guest interaction should play a critical role in adsorptive behaviors in this low-pressure region. By contrast, at a high loading (0.945 g/g), our results show that most of the water are located near the center (within 14 Å of the center of the cage). Specifically, approximately 80% of the total water molecules are adsorbed away from the cage surface and therefore, guest-guest interactions should dominate the total energies. Interestingly, our analysis shown in Figures S1 and S2 also indicates that the density of adsorbed water in both cages can be notably different. It was found that the adsorbed amount of water molecules in the small cages is greater than that in


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the large cages, leading to a denser adsorbed phase in the small cages. In agreement with a recent study by Choi et al., the density of adsorbed phase may differ drastically, which depends on the pore structure and surface chemistry.55 Furthermore, for the guest-guest interactions shown in Figure 5b, it was found that water molecules in MIL-100(Fe)_F have the least favorable interactions (i.e., highest energy values). This may be attributed to the weaker guest-host interaction found in MIL-100(Fe)_Cl and MIL-100(Fe)_OH, thus allowing the agglomeration of water molecules with a more favorable configuration. In other words, the less constrain imposed by the host-guest interactions may lead to more favorable guestguest contributions.

Figure 5a shows that there is an interesting local maximum energy in the guest-host interaction at the loading of approximately 0.1 g-water/g-MOF for all of the three structures, and a corresponding local minimum can be seen in the guest-guest interaction (Figure 5b). This behavior indicates a possible configuration change for water molecules adsorbed in the framework. At a low loading condition (less than 0.05 g/g, water molecules are dispersedly adsorbed in the framework), these water molecules are located in a way to maximize its interaction with the framework. When the concentration of adsorbed water increases, a change in the configuration for those initially adsorbed water molecules may be needed in order to gain strong water-water interactions and subsequently lower the overall energy,

The binding geometry and distance of water in MIL-100(Fe)_F, MIL-100(Fe)_Cl, and MIL100(Fe)_OH determined by NVT calculations at a low temperature are summarized in Figure 6. In all of the three MIL-100(Fe) structures, the water binding configuration was found to be located between an open-metal site and an anion terminal. In particular, one of the water hydrogen atoms was in a close proximity to the anion terminal while the oxygen atom was


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located near the open-metal site. These results appear reasonable as, from the classical force field point of view, the negatively charged anion can help stabilize the positively charged hydrogen atoms of water. Similarly, the same effect exists between the negatively charged oxygen atom of water and the positively charged open-metal site. The hydrogen (H2O)-anion distance is shorter in MIL-100(Fe)_F (1.87 Å) relative to that in MIL-100(Fe)_OH (2.19 Å) and MIL-100(Fe)_Cl (2.28 Å), demonstrating that F- possesses the strongest interaction with water. The oxygen (H2O)-metal site distance was found to be correspondingly larger in MIL100(Fe)_F (3.49 Å) relative to that in MIL-100(Fe)_OH (3.22 Å) and MIL-100(Fe)_Cl (3.17 Å).

Structural Stability and Defective MIL-100(Fe). To understand the structural stability of MIL-100(Fe)_F, MIL-100(Fe)_Cl, and MIL-100(Fe)_OH, the ground state energy of their representative clusters as introduced previously were calculated and compared. The relative energy can serve as a proxy to indicate the stability of the basic building unit in MIL-100(Fe). As shown in Figure 7, MIL-100(Fe)_F was found to be more stable than MIL-100(Fe)_OH by approximately 0.8 eV. This value is deemed as significant, which is far beyond the thermal energy of 0.025 eV at the room temperature. This finding supports the observation reported in the literature that MIL-100(Fe) synthesized in the absence of HF (i.e., structures with hydroxide) tends to be notably less stable.30,37 Interestingly, MIL-100(Fe)_Cl was identified to be potentially more stable than MIL-100(Fe)_F. However, we should note that our energy calculations do not consider the presence of adsorbed molecules. For instance, upon water vapor adsorption, the stronger interactions between H2O and metal trimers with Frelative to that between H2O and trimers with other anions may also influence the structural stability. Additionally, although MIL-100(Fe)_Cl is predicted to be more thermodynamically stable than MIL-100(Fe)_OH, the fact that MIL-100(Fe)_Cl structures were not observed


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experimentally in HF-free conditions suggests that solvent molecules involved in the synthesis may also play a role. Nonetheless, our calculations have, for the first time, shed light on the relative structural stability of MIL-100(Fe) with varying anions.

As noted above, based on our DFT calculations, the metal trimer cluster of MIL-100(Fe)_OH was identified to be notably less stable compared to that of MIL-100(Fe)_F and MIL100(Fe)_Cl. It is therefore expected that MIL-100(Fe)_OH may be potentially more prone to structural defects. In this study, considering its relatively unstable trimer clusters, we proposed a defective MIL-100(Fe)_OH structure that has metal-node missing defects (see Figure 8 for illustration) with a density of one missing metal node out of 68 nodes in the primitive cell of MOF-100(Fe). The atomic coordinates of the defect structure can be found in the Supporting Information. Similar types of defects have been also studied in another well-studied MOF, UiO66.56-58 Same as the analysis carried out for defect-free frameworks, we have also computed the adsorption isotherms using GCMC simulations in this metal-node missing structure. Figure 9 shows that the uptake in the defective MIL-100(Fe)_OH structure was predicted to be lower than that of the corresponding defect-free one. Specifically, the uptake of water in the defective MIL-100(Fe)_OH structure is nearly zero even at a relative pressure of approximately 1.0. In the absence of the metal nodes, the interactions between adsorbed water molecules and the framework are anticipated to become overall weaker. As shown in Supporting Information Figure S3, the host-guest interaction is weaker in the MIL100(Fe)_OH structure with metal-node missing defects, in particular in the low-pressure region. Given the host-guest interactions at a low loading can facilitate the condensation of water adsorbed in the framework, such defective structure accordingly shows a lower adsorption ability. Our calculations also infer that MIL-100(Fe)_OH may have a relatively poor cyclic stability with a reduction in adsorption ability, which is in good agreement with previously reported experimental results that MIL-100(Fe) synthesized in absence of HF


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suffered from a significant reduction in adsorption ability after aqueous water treatment.37 We should note that it remains unknown whether the proposed defective structure is stable in the presence of water. The defective structure is possibly an intermediate structure before the complete degradation of the materials. A detailed investigation of the structural degradation can be an important subject of future studies.


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Conclusion We employed Monte Carlo techniques (GCMC and NVT simulations) and density functional theory (DFT) calculations to investigate the water adsorption behaviors and relative structural stability of MIL-100(Fe) with different terminal anions (F-, Cl-, or OH-). Our calculated adsorption isotherm in MIL-100(Fe)_F is in agreement with that measured experimentally, and both simulation and experiment isotherms show an obvious plateau. By visualizing the atomic-level adsorption configurations at varying pressures, it was found that the plateau can be attributed to the difference in adsorption characteristics of two distinct types of cages in MIL-100(Fe). Small cages, which provide a stronger interaction to water molecules compared to large cages, were found to be completely filled with water at a notably lower pressure. Amongst these three structures, due to the strongest interaction between the terminal F- anion and water molecules, the adsorption isotherm in MIL-100(Fe)_F has the largest adsorption capability with the occurrence of water condensation at a much smaller relative pressure. From the ground state energy calculations by DFT, MIL-100(Fe)_F was identified to be significantly more stable than MIL-100(Fe)_OH. This result agrees with the experimental observations where MIL-100(Fe)_F was demonstrated to have higher crystallinity with better defined morphology. Considering the relatively poor stability of MIL-100(Fe)_OH, a possible defective MIL-100(Fe)_OH structure (i.e., metal-node missing defects) was proposed and studied. This proposed defective structure appears to have a lower water uptake compared to the defect-free one, inferring that these less stable materials are prone to a poor cyclic stability in adsorption. Overall, the outcomes of this study are in support of the experimental findings reported in the literature, while the atomistic understandings achieved have shed light on the water adsorption mechanism in MIL-100(Fe) with different anions.


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Figure 1. Illustration of the cage structure (left) and the building unit (right) of MIL-100(Fe). Three different anions are considered in this study: F-, Cl-, or OH-.


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Figure 2. Illustration of the cluster model used for the ground state energy calculations.


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Figure 3. GCMC-calculated and experiment-measured17,25 water adsorption isotherms at 300 K. The adsorption isotherm for the anhydrous framework of MIL-100(Fe)_F from a computational study is also included for comparison.38


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Figure 4. (a) Illustration of two distinct types of cages in MIL-100(Fe). (b) Snapshots of water configurations adsorbed in MIL-100(Fe)_F at various relative pressures, P/P0, ranging from 0.27 to 0.8.


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Host-Guest Energy (kJ/mol)

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Loading (g/g) Figure 5. (a) Host(framework)-guest(water), (b) guest-guest, and (c) total interaction energies as a function of the water uptake in MIL-100(Fe)_F, MIL-100(Fe)_Cl, and MIL100(Fe)_OH.


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Figure 6. The binding configurations of water molecules and the corresponding distances of H(H2O)-anions(framework) and O(H2O)-Fe(framework) in (a) MIL-100(Fe)_F, (b) MIL100(Fe)_Cl, and (c) MIL-100(Fe)_OH.


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Figure 7. The relative energies of MIL-100(Fe) clusters with different terminal anions (i.e., F-, Cl-, and OH-).


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Figure 8. Illustration of (a) defect-free MIL-100(Fe) and (b) metal-node missing MIL-100(Fe) structures. Note that hydrogen atoms are not displayed in this figure.


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Figure 9. The GCMC-calculated water adsorption isotherm of the metal-node missing MIL100(Fe)_OH structure.


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Associated Content Supporting Information Available. The supporting information includes additional figures referred in the main article (i.e., accumulative number of water adsorbed in MIL-100(Fe)_F and the host-guest interaction energy of water in the defective MIL-100(Fe)_OH structure) as well as the structural information of all structures studied in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Author Contributions This study was developed and completed through contributions by all authors.

Acknowledgement This work was supported by the Bureau of Energy of the Ministry of Economic Affairs, Taiwan. The authors also gratefully thank Ohio Supercomputer Center (OSC) for computational resources.59


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