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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Exploring the Potential of Defective UiO-66 as Reverse Osmosis Membranes for Desalination Qiang Lyu, Xuepeng Deng, Songqing Hu, Li-Chiang Lin, and W.S. Winston Ho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01765 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 7, 2019

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The Journal of Physical Chemistry

Exploring the Potential of Defective UiO-66 as Reverse Osmosis Membranes for Desalination Qiang Lyu†‡, Xuepeng Deng‡, Songqing Hu*†¶, Li-Chiang Lin*‡, and W.S. Winston Ho‡§ †School

of Materials Science and Engineering, China University of Petroleum (East

China), Qingdao, Shandong 266580, China ‡William

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

State University, Columbus, Ohio 43210, United States ¶Institute

of Advanced Materials, China University of Petroleum (East China), Qingdao,

Shandong 266580, China §Department

of Materials Science and Engineering, The Ohio State University, Columbus,

Ohio 43210, United States

*Corresponding authors: Songqing Hu ([email protected]); Li-Chiang Lin: ([email protected])

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Defects in metal-organic frameworks (MOFs) can play an important role in the development of MOFs as promising reverse osmosis (RO) membranes for water desalination. By employing molecular dynamics techniques, we explore the effects of experimentally relevant defects in UiO-66 on their desalination performance. Different defect types with varying densities and chemical compensations are studied. Our results show that defective membranes can possess substantially improved water permeability and enhanced water intrusion rate by orders of magnitude compared to defect-free one while still maintaining excellent ability to reject salts. Further, the relationship between adsorption energetics and transport kinetics of water is established to shed light on the permeation behaviors of MOF membranes at an atomic scale. The outcomes of this work suggest that controlling structural defects provides opportunities toward the optimization of MOFs as RO membranes for reduced energy and cost requirements in desalination.


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1. INTRODUCTION As clean water has become increasingly limited, its production from unconventional sources is essential. Water desalination via reverse osmosis (RO) to produce drinkable water from abundant saline sources could represent a promising solution to water scarcity, but a major bottleneck toward its large-scale deployment is the high associated energy and cost requirements.1 These requirements are directly related to the intrinsic properties of RO membranes.2,3 To enable more affordable desalinated water, developing new RO membranes that can offer substantially enhanced water permeability while remaining excellent ability to reject salts is therefore of utmost importance.4 Metal-organic frameworks (MOFs), crystalline structures composed of metal clusters and organic linkers, have drawn considerable attention for their potential in a variety of applications.5–7 The well-defined pore structure and highly tunable nature of MOFs have also made them promising candidates as RO membranes for water desalination. Although some MOFs degrade in the presence of humid conditions, MOFs synthesized by metals of higher valance charges and organic linkers of a higher acidity (i.e., lower pKa) can be stable under aqueous conditions, owing to the strong bonding between the metals and the linkers.8–11 The first structure reported in this water-stable sub-class is UiO-66 having zirconium (Zr) clusters (i.e., ZrO4(OH)4) and each of which is connected by twelve benzene-1,4-dicarboxylate (BDC) linkers (see Figure 1a).12 Recently, a continuous polycrystalline

UiO-66

membrane

has

been

synthesized

and

experimentally

demonstrated for its potential in water filtration applications,13,14 and studies have also reported various techniques, including mechanical and chemical exfoliation,15,16 interfacial synthesis,17 surfactant-assisted synthesis,18 vapor-assisted conversion (VAC)



method,19 etc., to achieve 2D ultrathin-film MOF nanosheets,20 which allow MOF membranes to have ultra-short diffusion pathways for fast permeation.

Figure 1. Illustration of (a) defect-free and (b−c) defective UiO-66 structures. Defective structures shown in (b) and (c) contain two missing linkers and one missing cluster per unit cell, respectively. (d−h) Schemes for compensating the unsaturated metal sites introduced by the surface cleavage and defects, which respectively show one defect center (i.e., two unsaturated zirconium atoms) coordinated with trifluoroacetate (CF3COO–), acetate (CH3COO–), formate (HCOO–), hydroxide/water (OH–/H2O), and chloride/water (Cl–/H2O)). All structures are represented as ball and stick with color codes of carbon: grey, hydrogen: white, oxygen: red, zirconium: cyan, chlorine: green, and fluorine: magenta.

While MOFs can be designed via the choices of metal clusters and organic linkers, exploiting their structural defects that prevalently exist in MOFs (i.e., defective MOFs) should also be a key component in the development of MOF membranes. Defective MOFs in this work refer to structures that are based on a specific MOF of a given combination of metal clusters and organic linkers with missing linkers and/or missing


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clusters (see below for more details). That is, defective MOFs are structures based on their parent frameworks but possessing imperfections. Defects have been demonstrated to fundamentally change a variety of MOFs’ properties and have drawn increasing attention in the past few years.21–26 For instance, MOFs can contain defects to a high density,23,27,28 making them notably hydrophilic for strong water adsorption and as Lewisacid catalysis.22,25,29,30 Defective MOFs have recently also been incorporated into polymer matrix for mixed matrix membranes with improved separation performance.31 Using UiO66 as an example, modulators such as monocarboxylic acid (e.g., formic acid, trifluoroacetic acid, etc.) or concentrated hydrochloric acid (HCl) are added in the synthesis to achieve a more controlled crystal growth.32 Monocarboxylic acids act primarily as monodentate modulators to compete with bidentate BDC linkers for coordinating Zr clusters and therefore decelerate the crystal nucleation to assist in forming large crystal particle. During such process, inherent defects form due to the incomplete exchange of the pre-loaded modulators.23,33 On the other hand, the addition of concentrated HCl can facilitate in situ hydrolysis of ZrCl4 into ZrO4(OH)4, thus accelerating the crystallization kinetics. This however also promotes the formation of defects caused by either misconnections or dislocations.34,35 Both cases typically yield two defect types: missing BDC linkers or missing Zr clusters with their associated linkers (i.e., denoted respectively as missing-linker defects and missing-cluster defects hereafter, see Figure 1b and 1c). Studies have shown that the density and chemical functionality (see Figure 1d–h) of defects can be controlled by using certain modulators under properly designed conditions.23,33 Therefore, engineering MOF membranes via controllable defects for optimized RO desalination performance should represent a promising



direction, but the complex effects of defects in MOFs on their desalination performance have not yet been systematically explored to date. In this study, we employ molecular simulation techniques to explore the effects of experimentally relevant defects of MOFs on the RO desalination performance at an atomic level. UiO-66 is particularly studied herein. Our results demonstrate that defectcontaining membranes can indeed offer substantially enhanced separation performance. The type, density, and modulated functionality of defects in MOFs are found to significantly affect their permeation flux, selectivity, and water intrusion behavior, serving as a tunable platform for performance optimization. The outcomes of this work can provide useful guidelines for the design of defective MOFs as RO membranes toward energy-efficient and cost-effective desalination process.

2. COMPUTATION DETAILS Classical molecular dynamics (MD) simulations are carried out using the opensource LAMMPS package36 to investigate the desalination performance of defect-free and defective UiO-66 membranes. A sandwich-type simulation system as schematically depicted in Figures 2 is utilized to mimic the separation process, a setup that was also used in our prior work.37–40 The system contains saline solution on the feed side and pure water on the permeate side, separated by a slab of UiO-66 structure (i.e., active membrane layer) with a surface termination parallel to the [1 1 1] crystal plane (see SI Figure S1) and a thickness of ~35 Å. A recent experimental synthesis of UiO-66 films shows a [1 1 1] surface termination, suggesting that this orientation may be a relatively


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more stable phase.19 Two graphene sheets, which acted as rigid pistons, are placed on both sides of the system to modulate the transmembrane pressure. Both defect-free and defective membranes are studied as shown in Figure 1a–c. For defective structures, the unsaturated metal sites resulting from the surface cleavage and defects are terminated by chloride (Cl–), hydroxide (OH–), or monocarboxylates (including formate (HCOO–), acetate (CH3COO–), and trifluoroacetate (CF3COO–)). These have been proposed as charge compensations based on experimental observations and/or speculations in previous studies (see Figure 1d−h).21,27,41–44 We note that, for the Cl– and OH– cases, each defect center is practically terminated with an anion (i.e., Cl– or OH–) and also a water molecule (see Figure 1g, 1h, and SI Figure S2). All of the defect centers are terminated by chemical compensations, i.e., there are no open metal sites existing in defective membranes, and for each defect scenario only one type of compensation groups is considered for simplicity (see SI Figure S3). Combinations of different compensation types may possibly exist, and their implications are out of the scope of this work. We also note that some defective structures may be metastable phases during crystallization process,45 but the stability of the defective structures is not considered herein. Each studied membrane structure is relaxed using the PM7 semi-empirical method46 implemented in MOPAC 2016.47 For simplicity, these structures have the same cell parameter as experimentally determined in a previous work. 12 The feed side initially contains 20 Na+/Cl– ion pairs solvated by 2000 water molecules, corresponding to a salinity of 0.555 M (i.e., similar to that in natural seawater (0.559 M)), while 1200 water molecules are placed on the permeate side. Periodic boundary conditions are applied in all three directions, and a vacuum region with a dimension of 80 Å along the z-direction



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(i.e., permeation direction) is included to minimize the interactions between periodic images.

Salt water

Fresh water

Pf

Pp

Vacuum

y z

Piston

Cl-

Na+

UiO-66 membrane

Piston

Figure 2. Schematic illustration of the simulation system. In this figure, defect-free UiO66 is used as an example RO active layer to desalinate saline sources via an applied transmembrane pressure of Pf – Pp modulated by two pistons. Color codes of carbon: grey, hydrogen: white, oxygen: red, zirconium: cyan, sodium: purple, and chlorine: green.

In these calculations, 6-12 Lennard-Jones (L-J) potentials and long-range Coulombic interactions using point charge models are adopted to describe intermolecular interactions.48 The rigid SPC/E model49 is used for water molecules with the SHAKE algorithm.50 For membrane atoms, the L-J parameters are taken from the DREIDING force field51 except for Zr atoms whose parameters are instead from the Universal force field (UFF).52 Such choices have been commonly made to describe the permeation behaviors of water and ions in membranes, including MOF-related materials.53,54 The atomic charges of the framework atoms are derived from the PM7 calculations based on population analysis. The non-bonded potentials developed by Joung et al. and Werder et al. are used for salt ions55 and piston atoms,56 respectively. The Lorentz-Berthelot mixing rule is applied to estimate the cross pair-wise L-J parameters between atoms. The L-J


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potentials are truncated and shifted to zero at a cutoff distance of 12 Å, while the longrange Coulombic interactions are calculated by the particle-particle particle-mesh (PPPM) algorithm57 with a precision of 10-6. All of the non-bonded potential parameters are summarized in Tables S1. To compute the water permeability and salt rejection of each membrane candidate, pressure-driven MD simulations are performed in a canonical ensemble (i.e., NVT) at a temperature of 300 K with a time step of 1 fs. The temperature is modulated using the Nosé-Hoover thermostat with a damping factor of 100 time steps (i.e., 100 fs).58,59 For each simulation, an energy minimization is first carried out to preliminarily relax the system. Subsequently, a hydrostatic pressure of 1 atm is applied on both pistons (i.e., toward the membrane), and MD simulations are performed for 1 ns to bring the liquid density closer to its equilibrium value (i.e., 1.0 g/cm 3). Pressures of 60 MPa and 1 atm are then applied to the feed and the permeate piston (i.e., toward the membrane), respectively, to push the salt solution across the membrane for 20 ns. The transmembrane pressure of approximately 60 MPa, which is significantly greater than the typical value applied in RO plants, is used in order to achieve more meaningful statistics within a finite simulation time. In these calculations, all of the studied membranes are assumed to be rigid. We note that, before sampling the water permeability and salt rejection of each membrane, the first 5 ns is used to ensure a steady-state flow has been achieved. To obtain meaningful statistics, more than five independent simulations are conducted for each studied membrane.



3. RESULTS AND DISCUSSION 3.1 Water Permeability. Water permeability of all studied membranes are summarized in Figure 3. The calculated water permeability of defect-free UiO-66 membrane is 51.05 L·m2·h-1·bar-1. If the permeability is assumed to be perfectly inversely proportional to the thickness, the thickness normalized permeability can be accordingly extrapolated to be 0.18 L·m2·h-1·bar-1·μm. This is on the same order as the value of 0.28 L·m2·h-1·bar-1·μm of a polycrystalline UiO-66 membrane experimentally synthesized by Liu at al.13 We should note, however, that the laboratory-measured permeability may be influenced by many factors such as crystal orientations, residue molecules and macroscopic cracks existing in the synthesized membranes, and intrinsic microscopic defects. As a result, a direct comparison may be still deemed impossible. When microscopic defects are present, our calculations show that membrane’s permeation rates can be significantly improved. Defective UiO-66 membranes, in particular for those with missing-cluster defects, are identified to offer faster flow by more than 20 times to be as large as 1200 L·m2·h-1·bar-1. To understand such an enhancement, provided that geometric characteristics of membranes have been found to closely relate to their permeation behaviors, we first quantify the accessible permeation area (i.e. Aacc) and the diameter of the largest free sphere (i.e., Dacc) at each cross section of the membrane along the permeation direction (i.e., z-direction; Aacc (z) and Dacc (z)), based on the available space excluding the van der Waals surface of each atom.60


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100 Defect-free Missing-linker Missing-cluster

1200

80

O

C H

O

C

H

3C

O 3C

F C

O

2

1/2

0

2O

1/2

l /H

1/2

C

1/2

H/H

1/2

O

1/4 H/H

1/8

H/H O

H/H

2O

0

O

20

O-

300

O-

40

O-

600

2O

60

2O

900

Salt rejection (%)

1500

O

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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Water permeability (L·m-2·h-1·bar-1)

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Figure 3. Water permeability (left axis; presented by bars) and salt rejection (right axis; presented by closed circles) of defect-free and defective membranes. The density of BDC vacancies for each studied membrane is given in the figure, which varies from 1/8, 1/4, to 1/2 (i.e., missing-cluster structures), corresponding respectively to 3, 6, and 12 linker vacancies per unit cell. These data are also available in SI Table S2.

Figure 4 clearly shows that, compared to the defect-free membrane, defective membranes have larger Aacc and Dacc values to allow an enlarged flux. Specifically, compared to defect-free and missing-linker defective structures, the pore openings become notably larger for structures containing missing-cluster defects. The smallest values of Aacc and Dacc for the missing-cluster structure terminated by OH-/H2O are determined to be 834.99 Å2 and 9.47 Å, respectively, larger than that of 475.76 Å2 and 4.12 Å for the defect-free one (see SI Table S2). We have also analyzed the free energy barriers for the water passage through membranes. Consistent with the geometry analysis, the largest transport barriers for the defect-free membrane, the missing-linker membrane with a defect density of 1/8, and the missing-cluster membrane terminated with OH-/H2O are computed to be 5.41, 1.18 and 0.60 kcal·mol-1, respectively (see SI



Figure S4). Additionally, the enlarged aperture size as the density of missing-linker defects increases from 1/8 to 1/4 (see Figure 4a and 4c) also reduces the barrier of water transport from 1.18 to 0.91 kcal·mol-1 (see SI Figure S4), which accordingly results in higher water permeability. We note that, for the membranes with the same density of linker vacancies, the spatial arrangement of missing linkers may likely also influence the permeation behavior. Systematically investigating this effect is out of the scope of this work, which should be an important subject of future studies.

(a)

(b) 18

18 Defect-free_0 Missing-linker_1/8 Missing-linker_1/4 Missing-cluster_1/2 Compensation group: OH-/H2O

14 12 10

14

6

6

-10

-5

0

5

10

15

4 -20

20

HCOO-

OH-/H2O

Cl-/H2O

10 8

-15

CH3COO-

12

8

4 -20

CF3COO-

Missing-cluster 16

Aacc (102·Å2)

Aacc (102·Å2)

16

-15

-10

Relative distance (Å)

-5

0

5

10

15

20

15

20


(c)

(d) 28

28 Defect-free_0 Missing-linker_1/8 Missing-linker_1/4 Missing-cluster_1/2 Compensation group: OH-/H2O

20

20

16 12

4

4

-10

-5

0

5

10

15

20

HCOO-

OH-/H2O

Cl-/H2O

12 8

-15

CH3COO-

16

8

0 -20

CF3COO-

Missing-cluster 24

Dacc (Å)

24

Dacc (Å)

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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 -20

-15

-10


-5

0

5

10


Figure 4. Accessible permeation area (Aacc, a–b) and the diameter of the largest free sphere (Dacc, c–d) profiles of defect-free and defective membrane structures along the permeation direction (i.e., z-direction). All defect centers of the membrane structures in (a, c) are saturated with OH–/H2O groups. Defective structures shown in (b, d) only contain missing-cluster defects that are terminated with different compensation groups. The two vertical grey dotted lines show the boundaries of the studied membrane,


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determined by the position of the furthest atoms from the membrane center. The horizontal red, purple, and green dotted lines in (c, d) indicate the diameters of water, hydrated sodium and hydrated chlorine, respectively.

As noted above, varying modulators have been introduced during the synthesis process, and the resulting different charge compensation anions are anticipated to affect the permeation properties to a large extent. For clear presentation, we hereafter focus mainly on the missing-cluster membranes. Figure 3 indicates that different compensation anions indeed lead to a notable variation in water permeability by as large as 68%. The membranes with terminations of smaller groups (e.g., OH–/H2O and HCOO–) display much higher water permeability than those with larger groups (e.g., CH3COO– and CF3COO–). This result appears reasonable; as can been seen in Figure 4b and 4d, larger compensation groups effectively result in smaller pore openings. However, these simple geometry descriptions are unable to fully explain the observed trend in permeability. For instance, the membrane by CF3COO– possesses the narrowest aperture, but it does not exhibit the lowest water permeability. Moreover, the permeation rate of the OH–/H2O terminated membrane is 47% higher than that by Cl–/H2O, although their geometric characteristics are nearly identical. To better understand the water permeation properties of membranes with different compensation anions, we have also quantitatively evaluated the water dynamics crossing the membranes. Figure 5a shows the net velocity of water residing in the membranes along the permeation direction. The net velocity of water generally correlates with the observed water permeation as shown in Figure 3, which is reasonable as one would expect that faster water dynamics (i.e., larger net velocity) leads to a more efficient



permeation thus higher permeability. For example, the net velocity of water in membranes by CF3COO– and OH–/H2O is found to be larger than that of membranes by CH3COO– and Cl–/H2O, respectively, resulting in relatively larger permeability of the former two structures. However, interestingly, although the net velocity of water in membrane by HCOO– is the highest among these missing-cluster membranes, it does not possess the largest permeability. In fact, while the transport dynamics of water is a key factor in determining the membrane’s permeability, the volumetric water capacity of the membrane also plays an important role. The membrane’s permeability (Q) should be proportional to the product of water capacity (C) of the membrane and net velocity (V) of water inside the membrane (i.e., Q ∝ C·V; see SI Figure S5). Figure 5b therefore shows the volumetric water capacity of each missing-cluster structure. It is indeed found that the HCOO– terminated membrane does not possess the largest capacity. The water capacity of membranes is sensitive to the energetics of water adsorption. A structure with more favorable adsorption energetics is anticipated to be capable of capturing more water per unit volume. We have herein also quantified the adsorption energy per water molecule as well as its detailed decomposition of water-membrane and water-water contributions as summarized in Figure 5c (see SI for calculation details), and the result is indeed consistent with that shown in Figure 5b for the water capacity (see SI Figure S6). While the adsorption energetics is shown to influence the adsorption capability of the membrane, the observed water dynamics is also found to strongly relate to the adsorption energetics. The slower water dynamics in the Cl–/H2O compensated membrane relative to that by OH–/H2O can be attributed to the more favorable adsorption energy, mainly due to the stronger water-membrane interaction (see Figure 5c for the decomposed energies).


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Similarly, the slower kinetics in the CH3COO– terminated structure as compared to CF3COO– can also be rationalized, caused instead by its relatively more stable water−water interaction. We should note that the above comparisons can only be made for structures with similar geometric features (e.g., Cl–/H2O vs. OH–/H2O). Although the Cl–/H2O terminated membrane has a stronger affinity (i.e., lower adsorption energy) toward water than the membrane by CH3COO–, the smaller pore aperture of the latter membrane imposes a higher barrier for water diffusion and therefore result in a marginally larger net velocity of water than the former membrane. Finally, as shown in Figure 5d, we have also computed the passage time distribution of a water to permeate through the membrane. The trend of water kinetics (i.e., net velocity) is, as expected, generally reciprocally reflected by the probability distribution of water passage time. A notable exception case, though, is that the Cl–/H2O compensated membrane has a shorter passage time than that by CF3COO– but the former has a slightly lower net velocity. This may be attributed to a portion of the adsorbed water molecules trapped in the relatively strongly adsorbing Cl–/H2O terminated membrane, thus resulting in a lower average net velocity. Overall, our results show that membranes with more favorable water adsorption energetics can have notably slower water kinetics as well as a higher adsorption capacity and consequently potentially lead to overall lower water permeability. This finding is consistent with a recent study by Jiang et al. on zeolitic imidazolate framework membranes, but the relationship among geometric features, volumetric capacities, adsorption energetics, and transport kinetics was not established in the prior work.54 With such energetics and dynamics information together with the geometric characteristics at



our disposal, the observed trend in water permeability can be well explained. The outcomes of the above analysis can guide the future rational design of better MOF membranes for water desalination.

Figure 5. Permeation mechanism of water crossing the missing-cluster membranes. (a) Net velocity of water residing in the membranes along the permeation direction. (b) Volumetric water capacity of the membranes. (c) Interaction energy per water molecule inside the membranes as well as its decomposed energy contributions of water−membrane and water−water. A negative energy value represents an attractive interaction. Details of the energy calculations can be found in the SI. (d) Probability distribution of water passage time for the membranes.


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3.2 Salt Rejection. To achieve effective desalination processes, membranes have to be nearly semipermeable. Given that the diameters of hydrated sodium and chloride are 7.16 Å and 6.64 Å, respectively,61 membranes with aperture sizes in the vicinity of or smaller than these values may be appealing candidates to block the passage of salt ions. Figure 3 shows that both the defect-free structure and the missing-linker membranes possess a salt rejection rate of 100%; the narrowest pore diameters of these membranes (see Figure 4c) are smaller than 6.64 Å. This is an important finding as it suggests that the defect density of missing-linker membranes can be optimized to increase the water permeability while maintaining 100% salt rejection. We note though, the reduced mechanical strength of MOFs with a high defect density needs to be considered in the design of membranes.62,63 On the other hand, all of the studied missing-cluster membranes, although not perfect, also exhibit promising salt rejection rates of >90%. The salt rejection of missing-cluster membranes is generally higher when their pore sizes are smaller, and the one terminated with the largest groups (i.e., CF3COO–) demonstrates a nearly complete rejection of 99.29%. However, the salt rejection of HCOO– terminated membrane is surprisingly found to be lower compared to those by OH–/H2O or Cl–/H2O. This counterintuitive observation can be also attributed to the structural hydrophilicity. Due to the strong affinity between water and the OH– and Cl–, hydrated ions can possibly be better stabilized in these membranes and therefore experience a higher transport barrier, thus leading to improved salt rejection. In other words, the stronger surface hydrophilicity effectively reduces the pore size available for ion permeation. At this point, it should be noted that, while the salt rejection rates predicted for these missing-cluster membranes except that by CF3COO– may not be sufficiently high for one-step seawater



desalination, the high pressure of 60 MPa applied in our calculations can underestimate their ability to reject salts. Under the typical operation condition with an applied pressure of below 10 MPa, the salt rejection of these membranes may be better.

3.3 Water intrusion. Thus far, our results have shown that controlling the hydrophobicity (or hydrophilicity) of defective membranes can substantially affect their permeability and salt rejection. From a practical viewpoint, it remains of importance to also understand the water intrusion behavior of membranes with different chemical characteristics. For this, simulations are carried out to probe the water uptake in membranes as a function of the applied pressure. In these calculations, the number of water molecules inside initially empty membranes was counted as a function of simulation time with an applied pressure increasing by 5 MPa per ns. In agreement with the trend in the adsorption energetics reported in Figure 5c, Figure 6 shows that defective membranes with a stronger water affinity possess a notably higher water intrusion rate and therefore require a much lower applied pressure to reach saturation. By contrast, saturating defect-free UiO-66 with water can be a challenge even under an extremely high pressure of >100 MPa (see the inset of Figure 6). Overall, while hydrophilic membranes may slower down the kinetics of water permeating through the membrane, the stronger water affinity can improve their ability to reject salts and facilitate the intrusion of water.


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Figure 6. Cumulative amount of water molecules in defect-free and missing-cluster membranes as a function of simulation time. The results shown here are averaged from five independent simulations. Molecular simulations were carried out using the NVT ensemble at 300 K with a gradually increasing applied pressure at an increment of 5 MPa per ns. The value of applied pressure can be seen at the upper axis of the figure. The inset shows the evolution of water uptake in the defect-free membrane with an applied pressure up to 200 MPa.

4. CONCLUDING REMARKS In summary, by employing classical molecular dynamics simulations and as a proof of concept, this work presents the very first attempt to systematically explore the effects of structural defects, including defect types, densities, and chemical compensations, on the desalination performance of MOF membranes and demonstrates the great potential of defective structures, if designed properly. Moreover, this study has also shed light on the complex interplays between the geometric features of the membranes and the kinetics



and energetics of water inside the membranes, as well as their effects on the permeation behaviors of water and ions. Via designing defective MOFs, both the aperture size and water affinity of UiO-66 membranes can be optimized to offer a substantially improved water permeability by an order of magnitude while maintaining excellent ability to reject salts. The single-layer missing-cluster membrane by CF3COO– possesses a water permeability of as high as 800 L m−2 h−1 bar−1, as compared to approximately 50 L m−2 h−1 bar−1 for the defect-free membrane, and it remains a nearly complete salt rejection of >99%. Furthermore, the water intrusion of defective membranes can be enhanced by introducing hydrophilic compensation groups at the defective sites; the required pressure for membrane saturation can be lower by more than 10 times. It is important to note though that, in this work, the simulation setup implicitly assumes a correlated distribution of defects (i.e., spatially ordered defects) across the whole membrane when its thickness is more than one single layer. This has been in fact also observed in experiment; ordered defect nanoregions with a domain size of ~7 nm can emerge in the primary framework net.42 However, the spatial ordering of defects in real MOF membranes might still remain rather complicated and could have impacts on their permeation properties. Exploring such effect is out of the scope of this study and should be a subject of future studies. It is also worth mentioning that, owing to the limitation of classical force fields, the detachments of coordination bonds between organic linkers or compensation groups (e.g., OH–/H2O, CH3COO–) and metal clusters are not considered. A recent study by Vandichel et al. has also shown that hydroxyl groups can be detached from the zirconium cluster, especially for defective structures.64 Similarly, other possible effects such as the dynamic exchange between OH– and H2O, the replacement of OH– by Cl– (salt ions), and mechanical


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deformation of the framework structure under an applied pressure are not considered. Employing bond-order based force fields (e.g., reactive force field (ReaxFF)) may represent a promising direction to further understand these effects on the permeation performance of defective MOF membranes.65 For this, a detailed parameterization and validation of ReaxFF will be needed. Nonetheless, although the models employed herein are relatively ideal, the influences of defects on the desalination performance of MOF membranes have been extensively explored and the outcomes obtained in this study can contribute to a new venue in utilizing defects in MOFs as well as provide guidelines toward the rational design of MOF membranes with significantly improved performance for more energy-efficient and cost-effective desalination processes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DIO: XXXXXX. Additional details for surface cleavages, compensation strategies of defect centers, nonbonded interaction parameters, tabulated summary for membranes’ performance, free energy profiles of water in OH–/H2O terminated membranes, computational details of the interaction energy per water inside the membranes, supplementary of water permeation mechanism, and the structural files of all studied membranes.

AUTHOR INFORMATION



Corresponding Authors *E-mail: [email protected] (S.H.) *E-mail: [email protected] (L.-C.L.)

Author Contributions The study was developed and completed through contributions by all authors. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank Eun Hyun Cho for useful discussion. The authors also thank James Stewart for providing the MOPAC license. This research was sponsored by the Ohio State University and the Fundamental Research Funds for the Central Universities (18CX06002A). Q.L. also acknowledges the financial support from the China Scholar Council (CSC). The authors gratefully acknowledge the Ohio Supercomputer Center (OSC)66 for providing computational resources.

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TOC Graphic Missing-linker

Compensation groups

Missing-cluster

100

Defect-free Missing-linker Missing-cluster

1200

80

900

60

600

40

300

20

O-

1/2

O

2

l-

/

O

/H

0

O

2

C

C H

H

1/2

HH

O

O-

O-

O

2O

1/2

1/2

3C

C

H O

F

-

3C

/H

2

O

O

1/2

C

2O

/

H O

H

-

1/4

O

1/8

0

Salt rejection rate (%)

1500

H/H

Water permeability (L·cm-2·h-1·bar-1)

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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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