Seawater Pervaporation through Zeolitic Imidazolate Framework

May 19, 2016 - (13, 14) Therefore, increasing interest is to explore new PV membranes that exhibit superior performance for water desalination. Over t...
1 downloads 18 Views 4MB Size
Download PDF
Research Article www.acsami.org

Seawater Pervaporation through Zeolitic Imidazolate Framework Membranes: Atomistic Simulation Study Krishna M. Gupta, Zhiwei Qiao, Kang Zhang, and Jianwen Jiang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore S Supporting Information *

ABSTRACT: An atomistic simulation study is reported for seawater pervaporation through five zeolitic imidazolate framework (ZIF) membranes including ZIF-8, -93, -95, -97, and -100. Salt rejection in the five ZIFs is predicted to be 100%. With the largest aperture, ZIF-100 possesses the highest water permeability of 5 × 10−4 kg m/(m2 h bar), which is substantially higher compared to commercial reverse osmosis membranes, as well as zeolite and graphene oxide pervaporation membranes. In ZIF-8, -93, -95, and -97 with similar aperture size, water flux is governed by framework hydrophobicity/hydrophilicity; in hydrophobic ZIF-8 and -95, water flux is higher than in hydrophilic ZIF-93 and -97. Furthermore, water molecules in ZIF-93 move slowly and remain in the membrane for a long time but undergo to-and-fro motion in ZIF-100. The lifetime of hydrogen bonds in ZIF-93 is found to be longer than in ZIF-100. This simulation study quantitatively elucidates the dynamic and structural properties of water in ZIF membranes, identifies the key governing factors (aperture size and framework hydrophobicity/hydrophilicity), and suggests that ZIF-100 is an intriguing membrane for seawater pervaporation. KEYWORDS: pervaporation, ZIFs, permeability, salt rejection, atomistic simulation

1. INTRODUCTION Shortage of freshwater has escalated as a global concern due to increasing population, energy demand, and industrialization.1,2 Currently, there is considerable interest to desalinate seawater, which constitutes over 95% of water on the Earth.3,4 Among several desalination techniques, multistage flash distillation (MSFD) and reverse osmosis (RO) have been majorly utilized.5 However, phase transition occurs in energy intensive thermal-based MSFD; and RO needs pressurization associated with easy fouling. Recently, pervaporation (PV) has emerged as a relatively new, economically viable separation technique.6 As a unique integration of membrane permeation and evaporation, PV offers several advantages like low energy consumption, high separation capability, and easy scaling-up. Polymer membranes such as cellulose diacetate, suthatlfonated and quaternized polyethylenes, polyether amide and polyether ester,7,8 as well as zeolitic membranes9,10 were tested for water PV. Nevertheless, they possess low water flux or salt rejection. To improve PV performance, nanofiber composite membranes were developed with water flux ranging from 5 to 8 L m−2 h−1 and nearly 100% salt rejection.11 Graphene oxide/ polyacrylonitrile composite membranes were found to have a relatively higher water flux (∼65.1 L m−2 h−1).12 To date, most PV membranes still suffer from low mass transfer (flux or permeability) compared to graphene and carbon nanotube (CNT) membranes. However, it is substantially challenging to produce graphene and CNT membranes in a large scale with high-density pores and sufficient durability.13,14 Therefore, © XXXX American Chemical Society

increasing interest is to explore new PV membranes that exhibit superior performance for water desalination. Over the past few decades, metal−organic frameworks (MOFs) have emerged as a novel class of porous materials. Enormous metal oxides and organic linkers can be used to synthesize MOFs with customizable pore size, surface area, and functionality. Consequently, MOFs are considered versatile materials for storage, separation, and many other potential applications.15 However, most experimental and theoretical studies for MOFs have been focused on gas storage and separation, particularly the storage of low carbon footprint energy carriers (e.g., H2 and CH4) and the separation of CO2containing gas mixtures for CO2 capture.16−22 Currently, there are scarce studies examining MOF membranes for water desalination. Notably, we conducted a molecular simulation study to explore zeolitic-imidazolate framework-8 (ZIF-8) as a RO membrane for seawater desalination.23 ZIFs are a subclass of MOFs with exceptional chemical and thermal stability. Moreover, the pore size and affinity of ZIFs are readily tunable and thus a large opportunity exists to potentially utilize ZIFs for water desalination. Our simulation study predicted that ZIF-8 has a superior desalination performance with water permeance of 160 kg/ (m2 h bar) and 100% salt rejection.23 This proof-of-concept Received: February 6, 2016 Accepted: May 12, 2016

A

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Atomic structures of ZIF-8, -93, -95, -97, and -100. ZnN4 cluster, orange polyhedron; C, cyan; O, red; N, blue; Cl, green; and H, white. The structures are not scaled to their actual sizes. are connected to form SOD cages.27 ZIF-93 and -97 belong to RHOtopology dually functionalized at positions 4 and 5.28 ZIF-95 and -100 possess POZ and MOZ-topologies, respectively.29 The pore sizes in the five ZIFs were estimated using Zeo++.30 Table 1 lists the diameters

prediction was supported by a recent experiment, in which 0.4% of ZIF-8 was added into a polyamide membrane and found to increase water permeance by 162%.24 Furthermore, we examined seawater desalination through ZIF-25, -71, -93, -96, and -97 membranes via RO. The permeance was found to range between 27−710 kg/(m2 h bar), approximately 1 to 2 orders of magnitude higher than commercial RO membranes. It was revealed that RO desalination performance is governed by the aperture size and polarity of functional group in the ZIFs.25 We report here a systematic simulation study using a series of ZIFs as PV membranes for seawater desalination. As mentioned above, PV is considered to be superior to RO; however, there is only one recent experimental and simulation study,26 to our best knowledge, using ZIFs (ZIF-7, -8, and -90) for seawater PV. Therefore, this study will provide fundamental insight into the microscopic properties of water and ions in ZIFs and assist in the rational design of new ZIFs for better seawater PV. The ZIFs examined are ZIF-8, -93, -95, -97, and -100 with different topologies and organic linkers. Following this introduction, the models of the ZIFs and seawater as well as the methods used are briefly described in Section 2. In Section 3, salt rejection and water permeability through the ZIF membranes are presented and compared to other RO and PV membranes; furthermore, the dynamics and structure of water in the ZIF membranes are discussed. Finally, the concluding remarks are summarized in Section 4.

Table 1. Structural Characteristics of ZIF-8, -93, -95, -97, and -100

of cage (dc) and aperture (da) in each ZIF. The dc varies widely from 11.4 to 36.4 Å. In ZIF-8, -93, -95, and -97, the da is close (3.4−3.7 Å), while it is 4.7 Å in ZIF-100. We shall find below that the da is an important parameter governing water transport through the ZIFs. A PV system consists of a feed chamber and a permeate chamber separated by a membrane. The driving force for mass transfer is the difference of chemical potentials in the two chambers, which is provided by either a vacuum or a flow of air.12 Figure 2 schematically illustrates the simulation system for seawater PV through a ZIF membrane. One chamber (left) contains NaCl solution and the other (right) is initially a vacuum. The concentration of NaCl is 2 wt % (about 0.35 M) representing seawater. A graphene plate in the left chamber can adjust its position during simulation under atmospheric pressure (1 bar). In the right chamber with a length of 500 Å along the z direction, an adsorbing plate is placed and it can adsorb water molecules after they pass through the membrane; this is to maintain a low pressure in the permeate side. The membrane thicknesses are 58.86, 56.71, 56.45, 56.86, and 83.06 Å for ZIF-8, -93, -95, -97, and -100, respectively. The pore of each ZIF membrane is aligned along the z direction, and periodic boundary conditions are applied in all the directions. Thus, the membrane is mimicked to be infinitely large perpendicular to the z direction.

2. MODELS AND METHODS ZIFs possess structures similar to zeolites, wherein the tetrahedral Si/ Al nodes and O bridges are replaced by metal ions (usually Zn or Co) and imidazolate linkers, respectively. Figure 1 depicts the atomic structures of ZIF-8, -93, -95, -97, and -100. With the identical tetrahedral metal clusters (ZnN4), however, the imidazolate linkers are different. The linkers are 2-methyl imidazolate (meIm), aldehydemethyl imidazolate (almeIm), and hydroxymethyl imidazolate (hymeIm) in ZIF-8, -93, and -97, respectively; and chlorobenzimidazole (cbIm) in both ZIF-95 and -100. The topologies are also different in the five ZIFs. Specifically, ZIF-8 has SOD-topology in which meIm is singly functionalized at position 2, while 4- and 6-membered rings B

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Simulation system for seawater pervaporation through a ZIF membrane. An aqueous NaCl solution with 2 wt % NaCl and a vacuum are on the left and right chambers of the membrane, respectively. A graphene plate is exerted under atmospheric pressure p in the left and an adsorbing plate is placed in the right chamber. Blue, Na+; green, Cl−; white, H of H2O; red, O of H2O.

Figure 3. Number distributions of Na+ and Cl− ions along the z-axis at 14 ns. Each membrane is between the two dotted lines. The framework atoms of ZIFs were described by Lennard−Jones (LJ) and electrostatic potentials

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij σij U = ∑ 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

∑

Ewald method was used to evaluate the electrostatic interactions with grid spacing of 1.2 Å and real-space cutoff of 14 Å. A time step of 2 fs was used to integrate the equations of motion by leapfrog algorithm. The simulation duration was 14 ns and the trajectory was saved every 4 ps. The simulation was conducted using GROMACS v.5.0.6.39

qiqj 4πε0rij

(1)

3. RESULTS AND DISCUSSION The performances of seawater PV through the five ZIF membranes are evaluated on the basis of salt rejection and water permeability and compared to RO and PV membranes. Furthermore, the dynamic and structural properties of water in the membranes are presented. 3.1. Salt Rejection and Water Permeability. After the simulation initiates, water in the left chamber (feed) enters into the membrane and then passes into the right chamber (permeate). The number of water molecules in the permeate chamber increases along with time, implying the occurrence of PV. A movie is provided in the Supporting Information to visualize the PV process through ZIF-100 membrane within the initial 3 ns. Along with water transport, Na+ and Cl− ions also move toward the membrane. Figure 3 shows their number distributions along the z-axis in the five systems at 14 ns (the final stage). Most Na+ and Cl− ions remain in the feed (NaCl solution), whereas a few ions cross the seawater/membrane interface because open cages exist at the interface. In ZIF-8, -93, -95 and -97, the ions cannot enter into the membrane interior due to the restriction of small aperture. In ZIF-100, however, both Na+ and Cl− ions can enter. Specifically, Na+ ion is able to move in the entire pore of ZIF-100. With a larger size, nevertheless, Cl− ion can only enter until the central region of ZIF-100 (z ≈ 16 nm), at which the aperture exists and blocks Cl− ion. An interesting observation in Figure 3 is that Na+ ion residing in ZIF-100 cannot pass through the membrane into the permeate side. To elucidate, the interaction of Na+ ion with

where εij and σij are the well depth and collision diameter, rij is the distance between atoms i and j, qi is the atomic charge of atom i, and ε0 = 8.8542 × 10−12 C2(N m2) is the permittivity of vacuum. As a large number of simulation studies have shown that the universal force field (UFF)31 can well predict the adsorption and diffusion of guests in various MOFs,32−34 thus the LJ parameters were taken from the UFF as listed in Table S1. The atomic charges of ZIFs were calculated using density functional theory (DFT) based on fragmental clusters (see Figure S1). The DFT calculations were conducted using Gaussian 09,35 based on the LANL2DZ basis set for Zn atoms and 6-31G(d) basis set for other atoms. By fitting the electrostatic potentials, the atomic charges were estimated as listed in Table S2. Water was modeled by the TIP3P,36 and Na+ and Cl− ions were described as charged LJ particles by the AMBER force field.37 The carbon atoms in the graphene plate were mimicked by the LJ potential as used for CNTs,38 while ε/kb = 500 K was assigned to the atoms in the adsorbing plate. It should be noted that the dangling bonds on the ZIF membrane surfaces were unsaturated. These unsaturated dangling bonds were indeed to mimic the polar groups that usually exist on a membrane surface. To quantitatively examine the effect of dangling bonds, the surfaces of ZIF-8 (hydrophobic) and ZIF-93 (hydrophilic) were saturated, as illustrated in Figure S2. Water transfer rates were evaluated with both saturated and unsaturated surface dangling bonds. Initially, each simulation system was subjected to energy minimization using the steepest descent method with a maximum step size of 0.1 Å and a force tolerance of 1 kJ/(mol Å). Then, velocities were assigned according to the Maxwell−Boltzmann distribution at 75 °C. Finally, molecular dynamics (MD) simulation was carried out at 75 °C. Temperature was controlled by the velocityrescaled Berendsen thermostat with a relaxation time of 0.1 ps. The ZIF membrane was assumed to be rigid during simulation. A cutoff of 14 Å was used to calculate the LJ interactions, and the particle-mesh C

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

the accumulative average of Nw within 2 ns duration. The hierarchy of Nw in the five ZIFs is ZIF-100 ≫ -8 > -95 ≥ -93 > -97. Compared to the other four, the Nw in ZIF-100 is substantially greater due to the presence of an appreciably larger aperture (4.7 Å). In ZIF-8, -93, -95 and -97, the aperture size da is 3.4, 3.7, 3.6, and 3.5 Å, respectively. Apparently, the Nw in these four ZIFs does not simply follow the da. Even though ZIF-8 has the smallest aperture (3.4 Å), it exhibits greater Nw than the other three. As also observed in our recent simulation study for water desalination by RO,25 this counterintuitive phenomenon is attributed to different organic linkers in the ZIFs. ZIF-93 and -97 contain hydrophilic almeIm and hymeIm linkers, which have high affinity for water and impede the flow of water molecules once entering into ZIF-93 and -97 membranes. Particularly, the highly polar −OH groups in ZIF-97 strongly interact with water molecules, leading to vanishingly small Nw in ZIF-97. In ZIF-8 and -95 (as well as ZIF-100), however, meIm and cbIm linkers are relatively hydrophobic; water molecules therein thus experience weaker interaction and can transport faster. On the basis of the simulation of water adsorption, it was shown ZIF-97 is hydrophilic, whereas ZIF-8 is hydrophobic.40 Therefore, the hierarchy of Nw observed in the five ZIFs is governed by primarily the aperture size and then framework hydrophobicity/hydrophilicity. This finding is essentially similar to our recent study.25 The Nw in each ZIF increases almost linearly with time. From the slope, water flux Jw can be calculated by

ZIF-100 framework is analyzed in terms of the radial distribution function g(r) gij(r ) =

Nij(r , r + Δr )V 4πr 2ΔrNN i j

(2)

where r is the distance between atoms i and j, Nij(r,r + Δr) is the number of atom j around i within a shell from r to r + Δr, V is the system volume, and Ni and Nj are the numbers of atoms i and j, respectively. As shown in Figure 4, sharp peaks are seen

Figure 4. Radial distribution functions g(r) of Na+ ion around the framework atoms of ZIF-100. The notations of the framework atoms are in Figure S1.

at 2.3 and 3.4 Å for Na+ around the N and Cl atoms of ZIF-100. This indicates favorable interaction between Na+ ion and ZIF100 membrane, thus Na+ ion preferentially resides in the membrane. If we define salt rejection as (Cleft − Cright)/Cleft × 100%, where Cleft and Cright are ion concentrations in the left and right chambers, respectively, the results in Figure 3 suggest that all the five ZIFs would achieve 100% salt rejection for seawater PV. To evaluate the effect of surface dangling bonds, Figure S3 shows the numbers of water molecules Nw passing through ZIF-8 and -93 membranes. With saturated and unsaturated dangling bonds, the Nw are close despite certain fluctuations. This suggests that the surface dangling bonds play an insignificant role because the surface environment is similar in the two cases. For this reason, all the simulations were conducted for the ZIF membranes with unsaturated surface dangling bonds. Figure 5 shows the numbers of water molecules Nw passing through all the five ZIF membranes. Each point in the figure is

Jw = Nw /(A × t )

(3)

where A is the membrane area and t is time duration. The Jw are 11 909, 5 666, 3 584, 2 777, and 0 kg/(m2 h) in ZIF-100, -8, -95, -93, and -97, respectively. Water fluxes in these ZIF membranes (except ZIF-97) at 75 °C are 1 to 2 orders of magnitude higher than 65 kg/(m2 h), which was reported recently in the graphene oxide/polyacrylonitrile composite membrane at 90 °C.12 We should note that the Jw in ZIF-8 predicted here is nearly 4.5 times of 1255 kg/(m2 h) in our recent study.26 This is because the right chamber under the current study is 50 nm along the z direction, much greater than 12 nm in the earlier study. Additionally, an adsorbing plate is added here. Therefore, the permeate pressure here is much lower leading to a higher flux. It has been experimentally demonstrated that flux in a PV process drastically depends on permeate pressure.41 More detailed quantitative evaluation for the dependence of permeate pressure is desirable from a microscopic scale and will be further investigated in our future study. The performances of ZIF membranes are quantified in terms of salt rejection and water permeability. The latter is defined as

P = Jw S /Δp

(4)

where S is membrane thickness and Δp is pressure difference between the feed side pfeed and permeate side pperm. The pfeed is maintained at 1 bar, while pperm increases as water passes through the membrane. On the basis of the number of water molecules in the permeate side (excluding those adsorbed by the adsorbing plate), the pperm is estimated by assuming ideal gas behavior. Figure S4 shows pperm increases with simulation time due to the accumulation of water molecules in the permeate side. Thus, the pressure difference Δp drops and the driving force is reduced along with simulation. We should note

Figure 5. Numbers of water molecules passing through ZIF membranes. D

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 6. Performances of ZIFs as PV membranes in this study, along with (a) RO membranes in the literature (functionalized and pristine CNTs,13 ZIFs,25 graphyne,43 MFI zeolite and commercial RO,42 graphene,44 and UiO-6646 at 20−27 °C). (b) PV membranes in the literature (NaA and SOD zeolites,9,10 and graphene oxide45 at 75 °C).

Figure 7. Trajectories of selected water molecules through ZIF-93 and -100 membranes. The membrane is between the two dotted lines.

that in a practical PV process, a very low pperm (e.g., 1 mbar) is usually kept by a vacuum pump and the driving force remains nearly a constant. It is intriguing on how to more realistically maintain a very low pperm during simulation. Figure 6 illustrates the performances of ZIFs as PV membranes as well as RO and PV membranes reported in the literature. Compared to commercial seawater RO, brackish RO, and high-flux RO membranes, 42 the ZIFs show significantly higher permeability ranging from 1.9 × 10−5 to 5.0 × 10−4 kg m/(m2 h bar) in ZIF-8, -93, -95, and -100. Their performances are also better than functionalized CNT,13 graphyne43 and graphene.44 Furthermore, the ZIFs outperform over SOD zeolite9 and graphene oxide membranes45 in terms of PV performance; particularly, ZIF-100 exhibits the highest permeability, higher than NaA zeolite.10 As mentioned earlier, most of the existing PV membranes suffer from low permeability; however, the ZIFs examined here are predicted to possess superior performance surpassing many other counterparts and hence could be interesting candidates for seawater PV. 3.2. Water Dynamics and Structure. It is interesting to examine how water transports through a ZIF membrane. Figure 7 shows the trajectories of randomly selected water molecules through ZIF-93 (with the lowest flux except ZIF-97) and ZIF100 (with the highest flux). After water molecules enter into ZIF-93 membrane, they remain there with slow local motion (nearly constant z) for over 10 ns due to small aperture and strong interaction with hydrophilic framework and then leave the membrane entering the permeate side. Nevertheless, some

water molecules in the permeate side can move back to the membrane surface. In contrast, water molecules exhibit to-andfro motion in ZIF-100 membrane due to a comparatively larger aperture and weaker interaction with a hydrophobic framework. Once leaving the ZIF-100 membrane, water molecules in the permeate side move toward the adsorbing plate and cannot move back, due to the strong attraction of many water molecules adsorbed on the adsorbing plate. To further elucidate water transport, water structure in ZIF93 and -100 membranes is characterized by hydrogen bonding. Specifically, two geometrical criteria were implemented to define a hydrogen bond: (1) the distance between a donor and an acceptor ≤0.35 nm and (2) the angle of hydrogen−donor− acceptor ≤30°.47 On average, one water molecule forms 2.3 hydrogen bonds in ZIF-93, lower than 2.8 in ZIF-100; this is because ZIF-100 has a larger cage and a weaker affinity for water, thus water therein is less confined. These values are lower than 3.1 in bulk water at 75 °C, suggesting energetically unfavorable for water transporting from bulk phase to the ZIF membrane. Nevertheless, the energy loss of weakened hydrogen bonding is compensated by the interaction with framework. Moreover, the relaxation of hydrogen bonding was examined using the autocorrelation function47 c(t ) =

⟨h(0)h(t )⟩ ⟨h⟩

(5)

where h(t) = 1 if two water molecules are hydrogen bonded at time t and h(t) = 0 otherwise. The ensemble average ⟨...⟩ is on E

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces all the pairs of hydrogen boned water molecules. Physically, c(t) measures the time duration of hydrogen bonding, i.e., the probability for two water molecules remaining hydrogen bonded at time t = 0 as well as time t. When time approaches infinity, c(t) approaches to zero. The lifetime of hydrogen bonding τHB is estimated by c(t = τHB) = e−1.48 Figure 8 shows

performance deterioration. (3) The ZIF model membranes were very thin and cannot be straightforwardly compared to real membranes. (4) The permeate pressure increases along with simulation and is not kept at a very low value. Therefore, more simulation endeavors are desired by incorporating practical situation and condition. Despite these limitations, this simulation study provides atomistic insight into water transport through the ZIFs and suggests that the ZIFs (particularly ZIF-100) are interesting candidates for seawater pervaporation.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01626. Fragmental clusters, force field parameters, atomic charges, saturated surface dangling bonds of ZIF-8 and -93, numbers of water molecules passing through ZIF-8 and ZIF-93 membranes with saturated and unsaturated surface dangling bonds, and permeate pressure versus simulation time (PDF) Movie visualizing the PV process through ZIF-100 membrane within the initial 3 ns (AVI)

Figure 8. Autocorrelation functions of hydrogen bonding in ZIF-93 and -100 membranes and in bulk phase, respectively.

the c(t) in ZIF-93 and -100 membranes, as well as in bulk water, at 75 °C. At a given time, c(t) in the ZIFs is greater than in bulk water, which implies the confinement and framework interaction restrict water motion in the membranes and lead to longer existence of hydrogen bonds. Compared to ZIF-93, ZIF100 possesses a larger cage and a relatively more hydrophobic framework; therefore, the relaxation of hydrogen bonds is faster. The τHB are estimated to be 1.6 and 1.0 ps in ZIF-93 and -100, respectively, which are close to those in RO membranes.25 Nevertheless, the τHB in ZIF-93 and -100 are considerably shorter than in a (10, 0) CNT with τHB > 15 ps.49 This is because water molecules are packed more compactly in onedimensional small CNT than in three-dimensional network of ZIFs, thus a longer time is required for hydrogen bonds to relax in CNT.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge the A*star of Singapore (Grant R279-000-431-305) for financial support and Prof. Neal Chung and Dr. Zhongqiao Hu for helpful discussions.

■

REFERENCES

(1) Schiermeier, Q. Water: Purification with a Pinch of Salt. Nature 2008, 452, 260−261. (2) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and Technology for Water Purification in the Coming Decades. Nature 2008, 452, 301−310. (3) Service, R. F. Desalination Freshens Up. Science 2006, 313, 1088−1090. (4) Elimelech, M.; Phillip, W. A. The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333, 712−717. (5) Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J. M. Advances in Seawater Desalination Technologies. Desalination 2008, 221, 47−69. (6) Van der Bruggen, B.; Luis, P. Pervaporation as a Tool in Chemical Engineering: A New Era? Curr. Opin. Chem. Eng. 2014, 4, 47−53. (7) Quiñones-Bolaños, E.; Zhou, H.; Soundararajan, R.; Otten, L. Water and Solute Transport in Pervaporation Hydrophilic Membranes to Reclaim Contaminated Water for Micro-Irrigation. J. Membr. Sci. 2005, 252, 19−28. (8) Zwijnenberg, H. J.; Koops, G. H.; Wessling, M. Solar Driven Membrane Pervaporation for Desalination Processes. J. Membr. Sci. 2005, 250, 235−246. (9) Khajavi, S.; Jansen, J. C.; Kapteijn, F. Production of Ultra Pure Water by Desalination of Seawater Using a Hydroxy Sodalite Membrane. J. Membr. Sci. 2010, 356, 52−57. (10) Cho, C. H.; Oh, K. Y.; Kim, S. K.; Yeo, J. G.; Sharma, P. Pervaporative Seawater Desalination Using NaA Zeolite Membrane:

4. CONCLUSIONS Seawater pervaporation through ZIF-8, -93, -95, -97, and -100 membranes has been investigated. Water flux decreases in the order of ZIF-100 ≫ -8 > -95 > -93 > -97. With the largest aperture (4.7 Å), in addition to hydrophobic framework, ZIF100 exhibits the highest water flux. Although the aperture size is close (3.4−3.7 Å) in ZIF-8, -93, -95, and -97, water flux in ZIF8 and -95 is higher than in ZIF-93 and -97. This is because ZIF8 and -95 are hydrophobic with weak affinity for water; however, hydrophilic ZIF-93 and -97 interact strongly with water thus leading to slower water transport. We infer that water flux in the ZIF membranes is governed by both aperture size and framework hydrophobicity/hydrophilicity. In ZIF-8, -93, -95, and -100, water permeability ranges from 1.9 × 10−5 to 5.0 × 10−4 kg m/(m2 h bar). Moreover, salt rejection in all the five ZIFs is predicted to be 100%, as attributed to the sieving of small apertures. Compared to many RO and PV membranes, the ZIFs possess significantly higher permeability. We should note several limitations associated with this simulation study: (1) The structures of ZIFs were considered as rigid. However, ZIFs usually exhibit structural flexibility, which may affect salt rejection as well as water flux. (2) The ZIFs were assumed to be chemically stable in water, but realistically they may not remain stable for a long duration thus leading to F

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Mechanisms of High Water Flux and High Salt Rejection. J. Membr. Sci. 2011, 371, 226−238. (11) Liang, B.; Pan, K.; Li, L.; Giannelis, E. P.; Cao, B. High Performance Hydrophilic Pervaporation Composite Membranes for Water Desalination. Desalination 2014, 347, 199−206. (12) Liang, B.; Zhan, W.; Qi, G.; Lin, S.; Nan, Q.; Liu, Y.; Cao, B.; Pan, K. High Performance Graphene Oxide/Polyacrylonitrile Composite Pervaporation Membranes for Desalination Applications. J. Mater. Chem. A 2015, 3, 5140−5147. (13) Das, R.; Ali, M. E.; Hamid, S. B. A.; Ramakrishna, S.; Chowdhury, Z. Z. Carbon Nanotube Membranes for Water Purification: A Bright Future in Water Desalination. Desalination 2014, 336, 97−109. (14) Liu, G.; Jin, W.; Xu, N. Graphene-Based Membranes. Chem. Soc. Rev. 2015, 44, 5016−5030. (15) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 1230444. (16) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective Gas Adsorption and Separation in Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1477−1504. (17) Keskin, S.; Liu, J.; Rankin, R. B.; Johnson, J. K.; Sholl, D. S. Progress, Opportunities, and Challenges for Applying Atomically Detailed Modeling to Molecular Adsorption and Transport in MetalOrganic Framework Materials. Ind. Eng. Chem. Res. 2009, 48, 2355− 2371. (18) Jiang, J. W.; Babarao, R.; Hu, Z. Q. Molecular Simulations for Energy, Environmental and Pharmaceutical Applications of Nanoporous Materials: From Zeolites, Metal-Organic Frameworks to Protein Crystals. Chem. Soc. Rev. 2011, 40, 3599−3612. (19) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D. W. Hydrogen Storage in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 782− 835. (20) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T. H.; Long, J. R. Carbon Dioxide Capture in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 724− 781. (21) Getman, R. B.; Bae, Y. S.; Wilmer, C. E.; Snurr, R. Q. Review and Analysis of Molecular Simulations of Methane, Hydrogen and Acetylene Storage in Metal−Organic Frameworks. Chem. Rev. 2012, 112, 703−723. (22) Yang, Q. Y.; Liu, D. H.; Zhong, C. L.; Li, J. R. Development of Computational Methodologies for Metal−Organic Frameworks and Their Application in Gas Separations. Chem. Rev. 2013, 113, 8261− 8323. (23) Hu, Z. Q.; Chen, Y. F.; Jiang, J. W. Zeolitic Imidazolate Framework-8 as a Reverse Osmosis Membrane for Water Desalination: Insight from Molecular Simulation. J. Chem. Phys. 2011, 134, 134705. (24) Duan, J.; Pan, Y.; Pacheco, F.; Litwiller, E.; Lai, Z.; Pinnau, I. High-Performance Polyamidethin-Film-Nanocomposite Reverse Osmosis Membranes Containing Hydrophobic ZIF-8. J. Membr. Sci. 2015, 476, 303−310. (25) Gupta, K. M.; Zhang, K.; Jiang, J. W. Water Desalination through Zeolitic Imidazolate Framework Membranes: Significant Role of Functional Groups. Langmuir 2015, 31, 13230−13237. (26) Zhu, Y.; Gupta, K. M.; Liu, Q.; Jiang, J. W.; Caro, J.; Huang, A. S. Synthesis and Seawater Desalination of Molecular Sieving Zeolitic Imidazolate Framework Membranes. Desalination 2016, 385, 75−82. (27) Park, K. S.; Ni, Z.; Côté, A. P.; Choi, J. Y.; Huang, R.; UribeRomo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10186−10191. (28) Morris, W.; Leung, B.; Furukawa, H.; Yaghi, O. K.; He, N.; Hayashi, H.; Houndonougbo, Y.; Asta, M.; Laird, B. B.; Yaghi, O. M. A Combined Experimental−Computational Investigation of Carbon Dioxide Capture in a Series of Isoreticular Zeolitic Imidazolate Frameworks. J. Am. Chem. Soc. 2010, 132, 11006−11008.

(29) Wang, B.; Cote, A. P.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Colossal Cages in Zeolitic Imidazolate Frameworks as Selective Carbon Dioxide Reservoirs. Nature 2008, 453, 207−211. (30) Willems, T. F.; Rycroft, C. H.; Kazi, M.; Meza, J. C.; Haranczyk, M. Algorithms and Tools for High-Throughput Geometry-Based Analysis of Crystalline Porous Materials. Microporous Mesoporous Mater. 2012, 149, 134−141. (31) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. UFF, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024−10035. (32) Dubbeldam, D.; Frost, H.; Walton, K. S.; Snurr, R. Q. Molecular Simulation of Adsorption Sites of Light Gases in the Metal-Organic Framework IRMOF-1. Fluid Phase Equilib. 2007, 261, 152−161. (33) Babarao, R.; Jiang, J. W. Molecular Screening of Metal-Organic Frameworks for CO2 Storage. Langmuir 2008, 24, 6270−6278. (34) Liu, B.; Yang, Q.; Xue, C.; Zhong, C. L.; Smit, B. Enhanced Adsorption Selectivity of Hydrogen/Methane Mixtures in Metal− Organic Frameworks with Interpenetration: A Molecular Simulation Study. J. Phys. Chem. C 2008, 112, 9854−9860. (35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 09, revision D.01 ed.; Gaussian, Inc.: Wallingford, CT, 2009. (36) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926−935. (37) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. A 2nd Generation Force-Field for the Simulation of Proteins, Nucleic-Acids, and Organic-Molecules. J. Am. Chem. Soc. 1995, 117, 5179−5197. (38) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water Conduction through the Hydrophobic Channel of a Carbon Nanotube. Nature 2001, 414, 188−190. (39) van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. Gromacs: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26, 1701−1718. (40) Zhang, K.; Nalaparaju, A.; Chen, Y.; Jiang, J. W. Biofuel Purification in Zeolitic Imidazolate Frameworks: The Significant Role of Functional Groups. Phys. Chem. Chem. Phys. 2014, 16, 9643−9655. (41) Wang, Y.; Chung, T. S.; Neo, B. W.; Gruender, M. Processing and Engineering of Pervaporation Dehydration of Ethylene Glycol via Dual-Layer Polybenzimidazole/Polyetherimide Membranes. J. Membr. Sci. 2011, 378, 339−350. (42) Pendergast, M. T. M.; Hoek, E. M. V. A Review of Water Treatment Membrane Nanotechnologies. Energy Environ. Sci. 2011, 4, 1946−1971. (43) Kou, J.; Zhou, X.; Lu, H.; Wu, F.; Fan, J. Graphyne as the Membrane for Water Desalination. Nanoscale 2014, 6, 1865−1870. (44) Cohen-Tanugi, D.; Grossman, J. C. Water Desalination across Nanoporous Graphene. Nano Lett. 2012, 12, 3602−3608. (45) Xu, K.; Feng, B.; Zhou, C.; Huang, A. S. Synthesis of Highly Stable Graphene Oxide Membranes on Polydopamine Functionalized Supports for Seawater Desalination. Chem. Eng. Sci. 2016, 146, 159− 165. (46) Liu, X.; Demir, N. K.; Wu, Z.; Li, K. Highly Water-Stable Zirconium Metal−Organic Framework UiO-66 Membranes Supported on Alumina Hollow Fibers for Desalination. J. Am. Chem. Soc. 2015, 137, 6999−7002. G

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (47) Luzar, A.; Chandler, D. Hydrogen-Bond Kinetic in Liquid Water. Nature 1996, 379, 55−57. (48) Xu, H. F.; Berne, B. J. Hydrogen-Bond Kinetics in the Solvation Shell of a Polypeptide. J. Phys. Chem. B 2001, 105, 11929−11932. (49) Suk, M. E.; Aluru, N. R. Water Transport through Ultrathin Graphene. J. Phys. Chem. Lett. 2010, 1, 1590−1594.

H

DOI: 10.1021/acsami.6b01626 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX