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Fast Desalination by Multilayered Covalent Organic Framework (COF) Nanosheets Wei Zhou, Mingjie Wei, Xin Zhang, Fang Xu, and Yong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01883 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Materials & Interfaces

Fast Desalination by Multilayered Covalent Organic Framework (COF) Nanosheets

Wei Zhou, Mingjie Wei*, Xin Zhang, Fang Xu and Yong Wang*

State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, and College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, Jiangsu (P. R. China)

*Corresponding author. Tel.: +86-25-8317 2247; Fax: +86-25-8317 2292. E-mail: [email protected] (M. Wei); [email protected] (Y. Wang)

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Abstract Covalent organic frameworks (COFs) are penetrated with uniform and ordered nanopores, implying their great potential in molecular/ion separations. As an imine-linked, stable COF, TpPa-1 is receiving tremendous interest for molecular

sieving

membranes.

Theoretically,

atomically

thin

TpPa-1

monolayers exhibit extremely high water permeance but unfortunately no rejection to ions because of its large pore sizes (~1.58 nm). COF monolayers tend to stack to form laminated multilayers, but how this stacking influence water transport and ion rejections remain unknown. Herein, we investigate the transport behavior of water and salt ions through multilayered TpPa-1 COFs by nonequilibrium molecular dynamics simulations. By analyzing both the interfacial and interior resistance for water transport we reveal that with rising stacking numbers COF multilayers exhibit increasing ion rejections at the expense of water permeance. More importantly, stacking in the offset eclipsed fashion significantly reduces the equivalent pore size of COF multilayers to 0.89 nm and ion rejection is correspondingly increased. Remarkably, 25 COF monolayers stacked in this fashion give 100% MgCl2 rejection while the water permeance remain 1 to 2 orders of magnitude higher than that of commercial nanofiltration membranes. This work demonstrates the rational design of fast membranes for desalination by tailoring stacking number and fashion of COF monolayers.

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Keywords: membrane separation; transport resistance; nonequilibrium molecular dynamics; covalent organic frameworks (COFs); desalination

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1. Introduction Two dimensional (2D) nanosheets with atomically thin thickness have attracted extensive interests in the fabrication of highly permeable membranes1-3. Covalent organic frameworks (COFs)4 are distinguished from other 2D nanosheets because of the presence of ordered uniform sub-5 nm pores penetrating through their 2D planes. The covalently bonded framework restricts COFs into 2D single-layered sheets which are then stacked together via van der Waals forces to form a laminar structure5-6. Importantly, the designable chemical compositions and structures as well as the robust chemical and thermal stability make COFs highly appealing for membrane separations 4, 7-11. To achieve nanosheets with ultralow thicknesses, bulk COF particles are usually exfoliated into ultrathin flakes, which are then deposited onto substrates to form thin layers for separations12-16. Banerjee et al. delaminated imine-based COF particles with high crystallinity and chemical stability to produce nanosheets of 10-30 COF monolayers13, and very recently the exfoliated microporous COF nanosheets were deposited on inorganic substrates to prepare composite membranes16. Alternatively, several methods have also been developed to grow COF layers on various substrates17-19. Dey and coworkers reported a bottom-up interfacial crystallization strategy to fabricate large-area thin films with sub-100 nm thickness20. The COF layers synthesized by interfacial polymerization were transferred to polymeric 4


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substrates to prepare thin-film composite nanofiltration membranes21. Matsumoto et al. further prepared free-standing COF films with thickness tunable from 2.5 nm to 0.1 mm via the similar strategy of interfacial synthesis22. In addition, our group used the layer-by-layer23 and interfacial polymerization24 methods to in-situ grow COFs on porous substrates. However, in these cases, COF particulates or films composed of a large number of stacked COF monolayers were produced and separations were realized by transport through the pores along the multilayered COF sheets. Although there are a few reports on the preparation of COF monolayers on solid substrates25-26, it is extremely challenging to synthesize or transfer single-layered COF sheets with reasonable lateral sizes on porous substrates required for membrane separations. Therefore, the simulation studies on COFs should focus on the multilayer which is close to the real-world preparations and applications. However, molecular dynamics (MD) simulations on 2D materials were predominantly applied to atomically thick monolayers. Cohen-Tanugi et al. showed that nanometer-scale pores in single-layered freestanding graphene can effectively filter NaCl salt from water27. Simulation also indicated that graphynes exhibited water permeance several orders of magnitude higher than conventional reverse osmosis membranes28. For 2D COFs, Lin et al. demonstrated that COF nanosheets were promising building blocks to produce highly permeable reverse osmosis (RO) membranes29. Zhang et al. also reported that a series of 2D functional COF membranes were favored for water 5



desalination

30.

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These simulation works indicate the extremely high water

permeance through the 2D materials, however, this high permeance only occurs when the 2D materials are in the form of atomically thick monolayers. Unfortunately, the simulation findings can hardly be realized in real world because of the difficulties in the experimental accessibility of COF monolayers. Moreover, the exceptionally high permeance observed by simulations on atomic monolayers cannot directly explain or predict the permeance of multilayered COF films in which the stacking number and the stacking fashion of COF monolayers are expected to play significant roles.

we have to turn to

multilayered COFs for real-world preparations and applications of COF membranes. To this end, it is required to understand the transport mechanism through multilayered COFs and to develop a reliable method to predict the permeance and rejections of COFs with a specific stacking mode. In this work, we conduct systematic nonequilibrium molecular dynamics (NEMD)

simulations

on

water

transport

through

single-layered

and

multilayered TpPa-1, which is an extensively used, highly water-stable COF13, 31-32.

To explore the transport resistance and its composition, we develop an

equation to describe transport resistance in which both the interior and interfacial resistance are considered. We identify the origin of the two contributions to the transport resistance, and elucidate their relationships with the number of COF layers. It is found that the offset eclipsed COF layers, which are energy favorable, give much smaller equivalent pores compared to 6


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the pores in the COF monolayers, thus delivering 100% rejection to MgCl2 while the permeance remains 1-2 orders of magnitude higher than that of commercial nanofiltration membranes.

2. Simulation details 2.1 Construction of molecular models TpPa-1

is

synthesized

from

1,3,5-triformylphloroglucinol

and

p-phenylenediamine. Its atomistic structure is shown in Fig. 1a. The center-to-center diameter of TpPa-1 is 1.8 nm and the corresponding effective diameter is approximately 1.58 nm. The length of the periodic cell in x and y directions are 3.9 and 4.6 nm, respectively. If the TpPa-1 monolayers are stacked in the fully eclipsed fashion, the formed multilayered nanosheets will exhibit cylindrical pores that run parallel to the direction of stacking (Fig. 1). A single-layered or multilayered TpPa-1 nanosheet was then placed in the direction parallel to the xy plane with a water reservoir on both sides (Fig. 1b), similar to most NEMD works27. The thickness of water reservoirs in this work was set to 2.5 nm and the dark blue area that is encompassed with dashed line was the region of the applying external force. The external force added on each water molecule that creates a pressure drop (ΔP) across the membrane is given by: (1)

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where A is the area of the membrane, and n is the total number of water molecules in the region of the external force. NEMD has been applied to establish a constant ΔP and hence maintain a steady state water permeation across the TpPa-1 nanosheet. ΔPs, up to 50 MPa, usually used in the NEMD simulations are higher than experimental values because large ΔPs lead to high signal-to-noise ratios, so that the sampling time is controlled in the simulation scale. For each simulation, an energy

minimization

was

first

carried

out,

followed

by

a

1

ns

isothermal–isobaric ensemble (NPT) simulation to equilibrate the system at 1 atm and 298 K. A 20 ns canonical ensemble (NVT) simulation with a time step of 1 fs was conducted while the first 10 ns was used to stabilize the flow and the last 10 ns were used for data analysis. The water or ion flux was calculated by directly counting the net number of water molecules passing through the TpPa-1 nanosheet during a span of simulation time.

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Figure 1. (a) The periodic structure of TpPa-1, O: red, N: blue, C: cyan, H: white; (b) The snapshot of the simulation process of water transport through multilayered TpPa-1 stacked in the fully eclipsed fashion. The thickness of water reservoirs was set to 2.5 nm and the dark blue area was where the external force region was located.

2.2 Details of Non-equilibrium molecular simulations All the simulations were performed with the LAMMPS package. The water model of SPC/E was adopted and the SHAKE algorithm was employed to maintain the rigidity of the water molecules and save simulation time. Non-bonding interactions were given as: (2

)

where εij and σij are the well depth and collision diameter, respectively, rij is the distance between atoms i and j, qi is the partial charge of atom i, the cutoff of the LJ and the electrostatic interaction distance is 1.0 nm. The LJ potential parameters for the COF atoms were taken from the Dreiding force field that has been widely used in the studies of COF or metal organic framework materials29,

33-35.

The particle-particle particle-mesh with the

accuracy of 10-4 was employed to calculate the long-range electrostatic 9



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interactions. Periodic boundary conditions were applied in all the three directions.

The

pressure

and

temperature

were

modulated

by

the

Nosé-Hoover barostat and thermostat, respectively. The external forces would bring a constant energy input. In order to dissipate excess energy and to perturb the dynamics of water molecules as litter as possible, Berendsen thermostats were applied to the water molecules. The temperature of water is calculated after subtracting the center-of-velocity while applying the NVT at 300 K on water molecules,

which has been validated before 36.

3. Results and Discussion 3.1 Permeance of TpPa-1 monolayer Pure water fluxes (PWFs) of the single-layered TpPa-1 are firstly plotted as a function of ΔPs in Fig. 2a. The water flux was calculated by directly counting the net number of water molecules passing through the TpPa-1 nanosheet during a nanosecond. The PWF rises from 0.91 × 103 to 5.95 × 105 L m-2 h-1 while ΔP increases from 50 to 250 bar. There is a clear proportional relationship between PWF and ΔP. Such relationship obeys the Darcy’s law in which the PWF should be proportional to ΔP37. The similar relationship was also observed in carbon nanotube pores38-39 and graphene silt pores40. After fitting the data in Fig. 2a, the slope of fitting line represents the permeance of the single-layered TpPa-1, which is 23798 L m-2 h-1 bar-1 (LMHB). Zhang et al. also reported that the permeance of a series of 2D functional single-layered TpPa-1 membranes for water desalination is about 1000 (LMHB), since the 10


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pore diameters of TpPa-1 are reduced by the functional groups41. Due to the atomic thickness of the single-layered TpPa-1, its water permeance is three orders of magnitude higher than commercial nanofiltration membranes with similar pore sizes, whose water permeance is less than 20 LMHB while their thickness is around 100 nm. Moreover, compared to single-layered porous graphene (water permeance: ~100 LMHB)42, the single-layered TpPa-1 also exhibits hundreds times higher permeance because of its extremely high porosity (TpPa-1: 44%, Graphene: 3%). The exceptional water permeance may imply the significant potential of TpPa-1 in water treatment and desalination, while the pore size is unchanged and ion rejection might consequently keep as high as those of thick membranes.

Figure 2. (a) Pure water flux (PWF) of the single-layered TpPa-1 as a function of pressure drops; (b) Resistance for the multilayered TpPa-1 as a function of the number of monolayers.

3.2 Transport resistance of multilayered TpPa-1 11



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As we discussed above, multilayered COF nanosheets are the realistic and reliable choice from the standing point of real-world preparations and applications. We then investigate transport behavior inside multilayered COF structures. As described in our previous work43, there is a linear relationship between membrane thickness and resistance. Therefore, the resistance outside the range of our simulations can also be estimated because of the linear relationship. The equation developed from this linear relationship can predict the PWF at a wider range of the membrane thickness. The total resistance of membranes can be defined as44 (3)

where J is PWF, m3 s-1. ΔP is a pressure drop across the membrane, Pa, R is the total resistance, Pa s m-3. Our previous work43 demonstrated that the total resistance (Rtotal) is composed of the interfacial resistance (Rinterfacial) and the interior resistance (Rinterior): (4) where Rinterior is related with the membrane thickness while Rinterfacial is independent of thickness. According to Sampson’ equation38-39,

45,

the

interfacial resistance can be described as, (5)

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where r is the pore radius, μ = 0.91 × 10-3 Pa s is the dynamic viscosity for SPC/E water and C is the loss coefficient that is a constant related physical and chemical properties of TpPa-1 in this work. The interior resistance is defined by Darcy’s law40, which can be written in the resistance form as (6)

where κ is intrinsic permeability of the membrane material and lmem is the thickness of the membrane, which is depended on the number of TpPa-1 monolayers in this work. Therefore, the total resistance can be described as a function of layer numbers: (7)

By plotting the ΔP/J as function of layer numbers, the slope and intercept can be obtained by fitting the data and then the total resistance can be estimated In Fig. 2b, the total resistance is actually linear with the number of TpPa-1 monolayers. Moreover, there is an evident intercept while the layer number extends to zero, confirming the existence of both interfacial and interior resistance. By fitting the data in Fig. 2b, we obtain the relationship between resistance and the membrane thickness as (8) Correspondingly, the values of

and C are 7.03 × 1019 m2 and 3.30 × 108 13



respectively, while r is 0.79 nm according to the diameter of TpPa-1. lmem is the thickness of the membrane, which is depended on the number of TpPa-1 monolayers, thus, the unit of lmem is defined as nanometer in this work. Eq. 7 is the description of total resistance for all kinds of membranes, in which the interior resistance is proportional to the membrane thickness while the interfacial one has no relationship with the membrane thickness. Eq. 8 is the fitting result of Fig. 2b base on Eq. 7 The resistance distributions for TpPa-1 nanosheets with changing layer numbers are calculated and shown in Fig. 3. Blue pillars on the bottom denoted the interfacial resistance part while cyan pillars on the top denoted the interior resistance. The value of Rinterfacial is dominant in the single-layered COF nanosheet. When the layer number increases from 1 to 10, the Rinterior linearly rises from 2.15 × 1023 to 2.26 × 1024 Pa s m-3, but the Rinterfacial keeps unchanged. Correspondingly, the ratio of Rinterfacial/Rtotal rises from 26% to 78%. Importantly, by solving Eq. (8) we can conveniently predict the PWF of a specific TpPa-1 membrane if the layer numbers or thickness of the TpPa-1 nanosheet is known.

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Figure 3. The resistance distribution for TpPa-1 nanosheets with changing numbers of stacked monolayers

3.3 Molecular details of transport resistance To understand the source of the interfacial and interior resistances, it is required to figure out the molecular details of water molecules while they transport through the TpPa-1 nanosheets. Our previous work demonstrates that larger pores exhibit lower interfacial resistance43. It is also found that more polar groups, which lead to stronger hydrophilic pore walls, result in the reducing interfacial resistance on one hand. At the same time, the hydrophilic pore walls rise the interior resistance due to stronger electrical interactions of polar groups with water molecules on the other46. The TpPa-1 nanosheets 15



contain partially charged oxygen and nitrogen atoms along the pore wall, which are expected to have strong interactions with water molecules. Taking the cases of 5-layered TpPa-1 as examples, the xy planar density distributions of water molecules at layered positions and interstices between two layers are plotted in Fig. 4a and Fig. 4b, the water density at positions of the six corners of the hexagon is much higher than other regions. Moreover, the high-density areas exactly correspond to areas near oxygen atoms on inner pore wall. This is because the electrostatic interactions will result in the aggregation of water molecules47. It is clear, from Fig. 4b, that the six-corner high-density region extends to other area near the pore wall compared to Fig. 4a, because the interstices between monolayers provide larger space for water molecules to stay. However, these water molecules need to aggregate so as to cross over the edge of the pore at positions of layered TpPa-1, and thus consume additional energy cost compared to smooth pore walls like carbon nanotubes. It is concluded that the electrostatic interactions between water molecules and the hydrophilic atoms of TpPa-1 nanosheets certainly bring additional frictional force against water transport48. Besides, the changes of water density at the layered positions and interstices between TpPa-1 layers might attribute to the interior resistance because of the frequent forming and breaking of hydrogen bonds (HBs). To verify this assumption, the average numbers of HBs, NHB are calculated along the z-axis in Fig. 4c and Fig.4d, while the HB is defined by the 16


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geometrical criterion40, 49. It is obvious that NHB maintains at 3.5 in bulk water reservoirs but reduces to around 3.1 at the entrance of TpPa-1 pores. This observation interprets the breaking of HBs for water molecules. This HB breaking is similar to the dehydration of ions, thus indicates the energy loss at the entrance and likely the source of the interfacial resistance. Furthermore, curves of NHB oscillate regularly while the valleys occur at the position of each TpPa-1 monolayer and the peaks are usually located at interstices between two TpPa-1 monolayers. These changes of NHB can be ascribed to the frequent forming and breaking of HBs, which results in the interior resistance. Potential of mean force (PMF) is usually used to evaluate the blocking capability of pores to the passage of fluids48. The PMF along the z axis for water molecules was further calculated by using the average densities in equilibrium simulations using the fact that PMF(z)=-kTln(ρ(z)/ρo), where ρ(z), ρo are water density at position z and in bulk, respectively48. We calculated the average density of water inside the pore is about 0.5 g cm-3, thus the average PMF of water inside the pore is about 0.4 kcal mol-1. It should be noted that the density profile was calculated based on the volume including TpPa-1 atoms. The energy changes between interior and outside of TpPa-1 nanosheets represent the barriers of water molecules entering into pores. In Fig. 4c and Fig. 4d, the energy barriers (the height of peaks) for the 5 and 10 layers of TpPa-1 are almost the same, in accord with the same interfacial resistance for both TpPa-1 nanosheets. Both NHB and PMF results confirmed the additional 17



interior resistance due to the roughness of multilayered COF membranes.

Figure 4. The maps of water molecule density distribution in xy plates of (a) the layered position and (b) the interstices between two layers. The density values are shown in colors while red represent 5.88 g cm-3 and blue represents 0 g cm-3. The carbon atoms are replaced in grey for clarity while colors of other atoms are the same as those in Fig. 1. Average hydrogen-bond numbers and the potential of mean force (PMF) of water molecules along the z-axis cross the pores of TpPa-1 nanosheets with (c) 5 and (d) 10 TpPa-1 monolayers. The dashed lines represent the positions of the TpPa-1 monolayers.

3.4 Performance of multilayered TpPa-1 in offset eclipsed fashion Although the crystal structure of TpPa-1 is well characterized by powder 18


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X-ray diffraction, the detailed stacked structure of the TpPa-1 monolayers is not yet fully elucidated. It has been commonly assumed that the COF monolayers are stacked in the fully eclipsed manner. However, a recent work showed that stacking with small horizontal offsets was more stable than that of the fully eclipsed stacking despite the same patterns in simulated powder X-ray diffraction13. As illustrated in Fig. 5a, we constructed multilayered TpPa-1 nanosheets with offsets, termed as offset eclipsed structure, by performing an energy optimization process via MD simulations. Specifically, the energy optimization process allows nanosheets to move freely in all directions but no rotation is allowed. Fig. 5a shows the front and top view of the multilayered TpPa-1 structures before and after energy optimization. It is clear that there are offsets between TpPa-1 monolayers the shape of the pore mouth also slightly changed after energy optimization as observed in a previous work13. The average offsets between two adjacent monolayers are estimated to be 0.07 nm13. As shown in Fig. 1a, the TpPa-1 monolayers have the pore size of 1.58 nm, which will result in low rejection to ions. From the top view of Fig. 5a, the pore size is measured to be 0.89 nm for the offset eclipsed nanosheets, which are expected to give higher salt rejections50-51. To compare the desalination performance of the COF nanosheets stacked in the two different structures, the MgCl2 aqueous solution with the concentration of 0.5 M was selected as the feed solution. The rejection was calculated by the following equation52: 19



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(9 )

where Jion and Jwater are flux of ions and water, respectively, which are calculated by directly counting the net number of ions or water molecules passing through the TpPa-1 nanosheet during the span of simulation time. Cwater and Cion are concentrations of water and ion in the bulk phase, respectively. The value of Cwater/Cion is 90.91, which is corresponding to the concentration of 0.5 M. TpPa-1 nanosheets composed of 5, 10, 15, 20, 25 monolayers stacked in the fully eclipsed and offset eclipsed fashion were constructed for comparisons.

Figure 5. (a) The multilayered TpPa-1 structures before and after energy 20


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optimization. The colors of atoms are defined the same as Fig. 1; (b) The permeance and MgCl2 rejection of the multilayered TpPa-1 as a function of the number of layers; (c) The resistance as a function of the length of TpPa-1; (d) Average number of hydrogen bond profiles along the flow direction (z-axis). The dashed lines represent the entrance and exit of the membrane.

The permeance and salt rejection for both fully eclipsed and offset eclipsed COF nanosheets are plotted in Fig. 5b. For the fully eclipsed structure, the permeance reduces from 11876 to 3207 LMHB with the increase of layer numbers from 5 to 25 while the MgCl2 rejection rises from 0% to 42%. The offset eclipsed structure exhibits lower permeance due to increased transport resistance. However, the MgCl2 rejection performs much better as the rejection reaches 100% while the stacking number is 25. To find the reason behind, we also turn to the analysis of transport resistance. The relationships between the resistance and the number of monolayers of the membrane for both cases are plotted in Fig. 5c. The linear relationship remains as expected while the layer number reaches 25. The fitting equation for the offset eclipsed structure is given in Eq. (10). (10) The intercepts of the fitting line of the offset eclipsed and fully eclipsed structure are supposed to be close. However, it should be noted that the interfacial resistance obtained from Eq.7 and Fig. 2b (or Fig. 5c) has large 21



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systematic residuals. While the thickness reaches to 2 nm or larger, the interfacial resistance contributes very little to the total resistance. Therefore, it is very possible that the systematic residuals might be as large as or even much larger than the values of interfacial resistance while fitting the data. In addition, the slopes are inversely proportional to the square of radius according to Eq. (7). (13) As mentioned above, the effective radius of fully eclipsed nanosheets (rfully) is 0.79 nm. Therefore, the equivalent radius of offset eclipsed nanosheets (roffset) can be calculated as 0.445 nm. That is, the effective diameter of offset eclipsed nanosheets is reduced by 44%, which explains the rise of MgCl2 rejection from 42% to 100%. For the offset eclipsed TpPa-1 nanosheet with 25 monolayers, it exhibits a complete rejection to MgCl2 and a water permeance of 1118 LMHB, which is still 1 to 2 orders of magnitude higher than polyamide nanofiltration membranes. In addition, the estimated permeance of TpPa-1 membranes at experimental condition (250 nm) is 116 LMHB, which is slightly higher than the reported results, 85 LHMB24. NHB distributions along the flow direction for these two stacking structures are shown in Fig. 5d. The vibration amplitude of the shifting-eclipsed structure is much larger than the fully eclipsed one. Such large vibration amplitude origins from more frequent forming and breaking of HBs inside offset eclipsed pores, consequently leads 22


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to higher interior resistance.

4. Conclusions Water transporting through TpPa-1 nanosheets is investigated via NEMD simulations. The permeance of single-layered TpPa-1 is several orders of magnitude higher than conventional nanofiltration membranes. However, the multilayered TpPa-1 nanosheets, which are the realistic choice for the real-world applications, exhibits reduced permeability with rising numbers of stacked COF monolayers. Resistance of water transport through COF nanosheets is identified to be contributed by the interfacial part and the interior part. Forming/breaking of hydrogen bonds as well as the interaction between oxygen and nitrogen atoms on pore walls and water molecules determines the interior resistance while the breaking/forming of hydrogen bonds contributes to the interfacial resistance. Considering both the interior and interfacial resistance, we develop an equation to predict water permeance in multilayered TpPa-1 nanosheets. The stacking fashion of the COF monolayer also significantly influences the permselectivity of multilayered COFs. The permeance of offset eclipsed TpPa-1 is smaller than the fully eclipsed one because the equivalent diameter of pores is reduced from 1.58 to 0.89 nm. However, MgCl2 rejection is significantly increased from 42% to 100% while the permeance remains as high as 1118 L m-2 h-1 bar-1, implying that offset eclipsed multilayered COFs deliver customizable nanofiltration performances 23



depending on the number of stacked COF monolayers and their stacking fashion. Acknowledgement Financial support from the National Key Research and Development Program of China (2017YFC0403902), the National Basic Research Program of China (2015CB655301), the National Natural Science Foundation of China (21825803), and the Jiangsu Natural Science Foundations (BK20150063). We are also grateful to the High Performance Computing Center of Nanjing Tech University for supporting the computational resources. This work is also supported by the Program of Excellent Innovation Teams of Jiangsu Higher Education Institutions and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Pure water flux as a function of the membrane thickness; Density of water molecules along the z-axis; Density and potential of mean force of water molecules along the z-axis when excluding the volume of TpPa-1; NaCl

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(COF) Nanosheets - ACS Publications - American Chemical Society

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