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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Computational Insights on the Role of Nanochannel Environment in the CO/CH and H/CH Separation Using Restacked Covalent Organic Framework Membranes 2

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Minman Tong, Yadong Zhang, Tongan Yan, Guojian Chen, Zhouyang Long, Zhenglong Qin, Qingyuan Yang, and Chongli Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05183 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Computational Insights on the Role of Nanochannel Environment in the CO2/CH4 and H2/CH4 Separation Using Restacked Covalent Organic Framework Membranes Minman Tonga,*, Yadong Zhanga, Tongan Yanb, Guojian Chena,*, Zhouyang Longa,*, Zhenglong Qina, Qingyuan Yangb, Chongli Zhongb,c a. School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221116, China. b. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. c. School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China.

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ABSTRACT

To fabricate high performing ultrathin membranes, the interlayer nanochannel environment, which is hard to be observed and characterized in experiments, plays a key role in membrane performance.

In this work, a series of restacked ultrathin 2D-COF membranes are

computationally constructed to explore the influence of nanochannel microenvironment on the separation of CO2/CH4 and H2/CH4. The results show that the molecular sieving property can be achieved for both CO2/CH4 and H2/CH4 mixtures through the size control by tuning the nanochannels with different size. However, when fixing the size of the nanochannel, the energy control through changing the stacking modes of the few more layer restacked nanosheets is only effective in improving the membrane selectivity for CO2/CH4 but not for H2/CH4. Under energy control, CH4 permeance plays the key role in the CO2/CH4 separation performance, and heterogeneous energetic microenvironment with interlayer diffusion inlets and outlets in high potential energy is beneficial to decreasing the CH4 permeance and thus enhancing the membrane selectivity. The knowledge obtained in this work will enrich the understanding of the role of nanochannel microenvironment in separation, which can guide the discovery of ultrathin membranes with improved separation performance.


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Introduction Membrane separation technology bears the distinct merits including energy efficiency and environmental friendliness compared with the traditional separation technologies for gas separation.1 Membrane materials having a combined high gas permeance and selectivity are necessary to allow separation of the massive quantities of the released gases,2 which is still a challenge as for the fields of materials, chemistry and separation.3 To overcome the trade-off between permeability and selectivity which was usually encountered by polymer membranes,4-8 ultrathin membranes fabricated from various materials, such as diamond-like carbon nanosheets,9 zeolites,10 graphene and graphene oxide (GO) nanosheets,11 WS2 nanosheets,12 single-walled carbon nanotubes (SWCNTs),13 and MOFs,14 have attracted a burgeoning interest and emerged to show exhilarating performance in different separation applications. Currently, the related studies on ultrathin membranes are mainly focused on exploring the fabrication method of highly permeable and selective membranes, which include but not limited to the following aspects: a. preparation of thin and dense membrane with regard to pristine materials with suitable pore size,15,16 b. restacking the films or nanosheets of original materials with over-sized pores or defects to reduce the pore size,17 c. drilling holes on the nonporous sheets or introducing supports or linkers to prop up the layered structures to create appropriate gaps.2,18 Two-dimensional (2D) covalent organic frameworks (COFs) are a novel class of crystalline nanoporous materials with highly-ordered networks.19-21 The stacked 2D sheets are restrained via van der Waals forces to form a laminar structure.22,23 With the inherent well-defined pores of 2DCOFs, they can be directly used without efforts of drilling holes on the nanosheets. To date, single- or few-layered 2D-COF films have be grown or restacked on various substrates;24-28


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moreover, selfstanding porous membranes were also successfully derived from 2D-COFs,29 showing outstanding performance in gas separation, desalination and water purification. During separation, the nanochannel in these ultrathin membranes plays the dominant role in the separation performance.2,30 However, since the observation and characterization of the nanochannels are not easy to be carried out in experiments, studies on the effect of nanochannel physicochemical microenvironment on guest transport behaviors are scarce.2,30,31 In this work, the typical 2D COF-632 is chosen as model material to computationally construct outperforming few-layered 2D-COF membranes. From the perspective of size control and energy control, the influence of nanochannel microenvironment on the separation of industrial mixtures of CO2/CH4 and H2/CH4, which are two systems with quite dissimilar competitive adsorption and diffusion behaviors, are explored for the restacked ultrathin membranes.

Computational Details Membrane models The few layer stacked COF-6 membranes were constructed by using rectangular nanosheets with xy dimensions of 75.5 × 78.4 Å2 which were placed in the center of the simulation box with a height of 200 Å, as shown in Figure 1. A He wall composed of 812 He atoms was placed at the bottom of the permeate side to avoid molecules diffusing between the feed phase and permeate phase without passing through the membranes due to the periodicity in the z axis. A very weak interaction between the adsorbates and He atoms was assigned to guarantee no influence on the results.33


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He wall feed nanosheets

permeate He wall Figure 1. Illustration of the membrane models. Force fields In our work, the adsorbate–adsorbate and adsorbate–COF interactions were described using a combination of Lennard-Jones (LJ) and Coulombic potentials. CO2 was modelled as a rigid linear molecule with potential parameters taken from the EPM2 force field.34 Single-site spherical LJ 12-6 potential was used to model H235 and CH436 molecules. The LJ potential parameters for the framework atoms were taken from the Dreiding force field,37 and the DFT partial charges were taken from our previous work.38 The above set of force fields were validated to be reliable by well reproduction of the meausred CO2, CH4 and H2 adsorption isotherms (see Figure S1). Molecular dynamics simulations


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The transport behaviors of CO2/CH4 and H2/CH4 mixture through restacked COF-6 ultrathin membranes were studied by performing

molecular dynamics (MD) simulations in the NVT

ensemble at 300 K using the DL_POLY program.39,40 The feed side of each membane was filled up with equimolar CO2/CH4 or H2/CH4 mixtures. Initially, the number of molecules randomly distributed in the feed region are 75:75 for CO2/CH4 and 72:72 for H2/CH4, corresponding to the mixture with bulk pressure of 10 bar calculated by using Peng-Robinson equation of state to convert the number of molecules to pressure. The permeate region underneath the membrane was initially empty. To mimic the practical constant-pressure-gradient separation process, permeated molecules that successfully pass to the permeate side were removed every 50 ps, and certain number of molecules were added to the feed region according to the same initial composition.41 Therefore, the pressure drop of each gas component is considered as a constant equal to 5 bar. In the meantime, the number of molecules that successfully pass to the permeate side were monitored against a total simulation time of 20 ns. Note that the number of molecuels is constant in each 50 ps but variable in total 20 ns run. Nosé-Hoover thermostat was used to maintain the temperature condition constant. All the LJ interactions were calculated with a cutoff radius of 14.0 Å. The membrane structures were treated as rigid. The Ewald summation method used to evaluate the long-range Coulombic interactions. Periodic boundary conditions were applied in the three directions. The time step was taken as 1.0 fs in the MD simulations. Statistics of gas molecules successfully crossing the membranes During the simulations, gas molecules will be adsorbed on the membranes to form adsorbed layers due to strong interactions. To determine the adsorption-layer zones, distributions of CO2, CH4 and H2 molecules in the simulation box were plotted as a function of the distance from the membranes through the

statistics of the MD simulated trajectories (see Figures S3 and S4). It can


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be found that all the CO2, CH4 and H2 form strong adsorption layers on the membrane surface within a distance of about 6 Å, that is, the regions defined by the two green dotted lines, and hence these regions were considered as the adsorption-layer zone in this work. The molecules successfully passing across the adsorption-layer zone in the permeate phase were considered as the permeated molecules. Gas permeances were calculated using

Pi (mol m -2 s -1 Pa -1 ) =

Fi Δp i × A

(1)

where 𝑃" and 𝐹" are respectively the permance and molar flux of component i, ∆𝑝" is the pressure drop of gas i across the membrane. 𝐴 is the built membrane area. The membrane selectivity ( 𝑆(/* ) for component i over j was defined as the ratio of the permeances of the two components:

S i/j =

Pi Pj

(2)

Validation of the simulation model The CO2, H2 and CH4 gas permeability of COF-300 membrane at room temperature, which has been measured in experiments,42 was calcultaed to validate the reliability of our simulation model. The prepared COF-300 membrane is composed of roundish nanocrystallites and shows a continuous and uniform layer with thickness of 45 µm. The area of the simulated COF-300 membrane is 56.3 Å × 56.3 Å with a thickness of 44.4 Å. There are no binary mixture seapration data for CO2/CH4 and H2/CH4 in experiment, so we calcualte the single gas permeability (Figure


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2). The simulated gas permeability is slightly higher than the experimental value. The simulated selectivities for CO2/CH4 (0.80) and H2/CH4 (2.11) agree well the experimental values for CO2/CH4 (0.55) and H2/CH4 (3.25), demonstrating the simulation model is reasonable to predict the membrane separation property. The effect of time interval on the number of permeated molecules in COF-300 membrane was also evaluated, as shown in Figure S2. When the time interval decreased to 25 ps, the gas permeability and selectivity are close to that with interval of 50 ps. Therefore, time interval of 50 ps was chosen throughout this study.

7

10

0

Experimental selectivity 4 6

2

8

10 10

selectivity permeability

6

10

CO2 CH4

5

10

8 H2

6

4

4

10

3

10

H2/CH4

2

CO2/CH4

Simulated selectivity

(b) Simulated permeability (Barrer)

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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2

10

2

10

3

4

5

6

10 10 10 10 Experimental permeability (Barrer)

7

10

Figure 2. (a) The calculated number of permeated gas molecules as a function of time in COF300 membrane, the linear fit is used to calculate the gas permeance in the steady state of permeation, (b) comparison of simulated memnbrane properties with experimental values42 for COF-300 membrane. Potential energy distribution The 2D potential energy distributions in the layers of the few layer stacked membranes were calculated by using a simple Monte Carlo technique. Specifically, one pore of the 2D-COF sheet was divided into 20 circles and then the potential energies were scanned at an interval of two


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degree along the circumference of each circle. To obtain the lowest potential energies, 105 orientations were randomly generated at each scanning point for CO2, CH4 and H2 molecules.

Results and discussion Size control on two-layered COF-6 membranes COF-6 is a boron-containing COF that has one-dimensional pore with size of 8.6 Å. The pore is oversized for the small CO2, CH4 and H2 molecules with respectively kinetic diameter of ~3.3, 3.8 and 2.8 Å, leading to the Knudsen type transport of gases through the one-layered COF membrane No. 1, and thus the CO2/CH4 and H2/CH4 selectivities (respectively 0.8 and 2.5) are not high. According to the commonly used method of restacking the nanosheets of 2D nanomaterials, ultrathin COF-6 membrane with high gas permeance and high selectivity can be achieved for both CO2/CH4 (membranes No. 20-24) and H2/CH4 (membranes No. 25-29) separation system through elaborate shift of the stacked nanosheet, as shown in Figure 3.


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(a)

H1M distance (Å)

Membrane selectivity

CO2 or H2 permeance (GPU)

1

/

0.8

1.57×107

20

5.644

6.5

7.04×106

5.626

10.5

6.55×106

5.559

21.9

5.82×106

23

5.519

99.0

5.25×106

24

5.488

∞

4.40×106

1

/

2.5

4.53×107

H1

25

5.749

5.4

1.24×107

90° H3 M

26

5.712

9.1

1.17×107

5.651

15.9

1.11×107

28

5.546

81.1

1.02×107

29

5.432

∞

9.40×106

(b)

Membrane No.

21 22

H1 O1 H2

H3 H2

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27

Mixture

CO2/CH4

H2/CH4

Figure 3. (a) The two-layered COF-6 membranes after the shift of one nanosheet. H1, H2, H3 and O1 in framework are the positioning atoms for the precise control of the interlayer nanochannels. M represents the midpoint between the position of atom H2 and H3. During the shift, the distance between atom H3 and O1 is variable but the angle formed by H1MH3 is fixed at 90°. The white areas surrounded by the positioning atoms are the newly generated narrow interlayer nanochannels. The crimson arrows represent the diffusion inlet and outlet in the membrane, (b) separation performance of the one-layered COF-6 membrane No.1 and twolayered membranes No.20-29. 1 GPU gas permeance unit = 3.35× 10-10 mol m-2 s-1 Pa-1 To construct a two-layered ultrathin membrane with expected performance, the precise control on the size of the interlayer nanochannel should be accurate to less than 1 Å (as can be seen from Figure 3b), which is extremely difficult for experimentalist to operate during restacking the nanosheets.43 In practical membrane fabrication, usually, researchers will deposit more sheets on the ultrathin membrane to create more interlayer nanochannels, with the hope of


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realizing improvement on the separation performance. Hence, there currently is a high demand on revealing the effect of nanochannel physicochemical microenvironment on separation performance in few-layered membranes beyond the scope of molecular sieving.44 For this reason, membrane No. 21 for CO2/CH4 and membrane No. 26 for H2/CH4 with low selectivities are chosen to be deposited with more layers for further investigations. To reduce the influence of pore size effect when exploring the transport mechanisms in the few-layered membranes, the shape and size of the narrow interlayer nanochannels in the lately built few-layered membranes are kept unchanged compared with that in the pristine two-layered menbranes No. 21 and No. 26. The interlayer distances in all the restacked membranes are kept the same as that in the pristine structure of COF-6 (3.6 Å) according to the experimental observations on the structures of some few-layered 2D-COFs45,46 and layer-stacked graphene membranes16. The content on few-layered

COF-6 membrane for CO2/CH4 and H2/CH4

separation will be discussed separately. Energy control on few-layered COF-6 membrane for CO2/CH4 separation Initially, one more layer is added and shifted on the selected two-layered membrane but with

the

size of the narrow interlayer nanochannel remaining unchanged. Four three-layered membranes with different stacking modes, i.e. different orientations of the interlayer nanochannel, are derived based on the two-layered membrane No. 21. The channel structures of the obtained membranes and the corresponding separation performance are given in Table 1. The flow passages channels in these restacked membranes consist of alternate wide pores and narrow interlayer nanochannels. Due to the increasing thickness, all the three-layered membranes show approximately half decrease in CO2 permeance compared with the membrane No. 21. However,


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the CO2/CH4 membrane selectivity of the three-layered membranes are all improved, which can be ascribed to the synergistic effect from the prominently increased CO2 molecules adsorbed on the three-layered membranes. Table 1. Channel structures and the CO2/CH4 separation properties of the four three-layered COF membranes constructed based on two-layered membrane No. 21. Membrane No.

21

31

32

33

34

Top view a

Front view Adsorbed CO2 b

33

67

65

65

62

CH4 b

11

16

17

17

16

Permeated CO2 c

782

416

374

397

378

74

24

24

26

29

17.3

15.6

15.3

13

CH4 c

CO2/CH4 Membrane 10.5 selectivity aThe

black line in the top view represent the diffusion path in the membrane, and the dots and the arrows respectively locate at the middle layer and the last layer of the membrane. bThe statistical average values of the CO and CH molecules in the adsorption-layer zone after 5 ns. 2 4 cThe total number of CO and CH molecules appeared in the permeate phase in unit of molecules per 10 ns. 2 4

In light of the improved performance by introducing the third layer, the fourth layer is further added and shifted on the No. 31 membrane which shows the highest selectivity in the designed three-layered membranes. The rule and method for shifting the fourth layer are the same with that for previously getting three-layered membranes. Table 2 shows the channel structures, number of adsorbed molecules and membrane separation performance of the three so-obtained


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four-layered membranes (No. 41, 42 and 43). The numbers of adsorbed CO2 and CH4 molecules in membranes No. 41-43 are close to each other and are higher than that of the No. 31 membrane. The CO2 permeances are similar between the three four-layered membranes, but the CH4 permeance of membrane No. 43 is much higher than that of membranes No. 41 and 42, and thus enhancement on membrane selectivity are found in membranes No. 41 and 42 but not for membrane No. 43. Table 2. Channel structures and the CO2/CH4 separation properties of the four-layered COF membranes No. 41-43 constructed from three-layered membrane No. 31 and the further designed membrane No. 44.

Membrane No.

31

41

42

43

44

Top viewa

Front view Adsorbed CO2 b

67

88

88

87

89

CH4 b

16

22

20

23

20

Permeated CO2 c

416

244

238

234

220

8

8

14

6

32.5

31.7

16.7

35.2

CH4 c

24 CO2/CH4 membrane 17.3 selectivity aThe

black line in the top view represent the diffusion path in the membrane, and the dots and the arrows respectively locate at the middle layer and the last layer of the membrane. bThe statistical average values of the CO and CH molecules in the adsorption-layer zone after 5 ns. 2 4 cThe total number of CO and CH molecules appeared in the permeate phase in unit of molecules per 10 ns. 2 4


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To understand this result, we calculate the 2D potential energy maps for CO2 and CH4 in the pores of the 2nd and 3rd layers of the No. 41-43 membranes, as shown in Figure 4. The rose red (with lowest potential energy) and blue area (with highest potential energy) are respectively the most attractive and repulsive region for the calculated molecules. Since the 1st to 3rd layer of membranes No. 41-43 are the same, the potential energy maps of CO2 (Figure S5a-c supporting information) as well as CH4 (Figure S5e-g in supporting information) in the pores of the 2nd layer are the same for the No. 41-43 membranes. As to the potential energy maps of CO2 (Figure 4a-c) and CH4 (Figure 4e-g) in the pores of the 3rd layer, the layouts of membranes No. 41 and 42 are similar, especially the potential energy of the inlets and outlets marked by the dashed grey ellipses are the same, leading to the similar separation performance of membranes No. 41 and 42; however, membrane No. 43 shows a totally different energetic microenviroment. Considering the difference on membrane selectivity is mainly coming from the discrepancy in CH4 permeance, further analysis will be focused on CH4 molecules.


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CO2

3rd layer 26 25 24

25

a y[A]

23

22 21

18

19

26

b

23

23

22

23

24

25

-23.50 -21.90 -20.30 -18.70 -17.10 -15.50 -13.90 -12.30 -10.70 -9.100 -7.500 -5.900 -4.300 -2.700 -1.100

x[A]

j

E = -20.3

high

20

18

El = -34.2 20 21 22 23 ΔE = 32.2 x[A]

19

26

19 24

25

26

c

18

19

24 23

y[A]

23 22 21

El = -23.5 22 23 ΔE 21= 22.4 x[A]

20 -32.50 -30.40 -28.30 -26.20 -24.10 -22.00 -19.90 -17.80 -15.70 -13.60 -11.50 -9.400 -7.300 -5.200 -3.100 -1.000

g

25

24

y[A]

f

20 -34.20 -31.90 -29.60 -27.30 -25.00 -22.70 -20.40 -18.10 -15.80 -13.50 -11.20 -8.900 -6.600 -4.300 -2.000

21

19

24

25

k

E = -21.7

-23.20 -21.70 -20.20 -18.70 -17.20 -15.70 -14.20 -12.70 -11.20 -9.700 -8.200 -6.700 -5.200 -3.700 -2.200 -0.7000

low

22

Unit, kJ/mol

21

20

20

El = -32.5 ΔE = 31.5 x[A]

19 18

19

20

30

21

22

31

23

24

18

19

h

29

d

28 27

y[A]

26

26

25

25

24

24

23

12

13

El = -32.3 15 16 ΔE14 = 31.5 x[A]

El = -23.2 22 23 ΔE 21= 22.5

19 25

30

27

No. 44

19

22

20

28

18

25

y[A]

y[A]

25 26

24

29

El = -23.5 21 22 ΔE = 22.4

19 24

21

No.43

E = -20.3

22

24

25

i

20

El = -34.2 20 21 22 23 ΔE = 32.2 x[A]

19

No.42

-23.50 -21.90 -20.30 -18.70 -17.10 -15.50 -13.90 -12.30 -10.70 -9.100 -7.500 -5.900 -4.300 -2.700 -1.100

21

20

25

e

24

Structure

CH4

-34.20 -31.90 -29.60 -27.30 -25.00 -22.70 -20.40 -18.10 -15.80 -13.50 -11.20 -8.900 -6.600 -4.300 -2.000

26

23

y[A]

No.41

y[A]

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


20

-32.30 -30.20 -28.10 -26.00 -23.90 -21.80 -19.70 -17.60 -15.50 -13.40 -11.30 -9.200 -7.100 -5.000 -2.900 -0.8000

1822

-22.40 -21.00 -19.60 -18.20 -16.80 -15.40 -14.00 -12.60 -11.20 -9.800 -8.400 -7.000 -5.600 -4.200 -2.800 -1.400

25

l

E = -18.2 El = -22.4 ΔE = 20.0

23 17

24

x[A]

19

11

12

13

14

15

16

17

18

19

20

x[A]

Figure 4. Contour plots of the 2D potential energy distributions for (a-d) CO2 and (e-h) CH4 in the pores in the 3rd layers of the four-layered membranes No. 41-44. For subfigure (a-h), the two numbers (units: kJ/mol) under each map represent the values of the lowest potential energy (El, the most negative one) and the difference between the highest and lowest energies (ΔE). (i-l) The local structures that compose the pores in the 3rd layers of membranes No. 41-44. (2nd layer, pink; 3rd layer, green; 4th layer, aquamarine) The regions enclosed by the dashed grey ellipses in (a-h) and the blue ellipsoid in (i-l) are the inlets and outlets for interlayer diffusion. The number


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E written near the dashed grey ellipse represents the potential energy of the main area in the ellipse. For CH4 molecules in membranes No. 41 and No. 42, the inlets and outlets locate at the region mainly with color of green, white and pink where less CH4 molecules are absorbed, the regions away from the inlets and outlets show low potential energy where attract the major amount of CH4 molecules, these molecules are inclined to be stayed here rather than diffuse to the outlets of the membrane, and thus the probability for CH4 to escaping from the membrane is low. The potential energy of the pink area is -20.3 kJ/mol. For membrane No. 43, the inlets and outlets locate at the region mainly with color of rose red ( E = -21.7 kJ/mol ), where the energetic microenvironment is more favorable for the adsorption of CH4. Figure 4i-k depicts the framework atoms that form the pore in the 3rd layer of membranes No. 41-43. For membranes No.41 and 42, we can see the framework atoms in the area enclosed by the black dashed line are denser than other areas. The more framework surfaces here impose stronger van der Waals interactions on CH4 molecules, corresponding to the area with lowest potential energy (in color of rose red) and evident gradient change in energy shown in Figure 4e and f, acting as an energy trap that hinders the CH4 molecules from diffusing out. In contrast, the distribution of framework atoms in membrane No.43 is relatively homogeneous, leading to the smooth energetic environment in the pore. That is to say, the lower potential energy of the inlets and outlets and the smoother energetic environment in membrane No.43 together attract more CH4 molecules to pass by and then increase the CH4 permeance. Therefore, for CO2/CH4 separation in restacked few-layered COF-6 membrane, the control on CH4 permeance is of high significance in enhancing the membrane separation performance which can be achieved by constructing heterogeneous energetic environment through changing


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the framework atom distribution. According to this deduction, we further construct a fourlayered membrane No. 44 (see Table 2) with higher heterogeneity in energetic environment. The inlets and outlets are overlapped in each pore, as shown in Figure 4d, h and l. The gas-attractive framework atoms of the 2nd and 4th layer are concentrated in the area away from the inlet and outlet (Figure 4l), leading to the high potential energy of the inlet and outlet (mainly in color of light pink, E = -18.2 kJ/mol) as well as the high energetic heterogeneity in the pore (Figure 4h), and thus CH4 molecules are hard to escape from the membrane. In our previous study on fewlayered CTF-1 membrane for CO2/N2 separation,31 we found that CO2/N2 selectivity can also be improved by depositing more sheets; however, the control on permeance of faster CO2 rather than N2 plays the determining role in separation performance, and CO2 permeance is highly dependent on the membranes structure, demonstrating the different transport behavior is dependent on both the species of gas mixtures and the structure of pristine 2D-COFs. Figure 5 shows a comparison of the CO2/ CH4 separation performance of the designed twoand four-layered COF-6 membranes with some typical zeolite and MOF membranes.47,

48, 49

The

permeances of the few-layered membranes are significantly higher than the other membranes; moreover, both the size control (on one to two- layered membranes) and energetic control (on two to four- layered membranes) are effective in enhancing the membrane selectivity.


17


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Size control Energy control

Figure 5. Gas separation performance of the few-layered COF-6 membranes and some typical membranes.47-49 The black line represents the 2008 Robeson upper bound50 for CO2/CH4 separation. Blue and red lines represent the upper bounds for separation of CO2/CH4 (20:80) and CO2/CH4 (80:20) mixtures, respectively.51 The four-layered membranes are derived through depositing more layers on the two-layered membrane No. 21. A thickness of 1 μm is used in the conversion for the other compared typical membranes as their CO2 permeability is given in unit of Barrer in the literature. Energy control on few-layered COF-6 membrane for H2/CH4 separation Similar with the operation in constructing three-layered membranes for CO2/CH4 separation, four three-layered membranes with different stacking modes are derived based on the two-layered membrane No.26 (Table 3). With the addition of the third layer, the adsorption amount of CH4 increases from 15 to ~40 while the amount of H2 keeps unchanged as 3; moreover, both the gas (H2 and CH4) permeance and the H2/CH4 selectivity are decreased.


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Table 3. Channel structures and the H2/CH4 separation properties of the four three-layered COF membranes constructed based on two-layered membrane No. 26.

Membrane No.

26

35

36

37

38

Top view a

Front view Adsorbed H2 b CH4 b Permeated H2 c CH4 c H2/CH4 membrane selectivity

3

3

3

3

3

15

40

40

39

39

1396

531

670

556

347

154

69

111

131

129

9.1

7.7

6.0

4.2

2.9

aThe

black line in the top view represent the diffusion path in the membrane, and the dots and the arrows respectively locate at the middle layer and the last layer of the membrane. b The statistical average values of the H and CH molecules in the adsorption-layer zone after 5 ns. 2 4 c The total number of H and CH molecules appeared in the permeate phase in unit of molecules per 10 ns. 2 4

To figure out the effect of film thickness on H2/CH4 separation, the fourth layer is further added and shifted on the membrane No.35 which shows relatively better performance in membranes No. 35-38. With the restacking of the fourth layer, the adsorption amount of CH4 in the three obtained four-layered membranes increases from ~40 to 51~56 while the amount of H2 still keeps unchanged as 3 (Table 4). H2 permeance continues to decrease, but, on the contrary, CH4 permeance increases in membranes No. 46 and 47. Hence, the performance of membranes No. 46 and 47 is even worse than that of membrane No.35. The H2/CH4 selectivity of membrane No.


19


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45 is slightly higher than that of membranes No.35, 46 and 47 but still worse than the pristine two-layered membrane No. 26. In the H2/CH4 mixture, light H2 is the weakly adsorbed and fastly diffused species during permeation. The increasingly adsorbed CH4 molecules would occupy the pores in the membranes and prevent H2 from diffusing. That is to say, restacking more layers on two-layered membrane is ineffective in enhancing the H2/CH4 separation performance if the size of the interlayer nanochannel can not be narrowed down. Table 4. Channel structures and the H2/CH4 separation properties of the three four-layered COF membranes constructed based on two-layered membrane No. 35.

Membrane No.

35

45

46

47

Top view

Front view Adsorbed H2 a

3

3

3

3

CH4 a

40

51

55

56

Permeated H2 b

531

439

410

379

CH4 b

69

51

78

75

H2/CH4 membrane selectivity

7.7

8.7

5.3

5.0

aThe

black line in the top view represent the diffusion path in the membrane, and the dots and the arrows respectively locate at the middle layer and the last layer of the membrane. b The statistical average values of the H and CH molecules in the adsorption-layer zone after 5 ns. 2 4 c The total number of H and CH molecules appeared in the permeate phase in unit of molecules per 10 ns. 2 4


20

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With regard to the comparison between membranes No. 45-47, Figure 6 presents the 2D potential energy maps for CH4 in their pores of the 3rd layers. The inlets and outlets in membrane No. 45 are overlapped and located at the region with high potential energy (mainly with color in white) for CH4, but for membranes No. 46 and 47, the inlets and outlets are located at the region with low potential energy (mainly with color in pink). In this case, CH4 molecules are not easy to appear near the inlets and outlets in membrane No. 45, and hence the probability for CH4 molecules to diffusing out is lower than that of membranes No. 46 and 47. No.45

No.46

No.47

-1.3

3rd layer -22.3 Unit, kJ/mol

Figure 6. Contour plots of the 2D potential energy distributions for CH4 in the pores in the 3rd layers of the four-layered membranes No. 45-47. The regions enclosed by the dashed grey ellipses are the inlets and outlets for interlayer diffusion. For membrane No. 45, the inlets and outlets are overlapped. The comparison between the H2/CH4 separation performance of the designed two- and fourlayered COF-6 membranes and some typical zeolite and MOF membranes47,52 are shown in Figure 7. Similar with the condition in CO2/CH4, the high gas permeance is a distinct advantage of the few-layered membranes; however, only the size control (on one to two- layered membranes) is effective in improving the membrane selectivity, while and energetic control (on


21


Page 22 of 32

two- to four- layered membranes) through regulating the stacking modes is invalid and may even decrease the membrane selectivity.

Size control

Energy control

Figure 7. Gas separation performance of the few-layered COF-6 membranes and some typical membranes.47,52 The black line represents the 2008 Robeson upper bound50 for H2/CH4 separation. The four-layered membranes are derived through depositing more layers on the twolayered membrane No. 26. A thickness of 1 μm is used in the conversion for the other compared typical membranes as their H2 permeability is given in unit of Barrer in the literature.

Conclusion In summary, a series of restacked COF-6 ultrathin membranes are constructed for industrial CO2/CH4 and H2/CH4 separation. Molecular sieving ability can be achieved by regulating the size of interlayer nanochannels, indicating the potential of COF-6 serving as ideal platform for fabricating highly permeable and highly selective membranes. Through depositing more sheets


22

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on the two-layered COF-6 membranes, the derived membrane performances are completely different with regard to the different targeted separation systems. For CO2/CH4 separation, in the premise of not minishing the size of the interlayer nanochannels, the control on the permeance of slow CH4 plays the key role in the membrane separation performance when stacking more layers. Heterogeneous energetic microenvironment with interlayer diffusion inlets and outlets in high potential energy is beneficial to decreasing the CH4 permeance and thus enhancing the membrane selectivity. To create this energetic microenvironment, it is suggested to increase the imbalance of the framework density around the interlayer nanochannels and to decrease the framework density near the diffusion inlets and outlets. This finding is totally different from our previous results on few-layered CTF-1 membrane for CO2/N2 separation, in which we found CO2 permeance is highly dependent on the membranes structure and the control on the permeance of fast CO2 plays the determining role in membrane separation performance. For H2/CH4 separation, the membrane selectivity can not be improved by tuning the stacking modes of the added layers once the size of the interlayer nanochannel is fixed, since the increasingly adsorbed CH4 molecules after restacking more layers will severely prevent H2 from diffusion. The information obtained in this work will enrich the understanding of the role of nanochannel microenvironment in the separation, and thus provide useful guidance for the discovery of ultrathin membranes with improved separation performance.

ASSOCIATED CONTENT Supporting

Information.

The Supporting Information is available free of charge on the ACS

Publications website. Probability density distributions of CO2, CH4 and H2 molecules in ultrathin


23


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COF-6 membrane; 2D potential energy distributions for CO2 and CH4 in the pores in the 2nd layers of the four-layered membranes.

AUTHOR INFORMATION Corresponding Author Email: [email protected]; [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully thank the Natural Science Foundation of China (Nos. 21706106, 21603089, 21503098, 21536001 and 21878229), the Jiangsu Province Science Foundation for Youths (BK20160209), the Natural Science Foundation of Jiangsu Normal University (16XLR011) and Priority Academic Program Development of Jiangsu Higher Education Institutions for financial support.

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[52] Altintas, C.; Avci, G.; Daglar, H.; Gulcay, E.; Erucar, I.; Keskin, S. Computer Simulations of 4240 MOF Membranes for H2/CH4 Separations: Insights into Structure–Performance Relations. J. Mater. Chem. A 2018, 6 (14), 5836. TOC Graphic Size control

Both CO2/CH4 and H2/CH4 selectivity CO2

Energy control

Only CO2/CH4 selectivity

CH4 H2


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