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Adsorption and Membrane-Based CH4/N2 Separation Performances of MOFs Zeynep Sumer, and Seda Keskin Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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Industrial & Engineering Chemistry Research

Adsorption and Membrane-Based CH4/N2 Separation Performances of MOFs Zeynep Sumer and Seda Keskin* Department of Chemical and Biological Engineering, Koc University, Rumelifeneri Yolu, Sariyer, 34450, Istanbul, Turkey Submitted to Industrial and Engineering Chemistry Research Abstract Metal organic frameworks (MOFs) have been widely studied as adsorbents and membranes for gas separation applications. Considering the large number of available MOFs, it is not possible to fabricate and test the gas separation performance of every single MOF using purely experimental manners. In this study, we used molecular simulations to assess both adsorption-based and membrane-based CH4/N2 separation performances of 102 different MOFs. This is the largest number of MOF adsorbents and membranes studied to date for separation of CH4/N2 mixtures. Several adsorbent evaluation metrics such as adsorption selectivity, working capacity and regenerability were predicted and the top performing adsorbents were identified. Several MOFs were predicted to exhibit higher adsorption selectivities than the traditional adsorbents such as zeolites and activated carbons. Relation between adsorption-based separation performances of MOFs and their structural properties were also investigated. Results showed that MOFs having largest cavity diameters in the range of 4.6-5.4 Å, pore limiting diameters in the range of 2.4-3.7 Å, surface areas less than 2000 m2/g and porosities less than 0.5 are promising adsorbents for CH4/N2 separations. We then combined adsorption and diffusion data obtained from molecular simulations and predicted both membrane selectivities and gas permeabilities of MOFs for separation of CH4/N2 mixtures. A significant number of MOF membranes was identified to be CH4 selective in contrast to the traditional membrane materials which are generally N2 selective. Several MOFs exceeded the upper bound established for the polymeric membranes and many MOFs exhibited higher gas permeabilities than zeolites. The results of this study will be useful to guide the experiments to the most promising MOF adsorbents and membranes for efficient separation of CH4/N2 mixtures. Keywords: metal organic framework, molecular simulation, adsorption, diffusion, membrane, selectivity *Corresponding author: [email protected] 1 ACS Paragon Plus Environment


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1. Introduction: The main component of natural gas, methane (CH4), has been considered as a strong alternative to the petroleum.1 Natural gas extracted from unconventional hydrocarbon sources such as landfill gas and shale gas contains major impurities, mainly N2 and CO2. These impurities are generally separated by cryogenic distillation, however this process is energyintensive with high operating costs. Adsorption-based and membrane-based gas separations offer very large reductions in the energy consumption and costs of the separation processes. The greatest limitation in the applications of these separation technologies is the low selectivity of the materials used as adsorbents and/or membranes. Separation of CH4 from N2 by an adsorbent or a membrane is particularly difficult because similar molecule sizes of these gases leads to a low selectivity.2 For example, adsorption-based CH4/N2 selectivities of carbon molecular sieves (CMSs) (1.9),3 silicalite pellets (3.4),4 and activated carbons (3.0-4.0)5 have been reported to be low. Similarly, selectivities of membrane materials such as glassy polymers (4) than MOFs with larger pore sizes. As the LCD increases, selectivity generally decreases. MOFs that have large LCDs (>10 Å) show lower selectivities (1000 m2/g and porosities >0.5 have higher working capacities. These results highlight the importance of the operation conditions and different material requirements of the VSA and PSA-based separation processes. 3.2. Separation performances of MOF membranes: We also investigated membrane-based CH4/N2 separation performances of MOFs. In order to predict the membrane-based separation properties of MOFs, EMD simulations were performed to assess diffusivities of CH4/N2 mixtures in MOFs. Diffusion is not accessible at the nanosecond scale by EMD simulations if the computed self-diffusivities of gases are less than 10-8 cm2/s. We eliminated 5 MOFs in which diffusion was slower than 10-8 cm2/s and examined 97 MOFs as membranes. In Case-1, we set the feed pressure of the membrane to 0.01 bar to represent dilute conditions and in Case-2 we set the feed pressure to 10 bar to represent industrial operating conditions. Selectivity and gas permeability of the MOF membranes were examined for both cases. Figure 6 shows N2/CH4 selectivity of MOF membranes as a function of N2 permeability. Different from the adsorption-based gas separation, we presented N2 selectivity instead of CH4 selectivity in order to compare MOFs with the polymeric membranes which are generally N2 selective.45 The Robeson’s upper bound, which was prepared using the empirical permeability data of numerous polymeric membranes for the N2/CH4 separation, is also shown in Figure 6.45 Polymers have N2 selectivity ranging from 0.2 to 9 and their N2 permeabilities vary between 0.1 and 104 Barrer. MOFs we considered in this work have N2 selectivities ranging between 0.08 and 26 and their N2 permeabilities vary between 102 and 106 Barrer. MOF 13 ACS Paragon Plus Environment


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membranes generally exhibit lower N2 selectivities than the polymeric membranes but many of them (38 out of 97 in Figure 6(a), 28 out of 97 in Figure 6(b)) exceed the upper bound due to their high N2 permeabilities. The high gas permeability of MOFs can be explained by the highly porous structure of MOFs compared to polymers.25 The dashed line in Figure 6 represents the gas preference of the membranes. 9 (8) MOF membranes are N2 selective whereas 88 (89) MOF membranes are CH4 selective at 0.01 bar (10 bar) as shown in Figure 6a (b). In order to better understand the effects of adsorption and diffusion on the membrane selectivity of MOFs, we examined the adsorption and diffusion selectivities in detail and showed their relations in Figure 7. All the MOFs considered in this work are CH4 selective over N2 in the adsorption process as we discussed before. Therefore, N2/CH4 adsorption selectivities are always less than 1 in Figure 7. On the other hand, diffusion selectivity can favor either CH4 or N2. These two molecules have similar sizes and weights, therefore depending on the pore structure of the MOF, one can diffuse faster than the other or the two molecules can have similar diffusion rates. We color-coded the diffusion selectivities of the MOFs in Figures 6 and 7. Red colors represent MOFs that are CH4 selective in diffusion (N2/CH4 diffusion selectivity between 0.4-1 for Case-1 and 0.5-1 for Case-2), green colors represent MOFs that are N2 selective in diffusion (N2/CH4 diffusion selectivity between 1-4) and blue colors represent MOFs that are strongly N2 selective in diffusion (N2/CH4 diffusion selectivity between 4-80 for Case-1 and 4-131 for Case-2). As a result of these different diffusion selectivities, MOF membranes are categorized into three to discuss their separation performances: (i) MOFs in which both diffusion and membrane selectivity favor N2, (ii) MOFs in which both diffusion and membrane selectivity favor CH4, (iii) MOFs in which diffusion selectivity favors N2 but membrane selectivity favors CH4. (i) For 8 MOFs, shown by blue color in Figures 6 and 7, both diffusion and membrane selectivities favor N2. Strong diffusion selectivities of these MOFs for N2 dominate their low adsorption selectivities for N2 and make the membrane N2 selective. Three MOFs, HAJKOU, DEJROB and BOWSIQ are promising due to their high N2 selectivities and high N2 permeabilities and they exceed the upper bound as shown in Figure 6. (ii) 26 (29) MOFs have CH4 diffusion selectivities around 1-2 as shown in Figure 7a (b). Since both adsorption and diffusion favor CH4, these MOF membranes are highly CH4 selective. 14 ACS Paragon Plus Environment

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EHALOP and EYOQAL are promising membranes due to their high CH4 selectivities, 11.78 (10.26) and 12.51 (8.72), respectively as shown in Figure 7a (b). (iii) Majority of the MOFs (63 among 97 in Figure 7(a) and 60 among 97 in Figure 7(b)) weakly favors N2 in diffusion. High adsorption selectivity for CH4 (1.3-9) dominates the weak diffusion selectivity for N2 (1-4) in these MOFs and they become CH4 selective membranes. Overall analysis of these results can be summarized as follows: Adsorption selectivity generally favors CH4 in MOFs and membrane selectivity is governed by the diffusion selectivity: if diffusion selectivity strongly (weakly) favors N2, the MOF becomes highly N2 (weakly CH4) selective membrane. On the other hand, if diffusion selectivity favors CH4 in addition to the adsorption selectivity, then the MOF becomes a highly CH4 selective membrane. The most promising MOF membranes are listed in Tables 3 and 4. We considered MOFs that have N2/CH4 membrane selectivities greater than 2 and ranked the first 5 MOFs with the highest N2 permeabilities for each case in Table 3. The best performing MOF membranes are the same for the two cases. These MOFs are N2 selective and N2 permeable in a membrane-based separation process due to their high diffusion selectivities which dominate their low adsorption selectivities. Top 5 MOF membranes that are CH4 selective over N2 are listed in Table 4. MOFs with CH4/N2 selectivities greater than 5 were ranked based on their CH4 permeabilities in Table 4. These MOFs show low diffusion selectivity for CH4 and their membrane selectivities are dominated by the high CH4 adsorption selectivities. We compared the membrane performances of the MOFs we considered in this work for Case-1 (0.01 bar) with the results of Qiao and coworkers,22 who studied 17,257 hypothetical MOFs at dilute conditions. Before making a comparison, it is important to note that we considered a small number of experimentally synthesized, real MOFs with PLDs in the range of 1.8-14.9 Å whereas they studied a large number of hypothetical MOFs with PLDs ranging between 3-4 Å. They computed self-diffusivity of N2 and CH4 as 2×10-8-5×10-5 cm2/s and 10-85×10-5 cm2/s, respectively in the pores of hypothetical MOFs at infinite dilution. Our computed gas diffusivities for real MOFs were 4.8×10-6-1.7×10-3 cm2/s for N2 and 8×10-8-2.8×10-3 cm2/s CH4 at 0.01 bar. Higher gas diffusivities of real MOFs compared to hypothetical MOFs can be explained by the larger pore sizes of the real MOFs. N2 selectivities and permeabilities of hypothetical MOFs were reported to be 0.04-1000 and 0.1-105 Barrer, respectively. Our 15 ACS Paragon Plus Environment


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predicted N2 selectivities and permeabilities for real MOFs were between 0.08-11.66 and 2.9×102-2.8×106 Barrer. This comparison shows that N2 selectivities of real MOFs are lower than those of hypothetical MOFs, whereas N2 permeabilities of real MOFs are higher than those of hypothetical MOFs. This result can be explained by the very narrow pore sizes of the hypothetical MOFs, which lead to higher selectivity but lower permeability of N2. Similarities between membrane selectivities and permeabilities of real and hypothetical MOFs suggest that MOFs, real or computer-generated, have some common chemical and structural properties that lead to similar separation performances. By studying the MOFs for both adsorption-based and membrane-based gas separations, we also showed that a MOF with excellent properties as an adsorbent may have less exciting properties as a membrane. For example, QIFLOI has a high CH4 adsorption selectivity (6.58) at 10 bar, but its membrane selectivity for CH4 is too low, 0.45. In fact, this MOF is weakly N2 selective membrane (2.2) and it cannot be considered as a promising membrane. At that point, it is important to note that materials that we identified as promising adsorbents and membranes must be tested under real operating conditions and stability of the MOFs that will be used as adsorbents and/or membranes must be examined. We checked the stability information for BERGAI01, EHALOP, HAJKIO and DEJROB since they were identified as top adsorbents and membranes in this work. These MOFs were reported to be stable: BERGAI01 shows high thermo-stability,49 EHALOP preserves its framework integrity at high temperature,50 HAJKIO exhibits high thermal stability and permanent porosity,51 and DEJROB has a thermally stable structure at ambient conditions.52 4. Conclusion: Due to the similarity in physical and chemical properties of CH4 and N2 molecules, it is very difficult to find an adsorbent or membrane material that can efficiently separate CH4/N2 mixtures. In this study, we investigated adsorption and diffusion of CH4/N2 mixtures in 102 different MOFs and predicted their adsorption-based and membrane-based separation performances. Several metrics such as adsorption selectivity, working capacity and regenerability which are required to evaluate the efficiency of an adsorption-based separation process were calculated and the top performing MOF adsorbents were identified based on these metrics. Three MOFs, BERGAI01, PEQHOK and GUSLUC were found to show the best 16 ACS Paragon Plus Environment

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adsorption selectivities, even higher than those of traditional adsorbents, such as zeolites and activated carbons. MOFs having LCDs in the range of 4.6-5.4 Å, PLDs in the range of 2.4-3.7 Å, surface areas less than 2000 m2/g and porosities less than 0.5 were found to be promising adsorbents. In order to evaluate the membrane-based separation performances of MOFs, we computed diffusivity of CH4/N2 mixtures through the MOFs’ pores and reported the diffusion selectivity of each material. Our results showed that if the diffusion selectivity strongly (weakly) favors N2, the MOF becomes highly N2 (weakly CH4) selective membrane whereas if the diffusion selectivity also favors CH4 similar to the adsorption selectivity, then the MOF becomes a highly CH4 selective membrane. HAJKOU and DEJROB were identified as the top two N2 selective membranes with selectivities of 11.66 and 8.78 (25.63 and 8.68) whereas EYOQAL and EHALOP were identified as the top two CH4 selective membranes with selectivities of 12.51 and 11.78 (8.72 and 10.26) at 0.01 bar (10 bar). Our results also show that MOFs can outperform polymer and zeolite membranes reported in the literature in terms of gas permeability. It is important to note that only 102 selected MOFs were studied in this work and many other MOFs which may show better gas separation performance may exist in the database. The aim of our work was to initially perform a large-scale material screening in order to identify the most promising materials for CH4/N2 separations before extensive experimental efforts. We believe that results of this work will be a guide for future computational and experimental studies to further investigate more MOFs for CH4/N2 separation. Supporting Information: Data for experimental studies for N2 uptake of MOFs used in Figure 1(a). Data for experimental and computational studies for CH4/N2 adsorption selectivity of MOFs used in Figure 1(b). References: (1) Simon, C. M.; Kim, J.; Gomez-Gualdron, D. A.; Camp, J. S.; Chung, Y. G.; Martin, R. L.; Mercado, R.; Deem, M. W.; Gunter, D.; Haranczyk, M., The materials genome in action: identifying the performance limits for methane storage. Energy Environ. Sci. 2015, 8, 11901199. (2) Bhadra, S.; Farooq, S., Separation of methane–nitrogen mixture by pressure swing adsorption for natural gas upgrading. Ind. Eng. Chem. Res. 2011, 50, 14030-14045. (3) Cavenati, S.; Grande, C. A.; Rodrigues, A. E., Separation of methane and nitrogen by adsorption on carbon molecular sieve. Sep. Sci. Technol. 2005, 40, 2721-2743. (4) Delgado, J. A.; Uguina, M. A.; Sotelo, J. L.; Ruiz, B., Modelling of the fixed-bed adsorption of methane/nitrogen mixtures on silicalite pellets. Sep. Purif. Technol. 2006, 50, 192-203. 17 ACS Paragon Plus Environment


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(57) Xu, X.; Zhao, X.; Sun, L.; Liu, X., Adsorption separation of carbon dioxide, methane, and nitrogen on Hβ and Na-exchanged β-zeolite. J. Nat. Gas Chem. 2008, 17, 391-396.



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

Page 22 of 31

Table 1. CH4/N2 selectivities of different adsorbents. Adsorbents

Sads, CH4/N2

Condition

Method

Reference

BERGAI01 PEQHOK GUSLUC Linde 4A zeolite F30-470 Degussa Activated Carbon H+ mordenite zeolite SAPO-34 zeolite BPL Calgon Carbon Na-SAPO-34 zeolite Linde 5A zeolite Hβ zeolite Bayer KEL2200 5A molecular sieve Chabazite zeolite Naβ zeolite

8.80 (8.86) 8.40 (8.24) 7.70 (8.28) 3.40 3.20 3.20 3.00 2.80 2.56 2.20 2.00 1.90 1.90 1.75

298 K, 10 bar (1 bar) 298 K, 10 bar (1 bar) 298 K, 10 bar (1 bar) 302 K, 1 bar 303 K, 10 bar 302 K, 1 bar 298 K, 10 bar 298 K, 1 bar 298 K, 1 bar 298 K, 10 bar 303 K, 1 bar 303 K, 10 bar 302 K, 1 bar 303 K, 1 bar

a a a b c b d b b d e c b e

This work This work This work 53 54 53 5 55 56 5 57 54 53 57

(a)Mixture simulations (CH4/N2:50/50), (b)Single-component adsorption experiments, (c)Single-component adsorption experiments and IAST calculations (CH4/N2:40/60), (d)Breakthrough experiments (CH4/N2:50/50), (e)Ratio of the Henry’s coefficients obtained from the single-component adsorption experiments.

Table 2. Top performing MOF adsorbents for CH4/N2 separation.

QIFLOI EYOQAL HAJKOU EMIVAY ACODED

Sads, CH4/N2 7.20 6.71 6.33 6.11 5.91

Case-1 ∆N (mol/kg) 1.03 1.10 1.04 1.44 1.04

R% 74.17 74.63 78.17 82.59 86.35

EHALOP HASSUR KARLAS DIDBID EBAMOL


Sads, CH4/N2 6.71 4.74 4.64 4.61 4.58

Case-2 ∆N (mol/kg) 3.64 3.69 2.55 3.46 2.56

R% 78.72 78.08 75.46 80.06 75.39

Page 23 of 31

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


Table 3. Top performing MOF membranes for selective separation of N2 from CH4. All selectivities are reported for N2 over CH4. Case-1 HAJKOU DEJROB BOWSIQ NUJCIE MOCKAR

Sads 0.15 0.11 0.23 0.32 0.33 Sads

HAJKOU DEJROB BOWSIQ NUJCIE MOCKAR

0.20 0.17 0.22 0.31 0.32

Sdiff 79.35 79.33 38.08 8.15 6.29 Sdiff

Smem 11.66 8.78 8.75 2.57 2.05

P, N2 (Barrer) 9887 3559 9482 287 18286

P, CH4 (Barrer) 847 405 1083 112 8919

Smem

Case-2 P, N2 (Barrer)

P, CH4 (Barrer)

1259 751 628 193 13264

49 86 199 89 6600

130.41 25.63 51.65 8.68 14.19 3.15 7.02 2.16 6.36 2.01

Table 4. Top performing MOF membranes for selective separation of CH4 from N2. All selectivities are reported for CH4 over N2. Case-1 EYOQAL EHALOP EYOPUE EMIVAY AJIHOQ

Sads 7.67 5.18 5.47 5.58 5.25 Sads

EHALOP EYOQAL IDIWOH EMIVAY DIDBID

6.71 5.71 3.73 6.15 4.61

Sdiff 1.63 2.27 1.36 0.96 0.96 Sdiff

Smem 12.51 11.78 7.45 5.33 5.05

P, N2 (Barrer) 2.58×106 6.88×105 8.94×105 4.02×105 3.25×105

P, CH4 (Barrer) 3.22×107 8.10×106 6.66×106 2.15×106 1.65×106

Smem

Case-2 P, N2 (Barrer)

P, CH4 (Barrer)

5

1.53 10.26 1.53 8.72 1.55 5.76 0.93 5.73 1.11 5.12

2.55×10 1.08×105 4.05×105 1.87×104 1.04×105


2.61×106 9.44×105 2.33×106 1.07×105 5.33×105

5

(a) 2 R =0.99

Sads(CH4/N2) from literature

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

N2 uptake from literature (mol/kg)


0

10

BioMOF-11 BioMOF-12 CuBTC CuTDPAT IRMOF-1 MIL-53(Al) ZIF-8 ZIF-78 ZnMOF-74

-1

10

-2

10

-2

10

-1

10

Page 24 of 31

(b) 2 R =0.88

4

3 CuBTC CuBTC IRMOF-1 IRMOF-14 NiMOF-74 ZIF-68 ZIF-79

2

1

0

1

10

2

3

4

5

Sads(CH4/N2) from simulations

N uptake from simulations (mol/kg) 2

Figure 1. (a)Comparison of our molecular simulations with the experiments for N2 uptake of MOFs at 298 K, 0.1-15 bar. (b)Comparison of our predicted adsorption selectivity with the experimentally/computationally reported selectivity for CH4/N2 separation at 298 K and 10 bar.


Page 25 of 31

10

(a) QIFLOI

8

EMIVAY

HAJKOU

6 ACODED

4 2 0 0.0

(b)

BERGAI01 PEQHOK GUSLUC

8

EYOQAL

Sads(CH4/N2)

10

Sads(CH4/N2)

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


EHALOP

6

HASSUR GALHUY

4

HECQUB

2 0

0.5

∆N

CH4

1.0

1.5

0

1

2

∆N

(mol/kg)

CH4

3

4

(mol/kg)

Figure 2. Adsorption selectivity and working capacity of MOFs for (a)Case-1 (b)Case-2.


5


100

100

(a)

75

75

50

50

R%

R%

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

25

25

0

0 0

2

4

6

8

10

Page 26 of 31

(b)

0

2

4

6

8

10

Sads(CH4/N2)

Sads(CH4/N2)

Figure 3. Regenerability and adsorption selectivity of MOFs for (a)Case-1 (b)Case-2. Dashed line represents 75% regenerability, which was set as the minimum desired R%.


Page 27 of 31

10

10

(a)

8

(b)

8

6

4

2

6 PLD (Å)

Sads(CH4/N2)

Sads(CH4/N2)

1.8

4

2

0

5.3 15

4

8

12

16

20

24

0

4

8

LCD (Å)

12

16

20

24

LCD (Å)

10

(c)

8

Sads(CH4/N2)

8

(d)

6

4

2

0.18 porosity (φ)

10

3.8

0 0

Sads(CH4/N2)

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


6

4

0.50

0.88 2

0

0 0

2000

4000

6000

0

2

2000

4000

6000

2

Surface area (m /g)

Surface area (m /g)

Figure 4. Adsorption selectivities of MOFs as a function of LCDs and PLDs for (a)Case-1 (b)Case-2 and as a function of surface area and porosity for (c)Case-1 (d)Case-2.



5

(a)

(b)

1.5

∆NCH4 (mol/kg)

1.0

0.5

1.8

3 PLD (Å)

∆NCH4 (mol/kg)

4

2 1

3.8 5.3 15

0

0.0 4

8

12

16

20

24

4

8

12

16

20

24

LCD (Å)

LCD (Å) 5

(c)

(d)

1.5

1.0

0.5

0.18 porosity (φ)

∆NCH4 (mol/kg)

4

∆NCH4 (mol/kg)

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

Page 28 of 31

3

2

0.50 0.88

1 0.0

0 0

2000

4000

6000

0

2

2000

4000

6000

2

Surface area (m /g)

Surface area (m /g)

Figure 5. Working capacities of MOFs as a function of LCD and PLD for (a)Case-1 (b)Case-2 and as a function of surface area and porosity for (c)Case-1 (d)Case-2.


Page 29 of 31

(a)

(b) HAJKOU HAJKOU BOWSIQ

10

Smem(N2/CH4)

10

Smem(N2/CH4)

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


DEJROB

1

N2 Selective CH4 Selective

BOWSIQ DEJROB

1

N2 Selective CH4 Selective

EYOPUE

0.1 -1

0

1

2

3

4

5

IDIWOH

0.1

EHALOP EYOQAL 6

10 10 10 10 10 10 10 10 10

7

Permeability, PN2 (Barrer)

EHALOP -1

0

1

2

3

4

EYOQAL 5

6

10 10 10 10 10 10 10 10 10

7

Permeability, PN2 (Barrer)

Figure 6. Membrane selectivity and permeability of MOFs for (a)Case-1 (b)Case-2. Solid lines represent the Robeson’s upper bound for N2/CH4 separation.45 Color coding is given in Figure 7.



(a)

(b)

10

1

1.00 4.00 79.40

0.1

0.50

Sdiff(N2/CH4)

0.40

Smem(N2/CH4)

10

Sdiff(N2/CH4)

Smem(N2/CH4)

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

Page 30 of 31

1

1

10

4.00 130.50

0.1 0.1

1.00

0.1

1

10

Sads(N2/CH4)

Sads(N2/CH4)

Figure 7. Adsorption, diffusion and membrane selectivity of MOFs for (a)Case-1 (b)Case-2.


Page 31 of 31

TOC Graph:

(a)

(b)

10

1

1.00 4.00 79.40

0.1

0.50

Sdiff(N2/CH4)

0.40

Smem(N2/CH4)

10

Sdiff(N2/CH4)

Smem(N2/CH4)

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


1

1

10

4.00 130.50

0.1 0.1

1.00

0.1

1

Sads(N2/CH4)

Sads(N2/CH4)


10

N2 ... - ACS Publications

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