Enhancing Higher Hydrocarbons Capture for Natural Gas Upgrading

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Enhancing Higher Hydrocarbons Capture for Natural Gas Upgrading by Tuning Van der Waals Interactions in fcu-Type Zr-MOFs Guopeng Han, Keke Wang, Yaguang Peng, Yuxi Zhang, Hongliang Huang, and Chongli Zhong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03341 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Industrial & Engineering Chemistry Research

Enhancing Higher Hydrocarbons Capture for Natural Gas Upgrading by Tuning Van der Waals Interactions in fcu-Type Zr-MOFs

Guopeng Han1, Keke Wang1, Yaguang Peng1, Yuxi Zhang1, Hongliang Huang*, 1, 2 and Chongli Zhong* , 1, 2, 3

1

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

2

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China

3

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing 100029, China *Correspondence to: [email protected]; [email protected]

Abstract Higher hydrocarbons in natural gas must be removed for safe storage, transport and application of natural gas. Considering the C3H8 and CH4 are nonpolar molecules, electrostatic interactions between C3 and MOFs are relative weak while they could be sensitive to the Van der Waals interactions. Thus, it is an effective method to greatly enhance the separation performance by impoving the Van der Waals interactions through tuning the pore size of MOFs. Herein, we synthesized a series of isostructural Zr-MOFs with different pore size and the separation performances of these materials for C3/C1 were systematically studied. The results indicate that 1

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pore size plays an important role in the C3 storage and C3/C1 separation in MOFs. Specifically, Zr-BPDC with large surface area and pore volume has the highest C3H8 and C3H6 adsorption capacity (159.2 cc/g and 161.5 cc/g at 298 K 1 bar, respectively) while Zr-FUM with the smallest surface area and pore volume has the highest adsorption heat for C3 as well as C3/C1 selectivities (292.0 and 242.2 at 298 K and 1 bar for C3H8/CH4 and C3H6/CH4, respectively) among the five Zr-MOFs. In addition, defective structure in MOFs can largely improve C3 adsorption capacity for its higher surface area and pore volume while functional groups in Zr-MOF will not obviously affect the C3 adsorption and C3/C1 separation performance. These work shows that Van der Waals interactions in MOFs are predominant for C3 adsorption and C3/C1 separation and it can be efficiently tuned by changing the surface area and pore volume in MOFs. More importantly, these information could help design and synthesize novel adsorbent to separate C3/C1 mixtures. Key words: C3/C1; metal-organic frameworks; zirconium; pore size; adsorption; separation.

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1. Introduction The atmospheric CO2 mainly coming from burning fossil fuels is a public concern associated with global warming.1 Compared to the traditional fossil fuels, natural gas is a much cleaner energy and it has been widely applied.2 The main component in natural gases is methane, while there are a small quantity of higher hydrocarbons such as C3H8, C3H6. On one hand, although the proportion of higher hydrocarbon impurities is small, the existence of them in natural gas will reduce conversion rate and energy content as well as affect cyclic steady state in the storage process and safe transmission in the pipe.3-6 Since higher hydrocarbons adsorb much strongly in microporous materials than methane in storage tanks, there is an accumulation of the higher hydrocarbons in the adsorbents over many cycles and the formation of a cyclic steady state, which will greatly decrease the methane storage capacity. On the other hand, the higher hydrocarbons are important chemical raw materials for the manufacture of polymers.7, 8 Thus, it is essential to separate these higher hydrocarbons from natural gas. Presently, there are several commonly used approaches have been proposed for C3/C1 separation, such as membrane separation, pressure swing adsorption (PSA) technology and low-temperature distillation, etc.9 Compared to the other approaches, adsorption process based on solid adsorbent owns more merits such as energy conservation.10, 11 The key issue of utilizing adsorption technology to C3/C1 separation is to design materials with high adsorptive selectivity, high stability and low cost. As a new kind of porous materials consisting of metal ions linked together by organic ligands, MOFs have received increasing attention for their structural merits such as large and tunable specific surface area and pore volume, tunable function, in particular at the molecular level.12-16 3



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Furthermore, they have shown potential applications in many fields, such as gas storage and separation, catalysis, drug delivery, and sensing.17-28 Therefore, MOFs could be the potential material for separating C3 and C1. It should be noted that electrostatic interactions and Van der Waals interactions between gas molecules and materials play an important role on the adsorption and separation performance of gases mixture in MOFs.29 The electrostatic interactions of the MOFs can be adjusted by introducing different polar functional groups, while the Van der Waals interactions can be adjusted by changing the surface area, pore size and pore volume of the material. Some previous works have widely proved the fact that introduction of polar functional groups or unsaturated metal sites into MOFs can greatly improve the binding energy between MOFs and the electrostatic sensitive gas such as CO2 and H2S, thus greatly enhancing the separation performances for CO2/CH4, CO2/N2, H2S/CH4 and so on.30-32 Considering the C3H8 and CH4 are nonpolar molecules, electrostatic interactions between C3 and MOFs are relative weak. Thus, introduction of polar functionalities or unsaturated metal sites cannot be regarded as an efficient means to improve the selectivity of C3/C1 mixture. However, in view of their great difference on the length of carbon chain, they could be sensitive to the Van der Waals interactions and their separation property can be greatly enhanced through tuning the pore size and specific surface area of MOFs. Hence, we put forward the idea that we could regulate the Van der Waals interactions and improve the separation performance of C3/C1 by changing the specific surface area and pore size of materials. Herein, as a proof of concept, a series of isostructural Zr-MOFs with different pore size and the same topology were selected for C3/C1 separation since Zr-MOFs have showed great chemical and mechanical stability.33-35 The results shows that the Zr-FUM with small pore size owns the excellent separation selectivity and high adsorption heat, and the selectivities for C3H8/CH4 and C3H6/CH4 are 578.3 and 316.8, respectively, which are much higher than most reported results in references. As a 4


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control-experiment, we also examined the C3/C1 performances of another three Zr-MOFs with different polar functional groups. The results indicate that electrostatic interactions in MOFs will not obviously affect the C3/C1 separation. These results demonstrate that the C3/C1 separation in MOFs can be efficiently improved by tuning the Van der Waals interactions. More importantly, the strategy is facile and inexpensive by using commercially available ligands, which provides a guideline for the design of new porous materials toward C3 capture for natural gas upgrading.

H2FUM

H2-1, 4-NDC

Zr-FUM

H2BDC H2-2, 6-NDC H2BPDC

Zr-1, 4-NDC

Zr-BDC

Zr6 cluster

Zr-2, 6-NDC

Zr-BPDC

Figure 1. Illustration of the crystal structure of Zr-MOFs and their organic ligand and inorganic cluster. The large spheres represent the void regions inside the cages (Zr polyhedra: yellow for octahedral cage, blue for tetrahedral cages; C, gray; O, red; H atoms were omitted for clarity). The extended organic linkers are also presented.

2. Experimental Section All reagents and solvents used in this work were commercially available and used as supplied without further purification. The details for the synthesis of Zr-MOFs are provided in the Supporting Information.

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The X-ray diffraction (XRD) analysis of the powder samples patterns were collected at room temperature. It was carried out on a D8 ADVANCE XRD-6000 X-ray diffractometer in reflection mode using Cu Kα radiation ( λ = 1.54056 Å ). The 2 θ range from 5° to 50° was scanned with a step size of 0.02°. Thermogravimetric analyses were recorded on a TGA/DSC 1/1100 SF STA at a heating rate of 10 ℃/min under nitrogen atmosphere from 293 K to 973 K. The data of gas uptakes were carried out on Autosorb-iQ-MP automated gas sorption analyzer (Quantachrome Instruments).

3. Results and Discussion 3.1. Characterization of MOFs. To systematic regulate the pore size and surface area of MOFs, here we synthesized five isostructural Zr-MOFs based on some linear ligands with different length or width, as shown in Figure 1. These Zr-MOFs with three-dimensional (3D) cubic network share the same Zr6O4(OH)4(NDC)6 SBU as nodes.36 The powder XRD was employed to characterize the structural of the Zr-MOFs, as shown in Figure 2. For all the Zr-MOFs, the experimental XRD data show the similar patterns to those of simulated one, indicating that all the obtained materials are well crystallized.

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(b)

(a)

Sim. Exp.

Intensity

Intensity

Sim. Exp.

0

10

20

30

40

0

50

10

20

30

40

50

2 θ (degree)

2 θ (degree)

(c)

(d) Sim. Exp. Intensity

Intensity

Sim. Exp.

0

10

20

30

40

0

50

10

2 θ (degree)

20

30

40

50

2 θ (degree)

(e) Sim. Exp. Intensity

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


0

10

20

30

40

50

2 θ (degree)

Figure 2. Comparison of experimental and simulated powder XRD patterns of five different Zr-MOFs: (a) Zr-FUM; (b) Zr-1, 4-NDC; (c) Zr-BDC; (d) Zr-2, 6-NDC; (e) Zr-BPDC. Furthermore, the adsorption isotherms of N2 of these MOFs were also investigated at 77 K to evaluate the permanent porosity of these materials. As shown in Figure 3, all the adsorption isotherms show the typical characteristics of Ⅰ type adsorption and a very sharp uptake at P/P0 < 0.005, which indicates that these MOFs have micro-porous structure. The Brunauer−Emmett−Teller (BET) surface area and pore volume from the N2 adsorption data are showed in Table 1. Obviously, the surface area and pore volume in there Zr-MOFs were successfully tuned by introducing the 7



organic ligands with different length or width.

N2 uptake at 77 K (cc/g)

200

160

120 0.0

(c) 400 N2 uptake at 77 K (cc/g)

(b) 320

Adsorption Desorption

0.2

0.4

P/P0

0.6

0.8

1.0

(d)


200

100

0.2

0.4

160

80 0.2

0.6

0.8

560

0.6

0.8

0.6

0.8

1.0

420

280

140

0 0.0

1.0


0.2

0.4

1.0

P/P0

P/P0

(e) 800

0.4

P/P0

300

0 0.0


240

0.0



(a) 240


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600

400

200

0 0.0

0.2

0.4

P/P0

0.6

0.8

1.0

Figure 3. N2 adsorption isotherms of five different Zr-MOFs at 77 K: (a) Zr-FUM; (b) Zr-1,4-NDC; (c) Zr-BDC; (d) Zr-2,6-NDC; (e) Zr-BPDC.

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Table 1.. BET surface areas and pore volumes of five Zr-MOFs. α= β = γ

Zr-MOFs

a=b=c (Å)

Zr-FUM Zr-1,4-NDC Zr-BDC Zr-2,6-NDC Zr-BPDC

17.943 20.7786 20.7651 23.7853 26.8445

90 90 90 90 90

Pore size (Å)

BET surface area (m2/g)a

5/7 6/8 8/11 11/14 12/16

a

Calculated from N2 adsorption isotherms at 77 K in range of P/P0 = 0.005～0.05;

b

Calculated at N2 adsorbed amounts at P/P0 = 0.95

735 876 1200 1717 2500

Pore volume （cc/g)b 0.34 0.43 0.59 0.74 1.10

3.2. Higher Hydrocarbons Uptake Studies. Based on these MOFs we have synthesized, we measured the sorption isotherm of C3H8, C3H6 and CH4 at 298 K. As we can see from the Figure 4, C3H8 and C3H6 adsorption capacity in Zr-BPDC at 1 bar are 159.2 and 161.5 cc/g, respectively, which are much higher than those in other Zr-MOFs and some reported works such as JUC-100, JUC-103, JUC-106 ( 136, 122 and 114 cc/g, respectively),7 and LIFM-38( 55 cc/g).37 It is worth noting that adsorption of Zr-FUM closes to saturation while Zr-BPDC still far away from saturation at 1 bar, indicating that entropy effect is predominant for C3 adsorption capacity in microspore MOFs. The order of adsorption capacity (298 K, 1 bar) in the five Zr-MOFs follows the sequence of specific surface area and pore size of MOFs: Zr-BPDC> Zr-2, 6-NDC> Zr-BDC> Zr-1, 4-NDC> Zr-FUM. This fact indicates that the MOFs with larger surface area and pore volume will own greater gas uptake at 1 bar, as shown in Figure 4f and Figure S7. To further confirm the effect of surface area and pore volume of MOF on C3 adsorption, we performed the C3 adsorption in defective Zr-BDC which was prepared by using benzoic acid as a modulator. The characterizations of the Zr-BDC with defect were provided in Figure S1b, Figure S2i, and Figure S3d in Supporting Information. As shown in Table S1 and Figure S6a, the C3H8 and C3H6 adsorbed amounts are 100.7 and 105.9 cc/g, respectively, which are much larger than that in Zr-BDC (69.4 and 80.8 cc/g), suggesting the existence of defect in MOF is beneficial for C3 adsorption. In addition, we also tested the effect of functional groups in MOFs for 9



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C3 adsorption. Here three Zr-MOFs with different polar groups (Zr-BDC-NH2, Zr-BDC-NO2 and Zr-BDC-Br) based on Zr-BDC were synthesized and characterized (Figure S1a, Figure S2f-h and Figure S3a-c). Figure S5 shows that the polar groups in Zr-BDC will not obviously affect the C3 adsorption in Zr-MOFs, implying the C3 adsorption in MOFs is not sensitive for electrostatic interaction sensitive. All the results confirmed that C3 adsorption in MOFs are closely related to Van der Waals interactions and enlarging the surface area and pore volume of MOFs can efficiently improve the C3 adsorption capacity.

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(b) 60

C3H8 C3H6 CH4

60

Gas Uptake at 298 K (cc/g)


(a)

40

20

0 0.0

0.2

0.4

0.6

0.8

40

20

0 0.0

1.0

C3H8 C3H6 CH4

0.2

0.4

Pressure (bar)

60

C3H8 C3H6 CH4

(d)

40

20

0 0.0

0.2

0.4

0.6

0.8

120

120

C 3 H6 CH4

40

0.2

0.4

(f) 160

40

0.2

0.4

0.6

0.6

0.8

1.0

Pressure (bar)

80

0 0.0

1.0

80

0 0.0

1.0

C3 uptake at 298 K (cc/g)

C 3 H8

0.8

C3H8 C3H6 CH4

Pressure (bar)

(e)160

0.6

Pressure (bar)



(c) 80


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0.8

C3H6 120

80

40

1.0

C3H8

0.4

Pressure (bar)

0.6

0.8

1.0

1.2

Pore volume (cc/g)

Figure 4. Adsorption isotherms of CH4, C3H6 and C3H8 in five different Zr- MOFs at 298 K: (a) Zr-FUM; (b) Zr-1, 4-NDC; (c) Zr-BDC; (d) Zr-2, 6-NDC; (e) Zr-BPDC; (f) C3 uptake in Zr-MOFs as a function of different pore volume.

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3.3. Selectivities of C3/C1 in Zr-MOFs. Table 2.. Adsorption amount and the limiting selectivities of C3/C1in Zr-MOFs. C3 uptake (cc/g)

compound Zr-FUM Zr-1,4-NDC Zr-BDC Zr-2,6-NDC Zr-BPDC

C3H8

Qst

C3H6

298 K

273 K

298 K

273 K

53.3 53.8 69.4 127. 9 159.2

56.5 64.7 85.5 140.9 170.0

57.9 58.4 80.8 133.3 161.5

62.0 70.2 100.4 149.8 175.1

C3H8 40.6 37.2 35.2 25.4 22.3

a

(kJ/mol)

C3H6 41.6 38.4 37.0 28.7 27.0

Selectivityb CH4

C3H8/CH4

C3H6/CH4

21.6 20.1 19.4 14.6 12.5

578.3 247.1 168.7 141.1 65.8

316.8 210.1 145.4 50.4 50.2

a

Heat of adsorption calculated using the ideal adsorption solution theory.

b

Selectivity was calculated from the ratio of initial slopes of C3H8 or C3H6 and CH4 adsorption isotherms at 298 K.

In order to study the influence of pore size and surface area of MOFs on C3/C1 separation, the limiting selectivities of C3/C1 in these materials under the condition of infinite dilution were calculated and the results are shown in Table 2. Obviously, the order of adsorption selectivities of C3/C1 in the five Zr-MOFs is opposite with that of adsorption capacity: Zr-FUM > Zr-1, 4-NDC > Zr-BDC> Zr-2, 6-NDC > Zr-BPDC. The results indicate that the C3/C1 adsorptive selectivity can also be tuned by changing the surface area (Figure S8) and pore size (Figure 5d, Figure S10) of MOFs. Interestingly, limiting selectivities of C3H8/CH4 and C3H6/CH4 in Zr-FUM reach values of 578.3 and 316.8, respectively, which are much higher than other Zr-MOFs and some reported MOFs such as tbo-MOFs(43-143),38 UTSA-53a(80-90) with open metal sites,39 interpenetrated JLU-Liu37 (206),6 and JLU-Liu34 (45.9) on base of Zr-MOFs with functional groups.40 The high C3/C1 selectivities of Zr-FUM can be ascribed to the ultramicropores (pores of width < 7 Å), which are generally beneficial for C3 molecules since small pore size could lead to deep overlap of potential and thus strong Van der Waals interactions between C3 and Zr-FUM. Since the values calculated from Henry constant ratio only represent selectivities close to zero pressure, the ideal adsorption solution theory (IAST) model was additionally used to predict C3/C1 12


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separation performance of Zr-MOFs at the whole pressure region. Again, Zr-FUM with smaller pore size owns much higher separation performance than other MOFs in the whole pressure region. More impressively, as far as we know, Zr-FUM owns excellent C3H8/CH4 selectivity among all reported MOFs, as shown in Table 3. These results suggested Zr-FUM could be a potential material for the selective remove C3H8 and C3H6 from natural gas. In order to further distinguish the effect of electrostatic interactions and Van der Waals interactions for the contribution of C3/C1 separation in Zr-MOFs, the selectivities of C3/C1 in Zr-BDC with polar groups were examined as a control-experiment, as shown in Figure 5c. The adsorption selectivities of C3/C1 in MOFs with functional groups are close to that of Zr-BDC, indicating the electrostatic interactions have no dramatic effect on C3/C1 separation in MOFs. Some reported work have shown that polar functional groups in MOFs are benefit for the separation of polar gas such as CO2/N2, H2S/CH4 and so on25, 27, 40, indicating electrostatic interactions are playing an important role for these polar gases mixture. In order to further explain the effect of Van der Waals interactions for the contribution of C3/C1 separation in Zr-MOFs, the selectivities of C3/C1 in Zr-BDC with defect were examined as a control-experiment, as shown in Figure S10. The adsorption selectivities of C3/C1 in MOFs with defect are higher than Zr-BDC since Zr-BDC with defect has larger pore volume and BET surface area. These results clearly suggested that the Van der Waals interactions have dramatic effect on the separation of nonpolar C3/C1 separation in MOFs.

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(b)


400

Selectivity of C3H6/CH4 at 298 K

Selectivityo f C3H8/CH4 at 298 K

(a) 600

200

0.0

0.2

0.4

0.6

0.8

400


300

200

100

0 0.0

1.0

0.2

0.6

0.8

1.0

600

(d) 600

Zr-FUM Zr-BDC-Br Zr-BDC-NO2 Zr-BDC-NH2 Zr-BDC

450

Selectivity of C3/C1 at 298 K

(c)

0.4

Absolute pressure (bar)


Selectivity of C3H8/CH4 at 298 K

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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300

150

0.0

0.2

0.4

0.6

0.8


C3H8/CH4 C3H6/CH4

400

200

0

1.0

0.4

0.6

0.8

1.0

1.2

Pore volume (cc/g)

Figure 5. IAST-predicted adsorption selectivity of five different Zr- MOFs at 298 K: (a) C3H8/CH4; (b) C3H6/CH4; (c) selectivities of C3H8/CH4 in Zr-BDC with different polar groups. (d) The limiting selectivity of C3/C1 in Zr-MOFs as a function of pore volume.

Table 3.. The summary of Selectivities of C3/C1 in MOFs. MOFs ID

Name of MOFs

T/K SC3H8/CH4

a

SC3H6/CH4

a

ref

1

Zr-FUM

298

292.0

242.2

this work

2

Zr-1,4-NDC

298

73.5

77.4

this work

3

Zr-BDC

298

71.5

68.7

this work

4

Zr-2,6-NDC

298

49.2

52.8

this work

5

Zr-BPDC

298

65.0

52.5

this work

d

42

b

43

6

UiO-67

298

7

JXNU-4

273

144

8

FIR-51

294

75

44

9

JLU-Liu5

298

107.8

3

10

JLU-Liu6

298

274.6

3

11

JLU-Liu18

298

108.2

4

69.3

14


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12

JLU-Liu22

298

271.5

45

13

JLU-Liu34

298

45.9

40

14

JLU-Liu36

298

40

40

15

JLU-Liu37

298

206

6

16

JLU-Liu38

298

98

6

17

UTSA-35a

296

85

18

FIR-7a-ht

298

78.8

19

FJI-C1

298

471

47

20

FJI-C4

298

293.4

5

21

1-mim

297

200

48

22

1-eim

297

80

48

23

1-pim

297

75

48

24

1-buim

297

50

48

25

JUC-100

298

65

7

26

JUC-103

298

65

7

27

JUC-106

298

65

7

28

MFM-202a

293

87

29

tbo-MOF-1

298

30

tbo-MOF-2

298

31

tbo-MOF-3

298

32

Zn6 (2-mbim)2

285

90

33

InOF-1

298

80

34 a

LIFM-38

298

43

39

90 c

46

49

70 b

143

38 b

75

38

178

38 50 51

35.8

c

34.4

c

37

SC3/C1 calculated using the ideal adsorption solution theory under the condition of equimolar binary mixtures and 1bar.

b

SC3/C1 calculated using the ideal adsorption solution theory under the condition of C3/C1 = 0.05/0.95 and 1bar.

c

SC3/C1 calculated using the ideal adsorption solution theory under the condition of 1bar (unspecified Molar ratio).

d

SC3/C1 calculated using the ideal adsorption solution theory under the condition of C3/C1 = 5/85 and 1bar.

For better understanding the relationship between C3 affinity energy and the Van der Waals interactions in MOFs, the isosteric heats of adsorption (Qst) of these Zr-MOFs were calculated by using Clausius-Clapeyron equation. The isosteric heats of C3H8, C3H6 and CH4 adsorption are obtained based on the adsorption data collected at 273 (Figure S4) and 298 K (Figure 4). As shown in Figure 6a, the Qst values for Zr-FUM (40.6 kJ/mol) are higher than the Qst values of the other four materials (37.2, 35.2, 25.4, 22.3 kJ/mol) and other materials such as LIFM-38 (28.3 kJ/mol),37 1-buim(27.97 kJ/mol)47 and FIR-51(26.0 kJ/mol)44, which can be attributed to the stronger Van der Waals interactions between C3 molecules and the Zr-FUM with smaller pore size. And the order of Qst values perfectly matches the order of C3/C1 selectivities of MOFs very well (Figure S9). All

15



these experimental results imply that the Van der Waals interactions play an important role on C3/C1 separation in MOFs, and the adsorptive selectivity can be easily tuned by changing the pore size and surface area of MOFs. From the energy saving aspect, as shown in Figure 4 and Table 2, the steep slope of the isotherms and large heat of adsorption of C3 in Zr-MOFs indicating the energy consumption for VSA process will be high. However, compared with cryogenic distillation, VSA and PSA are considered as more energy-efficient processes.


Qst (kJ/mol)

40

32

24

(b)

40


35

Qst (kJ/mol)

(a)

30

25

20

16 0

20

40

60

80

100

120

20

Uptake (cc/g)

40

60

80

C3H8

16


8 0

3

6

9

12

Qst (kJ/mol)

(d) 40

20

12

100

Uptake (cc/g)

(c) Qst (kJ/mol)

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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C3H6 CH4

30

20

10

15

Uptake (cc/g)

0.4

0.6

0.8

1.0

1.2

Pore volume (cc/g)

Figure 6. Heat of adsorption of (a) C3H8, (b) C3H6 and (c) CH4 in Zr- MOFs at 298 K, (d) Qst at zero coverage in Zr-MOFs as a function of pore volume of MOFs.

4. Conclusions In this work, a series of isostructural Zr-MOFs with different pore size and surface area were used to systematically study the effect der Waals interactions in MOFs for C3/C1 separation. It is 16


Page 17 of 23

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found that surface area and pore size play an important role in the C3 storage and C3/C1 separation in MOFs. Specifically, Zr-BPDC with large surface area and pore volume has the highest C3 adsorption capacity while Zr-FUM with the smallest surface area and pore volume has the highest adsorption heat for C3 as well as C3/C1 selectivities. In addition, polar groups in MOFs will not obviously affect the C3 adsorption and C3/C1 separation performance, indicating the electrostatic interactions have no dramatic effect on C3 storage and C3/C1 separation in MOFs. This work shows that Van der Waals interactions in MOFs are predominant for C3 adsorption and C3/C1 separation and it can be efficiently tuned by changing the surface area and pore volume in MOFs. This information could help design and synthesis novel MOFs toward C3 capture for natural gas upgrading.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publication website. Synthesis of Zr-MOFs; ideal adsorption solution theory (IAST) calculations; calculation of isosteric heats of adsorption; PXRD of the Zr-MOFs; TGA of Zr-MOFs; N2 adsorption of Zr-MOFs; C3 adsorption in Zr-MOFs; the relationship between C3 uptake and the BET surface area of Zr-MOFs; the relationship between limiting C3/C1 selectivities and the BET surface area of Zr-MOFs; the relationship between Qst at zero coverage and the BET surface area of Zr-MOFs; IAST-predicted C3/C1 selectivities in Zr-BDC and Zr-BDC with defect.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]；[email protected]

Notes 17



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The authors declare no competing financial interest.

Acknowledgment This work is supported by National Key R&D Program of China (2016YFB0600901) a nd the Natural Science Foundation of China (NO. 21536001 and 21606007).

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Table of Contents 600

Selectivityo f C3/C1 at 298 K

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



400

200

0.0

0.2

0.4

0.6

0.8


23


1.0

Enhancing Higher Hydrocarbons Capture for Natural Gas Upgrading

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