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Adsorption Equilibrium and Kinetics of Nitrogen, Methane and Carbon Dioxide Gases onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 Ahmed Awadallah-F, Febrian Hillman, Shaheen A Al-Muhtaseb, and Hae-Kwon Jeong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05892 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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Industrial & Engineering Chemistry Research

Adsorption Equilibrium and Kinetics of Nitrogen, Methane and Carbon Dioxide Gases onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8

Ahmed Awadallah-F (a),1, Febrian Hillman(b), Shaheen A. Al-Muhtaseb(a),* Hae-Kwon Jeong (b), (c) (a) Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar (b) Artie McFerrin Department of Chemical Engineering and (c) Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3122, United States

ABSTRACT: Various ZIFs (ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8) were exploited in the adsorption of N2, CH4 and CO2 gases over a temperature range from 25 to 55 °C. The dual site Langmuir model was used to describe the measured adsorption equilibria. Overall, N2 exhibited the lowest adsorption capacity onto all adsorbents, whereas CO2 gas exhibited the highest adsorbed amount. Among the three adsorbents, the highest adsorption capacities of all gases was onto Cu10%/ZIF-8. The adsorption isosteric heats of different systems were determined. Furthermore, the overall mass transfer coefficients for adsorbing N2, CH4 and CO2 gases on each adsorbent were studied at different temperatures.

Keywords: ZIFs; adsorption isotherms, overall mass transfer coefficient, Isosteric heats

*Corresponding Author. Tel.(+974) 4403-4139; Fax: (+974) 4403-4131; E-mail: [email protected] 1 On leave from the Radiation Research of Polymer Department, National Centre for Radiation Research and Technology, Atomic Energy Authority, P.O. Box 29, Nasr City, Cairo, Egypt

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Industrial & Engineering Chemistry Research 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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1. INTRODUCTION The separation of N2 from CH4 is a challenging issue in the development of natural gas fields including high level of N2 gas and in the enrichment of coal bed methane.1 Worldwide joint efforts attempt to put a wise strategy to develop promising technologies for removing CO2 gas from post combustion gas mixtures and for separating CO2 gas from CH4 gas mixtures.2-4 Processes such as cryogenic distillation, membrane separation, absorption and adsorption have been utilized to separate CO2 from CH4 and N2 mixtures.5-8 Zeolitic imidazolate frameworks (ZIFs) are typical materials to be used in the separation of gas mixtures. This is due to their unique features such as their excellent chemical and hydrothermal stability, high surface area, microporosity and framework flexibility.9,10 ZIF-8 includes a Zn center bonded by the imidazole linker, and exhibits big pore widths of 11.6 Å and a diminutive aperture of 3.4 Å.11 Lately, the microwave field attracted a huge attention in the chemical preparation of nanoporous substances.12,13 The microwave technique is characterized by uniform and rapid heating with a handy rise rate,14 which has effectively minimized the preparation period and improved the outcome.15 Up to now, only some works addressed the microwave-supported preparation of ZIF-8.16, 17

The production of ZIF-8 particles necessitates two steps: nucleation and crystallization.18 Various

parameters (i.e., media temperature, solvent evaporation and solution molar composition) influence the particle properties after the microwave-supported preparation process.19 Due to the physical features of ZIFs such as porosity, intra-pore ion exchange capability, in-pore changeable acidity, molecular sized pore dimensionality and controllable diffusivity of gas molecules; zeolites have been widely examined for the adsorption and separation of CO2 gas either in bench scale or mass production.20 Liang et al mentioned in their work that the CO2, N2 and CH4 gases adsorption onto (Zn2(bdc)2(dabco)(H2O)0.5(DMF)4 and (Ni2(bdc)2(dabco)(H2O)0.5(DMF)4 at pressures up to ~1.5 MPa. The results showed that it is possible to use the (Zn2(bdc)2(dabco)(H2O)0.5(DMF)4 in the adsorption of CO2, N2 and CH4 gases.21 Sumida et al reported that Cu(II) benzene-1,3,5-tricarboxylate) performed satisfactorily for the selective adsorption of carbon dioxide over methane and nitrogen gases.22 Burd et al studied the adsorption of CO2 and CH4 gases on [Cu(bpy-1)2(SiF6)] and reported a CO2 uptake of 23.1 wt.% and a CO2/CH4 selectivity of 10.5 at 25oC and 0.1 MPa.23 The development of ultramicroporosity of materials is necessary to enhance the CH4 uptake capacity as well as the effective separation of CH4 from N2. Saha et al studied the adsorption of carbon dioxide, methane and nitrogen gases onto MOF-5, MOF-177, and Zeolite 5A. They found that MOF adsorbents have higher 2 ACS Paragon Plus Environment

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adsorption capacities for CO2 and CH4 than zeolite 5A at high pressures, and that the MOF-5 and MOF-177 are better adsorbents for CO2 and CH4.24 Abid et al studied the adsorption of CO2 and CH4 gases onto zirconium-metal organic frameworks (Zr-MOFs) and reported adsorption capacities of CO2 and CH4 as 8.1 and 3.6 mmol/g, respectively.25 Couck et al reported the synthesis of an aminofunctionalized MIL-53 MOF in order to separate CO2 and CH4 gases.8 Bao et al examined the separation of methane and carbon dioxide gases by using a copper MOF, and indicated a potential to use this adsorbent in separation processes. 26,27 The purpose of this work is to use ZIF-8, in its pure form and after partial exchange of zinc ions with copper ions, namely Cu10%/ZIF-8 and Cu30%/ZIF-8, for adsorption of N2, CH4 and CO2 gases at 25, 35, 45 and 55 °C; at pressures up to ~1 MPa. The impact of compositional structure of the adsorbents on their adsorption behavior was investigated. The dual site Langmuir model was exploited to fit the adsorption equilibrium data in terms of pressure and temperature; and the isosteric heats of adsorption for these gases onto ZIF samples were evaluated. Furthermore, the overall mass transfer coefficients for adsorbing N2, CH4 and CO2 gases onto these adsorbents were investigated at different temperatures.

2. EXPERIMENTAL SECTION 2.1. Materials. Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, 98%, Sigma Aldrich) and copper nitrate hemi(pentahydrate) (Cu(NO3)2.2.5H2O, ≥ 99.99%, Sigma Aldrich) were utilized as metal sources. 2Methylimidazole (C4H5N2, 97%, Sigma Aldrich) was utilized as a ligand and CH3OH (99.8%, Alfa Aesar) was utilized as a reaction medium. Various gases utilized (namely, methane, carbon dioxide, nitrogen and helium) were purchased from the National Industrial Gas Plants (NIGP, Qatar) with a Grade 5 purity (99.999%). All reagents were used as supplied without extra treatment.

2.2. Microwave preparation of mixed metal Cu/ZIF-8. The amounts of (4.45-) mmol of Zn(NO3)2.6H2O and  mmol of Cu(NO3)2.2.5H2O were added to a 15 ml of CH3OH. The quantity of  ranged from 0 to 2.23 in order to adjust the ratio of Cu(NO3)2.2.5H2O/Zn(NO3)2.6H2O in the mixed metal solution. A ligand solution was prepared from 39.5 mmol of 2-methylimidazole in 15 mL of methanol. The metal solution was then added to the ligand solution while stirring the mixture for 1 minute. The mixed solution was then irradiated with a microwave power of 100 W for 1.5 minutes. 3 ACS Paragon Plus Environment


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Full synthesis detail are found elsewhere.28 The resulting samples with initial compositions of 10% and 30% copper were denoted as Cu10%/ZIF-8and Cu30%/ZIF-8, respectively.

2.3. Characterization. The FT-Raman spectra were determined by using a Bruker FT-Raman spectrometer, which is attached to a Bruker-IFS 66/S spectrometer that provides high resolution to better than 0.10 cm-1. Foureir transform infra-red (FTIR, NICOLET, Thermo-Scentific) was utilized to assess the chemical structure of preapred specimens. The morphology of ZIF-8 speciemens was scanned with a FEI Nova™ nanoscanning electron microscopy 450 (Nova NanoSEM). The elemental compositions of specimens were examined by Energy-dispersive X-ray spectroscopy (EDX) connected to NanoSEM. X-ray photoelectron spectroscopy (XPS) was conducted via a Thermo Scientific K-alpha photoelectron spectrometer with monochromatic Al K- radiation. X-ray diffraction (XRD) tests were done by a Miniflex II Benchtop XRD, manufactured by Rigaku Corporation Japan. Thermogravimetric analyses (TGA) were carried out under an N2 gas with a heating rate of 10 °C/min in the range of 30 to 800 °C (PerkinElmer Pyris 6 TGA).29 All these chataerizations results are found in a preveuios work.28 Before measuring the adsorption/desorption isotherms, the samples were degassed under vacuum at 110 °C for 24 h. The gas adsorption/desorption isotherms of carbon dioxide, methane and nitrogen were meausred by a Hygra magnetic suspension microbalance (MSB, Rubotherm), following same procedures and calculations as explained elsewhere.30 The measurement uncertainties of the MSB are: weight measurement (±10 µg), temperature (±10 mK) and pressure (± 0.01 MPa).

3. THEORY 3.1. Adsorption equilibria. The multi-site Langmuir adsorption model considers a heterogeneous adsorbent surface that is formed from a number of sites of distinct characteristic adsorption energies. Every site is, hence, thought as a homogeneous patch of the material surface; and the overall quantity adsorbed of a gas onto a number (J) of surface patches can be defined as 𝐽

𝑛𝑎𝑑𝑠 =

𝑚 𝑗 𝑏𝑗 𝑃

∑1 + 𝑏 𝑃

𝑗=1

𝑗

4 ACS Paragon Plus Environment

(1)

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where J is usually set to 2, depending on the extent of heterogeneity, which corresponds to the dualsite Langmuir (DSL)). P is pressure, mj is adsorption capacity of the component on the adsorption site (j); and bj is the corresponding adsorption affinity on the same site, which is given by: 𝜀𝑗

( )

𝑏𝑗 = 𝑏0𝑗 𝑒𝑥𝑝

(2)

𝑅𝑇

where 𝑏0𝑗 is the corresponding adsorption affinity, 𝜀𝑗 is the corresponding adsorption energy, R is the universal gas constant and T is temperature. 3.2. Isosteric heat of adsorption. The Clausius-Clapeyron equation31 can be used to estimate the isosteric heats of adsorption (-∆𝑸𝒔𝒕) as

∆𝑄𝑠𝑡 = ―𝑅

[ ] ∂𝑙𝑛𝑃 1 ∂( ) 𝑇

(3) 𝑛

The change of isosteric heats of adsorption with the amounts adsorbed is an indirect estimation of the interactions among adsorbed gas molecules, and between gas molecules and the ZIF surface. Also, it refers to the energetic heterogeneity of ZIF surface.32 3.3. Rate of Adsorption. The adsorption rate can be evaluated by the linear driving force (LDF) formula,33 which gives

() 𝑛𝑡

𝑛𝑒

= (1 ― 𝑒 ―𝑘𝑡 )

(4)

where nt (mole/kg) indicates the amount adsorbed at time t (sec), ne (mole/kg) indicates the equilibrium amount adsorbed at P and T; and k is the overall mass transfer coefficient (sec-1).

(

𝑛𝑡

)

Eq.(4) is linearized as 𝑙𝑛 1 ― 𝑛𝑒 = ―𝑘𝑡 and the data of each adsorption segment (amount adsorbed

(

𝑛𝑡

)

versus time) before reaching equilibrium is plotted as 𝑙𝑛 1 ― 𝑛𝑒 versus t, where the slope is determined as -k for the corresponding pressure segment at the studied temperature. The limitations of the LDF model were discussed elsewhere.34-37

3.4. Error analysis. Error analysis was used to define the best fitting relationship that quantifies the distribution of adsorbates, and also for verifying the precision of adsorption models. The least sum of 5 ACS Paragon Plus Environment


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square of errors (LSSE) is said to be the most widely used error function for regression purposes. This method can be represented by the following expression38

[∑

]

𝑁

𝐿𝑆𝑆𝐸 = 𝑀𝑖𝑛𝑖𝑚𝑢𝑚

(𝑛𝑒,𝑖,𝐶𝑎𝑙𝑐 ― 𝑛𝑒,𝑖, 𝑒𝑥𝑝)2

𝑖=1

(5)

where i refers to the experimental point number, N refers to the count of experimental points; and 𝑛𝑖,𝐶𝑎𝑙𝑐 and 𝑛𝑖, 𝑒𝑥𝑝 represent, respectively, the calculated and experimental amounts adsorbed.

The average relative error percentage (ARE%) was used to evaluate the precision of the regression model over the corresponding range. It is given by39

100% 𝐴𝑅𝐸(%) = 𝑁

𝑁

∑[

𝑖=1

]

𝑛𝑖,𝐶𝑎𝑙𝑐 ― 𝑛𝑖, 𝑒𝑥𝑝 𝑛𝑖, 𝑒𝑥𝑝

(6)

4. RESULTS AND DISCUSSION 4.1. Adsorption isotherms Figures (1-3) show the adsorption isotherms of N2, CH4 and CO2 on ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8, respectively, at the temperatures of 25, 35, 45 and 55 °C. Overall, from the three figures, it was observed that the adsorption equilibrium capacity deceases by increasing the temperature. Further, N2 exhibits the lowest equilibrium adsorption amount whereas CO2 exhibits to the highest adsorption equilibrium capacity at each temperature. The DSL model was used to fit the experimental adsorption data of N2, CH4 and CO2 gases in terms of pressure and temperature. The fitting results found in Table 1 show a satisfactory representation of the experimental data as noticed by the lines in Figures 1-3 and the ARE% values listed in Table 1. The adsorption isotherms of N2, CH4 and CO2 gases onto ZIF-8 were shown in Figure 1. It was seen that the adsorption of these gases is fitted well by the DSL model over the entire ranges of temperature and pressure. Furthermore, the adsorption of these gases on ZIF-8 shows almost a straight-line behaviors that do not approach monolayer saturation within the studied range of pressure. Figure 2 illustrates the adsorption of N2, CH4 and CO2 gases onto Cu10%/ZIF-8. It was noticed that the experimental data of N2 and CO2 gases adsorption has deviated slightly from DSL model in minor behavior. Figure 2b shows that the fitting of CH4 exhibits some deviations between the experimental points and the DSL correlation, especially at 35 °C. Overall, 6 ACS Paragon Plus Environment

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it is seen that the adsorption data of N2, CH4 and CO2 gases exhibit almost straight-line behaviors at all temperatures. Figure 3 shows the adsorption of N2, CH4 and CO2 gases gas onto Cu30%/ZIF-8. It was noticed that some deviations between experimental data and the DSL model has occurred at both low and high temperatures. Figure 3c shows that the deviation between experimental points and the DSL correlation was somewhat significant at low pressures, especially at 35, 45 and 55 °C. Overall, it is seen that the adsorption data of N2, CH4 and CO2 gases exhibit straight-line behaviors at low temperatures, whereas they show a slight approach to the monolayer saturation limit for the N2 and CH4 adsorption at elevated temperatures. Moreover, Figure 3 may have exhibited different isotherm shapes in comparison to the other samples. This may be due to the high ratio of Cu2+ ions into the matrix of ZIF-8, which leads to this behavior. Table 2 exposes the adsorption capacities of N2, CH4 and CO2 on ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 compared with other adsorbents, and it shows that the adsorption capacities in the present study are higher than those onto ZIF-8, ZIF-67, Zeolite-13X, MOF-505@2GO and BPL activated carbon. Nonetheless, they are lower than those on MFM-300(In), NiDABCO and ZnDABCO. It is noteworthy to mention that ZIF-8 was studied by Liu et al41 and Kong et al42 at 25 °C and 1 bar, while the current work considers pressures up to 10 bar.Such differences between adsorbents can be attributed to different reasons; including the nature of the adsorbent surface, functional groups and pore characteristics. The relationship between the gas type and LSSE and ARE(%) values across the range of adsorption pressures are shown in the Supplementary Data File (Fig.S1) and a comparison of LSSE and ARE(%) values obtained in this work against those reported in literature is shown in Table S1 (Supplementary Data File).



0.8

a

(mole/kg)

n

N2

0.6

25oC 35oC 45oC 55oC

0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 1.8 1.6

(mole/kg)

n

CH4

1.4

25oC

b

35oC 45oC

1.2

55oC

1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 7

(mole/kg)

CO2

6

n

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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5

c

25oC

35oC

4

45oC 55oC

3 2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) Figure 1. Adsorption isotherms of (a) N2, (b) CH4 and (c) CO2 onto ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols represent experimental data, and lines represent DSL adsorption model fitting. 8 ACS Paragon Plus Environment

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a

2.5

(mole/kg)

n

N2

2.0

25oC

1.5 35oC

1.0

45oC 55oC

0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa)

(mole/kg)

n

CH4

3

b

25oC

2

35oC

1

o 45 C o 55 C

0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 8

c

(mole/kg)

CO2

6

n

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


25oC

35oC

4

45oC 55oC

2 0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) Figure 2. Adsorption isotherms of (a) N2, (b) CH4 and (c) CO2 onto Cu10%/ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols represent experimental data and lines represent DSL adsorption model fitting. 9 ACS Paragon Plus Environment


a

1.8 1.6

(mole/kg)

n

N2

1.4

25oC

1.2 1.0

35oC

0.8 0.6

45oC

0.4

55oC

0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 3.0

b

(mole/kg)

CH4

2.5

n

25oC

2.0

35oC

1.5

45oC 55oC

1.0 0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 7

(mole/kg)

CO2

6

n

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 10 of 42

c

o 25 C

5

o 35 C

4

o 45 C

3

o 55 C

2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) Figure 3. Adsorption isotherms of (a) N2, (b) CH4 and (c) CO2 onto Cu30%/ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols represent experimental data and lines represent DSL adsorption model fitting.


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Figure 4(a-c) displays the effect of various ZIFs, namely, ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 , toward the adsorption of N2, CH4 and CO2 gases at 25 °C (note that 25 °C was chosen as an example temperature for the sake of brevity and to avoid data crowding). It was observed that the amount adsorbed of each gas onto the Cu10%/ZIF-8 adsorbent is the highest. Further, the sequential order of the amount adsorbed is Cu10%/ZIF-8 > Cu30%/ZIF-8 > ZIF-8. Therefore, it can be concluded that an intermediate copper concentration doped into ZIF-8 (e.g., Cu10%/ZIF-8) shows the highest adsorption capacity of N2, CH4 and CO2, whereas an increased concentration of copper can reduce the adsorption capacity for these gases. Nonetheless, it remains to be higher than that of the un-doped ZIF-8.



2.5

a

nN2 (mole/kg)

2.0

/Z I

8

F-

0% Cu 1

1.5

/Z I

1.0

F-8

0% Cu 3

ZIF-8

0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 4

b

nCH4 (mole/kg)

3

/Z I

F-8

0% Cu 1

/Z I

F-8

0% Cu 3

2 ZIF-8

1 0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) 8 6

nCO2 (mole/kg)

c

/Z I

F-8

u 10%

C

/Z I

4

F-8

u 30%

C

F8

2 ZI

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 12 of 42

0 0.0

0.2

0.4

0.6

0.8

1.0

P (MPa) Figure 4. Effect of different adsorbents of ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 on the adsorption of (a) N2, (b) CH4 and (c) CO2 gases at 25 °C. Symbols represent experimental data and lines represent DSL adsorption model fitting.


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Table1. Fitting parameters of DSL model for the adsorption of N2, CH4 and CO2 gases onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8.

ZIF-8

Parameters

m1 (mole/kg) b0 (MPa-1) E/R (K) m2 (mole/kg) b0 (MPa-1) E/R (K) SSE ARE (%)

Adsorbent Cu10%/ZIF-8

Cu30%/ZIF-8

Adsorbate N2

CH4

CO2

N2

CH4

CO2

N2

CH4

CO2

38359

9.58

97.28

461.66

10.43

12.032

33.62

16.86

33871

1.8410-6

9.9810-4

2.1410-9

3.2010-6

4.4310-14

2.1910--13

1.4410-16

8.7510-6

1.7310-8

693.55

1582

5038

1927

8643

8375

9705

2899

2730

0

0

27.89

34.17

1567

109.93

75.08

5.70

1.09

0

0

7.6110-2

3.3810-15

3.2210-6

4.3810-3

3.2410-6

6.1310-7

1.7110-8

0

0

0

8769

1574

490.58

2323

412

0

0.020 8.82

0.0082 3.54

1.07 7.46

0.32 34.10

2.76 36.30

0.089 3.20

0.56 46.29

1.52 16.49

1.98 20.15



Page 14 of 42

Table 2. A comparison of adsorption saturation limits of N2, CH4 and CO2 gases onto different adsorbents. Gas type ZIF-8 Cu10%/ZIF-8 Cu30%/ZIF-8 Zeolite-13X ZIF-8 ZIF-8 ZIF-67 Zr-MOF Cu/ZIF-67 MFM-300(In) NiDABCO MOF-505@2GO Coffee grounds (activated carbon) Chitosan (activated carbon) Coconut shell (activated carbon) Olive stones (activated carbon) Celtuce leaves (activated carbon) ZnDABCO

N2 CH4 CO2 N2 CH4 CO2 N2 CH4 CO2 N2 CH4 CO2 N2 CO2 CH4 N2 CH4 CO2 CO2 N2 CO2 CO2 N2 CH4 CO2 N2 CH4 CO2 N2 CH4 CO2

Adsorption (mmol/g) 0.71 1.74 6.19 2.08 3.40 6.76 1.52 2.51 6.06 0.2 0.3 1.7 0.08 0.66 0.28 0.07 0.2 0.9 0.73 2.5 3 1.17 4 6.5 9 4 8 13 0.25 0.8 3.4

CO2

3

Plaza et al49

CO2

3.8

Gillard et al50

CO2

3.9

Ello et al51

CO2

2.4

Plaza et al52

CO2

4.3

Wang et al53

N2 CH4 CO2

3.5 7.5 12.5

Mishra et al54

Gas


Reference Present study Present study Present study McWen et al40 Liu et al 41 Kong et al 42 Zhong et al43 Huang et al44 Yang et al 45 Wu et al46 Mishra et al47 Chen et al48

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4.2. Overall mass transfer coefficient Figures (5-7) show the relationship between the overall mass transfer coefficients (k, sec-1) versus the reciprocal of pressure (1/P) at different temperature; namely 25, 35, 45 and 55 °C for the adsorption of N2, CH4 and CO2 gases onto the three adsorbents; ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8. The symbols and lines represent the experimental data and regression. The regression parameters are listed in Table 3. Overall, it is noticed that k values increase by increasing temperature and by decreasing pressure (or increasing 1/P). Moreover, regardless of temperature, k values diminish similarly at very high P (when 1/P → 0) for all three gases. Figure 5(a-c) displays the variation of the overall mass transfer coefficients for the adsorption of N2, CH4 and CO2 gases onto ZIF-8 adsorbent against 1/P at various temperatures, respectively. It was observed that the adsorption of CO2 gas onto ZIF-8 adsorbent is slowest among the three gases while that of CH4 gas is the fastest. Figure 6(a-c) shows the variation of k values for the adsorption of different gases onto Cu10%/ZIF-8 versus 1/P at various temperatures. Through results, it could be deduced that the adsorption of CO2 onto Cu10%/ZIF-8 is faster than that of CH4 and N2 molecules during adsorption process. Figure 7(a-c) illustrates that the adsorption of N2 gas onto Cu30%/ZIF-8 was the fastest among the three gases while that of CO2 was the slowest.



Figure 5. Overall mass transfer coefficients for gas adsorption of (a) N2, (b) CH4 and (c) CO2 gases onto ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols and lines represent experimental data and regression, respectively. 16 ACS Paragon Plus Environment

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Figure 6. Overall mass transfer coefficients for the gas adsorption of (a) N2, (b) CH4 and (c) CO2 gases onto Cu10%/ZIF-8 adsorbents at 25, 35, 45 and 55 °C. Symbols and lines represent experimental data and regression, respectively. 17 ACS Paragon Plus Environment


Figure 7. Overall mass transfer coefficients for the gas adsorption of (a) N2, (b) CH4 and (c) CO2 gases onto Cu30%/ZIF-8 adsorbents at 25, 35, 45 and 55 °C. Symbols and lines represent experimental data and regression, respectively.


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Figure 8 shows the effect of different adsorbents; ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF8 on the k values for the adsorption of N2, CH4 and CO2 at 25 and 55 °C (note that the temperature of 25 and 55 °C were chosen as examples for the sake of brevity and to avoid data crowding). It is noteworthy to mention that the regression line equations are as listed in Table 3. It was found that, at both 25 and 55 °C, the adsorption of the three gases was the fastest onto ZIF-8; where it was the slowest on Cu10%/ZIF-8. It is noteworthy to mention that a comparison between the k values found in this work and those reported elsewhere is found in Table S2 (Supplementary File Data).



70

a

50

15

-8

10

-1

/ZIF % Cu 30

IF-8 Cu 10%/Z

5

b

60

(sec )

-1

(sec )

4

20

kN2 x 10

-8

4

25

F ZI

kN2 x 10

30

ZIF

40

/ZIF Cu 30%

30 20

1

2

3

4

5

0

6

1

2

35

8 FZI

Cu30%/ZIF-8

-1

15

5 0 1

2

3

4

5

ZIF

Cu 10%/ZIF-8

6

0

1

2

200

-8

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Cu 10%/ZIF-8

0 0

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4

6

8

10

(sec )

/ZIF % Cu 30

4

5

6

-8

100 50

Cu30%/ZIF-8

0 12

ZIF

150

-1

6

4

f

8

kCO2 x 10

-1

(sec )

Z

IF

4

8

3

1/P (MPa) 250

e

10

-8

20

1/P (MPa) 12

6

8 /ZIFCu 30%

40

0

Cu10%/ZIF-8

0

5

60

(sec )

kCH4 x 10

-1

(sec )

20

10

4

d

80

4

25

-4

100

c

30

3

1/P (MPa)

1/P (MPa)

kCH4 x 10

-8

0 0

-4

-8

Cu 10%/ZIF-8

10

0

kCO2 x 10

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 20 of 42

Cu10%/ZIF-8

0

2

4

6

8

10

1/P (MPa)

1/P (MPa)

Figure 8. Effect of different adsorbents; namely, ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8, on the overall mass transfer coefficients for the gas adsorption of (a, b) N2, (c, d) CH4 and (e, f) CO2 gases at 25 and 55 °C, respectively. Symbols and lines represent experimental data and regression, respectively. The symbols of (□) in subfigure 8f indicates to Cu10%/ZIF-8, which overlaps with Cu30%/ZIF-8 (black symbols).


12

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Table 3. Regression of data of overall mass transfer coefficient (104×𝑘 = 𝑎/𝑃 + 𝑏) and determination coefficients (R2) for the adsorption of N2, CH4 and CO2 onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 at 25, 35, 45 and 55 °C. The units of a and b are MPa.s-1 and s-1, respectively. Adsorbent

T (°C) 25 35

ZIF-8

45 55 25 35

Cu10%/ZIF-8

45 55 25 35

Cu30%/ZIF-8

45 55

N2

CH4

CO2

5.51/P-4.41 (R2= 0.95) 5.90/P +5.97 (R2= 0.93) 6.16/P +6.86 (R2= 0.97) 7.18/P +5.79 (R2= 0.98) 0.47/P-0.17 (R2=0.98) 0.45/P +0.96 (R2=0.90) 0.93/P+0.93 (R2=0.97) 0.94/P +1.15 (R2=0.98) 1.75/P +1.37 (R2=0.90) 7.18/P +9.24 (R2=0.93) 8.97/P +6.56 (R2=0.92) 10.43/P +4.06 (R2=0.98)

8.96/P-0.42 (R2=0.90) 10.63/P+0.84 (R2=0.99) 12.88/P -1.82 (R2=0.99) 16.48/P -2.94 (R2=0.99) 0.18/P +2.13 (R2=0.90) 0.74/P +1.82 (R2=0.96) 3.09/P-2.94 (R2=0.91) 3.18/P +0.23 (R2=0.99) -0.30/P +4.12 (R2=0.89) 1.39/P +1.62 (R2=0.93) 3.13/P+0.46 (R2=0.92) 4.35/P +3.14 (R2=0.96)

0.40/P +0.06 (R² = 0.90) 0.45/P +0.33 (R² = 0.95) 0.47/P+ 0.34 (R² = 0.98) 0.95/P- 0.44 (R² = 0.86) 0.20/P+0.24 (R2=0.91) 7.69/P-6.66 (R2=0.98) 10.15/P+2.24 (R2=0.98) 20.35/P -12.35 (R2=0.97) 0.65/P -0.58 (R2=0.99) 0.83/P -0.38 (R2=0.98) 1.39/P -1.11 (R2=0.97) 2.08/P -1.54 (R2=0.97)

4.3. Isosteric heats of adsorption Figure 9(a-c) represents the isosteric heats of adsorption (Qst) of N2, CH4, and CO2 gases onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 adsorbents. Overall, it is noted that the isosteric heats of all gases increase with different extents when increasing the loading of adsorbed species, which indicates favorable interactions among the adsorbed molecules on the surfaces of the adsorbents.32 Nonetheless, it is seen from Figure 9a that the isosteric heat of adsorption of N2 on ZIF-8 increases by increasing the loading up to ~0.3 mole/kg to reach ~11 kJ/mole, then levels off at around this value, which 21 ACS Paragon Plus Environment


indicates adsorption on a virtually homogeneous surface of the adsorbent 32 or a balance between favorable and unfavorable interactions. The maximum Qst value of CO2 is ~33 kJ/mole at a corresponding loading of ~3 mole/kg. On the other hand, the Qst value of CH4 increases to ~ 31 kJ/mole at corresponding loading of ~1 mole/kg. Overall, it was observed from Figure 9a that the order of the isosteric heat of adsorption of different components (within the range of their common loading) on ZIF-8 is CH4 > CO2 > N2, which also indicates the order of adsorption affinity of ZIF-8 towards these gases. In contrary to ZIF-8, Figure 9b shows that the isosteric heat of adsorption of N2 onto Cu10%/ZIF-8 increases by increasing the loading significantly. The maximum Qst value of N2 gas reaches ~51 kJ/mole at a loading of ~ 0.8 mole/kg. The Qst value of CO2 gas increases very slightly with loading, up to a maximum of ~35 kJ/mole. Further, it was noticed that the Qst of CH4 reaches ~50 kJ/mole a loading of ~1 mole/kg. Overall, the order of isosteric heats of adsorption onto Cu10%/ZIF-8, which relates to the adsorption affinity, can be categorized roughly into three different regions. First, at low loadings of each component adsorbed (i.e., up to a loading of ~0.3 mole/kg), the order is CH4 > CO2 > N2. Then, up to a loading of ~0.7 mole/kg, the order is CH4 > N2 > CO2. After that loading, the order becomes N2 > CH4 > CO2. Figure 9c indicates to the Qst value of N2 gas adsorption onto Cu30%/ZIF8 increases by increasing the loading up to ~27 kJ/mole at a loading of ~0.3 mole/kg. Up to that loading, the isosteric heat of N2 is almost equal to (just slightly less than) that of CO2. In turn, the Qst value of CO2 gas increases significantly up to ~72 kJ/mole at a loading of ~ 0.9 mole/kg. After this value, the increase becomes more moderate up to ~102 kJ/mole at a loading of ~3 mole/kg. Furthermore, the Qst value of CH4 gas increases up to ~80 kJ/mole at a loading of ~0.9 mole/kg. Overall, the order of the isosteric heats of adsorption of different components (within the range of their common loading) is CH4 > CO2 > N2. Figure 10(a-c) shows a comparison between the Qst values of each gas onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8. Through results, it was observed from Figure 10a that the Cu10%/ZIF-8 adsorbent exhibits the highest Qst value of N2 whereas the ZIF-8 adsorbent exposes the lowest Qst value of N2. Furthermore, the isosteric heat of adsorption of N2 increases steadily with the amount adsorbed for both of the two Cu/ZIF-8 adsorbents, while it is almost constant for the ZIF-8 (without copper). 22 ACS Paragon Plus Environment

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Additionally, it could be deduced form Fig. 10(b, c) that the Cu30%/ZIF-8 adsorbent represents the highest Qst values of both CH4 and CO2 gases, whereas ZIF-8 exhibits the lowest value. Therefore, in general, it can be concluded that the presence of copper with different ratios into the matrix of ZIF-8 has a significant impact on the isosteric heats of adsorption of N2, CH4 and CO2.



35

C

H 4

Zn-ZIF-8

30

C

O 2

25 20 15

ZIF-8

N 2

Qst (kJ/mole)

a

10 5 0.0

0.5

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Loading (mole/kg)

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CH 4

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ZnCu-ZIF-8 (10%)

Qst (kJ/mole)

45 40 CO2

35 30 25 20

N2

Cu10%/ZIF-8

15 0.0

c Qst (kJ/mole)

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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0.5

1.0

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Loading (mole/kg) 100

CO2

ZnCu-ZIF-8 (30%) CH 4

80 60 40 20

Cu30%/ZIF-8 N2

0 0

1

2

3

4

5

Loading (mole/kg) Figure 9. Isosteric heats (Qst) for the adsorption of N2, CH4, and CO2 gases onto (a) ZIF-8, (b) Cu10%/ZIF-8and (c) Cu30%/ZIF-8 adsorbents. 24 ACS Paragon Plus Environment

Page 25 of 42

Qst (kJ/mole)

a

60

N2

50

/ZIF Cu 10%

40 30

-8

/ZIF Cu 30%

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-8

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10 0 0.0

0.2

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Loading (mole/kg)

b

90

CH4

80

/ZIF % 0 u 3 C

-8

Qst (kJ/mole)

70 60

Cu 10%/ZIF-8

50 40 ZIF-8

30 20 10 0.0

c

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Loading (mole/kg) 120 CO2

100

Qst (kJ/mole)

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


/ZIF

-8

Cu 30%

80 60 Cu10%/ZIF-8

40

ZIF-8

20 0 0

1

2

3

4

Loading (mole/kg) Figure 10. Isosteric heats of adsorption of (a) N2, (b) CH4 and (c) CO2 onto ZIF-8, Cu10%/ZIF-8and Cu30%/ZIF-8. 25 ACS Paragon Plus Environment


5. CONCLUSIONS In this study, copper was exchanged with zinc of ZIF-8 with different ratios by a novel technique of microwave-irradiation. According to the authors' knowledge, this technique is novel for these specific products; and it is featured with a fast production rate, which saves time and efforts. These samples are ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8. These ZIF materials were tested for the adsorption of N2, CH4 and CO2 gases at 25, 35, 45 and 55 °C. The dual site Langmuir (DSL) model was used to fit the experimental adsorption data for each adsorbent-adsorbate system as functions of temperature and pressure. The Cu10%/ZIF-8 represents the highest adsorption capacity towards N2, CH4 and CO2 gases at all temperatures. Further, the ZIF-8 represents the lowest adsorbent of adsorption capacity. The overall mass transfer coefficients were determined as an indicator on the rates of adsorption. In general, the rates of adsorption increased when increasing temperature or decreasing pressure. Overall, the rate of adsorption of different gases was in the order ZIF-8 > Cu30%/ZIF-8 > Cu10%/ZIF-8. These variations can be used to enhance the kinetic separation of these components from their mixtures. Moreover, the isosteric heats of adsorption of N2, CH4 and CO2 gases onto ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 adsorbents were determined and analyzed.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This publication was made possible by the NPRP awards (NPRP 08-014-2-003 and NPRP-8-001-2-001) from the Qatar National Research Fund (a member of The Qatar Foundation). H.K.-J. acknowledges support from the National Science Foundation (CMMI-1561897). The statements made herein are solely the responsibility of the authors. Technical support from the Department of Chemical Engineering, the Central Laboratory Unit (CLU) and the Gas Processing Centre (GPC) at Qatar University is also acknowledged.


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Figure 1. Adsorption isotherms of (a) N2, (b) CH4 and (c) CO2 onto ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols represent experimental data, and lines represent DSL adsorption model fitting. 174x376mm (150 x 150 DPI)


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Figure 2. Adsorption isotherms of (a) N2, (b) CH4 and (c) CO2 onto Cu10%/ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols represent experimental data and lines represent DSL adsorption model fitting. 175x375mm (150 x 150 DPI)



Figure 3. Adsorption isotherms of (a) N2, (b) CH4 and (c) CO2 onto Cu30%/ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols represent experimental data and lines represent DSL adsorption model fitting. 177x379mm (150 x 150 DPI)


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Figure 4. Effect of different adsorbents of ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8 on the adsorption of (a) N2, (b) CH4 and (c) CO2 gases at 25 °C. Symbols represent experimental data and lines represent DSL adsorption model fitting. 174x379mm (150 x 150 DPI)



Figure 5. Overall mass transfer coefficients for gas adsorption of (a) N2, (b) CH4 and (c) CO2 gases onto ZIF-8 adsorbent at 25, 35, 45 and 55 °C. Symbols and lines represent experimental data and regression, respectively. 195x379mm (150 x 150 DPI)


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Figure 6. Overall mass transfer coefficients for the gas adsorption of (a) N2, (b) CH4 and (c) CO2 gases onto Cu10%/ZIF-8 adsorbents at 25, 35, 45 and 55 °C. Symbols and lines represent experimental data and regression, respectively. 177x386mm (150 x 150 DPI)



Figure 7. Overall mass transfer coefficients for the gas adsorption of (a) N2, (b) CH4 and (c) CO2 gases onto Cu30%/ZIF-8 adsorbents at 25, 35, 45 and 55 °C. Symbols and lines represent experimental data and regression, respectively. 176x366mm (150 x 150 DPI)


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Figure 8. Effect of different adsorbents; namely, ZIF-8, Cu10%/ZIF-8 and Cu30%/ZIF-8, on the overall mass transfer coefficients for the gas adsorption of (a, b) N2, (c, d) CH4 and (e, f) CO2 gases at 25 and 55 °C, respectively. Symbols and lines represent experimental data and regression, respectively. The symbols of (□) in subfigure 8f indicates to Cu10%/ZIF-8, which overlaps with Cu30%/ZIF-8 (black symbols). 354x382mm (150 x 150 DPI)



Figure 9. Isosteric heats (Qst) for the adsorption of N2, CH4, and CO2 gases onto (a) ZIF-8, (b) Cu10%/ZIF-8and (c) Cu30%/ZIF-8 adsorbents. 178x382mm (150 x 150 DPI)


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Figure 10. Isosteric heats of adsorption of (a) N2, (b) CH4 and (c) CO2 onto ZIF-8, Cu10%/ZIF-8and Cu30%/ZIF-8. 172x381mm (150 x 150 DPI)



254x190mm (96 x 96 DPI)


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Adsorption Equilibrium and Kinetics of Nitrogen ... - ACS Publications

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