CH4 on titanium based MOFs

Fuel 160 (2015) 318–327

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Effects of amino functionality on uptake of CO2, CH4 and selectivity of CO2/CH4 on titanium based MOFs Zana Hassan Rada a, Hussein Rasool Abid b, Jin Shang c, Yingdian He c, Paul Webley c, Shaomin Liu a, Hongqi Sun a, Shaobin Wang a,⇑ a b c

Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia Department of Environmental Health, Applied Medical Science College, Karbala University, Iraq Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Metal organic frameworks,

MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti), were synthesized.  CH4 and CO2 adsorption was tested on the above Ti-based adsorbents.  NH2-MIL-125(Ti) presented the highest adsorption capacity.  MIX-MIL-125(Ti) presented better adsorption and thermal stability than MIL-125(Ti).

a r t i c l e

i n f o

Article history: Received 16 March 2015 Received in revised form 27 July 2015 Accepted 28 July 2015 Available online 4 August 2015 Keywords: MIL-125(Ti) MIX-MIL-125(Ti) CO2 adsorption CH4 adsorption

a b s t r a c t This study examines adsorption of CO2 and CH4 gases and CO2/CH4 selectivity on titanium based MOFs such as MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) at high pressures up to 10 bar. Characterization and structural analysis of the samples were studied using FT-IR, XRD, TGA, SEM and N2 adsorption/desorption. The effects of double linkers in the synthesis process and addition of amino-functionalised linker in the structure on adsorption of CO2 and CH4 have been discussed. It was found that addition of NH2 functional group will increase surface area and micropore volume, but reduce particle size. Meanwhile, both CO2 and CH4 adsorption will also be increased. Using binary linkers, thermal stability of MIX-MIL-125(Ti) will also be improved. NH2-MIL-125(Ti) showed the highest CO2 and CH4 adsorption capacities. The adsorption heats of CO2 on MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) were not changed significantly, while the adsorption heats of CH4 reduced after amino functionalization. The selectivity factor of CO2/CH4 in MIX-MIL-125 was lower than MIL-125 but higher than NH2-MIL-125. Compared with other adsorbents such as other MOFs, zeolite 13X and activated carbon, MIL-125 demonstrated a higher selectivity factor. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.fuel.2015.07.088 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

In recent years, natural gas becomes one of the alternative resources for transportation and other daily uses of energy.

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Intensity (a.u)

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dioxide (CO2) from natural gas, is one of the important aims in natural gas processing [4]. Separation of carbon dioxide from methane in natural gas can take place through some technologies such as adsorption and absorption [5,6]. Absorption approach in natural gas industry has been used broadly using aqueous amine solutions through chemisorption. Nonetheless, limitations in absorption process are high operation cost and huge amount of energy requirement for amine regeneration [7]. Porous materials such as zeolites [8] and activated carbon [9] have been used as adsorbents in two main adsorption processes, pressure swing adsorption (PSA) and temperature swing adsorption (TSP) [4,10]. However, selectivities for such adsorbents are not good enough for CO2 separation from flue gases. Thus, investigation and researching to develop new porous materials with high selectivity and high CO2 adsorption capacity have been carried out. Metal organic frameworks (MOFs) are one class of porous materials being recognized for enhancing gas storage and selectivity over the traditional adsorbents [11]. MOFs materials are synthesized through coordination bonds between metal clusters and organic ligands, acquisition a high specific surface area, high pore volume and low density [12–14]. These materials have received significant attention by many

a b c

NH2-MIL-125-(Ti) MIX-MIL-125-(Ti) MIL-125-(Ti)

a b c

0

10

20

30

40

50

60

70

80

2θ

Fig. 1. XRD patterns of NH2-MIL-125(Ti) (a), MIX-MIL-125(Ti) (b) and MIL-125(Ti) (c) samples.

Natural gas is generally composed of up to 90% of methane and various proportions of impurities such as CO2, CO, and other hydrocarbons depending on the source of the gas [1–3]. Removing and separation of non-hydrocarbon gas contaminants, mainly carbon

(a)

200

Transmittance (a.u)

180

MIL-125(Ti)

160

140

MIX-MIL-125(Ti)

120

NH 2 -MIL-125(Ti)

100

80 4000

3000

2000

1000

Wavenumber (cm-1 )

(b)

101

MIL-125(Ti)

Transmittance (a.u)

100

MIX-MIL-125(Ti)

99 3470 cm-1 -1

3375 cm

98

NH2 -MIL-125(Ti)

97

96

95 4000

3800

3600

3400

3200

3000

2800

2600

Wavenumber (cm-1) Fig. 2. FTIR spectra of MIL-125(Ti), MIX-MIL-125(Ti) and NH2- MIL-125(Ti) (a) and magnification part for amino groups of MIL-125(Ti), MIX-MIL-125(Ti) and NH2- MIL125(Ti) (b)

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Z.H. Rada et al. / Fuel 160 (2015) 318–327 100

a

90 MIL-125 (Ti)

(TGA)

80

b

90 Remaining weight (%)

70

MIX-MIL-125 (Ti)

80 70

TGA

60 50 40

60 0.0000 -0.0005 630 K

-0.0010 740 K

-0.0015 -0.0020 -0.0025

345 K

0.000

Derivatived weight (%/ min)

(DTG)

DTG 635 K

-0.001 -0.002 770 K

-0.003 -0.004

355 K

-0.005

-0.0030 400

600

800

Derivatived weight (%/ min)

Remaining weight (%)

100

400

1000

600

800

1000

Temperature K

Temperature K

100

c

Remaining weight (%)

90 80 70

TGA

60

NH 2-MIL-125 (Ti)

50 40

DTG

-0.001 628 K

-0.002 725 K

-0.003 -0.004

355 K

400

600

800

1000

Derivatived weight (%/ min)

0.000

-0.005

Temperature K Fig. 3. TGA and DTG profiles of activated MIL-125(Ti) (a), MIX-MIL-125(Ti) (b), and NH2-MIL-125(Ti) (c).

researchers regarding capture of CO2 and separation of CO2 from CH4 [15,16]. Some of them can be achieved during the synthesis process, while others may be done via post–synthesis [17,18]. Functionalized MIL-53(Al) has been found to show good separation of CO2/CH4. Recently, mixed linkers MOFs and mixed metal MOFs have been studied for enhanced adsorption capacity and selectivity of mixtures gases. High separation of CO2/CH4 has been found on mixed linkers MOFs in recent years [19]. Uptakes of CO2 have been

investigated on mixed metals such as ZnATi and ZnAW [20,21] as well as on mixed linkers in Al-MOF [22]. Ti-based MOFs (MIL-125) have been prepared by several researchers for different applications such as gas separations and photocatalysis [23,24]. Moreira et al. used Ti-based MOFs for separation [23] and Zhang et al. tested them as a humidity sensor [25]. However, few investigations have been reported for Ti-based MOFs for CO2/N2 or CO2/CH4 separation. Wiersum et al.

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Z.H. Rada et al. / Fuel 160 (2015) 318–327 500

a NH 2-MIL-125 (Ti)

Nitrogen adsorption (cc/g STP)

450

400 MIX-MIL-125 (Ti)

350

300

250 MIL-125 (Ti)

200

150 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P 0 )

b

0.4

dv/dD

0.3

0.2

a

MIL-125 (T i)

b

MIX-MIL-125 (T i)

c

NH2 -MIL-125 (T i)

a b c

0.1

0.0

5

10

15

20

25

30

35

40

Pore Diameter (nm) Fig. 5. N2 adsorption/desorption isotherms (a) and pore size distributions (b) of TiMOFs samples.

[MIX-MIL-125(Ti)] and investigate adsorption capabilities of the MIX-MIL-125(Ti) as well as NH2-MIL-125(Ti) and MIL-125(Ti) toward CO2 and CH4 at different temperatures and high pressures to understand the effect of amino functionality on adsorption and separation. Fig. 4. SEM images (a) MIL-125(Ti), (b) MIX-MIL-125(Ti), and (c) NH2-MIL-125(Ti).

proposed an adsorbent performance indicator (API) and evaluated uptake of carbon dioxide and methane adsorptions on various MOFs including Ti-based MOFs [26]. Generally, it is believed that amine-functionalization can improve CO2 adsorption. However, it may also affect adsorption of other gases, influencing separation efficiency. In addition, Ti-MOF by mix-linker has not been reported. Therefore, in this study, we synthesized a mixed linker Ti-MOF

2. Experimental 2.1. Chemicals All chemicals including titanium isoproproxide (Ti(OiPr)4, 99.9%), N, N-dimethylformamide (DMF, C3H7NO, 98%), methanol (Analytic grade, CH3OH, 99%), 1,4-benzenedicarboxylic acid (BDC), 2-aminoterephthalic acids (H2BDC-NH2, 99%), were supplied by Sigma–Aldrich without further purification. Also,

Table 1 Porous structure of MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) samples. Sample

SNU-31 MIL-125(Ti) MIX-MIL-125(Ti) NH2-MIL-125(Ti) NH2-MIL-125(Ti) NH2-MIL-125(Ti) MOF-505 SNU-71

Textural properties

References

SBET (m2 g1)

Micropore volume (cm3 g1)

Micropore area% (t-plot method)

308 714 1488 1660 1360 1469 1661 1770

0.14 0.16 0.37 0.57 0.50 0.60 0.67 0.70

– 51.5% 73.2% 90.1% – – – –

[46] This work This work This work [27] [28] [2] [47]

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commercial activated carbon and zeolite 13X samples were supplied by ChemFFX and UOP, respectively. 2.2. Synthesis and activation of various samples MIL-125(Ti) and NH2-MIL-125(Ti) were synthesized based on the previous reports with some modifications [27,28]. MIL-125(Ti) was synthesized by dissolving 2.95 mmol of BDC in the mixture of solvents consisting of 8.8 mL of DMF and 0.98 mL of methanol, and then 1.9 mmol of titanium isoproproxide was added to the solution. After stirring for around 30 min, the solution was placed inside a Parr PTFE-lined digestion vessel (autoclave) of 45 mL, and then the autoclave was closed and tightly sealed. Next, it was heated in an oven at 423 K for 16 h, finally, the autoclave was cooled down to room temperature. NH2-MIL-125(Ti) was produced by adding 4 mmol of H2BDC-NH2 in the mixture of 7.5 mL of DMF and 7.5 mL of methanol, then 2 mmol of titanium isoproproxide were added to the solution under stirring for around 30 min. The homogeneous mixture was placed inside a Parr PTFE-lined digestion vessel of 45 mL and heated in an oven to 423 K for 16 h. Finally, very uniform white and yellow crystals of MIL-125(Ti) and NH2-MIL-125(Ti), respectively, were collected by vacuum filtration. MIX-MIL-125(Ti) was synthesized using 1.37 mmol of titanium isoproproxide, 2.29 mmol of BDC and 0.229 mmol of H2BDC-NH2 mixed together in 7 mL of DMF and 0.75 mL of methanol. The mixture was stirred for around 1 h and then the homogeneous mixture

was placed inside a Parr PTFE-lined digestion vessel of 45 mL. The digestion vessel was placed in an oven preheated to 423 K for 16 h. After that, the autoclave was cooled down to room temperature. Finally, very uniform light yellow crystals were collected by vacuum filtration. After synthesis of these samples, activation were carried out using the method previously reported [29]. Crystalline product was immersed in 50 mL of DMF and stirred for 24 h and then filtered. After that, the same procedure was repeated using 50 mL methanol. Finally, these materials were dried by vacuum filtration and then heated under vacuum at 423 K overnight. 2.3. Characterizations of various MIL-125(Ti) samples All samples have been checked by a FTIR spectrometer (Perkin– Elmer 100 FT-IR) for intensities of functional groups. The spectrum was scanned from 600 to 4000 cm1 with a resolution of 4 cm1 using an attenuated total reflectance (ATR) technique. Thermal stability of all samples was evaluated by a thermogravimetric analysis (TGA) instrument (TGA/DSC1 STARe system, METTLER-TOLEDO). Typically, 10–20 mg of each sample was loaded in an alumina pan and then placed automatically in the TGA furnace to heat up to 1100 K at a heating rate of 10 K/min in an Argon atmosphere with a flow rate of 10 mL/min. XRD diffractometer (Diffractometer D8 Advance- BrukeraXS) with Cu Ka radiation (k = 1.5406 Å) was used to determine crystalline structure. The morphologies of the samples were determined by a SEM machine (Zeiss NEON 40 EsB

10 At 298 K

CO2 Adsorption (mmole/g)

8

NH2 -MIL-125-(Ti)

6 MIX-MIL-125-(Ti)

4

MIL-125-(Ti)

2

0

0

200

400

600 Pressure (kPa)

800

1000

12 At 273 K

10 CO2 Adsorption (mmole/g)

NH 2 -MIL-125(Ti)

8 MIX-MIL-125 (Ti)

6

4 MIL-125(Ti)

2

0 0

200

400

600

800

1000

Pressure (kPa)

Fig. 6. CO2 adsorption on MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) at 296 K and 273 K.

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Cross-Beam) for particle size and shape of crystals. Surface area and pore size distribution by the BET and BJH methods, respectively, were measured on a Micromeritics Tristar 3000. 2.4. Adsorption study The adsorption isotherms of carbon dioxide (99.995%) and methane (99.995%) at high pressure (up to 1000 kPa) and temperatures of 273 and 295 K were measured by a MicoromeriticsASAP2050. Prior to isotherm measurements, the samples were thoroughly dehydrated and degassed on a Micromeritics ASAP2050 accelerated surface area and porosity analyzer by heating stepwisely at 1 K/min up to 423 K and holding at this temperature for 8 h under high vacuum. An equilibrium interval of 20 s was adopted in all the isotherm measurements. Carbon dioxide (99.995%) and methane (99.995%) were supplied by Coregas. 3. Results and discussion XRD patterns of all samples are shown in Fig. 1. It can be seen that the profiles of the as-synthesized samples are similar to the patterns of MIL-125(Ti) and NH2-MIL-125(Ti) as reported in the previous studies [27,30]. MIX-MIL-125(Ti) sample also showed a similar pattern of MIL-125(Ti) and NH2-MIL-125(Ti). The presence

of NH2 groups in the organic linkers in MIX-MIL-125(Ti) does not affect the structure of MIL-125(Ti). FTIR spectra of samples are shown in Fig. 2. In Fig. 2a, the bands at 1420 and 1545 cm1 occurring for all samples are referred to symmetrical stretching vibration of carboxylates groups (vc@o) and the band at 1665 cm1 is attributed to carboxyl group from free aromatic carboxylic acid. OATiAO vibrations are observed in 400–800 cm1 [31–34]. Bending terephthalate group to titanium metal is found at wavenumber of 1390 cm1 [35]. In Fig. 2b, a magnification part of the spectra of all MIL-125(Ti) samples in a range of 2600–4000 cm1 is displayed. The band at 3370 cm1 shows the primary amines-NH2 on the organic linker and low intensity of this peak may be attributed to the strong interconnecting between the amino groups on the coordinated linker and bridging OH group on the metal center [36,37]. TGA and DTG profiles of samples are shown in Fig. 3. It can be seen that all samples have three stages of weight loss. The first weight loss occurs from 308 to 355 K, corresponding to the removal of moisture, CH3OH and DMF, and the weight loss ratios are around 16%, 22% and 20% for MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti), respectively. The second weight loss corresponding to removal of the non-coordinated organic linkers, which are burnt at 625–630 K for all samples. The decay of these MOFs and collapse of their frameworks to produce TiO2 starts at different

4

CH 4 Adsorption (mmole/g)

At 298 K

3

NH2 -MIL-125(Ti)

2 MIX-MIL-125 (Ti)

1

MIL-125(Ti)

0 0

200

400

600

800

1000

Pressure (kPa) 6

At 273K

CH4 Adsorption (mmole/g)

5

NH2 -MIL-125(Ti)

4

3 MIX-MIL-125 (Ti)

2

1

MIL-125(Ti)

0 0

200

400

600

800

1000

Pressure (kPa) Fig. 7. CH4 adsorption on MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) at 298 K and 273 K.

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temperature ranges, which are (630 and 740 K), (628 and 725 K) and (635 and 770 K) for MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti), respectively. Moreover, NH2-MIL-125 sample shows framework decay at a higher temperature than that reported by Kim et al. [28]. Also, it can be seen that thermal behavior of MIX-MIL-125(Ti) shows a similar thermal stability to NH2-MIL-125 while it has higher thermal stability than MIL-125(Ti) sample.

Fig. 4 shows SEM images of three samples. The three samples presented the similar crystal shape in a round plate. However, NH2-MIL-125(Ti) has a smaller crystal size than MIX-MIL-125(Ti) or MIL-125(Ti) with size of 0.5 lm, 1.3 lm and 2 lm, respectively, suggesting that addition of amino group will decrease the particle size. Porous structures and pore size distributions of MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) samples were determined

28

CO2 Heat of adsorption

29

24

CH 4 Heat of adsorption

26

MIL-125 (Ti)

22

20

27 MIL-125 (Ti)

25

23

5

10

15

20

25

5

10

15

20

3

Adsorption (cm3/g)

27.0

22

26.5

21 CH 4 Heat of adsorption

CO2 Heat of adsorption

Adsorption (cm /g)

MIX-MIL-125 (Ti)

26.0

25.5

25.0

MIX-MIL-125 (Ti)

19

18

24.5 5

10

15

Adsorption

20

17

25

5

(cm3/g)

27

24

26

22

NH2-MIL-125 (Ti) 25

24

23 5

10

15

Adsorption (cm3/g)

10

15

20

Adsorption (cm3/g)

20

25

CH 4 Heat of adsorption

CO2 Heat of adsorption

20

NH2-MIL-125 (Ti) 20

18

16

5

10

15

20

Adsorption (cm3/g)

Fig. 8. Variations in adsorption heat of CO2 (left) and CH4 (right) for MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti) samples at different adsorbed volume.

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12

Table 3 Separation selectivity of CO2/CH4 on various MOFs, zeolite and activated carbon.

at 298 K

MOFs

CO2/CH4 at 298K

CO2/CH4 at 273K

MIL-125(Ti)

6.0 (1 bar) 4.4 (9.8 bar) 4.2 (1 bar) 2.9 (9.8bar) 3.9 (1 bar) 2.5 (9.8 bar) 3.8 (1bar) 3.6 (1 bar) 5.9 (1 bar) 2.2 (9.8 bar) 3.4 (1bar) 3.2 (1bar) 2.3 (1 bar) 1.92 (9.8 bar) 2.2 (1 bar) 3.39 (9.8 bar) – – – 1.47 (17.5 bar) 1.8 (17.5 bar)

5.1 (1 bar) (9.8bar) 4.6 (1 bar) (9.8 bar) 4.1 (1 bar) (9.8 bar) 5.0 (1 bar) – 4.6 (1 bar) (9.8 bar) 5.1 (1 bar) 5.2 (1 bar) 2.2 (1 bar) (9.8 bar) –

References

Selectivity (CO2/CH 4)

10

MIX-MIL-125(Ti)

8 MIL-125 (Ti)

NH2-MIL-125(Ti)

MIX-MIL-125 (Ti)

6

NH2 -MIL-125 (Ti)

ZIF 68 Zeolite 13X Zeolite 13X (UOP)

4

ZIF-69 ZIF-70 Activated carbon (ChemFFX) Activated carbon, A35/4,

2 0

200

400

600

800

1000

Pressure (kPa)

[H3O][Zn7(l3-OH)3(bbs)6] Mn(ndc) Cu(H-pymo)2 Zn2(bttb) Zn2(2,6-ndc)2(dpni) microwave sample

8 at 273 K

Selectivity (CO2 /CH4 )

7 6

3.1

This work

2.6

This work

2.2

This work

1.9

[54] [3] This work

1.9

[54] [54] This work

3.5 (1 bar) 1.87 (1 bar) 1.57 (28 bar) – –

[55] [56] [57] [58] [10] [19]

5 MIL-125 (Ti) MIX-MIL-125 (Ti)

4

NH2 -MIL-125 (Ti)

3 2 0

200

400 600 Pressure (kPa)

800

1000

Fig. 9. Selectivities of CO2/CH4 at varying pressures and (298 K and 273 K).

Table 2 Heat capacities of CH4 and CO2 adsorption on various MOFs. MOFs

CO2 heat of adsorption (kJ/mol)

CH4 heat of adsorption (kJ/mol)

References

PCN-14 MIL-125(Ti) Ni-MOF-74 Co-MOF-74 MIX-MIL-125(Ti) Mg-MOF-74 NH2-MIL-125(Ti) NOTT-140 PCN-11 MOF-5(IRMOF-1) COF-6

– 21.5–26.8 42 37 24.5–26.5 47 24.3–25.5 25 – 34 –

30 24.5–28.7 20.2 19.6 18.1–21.2 18.5 18.2–23.4 16.6 14.6 12.2 7

[48] This work [49] [49] This work [49] This work [50] [51] [52] [53]

by nitrogen adsorption/desorption isotherms (Fig. 5) and the textural parameters of the Ti-MOFs are presented in Table 1. The three samples demonstrate type I isotherms at 77 K with a hysteresis loop at P/P0 = 0.50–0.95, indicating the presence of mesoporous structure with the micropores. Fig. 5 shows that pore size distributions of the three samples have one peak centered at around 4 nm. The BET surface areas of NH2-MIL-125(Ti), MIX-MIL-125(Ti) and MIL-125(Ti) are 1660 m2 g1, 1488 m2 g1 and 714 m2 g1, respectively. The BET surface area of NH2MIL-125(Ti) is higher than those reported previously (1360– 1469 m2 g1) [27,28]. MIX-MIL-125(Ti) gives a twice BET surface

area value as MIL-125(Ti), which suggests that a small portion of H2BDC-NH2 provides much more connection sites with metals. In terms of micropore portion, MIL-125(Ti), MIX-MIL-125(Ti) and NH2-MIL-125(Ti) has 51%, 73% and 90% of total pore area, respectively, and MIL-125(Ti) has the highest average pore size. Therefore, amino group will result in reduced pore size of MIL-125(Ti). CO2 and CH4 adsorption profiles of all Ti-MOFs samples at varying temperatures (273 K and 296 K) are showed in Figs. 6 and 7, respectively. Fig. 6 shows that the highest CO2 uptake was achieved on NH2-MIL-125(Ti) with adsorption capacities of 8.9 and 10.76 mmol/g at 298 K and 273 K, respectively. Furthermore, MIX-MIL-125(Ti) sample presented adsorption capacities of 6.25 and 7.71 mmol/g at 298 K and 273 K, respectively which are higher than those of MIL-125(Ti). Adding amino-functional group to MIL-125(Ti) using mixed linkers can lead to more surface area and sites for CO2 uptake [33,36]. In addition, it is believed that the amino groups also increase the affinity toward CO2 adsorption [38]. Fig. 7 displays that the highest CH4 uptake can also be achieved on NH2-MIL-125(Ti) with capacities of 3.5 and 4.9 mmol/g at 298 K and 273 K, respectively. Similarly, MIX-MIL-125(Ti) sample exhibited higher CH4 uptake than MIL-125(Ti). This could be attributed to higher surface area of MIX-MIL-125(Ti) sample which has been gained by using mixed linkers system into the structure of MIL-125(Ti) [39,40]. Compared with CO2 adsorption (Figs. 6 and 7), all samples presented higher CO2 adsorbed than that of CH4, due to the differences in polarity between CO2 and CH4 and the greater quadrupole moment of CO2 (13.4  1040 cm3) than CH4 [41]. Adsorption isotherms of CO2 and CH4 on commercial zeolite 13X and activated carbon samples are shown in Figs. S1 and S2 (ESI) and the adsorption capacities are compared with MIL-125(Ti) as shown in Table S1 (ESI). As seen from the data, MOF-125(Ti) sample exhibited comparable or better performance in CO2 adsorption than zeolite 13X and activated carbon, but lower adsorption in CH4, suggesting the better potential of selective adsorption. In Fig. 8, the isosteric heats of adsorption (Qst) for CO2 and CH4 are presented based on the Clausius–Claperyron equation ðdp=p ¼ DHdT=RT2Þ. The heat values of CO2 adsorption at different

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CO2 coverages on MIL-125(Ti), MIX-MIL-125(Ti) and NH2-MIL-125(Ti) were found to be (21–27 kJ/mol), (25.5–26.5 kJ/mol) and (24.5–26.1 kJ/mol), respectively. They are consistent with the previously reported values (30 kJ/mol) on NH2-MIL-125(Ti) and (26 kJ/mol) on MIL-125(Ti) [26]. Meanwhile, the heat values of CH4 adsorption at different CH4 coverages were obtained to be (24–29 kJ/mol), (18–21 kJ/mol) and (19–23 kJ/mol) on MIL-125(Ti), MIX-MIL-125(Ti) and NH2-MIL-125(Ti), respectively. Table 2 presents the adsorption heats of CH4 and CO2 on various MOFs. It can be seen that the values of adsorption heat for MIL-125(Ti), MIX-MIL-125(Ti) and NH2-MIL-125(Ti) samples are almost identical among these MOFs. High values of isosteric heats of adsorption of CO2 and CH4 promise a strong capture of the individual molecules as a result of differences in kinetic diameters for CO2 (3.3 Å) and CH4 (3.758 Å), respectively [42]. Fig. 9 shows the selectivities of CO2 over CH4 at temperatures, 298 K and 273 K, on Ti-MOFs samples, which were calculated by static adsorption at varying pressures up to 1000 kPa. The selectivities of CO2 over CH4 on commercial zeolite 13X and activated carbon are shown in Fig. S3 (ESI). The selectivity of CO2 over CH4 was obtained using the following equation.

selectivity ¼

X1=X2 Y1=Y2

ð1Þ

where Xi indicates the mole fraction of component i in the adsorbed phase and Yi indicate the mole fraction of component i in the bulk gas phase [43]. It can be seen that MIL-125(Ti) sample shows the best separation factors for CO2 over CH4, which are 4.4 and 3.1, respectively, whereas NH2-MIL-125(Ti) has the lowest separation power (2.5) and (2.2). In addition, at temperatures 298 K and 273 K, the selectivity of CO2 over CH4 drops sharply as gas pressure increases. At 298 K, a significant separation of CO2 on samples can occurs at pressures lower than 200 kPa, while at 273 K the best selectivity CO2/CH4 can be seen at pressures less than 400 kPa. Table 3 summarizes selectivities of CO2/CH4 on various porous materials. The comparison shows that, MIL-125(Ti) presented better values of separation of CO2 over CH4 at high pressure compared with various adsorbents including other MOFs, zeolite 13X and activated carbon. The highest selectivity of CO2/CH4 in MIL-125(Ti) may be related to the highest mesopores, which enhance the adsorption of CO2 rather than CH4. Meanwhile, interconnecting pores in microporous materials such as NH2-MIL-125(Ti) and MIX-MIL-125(Ti) can enhance CH4 uptake, which has be reported in NH2-MIL-125(Ti) [39,44,45]. 4. Conclusions Titanium-based metal organic frameworks, MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti), were successfully synthesized, characterized and tested as adsorbents for CO2 and CH4 adsorption under high pressure. It was found that addition of amino functional group will significantly improve CO2 and CH4 adsorption. NH2-MIL-125(Ti) can present the highest CO2 adsorption around 10.76 mmol/g and 8.93 mmol/g at 273 and 298 K, and pressure of 988 kPa. However, CH4 adsorptions are less than CO2 uptake giving values of 4.91 and 3.57 mmol/g at 273 and 298 K, respectively. In addition, MIX-MIL-125(Ti) showed better CO2 and CH4 uptakes than MIL-125(Ti) due to NH2 functional group in the structure. The heats of adsorption of CO2 on MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti), were estimated to be 21, 25.5 and 26.5 kJ/mol, respectively, while the values of adsorption heat for CH4 are 24.5, 19 and 17 kJ/mol, respectively. Finally, titanium-based metal organic frameworks showed reasonable selective CO2 adsorption to CH4, giving potentials to be as promising materials for natural gas separation.

Acknowledgments We thank Ms Elaine Miller for SEM measurements. We also acknowledge the Ministry of Higher Education and Minister of Natural Resources/Kurdistan regional government-Iraq for PhD scholarship.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.07.088.

References [1] Bao Z, Yu L, Ren Q, Lu X, Deng S. Adsorption of CO2 and CH4 on a magnesiumbased metal organic framework. J Colloid Interface Sci 2011;353:549–56. [2] Mason JA, Veenstra M, Long JR. Evaluating metal–organic frameworks for natural gas storage. Chem Sci 2014;5:32–51. [3] Liang Z, Marshall M, Chaffee AL. CO2 adsorption-based separation by metal organic framework (Cu-BTC) versus zeolite (13X). Energy Fuels 2009;23:2785–9. [4] Bae YS, Snurr RQ. Development and evaluation of porous materials for carbon dioxide separation and capture. Angew Chem Int Ed 2011;50:11586–96. [5] Rinker EB, Ashour SS, Sandall OC. Absorption of carbon dioxide into aqueous blends of diethanolamine and methyldiethanolamine. Ind Eng Chem Res 2000;39:4346–56. [6] Cavenati S, Grande CA, Rodrigues AE. Adsorption equilibrium of methane, carbon dioxide, and nitrogen on zeolite 13X at high pressures. J Chem Eng Data 2004;49:1095–101. [7] Aaron D, Tsouris C. Separation of CO2 from flue gas: a review. Sep Sci Technol 2005;40:321–48. [8] Siriwardane RV, Shen M-S, Fisher EP. Adsorption of CO2, N2, and O2 on natural zeolites. Energy Fuels 2003;17:571–6. [9] Guo B, Chang L, Xie K. Adsorption of carbon dioxide on activated carbon. J Nat Gas Chem 2006;15:223–9. [10] Bae Y-S, Farha OK, Hupp JT, Snurr RQ. Enhancement of CO2/N2 selectivity in a metal-organic framework by cavity modification. J Mater Chem 2009;19:2131–4. [11] Couck S, Denayer JF, Baron GV, Rémy T, Gascon J, Kapteijn F. An aminefunctionalized MIL-53 metal–organic framework with large separation power for CO2 and CH4. J Am Chem Soc 2009;131:6326–7. [12] Furukawa H, Cordova KE, O’Keeffe M, Yaghi OM. The chemistry and applications of metal-organic frameworks. Science 2013;341:1230444. [13] Férey G. Hybrid porous solids: past, present, future. Chem Soc Rev 2008;37:191–214. [14] Liu J, Chen L, Cui H, Zhang J, Zhang L, Su C-Y. Applications of metal–organic frameworks in heterogeneous supramolecular catalysis. Chem Soc Rev 2014. [15] Furukawa H, Yaghi OM. Storage of hydrogen, methane, and carbon dioxide in highly porous covalent organic frameworks for clean energy applications. J Am Chem Soc 2009;131:8875–83. [16] Seki K. Dynamic channels of a porous coordination polymer responding to external stimuli. Phys Chem Chem Phys 2002;4:1968–71. [17] Gascon J, Aktay U, Hernandez-Alonso MD, van Klink GP, Kapteijn F. Aminobased metal-organic frameworks as stable, highly active basic catalysts. J Catal 2009;261:75–87. [18] Shultz AM, Sarjeant AA, Farha OK, Hupp JT, Nguyen ST. Post-synthesis modification of a metal–organic framework to form metallosalen-containing MOF Materials. J Am Chem Soc 2011;133:13252–5. [19] Bae Y-S, Mulfort KL, Frost H, Ryan P, Punnathanam S, Broadbelt LJ, et al. Separation of CO2 from CH4 using mixed-ligand metal–organic frameworks. Langmuir 2008;24:8592–8. [20] Wang F, Liu ZS, Yang H, Tan YX, Zhang J. Hybrid zeolitic imidazolate frameworks with catalytically active TO4 building blocks. Angew Chem Int Ed 2011;50:450–3. [21] Hong K, Bak W, Chun H. Unique coordination-based heterometallic approach for the stoichiometric inclusion of high-valent metal ions in a porous metal– organic framework. Inorg Chem 2013;52:5645–7. [22] Reinsch H, Waitschat S, Stock N. Mixed-linker MOFs with CAU-10 structure: synthesis and gas sorption characteristics. Dalton Trans 2013;42:4840–7. [23] Moreira MA, Santos JC, Ferreira AF, Loureiro JM, Ragon F, Horcajada P, et al. Effect of ethylbenzene in p-xylene selectivity of the porous titanium amino terephthalate MIL-125 (Ti)-NH2. Microporous Mesoporous Mater 2012;158:229–34. [24] Nasalevich MA, Goesten MG, Savenije TJ, Kapteijn F, Gascon J. Enhancing optical absorption of metal–organic frameworks for improved visible light photocatalysis. Chem Commun 2013;49:10575–7. [25] Zhang Y, Chen Y, Zhang Y, Cong H, Fu B, Wen S, et al. A novel humidity sensor based on NH2-MIL-125 (Ti) metal organic framework with high responsiveness. J Nanopart Res 2013;15:1–6.

Z.H. Rada et al. / Fuel 160 (2015) 318–327 [26] Wiersum AD, Chang J-S, Serre C, Llewellyn PL. An adsorbent performance indicator as a first step evaluation of novel sorbents for gas separations: application to metal–organic frameworks. Langmuir 2013;29:3301–9. [27] Zlotea C, Phanon D, Mazaj M, Heurtaux D, Guillerm V, Serre C, et al. Effect of NH2 and CF3 functionalization on the hydrogen sorption properties of MOFs. Dalton Trans 2011;40:4879–81. [28] Kim S-N, Kim J, Kim H-Y, Cho H-Y, Ahn W-S. Adsorption/catalytic properties of MIL-125 and NH2-MIL-125. Catal Today 2013;204:85–93. [29] Vermoortele F, Maes M, Moghadam PZ, Lennox MJ, Ragon F, Boulhout M, et al. P-xylene-selective metal–organic frameworks: a case of topology-directed selectivity. J Am Chem Soc 2011;133:18526–9. [30] Moreira MA, Santos JoC, Ferreira AF, Loureiro JM, Ragon F, Horcajada P, et al. Toward understanding the influence of ethylbenzene in p-xylene selectivity of the porous titanium amino terephthalate MIL-125 (Ti): adsorption equilibrium and separation of xylene isomers. Langmuir 2012;28:3494–502. [31] Biemmi E, Bein T, Stock N. Synthesis and characterization of a new metal organic framework structure with a 2D porous system: (H2NEt2)2[Zn3(BDC)4]3DEF. Solid State Sci 2006;8:363–70. [32] Devic T, Horcajada P, Serre C, Salles F, Maurin G, Moulin B, et al. Functionalization in flexible porous solids: effects on the pore opening and the host–guest interactions. J Am Chem Soc 2009;132:1127–36. [33] Biswas S, Ahnfeldt T, Stock N. New functionalized flexible Al-MIL-53-X (X = –Cl, –Br, –CH3, –NO2, –(OH)2) solids: syntheses, characterization, sorption, and breathing behavior. Inorg Chem 2011;50:9518–26. [34] Dan-Hardi M, Serre C, Frot T, Rozes L, Maurin G, Sanchez C, et al. A new photoactive crystalline highly porous titanium (IV) dicarboxylate. J Am Chem Soc 2009;131:10857–9. [35] Sabo M, Böhlmann W, Kaskel S. Titanium terephthalate (TT-1) hybrid materials with high specific surface area. J Mater Chem 2006;16:2354–7. [36] Stavitski E, Pidko EA, Couck S, Remy T, Hensen EJ, Weckhuysen BM, et al. Complexity behind CO2 capture on NH2-MIL-53 [Al]. Langmuir 2011;27:3970–6. [37] Ahnfeldt T, Gunzelmann D, Loiseau T, Hirsemann D, Senker Jr, Férey G, Stock N. Synthesis and modification of a functionalized 3D open-framework structure with MIL-53 topology. Inorg Chem 2009;48:3057–64. [38] Cmarik GE, Kim M, Cohen SM, Walton KS. Tuning the adsorption properties of UiO-66 via ligand functionalization. Langmuir 2012;28:15606–13. [39] Konstas K, Osl T, Yang Y, Batten M, Burke N, Hill AJ, et al. Methane storage in metal organic frameworks. J Mater Chem 2012;22:16698–708. [40] Janiak C, Vieth JK. MOFs, MILs and more: concepts, properties and applications for porous coordination networks (PCNs). New J Chem 2010;34:2366–88. [41] Mason JA, Sumida K, Herm ZR, Krishna R, Long JR. Evaluating metal–organic frameworks for post-combustion carbon dioxide capture via temperature swing adsorption. Energy Environ Sci 2011;4:3030–40. [42] Comotti A, Bracco S, Distefano G, Sozzani P. Methane, carbon dioxide and hydrogen storage in nanoporous dipeptide-based materials. Chem Commun 2009:284–6.

327

[43] Xu X, Zhao X, Sun L, Liu X. Adsorption separation of carbon dioxide, methane, and nitrogen on Hb and Na-exchanged b-zeolite. J Nat Gas Chem 2008;17:391–6. [44] D’Alessandro DM, Smit B, Long JR. Carbon dioxide capture: prospects for new materials. Angew Chem Int Ed 2010;49:6058–82. [45] Mu B, Li F, Walton KS. A novel metal–organic coordination polymer for selective adsorption of CO2 over CH4. Chem Commun 2009:2493–5. [46] Park HJ, Cheon YE, Suh MP. Post-Synthetic Reversible incorporation of organic linkers into porous metal–organic frameworks through single-crystal-tosingle-crystal transformations and modification of gas-sorption properties. Chem – Eur J 2010;16:11662–9. [47] Prasad TK, Suh MP. Control of interpenetration and gas-sorption properties of metal–organic frameworks by a simple change in ligand design. Chem – Eur J 2012;18:8673–80. [48] Ma S, Sun D, Simmons JM, Collier CD, Yuan D, Zhou H-C. Metal-organic framework from an anthracene derivative containing nanoscopic cages exhibiting high methane uptake. J Am Chem Soc 2008;130:1012–6. [49] Wu H, Zhou W, Yildirim T. High-capacity methane storage in metal–organic frameworks M2 (dhtp): the important role of open metal sites. J Am Chem Soc 2009;131:4995–5000. [50] Tan C, Yang S, Champness NR, Lin X, Blake AJ, Lewis W, et al. High capacity gas storage by a 4,8-connected metal–organic polyhedral framework. Chem Commun 2011;47:4487–9. [51] Wang X-S, Ma S, Rauch K, Simmons JM, Yuan D, Wang X, Meijere Ad, et al. Metal–organic frameworks based on double-bond-coupled di-isophthalate linkers with high hydrogen and methane uptakes. Chem Mater 2008;20:3145–52. [52] Li H, Eddaoudi M, O’Keeffe M, Yaghi OM. Design and synthesis of an exceptionally stable and highly porous metal-organic framework. Nature 1999;402:276–9. [53] Mendoza-Cortes JL, Pascal TA, Goddard III WA. Design of covalent organic frameworks for methane storage. J Phys Chem A 2011;115:13852–7. [54] Phan A, Doonan CJ, Uribe-Romo FJ, Knobler CB, keeffe MO, Yaghi OM. Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks. Acc Chem Res 2010;43:58–67. [55] Heuchel M, Davies G, Buss E, Seaton N. Adsorption of carbon dioxide and methane and their mixtures on an activated carbon: simulation and experiment. Langmuir 1999;15:8695–705. [56] Neofotistou E, Malliakas CD, Trikalitis PN. Unprecedented sulfonefunctionalized metal–organic frameworks and gas-sorption properties. Chem – Eur J 2009;15:4523–7. [57] Moon HR, Kobayashi N, Suh MP. Porous metal-organic framework with coordinatively unsaturated MnII sites: sorption properties for various gases. Inorg Chem 2006;45:8672–6. [58] Galli S, Masciocchi N, Tagliabue G, Sironi A, Navarro JA, Salas JM, et al. Polymorphic coordination networks responsive to CO2, moisture, and thermal stimuli: porous cobalt (II) and zinc (II) fluoropyrimidinolates. Chem – Eur J 2008;14:9890–901.