Sorption of carbon dioxide, methane, nitrogen and carbon monoxide on MIL-101(Cr): Volumetric measurements and dynamic adsorption studies

Chemical Engineering Journal 195–196 (2012) 359–368

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Sorption of carbon dioxide, methane, nitrogen and carbon monoxide on MIL-101(Cr): Volumetric measurements and dynamic adsorption studies Kuppusamy Munusamy, Govind Sethia, Dinesh V. Patil, Phani B. Somayajulu Rallapalli, Rajesh S. Somani ⇑, Hari C. Bajaj ⇑ Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR–CSMCRI), Bhavnagar, 364 002 Gujarat, India

h i g h l i g h t s " Metal organic frameworks (MOFs) are promising in the field of Gas storage and separation. " In this study we examine the separation properties of MIL-101(Cr) via single column Breakthrough measurement. " Very high selectivity of CO2 over N2. " Regeneration of MIL-101(Cr) is not energy intensive.

a r t i c l e

i n f o

Article history: Received 9 February 2012 Received in revised form 20 April 2012 Accepted 23 April 2012 Available online 4 May 2012 Keywords: MIL-101 Adsorption Gas separation Carbon dioxide capture

a b s t r a c t Metal organic framework material, MIL-101(Cr), synthesized by hydrothermal method was granulated using starch and sodium salt of carboxyl methyl cellulose as binder. The equilibrium adsorption capacities of pure gases CO2, N2, CH4, and CO were measured at 288, 303 and 313 K up to 850 mm Hg on both powder and granules of MIL-101(Cr). The selectivity of CO2 over N2 in MIL-101(Cr) was found to be 12.6 at 303 K, more than those for CO2 over CH4 (5.69) and CO (2.90). The CO2 dynamic adsorption studies from binary gas mixture (CO2 + N2) with five different CO2 concentration in the range 5–50 vol.%, and with two different feed gas flow rates (50 and 100 mL/min), were carried out on MIL-101(Cr) granules. The regeneration of adsorbent was feasible at a relatively low temperature attributed to physical adsorption of CO2. The MIL-101(Cr) showed potential for CO2 adsorption compared with different types of carbon, silica and zeolite based adsorbents reported in the literature. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction A substantial increase in the global atmospheric CO2 concentration (391.8 ppm, 2011) has raised concern about climate change and led to worldwide efforts on reduction of CO2 emissions. The main CO2 emission sources include fossil fuel based power plants, vehicles, cement manufacturing plants, limestone calcinations, hydrogen and ammonia production and residential buildings. CO2 capture and sequestration from stationary sources is considered as the first step for the control of CO2 emission. Various CO2 capture technologies including absorption, adsorption, cryogenics and membrane separation have been investigated [1,2]. However, the present CO2 capture technologies are energy intensive. It is therefore important to look at new approaches for CO2 separation from ⇑ Corresponding authors. Tel.: +91 278 2471793. E-mail addresses: [email protected] (R.S. Somani), [email protected] (H.C. Bajaj). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.04.071

flue gases. The well-known potential adsorbents reported for CO2 separation are activated carbon [3,4] zeolites [5,6] silica based adsorbents [7,8] carbon nanotubes [9] and nanoporous silica-based molecular basket [10,11]. The adsorption processes and the development of a new generation adsorbent materials that would efficiently adsorb more quantity of CO2 will undoubtedly enhance the competitiveness of adsorptive separation. Recently, metal organic frameworks (MOFs) have attracted extensive interest as potential materials for storage of gases like CH4, H2 and CO2, due to their high surface area, tunable pore size and high micropore volume [12–25]. MIL-101(Cr) with molecular formula [Cr3F(H2O)2O[BDC]3 nH2O]; (BDC = 1,4-benzenedicarboxylate, n = 25), have been studied extensively [26–30]. In MIL-101(Cr), a super tetrahedral building unit (ST) is formed by terephthalate ligands and trimeric chromium (III) octahedral clusters. The four vertices of the ST are occupied by the trimers, and the organic linkers are located at the six edges of the ST. The connection between the ST is established through vertices to ensure a 3D network of ‘‘corner

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sharing’’ super tetrahedra with an augmented MTN zeotype architecture derived from the high-silica zeolite ZSM-39. The STs are micro porous (with 8.6 Å free aperture for the windows), and the resulting framework delimits two types of mesoporous cages filled with guest molecules. Its cubic structure exhibits several unprecedented features: a mesoporous zeotype architecture with a MTN topology, a giant cell volume (702,000 Å3), a large free aperture (12 Å from pentagonal windows and 16 Å  14.5 Å from hexagonal windows), mesoporous cages (with 29 Å and 34 Å diameter), large BET and Langmuir surface area (4100 ± 200 m2/g; 5900 ± 300 m2/g, respectively), and numerous unsaturated chromium sites (theoretically up to approximately 3.0 mmol/g). It has potential catalytic ability [31] and excellent gas adsorption capacity, especially for CO2 storage, up to 8 wt% at 6 MPa and 303 K [32]. Though MOFs are widely studied as gas storage materials and plenty of equilibrium adsorption data for various gases, even at high pressures are available in published literature [17], their dynamic gas adsorption property is still unexplored. Herein, we report dynamic gas adsorption properties of MIL-101(Cr) synthesized in our laboratory by reported method [12], using mixture of (CO2 + N2) with five different CO2 concentrations in the range 5– 50% by volume and at two different feed gas flow rates, 50 and 100 mL/min along with the equilibrium adsorption properties for pure gases CO2, CH4, CO & N2 at 288, 303 and 313 K up to 850 mm Hg. To the best of our knowledge no such study has been reported in the literature.

2. Experimental 2.1. Materials Chromium (III) nitrate, Cr (NO3)39H2O (98% pure), 1,4-benzenedicarboxylic acid, H2BDC (97% pure), Hydrofluoric acid, HF (48%), dimethyl formamide, DMF (99% pure) and methanol, CH3OH (95% pure) were procured from (S. d. Fine Chemicals) and used as received. CO2, CO, CH4, N2, and He (Inox India Ltd.) gases used for the sorption studies were of ultra high purity grade. Carboxymethylcellulose sodium salt (Rankem) and starch (S. d. Fine Chemicals) were used as a binder to make shaped body (granules) of MIL101(Cr).

2.2. Methods 2.2.1. Synthesis of MIL-101(Cr) Synthesis of MIL-101(Cr) was carried out using hydrothermal protocol as reported by Ferey et al. [12], 750 mL capacity stainless steel autoclave was charged with 40.0 g of Chromium(III) nitrate, 16.6 g of H2BDC, 500 mL deionized water and 4 mL of HF. The mixture was stirred for 10 min and the autoclave was heated to 493 K and maintained for 8 h. Thereafter the autoclave was allowed to cool to room temperature. The reaction mass was filtered, washed with water and dried at 353 K for overnight period. The crude product contained green colored powder of MIL-101(Cr) along with white needle shaped crystals of unreacted H2BDC. The unreacted and/or occluded H2BDC inside the pores of the MIL-101(Cr) was removed by solvent extraction method [33]. 9 g Of purified powdered MIL-101(Cr) was mixed with 0.5 g of starch and 0.5 g of sodium salt of carboxymethylcellulose and 2 mL of distilled water to make a dough type wet mass which was extruded, dried at 383 K in oven for 8 h, and manually broken in small pieces so as to obtain granular (3–4 mm length, 2 mm dia.) MIL-101(Cr).

2.2.2. Characterization of MIL-101(Cr) Powder X-ray diffraction study of purified MIL-101(Cr) was carried out using Philips X0 pert MPD system in the 2h range of 1–20° using Cu Ka1 (k = 1.54056 Å). Thermo gravimetric analysis was carried out from 303 to 873 K using a TGA/DTA analyzer (Mettler Toledo) under the argon atmosphere at the heating rate of 10 K/ min. The surface area measurement of the purified MIL-101(Cr) sample was carried out using static volumetric adsorption system (Model-ASAP 2020, Micromeritics Inc., USA) by obtaining N2 adsorption/desorption isotherms at 77.4 K. Prior to the adsorption measurement the sample was degassed overnight under vacuum (5  106 mm Hg) at 423 K. In a typical experiment of CO2-TPD (Temperature Programmed Desorption), 50 mg of MIL-101(Cr) was loaded in a quartz sample tube. Prior to CO2 adsorption, the adsorbent was activated in helium gas flow at 423 K for 3 h and then cooled to room temperature. The adsorption of CO2 was carried out by passing 20 mL/min of pure CO2 gas over the activated adsorbent for 1 h. Before the TPD run, the adsorbent was flushed with helium gas for 1 h at 323 K to remove the physisorbed gases. TPD was carried out in a helium gas flow of 30 mL/min with a temperature ramp of 10 K/min. The desorbed CO2 was monitored by the thermal conductivity detector (TCD) using an automated catalyst characterization system (model-Autochem-2920, Micromeritics Inc., USA). 2.2.3. Equilibrium adsorption measurements The adsorption isotherms for CO2, CO, CH4, and N2, were measured at 288, 303 and 313 K using the surface area and pore size analyzer (Model -ASAP-2020, Micromeritics Inc., USA). Prior to the adsorption measurements, the samples were dried at 423 K for 12 h in oven. The samples were further activated in situ by increasing the temperature at a heating rate of 1 K/min up to 423 K under vacuum (5  106 mm Hg) for 8 h before the sorption measurements. Adsorption capacity and selectivity were determined from the adsorption isotherms measured at different temperatures. 2.2.4. Dynamic adsorption measurements Single column breakthrough setup (Fig. 1) for studying the dynamic adsorption of desired gas from binary feed gas mixtures was designed and assembled in-house. The adsorbent column was packed with 6 g of granular MIL-101(Cr) and glass beads. The glass beads were used to reduce the dead volume in the column. The top and bottom of the adsorption column were plugged with glass wool. Prior to generating breakthrough curves, the sample was activated in situ by increasing the temperature at a heating

Fig. 1. Single column breakthrough set up.

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Intensity (a.u)

TCD Concentration (a.u)

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19.9

19.8

19.7 2

4

6

8

10

350

2θ

400

450

500

Temperature (K) Fig. 2. PXRD pattern of MIL-101(Cr). Fig. 4. CO2-Temperature programmed desorption profile of MIL-101(Cr).

rate of 1 K/min up to 423 K under helium flow (100 mL/min) and maintaining the temperature and flow for 12 h to remove moisture and all adsorbed gases. The CO2 concentration in the feed gas and its flow rate were controlled and maintained throughout the experiments using mass flow controllers. Different feed gas compositions in the range 5–50% CO2 by volume (balance N2), were ascertained by analyzing the feed gas using a gas chromatograph. After the activation, the dry weight of MIL-101(Cr) was 4.53 g. It was considered for calculating dynamic adsorption capacity. Desorption was carried out counter currently under N2 flow at the same temperature and pressure, to check the easiness of recovering CO2. No considerable change in temperature occurred during adsorption and desorption cycle. The raffinate was analyzed by gas chromatograph, (Chemito, model 7610) having molecular sieve column and thermal conductivity detector (TCD). Helium was used as carrier gas at 50 mL/min. To check the recyclability of MIL101(Cr) as adsorbent, several cycles of adsorption–desorption were carried out under identical conditions using 47% CO2 + 53% N2 with 50 mL/min feed gas flow rate. After the adsorbent bed got saturated with CO2 of feed gas mixture, it was desorbed counter currently by N2 purge at ambient temperature till no CO2 was

100 0.0

DTA -0.1

50

-0.2

dW/dT (%)

weight loss (%)

75

TGA 25

0 300

-0.3

450

600

750

-0.4 900

Temperature (K) Fig. 3. Thermo gravimetric and differential thermal analysis of MIL-101(Cr).

detected in the raffinate; and then consecutively adsorption runs were carried out.

3. Results and discussion The powder X-ray diffraction pattern for MIL-101(Cr) is shown in (Fig. 2). The diffraction pattern indicated that the synthesized material is well crystalline and the peak positions are in good agreement with those of the MIL-101(Cr) reported in the literature [12]. The TGA plot of MIL-101(Cr) showed a two-step weight loss (Fig. 3), the first, in the range 298–373 K relates to the loss of guest water (10%) molecules. The second weight loss observed between 548 and 773 K attributed to the removal of solvent molecules and/ or decomposition of MIL-101(Cr) framework (50%). This clearly indicates that, the MIL-101(Cr) is thermally stable up to 548 K and it should be activated below 548 K prior to adsorption and breakthrough measurements. The CO2-TPD measurements were carried out to determine strength and number of active adsorption sites. The CO2-TPD profile is shown in (Fig. 4). The desorption profile of the MIL-101(Cr) was characterized with low temperature CO2 desorption peak at Tmax = 373 K. This is attributed to the interaction of CO2 with weak basic sites present in the MIL-101(Cr). The corresponding CO2 desorption was 0.277 mmol/g. The N2 adsorption–desorption isotherm of MIL-101(Cr) at 77.4 K for both powder and granules exhibits two secondary uptakes near p/po = 0.1 and p/po = 0.2 (Fig. 5), indicating the presence of two nano porous windows in the framework [12]. The BET and Langmuir surface area of powder MIL-101(Cr), calculated from N2 adsorption–desorption data at 77.4 K using BET and Langmuir equations, were found to be 2471 m2/g and 3233 m2/g, respectively, with the total pore volume of 1.20 cm3/g at p/p0  0.99. However, with the addition of binder for preparing granules of MIL-101(Cr), both the BET and Langmuir surface area ofMIL101(Cr) decreased to 1642 m2/g and 2253 m2/g, respectively, and the total pore volume also decreased to 0.83 cm3/g. It is attributed to the blockage of small pores, which is evident from (Fig. 6), density functional theory (DFT) pore size distributions (PSD) analysis. The PSD profiles of MIL-101(Cr) powder and granule is well matched except the reduction in pore volume due to blockage of some pores in the micropore region.

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800

0.16

a b

3

Volume adsorbed(cm /g)

3

Increamental pore volume(cm /g)

a 600

b Ads Des Ads Des

400

200

0.12

0.08

0.04

0.00 0.0

0.2

0.4

0.6

0.8

1.0

10

Relative pressure (p/po)

20

30

40

50

Pore width (Å)

Fig. 5. Nitrogen adsorption–desorption isotherms of MIL-101(Cr) at 77 K, (a) powder and (b) granules.

The adsorption capacities of CO2, CO, CH4, and N2 measured at three different temperatures on both MIL-101(Cr) powder and granules up to 850 mm Hg are tabulated in Table 1. It can be observed that at a given pressure, adsorption capacity decreases with increasing temperature. This can be understood simply on the basis of the kinetic energy of the molecules which is proportional to temperature. Increasing the temperature provides additional energy that help molecules escape (desorb) faster from the surface, hence attains the equilibrium fast. The adsorption isotherms (Fig. 7) of different gases clearly showed that the adsorption capacity of CO2 is much higher than those of all other gases studied and the order of adsorption capacity is CO2 > CO > CH4 > N2. The powder sample showed higher adsorption capacity than the granules of MIL-101(Cr). This is due to the blockage of some pores of MIL101(Cr) by the binder. Even though the adsorption capacity of granules is reduced to half of powder samples, the selectivity of gases for powder and granular samples remained unchanged. Significantly lower uptake for CO, CH4 and N2 was observed at all the three temperatures and at 850 mm Hg. These results revealed that MIL-101(Cr) can preferentially adsorb CO2 over CH4, CO and N2. The higher uptake of CO2 was due to its higher quadrupole moment (13.4  1040 C m2) and polarizability (26.3  1025 cm3) as compared to CO, (12.3  1040 C m2 and 18.44  1025 cm3), CH4 (0 and 26.0  1025 cm3), and N2 (4.7  1040 C m2 and 17.6  1025 cm3). The higher uptake of methane over nitrogen was due to the higher polarizability of methane than nitrogen [19]. As such, CH4 is non-polar but due to asymmetric vibrations it has some polar character. When CH4 molecules are in close proximity to atom or ions within the structure, they causes an instantaneous shift in the time averaged neutral electrostatic field of CH4 and this induced polarity results into the high adsorption

Fig. 6. Pore size distribution of MIL-101(Cr) by N2 sorption at 77 K, (a) powder and (b) granules.

capacity of CH4. The selectivity of the above gases at three different temperatures for both powder and granules are calculated from equilibrium adsorption capacities (Table 2). With an increase in adsorption temperature the selectivity of CO2, CH4 and CO over nitrogen increases due to comparative poor electrostatic interactions between nitrogen and MIL-101(Cr) at higher temperature. The selectivity follows the order CO2/N2 > CO2/CH4 > CO2/CO. The MIL-101(Cr) showed comparable volumetric equilibrium adsorption capacity of CO2 (2.90 mmol/g) (see Table 3) with those adsorbents reported in the literature [34–40]. 3.1. Heat of adsorption The Henry’s constant was calculated from isotherm data fitted in the Virial equation (Eq. (1))

ln

  p ¼ A þ Bq þ Cq2 q

ð1Þ

where q is the amount of gas adsorbed (mmol per unit weight of the adsorbent), p is the equilibrium pressure (mm Hg), and A, B, and C are the 1st, 2nd and 3rd Virial coefficients, respectively. Henry’s constant, K, was calculated from 1st Virial coefficient (A) using in the following equation.

K ¼ ExpðAÞ

ð2Þ

The selectivity of a gas A over gas B was calculated using in the following equation.

a¼

qA qB

ð3Þ

where qA and qB are the quantity (mmol/g) of gas A and B adsorbed, respectively.

Table 1 Equilibrium adsorption capacities of N2, CH4, CO2 and CO at different temperatures on MIL-101(Cr) powder and granules at 850 mm Hg. Pure gas

Equilibrium adsorption capacities MIL-101(Cr) powder

CO2 CO CH4 N2

MIL-101(Cr) granules

288 K (mmol g1)

303 K (mmol g1)

313 K (mmol g1)

288 K (mmol g1)

303 K (mmol g1)

313 K (mmol g1)

3.80 1.13 0.58 0.31

2.90 1.00 0.51 0.23

2.55 0.89 0.46 0.12

2.60 0.66 0.46 0.24

1.68 0.58 0.32 0.14

1.63 0.50 0.29 0.14

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3

b

a Gas adsorbed(mmol.g )

2

N2 CH4 CO2 CO

-1

N2 CH4 CO2 CO

-1

Gas adsorbed(mmol.g )

1.5

1

1.0

0.5

0

0.0 0

200

400

600

0

800

200

Absolute pressure (mmHg)

400

600

800

Absolute pressure (mmHg)

c -1

Gas adsorbed(mmol.g )

1.5 N2 CH4 CO2 CO

1.0

0.5

0.0 0

200

400

600

800

Absolute pressure (mmHg) Fig. 7. Adsorption isotherms of N2, CH4, CO2 and CO on MIL-101(Cr) granules at (a) 288 K, (b) 303 K and (c) 313 K.

Isosteric heats of adsorption were calculated from adsorption data collected at different temperatures using Eq. (4), the Clausius–Clapeyron equation.

DH ¼ R

" # d ln P dðT1Þ

ð4Þ

h

where R is the gas constant (kJ mol1 K1), T is temperature (Kelvin), P is pressure (mm Hg), and h is the fraction of adsorbed sites at P and T.

Table 2 Equilibrium selectivity of CO2, N2, CH4, and CO over other gases on MIL-101(Cr) powder and granules. Adsorbate pair

Equilibrium capacity selectivity MIL-101(Cr) powder

CO2/N2 CH4/N2 CO/N2 CO2/CH4 CO/CH4 CO2/CO

MIL-101(Cr) granules

288 K

303 K

313 K

288 K

303 K

313 K

12.26 1.87 3.65 6.55 1.95 3.36

12.61 2.22 4.35 5.69 1.96 2.90

21.25 3.83 7.42 5.54 1.93 2.87

10.83 1.92 2.75 5.65 1.43 3.94

12.00 2.29 4.14 5.25 1.81 2.90

11.64 2.07 3.57 5.62 1.72 3.26

The variation in the heat of adsorption as a function of loading is shown in (Fig. 8). The isosteric heat of adsorption for CO, CH4 and N2 decreased with the increase in loading. The high heat of adsorption in the lower loading region is due to strong electrostatic interactions between gas molecules and coordinately unsaturated metal centers of MIL-101(Cr) [13]. The change in the heat of adsorption with loading may be due to the surface heterogeneity. The highest heat of adsorption for carbon monoxide among the studied gases was due to its polar nature and presence of permanent dipole moment (0.112  1018 esu cm). Carbon dioxide shows heat of adsorption higher than methane and nitrogen due to higher

Table 3 CO2 adsorption on various adsorbent materials. Adsorbent type

CO2 uptakecapacity (mmol g1) at 850 mm Hg, 298 K

Reference

Ordered mesoporous carbon Chemical activated carbon Physical activated carbon Co-MOF MIL-53(Al) Mg-MOF-74 MIL-101(Cr)

0.41 0.68 0.98 2.23 2.29 0.20 2.90

[34] [35] [36] [37] [38] [39] This study

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60

CO CO2 CH4 N2

-Δ H (kJ/mol)

-ΔH (kJ/mol)

45

60

A

30

45

B

CO CO2 CH4 N2

30

15 15 0.0025

0.0050

0.0075

0.0100

0.0010

Loading (mmol/g)

0.0015

0.0020

0.0025

Loading (mmol/g)

Fig. 8. Isosteric heat of adsorption of MIL-101(Cr) powder: (A) Loading up to 0.01 mmol/g and (B) loading less than 0.0025 mmol/g.

Table 4 Heat of adsorption and Henry’s constant for studied gases on MIL-101(Cr) powder and granules. Adsorbate

MIL-101(Cr) powder

MIL-101(Cr) granules

Henry’s constant (103 mmol g1 mm Hg1)

CO2 CO CH4 N2

288 K

303 K

313 K

11.08 16.11 0.55 0.21

15.16 23.94 0.77 0.31

38.54 72.15 0.83 0.43

Heat of Adsorption (DH) (kJ mol1)

38.81 41.92 30.07 19.66

Table 5 Dynamic adsorption parametersa for CO2 + N2 feed gas mixtures on MIL-101(Cr) granules. Feed gas composition (vol.%) CO2

N2

6 9 12 24 47 4 10 14 22 48

94 91 88 76 53 96 96 86 78 52

Flow rate (mL/min)

Breakthrough time (s)

Dynamic adsorption capacity (mmol/g)

50 50 50 50 50 100 100 100 100 100

300 240 240 180 180 180 180 180 120 0.60

0.305 0.370 0.488 0.713 1.388 0.130 0.307 0.414 0.435 0.472

a Column characteristic: column capacity = 140 mL, column length = 20 cm. Inner diameter = 3 cm, weight of adsorbent after activation = 4.53 g.

quadrupole moment and polarizability. The decreasing order of the heat of adsorption, CO > CO2 > CH4 > N2 shows the dominance of dipole–dipole and dipole-induced dipole interactions towards the total heat of adsorption. Henry’s constant is the measurement of strength of adsorption interactions between gas molecules and the adsorbent. The results for heat of adsorption and Henry’s constant for granulated MIL-101(Cr) are comparable to powder MIL101(Cr) and the slight difference may be due to presence of binder used for granulation given in Table 4. The Henry’s constant for CO is more than that of other gases. This is due to the electrostatic interaction of CO with MIL-101(Cr) are much stronger than CO2,

Henry’s constant (103 mmol g1 mm Hg1) 288 K

303 K

313 K

3.98 6.36 0.37 0.17

7.46 11.22 0.40 0.21

20.04 30.93 0.60 0.31

Heat of Adsorption (DH) (kJ mol1)

39.08 41.16 21.21 15.80

CH4 and N2. More than 50% reduction in Henry’s constant for MIL-101(Cr) granules due to hidden unsaturated chromium centers in the frameworks by use of binder. The equilibrium adsorption studies on MIL-101(Cr) showed potential for carbon dioxide separation from its mixture with other gases like N2, CH4, and CO with high CO2/N2 selectivity. Therefore, dynamic adsorption studies from mixed gases are carried out on MIL-101(Cr) granules. The breakthrough time and the capacity depend mainly on the adsorption capacity, adsorption selectivity, feed gas composition, feed gas flow rate, and pressure. In breakthrough experiments, the compositions of the feed gas mixture at the inlet of column remained constant but at column outlet it varies with the time. Starting with pure N2 coming out of the column (as raffinate) at the beginning of the breakthrough measurement, the N2 content gradually decreases and finally reaches the feed gas composition. The term ‘‘breakthrough time’’ can be defined as the time at which adsorbent breaks the barrier of adsorption before attaining the saturation. The breakthrough measurements for carbon dioxide from (CO2 + N2) binary gas mixtures on MIL-101 (Cr) were carried out at 303 K and 1 atm. pressure and feed flow of 100 mL/min. The effect of carbon dioxide concentration in (CO2 + N2) mixture on the dynamic adsorption of mixture components was also investigated. The breakthrough results are given in Tables 4 and 5. Figs. 9 and 10 shows the CO2 adsorption and desorption breakthrough curves from (CO2 + N2) binary gas mixture with feed flow of 50 and 100 mL/min, respectively. The CO2 breakthrough adsorption capacities for the 4%, 10%, 14%, 22%, and 48%, carbon dioxide concentration in binary gas mixture were 0.130, 0.307, 0.414, 0.435, and 0.472 mmol/g, respectively at 303 K for feed flow of 100 mL/min.

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A

B

1.0 0.8

0.6

0.6

2

0.8

(C/C0) CO

(C/C0) CO

2

1.0

0.4

CO2 Ads CO2 Des

0.2

0.4

CO 2 Ads CO 2 Des

0.2 0.0

0.0 0

300

600

900

1200

0

1500

300

Time (sec)

1200

1500

D

1.0

0.8

0.6

0.6

2

0.8

0.4

CO 2 Ads CO 2 Des

(C/C0)CO

2

(C/C0) CO

900

Time (sec)

C

1.0

600

0.2

0.4

CO2 Ads CO2 Des

0.2

0.0

0.0 0

300

600

900

1200

1500

0

150

Time (sec)

300

450

600

750

Time (sec)

E

1.0

(C/C0)CO

2

0.8

0.6

0.4

CO2 Ads CO2 Des

0.2

0.0 0

150

300

450

600

750

Time (sec) Fig. 9. CO2 adsorption–desorption breakthrough curves from binary gas mixture (CO2 + N2) having different CO2 concentrations on MIL-101(Cr) at feed gas flow rate of 50 mL/min (A) 6, (B) 9, (C) 12, (D) 24 and (E) 47 vol.% of CO2, balance N2.

To determine the effect of feed flow rate and diffusion of gas molecules from the mixtures the breakthrough measurements are also carried out at feed flow of 50 mL/min. The carbon dioxide breakthrough adsorption capacities for the 6%, 9%, 12%, 24%, and 47%, carbon dioxide in gas mixture were 0.305, 0.370, 0.488, 0.713, and 1.388 mmol/g, respectively, at 303 K for feed flow of 50 mL/

min. Both the breakthrough time and capacity of CO2 increased on decreasing the feed flow rate. This is due to the increase in the contact time for the adsorption of CO2 gas molecules and better diffusion of gas molecules inside the pores of MIL-101 (Cr). At feed flow of 100 mL/min the gas mixture moves faster through the adsorbent bed column and part of it passes without getting

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A

B

1.0

0.8

0.6

0.6

2

0.8

0.4

CO2 Ads CO2 Des

0.2

(C/C0) CO

(C/C0) CO

2

1.0

0.4

CO2 Ads CO2 Des

0.2

0.0

0.0 0

150

300

450

0

600

150

300

Time (sec)

0.8

0.4 0.2

2

CO2 Ads CO2 Des

0.6

(C/C0) CO

2

D

1.0

0.8

(C/C0) CO

600

Time (sec)

C

1.0

450

0.6

CO2 Ads CO2 Des

0.4

0.2

0.0

0.0 0

150

300

450

600

750

0

200

400

Time (sec)

600

800

Time (sec)

E

1.0

(C/C0) CO

2

0.8

0.6

0.4

CO2 Ads CO2 Des

0.2

0.0 0

150

300

450

600

Time (sec) Fig. 10. CO2 adsorption–desorption breakthrough curves from binary gas mixture (CO2 + N2) having different CO2 concentrations on MIL-101(Cr) at feed gas flow rate of 100 mL/min (A) 4, (B) 10, (C) 14, (D) 22 and (E) 48% of CO2, balance N2.

adsorbed by the adsorbent. The dynamic capacity of CO2 increased from 0.47 to 1.388 mmol/g when the feed gas (having 47% CO2 + 53% N2) flow rate was decreased from 100 to 50 mL/min. The breakthrough capacity increases with increase in CO2

concentration in the feed gas mixture. This increase is due to increase in partial pressure of carbon dioxide with increasing CO2 concentration. The same and nearly constant breakthrough time for different (CO2 + N2) mixtures indicated the weak

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adsorbent-adsorbate interactions. The diffusion of gases having linear isotherm is independent of their percentage in mixture. The adsorbed amount (loading) and the adsorbate concentration (partial pressure) increase by the same factor (i.e. constant relationship). The breakthrough time, steepness of breakthrough curve and mass transfer zone depends on the mass transfer coefficient of the CO2 and N2, which depends on the diffusion of gas molecules in the pores of MIL-101 (Cr), temperature, pressure and slope of isotherm. A faster mass transfer is highly desired for the more efficient use of adsorbent bed. The MIL-101 (Cr) shows steep adsorption–desorption breakthrough curve with high CO2 breakthrough capacity and easy desorption by counter current purging of N2 at 100 mL/min. In order to check the recyclability of the adsorbent MIL-101(Cr), a breakthrough experiment with feed gas composition of CO2 and N2 (47% CO2 and 53% N2) and flow rate of 50 mL/min was carried out. After the completion of the breakthrough run, the adsorbent was used in the subsequent breakthrough cycles without regeneration. The five successive adsorption/desorption breakthrough curves generated (Fig. 11). It is evident from the results that the adsorbent can be used up to five adsorption/desorption cycles without any change in dynamic adsorption capacity. The Breakthrough capacities for CO2 on various materials reported in the

367

literature are given in Table 6 indicating that the results derived from this study are comparable. 4. Conclusions The CO2-TPD measurements indicated the presence of weak to moderate basic sites on MIL-101(Cr) and the corresponding CO2 desorption was 0.277 mmol/g. with the increase in adsorption temperature, the selectivity of CO2, CH4 and CO over N2 in MIL101(Cr) increased. The selectivity of CO2 over CH4, CO over CH4 and CO2 over CO decreased with increase in adsorption temperature. The selectivity follows the order CO2/N2 > CO2/CH4 > CO/ N2 > CO2/CO > CH4/N2 > CO/CH4. The MIL-101(Cr) showed highest volumetric equilibrium selectivity value 12.6 for CO2 over N2. The dynamic adsorption capacity for CO2 was 1.388 mmol/g and more than eighty percent of adsorbed CO2 could be desorbed by counter current purge of N2 at 303 K without employing temperature. The dynamic adsorption capacity of CO2 increased with the increase in CO2 concentration in feed gas with minimizing the breakthrough time. The equilibrium and breakthrough/dynamic adsorption capacity of CO2 at 303 K obtained were 1.68 mmol/g and 1.388 mmol/g, respectively. Acknowledgements G.S. and P.B.S Rallapalli acknowledge CSIR for SRF and financial support under network Project NWP-021. Analytical science division, CSIR-CSMCRI, Bhavnagar is acknowledged for their help in instrumental characterization.

1.0

CO2 adsorption

References

0.8

(C/C0)CO

2

Run-I Run-II Run-III Run-IV Run-V

0.6

0.4

0.2

CO2 desorption 0.0 0

300

600

900

1200

1500

Time (sec) Fig. 11. Reproducibility of breakthrough adsorption – desorption cycle on MIL101(Cr) granules.

Table 6 Dynamic CO2 adsorption capacities of various adsorbent materials at atmospheric pressure. Adsorbent

Feed flow rate (mL/min)

Temp. (K)

Capacity (mmol/ g)

Reference

SBA-15 APS/SBA-15(III) AEAPS/SBA-15(III) TA/SBA-15(III) Zn- MOF-74 Mg-MOF-74 MWCNTs Granular Activated Carbon Zeolite-L MIL-101(Cr)

30 30 30 30 10 10 80 80

333 333 333 333 213 213 298 298

0.05 0.66 1.36 1.58 0.08 2.02 1.57 1.66

[8] [8] [8] [8] [39] [39] [40] [40]

80 50

298 303

1.44 1.39

[40] This study

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