Enhanced hydrogen sorption in single walled carbon nanotube incorporated MIL-101 composite metal–organic framework

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 7 5 9 4 e7 6 0 1

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Enhanced hydrogen sorption in single walled carbon nanotube incorporated MIL-101 composite metaleorganic framework K.P. Prasanth a, Phani Rallapalli a, Manoj C. Raj a, H.C. Bajaj a, Raksh Vir Jasra b,* a

Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G.B. Marg, Bhavnagar 364 021, Gujarat, India b Reliance Technology Group, Reliance Industries Limited, Vadodara Manufacturing Division, Vadodara 391 346, Gujarat, India

article info

abstract

Article history:

MetaleOrganic Frameworks (MOFs) have emerged as potential hydrogen storage media due to

Received 12 December 2010

their high surface area, pore volume and adjustable pore sizes. The large void space generated

Received in revised form

by cages in MOFs is not completely utilized for hydrogen storage application owing to weak

7 March 2011

interactions between the walls of MOFs and H2 molecules. These unutilized volumes in MOFs

Accepted 19 March 2011

can be effectively utilized by incorporation of other microporous materials such as single

Available online 22 April 2011

walled carbon nanotubes into the pores of MOFs which could effectively tune the pore size and pore volume of the material towards hydrogen sorption. Single walled carbon nanotubes

Keywords:

(SWNT) incorporated MIL-101 composite MOF material (SWNT@MIL-101) was synthesized by

MetaleOrganic framework (MOF)

adding purified single walled carbon nanotube (SWNT) in situ during the synthesis of MIL-101.

MIL-101

The powder X-ray diffraction patterns of SWNT@MIL-101 showed the structure of MOF was

SWNT

not disturbed by SWNT incorporation. Hydrogen sorption capacities of MIL-101 was observed

Hydrogen sorption

to increase from 6.37 to 9.18 wt% at 77 K up to 60 bar and from 0.23 to 0.64 wt% at 298 K up to 60 bar. The increment in the hydrogen uptake capacities of composite MOF materials was attributed to the decrease in the pore size and enhancement of micropore volume of MIL-101 by single walled carbon nanotube incorporation. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Hydrogen adsorption in porous materials is one of the possible methods proposed for on-board hydrogen storage applications. High pressure hydrogen sorption studies were performed on various porous materials such as zeolites [1e4], mesoporous materials [5], carbon nanomaterials [6,7] and metaleorganic frameworks (MOFs) [8e12]. Metaleorganic frameworks are crystalline solids consisting of multidentate organic ligands connecting metal ions or small metal-

containing clusters. Most of the MOFs have a three-dimensional framework that encloses uniform pores which are inter-connected to form an ordered network of channels. These are synthesized by a self assembly process in which different combinations of organic linkers and metal nodules lead to materials having a wide range of varying topologies and pore sizes. These offer a wider range of properties such as crystallinity, porosity, and functionality of metal ions and organic ligands [13]. After removal of retained solvent molecules, MOFs can show surface area values in the range of

* Corresponding author. Tel.: þ91 265 6996313/6993935; fax: þ91 265 6693934. E-mail address: [email protected] (R.V. Jasra). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.109

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 7 5 9 4 e7 6 0 1

1500e3000 m2 g1, but surface area higher than 5000 m2 g1 are also reported for some MOFs [14e17]. Pore volume usually ranges from 0.2 to 0.8 cm3 g1, but values well over 1.1 cm3 g1 were also reported for some MOFs [14,15,18,19]. MOFs possess exceptionally high porosity, uniform but tunable pore size (about 0.5e2 nm in diameter) and well-defined hydrogen occupation sites [20e25]. It has been reported that higher gas adsorption could be achieved on materials with a large volume of micropores with an appropriate pore diameter [26]. Earlier reports were directed towards the synthesis of highly porous frameworks, which could then be filled with hydrogen gas. Some examples include the ‘MIL’ series of MOFs ˚ [15], reported by Ferey et al., with pore sizes greater than 25 A and the isoreticular ‘IRMOF’ series with progressively larger pores reported by Yaghi et al. [14,27], all based on carboxylate ligands and possessing non interpenetrated networks. These materials possess pore volumes greater than 1.5 cm3 g1, and in turn adsorb a good amount of hydrogen at high pressures, e.g., 6.01 wt% for MIL-101 [15] and 6.7 wt% for IRMOF-20 [16] at 77 K up to 80 bar pressure. Even though these materials showed good adsorption capacities for hydrogen, nearly 80% of the volume of these materials was not utilized during adsorption process. Therefore, there is a scope to enhance hydrogen sorption capacities of these high surface area MOFs by utilization of these unused volumes for hydrogen occupation. Various strategies like optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of unsaturated open metal sites and lighter metals have been tried to enhance the hydrogen storage capacities in MOFs [20]. The highest hydrogen storage capacities for the MOFs were at liquid nitrogen temperature. However, moderate sorption capacities have been reported for MOFs at ambient temperature [15,28e30]. By optimization of the pore radius, enhanced adsorption potential due to the overlapping force fields of the opposite micropore walls could make the adsorption of hydrogen molecule more feasible even at room temperature. Thus, optimal storage can be achieved on maximization of the micropore volume. Specific surface area is not the only criteria for hydrogen adsorption in MOFs, the nature and size of the pores also are important factors affecting the hydrogen sorption capacity. To retain hydrogen in the tunnels and cages of porous solids, one needs strong interactions between the walls of the adsorbent (MOF) and the H2 molecules, provided that the pores are sufficiently large to accommodate H2 molecules. The ideal ˚ , or approximately 2.8e3.3 A ˚ pore size seems to be 4.5e5 A when the van der Waals radii of the atoms composing the pore walls are excluded that is comparable to kinetic diameter of H2 ˚ ). Pores of this size allow the hydrogen molecule to (2.89 A interact with multiple portions of the framework rather than just one secondary building unit (SBU) or organic linker, thereby increasing the interaction energy between the framework and H2. This is evidenced by recent work by Jhung et al. [31] on nanoporous aluminophosphates wherein it was shown that small pore size and large micropore volume are beneficial for high hydrogen uptake. The tuning of the pore size and enhancement in micropore volume in MOFs can be obtained by incorporation of other microporous materials such as carbon nanotubes into MOFs, which also have sufficient hydrogen sorption capacities.

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Composite materials of MOFs and multi walled carbon nanotubes with enhanced hydrostability were synthesized and were found effective in hydrogen adsorption [32,33]. Based on these observation single walled carbon nanotubes (SWNTs) were incorporated into the MIL-101. SWNT were selected for the incorporation as there have sufficient porosities and hydrogen sorption capacities both at 77 K and 303 K [34,35].

2.

Experimental

2.1.

Materials

Chromium (III) nitrate Cr(NO3)9H2O (98% pure), HNO3, H2O2, HF, NH4F (all from s.d. Fine Chemicals, India), 1, 4- Benzenedicarboxylic acid (BDC) (98% pure, Aldrich) were used for the synthesis of MIL-101. The single walled carbon nanotube (SWNT) obtained from Carbon Solutions Inc. (produced by electric arc method) with a narrow diameter distribution peaked at 1.4 nm was used for the preparation of SWNT@MIL-101 composite material. H2, N2, CO2 and He gases used for the adsorption studies and surface area measurements were of ultra high pure grade (99.999%) procured from Inox India Ltd, India.

2.2.

Synthesis of MIL-101 [Cr3F(H2O)2O[BDC]3nH2O]

The synthesis of MIL-101 consists of the hydrothermal reaction of BDC (166 mg, 2 mmol) with Cr(NO3)39H2O (400 mg, 1 mmol), fluorhydric acid, HF(1 mmol), and 4.8 mL of H2O (265 mmol) for 8 h at 220  C, producing highly crystallized green powder of the chromium terephthalate with formula Cr3F(H2O)2O[(O2C)e C6H4e(CO2)]3$nH2O (n w 25), based on chemical analysis. It was then filtered and washed with distilled water and refluxed in NH4F solution to remove the unreacted BDC molecules present as needles after synthesis. The yield of the final product obtained after purification was 55%.

2.3. Synthesis of SWNT@MIL-101 composite MOF material Commercially purchased carbon nanotubes contain impurities of residual nickel catalyst and amorphous carbon. The impure carbon nanotubes were purified using a two step process. In the first step, 100 mg of SWNT was dispersed in 200 mL of 70% nitric acid and refluxed with stirring for 12 h. The dispersion was filtered and washed with distilled water until the washings were neutral and dried at 80  C for 4 h. In the second step the acid treated SWNTs sample were immersed into 30 wt% H2O2 solution by ultrasonication in order to separate the nanotubes. The dispersed SWNTs were then stirred in an H2O2 solution for up to 7 days at room temperature in order to give the optimal time for the purification [36]. The suspension was filtered and washed with distilled water and ethanol, dried at 80  C for 4 h. The purified SWNTs were added in situ during the synthesis of MIL-101 along with the raw materials to synthesize SWNT@MIL-101 composite MOF material. The amount of purified SWNT was varied to synthesize 6 wt%, 8 wt% and 10 wt% single walled carbon nanotube incorporated MIL-101 samples.

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Characterization

The powder X-ray diffraction (PXRD) patterns of the samples before and after SWNT incorporation at ambient temperature were carried out using a PHILIPS X’pert MPD diffractometer system in the 2q range of 2e60 at a scan speed of 0.1 sec1 ˚ ) radiation to determine the crysusing CuKa1 (l ¼ 1.54056 A tallinity. The thermal stability of MIL-101 and SWNT incorporated MIL-101 samples were investigated using TGA/DTA (Mettler Toledo 851) analyzer at a heating rate of 10  C/min under argon atmosphere from room temperature to 900 K. The elemental composition of the samples was accomplished by CHN analyzer (Perkin Elmer-2400). The FTIR spectra of SWNT and purified SWNT were recorded in Perkin Elmer GXFTIR spectrometer. The morphology of the samples was studied by TEM analysis using HRTEM JEOL, JEM-2100 Electron microscope. A small amount of the sample was dispersed in ethanol by sonication and a drop of resultant suspension was placed on a Cu grid, dried and given for TEM analysis. The BET surface area, pore volume, pore diameter of the synthesized MOF materials were determined in a static volumetric system (Micromeritics Instrument Corporation, USA, model ASAP 2010) using N2 adsorption-desorption isotherm at 77 K up to 1 bar pressure. Prior to adsorption measurements the samples were activated by heating at a rate of 1 K min1, to 393 K under vacuum (5  103 mmHg) for 8 h. The weight of sample was determined before and after activation. The BET (BrunauereEmmetteTeller) equation was used to calculate the surface area and pore diameter was obtained using t-plot.

2.5.

High pressure hydrogen sorption measurements

High pressure hydrogen adsorption measurements at 77 K and 298 K up to 60 bar were carried out in an automated high pressure gas adsorption system BELSORP-HP, BEL Japan, Inc. Prior to the sorption isotherm measurements, the sample was activated at a heating rate of 1 K min1, to 393 K under vacuum (6.7  102 Pa). The temperature and vacuum was maintained for 10 h before the sorption measurements. After activation, the samples were allowed to cool down to 298 K. The amount of activated sample was determined from the weight of sample before and after activation. H2 sorption measurements at 77 K were performed by immersing the sample in liquid nitrogen Dewar with automatic liquid nitrogen level controller. H2 adsorptionedesorption studies at 298 K were performed by maintaining the temperature using an external water circulator (Poly Science, USA). The errors in the

SWNT

Intensity (a. u.)

2.4.

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SWNT@MIL-101-10% SWNT@MIL-101-8% SWNT@MIL-101-6% MIL-101

10

20

30 2θ

40

50

60

Fig. 1 e Powder X-ray diffraction patterns of MIL-101 and SWNT@MIL-101 composite materials.

measurements of hydrogen uptake values were within the range of 0.5%.

3.

Results and discussion

The PXRD patterns of SWNT incorporated MIL-101 samples showed the same diffraction patterns as of MIL-101 (Fig. 1). This confirms that the structure of MIL-101 was not affected by the SWNT incorporation. The elemental analysis shows increase in the percentage of carbon in SWNT@MIL-101 composite MOF material (Table 1). N2 adsorptionedesorption isotherms in SWNT incorporated MIL-101 composite materials at 77 K up to 1 bar pressure are shown in Fig. 2. BET and the Langmuir surface areas of the SWNT incorporated samples were calculated from nitrogen adsorptionedesorption data at 77 K (Table 1). There is not much variation in BET surface area values of the composite materials while the Langmuir surface area of modified MIL101 materials were found to be decreased compared to bare MIL-101. Decrease in the Langmuir surface area could be due to the decrease in pore width and pore blocking by SWNT in the pores of MIL-101 as evidenced from pore size distribution curves. The pore volume of MIL-101 was found to continuously decrease as the percentage of SWNT incorporation

Table 1 e Percentage of C and textural properties of MIL-101 and SWNT@MIL-101 composite materials. Material

MIL-101 SWNT@MIL-101-6% SWNT@MIL-101-8% SWNT@MIL-101-10%

% Of Carbon 29.72 33.19 34.95 37.72

BET Surface Area (m2/g) 2887 2884 2998 2835

   

62 46 43 47

Langmuir Surface Area (m2/g)

Pore Volume (BJH) (cm3/g)

t-Plot Micropore volume (cm3/g)

Pore Volume in ˚ (cm3/g) Pores <15 A (by DFT Method)

   

1.45 1.40 1.36 1.32

0.39 0.34 0.38 0.45

0.35 0.40 0.42 0.49

4350 3989 4162 3919

87 109 115 111

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1000 900

Voume adsorbed (cc/g)

800 700

MIL-101 SWNT SWNT@MIL-101-6 wt SWNT@MIL-101-8 wt SWNT@MIL-101-10 wt

600 500 400 300 200 100 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative pressure (P/P0) Fig. 2 e N2 adsorptionedesorption isotherms in MIL-101 and 6, 8 and 10 wt% SWNT incorporated MIL-101 composite materials.

increases. But the t-Plot micropore volume and the DFT pore ˚ increases as the percentage of SWNT volume of pores <15 A loading increases. This may due to the formation of additional micropores by SWNT incorporation in SWNT@MIL-101 composite material which was later confirmed by CO2 sorption studies at 273 K. The thermal behaviour of MIL-101 and SWNT@MIL-101 composite materials was investigated using thermo gravimetric analysis (TGA) to determine the thermal stability (Fig. 3) of the material. The TGA profiles reveal that MIL-101 is stable up to 300  C. MIL-101 shows the initial loss of weakly bound guest water molecules up to 100  C. From 100  C to 300  C the 10 to 14 percent weight loss was observed which may be due to loss of OH/F groups of MIL-101 [37]. After 350  C the structure of MIL-101 collapsed due to the decomposition of benzenedicarboxylic acid linkers. The thermo grams shows

that the amount of adsorbed water molecules reduced after the SWNT incorporation as the weight loss between 30  C and 100  C is observed to decrease. This may be due to the hydrophobic nature of SWNT incorporated in the MIL-101 metaleorganic framework. The thermal stability of the MIL101 was also found to increase by SWNT incorporation. The FTIR spectra (Fig. 4.) of SWNT samples before and after purification showed the presence of eOH and eCOOH functional groups on the surface of SWNT. The Non Local Density Functional Theory (NLDFT) method was used to calculate the pore size distribution curves in MIL101 and composite samples from N2 adsorption data measured at 77 K. Conventional, macroscopic, thermodynamic methods (e.g., methods based on the Kelvin equation such as BJH) assume bulk-fluid like behaviour for pore fluid and neglect details of the fluidewall interactions, but microscopic methods such as NLDFT takes into consideration the configuration of adsorbed molecules in pores on a molecular level. MIL-101 is a metaleorganic framework material with pore sizes in both micropore and mesopore region. NLDFT allows to obtain a more accurate and comprehensive pore size analysis over complete micro/meso pore size range compared to macroscopic, thermodynamic methods namely BJH, HK, SF, DR [29]. The pore size distribution curves in MIL-101 and SWNT@MIL-101 composite materials calculated by the NLDFT method from N2 adsorption data at 77 K are given in Fig. 5. The NLDFT pore size distribution curve of MIL-101 showed two ˚ and 30.5 A ˚ which decreased to 19.8 A ˚ and distinct pores at 23.3 A ˚ respectively in SWNT@MIL-101 composite materials. 28.2 A There was also an increase in micropore volume of ultra˚ ) as shown in Table 1. This micropore region (pore width <15 A difference in the pore volume in the two regions is an evidence for the incorporation of SWNTs into the pores of MIL-101. The pore size distribution curve in the ultramicropore region was not obtained by N2 adsorption at 77 K, hence CO2 sorption was carried out at 273 K to find out the micropore distribution curves. The DFT pore size distribution curves in the micropore region measured from CO2 adsorption at 273 K are shown in Fig. 6. CO2 adsorption at 273 K was preferred over

100 90

SWNT purified

transmittance (a. u.)

weight loss

80 70 60 50

SWNT@MIL-101-10wt SWNT@MIL-101-8wt SWNT@MIL-101-6wt MIL-101

40

-1

1700 cm

-1

1400 cm

SWNT

30 -1

50

100 150 200 250 300 350 400 450 500 550 600 o

Temperature ( C)

3500 cm 4000

3500

3000

2500

2000

1500

1000

500

-1

Fig. 3 e TGA plots of MIL-101 and 6, 8 and 10 wt% SWNT incorporated MIL-101 composite materials.

wave number(cm ) Fig. 4 e FTIR spectra of SWNT and purified SWNT samples.

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b

1.0

0.8

3

0.14

Cumulative pore volume (cm /g)

MIL-101 SWNT@MIL-101-6 wt SWNT@MIL-101-8 wt SWNT@MIL-101-10 wt

0.16

3

Incremental pore volume (cm /g)

a

0.12 0.10 0.08 0.06 0.04 0.02

0.6

0.4

MIL-101 SWNT@MIL-101-6wt SWNT@MIL-101-8wt SWNT@MIL-101-10wt

0.2

0.0

0.00 10

15

20

25

30

35

40

45

50

55

10

60

15

20

25

30

35

40

Pore width (Å)

Pore width (Å)

Fig. 5 e Non local density functional theory (NLDFT) pore size distribution of (a) incremental pore volume and (b) cumulative pore volume in MIL-101 and SWNT@MIL-101 samples by N2 sorption at 77 K.

nanotubes with w1.4 nm diameters. The TEM images of SWNT@MIL-101-8 wt% and SWNT@MIL-101-10 wt% were shown in Fig. 7c and d. The image SWNT inside the pores of MIL-101 was not obtained as expected. This may be due to the bigger size of MIL-101 crystals and thickness of the pore wall which prevents the electron beam to penetrate through the walls to obtain the image of composite material. Another difficulty is that MIL-101 is extremely sensitive to the electron beam and, in usual conditions; the structure could collapse after a few minutes [39]. The TEM image of SWNT@MIL-10110 wt% showed a layered morphology for the crystals of MIL101. This might be due to the growth of MIL-101 on the surface of the functional groups in purified SWNTs and likely lead to a polycrystalline MOF layer on the outside of the tubes on higher SWNT loading percentages. Hydrogen adsorption-desorption isotherms in MIL-101 and SWNT@MIL-101 composite materials at 77 K (Fig. 8) and 298 K (Fig. 9) were measured up to 60 bar pressure. At 77 K the isotherms are all highly reversible with respect to pressure. MIL-101 shows 6.37 wt% and 0.23 wt% of hydrogen sorption capacity at 77 K and 298 K respectively which are quite comparable with the earlier reported studies [15]. Adsorption isotherms depicts considerable enhancement in the hydrogen

N2 at 77 K for accessing the micro porosity because at elevated temperatures and higher absolute pressure (P0 ¼ 26,200 Torr) CO2 can access ultramicropores, which are not accessible to nitrogen molecules at 77 K [38]. The decrease in analysis time is also another advantage for using CO2 as an adsorbent. Due to higher diffusion rate equilibrium is achieved much faster as compared to nitrogen adsorption at 77 K and dramatic decrease in analysis time can be achieved (3e5 h for CO2 versus 30e50 h for N2). The DFT micropore distribution curves by CO2 adsorption showed a small decrease in the pore size in composite MOF material as compared to MIL-101. But an enhancement (w0.03 cm3/g) in the micropore volume of composite was clearly observed in the pore size distribution curve. There is also the formation of additional ultra˚ ) observed in SWNT@MIL-101 composite micropores (6.7 A material due to the incorporation of carbon nanotube inside the pores. The TEM images of MIL-101, SWNT and SWNT@MIL-1018 wt% and SWNT@MIL-101-10 wt% samples are shown in Fig. 7. The dimensions of the MIL-101 crystals range from 100 nm up to about 1 mm. The cubic symmetry of the MIL-101 is also reflected in the shape of the crystals (Fig. 7a) [30]. Fig. 7b shows the TEM image of SWNT appears as bundles of

a

Cumulative pore volume (cm /g)

0.05 0.04 0.03 0.02 0.01 0.00 5.0

MIL-101 SWNT@MIL-101-8 wt

0.12

3

3

Incremental pore volume (cm /g)

b 0.14

MIL-101 SWNT@MIL-101-8wt

0.06

5.5

6.0

6.5

7.0 7.5 8.0 Pore width (Å)

8.5

9.0

9.5

10.0

0.10 0.08 0.06 0.04 0.02 0.00 5.0

5.5

6.0

6.5

7.0 7.5 8.0 8.5 Pore width (Å)

9.0

9.5 10.0 10.5

Fig. 6 e Density functional theory (DFT) pore size distribution of (a) incremental pore volume and (b) cumulative pore volume in MIL-101 and SWNT@MIL-101-8 wt% samples by CO2 sorption at 273 K.

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Fig. 7 e TEM images of (a) MIL-101, (b) SWNT, (c) SWNT@MIL-101-8 wt% and (d) SWNT@MIL-101-10 wt% composite.

sorption capacities of MIL-101 samples by SWNT incorporation. 8 wt% SWNT incorporated MIL-101 showed the highest hydrogen storage capacity of 9.18 wt%. The room temperature hydrogen sorption capacity of the composite materials was

also higher compared to MIL-101 and single walled carbon nanotube. SWNT@MIL-101-8wt% sample showed the highest capacity of 0.64 wt% at 298 K. A small hysteresis was observed in the adsorptionedesorption isotherms in SWNT@MIL-101 composite material. 10 wt% SWNT incorporated sample showed decrease in hydrogen sorption capacities compared to

9 8 MIL-101 SWNT MIL-101@SWNT-6wt MIL-101@SWNT-8wt MIL-101@SWNT=10wt

0.6

7 6

0.5

H2 wt.

5 0.4

H2 wt.

4 3 2

0.3 0.2

1 0.1

0 0

5

10

15

20

25

30

35

40

45

50

55

60

Pressure (bar) MIL-101 SWNT MIL-101@SWNT-8 wt

0.0

MIL-101@SWNT-6 wt MIL-101@SWNT-10 wt

Fig. 8 e Hydrogen adsorptionedesorption isotherms in MIL-101 and 6, 8, 10 wt% SWNT@MIL-101 composite material at 77 K up to 60 bar.

0

10

20

30

40

50

Pressure (bar) Fig. 9 e Hydrogen adsorptionedesorption isotherms in MIL-101 and 6, 8, 10 wt% SWNT@MIL-101 composite material at 298 K up to 60 bar.

60

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Table 2 e Hydrogen sorption capacities in MIL-101, SWNT and MIL-101@SWNT composite at 77 K and 298 K up to 60 bar. Material

MIL-101 SWNT MIL-101@SWNT-6 wt% MIL-101@SWNT-8 wt% MIL-101@SWNT-10 wt%

H2 sorption H2 sorption capacity capacity at 77 K at 298 K up to 60 up to 60 bar (wt%) bar (wt%) 6.37 3.94 8.60 9.18 8.24

0.23 0.19 0.56 0.64 0.21

bare MIL-101 both at 77 K and 298 K. This decrease could be attributed to the pore filling by the higher concentration of SWNT inside the pores and also the decrease in pore width and pore blocking effect of additional SWNTs sitting on the pore openings. The decrease in pore size and increase in micropore volume due to the SWNT incorporation could be the reason for the enhancement of hydrogen sorption capacities in the composite MOF materials. The formation of new ultramicropores by SWNT incorporation increases the interaction between MOF surface and hydrogen molecules. ˚ in Generally, it is recognized that ultramicropores of 6e7 A diameter are more effective in high level of hydrogen uptake than the pores having other diameters [40]. MIL-101 has very large surface area but the pore diameter is usually in the range ˚ which reduces the interaction of hydrogen with the 20e30 A pore walls of MIL-101 despite of high surface area. With SWNT incorporation, the ultramicropore volume significantly increased compared with that of MIL-101. Thus, the higher hydrogen storage capacity of SWNT@MIL-101 composite materials contains a contribution from ultramicropores arising because of SWNT incorporation. It is important that, at 298 K and high pressure, the hybrid composite H2 storage capacity became much more apparent. At low pressure and 298 K, H2 storage capacity of SWNT@MIL-101 composite was only slightly higher than those of other materials examined and the isotherm slope indeed suggested enhanced H2 storage capacity at high pressure. Under higher pressure, up to about 60 bar at 298 K, SWNT and MIL-101 had H2 storage capacities of 0.19 and 0.23 wt%. However, the hybrid composite had almost three fold greater H2 storage capacity (0.64 wt%) than did MIL101 which was reversible. These observations imply that additional micropore development within SWNT@MIL-101 enhanced H2 storage capacity at 77 K, but particularly at 298 K, over a wide pressure range. Hydrogen sorption capacities at 77 K and 298 K measured for MIL-101 and SWNT incorporated MIL-101 composite samples are listed in Table 2.

4.

Conclusions

Single walled carbon nanotube incorporated MIL-101 composite metaleorganic framework was successfully synthesized and characterized. A significant enhancement in the hydrogen storage capacity at 77 K and 298 K was observed in the composite MOF materials. The approach of incorporation of SWNTs in high surface area metaleorganic frameworks and subsequent

utilization of the unutilized volume of MOFs presents new directions for achieving novel hybrid materials with higher hydrogen sorption capacities between the two highlighted materials, such as CNTs and MOFs.

Acknowledgements The authors are thankful to Analytical Science Discipline, CSMCRI for analytical support. The financial support by CSIR, New Delhi under CSIR Network Project NWP-022 is gratefully acknowledged. KPP and PR thanks to CSIR, New Delhi for financial support in the form of senior research fellowship.

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