MOF-5 and activated carbons as adsorbents for gas storage

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 7 ( 2 0 1 2 ) 2 3 7 0 e2 3 8 1

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MOF-5 and activated carbons as adsorbents for gas storage J.P. Marco-Lozar, J. Juan-Juan, F. Sua´rez-Garcı´a, D. Cazorla-Amoro´s, A. Linares-Solano* Grupo de Materiales Carbonosos y Medio Ambiente, Departamento de Quı´mica Inorga´nica, Universidad de Alicante, E-03080 Alicante, Spain

article info

abstract

Article history:

This paper reports comparatively the capacities of two activated carbons (ACs) and MOF-5

Received 22 July 2011

for storing gases. It analyzes, using similar equipments and experimental procedures, the

Received in revised form

density used to convert gravimetric data to volumetric ones, measuring the density (tap

4 November 2011

and packing at different pressures). It presents data on porosity, surface area and gas

Accepted 9 November 2011

storage (H2, CH4 and CO2) obtained under different temperatures (77 K and RT) and pres-

Available online 3 December 2011

sures (0.1, 4 and 20 MPa). MOF-5 presents lower volume of narrow micropores than both ACs, making its storage at RT lower, independently of the gas used (H2, CH4 and CO2) and

Keywords:

the basis of reporting data (gravimetric or volumetric). For H2 at 77 K the reliability of the

Actived carbons

results depends too much on the density used. It is shown that the outstanding volumetric

Metal-organic frameworks

performance of MOF-5, in relation to ACs, is due to the use of an unrealistic high density

Hydrogen storage

(crystal density) that, not including the adsorbent inter-particle space, gives an apparently

Gas storage

high volumetric gas storage capacity. When a density measured similarly in both types of

Material density

adsorbents is used (e.g. tap or packing densities) MOF-5 presents, for all gases and conditions studied, lower adsorption capacities on volumetric basis and storage capacities than ACs. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In the last decades, there has been an increasing interest in developing gas storage systems for different applications such as H2 storage for automotive applications or carbon dioxide capture due to concerns over greenhouse gas emissions. Currently, there are different methods for storing gases. Among the different approaches [1], high-pressure adsorption on highly porous adsorbents appears to be an interesting alternative to store gases [1e13]. For some of these storage applications (e.g. H2, CH4 and CO2), the basic adsorption principle has to be extended to high-pressure adsorption where these gases (except CO2) are in a supercritical state. In such adsorption process the molecules of gases to be stored (the adsorbate) are bonded by Van der Waals forces to the adsorbent which has to have a high porosity and surface area [14].

There is a large variety of available porous materials having quite different properties, such as zeolites, highly activated carbons, activated carbon nanotubes, zeolite template carbons, carbide-derived carbons, metal organic frameworks (MOFs) and covalent organic frameworks (COFs). In all of them, their porosity, surface area, morphology, size and shape are tuneable and, hence, able for further improvements. In this way, the development of new porous materials has been, and continues to be, intensively investigated. Hence, the number of publications reporting their subsequent performance improvement for gas storage has increased considerably. Among the highest reported values, two families of porous solids have shown to have better performances [1,15]: (i) the more “classical” one (i.e. activated carbons) and (ii) the more “recent and new” type of porous materials (i.e. MOFs and COFs).

* Corresponding author. Fax: þ34 965903454. E-mail address: [email protected] (A. Linares-Solano). 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.2011.11.023

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 7 ( 2 0 1 2 ) 2 3 7 0 e2 3 8 1

Among these two types of adsorbents, the superior gas storage performance of one in relation to the other needs, in our opinion, further investigation. This is due because gas storage values are not always reported in a similar way on both families of adsorbents. Generally, the results are mostly expressed on gravimetric basis and when volumetric storage capacities are reported they do not always use comparable material densities. Therefore, a conclusive opinion about “the best adsorbent” for gas storage application is still lacking and more attention is required performing suitable comparatives analysis. Papers dealing with high-pressure gas adsorption storage should report their results both per unit of weight and volume of adsorbent (i.e. gravimetric and volumetric basis respectively), as was pointed out long time ago (1994) by Chahine et al. [16]. This should be done considering that the adsorbent has to be confined on the limited tank volume (a vehicle for an on-board storage application in transportation). Unfortunately, the results on a volumetric basis are not frequently reported and/or the type of density used is not well defined. Thus, those papers that convert their gravimetric results to volumetric ones do not always give details of the experimental procedure used to measure the density of the adsorbent investigated. This is especially important in the case of the above mentioned “recent and new” porous materials (MOFs and COFs) that mostly use a “calculated” density (e.g. crystal density). Using such crystal density many papers have reported very high volumetric storage capacities [17e22]. Consequently, the current feeling of most scientists dealing with high-pressure adsorption gas storage is that MOFs are the most outstanding adsorbents among the different existing ones. However, in a previous paper [23], we analyzed and demonstrated that the use of crystal density gives unrealistic high capacity values since this theoretical density does not include the inter-particle space and, thus, leads to an overestimated adsorption value. For this reason, in this paper, the objective is to deepen into the claimed superior capacity of MOFs in relation to activated carbons (ACs), reviewing and extending our comparative work [23] on the field of gas storage (H2, CH4 and CO2), with new data, paying attention to the data reported in the literature (on gravimetric and volumetric basis), as well as to the suitability of different densities employed.

2.

Experimental

2.1.

Adsorbent materials

Three adsorbents (one MOF and two activated carbons) have been selected in this investigation [23]. These adsorbents were selected because all of them have high and very similar adsorption capacities (per unit of gram), enough amount of sample are available (two of them from companies), and they are among the samples most studied having high gas storage capacities [8,9,16,17,24e30]. These samples are: (i) MOF-5 (also know as IRMOF-1) supplied by BASF; (ii) a KOH activated carbon known as Maxsorb3000 (also as MaxsorbMSC30) supplied by Kansai Coke & Chemical Co. This sample, named in this work as Max3, is among the commercial activated

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carbons, one with the better performance; and (iii) a second KOH activated carbon (AC1) which was prepared in our labs from a Spanish anthracite, following the experimental procedure previously described [31].

2.2.

Textural characterization

Prior to gas storage measurements, porous texture characterization of all the samples was performed by physical adsorption of N2 at 77 K and CO2 at 273 K (using an Autosorb-6, Quantachrome, equipment). High resolution N2 isotherms at 77 K were also carried out in an automatic adsorption system ASAP 2020 (Micromeritics). For these measurements, about 150 mg of sample were previously degassed at 423 K for 4 h. From N2 adsorption isotherms, the apparent BET surface areas, SBET, as well as the total micropore volumes, VDR(N2), were calculated. In the same way, the volumes of narrow micropores smaller than 0.7 nm, VDR(CO2), were obtained from CO2 adsorption isotherms. Both, total and narrow micropore volumes were calculated using the DR (DubininRadushkevich) equation.

2.3.

Densities

Density of the powder samples was determined by four different methods: (i) filling and vibrating a container with a known weight of sample, obtaining the occupied volume (i.e., tap density, also called bulk or apparent density); (ii) pressing a given amount of sample in a mould at a pressure of 415 kg/cm2 (packing density) [24], although additional measurements at higher pressures were done; (iii) the real or true density (helium density) which was determined using a Micromeritics Accupyc 1330 pycnometer; and (iv) the crystal density that was estimated as the sum of the volume occupied by the atoms of the material (i.e. 1/He density) plus the total pore volume obtained from the N2 adsorption isotherm at P/ Po ¼ 0.99. (Note: activated carbons are not crystalline materials, so, their crystal densities have no physical meaning. They are only used for comparative proposes.)

2.4.

Gas storage experiments

The storage of the different gases studied (hydrogen, methane and carbon dioxide) has been carried out in different equipments, depending on the pressure required (from 0.1 to 20 MPa) and the temperature used (room temperature and 77 K). For hydrogen storage three equipments were used depending on the range of pressures analyzed: i) for subatmospheric pressures at 77 K, an automatic volumetric adsorption apparatus, (ASAP 2020; Micromeritics). The sample was outgassed at 423 K under vacuum for at least 4 h before measuring their isotherms, ii) for high pressures (up to 4 MPa) at 77 K, a Sartorius 4406- DMT high-pressure. Approximately 200 mg of sample was outgassed “in situ” at 423 K during 4 h under vacuum. The experimental results were corrected for buoyancy effects related to the displacement of gas by the sample, sample holder and pan [32] and iii) for very high pressures (up to 20 MPa) at room temperature (298 K), a fully automated volumetric apparatus, designed and built up at University of Alicante [24,33]. About 700 mg of sample were

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degassed at 423 K during 4 h under vacuum and the weight of the degassed sample was measured. After that, the sample was located in the sample holder, where it was degassed at 423 K during another 4 h in vacuum, in order to prepare the sample for the measurement. The bulk gas densities were calculated by the equation of state of Modified-Benedic-WebbRubin proposed by Younglove [34]. For CH4 and CO2 their storages were studied at room temperature up to 3 and 4 MPa respectively, in the same Sartorius device. In both cases, about 150 mg of sample were placed in the sample holder and degassed “in situ” at 423 K under vacuum until a constant weight was measured. The experimental results were corrected for buoyancy effects related to the displacement of gas by the sample, sample holder and pan [32].

3.

Results and discussion

3.1.

Sample characterization

Fig. 1a shows the N2 adsorption/desorption isotherms at 77 K for the three studied samples (AC1, Max3 and MOF-5) together with a MOF-5 isotherm taken from the literature [35]. The Fig. 1a shows the shapes and type of isotherms (type I of the IUPAC classification). From it, it can be concluded that all the samples have quite comparable adsorption capacity and are essentially microporous. However they present some differences: 1) MOF-5 adsorbs more N2 than the ACs at low relative pressures and less at high relative pressures than both ACs and 2) the isotherms of the two ACs present much wider knee than the MOF-5 isotherm. These two observations indicate that MOF-5 has a lower but more homogeneous microporous network system than the two ACs used. Fig. 1b presents the sub-atmospheric CO2 adsorption isotherms at 273 K of the three selected samples. It includes a MOF-5 isotherm redrawn from the very scarce literature reporting such type of data [36]. Because sub-atmospheric CO2 adsorption at 273 K is related with narrow microporosity (i.e., pore width < 0.7 nm [37,38]), it can be stated that: i) the three materials have narrow micropores and ii) the amount of narrow microporosity is lower for MOF-5 than for the two ACs.

Sample Max3 AC-1 MOF-5

b

5

800

4

Max3 AC-1 MOF-5 MOF-5 [*]

400 200 0 0.0

0.2

0.4

0.6 P/Po

0.8

1.0

VDR(N2) (cc/g)

VDR(CO2) (cc/g)

3180 3120 2800

1.31 1.25 1.13

0.70 0.72 0.44

6

1000

600

SBET (m2/g)

Table 1 summarizes the data obtained from the analysis of the N2 and CO2 adsorption isotherms (BET and DR equations). It can be seen that the three adsorbents studied have comparable N2 adsorption capacities per unit of weight and, hence, BET surface area and micropore volumes. However, MOF-5 (having BET surface area in agreement with data published by other authors [17,23,28,39,40]) has much lower CO2 adsorption capacity than the ACs, as commented above. At a first glance, according to the shape of the low relative pressure range of the N2 adsorption isotherm (Fig. 1a) one would expect that the VDR(CO2) value of Table 2 for MOF-5 were even higher than those for the ACs. To confirm such low VDR(CO2) value (that will affect its storage capacity) a N2 adsorption isotherm at 77 K, covering the same low relative pressure range as CO2 (i.e. 103 to 105) was carried out (Micromeritics; ASAP 2020) as shown in Fig. 2. MOF-5 has an unusual sigmoidal shape which is not observed on the ACs. Such behaviour seems to be related to the lower VDR(CO2) of MOF-5 in relation to both ACs. Following with samples characterization, Table 2 summarizes the different sample densities measured and used in this study. As expected, the density differs according to the experimental procedure used to assess it. It increases in the following sequence: tap density < packing density < crystal density < true density. Consequently the type of density used to convert the gravimetric results to volumetric ones, will affect very much, and presumably in an unrealistic manner, the volumetric storage results. This highlights the importance of reporting the density of the material and the need to well describe how it has been measured. In relation to the MOF-5 used in this study, it has to be pointed out that the obtained densities of Table 2 agree very well with reported data: a tap density 0.3 g/cm3 by Mueller et al. [28] and a crystal density 0.59 g/cm3 by Wong-Foy et al. [17].

mmol/g

V(N2) (cc/g)

a 1200

Table 1 e Porous texture characterization results for the selected materials.

Max3 AC-1 MOF-5 MOF-5 [*]

3 2 1 0 0.000

0.005

0.010

0.015 P/Po

0.020

0.025

0.030

Fig. 1 e (a) N2 adsorption/desorption isotherms at 77 K for the three selected samples and a MOF-5 [*] redrawn from published work [35] and (b) Sub-atmospheric CO2 adsorption isotherms at 273 K for the three selected samples and a MOF-5 [*] redrawn from published work [36].

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Table 2 e Values for the different densities of the samples studied. Sample

True density (g/cm3)

Tap density (g/cm3)

Packing density (g/cm3)

Crystal density (g/cm3)

2.1 2.1 2.0

0.36 0.38 0.30

0.41 0.50 0.57

0.48 0.52 0.58

Max3 AC-1 MOF-5

Tap and packing densities include the particle volume (i.e. the volume occupied by atoms and the internal pore volume of the material), and the inter-particle space volume. The difference between them is that in the second case, by applying pressure, the particles are forced to settle down and, therefore, the inter-particle space is reduced, increasing the density. The higher the packing pressure used the higher will be resulting packing density [23,41] (more data and discussion will be presented later on, after analyzing hydrogen storage). Crystal density of the materials includes the volume occupied by the atoms and the internal pore volume but it does not include the inter-particle space [17,23,42]. Hence, it assumes that the adsorbent is a monocrystal. Helium or true density, having no sense to be used for expressing volumetric storage capacity, is only used for calculation of the crystal density.

example that confirms the relevant contribution that the narrow microporosity has on the adsorption of H2 at these conditions of low pressures and temperature [6,24]. For gas storage applications we have highlighted the importance of expressing the amount of hydrogen adsorbed per unit of volume. We have also pointed out the importance of the density and the need of assessing the density in a similar way to carry out a comparison without misunderstandings. Fig. 3b plots the above results but now expressed on a volumetric basis using, for the three samples, the same type of density (tap) that is obtained similarly. Of course, such comparison carried out mixing different types of density will give different, and wrong, results. The Figure shows, without doubt, that for low pressure H2 adsorption at 77 K both ACs behave much better than MOF-5.

3.2.

3.2.2. 77 K

Hydrogen adsorption

3.2.1. H2 adsorption at sub-atmospheric pressures (<0.1 MPa) and 77 K Fig. 3a plots, on a gravimetric basis, the H2 adsorption/ desorption isotherms obtained for the three studied adsorbents at 77 K and sub-atmospheric pressures. The figure also includes a MOF-5 isotherm taken from the literature [43]. It can be observed that: (i) the adsorption capacities of both MOF-5 are identical, (ii) all these hydrogen adsorption isotherms are clearly of Type I and (iii) all are completely reversible. However, comparisons of H2 adsorption capacities reveal substantial differences between the two ACs and both MOF-5. The amount of hydrogen adsorbed changes between 1.3 wt% (MOF-5 samples) to 3.3 wt% (ACs samples). These differences are consistent with the previous comments about the low narrow micropore volume of MOF-5, and are the first

700 600

V(N2) (cc/g)

500 400 300 200

Max3 AC-1 MOF-5

100 0 0.000

0.005

0.010 P/Po

0.015

0.020

Fig. 2 e N2 adsorption isotherms at 77 K for the three selected materials at low relative pressures.

H2 adsorption at moderate pressures (<4 MPa) and

Fig. 4a shows the excess H2 adsorption isotherms at 77 K (up to 4 MPa) for these three samples, including the isotherm of MOF-5 (IRMOF-1) reported by Wong-Foy et al [17]. Two points should be remarked from this Figure: (i) the H2 uptakes expressed on a gravimetric basis (wt%) for the two ACs and MOF-5 are quite similar. This confirms, independently of the type of adsorbent used (ACs or MOF), that the apparent BET surface area and/or the micropore volume control the H2 uptake at 77 K and at high pressures [8,9,24,44e46] and (ii) it is concluded that both MOF (IRMOF-1 from reference [17] and MOF-5 used in this work) are very similar because their H2 adsorption isotherms are very close. For the purpose of this work, to point out such similarity of both MOF-5 samples is important. H2 adsorption isotherms on volumetric basis have been plotted in Fig. 4b by using the tap density of the sorbents. These isotherms have been plotted together with the isotherm reported by Wong-Foy et al [17] calculated using the crystal density of MOF-5. Firstly, contrarily to the similar results observed in Fig. 4a, the volumetric results obtained using the tap density for the three samples are quite different. Their performances increase as follows: MOF-5 < Max3 < AC1 because of their different inherent densities (Table 2). Secondly, the data for the MOF-5, obtained from the literature, has almost twice volumetric uptake of H2 than the MOF-5 used in this study. This is so because to convert their similar gravimetric uptakes to volumetric ones, we use the tap density (0.30 g/cm3) whereas for the reported MOF-5 a density two times higher is used (crystal density of 0.59 g/cm3 [17]). This is another example, showing the significant differences that can exist using different types of densities and, hence, the wrong conclusion that can be extracted comparing adsorbents. These results highlight that much more work is needed before being able to get a conclusive statement about

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a

3.0

14 12

2.5

H adsorbed (g/L)

H adsorbed (wt%)

b

Max3 AC-1 MOF-5 MOF-5 [*]

3.5

2.0 1.5 1.0 0.5

Max3 AC-1 MOF-5

10 8 6 4 2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

0 0.0

1.2

0.2

0.4

P (bar)

0.6

0.8

1.0

1.2

P (bar)

Fig. 3 e (a) H2 adsorption isotherms at sub-atmospheric pressure on the selected adsorbents at 77K in gravimetric basis (wt%) and a MOF-5 [*] redrawn from published work [43] and (b) H2 adsorption isotherms at sub-atmospheric pressure at 77 K on volumetric basis using the tap density of the adsorbents (g/L).

adsorbent with the best performance for gas storage applications. This is especially relevant for MOFs in which unrealistically high adsorption values are obtained using crystal density, as it will be shown next.

3.3.

H2, CO2 and CH4 adsorption at room temperature

Nowadays, the room temperature storage under pressure of H2, CH4 (both alternative fuels to conventional gasoline and diesel) and CO2 (storage and transport for short-term CO2 storage for CCS technologies) is of a great interest and two families of adsorbents give the best results; ACs and MOFs. Hence, in the following we comparatively analyze these two types of adsorbents for storing H2, CH4 and CO2 at RT. Before such analysis, it has to be remembered that their storage take place by a high pressure adsorption process. Hence, both the properties of the adsorbents (mainly surface area, micropore volume and narrow micropore volume) and the “relative pressure” reached (depending on the physical properties of the gas analyzed) will control the storage capacity of these adsorbents. Under the experimental conditions used in this study, hydrogen and methane are adsorbed as supercritical gases but carbon dioxide is not. Hence, the concept of relative pressure (P/Po, being Po the saturation

3.3.1.

H2 adsorption at high pressure (<20 MPa)

The excess H2 adsorption isotherms, expressed on gravimetric basis, and measured at 298 K and up to 20 MPa, are shown in Fig. 5a. The amount of hydrogen adsorbed on gravimetric basis increases in the following order MOF-5 < Max3 < AC-1, being 0.86, 1.07 and 1.17 wt%, respectively. The Figure also

6

b 35

5

30 25

4

Uptake (gH 2 /l)

H2 adsorbed (wt%)

a

pressure) can only be used for CO2 but not for H2 and CH4. For them, a “relative pressure” calculated as “P/Pcs” being Pcs the saturation pressure calculated using the critical pressure and the empirical equation proposed by Dubinin [32,47] has to be used. The “P/Pcs” value reached with the different gases, pressures and temperatures have to be considered when comparing results. H2 at 77 K has a Pcs value of 7 MPa, hence the results above discussed, going from sub-atmospheric pressure to 4 MPa, cover a relative “P/Pcs” range from 0.014 (at 0.1 MPa) to 0.57 (at 4 MPa). At RT its Pcs is 106 MPa and hence at the pressure analyzed (20 MPa) its “P/Pcs” is <0.19. In the case of CH4 at RT, its Pcs at RT is 11.25 MPa and the pressure analyzed is up to 3 MPa, hence its “P/Pcs” is <0.27 at 3 MPa. Finally, for CO2 at RT, its Po is 6.41 MPa and the pressure analyzed is up to 4 MPa, hence, its P/Po is <0.6, which is slightly higher than 0.57 for H2 at 77 K and 4 MPa.

3 Max3 AC-1 MOF-5 UA MOF-5 [*]

2 1

20 15 Max3 AC-1 MOF-5 MOF-5 [*]

10 5 0

0 0

1

2 P (MPa)

3

4

0

1

2

3

4

P (MPa)

Fig. 4 e (a) Excess adsorption isotherms for hydrogen at 77 K on the different materials up to 4 MPa in gravimetric basis (wt%) and on a MOF-5 [*] redrawn from published work [17] and (b) H2 adsorption isotherms for the selected samples measured at 77 K in volumetric units (g/L) and for a MOF-5 [*] redrawn from published work [17].

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a

b Max3 AC-1 MOF-5 MOF-5 [*]

H2 adsorbed (wt%)

1.0

5 Max3 AC-1 MOF-5 MOF-5 [*]

4 H2 adsorption (g/L)

1.2

0.8 0.6 0.4 0.2

3 2 1 0

0.0 0

5

10

15

0

20

5

10 P (MPa)

P (MPa)

15

20

Fig. 5 e Excess adsorption isotherms for hydrogen at 298 K on the different materials studied and a MOF-5 [*] redrawn from published work [39], (a) on gravimetric basis (wt%) and (b) on volumetric basis (g/L) and a MOF-5 [*] redrawn from published work [39] using its crystal density.

presents an adsorption isotherm previously published in the literature [39]. Similarly to what happens in Fig. 4, the amounts of H2 adsorbed by both MOF-5 agree very well. The lower capacity of both MOF-5 in relation to the ACs can again be explained considering its lower VDR(CO2). Additionally, this lower performance should be expected, in conformity with reported data obtained under similar H2 storage conditions [24] where it was shown that the narrow micropore volume (<0.7 nm) controls the H2 storage at room temperature and a pressures lower than 20 MPa [6,24,48,49] which is also in agreement with the reported value for the optimal pore size for hydrogen storage (about 0.6e0.7 nm [48,50,51]). Fig. 5b presents the excess adsorption calculated on a volumetric basis using the tap density of Table 2. It also includes the volumetric adsorption isotherm of a MOF-5 redrawn from Kaye et al. [39] using its crystal density. It can be seen that their performances maintain the same order; MOF-5 < Max3 < AC1 which is slightly increased due to the effect of their different inherent tap densities. When crystal density is used, the resulting volumetric capacity is apparently twice, in relation to the use of tap density (compare both MOF-5 of Fig. 5b).

3.3.3. Carbon dioxide adsorption at moderate pressures (<4 MPa) Fig. 7a plots the CO2 adsorption of the three samples studied and it includes a MOF-5 extracted from the literature [27], for comparative purposes. It can be seen that: (i) both ACs have higher adsorption than both MOF-5, (ii) on a gravimetric basis, the MOF-5 used in this study is quite comparable to the one

b

90

14

80

12

70

10 8 Max3 AC-1 MOF-5 [*]

6 4 2 0 0.0

Methane at moderate pressures (<3 MPa)

Fig. 6 presents the methane isotherms for the two activated carbon samples measured up to 3 MPa and a MOF-5 redrawn from [52], expressed on a gravimetric basis (Fig. 6a) and expressed on a volumetric basis (Fig. 6b), using the tap density of Table 2. The Fig. 6b also includes the methane isotherm on MOF-5 on a volumetric basis using its crystal density. Fig. 6(a and b) allows to see that ACs can store higher amounts of CH4 than MOF-5, as it also happens for H2 storage at RT up to 20 MPa. The low VDR(CO2) of MOF-5, in relation to the ACs, can explain its lower performance to store H2 and CH4 at RT because of the low “P/Pcs” reached in both adsorbates. When the crystal density is used (Fig. 6b) MOF-5 becomes the material with the best artificial performance.

CH4 adsorbed (g/L)

CH4 adsorbed (wt%)

a 16

3.3.2.

60

Max3 AC-1 MOF-5 (a) MOF-5 (b)

50 40 30 20 10

0.5

1.0

1.5 P (MPa)

2.0

2.5

3.0

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

P (MPa)

Fig. 6 e CH4 adsorption isotherms at 298 K for the three adsorbents (the MOF-5 [*] is redrawn from published work [52]); (a) on gravimetric basis (wt%) and (b) on a volumetric basis using theirs tap densities for AC samples and MOF-5 (a) and using its crystal density for sample MOF-5 (b).

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Max3 AC-1 MOF-5 MOF-5 [*]

120 CO uptake (wt%)

b

140

100

Max3 AC-1 MOF-5 MOF-5 [*]

600 500 CO uptake (g/L)

a

80 60 40 20

400 300 200 100

0 0

10

20 P (bar)

30

40

0 0

10

20 P (bar)

30

40

Fig. 7 e CO2 adsorption isotherms at 298 K for the three adsorbents. (a) on a gravimetric basis (wt%); MOF-5 [*] redrawn from [27] and (b) on a volumetric basis; MOF-5 [*] redrawn from [27] by using crystal density.

from the literature, and (iii) MOF-5 shows an unusual S-shaped isotherm not found for both ACs. Fig. 7b plots the CO2 adsorption isotherms per litre of adsorbent, using the tap density (Table 2). The same conclusion can be extractewhen the crystal density is used, MOF-5 an artificial remarkable increase in its volumetric uptake is observed, becoming now the material with the best performance. CO2 storage at RT up to 4 MPa, adsorbed as a subcritical gas, reaches a relative pressure (P/Po) of 0.63 which means that a considerable porosity of the adsorbent is used. Hence, from Fig. 7, it can be noted: i) the adsorbent porosity development, controls CO2 adsorption capacity per unit of weight of adsorbent and per unit of volume; ii) in the latter case, the porosity is not the only parameter that affects adsorption, since adsorbent density is also important, and it increases with material density, iii) using tap density, the three adsorbents behave as expected according to their porosity, MOF5 < Max3 < AC1, and iv) the highest adsorption capacity of MOFs compared to ACs, claimed by different authors, is only related to the use of a higher density value (crystal density). In summary, the results obtained at RT point out the importance of preparing adsorbents having a good balance between porosity development and density to maximize their volumetric storage capacity.

3.4. Volumetric gas adsorption capacities using different packing densities Up to this moment, volumetric hydrogen storage has been presented and discussed by using only the tap density. In this

section, the effect of increasing the packing pressure will be analyzed on the commercial samples Max3 and MOF-5. As mentioned above, by applying pressure, the particles of the adsorbent are forced to settle down and, therefore, the interparticle space is reduced. The higher the packing pressure used, the higher will be the resulting packing density and its corresponding gas storage value. Hence, up to a certain limit, this is a way to increase the filling of a given thank volume with the adsorbent. Table 3 summarizes the packing densities for the samples Max3 and MOF-5 at different applied pressures. As it is expected, the packing density of the samples increases with the applied pressure. As it can be observed, the evolution of the packing density with pressure is different for both samples. The packing density of Max 3 increases slowly with pressure (from 0.41 to 0.47 g/cm3) tending to reach its theoretical crystal density (0.48 g/cm3) at the maximum pressure studied. Contrarily, MOF-5 is more easily compacted than the activated carbon, its packing density increases with pressure (sharply up to 830 kg/ cm2 and slower above), reaching its crystal density value (0.57 g/cm3) at pressure of 415 kg/cm2. However, at the highest pressure used, an unreasonable high value (1.48 g/cm3) is reached, even higher that its crystal density (maximum theoretical value), which indicates a possible structural change for MOF-5. When a sample is subjected to high axial pressure, it is first necessary to check that its structure can support such pressure and that its porous texture does not change. Therefore, to check it, both adsorbents have been analyzed by N2 adsorption at 77 K and their adsorption isotherms are plotted in Fig. 8.

Table 3 e Packing density of the adsorbents as a function of the pressures. Sample

Pressure (kg/cm2)

Density (g/cm3)

Sample

Pressure (kg/cm2)

Density (g/cm3)

Max3-tap Max3-P1 Max3-P2 Max3-P3 Max3-P4

0 415 830 1452 2075

0.36 0.41 0.42 0.46 0.47

MOF5-tap MOF5-P1 MOF5-P2 MOF5-P3 MOF5-P4

0 415 830 1452 2075

0.30 0.57 1.17 1.31 1.48

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a

25

600 400 200

0.2

0.4

0.6

0.8

1.0

P/Po

15 6 5 4 3 2 Max3

b 800

V(N2) (cc/g)

20

298K 20 MPa

Max3 Max3-P1 Max3-P2 Max3-P3 Max3-P4

800

H2 uptake (g/L)

V(N2) (cc/g)

1000

0 0.0

77K 4 MPa

30

1200

600

MOF5 MOF5-P1 MOF5-P2 MOF5-P3 MOF5-P4

400

AC-1

MOF-5

Fig. 9 e Excess volumetric uptake of hydrogen at 77 K and 4 MPa (empty symbols) and at 298 K and 20 MPa (full symbols) calculated from the gravimetric data using different densities: (squares) tap density; (circles) packing density at 415 kg/cm2 and (triangles) crystal density.

200

0.0

0.2

0.4

0.6

0.8

1.0

P/Po

Fig. 8 e Effect of the applied pressure on N2 adsorption isotherms. (a) Max3 and (b) MOF-5.

The isotherms of Max3 (Fig. 8a) remain practically unchanged by increasing the packing pressure which indicates that its porosity is not significantly modified (the surface area decreases only from 3180 m2/g (fresh sample) to 3000 m2/ g (sample subjected to a packing pressure of 2075 kg/cm2). Contrarily, the isotherms of MOF-5 (Fig. 8b) are strongly affected by the applied pressure; N2 adsorption decreases drastically above 415 kg/m2 and hence its apparent BET surface area (from 2800 m2/g to only 85 m2/g) proving that its texture is completely damaged (collapsed). Similar results were recently reported [41]. The effect of the pressure on its porosity depends on the pressure range applied; at pressures lower than 415 kg/cm2, the inter-particle space decreases, hence the packing density approaches the crystal density and the porosity remains almost unchanged (see in Fig. 8b) at higher pressures, the pore volume noticeably decreases (collapses). From the obtained results, it can be concluded that, for both materials, it is possible to increase their density by decreasing the inter-particle space and consequently their volumetric gas storage. However, in the case of MOF-5, the packing pressure used has to be limited to 415 kg/m2 because, above that pressure, there are irreversible textural damages. By using the tap density or the packing density, measured at pressure conditions in which the structure of the material does not collapses, the volumetric amount of gas adsorbed as a function of the density used has been calculated for H2 in Fig. 9 and for CO2 in Fig. 10. From the values shown in these figures, we can conclude that, for all conditions and gases studied here, the amount

650 600 550 CO2 uptake (g/L)

0

adsorbed in volumetric basis strongly depends on the density of the material used for the calculation and it increases with decreasing the inter-particle space. Therefore the amount of gas adsorbed in volumetric basis increases in the order: tap density < packing density < crystal density. The amount adsorbed tends to a theoretical maximum value for each material which is obtained when its crystal density is used for the calculation. Of course, this theoretical maximum can never be achieved in a real application because it is impossible to reduce to zero the inter-particle space without modifying or destroying the structure of the material. Thus, the crystal density of the material should not be used for the calculation of the amount adsorbed in volumetric basis. The actual amount of gas adsorbed in volumetric basis should be between that obtained with the tap density and that obtained with the high packing density that can be measured at the highest pressure that can withstand the material. Therefore, it is very important to indicate the experimental

500 450 400 350 300 250 50 0 Max3

AC-1

MOF-5

Fig. 10 e Absolute volumetric uptake of CO2 at 298 K and 4 MPa calculated from the gravimetric data using different densities: (squares) tap density; (circles) packing density at 415 kg/cm2 and (triangles) crystal density.

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conditions at which these densities are measured. Thus, MOF5 presents lower adsorption capacities on volumetric basis than ACs when the tap density is used. These differences between MOF and ACs can be explained considering the lower tap density value of MOF-5 compared with those of the ACs. When packing density is used, although the values reached by the MOF-5 and AC-1 are very close, AC-1 has higher adsorption capacities than the MOF-5 for the gases and adsorption temperatures studied.

3.5.

ne ¼ na  rgas Vad

Samples stability

For gas store applications, the adsorbent used has to have a suitable mechanical and thermal stability as well as an appropriate chemical inertness. These characteristics, well performed by most of the members of the porous carbon family, present some constrains in MOFs family, related to the synthesis step, to its handling (air exposure) and to the presence of humidity [39] that would require special dry conditions or pre-dried gas streams. In order to analyze the stability of the selected materials, two samples (AC-1 and MOF-5) were exposed to an atmosphere of air with a 70% relative humidity (RH) for 72 h. Their textural properties were analyzed before and after the exposure using N2 adsorption at 77 K as shown in Fig. 11a. It can be seen that the exposure of AC1 to humidity has no effect on its porosity. However, in the case of MOF-5, humidity causes total destruction of its porosity. Similar results were published for other MOF, such as MOF-177 [53], despite its very interesting adsorption capacity. Knowing the negative effect of humidity, the MOF-5 received was kept in a closed bottle and handled with care trying to avoid as much as possible the exposure to air. However, after a period of two years, an important structural change was observed based on a noticeable change on its textural porosity, loosing most of its adsorption capacity, as it is shown in Fig. 11b, and hence its capacity to store gases.

3.6.

the material plus the compressed gas in the space not occupied by atoms of the material, including the inter-particle space and the porosity of the sample. As it is known, the isotherm obtained experimentally is the excess adsorption isotherm or Gibbs isotherm, i.e. the amount of adsorbed gas which density is higher than the density of the gas at the same pressure and temperature. The relationship between the excess, ne and the absolute adsorption amount, na, is given by the following equation:

Total storage capacities

The parameter of greatest interest from a practical point of view is the total amount of gas stored, since this is the amount of gas that we can load in a tank filled with an adsorbent. This parameter includes the amount of gas adsorbed in the pores of

a

(1)

Where, rgas is the gas density and Vad is the volume of the adsorbed phase. Both ne and rgas are experimentally measurable parameters, while na and Vad cannot be directly measured. To obtain the total amount of gas stored in a tank filled with an adsorbent it is not necessary to calculate the absolute isotherm. If we consider the definition of excess isotherm or Gibbs isotherm (Eq. (1)), which defines the relationship between the excess and absolute adsorption isotherm, we see that the amount of gas that cannot be measured experimentally is one that is in the pores of the sample and has a density equal to the gas that occupies the free space of the adsorption cell (i.e. rgas). Therefore, to obtain the total amount of gas stored in a tank filled with an adsorbent, in addition to the excess adsorption isotherm, we only need to know the space occupied by the skeleton of the material and the amount of material that can be loaded in the tank. The volume occupied by the skeleton of the material (VS) can be measured experimentally by helium pycnometry (Eq. (2)): Vs ¼

1 rHe

(2)

and the amount of material (W ) that can be loaded in the tank can be obtained from its tap density or its packing density (both densities are the mass per unit volume): W ¼ Vtank $ra

(3)

where, Vtank is the tank volume and ra is tap or packing density. Therefore, the free volume or volume not occupied by the skeleton of the material in a tank is:   r Vf ¼ Vtank 1  a rHe

(4)

b 1000

800

600 AC-1 AC-1-H O MOF-5 MOF-5-H O

400 200 0 0.0

0.2

0.4

0.6 P/Po

0.8

1.0

V(cc/g) STP

V(N ) (cc/g)

800 600

original after 2 years

400

200

0 0.0

0.2

0.4

0.6 P/Po

0.8

1.0

Fig. 11 e (a) N2 adsorption/desorption isotherms at 77 K on AC-1 and MOF-5 samples before and after exposing them to air with a 70% RH for 72 h and (b) after two year of the sample reception maintained in a closed bottle.

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MOF-5. In addition, the storage of CO2 at 298 K and H2 at 77 K both up to 4 MPa by adsorption on porous solids achieves high values, demonstrating the feasibility of this method.

4.

Conclusions

From the comparative work carried out with two activated carbons and MOF-5 for storing gases (H2, CH4 and CO2) at different temperatures (77 K and RT) and pressures (from 0.1 MPa to 20 MPa), the following conclusions have been obtained:

Fig. 12 e Total gas stored capacities for Max3, AC-1 and MOF-5 calculated with Eq. (5) by using their packing densities at 415 kg/cm2.

Finally, the total amount of gas stored (nS) in a tank of 1 L is:   r nS ¼ ne þ rgas 1  a rHe

(5)

where, nS and ne are on volumetric basis. With this simple equation and using only experimentally measurable parameter the total amount of gas stored by adsorption can be obtained both at subcritical and supercritical conditions. Using Eq. (5), the total amounts of H2 both at 77 and 298 K and CO2 at 298 K stored by the materials studied in this work, have been obtained (Fig. 10). In this case we have used the packing density at 415 kg/cm2 to calculate the excess adsorbed amount on volumetric basis and to obtain the volume not occupied by the skeleton of the material (Eq. (4)). In the case of CO2 at 298 K and 4 MPa, it can be clearly seen in Fig. 12, that a tank filled with the activated carbons studied in this work (Max3 or AC-1) would store more CO2 than the same tank filled with MOF-5 (about 40 g/L more). Compared to the amount of CO2 stored only by compression (CO2 density at 298 K and 4 MPa is 93.46 g/L) in an empty tank, more than 6 times of CO2 can be stored in a tank filled with Max3 or AC-1. In the case of H2 storage, sample AC-1 reaches higher values than MOF-5 at both temperatures. At room temperature, in a tank of 1 L filled with AC-1, 18.6 g of H2 would be stored. This is 2.4 g more than the amount of hydrogen stored by compression (H2 density at 298 K and 20 MPa is 14.5 g/L). On the other hand, at 77 K and 4 MPa sample AC-1 can store 40.4 g of H2/L. This value is three times higher than that obtained by compression (H2 density at 77 K and 4 MPa is 13.2 g/L). From these results, we can conclude that the activated carbons studied here have a higher storage capacity than

 Independently of the type of adsorbents used, gas storages (sub-atmospheric and high pressure) occur via a physisorption process which is controlled by both the adsorbents characteristics (porosity and density), by the nature of the gas and the experimental conditions used to store them.  The lower is the relative pressure achieved (P/Pcs or P/Po), the more important will be the contribution of the narrow microporosity (<0.7 nm) of the samples and this narrow microporosity is higher for both ACs than for MOF-5.  At higher relative pressures, most of the sample porosity will be used for the storage; hence the micropore volume or the sample surface area will control its extension, as it happens for example with H2 at 77 K up to 4 MPa. Because the three samples have comparable surface area, their capacities to store hydrogen are similar, especially if they are expressed per unit of weight.  When gravimetric results are converted to volumetric ones the inherent density of the adsorbents (higher for both ACs than for MOF-5) will give higher storing capacity results for both CAs than for MOF-5.  Because, the higher the density is, the higher will be the storage results, there is a need of using similar densities when comparing the storage capacity of adsorbents. Using a density measured similarly in the three samples (e.g. tap density), MOF-5 presents, for all gases and conditions studied lower adsorption capacities in volumetric basis than ACs, due to its lower tap density value and also lower adsorption capacities than AC-1 when packing density is used.  MOF-5 can achieve packing densities higher than their crystal densities but its porous structure is lost. Additionally, this sample shows stability problems in the presence of humidity and with the time.  All samples have high gas store capacities, achieving values higher than the case of the gas compressed in the same tank. For the best sample (AC-1), more than 6 times for CO2 at 298 K and 4 MPa, 3 times for H2 at 77 K and 4 MPa and 1.2 times for H2 at 298 K and 20 MPa. Comparing the three adsorbent, one can conclude that MOF-5 achieves lower store capacities of CO2 and H2 at room temperature than ACs samples and also lower store capacity of H2 at 77 K than AC-1.  The outstanding adsorption capacities on volumetric basis and storage capacities of MOFs in relation to ACs, frequently claimed in the literature, is due to the use of a higher density; the crystal density which does not include the inter-particle space of the adsorbents.

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Acknowledgements The authors thank BASF- the Chemical Company- and Kansai Coke & Chemicals for supplying MOF-5 and Maxsorb3000, respectively. The authors thank Generalitat Valenciana (PROMETEO/2009/047) and FEDER and the Spanish Ministerio de Ciencia e Innovacio´n (MICINN) and PLAN E funds (Projects CTQ2009-10813/PPQ) for financial support.

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