H2 in MIL-101s by molecular simulation study

Chemical Engineering Science 98 (2013) 246–254

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Adsorption and separation of CH4/H2 in MIL-101s by molecular simulation study Defei Liu a,b, Y.S. Lin b, Zhong Li a, Hongxia Xi a,n a b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe 85287, AZ, USA

H I G H L I G H T S







A new MTN structure: MIL-101_R7-BDC was constructed and characterized. CH4/H2 adsorption behavior and separation selectivity on MIL-101, MIL-101_R7-BDC and MIL-101_NDC were studied by Monte Carlo simulation. Adsorption amount of CH4 and H2 on MIL-101_NDC is higher than that on the other two MIL-101s at 2981 K. MIL-101_R7-BDC had the highest CH4 selectivity due to the high Δqst (adsorption heat) and low Sacc (accessible surface area)which are affected by organic ligands.

art ic l e i nf o

a b s t r a c t

Article history: Received 18 February 2013 Received in revised form 16 April 2013 Accepted 4 May 2013 Available online 28 May 2013

MIL-101s offer potential application for CH4/H2 adsorption separation owing to its high porosity and excellent chemical stability. In this work, the grand canonical Monte Carlo method was employed to study the adsorption and separation of CH4/H2 on a novel series of MIL-101s materials, such as MIL-101, MIL-101_NDC and new constructed MIL-101_R7-BDC. The simulation results showed that CH4 was preferentially adsorbed over H2 in all MIL-101s. The adsorption capacities of pure CH4 and H2 on MIL101_NDC were much higher than that in the other two MIL-101s, due to its largest Sacc (accessible surface area) and Vp (pore volume). The adsorption-site results suggested that the super-tetrahedras were the main adsorption site for CH4 molecules in all studied MIL-101s. Moreover, the additional benzene rings in MIL-101_R7-BDC introduced several new CH4 adsorption sites which were close to the edge of large and small cages. Finally, we found that MIL-101_R7-BDC had the highest CH4 selectivity. CH4 selectivity of MIL-101s was significantly affected by Δqst (adsorption heat) and Sacc (accessible surface area) in terms of different organic ligands. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Adsorption Separations Simulation Selectivity MIL-101s CH4/H2

1. Introduction Hydrogen is an alternative energy carrier with potential to improve energy efficiencies and reduce pollutant emissions. In order to produce hydrogen, methane dry reforming and methane steam reforming are the most widely used techniques (Dubbeldam et al., 2009). Prior to the process, hydrogen should be separated from other impurities, such as methane. Pressure swing adsorption (PSA) which is simply operated, low operating costs and with a high degree of H2 purification is generally used to resolve this problem. Hence, large numbers of porous materials have been studied for CH4/H2 adsorption separation, such as activated carbon and zeolite 5A (Ralph, 2003; Yang et al., 2008; Lopes et al., 2011).

n

Corresponding author. Tel.: +86 2087280391. E-mail address: [email protected] (H. Xi).

0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2013.05.009

Metal organic frameworks (MOFs) have been investigated as good candidates for H2 and CH4 adsorption (Furukawa et al., 2010; Garcia-Perez et al., 2009; Kitagawa et al., 2004; Rowsell et al., 2005; Sudik et al., 2005) or CH4/H2 separation (Bux et al., 2009; Guo et al., 2009; Li et al., 2010), due to their well-defined pores, extremely high surface areas, and desired chemical functionalization. In recent years, the combination of different organics and metal units have given rise to thousands of MOF structures (Eddaoudi et al., 2002; Rosi et al., 2003). However, the unknown structure–property relationship of new MOFs impedes its development. Molecular simulations, especially the grand canonical Monte Carlo simulation (GCMC), provide an important way to complement experimental methods for designing and screening. Gallo and Glossman-Mitnik (2009), calculated the adsorption and separation of CH4/H2 in five MOFs (MOF-5, MOF-177, IRMOF-11, MOF-14, and MOF-74) using GCMC simulations, the results showed that the CH4/H2 selectivity values for MOF-74 was as high

D. Liu et al. / Chemical Engineering Science 98 (2013) 246–254

as 25 at lower pressure. Yang and Zhong (2006) found out that CH4/H2 adsorption selectivity was about 20 for Cu-BTC, compared to 5 for MOF-5. Liu et al. (2008), studied the adsorptive separation of CH4/H2 mixtures at room temperature, and the CH4/H2 adsorption selectivity was on the order of 35 for equimolar mixtures of these compounds in the catenated IRMOF-11, while the CH4/H2 adsorption selectivity was about 5 for nonecatenated IRMOF-11. The increase of CH4 selectivity can be attributed to catenated IRMOFs. Guo et al. (2010) investigated the adsorption and separation properties of CH4/H2 in different topology MOFs: ZIF-8,−67, −0, −60, and −3. They found that adsorption selectivity of ZIF-3 was up to 15. Moreover, Sholl and co-workers (Keskin et al., 2008; Keskin and Sholl, 2009; Liu et al., 2011,) used GCMC combined with equilibrium molecular dynamics (EMD) simulations to predict CH4/H2 mixtures in several types of MOF materials, through their calculation, the important properties of the MOF materials, such as adsorption selectivity, diffusion selectivity, and membrane selectivity can be also obtained. To date, there have been many studies about CH4/H2 adsorption and separation on MOFs material. However, almost none of them have exhibited high porosity combined with high thermal and chemical stability. MIL-101s (material from Institute Lavoisier) are a set of MOFs that are produced by different terephthalate ligands and trimeric metal(III) octahedral clusters. Due to the inertness of the trivalent metal ion, they showed high thermal and chemical stability. They also possessed several unprecedented pore features such as the mesoporous zeotype architecture with mesoporous cages, a giant cell volume, huge surface area and large pore volume. MIL-101 and MIL101_NDC are two typical MIL-101s which were constructed with two different terephthalate ligands: a shorter carboxylate acid (H2BDC) ligand and a longer 2, 6-naphthalenedicarboxylic acid (H2NDC) ligand. The huge numbers of benzene rings inside terephthalate ligands represents their great potential ability for CH4/H2 separation (Rosenbach et al., 2008; Salles et al., 2009). Furthermore, compared with MIL-101, MIL-101_NDC has higher surface area, lower crystal density, larger pore volume and more benzene rings inside the structure (Sonnauer et al., 2009). These properties showed that MIL101_NDC should be a much better candidate for CH4/H2 adsorption and separation than MIL-101. However, for these two MIL-101s studies, there are only a few experimental and simulation studies about CH4 and H2 single gas adsorption on MIL-101 (Llewellyn et al., 2008; Latroche et al., 2006; Chen et al., 2010). Moreover, for MIL101_NDC, no experimental and simulation gas data are available in literature, and by virtue of this, the CH4/H2 adsorption and separation mechanisms on these two MIL-101s are still not clear. In this work, GCMC simulation was used to study the adsorption behaviors including adsorption isotherms, heats of adsorption, adsorption-site of MIL-101 and MIL-101_NDC. At the same time, we constructed and characterized a new MTN (mobil-thirty-nine zeolite socony topology) structure: MIL-101_R7-BDC by adding one more benzene ring into each carboxylate ligands of MIL-101. We also carried out the same GCMC simulation as above to MIL-101_R7-BDC for a comparative study. This was followed by another comparative study to investigate the CH4/H2 adsorption selectivity of these three MIL-101s. The objective of this work is (1) to predict the CH4/H2 adsorption capacity and adsorption selective of MIL-101, MIL-101_R7BDC and MIL-101_NDC, and (2) to examine the effects of organic ligands on the CH4/H2 adsorption selectivity for MIL-101s.

frameworks were taken from experimental XRD (X-ray diffraction) data (Ferey et al., 2005; Sonnauer et al., 2009). The structure model of MIL-101_R7-BDC was constructed by molecular simulation techniques using the Visualizer module of Materials Studio Version 5.0 (Accelrys, 2010). We constructed the MIL-101_R7-BDC unit cell using a three-step process based on computational methods. Firstly, the set of fractional coordinates found in MIL101 was used as the initial model, and then the benzene linkers were removed, only retaining only the trimeric units (Cr3(OH) (H2O)2) and carboxylate moiety atoms (CO2−). Secondly, the new organic linkers such as naphthalene rings were inserted in order to construct the MIL-101_R7-BDC crystal cell. Finally, the universal force field (UFF) (Rappe et al., 1992) that is implemented by the Forcite module of Materials Studio 5.0 was used to optimize the crystal structure including energy minimization and unit cell dimensions. The parameters for the optimization were respectively, 1  10−5 kcal mol−1 (energy), 0.0005 kcal mol−1 Å−1 (forces) and 5.0  10−6 Å (displacement). 2.2. Force field To describe the adsorption of CH4 and H2 molecules in the three MIL-101 structures, the following models were used for the pure components. CH4 was modeled as a single-center Lennard– Jones (LJ) molecule. The potential parameters were taken from the TraPPE force field (Martin and Siepmann, 1998). H2 was described as a pseudo atom a method which was developed by Buch (1994). The dispersive interactions of all of the atoms in the MIL-101 structures were taken from the UFF force field (Rappe et al. 1992). The parameters that we used are shown in Table 1. For all the MIL-101 structures, the site-site LJ was used to calculate the interactions between adsorbate molecules and adsorbents. In our simulations, all of the LJ cross interaction parameters were determined by the Lorentz–Berthelot mixing rules: sij ¼

1 ðs þ sj Þ 2 i

ð1Þ

εij ¼

pffiffiffiffiffiffiffi εi εj

ð2Þ

Since the adsorbates (CH4 and H2 treated as single spheres) do not carry any dipole, octupol and the MOF atoms (which carry partial charges) are frozen, the long-range electrostatic contribution is constant and can be omitted. However, van der Waals (VDW) interactions between the frameworks and the adsorbed molecules were treated with a 12–6 Lennard–Jones (LJ) potential: "   6 # sij 12 sij uLJ ¼ 4εij − ð3Þ r ij r ij 2.3. Simulation method The grand-canonical Monte Carlo (GCMC) method at fixed chemical potential μ, volume V, and temperature T (Vandoorn et al., 1994) Table 1 LJ potential parameters for adsorbate–adsorbate and adsorbate-MILs. Atom

s (Å)

ε/kB (K)

Frameworks

MIL_C MIL_O MIL_H MIL_C MIL_F

3.430 3.120 2.570 2.690 2.997

52.84 30.19 22.14 7.550 25.15

Adsorbate

CH4 H2

3.730 2.960

2. Models and Methods 2.1. MIL-101s structure construction In this work, MIL-101, MIL-101_R7-BDC and MIL-101_NDC were studied. The unit cell of the MIL-101 and MIL-101_NDC

247

148.0 34.20

248

D. Liu et al. / Chemical Engineering Science 98 (2013) 246–254

was employed to calculate the pure component CH4 and H2 adsorption isotherms as well as mixtures of CH4/H2 adsorption isotherms. All the calculations were carried out with the code MuSIC (Gupta et al., 2003). Chemical potentials were converted to fugacity with the Peng–Robinson (PR-EOS) equation (Cho et al., 1998). p¼

RT aα − V m −b V m 2 þ 2bV m −b2

ð4Þ

where Vm is molar volume, a, b and α showed in Table 2 are the equation parameters calculated from the critical property parameters of CH4 and H2 (John, 1991). The MIL-101 structures studied in this work were treated as rigid frameworks with atoms frozen at their crystallographic positions during the simulations. It has already been shown that the flexibility of the MOF framework has a negligible influence on the adsorption of gases(Chen et al., 2010), although it does have a significant effect on gas diffusivity(Dubbeldam et al., 2007). In this work, we studied the adsorption behavior of CH4 and H2. Therefore, the treatment using a rigid framework is reasonable. In this work, the simulation box contained one unit cell of each of the MIL-101s studied. Periodic boundary conditions were applied in all three dimensions. The cutoff radius of 20.0 Å for the three MIL-101s was applied to all of the LJ interactions. The number of Monte Carlo moves used in the simulation was 2  107 where the first half of these moves were used for equilibration and the remaining half steps were used for calculating the ensemble averages. For pure CH4 and H2, three types of moves (translation, random insertion, and deletion) were used. For binary mixtures, an additional move named identity-swap was included in order to accelerate the convergence and reduce statistical errors (Fuchs and Cheetham, 2001). The statistical uncertainty was estimated by dividing each run into 20 blocks and calculating the standard deviation from the block averages. The standard deviation was within 7 5% for every simulation. In many cases, the experimental data are usually reported as the excess amount of adsorption Nex, However, the μVT GCMC simulation gives only the absolute number of molecules adsorbed Nab. In this paper, in order to make a comparison with the experimental, the Nab was converted into experimental Nex by N ex ¼ N ab −V P ρ

ð5Þ

where ρ is the density of the bulk gas. It is calculated by using the reciprocal of the Vm that can be obtained from PR-EOS Eq. (4). The VP is the pore volume within the adsorbent for adsorption estimated from the GCMC simulations described by Talu and Myers (2001). The pore volume detected by helium, which is Table 2 Peng–Robinson (PR-EOS) state equation parameters for CH4 and H2 calculation.

CH4 H2

ω

Tc (K)

Pc (MPa)

α

a

b

0.012 −0.219

190.4 33.2

4.599 1.28

0.805 0.906

2491689 272160.9

26.78 16.78

assumed to be a nonadsorbing gas, at low pressure (P) and usually at room temperature (T0) is chosen as the calculating parameters. The pore volume can be calculated from VP ¼

N a N m kT 0 Pmm

ð6Þ

Here k refers to the Boltzmann constant, Na is the Avogadro constant and Nm is the number of He atoms per molar mass mm of the adsorbent. The accessible surface area (Sacc) is the closest approach to the BET surface area determined by experimental methods. We used a simple Monte Carlo simulation program obtained from Duren and Snurr (2004) to obtain the surface area accessible with He molecules. The simulated results of the pore volume (VP) and accessible surface area (Sacc) are showed in Table 3. The isosteric heat of adsorption rather than the adsorption isotherm is usually used to ascertain the adsorption mechanism because the isosteric heat is more sensitive to the changes of adsorption energy. The isosteric heat qst is calculated by   ∂〈ν〉 ð7Þ qst ¼ RT− ∂〈N〉 T where 〈ν〉 is the average potential energy of the adsorbed phase, 〈N〉 is the average number of molecules adsorbed, T is the temperature, and R is the ideal gas constant. The isosteric heats are shown in Table 3. In gas separation processes, a good indication of the efficiency of separation is the adsorption selectivity for different components. The adsorption selectivity for CH4 relative to H2 is defined as SðCH4 =H2 Þ ¼

xCH4 =xH2 yCH4 =yH2

ð8Þ

where x and y are the molar fractions of the adsorbed and bulk phase, respectively.

3. Result and discussion 3.1. Structure of MIL-101s and validation of the force field MIL-101, MIL-101_R7-BDC and MIL-101_NDC have the same primitive MTN topology. The MIL-101s structures were consisted of tetrahedral ST building units and the organic ligands (as shown in Fig. 1). Noted that all of the MIL-101 structures (MIL-101, MIL101_R7-BDC and MIL-101_NDC) have two types of mesoporous cage, larger hexacaidecahedral 51264 cages and smaller dodecahedral 512 cages; these are present in a ratio of 2:1. The smaller dodecahedral 512 cages are formed by 12 pentagonal rings, while the larger hexacaidecahedral 51264 cages are formed by 12 pentagonal rings and 4 hexagonal rings. The guest-free crystal structures of the three MIL-101s are given in Fig. 1, and key parameters are summarized in Table 4. The X-ray diffraction patterns of the MIL-101s were back calculated with the reflex module of Materials Studio Version 5.0 (Keskin et al., 2008). The simulated X-ray diffraction patterns are

Table 3 CH4 and H2 adsorption isosteric heats at 1000 kPa and pore volumes for three MIL-101s. Framework

VP (cm3/g) GCMC

VP (m3/g) Exp

qst(CH4) (kJ/mol)

qst(H2) (kJ/mol)

Sscc (m2/g) GCMC

Sscc (m2/g) Exp

MIL-101 MIL-101_R7-BDC MIL-101_NDC

1.86 1.41 2.64

1.74a

17.43 17.71 17.69

7.62 6.13 7.31

3434.35 2807.37 4193.28

3211a

a b

Llewellyn et al, 2008. Sonnauer et al., 2009.

2.345b

3851b

D. Liu et al. / Chemical Engineering Science 98 (2013) 246–254

249

Fig. 1. Unit cell of (A, a) MIL-101, (B, b) MIL-101_R7-BDC and (C, c) MIL-101_NDC. Color code: Cr, yellow; F, green; C, gray; O, red; H, white. The modified organic ligands are shown for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

shown in Fig. 2. The simulated XRD patterns show that MIL-101 and MIL-101_R7-BDC have similar diffraction peaks; however, the diffraction peaks of MIL-101_NDC were shifted to smaller angles. This is because MIL-101 and MIL-101_R7-BDC have similar unit cell side lengths as well as the same topology (MTN), but for MIL101_NDC, the 2, 6-connected naphthalene ring structure makes the side length of the unit cell larger than in the other two MIL-101s.

To study the adsorption properties of CH4 and H2 in MIL-101, MIL101_R7-BDC, and MIL-101_NDC, the force field must be validated. Simulated adsorption isotherms of CH4 and H2 on MIL-101 were used to compare with available experimental data (Latroche et al., 2006, Llewellyn et al., 2008) and the results are shown in Fig. 3. The results show that the force field based on the LJ parameters taken from this work is reasonable for further study of the MIL-101s.

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Table 4 Structural Properties for MIL-101, MIL-101_R7-BDC and MIL-101_NDC studied in this work.

MIL-101 MIL-101_R7-BDC MIL-101_NDC

a b c

dcage (Å)

Cage

Cell angle (deg)

Unit cell (Å)

ρcrysa (g/cm3)

Porosity

dlarge ¼ 34a dsmall ¼29a dlarge o 34b dsmall o 29b dlarge ¼ 46c dsmall ¼39c

51264, 512a

α¼ β ¼γ ¼ 901a

a ¼b ¼ c¼ 88.198a

0.45

0.83

51264, 512b

α¼ β ¼γ ¼ 901b

a ¼b ¼ c¼ 87.752b

0.56b

0.79

c

0.87

12 4

5 6 ,5

12c

c

α¼ β ¼γ ¼ 901

a ¼b ¼ c¼ 104.126

c

0.33

Taken from Latroche et al. (2006). Obtained from this work. Taken from Llewellyn et al. (2008).

Fig. 2. Simulated single-crystal PXRD of MIL-101, MIL-101_R7-BDC, and MIL101_NDC.

3.2. Pure CH4 and H2 adsorption isotherms. From Table 3, it can be seen that both the pore volumes (Vp) and the accessible surface areas (Sacc) decrease in the order MIL101_NDC 4MIL-101 4MIL-101_R7-BDC. In this work, we cannot find a proper simulation method to measure the pores size of MIL101_R7-BDC, but due to the steric hindrance of the organic ligands, R7-BDC should, in principle, follow the order of pore sizes as: MIL101_NDC 4MIL-101 4MIL-101_R7-BDC. The adsorption capacity of a material is affected by the accessible surface area (Sacc), adsorbent framework density (ρcrys), free volumes (Vfree), and the interactions between adsorbate and adsorbent at low loading (Hou et al., 2000). Thus, a high quality adsorbent should also possess a large accessible surface area, ideal pore topology, low adsorbent framework density, as well as high pore volume and a strong affinity for the adsorbate molecules. Obviously, the CH4 and H2 adsorption isotherms of methane and hydrogen in the MIL-101s meet these requirements. From Fig. 4(a), it is seen that none of the materials studied displayed methane absolute uptakes that reached saturation at 298 K at the pressures studied. Of all the materials studied, MIL101_NDC has the highest methane absolute uptake of 13.38 mmol/g at 35 bar and 298 K, which is reasonable since MIL-101_NDC has the largest accessible surface area and free volumes (Vfree). At a pressure lower than 25 bar, the absolute uptake of MIL-101_R7BDC is larger than that of MIL-101 However, above 25 bar the absolute uptake of MIL-101 is larger. This can be attributed to the fact that at low pressures, for approximate lattice constant materials, a small pore size plays a leading role for CH4 adsorption ability. However, at high pressure, high pore volume and surface area dominated. Fig. 4(b) shows the hydrogen absolute uptakes at

Fig. 3. Comparison of simulated and experimental adsorption isotherms on MIL101 for (a) CH4 (Latroche et al., 2006) and (b) H2 (Llewellyn et al., 2008) at 298 K.

298 K. It can be seen that hydrogen in the pure states has not reached saturation for any of the MIL-101s at the pressures studied. The adsorption capacities for H2 follow the order of decreasing pore volume and accessible surface area as MIL101_NDC 4 MIL-101 4MIL-101_R7-BDC. This indicates that the MIL-101s, which have a higher pore volume and accessible surface, can take more H2. From Eq. (5), we can calculate the bulk gas uptakes of the adsorbent using ρVp (Zhou et al., 2007). The CH4 and H2 bulk gases uptake of MIL-101s materials are also showed in Fig. 4a and b. Fig. 4a show that the absolute uptakes of CH4 are much higher that the CH4 bulk gas uptake. It indicated that the CH4 have strong interaction with MIL-101 materials, the CH4 was adsorbed a lot on the pore surface of MIL-101 materials. However, in Fig. 4b,

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the absolute uptakes of H2 in all MIL-101s are mainly attributed to the bulk gas uptakes. This is due to the relatively weak interactions between MIL-101s and H2 molecules. Moreover, by comparing the bulk gas uptake with the absolute uptake of both CH4 and H2, we found that there is not only simply the gas corresponding to the bulk equation of state in the porous volume but also the adsorption in each MIL-101s. 3.3. Adsorption selectivity for CH4/H2 mixtures The adsorption selectivities of MIL-101s for CH4/H2 mixtures with 50%, 70%, and 90% CH4 in the bulk phase at 298 K are shown in Fig. 5. As the pressure increases, the CH4 adsorption selectivity of all the MIL-101s gradually declined. The selectivity for CH4 in MIL-101 is almost of the same as MIL-101_NDC for all mixture compositions over the entire pressure range. For total pressures up

Fig. 4. Simulated absolute isotherms and gas bulk uptake in MIL-101, MIL-101_R7BDC, and MIL-101_NDC at 298 K for (a) CH4 and (b) H2.

Fig. 6. Differences of isosteric heats(Δqst) between CH4 and H2 over different frameworks for equimolar CH4/H2 mixtures.

Fig. 5. Influence of pressure on CH4 selectivity for three gas compositions: (a) yCH4/yH2 ¼1:1, (b) yCH4/yH2 ¼7:3, and (c) yCH4/yH2 ¼ 9:1.

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to 3.5 MPa, the selectivity of MIL-101_R7-BDC is larger than that of MIL-101 and MIL-101_NDC. This is due to the benzene rings added to the BDC ligands, which gives rise to additional interactions (van der Waals interactions) with organic probe molecules such as hydrocarbons and aromatics which can enhance the CH4 adsorption selectivity (Rosenbach et al., 2008). However, the same behavior is not observed for MIL-101_NDC, because NDC ligands also lead to an increase of pore volume, pore size and Sacc. In this work, Δqst was also calculated to investigate the effect of different organic ligands on CH4 selectivity and the difference between the isosteric heats of adsorption of CH4 and H2. Generally, a large Δqst indicates good CH4 selectivity (Sircar and Cao, 2002). Therefore, the value of Δqst provides a measure of how different organic ligands affect the CH4 selectivity of MIL-101s. Fig. 6 illustrates Δqst over different sorbents for equimolar CH4/H2

mixtures at different pressures. Apart from MIL-101_NDC, the order of Δqst for the frameworks is MIL-101_R7-BDC≈MIL101_NDC 4 MIL-101. The reason for this is for MIL-101_R7-BDC and MIL-101_NDC, the naphthalene structure of the R7-BDC and the 2, 6-NDC organic ligands result in an approaching large Δqst. However, another influencing factor is that the large Sacc of MIL101_NDC also affects the CH4 selectivity. It is observed that selectivities for CH4/H2 mixtures for same topology MIL-101s (MTN) are independent of the different constituent organic ligands. The reason for this might be that the different constituted organic ligands lead to different Δqst and Sacc. The high selectivities for CH4/H2 mixtures are dependent on having a large Δqst and small Sacc. However, the highest selectivity for CH4 with MIL-101_R7-BDC and relatively low selectivity for CH4 with MIL-101_NDC indicates that Sacc is the dominant factor.

Fig. 7. The density distribution contours of CH4 adsorbed at 500 kPa and 2500 kPa on (100) plane of the clusters in (A) MIL-101, (B) MIL-101_R7-BDC, and (C) MIL-101_NDC.

D. Liu et al. / Chemical Engineering Science 98 (2013) 246–254

253

Fig. 8. Equimolar CH4/H2 mixture adsorbed in (I) MIL-101, and (II) MIL-101_R7-BDC at 1000 kPa. Blue dot: CH4; red dot: H2. For better view of the density distribution of gas molecules (a) in the pentagonal window, (b) inside the supertetrahedra, (c) on the edge of large cages, and (d) small cages. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.4. Adsorption sites To identify the favorable CH4 adsorption sites on MIL-101s with different organic ligands, the density distribution contours on the (100) plane of the clusters in MIL-101, MIL-101_R7-BDC and MIL101_NDC at 500 kPa and 2500 kPa, respectively, are shown in Fig. 7. This also shows some of the special features of MIL-101s, namely the pentagonal windows (the gate between 51264 cages and 512 cages), the middle 512 cages, the large 51264 cages and the supertetrahedral cages. The contours are generated by accumulating the center-of mass of CH4 molecules in 1000 equilibrium configurations. At 500 kPa, CH4 in MIL-101, MIL-101_R7-BDC, and MIL-101_NDC is preferentially adsorbed in hybrid supertetrahedras. Because of the strong surface potentials of trimeric chromium octahedral clusters, the microporous supertetrahedra are the most favorable adsorption sites. However, the adsorption in MIL-101_R7-BDC that occurs in the superterahedras is much stronger than in MIL-101 because, at lower pressure, the hindrance of the organic ligand, R7-BDC, reduces the pore size of supertetrahedra, and the smaller pores size also leads to stronger adsorption ability. In MIL-101_R7-BDC, there are new CH4 adsorption sites found on the edge of the large cages and small cages at low pressure due to the improved CH4 adsorption brought about by additional benzene rings. At 2500 kPa, CH4 molecules not only appear in the supertetrahedras but also in the pentagonal window and on the edge of the middle 512 cages and large 51264 cages. At very high pressures, the micropore cages (supertetrahedra) are gradually filled, while more CH4 molecules begin to populate the mesoporous areas (such as the edges of the large cages, small cages and pentagonal windows). At this time, the surface area begins to play a leading role in affecting the adsorption capacity. With respect to the pure CH4 adsorption isotherms of MIL-101, MIL-101_R7-BDC, and MIL101_NDC as shown in Fig. 3a, this phenomenon can also prove that when the pressure up to 2500 kPa, the order of absolute uptakes becomes MIL-101_NDC4MIL-1014MIL-101_R7-BDC. Fig. 8 gives an example of the adsorption of an equimolar CH4/ H2 mixture in MIL-101 and MIL-101_R7-BDC by presenting equilibrium snapshots at 1000 kPa. In MIL-101 and MIL-101_R7-BDC, CH4 favorably occupies sites near (a) the pentagonal window, (b) inside supertetrahedra, (c) on the edge of large cages and (d) within the small cages. However, the density distribution contours of H2 molecules are too low to be observed in either MIL-101 and MIL-101_R7-BDC. We simply observe random adsorption of all of the H2 molecules in both structures. Fig. 8 II shows

that CH4 molecules are more preferably adsorbed on the edge of (c) the large cages, and the (d) the small cages of MIL-101_R7-BDC as compared with those of MIL-101. This is because the constituent organic ligands of MIL-101_R7-BDC not only lead to a smaller pore size and increasing surface area, but also increase the number of benzene rings inside the structure, subsequently evoking stronger adsorption sites for CH4.

4. Conclusions GCMC simulations were used to predict the adsorption and separation of CH4/H2 on MIL-101, MIL-101_NDC and the newly constructed MIL-101_R7-BDC. The simulated pure CH4 and H2 isotherms in MIL-101 agree with experimental data, indicating that the force field taken from this work can describe the adsorption of these gases in MIL-101. The simulated CH4 and H2 adsorption isotherms of MIL-101_NDC showed that the large lattice constants for MIL-101s had good CH4 adsorption ability. For MIL-101s of approximately similar lattice constant, MIL-101 and MIL-101_R7-BDC, the smaller pore sizes favor adsorption of CH4, at lower pressure,. At higher pressure, MIL-101s with higher surface area and larger pore volume adsorbed more CH4. However, H2 isotherms in different MIL-101s followed the order of decreasing pore volume and accessible surface area. At 5 bar, the preferred adsorption sites of all MIL-101s were resided inside supertetrahedra, but the additional benzene rings in MIL-101_R7-BDC provided some new CH4 adsorption sites near the edges of the large and small cages. MIL-101_R7-BDC, which had a higher heat of adsorption (Δqst) and smaller accessible surface area (Sacc), had higher CH4/H2 selectivity. Both Δqst and Sacc were mainly influenced by the additional benzene rings in the organic ligands.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 20936001 and 21176084), the National High Technology Research and Development Program of China (No.2013AA065005), Guangdong Natural Science Foundation (S2011030001366) and the over study program of GuangZhou Elite projet are gratefully acknowledged.

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