CO2 separation

Journal of Membrane Science 454 (2014) 126–132

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Synthesis of highly hydrophobic and permselective metal–organic framework Zn(BDC)(TED)0.5 membranes for H2/CO2 separation Aisheng Huang a,n, Yifei Chen b, Qian Liu a, Nanyi Wang c, Jianwen Jiang b, Jürgen Caro c a

Institute of New Energy Technology, Ningbo Institute of Material Technology and Engineering, CAS, 519 Zhuangshi Road, 315201 Ningbo, PR China Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576 Singapore, Singapore c Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstraße 3-3A, D-30167 Hannover, Germany b

art ic l e i nf o

a b s t r a c t

Article history: Received 14 July 2013 Received in revised form 3 December 2013 Accepted 6 December 2013 Available online 16 December 2013

Metal–organic frameworks (MOFs), as a newly developed family of crystalline microporous materials, have attracted considerable attention as promising candidates for the fabrication of superior molecular sieve membranes. For the synthesis of MOF membranes, the MOF's characteristics, such as pore size, stability, and adsorption affinity, have to be considered carefully. In this work, we prepare a highly hydrophobic and permselective Zn(BDC)(TED)0.5 membrane for H2/CO2 separation through secondary growth method with seeding. Attributed to the preferential adsorption affinity and capacity to CO2 as well as a highly porous structure with large channels, the Zn(BDC)(TED)0.5 membrane displays a high H2/ CO2 permselectivity. For the separation of an equimolar H2/CO2 mixture at 180 1C and 1 bar, a H2 permeance of 2.65
10  6 mol m  2 s  1 Pa  1 and a H2/CO2 selectivity of 12.1 are obtained. Furthermore, because of its high hydrophobicity, the pore volume of Zn(BDC)(TED)0.5 is not blocked in the presence of steam, which is promising for potential applications of hydrogen separation and purification. & 2013 Elsevier B.V. All rights reserved.

Keywords: Molecular sieve membrane Metal–organic frameworks Zn(BDC)(TED)0.5 membrane Simulation study H2/CO2 separation

1. Introduction Microporous metal–organic frameworks (MOFs) have attracted much attention in the past decade due to their potential applications in gas adsorption and storage, drug delivery, and catalysis [1–8]. Furthermore, supported MOF layers are of interest for the fabrication of molecular sieve membranes due to their highly diversified pore structures and pore sizes as well as specific adsorption affinities. From the past 5 years, a great deal of research effort has been expended on the preparation of supported MOF membranes [9–28]. In particular, zeolitic imidazolate frameworks (ZIFs), a subfamily of MOFs based on transition metal ions and imidazolate linkers, have emerged as a novel type of crystalline porous material for the fabrication of molecular sieve membranes due to their zeolite-like properties such as permanent porosity, uniform pore size, and exceptional thermal and chemical stability [29,30]. Very recently, a series of supported ZIF membranes have been reported for single gas permeation and gas mixture separation [15–28]. It is reasonable that both the growth of dense polycrystalline MOF layers and the stability of the MOF membranes have to be considered for practical applications. Recently, a novel MOF named Zn(BDC) (TED)0.5 (BDC¼ benzenedicarboxylate, TED¼triethylenediamine) was prepared and characterized [31,32]. Zn(BDC)(TED)0.5 is not only

n

Corresponding author. Tel.: þ 86 574 86382530; fax: þ 86 574 86685043. E-mail address: [email protected] (A. Huang).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.12.018

thermally stable up to 282 1C, but also shows a high porosity (61.3%) with large channels (7.5 Å
7.5 Å along c-axis and 4.8 Å
3.2 Å along a- and b-axes) [32]. In addition, Zn(BDC)(TED)0.5 is highly hydrophobic which is helpful in keeping the pore volume unblocked in the presence of steam for separation [32,33]. Furthermore, Zn(BDC)(TED)0.5 displays a high CO2/CH4 adsorption selectivity since CO2 has stronger dispersion and electrostatic interactions with the framework [33]. According to the simulated adsorption of an equimolar H2/CO2 mixture in Zn(BDC)(TED)0.5 pores (Fig. 1), CO2 is predominantly adsorbed over H2. CO2 has a far stronger interaction with Zn(BDC)(TED)0.5 than does H2, which is helpful in trapping CO2 at the sorption sites, while H2 can diffuse easily through the wide pores. Owing to these merits, Zn(BDC)(TED)0.5 can be expected to be a promising candidate for the fabrication of a hydrogen selective membrane for the separation of H2/CO2 mixtures [28]. The separation of H2 from CO2 is important for the production of hydrogen by steam–methane reforming (SMR). Currently, the majority of hydrogen is produced by SMR followed by a water–gas shift (WGS) strategy. The hydrogen has to be purified from the resulting SMR gas mixture, which mainly contains CO2, before hydrogen can be used in fuel cell or other advanced applications. In comparison with conventional separation methods like pressure swing adsorption (PSA), membrane-based separation has been considered to be the most promising alternative because of its low energy consumption, ease of operation, and cost effectiveness [34]. Furthermore, the hydrogen could also be removed in situ during SMR in catalytic membrane reactors. Inorganic membranes are more promising than

A. Huang et al. / Journal of Membrane Science 454 (2014) 126–132

127

by ultrasonic treatment in ethanol to obtain a 0.5 wt% seed suspension. The seed suspension was given to the horizontally oriented supports in a Teflon holder. After ethanol evaporation at room temperature for 20 h, a homogeneous Zn(BDC)(TED)0.5 seed coating is formed on the support surface. The seeded support was fixed on a Teflon holder and placed horizontally in the Teflon-lined stainless steel autoclave, and then the synthesis solution was poured into the autoclave. After heating in an oven for 24 h at 150 1C with air circulation, the autoclave was cooled down, and the solution was decanted off. The membrane was washed with DMF to remove the loose Zn(BDC)(TED)0.5 powder and then dried at 60 1C overnight. 2.3. Characterization of Zn(BDC)(TED)0.5 membranes

Fig. 1. Snapshot of adsorbed CO2 and H2 molecules in Zn(BDC)(TED)0.5 cages at 25 1C and 100 kPa. CO2: yellow-violet stick and H2: green ball. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

organic polymer membranes under harsh separation conditions. In the recent 20 years, hydrogen permselective inorganic membranes, such as dense Pd-based metal membranes [35], microporous silica membranes [36], carbon membranes [37], and zeolite membranes [38–41] have been developed for the separation of H2 from CO2. Recently, MOFs have drawn much interest in application in CO2 capture due to its high specific CO2 adsorption properties. Many comprehensive overviews associated with CO2 capture by using MOF have been published [42,43]. High CO2 selective MOF membrane was also reported recently [12,14,16]. However, there are only a few reports of highly permselective MOF membranes for the separation of H2/CO2 [26,28,44]. In the present work, we report the synthesis of highly hydrophobic and permselective Zn(BDC)(TED)0.5 membranes through a secondary growth method for H2/CO2 separation.

2. Experimental 2.1. Materials Chemicals were used as received: 1,4-benzenedicarboxylic acid (BDC, 98%, Aldrich), triethylenediamine (TED, Z99%, Sigma-Aldrich); N,N-dimethylformamide (DMF, water o50 ppm, Acros). Porous α-Al2O3 disks (Fraunhofer Institute IKTS, former HITK/Inocermic, Hermsdorf, Germany: 18 mm in diameter, 1.0 mm in thickness, 70 nm particles in the top layer) were used as supports. 2.2. Preparation of Zn(BDC)(TED)0.5 membranes The Zn(BDC)(TED)0.5 membranes were prepared by a secondary growth method on seeded supports. The solution for the synthesis of the Zn(BDC)(TED)0.5 membranes was prepared according to the procedure reported elsewhere [32,33]. A mixture of zinc (II) nitrate hexahydrate (1.194 mmol), BDC (1.228 mmol), TED (0.642 mmol), and 30 ml of DMF were put in a 50 ml vessel. A clear solution was obtained after ultrasonic treatment. Before solvothermal synthesis, the top surface of the Al2O3 support was coated with Zn(BDC) (TED)0.5 seeds using a dipping-evaporation strategy [41]. Zn(BDC) (TED)0.5 seeds, which were prepared by microwave heating at 150 1C for 4 h with the same synthesis solution, were dispersed

The morphology and thickness of the Zn(BDC)(TED)0.5 membranes were characterized by field emission scanning electron microscopy (FESEM). FESEM micrographs were taken on an S-4800 (Hitachi) with a cold field emission gun operating at 4 kV. The phase purity and crystallinity of the Zn(BDC)(TED)0.5 membranes were confirmed by X-ray diffraction (XRD). The XRD patterns were recorded at room temperature under ambient conditions with a Bruker D8 ADVANCE X-ray diffractometer with CuKα radiation. Thermogravimetric analysis (TGA) was recorded with Pyris Diamond TG/DTA (Prkin-Elmer). 2.4. Evaluation of single gas permeation and mixed gas separation The Zn(BDC)(TED)0.5 membranes supported on α-Al2O3 disks were evaluated by single gas permeation and mixed gas separation. For the permeation studies, the supported Zn(BDC)(TED)0.5 membrane was sealed in a permeation module with silicone O-rings. Before measurements, the Zn(BDC)(TED)0.5 membrane was on-stream activated to remove DMF solvent at 180 1C by using an equimolar H2–CO2 mixture as feed in a Wicke–Kallenbach permeation apparatus. The N2 sweep gas was fed on the permeate side to keep the concentration of the permeating gas as low as possible thus providing a driving force for permeation. Atmospheric pressure was maintained on both sides of the membrane as detailed elsewhere [21]. After activation, the permeation of the single gases H2, CO2, CH4, and C3H8 as well as the separation of equimolar binary mixtures of H2 with CO2, CH4 and C3H8 were studied using the Wicke–Kallenbach technique with N2 as sweep gas on the permeate side. In order to investigate the effect of H2O or CH3OH on the H2–CO2 separation, the volumetric flow rates of the equimolar H2–CO2 mixture were saturated with H2O or CH3OH at room temperature before they were fed to the permeation module, and about 3.0 wt% H2O or CH3OH are present in the feed [45,46]. The fluxes of feed and sweep gases were determined using mass flow controllers, and a calibrated gas chromatograph (HP6890) was used to measure the gas concentrations. The separation factor αi,j of a binary mixture permeation is defined as the quotient of the molar ratios of components (i, j) in the permeate, divided by the quotient of the molar ratio of components (i, j) in the retentate, as shown in Eq. (1). αi=j ¼

yi;Perm =yj;Perm yi;Ret =yj;Ret

ð1Þ

2.5. Simulation models and methods Zn(BDC)(TED)0.5 possesses a paddle-wheel structure with metal-oxides Zn2(COO)4 bridged by BDC linkers to form a 2D square-grid net [Zn2(1,4-BDC)2]. The TED pillars occupy the axial sites to extend the 2D layers into a 3D framework [31,32]. Fig. 2 illustrates the crystal structure of Zn(BDC)(TED)0.5. The atomic charges in the Zn(BDC)(TED)0.5 framework were calculated from density functional theory (DFT), as reported in our previous study

128

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XY plane

XZ plane

YZ plane

Fig. 2. Crystal structure of Zn(BDC)(TED)0.5. Color code: Zn, pink polyhedra; N, green; C, cyan; O, red; H, white. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

[33]. Using the Becke exchange plus Lee–Yang–Parr functional, the DFT calculation was carried out by Gaussian 03 [47]. The 6-31G(d) basis set was used for all atoms except Zn atoms, for which the LANL2DZ basis set was used. The dispersion interactions of the framework atoms were modeled by the Lennard–Jones (LJ) potential with parameters from the DREIDING force field [48], which has been commonly adopted in the simulation of MOFs. H2 was modeled as a two-site rigid molecule with a bond length of 0.74 Å [49]. CO2 was represented as a three-site rigid molecule and its intrinsic quadrupole moment was described by a partialcharge model [50]. The partial charges on C and O atoms were qC ¼0.576e and qO ¼  0.288e (e¼ 1.6022
10  19, the elementary charge), respectively. The C–O bond length was 1.18 Å and the bond angle O–C–O was 1801. H2O was mimicked by the three-point transferable interaction potentials (TIP3P) model [51], in which the O–H bond length was 0.9572 Å and the ∠HOH angle was 104.521. It has been shown that the TIP3P model gives a reasonably good interaction potential compared to experimental value [52]. The intermolecular interactions of CO2 and H2O were modeled by the additive LJ and Columbic potentials, as shown in Eq. (2). ( "
) 
 # qα qβ sαβ 12 sαβ 6  uij ðrÞ ¼ ∑ 4εαβ þ ð2Þ r αβ r αβ 4πε0 r αβ αAi

Table 1 Potential parameters of H2, CO2, H2O, and CH3OH for simulation study. Species LJ and Columbic potential

Bond length

Site s (Å) ε⧸kΒ (K) q (e) H2

H

2.59

CO2

C O

2.789 29.66 3.011 82.96

þ0.576  0.288

rC–O ¼1.18 Å

H2O

O H

3.151 0

76.47 0

 0.834 þ0.417

rH–O ¼0.9527 Å θ∠HOH ¼104.521

98.0 93.0 0

þ0.265  0.700 þ0.435

rCH3–O ¼1.43 Å rO–H ¼0.945 Å

CH3OH CH3 3.75 O 3.02 H 0

12.5

0

Bending angle and force constant

0.74 θ∠OCO ¼ 1801

θ∠CH3 OH ¼ 108:51 kθ/kB ¼ 55,400 K

in a typical GCMC simulation was 2
107, in which the first 107 moves were used for equilibration and the second 107 moves for ensemble averages. Six types of trial moves were randomly attempted in the GCMC simulation: displacement, rotation, and partial regrowth at a neighboring position; entire re-growth at a new position; and swap with reservoir including creation and deletion at equal probability; the exchange of molecular identity.

βAj where ε0 ¼8.8542
10  12 C2 N  1 m  2 is the permittivity of the vacuum, sαβ and εαβ are collision diameter and potential well depth, respectively. CH3OH was represented by a united-atom model with CH3 as a single interaction site, with the parameters adopted from the transferable potentials for the phase equilibria (TraPPE) force field [53]. In addition to LJ interaction and Columbic interaction, bond bending interaction was also taken into account for CH3OH, as shown in Eq. (3). ubending ðθÞ ¼ 0:5kθ ðθ  θ0 Þ2

ð3Þ

Table 1 lists the potential parameters for H2, CO2, H2O, and CH3OH. The adsorption of an equimolar H2/CO2 mixture in Zn(BDC) (TED)0.5 was simulated at 180 1C by the grand canonical Monte Carlo (GCMC) method [54]. To examine the effect of H2O and CH3OH, the adsorption of H2/CO2 mixture in the presence of 3.1% H2O and 16.8% CH3OH (in terms of mole fraction) was also simulated. The framework atoms in Zn(BDC)(TED)0.5 were assumed to be rigid and the potential energies between adsorbate atoms and framework were pre-tabulated. The LJ interactions were evaluated with a spherical cutoff of 13 Å with the long-range corrections added; the Columbic interactions were calculated using the Ewald summation. The real/ reciprocal space partition parameter and the cutoff for reciprocal lattice vectors were chosen to be 0.2 Å  1 and 8, respectively, to ensure the convergence of the Ewald sum. The number of trial moves

3. Results and discussion 3.1. Synthesis and characterization of Zn(BDC)(TED)0.5 membranes The organic linkers of Zn(BDC)(TED)0.5 cannot provide additional linkage groups to form covalent bonds with surface OH groups of the Al2O3 support, which causes a problem for the heterogeneous nucleation of Zn(BDC)(TED)0.5 on the alumina support surface. As shown in Fig. 3a, in our first attempt to synthesize a Zn(BDC)(TED)0.5 membrane by the in-situ synthesis method, we failed to obtain a continuous Zn(BDC)(TED)0.5 membrane on the non-seeded Al2O3 support. Therefore, seed-coating of the support was performed to direct the nucleation and growth of the Zn(BDC)(TED)0.5 layer. As shown in Fig. 3b, a homogeneous seed layer was formed on the α-Al2O3 support surface using a dipping-evaporation strategy. Fig. 3c shows the FESEM top view of the Zn(BDC)(TED)0.5 membrane prepared on the seeded α-Al2O3 support. It can be seen that after secondary growth for 24 h at 150 1C, the support surface is completely covered by compact cubic crystals, and no visible cracks, pinholes or other defects are observed. From the cross-section shown in Fig. 3d, it can be seen that the former seeds have grown into a well-intergrown continuous layer with a thickness of about 25 μm.

A. Huang et al. / Journal of Membrane Science 454 (2014) 126–132

20 µm

129

20 µm

Zn(BDC)(TED)0.5 layer

Porous Al2O3 support

5 µm

20 µm

Fig. 3. Top view FESEM image of the Zn(BDC)(TED)0.5 membrane layer prepared without seeding (a), top view FESEM image of the Zn(BDC)(TED)0.5 seeding layer (b), and top view (c) and cross-section (d) FESEM images of the Zn(BDC)(TED)0.5 membrane layer prepared with seeding on an Al2O3 disk.

3.2. Gas permeation behavior of Zn(BDC)(TED)0.5 membranes

Fig. 4. XRD patterns of Zn(BDC)(TED)0.5 powder (a), Zn(BDC)(TED)0.5 seeding layer on porous Al2O3 support (b), Zn(BDC)(TED)0.5 membrane before permeation measurement on porous Al2O3 support (c), and Zn(BDC)(TED)0.5 membrane after permeation measurement at 180 1C with H2O (d). (●): Al2O3 support, (not marked): Zn(BDC)(TED)0.5.

Fig. 4 shows XRD patterns of the Zn(BDC)(TED)0.5 membrane and powder collected from the bottom of the autoclave during membrane synthesis. It can be seen that all peaks of the membrane match well with those of Zn(BDC)(TED)0.5 powder reported previously besides Al2O3 signals from the support [31,32], indicating that a phase-pure Zn(BDC)(TED)0.5 membrane with high crystallinity has been formed on the support. It should be noted that, in comparison with the XRD pattern of the powder and of the seed layer, which are assumed to represent a random crystal orientation, the XRD pattern of the Zn(BDC)(TED)0.5 membrane exhibits a strongly increased relative intensity of the (001) reflection, which indicates a preferred crystal orientation of the (001) planes parallel to the support.

Before measurements, the Zn(BDC)(TED)0.5 membrane was onstream activated to remove DMF solvent at 180 1C. According to TGA data of the Zn(BDC)(TED)0.5 before and after activation (Fig. 5), Zn (BDC)(TED)0.5 loses its guest molecules in the temperature range of 100–180 1C. Therefore, the Zn(BDC)(TED)0.5 membranes are permeable after on-stream activation at 180 1C. Fig. 6 shows the permeances of the single gases through the activated Zn(BDC)(TED)0.5 membrane as a function of the kinetic diameters of the permeating molecules. As shown in Fig. 6, H2 has the highest permeance of 3.95
10  6 mol m  2 s  1 Pa  1 due to its small kinetic diameter of 0.29 nm. The single gas permeances through the Zn(BDC)(TED)0.5 membrane follow the order: H2 4CH4 4CO2 4C3H8, which mainly corresponds to their kinetic diameters with the exception of CO2. As reported previously [33], CO2 has stronger dispersion and electrostatic interactions with the Zn(BDC)(TED)0.5 framework, leading to a strong adsorption interaction but a lower diffusional mobility, which results in a lower CO2 permeance by the end than that of CH4. The strong adsorption affinity of Zn(BDC)(TED)0.5 to CO2 is in good agreement with the simulation study of gas adsorption (Fig. 1). Since the kinetic diameter of C3H8 is comparable with the small channels of Zn(BDC)(TED)0.5 running along the a- and b-axes with a cross section of 4.8 Å
3.2 Å, the diffusion of C3H8 is restricted to the channel system, leading to the lowest permeance. The ideal separation factors of H2 from CO2, CH4, and C3H8 are 16.9, 8.9, and 33.5, which by far exceeds the corresponding Knudsen coefficients, indicating that the Zn(BDC)(TED)0.5 membrane displays a high hydrogen permselectivity. The molecular sieve performance of the Zn(BDC)(TED)0.5 membrane was confirmed by the separation of equimolar gas mixtures at 180 1C and 1 bar. As shown in the inset of Fig. 6, for the 1:1 binary mixtures, the mixture separation factors for H2/CO2, H2/CH4, and H2/C3H8 are 12.1, 7.3, and 20.4, which also exceed the corresponding

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A. Huang et al. / Journal of Membrane Science 454 (2014) 126–132

0.35

110

CO2

Zn(BDC)(TED)0.5 before activation

100

Zn(BDC)(TED)0.5 after activation

H2

0.30

90

N (mmol/g)

Weight / %

CO2 with water

0.25

80 70 60 50

H2 with water CO2 with methanol

0.20

H2 with methanol 0.15 0.10

40

0.05

30

0.00

20 0

200

400

600

800

1000

Temperature / °C Fig. 5. TGA trace of the synthesized Zn(BDC)(TED)0.5 and activated Zn(BDC)(TED)0.5 heated at 180 1C.

Fig. 6. Single gas permeance of different gases through the Zn(BDC)(TED)0.5 membrane as a function of the gas kinetic diameter at 180 1C and 1 bar pressure difference. The inset shows the corresponding ideal separation factors derived from the single gas permeances and the mixture separation factors of the Zn(BDC) (TED)0.5 membrane. The data represent the average of 3 independent membranes.

Knudsen coefficients. The decrease of the separation selectivity in comparison with the ideal selectivity can be explained by the fact that the faster diffusing H2 molecules are inhibited by the stronger adsorbed molecules (CO2, CH4, and C3H8). The high H2/CO2 selectivity has also been confirmed by a simulation [54]. According to the simulated adsorption isotherms of the equimolar H2/CO2 mixture in Zn(BDC)(TED)0.5 pores at 180 1C (Fig. 7), CO2 uptake is much higher than H2 uptake. CO2 has a very strong interaction with the Zn(BDC)(TED)0.5 framework, which constrains the CO2 diffusion, and therefore, enhances H2/CO2 selectivity. Recently, we reported a highly permselective ZIF-95 molecular sieve membrane with a H2/CO2 selectivity of 25.7 making use of its preferred adsorption affinity and capacity for CO2 [28]. Since the pore size of Zn(BDC) (TED)0.5 is larger (7.5 Å
7.5 Å along the c-axis, and 4.8 Å
3.2 Å along the a- and b-axes) than that of ZIF-95 (3.7 Å), the Zn(BDC)(TED)0.5 membrane shows a lower H2/CO2 selectivity but a higher H2 permeance than ZIF-95 membrane. Further, in comparison with the previously reported MOF membranes (Table 2), the Zn(BDC)(TED)0.5 membranes developed in this study display a higher H2 permeance with comparable separation selectivity. 3.3. Stability tests: long time permeation with humid feeds at 180 1C Fig. 8 shows the mixed gas permeances and H2/CO2 selectivity of the Zn(BDC)(TED)0.5 membrane as function of permeation temperature at 1 bar. H2 permeance increases from 5.0
10  7 to 2.7
10  6 mol m  2 s  1 Pa  1 when the temperature increases from

0

50

100

150

200

P / kPa Fig. 7. Adsorption isotherms of CO2 and H2 in CO2/H2, CO2/H2/H2O, and CO2/H2/ CH3OH mixtures at 180 1C simulated by the grand canonical Monte Carlo method [54].

25 to 175 1C, while CO2 permeance only slightly increases from 6.9
10  8 to 2.3
10  7 mol m  2 s  1 Pa  1 and the H2/CO2 mixture separation factor rises from 7.2 to 11.7. This phenomenon can be explained by an adsorption–diffusion model. As shown above, Zn (BDC)(TED)0.5 has a high affinity to CO2 (Fig. 7). Therefore, at low temperature, mainly CO2 is adsorbed in the Zn(BDC)(TED)0.5 pores, thus blocking the diffusion of the rarely adsorbed and highly mobile H2. When the temperature increases, less CO2 becomes adsorbed and thus more H2 can diffuse in the resulting free volume, leading to large enhancement of H2 permeance, and consequently, enhancement of H2 selectivity. The Zn(BDC)(TED)0.5 membrane has been tested at 180 1C for 36 h, and no decrease in its high separation performances with a H2 permeance of about 2.7
10  6 mol m  2 s  1 Pa  1 and a H2/CO2 selectivity of 12.1 could be found (Fig. 9). Further, the Zn(BDC)(TED)0.5 membrane can keep its high H2 permselectivity when the H2 partial pressure increases from 0.5 to 1.0 bar (Fig. 10). Fig. 11 shows the gas permeances and H2/CO2 selectivity of the Zn(BDC)(TED)0.5 membrane at 180 1C and 1 bar when the H2/CO2 feed mixture was additionally saturated with H2O at 25 1C before it was fed to the permeation module. It can be seen that the presence of steam in the feed mixture has only a slightly negative effect on the separation performance of the Zn(BDC)(TED)0.5 membrane. Both H2 permeance and H2/CO2 selectivity are almost unchanged for 48 h, indicating that the pore volume of the Zn (BDC)(TED)0.5 was not blocked by adsorbed water. The slight reduction of H2 permeance can be attributed to the parallel permeation of H2O and H2 through the Zn(BDC)(TED)0.5 membrane since the kinetic diameter of H2O is only 0.26 nm, which is much smaller than the pore size of Zn(BDC)(TED)0.5. Furthermore, XRD analysis shows that all peaks of the spent Zn(BDC)(TED)0.5 membrane in steam match well with those of the as-prepared Zn (BDC)(TED)0.5 membrane (Fig. 4d), indicating that the Zn(BDC) (TED)0.5 membrane has a high hydrothermal stability. On the contrary, in the presence of CH3OH in the feed mixture, both H2 permeance and H2/CO2 selectivity decrease strongly (Fig. 11). Specifically, the H2 permeance decreases from 2.7
10  6 mol m  2 s  1 Pa  1 to 2.0
10  6 mol m  2 s  1 Pa  1 and the H2/CO2 selectivity decreases from 12.1 to 8.1. Due to the high hydrophobicity of the Zn(BDC)(TED)0.5 framework [32,33], the strong adsorption of CH3OH blocks the diffusion of the rarely adsorbed H2 more than that of CO2, leading to a decrease of H2 permeance, and consequently, to a decrease of H2 selectivity. The high hydrophobicity of Zn(BDC)(TED)0.5 makes it a promising candidate for the separation of alcohol/water mixtures by membrane permeation. The separation of alcohol/water mixtures by pervaporation is in progress and will be reported soon.

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Table 2 Comparison of H2 permeances and H2/CO2 selectivity for Zn(BDC)(TED)0.5 membrane with the previously reported MOF membranes. ZIF membranes

Pore size (nm)

Thickness (mm)

H2 permeances (mol m  2 s  1 Pa  1)

H2/CO2 selectivity

References

ZIF-7 ZIF-8 ZIF-22 ZIF-69 ZIF-90 MIL-53 KUST-1 ZIF-95 Zn(BDC)(TED)0.5

0.30 0.34 0.30 0.44 0.35 0.73
0.77 0.9 0.37 0.75
0.75 0.48
0.32

1.5 30 40 50 20 8 60 30 25

7.71
10  8 6.04
10  8 1.66
10  7 6.50
10  8 2.37
10  7 5.01
10  7 1.00
10  6 1.95
10  6 2.7
10  6

6.48 4.54 7.2 2.7 7.3 6.8 2.7 25.7 12.1

[15] [17] [21] [22] [24] [13] [10] [28] This study

3.0x10-6

20

-6

2.0x10-6

15

1.0x10-6

10

0.0

Permeance / mol m-2s-1Pa-1

Mixture separation factor

Mixture separation factor

Permeance / mol m-2s-1Pa-1

CO2 permeance

2.5x10

-6

2.0x10

-6

1.5x10

-6

1.0x10

-6

5.0x10

-7

5

20

15

10 H2 permeance CO2 permeance

5

Separation factor

0.0

0

50

100

150

0 0.50

200

0.75

1.00

H2 partial pressure / bar

Temperature / °C

Fig. 10. Gas permeances and H2/CO2 selectivity of the Zn(BDC)(TED)0.5 membrane as a function of the H2 partial pressure at 180 1C.

-2 -1

Permeance / mol m s Pa

3.0x10-6

16

2.5x10-6

14

2.0x10-6

12 10

1.5x10-6

8

1.0x10-6

H2 permeance

H2 permeance

CO2 permeance

5.0x10-7

CO2 permeance

Separation factor

6

Mixture separation factor

-1

Fig. 8. Permeances of mixed gas and H2/CO2 selectivity of the Zn(BDC)(TED)0.5 membrane as a function of permeation temperature at 1 bar.

Mixture separation factor

3.0x10

H2 permeance

Separation factor 4

0.0 0

8

16

24

32

40

48

Time / h

Fig. 9. Gas permeances and H2/CO2 selectivity of the Zn(BDC)(TED)0.5 membrane as a function of the permeation time at 180 1C and 1 bar.

Fig. 11. Gas permeances and H2/CO2 selectivity of the Zn(BDC)(TED)0.5 membrane at 180 1C and 1 bar when the H2/CO2 feed mixtures were saturated at 25 1C by H2O (filled symbols ■, ● and ▲) or CH3OH (open symbols □, ○ and △) before they were fed to the permeation module.

4. Conclusion In conclusion, in the present work we have developed a highly hydrophobic and permselective Zn(BDC)(TED)0.5 membrane for the separation of H2 from CO2. Attributed to its preferred adsorption affinity and capacity for CO2 as well as highly porous and oriented structure with large channels in the (001) direction, the Zn(BDC)(TED)0.5 membrane displays a high H2/CO2 permselectivity. For the separation of an equimolar H2/CO2 mixture at 180 1C and 1 bar, a high H2 permeance of 2.7
10  6 mol m  2 s  1 Pa  1 is obtained due to the highly porous structure and rather wide channels. The mixture separation factor of H2/CO2 is 12.1, which by far exceeds the corresponding Knudsen coefficient. Furthermore, the

Zn(BDC)(TED)0.5 membrane shows a high thermal and hydrothermal stability, thus offering a potential application in hydrogen separation and purification in the presence of steam.

Acknowledgments Financial support by the National Natural Science Foundation of China (Grant number: 21276262), Chinese Academy of Science Visiting Professorship for Senior International Scientists (Grant number: 2013T1G0047), and the Starting Research Fund of Team Talent (Grant number: Y20808A05) from NIMTE is acknowledged.

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References [1] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Design and synthesis of an exceptionally stable and highly porous metal–organic framework, Nature 402 (1999) 276–279. [2] J.S. Seo, D. Whang, H. Lee, S.I. Jun, J. Oh, Y.J. Jeon, K. Kim, A homochiral metal– organic porous material for enantioselective separation and catalysis, Nature 404 (2000) 982–986. [3] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular synthesis and the design of new materials, Nature 423 (2003) 705–714. [4] M. Dincâ, A.F. Yu, J.R. Long, Microporous metal–organic frameworks incorporating 1,4-benzeneditetrazolate: syntheses, structures, and hydrogen storage properties, J. Am. Chem. Soc. 128 (2006) 8904–8913. [5] L.J. Murray, M. Dincâ, J.R. Long, Hydrogen storage in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1294–1314. [6] E. Biemmi, C. Scherb, T. Bein, Oriented growth of the metal organic framework Cu3(BTC)2(H2O)3  xH2O tunable with functionalized self-assembled monolayers, J. Am. Chem. Soc. 129 (2007) 8054–8055. [7] S. Hermes, F. Schroder, R. Chelmowski, C. Woll, R.A. Fischer, Selective nucleation and growth of metal–organic open framework thin films on patterned COOH/CF3-terminated self-assembled monolayers on Au(111), J. Am. Chem. Soc. 127 (2005) 13744–13745. [8] G. Lu, J.T. Hupp, Metal–organic frameworks as sensors: a ZIF-8 based Fabry– Pérot device as a selective sensor for chemical vapors and gases, J. Am. Chem. Soc. 132 (2010) 7832–7833. [9] R. Ranjan, M. Tsapatsis, Microporous metal organic framework membrane on porous support using the seeded growth method, Chem. Mater. 21 (2009) 4920–4924. [10] H. Guo, G. Zhu, I.J. Hewitt, S. Qiu, “Twin copper source” growth of metal– organic framework membrane: Cu3(BTC)2 with high permeability and selectivity for recycling H2, J. Am. Chem. Soc. 131 (2009) 1646–1647. [11] Y. Liu, Z. Ng, E.A. Khan, H. Jeong, C. Ching, Z. Lai, Synthesis of continuous MOF5 membranes on porous α-alumina substrates, Microporpous Mesoporous Mater. 118 (2009) 296–301. [12] A. Bétard, H. Bux, S. Henke, D. Zacher, J. Caro, R.A. Fischer, Fabrication of a CO2selective membrane by stepwise liquid-phase deposition of an alkylether functionalized pillared-layered metal–organic framework [Cu2L2P]n on a macroporous support, Microporous Mesoporous Mater. 150 (2012) 76–82. [13] Y. Hu, X. Dong, J. Nan, W. Jin, X. Ren, N. Xu, Y.M. Lee, Metal–organic framework membranes fabricated via reactive seeding, Chem. Commun. 47 (2011) 737–739. [14] S. Aguado, C.H. Nicolas, V. Moizan-Baslé, C. Nieto, H. Amrouche, N. Bats, N. Audebrandd, D. Farrusseng, Facile synthesis of an ultramicroporous MOF tubular membrane with selectivity towards CO2, New J. Chem. 35 (2011) 41–44. [15] Y. Li, F. Liang, H. Bux, A. Feldhoff, W. Yang, J. Caro, Molecular sieve membrane: supported metal–organic framework with high hydrogen selectivity, Angew. Chem. Int. Ed. 49 (2010) 548–551. [16] S.R. Venna, M.A. Carreon, Highly permeable zeolite imidazolate framework-8 membranes for CO2/CH4 separation, J. Am. Chem. Soc. 132 (2010) 76–78. [17] H. Bux, F. Liang, Y. Li, J. Cravillon, M. Wiebcke, J. Caro, Zeolitic imidazolate framework membrane with molecular sieving properties by microwaveassisted solvothermal synthesis, J. Am. Chem. Soc. 131 (2009) 16000–16001. [18] H. Bux, A. Feldhoff, J. Cravillon, M. Wiebcke, Y. Li, J. Caro, Oriented zeolitic imidazolate framework-8 membrane with sharp H2/C3H8 molecular sieve separation, Chem. Mater. 23 (2011) 2262–2269. [19] M.C. McCarthy, V. Varela-Guerrero, G.V. Barnett, H.-K. Jeong, Synthesis of zeolitic imidazolate framework films and membranes with controlled microstructures, Langmuir 26 (2010) 14636–14641. [20] Y. Pan, Z. Lai, Sharp separation of C2/C3 hydrocarbon mixtures by zeolitic imidazolate framework-8 (ZIF-8) membranes synthesized in aqueous solutions, Chem. Commun. 47 (2011) 10275–10277. [21] A. Huang, H. Bux, F. Steinbach, J. Caro, Molecular-sieve membrane with hydrogen permselectivity: ZIF-22 in LTA topology prepared with 3aminopropyltriethoxysilane as covalent linker, Angew. Chem. Int. Ed. 49 (2010) 4958–4961. [22] Y. Liu, G. Zeng, Y. Pan, Z. Lai, Synthesis of highly c-oriented ZIF-69 membranes by secondary growth and their gas permeation properties, J. Membr. Sci. 379 (2011) 46–51. [23] X. Dong, Y.S. Li, Synthesis of an organophilic ZIF-71 membrane for pervaporation solvent separation, Chem. Commun. 49 (2013) 1196–1198. [24] A. Huang, W. Dou, J. Caro, Steam-stable zeolitic imidazolate framework ZIF-90 membrane with hydrogen selectivity through covalent functionalization, J. Am. Chem. Soc. 132 (2010) 15562–15564. [25] A. Huang, J. Caro, Covalent post-functionalization of zeolitic imidazolate framework ZIF-90 membrane for enhanced hydrogen selectivity, Angew. Chem. Int. Ed. 50 (2011) 4979–4982. [26] A. Huang, N. Wang, K. Kong, J. Caro, Organosilica-functionalized zeolitic imidazolate framework ZIF-90 membrane with high gas-separation performance, Angew. Chem. Int. Ed. 51 (2012) 10551–10555. [27] A.J. Brown, J.R. Johnson, M.E. Lydon, W.J. Koros, C.W. Jones, S. Nair, Continuous polycrystalline zeolitic imidazolate framework-90 membranes on polymeric hollow fibers, Angew. Chem. Int. Ed. 51 (2012) 10615–10618.

[28] A. Huang, Y. Chen, N. Wang, Z. Hu, J. Jiang, J. Caro, A highly permeable and selective zeolitic imidazolate framework ZIF-95 membrane for H2/CO2 separation, Chem. Commun. 48 (2012) 10981–109833. [29] K.S. Park, Z. Ni, A.P. Côté, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O’Keeffe, O.M. Yaghi, Exceptional chemical and thermal stability of zeolitic imidazolate frameworks, Proc. Natl. Acad. Sci. USA 103 (2006) 10186–10191. [30] A. Phan, C.J. Doonan, F.J. Uribe-Romo, C.B. Knobler, M. O’Keeffe, O.M. Yaghi, Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks, Acc. Chem. Res. 43 (2010) 58–67. [31] D.N. Dybtsev, H. Chun, K. Kim, Rigid and flexible: a highly porous metal– organic framework with unusual guest-dependent dynamic behavior, Angew. Chem. Int. Ed. 43 (2004) 5033–5036. [32] J.Y. Lee, D.H. Olson, L. Pan, T.J. Emge, J. Li, Microporous metal–organic frameworks with high gas sorption and separation capacity, Adv. Funct. Mater. 17 (2007) 1255–1262. [33] Y.F. Chen, J.Y. Lee, R. Babarao, J. Li, J.W. Jiang, A highly hydrophobic metal– organic framework Zn(BDC)(TED)0.5 for adsorption and separation of CH3OH/ H2O and CO2/CH4: an integrated experimental and simulation study, J. Phys. Chem. C 114 (2010) 6602–6660. [34] N.W. Ockwig, T.M. Nenoff, Membranes for hydrogen separation, Chem. Rev. 107 (2007) 4078–4110. [35] S. Uemiya, T. Matsuda, E. Kikuchi, Hydrogen permeable palladium–silver alloy membrane supported on porous ceramics, J. Membr. Sci. 56 (1991) 315–325. [36] R.M. de Vos, H. Verweij, High-selectivity, high-flux silica membranes for gas separation, Science 279 (1998) 1710–1711. [37] M.B. Shiflett, H.C. Foley, Ultrasonic deposition of high-selectivity nanoporous carbon membranes, Science 285 (1999) 1902–1905. [38] M. Hong, S. Li, J.L. Falconer, R.D. Noble, Hydrogen purification using a SAPO-34 membrane, J. Membr. Sci. 307 (2008) 277–283. [39] M. Kanezashi, J. O’Brien-Abraham, Y.S. Lin, K. Suzuki, Gas permeation through DDRtype zeolite membranes at high temperatures, AIChE J. 54 (2008) 1478–1486. [40] J. van den Bergh, A. Tihaya, F. Kapteijn, High temperature permeation and separation characteristics of an all-silica DDR zeolite membrane, Microporous Mesoporous Mater. 132 (2010) 137–147. [41] A. Huang, F. Liang, F. Steinbach, T. M. Gesing, J. Caro, Neutral and cation-free LTA-type aluminophosphate (AlPO4) molecular sieve membrane with high hydrogen permselectivity, J. Am. Chem. Soc. 132 (2010) 2140–2141. [42] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.H. Bae, J.R. Long, Carbon dioxide capture in metal–organic frameworks, Chem. Rev. 112 (2012) 724–781. [43] J.-R. Li, Y. Ma, M.C McCarthy, J. Sculley, J. Yu, H.-K. Jeong, P.B. Balbuena, H.-C. Zhou, Carbon dioxide capture-related gas adsorption and separation in metal–organic frameworks, Coord. Chem. Rev. 255 (2011) 1791–1823. [44] Y. Pan, B. Wang, Z. Lai, Synthesis of ceramic hollow fiber supported zeolitic imidazolate framework-8 (ZIF-8) membranes with high hydrogen permeability, J. Membr. Sci.421–422 (2012) 292–298. [45] Y. Li, F. Liang, H. Bux, W. Yang, J. Caro, Zeolitic imidazolate framework ZIF-7 based molecular sieve membrane for hydrogen separation, J. Membr. Sci. 354 (2010) 48–54. [46] A. Huang, J. Caro, Steam-stable hydrophobic ITQ-29 molecular sieve membrane with H2 selectivity prepared by secondary growth using Kryptofix 222 as SDA, Chem. Commun. 46 (2010) 7748–7750. [47] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A. D. Rabuck, K. Raghavachari, J.B. Foresman, J. Ortiz, J.V. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B.G. Johnson, W. Chen, M. W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03, Revision D.01 ed., Gaussian Inc., Wallingford, CT, 2004. [48] S.L. Mayo, B.D. Olafson, W.A. Goddard, Dreiding: a generic force field for molecular simulations, J. Phys. Chem. 94 (1990) 8897–8909. [49] R.F. Cracknell, Molecular simulation of hydrogen adsorption in graphitic nanofibres, Phys. Chem. Chem. Phys. 3 (2001) 2091–2097. [50] A. Hirotani, K. Mizukami, R. Miura, H. Takaba, T. Miya, A. Fahmi, A. Stirling, M. Kubo, A. Miyamoto, Grand canonical Monte Carlo simulation of the adsorption of CO2 on silicate and NaZSM-5, Appl. Surf. Sci. 120 (1997) 81–84. [51] W.L. Jorgensen, J. Chandrasekhar, J.D. Madura, R.W. Impey, M.L. Klein, Comparison of simple potential functions for modelling water, J. Chem. Phys. 79 (1983) 926–935. [52] P. Mark, L. Nilsson, Structure and dynamics of the tip3p, spc, and spc/e water models at 298k, J. Phys. Chem. A 105 (2001) 9954–9960. [53] B. Chen, J.J. Potoff, J.I. Siepmann, Monte Carlo calculations for alcohols and their mixture with alkanes. Transferable potential for phase equilibria. 5. United atom description of primary, secondary, and tertiary alcohols, J. Phys. Chem. B 105 (2001) 3093–3104. [54] D. Frenkel, B. Smit, Understanding Molecular Simulation: From Algorithms to Applications, Academic Press, San Diego, 2002.