Zeolite Imidazolate FrameworkâIonic Liquid Hybrid Membranes for

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Zeolite Imidazolate Framework - Ionic Liquid Hybrid Membranes for Highly Selective CO2 Separation Ourania Tzialla, Charitomeni M Veziri, Xenophon Papatryfon, Konstantinos Beltsios, Anastasios Labropoulos, Boyan Iliev, Gabriela Adamova, Thomas J. S. Schubert, Maaike C. Kroon, Maria Francisco Casal, Lawien F Zubeir, George E Romanos, and Georgios Karanikolos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4051287 • Publication Date (Web): 02 Aug 2013 Downloaded from http://pubs.acs.org on August 4, 2013

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The Journal of Physical Chemistry

Zeolite Imidazolate Framework - Ionic Liquid Hybrid Membranes for Highly Selective CO2 Separation O. Tzialla1,2,#, Ch. Veziri1,#, X. Papatryfon1,2, K.G. Beltsios2, A. Labropoulos1, B. Iliev3, G. Adamova3, T.J.S. Schubert3, M.C. Kroon4, M. Francisco4, L.F. Zubeir4, G.E. Romanos1,*, G. N. Karanikolos1,5*

1

Department of Physical Chemistry, Institute of Advanced Materials, Physicochemical

Processes, Nanotechnology, and Microsystems (IAMPPNM), Demokritos National Research Center, Athens 15310, Greece 2

Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece 3

4

IoLiTec GmbH, Salzstrasse 184, 74076 Heilbronn, Germany

Separation Technology Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, Netherlands 5

Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates

#

These authors contributed equally to the work

*

Corresponding authors e-mail: [email protected], [email protected]

*

Corresponding authors phone numbers: +30-2106503977, +30-2106503973

1 ACS Paragon Plus Environment


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Abstract Zeolitic imidazolate framework ZIF-69 membranes were grown on porous α-alumina substrates via seeded secondary growth and further functionalized by a CO2-selective tricyanomethanide anion/alkylmethylimidazolium cation based ionic liquid (IL), as to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium exhibiting high selectivity for CO2 itself. As a result, the selectivity of the hybrid membrane was significantly higher than that through as-grown ZIF membranes and the permeability was higher than that corresponding to bulk IL due to of the existence of the ZIF channels. Specifically, CO2 permeated 20 times faster than N2 through the intact ZIF pores, and 65 times faster through the bulk IL phase. Indicatively, the developed membranes at room temperature and under a 2-bar trans-membrane pressure exhibited CO2 permeance of 5.6x10-11 and 3.7x10-11 mol/m2/sec/Pa and real CO2/N2 selectivities of 44 and 64 for CO2/N2 mixtures consisting of 44 % and 75 % (vol/vol) CO2, respectively. In addition, based on the experimental evidence from the hybrid membranes, predictions were made on the expected performance of an ideal crack-free and homogeneous ZIF-69 membrane. This work provides a promising solution to face the challenges associated with defect formation upon growth of ZIF but also of other zeolite and inorganic membranes to be applied for CO2 separation.

Keywords: ZIF-69, membranes, ionic liquids, performance prediction, CO2/N2 selectivity, MOF


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1. Introduction In almost all CO2 absorption and adsorption processes, the separation step entails the formation of molecular complexes that must be reversed through significant increase in temperature.1-2 The heating and subsequent cooling of sorbents to prepare them for the next sorption cycle is highly energy consuming.3-7 One promising direction to minimize these large thermal swings is the development of adsorbents with well-defined cavities and pores, and tunable functionality, as found in zeolite imidazolate frameworks (ZIFs).8 The high versatility of ZIF structures creates new possibilities for the development of novel materials with optimized adsorption capacity and selectivity. To this extent, achieving the growth of dense, defect-free layers of ZIF crystals on the surface of porous supports can lead to the development of high flux/high selectivity CO2 membranes, the application of which may bring great potential towards reducing the parasitic energy loss associated with CO2 separation. ZIFs combine highly desirable properties from both zeolites and metal-organic frameworks (MOFs), such as microporosity, high surface area, tailored pore functionality, thermal and chemical stability, and tunable metal clusters and organic ligands.9-12 CO2 uptake capacity is primarily influenced by functionality effects particularly concerning attraction between the polar functional groups in the pores, such as –Cl, -CN, -Me, -Br, -NO2, or -C6H6, and the CO2 molecules.9 Because of the potential of these materials, several types of ZIF membranes have been reported including ZIF-7,13 ZIF-8,14-15 ZIF-22,16 ZIF-69,17-18 and ZIF-90,19 which were grown by various techniques mainly adapted from zeolite membrane fabrication. Despite the significant advances, development of well intergrown, highly selective membranes without intercrystalline



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defects still remains a challenge. This is a major hurdle that is also common to zeolites and other inorganic membranes, where it is highly demanded that the crystal intergrain defects are eliminated such that there are no continuous paths allowing easy crossing without sieving action. We report here a composite membrane fabricated through the filling of intercrystalline gaps of ZIF-69 membranes, grown by the seeded technique, with a CO2 selective,

bulky

ionic

liquid

(IL),

namely,

the

1-octyl-3-methyl-imidazolium

tricyanomethanide ([omim][TCM]). ILs are organic salts with melting points usually near room temperature exhibiting superior characteristics such as extremely low vapour pressures, wide liquid ranges, thermal stability, chemical tunability, hydrophobicity, and surface tension values between those of common organic solvents and water (24-55 mNm-1). These important characteristics are combined with a large CO2 absorption capacity and selectivity.20-22 The composite membranes developed here, combine the superior CO2 performance of ILs with ZIF-69, which is one of the best performing ZIFs for CO2 capture and separation.9 ZIF-69 has a pore size of 7.8 Ǻ and exhibits GME topology consisting of 12 membered ring straight channels along the c-axis and 8 membered ring channels along a-, b- axes.23 The resulting performance is in agreement to recent computational studies revealing that CO2 capture properties of a composite MOF/IL membrane could act favourably in enhancing separation capability due to the synergistic role of the two materials.24-25 In addition, based on gas absorption and diffusivity data of the bulk IL in combination to the performance data of the hybrid membranes, predictions were made for the CO2 permeance and CO2/N2 selectivity of an ideal, defect-free ZIF-69 membrane. It


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was found that, despite significant progress in the area of fabrication of ZIF-69 membranes, the best performances achieved so far (a 2.2 ideal CO2/N2 separation factor and a real CO2/N2 mixture selectivity of 6.3 at room temperature and under a 1-bar transmembrane pressure17-18) fall far behind the values predicted here, in agreement to gas adsorption measurements on ZIF-69 crystals in powder form reporting an ideal selectivity of 20 at 298 K.9 This supports that fabrication of defect-free ZIF membranes remains a challenge and needs to be overcome in order to realize the full potential of these materials for CO2 separation applications.

3. Experimental section 3.1. Preparation of ZIF-69 membranes ZIF-69 membranes were grown by secondary seeded growth.26-27 Macroporous αalumina discs with a diameter of 22 mm and a thickness of 2 mm made by pressing high purity alumina powder (Baikowski, CR-6) were used as supports.28 One side of the support was polished using a 1000 grit sandpaper. The disc was then cleaned with DI water and dried at 100 °C overnight before use. ZIF-69 crystal seeds were in-situ formed on α-alumina support by microwave heating. The reaction solution was prepared based on previous procedures.9,

18

Initially, 2 ml of a 0.3 M 2-nitroimidazole (98 %, Acros

Organics) solution in N,N dimethylformamide (DMF, 99.5%, Merck) and 2 ml of a 0.3 M 5-chlorobenzimidazole (96 %, Aldrich) solution in DMF were mixed in a 15 ml vial. 2 mL of a 0.15 M zinc nitrate hexahydrate (98% Aldrich) solution in DMF was subsequently added into the above solution under stirring. The reaction solution was then transferred into a 45 ml polytetrafluoroethylene (Teflon) beaker in which the cleaned and



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dried support was placed horizontally with the polished side facing upwards. Microwaveassisted solvothermal synthesis was carried out in order to deposit seeds on the support. The Teflon beaker was heated in a conventional microwave oven (operating at 10 % of total power) for 4 h at 100 °C. The above seeding synthesis cycle was repeated twice. After cooling to room temperature, the membrane was washed with methanol and dried overnight at room temperature. Subsequently the seeded supports were exposed to secondary growth. A precursor with same composition as the one used for seeding was loaded in a 45 ml Teflon-lined stainless steel autoclave in which the support was immersed horizontally with the seeded surface facing upwards. Secondary growth took place in a conventional oven by heating at 100 °C for 72 h. After cooling to room temperature, the ZIF-69 membrane was thoroughly washed with methanol and dried under vacuum. ZIF-69 crystals used for powder experiments were synthesized according to the following procedure:9, 23 2 mL 2-nitroimidazole stock solution (0.2 M) and 2 ml 5chlorobenzimidazole solution (0.2 M) were mixed in a 15 ml vial. 2 ml Zn(NO3)2•4H2O solution (0.2 M) was added to the above solution and the vial was capped and heated to 85 ºC for 96 h. The resulting suspension was decanted and yellow crystals of ZIF-69 were washed with DMF three times, collected by filtration, and dried in air. 3.2. Synthesis of the [omim][TCM] IL 1 mol of the 1-methyl-3-octylimidazolium chloride (>98%, Iolitec) was dissolved in 1,5L of dry dichloromethane. 1.05 mol of sodium tricyanomethide was added at once and the reaction mixture was stirred for 48h at room temperature. The mixture was then filtered through Celite, the mother liquor evaporated under reduced pressure, and the


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product dried under high vacuum for 24h at 40°C. Ionic chromatography showed Cl content of below 0.5 %. Achieved yield was 92%. 2.3. ZIF-69 membrane loading with IL The ZIF membranes were loaded with the [omim][TCM] IL using a homemade apparatus where bulk IL was brought into contact with the side of the support on which the ZIF layer for 1 day without applying any pressure gradient. After loading, the excess IL was wiped off the surface of the membrane with a filter paper prior to membrane characterization. 2.4. Characterization Physicochemical and thermophysical properties of the IL were evaluated as described in the supporting information. Permeance measurements of the prepared membranes were carried out using a home-made apparatus operating in the WickeKallenbach mode, by applying helium as the gas phase that sweeps the permeate side of the membrane. The permeating gases (CO2, N2) were transferred to the sampling loop of a gas chromatograph for analysis. Scanning electron microscopy (SEM) imaging was carried out using a JEOL-7401 Field-Emission Gun scanning electron microscope.

3. Results and Discussion ZIF-69 membrane fabrication by in-situ crystallization was first attempted in order to obtain a continuous ZIF-69 layer on α-alumina. The polished support was introduced in the autoclave and exposed to hydrothermal treatment without involvement of seeds. The composition of the reaction mixture was the same as the one used by Yaghi and co-workers to synthesize bulk ZIF-69 crystals.9 According to SEM analysis, the



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obtained film consisted of hexagonal ZIF-69 prisms elongated along c-axis and oriented parallel to the support surface (supporting information), while permeability experiments indicated the existence of intercrystalline gaps, as fluxes through the microporous layer (a)

5 µm

(b)

10 µm

(c)


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Figure 1. (a) ZIF-69 seed layer on α-alumina after 2-cycle microwave seeding. (b) ZIF69 membrane produced by secondary growth using the seeded support shown in panel (a). (c) XRD pattern of the as-grown membrane confirming the ZIF-69 structure.

resembled the ones for the bare alumina support. To improve film orientation and continuity, seeded growth was employed as to allow for decoupling of the nucleation and growth steps.26-27,

29-30

In-situ seeding took place using microwave heating into two

isochronous steps. Fig. 1a shows the seed morphology on the support after the two seeding cycles, and reveals that the particles are crystalline with a size of ~ 1 µm and are deposited throughout the support surface with an approximate inter-seed separation distance of 1 µm. Upon secondary growth, the crystals merged and formed a continuous film, as indicated in Fig. 1b. Cracks remain, though their total extent was substantially lower compared to the seedless film growth. These cracks were repaired by IL filling, as was the primary scope of this work. The selection of the [omim][TCM] IL was based on its large molecular size so as to prevent its imbibition into the ZIF-69 channels (pore diameter 0.78 nm, pore aperture 0.44 nm). Moreover, due to the existence of the octyl chain, the specific IL exhibits higher hydrophobicity and lower surface tension compared to other ILs of the same family (data given in supporting information), thus preventing IL imbibition into the pores of the hydrophilic α-alumina support. In addition, the enhanced thermal stability, and low viscosity, as well as the easiness of synthesis, and the relatively low cost compared to other IL candidates made the selection of [omim][TCM] favourable.



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The gas permeance results of the composite ZIF/IL membranes were interpreted via the employment of an electric analogue consisting of two resistances arranged in parallel and one resistance in series (Fig. 2). In such a configuration, it is considered that the mass transport of CO2 takes place simultaneously, though at different transport rates, through the pores of the ZIF-69 crystals and the IL phase hosted in the gaps in between the crystals of the deposited ZIF layer. CO2 transport through the pores of the macroporous α-alumina support follows immediately after CO2 permeation through the ZIF/IL layer. As illustrated in Fig. 2, the two resistances in parallel, RZIF and RIL,

Figure 2. Schematic illustration of ZIF-69 membrane morphologies. (a) As-grown membrane where the intercrystalline voids are evident, accompanied by a cross-section SEM image of the membrane. (b) IL/ZIF-69 hybrid membrane. Left: IL fills only the intercrystalline gaps. Resistance to the flow is through the ZIF crystals, the intercrystal-


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located IL, and the support. Right: IL fills the intercrystalline gaps and also forms an additional layer on top of the crystalline film creating an additional resistance to the flow. The hydrophobic nature of the IL does not allow for diffusion and uniform distribution into the pores of the hydrophilic support.

correspond to the ZIF channels (intra-crystalline diffusion) and IL phase (diffusion through the intercrystallline gaps), respectively, and the resistance in series RSUP, corresponds to the pore structure of the α-alumina support. Given that the total permeance (PeTOT) can be determined experimentally and expressed as PeTOT=1/RTOT, and the permeance through the pristine support (PeSUP) can be expressed as PeSUP =1/RSUP, it is possible to derive a correlation function between the permeance through the ZIF channels and the IL phase as follows:

(1/PeTOT)-(1/PeSUP) = 1/(PeIL+PeZIF)

(1)

Permeance experimental data of the as-prepared ZIF-69 membranes are shown in Fig 3. The as-grown ZIF-69 membrane exhibits a 2 to 2.5 fold lower permeance compared to the pristine α-alumina support (Fig. 3a) due to the formation of the ZIF-69 film on the support surface. However, the absence of a CO2/N2 separation capability (Fig. 3b) suggests the existence of defects in the ZIF layer, which originate from inter-crystal gaps. The application of a higher trans-membrane pressure difference (up to 2 bars, Fig. 3c, 3d) did not improve the selectivity. Notably, an almost linear correlation between permeance and trans-membrane pressure was observed, which is compatible with prevailing



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Poiseuille flow in macropores and constitutes another piece of evidence that inter-crystal, macro-sized defects exist. If the film was continuous, the permeation would have been fully controlled by the ZIF-69 channels, where the strong and CO2-selective adsorption would give rise to internal surface diffusion phenomena resulting in enhancement of the CO2 flux over N2. The performance of the [omim][TCM]-casted membrane in comparison to the pristine ZIF-69 one is presented in Fig. 4. Loading with the IL resulted in significant enhancement of the CO2/N2 selectivity, which reached the values of 44 and 64 for CO2 concentrations of 45 and 75% (vol/vol) in the feed stream, respectively, accompanied

Figure 3. (a) CO2 and N2 permeance as a function of CO2 concentration in the feed for the α-alumina support and the pristine ZIF-69 membrane. (b) CO2/N2 selectivity vs. CO2


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concentration in the feed. (c) Effect of trans-membrane pressure difference on the CO2 and N2 permeance for ZIF-69 at 30 °C. (d) Effect of trans-membrane pressure difference on the CO2 and N2 permeance for ZIF-69 at 50 °C.

with lower flux through the membrane. These findings indicate that IL plugged and repaired effectively the membrane cracks. Interestingly, the permeance of the IL-casted membrane declined with increasing CO2 content. This was an indication that gas diffusion through the IL-modified membrane was controlled by the IL phase. Indeed, a similar trend was observed in experiments where diffusion of CO2 through a thin layer of [omim][TCM] was studied. During these experiments, the diffusion coefficient D (m2/sec) and the solubility S (mol/m3) of CO2 in the IL were obtained by gravimetric measurements, and the permeability factor (Pe) was calculated via the equation Pe=DxS. The thickness of the IL layer was 0.24 cm, as defined by the mass and density of the IL



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Figure 4. (a) CO2 and N2 permeance of the pristine ZIF-69 membrane and of the ZIF-69 membrane after IL casting. Square marks show the respective permeances of an IL-casted α-alumina support. (b) Enhancement of the CO2/N2 selectivity of the ZIF 69 membrane after IL casting.

and the geometrical dimensions of the sample pan of the gravimetric system. More details on the experimental setup, the measurement procedure, and the calculation of the solubility and diffusivity properties are included in the supporting information. The respective results are presented in Table 1 and show the trend of decreasing permeability as the CO2 pressure increases. Furthermore, for comparison, a blank α-alumina support was loaded by IL under the same procedure as the one used to load the ZIF-69 membranes. The resulting IL film exhibited a 2.5 times lower permeance compared to the IL-casted ZIF-69 membrane (Fig. 4a), due to the absence of the ZIF crystals which exhibit higher permeability than the bulk IL phase. Using an average ZIF-69 layer thickness of 60 µm, as estimated by cross-section SEM, the permeances of the IL-modified membranes (Fig. 4a) were converted to permeabilities, assuming that the IL only plugged the intercrystalline gaps and did not form an additional layer of bulk IL on the top of the ZIF layer. Under this assumption, the CO2 permeability at a 2-bar trans-membrane pressure and a CO2 feed content of 44% was calculated to be 3.4x10-15 mol·m/m2/sec/Pa (or 11.4 Barrer) and that of N2 7.8x10-17 mol·m/m2/sec/Pa (or 0.24 Barrer). The extracted permeability values are relatively low yet a CO2/N2 selectivity of 44 was achieved, which is considered significant for a ZIFbased membrane. The calculated CO2 permeability was several times lower than that


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through the 0.24 cm thick bulk IL layer described above (8.77x10-14 mol·m/m2/sec/Pa, Table 1). This indicates that the IL had not only filled the inter-crystal gaps, but had further formed a quite thick layer on the top of the ZIF crystals. Moreover, the case that the IL has plugged the pore structure of the α-alumina support was also considered. In order to explore this, we performed water vapour absorption measurements of the Table 1. Permeability values of a thin [omim][TCM] layer calculated from the CO2 and N2 solubilities and diffusivities. Gas

CO2

N2

Pres.

S

D

Permeability

Permeance

Pa

mol/m3

1010 · (m2/sec)

1014·(mol·m/m2/sec/Pa)

1013·(mol/m2/sec/Pa)

10660 30150 50990 79960 96640 21740 41670 61580 81520

13.27 37.1 61.32 94.5 122.1 0.15 0.29 0.43 0.57

1.19 0.878 0.792 0.785 0.694 0.517 0.708 0.897 1.95

14.8 10.8 9.53 9.28 8.77 0.0364 0.0498 0.0632 0.137

624 455 401 390 369 1.53 2.10 2.66 5.76

[omim][TCM] and found that the IL is highly hydrophobic (Fig. S1 in supporting information). Indeed, the resulting absorption isotherm was of type III indicating that there is no specific interaction between water and IL. As a consequence, wetting of the hydrophilic α-alumina was not expected to occur, a conclusion that is further supported by the fact that no external pressure gradient is applied during IL loading, and the access of the IL to the support external surface is rather limited to sporadic spots corresponding to those ZIF film cracks that span the whole thickness of the film reaching the alumina surface (cross-section SEM image in Fig. 2). Therefore, formation of a uniform IL phase



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within the alumina support capable of providing significant resistance to the transport is not considered to be the case here. Based on the above, the electric analogue model, as described by Eq. 1 and represented in Fig. 2, was modified with the introduction of a second resistance in series corresponding to the continuous IL layer formed on the top of the ZIF film (Fig. 2b, right). In such a configuration, Eq. 1 becomes: (1/PeTOT)-(1/PeSUP)-(1/PeIL)=1/(PeIL+PeZIF)

(2)

Applying the experimentally defined permeance values of the support (PeSUP), the IL (PeIL), and the total corresponding to the IL- modified membrane (PeTOT) to Eq. 2, it was possible to calculate the unknown permeances through the ZIF film (PeZIF) and the CO2/N2 selectivity corresponding to the ZIF crystals, which are presented in Table 2. The values of Table 2 for the permeance and selectivity corresponding to ZIF are also the ones expected if an ideal, homogeneous ZIF-69 membrane without any defect could be formed. Notably, the calculated CO2/N2 selectivity is about five times higher than the upto-date highest-performance ZIF-69 membranes,18 and the CO2 and N2 permeances are one order of magnitude lower. The values calculated here are shown in the latest Robeson31 plot (2008) for CO2/N2 separation (Fig. 5). The abscissa is the CO2 permeability of 4.88x10-13 mol·m/m2/sec/Pa, as calculated above and shown in Table 3 after being converted to Barrer units, and the ordinate is the calculated CO2/N2 selectivity of 27. The filled black circle in the plot corresponds to the features calculated herein for an ideal ZIF-69 membrane for CO2/N2 separation. Notably, the permeance of such a target membrane can be even higher if optimum orientation of the crystalline film along the c-direction can also be achieved.


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Table 2. Experimentally defined CO2 and N2 permeance values that were used for the calculation of the permeances of an ideal ZIF-69 membrane with a layer thickness of 60 µm

CO2

N2

CO2/N2

Parameters defined by experiment

107·(PeSUP)

2.6

2.96

0.87

Fig. 3a

1013·(PeIL) 1013·(PeTOT)

369 3.67

5.76 5.75

64 63.8

Table 1 Fig. 4a

Calculation

1010·(PeZIF)

81.3

2.99

27.2

Figure 5. Robeson plot for CO2/N2 separation (2008) depicting the performance of a defect-free ZIF-69 membrane as estimated by the sequential flow resistance model applied to the fabricated IL/ZIF-69 hybrid membranes.



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To further support the calculated values, the permeability factor of a ZIF-69 crystal powder was derived as the product of CO2 solubility (S, mol/m3) and diffusivity (D, m2/sec), which were experimentally defined from the CO2 adsorption isotherm obtained using a gravimetric system. The adsorption isotherms at 35 °C, the adsorption transient curves for successive pressure steps, and the respective fits of the solution of the transient adsorption equation, which was used for the calculation of D, as well as details on the procedure, are given in the supporting information. The data in Table 3 reveal the agreement between the CO2 permeability factors of ZIF-69 crystals obtained from the adsorption isotherm, and those extracted from the experiments with the IL-casted ZIF-69 membrane, which were derived as the product of permeance (Table 2) with the thickness of the ZIF layer (60 µm).

Table 3. CO2 and N2 permeability of ZIF-69 as derived by the adsorption/diffusivity measurements on bulk crystals (Pe1), and as calculated based on the IL-casted ZIF-69 membrane applying the sequential flow resistance model (Pe2). Data of the recent literature are included for comparison (Pe3). Gas

CO2 N2

Pres.

S

D

Pe1[a]

Pe2[b]

Pe3[c]

Pa

mol/m3

1012·(m2/sec)

1013·(mol·m/m 2 /sec/Pa)

1013·(mol·m/m 2 /sec/Pa)

107·(mol·m/m2 /sec/Pa)

568 712 51.9 69.8

6.48 5.04 39.5 39.3

7.28 5.25 0.56 0.54

4.88

1.03

0.179

0.163

31600 42700 36530 50710

[a] Pe1=D•S [b] Pe2=(PeZIF)•ℓ, ℓ=ZIF layer thickness [c] Pe3 = Permeability from literature for a ZIF-69 membrane18


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4. Conclusions Hybrid IL/ZIF-69 membranes were fabricated by exposing ZIF-69 films grown by seeded growth on macroporous α-alumina supports to a tricyanomethanide anion / alkylmethylimidazolium cation based IL, resulting, through the combination of the superior CO2 properties of the two materials, in membranes exhibiting higher CO2 selectivity and permeability compared to as-grown ZIF-69 membranes and bulk IL, respectively. Using a sequential flow resistance model, we were able to estimate the permeability through the ZIF crystals and the selectivity attributed to the ZIF portion, as well as to predict the performance of an ideal, defect-free ZIF-69 membrane. This methodology can be generally applied to other CO2 separation membranes (zeolite, MOF, inorganic) and offers useful target performance guides for membrane fabrication. Optimization experiments are currently underway in our group as to restrict IL deposition exclusively in the intercrystalline gaps of the hybrid membranes in order to enhance permeability without sacrificing selectivity by eliminating the additional, slow-diffusivity path caused by the IL layer on the top of the ZIF film. To this extent, gentle polishing of the top layer and fine control of the IL loading time are under consideration. The hybrid IL/ZIF-69 membranes developed combine the enhanced CO2 selectivity of ILs with the enhanced CO2 permeability of ZIFs into one composite membrane, thus avoiding tedious and often non-reproducible efforts to prepare crackless homogeneous porous films, and providing

more

degrees

of

freedom

to

optimize

and

achieve

customized

selectivity/permeability combinations towards highly efficient CO2 separation processes.



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Acknowledgement Financial support by the EU project IOLICAP (FP7, grant agreement n° 283077) is acknowledged.

Supporting Information Available: Physicochemical and thermophysical properties of the [omim][TCM] IL, transient curves – definition of N2 and CO2 diffusivity (D) in the [omim][TCM] and the ZIF-69 crystals,

ZIF-69 membrane morphology by in–situ

growth, gravimetric apparatus and measurement procedure, accuracy of the gas absorption/adsorption measurements and assumptions made for the derivation of the solution of the transient absorption equation for the IL. This material is available free of charge via the Internet at http://pubs.acs.org.

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