Thin Ionic Liquid Membranes Based on Inorganic Supports with

Apr 24, 2014 - Baltus , R. E.; Counce , R. M.; Culbertson , B. H.; Luo , H.; Depaoli , D. W.; Dai , S.; Duckworth , D. C. Examination of the potential...
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Thin Ionic Liquid Membranes Based on Inorganic Supports with Different Pore Sizes Jonathan Albo and Toshinori Tsuru* Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagayami-yama, Higashi-Hiroshima, Hiroshima 739-8527, Japan S Supporting Information *

ABSTRACT: Nanoporous TiO2 and SiO2−ZrO2 membranes with controlled pore sizes were prepared by sol−gel processing and combined with a high CO2 solubility ionic liquid, 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), by hand-coating the ionic liquid onto the membrane surface, and then by a spray-coating procedure. The CO2 permeation through SILMs based on a support with 20 nm pores (SILM-20), PCO2 = 2.28 ± 0.16 × 10−8 mol/(m2·s·Pa), was 62% higher than that permeance obtained for SILMs with 1 nm pore sizes (SILM-1). Besides, the activation energy for permeation increased for membranes with smaller pore sizes, which suggested that the properties of the ionic liquid were influenced by the interaction with the membrane pore surface. The membranes prepared by spray coating revealed a CO2 permeance as high as PCO2 = 4.31 ± 0.13 × 10−8 mol/(m2·s· Pa), with an ideal CO2/N2 selectivity of α(CO2/N2) = 31.18 ± 1.34, which outperformed the current state-of-the-art materials for CO2/N2 separation.

1. INTRODUCTION The control of anthropogenic CO2 emissions is one of the most challenging environmental issues facing industrialized countries because of the undesirable implications of global warming. Today, the burning of fossil fuels is responsible for the majority of these CO2 emissions,1 and there is significant interest in developing technologies to mitigate emissions and therefore reduce atmospheric CO2 levels. The conventional method for capturing CO2 emissions uses a reversible solvent absorption process, which causes solvent losses from the direct contact between gas and liquid phases and necessitates a high expenditure of energy.2,3 As an alternative, gas separation using membranes in combination with ionic liquids (ILs), the so-called supported ionic liquid membranes (SILMs), is extremely attractive due to energy efficiency and operational simplicity in a compact equipment.1,4 In this configuration, the IL is immobilized in a membrane support, reducing both the amount of IL required and the energy consumption for pumping by comparison with other nondispersive membrane-based technologies, such as a combination of hollow fiber membrane contactors and ILs that can be used for CO2 absorption.2−10 In SILMs, the CO2 dissolves into the membrane at the feed side, diffuses through the membranes, and is desorbed at the opposite membrane surface.11−17 The critical performance metrics of the SILM systems include the CO2 permeation and CO2/N2 selectivity, which are generally thought to depend more on the IL anion than on the cation. The literature shows that ILs containing the acetate anion possess a high degree of absorption for CO2 across a wide range of temperatures and applied pressures.18,19 Recently, Santos et al.16 have extensively studied ILs based on acetate anions, and they have demonstrated desirable separation properties, which include a high degree of CO2 solubility. Therefore, it is possible to design tailor-made membrane systems based on task-specific ILs with an enhanced selectivity toward CO2. © 2014 American Chemical Society

The most widely investigated SILM substrates for CO2 separation are polymers. Their application, however, is limited under industrially relevant conditions because of the low flux and a lack of high-temperature stability. As a potential alternative, porous ceramic membranes, such as alumina (Al2O3) or titania (TiO2), have received a great deal of attention, because of their excellent thermal and mechanical stability,20 although a limited number of reports can be found in the literature for the application of ceramic supports in SILMs.2,17,21−23 In particular, in our previous study the combination of an Al2O3/TiO2 substrate and an IL with high CO2 solubility, [emim][Ac], led to a CO2 permeance as high as PCO2 = 2.78 ± 0.11 × 10−8 mol/(m2·s·Pa) with an ideal CO2/ N 2 selectivity of α(CO2/N2 ) = 30.72 ± 0.86, which outperformed the state-of-the-art for CO2/N2 separation processes that use polymeric materials. Also, the membrane stability tests demonstrated that the ceramic support was very effective for the immobilization of the liquid phase in the membrane for a period of 25 h and for feed pressures of up to 4 bar.17 The separation performance of SILMs is generally attributed to the imbibed IL characteristics, rather than to the membrane substrate. Even so, recent reports have suggested that the state of the ILs inside the support pores and the presence of a solid/ liquid interface may be an important part of the permeation mechanism for SILMs.11,14,23,24 In particular, Scovazzo et al. has reported that, when combined with a poly(ether sulfone) support, the CO2 permeance of [emim][CF3SO3] was 2-fold that of the same IL impregnated within a hydrophilic PVDF membrane.24 Neves et al. examined gas separation in SILMs Received: Revised: Accepted: Published: 8045

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prepared with imidazolium-based ILs and two different PVDF support membranes, one hydrophobic and the other hydrophilic.11 Results showed that SILMs prepared with the more hydrophobic support were more stable than those prepared with the hydrophilic membrane. The same observation was made when combining magnetic ILs and PVDF substrates, possibly due to a better affinity between the ILs and the hydrophobic substrate.14 Close et al. studied CO2 permeation in SILMs based on Al2O3 supports and [emim][Tf2N] and [C6mim][Tf2N] ILs.23 The results were 1.55 and 1.59 times higher than the values obtained when these ILs were included in porous glass fiber supports.25 They speculated that the difference arises from the influence of the support pore size and the IL-ceramic interactions on SILM performance. Recently, Banu et al. evaluated the effect of the confinement of imidazolium-based ILs in Al2O3 membranes (pore size: 20 nm).26 The CO2 permeability for confined ILs was increased compared with the bulk phase by a factor of 15.7, which was attributed to changes in the physical properties of the IL on a pore surface through a reorganization of the cation and anion at the interface. Moreover, a number of other studies have examined the interfacial properties, structural organization, and phase transitions of ILs confined within porous networks or near solid surfaces via a number of different techniques with mixed results.27−33 Some of those studies revealed organized structures of the ILs in the vicinity of a solid interface and pore size-dependent phase transitions; others indicated liquid properties similar to those of the bulk ILs. As a consequence, a clear understanding of the effect of support pore surface and the interactions with the imbibed ILs has not yet been achieved in SILM systems. In this work, nanoporous TiO2 and SiO2−ZrO2 membranes with controlled pore sizes ranging from 1 to 20 nm were prepared by sol−gel processing and combined with an IL with high CO2 solubility, 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), in SILMs. The main objectives of the present study were as follows: (1) to determine the performance of the prepared ceramic-based SILMs with an increasing number of by-hand coatings; (2) to elucidate the contribution to gas permeation of, first, membrane effective IL thickness and, second, support pore size at increasing temperatures; and (3) to test a spray-coating procedure to immobilize a thinner IL layer in the ceramic support. The results are compared with those of the current best performing membrane materials for CO2/N2 separation.

Figure 1. Schematic representation of the preparation of the inorganic membranes (M-20, M-2.5, and M-1).

respectively. This coating procedure was repeated several times for each sol solution and was carried out for sols from a larger diameter to a smaller diameter. As a result, three membrane samples with different average pore sizes were obtained and used in the present study (M-20, M-2.5, and M-1). Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800 field-emission scanning electron microscope (Japan) using an accelerating voltage of 4 kV. Samples were sputter-coated with palladium/platinum to minimize charging in a JEOL JFC-1300 Auto Fine Coater. The pore diameters of the membranes were determined by nanopermporometry (NPP), which is an adequate methodology to measure nanosized pores ranging from 0.6 to 30 nm.36,37 The basic principle of this methodology lies in the capillary condensation of a condensable vapor in the membrane pores at a given humidity and in the blocking of noncondensable gas permeation through the condensed pores. The experimental procedure has been explained elsewhere.37 Basically, NPP was initiated by measuring the steady permeance of the dry gas as an initial reference point. The vapor pressure was then increased gradually by controlling the dry and wet gas flow rates until gas permeation was blocked by capillary condensation. N2 was used as noncondensable gas, and hexane has been applied as condensable vapor since the use of nonpolar compounds is appropriate for measuring free-volume pore size distributions smaller than 1 nm in ceramic membranes.36,37 In the small pores, vapor condensation at vapor pressure, p, lower than the saturated vapor pressure, ps, occurs, as it is represented in the Kelvin equation

2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. Preparation and Characterization of the Inorganic Supports. Porous inorganic membranes were prepared via a sol−gel process. TiO2 and SiO2−ZrO2 sols with different colloidal diameters were used for membrane preparation. The membranes were prepared by coating the sols on Al2O3 tubes (porosity: ∼46%; average pore size: 150 nm; length: 70 mm; outer diameter: 3.0 mm; inner diameter: ∼2.2 mm (NOK, Japan)). First, 10 mm of each end of the support was glazed for sealing, resulting in an active length of 50 mm, and then, the sols were coated according to the scheme shown in Figure 1. The TiO2 sol used for the first coating layer was commercially available (STS01 was kindly supplied by Ishihara Sangyo Kaisha, Japan). On the other hand, the TiO2 and SiO2−ZrO2 sols for the outer layers were synthesized, resulting in particle sizes of 20−30 nm34 and 11 nm,35

⎛P⎞ σ cos θ RT ln⎜ ⎟ = 4v dk ⎝ Ps ⎠ 8046

(1)

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Table 1. Characteristics of [emim][Ac] Ionic Liquid molecular formula

mol wt, M (g/mol)

density at 25 °C, ρ (g/cm3)

viscosity at 25 °C, μ (cP)

decomposition temp, Td (°C)

CO2 solubility at 25 °C, 2 bar, S (molCO2/LIL, mole fraction)

C8H14N2O2

170.21

1.03

143.61

204.8

2.75, 0.313

where v, σ, and θ are the molar volume, surface tension, and contact angle, respectively. This equation permits calculation of the Kelvin diameter, dk. 2.1.2. Ionic Liquid (IL). The IL 1-ethyl-3-methylimidazolium acetate ([emim][Ac]) was selected from among the commercially available ILs because of its high degree of CO2 solubility18,19 and was supplied by Sigma-Aldrich (97% purity) and used without further purification. The supplier provided the main characteristics that are presented in Table 1. CO2 solubility, viscosity, and decomposition temperature data are from the literature.16,18,19 2.2. Preparation of Supported Ionic Liquid Membranes (SILMs). The SILMs were prepared by coating the IL onto the TiO2 or SiO2−ZrO2 surface of the inorganic membranes following two different impregnation procedures. First, the IL was progressively coated by hand with wetted gauzes, as described in our previous work.17 The excess IL was wiped with blotting papers, and the membrane was ready to use. The coating-by-hand technique effectively prepared very thin IL membranes, although some pores in the thin effective layer might not have been completely impregnated. In an attempt to reduce membrane thickness and ensure the complete impregnation of the membrane pores in the top layer, a spray-coating procedure was applied. Membrane substrates were placed in a 5,000 rpm rotating support, and the IL was applied with a spray gun for 6 s in each coating step. The temperature was maintained at 120 °C. The IL excess was carefully removed from the surface with blotting papers. As a result, the effective membrane thickness was reduced, thereby reducing both the amount of IL required and the membrane resistance to gas permeation. The samples were weighted before and after the coating procedure using an analytical balance (Mettler Toledo, USA). 2.3. Gas Permeation in the Inorganic-Based SILMs. Gas permeation through the SILMs was measured in a stainless permeation module. The inorganic supports were first evaluated for the permeation of He, H2, CO2, and N2 in single-gas permeation tests, while the study was focused on CO2 and N2 permeation for the SILMs. The effective membrane area was 4.71 cm2. A schematic drawing of the experimental apparatus appears in Figure 2. The temperature was progressively increased from room temperature (RT (25 ± 1 °C)) to 100 °C and controlled by a thermocouple. The feed gas pressure was set at 2 bar, and the permeate was open to the atmosphere. The flow rate of the permeating gas was measured using a bubble flow meter. Prior to the measurement, membranes were outgassed in a He flow under vacuum for 1 h. Additionally, samples were under vacuum for 5 min between different gas measurements in order to remove the gas from within the gas permeation equipment. 2.3.1. Calculations. The permeance of the gas was calculated using the following equation P(i) =

J (i ) Δp(i)

Figure 2. Schematic drawing of the gas experimental setup.

difference (Pa). Besides, the permeance of i, P(i), can be normalized by membrane thickness in a permeability term, P*(i) (mol·μm/(m2·s·Pa)). The ability of the membrane to separate different gases, depending on the separating layer properties, was quantified by the membrane ideal selectivity, α(i/j), which can also be expressed by the solubility, S, and diffusion, D, contribution of each gas, as follows α ( i / j) =

P*(i) S(i) ·D(i) = P*(j) S(j) ·D(j)

(3)

where α is the membrane selectivity for gas i relative to gas j.

3. RESULTS AND DISCUSSION 3.1. SEM and Pore Size Distribution in the Supports As Determined by NPP. Figures 3a and 3b show a cross section of M-1 (which contains all coating layers) at 250 and 50,000 magnification, respectively. Fingerlike voids, which would be preferred for the reduction of the resistance of gas permeation, were observed (Figure 3a). The Al2O3 substrate showed an overall thickness of ∼380 μm. The TiO2 layer prepared with 0.2 and 0.72 wt % aqueous solutions, which were coated on the outer surface of the substrate, had thicknesses of ∼250 and ∼200 nm, respectively. Finally, the top SiO2−ZrO2 sol formed a thin layer of ∼100 nm (Figure 3b). Pore size distribution was measured by NPP. Figure 4 shows the dimensionless permeance of N2 as a function of Kelvin diameter, dk (eq 1), when using hexane as a condensable vapor. The dimensionless permeance of N2 (N2 permeance at a specific feed humidity normalized with dry nitrogen permeance) was plotted as a function of the Kelvin diameter, which was calculated by assuming a contact angle of zero, and a molar volume and surface tension of v = 1.3 × 10−4 m3/mol and σ = 18.4 mN/m for hexane, respectively. The curves may correspond to a pore size distribution curve in the inorganic samples. The average pore size for the three samples, defined as 50% of the dimensionless permeance of N2, is presented in Table 2. 3.2. Gas Permeation in the Inorganic Supports. Figure 5 shows the single-gas permeance, P, of all gases tested at room

(2)

where P(i) is the permeance of i (mol/(m2·s·Pa)), J(i) is the permeate flux (mol/(m2·s)), and Δp(i) is the partial pressure 8047

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Figure 3. SEM cross-section images of M-1 at magnifications of a) 250 and b) 50,000.

was less than 9.3% (Figure 6) and 14.3% (Figure 7) of the averaged values shown. The percentage errors can be explained by the difficulties in preparing the ceramic substrates. As expected, the membrane resistance to gas permeation, 1/ PCO2, increased with increases in the number of coating steps (Figure 6). The CO2 permeance through SILM-20 after a first coating step, was PCO2 = 2.82 ± 0.24 × 10−8 mol/(m2·s·Pa), which agreed well with the value previously reported for Al2O3/ TiO2 tubes with 20 nm pores combined with [emim][Ac], PCO2 = 2.78 ± 0.11 × 10−8 mol/(m2·s·Pa).17 The obtained results were 1−2 orders of magnitude higher than those obtained in the empty inorganic supports (M-20, M2.5, and M-1), which demonstrated the effective impregnation of the IL in the pores of the support layer. However, as Figure 7 shows, the averaged CO2/N2 selectivities in SILM-20 and SILM-2.5 values were slightly lower after the first step coating. This can probably be attributed to the presence of some nonimpregnated pores in the very thin IL layer. After a second coating step, the selectivity toward CO2 remained approximately in a constant value, and, therefore, the membrane pores could be completely filled. Table 4 presents the averaged CO2 permeation and CO2/N2 selectivity values for the three samples after a second by-hand coating had ensured the complete impregnation. The results after a second-step coating were used for the remainder of the analysis in the present study. The performance of SILM systems can generally be attributed to the IL, rather than to the characteristics of the substrate. If the IL properties do not change when imbibed

temperature as a function of kinetic diameter in the inorganic supports (M-20, M-2.5, and M-1). Three samples were measured for each membrane type. A pseudo-steady-state was obtained after 1 h of experimental time. As observed, the permeance through the M-20 support was about 1 order of magnitude higher than the values obtained in the M-2.5 sample, which provided a higher permeance than the M-1 sample, as expected by the pore size diameters of the samples. The separation characteristics of ceramic membranes are commonly attributed to membrane pore size, showing a reasonable agreement between the pore size and gas permeation through the membranes.36,38 The membrane average selectivity (α), calculated using eq 3, for each gas with respect to N2 is presented in Table 3. The gas selectivity values for M-20 and M-2.5 supports were consistent with the Knudsen characteristics for the measured gases. Nevertheless, a higher selectivity value was obtained for He (2.6 Å), H2 (2.9 Å), and CO2 (3.3 Å) with respect to N2 (3.64 Å), in M-1. This may indicate a partial separation of N2 by molecular sieving in the supports with an average pore size of 1 nm. 3.3. Gas Permeation in the SILMs. Figures 6 and 7 present the membrane resistance to CO2 permeation, 1/PCO2 ((m2·s·Pa)/mol), and CO2/N2 selectivity, respectively, after applying an increasing number of by-hand coatings of IL to the inorganic supports, which resulted in the SILM-20, SILM-2.5, and SILM-1 samples. Three SILMs were prepared and measured for each membrane type. The experimental error 8048

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Figure 4. N2 dimensionless permeance as a function of Kelvin diameter, dk, in a) M-20, b) M-2.5, and c) M-1 (hexane, 40 °C).

membrane support used. The averaged PCO2 results through the three SILMs samples, however, suggest the contrary. One possible explanation for this observation may be the differences in the porosity and tortuosity of the separating layers of each sample. Previous studies compared the CO2 permeability through bulk imidazolium-based ILs and that permeability through the same ILs confined in alumina supports.23,26 The results showed that the experimentally determined porosity and tortuosity of the membranes were insufficient to explain the permeation differences between the bulk and SILM systems. Similarly in this work, it is unlikely that differences in porosity/ tortuosity between TiO2 and SiO2−ZrO2 layers are enough to explain the large differences in CO2 permeation observed between the samples. The influence of water content in each SILM system can be initially considered as a partial explanation for the observed differences in CO2 permeation. Following increasing/decreasing temperature tests, our previous work denoted the importance of considering the effect of water content in the permselectivity properties of Al2O2/TiO2-[emim][Ac] membranes.17 Table 5 compares the CO2 permeation and CO2/N2 selectivity for the SILMs before and after a high-temperature drying step (120 °C, 1 h). The PCO2 was slightly reduced for every membrane type, which led to an enhancement in α(CO2/N2) after exposure to 120 °C. In fact, the PCO2 in the SILM-20 system was 62% higher than the obtained results for the SILM based on a 1 nm pore size substrate (SILM-1). These results can be explained as the

Table 2. Pore Size Characterization of the Three Inorganic Samples membrane

type of sol in the top layer

av pore diameter, d [nm]

M-20 M-2.5 M-1

TiO2 (0.2 wt %) TiO2 (0.72 wt %) SiO2−ZrO2 (0.2 wt %)

∼20 ∼2.5 ∼1

Figure 5. Gas permeation of the three supports. Permeance was measured at RT (25 ± 1 °C) and at 2 bar.

within the inorganic support, then the membrane performance in these systems should be independent of the inorganic 8049

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Table 3. Gas Selectivity Values for the Three Samples (M-20, M-2.5, and M-1) gas selectivity, α [-] support

permeance, PN2 [10

−7

mol/(m ·s·Pa)] 2

83.5 ± 3.5 4.94 ± 0.30 0.63 ± 0.04

M-20 M-2.5 M-1 Knudsen

He/N2

H2/N2

CO2/N2

SF6/N2

2.64 2.66 4.45 2.64

3.73 3.76 5.83 3.73

0.80 0.79 3.49 0.8

0.44 0.43 0.44 0.44

Hence, two probable explanations for this observation are proposed in the present study: a) Membrane thickness: The by-hand coating procedure may lead to different IL effective membrane thicknesses after a second coating step, defining the gas permeation resistance of the different SILM systems. b) Pore size: The properties of the IL may be influenced by the presence of a solid/liquid interface in the porous system, which may depend on the pore size and surface chemistry of the inorganic membrane supports. Those possibilities are assessed in the following sections. 3.4. Effect of Membrane Thickness on SILM Performance. A theoretical IL effective membrane thickness, δt, for each sample can be estimated as the relationship of the expected permeability of CO2 in the bulk [emim][Ac], P*bulk (mol·μm/(m2·s·Pa)), and the experimentally measured permeance for the SILMs, PCO2 (mol/(m2·s·Pa)). The experimental CO2 permeance values were taken after the application of a drying pretreatment step (120 °C, 1h) and corrected for both alumina support porosity (ε = 46%41) and pore tortuosity (τ = 342). Tortuosity usually ranges between 2 and 6, averaging about 3, which is a reasonable value for inorganic mesoporous membranes.43 It should be noted that the network effect expressed into a single tortuosity factor is an approximation, since it depends on the implicit assumption that the effects of pore structure are the same regardless of the pore size distribution. The equation is as follows:

Figure 6. Membrane resistance to CO2 permeation, 1/PCO2, in SILM20, SILM-2.5, and SILM-1 as a function of number of coatings; at RT and 2 bar.

δt =

Table 4. CO2 Permeance and CO2/N2 Selectivity in the SILMs after a Second Coating Step CO2 permeance, PCO2, [10−8 mol/(m2·s·Pa)]

SILM-20 SILM-2.5 SILM-1

2.58 ± 0.24 2.26 ± 0.19 1.75 ± 0.15

(4)

According to the solution-diffusion model (eq 3), the CO2 permeability in the bulk IL, P*bulk, can be calculated as the product of CO2 solubility (Table 1) and the diffusion coefficient, DCO2 (cm2/s), which can be estimated using the correlation proposed by Morgan et al. for imidazolium-based ILs44

Figure 7. CO2/N2 selectivity in the SILMs, as a function of number of coatings; at RT and 2 bar.

membrane

P*bulk ε · PCO2 τ

DCO2 = 2.66 × 10−3·

gas selectivity, α(CO2/N2), [-] 33.17 ± 4.81 33.98 ± 5.09 34.09 ± 4.34

1 μIL0.66 ·VCO21.04

(5)

where μIL is the IL viscosity (Table 1), and VCO2 is the molar volume of carbon dioxide at the normal boiling point (37.22 cm3/mol). The diffusion coefficient of CO2 in the IL resulted in a DCO2= 2.33 × 10−6 cm2/s. Table 6 presents the estimated IL effective membrane thicknesses, δt, (μm) for each sample. In addition, the amount of immobilized IL in the samples was weighted before and after the coating procedure, and the IL impregnation thicknesses, δ, were calculated according to membrane area and IL density (Table 1). The averaged results are presented in the table.

removal of the water content in the IL, as well as the absorbed water in the ceramic supports, which led to a higher IL viscosity as conventionally reported in literature.39 The effect of water in the SiO2−ZrO2 top layer sample (SILM-1) was more pronounced, as expected for the higher water sorption of SiO2-based materials in an open atmosphere.40 In any case, the effect of water alone seems insufficient to explain the differences in CO2 permeation through the samples. 8050

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Table 5. PCO2 and α(CO2/N2) at RT before/after Exposure to 120 °C CO2 permeance, PCO2 [10−8 mol/(m2·s·Pa)]

gas selectivity, α(CO2/N2) [-]

membrane

RT

RT (dried at 120 °C)

PCO2 reduction [%]

RT

RT (dried at 120 °C)

SILM-20 SILM-2.5 SLIM-1

2.58 ± 0.24 2.26 ± 0.19 1.75 ± 0.15

2.28 ± 0.16 1.98 ± 0.12 1.41 ± 0.07

11.62 12.12 17.64

33.17 ± 3.56 33.98 ± 2.92 34.09 ± 3.35

34.15 ± 0.33 34.27 ± 0.48 35.08 ± 0.67

Table 6. Estimated Thicknesses from Gas Permeance, δt, and from the Impregnation Amount of IL, δ, for SILM-20, SILM2.5, and SILM-1 membrane

thickness (from gas permeance), δt (P*bulk/PCO2) [μm]

thickness (from impregnation amount), δ [μm]

SILM-20 SILM-2.5 SILM-1

0.68 ± 0.05 0.78 ± 0.08 1.1 ± 0.08

1.39 ± 0.13 1.52 ± 0.10 1.58 ± 0.19

determined thicknesses (by weight) presented in Table 6 are not enough to explain the differences in CO2 permeation through the samples. It appears that the IL properties were different when confined to the inorganic supports with different pore sizes. Recent studies23,26 have reported the effect of imidazoliumbased ILs confinement in Al2O3 membranes (pore size: 20 nm). The CO2 permeability for confined ILs was found to increase over that of bulk IL by a factor of 15.7,26 which was attributed to a change in physical properties of the IL at the presence of a pore surface due to a reorganization of the cation and anion at the interface. If this is the case, the surface effect would be expected to be more important for smaller pore systems compared to the bulk IL, leading to higher CO2 permeabilities with decreases in support pore size. The experimental results from this study, however, showed the opposite and the CO2 permeabilities in SILM-20 and SILM-2.5 were considerably higher than those in the SILM-1 system. This agreed with the results reported by Close et al.,23 where the CO2 permeance measured for an Al2O3 support with 100 nm nominal pores, in combination with [emim][Tf2N] (PCO2 = 2.65 × 10−8 mol/ (m2·s·Pa)), was larger than the values measured with membranes prepared with 20 nm pores using the same material (PCO2 = 1.05 × 10−8 mol/(m2·s·Pa)), where the IL effective thickness of the SILMs were assumed to be equal to the thickness of the alumina support (60 μm). The results from the present study may also be explained by the significance in comparing the affinity (i.e., hydrogen bonding or electrostatic interactions) between the [emim][Ac] and the SiO2−ZrO2 top layer (SILM-1) with the affinity between the IL and TiO2 layer (SILM-20 and SILM-2.5). This may define the cation and anion reorganization at the wall of the pores and the penetration depth of the IL organization in the pores. The importance of considering the affinity of the IL and a substrate material has been previously proven in the literature when evaluating the stability of imidazolium-based ILs in polymeric supports with different hydrophobicities.11,14 3.5. Effect of Support Pore Size on the Performance of SILMs. The interest in the phase behavior or property variation of ILs immobilized in nanosized surfaces or porous networks has increased in recent years.27−33 The results have sometimes revealed that some ILs have organized structures in the vicinity of a solid interface that depends on membrane pore size,27,30 which may influence the permeation of compounds. CO2 permeation through SILMs is commonly attributed to the IL viscosity, while selectivity is scaled according to IL molar volume.25 Therefore, one reasonable explanation for the different permeation values obtained in this work may be the different viscosity of the IL when imbibed in substrates with different pore sizes, as previously reported for water systems.34,45 Ternan assumed that the viscosity in a small pore could be divided into two partsbulk viscosity and an incremental contribution caused by proximity of the pore walland successfully explained the temperature dependency

The calculated thicknesses by weight measurements, δ, resulted in values that were higher than those theoretically predicted from solubility and diffusivity in the bulk IL, δt. The thicknesses obtained may indicate the complete impregnation of the SiO2−ZrO2 top layer (∼100 nm) and TiO2 layer thicknesses (∼200 and ∼250 nm), as well as the partial wetting of the Al2O3 substrate, as observed via SEM (Figure 4). Besides, the thicknesses experimentally obtained by weight denoted that the IL was mainly impregnated in the alumina support. Therefore, CO2 permeation through the samples may be on dependence not only to porosity/tortuosity of the outer layer but mainly to the characteristics of the alumina substrate (ε = 46%, τ = 3). In order to include the membrane thickness in the analysis, Figure 8 shows the experimental CO2 permeability, P*CO2

Figure 8. CO2 permeability, P*CO2, calculated using experimental CO2 permeance and the thicknesses obtained by impregnation weight (δ).

(mol·μm/(m2·s·Pa)), as calculated according to the CO2 permeance experimental results and the measured IL effective thicknesses (by weight), δ. In the SILM-20 and SILM-2.5 samples, the experimental CO2 permeability values were consistently higher than those obtained in the IL membrane based on substrates with 1 nm pores (SILM-1). In fact, the average CO2 permeability for SILM-20, P*CO2 = 2.08 ± 0.21 × 10−7 mol·μm/(m2·s·Pa) was 48% higher than the obtained values for the SILM-1 sample. Therefore, as observed in the figure, the experimentally 8051

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Figure 9. Temperature dependence of a) CO2 permeation and b) CO2/N2 selectivity, for SILM-20, SILM-2.5, and SILM-1 samples. Dotted lines in b) are eye-guides.

Figure 10. Normalized: a) CO2 permeability (P*CO2/P*(25 °C)) and b) (P*N2/P*(25 °C)) for SILMs with different pore sizes as a function of temperature. Dotted line in a) shows estimated values (predicted from solubility and diffusivity in the bulk IL).

that permeability at 25 °C (P*/P*(25 °C)). These values were compared to the normalized CO2 permeability in the bulk IL predicted from solubility and diffusivity (eq 3). The theoretical line was based on the experimental CO2 solubility16 and viscosity measurements49 at different temperatures reported for [emim][Ac] in the literature. These values for CO2 are shown in the Supporting Information (SI). There are no consistent experimental data in the literature for N2 solubility in [emim][Ac]. The normalized permeabilities should be the same for the three samples and for the bulk phase if the transport is defined only by the IL. However, normalized permeabilities increased differently. The inorganic membrane with the smallest pore size (SILM-1) clearly showed a larger slope for CO2 and N2, while the values in SILM-20 and SILM-2.5 systems agreed well with those estimated for the bulk phase. These results may reveal the importance of considering the solid/liquid interactions in the nanosized pores of the membranes when evaluating the performance of the SILM systems. Table 7 summarizes the average activation energy for CO2 and N2 permeation, Ep (kJ/mol) in the SILM systems and bulk phase, as obtained from the Arrhenius equation. As observed, the Ep increased for membranes with smaller pore sizes, as previously found when evaluating ceramic membranes with different pore sizes for water systems.34 In

of diffusivity through absorbents by relating diffusivity of solutes and solution viscosity.46 Figures 9a and 9b show the temperature dependency for CO2 permeation and CO2/N2 selectivity, respectively (average results from three replicates were within 5.8% of the values shown). Membranes were dried at 120 °C for 1 h before the tests. With an increase in the permeation temperature from RT (25 ± 1 °C) to 100 °C, the CO2 permeance increased similarly in the three samples as observed in the figure. However, the experimental data showed an increase in permeation of 5.3- to 8.4-fold, depending on the support pore size, which may be attributed to a reduction in IL viscosity and a consequent reduction in the membrane gas permeation resistance.16,21,47,48 In particular, the CO2/N2 selectivity (Figure 9b) dependence on temperature was found to be independent of the membrane pore size, indicating that IL property changes resulting from confinement in the support do not appear to only impact CO2 but also impact N2 permeation in the same manner. It should be noted that all the SILMs showed a CO2 permeance value higher than 10−7 mol/(m2·s·Pa) with CO2/N2 selectivities higher than 20, confirming the possibilities for high temperature applications. To further understand the effect of membrane pore size, Figure 10a and 10b show the CO2 and N2 permeabilities, respectively, as a function of temperature when normalized by 8052

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Table 7. Activation Energy, Ep, for Permeation through the SILM Samples and the Bulk Phase Ep [kJ/mol] membrane

CO2

N2

SILM-20 SILM-2.5 SILM-1 Bulk IL

20.65 ± 0.17 21.55 ± 0.36 26.04 ± 0.23 20.49

25.34 ± 0.44 25.93 ± 0.83 30.84 ± 0.66 -

particular, the Ep in SILM-1 was considerably higher than expected for the bulk phase. These differences cannot be explained by the effect of a thermal expansion of the membrane pores in ceramic membranes, as it can also be encountered in polymeric materials. The evidence from the present study may suggest that the interaction between the membrane surface and the IL may be responsible for the IL properties (i.e., viscosity) on the liquid/solid interface, defining the gas permeabilities through SILM systems. Compared with the values for SILM-20, if we attribute all of the decrease in CO2 permeability in the SILM-1 system to an increase in viscosity, a 73% increase in IL viscosity would be needed to explain the change in diffusivity in the systems (eq 5). This result may indicate the presence of an additional phenomenon affecting the SILM performance. Banu et al. suggested that CO2 permeability through SILMs may be additionally defined by the presence of micropore channels in the IL structure.26 These channels may be an indicator of the IL free-volume and may define the CO2 permeability. The importance of the IL channels on SILM performance would not be unexpected with a reorganization of the IL structure in response to the presence of a pore wall, which may depend on surface-IL interactions. More study is needed in order to determine the effect of reorganization on ILs in confined systems. As a consequence, the most effective research on CO2 separation through SILMs would focus not only on IL properties but would also examine the effect of pore size and surface chemistry on the stability and permeation properties of SILMs. The findings from the present study may provide insights for future research on changes in IL properties after confinement in the pores of different substrates. 3.6. Reducing the Effective IL Membrane Thickness via Spray-Coating. For the development of high-permeance SILMs, a spray-coating procedure was applied as described in Section 2.2. The spray-coating methodology has proven to be adequate for the production of homogeneous and reproducible thin layers on porous materials.50 The SILM-2.5 sample was selected for a proper impregnation of the support pores with a lower amount of IL, simultaneously enabling a higher CO2 permeability (Figure 8). The samples were coated until the selectivity toward CO2 was similar to the previous data. In addition, in order to further reduce membrane thickness, the IL was progressively diluted in water and spray-coated in the same manner. The membranes were dried at 120 °C for 1 h before testing. The permeance and CO2/N2 selectivity results are presented in Figure 11 (averaged results from three replicates were within 7.6% of the values shown). Two spray-coating steps were needed in the case of pure [emim][Ac] and for a 50 wt % IL aqueous solution. Three steps were used to apply a 25 wt % IL aqueous solution. Further reductions in IL concentration led to a remarkable decrease in CO2/N2 selectivity after the application of five

Figure 11. CO2 permeance and CO2/N2 selectivity in SILM-2.5 after impregnation of [emim][Ac] aqueous solutions via spray coating; RT and 2 bar.

coating steps. Values are presented in permeance (mol/(m2·s· Pa)) due to the difficulties in accurately measuring the amount of impregnated liquid in a very thin-IL layer when spraying the diluted IL. It should be noted that the estimated thickness (by weight) was δ = 0.94 μm for the SILM-2.5 sample prepared with pure IL. This led to a CO2 permeability of P*CO2 = 1.89 × 10−7 mol·μm/(m2·s·Pa), which agreed well with values shown for the same membrane prepared via a hand-coating procedure, P*CO2 = 1.97 × 10−7 mol·μm/(m2·s·Pa), and confirmed the possibility for a reduction in the effective IL membrane thickness via spray coating. As Figure 11 shows, CO2 permeation can still be enhanced by reducing the effective IL membrane thickness via a spray method. In fact, when spraying pure IL, CO2 permeation was increased from PCO2= 3.09 ± 0.11 × 10−8 mol/(m2·s·Pa) to PCO2= 4.31 ± 0.13 × 10−8 mol/(m2·s·Pa) for a 25 wt % IL aqueous solution. This maximum CO2 permeance was achieved with no important impact on CO2 selectivity, α(CO2/N2)= 31.18 ± 1.34, and may correspond to a theoretical IL effective membrane thickness (δt = P*bulk/PCO2) of 0.36 μm. This value is in the expected range of the outer TiO2 layer thickness, as observed by SEM (Figure 3). Further dilution of the IL led not only to increases in CO2 permeation but also to a remarkable decrease in selectivity. This can be partially explained by the difficulties in the complete impregnation of the outer membrane pores upon further dilution of the IL. However, because the enhancements in CO2 permeance were not excessive, it was also probable that CO2 permeation was limited by the IL-CO2 chemical reactions that took place at the IL interface. A previous study determined that the acetate anion of [emim][Ac] is able to form chemical complexes with CO2 at a 1:2 ratio of CO2:RTIL.18 This may indicate that the performance of SILM-2.5 may not be uniquely controlled by diffusion across the membrane (dependent on membrane thickness) but also by the CO2-[emim][Ac] chemical reaction rate at the gas/liquid interface, as it was previously observed when testing the CO2 separation performance of EMISE ionic liquid and polypropylene membrane contactors.8 Therefore, CO2 separation properties (permeance and selectivity) may be highly influenced by the operation conditions, such as CO2 partial pressure, temperature, etc. In 8053

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supported ionic liquid membranes (SILMs) for CO 2/N2 separation. The samples were first prepared by a hand-coating procedure and then by spray coating, which led to a CO2 permeance as high as PCO2= 4.31 ± 0.13 × 10−8 mol/(m2·s·Pa) with an ideal CO2/N2 selectivity of α(CO2/N2)= 31.18 ± 1.34, which outperformed the state-of-the-art materials currently used for CO2/N2 separation. The main findings from this work are as follows: (a) The permeance of CO2 through membranes prepared on a support with 20 nm pores (SILM-20), PCO2 = 2.28 ± 0.16 × 10−8 mol/(m2·s·Pa), was 62% higher than that obtained for samples with 1 nm pores (SILM-1). The effect of the water content of the SILMs was insufficient to explain the differences in CO2 permeation between the samples. (b) Experimental CO2 permeability values through dried samples were higher for the SILM-20 system, P*CO2 = 2.08 ± 0.21 × 10−7 mol·μm/(m2·s·Pa), compared with those obtained using the SILM-1 system, which indicated that CO2 permeation differences through the samples cannot be uniquely explained by the ionic liquid effective membrane thicknesses of each SILM system. This suggests that the properties of the ionic liquid are different when confined in the inorganic supports with different pore sizes. (c) With an increase in temperature from 25 ± 1 to 100 °C, the permeation of CO2 increased 5.3- to 8.4-fold, depending on the support pore size of the substrate. The activation energy for the permeation of CO2, Ep, was remarkably higher for the membrane with the smallest pore size (SILM-1), Ep = 26.04 ± 0.23 kJ/mol. These results suggest that the interactions of [emim][Ac] and the inorganic supports defined the properties of the ionic liquid (by cation and anion reorganization) and, therefore, defined the degree of CO2 permeation through the membranes. In short, the most effective investigation on CO2 separation performance through SILMs should focus not only on the development of new task-specific ILs but also on an evaluation into the effect of pore size and membrane surface chemistry on the separation performance of SILM systems.

general, CO2 permeation controlled by a chemical reaction can be achieved at either a high diffusion rate (thin membrane thickness) or at a low reaction rate (low CO2 partial pressure, low temperature, etc.), while diffusion-control can be achieved at either a low diffusion rate (thick membrane thickness) or at a high reaction rate (high CO2 partial pressure, high temperature, etc.). CO2 permeation properties are higher in reaction-control than in diffusion-control systems. In our previous paper, the CO2 permeance of [emim][Ac] supported on TiO2/Al2O3 with an estimated IL thickness of 0.56 μm was independent of CO2 partial pressure when the pressure in the feed was higher than 1 bar and the permeate was open to the atmosphere, which confirmed that the diffusion-controlled CO2 permeation was dominant. However, the occurrence of a thin layer prepared via the spray coating of a 25 wt % IL aqueous solution with subsequent drying might have shifted the permeation resistance from diffusion-control to reaction-control. In this case, probably a dual-mode facilitated transport model would be more accurate to analyze the diffusion in the very thin layer-system prepared by spray-coating procedure. Finally, the CO 2/N 2 separation performance of the developed ultrathin IL inorganic membrane-based SILM-2.5 (25 wt % IL) was compared with that of the highest-performing CO2 permeation membrane materials. The permeance, PCO2 (mol/(m2·s·Pa)), values are presented as SI. Figure 12 graphically compares the results from this work with those previously reported in the literature.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 12. CO2/N2 selectivity as a function of PCO2 (mol/(m2·s·Pa) in SILM-2.5 prepared via spray-coating at different IL concentrations and by hand coating at 100 °C, compared with that of other materials. Spray-coating concentrations were as follows: 100 wt % IL (the highest selectivity), 50, 25, 10, 5, and 2.5 wt % IL (the lowest selectivity).

A table presenting the experimental CO2 solubility and viscosity values at different temperatures for [emim][Ac] and a table comparing the CO2/N2 separation performance of the developed ultrathin IL membrane-based SILM-2.5 (25 wt % IL) with literature are shown. This material is available free of charge via the Internet at http://pubs.acs.org.

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As observed from the permeance values and selectivity toward CO2, the very thin SILMs developed in the present study may offer strong competition to current polymeric materials, as well as to SILMs based on polymeric and ceramic supports. This reinforces the plausibility of using SILMs based on ceramic supports for certain applications, including hightemperature separation processes.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81 824 24 7714. Fax: +81 824 22 7191. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS Nanoporous TiO2 and SiO2−ZrO2 membranes with controlled pore sizes ranging from 1 to 20 nm were prepared by sol−gel processing and were combined with a high CO2 solubility ionic liquid, 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), in

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Japan Society for the Promotion of Science, under the Postdoctoral Fellowship for Foreign Researchers FY2012. 8054

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NOMENCLATURE d = pore diameter (nm) dk = Kelvin diameter (nm) D = diffusion coefficient (cm2/s) Ep = activation energy for permeation (kJ/mol) J = permeate flux (mol/(m2·s)) M = molecular weight (g/mol) p = partial pressure (Pa) P = gas permeance (mol/(m2·s·Pa)) P* = gas permeability (mol·μm/(m2·s·Pa)) S = gas solubility (molCO2/LIL, molar fraction) T = temperature (°C) v = molar volume (cm3/mol)

Greek letters

α σ δ θ μ ρ

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gas selectivity surface tension (mN/m) membrane thickness (μm) contact angle (deg) viscosity (cp) density (g/cm3)

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