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Facilitated O2 transport membrane containing Co(II)-salen complex-based ionic liquid as O2 carrier
Author’s Accepted Manuscript Facilitated O2 transport membrane containing Co(II)-salen complex-based ionic liquid as O2 carrier A. Matsuoka, E. Kamio, T. Mochida, H. Matsuyama www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(17)30861-X http://dx.doi.org/10.1016/j.memsci.2017.07.027 MEMSCI15432
To appear in: Journal of Membrane Science Received date: 24 March 2017 Revised date: 10 July 2017 Accepted date: 12 July 2017 Cite this article as: A. Matsuoka, E. Kamio, T. Mochida and H. Matsuyama, Facilitated O2 transport membrane containing Co(II)-salen complex-based ionic liquid as O2 carrier, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facilitated O2 transport membrane containing Co(II)-salen complex-based ionic liquid as O2 carrier a
A. Matsuoka , E. Kamioa, T. Mochidab, H. Matsuyamaa* aCenter
for Membrane and Film Technology, Department of Chemical Science and
Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan bDepartment
of Chemistry, Graduate School of Science, Kobe University, 1-1
Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan *Corresponding
author. Tel.: +81 78 803 6180; fax: +81 78 803 6180. [email protected]
Abstract Novel metal-containing ionic liquids (MCILs) with O 2 absorbability were synthesized by mixing a N,N'-bis(salicylidene)ethylenediamine cobalt(II) (Co(salen)) complex and ionic liquid-based
ligands.
The
couple
of
anions
of
the
ionic
liquid-based
ligands,
trihexyl(tetradecyl)phosphonium N-methylglycinate and trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide, coordinated to Co(salen) complex and gave negative charge to the neutral Co(salen). The negatively chaged Co(salen) complex with the ionic liquid-based ligands and the bulky trihexyl(tetradecyl)phosphonium cation allowed to form Co(salen) complex-based MCILs. The MCILs could chemically and selectively absorb a large amount of O2 and could function as an O 2 carrier in supported MCIL membranes. The O2 permeability of the MCIL-based facilitated O2 transport membranes were higher than those of conventional fixed-O2 carrier membranes as well as polymer membranes. The fast O2 transportation could be realized from the high diffusivity of the O 2 complex, which was formed via the reaction between MCIL and O 2, owing to the liquidity of the MCILs. Graphical Abstract
Keywords: Oxygen separation; Facilitated transport; Metal-containing ionic liquid; Cobalt(II)-salen complex; Ionic liquid-based ligands
1. Introduction Oxygen-enriched air is used in various medical, chemical, and environmental applications such as medical devices, steel manufacturing, and chemical manufacturing, as well as in coal-fired power plants utilizing a novel combustion technique called oxy-fuel combustion [1-4]. In all of these applications, the energy consumption and monetary cost of producing oxygen are mainly affected by the process efficiency. Because the current cryogenic distillation and pressure swing adsorption methods are energy and cost intensive [5, 6], alternative oxygen separation processes for oxygen-enriched air production are highly desired. One attractive alternative is membrane technology, which provides several advantages over the current separation technologies, including its high energy efficiency, ease of processing and scaling up, simple and compact modular equipment, and low capital cost [3, 6, 7]. Polymer membranes have been widely investigated for O2/N2 separation. However, in the current stage, the O2 permeability and O2/N2 selectivity of polymer membranes are insufficient for practical applications [8]. Moreover, there is trade-off limitation between the O2 permeability and O2/N2 selectivity. As inorganic membranes, perovskite-based O2 transport membranes have recently been investigated [9-11]. Although a perovskite membrane allows the permeation of oxygen ions with an infinite O2/N2 selectivity, sufficient O2 permeability was achieved at a highly elevated temperature of more than 973 K [3, 10, 11]. The development of a membrane with promising O2 permeability at a lower operating temperature is a critical issue to realize an O2 separation process that conserves energy. Facilitated transport membranes (FTMs) are a well-known class of functional membranes that consist of a polymer matrix and specific chemical compound, the so-called “carrier,” which can chemically and selectively react with a specific gas molecule [12-15]. Because of the effect of the carrier, FTMs can realize the rapid and highly selective permeation of the target gas. This concept was first used for O2 separation in a supported liquid membrane with hemoglobin and then with a cobalt Schiff base complex [16, 17]. Although such metal complexes are crystalline solids, they were used as the mobile O2 carriers in some preferable solvent. However, because of the poor O2 permeation performance due to the poor solubility limitation of the carrier and low stability due to the solvent vaporization, the liquid membrane was altered using a solid polymer membrane with fixed metal complexes on the polymer matrix as the O2 carrier [18-24]. This fixed O2 carrier membrane improved the stability of the facilitated O2 transport membrane. However, the O2 permeability was still insufficient because of the large diffusion resistance of the solid membrane [15]. To improve the stability of a mobile carrier membrane, we previously proposed a novel type of FTM that utilized a task-specific ionic liquid (TSIL) for CO2 separation [25-29]. The TSIL played the role of the mobile carrier, as well as the diffusion medium in the membrane. Because an ionic
liquid (IL) has a non-volatile property, it overcomes the stability limitation of the liquid-type FTM. Moreover, the liquidity of an IL overcomes the mobility issue of fixed carrier membranes. Based on the same concept, an O2 reactive TSIL could be expected to be a promising carrier for a facilitated O2 transport membrane. The major challenge is the development of the O2 reactive IL. A straightforward strategy to create an O2 reactive IL is the utilization of a metal complex with O2 reactivity. Recently, metal-containing ionic liquids (MCILs) have attracted great attention as novel functional ILs with a specific property of the metal complexes [30-32]. In 2015, a Co(II)-based-MCIL with selective O2 absorbability was firstly reported by Kohno et al. [33]. However, until now, there has been no report on the utilization of O2 reactive MCILs in an FTM. This might be because of the insufficient properties of MCILs as mobile O2 carriers. If the physico-chemical properties of an MCIL, such as its O2 reactivity and viscosity, could be easily tuned, FTMs with outstanding O2 permeability, O2 permselectivity, and long-term stability could be developed using MCILs. In general, the properties of ILs, including MCILs, are controlled by designing their molecular structures via organic synthesis, which is often complicated. Therefore, the main challenge for an MCIL-based facilitated O2 transport membrane is the development of a simple synthesis method for the MCIL. One powerful option is a solvate IL, which is a novel type of IL composed of a metal ion and some ligands [34, 35]. Solvate ILs can easily be synthesized by simply mixing the metal salt with the ligands. The properties of a solvate IL could also be easily controlled by changing the kind of ligands. Here, based on the concept of solvate ILs, we present novel O2 reactive MCILs that can be easily synthesized by mixing an O2 reactive metal complex with an IL-based ligand. 2. Experimental 2.1 Synthesis and characterization of Co(salen) complex-based ILs N,N'-bis(salicylidene)ethylenediamine cobalt(II) (Co(salen); >95%, purchased from Tokyo Chemical Industry Co. (Tokyo, Japan) and used as received) was selected as the O2 reactive metal complex. It is well known that the Co(salen) complex has a square planar geometry, to which axial ligands can also be coordinated. When an IL composed of a bulky cation and metal coordinating anion (hereafter the IL is written as IL-based ligand) is mixed with the Co(salen) complex, it is expected that the anion would coordinate with the Co(salen) complex as the axial ligand and give it a negative charge. The resulting Co(salen) complex with the negatively charged ligand could act as an anion to form an MCIL with a bulky counter cation. In this proof-of-concept study, we selected IL-based ligands composed of N-methylglycinate (N-mGly-) and bis(trifluoro-methanesulfonyl)imide (Tf2N-) as the coordinating anions and the trihexyl(tetradecyl)phosphonium cation (P66614+, where the subscripts “6” and “14” represent the carbon numbers of the alkyl chains). The detailed explanation of the synthesis of IL-based ligands,
including the reagents used, is provided in the Supporting Information.
2.1.1 Synthesis of [P66614]2[Co(salen)(N-mGly)2] The solid Co(salen) complex and [P66614][N-mGly] in a 1:2 molar ratio were mixed at room temperature in a nitrogen atmosphere. A certain amount of Co(salen) was agitated with ethanol in a flask under a nitrogen atmosphere. It is worth noting that the Co(salen) was hardly dissolved in ethanol, but was suspended in the ethanol. In a different batch, a certain amount of synthesized [P66614][N-mGly] was dissolved in ethanol. The [P66614][N-mGly] solution was added to the Co(salen)/ethanol suspension by dropwise addition. When the Co(salen)/ethanol suspension was mixed with the [P66614][N-mGly] solution, the solid Co(salen) was gradually dissolved. The mixture was continuously agitated for 3 h at 303 K. Finally, the product was kept in vacuo at 323 K for 12 h to remove the ethanol.
2.1.2. Synthesis of [P66614]2[Co(salen)(N-mGly)(Tf2N)] A certain amount of Co(salen) was agitated with ethanol in a flask under an air atmosphere. In a different batch, an equimolar amount of synthesized [P66614][N-mGly] (relative to Co(salen)) was dissolved in ethanol. The [P66614][N-mGly] solution was added to the Co(salen)/ethanol suspension by dropwise addition. When the Co(salen)/ethanol suspension was mixed with the [P66614][N-mGly] solution, the solid Co(salen) was gradually dissolved. The mixture was continuously agitated for 3 h at 303 K. Under an air atmosphere, the Co(salen) was fully dissolved in the solution, even though an equimolar amount of [P66614][N-mGly] was added to the system. In this case, O2 would be coordinated with [P66614][Co(salen)(N-mGly)] and form a stable six-coordinated octahedral geometry. After 3 h of agitation, an ethanol solution containing an equimolar amount of synthesized [P66614][Tf2N] (relative to Co(salen)) was added to the mixture and agitated for an additional 1 h at 303 K. Finally, the ethanol in the product was evaporated at 313 K for 3 h.
2.1.3. Characterization of [P66614]2[Co(salen)(N-mGly)2] The synthesized MCILs were characterized using Fourier transform infrared spectroscopy (FT-IR spectrometer, NIKOLET iS5 with id5 ATR accessory, Thermo Fisher Scientific Co. Ltd.), CHN elementary analysis, mass spectrometry (ESI-MS, LTQ Orbitrap Discovery, Thermo Fisher Scientific Co. Ltd.), UV/vis absorption spectrometry (UV-vis spectrophotometer, V-650, JASCO Co.), thermogravimetry-differential thermal analysis (TG-DTA, Thermo plus EVO II TG8120, Rigaku Co.), and viscosity measurements (rheometer, MCR501, Anton Paar Co. Ltd.).
2.2 Gas absorption experiments The amounts of O2 and N2 absorbed into the MCILs were measured using our previously
reported method and apparatus [28]. The apparatus, which consisted of stainless steel tubes, and reference and sample cells, is illustrated in Fig. S4. The MCILs used in the gas absorption test were first degassed to remove the considerable amount of O2 absorbed from the air. The temperature of the system was kept constant at 303 ± 0.1 K by a water bath (T-105B; Thomas Kagaku Co., Ltd., Tokyo, Japan). The reference and sample cells were degassed under vacuum for more than 1 h to remove air, charged with pure N2 to completely remove the considerable amount of O2 that remained in both cells, and again degassed under vacuum for more than 1 h. Then, the MCIL was loaded through the septum using a syringe. The weight of the loaded MCIL was calculated from the weight change of the syringe before and after the MCIL loading. After the MCIL loading, the system was fully evacuated again to completely remove the gas in the system. After the degasification of the cells, the valve separating the two cells was closed. Then, the reference cell was pressurized to charge a known amount of O2 or N2. After the gas was charged, the stirrer was turned on, and the MCIL was then constantly stirred throughout the experiment. The gas absorption began when the valve connecting the two cells was opened, and the gas fed into the reference cell was transferred into the sample cell to come into contact with the MCIL. The pressure drop from the absorption of the gas by the MCIL was measured with a digital pressure gauge (Model AM-756 digital manometer; GE Sensing & Inspection Technologies Co., Ltd.). The pressure was monitored until it remained constant. Equilibration was usually attained within 12 h. After equilibration was attained, the final pressure was measured, and the amount of gas absorbed was determined from the observed pressure change.
2.3 Gas permeation test We prepared supported IL membranes (SILMs) using the two MCILs and evaluated their O2 permeabilities. Although the viscosities of the MCILs synthesized in this study were high, they could be impregnated in a porous support membrane. The MCIL-based SILMs were prepared as follows. A hydrophilic PTFE microporous support membrane with a thickness of 35 μm and pore size of 100 nm, purchased from Toyo Roshi Kaisha, Ltd., Japan, was immersed into the synthesized MCIL ([P66614]2[Co(salen)(N-mGly)2]
or
[P66614]2[Co(salen)(N-mGly)(Tf2N)])
and
immediately
decompressed for 3,600 s to completely exchange the air in the PTFE membrane with the MCIL. The MCIL-impregnated PTFE membrane was taken out, and the excess MCIL was wiped off the surface. The permeation test was carried out using an O2/N2 mixture with the sweep method. O2 and N2 gases with a purity of 99.9% were used to prepare the O2/N2 mixture with desired O2 partial pressure. A schematic of the experimental setup of the gas permeation test is shown in Fig. S5(a). The gas transport properties of the membrane were measured using a flat-type permeation cell (Fig. S5(b)) placed in a thermostatically controlled oven adjusted to 303 K. The permeation cell was made of
stainless steel and had an effective permeation area of 2.88 cm2. A model feed gas was prepared by mixing the O2 and N2 under a dry condition. The total pressure on the feed side was maintained at atmospheric pressure. The flow rates of the O2 and N2 were controlled using mass flow controllers (Hemmi Slide Rule Co. Ltd., Japan) to adjust the O2 and N2 partial pressures as desired. The total flow rate of the feed gas was adjusted to 7.45 × 10-5 mol/s at 298 K and 101.3 kPa. The flow rates of the feed streams were measured using soap-film flow meters (HORIBA STEC Ltd., Japan). Helium was supplied to the permeate side of the cell as a sweep gas. The flow rate was 2.98 × 10-5 mol/s at 298 K and 101.3 kPa. The flow rate of the sweep gas was also measured using a soap-film flow meter. The pressure on the sweep side was maintained at atmospheric pressure. The conditions for the gas permeation test were summarized in Table 1. The sweep gas containing the O2 and N2 permeated through the MCIL-based supported liquid membranes was introduced to a gas chromatograph (VARIAN µGC 490-GC, column: Molsieve 5A, 10 m) to determine the composition of the permeated gases.
Table 1 Experimental conditions of gas permeation test Conditions Gases 303 Temperature Pressure Feed 101.3 (atmospheric) Sweep 101.3 (atmospheric) Pressure difference 0 Gas flow rate Feed total 100 O2 1, 2.5, 5, 10, 15, 20 N2 99, 97.5, 95, 90, 85, 80 Sweep He 40 Relative humidity Feed 0 (dry) Sweep 0 (dry)
Unit K kPaA kPaA kPaA mL/min mL/min mL/min mL/min % %
3. Results and discussion 3.1 Synthesized [P66614]2[Co(salen)(N-mGly)2] As expected, the obtained material was liquid at room temperature. A photograph of the synthesized
MCILs
are
shown
in
Fig.
1(a).
The
product
synthesized
with
the
[P66614][N-mGly]/Co(salen) molar ratio of 1/2 was analyzed using FT-IR. The FT-IR spectrum of the product is shown in Fig. 2, along with those of [P66614][N-mGly] and the Co(salen) complex. The assigned peaks are listed in Table 2. Based on the results, it was confirmed that the obtained liquid was composed of the Co(salen) complex and [P66614][N-mGly]. The C/H/N mole ratio of the liquid, as determined by the elemental analysis, was 68.9/11.0/3.6, as listed in Table S4. This C/H/N mole ratio indicated that the liquid consisted of one mole of the Co(salen) complex and two moles of [P66614][N-mGly]; in this case, the theoretical C/H/N mole ratio was 70.3/11.1/3.8. To investigate
whether [P66614][N-mGly] coordinated with the Co(salen) to form the MCIL or acted as a solvent of the Co(salen), we also synthesized liquids by mixing the Co(salen) complex and [P66614][N-mGly] in 1:1 and 1:1.3 molar ratios. It should be noted that no solid remained when the added amount of [P66614][N-mGly] was larger than double the equimolar amount of Co(salen). On the other hand, when the amount of [P66614][N-mGly] was less than double the equimolar amount of Co(salen), some amount of solid remained in the solution. In such cases, the solid residue was filtered and collected. The filtrate was washed with ethanol several times and dried in a vacuum oven. Small amount of the solid was dissolved in ethanol and analyzed using electrospray ionization-mass spectrometry. It was confirmed that the solid residue was unreacted Co(salen); ESI-MS: m/z calc for Co(salen)-: 325.0384; found: 325.0381 (Fig. S4).
(a)
Fig.
1
Photos
of
(b)
synthesized
MCILs:
(a)
[P66614]2[Co(salen)(N-mGly)2]
and
[P66614]2[Co(salen)(N-mGly)(Tf2N)]
[P66614]2[Co(salne)(N-mGly)2]
Tranmittance / a.u.
[P66614]2[Co(salne)(N-mGly)(Tf2N)]
[P66614][N-mGly]
[P66614][Tf2N] Co(II)-salen complex
3700
3200
2700
2200
Wavenumber / Fig.
2
FT-IR
spectra
of
synthesized
1700
1200
700
cm-1
[P66614]2[Co(salen)(N-mGly)2],
(N-mGly)(Tf2N)], [P66614][N-mGly], [P66614][Tf2N], and Co(salen) complex
[P66614]2[Co(salen)-
Table 2 Assignments of FT-IR absorption spectra of synthesized [P66614]2[Co(salen)(N-mGly)(Tf2N)], Co(salen) complex, [P66614][N-mGly], and [P66614][Tf2N] Wavenumber / cm-1 [P66614]2[Co(salne)- [P66614]2[Co(salne)Co(II)-salen (N-mGly)2] (N-mGly)(Tf2N)]
[P66614][N-mGly] [P66614][Tf2N]
Assignment
2920
2920
2920
2920
-CH2- symmetric stretching
2850
2850
2850
2850
-CH3 symmetric stretching
1635
1635
1600
1600
1600
1470
1470
1470
1380
1380
1380
1635
-C=N- stretching[36] -COO- asymmetric stretching[37] 1470
-CH3 degeneracy bending -NH wagging[37]
1350
1350
-SO2 asymmetric stretching[38]
1180
1180
-CF3 asymmetric stretching[38]
1140
1140
-SO2 symmetric stretching[38]
1060
1060
S-N-S symmetric stretching[38]
On the other hand, the liquid phases obtained in mixtures of the [P66614][N-mGly]/ethanol solution and the Co(salen)/ethanol suspension, which had [P66614][N-mGly]/Co(salen) molar ratios of 1/1, 1/1.3, and 1/2, were kept in vacuo at 323 K for 12 h to remove the ethanol and then characterized using elemental analyses. The C/H/N ratios of the samples are listed in Table S4. The calculated C/H/N ratio based on the chemical structure of the MCIL composed of one Co(salen) molecule coordinated with two [P66614][N-mGly] ([P66614]2[Co(salen)(N-mGly)2]) is also shown. The experimentally obtained C/H/N ratios for the three samples were almost the same as the theoretical values. In order to confirm the coordination bond formation and number of [P66614][N-mGly] coordinated to Co(II), the UV spectra of the Co(salen)/[P66614][N-mGly]/methanol mixtures with different molar Co(salen)/[P66614][N-mGly] ratios were measured. Because a relatively large amount of the Co(salen) complex could be dissolved in methanol (although not fully), methanol was used as the solvent. The samples were prepared under a nitrogen atmosphere (the details of the preparation of Co(salen)/[P66614][N-mGly]/methanol samples were written in Supporting Information) and the UV spectrum of each sample was measured using a UV-vis spectrophotometer. The results are shown in Fig. 3. Monotonic absorbance changes were observed at 595 nm (d-d transition) and around 390 nm (π→π* transition (C=N)) [39]. The absorption band at 595 nm shown in Fig. 3(b)
and (c) is evidence of the octahedral geometry of the compound.[40, 41] The intensity of the absorption band monotonically increased with an increase in the mole ratio of the [P66614][N-mGly]/Co(salen) complex from 0 to 2. This intensity change was due to the coordination of [P66614][N-mGly] to Co(II). In addition, the absorption band around 390 nm was slightly blue shifted with an increase in the [P66614][N-mGly]/Co(salen) mole ratio from 0 to 2 (Fig. 3(d) and (e)), which would have occurred as a result of the decrease in the electron density of the conjugated C=N bond with the coordination of [P66614][N-mGly] to Co(II). These characterizations strongly suggested that the synthesized compound was an MCIL composed of one Co(salen) molecule coordinated with two couples of [P66614][N-mGly]. From the results, it was suggested that the Co(salen) complex was coordinated by two [P66614][N-mGly] molecules and stabilized in an octahedral geometry; i.e. the chemical structure of the obtained product was estimated as [P66614]2[Co(salen)(N-mGly)2]. The expected structural formula of [P66614]2[Co(salen)(N-mGly)2] is shown in Fig. 4(a). The MCIL had good thermal stability, with a decomposition temperature of approximately 200 °C (Fig. 5) and a high viscosity (46350 ± 3350 mPa·s).
3
mole ratio of Co(II)-salen complex : [P66614][N-mGly]
(a)
Absorbance / -
2.5
0:1 2
1:0 1:0.5
1.5
1:1
1
1:1.5 1:2
0.5
1:3
0
1:4 250
350
450
550
650
750
wavelength / nm 0.1
0.06
(b)
0.09 0.08 0.07
∆A595nm
Absorbance / -
(c)
0.05
0.06 0.05
0.04 0.03 0.02
0.04 0.03
0.01
0.02
0
0.01 500
600
0
700
1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5
1 2 3 4 mol-IL/mol-Co(salen)
5
391
(d)
(e)
390 λmax around 390nm
Absorbance / -
wavelength / nm
389 388 387 386 385 384
350
400
wavelength / nm
450
0
1 2 3 4 mol-IL/mol-Co(salen)
5
Fig. 3 UV absorption spectra of Co(salen)/[P66614][N-mGly]/methanol solutions with different [P66614][N-mGly] concentrations: (a) whole wavelength range, (b) absorbance around 600 nm, (c) relationship between sample and [P66614][N-mGly]/methanol solution absorbance difference at 595 nm (wavelength at peak top) and [P66614][N-mGly] to Co(salen) molar ratio, (d) absorbance around 400 nm, and (e) relationship between wavelength at peak top and molar ratio of [P66614][N-mGly] to Co(salen)
(b)
(a) P66614+
P66614+
P66614+
P66614+
(c)
(d) P66614+
P66614+ P66614+
Fig.
4
Predicted
chemical
structures
of
(a)
[P66614]2[Co(salen)(N-mGly)2],
(b)
[P66614]2[Co(salen)(N-mGly)(Tf2N)], (c) O2 complex with 1:1 stoichiometry, and (d) O2 complex
Weight loss (m|T=125°C basis) / %
with 1:2 stoichiometry
100 80 60 40
20
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
0
100 150 200 250 300 350 Temperature / oC Fig. 5 TGA profiles of [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] under N2 using temperature ramp rate of 10 oC/min
3.2 Synthesized [P66614]2[Co(salen)(N-mGly)(Tf2N)] An MCIL was also synthesized by mixing the Co(salen) complex with an equimolar mixture of [P66614][N-mGly] and [P66614][Tf2N] (the mole ratio of Co(salen)/[P66614][N-mGly]/ [P66614][Tf2N] was 1/1/1). A photograph of the synthesized MCIL, [P66614]2[Co(salen)(N-mGly)(Tf2N)], is shown in Figs. 1(b). It is worth mentioning that the product retained its liquid state, and no solid was precipitated after evaporation. If no [P66614][Tf2N] was added to the solution, the solid Co(salen) complex was precipitated because of the removal of coordinated O2 from the product during the evaporation process. The O2 removal was qualitatively confirmed from the color change of the product before and after evaporation. When O2 was detached from the product, the coordination number of Co(II) became five, and [P66614][N-mGly] abstraction occurred. As a result, half of the Co(salen) complex formed the six-coordinated [P66614]2[Co(salen)(N-mGly)2], and the other half precipitated in the solution. On the other hand, when [P66614][Tf2N] coexisted in the solution, it could coordinate to the Co(salen) complex instead of O2. As a result, after the removal of the O2, [P66614]2[Co(salen)(N-mGly)(Tf2N)] could be formed, which had a stable octahedral geometry with six coordination bonds. Therefore, no Co(salen) precipitation was observed in the solution after evaporation. The FT-IR spectrum of the product is shown in Fig. 2, along with those of [P66614][N-mGly], [P66614][Tf2N], and the Co(salen) complex. The assigned peaks are listed in Table 1. It was confirmed that the product was composed of [P66614][N-mGly], [P66614][Tf2N], and the Co(salen) complex. The estimated structural formula of [P66614]2[Co(salen)(N-mGly)(Tf2N)] is shown in Fig. 4(b). The thermal stability of the synthesized [P66614]2[Co(salen)(N-mGly)(Tf2N)] was also examined. The TGA data are shown in Fig. 5, along with the data for [P66614]2[Co(salen)(N-mGly)2]. It was confirmed that the thermal decomposition temperature of [P66614]2[Co(salen)(N-mGly)(Tf2N)] was also greater than 200 °C. Interestingly, the viscosity of [P66614]2[Co(salen)(N-mGly)(Tf2N)] (7504 ± 1855 mPa·s) was significantly smaller than that of [P66614]2[Co(salen)(N-mGly)2] (46350 ± 3350 mPa·s). Based on these results, it can be expected that different IL-based ligands would provide numerous opportunities to create various MCILs with diverse physico-chemical properties. Designing the structures of IL-based ligands and changing the combination of them could be a useful methodology to control the properties of the MCILs.
3.3 Selective O2 absorbability of Co(salen) complex-based ILs One of the most important properties of the synthesized MCILs was the O2 absorbability. Fig. 6 shows
the
O2
and
N2
absorption
isotherms
of
[P66614]2[Co(salen)(N-mGly)2]
and
[P66614]2[Co(salen)(N-mGly)(Tf2N)]. The absorption isotherms with the unit of mmol-O2/g-MCIL are shown in Fig. S8. As clearly shown in Fig. 6, the MCILs absorbed O2 chemically and N2 physically. It was suggested that O2 was absorbed in the MCILs via ligand exchange reaction; i.e. the anion of the ionic liquid-based ligand would be released from Co(salen) complex and O2 coordinated to the complex alternatively. Below atmospheric pressure, the O2 absorption amounts for both MCILs were much higher than those for N2 under the same pressure conditions. The higher O2 absorption amounts were because of the contributions of the chemical reactions between the MCILs and O2. Thus, it was found that the MCILs retained the O2 reactivity of the Co(salen) complex. A comparison of the saturated O2 absorption amounts of the two MCILs showed that they had completely different stoichiometries,
i.e.,
1
mol-O2/mol-MCIL
for
[P66614]2[Co(salen)-(N-mGly)2]
and
0.5
mol-O2/mol-MCIL for [P66614]2[Co(salen)(N-mGly)(Tf2N)] (as shown in Fig. 6(a)). These results suggested that an O2 unit molecule reacted with a [P66614]2[Co(salen)(N-mGly)2] unit molecule (1:1 complex formation) and two [P66614]2[Co(salen)(N-mGly)(Tf2N)] molecules (1:2 complex formation). The predicted structure of the 1:1 and 1:2 complexes are shown in Fig. 4(c) and (d), respectively. Regarding the complex formation mechanism, it was reported that Co(salen) complex reacts with O2 according to the following sequential reaction [42].
Co(salen) A O 2
Co(salen) A O 2
Co(salen) A O 2 Co(salen) A
(1)
Co(salen) A2 O 2
(2)
where "A" expresses the axial ligand. According to the sequential reaction, it can be considered that 1:2 complex, (Co(salen)·A)2·O2, could be formed when considerable amount of Co(salen)·A is formed and the reaction (2) progresses. On the other hand, If formation of Co(salen)·A is hardly occurred, reaction (2) could not progressed. In this case, 1:1 complex, Co(salen)·A·O2, could be mainly formed. According to the reactions, the complex formation between [P66614]2[Co(salen)(N-mGly)2] and O2 could be expressed as the following sequential reactions.
Co(salen) A 2
Co(salen) A 2 O 2
Co(salen) A A
(3)
Co(salen) A O 2 A
Co(salen) A O 2 Co(salen) A
(4)
Co(salen) A2 O 2
(5)
where "A" in Eqs. (3) – (5) means [P66614][N-mGly]. Regarding amino group, it is worth mentioning that high donor number (DN ≈ 57 – 61) was reported [43]. The high DN indicates that N-methylglycinate could strongly coordinate to Co(salen) complex. Therefore, in this case, it could be considered that the reaction (3) was hardly occurred. On the other hand, because of the strong coordination ability of O2 with Co(salen) complex, reaction (4) could progresses and Co(salen)·A·O2 could be formed when O2 was existed in the system. However, because reaction (3) hardly occurred,
negligible amount of Co(salen)·A would be existed. As the result, reaction (5) hardly occurred and considerable amount of (Co(salen)·A)2·O2 could not be formed. Thus, 1:1 complex of Co(salen)·A·O2, which corresponds to [P66614][Co(salen)(N-mGly)·O2], could be mainly formed. On the other hand, the complex formation between [P66614]2[Co(salen)(N-mGly)(Tf2N)] and O2 could be expressed as the following sequential reactions. Co(salen) A B Co(salen) A B
Co(salen) A B O 2
(6)
Co(salen) A O 2 B
Co(salen) A O 2 Co(salen) A
(7)
Co(salen) A2 O 2
(8)
where "A" and "B" in Eqs. (6) – (8) express [P66614][N-mGly] and [P66614][Tf2N], respectively. In this case, reaction (6) could progress and Co(salen)·A could be formed in the system because of the weak coordination ability of Tf2N anion to the Co(salen) complex (DN ≈ 5.6 [43]). In addition, reaction (7) could also progress because of the strong coordination ability of O2 with Co(salen) complex. Therefore,
1:2
complex
of
(Co(salen)·A·O2)2,
which
corresponds
to
[P66614]2[(Co(salen)(N-mGly))2·O2], could be formed via reaction (8). Interestingly, the viscosity of [P66614]2[Co(salen)(N-mGly)2] was increased after O2 absorption (from 46350 mPa·s before O2 absorption to 81530 mPa·s after O2 absorption) although that of [P66614]2[Co(salen)(N-mGly)(Tf2N)] showed no considerable change (from 7500 mPa·s before O2 absorption to 7380 mPa·s after O2 absorption). Because [P66614]2[Co(salen)(N-mGly)2] formed 1:1 complex with O2, there is free space around the O2 which coordinated to Co(salen) complex. Therefore, N-methylglycinate which coordinated with a Co(salen) complex can approach to an O2 which coordinated with another Co(salen). The hydrogen atom on the secondary amino group of the N-methylglycinate which coordinated with a Co(salen) complex could form hydrogen bonding with the O2 on another Co(salen) complex. As the result, hydrogen bonding network between the [P66614]2[Co(salen)(N-mGly)2]-based
O2
complexes
could
mGly-Co(salen)-O2···mGly-Co(salen)-O2···mGly-Co(salen)-O2
where
be "-"
formed; and
"···"
i.e. mean
coordination bond and hydrogen bond, respectively. Because of the hydrogen bonding network, the viscosity of [P66614]2[Co(salen)(N-mGly)2] would be increased after O2 absorption. On the other hand, because [P66614]2[Co(salen)(N-mGly)(Tf2N)] formed 1:2 complex with O2 and the coordinated O2 was sandwiched by the Co(salen) complexes from upper and lower sides, there is no free space around the coordinated O2. Therefore, no hydrogen bonding between the coordinated O2 and N-methylglycinate was not formed and no considerable viscosity change was observed. The hypothesis on the hydrogen bonding formation between N-methylglycinate and O2 in the [P66614]2[Co(salen)(N-mGly)2]-based
O2
complex
was
checked
using
an
MCIL
with
trihexyl(tetradecyl)phosphonium dimethylglycinate ([P66614][dmGly]) as the axial ligands; [P66614]2[Co(salen)(dmGly)2]. Because the amino group of dimethylglycinate has no hydrogen atom
which directly bonded to the nitrogen atom of the tertiary amine group, dimethylglycinate could not form hydrogen bonding with the O2 which coordinated with Co(salen). Therefore, it is predicted that no considerable viscosity change of [P66614]2[Co(salen)(dmGly)2] before and after O2 absorption would be occurred. Before the measurement of the viscosity change of [P66614]2[Co(salen)(dmGly)2] before and after O2 absorption, we checked that [P66614]2[Co(salen)(dmGly)2] could absorb considerable amount of O2. Then, we measured the viscosities of [P66614]2[Co(salen)(dmGly)2] before and after O2 bubbling. As the result, it was confirmed that the viscosity of [P66614]2[Co(salen)(dmGly)2] before O2 absorption (2606 mPa·s) was almost same as that after O2 absorption (3074 mPa·s). This result suggested that the suitability of our assumption on the viscosity enhancement by the hydrogen bonding formation between the [P66614]2[Co(salen)(N-mGly)2]-based O2 complexes. As shown in Fig. S7, the saturated absorption amounts of O2 in [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] were about 0.7 mmol/g and 0.3 mmol/g, respectively. These values were much higher than that of an MCIL reported by Kohno et al. (ca. 0.12 mmol/g), which was obtained by dosing an MCIL-impregnated porous material using an O2 atmosphere of 0.9 bar at 298 K for 2 h
[33]. Our developed MCILs could easily be synthesized and could absorb large
amounts of O2. These are the attractive advances of our developed MCILs.
Amount of O2 absorbed / mol-O2·mol-MCIL-1
2.0
(a) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
1.5 1.0 0.5 0.0
0 5 10 15 20 25 Equilibrium pressure (O2) / kPa
Amount of N2 absorbed / mol-N2·mol-MCIL-1
2.0
(b) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
1.5 1.0 0.5 0.0
0 100 200 300 400 500 Equilibrium pressure (N2) / kPa
Fig. 6 O2 (a) and N2 (b) absorption isotherms of [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] at 303 K
3.4 Gas permeation properties of the SILM containing Co(salen) complex-based ILs Fig. 7 shows the partial pressure dependency on the O2 and N2 permeabilities and O2/N2 selectivities of the prepared membranes. We can evaluate the gas permeation mechanism based on the trends, i.e., the profiles with and without partial pressure dependency indicate that the gas permeation mechanisms are facilitated transport and solution-diffusion, respectively.[13, 18, 26] The marked dependency on the partial pressure for FTMs reflects carrier saturation with the gas absorption in the membrane. As shown in Fig. 7(a), the O2 permeabilities drastically decreased with the increase of the O2 partial pressure from 1 kPa to about 5 kPa and then became constant above 5 kPa. This strong dependency of the O2 permeability on O2 partial pressure is the evidence supporting the facilitated O2 transport by MCIL-based SILMs; i.e. the MCIL acted as an O2 carrier in the membrane. As shown in Fig. 6(a), the O2 absorption amounts also became plateau above 5 kPa of the O2 pressure, i.e. the trend shows good agreement with that of the O2 permeability. When the
MCIL-based SILMs contacted with the gas containing more than 5 kPa of O2, MCILs were saturated with O2. Therefore, the facilitation effect of the MCIL-based SILMs at more than 5 kPa of O2 partial pressure became constant. In other words, when the MCIL-based SILMs were exposed to the gas with less than 5 kPa of O2 partial pressure, the driving force of O2 permeation was mainly dominated by and strongly depended on the chemically absorbed O2. On the other hand, the N2 permeabilities indicated no dependency on N2 and O2 partial pressures because of the Henry type absorption (as shown in Fig. 6(b)). N2 partial pressure / kPa 100 95 90 85 80
100
(a)
(b)
100
10
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (N₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (N₂)
O2/N2 selectivity
Gas permeability / Barrer
1000
N2 partial pressure / kPa 100 95 90 85 80
10
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
1
1 0
5 10 15 20 O2 partial pressure / kPa
0
N2 partial pressure / kPa 100 95 90 85 80
N2 partial pressure / kPa 100 95 90 85 80
100
(c)
(d) O2/N2 selectivity
Gas permeability / Barrer
1000
100
10
[P₆₆₆₁₄][N-mGly] (O₂) [P₆₆₆₁₄][Tf₂N] (O₂) [P₆₆₆₁₄][N-mGly] (N₂) [P₆₆₆₁₄][Tf₂N] (N₂)
1
7
O2
[P₆₆₆₁₄][N-mGly] [P₆₆₆₁₄][Tf₂N]
10
1 0
Fig.
5 10 15 20 O2 partial pressure / kPa
5 10 15 20 O2 partial pressure / kPa
permeation
properties
[P66614]2[Co(salen)(N-mGly)(Tf2N)]-based
of
0
(a,b)
SILMs,
and
5 10 15 20 O2 partial pressure / kPa
[P66614]2[Co(salen)(N-mGly)2](c,d)
[P66614][N-mGly]-,
and and
[P66614][Tf2N]-based SILMs, along with O2 partial pressure dependency on (a,c) O2 and N2 permeabilities and (b,d) O2/N2 selectivities. Experimental conditions: total pressure at feed and permeate sides was atmospheric pressure, temperature was 303 K, and atmosphere was dry.
100
polymer membranes
O2/N2 selectivity
Robeson upper bound 2008 (Ref.8)
p O2
Fixed carrier membranes (Refs.18-23)
1 kPa
10 2.5 kPa
15 kPa 20 kPa
5 kPa 10 kPa 15 kPa 20 kPa
1 0.01
Fixed carrier membrane A (Ref. 24) O₂ partial pressure dependency Fixed carrier membrane B (Ref. 24) O₂ partial pressure dependency this work ([P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂]) this work ([P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)])
1 100 10000 O2 permeability / barrer
Fig. 8 Comparison of O2 separation performances with various O2 separation membranes.[8, 18-24] In addition, to confirm the effect of Co(salen) complex on the facilitated O2 transport, the O2 and N2 permeabilities and O2/N2 selectivities of the SILMs containing the IL-based ligands ([P66614][N-mGly] and [P66614][Tf2N]) were evaluated (Fig. 7(c) and (d)). It was clearly shown that the SILMs without Co(salen) complex demonstrated no facilitation effect on the O2 transport. From the results shown in Figs. 6 and 7, there is no doubt that the Co(salen) complex-based MCILs facilitated O2 permeation. On the other hand, regarding the O2 permeabilities of the two MCILs-based SILMs, [P66614]2[Co(salen)(N-mGly)(Tf2N)]-based SILM showed 1.26 – 1.5 times higher O2 permeability than that of [P66614]2[Co(salen)(N-mGly)2]-based SILM although the O2 absorption amount of [P66614]2[Co(salen)(N-mGly)(Tf2N)] was two times lower than that of [P66614]2[Co(salen)(N-mGly)2]. The lower diffusivity of the O2 complex with [P66614]2[Co(salen)(N-mGly)2] would be the reason of the lower O2 permeability. It is considered that the higher viscosity of [P66614]2[Co(salen)(N-mGly)2] (ca. 81500 mPa·s after O2 absorption) than that of [P66614]2[Co(salen)(N-mGly)(Tf2N)] (ca. 7380 mPa·s after O2 absorption) would the main factor of the lower diffusivity. Additionally, the molecular volumes of the O2 complexes, which are the diffusates in the MCILs, would also affect the diffusivity. Because [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] made the O2 complexes with 1:1 and 1:2 stoichiometries, respectively, the molecular volume of the O2 complex with [P66614]2[Co(salen)(N-mGly)2] is approximately two times larger than that of the O2 complex with [P66614]2[Co(salen)(N-mGly)(Tf2N)]. According to Arnold equation [44], which can relatively accurately predict the diffusivity of solute in ILs [45], the diffusion coefficient of solute has proportional relationship with (1/Mcom+1/MIL)0.5/(η0.5·(Vcom1/3+VIL1/3)2), where Mi, η, and Vi are molecular weight, viscosity of the MCIL and molecular volume, respectively. Subscripts "com" and "IL" shows of the O2 complex and MCIL, respectively. In addition, the O2 permeability, which can
be expressed by (ε/τ)·Dcom·ΔCcom where ε is porosity of the support, τ is the tortuosity of the pore of the support, Dcom is the diffusion coefficient of the O2 complex as discussed above and ΔCcom is the trans-membrane concentration difference of the O2 complex which can be considered as same as the equilibrium concentration of the O2 complex because of the sweep method used in this study. Considering the differences of the Dcom and ΔCcom, it was confirmed that the permeability of the [P66614]2[Co(salen)(N-mGly)(Tf2N)]-based SILM is about 1.1 times higher than that of the [P66614]2[Co(salen)(N-mGly)2]-based SILM. The opposite trend between the O2 absorption amounts and the O2 permeabilities of the two MCILs could be explained by the effect of diffusivity. Although further investigations are needed, it could be considered that the O2 permeability of the MCIL-based SILMs would be strongly affected by the diffusivity of the O2 complex. A comparison of the performances of the developed MCIL-based SILMs and those of the previously reported FTMs with fixed O2 carrier along with those of polymer membranes are shown in Fig. 8. As shown in this figure, the O2 permeation performance of the MCIL-based FTMs shows an O2 partial pressure dependency similar to those of the fixed carrier type membranes (shown in the square symbols). The similar trends also supports the facilitated O2 transport mechanism of the MCIL-based SILMs. From another point of view, the O2 permeabilities of the MCIL-based SILMs were higher than those of the fixed carrier membranes. The superior O2 permeability of the MCIL-based SILMs might be the result of the higher mobility of the MCIL as the O2 carrier. Comparing the performances of the MCIL-based SILMs and those of the polymer membranes [8], the MCIL-based SILMs demonstrated much higher O2/N2 selectivities under low O2 partial pressure conditions. On the other hand, at a high O2 partial pressure, the performance was still lower than those of some polymer membranes. The low O2 permeability at high O2 partial pressure limited the O2/N2 selectivity. Improving the O2 permeabilities of the MCIL-based SILMs is our next challenge. Finally, we investigated the stability of the MCIL-based SILMs. For previously reported facilitated O2 transport membranes, one of the major problems was the instability of the O2 carrier against irreversible oxidation [15]. The stability of the MCIL-based SILMs is indicated in Fig. 9. As demonstrated in Fig. 9, the high O2 permeabilities and N2 barrier properties of the MCIL-based SILMs were maintained for more than 80 h. These results suggested that the developed MCILs retained their selective O2 absorbability because of their good stability.
Gas permeability / Barrer
300
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (N₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (N₂)
(a)
250 200 150 100 50
0
O2/N2 selectivity
0 10 9 8 7 6 5 4 3 2 1 0
20
40 60 Time / h
80
100
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂]
(b)
0
20
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
40 60 Time /h
80
100
Fig. 9 Stability of [P66614]2[Co(salen)(N-mGly)2]- and [P66614]2[Co(salen)(N-mGly)(Tf2N)]-based SILMs; (a) O2 and N2 permeabilities, and (b) O2/N2 selectivities. Experimental conditions: total pressure at feed and permeate sides was atmospheric pressure, O2 partial pressure of feed was 2.5 kPa, temperature was 303 K, and atmosphere was dry.
To the best of our knowledge, this is the first report on an IL-based mobile O2 carrier membrane. Although the MCILs developed in this study are synthesized from relatively expensive raw materials, the simple synthesis process could reduce the production cost. Furthermore, fortunately small amount of MCILs is needed to prepare the facilitated transport membrane with very thin O2 transport layer. Therefore, membrane is one of the best media for the MCILs. In addition, the good stability of the MCIL-based membrane allows the long-term use, which could reduce the total operation cost. On the other hand, although the developed membrane had high and selective O2 permeability and good stability, the performance at high O2 partial pressure is still insufficient. As previously indicated, the physico-chemical properties of the MCILs could easily be tuned by designing the IL-based axial ligands. For instance, an MCIL with a smaller molecular size would have a higher
Co(salen) complex density and lower free volume. It is expected that such a small MCIL could provide a larger amount of O2 absorption, lower N2 solubility, and lower viscosity [46, 47]. Combination of main and auxiliary IL-based ligands also has potential for optimizing the properties of an MCIL. MCILs have considerable potential for performance improvement. We believe that the optimization of the physico-chemical properties of MCILs could provide a new class of O2 separation membrane, which could provide opportunities for designing novel processes and devises.
4. Conclusions Novel MCILs containing O2 reactive Co(salen) complex were synthesized according to the concept of a solvate IL. Simple mixing of Co(salen) complex and an ILs, which was composed of coordinating anion and bulky cation, in ethanol allowed to prepare O2 reactive MCILs. The developed MCILs chemically and selectively absorbed a large amount of O2 and acted as an O2 carrier of an FTM. The facilitated O2 transportation of the FTM was confirmed from the O2 partial pressure dependency on the O2 permeabilities. Owing to the liquidity of the MCILs, the FTMs showed higher O2 permeability than those of fixed O2-carrier membranes as well as polymer membranes. In addition, the FTMs overcame the instability issue, which was one of the major challenges of conventional facilitated O2 carrier membranes. The findings of the present proof-of-concept study suggested that the developed MCILs have great potential as an O2 separation media because the MCILs could be easily tuned the structure and physico-chemical properties by selecting the combination of main and auxiliary IL-based ligands.
Acknowledgements A part of this research was supported by the Matching Planner Program (MP28116808274) from the Japan Science and Technology Agency.
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414 (2005) 378-383. [44] J.H. Arnold, Diffusion. II. Kinetic theory of diffusion in liquid systems, J. Am. Chem. Soc., 52 (1930) 3937-3955. [45] D. Morgan, L. Ferguson, P. Scovazzo, Diffusivities of Gases in Room-Temperature Ionic Liquids: Data and Correlations Obtained Using a Lag-Time Technique, Ind. Eng. Chem. Res., 44 (2005) 4815-4823. [46] S. Kasahara, E. Kamio, H. Matsuyama, Improvements in the CO2 permeation selectivities of amino acid ionic liquid-based facilitated transport membranes by controlling their gas absorption properties, J. Membr. Sci., 454 (2014) 155-162. [47] A. Otani, Y. Zhang, T. Matsuki, E. Kamio, H. Matsuyama, E.J. Maginn, Molecular Design of High CO2 Reactivity and Low Viscosity Ionic Liquids for CO2 Separative Facilitated Transport Membranes, Ind. Eng. Chem. Res., 55 (2016) 2821-2830.
Highlights:
Novel O2 reactive metal-containing ionic liquids (MCILs) were synthesized.
The MCILs could have a potential to tune the physico-chemical property easily.
The MCILs absorbed a large amount of O2 selectively and acted as an O2 carrier.
The MCIL-based membranes permeate O2 based on facilitated transport mechanism.
Owing to the liquidity of the MCILs, the membrane exhibited high O2 permeability.
PII: DOI: Reference:
S0376-7388(17)30861-X http://dx.doi.org/10.1016/j.memsci.2017.07.027 MEMSCI15432
To appear in: Journal of Membrane Science Received date: 24 March 2017 Revised date: 10 July 2017 Accepted date: 12 July 2017 Cite this article as: A. Matsuoka, E. Kamio, T. Mochida and H. Matsuyama, Facilitated O2 transport membrane containing Co(II)-salen complex-based ionic liquid as O2 carrier, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facilitated O2 transport membrane containing Co(II)-salen complex-based ionic liquid as O2 carrier a
A. Matsuoka , E. Kamioa, T. Mochidab, H. Matsuyamaa* aCenter
for Membrane and Film Technology, Department of Chemical Science and
Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan bDepartment
of Chemistry, Graduate School of Science, Kobe University, 1-1
Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan *Corresponding
author. Tel.: +81 78 803 6180; fax: +81 78 803 6180. [email protected]
Abstract Novel metal-containing ionic liquids (MCILs) with O 2 absorbability were synthesized by mixing a N,N'-bis(salicylidene)ethylenediamine cobalt(II) (Co(salen)) complex and ionic liquid-based
ligands.
The
couple
of
anions
of
the
ionic
liquid-based
ligands,
trihexyl(tetradecyl)phosphonium N-methylglycinate and trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide, coordinated to Co(salen) complex and gave negative charge to the neutral Co(salen). The negatively chaged Co(salen) complex with the ionic liquid-based ligands and the bulky trihexyl(tetradecyl)phosphonium cation allowed to form Co(salen) complex-based MCILs. The MCILs could chemically and selectively absorb a large amount of O2 and could function as an O 2 carrier in supported MCIL membranes. The O2 permeability of the MCIL-based facilitated O2 transport membranes were higher than those of conventional fixed-O2 carrier membranes as well as polymer membranes. The fast O2 transportation could be realized from the high diffusivity of the O 2 complex, which was formed via the reaction between MCIL and O 2, owing to the liquidity of the MCILs. Graphical Abstract
Keywords: Oxygen separation; Facilitated transport; Metal-containing ionic liquid; Cobalt(II)-salen complex; Ionic liquid-based ligands
1. Introduction Oxygen-enriched air is used in various medical, chemical, and environmental applications such as medical devices, steel manufacturing, and chemical manufacturing, as well as in coal-fired power plants utilizing a novel combustion technique called oxy-fuel combustion [1-4]. In all of these applications, the energy consumption and monetary cost of producing oxygen are mainly affected by the process efficiency. Because the current cryogenic distillation and pressure swing adsorption methods are energy and cost intensive [5, 6], alternative oxygen separation processes for oxygen-enriched air production are highly desired. One attractive alternative is membrane technology, which provides several advantages over the current separation technologies, including its high energy efficiency, ease of processing and scaling up, simple and compact modular equipment, and low capital cost [3, 6, 7]. Polymer membranes have been widely investigated for O2/N2 separation. However, in the current stage, the O2 permeability and O2/N2 selectivity of polymer membranes are insufficient for practical applications [8]. Moreover, there is trade-off limitation between the O2 permeability and O2/N2 selectivity. As inorganic membranes, perovskite-based O2 transport membranes have recently been investigated [9-11]. Although a perovskite membrane allows the permeation of oxygen ions with an infinite O2/N2 selectivity, sufficient O2 permeability was achieved at a highly elevated temperature of more than 973 K [3, 10, 11]. The development of a membrane with promising O2 permeability at a lower operating temperature is a critical issue to realize an O2 separation process that conserves energy. Facilitated transport membranes (FTMs) are a well-known class of functional membranes that consist of a polymer matrix and specific chemical compound, the so-called “carrier,” which can chemically and selectively react with a specific gas molecule [12-15]. Because of the effect of the carrier, FTMs can realize the rapid and highly selective permeation of the target gas. This concept was first used for O2 separation in a supported liquid membrane with hemoglobin and then with a cobalt Schiff base complex [16, 17]. Although such metal complexes are crystalline solids, they were used as the mobile O2 carriers in some preferable solvent. However, because of the poor O2 permeation performance due to the poor solubility limitation of the carrier and low stability due to the solvent vaporization, the liquid membrane was altered using a solid polymer membrane with fixed metal complexes on the polymer matrix as the O2 carrier [18-24]. This fixed O2 carrier membrane improved the stability of the facilitated O2 transport membrane. However, the O2 permeability was still insufficient because of the large diffusion resistance of the solid membrane [15]. To improve the stability of a mobile carrier membrane, we previously proposed a novel type of FTM that utilized a task-specific ionic liquid (TSIL) for CO2 separation [25-29]. The TSIL played the role of the mobile carrier, as well as the diffusion medium in the membrane. Because an ionic
liquid (IL) has a non-volatile property, it overcomes the stability limitation of the liquid-type FTM. Moreover, the liquidity of an IL overcomes the mobility issue of fixed carrier membranes. Based on the same concept, an O2 reactive TSIL could be expected to be a promising carrier for a facilitated O2 transport membrane. The major challenge is the development of the O2 reactive IL. A straightforward strategy to create an O2 reactive IL is the utilization of a metal complex with O2 reactivity. Recently, metal-containing ionic liquids (MCILs) have attracted great attention as novel functional ILs with a specific property of the metal complexes [30-32]. In 2015, a Co(II)-based-MCIL with selective O2 absorbability was firstly reported by Kohno et al. [33]. However, until now, there has been no report on the utilization of O2 reactive MCILs in an FTM. This might be because of the insufficient properties of MCILs as mobile O2 carriers. If the physico-chemical properties of an MCIL, such as its O2 reactivity and viscosity, could be easily tuned, FTMs with outstanding O2 permeability, O2 permselectivity, and long-term stability could be developed using MCILs. In general, the properties of ILs, including MCILs, are controlled by designing their molecular structures via organic synthesis, which is often complicated. Therefore, the main challenge for an MCIL-based facilitated O2 transport membrane is the development of a simple synthesis method for the MCIL. One powerful option is a solvate IL, which is a novel type of IL composed of a metal ion and some ligands [34, 35]. Solvate ILs can easily be synthesized by simply mixing the metal salt with the ligands. The properties of a solvate IL could also be easily controlled by changing the kind of ligands. Here, based on the concept of solvate ILs, we present novel O2 reactive MCILs that can be easily synthesized by mixing an O2 reactive metal complex with an IL-based ligand. 2. Experimental 2.1 Synthesis and characterization of Co(salen) complex-based ILs N,N'-bis(salicylidene)ethylenediamine cobalt(II) (Co(salen); >95%, purchased from Tokyo Chemical Industry Co. (Tokyo, Japan) and used as received) was selected as the O2 reactive metal complex. It is well known that the Co(salen) complex has a square planar geometry, to which axial ligands can also be coordinated. When an IL composed of a bulky cation and metal coordinating anion (hereafter the IL is written as IL-based ligand) is mixed with the Co(salen) complex, it is expected that the anion would coordinate with the Co(salen) complex as the axial ligand and give it a negative charge. The resulting Co(salen) complex with the negatively charged ligand could act as an anion to form an MCIL with a bulky counter cation. In this proof-of-concept study, we selected IL-based ligands composed of N-methylglycinate (N-mGly-) and bis(trifluoro-methanesulfonyl)imide (Tf2N-) as the coordinating anions and the trihexyl(tetradecyl)phosphonium cation (P66614+, where the subscripts “6” and “14” represent the carbon numbers of the alkyl chains). The detailed explanation of the synthesis of IL-based ligands,
including the reagents used, is provided in the Supporting Information.
2.1.1 Synthesis of [P66614]2[Co(salen)(N-mGly)2] The solid Co(salen) complex and [P66614][N-mGly] in a 1:2 molar ratio were mixed at room temperature in a nitrogen atmosphere. A certain amount of Co(salen) was agitated with ethanol in a flask under a nitrogen atmosphere. It is worth noting that the Co(salen) was hardly dissolved in ethanol, but was suspended in the ethanol. In a different batch, a certain amount of synthesized [P66614][N-mGly] was dissolved in ethanol. The [P66614][N-mGly] solution was added to the Co(salen)/ethanol suspension by dropwise addition. When the Co(salen)/ethanol suspension was mixed with the [P66614][N-mGly] solution, the solid Co(salen) was gradually dissolved. The mixture was continuously agitated for 3 h at 303 K. Finally, the product was kept in vacuo at 323 K for 12 h to remove the ethanol.
2.1.2. Synthesis of [P66614]2[Co(salen)(N-mGly)(Tf2N)] A certain amount of Co(salen) was agitated with ethanol in a flask under an air atmosphere. In a different batch, an equimolar amount of synthesized [P66614][N-mGly] (relative to Co(salen)) was dissolved in ethanol. The [P66614][N-mGly] solution was added to the Co(salen)/ethanol suspension by dropwise addition. When the Co(salen)/ethanol suspension was mixed with the [P66614][N-mGly] solution, the solid Co(salen) was gradually dissolved. The mixture was continuously agitated for 3 h at 303 K. Under an air atmosphere, the Co(salen) was fully dissolved in the solution, even though an equimolar amount of [P66614][N-mGly] was added to the system. In this case, O2 would be coordinated with [P66614][Co(salen)(N-mGly)] and form a stable six-coordinated octahedral geometry. After 3 h of agitation, an ethanol solution containing an equimolar amount of synthesized [P66614][Tf2N] (relative to Co(salen)) was added to the mixture and agitated for an additional 1 h at 303 K. Finally, the ethanol in the product was evaporated at 313 K for 3 h.
2.1.3. Characterization of [P66614]2[Co(salen)(N-mGly)2] The synthesized MCILs were characterized using Fourier transform infrared spectroscopy (FT-IR spectrometer, NIKOLET iS5 with id5 ATR accessory, Thermo Fisher Scientific Co. Ltd.), CHN elementary analysis, mass spectrometry (ESI-MS, LTQ Orbitrap Discovery, Thermo Fisher Scientific Co. Ltd.), UV/vis absorption spectrometry (UV-vis spectrophotometer, V-650, JASCO Co.), thermogravimetry-differential thermal analysis (TG-DTA, Thermo plus EVO II TG8120, Rigaku Co.), and viscosity measurements (rheometer, MCR501, Anton Paar Co. Ltd.).
2.2 Gas absorption experiments The amounts of O2 and N2 absorbed into the MCILs were measured using our previously
reported method and apparatus [28]. The apparatus, which consisted of stainless steel tubes, and reference and sample cells, is illustrated in Fig. S4. The MCILs used in the gas absorption test were first degassed to remove the considerable amount of O2 absorbed from the air. The temperature of the system was kept constant at 303 ± 0.1 K by a water bath (T-105B; Thomas Kagaku Co., Ltd., Tokyo, Japan). The reference and sample cells were degassed under vacuum for more than 1 h to remove air, charged with pure N2 to completely remove the considerable amount of O2 that remained in both cells, and again degassed under vacuum for more than 1 h. Then, the MCIL was loaded through the septum using a syringe. The weight of the loaded MCIL was calculated from the weight change of the syringe before and after the MCIL loading. After the MCIL loading, the system was fully evacuated again to completely remove the gas in the system. After the degasification of the cells, the valve separating the two cells was closed. Then, the reference cell was pressurized to charge a known amount of O2 or N2. After the gas was charged, the stirrer was turned on, and the MCIL was then constantly stirred throughout the experiment. The gas absorption began when the valve connecting the two cells was opened, and the gas fed into the reference cell was transferred into the sample cell to come into contact with the MCIL. The pressure drop from the absorption of the gas by the MCIL was measured with a digital pressure gauge (Model AM-756 digital manometer; GE Sensing & Inspection Technologies Co., Ltd.). The pressure was monitored until it remained constant. Equilibration was usually attained within 12 h. After equilibration was attained, the final pressure was measured, and the amount of gas absorbed was determined from the observed pressure change.
2.3 Gas permeation test We prepared supported IL membranes (SILMs) using the two MCILs and evaluated their O2 permeabilities. Although the viscosities of the MCILs synthesized in this study were high, they could be impregnated in a porous support membrane. The MCIL-based SILMs were prepared as follows. A hydrophilic PTFE microporous support membrane with a thickness of 35 μm and pore size of 100 nm, purchased from Toyo Roshi Kaisha, Ltd., Japan, was immersed into the synthesized MCIL ([P66614]2[Co(salen)(N-mGly)2]
or
[P66614]2[Co(salen)(N-mGly)(Tf2N)])
and
immediately
decompressed for 3,600 s to completely exchange the air in the PTFE membrane with the MCIL. The MCIL-impregnated PTFE membrane was taken out, and the excess MCIL was wiped off the surface. The permeation test was carried out using an O2/N2 mixture with the sweep method. O2 and N2 gases with a purity of 99.9% were used to prepare the O2/N2 mixture with desired O2 partial pressure. A schematic of the experimental setup of the gas permeation test is shown in Fig. S5(a). The gas transport properties of the membrane were measured using a flat-type permeation cell (Fig. S5(b)) placed in a thermostatically controlled oven adjusted to 303 K. The permeation cell was made of
stainless steel and had an effective permeation area of 2.88 cm2. A model feed gas was prepared by mixing the O2 and N2 under a dry condition. The total pressure on the feed side was maintained at atmospheric pressure. The flow rates of the O2 and N2 were controlled using mass flow controllers (Hemmi Slide Rule Co. Ltd., Japan) to adjust the O2 and N2 partial pressures as desired. The total flow rate of the feed gas was adjusted to 7.45 × 10-5 mol/s at 298 K and 101.3 kPa. The flow rates of the feed streams were measured using soap-film flow meters (HORIBA STEC Ltd., Japan). Helium was supplied to the permeate side of the cell as a sweep gas. The flow rate was 2.98 × 10-5 mol/s at 298 K and 101.3 kPa. The flow rate of the sweep gas was also measured using a soap-film flow meter. The pressure on the sweep side was maintained at atmospheric pressure. The conditions for the gas permeation test were summarized in Table 1. The sweep gas containing the O2 and N2 permeated through the MCIL-based supported liquid membranes was introduced to a gas chromatograph (VARIAN µGC 490-GC, column: Molsieve 5A, 10 m) to determine the composition of the permeated gases.
Table 1 Experimental conditions of gas permeation test Conditions Gases 303 Temperature Pressure Feed 101.3 (atmospheric) Sweep 101.3 (atmospheric) Pressure difference 0 Gas flow rate Feed total 100 O2 1, 2.5, 5, 10, 15, 20 N2 99, 97.5, 95, 90, 85, 80 Sweep He 40 Relative humidity Feed 0 (dry) Sweep 0 (dry)
Unit K kPaA kPaA kPaA mL/min mL/min mL/min mL/min % %
3. Results and discussion 3.1 Synthesized [P66614]2[Co(salen)(N-mGly)2] As expected, the obtained material was liquid at room temperature. A photograph of the synthesized
MCILs
are
shown
in
Fig.
1(a).
The
product
synthesized
with
the
[P66614][N-mGly]/Co(salen) molar ratio of 1/2 was analyzed using FT-IR. The FT-IR spectrum of the product is shown in Fig. 2, along with those of [P66614][N-mGly] and the Co(salen) complex. The assigned peaks are listed in Table 2. Based on the results, it was confirmed that the obtained liquid was composed of the Co(salen) complex and [P66614][N-mGly]. The C/H/N mole ratio of the liquid, as determined by the elemental analysis, was 68.9/11.0/3.6, as listed in Table S4. This C/H/N mole ratio indicated that the liquid consisted of one mole of the Co(salen) complex and two moles of [P66614][N-mGly]; in this case, the theoretical C/H/N mole ratio was 70.3/11.1/3.8. To investigate
whether [P66614][N-mGly] coordinated with the Co(salen) to form the MCIL or acted as a solvent of the Co(salen), we also synthesized liquids by mixing the Co(salen) complex and [P66614][N-mGly] in 1:1 and 1:1.3 molar ratios. It should be noted that no solid remained when the added amount of [P66614][N-mGly] was larger than double the equimolar amount of Co(salen). On the other hand, when the amount of [P66614][N-mGly] was less than double the equimolar amount of Co(salen), some amount of solid remained in the solution. In such cases, the solid residue was filtered and collected. The filtrate was washed with ethanol several times and dried in a vacuum oven. Small amount of the solid was dissolved in ethanol and analyzed using electrospray ionization-mass spectrometry. It was confirmed that the solid residue was unreacted Co(salen); ESI-MS: m/z calc for Co(salen)-: 325.0384; found: 325.0381 (Fig. S4).
(a)
Fig.
1
Photos
of
(b)
synthesized
MCILs:
(a)
[P66614]2[Co(salen)(N-mGly)2]
and
[P66614]2[Co(salen)(N-mGly)(Tf2N)]
[P66614]2[Co(salne)(N-mGly)2]
Tranmittance / a.u.
[P66614]2[Co(salne)(N-mGly)(Tf2N)]
[P66614][N-mGly]
[P66614][Tf2N] Co(II)-salen complex
3700
3200
2700
2200
Wavenumber / Fig.
2
FT-IR
spectra
of
synthesized
1700
1200
700
cm-1
[P66614]2[Co(salen)(N-mGly)2],
(N-mGly)(Tf2N)], [P66614][N-mGly], [P66614][Tf2N], and Co(salen) complex
[P66614]2[Co(salen)-
Table 2 Assignments of FT-IR absorption spectra of synthesized [P66614]2[Co(salen)(N-mGly)(Tf2N)], Co(salen) complex, [P66614][N-mGly], and [P66614][Tf2N] Wavenumber / cm-1 [P66614]2[Co(salne)- [P66614]2[Co(salne)Co(II)-salen (N-mGly)2] (N-mGly)(Tf2N)]
[P66614][N-mGly] [P66614][Tf2N]
Assignment
2920
2920
2920
2920
-CH2- symmetric stretching
2850
2850
2850
2850
-CH3 symmetric stretching
1635
1635
1600
1600
1600
1470
1470
1470
1380
1380
1380
1635
-C=N- stretching[36] -COO- asymmetric stretching[37] 1470
-CH3 degeneracy bending -NH wagging[37]
1350
1350
-SO2 asymmetric stretching[38]
1180
1180
-CF3 asymmetric stretching[38]
1140
1140
-SO2 symmetric stretching[38]
1060
1060
S-N-S symmetric stretching[38]
On the other hand, the liquid phases obtained in mixtures of the [P66614][N-mGly]/ethanol solution and the Co(salen)/ethanol suspension, which had [P66614][N-mGly]/Co(salen) molar ratios of 1/1, 1/1.3, and 1/2, were kept in vacuo at 323 K for 12 h to remove the ethanol and then characterized using elemental analyses. The C/H/N ratios of the samples are listed in Table S4. The calculated C/H/N ratio based on the chemical structure of the MCIL composed of one Co(salen) molecule coordinated with two [P66614][N-mGly] ([P66614]2[Co(salen)(N-mGly)2]) is also shown. The experimentally obtained C/H/N ratios for the three samples were almost the same as the theoretical values. In order to confirm the coordination bond formation and number of [P66614][N-mGly] coordinated to Co(II), the UV spectra of the Co(salen)/[P66614][N-mGly]/methanol mixtures with different molar Co(salen)/[P66614][N-mGly] ratios were measured. Because a relatively large amount of the Co(salen) complex could be dissolved in methanol (although not fully), methanol was used as the solvent. The samples were prepared under a nitrogen atmosphere (the details of the preparation of Co(salen)/[P66614][N-mGly]/methanol samples were written in Supporting Information) and the UV spectrum of each sample was measured using a UV-vis spectrophotometer. The results are shown in Fig. 3. Monotonic absorbance changes were observed at 595 nm (d-d transition) and around 390 nm (π→π* transition (C=N)) [39]. The absorption band at 595 nm shown in Fig. 3(b)
and (c) is evidence of the octahedral geometry of the compound.[40, 41] The intensity of the absorption band monotonically increased with an increase in the mole ratio of the [P66614][N-mGly]/Co(salen) complex from 0 to 2. This intensity change was due to the coordination of [P66614][N-mGly] to Co(II). In addition, the absorption band around 390 nm was slightly blue shifted with an increase in the [P66614][N-mGly]/Co(salen) mole ratio from 0 to 2 (Fig. 3(d) and (e)), which would have occurred as a result of the decrease in the electron density of the conjugated C=N bond with the coordination of [P66614][N-mGly] to Co(II). These characterizations strongly suggested that the synthesized compound was an MCIL composed of one Co(salen) molecule coordinated with two couples of [P66614][N-mGly]. From the results, it was suggested that the Co(salen) complex was coordinated by two [P66614][N-mGly] molecules and stabilized in an octahedral geometry; i.e. the chemical structure of the obtained product was estimated as [P66614]2[Co(salen)(N-mGly)2]. The expected structural formula of [P66614]2[Co(salen)(N-mGly)2] is shown in Fig. 4(a). The MCIL had good thermal stability, with a decomposition temperature of approximately 200 °C (Fig. 5) and a high viscosity (46350 ± 3350 mPa·s).
3
mole ratio of Co(II)-salen complex : [P66614][N-mGly]
(a)
Absorbance / -
2.5
0:1 2
1:0 1:0.5
1.5
1:1
1
1:1.5 1:2
0.5
1:3
0
1:4 250
350
450
550
650
750
wavelength / nm 0.1
0.06
(b)
0.09 0.08 0.07
∆A595nm
Absorbance / -
(c)
0.05
0.06 0.05
0.04 0.03 0.02
0.04 0.03
0.01
0.02
0
0.01 500
600
0
700
1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5
1 2 3 4 mol-IL/mol-Co(salen)
5
391
(d)
(e)
390 λmax around 390nm
Absorbance / -
wavelength / nm
389 388 387 386 385 384
350
400
wavelength / nm
450
0
1 2 3 4 mol-IL/mol-Co(salen)
5
Fig. 3 UV absorption spectra of Co(salen)/[P66614][N-mGly]/methanol solutions with different [P66614][N-mGly] concentrations: (a) whole wavelength range, (b) absorbance around 600 nm, (c) relationship between sample and [P66614][N-mGly]/methanol solution absorbance difference at 595 nm (wavelength at peak top) and [P66614][N-mGly] to Co(salen) molar ratio, (d) absorbance around 400 nm, and (e) relationship between wavelength at peak top and molar ratio of [P66614][N-mGly] to Co(salen)
(b)
(a) P66614+
P66614+
P66614+
P66614+
(c)
(d) P66614+
P66614+ P66614+
Fig.
4
Predicted
chemical
structures
of
(a)
[P66614]2[Co(salen)(N-mGly)2],
(b)
[P66614]2[Co(salen)(N-mGly)(Tf2N)], (c) O2 complex with 1:1 stoichiometry, and (d) O2 complex
Weight loss (m|T=125°C basis) / %
with 1:2 stoichiometry
100 80 60 40
20
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
0
100 150 200 250 300 350 Temperature / oC Fig. 5 TGA profiles of [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] under N2 using temperature ramp rate of 10 oC/min
3.2 Synthesized [P66614]2[Co(salen)(N-mGly)(Tf2N)] An MCIL was also synthesized by mixing the Co(salen) complex with an equimolar mixture of [P66614][N-mGly] and [P66614][Tf2N] (the mole ratio of Co(salen)/[P66614][N-mGly]/ [P66614][Tf2N] was 1/1/1). A photograph of the synthesized MCIL, [P66614]2[Co(salen)(N-mGly)(Tf2N)], is shown in Figs. 1(b). It is worth mentioning that the product retained its liquid state, and no solid was precipitated after evaporation. If no [P66614][Tf2N] was added to the solution, the solid Co(salen) complex was precipitated because of the removal of coordinated O2 from the product during the evaporation process. The O2 removal was qualitatively confirmed from the color change of the product before and after evaporation. When O2 was detached from the product, the coordination number of Co(II) became five, and [P66614][N-mGly] abstraction occurred. As a result, half of the Co(salen) complex formed the six-coordinated [P66614]2[Co(salen)(N-mGly)2], and the other half precipitated in the solution. On the other hand, when [P66614][Tf2N] coexisted in the solution, it could coordinate to the Co(salen) complex instead of O2. As a result, after the removal of the O2, [P66614]2[Co(salen)(N-mGly)(Tf2N)] could be formed, which had a stable octahedral geometry with six coordination bonds. Therefore, no Co(salen) precipitation was observed in the solution after evaporation. The FT-IR spectrum of the product is shown in Fig. 2, along with those of [P66614][N-mGly], [P66614][Tf2N], and the Co(salen) complex. The assigned peaks are listed in Table 1. It was confirmed that the product was composed of [P66614][N-mGly], [P66614][Tf2N], and the Co(salen) complex. The estimated structural formula of [P66614]2[Co(salen)(N-mGly)(Tf2N)] is shown in Fig. 4(b). The thermal stability of the synthesized [P66614]2[Co(salen)(N-mGly)(Tf2N)] was also examined. The TGA data are shown in Fig. 5, along with the data for [P66614]2[Co(salen)(N-mGly)2]. It was confirmed that the thermal decomposition temperature of [P66614]2[Co(salen)(N-mGly)(Tf2N)] was also greater than 200 °C. Interestingly, the viscosity of [P66614]2[Co(salen)(N-mGly)(Tf2N)] (7504 ± 1855 mPa·s) was significantly smaller than that of [P66614]2[Co(salen)(N-mGly)2] (46350 ± 3350 mPa·s). Based on these results, it can be expected that different IL-based ligands would provide numerous opportunities to create various MCILs with diverse physico-chemical properties. Designing the structures of IL-based ligands and changing the combination of them could be a useful methodology to control the properties of the MCILs.
3.3 Selective O2 absorbability of Co(salen) complex-based ILs One of the most important properties of the synthesized MCILs was the O2 absorbability. Fig. 6 shows
the
O2
and
N2
absorption
isotherms
of
[P66614]2[Co(salen)(N-mGly)2]
and
[P66614]2[Co(salen)(N-mGly)(Tf2N)]. The absorption isotherms with the unit of mmol-O2/g-MCIL are shown in Fig. S8. As clearly shown in Fig. 6, the MCILs absorbed O2 chemically and N2 physically. It was suggested that O2 was absorbed in the MCILs via ligand exchange reaction; i.e. the anion of the ionic liquid-based ligand would be released from Co(salen) complex and O2 coordinated to the complex alternatively. Below atmospheric pressure, the O2 absorption amounts for both MCILs were much higher than those for N2 under the same pressure conditions. The higher O2 absorption amounts were because of the contributions of the chemical reactions between the MCILs and O2. Thus, it was found that the MCILs retained the O2 reactivity of the Co(salen) complex. A comparison of the saturated O2 absorption amounts of the two MCILs showed that they had completely different stoichiometries,
i.e.,
1
mol-O2/mol-MCIL
for
[P66614]2[Co(salen)-(N-mGly)2]
and
0.5
mol-O2/mol-MCIL for [P66614]2[Co(salen)(N-mGly)(Tf2N)] (as shown in Fig. 6(a)). These results suggested that an O2 unit molecule reacted with a [P66614]2[Co(salen)(N-mGly)2] unit molecule (1:1 complex formation) and two [P66614]2[Co(salen)(N-mGly)(Tf2N)] molecules (1:2 complex formation). The predicted structure of the 1:1 and 1:2 complexes are shown in Fig. 4(c) and (d), respectively. Regarding the complex formation mechanism, it was reported that Co(salen) complex reacts with O2 according to the following sequential reaction [42].
Co(salen) A O 2
Co(salen) A O 2
Co(salen) A O 2 Co(salen) A
(1)
Co(salen) A2 O 2
(2)
where "A" expresses the axial ligand. According to the sequential reaction, it can be considered that 1:2 complex, (Co(salen)·A)2·O2, could be formed when considerable amount of Co(salen)·A is formed and the reaction (2) progresses. On the other hand, If formation of Co(salen)·A is hardly occurred, reaction (2) could not progressed. In this case, 1:1 complex, Co(salen)·A·O2, could be mainly formed. According to the reactions, the complex formation between [P66614]2[Co(salen)(N-mGly)2] and O2 could be expressed as the following sequential reactions.
Co(salen) A 2
Co(salen) A 2 O 2
Co(salen) A A
(3)
Co(salen) A O 2 A
Co(salen) A O 2 Co(salen) A
(4)
Co(salen) A2 O 2
(5)
where "A" in Eqs. (3) – (5) means [P66614][N-mGly]. Regarding amino group, it is worth mentioning that high donor number (DN ≈ 57 – 61) was reported [43]. The high DN indicates that N-methylglycinate could strongly coordinate to Co(salen) complex. Therefore, in this case, it could be considered that the reaction (3) was hardly occurred. On the other hand, because of the strong coordination ability of O2 with Co(salen) complex, reaction (4) could progresses and Co(salen)·A·O2 could be formed when O2 was existed in the system. However, because reaction (3) hardly occurred,
negligible amount of Co(salen)·A would be existed. As the result, reaction (5) hardly occurred and considerable amount of (Co(salen)·A)2·O2 could not be formed. Thus, 1:1 complex of Co(salen)·A·O2, which corresponds to [P66614][Co(salen)(N-mGly)·O2], could be mainly formed. On the other hand, the complex formation between [P66614]2[Co(salen)(N-mGly)(Tf2N)] and O2 could be expressed as the following sequential reactions. Co(salen) A B Co(salen) A B
Co(salen) A B O 2
(6)
Co(salen) A O 2 B
Co(salen) A O 2 Co(salen) A
(7)
Co(salen) A2 O 2
(8)
where "A" and "B" in Eqs. (6) – (8) express [P66614][N-mGly] and [P66614][Tf2N], respectively. In this case, reaction (6) could progress and Co(salen)·A could be formed in the system because of the weak coordination ability of Tf2N anion to the Co(salen) complex (DN ≈ 5.6 [43]). In addition, reaction (7) could also progress because of the strong coordination ability of O2 with Co(salen) complex. Therefore,
1:2
complex
of
(Co(salen)·A·O2)2,
which
corresponds
to
[P66614]2[(Co(salen)(N-mGly))2·O2], could be formed via reaction (8). Interestingly, the viscosity of [P66614]2[Co(salen)(N-mGly)2] was increased after O2 absorption (from 46350 mPa·s before O2 absorption to 81530 mPa·s after O2 absorption) although that of [P66614]2[Co(salen)(N-mGly)(Tf2N)] showed no considerable change (from 7500 mPa·s before O2 absorption to 7380 mPa·s after O2 absorption). Because [P66614]2[Co(salen)(N-mGly)2] formed 1:1 complex with O2, there is free space around the O2 which coordinated to Co(salen) complex. Therefore, N-methylglycinate which coordinated with a Co(salen) complex can approach to an O2 which coordinated with another Co(salen). The hydrogen atom on the secondary amino group of the N-methylglycinate which coordinated with a Co(salen) complex could form hydrogen bonding with the O2 on another Co(salen) complex. As the result, hydrogen bonding network between the [P66614]2[Co(salen)(N-mGly)2]-based
O2
complexes
could
mGly-Co(salen)-O2···mGly-Co(salen)-O2···mGly-Co(salen)-O2
where
be "-"
formed; and
"···"
i.e. mean
coordination bond and hydrogen bond, respectively. Because of the hydrogen bonding network, the viscosity of [P66614]2[Co(salen)(N-mGly)2] would be increased after O2 absorption. On the other hand, because [P66614]2[Co(salen)(N-mGly)(Tf2N)] formed 1:2 complex with O2 and the coordinated O2 was sandwiched by the Co(salen) complexes from upper and lower sides, there is no free space around the coordinated O2. Therefore, no hydrogen bonding between the coordinated O2 and N-methylglycinate was not formed and no considerable viscosity change was observed. The hypothesis on the hydrogen bonding formation between N-methylglycinate and O2 in the [P66614]2[Co(salen)(N-mGly)2]-based
O2
complex
was
checked
using
an
MCIL
with
trihexyl(tetradecyl)phosphonium dimethylglycinate ([P66614][dmGly]) as the axial ligands; [P66614]2[Co(salen)(dmGly)2]. Because the amino group of dimethylglycinate has no hydrogen atom
which directly bonded to the nitrogen atom of the tertiary amine group, dimethylglycinate could not form hydrogen bonding with the O2 which coordinated with Co(salen). Therefore, it is predicted that no considerable viscosity change of [P66614]2[Co(salen)(dmGly)2] before and after O2 absorption would be occurred. Before the measurement of the viscosity change of [P66614]2[Co(salen)(dmGly)2] before and after O2 absorption, we checked that [P66614]2[Co(salen)(dmGly)2] could absorb considerable amount of O2. Then, we measured the viscosities of [P66614]2[Co(salen)(dmGly)2] before and after O2 bubbling. As the result, it was confirmed that the viscosity of [P66614]2[Co(salen)(dmGly)2] before O2 absorption (2606 mPa·s) was almost same as that after O2 absorption (3074 mPa·s). This result suggested that the suitability of our assumption on the viscosity enhancement by the hydrogen bonding formation between the [P66614]2[Co(salen)(N-mGly)2]-based O2 complexes. As shown in Fig. S7, the saturated absorption amounts of O2 in [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] were about 0.7 mmol/g and 0.3 mmol/g, respectively. These values were much higher than that of an MCIL reported by Kohno et al. (ca. 0.12 mmol/g), which was obtained by dosing an MCIL-impregnated porous material using an O2 atmosphere of 0.9 bar at 298 K for 2 h
[33]. Our developed MCILs could easily be synthesized and could absorb large
amounts of O2. These are the attractive advances of our developed MCILs.
Amount of O2 absorbed / mol-O2·mol-MCIL-1
2.0
(a) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
1.5 1.0 0.5 0.0
0 5 10 15 20 25 Equilibrium pressure (O2) / kPa
Amount of N2 absorbed / mol-N2·mol-MCIL-1
2.0
(b) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
1.5 1.0 0.5 0.0
0 100 200 300 400 500 Equilibrium pressure (N2) / kPa
Fig. 6 O2 (a) and N2 (b) absorption isotherms of [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] at 303 K
3.4 Gas permeation properties of the SILM containing Co(salen) complex-based ILs Fig. 7 shows the partial pressure dependency on the O2 and N2 permeabilities and O2/N2 selectivities of the prepared membranes. We can evaluate the gas permeation mechanism based on the trends, i.e., the profiles with and without partial pressure dependency indicate that the gas permeation mechanisms are facilitated transport and solution-diffusion, respectively.[13, 18, 26] The marked dependency on the partial pressure for FTMs reflects carrier saturation with the gas absorption in the membrane. As shown in Fig. 7(a), the O2 permeabilities drastically decreased with the increase of the O2 partial pressure from 1 kPa to about 5 kPa and then became constant above 5 kPa. This strong dependency of the O2 permeability on O2 partial pressure is the evidence supporting the facilitated O2 transport by MCIL-based SILMs; i.e. the MCIL acted as an O2 carrier in the membrane. As shown in Fig. 6(a), the O2 absorption amounts also became plateau above 5 kPa of the O2 pressure, i.e. the trend shows good agreement with that of the O2 permeability. When the
MCIL-based SILMs contacted with the gas containing more than 5 kPa of O2, MCILs were saturated with O2. Therefore, the facilitation effect of the MCIL-based SILMs at more than 5 kPa of O2 partial pressure became constant. In other words, when the MCIL-based SILMs were exposed to the gas with less than 5 kPa of O2 partial pressure, the driving force of O2 permeation was mainly dominated by and strongly depended on the chemically absorbed O2. On the other hand, the N2 permeabilities indicated no dependency on N2 and O2 partial pressures because of the Henry type absorption (as shown in Fig. 6(b)). N2 partial pressure / kPa 100 95 90 85 80
100
(a)
(b)
100
10
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (N₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (N₂)
O2/N2 selectivity
Gas permeability / Barrer
1000
N2 partial pressure / kPa 100 95 90 85 80
10
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
1
1 0
5 10 15 20 O2 partial pressure / kPa
0
N2 partial pressure / kPa 100 95 90 85 80
N2 partial pressure / kPa 100 95 90 85 80
100
(c)
(d) O2/N2 selectivity
Gas permeability / Barrer
1000
100
10
[P₆₆₆₁₄][N-mGly] (O₂) [P₆₆₆₁₄][Tf₂N] (O₂) [P₆₆₆₁₄][N-mGly] (N₂) [P₆₆₆₁₄][Tf₂N] (N₂)
1
7
O2
[P₆₆₆₁₄][N-mGly] [P₆₆₆₁₄][Tf₂N]
10
1 0
Fig.
5 10 15 20 O2 partial pressure / kPa
5 10 15 20 O2 partial pressure / kPa
permeation
properties
[P66614]2[Co(salen)(N-mGly)(Tf2N)]-based
of
0
(a,b)
SILMs,
and
5 10 15 20 O2 partial pressure / kPa
[P66614]2[Co(salen)(N-mGly)2](c,d)
[P66614][N-mGly]-,
and and
[P66614][Tf2N]-based SILMs, along with O2 partial pressure dependency on (a,c) O2 and N2 permeabilities and (b,d) O2/N2 selectivities. Experimental conditions: total pressure at feed and permeate sides was atmospheric pressure, temperature was 303 K, and atmosphere was dry.
100
polymer membranes
O2/N2 selectivity
Robeson upper bound 2008 (Ref.8)
p O2
Fixed carrier membranes (Refs.18-23)
1 kPa
10 2.5 kPa
15 kPa 20 kPa
5 kPa 10 kPa 15 kPa 20 kPa
1 0.01
Fixed carrier membrane A (Ref. 24) O₂ partial pressure dependency Fixed carrier membrane B (Ref. 24) O₂ partial pressure dependency this work ([P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂]) this work ([P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)])
1 100 10000 O2 permeability / barrer
Fig. 8 Comparison of O2 separation performances with various O2 separation membranes.[8, 18-24] In addition, to confirm the effect of Co(salen) complex on the facilitated O2 transport, the O2 and N2 permeabilities and O2/N2 selectivities of the SILMs containing the IL-based ligands ([P66614][N-mGly] and [P66614][Tf2N]) were evaluated (Fig. 7(c) and (d)). It was clearly shown that the SILMs without Co(salen) complex demonstrated no facilitation effect on the O2 transport. From the results shown in Figs. 6 and 7, there is no doubt that the Co(salen) complex-based MCILs facilitated O2 permeation. On the other hand, regarding the O2 permeabilities of the two MCILs-based SILMs, [P66614]2[Co(salen)(N-mGly)(Tf2N)]-based SILM showed 1.26 – 1.5 times higher O2 permeability than that of [P66614]2[Co(salen)(N-mGly)2]-based SILM although the O2 absorption amount of [P66614]2[Co(salen)(N-mGly)(Tf2N)] was two times lower than that of [P66614]2[Co(salen)(N-mGly)2]. The lower diffusivity of the O2 complex with [P66614]2[Co(salen)(N-mGly)2] would be the reason of the lower O2 permeability. It is considered that the higher viscosity of [P66614]2[Co(salen)(N-mGly)2] (ca. 81500 mPa·s after O2 absorption) than that of [P66614]2[Co(salen)(N-mGly)(Tf2N)] (ca. 7380 mPa·s after O2 absorption) would the main factor of the lower diffusivity. Additionally, the molecular volumes of the O2 complexes, which are the diffusates in the MCILs, would also affect the diffusivity. Because [P66614]2[Co(salen)(N-mGly)2] and [P66614]2[Co(salen)(N-mGly)(Tf2N)] made the O2 complexes with 1:1 and 1:2 stoichiometries, respectively, the molecular volume of the O2 complex with [P66614]2[Co(salen)(N-mGly)2] is approximately two times larger than that of the O2 complex with [P66614]2[Co(salen)(N-mGly)(Tf2N)]. According to Arnold equation [44], which can relatively accurately predict the diffusivity of solute in ILs [45], the diffusion coefficient of solute has proportional relationship with (1/Mcom+1/MIL)0.5/(η0.5·(Vcom1/3+VIL1/3)2), where Mi, η, and Vi are molecular weight, viscosity of the MCIL and molecular volume, respectively. Subscripts "com" and "IL" shows of the O2 complex and MCIL, respectively. In addition, the O2 permeability, which can
be expressed by (ε/τ)·Dcom·ΔCcom where ε is porosity of the support, τ is the tortuosity of the pore of the support, Dcom is the diffusion coefficient of the O2 complex as discussed above and ΔCcom is the trans-membrane concentration difference of the O2 complex which can be considered as same as the equilibrium concentration of the O2 complex because of the sweep method used in this study. Considering the differences of the Dcom and ΔCcom, it was confirmed that the permeability of the [P66614]2[Co(salen)(N-mGly)(Tf2N)]-based SILM is about 1.1 times higher than that of the [P66614]2[Co(salen)(N-mGly)2]-based SILM. The opposite trend between the O2 absorption amounts and the O2 permeabilities of the two MCILs could be explained by the effect of diffusivity. Although further investigations are needed, it could be considered that the O2 permeability of the MCIL-based SILMs would be strongly affected by the diffusivity of the O2 complex. A comparison of the performances of the developed MCIL-based SILMs and those of the previously reported FTMs with fixed O2 carrier along with those of polymer membranes are shown in Fig. 8. As shown in this figure, the O2 permeation performance of the MCIL-based FTMs shows an O2 partial pressure dependency similar to those of the fixed carrier type membranes (shown in the square symbols). The similar trends also supports the facilitated O2 transport mechanism of the MCIL-based SILMs. From another point of view, the O2 permeabilities of the MCIL-based SILMs were higher than those of the fixed carrier membranes. The superior O2 permeability of the MCIL-based SILMs might be the result of the higher mobility of the MCIL as the O2 carrier. Comparing the performances of the MCIL-based SILMs and those of the polymer membranes [8], the MCIL-based SILMs demonstrated much higher O2/N2 selectivities under low O2 partial pressure conditions. On the other hand, at a high O2 partial pressure, the performance was still lower than those of some polymer membranes. The low O2 permeability at high O2 partial pressure limited the O2/N2 selectivity. Improving the O2 permeabilities of the MCIL-based SILMs is our next challenge. Finally, we investigated the stability of the MCIL-based SILMs. For previously reported facilitated O2 transport membranes, one of the major problems was the instability of the O2 carrier against irreversible oxidation [15]. The stability of the MCIL-based SILMs is indicated in Fig. 9. As demonstrated in Fig. 9, the high O2 permeabilities and N2 barrier properties of the MCIL-based SILMs were maintained for more than 80 h. These results suggested that the developed MCILs retained their selective O2 absorbability because of their good stability.
Gas permeability / Barrer
300
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (O₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂] (N₂) [P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)] (N₂)
(a)
250 200 150 100 50
0
O2/N2 selectivity
0 10 9 8 7 6 5 4 3 2 1 0
20
40 60 Time / h
80
100
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)₂]
(b)
0
20
[P₆₆₆₁₄]₂[Co(salen)(N-mGly)(Tf₂N)]
40 60 Time /h
80
100
Fig. 9 Stability of [P66614]2[Co(salen)(N-mGly)2]- and [P66614]2[Co(salen)(N-mGly)(Tf2N)]-based SILMs; (a) O2 and N2 permeabilities, and (b) O2/N2 selectivities. Experimental conditions: total pressure at feed and permeate sides was atmospheric pressure, O2 partial pressure of feed was 2.5 kPa, temperature was 303 K, and atmosphere was dry.
To the best of our knowledge, this is the first report on an IL-based mobile O2 carrier membrane. Although the MCILs developed in this study are synthesized from relatively expensive raw materials, the simple synthesis process could reduce the production cost. Furthermore, fortunately small amount of MCILs is needed to prepare the facilitated transport membrane with very thin O2 transport layer. Therefore, membrane is one of the best media for the MCILs. In addition, the good stability of the MCIL-based membrane allows the long-term use, which could reduce the total operation cost. On the other hand, although the developed membrane had high and selective O2 permeability and good stability, the performance at high O2 partial pressure is still insufficient. As previously indicated, the physico-chemical properties of the MCILs could easily be tuned by designing the IL-based axial ligands. For instance, an MCIL with a smaller molecular size would have a higher
Co(salen) complex density and lower free volume. It is expected that such a small MCIL could provide a larger amount of O2 absorption, lower N2 solubility, and lower viscosity [46, 47]. Combination of main and auxiliary IL-based ligands also has potential for optimizing the properties of an MCIL. MCILs have considerable potential for performance improvement. We believe that the optimization of the physico-chemical properties of MCILs could provide a new class of O2 separation membrane, which could provide opportunities for designing novel processes and devises.
4. Conclusions Novel MCILs containing O2 reactive Co(salen) complex were synthesized according to the concept of a solvate IL. Simple mixing of Co(salen) complex and an ILs, which was composed of coordinating anion and bulky cation, in ethanol allowed to prepare O2 reactive MCILs. The developed MCILs chemically and selectively absorbed a large amount of O2 and acted as an O2 carrier of an FTM. The facilitated O2 transportation of the FTM was confirmed from the O2 partial pressure dependency on the O2 permeabilities. Owing to the liquidity of the MCILs, the FTMs showed higher O2 permeability than those of fixed O2-carrier membranes as well as polymer membranes. In addition, the FTMs overcame the instability issue, which was one of the major challenges of conventional facilitated O2 carrier membranes. The findings of the present proof-of-concept study suggested that the developed MCILs have great potential as an O2 separation media because the MCILs could be easily tuned the structure and physico-chemical properties by selecting the combination of main and auxiliary IL-based ligands.
Acknowledgements A part of this research was supported by the Matching Planner Program (MP28116808274) from the Japan Science and Technology Agency.
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Highlights:
Novel O2 reactive metal-containing ionic liquids (MCILs) were synthesized.
The MCILs could have a potential to tune the physico-chemical property easily.
The MCILs absorbed a large amount of O2 selectively and acted as an O2 carrier.
The MCIL-based membranes permeate O2 based on facilitated transport mechanism.
Owing to the liquidity of the MCILs, the membrane exhibited high O2 permeability.