Application of Protic Ionic Liquids to CO2 Separation in a Sulfonated

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Application of protic ionic liquids to CO2 separation in a sulfonated polyimide-derived ion gel membrane Eri Hayashi, Morgan L. Thomas, Kei Hashimoto, Seiji Tsuzuki, Akika Ito, and Masayoshi Watanabe ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00383 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Polymer Materials

Application of Protic Ionic Liquids to CO2 Separation in a Sulfonated Polyimide-derived Ion Gel Membrane Eri Hayashi,a Morgan L. Thomas,a iD Kei Hashimoto,a iD Seiji Tsuzuki,b iD Akika Ito,a and Masayoshi Watanabe *, a iD

a

Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai,

Hodogaya-ku, Yokohama 240-8501, Japan. b

Research Center for Computational Design of Advanced Functional Materials (CD-FMat),

National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japan

KEYWORDS: ionic liquid, protic ionic liquid, ion gel, carbon dioxide separation membrane, sulfonated polyimide

ACS Paragon Plus Environment

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ABSTRACT

It is of great interest to determine whether a relatively easily prepared and inexpensive protic ionic liquid (PIL) could be used in place of an aprotic ionic liquid (AIL) in a composite membrane for CO2 separation. We prepared mechanically reliable sulfonated polyimide (SPI) composite membranes

with

a

PIL,

[N124][NTf2]

(N,N,N-butylethylmethylammonium

bis(trifluoromethylsulfonyl)amide) by a solution casting method, and their mechanical, thermal and gas separation (CO2 and N2) properties were compared with those of membranes incorporating an isomeric AIL, [N1123][NTf2] ([N1123]+: N,N,N,N-ethyldimethylpropylammonium). We discussed the influence of the presence or absence of active protons in the ILs on CO2 separation characteristics and the applicability of the PIL to a CO2 separation membrane. The SPI/PIL membrane exhibited a slightly lower selectivity, but higher permeability, than the SPI/AIL membrane. It is supposed that these differences originate from the difference in membrane structure and CO2 interactions between PIL and AIL, which was investigated through a combination of ab initio calculations, CO2 solubility measurements and dynamic mechanical analysis (DMA). The [N124][NTf2](PIL)/SPI membranes showed a CO2 permeability (PCO2 = 240 Barrer) with a relatively high selectivity (αCO2/N2 = 23) at 30 °C, which indicates that a PIL/SPI composite membrane can be employed for CO2 separation. We also explored CO2 separation under elevated temperatures and humidified conditions for confirming the applicability of the SPI/PIL membrane. Finally, the effect of PIL structures was studied by using [N122][NTf2] ([N122]+: N,N,N-diethylmethylammonium)

in

comparison

with

[N124][NTf2].

Composite

[N122][NTf2]/SPI membranes exhibited a comparable selectivity, and an improved permeability. We anticipate these results will unveil a new paradigm in functional ion gels for critical CO2 separation technology, towards utilization of effective, benign materials.


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INTRODUCTION

The reduction of CO2 emitted by the combustion of fossil fuels is known as a critical issue due to its large impact on the climate change and related environmental problems, including global warming, enhancing heat stress, higher frequency of storms, increasing ocean acidity, sea level rise, and the melting of glaciers.1-3 In order to reduce CO2 emissions, carbon capture and storage (CCS) is required, which is a technology for isolating and collecting the emitted CO2, and artificially storing it underground at a depth of 1,000 to 1,500 m.4 However, one problem with this approach is the high energy cost for separation. Therefore, a low cost CO2 separation technology is required for putting CCS into practical use.5 Considering the capture of CO2 from industrial processes, we have focused on CO2 capture processes using membranes. The membrane separation strategy involves the separation of gases using a dense membrane having gas permeability and CO2 selectivity.6-10 Among various investigated CO2 separation methods (e.g., chemical and physical absorption,11-13 solid adsorption,14-15 chemical looping16-18 and cryogenic separation)19, the membrane separation technic is regarded as an emerging separation technology,7-10, 20 which can save energy and reduce environmental impact on the climate due to the intrinsically simple separation mechanism, i.e., gas diffusion driven by the partial pressure difference across the membrane.21 However, this technology is still under development, and amine absorption method is the currently major technology. In CO2 separation market, amine absorption occupies 90% of the share while the membrane separation has only ca. 10% mainly for natural gas sweetening and biogas upgrading.22 One reason for this is the low CO2 permeability and CO2/N2 selectivity of the membrane separation. Robeson investigated the CO2 separation ability of various polymeric membranes to


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report that the CO2 permeability and CO2/N2 selectivity are in a trade-off relationship,23 which is the main obstacle for achieving high CO2 permeability along with high CO2/N2 selectivity. Therefore, bestowing high CO2/N2 selectivity in addition to high permeability to the polymeric membranes is necessary for the development of economically viable separation membranes. In this regard, ionic liquids (ILs), i.e., salts composed only of anions and cations, which are liquid above room temperature, are one of the emerging CO2 separation media because they have various distinctive features, e.g., negligible vapor pressure, high thermal and chemical stability, CO2 absorption capability, and tunable chemical nature.24-28 It was reported that supported IL membranes (SILMs), which are porous membranes incorporating ILs by a capillary force, exhibit excellent CO2 selectivity and permeability.27 Although SILMs showed great CO2 separation performance, one of the most critical problems is the leakage of ILs. Even at low pressure differences across the membrane (~1 atm), ILs confined in the porous supports can leak, which limits the industrial application of SILMs. In order to overcome the mechanical and processing problems of IL, poly-ILs were developed by direct polymerization of IL monomers. Although poly-IL materials exhibit high CO2 selectivity inherited from IL,29 the low permeability (i.e., poor CO2 diffusivity), which results from the low mobility of IL structure attached to polymer side chains, is a serious problem.30 Therefore, in recent years, development of an ion gel material combining characteristics of ILs and poly-ILs for producing membranes having high CO2 permeability, CO2/N2 selectivity, and processability has been conducted. Ion gels generally have both mechanical properties derived from the polymer network and the CO2 absorption characteristics derived from the IL. Unlike polyILs, ILs entrained in the polymer network can freely diffuse, thus, they do not interrupt the CO2 permeation, resulting in high gas permeability.30 To fabricate high-performance CO2 separation


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membranes with good mechanical properties, numerous types of IL-based membranes have been proposed.31-35 It has been reported that a CO2 separation membrane using a tetra-PEG network (i.e., a homogeneous polymer network formed by tetra-arm poly(ethylene glycol)) ion gel achieves permeability close to that of the IL itself because of its ability to support a high ionic liquid loading. In such membranes, the polymer content is rather low, enabling ionic liquid-like CO2 transport. However, its mechanical strength (elastic modulus ~10 kPa) is insufficient for preparing a thin membrane (10 MPa) even with a high loading of IL (IL content >75 wt%).38 The SPI/IL composite membranes have been applied to non-humidified intermediate temperature fuel cells39 and actuators.40 We further reported these composite membranes consisting of the IL 1butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][NTf2]) can be used for a CO2 separation membrane.41-42 　 Notably, such a composite membrane containing 75 wt% [C4mim][NTf2] exhibited CO2 permeability (PCO2) of 412 Barrer at 30 °C and selectivity against N2 (αCO2) of 27.41 However, the optimization of the chemical structure of IL is under development,42 and there is scope for consideration of alternative structures. Protic ILs (PILs), which are typically prepared by the simple neutralization reaction of equimolar amounts of a Brønsted acid and base, are a category of ILs having an active proton in the cation. Their straightforward preparation can eliminate or reduce multiple reaction steps such as solvent separation and ion-exchange reactions, energy, and time, leading to the less expensive production than common aprotic ILs (AILs),43 and


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indeed the use of shorter synthetic pathways has been identified as an important factor towards reducing the overall environmental impacts of ionic liquid production.44 Moreover, PILs generally exhibit low toxicity and biodegradability with respect to the corresponding AILs. 45 Thus, the use of separation membranes comprising PILs may enable a simple and low-cost separation technique.46 Generally, PILs contain some proportion of the neutral acid and base due to the selfdissociation reaction following acid-base equilibrium, resulting in low thermal stability due to the evaporation of acid and base. The proportion of neutral acid and base strongly depends on the difference of acid dissociation constant between acid and protonated base (ΔpKa).47 In recent years, it has been reported that PILs with a ΔpKa of more than 15 exhibit good thermal stability, analogous to typical AILs, because the amount of neutral acid and base is negligible.48 Moreover, this criterion might be lowered to a ΔpKa > 10 for [NTf2]-based PILs .49 Therefore, by selecting an appropriate acid and base to overcome thermal instability limitations, successful application of PIL to CO2 separation technology is anticipated. In this work, to clarify the feasibility of using such a PIL derived ion gel for a CO2 separation membrane, we initially assessed the physicochemical properties of two structurally isomeric ILs (one protic, another aprotic), and we then investigated the thermal, mechanical and CO2 separation properties of the corresponding ion gel membranes. The gas permeability, thermal stability, mechanical properties and CO2 solubility of IL were investigated for these composite membranes, and we discussed the influence of the presence or absence of active protons on CO2 separation characteristics and the applicability of the PIL to a CO2 separation membrane.

EXPERIMENTAL SECTION


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Materials. Lithium bis(trifluoromethanesulfonyl)amide (Li[NTf2]) was provided by Morita Chemical, Japan. N,N-dimethylformamide (DMF), tetrahydrofuran (THF), 1-bromobutane, 1bromopropane,

triethylamine,

benzoic

acid

(Wako

Chemicals,

Japan)

and

bis(trifluoromethanesulfonyl)amide acid (H[NTf2]; Kanto Chemical, Japan)) were used as received. Diethylmethylamine, (dema, N122; Tokyo Chemical Industry Co., Japan) and m-cresol (Kanto Chemical, Japan.) were used after distillation. Bis[4-(3-aminophenoxy)-phenyl]sulfone (3BAPPS; Tokyo Chemical Industry Co., Japan) was used after recrystallization with ethanol/water. 1,4,5,8-Naphthalene-tetracarboxylic dianhydride (NTDA; Sigma-Aldrich, USA) was dispersed in DMF and the resulting suspension was refluxed at 60 °C for 1 day to remove soluble impurities. The resulting solid was washed several times with acetone and dried under vacuum. 2,2-Benzidinedisulfonic acid (BDSA; Tokyo Chemical Industry Co., Japan) was dissolved in deionized water containing an excess amount of triethylamine and solid impurities were removed by filtering the solution. 1 M H2SO4 aqueous solution was added to the solution and pure BDSA was precipitated as a white powder. The obtained white powder was washed several times with water and dried under vacuum. Ionic liquids. The PIL, [N124][NTf2] was synthesized according to the following procedure, adapted from the previously reported synthetic route (quaternization, Hoffman elimination, and neutralization).50 To a 1-bromobutane (1.27 g, 0.601 mol) solution in DMF (100 mL), N122 (60 mL, 0.488 mol) was added dropwise with stirring at room temperature. After stirring for 24 h at 40 °C, the solvent was evaporated to obtain a residue. The residue was washed with ethyl acetate several


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times and vacuum dried for 24 h to obtain a white ammonium salt, [N1224]Br (mass yield: 92.2%). [N1224]Br (101.11 g, 0.453 mol) and granular KOH (33.9 g, 0.604 mol) were dissolved in water (15 ml) and stirred at 140 °C to perform the Hofmann elimination reaction. The resulting droplets of water were trapped by the Liebig condenser during the reaction. To dry the solution, an appropriate amount of molecular sieve was added to the obtained solution, followed by the addition of CaH2 and subsequent distillation. Finally, n-butylethylmethylamine (N124) (mass yield: 57%) was obtained. [N124][NTf2] was prepared by the neutralization reaction of equimolar amounts of the Brønsted acid HNTf2 and Brønsted base N124, according to the reported method.50 After the reaction, the obtained liquid was vacuum-dried at 80 °C for 48 h. It was confirmed that the water content was < 20 ppm by Karl-Fischer titration. The 1H NMR spectrum of [N124][NTf2] is provided in Figure S1. The AIL, [N1123][NTf2] was synthesized according to the following procedure. To a 1bromopropane (64 mL, 0.707 mol) solution in THF (100 mL), ethyldimethylamine (N211, 60 mL, 0.589 mol) was gradually added dropwise using a dropping funnel. After the dropwise addition, the mixture was stirred at room temperature for 12 h, and further stirred at 50 °C for 6 h. Solvent (THF) and excess 1-bromopropane were removed by evaporation and then vacuum dried at 80 °C for 3 h. The obtained white solid was purified by three cycles of (i) filtration at 90 °C and (ii) recrystallization at −20 °C in acetonitrile, followed by vacuum drying for 6 h to obtain [N1123]Br (mass yield: 41%) as a white crystalline solid. The obtained [N1123]Br (47.1 g, 0.240 mol) and Li[NTf2] (72.5 g, 0.252 mol) were dissolved in water, respectively. Thereafter, the Li[NTf2] aqueous solution was added dropwise to


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the aqueous [N1123]Br solution, and the mixture was stirred for 12 h at room temperature. The reaction solution spontaneously separated into two liquid phases according to previous report,51 and the lower ionic liquid phase with high density was extracted. The obtained ionic liquid was washed 5 times with pure water using a separating funnel. Finally, the obtained liquid was vacuum dried (80 °C, 48 h) to remove water to obtain [N1123][NTf2] (yield: 32%). It was confirmed that the water content was < 20 ppm by Karl-Fischer titration. The 1H spectrum of [N1123][NTf2] is provided in Figure S2. The PIL, [N122][NTf2] was prepared by the neutralization reaction of an equimolar amount of the Brønsted acid H[NTf2] and Brønsted base N122, according to the reported method.50 After the reaction, the obtained liquid was vacuum-dried at 80 °C for 48 h. It was confirmed that the water content was < 20 ppm by Karl-Fischer titration. Sulfonated polyimide. [N122]-type-SPIs was synthesized in two steps of polyaddition and chemical imidization reaction of BDSA and 3BAPPS (BDSA ： 3BAPPS = 4:1) with the equivalent amounts of NTDA according to the previously reported method.36 Owing to the poor solubility of SPI in other solvents, m-cresol was employed as the synthetic solvent. Molecular weight of the SPI (Mn = 47 kDa; Mw = 96 kDa; Mw / Mn = 2.1; standard: polystyrene) was estimated by gel permeation chromatography (GPC) on a Shimazu LC-20 series, with a Tosoh TSKgel G3000Hxl column (40 °C, UV-vis detector, 100 μL injection loop, DMF mobile phase). Membrane preparation. All [N124][NTf2]/SPI, [N1123][NTf2]/SPI, and [N122][NTf2]/SPI composite membranes (IL content: 0, 33, 50, 66 and 75 wt%; only 75 wt% for [N122][NTf2]/SPI composite membrane) were prepared by a solution casting method in m-cresol as a solvent (the asprepared SPI has a poor solubility in other solvents) according to the previously reported method.36


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Figure 1 Photographs of composite membranes comprised of SPI at 75wt% of IL content, together with corresponding chemical structures of the ILs.

The membranes used in this study are denoted as SPI-A(X), where A corresponds to the chemical structure of the cation within the IL, and X corresponds to the content of the ILs (wt%). Figure 1 shows the appearance of several composite membranes (thickness: 30–190 μm) described in this study. For all compositions, uniform, flexible and transparent membranes were obtained without any leakage of ILs. Thermal analyses. Thermogravimetric analysis was performed using a Seiko Instruments TGDTA 7200C (Hitachi High-Technologies, Tokyo, Japan). The sample were placed in the aluminum pan, and weighed. For the removal of included water, they were first dried at 100 °C under N2 atmosphere for 30 min. Successively, they are cooled to 25 °C and heated from 25 °C to 550 °C at a heating rate of 10 °C min−1. The thermogravimetric curve in the heating step was recorded. Differential scanning calorimetry (DSC) measurements were performed using a Hitachi High-Tech DSC 6220 (Hitachi High-Technologies, Tokyo, Japan). IL samples were dropped into an aluminum pan and tightly sealed in an Ar-filled glove box ([H2O] 100 °C).60 Thus for the purposes of comparison in the initial stage of this study, we selected [N124][NTf2] (Tm = −6 °C) for the PIL and [N1123][NTf2] (Tm = −13 °C) for the AIL, which are both room temperature ionic liquids, with identical molecular masses, and near identical densities, as shown in Figure 2 and Table S1. Considering the tertiary amines as the starting point for the synthesis, a PIL may be produced via a simple neutralization reaction, with simple work-up, whilst a corresponding AIL requires an initial alkylation, followed by anion exchange reaction, with multiple purification steps. The sulfonated polyimide (SPI) with diethylmethylammonium cations shown in Figure 2, prepared according to our previous study,38 was then combined with the ionic liquids to form ion gels.


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N

+ F 3C

O

N

S O

N

F 3C

H

O S

H N

O

PIL [N124][NTf2]

O S

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Br

CF3

N

+

O O S

N

CF3

Br

Li[NTf2]

O

LiBr

structurally isomeric cations: C7H17N [N124]+ or [N1123]+ [NTf2]molecular mass : 396.4 g mol-1

O

O

N

N

O

O

S O

O3S H

O

N

AIL [N1123][NTf2]

N SO3 H

N

F 3C

N

O

O

N

N

O

O

0.8

O S

CF3

O

O O

S O

O 0.2

SPI

Figure 2 Syntheses (from amine), structures and images of the protic and aprotic ionic liquids (PIL and AIL) employed in this study, and the structure of the sulfonated polyimide (SPI) employed in this study.

Thermal analyses. The DSC curves of [N124][NTf2] and [N1123][NTf2] and their ion gels formed upon mixing and casting with SPI, are shown in Figure S5(a) and Figure S5(b), respectively. Melting temperature (Tm) and glass transition temperature (Tg) are listed in Table S2. We note here that phase transitions observed for the neat ILs were not observed for the composite membranes, as was noted in our previous study for an SPI/imidazolium-type AIL composite, and speculated to be due to confinement of the IL in nano-order ionic domains.41 Figure S6(a) shows the TG curves for SPI-N124(0-75) and SPI-N1123(0-75). The thermal decomposition temperatures (Td), defined here as the temperature at 5% weight loss, are listed in Table S3 and are also shown in Figure S6(b). From the result, the AIL membranes in all membranes had higher


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thermal stability than those observed for the PIL membranes. However, the thermal decomposition temperature of the PIL membranes also showed relatively high thermal stability of more than 200 °C. We assume this is because free acid and base were scarcely present in the PIL with a high ΔpKa (see Introduction), therefore, although PILs are generally considered to have low thermal stability, this combination of cation, anion and polymer showed sufficiently high thermal stability. Volume expansion and CO2 solubility. Here, we utilized moderate pressure CO2 expansion and solubility measurements for the ILs. We note here that although the conditions for membrane operation do not approach these pressures, and thus the expansion/solubility measurements do not provide direct understanding of the gas separation performance (vide infra), the measurement in this range does provide meaningful insights into the properties/interactions of the PIL and AIL in the presence of CO2. Figure 3(a) shows a plot of the coefficient of volumetric expansion versus the mole fraction. In both systems, the volume of the liquid phase (VIL) increased as the mole fraction of CO2 increased. Even with high CO2 mole fraction (XCO2~0.6), the volume expansions of AIL and PIL were small (

Application of Protic Ionic Liquids to CO2 Separation in a Sulfonated

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