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Cite This: ACS Appl. Polym. Mater. 2019, 1, 1579−1589
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*,† †
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Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ 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 S Supporting Information *
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. 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, toward utilization of effective, benign materials. KEYWORDS: ionic liquid, protic ionic liquid, ion gel, carbon dioxide separation membrane, sulfonated polyimide
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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 1000−1500 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 © 2019 American Chemical Society
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 looping,16−18 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 an amine absorption method is the current major technology. In the CO2 separation market, amine absorption Received: April 23, 2019 Accepted: May 7, 2019 Published: May 7, 2019 1579
DOI: 10.1021/acsapm.9b00383 ACS Appl. Polym. Mater. 2019, 1, 1579−1589
Article
ACS Applied Polymer Materials
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 indeed the use of shorter synthetic pathways has been identified as an important factor toward 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 self-dissociation 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 the 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.
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 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 poly-ILs, 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 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 a 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 nonhumidified intermediate temperature fuel cells39 and actuators.40 We further reported these composite membranes consisting of the IL 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-
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EXPERIMENTAL SECTION
Materials. Lithium bis(trifluoromethanesulfonyl)amide (Li[NTf2]) was provided by Morita Chemical, Japan. N,N-Dimethylformamide (DMF), tetrahydrofuran (THF), 1-bromobutane, 1-bromopropane, 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 1580
DOI: 10.1021/acsapm.9b00383 ACS Appl. Polym. Mater. 2019, 1, 1579−1589
Article
ACS Applied Polymer Materials
Figure 1. Photographs of composite membranes comprised of SPI at 75 wt % of IL content, together with corresponding chemical structures of the ILs. confirmed that the water content was
