Research Note pubs.acs.org/IECR
High-Permeance Room-Temperature Ionic-Liquid-Based Membranes for CO2/N2 Separation Jinsheng Zhou,*,† Michelle M. Mok,† Matthew G. Cowan,‡,§ William M. McDanel,‡ Trevor K. Carlisle,‡ Douglas L. Gin,‡,§ and Richard D. Noble‡ †
3M Corporate Research Process Laboratory, 3M Company, St. Paul, Minnesota 55144, United States Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, United States § Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ‡
S Supporting Information *
ABSTRACT: We have developed and fabricated thin-film composite (TFC) membranes with an active layer consisting of a room-temperature ionic liquid/polymerized (room-temperature ionic liquid) [i.e., (RTIL)/poly(RTIL)] composite material. The resulting membrane has a CO2 permeance of 6100 ± 400 GPU (where 1 GPU = 10−6 cm3/(cm2 s cmHg)) and an ideal CO2/N2 selectivity of 22 ± 2. This represents a new membrane with state-of-the-art CO2 permeance and good CO2/N2 selectivity. To our knowledge, this is the first example of a TFC gas separation membrane composed of an RTIL-containing active layer.
1. INTRODUCTION
and a CO2/N2 selectivity >30. These new membrane systems are currently being tested at an industrially relevant scale.9 Membranes comprised of room-temperature ionic liquids (RTILs) and polymerized room-temperature ionic liquids (poly(RTIL)s) have high CO2 permeability (i.e., pressureand membrane-active layer thickness-normalized flux typically measured in barrers, where 1 barrer =10−10 cm3(STP) cm/(cm2 s cmHg)) and good CO2/light gas selectivities.10−14 Reported RTIL- and poly(RTIL)/RTIL-based composite membranes have thick active layers (>50 μm), which hinder the measured permeance values significantly. The practical challenge is fabricating these RTIL-based liquid or gel materials into a defect-free thin-film membrane on the surface of a porous support in order to achieve high CO2 fluxes and good selectivity. Herein, we describe the fabrication and room-temperature performance of a defect-free, poly(RTIL)/RTIL thin-film composite (TFC) membrane with a pure-gas CO2 permeance of 6100 ± 400 GPU and a CO2/N2 ideal (i.e., single-gas) selectivity of 22 ± 2. This TFC membrane consists of a composite poly(RTIL)/RTIL active layer (96 ± 4 nm thick) on a proprietary gutter layer supported by a commercially available ultrafiltration membrane. To our knowledge, this is the first report of an RTIL-containing TFC membrane and one of the most CO2-permeable membranes with a CO2/N2 selectivity of ≥20.
Carbon dioxide (CO2) comprises 78% of anthropogenic greenhouse gas production and has been linked to global climate change and ocean acidification.1,2 Therefore, as climate change becomes an increasingly unavoidable environmental, industrial, and political issue, it is desirable to develop technology capable of economically separating and sequestering large amounts of CO2.3 This separation and sequestration technology is also likely to play a role in maintaining the economic value of fossil fuels in the event of a concerted global CO2 mitigation effort.4 The initial step toward this technology is to identify and develop an economically viable method of separating CO2 from flue gas at large point sources such as coal-fired power plants. The U.S. Department of Energy (DOE) listed its 2013 CO2 separation cost targets as $40/ tonCO2 captured by 2020−2025 and $10/tonCO2 captured by 2030−2035 with commercial deployments in 2025 and 2035, respectively.5 Estimates from 2013 show that if CO2 capture by amine scrubbing, the currently available technology, is performed on a new coal-fired power plant, the cost of electricity would increase by ca. 80% and the power output would decrease by 30%.5 As an alternate CO2 capture technology, polymer-based membrane separations offer relatively low operating costs with low-maintenance requirements. Key parameters for membrane technology are permeance (i.e., pressure-normalized flux typically reported in GPU, where 1 GPU = 10−6 cm3/(cm2 s cmHg)) and ideal selectivity (i.e., the ratio of the permeances of the gases when tested individually, in this case, CO2 and N2).6,7 Process modeling has shown that high CO2 permeance (e.g., >2000 GPU) plays the key role in reducing the cost of CO2 capture for membrane-based separations, with only moderate CO2/N2 selectivities of 20−40 required.8 Recently, Membrane Technology and Research, Inc. (MTR) reported the Polaris membrane systems, which have a CO2 permeance of 3000 GPU © 2014 American Chemical Society
2. RESULTS AND DISCUSSION This high-permeance TFC membrane was prepared by modification of a previously reported two-step fabrication procedure.11 In this method, a thin layer of poly(RTIL) (1) is Received: Revised: Accepted: Published: 20064
October 14, 2014 November 23, 2014 December 3, 2014 December 9, 2014 dx.doi.org/10.1021/ie5040682 | Ind. Eng. Chem. Res. 2014, 53, 20064−20067
Industrial & Engineering Chemistry Research
Research Note
first deposited onto a support and then swelled in a second coating step with an isopropanol solution containing free RTIL (2) to form a poly(RTIL)/RTIL composite film. For this work, poly(RTIL) 1 (poly(vinylhexylimidazolium bis(trifluoromethylsulfonyl)imide)) and RTIL 2 (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) (illustrated in Figure 1) were prepared using previously described methods.15
Figure 1. Structures of the poly(RTIL) 1 (left) and RTIL 2 (right) used in this work. Figure 3. Cross-sectional SEM image of the TFC membrane showing the poly(RTIL)/RTIL active layer on the porous support. The active layer thickness is estimated at 96 ± 4 nm.
This two-step coating process was employed because initial attempts at casting a mixture containing both 1 and 2 resulted in membranes with low CO2 permeance. The membrane was prepared using a proprietary pilot casting line at the 3M Company (Figure 2). A 3M proprietary gutter
Table 1. CO2 Permeance and Single-Gas CO2/N2 Ideal Selectivity Values of the Pristine Gutter Support, Four TFC Poly(RTIL)/RTIL Membranes Prepared on the Gutter Support (A−D), and Selected High-Permeance CO2/N2Selective Membranes Reported in the Literature membrane gutter supporta
CO2 permeance (GPU)
CO2/N2 selectivity
45 000 ± 1000
5
reference this work this work this work this work this work 9 16 17
6500 ± 100
21
b
B
5800 ± 80
22
Figure 2. General representation of the processing line for a coating of poly(RTIL) (1).
Cb
5700 ± 50
19
Db
6400 ± 100
25
layer material was prepared as a nonporous highly permeable film on the porous support. The CO2 permeance of this gutter support was measured by a dead-end cell to be (45 000 ± 1000) GPU while the CO2/N2 ideal selectivity was measured to be 5 (see Table 1 and the Supporting Information for details). A 1 wt % solution of 1 in methanol was coated onto the gutter layer, at a calculated thickness of 82 nm, using a pilotscale coating system that deposits a 10.2-cm-wide active area at a line speed of 5.1 cm/s. The resulting membrane was then dried at 77 °C in a 3.05-m-long oven. The dry membrane was then coated with a 1 wt % solution of 2 in isopropanol using a No. 5 Mayer rod and allowed to dry at room temperature for at least 24 h. This produced a membrane with a calculated loading level of 2 of 58 wt %. Because of the small amounts of materials and thin films involved in this coating procedure, the exact amount of 2 in the coating could not be determined by mass difference before and after applying the solution of 2. No further treatment or cleaning was performed before membrane testing. Scanning electron microscopy (SEM) imaging confirmed that the membranes had coating thicknesses of 92−100 nm (see Figure 3). The TFC membranes were tested for single-gas (i.e., ideal) CO2 and N2 permeance using a bubble flow meter (see the Supporting Information for full details). The results of these measurements are listed in Table 1 and show an average CO2 permeance of 6100 ± 400 GPU and an average ideal CO2/N2
Polaris NaA zeolite/carbon Amine-functionalized SAPO-34 TiO2/SiO2/ZrO2/[EMIM] [Tf2N] Al2O3/TiO2/[EMIM] [Tf2N] gelled RTIL
3000 1100 900
>30 6 40
140
31
18
90
35
19
50
20−30
20
Ab
a
The CO2 permeance and CO2/N2 ideal selectivity values of the pristine gutter support were measured using a dead-end cell. b Permeances for TFC membranes A−D were measured using an ideal gas bubble flow unit (see the Supporting Information for details). The permeance values shown are the average of three independent measurements with standard deviation error bars.
selectivity of 22 ± 2 for the TFC poly(RTIL)/RTIL membranes. Also shown in Table 1 are the reported CO2 permeance and CO2/N2 selectivity values of several highpermeance CO2/N2-selective membranes reported in the literature. The average observed CO2 permeance of our new TFC poly(RTIL)/RTIL membranes is approximately double that of the recently reported MTR Polaris membrane, although the selectivity is lower. Compared to the other selected membranes shown in Table 1, the membranes reported in this work have much higher permeance values (5700−6500 vs 50− 20065
dx.doi.org/10.1021/ie5040682 | Ind. Eng. Chem. Res. 2014, 53, 20064−20067
Industrial & Engineering Chemistry Research
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1000 GPU) and slightly lower CO2/N2 selectivity (19−25 vs 6−40).
ASSOCIATED CONTENT
S Supporting Information *
Experimental details on the poly(RTIL) and RTIL coating materials. Procedures for performing the gas permeance measurements using a bubble-flow meter and a dead-end pressure cell. A discussion regarding the calculated CO2 permeability of the poly(RTIL)/RTIL active layer of the TFC membranes. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors have approved the final manuscript version. M.G.C. and W.M.M. contributed equally to this work. Notes
The authors declare no competing financial interest.
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REFERENCES
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3. SUMMARY By coating a CO2-selective poly(RTIL)/RTIL composite layer at a thickness of ca. 100 nm, TFC membranes are obtained that exhibit CO2 permeances ∼2 orders of magnitude higher than those reported for typically fabricated thicker membranes.10−14 In addition, these new TFC membranes showed little CO2/N2 selectivity change, compared to the bulk poly(RTIL)/RTIL material, indicating that they are able to fully utilize the permselectivity of the applied coating material. Our best mode of preparation so far generates TFC membranes with a pure-gas CO2 permeance of 6100 ± 400 GPU and an ideal CO2/N2 selectivity of 22 ± 2. To the best of our knowledge, this is among the highest CO2 permeance values reported in the literature for a membrane with a pure-gas CO2/N2 selectivity of >20. This finding suggests that poly(RTIL)/RTIL TFC membranes are promising candidates for post-combustion CO2 separation applications and can potentially reduce the cost for capturing CO2 from a flue gas stream to
