Abstract
Supported ionic liquid membranes (SILMs), containing aprotic N-heterocyclic anion ionic liquids (AHA ILs) in an inorganic inert support, exhibit CO2/N2 permselectivity values as high as 640 at 35.0 °C and 0.03 bar CO2, which represents conditions similar to post-combustion carbon capture (PCCC) from a natural gas power plant. A Fickian model fit to the experimental data estimates CO2 permeability at direct air capture (DAC) conditions of 10,400 barrer and a CO2/N2 permselectivity of 4000 for the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P2228][4-BrPyra]). The most important criterion for high selectivity is a large equilibrium constant for binding between the IL and CO2, which results in high CO2 solubility. ILs with smaller molar volumes and with no fluoroalkyl chains enhance N2 rejection. Low viscosity and high IL molar density also enhance CO2/N2 permselectivity and CO2 permeance.
| Original language | English |
|---|---|
| Article number | 122758 |
| Number of pages | 13 |
| Journal | Journal of Membrane Science |
| Volume | 702 |
| DOIs | |
| Publication status | Published - May 2024 |
| Externally published | Yes |
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In: Journal of Membrane Science, Vol. 702, 122758, 05.2024.
Research output: Contribution to journal › Article › Research › peer-review
TY - JOUR
T1 - Amine functionalized supported ionic liquid membranes (SILMs) for CO2/N2 separation
AU - Chamoun-Farah, Antoine
AU - Keller, Austin N.
AU - Balogun, Mariam Y.
AU - Cañada, Louise M.
AU - Brennecke, Joan F.
AU - Freeman, Benny D.
N1 - Funding Information: Supported ionic liquid membranes (SILMs), containing aprotic N-heterocyclic anion ionic liquids (AHA ILs) in an inorganic inert support, exhibit CO2/N2 permselectivity values as high as 640 at 35.0 \u00B0C and 0.03 bar CO2, which represents conditions similar to post-combustion carbon capture (PCCC) from a natural gas power plant. A Fickian model fit to the experimental data estimates CO2 permeability at direct air capture (DAC) conditions of 10,400 barrer and a CO2/N2 permselectivity of 4000 for the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P2228][4-BrPyra]). The most important criterion for high selectivity is a large equilibrium constant for binding between the IL and CO2, which results in high CO2 solubility. ILs with smaller molar volumes and with no fluoroalkyl chains enhance N2 rejection. Low viscosity and high IL molar density also enhance CO2/N2 permselectivity and CO2 permeance.In this study, we aim to advance the use of a variety of AHA ILs in SILMs and determined the influence of anion type on the CO2/N2 permselectivity in five triethyl(octyl)phosphonium [P2228]+ based SILMs in an inert inorganic support. The [P2228]+ cation was chosen due to higher thermal stability of the IL (when compared to an ammonium cation) and high fluidity, relative to larger phosphonium cations [40]. Permeability measurements were conducted at 35.0 \u00B0C and pressures ranging from 0.03 bar to 1.5 bar. We use thermophysical property characterization, along with a proposed Fickian model, to correlate differences in permeability with IL properties. We also studied the influence of the cation on CO2/N2 permselectivity by preparing SILMs with ILs composed of a large trihexyl(tetradecyl)phosphonium [P66614]+ cation and a smaller [P2228]+ cation.Furthermore, the individual solubility and diffusivity contributions to permselectivity are commonly considered. By substituting Eq. (1) into Eq. (3), the diffusivity selectivity, DA/DB, and solubility selectivity, SA/SB, can be obtained [44\u201346]. As mentioned above, for supported liquid membranes the support is normally chosen to be porous. As a result, dissolution and diffusion through the membrane only occur for the porous part of the membrane that is filled with liquid. While we have estimated \u03C6/\u03C4 to be 0.354 for the ceramic supports used in this study (see Section 1 of the SI), this value is not known precisely, so examination of permselectivity, \u03B1AB, and diffusivity selectivity, DA/DB, is particularly helpful since the porosity and tortuosity cancel out for these quantities.The structures and abbreviations for the six phosphonium AHA ILs considered in this study are recorded in Table 1. The ILs were synthesized using a method consistent with our previous work [61]. Specific details of the synthesis are provided in Section 2 of the SI, along with the bromide content, which was determined by ion chromatography. An additional imidazolium-based ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [hmim][Tf2N], was purchased from IoLiTec (99 % purity, product #: IL-0098-HP) to serve as a standard to characterize the properties of the support, but solely for this purpose as its membrane transport properties are well understood, and it does not chemically bind CO2. All ILs were dried under vacuum at 40 \u00B0C and tested with a Metrohm 831 Karl Fischer Coulometer (\u00B13\u03BCg water resolution) until the water content was below 1000 ppm. To confirm IL structure and purity, 1H, 13C and 31P Nuclear Magnetic Resonance (NMR) spectra were obtained using a 400 MHz spectrometer. All ILs synthesized had greater than 98 % purity. 1H, 13C and 31P NMR spectra for all the neat ILs synthesized are attached in Sections 3-5 of the SI (Figs. S1\u2013S3). For the ILs containing fluorine ([P2228][Tf2N] and [P2228][3-CH3-5-CF3-Pyra]), 19F spectra are attached in Section 6 of the SI (Fig. S4).Zhang et al. made supported diamine-monocarboxylate-based protic IL-FTMs on a hydrophilic polyethersulfone (PES) support and studied the effect of humidity on their SILMs. The SILM tested with 15 wt% water showed significantly higher CO2 permeability when compared to SILMs tested under dry conditions. Their best performing SILMs, 3-dimethylamino-1-propylamine trifluoroacetate [DMAPAH][TFA] (membrane 14) and [DMAPAH][EOAc] (membrane 15) (methoxyacetate) are represented in Fig. 3 [73]. Similarly, Tu et al. made fluoride ion-based protic IL-FTMs on a PES support and studied the effect of humidity on CO2 permeation. Increasing the water content from 20 to 40 wt% facilitated the transport of CO2 and increased permeability. Their best performing SILMs, [DMAEAH][F] (membrane 16) and [DMAPAH][F] (membrane 17) are presented in Fig. 3. Thin film composite FTMs from Lee and Gurkan's literature review are compiled here and presented in Fig. 3 (membranes 18\u201324) [23,74]. Zhou et al. 2017 (membrane 22) used a sub 20 nm thick graphene oxide-based hollow fiber membrane with piperazine as the CO2 carrier and had the highest CO2 permeability reported (51,000 barrer) of any of the membranes shown in Fig. 3 [70]. However, their membrane requires high humidity (100 % RH) and temperature (50 \u00B0C) to operate. Lastly, Kohno et al. [28] reported the highest permeability and permselectivity combination of any materials in this class using SILMs of diamine-based IL mixtures on a PTFE support. Their best performing SILM mixture of [AEEA][AcO] and [Emim][AcO] (molar ratio 15:85) is shown (membrane 11) alongside their measurement of [P4444][Pro] (membrane 12), a benchmark SILM that they ran for reference [28]. Sei et al. also ran [P4444][Pro] on a PTFE support (membrane 13) and reported a permeability of 15,500 barrer and permselectivity of 3600 with a dry mixed-gas feed of 400 ppm CO2 balanced with N2 at 1 bar [29]. However, the permeability and permselectivity reported by Kohno et al. [28] (membrane 12) for [P4444][Pro] is higher than that measured by Sei et al. [29] (membrane 13) at the same operating conditions.This material is based upon work primarily supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0023343. In addition, ionic liquid synthesis and transport modeling were supported by the Robert A. Welch Foundation (Grant F-1945). Publisher Copyright: © 2024 Elsevier B.V.
PY - 2024/5
Y1 - 2024/5
N2 - Supported ionic liquid membranes (SILMs), containing aprotic N-heterocyclic anion ionic liquids (AHA ILs) in an inorganic inert support, exhibit CO2/N2 permselectivity values as high as 640 at 35.0 °C and 0.03 bar CO2, which represents conditions similar to post-combustion carbon capture (PCCC) from a natural gas power plant. A Fickian model fit to the experimental data estimates CO2 permeability at direct air capture (DAC) conditions of 10,400 barrer and a CO2/N2 permselectivity of 4000 for the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P2228][4-BrPyra]). The most important criterion for high selectivity is a large equilibrium constant for binding between the IL and CO2, which results in high CO2 solubility. ILs with smaller molar volumes and with no fluoroalkyl chains enhance N2 rejection. Low viscosity and high IL molar density also enhance CO2/N2 permselectivity and CO2 permeance.
AB - Supported ionic liquid membranes (SILMs), containing aprotic N-heterocyclic anion ionic liquids (AHA ILs) in an inorganic inert support, exhibit CO2/N2 permselectivity values as high as 640 at 35.0 °C and 0.03 bar CO2, which represents conditions similar to post-combustion carbon capture (PCCC) from a natural gas power plant. A Fickian model fit to the experimental data estimates CO2 permeability at direct air capture (DAC) conditions of 10,400 barrer and a CO2/N2 permselectivity of 4000 for the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P2228][4-BrPyra]). The most important criterion for high selectivity is a large equilibrium constant for binding between the IL and CO2, which results in high CO2 solubility. ILs with smaller molar volumes and with no fluoroalkyl chains enhance N2 rejection. Low viscosity and high IL molar density also enhance CO2/N2 permselectivity and CO2 permeance.
UR - https://www.scopus.com/pages/publications/85191574036
U2 - 10.1016/j.memsci.2024.122758
DO - 10.1016/j.memsci.2024.122758
M3 - Article
AN - SCOPUS:85191574036
SN - 0376-7388
VL - 702
JO - Journal of Membrane Science
JF - Journal of Membrane Science
M1 - 122758
ER -