Facilitated Separation of CO2 by Liquid Membranes and Composite

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Facilitated separation of CO2 by liquid membrane and composite membrane with task-specific ionic liquids Wei He, Fan Zhang, Zhi Wang, Wei Sun, Zhiyong Zhou, and Zhongqi Ren Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Industrial & Engineering Chemistry Research

Facilitated separation of CO2 by liquid membrane and composite membrane with task-specific ionic liquids Wei He, Fan Zhang, Zhi Wang, Wei Sun, Zhiyong Zhou*, Zhongqi Ren* College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

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ACS Paragon Plus Environment


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ABSTRACT: Carbon capture and sequestration (CCS) has become an increasingly important technology for environmental protection and resource utilization. In this work, four task-specific ionic liquids (TSILs), NH2-functionalized IL and three ether-functionalized ILs, were synthesized for CO2 absorption. CO2 absorption experiments of these task-specific ionic liquids (TSILs) showed that the absorption performance of NH2-functionalized IL was higher than that of ether-functionalized ILs. The recycling experiments of CO2 absorption demonstrated a good reusability of synthesized TSILs for CO2 absorption. Supported ionic liquid membranes (SILMs) with synthesized TSILs were prepared and the CO2/CH4 and CO2/N2 separation performance of SILMs were examined. Results indicated that the SILMs with ether-functionalized ILs have slightly lower permeability but much higher selectivity (up to 9.7) than the SILMs with NH2-functionalized IL. Poly (RTIL)-RTIL composite membranes were prepared using ether-functionalized ILs. The effect of crosslinking monomer content and non-polymerizable IL (“free” IL) content in poly (RTIL)-RTIL membranes on the CO2/CH4 and CO2/N2 separation performance were investigated. The composite membranes have higher gas permeability with little or no sacrifice in selectivity compared with SILMs. The stability evaluation and comparison of the SILMs and poly (RTIL)-RTIL membranes were conducted, the results of which verified the stronger structural stability of the poly (RTIL)-RTIL membranes. This suggests the potential of poly (RTIL)-RTIL membranes in future CO2-selective membrane separation.

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1. INTRODUCTION Increasing carbon dioxide (CO2) emissions from the burning of fossil fuels have contributed more than 60% of the greenhouse effect,1,2 which has brought serious environmental problems. On the other hand, CO2 is also an abundant and non-toxic carton resource.3 Thus, carbon capture and sequestration (CCS) has been considered as one hot topic of research in 21st century. The traditional method of CO2 separation is solvent absorption,4,5 but it is of environmental concern because of the volatile and corrosive nature of traditional solvents. 4,6,7 As a kind of novel green solvents, room temperature ionic liquids (RTILs) have gained increasing attention in recent decades, due to their unique physicochemical properties, such as good thermal stability, extremely low volatility, non-ﬂammability, and diversity of structure.8,9 Especially, the properties of ionic liquids can be tuned by introducing functional groups with specific structures, namely, task-specific ionic liquids(TSILs). The goal of better absorption and separation of CO2 can be achieved through physical or chemical interaction between functional groups and CO2.10 The best-known way to functionalize ILs for facilitated CO2 transport is incorporating amine groups into ionic liquids, by which CO2 can be captured chemically.11,12 Bates et al.11 synthesized a primary amine-functionalized ionic liquid (NH2-RTIL), CO2 solubility of which is much greater than that of common ILs. But NH2-functionalized ILs usually have relatively higher viscosity and additional energy input could be required to decomplex chemically bound CO2.12 Another method for improving CO2 solubility and selectivity is by the 3



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introduction of polar groups, such as ether and nitrile groups, which can strengthen the physical forces between ionic liquids and CO2.5,12 Zeng et al.13 designed and synthesized a series of ether-functionalized ILs [EnPy][Tf2N] and the corresponding non-functionalized analogues [CmPy][Tf2N] for comparing the separation of CO2 from CH4. The results showed that the existence of ether groups on the cation has weak influence on CO2 solubility of the ILs, whereas it contributes to much lower CH4 solubility, which results in higher selectivity of CO2/CH4. In spite of the attractive properties of ILs, their viscosities and relatively high costs limit their industrial application to some extent. In a growing number of research, RTILs are combined with membranes for CO2 separation.14,15 At present, membrane separation technology occupies a small, yet increasing share in the market of CO2 separation.16,17 It can be economically and environmentally attractive because of its lower energy consumption and capital cost, simpler operation and better safety, easier combination with other separation processes.5 Among various membrane processes for CO2 separation, supported ionic liquid membranes (SILMs) have received considerable attention not only for their remarkably high permeability compared to polymeric membranes,18 but also for their solvent evaporation free, lower toxic effects, and increased stability in process compared to conventional supported liquid membranes (SLMs).19 Many studies have been conducted on CO2 separation by SILMs. Some researchers20-22 studied the influences of environmental factors, operating conditions and mixed gas operation on the separation performances of these SILMs, which exhibited a long-term stability of 4


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SILMs in low pressure operations. However, SILMs suffer from limited tolerance for pressure difference across the membrane in gas separation processes.22 Poly (RTIL)-RTIL composites membranes can be a valid alternative, in which the non-polymerizable IL (“free” IL) is incorporated into the poly (RTIL) matrix, as shown in Figure 1. Though they may be regarded as a type of SILMs configuration, the stability of poly(RTIL)-RTIL membranes is much better due to the Coulombic attractions between “free” ILs and poly(RTILs).23 Camper et al.24 incorporated 20 mol% of non-polymerizable RTILs with various anions into poly(RTILs) matrix to enhance gas separation performance. It was found that the CO2 permeability of these poly (RTIL)-RTIL composites could increase up to 400% with little or no decrease in selectivity for CO2/CH4 and CO2/N2 relative to that in the neat poly (RTIL) membrane. In this work, NH2-functionalized ILs and three ether-functionalized ILs containing polymerizable vinyl groups were synthesized. CO2 absorption performance using these TSILs was evaluated. SILMs with synthesized task-specific ionic liquids were prepared. The effect of transmembrane pressure and the types of ILs on CO2/CH4 and CO2/N2 separation performance of SILMs was studied. Poly (RTIL)-RTIL membranes with the synthesized ether-functionalized ILs were prepared. The effect of crosslinking monomer and “free” IL content on the permeability and selectivity of CO2/CH4 and CO2/N2 were investigated. The study of stability of the SILMs and poly(RTIL)–RTIL membranes were investigated too.

2. EXPERIMENTAL SECTION 5



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2.1. Materials. N-methyl imidazole (purity > 99%), Tetrahydrofuran (THF, purity > 99.5%), sodium hydroxide (NaOH, purity > 99%), and sodium iodide (NaI, purity > 99%) were supplied by Beijing HWRK Chem. Co. LTD. Tetraethylene glycol, 2-methoxyethanol, 2-(2-methoxyethoxy)-ethanol and paratoluensulfonyl chloride were supplied by Tianjin Fu Chen Chemical Reagents Factory with purities higher than 99%. Bis (trifluoromethane sulfonimide) lithium salt （ LiTf2N ） and 1-butyl-3-methylimdazoliuim bis-trifluoromethyl-sulfony limide ([Bmim][Tf2N]) were purchased from ShangHai Cheng Jie Chemical Co. LTD with purity of 99%. Ethyl acetate (purity > 99.8%) and anhydrous ether (purity > 99%) were purchased from Beijing Chemical Works. N-vinylimidazole (purity > 99%) was brought from Sun Chemical Technology (Shanghai) Co., Ltd. with purity higher than 99%. Azodiisobutyronitrile (AIBN, purity > 99% ） and 3-bromine propylamine hydrobromide (purity > 99%）were purchased from Aladdin Industrial Corporation, Shanghai, China. Gases, including carbon dioxide, nitrogen and methane, were purchased from Beijing Shunanqite Gas Co. ltd. with purities higher than 99.95%. 2.2. Synthesis and Characterization of TSILs The designations and properties of the four synthesized TSILs and non-functionalized IL ([Bmim][Tf2N]) are listed in Table 1. The viscosity of the NH2-functionalized IL was measured by an Anton Paar MCR 702 rheometer (2EC-SMT mode) with 25 mm parallel plates and a gap size of 1.0 mm, covering a frequency range of 0.01 rad/s to 10 rad/s, and the viscosities of the other ILs were measured by falling ball viscometer (Anton Paar Lovis 2000 M/ME, Austria). The 6


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synthesized ILs were characterized by nuclear magnetic resonance (1H NMR) spectroscopy (Bruker AV400/600MHz, Germany) and Fourier transform infrared (FTIR) spectrometer (Thermo Electron, NEXUS8700, USA). 2.2.1. Synthesis of NH2-Functionalized Ionic Liquids. [apmim]Br The NH2-functionalized IL was synthesized according to the literature,24 as shown in Scheme 1. 1HNMR (600MHz, DMSO-d6): δ=9.490 (s,1H,2H); 7.951 (s,1H,4H,5H); 7.862 (s,1H,5H,4H); 4.259 (s,2H,6H); 3.878 (m,3H,9H); 2.358 (s,2H,8H); 1.793 (s,2H,9H). Corresponding FTIR spectrum was showed in Supporting Information (Figure S1). 2.2.2. Synthesis of Ether-Functionalized Ionic Liquids. Three polymerizable TSILs with ether groups were synthesized on the basis of relevant literatures.11,25,26 The synthesis of ether-functionalized ionic liquids of 1-3 is depicted in Scheme 2. Corresponding FTIR spectra were showed in Supporting Information (Figure S2). 1-vinyl-3-(2-methoxyethyl) imidazolium-bis-trifluoromethyl-sulfony limide (Eh-1): 2-methoxyethanol and p-toluenesulfonyl chloride (TsCl) (in the mole ratio of 1:1) were dissolved in THF separately. The mixture was reacted in the NaOH solution at 0 °C for 3 h，and reacted at room temperature for 24 h. NaI was added into the acetone solution of the obtained intermediate, and the mixture reacted in dark place for 24 h, and filtered to remove the precipitate of sodium p-toluenesulfonate. Then the target product was synthesized through the two-step method, reacting with N-vinylimidazole and then having anion exchange with LiTf2N (Scheme 2a).

1

H

NMR (400 MHz, DMSO-d6): δ=3.257 (s, 3H), 3.712 (t, 2H), 4.378 (t, 2H), 5.393 (dd, 7



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1H), 5.959 (dd, 1H), 7.315 (dd, 1H), 7.822 (t, 1H), 8.136 (t, 1H), 9.438 (t,1H). 1-vinylimidazole-3-(2-(2-methoxy-ethoxy))-ethyl-imidazolium-bis-trifluoro methyl-sulfonylimide (Eh-2): 2-(2-Methoxyethoxy)-ethanol and p-toluenesulfonyl chloride (TsCl) (in the mole ratio of 1:1) were dissolved in THF separately. Other procedures were same as those for 1 (Scheme 2a).

1

H NMR (400 MHz, DMSO-d6):

δ=3.204 (s, 3H), 3.423 (m, 2H), 3.552 (m,2H), 3.805 (t, 2H), 4.388 (t, 2H), 5.431 (dd, 1H), 5.940 (dd,1H), 7.311 (dd, 1H), 7.861 (t, 1H), 8.179 (t, 1H), 9.436 (t, 1H). Bis(1-vinyl-3-ethoxy-imidazolium bis-trifluoromethyl-sulfonylimide) ether (Eh-3): Tetraethylene glycol and p-toluenesulfonyl chloride (TsCl) (in the mole ratio of 1:2) were dissolved in THF separately. Other procedures were same as those for 1 (Scheme 2b). 1HNMR (400 MHz, DMSO-d6): δ=3.495 (m, 8H), 3.810 (t, 4H), 4.386 (t, 4H), 5.412 (dd, 2H), 5.950 (dd,2H), 7.295 (dd, 2H), 7.822 (t, 2H), 8.123 (t, 2H), 9.410 (t, 2H). 2.3. Gas Absorption Experiments. The CO2 absorption experiments of four synthesized TSILs were carried out. Especially, the CO2 absorption experiment of [apmim][Br] was conducted in 45 wt% aqueous solutions, considering its relatively higher viscosity.11 A schematic diagram of the experimental setup was provided in Figure 2. By recording the time and the distance of the soap film moving within soap-film flowmeter, the absorption rate and absorption capacity of CO2 with ILs were measured at room temperature and atmospheric pressure. The location of the soap film could remain unchanged when the IL or IL solution reached saturation with CO2. The absorption performance was determined repeatedly after CO2 in the 8


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saturated ILs was desorbed under vacuum and at elevated temperature, to verify the recyclability of these TSILs. The absorption rate (q, mol CO2/s·mol ILs) and absorption capacity (L, mol CO2/mol ILs) of CO2 with TSILs were calculated by the following equations: q=

L=

π 4

d2 ⋅

∆L p M ILs ∆t RT mILs

（1）

m1 − m0 M ILs mILs M CO2

（2）

where d is the insidediameter of the soap-film flowmeter (cm), △t is the time interval (s), △L is the distance of the soap-film moving in △t (cm), T is the Kevin temperature (K), R is the molar gas constant. And mILs is the mass of the IL (g), m0 and m1 is the mass (g) of the system before absorption and after absorption, respectively. MILs and MCO2 is molar mass of the IL and CO2, respectively.

2.4. Membrane Preparation and Characterization 2.4.1. SILMs Preparation. The hydrophobic polyvinylidene fluoride (PVDF) membrane was selected as the support for ILs according to the result of contact angle analysis, as shown in Figure 3. Compared with nylon (JN) membrane and polytetrafluoroethylene (PTFE) membrane, PVDF materials had a better affinity with synthesized TSILs. The SILMs was prepared by casting moderate ILs onto the PVDF porous membrane with 0.22 µm pores. The coated membrane was put in a vacuum oven at 60 °C, 0.08 Mpa negative pressure for 12 h. Elevated temperature would lead to the decrease of the viscosity of ILs, which makes it easier for ILs to enter the porous 9



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PVDF membrane. Then the membrane was put in a vacuum oven at room temperature for 12 h. In order to ensure that the membranes were saturated with ILs, the procedure of casting ILs onto the porous supports needed to be repeated for several times. Excessive ILs on the surface of membrane was carefully wiped up with filter papers. The amount of IL loading in the membrane was determined gravimetrically. 2.4.2. Poly (RTIL)-RTIL Membranes Preparation and Characterization. To synthesize

the

poly

(RTIL)-RTIL membranes,

two

kinds

of

synthesized

ether-functionalized ILs, Eh-1 and Eh-3 acted as a monomer and a crosslinking monomer in the process of polymerization, respectively. [Bmim][Tf2N] was chosen as the non-polymerizable IL (“free” IL) for the composite membranes. A typical procedure for the preparation of poly (RTIL)-RTIL membranes was as follows. The monomer,

crosslinking

monomer,

[Bmim][Tf2N],

and

thermal

initiator

(azodiisobutyronitrile, AIBN) were mixed according to a certain proportion. And ethanol was added as a solvent. The mixture was homogenized by ultrasonic mixing and then let it sit for 0.5 h. The solution was cast onto a PVDF porous membrane, then put in a vacuum oven and heated at 60 °C for 24 h. The prepared poly(RTIL)-RTIL membranes were characterized by Wide-angle X-ray diffraction (XRD) spectroscopy (D8 ADVANCE, Germany), Fourier transform infrared (FTIR) spectrometer (Thermo Electron, Nicolet 8700, USA), and Thermogravimetric analysis (TGA) scans (TGA/DSC 1/1100 SF, METTLRR TOLEDO) using a heating rate of 5 °C/min under nitrogen atmosphere.

2.5. Gas Permeation Measurements. Single gas permeabilities through 10


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prepared SILMs and poly(RTIL)-RTIL membranes were measured through a constant-pressure volume increase method. A diagram of the experimental apparatus was shown in Figure 4. The membranes were placed in a stainless steel cell with an effective membrane area of about 4.91cm2. The permeate flux of single gas (Ji, cm3/(STP)/(cm2·s·cm Hg)) was determined using the following equations:

Ji =

QV A

QV =

π 4

(3) d2 ⋅

∆L 273.15 ∆t T

(4)

where QV is the volume flow rate of single gas at the permeate side under steady state (cm3/(STP)/s). A is the effective membrane area (cm2). The ideal permeability (Pi, GPU, 1GPU=10-6 cm3/(STP) /(cm2·s·cm Hg)) of single gas was then calculated according to the following equation: Pi =

Ji Q = i ∆p A∆p

(5)

where △p is the transmembrane pressure, cm Hg. The ideal permeability selectivity (or permselectivity), αi/j, was obtained by dividing the permeability of the more permeable component i to the permeability of the less permeable component j, shown in the equation below.

α i/j =

Pi Pj

(6)

2.6. Stability of SILMs and Poly (RTIL)-RTIL Membranes The study of stability of the membranes was carried out in the gas permeation separation experimental setup, as shown in Figure 4. The applied pressure was adjusted from 0.10 MPa to 0.25 MPa by nitrogen gas (N2). In order to evaluate the 11



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stability of the membranes during separation process, the membranes were weighed using an analytical balance (METTLRR TOLEDO XP205, Switzerland) at regular periods of time to record the change in the mass of membranes with the increase of N2 pressure.

3. RESULTS AND DISCUSSION 3.1. CO2 Absorption Performance of Task-Specific Ionic Liquids 3.1.1. Absorption Rate of CO2 with TSILs. The absorption rates of CO2 with four synthesized TSILs were shown in Figure 5. It could be observed that the absorption rate of CO2 with [apmim][Br] was much greater than those with ether-functionalized ILs, which could be benefited from the chemical reactions between CO2 and amine groups of [apmim][Br]27,28 (Figure 6), whereas ether-functionalized ILs absorbs CO2 only through a physical mechanism. Compared with the physical absorption which is only dominated by weak Van der Waals forces and hydrogen bondings, a faster reaction rate and greater absorption capacity of the gas can be obtained by chemical absorption.29,30 Besides, the viscosity of [apmim][Br] (25%) diluted in aqueous solution is lower than that of neat ether-functionalized ILs (Table 1). On the basis of double film theory, the viscosity of the liquid is one of the important factors influencing the mass transfer rate owing to the significant effect of viscosity of the liquid on the liquid film coefficient for mass transfer.31,32,33 The relatively lower viscosity of [apmim][Br] aqueous solution is beneficial to obtaining a greater mass transfer rate of CO2. Among three ether-functionalized ILs, the absorption rate of CO2 with Eh-3 was 12


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greater than those with the other two, while there was no significant difference between the other two ILs. Some studies had showed that the effect of anions of ionic liquids on CO2 solubility was found to be more pronounced than the effect of cationic strutures.34,35 Three ILs have similar cationic structures and the same anion, [Tf2N]-. However, the quantity of [Tf2N]- in per mole of Eh-3 is as twice as that in per mole of Eh-1 and Eh-2. Greater quantity of anions can supply more action sites for CO2, leading to much stronger affinity for CO2.36 3.1.2. Absorption Capacity of CO2 with TSILs. The absorption capacities of CO2 with these synthesized TSILs were shown in Figure 7. As it was observed, the value of [apmim][Br] was much higher than those of the other three, and there existed no much difference among three ether-functionalized ILs. Among these three ether-functionalized ILs, although Eh-3 showed a greater absorption rate of CO2 than the other two ILs, the absorption capacity of CO2 with Eh-3 was very close to that with the other two ILs. This could be explained by the fact that the viscosity of Eh-3 was much higher than that of the other two ILs due to the higher symmetry and complexity of molecular structure of Eh-3, which is not beneficial to absorption process. As shown in Figure 5, there was no obvious difference between Eh-1 and Eh-2 for the absorption rate of CO2 due to their similar structures. However, the absorption capacity of CO2 with Eh-2 was slightly higher than that with Eh-1, as shown in Figure 7. It is probably because that there is one more 2-methoxy-ethoxy group existed in Eh-2, which provides more action sites for CO2, resulting in the increase of CO2 13



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solubility. The comparison of the results obtained in this work with other studies in CO2 absorption was listed in Table 2. As seen, for physical absorption, all the absorption capacities of CO2 with the ILs reported in other studies were lower than 0.06 mol CO2/mol ILs. However, in this work, the highest values of three ether-functionalized ILs, Eh-1, Eh-2, Eh-3, were 0.0617, 0.0695, 0.0882 mol CO2/mol ILs, respectively. For chemical absorption, the highest absorption capacity of CO2 with [apmim]Br used in this work was 0.4853 mol CO2/mol ILs, which was larger than most of the results reported in other studies. These results demonstrated the advantages of the synthesized TSILs in this work on CO2 absorption, whether for physical absorption or chemical absorption. 3.1.3 Recyclability of TSILs in CO2 Absorption Process. The industrial production of ionic liquid suffers from its relatively high cost. The recycle of ionic liquids can significantly save cost if the stability of the absorption performance of ILs can be guaranteed. In our work, the recyclability of TSILs in CO2 absorption process were investigated with the method of absorption–desorption. The CO2 absorption with synthesized TSILs was conducted at 298.15 K and 0.1 MPa, and the CO2 desorption was carried out at 343.15 K under a vacuum of about 0.08 MPa for 2 hours. The IL was weighed with the mass of IL before and after absorption to judge whether the CO2 absorbed with ILs has been completely desorbed. The results of the four TSILs are shown in Figure 8, Figure 9, Figure 10 and Figure 11, respectively. It was observed that the synthesized TSILs could be repeatedly used for 20 times, and the 14


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absorption rate and capacity of CO2 with TSILs kept almost no change after twenty absorption−desorption cycles, indicating that this process was highly recyclable.

3.2. Gas Separation Performance of Supported Ionic Liquid Membranes (SILMs) 3.2.1. Effect of Types of Ionic Liquids. The influences of different ILs on the separation performance of SILMs were examined, including previous synthesized TSILs and the non-functionalized IL, [bmim][Tf2N]. As shown in Figure 12, the CO2 permeation rate of Eh-1 was close to that of SILMs with Eh-2, which was consistent with the results of CO2 absorption (Figure 5 and Figure 6). However, the permeation selectivity of SILMs with Eh-2 was significantly lower than that with Eh-1. According to the above discussions on CO2 absorption, the small difference of the structures of Eh-1 and Eh-2 had little influence on CO2 solubility. Thus, no obvious change of the CO2 permeation rate of SILMs with these two ether-functionalized ILs could be observed. However, compared with Eh-1, there were longer side chains on the imidazole rings of Eh-2 due to the existence of one more 2-methoxy-ethoxy group, leading to a larger free volume. It is beneficial to the diffusion of small permeable components, leading to the larger permeation rate of CH4 or N2. It could explain the corresponding lower permeation selectivity of CO2/CH4 and CO2/N2 of SILMs with Eh-2 than Eh-1. As discussed above, Eh-3 showed higher CO2 absorption performance than Eh-1 and Eh-2. However, as shown in Figure 12, the CO2 permeation rate of SILMs with Eh-3 was clearly lower than those of SILMs with the other two ether-functionalized 15



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ILs, whereas the selectivity was relatively higher. Gas transport through membranes is different from gas absorption process, which is usually assumed to follow a solution-diffusion mechanism.48,49 Among three ether-functionalized ILs, although the solubility of CO2 in Eh-3 was slight larger than that in the other two ILs due to the influence of more quantity of anions, the diffusion of CO2 through the membrane was greately hindered by the relatively higher viscosity of Eh-3 (Table 1). The limit in the diffusivity of CO2 is enough to offset the contribution of solubility, therefore resulting in lower gas permeability than the other two ether-functionalized ILs. Likewise, the higher viscosity of Eh-3 could also impact the diffusion of small permeable components including CH4 or N2, improving the permeation selectivity of CO2/CH4 and CO2/N2. Interestingly, the permeation rate of CO2 of SILMs with [apmim][Br] was lower than that of CH4 and N2. As discussed in CO2 absorption section, the CO2 solubility in NH2-functionalized IL was much larger than that in other ILs due to the chemical interaction between amine groups and CO2. However, a larger gas diffusion resistance could be formed by the much higher viscosity of [apmim][Br], which would even further increase after the complexation of [apmim][Br] and CO2. Most importantly, the diffusion of CO2 through the SILMs was greatly restricted by the difficulty to remove the chemically bound CO2. Therefore, compared with CH4 or N2, CO2 is easier to be dissolved in the [apmim][Br] filled in the membrane pores and yet much harder to be transported to the permeate side. From another point of view, the SILMs with NH2-functionalized ILs might be suitable for the separation of CH4 or N2 from 16


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CO2. In addition, compared with the non-functionalized IL ([bmim][Tf2N]), the SILMs with three ether-functionalized ILs had slightly lower permeation rates of CO2 but much higher selectivity, due to the much lower permeation rates of CH4 and N2. It was mainly attributed to the existence of ether groups on the cation of ether-functionalized ILs, resulting in much lower CH4 or N2 solubility, which was consistent with the results reported by Zeng et al.13 According to the correlation between free volume of ILs and CO2 selectivity revealed by Bara and co-workers,50,51 the flexible ether group resulted in remarkable decrease of intrinsic free volume of ILs, leading to the increase of CO2 selectivity against N2 and CH4, which indicates that the ether functionalization is one of promising approaches to increase the membrane selectivity for CO2 and the synthesized ether-functionalized ILs in this work show a good potential in the membrane separation of CO2 from N2 or CH4. Combining the process of absorption with membrane separation, it could be seen that both NH2-functionalized and ether-functionalized ILs had their own pros and cons in different ways for CO2 capture. In the field of CO2 absorption, NH2-functionalized ILs could highlight their advantages compared with physical absorption. However, the ether functionalization could be a better choice for ILs applied in the membrane separation of CO2. 3.2.2. Effect of Transmembrane Pressure. The influences of different transmembrane pressures on the gas separation performance of SILMs were measured through a ﬁxed-volume pressure increase method, by which the transmembrane 17



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pressure was regulated by changing the pressure at the feed side, while the pressure at the permeate side was kept at atmospheric pressure. According to the above comparison in the types of ILs, Eh-1 was chosen for SILMs because of its proper viscosity and better performance of CO2 separation. The result shown in Figure 13 indicated that the permeation rates of CO2, N2, and CH4 all kept increasing with the increase of transmembrane pressure, while the selectivity of CO2/N2 and CO2/CH4 gradually decreases. Pressure plays a significant role in CO2 dissolution in the ether-functionalized ILs filled in the SILMs. CO2 solubility in the IL increased with increasing pressure. On one hand, the solubility of CO2 in the SILMs was much larger than that of CH4 and N2. With the increasing transmembrane pressure, CO2 permeation could be limited by concentration polarization for higher concentrations of CO2 on the membrane surface.52 Thus the increase of the permeation rate of CO2 was slower than that of CH4 and N2, which could lead to the decrease of the selectivity of CO2/N2 and CO2/CH4. On the other hand, it is inevitable that the membrane liquid loss of the SILMs increases with increasing the transmembrane pressure from 0.1 MPa to 0.18 MPa. The slight loss of the ILs from the membrane pores is beneficial to decreasing the gas diffusion resistance, especially for small permeable components (CH4 and N2), leading to the decrease of the selectivity of CO2/N2 and CO2/CH4. As shown in Figure 13, the permeation rates of all three testing gases showed a stable linear relationship with the transmembrane pressure, indicating that the slight membrane liquid loss could not lead to the decomposition of the SILMs at the testing pressure. 18


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Compared with other SILMs for CO2 separation in previous studies (shown in Table 3), the SILMs synthesized in this work showed the CO2 permeability of 1541.73 barrer combined with the selectivity of 9.73 for CO2/CH4, both of which were much higher than those obtained in other previous studies. Although the selectivity for CO2/N2 was 6.73, which was not very prominent, the excellent CO2 permeability could be a great advantage of the SILMs with synthesized ether-functionalized ILs towards the industrial application for CO2 separation from other gases.

3.3. Gas Separation Performance of Poly(RTIL)–RTIL Membranes 3.3.1. Effect of Crosslinking Monomer Content. The network structure and crosslinking density of the membrane were directly affected by the crosslinking degree of polymer, which could influence the gas permeation performance of polymer membranes. As shown in Figure 14, the permeation rate of CO2 slightly increased and then decreased gradually while that of CH4 and N2 kept decreasing, with the increase of crosslinking monomer content. When the content of crosslinking monomer was lower, the increasing crosslinking degree contributed to the denser action sites of CO2 on the composite membranes, causing a slightly larger solubility with CO2. The further increase of crosslinking monomer content could bring the increase in polymer glass transition temperature (Tg) and the decrease in fractional free volume, as polymer chains become more restricted with increased crosslinking between chains.70 Then, the increased diffusion resistance would be resulted, and therefore lead to the decreased permeation rate of all gases. 19



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With further increasing crosslinking degree, the permeation rate of N2 and CH4 remained at relatively lower values but meanwhile the permeation rate of CO2 decreased rapidly, which could lead to the decrease of selectivity. 3.3.2. Effect of “Free” Ionic Liquid Content. As mentioned before, the poly (RTIL) matrix was made more permeable by incorporating the “free” RTIL component, similar to liquid phase filled in SILMs. The influence of “free” ionic liquid content on gas separation performance of the composite membranes was investigated, as shown in Figure 15. The results indicated that the permeation rates of CO2, CH4 and N2 all increased first and then decreased with the increase of [bmim][Tf2N] content, while the selectivities of CO2/CH4 and CO2/N2 kept increasing. When ionic liquid content was lower, the “free” RTIL essentially served as non-volatile plasticizer for reducing the crystallinity of the polymer matrix and opening up the network structure.71 The increased free volume contributed to the gas diffusion, resulting in the growing permeation rates of all gases. However, no further obvious change on the network structure could be made if additional ionic liquid content was increased. Instead, larger proportion of “liquid phase” filled in the polymer matrix would cause the increase of diffusion resistance for all gases. As the membranes had stronger affinity with CO2 than that with CH4 and N2, the drop of the permeation rate of CO2 was slower than that of CH4 and N2 with the increasing content of “free” IL, which could explain the continuous increase of the CO2/CH4 and CO2/N2 selectivity. 20


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It could be noticed that when compared with the results of SILMs, poly(RTIL)-RTIL membranes had the higher CO2 permeability with little or no sacrifice in selectivity for CO2/N2 and CO2/CH4, which displayed more significant advantage for CO2 separation than SILMs. The results of CO2 separation with many similar types of membranes in other studies were listed in Table 3. In this work, when the“free” IL ratios were 28% and 35%, the CO2 permeability of the poly(RTIL)–RTIL membranes reached up to 3058.37 and 2528.91 barrer, respectively, both of which were even two orders of magnitude higher than that reported in previous studies. Meanwhile, the selectivities for CO2/N2 and CO2/CH4 reached up to 8.32 and 6.18 (when the“free” IL ratio was 35%), both of which were at a medium level when compared with her results in other studies. Therefore, this advantage of the poly(RTIL)-RTIL membranes on CO2 permeability would be beneficial to the industrial application of CO2 membrane separation.

3.4. Stability of SILMs and Poly (RTIL)-RTIL Membranes 3.4.1. Stability of SILMs. As discussed in the effect of transmembrane pressure on the gas separation performance of SILMs, the loss of membrane liquid happened during the separation process with SILMs. Therefore, the study of stability of the membranes was carried out to determine the membrane liquid loss degree. As shown in Figure 16, little change of the weight of the SILMs could be found at 0.1 MPa pressure. When the applied pressure increased to 0.15 MPa, a slight decrease of the membrane weight 21



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was observed. With increasing the applied pressure to 0.2 MPa, the decrease of the membrane weight kept a stable trend and the loss percentage was less than 5%. However, the membrane weight decreased sharply with increasing the applied pressure from 0.20 MPa to 0.25 MPa, Indicating that 0.25 MPa might have exceeded the breakthrough pressure associated with the surface tension of the RTIL within the membrane pores. In

general,

the

membrane

weight

of

SILMs

with

the

synthesized

ether-functionalized IL basically kept stable at the testing pressure below 0.2 MPa, showing a good stability of the SILMs prepared in this work. It could be attributed to the relatively higher viscosity of the synthesized ether-functionalized IL and stronger affinity between the IL and support membrane. 3.4.2. Stability of Poly(RTIL)–RTIL Membranes. It is generally considered that poly (RTIL)-RTIL membranes have stronger structural stability due to the coulombic attractions between “free” ILs and poly(RTILs), compared with the SILMs depending on the capillary force between RTILs and support membrane.23,72 In order to verify the concept, the study of stability of the poly (RTIL)-RTIL membranes was carried out under the same testing conditions with the SILMs. As expected, as shown in Figure 17, the weight decrease percentage of the poly (RTIL)-RTIL membranes was less than 3%, which was much smaller than that of the SILMs. Besides, no obvious decrease of the weight of poly (RTIL)-RTIL membranes could be observed even under 0.25 MPa applied pressure, indicating that the poly(RTIL)–RTIL membranes had stronger structural stability in 22


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comparison with the SILMs.”

3.5. Characterization of Poly (RTIL)–RTIL Membranes 3.5.1. FT-IR Studies. As shown in Figure 18, in the spectrum of poly(RTIL)-RTIL membrane, the peak at 1660cm-1 related to the C=C stretching vibration was obviously weakened compared with that of the monomer, as a evidence of polymerization reaction. Bands observed at 1570 and 1190 cm-1 belonged to the in-plane flexural vibration by the backbone of the imidazole ring, bands at 740 cm-1 belonged to the C-H out-of-plane flexural vibration on the imidazole ring, and bands at 1140 cm-1 corresponded to the C-O-C asymmetric stretching vibration, which proved the existence of the imidazole ring and ether group. In addition, the characteristic peaks belonging to the PVDF membrane did not appear in the spectrum of poly(RTIL)-RTIL membrane, proving that the surface of the support membrane was covered by the polymer layer. 3.5.2. TGA Studies. As the TG curve shown in Figure 19, the weight loss of the membrane in the temperature range from 50 °C to 300 °C was less than 3%. The polymer started to significantly decompose at 350°C with a rapid weight loss from 350 °C to 500 °C. According to the TGA results, the poly (RTIL)-RTIL was stable for operating temperature up to 350 °C, which exhibited high thermal stability. 3.5.3. XRD Studies. The X-ray diffraction measurement was performed to characterize the crystalline nature of the membrane. As shown in Figure 20, the sharp peaks observed at around 5° to 20° were assigned to the crystalline region, and the broad hump between 35° and 50° indicated amorphous contributions. The result 23



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exhibited that the poly (RTIL)-RTIL composite membranes possessed a semi– crystalline nature. The amorphous characteristic of the membranes could greatly enhance the gas permeability due to the increase of chain mobility in the polymer matrix.73 Hence, the gas separation performance of the composite membrane could be directly influenced by the crystal fraction in the polymer matrix, which could be adjusted by changing the composition and ratio of the casting solution.

4. CONCLUSION Three ether-functionalized ILs and NH2-functionalized IL were synthesized. CO2 absorption experiments with synthesized TSILs were conducted. The absorption rate and absorption capacity of CO2 with NH2-functionalized IL were higher than those with ether-functionalized ILs. Compared with the results reported in previous studies, all the four synthesized TSILs showed better absorption performance of CO2 whether by physical absorption or chemical absorption. By comparing the results obtained with the ether-functionalized ILs synthesized in this work with other studies, it could be concluded that both the anion and viscosity of ILs were crucial factors affecting the gas absorption behavior of ILs. Besides, the 20 cycles of absorption−desorption process showed a good recyclability of these synthesized TSILs for CO2 absorption. The SILMs with these task-specific ionic liquids were prepared and tested for their gas separation performance with CO2/N2 and CO2/CH4. The SILMs with Eh-3 showed a clearly lower permeation rate of CO2 and relatively higher selectivity compared with the other two ether-functionalized ILs owing to the higher viscosity of Eh-3.

Moreover,

the

CO2 permeation

rates

of

the

24


SILMs

with

three

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ether-functionalized ILs were slightly lower than the non-functionalized IL, [bmim][Tf2N], but the selectivities of CO2/N2 and CO2/CH4 were much higher, indicating that the flexible ether group had significant influence on the CO2 selectivity. Furthermore, with the increase of transmembrane pressure, the permeation rates of CO2, N2, and CH4 all kept increasing while the selectivity of CO2/N2 and CO2/CH4 gradually decreased. Poly (RTIL)-RTIL composite membranes were prepared using synthesized ether-functionalized ILs. The permeation rate of gases basically kept decreased, while the selectivity of CO2/N2 and CO2/CH4 first increased and then decreased gradually with the increase of crosslinking monomer content. With the increase of “free” IL concentration, the permeation rates of CO2, CH4 and N2 all increased first and then decreased, while the selectivity of CO2/CH4 and CO2/N2 kept increasing. The composite membranes were characterized by XRD, FTIR and TGA. The results of characterization proved the existence of the polymer layer forming on the surface of PVDF support, the semi-crystalline nature and high thermal stability of the membrane. Although the selectivities of CO2 from other gases were not very prominent, the SILMs and poly(RTIL)–RTIL membranes in this work showed much higher CO2 permeability than most of the results in other previous studies, which could be regarded as one of the advantages when applied in industry. The study of stability of the SILMs and poly(RTIL)–RTIL membranes were carried out. The results showed a good stability of the SILMs in this work under the 25



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pressure below 0.2 MPa. Meanwhile, compared with the SILMs, the poly(RTIL)– RTIL membranes with a stronger structural stability is beneficial to maintaining the ILs in the membrane pores. Poly (RTIL)-RTIL membranes have better structural stability and higher gas permeability without loss of selectivity compared with SILMs, which can be a promising alternative in membrane separation of CO2 from other gases. However, there is still a big margin for the further improvement on the performance of poly (RTIL)-RTIL membranes. More research can be done in our further works from the following aspects: (1) To carry out deeper research of interaction theory between task-specific ILs and CO2, better understanding of structure-property relationship of ILs and desired properties which are suitable for membrane separation of CO2 (e.g. higher CO2 absorption capacity, lower viscosity) and design and synthesize new task-specific ILs used in the poly (RTIL)-RTIL membranes. (2) To carry out further investigation of process performance of poly (RTIL)-RTIL membranes under realistic operating conditions (e.g. elevated temperature and mixed-gas feed). (3) To select poly(RTIL) with better mechanical strength and stronger interaction with “free” ILs to enhance the stability and gas separation performance of poly (RTIL)-RTIL membranes.

AUTHOR INFORMATION Corresponding Authors 26


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*E-mail: [email protected] (Zhongqi Ren); [email protected] (Zhiyong Zhou)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (21276012 and

21576010),

National

(2013ZX09201006001,

Science

and

2013ZX09202005

Technology

and

Major

Project

2014ZX09201001-006-003),

Fundamental Research Funds for the Central Universities (BUCTRC-201515), and BUCT Fund for Disciplines Construction and Development (XK1508). The authors gratefully acknowledge these grants.

ASSOCIATED CONTENT Supporting Information FT-IR spectra of [apmim]Br and ether-functionalized vinylimidazolium ionic liquids

(Figure

S1

and

1-vinyl-3-(2-methoxyethyl)

Figure

S2);

imidazolium

1

H

NMR

spectra

of

[apmim]Br,

bis-trifluoromethyl-sulfonylimide,

1-vinylimidazole-3-(2-(2-methoxy-ethoxy))-ethyl-imidazolium-bis-trifluoromethyl-su lfonylimide and bis(1-vinyl-3-ethoxy-imidazolium bis-trifluoromethyl-sulfonylimide) ether (Figure S3, Figure S4, Figure S5 and Figure S6).

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Diffusivities

of

Gases

in

Room-Temperature Ionic Liquids: Data and Correlations Obtained Using a Lag-Time Technique. Ind. Eng. Chem. Res. 2005, 44, 4815-4823. (50)

Horne, W. J.; Shannon, M. S.; Bara, J. E. Correlating fractional free volume to CO2 selectivity in [Rmim][Tf2N] ionic liquids. J. Chem. Thermodyn. 2014, 77, 190-196.

(51)

Shannon, M. S.; Tedstone, J. M.; Danielsen, S. P. O.; Hindman, M. S.; Irvin,

A.

C.; Bara, J. E. Free Volume as the Basis of Gas Solubility and Selectivity in Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 5565-5576. (52)

Avila, A. M.; Funke, H. H.; Zhang, Y.; Falconer, J. L.; Noble, R. D. Concentration polarization in SAPO-34 membranes at high pressures. J. Membr. Sci. 2009, 335, 32-36.

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Scovazzo, P.; Kieft, J.; Finan, D. A.; Koval, C.; DuBois, D.; Noble, R. Gas separations using non-hexafluorophosphate (PF6)−, anion supported ionic liquid

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membranes. J. Membr. Sci. 2004, 238, 57-63. (54) Jiang, Y. Y.; Zhou, Z.; Jiao, Z.; Li, L.; Wu, Y. T.; Zhang, Z. B. SO2 gas separation using supported ionic liquid membranes. J. Phys. Chem. B, 2007, 111, 5058-61. (55) Cserjési, P.; Nemestóthy, N.; Bélafi-Bakó, K. Gas separation properties of supported liquid membranes prepared with unconventional ionic liquids. J. Membr. Sci. 2010, 349, 6-11. (56) Bhavsar, R. S.; Kumbharkar, S. C.; Kharul U K. Investigation of gas permeation properties of film forming polymeric ionic liquids (PILs) based on polybenzimidazoles. J. Membr. Sci. 2014, 470,494-503. (57) Shaligram, S. V.; Wadgaonkar, P. P.; Kharul, U. K. Polybenzimidazole-based polymeric ionic liquids (PILs): Effects of 'substitution asymmetry' on CO2 permeation properties. J. Membr. Sci. 2015, 493,403-413. (58) Cowan, M. G.; Masuda, M.; Mcdanel, W. M.; Kohno, Y.; Gin, D. L.; Noble, R. D. Phosphonium-based poly(Ionic liquid) membranes: The effect of cation alkyl chain length on light gas separation properties and Ionic conductivity. J. Membr. Sci. 2015, 498,408-413. (59) Tomé, L. C.; Gouveia, A. S. L.; Freire, C. S. R.; Mecerreyes, D.; Marrucho, I. M. Polymeric ionic liquid-based membranes: Influence of polycation variation on gas transport and CO2, selectivity properties. J. Membr. Sci. 2015, 486,40-48. (60) Singh, Z. V.; Cowan, M. G.; Mcdanel, W. M.; Luo, Y.; Zhou, R.; Gin, D. L.; Determination and optimization of factors affecting CO2/CH4, separation performance in poly(ionic liquid)-ionic liquid-zeolite mixed-matrix membranes. J. 34


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Membr. Sci. 2016, 509,149-155. (61) Achalpurkar, M. P.; Kharul, U. K.; Lohokare, H. R.; Karadkar, P. B. permeation

in

amine

functionalized

silicon

rubber

membranes.

Gas Sep.

Purif .Technol. 2007, 57(2),304-313. (62) Li, H.; Freeman, B. D.; Ekiner, O. M.; Gas permeation properties of poly(urethane-urea)s containing different polyethers. J. Membr. Sci. 2011, 369(1-2);49-58. (63) Ostwal,

M.;

Singh,

R.

P.;

Dec,

S.

F.;

Lusk,

M.

T.;

Way,

J.

D.3-Aminopropyltriethoxysilane functionalized inorganic membranes for high temperature CO2/N2, separation. J. Membr. Sci. 2011, 369,139-147. (64) Asadollahi, M.; Bastani, D.; Kazemian, H.; Permeation of single gases through TEG liquid membranes modified by Na-Y nano-zeolite particles. Sep. Purif .Technol. 2010, 76,120-125. (65) C. Camacho-Zuñiga, F.A.; Ruiz-Treviño, S.; Hernández-López, M.G.; Zolotukhin, F.H.J. Maurer.; A. González-Montiel. Aromatic polysulfone copolymers for gas separation membrane applications. J. Membr. Sci. 2009, 340,221-226. (66) Liang, L.; Quan, G.; Nancarrow, P. Composite ionic liquid and polymer membranes for gas separation at elevated temperatures. J. Membr. Sci. 2014, 450, 407-417. (67) Kanehashi, S.; Kishida, M.; Kidesaki, T.; Shindo, R.; Sato, S.; Miyakoshi, T. CO2, separation properties of a glassy aromatic polyimide composite membranes containing

high-content

1-butyl-3-methylimidazolium 35



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bis(trifluoromethylsulfonyl)imide ionic liquid. J. Membr. Sci. 2013, 430, 211-222. (68) Hwang, S.; Chi, W. S.; Su, J. L.; Sang, H. I.; Kim, J. H.; Kim, J. Hollow ZIF-8 nanoparticles improve the permeability of mixed matrix membranes for CO2/CH4, gas separation. J. Membr. Sci. 2015, 480, 11-19. (69)

Nafisi, V.; Hägg, M. B. Development of dual layer of ZIF-8/PEBAX-2533 mixed matrix membrane for CO2 capture. J. Membr. Sci. 2014, 459, 244-255.

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Lin, H. Q.; Freemana, B. D.; Kalakkunnath, S.; Kalika, D. S. Effect of copolymer composition, temperature, and carbon dioxide fugacity on pure- and mixed-gas permeability in poly(ethylene glycol)-based materials: Free volume interpretation. J. Membr. Sci. 2007, 291, 131-139.

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Bara, J. E.; Hatakeyama, E. S.; Gin, D. L.; Noble, R. D. Improving CO2 permeability in polymerized room-temperature ionic liquid gas separation membranes through the formation of a solid composite with a room-temperature ionic liquid. Polym. Advan. Technol. 2008, 19, 1415-1420.

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Cowan, M. G.; Gin, D. L.; Noble, R. D. Poly(ionic liquid)/Ionic Liquid Ion-Gels with High "Free" Ionic Liquid Content: Platform Membrane Materials for CO2 / Light Gas Separations. Acc. Chem. Res., 2016, 49, 724-732.

(73) Bitter, J. G. A. Effect of crystallinity and swelling on the permeability and selectivity of polymer membranes. Desalination 1984, 51, 19-35.

36


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Scheme Captions Scheme 1. Synthesis of [apmim]Br. Scheme 2. Synthesis of ether-functionalized RTILs.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. Synthesis of [apmim]Br.

38


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Scheme 2. Synthesis of ether-functionalized RTILs (a. n=1: Eh-1; n=2: Eh-2; b. Eh-3)

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table Captions Table 1. Physical properties of the ILs used in this work. Table 2. Comparison of absorption capacity of CO2 in this work with previous studies.

Table 3. Comparison of the separation properties of CO2/N2 and CO2/CH4 in this work with those reported in previous literatures.

40


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Table 1. Physical properties of the ILs used in this work. Name

Abbreviation

Molecular structure

Mw

Viscosity (mPa.S-1 298K)

1-(3-amino-propyl)-3methylimidazole bromine

[apmim][Br]

220.0

1-vinyl-3-(2-methoxyethyl) imidazolium bis-trifluoromethyl-sulfony limide

Eh-1

433.3

67670 1.661(in25% solution) 104.7

1-vinylimidazole-3-(2-(2-met hoxy-ethoxy))-ethyl-imidazol ium bis-trifluoro methyl-sulfonylimide

Eh-2

477.4

54.10

bis(1-vinyl-3-ethoxy-imidazo lium bis-trifluoromethyl-sulfonyli mide) ether

Eh-3

908.7

777.1

1-butyl-3-methylimdazoliuim bis-trifluoromethyl-sulfony limide

[Bmim][Tf2N]

419.4

78.22

41


，


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 68

Table 2. Comparison of absorption capacity of CO2 in this work with previous studies. Conditions IL

Physical absorption

Chemical absorption

Absorption (molCO2/mol ILs)

Refs.

Pressure

temperature

[BMIM][BF4]

213KPa

298K

0.033

(37)

[EMIM][BF4] [bmim][NTFS] [bmim][TF] [bmim][PFOS] [bmim][1-naf] [bmim][PhenF5] [bmim][Ox] (1:1) [bmim][MS] [EMIM][PF6] [BMIM][BF4]-H2O (bheea)-H2O DBAB Eh-1 Eh-2 Eh-3

161KPa 100KPa 100KPa 100KPa 100KPa 100KPa 100KPa 100KPa 163KPa 351KPa 382KPa 100KPa 100KPa 100KPa 100KPa

298K 303 K 303 K 303 K 303 K 303 K 303 K 303 K 298K 298K 298K 313K 285K 285K 285K

0.022 0.031 0.031 0.042 0.01 0.02 0.042 0.053 0.015 0.0367 0.0534 0.04 0.0617 0.0695 0.0882

(37) (38) (38) (38) (38) (38) (38) (38) (37) (39) (39) (40) This work This work This work

[BMIM][AC]-MDEA-H2O [N4444][DOC] AMP + [N1111][Gly] [gua][OTf] [gua][FAP]-MDEA-H2O [N1111][Gly]-H2O [BMIM][PF6]-MEA [EMIM][BF4]-MEA MEA-MDEA-H2O DBAB DPAB MDEA+ [N1111][Gly] [apmim]Br

100KPa 100KPa 100KPa 100KPa 159KPa 98KPa 100KPa 100KPa 98KPa 100KPa 100KPa 100KPa 100KPa

303K 333.5K 298K 303.2K 313K 298K 298K 298K 298K 298K 298K 323K 298K

0.121 0.188 0.255 0.41 0.377 0.402 0.371 0.508 0.363 0.35 0.56 0.43 0.4853

(41) (42) (43) (44) (45) (46) (37) (37) (46) (40) (40) (47) This work

42


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Table 3. Comparison of the separation properties of CO2/N2 and CO2/CH4 in this work with those reported in previous literatures. membranes

Support

SILMs

Permeability (barrer)

Selectivity

Ref.

CO2

N2

CH4

CO2/N2

CO2/CH4

89

14.58

(53)

IL

Hydrophilic PES

[P66614][Cl]

350±20

24±2

Hydrophilic PES

[emim][BF4]

480 ±15

42 ± 3

11.43

(54)

[hmim][BF4]

520 ±11

45 ± 2

11.56

(54)

hydrophobic PVDF

ECOENG TM 1111P

127

15.6

8.14

(55)

hydrophobic PVDF

Cyphos 102

637

76.5

8.32

(55)

hydrophobic PVDF

Cyphos 103

487

65.1

7.48

(55)

hydrophobic PVDF

Cyphos 104

642

113

5.68

(55)

hydrophobic PVDF

ECOENGTM 1111P

127

11.6

15.6

10.94

8.14

(55)

hydrophobic PVDF

Eh-2

1541.73

228.97

158.42

6.73

9.73

This work

[TMPBI-BuI][HFB]

6.52

0.49

0.21

13.31

(56)

PBI-HFA based PILs

(57)

[DBzDMPBI-HFA][I]

11

0.5

22

(57)

[DBzDMPBI-HFA][Tf2N]

22.2

1

22.2

(57)

[DBzDMPBI-HFA][HFB]

23.2

1.01

22.9

(57)

PBI-BuI based PILs

PIL

PIL-IL

(57)

[DBzDMPBI-BuI][I]

11.3

0.51

22.6

(57)

[DBzDMPBI-BuI][Tf2N]

24.5

0.9

27.2

(57)

[DBzDMPBI-BuI][HFB]

24.9

0.95

26.2

(57)

poly([P444VB] [Tf2N])

51±1

2.7±0.2

4.5±0.2

18.9

11.3

(57)


120±4

8.5±0.1

19±1

14.1

6.3

(58)


186±6

12.0±0.3

73±1

15.5

2.55

(58)

PIL imidazolium–10 IL

14.9±0.12

23.7±0.5

25.1±1.0

(59)

PIL pyridinium–10 IL

20.4±0.07

20.1±0.8

25.8±0.3

(59)

43



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

poly([SMIM][Tf2N]-

Page 44 of 68

44±3

27±1

(60)

6.1

This

20%[EMIM][Tf2N] poly[Ether-IL]-

3058.37

775.9

501.75

3.94

28%BMIMNTf2a

work 2528.91

poly[Ether-IL]-

408.98

303.9

8.32

6.18

35%BMIMNTf2b

This work

composite membranes based on amine substituted silicon

4188

364

1294

3.2443

(61)

3628

405

1149

3.16

(61)

PDMS

3800

460

1200

3.17

(61)

poly(ethylene glycol) (PEG) 2000

190

11

34

5.60

(62)

poly(propylene glycol) (PPG) 2700

170

9.2

25

6.8

(62)

poly(tetramethylene ether glycol)

170

8.5

20

8.5

(62)

Terathane ®2900

150

7.2

22

6.8218

(62)

PEG 2000 and Terathane ® 2000

69

2.4

9.1

7.58

(62)

rubber composite membranes based on amine unsubstituted silicon rubber

(Terathane ® ) 2000

Other

3-Aminopropyltriethoxysilane

types

membranes

functionalized inorganic

< 855

< 10

(63)

PT

24.92

6.33

3.94

(64)

PTY-5

41.57

9.33

4.46

(64)

PTY-10

63.45

10.46

6.07

(65)

[PSF(TMHF-NPSF)

6.97

0.26

26.81

(55)

Pure PMDA-ODA PI

3

0.067

44.6

(66)

7.9

0.14

54.9

(66)

34.4

1.43

24.1

(67)

membrane PMDA-ODA PI with 30 wt% [C4mim][NTf2] 6FDA-TeMPD PI with 35 wt% [C4mim][NTf2]

44


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZIF-8/PVC-g-POEM

623

57

11.2

(68)

ZIF-8/Pebax 2533

1287

143

9

(69)

45



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions Figure 1. Schematic illustration of (a) neat poly(RTIL) membrane; (b) poly(RTIL)-RTIL composite membrane.

Figure 2. Schematic diagram of experimental setup for the CO2 absorption process. Figure 3. Contact angle between the support membrane and the Eh-1 ((a) JN membrane, (b) PVDF membrane, (c) PTFE membrane).

Figure 4. Flow diagram of the gas permeation separation experimental setup. Figure 5. Absorption rate of CO2 in the task-specific ionic liquids (a. Eh-1; b. Eh-2; c. Eh-3). Figure 6. Reaction schematics of CO2 with [apmim][Br]. Figure 7. Absorption capacity of CO2 in the task-sepecific ionic liquids (a. Eh-1; b. Eh-2; c. Eh-3).

Figure 8. Effect of recycle time on absorption rate (a) and absorption capacity (b) of CO2 with [apmim]Br.

Figure 9. Effect of recycle time on absorption rate (a) and absorption capacity (b) of CO2 with Eh-1.



Figure 12. Effect of the types of ionic liquids on the permeation rate (a) and permeation selectivity (b) of membranes (1. Eh-1; 2. Eh-2; 3. Eh-3; 4. [apmim]Br; 5. [bmim][Tf2N]). 46


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Figure 13. Effect of transmembrane pressure on the permeation rate (a) and permeation selectivity (b) of membranes.

Figure 14. Effect of crosslinking monomer content on the permeation rate (a) and permeation selectivity (b) of membranes (Conditions: transmembrane pressure: 0.10 MPa; room temperature; the mass ratio of monomer, crosslinking monomer, [bmim][Tf2N], AIBN and alcohol is 2-x:x:0.12:0.04:4).

Figure 15. The effect of [bmim][Tf2N] content on the permeation rate (a) and permeation selectivity (b) of membranes (Conditions: transmembrane pressure: 0.10 MPa; room temperature; the mass ratio of monomer, crosslinking monomer, [bmim][Tf2N], AIBN and alcohol is 0.6:1.4:x:0.04:4).

Figure 16. Relative membrane weight of the SILMs as a function of time under different applied pressures.

Figure 17. Relative membrane weight of the poly (RTIL)-RTIL membranes as a function of time under different applied pressures.

Figure 18. FT-IR spectrum of ether-functionalized monomer, poly(RTIL)-RTIL membrane and PVDF membrane.

Figure 19. TGA spectrum of poly (RTIL)-RTIL. Figure 20. XRD spectra of poly(RTIL)-RTIL.

47



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic illustration of (a) neat poly(RTIL) membrane; (b) poly(RTIL)-RTIL composite membrane.

48


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pressure regulator

Rotor flowmeter

U-type differential pressure gauge

Soap-film flowmeter

Erlenmeyer flask

CO2

Liquid seal

Magnetic stirrer

CO2 cylinder

Figure 2. Schematic diagram of experimental setup for the CO2 absorption process.

49



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

(c)

Figure 3. Contact angle between the support membrane and the Eh-1 ((a) JN membrane, (b) PVDF membrane, (c) PTFE membrane).

50


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pressure gauge Three-way valve

Pressure regulator

Ball valve

Membrane cell CO2

N2

CH4 Soap-film flowmeter

Gas cylinder

Figure 4. Flow diagram of the gas permeation separation experimental setup.

51



1000

q /(106mol CO2 /(s·mol ILs))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 68

a b c [apmim]Br

800

600

400

200

0 0

4000

8000

12000

16000

20000

24000

t/s

Figure 5. Absorption rate of CO2 in the task-specific ionic liquids (a. Eh-1; b. Eh-2; c. Eh-3).

52


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Figure 6. Reaction schematics of CO2 with [apmim][Br].

53



0.5

L / (mol CO2 / mol ILs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.4

0.3

0.2

0.1

0.0

a

b

c

[apmim]Br

The type of ILs

Figure 7. Absorption capacity of CO2 in the task-sepecific ionic liquids (a. Eh-1; b. Eh-2; c. Eh-3).

54


Page 54 of 68

Page 55 of 68

1000

0.7

1 2 3 4 5 10 15 20

600

0.6

0.5


800

400

6

q /(10 mol CO2 /(s⋅ mol ILs))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.3

0.2

200

0.1

0

0.0 0

2000

4000

6000

8000

10000

12000

0

5

t/s

10

15

20

Recycle time

(a)

(b)

Figure 8. Effect of recycle time on absorption rate (a) and absorption capacity (b) of CO2 with [apmim]Br.

55



0.10 120

1 2 3 4 5 10 15 20

80

0.09 0.08


100

q /(106mol CO2 /(s⋅ mol ILs))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 68

60

40

0.07 0.06 0.05 0.04 0.03 0.02

20 0.01 0

0.00 0

5000

10000

15000

20000

0

5

t/s

10

15

20

Recycle time

(a)

(b)


56


Page 57 of 68

0.10

100

60

40

0.09 0.08


1 2 3 4 5 10 15 20

80

q /(106mol CO2 /(s⋅ mol ILs))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.07 0.06 0.05 0.04 0.03 0.02

20

0.01 0.00

0 0

5000

10000

15000

0

20000

5

10

15

20

Recycle time

t/s

(a)

(b)


57



0.12

160

1 2 3 4 5 10 15 20

120 100

0.10


140

80 60

6

q /(10 mol CO2 /(s⋅ mol ILs))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 58 of 68

40

0.08

0.06

0.04

0.02

20 0.00

0 0

5000

10000

15000

0

20000

5

10

15

20

Recycle time

t/s

(a)

(b)


58


Page 59 of 68

0.20

CO2 CH4

0.16

N2

P/GPU

0.12

0.08

0.04

0.00 1

2

3

4

5

The types of ILs

(a)

CO2/CH4

10

CO2/N2 8

6

S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

2

0 1

2

3

4

5

The types of ILs

(b) Figure 12. Effect of the types of ionic liquids on the permeation rate (a) and permeation selectivity (b) of membranes (1. Eh-1; 2. Eh-2; 3. Eh-3; 4. [apmim]Br; 5. [bmim][Tf2N]).

59



0.25

CO2 CH4

0.20

N2

P/GPU

0.15

0.10

0.05

0.00 0.08

0.10

0.12

0.14

0.16

0.18

0.20

p/MPa

(a) 12 CO2/CH4

10

CO2/N2

8

S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 60 of 68

6

4

2

0 0.08

0.10

0.12

0.14

0.16

0.18

0.20

p/MPa

(b) Figure 13. Effect of transmembrane pressure on the permeation rate (a) and permeation selectivity (b) of membranes.

60


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0.30

0.25

P/GPU

0.20

0.15

0.10

CO2 0.05

CH4 N2

0.00 0.0

0.2

0.4

0.6

0.8

1.0

0.6

0.8

1.0

m/g (a) 2.5

CO2/CH4 2.0

CO2/N2

1.5

S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

0.5

0.0 0.0

0.2

0.4

m/g (b)

Figure 14. Effect of crosslinking monomer content on the permeation rate (a) and permeation selectivity (b) of membranes (Conditions: transmembrane pressure: 0.10 MPa; room temperature; the mass ratio of monomer, crosslinking monomer, [bmim][Tf2N], AIBN and alcohol is 2-x:x:0.12:0.04:4).

61



0.5

CO2 CH4

P/GPU

0.4

N2

0.3

0.2

0.1

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.8

1.0

1.2

m/g (a) 10

CO2/CH4 8

CO2/N2 6

S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 62 of 68

4

2

0 0.0

0.2

0.4

0.6

m/g

(b) Figure 15. The effect of [bmim][Tf2N] content on the permeation rate (a) and permeation selectivity (b) of membranes (Conditions: transmembrane pressure: 0.10 MPa; room temperature; the mass ratio of monomer, crosslinking monomer, [bmim][Tf2N], AIBN and alcohol is 0.6:1.4:x:0.04:4).

62


Page 63 of 68

100

80

Membrane weight/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

0.10 MPa 0.15 MPa 0.20 MPa 0.25 MPa

20

0 0

1

2

3

4

5

6

Time/h

Figure 16. Relative membrane weight of the SILMs as a function of time under different applied pressures.

63



100

80

Membrane weight/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60

40

0.10 MPa 0.15 MPa 0.20 MPa 0.25 MPa

20

0 0

1

2

3

4

5

6

Time/h

Figure 17. Relative membrane weight of the poly (RTIL)-RTIL membranes as a function of time under different applied pressures.

64


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 18. FT-IR spectrum of ether-functionalized monomer, poly(RTIL)-RTIL membrane and PVDF membrane.

65



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 19. TGA spectrum of poly (RTIL)-RTIL.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 20. XRD spectra of poly(RTIL)-RTIL.

67



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

146x108mm (220 x 220 DPI)


Page 68 of 68

Facilitated Separation of CO2 by Liquid Membranes and Composite

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