A Crown Ether-Containing Copolyimide Membrane with Improved

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

A Crown Ether-Containing Copolyimide Membrane with Improved Free Volume for CO2 Separation Dongyun Wu,† Chunhai Yi,*,† Cara M. Doherty,‡ Liping Lin,† and Zongli Xie‡ †

Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, P. R. China ‡ CSIRO Manufacturing, Private Bag 10, Clayton South, Victoria 3169, Australia

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S Supporting Information *

ABSTRACT: Membrane free volume is closely related to gas separation property. The free volume can be tuned by adjusting monomer structure for polyimide-based membrane materials, and then the gas separation property can be improved ulteriorly. In this work, crown ether (di(aminobenzo)-18-crown-6, DAB18C6)-based copolyimide membranes containing bulky and flexible diamine monomer (2,2′-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, HFBAPP) were synthesized for CO2 separation. The −C(CF3)2− and −O− groups in HFBAPP can improve the membrane free volume and affinity with CO2, respectively. The microcavity size and fractional free volume of HFBAPP/DAB18C6/4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) membranes were investigated by molecular dynamics simulation and positron annihilation lifetime spectroscopy testing. The result showed that fractional free volume was improved obviously due to the presence of −C(CF3)2− in HFBAPP, while the microcavity size only showed very insignificant increase. Correspondingly, these membranes exhibited much higher CO2 permeability and satisfied selectivity. Furthermore, the correlation between free volume and gas transport properties of crown ether-based copolyimide membranes was investigated to establish structure−property relationship. CO2/ CH4 and CO2/N2 mixed-gases separation properties of HFBAPP/DAB18C6/6FDA exceeded 2008 Robeson’s upper bounds.

1. INTRODUCTION CO2 emissions must be restrained to realize the atmospheric stabilization based on the International Panel on Climate Change.1 Flue gas streams from fossil-fuel-based power plants contain up to 40% of greenhouse gas emissions, which typically comprise of 4−20% CO 2 at atmospheric pressure. 2,3 Furthermore, CH4-based energy source can be a potential solution to meet this enormous energy demand. The raw natural gas usually contains 5−10% CO2, in which the CO2 content needs to be reduced to less than 2 vol % before using.4 Therefore, separating CO2 from N2 or CH4 is of great importance for industrial efficiency. Membrane technology is a promising candidate for CO2 separation from flue gas and natural gas.5−8 However, developing high-performance membranes, which are tailored specifically toward effective CO2/N2 and CO2/CH4 separation with enhanced permeability and selectivity simultaneously, is extremely urgent. Polyimide-based materials not only show outstanding gas permselectivity but also possess high chemical and thermal stabilities, which has aroused great interest for gas separation applications. Especially, glassy aromatic polyimides have been widely investigated and are regarded as state-of-the-art membranes. However, for the majority of polyimide-based gas separation membranes, the challenge to acquire both high © XXXX American Chemical Society

permeability and high selectivity still exists, which is wellknown as permeability−selectivity trade-off effect.9,10 To address this problem, various chemical modification methods such as substitution of monomers, cross-linking, or grafting bulky functional groups in the polyimide backbone have been explored.11−17 Alexis et al. modified the copolyimide by propanediol monoester cross-linking, and the cross-linked polymer showed permeabilities in excess of 75 barrer and CO2/CH4 selectivities of almost 45 for mixed gas trial.11 Shrimant et al. synthesized a new diamine with spirobisindane and phenazine units and then prepared a polyimide of intrinsic microporosity (PIM-PI-6FDA) (6FDA = 4,4′-hexafluoroisopropylidene bisphthalic dianhydride) membrane exhibiting high CO2 permeability of 185.4 barrer with CO2/N2 and CO2/ CH4 selectivity of 30.9 and 43.1, respectively.12 The gas separation properties of these polyimide-based membranes were still subject to the trade-off effect. Freeman and coworkers published an extensive review article about trade-off effect between membrane selectivity and permeability.18 Their Received: Revised: Accepted: Published: A

May 7, 2019 June 30, 2019 July 6, 2019 July 6, 2019 DOI: 10.1021/acs.iecr.9b02502 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

crown ether segments has potential application value in the field of CO2 separation. A crown ether-including copolyimide membrane for CO2 separation was proposed and synthesized in our previous work.35 The as-synthesized 4,4′-methylenedianiline/di(aminobenzo)-18-crown-6/4,4′(hexafluoroisopropylidene)diphthalic anhydride (MDA/ DAB18C6/6FDA) copolyimide membranes with crown ether exhibited good affinity with CO2 and strong rigid polymer chains, presenting superior CO2/N2 and CO2/CH4 selectivities but relative low permeability. Reported research has verified that the membrane with high FFV possessed high gas permeability. Therefore, in the present work, a crown ethercontaining copolyimide membrane with higher FFV is proposed by the further optimization of monomer structure. A bulky and flexible diamine monomer, 2,2-bis[4-(4aminophenoxy)phenyl]hexafluoropropanane (HFBAPP), was selected instead of MDA to improve the FFV of membrane and, finally, to enhance gas permeability. HFBAPP contains −O− and −C(CF3)2− groups. The −O− group has flexibility benefiting membrane-forming and good affinity with CO2. The −C(CF3)2− group has been shown to improve chain-to-chain spacing of polymer chain,25 which can result in FFV improvement of polymeric membrane. In addition, the molecular length of HFBAPP is longer than that of MDA used in our previous work, which may lead to an effective increase in membrane free volume and, in turn, lead to an increment in gas permeability. Hence, the introduction of HFBAPP in crown ether-containing copolyimide membrane is expected to increase the CO2 permeability. Another aim of this study is to expand on our preliminary data35 by optimizing the diamine structure and explore the correlations between structure and gas transport properties of as-synthesized crown ether-containing copolyimide membranes combining the membrane free volume obtained from PALS measurement and MDS modeling. The results in this study highlight the significance of comprehending membrane free volume-gas transport property correlations, thus allowing tailor-made polymer membrane with the desired permeation properties in the future.

research highlighted that traditional membranes for gas/water separations are mainly based on polymeric membranes and limited by the permeability−selectivity trade-off phenomenon. Many new materials and design approaches provide the prospect of better control over pore size and distribution, which can break the conventional upper bound effect. The transport of dense polymeric membranes generally follows the solution-diffusion mechanism. CO2 solubility and diffusivity can be improved via introduction of CO2-philic segments and amplification of membrane free volume. Generally, the −O− groups possess good affinity with CO2, and the polymers with a number of −O− groups are delivered to obtain the high CO2 permeability.19,20 Fractional free volume, the micropore size, and its distribution would have a direct influence upon the CO2 transport (CO2 diffusivity) through the dense polymeric membrane.21 Therefore, it will be important to study the free volume and gas permeability simultaneously to establish structure and properties relationship. Both molecular dynamics simulation (MDS) modeling and positron annihilation lifetime spectroscopy (PALS) measurement have been employed to evaluate the free volume of polymer membrane.22 In particular, PALS can demonstrate precisely this well-tuned size and size distribution of free volume, which correlate with gas transport properties.23−26 For example, Chung’s group prepared a series of 6FDA-based polyimides measuring pore size and size distribution of free volume using PALS and confirmed the reason for permselectivity improvement through PALS data.23,27,28 Some researches manifested that polymeric membranes with high fractional free volume (FFV) exhibited high gas permeability.29,30 Poly(trimethylsilyl-1-propyne) (PTMSP) as an ultrapermeable microporous polymer has large FFV of 0.32, and its CO2 permeability can reach to 20 000−30 000 barrer, while its gas selectivity is very low.29 McKewon’s group established an ultrapermeable polymer of intrinsic microporosity (PIM) membrane, PIM-TMN-Trip, that possessed high FFV of 0.314 due to the inefficient packing of the twodimensional (2D) chains.30 The high FFV resulted in the PIMTMN-Trip membrane with CO2 permeability of up to 33 300 barrer and CO2/N2 and CO2/CH4 selectivities of 14.9 and 9.7, respectively. For the above polymeric membranes with high FFV, the CO2 permeability is considerably high, but the CO2/ N2 and CO2/CH4 selectivities remain to be improved to enhance the entire economic feasibility in industrial application.7 Previous researches have revealed that the polyimide-based membranes containing −CF3 groups displayed remarkable gas permselectivity.31−34 This is because −CF3 groups can increase the chain stiffness and decrease chain packing, leading to enhanced gas permselectivity. Soroush’s group synthesized a PPImDA-6FDA polyimide with high FFV of 0.233, which possessed the highest CO2 permeability of 812 barrer among all reported pentiptycenebased polymers without sacrificing selectivity.34 The CO2/N2 and CO2/CH4 selectivities of PPImDA-6FDA were 24.0 and 27.4, respectively. Therefore, polyimide-based membranes with −CF3 groups are expected to enlarge the membrane free volume to improve the gas permeability and maintain the gas selectivity simultaneously. Through the study of the structure and property of crown ether, it was found that the crown ether has a non-coplanar rigid structure, cavity diameter of 18-crown-6 is equivalent to the dynamic diameter of CO2, and the ether groups in crown ether have good affinity with CO2. Hence, the membrane with

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-Hexafluoroisopropylidene bisphthalic dianhydride (6FDA), dibenzo-18-crown-6 (DB18C6), and 2, 2′-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropanane (HFBAPP) were provided by J&K Scientific Co., Ltd. Acetic anhydride was supplied by West Long Chemical Co., Ltd. trans-Di(aminobenzo)-18-crown-6 (DAB18C6) was prepared from DB18C6 (98%). Ethylene glycol monomethyl ether, Nmethyl-2-pyrrolidinone (NMP), N,N-dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), trichloromethane (CHCl3), N,N-dimethylformamide (DMF), methylene chloride (CH2Cl2), triethylamine, ethanol, 10 wt % Pd/C catalyst, nitric acid, and acetic acid were supplied by Sinopharm Chemical Reagent Co., Ltd. NMP, ethylene glycol monomethyl ether, DMF, and DMAc were purified using 5A molecular sieves prior to use. The others were applied without further purification. 2.2. Copolyimide Synthesis. The crown ether-based polyimide exhibits poor solubility and poor mechanical strength because of the strong rigid chain segment structure. Therefore, in this work the crown ether-based copolyimide materials with bulky and flexible diamine monomer (HFBAPP) and different molar contents of DAB18C6 (25, B

DOI: 10.1021/acs.iecr.9b02502 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthetic Method of Cyclic Crown Ether-Based Copolyimides

permeation property. The copolyimide materials were dissolved in DMAc to form a viscous solution of ∼10 wt % concentration. The homogeneous and viscous solutions were cast on a flat glass plate carefully, drying at 70 °C for 12 h without vacuum followed by 120 °C for at least 24 h with vacuum. The thickness of prepared copolyimide membranes was 115−125 μm. 2.4. Characterization. Fourier-transform infrared (FTIR, Thermo Fisher Nicolet iS50) analysis of synthesized copolyimides was executed ranging from 650 to 4000 cm−1 with the help of OMNI-sampler. 1H nuclear magnetic resonance (1H NMR, BrukerAvance 400 MHz) spectra analysis was performed using tetramethylsilane as the internal standard to record the chemical shifts. The glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC, Netzsch STA449F5) with nitrogen conditions conducted ranging from 30 to 500 °C. The thermal stability test was measured by thermogravimetric analysis (TGA, Netzsch STA449F5) with nitrogen conditions conducted ranging from 30 to 800 °C. The molecular weight of the synthesized copolyimide materials as well as their molecular distribution was tested by a gel permeation chromatography (GPC, Wyatt Technology Dawn Heleos-II) regarding DMF as the solvent. Polydispersity index (PDI) was determined by Mw/Mn. The intermolecular spacing between the intersegmental chains and amorphous structure was detected through wide angle X-ray diffraction (WAXD, Shimadzu XRD-6100). The 2θ angle ranged from 6 to 60° at step size of 10°/min with a copper cathode. Mechanical properties of these copolyimide membranes (dimension: 70 mm × 10 mm) were determined by a CMT-6503 electronic tensile machine at room temperature. PALS is regarded as an advanced approach to provide quantitative pore size and concentration of free volume within the polymer membranes. PALS uses ortho-positronium (o-Ps, the bound state of a positron and an electron with the same spin) as a probe to investigate the free volume in polymeric membranes. The o-Ps is attracted to areas of low electron density, and therefore its lifetime can be related to the average free-volume sizes of the membranes. The details of PALS test for the copolyimide membranes can be found in the

50, and 75 mol %) were synthesized via condensation polymerization and chemical imidization. DAB18C6 was synthesized from DB18C6 following a synthesis procedure as detailed in our previous study.36 Scheme 1 presents the chemical reaction process of these synthesized copolyimide materials. In this work, the copolyimide membranes were denoted on the basis of the molar ratio of diamine monomer as presented in Table 1. Taking the copolyimide HFBAPP/ Table 1. Designation of Copolyimide Membranes membranes (MDA)x-(DAB18C6)y6FDA

molar ratios of x/y 3:1

ref

3:1

this work

1:1 1:3

HDF11 HDF13

this work this work

1:1 1:3 (HFBAPP)x-(DAB18C6)y6FDA

abbreviation MDF31 (PI-25) MDF11 (PI-50) MDF13 (PI-75) HDF31

35 35 35

DAB18C6/6FDA containing 25 mol % DAB18C6 as an example, in the nitrogen atmosphere, 1.92 mmol of HFBAPP and 0.64 mmol of DAB18C6 were dissolved in 9.2 mL of DMAc. Following that, 2.56 mmol of 6FDA was added into the above solution. After the solution was stirred for 24 h at room temperature to form the high molecular polyamic acid solution, the chemical imidization procedure was conducted with acetic anhydride and triethylamine at room temperature and kept stirring for 24 h. The resulting highly viscous copolyimide solution was added into ethanol slowly, and it precipitated uniformly in a blender washing three times with fresh ethanol. And then the prepared copolyimide materials were dried for 12 h at 100 °C and over 24 h at 200 °C under vacuum. The other copolyimide materials were synthesized in a similar way to that of HFBAPP/DAB18C6/6FDA containing 25 mol % DAB18C6. 2.3. Membrane Preparation. The copolyimide membranes containing various diamine structures and contents were fabricated by a solution casting method to test the gas C


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Figure 1. (a) FTIR and (b) 1H NMR spectra of HDF copolyimides containing different diamine contents.

references.24,25 The o-Ps components were used to characterize the sample’s free volume,37 using a simple quantum mechanical model and assuming that the o-Ps was localized in a spherical potential well-surrounded by an electron layer of thickness ΔR of 1.66 Å. A correlation between the o-Ps lifetime (τ3) and the average free volume size can be established. The analysis of the PALS data was operated by calculating the average radius of holes (R) using the Tao−Eldrup equation:38,39 ÅÄ ÑÉ 1 ÅÅ R 1 ijj 2πR yzzÑÑÑÑ + sinjj τ3 = ÅÅÅÅ1 − zÑ 2 ÅÅÅÇ R0 2π jk R 0 zz{ÑÑÑÑÖ (1)

FAV = (V − VW )/V

where V and Vw are the simulation cell volume and van der Waals volume of copolyimide chains, respectively. The Vw is reckoned via Atom Volumes & Surfaces tool of MDS software,41 rather than the group contribution method.42,43 2.6. Gas Permeation Test. The mixed-gas permeability of synthesized copolyimide membranes was measured by a custom-built permeation system described elsewhere.35 Each membrane was tested at least three times to guarantee the relative standard deviation of less than 5%. The gas permeation fluxes of CH4 (NCH4), N2 (NN2), and CO2 (NCO2) were obtained through the flow rate of sweep gas as well as their compositions, and then mixed-gas permeability (Pi) was calculated as follows:

where τ3 (ns) is the longest positronium lifetime detected, corresponding to o-Ps “pick-off” annihilation. R (Å) is the average radius of the free volume, and R0 = R + ΔR. 2.5. Molecular Dynamics Simulations. Molecular dynamics simulation (MDS) method was used to evaluate the free volume of synthesized copolyimide materials in this study. The detailed MDS process can be obtained by consulting our previous paper.35 The fractional free volume (FFVMDS) is the representative of the polymer membrane void, and it can estimate polymer membrane permeation property,37 while FFVMDS of polymer membrane contains the dead volume that the penetrant molecule is impassability. The fractional accessible volume (FAV) of polymer membrane provides a more realistic estimation of available volume for penetrant molecule, because it excludes the dead volume. The analysis of the volume distribution via calculating the FAV instead of FFV is a more helpful approach to evaluate the packing efficiency of polymer chains. Through the Connolly task simulation, the probe, a hard spherical particle with a specific radius, is used to obtain available volume inside the simulation cell, and then the FAV is calculated. For the certain penetrant molecule, the probe radius is set as the dynamic radius of penetrant molecule to estimate the accessible volume and FAV, which the certain penetrant molecule can pass through. In this study, the synthesized copolyimide membranes were used for separating CO2 from N2 or CH4. Hence, the probe radius was set as 1.65, 1.82, and 1.9 Å in accord with the kinetic radius of CO2, N2, and CH4, respectively. In the process of FAV calculation, the mutual effects between polymer atoms and probe particle were not to take into account.40 The FFVMDS and FAV values of simulated copolyimide matrices were calculated as below: FFVMDS = (V − 1.3VW )/V

(3)

Pi =

Ni × l ΔPi

(4) −10

where Pi (barrer, 1 barrer = 1 × 10 cm (standard temperature and pressure (STP))·cm·cm−2·s−1·cmHg−1) refers to the gas permeability of component (i), Ni denotes the gas permeation flux of component (i) (cm3(STP)·cm−2·s−1), l is the membrane thickness (cm), and ΔPi denotes the pressure difference between the membrane upstream and the membrane downstream (cmHg). The real selectivity, αA/B, for component (A) and (B) was determined by the ratio of their permeabilities: αA/B =

PA PB

3

(5)

3. RESULTS AND DISCUSSION 3.1. Physical Properties of Copolyimides. In Figure 1a, FTIR spectra of HDF copolyimides show the symmetric imide carbonyl stretching at ∼1726 cm−1 and asymmetric imide carbonyl stretching at 1781 cm−1. Figure 1b shows a comparison of 1 H NMR spectra of resultant HDF copolyimides. The peaks at 8.03−7.89 and 6.98−6.91 ppm belong to the protons on the benzene ring of 6FDA and DAB18C6 moieties, respectively. The chemical shifts of protons in HFBAPP moieties are attributed to 7.43, 7.21, and 7.08 ppm, respectively. The increased peak intensity at δ = 4.20 and 4.04 ppm, corresponding to the −CH2− units on the crown ether ring, is indicative of the increased DAB18C6 content from HDF31 to HDF13. Meanwhile, the crown ether

(2) D


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Industrial & Engineering Chemistry Research contents in HDF31, HDF11, and HDF13 calculated from 1H NMR spectra are 25.3, 50.0, and 74.6 mol %, which was in agreement with the target values (25, 50, and 75 mol %). The 1 H NMR and FTIR measurements confirm the molecular structures precisely as presented in Scheme 1 and manifest the full imidization. The synthesized copolyimides have high Mw and narrow PDI of ∼1.3 as presented in Table 2. The Mw of HDF

in a variety of common solvents gives them an obvious advantage in the preparation of the gas separation membranes. 3.2. Free Volume Analysis. The free volume of synthesized HDF copolyimide was characterized by PALS and MDS. PALS is regarded as an advanced approach to provide quantitative pore size and concentration of free volume within the polymer membranes. The pore size and concentration of free volume in the membrane are characterized by o-Ps lifetime τ3 and intensity I3, respectively.45,46 However, the I3 values are quite low in polyimide membranes due to the strong affinity between electron and diimide moieties inhibiting the formation of positronium.47 The I3 values may not be representative of the real concentration of free volume in polyimide membranes. Therefore, the FFVMDS and FAV for CO2, N2, and CH4 are calculated by MDS in this work. Table 4 presents the PALS results of MDF and HDF copolyimide membranes containing o-Ps lifetime τ3, average

Table 2. Molecular Weight, PDI, and 2θ and d-Spacing of WAXD of Synthesized HDF Copolyimide Membranes Containing Different Diamine Contents GPC

WAXD

membranes

Mw (g/mol)

PDI

2θ (deg)

d-spacing (Å)

HDF31 HDF11 HDF13

157 600 127 600 81 570

1.289 1.186 1.171

15.36 15.94 16.74

5.76 5.56 5.29

Table 4. Pore Size Characterization of HDF Copolyimide Membranes Containing Different Diamine Structures and Contents by Positron Annihilation Lifetime Spectroscopy

copolyimides gradually increased with the increase of HFBAPP content. This can be attributed to the better reactivity of linear HFBAPP monomer. All of the copolyimide membranes are homogeneous with good flexibility and high mechanical strength (Table S1). Table 3 and Figure S1 show that the HDF copolyimides containing different diamine contents possessed high thermal stability with onset decomposition temperature up to ∼450 °C. The weight loss gradually decreased with the decrease of DAB18C6 content at onset decomposition temperature, which corresponded to the decomposition of the crown ether segments in BAB18C6 moieties. Table 3 presents the Tg values of synthesized copolyimide membranes. As it can be seen, the Tg decreased with the increasing crown ether content, which can be explained by the gradually decreased crystallinity of these polymers from HDF31 to HDF13. The good thermal and mechanical properties of resulting copolyimides confirmed that they can withstand gas permeation testing at high pressure and temperature. WAXD verified the microstructure of synthesized copolyimides in Table 2 and Figure S2. An obvious broad peak can be observed at ∼16° in HDF copolyimide membranes, implying that the synthesized polyimides are amorphous polyimides.23 The corresponding d-spacing is also calculated as listed in Table 2, which is a representative of the distance between polymer chains in the membrane and closely correlated to the free volume and consequent gas permeability.44 For the series of HDF copolyimides, the order of d-spacing is HDF31 > HDF11 > HDF13. This trend of d-spacing is in keeping with FFVMDS data presented in Table 5. Meanwhile, the synthesized copolyimide membrane materials are readily soluble in common solvents such as NMP, DMF, DMSO, DMAc, CHCl3, and CH2Cl2 at room temperature (Table S2). The favorable solubility of the copolyimides

membranes MDF31 MDF11 MDF13 HDF31 HDF11 HDF13

average radius of free volume (Å)

τ3 (ns) 1.998 2.009 1.853 2.286 2.149 1.895

± ± ± ± ± ±

0.042 0.152 0.076 0.033 0.060 0.081

2.852 2.862 2.715 3.102 2.986 2.755

± ± ± ± ± ±

0.038 0.139 0.074 0.027 0.052 0.077

I3 (%) 1.463 0.995 0.779 3.025 2.005 1.066

± ± ± ± ± ±

0.066 0.092 0.032 0.069 0.106 0.072

free volume radius, and pore intensity I3. The PALS data showed that there was only one o-Ps lifetime detected in the copolyimide membranes here. This means that the synthesized copolyimide membranes had a single microcavity size distribution, which is similar to previous results in the literature.23,27,28,48 Compared with WAXD and FFVMDS, PALS measurements present more direct and quantitative results of the microcavity size. As shown in Table 4, the τ3 and average free volume radius of MDF31 and MDF11 were similar, but I3 of MDF31 was much larger than that of MDF11. The τ3, average free volume radius and I3 of MDF13 were the smallest in series of MDF membranes. For HDF copolyimide membranes, the τ 3 decreased from 2.286 to 1.895 ns, while the average radius of free volume correspondingly changed from 3.102 to 2.755 Å with crown ether content increased from 25 to 75 mol %, which is in accordance with the d-spacing seen with WAXD (Table 2). Compared with MDF copolyimide membranes, the average free volume radius of HDF was slightly larger than that of MDF with the same crown ether content. This phenomenon can be explained by larger interchain spacing and the more

Table 3. Thermal Properties of Synthesized HDF Copolyimide Membranes Containing Different Diamine Contents T of weight lossb (°C) a

DTG (°C)

membranes

CY (%)

T5

T10

T20

T1

T2

Tg (°C)

HDF31 HDF11 HDF13

53.90 48.24 40.65

453.2 434.4 423.1

502.8 450.8 435.6

544.7 500.4 450.5

446.3 444.9 443.4

549.1 547.8 540.9

302.6 262.4 250.7

CY is char yield at 800 °C. bT5, T10, and T20 are the temperatures of weight loss indicated in percentage as the subscript.

a

E


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Figure 2. Three-dimensional representation of the accessible free volume distribution of HDF31with different probes: (a) CO2; (b) N2; (c) CH4.

Table 5. Free Volume Characterization of Copolyimide Membranes Containing Different Diamine Structures and Contents Simulated by Molecular Dynamics Simulations membranes

FFVMDS (%)

FAV of CO2 (%)

FAV of N2 (%)

FAV ratios of CO2 to N2

FAV of CH4 (%)

FAV ratios of CO2 to CH4

ref

MDF31 MDF11 MDF13 HDF31 HDF11 HDF13

20.27 19.04 17.81 21.25 20.04 19.10

11.46 9.86 7.95 12.19 10.79 9.23

9.44 7.95 6.05 10.13 8.84 7.24

1.214 1.240 1.314 1.203 1.221 1.275

8.60 7.16 5.30 9.26 8.04 6.45

1.333 1.377 1.500 1.316 1.342 1.431

35 35 35 this work this work this work

Table 6. Gas Permeation Performance of Synthesized Copolyimide Membranes Containing Different Diamine Structures and Contents CO2/N2 mixed-gasa membranes MDF31 MDF11 MDF13 HDF31 HDF11 HDF13

PCO2 88.53 82.15 38.68 152.9 102.3 68.92

± ± ± ± ± ±

PN2 1.72 2.68 0.01 4.98 0.16 1.83

1.23 0.98 0.41 2.09 1.28 0.73

± ± ± ± ± ±

CO2/CH4 mixed-gasa PCO2

selectivity 0.01 0.05 0.02 0.04 0.01 0.01

71.70 83.98 94.32 72.77 80.01 94.67

± ± ± ± ± ±

1.51 1.26 3.99 1.49 0.91 1.84

109.0 82.07 43.67 149.5 105.6 65.82

± ± ± ± ± ±

PCH4 0.57 0.08 0.30 1.13 0.44 0.64

1.17 0.78 0.40 1.96 1.26 0.69

± ± ± ± ± ±

0.05 0.03 0.02 0.10 0.04 0.01

selectivity

ref

± ± ± ± ± ±

35 35 35 this work this work this work

92.74 101.3 109.6 76.16 83.84 95.33

1.24 1.36 1.50 3.57 3.55 0.53

Note: unit of P, 1 barrer = 1 × 10−10·cm3 (STP)·cm/(cm2·s·cmHg) measured at 35 °C and 20 bar, CO2/N2 mixed-gas (20/80 vol %), CO2/CH4 mixed-gas (10/90 vol %). a

disrupted chain packing resulting from −CF3 groups. Hence, an effective tuning of the free volume cavity size of the synthesized copolyimides can be realized via adjusting the diamine monomer structures and crown ether contents. Figure S3 shows the CO2 accessible free volume distribution and morphology for MDF and HDF membranes. As can be seen from Figure S3a−c, the pore size and number of CO2 accessible volume decreased continuously with crown ether content’s increasing. The HDF membranes presented the same trend when enhancing the crown ether content (Figure S3d− f). Compared with MDF membranes, the HDF possessed more CO2 accessible free volume and larger pore size with the equal amounts of crown ether. This can be attributed to the −CF3 groups in HFBAPP monomer of HDF membranes. Figure 2 exhibits the accessible free volume of CO2, N2, and CH4 in an HDF31 model. The small CO2 probe can obtain access to more interconnected void structure. For N2 and CH4 probe, they can access free volume, but the void structure was discontinuous. This would result in the lower diffusivity of N2 and CH4 molecules as compared with CO2 molecule. The exact value of free volume characterization for copolyimide membranes, such as FFVMDS and FAV for CO2, N2, and CH4, calculated by MDS, is presented in Table 5. The

copolyimide membranes were applied for CO2/N2 and CO2/ CH4 mixed-gas separation, and therefore 1.65, 1.82, and 1.9 Å were selected as the probe radius in keeping with kinetic radius of CO 2, N 2, and CH4, respectively. The synthesized copolyimide membranes including rigid cyclic crown ether presented higher FFVMDS than Matrimid with 17%.49 For the MDF copolyimide membranes, FFVMDS decreased slightly when crown ether content increased from 25 to 75 mol %. This can be ascribed to the nature of crown ether structure with large volume. The FAV of CO2, N2, and CH4 also exhibited the decreasing trend with crown ether content, which is similar to FFVMDS. Moreover, the effect of crown ether content on the FAV of N2 and CH4 is more significant than that of CO2. The decrements in the FAV of CO2, N2, and CH4 were 30.6%, 35.9%, and 38.4%, respectively, when crown ether content increased from 25 to 75 mol %. Obviously, the variation of FAV with crown ether content is related to the kinetic diameter of the gases. As a result, the FAV ratios of CO2 to N2 (CH4) increased with crown ether content. Compared with MDF membranes, the simulated results of HDF membranes exhibited similar tendency for FFVMDS and FAV of different gases, while the FFVMDS and FAV values of HDF membranes were higher than those of MDF membranes F


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Figure 3. Effects of average free volume radius and (FAV)CO2 on CO2 permeability of copolyimide membranes containing different diamine structures and contents: (a) MDF; (b) HDF. (Note: * represents average free volume radius. Lines are to guide).

Figure 4. Effect of the FAV ratios of CO2 to light gas (N2 and CH4) on selectivity of copolyimide membranes containing different diamine structures and contents: (a) MDF; (b) HDF. Lines are to guide.

than that of MDF membranes. At the same time, HDF membrane exhibited excellent CO2/inert gas selectivity. HDF membrane contains the diamine monomer HFBAPP with ether groups and −CF3 groups. Ether groups possess strong CO2-philic property, and −CF3 groups have a tetrapole−dipole interaction with CO2 resulting in a higher CO2 solubility coefficient of HDF membranes. Also, the FAV of HDF membrane was higher than that of MDF membrane as shown in Table 5, which implied that HDF membrane would have greater gas diffusion coefficient. HDF membranes showed much higher CO2 permeability than MDF membrane as listed in Table 6. The higher CO2 permeability can be attributed to the high FFV of HDF membranes, while the excellent selectivity of HDF membranes may be aroused from the good affinity with CO2. Therefore, the HDF membrane combining HFBAPP with DAB18C6 performed better gas permselectivity than MDF membrane. Comparing the different feed gas mixtures, we find that the synthesized copolyimide membranes possessed higher CO2/ CH4 selectivity than CO2/N2. Luo et al. has confirmed that the gas diffusivity has a significant impact on gas transport.25 CH4 has the larger molecular size (kinetic diameter 3.80 Å) than N2 (kinetic diameter 3.64 Å), which leads to low gas diffusivity. Therefore, the copolyimide membranes displayed lower CH4 permeance and higher CO2/CH4 selectivity. In addition, the CO2/N2 and CO2/CH4 selectivities of copolyimide membranes in this work were much larger than that of reported polyimide membranes23,27,28 and increased gradually with increasing crown ether content presented in Table 6.

with the same crown ether content. This can be attributed to the −CF3− groups, which act as the chain-spacer. The higher FAV value indicates that the HDF membrane may have higher gas permeability. Moreover, the FAV ratios of CO2 to N2 (CH4) of HDF membranes increased with crown ether content. By contrast, the FAV ratios of CO2 to N2 (CH4) of HDF membranes were slightly lower than those of MDF membranes with the same crown ether content, which means that HDF membranes may possess lower selectivity. 3.3. Analysis of Gas Separation Performances. The HDF copolyimide membranes were tested in gas separation experiments with CO2/N2 and CO2/CH4 mixtures at 35 °C and 20 bar. The results are presented in Table 6. The effects of operating temperature and feed pressure on the HDF31 membranes’ separation performance were also studied at 25− 65 °C and 6−30 bar, respectively, and the results are shown in the Supporting Information (Figures S4 and S5). As shown in Table 6, gas permselectivities of MDF and HDF copolyimide membranes exhibited similar trends with crown ether content’s increasing. The permeabilities of CO2, N2, and CH4 decreased following MDF31 > MDF11 > MDF13 and HDF31 > HDF11 > HDF13. However, the selectivities of CO2/N2 and CO2/CH4 increased with an increased crown ether content, which is attributed to the enhancement of membrane rigidity due to the increase of chain stiffness and bulky nature.50 Compared with MDF membranes, the HDF membranes with HFBAPP exhibited superior gas permeability. The CO2 permeability of HDF membranes was 24.5%−78.2% higher G


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can not only effectively enhance free volume of membrane but also control free volume distribution. This is helpful to prepare membranes with both high permeability and high selectivity. And the membrane permselectivity is in good agreement with the MDS and PALS results. Therefore, the PALS and MDS results can assist effectively in understanding the correlations of membranes free volume and gas permselectivity. The crown ether-containing copolyimide membranes showed remarkable gas separation performance for both CO2/N2 and CO2/CH4 mixed gas. The performance is compared with the 2008 Robeson’s upper bounds illustrated in Figure 6.10 Outstanding CO2 separation performance was observed for HDF membranes derived from combination of HFBAPP and DAB18C6 diamines. Their CO2 permeability and CO2/N2 and CO2/CH4 selectivities surpassed the 2008 Robeson’s upper bounds. Although MDF membranes showed relatively low CO2 permeability, their gas separation properties were still comparable for CO2/N2 and CO2/CH4 mixed gases approaching and exceeding the 2008 Robeson’s upper bounds, respectively. Compared to the reported PIM-PI,51,52 KAUSTPI,53 and 6FDA-based PI15,54−61 membranes, the MDF and HDF membranes exhibited highest CO2/N2 and CO2/CH4 selectivities. HDF31 showed 1.8-fold higher CO 2 /CH4 selectivity (76.16 vs 43) and 14-fold higher CO2 permeability (149.52 vs 10.7 barrer) than the commercial polyimide membrane material (Matrimid), making it a potential candidate material for membrane-based CO2/CH4 separation in natural gas and biogas applications.62

The permselectivity of copolyimide membranes developed in this work is in good agreement with the PALS and MDS results. Gas transport behaviors in glassy polymers are mainly dependent on pore size and number of free volume. Free volume has previously been shown to have a direct relation with the membrane gas permeability.43 As shown in Figure 3, with the decreasing DAB18C6 content, the FAV values of CO2 for MDF and HDF copolyimide membranes increased, which led to the improvement of CO2 permeability. And the CO2 permeability is also in accordance with the average free volume radius measured by PALS for MDF and HDF membranes. With the increase of average free volume radius, the CO2 permeability was enhanced gradually. The average free volume radii of MDF31 and MDF11 were similar, but I3 of MDF31 was much larger than that of MDF11. Hence, CO 2 permeability of MDF31 membrane was larger than that of MDF11 membrane. In addition, the FAV ratios of CO2 to N2 (CH4) are vital for the gas selectivity. As can be seen from Figure 4, the FAV ratios of CO2 to N2 (CH4) increased gradually with an increase of crown ether content. Correspondingly, CO2/N2 and CO2/ CH4 selectivities were improved continuously. The FAV ratio of CO2 to CH4 was greater than the FAV ratio of CO2 to N2 due to the larger kinetic diameter of CH4. Therefore, MDF and HDF membranes possessed better CO2/CH4 selectivity than CO2/N2. Furthermore, the CO2/N2 and CO2/CH4 selectivities of HDF membranes showed an opposite change with the average free volume radius as shown in Figure 5. With the increase of

4. CONCLUSIONS In this work, an intensive study, with respect to the effects of diamine structures (MDA and HFBAPP) and contents on free volume and gas transport properties of crown ether-containing copolyimide membranes, was conducted to elucidate the correlations between free volume and gas transport properties. PALS measurement and MDS were performed to investigate free volume characterization, such as average free volume radius, FFVMDS, and FAV. PALS measurements revealed unimodal microcavity with average free volume radius of 2.755−3.102 Å for HDF membranes, and the average free volume radius of HDF membranes was larger than that of previous MDF. MDS data indicated that FFVMDS and FAV of CO2, N2, and CH4 decreased with crown ether content, while the FAV ratios of CO2 to N2 (CH4) increased. When the crown ether content was certain, FFVMDS and FAV of HDF membranes were larger and FAV ratios of CO2 to N2 (CH4) were smaller than MDF membranes. The gas separation performance was investigated based on CO2/N2 and CO2/CH4 mixed gas. With increasing DAB18C6 content, the CO2 permeability decreased, and selectivities of CO2 over N2 (CH4) increased for MDF and HDF membranes. The HDF membranes with HFBAPP exhibited superior 24.5− 78.2% higher than MDF membranes with MDA. This result indicated that introducing diamine monomer with ether groups and −CF3 groups into crown ether-containing copolyimide membranes can effectively enhance CO2 permeability of membrane. The permselectivity of copolyimides is in good agreement with the PALS and MDS results. With the increase of DAB18C6 content, the FFVMDS and average free volume radius decreased resulting in a significant decrease in CO2 permeability and an increase in CO2/N2 and CO2/CH4 selectivities. Although the MDF membranes possessed higher CO2/CH4 selectivity than HDF, the CO2 permeability was

Figure 5. Effect of average free volume radius on selectivity of HDF copolyimide membranes containing different diamine contents. Lines are to guide.

crown ether content, the average free volume radius decreased from 3.102 to 2.755 Å, which resulted in a significant increase of CO2/N2 and CO2/CH4 selectivities. Chung’s group synthesized 6FDA-based polyimide membranes presenting the same phenomenon between average free volume radius and selectivity. The average free volume radius was confirmed by PALS ranging from 3.75 to 4.06 Å with CO2/CH4 selectivity from 21.87 to 16.4.23,28 The results also showed an inverse proportional relationship between average free volume radius and selectivity. Compared with Chung’s work, the average free volume radius in this work was much smaller, and the selectivity was much larger. In this study, it is very clear that introducing different contents of diamine monomer with ether groups and −CF3 groups into crown ether-containing copolyimide membranes H


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Figure 6. Correlations between CO2 permeability and selectivity of synthesized copolyimide membranes containing different diamine structures and contents and reported PI membranes with 2008 Robeson’s upper bounds:10 (a) CO2/N2 mixture; (b) CO2/CH4 mixture. (2) Galizia, M.; Chi, W. S.; Smith, Z. P.; Merkel, T. C.; Baker, R. W.; Freeman, B. D. 50th anniversary perspective: Polymers and mixed matrix membranes for gas and vapor separation: A review and prospective opportunities. Macromolecules 2017, 50 (20), 7809−7843. (3) Capellán-Pérez, I.; Arto, I.; Polanco-Martínez, J. M.; GonzálezEguino, M.; Neumann, M. B. Likelihood of climate change pathways under uncertainty on fossil fuel resource availability. Energy Environ. Sci. 2016, 9 (8), 2482−2496. (4) Baker, R. W. Membrane technology and applications, 3rd ed.; John Wiley and Sons Ltd.: Oxford, UK, 2012; p 359. (5) Sholl, D. S.; Lively, R. P. Seven chemical separations to change the world. Nature 2016, 532, 435−437. (6) Ghanem, B. S.; McKeown, N. B.; Budd, P. M.; Selbie, J. D.; Fritsch, D. High-performance membranes from polyimides with intrinsic microporosity. Adv. Mater. 2008, 20 (14), 2766−2771. (7) Baker, R. W.; Lokhandwala, K. Natural gas processing with membranes: An overview. Ind. Eng. Chem. Res. 2008, 47, 2109−2121. (8) Bernardo, P.; Drioli, E.; Golemme, G. Membrane gas separation: A review/state of the art. Ind. Eng. Chem. Res. 2009, 48, 4638−4663. (9) Robeson, L. M. Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 1991, 62, 165−185. (10) Robeson, L. M. The upper bound revisited. J. Membr. Sci. 2008, 320 (1−2), 390−400. (11) Hillock, A. M. W.; Koros, W. J. Cross-linkable polyimide membrane for natural gas purification and carbon dioxide plasticization reduction. Macromolecules 2007, 40, 583−587. (12) Shrimant, B.; Dangat, Y.; Kharul, U. K.; Wadgaonkar, P. P. Intrinsically microporous polyimides containing spirobisindane and phenazine units: Synthesis, characterization and gas permeation properties. J. Polym. Sci., Part A: Polym. Chem. 2018, 56 (7), 766−775. (13) Weidman, J. R.; Luo, S.; Doherty, C. M.; Hill, A. J.; Gao, P.; Guo, R. Analysis of governing factors controlling gas transport through fresh and aged triptycene-based polyimide films. J. Membr. Sci. 2017, 522, 12−22. (14) Luo, S.; Stevens, K. A.; Park, J. S.; Moon, J. D.; Liu, Q.; Freeman, B. D.; Guo, R. Highly CO2-selective gas separation membranes based on segmented copolymers of poly(ethylene oxide) reinforced with pentiptycene-containing polyimide hard segments. ACS Appl. Mater. Interfaces 2016, 8 (3), 2306−2317. (15) Ma, X.; Abdulhamid, M.; Miao, X.; Pinnau, I. Facile synthesis of a hydroxyl-functionalized Tröger’s base diamine: A new building block for high-performance polyimide gas separation membranes. Macromolecules 2017, 50 (24), 9569−9576. (16) Wang, Z.; Wang, D.; Zhang, F.; Jin, J. Tröger’s base-based microporous polyimide membranes for high-performance gas separation. ACS Macro Lett. 2014, 3 (7), 597−601. (17) Tong, H.; Hu, C.; Yang, S.; Ma, Y.; Guo, H.; Fan, L. Preparation of fluorinated polyimides with bulky structure and their gas separation performance correlated with microstructure. Polymer 2015, 69, 138−147.

quite low. Overall, the combination of HFBAPP and DAB18C6 exhibited better permselectivity than the combination of MDA and DAB18C6. The whole copolyimide membranes displayed competitive gas separation performance, especially for CO2/CH4 mixed gas, which exceeded 2008 Robeson’s upper bound, making the synthesized copolyimide membrane materials show application potential for natural gas and biogas purification.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02502. TGA, DTG, and XRD of HDF copolyimide membranes; permeability coefficients and separation factors of HDF31 copolyimide membrane versus feed pressure for CO2/N2 (20/80 by volume) and CO2/CH4 (10/90 by volume) mixed gases; the temperature dependence of permeability of HDF31 copolyimide membrane for CO2/N2 mixed gas (20/80 by volume) and (b) CO2/ CH4 mixed gas (10/90 by volume); solubility and mechanical properties of HDF copolyimide membranes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-29-82668789. ORCID

Chunhai Yi: 0000-0002-3570-4353 Zongli Xie: 0000-0002-4610-0758 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This support provided by the National Key R&D Program of China (2017YFB0603402), the National Natural Science Foundation of China (21576217), and the Fundamental Research Funds for the Central Universities (xjj2016044). The calculations were performed by using supercomputers at the Shen-Zhen Cloud Computing Centre. C.M.D. is supported by the Australian Research Council (DE140101359) and a Veski Inspiring Women Fellowship.

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A Crown Ether-Containing Copolyimide Membrane with Improved

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