Synthesis and Gas Transport Properties of Hydroxyl-Functionalized

Apr 26, 2012 - A newly designed diamine monomer, 3,3,3′,3′-tetramethyl-1,1′-spirobisindane-5,5′-diamino-6,6′-diol, was successfully used to ...
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Synthesis and Gas Transport Properties of Hydroxyl-Functionalized Polyimides with Intrinsic Microporosity Xiaohua Ma, Raja Swaidan, Youssef Belmabkhout, Yihan Zhu, Eric Litwiller, Mustapha Jouiad, Ingo Pinnau,* and Yu Han* Advanced Membranes and Porous Materials Center, Chemical and Life Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia S Supporting Information *

ABSTRACT: A newly designed diamine monomer, 3,3,3′,3′tetramethyl-1,1′-spirobisindane-5,5′-diamino-6,6′-diol, was successfully used to synthesize two types of polyimides for membrane-based gas separation applications. The novel polymers integrate significant microporosity and polar hydroxyl groups, showing the combined features of polymers of intrinsic microporosity (PIMs) and functional polyimides (PIs). They possess high thermal stability, good solubility, and easy processability for membrane fabrication; the resulting membranes exhibit good permeability owing to the intrinsic microporosity introduced by the highly contorted PIM segments as well as high CO2/CH4 selectivity that arises from the hydroxyl groups. The membranes show CO2/CH4 selectivities of >20 when tested with a 1:1 CO2/CH4 mixture for feed pressures up to 50 bar. In addition, the incorporation of hydroxyl groups and microporosity in the polymers enhances their affinity to water, leading to remarkable water sorption capacities of up to 22 wt % at 35 °C and 95% relative humidity.

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BET surface area.13 Specifically, microporous poly(1-trimethylsilyl-1-propyne) (PTMSP) exhibits the highest gas and organicvapor permeabilities of all known glassy polymers to date.14 In addition, PTMSP shows the highest mixture organic-vapor/ permanent-gas selectivities of all currently existing polymers.15,16 Recently, a new class of polymers of intrinsic microporosity (PIMs) was reported.17,18 These materials contain contorted ladder-like backbones consisting of spiro-centers and rigid fused dioxane rings that prevent close polymer chain packing. The ladder-type PIMs exhibit excellent gas separation performance because their molecular structure gives rise to remarkable microporosity in a suitable pore size range of 6−10 Å, combining high permeability with moderate selectivity for separation of permanent gases, such as O2/N2 and CO2/ N2.19−22 A few mixed-gas permeation studies also confirmed that ladder-type PIM-1 is preferentially permeable to higher hydrocarbons such as n-butane over methane or hydrogen with selectivities similar to those of PTMSP.11,23,24 Since the first ladder-type PIMs were reported, much effort has been made to enhance their performance,25−32 and a successful example is the tetrazole-functionalized PIM-1 (TZPIM) membrane that exhibits very high CO2 permeability and good CO2/N2 selectivity even under typical polymer plasticization condi-

INTRODUCTION High-performance polymeric gas separation membranes have important applications in hydrogen recovery, natural gas purification, on-site nitrogen production from air, and carbon dioxide capture/separation.1,2 Ideal membranes should be both highly permeable and selective. However, there is an inherent trade-off between permeability (P) and selectivity (α), as illustrated by the Robeson upper-bound relationship that is frequently updated according to the performance of state-ofthe-art membranes.3,4 Rational molecular design of polymeric materials is the key to creating new membranes with superior gas separation properties.5,6 To enhance gas permeability, an effective strategy is to introduce intrinsic micropores into highly rigid, glassy polymers that can increase both gas solubility and gas diffusivity.6 Polymers with intrinsic microporosity have been known since the development of disubstituted polyacetylenes in the 1980s.7,8 These highly rigid glassy polymers contain alternating double bonds in the main chain and bulky side-chain substituents that prevent efficient chain packing, leading to high free volume. As a result, they exhibit the following general properties: (i) extremely high gas permeability and low selectivity for separation of small permanent gases;8,9 (ii) extremely high organic vapor permeability and high organic-vapor/permanent-gas selectivity;10 (iii) blocking of permanent gases by preferential sorption of condensable organic vapors in gas mixture permeation experiments;11 (iv) negative activation energy of permeation (e.g., increase in permeability with decreasing temperature);12 and (v) very high © XXXX American Chemical Society

Received: March 18, 2012 Revised: April 17, 2012

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Scheme 1. Synthetic Procedure and Molecular Structures of Two OH-Functionalized PIM-PIsa

a Reagents and conditions: (i) methylsulfonic acid, 135 °C, 5 h; (ii) HNO3 (3N, 2.0 equiv), HAc, 12 h, room temperature; (iii) dichlorotin, HCl, methanol, reflux for 6 h; (iv) PMDA/6FDA, pyridine, toluene, reflux for 12 h.

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tions.31 However, most of the modifications were achieved by post-treatment of PIM-1 with functional groups limited to carboxyl, tetrazole, and thioamide.32−34 The use of specially designed monomers containing functional groups is another tool to enhance the gas separation properties of spiro-based ladder-type PIMs. For example, Du et al. reported the synthesis of disulfone-based PIM homo- and copolymers.26 As commonly used polymers for membrane-based gas separation, polyimides (PIs) are characterized by high thermal and mechanical stability, good solution processability, high glass-transition temperatures, and low dielectric constants.35 Moreover, PIs can be easily functionalized to adjust their affinities to different gases. Previous work demonstrated that the introduction of hydroxyl groups into PI membranes significantly increases the solubility of CO2, leading to materials with some of the highest CO2/CH4 selectivity in polymer membranes.36,37 Recently, the concept of combining the advantages of PIMs and PIs has attracted much attention. Spiro-centers were introduced into polyimide main chains for the first time in 2007,38,39 and shortly thereafter PIM−PI membranes were reported that exhibited significantly increased permeability compared to conventional PI membranes.40,41 In this study, we designed and synthesized a novel diamine monomer 3 containing both hydroxyl groups and aromatic segments with a bicyclic quaternary carbon spiro-center, from which two types of hydroxyl-functionalized microporous polyimides were prepared for membrane-based gas separation (Scheme 1). Both membrane types exhibit excellent performance for CO2/CH4 separation. In addition, because of their hydrophilic hydroxyl groups, the polymers show very high water vapor sorption capacity, making them potentially useful candidate materials for membranes in gas dehydration applications.

EXPERIMENTAL SECTION

Materials. The monomers 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA, 99%) and pyromellitic dianhydride (PMDA, 97%) were obtained from Aldrich and purified by sublimation before use. N-Methylpyrrolidone (NMP) was refluxed over P2O5 for 4 h before vacuum distillation and, thereafter, stored over 4 Å molecule sieves. Pyridine was distilled freshly prior to use. Bisphenol A, methylsulfonic acid, dichlorotin hydrochloride solution, sodium carbonate, dichloromethane, ethyl acetate, ligroin, and nitric acid were obtained from Aldrich and used as received. Characterization Methods. NMR spectra of the newly synthesized monomer and polymers were recorded with a Bruker AVANCE-III spectrometer at a frequency of 500 MHz using a cryo probe, in either deuterated chloroform or deuterated dimethylsulfone with tetramethylsilane as an internal standard. Elemental analysis was carried out using a Thermo Flash EA2000 elemental analyzer. Mass spectroscopy (MS) was conducted on a Thermo LC/MS system with LTQ Orbitrap Velos detectors. Fourier transform infrared (FT-IR) spectra were acquired using a Thermo Nicolet iS10 infrared microspectrometer. Molecular weights and molecular weight distribution of the polymers were obtained by gel permeation chromatography (Agilent GPC 1200) with polystyrene as external standard. Singlecrystal X-ray diffraction was carried out on an Oxford supernova single-crystal X-ray diffractometer at liquid nitrogen temperature (77 K). X-ray scattering was conducted on a Bruker D8 Advance diffractometer with a scanning rate of 0.5°/min. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out under a N2 atmosphere using Netzsch TG 209 F1 and DSC 204 F1 Phoenix systems, respectively. Argon sorption isotherms were recorded at 87 K (Micrometrics ASAP 2420) after degassing the samples at 200 °C for 24 h. Water vapor adsorption isotherms were obtained using a TA step isotherm vapor adsorption instrument (VTISA) at 35 °C; the relative humidity was gradually increased by 10% after sorption equilibrium was reached for each data point. Synthesis of 3,3,3′,3′-Tetramethyl-1,1′-spirobisindane-6,6′diol (1). Bisphenol A (100 g, 0.439 mmol) and methanesulfonic acid B

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(10 mL) were mixed in a reaction flask and heated to 135 °C for 4 h. The resulting brown sticky oil was poured into water (2000 mL) under rigorous stirring. Finally, white needle-like crystals (25.0 g, yield: 55.5%) were obtained after recrystallization from mixed solvent of ethanol/water (40/60, w/w). TLC: dichloromethane/ligroin = 1/4, Rf = 0.25. 1H NMR (500 MHz, DMSO-d6): δ 8.99 (s, 2H), 7.00 (d, J = 8.4 Hz, 2H), 6.60 (dd, J = 8.4 Hz, 2.4 Hz, 2H), 6.10 (d, J = 2.0 Hz, 2H), 2.27 (d, J = 12.8 Hz, 2H), 2.11 (d, J = 12.8 Hz, 2H), 1.31 (s, 6H), 1.25 (s, 6H). HRMS: calcd for C21H20O4: 336.1362; found: 336.1365. 3,3,3′,3′-Tetramethyl-1,1′-spirobisindane-5,5′-dinitrol-6,6′diol (2). Compound 1 (308 mg, 1.0 mmol) was dissolved in acetic acid (10 mL), to which a mixed solution of HNO3 (4 N, 2.1 equiv, 0.53 mL) and acetic acid (5.0 mL) was added dropwise. The mixed slurry was stirred overnight and then cooled to 4 °C before filtration. Light yellow solid was obtained (0.159 g, yield 40%) by column separation. TLC:ethyl acetate/ligroin = 1/4, Rf = 0.7. 1H NMR (500 MHz, CDCl3): δ 10.60 (s, 2H), 7.91 (s, 2H), 6.53 (s, 2H), 2.43 (d, J = 13.2 Hz, 2H), 2.28 (d, J = 13.3 Hz, 2H). 13C NMR (125 MHz, CDCl3): δ 160.5, 155.3, 145.0, 133.5, 118.6, 114.8, 58.6, 58.2, 43.4, 31.6, 30.1. HRMS: calcd for C21H21N2O6− [M − H]−: 397.1405; found: 397.1402. Anal. Calcd for C22H24Cl2N2O6: C, 54.67; H, 5.00; N, 5.80. Found: C, 54.66; H, 4.87; N, 5.70. 3,3,3′,3′-Tetramethyl-1,1′-spirobisindane-5,5′-diamino-6,6′diol (3). Intermediate 2 (398 mg, 1.00 mmol) was added into a mixed solution of methanol (20 mL) and THF (20 mL). SnCl2 (aq, 1.0 mL) and HCl (12 N, 1 mL) were then added, and thereafter, the mixture was heated to reflux for 12 h. After the yellowish solution became colorless, ethyl acetate (40 mL) and water (40 mL) were added and, thereafter, potassium carbonate was used to adjust the pH to 9. The light yellow organic layer was separated from the solids by centrifugation, dried with magnesium sulfate, and then diluted with 100 mL of dichloromethane. Finally, white crystals (320 mg, yield 95%) were obtained by reprecipitation in ethyl acetate/dichloromethane and drying under vacuum at 80 °C. TLC: ethyl acetate/ ligroin = 1/1, Rf = 0.5. 1H NMR (500 MHz, DMSO-d6): δ 8.59 (s, 2H), 6.30 (s, 2H), 5.96 (s, 2H), 4.26 (s, 4H), 2.06 (d, J = 12.8 Hz, 2H), 1.91 (d, J = 12.8 Hz, 2H), 1.18 (s, 6H), 1.13 (s, 6H). 13C NMR (125 MHz, DMSO-d6): δ 144.2, 142.6, 139.2, 135.9, 109.6, 107.5, 60.2, 56.7, 42.8, 32.1, 31.1. HRMS: calcd for C21H27N2O2+ [M + H]+: 339.2067; found: 339.2067. Anal. Calcd for C21H26N2O2: C, 74.52; H, 7.74; N, 8.28. Found: C, 75.00; H, 7.86; N, 8.45. Synthesis of PIM-6FDA-OH. In a 50 mL three-neck round bottle flask equipped with a magnetic stirrer, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (444.2 mg, 1.00 mmol) was added in portions to the solution of 3 (338.4 mg, 1.00 mmol, in 5.0 mL of absolute NMP). After stirring at room temperature for 4 h, 1.0 mL of pyridine and 4.0 mL of toluene were added, and the mixture was then heated to 160 °C under N2. During the reaction, water was removed by a Dean-Stark trap. By repeated precipitation in methanol/water (1/1, v/v) and then drying under vacuum at 120 °C for 12 h, a light yellow powder (680 mg, yield 91%) was collected. 1H NMR (500 MHz, DMSO-d6): δ 9.60 (s, 2H), 8.14 (d, J = 7.8 Hz, 2H), 7.95 (s, 2H), 7.75 (s, 2H), 7.14 (s, 2H), 6.41 (s, 2H), 2.35 (s, 2H), 2.23 (s, 2H), 1.34 (s, 6H), 1.28 (s, 6H). FT-IR (powder, ν, cm−1): 3100−3500 (br, str O−H), 2955 (str, aromatic C−H), 2929, 2862 (str, aliphatic C−H), 1785 (asym str of imide CO), 1722 (sym str of imide CO), 1381 (str of imide C− N). Anal. Calcd for C, 64.17; H, 4.04; N, 3.74. Found: C, 64.40; H, 4.24; N, 3.54. Molecular weight: Mn = 8.54 × 104, Mw = 1.65 × 105, PDI = 1.94; onset decomposition temperature: Td = 380 °C; BET surface area = 225 m2/g; density = 1.22 g/cm3; fractional free volume (FFV) 0.23. Synthesis of PIM-PMDA-OH. The synthetic method of PIMPMDA-OH was the same as that used for PIM-6FDA-OH. The polymer was obtained as a dark yellow powder with a yield of 95%. 1H NMR (500 MHz, CDCl3): δ 9.72 (s, 2H), 8.36 (s, 2H), 7.12 (s, 2H), 6.48 (s, 2H), 2.41 (s, 2H), 2.28 (s, 2H), 1.39 (s, 6H), 1.33 (s, 6H). FTIR (powder, ν, cm−1): 3100−3500 (br, str O−H), 2955 (str, aromatic C−H), 2929, 2862 (str, aliphatic C−H), 1779 (asym str of imide C O), 1722 (sym str of imide CO), 1377 (str of imide C−N). Anal. Calcd for C, 71.25; H, 5.02; N, 5.36. Found: C, 71.42; H, 5.26; N,

5.10. Molecular weight: Mn = 1.3 × 105, Mw = 3.56 × 105, PDI = 2.74; Td = 380 °C; BET surface area: SBET = 190 m2/g; density = 1.18 g/ cm3; fractional free volume (FFV) = 0.20. Membrane Fabrication. The polymers were dissolved in THF (2−3% w/v, g/mL) and then purified using small 1.0 μm PTFE filter cartridges. The solution was carefully transferred into a stainless steel ring supported by a leveled glass plate; thereafter, the solvent was evaporated in an oven at 45 °C. After 2 days, the dry membranes (∼80 to 100 μm thick) were soaked in a mixture of n-hexane/dichloromethane (90/10) for 24 h, air-dried, and then heated at 120 °C for 24 h under high vacuum. The densities of the polymers were determined gravimetrically by their measured weight, area, and film thickness. The fractional free volume of the polymers was calculated by the equation FFV =

Vsp − 1.3Vw Vsp

where Vsp is the specific volume of the polymer determined by density measurement and Vw is the van der Waals volume calculated from Bondi’s group contribution method.42 Pure-Gas Permeation Measurements. The gas permeability of the membranes was determined using the constant-volume/variablepressure method.43 The membranes were degassed in the permeation system on both sides under high vacuum at 35 °C for at least 24 h. The increase in permeate pressure with time was measured by a MKS Baratron transducer. The permeability of all gases except CO2 (1 bar) was measured at 2 bar at 35 °C and was calculated by44 P = DS = 1010

Vdl dp pup TRA dt

where P is the permeability (barrers) (1 barrer =10−10 cm3 (STP) cm/ (cm2 s cmHg)), pup is the upstream pressure (cmHg), dp/dt is the steady-state permeate-side pressure increase (cmHg/s), Vd is the calibrated permeate volume (cm3), l is the membrane thickness (cm), A is the effective membrane area (cm2), T is the operating temperature (K), and R is the gas constant (0.278 cm3 cmHg/(cm3 (STP) K)). The apparent diffusion coefficient D (cm2/s) of the polymer membrane was calculated by D = l2/6θ, where l is the membrane thickness and θ is the time lag of the permeability measurement. The solubility coefficient S (cm3 (STP)/(cm3 cmHg)) was obtained from the relationship S = P/D. Mixed-Gas Permeation Measurements. The mixed-gas permeation properties of the OH-functionalized spiro-polyimides were measured at 22 °C using a setup as previously described by O’Brien et al.45 The feed gas mixture was CO2/CH4 in a molar ratio of 1:1; and the pressure was varied between 1 and 50 bar. The stage cut, that is, the permeate flow rate to feed flow rate, was set at 0.01. Under these conditions, the residue composition was essentially equal to that of the feed gas. CO2 and CH4 concentrations in the permeate were detected with a gas chromatograph (Agilent 3000A Micro GC) equipped with a thermal conductivity detector. The mixed-gas permeability was determined by

PCO2 = 1010

PCH4 = 1010

yCO Vdl

dp 2 xCO2pup ART dt

yCH Vdl

dp 4 xCH4pup ART dt

where y and x are the mole fractions in the permeate and feed, respectively. The CO2/CH4 selectivity was obtained from

αCO2 /CH4 =

yCO /yCH 2

4

xCO2/xCH4

Gas Solubility Measurements. High-pressure pure-gas sorption measurements were performed with N2, CH4, and CO2 at 35 °C using a gravimetric apparatus (Rubotherm, Bochum, Germany).46 The sorption isotherms were determined up to 25 bar. The gas solubility C

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(S) in a given membrane at a specific pressure was calculated from the gas uptake, mass, and density of the membrane.

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RESULTS AND DISCUSSION The newly designed diamine monomer 3 was synthesized from bisphenol A through three successive reactions (Scheme 1): (i) the rearrangement of two bisphenol A molecules to obtain 3,3,3′,3′-tetramethyl-1,1′-spirobisindane-6,6′-diol (1);47 (ii) the nitration reaction of 1 to yield 3,3,3′,3′-tetramethyl-1,1′spirobisindane-5,5′-dinitrol-6,6′-diol (2); and (iii) the reduction of 2 to produce the spiro diamine monomer with o-OH group 3 . P y r o m e l l i t ic d i a n h y d r i d e ( P M D A ) a n d 4 , 4 ′ (hexafluoroisopropylidene)diphthalic anhydride (6FDA) were selected to react with 3 for polymerization via solvothermal azeotropic imidization reaction (detailed synthetic procedures can be found in the Experimental Section). In this way, two hydroxyl-functionalized PIM-structured polyimides (PIM-PI) were obtained, which are denoted as PIM-PMDA-OH and PIM-6FDA-OH, respectively (Scheme 1). The intrinsic cavities in these PIM-PI polymers formed by inefficient intermolecular chain packing can be understood from the free volume observed in the crystals of intermediate 2 via single-crystal Xray crystallography (Figure 1). The cavities arising from the

Figure 2. 1H NMR spectra of PIM-6FDA-OH and PIM-PMDA-OH (DMSO-d6, 500 MHz).

and NMP, the PIM-PIs reported in this study can be easily dissolved in most common organic solvents including THF, acetone, and ethyl acetate due to the presence of OHcontaining spiro centers38 (Supporting Information Table S1). The two OH-functionalized PIM-PI polymers are thermally stable, having similar onset decomposition temperatures (Td) of 380 °C, as compared to PIM-1 (Table 1 and Figure 3). Online mass spectroscopy (MS) was used to monitor the thermal decomposition process of PIM-6FDA-OH. The results show that CO2 is first released at ∼400 °C, immediately followed by H2O at ∼420 °C (Figure 3), corresponding to the formation of benzoxazoles by chain rearrangement and decomposition of the polymer main chains, respectively.49 In addition, differential scanning calorimetry (DSC) analysis does not reveal an apparent glass transition temperature (Tg) before thermal decomposition (Figure S2, Supporting Information), as is characteristic of PIMs with rigid backbones.18 The OHfunctionalized PIM-PI polymers show typical type I Ar sorption isotherms that are featured by high uptake at very low relative pressure and indicate the presence of significant microporosity (Figure 4). An open hysteresis loop is formed by the irreversible adsorption and desorption branches, which is frequently observed in typical PIM polymers.17 The BET surface areas derived from the sorption isotherms are 225 m2/g for PIM-6FDA-OH and 190 m2/g for PIM-PMDA-OH (Table 1). These values are much lower than that of PIM-1 (BET surface area: 830 m2/g).20 In the wide-angle X-ray scattering patterns, the two OH-functionalized PIM-PI membranes both exhibit a broad halo centered at d ≈ 6.5 Å (Figure 5). The d spacing is usually considered to represent the distance between segments of different chains and related to the free volume and gas permeability of a polymer.50−53 In comparison with PIM6FDA−OH, PIM-PMDA-OH has an additional shoulder peak at d < 5 Å (Figure 5), suggesting the presence of smaller pores. This along with the lower surface area accounts for the comparatively lower gas permeability of PIM-PMDA-OH, as will be discussed below. To evaluate the gas separation performance of OHfunctionalized PIM-PI membranes, pure-gas permeabilities of H2, N2, O2, CH4, and CO2 were measured by the constant-

Figure 1. Crystal structure of 3,3,3′,3′-tetramethyl-1,1′-spirobisindane5,5′, dinitrol-6,6′-diol (2) in ball−stick model with the unit cell (CCDC No. of 838941). The structure is resolved by single crystal Xray diffraction. Red, gray, blue, white, and green balls represent oxygen, carbon, nitrogen, hydrogen, and chloride atoms, respectively. The spiro-centers are highlighted in yellow, and the trapped CH2Cl2 molecules are marked in the space-filling model.

rigid and contorted molecular structure are firmly locked by spiro-bisindane units with a torsion angle of 74.2°, which are able to trap solvent molecules with a kinetic diameter of 3.3 Å (i.e., dichloromethane in the structure shown in Figure 1). It can therefore be expected that the packing of long polymer chains can create three-dimensional microporosity with pore walls functionalized by OH groups. The presence of OH groups in the polymers is identified in FT-IR spectra (Figure S1, Supporting Information) and confirmed by 1H NMR spectra that exhibit strong aromatic hydroxyl resonances at ∼10.0 ppm (Figure 2), in accordance with previously reported o-OH-containing polyimides.48 High molecular weights of ∼1 × 105 g/mol with polydispersity indices (PDI) of 2−3 have been achieved for the two polymers, as determined by gel permeation chromatography (Table 1). Different from many conventional polyimides that are only soluble in strongly polar aprotic solvents such as DMF, DMSO, D

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Table 1. Properties of Functionalized 6FDA and PMDA OH-Based PIM Polymers and Reference PIM Polymers polymer

Mn (×104)a

Mw (×104)a

PDIa

SB (m2/g)b

Td (°C)c

ρd

FFVe

PIM-6FDA-OH PIM-PMDA-OH PIM-1f PIM-PI-3h

8.54 13 10.1 2.2

16.5 35.6 25.7 4.5

1.94 2.74 2.54 2.05

225 190 830 471

380 380 350g 420

1.22 1.18 1.07 1.26

0.23 0.20 0.26 0.23

a

Molecular weights and polydispersity index (PDI), measured by GPC using THF as solvent, and polystyrene as external standard. bBET surface area determined by Ar sorption at 87 K. cOnset decomposition temperature determined by TG analysis. dPolymer density. eFractional free volume of the polymer membrane calculated based on Bondi’s theory. fData from refs 19 and 21. gData from ref 18. hDate from ref 41.

Figure 5. Wide-angle X-ray scattering patterns of PIM-PMDA-OH and PIM-6FDA-OH polymer membranes.

volume/variable-pressure method. The permeability coefficient was calculated from the steady-state region of the permeate pressure−time curve, and the diffusion coefficient (D) was determined by the time-lag method (Tables 2 and 3). Owing to the microporosity introduced by the spiro structure in the polymers, the two OH-functionalized PIM-PI membranes are both more permeable than conventional OH-containing polyimides. For example, the CO2 permeability coefficients of PIM-6FDA-OH and PIM-PMDA-OH membranes are 263 and 198 barrers, respectively, whereas conventional OH-containing polyimides typically have CO2 permeabilities of less than 20 barrers.36,37 PIM-6FDA-OH is comparatively more permeable than PIM-PMDA-OH for all gases tested (Table 2) probably because of its larger surface area that is favorable for improving

Figure 3. Thermogravimetric analysis (TGA) of the two OHfunctionalized PIM-PIs and PIM-1 (top); TGA combined with mass spectroscopy of PIM-6FDA-OH (bottom).

Figure 4. Argon adsorption/desorption isotherms of PIM-PMDA-OH and PIM-6FDA-OH polymers measured at 87 K. E

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Table 2. Permeability and Selectivity of Various PIM Membranes for Different Gases at 35 °C permeability (barrers)a

ideal selectivity (α)

polymers

H2

N2

O2

CH4

CO2b

H2/N2

O2/N2

CH4/N2

CO2/N2

CO2/CH4

PIM-6FDA-OH PIM-PMDA-OH PIM-1c PIM-PI-3d

259 190 1300 360

10.8 6.9 92 23

45.2 30.5 370 85

9.1 7.7 125 27

263 198 2300 520

24 28 14 16

4.2 4.5 4.0 3.7

0.83 1.1 1.4 1.2

24 29 25 23

29 26 18 19

a 1 barrer = 10−10 cm3 (STP) cm cm−2 s−1 cm Hg−1 or 7.6 × 10−18 m3 (STP) m m−2 s−1 Pa−1. bPermeability of CO2 was obtained at the upstream pressure of 1 bar while those of other gases were measured at 2 bar. cData from ref 20. dData from ref 41.

Table 3. Diffusion Coefficient (D), Solubility Coefficient (S), Diffusion Selectivity (αD) and Solubility Selectivity (αS) for Different Gases of Various PIM Membranes D (10−8 cm2/s)

S (10−2 cm3/cm3 cmHg−1)

αD

αS

polymers

N2

CH4

CO2

N2

CH4

CO2

CO2/CH4

CO2/CH4

PIM-6FDA-OHa PIM-6FDA-OHb PIM-PMDA-OHa PIM-PMDA-OHb PIM-1c PIM-PI-3d

7.24 7.10 5.50 4.76 22 10

1.65 2.02 1.50 1.47 6.8 3

8.04 9.88 5.71 5.64 26 12

1.49 1.52 1.25 1.45 4.2 2.3

5.52 4.51 5.16 5.25 18 9.3

32.8 26.7 34.6 35.1 88 44

4.87 4.89 3.80 3.84 3.82 4.0

5.94 5.92 6.70 6.69 4.89 4.73

a

D is determined by constant volume time-lag method; S is deduced based on the equation P = DS. bS is measured by barometric sorption method; D is calculated from P = DS. cData from ref 20. dData from ref 41.

Figure 6. High-pressure gas (N2, CO2, and CH4) sorption isotherms of PIM-6FDA-OH (left) and PIM-PMDA-OH (right) at 35 °C.

1 (6.8 × 10−8 cm2/s) > PIM-PI-3 (3.0 × 10−8 cm2/s) > PIM6FDA-OH (1.65 × 10−8 cm2/s) > PIM-PMDA-OH (1.5 × 10−8 cm2/s), consistent with the order of their BET surface areas (Table 1). Despite the lower diffusion coefficients of OHfunctionalized membranes, they show comparable (PIMPMDA-OH) or higher (PIM-6FDA-OH) CO2/CH4 diffusion selectivity in comparison with PIM-1 and PIM-PI-3 (Table 3). On the other hand, the determining factors for the gas solubility in a membrane include the surface area of the membrane material as well as its affinity to gas molecules.54 Because CO2 molecules have a quadrupole moment arising from the strong dipole of the CO bonds, introducing polar OH groups can enhance the affinity of the polymers to CO2 molecules.36 As shown in Table 3, the OH-functionalized PIMPIs show rather high CO2 solubility coefficients, close to 50% of that of PIM-1, given their 4 times lower surface areas (Table 1). Accordingly, they exhibit increased CO2/CH4 solubility selectivity, as compared with previously reported PIMs and PIM-PIs (Table 3).41 Similar effects of enhancing CO2 selectivity were previously observed in OH-functionalized polyimides.36 In these cases, however, the incorporation of OH groups decreased the intrinsically moderate permeability of PIs and thus compromised the overall performance. Benefitting

both diffusivity and solubility as well as the presence of hexafluoroisopropylidene groups which reduces chain interactions. As summarized in Table 2, despite the smaller surface areas and, consequently, lower permeability, the OH-functionalized PIM-PI membranes have higher CO2/CH4 selectivity than conventional PIMs and PIM-PIs. For example, the measured CO 2 /CH 4 selectivity of the PIM-6FDA-OH membrane is 1.6 times higher that of PIM-1 (29 vs 18).20 Meanwhile, the performance of OH-functionalized PIM-PIs depends on the dianhydride type; PIM-6FDA-OH exhibits a higher permeability than PIM-PMDA-OH and also a slightly higher selectivity for CO2/CH4 separation (Table 2). The selectivity determined from pure-gas permeation measurements (αX/Y) involves the contributions from the solubility selectivity (SX/SY) and diffusion selectivity (DX/DY). To better understand the role of hydroxyl groups in increasing CO2/CH4 selectivity, diffusion coefficients (D) and solubility coefficients (S) of different membranes were measured using both the time-lag permeation method and high-pressure gravimetric sorption method (Figure 6). As shown in Table 3, the D and S values determined from the two methods are in excellent agreement. For a given gas (e.g., CH4), the diffusion coefficient values in different membranes follow the order PIMF

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bar. 56 The mixed-gas permeation results reinforce the conclusions obtained from the pure-gas tests; i.e., OHfunctionalized PIM-PI polymers of this study are more permeable than conventional low-free-volume PIs while more selective than unfunctionalized PIMs. For example, at a CO2 partial pressure of 10 bar, the CO2/CH4 selectivity of PIM-1 is about 12.57 The incorporation of hydroxyl groups in the PIM-PI polymers also enhances their affinity to water. As shown in Figure 8, the two OH-functionalized PIM-PIs exhibit

from the PIM segments in the molecular structures, the OHfunctionalized polymers reported in this study show increased selectivity while maintaining high permeability. Binary CO2/CH4 mixed-gas (1:1 molar mixture) permeation properties of the two OH-functionalized polymer membranes were determined at 22 °C, and the effect of CO2 partial pressure on CO2/CH4 selectivity was exploited (Figure 7). For

Figure 8. Water vapor sorption isotherms of the OH-functionalized PIM-PIs and PIM-1 collected at 35 °C.

significantly higher water sorption capacities than PIM-1. In particular, PIM-PMDA-OH gives a water vapor uptake of up to 300 cm3/cm3 polymer at 35 °C at 95% relative humidity, corresponding to 22 wt %. This value is significantly higher than those of conventional PIs (e.g., Matrimid 5218: < 50 cm3/ cm3;58 6FDA-Durene: ∼100 cm3/cm3 59) and is comparable to some highly polar sulfonated polyimides.60 There are two possible reasons for the relatively lower water uptake of PIM6FDA-OH: the lower density of OH groups in the polymer backbone and the presence of hydrophobic fluorine groups (Scheme 1). Because of its high water vapor sorption capacity, PIM-PMDA-OH has great potential as membrane material for air and natural gas dehydration.

Figure 7. Pure- and mixed-gas CO2/CH4 selectivity as a function of CO2 partial pressure. Measurements were carried out at 22 °C using a 1:1 molar CO2/CH4 mixture.

comparison, pure-gas CO2 and CH4 permeabilities were also measured at 22 °C over a pressure range of 1−15 bar (see Tables S2 and S4). Despite the competitive mixed-gas sorption effect55 that results in an approximate 10% (PIM-6FDA-OH) and 40% (PIM-PMDA-OH) drop in CO2 permeability as compared to the pure-gas test results, both polymers show high CO2/CH4 selectivity (>30) at low CO2 partial pressure of 20 (Figure 7 and Tables S3 and S5). This result indicates that the novel OH-functionalized spiro-polyimides of this study plasticize significantly less as compared to conventional 6FDAbased polyimides. For example, Staudt-Bickel and Koros showed that the mixed-gas CO2/CH4 selectivity of a 6FDAmPD polyimide membrane dropped drastically from 40 to less than 5 by increasing the CO2 partial pressure from 2 to 10

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CONCLUSIONS Two hydroxyl-functionalized PIM-PI polymers were successfully synthesized using newly designed 3,3,3′,3′-tetramethyl-1,1′spirobisindane-5,5′-diamino-6,6′-diol with 6FDA and PMDA, respectively. The novel polymers possess good solubility, processability, and high thermal stability and are more permeable than most conventional PIs owing to the intrinsic microporosity introduced by the OH-containing PIM segments. Because of the hydroxyl functionality in the spirobisindane unit, they show higher CO2/CH4 selectivity than previously reported PIMs. Both PIM-PMDA-OH and PIM6FDA-OH show better plasticization resistance as compared to many previously reported polyimides, as indicated by their high CO2 permeability and CO2/CH4 selectivity under highpressure mixed-gas conditions. In addition, PIM-PMDA-OH and PIM-6FDA-OH show remarkable water vapor sorption capacity, which makes them potential candidate membrane materials for gas dehydration applications. G

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra, DSC analysis, solubility, pure- and mixed-gas permeability and selectivity of the OH-functionalized PIM-PI polymers; CIF file of the intermediate compound 2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.P.); [email protected] (Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by KAUST baseline funding for Yu Han and Ingo Pinnau.

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