Amine-Functionalized MIL-53 MetalâOrganic Framework in Polyimide

Article pubs.acs.org/IECR

Amine-Functionalized MIL-53 Metal−Organic Framework in Polyimide Mixed Matrix Membranes for CO2/CH4 Separation Xiao Yuan Chen, Hoang Vinh-Thang, Denis Rodrigue,* and Serge Kaliaguine Department of Chemical Engineering, Université Laval, Quebec City, QC G1 V 0A6, Canada ABSTRACT: Flexible crystalline metal−organic framework (MOF) of nanosize particles (Al-MIL-53) and 6FDA-ODA polyimides were used to produce mixed matrix membranes (MMM). These MMM display excellent CO2/CH4 separation properties even without any compatibilization (amine grafting). The ideal selectivity and separation factor were found to increase with increasing MOF loading. Furthermore, these materials have an improved CO2/CH4 separation factor with increasing pressure, in contrast to traditional polymer membranes.

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INTRODUCTION Over the last decades, the scientific community tried to improve polymer membrane properties by dispersing inorganic particles such as zeolites and carbon molecular sieves (CMC) into different matrices.1−5 These heterogeneous membranes, namely mixed matrix membranes (MMM), were often found to have separation performance exceeding the upper-bound trade-off curve proposed by Robeson in 2008.6 MMM combine the potential advantages of both inorganic particles and polymer membranes such as superior permeability and selectivity of the former with good processability and mechanical properties of the latter. Furthermore, these hybrid membranes are mechanically more resilient, easier to produce, and provide lower manufacturing costs.3,7 Both zeolitic and nonzeolitic inorganic materials have been used as fillers to synthesize MMM. Nonzeolitic inorganic fillers are nonporous or porous materials including silica nanoparticles, alumina, non- and functionalized activated carbon, carbon molecular sieves, TiO2 nanoparticles, layered materials, and mesoporous molecular sieves.1−5 On the other hand, zeolitic inorganic particles are conventional zeolites, as well as AlPO- and SAPO-molecular sieves. Recently, researchers started applying new families of materials such as organic−inorganic hybrids, known as metal−organic frameworks (MOFs)8−12 and polymers with intrinsic microporosity (PIM),13−16 for MMM preparation. In this work, nonfunctionalized (Al-MIL-53) and amine-functionalized (Al-MIL-53-NH2) MOF nanosize particles were used as fillers, and 6FDA-ODA-based polyimides (PIs) as the continuous phase were used to prepare MOF-based mixed matrix membranes for CO2/CH4 separation. The separation performance of these MMM as a function of MOF loading in the polyimide matrix was investigated.

size, shape, and structure. Compared to other porous materials, which associate with only limited metals, MOFs accept almost all cations up to tetravalent. A large choice of organic linkers containing O or N donors such as carboxylates, phosphonates, sulfonates, cyanides, pyridine, imidazoles, gives rise to opportunities for functionalization and grafting. Moreover, the carbon subnetwork can itself be functionalized with halogeno- or amino groups. They are of special interest for catalysis, semiconductors, luminescence, magnetism, thin films, biological and medical applications, and especially for gas separation and storage.17−21 It was then just a short step until MOFs were incorporated into polymer membranes. Up to now, a limited number of MOF structures were used to prepare MMM.22 The first MOFbased MMM synthesis was reported by Yehia et al.23 using copper(II) biphenyl dicarboxylate-triethylenediamine filled in poly(3-acetoxyethylthiophene) or Matrimid polymer matrix. Since this pioneering paper, several types of MOF material were used to prepare MMM. Won et al.24 incorporated {[Cu2(PF6)(NO3)(4,4′bpy)4]·2PF6·2H2O}n into an amorphous glassy polysulfone (PSf) polymer to study the separation of H2/CH4 gas mixtures. Incorporation of MOF-5 or IRMOF-1 into Matrimid or Ultem polyimides increased permeabilities proportionally to loading for several gases such as H2, N2, O2, CO2, and CH4. The ideal and mixture selectivities remained, however, unchanged.25−27 Addition of Cu3(BTC)2 (BTC = benzene-1,3,5-tricarboxylate) in polydimethylsiloxane (PDMS) and polysulfone showed high permeability and selectivity for CO2 and H2, while Mn(HCOO)2 showed high sorption for H2. Higher loading reduced gas solubility, but increased permeability, indicating defective membranes with interfacial voids.28 Cu3(BTC)2 crystals were also embedded in different polymers such as Matrimid,27 Matrimid/ PSf,29 and poly(amic acid)30 to study the single gas performances of H2, O2, N2, CO2, and CH4, as well as the CO2/CH4 and CO2/ N2 selectivity. Zhang et al.31 reported an increase in gas permeabilites of Cu−BPY−HFS/Matrimid MMM (Cu−BPY− HFS = Cu−4,4′-bipyridine-hexafluororsilicate), but a decrease

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LITERATURE REVIEW MOFs have appeared as a new class of crystalline and porous materials formed by self-assembly of complex subunits comprising transition metal centers connected by various polyfunctional organic ligands to form one-, two-, and three-dimensional structures.8−12 These materials are chemically combined in many different ways, making it possible to reach interesting properties such as framework regularity, large surface areas, high porosities, low densities, and enormous flexibility in pore © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6895

February April 16, April 18, April 18,

17, 2012 2012 2012 2012

dx.doi.org/10.1021/ie3004336 | Ind. Eng. Chem. Res. 2012, 51, 6895−6906

Industrial & Engineering Chemistry Research

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result, breathing functionalized MIL-53-NH2 embedded in glassy polysulfone produced MMM exhibiting excellent separation properties, especially for CO2/CH4 selectivity. This was attributed to better hydrogen bonding between the amine and sulfone groups of both constituents at their interface. 6FDA-ODA polyimide containing 6FDA, (4,4′-(hexafluoroisopropylidene)-diphthalic anhydride) and ODA (4,4′-oxidianiline), display higher gas permeability due to the −C(CF3)2 group in the dianhydride and better selectivity due to the −O group in the diamine for CO2/CH4 separation.50−52 CO2 permeability was reported to be 18.6 barrer, and ideal selectivity was 53.1 at 35 °C and 150 psi of feed pressure. The separation factor for a mixed gas (50:50 CO2:CH4) is 48.9. Moreover, in a previous work, this polyimide was found to exhibit good compatibility with inorganic fillers (zeolite FAU/EMT) for CO2/CH4 separation.53

in ideal CO2/CH4 and H2/CO2 selectivities, probably due to creation of nonselective voids. Including MOP-18 into Matrimid polyimide increased transport properties for several gases such as H2, CO2, O2, N2, CH4, C3H6, and C3H8.32,33 A Cu−TPA/PVAc membrane (TPA = terephthalic acid, PVAc = poly(vinyl acetate)) showed improvement for gas performance over neat polymers.34 Composite membranes with a pure ZIF-8 crystal have been fabricated with Matrimid35,36 poly(1,4-phenylene ether−ether− sulfone) (PPEES)37 and 6FDA−DAM (6FDA = 2,2-bis (3,4carboxyphenyl) hexafluoropropane dianhydride, DAM = diaminomesitylene)38 polymers and showed high H2/CH4, CO2/CH4, C2H4/C2H6, and C3H6/C3H8 selectivities. The hybrid membranes fabricated from ZIF-7 nanoparticles and polybenzimidazole (PBI) exhibited characteristics of high transparency and mechanical flexibility, combined with enhanced H2 permeability and ideal H2/CO2 permselectivity surpassing both neat PBI and ZIF-7 membranes.39 Recently, using ZIF-90 particles, three kinds of ZIF90-based MMM have been synthesized using different polyimides such as Ultem, Matrimid, and 6FDA-DAM.40 Interestingly, these MMM showed much higher CO2 permeability (>700 barrer) than any other MOF-based MMM, without any loss in CO2/CH4 selectivity. Moreover, several studies used the models of Maxwell and Bruggeman to estimate the ideal gas selectivity of Zn(bdc)(ted)0.5 (bdc = benzene dicarboxylic acid, ted = triethylene diamine) confined within different types of polymers for CH4/ H2,41 CO2/H2, and CO2/CH441,42 separations, and the pure gas permeabilities of IRMOF-1 or Cu(hfipbb)−(H2hfipbb)2 (H2fipbb = 4,4′-(hexafluoroisopropylilene) bis(benzoic acid) embedded in Matrimid polyimide for CH4, N2, CO2, and H2, as well as the ideal selectivies of their gas pairs.43 Liquid-phase separations with MOFbased MMM were reported by Basu et al.44 with HKUST-1, MIL47, MIL-53, or ZIF-8 as fillers in a PDMS matrix for the separation of Rose Bengal (RB) from isopropyl alcohol (IPA), and by Takamizawa et al.45 with Cu2(bza)4(pyz) (bza = benzoate, pyz = pyrazine) incorporated in PDMS for MeOH and EtOH/water separation. Generally, the resulting MMM combine the high sorption properties of MOFs with good permeabilities and mechanical properties of a polymeric matrix. Although these MOFbased MMM exhibit a high degree of MOF/polymer interaction, their gas separation performance is still insufficient. Recently, Zornoza et al.46 first reported the effect of an interaction between MOFs and a polymer matrix on the separation of CO2 from CH4 at elevated pressures. In their paper, one type of MOF (MIL-53) developed by Férey’s group,47 was functionalized with amine groups to produce MMM. This particular MOF framework showed an abnormal “breathing effect” toward different guest species. Amine-functionalized MIL-53 frameworks have an analogue structure to that of MIL-53.48 The structure of MIL-53 is built up from infinite inorganic chains, consisting of trans-corner-sharing {MO4(OH,F)2} (M = Al3+ or Cr3+ or Fe3+ or In3+) octahedra cross-linked by bis-bidentate terephthalate linkers, creating 1D rhombic shaped tunnels. The “breathing effect” of the MIL-53 framework is mostly related to the flexible linkage between the metal centers and the carboxylate groups. Because of their high surface area, large pores, and good chemical stability, MIL-53 materials show outstanding selective adsorption capacities for a variety of small molecules including CO2, H2, H2S, and hydrocarbons.12 Amine-functionalized MIL-53 crystals are highly thermal (up to 400 °C) and chemically stable to water and common organic solvents. The introduction of amino functional groups drastically enhances CO2 affinity, resulting in a very large selectivity in CO2/CH4 separations.48,49 As an interesting

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EXPERIMENTAL SECTION 1. Materials. Materials for the synthesis of polyimide: 4,4′(hexafluoroisopropylidene)diphthalic anhydride (6FDA, >99% purity) was provided by Chriskey Company. 4,4′-Oxidianiline (ODA, 97% purity) was purchased from Sigma-Aldrich and purified by vacuum sublimation. 1-Methyl-2-pyrrolidone (NMP) was purchased from TCI America and purified by vacuum distillation. Acetic anhydride and triethylamine were obtained from Sigma-Aldrich. Chloroform (CHCl3, 99.8% assay by GC analysis) was obtained from VWR International LLC, and methanol was obtained from Fisher Scientific. Materials for the synthesis of aluminum MIL-53 MOF, aluminum nitrate nonahydrate (Al(NO3)3.9H2O, >98% purity) and 2-amino-acid or terephthalic acid (H2NC6H3-1,4-(CO2H)2, 98% purity) were purchased from Sigma-Aldrich. As the solvent, DMF (dimethylformamide, 99.8% purity) was also obtained from Sigma-Aldrich. 2. MOF and Polyimide Syntheses. MOF synthesis: The synthesis of Al-MIL-53, as well as Al-MIL-53-NH2, was performed as described elsewhere.48,54 First, aluminum nitrate nonahydrate dissolved in DMF and 2-amino-acid or terephthalic acid dissolved in DMF were mixed under stirring. The mixture molar composition was 1 Al(NO3)3:1.48 acid (aminoor terephthalic acid):184.5 DMF. The obtained solution was aged for 24 h and then transferred to a Teflon-lined stainless steel autoclave. The autoclave was heated in an oven at 130 °C for 3 days. After the mixtures were cooled, the yellow (Al-MIL53-NH2) or white (Al-MIL-53) products were collected by centrifugation at 5000 rpm for 15 min. To remove the incorporated 2-amino-acid or terephthalic acid, the as-synthesized powders were washed twice with anhydrous DMF in an autoclave at 130 °C for 24 h. Afterward, DMF was removed by acetone Soxhlet extraction for 20 h. Finally, the solids were dried at 100 °C under vacuum for 48 h. Polyimide synthesis: Polyimides were synthesized by a twostep method as described in a previous work.50 3. Preparation of MMM Membranes. MMM were prepared by a dense film casting method using a “priming” technique which consists of the addition of low amounts of polymer to the MOF suspension before incorporatation of the particles into the polymer film. This technique is believed to make the particles more compatible with the bulk polymer film and promote greater affinity between the filler and the polymer, resulting in improved MMM transport properties.55 A nascent film was cast with the solution onto a clean glass plate using a small metal container with a cover to delay solvent evaporation 6896



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permeation tests. The reported data correspond to averages of these results. The relative reproducibility of accepted tests was also within 2%. The separation factor (αAB * ), represents the ability of a membrane to separate a binary gas mixture, and is defined as

from the nascent membrane. After 24 h, the cover was removed to evaporate residual solvent for 24 h. Then, the films were placed in a vacuum oven at 230 °C and each membrane was annealed for 15 h. The films were finally slowly cooled in the oven from 230 °C to room temperature (RT) and stored in a desiccator before characterization. In the present work the membrane designate was 6FO to designate the 6FDA-ODA polyimide and either MIL or MILNH2 indicated Al-MIL-53 or Al-MIL-53-NH2. For example 6FO-MIL-NH2-10% designates a membrane having 10 wt % Al-MIL-53-NH2 in a matrix of 6FDA-ODA. 4. Characterization. The nonamine, amine-functionalized MOFs and their MMM composites with polyimide (6FDAODA) were characterized using Fourier transform infrared spectroscopy (FT-IR). FT-IR spectra were recorded using a Nicolet Magna 850 Fourier transform infrared spectrometer (Thermo Scientific, Madison, WI) equipped with a liquid-nitrogen-cooled narrow-band MCT detector using Golden-Gate (diamond IRE) ATR accessories (Specac Ltd., London, U.K.). Each spectrum was obtained from the acquisition of 128 scans at 4 cm−1 resolution from 4000 to 700 cm−1 using Happ−Genzel apodization. All spectral operations were executed using the GRAMS/AI 8.0 software (Thermo Galactic, Salem, NH). The results were used to confirm the presence of functional groups and aminefunctionalized grafting in the mixed matrix membranes. The weight loss curves (TGA−DTG) were recorded using a TA Instruments TGA model Q5000IR from 50 to 850 °C at a heating rate of 10 °C/min under air. The glass transition temperature (Tg) and tensile properties of the MMM were determined using dynamic mechanical thermal analyzer (TA Instruments, RSA-3, New Castle, DE) on samples having dimensions of 25 × 6 × 0.02 mm3. For Tg measurements, the temperature was increased from 30 to 350 °C at a rate of 10 °C/min with the strain set at 0.05% and a frequency of 1 Hz. For tensile measurements, the extension rate was set at 1 mm/min and 25 °C. SEM scanning electron micrographs (SEM) were performed to determine the crystal size and to characterize the morphology of the dispersed phase, using a JEOL JSM-840A operated at 15−20 kV. Nitrogen adsorption measurements were carried out to characterize the textural properties of the samples, including the total BET surface area and the micropore volume (Nova 2000, Quantachrome Corp., USA, Omnisorb-100 automatic analyzer at −196 °C). Powder X-ray diffraction patterns (XRD) of MOF and the membranes were recorded using a Siemens D5000 powder diffractometer with Cu Kα radiation (λ = 1.5406 Ȧ ). Finally, CO2 and CH4 adsorption isotherms were obtained by using an automatic apparatus (Autosorp-1 supplied by Quantachrome Corp., USA) at low pressure (0−1 atm) and 308 K. 5. Gas Permeation Measurements. The pure gas transport properties were measured using the variable pressure (constant volume) method,50 such as the permeability coefficient, P (cm3 (STP) cm/cm2 s cmHg) and the ideal selectivity. In the measurement the permeate pressure was allowed to vary from 10−3 to 30 Torr. For each membrane composition at least three or four different membranes were prepared in order to detect defective membranes which would yield inacceptable permeation rates. For all reported data the membrane properties were established for at least two replicas of the same membrane. For each replica the permeation, selectivity, and separation factor were averaged over four replicate

* = (y /y )/(xA /x B) αAB A B

(1)

where yA and yB are the mole fractions of the components in the permeate, while xA and xB are their corresponding mole fractions in the feed. The values of molar compositions are average values of at least five measurements after steady composition was reached, in an experiment in which permeates composition was measured by gas sampling and GC analysis at different times. The relative error on these measured values is less than 2%. Indeed, the time to reach stable composition was well over 12 time lags (12θ).56 The diffusion coefficient (D) was calculated using the timelag method as modified for mixed matrix membranes by Paul and Kemp57 to give D=

⎡ ⎤ ⎛V ⎞ l2 ⎢ 1 + ⎜⎜ d ⎟⎟Kf (y)⎥ ⎥⎦ 6θ ⎢⎣ ⎝ Vp ⎠

(2)

where Vd is the volume fraction of filler and Vp is the volume fraction of the polymeric continuous phase (Vp = 1 − Vd). Also, y = bp2 where p2 is the upstream pressure (150 psi here), while K and b are constants calculated from Henry’s law and Langmuir adsorption isotherms. The correction function f(y) is given by57 ⎤ ⎡1 f (y) = 6y−3 ⎢ y 2 + y − (1 + y) ln(1 + y)⎥ ⎦ ⎣2

(3)

Once P and D are determined, the apparent solubility coefficient S could be obtained as P = DS.50

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RESULTS AND DISCUSSION 1. Characterization of Amine-Functionalized MOFs. A scanning electron micrograph of the synthesized Al-MIL-53 crystals before and after amine-functionalization is shown in Figure 1. The average particle size in Figure 1A−B can be determined to be in the range of 100−150 nm, and the morphology of the Al-MIL-53 was not modified upon functionalization (Figure 1B). Figure 2 shows N2 sorption isotherms of Al-MIL-53 and AlMIL-53-NH2, and BET results are listed in Table 1. The specific surface area, mesopore surface area, and micropore and mesopore volume of the amine-functionalized Al-MIL-53-NH2 were lower than those of the nonfunctionalized Al-MIL-53. The BET surface varied from 1365 to 735 m2/g, while the micropore volume (VM) decreased from 0.433 to 0.240 cm3/g (Table 1), corresponding to a decrease of 46% and 45%, respectively. Mesopore surface area and mesopore volume show the same trend (43% and 60%). So, the amine-functionalized group did not change the morphology of Al-MIL-53, but modified the surface area and pore volume. Figure 3 compares XRD patterns of Al-MIL-53-NH2 powder and 6FO-MIL-53-NH2-25% membrane (A), as well as Al-MIL53 crystals and 6FO-MIL-53-25% membrane (B). Powders and membranes show the same peak positions in the XRD patterns. The spectrum of MMM membranes after thermal-treatment was cleaned of a few peaks, indicating that some impurities have been removed. A comparison of Figure 3 panels A and B suggests that the peak at 2θ = 12° corresponds to the stereo regular position of the −NH2 group. 6897



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Figure 1. Scanning electron micrograph of nonfunctionalized Al-MIL-53 (A) and amine-functionalized Al-MIL-53-NH2 (B).

Figure 2. Nitrogen sorption isotherms of Al-MIL-53 and Al-MIL-53NH2 at 77 K. Figure 3. XRD spectra of Al-MIL-53-NH2 and 6FO-MIL-NH2-25% membrane (A), and Al-MIL-53 and 6FO-MIL-NH2-25% (B).

Table 1. Physical Properties of Amine and Nonamine Functionalized Al-MIL-53 sample designation

SBET (m2/g)

Smeso (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Al-MIL-53 Al-MIL-53-NH2

1365 735

230 130

0.433 0.240

0.265 0.105

The CO2 and CH4 adsorption was investigated using the Autosorb-1 equipment at 308 K with increasing pressure up to 1 bar (atm). The results for Al-MIL-53 and Al-MIL-53-NH2 are presented in Figure 4. The adsorption isotherms indicate that amine-functionalized MOF adsorb less CO2 than nonfunctionalized MOF at 1 bar, which is confirmed by BET results. On the contrary, when the pressure is less than 0.75 bar, aminefunctionalized MOF adsorbed more CO2 than nonfunctionalized MOF. This is due to the presence of NH2 groups in aminefunctionalized MOF in which case the molecule polarizes and can attract CO2, leading to higher CO2 adsorption of aminefunctionalized than nonfunctionalized MOF. CH4 adsorption isotherms for both MOF have similar trends: increasing with pressure. The CO2/CH4 sorption selectivity of Al-MIL-53 and AlMIL-53-NH2 are respectively 7.20 and 9.64 at 1 atm (14.7 psi). 2. Thermal Analysis. The temperatures to reach 5% (Td5%) and 10% (Td10%) weight loss in TGA tests are usually reported to characterize the thermal stability of a material.58 Figure 5 showsTGA curves and their derivatives (DTG) for 6FDA-ODA

Figure 4. CO2 and CH4 adsorption isotherms at 308 K for Al-MIL-53 and Al-NH2-MIL-53.

polyimide and Al-MIL-53 MOF powders (A,B) and MOF-PI MMM (C,D). Characteristic temperatures from TGA curves, determined as the low temperature peak in the DTG analysis of the component materials are listed in Table 2. Similar information is reported as a function of MOF content in Table 3. DTG curves also give information on the pyrolysis rates. Figure 5 panels A and B show that the thermal stability of 6FDA-ODA is quite high with Td5% and Td10% up to 515 and 534 °C (see 6898



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Figure 5. TGA-DTG curves. TGA (A) and DTG (B) of 6FDA-ODA and Al-MIL-53 powders. TGA (C) and DTG (D) of neat polyimide and Al-MIL-53-PI mixed matrix membranes (All DTG curves but 6FO are shifted upward by 1.5, 2.0, 2.5%/°C, respectively).

temperature peak) corresponds to the release of the guest molecules (water 100 °C and solvent DMF around 200 °C) within the pores, while the second step (highest temperature peak) is related to the decomposition of terephthalic acid and amino-terephthalic acid (ligand) from the organic framework. As reported in the work of Marx et al.,59 thermal stability decreases with increasing NH2 content. In their case, a 90 °C reduction was obtained between pure MIL-53 (500 °C) vs MIL-53-NH2 (410 °C). From from the DTG curves obtained (Figure 5B), it can be seen that the first decomposition is around 568 °C for MIL-53 and 492 °C for MIL-53-NH2 (76 °C reduction), while the second decomposition related to conversion of carbonyl groups (CO) to either CO or CO2 is around 591 and 510 °C, respectively (81 °C reduction). Since MOF samples were synthesized and solventexchanged, a tentative molecular formula can be in the form of Al(OH)[BDC0.95DMF and Al(OH)[H2N-BDC]·0.95DMF. The observed weight loss of 4−5% water and 23% solvent DMF corresponds to 0.95 equiv of DMF molecules, which is also confirmed by elemental analysis.60 Finally, as temperature increased up to 650 °C, the observed 20.5% and 18.7% residual materials correspond to the aluminum oxide content in AlMIL-53 and Al-MIL-53-NH2, respectively. From Table 3 all the Td5% of MMM are above 420 °C. For amine-functionalized MIL-53-NH2, Td5% and Td10% are decreasing with increasing MOF loading. The thermal stability of the Al-MIL-NH2 MOFPI MMM is therefore considered sufficient for gas separation applications.61 For example, the highest temperature used in hydrogen separation is in the range 300−500 °C. At the natural gas well, the gas pressure is rather high (in excess of 1000 psi), and the gas temperature is on the order of 30−50 °C. In our

Table 2. Characteristic Temperatures of 6FDA-ODA Polyimide, Amine and Nonfunctionalized Al-MIL-53 Obtained from TGA and DTG material

residual mass (%)

6FDA-ODA AL-MIL-53 AL-MIL-53-NH2

0 20.5 18.5

Td5% (oC) Td10% (oC) 515 184 192

534 222 236

DTG peak (oC) 545 568 492

Table 3. Characteristic Temperatures of MMM Obtained from TGA and DTG mixed matrix membranea 6FO 6FO-MIL-25% 6FO-MIL-NH2-10% 6FO-MIL-NH2-15% 6FO-MIL-NH2-20% 6FO-MIL-NH2-25% 6FO-MIL-NH2-30% 6FO-MIL-NH2-32% 6FO-MIL-NH2-36%

MOF (%) Td5% (oC) Td10% (oC) 0 23.2 11.4 13.3 21.8 24.9 29.2 31.9 35.6

516 515 464 463 462 461 451 442 422

534 534 513 514 505 503 498 492 476

DTG peak (oC) 576 576 589 591 597 596 594 595 572

a

Abbreviations: 6FO, pure PI (6FDA-ODA) membrane; 6FO-MIL, MMM containing nonamine-functionalized Al-MIL-53 and PI; 6FOMIL-NH2, MMM containing amine-functionalized Al-MIL-53-NH2 and PI; n%, wt % loading of Al-MIL-53.

Table 2). Compared to that of 6FDA-ODA polyimide, the curves of the two MOF powders Al-MIL-53 and Al-MIL-53NH2 show a two-step weight loss. The first step (lowest 6899



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1394 cm−1).64 This shift is associated with the interaction between amine-functional and carboxyl groups. The absorption peak at 1690−1670 cm−1 is attributed to free terephthalic acid molecules in both powders, which are encapsulated within their pores in the protonated from (−CO2H). In the case of neat polyimide membranes, the appearance of absorption bands at 1720 cm−1 (asymmetric stretch of the carbonyl group, imide I band) and 1780 cm−1 (symmetric stretch of the carbonyl group, imide II band) and 1377 cm−1 (C−N stretch), confirms that 6FDA-ODA membranes have imides functional groups.65 The three carboxyl group absorption peaks, two imide bands, and C−N stretching band are observed in the three MOF/PI MMM spectra, whereas the bands of terephthalic acid vanished in all MMM. Figure 7 shows the spectra of the same materials as in Figure 6, over the higher frequency (3800−3000 cm−1). The bands at 3500 and 3397/3385 cm−1 are due to the NH2 group,46 as well as the amino terephthalic acid in the pores. As can be seen in Figure 7, both bands are present in the FTIR spectra of the Al-MIL-53-NH2 powder. In 6FO-MIL-53-NH2-35% and 6FOMIL-53-NH2-25% membranes, a 5−8 cm−1 upward shift of the 3397 cm−1 and a corresponding upward shoulder in the 3500 cm−1 peak is due to hydrogen bonding interaction of the amine group with remnant carboxylic acid groups of the polyimide. For 6FDA-ODA membranes (6FO), only one peak due to NH2 in the diamine moiety is observed. The absence of these bands in the non amine-functionalized Al-MIL-53 is also recorded. 4. Dynamic Mechanical Analysis. DMA testing is a versatile and sensitive technique enabling the complete exploration of the relaxation mechanisms in viscoelastic materials.66,67 The most common use of DMA is for the determination of the glass-transition temperature (Tg) where the maximum loss of applied energy is observed, usually as a peak, in the traces of the loss factor versus temperature. Young’s modulus is obtained by the slope of stress−strain curves at low deformation (linear elastic) and represents the rigidity of the material. Table 4

results, all the TGA curves show a plateau below 400 °C which indicates no remnant hydroxyl groups. All DTG plots of the polyimides show two peaks. The first one around 500−540 °C, while the second decomposition temperature is around 640 °C. At the latter temperature, most chemical decomposition reactions of the materials already took place. These weight losses are mostly due to conversion of carbonyl groups (CO) to either CO or CO2.62 There was no residual absorbed solvent or hydroxyl group which would have yielded low (100−250 °C) decomposition temperature. 3. Infrared Spectroscopy. The successful presence of metal−organic frameworks, nonfunctionalized Al-MIL-53, and amine-functionalized Al-MIL-53-NH2 powders, as well as the presence of the functional group in neat (6FDA−ODA) polyimide and MIL-53/PI mixed matrix membranes, were confirmed by Fourier transform infrared (FTIR) spectroscopy (Figures 6 and 7).

Figure 6. ATR-FTIR spectra between 2200 and 1000 cm−1 for AlMIL-53, Al-MIL-53-NH2, and Al-MIL-53-PI MMM (all spectra but MIL-53 are shifted upward by 25, 50, 75, 100, and 125%, respectively).

Table 4. Glass Transition Temperature and Young’s Modulus of PI Membrane and MOF-PI MMM

Figure 7. ATR-FTIR spectra between 3800 and 3000 cm−1 for Al-MIL53, Al-MIL-53-NH2 and Al-MIL-53-PI MMM (all spectra but MIL-53NH2 are shifted upward by 2.5, 5.0, 7.5, 10, and 12.5%, respectively).

membrane

Tg (oC)

Young’s modulus (GPa)

elongation at break (%)

6FO 6FO-MIL-25% 6FO-MIL-NH2-10% 6FO-MIL-NH2-15% 6FO-MIL-NH2-20% 6FO-MIL-NH2-25% 6FO-MIL-NH2-30% 6FO-MIL-NH2-32% 6FO-MIL-NH2-36%

290 288 293 291 293 293 292 292 288

1.82 2.80 2.07 2.44 2.53 3.04 3.05 3.29 4.00

12.4 0.8 5.0 3.4 3.3 3.1 2.6 1.5 0.8

summarizes the glass transition temperature of 6FDA-ODA, and MOF/PI mixed matrix membranes. 6FDA-ODA polyimide has aromatic groups in the main chain which dramatically reduce rotation and therefore yield a higher Tg (290 °C). The influence of amine-functionalized group on the glass transition temperature of MOF-PI MMM is shown in Table 4. Compared to the 6FDA-ODA membrane, the Tg of the MMM is in most cases increased by about 1−3 °C. The glass transition temperatures of most MMM have similar values which are higher (exception the 6FO-MIL-25% and 6FO-MIL-NH 2 -36% MMM) than that the of polyimide membrane. This Tg increase

Figure 6 shows these spectra over the lower frequency range (2200−1000 cm−1). Bands, which can be attributed to CO2 asymmetric stretching vibration at 1608 and 1505 cm−1, as well as to the CO2 symmetric stretching vibration at 1422 cm−1,63 are present in the spectra of non- and amine-functionalized solids. For a nonfunctionalized sample, FTIR spectra display three adsorption bands at 1572, 1500, and 1437 cm−1. On the other hand, for functionalized material, three adsorption bands slightly shifted to lower adsorption frequency (1572, 1500, and 6900



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through these pores. This may be the reason why both gas permeabilities are higher than for pure PI membranes. The results of CO2/CH4 selectivity and the separation factor of 6FO-MIL-53-25% are lower compared to the original PI membrane. On the contrary, the pore size in the −NH2modified MOF was reduced and led to less CO2 adsorption at high pressure, therefore leading to lower CO2 permeability than for unmodified MOF. Figure 11 shows the performance of MOF-PI MMM containing from 0 to 36 wt % Al-MIL-53-NH2 in the 6FDAODA phase. The performances of MMM were obtained by averaging values from four replicate permeation tests over each membrane. The results show that CO2 permeability of MMM stays almost constant with increasing amine-functionalized MOF loading, whereas CH4 permeability decreases. From Figure 11, at loadings up to 32 wt % the addition of MIL-53NH2 results in a moderate CO2 flow rate increase and a small decrease in CH4 permeation. This corresponds to a CO2/CH4 membrane selectivity increase by 2-fold for the best performing membrane (32 wt %). It is well-known that in MMM an optimal filler loading exists. MIL-53-NH2 at 25 wt % was found to be an optimal filler loading for polysulfone membranes as reported by Zornoza et al.46 In the case of inorganic fillers like zeolites or silicas, due to the usually poor filler−polymer interaction, the optimal loading was reported to be below 12%.46 By improving zeolite-polyimide interaction, it was possible to reach 25 wt % as the optimal loading.53 Moreover, in the present work, the nanosize of the MOF particles minimizes their sedimentation during MMM casting. In the case of functionalized MOF, the amine group interacts with the terminal acid groups of polyimide, thus avoiding the need for an additional compatibility agent.68 Indeed, better dispersion in the polymer phase and better MOF-polymer interface quality using amine-modified MOF particles result in better membrane performances (Figure 11). 6.2. Gas Diffusivity and Solubility Coefficients of MOF/PI MMM. Gas permeation involves two processes: dissolution and diffusion. The experimental values of diffusion coefficients (D) and solubility coefficients (S) and their respective CO2/CH4 selectivities for the original polyimide membranes and MOF-PI MMM are shown in Table 5. The diffusion coefficients were determined by the time-lag method and calculated using eq 2. In this work, the Langmuir parameters calculated from the information of adsorption isotherm at 35 °C and 0−1 atm gave a correction (bracket term) of only 0.02%. The value obtained is used to calculate the solubility coefficient at 35 °C and 150 psi upstream pressure. From Table 5, all MMM gas diffusivity coefficient selectivities (DCO2/DCH4) are smaller than for neat polymer membranes, and the diffusivity coefficient selectivity of MIL-53-NH2/PI MMM stays almost constant. In contrast, all MMM gas solubility coefficient selectivities are higher than for neat PI membranes. Moreover, the values increase with increasing MOF loading until 32%. Therefore, higher ideal selectivity is attributed to the increasing solubility coefficient selectivities. Another important difference is that neat polyimide membranes are nonporous dense materials mostly governed by the solution-diffusion theory. On the other hand, diffusion into MOF-PI membranes is mainly controlled by CO2 adsorption on the cation sites. These effects are believed to be the origin of increasing CO2/CH4 solubility selectivities of mixed matrix membranes compared to neat polymer membranes.

might be due to MOF restricting the movement of the polymer chains by the interaction with the amine-functional group. Therefore, the quality of contact between MOF particles and the polymer is increased. Interestingly, both MOF-containing membranes of 6FO-MIL-25% and 6FO-MIL-NH2-36% show lower Tg values than the polymer. This may be due to bad particle spatial distribution (see Figure 9). It seems that higher Tg upon MOF interaction in the membrane reflects an effective polymer−particle interaction associated with the quality of the interface. Figure 8 presents typical stress−strain curves for PI/MOF (0, 10%, 30% MOF) membranes. The mechanical properties of

Figure 8. Typical stress−strain curves for PI/MOF membranes.

MOF-PI MMM were established from DMA testing (Table 4). The data showed that Young’s modulus of all MMM compared with 6FDA-ODA original polyimide membrane have improved as the values increase with increasing loading. On the other hand, elongation at break decreases with increasing filler content. Overall, MOF-PI membranes are the most brittle. For example, the elongation at break of 6FO-MIL-25% and 6FOMIL-NH2-36% were very low with only 0.8%. 5. Morphology of MOF/PI Mixed Matrix Membranes. To investigate the dispersion of MOF within the polymer, cross-sectional micrographs of the MMM containing 25 wt % Al-MIL-53 (Figure 9A,B) and 20, 25, 30, 32, 36, and 40 wt % Al-MIL-53-NH2 (Figure 9C−H) were examined by SEM. Filler agglomeration was observed in MOF-PI without an aminefunctionalized group. Continuous MOF-PI phases are obtained at all loadings with amine-functionalized MMM, even in the case of 36−40 wt % loading (Figure 9G−H). Thus it may be suggested that the amine-functional group enhanced the quality of the interface between the filler and PI. Moreover, MOF particle size being quite small (100−150 nm), the distribution of both MOF materials is quite uniform and the interface between the fillers and the polymer matrix is of good quality. 6. Gas Transport Properties. 6.1. Permeability, Ideal Selectivity, and Separation Factor. The permeability, ideal selectivity, and separation factor of CO2, CH4, and their blends (50/50 CO2/CH4) were measured at 35 °C and 150 psi upstream pressure for MOF-PI MMM. Figure 10 compares the gas transport properties of three membranes: 6FDA-ODA, 6FO-MIL-53-NH2-25% (modified MOF), and 6FO-MIL-53− 25% (unmodified MOF). The membranes containing unmodified (6FO-MIL-53) MOF have high permeability for both gases and average selectivity. This means that, since the unmodified MOF pores size (0.7−0.8 nm) are larger than the kinetic diameter of CH4 (0.38 nm), both gases can pass 6901



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Figure 9. SEM pictures of the cross-section of MMM containing 25 wt % (A,B) Al-MIL-53 or 20 (C), 25 (D), 30 (E), 32 (F), 36 (G), and 40 wt % (H) of Al-MIL-53-NH2.

6.3. Effect of Feed Pressure. CO2 permeability and separation factor of 6FO and 6FO-MIL-NH2-25% as a function of feed pressure in gas blend (CO2:CH4 50:50) are shown in Figure 12. CO2 permeability of both membranes shows similar trends; that is, the values decrease with feed pressure50 (Figure 12A). The separation factor of pure polymer membrane is decreased with increasing feed pressure as reported in the literature.50,69,70 Interestingly, the separation factor of 6FOMIL-NH2-25% increases with feed pressure increasing from 150 to 300 psi (Figure 12B). Similar trends were found for

some mixed matrix membranes using MOF particles as reported in the literature.46 In the work of Zornoza et al.,46 the authors investigated the effect of MOF breathing on the adsorptive behavior of Al-MIL-53-NH2 for CO2 pressure up to 25 bar, and their results are displayed in Figure 12C where a large increase in CO2 adsorption capacity at CO2 pressure between 150 and 300 psi was observed. This behavior was ascribed to the breathing of the MOF lattice allowing a large increase in the equilibrium CO2 adsorption content. It is remarkable that a strong increase in CO2/CH4 separation factor 6902



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Figure 10. Comparison of the separation performances of 6FO, 6FOMIL-NH2-25%, and 6FO-MIL-25% membranes.

Figure 11. CO2, CH4 permeability, ideal selectivity (PCO2/PCH4) and separation factor (α* CO2:CH4 = 50:50) as a function of Al-MIL-53NH2 loading at 308 K and 150 psi.

Figure 12. CO2 permeability (A) and separation factor (α*) (B) for gas mixture (CO2:CH4 = 50:50) as a function of feed pressure for 6FO and 6FO-MIL-NH2-25% membrane at 308 K. CO2 adsorption isotherm of pure Al-MIL-53-NH2 MOF (C).

over the MMM for the same Al-NH2-MIL-53 is also observed over essentially the same pressure range. Note that CO2 permeability is not showing the same trend over that range. In other words, the observed increase in separation factor is essentially associated with a strong decrease in CH4 permeability in the mixed gas at these pressures. It thus appears that the effect of breathing, which involves a higher CO2 adsorption capacity, does not yield an increased CO2 diffusivity, but an exclusion of CH4 from Al-MIL-53-NH2 cages affecting CH4 solubility and diffusivity in MMM. Decreasing CO2 permeability with increasing pressure (increased CO2 content in the MOF) suggests that the adsorbed CO2 has a strong interaction with the Al-MIL-53-NH2 walls. This in turn suggests that most CO2 adsorbed molecules in the 150− 300 psi (10−20 bar) pressure range are interacting with the

amine groups of Al-MIL-53-NH2. Nevertheless, this is a very important property for applications like natural gas and biogas upgrading. 7. Comparison with Robeson’s Upper Bound Curve. Thus far, this work showed that MMM made from aminefunctionalized MOF Al-MIL-53 with polyimide of 6FDA-ODA have promising CO2/CH4 separation properties. The separation performances of the membranes for the CO2/CH4 gas pair on a permeability-selectivity chart are compared with Robeson’s upper bound curves6,71 in Figure 13. Both MOF-PI with high Al-MIL-53-NH2 content (30 and 32 wt %) yield data points above the 1991 limit, which is an important achievement of this work.

Table 5. Pure Gas Diffusion and Solubility Coefficients of PI Membrane and MOF-PI MMM D (×10−8 cm2 s−1)

S (×10−2 cm3(STP) cm−3 atm)

selectivity

membrane sample

CO2

CH4

CO2

CH4

DCO2/DCH4

SCO2/SCH4

6FO 6FO-MIL-25% 6FO-MIL-NH2-10% 6FO-MIL-NH2-15% 6FO-MIL-NH2-20% 6FO-MIL-NH2-25% 6FO-MIL-NH2-30% 6FO-MIL-NH2-32% 6FO-MIL-NH2-36%

2.25 3.92 2.22 1.88 1.55 1.48 1.78 1.13 2.22

0.23 0.48 0.46 0.40 0.33 0.33 0.38 0.25 0.46

6.40 5.33 6.44 7.68 9.31 9.70 8.16 13.0 6.44

1.33 0.97 0.63 0.71 0.79 0.68 0.52 0.75 0.63

10.0 8.11 4.79 4.74 4.68 4.56 4.66 4.47 4.79

4.80 5.48 10.3 10.8 11.8 14.3 15.6 17.2 10.3

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(4) Ismail, A. F.; Goh, P. S.; Sanip, S. M.; Aziz, M. Transport and separation properties of carbon nanotube-mixed matrix membrane. Sep. Purif. Technol. 2009, 70, 12. (5) Liu, C.; Kulprathipanja, S.; Hillock, A. M. W.; Husain, S.; Koros, W. J. In Advanced Membrane Technology and Applications; Li, N. N., Fane, A. G., Winston Ho, W. S., Matsuura, T., Eds.; John Wiley and Sons Ltd: Hoboken, NJ, 2008; p 789. (6) Robeson, L. M. The upper bound revisited. J. Membr. Sci. 2008, 320, 390. (7) Cong, H. L.; Radosz, M.; Towler, B. F.; Shen, Y. Q. Polymer− inorganic nanocomposite membranes for gas separation. Sep. Purif. Technol. 2007, 55, 281. (8) Yaghi, O. M.; Li, H. L. Hydrothermal Synthesis of a Metal− Organic Framework containing large rectangular channels. J. Am. Chem. Soc. 1995, 117, 10401. (9) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular synthesis and the design of new materials. Nature 2003, 423, 705. (10) Kitagawa, S.; Kitaura, R.; Noro, S. Functional porous coordination polymers. Angew. Chem., Int. Ed. 2004, 43, 2334. (11) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R. D.; UribeRomo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Exceptional chemical and thermal stability of zeolitic imidazolate frameworks. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10186. (12) Ferey, G. Hybrid porous solids: past, present, future. Chem. Soc. Rev. 2008, 37, 191. (13) McKeown, N. B.; Budd, P. M. Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage. Chem. Soc. Rev. 2006, 35, 675. (14) Budd, P. M.; McKeown, N. B. Highly permeable polymers for gas separation membranes. Polym. Chem. UK 2010, 1, 63. (15) McKeown, N. B.; Budd, P. M. Exploitation of intrinsic microporosity in polymer-based materials. Macromolecules 2010, 43, 5163. (16) Du, N.; Park, H. B.; Robertson, G. P.; Dal-Cin, M. M.; Visser, T.; Scoles, L.; Guiver, M. D. Polymer nanosieve membranes for CO2capture applications. Nat. Mater. 2011, 10, 372. (17) Wright, P. A. Microporous Framework Solids; RSC Materials Monographs; Royal Society of Chemistry: Cambridge, UK, 2008. (18) MacGillivray, L. R. Metal−Organic Frameworks: Design and Application; John Wiley and Sons, Inc.: Hoboken, NJ, 2010. (19) Fang, Q.; Sculley, J.; Zhou, H.-C. J.; Zhu, G. In Comprehensive Nanoscience and Technology; Andrews, D. L., Scholes, G. D., Wiederrecht, G. P., Eds.; Academic Press: New York, 2010; Vol. 5, pp 1−20. (20) Alaerts, L.; de Vos, D. E. In Novel Concepts in Catalysis and Chemical Reactors; Cybulski, A., Moulijn, J. A., Stankiewicz, A., Eds.: Wiley-VCH Verlag GmbH and Co.: Weinheim, Germany, 2010; p 73. (21) Farrusseng, D. Metal−Organic FrameworksApplications from Catalysis to Gas Storage; Wiley-VCH Verlag & Co.: Weinheim, Germany, 2011. (22) Vinh-Thang, H.; Kaliaguine, S. MOF-Based Mixed-MatrixMembranes for Industrial Application. Coordination Polymers and Metal Organic Frameworks; Oscar, L., Ortiz, L. Ramirez, D., Eds.; Nova Science Publishers, Inc.: Hauppauge, N.Y., 2011; Chapter 4. (23) Yehia, H.; Pisklak, T. J.; Ferraris, J. P.; Balkus, K. J.; Musselman, I. H. Methane facilitated transport using copper(II) biphenyl dicarboxylate-triethylenediamine/poly(3-acetoxyethylthiophene) mixed matrix membranes. Abstr. Paper Am. Chem. Soc. 2004, 227, U351. (24) Won, J. G.; Seo, J. S.; Kim, J. H.; Kim, H. S.; Kang, Y. S.; Kim, S. J.; Kim, Y. M.; Jegal, J. G. Coordination compound molecular sieve membranes. Adv. Mater. 2005, 17, 80. (25) Perez, E. V.; Balkus, K. J.; Ferraris, J. P.; Musselman, I. H. Mixed-matrix membranes containing MOF-5 for gas separations. J. Membr. Sci. 2009, 328, 165. (26) Liu, C.; Kulprathipanja, S.; Wilson, S. High flux mixed matrix membranes for separations. U.S. Patent Appl. 2007/0209505A1, 2007.

Figure 13. Performance of Al-MIL-53/PI mixed matrix membranes with respect to Robeson’s upper limit curves.

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CONCLUSION Nanosized (100−150 nm) Al-MIL-53-NH2 crystal MOFs were synthesized with narrow particle size distribution. The Al-MIL53-NH2 particles showed excellent adhesion with the 6FDAODA polyimide in mixed matrix membranes without the addition of any compatibilizing agent. For this reason, high filler loadings (up to 32 wt %) were possible due to hydrogen bonding interactions between the MOF and the polymer. Mixed matrix membranes containing Al-MIL-53-NH2 particles for CO2/CH4 separation display high ideal selectivity up to 77, and high separation factor up to 53. Compared to other inorganic particles such as zeolites, the MMM fabrication process without any additional compatibility agent was easy, which would allow the easy production of hollow fibers for industrial applications. Moreover MOF-PI MMM made from this “breathing” material shows a better CO2/CH4 separation factor with increasing pressure. Also, the mixed matrices membranes produced using amine-functionalized MOF with polyimide were shown to be above the Robeson 1991 trade-off limit. Finally, the separation factors obtained in this work are improved compared with those of neat polyimide membranes and the selectivity is believed to be high enough for industrial applications. Nevertheless, more work needs to be done to still improve on these results.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Natural Science and Engineering Research Council of Canada (NSERC) for financial support through a strategic grant. X. Y. Chen also thanks the Fonds Québécois de Recherche sur la Nature et les Technologies (FQRNT) for a scholarship.

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REFERENCES

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Amine-Functionalized MIL-53 MetalâOrganic Framework in Polyimide

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