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Particle size effects on gas transport properties of 6FDA-Durene/ZIF-71 mixed matrix membranes Susilo Japip, Youchang Xiao, and Tai-Shung Chung Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02811 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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Industrial & Engineering Chemistry Research

Particle size effects on gas transport properties of 6FDA-Durene/ZIF-71 mixed matrix membranes

Susilo Japip‡, Youchang Xiao‡,┴, Tai-Shung Chung‡,*

‡

Department of Chemical and Biomolecular Engineering,

National University of Singapore, Singapore 117585 ┴

Suzhou Faith & Hope Membrane Technology Ltd Co., SIP, Jiangsu, PRC

ACS Paragon Plus Environment


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Abstract A series of 6FDA-Durene-ZIF71 mixed matrix membranes (MMMs) comprising three different ZIF-71 particle sizes (i.e., 30, 200, and 600 nm) have been fabricated to investigate the effects of particle size on gas separation performance of MMMs. Compared to MMMs containing ZIF-71 particle size of > 200 nm, ZIF-71 embedded MMMs with particle sizes of ≤ 200 nm exhibited significantly enhanced gas separation performance. The O2 and N2 permeability of MMMs comprising ZIF-71 with particle sizes of ≤ 200 nm have higher permeability than the predicted ones using the Maxwell model. By considering the inefficiently packed polymer chain and the sieve-in-a-cage phenomena, the modified Maxwell model displayed comparable permeability over the experimental results suggesting non-ideal nature of MMMs. For the first time, it was found that ZIF-71 of 200 nm generated the best MMM for gas separation, particularly for H2/CH4 systems. Therefore, a trade-off between ZIF-71 particle size vs. gas separation performance must be taken into account when designing an MMM suitable for industrial gas separation applications.

Keywords: ZIF-71, MMMs, particle size, gas separation, Maxwell model

2


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1. INTRODUCTION Membrane-based gas separation has been identified as an energetically efficient and environmentally friendly technology since the first installation of industrial gas separation membrane system.1-3 Compared to other separation processes such as pressure swing adsorption, chemical absorption, and cryogenic distillation, membrane technology offers advantages of smaller footprint, simpler operation and no requirement of regeneration.1, 2, 4-10

However, polymer membrane technology still faces the trade-off constraint between

permeability and selectivity.11, 12 Hence, mixed matrix membranes (MMMs) consisting of polymeric and inorganic moieties have been proposed to surpass the limitations of traditional polymer membranes for gas separation.13-21

The formation of defect-free MMMs is still challenging because of the possible existence of non-selective interfacial voids, particle agglomeration, oversize particles, chain rigidification and pore blockage.5, 14, 15, 18, 19, 22 By replacing the conventional molecular sieve particles such as zeolites with the metal organic frameworks (MOFs), i.e., a hybrid organic-inorganic material, several of the challenges have been overcome due to the better affinity between MOFs and polymeric matrices.19, 23-31

Zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, are often constructed by tetrahedral Zn or Co metal atoms (M) using imidazolate (Im) linkers as bridges to form the M–Im–M structure.32-35 Due to their exceptional thermal and chemical stability, ZIF membranes and ZIF based MMMs have been explored for molecular separation.23-25, 33, 34, 36-50

It has been observed that MMMs embedded with nanometric particles can enhance the 3



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gas separation performance more substantially than their corresponding ones containing micrometric particles. Rodenas et al. reported that the addition of nanometric CuBDC into the Matrimid matrix had a better CO2/CH4 selectivity as compared to that embedded with micrometric CuBDC.30 Wee et al. fabricated both micrometric and sub-micrometric ZIF71 based MMMs and found that the MMM consisting of sub-micrometric ZIF-71 had a better separation performance than that comprising micrometric ZIF-71.47 Similarly, Bae et al. showed that a sub-micrometric ZIF-90 based MMM has a higher CO2 permeability and CO2/CH4 selectivity than a micrometric ZIF-90 one.51

In light of the aforementioned advantages of nanometric MOF particles, we aim to synthesize a series of ZIF-71 with three different particle sizes to elucidate the particle size effect on gas separation performance of MMMs. Recently, it has been reported that an increase in ZIF-8 particle size from 50 nm to 150 nm would enhance H2/CO2 separation performance of ZIF-8/PBI MMMs due to the agglomeration issue.50 In contrast, Knebel et al. observed the reduced H2/CO2 separation performance of nanometric MIL-96/Matrimid MMMs because of the formation of micrometric MIL-96 crystals upon thermal activation.31 Hence, both Maxwell and modified Maxwell models will be utilized in this study to examine the effect of ZIF-71 particle size on separation performance. As compared to ZIF-8 and ZIF-90 (pore cavity of 11.6 Å and 11.2 Å, respectively) of sodalite topology, ZIF-71 of rhombic topology was selected because it possesses a relatively larger pore cavity of 16.5 Å which may enhance gas separation performance of MMMs.32, 33 It should also be noted that 6FDA-Durene was selected as the polymeric matrix in this study owing to its high intrinsic gas separation performance. 4


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2. EXPERIMENTAL 2.1. Materials The fluorinated polyimide (6FDA-Durene) was synthesized in-house following our previous studies48,

52

and its monomers, 4,4’-(hexafluoroisopropylidene) diphthalic

anhydride (6FDA) and 2,3,5,6-tetramethyl-1,4-phenylenediamine (Durene diamine), were procured from Clariant (Germany) and Sigma-Aldrich (Singapore), respectively. Both 6FDA and Durene diamine were purified via vacuum sublimation at 300 °C and recrystallization in methanol (MeOH) of HPLC grade, respectively. N-methyl-2pyrrolidinone (NMP) of analytical grade was vacuum distilled at 65 °C before being used as the solvent for the polyimide synthesis. Both NMP and MeOH (HPLC and technical grade) were purchased from Merck (Germany), while acetic anhydride (>99.5%) and triethylamine (TEA, 99%) were obtained from Sigma-Aldrich (Singapore) and Fisher Scientific (UK), respectively. In ZIF-71 syntheses, Zn(CH3COO)2.2H2O (ACS reagent, >98%) and 4,5-dichloroimidazole (>99%) were acquired from Sigma-Aldrich (Singapore) and TCI (Japan), respectively. In order to have a better control over the particle size of ZIF-71, a mixed solvent of HPLC grade MeOH and N,N-dimethylformamide (DMF, HPLC grade, Fisher Scientific, UK) was utilized. All purified gases (at least 99.95%) were kindly supplied by SOXAL Pte. Ltd. (Singapore).

2.2. Syntheses of ZIF-71 with different particle sizes Different sizes of ZIF-71 particles were synthesized by utilizing mixed MeOH and DMF solvents with different volume ratios following the Wee et al. method.47 In a typical 5



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reaction, a prescribed amount of Zn(CH3COO)2.2H2O was dissolved in either pure DMF or MeOH, whilst another prescribed amount of 4,5-dichloroimidazole (dcIm) was dissolved in either pure MeOH or a mixture of MeOH and DMF. The molar ratio of Zn2+ ions to dcIm was kept constant at 1:4 and the molar concentration of Zn2+ ions in the final mixture was maintained at 0.08 M. After being stirred for around 15 min, both solutions were mixed and stirred for another 4 h at room temperature. The ZIF-71 particles were then collected by centrifugation and conserved in fresh DMF as colloidal ZIF-71 prior to use. Detailed synthesis parameters of ZIF-71 with different particle sizes were summarized in Table 1. Table 1. Synthesis parameters of ZIF-71 with different particle sizes. Concentration Concentration Volume ratio Particle size 2+ of dCIm (M) DMF : MeOH of Zn (M) 30 nm

0.08

0.32

2

200 nm

0.08

0.32

0.5

600 nm

0.08

0.32

0.0045

Temperature (°C)

Room temperature

2.3. Fabrication of ZIF-71 based mixed matrix membranes (MMMs) Solutions of 5 wt% 6FDA-Durene and 6FDA-Durene-ZIF71 in DMF were prepared to fabricate dense membrane films in petri dishes by the solution casting method. Typically, 6FDA-Durene of 0.44 g was dissolved in 8.36 g DMF overnight to prepare a pristine 6FDA-Durene membrane or in 5 g DMF to prepare a ZIF-71 based MMM. The 6FDADurene solution was pre-filtered by a 1.0 μm PTFE membrane prior to casting. To prepare MMMs, the colloidal ZIF-71 in DMF was pre-sonicated for 1 h and then mixed with the pre-filtered 6FDA-Durene solution with stirring for 30 min prior to casting. The solvent 6


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was evaporated in a pre-heated oven at 80 °C for at least 18 h. The nascent membrane was then peeled off and further dried in a vacuum oven at 200 °C for 18 h followed by natural cooling under vacuum.

2.4. Characterizations In order to confirm the formation of ZIF-71 with different particle sizes, both highresolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F) and field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) were employed to characterize particles. FESEM was also employed to study the cross-sectional morphologies of ZIF-71 based MMMs as a function of ZIF-71 size after the samples were cryogenically fractured in liquid N2.

Particle size distribution of ZIF-71 was investigated by dynamic light scattering (DLS) in a DMF solution at room temperature using a Brookhaven 90Plus Particle Size Analyzer. The counting rate of each measurement was kept between 200-600 kcps. Meanwhile, the surface area of ZIF-71 particles with different particle sizes was quantified by N2 adsorption–desorption isotherms at 77 K employing a Quantachrome NOVA-3000 system. All samples were degassed at 120 °C for 4 h under a N2 flow prior to measurements. The surface area was then determined by standard multipoint Brunauer–Emmett–Teller (BET) and Langmuir models utilizing QuantachromeTM NovaWin software.

The crystallinity of pristine ZIF-71 particles and embedded ZIF-71 particles in MMMs was verified by a Bruker D8 Advance X-ray diffractometer (XRD) utilizing Cu K-α as the X7



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ray radiation source with a wavelength of 1.54 Å. The XRD patterns of ZIF-71 particles were compared with the simulated XRD pattern of ZIF-71 particle obtained from the literature.32 The thermal stability of pristine ZIF-71 particles and embedded ZIF-71 particles in MMMs was also characterized as a function of particle size under both N2 and air atmosphere by thermogravimetric analyses (TGA), respectively. A Shimadzu Thermal Analyzer (DTG-60AH/TA-60WS/FC-60A) was used for these characterizations with a heating rate of 10 °C min-1 from 50 °C to 800 °C. The loading of ZIF-71 was calculated based on the following equation. Particle loading 

weight of particles  100% weight of particles  weight of polymer

(1)

In order to determine the molecular weight of synthesized 6FDA-Durene, a gel permeation chromatography (GPC) of the Waters system was utilized. The GPC system comprises a Waters 1515 isocratic HPLC pump connected to a Waters 2414 refractive index detector. The sample was injected via a Waters 717 plus auto-sampler using DMF of HPLC grade as the mobile phase, whilst polystyrene standard was selected for calibration. The sample injection volume was 100 µL and the mobile phase flow rate was 1 mL.min-1.

Density of the fabricated membranes was measured by using a Mettler Toledo balance and a density kit via liquid displacement method. The density was computed based on the Archimedean principle by the following equation (2):



Wair o Wair  WHEX

(2)

8


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where  is the density of the membranes (g.mL-1), Wair and WHEX signify the sample weights in air and in hexane, respectively, and  o represents the density of hexane at 25°C which was assumed to be 0.667 g.mL-1.

2.5. Measurements of gas transport properties Gas permeation properties of the fabricated ZIF-71-based MMMs were evaluated utilizing a variable-pressure constant-volume gas permeation cell following the order of H2, O2, N2, CH4, and CO2. The gas permeation properties were measured at 3.5 atm. The cell temperature was kept constant at 35 °C. The gas permeability coefficient across the membrane was computed according to the steady state pressure increment dp dt  in the following equation (3): P

273  1010 760

Vl

dp 76  dt  AT  p2   14.7  

(3)

where P denotes the gas permeability coefficient in Barrer (1 Barrer = 1 × 10 -10 cm3 (STP).cm.cm-2.s-1.cmHg-1), V refers to the volume of the downstream reservoir (cm3), A is the effective membrane area (cm2), l represents the membrane thickness (cm), T is the testing temperature (K) and p 2 is the upstream pressure of the system (psia). The reported results have been repeated with three different membranes from three different batches of ZIF-71. The membrane thickness was measured by a Mitutoyo digital micrometer and was in the range of 60 – 80 µm.

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The ideal selectivity of gas A over gas B,  A , was assessed according to the equation as B

follows:

A  B

PA PB

(4)

Where PA and PB refer to the permeability coefficients of gases A and B, respectively.

According to the solution-diffusion model, permeability is the product of diffusivity (D) and solubility (S). Therefore, the ideal permeability selectivity (𝛼𝑃 ) can be defined as the product of diffusivity selectivity (𝛼𝐷 ) and solubility selectivity (𝛼𝑆 ) as follows:

P  D S

 A B  P  D  S 

(5) DA S A  DB S B

(6)

where DA and DB are the diffusivity coefficients of gases A and B, respectively (cm2.s-1); SA, and SB are the solubility coefficients of gases A and B, respectively (cm3(STP).cm3

.cmHg-1).

In order to obtain the solubility coefficients, a dual-volume pressure decay device was employed to measure the gas concentration, C (cm3(STP).cm-3(polymer)), adsorbed in a sample. A detailed description of the homemade dual-volume sorption cells and the experimental procedure can be found elsewhere.53 The measurements were conducted at 35 °C and 3.5 atm using sample weights of at least 0.4 g. The measured C was used to calculate the solubility coefficient, S, based on the following equation (7):

10


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S

C p

(7)

3. RESULTS AND DISCUSSION 3.1. Syntheses of ZIF-71 particles with different particle size Table 1 summarizes the reaction conditions to synthesize ZIF-71 particles using different DMF/MeOH solvent ratios. The molar ratio of Zn:dCIm was kept at 1:4 because our previous works indicated that the particle size did not vary much if the ratio was in the range of 1:8 to 1:4.48, 52, 54 Figures 1, 2, and 3 display the colloidal ZIF-71 particles after 4h reaction and then characterized by FESEM, HR-TEM and DLS, respectively. As shown in Figures 1 and 2, the ZIF-71 particles synthesized from different DMF/MeOH ratios possess different particle sizes which range from around 30 nm up to 600 nm. These values compliment well with the hydrodymanic sizes measured by DLS. Figure 3 displays that all ZIF-71 particles have a mono-modal particle size distribution except that synthesized from a DMF/MeOH solvent ratio of 2.

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Figure 1. Morphology of ZIF-71 particles synthesized at room temperature for 4h in different volumetric ratios of DMF/MeOH (a) 2, (b) 0.5, and (c) 0.0045.

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Figure 2. HR-TEM images of ZIF-71 particles synthesized at room temperature for 4h in different volumetric ratios of DMF/MeOH (a) 2, (b) 0.5, and (c) 0.0045.

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Figure 3. Particle size distribution of ZIF-71 particles synthesized at room temperature for 4h in different volumetric ratios of DMF/MeOH (a) 2, (b) 0.5, and (c) 0.0045.

It is quite surprising that our obtained ZIF-71 particle sizes are much smaller than those reported by Wee et al.,47 particularly for the ones synthesized from DMF/MeOH volumetric ratios of 2 and 0.5. Under the same solvent ratios, their ZIF-71 particle sizes were 140 nm and 290 nm, respectively. These discrepancies in particle size may arise from different synthesis procedures between ours and theirs such as solution concentration, synthesis duration, and type of metal salt. As observed by Venna et al.55 on ZIF-8 syntheses in MeOH and Schweinefu et al.56 on ZIF-71 syntheses in 1-propanol, the particle size increased with increasing reaction duration. In addition, Zn(CH3COO)2.2H2O was utilized in this study as the salt precursor instead of Zn(CH3COO)2 used by Wee et al. The water

14


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content in the salt precursor may affect the synthesis process and kinetics particularly to generate nanometric particles of ZIF-71.

Figure 4 displays the results of N2 adsorption at 77 K, whilst Table 2 summarizes the surface areas measured by BET and Langmuir methods and the total pore volume of ZIF71 particles with different particle sizes. N2 gas was strongly adsorbed at low relative pressures and then a plateau was reached for each particle size. Irrespective of particle size, all adsorption follow a type I isotherm pattern which is typical for microporous materials. 57

The calculated BET and Langmuir surface areas and the total pore volume of ZIF-71

correspond well with the results reported by Wee et al.47 Interestingly, ZIF-71 with a particle size of 30 nm possesses the lowest volume of adsorbed N2, surface area and the total pore volume. This phenomenon may be caused by the substantially higher outer surface area to volume ratio for the 30 nm ZIF-71 as compared to other larger ZIF-71 particles. In addition, it may also be due to the fact that the synthesized ZIF-71 nanoparticles with a size of 30 nm might still be in their nucleation stage. A similar phenomenon was observed by Venna et al.55 on N2 adsorption upon ZIF-8 particles.

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Figure 4. N2 adsorption at 77K for ZIF-71 with different particle sizes.

Table 2. Surface area and total pore volume of ZIF-71 as a function of particle size. Surface area (m²/g) Total pore Sample Particle size Reference (nm) volume (cm³/g) BET Langmuir 30 727 864 0.31 200 841 1065 0.41 This work ZIF-71 600 844 1058 0.38 290 827 1148 0.41 Wee et al.47 * Both BET and Langmuir surface area were calculated by standard models in the QuantachromeTM NovaWin software.

Figure 5a presents XRD patterns of the newly synthesized ZIF-71 particles. Comparing with the XRD pattern of the simulated ZIF-71,32 all ZIF-71 possess the same peaks confirming the same crystalline structure. However, the XRD pattern of ZIF-71-30nm is slightly broader than the other XRD patterns. It should be noted that the broadening XRD peaks may be due to the smaller crystal size based on Scherrer equation.58 Figure 5b compares the thermal stability of ZIF-71 particles with different particle sizes. All ZIF-71 16


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particles start to decompose at temperature above 300°C. However, ZIF-71-30nm (i.e., the smallest particle size) has the lowest thermal stability. In addition, the thermal stability of ZIF-71 linearly increases as the particle size increases.

Figure 5. (a) XRD patterns and (b) thermal stability (in N2 atmosphere) of ZIF-71 with different particle sizes.

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3.2. Effect of ZIF-71 particle size on MMMs The effect of ZIF-71 particle size on MMMs morphology was investigated by FESEM, XRD and density measurement. Figure 6 presents the cross-sectional images of ZIF-71 based MMMs, whilst Figure 7 displays the XRD patterns of the pristine 6FDA-Durene as well as ZIF-71 based MMMs. MMMs containing small ZIF-71 particles of 30 – 600 nm can be homogeneously dispersed in 6FDA-Durene matrices. A comparison of Figures 6ac and Figures 1-3 indicates that the embedded ZIF-71 sizes correspond well with the assynthesized particle sizes. XRD patterns of all MMMs (Figure 7) further confirm that ZIF71 particles have been successfully incorporated inside 6FDA-Durene matrices. It should be noted that the broad amorphous peak of 6FDA-Durene becomes noticeable as the particle size of ZIF-71 increases. Meanwhile, Table 3 demonstrates the density results of the fabricated MMMs. It can be clearly seen that addition of ZIF-71 with different particle sizes at the same weight loading reduces the density of the MMMs as a function of ZIF-71 size. This phenomenon may imply the possibility of voids present in the ZIF-71 containing MMMs.

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Figure 6. Cross-sectional morphology of (a) 6FDA-Durene-ZIF71-30nm, (b) 6FDADurene-ZIF71-200nm, and (c) 6FDA-Durene-ZIF71-600nm MMMs.

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Figure 7. XRD patterns of the pristine 6FDA-Durene and 6FDA-Durene-ZIF71 MMMs with different particle sizes.

Table 3. Density of the fabricated membranes. -1 Membranes Density (g.mL ) 6FDA-Durene 1.279 6FDA-Durene-ZIF71-30nm 1.255 6FDA-Durene-ZIF71-200nm 1.230 6FDA-Durene-ZIF71-600nm 1.226

3.3. Gas transport properties of MMMs The effect of ZIF-71 particle size on gas transport properties of MMMs was investigated. In order to minimize the effect of ZIF-71 loadings, the MMMs were fabricated to contain 20 wt% ZIF-71 particles. The actual ZIF-71 loadings in MMMs were further quantified by TGA. As disclosed in Table S1 and Figure 8, the ZIF-71 loadings in all fabricated MMMs are comparably similar and close to 20 wt%. Table S1 and S2 further show their gas permeation properties. All gas permeability increases with the incorporation of 20 wt% 20


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ZIF-71 particles. To clearly elucidate the effect of ZIF-71 particle size, Figure 9 plots the gas permeation properties. All other gas pair selectivity drops as compared to the pristine 6FDA-Durene membrane. Table 4 summarizes both solubility and diffusivity coefficients as a function of ZIF-71 size for light gases except H2. Solubility coefficients were obtained by using equation (7), while diffusivity coefficients were calculated based on equation (5). Addition of ZIF-71 mainly enhances the diffusivity coefficient with small improvements on solubility coefficients of O2, N2 and CH4. Furthermore, the reduction in ideal selectivity is majorly contributed by the reduction of solubility selectivity. This behavior implies that ZIF-71 may have lower selectivity for light-gas pairs which can be ascribed to its larger aperture size and pore cavity than the kinetic diameters of these light gases.

Figure 8. Thermal stability (in air atmosphere) of 6FDA-Durene-ZIF71 MMMs with different particle sizes.

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Figure 9. Effect ZIF-71 particle size on (a) H2/CH4, (b) H2/N2, (c) H2/CO2, (d) O2/N2, (e) CO2/CH4 and (f) CO2/N2 transport properties (blue y-axes denote faster permeating gas permeability and black y-axes represent selectivity).

Table 4. O2, N2, CO4 and CH4 permeability, diffusivity, solubility coefficients as well as their corresponding ideal selectivity of the pristine 6FDA-Durene, 6FDA-Durene-ZIF7130nm, 6FDA-Durene-ZIF71-200nm, and 6FDA-Durene-ZIF71-600nm at 3.5 atm and 35 °C. a b c (a) P D S Membranes O2 N2 CO2 CH4 O2 N2 CO2 CH4 O2 N2 CO2 CH4 6FDA-Durene 6FDA-DureneZIF71-30 6FDA-DureneZIF71-200 6FDA-DureneZIF71-600

184

55

805 47.6 79.9 29.4 44.7 9.06 2.30 1.87 18.0 5.25

575 186 2560 181 222 79.2 153 29.9 2.60 2.35 16.7 6.06 643 207 2744 198 243 84.2 167 31.5 2.65 2.46 16.5 6.27 386 120 1656 109 146 53.4 96.7 18.5 2.65 2.24 17.1 5.87

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(b)

O2/N2

Membranes 6FDA-Durene 6FDA-Durene-ZIF71-30 6FDA-Durene-ZIF71-200 6FDA-Durene-ZIF71-600

CO2/N2

CO2/CH4

αP

αD

αS

αP

αD

αS

αP

αD

αS

3.35 3.10 3.10 3.18

2.71 2.80 2.88 2.73

1.23 1.11 1.08 1.18

14.7 13.8 13.2 13.5

1.52 1.93 1.98 1.81

9.64 7.13 6.68 7.65

17.0 14.2 13.9 14.7

4.93 5.13 5.29 5.22

3.43 2.76 2.62 2.92

a

permeability (10-10 cm3(STP) cm cm-2 s-1 cmHg-1), diffusivity coefficient (10-8 cm2 s-1), c solubility coefficient (10-2 cm3(STP) cm-3 cmHg-1) b

3.4. Maxwell and modified-Maxwell prediction of O2 and N2 permeability In general, gas permeation across a heterogeneous membrane can be described by the Maxwell’s model where particles are randomly distributed in the continuous phase.17, 18, 59 The effective permeability of an MMM, Peff, can be expressed by the following equation (8):

 P  2 PC  2 D PC  PD  Peff  PC  D   PD  2 PC   D PC  PD  

(8)

where PC and PD refer to the permeability of the continuous phase and the dispersed phase, respectively.  D represents the volume fraction of the dispersed phase which can be calculated from the ZIF-71 loading using equation (9) as follows:

D 

wt D   M

(9)

D

where wt D is the weight fraction of ZIF-71 inside the MMM (i.e., the actual loadings of ZIF-71 obtained from TGA, Table S1).  M and  D represent the densities of the MMM and the dispersed phase (ZIF-71), respectively. The density of MMM is obtained from

23



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Page 24 of 43

Table 3 whereas the density of ZIF-71 is assumed to be 1.184 g.cm-3.60 In addition, the permeability of O2 and N2 across ZIF-71 (PD) was estimated from the simulation results.39

Figures 10 and 11 compare the permeability of MMMs between the experimental and predicted values using the Maxwell equation. Only the MMM consisting of 600 nm ZIF71 has a comparable permeability with the Maxwell equation. The MMMs comprising ZIF71 of 30 nm and 200 nm have experimental permeability much higher than the predicted ones. This behavior may indicate a non-idealized nature of the fabricated MMMs. In addition, the original Maxwell equation (and the most up to date approaches) disregards particle size effect. Therefore, a modified-Maxwell model was utilized to describe the O2 and N2 permeability of MMMs with different particle sizes. This modified-Maxwell model was based on the studies of Mahajan and Koros as well as Li et al.61, 62 In this modifiedMaxwell model, the MMM was assumed to be a pseudo-three phase composite with the first phase of polymer matrix. The second phase consists of the third phase with a nonideal polymer region around the dispersed phase and the third phase is the dispersed phase with a sieve-in-a-cage model. The permeability of the third phase can be computed by utilizing the Maxwell equation for the sieve-in-a-cage model,61 equation (10), as follows:

 P  2 PG  23 PG  PD  P3rd  PG  D   PD  2 PG  3 PG  PD  

(10)

where P3rd represents the composite permeability of the third phase, PG signifies the permeability of the gap around the dispersed phase, and 3 is the volume fraction of the dispersed phase in the third phase, which can be quantified by the following equation:

24


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3 

D

(11)

 D  G

where G is the volume fraction of the gap around the dispersed phase in the total membrane which can be estimated with the aid of equation (12) by using the weight fraction of ZIF-71 ( wt D ), the densities of MMM (  M ), ZIF-71 (  D ) and polymer matrix (  C ) obtained from Table 3.

G  1 

wt D   M

D



1  wt D    M C

(12)

The permeability of the second phase ( P2 nd ) can be further calculated based on Maxwell equation by assuming the non-ideal polymer region as the continuous phase and the third phase as the dispersed phase.

 P  2 PR  2 2 PR  P3rd  P2 nd  PR  3rd   P3rd  2 PR   2 PR  P3rd  

(13)

where PR is the permeability of the non-ideal polymer region and 2 is the volume fraction of the third phase inside the second phase which can be estimated by the following equation:

2 

 D  G  D  G   R

(14)

where  R is the volume fraction of the non-ideal polymer region in the total membrane.

Finally, the effective permeability of an MMM ( PMMM ) can be obtained using Maxwell equation for the third time by regarding the polymer matrix as the continuous phase and the second phase as the dispersed phase as shown in the following equation: 25



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

 P  2 PC  2  D  G   R  PC  P2 nd  PMMM  PC  2 nd   P2 nd  2 PC   D  G   R  PC  P2 nd  

Page 26 of 43

(15)

As the gas may pass through the gap outside ZIF-71 in the sieve-in-a-cage model, PG is obtained by calculating the permeability of dispersed phase (PD) with a model parameter, γ, as shown in equation (16). On the other hand, the permeability of the non-ideal polymer region is estimated by dividing the permeability of continuous phase with a model parameter, β, as provided in equation (17).

PG  PD  

PR 

(16)

PC

(17)



26


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Figure 10. Comparison of experimental O2 permeability vs. Maxwell and modifiedMaxwell predicted permeability of ZIF-71 containing MMMs. ■, ● , and ▲ symbolize the experimental permeability of 30 nm, 200 nm and 600 nm-containing MMMs, respectively, whilst dashed and dotted lines represent the Maxwell and the modifiedMaxwell predicted gas permeability. Lines are for guiding eye purpose.

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Figure 11. Comparison of experimental N2 permeability vs. Maxwell and modifiedMaxwell predicted permeability of ZIF-71 containing MMMs. ■, ● , and ▲ symbolize the experimental permeability of 30 nm, 200 nm and 600 nm-containing MMMs, respectively, whilst dashed and dotted lines represent the Maxwell and the modifiedMaxwell predicted gas permeability. Lines are for guiding eye purpose.

In order to obtain the model parameters and volume fraction of non-ideal polymer region, equations (10)-(17) were fitted with O2 and N2 permeability of MMMs and the results were summarized in Table 5, whilst the comparison of modified-Maxwell and Maxwell models is displayed in Figures 10 and 11. As shown in both Figures 10 and 11, the modifiedMaxwell model has satisfactorily fitted the O2 and N2 permeability of MMMs with 28


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different particle sizes. On the other hand, the Maxwell model generates a similar gas permeability for all MMMs with different particle sizes. A further analysis of Table 5 demonstrates the effect of ZIF-71 particle size on O2 and N2 permeability. It can be clearly seen that both  R and γ follow the trend of gas permeability, whilst β has a larger effect on selectivity. This phenomenon suggests that reducing the particle size of ZIF-71 induced a higher possibility to generate voids around ZIF-71 particles, as inferred by the model parameter γ. However, it should be noted that the term “void” does not necessarily indicate the ideal polymer-particle void; instead the model parameter γ has directly taken into account the particle agglomeration, the inter-particle voids and other non-ideal factors which are not considered in the Maxwell model. Meanwhile, the volume fraction of nonideal polymer region,  R , also indicates the direct effect of ZIF-71 particle size, with ZIF71 of less than 200 nm having comparable values, which may be due to the higher surface area of smaller size ZIF-71. Owing to the smaller than 1 of β, it can be inferred that the non-ideal polymer region may consist of inefficiently packed polymer chains owing to the presence of ZIF-71, which has been reported by Lau et al.63 in their nanocomposite membranes. Table 5 displays the similar effect of ZIF-71 particle size on the formation of the inefficiently packed polymer chains with larger ZIF-71 possessing larger β value. A similar observation has been reported by Moore et al.17 Table 5. Model parameters of modified-Maxwell for predicting O2 and N2 permeability of ZIF-71 containing MMMs. ϕR Particle size γ β 30 nm

0.24

3.83

0.14

200 nm

0.27

12.5

0.15

600 nm

0.06

0.22

0.17 29



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3.5. Comparison with the upper bound Figure 12 illustrates the comparison of other gas pairs with Robeson (2008) upper bounds.12 The separation performance deteriorates and moves nearer to the upper bound as the particle size increases. It should be noted that our earlier work in 2014 with 100 nm ZIF-71 showed a different trend as compared to the current study.52 The molecular weights of 6FDA-Durene synthesized in this study, i.e., the weight average molecular weight (Mw) of 126,669 g.mol-1, the number average molecular weight (Mn) of 85,882 g.mol-1 and polydispersity (PDI = Mw/Mn) of 1.48, are different from our earlier work’s (Mw = 201,000 g.mol-1, Mn = 158,000 g.mol-1 and PDI = 1.27). This phenomenon is consistent with the observation in our earlier work that a higher molecular weight 6FDA-Durene will generate a membrane with a higher permeability.52 In addition, the method to synthesize ZIF-71 in this study is different from our previous work which may further generate ZIF-71 with different surface characteristics. A similar observation has also been reported by Yang et al when synthesized Mg-MOF-74 using two different methods.64

30


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Figure 12. Benchmarking of pure gas transport properties vs. Robeson (2008) upper bound for various gas pairs. (■ denotes the pristine 6FDA-Durene membrane, ▲, x, and ♦ symbolize 6FDA-Durene-ZIF71 MMMs with particle sizes of 30 nm, 200 nm, and 600 nm, respectively)

Further analysis on Figure 12 indicates that MMMs with 30 and 200 nm particles have better gas separation performance than their counterpart with larger particles. A similar phenomenon has also been observed by Rodenas et al.30 for CO2/CH4 separation. Despite the fact that smaller particles may have better gas separation performance, this study suggests that the 6FDA-Durene-ZIF71 MMM comprising ZIF-71 particles of 200 nm possesses the best gas separation performance. Hence, a trade-off between gas separation

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performances vs. particle size has to be taken into account when designing a suitable MMM for gas separation applications.

4. CONCLUSIONS A rapid 4-hour and simple mixed solvent method was utilized to synthesize ZIF-71 with particle sizes of 30 nm, 200 nm and 600 nm at room temperature. Both nitrogen adsorption at 77 K and XRD experiments reveal that ZIF-71 particles of 30 nm may still be in the nucleation stage as compared to other large particles. A series of 6FDA-Durene-ZIF71 MMMs have been fabricated. Gas permeation results show that the incorporation of ZIF71 into 6FDA-Durene mainly improves the permeability. However, the separation performance deteriorates and moves close to the upper bound as the particle size increases. The O2 and N2 permeability of MMMs comprising ZIF-71 with particle sizes of ≤ 200 nm have higher permeability than the predicted ones using the Maxwell model. By considering the inefficiently packed polymer chain and the sieve-in-a-cage phenomena, the modified Maxwell model displayed comparable permeability over the experimental results suggesting non-ideal nature of MMMs. A comparison of MMMs with three different particle sizes over the upper bound shows that the 6FDA-Durene-ZIF71 MMM comprising ZIF-71 particles of 200 nm has the best gas separation performance, particularly for H2/CH4 separations.

ASSOCIATED CONTENT Supporting Information

32


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Gas transport properties of the fabricated membranes and intrinsic gas transport properties of ZIF-71. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Fax: (65)-67791936. Email: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources National University of Singapore (NUS) under the project entitled “Membrane research for CO2 capture” (Grant number: R-279-000-404-133) and the Dean’s Office, Faculty of Engineering, NUS under the project title of Natural Gas Center (NUS grant number R-261508-001-646).

Notes The authors declare no competing financial interest.

33



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ACKNOWLEDGEMENTS The authors would like to thank the National University of Singapore (NUS) under the project entitled “Membrane research for CO2 capture” (Grant number: R-279-000-404133), and the Dean’s Office, Faculty of Engineering, NUS under the project title of Natural Gas Center (NUS grant number R-261-508-001-646) for funding this research. The authors also thank Ms. Jen Ga Neo, Dr. Xue Li, Ms. Yu Zhang, Mr. Chunfeng Wan, Dr. Wai Fen Yong, Dr. Lin Hao and Dr. Kuo-Sung Liao for their valuable assistance and fruitful discussion.

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For Table of Contents Only A series of 6FDA-Durene-ZIF71 mixed matrix membranes (MMMs) comprising three different ZIF-71 particle sizes (i.e., 30, 200, and 600 nm) have been fabricated to investigate the effects of particle size on gas separation performance of MMMs. Compared to MMMs containing ZIF-71 particle size of > 200 nm, ZIF-71 embedded MMMs with particle sizes of ≤ 200 nm exhibited significantly enhanced gas separation performance. The O2 and N2 permeability of MMMs comprising ZIF-71 with particle sizes of ≤ 200 nm have higher permeability than the predicted ones using the Maxwell model. By considering 42


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the inefficiently packed polymer chain and the sieve-in-a-cage phenomena, the modified Maxwell model displayed comparable permeability over the experimental results suggesting non-ideal nature of MMMs. It was found that ZIF-71 of 200 nm generated the best MMM for gas separation, particularly for H2/CH4 systems. Therefore, a trade-off between ZIF-71 particle size vs. gas separation performance must be taken into account when designing an MMM suitable for industrial gas separation applications.

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