Mixing Effect of Ligand on Carbon Dioxide Capture Behavior of

Subscriber access provided by University of Sunderland

Article

The Mixing Effect of Ligand on Carbon Dioxide Capture Behavior of Zeolitic Imidazolate Framework/Poly (amide-b-ethylene oxide) Mixed Matrix Membranes Chien-Chieh Hu, Chia-Her Lin, Yu-Hsuan Chiao, Yu-Ching Huang, Hui-An Tsai, Wei-Song Hung, Kueir-Rarn Lee, and Juin-Yih Lai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03789 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 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

ACS Sustainable Chemistry & Engineering

The Mixing Effect of Ligand on Carbon Dioxide Capture Behavior of Zeolitic Imidazolate

Framework/Poly

(amide-b-ethylene

oxide)

Mixed

Matrix

Membranes

Chien-Chieh Hu*,†,‡, Chia-Her Lin§, Yu-Hsuan Chiao║, Yu-Ching Huang‡, Hui-An Tsai*,‡, WeiSong Hung†,‡, Kueir-Rarn Lee‡, Juin-Yih Lai†,‡,# †Graduate

Institute of Applied Science and Technology, Applied Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and Technology, No.43, Keelung Rd., Sec.4, Da'an Dist., Taipei 10607, Taiwan. ‡R&D Center for Membrane Technology, Chung Yuan University, No.200, Chung Pei Road, Chung Li District, Taoyuan 32023, Taiwan. §Department of Chemistry, Chung Yuan University, No.200, Chung Pei Road, Chung Li District, Taoyuan 32023, Taiwan. ║Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States. #Department of Chemical Engineering, National Taiwan University of Science and Technology, No.43, Keelung Rd., Sec.4, Da'an Dist., Taipei 10607, Taiwan.

Corresponding Authors *Email: [email protected] (C-C.H.) *Email: [email protected](H-A.T)

ACS Paragon Plus Environment

1

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

Page 2 of 26

ABSTRACT Mixed matrix membranes (MMMs) were synthesised from poly(amide-b-ethylene oxide) (Pebax) and mixed-ligand zeolitic imidazolate framework (ZIF-8-90(x)). The separation performance and microstructure of the membranes with different organic linker fractions and ZIF-8-90(x) loadings were characterized. Energy-dispersive X-ray microanalysis and Field-emission scanning electron microscopy showed excellent adhesion between Pebax and ZIF-8-90(x), and no voids or clusters were observed. Single gas permeation measurements on MMMs revealed that the CO2/N2 selectivity of the membranes with ZIF-8-90(x) was better than that of the membrane with ZIF-8. Permeation analysis showed that the membranes with mixed ZIF-8 and ZIF-90 offers a tight microstructure. Overall, this study demonstrated that the gas separation behaviour of the Pebax membrane was significantly changed by ZIF-8-90(x) loading. The ZIF-8-90(x)/Pebax membranes can be a good candidate for CO2 capture applications. KEYWORDS: Mixed-ligand ZIF, Mixed matrix membrane, CO2 capture, Poly(amide-b-ethylene oxide)

▓ INTRODUCTION At present, fossil fuels are the main source of energy.1,2 However, carbon dioxide emissions resulting from fossil fuel combustion increase global warming. Hence, significant efforts have been devoted by different governments and research groups worldwide to develop methods for the separation of CO2 from other gases. The membrane separation of gases is widely recognized as a promising technology.3–5 Moreover, CO2 gas separation methods that employ membranes are more energy saving than other available absorption technologies.6,7


2

Page 3 of 26 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


Polymer-based materials possess several advantages, such as cost effectiveness and good mechanical properties.8 However, the industrial utilization of polymer membranes is limited by permeability and selectivity trade-off.9 Although inorganic membranes show superior performance in gas separation, an industry-scale fabrication of defect-free inorganic membrane is expensive and challenging. The creation of MMMs is introduced to overcome the limitations and disadvantages of polymer and inorganic membranes. Given their high performance and convenient processing, MMMs have become the superior candidate for gas separation.10–12 Inorganic fillers can alter the free volume and packing structure of polymeric materials to allow selective gas transport in the membrane. Incorporating inorganic molecular sieve fillers with polymers integrates the size selectivity of porous inorganic particles with the mechanical stability and processing ability of polymers.13–17 Although significant MMMs have been developed in the last two decades, the poor adhesion between inorganic fillers and organic polymers inhibits their industrial applicability. Lin and Freeman reported that polymers containing ethylene oxide units are a good choice for obtaining CO2 removal membranes with favourable CO2/light gas selectivity and CO2 permeability.18 Ethylene oxide containing block copolymers as Pebax (poly(amide-b-ether)) has been studied as a gas separation membrane.19–21 Previous works showed that Pebax is a promising candidate to remove CO2 from flue and natural gas. Pebax shows a high carbon dioxide permeability due to the high affinity of its rubbery ethylene oxide segment with the carbon dioxide molecule. In addition, the membrane possesses mechanical strength due to the crystalline polyamide phase. Moreover, the high-chain mobility promotes adhesion between polymer and fillers. As a result, the incorporation of inorganic fillers to the Pebax membrane increases


3


Page 4 of 26

permeability and/or selectivity. The separation performance of MMMs depends on the type and property of fillers. Metal-organic frameworks (MOFs) have gained significant interesting in CO2 separation due to the porous structure. MOFs possess sorption capacities, high specific surface area and selective affinities for gases,22–25 which render them interesting membrane materials for gas separation.26–30 Zeolitic imidazolate frameworks (ZIFs) are a special type of metal-organic frameworks with network topology and related properties different from those of conventional zeolites. Stability comparison studies suggest that ZIFs possesses better thermal and chemical robustness than conventional MOFs. X-ray diffraction reveals that ZIF-8 has a sodalite zeolite-type structure with 11.6 Å large cavities and 3.4 Å small aperture. Increasing evidence suggests that gases with molecular size larger than the aperture size of ZIF-8 can be efficiently separate by ZIF-8.31,32 Haldoupis et al. predicted that ZIF-8 has extraordinarily high selectivity for CO2/CH4 mixtures due to its rigid crystal structure.33 The framework modification of porous materials allows the materials properties to be adjusted for specific gas separation.34 Two synthetic approaches are used by Thompson et al. to introduce functionality into the ZIF: (i) mixed-ligand ZIF synthesis with a substitution ligand of 2-aminobenzimidazole and (ii) postsynthetic modification of a mixed-ligand ZIF-8-90-(50) with ethylenediamine.35 The experimental results showed turning the surface properties of ZIFs by ether mixed-ligand synthesis and postsynthetic modification may generate new ZIFs with improved gas separation properties. Thompson et al. also report a study of ZIF/Matrimid 5128 MMMs, containing mixed-ligand ZIFs with ZIF-8 crystal topologies but composed of different organic ligand composition.36 Single gas permeation testing reveal that inclusion of mixed-ligand ZIFs yields MMMs with better CO2/CH4 selectivity than MMMs containing ZIF-8. The properties of ZIFs can also be adjusted via the combination of different


4

Page 5 of 26 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


ligands and aperture size. The general route for accessing multifunctional ZIFs is organic linker metathesis in ZIF secondary-building units while retaining their parent topology. For the synthesized ZIF-8-90, the cubic crystal structure (ZIF-8) and the aldehyde functional group with large aperture (3.5 Å) (ZIF-90) of the parent frameworks are retained. Numerous studies analysed the gas separation ability of ZIFs and MMMs, but only a few focused on the performance of hybrid ZIFs and MMMs. Thus, the present study investigated the CO2 separation behaviour of MMMs containing ZIF-8-90(x) fillers, where x is the quantity of incorporated aldehyde groups. Pebax MH1657 served as the matrix due to its known and successive CO2 separation properties. The effective selectivity and permeability properties of ZIF-8-90(x)/Pebax MMMs were achieved to explore the mixing effect of ligand on gas separation behaviour of mixed matrix membranes. ▓ EXPERIMENTAL SECTION Materials. Arkema Plastics (France) supplied Pebax® MH1657. N,N-Dimethylformamide (DMF, 98%) was obtained from Merk. Sodium formate (NaCO2H, 98%) was obtained from Alfa Aesar. 2-Methylimidazole (MeIM, 99%), 2-imidazolecarboxaldehyde (CHO-IM, 97%), magnesium nitrate hexahydrate (Mg(NO3)2 · 6H2O, 98%), zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 98%), ethanol (EtOH, 99.5%) and methanol (MeOH, 99.5%) were purchased from Sigma– Aldrich. All chemicals directly used in the experiments without any additional purification. Synthesis of ZIF-8. A solution of NaCO2H (10 mmol) and MeIM (10 mmol) in 25 mL of MeOH was heated to 50 ℃ to obtain a clear solution and then cooled down to room temperature. A second solution containing Zn(NO3)2·6H2O (2.5 mmol) in 25 mL of deionized water was prepared. Then, both solutions mixed and stirred at room temperature for 1 h. The precipitate from the resulting solution was centrifuged for 5 min and then washed with MeOH for three times. Finally, the precipitate was dried at 85 ℃ overnight.


5


Page 6 of 26

Synthesis of ZIF-90. Equal molar NaCO2H (10 mmol) and COH-IM were dissolved in MeOH (10 mL) to obtain the COH-IM solution. Mg(NO3)2·6H2O (0.25 mmol) or Zn(NO3)2·6H2O (2.25 mmol) dissolved in 40 mL of DMF and 10 mL of deionised water to produce the Zn and Mg solutions, respectively. Afterwards, the three solutions were mixed by magnetic stirring for 1 h at room temperature. The resulting precipitate solution centrifuged at 8000 rpm for 5 min and then washed with methanol for three times. The resulting powder was dried at 85 ℃ overnight. Synthesis of ZIF-8-90(x). The ZIF-8-90(x) hybrids were synthesised in accordance with the process used by Thompson et al. with some modifications.36 The aldehyde group was controlled as x%. In brief, equal molar NaCO2H (10 mmol) and the COH-IM and MeIM (3:7, 5:5 and 7:3) mixture were dissolved in 25 mL of MeOH at 50 ℃ to obtain solution A. Zn(NO3)2·6H2O dissolved in 5 mL of DMF and 20 mL of deionised water to obtain solution B. Solution A (room temperature) was mixed with solution B for 1 h at room temperature. The outcome powder was washed with MeOH for three times and then dried at 85 ℃. The chemical structure and composition of ZIF-890(X) was shown in Figure S1. Membranes Preparation. ZIF powder was dried at 180 oC for 24 h to get the activated ZIF particle. The activated ZIF particle was dispersed in H2O/ethanol (25/75 by weight) solution by sonication for 2 h. Once the ZIF particles were well dispersed, Pebax® MH1657 was added and stirred to dissolve at 80 ℃ for 6 h to obtain a 3 wt% Pebax solution containing homogeneously dispersed ZIF particles. The ZIF loading was set to 0, 10, 20 and 30 wt% based on Pebax. The ZIF/Pebax dope solution was sonicated in a 50 ℃ water bath for another 2 h and then poured into a Petri dish. Subsequently, the solvent was allowed to evaporate at 85 ℃ for 6 h. The obtained membranes were placed in a vacuum oven at room temperature and left for 24 h to remove the trace solvent. The thickness of membranes varied from 60 μm to 90 μm. The membrane


6

Page 7 of 26 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


preparation process for the direct mixed ZIF MMMs is the same with above procedures. Both quantitative ZIF-8 and ZIF-90 particles dispersed in H2O/ethanol (25/75 by weight) solution by sonication for 2 h and then Pebax® MH1657 was added and stirred to dissolve at 80 ℃ for 6 h to obtain a 3 wt% Pebax casting solution. Characterization Methods. The linker composition in the ZIF framework was determined by using XPS (VG Microtech MT-500 ESCA). A field-emission scanning electron microscope (FESEM) (Model: S-4800N, Hitachi, Japan) was used to take the images of the MMMs. The energydispersive X-ray microanalysis (EDX) system (SEM-EDX model: S-3000N, Hitachi, Japan) was employed for analysing the dispersion of ZIF particles in the MMMs. A differential scanning calorimeter (Perkin-Elmer DSC-7) was used to measure the crystallinity of the MMMs in N2 atmosphere at 10 K/min scanning rate. The free volume characteristics of the MMMs were evaluated by specialized positron annihilation lifetime (PAL) spectroscopy. The spectrum fits into three lifetime components using the PATFIT software.37–39 In this work, a positron annihilation lifetime spectrum includes 2 x 106 counts; meanwhile, intensities (I1, I2 and I3) and lifetimes (τ1, τ2 and τ3) belong to the para-positronium, free positron and ortho-positronium annihilation. The most used o-Ps lifetime (τ3), the so-called pickoff annihilation, is 1–5 ns in polymers. The MELT software was used to get the annihilation lifetime distribution of the MMMs. Gas Permeation Measurement. The pure-gas permeability of the MMMs for N2, CO2 and CH4 at 35 °C (±0.5 °C) was measured using a gas permeation analyser (Yanaco GTR10). The permeability (P) unit is barrer [10–10 (cm3 (STP) cm)/(cm2 s cmHg)]. The ideal selectivity in terms of the permeability can be expressed as follows:

 A/ B 

PA PB

(1)


7


Page 8 of 26

where PA and PB are the permeability for gas A and B. A Cahn microbalance (model D202) was utilised to determine the solution coefficient (S) of ZIF-8-90(x) and the MMMs. Permeability is equal to the product of diffusion (D) and solution coefficient (S):

P  DS

(2)

By rearranging Equation (2), we determined the diffusion coefficient as follows:

D

P S

(3)

▓ RESULTS AND DISCUSSION Characteristics of ZIF-8-90(x) Fillers. ZIF-8-90(x) fillers containing varied proportions of CHO-IM and MeIM ligands was prepared to understand whether microstructural changes of mixed-ligand ZIF affect the gas separation performance of the ZIF-8-90(x)/Pebax MMMs. XPS was utilised to measure the composition of the as-prepared ZIF-8-90(x) (Table 1). The oxygen content increased with increasing quantity of aldehyde groups. The O/C ratios of the synthesised ZIF-8-90(x) were very close to the theoretical O/C ratios, showing that ZIF-8-90(x) was successfully synthesised. The CO2 and N2 sorption isotherms were used to investigate the gas sorption ability of single and mixed-ligand ZIFs because the gas sorption ability of fillers changes the gas separation performance of the MMMs. The CO2 adsorption isotherm (Figure 1a) of ZIF8-90(x) revealed that the CO2 uptake was in the order of ZIF-90 > ZIF-8-90(50) > ZIF-8 at a lowpressure value. Mixed-ligand ZIFs show an improvement in gas adsorption capacity and selectivity was demonstrated by Thompson et al..35 The aperture and CO2 affinity of ZIF-8-90(x) increased with increasing CHO-IM ratio, which enhanced the uptake of CO2. However, our previous research showed that the pore volume of ZIF-8-90(x) decreases with increasing CHO-IM ratio


8

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


(Table S1).34 The gas uptake dominated by the pore volume of the adsorbent at a high-pressure. The effect of pore volume on gas uptake may surpass the effect of affinity at a high-pressure. ZIF-8 shows higher pore volume which results in more CO2 uptake at a high-pressure. For N2 adsorption (Figure 1b), the sorption isotherms were very close to ZIF-8-90(x). This result indicates that the mixing of ligand does not obviously affect the N2 sorption behaviour of ZIF-8-90(x). Table 1. ESCA Results for the Fillers. Zn(%)

C(%)

N(%0

O(%)

O/C

ZIF-8

9.0

66.7

23.0

1.3

0.02

O/C Theoretical 0.00

ZIF-8-90(30)

9.8

62.0

22.2

6.0

0.10

0.08

ZIF-8-90(50)

11.0

60.2

21.1

7.7

0.13

0.13

ZIF-8-90(70)

11.3

57.4

22.7

8.6

0.15

0.18

ZIF-90

10.4

56.1

21.8

11.7

0.21

0.25

Sample

Figure 1. Sorption isotherms of the filler at 308 K. (a) CO2 uptake, (b) N2 uptake.


9


Page 10 of 26

(a)

(b)

(c)

(d)

Figure 2. Cross-sectional image of membranes: (a) pristine Pebax, (b) ZIF-8 (30 wt%), (c) ZIF-890(50) (30 wt%) and (d) ZIF-90 (30 wt%). Morphology Studies of Membranes. Figure 2 shows the SEM micrographs of the controlled Pebax membrane and ZIF-8-90(x)/Pebax MMMs. The morphological identification of the Pebax membrane confirms the well-defined dense membrane surface (Figure 2a). Notable, the membrane contained 30 wt% ZIF-8-90(x), which clearly shows the absence of interface voids and the improved adhesion property between the filler and the polymer phase. The good adhesion was due to the flexible chain mobility of Pebax and the hydrophobic nature of ZIF-8-90(x). Unlike the traditional zeolite-filled MMMs, ZIF-8-90(x) fillers do not require any pre-treatment for preventing interface cracks.


10

Page 11 of 26 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


MMMs can successfully maintain the characteristics of the filler through uniform filler dispersion. The particle agglomeration problem in most studies of MMMs was not observed in this study. Figure S2 a–c shows the SEM-EDX zinc mapping results for the ZIF-8-90(x)/Pebax MMMs. Each ZIF-8-90(x) particle was fully discrete in the MMMs, and no clusters formed. SEM images (Figure 2) also show the evidence that ZIF-8-90(x) particles well disperse in MMMs.

Figure 3. Gas separation performance of ZIF-8-90(x)/Pebax MMMs with different ZIF contents. Gas Separation Results of ZIF-8-90(x)/ Pebax MMMs. Figure 3a shows the permeability increased with increasing ZIF-8 content. The Pebax membrane achieved the CO2 permeability around 171 barrer, whereas the highest permeability of CO2 was 330 barrer for ZIF-8/Pebax MMM. The higher ZIF-8 loading may cause the interconnection of fillers which results in the permeability increased dramatically for 30 wt%. The gas diffusivity and solubility in the MMMs was in direct proportion to the amount of ZIF-8 loading because of the fixed pore structure of ZIF.


11


Page 12 of 26

According to the solution-diffusion model, the permeability of the Pebax membrane should be lower than that of the ZIF-8/Pebax MMMs. Membrane selectivity for both CO2/N2 and CO2/CH4 decreased with ZIF-8 content increasing, which may be due to the decreasing crystallinity of the membrane and the defects formed at the interface between ZIF-8 and Pebax. Table S2 shows that the crystallinity of the ZIF-8/Pebax membrane was lower than that of Pebax, while Figure 2 displays the lack of obvious interface defects. The reducing crystallinity of the membrane dominates the selectivity decrease. Figure 3b, c show that the permeability also increased with increasing filler content; nevertheless, the selectivity (CO2/N2) of the Pebax membrane was lower than that of the ZIF-90/Pebax membrane. The permeability mild increasing for 30 wt % ZIF-90 loading MMM is due to the bigger aperture size of ZIF-90 reduces the effect of filler interconnection on permeability increasing. Besides, the selectivity (CO2/CH4) decreased with increasing filler loading. ZIF-90 was composed of 2-imidazolecarboxaldehyde, and ZIF-8 was composed of 2-methylimidazole. The interaction of the carboxaldehyde group with the condensable gas (CO2) was much higher than that of the methyl group. The higher the fraction of CHO-IM linkers in ZIF-8-90(x), the higher the CO2 solubility. The solubility selectivity of ZIF-890(x) controls the selectivity of MMMs, causing the lower CO2/N2 selectivity of Pebax compared with the MMMs. For CO2/CH4 selectivity, the effect of decreasing crystallinity was higher than the effect of solubility selectivity. As a result, the CO2/CH4 selectivity of the Pebax membrane was higher than that of the MMMs. The above discussion indicates that the ZIF-8-90(x)/Pebax membranes are suitable for separating CO2 from N2. Figure 4 illustrates the permeation data of Pebax membrane and MMMs with 30 wt% ZIF-890(x). The permeability of the MMMs evidently increased when ZIF-8-90(x) was used as the filler in Pebax. The ZIF-8/Pebax membrane showed higher permeability than the ZIF-90/Pebax


12

Page 13 of 26 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


membrane. We hypothesise that the interaction between ZIF and Pebax increases as the MeIM ligands are substituted by COH-IM, creating a difficult diffusion interphase for gas molecules. In addition, the pore volume of ZIF-8 (0.698 cm3/g) is higher than that of ZIF-90 (0.572 cm3/g).33

Figure 4. Gas separation results of the Pebax membrane and MMMs contain 30 wt% ZIF-8-90(x).


13


Page 14 of 26

These two factors resulted in the higher permeability of the ZIF-8/Pebax membrane than the ZIF90/Pebax membrane. As the substitution fraction of MeIM in ZIF-8 was increased, the permeability of the ZIF-8-90(x)/Pebax MMMs, except that of the ZIF-8-90(70)/Pebax membrane, decreased (Figure 4a). The pore volume decreased and difficult diffusion interphase increased with increasing COH-IM fraction, which reduced the permeability of the ZIF-8-90(x)/Pebax membrane. Thompson et al. also presented that the mixed ligand approach controls the porosity and pore size of the mixed-ligand ZIFs, the characteristics of mixed-ligand ZIFs alter the gas separation performance of MMMs.36 Notably, the ZIF-8-90(70)/Pebax membrane showed abnormal permeability. The addition of ZIF-8-90(70) may form MMMs with special microstructure that prefer CO2 permeation. However, the underlying mechanism needs to be explored in the future. Figure 4b shows the ideal permselectivity of CO2/N2 increased as the fraction of COH-IM linkers in ZIF-8-90(x) was increased. Additionally, an obvious improvement of CO2/N2 selectivity was observed compared with that of the Pebax membrane. This result is due to the increased interaction between Pebax and ZIF-8-90 with increasing COH-IM linkers in the ZIF-8-90(x). Likewise, a denser interphase improved the ZIF-8-90(x) sieving effect, which enhanced the separation performance of the ZIF-8-90(x)/Pebax MMMs. The gas sorption capacity of polymer membranes can be increased by incorporating highsorption-capacity fillers into the polymer. The CO2 and N2 adsorption isotherm in Pebax and ZIF8-90(x)/Pebax membranes are shown in Figure 5. Comparing the adsorption isotherm of the Pebax membrane to that of the ZIF-8-90(70)/Pebax MMMs, we found that the addition of porous ZIF-890(x) fillers highly improved the gas sorption capacity of the Pebax membrane. As shown in Figure 5a,b, an overall sorption capacity of the membranes increased with the substitution of increasing COH-IM in the ZIF-8 framework. Increasing the crystallinity of the membrane may decrease the


14

Page 15 of 26 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


sorption capacity of the membrane. As shown in Table S2, the crystallinity of the membrane monotonically decreased with increasing fraction of COH-IM, which was consistent with the increasing gas sorption capacity of the membranes due to the polar functional groups. Interestingly, the CO2 adsorption of the ZIF-90/Pebax membrane was lower than that of the ZIF-8-90(70)/Pebax membrane. This finding shows that ZIF-8-90(70) can have a significant microstructure that allows the adsorption and diffusion of gas molecules, which confers the ZIF-8-90(70)/Pebax membrane with notable permeability. The relationship between permeability, solubility, and diffusivity of MMMs was shown as Table S3. The gases solubility of MMMs significantly increases with the substitution of increasing COH-IM in the ZIF-8 framework. The diffusivity may increase or decrease due to the addition of ZIF-8-90(x) and does not show a consistent trend. We can conclude that the permeability and selectivity increasing of MMMs was mainly owing to the improving of solubility by adding the ZIF-8-90(x) particles.


15


Page 16 of 26

Figure 5. Sorption isotherms of the membranes. (a) CO2 uptake, (b) N2 uptake. Microstructure of the Membranes. Free volume or microporosity is one of the key factors that affect the gas separation behaviour of membranes. Positron annihilation lifetime spectroscopy (PALS) has emerged as an advanced approach to study the free volume or microporosity of membranes. The PALS analysis of the controlled ZIF-8-90(x)/Pebax MMMs is shown in Table 2. The filler addition increased the 3 of the ZIF-8-90(x)/Pebax membranes compared with that of Pebax membrane. This result indicates that the free volume size of the ZIF-8-90(70)/Pebax membranes is larger than that in the Pebax membrane. Besides, the free volume size also reduced with increasing COH-IM linkers. The pore volume and surface area of the filler and the interaction between filler and polymer may affect the free volume size and fractional free volume (ffv) of MMMs. In this work, the interaction between filler and polymer should be the main factor which affects the free volume characteristics of MMMs. The interaction of COH-IM with Pebax was


16

Page 17 of 26 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


stronger than that of MeIM, which increased the molecular packing density of interphase with increasing COH-IM linkers. This phenomenon decreased the free volume size and ffv. Figure S3 shows the specific ATR-FTIR spectra for the Pebax and MMMs with 30 wt% ZIF. The characteristic transmittance bands at around 1100 cm-1 is the stretching vibrations of C−O group in Pebax chain. The bands shift to smaller wave number side when the fraction of the COH-IM ligand in ZIF-8-90(x) increasing. The bands shift is due to the strong interaction between Pebax and ZIF-8-90(x). Free volume size and ffv are consistent with the permeability (Figure 4a) and the gas sorption (Figure 5a). Figure 6 shows the free volume distribution of Pebax and the ZIF-890(x)/Pebax MMMs. The free volume size in the MMMs can be controlled by adding ZIF-8-90(x) into Pebax. The pore size distribution became broad when ZIF-8-90(x) was added into Pebax. The permeability of the MMMs increased with increasing free volume, but the selectivity of the MMMs increased with free volume decreasing. The free volume distribution results support the observation that both permeability and selectivity of the ZIF-8-90(x)/Pebax membranes improved. Table 2. PALS Results of Neat Pebax and MMMs with 30wt% ZIF-8-90(x). Sample

3 (ns)

I3 (%)

ffv(%)

Pebax

2.42

15.7

3.9

ZIF-8/Pebax

2.80

19.2

6.2

ZIF-8-90(70)/Pebax

2.50

13.7

3.6

ZIF-90/Pebax

2.48

13.4

3.5


17


Page 18 of 26

Figure 6. Positronium lifetime distribution of neat Pebax and MMMs with 30wt% ZIF-8-90(x).

Figure 7. Gas separation performance varied with operation temperature. (a) Pebax membrane (b) MMM with 30wt% ZIF-8-90(70). Temperature Effects. The relationship between the separation performance of the Pebax membrane and its operation temperature is shown in Figure 7a. The permeability increased with operation temperature increasing up to 55 ℃. The selectivity considerably decreased with operation temperature increasing. This result is similar with the trade-off behaviour in glassy polymer membranes. Two hypotheses can explain this result. First, an increase in operation temperature improves the polymer chain mobility and enhances the free volume, which causes high permeability but low selectivity. Second, an increase in temperature upgrades the kinetic energy of gas molecules that pass through the membrane. Figure 7b shows the gas separation


18

Page 19 of 26 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


performance of the MMMs with 30 wt% ZIF-8-90(70). The selectivity decreased but the permeability increased with increasing operation temperature, which is similar to the case of the Pebax membrane. This shows that the addition of ZIF-8-90(x) does not change the temperature effect on the Pebax membrane. A similar phenomenon has also been observed in Vapor CrossLinked 6FDA-Durene/ZIF-71 MMMs.40

Figure 8. CO2 capture performance of the membranes prepared in this work with comparison to Robeson’s upper bound presented in 2008. Comparative Study. The CO2 capture behaviour of the mixed-ligand ZIF (ZIF-8-90(x)) MMMs was discussed in this work. We determined whether the CO2 capture performance of the ZIF-8-90(x)/Pebax membrane is different from that of the MMM prepared by direct mixing ZIF8 and ZIF-90. The data shown in Table 3 are generated for direct comparison. The ZIF-8/ZIF90/Pebax membrane demonstrated significantly higher permeability than the ZIF-8-90(x)/Pebax membrane. Likewise, the selectivity of the ZIF-8/ZIF-90/Pebax membrane was lower than that of the ZIF-8-90(x)/Pebax membrane. The selectivity increased with increasing ZIF-90 fraction in the membranes prepared using ZIF-8-90(x) or ZIF-8 mixed with ZIF-90. As shown in Table 3, directly mixing ZIF-8 and ZIF-90 into Pebax may create a membrane with a looser microstructure. To evaluate the potential of the ZIF-8-90(x)/Pebax membranes as a CO2 capture membrane, we


19


Page 20 of 26

compared the permeability/selectivity of the ZIF-8-90(x)/Pebax membranes with that of 2008 Robeson’s upper bound. Figure 8 shows that the ZIF-90/Pebax and ZIF-8-90(70)/Pebax MMMs lie above the upper bound. The CO2 capture performance of the other ZIF-8-90(x)/Pebax membranes was below the upper bound. These data indicate that the ZIF-8-90(x)/Pebax MMMs are fit for CO2 capture. Table 3. CO2 Capture Performance for ZIF-8-90(x)/Pebax MMM and Physical Mixing ZIF8/ZIF-90/Pebax MMM. Membrane

PCO2 (Barrer)

αCO2/N2

ZIF-8-90(30)/Pebax

306.1

51.0

[70% ZIF-8 + 30% ZIF-90]/Pebax

348.3

42.9

ZIF-8-90(50)/Pebax

264.2

54.1

[50% ZIF-8 + 50% ZIF-90]/Pebax

334.6

48.1

■ CONCLUSIONS The gas separation performance and microstructure of the MMMs can be successfully adjusted by changing the organic ligand fraction in the mixed-ligand ZIF and ZIF-8-90(x) loading. The MMMs exhibited a quite good adhesion between Pebax and ZIF-8-90(x). Both permeability and selectivity of the membranes can be increased by mixing ZIF-8-90(x) into Pebax. Increasing the operation temperature increased the permeability but decreased the selectivity of ZIF-890(x)/Pebax. The mixed organic ligand approach offers a tight microstructure of the MMMs. The gas separation performance shows that ZIF-8-90(x) is a promising filler in MMMs and thus can be applied in CO2 capture.


20

Page 21 of 26 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


■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. SEM-EDX zinc mapping cross-sections of the MMMs; Crystallinity of the MMMs with 30wt% ZIF-8-90(x) and pristine Pebax membrane.

▓ AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (C-C.H.) *Email: [email protected](H-A.T)

▓ NOTES The authors declare no competing financial interest.

▓ ACKNOWLEDGMENTS The authors sincerely thank the Ministry of Economic Affairs and the Ministry of Science of Taiwan for their financial support on this work. The authors also thank Dr. Mani Sivakumar for editing the manuscript.

REFERENCES (1) MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C.S.; Williams, C. K.; Shah, N.; Fennell, P. An overview of CO2 capture technologies, Energy


21


Page 22 of 26

Environ. Sci. 2010, 3, 1645-1669, DOI 10.1039/c004106h. (2) Xian, S.; Peng, J.; Zhang, Z.; Xia. Q.; Wang, H.; Li, Z. Highly enhanced and weakened adsorption properties of two MOFs by water vapor for separation of CO2/CH4 and CO2/N2 Binary Mixtures, Chem. Eng. J. 2015, 270, 385-392, DOI 10.1016/j.cej.2015.02.041. (3) Robeson, L. M. Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 1991, 62, 165-185, DOI 10.1016/0376-7388(91)80060-j. (4) Koros, W. J.; Fleming, G. K. Membrane-based gas separation, J. Membr. Sci. 1993, 83, 1-80, DOI 10.1016/0376-7388(93)80013-n. (5) Zhuang, Y.; Seong, J. G.; Do, Y. S.; Lee, W. H.; Lee, M. J.; Guiver, M. D.; Lee, Y. M. Highstrength, soluble polyimide membranes incorporating Tröger’s base for gas separation, J. Membr. Sci. 2016, 504, 55-65, DOI.org/10.1016/j.memsci.2015.12.057. (6) Merkel, T. C.; Lin, H.; Wei, X.; Baker, R. Power plant post-combustion carbon dioxide capture: an opportunity for membranes, J. Membr. Sci. 2010, 359, 126-139, DOI.org/10.1016/j.memsci.2009.10.041. (7) Li, T.; Pan, Y.; Peinemann, K. V.; Lai, Z. Carbon dioxide selective mixed matrix composite membrane containing ZIF-7 nano-fillers, J. Membr. Sci. 2013, 425-426, 235-242, Doi.org/10.1016/j.memsci.2012.09.006. (8) Wang, S.; Li, X.; Wu, H.; Tian, Z.; Xin, Q., He, G.; Peng, D.; Chen, S.; Yin, Y.; Jiang, Z.; Guiver, M. D. Advances in high permeability polymer-based membrane materials for CO2 separations, Energy Environ. Sci. 2016, 9, 1863-1890, DOI 10.1039/c6ee00811a. (9) Robeson, L. M. The upper bound revised, J. Membr. Sci. 2008, 320, 390-400, DOI.org/10.1016/j.memsci.2008.04.030. (10) Chung, T. S.; Jiang, L. Y.; Li, Y.; Kulprathipanja, S. Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 2007, 32, 483-507, DOI.org/10.1016/j.progpolymsci.2007.01.008. (11) Moore, T. T.; Mahajan, R.; Vu, D. Q.; Koros, W. J. Hybrid membrane materials comprising organic

polymers

with

rigid

dispersed

phase,

Aiche

J.

2004,

50,

311-321,

DOI.org/10.1002/aic.10029. (12) Noble, R. D. Perspectives on mixed matrix membranes, J. Membr. Sci. 2011, 378, 393397, DOI.org/10.1016/j.memsci.2011.05.031. (13) Vu, D. Q.; Koros, W. J.; Miller, S. J. Mixed matrix membranes using carbon molecular


22

Page 23 of 26 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


sieve I. preparation and experimental results, J. Membr. Sci. 2003, 200, 311-334, DOI.org/10.1016/s0376-7388(02)00429-5. (14) Jia, M. D.; Peinemann, K. V.; Behling, R. D. Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas separation, J. Membr. Sci. 1991, 57, 289-296, DOI.org/10.1016/s0376-7388(00)80684-5. (15) Kim, J. H.; Lee, Y. M. Gas permeation properties of poly(amide-6-b-ethylene oxide) – silica hybrid membranes, J. Membr. Sci. 2001, 193, 209-225, DOI.org/10.1016/s03767388(01)00514-2. (16) Ismail, A. F.; Rahim, N. H.; Mustafa, A.; Matsuura, T.; Ng, B. C.; Abdullah, S.; Hashemifard, S. A. Gas separation performance of polyethersulfone/multi-walled carbon nanotubes

mixed

matrix

membranes,

Sep.

Purif.

Technol.

2011,

80,

20-31,

DOI.org/10.1016/j.seppur.2011.03.031. (17) Li, Y.; Chung, T. S. Molecular-level mixed matrix membranes comprising Pebax® and POSS for hydrogen purification via preferential CO2 removal, Int. J. Hydrog. Energy 2010, 35, 10560-10568, DOI.org/10.1016/j.ijhydene.2010.07.124. (18) Lin, H.; Freeman, B. D. Materials selection guidelines for membranes that remove CO2 from gas mixtures, J. Mol. Struct. 2005, 739, 57-74, DOI.org/10.1016/j.molstruc.2004.07.045. (19) Chen, J. C.; Feng, X.; Penlidis, A. Gas permeation through poly(ether-b-amide) (Pebax 2533)

block

copolymer

membranes,

Sep.

Sci.

Technol.

2004,

39,

149-164,

DOI.org/10.1081/ss-120027406. (20) Kim, J. H.; Ha, S.Y.; Lee, Y. M. Gas permeation of poly(amide-6-b-ethylene oxide) copolymer, J. Membr. Sci. 2001, 190, 179-193, DOI.org/10.1016/s0376-7388(01)00444-6. (21) Bondar, V.; Freeman, B. D.; Pinnau, I. Gas sorption, characterization of poly(ether-bamide) segmented block copolymers, J. Polym. Sci. Part B polym. Phys. 1999, 37, 2463-2475, DOI.org/10.1002/(sici)1099-0488(19990901)37:173.0.co;2-h. (22) Furukawa, H.; Yaghi, O. M. Storage of hydrogen, methane, and carbon dioxide in highly porous covalent organic frameworks for clean energy applications, J. Am. Chem. Soc. 2009, 131, 8875-8883, DOI 10.1021/ja9015765. (23) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. High H2 adsorption in a microporous metal-organic framework with open metal sites, Angew Chem. Int. Ed. 2005, 44, 4745-4749, DOI.org/10.1002/anie.200462787.


23


Page 24 of 26

(24) Wong-Foy, A. G.; Matzger, A. J. Yaghi, O. M.; Exceptional H2 saturation uptake in microporous metal-organic frameworks, J. Am. Chem. Soc. 2006, 128, 3494-3495, DOI 10.1021/ja058213h. (25) Ma, S.; Zhou, H. C. Gas storage in porous metal-organic frameworks for clean energy applications, Chem. Commun. 2010, 46, 44-53, DOI 10.1039/b916295j. (26) Yoo, Y.; Lai, Z.; Jeong, H. K. Fabrication of MOF-5 membranes using microwave-induced rapid seeding and solvothermal secondary growth, Microporous Mesoporous Mater. 2009, 123, 100-106, DOI.org/10.1016/j.micromeso.2009.03.036. (27) Zacher, D.; Shekhah, O.; Wöll, C. Fischer, R. A. Thim films of metal-organic frameworks, Chem. Soc. Rev. 2009, 38, 1418-1429, DOI 10.1039/b805038b. (28) Liu, Y.; Ng, Z.; Khan, E. A.; Jeong, H. K.; Ching, C. B.; Lai, Z. Synthesis of continuous MOF-5 membranes on porous -alumina substrates, Microporous Mesoporous Mater. 2009, 118, 296-301, DOI.org/10.1016/j.micromeso.2008.08.054. (29) Ploegmakers, J.; Japip S.; Nijmeijer, K. Mixed matrix membranes containing MOFs for ethylene/ethane separation Part A: Membrane preparation and characterization, J. Membr. Sci. 2013 428, 445-453, DOI.org/10.1016/j.memsci.2012.11.014. (30) Japip, S.; Wang, H.; Xiao, Y.; Chung, T. S. Highly permeable zeolitic imidazolate framework (ZIF)-71 nano-particles enhanced polyimide membranes for gas separation, J. Membr. Sci. 2014, 467, 162-174, DOI.org/10.1016/j.memsci.2014.05.025. (31) Bux, B. H.; Chmelik, C.; Baten, J. M. V.; Krishna, R.; Caro, J. Novel MOF-membrane for molecular sieving predicted by IR-diffusion studies and molecular modeling, Adv. Mater. 2010, 22, 4741-4743, DOI.org/10.1002/adma.201002066. (32) Zhang, C.; Dai, Y.; Johnson, J. R.; Karvan, O.; Koros, W. J. High performance ZIF8/6FDA-DAM mixed matrix membrane for propylene/propane separations, J. Membr. Sci. 2012, 389, 34-42, DOI.org/10.1016/j.memsci.2011.10.003. (33) Haldoupis, E.; Nair, S.; Sholl, D. S. Efficient calculation of diffusion limitations in metal organic framework materials: a tool for identifying materials for kinetic separation, J. Am. Chem. Soc. 2010, 132, 7528-7539, DOI 10.1021/ja1023699. (34) Liao, Y. T.; Dutta, S.; Chien, C. H.; Hu, C. C.; Shieh, F. K.; Lin, C. H.; Wu, K. C. W. Synthesis of mixed-ligand zeolitic imidazolate framework (ZIF-8-00) for CO2 adsorption, J. Inorg. Organomet. Polym. 2015, 25; 251-258, DOI 10.1007/s10904-014-0131-z.


24

Page 25 of 26 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


(35) Thompson, J. A.; Vaughan, J. T.; Brunelli, N. A.; Koros, W. J.; Jones, C. W.; Nair, S. Mixed-linker zeolitic imidazolate framework mixed-matrix membranes for aggressive CO2 separation from natural gas, Microporous Mesoporous Mater. 2014, 192, 43-51, DOI.org/10.1016/j.micromeso.2013.06.036. (36) Thompson, J. A.; Brunelli, N. A.; Lively, R. P.; Johnson, J. R.; Jones, C. W.; Nair, S. Tunable CO2 adsorbents by mixed-linker synthesis and postsynthetic modification of zeolitic imidazolate frameworks, J. Phys. Chem. C 2013, 117, 8198-8207, DOI 10.1021/jp312590r. (37) Dlubek, G.; Pionteck, J.; Bondarenko, V.; Pompe, G.; Taesler, C.; Petters, K.; KrauseRehberg, R. Positron annihilation lifetime spectroscopy (PALS) for interdiffusion studies in disperse blends of compatible polymers: a quantitative analysis, Macromolecules 2002, 35, 6313-6323, DOI 10.1021/ma020451z. (38) Xie, W.; Ju, H.; Geise, G. M.; Freeman, B. D.; Mardel, J. I.; Hill, A. J.; McGrat, J. E. Effect of free volume on water and salt transport properties in directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers, Macromolecules 2011, 44, 4428-4438, DOI 10.1021/ma102745s. (39) Chen, H.; Hung, W. S.; Lo, C. H.; Huang, S. H.; Cheng, M. L.; Liu, G.; Lee, K. R.; Lai, J. Y.; Sun, Y. M.; Hu, C. C.; Suzuki, R.; Ohdaira, T.; Oshima, N.; Jean, Y. C. Free-volume depth profile of polymeric membranes studied by positron annihilation spectroscopy: layer structure from interfacial polymerization, Macromolecules 2007, 40, 7542-7557, DOI 10.1021/ma071493w. (40) Japip, S.; Liao, K. S.; Chung T. S. Molecularly tuned free volume of vapor cross-linked 6FDA-durene/ZIF-71 MMMs for H2/CO2 Separation at 150 °C, Adv. Mater. 2017, 29, 1603833, DOI.org/10.1002/adma.201603833.


25


Page 26 of 26

TOC

The development of high performance ZIF-8-90(x)/Pebax membranes for CO2 capture is one of the key issues for sustainability.


26

Mixing Effect of Ligand on Carbon Dioxide Capture Behavior of

Recommend Documents