Novel ZIF-300 Mixed-Matrix Membranes for Efficient CO2 Capture

Oct 19, 2017 - This work demonstrated that the proposed ZIF-300/PEBA MMM could be a potential candidate for an efficient CO2 capture process...
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Novel ZIF-300 Mixed-Matrix Membranes for Efficient CO Capture Jianwei Yuan, Haipeng Zhu, Jiajia Sun, Yangyang Mao, Gongping Liu, and Wanqin Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12507 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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Novel ZIF-300 Mixed-Matrix Membranes for Efficient CO2 Capture Jianwei Yuan, Haipeng Zhu, Jiajia Sun, Yangyang Mao, Gongping Liu*, Wanqin Jin* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China Corresponding author: [email protected] (Prof. W. Jin); [email protected] (Dr. G. Liu)

Abstract Because of the high separation performance and easy preparation, mixed-matrix membranes (MMMs) which were consist of metal-organic frameworks have received much attention. In this contribution, we report a novel ZIF-300/PEBA MMM consisting of zeolite imidazolate framework (ZIF-300) crystals and polyether block amide (PEBA) matrix. The ZIF-300 crystal size was effectively reduced by optimizing the hydrothermal reaction condition from ~15 µm to ~1 µm. The morphology, physicochemical and sorption properties of the synthesized ZIF-300 crystals and as-prepared ZIF-300/PEBA MMMs were systematically studied. The results showed that ZIF-300 crystals with the size of ~1 µm maintained an excellent preferential CO2 sorption over N2 without degradation of the crystal structure in the MMMs. As a result, uniformly incorporated ZIF-300 crystals highly enhanced both the CO2 permeability and CO2/N2 selectivity of pure PEBA membrane. The optimized ZIF-300-PEBA MMMs with ZIF-300 loading of 30 wt% exhibited a high and stable CO2 permeability of 83 Barrer and CO2/N2 selectivity of 84, which are 59.2 % and 53.5 % higher than pure PEBA membrane, respectively. The obtained performance surpassed the upper bound of state-of-the-art membranes for CO2/N2 separation. This work demonstrated that the proposed ZIF-300/PEBA MMM could be a potential candidate for efficient CO2 capture process.

Keywords: ZIF-300, PEBA, mixed-matrix membranes, CO2 sorption, CO2/N2 1

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1. Introduction Owing to fossil fuel combustion and mankind activities, greenhouse gases (especially CO2) are becoming one of the major global issues. In order to achieve both environmental and economic benefits, these CO2 should be captured.1-2 Hence, highly efficient CO2 capture and storage from a huge amount of energy-intensive industrial gases have attracted much attention, including biogas, flue gas, natural gas and so on.1-4 Among different conventional technologies such as solvent adsorption, chemical looping and cryogenic distillation, membrane separation shows easy operation and energy effectiveness.5-6 Polymeric materials have been applied in the membrane CO2 capture, such as polyamides,7 polyimides,8 polycarbonates,9 polysilicone,10 poly(ethylene oxides)11. However, polymeric membranes generally suffer a “trade-off” between permeability and selectivity.12-13 Nowadays, metal-organic frameworks (MOFs) as a kind of membrane materials, which is connected by metal centers and organic ligands, have attracted much attention because of the distinct properties of variable pore/aperture size, large surface area and specific adsorption affinity.14-16 As a result, MOFs-based membranes are considered as a promising candidate for molecular separation.17-24 The fabrication of MOFs membranes is complicated, and some of them are subjected to non-selective defects, such as cracks and the inter-crystal gaps, resulting in compromised separation efficiency. Alternatively, mixed-matrix membranes that combine the high-performing filler with the easy-processable polymer matrix25-32 have shown great potential to realize the practical development of MOFs-based membranes. It has been demonstrated that incorporating CO2-philic MOF crystals into polymer is an effective approach to developing high-performance CO2 separation membranes. Gascon and co-workers33 incorporated functionalized NH2-MIL-53(Al) particles to fabricate mixed-matrix membranes exhibiting a high selectivity for CO2/CH4 separation. [Cu3(BTC)2] with triangular windows of 0.35 nm diameter were incorporated into Matrimid® matrix for the separation of mixed gases, which showed improved CO2 permeability as well as CO2/CH4 and CO2/N2 selectivity.34 Bae et al.27 incorporated 2

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submicrometer-sized ZIF-90 crystals into polyimide to enhance the CO2 permeability. Recently, our group21 developed NH2-UiO-66/PEBA mixed matrix membranes, showing excellent and stable CO2/N2 separation performance beyond the Robeson upper-bound of polymeric membranes. In addition, smaller particle size of MOF crystals would be favor of improving the interfacial adhesion between the filler and polymer matrix, as well as decreasing the separation layer thickness for the composite/asymmetric mixed-matrix membranes. The influence of the filler size on the performance of mixed-matrix membranes have been well studied in recent papers 35-37

. Recently, a new kind of CO2-philic zeolite imidazolate frameworks 300 (ZIF-300,

Zn(2-mIm)0.86(bbIm)1.14) with chabazite (CHA) topology was synthesized by introducing two imidazolate links.38 It was facilely prepared by reacting zinc nitrate hexahydrate and 2-methylimidazolate with the 5(6)-bromobenzimidazolate link in N, N-dimethylformamide (DMF)/water mixtures. Tetrahedral Zn atoms are surrounded by four ditopic imidazolate links and linked together by imidazolate bridges to generate a three-dimensional CHA network. It has been reported that ZIF-300 shows significantly higher sorption capacity and affinity for CO2 over N2, enabling effectively dynamic separation of CO2 from N2. There is no performance loss over several sorption-desorption cycles in the ZIF-300 sorbents and they can be easily regenerated by purging N2 flow under room temperature. In view of these attractive features, we expected that ZIF-300 could be a promising membrane material for CO2 capture. However, to our best knowledge, it has not been realized for membrane separation until now. This paper, for the first time, presents the design and fabrication of high-performing ZIF-300-based membranes. As shown in Figure 1, ZIF-300 crystals were synthesized by solvothermal method and the crystal size was tuned by varying the synthesis condition. The obtained fine ZIF-300 particles were incorporated into a commercial polymer matrix, polyether block amide (PEBA), to fabricate ZIF-300 mixed-matrix membranes. PEBA, which consists of ethylene oxide as soft segments and amide groups as hard segments, was chosen to provide high CO2 permeability, as 3

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well as and sufficient mechanical strength.3 The as-prepared ZIF-300/PEBA MMM exhibited enhancement in both CO2 permeability and CO2/N2 selectivity than that of pure PEBA membrane. The membranes with optimized ZIF-300 loading showed an excellent and stable CO2/N2 separation performance beyond the upper-bound for state-of-the-art membranes. Moreover, the transport mechanism of the membrane were interpreted by analyzing the coefficients and selectivity of sorption and diffusion, thereby elaborating the CO2/N2 separation mechanism of ZIF-300/PEBA MMM.

Figure 1. Schematic illustration of membrane preparation process.

2. Experimental 2.1. Synthesis of ZIF-300 crystals The preparation of ZIF-300 MOFs was fabricated following the procedures in recent work.38 For ZIF-300, 2-mImH (11.7 mg, 0.143 mmol), bbImH (28.2 mg, 0.143 mmol), and Zn (NO3)4H2O (29.8 mg, 0.114 mmol) were dissolved in a solvent mixture of DMF (4.75 mL) and water (0.25 mL) in a PTFE reaction still (10 mL). Then, the reaction still was heated at 120 oC for nearly 3-6 days. The yellow powder was achieved by using centrifugal machine at 10000 rpm for 10 min and washed by DMF for at least three times. After the initial DMF washing, as-synthesized samples of ZIF-300 were soaked in methanol for solvent exchange for 3 days under room 4

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temperature. Finally, at the third day, the samples was evacuated at 10 mTorr at ambient temperature for 24 h, followed by subsequent heating at 180 oC for 24 h.

2.2. Fabrication of membranes ZIF-300 powder was dispersed in 7/3 (wt) ethanol/water mixed-solvent, follow by subsequent probe-type sonicator (SCIENTZ) at 300 W power for 10 min in ice bath. Then, a certain amount of dried PEBA (PEBAX MH 1657, Arkema, France) were added into the mixture solution containing ZIF-300 powder followed by stirring and refluxing at 80 oC for 12 h. Then, the solution was casted on a Teflon dish and left to evaporate the solvent for 3days under room temperature. Subsequently, the membrane was dried in an oven under 70 oC for 12 h. As a control, pure PEBA was also prepared with the same procedure as described above.

2.3. Characterizations Morphologies of the ZIF-300 MOFs crystals and ZIF-300/PEBA mixed matrix membranes were characterized by scanning electron microscope (SEM, S4800, Hitachi, Japan). All samples were treated with gold under vacuum. The crystalline structures of ZIF-300 MOFs crystals and ZIF-300/PEBA membranes were characterized by X-ray diffractometer (XRD, D8-advance, Bruker, Germany) using Cu K radiation, in the range of 2.5-50o with an increment of 0.05o at the room temperature. Fourier transform infrared (FT-IR, Nicolet8700, Nicolet, USA) spectra were recorded in spectrophotometer with the range of 400–4000 cm-1, using the KBr disk technique. The adsorption measurements of the ZIF-300 MOFs crystals were conducted by ASAP 2460 with CO2 (298 K) and N2 (298 K, 77K). BET surface area was calculated from the N2 isotherms at 77 K. The sorption properties of the ZIF-300/PEBA MMMs were measured by high-pressure adsorption apparatus (Belsorp-HP) at 293 K. The thermal property of ZIF-300 MOFs was carried out by thermogravimetric analysis (TGA, NETZSCH STA 449F3) from 25 oC to 800 oC with the rate of 10 oC/min. Differential scanning calorimetry (DSC, Q2000, TA instruments, USA) measurements were operated from -80 to -40 oC in the N2 atmosphere to obtain 5

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the glass transition temperature (Tg) of the ZIF-300/PEBA MMMs.

2.4. Gas permeation measurement Gas permeation test was carried out by constant-volume, variable-pressure method with a transmembrane pressure of 0.4 MPa at 20 °C. The membrane sample was cut into circular disks and mounted onto the home-made permeation cell. The effective permeation area was 3.2 cm2. The transport of gases through gas separation membrane follows the solution-diffusion model.39 Gas permeation is characterized by the permeability, P. Each measurement was repeated at least six times. After the system reached steady state, we use the following equation (1):







 = 3.5930 ∙ ∆ ∙ .  ∙  ∙ (  )

(1)

where  is the gas permeability [1 Barrer = 10-10 cm3 (STP) cm/ (cm2·s·cmHg)], is

∆ the transmembrane pressure (MPa), l is the thickness of the membrane, T is the room temperature (°C). A is the effective permeation area (cm2), and dp/dt is the pressure displacement rate. Permeability can also be expressed as the product of the diffusion coefficient, D, and the sorption coefficient, S, of a given gas i within the membrane, as shown in equation (2):

 =  ∙ 

(2)

where D and S characterize the kinetic and thermodynamic contribution to transport, respectively. S can be expressed by equation (3): 

 = 

(3)



where



is the gas concentration absorbed in the membrane, and ! is the fugacity

driving force of component i. The selectivity is the ratio of permeability of the individual gas and can be expressed by equation (4):

&

'

"$ =  = &  ∙ '  = "& ∙ "' #

%

%

%

where "& is the diffusion selectivity and "' is the sorption selectivity. 6

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(4)

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3. Results and discussions 3.1. Synthesis of ZIF-300 crystals Since particle size of the filler has a significant influence on the mixed matrix membranes, we firstly studied how to synthesize ZIF-300 with desired particle size. It was found that the particle size of ZIF-300 could be easily controlled by varying the synthesis time. SEM images of ZIF-300 particles with various synthesis time are shown in Figure 2. Homogeneous crystal morphology was observed in all the ZIF-300 samples synthesized under different hydrothermal synthesis time at 120 oC. It suggests high phase purity, which is further confirmed by XRD analysis discussed later. Generally, shorter synthesis time produces smaller ZIF-300 crystals. In our case, as the synthesis time was decreased by half (from 144 h to 72 h), the average size of ZIF-300 crystals was dramatically reduced by ~15 times (from ~15 µm to ~1 µm). Further lowering the synthesis time, which was expected to produce ZIF-300 crystals with submicron size, however failed to form the right crystal structure, namely is unable to obtain ZIF-300 crystals in current study. Some other parameters such as hydrothermal temperature or reactant concentration might need optimizations to further reduce the crystal size of ZIF-300, which are out of the scope of this study. Here, ZIF-300 crystals with ~1 µm particle size were used for the fabrication of mixed-matrix membranes.

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Figure 2. SEM images of ZIF-300 MOFs with different synthesis time at 120 oC: (a) 144 h, (b) 120 h, (c) 96 h, (d-e) 72 h; (f) effect of synthesis time on the average crystal size of ZIF-300. The powder X-ray diffraction (PXRD) patterns of ZIF-300 crystals synthesized by different hydrothermal time are shown in Figure 3a. It suggests that all these ZIF-300 MOFs crystals were with highly crystalline structure that are agreed with the simulation result and report in literature.38 Furthermore, the thermal stability and pore structures of ZIF-300 crystal synthesized by 72 h at 120 oC were investigated by thermogravimetric analysis and N2 adsorption-desorption isotherms, respectively. As shown in Fig. 4(b), the thermal decomposition occurs at ~500 oC, which is higher than the typical values of 350 oC for other MOFs.40-41 The obtained highly thermal stable property of ZIF-300 crystals could be owing to the intrinsic characteristic of the CHA-type ZIFs structure. Furthermore, no weight loss was observed below 300 oC also confirms the fully removal of guest molecules for ZIF-300 via the activation 8

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process (180 oC for 24 h under vacuum). It ensures the subsequent gas separation application for accessible pores within the ZIF-300 framework.

Figure 3. (a) XRD patterns of the ZIF-300 MOFs crystals synthesized by different hydrothermal reaction time and the simulated result. (b) TGA curve for ZIF-300 crystals, and the inset is the DTG curve. N2 sorption analysis was used to characterize the permanent porosity of ZIF-300 crystals and the result is shown in Figure 4a. Before the adsorption test, ZIF-300 MOFs crystals were activated at 120 oC for 24 h under vacuum for degassing. It followed type I isotherms with no hysteresis. The Langmuir (Brunauer-Emmet-Teller) BET surface area of ZIF-300 is calculated as 546 m2/g, which matches well with the reported value.38 The micropore volume and the micropore size for ZIF-300 crystals were 0.374 cm3/g and 0.79 nm, respectively. The relative large pore might be unable to offer substantial diffusion selectivity for gas pairs with molecular diameters smaller than the pore size, for instance: CO2 and N2 with kinetic diameter of 0.33 nm and 0.364 nm, respectively. Nevertheless, ZIF-300 shows higher sorption capacity and affinity for CO2 over N2,38 as proven by the thermodynamic CO2 and N2 uptake at 298 K for ZIF-300 crystals shown in Figure 4b. The isotherms for both CO2 and N2 exhibit a reversible adsorption and desorption process. At 1.05 bar, the sorption capacities of CO2 and N2 are 25.4 and 2.9 cm3 (STP)/g, respectively, clearly indicating a high sorption selectivity for CO2/N2 gas pair. It is also noticed that the initial slope of the CO2 isotherm is steeper than that for N2, indicating ZIF-300 has higher affinity for CO2 over N2. These features enable ZIF-300 as a favorable CO2-philic filler to modify the polymeric membranes for CO2/N2 separation. 9

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Figure 4. (a) N2 adsorption and desorption isotherms at 77 K for ZIF-300 crystals. (b) CO2 and N2 adsorption and desorption isotherms at 298 K for ZIF-300 crystals.

3.2. Membrane characterization 3.2.1. Morphology Morphology of the as-prepared pure PEBA membrane and ZIF-300/PEBA MMM was investigated by SEM. As displayed in Figure 5a, the cross-section of pure PEBA membrane is very smooth without any structural bulges. The cross-sectional SEM images of ZIF-300/PEBA MMMs varying ZIF-300 loading from 10 wt% to 40 wt% are shown in Figure 5b-f. It can be observed a homogenous morphology in the cross-section of PEBA membrane filled with ZIF-300 crystals up to 30 wt%. The enlarged image of 30 wt% sample clearly shows intact filler-polymer interfaces even exposed to high-energy electron beam that produced cracks in the PEBA matrix, as shown in Figure 5e. It suggests that ZIF-300 is compatible with PEBA, which is presumably attributed to the interactions between the organic linkers on the MOF and polymer chains. 42 However, further increasing the ZIF-300 loading to 40 wt%, the ZIF-300 crystals began to aggregate and some of them settled at the bottom of the membrane (Figure 5f), resulting in a poor ZIF-300-PEBA interfacial morphology with several large voids damaging the integrity of the mixed-matrix membrane.

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Figure 5. Cross-sectional SEM image of (a) pure PEBA; (b) 10 wt%, (c) 20 wt%, (d-e) 30 wt% and (f) 40 wt% ZIF-300/PEBA MMM. 3.2.2. Physicochemical properties The XRD patterns of ZIF-300/PEBA MMMs with various ZIF-300 loadings are shown in Figure 6a. Pure PEBA is a semi-crystalline copolymer consisting of both crystalline (PA6) and amorphous (PEO) phases. It thus showed characteristic peak ranging from 15o to 25o.43 This broad peak is remained in the MMMs with ZIF-300 loading from 10 to 40 wt%, suggesting the amorphous structure of PEBA matrix at room temperature. Meanwhile, the intrinsic characteristic peaks of ZIF-300 are also observed in all the MMM samples and their intensities are enhanced with the increase loading of ZIF-300. This result suggests that the physical incorporation did not affect the crystal structure of ZIF-300,31 ensuring the pore structure of ZIF-300 for molecular transport.

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Figure 6. (a) XRD patterns of pure PEBA membrane and ZIF-300/PEBA MMMs; (b) FT-IR spectra of ZIF-300 crystals, pure PEBA membrane and ZIF-300/PEBA MMMs.

FT-IR spectra of the PEBA and ZIF-300/PEBA MMMs were presented in Figure 6b. The characteristic peak at 1094 cm-1 is corresponding to the C-O-C stretching vibration from PEO part.43 The peaks at 1636 and 1732 cm-1 are attributed to H–N– C=O and O–C=O groups, respectively, resulting from the PA part.44 It’s reasonable to observe that the intensity of characteristic peaks from PEBA was reduced by increasing the amount of ZIF-300 crystals in the membrane. Besides of the respective characteristic peaks from ZIF-300 and PEBA, no new peak appears in these MMMs, further confirming the physical incorporation without chemical bonding between ZIF-300 crystals and PEBA chains. 3.2.3. Thermal properties The chain packing and flexibility of the PEBA membranes with different ZIF-300 loading were studied by differential scanning calorimetry (DSC) measurement., The Tg of pure PEBA membrane is ca. -52.3 oC, which agrees well with the literature (Figure 7).45 With the increase of the ZIF-300 loading in the PEBA matrix, the Tg was decreased gradually. The incorporation of ZIF-300 particles might inhibit the regular packing of PEBA chains, causing the decline of Tg. The looser structure of the matrix is expected to provide more pathways for gas diffusion through the membrane. 12

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Figure 7. Glass transition temperatures of ZIF-300/PEBA MMMs with different ZIF-300 loading 3.2.4. Gas sorption properties In order to investigate sorption properties of the ZIF-300/PEBA MMMs under the permeation condition (0.4 MPa, 20 oC), high-pressure sorption (up to 1.2 MPa) measurement was carried out to obtain the sorption isotherms of the MMMs for CO2 and N2 at 20 oC. ZIF-300-PEBA MMMs were treated at 70 oC for 24 h under vacuum for degassing. As shown in Figure 8, pure PEBA show higher sorption capacity for CO2 over N2 due to the higher condensability of CO2 and the affinity of PEBA chain towards CO2. The linear relationship between sorption amount and pressure indicates that CO2 or N2 sorption in rubbery PEBA generally follows the Henry’s law.46 By incorporating the porous CO2-philic ZIF-300 crystals, the CO2 sorption was significantly enhanced along with only a slight increase in N2 sorption capacity. For instance, in the 30 wt% ZIF-300 filled PEBA MMM, the CO2 sorption coefficient and CO2/N2 sorption selectivity of pure PEBA membrane at 0.4 MPa were increased from 9.8 to 17.8 ×10-2 cm3 (STP) cm-3 cmHg-1 and from 6.66 to 10.08, respectively. The sorption amount can be further increased by incorporating more ZIF-300 crystals in the membrane, as indicated by the isotherms of 40 wt% ZIF-300/PEBA MMM. These obtained sorption coefficients can also be used to calculate the diffusion coefficients by using the measured permeability of the membrane, which will be discussed in the next section. Moreover, it is observed that the sorption isotherm of the ZIF-300/PEBA MMM is likely the combination of PEBA matrix and ZIF-300 filler, suggesting that 13

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the highly porous structure of ZIF-300 was not affected by the surrounded PEBA chains.

Figure 8. CO2 and N2 sorption isotherms of pure PEBA membrane and ZIF-300/PEBA MMM with 30 wt% or 40 wt% ZIF-300 at 0-1.2 MPa and 20 oC.

3.2. Gas separation performance 3.2.1. Effect of ZIF-300 loading Gas permeation measurement was carried out to investigate the separation performance of the as-prepared ZIF-300/PEBA MMMs. According to the preferential sorption of CO2 over N2 in ZIF-300 as well as PEBA, CO2/N2 gas pair is chosen as the separation purpose since it is an important process for CO2 capture from flue gas. The effect of ZIF-300 loading on the CO2/N2 separation performance of ZIF-300/PEBA MMMs was investigated, as shown in Figure 9. When the loading of ZIF-300 increases up to 30 wt%, the CO2 permeability and CO2/N2 selectivity increased simultaneously, breaking the permeability-selectivity “trade-off” in conventional polymeric membranes.12-13 Compared to the pure PEBA membrane, CO2 permeability in the 30 wt% ZIF-300/PEBA MMM was increased by 59%, and meanwhile the CO2/N2 selectivity was improved by 54%. The significant improvement in permeability can be mainly attributed to the uniformly incorporated high-performing ZIF-300 filler. In addition, the disturbed packing of PEBA chains, as revealed by the decreased Tg in the ZIF-300/PEBA MMMs, might also reduce the gas transport resistance through the PEBA matrix. For another, the kinetic diameters of CO2 and N2 are 0.33 nm and 0.364 nm respectively, which are smaller than the 14

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micropore size of ZIF-300 is 0.79 nm (BET result in Fig. 4a). So the highly enhanced CO2/N2 selectivity is presumably owing to the excellent CO2 preferential sorption over N2 within ZIF-300 crystals, as displayed in Figures 4b and 8. When the ZIF-300 loading was increased to 40 wt%, CO2/N2 selectivity was sharply decreased back to the value of pure PEBA membrane. This reveals the present of non-selective defects that probably generated by the aggregation of ZIF-300 crystals in the PEBA matrix observed under SEM (Figure 5f). Previous studies21, 27, 29 also demonstrated that it remains challenging to fabricate defect-free mixed-matrix membranes with filler loading above 30 wt%. In addition, we also tried large sized ZIF-300 crystals for fabricating PEBA MMMs, while the preliminary result showed that they were prone to generate defects in the MMMs, thereby hindering the performance improvement and/or reproducible permeation results. Similar phenomena have been found in literature.47-50

Figure 9. Effect of ZIF-300 loading on the CO2/N2 separation performance of ZIF-300/PEBA MMMs. Pure gas permeation test was measured at 0.4 MPa and 20 oC.

The permeability (P) can be defined as the product of diffusion coefficient (D) and sorption coefficient (S).39 D was calculated by using the measured P and S in the permeation and sorption experiment, respectively. To better understand the role of ZIF-300 crystals in the gas transport of ZIF-300/PEBA MMMs, the sorption coefficient and diffusion coefficient of CO2 and N2, as well sorption selectivity ("' ) and diffusion selectivity ("& ) for CO2/N2 were studied in detail. As is shown in Figure 10, in the pure PEBA membrane, both sorption coefficient and diffusion coefficient 15

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for CO2 are higher than N2, owing to the higher condensability and smaller molecular size of CO2. By introducing ZIF-300 crystals in PEBA membrane, the sorption coefficient was increased while the diffusion coefficient was decreased, for both 30 wt% and 40 wt% loading. Owing to the CO2-philic feature of ZIF-300 crystals, the improvement of sorption coefficient for CO2 is higher than N2, leading to the enhanced CO2/N2 sorption selectivity in ZIF-300/PEBA MMMs. As discussed above, the relative large pore size of ZIF-300 could hardly contribute to discrimination for CO2 and N2, thus the CO2/N2 diffusion selectivity was not improved in ZIF-300/PEBA MMMs. The lower diffusion coefficient in the MMMs may be caused by reduced diffusivity through the MOF-polymer inter-phase and/or irregular stacking of ZIF-300 transport channels.51 Compared with 30 wt% loading, 40 wt% ZIF-300 filled PEBA membrane had interfacial voids that hindered the increase of transport selectivity. Overall, the sorption and diffusion analysis clearly confirm our speculation that the excellent preferential sorption of CO2 over N2 enabled ZIF-300 highly improving the permeability and selectivity of PEBA membrane for CO2/N2 separation.

Figure 10. (a) Sorption coefficient (S) and diffusion coefficient (D); and (b) sorption selectivity ("' ) and diffusion selectivity ("& ) of ZIF-300/PEBA MMMs with different ZIF-300 loading for CO2 and N2 at 0.4 MPa and 20 oC. 3.2.2. Effect of operating temperature The effect of operating temperature on the CO2/N2 separation performance of pure PEBA membrane and optimized ZIF-300/PEBA MMM with 30 wt% loading was studied. As is shown in Figure 11, gas permeability increased while CO2/N2 selectivity decreased with the operation temperature increased from 20 oC to 60 oC. This phenomenon agrees well with literature.52 At elevated temperature, the increased 16

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polymer chain mobility increases the diffusion coefficients for both CO2 and N2. However, N2 with larger molecular size is affected more significantly over CO2, thus higher temperature decreases the diffusion advantage of CO2 over N2..Furthermore, the sorption advantage of CO2 over N2 is also reduced when increasing the temperature. This phenomenon indicates that higher operating temperature may lead to a higher productivity but lower product purity.

Figure 11. (a) Effect of operating temperature on CO2/N2 separation performance and (b) Arrhenius plots for CO2 and N2 permeability of 30 wt% ZIF-300/PEBA MMM at 0.4 MPa. The temperature dependence of permeability can be expressed by the Arrhenius equation as follows12:  = ,) exp .

/01 2

3

(5)

where Pi is the permeability of component i, Ep it the activation energy, Pi,0 is pre-exponential factor, R is the gas constant; T is the operation temperature in Kelvin. The Arrhenius plot is shown in Figure 11b, in which a linearity between log (Pi) and 1/T suggests that the experimental permeation follows the theoretical model shown in equation (5). The apparent activation energy values for CO2 and N2 were calculated as 18.61 kJ/mol and 35.54 kJ/mol, respectively. It suggests that the transport of N2 molecule across the membrane is more sensitive than CO2 molecule to operating temperature. 3.2.3. Long-term permeation Long-term permeation was also tested to investigate the membrane structural stability. As shown in Figure 12, during 100 h continuous operation, the CO2/N2 17

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separation performance of 30 wt% ZIF-300/PEBA MMM is high and stable, showing average CO2 permeability of 83 Barrer and CO2/N2 selectivity of 84. The good structural stability of the MMM is assumed to the favorable physical interaction between ZIF-300 filler and PEBA matrix, which can also be inferred from the uniform dispersion of ZIF-300 in the membrane up to 30 wt% as displayed in the SEM images (Figure 5b-d).

Figure 12. Long-term permeation of 30 wt% ZIF-300/PEBA MMM at 0.4 MPa and 20 oC.

3.2.4. Performance comparison with literature The CO2/N2 separation performance of our ZIF-300/PEBA MMMs was plot in 2008 Robeson upper-bound, as given in Figure 13. As discuss above, both CO2 permeability and CO2/N2 selectivity were improved with the increase of ZIF-300 loading in the PEBA matrix. As the loading is up to 30 wt%, the performance of the ZIF-300/PEBA MMM surpassed the 2008 Robeson upper-bound for CO2/N2 separation.53-54

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Figure 13. Comparison of CO2/N2 separation performance of as-prepared ZIF-300/PEBA MMMs with 2008 Robeson upper-bound.

4. Conclusion In this work, a novel ZIF-300 MOF material was realized for membrane separation for the first time. The membrane was prepared by incorporating ZIF-300 crystals into PEBA matrix to form ZIF-300/PEBA MMM that was applied for CO2 capture. It was found that smaller ZIF-300 crystals can be produced by rationally reducing the time of hydrothermal reaction. The obtained ZIF-300 crystals were uniformed dispersed in the PEAB matrix with well-preserved pore structure, showing an excellent CO2 sorption capacity and selectivity over N2. Thus, the CO2 permeability and CO2/N2 selectivity of the pure PEBA membrane were simultaneously improved by introducing ZIF-300 crystals up to 30 wt%. A high and stable CO2/N2 separation performance was achieved in the resulting 30 wt% ZIF-300 filled PEBA membrane that transcended the 2008 Robeson upper-bound for state-of-the-art membranes. Our work showed the potential of ZIF-300 for developing high-performance mixed-matrix membrane that could be promising for application of CO2 capture.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (grant nos. 21406107, 21490585, 21301148, 21776125), National Key Basic Research Program (2017YFB0602500), the Innovative Research Team Program by the Ministry of Education of China (grant no. IRT_17R54), the Natural Science Foundation of Jiangsu Province (no. BK20140930), Topnotch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), and the Innovation Project of Graduate Research by Jiangsu Province (grant no. KYLX_0764).

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