N2 separation

Journal Pre-proof Functionalized ZIF-7/Pebax® 2533 Mixed Matrix Membranes for CO2/N2 Separation Jie Gao, Haizhuo Mao, Hua Jin, Chen Chen, Armin Feldhoff, Yanshuo Li PII:

S1387-1811(20)30033-0

DOI:

https://doi.org/10.1016/j.micromeso.2020.110030

Reference:

MICMAT 110030

To appear in:

Microporous and Mesoporous Materials

Received Date: 30 August 2019 Revised Date:

12 January 2020

Accepted Date: 13 January 2020

Please cite this article as: J. Gao, H. Mao, H. Jin, C. Chen, A. Feldhoff, Y. Li, Functionalized ZIF-7/ Pebax® 2533 Mixed Matrix Membranes for CO2/N2 Separation, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2020.110030. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Inc.

Jie

Gao: Data curation

(mainly the

preparation

and

characterization of MMMs). Haizhuo Mao: Data curation (mainly the gas permeability measurements of MMMs). Hua Jin: Writing- Original draft, Supervision. Chen Chen: Data analysis. Armin Feldhoff: review & editing. Yanshuo Li: Idea, Funding acquisition, Supervision.

Functionalized ZIF-7/Pebax® 2533 Mixed Matrix Membranes for CO2/N2 Separation Jie Gao a, Haizhuo Mao a, Hua Jin a,*, Chen Chen a, Armin Feldhoff b, Yanshuo Li a,*

a

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211,

China b

Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover,

Callinstr.3, D-30167 Hannover, Germany

*Corresponding author. E-mail addresses: [email protected] (H. Jin), [email protected] (Y. Li).

1

Abstract Membrane-based separation technology has evolved as a competitive approach for CO2 capture from flue gas (mainly N2). To achieve high separation performance, three partially NH2-, OH- and CH3OH- functionalized mixed-linker-ZIF-7 were successfully synthesized, and incorporated into polyether-block-amide (Pebax® 2533) polymer to form mixed-matrix membranes (MMMs). As evidenced by the CO2 adsorption isotherms, introducing functional groups in the ZIF-7 framework was indeed beneficial for CO2 adsorption. All MMMs composed of ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH offered better CO2/N2 separation performance than the parent ZIF-7-Pebax® 2533 membrane, suggesting the positive effect of functionalized ZIF-7 fillers on the gas separation performance. Among the three functionalized ZIF-7 based MMMs, the ZIF-7-OH-Pebax MMMs exhibited the best performance for CO2/N2 separation, which might be ascribed to the highest adsorption selectivity of CO2 over N2 predicted by ideal adsorbed solution theory (IAST) for ZIF-7-OH fillers. The 14% ZIF-7-OH-Pebax MMM showed high CO2 permeability of 273 Barrer and CO2/N2 separation factor of 38, which increased by 60% and 145% as compared with the neat Pebax membrane. The strategy of preparing functionalized MOFs with strong affinity for CO2 provides an effective method to develop MMMs for highly efficient CO2 separation.

Keywords: Metal-organic framework; Mixed-matrix membranes; Functionalized ZIF-7; CO2 separation; Pebax

2

1. Introduction Carbon dioxide (CO2) capture and storage (CCS) has already become an important means of dealing with global warming. Technologies for achieving CO2 separation from power plant flue gas (mainly N2) with high e ciency are in high demand. Nowadays, the most mature technology for post-combustion capture is absorption by aqueous amine-solutions [1,2]. However, the large deployment of this technology is severely hindered due to the high energy requirement and potential amine degradation. Membrane-based separation technology has evolved as a competitive approach for CO2 capture from flue gas, owing to its inherent advantages such as high energy e ciency and continuous operation [3]. Polymeric membranes have made progress towards large-scale industrial application in gas separation, and have been proposed for CO2 removal. However, polymeric membranes are suffering from numerous shortcomings such as the trade-o

between permeability and

selectivity (i.e., the so called Robeson upper bound limit), membrane plasticization and aging e ects [4,5]. Mixed-matrix membranes (MMMs) consisting of filler particles in a polymeric matrix have been proven to provide the solution to overcome the above-mentioned shortcomings of polymeric membranes [6,7]. Indeed, enormous research e ort has been devoted to the development of potential fillers in the polymer matrix. Metal-organic frameworks (MOFs) has attracted significant attention with great potential in adsorption and membrane separation processes [8-11]. Owing to the large choice of building blocks and post-synthetic modification, MOFs can offer great structural diversity and chemical variety. Thus, MOFs can be fine-tuned for selective adsorption of strategic gases, which makes them more attractive as inorganic fillers in MMMs as compared to conventional fillers (zeolites, silicas or activated carbons) [12]. More importantly, the significant improvement of the compatibility between filler and matrix for MOF-based MMMs is predictable because of the organic moiety of MOFs. Therefore, MOFs-based MMMs are regarded as promising candidates for energy-efficient CO2 separation [13-18]. Ge et al. [15] fabricated Cu-BTC/PPO MMMs incorporated with differently sized Cu-BTC. The as-synthesized MMMs exhibited both enhanced gas permeability and selectivity, with the CO2 permeability of 87 Barrer and ideal separation factor of around 23 for CO2/N2 mixture. MOFs improved both diffusivity and solubility of gas molecules, which accounted for the excellent membrane performance. Li et al [16] prepared a high performance MMM containing ultra-small ZIF-7 nano-particles (around 30-35nm) as inorganic filler in Pebax® 1657 polymer matrix. The composite membrane exhibited very good adhesion between the 3

two phases (Pebax and ZIF-7). Both the selectivity and permeability for CO2/N2 increased by incorporation of ZIF-7 filler, which could be ascribed to the combination of molecular sieving effect from ZIF-7 filler and the high solubility for CO2 in Pebax®1657. Adsorption controlled permeation mechanism is generally involved in the MOF-based MMMs for CO2 separation. Therefore, MOFs with functional groups that have strong affinity for CO2 are potential fillers for MMMs, which enable improved CO2 adsorptive selectivity. Anjum et al. [19] developed two different MMMs by adding MIL-125 and NH2-MIL-125 fillers to Matrimid® polyimide. The NH2-MIL-125 based MMMs outperformed MIL-125 MMMs both in permeability and CO2/CH4 selectivity. The results could be ascribed to the presence of the amine groups which form extra CO2 selective sorption sites, offering favourable hydrogen bonding with CO2. Similarly, amine functionalization of UiO-66 provided the so-prepared UiO-66-NH2-PEBA mixed matrix membrane with higher CO2/N2 selectivity and slightly decreased CO2 permeability than those of UiO-66-PEBA membrane [20]. Xiang et al. [21] demonstrated that the incorporation of amino-functionalized ZIF-7 (ZIF-7-NH2) fillers enabled the neat crosslinked poly(ethylene oxide) rubbery polymer membrane with significantly improved gas permeability, selectivity, and operating stability for CO2/CH4 separation. The remarkably enhanced intrinsic separation ability (i.e., improved CO2 uptake capacity and diffusion selectivity) for CO2/CH4 on ZIF-7-NH2 accounts for the outstanding performance of MMMs. Generally, amino-functional MOFs are most widely used for preparing high performance MMMs for CO2 separation. The development of MOFs with other functional groups for enhancing CO2 affinity are still in its infancy. In this work, three partially NH2-, OH- and CH3OH- functionalized mixed-linker-ZIF-7 were successfully synthesized. The partially OH-functionalized ZIF-7 exhibited the highest adsorption selectivity for CO2 over N2. All the three functionalized mixed-linker-ZIF-7 along with parent ZIF-7 were then incorporated into a polyether-block-amide (Pebax) polymer (a nice potential for CO2 separation) to fabricate MMMs. Single gas permeation results showed that both CO2 permeability and CO2/N2 selectivity of MMMs increased as compared to the neat Pebax membrane. 2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%) and benzimidazole (98%) were supplied by Sigma-Aldrich. 2-Aminobenzimidazole (97%), 2-hydroxymethylbenzimidazole (97%) and 2-hydroxybenzimidazole (≥98%) were purchased from Aladdin. Methanol 4

(≥99.7%), N,N-dimethylformamid (DMF, ≥99.5%) and 1-butanol (≥99.5%) were received from the Sinopharm Chemical Reagent Co., Ltd. Pebax® 2533 containing 80 wt% flexible poly(tetramethylene oxide) [PTMO] and 20 wt% rigid polyamide 12 [PA12] was purchased from Arkema Inc., France. All chemicals and solvents were used as received without further purification. 2.2. Preparation of ZIF-7-X (70%) crystals Mono-ligand ZIF-7 nanoparticles were prepared according to the method developed by Peng et al [22]. A solution of Zn(NO3)2·6H2O (2.387 g) in 400 mL of DMF was rapidly added into another solution of benzimidazole (6.156 g) in 400 mL of DMF. After the mixture has been stirred at room temperature for 72 h, the product was collected by centrifugation and washed by plenty of methanol. The synthesis of three partially NH2-, OH- and CH3OHfunctionalized mixed-linker-ZIF-7 (denoted as ZIF-7-NH2, ZIF-7-H-OH and ZIF-7-CH3OH) were carried out by the modified protocol [23]. As for the synthesis of ZIF-7-NH2 crystals, a solution of Zn(NO3)2·6H2O (1.487 g) in 50 mL of methanol was rapidly added into another solution of benzimidazole (0.3540 g) and 2-aminobenzimidazole (0.9320 g) in 50 mL of DMF. The molar ratio of benzimidazole and 2-aminobenzimidazole was 3:7. After the mixture was stirred after 6 h, the product was collected by centrifugation and washed by methanol. When the percentage of 2-aminobenzimidazole to the total imidazole ligands existed in the starting synthesis solution higher than 70%, ZIF-7-NH2 crystals with high degree of crystallization couldn’t be obtained [21]. The phenomenon was speculated to be caused by the steric effect of large size of functional groups. In the case of preparing ZIF-7-OH and ZIF-7-CH3OH crystals, all the synthesis conditions remained the same except the 2-hydroxybenzimidazole (0.9388 g) and 2-hydroxymethylbenzimidazole (1.0370 g) were used. 2.3. Fabrication of MMMs The ZIF-7-X /Pebax® 2533 MMMs were fabricated by a solution-casting method followed by solvent evaporation. Firstly, Pebax® 2533 was dissolved to 1-butanol under stirring at 75 oC for 5 h to get a 7.5 wt.% Pebax® 2533 solution. Then specific amounts of ZIF-7-X crystals were dispersed in 1-butanol to produce homogeneous ZIF-7-X suspensions. The obtained ZIF-7-X suspension was added to the above Pebax® 2533 solution under stirring. The mixed solution with settled Pebax® 2533 concentration of 5 wt.% was stirred for another 3 h to remove any gas bubble. Thereafter, the solution was immediately poured into a clean home-made glass dish. The cast membrane was dried at 70 °C for 24 h and then peeled 5

from the glass plate. For comparison, pure Pebax® 2533 membranes were prepared using similar procedures. The dry thickness of all membranes used for gas separation was 30±3µm. 2.4. Characterization The X-ray powder diffraction (XRD) measurements were carried out on a Bruker D8 Advance with Cu Kα radiation (λ=0.154 nm at 40 kV and 40mA). The morphology of functionalized ZIF-7 crystals and the corresponding MMMs were observed by scanning electron microscope (SEM, Nova™ NanoSEM 50, FEI Co., 5kv). The adsorption/desorption isotherm of mono-ligand ZIF-7 and mixed-linker ZIF-7-X crystals for CO2, CH4 and N2 were measured on a Micromeritics 3Flex Surface Characterization Analyzer. Thermogravimetric analysis (TGA) of functionalized ZIF-7 crystals and the corresponding MMMs was performed on a Perkin Elmer Diamond TG-DTA at a heating rate of 10 °C/min from 25 oC to 800 °C with the helium and air flow rate of 50 mL/min, respectively. 2.5. Ideal adsorbed solution theory selectivity Ideal adsorbed solution theory (IAST) is applied to calculate the selectivity parameters based on the single component isotherms at 298 K. All the CO2 and N2 isotherms were fitted using a Langmuir-Freundlich (L-F) model [24]:

bp1 n q=q m 1+bp1 n

(1)

where q is the adsorbed amount per mass of adsorbent (mol/kg), p the pressure of the bulk gas at equilibrium (kPa), q m the saturation capacities of sites (mol/kg), b the affinity coefficients of the sites (1/kPa), n the measure of the deviations from an ideal homogeneous surface. The IAST selectivity Sads is calculated as follows: Sads =

q1 p1 q2 p2

(2)

2.6. Gas permeability measurements The unsteady state gas permeability of the as-synthesized membranes was measured on a fixed-volume pressure increase instrument (Fig. 1a). The tests were carried out using single gases (CO2 or N2) at 25 oC with the feed pressure of around 4.5 bar. Permeability coefficient (P) [25,26] can be calculated by Eq. (3):

pt = p 0 + (dp/dt)0 ⋅ t +

RT ⋅ A p f ⋅ P l2 ⋅ (t − ) V p ⋅ Vm l 6D

(3)

6

where pt (bar) is the permeate pressure at time t (s), p0 the starting pressure (bar), (dp / dt )0 the baseline slope (bar/s), p f the feed pressure (bar), R the universal gas constant [8.314×10−5m3 bar/(mol K)], T the temperature (K), A the tested membrane area (m2), Vp the permeate volume (m3), Vm the molar volume of a gas at standard temperature and pressure (22.4×10-3 m3/mol at 0 oC and 1 atm), l the membrane thickness (m) and D the diffusion coefficient (m2/s). In the case of well evacuated defect free samples, the starting pressure and the baseline slope p0 + (dp / dt )0 can be negligible. The ideal selectivity ( α ijideal ) of membrane is calculated from the single gas permeability

Pi and Pj as in the following equation.

α ijideal =

Pi Pj

(4)

The mixture gas permeation measurements were measured by the Wicke-Kallenbach method (Fig. 1b). Equimolar CO2/N2 mixture with total flow rate of 40 mL min-1 was used as feed gas, and helium with flow rate of 20 mL min-1 was used as sweep gas. The composition of the permeation was analyzed by gas chromatography (Agilent 7890B). The gas permeability Pi (1 Barrer = 10-10 cm3 (STP) cm cm-2 s-1 cm Hg-1 ) can be calculated by Eq. (5): Pi =

Nil A∆Pi

(5)

where N i is molar flow rate of the permeate component i (cm3 s-1) at standard temperature and pressure (STP), l the membrane thickness (cm), A is the effective membrane area (cm2) and ∆Pi the transmembrane pressure difference (cm Hg). The mixed gas separation factor is determined by Eq. (6) Sij =

yi / y j xi / x j

(6)

where xi ( x j ) and yi ( y j ) represent the molar fraction of component i (component j) in the feed side and permeate side, respectively.

7

Fig. 1. Schematic diagram of (a) fixed volume/pressure increase time-lag setup and (b) Wicke-Kallenbach setup.

3. Results and discussion 3.1. Synthesis and characterization of functionalized ZIF-7 The crystal structure of the as-synthesized functionalized ZIF-7 was characterized by XRD (Fig. 2a). All the diffraction peaks of three functionalized ZIF-7 matched well with that of parent ZIF-7, indicating the successfully preparation of mixed-linker ZIF-7 crystals with high phase purity. The thermal stability was investigated by thermogravimetric analysis (TGA) and the results were shown in Fig. 2b. All the functionalized ZIF-7 demonstrated excellent thermal stability in the He atmosphere with the decomposition temperature higher than 600 oC. Fig. 3 showed SEM images of parent and mixed-linker ZIF-7 crystals with near spherical morphology. The average particle sizes of ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH were 44, 36, 85 and 682 nm, respectively.

Fig. 2. (a) XRD patterns and (b) TGA curves of monoligand ZIF-7 (black), amino-functionalized

ZIF-7

(blue),

hydroxy-functionalized

ZIF-7

(olive)

and

hydroxymethyl-functionalized ZIF-7 crystals (dark yellow).

Fig. 3. SEM images of ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH. 3.2. Adsorption properties of functionalized ZIF-7 The N2 sorption isotherms on ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH crystals at 77 K were given in Fig. 4. The calculated BET surface area (pore volume) in Table 1 for ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH crystals were 201 (0.100), 361 (0.163), 28.2 (0.011) and 4.86 m2/g (0.002 cm3/g), respectively. The BET surface areas and total pore volumes of ZIF-7 and ZIF-7-NH2 were basically consistent with previously reported [21,27]. The BET surface area and pore volume of ZIF-7-OH and ZIF-7-CH3OH were significantly lower than that of parent ZIF-7, which was mainly due to the occupation of functional groups in the channel pores.

Fig. 4. N2 sorption isotherms on ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH crystals at 77 K. 8

Table 1. Physical properties of ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH. Samples

SBET (m2/g)

Vmicro (cm3/g)

Average particle size (nm)

ZIF-7

201

0.100

44

ZIF-7-NH2

361

0.163

36

ZIF-7-OH

28.2

0.011

85

ZIF-7-CH3OH

4.86

0.002

682

The adsorption isotherms of CO2 and N2 on the parent and mixed-linker ZIF-7 crystals were also performed at 298 K (Fig. 5). The equilibrium CO2 adsorption amount of each functionalized ZIF-7 was higher than that of parent ZIF-7, indicating that the introduction of functional group in ZIF-7 framework was indeed beneficial for CO2 adsorption. An obvious hysteresis loop was found in the CO2 adsorption isotherms on ZIF-7-OH and ZIF-CH3OH, indicating the gate-opening effect, i.e., the reorganization of adsorbed gas molecules at a certain threshold pressure [28]. The N2 adsorption uptakes decreased in the order ZIF-7-NH2 > ZIF-7 > ZIF-7-CH3OH > ZIF-7-OH, which was basically consistent with their BET surface areas. The ideal selectivities of CO2 over N2 by the ideal adsorbed solution theory (IAST) method were shown in Fig. 6. The ideal CO2/N2 selectivities of all three functionalized ZIF-7 samples exceeded that of parent ZIF-7, demonstrating the positive effect of introducing functional groups in the ZIF-7 framework for CO2 separation. Particularly, ZIF-7-OH and ZIF-CH3OH possessed much higher ideal CO2/N2 selectivities than parent ZIF-7. The significant hindering effect for N2 adsorption induced by the introduction of large size -OH and -CH3OH group in the ZIF-7 framework might accounted for this totally different phenomenon. Therefore, ZIF-7-OH and ZIF-CH3OH have been identified as promising fillers in MMMs for CO2/N2 separation.

Fig. 5. Adsorption isotherms of (a) CO2 and (b) N2 on ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH at 298 K.

Fig. 6. IAST predicted selectivities of ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH crystals for CO2/N2. 3.3. Membranes characterizations 3.3.1 XRD patterns and SEM morphology 9

The XRD patterns of the neat Pebax® 2533 membrane and functionalized ZIF-7/Pebax® 2533 MMMs with the MOF filler content of 10% were given in Fig. 7. The typical broad peaks around 20° in the pattern of neat Pebax® 2533 membrane suggested the amorphous structure of Pebax® 2533. The structure of functionalized ZIF-7 crystals maintained in their MMMs as evidenced by the typical diffraction peaks. The surface and cross-section SEM images of the neat Pebax® 2533 membrane and MMMs were shown in Fig. 8. Except the ZIF-7-CH3OH fillers with micron size, a homogeneous dispersion of ZIF-7, ZIF-7-NH2, ZIF-7-OH nanoparticles into Pebax® 2533 polymer matrix was observed. In addition, ideal interfacial adhesion between polymer chains and fillers without obvious voids were acquired for ZIF-7, ZIF-7-NH2, ZIF-7-OH nanoparticles. However, certain voids inevitably occurred in the interface between Pebax® 2533 polymer matrix and ZIF-7-CH3OH fillers, which might have bad effects on the gas separation performance of its MMMs.

Fig. 7. XRD patterns of neat Pebax® 2533 membrane and its MMMs containing ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH fillers.

Fig. 8. SEM images of the surface and cross-section of (a, b) neat Pebax® 2533 membrane and its MMMs containing the following fillers: (c, d) ZIF-7, (e, f) ZIF-7-NH2, (g, h) ZIF-7-OH and (i, j) ZIF-7-CH3OH fillers. 3.3.2 Gas permeation of MMMs The single gas permeability and the ideal selectivity of CO2 and N2 through the functionalized ZIF-7/Pebax® 2533 MMMs were measured at 25 °C and 4.5 bar, and the results were given in Table 2. All MMMs composed of ZIF-7, ZIF-7-NH2, ZIF-7-OH and ZIF-7-CH3OH fillers possessed higher CO2 permeability and CO2/N2 selectivity than the neat Pebax® 2533 membrane, suggesting the positive effect of fillers on the membrane performance. Moreover, the ZIF-7-OH-Pebax MMMs exhibited the best performance for CO2/N2 separation, which might be attributed to the highest CO2/N2 adsorption selectivity of ZIF-7-OH fillers. The larger particle sizes of ZIF-7-CH3OH crystals are adverse to its MMM performance through formation of non-selective voids, resulting to the much higher CO2 permeability and lower CO2/N2 selectivity. To assess the reproducibility of the functionalized ZIF-7/Pebax® 2533 MMMs for CO2/N2 separation, three batches of ZIF-7-NH2-Pebax 10

MMMs as the representative one (i.e., ZIF-7-NH2 yield (~35%) was much larger than ZIF-7-OH (~1.7%) and ZIF-7-CH3OH (~2.4%)) were synthesized under the exactly same condition. The CO2 permeabilities were 206, 205 and 227, and the corresponding CO2/N2 selectivities were 27.3, 27.4 and 26.9, respectively. The results demonstrated that the ZIF-7-NH2-Pebax MMMs possessed good reproducibility for CO2 separation.

Table 2. Single gas permeability of the as-synthesized membranes measured on a fixed-volume pressure increase instrument. Permeability (Barrer) α (CO2/N2)

Membranes CO2

N2

Pure Pebax

171

11.0

15.5

ZIF-7-Pebax

198

8.74

22.6

ZIF-7-NH2-Pebax

206

7.53

27.3

ZIF-7-OH-Pebax

249

7.66

32.5

ZIF-7-CH3OH-Pebax

562

29.6

19

3.3.3 Effect of ZIF-7-OH loading Since the ZIF-7-OH-Pebax MMMs possessed highest CO2/N2 ideal selectivity, the e ect of ZIF-7-OH loading was further investigated. The accurate loadings of ZIF-7-OH in MMMs (M1-M4) were determined from TGA curves (Fig. 9). As can be seen from the TG curve of ZIF-7-OH, a sharp weight loss took place after 500 °C owing to the framework structural decomposition. The TGA curve tended to follow a plateau when the temperature was over 600 °C, and the residue of the heated sample was ZnO with weight percent of approximate 31.5%. The remaining weight percent of neat Pebax® 2533 membrane was nearly 0.34%. Therefore, the calculated ZIF-7-OH loading in MMMs (M1-M4) were 3.4%, 8.0%, 14% and 23%, respectively.

Fig. 9. TGA curves of (a) ZIF-7-OH, (b) Pebax® 2533 and (c-f) ZIF-7-OH-Pebax MMMs. Fig. 10 gave the performance of the neat Pebax® 2533 membrane and ZIF-7-OH-Pebax MMMs for CO2/N2 separation. A continuous and remarkable improvement in both CO2 permeability and CO2/N2 separation factors was observed as the loading of ZIF-7-OH fillers in Pebax polymer matrix increased from 0 wt% to 14 wt%. Specifically, the highest performance at ZIF-7-OH filler content of 14 wt% exhibited CO2 permeability of 273 Barrer 11

and CO2/N2 separation factor of 38, respectively, which increased by 59.6% and 145% as compared with the neat Pebax® 2533 membrane. Nevertheless, the CO2/N2 separation factor showed a moderate decrease when the ZIF-7-OH loading further increased to 23 wt%. As we all known, the agglomeration of inorganic fillers in polymer matrix is usually inevitable especially at high filler content for MMMs prepared by conventional solution-blending method. Therefore, the performance degradation of MMMs at ZIF-7-OH content of 23 wt% was mainly ascribed to the nonselective interfacial defects formed by the agglomeration of ZIF-7-OH nanoparticles in Pebax matrix. The mixed gas separation performances of pure Pebax® 2533 membrane and ZIF-7-OH-Pebax MMM with filler content of 14 wt% were further investigated by the Wicke-Kallenbach method. Fig. 11 showed the e ect of feed pressure on the membrane performance for CO2/N2 mixture separation. Both the CO2 permeability of pure Pebax membrane (143 to 188 Barrer) and ZIF-7-OH-Pebax MMM (226 to 354 Barrer) increased with increasing feed pressure from 1 to 10 bar. In contrast, the CO2/N2 separation factor exhibited very small changes for the two membranes. Moreover, it was found that both the CO2 permeability and CO2/N2 separation factor of the ZIF-7-OH-Pebax MMM were higher than that of pure Pebax membrane at any feed pressure, demonstrating the significant improvement of ZIF-7-OH filler on the Pebax membrane for CO2/N2 separation. The separation performance of functionalized ZIF-7 incorporated MMMs for CO2/N2 were compared with other literatures [16, 29-33]. The data points of functionalized ZIF-7 based MMMs are relatively closer to the upper bound than those of the neat Pebax membrane (Fig. 12), indicating the important impact of functionalized ZIF-7 fillers on the improved gas separation performance of MOF-based MMMs. In addition, the CO2/N2 separation performance of the ZIF-7-OH-Pebax MMM was comparable to that of other MOF incorporated Pebax membranes.

Fig. 10. The e ect of ZIF-7-OH loadings on single gas permeability of ZIF-7-OH-Pebax MMMs from time-lag permeation measurements.

Fig. 11. The e ect of pressure on the CO2 permeability and CO2/N2 separation factor of the pure Pebax membrane and ZIF-7-OH-Pebax MMM.

12

Fig. 12. Comparison of CO2/N2 separation performance of functionalized ZIF-7 based MMMs with other literatures.

4. Conclusions In summary, three mixed-linker ZIF-7 crystals with high phase purity were successfully prepared. The CO2 adsorption on each functionalized ZIF-7 was higher than that on parent ZIF-7, demonstrating that the introduce of functional group in ZIF-7 framework was indeed beneficial for CO2 adsorption. Alongside with ZIF-7, the three functionalized ZIF-7 was used as fillers to prepare Pebax MMMs. Due to the much higher CO2/N2 adsorption selectivity of ZIF-7-OH fillers, its MMMs outperformed other MMMs for CO2/N2 separation. The ZIF-7-OH-Pebax MMM with filler content of 14 wt% exhibited highest performance with CO2 permeability of 273 Barrer and CO2/N2 separation factor of 38, respectively, which increased by 60% and 145% as compared with the neat Pebax membrane. This work demonstrates that functionalized ZIF-7 with high affinity CO2 adsorption is promising materials to develop mixed matrix membranes for CO2 separation.

Acknowledgment This work was supported by the National Natural Science Foundation of China (21808113, 21622607, 21761132009), National Natural Science Foundation of Zhejiang (no. LR18B060002) and the K. C. Wong Magna Fund in Ningbo University. Gratefully acknowledged is funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - FE928/15-1.

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Partially NH2-, OH- and CH3OH- functionalized mixed-linker-ZIF-7 were synthesized. ZIF-7-OH exhibited highest adsorption selectivity of CO2 over N2 predicted by IAST. Improved CO2 permeability and CO2/N2 selectivity of mixed-linker ZIF-7/Pebax MMM. The ZIF-7-OH-Pebax MMMs exhibited the best performance for CO2/N2 separation.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: