novel zif-300 mixed-matrix membranes for efficient co2 capture · novel zif-300 mixed-matrix...

Novel ZIF-300 Mixed-Matrix Membranes for Efficient CO2 CaptureJianwei Yuan, Haipeng Zhu, Jiajia Sun, Yangyang Mao, Gongping Liu,* and Wanqin Jin*

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National SynergeticInnovation Center for Advanced Materials, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, People’s Republic of China

ABSTRACT: Because of the high separation performance and easy preparation, mixed-matrix membranes (MMMs) consistingof metal−organic frameworks have received much attention. In this article, we report a novel ZIF-300/PEBA MMM consisting ofzeolite imidazolate framework (ZIF-300) crystals and polyether block amide (PEBA) matrix. The ZIF-300 crystal size waseffectively reduced by optimizing the hydrothermal reaction condition from ∼15 to ∼1 μm. The morphology andphysicochemical and sorption properties of the synthesized ZIF-300 crystals and as-prepared ZIF-300/PEBA MMMs weresystematically studied. The results showed that ZIF-300 crystals with a size of ∼1 μm maintained excellent preferential CO2sorption over N2 without degradation of the crystal structure in the MMMs. As a result, uniformly incorporated ZIF-300 crystalshighly enhanced both the CO2 permeability and the CO2/N2 selectivity of pure PEBA membrane. The optimized ZIF-300-PEBAMMMs with a ZIF-300 loading of 30 wt % exhibited a high and stable CO2 permeability of 83 Barrer and CO2/N2 selectivity of84, which are 59.2% and 53.5% higher than pure PEBA membrane, respectively. The obtained performance surpassed the upperbound of state-of-the-art membranes for CO2/N2 separation. This work demonstrated that the proposed ZIF-300/PEBA MMMcould be a potential candidate for an efficient CO2 capture process.

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

1. INTRODUCTION

Owing to fossil fuel combustion and mankind activities,greenhouse gases (especially CO2) are becoming a majorglobal issues. In order to achieve both environmental andeconomic benefits, this CO2 should be captured.1,2 Hence,highly efficient CO2 capture and storage from a huge amount ofenergy-intensive industrial gases have attracted much attention,including biogas, flue gas, natural gas, and so on.1−4 Amongdifferent conventional technologies such as solvent adsorption,chemical looping, and cryogenic distillation, membraneseparation shows easy operation and energy effectiveness.5,6

Polymeric materials have been applied in the membrane CO2capture, such as polyamides,7 polyimides,8 polycarbonates,9

polysilicone,10 and poly(ethylene oxides).11 However, poly-meric membranes generally suffer a “trade-off” betweenpermeability and selectivity.12,13

Nowadays, metal−organic frameworks (MOFs) as a kind ofmembrane material, which is connected by metal centers andorganic ligands, have attracted much attention because of thedistinct properties of variable pore/aperture size, large surfacearea, 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 mem-branes is complicated, and some of them are subjected tononselective defects, such as cracks and the intercrystal gaps,resulting in compromised separation efficiency. Alternatively,mixed-matrix membranes that combine the high-performingfiller with the easily-processable polymer matrix25−32 haveshown great potential to realize the practical development ofMOFs-based membranes. It has been demonstrated thatincorporating CO2-philic MOF crystals into a polymer is aneffective approach to developing high-performance CO2

separation membranes. Gascon and co-workers33 incorporatedfunctionalized NH2-MIL-53(Al) particles to fabricate mixed-matrix membranes exhibiting high selectivity for CO2/CH4

separation. [Cu3(BTC)2] with triangular windows of 0.35 nmdiameter were incorporated into Matrimid matrix for theseparation of mixed gases, which showed improved CO2

permeability as well as CO2/CH4 and CO2/N2 selectivity.34

Bae et al.27 incorporated submicrometer-sized ZIF-90 crystals

Received: August 20, 2017Accepted: October 19, 2017Published: October 19, 2017

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.7b12507ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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into polyimide to enhance the CO2 permeability. Recently, ourgroup21 developed NH2-UiO-66/PEBA mixed-matrix mem-branes showing excellent and stable CO2/N2 separationperformance beyond the Robeson upper bound of polymericmembranes. In addition, the smaller particle size of MOFcrystals would favor improving the interfacial adhesion betweenthe filler and the polymer matrix as well as decreasing theseparation layer thickness for the composite/asymmetricmixed-matrix membranes. The influence of the filler size onthe performance of mixed-matrix membranes has been wellstudied in recent papers.35−37

Recently, a new kind of CO2-philic zeolite imidazolateframeworks 300 (ZIF-300, Zn(2-mIm)0.86(bbIm)1.14) withchabazite (CHA) topology was synthesized by introducingtwo imidazolate links.38 It was facilely prepared by reacting zincnitrate hexahydrate and 2-methylimidazolate with the 5(6)-bromobenzimidazolate link in N,N-dimethylformamide(DMF)/water mixtures. Tetrahedral Zn atoms are surroundedby four ditopic imidazolate links and linked together byimidazolate bridges to generate a three-dimensional CHAnetwork. It has been reported that ZIF-300 shows significantlyhigher sorption capacity and affinity for CO2 over N2, enablingeffective dynamic separation of CO2 from N2. There is noperformance loss over several sorption−desorption cycles inthe ZIF-300 sorbents, and they can be easily regenerated bypurging N2 flow at room temperature. In view of theseattractive features, we expected that ZIF-300 could be apromising membrane material for CO2 capture. However, tothe best of our knowledge, it has not been realized formembrane separation until now.This paper, for the first time, presents the design and

fabrication of high-performing ZIF-300-based membranes. Asshown in Figure 1, ZIF-300 crystals were synthesized by the

solvothermal method and the crystal size was tuned by varyingthe synthesis conditions. The obtained fine ZIF-300 particleswere incorporated into a commercial polymer matrix, polyetherblock amide (PEBA), to fabricate ZIF-300 mixed-matrixmembranes. PEBA, which consists of ethylene oxide as softsegments and amide groups as hard segments, was chosen toprovide high CO2 permeability as well as and sufficientmechanical strength.3 The as-prepared ZIF-300/PEBA MMM

exhibited enhancement in both CO2 permeability and CO2/N2selectivity over that of pure PEBA membrane. The membraneswith optimized ZIF-300 loading showed excellent and stableCO2/N2 separation performance beyond the upper bound forstate-of-the-art membranes. Moreover, the transport mecha-nism of the membrane was interpreted by analyzing thecoefficients and selectivity of sorption and diffusion, therebyelaborating the CO2/N2 separation mechanism of ZIF-300/PEBA MMM.

2. EXPERIMENTAL SECTION2.1. Synthesis of ZIF-300 Crystals. The preparation of ZIF-300

MOFs was fabricated following the procedures in recent work.38 ForZIF-300, 2-mImH (11.7 mg, 0.143 mmol), bbImH (28.2 mg, 0.143mmol), and Zn (NO3) 4H2O (29.8 mg, 0.114 mmol) were dissolvedin a solvent mixture of DMF (4.75 mL) and water (0.25 mL) in aPTFE reaction still (10 mL). Then the reaction still was heated at 120°C for nearly 3−6 days. The yellow powder was achieved by using acentrifugal machine at 10 000 rpm for 10 min and washed by DMF atleast three times. After the initial DMF washing, as-synthesizedsamples of ZIF-300 were soaked in methanol for solvent exchange for3 days under room temperature. Finally, at the third day, the sampleswas evacuated at 10 mTorr at ambient temperature for 24 h, followedby subsequent heating at 180 °C for 24 h.

2.2. Fabrication of Membranes. ZIF-300 powder was dispersedin 7/3 (wt) ethanol/water mixed solvent followed by a subsequentprobe-type sonicator (SCIENTZ) at 300 W power for 10 min in an icebath. Then a certain amount of dried PEBA (PEBAX MH 1657,Arkema, France) was added into the mixture solution containing ZIF-300 powder followed by stirring and refluxing at 80 °C for 12 h. Thesolution was cast on a Teflon dish and left to evaporate the solvent for3days under room temperature. Subsequently, the membrane wasdried in an oven under 70 °C for 12 h. As a control, pure PEBA wasalso prepared with the same procedure as described above.

2.3. Characterization. Morphologies of the ZIF-300 MOFscrystals and ZIF-300/PEBA mixed-matrix membranes were charac-terized by a scanning electron microscope (SEM, S4800, Hitachi,Japan). All samples were treated with gold under vacuum. Thecrystalline structures of ZIF-300 MOFs crystals and ZIF-300/PEBAmembranes were characterized by an X-ray diffractometer (XRD, D8-advance, Bruker, Germany) using Cu Kα radiation in the range of 2.5−50° with an increment of 0.05° at room temperature. Fouriertransform infrared (FT-IR, Nicolet8700, Nicolet, USA) spectra wererecorded in a spectrophotometer with a range of 400−4000 cm−1

using the KBr disk technique. The adsorption measurements of theZIF-300 MOFs crystals were conducted by ASAP 2460 with CO2 (298K) and N2 (298 K, 77 K). BET surface area was calculated from the N2isotherms at 77 K. The sorption properties of the ZIF-300/PEBAMMMs were measured by a high-pressure adsorption apparatus(Belsorp-HP) at 293 K. The thermal property of ZIF-300 MOFs wascarried out by thermogravimetric analysis (TGA, NETZSCH STA449F3) from 25 to 800 °C with a rate of 10 °C/min. Differentialscanning calorimetry (DSC, Q2000, TA Instruments, USA) measure-ments were operated from −80 to −40 °C in N2 atmosphere to obtainthe glass transition temperature (Tg) of the ZIF-300/PEBA MMMs.

2.4. Gas Permeation Measurement. Gas permeation test wascarried out by a constant-volume, variable-pressure method with atransmembrane pressure of 0.4 MPa at 20 °C. The membrane samplewas cut into circular disks and mounted onto the homemadepermeation cell. The effective permeation area was 3.2 cm2.

The transport of gases through the gas separation membranefollows the solution-diffusion model.39 Gas permeation is characterizedby the permeability, P. Each measurement was repeated at least sixtimes. After the system reached steady state, we use eq 1

= ·Δ

·+

· ·⎜ ⎟⎛⎝

⎞⎠P

p TlA

Pt

3.59301 1

273.15dd (1)

Figure 1. Schematic illustration of the membrane preparation process.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.7b12507ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

B


where P is the gas permeability [1 Barrer = 10−10 cm3 (STP) cm/(cm2·s·cmHg)], Δp is the transmembrane pressure (MPa), l is the thicknessof the membrane, T is the room temperature (°C), A is the effectivepermeation 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 themembrane, as shown in eq 2

= ·P D Si i i (2)

where D and S characterize the kinetic and thermodynamiccontribution to transport, respectively. S can be expressed by eq 3

=Scfii

i (3)

where ci is the gas concentration absorbed in the membrane and f i isthe fugacity driving force of component i.The selectivity is the ratio of permeability of the individual gas and

can be expressed by eq 4

α α α= = · = ·p

pDD

SSi j

i

j

i

j

i

jS/ D

(4)

where αD is the diffusion selectivity and αS is the sorption selectivity.

3. RESULTS AND DISCUSSION3.1. Synthesis of ZIF-300 Crystals. Since the particle size

of the filler has a significant influence on the mixed-matrixmembranes, we first studied how to synthesize ZIF-300 withthe desired particle size. It was found that the particle size ofZIF-300 could be easily controlled by varying the synthesistime. SEM images of ZIF-300 particles with various synthesistimes are shown in Figure 2. A homogeneous crystalmorphology was observed in all ZIF-300 samples synthesizedunder different hydrothermal synthesis times at 120 °C. Itsuggests high phase purity, which is further confirmed by XRDanalysis discussed later. Generally, a shorter synthesis time

produces smaller ZIF-300 crystals. In our case, as the synthesistime was decreased by one-half (from 144 to 72 h), the averagesize of ZIF-300 crystals was dramatically reduced ∼15 times(from ∼15 to ∼1 μm). Further lowering the synthesis time,which was expected to produce ZIF-300 crystals withsubmicrometer size, however failed to form the right crystalstructure, namely, we were unable to obtain ZIF-300 crystals inthe current study. Some other parameters such as hydrothermaltemperature or reactant concentration might need optimizatedto further reduce the crystal size of ZIF-300, which is out of thescope of this study. Here, ZIF-300 crystals with ∼1 μm particlesize were used for the fabrication of mixed-matrix membranes.The powder X-ray diffraction (PXRD) patterns of ZIF-300

crystals synthesized by different hydrothermal times are shownin Figure 3a. It suggests that all ZIF-300 MOFs crystals werewith a highly crystalline structure, which agrees with thesimulation result and report in the literature.38 Furthermore,the thermal stability and pore structures of the ZIF-300 crystalsynthesized by 72 h at 120 °C were investigated bythermogravimetric analysis and N2 adsorption−desorptionisotherms, respectively. As shown in Figure 4b, thermaldecomposition occurs at ∼500 °C, which is higher than thetypical values of 350 °C for other MOFs.40,41 The obtainedhigh thermal stability property of ZIF-300 crystals could be dueto the intrinsic characteristic of the CHA-type ZIFs structure.Furthermore, no weight loss was observed below 300 °C, alsoconfirming full removal of guest molecules for ZIF-300 via theactivation process (180 °C for 24 h under vacuum). It ensuresthe subsequent gas separation application for accessible poreswithin the ZIF-300 framework.N2 sorption analysis was used to characterize the permanent

porosity of ZIF-300 crystals, and the result is shown in Figure4a. Before the adsorption test, ZIF-300 MOFs crystals wereactivated at 120 °C for 24 h under vacuum for degassing. Itfollowed type I isotherms with no hysteresis. The Langmuir(Brunauer−Emmet−Teller) BET surface area of ZIF-300 iscalculated as 546 m2/g, which matches well with the reportedvalue.38 The micropore volume and micropore size for ZIF-300crystals were 0.374 cm3/g and 0.79 nm, respectively. Therelatively large pore might be unable to offer substantialdiffusion selectivity for gas pairs with molecular diameterssmaller than the pore size, for instance, CO2 and N2 withkinetic diameters of 0.33 and 0.364 nm, respectively. Never-theless, ZIF-300 shows higher sorption capacity and affinity forCO2 over N2,

38 as proven by the thermodynamic CO2 and N2uptake at 298 K for ZIF-300 crystals shown in Figure 4b. Theisotherms for both CO2 and N2 exhibit a reversible adsorptionand desorption process. At 1.05 bar, the sorption capacities ofCO2 and N2 are 25.4 and 2.9 cm

3 (STP)/g, respectively, clearlyindicating a high sorption selectivity for CO2/N2 gas pair. It isalso noticed that the initial slope of the CO2 isotherm is steeperthan that for N2, indicating ZIF-300 has a higher affinity forCO2 over N2. These features enable ZIF-300 as a favorableCO2-philic filler to modify the polymeric membranes for CO2/N2 separation.

3.2. Membrane Characterization. 3.2.1. Morphology.The morphology of the as-prepared pure PEBA membrane andZIF-300/PEBA MMM was investigated by SEM. As displayedin Figure 5a, the cross-section of pure PEBA membrane is verysmooth without any structural bulges. The cross-sectional SEMimages of ZIF-300/PEBA MMMs varying ZIF-300 loadingfrom 10 to 40 wt % are shown in Figure 5b−f. A homogeneousmorphology can be observed in the cross-section of PEBA

Figure 2. SEM images of ZIF-300 MOFs with different synthesis timesat 120 °C: (a) 144, (b) 120, (c) 96, (d and e) 72 h. (f) Effect ofsynthesis time on the average crystal size of ZIF-300.



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membrane filled with ZIF-300 crystals up to 30 wt %. Theenlarged image of 30 wt % sample clearly shows intact filler−polymer interfaces even exposed to a high-energy electronbeam that produced cracks in the PEBA matrix, as shown inFigure 5e. It suggests that ZIF-300 is compatible with PEBA,which is presumably attributed to the interactions between theorganic linkers on the MOF and the polymer chains.42

However, on further increasing the ZIF-300 loading to 40 wt%, the ZIF-300 crystals began to aggregate and some of themsettled at the bottom of the membrane (Figure 5f), resulting in

a poor ZIF-300-PEBA interfacial morphology with several largevoids damaging the integrity of the mixed-matrix membrane.

3.2.2. Physicochemical Properties. The XRD patterns ofZIF-300/PEBA MMMs with various ZIF-300 loadings areshown in Figure 6a. Pure PEBA is a semicrystalline copolymerconsisting of both crystalline (PA6) and amorphous (PEO)phases. It thus showed a characteristic peak ranging from 15° to25°.43 This broad peak remains in the MMMs with ZIF-300loading from 10 to 40 wt %, suggesting the amorphousstructure of PEBA matrix at room temperature. Meanwhile, theintrinsic characteristic peaks of ZIF-300 are also observed in all

Figure 3. (a) XRD patterns of the ZIF-300 MOFs crystals synthesized by different hydrothermal reaction times and the simulated result. (b) TGAcurve for ZIF-300 crystals; (inset) DTG curve.

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 Kfor ZIF-300 crystals.

Figure 5. Cross-sectional SEM image of (a) pure PEBA and (b) 10, (c) 20, (d and e) 30, and (f) 40 wt % ZIF-300/PEBA MMM.



D


the MMM samples, and their intensities are enhanced with theincreased loading of ZIF-300. This result suggests that physicalincorporation did not affect the crystal structure of ZIF-300,31

ensuring the pore structure of ZIF-300 for molecular transport.FT-IR spectra of the PEBA and ZIF-300/PEBA MMMs are

presented in Figure 6b. The characteristic peak at 1094 cm−1

corresponds to the C−O−C stretching vibration from the PEOpart.43 The peaks at 1636 and 1732 cm−1 are attributed to H−N−CO and O−CO groups, respectively, resulting fromthe PA part.44 It is reasonable to observe that the intensity ofthe characteristic peaks from PEBA was reduced by increasingthe amount of ZIF-300 crystals in the membrane. Besides therespective characteristic peaks from ZIF-300 and PEBA, no newpeak appears in these MMMs, further confirming physicalincorporation without chemical bonding between ZIF-300crystals and PEBA chains.3.2.3. Thermal Properties. The chain packing and flexibility

of the PEBA membranes with different ZIF-300 loading werestudied by differential scanning calorimetry (DSC) measure-ment., The Tg of pure PEBA membrane is ca. −52.3 °C, whichagrees well with the literature (Figure 7).45 With the increase ofthe ZIF-300 loading in the PEBA matrix, the Tg was decreasedgradually. The incorporation of ZIF-300 particles might inhibitthe regular packing of PEBA chains, causing the decline of Tg.

The looser structure of the matrix is expected to provide morepathways for gas diffusion through the membrane.

3.2.4. Gas Sorption Properties. In order to investigate thesorption properties of the ZIF-300/PEBA MMMs under thepermeation conditions (0.4 MPa, 20 °C), high-pressuresorption (up to 1.2 MPa) measurement was carried out toobtain the sorption isotherms of the MMMs for CO2 and N2 at20 °C. ZIF-300-PEBA MMMs were treated at 70 °C for 24 hunder vacuum for degassing. As shown in Figure 8, pure PEBA

shows higher sorption capacity for CO2 over N2 due to thehigher condensability of CO2 and the affinity of the PEBAchain toward CO2. The linear relationship between the sorptionamount and the pressure indicates that CO2 or N2 sorption inrubbery PEBA generally follows Henry’s law.46 By incorporat-ing the porous CO2-philic ZIF-300 crystals, CO2 sorption wassignificantly enhanced along with only a slight increase in N2sorption capacity. For instance, in the 30 wt % ZIF-300-filledPEBA MMM, the CO2 sorption coefficient and CO2/N2sorption selectivity of pure PEBA membrane at 0.4 MPa wereincreased 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 canbe further increased by incorporating more ZIF-300 crystals inthe membrane, as indicated by the isotherms of 40 wt % ZIF-300/PEBA MMM. These obtained sorption coefficients canalso be used to calculate the diffusion coefficients by using the

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.

Figure 7. Glass transition temperatures of ZIF-300/PEBA MMMswith different ZIF-300 loading.

Figure 8. CO2 and N2 sorption isotherms of pure PEBA membraneand ZIF-300/PEBA MMM with 30 or 40 wt % ZIF-300 at 0−1.2 MPaand 20 °C.



E


measured permeability of the membrane, which will bediscussed in the next section. Moreover, it is observed thatthe sorption isotherm of the ZIF-300/PEBA MMM is likely thecombination of PEBA matrix and ZIF-300 filler, suggesting thatthe highly porous structure of ZIF-300 was not affected by thesurrounding PEBA chains.3.3. Gas Separation Performance. 3.3.1. Effect of ZIF-

300 Loading. Gas permeation measurement was carried out toinvestigate the separation performance of the as-prepared ZIF-300/PEBA MMMs. According to the preferential sorption ofCO2 over N2 in ZIF-300 as well as PEBA, the CO2/N2 gas pairis chosen for separation since it is an important process for CO2capture from flue gas. The effect of ZIF-300 loading on theCO2/N2 separation performance of ZIF-300/PEBA MMMswas investigated, as shown in Figure 9. When the loading of

ZIF-300 increases up to 30 wt %, the CO2 permeability andCO2/N2 selectivity increased simultaneously, breaking thepermeability−selectivity “trade-off” in conventional polymericmembranes.12,13 Compared to the pure PEBA membrane, CO2permeability in the 30 wt % ZIF-300/PEBA MMM wasincreased by 59%, and meanwhile, the CO2/N2 selectivity wasimproved by 54%. The significant improvement in permeabilitycan be mainly attributed to the uniformly incorporated high-performing ZIF-300 filler. In addition, the disturbed packing ofPEBA chains, as revealed by the decreased Tg in the ZIF-300/PEBA MMMs, might also reduce the gas transport resistancethrough the PEBA matrix. Also, the kinetic diameters of CO2

and N2 are 0.33 and 0.364 nm respectively, which are smallerthan the micropore size of ZIF-300 which is 0.79 nm (BETresult in Figure 4a). Thus, the highly enhanced CO2/N2selectivity is presumably owing to the excellent CO2preferential sorption over N2 within ZIF-300 crystals, asdisplayed in Figures 4b and 8. When the ZIF-300 loadingwas increased to 40 wt %, CO2/N2 selectivity was sharplydecreased back to the value of pure PEBA membrane. Thisreveals the presence of nonselective defects that were probablygenerated by the aggregation of ZIF-300 crystals in the PEBAmatrix observed under SEM (Figure 5f). Previous studies21,27,29

also demonstrated that it remains challenging to fabricatedefect-free mixed-matrix membranes with a filler loading above30 wt %.In addition, we also tried large-sized ZIF-300 crystals for

fabricating PEBA MMMs, while the preliminary result showedthat they were prone to generate defects in the MMMs, therebyhindering the performance improvement and/or reproduciblepermeation results. Similar phenomena have been found in theliterature.47−50

The permeability (P) can be defined as the product of thediffusion coefficient (D) and the sorption coefficient (S).39 Dwas calculated by using the measured P and S in the permeationand sorption experiment, respectively. To better understand therole of ZIF-300 crystals in the gas transport of ZIF-300/PEBAMMMs, the sorption coefficient and diffusion coefficient ofCO2 and N2 as well as the sorption selectivity (αS) anddiffusion selectivity (αD) for CO2/N2 were studied in detail. Asshown in Figure 10, in the pure PEBA membrane, both thesorption coefficient and the diffusion coefficient for CO2 arehigher than N2, owing to the higher condensability and smallermolecular size of CO2. By introducing ZIF-300 crystals inPEBA membrane, the sorption coefficient was increased whilethe diffusion coefficient was decreased for both 30 and 40 wt %loading. Owing to the CO2-philic feature of ZIF-300 crystals,the improvement of the sorption coefficient for CO2 is higherthan that for N2, leading to the enhanced CO2/N2 sorptionselectivity in ZIF-300/PEBA MMMs. As discussed above, therelatively large pore size of ZIF-300 could hardly contribute todiscrimination for CO2 and N2; thus, the CO2/N2 diffusionselectivity was not improved in ZIF-300/PEBA MMMs. Thelower diffusion coefficient in the MMMs may be caused byreduced diffusivity through the MOF−polymer interphase and/or irregular stacking of ZIF-300 transport channels.51

Compared with 30 wt % loading, 40 wt % ZIF-300-filledPEBA membrane had interfacial voids that hindered the

Figure 9. Effect of ZIF-300 loading on the CO2/N2 separationperformance of ZIF-300/PEBA MMMs. Pure gas permeation test wasmeasured at 0.4 MPa and 20 °C.

Figure 10. (a) Sorption coefficient (S) and diffusion coefficient (D) and (b) sorption selectivity (αS) and diffusion selectivity (αD) of ZIF-300/PEBAMMMs with different ZIF-300 loading for CO2 and N2 at 0.4 MPa and 20 °C.



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increase of transport selectivity. Overall, the sorption anddiffusion analyses clearly confirm our speculation that theexcellent preferential sorption of CO2 over N2 enabled ZIF-300to highly improve the permeability and selectivity of the PEBAmembrane for CO2/N2 separation.3.3.2. Effect of Operating Temperature. The effect of

operating temperature on the CO2/N2 separation performanceof pure PEBA membrane and optimized ZIF-300/PEBA MMMwith 30 wt % loading was studied. As shown in Figure 11, thegas permeability increased while CO2/N2 selectivity decreasedwith the operation temperature increasing from 20 to 60 °C.This phenomenon agrees well with literature.52 At elevatedtemperature, the increased polymer chain mobility increases thediffusion coefficients for both CO2 and N2. However, N2 withlarger molecular size is affected more significantly over CO2;thus, higher temperature decreases the diffusion advantage ofCO2 over N2. Furthermore, the sorption advantage of CO2 overN2 is also reduced when increasing the temperature. Thisphenomenon indicates that higher operating temperature maylead to a higher productivity but lower product purity.The temperature dependence of permeability can be

expressed by the Arrhenius equation as follows12

=−⎛

⎝⎜⎞⎠⎟P P

E

RTexpi i ,0

p

(5)

where Pi is the permeability of component i, Ep is the activationenergy, Pi,0 is pre-exponential factor, R is the gas constant, andT is the operation temperature in Kelvin. The Arrhenius plot isshown in Figure 11b, in which a linearity between log (Pi) and1/T suggests that the experimental permeation follows thetheoretical model shown in eq 5. The apparent activationenergy values for CO2 and N2 were calculated as 18.61 and35.54 kJ/mol, respectively. It suggests that the transport of theN2 molecule across the membrane is more sensitive than theCO2 molecule to operating temperature.3.3.3. Long-Term Permeation. Long-term permeation was

also tested to investigate the membrane structural stability. Asshown in Figure 12, during 100 h of continuous operation, theCO2/N2 separation performance of 30 wt % ZIF-300/PEBAMMM is high and stable, showing an average CO2 permeabilityof 83 Barrer and CO2/N2 selectivity of 84. The good structuralstability of the MMM is assumed to the favorable physicalinteraction between ZIF-300 filler and the PEBA matrix, whichcan also be inferred from the uniform dispersion of ZIF-300 inthe membrane up to 30 wt % as displayed in the SEM images(Figure 5b−d).

3.3.4. Performance Comparison with the Literature. TheCO2/N2 separation performance of our ZIF-300/PEBA MMMswas plotted in 2008 Robeson upper bound, as given in Figure13. As discussed above, both the CO2 permeability and the

CO2/N2 selectivity were improved with the increase of ZIF-300loading in the PEBA matrix. As the loading is up to 30 wt %,the performance of the ZIF-300/PEBA MMM surpassed the2008 Robeson upper bound for CO2/N2 separation.

53

4. CONCLUSIONIn this work, a novel ZIF-300 MOF material was realized formembrane separation for the first time. The membrane was

Figure 11. (a) Effect of operating temperature on CO2/N2 separation performance. (b) Arrhenius plots for CO2 and N2 permeability of 30 wt %ZIF-300/PEBA MMM at 0.4 MPa.

Figure 12. Long-term permeation of 30 wt % ZIF-300/PEBA MMMat 0.4 MPa and 20 °C.

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



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prepared by incorporating ZIF-300 crystals into the PEBAmatrix to form ZIF-300/PEBA MMM that was applied for CO2capture. It was found that smaller ZIF-300 crystals can beproduced by rationally reducing the time of the hydrothermalreaction. The obtained ZIF-300 crystals were uniformlydispersed in the PEAB matrix with well-preserved porestructure, showing an excellent CO2 sorption capacity andselectivity over N2. Thus, the CO2 permeability and CO2/N2selectivity of the pure PEBA membrane were simultaneouslyimproved by introducing ZIF-300 crystals up to 30 wt %. Ahigh and stable CO2/N2 separation performance was achievedin the resulting 30 wt % ZIF-300-filled PEBA membrane thattranscended the 2008 Robeson upper bound for state-of-the-artmembranes. Our work showed the potential of ZIF-300 fordeveloping high-performance mixed-matrix membrane thatcould be promising for application of CO2 capture.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: [email protected].*E-mail: [email protected].

ORCIDWanqin Jin: 0000-0001-8103-4883NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was financially supported by the National NaturalScience Foundation of China (grant nos. 21406107, 21490585,21301148, and 21776125), the National Key Basic ResearchProgram (2017YFB0602500), the Innovative Research TeamProgram by the Ministry of Education of China (grant no.IRT_17R54), the Natural Science Foundation of JiangsuProvince (no. BK20140930), the Topnotch Academic Pro-grams Project of Jiangsu Higher Education Institutions(TAPP), and the Innovation Project of Graduate Research byJiangsu Province (grant no. KYLX_0764).

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