Journal of Membrane Science 513 (2016) 155–165

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Journal of Membrane Science

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UiO-66-polyether block amide mixed matrix membranes for CO2

separation

Jie Shen, Gongping Liu, Kang Huang, Qianqian Li, Kecheng Guan, Yukai Li, Wanqin Jin n

State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing TechUniversity (former Nanjing University of Technology), 5 Xinmofan Road, Nanjing 210009, PR China

a r t i c l e i n f o

Article history:Received 3 February 2016Received in revised form18 April 2016Accepted 18 April 2016Available online 22 April 2016

Keywords:UiO-66Amine functionalizationMixed matrix membraneCarbon dioxide capturePolyether block amide

x.doi.org/10.1016/j.memsci.2016.04.04588/& 2016 Elsevier B.V. All rights reserved.

esponding author.ail address: wqjin@njtech.edu.cn (W. Jin).

a b s t r a c t

Mixed matrix membranes containing metal-organic frameworks have attracted large attention owing tothe combined advantages of high separation performance and easy processability. In this work,CO2-philic zirconium metal organic framework UiO-66 and UiO-66-NH2 nanocrystals were synthesizedand embedded into polyether block amide (PEBA) polymer membranes for CO2 separation. It can befound that amine-functionalization endowed UiO-66-NH2 nanoparticles with stronger CO2 affinitycompared with that of UiO-66. Also, the hydrogen bonding frameworks between UiO-66-NH2 and PEBAwere enhanced, leading to improved dispersibility in polymer matrix. Both the UiO-66-PEBA andUiO-66-NH2-PEBA mixed matrix membranes showed much higher CO2 separation performance than thatof pure PEBA membrane. Especially, amine functionalization of the porous frameworks provided the so-prepared UiO-66-NH2-PEBA mixed matrix membrane with higher CO2/N2 selectivity and slightly de-creased CO2 permeability than those of UiO-66-PEBA membrane. The developed UiO-66-NH2-PEBAmixed matrix membrane (with MOFs loading of 10 wt%) was tested in humid state and showed excellentand stable CO2/N2 separation performance (CO2 permeability of 130 Barrer, CO2/N2 selectivity of 72)surpassing the upper bound of polymer membranes. This type of UiO-66 based mixed matrix membranesfeaturing with excellent structural stability and significantly improved gas separation performance offerpromising potential for CO2 capture.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

The excessive emission of greenhouse gases (especially CO2)from fossil fuel combustion and other mankind economic andsocial activities has been causing serious environmental problems.Global attention has been focused on efficient CO2 capture andstorage (CCS) from a huge amount of energy-intensive industrialgases, including natural gas, biogas, flue gas and so on [1–4].Compared with traditional technologies such as absorption, cryo-genic purification and adsorption, membrane based separation hasbeen considered to be a prospective alternative, with potentiallyhigh efficiency, low energy consumption, ease of scale-up andenvironmental friendliness. Polymeric materials are the mostwidely used membrane separation materials. Many classes ofpolymers were applied for CO2 separation, such as polyamides [5],polyimides [6], polycarbonates [7], polysilicone [8], poly(ethyleneoxides) [9], etc. However, these polymeric membranes often sufferfrom the trade-off between permeability and selectivity [10,11],

which significantly restrict the separation performance. Many ef-forts have been taken to further improve the permeability andselectivity by developing next-generation membranes throughtuning cavity size of membrane materials and chemical affinitytowards guest molecules [12].

Metal–organic frameworks (MOFs) are a large emerging class ofhybrid materials with porous crystalline structures that combinethe connectivity of metal centers with the bridging ability of or-ganic ligands. Judicious choice of metal and linker allows newstructures to be designed and synthesized with the desired func-tionalities, pore sizes and pore shapes [13] for a vast range ofpotential applications such as gas storage [14], catalysis [15],sensing [16] and drug delivery [17]. Particularly, with the uniqueproperties of tunable pore size, large surface areas and specificadsorption affinities, MOFs can be developed to be potentialmembrane materials for molecular separation [18–20]. It has beenreported that pure MOFs membranes can show excellent gas se-paration performances [21–28]. However, pure MOFs membranesare often subjected to the difficulty of being satisfactory for in-dustrial-scale applications. Defects in the membranes such ascracks and inter-crystal gaps can cause non-selective permeation

Fig. 1. Schematic structures of UiO-66-NH2, PEBA and UiO-66-NH2-PEBA mixedmatrix membrane.

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165156

of gases. Another challenge is the large-area fabrication of pureMOFs membranes for being applied to industrial-scale gas se-paration processes. Mixed matrix membrane is a promising alter-native route to develop applicable membranes, which combinesthe high separation performance of the incorporated fillers withthe good processability and mechanical stability of polymers [29–36]. Incorporating CO2-selective porous frameworks into polymermembranes has been proven to be an effective strategy to developMOFs-based mixed matrix membranes owing to the wide range ofapplication such as CO2 capture, natural gas treatment, biogasseparation and hydrogen purification [1,37,38]. ZIF-90 with poresize of about 3.5 Å was added into polymers to improve themembranes CO2 permeability [31]. Also, Song et al. [39] fabricatednanocomposite membranes by incorporating ZIF-8 (with pore sizeof 3.4 Å) nanoparticles into Matrimids, showing enhanced CO2

permeability and CO2/CH4 selectivity even at high ZIF-8 loading.These works focused on incorporating MOFs with small pore sizeto tuning the molecular sieving characteristic of the membranes.In addition, CO2-philic frameworks were also selected to preparemixed matrix membranes because of the strong interaction be-tween the functionalized groups of MOFs and CO2 molecules. Longet al. [40] chose the synthesized Mg2(dobdc) as the fillers, whichexhibited excellent CO2 adsorption capacity. Hence, the CO2/N2

selectivity of Mg2(dobdc)-polymer mixed matrix membranes wasimproved obviously compared with those of pure polymer mem-branes. Recently, Zn(pyrz)2(SiF6) metal-organic framework withpore size of 3.8 Å was demonstrated to show strong affinity to CO2

molecules [41]. After introducing into polyethylene oxide matrix,the mixed matrix membranes showed improved CO2 permeabilityas well as CO2/N2 and CO2/CH4 selectivity [42]. It can be found thatCO2-philic metal-organic frameworks with appropriate pore sizeare promising for developing highly efficient CO2 separationmembranes.

A new class of Zirconium-based porous MOF UiO-66 (UiO:University of Oslo), has attracted great interest recently [43]. In itsface-centered-cubic crystal structure, each zirconium metal centeris connected to 12 benzene-1,4-dicarboxylate (BDC) linkers toform the a highly connected framework, which is believed to bethe main reason for the high structural stabilities. It has centricoctahedral cages, which is linked with eight corner tetrahedralcages by means of triangular windows with the size of 6 Å or so.Many studies discovered that UiO-66 can exhibit strong affinity toCO2 molecules owing to the -OH groups coordinated to Zr cluster[44–47]. A series of functionalized UiO-66 frameworks can beconveniently synthesized by simply incorporating different func-tional groups, as to finely tailor the pore structure and chemicalproperties [45,46]. MMMs with Ti exchanged UiO-66 and poly-mers of intrinsic microporosity (PIMs) [48] have been proven toexhibit high CO2 permeability of 13540 Barrer for the separation ofCO2/N2 mixture. Also, functionalized UiO-66 and polyimide (PI)MMMs showed enhanced CO2/CH4 separation performances[49,50]. Amine-functionalization is an effective strategy for en-hancing the CO2 affinity of MOFs crystals [49,51–53]. As a result,UiO-66-NH2 was reported to show stronger CO2 adsorption ca-pacity than that of pristine UiO-66 [46], which offers great po-tential for fabricating CO2-separation mixed matrix membranes.Su et al. [54] fabricated UiO-66-NH2/Polysulfone (PSF) membranewith CO2/N2 ideal selectivity of 26 and dramatically increased CO2

permeability of 46 Barrer. Moreover, phenyl acetyl functionalizedUiO-66-NH2 particles were synthesized and incorporated intoMatrimid

s

polymer [55]. The membrane showed CO2 permeabilityof about 37 Barrer (increased by 200%) and CO2/N2 ideal selectivityof about 30.

In this work, we report on the design and fabrication of UiO-66based metal-organic frameworks mixed matrix membranes forCO2 capture application. Nano-sized UiO-66 crystals were

synthesized and functionalized by amine-groups, which is aimedat maximizing the interaction between UiO-66 frameworks andpolymer chains as well as improving the CO2 affinity of mem-branes. We chose polyether block amide (PEBA) as the polymermaterial. It is composed of ethylene oxide and amide groups intheir soft and hard segments, respectively, which provide high CO2

permeability and mechanical strength at the same time [2].Thereby, for the first time, we fabricated UiO-66 (NH2)-PEBAmixed matrix membranes as shown in Fig. 1. The membranesmicrostructures as well as MOF-polymer interaction were sys-tematically characterized and studied. Compared with previousworks of UiO-66 based mixed matrix membranes [49,53–56], ourUiO-66 (NH2)-PEBA membranes showed more selective transportof CO2 molecules and simultaneous enhancement of CO2 perme-ability and CO2/N2 selectivity than those of pure PEBA membrane.The as-prepared membrane also showed excellent long term sta-bility in humidified state, which shows promising potential for CO2

capture application.

2. Experimental

2.1. Materials

For the synthesis of UiO-66 MOFs, zirconium (IV) chloride(ZrCl4) powder was supplied by Alfa Aesar, Terephthalic acid and2-Aminoterephthalic acid were provided by Sigma-Aldrich, N,N-Dimethylformamide (DMF) was purchased by Sinopharm Chemi-cal Reagent Co., Ltd, formic acid was received from Xilong Che-mical Co., Ltd., ethanol was obtained by Wuxi City Yasheng Che-mical Co., Ltd., PEBAX MH 1657 was purchased from Arkema,France. N2 and CO2 with purity 99.999% was supplied by NanjingSpecial Gases Company. Deionized water was used in all the ex-periments. All of the materials were used without furtherpurification.

2.2. Preparation of UiO-66 MOFs

The preparation of UiO-66 and UiO-66-NH2 were carried outfollowing the procedures in previous works [43,57]. For UiO-66,ZrCl4 (4.49 mmol, 1.06 g), Terephthalic acid (4.49 mmol, 0.67 g),5 mL formic acid were dissolved in 50 mL DMF and then heated at120 °C for 24 h. The white powder was obtained by centrifugation

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165 157

at 8000 rpm for 10 min and were immersed in methanol for sol-vent exchange for three days and activated under vacuum at100 °C. Similar process was carried out for the synthesis ofUiO-66-NH2 through replacing the Terephthalic acid (4.49 mmol,0.67 g) with 2-aminoterephthalic acid (4.49 mmol, 0.81 g).

2.3. Membrane fabrication

A certain amount of Zr-MOFs (UiO-66 or UiO-66-NH2) powderswere ground and dispersed in ethanol and water mixed solventwith mass ratio of 7:3, followed by sonication for 10 min Then,PEBA were added into the dispersion containing Zr-MOFs nano-particles and heated with stirring and refluxing under 80 °C for12 h. The mass ratios of PEBA to solvent is 5:95 while the contentsof Zr-MOFs in polymer matrix were varied from 5 to 20 wt%. Theflat membrane was prepared by casting the mixed solution onPVDF supports, which were pretreated by being immersed inwater for 24 h. The free-standing membranes for characterizationand analysis were prepared by casting the corresponding mixedsolutions on the glass substrates. The membranes were finallyfabricated after evaporating the solvent at room temperature for2 days and dried in a vacuum oven at 70 °C for 12 h. For com-parison, the neat PEBA was prepared following the similarprocedure.

2.4. Characterization

Morphologies of the as-prepared UiO-66 MOFs nanoparticlesand membranes were characterized by scanning electron micro-scope (SEM, S4800, Hitachi, Japan) together with energy-dis-persive X-ray spectroscopy (EDXS). The crystalline structures ofUiO-66 MOFs were analyzed through X-Ray Diffraction (XRD,Bruker, D8 Advance) using Cu Kα radiation (λ¼1.54 Å) at 40 kVand 15 mA at room temperature. The functional groups of syn-thesized UiO-66 MOFs and hydrogen bonds between UiO-66 MOFsand PEBA polymer were characterized by Fourier transform in-frared (FTIR, AVATAR-FT-IR-360, Thermo Nicolet, USA) spectrawith the range of 600–4000 cm�1. The thermal properties of thesynthesized UiO-66 MOFs were characterized by thermogravi-metric analysis (TGA, NETZSCH STA 449F3) in the range of roomtemperature to 800 °C with the rate of 10 °C min�1. Adsorptionexperiments of the UiO-66 MOFs nanocrystals were carried out byASAP 2460 with CO2 (298 K) and N2 (77 K). BET surface areas werecalculated from the N2 isotherms at 77 K. The morphologies ofUiO-66-PEBA and UiO-66-NH2-PEBA membranes were obtainedby atomic force microscopy (AFM, XE-100, Park SYSTEMS, SouthKorea) operated in the Non-contact mode. Differential scanningcalorimetry (DSC, Q2000, TA Instruments, USA) measurementswere performed from �80 to �40 °C in N2 atmosphere to studythe glass transition temperatures (Tg) of the membranes. Themechanical properties of UiO-66-PEBA and UiO-66-NH2-PEBAmembranes were analyzed using nanoindentation technique(Nano-Test Vantage, Micro Materials, UK). The diameter of thenanoindenter is 50 nm. For each membrane, 36 testing pointswere applied and distributed uniformly in the testing area of90 mm�90 mm. The testing points with large deviation were re-moved, and the average value was finally calculated.

2.5. Gas separation experiments

Gas transport behaviors were both measured by pure gas andmixed gas permeation tests using constant pressure/variable vo-lume technique (see the schematic in Supplementary Fig. 1). Forpure gas permeation tests of CO2 and N2, the pressure and tem-perature were set as 0.3 MPa and 25 °C, respectively. After thesystem reached steady-state, all the gas permeation

measurements were performed more than five times, and the gaspermeability can be calculated using the following equation:

=∆ ( )PQLPA 1

where P is the gas permeability [1 Barrer¼10�10 cm3 (STP)cm/(cm2 s cmHg)], Q is the volume permeate rate of gas (cm3/s) atstandard temperature and pressure (STP), L is the membranethickness (cm),ΔP is the transmembrane pressure (cmHg) and A isthe effective membrane area (12.56 cm2 in this work). The idealselectivity of CO2/N2 can be calculated by the ratio of the perme-ability of the individual gas which can be expressed as follows:

α =( )

PP 2

A

B

Mixture of CO2 and N2 (50 vol%:50 vol%) was used as feed gasand CH4 was chosen as sweep gas (Supplementary Fig. 1). The gaspermeability was also calculated by Eq. (1) while selectivity can beobtained by using Eq. (3):

α =( )

y yx x

// 3

A BA B

A B/

where x and y are the volumetric fractions of the one compo-nent in the feed and permeate, respectively. When the membraneswere tested in humid state, the feed and sweep gases were bothhumidified by water bottles. The relative humidity in dry state wasabout 20%, while after humidifying, the relative humidity in-creased to about 85%.

3. Results and discussion

3.1. Physicochemical properties of UiO-66 MOFs

Scanning electron microscope (SEM) showed that the synthe-sized UiO-66 and UiO-66-NH2 both crystallized small octahedrallycubic nanocrystals with size in the range of 60–80 nm (Fig. 2).These nano-sized nanoparticles are considered to be excellentfillers to prepare thin and homogenous mixed matrix membranes[58]. EDXS was also carried out to analyze the elemental compo-sition of synthesized UiO-66 MOFs samples. It can be seen thatUiO-66 is composed of Zr, C and O elements (Fig. 2(b)), which is inagreement with its crystal structure, the Zr6-cluster coordinatedwith terephthalate ligands. For UiO-66-NH2, a new peak of Nelement appeared, indicating the amino-functionalization of UiO-66 by substituting the linker.

The powder X-ray diffraction (PXRD) patterns (Fig. 3(a))showed that the synthesized UiO-66 MOFs were with highlycrystalline structure, matching well with the reported literature[43,59]. It should be noted that the introduction of amine func-tional groups has neglectable influence on the crystal structure ofUiO-66 MOFs. The size of crystallites estimated by the Schererequation [40] was about 66 nm, which is approximate to the sizeof UiO-66 MOFs nanocrystals observed in SEM (Fig. 2). Also, thestability of synthesized UiO-66 and UiO-66-NH2 nanoparticleswere evaluated by PXRD after heating in ethanol/water mixedsolvents (ωethanol:ωwater¼70:30), which is required by membranefabrication process. As shown in Fig. 3, both UiO-66 andUiO-66-NH2 maintain highly crystalline structure after solvother-mal treatment, demonstrating that the 3D framework of UiO-66MOFs is highly stable. The chemistry structure was analyzed byFourier transform infrared (FTIR) spectra (Fig. 3(b)). The adsorp-tion bands from 1503 to 1600 cm�1 can be ascribed to stretchingvibration of C4O group. The peak at 1395 cm�1 is assigned to thestretching vibration of C-O group. Especially, for UiO-66-NH2, a

Fig. 2. SEM images of synthesized (a) UiO-66 nanoparticles and (c) UiO-66-NH2 nanoparticles. (b) and (d) are the corresponding EDXS data of UiO-66 and UiO-66-NH2

nanoparticles, respectively. Inserted are the images of elemental maps.

Fig. 3. (a) PXRD patterns and (b) FTIR spectra of (i) UiO-66 (ii) UiO-66 after heating at 80 °C for 10 h in ethanol/water mixed solvent (iii) UiO-66-NH2 (iv) UiO-66-NH2 afterheating at 80 °C for 10 h in ethanol/water mixed solvent. (c) TGA curves for UiO-66 and UiO-66-NH2 powders.

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165158

new peak at 1680 cm�1 appeared, which clearly indicates thepresence of –NH2 group. It has been well demonstrated thatamino-functionalized groups can intensify the materialsCO2-philic property [60]. Also, UiO-66 and UiO-66-NH2 powdersbeing heated in ethanol/water mixed solvent were tested by FITRto further investigate the stability. All the chemical functionalizedgroups essentially remain unchanged, which again proves theexcellent stability of the synthesized UiO-66 MOFs.

The 3D framework stability and structural integrity of UiO-66MOFs were further studied by Thermogravimetric analysis (TGA)as shown in Fig. 3(c). Both UiO-66 and UiO-66-NH2 powders un-derwent slight weight loss from 100 to 180 °C. Then, the weightloss from 180 to 280 °C can be ascribed to the dehydroxylation ofZr6O4(OH)4 cornerstone into Zr6O6. For UiO-66, the decompositiontemperature is about 500 °C, which is higher than the typical valueof 350 °C of other MOFs [43,61]. After 500 °C, the benzene frag-ments lost gradually and the residue materials became to ZrO2 atlast. However, the crystalline structure of UiO-66-NH2 maintainsup to 290 °C, which is lower compared with UiO-66.

It can be seen that the N2 adsorption isotherms at 77 K for UiO-66 and UiO-66-NH2 both followed type I isotherm with no hys-teresis. BET surface areas of UiO-66 and UiO-66-NH2 were 847 and822 m2/g, respectively, which are in consistence with literature[62,63]. The micropore volumes for UiO-66 and UiO-66-NH2 were0.43 and 0.38 cm3/g, respectively. These indicate that the sub-stitution of ligand with amine functionalized groups has no ob-vious influence on the pore structure. The micropore size was0.59 nm, which is close to the theoretical value (0.6 nm) of thetriangular windows in UiO-66 framework. It should be noted thateven with slightly lower BET surface area and micropore volume,UiO-66-NH2 still showed enhanced CO2 adsorption ability (in-creased 20% or so) than that of pristine UiO-66 (Fig. 4(c)). Thisfurther proves the CO2-philic property of amino groups, which isexpected to be beneficial for preferential CO2 permeation.

Fig. 4. (a) and (b) are the N2 adsorption and desorption isotherms at 77 K for UiO-66 and UiO-66-NH2 nanoparticles, respectively. (c) CO2 adsorption isotherms at 298 K forUiO-66 and UiO-66-NH2 nanoparticles.

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165 159

3.2. Membrane characterization

3.2.1. Morphology of MMMsThe morphology of as-prepared membranes was characterized

by SEM. Membranes were fabricated on flat PVDF substrate usingcasting method. Fig. 5(a) and (e) show the surface and cross-sec-tion SEM images of pure PEBA membrane. It can be seen that purePEBA membrane is defect free and highly smooth without anystructural bulges on membrane surface. UiO-66-PEBA andUiO-66-PEBA-NH2 membranes with different Zr-MOFs loadingswere demoted as UiO-66-PEBA (X) and UiO-66-NH2-PEBA (X),respectively, in which “X” indicates that the mass fraction of UiO-66 MOFs in polymeric matrix is X wt%. As shown in Fig. 5(b), UiO-66-PEBA (10) membrane showed rough surface with some largebulges, which is due to the aggregation of UiO-66 nanoparticles inthe polymeric matrix. From the cross-section SEM image, we canclearly see the agglomerate of UiO-66 particles with size of severalmicrometers. In addition, the poor adhesion at MOF-polymer in-terface is observed. The poor dispersion property of UiO-66 na-noparticles is attributed to the weak interaction between UiO-66frameworks and polymer chains. Fortunately, the incorporation ofamino functionalized groups into the UiO-66 crystalline structuremade the as-synthesized UiO-66-NH2 nanocrystals be morecompatible with polymeric matrix. Fig. 5(c) and (g) exhibit highquality membrane [UiO-66-NH2-PEBA (10)] with homogenousdistribution of UiO-66-NH2 nanocrystals. Large conglomerates of

Fig. 5. Surface SEM images of (a) pure PEBA, (b) UiO-66-PEBA (10), (c) UiO-66-NH2-PEBimages of (e) pure PEBA, (f) UiO-66-PEBA (10), (g) UiO-66-NH2-PEBA (10) and (h) UiO-enlarged surface images of UiO-66-PEBA (10) and UiO-66-NH2-PEBA (10) membranes, r

MOF particles as well as visible voids at MOF-polymer interfaceare not observable in SEM images. This further demonstrates theenhanced interface interaction between MOF and polymer, whichis because that the introduced –NH2 groups can effectively im-prove the hydrogen bonding frameworks with PEBA. Similarphenomenon was also observed in the work of amine-functiona-lized MIL-53 filled polysulfone MMMs [64]. However, severe ag-gregation appeared when the content of UiO-66-NH2 increased to20 wt% (Fig. 5(d) and (h)).

In addition, AFM characterization further compared the mem-branes surface roughness. Pure PEBA membrane showed smallestsurface roughness parameters (Rq and Ra, as shown in Table 1).However, the roughness of UiO-66-PEBA (10) membrane increasedalmost two times than that of pure PEBA membrane. ForUiO-66-NH2-PEBA (10) membrane, the surface became muchsmoother, showing a better compatibility. It is considered thatsuch MOF-polymer compatibility not only influences the mem-brane morphology, but also rationally differs the membranes mi-crostructures as well as gas separation performances.

3.2.2. Microstructure of MMMsXRD characterization was carried out to further analyze the

microstructure of as-prepared UiO-66 MOFs MMMs. Pristine PEBAis a semicrystalline copolymer which consists of both crystallineand amorphous PEO and PA6 phases, which showed characteristicpeak ranging from 15° to 25° [65]. Generally, the sharp X-ray

A (10) and (d) UiO-66-NH2-PEBA (20) membranes, respectively. Cross-section SEM66-NH2-PEBA (20) membranes, respectively. Inserted images in (b) and (c) are theespectively.

Table 1Surface roughness parameters for pure PEBA, UiO-66-PEBA (10) andUiO-66-NH2-PEBA (10) membranes, respectively.

Membranes Pure PEBA UiO-66-PEBA (10) UiO-66-NH2-PEBA (10)

Rq (nm) 34.847 97.478 44.304Ra (nm) 26.677 75.106 34.204

Fig. 6. XRD patterns for pure PEBA, UiO-66-PEBA and UiO-66-NH2-PEBAmembranes.

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165160

diffraction peak represents the crystalline region of the polymerwhile the amorphous polymer exhibits a broad peak. XRD patternsfor UiO-66 MOFs MMMs with different loadings of MOFs are alsopresented in Fig. 6. It is clear that both the UiO-66 and UiO-66-NH2

nanoparticles maintain their crystallinity and topology in polymermatrix. The crystal sizes calculated by Scherer equation were inthe range of 52–59 nm, which were slightly smaller than thoseestimated by XRD characterization of UiO-66 MOFs powders. Itshould be noted that the peak position of PA6 varied by introdu-cing different UiO-66 MOFs nanoparticles. Pure PEBA membraneshowed the peak of PA6 at about 2θ¼23.8 °. For UiO-66-PEBAMMMs, peak position of PA6 almost remained unchanged. How-ever, after incorporating UiO-66-NH2 nanocrystals, this peak

Fig. 7. FTIR spectra for pure PEBA, UiO-66-PEBA and UiO-66-NH2-PEBA membranes. (a) WN–H–) and (c) wave number from 1550 to 1725 cm�1 (H–N–C¼O).

gradually shifted to higher degree. Thus, the calculated d-spacingwas reduced, indicating the decrease of free volume cavities inpolymer chains. This again suggests the enhanced hydrogenbonding frameworks between amine-functionalized UiO-66 MOFsnanoparticles with PEBA molecules.

In order to further characterize and analyze the hydrogenbonding frameworks between incorporated UiO-66 MOFs andpolymer chains, FTIR spectra were conducted (Fig. 7). The ob-served peak at 1094 cm�1 is attributed to the stretching vibrationof the C-O-C group of the soft segment part of PEO [65]. The peakcorresponding to –N–H– linkages is approximately at 3298 cm�1

while the characteristic peaks at 1636 and 1732 cm�1 are assignedto H–N–C4O group and O–C¼O group, respectively. The abovethree functional groups are indicates the presence of the hardsegment of PA chains [66]. All the membranes show no new ab-sorbance peaks, which demonstrates that there were no strongchemical interaction between Zr-MOFs nanoparticles and PEBAchains. Moreover, it should be noticed that the peaks at around1636 and 3298 cm�1 shifted gradually to lower frequency forUiO-66-NH2-PEBA membranes while the peaks of UiO-66-PEBAmembranes remained unchanged. The hydrogen bonding frame-works was obviously improved, which is demonstrated by theredshift of IR characteristic peaks of PEBA [67].

Differential scanning calorimetry (DSC) measurement (Fig. 8(a)) was carried out to characterize the polymer thermodynamicproperties. Pure PEBA membrane showed a glass transition tem-perature (Tg) of �54.6 °C that agrees well with the literature [68].It can be noted that gradually adding UiO-66-NH2 nanoparticlesmade Tg in the trend of increase, which indicates that the mem-branes were likely undergoing glass transition. As a result, themechanical property of PEBA membranes can be improved (Fig. 8(b)). For UiO-66-PEBA MMMs, the hardness and elastic modulusjust increased slightly. However, for UiO-66-NH2-PEBA MMMs, themechanical property was obviously improved. Compared withpure PEBA membrane, UiO-66-NH2-PEBA (20) membrane showed52% and 29.4% enhancement in hardness and elastic modulus,respectively. This can be attributed to the significant aggregationof UiO-66 particles in membranes, resulting in less contact be-tween UiO-66 particles and polymer matrix. Similar phenomenoncan also be observed in other reports [49,69]. It should be notedthat in our experiments, when adding too many UiO-66-NH2 na-noparticles (more than 20 wt%), the composite membranes be-came brittle, which further suggests the rigidification of the

ave number from 700 to 3500 cm�1, (b) wave number from 3000 to 3500 cm�1 (–

Fig. 8. (a) Glass transition temperatures and (b) comparison of hardness and elastic modulus of pure PEBA, UiO-66-PEBA and UiO-66-NH2-PEBA membranes.

Fig. 9. Effect of UiO-66 MOFs loading on the CO2/N2 separation performances ofUiO-66-PEBA and UiO-66-NH2-PEBA mixed matrix membranes. Pure gas permea-tion tests were measured at 0.3 MPa and 20 °C, 1 Barrer¼10�10 cm3 (STP)cm/(cm2 s cmHg).

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165 161

polymer chains.

3.3. Gas separation performances

3.3.1. Effect of UiO-66 MOFs loadingGas permeation measurements were carried out to further

study the molecular separation behaviors of as-prepared UiO-66-PEBA and UiO-66-NH2-PEBA mixed matrix membranes. First, theeffect of UiO-66 MOFs loading on the membranes CO2/N2 se-paration performances were investigated (Fig. 9). For UiO-66-PEBAmixed matrix membranes, it can be seen that when UiO-66loading increased to 7.5 wt%, both of the CO2 permeability andCO2/N2 selectivity increased at the same time, breaking the per-meability/selectivity trade-off relationship in polymeric mem-branes [10,11]. Compared to pure PEBA membrane, CO2 perme-ability of UiO-66-PEBA (7.5) increased by 72.5%, however, CO2/N2

selectivity only increased by 31.1%. This can be due to the trian-gular window of UiO-66 is 0.6 nm, which is larger than theoreticalfree volume of polymer chains [70]. However, the size of 0.6 nm isalso larger than the kinetic diameters of CO2 (0.33 nm) and N2

(0.36 nm). The enhanced selectivity is ascribed to the CO2-philiccharacteristic of UiO-66 frameworks [45]. CO2 permeability keptincreasing while the selectivity decreased with UiO-66 loading. Itcan be seen that when the concentration of UiO-66 rose to 20 wt%,CO2/N2 selectivity dropped to about 20, which is lower than that ofpure PEBA. This is because that severe agglomeration occurredwhen large amount of UiO-66 particles were added into polymericmatrix. As a result, non-selective interface defects were generated.UiO-66-NH2-PEBA mixed matrix membranes showed similar var-iation tends in CO2 permeability and CO2/N2 selectivity with thoseof UiO-66-PEBA membranes. However, owing to amino functio-nalized microstructure, UiO-66-NH2-PEBA mixed matrix mem-branes showed higher CO2/N2 selectivity of nearly 80 (with 10 wt%UiO-66-NH2) than that of UiO-66-PEBA (7.5). It can also be at-tributed to the homogenous dispersion of UiO-66-NH2 nanocrys-tals in PEBA membrane.

3.3.2. Effect of operation temperatureThe effect of operation temperature was also studied (Fig. 10). It

can be seen that gas permeability significantly increased withoperation temperature while CO2/N2 selectivity decreased.UiO-66-NH2-PEBA (10) mixed matrix membrane apparentlyshowed higher CO2 permeability than that of pure PEBA mem-brane under temperature ranging from 20 to 80 °C. This can beascribed to the 3D porous framework of UiO-66-NH2, providing

fast diffusion channels for molecules. Fig. 10(b) showed thatUiO-66-NH2-PEBA (10) mixed matrix membrane also possesshigher CO2/N2 selectivity than that of pure PEBA membrane athigh temperature. At 40 °C, CO2/N2 selectivity can still be as highas 68, which is almost twice as that of pure PEBA. As a result, the

Fig. 10. Effect of operation temperature on (a) CO2 permeability and (b) CO2/N2

selectivity of pure PEBA and UiO-66-NH2-PEBA (10) membranes. Pure gas per-meation tests were measured at 0.3 MPa.

Fig. 11. Effect of humidity on pure PEBA and UiO-66-NH2-PEBA membranes. Mixedgas permeation tests were measured at atmosphere pressure and 20 °C. Beforehumidifying, the relative humidity was about 20%, while after humidifying, therelative humidity was about 85%.

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165162

developed UiO-66-NH2-PEBA mixed matrix membrane offers po-tential opportunity for practical separation.

3.3.3. Effect of humidityNowadays, MOFs materials have been widely investigated to be

utilized in adsorption/separation, however, many of them sufferedfrom their low structural stabilities when being treated withmoisture, heat or acid-base [71–74]. This hinders their applicabilityin industry. Because of the exceptional chemical stability, UiO-66materials exhibit outstanding water stability. UiO-66 frameworkscan still remain good crystallinity even being immersed in waterfor several months [47]. In addition, UiO-66 with hydrophilicsurface showed affinity for water molecules, which also widens itspractical application such as being used as environmental ad-sorbents [57]. Generally, separating CO2 molecules from flue gas isaccompanied with water vapor [75]. Thus the effect of humiditywas studied, as shown in Fig. 11. Pure PEBA membrane showedincreased CO2 permeability while almost no change to CO2/N2

selectivity. This indicates that PEBA membrane can show water-facilitated CO2 separation behaviors [2]. For UiO-66-NH2-PEBAmixed matrix membranes, CO2 permeability significantly in-creased after humidifying. Also, when UiO-66-NH2 loading variedfrom 5 to 10 wt%, CO2/N2 selectivity slightly increased, which is

owing to the hydrophilic as well as CO2-philic properties ofUiO-66-NH2. Water vapor facilitated CO2 transport in membranesinstead of showing hindering effect. By comparing separationperformances of membranes in Table 2, we can find that UiO-66-PEBA (10) membrane showed higher CO2 permeability whilelower CO2/N2 selectivity than those of UiO-66-NH2-PEBA (10)membrane. These are attributed to the enhanced polymer-MOFsinteraction and the CO2-philic effect of amino functionalizedgroups. When the concentration of UiO-66-NH2 nanoparticlescontinued to increase, CO2/N2 selectivity dropped dramaticallybecause of the particles aggregation, which is shown in Fig. 5(d)and (h). It can be seen that the developed UiO-66-NH2-PEBAmixed matrix membrane showed enhanced CO2/N2 separationperformance in humid state. The membranes structure and gaspermeation behaviors under more relative humidity are needed tobe studied in detail, so as to further investigate the effects of watermolecules.

3.3.4. Long term operationLong term operation test of mixed gas permeation in humid

state was carried out to further investigate the membrane struc-tural stability (Fig. 12). The data were recorded after the whole

Table 2Comparison of mixed gas separation performances of membranes.

Membrane Dry state CO2 permeability(Barrer)

Dry state CO2/N2 selectivity (–) Humid state CO2 permeability(Barrer)

Humid state CO2/N2 selectivity (–)

Pure PEBA 51.571.1 42.171.9 71.672.7 43.271.2UiO-66-PEBA (10) 96.371.8 56.671.9 139.773.8 61.172.0UiO-66-PEBA-NH2 (10) 87.071.8 66.172.2 130.273.5 72.272.6

Fig. 12. Long term operation test on UiO-66-NH2-PEBA membrane in humid state.Mixed gas permeation tests were measured at atmosphere pressure and 20 °C. Therelative humidity was about 85%.

Fig. 13. Comparison of CO2/N2 mixed gas separation performances of pure PEBA,UiO-66-PEBA and UiO-66-NH2-PEBA membranes with Robeson upper bound. Thesolid data points are in humid state while the hollow data points are in dry state.

J. Shen et al. / Journal of Membrane Science 513 (2016) 155–165 163

permeation system was steady. During 100 h operation test, theUiO-66-NH2-PEBA (10) membranes showed high and stable se-paration performance with average CO2 permeability of 130 Barrerand CO2/N2 selectivity of 72. It also proves that the as-preparedUiO-66-NH2-PEBA mixed matrix membrane showed good struc-tural stability with the presence of water vapor, which is pro-mising for application of CO2 capture from flue gas.

3.3.5. Comparison with upper boundAs is shown in Fig. 13, both UiO-66-PEBA and

UiO-66-NH2-PEBA mixed matrix membranes exhibited improvedCO2/N2 mixed gas separation performance compared with that ofpure PEBA membrane. UiO-66-PEBA membrane showed higherCO2 permeability while UiO-66-NH2-PEBA membrane possessed

higher CO2/N2 selectivity. In addition, it can be seen thatUiO-66-NH2-PEBA (10) membrane possessed enhanced CO2/N2

mixed gas separation performance in humid state, surpassing theupper bound for state-of-the-art membranes. This endows thedeveloped UiO-66 MOFs mixed matrix membrane with potenti-ality for practical application.

4. Conclusion

In summary, we report the design and fabrication of UiO-66based metal-organic frameworks-PEBA mixed matrix membranes.Compared with UiO-66, UiO-66-NH2 showed enhanced MOF-polymer interactions, which led to excellent dispersibility inpolymer matrix. Moreover, stronger affinity with CO2 moleculeswas realized after functionalized by amine groups. The gas per-meation behaviors of the as-prepared membranes were also in-vestigated. The results showed that both of the UiO-66-PEBA andUiO-66-NH2-PEBA mixed matrix membranes showed significantlyimproved CO2 separation performance than that of pure polymermembrane. However, UiO-66-NH2-PEBA membranes possessedhigher CO2/N2 selectivity and slightly lower CO2 permeability thanthose of UiO-66-PEBA membrane, which can be ascribed to thefunctionalities of amine groups. The as preparedUiO-66-NH2-PEBA exhibited high and stable CO2/N2 separationperformance (CO2 permeability: 130 Barrer, CO2/N2 selectivity: 72)in humid state, transcending the upper bound for state-of-the-artpolymeric membranes. This work demonstrated that UiO-66 basedmetal-organic frameworks are promising materials for developingmixed matrix membranes, which offer promising potential for CO2

capture.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Grant nos. 21476107, 21490585,21406107), the Innovative Research Team Program by the Ministryof Education of China (Grant no. IRT13070), a Project funded by thePriority Academic Program Development of Jiangsu Higher Edu-cation Institutions (PAPD) and the Innovation Project of GraduateResearch by Jiangsu Province (Grant no. KYLX_0764).

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.memsci.2016.04.045.

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