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1 Author s Accepted Manuscript UiO-66-Polyether Block Amide Mixed Matrix Membranes for CO 2 Separation Jie Shen, Gongping Liu, Kang Huang, Qianqian Li, Kecheng Guan, Yukai Li, Wanqin Jin PII: DOI: Reference: To appear in: S (16) MEMSCI14445 Journal of Membrane Science Received date: 3 February 2016 Revised date: 18 April 2016 Accepted date: 18 April 2016 Cite this article as: Jie Shen, Gongping Liu, Kang Huang, Qianqian Li, Kecheng Guan, Yukai Li and Wanqin Jin, UiO-66-Polyether Block Amide Mixed Matrix Membranes for CO 2 Separation, Journal of Membrane Science This is a PDF file of an unedited manuscript that has been accepted fo publication. As a service to our customers we are providing this early version o the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form 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

2 UiO-66-Polyether Block Amide Mixed Matrix Membranes for CO 2 Separation Jie Shen, Gongping Liu, Kang Huang, Qianqian Li, Kecheng Guan, Yukai Li and Wanqin Jin* State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University (former Nanjing University of Technology), 5 Xinmofan Road, Nanjing , P.R. China. *To whom all correspondence should be addressed. Contact information: Prof. Wanqin Jin State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, 5 Xinmofan Road, Nanjing , P. R. China Tel.: ; Fax: ; wqjin@njtech.edu.cn 1

3 Abstract Mixed matrix membranes containing metal-organic frameworks have attracted large attention owing to the combined advantages of high separation performance and easy processability. In this work, CO 2 -philic zirconium metal organic framework UiO-66 and UiO-66-NH 2 nanocrystals were synthesized and embedded into polyether block amide (PEBA) polymer membranes for CO 2 separation. It can be found that amine-functionalization endowed UiO-66-NH 2 nanoparticles with stronger CO 2 affinity compared with that of UiO-66. Also, the hydrogen bonding frameworks between UiO-66-NH 2 and PEBA were enhanced, leading to improved dispersibility in polymer matrix. Both the UiO-66-PEBA and UiO-66-NH 2 -PEBA mixed matrix membranes showed much higher CO 2 separation performance than that of pure PEBA membrane. Especially, amine functionalization of the porous frameworks provided the so-prepared UiO-66-NH 2 -PEBA mixed matrix membrane with higher CO 2 /N 2 selectivity and slightly decreased CO 2 permeability than those of UiO-66-PEBA membrane. The developed UiO-66-NH 2 -PEBA mixed matrix membrane (with MOFs loading of 10 wt%) was tested in humid state and showed excellent and stable CO 2 /N 2 separation performance (CO 2 permeability of 130 Barrer, CO 2 /N 2 selectivity of 72) surpassing the upper bound of polymer membranes. This type of UiO-66 based mixed matrix membranes featuring with excellent structural stability and significantly improved gas separation performance offer promising potential for CO 2 capture. Keywords: UiO-66, amine functionalization, mixed matrix membrane, carbon dioxide capture, polyether block amide 2

4 1. Introduction The excessive emission of greenhouse gases (especially CO 2 ) from fossil fuel combustion and other mankind economic and social activities has been causing serious environmental problems. Global attention has been focused on efficient CO 2 capture and storage (CCS) from a huge amount of energy-intensive industrial gases, including natural gas, biogas, flue gas and so on [1-4]. Compared with traditional technologies such as absorption, cryogenic purification and adsorption, membrane based separation has been considered to be a prospective alternative, with potentially high efficiency, low energy consumption, ease of scale-up and environmental friendliness. Polymeric materials are the most widely used membrane separation materials. Many classes of polymers were applied for CO 2 separation, such as polyamides [5], polyimides [6], polycarbonates [7], polysilicone [8], poly(ethylene oxides) [9], etc. However, these polymeric membranes often suffer from the trade-off between permeability and selectivity [10, 11], which significantly restrict the separation performance. Many efforts have been taken to further improve the permeability and selectivity by developing next-generation membranes through tuning cavity size of membrane materials and chemical affinity towards guest molecules [12]. Metal organic frameworks (MOFs) are a large emerging class of hybrid materials with porous crystalline structures that combine the connectivity of metal centers with the bridging ability of organic ligands. Judicious choice of metal and linker allows new structures to be designed and synthesized with the desired functionalities, pore sizes and pore shapes [13] for a vast range of potential applications such as gas storage [14], catalysis [15], sensing [16] and drug delivery [17]. Particularly, with the unique properties of tunable pore size, large surface areas and specific adsorption affinities, MOFs can be developed to be potential membrane materials for molecular 3

5 separation [18-20]. It has been reported that pure MOFs membranes can show excellent gas separation performances [21-28]. However, pure MOFs membranes are often subjected to the difficulty of being satisfactory for industrial-scale applications. Defects in the membranes such as cracks and inter-crystal gaps can cause non-selective permeation of gases. Another challenge is the large-area fabrication of pure MOFs membranes for being applied to industrial-scale gas separation processes. Mixed matrix membrane is a promising alternative route to develop applicable membranes, which combines the high separation performance of the incorporated fillers with the good processibility and mechanical stability of polymers [29-36]. Incorporating CO 2 -selective porous frameworks into polymer membranes has been proven to be an effective strategy to develop MOFs-based mixed matrix membranes owing to the wide range of application such as CO 2 capture, natural gas treatment, biogas separation and hydrogen purification [1, 37, 38]. ZIF-90 with pore size of about 3.5 Å was added into polymers to improve the membranes CO 2 permeability [31]. Also, Song et al. [39] fabricated nanocomposite membranes by incorporating ZIF-8 (with pore size of 3.4 Å) nanoparticles into Matrimid, showing enhanced CO 2 permeability and CO 2 /CH 4 selectivity even at high ZIF-8 loading. These works focused on incorporating MOFs with small pore size to tuning the molecular sieving characteristic of the membranes. In addition, CO 2 -philic frameworks were also selected to prepare mixed matrix membranes because of the strong interaction between the functionalized groups of MOFs and CO 2 molecules. Long et al. [40] chose the synthesized Mg 2 (dobdc) as the fillers, which exhibited excellent CO 2 adsorption capacity. Hence, the CO 2 /N 2 selectivity of Mg 2 (dobdc)-polymer mixed matrix membranes was improved obviously compared with those of pure polymer membranes. Recently, Zn(pyrz) 2 (SiF 6 ) metal-organic framework with pore size of 3.8 Å was demonstrated to show strong affinity to CO 2 molecules [41]. After 4

6 introducing into polyethylene oxide matrix, the mixed matrix membranes showed improved CO 2 permeability as well as CO 2 /N 2 and CO 2 /CH 4 selectivity [42]. It can be found that CO 2 -philic metal-organic frameworks with appropriate pore size are promising for developing highly efficient CO 2 separation membranes. A new class of Zirconium-based porous MOF UiO-66 (UiO: University of Oslo), has attracted great interest recently [43]. In its face-centered-cubic crystal structure, each zirconium metal center is connected to 12 benzene-1,4-dicarboxylate (BDC) linkers to form the a highly connected framework, which is believed to be the main reason for the high structural stabilities. It has centric octahedral cages, which is linked with eight corner tetrahedral cages by means of triangular windows with the size of 6 Å or so. Many studies discovered that UiO-66 can exhibit strong affinity to CO 2 molecules owing to the -OH groups coordinated to Zr cluster [44-47]. A series of functionalized UiO-66 frameworks can be conveniently synthesized by simply incorporating different functional groups, as to finely tailor the pore structure and chemical properties [45, 46]. MMMs with Ti exchanged UiO-66 and polymers of intrinsic microporosity (PIMs) [48] have been proven to exhibit high CO 2 permeability of Barrer for the separation of CO 2 /N 2 mixture. Also, functionalized UiO-66 and polyimide (PI) MMMs showed enhanced CO 2 /CH 4 separation performances [49, 50]. Amine-functionalization is an effective strategy for enhancing the CO 2 affinity of MOFs crystals [49, 51-53]. As a result, UiO-66-NH 2 was reported to show stronger CO 2 adsorption capacity than that of pristine UiO-66 [46], which offers great potential for fabricating CO 2 -separation mixed matrix membranes. Su et al. [54] fabricated UiO-66-NH 2 /Polysulfone (PSF) membrane with CO 2 /N 2 ideal selectivity of 26 and dramatically increased CO 2 permeability of 46 Barrer. Moreover, phenyl acetyl functionalized UiO-66-NH 2 particles were synthesized and 5

7 incorporated into Matrimid polymer [55]. The membrane showed CO 2 permeability of about 37 Barrer (increased by 200 %) and CO 2 /N 2 ideal selectivity of about 30. In this work, we report on the design and fabrication of UiO-66 based metal-organic frameworks mixed matrix membranes for CO 2 capture application. Nano-sized UiO-66 crystals were synthesized and functionalized by amine-groups, which is aimed at maximizing the interaction between UiO-66 frameworks and polymer chains as well as improving the CO 2 affinity of membranes. We chose polyether block amide (PEBA) as the polymer material. It is composed of ethylene oxide and amide groups in their soft and hard segments, respectively, which provide high CO 2 permeability and mechanical strength at the same time [2]. Thereby, for the first time, we fabricated UiO-66 (NH 2 )-PEBA mixed matrix membranes as shown in Fig. 1. The membranes microstructures as well as MOF-polymer interaction were systematically characterized and studied. Compared with previous works of UiO-66 based mixed matrix membranes [49, 53-56], our UiO-66 (NH 2 )-PEBA membranes showed more selective transport of CO 2 molecules and simultaneous enhancement of CO 2 permeability and CO 2 /N 2 selectivity than those of pure PEBA membrane. The as-prepared membrane also showed excellent long term stability in humidified state, which shows promising potential for CO 2 capture application. 6

8 Fig. 1. Schematic structures of UiO-66-NH 2, PEBA and UiO-66-NH 2 -PEBA mixed matrix membrane. 2. Experimental 2.1 Materials For the synthesis of UiO-66 MOFs, zirconium (IV) chloride (ZrCl 4 ) powder was supplied by Alfa Aesar, Terephthalic acid and 2-Aminoterephthalic acid were provided by Sigma-Aldrich, N,N-Dimethylformamide (DMF) was purchased by Sinopharm Chemical Reagent Co., Ltd, formic acid was received from Xilong Chemical Co., Ltd., ethanol was obtained by Wuxi City Yasheng Chemical Co., Ltd., PEBAX MH 1657 was purchased from Arkema, France. N 2 and CO 2 with purity % was supplied by Nanjing Special Gases Company. Deionized water was used in all the experiments. All of the materials were used without further purification. 2.2 Preparation of UiO-66 MOFs The preparation of UiO-66 and UiO-66-NH 2 were carried out following the procedures in previous works [43, 57]. For UiO-66, ZrCl 4 (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 at 120 ºC for 24 h. The white powder was obtained by centrifugation at 8000 rpm for 10 min and were immersed in methanol for solvent exchange for three days and activated under vacuum at 100 ºC. Similar 7

9 process was carried out for the synthesis of UiO-66-NH 2 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-NH 2 ) powders were ground and dispersed in ethanol and water mixed solvent with mass ratio of 7:3, followed by sonication for 10 min. Then, PEBA were added into the dispersion containing Zr-MOFs nanoparticles and heated with stirring and refluxing under 80 ºC for 12 h. The mass ratios of PEBA to solvent is 5:95 while the contents of Zr-MOFs in polymer matrix were varied from 5 to 20 wt%. The flat membrane was prepared by casting the mixed solution on PVDF supports, which were pretreated by being immersed in water for 24h. The free-standing membranes for characterization and analysis were prepared by casting the corresponding mixed solutions on the glass substrates. The membranes were finally fabricated after evaporating the solvent at room temperature for 2 days and dried in a vacuum oven at 70 ºC for 12 h. For comparison, the neat PEBA was prepared following the similar procedure. 2.4 Characterization Morphologies of the as-prepared UiO-66 MOFs nanoparticles and membranes were characterized by scanning electron microscope (SEM, S4800, Hitachi, Japan) together with energy-dispersive X-ray spectroscopy (EDXS). The crystalline structures of UiO-66 MOFs were analyzed through X-Ray Diffraction (XRD, Bruker, D8 Advance) using Cu K radiation ( =1.54 Å) at 40 kv and 15 ma at room temperature. The functional groups of synthesized UiO-66 MOFs and hydrogen bonds between UiO-66 MOFs and PEBA polymer were characterized by Fourier transform infrared (FTIR, AVATAR-FT-IR-360, Thermo Nicolet, USA) spectra with the range of cm -1 ). The thermal properties of the synthesized UiO-66 MOFs were characterized by 8

10 thermogravimetric analysis (TGA, NETZSCH STA 449F3) in the range of room temperature to 800 ºC with the rate of 10 ºC min -1. Adsorption experiments of the UiO-66 MOFs nanocrystals were carried out by ASAP 2460 with CO 2 (298K) and N 2 (77K). BET surface areas were calculated from the N 2 isotherms at 77 K. The morphologies of UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes were obtained by atomic force microscopy (AFM, XE-100, Park SYSTEMS, South Korea) operated in the Non-contact mode. Differential scanning calorimetry (DSC, Q2000, TA Instruments, USA) measurements were performed from 80 to 40 ºC in N 2 atmosphere to study the glass transition temperatures (Tg) of the membranes. The mechanical properties of UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes were analyzed using nanoindentation technique (Nano-Test Vantage, Micro Materials, UK). The diameter of the nanoindenter is 50 nm. For each membrane, 36 testing points were applied and distributed uniformly in the testing area of 90 μm 90 μm. The testing points with large deviation were removed, and the average value was finally calculated. 2.5 Gas separation experiments Gas transport behaviors were both measured by pure gas and mixed gas permeation tests using constant pressure/variable volume technique (see the schematic in Supplementary Fig.1). For pure gas permeation tests of CO 2 and N 2, the pressure and temperature were set as 0.3 MPa and 25 C, respectively. After the system reached steady-state, all the gas permeation measurements were performed more than five times, and the gas permeability can be calculated using the following equation: P = QL PA (1) where P is the gas permeability [1 Barrer = cm 3 (STP) cm/ (cm 2 s cmhg)], Q is the volume permeate rate of gas (cm 3 /s) at standard temperature and pressure (STP), L is the membrane 9

11 thickness (cm), ΔP is the transmembrane pressure (cmhg) and A is the effective membrane area (12.56 cm 2 in this work). The ideal selectivity of CO 2 /N 2 can be calculated by the ratio of the permeability of the individual gas which can be expressed as follows: α = P A P B (2) Mixture of CO 2 and N 2 (50 vol% : 50 vol%) was used as feed gas and CH 4 was chosen as sweep gas (Supplementary Fig.1). The gas permeability was also calculated by equation (1) while selectivity can be obtained by using equation (3): α A/B = y A/y B x A /x B (3) where x and y are the volumetric fractions of the one component in the feed and permeate, respectively. When the membranes were tested in humid state, the feed and sweep gases were both humidified by water bottles. The relative humidity in dry state was about 20 %, while after humidifying, the relative humidity increased to about 85 %. 3 Results and discussion 3.1 Physicochemical properties of UiO-66 MOFs Scanning electron microscope (SEM) showed that the synthesized UiO-66 and UiO-66-NH 2 both crystallized small octahedrally cubic nanocrystals with size in the range of nm (Fig. 2). These nano-sized nanoparticles are considered to be excellent fillers to prepare thin and homogenous mixed matrix membranes [58]. EDXS was also carried out to analyze the elemental composition of synthesized UiO-66 MOFs samples. It can be seen that UiO-66 is composed of Zr, C and O elements (Fig. 2b), which is in agreement with its crystal structure, the Zr 6 -cluster coordinated with terephthalate ligands. For UiO-66-NH 2, a new peak of N element appeared, indicating the amino-functionalization of UiO-66 by substituting the linker. 10

12 Fig. 2. SEM images of synthesized (a) UiO-66 nanoparticles and (c) UiO-66-NH 2 nanoparticles. (b) and (d) are the corresponding EDXS data of UiO-66 and UiO-66-NH 2 nanoparticles, respectively. Inserted are the images of elemental maps. The powder X-ray diffraction (PXRD) patterns (Fig. 3a) showed that the synthesized UiO-66 MOFs were with highly crystalline structure, matching well with the reported literature [43, 59]. It should be noted that the introduction of amine functional groups has neglectable influence on the crystal structure of UiO-66 MOFs. The size of crystallites estimated by the Scherer equation [40] was about 66 nm, which is approximate to the size of UiO-66 MOFs nanocrystals observed in SEM (Fig. 2). Also, the stability of synthesized UiO-66 and UiO-66-NH 2 nanoparticles were evaluated by PXRD after heating in ethanol/water mixed solvents (ω ethanol : ω water =70:30), which is required by membrane fabrication process. As shown in Fig. 3, both UiO-66 and UiO-66-NH 2 maintain highly crystalline structure after solvothermal treatment, demonstrating that the 3D framework of UiO-66 MOFs is highly stable. The chemistry structure was analyzed by Fourier transform infrared (FTIR) spectra (Fig. 3b). The adsorption bands from cm -1 can be ascribed to stretching vibration 11

13 of C=O group. The peak at 1395 cm -1 is assigned to the stretching vibration of C-O group. Especially, for UiO-66-NH 2, a new peak at 1680 cm -1 appeared, which clearly indicates the presence of NH 2 group. It has been well demonstrated that amino-functionalized groups can intensify the materials CO 2 -philic property [60]. Also, UiO-66 and UiO-66-NH 2 powders being heated in ethanol/water mixed solvent were tested by FITR to further investigate the stability. All the chemical functionalized groups essentially remain unchanged, which again proves the excellent stability of the synthesized UiO-66 MOFs. Fig. 3. (a) PXRD patterns and (b) FTIR spectra of (i) UiO-66 (ii) UiO-66 after heating at 80 ºC for 10 h in ethanol/water mixed solvent (iii) UiO-66-NH 2 (iv) UiO-66-NH 2 after heating at 80 ºC for 10 h in ethanol/water mixed solvent. (c) TGA curves for UiO-66 and UiO-66-NH 2 powders. The 3D framework stability and structural integrity of UiO-66 MOFs were further studied by Thermogravimetric analysis (TGA) as shown in Fig.3c. Both UiO-66 and UiO-66-NH 2 powders underwent slight weight loss from 100 to 180 C. Then, the weight loss from 180 to 280 C can be ascribed to the dehydroxylation of Zr 6 O 4 (OH) 4 cornerstone into Zr 6 O 6. For UiO-66, the decomposition temperature is about 500 C, which is higher than the typical value of 350 C of other MOFs [43, 61]. After 500 C, the benzene fragments lost gradually and the residue materials became to ZrO 2 at last. However, the crystalline structure of UiO-66-NH 2 maintains up to 290 C, which is lower compared with UiO

14 Fig. 4. (a) and (b) are the N 2 adsorption and desorption isotherms at 77 K for UiO-66 and UiO-66-NH 2 nanoparticles, respectively. (c) CO 2 adsorption isotherms at 298 K for UiO-66 and UiO-66-NH 2 nanoparticles. It can be seen that the N 2 adsorption isotherms at 77K for UiO-66 and UiO-66-NH 2 both followed type I isotherm with no hysteresis. BET surface areas of UiO-66 and UiO-66-NH 2 were 847 and 822 m 2 /g, respectively, which are in consistence with literature [62, 63]. The micropore volumes for UiO-66 and UiO-66-NH 2 were 0.43 and 0.38 cm 3 /g, respectively. These indicate that the substitution of ligand with amine functionalized groups has no obvious influence on the pore structure. The micropore size was 0.59 nm, which is close to the theoretical value (0.6 nm) of the triangular windows in UiO-66 framework. It should be noted that even with slightly lower BET surface area and micropore volume, UiO-66-NH 2 still showed enhanced CO 2 adsorption ability (increased 20 % or so) than that of pristine UiO-66 (Fig. 4c). This further proves the CO 2 -philic property of amino groups, which is expected to be beneficial for preferential CO 2 permeation. 3.2 Membrane characterization Morphology of MMMs The morphology of as-prepared membranes was characterized by SEM. Membranes were fabricated on flat PVDF substrate using casting method. Fig. 5a and 5e show the surface and cross-section SEM images of pure PEBA membrane. It can be seen that pure PEBA membrane is 13

15 defect free and highly smooth without any structural bulges on membrane surface. UiO-66-PEBA and UiO-66-PEBA-NH 2 membranes with different Zr-MOFs loadings were demoted as UiO-66-PEBA (X) and UiO-66-NH 2 -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. 5b, UiO-66-PEBA (10) membrane showed rough surface with some large bulges, which is due to the aggregation of UiO-66 nanoparticles in the polymeric matrix. From the cross-section SEM image, we can clearly see the agglomerate of UiO-66 particles with size of several micrometers. In addition, the poor adhesion at MOF-polymer interface is observed. The poor dispersion property of UiO-66 nanoparticles is attributed to the weak interaction between UiO-66 frameworks and polymer chains. Fortunately, the incorporation of amino functionalized groups into the UiO-66 crystalline structure made the as-synthesized UiO-66-NH 2 nanocrystals be more compatible with polymeric matrix. Fig. 5c and 5g exhibit high quality membrane [UiO-66-NH 2 -PEBA (10)] with homogenous distribution of UiO-66-NH 2 nanocrystals. Large conglomerates of MOF particles as well as visible voids at MOF-polymer interface are not observable in SEM images. This further demonstrates the enhanced interface interaction between MOF and polymer, which is because that the introduced NH 2 groups can effectively improve the hydrogen bonding frameworks with PEBA. Similar phenomenon was also observed in the work of amine-functionalized MIL-53 filled polysulfone MMMs [64]. However, severe aggregation appeared when the content of UiO-66-NH 2 increased to 20 wt% (Fig. 5d and 5h). 14

16 Fig. 5. Surface SEM images of (a) pure PEBA, (b) UiO-66-PEBA (10), (c) UiO-66-NH 2 -PEBA (10) and (d) UiO-66-NH 2 -PEBA (20) membranes, respectively. Cross-section SEM images of (e) pure PEBA, (f) UiO-66-PEBA (10), (g) UiO-66-NH 2 -PEBA (10) and (h) UiO-66-NH 2 -PEBA (20) membranes, respectively. Inserted images in (b) and (c) are the enlarged surface images of UiO-66-PEBA (10) and UiO-66-NH 2 -PEBA (10) membranes, respectively. In addition, AFM characterization further compared the membranes surface roughness. Pure PEBA membrane showed smallest surface roughness parameters (Rq and Ra, as shown in Table 1). However, the roughness of UiO-66-PEBA (10) membrane increased almost two times than that of pure PEBA membrane. For UiO-66-NH 2 -PEBA (10) membrane, the surface became much smoother, showing a better compatibility. It is considered that such MOF-polymer compatibility not only influences the membrane morphology, but also rationally differs the membranes microstructures as well as gas separation performances. Table 1. Surface roughness parameters for pure PEBA, UiO-66-PEBA (10) and UiO-66-NH 2 -PEBA (10) membranes, respectively. Membranes Pure PEBA UiO-66-PEBA (10) UiO-66-NH 2 -PEBA (10) Rq (nm) Ra (nm)

17 3.2.2 Microstructure of MMMs XRD characterization was carried out to further analyze the microstructure of as-prepared UiO-66 MOFs MMMs. Pristine PEBA is a semicrystalline copolymer which consists of both crystalline and amorphous PEO and PA6 phases, which showed characteristic peak ranging from [65]. Generally, the sharp X-ray diffraction peak represents the crystalline region of the polymer while the amorphous polymer exhibits a broad peak. XRD patterns for UiO-66 MOFs MMMs with different loadings of MOFs are also presented in Fig. 6. It is clear that both the UiO-66 and UiO-66-NH 2 nanoparticles maintain their crystallinity and topology in polymer matrix. The crystal sizes calculated by Scherer equation were in the range of nm, which were slightly smaller than those estimated by XRD characterization of UiO-66 MOFs powders. It should be noted that the peak position of PA6 varied by introducing different UiO-66 MOFs nanoparticles. Pure PEBA membrane showed the peak of PA6 at about 2θ=23.8. For UiO-66-PEBA MMMs, peak position of PA6 almost remained unchanged. However, after incorporating UiO-66-NH 2 nanocrystals, this peak gradually shifted to higher degree. Thus, the calculated d-spacing was reduced, indicating the decrease of free volume cavities in polymer chains. This again suggests the enhanced hydrogen bonding frameworks between amine-functionalized UiO-66 MOFs nanoparticles with PEBA molecules. 16

18 Fig. 6. XRD patterns for pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes. In order to further characterize and analyze the hydrogen bonding frameworks between incorporated UiO-66 MOFs and polymer chains, FTIR spectra were conducted (Fig. 7). The observed peak at 1094 cm -1 is attributed to the stretching vibration of the C-O-C group of the soft segment part of PEO [65]. The peak corresponding to N H linkages is approximately at 3298 cm -1 while the characteristic peaks at 1636 and 1732 cm -1 are assigned to H N C=O group and O C=O group, respectively. The above three functional groups are indicates the presence of the hard segment of PA chains [66]. All the membranes show no new absorbance peaks, which demonstrates that there were no strong chemical interaction between Zr-MOFs nanoparticles and PEBA chains. Moreover, it should be noticed that the peaks at around 1636 and 3298 cm -1 shifted gradually to lower frequency for UiO-66-NH 2 -PEBA membranes while the peaks of UiO-66-PEBA membranes remained unchanged. The hydrogen bonding frameworks was obviously improved, which is demonstrated by the redshift of IR characteristic peaks of PEBA [67]. 17

19 Fig. 7. FTIR spectra for pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes. a) Wave number from cm -1, b) wave number from cm -1 ( N H ) and c) wave number from cm -1 (H N C=O). Differential scanning calorimetry (DSC) measurement (Fig. 8a) was carried out to characterize the polymer thermodynamic properties. Pure PEBA membrane showed a glass transition temperature (T g ) of 54.6 C that agrees well with the literature [68]. It can be noted that gradually adding UiO-66-NH 2 nanoparticles made T g in the trend of increase, which indicates that the membranes were likely undergoing glass transition. As a result, the mechanical property of PEBA membranes can be improved (Fig. 8b). For UiO-66-PEBA MMMs, the hardness and elastic modulus just increased slightly. However, for UiO-66-NH 2 -PEBA MMMs, the mechanical property was obviously improved. Compared with pure PEBA membrane, UiO-66-NH 2 -PEBA (20) membrane showed 52 % and 29.4 % enhancement in hardness and elastic modulus, respectively. This can be attributed to the significant aggregation of UiO-66 particles in membranes, resulting in less contact between UiO-66 particles and polymer matrix. Similar phenomenon can also be observed in other reports [49, 69]. It should be noted that in our experiments, when adding too many UiO-66-NH 2 nanoparticles (more than 20 wt%), the composite membranes became brittle, which further suggests the rigidification of the polymer chains. 18

20 Fig. 8. (a) Glass transition temperatures and (b) comparison of hardness and elastic modulus of pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes. 3.3 Gas separation performances Effect of UiO-66 MOFs loading Gas permeation measurements were carried out to further study the molecular separation behaviors of as-prepared UiO-66-PEBA and UiO-66-NH 2 -PEBA mixed matrix membranes. First, the effect of UiO-66 MOFs loading on the membranes CO 2 /N 2 separation performances were investigated (Fig. 9). For UiO-66-PEBA mixed matrix membranes, it can be seen that when UiO-66 loading increased to 7.5 wt%, both of the CO 2 permeability and CO 2 /N 2 selectivity increased at the same time, breaking the permeability/selectivity trade-off relationship in polymeric membranes [10, 11]. Compared to pure PEBA membrane, CO 2 permeability of UiO-66-PEBA (7.5) increased by 72.5 %, however, CO 2 /N 2 selectivity only increased by 31.1 %. This can be due to the triangular window of UiO-66 is 0.6 nm, which is larger than theoretical free volume of polymer chains [70]. However, the size of 0.6 nm is also larger than the kinetic diameters of CO 2 (0.33 nm) and N 2 (0.36 nm). The enhanced selectivity is ascribed to the CO 2 -philic characteristic of UiO-66 frameworks [45]. CO 2 permeability kept increasing while the selectivity decreased with UiO-66 loading. It can be seen that when the concentration of UiO-66 rose to 20 wt%, CO 2 /N 2 selectivity dropped to about 19

21 20, which is lower than that of pure PEBA. This is because that severe agglomeration occurred when large amount of UiO-66 particles were added into polymeric matrix. As a result, non-selective interface defects were generated. UiO-66-NH 2 -PEBA mixed matrix membranes showed similar variation tends in CO 2 permeability and CO 2 /N 2 selectivity with those of UiO-66-PEBA membranes. However, owing to amino functionalized microstructure, UiO-66-NH 2 -PEBA mixed matrix membranes showed higher CO 2 /N 2 selectivity of nearly 80 (with 10 wt% UiO-66-NH 2 ) than that of UiO-66-PEBA (7.5). It can also be attributed to the homogenous dispersion of UiO-66-NH 2 nanocrystals in PEBA membrane. Fig. 9. Effect of UiO-66 MOFs loading on the CO 2 /N 2 separation performances of UiO-66-PEBA and UiO-66-NH 2 -PEBA mixed matrix membranes. Pure gas permeation tests were measured at 0.3 MPa and 20 C, 1 Barrer=10-10 cm 3 (STP) cm/(cm 2 s cmhg). 20

22 3.3.2 Effect of operation temperature The effect of operation temperature was also studied (Fig. 10). It can be seen that gas permeability significantly increased with operation temperature while CO 2 /N 2 selectivity decreased. UiO-66-NH 2 -PEBA (10) mixed matrix membrane apparently showed higher CO 2 permeability than that of pure PEBA membrane under temperature ranging from 20 to 80 C. This can be ascribed to the 3D porous framework of UiO-66-NH 2, providing fast diffusion channels for molecules. Fig. 10b showed that UiO-66-NH 2 -PEBA (10) mixed matrix membrane also possess higher CO 2 /N 2 selectivity than that of pure PEBA membrane at high temperature. At 40 C, CO 2 /N 2 selectivity can still be as high as 68, which is almost twice as that of pure PEBA. As a result, the developed UiO-66-NH 2 -PEBA mixed matrix membrane offers potential opportunity for practical separation. 21

23 Fig. 10. Effect of operation temperature on (a) CO 2 permeability and (b) CO 2 /N 2 selectivity of pure PEBA and UiO-66-NH 2 -PEBA (10) membranes. Pure gas permeation tests were measured at 0.3 MPa Effect of humidity Nowadays, MOFs materials have been widely investigated to be utilized in adsorption/ separation, however, many of them suffered from their low structural stabilities when being treated with moisture, heat or acid-base [71-74]. This hinders their applicability in industry. Because of the exceptional chemical stability, UiO-66 materials exhibit outstanding water stability. UiO-66 frameworks can still remain good crystallinity even being immersed in water for several months [47]. In addition, UiO-66 with hydrophilic surface showed affinity for water molecules, which also widens its practical application such as being used as environmental adsorbents [57]. Generally, separating CO 2 molecules from flue gas is accompanied with water vapor [75]. Thus the effect of humidity was studied, as shown in Fig. 11. Pure PEBA membrane showed increased CO 2 permeability while almost no change to CO 2 /N 2 selectivity. This indicates that PEBA membrane can show water-facilitated CO 2 separation behaviors [2]. For UiO-66-NH 2 -PEBA mixed matrix membranes, CO 2 permeability significantly increased after humidifying. Also, when UiO-66-NH 2 loading varied from 5 10 wt%, CO 2 /N 2 selectivity slightly increased, which is owing to the hydrophilic as well as CO 2 -philic properties of UiO-66-NH 2. Water vapor facilitated CO 2 transport in membranes instead of showing hindering effect. By comparing separation performances of membranes in Table 2, we can find that UiO-66-PEBA (10) membrane showed higher CO 2 permeability while lower CO 2 /N 2 selectivity than those of UiO-66-NH 2 -PEBA (10) membrane. These are attributed to the enhanced polymer-mofs interaction and the CO 2 -philic effect of amino functionalized groups. When the concentration of UiO-66-NH 2 nanoparticles continued to increase, 22

24 CO 2 /N 2 selectivity dropped dramatically because of the particles aggregation, which is shown in Fig. 5d and 5h. It can be seen that the developed UiO-66-NH 2 -PEBA mixed matrix membrane showed enhanced CO 2 /N 2 separation performance in humid state. The membranes structure and gas permeation behaviors under more relative humidity are needed to be studied in detail, so as to further investigate the effects of water molecules. Fig. 11. Effect of humidity on pure PEBA and UiO-66-NH 2 -PEBA membranes. Mixed gas permeation tests were measured at atmosphere pressure and 20 C. Before humidifying, the relative humidity was about 20 %, while after humidifying, the relative humidity was about 85 %. Table 2. Comparison of mixed gas separation performances of membranes. Membrane Dry state CO 2 permeability Dry state CO 2 /N 2 Humid state CO 2 permeability Humid state CO 2 /N 2 23

25 (Barrer) selectivity (-) (Barrer) selectivity (-) Pure PEBA 51.5 ± ± ± ± 1.2 UiO-66-PEBA (10) 96.3 ± ± ± ± 2.0 UiO-66-PEBA-NH 2 (10) 87.0 ± ± ± ± Long term operation Long term operation test of mixed gas permeation in humid state was carried out to further investigate the membrane structural stability (Fig. 12). The data were recorded after the whole permeation system was steady. During 100 h operation test, the UiO-66-NH 2 -PEBA (10) membranes showed high and stable separation performance with average CO 2 permeability of 130 Barrer and CO 2 /N 2 selectivity of 72. It also proves that the as-prepared UiO-66-NH 2 -PEBA mixed matrix membrane showed good structural stability with the presence of water vapor, which is promising for application of CO 2 capture from flue gas. Fig. 12. Long term operation test on UiO-66-NH 2 -PEBA membrane in humid state. Mixed gas permeation tests were measured at atmosphere pressure and 20 C. The relative humidity was about 85 % Comparison with upper bound As is shown in Fig. 13, both UiO-66-PEBA and UiO-66-NH 2 -PEBA mixed matrix membranes exhibited improved CO 2 /N 2 mixed gas separation performance compared with that of pure PEBA 24

26 membrane. UiO-66-PEBA membrane showed higher CO 2 permeability while UiO-66-NH 2 -PEBA membrane possessed higher CO 2 /N 2 selectivity. In addition, it can be seen that UiO-66-NH 2 -PEBA (10) membrane possessed enhanced CO 2 /N 2 mixed gas separation performance in humid state, surpassing the upper bound for state-of-the-art membranes. This endows the developed UiO-66 MOFs mixed matrix membrane with potentiality for practical application. Fig. 13. Comparison of CO 2 /N 2 mixed gas separation performances of pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes with Robeson upper bound. The solid data points are in humid state while the hollow data points are in dry state. 4 Conclusion In summary, we report the design and fabrication of UiO-66 based metal-organic frameworks-peba mixed matrix membranes. Compared with UiO-66, UiO-66-NH 2 showed enhanced MOF-polymer interactions, which led to excellent dispersibility in polymer matrix. Moreover, stronger affinity with CO 2 molecules was realized after functionalized by amine groups. The gas permeation behaviors of the as-prepared membranes were also investigated. The results showed that both of the UiO-66-PEBA and UiO-66-NH 2 -PEBA mixed matrix membranes showed significantly improved CO 2 separation performance than that of pure polymer membrane. However, UiO-66-NH 2 -PEBA membranes possessed higher CO 2 /N 2 selectivity and slightly lower CO 2 25

27 permeability than those of UiO-66-PEBA membrane, which can be ascribed to the functionalities of amine groups. The as prepared UiO-66-NH 2 -PEBA exhibited high and stable CO 2 /N 2 separation performance (CO 2 permeability: 130 Barrer, CO 2 /N 2 selectivity: 72) in humid state, transcending the upper bound for state-of-the-art polymeric membranes. This work demonstrated that UiO-66 based metal-organic frameworks are promising materials for developing mixed matrix membranes, which offer promising potential for CO 2 capture. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos , , ), the Innovative Research Team Program by the Ministry of Education of China (Grant No. IRT13070), a Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Innovation Project of Graduate Research by Jiangsu Province (Grant No. KYLX_0764). Reference [1] K. Sumida, D.L. Rogow, J.A. Mason, T.M. McDonald, E.D. Bloch, Z.R. Herm, T.-H. Bae, J.R. Long, Carbon Dioxide Capture in Metal Organic Frameworks, Chemical Reviews, 112 (2012) [2] Y. Li, Q. Xin, H. Wu, R. Guo, Z. Tian, Y. Liu, S. Wang, G. He, F. Pan, Z. Jiang, Efficient CO2 capture by humidified polymer electrolyte membranes with tunable water state, Energy & Environmental Science, 7 (2014) [3] N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C.S. Adjiman, C.K. Williams, N. Shah, P. Fennell, An overview of CO2 capture technologies, Energy & Environmental Science, 3 (2010) [4] N. Du, H.B. Park, M.M. Dal-Cin, M.D. Guiver, Advances in high permeability polymeric membrane materials for CO2 separations, Energy & Environmental Science, 5 (2012) [5] R.A. Hayes, Aromatic polyamides; selective, in, Google Patents,

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34 Table 2. Comparison of mixed gas separation performances of membranes. Figure captions Fig. 1. Schematic structures of UiO-66-NH 2, PEBA and UiO-66-NH 2 -PEBA mixed matrix membrane. Fig. 2. SEM images of synthesized (a) UiO-66 nanoparticles and (c) UiO-66-NH 2 nanoparticles. (b) and (d) are the corresponding EDXS data of UiO-66 and UiO-66-NH 2 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 ºC for 10 h in ethanol/water mixed solvent (iii) UiO-66-NH 2 (iv) UiO-66-NH 2 after heating at 80 ºC for 10 h in ethanol/water mixed solvent. (c) TGA curves for UiO-66 and UiO-66-NH 2 powders. Fig. 4. (a) and (b) are the N 2 adsorption and desorption isotherms at 77 K for UiO-66 and UiO-66-NH 2 nanoparticles, respectively. (c) CO 2 adsorption isotherms at 298 K for UiO-66 and UiO-66-NH 2 nanoparticles. Fig. 5. Surface SEM images of (a) pure PEBA, (b) UiO-66-PEBA (10), (c) UiO-66-NH 2 -PEBA (10) and (d) UiO-66-NH 2 -PEBA (20) membranes, respectively. Cross-section SEM images of (e) pure PEBA, (f) UiO-66-PEBA (10), (g) UiO-66-NH 2 -PEBA (10) and (h) UiO-66-NH 2 -PEBA (20) membranes, respectively. Inserted images in (b) and (c) are the enlarged surface images of UiO-66-PEBA (10) and UiO-66-NH 2 -PEBA (10) membranes, respectively. Fig. 6. XRD patterns for pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes. 33

35 Fig. 7. FTIR spectra for pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes. a) Wave number from cm -1, b) wave number from cm -1 ( N H ) and c) wave number from cm -1 (H N C=O). Fig. 8. (a) Glass transition temperatures and (b) comparison of hardness and elastic modulus of pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes. Fig. 9. Effect of UiO-66 MOFs loading on the CO 2 /N 2 separation performances of UiO-66-PEBA and UiO-66-NH 2 -PEBA mixed matrix membranes. Pure gas permeation tests were measured at 0.3 MPa and 20 C, 1 Barrer=10-10 cm 3 (STP) cm/(cm 2 s cmhg). Fig. 10. Effect of operation temperature on (a) CO 2 permeability and (b) CO 2 /N 2 selectivity of pure PEBA and UiO-66-NH 2 -PEBA (10) membranes. Pure gas permeation tests were measured at 0.3 MPa. Fig. 11. Effect of humidity on pure PEBA and UiO-66-NH 2 -PEBA membranes. Mixed gas permeation tests were measured at atmosphere pressure and 20 C. Before humidifying, the relative humidity was about 20 %, while after humidifying, the relative humidity was about 85 %. Fig. 12. Long term operation test on UiO-66-NH 2 -PEBA membrane in humid state. Mixed gas permeation tests were measured at atmosphere pressure and 20 C. The relative humidity was about 85 %. Fig. 13. Comparison of CO 2 /N 2 mixed gas separation performances of pure PEBA, UiO-66-PEBA and UiO-66-NH 2 -PEBA membranes with Robeson upper bound. The solid data points are in humid state while the hollow data points are in dry state. 34

36 Highlights UiO-66 based MOFs-PEBA MMMs were designed and fabricated. Amine functionalization improved the crystals dispersibility and CO 2 affinity. UiO-66-NH 2 -PEBA membrane showed significantly enhanced CO 2 separation performance. UiO-66-NH 2 -PEBA membrane exhibited good stability under humid state. Table 1. Surface roughness parameters for pure PEBA, UiO-66-PEBA (10) and UiO-66-NH 2 -PEBA (10) membranes, respectively. Membranes Pure PEBA UiO-66-PEBA (10) UiO-66-NH 2 -PEBA (10) Rq (nm) Ra (nm) Membrane Table 2. Comparison of mixed gas separation performances of membranes. Dry state CO 2 permeability (Barrer) Dry state CO 2 /N 2 selectivity (-) Humid state CO 2 permeability (Barrer) Humid state CO 2 /N 2 selectivity (-) Pure PEBA 51.5 ± ± ± ± 1.2 UiO-66-PEBA (10) 96.3 ± ± ± ± 2.0 UiO-66-PEBA-NH 2 (10) 87.0 ± ± ± ±