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Aug 13, 2018 - polymer membranes for the gas separation have a near-universal trade-off ... decreases the overall gas separation efficiency of the membrane.
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High Performance Gas Separation Mixed Matrix Membrane Fabricated by Incorporation of Functionalized Submicrometer-Sized Metal-Organic Framework Baosheng Ge 1 ID , Yanyan Xu 2,3 , Haoru Zhao 2 , Haixiang Sun 1,2, * Wenguang Wang 2 1 2 3

*

ID

, Yaoli Guo 1 and

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, China; [email protected] (B.G.); [email protected] (Y.G.) College of Science, China University of Petroleum (East China), Qingdao 266580, China; [email protected] (Y.X.); [email protected] (H.Z.); [email protected] (W.W.) Guangzhou Special Pressure Equipment Inspection and Research Institute, Guangzhou 510663, China Correspondence: [email protected]

Received: 29 June 2018; Accepted: 8 August 2018; Published: 13 August 2018

 

Abstract: Mixed matrix membranes (MMMs) attract great attention due to their outstanding gas separation performance. The compatibility between the fillers and the polymer matrix is one of the key points for the preparation of high-performance MMMs. In this work, MMMs consisting of metal-organic frameworks (MOFs) of amine-modified Cu-BTC (NH2 -Cu-BTC; BTC = 1,3,5benzenetricarboxylic acid) and submicrometer-sized amine-modified Cu-BTC (sub-NH2 -Cu-BTC) incorporated into a Pebax-1657 polymer were fabricated for the gas separation. The SEM image and Fourier transform infrared spectroscopy (FTIR) spectra showed an increase in the surface roughness of MOFs and the presence of amino groups on the surface of Cu-BTC after the amination modification, and a decrease in the size of MOFs crystals after the submicrometer-sized aminated modification. Gas adsorption analysis indicated that NH2 -Cu-BTC and sub-NH2 -Cu-BTC had a higher gas adsorption capacity for CO2 compared to the unmodified Cu-BTC. The scanning electron microscopy (SEM) image showed that NH2 -Cu-BTC and sub-NH2 -Cu-BTC, especially sub-NH2 -Cu-BTC, had a better compatibility with a polyether-block-amide (Pebax) matrix in the MMMs. The gas separation performance indicated that the Pebax/sub-NH2 -Cu-BTC MMMs evidently improved the CO2 /N2 and CO2 /CH4 selectivity at the expense of a slight CO2 permeability. The results reveal that modified MOF-filled MMMs possess great potential for applications in the CO2 separation field. Keywords: mixed matrix membranes; metal–organic framework; gas separation; amination; submicron

1. Introduction Carbon dioxide (CO2 ) is one of the main gases which cause global warming, corrode the natural gas pipelines, and reduce the combustion efficiency of natural gas. To overcome these problems, the effective separation of CO2 /N2 and CO2 /CH4 has become a hot research topic in recent years. Compared to other separation technologies such as cryogenic techniques, chemical adsorption [1], physical adsorption [2], and adsorption–separation [3], membrane separation has evolved as an important CO2 separation method owing to its low energy consumption, simple operation, low cost, and the absence of a phase transition during the separation process [4–6]. Polymer membranes are regarded as an effective media for the separation of gaseous mixtures accounting for a high separation efficiency, low running costs, and simple operating procedures. Nevertheless, the traditional Materials 2018, 11, 1421; doi:10.3390/ma11081421

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polymer membranes for the gas separation have a near-universal trade-off phenomenon between the permeability and selectivity, as demonstrated by Robeson [7]. Mixed matrix membranes (MMMs) consisting of a polymer as the continuous phase and an inorganic filler as the dispersed phase can overcome Robeson’s upper-bound trade-off limit. The fillers embedded in the polymer matrix are classified into conventional fillers (zeolites [8], silicas [9], and metal oxides) and alternative fillers (carbon nanotubes (CNTs), metal–organic frameworks (MOFs), and graphene [8,10–18]). These fillers may lead to the development of high-performance membranes based on the properties and functionalization of fillers. However, still many problems should be solved for large-scale industrial production of MMMs. One of the most important problems is the generation of non-selective voids because of the incompatibility between the fillers and polymer matrix at the interface, which decreases the overall gas separation efficiency of the membrane. MOFs are highly attractive for applications in the gas separation membranes because of high surface areas, large porosity, and high functionality compared to other fillers [19–21]. Moreover, the organic linkers in MOFs have a better affinity for the polymer chains compared to other fillers, and the surface properties of MOFs can be easily regulated by functionalization with various organic molecules [22]. The addition of MOFs in mixed matrix membranes can effectively control the interface morphology between the fillers and polymer matrix; this decreases the occurrence of defective morphologies [23]. However, there were still some nonselective voids present at the interface of MOFs and polymers attributed to the artifacts created during the sample preparation [24], which were unfavorable for the gas separation. To improve the MOF–polymer interfacial interactions, effective MOF modification techniques have been developed in the past few years, including the use of silane coupling agents [25] and particle fusion [26]. However, it is still challenging to achieve the desired interfacial morphology and improve the gas separation properties of MMMs. Synthesis of amine-modified and submicrometer-sized MOF crystals provides effective approaches to solve these problems [27–29]. Besides the fillers, it is equally important to select a suitable polymer for enhancing the separation properties of prepared membranes. Polyether-block-amide (Pebax) as a commercial thermoplastic elastomer is known for its selective penetration of CO2 over other light gases such as H2 , N2 , and CH4 based on the interactions between ethylene oxide (EO) units and CO2 [30]. Furthermore, Pebax is composed of polyoxyalkylene glycols (such as poly(ethylene oxide) (PEO) and poly(tetramethylene oxide) (PTMO)) and dicarboxylic acid terminated aliphatic polyamides (such as nylon-6 (PA6) and nylon-12 (PA12)). It has been known that the hard polyamide segment is favorable to improve the mechanical properties of the membranes, and the flexible polyether segment provides the passageway for the gas to permeate [31,32], therefore, different grades of Pebax can be obtained by varying the type and content of the segment. In this study, Cu-BTC crystals were fabricated using the hydrothermal method. Then aminemodified Cu-BTC (NH2 -Cu-BTC) was prepared using the replacing ligands, and submicrometer-sized amine-modified Cu-BTC (sub-NH2 -Cu-BTC) was prepared using the “coordination modulation method” (with sodium formate as the modulator). Three kinds of MOFs and Pebax-1657 were employed as the dispersed phase and the continuous phase, respectively, to prepare Pebax/Cu-BTC, Pebax/NH2 -Cu-BTC, and Pebax/sub-NH2 -Cu-BTC MMMs using the solution-casting method. It was expected that the dispersion of NH2 -Cu-BTC and sub-NH2 -Cu-BTC would be more homogeneous by amino functionalization, and the size of the crystals would be reduced. The chemical structure and morphology of the MOFs were confirmed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analysis. The aggregate morphology of MOFs in a Pebax matrix was studied by SEM analysis. The permeability of pure gases in a Pebax membrane and Pebax/MOFs MMMs was determined by the constant volume–variable pressure approach. The corresponding separation mechanism of MOFs in MMMs for the gas separation was also discussed. Moreover, the effects of functionalized Cu-BTC on the solubility and diffusivity selectivity were systematically investigated.

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2. Materials and Methods 2.1. Materials Pebax-1657 was purchased from Arkema Inc. (Paris, France). The main ligands 1,3,5benzenetricarboxylic acid (BTC) and 2-aminoterephthalic acid (NH2 -BDC) with 98% purity were supplied by Aladdin (Shanghai, China) and Macklin (Shanghai, China), respectively. Copper (II) nitrate trihydrate (Cu(NO3 )2 ·3H2 O) and sodium formate (HCOONa) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ethanol and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as solvents without any purification. 2.2. Synthesis of MOFs Cu-BTC was synthesized by adding 2.4 g Cu(NO3 )2 ·3H2 O, 1.2 g BTC, 20 mL bi-distilled water, 20 mL ethanol, and 20 mL DMF in a beaker, followed by stirring for 30 min and heating in a Teflon-lined steel autoclave at 85 ◦ C for 20 h. Then, the obtained crystals were washed with DMF and first dried at 60 ◦ C overnight and then dried at 120 ◦ C for a day. NH2 -Cu-BTC was synthesized in a similar manner as Cu-BTC except by replacing 25 wt.% BTC ligands with NH2 -BDC in the initial synthesis compound using a mixture of 0.3 g NH2 -BDC and 0.9 g BTC as the ligands. Sub-NH2 -Cu-BTC was synthesized in a similar manner as NH2 -Cu-BTC except by adding 0.1 g sodium formate to the reaction mixture. 2.3. Fabrication of MMMs Pristine Pebax membrane and Pebax/MOFs MMMs were prepared using the conventional solution-casting method. To prepare a 4 wt.% solution of Pebax, 1.61 g Pebax pellets were dissolved in a mixture of 70 wt.% ethanol and 30 wt.% water under reflux at 80 ◦ C for 2 h. Then, various quantities of MOFs (0, 1, 2, 3 and 4 wt.%) treated with different processes were sonicated in the Pebax solution described above for 30 min. The mixture was stirred for an additional 2 h at room temperature to obtain homogeneous dispersions containing different amounts of Cu-BTC, NH2 -Cu-BTC, and sub-NH2 -Cu-BTC. Then, the solutions were degassed by keeping them at room temperature for 10 h and cast on a flat glass plate. The obtained membranes were air-dried at ambient temperature for 24 h to complete the solvent evaporation. Finally, the membranes were placed in a vacuum oven for an additional 24 h to further ensure the complete removal of the residual solvent. The thickness of all MMMs was approximately 15–20 µm. 2.4. Characterization The morphology of both MOF particles and MMMs was observed using a scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan). The samples were each coated with gold before the SEM measurements. To observe the cross-sections, the MMMs were cryogenically fractured in liquid nitrogen prior to coating with gold. The FTIR spectra of pristine and modified Cu-BTC were obtained using a BRUKER TENSORII FT-IR spectrometer (Billerica, MA, USA) in the wavenumber range 4000−600 cm−1 . The XRD spectra of pristine and functionalized Cu-BTC, pristine Pebax membrane, and the MMMs incorporated with MOFs were obtained using a PANalytical X’Pert PRO Materials Research diffractometer (Almelo, The Netherlands) using a voltage of 40 kV and a current of 40 mA. The X-rays (λ = 1.5418 Å) were generated from a Cu Kα source. Each pattern was collected in the 2θ range 5–40◦ in the repetition mode (three times) with a total duration of approximately 0.4 h at selected times of hydration. To investigate the thermal stability of MOFs, thermogravimetric analysis (TGA) was carried out under air flow using a TA instrument (NETZSCH STA 449F5, Selb, Germany). All MOFs were heated from 25 ◦ C to 1000 ◦ C at a heating rate of 10 ◦ C/min. The adsorption capacity of CO2 , N2 , and CH4 for the MOFs were measured by ASAP-2020 gas adsorption tester produced by Ningbo Oppe Instruments Co., Ltd. (Ningbo, China). The crystals were immersed in dichloromethane and methanol for three days (the solvents were changed once a day), then filtered for 5 h at 60 ◦ C. Then the adsorption

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amount of CO2 , N2 , and CH4 of the MOFs was measured at 308 K. The nitrogen adsorption of the MOFs was characterized by ASAP2020M specific surface area and a micropore physical adsorption analyzer produced by Micromeritics Company (Atlanta, GA, USA) at 77 K, with a sample quantity of 0.617 g. Considering the influence of MOFs’ gas adsorption capacity on the selective CO2 permeability performance of MMMs, the gas uptake of pristine and functionalized Cu-BTC was also measured. Before the measurement, the as-synthesized crystal samples were immersed in methanol to exchange the uncoordinated DMF, ethanol, and water molecules. Then, the samples were dried at 60 ◦ C under high vacuum for 3 h to obtain the activated samples. 2.5. Procedure for Gas Permeability Measurements Pure gas (CO2 , N2 , and CH4 ) permeability measurements were carried out using a pressure permeability tester (Labthink Instruments, Jinan, China). The experiments were performed at 25 ◦ C. A circular membrane with an effective area of 4.95 cm2 was mounted in a permeation cell prior to degassing the entire apparatus. Gas permeability coefficients (P) can be calculated as follows [33]: P=

273 VL dp × × 76 AT p0 dt

(1)

where P is the permeability of a membrane to a gas, and its unit is Barrer (1 Barrer = 1 × 10−10 cm3 cm cm−2 s−1 cm Hg−1 ); V is the volume of the down-stream chamber (cm3 ); L and A are the thickness (cm) and effective area of the membrane (cm2 ), respectively; T is the experimental temperature (K); p0 is the pressure in cmHg of the gas in the upstream chamber, and dp/dt is the rate of pressure increase at the low-pressure chamber (the permeate side). The absolute temperature of 0 ◦ C and standard atmospheric pressure are 273 K and 76 cm Hg, respectively. For the permeation measurement, the values were repeated at least three times to verify the reproducibility, and the relative standard deviation was within 5%. The apparent diffusion coefficient (D) was obtained from the Equation (2): D=

L2 6θ

(2)

where θ is the lag time when a steady dp/dt rate is obtained on the downstream side. The ideal selectivity αA/B can be defined as follows: α A/B =

PA PB

(3)

where PA and PB represent the permeability of gases A and B, respectively. Here, PA and PB are the permeability of pure gas CO2 and N2 , or CO2 and CH4 (Barrer), respectively. 3. Results and Discussion 3.1. Characterization of MOFs 3.1.1. SEM The morphological features of the pristine and functionalized MOFs were investigated by SEM, shown in Figure 1. The pristine Cu-BTC is in the form of cube-shaped crystals [23] with a particle size of ~18 µm. After the amino functionalization, the crystal size and shape of NH2 -Cu-BTC displays no substantial change compared to the Cu-BTC crystal, whereas some whisker-like roughness is observed on the crystal surface [27]. The roughness of the crystal surface is mainly due to the replacement of a part of the BTC ligand with NH2 -BDC, and the addition of NH2 -BDC leads to the secondary nucleation on the crystal surface, which causes the crystal surface become rougher. With the addition of a certain

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amount of from sodium the size sub-NH crystals leads totexture a decrease from the 2 -Cu-BTC a decrease theformate, original size of 18oftothe 2 μm, whereas the shape and surface of crystals has original size of 18 to 2 µm, whereas the shape and surface texture of crystals has no apparent variation. no apparent variation.

Figure 1. Scanning (SEM) images of the surface of (a,b) pristine Cu-BTC, (c,d) Figure Scanningelectron electronmicroscope microscope (SEM) images of the surface of (a,b) pristine Cu-BTC, NH2-Cu-BTC and (e,f) 2-Cu-BTC. Note BTC = 1,3,5-benzenetricarboxylic acid (c,d) NH2 -Cu-BTC and sub-NH (e,f) sub-NH -Cu-BTC. Note BTC = 1,3,5-benzenetricarboxylic acid. 2

3.1.2. FTIR 3.1.2. The FTIR FTIR experiment experimentwas wasconducted conductedtotodetermine determinethe thefunctional functional groups present surface The groups present onon thethe surface of of the MOFs. Figure 2 shows the FTIR spectra of pristine Cu-BTC, NH2-Cu-BTC, and sub-NH 2-Cuthe MOFs. Figure 2 shows the FTIR spectra of pristine Cu-BTC, NH2 -Cu-BTC, and sub-NH 2 -Cu-BTC 1 . −1For BTC powders the wavenumber range of 3500–700 . Forthe thepristine pristineCu-BTC, Cu-BTC,the thepeaks peaks at at the the powders in theinwavenumber range of 3500–700 cm−cm −11and 1448 cm−1−correspond − 1 wavenumber 1645 cm to the C-C skeletal vibration of benzene groups in wavenumber and 1448 cm correspond to the C-C skeletal vibration of benzene groups − thethe BTC at 1110 1110cm cm−11 in BTClinker linkerand andthe theasymmetric asymmetric COO COO stretching, stretching, respectively [34]. The The peak at corresponds to to C-O-Cu C-O-Cu stretching stretching[35], [35],and andthe theband bandaround around764 764cm cm−−11 represents Cu substitution on on corresponds −1 − 1 benzene groups. groups. The The new newpeak peakappearance appearanceatat1594 1594cm cm in the NH22-Cu-BTC attributed benzene -Cu-BTC crystal can be attributed to the theasymmetric asymmetric COO band of carboxylate the carboxylate in the of presence of confirming DMF [36], to COO band of the groupgroup of BDCofin BDC the presence DMF [36], 1 is representative confirming of BDC linker in NH 2-MOF. Thepeak middle peakcm at −1545 cm−1 is representative the presencethe ofpresence BDC linker in NH The middle at 1545 of the 2 -MOF. of the C-N and bond, theintensity peak intensity indicates the existence of primary amine no secondary C-N bond, theand peak indicates the existence of primary amine and no and secondary amine. −1 in correspondence to the stretching of C-N bond, and −1 in amine. In addition, peak appears 1153 cmcorrespondence In addition, a peak a appears at 1153atcm to the stretching of C-N bond, and the −1 and 3231 cm −1 are attributed to N-H, further proving the existence of amino − 1 − 1 the two peaks at 3148 cm two peaks at 3148 cm and 3231 cm are attributed to N-H, further proving the existence of amino groupsininNH NH 2-Cu-BTC. sub-NH 2-Cu-BTC powder shown in Figure (c) of 2,Figure 2, spectrum the FTIR groups ForFor thethe sub-NH powder shown in (c) of the FTIR 2 -Cu-BTC. 2 -Cu-BTC 2-Cu-BTC powder, which indicates thatsubmicrometer the modified spectrum is same almostasthe as2 -Cu-BTC that of NH is almost the thatsame of NH powder, which indicates that the modified submicrometer method reduces the crystal size without changing the functional groups on the method only reduces theonly crystal size without changing the functional groups on the crystal surface. crystal surface.

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(a) 1110 764

(a) 1645

1448

(b)

Transmittance

Transmittance

1645

1110 764

1448

(b)

1153 1153

3148

1545

3231

3148

1545 1594 1594

3231

(c) (c)

764764

3200 2800 2800 3200

2400 2400

2000 2000

1600 1600

1200 1200

800800

-1

Wavenumber(cm -1)) Wavenumber(cm Figure 2. Fourier transform infrared spectroscopy (FTIR) spectra of (a) Cu-BTC, (b) NH2-Cu-BTC and

Figure 2. Fourier transform infrared infrared spectroscopy spectroscopy (FTIR) (FTIR) spectra spectra of of (a) (a) Cu-BTC, Cu-BTC, (b) (b) NH NH22-Cu-BTC and (c) sub-NH2-Cu-BTC. (c) (c) sub-NH sub-NH22-Cu-BTC.

3.1.3. XRD

3.1.3. XRD

To investigate the effects of different modification methods on the crystal structure of MOFs, the

To investigate the different methods onon the crystal structure the as-synthesized MOFs wereof characterized by XRD. Figure 3 indicates that the three types of MOFs, MOFs To the effects effects of differentmodification modification methods the crystal structure of MOFs, have highly crystalline structures. The sharp peaks at 2θ 5.8, 6.7, 9.4, 11.6, 13.4, 17.4, and 19.0° confirm as-synthesized MOFs were characterized byby XRD. Figure the as-synthesized MOFs were characterized XRD. Figure3 3indicates indicatesthat thatthe thethree three types types of MOFs the ordered crystalline structure of sharp Cu-BTC, which with the13.4, reported of have highly highly crystalline structures. The sharp peaks peaks at is 2θconsistent 5.8, 6.7, 6.7, 9.4, 9.4, 11.6, 13.4, 17.4,XRD and spectra 19.0°◦ confirm have crystalline structures. The at 2θ 5.8, 11.6, 17.4, and 19.0 MOFs in the literature [27]. The addition of a new ligand containing an –NH 2 functional group in the the ordered crystalline crystalline structure of Cu-BTC, Cu-BTC, which is consistent consistent with the reported reported XRD spectra spectra of spectra of the NH2-Cu-BTC and sub-NH2-Cu-BTC has no obvious effect on the XRD spectra, which ligand containing anan –NH 2 functional group in the MOFs in the the literature literature[27]. [27].The Theaddition additionofofa anew new ligand containing –NH functional group in indicates that the modification does not affect the crystal structure of the 2 MOFs. After the spectra of the NH 2NH -Cu-BTC and sub-NH 2-Cu-BTC has no obvious effect on the XRD spectra, which the spectra of the -Cu-BTC and sub-NH -Cu-BTC has no obvious effect on the XRD spectra, 2 functionalization modification, the intensity 2of Cu-BTC significantly increased, indicating that the indicates that the modification does not not affect thethe crystal which indicates that the modification does affect crystal structure ofthe the MOFs. MOFs. After crystallinity of the MOFs has been improved. The reason for the structure increase of of the peak intensityAfter may the functionalization modification, the intensity of Cu-BTC significantly increased, indicating that be due to the increase of crystallinity caused by the two nucleation process after the addition of NH 2- the crystallinity of the MOFs has been improved. The reason for the increase of the peak intensity may crystallinity the MOFs has been improved. The reason for the increase of the peak intensity may be BDC as of ligands. be due to the increase of crystallinity caused by two the two nucleation process the addition of2 -BDC NH2due to the increase of crystallinity caused by the nucleation process afterafter the addition of NH BDC as ligands. as ligands. 11.6

Intensity

Intensity

11.6

6.7

9.4

17.4 19

13.4 20.2

5.8

6.7 5 5.8

17.4 19

9.4

13.4 10

15

20.2 20

(c) (b) (a) 25

(c) (b) Figure 3. The X-ray diffraction (XRD) patterns of (a) Cu-BTC, (b) NH2-Cu-BTC and(a) (c) sub-NH2-CuBTC.

5

2Theta/degree

10

15

20

25

2Theta/degree Figure 3. 3. The TheX-ray X-raydiffraction diffraction(XRD) (XRD) patterns of (a) Cu-BTC, (b) 2NH 2-Cu-BTC andsub-NH (c) sub-NH 2-CuFigure patterns of (a) Cu-BTC, (b) NH -Cu-BTC and (c) 2 -Cu-BTC. BTC.

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3.1.4. Thermal Properties TGA is an important method to investigate the effect of different modification methods on the thermal stability of MOFs. The The weight weight loss loss curves curves of of the the as-prepared as-prepared MOFs MOFs are are shown shown in in Figure Figure 4. 4. The weight weightloss losscurve curve of pristine Cu-BTC and NH 2 -Cu-BTC showed that about 10 wt.% of the of pristine Cu-BTC and NH2 -Cu-BTC showed that about 10 wt.% of the material ◦ C.atThis material was 100 can °C. This can be attributed to the solvent trappedinsolvent in the pores and the presence was lost at 100lost be attributed to the trapped the pores and the presence of water ◦ C, of on the of surface of the crystals. The crystals up to 300 as reported earlier onwater the surface the crystals. The crystals remainremain stable stable up to 300 as°C, reported earlier [37], [37], and and above this temperature, the crystals start to decompose. Submicrometer-sized NH 2 -Cu-BTC above this temperature, the crystals start to decompose. Submicrometer-sized NH2 -Cu-BTC crystals crystals similar trendthat except the loss weight loss◦at 100 °C is slightly The reason follow afollow similara trend except thethat weight at 100 C is slightly smaller.smaller. The reason for thisfor is this the amount of residual in the sub-NH 2-Cu-BTC crystals is lower than that of the that is thethat amount of residual solvent solvent in the sub-NH -Cu-BTC crystals is lower than that of the Cu-BTC 2 Cu-BTC NH2-Cu-BTC due to thepore smaller pore of volume of crystals. and NH2and -Cu-BTC due to the smaller volume crystals.

100 (c)

(a)

Weight(%)

80

(b)

60

40

20 100

200

300

400

500

600

o

Temperature ( C) Figure 4. Thermogravimetric Thermogravimetric analysis (TGA) curves of (a) Cu-BTC, (b) NH22-Cu-BTC and (c) sub-NH22-Cu-BTC.

3.1.5. 3.1.5. Gas Gas Adsorption Adsorption Measurements Measurements Figure 5 shows shows the the CO CO22,, N Figure 5 N22,, and and CH CH44 adsorption adsorptionisotherms isothermsof ofthe theMOFs MOFsat at 308 308 K, K, and and Table Table 11 shows the physical and textural properties of pristine and functionalized Cu-BTC including the shows the physical and textural properties of pristine and functionalized Cu-BTC including the Brunauer-Emmett-Teller (BET) surface surface area, area, Langmuir Langmuir surface surface area, area, pore pore volume, volume, and and the the maximum Brunauer-Emmett-Teller (BET) maximum adsorption amounts of CO 2 , CH 4 , and N 2 . According to the results of the N 2 adsorption isotherms at adsorption amounts of CO2 , CH4 , and N2 . According to the results of the N2 adsorption isotherms at 77 77 K, the amount of N 2 adsorbed by Cu-BTC is significantly higher than that absorbed by NH 2 -CuK, the amount of N2 adsorbed by Cu-BTC is significantly higher than that absorbed by NH2 -Cu-BTC BTC and sub-NH 2-Cu-BTC. This indicates that Cu-BTC has a higher BET surface area and micropore and sub-NH 2 -Cu-BTC. This indicates that Cu-BTC has a higher BET surface area and micropore volume, as shown in Table Table 1. 1. The The CO CO22 uptake volume, as shown in uptake of of three three types types of of MOFs MOFs significantly significantly increases increases with with the the increase in adsorption owing to to the the strong strong interactions interactions between between the the positive positive charges on the the increase in adsorption pressure pressure owing charges on unsaturated openCu Cumetal metaland andthe the quadrupolar 2 molecules. The amount of CO2 adsorbed per unsaturated open quadrupolar COCO 2 molecules. The amount of CO2 adsorbed per unit unit mass is higher than that of N 2 and CH 4 , and the amounts of2 CO 2 adsorbed by sub-NH 2-Cu-BTC mass is higher than that of N2 and CH4 , and the amounts of CO adsorbed by sub-NH 2 -Cu-BTC and and 2-Cu-BTC are 23.99 21.01 cc/g, respectively, higher than amount of CO 2 absorbed NH2NH -Cu-BTC are 23.99 cc/gcc/g andand 21.01 cc/g, respectively, higher than the the amount of CO 2 absorbed by by Cu-BTC (11.62 cc/g). This behavior is inconsistent with the BET results shown in Table 1, Cu-BTC (11.62 cc/g). This behavior is inconsistent with the BET results shown in Table 1, suggesting suggesting that the micropore volume of NH 2 -Cu-BTC and sub-NH 2 -Cu-BTC reduced compared to that the micropore volume of NH2 -Cu-BTC and sub-NH2 -Cu-BTC reduced compared to Cu-BTC Cu-BTC due to thesubstitution partial substitution of BTC linker NH 2-BDC. This can be attributed to the due to the partial of BTC linker with NH2with -BDC. This can be attributed to the presence presence of primary amino groups in the porous structure, creating an electric field inside the pores of primary amino groups in the porous structure, creating an electric field inside the pores against against more polarizable adsorbates. In addition, the free amine primary amineinpresent in NH2formed -Cu-BTC more polarizable adsorbates. In addition, the free primary present NH2 -Cu-BTC a formed a carbamate with the CO 2 , providing a high energy efficiency [38], which is favorable to carbamate with the CO2 , providing a high energy efficiency [38], which is favorable to improve the improve the CO2 separation. The increase of N2 and CH4 adsorption capacity is due to the increase of the surface roughness after the amine functionalization.

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Table 1. Physical and textural properties of Cu-BTC. CO2 Pore CH4 Volume Adsorption Adsorption (m3/g) Amount (cc/g) Amount (cc/g) Cu-BTC 1018 1191 0.48 11.62 6.05 CO2 separation. of847 N2 and CH capacity is due to the NH2-Cu-BTC The increase 797 0.394 adsorption 21.01 8.61 718 functionalization. 724 0.35 23.99 8.94 sub-NH2-Cu-BTC roughness after the amine SLangmuir (m2/g)

SBET (m2/g)

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N2 Adsorption 8 of 16 Amount (cc/g) 0.67 increase of1.11 the surface 1.87

25 sub-NH2-Cu-BTC NH2-Cu-BTC

Adsorbed amount(cc/g)

20

CO2 15 Cu-BTC

10

sub-NH2-Cu-BTC NH2-Cu-BTC

CH4

Cu-BTC

5

sub-NH2-Cu-BTC

NH2-Cu-BTC Cu-BTC

0 0

20

40

60

80

100

120

N2 140

P(kPa)

Figure5.5.CO CO22,,N N22,, and and CH CH44 adsorption isotherms of Cu-BTC at 308 K. Figure

3.2. Characterization of MMMs Table 1. Physical and textural properties of Cu-BTC. 3.2.1.Sample SEM

SBET (m2 /g)

SLangmuir (m2 /g)

Pore Volume (m3 /g)

CO2 Adsorption Amount (cc/g)

CH4 Adsorption Amount (cc/g)

N2 Adsorption Amount (cc/g)

The dispersion of MOF particles within the polymer matrix and the 6.05 internal microstructure of 0.48 11.62 0.67 0.39 21.01 8.61 1.11 the cross-section SEM images shown in Figure 6. It can clearly be 0.35 23.99 8.94 1.87 seen that the Pebax membrane is smooth and defect-free. The introduction of Cu-BTC has a significant effect on the membrane morphology. The void spaces emerge between the Cu-BTC 3.2. Characterization of MMMs particles and polymeric matrix based on the poor distribution of the unmodified Cu-BTC particles in the polymer matrix and have poor compatibility with the polymer matrix. The cross-section SEM 3.2.1. SEM images of the MMMs incorporated with NH2-Cu-BTC and sub-NH2-Cu-BTC show that the MOFs The of MOF within polymer matrix and the internal microstructure inside thedispersion polymer matrix areparticles defect-free and the have good compatibility. A better adhesion of the NHof2MMMs characterized thematrix cross-section imagesto shown in Figure 6.ofIthydrogen can clearlybonding be seen Cu-BTCwere surface with the from Pebax can beSEM attributed the formation that the Pebax membrane is smooth and defect-free. introduction has aindicates significant effect between the amine groups of NH2-Cu-BTC and theThe hydroxyl groupsofofCu-BTC Pebax. This that the on the membrane morphology. The void emerge between the Cu-BTC and polymeric two modification methods improved thespaces interface compatibility between theparticles Pebax matrix and filler matrix based on the the poorMOFs distribution of the unmodified Cu-BTC particles in theinpolymer matrix6 particles. However, agglomerations and clusters are clearly observed (f) of Figure and have poor compatibility with the polymer matrix. The cross-section SEM images of the MMMs when 4 wt.% sub-NH2-Cu-BTC is incorporated into the Pebax matrix, indicating that MOF is incorporated with NH and showaggregation that the MOFs inside the polymer matrix poorly dispersed in the polymer at sub-NH higher loadings. of MOFs would increase the 2 -Cu-BTC 2 -Cu-BTC This are defect-free and havethe good compatibility. better adhesion of which the NHimproves -Cu-BTC surface with the void spaces between polymer matrix A and nanoparticles, the CO 2 and N2 2 Pebax matrix at canthe besame attributed to the formation of hydrogen between the amine groups of permeability time, leading to the deterioration of bonding the gas separation selectivity. NH2 -Cu-BTC and the hydroxyl groups of Pebax. This indicates that the two modification methods improved the interface compatibility between the Pebax matrix and filler particles. However, the MOFs agglomerations and clusters are clearly observed in (f) of Figure 6 when 4 wt.% sub-NH2 -Cu-BTC is incorporated into the Pebax matrix, indicating that the MOF is poorly dispersed in the polymer at higher loadings. This aggregation of MOFs would increase the void spaces between the polymer matrix and nanoparticles, which improves the CO2 and N2 permeability at the same time, leading to the deterioration of the gas separation selectivity. Cu-BTC 1018 1191 NH2 -Cu-BTC 797 847 MMMs were characterized from sub-NH2 -Cu-BTC 718 724

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Figure 6. The SEM images of cross-section of (a) polyether-block-amide (Pebax) membrane, (b) Figure 6. The SEM images of cross-section of (a) polyether-block-amide (Pebax) membrane, Pebax/Cu-BTC mixed matrix membranes (MMMs) (3 wt.% MOFMOF content in the MMMs), (c) (b) Pebax/Cu-BTC mixed matrix membranes (MMMs) (3 wt.% content in the MMMs), 2-Cu-BTC MMMs (3wt.% MOF content in the MMMs), (d) Pebax/sub-NH2-Cu-BTC MMMs Pebax/NH (c) Pebax/NH2 -Cu-BTC MMMs (3wt.% MOF content in the MMMs), (d) Pebax/sub-NH2 -Cu-BTC (3 wt.% (3 metal-organic framework (MOF) content in the MMMs), (e) the low magnification of (d) and MMMs wt.% metal-organic framework (MOF) content in the MMMs), (e) the low magnification of 2-Cu-BTC MMMs (4 wt.% MOF content in the MMMs). (f) Pebax/sub-NH (d) and (f) Pebax/sub-NH2 -Cu-BTC MMMs (4 wt.% MOF content in the MMMs).

3.2.2. XRD 3.2.2. XRD Figure 7 shows the XRD patterns of pure Pebax and Pebax/MOFs membranes between 5° and Figure 7 shows the XRD patterns of pure Pebax and Pebax/MOFs membranes between 5◦ and 35°◦ to determine their crystalline structure. The pristine Pebax membrane shows a broad peak at 16.6° 35 to determine their crystalline structure. The pristine Pebax membrane shows a broad peak at 16.6◦ of 2θ and a narrow diffraction peak at 22.6°, suggesting the semicrystalline structure of the sample of 2θ and a narrow diffraction peak at 22.6◦ , suggesting the semicrystalline structure of the sample [39] [39] similar to the reported XRD pattern for the Pebax membrane in the literature [40]. In addition, similar to the reported XRD pattern for the Pebax membrane in the literature [40]. In addition, the peak the peak at 2θ = 22.6° can be attributed to the crystalline region of polyamide (PA) segment formed at 2θ = 22.6◦ can be attributed to the crystalline region of polyamide (PA) segment formed by hydrogen by hydrogen bonding [41]. For the diffraction patterns of Pebax/MOF membranes, the intensity and bonding [41]. For the diffraction patterns of Pebax/MOF membranes, the intensity and position of position of characteristic peaks of the Pebax membrane are almost constant with the incorporation of characteristic peaks of the Pebax membrane are almost constant with the incorporation of MOFs, MOFs, indicating that the chain packing and intersegmental spacing of the Pebax polymer had no indicating that the chain packing and intersegmental spacing of the Pebax polymer had no substantial substantial change. Two new characteristic peaks of MOFs at 2θ = 11.6° and 17.4°, corresponding to change. Two new characteristic peaks of MOFs at 2θ = 11.6◦ and 17.4◦ , corresponding to (222) and (222) and (422), are observed at a low loading. Furthermore, Figure 7 also shows that the intensity of the two characteristic peaks of MMMs doped with NH2-Cu-BTC and sub-NH2-Cu-BTC at 2θ = 16.6°

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(422), are observed at a low loading. Furthermore, Figure 7 also shows that the intensity of the two characteristic peaks of MMMs doped with NH2 -Cu-BTC and sub-NH2 -Cu-BTC at 2θ = 16.6◦ and 22.6◦ and 22.6° slightly decreased compared to those doped with Cu-BTC. This is because the polymer slightly decreased compared to those doped with Cu-BTC. This is because the polymer crystallinity crystallinity decreased due to the formation of hydrogen bonds caused by the interactions between decreased due to the formation of hydrogen bonds caused by the interactions between the amine the amine functional groups on the surface of NH2-Cu-BTC and sub-NH2-Cu-BTC with the PEO functional groups on the surface of NH2 -Cu-BTC and sub-NH2 -Cu-BTC with the PEO component component of Pebax. of Pebax.

11.6

16.6

17.4 22.6

Intensity

14.2

(d)

(c) (b) (a) 10

20

30

40

2Theta/degree Figure7.7. The TheXRD XRDpatterns patternsof of(a) (a)pure purePebax Pebaxmembrane, membrane,(b) (b)Pebax/Cu-BTC Pebax/Cu-BTCMMMs, MMMs,(c) (c)Pebax/NH Pebax/NH22Figure Cu-BTCMMMs MMMsand and(d) (d)Pebax/sub-NH Pebax/sub-NH22-Cu-BTC Cu-BTC -Cu-BTC MMMs MMMs (3 (3 wt.% wt.% MOFs content in the MMMs).

3.3. Gas Gas Separation Separation Performances Performances 3.3. The gas gas permeation byby thethe interface morphology of the The permeation properties propertiesof ofMMMs MMMsare aremainly mainlyaffected affected interface morphology of continuous and dispersed phases. To investigate the effect of different types of MOFs and filler the continuous and dispersed phases. To investigate the effect of different types of MOFs and filler loadings on on the MMMs with different loadings of Culoadings the membrane membraneseparation separationproperties, properties,Pebax/MOFs Pebax/MOFs MMMs with different loadings of BTC, NHNH 2-Cu-BTC, or sub-NH 2-Cu-BTC were fabricated. The gas permeability and selectivity of Cu-BTC, -Cu-BTC, or sub-NH -Cu-BTC were fabricated. The gas permeability and selectivity 2 2 ◦ CO 2 /N 2 and CO 2 /CH 4 at 25 °C and 0.1 MPa of the feed pressure are shown in Figures 8 and 9, of CO2 /N2 and CO2 /CH4 at 25 C and 0.1 MPa of the feed pressure are shown in Figures 8 and 9, respectively.All Allexperiments experiments were performed at least the average results are respectively. were performed at least three three times, times, and theand average results are reported. reported. A comparable increase in the CO 2 permeability of MMMs fabricated with different types of A comparable increase in the CO2 permeability of MMMs fabricated with different types of MOFs MOFs with the Pebax membrane results in an increase in the selectivity of CO 2 /N 2 and CO 2 /CH 4 than with the Pebax membrane results in an increase in the selectivity of CO2 /N2 and CO2 /CH4 than that of of the the pristine pristine Pebax with the increase of that Pebax membrane. membrane. The TheCO CO2 2permeability permeabilityofofMMMs MMMsincreased increased with the increase Cu-BTC loading, and the improved permeability can be mainly attributed to the defects at the of Cu-BTC loading, and the improved permeability can be mainly attributed to the defects at the inorganic–polymer interface. interface. Moreover, Moreover, the the strong strong quadrupole quadrupole moment moment of of CO CO22 has has aa higher higher affinity affinity inorganic–polymer withunsaturated unsaturatedCu Cusites sites than 2 and 4, but the increase in the N 2 CH and CH 4 permeability isclear. not with than N2Nand CHCH , but the increase in the N and permeability is not 4 2 4 clear. The results are consistent with the results of N 2 , CO 2 , and CH 4 adsorption measurements. The results are consistent with the results of N2 , CO2 , and CH4 adsorption measurements. Furthermore, Furthermore, the CHis4 permeability slightly higher thanis that of N2because . This isthe probably because the the CH4 permeability slightly higheristhan that of N2 . This probably critical temperature critical temperature of gases decreases in the following order: CO 2 (304.2 K) > CH 4 (190.7 K) 2 (126.1 of gases decreases in the following order: CO2 (304.2 K) > CH4 (190.7 K) > N2 (126.1 K).>ANhigher K). A higher condensability shows a higher solubility of the gas in the polymer matrix [32], which condensability shows a higher solubility of the gas in the polymer matrix [32], which results in a higher results in a higher permeability. Therefore, the selectivity of CO2/N2 is higher than that of CO2/CH4. permeability. Therefore, the selectivity of CO 2 /N2 is higher than that of CO2 /CH4 . The selectivity The selectivity of CO 2 /N 2 and CO 2 /CH 4 also increased with the increase loading. in Cu-BTC loading. above However, of CO2 /N2 and CO2 /CH4 also increased with the increase in Cu-BTC However, the above the Cu-BTC of 4 wt.%, both the selectivity CO 2CO /N 2 and CO 2/CH 4 of MMMs decreased Cu-BTC loading of loading 4 wt.%, both the selectivity of CO2 /Nof and /CH of MMMs decreased due to 2 2 4 due to poor compatibility and presence of nonselective at the inorganic-polymer interface. poor compatibility and presence of nonselective voids atvoids the inorganic-polymer interface.

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140 140

CO Permeabilty(barrers) CO 2 2 Permeabilty(barrers)

25 25

Cu-BTC Cu-BTC NH2-Cu-BTC NH2-Cu-BTC sub-NH2-Cu-BTC sub-NH2-Cu-BTC Cu-BTC Cu-BTC NH2-Cu-BTC NH2-Cu-BTC sub-NH2-Cu-BTC sub-NH -Cu-BTC Cu-BTC2 Cu-BTC NH2-Cu-BTC NH2-Cu-BTC sub-NH2-Cu-BTC sub-NH2-Cu-BTC

120 120 100 100

20 20

15 15

80 80 60 60

CH CH44

40 40

N N22

20 20 0 0

1 1

2 2

MOFs content (wt.%) MOFs content (wt.%)

3 3

10 10

5 5

NN /CH4 Permeabilty(barrers) Permeabilty(barrers) 2 2/CH 4

160 160

11 of 16 15 11 11 of of 15

0 0

4 4

Figure 8. Effects of the MOFs loading level on the gas permeability of MMMs. Figure 8. 8. Effects of the the MOFs MOFs loading loading level level on on the the gas gas permeability permeability of of MMMs. MMMs. Figure Effects of

Figure 9 and Table 2 show that the MMMs incorporated with NH2-Cu-BTC exhibit a higher Figure 9 and Table 2 show that the MMMs incorporated with NH2-Cu-BTC exhibit a higher Figureof9 CO and2/N Table 2 CO show that the MMMs incorporated with NH2 -Cu-BTC exhibit a higher selectivity 2 and 2/CH 4 compared to that doped with pristine Cu-BTC at the expense of selectivity of CO2/N2 and CO2/CH4 compared to that doped with pristine Cu-BTC at the expense of selectivity of CO /N and CO /CH compared to that doped with pristine Cu-BTC at the expense of 2 The 2 decreased 2 4 CO2 permeability. permeability can be attributed to the good compatibility between CO2 permeability. The decreased permeability can be attributed to the good compatibility between CO permeability. The decreased permeability can interaction be attributed to the CO good compatibility between the2amine functionalized fillers and Pebax [28]. The between 2 molecules and the amino the amine functionalized fillers and Pebax [28]. The interaction between CO2 molecules and the amino the amine and Pebax [28]. interaction between CO 2 molecules groups onfunctionalized the surface offillers NH2-Cu-BTC and the The decrease of nonselective voids increase and the the gas groups on the surface of NH2-Cu-BTC and the decrease of nonselective voids increase the gas amino groups on the surface of NH -Cu-BTC and the decrease of nonselective voids increase the gas 2 separation selectivity. separation selectivity. separation selectivity.

Selectivity(CO /N2)2) Selectivity(CO 2 2/N

64 64

48 48

Cu-BTC Cu-BTC NH2-Cu-BTC NH2-Cu-BTC sub-NH2-Cu-BTC sub-NH2-Cu-BTC Cu-BTC Cu-BTC NH2-Cu-BTC NH2-Cu-BTC sub-NH2-Cu-BTC sub-NH2-Cu-BTC

40 40

56 56

32 32

48 48

24 24

40 40

Selectivity(CO /CH4)4) Selectivity(CO 2 2/CH

72 72

16 16

32 32 0 0

1 1

2 2

MOFs content (wt.%) MOFs content (wt.%)

3 3

4 4

Figure 9. 9. Effects Effects of of MOFs MOFs loading loading level level on on the gas gas separation selectivity. selectivity. Figure Figure 9. Effects of MOFs loading level on the the gas separation separation selectivity.

Furthermore, Figure 8 and Table 2 show that the CO2 permeability of MMMs incorporated with Furthermore, Furthermore, Figure Figure 88 and and Table Table 22 show show that that the the CO CO22 permeability of MMMs incorporated with sub-NH2-Cu-BTC is higher than that of MMMs doped with NH2-Cu-BTC, but worse than that of sub-NH22-Cu-BTC -Cu-BTC is is higher higher than that of MMMs doped with NH NH22-Cu-BTC, but worse than that of pristine Cu-BTC. The presence of nanoparticles in the polymer matrix disrupts the chain packing and pristine pristine Cu-BTC. Cu-BTC. The The presence presence of of nanoparticles nanoparticles in in the the polymer polymer matrix matrix disrupts disrupts the the chain chain packing packing and changes the structural regularity of the polymer–nanoparticle interface. Void spaces are formed due changes the structural regularity of the polymer–nanoparticle interface. Void spaces are formed due to changes the structural regularity of the polymer–nanoparticle interface. Void spaces are formed due to weak compatibility at the interface of the polymer and nanoparticle phases that serve as channels weak compatibility at the interface of the polymer andand nanoparticle phases that serve as channels to to weak compatibility at the interface of the polymer nanoparticle phases that serve as channels to transport gas molecules more effectively. As shown in Figure 6, the MMMs incorporated with to transport gas molecules more effectively. As shown in Figure 6, the MMMs incorporated with

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transport gas molecules more effectively. As shown in Figure 6, the MMMs incorporated with pristine Cu-BTC had worse compatibilities with the polymer matrix, which exhibited a higher CO2 permeability compared to the functionalized Cu-BTC. The permeability of MMMs doped with sub-NH2 -Cu-BTC can be attributed to the smaller size of fillers after the submicrometer treatment and the presence of more amino and carboxyl groups on the crystal surface at the same loadings. Correspondingly, the MMMs incorporated with sub-NH2 -Cu-BTC show a better selectivity of CO2 /N2 and CO2 /CH4 . However, at a higher MOFs loading ratio, such as the loading above 4 wt.%, the increase in the rate of CH4 and N2 permeability is higher than that of the CO2 permeability based on the void spaces between the polymer matrix and nanoparticles, which leads to the deterioration of the gas separation selectivity. Compared with results reported in other literature, which directly incorporated pristine MOFs or amine functionalized MOFs [19,42], we found that the dispersion of functionalized MOFs became better in the Pebax matrix if the crystal size was effectively regulated. The CO2 permeability in the Pebax/sub-NH2 -Cu-BTC was 303% higher than neat Pebax due to the fine dispersion and the presence of groups with a superior CO2 -philicity in the framework. The ideal selectivity of CO2 /N2 and CO2 /CH4 was equally higher than that of the neat matrix. Table 2. Gas permeability and CO2 /N2 and CO2 /CH4 selectivity of the prepared membranes. Permeability (Barrer)

Type of Membrane Pebax Pebax/Cu-BTC Pebax/NH2 -Cu-BTC Pebax/sub-NH2 -Cu-BTC

Selectivity

N2

CH4

CO2

CO2 /N2

CO2 /CH4

0.71 2.16 1.42 1.64

1.89 7.35 3.33 3.73

26.89 119.3 86.58 108.5

38.00 55.13 60.88 66.27

14.24 16.23 25.97 29.05

3 wt.% MOFs content in MMMs.

According to the solution-diffusion model, gas permeation through a dense membrane depends on the diffusivity coefficient (D) and solubility coefficient (S). To further elucidate the role of fillers in gas permeation, the D value was calculated according to the Equation (2) and S values of the membranes were calculated according to the equation S = P/D. The diffusivity and solubility selectivities of CO2 /N2 and CO2 /CH4 of a pristine Pebax membrane and MMMs at 25 ◦ C and 0.1 MPa are shown in Table 3. It can clearly be seen that both the CO2 /N2 and CO2 /CH4 diffusivity selectivities of these MMMs containing MOFs have no improvement compared to a pristine Pebax membrane. However, the solubility selectivities of CO2 /N2 and CO2 /CH4 are higher than those of a pristine Pebax membrane. Thus, the separation efficiency is based on the gas solubility, but not on the size differences. The unsaturated Cu sites and carboxyl groups of Cu-BTC enhance the affinity with CO2 molecules, and after the amination functionalization, the amide groups of the MOFs further increase the solubility selectivity of the MMMs due to the Lewis acid-base interactions with CO2 molecules. Besides, it is important to note that the introduction of primary amino groups on the MOFs leads to the formation of reactivity-selective membranes. Therefore, the extremely high CO2 /N2 and CO2 /CH4 selectivities of the MMMs can be attributed to the solubility selectivity. Table 3. Diffusivity and solubility selectivity of CO2 /N2 and CO2 /CH4 . Type of Membrane

DCO2 /DN2

DCO2/CH4

SCO2/N2

SCO2/CH4

Pebax Pebax/Cu-BTC Pebax/NH2 -Cu-BTC Pebax/sub-NH2 -Cu-BTC

2.01 2.04 1.98 2.01

3.13 3.41 3.77 3.92

19.02 27.02 30.75 32.97

4.55 4.76 6.89 7.41

3 wt.% MOFs Content in MMMs.

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Figure 10 shows the trade-off lines between the selectivity and permeability for CO2 /N2 separation (Robeson plot). The performance data of CO2 and N2 of a pristine Pebax membrane Materials 2018, 11, x FOR PEER REVIEW 13 of 15 falls far below the updated upper bound reported in 2008 [7]. The Cu-BTC and NH2 -Cu-BTC containing membranes are close to or surpass the upper bound plot, and the MMMs incorporated Cu-BTC clearly surpass that line. This demonstrates the promising performance of submicrometerwith sub-NH2 -Cu-BTC clearly surpass that line. This demonstrates the promising performance of functionalized Cu-BTC compared to the untreated Cu-BTC in the CO2 separation field. submicrometer-functionalized Cu-BTC compared to the untreated Cu-BTC in the CO2 separation field. 100 p-Pebax Pebax/Cu-BTC Pebax/NH2-Cu-BTC Pebax/sub-NH2-Cu-BTC

80

CO2/N2 Selecility

60

40

20

10

100

1000

10000

CO2 Permeability(Barrer)

Figure 2 separation performance of pure Pebax (□), Pebax/Cu-BTC (▲), Pebax/NH2-CuFigure 10. 10. CO CO2/N 2 /N2 separation performance of pure Pebax (), Pebax/Cu-BTC (N), Pebax/NH2 2-Cu-BTC (★) MMMs in Robeson upper bound plot (2008). BTC (◆) (u) andand Pebax/sub-NH Cu-BTC Pebax/sub-NH 2 -Cu-BTC (H) MMMs in Robeson upper bound plot (2008).

4. 4. Conclusions Conclusions In based on on Pebax Pebax and and Cu-BTC Cu-BTC particles particles (including (including Cu-BTC, Cu-BTC, NH NH2-Cu-BTC, In this this work, work, MMMs MMMs based 2 -Cu-BTC, and sub-NH 2-Cu-BTC) were fabricated using the solution-casting method. SEM analysis showed that and sub-NH2 -Cu-BTC) were fabricated using the solution-casting method. SEM analysis showed that the roughness of of NH NH2-Cu-BTC clearly increased, and the size of sub-NH2-Cu-BTC clearly the surface surface roughness 2 -Cu-BTC clearly increased, and the size of sub-NH2 -Cu-BTC clearly decreased. The resulting fine dispersion of MOFs throughout the matrix Pebax and matrix and interfacial a strong decreased. The resulting fine dispersion of MOFs throughout the Pebax a strong interfacial adhesion between the modified MOFs and the polymer matrix were obtained. FTIR adhesion between the modified MOFs and the polymer matrix were obtained. FTIR analysis confirmed analysis confirmed that the amine modification introducedgroups amino(–NH functional groups (–NH2) on the that the amine modification introduced amino functional 2 ) on the surface of Cu-BTC. surface of Cu-BTC. The XRD patterns obtained to establish the consistency of theindicated crystal structure The XRD patterns obtained to establish the consistency of the crystal structure that the indicated that the modification process had no influence on the crystal structure of the MOFs. The modification process had no influence on the crystal structure of the MOFs. The order of different order of different MOFs’ adsorption capacities of N 2, CH4, and CO2 obtained by gas adsorption MOFs’ adsorption capacities of N2 , CH4 , and CO2 obtained by gas adsorption measurements at 308 K measurements at 308 K indicated amine modification and submicrometer-sized amine indicated that amine modification andthat submicrometer-sized amine modification were favorable for modification were favorable for the increase of the gas adsorbability of MOFs. TGA showed that the the increase of the gas adsorbability of MOFs. TGA showed that the crystals of three kinds of MOFs crystals ofstable three up kinds of MOFs stable up to 300showed °C. Gasthat separation performance showed ◦ C. Gasremained remained to 300 separation performance the MMMs with NH2 -Cu-BTC that the MMMs with NH 2-Cu-BTC and sub-NH2-Cu-BTC represented a better gas separation and sub-NH2 -Cu-BTC represented a better gas separation selectivity than that of MMMs incorporated selectivity than that of MMMs incorporated the pristine Cu-BTC, andincrease the separation with the pristine Cu-BTC, and the separation with selectivity increased with the in MOF selectivity loading at increased with the increase in MOF loading at low MOF contents. This can be attributed the fine low MOF contents. This can be attributed to the fine dispersion of NH2 -Cu-BTC and sub-NHto 2 -Cu-BTC dispersion of NH 2-Cu-BTC and sub-NH2-Cu-BTC and the presence of amino functional groups. and the presence of amino functional groups. However, the gas permeability of NH2 -Cu-BTC and However, the gas permeability of NH2-Cu-BTC sub-NH 2-Cu-BTC was slightly sacrificed because sub-NH2 -Cu-BTC was slightly sacrificed becauseand of the decrease in interfacial voids between the MOFs of the decrease in interfacial voids between the MOFs and the Pebax matrix. The promising and the Pebax matrix. The promising performance of submicrometer-functionalized Cu-BTC in the performance of submicrometer-functionalized Cu-BTC in the fabrication of polymer membranes fabrication of polymer membranes for effective CO2 /N2 and CO2 /CH4 separation was demonstrated. for effective CO2/N2 and CO2/CH4 separation was demonstrated. Author Contributions: H.S. conceived and designed the experiments; B.G., Y.X. and H.Z. performed the experiments; B.G. and H.S. analyzed the data; Y.G. and W.W. contributed reagents/materials/analysis tools;the all Author Contributions: H.S. conceived and designed the experiments; B.G., Y.X. and H.Z. performed authors contributed to H.S. the writing of the experiments; B.G. and analyzed the paper. data; Y.G. and W.W. contributed reagents/materials/analysis tools; all authors contributed to the writing of the paper.

Funding: This research was funded by the National Key Research and Development Plan of China (2016YFE0106700), the National Natural Science Foundation of China (21406268), the Province Key Research and Development Program of Shandong (2016GSF115032), and the Fundamental Research Funds for the Central Universities (18CX05006A & 16CX05009A).

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Funding: This research was funded by the National Key Research and Development Plan of China (2016YFE0106700), the National Natural Science Foundation of China (21406268), the Province Key Research and Development Program of Shandong (2016GSF115032), and the Fundamental Research Funds for the Central Universities (18CX05006A & 16CX05009A). Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12.

13. 14.

15.

16. 17. 18. 19.

20.

Yue, M.B.; Chun, Y.; Cao, Y.; Dong, X.; Zhu, J.H. CO2 capture by as-prepared SBA-15 with an occluded organic template. Adv. Funct. Mater. 2006, 16, 1717–1722. [CrossRef] Satyapal, S.; Filburn, T.; Trela, J.; Strange, J. Performance and properties of a solid amine sorbent for carbon dioxide removal in space life support applications. Energy Fuels 2001, 15, 250–255. [CrossRef] Lua, A.C.; Shen, Y. Preparation and characterization of polyimide-silica composite membranes and their derived carbon–silica composite membranes for gas separation. Chem. Eng. J. 2013, 220, 441–451. [CrossRef] Vorotyntsev, V.M.; Drozdov, P.N.; Vorotyntsev, I.V. High purification of substances by a gas separation method. Desalination 2009, 240, 301–305. [CrossRef] Zornoza, B.; Irusta, S.; Téllez, C.; Coronas, J. Mesoporous silica sphere-polysulfone mixed matrix membranes for gas separation. Langmuir 2009, 25, 5903–5909. [CrossRef] [PubMed] Ho, W.S.W.; Sirkar, K.K. Membrane Handbook; Kluwer Academic Publisher: New York, NY, USA, 1992; pp. 1–998. Robeson, L.M. The upper bound revisited. J. Membr. Sci. 2008, 320, 390–400. [CrossRef] Barankova, E.; Pradeep, N.; Peinemann, K.V. Zeolite-imidazolate framework (ZIF-8) membrane synthesis on a mixed-matrix substrate. Chem. Commun. 2013, 49, 9419–9421. [CrossRef] [PubMed] Zornoza, B.; Téllez, C.; Coronas, J. Mixed matrix membranes comprising glassy polymers and dispersed mesoporous silica spheres for gas separation. J. Membr. Sci. 2011, 368, 100–109. [CrossRef] Wang, S.; Liu, Y.; Huang, S.; Wu, H.; Li, Y.; Tian, Z.; Jiang, Z. Pebax-PEG-MWCNT hybrid membranes with enhanced CO2 capture properties. J. Membr. Sci. 2014, 460, 62–70. [CrossRef] Liu, Y.; Peng, D.; He, G.; Wang, S.; Li, Y.; Wu, H.; Jiang, Z. Enhanced CO2 permeability of membranes by incorporating polyzwitterion@CNT composite particles into polyimide matrix. ACS Appl. Mater. Interfaces 2014, 6, 13051–13060. [CrossRef] [PubMed] Lin, R.; Ge, L.; Hou, L.; Strounina, E.; Rudolph, V.; Zhu, Z. Mixed matrix membranes with strengthened MOFs/polymer interfacial interaction and improved membrane performance. ACS Appl. Mater. Interfaces 2014, 6, 5609–5618. [CrossRef] [PubMed] Gomes, D.; Nunes, S.P.; Peinemann, K.V. Membranes for gas separation based on poly(1-trimethylsilyl-1propyne)-silica nanocomposites. J. Membr. Sci. 2005, 246, 13–25. [CrossRef] Ahn, J.; Chung, W.J.; Pinnau, I.; Song, J.; Du, N.; Robertson, G.P.; Guiver, M.D. Gas transport behavior of mixed-matrix membranes composed of silica nanoparticles in a polymer of intrinsic microporosity (PIM-1). J. Membr. Sci. 2010, 346, 280–287. [CrossRef] Arjmandi, M.; Pakizeh, M. Mixed matrix membranes incorporated with cubic-MOF-5 for improved polyetherimide gas separation membranes: Theory and experiment. J. Ind. Eng. Chem. 2014, 20, 3857–3868. [CrossRef] Zhao, Y.; Jung, B.T.; Ansaloni, L.; Ho, W.S.W. Multiwalled carbon nanotube mixed matrix membranes containing amines for high pressure CO2 /H2 separation. J. Membr. Sci. 2014, 459, 233–243. [CrossRef] Cao, L.; Tao, K.; Huang, A.; Kong, C.; Chen, L. A highly permeable mixed matrix membrane containing CAU-1-NH2 for H2 and CO2 separation. Chem. Commun. 2013, 49, 8513–8515. [CrossRef] [PubMed] Vanherck, K.; Aerts, A.; Martens, J.; Vankelecom, L. Hollow filler based mixed matrix membranes. Chem. Commun. 2010, 46, 2492–2494. [CrossRef] [PubMed] Li, H.; Tuo, L.; Yang, K.; Jeong, H.; Dai, Y.; He, G.; Zhao, W. Simultaneous enhancement of mechanical properties and CO2 selectivity of ZIF-8 mixed matrix membranes: Interfacial toughening effect of ionic liquid. J. Membr. Sci. 2016, 511, 130–142. [CrossRef] Erucar, I.; Yilmaz, G.; Keskin, S. Recent advances in metal–organic framework-based mixed matrix membranes. Chem. Asian J. 2013, 8, 1692–1704. [CrossRef] [PubMed]

Materials 2018, 11, 1421

21.

22. 23. 24. 25. 26.

27. 28.

29.

30. 31.

32.

33.

34. 35. 36.

37.

38. 39. 40.

15 of 16

Seoane, B.; Coronas, J.; Gascon, I.; Benavides, M.E.; Karvan, O.; Caro, J.; Kapteijn, F.; Gascon, J. Metal-organic framework based mixed matrix membranes: A solution for highly efficient CO2 capture? Chem. Soc. Rev. 2015, 44, 2421–2454. [CrossRef] [PubMed] Wang, Z.Q.; Cohen, S.M. Postsynthetic modification of metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1315–1329. [CrossRef] [PubMed] Basu, S.; Cano-Odena, A.; Vankelecom, I.F.J. Asymmetric Matrimid® /[Cu3 (BTC)2 ] mixed-matrix membranes for gas separations. J. Membr. Sci. 2010, 362, 478–487. [CrossRef] Basu, S.; Cano-Odena, A.; Vankelecom, I.F.J. MOF-containing mixed-matrix membranes for CO2 /CH4 and CO2 /N2 binary gas mixture separations. Sep. Purif. Technol. 2011, 81, 31–40. [CrossRef] Basu, S.; Maes, M.; Cano-Odena, A.; Alaerts, L.; Vos, L.A.D.; Vankelecom, I. Solvent resistant nanofiltration (SRNF) membranes based on metal-organic frameworks. J. Membr. Sci. 2009, 344, 190–198. [CrossRef] Shahid, S.; Nijmeijer, K.; Nehache, S.; Vankelecom, I.; Deratani, A.; Quemener, D. MOF-mixed matrix membranes: Precise dispersion of MOF particles with better compatibility via a particle fusion approach for enhanced gas separation properties. J. Membr. Sci. 2015, 492, 21–31. [CrossRef] Nik, O.G.; Chen, K.X.Y.; Kaliaguine, S. Functionalized metal organic framework-polyimide mixed matrix membranes for CO2 /CH4 separation. J. Membr. Sci. 2012, 413–414, 48–61. [CrossRef] Bae, T.H.; Lee, J.S.; Qiu, W.; Koros, W.J.; Jones, C.W.; Nair, S. A high-performance gas-separation membrane containing submicrometer-sized metal-organic framework crystals. Angew. Chem. Int. Ed. 2010, 49, 9863–9866. [CrossRef] [PubMed] Khan, N.A.; Haque, M.M.; Jhung, S.H. Accelerated syntheses of porous isostructural lanthanidebenzenetricarboxylates (Ln–BTC) under ultrasound at room. Eur. J. Org. Chem. 2010, 2010, 4975–4981. [CrossRef] Lin, H.Q.; Freeman, B.D. Materials selection guidelines for membranes that remove CO2 from gas mixtures. J. Mol. Struct. 2005, 739, 57–74. [CrossRef] Tocci, E.; Gugliuzza, A.; Lorenzo, L.D.; Macchione, M.; Luca, G.D.; Drioli, E. Transport properties of a co-poly(amide-12-b-ethyleneoxide) membrane: A comparative study between experimental and molecular modelling results. J. Membr. Sci. 2008, 323, 316–327. [CrossRef] Reijerkerk, S.R.; Knoef, M.H.; Nijmeijer, K.; Wessling, M. Poly(ethyleneglycol) and poly(dimethylsiloxane): Combining their advantages into efficient CO2 gas separation membranes. J. Membr. Sci. 2010, 352, 126–135. [CrossRef] Sun, H.; Ma, C.; Yuan, B.; Wang, T.; Xu, Y.; Xue, Q.; Li, P.; Kong, Y. Cardo polyimides/TiO2 mixed matrix membranes: Synthesis, characterization, and gas separation property improvement. Sep. Purif. Technol. 2014, 122, 367–375. [CrossRef] Hadjiivanov, K.I.; Vayssilov, G.N. Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule. Adv. Catal. 2002, 47, 307–511. Hu, J.; Cai, H.; Ren, H.; Wei, Y.; Xu, Z.; Liu, H.; Hu, Y. Mixed-matrix membrane hollow fibers of Cu3 (BTC)2 MOF and polyimide for gas separation and adsorption. Ind. Eng. Chem. Res. 2010, 49, 12605–12612. [CrossRef] Carson, C.G.; Hardcastle, K.; Schwartz, J.; Liu, X.; Hoffmann, C.; Gerhardt, R.A.; Tannenbaum, R. Synthesis and structure characterization of copper terephthalate metal-organic frameworks. Eur. J. Inorg. Chem. 2009, 16, 2338–2343. [CrossRef] Wang, Q.M.; Shen, D.; Bulow, M.; Lau, M.L.; Deng, S.; Fitch, F.R.; Lemcoff, N.O.; Semanscin, J. Metallo-organic molecular sieve for gas separation and purification. Microporous Mesoporous Mater. 2002, 55, 217–230. [CrossRef] Zhao, Y.; Ho, W.S.W. Steric hindrance effect on amine demonstrated in solid polymer membranes for CO2 transport. J. Membr. Sci. 2012, 415–416, 132–138. [CrossRef] Xiang, L.; Pan, Y.; Zeng, G.; Jiang, J.; Chen, J.; Wang, C. Preparation of poly(ether-block-amide)/attapulgite mixed matrix membranes for CO2 /N2 separation. J. Membr. Sci. 2016, 500, 66–75. [CrossRef] Zhao, D.; Ren, J.; Li, H.; Li, X.; Deng, M. Gas separation properties of poly(amide-6-b- ethyleneoxide)/amino, modified multi-walled carbon nanotubes mixed matrix membranes. J. Membr. Sci. 2014, 467, 41–47. [CrossRef]

Materials 2018, 11, 1421

41. 42.

16 of 16

Kim, J.H.; Ha, S.Y.; Lee, Y.M. Gas permeation of poly(amide-6-b-ethyleneoxide) copolymer. J. Membr. Sci. 2001, 190, 179–193. [CrossRef] Meshkat, S.; Kaliaguine, S.; Rodrigue, D. Mixed matrix membranes based on amine and non-amine MIL-53(Al) in Pebax® MH-1657 for CO2 separation. Sep. Purif. Technol. 2018, 200, 177–190. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).