Metal-organic framework-graphene oxide composites: A facile method to highly improve the CO2 separation performance of mixed matrix membranes

Journal of Membrane Science 520 (2016) 801–811

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Metal-organic framework-graphene oxide composites: A facile method to highly improve the CO2 separation performance of mixed matrix membranes Liangliang Dong, Mingqing Chen n, Jie Li, Dongjian Shi, Weifu Dong, Xiaojie Li, Yunxiang Bai Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 May 2016 Received in revised form 19 July 2016 Accepted 23 August 2016 Available online 24 August 2016

Mixed matrix membranes (MMMs) were fabricated by incorporating ZIF-8@GO into Pebax matrix to improve CO2 separation performance. The ZIF-8@GO played multiple roles in enhancing membrane performance. First, the high-aspect ratio GO nanosheets in polymer matrix increased the length of the tortuous path of gas diffusion, restricting the diffusion of larger molecules and favoring the diffusion of small molecules with less resistance, which enhanced the diffusivity selectivity. Second, the inherent high permeability of ZIF-8 with ultra-microporosity was anticipated to optimize fractional free volume and enhance the gas permeability and solubility selectivity of MMMs. The MMMs doped with ZIG-8@GO had better gas separation performance than those doped with only ZIF-8 or GO. The membrane containing 6 wt% of ZIF-8@GO (Peabx/ZIF-8@GO-6) exhibited the optimum performance with a CO2 permeability of 249 Barrer and a CO2/N2 selectivity of 47.6. Compared with the pure Pebax membrane, the CO2 permeability and CO2/N2 selectivity of the Pebax/ZIF-8@GO-6 MMMs were increased by 191% and 174%, respectively. The strategy of growing ZIFs on GO may provide an effective method to further develop MMMs performance through the modification of ZIFs on existing fillers which had larger adsorption differences to specific gases. & 2016 Elsevier B.V. All rights reserved.

Keywords: ZIF-8 GO Mixed matrix membrane Peabx CO2 separation

1. Introduction Carbon dioxide capture from power plant flue gas and subsequent sequestration is expected to play a key role in mitigating global climate change. Membrane separation is one of the potential methods to remove CO2 from flue gas [1,2]. Compared with traditional separation techniques such as cryogenic distillation or absorption, membrane-separation technology has advantages of low energy consumption, mechanical simplicity, ease to scale up and smaller footprint [3]. Polymeric membranes suffer from a trade-off between permeability and selectivity, i.e., polymers with high gas permeability generally have low gas selectivity and vice versa [4]. Mixed matrix membranes (MMMs) consisted of organic polymer and inorganic particles have been recognized as an effective way to overcome the curse of trade-off effect. They combine the merits of polymer membrane of easy processability and inorganic membrane of superior permeability and selectivity [5]. Among the fillers, graphene oxide (GO), a typical sheet-shaped n

Corresponding author. E-mail address: [email protected] (M. Chen).

http://dx.doi.org/10.1016/j.memsci.2016.08.043 0376-7388/& 2016 Elsevier B.V. All rights reserved.

material, has attracted much attention because of high aspect ratio (41000), facile synthesis, tunable surface functionalization, and high mechanical and thermal properties [6]. In GO-based MMMs, the high aspect ratio of GO nanosheets forms long and tortuous paths which restricts the diffusion of larger molecules but favors the diffusion of small molecules with less resistance, thus improving gas diffusivity and diffusivity selectivity [7,8]. However, the ability of recognizing the penetrants based on molecular size drops off with the increase of the GO content in the matrix. Furthermore, the improvement of gas separation performance of GOdoped membranes is always limited, due to only enhancing a single permselectivity (diffusivity selectivity). To address these issues, various surface modification strategies have been attempted. One feasible approach is to graft functional groups which have strong affinity with polar gases like CO2 on the GO [6,9]. This method not only enhances solubility selectivity of MMMs, but also improves the dispersion of GO in the MMMs. However, this method sometimes damages the integrity of the GO, making them inappropriate for use as the filler for MMMs. Apart from grafting organic functional groups, an alternative is to decorate metal-organic frameworks (MOFs) on GO, which aims

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L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

Scheme 1. Illustration of the preparation of ZIF-8@GO.

to improve the adsorption selectivity of GO. Furthermore, previous studies had confirmed that the incorporation of MOF could improve the dispersion of other nanofillers in MMMs [10–12]. As a novel porous hybrid material, MOFs can be grown from the precursors of metal and organic linkers, which possess excellent thermal and chemical stability, high porosity, strong affinity toward certain gas molecules and convenient synthesis [13]. Recently, Hu et al. [14] reported ZIF-8/GO molecular sieving membranes for gas separation, which showed excellent molecular sieving gas separation properties, such as with a high CO2/N2 selectivity of 7.0. However, research on gas separation performance of MOF@GO-based MMMs is still rare, as a majority of applications of MOF@GO composites are more related to adsorption [15–18]. In this study, we reported a novel MMM derived from in situ growth of MOFs on the surface of GO to improve gas separation performance. ZIF-8 was one of the most studied prototypical and common zeolitic imidazolate frameworks (ZIFs) compounds which were a subclass of MOFs, due to its ultra-microporosity, high thermal and chemical stability and high kinetic selectivity of CO2/N2. As-synthesized ZIF-8@GO in this work had three obvious advantages. First, the inherent high CO2 sorption of ZIF-8 with ultra-microporosity was anticipated to enhance the solubility selectivity of MMMs. Second, the high aspect ratio of GO nanosheets acted as a selective barrier to render diffusivity selectivity. At last, the ZIF-8@GO exhibited good compatibility with polymer matrix. The as-prepared ZIF-8@GO was investigated by a series of characterization methods e.g. XRD, FTIR, TGA, TEM etc. Then, Pebax/ZIF-8@GO MMMs with different filler loadings were made via solution-casting method. Different characterization techniques had been applied to gain insight into the thermal, mechanical and structural properties of the Pebax/ZIF-8@GO MMMs, and their correlation with the transport properties were analyzed, in view of the potential use of such membrane in CO2 recovery and purification.

2. Experimental 2.1. Material Pebax (grade 2533) copolymer (comprise 80 wt% of poly (tetramethylene oxide) [PTMO] and 20 wt% Nylon-12 [PA12]) was supplied by Arkema. Fake graphite (44 mm average particle diameter) was purchased from Qingdao Jinrilai Graphite Co. Ltd. Zinc

nitrate hexahydrate (Zn(NO3)2
6H2O) and 2-methylimidazole (HMeIM) were purchased from Sigma-Aldrich. Potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (HCl), sulfuric acid (H2SO4, 98 wt%), hydrogen peroxide aqueous solution (H2O2, 30 wt%), n-butanol and methyl alcohol were of analytical grade and purchased from National Pharmaceutical Group Chemical Reagent Co. Ltd. CO2 and N2 were supplied by Wuxi Xinnan Chemical Gas Co. Ltd., China, and were of at least 99.99% purity. Gases were used without further purification. 2.2. Synthesis of ZIF-8 ZIF-8 particles were synthesized as modified literature procedure [19]. Briefly, Zn(NO3)2
6H2O and H-MeIM with a 1:4 mol ratio were dissolved in 50 mL of methyl alcohol, respectively. The zinc nitrate solution was then mixed with the H-MeIM solution under sustained stirring for 12 h at room temperature. After that, stirring was stopped and stood for 24 h at room temperature. Washing with fresh MeOH and centrifugation was repeated three times. The product was dried at 50 °C under reduced pressure. 2.3. Synthesis of ZIF-8@GO GO was synthesized by a modified Hummers method [20]. Flaky graphite (1.25 g) and NaNO3 (0.94 g) were dissolved in concentrated H2SO4 (93.75 mL) with stirring in an ice-water bath. Then KMnO4 (5.63 g) was slowly added to the mixture under vigorous stirring and the mixture was stirred for another 2 h. After keeping vigorous stirring at 25 °C for 4 days, 5 wt% H2SO4 aqueous solution (175 mL) was slowly added to the mixture, and then stirring was continued for 2 h at 98 °C. After the system cooled to 60 °C, 30 wt% H2O2 (7.5 mL) was added, and the mixture was kept stirring for 2 h. Finally, the mixture obtained was centrifuged and washed with a 3 wt% H2SO4/0.5 wt% H2O2 aqueous solution (500 mL) for 20 times. The solids at the bottom were washed with a 3 wt% HCl aqueous (200 mL) and deionized H2O solution (300 mL). The final product was obtained by centrifugation to remove the large and not fully exfoliated parts. The schematic illustration of the preparation of ZIF-8@GO was shown in Scheme 1. To prepare the ZIF-8@GO, 40 mg GO and Zn(NO3)2
6H2O (297 mg, 1.00 mmol) were added into 70 mL MeOH under ultrasound for 8 h, afterwards, 2-methylimidazole (328 mg, 4.65 mmol) was added. After ultrasound at room temperature for 12 h, the resulting dark gray powder was filtered and washed with MeOH thoroughly. Then was collected by filtration

L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

where ρ (g/cm3) was the density of pure or blended membranes, VW (cm3/g) was the van der Waals volume calculated using the group contribution method of Bondi [22].

and finally dried at 100 °C under vacuum for 12 h. 2.4. Membrane preparation Pebax was firstly dissolved at a concentration of 7 wt% in nbutanol for 3 h at 80 °C. Then designated amount of additives which were beforehand sonicated in n-butanol for 30 min was added to the polymer solutions. Finally, the solutions were stirred for another 12 h to ensure homogeneous mixtures. Membranes were prepared by controlled solvent evaporation of the above- mentioned solutions. The polymer solutions were cast onto clean Teflon plates and dried at room temperature for at least 48 h under a fume hood. In order to guarantee a complete removal of n-butanol, they were further dried at vacuum drying oven at 40 °C for another 12 h. The obtained membranes were designated as Pebax/X-Y, where X is ZIF-8@GO, ZIF-8 and GO, Y is the wt% of additives out of the mass of Pebax. The thickness of prepared membranes measured by a digital micrometer (0–1 mm, Guilin Measure & Cutting Tool Co. Ltd.) varied from 55 to 65 mm. 2.5. Nanofiller and membrane characterization The chemical structures of ZIF-8@GO, ZIF-8 and GO were characterized by a FTLA 2000 type Fourier transform infrared (FTIR) spectrometer with scan range of 4000–400 cm  1 and resolution of 1.93 cm  1. Raman spectra were obtained on a confocal microscopic Raman spectrometer (Renishow In-Via, USA) with 532 nm laser light irradiation from 400 to 3000 cm  1 at a duration time of 10 s. The morphologies of ZIF-8@GO, ZIF-8 and GO were observed by a JEM-2100 transmission electron microscopy. Nitrogen physisorption isotherms were measured at 77 K on an automatic volumetric adsorption apparatus (ASAP 2020). Scanning electron micrographs and EDAX images of ZIF-8@GO, Pebax and its corresponding MMMs were performed on a Hitachi S4800 scanning electron microscope (SEM) instrument with an EDAX system (Octane Super). Thermal stability of ZIF-8, ZIF-8@GO and its corresponding MMMs were examined with a METTLER 1/1100SF Thermogravimetric analyzer (TGA). Mechanical property of the membranes was studied using an electronic universal testing machine (Jinan, China). X-ray diffraction measurements were performed using bruker D8-Advance diffractometer with Cu-Kα radiation (λ ¼ 0.15406 nm). Thermal properties of membranes were determined by a Mettler DSC 7 differential scanning calorimeter. Temperature range of  50 to 170 °C with a heating rate of 10 °C/min in nitrogen atmosphere was applied to record DSC thermograms. The densities of prepared membranes were determined by specific pycnometer at 25 °C and three parallel measurements were carried out for each sample. The density was accurate to four decimal places. Prior to density measurement, the films samples were dried in vacuum oven at 25 °C for 3 days. The film density (ρ, g/cm3) was calculated by using the following equation:

m2 ρ0 ρ= m2 + m1 − m3

(1)

where m1 was the weight of pycnometer containing auxiliary liquid (g), m2 was the dry film weight (g), and m3 was the total weight of dry film weight and pycnometer containing auxiliary liquid (g), ρ0 was the density of the auxiliary liquid (g/cm3). Nheptane was used as the auxiliary liquid in our study. The fractional free volume (FFV) (vol%) of the MMMs played an important role in determining their permeability, which was estimated by the following equation [21]:

FFV =1 − 1. 3VW ρ

803

(2)

2.6. Gas sorption and permeability measurements CO2 adsorption isotherms for ZIF-8, ZIF-8@GO and GO were conducted on an adsorption analyzer (V-Sorb 2800TP, Gold APP Instruments Corporation, China) at room temperature. The apparatus used to measure the gas adsorption isotherms was illustrated in Fig. S1. The gas sorption isotherms of the prepared membranes were investigated using the barometric pressure decay method with a dual-volume, comprised of a sample container and a reference volume where the chambers were connected in series. The volumes of both chambers were carefully calibrated before the measurements. The device was placed in a temperature-controlled water bath to minimize the effect of environmental temperature on the gas sorption behavior. In this study, the temperature was maintained at 25 °C. The membranes were introduced into the sample chamber and degassed for more than 3 h to remove all adsorbed species. Feed gas was firstly charged into the reference chamber and then the pressure decay was immediately initiated after opening the valve between the sample and reference chamber. The gas sorption reached equilibrium once no further pressure decay was observed. The concentration of gas adsorbed in prepared membranes was evaluated by using the equation of state and the Soave–Redlich–Kwong (SRK) equation of state [23]. After concentration measurement, solubility can be evaluated by following equation:

S=

C P

(3)

where, S was solubility coefficients and P was the operating pressure. Gas permeability experiments were evaluated for CO2 and N2 using the constant pressure/variable volume method at 25 °C. Fig. S2 showed the schematic representation of the gas permeation equipment. A circular sample with 13.80 cm2 effective areas (A) was cut from membranes samples and placed in the stainless steel membrane module. The gas permeability of membranes was calculated by solution-diffusion mechanism as follows.

P=

F A*Δp/L

(4) 3

where F was the flux (cm (STP)/min), L was membrane thickness (cm), A was the membrane effective area (cm2), ΔP was the pressure difference of membranes on both sides and P was permeability expressed in Barrer. Diffusion coefficients (D) can be also calculated based on solution-diffusion transport mechanism through the membrane, as follows:

D=

P S

(5)

The permeability ratio for gas A and gas B which was called gas pair selectivity, was determined as follows.

αA / B =

PA PB

(6)

where PA and PB were permeability of gas A and gas B, respectively. αA / B was gas selectivity of gas A and gas B. Mixed gas test (CO2/N2 ¼15/85) was conducted with the permeation system (G2-110, i-Sepamate 7800) produced by Labthink (Jinan, China). After installing a membrane in the testing cell, both the permeation system and the membrane were evacuated to

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remove all residual gases. Next, the upstream chamber of the cell was pressurized to 1 bar with the feed gas. The downstream gas permeated through the membrane was swept by He and fed to Gas Chromatograph (GC) to analyze the composition. The permeability of individual component was measured using the flow rate of sweeping gas (He), the composition of permeated gas was calculated by GC, and the properties of membrane such as effective area and thickness. Throughout the whole operation, the temperature of entire system was maintained at 25 °C.

3. Results and discussion 3.1. Nanofiller characterization The sizes and morphologies of GO, ZIF-8, and ZIF-8@GO were characterized by TEM, as shown in Fig. 1. The GO sheet exhibited a typical sheet-shaped morphology with the sizes of 200 nm. Furthermore, the TEM images of GO showed wrinkles and folding on the edges, which was in accordance with the previous report [24]. The as-synthesized ZIF-8 (without GO) exhibited hexagonal morphology with the particle size of 50 nm as shown in Fig. 1b. Similar hexagonal morphology and particle size were also found for ZIF8@GO (Fig. 1c). The size of ZIF-8 in ZIF-8@GO was relatively uniform. Because of the anchoring groups of GO, such as the carboxyl groups, ZIF-8 grew in situ on the surface of GO sheet. In detail, the carboxyl groups could anchor the Zn2 þ ion, and therefore, the GO sheet provided a platform for the nucleation and growth of ZIF-8 [25]. Comparison of X-ray diffraction (XRD) patterns taken from GO, ZIF-8 and ZIF-8/GO were shown in Fig. 2(a). The pristine GO sheets revealed a strong peak at 2θ ¼10.00°, corresponding to the (001) reflection [9]. All diffraction peaks of as-synthesized ZIF-8 were assigned to typical crystal structure of ZIF-8, indicating successful formation of pure-phase ZIF-8 material [19]. The diffraction patterns of the ZIF-8/GO were consistent with that of pure ZIF-8, indicating the existence of well-defined ZIF-8 in as-synthesized ZIF8@GO. This confirmed that the presence of GO did not destroy the formation of the ZIF-8 crystal structure. The EDAX analysis revealed the presence of Zn, O, N and C elements in ZIF-8@GO (Fig. S4 and Table S1). Raman spectroscopy was a useful method to measure the homogeneity and authenticity of the nanocarbon composites [16]. As shown in Fig. 2(b), the Raman spectrum of ZIF8 displayed bands associated with the vibration modes of 2-methylimidazole linker, while GO exhibited bands at 1330 cm  1 and 1590 cm  1 due to the D and G bands respectively. The ZIF-8@GO exhibited bands associated with the ZIF-8 as well as D and G

bands. The FT-IR spectra of GO, ZIF-8 and ZIF-8@GO were shown in Fig. 2(c). In the spectra of ZIF-8, the peaks at 2925 cm  1, attributed to C-H stretching vibration. The CQN stretching vibrations appeared at 1588.1 cm  1 and the C-N stretching vibrations appeared at 1179.5 cm  1. Moreover, the peaks at 754.8 and 699.1 cm  1 were assigned to Zn-O and Zn-N of ZIF-8, respectively [25]. In the spectra of GO, the spectra around 3390.7 cm  1 and 1735.1 cm  1 were assigned to O-H stretching vibration and CQO stretching vibration of carboxyl group in the GO, respectively [25]. However, in the spectra of ZIF-8@GO, the intensity of the peak at 1735.1 cm  1 decreased, indicating that the carboxyl groups of GO interacted with Zn2 þ and further confirming the successful growth of ZIF-8 on the GO sheet [26]. Thermogravimetric analysis (TGA) of ZIF-8 and ZIF-8@GO was conducted and the resultant curves were shown in Fig. 2(d). ZIF-8 revealed a little weight loss (6%) before 270 °C, which was ascribed to the removal of residual solvent (e.g., methanol) or some guest molecules (e.g., Hmim) in ZIF-8 [27]. A sharp weight loss appeared between 350 °C and 550 °C, indicating the decomposition of the organic linker molecules and the collapse of the ZIF-8 structure [27]. Compared with ZIF-8, decomposition of ZIF-8@GO started at 200 °C, which has previously been assigned to removal of the functional groups of GO sheets [14]. As the final white solid residue (37.52%) in ZIF-8/ GO after 700 °C calcination in O2 was identified as ZnO from ZIF-8 structure and GO had been burned off, the content of ZIF-8 and GO in the composites can be calculated from the residual weight percentages (detailed calculations were shown in SI). The actual percentage of ZIF-8 in ZIF-8@GO is calculated to be 69.8 wt%. 3.2. Membrane characterization Membrane cross section morphology and the particle-polymer interface were investigated by SEM, as shown in Fig. 3. Pure Pebax membrane (shown in Fig. S5(a)) showed a homogeneous and smooth morphology without any noticeable defects, which was characteristic of a dense Pebax membrane. For Pebax/ZIF-8@GO MMMs, the ZIF-8@GO particles homogeneously distributed in the Pebax matrix as the ZIF-8@GO content was 2–6 wt%. From the high magnification image (Fig. S5(b)), it can be found that ZIF-8@GO was wrapped tightly by the Pebax matrix on 6 wt% Pebax/ZIF8@GO MMMs. No visible voids were observed, suggesting the well adhesion between polymer and ZIF-8@GO fillers, which was most probably because of the optimized mixing protocol for membrane preparation and good affinity of the ZIF-8@GO to the hydrophilic Pebax chains. However, as ZIF-8@GO loading exceeded 8 wt%, it was difficult to achieve a well dispersion of fillers in the polymer matrix and appeared slight agglomeration, which was possibly

Fig. 1. TEM images of GO (a), ZIF-8 (b) and ZIF-8@GO (c).

L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

805

GO

GO ZIF-8@GO

ZIF-8 ZIF-8

ZIF-8@GO

5

10

15

20

25 30 2θ/degree

35

40

45

200 400 600 800 1000 1200 1400 1600 1800 2000 Raman Shift

50

Weight/%

1 1735.1 3390.7

754.8 1588.1

ZIF-8@GO O ZIF-8 GO

4000

3500

2925.7

30 000

2500

2000

1500

1179.1 6999.1

1000

500

1110 1000 90 9 80 8 70 7 60 6 50 5 40 4 30 3 20 2 10 1 0

48.87% %

62.48%

ZIF-8@G GO ZIF-8

100

200

300

400

500

600

700

Wavenumber Fig. 2. XRD patterns (a), Raman spectra (b), FT-IR spectra (c) and TGA under O2 (d) of GO, ZIF-8 and ZIF-8@GO.

due to force of gravity and the quite different physical property between ZIF-8@GO and polymer (Fig. 3(d)). Furthermore, through the comparison of SEM images of the different type of MMMs containing 6 wt% fillers (Fig. 3(c), (e) and (f)), it can be clearly seen that incorporation of ZIF-8@GO could obviously improve the

interface compatibility between fillers and the Pebax matrix. This would help to improve the CO2/N2 selectivity, which would be discussed in detail in gas permeation properties. Fig. 4 showed the thermal stability of membranes. The pure Pebax had a major weight loss stage starting from approximately

Fig. 3. SEM images of ZIF-8@GO MMMs, GO MMMs and ZIF-8 MMMs.

L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

endo

806

100

Pebax/ZIF-8@GO-8

Pebax/ZIF-8@GO-6

Pebax/ZIF-8@GO-6 Pebax/GO-6 Pebax/ZIF-8-6 Pebax

60

Heat flow (mw/mg)

Weight (%)

80

40 20

Pebax/ZIF-8@GO-4 Pebax/ZIF-8@GO-2 Pebax2533 Pebax/ZIF-8-6 Pebax/GO-6

0 100

200

300 Temperature

400

500

600

-40 -20

o

C

0

20

40

60

80 100 120 140 160

Temperature o

Fig. 4. TGA curves of Pebax control and MMMs.

Fig. 6. DSC curves of Pebax control and MMMs.

320 °C, which was attributed to the decomposition of the polymer main chains. Compared with the pure Pebax membrane, the MMMs displayed higher thermal-decomposition temperature except Pebax/GO MMMs, which suggested that incorporation of ZIF8 and ZIF-8@GO improved the thermal stability of composite membranes. The increased decomposition temperature was mainly because the presence of ZIF-8 and ZIF-8@GO in MMMs limited the thermal motion of polymer. This limited motion enhanced the energy needed for the movement and segmentation of polymer chains, which ultimately increased the thermal stability of the membranes [28]. The Pebax/GO MMMs had two major weight loss stages. The first weight loss in the temperature range of 180–250 °C was caused by the decomposition of GO [14]. The second weight loss starting from approximately 320 °C was attributed to the decomposition of the polymer main chains, which was similar to pure Pebax membrane. To investigate the influence of fillers on the arrangement of polymer chains, the crystalline structure of MMMs was studied by XRD. Fig. 5 showed the XRD patterns of MMMs doped with ZIF-8, ZIF-8@GO and GO. MMMs displayed a strong and broad peak at diffraction angles (2θ) of 20.0°, which was a characteristic peak of Pebax. The characteristic peaks belong to ZIF-8 and ZIF-8@GO could be found in their MMMs, indicating that membrane preparation procedure did not destroy the fillers's crystallinities. MMMs doped with GO displayed a peak at a 2θ of 6.4°, indicating that some GO nanosheets retained their stack structure and were not homogeneously dispersed into the polymer matrix [9,29]. Compared with Pebax/GO MMMs, no characteristic peak of GO

Pebax/ZIF-8-6

Intensity

Pebax/GO-6 Pebax/ZIF-8@GO-8 Pebax/ZIF-8@GO-6 Pebax/ZIF-8@GO-4 Pebax/ZIF-8@GO-2 Pebax

5

10 15 20 25 30 35 40 45 50 55 60 ο 2θ ( ) Fig. 5. XRD patterns of Pebax control and MMMs.

was observed in Pebax/ZIF-8@GO MMMs, indicating that GO sheets were fully exfoliated into individual nanosheets and dispersed in the polymer matrix homogeneously. This result confirmed that the presence of ZIF-8 was helpful to exfoliate GO nanosheets in the polymer matrix. The XRD pattern of pure Pebax2533 membrane showed a d-spacing of 0.427 nm by using Bragg’s law, which was in agreement with that reported in the literatures for Pebax2533 [30]. It can be seen that the d-spacing of MMMs did not change distinctly with the addition of ZIF-8@GO and ZIF-8. However, compared with pure Pebax membrane, the intensity of these peaks in the MMMs showed broadening and decline, indicating the enhancement of the amorphous region of MMMs. Such results were ascribed to the presence of inorganic fillers, which disrupted the ordered packing of Pebax chains and then decreased the crystallization of the backbone, which would improve the gas permeability of the membrane [31]. To further confirm the effects of different fillers on the crystallinity of their corresponding membranes. DSC test was conducted to provide detailed crystallinity information about membranes. Fig. 6 showed DSC curves of pure Pebax membrane and its corresponding MMMs. In order to remove effects of less favorable crystal phases formed during the preparation procedure, the second heating run was regarded as the base of the studies. For the pure Pebax membrane, two characteristic Tm were present at 9.1 °C and 137.9 °C, which was in consistent with the microphase separated structure in block copolymers. The low Tm was ascribed to the melt of PTMO crystals and the high Tm values was corresponding to the melt of PA crystalline phase. From DSC experiment, the heat of fusion (ΔHm) and the degree of crystallization (fc) of all membranes were obtained, which were listed in Table 1. The degree of crystallinity of both the PA and PEO segments was calculated as follows:

fc =

0 ΔHm w ΔHm

(7)

where ΔHm was the enthalpy of formation of the crystalline PEO or PA phase (J/g), ΔHm0 was the enthalpy of formation of the pure crystal (for PEO 196.6 J/g and for PA 246 J/g) [32], and w was the weight percent PEO or PA present in the blend. Regarding Jansen's study for the pure Pebax2533 membrane, the crystallinity for PEO and PA segments were approximately 14.43% and 12.60%, respectively [33], which were in conformity with our results (13.27 for PA segments and 15.49 for PEO segments). As shown in Table 1, all MMMs displayed a lower degree of crystallinity (fc) than that of pure Pebax membrane, indicating that the ZIF-8@GO, GO and ZIF-8 were added to weaken the force

L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

807

Table 1 Density, FFV, heat of fusion (ΔHm) and degree of crystallization (fc) of Pebax and its corresponding MMMs. Sample

ΔHm,

Pebax2533 Pebax/ZIF-8@GO-2 Pebax/ZIF-8@GO-4 Pebax/ZIF-8@GO-6 Pebax/ZIF-8@GO-8 Pebax/ZIF-8-6 Pebax/GO-6

24.36 19.03 16.43 15.89 17.79 16.69 16.93

PEO

(J g  1)

ΔHm,

PA

(J g  1)

6.53 6.34 6.09 5.42 5.28 6.12 5.81

Table 2 Mechanical properties of Pebax and its corresponding MMMs. Membrane

Tensile strength (MPa)

Young's modulus

Elongation at break (%)

Pebax Pebax/ZIF-8@GO-2 Pebax/ZIF-8@GO-4 Pebax/ZIF-8@GO-6 Pebax/ZIF-8@GO-8 Pebax/ZIF-8-6 Pebax/GO-6

21.43 18.57 17.99 17.66 16.27 18.80 15.67

1.16 1.33 1.34 1.58 1.50 1.41 1.31

1849.5 1393.1 1341.0 1118.2 1082.6 1330.3 1200.2

between the molecular chains and interrupt the ordered chain packing of the polymer, inducing the smaller crystallization of the polymer chains. This result was in good accordance with the aforementioned XRD analysis. The decreased crystallinity of membranes is good for gas transport, as the smaller the crystallinity in the polymer matrix, the easier the penetrant diffusion is. In order to research the effect of fillers on mechanical

fc,

PEO

15.49 12.34 10.88 10.74 10.22 11.29 11.45

(%)

fc,

PA

(%)

13.27 13.14 12.89 11.72 11.66 13.23 12.56

Density (g/cm3)

FFV (vol%)

1.041 0.915 0.874 0.823 0.843 0.886 0.989

12.56 23.14 26.56 30.89 29.16 25.55 16.89

properties, the tensile strength, Young's modulus and maximum deformation of Pebax membrane and its corresponding MMMs were measured, as shown in Table 2. All membranes were obtained as flexible dense membranes with sufficient mechanical resistance to be handled without difficulties. It can be clearly seen that all MMMs had a higher Young’s modulus than the pure Peabx membrane. Furthermore, with the ZIF-8@GO content from 2 wt% to 6 wt%, a rising trends of Young's modulus were observed. These phenomena indicated a good dispersion of ZIF-8@GO throughout the membrane as well as a good interaction between the ZIF8@GO and polymer matrix. However, at 8 wt%, the Young's Modulus decreased, which was due to less interfacial interaction between the polymer and ZIF-8@GO resulting from the aggregation of the particles as shown in Fig. 3(d). Moreover, compared with Pebax/GO-6 and Pebax/ZIF-8-6 membranes, the Pebax/ZIF-8@GO6 membrane showed increased Young's modulus, which was mainly due to the increased interfacial interaction between ZIF8@GO and Pebax.

Fig. 7. Sorption isotherms of CO2 and N2 on pure Pebax and MMMs at 25 °C.

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L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

3.3. Gas sorption

250

60

3.4. Gas separation performance of Pebax and its MMMs Permeability of pure gas (CO2 and N2) was investigated for the Pebax and its MMMs at1 bar and 25 °C. It can be seen that the presence of ZIF-8@GO strongly affected the transport properties of the membranes. As shown in Fig. 8, the permeability of CO2 increased gradually when the filler content in membranes rose, while the permeability of N2 slightly increased. For example, a 300

60 PCO2 αCO2/N2

50

PN2

200 40 150 100

30

15 10 5 0

20 10 0

2

4 6 Filler content (wt%)

8

Fig. 8. Gas separation performance of Pebax/ZIF-8@GO MMMs.

CO2/N2 selectivity

Gas permeability (Barrer)

250

PN2

200

50

αCO2/N2

40 150 30 100 20 15 10 5 0 Pe

CO2/N2 selectivity

Fig. 7 showed the sorption isotherms of CO2 and N2 in the neat and MMMs at 25 °C and different pressures from 10 to 100 KPa. It can be seen that the both CO2 and N2 sorption uptake increased with the increasing pressure. Both the isotherms of CO2 and N2 were linear with respect to the gas phase pressure. Analogous phenomenon was reported by Merkel et al. when they studied the sorption of hydrocarbons or other organic vapors in rubbery polymers [34]. Their studies indicated that at relatively low pressure, the sorption isotherms were almost linear, which approximately complied with the Henry's law. In this case, the pressure had little influence on the gas solubility. Compared with pure Pebax membrane, filler addition led to more CO2 sorption in the membranes. This was due to the followed three reasons. First, in pure Pebax membrane, dense polymeric chains were entangled with each other, which made CO2 gas unable to diffuse and solubilize easily in membrane, causing low CO2 sorption. Second, it was well known that ZIF-8 had high CO2 adsorption due to high surface area and ultra-microporosity. Therefore, incorporation of ZIF-8 and ZIF-8@GO led to high CO2 sorption (shown in Fig. S6(b)). On the other hand, different polar functional groups of GO provided specific interaction with CO2, which enhanced CO2 sorption [15]. These experimental findings were also supported by previously published results in which MOF@GO had larger CO2 storage capacity than that of MOF [15–18]. This reason also explained the phenomenon that Pebax/ZIF-8@GO membranes had larger CO2 sorption than Pebax/ZIF-8 MMMs and Pebax/GO MMMs (Fig. 7c). Finally, addition of ZIF-8, GO and ZIF-8@GO could increase the free volume of membranes (shown in Table 1), which was helpful in diffusion and sorption of CO2 in polymeric matrix. Although the free volumes of Pebax/ZIF-8@GO MMMs were decreased at high filler content (8 wt%), the degree of increase of CO2 sorption caused by increased ZIF-8@GO content was bigger than the degree of decrease of CO2 sorption caused by decreased free volume, which ultimately enhanced the CO2 sorption. What’s more, CO2 showed a higher sorption than N2, which was due to multiple effects of high critical temperature (Tc) and condensability of CO2 (Tc-CO2: 304.2 K and Tc-N2: 126.1 K) and favorable quadrupolar interactions of CO2 [35].

Gas permeability (Barrer)

PCO2

10 0 b

2 ax

53

Pe

3

ba

I x /Z

F-

8@

GO

6 b Pe

ax

/Z

I

8F-

6 Pe

ba

x/

GO

6

Fig. 9. Gas separation performance of Pebax and the MMMs with different fillers.

loading at 6 wt% the CO2 permeability of ZIF-8@GO-based MMMs increased by a factor up to 1.9 as compared with that of the pure polymeric membrane. The permeability enhancement of MMMs can be attributed to the following reasons. First, the high porosity and flexible framework of ZIF-8 facilitated CO2 diffusion, and thus enhanced the CO2 permeability [36,37]. Second, polar functional groups of GO provided specific interaction with CO2, facilitating CO2 transport. Last but not least, the disrupted arrangement of polymer chain packing reduced the crystallization of MMMs (shown in Table 1), making it easier to pass gas molecule through the membranes. Finally, the larger free volume introduced by the incorporation of ZIF-8@GO also facilitated transport of CO2 in the membranes. Although the last two reasons also improved the N2 permeability, this increase was smaller than that of CO2 permeability due to larger kinetic diameter of N2 (3.6 Å) than that of CO2 (3.3 Å). Meanwhile, the first two reasons only selectively enhanced the CO2 permeability. As a result, a more profound increase in CO2 permeability was observed when compared with the N2. As shown in Fig. 9, the Pebax/ZIF-8@GO MMMs had a higher CO2 permeability than Pebax/ZIF-8 MMMs and Pebax/GO MMMs (Fig. 9), which was due to multiple effects of higher free volumes, higher CO2 adsorption and lower crystallinity. Furthermore, the lower gas permeability of Pebax/GO MMMs than pure Pebax membrane was mainly due to the increased tortuosity of diffusion paths for gas molecules, leads to reduced gas diffusion [6,9,29]. In contrast, the CO2/N2 selectivity of the MMMs increased firstly and then decreased slightly with the ZIF-8@GO content. The decreased CO2/N2 selectivity at high filler content (8 wt%) was probably due to an occurrence of some cracks or big interfacial holes between ZIF-8@GO and Pebax matrix, the reason of which was supported by the decline in Young's modulus (shown in Table 2) [38]. Similar observations of reduced selectivity by incorporating functionalized GO into polymer matrix have also been found previously [9]. Additionally, as shown in Fig. 9, a higher CO2/N2 selectivity was achieved in Pebax/ZIF-8@GO MMMs at the same filler content. We thought it was mainly due to the synergistic effect of ZIF-8 and GO. That is, the high porosity and flexible framework of the ZIF-8 could enhance the CO2/N2 solubility selectivity. The high aspect ratio of GO sheet led to highly tortuous diffusion paths for larger molecules in the polymer matrix, which increased the CO2/N2 diffusivity selectivity. To further confirm our hypothesis, the diffusivity coefficient (D) and the solubility coefficient (S) of membranes were measured according to Eqs. (3) and (5), as shown in Table 3. It can be seen that both CO2 and N2 diffusivity coefficients increased firstly and then decreased slightly with the ZIF-8@GO content. The increased

L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

matrix [35].

Table 3 Gas diffusivity and solubility coefficients of the membranes. Membrane

DCO2a

DN2a

SCO2b

SN2b

DCO2/DN2

SCO2/SN2

Pebax Pebax/ZIF-8@GO-2 Pebax/ZIF-8@GO-4 Pebax/ZIF-8@GO-6 Pebax/ZIF-8@GO-8 Pebax/ZIF-8-6 Pebax/GO-6

57.69 60.87 84.24 90.31 84.50 70.59 49.98

50.25 50.74 51.15 51.31 50.62 50.54 39.94

2.27 2.41 2.53 2.76 3.05 2.55 2.41

0.095 0.098 0.100 0.102 0.118 0.101 0.097

1.15 1.19 1.64 1.76 1.67 1.39 1.25

23.89 24.59 25.30 27.06 25.85 25.25 24.84

a b

CO2 and N2 diffusivity coefficient at ZIF-8@GO loading from 2 wt% to 6 wt% was attributed to the following two reasons. On the one hand, the larger free volumes after addition of ZIF-8@GO caused higher the diffusion of gases. On the other hand, based on the DSC results, the addition of ZIF-8@GO led to a lower crystallinity of MMMs, which resulted in more permeable amorphous phase in polymer matrix, thus the diffusion of gases increased. The decreased CO2 and N2 diffusivity coefficient at high ZIF-8@GO loading (8 wt%) was mainly due to the decline of free volume and increased density of MMMs (Table 1). Compared with the Pebax/ZIF8 MMMs and Pebax/GO MMMs, the Pebax/ZIF-8@GO MMMs showed a higher CO2 diffusivity coefficient, which mainly because of multiple effects of higher free volumes, lower crystallinity and specific interaction with CO2. Furthermore, the CO2/N2 diffusivity selectivities of Pebax/ZIF-8@GO MMMs were bigger than that of pure Pebax membrane and Pebax/ZIF-8 MMMs, which was because the layer structures of GO sheet led to the highly tortuous diffusion path in the polymer matrix, restricting the diffusion of larger molecules (N2, 3.6 Å) and favoring the diffusion of small molecules (CO2, 3.3 Å) with less resistance, improving CO2/N2 diffusivity selectivity. In contrast with the diffusivity, all MMMs showed an increased solubility with increasing filler loading and the solubility coefficient of CO2 increased more quickly than solubility coefficient of N2. The CO2 solubility coefficient increased from 2.27  10  2 cm3(STP)/(cm3cmHg) for pure Pebax to 3.05  10  2 cm3(STP)/(cm3cmHg) for Pebax/ZIF-8@GO MMMs at the loading of 8 wt%. The increased CO2 solubility coefficient was mainly ascribed to increased CO2 sorption of MMMs (shown in Fig. 7). The higher solubility coefficient of CO2 than that of N2 was due to their relative condensability, which was characterized by critical temperature. The critical temperatures of gases decreased in the following order: CO2 (304.2 K) 4N2 (126.1 K). The higher condensability had the higher solubility of the gas in the polymer

P = P0 exp ( − Ep/RT )

3.6. Mixed gas separation performance To evaluate the practical separation performance of membranes, binary gas mixtures (CO2/N2 ¼15/85 vol%) were tested. Fig. 11 and Fig. S7 illustrated pure and mixed gas permeation of CO2/N2 pairs through the pure Pebax, Pebax/ZIF-8, Pebax/GO and Pebax/ZIF-8@GO MMMs. As can be seen, the trends of both the mixed CO2 and N2 permeability were similar to that of pure CO2 and N2 permeability in all membranes. But the mixed CO2 permeability was slightly lower than the pure CO2 permeability. In contrast, the mixed N2 permeability improved in comparison with the pure N2 permeability. The observed decrease in the mixed CO2 permeability was due to the prevented plasticization effect of CO2 because of the presence of N2 in the feed gas, while the enhancement in the mixed N2 permeability was ascribed to the plasticization effect of CO2 [40]. In addition, the mixed gas CO2/N2 selectivities of all membranes were lower than their corresponding ideal selectivities. This decline in selectivity was due to competitive adsorption in separating gas mixtures [6,9,29]. The influence of feed gas pressure was studied by using Pebax and Pebax/ZIF-8@GO-6 membranes for CO2/N2 (15/85 vol%) mixed gas separation and shown in Fig. S8. With increasing feed pressure, pure Pebax and Pebax/ZIF-8@GO-6 membranes showed an

6.0

56

5.8

52

5.6

48

5.4 5.2 5.0

4.6 4.4 4.2 3.00

Pebax/ZIF-8@GO-6 Pebax/ZIF-8-6 Pebax Pebax/GO-6

3.05

3.10

3.15 3.20

(10)

where P was the permeability of the gas, P0 was the pre-exponential factor, R was the gas constant and T was the absolute temperature. The incorporation of filler in the MMMs decreased Ep of CO2 from 12.85 kJ/mol for pure Pebax membrane to 8.79, 8.21 and 4.64 kJ/mol for Pebax/GO-6, Pebax/ZIF-8-6 and Pebax/ZIF8@GO-6 MMMs, respectively. The activation energies of permeability (Ep) obviously decreased for the MMMs at the loading of 6 wt% in comparison with pure Pebax membrane.

CO2/N2 selectivity

ln PCO2(Barrer)

3.5. Effect of operating temperature Fig. 10 revealed the effect of temperature on CO2 separation performance for MMMs. The CO2 permeability increased with the increase of temperature, while the CO2/N2 selectivity decreased. The higher temperature resulted in the rise in CO2 permeability, which was due to the increased chains flexibility and increased gas diffusivity [39]. The effect of temperature could be further elaborated by using the Arrhenius equation which related the gas permeability to the operating temperature via the activation energy of permeability (Ep) as expressed by the following:

Diffusivity coefficient [cm2/s]  108. Solubility coefficient [cm3(STP) per cm3 per cm Hg]  102.

4.8

809

44

Pebax/ZIF-8@GO-6 Pebax/ZIF-8-6 Pebax/GO-6 Pebax

40 36 32 28 24 20

3.25 -1

3.30

3.35

3.40

16 3.00

3.05

3.10

3.15

3.20 1000/T

3.25

3.30

3.35

3.40

-1

Fig. 10. Effect of operating temperature on (a) LnPCO2 (Barrer) and (b) CO2/N2 selectivity. Permeation tests were performed at 1 bar feed pressure.

810

L. Dong et al. / Journal of Membrane Science 520 (2016) 801–811

50

4. Conclusion

200

45

150

40

In summary, we reported high-performance MMMs for efficient separation of CO2/N2 by taking advantage of novel ZIF-8@GO composite filler. The synthesized ZIF-8@GO fillers showed good adhesion with the polymer matrix. Compared with the pure polymer membranes, these novel MMMs effectively improved the permeability and selectivity of CO2. The CO2 permeability reached 249 Barrer with the CO2/N2 selectivity of 47.6 at 25 °C with the feed pressure of 0.1 MPa, which was much closer to Robeson upper bound. ZIF-8 and GO showed a favorable synergistic effect on the gas separation performance. The presence of ZIF-8 enhanced the gas solubility selectivity. Simultaneously, the presence of GO nanosheets improved the gas diffusivity selectivity in MMMs. We believe that our strategy may provide a more effective way to further improve MMMs performance through the modification of ZIFs on existing fillers which have larger adsorption differences to specific gases.

100 35

50 6

30

5

25

4

0

2

4 6 Filler content (wt%)

8

CO2/N2 selectivity

Gas permeability (Barrer)

250

20

Fig. 11. Pure (the dark dotted lines) and mixed (the blue solid lines) permeability and permeation selectivity of CO2/N2 at 25 °C (squares: CO2 permeability. Circle: N2 permeability. Triangles: CO2/N2 selectivity). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

100 80

CO2/N2 selectivity

60 40

Pebax Pebax/ZIF-8@GO-2 Pebax/ZIF-8@GO-4 Pebax/ZIF-8@GO-6 Pebax/ZIF-8@GO-8

Acknowledgment

2008 Robeson upper

This study was supported by the National Natural Science Foundation of China (Nos. 51173072 and 21571084), the Fundamental Research Funds for the Central Universities (JUSRP51408B), and MOE & SAFEA for the 111 Project (B13025).

Appendix A. Supplementary material

20

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2016.08. 043.

10

100 CO2 permeability (Barrer)

1000

Fig. 12. Robeson plot for CO2/N2 upon variation of the ZIF-8@GO addition in the membranes.

increase in CO2 and N2 permeability and a decrease in CO2/N2 selectivity. The enhanced CO2 and N2 permeability can be attributed to CO2-induced plasticization, which caused the polymer matrix to swell, increased the FFV and increased gas diffusivity [41]. The reduction in CO2/N2 selectivity was because the change in diffusivity is greater for the lighter gas molecules (i.e., N2). A 100 h long-term stability experiment of the Pebax/ZIF-8@GO6 MMMs for CO2/N2 was shown in Fig. S9. During the entire test period, the CO2 permeability and the selectivity of CO2/N2 kept stable, indicating good operation stability. Fig. 12 plotted the separation performance against the Robeson upper bound 2008 as a function of ZIF-8@GO loading [42]. Although the gas separation performance of the Pebax/ZIF-8@GO MMMs in current study was much closer to the upper bound, the overall performance of the MMMs prepared did not overcome the Robeson’s upper bound. We attributed it to the relatively low separation performance of pure Pebax membrane. Recently, our lab fabricated a high performance and CO2-philic, comb copolymer composite membranes consisting of poly(ethylene glycol) methyl ether methacrylate (PEGMA), polymethyl methacrylate (PMMA) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with the CO2 permeability of 308 Barrer and CO2/N2 selectivity of 38 at 0.1 MPa, which was larger than those of Pebax membranes (Fig. S10, detailed information is spelled out in another article being revised). Therefore, we believed that incorporation of ZIF8@GO into this comb copolymer would effectively improve CO2/N2 separation performance, even would surpass the upper bound.

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