Development of dual layer of ZIF-8/PEBAX-2533 mixed matrixmembrane for CO2 capture

Vajiheh Nafisi, May-Britt Hägg n

Department of Chemical Engineering, Faculty of Natural Science and Technology, Norwegian University of Science and Technology (NTNU),NO-7491 Trondheim, Norway

a r t i c l e i n f o

Article history:Received 17 December 2013Received in revised form30 January 2014Accepted 1 February 2014Available online 19 February 2014

Keywords:Dual layer mixed matrix membraneZIF-8MOFCO2 capturePEBAX-2533

a b s t r a c t

A new kind of self-supported dual layer mixed matrix membrane was developed in this work using ZIF-8as inorganic filler in PEBAX-2533 polymer matrix. The developed dual layer flat sheet mixed matrixmembrane was characterized to investigate the morphology of organic-based and inorganic-based layer.The gas separation properties of the mixed matrix membranes were tested using single gases CO2, CH4,N2, and O2 and a mixture of CO2 and N2 in dry and humidified conditions. The permeability of allexamined gases increased as the inorganic filler content increased in the matrix membranes, andspecifically it increased dramatically for CO2 in all cases, single feed gas and dry and humidified mixedgas. The CO2/N2 selectivity decreased slightly from 33.8 for the pure PEBAX membrane to 32.3 for themixed matrix membrane with 35% ZIF-8 loading, while more significant drop in CO2/N2 selectivity wasobserved in experiments using mixed gases.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The effect of CO2 concentration increment on global warming iscommonly accepted [1]. It is predicted that the concentration ofCO2 will double around 2050, if no special action is taken. Differentoptions are being considered to combat this global problem suchas improved energy efficiency, renewable energy and CO2 captureand storage (CCS) [2].

Among the different methods discussed for CO2 capture,membrane processes have received increased attention due totheir advantages such as small footprint, high energy efficiencyand environmental sustainability [3]. Despite all the investigationsand studies, polymeric membrane separation is still restricted bythe famous trade-off between gas permeability and selectivity, asintroduced by Robeson [4]. Basically this means that any mod-ification of the membrane material which leads to improvement inpermeability, will lead to decrease in selectivity and vice versa [5].

Inorganic membrane materials show high gas perm-selectivityon laboratory scale, however because of their expense and fragilestructures they are difficult to fabricate on a larger scale [6]. Thecombination of polymeric membrane materials and inorganicparticles, which is called mixed matrix membrane (MMM), have

been introduced and investigated for gas separation to cross to theupper bound trade-off [7]. For instance, adding inorganic particleto polymer membranes shows improvement in gas permeabilitieswhile it shows similar or even improved gas selectivities com-pared to the corresponding pure polymer membranes [8–13].

During the last decades, mixed matrix membranes (MMM) forgas separation have been investigated with different kinds ofinorganic fillers [14–18]. Fillers such as zeolites [17,19], carbonmolecular sieves [15], nano-sized inorganic particles like silica andTiO2 [8,20–23] or specific organic compounds like polyethyleneglycol have been used in different polymeric matrices to developnew MMMs and improve their permeation properties. Chemicalmodification of inorganic fillers has likewise been used in order toimprove not only dispersion of inorganic fillers, but also theseparation properties [24–26].

Ethylene oxide (EO) units have this ability to achieve highpermeability with a reasonable high CO2/light gas selectivity dueto its polar ether group which has high affinity to CO2 [27].However, because of its semi-crystalline structure and its weakmechanical strength, its application has been restricted [28].Recently, PEO has been considered in different works due to itsfeasibility to make membranes for CO2 separation [27–34]. Toovercome the drawback of PEO and improve gas separationproperties, some strategies have been investigated such as incor-poration of PEO with other monomers as copolymers or aspolymer blends possibly with crosslinking [28,32].

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Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.02.0020376-7388 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ47 73594033; fax: þ47 73594080.E-mail address: may-britt.hagg@chemeng.ntnu.no (M.-B. Hägg).

Journal of Membrane Science 459 (2014) 244–255

Polyether block polyamide (PEBA) resin; a thermoplastic elas-tomer made of rigid polyamide segments and flexible polyethersegments, is best known under the trademark PEBAX. PEBAX blockcopolymers, which are synthesized from dicarboxylic acid termi-nated aliphatic polyamides and polyoxyalkene glycols, have beeninvestigated for gas separation properties and have shown a nicepotential for CO2 separation [35]. Gas pass through the softamorphous polyether block and the mechanical stability of themembrane material is provided by the hard polyamide phase[35,36].

The structure of the PEBA repeating unit is:

where PE is a soft segment which is an amorphous polyether,either poly(ethylene oxide) or poly(tetramethylene oxide), and PAor hard segment is an aliphatic polyamide. A blend of properties ofthermoplastics and rubbers are provided by this crystalline/amor-phous structure – this is more closely discussed in Section 4. Forinstance these materials show two Tg points corresponding to bothPE and PA segments.

PEBAX material has been investigated in different membranesfor gas separations. Rezac et al. has investigated the PEBAX seriesinclude Nylon-12 as hard segment and polyetetramethylene oxideas soft segment in different ratios. Four different grades of PEBAX(2533, 3533, 5533, 6333) were investigated and among themPEBAX-2533 showed promising high permeation rates [37]. Inthe current work MMMs consisting of PEBAX and ZIF-8 particleswere prepared and investigated for their separation propertieswhere CO2 is present in the gas stream – these materials aretherefore further reviewed in Section 2.

2. Background

Potreck et al. has investigated the transport behavior of PEBAX-1074 as membrane material for water removal from light gases.As water vapor activity increases, the water vapor permeabilityincreases exponentially whereas the nitrogen permeability slightlydecreases. Consequently, the water over nitrogen selectivityincreases with increasing water vapor activity [38]. Reijerkerket al. reported gas separation properties of blend of PEBAX MH-1657 and a poly ethylene glycol (PEG) base additive consist of 80%PEG and 20% of poly dimethyl siloxane (PDMS) [30]. Kim et al.reported strong affinity of polar gases to the PE block which causehigh permeability and permselectivity of gases like CO2 and SO2

through the PEBAX membranes [13].Bondar et al. investigated four grades of PEBAX (2533, 4011,

1074 and 4033) for gas separation properties. The permeation andsorption results showed strong interaction between soft segment,PE block, and CO2 gas [35], while Chen et al. investigated the gasseparation properties of PEBAX-2533 for different organic gas suchas ethane, ethylene, propane and propylene at different tempera-ture and pressures. PEBAX-2533 shows high hydrocarbon perme-ability. Permeation properties obtained with propane andpropylene indicated strong plasticization effects on the PEBAX-2533 polymer at higher pressure [39].

Barbi et al. studied the nanostructure of different PEBAXmembranes material (PEBAX-2533, 3533 and 4533) and foundthat diameters of the hard domains are almost the same for all thestudied materials, however a considerable increase of the softdomain size causes improvement in gas transport [36]. Schmidtet al. evaluated the gas permeation properties of two differentgrades of PEBAX (1074 and 2533) prepared on a support layer for aspecial application. Their result shows the PEBAX grade 2533

shows better permeability while the other grade shows betterCO2 over O2 selectivity [40].

Recently metal organic frameworks (MOFs), a new class ofporous materials, have received interest and gained rapid devel-opment. The MOFs are built up of metal ions or metal ion clusterslinked to multidentate organic ligands [41]. Recent investigationsof MOFs show promising application as drug delivery carrier,adsorbent for separation and as catalysts [42–45]. Due to theirsurface area [46], there is a potential to control porosity andaffinity towards certain gases [47,48]. Some MOFs have beenevaluated for gas separation properties. The sorption selectivityand CO2 uptake is much higher compared to corresponding valuesof zeolites [48].

Among the zeolitic imidazolate frameworks (ZIFs), a newsubclass of MOFs, there are some ZIFs that show high thermaland chemical stability [49,50]. Due to their organic linkage, thesemicro-porous materials, MOFs and ZIFs, are of potential use inMMMs with particular interest in applications for such as CO2

capture [16,51,52]. In this regard, ZIF-8 having formula Zn(methy-limidozolate)2, is not only highly stable, but it also has a pore size3.4 Å due to the narrow size of six membered ring, thus it is a goodcandidate for CO2 capture with a L–J diameter of 3.3 Å [53,54]. Buxet al. have investigated ZIF-8 membrane separation properties fordifferent gases such as H2, CO2, O2, N2 and CH4. The volumetricflow rates of the single before mentioned gases and of a 1:1mixture of H2 and CH4 were measured. In Fig. 1, the calculatedpermeance of various gases are shown as a function of Kineticdiameter of the gases [53].

Chmelik et al. has investigated the potential of ZIF-8 for CO2

capture. In their study, the uptake of CO2/CH4 mixture diffusion inZIF-8 was monitored by infra-red microscopy (IRM). Their researchshowed that due to CH4 molecules in ZIF-8 are hindered by CO2

molecules that are preferentially located at the cage region. It isexpected that this hindering effect can be advantageous in CO2

capture applications [55].Ordonez et al. developed MMMs of matrimid and ZIF-8. The

permeability properties of the ZIF-8/Matrimids MMMs weretested in their research for H2, CO2, O2, N2, CH4, C3H8, and gasmixtures of H2/CO2 and CO2/CH4. The permeability valuesincreased as the ZIF-8 loading increased. However, at higherloadings of ZIF-8 the permeability decreased for all gases whilethe selectivities increased consistently. Both the ability of the ZIF-8material to selectively transport H2 and CO2 molecules and its hightemperatures resistance makes it a promising material for gasseparation processes at higher pressures and temperatures [54].

In this work MMMs of PEBAX-2533 and ZIF-8 were prepared,characterized and tested for their permeability and selectivityproperties for specific single gases as well as mixed gas in dry

Fig. 1. Single (squares) and mixed (triangles) gas permeance for a ZIF-8 membranevs. kinetic diameters [55].

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255 245

and humidified condition. Other characterizations of the preparedMMMs were tested to investigate influence of loading ZIF-8 onmembrane properties.

3. Theory

Mass transport in dense polymeric membranes is based on thesolution–diffusion mechanism as shown in Eq. (1)and has beenwell documented elsewhere [56]:

PA ¼DASA ð1Þ

where PA is the steady state permeability of gas A [cm3 (STP) cm(cm2 s cm Hg)�1] or Barrer, (1Barrer¼[10�10 cm3(STP) cm/(cm2 scm Hg)] or [7.5�10�18 (m3 (STP) m/ (m2 s Pa)])), DA is the averageeffective diffusion coefficient (cm2/s) and SA is the solubilitycoefficient (cm3 (STP)/cm3 cm Hg).

The ideal selectivity of membrane for gases A and B is given asfollows:

αA=B ¼PA

PB¼DA

DB� SA

SBð2Þ

where DA/DB is the diffusivity selectivity and SA/SB is the solubilityselectivity.

Fig. 2. SEM images of ZIF-8 in ethanol solution. Strong aggregation happens quickly after pouring the solution on TEM grid.

Fig. 3. The SEM images of different and newmorphology of PEBAX-2533/ZIF-8 MMMmade in this work. The membrane is made of two different layers. There is an inorganicbase layer, shown in the lighter color in this figure and one polymer base layer, shown in the darker color.

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255246

Generally, gas diffusivity increase by decreasing penetrant size,enhancing polymer fractional free volume, decreasing penetrant–polymer interactions and increasing polymer chain flexibility (canbe identified by a decrease in glass transition temperature) [57].Gas solubility is enhanced by increasing condensability of pene-trant gas (i.e. higher normal boiling point or higher criticaltemperature) and more favorable penetrant–polymer interactions[57]. A polyether based block copolymers like PEBAX, shows highdiffusivity due to their low Tg, varied in the range �50 to �80 1Ccorresponding to PEO segment. However, the diffusivity selectivityis low due to their low size sieving ability, but the high CO2/lightgas selectivity is gained by their high CO2/light gas solubility[28,35,58].

The mixed gas permeance of CO2 and N2 was calculated usingcomplete mixing model from the total permeate flow (see Eqs.(3) and (4)). The details about the mixed gas permeation set up are

well described in previous works in the MEMFO group [59].

QA ¼JA

xf A Ph�xpA Plð3Þ

αA=B ¼xpA

1�xpA

xfA1�xfA

�ð4Þ

where QA represents the permeance (m3(STP)/(m2 bar h)) of com-ponent A (CO2 or N2), JA is the flux (m3(STP)/m2 h), XfA and Xp,A

represent molar fraction of the component on feed and permeateside respectively and Ph and Pl (bar) are the absolute pressure onfeed and permeate side. However, the permeability of the mem-branes with various loadings was calculated based on the mea-sured thickness. Two CO2/N2 selectivity parameters are reported inthis work: concentration CO2/N2 selectivity which represents theratio of CO2/N2 concentrations on permeate side to CO2/N2

Fig. 4. The SEM images of polymer-based layer. In the polymer the inorganic ZIF-8 particles dispersed both as single particles and as agglomerated particles.

Fig. 5. The SEM images of inorganic layer in MMM as a porous layer while the polymer keeps the inorganic particles connected. In the upright image the two layersare shown.

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255 247

concentration on feed side (Eq. (4)), and membrane CO2/N2

selectivity which corresponds to the ratio of the permeances.

4. Experimental

4.1. Materials

PEBAX-2533, kindly supplied by Arkema (France), is a blockcopolymer contain 80 wt% poly(ethylene oxide) (PEO) and 20 wt%polyamide (PA12, nylon-12) and will be further referred to asPEBAX-2533. Based on some research done by Rezac et al. [37], thenumber of PEO and specifically in the PEBAX they used, polyte-tramethylene oxide (PTMO) group per repeat unit is 2.68 while thenumber of PA12 per repeat unit is 27.80, which means weightpercentage of PA in the block copolymer is 21.6%. The Tg of PTMO is�76 1C and Tm of crystalline PTMO is 12 1C while the Tg and Tm ofPA is around 70 1C and 137 respectively. Xc, crystallinity in PA blockwas reported 14% in some other works [35]. ZIF-8 with the

trademark name of Basolite and technical ethanol (96%) wasobtained from Merk.

4.2. Membrane preparation

PEBAX-2533 was prepared as a flat film membrane by thesolvent evaporation method. Ethanol, certified technical grade,was used to make a 3 wt% PEBAX-2533 solution. The PEBAX wasdissolved under reflux condition and continues stirring at 70 1C for3–4 h. After the polymer was completely dissolved, the polymersolution was cooled to ambient temperature; a homogeneoussolution without gelation was obtained. The dense membrane ofpure PEBAX-2533 was obtained by casting 3% polymer solution onTeflon Petri dish. MMM solutions were prepared by adding properamount of ZIF-8 to the polymer solution, and the inorganicparticles were dispersed in the solution for 20–30 min usingultrasonic mixing. The solution was then poured into a TeflonPetri dish which was covered with a funnel to avoid any dust insolution and allow solvent evaporation. The casted membrane wasdried at room temperature during one day and then placed in

Fig. 6. The SEM images of PEBX-2533/ZIF-8 MMM. There is a polymeric connection between two layers.

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255248

vacuum oven at room temperature for 10 h, after which thetemperature was increased gradually up to 70 1C and was kept atthis temperature overnight to completely remove residual solvent.Finally the temperature was raised to 80 1C and was kept there forone hour before it was slowly cooled down to room temperature.The thickness of achieved membrane varied between 40 and60 mm. Mixed Matrix ZIF-8/PEBAX 2533 flat sheet membraneswith area of 5 cm2 and 20 cm2 for single gas and mixed gaspermeation tests respectively, were used for gas separationcharacterization.

4.3. Membrane characterization

4.3.1. Gas permeabilityGas separation performance for MMMs was measured using

single gas (N2, O2, CH4 and CO2) and binary gas mixture (10% CO2

and 90% N2). Single gas permeation measurements were donewith a standard constant volume/variable pressure technique,applying vacuum on the permeate side. The gas permeance wascalculated from the rate of increase of pressure over time on thepermeate side. The single gas measurements were done at roomtemperature at 2 and 6 bar. The gas permeability values wereobtained by considering the thickness of samples and the mea-sured permeance.

The CO2 permeability and CO2/N2 mixed gas selectivity valuesof ZIF-8/PEBAX-2533 mixed matrix membranes were determinedby measuring the steady state flux of two components in a mixedgas stream permeating through membrane, where all the processvariables such as pressure, relative humidity (RH%) of gases, gasflow rate, temperature, gas composition were continuously andsimultaneously registered by a Lab View program. The tempera-ture and RH% were measured directly inside feed and sweep gas

Fig. 7. The SEM images of PEBAX-2533/ZIF-8 MMM. ZIF-8 loading percentage in MMM is (a) 5%, (b)10%, (c)20%, (d)30%, (e)40% and (f)50%. The inorganic thickness increase asthe ZIF-8% increase in MMM.

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255 249

side close to the permeation cell. A gas permeation cell with20 cm2 membrane active area was used for mixed gas permeationtests. A mixed gas with composition of 10% CO2/90% N2 was usedas feed gas and He was used as a sweep gas. The composition ofthe permeate gas was analyzed continuously by a micro GC;Agilent 3000. All mixed gas permeation experiment were per-formed at 25 1C, relative humidity was 100%, feed pressure 2.6 barand sweep pressure 1.1 bar if not otherwise is mentioned.

4.3.2. Wide angle X-ray diffractionIn this work, wide angle X-ray diffraction was used to identify

the phase composition of the film, the texture of the film, presenceof crystals and amorphous regions in film. Wide angle x-raydiffraction of pure polymer and MMMs were recorded using CuKα radiation of wavelength λ¼1.54060 Å with a graphite mono-chromator produced by Bruker AXS D8 focus advanced X-raydiffraction meter (Rigaku, Japan, Tokyo) with Ni-filtered. TheX-ray scans were taken with a scanning speed 11/mm and stepsize of 0.011. The angle diffraction 2θ was varied from 5 to 701 to

identify any change in the crystal structure of intermoleculardistance between inter segmental chains.

4.3.3. Scanning electron microscopyIn this work Hitachi VP-SEM S-3400N and Hitachi S5500 STEM

were used to investigate cross section and surface of puremembranes and MMMs containing ZIF-8. To avoid charging ofsamples with electron beam all samples were covered withconducting gold coating.

4.3.4. Differential scanning calorimetry(DSC)Thermal properties of the pure membranes and MMMs were

investigated using differential scanning calorimeter (TA Q100).Sample of 10 mg was put in an aluminum pan covered with aproper lid together with the standard empty pan into the DSCsample holder and heated at the rate of 10 1C/min under N2

atmosphere.

4.3.5. Thermogravimetric analysisThermogravimetric analysis Instrument (TGA) model Q500,

New Castel USA, was used to determine the amount or rate ofweight changes as a function of temperature over time incontrolled atmosphere. About 10 mg of the sample was put insample holder, and analyzed by the TGA instrument. He was usedas the balance and purge gas. The heating rate was 10 1C/min.

5. Result and discussion

5.1. Morphology

SEM was used to investigate the membrane cross sectionmorphology and the interface of the polymer and inorganic filler.Fig. 2 shows sample images of sonicated Pure ZIF-8 in ethanolsolution. According to the SEM images considerable agglomerationhas taken place for pure ZIF-8. However, the images of the filler inpolymer phase (Figs. 3 and 4) show that sonication during themembrane preparation partly destroys the aggregates of ZIF-8particles.

Fig. 3 shows a different morphology of inorganic–organicmatrix membranes compared to the common ones in which

Fig. 8. The SEM images of PEBAX-2533/ZIF-8 MMMM with weak mechanicalproperties. It is made of only one brittle layer.

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100110120130140150160170180190

210

CP

S

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

2Theta (Coupled TwoTheta/Theta) WL=1,54060

Fig. 9. XRD pattern of polymer layer (black line) and inorganic layer (red line). The results belong to both sides of PEBAX-2533/ZIF-8(15%) MMM. (For interpretation ofreferences to color in this figure legend, the reader is referred to the web version of this article.)

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255250

inorganic fillers are more homogenously dispersed in the polymermatrix. The prepared mixed matrix membrane in this work showsa two layer membrane in which there is one polymer-based layerwith a few inorganic particles dispersed (see Fig. 4) and oneinorganic-based layer (see Fig. 5). In the inorganic-based layer,particles seems to be closely connected through a network held totogether by the polymer, as shown in Figs. 5 and 6. The two layersare completely connected through the polymer as shown in Fig. 6,and the two layers cannot be separated by peeling off.

Fig. 7a–f respectively shows the cross section SEM images oftwo layer mixed matrix membranes with 5, 10, 20, 30, 40, and50 wt% of inorganic filler in polymer matrix. As shown in Fig. 7, thethickness of the inorganic-based layer compared to the thicknessof organic-based layer increases as the percentage of inorganicfiller increases. However there is a limit to the amount of loadingZIF-8 in MMM, meaning that if more than �50% of ZIF-8 is added

to the polymer will result in one porous layer with very weakmechanical properties as shown in Fig. 8. Although there was anattempt to achieve data for mechanical properties of the devel-oped MMMs and pure membrane, however, due to the largeelongation of the membranes this was not successful.

The formation of two layers inorganic–organic mixed matrixmembrane may be due to the aggregation of inorganic fillers in thesolution during the solvent evaporation process. The aggregatedparticles will settle more easily than the individual small particles,hence there will be a formation of two phases in the solution. Theresult will be one phase organic-based polymer solution with non-agglomerated inorganic fillers, and the other phase inorganic-based with agglomerated inorganic fillers and less polymer solu-tion. It is impossible to see the two phases in the membrane withthe naked eye; however the polymer layer has a more shinyappearance.

0

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CP

S

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

2Theta (Coupled TwoTheta/Theta) WL=1,54060

Fig. 10. XRD pattern of polymer layer (black line) and inorganic layer (red line). The results belong to both sides of PEBAX-2533/ZIF-8(30%) MMM. (For interpretation ofreferences to color in this figure legend, the reader is referred to the web version of this article.)

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S

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

2Theta (Coupled TwoTheta/Theta) WL=1,54060

Fig. 11. XRD pattern of PEBAX-2533/ZIF-8 MMM with different amount of ZIF-8 loading in MMM. Loading percentage of ZIf-8 in MMM is shown as 5%—blue, 10%—black, 20%—red and 30%—green. (For interpretation of references to color in this figure legend, the reader is referred to the web version of this article.)

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255 251

The pure PEBAX-2533 thin film membrane is rubbery andsticky which makes it difficult to use without a support, howeverthese properties seem to be favorable for creating a good networkwhen mixed with the ZIF-8 particles. The dual layer mixed matrixmembrane of ZIF-8/PEBAX-2533 improves the physical propertiesof PEBAX-2533 significantly, and is easy to handle.

In addition to SEM imaging, other methods such as XRD andFTIR were used to verify the formation of dual layer MMM. XRD isa common technique to confirm the presence of crystals inamorphous matrix [60].

The XRD results of both sides of one membrane for twosamples of mixed matrix with 15 wt% and 30 wt% loaded inorganicparticles are shown respectively in Figs. 9 and 10. In both figuresthe black line is corresponding to the scattered pattern of thepolymer-based layer of the membrane while the red line iscorresponding to the one of the inorganic-based line.

In general, adding more ZIF-8 inorganic particles changes themorphology of the matrix. As shown in Fig. 11 the more loadingsof ZIF-8 in MMM, the less wide scattering about 15–25 of 2θ – thisis corresponding to amorphous polymer region as shown for purepolymer in Fig. 12.

FTIR spectra show some changes which can be seen in theintensities of the bands of strongest absorption. This occurs whenthe spectra were measured directly from the polymer side and theinorganic side of PEBAX-2533/ZIF-8 MMM with different loadings.The change can be corresponding to different amounts of polymerin inorganic layer and inorganic particles in polymer layer.Specifically the peak at wavenumber 3135 cm�1 correspondingto C–H stretch of imidazole of ZIF-8 is not clear in polymer sidewhile it is present in inorganic side of the same membrane.Additionally intensity of the peak at wavenumber 3300 cm�1,corresponding to C–N stretch of amide group in polyamide block,reduces in the inorganic layer as the amount of loading ZIF-8increases (see Figs. 13 and 14) for loading 10% and 50% of ZIF-8 inMMM respectively. The FTIR spectra of pure PEBAX-2533 mem-brane and ZIF-8 are presented in Fig. 15.

TGA results show that decomposition temperature of PEBAX-2533 is around 350 1C while adding ZIF-8 increase decompositiontemperature to 380 1C. There is a weight loss around 350 1C but

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2Theta (Coupled TwoTheta/Theta) WL=1,54060

Pure Pebax-F4.raw

Fig. 12. XRD pattern of Pure PEBAX-2533 membrane.

Fig. 13. FTIR spectra of PEBAX-2533/ZIF-8 (10%) of polymer side (red) and inorganicside (blue). (For interpretation of references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 14. FTIR spectra of PEBAX-2533/ZIF-8 (50%) of polymer side (red) andinorganic side (blue). (For interpretation of references to color in this figure legend,the reader is referred to the web version of this article.)

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255252

the main weight loss and decomposition is moved up to thetemperature 380 1C is reached (see Fig. 16). According to theresults of DSC there is no significant change of Tg and Tm ofMMM compared to pure PEBAX-2533.

In summary the formation of dual layer membrane occurs dueto the aggregation of inorganic particles where the polymernetworks keep the particles nicely connected in the inorganic-based layer. The polymer-based layer is more like the commonMMM where inorganic particles are dispersed in the polymermatrix. SEM images show the connection of two layers. In both

layers there are inorganic particles as confirmed by the XRD andFTIR, while the layers have completely different morphology.

5.2. Permeability

In Fig. 17 the CO2 pure gas permeability of the PEBAX-2533/ZIF-8 mixed matrix membrane up to 35 wt% of ZIF-8 as a porousinorganic particle is presented. The separation coefficients ofnitrogen, oxygen, methane and carbon dioxide in pure PEBAX-2533 and PEBAX-2533/ZIF-8 mixed matrix membrane is presentedin Table 1. In Figs. 18–20 the results of mixed gas experiments ofCO2 and N2 (10% CO2) are shown. Fig. 18 summarizes the CO2

permeability and selectivity of the PEBAX-2533/ZIF-8 mixedmatrix membrane up to 40 wt% ZIF-8 using dry feed and sweepgas. Fig. 19 shows the CO2 permeability and selectivity of thePEBAX-2533/ZIF-8 mixed matrix membrane up to 50 wt% ZIF-8using 100% humidified feed and sweep gas. Fig. 20 shows thestability of results of MMM over two days of humidified mixed gaspermeation measurement.

As documented in the reported figures and table, the perme-ability of CO2 in single gas measurements is increasing from 351Barrer (for pure PEBAX-2533) to 1287 Barrer for MMM with 35%loading of ZIF-8 while there is no considerable increase in CO2/N2

and CO2/CH4 selectivity. Mixed gas separation properties showsimilar results. In the case of dry mixed gas measurement the CO2

permeability changes from 188 Barrer for pure PEBAX-2533membrane to 1098 Barrer for MMM of 40% loading ZIF-8 andthe CO2/N2 selectivity drop from 64 to 33. In the case of humidifiedmixed gas properties the CO2 permeability increase from 200Barrer for pure membrane to 993 Barrer for MMM with a 50%loading of ZIF-8. A less dramatic decrease takes place for CO2/N2

going from 49 for the pure membrane to 33 for MMM with 50% ofZIF-8 loading. The stability of the performance of the membraneunder humidified conditions for mixed gas conditions is shown asa function of time (min), see Fig. 20.

Presence of two layers with different structure in the mem-brane changes the gas performance of the membrane completely.The polymer-based layer includes some inorganic filler which isexpected to cause an increase both in permeability and selectivity.The inorganic-based layer which is formed mainly from ZIF-8 isalso expected to cause an increase in CO2 selectivity due to its CO2

capacity; however, the porous structure of this layer caused a dropin selectivity. A dramatic increase in permeability was howeverobserved. The increase in permeability may be due to bothinterrupted chain packing in polymer matrix by the dispersedfillers in polymer-based layer and also the presence of the porousinorganic-based layer. In single gas measurement a drop in CO2/N2

selectivity was observed with first adding inorganic fillers whichmay occur due to the formation of the dual layer. With furtherincrement of the loading of the inorganic filler, selectivityincreased most likely due to the increase of the amount of theselective particles in both polymer-based layer and inorganic-based layer. However, a more significant drop in CO2/N2 selectivitywas observed in mixed gas measurements in both dry andhumidified conditions which is a result of that N2 will competewith CO2 in permeation, especially in the porous layer. However,the decrease in selectivity can be a result of having relativelythinner polymer-based layer as the loading percentage of inor-ganic filler increase as it was mentioned earlier in morphologysection and specifically in Fig. 7.

6. Conclusion

The present study addresses the development of a novel duallayer mixed matrix membrane of PEBAX-2533 and ZIF-8 used for

Fig. 15. FTIR spectra of PEBAX-2533 (blue) and ZIF-8 (red). (For interpretation ofreferences to color in this figure legend, the reader is referred to the web version ofthis article.)

80

90

100

110

120

Wei

ght (

%)

300 320 340 360 380 400 420

Temperature (°C)

Sample203-PZ3%Sample205-PZ5%Sample207-PZ7%Sample210-PZ10%Sample220-PZ20%Sample215-PZ15%Sample225-PZ25%Pure Pebax- B

Universal V4.7A TA Instruments

Fig. 16. TGA result of PEBAX-2533/ZIF-8 MMM with different loading amounts ofZIF-8.

Fig. 17. Single gas permeability and selectivity as a function of amount of loadingZIF-8 in MMM.

V. Nafisi, M.-B. Hägg / Journal of Membrane Science 459 (2014) 244–255 253

gas separation with the specific for removal of CO2 from mixed gasstreams. SEM images confirm formation of two different layerswhich are connected strongly by the polymer matrix. One layer

has the polymer-based while the other layer has the inorganic-based confirmed by FTIR. The crystalline and amorphous scatter-ing pattern of each side of one membrane is different while it is avariable loading of inorganic particles. The inorganic-based layerthickness increases as the loading percentage of inorganic materialincreases. However, there is an upper limit for loading inorganicparticles which results in one layer membrane with mechanicallyweak properties. Thermal studies done by DSC and TGA show nochange of the Tg, while there will be some increase on decom-position temperature. Gas separation performance of developedmixed matrix membranes was done for single and mixed gas.The mixed gas measurement was done both in dry and humidifiedcondition. The studies show as the loading percentage of inorganicmaterial increases, there is a considerable increase in permeabilityfor all cases, while the CO2/N2 selectivity is constant or slightlydecreases.

Acknowledgment

This work was financially supported by BIGCCS project as a partof PhD work of the first author.

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