N2 separation

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JIEC-2374; No. of Pages 17 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

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Gas permeation and sorption properties of poly(amide-12-b-ethyleneoxide)(Pebax1074)/SAPO-34 mixed matrix membrane for CO2/CH4 and CO2/N2 separation Hesamoddin Rabiee a, Shadi Meshkat Alsadat a, Mohammad Soltanieh a,*, Seyyed Abbas Mousavi a, Ali Ghadimi b a b

Department of Chemical and Petroleum Engineering, Sharif University of Technology, Azadi Avenue, P.O. Box 11155 9465, Tehran, Iran National Petrochemical Company, Petrochemical Research and Technology Company, P.O. Box 1435884711, Tehran, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 August 2014 Received in revised form 5 December 2014 Accepted 30 December 2014 Available online xxx

Zeolite SAPO-34 was used for fabrication of mixed matrix membranes (MMMs) to improve the CO2/CH4/ N2 gas separation performance of the neat Pebax1074 membrane. Permeability and selectivity of the MMMs were studied at different temperatures of 25–65 8C and pressures of 4–24 bars. Also sorption of different gases in MMMs was measured at 35 8C and different pressures, which showed enhanced solubility coefficients. Moreover, thermal, morphological and mechanical properties of MMMs were characterized by differential scanning calorimetry (DSC), scanning electron microscope (SEM) and tensile analysis. The results showed excellent improvement in CO2/CH4 selectivity (about 70%) and CO2/N2 selectivity (about 15%) at 20 wt% SAPO-34 loading. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Membrane gas separation Pebax1074 Zeolite SAPO-34 Mixed matrix membrane CO2 capture

Introduction Gas separation by membrane technology has many applications in chemical industry such as: CO2 and H2S removal from CH4 (natural gas sweetening), H2/CO ratio adjustment for syngas, hydrogen purification in process streams, N2/O2 separation and dehydration [1]. Membrane processes have obtained great attention due to their high energy and separation efficiency, such as simple operating process and low running costs in comparison with conventional methods [2]. Among various types of membranes, polymeric membranes have reasonable mechanical property to work in high pressure and temperature conditions [3,4]. However, it is necessary to fulfill the trade-off between gas permeability and selectivity and overcome ‘‘Robeson upper bond’’ curve in polymeric membranes to extend their usage in separation processes because it has been found that membranes with high permeability are less selective and vice versa [5]. Thus, there is a considerable demand to develop new methods or materials for membrane preparation. Several attempts have been demonstrated

* Corresponding author. Tel.: +98 21 6616 5417; fax: +98 21 6602 2853. E-mail address: [email protected] (M. Soltanieh).

to exceed this limitation such as mixed matrix membrane (MMM) and block copolymers [6]. MMMs are result of incorporation of inorganic fillers like zeolites, carbon molecular sieves and non-porous particles (like silica (SiO2), MgO and TiO2) into polymer matrix to prepare a membrane with polymer as the continuous phase and inorganic fillers as the dispersed phase to enhance permselectivity [7–9]. Many variables should be regarded to prepare membrane with demanded ability in separation because addition of fillers on the one hand can interrupt the uniformity of polymer structure which leads to better permeability but on the other hand fillers may hinder gas permeation in membranes [6,10,11]. Several significant reviews have been published to show the promising role of this type of membranes in separation processes and state some practical suggestions to improve their performance [6,12–15]. Both glassy and rubbery polymers can be considered in MMMs. Rubbery polymers are flexible enough to provide appropriate adhesion between polymer and inorganic phases and thus uniform and defect-free membranes can be prepared with these polymers as continuous phase. Rezakazemi et al. have recently succeeded to prepare H2-selective mixed matrix membrane with poly (dimethylsiloxane) (PDMS) [16], even though PDMS had been used earlier to prepare MMMs for gas separation [17,18]. However the number of rubbery polymers which can be

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used as a membrane in gas separation is not as much as glassy ones. Furthermore, rubbery membranes suffer from low selectivity. Consequently, using glassy polymers as a continuous phase has been considered to achieve better separation results in many researches due to diffusion selectivity by sieving molecules based on differences in molecular size [19–22]. Some of the main hindrances in MMMs fabrication are: sedimentation and aggregation of particles during preparation, surface pattern effect and non-selective permeation through polymer–inorganic interface due to the poor contact [23,24]. The first problem can be solved by sonication of solution and making more viscous slurries [8,25,26]. Surface pattern, which is migration of particles toward top of membrane and formation of top layer, arose by rapid evaporation of solvent so can be solved by slow/controlled evaporation or heating the membrane from the top [25–27]. Poor polymer–particle adhesion can be dealt with different approaches such as surface modification by coupling agents like silane or amine groups [28–30], low molecular weight additives [22,31,32], using polymer with medium glass transition temperature to avoid problems associated with soft and rigid polymers [8] or using of copolymers which contains both rigid and flexible chains. In copolymers soft segment provides good contact with filler and hard segment gives good mechanical resistance [33]. It is well known that at high zeolite loading, both polymer and inorganic phases participate in separation. Thus material selection for both phases should be taken into consideration to improve separation performance of MMMs. One of the most promising copolymers for gas separation which has been recently used for membrane preparation is poly(ether-b-amide), commercially under the name of Pebax. Lin et al. showed the interesting gas separation properties of poly(ethylene oxide) (PEO), as the soft segment of Pebax, for acid gas removal from non-polar gases

[34,35]. Therefore, PEO (soft segment) on the one hand provides enough adhesion between two phases and on the other hand has high affinity with sour gases. This fact has been considered to use PEO to fabricate other co-polymer membranes [36–38]. Recently, CO2 separation from CH4 and N2 by Pebax1657/ZIF-7 mixed matrix membranes was studied by Li et al. [39]. Their results show that addition of ZIF-7 up to 34%, increases CO2/CH4 and CO2/N2 selectivity more than 3 times due to molecular sieving effect of ZIF7 nanoparticles. Friess et al. investigated gas permeation properties of Pebax4033 membrane filled by two types of ZSM-5 zeolites and found that zeolite fillers increased CO2 permeation but CO2/O2 and CO2/N2 ideal selectivity did not enhance very much and in some cases decreased [40]. Table 1 summarizes Pebax-based organic–inorganic membranes, synthesized for gas separation. Zeolites, as porous filler in MMM for gas separation, have been regarded in many works [8,16,31]. Since diffusion rate of gas molecules in zeolite pores is associated with pore size, molecules with the same or larger size compared to the zeolite pore size, diffuse much harder and gas separation by molecular sieving occurs [33]. According to the properties of the commonly applied zeolites, SAPO-34 with pore size of 3.8 A˚ is able to pass CO2 molecule and resist CH4 permeation (kinetic diameter of CO2 and CH4 are 3.3 A˚ and 3.8 A˚, respectively) [41]. Different grades of Pebax possess particular properties and their usage in industrial application is related to these properties. For example, Pebax1657 is the best option among other grades for the case of H2 purification and syngas application [42–46]. However, for natural gas sweetening, CO2/CH4 separation, Pebax1074 shows interesting characteristics among other grades [1,47] and few studies have concentrated on the gas separation properties of this grade. That is why Pebax1074 has been selected for MMM preparation in this study. Pebax1657 for MMM preparation with SAPO-34 has been considered very recently, however their results,

Table 1 Pebax-based nano composite and MMMs for gas separation. Reference

Pebax grade

Filler

Result

Yu et al. [49]

1657

Silica/CNT/polystyrene (PS) colloids

Li et al. [39]

1657

ZIF-7

Friess et al. [40] Rahman et al. [50]

4033 1657, 2533

Two types ZSM-5 zeolites PEG functionalized POSS

Wang et al. [51]

2533

AgBF4

Sridhar et al. [52]

2533

Ag(Silver)

Murali et al. [53]

1657

MWNT

Li et al. [54]

1657

POSS

Sforc¸a et al. [55]

4033

Silica

Kim et al. [56]

1657

Silica

Zhao et al. [48]

1657

SAPO-34

CO2 permeation decreased with silica and PS addition but CNT enhanced CO2 permeation up to 5% loading then it declined. CO2 selectivity over N2 and H2 did not improve significantly Selectivity and permeability for CO2/CH4 and CO2/N2 increased by addition of ZIF-7 up to 22% loading due to molecular sieving of filler however, with more ZIF-7 loading, polymer chain rigidified and permeation reduced CO2 permeation increased but selectivity did not improve CO2 permeability for Pebax1657 MMM doubled up to 30% filler addition but CO2 selectivity over CH4, N2, H2, and O2 did not change significantly. For Pebax2533 MMM CO2 permeation increased up to 30% loading but it was not as significant as in case of Pebax1657. With more loading, it decreased due to reduction in diffusion Silver salts such as AgBF4 are olefin selective and lead to facilitated transport of propylene so after threshold silver concentration propylene permeation increases and selectivity greatly enhances Ag salt addition increased diffusion selectivity of membrane thus selectivity increased two and half times but CO2 permeation decreased. After MWNT loading, CO2, N2, H2, O2 permeability increased but increase in CO2 permeation was the most thus CO2/N2, CO2/H2 improved. CO2 permeation enhanced due to large cavity of POSS structure at small loading then it declined because of pore blockage and reduction in the mobility of polymer chains caused by presence of H-bonding interaction between polymer and POSS. Permeability of all tested gases (H2, N2, O2, CH4, CO2) increased with SiO2 addition so except for CO2/H2, other selectivity values decreased Permeabilities and permselectivities enhanced for organic–inorganic hybrid membrane prepared via sol–gel process especially at elevated temperature particularly for CO2 due to strong interaction between CO2 molecules and SiO2 domains Increment in permeation of N2, H2, CH4, CO2 was observed with zeolite addition while CO2 pair selectivities did not improve significantly

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especially for CO2/CH4 separation, differs from the obtained results of this study [48] which will be discussed in permeation results. Preparation of MMM with Pebax1074 and zeolite SAPO-34 for gas separation has not been worked on. Thus the present work shows fabrication of MMMs using Pebax1074 as the base polymer matrix withSAPO-34 zeolite. Sorption, diffusion and permeation properties are studied precisely to investigate the influence of zeolite addition which leads to some different results, compared to another similar work. Also, the effects of various operational conditions (temperature and pressure) on separation and transport properties of the synthesized membranes for CO2 capture from N2 and CH4 is investigated. Gas sorption properties of the prepared membranes were investigated at 35 8C and different operating pressures. Also, for the first time for the case of Pebax-based membranes the Flory–Huggins (F–H) theory was applied to investigate changes in membrane–penetrant interaction caused by zeolite. Gas permeability of MMMs was measured at wide range of operating conditions from 25 to 65 8C (25, 35, 45, 55, 65 8C) and 4 to 24 bar (4, 8, 12, 16, 20 and 24 bar) in order to evaluate separation performance of the prepared membranes near industrial conditions. MMMs were characterized by SEM, DSC, and Tensile analysis to find the influence of the applied zeolite on morphological, thermal and mechanical properties of the prepared membranes, respectively. The obtained results showed excellent increment in CO2 permeation and CO2/CH4 selectivity due to incorporation of SAPO-34 zeolites which tends to facilitate permeation of CO2 and resist against CH4 permeation. For example at 4 bar and 25 8C, CO2 permeation increased 33% and CO2/CH4 selectivity almost doubled that is mainly because of the effect of SAPO-34 in separation properties of the membranes. This improvement in CO2/CH4 selectivity leads to separation performances of the MMMs to pass the prior Robeson upper bound and moves toward the present upper bound. The increase in N2 permeation was less compared to that of CO2, thereby performance of the prepared MMMs for CO2/ N2 separation enhanced and moved across the Robeson upper bound limit. The separation performance of the prepared membranes shows suitable development in greenhouse gas control and CO2 capture.

Experimental Materials Pebax1074 (comprising 55 wt% of poly(ethylene oxide) (PEO) and 45 wt% polyamide (PA-12)) was supplied by Arkema Inc., France, and used as received. 1-Butanol, used as solvent, was purchased from MERCK, Germany. Zeolite SAPO-34 (molar ratio of P:Si:AL = 1:1:0.4, r = 0.75 g/cm3) particles [57] were dedicated from Research Institute of Petroleum Industries, Iran. Some Teflon petri dish used in order to prepare MMMs. A vacuum oven (Wisd, WiseVen), an oven (Memmert), a stirrer (F20 FALC, Italy), a vacuum pump (DV.3 E JB Eliminator, USA), a digital microbalance (Percisa, 310M) and a pressure sensor (Lutron, VC-9200) were also used. Mitutoyo digital micrometer (1 mm

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accuracy) was used to measure the membranes thickness. N2, CO2, and CH4 gases with purity of 99.9% were supplied by Technical Gas Services, Inc. Membrane preparation Zeolite activation is of great importance which affects gas permeation properties. Accordingly, zeolite powder was dried in vacuum oven at 180 8C for 24 h in order to remove water from its cavities. Weight comparison of the zeolite before and after drying in vacuum oven shows that the dried powder was almost 50% after activation. Activated particles were put in desiccator to be away from humidity and other organic sorption. In order to have defect-free membranes some parameters should be considered. Polymer concentration in the solution is the most important factor that can lead to good dispersion of zeolite and eliminate sedimentation in the prepared membranes. But increased polymer content in the solution, leads to higher solution viscosity and weaker homogenous dispersion of particles, resulting in thicker membrane with lower permeance. In this case the amount of casting solution should be reduced. In this research, 15 wt% polymeric solution was prepared by dissolving Pebax1074 in 1-buthanol at 90–100 8C under reflux for 24 h. For preparation of MMMs, in the first step zeolite was added to solvent and the mixture was stirred for 2 h. Then 10–15 wt% of polymer was added to mixture in order to prime the solution. This priming causes coating of fillers with polymer in dilute solution and helps dispersion and adhesion between polymer and filler [25]. After 24 h, a homogeneous solution was achieved and the remaining amount of the polymer was added to the solution to have a 15 wt% polymer solution and zeolite content ranging from 10 wt% to 35 wt% of polymer. MMMs with 35 wt% zeolite loading were fabricated but they were too brittle for performance and characterization tests, thereby they were not used. Subsequently, the Pebax-zeolite suspension was casted on Teflon coated petri dish, warmed at 60 8C. Petri dish was held in oven at 60 8C, overnight. After solvent evaporation MMM was easily peeled off from petri dish and membrane was put in a vacuum oven at 50 8C for another 24 h to remove residual solvent. Finally, the fabricated membrane was allowed to cool to room temperature, followed by storing in desiccator for further experiments. Thickness of the films was determined using a digital micrometer and was about 68 mm for the neat one, 80 mm for 10%MMM, 93 mm for 20%MMM and 105 mm for 30%MMM. Fig. 1 shows the step-by-step methodology of MMM preparation.

Membrane characterization Differential scanning calorimetry (DSC) The instrument METTLER TOLEDO DSC 822 (Switzerland) was used to investigate thermal properties of the prepared membranes by differential scanning calorimeter (DSC). The tests were done in the range of 100 8C to 220 8C under nitrogen atmosphere and heating rate was adjusted at 10 8C/min.

Fig. 1. Mixed matrix membrane preparation methodology.

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Fig. 2. Schematic diagram of gas permeation and sorption set up.

Mechanical properties In order to investigate tensile modulus and tensile strain of the neat and mixed matrix membranes, INSTRON (5566, England) instrument was used at room temperature. Tests were carried out at least 5 times for each fabricated membranes to assure reproducibility of the results. Operating head load was adjusted at 5 kN for samples, having length of 5 cm (grip to grip length), while testing speed was 12.5 mm/min. Scanning electron microscope (SEM) Investigation the morphological structure of the neat and MMMs was done using scanning electron microscopy (VEGA\\TESCAN SEM, Czech Republic). The samples were fractured in liquid nitrogen and coated with gold before SEM analysis. Gas permeation and sorption apparatus The permeation properties of the prepared membranes were studied by the set-up which is capable to control temperature and pressure (Fig. 2). The set-up, pipe lines and cell are immersed in a bath and the temperature of the water of the bath is controlled by a thermostat and water circulator. The permeation coefficients

(P) of the prepared membranes were measured by constant volume method, as follows: P¼

V 273:15 l dp    A T ps D p dt

(1)

where, V, A, ps are volume of permeate (cm3), membrane area (cm2), standard pressure (1 bar) and thickness of membrane (cm), respectively. Dp and ddtp are pressure difference across the membrane (mmHg) and pressure increment with time (bar/s), respectively. T represents the operating temperature. Permeation obtained from Eq. (1) has unit of Barrer (1 Barrer = 1010 cm3 (STP) cm cm2 s1 cm Hg1). The membrane cell is of cross-flow type which consists of two detachable parts as shown in Fig. 3. Its effective area is about 20 cm2 and two concentric rubber O-rings were used to seal cell parts from gas leakage and water wetting. The apparatus is also designed to measure concentration (C) of the tested gases using the following equation: C¼

22414 Vm ð p1  p2 Þ RT Vp

Fig. 3. Membrane module.

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(2)

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where, Vm and Vp are the volumes of the module and membrane sample (cm3), respectively. 22,414, R and T are the Avogadro’s (number of gas molecules per mole at standard condition), gas constant and the absolute temperature, respectively. p1 and p2 refer to initial and final pressure in the module, respectively. After concentration measurement, solubility can be obtained by following equation: S¼

C p

(3)

where, S is solubility coefficients and P is the operating pressure. Diffusion coefficients (D) can be also calculated by considering solution–diffusion transport mechanism through the membrane, as follows: D¼

P S

(4)

Results and discussion Characterization Morphology of Pebax1074/SAPO-34 MMMs Dispersion of fillers in the polymeric matrix is of a great importance to fabricate a MMM with high separation properties. SEM images were used to analyze the morphology cross-section of the neat and MMMs, to demonstrate dispersion and interfacial adhesion of the polymer and zeolite particles. SEM images show distribution of zeolite particles in membrane cross-section without aggregation and voidage in organic–inorganic interface (Fig. 4). Particles size of the applied zeolites, which is in the range of 2– 4 mm, could be obtained from SEM images. The SEM images show cross-sectional morphology without any crack or defects which confirms the applied preparation method is suitable for these materials. As it can be seen in Fig. 4f, the particle is completely surrounded by polymer and no interfacial defect can be observed. It confirms desirable interfacial adhesion between two phases which is mainly due to rubbery nature of continuous phase (polymer) and existence of the soft segment in polymer [8,16,33]. DSC The glass transition temperature of the neat and MMMs were found by DSC to study the effect of the filler on thermal properties of membranes. As can be seen in Fig. 5, the peaks attributed to Tg of soft segment in Pebax (peaks are circled for clarification) increases as the zeolite content increases. The obtained results show that Tg goes up from 55 8C for the neat Pebax1074 to around 47 8C for membrane containing 30 wt% SAPO-34. Generally, Pebax1074 shows a very smooth peak related to glass transition and sharp peaks around 16 8C and 159 8C, which are ascribed to melting points, Tm, of PEO and PA, respectively. Table 2 summarizes Tg and Tm of the neat and MMMs, which are in excellent agreement with those reported in the literature [43,58,59]. The main reason that Tg is affected in nano-composite and MMMs, is chain rigidification of polymer chains around particles. The other reason is physical cross-linking in membrane matrix, resulted from existence of the particles, which is also reported in other studies [8,16]. Ghadimi et al. reported increasing Tg due to cross-linking in membrane matrix, caused by addition of low molecular weight additive and observed better separation factor, compared to the neat membrane [44]. However, due to very low value of Tg for Pebax1074, which is totally in rubbery state, this rigidification and physical cross-linkage does not cause any voidage or defect in polymer–particle interface (as observed in the SEM images). However, zeolite particle loading leads to enhancement in mechanical properties of membranes as discussed

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below in Section ‘Mechanical properties’. Melting points of PA and PEO did not change very much by zeolite loading as it can be seen in Fig. 5 and Table 2. Another feature that could be obtained from DSC analysis is the amount of crystallinity in the polymeric matrix which is calculated as follows: X crystallinity ¼

DH m  100 0 DHm

(5)

where, DHm (J/g) is the heat of melting of samples that is determined by integrating the area under the melting peaks, and 0 DHm is the heat of fusion if the polymer were 100% crystalline. 0 DHm for PEO and PA12 are 166.4 J/g and 209.3 J/g, respectively [44,60]. The total crystallinity of samples could be estimated by considering 55% of PEO-crystallinity and 45% of PA12-crystallinity. Thermal properties of neat Pebax1074 are listed in Table 3 and our results are in accordance with the similar studies [43,44,46]. Existence of crystallinity in a polymeric matrix could provide better mechanical properties for the operation at high pressures. It should be noted that the size of crystal of Pebax is in the order of nano-meter (about 8 nm) [44], thereby micro-size particles cannot locate among these crystals and changing in the membrane crystallinity by zeolites cannot be expected. Mechanical properties Tensile analysis was applied to investigate mechanical strength of the neat and MMMs. All the prepared membranes, containing zeolite up to 30 wt%, had sufficient stability for characterization and permeation tests up to 24 bar. Tensile modulus and strain for the prepared membranes are listed in Table 4. As can be seen from this table, tensile modulus and strain increase with addition of more zeolite into the polymer matrix. The main reason for this observation is that zeolites work as physical crosslinking in the membrane body (as stated in the DSC results)and helps to have membranes with higher mechanical strength. This phenomenon can also prevent membranes from compactness at high pressure permeation testing. However, at 30 wt% zeolite loading tensile strain and modulus decrease which is probably due to higher chance of particles to agglomerate. It should be mentioned that in order to ensure about the repeatability of the results, the tests were repeated 4 times for each membrane.

Transport properties Solubility and diffusion The prepared membranes were tested to evaluate their capacity to dissolve the tested gases at 35 8C and different operating pressures from 4 to 24 bar. The effect of SAPO-34 loading on sorption isotherms of CO2, CH4 and N2 in the neat and MMMs is shown in Fig. 6. As it can be seen SAPO-34 addition leads to more gas sorption and the higher zeolite loading, the more gas sorption in the membranes. The isotherms for all the penetrants are linear or nearly linear which is in accordance with the reported gas sorption isotherms in Pebax1074 [42] and also in rubbery polymers [61]. Sorption isotherms of different gases on SAPO-34 zeolite show that the SAPO-34 affinity toward CO2 is stronger than CH4 and much stronger than N2 [57,62,63]. Also, these sorption isotherms are temperature-dependent. Thereby, as gas solubility capacity of zeolite SAPO-34 is high, compared to the neat Pebax1074 membrane, SAPO-34 addition leads to higher gas solubility of the MMMs. The order of gas sorption in MMMs is: Pebax1074 < 5 wt% MMM < 10 wt% MMM < 20 wt% MMM < 30%wt MMM < SAPO-34.

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Fig. 4. SEM cross sectional morphology of the neat and MMMs: (a) neat Pebax1074, (b) and (d) 10 wt% MMM, (c) and (e) 30 wt% MMM, (f) example of good interfacial adhesion.

The isotherm sorption of gas in zeolites can be obtained from Langmuir-type sorption model, as follows: CF ¼

C 0A b p 1þ bp

(6)

where, CF, C 0A , b, and p are the sorbed concentration (cm3STP gas sorbed=cm3 zeolite), the Langmuir capacity constant, the Langmuir affinity parameter, operating pressure, respectively.

The gas concentration dissolved in polymer phase can be determined, using Henry’s law equation, as follows: C p ¼ KD p

(7)

where, KD is the equilibrium Henry’s law coefficient for the pure polymer and the gas. Thereby it can be concluded that the gas solubility in MMMs follows a general form through the number of gas moles (n) per

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Table 3 Thermal properties of Pebax1074 neat membrane obtained from DSC diagram. Sample weight (mg)

Integral of PEO melting peak (mJ)

Integral of PA melting peak (mJ)

XC (PEO) (%)

XC (PA) (%)

XC (total) (%)

5.2

75.2

160.5

15.8

32.7

23.5

Table 4 Mechanical properties of the neat and MMMs.

Fig. 5. DSC thermograms of the neat and SAPO-34. Table 2 Glass transition temperatures and melting point of the neat and MMMs. Membrane

Tg (8C)

Tm-PEO (8C)

Tm-PA (8C)

Neat Pebax1074 10%SAPO-34MMM 20%SAPO-34MMM 30%SAPO-34MMM

55.1 53.3 51.2 47.4

15.7 16.1 16.3 15.5

159.6 159.3 158.6 158.9

Tensile strain (%)

Tensile modulus (MPa)

Neat Pebax1074 5% SAPO-34 MMM 10% SAPO-34 MMM 20% SAPO-34 MMM 30% SAPO-34 MMM

155.2 162.4 195.1 212.3 176.9

256.4 263.3 281.5 301.8 265.7

As sorption isotherm of MMMs is similar to zeolite sorption behavior, thus it is possible to calculate total concentration of a gas in a MMM, using the following semi-empirical equation [64]: 0

C ¼ y p C P þ yF C F ¼ y p K D p þ yF

unit volume of the membrane as follows: C ¼ ð1  yF ÞC p þ yF C F

Membrane

(8)

where, yF is the volume fraction of filler and C is the gas solubility per unit volume of the MMM.

CA b p 1 þ bp

(9)

The above-mentioned model was correlated with the experimental data of sorption of the tested gases in the prepared membranes and the results are presented in Table 5. As it can be seen the error of calculation which is the ratio of difference between calculated and experimental data on experimental data, is

Fig. 6. Sorption isotherms of the tested gases in the prepared membranes at different operating pressures and 35 8C.

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Table 5 The sorption parameters for the tested gases based on the proposed model. Gas

KD ðcm3STP =cm3 barÞ

C 0A ðcm3STP =cm3 Þ

b (bar1)

Error (%)

CO2 CH4 N2

1.3121 0.0839 0.0130

177.5436 8.5781 1.4579

0.0162 0.0346 0.0273

4.6 5.1 2.3

fairly low and acceptable, thereby the proposed model predicts sorption of gases in the membranes precisely. Also, the solubility coefficients of the tested gases were calculated using sorption isotherms and Eq. (3). As it is shown in Fig. 7, solubility of CO2 increases with pressure, whereas it decreases for N2 and remains almost steady for CH4. The increment in CO2 solubility is mainly attributed to high affinity between this gas and PEO blocks of polymer matrix, thus the slope of increasing concentration is sharper than that of increasing pressure, which based on Eq. (3), leads to higher solubility coefficient. However, for the less soluble gases (CH4 and N2), as they are not very condensable, increment in solubility coefficient is not observed and even for N2, it reduces. This is because of lower increase in concentration with pressure compared to CO2 and CH4. Solubility selectivity of the prepared membranes as a function pressure also shows that as the pressure increases, CO2/CH4 and CO2/ N2 solubility selectivity steadily enhances. CO2/CH4 and CO2/N2 solubility selectivity of the neat Pebax1074 and 20 wt% MMM are shown in Fig. 8. As it can be seen from this figure, increment of solubility selectivity for CO2/N2 is higher than that for CO2/CH4, which is due to lower condensability of N2, compared to that of CH4. However, effect of SAPO-34 zeolite on CO2/CH4 and CO2/N2 pair solubility selectivity show that these values do not change

significantly as zeolite content increases. Fig. 9 illustrates the effect of zeolite loading on CO2/CH4 and CO2/N2 solubility selectivity at 8 bar and 35 8C. Therefore, the probable changes in permeation selectivity would be mainly due to changes in diffusion selectivity, which will be discussed in the permeation results. As it was mentioned above, solubility of CO2 in a Pebax-based membrane is much higher than that of the other gases due to affinity between CO2 and oxygen ether in soft segment (PEO) of polymer. Thereby, it is reported that permeation through Pebax is mainly dominated by solubility. Kim et al. observed that the molecular-size dependence of Pebax is very similar to that of silicon rubber, thus gas flows mainly through rubbery PEO block which has high affinity to quadrupolar gases, like CO2 [65]. In this study the Flory–Huggins theory, which is for evaluation the solubility of more soluble gases in rubbery polymers [66,67], is applied to investigate solubility of gases theoretically, as follows: 2

Ln a ¼ Ln F2 þ ð1  F2 Þ þ xð1  F2 Þ

(10)

where, a is the penetrant activity in the vapor phase. F2 and x are the volume fraction of the sorbed penetrant and the Flory–Huggins interaction parameter, respectively. The volume fraction of the sorbed gases, F2, could be obtained from the equilibrium penetrant concentration in the polymer, as follows: 

F2 ¼ 1 þ

 22414 1 C V¯ 2

(11)

where, V¯2 ðcm3 =molÞ is the penetrant partial molar volume which was estimated and reported by Merkel et al. for rubbery polymers

Fig. 7. Gas solubility in the prepared membranes at different operating pressures and 35 8C MMMs.

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Fig. 8. Pair solubility selectivity at 35 8C and different operating pressures in: (A) the neat Pebax1074 and (B) 20 wt% SAPO-34.

[68]. 22414 is the Avogadro’s number as a conversion factor (cm3 (STP)/mol). It should be mentioned that as the experiments are done at operating pressures lower than 25 bar, non-ideal behavior of gases is negligible. Thereby, the activity coefficient which is generally defined as a = f/fsat can be written as a = P/Psat [69]. These parameters for gases are presented in Table 6. As the critical temperature of the gases, is lower than the experimental temperature (35 8C), the saturation vapor pressure at the experimental temperature is unknown. Hence, they can be estimated by extrapolation of the vapor pressure curve to 35 8C [66]. For each sorption value of the gases, x was calculated as a function of transmembrane pressure and is shown in Fig. 10. As it can be seen from this figure, x is almost constant for all the gases at different pressures, thereby the average values of x can be used to

solve Flory–Huggins equation. However, as shown in this figure, x decreases as zeolite content increases. Generally, the smaller x value means higher affinity between the penetrants and polymer matrix or it can be said that x is mainly affected by similarity or dissimilarity of the gases with the polymer (membrane)[66]. Thus, if the penetrant molecules and polymer matrix are dissimilar from chemical point of view, without any specific interaction, the x value is higher than when the membrane matrix and penetrant are similar and membrane-interaction exists. That is why x for CO2 is lower than that for other gases due to CO2-plilic ether functional groups in the structure of Pebax [70,71]. However, x value decreases as zeolite content increases (Fig. 10). It indicates that MMMs show more affinity to the tested gases and the capacity of the membranes to dissolve penetrants in its structure increases. Thereby, as it was observed earlier in the results of concentration, MMMs sorb gases higher than the neat Pebax1074 membrane. Solubility in the polymeric membranes is determined by condensability (related to critical temperature) and polymer–gas

Fig. 9. Pair solubility selectivity at different zeolite loading and 8 bar, 35 8C MMM.

Table 6 Gas physical properties including critical properties, saturation vapor pressure at 35 8C, partial molar volume and kinetic diameter. Gas

Tc (K)

Vc (cm3/mol)

Psat (atm)

V¯ 2 (cm3/mol)

Kinetic diameter (A˚)

CO2 N2 CH4

304.1 126.2 190.5

94 86.2 98.6

81.9 902 359

45 48 46

3.3 3.64 3.8

Fig. 10. The Flory–Huggins interaction parameter, x, at different operating pressures and zeolite loading.

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Table 7 The infinite dilution solubility at 35 8C in the neat Pebax1074 and MMMs. Membrane

CO2

N2

1

Neat Pebaxl074 5% MMM 10% MMM 20% MMM 30% MMM

1

S Experimental

S F–H

S Experimental

S F–H

S1 Experimental

S1 F–H

1.0785 1.1363 1.1856 1.3070 1.3599

1.1103 1.1537 1.2277 1.3297 1.4019

0.0148 0.0163 0.0174 0.0190 0.0207

0.0136 0.0148 0.0157 0.0172 0.0186

0.0871 0.0903 0.0975 0.1110 0.1308

0.0860 0.0890 0.0943 0.1079 0.1277

interaction (characterized by x values) and the trade-off between them, specifies the overall solubility of the gas [68]. The order of x value for the tested gases is: N2 > CH4 > CO2, which is exactly opposite to condensability. It means that both condensability and polymer–penetrant interaction are consistent and lead to higher solubility for CO2, as observed in the solubility results. As it was seen, the solubility coefficients of the tested gases change with pressure (Fig. 7). Generally, solubility coefficient for all gases can be expressed by a linear function of pressure, as follows: S ¼ S1 þ nP

CH4

1

(12)

where, n describes the pressure dependence of solubility and S1 is the infinite dilution solubility which can be calculated as

1

follows: S1 ¼ lim p ! 0

  C P

(13)

In addition, S1 can also be determined by Flory–Huggins (F–H) theory, as follows: S1 Psat ¼

22414 V¯ 2 exp ð1 þ xÞ

(14)

The results show that for low-sorbing gases (N2 and CH4), the solubility is almost independent from pressure and their n values for the neat Pebax1074 membrane is near zero (5  106 and 9  105 for CH4 and N2, respectively). However, for CO2, as the most soluble gas in this study with high affinity to membrane matrix, it was seen that solubility increases with pressure, thus

Fig. 11. Effect of zeolite loading on permeability and ideal selectivity of MMMs at T = 298 K.

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Fig. 12. Pair diffusion selectivity at different zeolite loading and 8 bar, 35 8C.

11

Fig. 13. Comparison between CO2 permeation obtained from experimental results and Maxwell equation at 25 8C and 4 bar.

Permeability larger n value is obtained and for the neat Pebax1074 membrane, n is about 1.27  102. Also, as it is presented in Table 7, the values for S1, from experimental and the F–H theory are fairly close. The observations for S1 values of all the prepared membranes shows that the order of solubility follows the condensability of gases, characterized by their critical temperature (CO2 > CH4 > N2).

Effect of zeolite loading. Fig. 11 shows the effect of zeolite loading on permeability and ideal selectivity of CO2/N2, CO2/CH4 for the neat and MMMs, up to 30 wt% zeolite content. As it is clear, at low zeolite loading (5 wt%), there are no significant changes in permeability because gas transport takes place mainly in polymeric matrix [72,73]. Increasing the zeolite content up to

Fig. 14. Effect of operating pressure on permeability and ideal selectivity of MMMs at various zeolite loading and T = 298 K 35 8C.

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30 wt% of the polymer weight leads to increment in permeability of CO2 and N2, whereas CH4 permeability shows an opposite trend. The pore structure of porous fillers and adsorption capacity of inorganic fillers are the key factors for separation performance of fabricated MMMs [74]. Molecular sieving effect of zeolite on the one hand, lets CO2 which possess smaller kinetic diameter than zeolite pore size, to pass easier and on the other hand, high sorption capacity of SAPO-34 for CO2 [62,75], lead to increase CO2 permeability, and consequently higher permeation selectivity over N2 and CH4. However, CH4 and N2 with higher kinetic diameter, diffuse slower than CO2, so as the zeolite loading increases, CH4 permeation decreases and N2 permeation slightly rises. Increasing in N2 permeation is because of its kinetic diameter that is somehow smaller than SAPO-34 pore size, but in comparison with CO2, SAPO-34 provides favorable molecular sieving contribution that leads to better selectivity for CO2 over N2, compared to the neat membrane. However, permeation of CH4, with the highest kinetic diameter, decreases continuously as filler loading increases. These observations are fairly consistent with other work done on SAPO34 MMM [72]. They also observed increment in CO2 permeation along with reduction of CH4 permeation, which is related to zeolite pore size and gases molecular size. Some studies have reported increment in permeation along with reduction in permselectivity which is resulted by nonselective voidage at polymer–filler interface [24,76]. But, the growing trend of permeation in this research for CO2 and N2, had no negative effect on selectivity values which confirms the contribution of molecular sieving mechanism of zeolites in separation and fabrication defect-free membrane [77].

As it can be seen in Fig. 11, addition of more zeolite to 30 wt% reduces the slope of CO2 and N2 permeation increment and leads to almost constant CO2/N2 selectivity value. The reason is probably due to long tortuosity of diffusion path or agglomeration of particles at high loading that decreases the rate of diffusion of gases in pores which results in less increment of permeation, compared to the membranes with lower zeolite content [16]. However, CH4 permeation followed its previous trend and decreased even more than lower loading, resulting in increment in CO2/CH4 pair selectivity. Although, CH4 has an equal kinetic diameter to SAPO-34 pore size, but it can diffuse through the pores slightly. But high zeolite content causes long diffusion path and makes CH4 diffusion almost impossible through inorganic phase. Unlike lower loadings that CH4 diffuses slowly, at high loading, CH4 diffusion through the zeolite pores is negligible [72]. Ideal selectivity of CO2 over N2 and CH4 is shown in Fig. 11 (red graphs). As an overall trend, it is quite evident that both CO2/N2 and CO2/CH4 selectivities enhances with increment in zeolite loading. Improvement of CO2/CH4 selectivity is quite interesting and for example at 4 bar this value increases from almost 18 to near 30 for 20% SAPO-34 MMM which is because of high increment in CO2 permeation and decline in CH4 permeation. This fairly high enhancement in CO2/CH4 selectivity indicates contribution of molecular sieving in separation which is reported elsewhere [72]. Also, as CO2 permeability increases more than that of N2, relatively, CO2/N2 ideal selectivity improves, either and for the same membrane as above said, selectivity improves from 60 to near 69. However, in a similar study in which another grade of Pebax (Pebax1657) has been used to prepare a MMM, increment in CH4 permeability is also observed [48]. In this study reduction in CH4

Fig. 15. Effect of temperature on permeability of the neat and MMMs at 8 bar.

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zeolite loading; rather the effect of improvement in diffusion selectivity is more dominant. As it is shown in Fig. 12, CO2/N2 and CO2/CH4 diffusion selectivity at 35 8C and 8 bar (the same operating condition as Fig. 9), increases continuously as zeolite content increases. It could be concluded that this is due to molecular sieving effect of the dispersed phase that leads to higher diffusion coefficients for gases with smaller molecular size and consequently, enhancement in diffusion selectivity. Increasing in diffusion selectivity is about 1.8 and 1.3 times for CO2/CH4 and CO2/N2, respectively. This indicates dominant effect of diffusion selectivity on permselectivity and also higher improvement in CO2/CH4 diffusion selectivity, which is consistent with zeolite pore size and gases molecular size. Maxwell equation, as a well-known derivation, was used to predict the permeability of CO2 through the neat and MMMs [78], as follows: PMMM ¼ P c Fig. 16. Effect of temperature on deviation of CO2 permeation in MMMs from neat Pebax1074 membrane at 8 bar.

permeability is observed, owing to molecular sieving effect of zeolites that resists against CH4 permeation. In fact, this is the aim of using SAPO-34 as filler, is selected permeation which is also reported elsewhere [72], in which they observed about 97% and 55% increment in CO2/CH4 ideal selectivity and CO2 permeation. Additionally, the membranes were tested 4 times to ensure the observations and results obtained in this study are supported by changes in transport properties and diffusion selectivity, as mentioned below. It was mentioned in Section ‘Solubility and diffusion’ and Fig. 9, solubility selectivity did not change considerably with SAPO-34

  Pd þ 2P c  2FðPc  P d Þ P d þ 2Pc þ FðP c  Pd Þ

(15)

where, PMMM, PC and Pd are permeability of MMM, polymer phase (continuous phase) and dispersed phase (zeolite), respectively. F is the volume fraction of the dispersed phase. Permeability of CO2 through SAPO-34 membrane were obtained from the literature which is about 500–550 Barrer [57,62,63,75]. As it can be seen in Fig. 13, the trend of changing in permeability from Maxwell equation is similar with experimental results and CO2 permeation increases with increment of zeolite content that shows permeation through the prepared MMM can be correlated with permeation of pure components. However, the difference between the permeabilities can be due to chain rigidification and physical cross-linking (as observed in the results of membrane characterization) and slight agglomerations of particles, particularly at higher

Fig. 17. Long time performance of the neat and 20%MMMs for 48 h.

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zeolite content (at higher loading this difference is more evident). Also, the estimation of permeability through SAPO-34 membrane was not quite accurate. In addition this equation is used for ideal dispersion of particles in polymer matrix and some defects like chain rigidification or partial pore blockage are not considered in the results of prediction [79]. Effect of operating pressure. Effect of operating pressure on permeation properties of MMMs is shown in Fig. 14. As it can be seen from this figure, as low pressure increases from (4 bar) up to 24 bar; permeation of the tested gases also increases. Permeation for dense membranes is based on solution–diffusion mechanism in which permeants after dissolving in membrane matrix, diffuse across the membrane from high pressure side to low pressure side. Both solution and diffusion have important effect on permeation of membranes based on Eqs. (3) and (4). Increasing pressure leads to higher concentration of penetrants in polymer matrix but based on Eq. (3), it can cause reduction in solubility of non-condensable gases (like N2 as observed in Section ‘Solubility and diffusion’). However, increment in concentration, caused by increasing pressure for condensable gases like CO2, is much higher than for non-soluble and light gases, such as N2, CH4 and H2, which results in enhanced CO2 solubility. In contrast, for non-condensable gases, the effect of pressure on solubility is more dominant and as shown in Fig. 7, solubility decreases or remains constant. Increasing pressure also acts as a driving force, leading to higher movement of gases through polymer matrix. Although, high pressures also can cause compactness in polymer chain, which results in lower free volume in membrane structure and reduction in diffusion [16]. Thereby, for discussion about the influence of pressure on permeation, these different effects are involved. For the effect of pressure on gas permeability of polymeric membranes in the literature, the most observed results are either increasing trend for the condensable gases like CO2 or declining/ stable trend for non-condensable gases [8,16,80,81]. However, as it has been reported recently [44,46], for the case of Pebax, permeation of both condensable and non-condensable gases increase. The main reason for this phenomenon is that Pebax is a semi-crystalline polymer which possesses about 23.5% crystal in its hard and soft segments. These crystals are like rigid parts and physical crosslinking in the membrane body, which can provide mechanical strength and resist against compactness and reduction in chain mobility of membrane, caused by high pressures. Although, this increasing effect for light gases is not significant in comparison with the effect on CO2, however, it influences the pair selectivities. Moreover, increase in concentration of condensable gases with pressure in membrane leads to plasticization of polymer matrix and more movement in polymer chains. Consequently, molecular-size hole in polymer matrix is created easier with lower energy for solution and diffusion, thereby, permeability enhances for CO2 quite obviously [82]. For MMMs a similar trend was observed. Increase in pressure is an effective driving force to enhance diffusion of gases, particularly those with kinetic diameter less than the zeolite pore size (CO2, N2). Therefore, gas diffusion through zeolite particles in membrane matrix increases with pressure. In addition, it should be mentioned that zeolite existence in membrane structure provides additional physical cross-linking which makes membranes more resistant for testing at high pressures, as observed from the results of tensile. CO2 selectivity over CH4 and N2 increases monotonously with increasing pressure as shown in Fig. 14 (red graphs). It is due to more increment of CO2 permeation because of the following two reasons: (1) higher tendency of CO2 for condensation and (2) lower molecular size of CO2 which causes easier diffusion. For example for membrane containing 20 wt% SAPO-34, permeability increases about 62%, 32% and 25% for CO2, N2 and CH4, respectively which indicates increment in pair selectivities.

Effect of operating temperature. Effect of operating temperature on permeability and selectivity of MMMs were studied from 25 8C to 65 8C at the pressure of 8 bar. Fig. 15 shows that permeability of all the tested gases increases with temperature. The reason can be deduced by looking at Arrhenius types of equation that are used to describe temperature dependency of transport properties through dense polymeric membranes as follows:   Ep P ¼ P0 exp  (16) RT   DH s S ¼ S0 exp  RT

(17)

  E D ¼ D0 exp  d RT

(18)

E p ¼ Ed þ DHs

(19)

DHs ¼ DHCondensation þ DHMix

(20)

Fig. 18. Selectivity of CO2 over N2 and CH4 and N2 versus CO2 permeability for the neat and MMMs at 25 8C and 4 bar in the presence of Robeson upper bound.

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where, Ep, Ed, DHs are activation energy for permeation and diffusion and the heat of solution, respectively. DHCondensation is the molar heat of condensation of the permeant and DHMix is the partial molar heat of mixing.

15

Ed is usually positive, thereby, diffusion increases with temperature for all the gases [65]. For condensable gases, like CO2, H2S and vapors, DHCondensation is negative and dominant, thus DHs is negative and consequently solubility decreases with rising

Fig. 19. Performance of Pebax-based nano-composite and MMMs in this work for CO2/CH4 and CO2/N2 separation at 25 8C and 4 bar and comparison with the literature results: Li et al. [39], Sridhar et al. [52], Rahman et al. [50], Sforc¸a et al. [55], Friess et al. [40], Yu et al. [49], Murali et al. [53].

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temperature. For these gases the sorption energy evolved exceeds the energy needed to make molecular scale hole in the polymer [82]. Unlike condensable gases, for small and non-interacting gases like N2, H2, CH4, which are all above their critical temperature at room temperature, DHCondensation is small and DHs is mainly governed by DHMix which is positive for these gases because of weak interaction between gas and polymer. Hence, DHs is positive and solubility increases when temperature goes up. Thus, based on Eq. (19), Ep for CO2 is lower than that for CH4 and N2 and that is why permeability of CO2 is higher than that of CH4 and N2 (based on Eq. (16)). But, increment in permeation of gases with higher Ep (N2 and CH4), is higher than that of CO2 with temperature. However, diffusion, which increases with temperature, is more temperaturedependent compared to solubility, consequently, in the most cases permeability enhances with increasing temperature. Although, in some cases opposite results have been reported especially near the glass transition temperature of polymer [83]. For MMMs the same trend in permeability with temperature is observed and increasing in permeability for all tested gases in MMMs, is somehow more than that for the neat Pebax1074. This is probably because of two main reasons: (1) incorporation of molecular sieving mechanism into membrane matrix instead of solubility by zeolite addition (2) temperature elevation increases the chain mobility of polymer around filler and makes them more activated for gases to penetrate. Temperature elevation increases mobility of gases, hence diffusion enhances. Diffusion in zeolite pores also follows an Arrhenius type equation like diffusion in polymers. On the one hand, activation energy of diffusion in zeolites is higher for molecules with larger kinetic diameter [84], thereby more increment in permeation with temperature is expected for CH4 and N2 in comparison to CO2. On the other hand, based on Le Chatelier’s principle, it has been observed that higher temperature decreases adsorption of gases in zeolites and vice versa [62]. Thus, another possibility for increasing in permeation with rising temperature can be due to less sorption of gases on the internal surface of cavities of zeolites at higher temperature [62]. High affinity between permeants and zeolite surface lead to adsorption and entrapment of gases into the zeolite pores which can block zeolite pores. However, at higher temperatures, gas mobility as well as the polymer chain mobility around zeolites increases, hence gas molecules can leave pores (desorb) easier. It must be pointed out that this assumption cannot be accepted with certainty because several factors in both phases of MMMs are changing with temperature simultaneously and their influences have not been studied separately. As it can be seen from Fig. 15, the slope of increasing permeation for N2 and CO2 after 45 8C, especially for MMM at higher loadings, is sharper than that at lower temperatures. Also, unlike at low temperatures that permeation for membranes with 20 wt% and 30 wt% zeolite, was almost similar and slight particle agglomeration or polymer chain rigidification was suggested as the probable reason, at higher temperatures permeation through membrane with 30 wt% loading is higher than that of 20 wt%. This proves reduction in chain rigidification and improvement in polymer chain mobility around particles that makes them more active [8]. Fig. 15 (red graphs) shows the effect of temperature on CO2/N2 and CO2/CH4 ideal selectivity at 8 bar and various SAPO-34 zeolite loading. As shown in Fig. 15, CO2 selectivity over CH4 and N2 decreases with rising temperature. This is because of more increment in permeation of CH4 and N2 in comparison with CO2, which was discussed above. In order to investigate the effect of temperature on membrane properties, a term, (PMMM  Pneat)/Pneat, which is in fact fractional change in permeability with temperature, at different loading and temperatures for CO2 was calculated. As shown in Fig. 16 this parameter is generally on the growth with temperature for all

zeolite loadings but its increasing trend is sharper after 45 8C especially for 30% zeolite loading. This can confirm our previous assumption about more mobility for polymeric chains and gases at higher temperature. In order to investigate the long time performance of the prepared membranes, the permeability of neat and 20%MMMs were tested for 48 h. The results in Fig. 17 show that permeation of the gases through membranes changes slightly which is probably due to error of repeatability, measurements or changing membranes. Also, changes in the selectivity ratios are not very significant, thereby it can be concluded that performance of the membranes do not alter with time, considerably. Separation performance of prepared MMMs in order to pass ‘‘Robeson upper bound limit’’ was studied to find out enhancement in selectivity and permeability, as shown in Fig. 18. For the case of CO2/N2 separation, the fabricated membranes are able to pass the upper bound, while the neat membrane could not meet the limit. For CO2/CH4, the neat membrane is under even prior upper bond, presented in 1991 [85]; however, with incorporation of SAPO-34 in polymer matrix, the membrane separation performance trend overcomes the prior limit and approaches the present upper bound [5], which means excellent enhancement of the prepared membranes for this application that can provide sufficient permeability and selectivity. Comparison between CO2/CH4 and CO2/N2 selectivity in this work and other Pebax-based membranes, modified by inorganic particles indicates great improvement, especially for CO2/CH4. As it is shown in Fig. 19, incorporation of SAPO-34 in pebax1074 matrix leads to fairly interesting performance of membranes for CO2/CH4 and CO2/N2 separation, compared to similar works.

Conclusion Mixed matrix membranes were fabricated using Pebax1074 and zeolite SAPO-34 for gas separation. Permeation of CO2, N2 and CH4 was measured at wide range of operating conditions form 4 to 24 bar and 25 8C to 65 8C. Molecular sieving effect of zeolite led to increment in CO2 and N2 permeation due to lower molecular size of CO2 and N2 compared to zeolite pore size. In contrast, CH4 permeation was reduced and resulted in excellent enhancement in CO2/CH4 separation which moved membrane performance toward Robeson upper bound. CO2/N2 selectivity also was improved and crossed Robeson upper bound for this separation. Permeability of gases improved more slightly after 20 wt% zeolite loading which indicates that MMM preparation with the current method and materials, gives the best results at this zeolite content with almost 70% and 15% improvement in CO2/CH4 and CO2/N2 selectivity, respectively. SAPO-34 addition led to higher solubility coefficients, compared to the neat membrane. The results showed that diffusion selectivity enhanced whereas solubility selectivity did not change remarkably, which is due to molecular sieving effect and contribution of SAPO-34 on separation properties of the prepared membranes. The interesting observations for CO2/CH4 and CO2/N2 selectivities show the possibility of using the fabricated membranes in natural gas sweetening and post-combustion carbon capture. Also, a semi-empirical model was proposed to correlate sorption isotherms with pressure and value of added zeolite and the calculations showed fairly minor error between experimental results and predicted data from model. Acknowledgment The authors gratefully acknowledge the financial support of this project by New Technology Group, Petrochemical Research and Technology Company (Grant No. 0870289106).

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