Comparison between ZIF-67 and ZIF-8 in Pebax® MH-1657 mixed matrix membranes for CO2 separation

Journal Pre-proofs Comparison between ZIF-67 and ZIF-8 in Pebax ® MH-1657 mixed matrix membranes for CO2 separation Shadi Meshkat, Serge Kaliaguine, Denis Rodrigue PII: DOI: Reference:

S1383-5866(19)32260-9 https://doi.org/10.1016/j.seppur.2019.116150 SEPPUR 116150

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

30 May 2019 1 October 2019 1 October 2019

Please cite this article as: S. Meshkat, S. Kaliaguine, D. Rodrigue, Comparison between ZIF-67 and ZIF-8 in Pebax ® MH-1657 mixed matrix membranes for CO2 separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116150

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Comparison between ZIF-67 and ZIF-8 in Pebax MH-1657 mixed matrix membranes for CO2 separation Shadi Meshkat, Serge Kaliaguine, Denis Rodrigue Department of Chemical Engineering, Université Laval, Quebec City (QC), G1V 0A6, Canada. [email protected]

Abstract In this study, mixed matrix membranes (MMM) based on poly(ether-b-amide) or Pebax® were prepared using a synthetized ZIF-67 and a commercial ZIF-8 (Basolite® Z1200) to determine the effect of particle content (0, 2, 3, 4 and 5 wt.%) on CO2, CH4 and N2 single gas permeability as well as CO2/CH4 and CO2/N2 ideal selectivity. The MMM morphology was evaluated first by scanning electron microscopy (SEM) where excellent dispersion without aggregation was observed. The thermal properties were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showing the disruptive role of fillers on polymer chain mobility. Fourier transform infrared spectroscopy (FTIR) showed that no significant chemical interaction between the polymer and ZIF particles occurred. Finally, gas permeation results at 35 °C and 11 bar revealed higher CO2 permeability for all MMM, but especially for Pebax/ZIF-67 with a 130% increase (162 Barrer) compared to the pristine Pebax membrane (70 Barrer), while the Pebax/ZIF-8 produced a lower (85%) increase (130 Barrer). Due to the smaller pore aperture of ZIF-67, CO2 selectivity over CH4 and N2 was higher compared to ZIF-8. Overall, the Pebax/ZIF-67 system was able to overcome the Robeson upper bound for the CO2/N2 separation. Keywords: Pebax; mixed matrix membrane; ZIF-67; ZIF-8; CO2 separation.

Declaration of interest: None.

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1. Introduction During the last decades, membranes have been studied and improved to efficiently separate gases and become substitute for more conventional energy consuming separation techniques such as amine absorption and cryogenic distillation. Currently, membranes are favored over other separation techniques due to their simplicity, low energy consumption and scalability. However they still suffer from low productivity, as well as low thermal and mechanical stability [1]. The basic materials used for most of the successful gas separation membranes are polymers. In addition to technical challenges associated to polymer membranes fabrication, polymeric membranes are subject to a trade-off behavior between permeability and selectivity, which is well represented by the Robeson upper-bound curves for various gas pairs [2]. To overcome this limitation of neat polymer membranes, fillers are introduced to get mixed matrix membranes (MMM). Furthermore, as CO2 separation from other light permanent gases such as N2, CH4 and H2 is of interest, both the polymer and filler should have suitable functional groups to differentiate between the acidic polar CO2 from nonpolar gases [3]. This is why polymers containing ether, carboxylate, amine or acetate groups are more interesting as their functional groups can interact with CO2 to increase the membrane solubility selectivity [4]. On the other hand, the selected fillers should be carefully chosen not only to display good adhesion with the polymer matrix to avoid interfacial defects, but also to introduce a controlled porosity into the dense polymer to improve the membrane permeability. Besides this trade-off, plasticization induced by the presence of CO2 inside a polymer matrix might deteriorate the membrane selectivity in case of mixed-gas separation, which can also be tackled by polymer cross-linking or fillers addition into the matrix [5]. Polymers based on poly(ethylene oxide) (PEO), because of their polar ether groups leading to enhanced CO2 solubility selectivity, have recently gained growing attention. However, PEO suffers from high crystallinity and low mechanical strength when used alone in a membrane [6]. Among the most known PEO-containing polymers, poly(ether-block-amide) (PEBA) is a block copolymer composed of a rubbery PEO block providing interaction with CO2 and a glassy block of polyamide (PA) providing mechanical strength [7]. PEBA is commercially available under the trade name of Pebax which is now widely investigated as a matrix for the production of mixed matrix membranes [8]. Although a wide range of particles has been used to produce MMM, porous fillers like metalorganic frameworks (MOF) and covalent-organic frameworks (COF) have been shown to have exceptional properties such as tunable pore size, shape or functionality, low density, high CO2 adsorption capacity, high porosity and specific surface area combined with moderate thermal and chemical stability [9–11]. MOF are composed of a metal cluster connected by organic linkers, in which the organic moiety is responsible for improving the wetting properties between the metallic and organic phases as well as enhancing filler interaction with the polymer [12]. Zeolitic imidazolate frameworks (ZIF) are a subfamily of MOF with a zeolite topology in which the metal clusters are connected by imidazole linkers. Their most important feature is their large diameter cages with narrow pore apertures in the range of the CO2 kinetic diameter [13]. The addition of MOF in polymers to produce MMM is currently an active area in gas separation membrane research and several new MMM have been recently introduced and studied.

3 ZIF-8, the most studied ZIF in gas separation membranes, has a framework of Zn(II) metal centers bridged to the nitrogen atoms at the 1,3-positions of five-membered imidazolate rings to form a sodalite topology [14–20]. The metal-substituted counterpart of ZIF-8 is known as ZIF-67 where Zn is substituted by Co. It was first introduced by Banerjee et al. [21] and is reported to have stiffer Co-N bonds than Zn-N. Although several groups have investigated ZIF-67 for various applications such as ethanol recovery via pervaporation [22], adsorptive propane/propylene separation [23], gas sensing [24] and adsorptive removal of azo dyes [25], very few researches studied the effect of ZIF-67 in membranes and more especially for gas separation [26,27]. As a pioneering work in 2015, Kwon et al. [28] studied ZIF-67 membranes prepared by the heteroepitaxial growth of ZIF-67 on ZIF-8 seeds for propylene/propane separation and reported a separation factor as high as 209. The first mixed matrix membrane based on ZIF-67 was also introduced by the same group in 2017 when they produced 6FDA-DAM/ZIF-67 MMM and studied the separation of propylene/propane [29]. Lee et al. [30] synthetized a hollow ZIF-8 nanocrystal via the use of ZIF-67 as sacrificial templates and dispersed it in a home-made graft copolymer to get mixed matrix membranes. They claimed that subsequent to the filler inclusion, the gas diffusion resistance was substantially decreased leading to a five-fold increase of CO2 permeability, while the CO2/CH4 selectivity remained constant. For low permeability polymers, Vega et al. [31] studied the effect of ZIF-8, ZIF-67 and their mixture in polyetherimide (PEI). The results showed that PEI/ZIF-8 displayed the highest CO2/N2 separation factor due to a size exclusion effect (46), while PEI/ZIF mixture produced the highest CO2 permeability (9.4 Barrer). But for high permeability polymers, Wu et al. [32] conducted a thorough investigation on PIM1/ZIF-67 MMM and reported good ZIF adhesion and dispersion inside PIM-1, coupled with a more pronounced molecular sieving effect for CO2 compared to ZIF-8 due to ZIF-67 smaller pore aperture, with CO2 permeability of 6700 Barrer and CO2/N2 ideal selectivity of 24. Based on the promising results reported on the incorporation of ZIF-67 in polymer matrices to improve the gas separation properties of neat membranes, the main objective of this work is to investigate the effect of particle content on CO2 separation performances of Pebax and to compare the results with the more popular ZIF-8. To the best of our knowledge, this is the first time that Pebax/ZIF-67 MMM are produced. As a first step, only single gas permeation tests are performed for a CO2 separation from N2 and CH4. Each MMM was thoroughly characterized in terms of morphological and thermal properties to determine the effect of particle content.

2. Experimental 2.1. 

Materials

Pebax MH-1657 (abbreviated as Pebax) was provided by Arkema Inc. (USA). The pellets were dried for at least 7 h at 70 °C prior to use. Ethanol (95%, GreenField Speciality Alcohols Inc, Canada) was used as the solvent. Commercially available ZIF-8 (Basolite Z1200, SigmaAldrich) was used in this work. The materials used for the synthesis of ZIF-67 nanoparticles were cobalt nitrate hexahydrate (Co(NO3)2.6H2O, 98%, Sigma-Aldrich), 2-methylimidazole (2-mIM, 99%, Sigma-Aldrich), methanol (ACS 98%, Fisher Scientific) and chloroform (99.8%, VWR Chemicals). All of them were used as received.

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2.2.

Synthesis of ZIF-67

The ZIF-67 synthesis was adapted from Tan et al. [33], in which solutions of 0.359 g of cobalt nitrate hexahydrate in 25 mL of methanol and 0.811 g of 2-mIM in 25 mL methanol were prepared separately. Then, both solutions were rapidly mixed under vigorous stirring for 1 min to give a dark purple solution. Subsequently, the solution was left to stand overnight. To collect the resulting powder, the solution was centrifuged and washed three times with methanol and one time with chloroform before being oven dried overnight at 100 °C.

2.3.

Mixed Matrix Membranes Fabrication

Neat Pebax membranes were prepared first as reported in our previous study [8]. A similar procedure was adopted for the preparation of Pebax/ZIF-8 and Pebax/ZIF-67 MMM. Firstly, a solution of 5 wt.% polymer in two-third of the final solvent amount of EtOH/H2O (7:3) was prepared at 90 °C under reflux and vigorous stirring. Separately, the different nanoparticle loading (2, 3, 4 and 5 wt.% of the polymer mass) was dispersed in one-third of the final solvent amount and sonicated (Hielscher UP100H Ultrasonic Processor) for 30 min at 100% amplitude. Then, the nanoparticle suspension was added to the Pebax solution and further stirred for 24 h under the same conditions. Next, the homogeneous solution was casted on a glass plate and the solvent was evaporated at room temperature for 48 h. Finally, the membranes were carefully peeled off the glass plate and further dried overnight in a vacuum oven at 70 °C. The samples are coded based on the weight fraction of filler in parenthesis (e.g. Pebax/ZIF-67 (2) is Pebax with 2 wt.% of ZIF-67). It is important to mention that filler loadings above 5 wt.% were not possible to produce as the membranes broke during the peel off stage.

2.4.

Gas Permeation Test

Gas separation performance of the membranes was tested in a custom-built setup as reported elsewhere [34]. Circular flat dense membranes of 21 cm2 were mounted in a blind flange and sealed by silicon O-rings. The cell was placed in an oven with temperature control. For each test, both sides of the membrane were evacuated for several hours and kept at 35 °C. To start the test, the test gas was introduced on the upper side of the membrane at a pressure of 11 bar, while the permeate side was kept evacuated. The pressure variation with time on the permeate side was recorded until a steady-state was reached (normally after 1 h). The permeability of each single gas in the membranes is determined by Equation (1) in Barrer (10-10 cm3 (STP) cm cm-2 s-1 cmHg-1): Eq. (1) where A is the membrane area (21 cm2), V is the constant volume of the permeate side of the cell (30 cm3), Q is the flow rate (cm3(STP) s-1), R is the universal gas constant (6236.56 cm3 cmHg mol-1 K-1), T is the absolute operating temperature (308 K), l is the membrane thickness (0.0095±0.0005 cm, as displayed in Figure 3d), p2 and p1 are the feed pressure and permeate pressure (psi) respectively, and

is the rate of pressure increase on the permeate side (cmHg s-1).

Ideal selectivity (ij) is defined as the ratio between the permeabilities of the more permeable gas (i) to the less permeable one (j). It also represents the product between the diffusivity selectivity and the solubility selectivity as based on the solution-diffusion theory [35]:

5 Eq. (2)

3. Results and discussion 3.1.

Characterization of ZIF-67

The detailed characterization techniques as well as the corresponding figures are reported as Supplementary Information. Figure 1 presents the chemical structure of both ZIF with a general formula of M(2-methylimidazole) or briefly M(2-mIM)2 where M is Zn for ZIF-8 and Co for ZIF67. These structures have a sodalite (SOD) topology of interconnected 6-ring windows with a theoretical aperture of 3.4 Å and a pore cavity of 11.6 Å [36,37]. The metal-imidazolate-metal angle is 145, similar to the Si-O-Si angle in most zeolites and the five-membered imidazolate ring bridging the nitrogen atoms at the 1,3-positions with the tetrahedral metal center [38]. The morphology of ZIF-8 and ZIF-67 was investigated by SEM (Figure 2) where both nanoparticles exhibit uniform particle sizes. Nitrogen adsorption test confirms a type I isotherm for ZIF-67 with a BET surface area of 1465 m2/g and a total pore volume of 0.59 cc/g (Figure S1), while according to the literature the values for ZIF-8 are 1509 m2/g and 0.54 cc/g [39], respectively. As expected, the surface area of ZIF-67 is lower than ZIF-8, while its pore volume is slightly higher [27]. The dynamic light scattering (DLS) results show that the average particle size of ZIF-8 and ZIF-67 are 700 and 300 nm respectively, both presenting a range of particle size distribution of 500-1100 nm for ZIF-8, compared to 140-700 nm for ZIF-67 (Figure S2). Based on the thermogravimetric analysis curves in Figure S3, no significant weight loss is observed below 320 C for ZIF-8 and 500 C for ZIF-67, which confirms the successful removal of solvent and unreacted linker from the MOF. At higher temperatures, the frameworks start to disintegrate and totally collapse at 560 C and 370 C for ZIF-67 and ZIF-8 respectively, which corresponds to the highest peak in the DTG curves associated to the main weight loss stage. Consequently, ZIF-67 seems to be more thermally stable than ZIF-8. Furthermore, while ZIF-8 is completely decomposed at 420 C, the residual weight of ZIF-67, corresponding to the metal center, is still 51% up to 800 C, which is related to the higher decomposition temperature of cobalt imidazolate [29,32]. In terms of elemental analysis, the FTIR spectra for ZIF-8 and ZIF-67 (Figure S4) are in agreement with the literature with the characteristic peaks of ZIF: 1585 cm-1, 990 cm-1 and 1145 cm-1 where the former corresponds to C=N stretching vibration and the latter two are ascribed to C-N stretching [18,40]. Furthermore, according to Bustamante et al., the absence of an absorption band at 1850 cm-1 confirms the formation of imidazolate [41]. In terms of phase analysis, Figure S5 displays the XRD spectra of ZIF-8 and ZIF-67 showing their crystalline structure. The diffraction peaks of ZIF-8 and ZIF-67 are consistent with a sodalite structure as reported elsewhere [25,29]. It is worth noting that, in order to get a proper XRD pattern, the samples require to be thoroughly washed and dried to remove all the impurities from the pores.

6

Co(mIM)

Zn(mIM)2

2

Figure 1. The chemical structure of ZIF-8, ZIF-67 and their crystal.

(a)

15 KV 15,000 X

(b)

1 µm

Figure 2. SEM images of: (a) ZIF-8 and (b) ZIF-67 nanoparticles.

3.2. Characterization of Pebax/ZIF-8 and Pebax/ZIF-67 MMM 3.2.1. Morphology analysis (SEM) The cross-sectional view of Pebax/ZIF-67 (2) and (5) membranes are presented in Figure 3a and 3b, respectively. Based on these images, the particles are embedded relatively far from one another suggesting excellent dispersion, and no aggregation or microvoid can be observed. The reason behind this homogeneous particle dispersion is the pre-sonicating step in the membrane production, in which the particles are well-dispersed in the solvent by intense sonication prior to being introduced in the polymer solution [42,43]. In addition, the SEM images validate the ZIF-67 particle size which was previously determined by DLS (Figure S2). In order to test their stability, the casting solutions of Pebax/ZIF-67 were left to rest and no precipitation or deposit was observed after 24 h (Figure 3c).

7 (b)

(a)

15 KV 15,000 X

(c)

1 µm

15 KV 15,000 X

1 µm

(d)

15 KV 1,000 X

10 µm

Figure 3. Cross-section SEM image of: (a) Pebax/ZIF-8 (2) and (b) Pebax/ZIF-67 (5). (c) Pebax/ZIF-67 (5) casting solution after resting for 24 h showing good stability. (d) Lowmagnitude cross-section SEM image of Pebax/ZIF-67 (5) to display membrane thickness.

3.2.2. Thermal analysis (TGA-DTG and DSC) Thermal analysis of the membranes provides invaluable information about the modifications induced to the polymer host by the filler, as well as the thermal stability of the as-synthetized MMM. The Tg corresponding to the PEO segment, as that of PA cannot be detected by DSC [44]. The Tg of neat Pebax is -53.1C, which is in good agreement with the literature [45]. Due to the multiphase-separated structure of Pebax, all the MMM display two endothermic peaks for melting points (Tm), the lower for PEO and the higher for PA, which are 11 C and 202 C for the neat Pebax. In order to quantitatively analyze the degree of crystallinity of both PEO and PA phases in Pebax, the integral of endothermic peaks, attributed to the fusion of the crystalline fraction of phases, were obtained from the DSC thermograms. The degree of crystallinity ( ) of each phase was calculated as [46]: Eq. (3) where (J/g) is the enthalpy of fusion of the corresponding phase, as determined from the area of the melting peak, and (J/g) is the enthalpy of fusion when the polymer phase is purely crystalline as obtained from the literature (166.4 J/g and 23 J/g for PEO and PA, respectively) [47]. The total degree of crystallinity can be subsequently calculated based on the fact that Pebax is composed of 60 wt.% PEO and 40 wt.% PA blocks. Based on Figure 4, the Tg values are only

8 slightly decreased, suggesting that unlike most of polymer-MOF membranes, the Pebax chains are not significantly rigidified by ZIF particles [48]. This phenomenon either indicates weak or absence of polymer/filler interaction, or may simply be due to the low filler content used, which hardly affects the glass transition temperature of the polymer host [49]. Nevertheless, the following speculation is more likely while taking into account other characterizations. Slightly reduced Tg values in MMM may be due to the disturbing role of the ZIF particles for the ordered chain arrangements of the polymer matrix leading to higher fractional free volume (FFV). A decrease in Tg is generally indicative of higher chain flexibility and consequently the creation of more FFV, according to: Eq. (4) where FFV and FFVg are the fractional free volume at T and Tg, while is the thermal expansion coefficient of the PEO rubbery segment, which has a value of 8.4 ± 2.6×10-4 K-1 [50,51]. The enhanced experimental FFV values of MMM seem to validate this explanation (Table 2). This hypothesis is further confirmed by means of the degree of crystallinity. As shown in Table 1, the crystallinity decreases after ZIF inclusion, which indicates a transition to a more rubbery state of the MMM. In fact, despite the increasing melting point of PEO, the MMM are not prone to significant rigidification and consequently restricted chain mobility. In general, rigidification occurs as a result of strong interactions between the polymer and fillers [52,53]. Keeping this in mind as well as considering the decreased Tg values and FTIR spectra (Table 1 and Figure 6), one can speculate that although the ZIF particles do not participate in any strong chemical interactions with the polymer, they substantially modify the ordered chain arrangements, leading to higher free volume [54]. The same phenomenon was reported in other Pebax-based MMM [55,56].

-54.0˚C

Tm (PA)

Tm (PEO)

Pebax/ZIF-8 (5) -54.4˚C -56.8˚C -56.7˚C

Pebax/ZIF-8 (3) Pebax/ZIF-67 (5) Pebax/ZIF-67 (3)

-53.1˚C

Pebax

Temperature (oC)

Figure 4. DSC curves of neat Pebax and typical Pebax/ZIF-8 and Pebax/ZIF-67 MMM. The curves are shifted vertically for clarity.

9

Membrane

Table 1. DSC analysis and thermal properties of Pebax-based MMM. Tg Tm Tm Integral of melting peak XPEO XPA (°C) (°C) (°C) (J/g) (%) (%) (PEO) (PA) PEO PA

Neat Pebax Pebax/ZIF-8 (3) Pebax/ZIF-8 (5) Pebax/ZIF-67 (3) Pebax/ZIF-67 (5)

-53.1 -54.4 -54.0 -56.7 -56.8

11.0 17.5 17.0 25.5 22.1

201.8 202.0 197.5 202.6 203.2

19.6 15.4 12.2 19.8 16.4

24.1 19.2 18.0 17.8 16.2

19.6 15.4 12.2 19.8 16.4

26.2 20.1 19.5 19.3 17.6

Xtotal (%) 22.2 17.6 15.2 19.6 16.9

To further investigate the thermal properties, the TGA-DTG curves are presented in Figure 5. Firstly, negligible weight loss below 250 °C confirms the successful removal of any residual solvent in MMM. Secondly, the onset decomposition temperatures (Tonset) of all MMM is substantially lower than for the neat Pebax (~250 °C for MMM vs. ~300 °C for neat Pebax), which for ZIF-8 is due to the lower intrinsic thermal stability of the filler [57]. However, considering the Tonset of the neat ZIF-67 (450 °C), the much lower thermal stability of Pebax/ZIF67 MMM may be a sign of the highly disturbed chain arrangement of the polymer with the addition of ZIF-67. For both MMM, the weight loss above 375 °C is due to the carbonization of the polymer. Nevertheless, the overall thermal stability of the membranes, considering either the onset decomposition temperature or the highest weight loss step, is still sufficiently high for all MMM applications. So a clear distinction exists between Pebax/ZIF-67 and Pebax/ZIF-8 MMM in terms of their thermal properties. Both Tg for the neat polymer (~3.6 °C decrease) and Tonset for ZIF-67 (~200 °C decrease) are more affected by the addition of ZIF-67 compared to ZIF-8. This observation confirms the hypothesis that the presence of ZIF-67 in Pebax is more effective than ZIF-8 in terms of modifying the chain arrangements.

10

W ei gh t ( % )

(a)

(b)

W ei gh t ( % )

(c)

(d)

Figure 5. TGA-DTG curves of neat Pebax and typical Pebax/ZIF-8 (a,b) and Pebax/ZIF-67 (c,d) from 200 to 450 °C in nitrogen.

11

3.2.3. FTIR Figure 6 presents the FTIR spectra to determine possible chemical interactions between MOF and Pebax. The neat Pebax spectrum is similar to the ones reported in literature. All the characteristic bands remained unchanged after particle addition [58,59]. The characteristic peaks of the neat Pebax are: 1099 cm-1, 1637 cm-1, 1665 cm-1 and 1734 cm-1 attributed to the carbonyl in saturated ester, the hydrogen-bonded amides, the free amides and the –C-O bond in the polyether segment, respectively [8]. No significant peak modification or the generation of new peaks is observed in the MMM spectra. This observation indicates that no strong chemical interaction occurs between ZIF-67 and Pebax, which was also reported when incorporating ZIF-67 in PIM-1 and 6FDA-DAM [29,32]. This is not surprising for ZIF-67 as this phenomenon was also previously reported in the literature for ZIF-8, and is associated to the 2-mIM organic ligand, common in ZIF-8 and ZIF-67 [60]. However, a minor change is detected at 3300 cm-1 for both types of MMM, corresponding to -N-H bond in the secondary amide of Pebax, suggesting a weak interaction between the metal lone pair and nitrogen in amide [61].

(a)

A bs or ba nc e (a. u. )

ZIF-8

5%

4% 3% 2%

Pebax

900

3500

12

(b)

ZIF-67

5%

Absorbance (a.u.)

4% 3% 2%

Pebax

900

3500

Figure 6. FTIR spectra of neat Pebax, Pebax/ZIF-8 (a) and Pebax/ZIF-67 (b) MMM. The spectra between 1800 and 2700 cm-1 was removed due to the absence of significant peaks in this region. The curves are shifted vertically for clarity.

3.2.4. Density and FFV To further investigate the effect of ZIF on the polymer matrix rigidity, the fractional free volume (FFV) and density are reported in Table 2. As mentioned earlier in the thermal analysis section, FFV is a critical parameter to study the effect of filler addition in a polymer matrix as it plays a crucial role in the gas permeation performance of a membrane [62]. The particle density of ZIF-8 and ZIF-67 are 0.942 and 0.912 g/cc respectively, as calculated based on the number of atoms in a cubic unit cell of ZIF [37]. The neat Pebax density is 1.15 g/cc which is in agreement with the literature [63,64]. As shown in Table 2, the density and FFV of the membranes increased with ZIF content. On the one hand, higher FFV is related to the porous nature of ZIF, and the presence of ZIF in the polymer matrix, regardless of being chemically interactive with the polymer, contributes to higher free volume in the membrane. Hence, as the ZIF content is low in MMM, the FFV is slightly improved. On the other hand, based on the DSC results (~3.6 oC lower Tg and ~ 5% lower crystallinity values), the increased intersegmental mobility impeded by the presence of ZIF particles, leads to higher FFV.

13 Table 2. Density and FFV values of neat Pebax, Pebax/ZIF-8 and Pebax/ZIF-67 membranes. Membrane ZIF content (wt.%) Density (g/cc) FFV Neat Pebax 0 1.148 0.210 2 1.185 0.217 3 1.239 0.221 Pebax/ZIF-8 4 1.266 0.224 5 1.307 0.228 2 1.192 0.219 3 1.203 0.223 Pebax/ZIF-67 4 1.194 0.228 5 1.223 0.232 *The density test was repeated three times and the average result is reported here. The standard deviation is less than ±0.014.

3.3. Gas permeation results Single gas permeability tests were performed at 35 °C and 11 bar for N2, CH4 and CO2. For all the membranes, nitrogen was the first and carbon dioxide was the last gas to test. As reproducibility is important in mixed matrix membranes, three replicas were tested for each membrane and the results are reported in terms of the average and standard deviation. To shed light on the solubility and diffusivity contributions of the gas permeation results, the time lag method was used to calculate the gas diffusivity coefficient (D) in each membrane [65]. Based on the solution-diffusion theory: Eq. (5) where the gas solubility coefficient (S) can also be easily determined [8]. The CO2 diffusivity and solubility coefficients of the membranes will be discussed later in Figure 8 and Table 3.

3.3.1. Permeability and ideal selectivity Figure 7 and Table 3 present the single gas permeability and ideal selectivity of the membranes. All the MMM have improved permeability regardless of the type ZIF or gas suggesting that the filler addition was successful in improving the gas transport. For Pebax/ZIF-67 (5), the CO2 permeability increased by 130% (162 Barrer) compared to the neat Pebax (70 Barrer), while for CH4 and N2, the values are lower with 58% and 43%, respectively. The increased CO2 permeability is lower for Pebax/ZIF-8 membranes. At the highest ZIF-8 content (5 wt.%), the CO2 permeability is only 85% (130 Barrer), while the values are 70% and 100% for CH4 and N2, respectively. As expected, the ideal selectivity of CO2/CH4 and CO2/N2 was improved by 60% (27.6 and 81.0, respectively) with ZIF-67 addition. In contrast, for Pebax/ZIF-8 MMM, the CO2/CH4 selectivity increased by 25% (21.5) at 2 wt.% before dropping at higher content. For the Pebax/ZIF-8 CO2/N2 selectivity, the results are not good as small increase at 2 wt.% ZIF-8 is observed while it decreases below the neat Pebax at higher filler loading. Based on these permeation results and the characterization results presented, ZIF-8 is less effective in improving CO2 permeability and selectivity over CH4 and N2. Although both ZIF-8 and ZIF-67 have identical linkers, their difference in the metal centers (Co vs. Zn) indicates that the metal atom effect needs to be further investigated in the future.

14 Table 3. Gas permeability, ideal selectivity, CO2 diffusivity and CO2 solubility coefficient at 35 °C and 11 bar. ZIF Membrane content DCO2* SCO2** (Barrer) (Barrer) (Barrer) (wt.%) Neat Pebax 0 70.1±1.5 4.1±0.1 1.4±0.1 17.1±0.1 50.0±2.7 6.6 10.6 2 117.9±2.8 5.5±1.0 2.0±0.3 21.4±2.1 59.0±2.4 7.8 15.2 3 125.7±4.2 6.3±0.7 2.7±0.2 20.0±2.1 46.6±2.1 8.2 15.3 Pebax/ZIF4 128.4±3.1 6.5±0.5 2.7±0.4 19.8±1.7 47.6±3.8 8.1 15.9 8

Pebax/ZIF67

5

130.0±2.5 7.0±0.9

2.8±0.2

18.6±2.3

46.4±2.3

10.1

12.8

2 3 4 5

110.5±2.8 154.3±5.9 160.0±5.6 162.0±8.4

2.1±0.1 2.1±0.2 2.2±0.1 2.0±0.1

19.7±1.7 27.1±3.0 27.6±3.2 24.9±2.8

52.6±2.6 72.5±5.1 72.7±5.4 81.0±7.8

10.1 10.8 12.3 13.2

10.9 14.3 13.0 12.3

5.6±0.5 5.7±0.5 5.8±0.4 6.5±0.5

*Diffusivity coefficient of CO2: 10-7 cm2 s-1 **Solubility coefficient of CO2: 10-3 cm3 (STP) cm-3 cmHg-1

On the one hand, the contribution of kinetic diffusional parameters such as (a) filler intrinsic porosity (large internal cavity of 11.6 Å for ZIF-67), (b) penetrant kinetic diameter (CO2 = 3.3Å, N2 = 3.6Å and CH4 = 3.8Å), and (c) the disturbing role of filler on polymer chain arrangement (increased FFV and decreased crystallinity) should be taken into consideration while explaining the gas permeability results [66]. The latter can also be interpreted via higher chain mobility leading to higher FFV, with more permeable amorphous phase based on lower crystallinity degree. On the other hand, the thermodynamic solubility parameters are not to be neglected on improving the selective gas transport [67]. More precisely, two important thermodynamic characteristics of CO2 are highlighted here: the condensability and the quadrupole moment [8]. The higher condensability of CO2 compared to CH4 and N2 (304.2 K vs. 191.5 K and 126.2 K critical temperature) improves the initial step of gas transport in the membrane, which is the dissolution of penetrant molecules [68]. In terms of CO2 interactions with the membrane, first the dipolequadrupole interactions established by the ether oxygen of Pebax and CO2, as well as the electrostatic interactions between uncoordinated nitrogen on the imidazolate linker and unsaturated metal sites of ZIF with CO2 molecules, leads to CO2 being the more soluble species [69]. Although CO2 is a nonpolar molecule, the polar nature of its C-O bonds favors its preferential adsorption on polar ZIF walls, which are intrinsically polarized by the uncoordinated nitrogen atoms of the imidazolate linkers [70]. Overall, the CO2 concentration in the matrix is higher than CH4 or N2 under similar conditions, due to the smaller size, higher condensability and polarizability, as well as intrinsic electrostatic properties of the ether oxygen in the polymer, as well as the metal and linker in the ZIF framework [71]. The CO2 diffusivity and solubility coefficients are presented in Table 3. It can be seen that D and S are increasing in both series of MMM compared to the neat Pebax. However, based on Figure 8, the CO2 diffusivity coefficient increases more rapidly for Pebax/ZIF-67 compared to Pebax/ZIF-8. In fact, ZIF-8 contributes more to superior CO2 solubility, while ZIF-67 mainly improves CO2

15 diffusivity. ZIF-8 contribution to CO2 solubility is probably due to a higher number of accessible adsorption sites to CO2 molecule. The overall effect of ZIF-67 on the membranes CO2 permeability shows that subsequent to strong CO2 dissolution in the membrane (higher solubility coefficient values), CO2 molecules are not only able to perform diffusional jumps from one free volume element to another (based on FFV results), but also ZIF-67 pores provide additional diffusion pathways for CO2. As a result, the substantially improved CO2 permeability in Pebax/ZIF-67 is a consequence of simultaneous increase in diffusivity and solubility. The same trend was reported in PDMS/ZIF-67, PIM-1/ZIF-67 and 6FDA-DAM/ZIF-67 MMM [22,29,32]. For Pebax/ZIF-8, the CO2 diffusivity slightly increases. In this case, higher solubility coefficients indicate stronger CO2 adsorption leading to slower diffusion, coupled with a denser structure of Pebax/ZIF-8 MMMs validated by density values (Table 2) limiting CO2 diffusion [72]. Li et al. [69] and Xu et al. [53] also reported small improvement of CO2 diffusivity and solubility for Pebax/ZIF-8 MMM at 5 wt.%. (b)

(a)

0

0

Figure 7. Effect of filler loading on CO2 permeability and CO2/N2 and CO2/CH4 ideal selectivity of: (a) Pebax/ZIF-8 and (b) Pebax/ZIF-67 at 35 °C and 11 bar.

Figure 8. CO2 diffusivity coefficient of the MMM as a function of filler content.

16

3.3.2. Comparison of ZIF-8 and ZIF-67 in MMM The theoretical pore aperture of ZIF-67 and ZIF-8 is 3.4 Å. Nevertheless their effective pore aperture is reported to be rather different. For ZIF-8, Zhang et al. [73] showed that due to the flexible framework and the rotations of 2-mIM linkers, the effective aperture is between 4.0 and 4.2 Å. For ZIF-67, a similar effective aperture is expected as the linker is identical in both ZIF. However, Kwon et al. [28] showed via FTIR that the coordination bonding of Co-N in ZIF-67 is stiffer than Zn-N in ZIF-8, leading to a contraction in the lattice due to restricted flipping motions of the linker. In other words, the CO2 molecular sieving property of ZIF-67 is improved by the metal substitution (cobalt instead of zinc) and its narrower effective pore aperture (approximately 3.3 Å vs. 3.4 Å for ZIF-8) allowing more efficient discrimination of CO2 [26]. Regarding the effect of the metal atom on CO2 adsorption, the coordination unsaturated metal centers of MOF are also known to be effective on the overall CO2 binding to the MOF, thereby several researchers have studied the heat of adsorption of CO2 by substituting various metal centers in specific MOF [74–77]. According to Yu et al. [78], as the charge transfer from the CO2 molecule to the metal ions is relatively small, the nature of the CO2 adsorption on the metal is via electrostatic interactions through particular CO2-open metal site orientations. Hence, the effective charge of the metal atom is directly related to the CO2-metal binding strength. The effective charge of the metal atom is induced by the incomplete screening of the 3d orbital electrons of the metal atom to the adjacent atoms in the linker, as well as the CO2 molecule [79]. Based on the effective metal charge calculations in metal-substituted variants of different MOF (M-DOBDC, M-HKUST-1, M-CPO-27, and M-MOF-74, M=Zn or Co), zinc has higher effective charge compared to cobalt leading to higher binding strength with CO2 [78,80,81]. As a result, while the cobalt in ZIF-67 does not significantly contribute to the superior CO2 permeability, the higher CO2 solubility coefficient of Pebax/ZIF-8 MMM stems from the strong electrostatic interactions of zinc with CO2 molecules. In other words, the higher CO2-metal interactions in ZIF-8 slow down the gas transport through the membrane, resulting in lower CO2 permeability values. Hence, the induced free volume formed by the disturbing role of ZIF-67 for Pebax chain arrangements, as well as appropriate pore aperture size are the main reasons behind superior CO2 separation properties of Pebax/ZIF-67. To illustrate the performances of these MMM, their separation performances are compared on a Robeson plot in Figure 9. It is clear that ZIF-67 performs much better than ZIF-8 for the range of conditions investigated.

17 (a)

(b)

Figure 9. Performance of the MMM on Robeson plots for: (a) CO2/CH4 and (b) CO2/N2.

4. Conclusion In this work, two isoreticular ZIF based on M(2-mIM)2 (M=Zn, Co), known as ZIF-8 and ZIF-67, were added at different concentrations (0-5 wt.%) into Pebax MH-1657 as the matrix. Based on the samples produced, the CO2 separation from CH4 and N2 was investigated for these mixed matrix membranes. It was found that no chemical interaction between ZIF and Pebax was detected by FTIR, while TGA, DSC and density measurements reported lower crystallinity and higher free volume, indicating a more rubbery state resulting from the disruptive role of both fillers between the ordered polymer chains. Also, all the MMM are thermally stable up to 250 °C based on TGA. For CO2 permeability, ZIF-67 and ZIF-8 addition increased the value of neat Pebax by 130% and 85%, respectively. In terms of ideal selectivity, CO2/N2 and CO2/CH4 were both improved but ZIF-67 (81 and 25) was more effective than ZIF-8 (46 and 19). Based on the diffusivity and solubility data, ZIF-67 is more effective in facilitating CO2 diffusion, while ZIF-8 contributes to improve CO2 solubility. This behavior is due to stronger electrostatic interactions between the Zn metal center of ZIF-8 and CO2 molecules limiting fast CO2 diffusion. The facilitating role of ZIF67 on CO2 transport through the membrane originates from its appropriate effective pore aperture as well as relatively large pore cavities able to accommodate the CO2 molecules, while having weaker metal-gas interactions. Overall, Pebax with 5 wt.% of ZIF-67 MMM was able to overcome the 2008 upper bound of Robeson for CO2/N2 while being above the 1991 upper bound for CO2/CH4.

Acknowledgements The authors acknowledge the financial support of the Natural Science and Engineering Research Council of Canada (NSERC). Pebax samples were kindly provided by Arkema.

18

References [1]

[2] [3]

[4]

[5]

[6] [7]

[8]

[9]

[10]

[11]

[12]

[13] [14]

[15]

[16]

[17]

X.Y. Chen, S. Kaliaguine, D. Rodrigue, A Comparison between Several Commercial Polymer Hollow Fiber Membranes for Gas Separation, J. Membr. Sep. Technol. 6 (2017) 1–15. L.M. Robeson, The upper bound revisited, J. Memb. Sci. 320 (2008) 390–400. doi:10.1016/j.memsci.2008.04.030. S.L. Liu, L. Shao, M.L. Chua, C.H. Lau, H. Wang, S. Quan, Recent progress in the design of advanced PEO-containing membranes for CO2 removal, Prog. Polym. Sci. 38 (2013) 1089–1120. doi:10.1016/j.progpolymsci.2013.02.002. M. Wang, Z. Wang, S. Zhao, J. Wang, S. Wang, Recent advances on mixed matrix membranes for CO2 separation, Chinese J. Chem. Eng. 25 (2017) 1581–1597. doi:10.1016/j.cjche.2017.07.006. P.M. Rezakazemi, M. Sadrzadeh, T. Matsuura, Thermally stable polymers for advanced high-performance gas separation membranes, Prog. Energy Combust. Sci. 66 (2018) 1–41. doi:10.1016/j.pecs.2017.11.002. J. Liu, X. Hou, H.B. Park, H. Lin, High-Performance Polymers for Membrane CO2/N2 Separation, Chem. Eur. J. 22 (2016) 1–12. doi:10.1002/chem.201603002. S. Wang, X. Li, H. Wu, Z. Tian, Q. Xin, G. He, D. Peng, S. Chen, Y. Yin, Z. Jiang, M.D. Guiver, Advances in high permeability polymeric membrane materials for CO2 separations, Energy Environ. Sci. 9 (2016) 1863–1890. doi:10.1039/C6EE00811A. S. Meshkat, S. Kaliaguine, D. Rodrigue, Mixed matrix membranes based on amine and non-amine MIL-53(Al) in Pebax® MH-1657 for CO2 separation, Sep. Purif. Technol. 200 (2018) 177–190. doi:10.1016/j.seppur.2018.02.038. B. Zornoza, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Metal organic framework based mixed matrix membranes: An increasingly important field of research with a large application potential, Microporous Mesoporous Mater. 166 (2013) 67–78. doi:10.1016/j.micromeso.2012.03.012. B. Seoane, J. Coronas, I. Gascon, M.E. Benavides, O. Karvan, J. Caro, F. Kapteijn, J. Gascon, Metal–organic framework based mixed matrix membranes: a solution for highly efficient CO2 capture?, Chem. Soc. Rev. 44 (2015) 2421–2454. doi:10.1039/C4CS00437J. H. Vinh-Thang, S. Kaliaguine, MOF-based mixed matrix membranes for industrial applications, in: O.L. Ortiz, L.D. Ramírez (Eds.), Coord. Polym. Met. Org. Fram., Nova Science Publishers, Inc., 2011. M. Vinoba, M. Bhagiyalakshmi, Y. Alqaheem, A.A. Alomair, A. Pérez, M.S. Rana, Recent progress of fillers in mixed matrix membranes for CO2 separation: A review, Sep. Purif. Technol. 188 (2017) 431–450. doi:10.1016/j.seppur.2017.07.051. W.J. Koros, C. Zhang, Materials for next-generation molecularly selective synthetic membranes, Nat. Mater. 16 (2017) 289–297. doi:10.1038/NMAT4805. G.M. Shi, T. Yang, T.S. Chung, Polybenzimidazole (PBI)/zeolitic imidazolate frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of alcohols, J. Memb. Sci. 415–416 (2012) 577–586. doi:10.1016/j.memsci.2012.05.052. M. Barooah, B. Mandal, Synthesis, characterization and CO2 separation performance of novel PVA/PG/ZIF-8 mixed matrix membrane, J. Memb. Sci. 572 (2019) 198–209. doi:10.1016/j.memsci.2018.11.001. C. Zhang, Y. Dai, J.R. Johnson, O. Karvan, W.J. Koros, High performance ZIF-8/6FDADAM mixed matrix membrane for propylene/propane separations, J. Memb. Sci. 389 (2012) 34–42. doi:10.1016/j.memsci.2011.10.003. Y. Dai, J.R. Johnson, O. Guz Karvan, D.S. Sholl, W.J. Koros, Ultem®/ZIF-8 mixed matrix hollow fiber membranes for CO2/N2 separations, J. Memb. Sci. 401–402 (2012) 76–82.

19 [18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

doi:10.1016/j.memsci.2012.01.044. M.J.C. Ordonez, K.J. Balkus, J.P. Ferraris, I.H. Musselman, Molecular sieving realized with ZIF-8/Matrimid® mixed-matrix membranes, J. Memb. Sci. 361 (2010) 28–37. doi:10.1016/j.memsci.2010.06.017. J. Sánchez-Laínez, B. Zornoza, S. Friebe, J. Caro, S. Cao, A. Sabetghadam, B. Seoane, J. Gascon, F. Kapteijn, C. Le Guillouzer, G. Clet, M. Daturi, C. Téllez, J. Coronas, Influence of ZIF-8 particle size in the performance of polybenzimidazole mixed matrix membranes for pre-combustion CO2 capture and its validation through interlaboratory test, J. Memb. Sci. 515 (2016) 45–53. doi:10.1016/j.memsci.2016.05.039. V. Nafisi, M.B. Hägg, Development of dual layer of ZIF-8/PEBAX-2533 mixed matrix membrane for CO2 capture, J. Memb. Sci. 459 (2014) 244–255. doi:10.1016/j.memsci.2014.02.002. R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe, O.M. Yaghi, High-throughput synthesis of zeolitic imidazolate frameworks and application to CO2 capture, Science (80-. ). 319 (2008) 939–943. doi:10.1126/science.1151614. A. Khan, M. Ali, A. Ilyas, P. Naik, I.F.J. Vankelecom, M.A. Gilani, M. Roil Bilad, Z. Sajjad, A. Laeeq Khan, ZIF-67 filled PDMS mixed matrix membranes for recovery of ethanol via pervaporation, Sep. Purif. Technol. 206 (2018) 50–58. doi:10.1016/j.seppur.2018.05.055. E. Andres-Garcia, L. Oar-Arteta, J. Gascon, F. Kapteijn, ZIF-67 as silver-bullet in adsorptive propane/propylene separation, Chem. Eng. J. 360 (2019) 10–14. doi:10.1016/j.cej.2018.11.118. D. Matatagui, A. Sainz-Vidal, I. Gràcia, E. Figueras, C. Cané, J.M. Saniger, Chemoresistive gas sensor based on ZIF-8/ZIF-67 nanocrystals, Sensors Actuators B Chem. 274 (2018) 601–608. doi:10.1016/j.snb.2018.07.137. Z.-H. Zhang, J.-L. Zhang, J.-M. Liu, Z.-H. Xiong, X. Chen, Selective and Competitive Adsorption of Azo Dyes on the Metal–Organic Framework ZIF-67, Water, Air, Soil Pollut. 227 (2016) 471–491. doi:10.1007/s11270-016-3166-7. P. Krokidas, M. Castier, I.G. Economou, Computational Study of ZIF‑ 8 and ZIF-67 Performance for Separation of Gas Mixtures, J. Phys. Chem. C. 121 (2017) 17999–18011. doi:10.1021/acs.jpcc.7b05700. X. Wu, W. Liu, H. Wu, X. Zong, L. Yang, Y. Wu, Y. Ren, C. Shi, S. Wang, Z. Jiang, Nanoporous ZIF-67 embedded polymers of intrinsic microporosity membranes with enhanced gas separation performance, J. Memb. Sci. 548 (2018) 309–318. doi:10.1016/j.memsci.2017.11.038. H. Taek Kwon, H.-K. Jeong, A.S. Lee, H. Seong An, J. Suk Lee, Heteroepitaxially Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane Separation Performances, J. Am. Chem. Soc. 137 (2015) 12304–12311. doi:10.1021/jacs.5b06730. H. An, S. Park, H.T. Kwon, H.-K. Jeong, J.S. Lee, A new superior competitor for exceptional propylene/propane separations: ZIF-67 containing mixed matrix membranes, J. Memb. Sci. 526 (2017) 367–376. doi:10.1016/j.memsci.2016.12.053. J.H. Lee, H.T. Kwon, S. Bae, J. Kim, J.H. Kim, Mixed-matrix membranes containing nanocage-like hollow ZIF-8 polyhedral nanocrystals in graft copolymers for carbon dioxide/methane separation, Sep. Purif. Technol. 207 (2018) 427–434. doi:10.1016/j.seppur.2018.06.076. J. Vega, A. Andrio, A.A. Lemus, J.A.I. Díaz, L.F. del Castillo, R. Gavara, V. Compañ, Modification of polyetherimide membranes with ZIFs fillers for CO2 separation, Sep. Purif. Technol. 212 (2019) 474–482. doi:10.1016/j.seppur.2018.11.033.

20 [32]

[33]

[34] [35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

X. Wu, W. Liu, H. Wu, X. Zong, L. Yang, Y. Wu, Y. Ren, C. Shi, S. Wang, Z. Jiang, Nanoporous ZIF-67 embedded polymers of intrinsic microporosity membranes with enhanced gas separation performance, J. Memb. Sci. 548 (2018) 309–318. doi:10.1016/j.memsci.2017.11.038. Y.C. Tan, H.C. Zeng, Self-templating synthesis of hollow spheres of MOFs and their derived nanostructures, Chem. Commun. 52 (2016) 11591–11594. doi:10.1039/c6cc05699g. X.Y. Chen, S. Kaliaguine, Mixed gas and pure gas transport properties of copolyimide membranes, J. Appl. Polym. Sci. 128 (2013) 380–389. doi:10.1002/app.37728. T. Graham, On the Absorption and Dialytic Separation o f Gases by Colloid Septa, Philosophical Transactions of the Royal Society of London, London, UK, 1866. doi:10.1098/rstl.1866.0018. C.-W. Tsai, R.E. Kroon, H.C. Swart, J.J. Terblans, R.A. Harris, Photoluminescence of metal-imidazolate complexes with Cd(II), Zn(II), Co(II) and Ni(II) cation nodes and 2methylimidazole organic linker, J. Lumin. 207 (2019) 454–459. doi:10.1016/j.jlumin.2018.11.026. P. Krokidas, M. Castier, S. Moncho, D.N. Sredojevic, E.N. Brothers, H.T. Kwon, H.-K. Jeong, S.J. Lee, I.G. Economou, ZIF-67 framework: a promising new candidate for propylene/ propane separation. Experimental data and molecular simulations, J. Phys. Chem. C. 120 (2016) 8116–8124. doi:10.1021/acs.jpcc.6b00305. K.S. Park, Z. Ni, A.P. Cô, J.Y. Choi, R. Huang, F.J. Uribe-Romo, H.K. Chae, M. O’keeffe, O.M. Yaghi, Exceptional chemical and thermal stability of zeolitic imidazolate frameworks, Proc. Natl. Acad. Sci. United States Am. 103 (2006) 10186–10191. doi:10.1073/pnas.0602439103. D.N. Ta, H.K.D. Nguyen, B.X. Trinh, Q.T.N. Le, H.N. Ta, H.T. Nguyen, Preparation of nano-ZIF-8 in methanol with high yield, Can. J. Chem. Eng. 96 (2018) 1518–1531. doi:10.1002/cjce.23155. H. Kaur, G.C. Mohanta, V. Gupta, D. Kukkar, S. Tyagi, Synthesis and characterization of ZIF-8 nanoparticles for controlled release of 6-mercaptopurine drug, J. Drug Deliv. Sci. Technol. 41 (2017) 106–112. doi:10.1016/j.jddst.2017.07.004. E.L. Bustamante, J.L. Fernández, J.M. Zamaro, Influence of the solvent in the synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals at room temperature, J. Colloid Interface Sci. 424 (2014) 37–43. doi:10.1016/j.jcis.2014.03.014. M. Zamidi Ahmad, M. Navarro, M. Lhotka, B. Zornoza, C. Téllez, V. Fila, J. Coronas, Enhancement of CO2/CH4 separation performances of 6FDA-based co-polyimides mixed matrix membranes embedded with UiO-66 nanoparticles, Sep. Purif. Technol. 192 (2018) 465–474. doi:10.1016/J.SEPPUR.2017.10.039. M. Jia, Y. Feng, J. Qiu, X.-F. Zhang, J. Yao, Amine-functionalized MOFs@GO as filler in mixed matrix membrane for selective CO2 separation, Sep. Purif. Technol. 213 (2019) 63– 69. doi:10.1016/J.SEPPUR.2018.12.029. A. Car, C. Stropnik, W. Yave, K. V. Peinemann, PEG modified poly(amide-b-ethylene oxide) membranes for CO2 separation, J. Memb. Sci. 307 (2008) 88–95. doi:10.1016/j.memsci.2007.09.023. J.E. Shin, S.K. Lee, Y.H. Cho, H.B. Park, Effect of PEG-MEA and graphene oxide additives on the performance of Pebax®1657 mixed matrix membranes for CO2 separation, J. Memb. Sci. 572 (2019) 300–308. doi:10.1016/j.memsci.2018.11.025. H. Rabiee, S.M. Alsadat, M. Soltanieh, S.A. Mousavi, A. Ghadimi, S. Meshkat Alsadat, M. Soltanieh, S.A. Mousavi, A. Ghadimi, Gas permeation and sorption properties of poly(amide-12-b-ethyleneoxide)(Pebax1074)/SAPO-34 mixed matrix membrane for

21

[47]

[48]

[49]

[50] [51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

CO2/CH4 and CO2/N2 separation, J. Ind. Eng. Chem. 27 (2015) 223–239. doi:10.1016/j.jiec.2014.12.039. Y. Wang, H. Li, G. Dong, C. Scholes, V. Chen, Effect of Fabrication and Operation Conditions on CO 2 Separation Performance of PEO−PA Block Copolymer Membranes, Ind. Eng. Chem. Res. 54 (2015) 7173–7283. doi:10.1021/acs.iecr.5b01234. M.W. Anjum, F. Vermoortele, A.L. Khan, B. Bueken, D.E. De Vos, I.F.J. Vankelecom, Modulated UiO-66-Based Mixed-Matrix Membranes for CO2 Separation, ACS Appl. Mater. Interfaces. 7 (2015) 25193–25201. doi:10.1021/acsami.5b08964. W. Zheng, R. Ding, K. Yang, Y. Dai, X. Yan, G. He, ZIF-8 nanoparticles with tunable size for enhanced CO2 capture of Pebax based MMMs, Sep. Purif. Technol. 214 (2019) 111– 119. doi:10.1016/j.seppur.2018.04.010. H. Lin, B.D. Freeman, Materials selection guidelines for membranes that remove CO2 from gas mixtures, J. Mol. Struct. 739 (2005) 57–74. doi:10.1016/j.molstruc.2004.07.045. S.R. Reijerkerk, M. Wessling, K. Nijmeijer, Pushing the limits of block copolymer membranes for CO 2 separation, J. Memb. Sci. 378 (2011) 479–484. doi:10.1016/j.memsci.2011.05.039. L. Yang, S. Zhang, H. Wu, C. Ye, X. Liang, S. Wang, X. Wu, Y. Wu, Y. Ren, Y. Liu, N. Nasir, Z. Jiang, Porous organosilicon nanotubes in pebax-based mixed-matrix membranes for biogas purification, J. Memb. Sci. 573 (2019) 301–308. doi:10.1016/j.memsci.2018.12.018. L. Xu, L. Xiang, C. Wang, J. Yu, L. Zhang, Y. Pan, Enhanced permeation performance of polyether-polyamide block copolymer membranes through incorporating ZIF-8 nanocrystals, Chinese J. Chem. Eng. 25 (2017) 882–891. doi:10.1016/j.cjche.2016.11.007. J.E. Shin, S.K. Lee, Y.H. Cho, H.B. Park, Effect of PEG-MEA and graphene oxide additives on the performance of Pebax®1657 mixed matrix membranes for CO2 separation, J. Memb. Sci. 572 (2019) 300–308. doi:10.1016/j.memsci.2018.11.025. D. Zhao, J. Ren, Y. Wang, Y. Qiu, H. Li, K. Hua, X. Li, J. Ji, M. Deng, High CO2 separation performance of Pebax®/CNTs/GTA mixed matrix membranes, J. Memb. Sci. 521 (2016) 104–113. doi:10.1016/j.memsci.2016.08.061. Y. Li, T.S. Chung, Molecular-level mixed matrix membranes comprising Pebax® and POSS for hydrogen purification via preferential CO2 removal, Int. J. Hydrogen Energy. 35 (2010) 10560–10568. doi:10.1016/j.ijhydene.2010.07.124. X.Y. Chen, V.-T. Hoang, D. Rodrigue, S. Kaliaguine, Optimization of continuous phase in amino-functionalized metal-organic framework (MIL-53) based co-polyimide mixed matrix membranes for CO2/CH4 separation, RSC Adv. 3 (2013) 24266–24279. doi:10.1039/c3ra43486a. Y. Cheng, L. Zhai, Y. Ying, Y. Wang, G. Liu, J. Dong, D.Z.L. Ng, S.A. Khan, D. Zhao, Highly efficient CO2 capture by mixed matrix membranes containing three-dimensional covalent organic framework fillers, J. Mater. Chem. A. 7 (2019) 4549–4560. doi:10.1039/c8ta10333j. S. Jeong, H. Sohn, S.W. Kang, Highly permeable Pebax-1657 membranes to have longterm stability for facilitated olefin transport, Chem. Eng. J. 333 (2018) 276–279. doi:10.1016/j.cej.2017.09.125. Q. Song, S.K. Nataraj, M. V. Roussenova, J.C. Tan, D.J. Hughes, W. Li, P. Bourgoin, M.A. Alam, A.K. Cheetham, S. a. Al-Muhtaseb, E. Sivaniah, Zeolitic imidazolate framework (ZIF-8) based polymer nanocomposite membranes for gas separation, Energy Environ. Sci. 5 (2012) 8359–8369. doi:10.1039/c2ee21996d. H.R. Amedi, M. Aghajani, Aminosilane-functionalized ZIF-8/PEBA mixed matrix membrane for gas separation application, Microporous Mesoporous Mater. 247 (2017)

22 [62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70] [71]

[72]

[73]

[74]

[75]

[76]

[77]

124–135. doi:10.1016/j.micromeso.2017.04.001. C.A. Scholes, B.D. Freeman, Thermal rearranged poly(imide-co-ethylene glycol) membranes for gas separation, J. Memb. Sci. 563 (2018) 676–683. doi:10.1016/j.memsci.2018.06.027. S.A. Habibiannejad, A. Aroujalian, A. Raisi, Pebax-1657 mixed matrix membrane containing surface modified multi-walled carbon nanotubes for gas separation, RSC Adv. 6 (2016) 79563–79577. doi:10.1039/C6RA14141B. H. Sanaeepur, R. Ahmadi, A. Ebadi Amooghin, D. Ghanbari, A novel ternary mixed matrix membrane containing glycerol-modified poly(ether-block-amide) (Pebax 1657)/copper nanoparticles for CO2 separation, J. Memb. Sci. 573 (2019) 234–246. doi:10.1016/j.memsci.2018.12.012. O.G. Nik, M. Sadrzadeh, S. Kaliaguine, Surface grafting of FAU/EMT zeolite with (3aminopropyl)methyldiethoxysilane optimized using Taguchi experimental design, Chem. Eng. Res. Des. 90 (2012) 1313–1321. doi:10.1016/j.cherd.2011.12.008. C. Hon Lau, P. Li, F. Li, T.-S. Chung, D.R. Paul, Reverse-selective polymeric membranes for gas separations, Prog. Polym. Sci. 38 (2013) 740–766. doi:10.1016/j.progpolymsci.2012.09.006. M. Gholami, T. Mohammadi, S. Mosleh, M. Hemmati, CO2/CH4 separation using mixed matrix membrane-based polyurethane incorporated with ZIF-8 nanoparticles, Chem. Pap. 71 (2017) 1839–1853. doi:10.1007/s11696-017-0177-9. L.M. Robeson, Z.P. Smith, B.D. Freeman, D.R. Paul, Contributions of diffusion and solubility selectivity to the upper bound analysis for glassy gas separation membranes, J. Memb. Sci. 453 (2014) 71–83. doi:10.1016/j.memsci.2013.10.066. M. Li, X. Zhang, S. Zeng, L. Bai, H. Gao, J. Deng, Q. Yang, S. Zhang, Pebax-based composite membranes with high gas transport properties enhanced by ionic liquids for CO2 separation, RSC Adv. 7 (2017) 6422. doi:10.1039/c6ra27221e. H. Hayashi, A.P. Cote, H. Furukawa, O.M. Yaghi, Zeolite A imidazolate frameworks, Nat. Mater. 7 (2007) 501–506. doi:10.1038/nmat1927. A. Ghadimi, M. Amirilargani, T. Mohammadi, N. Kasiri, B. Sadatnia, Preparation of alloyed poly(ether block amide)/poly(ethylene glycol diacrylate) membranes for separation of CO 2 /H 2 (syngas application), J. Memb. Sci. 458 (2014) 14–26. doi:10.1016/j.memsci.2014.01.048. C. Zhang, R.P. Lively, K. Zhang, J.R. Johnson, O. Karvan, W.J. Koros, Unexpected Molecular Sieving Properties of Zeolitic Imidazolate Framework‑ 8, J. Phys. Chem. Lett. 3 (2012) 2130–2134. doi:10.1021/jz300855a. C. Zhang, R.P. Lively, K. Zhang, J.R. Johnson, O. Karvan, W.J. Koros, Unexpected Molecular Sieving Properties of Zeolitic Imidazolate Framework‑ 8, J. Phys. Chem. Lett. 3 (2012) 2130–2134. doi:10.1021/jz300855a. R. Poloni, K. Lee, R.F. Berger, B. Smit, J.B. Neaton, Understanding Trends in CO2 Adsorption in Metal−Organic Frameworks with Open-Metal Sites, J. Phys. Chem. Lett. 5 (2014) 861–865. doi:10.1021/jz500202x. H. Seung Koh, M. Kumar Rana, J. Hwang, D.J. Siegel, Thermodynamic screening of metalsubstituted MOFs for carbon capture, Phys. Chem. Chem. Phys. 15 (2013) 4573–4581. doi:10.1039/c3cp50622c. J. Park, H. Kim, S.S. Han, Y. Jung, Tuning Metal−Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO 2 Affinity by Metal Substitution, J. Phys. Chem. Lett. 3 (2012) 826–829. doi:10.1021/jz300047n. D. Yu, A.O. Yazaydin, J.R. Lane, P.D.C. Dietzel, R.Q. Snurr, A combined experimental and quantum chemical study of CO 2 adsorption in the metal-organic framework CPO-27

23 [78]

[79]

[80]

[81]

with different metals, Chem. Sci. 4 (2013) 3544–3556. doi:10.1039/c3sc51319j. D. Yu, A.O. Yazaydin, J.R. Lane, P.D.C. Dietzel, R.Q. Snurr, A combined experimental and quantum chemical study of CO 2 adsorption in the metal-organic framework CPO-27 with different metals, Chem. Sci. 4 (2013) 3544–3556. doi:10.1039/c3sc51319j. Y. Pan, H. Li, X.-X. Zhang, Z. Zhang, X.-S. Tong, C.-Z. Jia, B. Liu, C.-Y. Sun, L.-Y. Yang, G.-J. Chen, Large-scale synthesis of ZIF-67 and highly efficient carbon capture using a ZIF-67/glycol-2-methylimidazole slurry, Chem. Eng. Sci. 137 (2015) 504–514. doi:10.1016/j.ces.2015.06.069. H. Seung Koh, M. Kumar Rana, J. Hwang, D.J. Siegel, Thermodynamic screening of metalsubstituted MOFs for carbon capture, Phys. Chem. Chem. Phys. 15 (2013) 4573–4581. doi:10.1039/c3cp50622c. J. Park, H. Kim, S.S. Han, Y. Jung, Tuning Metal−Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO 2 Affinity by Metal Substitution, J. Phys. Chem. Lett. 3 (2012) 826–829. doi:10.1021/jz300047n.

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Highlights   

Mixed matrix membranes based on Pebax and ZIF-67 or ZIF-8 were succesfully produced. CO2 permeability, as well as CO2/CH4 and CO2/N2 selectivity, were improved with ZIF addition. ZIF-67 is more effective than ZIF-8 for CO2 separation.

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Conflict of interest

None.