N2 permeation of carbon molecular sieving membranes

Microporous and Mesoporous Materials 285 (2019) 142–149

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The positive/negative effects of bentonite on O2/N2 permeation of carbon molecular sieving membranes

T

Bing Zhanga,∗, Chen Yanga, Shanshan Liua, Yonghong Wua, Tonghua Wangb, Jieshan Qiub,c a

School of Petrochemical Engineering, Shenyang University of Technology, No. 30, Guanghua Street, Hongwei District, Liaoyang, 111003, China Carbon Research Laboratory, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, No. 2, Linggong Road, Dalian, 116024, China c College of Chemical Engineering, Beijing University of Chemical Technology, No.15, North Third Ring Road, Chaoyang District, Beijing, 100029, China b

ARTICLE INFO

ABSTRACT

Keywords: Polyimide Carbon membranes Support Bentonite Gas permeation

In this work, bentonite (BT) was first developed as additives to modify carbon molecular sieving (CMS) membranes. The membrane samples were characterized by the techniques of thermogravimetric analysis, differential scanning calorimetry, infrared spectroscopy, X-ray diffraction, nitrogen adsorption, scanning electron microscope and transmission electron microscopy. The effects of BT amount, transmembrane pressure and permeation temperature on the microstructure and gas permeation of CMS membranes were investigated. It finds that the incorporation of BT improves the thermal stability of precursors and the microporous volume of CMS membranes. The graphitization degree and structural compactness of the CMS membranes increase with elevating the BT amount. At the same time, the gas permeability first increases then decreases. In comparison, BT-doped CMS membranes are more permeable for oxygen and nitrogen. Moreover, the permeability of CMS membranes increases by rising permeation temperature or reducing transmembrane pressure. The CMS membranes can directly enrich the mole fraction of O2 to 78% from compressed air at one time.

1. Introduction Membrane-based gas separation is regarded as one of the most promising separation technologies in the 21st century [1]. It takes advantages of high energy efficiency, low investment and maintenance cost, small footprint and easy operation, in comparison to traditional cryogenic distillation and pressure swing adsorption. Therefore, membrane technology has attracted much attention for potential applications of hydrogen recovery, oxygen/nitrogen enrichment, olefin/paraffin separation, and so on [2]. At present, a large number of installations of polymeric membranes have been extensively established for industrial application, with the outcome of enormously economic and social benefits [3]. Nevertheless, some drawbacks of polymeric membranes, including low permeation, poor thermal resistance, inferior chemical stability, and liable degradation in harsh medium, have restricted their further wide applications. Besides, their separation performance is also confined to the well-known Robeson's trade-off upper bound [4]. Therefore, numerous efforts have been concentrated on inorganic membranes, such as metal, silica, ceramics, zeolites and carbon [5,6]. Of them, carbon molecular sieving (CMS) membranes have gained considerable interest for promising candidates of

∗

membrane technology [7]. CMS membranes feature with finely controllable pore size distribution, high thermal stability, chemical inertness, and outstanding molecular recognition ability for gases with approaching dimensions [8,9]. The gas separation property of CMS membranes heavily depends upon their porous structure since the permeation process is mainly dominated by molecular sieving mechanism [10]. As such, the fine control of porous structure of CMS membranes is of great significance for optimizing the gas separation performance [11]. So far, many methods, including polymer blending, pre-oxidation, doping fillers, post-oxidation and activation, can be applied to modify the porous structure [12,13]. Among those, doping fillers (such as, zeolites [14], silicon [15], metal [16], etc.) has proved to be a simple and effective strategy. However, the resultant CMS membranes are sometimes suffered from the defect formation due to the incompatibility of dispersive fillers with continuous carbon matrix [17]. To mitigate those deficiencies, it is essential to develop suitable fillers for the fabrication of high performance CMS membranes. Bentonite (BT) is a kind of aluminium phyllosilicate clay containing closely packed platelets, with the characteristics of high porosity, large specific surface area, molecular sieving structure, superior dispersivity and large deposition around the world [18,19]. It has found that BT is a

Corresponding author. E-mail address: [email protected] (B. Zhang).

https://doi.org/10.1016/j.micromeso.2019.04.070 Received 22 January 2019; Received in revised form 19 March 2019; Accepted 29 April 2019 Available online 30 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.

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prominent absorbent for gas/liquid separation or purification [20], as well as a good additive for polymer-clay nanocomposites because of super compatibility [21]. Considering those points, here BT was developed as fillers to fabricate CMS membranes for the first time, aiming at modifying the prous structure and property of CMS membranes for oxygen/nitrogen enrichment.

thermogravimetric analyzer (Perkin Elmer) under 20 mL/min flowing nitrogen atmosphere, at a heating rate of 20 °C/min from room temperature to 800 °C. The melting behavior of precursors was evaluated by a differential scanning calorimeter instrument (DSC, Mettler, DSC 822e, Im Langacher, Greifensee, Switzerland) in the temperature range of 0–280 °C at a heating rate of 10 °C/min under a nitrogen purge atmosphere. The evolution on the surface chemical groups of membrane samples was detected by a Bruke TENSOR II Fourier transform infrared spectroscopy (FTIR) in a wavenumber range of 4000–400 cm−1. The microstructure of membrane samples was analyzed by a Rigaku Ultima IV model X-ray diffractometer (XRD) at 40 kV and 40 mA in a scanning range of 10–60°. The structural parameter interlayer spacing d002 values were calculated on the basis of the diffraction angle and famous Bragg equation (λ = 0.1541 nm). The morphology of CMS membranes was observed by TM-3000 tabletop/SU8010 cold field-emission scanning electron microscope (SEM) (Hitachi) at an accelerating voltage of 15/5 kV on standard observation mode. Besides, transmission electron microscopy (TEM) was also applied to insight into the microcrystal structure by a JEM-2100 TEM from JEOL. The adsorption-desorption isotherms of CMS membranes were measured by a micromeritics ASAP 2420 nitrogen adsorption instrument at liquid nitrogen temperature (−196 °C). Depending on the isotherms and standard calculation methods, BET specific surface area, porous volume and pore size distribution were obtained, respectively.

2. Experimental 2.1. Raw materials The precursor for supports production is an industrial grade phenolic resin (PR) supplied by Xinxiang Boma Fengfan Industrial Ltd. Co. of China. In addition, commercial hexamethylenetetramine (HMTA) and sodium carboxymethyl cellulose (NaCMC) were used as a curing agent and binders, respectively. A homemade poly(amic acid) solution of a solid content of 15% in N, N-dimethylformamide (DMAc) was applied as the precursor of CMS membranes. It was the prepolymer of 1,4- bis (4- amino-2 –trifluoromethyl -phenoxyl) benzene- 1, 2, 3, 4- cyclobutane tetracarboxylic dianhydride) type polyimide synthesized as per the references [22,23]. The white BT powder was produced from Hebei province in China, with montmorillonite content above 98%, mean particle diameter of 11 μm and density of 2.4g/cm3. 2.2. Preparation of CMS membranes First of all, a uniform dispersion was prepared by adding BT powder into the poly(amic acid) solution in a mass fraction of 0–0.4% by vigorous stirring for 2 h and ultrasonic dispersion for 15 min. Then, it was set aside overnight at room temperature to get rid of any air bubbles. Last, a homogeneously yellowish membrane-forming solution was obtained. At the same time, pristine phenolic resin-based disk supports were also produced prior to membrane coating, of which the detailed process was given in our previous work [24]. The barely supports had an average pore size of 0.84 μm, diameter of 3.0 cm and porosity of 47.2% after subsequent pyrolysis. The spin-coating was performed by dropping 0.4g membrane-forming solution on the upper side of support fixed horizontally in a KW-4A-type coating instrument (Institute of Microelectronics, Chinese Academy of Sciences). Then, the samples were rotated at 1500 r/min for 30 s, followed by drying for 6 h at ambient condition. In the same way, another two rounds of coating-drying were also performed with the last drying time for 4–5 days. The pyrolysis was subsequently performed in a horizontally tubular furnace with a programmable temperature controller. The step-heating process was conducted from room temperature to 100 °C, 200 °C, 300 °C and 400 °C at a heating rate of 2 °C/min under the purging of 150 mL/ min nitrogen. At each stagnation temperature, the holding time was 30min. Afterwards, the temperature was further elevated to the final temperature of 650 °C at 1 °C/min. After holding for 60 min, it was cooled down to ambient temperature. Finally, the supported CMS membranes were collected and denoted with the symbols of CMS-x%, where the letter “x” referred to the mass fraction of BT. In addition, the corresponding freestanding CMS membranes were also made for characterization, of which the preparation condition was the same to the aforementioned supported ones, except for the exclusion of coating step. The precursor membranes were formed by directly casting the membrane-forming solution on the surface of horizontally sanitary glass plates. Besides, the precursors referred in the latter section were collected after heat treatment at 200 °C with the symbols of PB-x%.

2.4. Gas permeation test Single gas permeation of CMS membranes was performed by the traditional constant pressure-variable volume method. Ultra-purified gases, O2 and N2 (> 99.999%) were directly supplied from gas cylinders. The CMS membranes were housed in stainless membrane cells. The permeability of single gases was determined according to the detailed test procedure as described in literature [25]. The permeation test of mixed gas was performed by feeding compressed air. During measurement, the pressures were kept at a preset value at feed side and the atmospheric at permeate side. Meanwhile, the permeate side was kept sweeping by 4 mL/min flowing helium. The gas composition was determined by a FULI-9790 gas chromatography equipped with a packed 5 Å column and a thermal conductivity detector. The gas permeability of gas species i for CMS membranes were calculated by Equation (1).

Pi =

Fi A × P /L

(1)

Where, the symbols P, F, ΔP, A and L were the permeability in a standard unit of Barrer (1 Barrer = 1 kmol mm−2 s−1 Pa−1), gas flux permeating through the membrane, trans-membrane pressure differential of feed side and permeate side, effective membrane area and membrane thickness, respectively. The precision of permeability was estimated to be within ± 10%. The ideal separation selectivity of single gas pairs for O2/N2 was obtained by Equation (2). O2/ N 2

=

PO2 PN 2

(2)

Meanwhile, the separation factor of mixed gas (O2/N2) was calculated by Equation (3). Where, xO2 and xN2 were the mole fractions at permeate side for O2 and N2, respectively. The subscript of “0” referred to the mole fractions at feed side.

2.3. Characterization O2/ N 2

The thermal stability of precursors was examined by a TGA-4000 143

=

xO2 / xN 2 xO2,0 / xN 2,0

(3)

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CMS membranes [30]. Therefore, it concludes that the present pyrolysis temperature 650 °C can ensure the well evolution of porous structure in CMS membranes. Overall, the three membranes have similar profiles of TGA and DTG. Nevertheless, the temperatures corresponding to the maximum degradation rates obey the order of PB-0.4% (249.3 °C) > PB-0.2% (242.5 °C) > PB-0% (240.7 °C) at the second stage, together with a close value of 476.5 °C at the third stage. In addition, the final weight remaining of the membranes at 800 °C increases by incorporation of BT, i.e., PB-0.2% (42.3%), PB-0.4% (40.3%) and PB-0% (35.2%). This result agrees with Jose et al.‘s report [31]. Obviously, BT addition improves the thermal stability of precursor membranes. Simultaneously, a strong interaction is probably formed between BT and polymer chains via the abundant functional groups on BT surface [21]. This can also be verified by the DSC curves (Fig. 1(b)). It shows that all the three precursor membranes have a strong endothermic peak of melting process around 210 °C, i.e. PB-0% (213.6 °C), PB-0.2% (209.8 °C) and PB-0.4% (216.5 °C). The variation of thermal behavior of precursor membranes is probably due to the influence of thermal stability and crystallite degree by the BT interference on the molecular motion of polyimide [32,33]. As the result, BT addition tends to improve the thermal stability, mechanics, gas permeation properties of the membranes [34]. Regarding the inherent relationship between CMS membranes and their precursor membranes, it is anticipated that the gas permeation of derived CMS membranes could certainly be modified by incorporating BT in precursor membranes. 3.2. Evolution of functional groups Fig. 2 is the FTIR spectra of the membrane samples. For BT (Fig. 2(a)), a strong reflectance peak can be easily identified at 1004 cm−1 corresponding to Si–O–Si asymmetric stretching vibrations of the SiO4 tetrahedron [35]. Besides, the peaks at 1450 cm−1, 790 cm−1, 577 cm−1 and 463 cm−1 indicate the grafting of organic sulfo acid groups on the surface of BT [36], the deformation and bending modes of Si–O bond [37], the bending vibration of Si–O–Al called a “sensitive peak” for iron replacement, and the bending vibration of Si–O–Si [38], respectively. In the spectra of PB-0%, it clearly shows the typically characteristic peaks at 1715 cm−1 (asymmetric stretching vibration of –C=O), 1484 cm−1 ( vibration), 1430 cm−1 (stretching vibration of -C-N in imide), 1163 cm−1 and 1122 cm−1 (stretching vibration of C–F in –CF3 substitute group) [39], 1043 cm−1 and 1223 cm−1 (stretching vibration of C–O), 740 cm−1 (out-plane blending vibration of C–H in aromatic rings) [40], and 843 cm−1 ( deformation vibration in phenyl ether). Furthermore, another two characteristic peaks are also available at 1617 cm−1 and 1548 cm−1 corresponding to the C=O and N–H bending vibration of the amide group, respectively. It implies that a small portion of poly(amic acid) still remains in the membrane matrix even after thermal imidization [25]. This is consistent with the above TGA analysis. In comparison to the free BT, the intensity of characteristic peaks of BT is weakened for BT-doped precursors due to the restriction effect of molecular motion induced around polyimide molecules. After pyrolysis, all the aforementioned peaks diminish on the surface of CMS membranes, except for the aromatic structure. It is mainly attributed to the decomposition of chemical groups from the main molecular chains of polyimide at high temperature pyrolysis, accompanied by the evolution of a graphite-like amorphous carbon structure in the membrane matrix [41].

Fig. 1. The thermal stability analysis of precursor membranes. (a) TGA and DTG, (b) DSC.

3. Results and discussion 3.1. Thermal stability of precursors The curves of thermal weight loss and their derivatives of precursors are shown in Fig. 1(a). It notices that the used BT is thermally stable enough with the temperature at 5% weight loss being greater than 600 °C. After that, there is only a minor weight loss for BT with approximately 12.7% up to 800 °C owing to the dehydration and structural reorganization [26]. In contrast, the precursor membranes have two major weight loss stages. The first one happens beneath 350 °C because of the emission of a tiny amount of water derived from the thermal imidization of residual poly(amic acid) and the volatilization of remaining solvent DMAc [25]. The second one occurs at 350–650 °C, corresponding to the remarkably thermal degradation of molecular chains of polyimide. As the consequence, abundant porous structures are evolved in the membrane matrix along with the release of derived gases or vapors, i.e. H2, CO2, O2, CH4, etc [27,28]. When the temperature is higher than 650 °C, the profiles of weight loss curves for precursor membranes gradually turn to flat. It means that the predominant thermal reaction has shifted from decomposition to structural rearrangement [29]. In another word, the final pyrolysis temperature takes great effect on the development of resultant porous structure in

3.3. Microstructure analysis Fig. 3 gives the XRD patterns of precursor membranes and CMS membranes. For BT, the diffraction peaks could be clearly assigned to the existence of quartz at 19.5°, 25.3°, 26.3°, 27.7° and 29.6°, and the 144

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Fig. 3. XRD patterns of (a) precursor membranes and (b) CMS membranes. Fig. 2. FTIR spectra of (a) precursor membranes and (b) CMS membranes.

major chemical composition of SiO2 at 20.8°, 39.4°and 50.0° [42]. In Fig. 3(a), the three precursor membranes present a large shoulder shape peak at 19° and 23°, reflecting the crystal and amorphous phases, respectively. Moreover, the peak positions slightly shift toward higher diffraction degree with the BT amount increasing from 0% to 0.4%, along with the interlayer distance d002 values reducing from 0.454 nm to 0.448 nm. Additionally, another two diffraction peaks also appear at 26.3° and 29.6° for precursor membranes containing higher content of BT (e.g., PB-0.4%), which are the representatives of typical character of BT. After pyrolysis, the originally intense diffraction peak around 20° is replaced by a quite broad peak at 20–25°, suggesting the evolution of an amorphous carbon structure. Additionally, the presence of a weak diffraction peak at approximately 43.6° also verifies the formation of an amorphous graphite-like carbon structure in CMS membranes [43,44]. Moreover, the d002 values are significantly reduced with the increment of BT amount, as shown in Fig. 3 (b). It suggests that the addition of BT remarkably enhances the structural compactness of CMS membranes [45,46]. This result is associated with the contribution of the innate dense framework of BT, although it might probably weaken the cohesive energy of polyimide molecules resulting into the formation of loosely local region around BT particles after pyrolysis [47].

Fig. 4. Nitrogen adsorption isotherms and pore size distribution curves of CMS membranes.

3.4. Microporous structure analysis In Fig. 4, it shows that the adsorption isotherms for all CMS membranes are assigned to the typical Type I as per the IUPAC classification, 145

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Table 1 The porous structure parameters of CMS membranes. Samples

BET specific surface area (m2/g)

Pore volume (cm3/g)

Average pore width (nm)

BT CMS-0% CMS-0.2% CMS -0.4% Ref [49] Ref [50]

34.7 412.3 613.7 581.6 591.0 560.0

0.05 0.19 0.26 0.24 – 0.32

6.14 1.81 1.69 1.65 5.60 2.37

which has an abrupt increase of adsorption quantity at low relative pressures. That implies the existence of abundant microporous structure in CMS membranes [48]. Although the isotherm profiles of the three CMS membranes are quite similar, their adsorption quantities follow the order of CMS-0.2% > CMS-0.4% > CMS-0%. As listed in Table 1, the porous parameters of as-obtained CMS membranes are comparable to those in reports [49,50]. Besides, the BET specific surface area and porous volume of CMS membranes first

Fig. 6. Effect of BT amount on the gas permeation of CMS membranes at 0.05 MPa and 30 °C.

Fig. 5. SEM images of supported CMS membranes. (a) (d) CMS-0%, (b) (e) CMS-0.2%, (c) (f) CMS-0.4%, (g)(h) locally magnified images and (i) TEM image of CMS0.4%.

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Fig. 7. Schematic of the evolution of gas penetrants through CMS membranes with the variation of BT amount.

Fig. 10. Effect of permeation pressure on the gas separation performance of CMS membranes at 0.05 MPa and 30 °C. Ref. [62], Ref. [63].

Fig. 8. Effect of transmembrane pressure on the gas separation performance of CMS membranes at 30 °C.

increase then decrease with elevating the BT amount from 0% to 0.4%. Meanwhile, the average pore size monotonously decreases. It can be sure that the adjustment of porous structure by BT addition would significantly influence the gas permeation of CMS membranes. 3.5. Morphology observation As shown in the SEM images (Fig. 5), the top surface layers of all CMS membranes are uniform without any cracks or pinholes. In the cross-section, the CMS membranes possess a clearly asymmetric structure that includes a dense surface layer and a macroporous support. The dense surface layer determines the separation properties of the membranes, whereas the porous substrate provides mechanical support [51]. Besides, the irregular boundary line between the surface layer and support indicates that partial surface layer has extended into the support matrix during the membrane formation. This is beneficial for improving the mechanical property of the surface layer for CMS membranes. At the same time, it also notices that two absolutely different states, i.e., loose domains and dense domains, could be clearly found for BT in the matrix from the locally magnified images (Fig. 5(h)). When the BT amount is small in CMS membranes, the good dispersion is favorable for forming more loose domains. On the contrary, it is prone to yield more dense domains. This effect on gas separation performance will be further discussed in the following section. In addition, it is clearly found that bentonite with parallel layered microcrystalline

Fig. 9. Correlation of permeation temperature with gas permeation of CMS0.2% at 0.05 MPa.

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Table 2 The O2/N2 separation data of CMS membranes measured at 30 °C and 0.15 MPa. Membrane codes

CMS-0% CMS-0.4%

Feed gas

Mixed Single Mixed Single

Permeability (Barrer)

Selectivity

Mole fraction (%)

O2

N2

O2/N2

O2

N2

52.07 45.1 137.87 218.6

3.50 4.6 14.37 44.0

14.1 9.8 9.1 5.0

78.9 – 70.6 –

21.1 – 29.4 –

structure is embedded in the amorphous carbon matrix from the TEM image (Fig. 5(i)).

The as-obtained Ea values are given in Fig. 9. The values are comparable to the literature reports [23,59,60]. It is considered that the activation energy of permeation is comprised of the isosteric heat of adsorption and the activation energy of the micropore diffusion [61]. Thus, the gas molecules with a larger kinetic diameter are more dependant on the permeation temperature, owing to their higher diffusion resistance and permeation resistance [55].

3.6. Gas separation performance of CMS membranes 3.6.1. Dependence of BT amount Fig. 6 presents the variation of gas permeation of CMS membranes with the BT amount. Obviously, the permeability follows the order of CMS-0.2% > CMS-0.4% > CMS-0%, which is in the same sequence of the adsorption quantity as has been given previously. It is attributed to the pore-formation effect of BT in the matrix of CMS membranes. On the other hand, another negative pore-blockage effect would also become more evident and even take over the dominant role of pore-formation when the BT amount is high enough, for instance 0.4%. In brief, CMS-0.2% exhibits the highest permeability among all the CMS membranes in this work. Fig. 7 illustrates the difference in microstructure-permeation relationship of the three CMS membrans. For Case 1, gases strive to permeate CMS membranes from one side to the other side by overcoming a higher potential barrier of the complex porous carbon structure formed by the interconnectivity of larger pore openings and smaller constrictions [52,53]. For Case 2, the potential barrier of permeation would be reduced to a certain degree due to the generation of some loose local domains in the interfacial region between BT and carbon matrix [3,53]. Consequently, the gas permeability of resultant CMS membranes is increased. For Case 3, the situation is changed to the opposite direction, where the higher amount of BT tends to form larger agglomerated particles. Since the inner pores of BT are mostly impermeable and dead-end, gases have to bypass the agglomerated particles by surmounting a slightly higher potential energy due to the extended length of permeating passages [54]. This has been verified by the previous microstructural analysis and morphology observation. As the result, the gas permeability decreases to some extent. Therefore, it would be necessary to modify the porous structure and surface properties of BT in the future, in order to get more permeable and cognitive separation performance of resultant BT hybrid CMS membranes.

3.6.4. Comprehensive evaluation of separation performance A famous Robeson's plot is commonly adopted to assess the comprehensive separation performance of membrane materials because of the existence of an indistinguishable trade-off relationship between the permeability and selectivity. As shown in Fig. 10, all the as-prepared CMS membranes in this work have surpassed the upper bound with locating in the recommended commercially attractive region. In contrast, the gas permeability of present CMS membranes is much superior to those of the zeolite and Ni-modified CMS membranes [62,63]. This demonstrates that the incorporation of BT is a feasible and effective way for modifying the permeation property of CMS membranes. 3.6.5. Separation of mixed gas Table 2 lists the gas permeation data of CMS-0% and CMS-0.4%. In comparison to single gases, the gas permeability is depressed on the whole under the situation of mixed gas feed. Oppositely, the selectivity is significantly improved. This phenomenon has also been found by some other researchers [64]. The reason is associated with the competitive permeation between O2 and N2 for the limited adsorption sites and diffusive passages in CMS membranes. Once permeation through CMS membranes, the mole fraction of O2 molecule is greatly improved for the air, i.e. from 21% to 78.9% of CMS-0% or 70.6% of CMS-0.4%. In summary, the results have shown that the present CMS membranes are very promising for enrichment of O2 from the air. 4. Conclusions Bentonite (BT) composite CMS membranes were successfully prepared by the processes of membrane formation and pyrolysis. The addition of BT increases the thermal stability of precursor membranes. The graphitization degree and structural compactness of CMS membranes are elevated with increasing the BT content. The BT composite CMS membranes exhibit an increased microporous volume in comparison to the pure ones. The BT addition increases the gas permeability of CMS membranes at a slight loss of selectivity. With elevating the BT content, two opposite effects (i.e., the positive and the negative) play essential roles in determination of the gas permeability and selectivity of CMS membranes. The highest gas permeability is assigned to CMS membranes made by the precursor containing 0.2% of BT, which reaches to 434.7 Barrer for O2, along with the ideal O2/ N2 selectivity of 7.2.

3.6.2. Dependence of transmembrane pressure In Fig. 8, it shows that both gas permeability and selectivity are reduced with the transmembrane pressure elevating from 0.05 MPa to 0.15 MPa. The results validate that the as-prepared CMS membranes are free of visible defects with the dimension of 50–100 nm [55]. What's more, the superior O2/N2 selectivity has demonstrated that the molecular sieving mechanism dominates the permeation process of gases since it is far beyond the Knudsen diffusion (0.94). In fact, on the basis of the inversely proportional relation between gas permeability and their kinetic diameters, O2 (0.346 nm) > N2 (0.364 nm), it can also come to the conclusion that the molecular sieving is the dominant permeation mechanism of CMS membranes [56]. 3.6.3. Dependence of permeation temperature Generally, the gas permeation through CMS membranes is a temperature-activation process [57]. The activation energy (Ea) could be calculated through the linear formula of Arrhenius equation, E ln P = ln P0 RTa [58].

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 20906063) and the Liaoning BaiQianWan Talents Program (No. 2018921046). 148

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