Highly Selective Oxygen/Nitrogen Separation Membrane Engineered Using a Porphyrin-Based Oxygen Carrier

Air separation is very important from the viewpoint of the economic and environmental advantages. In this work, defect-free facilitated transport membranes based on poly(amide-12-b-ethylene oxide) (Pebax-2533) and tetra(p-methoxylphenyl)porphyrin cobalt chloride (T(p-OCH3)PPCoCl) were fabricated in systematically varied compositions for O2/N2 separation. T(p-OCH3)PPCoCl was introduced as carriers that selectively and reversibly interacted with O2 and facilitated O2 transport in the membrane. The T(p-OCH3)PPCoCl had good compatibility with the Pebax-2533 via the hydrogen bond interaction and formed a uniform and thin selective layer on the substrate. The O2 separation performance of the thin film composite (TFC) membranes was improved by adding a small amount of the T(p-OCH3)PPCoCl and decreasing the feed pressure. At the pressure of 0.035 MPa, the O2 permeability and O2/N2 selectivity of the 0.6 wt % T(p-OCH3)PPCoCl/Pebax-2533 was more than 3.5 times that of the Pebax-2533 TFC membrane, which reached the 2008 Robeson upper bound. It provides a candidate membrane material for O2/N2 efficient separation in moderate conditions.


Introduction
Air separation is an essential process, in which the obtained oxygen-enriched air can be used to assist combustion by increasing the burning velocities [1], regenerate catalysts in the fluid catalytic cracking [2], and improve indoor air quality [3]; meanwhile, the obtained nitrogen-enriched air can be applied in keeping food fresh, preventing fires, oil recovery, and draining water [4,5]. Currently, cryogenic distillation is the commercial and mature technology for air separation, which can produce a large amount of high purity (>99%) oxygen and nitrogen. However, this process is complex, as well as cost and energy intensive [6]. Pressure swing adsorption (PSA) is another commonly used air separation method that can produce high purity (≈95%) oxygen with medium production, but the larger space, higher investment, and energy consumption of the process are still challenging [7]. Compared with these air separation technologies, the membrane gas separation method is a green and sustainable process because of its continuous production, easy operation, small space, environmental friendliness, Chart 1. Chemical structures of Pebax-2533 (a) and T(p-OCH3)PPCoCl (b).

Membrane Preparation
Free-standing Pebax-2533 membrane and T(p-OCH3)PPCoCl/Pebax-2533 mixed matrix membranes (MMMs) were prepared using a solution-casting method. Pebax-2533 pellets were dissolved in ethanol (10 wt%) with magnetic stirring at 80 °C for 3 h. A predetermined amount of T(p-OCH3)PPCoCl was then added and stirred at 40 °C for 2 h. After that, the solution was cast onto a glass dish and dried at room temperature for 24 h to form a sheet of membrane, which was further dried under vacuum at 60 °C for 8 h.
TFC membranes of T(p-OCH3)PPCoCl/Pebax-2533 on PVDF supports were prepared using a dip-coating method. Pre-weighed amounts of Pebax-2533 pellets were dissolved in hot ethanol at 80 °C to obtain a homogeneous solution (2 wt%). Then, T(p-OCH3)PPCoCl was added and the mixture was stirred continuously at 40 °C until it became homogeneous. The solution was degassed and poured into a glass cell. After that, the prefixed substrate PVDF on a glass pane was vertically immersed into the glass cell with the T(p-OCH3)PPCoCl/Pebax-2533 solution and then dried at ambient temperature for 2 h. Finally, the composite membranes were dried in a vacuum oven at 60 °C for 8 h to remove any solvent residue.

Synthesis of Cobalt Porphyrin
Tetra(p-methoxylphenyl)porphyrin (T(p-OCH 3 )PP) was prepared according to the literature [25,26] with some revisions. In a three-necked flask, propionic acid (20 mL), acetic acid (10 mL), and o-nitrotoluene (10 mL) were added. The mixture was heated to reflux, and then p-anisaldehyde (10 mmol) was added into the refluxing mixture and stirred for 5-6 min. Pyrrole (10 mmol) dissolved in o-nitrotoluene (10 mL) was added dropwise and reacted for 1 h. The mixture was allowed to cool to 60 • C, and then 30 mL of methanol was added slowly and set aside for recrystallization. The blue crystals were filtered and washed three times with water and methanol. The resulting T(p-OCH 3 )PP product (0.34 g, 18.5% yield) was obtained and dried at 60 • C under vacuum for 8 h. 1 [27]. T(p-OCH 3 )PP (147 mg, 0.2 mmol), DMF(30 mL) and acetic acid (3 mL) were added into a three-necked flask and heated to reflux for 30 min. Then, CoCl 2 (156 mg, 1.2 mmol) was added with five portions and reacted for 5 h. After the mixture was cooled down to room temperature, hydrochloric acid aqueous solution (40 mL, ≈19 wt %) was poured into the flask and held overnight for recrystallization. The crystals were filtered, washed thoroughly with hydrochloric acid aqueous solution (200 mL, ≈8 wt %), and dried at 60 • C under vacuum for 8 h to obtain purple crystals of T(p-OCH 3 )PPCoCl (160 mg, yield 97%).

Membrane Preparation
Free-standing Pebax-2533 membrane and T(p-OCH 3 )PPCoCl/Pebax-2533 mixed matrix membranes (MMMs) were prepared using a solution-casting method. Pebax-2533 pellets were dissolved in ethanol (10 wt %) with magnetic stirring at 80 • C for 3 h. A predetermined amount of T(p-OCH 3 )PPCoCl was then added and stirred at 40 • C for 2 h. After that, the solution was cast onto a glass dish and dried at room temperature for 24 h to form a sheet of membrane, which was further dried under vacuum at 60 • C for 8 h.
TFC membranes of T(p-OCH 3 )PPCoCl/Pebax-2533 on PVDF supports were prepared using a dip-coating method. Pre-weighed amounts of Pebax-2533 pellets were dissolved in hot ethanol at 80 • C to obtain a homogeneous solution (2 wt %). Then, T(p-OCH 3 )PPCoCl was added and the mixture was stirred continuously at 40 • C until it became homogeneous. The solution was degassed and poured into a glass cell. After that, the prefixed substrate PVDF on a glass pane was vertically immersed into the glass cell with the T(p-OCH 3 )PPCoCl/Pebax-2533 solution and then dried at ambient temperature for 2 h. Finally, the composite membranes were dried in a vacuum oven at 60 • C for 8 h to remove any solvent residue.

Characterization Methods
Ultraviolet-visible (UV-Vis) spectra of T(p-OCH 3 )PP and T(p-OCH 3 )PPCoCl were recorded in toluene on a Shimadzu UV-2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan) in the range of 350-680 nm. The Fourier transform infrared (FTIR) spectrum of T(p-OCH 3 )PPCoCl and the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of Pebax-2533 and MMMs were obtained using a Thermo Nicolet 380 spectrometer (Thermo Electron Corporation, Madison, WI, USA) in the range of 650-4000 cm −1 under ambient conditions. Wide angle X-ray diffraction (WAXRD) patterns of the MMMs were measured using a Smartlab (9 kW) diffractometer (Rigaku Corporation, Tokyo, Japan) at a scan rate of 15 • min −1 ; the d-spacing values were calculated based on the diffraction peak maxima using Bragg's equation. The surface and cross-sectional morphologies of the TFC membranes were observed using SU8020 scanning electron microscopy (SEM) (Hitachi High-Technologies Corporation, Tokyo, Japan). Before scanning, the membranes were fractured in liquid nitrogen and sputtered with gold.
The O 2 and N 2 permeation tests were carried out at ambient temperature using the standard constant-pressure, variable-volume method [28]. The feed pressure varied from 0.035 to 0.8 MPa in gauge mode, and the permeate pressure was kept at atmospheric pressure. Gas permeability was calculated using the following equation: where P is the gas permeability (barrer, 1 barrer = 10 −10 (cm 3 (STP) cm cm −2 s −1 cmHg −1 ), Q is the volume of permeated gas (cm 3 (STP)), l is the film thickness (cm), t is the permeation time (s), A is the effective membrane area (cm 2 ), and p 1 and p 2 are the upstream and permeate side pressures (cmHg), respectively. The ideal selectivity (S) is defined as the ratio of the pure gas permeability of the faster gas O 2 (P A ) over that of the slower permeant N 2 (P B ):

Molecular Modeling
Molecular geometries (O 2 , N 2 , and T(p-OCH 3 )PPCoCl) were optimized using density functional theory (DFT) calculations with Gaussian 09 software (Gaussian, Inc., Wallingford, CT, USA) [29]. The formation of complexes between T(p-OCH 3 )PPCoCl and N 2 /O 2 with several different structures were fabricated by locating N 2 or O 2 at different positions around T(p-OCH 3 )PPCoCl, and the energy minimizations of all complexes were implemented to determine the optimized geometry. All of the optimizations were explored at the B3LYP/6-311G (d, p) level. The interaction energies between T(p-OCH 3 )PPCoCl and N 2 /O 2 were calculated at the same level.

Synthesis and Characterization of T(p-OCH 3 )PPCoCl
T(p-OCH 3 )PP was prepared in a mixed solvent of propionic acid, acetic acid, and o-nitrotoluene under an air atmosphere. Propionic acid and acetic acid were used as catalysts for the condensation of pyrrole and p-anisaldehyde to obtain porphyrinogen, which was further oxidized by the oxygen (air) and o-nitrotoluene to produce T(p-OCH 3 )PP with a yield of 18.5%. Compared with other metalloporphyrins, cobalt porphyrin has a higher stability [30]. Therefore, T(p-OCH 3 )PPCoCl was used as the O 2 carrier and prepared in DMF with a high yield (97%). The obtained T(p-OCH 3 )PP and T(p-OCH 3 )PPCoCl were characterized using UV-vis and the spectra are shown in Figure 1. T(p-OCH 3 )PP exhibited a characteristic major Soret band at 422.8 nm owing to the π→π* transition, and four visible Q bands at 518.2, 554.7, 595.2, and 653.3 nm. The Soret band of T(p-OCH 3 )PPCoCl shifted to a longer wavelength of 441.7 nm and two Q bands emerged at 556.8 nm and 596.1 nm. The decreased amount of Q bands can be ascribed the metalation of T(p-OCH 3 )PP, which increased the molecular symmetry [31].

Membranes Properties and Characterizations
The interaction between Pebax-2533 matrix and T(p-OCH 3 )PPCoCl was investigated using FTIR, and the spectra of T(p-OCH 3  respectively. The spectrum of neat Pebax-2533 membrane was in accordance with the literature [32,33], in which the bands at 3307.9, 1734.7, 1638.2, and 1104.0 cm −1 represent the N-H, C=O, H-N-C=O, and C-O stretching, respectively. The peak at 1248.8 cm −1 corresponds to the -C-O-C stretching vibration, which showed an increasing intensity with the increasing of the T(p-OCH 3 )PPCoCl content in the MMMs, as expected. More importantly, the -C-O-C stretching vibration in T(p-OCH 3 )PPCoCl shifted to a longer wavelength from 1248.8 to 1243.9 cm −1 in MMMs and the N-H stretching vibration in Pebax-2533 similarly shifted to a longer wavelength from 3307.9 to 3296.8 cm −1 , confirming substantial hydrogen bonding was formed in the composite membranes [34]. The formation of hydrogen bonding could improve the affinity between T(p-OCH 3 )PPCoCl and Pebax-2533, thus avoiding interfacial defects.

Membranes Properties and Characterizations
The interaction between Pebax-2533 matrix and T(p-OCH3)PPCoCl was investigated using FTIR, and the spectra of T(p-OCH3)PPCoCl, neat Pebax-2533 membrane, and T(p-OCH3)PPCoCl/Pebax-2533 MMMs are given in Figure 2. The absorption peaks at 2834.0, 1606.2, 1351.2, 1248.8, and 1001.7 cm −1 for T(p-OCH3)PPCoCl were assigned to C-H, C=C, C=N, -C-O-C, and Co-N stretching vibrations, respectively. The spectrum of neat Pebax-2533 membrane was in accordance with the literature [32,33], in which the bands at 3307.9, 1734.7, 1638.2, and 1104.0 cm −1 represent the N-H, C=O, H-N-C=O, and C-O stretching, respectively. The peak at 1248.8 cm −1 corresponds to the -C-O-C stretching vibration, which showed an increasing intensity with the increasing of the T(p-OCH3)PPCoCl content in the MMMs, as expected. More importantly, the -C-O-C stretching vibration in T(p-OCH3)PPCoCl shifted to a longer wavelength from 1248.8 to 1243.9 cm −1 in MMMs and the N-H stretching vibration in Pebax-2533 similarly shifted to a longer wavelength from 3307.9 to 3296.8 cm −1 , confirming substantial hydrogen bonding was formed in the composite membranes [34]. The formation of hydrogen bonding could improve the affinity between T(p-OCH3)PPCoCl and Pebax-2533, thus avoiding interfacial defects. To investigate the effect of T(p-OCH 3 )PPCoCl on the chain packing structure of Pebax-2533, XRD patterns of the Pebax-2533 membrane, 0.6 wt % T(p-OCH 3 )PPCoCl/Pebax-2533 MMM and 2 wt % T(p-OCH 3 )PPCoCl/Pebax-2533 MMM were measured, as shown in Figure 3. The neat Pebax-2533 membrane had a broad peak located at 2θ = 19.8 • , which corresponds to a d-spacing value of 4.5 Å. The broad halo was identified as the inter-chain distance of amorphous regions [35][36][37]. Adding T(p-OCH 3 )PPCoCl into Pebax-2533 matrix did not change the polymer chain packing structure, as evidenced in Figure 3, where the d-spacing values of the MMMs were the same as that of the pure Pebax-2533 membrane. No crystal peak of T(p-OCH 3 )PPCoCl was observed in the WAXD patterns. This could have originated from a small weight fraction of the T(p-OCH 3 )PPCoCl and suggests a good compatibility in the T(p-OCH 3 )PPCoCl/Pebax-2533 composite membranes.
Membranes 2019, 9, x FOR PEER REVIEW 7 of 14 membrane had a broad peak located at 2θ = 19.8°, which corresponds to a d-spacing value of 4.5 Å .
The broad halo was identified as the inter-chain distance of amorphous regions [35][36][37]. Adding T(p-OCH3)PPCoCl into Pebax-2533 matrix did not change the polymer chain packing structure, as evidenced in Figure 3, where the d-spacing values of the MMMs were the same as that of the pure Pebax-2533 membrane. No crystal peak of T(p-OCH3)PPCoCl was observed in the WAXD patterns. This could have originated from a small weight fraction of the T(p-OCH3)PPCoCl and suggests a good compatibility in the T(p-OCH3)PPCoCl/Pebax-2533 composite membranes.   (Figure 4c) of Pebax-2533 shows two layers, i.e., the top selective layer with a film thickness of 1 μm and the PVDF substrate. As shown in Figure 4b,d, no particulate clusters were observed, indicating that T(p-OCH3)PPCoCl was well dispersed at the molecular level in the   (Figure 4c) of Pebax-2533 shows two layers, i.e., the top selective layer with a film thickness of 1 µm and the PVDF substrate. As shown in Figure 4b,d, no particulate clusters were observed, indicating that T(p-OCH 3 )PPCoCl was well dispersed at the molecular level in the Pebax-2533 matrix without obvious agglomerations.

Molecular Modeling of O2/N2 and T(p-OCH3)PPCoCl Interactions
The optimized structures of the T(p-OCH3)PPCoCl and the interaction between T(p-OCH3)PPCoCl and gas molecules were studied using molecular modeling, and the results are shown in Figure 5. T(p-OCH3)PPCoCl exhibited a three-dimensional and distorted structure (Figure 5a), and it may have increased the fractional free volume and gas permeability of the membranes by disrupting the polymer chain packing. In the T(p-OCH3)PPCoCl-N2 complex (Figure 5b), the distance between 97Co and 120N was 3.25 Å , and there was no obvious bond, confirming there was no interaction between T(p-OCH3)PPCoCl and N2. However, in the T(p-OCH3)PPCoCl-O2 complex (Figure 5c), O2 interacted with the central cobalt ion of T(p-OCH3)PPCoCl to form a six-coordinate complex (Figure 5c). The distance of 97Co-119O was 1.89 Å , which is the same as the length of the reported Co-O coordinate bond [38], showing the coordinate bond was formed between 97Co and 119O. It indicates there was an interaction between T(p-OCH3)PPCoCl and O2. At the same time, the distance of 97Co-118Cl (2.31 Å ) in the T(p-OCH3)PPCoCl-O2 complex was longer than that (2.20 Å ) in T(p-OCH3)PPCoCl and shows the strength of the 97Co-118Cl bond decreased during the T(p-OCH3)PPCoCl-O2 complex formation. Their interaction energies between T(p-OCH3)PPCoCl and N2 or O2 estimated using quantum chemical calculations were −1.03 kcal/mol and −23.54 kcal/mol, respectively. It shows the interaction between T(p-OCH3)PPCoCl and O2 was higher than that between T(p-OCH3)PPCoCl and N2 and they may be reversible chemical and physical interactions separately. The reversible interaction makes T(p-OCH3)PPCoCl selectively adsorb oxygen and facilitate its transport in the membrane.

Molecular Modeling of O 2 /N 2 and T(p-OCH 3 )PPCoCl Interactions
The optimized structures of the T(p-OCH 3 )PPCoCl and the interaction between T(p-OCH 3 )PPCoCl and gas molecules were studied using molecular modeling, and the results are shown in Figure 5. T(p-OCH 3 )PPCoCl exhibited a three-dimensional and distorted structure (Figure 5a), and it may have increased the fractional free volume and gas permeability of the membranes by disrupting the polymer chain packing. In the T(p-OCH 3 )PPCoCl-N 2 complex (Figure 5b), the distance between 97Co and 120N was 3.25 Å, and there was no obvious bond, confirming there was no interaction between T(p-OCH 3 )PPCoCl and N 2 . However, in the T(p-OCH 3 )PPCoCl-O 2 complex (Figure 5c), O 2 interacted with the central cobalt ion of T(p-OCH 3 )PPCoCl to form a six-coordinate complex (Figure 5c). The distance of 97Co-119O was 1.89 Å, which is the same as the length of the reported Co-O coordinate bond [38], showing the coordinate bond was formed between 97Co and 119O. It indicates there was an interaction between T(p-OCH 3 )PPCoCl and O 2 . At the same time, the distance of 97Co-118Cl (2.31 Å) in the T(p-OCH 3 )PPCoCl-O 2 complex was longer than that (2.20 Å) in T(p-OCH 3 )PPCoCl and shows the strength of the 97Co-118Cl bond decreased during the T(p-OCH 3 )PPCoCl-O 2 complex formation. Their interaction energies between T(p-OCH 3 )PPCoCl and N 2 or O 2 estimated using quantum chemical calculations were −1.03 kcal/mol and −23.54 kcal/mol, respectively. It shows the interaction between T(p-OCH 3 )PPCoCl and O 2 was higher than that between T(p-OCH 3

Gas Permeation Properties
The influence of the T(p-OCH3)PPCoCl content on O2 and N2 permeability and O2/N2 selectivity were systematically investigated. It can be seen from Figure 6a that the N2 permeability increased with the increasing T(p-OCH3)PPCoCl content. Given that there was almost no interaction between N2 and T(p-OCH3)PPCoCl (interaction energy of −1.03 kcal/mol), the increase of the N2 permeability was mainly due to the increase of the free volume resulting from adding the twisted T(p-OCH3)PPCoCl into the membrane. However, the O2 permeability first increased and then decreased

Gas Permeation Properties
The influence of the T(p-OCH 3 )PPCoCl content on O 2 and N 2 permeability and O 2 /N 2 selectivity were systematically investigated. It can be seen from Figure 6a that the N 2 permeability increased with the increasing T(p-OCH 3 )PPCoCl content. Given that there was almost no interaction between N 2 and T(p-OCH 3 )PPCoCl (interaction energy of −1.03 kcal/mol), the increase of the N 2 permeability was mainly due to the increase of the free volume resulting from adding the twisted T(p-OCH 3 )PPCoCl into the membrane. However, the O 2 permeability first increased and then decreased with the increasing T(p-OCH 3   Gas feed pressure also has a great influence on the O2 separation performance, and the results of the investigation is shown in Figure 7a,b. The O2 and N2 permeability of the membrane without T(p-OCH3)PPCoCl almost remained constant when varying the feed pressure, indicating the change of pressure had no obvious influence. For the TFC membrane containing 0.6 wt% T(p-OCH3)PPCoCl, the N2 permeability increased slightly with the increasing feed pressure. The O2 permeability also remained constant when the feed pressure was higher than 0.1 MPa, but interestingly, the O2 permeability increased significantly from 8.0 barrer to 12.2 barrer, and the O2/N2 selectivity increased from 4.2 to 7.6 by lowering the feed pressure from 0.1 MPa to 0.035 MPa. This indicates that the lower pressure was beneficial for O2 separation because the chemical interaction between O2 and the carrier was predominant at low feed pressure [39], hence promoting the oxygen transport by T(p-OCH3)PPCoCl. Gas feed pressure also has a great influence on the O 2 separation performance, and the results of the investigation is shown in Figure 7a,b. The O 2 and N 2 permeability of the membrane without T(p-OCH 3 )PPCoCl almost remained constant when varying the feed pressure, indicating the change of pressure had no obvious influence. For the TFC membrane containing 0.6 wt % T(p-OCH 3 )PPCoCl, the N 2 permeability increased slightly with the increasing feed pressure. The O 2 permeability also remained constant when the feed pressure was higher than 0.1 MPa, but interestingly, the O 2 permeability increased significantly from 8.0 barrer to 12.2 barrer, and the O 2 /N 2 selectivity increased from 4.2 to 7.6 by lowering the feed pressure from 0.1 MPa to 0.035 MPa. This indicates that the lower pressure was beneficial for O 2 separation because the chemical interaction between O 2 and the carrier was predominant at low feed pressure [39], hence promoting the oxygen transport by T(p-OCH 3  The O2/N2 separation performances of the T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane were plotted against the 2008 Robeson upper bound, as shown in Figure 8. Compared with the membrane without T(p-OCH3)PPCoCl, the adding of T(p-OCH3)PPCoCl could enhance the separation performance mainly because T(p-OCH3)PPCoCl could efficiently facilitate O2 transport in the membrane, as well as the non-planar T(p-OCH3)PPCoCl increasing the gas permeability by increasing the fractional free volume. As the feed pressure was reduced, the separation performance was promoted gradually. The O2 permeability and O2/N2 selectivity were 12.2 barrer and 7.6, respectively, at 0.035 MPa, which is near the 2008 Robeson upper bound. For the facilitated membranes, lowering the feed pressure could enhance the separation performance. However, the separation performance of the 0.6 wt% T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane could reach the 2008 upper bound at a feed pressure of 35 kPa, which is higher than the feed pressures (<20 kPa) of the reported lectures [15,19,[21][22][23]. It shows the separation performance of the membrane could more easily reach the upper bound than the reported membranes by reducing the feed pressure. The separation performances of the reported membranes are presented in Table 1 for comparison. The data shows the 0.6 wt% T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane had satisfactory separation performance. Its O2/N2 selectivity was lower than only that of the 1 wt% CoPc/Pebax-1657 membrane and 20 wt% CoFPP Nafion membrane, but its O2 permeance was much higher than theirs. Thus, the T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane is a promising candidate for O2/N2 separation.    Figure 8. Compared with the membrane without T(p-OCH 3 )PPCoCl, the adding of T(p-OCH 3 )PPCoCl could enhance the separation performance mainly because T(p-OCH 3 )PPCoCl could efficiently facilitate O 2 transport in the membrane, as well as the non-planar T(p-OCH 3 )PPCoCl increasing the gas permeability by increasing the fractional free volume. As the feed pressure was reduced, the separation performance was promoted gradually. The O 2 permeability and O 2 /N 2 selectivity were 12.2 barrer and 7.6, respectively, at 0.035 MPa, which is near the 2008 Robeson upper bound. For the facilitated membranes, lowering the feed pressure could enhance the separation performance. However, the separation performance of the 0.6 wt % T(p-OCH 3 )PPCoCl/Pebax-2533 TFC membrane could reach the 2008 upper bound at a feed pressure of 35 kPa, which is higher than the feed pressures (<20 kPa) of the reported lectures [15,19,[21][22][23]. It shows the separation performance of the membrane could more easily reach the upper bound than the reported membranes by reducing the feed pressure. The separation performances of the reported membranes are presented in Table 1 for comparison. The data shows the 0.6 wt % T(p-OCH 3 )PPCoCl/Pebax-2533 TFC membrane had satisfactory separation performance. Its O 2 /N 2 selectivity was lower than only that of the 1 wt % CoPc/Pebax-1657 membrane and 20 wt % CoFPP Nafion membrane, but its O 2 permeance was much higher than theirs. Thus, the T(p-OCH 3 )PPCoCl/Pebax-2533 TFC membrane is a promising candidate for O 2 /N 2 separation. The O2/N2 separation performances of the T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane were plotted against the 2008 Robeson upper bound, as shown in Figure 8. Compared with the membrane without T(p-OCH3)PPCoCl, the adding of T(p-OCH3)PPCoCl could enhance the separation performance mainly because T(p-OCH3)PPCoCl could efficiently facilitate O2 transport in the membrane, as well as the non-planar T(p-OCH3)PPCoCl increasing the gas permeability by increasing the fractional free volume. As the feed pressure was reduced, the separation performance was promoted gradually. The O2 permeability and O2/N2 selectivity were 12.2 barrer and 7.6, respectively, at 0.035 MPa, which is near the 2008 Robeson upper bound. For the facilitated membranes, lowering the feed pressure could enhance the separation performance. However, the separation performance of the 0.6 wt% T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane could reach the 2008 upper bound at a feed pressure of 35 kPa, which is higher than the feed pressures (<20 kPa) of the reported lectures [15,19,[21][22][23]. It shows the separation performance of the membrane could more easily reach the upper bound than the reported membranes by reducing the feed pressure. The separation performances of the reported membranes are presented in Table 1 for comparison. The data shows the 0.6 wt% T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane had satisfactory separation performance. Its O2/N2 selectivity was lower than only that of the 1 wt% CoPc/Pebax-1657 membrane and 20 wt% CoFPP Nafion membrane, but its O2 permeance was much higher than theirs. Thus, the T(p-OCH3)PPCoCl/Pebax-2533 TFC membrane is a promising candidate for O2/N2 separation.

Conclusions
T(p-OCH 3 )PPCoCl/Pebax-2533 TFC membranes with a thin and defect-free active layer were prepared. The T(p-OCH 3 )PPCoCl and Pebax-2533 had good compatibility due to the formation of the hydrogen bond, improving the dispersion of the T(p-OCH 3 )PPCoCl in Pebax-2533. T(p-OCH 3 )PPCoCl as an oxygen carrier could not only facilitate oxygen transport due to the reversible interaction between T(p-OCH 3 )PPCoCl and O 2 , but also increase the membrane free volume, enhancing the O 2 and N 2 permeability and O 2 /N 2 selectivity. The T(p-OCH 3 )PPCoCl content and feed pressure had a great influence on membrane separation performance. The 0.6 wt % T(p-OCH 3

Conflicts of Interest:
The authors declare that they have no conflict of interest.