Preparation of Pebax 1657/MAF-7 Mixed Matrix Membranes with Enhanced CO2/N2 Separation by Active Site of Triazole Ligand

Fillers play a critical role in the performance of mixed matrix membranes (MMMs). Microporous metal azolate frameworks (MAFs) are a subclass material of metal–organic frameworks (MOFs). Due to the uncoordinated nitrogen of the organic ligands, MAF-7 (SOD-[Zn(mtz)2], Hmtz = 3-methyl-1,2,4-triazole, window: d = 0.34 nm) shows excellent CO2 adsorption performance. In this work, Pebax 1657/MAF-7 MMMs were prepared by a sample solution casting method with MAF-7 particles as fillers for the first time. By means of X-ray diffraction (XRD), scanning electron microscope (SEM), infrared radiation (IR), and thermogravimetry (TG), the compositional and structural properties of the mixed matrix membrane with different filler content were analyzed. The results show that the compatibility of MAF-7 and Pebax is good with a filler content of 5 wt.%. The pure gas testing showed that mixed matrix membrane has a high ideal CO2/N2 selectivity of 124.84 together with a better CO2 permeability of 76.15 Barrer with the optimized filler content of 5 wt.%. The obtained membrane showed 323.04% enhancement in selectivity of CO2/N2 and 27.74% increase in the permeability of CO2 compared to the pristine membrane at 25 °C and 3 bar. The excellent separation performance may be due to the ligands that can afford a Lewis base active site for CO2 binding with the uniform dispersion of MAF-7 particles in Pebax and the favorable interface compatibility. The obtained membrane overcomes the Robeson’s upper bound in 2008 for CO2/N2 separation. This work provides a new strategy by utilizing MAFs as fillers with triazole ligand to enhance the gas separation performance of mixed matrix membranes.


Introduction
With the growing demand for energy in the modern society, the excessive emission of CO 2 from fossil fuel combustion has been causing environmental problems. Hence, the CO 2 capture and storage has attracted more and more attention in the past decades [1,2]. Compared with conventional separation such as adsorption and cryogenic purification, membrane separation technology plays an important role in the field of gas separation with the advantages of low energy consumption, low running costs, and environmental friendliness [3,4]. Many kinds of polymers have been adopted for preparing polymer membrane, such as cellulose acetate (CA), polydimethylsiloxane (PDMS), and polyethylene oxide (PEO). However, polymer membranes often suffer from the Robeson upper bound that is the trade-off between selectivity and permeability [5,6]. Therefore, it is very important The SEM images showed that the MAF-7 particles were homogeneously dispersed in the organic phase, and there were no obvious interface defects between the fillers and the Pebax, combining the uncoordinated nitrogen of the organic ligands, the resulting MMM shows excellent CO 2 /N 2 selectivity. This work provides a strategy by utilizing MAFs as fillers with triazole ligand to enhance the gas separation performance of mixed matrix membranes.

Synthesis of MAF-7 Particles
MAF-7 particles were fabricated based on the reference with a slight modification [38]. In detail, first, 0.672 g Hmtz and 0.297 g Zn(NO 3 ) 2 ·6H 2 O were dissolved in two vials containing 5 mL deionized water, respectively. Second, the vials were stirred continuously for about 10 min followed by adding 1 mL ammonia to the zinc source solution and stirred for 5 min. Third, the above solutions were mixed and 0.100 g PVP was added into the solution with stirring for 10 min. Last, the solution above was transferred into the autoclave and put it into the oven at 120 • C for 4 h. The obtained crystals were washed with methanol by centrifugation three times. The MAF-7 powders were dried and activated in a 60 • C vacuum oven overnight.

Preparation of Pebax 1657/MAF-7 Mixed Matrix Membranes
The mixed matrix membranes based on MAF-7 crystals were made by the solution casting method. The prepared MAF-7 powders with different loading of 0 wt.%, 1 wt.%, 3 wt.%, 5 wt.%, 7 wt.%, and 9 wt.% were uniformly dispersed in the mixed solvent of ethanol and deionized water (mass ratio 70/30) under continuous reflux for 0.5 h at 80 • C. Then the suspension was stirred for 5 min, and ultrasounded for 1 h. Half of the Pebax pellets were added into the mixed solution and refluxed to induce dissolution, then the rest of the polymers were added into the flask and sonicated for another 1 h. Next, the flask was stirred in an oil bath under reflux at 80 • C for 24 h to obtain a uniform mixed Pebax/MAF-7 membrane solution. A stainless-steel ring was fixed on a glass pane to construct MMMs. The casting solution was added by a pipette into the stainless-steel ring in the clean room, then the membrane was transferred to the 60 • C vacuum oven for drying overnight, peeled off carefully from the substrate, and stored in a desiccator. Based on the MOF loading, the fabricated membrane abbreviations are given in Table 1.

Characterization Techniques
The crystalline phase of MAF-7 powders, pure membrane, and Pebax 1657/MAF-7 mixed matrix membranes were confirmed by X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan) with Cu-Kα (λ = 1.54 Å) radiation in the 2θ range between 5 • and 45 • . The morphology and size of the above samples were observed by field emission scanning electron microscope (SEM, SU8020, Hitachi, Tokyo, Japan), with an accelerating voltage of 5 kV. The MMM samples were prepared by freeze cutting under liquid nitrogen followed by a sputter coating of the gold layer using a sputter coater. Fourier infrared characterization of membrane samples using diffuse reflectance conditions, and the wavenumber range was 4000-400 cm −1 . The high temperature thermogravimetry-differential thermal synchronous analyzer Diamond TG/DTA used a temperature of 30-800 • C, a nitrogen atmosphere with the temperature increase rate of 10 • min −1 . Prior to tests, the membranes were vacuum dried at 70 • C overnight.

Gas Permeation Test
The gas permeation performance of the mixed matrix membranes was tested by the pure gas constant volume-variable pressure method in the gas permeation in a homemade device, the schematic diagram of the plant is shown in Figure 1. Every membrane was tested three times to confirm the reproducibility of the testing. Before testing, membranes were activated in a vacuum oven at 70 • C for 24 h. Then, the membranes were assembled and placed in an infiltration mold. The gas performance was tested at 25 • C and under 3 bar feed pressure. The definition of permeability is as follows: where P is the permeability of the gas (1 Barrer = 1 × 10 −10 cm 3 (STP) cm cm −2 ·s −1 ·cmHg −1 ), V is the volume of the container to be tested (cm 3 ), L is the thickness of the membrane (cm), A is the effective area of the membrane (cm 2 ), T is the experimental temperature (K), p is the feed pressure (cmHg), and d p /d t is the rate of change of pressure over time (cmHg·s −1 ). Another parameter characterizing the gas separation performance of membrane is the ideal selectivity α, which is defined as the ratio of permeability of two gases.
Membranes 2022, 12, x FOR PEER REVIEW 5 of 16 Figure 1. Schematic diagram of a constant volume/variable pressure set-up for the gas permeation measurement.

XRD Patterns and SEM Image of MAF-7 Powders
As shown in Figure 2, the characteristic peaks of synthesized MAF-7 at 2θ = 7.3 • , 10.3 • , 12.7 • , 14.7 • , 16.5 • , 18 • are in good agreement with those of the simulated MAF-7. Proving that we have successfully synthesized the pure phase MAF-7 powders (CCDC: 787,579). Figure 3 shows a regular crystal morphology of MAF-7 with the crystals size about 4 µm in average.

XRD Patterns and SEM Image of MAF-7 Powders
As shown in Figure 2, the characteristic peaks of synthesized MAF-7 at 2θ = 7.3°, 10.3°, 12.7°, 14.7°, 16.5°, 18° are in good agreement with those of the simulated MAF-7. Proving that we have successfully synthesized the pure phase MAF-7 powders (CCDC: 787,579). Figure 3 shows a regular crystal morphology of MAF-7 with the crystals size about 4 μm in average.

XRD characterization of MAF-7/Pebax MMMs
The crystal phase of the membranes was analyzed by XRD technique. Pebax-1657 was a block copolymer, with the characteristic peak of crystalline PA appeared at 23.9° [24]. As shown in Figure 4, with a lower filler content (1% and 3%), the characterized MAF-7 peaks did not show obviously in the MMMs, this may be due to a lower filler loading of the membrane with a weak peak intensity. As the loading of MAF-7 increased, the characteristic peak intensities of MAF-7 became more and more obvious. Comparing the simulated crystal structure of MAF-7, we can observe that the characteristic peak of filler at 7.3° in the mixed matrix membrane was gradually enhanced, which indicates that MAF-7 was successfully incorporated into organic phase.

XRD characterization of MAF-7/Pebax MMMs
The crystal phase of the membranes was analyzed by XRD technique. Pebax-1657 was a block copolymer, with the characteristic peak of crystalline PA appeared at 23.9 • [24]. As shown in Figure 4, with a lower filler content (1% and 3%), the characterized MAF-7 peaks did not show obviously in the MMMs, this may be due to a lower filler loading of the membrane with a weak peak intensity. As the loading of MAF-7 increased, the characteristic peak intensities of MAF-7 became more and more obvious. Comparing the simulated crystal structure of MAF-7, we can observe that the characteristic peak of filler at 7.3 • in the mixed matrix membrane was gradually enhanced, which indicates that MAF-7 was successfully incorporated into organic phase.

XRD characterization of MAF-7/Pebax MMMs
The crystal phase of the membranes was analyzed by XRD technique. Pebax-1657 was a block copolymer, with the characteristic peak of crystalline PA appeared at 23.9° [24]. As shown in Figure 4, with a lower filler content (1% and 3%), the characterized MAF-7 peaks did not show obviously in the MMMs, this may be due to a lower filler loading of the membrane with a weak peak intensity. As the loading of MAF-7 increased, the characteristic peak intensities of MAF-7 became more and more obvious. Comparing the simulated crystal structure of MAF-7, we can observe that the characteristic peak of filler at 7.3° in the mixed matrix membrane was gradually enhanced, which indicates that MAF-7 was successfully incorporated into organic phase.

SEM Characterization of Pebax 1657/MAF-7 MMMs
The morphologies of prepared Pebax1657/MAF-7 MMMs containing varied loadings of MAF-7 particles were observed as shown in Figure 5. Figure 5a,b demonstrated that the top-view and cross-section morphology of pure Pebax membrane was compact and uniform, and no obvious interface defects were observed. With a lower filler content (from 1% to 5%), the SEM analyses displayed a smooth surface of the obtained MMMs. Due to the low content having good compatibility with the MAF-7 powders in the membrane matrix, no obvious voids and/or cracks were observed in the surface of the Pebax/MAF-7 MMMs, indicating the MAF-7 particles in the membrane matrix have good dispersion and stability. When the filler contents were 7-9 wt.%, the MAF-7 particles did not embed completely in the Pebax matrix. In addition, the self-aggregation of MAF-7 particles was also observed on the membrane surface. When the filler content exceeds a critical loading (7 wt.% here), the particles tend to agglomerate, due to the polymer chains not being able to intimately cover them. In this work, the micrometer size of MAF-7 limits the high loading level [26]. The nonuniform distribution leads to the interfacial defects which is disadvantageous to the gas separation performance of the membrane, as shown in Figure 5i-l.

Thermodynamic Properties of Membranes
The TGA curves of the pure Pebax membrane and the Pebax 1657 /MAF-7 MMMs with different filler contents are given in Figure 6 to evaluate the thermodynamic properties. The entire thermal decomposition process consists of three major stages of mass loss. The first mass loss stage is about at 100 • C, it ascribes to the evaporation of residual solvent in the membrane; and the deacetylation and depolymerization of Pebax 1657 lead to the second mass loss stage between 100 and 400 • C; for the last mass loss stage, it is due to the residual decomposition of Pebax 1657. This result is consistent with the previous work [32,40]. In detail, with the addition of MAF-7 filler, the initial thermal decomposition temperature of Pebax 1657/MAF-7 MMMs decreased slightly. According to the previous report, the pyrolysis temperature of MAF-7 powder was about 270 • C [38]. Hence, by blending with Pebax and MAF-7 to form a membrane, the final decomposition temperature of Pebax 1657/MAF-7 MMMs is only slightly lower than that of a pristine Pebax membrane. The results demonstrated that the obtained membranes have comparable thermal stability which is sufficient for conventional gas separation applications.

Fourier Transform Infrared Spectroscopy of Membranes
FT-IR analysis was conducted to verify the successful blending of Pebax 165 7 crystals. The FT-IR results of pure Pebax-1657 membrane and Pebax-1657/MA with varied MAF-7 loadings are shown in Figure 7. As demonstrated in Figure 7a teristic peaks of membranes located at 1091, 1636, 2869, and 3297 cm −1 are ascr stretching vibration of the ether group, C=O, C-H, and N-H groups [22,41], respect pared with a pure Pebax membrane, when the loading of filler is 5 wt.%, Pebax 1 MMMs began to show obvious MAF-7 characteristic peaks in Figure 7b. The ch peak at 420 cm −1 is attributed to the Zn-N vibration of the mixed matrix membran we successful blended MAF-7 crystals into the membrane matrix [42,43].   [22,41], respectively. Compared with a pure Pebax membrane, when the loading of filler is 5 wt.%, Pebax 1657/MAF-7 MMMs began to show obvious MAF-7 characteristic peaks in Figure 7b. The characteristic peak at 420 cm −1 is attributed to the Zn-N vibration of the mixed matrix membranes, proving we successful blended MAF-7 crystals into the membrane matrix [42,43].

Fourier Transform Infrared Spectroscopy of Membranes
with varied MAF-7 loadings are shown in Figure 7. As demonstrated in Figure 7a, the characteristic peaks of membranes located at 1091, 1636, 2869, and 3297 cm −1 are ascribed to the stretching vibration of the ether group, C=O, C-H, and N-H groups [22,41], respectively. Compared with a pure Pebax membrane, when the loading of filler is 5 wt.%, Pebax 1657/MAF-7 MMMs began to show obvious MAF-7 characteristic peaks in Figure 7b. The characteristic peak at 420 cm −1 is attributed to the Zn-N vibration of the mixed matrix membranes, proving we successful blended MAF-7 crystals into the membrane matrix [42,43].

Effect of MAF-7 Loading in MMMs
The pure gas permeation results of the membranes with different filler loading under 3 bar feed pressure and 25 °C are shown in Figure 8. With the increasing content of MAF-7 fillers to 5 wt.%, the CO2 permeability and ideal selectivity of Pebax 1657/MAF-7 MMMs increased simultaneously. For dense and nonporous pristine Pebax membrane, the transport of gases through the membrane obeys a solution diffusion mechanism, the PEO section can provide strong affinity with CO2, leading to CO2 preferentially dissolving in the upstream face of a membrane, diffusing across the membrane, and desorbing from the downstream face of the membrane. Because of the dense membrane layer, the pristine Pebax membrane usually shows a low permeability. As a highly porous framework, MAF-7 has a pore widow of 0.34 nm, which is a suitable size for sieving CO2 (0.33 nm) and N2 (0.36 nm) [38]. The increasing MAF-7 contents in the membrane matrix could promote the diffusion rate of CO2 and have less effect on small sized N2, which leads to CO2 diffusion being easier than N2. On the other hand, the uncoordinated N donor in the MAF-7 threedimensional framework can adsorb carbon dioxide due to the Lewis acid-Lewis base interaction between the negative charged oxygen atoms of CO2 and N-containing organic heterocyclic molecules, which results in an increase in the permeability of the membrane

Effect of MAF-7 Loading in MMMs
The pure gas permeation results of the membranes with different filler loading under 3 bar feed pressure and 25 • C are shown in Figure 8. With the increasing content of MAF-7 fillers to 5 wt.%, the CO 2 permeability and ideal selectivity of Pebax 1657/MAF-7 MMMs increased simultaneously. For dense and nonporous pristine Pebax membrane, the transport of gases through the membrane obeys a solution diffusion mechanism, the PEO section can provide strong affinity with CO 2 , leading to CO 2 preferentially dissolving in the upstream face of a membrane, diffusing across the membrane, and desorbing from the downstream face of the membrane. Because of the dense membrane layer, the pristine Pebax membrane usually shows a low permeability. As a highly porous framework, MAF-7 has a pore widow of 0.34 nm, which is a suitable size for sieving CO 2 (0.33 nm) and N 2 (0.36 nm) [38]. The increasing MAF-7 contents in the membrane matrix could promote the diffusion rate of CO 2 and have less effect on small sized N 2 , which leads to CO 2 diffusion being easier than N 2 . On the other hand, the uncoordinated N donor in the MAF-7 three-dimensional framework can adsorb carbon dioxide due to the Lewis acid-Lewis base interaction between the negative charged oxygen atoms of CO 2 and Ncontaining organic heterocyclic molecules, which results in an increase in the permeability of the membrane to CO 2 and the ideal CO 2 /N 2 selectivity [44]. When the filler loading is 5 wt.%, the CO 2 permeability is 76.15 Barrer, and the CO 2 /N 2 ideal selectivity reaches the highest value of 124.84. Compared with the pure membrane, the ideal selectivity of CO 2 permeability and CO 2 /N 2 of Pebax 1657/MAF-7 MMMs has been increased by 27.75% and 323.04%, respectively. Afterwards, as the MAF-7 filler continues to increase, the fillers tend to agglomerate, and the membrane has more interface defects, resulting in a decrease in CO 2 /N 2 selectivity. However, even with the filler loading increase to 9 wt.%, the CO 2 /N 2 ideal selectivity of MMMs is still higher than that of the pure Pebax membrane. Obviously, the introduction of MAF-7 filler in Pebax matrix can improve the gas separation performance and the results also show the great potential of MAF-7 filler-based membrane in the field of gas separation.  Figure 9 illustrates the relationship between feed pressure and separation performance on membranes with 5 wt.% MAF-7 loading. The gas permeability of CO2 and N2 also showed the regularity of increasing as the feed pressure increased, but the CO2/N2 selectivity showed the opposite changing regularity. The separation principle of the rubbery polymer membrane was followed by the solution-diffusion mechanism. When the pressure was increased, the membrane increased the diffusion rate and solubility of the gas, resulting in an increase in the permeability of both gases [45]. However, the N2 molecules were less affected than CO2 molecules with increasing feed pressure, resulting in a decrease in selectivity. The phenomenon of the increasing feed pressure was consistent with previously studies [46].  Figure 10 illustrates the influence of the operating temperatures on the separation performance of the membrane. As the testing temperature increased, the gas permeability increased, and the selectivity showed the same variation trend. Generally, the gas diffuses among the MMM layer through the porous and free volumes in the membrane matrix,  Figure 9 illustrates the relationship between feed pressure and separation performance on membranes with 5 wt.% MAF-7 loading. The gas permeability of CO 2 and N 2 also showed the regularity of increasing as the feed pressure increased, but the CO 2 /N 2 selectivity showed the opposite changing regularity. The separation principle of the rubbery polymer membrane was followed by the solution-diffusion mechanism. When the pressure was increased, the membrane increased the diffusion rate and solubility of the gas, resulting in an increase in the permeability of both gases [45]. However, the N 2 molecules were less affected than CO 2 molecules with increasing feed pressure, resulting in a decrease in selectivity. The phenomenon of the increasing feed pressure was consistent with previously studies [46].  Figure 9 illustrates the relationship between feed pressure and separation performance on membranes with 5 wt.% MAF-7 loading. The gas permeability of CO2 and N2 also showed the regularity of increasing as the feed pressure increased, but the CO2/N2 selectivity showed the opposite changing regularity. The separation principle of the rubbery polymer membrane was followed by the solution-diffusion mechanism. When the pressure was increased, the membrane increased the diffusion rate and solubility of the gas, resulting in an increase in the permeability of both gases [45]. However, the N2 molecules were less affected than CO2 molecules with increasing feed pressure, resulting in a decrease in selectivity. The phenomenon of the increasing feed pressure was consistent with previously studies [46]. 3.6.3. Effect of Testing Temperatures Figure 10 illustrates the influence of the operating temperatures on the separation performance of the membrane. As the testing temperature increased, the gas permeability increased, and the selectivity showed the same variation trend. Generally, the gas diffuses  Figure 10 illustrates the influence of the operating temperatures on the separation performance of the membrane. As the testing temperature increased, the gas permeability increased, and the selectivity showed the same variation trend. Generally, the gas diffuses among the MMM layer through the porous and free volumes in the membrane matrix, and the free volumes are randomly formed by the motions of the organic chains. The increased testing temperature induced the increased flexibility of the polymer chains, leading to more free volumes of the membrane and faster diffusion of the gas [47]. Thus, the increasing permeability of CO 2 and N 2 was mainly due to the increased diffusion of gas molecules at high temperature, and the increased flexibility of the membrane matrix [48]. However, at high testing temperature, the membrane showed a low affinity for gases, especially for CO 2 . That is not conducive to promoting separation through preferential adsorption on the membrane. Obviously, as the temperature rises, it resulted in a decrease in CO 2 /N 2 selectivity [47,49]. A similar result was also observed in MOF-801 mixed matrix membranes [32].

Effect of Testing Temperatures
Membranes 2022, 12, x FOR PEER REVIEW 12 of 16 and the free volumes are randomly formed by the motions of the organic chains. The increased testing temperature induced the increased flexibility of the polymer chains, leading to more free volumes of the membrane and faster diffusion of the gas [47]. Thus, the increasing permeability of CO2 and N2 was mainly due to the increased diffusion of gas molecules at high temperature, and the increased flexibility of the membrane matrix [48]. However, at high testing temperature, the membrane showed a low affinity for gases, especially for CO2. That is not conducive to promoting separation through preferential adsorption on the membrane. Obviously, as the temperature rises, it resulted in a decrease in CO2/N2 selectivity [47,49]. A similar result was also observed in MOF-801 mixed matrix membranes [32]. The results indicate that the gas permeability through the MMMs is a temperature activation process; thus, the activated energies of the gas permeability can be obtained by correlation with the Arrhenius equation: where P is the permeability coefficient of the corresponding gas (Barrer), P0 is the preexponential factor (Barrer), Ep is the permeation activation energy (kJ mol −1 ), R is the gas universal constant (J mol −1 K −1 ), and T is the absolute temperature (K). By linear fitting and regressing, the Ep of CO2 is 51.05 ±2.03 kJ mol −1 , which is significantly lower than that of N2 (114.32 ±5.16 kJ mol −1 ), indicating that CO2 permeability is less sensitive to the temperature change than N2 permeability, because CO2 has a smaller kinetic diameter, and a larger diffusion coefficient through membrane than that of N2. Hence, with an increase in the testing temperature, the increase in both CO2 and N2 permeability led to a decrease in CO2/N2 selectivity.

Comparison with Previous Research
The gas permeation performance of the synthesized Pebax 1657/MAF-7 MMMs was compared with the Pebax-based mixed matrix membranes in previous publications. As shown in Table 2  The results indicate that the gas permeability through the MMMs is a temperature activation process; thus, the activated energies of the gas permeability can be obtained by correlation with the Arrhenius equation: where P is the permeability coefficient of the corresponding gas (Barrer), P 0 is the preexponential factor (Barrer), Ep is the permeation activation energy (kJ mol −1 ), R is the gas universal constant (J mol −1 K −1 ), and T is the absolute temperature (K). By linear fitting and regressing, the E p of CO 2 is 51.05 ± 2.03 kJ mol −1 , which is significantly lower than that of N 2 (114.32 ± 5.16 kJ mol −1 ), indicating that CO 2 permeability is less sensitive to the temperature change than N 2 permeability, because CO 2 has a smaller kinetic diameter, and a larger diffusion coefficient through membrane than that of N 2 . Hence, with an increase in the testing temperature, the increase in both CO 2 and N 2 permeability led to a decrease in CO 2 /N 2 selectivity.

Comparison with Previous Research
The gas permeation performance of the synthesized Pebax 1657/MAF-7 MMMs was compared with the Pebax-based mixed matrix membranes in previous publications. As shown in Table 2, Deng et al. reported a Pebax/ZIF-C (zeolitic imidazolate framework cuboid) membrane, which showed high CO 2 permeability accompanied with a CO 2 /N 2 selectivity of 47.1 [50]. Liu et al. published about COF-5 as fillers to fabricate MMMs that exhibited an enhanced CO 2 permeability of 493 Barrer together with a CO 2 /N 2 selectivity of 49.3 [51]. The above research followed a trade-off effect between permeability and selectivity. In comparison, the compactness of the membrane which is represented by selectivity is critical for high quality membrane. Therefore, it is desired to fabricate membranes possessing high selectivity and comparable permeability. We used a simple and feasible casting method by incorporating MAF-7 crystals into Pebax membrane to construct a high-performance membrane which exhibited improvements in CO 2 permeability with remarkable elevated CO 2 /N 2 selectivity of 124.84. Finally, the gas separation performance of the prepared Pebax 1657/MAF-7 mixed matrix membrane was compared with the Robeson upper bound in 2008, and the results are shown in Figure 11. Compared with some Pebax-based mixed matrix membranes, the Pebax 1657/MAF-7 mixed matrix membrane has better selectivity. After the addition of MAF-7 filler, the gas separation performance of the prepared Pebax 1657/MAF-7 MMMs is higher than that of pure Pebax membrane. The separation performance reaches the highest when the filler content is 5 wt.%, which has exceeded the Robeson upper bound in 2008. The results show that Pebax 1657/MAF-7 mixed matrix membrane is an alternative membrane material with potential advantages in the field of CO 2 separation and capture.  [51]. The above research followed a trade-off effect between permeability and selectivity. In comparison, the compactness of the membrane which is represented by selectivity is critical for high quality membrane. Therefore, it is desired to fabricate membranes possessing high selectivity and comparable permeability. We used a simple and feasible casting method by incorporating MAF-7 crystals into Pebax membrane to construct a high-performance membrane which exhibited improvements in CO2 permeability with remarkable elevated CO2/N2 selectivity of 124.84. Finally, the gas separation performance of the prepared Pebax 1657/MAF-7 mixed matrix membrane was compared with the Robeson upper bound in 2008, and the results are shown in Figure 11. Compared with some Pebax-based mixed matrix membranes, the Pebax 1657/MAF-7 mixed matrix membrane has better selectivity. After the addition of MAF-7 filler, the gas separation performance of the prepared Pebax 1657/MAF-7 MMMs is higher than that of pure Pebax membrane. The separation performance reaches the highest when the filler content is 5 wt.%, which has exceeded the Robeson upper bound in 2008. The results show that Pebax 1657/MAF-7 mixed matrix membrane is an alternative membrane material with potential advantages in the field of CO2 separation and capture.

Conclusions
In summary, Pebax 1657/MAF-7 MMMs were successfully prepared by blending with Pebax 1657 and MAF-7 fillers in this work. As a result of the organic ligands in the MAF-7 structure, the MAF-7 crystals showed good compatibility with Pebax polymers, combining with the uncoordinated N donor in the MAF-7 crystals, the obtained membrane showed an increasing ideal selectivity of CO 2 /N 2 . The permeance results showed that Pebax 1657/MAF-7 MMMs have a better pure gas permeability performance with the filler content of 5 wt.%; CO 2 permeability reached 76.15 Barrer with a CO 2 /N 2 ideal selectivity of 124.84, which exceeds the Robeson upper bound in 2008. The obtained membrane showed 323.04% enhancement in selectivity of CO 2 /N 2 and 27.74% increase in permeability of CO 2 compared to the pristine membrane at 25 • C and 3 bar. Compared with the complicated chemical methods to modify fillers for improving the chemical affinity toward guest molecules and the compatibility between fillers and Pebax, MAF-7 (other MAFs as well) crystals may be ideal candidate fillers to fabricate high performance membrane for CO 2 /N 2 separation in practical applications.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.