Magnetically Aligned and Enriched Pathways of Zeolitic Imidazolate Framework 8 in Matrimid Mixed Matrix Membranes for Enhanced CO2 Permeability

Metal-organic frameworks (MOFs) as additives in mixed matrix membranes (MMMs) for gas separation have gained significant attention over the past decades. Many design parameters have been investigated for MOF based MMMs, but the spatial distribution of the MOF throughout MMMs lacks investigation. Therefore, magnetically aligned and enriched pathways of zeolitic imidazolate framework 8 (ZIF−8) in Matrimid MMMs were synthesized and investigated by means of their N2 and CO2 permeability. Magnetic ZIF−8 (m–ZIF−8) was synthesized by incorporating Fe3O4 in the ZIF−8 structure. The presence of Fe3O4 in m–ZIF−8 showed a decrease in surface area and N2 and CO2 uptake, with respect to pure ZIF−8. Alignment of m–ZIF−8 in Matrimid showed the presence of enriched pathways of m–ZIF−8 through the MMMs. At 10 wt.% m–ZIF−8 incorporation, no effect of alignment was observed for the N2 and CO2 permeability, which was ascribed anon-ideal tortuous alignment. However, alignment of 20 wt.% m–ZIF−8 in Matrimid showed to increase the CO2 diffusivity and permeability (19%) at 7 bar, while no loss in ideal selectivity was observed, with respect to homogeneously dispersed m–ZIF−8 membranes. Thus, the alignment of MOF particles throughout the matrix was shown to enhance the CO2 permeability at a certain weight content of MOF.


Introduction
Polymeric membranes for gas separation are used for purification, generation and separation of gaseous streams, besides other methods, such as (cryogenic) distillation, pressure swing adsorption and amine scrubbing [1]. In comparison with these other technologies, gas separation membranes may simplify the operational process, provide a relatively easy possibility for upscaling and require no phase change [1]. Despite these advantages, polymeric membranes exhibit a negative correlation between selectivity and permeability, which is known as Robeson's upper bound [2,3]. The incorporation of additives in polymeric membranes, i.e., a mixed matrix membrane (MMM), can result in shifting the membrane performance towards this upper bound or even surpassing it [4][5][6][7]. Additives that are used to enhance the MMM separation performance and have gained significant attention over the past decades are metal-organic frameworks (MOFs) [8]. MOFs are microporous crystalline networks where metal ions, i.e., the nodes, are coordinated by organic linkers. Like other microporous materials, such as zeolites, MOFs have large surface areas. The chemical affinity, surface area and pore size of MOFs are highly tunable by varying the linker between the metal nodes. These exceptional properties make MOFs promising candidates for the application in the field of gas separation and storage [9]. through these permeation paths. This improved use of the MOFs could hypothetically reduce the amount of MOFs required in MMMs to obtain a desired permeability.
Therefore, this research aims to align a three-dimensional isotropic additive in an MMM in such a manner that the MOF will form local clusters which will lie parallel to the gas permeation path. Since it has been shown that magnetic properties can be introduced in ZIF−8 and that ZIF−8 as an additive in MMMs increases permeability, this MOF is selected as additive for the MMM. The polyimide Matrimid ® 5218 is selected as polymer matrix. Nanoscale sized magnetite (Fe 3 O 4 ), an iron oxide with a significant higher magnetic susceptibility than other iron oxides, will be used as ferrimagnetic material to magnetize ZIF−8. Additive alignment will be done in a magnetic field, since other magnetic isotropic particles have already been successfully aligned in the direction of the magnetic field lines by magnetic fields in composite materials [35,36]. Lastly, the MMMs will be evaluated for their single gas permeability of CO 2 and N 2 . This gas pair is selected since it may resemble partially a flue gas composition [37]. Moreover, N 2 behaves similar to CO by means of thermal diffusion [38]. Furthermore, the kinetic diameters and critical temperatures both gases are in the same range (d N 2 = 3.64 Å, d CO = 3.76 Å, T c,N 2 = −147 • C, T c,CO = −140 • C). Therefore, CO 2 /N 2 gas mixtures resemble partially the output stream of CO 2 plasma conversion into CO as well [39].

Magnetic ZIF−8 Synthesis
Magnetic ZIF−8 (m-ZIF−8) and ZIF−8 were prepared by an aqueous room temperature method. First, 10 mL ultrapure water was added to 20 mg Fe 3 O 4 . The suspension was sonicated with a Sonics Vibra-Cell ultrasonication probe (Sonics and Materials Inc., Newtown, Connecticut, USA) at 35% probe intensity for 10 min. Meanwhile, 500 mg PEG, which is used as surfactant, was dissolved in 10 mL ultrapure water. The sonicated Fe 3 O 4 suspension and PEG solution were added together and the mixture was stabilized for 15 min. Again, the mixture was sonicated with a sonication probe for 10 min and stabilized for 15 min. The PEG stabilized Fe 3 O 4 was washed three times with ultrapure water to remove the excess PEG by simply containing the Fe 3 O 4 with a magnet. 10 mL of ultrapure water was added to the magnetic particles. 150 mg Zn(NO 3 ) 2 · 6 H 2 O (0.50 mmol) was added to the stabilized Fe 3 O 4 suspension. A solution of 1.64 g mIm (20 mmol) in 15 mL ultrapure water was added to the mixture. The mixture was stirred for 30 min while ZIF−8 formation occurred. The product was washed three times by containing the product with a magnet. M-ZIF−8 was dried in a vacuum oven overnight at 75 • C and 350 mbar. Lastly, the product was further dried for 24 hr at 100 • C and 0 mbar. Traditional ZIF−8 was synthesized as a reference by the same method as the m-ZIF−8 synthesis, but without the presence of PEG stabilized Fe 3 O 4 . Since ZIF−8 is not magnetic by nature, washing was performed by centrifuging the ZIF−8 suspension with a CompactStar CS 4 (VWR, Amsterdam, the Netherlands) for 20 min at 6500 rpm three times.

Native Matrimid Membranes
A total of 5 g Matrimid was dissolved in 20 g DMF overnight, resulting in a 20 wt.% total solids polymer solution. This solution was cast on a glass plate with a 0.5 mm casting knife. Evaporation of the DMF in a nitrogen box for two days solidifies the polymer membrane. To completely dry the cast flat sheet membrane, the membrane was put in a nitrogen oven at 80 • C for two days and subsequently in a vacuum oven at 125 • C and 0 mbar for two days.

Homogeneous m-ZIF−8/Matrimid MMMs
The required m-ZIF−8 amount was added to 20.5 g DMF (0.45 g for 10 wt.%, 0.90 g for 20 wt.%). The suspension was sonicated in a Branson 3510 sonication bath for 30 min. Matrimid was added to the MOF suspension in three stages (4.05 g in total for 10 wt.% and 3.6 g in total for 20 wt.%), which results in casting solutions with 18 wt.% total solids. The MMM solutions were stirred overnight. MMMs were cast from these solutions on a glass plate with a 0.5 mm casting knife. The obtained homogeneous MMMs were dried in the same manner as the pure Matrimid membranes. The obtained 10 and 20 wt.% homogeneous (H) m-ZIF−8/Matrimid MMMs are abbreviated to 10 wt.% H and 20 wt.% H, respectively.

Magnetically Aligned m-ZIF−8/Matrimid MMMs
Magnetically aligned m-ZIF−8/Matrimid MMMs were made similarly to their homogeneous counterpart, where the only difference is the first day of the solvent evaporation. For the homogenous MMM preparation, solvent evaporation in a nitrogen box from the cast polymer solution forms the membrane, whereas for the preparation of the aligned MMMs the nitrogen box was placed in the middle of a permanent magnetic field for one day ( Figure 1A). To prevent excessive m-ZIF−8 migration, an aluminum ring (d = 25 mm) was placed in the center of the alignment setup. After one day of solidification in the magnetic field, the membrane was placed in a normal nitrogen box and the previously mentioned drying procedure was followed. This approach of the MOF alignment in MMMs should also be suitable for extension to other types of MOFs and matrices. However, if the MOF contains a one-dimensional pore system, other strategies, such as exterior decoration with Fe 3 O 4 , should be considered, as the incorporation of Fe 3 O 4 in a one-dimensional system might result in pore blocking. The obtained 10 and 20 wt.% aligned m-ZIF−8/Matrimid MMMs are abbreviated to 10 wt.% Al and 20 wt.% Al, respectively. Higher weight percentages of m-ZIF−8/Matrimid were also attempted to be aligned, but because of the high M-ZIF−8 volume fraction this did not result in defect free membranes.
The magnetic field was created by two N45 grade permanent magnets (d = 70 mm, h = 60 mm, Nb-Fe-B axially magnetized). The magnetic field strength in the middle of the magnets was 0.22 T when the gap between the magnets was set to 80 mm. The authors are aware that the obtained magnetic field is non-homogeneous, i.e., the field strength decreases from the middle point towards the outer parts of the setup ( Figure 1B). However, experiments were also performed in homogeneous magnetic fields created by an electromagnet, but the heat generated by the coils of the electromagnet interfered with the solidification process.

Characterization
The crystal structure of MOF contains a one-dimensional pore system, other strategies, such as exterior decoration with Fe3O4, should be considered, as the incorporation of Fe3O4 in a one-dimensional system might result in pore blocking. The obtained 10 and 20 wt.% aligned m-ZIF−8/Matrimid MMMs are abbreviated to 10 wt.% Al and 20 wt.% Al, respectively. Higher weight percentages of m-ZIF−8/Matrimid were also attempted to be aligned, but because of the high M-ZIF−8 volume fraction this did not result in defect free membranes.  The elemental composition and dispersion of ZIF−8, m-ZIF−8 and the MMMs were analyzed with energy dispersive X-ray spectroscopy (EDS, JEOL JSM-IT100, Tokyo, Japan). Samples were prepared in the same manner as the SEM analysis.
High pressure gas sorption of N 2 and CO 2 was performed at 35 • C for ZIF−8, m-ZIF−8, Matrimid and the MMMs (Rubotherm series IsoSORP ® sorption instrument, Bochum, Germany). The equipment measures the sorbed weight of the gas with a magnetically suspended balance. Prior to each sorption measurement, the weight and volume of the sample is determined with a helium reference measurement. With the obtained sample weight and volume, the measured sorbed weight is corrected for the buoyancy of the measurement gases according to Equation (1), where m corrected is the corrected weight (g), m measured the measured weight (g), ρ gas the density of the measuring gas (g/cm 3 ) and V sample the sample volume (cm 3 ).
The single gas permeability of Matrimid and the MMMs for N 2 and CO 2 was determined at 35 • C in triplo by measuring permeate pressure increase over time in a calibrated volume at pressures ranging from 7 to 27 bar according to Equation (2). In Equation (2), P i is the permeability of gas species i (Barrer), ∆p permeate the increase in permeate pressure (Pa) per time interval ∆t (s), V c the calibrated permeate volume (m 3 ), R the gas constant (J/(K·mol)), T the permeate temperature (K), V m the molar volume at STP (cm 3 /mol), L the membrane thickness (cm), a the membrane area (cm 2 ) and ∆p the transmembrane pressure (cmHg).
Prior to each single gas permeation measurement, the membranes were first conditioned for 8 hr at 35 • C and 2 bar N 2 . Hereafter, first, the N 2 single gas permeation measurements were performed. Then, the CO 2 single gas permeation measurements were performed, since the CO 2 measurements plasticize the membranes. The ideal CO 2 /N 2 selectivity, α CO 2 /N 2 (−), was calculated according to Equation (3).
The CO 2 diffusion coefficient of Matrimid and the MMMs was determined according to Equation (4), where D is the CO 2 diffusion coefficient (cm 2 /s), P the CO 2 permeability (Barrer) and S the CO 2 solubility coefficient (cm 3 STP/(cm 3 ·cmHg)). The permeability was experimentally determined by permeation experiments whereas the CO 2 solubility coefficient was experimentally obtained by the high pressure sorption measurements using a magnetic suspension balance.

ZIF−8, m-ZIF−8 and Fe 3 O 4 Analysis
The XRD patterns ( Figure 2) show the typical diffraction peaks for both Fe 3 O 4 and ZIF−8. The typical diffraction peaks of both materials are in agreement with the crystal structures found in literature [16,40]. It is clear that the diffraction pattern of m-ZIF−8 shows both the diffraction peaks of ZIF−8 and Fe 3 O 4 , which means that the crystal structures of both particles are present in the composite m-ZIF−8 crystal. The ratio of the (211) ZIF−8 peak and the (440) Fe 3 O 4 peak in m-ZIF−8 is higher than the ratio found in literature for ZIF−8/Fe 3 O 4 composites where a significant lower relative amount of Fe 3 O 4 is used in the synthesis mixture [19,21]. On the other hand, the peak ratio of m-ZIF−8 is lower than the ratio found in literature for ZIF/Fe 3 O 4 composites, where the Fe 3 O 4 particle size was significantly bigger [20]. This observation indicates that a considerable amount of Fe 3 O 4 nanoparticles is present in the m-ZIF−8 crystals.
Membranes 2020, 10, x FOR PEER REVIEW 6 of 17 The CO2 diffusion coefficient of Matrimid and the MMMs was determined according to Equation 4, where D is the CO2 diffusion coefficient (cm 2 /s), P the CO2 permeability (Barrer) and S the CO2 solubility coefficient (cm 3 STP/(cm 3 ·cmHg)). The permeability was experimentally determined by permeation experiments whereas the CO2 solubility coefficient was experimentally obtained by the high pressure sorption measurements using a magnetic suspension balance.

ZIF−8, m-ZIF−8 and Fe3O4 Analysis
The XRD patterns ( Figure 2) show the typical diffraction peaks for both Fe3O4 and ZIF−8. The typical diffraction peaks of both materials are in agreement with the crystal structures found in literature [16,40]. It is clear that the diffraction pattern of m-ZIF−8 shows both the diffraction peaks of ZIF−8 and Fe3O4, which means that the crystal structures of both particles are present in the composite m-ZIF−8 crystal. The ratio of the (211) ZIF−8 peak and the (440) Fe3O4 peak in m-ZIF−8 is higher than the ratio found in literature for ZIF−8/Fe3O4 composites where a significant lower relative amount of Fe3O4 is used in the synthesis mixture [19,21]. On the other hand, the peak ratio of m-ZIF−8 is lower than the ratio found in literature for ZIF/Fe3O4 composites, where the Fe3O4 particle size was significantly bigger [20]. This observation indicates that a considerable amount of Fe3O4 nanoparticles is present in the m-ZIF−8 crystals. The influence of Fe3O4 incorporation in ZIF−8 on the N2 adsorption is visible in Figure 3. The BET surface area of 943 m 2 /g of ZIF−8 is comparable to BET surfaces areas reported in literature for ZIF−8 aqueous syntheses at room temperature and confirms the presence of permanent microporous system [14,41]. The BET surface area of m-ZIF−8 (625 m 2 /g) is clearly lowered by the presence of nonmicroporous Fe3O4 (40 m 2 /g) in comparison with pure ZIF−8, i.e., a decrease of 33.7%. ZIF−8/Fe3O4 composites with relatively lower amounts of Fe3O4 in the synthesis mixture show less decrease in surface area [21]. The influence of Fe 3 O 4 incorporation in ZIF−8 on the N 2 adsorption is visible in Figure 3. The BET surface area of 943 m 2 /g of ZIF−8 is comparable to BET surfaces areas reported in literature for ZIF−8 aqueous syntheses at room temperature and confirms the presence of permanent microporous system [14,41]. The BET surface area of m-ZIF−8 (625 m 2 /g) is clearly lowered by the presence of  SEM/EDS analysis of the morphology and elemental composition of ZIF−8 and m-ZIF−8 shows two clear differences between the reference and the magnetized ZIF−8 samples ( Figure 5). First, the particle size and range of particle size is increased when Fe3O4 is added to the synthesis mixture of ZIF−8. The ZIF−8 particle size from ranges 0.6 to 1 µm and the m-ZIF−8 particle size ranges from 1 to 1.6 µm. Apparently, the presence Fe3O4 in the m-ZIF−8 synthesis acts as nucleation point, which causes a slightly increased particle size. Secondly, and obviously, no Fe is detected by EDS for ZIF−8 and Fe is detected for the m-ZIF−8.  SEM/EDS analysis of the morphology and elemental composition of ZIF−8 and m-ZIF−8 shows two clear differences between the reference and the magnetized ZIF−8 samples ( Figure 5). First, the particle size and range of particle size is increased when Fe3O4 is added to the synthesis mixture of ZIF−8. The ZIF−8 particle size from ranges 0.6 to 1 µm and the m-ZIF−8 particle size ranges from 1 to 1.6 µm. Apparently, the presence Fe3O4 in the m-ZIF−8 synthesis acts as nucleation point, which causes a slightly increased particle size. Secondly, and obviously, no Fe is detected by EDS for ZIF−8 and Fe is detected for the m-ZIF−8.  SEM/EDS analysis of the morphology and elemental composition of ZIF−8 and m-ZIF−8 shows two clear differences between the reference and the magnetized ZIF−8 samples ( Figure 5). First, the particle size and range of particle size is increased when Fe 3 O 4 is added to the synthesis mixture of ZIF−8. The ZIF−8 particle size from ranges 0.6 to 1 µm and the m-ZIF−8 particle size ranges from 1 Membranes 2020, 10, 155 8 of 17 to 1.6 µm. Apparently, the presence Fe 3 O 4 in the m-ZIF−8 synthesis acts as nucleation point, which causes a slightly increased particle size. Secondly, and obviously, no Fe is detected by EDS for ZIF−8 and Fe is detected for the m-ZIF−8. The effect of the incorporation of Fe3O4 on the N2 and CO2 sorption in m-ZIF−8 is shown in Figure 6 and confirms the observation made for the decrease in BET surface area of m-ZIF−8. Since Fe3O4 has no microporosity, a decrease in both N2 and CO2 sorption is observed when Fe3O4 is incorporated in ZIF−8. The sorption of N2 in ZIF−8 and m-ZIF−8 shows Henry sorption behavior, since almost linear sorption behavior is observed. On the other hand, Langmuir sorption behavior is observed for CO2 in both ZIF types, since the slope of sorption decreases over the whole pressure range. Clearly, higher quantities of CO2 are sorbed in both ZIF structures than N2. This indicates that the sorption of CO2 is thermodynamically favored over N2 sorption, which was also observed for another ZIF type [42]. Moreover, the observed sorption ratio of the two gases corresponds with simulated ratios of sorption of ZIF−8 at 25 °C [43].

Membrane Analysis
TGA confirms that the drying method was sufficient for Matrimid and the MMMs (Figure 7). The membranes contained no solvent, since no weight loss observed in the trajectory from approximately 150 to 200 °C (boiling point DMF = 153 °C). All membranes have a degradation temperature of approximately 500 °C. The degradation temperature of m-ZIF−8 (600 °C, Figure 4) is 100 °C higher in comparison with Matrimid that has a degradation temperature of 500 °C (Figure 7). The effect of the incorporation of Fe 3 O 4 on the N 2 and CO 2 sorption in m-ZIF−8 is shown in Figure 6 and confirms the observation made for the decrease in BET surface area of m-ZIF−8. Since Fe 3 O 4 has no microporosity, a decrease in both N 2 and CO 2 sorption is observed when Fe 3 O 4 is incorporated in ZIF−8. The sorption of N 2 in ZIF−8 and m-ZIF−8 shows Henry sorption behavior, since almost linear sorption behavior is observed. On the other hand, Langmuir sorption behavior is observed for CO 2 in both ZIF types, since the slope of sorption decreases over the whole pressure range. Clearly, higher quantities of CO 2 are sorbed in both ZIF structures than N 2 . This indicates that the sorption of CO 2 is thermodynamically favored over N 2 sorption, which was also observed for another ZIF type [42]. Moreover, the observed sorption ratio of the two gases corresponds with simulated ratios of sorption of ZIF−8 at 25 • C [43]. The effect of the incorporation of Fe3O4 on the N2 and CO2 sorption in m-ZIF−8 is shown in Figure 6 and confirms the observation made for the decrease in BET surface area of m-ZIF−8. Since Fe3O4 has no microporosity, a decrease in both N2 and CO2 sorption is observed when Fe3O4 is incorporated in ZIF−8. The sorption of N2 in ZIF−8 and m-ZIF−8 shows Henry sorption behavior, since almost linear sorption behavior is observed. On the other hand, Langmuir sorption behavior is observed for CO2 in both ZIF types, since the slope of sorption decreases over the whole pressure range. Clearly, higher quantities of CO2 are sorbed in both ZIF structures than N2. This indicates that the sorption of CO2 is thermodynamically favored over N2 sorption, which was also observed for another ZIF type [42]. Moreover, the observed sorption ratio of the two gases corresponds with simulated ratios of sorption of ZIF−8 at 25 °C [43].

Membrane Analysis
TGA confirms that the drying method was sufficient for Matrimid and the MMMs (Figure 7). The membranes contained no solvent, since no weight loss observed in the trajectory from approximately 150 to 200 °C (boiling point DMF = 153 °C). All membranes have a degradation temperature of approximately 500 °C. The degradation temperature of m-ZIF−8 (600 °C, Figure 4) is 100 °C higher in comparison with Matrimid that has a degradation temperature of 500 °C (Figure 7).

Membrane Analysis
TGA confirms that the drying method was sufficient for Matrimid and the MMMs (Figure 7). The membranes contained no solvent, since no weight loss observed in the trajectory from approximately  Figure 4) is 100 • C higher in comparison with Matrimid that has a degradation temperature of 500 • C (Figure 7). Since the MMMs contain at least 80 wt.% Matrimid, the difference between pure Matrimid and the MMMs is marginally visible.
Membranes 2020, 10, x FOR PEER REVIEW 9 of 17 Since the MMMs contain at least 80 wt.% Matrimid, the difference between pure Matrimid and the MMMs is marginally visible.   . It is visible for the aligned MMMs that the used magnetic field is inhomogeneous, since alignment of the MMMs is not perpendicular to the membrane surface, which increases the MOF path length through the matrix.
Alignment and clustering of m-ZIF−8 in the MMMs is further substantiated by analyzing the particle count of Zn (Figure 9). The morphology and elemental distribution of cross-sections of the homogeneous and aligned MMMs are shown in Figure 8.  In Figure 9, the vertical particle count of the respective MMM is visualized by the graph which overlays the Zn EDS map. Moreover, the aligned MMMs are rotated such that the direction of alignment, as indicated by the arrows in Figure 8, is positioned vertically. Both the 10 and 20 wt.% H MMMs show that the particle count throughout the membranes is evenly distributed, since the particle count graphs are approximately horizontal. On the other hand, clear peaks are present in the In Figure 9, the vertical particle count of the respective MMM is visualized by the graph which overlays the Zn EDS map. Moreover, the aligned MMMs are rotated such that the direction of alignment, as indicated by the arrows in Figure 8, is positioned vertically. Both the 10 and 20 wt.% H MMMs show that the particle count throughout the membranes is evenly distributed, since the particle count graphs are approximately horizontal. On the other hand, clear peaks are present in the particle graphs of the 10 and 20 wt.% Al MMMs that occur periodically (indicated by the curly brackets in Figure 9). As stated previously, the aligned pathways and enriched regions in the 10 wt.% Al MMM are narrower and smaller than the 20 wt.% Al MMM, which is visualized by the smaller periodicity of the 10 wt.% MMM (i.e., the smaller curly brackets). Lastly, the peaks of the particle count graphs are more delicately pronounced in the 10 wt.% Al MMM than in the 20 wt.% Al MMM, which is attributed to the increased brittleness of the locally m-ZIF−8 aligned pathway. As an increasing MOF content in MMMs results in an increased brittleness, it is more difficult to keep an aligned m-ZIF−8 path intact when the sample is cryogenically fractured for the SEM/EDS analysis [30,44]. N 2 and CO 2 sorption isotherms of Matrimid and the MMMs are shown in Figure 10.
Membranes 2020, 10, x FOR PEER REVIEW 11 of 17 periodicity of the 10 wt.% MMM (i.e., the smaller curly brackets). Lastly, the peaks of the particle count graphs are more delicately pronounced in the 10 wt.% Al MMM than in the 20 wt.% Al MMM, which is attributed to the increased brittleness of the locally m-ZIF−8 aligned pathway. As an increasing MOF content in MMMs results in an increased brittleness, it is more difficult to keep an aligned m-ZIF−8 path intact when the sample is cryogenically fractured for the SEM/EDS analysis [30,44]. N2 and CO2 sorption isotherms of Matrimid and the MMMs are shown in Figure 10. The sorption of N2 in Matrimid and the MMMs exhibits predominantly Henry sorption behavior. It is notable that the aligned MMMs have an increased sorption of N2, with respect to both Matrimid and the homogeneous MMMs. Since m-ZIF−8 ( Figure 6) has a higher N2 uptake than Matrimid (Figure 10), it is expected that all MMMs should have an increased N2 uptake. However, this is only observed for the aligned MMMs, which means that the N2 sorption sites of m-ZIF−8 in the aligned MMMs are more readily accessible than in the homogeneous MMMs. Therefore, it is concluded that ZIF−8 alignment and local enrichment effectively decrease the polymer content between the MOF particles. The uptake of CO2 in Matrimid and the MMMs occurs according to the dual-mode sorption model (i.e., Henry and Langmuir sorption. The aligned MMMs show a lower The sorption of N 2 in Matrimid and the MMMs exhibits predominantly Henry sorption behavior. It is notable that the aligned MMMs have an increased sorption of N 2 , with respect to both Matrimid and the homogeneous MMMs. Since m-ZIF−8 ( Figure 6) has a higher N 2 uptake than Matrimid (Figure 10), it is expected that all MMMs should have an increased N 2 uptake. However, this is only observed for the aligned MMMs, which means that the N 2 sorption sites of m-ZIF−8 in the aligned MMMs are more readily accessible than in the homogeneous MMMs. Therefore, it is concluded that ZIF−8 alignment and local enrichment effectively decrease the polymer content between the MOF particles. The uptake of CO 2 in Matrimid and the MMMs occurs according to the dual-mode sorption model (i.e., Henry and Langmuir sorption. The aligned MMMs show a lower CO 2 sorption than the homogeneous MMMs at elevated pressures. This decrease in solubility is ascribed to increased CO 2 uptake at the ZIF/matrix interface, since it was demonstrated that at elevated CO 2 pressure (starting at 10 bar) matrix derigidification at the interface causes an increased CO 2 uptake [45]. The aligned and enriched MMMs have a decreased polymer content between the MOF particles, causing a decreased CO 2 induced rigidification at elevated pressures. This results in a slightly lowered CO 2 sorption relative to the homogeneous MMM. Increasing the m-ZIF−8 content in the MMMs results in a higher CO 2 sorption, due to the high CO 2 sorption capacity of m-ZIF−8 ( Figure 6) in comparison with Matrimid ( Figure 10).
The influence of m-ZIF−8 content in the MMMs and effect of alignment on the CO 2 single gas permeability is shown in Figure 11.
Membranes 2020, 10, x FOR PEER REVIEW 12 of 17 CO2 uptake [45]. The aligned and enriched MMMs have a decreased polymer content between the MOF particles, causing a decreased CO2 induced rigidification at elevated pressures. This results in a slightly lowered CO2 sorption relative to the homogeneous MMM. Increasing the m-ZIF−8 content in the MMMs results in a higher CO2 sorption, due to the high CO2 sorption capacity of m-ZIF−8 ( Figure 6) in comparison with Matrimid ( Figure 10). The influence of m-ZIF−8 content in the MMMs and effect of alignment on the CO2 single gas permeability is shown in Figure 11. The CO2 permeability of Matrimid increases from 6.4 Barrer at 11 bar to 8.1 Barrer at 27 bar once its plasticization pressure is reached, which lies between 10 and 12 bar CO2 pressure [46]. The Matrimid CO2 permeability is slightly lower than values reported elsewhere in literature [29,44,46]. This observation is attributed to the difference in annealing protocol, which strongly influences the permeability [29]. The incorporation of 10 wt.% m-ZIF−8 results in an increased CO2 permeability of the homogeneous and aligned MMMs relative to pure Matrimid. Counterintuitively, since pure ZIF−8 has a high CO2 permeability, this increased CO2 permeability is caused by an increased CO2 solubility, while the CO2 diffusivity decreases (Table 1) [25]. This decrease in diffusivity is ascribed to matrix rigidification around the MOF particles [46,47]. Alignment at 10 wt.% m-ZIF−8 content does not influence the CO2 permeability, with respect to the homogeneous MMM. The CO2 permeability of both 10 wt.% MMMs starts to increase from 15 bar CO2 feed pressure, i.e., plasticization occurs. This increased plasticization pressure in MOF/Matrimid MMMs is in agreement with literature [46]. The CO 2 permeability of Matrimid increases from 6.4 Barrer at 11 bar to 8.1 Barrer at 27 bar once its plasticization pressure is reached, which lies between 10 and 12 bar CO 2 pressure [46]. The Matrimid CO 2 permeability is slightly lower than values reported elsewhere in literature [29,44,46]. This observation is attributed to the difference in annealing protocol, which strongly influences the permeability [29]. The incorporation of 10 wt.% m-ZIF−8 results in an increased CO 2 permeability of the homogeneous and aligned MMMs relative to pure Matrimid. Counterintuitively, since pure ZIF−8 has a high CO 2 permeability, this increased CO 2 permeability is caused by an increased CO 2 solubility, while the CO 2 diffusivity decreases (Table 1) [25]. This decrease in diffusivity is ascribed to matrix rigidification around the MOF particles [46,47]. Alignment at 10 wt.% m-ZIF−8 content does not influence the CO 2 permeability, with respect to the homogeneous MMM. The CO 2 permeability of both 10 wt.% MMMs starts to increase from 15 bar CO 2 feed pressure, i.e., plasticization occurs. This increased plasticization pressure in MOF/Matrimid MMMs is in agreement with literature [46]. Further increasing the m-ZIF−8 content resulted in an increased CO 2 permeability. For both the 20 wt.% H and Al MMMs, a further increase in CO 2 solubility is observed (Table 1)  At low feed pressure, pure Matrimid has a higher ideal selectivity than the MMMs. This means that m-ZIF−8 enhances next to the CO2 permeability ( Figure 11) also the N2 permeability, but the relative increase in N2 permeability is higher than the relative increase in CO2 permeability. Since single crystal ZIF−8 has a selectivity which is at maximum equal to the selectivity of Matrimid, it can be concluded that non idealities between the matrix-MOF interface are present [25]. Nonetheless, aligning m-ZIF−8 has a positive response on the permeability without compromising the ideal selectivity up to 15 bar, with respect to the homogeneous MMMs. The facts that overall, no significant differences are present between the ideal selectivity of the homogeneous and aligned MMMs ( Figure  12) and that the solubility of N2 is higher in the aligned MMMs than in the homogeneous MMMs ( Figure 10) prove that the aligned MMMs enhance the CO2 diffusivity more than the N2 diffusivity, relative to the homogeneous MMMs. It is argued that this enhanced CO2 diffusivity is caused by the At low feed pressure, pure Matrimid has a higher ideal selectivity than the MMMs. This means that m-ZIF−8 enhances next to the CO 2 permeability (Figure 11) also the N 2 permeability, but the relative increase in N 2 permeability is higher than the relative increase in CO 2 permeability. Since single crystal ZIF−8 has a selectivity which is at maximum equal to the selectivity of Matrimid, it can be concluded that non idealities between the matrix-MOF interface are present [25]. Nonetheless, aligning m-ZIF−8 has a positive response on the permeability without compromising the ideal selectivity up to 15 bar, with respect to the homogeneous MMMs. The facts that overall, no significant differences are present between the ideal selectivity of the homogeneous and aligned MMMs ( Figure 12) and that the solubility of N 2 is higher in the aligned MMMs than in the homogeneous MMMs ( Figure 10) prove that the aligned MMMs enhance the CO 2 diffusivity more than the N 2 diffusivity, relative to the homogeneous MMMs. It is argued that this enhanced CO 2 diffusivity is caused by the different kinetic diameters of CO 2 (3.30 Å) and N 2 (3.64 Å) and the effect of the aligned and enriched ZIF−8 pathways.
Since the pore aperture of ZIF−8 is 3.4 Å, CO 2 (with a kinetic diameter smaller than the ZIF−8 pore aperture) will diffuse more easily through the MOF than N 2 (with a kinetic diameter bigger than the ZIF−8 pore aperture). Moreover, the increased ZIF−8 interconnectivity in the aligned MMMs causes preferential permeation through the MOF relative to the matrix, since the permeability of pure ZIF−8 is significantly higher than the permeability of Matrimid. As a result, the CO 2 diffusivity will thus relatively be promoted more than the N 2 diffusivity in the aligned MMMs than in the homogeneous MMMs. With increasing CO 2 pressure, the effect of plasticization on the selectivity is observed. Due to the plasticization induced increased CO 2 permeability, the ideal selectivity of all membranes increase (an opposite trend is expected for mixed gas permeation experiments above the plasticization pressure). Clearly, Matrimid plasticizes most pronounced at 11 to 15 bar feed pressure, while the MMM start to plasticize at 15 bar feed pressure. Due to the limitation of a permanent magnetic field lines, it was observed that the alignment was not fully perpendicular to the membrane surface, which increased the effective length of the m-ZIF−8 pathway through the MMM. At 20 wt.% m-ZIF−8 incorporation in Matrimid, alignment was proven to increase the CO 2 permeability at maximum with 19% at 7 bar, while no CO 2 /N 2 selectivity loss of was observed, with respect to the homogeneous 20 wt.% MMM. Therefore, it was concluded that at 20 wt.% m-ZIF−8 content, the alignment results in sufficiently m-ZIF−8 enriched aligned clusters throughout the MMM matrix that established an increase in diffusivity and subsequently permeability. The use of a homogeneous magnetic field will straighten the percolation path and will result in a further improved membrane performance at even lower weight percentages of magnetic MOFs. Nonetheless, this paper shows that alignment of MOF particles throughout the matrix creating permeation highways is beneficial to enhance the CO 2 permeability without adversely affecting the selectivity. Funding: This work is part of the research programme 'Fuel feedstock production by a combined approach of controlled plasma conversion and membrane separation' with project number 13584 which is partly financed by the Dutch Research Council (NWO).

Conclusions
Membranes 2020, 10, x FOR PEER REVIEW diffusivity will thus relatively be promoted more than the N2 diffusivity in the homogeneous MMMs. With increasing CO2 pressure, the effe selectivity is observed. Due to the plasticization induced increased C selectivity of all membranes increase (an opposite trend is expected experiments above the plasticization pressure). Clearly, Matrimid plastic to 15 bar feed pressure, while the MMM start to plasticize at 15 bar feed p

Conclusions
ZIF−8 was successfully magnetized by the incorporation of Fe3O4. T ZIF−8 showed a decrease in BET surface area and CO2 and N2 solubility, Alignment of m-ZIF−8 in Matrimid was visually present by SEM-EDS an ZIF−8 enriched pathways and regions in the aligned and clustered MM Due to the limitation of a permanent magnetic field lines, it was observed fully perpendicular to the membrane surface, which increased the effec pathway through the MMM. At 20 wt.% m-ZIF−8 incorporation in Matri to increase the CO2 permeability at maximum with 19% at 7 bar, while n was observed, with respect to the homogeneous 20 wt.% MMM. Therefo 20 wt.% m-ZIF−8 content, the alignment results in sufficiently m-ZIF− throughout the MMM matrix that established an increase in diff permeability. The use of a homogeneous magnetic field will straighten th result in a further improved membrane performance at even lower weig MOFs. Nonetheless, this paper shows that alignment of MOF particles thr permeation highways is beneficial to enhance the CO2 permeability with selectivity.