Highly CO2 Selective Microporous Metal-Imidazolate Framework

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Highly CO2 Selective Microporous Metal-Imidazolate Framework (MMIF) Based Mixed Matrix Membranes Mahdi Ahmadi, Ender Tas, Ayse Kilic, Volkan Kumbaraci, Naciye Talinli, M. Goktug Ahunbay, and S. Birgul Tantekin-Ersolmaz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13054 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 2017

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ACS Applied Materials & Interfaces

Highly CO2 Selective Microporous MetalImidazolate Framework (MMIF) Based Mixed Matrix Membranes Mahdi Ahmadi1, Ender Taş1, Ayşe Kılıç1, Volkan Kumbaracı2, Naciye Talınlı2, M. Göktuğ Ahunbay1*, S. Birgül Tantekin-Ersolmaz1*, 1

Istanbul Technical University, Department of Chemical Eng., Maslak, Istanbul 34469, Turkey

2

Istanbul Technical University, Department of Chemistry, Maslak, Istanbul, 34469, Turkey

*Corresponding authors: [email protected], [email protected]

KEYWORDS: Mixed Matrix Membrane; MOF; CO2 Separation; MMIF, Polyimide, Matrimid

ACS Paragon Plus Environment

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ABSTRACT

Microporous metal-imidazolate framework (MMIF), a highly CO2 selective MOF, was incoporated into a polymeric membrane for separation of CO2 from CH4 and N2 for the first time. MMIF nanoparticles of 50-200 nm were synthesized using the sonication method and dispersed into Matrimid, a commercial polyimide, with MOF loading of 10 and 20 % by weight to fabricate mixed matrix membranes (MMMs). Morphology, thermal behavior, and

glass transition

temperature of the membranes were characterized, and single and mixed gas permeation measurements at 35C and 4 bar feed pressure were carried out to reveal their separation performance. Both 10% and 20% MMIF containing Matrimid membranes exhibited enhanced gas permeabilities for all three gases. Contrary to expectations, ideal selectivity of membranes was not improved possibly due to the flexible framework of MMIF. On the other hand, mixed gas permeability measurements showed significant improvement in CO2/CH4 separation factor by 130% and CO2/N2 separation factor by 79% owing to competitive adsorption favoring CO2.


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1. Introduction The drastic climate change since 1750s, which has stemmed from the emergence of new industries and man-made activities such as fossil fuel burning, cement and lime production, and deforestation, has become a frequent topic of discussion in the international political agenda, and CO2, as a result of these activities, was introduced as the chief culprit and largest single contributor of global warming effect1-4. Carbon dioxide together with hydrogen sulfide in natural gas are considered as impurities that should be removed before transferring because the impurities have the potential to produce acids and corrode pipelines and equipment in the presence of water5. Natural gas sweeting target, which is commonly CO2 removal from CH4, is achieved if CO2 concentration is below 2%. From natural gas purification to combustion processes (pre-, postcombustion, and oxyfuel combustion) CO2 can put obstacles in the way of clean energy. To mitigate CO2 emissions, CO2 capture and sequestration (CCS) is the primary strategy in depletion of CO2 sources focusing on pre-combustion, post-combustion, and oxyfuel combustion processes6,7. Among all of CO2 capture applications, post-combustion CO2 capture is the most challenging because CO2/N2 gas stream is hot and wet2. Conventional methods in gas separations are physical/chemical adsorption (TSA, PSA, VSA), physical/chemical absorption, and cryogenic distillation2. These conventional industrial methods need complicated equipment, high capital and operating costs8. Compared to those conventional methods, membrane based gas separation is an alternative which offers several advantages over traditional methods. Energy efficiency, ease of scale-up, low capital and operating costs, and simple process design are the promising features of membrane gas separations which have been of great interests over the course of few decades8. Since 1970s, polymeric membrane based gas separation has scaled-up commercially for several gas mixtures in industries9. To this end, cellulose acetate, polyimides, and


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perfluoropolymers have been utilized for natural gas treating and post combustion CO 2 separation10. Polyimides, as glassy polymers with glass transition temperatures above 300°C, have become a competitor for cellulose acetate in the market10. Unfortunately, the trade-off between selectivity and permeability of polymeric membranes, which is frequently reported as the Robeson upper-bound, created a challenge for membrane based gas separation processes9,11. This trade-off exhibits that an increase in permeability is at expense of selectivity9. Therefore, polymeric membranes generally suffer from this limitation. To surpass and overcome the upper-bound limitations and meet the industrial targets, researchers have focused on inorganic and hybrid membranes. Although permeability and selectivity of inorganic membranes are substantially higher than those of polymeric membranes, large-scale fabrication of inorganic membranes is a challenge and they are not robust enough to tolerate the high pressure12. There is a critical need for new membranes to exceed the upper bound, therefore, many efforts have been spent to develop hybrid membranes known as mixed matrix membranes (MMMs)12. Mixed matrix membranes are a combination of an organic part, which is the polymer matrix, and an inorganic part mainly molecular sieves, such as zeolites and metal organic frameworks (MOFs)

13,14

. MMMs take advantages of both phases, the extraordinary transport

properties of the inorganic phase and the processability and mechanical strength as well as thermal and chemical stability of the polymer matrix14. According to this approach, mixed matrix membranes incorporating fillers with excellent sorption and diffusion properties are required. The dispersion and adhesion of fillers in the matrix to form a homogenous membrane is of considerable importance in membrane preparation. Zeolite and carbon molecular sieve based MMMs have been studied for the last three decades and much effort has been spent to ensure good adhesion between


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organic and inorganic phases15. The affinity and compatibility between MOFs and polymers are more pronounced in comparison to that of zeolites and polymers. Therefore, controlling the interface morphology between the polymer matrix and filler is easier relative to zeolites16-18. MOFs have drawn considerable attention and become a frequent topic of discussion in a wide variety of applications such as gas storage2,19-24, catalysis25-30, drug delivery 31-33, fuel cell 34, chromatography35,36, and membranes 7,15,37-42. MOFs are mainly considered as a fascinating class of coordinated hybrid materials consisting of metal ions and organic bridging ligands 2, 43, 44. The most promising feature of MOFs is their tunable chemistry and framework, which gives rise to high porosity and excellent surface area compared to traditional porous materials such as zeolites and carbon molecular sieves36,45. The unlimited diversity of frameworks stems from the large number of metal ions or clusters and broad range of organic linkers. This diversity provides an inherent character in topologies with desired porosity resulted in choosing gas molecules selectively 43. MOFs either can be rigid or flexible materials; this property highly affects the performance of MOFs especially in their use as adsorbent in gas sorption technology such as pressure-swing adsorption and as filler in mixed matrix membranes. Rigid MOF frameworks are stable and robust with permanent porosity. However, flexible MOF framework is shown to undergo a change in the unit cell volume 46. The prominent feature of flexibility would be breathing that is responsible for a drastic change in pore volume of MOFs upon external stimuli such as guest molecule adsorption. Flexible MIL-88 and MIL-53 were reported by Ferey et al. for the first time in 200247. The insertion of metal organic frameworks into a polymer matrix, which is placed close to the upper bound, can yield a promising solution to polymeric membrane limitations 15. Researchers


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mainly have focused on well-known classic MOFs such as ZIFs. However, there are many other interesting MOFs in the Cambridge Structural Database (CSD) 48. Due to lack of experimental gas transport data of MOFs in the literature, several new MOFs were evaluated in terms of their gas sorption and permeability performances by molecular simulation studies49-51. The analysis of the new MOFs reported in the literature revealed a new MOF structure as a promising filler for MMM formation: Microporous Metal Imidazolate Framework known as MMIF has the potential for separation of CO2/CH4 and CO2/N2. MMIF is a new class of imidazolebased MOF like ZIFs with chemical formula of C12H8CuN4 and density of 1.519 g/cm3 which has been first synthesized and characterized by Chen et al. in 2010 with hydrothermal method using 1,4-di(1 H-imidazole-4-yl)benzene (H2L) as the ligand22. In another study, Liu et al in 2013 changed the synthesis method (coordination modulation method) using an ultrasound bath and was able to obtain size controlled nanoscale MMIF 52. MMIF is designated as EHUFAP in Cambridge Structural Database (CSD). The ellipsoid shaped MMIF is a highly stable 3D copper(II) microporous framework with one-dimensional cylindrical channels constructed with rigid 4-imidazole-containing ligand and pore limiting diameter of 3.3-3.4 Å

22,49

. The structure of MMIF obtained from CSD is seen in

Figure 1. The BET surface area was reported to be around 420 m2/g for particles with the size ranging from 200 nm to 600 nm

52

. Single gas sorption analysis at 195 K showed that CO2 and

CH4 sorption of nano-sized MMIF was 117.52 cm3/g (STP) and 73.17 cm3/g (STP), respectively 52

. Compared to the bulk MMIF, the nanosized MMIF provides higher sorption capacity which is

attributed to the additional surface area of micropores and mesopores. CO2/N2 selectivity of nanosized MMIF was 62 at 273K and 48 at 298K52. However, the CO2/N2 sorption selectivity was reported for bulk MMIF as 35 at 273K and 22 at 298K 22. Single gas sorption measurement shows


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that the MMIF is a promising candidate for CO2/CH4 and CO2/N2 53. No experimental sorption data for mixed gas was reported in the literature for MMIF.

Figure 1: MMIF Structure (simulated using the data available on CSD48). According to previous studies, MMIF has higher affinity toward CO2 than CH4 and N2 as supported by single gas adsorption isotherms

22,52,53

. The simulation study by Keskin

53

showed

also that MMIF favors CO2 over CH4 and N2 in a gas mixture due to the strong interaction between unsaturated metal sites and CO2. Furthermore, the rigid framework assumption used in the study led to the conclusion that the narrow window (pore limiting diameter is 3.4 Å) of MMIF impedes CH4 and N2 diffusion and facilitates CO2 diffusion, the latter being several orders of magnitude higher than diffusion of CH4 and N2. According to the study, the MMIF outperforms some frequently used MOFs such as ZIF-10, IRMOF-1, -8, -9, -10, and -14 at low pressures (0-5 bar), and exhibits a selectivity of 10 for an equimolar mixture of CO2/CH4 at 10 bar and 298K. The selectivity of CO2/N2 (50/50) was reported to be around 40 at 298 K and 5-20 bar 53.


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In another theoretical study, using atomically detailed simulations and continuum modeling method, Yilmaz and Keskin 51 in 2014 predicted the effect of MMIF on the performance of polymeric membranes for the CO2/N2 separation performance improvement. According to their results, the rigid MMIF has the permeability of 1822 Barrer and selectivity of around 105. In the case of flexible structure, the CO2/N2 selectivity of MMIF was assumed to be reduced by one order of magnitude. The Maxwell model predictions of 20% MMIF containing Matrimid MMM was calculated for both rigid and flexible MMIF cases. Considering the rigid structure, MMM selectivity of CO2/N2 was predicted to be improved by 67.33%, whereas the flexible MMIF was able to improve selectivity by only 10.5%. The goal of this study is to explore the potential of MMIF in MMM formation. To this end, Matrimid® 5218, consisting of 3,3’,4,4’-benzophenone tetracarboxylic dianhydride and diamiophenylindane monomers, was chosen to be employed as the continuous phase because of its commercial availability and thermal stability to test the performance potential of the selected MOFs as dispersed filler. In addition, there are several studies reported in the literature which uses Matrimid as the continuous phase and other MOFs as the dispersed phase, therefore it provides a good basis for comparison before a better polyimide match is tested. The molecular simulation studies in the literature revealed MMIF could be an outstanding candidate for MOF-based MMMs. Gas sorption data also confirmed the high separation performance of MMIF. However, no experimental data is available in the literature to confirm the predicted performances of this MOF; in particular, no MMM studies using this MOF are reported. In this work, the gas transport properties of MMMs produced by MMIF as the dispersed phase were investigated for the first time experimentally.


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2. Experimental 2.1. Materials Matrimid® 5218 (d = 1.23 g/cm3, Tg = 308°C) was supplied by Huntsman Advanced Materials Americas Inc. MMIF crystals and Matrimid® were dried overnight in a vacuum oven at 150°C and 120°C, respectively, prior to membrane preparation. Table 1 summarizes the chemicals used for MMIF synthesis. All of the materials were utilized as-recieved without further purification and kept in a desicator. CO2, CH4, and N2 were provided by the Linde Group with purities above 99.5%. Table 1: The list of chemicals used for MMIF synthesis. Materials

Purity

Supplier

Cobalt (II) chloride hexahydrate

98%

Aldrich

1,4-di(1-H-imidazol-4-yl)benzene (H2L)

--

Synthesized*

Ethyl Alcohol Absolute

--

Merck

Ammonia Solution 25% Merck * Materials for H2L synthesis, method, and characterization are given in detail in Supporting Information.

2.2. Synthesis of Microporous Metal-Imidazolate Framework (MMIF) The ligand H2L.2HBr (1,4-di[4(5)-imidazolyl)] benzene dihydrobromide) was synthesized as described in the literature54-56, for which the details are provided in Supporting Information. A sample of 85 mg of CuCl2.2H2O (0.5 mmol) was dissolved in 125 mL of deionized water and stirred for 15 minutes. 105 mg of H2L (0.5 mmol) was added to 125 mL of ethanol and left under sonication for 15 minutes. Then, the solution of H2L.2HBr and ethanol was added to CuCl2.2H2O


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and water solution slowly and stirred for 15 minutes to obtain a clear solution. 1 mL of ammonia solution (25%) was then introduced to this solution under sonication wave and the solution was left under sonication for 30 minutes. MMIF nanoparticles in the form of brown colored powder were collected after centrifuging and washing 3 times with water and ethanol. The powders dried at 60°C under vacuum for 5 hours and activated at 150°C under vacuum for 24 hours52. The output power and frequency of the ultrasound bath at all sonication steps were 300 KW and 35KHz. 2.3. Preparation of pure Matrimid and MMIF/ Matrimid mixed matrix membranes Prior to pure membrane and MMM preparation, Matrimid and MMIF were dried at 120°C and 150°C, respectively, under vacuum overnight. To prevent MMIF and Matrimid  from hydration, they were kept in capped vials in a desiccator. A weighed amount of dried Matrimid was dissolved in enough solvent (N,N-dimethylformamide, DMF) to obtain a 12% polymer solution. The solution was stirred overnight at ambient temperature. A conventional solution casting system (Doctor’s blade) was used for film formation on the smooth surface of a glass plate. The initial film thickness was 500 microns. The film on the glass plate was placed in an oven at 80°C for 2 hours. The membrane was then peeled off the glass and dried according to the same drying protocol developed for MMMs, which is explained later in this section. MMMs were prepared at two different MOF contents, 10% and 20% by weight. In order to prepare the casting solution, dry MOF was first added to DMF to obtain a slurry. The slurry was magnetically stirred for 1 hour, then, first, 1 hour of sonication, and then 24 hours of magnetic stirring was employed. Matrimid was added to this slurry in several portions in order to obtain a good dispersion of MOF in the polymer and prevent agglomeration. First, 10% of the total dry polymer was added while stirring. The solution was then sonicated and stirred for 10 minutes. This


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sonication-stirring procedure was repeated for 5 times. The solution was then left to be magnetically stirred overnight. After overnight stirring, the rest of the polymer was added to the solution in 4-5 portions at 2-hour intervals while stirring. After the addition of the last portion of the polymer, the solution left under stirring overnight followed by sonication for 10 minutes. Membrane was cast similar to dense polymer membrane preparation described before in this subsection. Then it was peeled off the glass surface and dried according to a post-synthesis drying protocol. The post-synthesis drying protocol to be employed was designed based on the thermal and structural stability of the MOF. As will be discussed later, MMIF was determined to be stable up to 150°C based on XRD analysis. Since the boiling point of DMF was 153°C, the final drying temperature was chosen to be 150°C for 10% MMIF containing membrane. The drying was carried out under vacuum. As the MOF content was increased from 10% to 20%, some void formation was observed at the MOF-polymer interface possibly due to thermal stresses occurring during drying. Therefore, the final drying temperature was reduced to 120°C for the 20% MOF containing membrane. In order to avoid any significant thermal stresses, the membrane was brought to the final drying temperature gradually. After the membrane was peeled off the glass surface, it was returned back to the oven and kept at 80°C overnight. Then the oven temperature was raised to 100°C and 120°C, (150°C in case of 10% MMIF content), at 24 h intervals. Then, 10% and 20% MMIF containing membranes were dried for two more days at 150°C and 120°C under vacuum, respectively. All membranes were kept in a desiccator for characterization. In order to test membrane reproducibility, two separate MMMs with 10% MMIF loading were prepared from the same MMIF synthesis batch. These membranes are coded as 10% MMIFMatrimid-1 and 10% MMIF- Matrimid-2.


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2.4. Characterization of MMIF nanoparticles Structural and crystalline stability of MMIF was characterized using X-ray powder diffraction (XRD) method. The XRD patterns were recorded on a Bruker D8 Advance using CuKα (λ=1.54 Å) radiation at room temperature in the 2θ range between 5° and 50°. Scanning electron microscopy (SEM) was used for morphological characterization of MMIF samples. SEM analysis was carried out using a QUANTA FEG 250 instrument to determine the shape and size of the MOF samples. Dry MOF samples were attached to a sample holder using a carbon foil with adhesive on both sides and then coated with gold/palladium (Au/Pd) for 45 s at 20 mA using a SC7620 mini sputter coater to obtain a conductive surface. Thermogravimetric analysis (TGA) was employed to examine the thermal stability and decomposition of MMIF. The TGA instrument used in this study was a Perkin Elmer Diamond TG/DTA. The analysis was carried out between 50°C and 550°C with a scanning rate of 10°C/min under flow of nitrogen at 100 ml/min flow rate. The gas sorption capacity of the MMIF sample was analyzed using the Intelligent Gravimetric Analyzer (IGA-001 Hiden Isochema Advanced Sorption Analysis) instrument. The information of gas sorption is based on the measurement of material’s weight at precise temperature and pressure at equilibrium condition. The data can be plotted as isotherms for selected gases. The pressure range used in this study was 1-8 bar at constant temperature of 35°C. The MMIF was activated and decontaminated at 150°C for 24 hours under vacuum in the IGA chamber before a gas sorption measurement. 2.5. Membrane characterization X-ray diffraction analysis was employed to ensure the structural stability of MMIF during MMMs preparation. Surface and cross-sectional morphology of MMMs were determined by


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scanning electron microcopy (SEM). A piece of membrane sample was fractured in liquid nitrogen to obtain a sample for examination of the membrane cross-section. The membrane was then attached to the sample holder with an adhesive carbon film and coated with gold/palladium (Au/Pd) before SEM analysis. SEM images revealed information on the dispersion and homogeneity of the MOF particles, adhesion between the MOF and polymer, and defects if any. TGA under N2 atmosphere was used to determine the thermal behavior of the membrane samples. Differential scanning calorimetry (DSC) was used to determine the glass transition temperature (Tg) of the membrane samples. The changes in the Tg can give information on how good the adhesion between the MOF and polymer is. The heating was carried out under N2 flow. For the first run, sample was heated from room temperature to 310°C (slightly above the Tg of Matrimid) at a rate of 10°C/min. The sample was kept at 310°C for 5 minutes and then cooled down to 250°C at a rate of 30°C/min. In the second heating run the sample was heated from 250°C to 400°C at a rate of 10°C/min. The Tg was determined by using the half Cp extrapolation method. Gas permeation through membranes were examined in a constant volume-variable pressure system at 4 bar feed gas pressure and 35°C. The constant volume downstream (permeate) side of the permeation cell comprises of a sensitive pressure transducer, a relief valve, and tubes connecting the cell to vacuum pump. The gas transducer detects the pressure rise in the constant volume due to gas permeation in time. Prior to permeability measurements, the rate of leak was calculated using an impermeable aluminum sheet. Membranes were masked, placed in the stainless steel permeation cell, and both upstream and downstream sides were evacuated for 24 hours to remove residual solvent and any adsorbed gases. The desired gas pressure then was applied to the upstream (feed) side while the downstream side of the chamber was kept under dynamic vacuum for a few hours more. Both pressures at downstream and upstream sides of chamber was recorded


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in every minute to calculate the gas permeability of membranes. To ensure steady-state condition, the pressure increase in the calibrated constant volume with time was followed and the pressure/time ratio was waited to remain unchanged for the alternate measurements. After reaching steady-state, the permeability measurement for each gas was repeated for 3 times and the average was reported. The permeability tests were carried out in a specific order for single gases: N2, CH4 and CO2, with vacuum employment for 24 hours between each gas. To measure mixed gas permeabilities and separation factors, the total feed side pressure were adjusted at a total pressure of 4 bar by feeding an equimolar CO2/N2 and CO2/CH4 gas mixture and the downstream gas composition was determined by using gas chromatography (GC). The mixed gas selectivities were calculated using the following relationship:

𝛼𝑖/𝑗 =

𝑦𝑖 ⁄𝑦𝑗

(1)

𝑥𝑖 ⁄𝑥𝑗

where 𝑦𝑖 and 𝑦𝑗 are the mole fractions of components in the permeate side, 𝑥𝑖 and 𝑥𝑗 are the mole fractions of the corresponding components in the feed side. 3. Results and Discussion 3.1. Characterization of MMIF To confirm the crystal structure, simulated XRD pattern of MMIF obtained from Materials Accelerys Studio software, using crystal structural data obtained from Cambridge structural database (CSD)48, is compared to the XRD patterns of the MMIF samples synthesized in this work. As can be seen from Figure 2(a), the XRD pattern of the product of each synthesis batch is in good agreement with the simulated pattern.


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(a)

(b) MMIF

As Synthesized MMIF-2 MMIF-Derivative As Synthesized MMIF-1

H2L.2HBr Simulated MMIF

(c)

(d)

MMIF-1, 200°C, 24 hr Vacuum

MMIF-1, 150°C, 24 hr Vacuum

MMIF-1, As Synthesized

Figure 2: (a) Powder XRD patterns of simulated MMIF and as-synthesized products, (b) TGA thermographs of MMIF and the ligand, (c) XRD patterns of MMIF samples heat treated at different temperatures under vacuum for 24 hours, and (d) Molecular structure of MMIF (simulated using data obtained from CSD48). The weight loss curve (TGA) in Figure 2(b) reveals that MMIF loses 4.4% of its weight from 39°C to 160°C, indicating that the MOF had some trapped water and ethanol inside its pores. The MMIF decomposition can be seen around 350-360°C up to 540°C. The turning point occurred at


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450°C. This step involves the decomposition of the ligand. It is clear to say that the ligand pyrolysis is happening at 330-470°C which is in good accordance with the degradation of MMIF around 400°C. The TGA derivative curve of MMIF shows that MMIF goes through another transition at 160°C, where possibly some ligand loss starts. Hence, the activation temperature of MMIF should not exceed 160°C. In order to check the stability of MMIF in terms of crystallinity, two samples were dried, one up to 150°C and the other up to 200°C under vacuum overnight separately and characterized by XRD. Figure 2(c) shows the XRD patterns of these samples in comparison to an untreated sample. The patterns indicate that MMIF is stable up to 150°C and the framework collapses at a temperature between 150-200°C as can be seen from the pattern of the sample heated at 200°C. The collapse of the framework leads to a crystalline-to-amorphous transition of the MMIF structure. Lui et al. also reported that MMIF was activated at 160°C under vacuum for about 10 hours 52 and they did not mention any stability issue. Morphological analysis of as-synthesized MMIF powders were performed using SEM analysis. Figure 3 presents the SEM images for the products of the two separate synthesis batches, MMIF-1 and MMIF-2. The images show well defined elliptic particles, which are in good accordance with the report of Lui et al. 52. The average particle size (as determined by SEM) for the batches were 40-50 nm for MMIF-1 (a and b) and 220 nm for MMIF-2 (c and d) and the particles were agglomerated. Figure 4 shows the gas sorption isotherms of MMIF-1 at 35°C (308 K) and pressures up to 8 bar. As revealed by the simulation studies in the literature, MMIF adsorbs CO2 more than CH4 and N2. The CO2 simulated uptake at 4 bar (membrane operating condition) is around 3.3


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molecules/unit cell and the CH4 simulated uptake is around 1.3 molecules/unit cell at the same operating condition53. The BET analysis for MMIF is available in the literature with N2 at 77 K 52. As it was reported, the adsorption isotherm is type I indicating a microporous structure and the sorption mechanism is monolayer. The sorption data in Figure 4 agree well with the sorption isotherms reported in the literature 25,53.

Figure 3: SEM images of nanosized MMIF, (a) MMIF-1, 400000X, (b) MMIF-1, 200000X, (c) MMIF-2, 100000X, (d) MMIF-2, 200000X. 3.2. Membrane characterization 3.2.1. Structural and thermal properties In order to characterize MMIF stability under sonication in DMF and thermal treatment during membrane preparation, XRD analysis of membranes was conducted to compare the patterns


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with that of crystalline MOF. Figure 5(a) presents the XRD patterns of as-synthesized MMIF-1 sample and MMM with 10% MMIF-1 loading. It is evident from the comparison of the two XRD patterns that the MMIF crystals maintained their structure under mechanical stirring, ultrasound operation, and thermal treatment.

Figure 4: Gas sorption analysis of MMIF-1 at 308 K. Figure 5(b) shows TGA curves of membranes, neat polymer, and MMMs with 10% and 20% MMIF loading. The neat polymer membrane and MMM containing 10% MMIF was dried at 150°C, however, the MMM containing 20% MMIF was dried at 120°C due to its fragile nature. All TGA curves shows approximately 1% cumulative weight loss up to 120°C. There was no further considerable weight loss (additional 0.1%) up to 153°C, which is the boiling point of the solvent indicating that water and residual solvent removal was successful. From 153°C to 300°C, the weight loss in MMMs was more than that of neat Matrimid, and the weight loss increased as the MMIF loading increased. This indicates that some residual solvent is trapped inside the pores


18

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of MMIF particles. The weight loss between 300°C and 450°C was 3.95% and 6.04% for 10% and 20% MMIF containing MMMs, respectively, which attributes to MMIF loading and the MMIFligand decomposition. Thermal decomposition of the polymer matrix starts at around 475°C.

(a)

(b)

MMIF/Matrimid (10% MMIF)

As Synthesized MMIF

Figure 5: (a) XRD patterns of MMIF/ Matrimid (10% MMIF) MMM and MMIF powder, (b) TGA thermograms of MMIF/ Matrimid MMMs and pure Matrimid membrane. Figure 6 (a, b, c, and d) and Figure 6 (e, f, g, and h) display the SEM images of cross sections of MMIF/ Matrimid MMMs containing 10% and 20%, respectively. The MMM containing 10% MMIF was prepared with the MMIF-1 sample and its final thickness was 40 microns whereas the MMM containing 20% MMIF was prepared with the MMIF-2 sample and its final thickness was 60 microns. The SEM images show generally a uniform distribution of MMIF over the cross section of both membranes. The adhesion between the MOF and polymer is generally good. Some interfacial voids are detected in the images, which may be the result of thermal stresses occurring during the membrane annealing process. It also may be attributed to hydrophilicity of MMIF, which is not


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compatible with hydrophobic Matrimid. Although MOF is well dispersed, some MMIF particles were agglomerated, especially in the membrane containing 20% MMIF. Considering that the particle size was 40-50 nm for MMIF-1 and 220 nm for MMIF-2 and the crystals were already agglomerated before incorporation into the polymer, this is not surprising. In fact, the dispersion obtained is very good in spite of these disadvantages.

Figure 6: SEM images of 10% MMIF-1 containing Matrimid membrane:(a) 4000X, (b) 30000X, (c) 60000X, and (d) 160000X. SEM images of 20% MMIF-2 containing Matrimid membrane: (e) 2500X, (f) 50000X, (g) 50000X, and (h) 100000X. The glass transition temperature (Tg) of membranes that were measured using DSC are presented in Table 2. While the Tg of pure Matrimid was around 310°C, it increased slightly as the 10% MMIF was added to the polymer. However, further increase in MMIF content (20%) resulted the Tg to decrease below its value for neat Matrimid. This behavior may be explained by an initial rigidification of the polymer chains at the polymer-MOF interface due to strong


20

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interactions between the dispersed and continuous phases when 10% MMIF was incorporated, hence confining the segmental motion of polymer chains around the MOF particles and leading the Tg to increase. However, further increase of the MMIF content to 20% may have caused agglomeration and interfacial defects (as can be seen in Figure 6), weakening the interactions between two phases and increasing the polymer chain mobility, hence resulting in a Tg lower than that of pure Matrimid 57. Table 2: Glass transition temperatures of pure Matrimid and MMMs.

Membrane

Tg (°C)

Matrimid

308-310

10% MMIF- Matrimid

315-318


300

3.2.2. Gas separation properties Single and mixed gas permeability measurements were conducted at 35°C and 4 bar feed pressure to examine the impact of MMIF filling on Matrimid. Table 3 shows single gas permeability data obtained for MMMs with 10% and 20% (wt.) MMIF loading for CO2, CH4, and N2. Table 4 and 5 present the mixed gas (50:50) permeabilities and selectivities of the same MMMs for CO2/CH4 and CO2/N2 separations, respectively. In the case of 10% MMIF loading in Matrimid, the CO2 permeability increased by 19% to 8.1 Barrer. Increasing MMIF loading to 20% in Matrimid resulted in a CO2 permeability improvement by 27%. The permeability increase for CH4 and N2 was 13-16% for 10% MMIF addition and 23-31% for 20% MMIF addition, respectively. The differences in permeability


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Page 22 of 38

increases are clearly due to the differences in the kinetic diameter of the gases, i.e. CH4 > N2 > CO2. Hence, an enhancement in selectivity might be expected due to pore limiting diameter of 3.3-3.4 Å 22,49. However, only a slight improvement in ideal selectivity for CO2/CH4 was obtained for 10% MMIF loading since both gas molecules were able to enter the cages of MMIF possibly due to the flexible nature of its framework. On the other hand, a 3.5% decrease in CO2/CH4 selectivity was observed when 20% MMIF was incorporated into Matrimid. This could be due to presence of some interfacial defects in this membrane which were also identified on SEM images. The CO2/N2 selectivity also showed no significant change with an average of 8.6% improvement for the case of 10% MMIF content with respect to pure Matrimid. This is in line with the predictions of Yilmaz and Keskin 51 who reported 10.5% improvement for the case of 20% MMIF content using flexible framework assumption. These results were compared on the Robeson plots in Figures 7(a) and 8 in comparison to available single gas permeability data in the literature. Table 3: Single gas permeabilities and ideal selectivities of mixed matrix membranes for different gases at 4 bar feed pressure and 35°C. The subscripts indicate the uncertainty in the last digits of the permeability data.

Membrane

Permeability (Barrer)

Ideal Selectivity

CO2

CH4

N2

CO2/CH4

CO2/N2

Pure Matrimid

6.822

0.1903

0.2608

35.9

26.2

10% MMIF- Matrimid-1

8.105

0.2101

0.2962

36.9

27.3

10% MMIF- Matrimid-2

7.251

0.1872

0.2451

38.8

29.6


8.651

0.2541

0.3212

34.6

27.0


22

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Table 4: Mixed gas (50:50) permeabilities and selectivities of MMMs for CO2/CH4 separation at 4 bar feed pressure and 35°C Permeability (Barrer)

Selectivity

CO2

CH4

CO2/CH4

Pure Matrimid®

7.95

0.210

38.3

10% MMIF- Matrimid®-1

9.74

0.120

81.0

10% MMIF- Matrimid®-2

7.33

0.103

70.8

20% MMIF- Matrimid®

10.1

0.117

88.0

Membrane

Table 5: Mixed gas (50:50) permeabilities and selectivities of MMMs for CO2/N2 separation at 4 bar feed pressure and 35°C Permeability (Barrer)

Membrane Pure Matrimid® 10% MMIF- Matrimid®-1

CO2

N2

CO2/N2

7.06

0.22

32.3

8.17

0.21

38.9

®

8.13

0.23

35.7

®

11.7

0.20

58.0

10% MMIF- Matrimid -2 20% MMIF- Matrimid

Selectivity

On the other hand, mixed gas (50/50) selectivity data, presented in Tables 4 and 5, exhibit that MMIF has an extraordinary potential for both CO2/CH4 and CO2/N2 separations. Incorporation of 10% MMIF into Matrimid resulted in 98% change in CO2/CH4 selectivity from 38.3 for pure Matrimid to 76 (average of two samples) for MMM. The 20% MMIF loading improved the CO2/CH4 selectivity by 130% from 38.3 for pure Matrimid to 88 for MMM.


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(a)

(b)

Figure 7. (a) Single gas permeability and (b) 50:50 mixed gas permeability data of CO 2/CH4 pair for 20% MMIF- Matrimid® MMMs on Robeson plot in comparison with available data for MOFMatrimid® MMMs in the literature.


24

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Figure 8: Single gas and 50:50 mixed gas permeability data of CO2/N2 pair for 20% MMIFMatrimid® MMMs on Robeson plot in comparison with available (single gas) data for MOFMatrimid® MMMs in the literature. The comparison of single gas and mixed gas separation performances of MMIF/ Matrimid® membranes presented in Tables 3 and 4 unravels the dominant mechanism in MMIF for CO2 separation. It is clear from the single gas measurements that a molecular sieving mechanism is not applicable, since only a slight increase in the ideal selectivities was observed, in contrast to the previous simulation study reporting selectivities of several orders of magnitude

53

. This

discrepancy may be attributed to the framework flexibility that was ignored in the simulation study. However, the significant increase in separation factors in mixed gas measurements indicate a competitive mechanism favoring CO2 over CH4 or N2. A similar mechanism was observed previously for MOFs favoring CO2 due to competitive adsorption resulted from strong electrostatic interactions between the framework and CO2 molecules due to strong quadrupole moment of CO2


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15,58

Page 26 of 38

. The presence of open metal sites in MMIF promotes CO2 adsorption over CH4 or N2 in the

same way, leading to significantly high separation factors. These results are supported by the molecular simulation studies of Keskin 53 who reported an about three- and four-fold increase in adsorption selectivity in favor of CO2 over CH4 and N2, respectively, between single-gas and mixed-gas adsorption cases at 4 bar and 298 K. The results show that MMIF enhanced significantly the separation performance of Matrimid® for both CO2/CH4 and CO2/N2 gas pairs in comparison to other MOFs. This may be attributed to the competitive adsorption mechanism in MMIF favoring CO2 over CH4 and N2. Hence, CO2 molecules are adsorbed preferentially into the pores of MMIF and hinder CH4 and N2 permeation through the framework, increasing overall CO2 selectivity of the MMM. MMIF based MMMs may be one of the few examples where membrane selectivity improvement relies on the filler’s sorption selectivity 69. Mixed-gas sorption studies of both MMIF and MMMs are needed to further explore this finding. 4. Conclusions This study focused on the development of MOF-polymer mixed matrix membranes (MMMs) for CO2/CH4 and CO2/N2 separation applications. Microporous metal imidazolate framework (MMIF) has been successfully synthesized and characterized in order to investigate its potential use in mixed matrix membrane fabrication for CO2 separation, for the first time in the literature. The crystal framework structure of MMIF was shown by XRD analysis to be stable up to 150°C under vacuum although TGA analysis indicated thermal stability up to around 300°C. MMIF with particle size of 50-200 nm were incorporated into Matrimid® 5218 as the dispersed phase to prepare MMMs containing 10% and 20% MMIF. Single gas permeability measurements


26

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at 35°C and 4 bar showed a maximum permeability improvement of 19% and no significant improvement in selectivity. This behavior is typical for several MOFs since their framework is usually flexible and molecules larger than the limiting pore diameter of the MOF can enter their cages easily. These results are in good agreement with the Maxwell model predictions accounting framework flexibility 51 and indicates that MMIF is a flexible MOF. On the other hand, mixed gas selectivity for CO2/CH4 improved significantly for both loadings. The mixed gas CO2/CH4 (50:50) selectivity enhanced from 38 for neat Matrimid® to 81 for MMM containing 10% MMIF and to 88 for MMM containing 20% MMIF. Similarly, the mixed gas CO2/N2 (50:50) selectivity enhanced from 32 for neat Matrimid® to 39 for MMM containing 10% MMIF and to 58 for MMM containing 20% MMIF. This can be mainly attributed to competitive adsorption favoring CO2 over CH4 and N2 which is to be further investigated by mixed-gas sorption measurements in future work. This study confirms that the actual potential of MOFs in MMMs could only be explored by mixed-gas measurements. Single gas measurements are not enough to characterize the full potential of MOFs. This is particularly necessary for MOFs identified as promising MMM material based on simulation studies since many simulation studies make simplifying assumptions such as rigid framework. While Matrimid® serves as a reference material in this proof-of-concept study to evaluate separation performance of MMIF in MMM fabrication relative to other MOFs, it is not the best polymer choice for obtaining a MMM for the specific applications targeted in this study. The permeability and selectivity of Matrimid® for the gas pairs in this study are significantly below the upper bound line. The results of this study suggest that selecting a glassy polymer with high permeability close to Robeson’s 2008 upper bound line as the continuous phase may lead to a


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membrane with significantly improved separation performance as a candidate to be tested for commercial use. ASSOCIATED CONTENT The following files are available free of charge. Supporting Information (PDF). Synthesis and characterization of the organic ligand H2L.2HBr. AUTHOR INFORMATION *Corresponding authors: [email protected], [email protected] The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) through Grant no. 113M776. REFERENCES (1)

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(2) Pera-Titus, M. Porous Inorganic Membranes for CO2 Capture: Present and Prospects. Chem. Rev. (Washington, DC, U. S.). 2014, 114, 1413-1492.


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TOC Graphic

MMIF

MMIF

MMIF in Matrimid®


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MMIF

MMIF

MMIF in Matrimid®


Highly CO2 Selective Microporous Metal-Imidazolate Framework

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