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Facile fabrication of mixed matrix membranes containing 6FDA-durene polyimide and ZIF-8 nanofillers for CO2 capture
Accepted Manuscript Title: Facile Fabrication of Mixed Matrix Membranes Containing 6FDA-Durene Polyimide and ZIF-8 Nanofillers for CO2 Capture Author: Norwahyu Jusoh Yin Fong Yeong Cheong Weng Leong Kok Keong Lau Azmi M. Shariff PII: DOI: Reference:
S1226-086X(16)30306-9 http://dx.doi.org/doi:10.1016/j.jiec.2016.08.030 JIEC 3065
To appear in: Received date: Revised date: Accepted date:
23-6-2016 25-8-2016 26-8-2016
Please cite this article as: Norwahyu Jusoh, Yin Fong Yeong, Cheong Weng Leong, Kok Keong Lau, Azmi M.Shariff, Facile Fabrication of Mixed Matrix Membranes Containing 6FDA-Durene Polyimide and ZIF-8 Nanofillers for CO2 Capture, Journal of Industrial and Engineering Chemistry http://dx.doi.org/10.1016/j.jiec.2016.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile Fabrication of Mixed Matrix Membranes Containing 6FDA-Durene Polyimide and ZIF-8 Nanofillers for CO2 Capture
Norwahyu Jusoh, Yin Fong Yeong*, Cheong Weng Leong, Kok Keong Lau and Azmi M. Shariff Chemical Engineering Department, Universiti Teknologi PETRONAS 32610 Bandar Seri Iskandar, Perak, Malaysia. *Corresponding Author: Yeong Yin Fong, Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. Tel: +605-3687564, Fax: +605-3656176, E-mail: [email protected]
Graphical abstract
Abstract: Membrane separation has been used successfully in numbers of industrial applications especially in CO2 removal from CH4 because it’s involved less energy consumption and low maintenance. The aim of this research is to fabricate ZIF-8/6FDAdurene mixed matrix membranes for enhancement of CO2 capture and consequently to optimize its fabrication method. The results showed that membrane fabricated using total dispersion duration of 3 hours with filler priming procedure demonstrated homogenous distribution of ZIF-8 in 6FDA-durene matrix and improved the separation performance. Therefore, the membrane is potential for the large scale production via the method optimized in this work.
Keywords: Optimization, ZIF-8, 6FDA-durene, CO2 Capture
1.0 INTRODUCTION
Recently, the concerns pertaining natural gas purification have been rising significantly mainly due to the presence of high CO2 content and impurities in the reservoirs. In Southeast Asia, the gas reservoirs that contain CO2 up to 76 mol% have been discovered in Natuna Field, Indonesia [1]. In Malaysia, over 13 trillion cubic feet of natural gas reserves are undeveloped due to high CO2 content up to 87% [2]. At present, natural gas wells with high CO2 contents are undeveloped due to the difficulties in the operation, production and technical activity. Thus, an efficient separation process should be developed to maintain a continuous supply of natural gas demand. The CO2 capture from natural gas can be realized using a number of established technologies in industrial applications, including absorption, adsorption, cryogenic and membrane separations. Among these technologies, membrane is
preferable because it requires small space and weight, low labor intensity and maintenance as well as minimum utility requirements [3].
Membrane separation technology has been continued to progress rapidly due to its economic and environmental advantages over the other separation technologies. Gas separation membranes can be generally classified into three categories; polymeric, inorganic and mixed matrix membranes (MMMs) [4]. Polymeric membranes have been widely utilized in industrial application because it is relatively cheap, flexible and easy to fabricate. However, the separation performance of pure polymeric membrane is suffer from the upper bound limit, where high permeability and high selectivity cannot be simultaneously attained [5]. On the other hand, inorganic membrane offers higher permeability and selectivity with great chemical and thermal stability. However, it is limited by the fabrication cost and mechanical strength of the membrane material [6].
In order to overcome the above aforementioned drawbacks, MMM has been introduced by incorporating fillers into the polymer phase [7]. The separation performance of MMM may surpassed the upper bound line and consequently can be applied for industrial application by combination of fillers and polymer materials. Polymers commonly used for MMM fabrication include cellulose acetate, polycarbonate, polysulfone and polyimides. Polyimide is ultimately used for CO2/CH4 separation compared to other type of polymers due to their good separation performance, high chemical and mechanical stability [8]. Current trend shows that fluorinated polyimides containing 6FDA is attractive for CO2/CH4 separation because of the presence of –C(CF3)2- and bulky methyl group which contributes to the restriction of intrasegmental mobility, interruption of the interchain packing and consequently, resulted in higher gas permeability and gas pair selectivity [9].
The beginning of MMM era have been concentrated on the incorporation of traditional fillers such as zeolites [10-12] and silicas [13, 14] into the commercial type of polymer phase. Notably, the recent years has focused on the incorporation of metal organic frameworks (MOFs) and a subclass of MOF, zeolitic imidazolate frameworks (ZIFs) as the filler in the polymer matrix for CO2 capture [15]. The presence of imidazolate linker in their framework could increase the interfacial properties between filler and polymer matrix and consequently enhance the compatibility between these two materials [16].
Among ZIF materials, zeolitic imidazolate frameworks-8 (ZIF-8) has emerged as an attractive filler for the fabrication of MMM for CO2/CH4 separation. ZIF-8 is built up from zinc (II) cations and 2-methylimidazole anions, giving a sodalite (SOD) zeolite type structure with two times larger of pore size compared to SOD zeolites [17]. Besides, ZIF-8 consists of six-ring β-cages with aperture pore size of 3.4Å. The framework and chemical structure of ZIF-8 is presented in Figure 1, with tetrahedral Zn ions, bridged by imidazolate (Im) inside the cage to represent the pore cavity [18, 19]. The characteristics of ZIF-8 such as high surface area, highly porous open framework structure and great chemical and thermal stability, make it an attractive candidate for gaseous separation, especially for CO2/CH4 separation [20]. Representative examples include the works reported by Bushell et al [21], whose fabricated ZIF-8/PIM-1MMM for CO2, CH4, O2, N2, He and H2 pure gas permeation studies, while later Nordin et al. [22] further investigated the permeation studies of CO2/CH4 and CO2/N2 binary gas mixtures using ZIF-8/polysulfone membranes. Both studies have demonstrated great implications to MMM development, where their findings showed that the incorporation of ZIFs or MOFs in polymer phase enhanced gas separation performance especially for CO2 capture from CH4.
However, most of the studies reported on MMMs were focused solely on the separation performance, concerning with the improvement of permeability and selectivity of the studied MMMs [23-25]. In contrast, the studies in finding the established approaches and the standard techniques for the formation of MMM with minimum defects through the optimization of the fabrication parameters are remained scarce. A great effort in investigating these fabrication parameters in details could contribute a significant impact on the production of high quality MMMs. The ideal MMMs normally were hindered by poor compatibility, phase separation and poor adhesion between the filler and polymer phase, and thus, worsen the thermophysical properties of membrane and caused severe reduction of gas separation performance [26-28]. Furthermore, the fabrication of defect-free MMM is normally requires a strict processing conditions, in which the techniques to prepare MMM is relatively time consuming, cumbersome, and often not economically viable.
Therefore, the present work demonstrates in-depth study to elucidate the basic parameters for fabricating MMM. The effects of dispersion duration of fillers, including stirring and sonication as well as priming and non-priming of fillers procedures were investigated.
The properties of the resultant membranes were characterized by various
analytical tools. In addition, permeation properties of the resultant MMMs in CO2/CH4 separation were studied in order to investigate the effect of fabrication parameters towards the separation performance.
2.0 EXPERIMENTAL METHOD
2.1 Materials Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, > 98% purity, Sigma Aldrich) and 2methylimidazole (Hmim, 98% purity, Sigma Aldrich), methanol (99.8% purity, Merck) were used without further purification. 4, 4’ – (Hexafluoroisopropylidene) diphthalic anhydride
(6FDA, 99% purity, Sigma Aldrich) monomers were purified by vacuum sublimation prior to use. 2, 3, 5, 6- Tetramethyl-p-phenylenediamine (durene diamine, 99% trace metal basis, Sigma Aldrich) monomers were purified by using re-crystallization method in methanol. Nmethyl-2-pyrrolidone (NMP) was purified through vacuum distillation. Propionic anhydride (PA, ≥ 98% purity, Merck), triethylamine (TEA, ≥ 99% purity, Merck), methanol (≥ 99.9% purity, Merck) and dichloromethane (DCM, ≥ 99.8% purity, Sigma Aldrich) solvent were used as received. Gases for permeation experiments, such as carbon dioxide (CO2, 99.99%) and methane (CH4, 99.99%) were purchased from Gas Walker Sdn Bhd and used as received.
2.2 Synthesis of ZIF-8 nanocrystals
ZIF-8 nanofiller was synthesized at room temperature by mixing zinc nitrate hexahydrate (Zn (NO3)2.6H2O) as zinc source and 2-methylimidazole (2-MeIM) as organic ligand in methanol (MeOH) with molar composition of 1 Zn2+: 8 Hmim: 1002 MeOH under stirring for 1 h [29]. The mixture was slowly turned into a cloudy solution and then nanocrystals were separated from the solution by centrifuging at 7800 rpm for 5 minutes. The solid was then washed with methanol for several times and dried in the oven at 60 oC for 24 h prior to use.
2.3 Preparation of 6FDA-durene Polymer
6FDA-durene polymer was synthesized using chemical imidization method reported by Liu et al. [30]. Equal mole of durene-diamine and 6FDA monomers were dissolved in purified NMP. The mixture was stirred for 24 h under nitrogen purge to obtain polyamic acid (PAA) solution. The mole ratio of propionic anhydride (PA) / triethylamine (TEA) to 6FDA of 4:1 were added to the PAA solution for chemical imidization to form polyimide. The polyimide was precipitated in methanol and then washed with methanol for 3 times. The
resultant polymer was dried at 80°C in vacuum oven for 24 h and stored in a desiccator prior to use.
2.4 Fabrication of Pure and ZIF-8/6FDA-durene Mixed Matrix Membrane
6FDA-durene flat sheet membrane was prepared by solvent evaporation method. A 2% w/v solution of polymer was dissolved in DCM before filtered using a 1.0 μm PTFE membrane filter and cast on a Petri dish. The cast film was dried in an oven at 60°C for 24 h followed by another 24 h under vacuum. The oven temperature was then increased from 60°C to 250°C at a heating of 25°C/h before annealed at 250°C for 24 h. On the other hand, MMMs contained 5 wt% of ZIF-8 in 6FDA-durene polymer phase was fabricated using solution blending method by manipulating the fabrication parameters including dispersion of ZIF-8 in DCM under different stirring and sonication durations and priming or non-priming of fillers. First, both solutions of polymer and ZIF-8 were prepared in two separate vials. 5 wt% of ZIF-8 crystals were added into DCM, stirred and sonicated alternately to disperse ZIF-8 in DCM solution. At the same time, 6FDA-durene polymer was stirred in DCM until dissolved and the solution is filtered in order to remove insoluble particles such as dust and impurities. After that, sonication was done in an ultrasonication water bath operating at 120 W and 40 kHz. Then, the ZIF-8 particles were primed by adding 10 wt% of 6FDA-durene polymer solution into the ZIF-8 solution, which it was further stirred and sonicated. After thorough mixing, the remaining bulk polymer solution was added and the mixture was again further stirred and sonicated. The mixture was then stirred vigorously for 1 h before cast into a Petri dish on a leveled glass plate. The resulting membrane was immediately covered with a clean glass plate for slow solvent evaporation in ambient air for overnight. The membrane was then peeled off and dried in an oven at 60°C for 24 h followed by another 24 h under vacuum. Oven temperature was then increased from 60 to 250°C at a heating rate of 25°C/h.
The nascent film was annealed at 250°C for 24 h followed by natural cooled down. Table 1 summarized the MMMs fabricated in the present work and total seven membrane samples were prepared by manipulating the fabrication parameters.
2.5 Characterization of ZIF-8 Particle and Membranes
The crystal structure of the ZIF-8 particle was examined using X-ray Diffraction (XRD), Bruker (USA) D8 Advance diffractometer equipped with CuKα radiation (λ=1.54059Å). The XRD pattern was obtained in 2 theta range of 5-28o at a step size of 0.02 o
. The morphology and crystal size of the ZIF-8 particles were identified using Hitachi
SU8000 field emission scanning electron microscope (FESEM). ZIF-8 particle was placed on top of the FESEM surface holder by using a double-sided carbon tape before analysis. Perkin Elmer Pyris 1 Thermogravimetric analysis (TGA) was used to determine the thermal stability and decomposition temperature of ZIF-8 particles. Samples weighing an average of 15-20 mg were loaded into an alumina pan followed by thermal scanning under air environment in a range of 50°C to 800°C, at heating rate of 10°C min-1. Chemical functionalities of ZIF-8 particle were measured by using Fourier Transform Infra-Red (FTIR), Perkin Elmer One spectrometer through transmittance method. The ZIF-8 particle in powder form was blended with KBr and compressed into the pellet. The analysis was then conducted in a transmittance mode from 650 to 4000 cm-1 using 50 scans at room temperature.
The resultant membranes were characterized by XRD, FESEM, EDX, TGA, DSC and ATR-FTIR. The membranes samples were cut and used for XRD analysis using the same parameters as previously described for ZIF-8 particle analysis. Furthermore, the images of the cross section and the elemental compositions of the resultant membranes were obtained by using FESEM and EDX, respectively. Cross sections of the membranes were prepared by freeze-fracture after immersion in liquid nitrogen for several minutes. Prior to imaging, the
membrane samples were coated with platinum target using Quorum Q150R S sputter coater under vacuum condition for 60s to prevent charging of the sample which could occur during imaging analysis. The elemental compositions of the MMMs and the distribution of the fillers in MMMs were verified using Bruker XFlash Detector 6/30 energy dispersion of X-ray (EDX).
The thermal stability of the membranes was determined by using TGA as previously described in ZIF-8 particle analysis. The measurement was conducted with heating ramp of 10°C rise per minute from 50°C to 800°C under air atmosphere, using 15 mg of sample. The differential scanning calorimetry (DSC, Q2000) was used to evaluate the glass transition temperature (Tg) of membrane samples. The membrane samples were heated under nitrogen environment to 450oC at a heating rate of 10°C/ min during the first cycle in order to remove the thermal history of membrane. Then, the membrane samples were quenched at a rate of 10°C/ min, and reheated at 10°C/ min to complete the second cycle of heating. The Tg of samples were determined from the second cycle. The functional groups in the MMMs were investigated using FTIR spectroscopy operated in the attenuated total reflection mode (ATR) equipped with a diamond crystal. The membrane films were scanned for 50 times for each sample and the scanning was from 650 to 4000 cm-1 at room temperature.
2.6 Gas Permeation Measurements
Single gas permeation and mixed gas separation performance of the membranes were conducted using custom-built gas permeation test rig. The detailed explanation on the testing setup and procedure can be found elsewhere [31]. For each run, at least 2 films, 1 piece from 2 separately cast membranes with the same fabrication parameters, were investigated for the reproducibility test. The permeability of pure CO2 and CH4 gases were measured at 30°C and 3.5 bar and calculated based on the equation (1) as follows [32]:
PA
Vpt
Am ph pl
(1)
Where PA is the permeability of membrane (Barrer), Vp is the permeate flow rate (cm3(STP)/s), t is the thickness of membrane (cm), Am is the membrane area (cm2), ph and pl are the pressure in feed side and permeate side, respectively (cmHg), subscript A is representing CO2 or CH4. The permeability of the membranes is reported in the unit of Barrer (1 Barrer =1×10-10cm3(STP).cm/s.cm2.cmHg).
The ideal selectivity of the membrane can be obtained by dividing permeability of CO2 over permeability of CH4 as shown in equation (2) as follows:
CO
2 / CH 4
PCO2 PCH4
(2)
Where α indicates the ideal selectivity of CO2/CH4 and P is the permeability (Barrer).
Meanwhile, the plasticization behavior of resultant MMMs were studied by intermittently ramped CO2 pure gas pressure from 3.5 to 20 bar. Since the pressure was ramped to the elevated pressure, non-ideal gas condition was considered. The pressure difference across the membrane as described in the equation (1) was changed to the fugacity as the permeation driving force. The fugacity can be calculated using a non-ideal equation of state (EOS), Peng-Robinson EOS where the detailed calculation on fugacity can be found elsewhere [33, 34]. The permeability of CO2 can be determined by the following equation:
PCO2
Vpt
Am h l
where φ is the fugacity of CO2 in the feed stream.
(3)
In addition, mixed gas permeation of MMMs were conducted using 50% CO2 and 50% CH4 binary mixtures at pressure of 3.5 bar and 30°C of temperature. The permeability of components in gas mixture was calculated based on the equations (4-5) as follows:
PCO2
PCH4
V p yCO2 t
Am ph xCO2 p1 yCO2
V p yCH4 t
Am ph xCH4 p1 yCH4
(4)
(5)
Where PCO2 and PCH4 is the permeability of CO2 and CH4, respectively, and x and y refer to the mole fraction of component in feed and retentate side, respectively.
The selectivity for mixed gas measurement can be determined based on equation (6) as follows:
CO
2 / CH 4
yCO2 / yCH4 xCO / xCH 2 4
(6)
3.0 RESULTS AND DISCUSSION
3.1 Characterization of pure 6FDA-durene and mixed matrix membranes
3.1.1 Physical appearance of the membranes
The physical appearance of the pure 6FDA-durene membrane and 5 wt% ZIF8/6FDA-durene MMMs (S1-S7) fabricated using different fabrication parameters are shown in Figure 2. It can be seen from Figure 2 that all the resultant MMMs turn to slightly blur compared to the transparent pure 6FDA-durene membrane. This may be attributed to the dispersion of ZIF-8 particles in polymer matrix and this phenomenon has also been reported
for the other MMMs loaded with different combination of polymer and fillers [35]. However, all the resultant MMMs are still able to bend, flexible, resilient and non-brittle even though they were fabricated using different fabrication parameters.
3.1.2 X-Ray Diffraction (XRD) The XRD patterns for ZIF-8 particles and membranes at 2θ range of 5° to 30° are shown in Figures 3 and 4, respectively. Referring to Figure 3, the XRD pattern for ZIF-8 sample matches with the reported XRD pattern of ZIF-8 reported by Cravillon et al. [36] with the peaks at 2θ = 7.30o, 10.36o, 12.68o, 16.40o and 17.98o. This result confirmed the formation of crystalline ZIF-8 phase. A sharp peak at 2θ of 7.30º was observed in the XRD pattern of the ZIF-8, signifying that a highly crystalline material is successfully obtained. Based on Figure 3 also, the XRD pattern of pure 6FDA-durene demonstrates a broad peak from 8° to 22°, implies the amorphous structure of polymer phase. This observation is correlated well with the reported XRD pattern for 6FDA-durene reported in the literature [16, 37].
On the other hand, the XRD patterns of all MMMs shown in Figure 4 are matched well with the XRD pattern of ZIF-8/6FDA-durene MMM reported in the literature [16]. It was observed that all the membrane samples demonstrate peaks at 2θ values of 7.3°, matching with the characteristic peaks of ZIF-8 crystal. However, the peaks of ZIF-8 crystals at 10.36o, 12.68o, 16.40o and 17.98o are not well pronounced because it is masked by the amorphous phase of 6FDA-durene polymer [38]. Thus, the diffraction patterns of ZIF-8/ 6FDA-durene MMMs verified the presence of ZIF-8 structures and 6FDA-durene phase in the resultant MMM via the intense crystalline peak and the amorphous halo, respectively.
3.1.3 Field Emission Scanning Electron Microscopy (FESEM)
Figures 5 and 6 show the FESEM images of ZIF-8 particles and the resultant MMMs. Meanwhile, Figure 7 displays the EDX mapping of the resultant membranes. Since the ZIF-8 nanoparticles is comprises of zinc element only, the EDX analysis was applied by mapping the presence of Zn element in the membrane in order to further confirm the distribution of ZIF-8 particles in polymer phase.
Based on Figure 5, well-shaped of ZIF-8 with the average particle size of 50 nm is observed. In addition, the FESEM images of all the MMM samples show that the ZIF-8 crystals are encapsulated in 6FDA-durene polymer phase. Referring to the FESEM images of S1 membrane, it is found that the ZIF-8 nanoparticles are well distributed in the polymer phase with the absence of interfacial gaps between the particle and polymer phase. This indicates that a good compatibility between ZIF-8 and 6FDA-durene phase has been achieved. A good dispersion of ZIF-8 particles in the membrane matrix was further confirmed by the uniform dispersion of Zn element in EDX mapping shown in Figure 7(a). However, as the filler dispersion duration increases from 3 h to 6 h (S2), the FESEM images of S2 membrane show the presence of sub-micron particles of ZIF-8 due to the aggregation of ZIF-8 nanoparticles in the polymer phase. In addition, formation of void space between ZIF-8 particles and polymer phase can be observed from the FESEM images. Despite the unselective voids are formed with the presence of sub-micron aggregates in the membrane, it can be seen that the ZIF-8 particles are still homogenously distributed within the 6FDAdurene polymer matrix, as verified in EDX mapping displays in Figure 7(b). The aggregation of nanosized particles to the submicron particles observed in S2 is expected due to the ripening effect which induced by the sonication during membrane fabrication. During sonication process, the acoustic cavitation of liquid which create high pressure and
temperature in the liquid encourages the dissolution of smaller particles and grow to the larger particles [39].
Furthermore, prolonging the total duration of stirring and sonication to 8 h (S3 and S4 membranes) increases the aggregation of ZIF-8 particles up to several sub-microns in size with the evidence of minor agglomeration in polymer matrix. In fact, this result is reasonable since the increase in the stirring and sonication durations generally increases the formation of larger particles due to the ripening effect. Although both of the S3 and S4 were prepared at the same total duration of fillers dispersion, however, S3 was subjected to 2 hours dispersion of ZIF-8 in DCM and 6 hours dispersion of ZIF-8 in 6FDA-durene polymer, whereas S4 was subjected to the 4 hours dispersion of ZIF-8 in DCM and 4 hours dispersion of ZIF-8 in polymer. According to Figures 5 and 6, it can be seen that prolonging the duration of stirring and sonication of ZIF-8 in DCM resulted in larger aggregation of ZIF-8 particles in S4 as compared to the aggregation of ZIF-8 particles in S3. This result might be due to the lower kinetic barrier to Ostwald ripening of ZIF-8 in DCM as compared to the kinetic barrier to Ostwald ripening in the polymer solution [40, 41]. Therefore, enhance the duration of sonication of ZIF-8 in DCM contributes to the larger particles. Meanwhile, referring to the EDX mapping in Figure 7(c) and (d), it can be seen that some of the ZIF-8 particles are migrate and cluster at the surface of S3, whereas a minor agglomeration of ZIF-8 particles in polymer phase can be observed in S4. These results indicate that relatively poor particles distributions of ZIF-8 particles in 6FDA-durene polymer phase were found for S3 and S4.
However, as the duration of fillers dispersion increases to 10 h (S5), obvious agglomeration of the particles in 6FDA-durene polymer phase with the presence of unselective voids and pinholes can be observed in the membrane. This result indicates that the formation of heterogeneity ZIF-8 particles is found in the polymer matrix, which could
consequently create the paths for unselective transport of gases. A poor dispersion of ZIF-8 particles in polymer matrix is further confirmed in Figure 7(e) by EDX-mapping image. Based on the results obtained, the shortest fillers dispersion duration of 3 h produces better homogeneity and blending of ZIF-8 in polymer phase, as compared to the longer stirring and sonication duration.
Besides, Figures 5 and 6 also displayed the FESEM images of the MMMs prepared without using priming procedure at fillers dispersion duration of 3 h (S6) and 6 h (S7). Both samples of S6 and S7 demonstrate poor dispersion of ZIF-8 particles in 6FDA-durene polymer matrix as compared to the samples fabricated using priming procedure. The FESEM images of sample S6 shows the presence of submicron particles with minor agglomeration as compared to sample S1 when subjected to the same total stirring and sonication durations. The EDX-mapping in Figure 7(f) further confirmed the poor dispersion of ZIF-8 particles in the polymer matrix. On the other hand, sample S7 shows severe agglomeration with unselective voids as compared to sample S2. It can be clearly seen from Figure 7(g) that the ZIF-8 particles were heterogeneously dispersed in the polymer phase. This result indicates that the priming procedure is important to assist the distribution of ZIF-8 particle by coating a thin layer of polymer to the surface of ZIF-8 and consequently preventing the formation of ZIF-8 agglomerates that would appear in the resulting membrane [42]. Therefore, in the present work, the optimum mixing duration of ZIF-8 particles in 6FDA-durene polymer phase is 3 h, by including the priming procedure.
3.1.4 Thermogravimetric Analysis (TGA)
The TGA results of ZIF-8 particles and resultant membranes are shown in Figure 8. The degradation temperature of the samples was determined at the 5% weight loss [43]. Based on Figure 8, minor weight loss of ~4wt% from 100°C to 400°C is found in the ZIF-8
crystals TGA curve, which may be contributed from the elimination of residual solvents or moisture molecules trapped in the ZIF-8 [22]. In addition, the framework decomposition of ZIF-8 crystal is occurred in the range of 400°C and 450°C, which is in agreement with the reported literature data [22, 44]. On the other hand, pure 6FDA-durene membrane demonstrates thermal stability up to 500°C, which is in agreement with the thermal decomposition of 6FDA-durene reported by Lin et al [45]. The decomposition of pure 6FDAdurene comprises the first degradation of polymer chain with the elimination of CO2, CH4, CO, and H2 gases and the complete degradation of polymer matrix including carbon and nitrogen [46].
Referring to Figure 8, all the MMMs (S1-S7) demonstrate thermal stability up to ~500°C under air atmosphere. The decomposition of MMMs is contributed to the ZIF-8 nanoparticles framework decomposition and 6FDA-durene matrix. Since the decomposition temperature of ZIF-8 and 6FDA-durene is close to each other, there is no obvious stage of weight loss can be observed. A similar behavior of thermal decomposition temperature of ZIF-8/6FDA-durene MMM was reported by Wijenayake et al.[47]. Furthermore, all the fabricated MMMs (S1-S7) exhibit insignificant changes in the decomposition temperature (~518 - 520°C). This result shows that prolonging the duration of fillers dispersion from 3 h to 10 h with and without priming procedure did not affect the thermal stability of membrane. This is might be due to the similar loading of ZIF-8 nanofillers (5 wt%) incorporated in 6FDA-durene polymer for all the membranes.
3.1.5 Differential Scanning Calorimetry (DSC) Analysis
The DSC results and glass transition temperatures (Tg) of all the membranes are presented in Figure 9. Referring to Figure 9, the Tg of pure 6FDA-durene membrane is 410ºC.
Meanwhile, all MMMs demonstrate higher Tg value of up to 423.50°C as compared to the pure membrane, demonstrating that the presence of ZIF-8 fillers has changed the polymer packing structure of the membrane [16]. As the total duration of stirring and sonication increases, the value of Tg decrease from 423.50°C to 414.96°C (S1-S5), indicating that the rigidity and the pore volume of the membranes are affected [48]. Same results were also obtained for MMMs without the priming procedure where S6 and S7 membranes demonstrated lower Tg as compared to MMM fabricated via priming procedure, indicating that the polymer chain rigidification is reduced.
3.1.6 Attenuated Total Reflection Fourier Transform Infra-Red (ATR-FTIR)
The functional groups presence in ZIF-8 and the resultant membranes are shown in Figure 10. Based on Figure 10, ZIF-8 particles comprises aromatic C-H stretch and aliphatic C-H stretch of the imidazole at 3130 and 2915 cm-1, respectively. The absorption bands at 1580 cm-1 are detected for the C=N stretch. The C-N bonds are characterized by peaks in the region of 1145-1400 cm-1. The characteristic peaks of ZIF-8 sample are consistent with those IR peaks reported in the literature for ZIF-8 structure [49]. Referring to Figure 10, 6FDAdurene membrane exhibits peaks at 1785, 1715, 1352, 1250 and 718 cm-1. The bands at 1715 cm-1 and 1785 cm-1 are attributed to the C=O symmetric and asymmetric stretch of imide group, respectively. The peak at 1352 cm-1 and 1250 cm-1 are assigned to the imide group CN stretch and C-F stretch in CF3 group, respectively. The peak at 718 cm-1 is attributed to the formation of the imide ring or the imide carbonyl group. The characteristic peaks of 6FDAdurene membrane are consistent with those IR peaks reported in the literature for 6FDAdurene structure [30, 45, 50].
Meanwhile, the spectra of MMMs demonstrate the characteristics of both ZIF-8 and 6FDA-durene with no obvious shifts in absorption bands when compared with ZIF-8
nanoparticles and pure 6FDA-durene membrane. This result indicates that a strong chemical interaction between functional group of polymer and ZIF8 particles was not occurred in the resultant MMMs.
3.1.7 Pure gas permeation tests
Figures 11 and 12 show the CO2 and CH4 gas permeabilities and CO2/CH4 selectivity of all the membranes fabricated in the present work. The gas separation performance of the pure 6FDA-durene membrane obtained in this study is lower than those results reported in the literature [16, 51, 52]. This could be due to solvent used in this work for the membrane fabrication.
Different
types
of
solvents
including
dichloromethane
(DCM),
dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and chloroform (CHCl3) have been reported in the literature for the fabrication of 6FDA-durene membrane [16, 37]. These solvents show dissimilar polarity, dielectric constant and dipole moment which can consequently change the properties of membrane and thus, affect the gas separation performance of the resultant membranes [53, 54]. In addition, the lower Tg obtained in this study as compared to the reported data support the lower separation performance observed in this study.
Referring to Figure 11, all the resultant MMMs demonstrate higher CO2 permeability and lower CH4 permeability as compared to pure membrane. The MMMs show CO2 permeability of 693.54 Barrer, 541.24 Barrer, 515.72 Barrer, 520.53 Barrer, 473.54 Barrer, 501.44 Barrer and 489.35 Barrer for S1, S2, S3, S4, S5, S6 and S7, respectively, whereas , for pure membrane, CO2 permeability of 468.01 Barrer is obtained. Besides, based on Figure 12, all the MMMs demonstrate higher CO2/CH4 selectivity as compared to pure membrane, with CO2/CH4 selectivity of only 7.03.
The increment of gas performance of MMM is contributed to the addition of nanoparticles of ZIF-8 in the polymer phase which has interrupted the polymer chain packing and consequently increase the free volume and the diffusion pathways for gas penetration [55]. Furthermore, the combined effects of the pore aperture sizes of ZIF-8 that able to separate smaller gas molecule, CO2 (3.3 Å) from CH4 (3.8 Å) through the interior cavities of ZIF-8 and strong quadrupolar interaction of CO2 with the imidazolate linker in the ZIF-8 particles framework facilitates the transport of gases [56] [22].
In addition, it can be seen from Figures 11 and 12 that the increment of duration of filler dispersion from 3 h (S1) to 10 h (S5) decreases CO2 permeability from 693.54 Barrer to 473.54 Barrer, respectively. On the other hand, the shortest duration of stirring and sonication of 3 h (S1) demonstrates the highest increment of CO2 permeability and CO2/CH4 selectivity. Compared to pure 6FDA-durene membrane, an increase in CO2 permeability of 48%, a reduction of 34% for CH4 permeability and 135% enhancement of CO2/CH4 selectivity are observed for sample S1. The increment of CO2 permeability and CO2/CH4 selectivity of S1 is also supported by the EDX-mapping and FESEM images shown in Figures 5-7, where the excellent dispersion of ZIF-8 particles in the polymer phase and the absence of interfacial gaps between the particle and polymer matrix were attained in sample S1. Besides, as revealed in Figure 9, the highest Tg value obtained for S1 membrane also supports its higher gas separation performance [57]. Moreover, the Tg also can be correlated with the fractional free volume (FFV) where the increment in the intersegmental mobility (Tg) is normally simultaneously accompanied with the enhancement of the intersegmental spacing [58]. Hence, the penetration of gas increases. However, prolong the fillers dispersion duration from 3 h to 10 h resulted in the increment of CH4 permeability from 41.92 Barrer to 65.63 Barrer,
respectively. This might be due to the presence of unselective voids and pronounced agglomeration of ZIF-8 particles in polymer phase and thus contribute to the penetration of larger molecules of CH4 to pass through the membrane matrix.
On the other hand, it is clearly shown in Figures 11 and 12 that membranes prepared at longer duration of fillers dispersion (S2 - S5) exhibit lower CO2 permeability and CO2/CH4 selectivity compared to S1. This result could be due to the presence of unselective voids occupied by the aggregates of ZIF-8 nanoparticles (Figures 5-7). In addition, the reduction of permeability and selectivity might be attributed to the formation of partial pore blockage of inorganic filler by polymer chains [7]. Even though polymer chains are difficult to clog the ZIF-8 pores directly, however the polymer chains may hinder a part of pores by sealing to the particle surfaces [48, 59]. Hence, the gas separation performance reduces.
Meanwhile, the results of pure gas permeation measurements of the MMM fabricated without priming procedure (S6 and S7) can also be observed from Figures 11 and 12. Referring to Figures 11 and 12, both samples S6 and S7 demonstrate the reduction of CO2 permeability and CO2/CH4 selectivity as compared to MMM fabricated with priming procedure (S1 and S2) at the same total duration of stirring and sonication. The reduction of gas separation performance can be explained by the formation of aggregation and agglomeration of the uncoated ZIF-8 particles in the polymer matrix, and thus allow the nonselective permeation of gases through the interstitial spaces [60]. The result obtained was in agreement with the FESEM and EDX-mapping images obtained in the earlier section. In addition, this result again confirmed that the priming procedure is important to provide an initial thin coating on the surface of filler in order to improve the compatibility of particles and polymer matrix. Therefore, in the present work it can be concluded that the shortest
duration of stirring and sonication of 3 h together with the incorporation of priming procedure is the optimum fabrication method for fabricating ZIF-8/6FDA-durene MMM for CO2/CH4 separation performance.
3.1.8 Mixed gas permeation tests and plasticization resistance
Since sample S1 demonstrated the highest pure gas permeation results among the MMMs fabricated in the present work, it was further analyzed for mixed gas permeation test and CO2 plasticization test up to 20 bar. Table 2 shows the gas permeability and selectivity of 6FDA-durene and S1 membranes. As shown in Table 2, both membranes exhibit lower gas separation performance under mixed gas condition compared to pure gas. The reduction in permeability and selectivity in mixed gas condition for both membranes is expected due to the competitive effect between the penetrants permeating through the membrane. Furthermore, the presence of larger molecule of CH4 may blocks the pore aperture of ZIF-8 and consequently inhibits the transport of CO2 through the membranes [61]. In addition, a significant deviation in permeability and selectivity of mixed gas from pure gas might be contributed from the non-ideality of gas phase and gas polarization effect [62].
Figure 13 demonstrates the CO2-induced plasticization behaviours of pure 6FDAdurene and S1 membranes. According to Figure 13, pure 6FDA-durene membrane demonstrates an increment in permeability at 5 bar due to swelling by the condensable CO2. On the other hand, S1 membrane shows plasticization resistance at a CO2 feed pressure of around 10 bar. Therefore, it can be concluded that the S1 MMM demonstrated higher resistance against CO2 plasticization due to the improved molecular interactions between ZIF-8 and polymer matrix.
CONCLUSIONS
In the present work, 5wt%-ZIF-8/6FDA-durene MMMs obtained using different fabricated parameters were characterized using different analytical tools and their performance in CO2/CH4 separation were tested. The structure and morphology of fabricated MMMs were confirmed by the characterization study using XRD, FESEM, EDX, FTIR, TGA and DSC. The separation performance of MMMs was tested in both pure and mixed gas systems including CO2-induced plasticization study. From the results, it was found that MMM fabricated using shortest fillers dispersion duration of 3 h with priming procedure demonstrated better dispersion of particle in polymer phase, absence of the interfacial gaps between ZIF-8 and polymer matrix, the highest value of glass transition temperature, Tg, as well as demonstrated highest value of CO2 permeability and CO2/CH4 selectivity among the membranes. In addition, the membrane also able to increase the CO2 plasticization resistance up to 10 bar as compared to pure 6FDA-durene membrane, with CO2 plasticization pressure of only 5 bar. In summary, the shortest fillers dispersion of 3h with the priming procedure is the optimum fabrication parameters in the present work to yield the improved separation characteristics of MMM, along with enhanced membrane properties. Therefore, a benchmark in fabricating the next generation of MMMs with enhanced characteristics can be anticipated. The successful development of this membrane material can result in a promising way towards mass production of MMM in CO2 capture which beneficial to oil-gas and energy industry.
ACKNOWLEDGEMENTS
The financial and technical supports provided by CO2 Management (MOR) research group, Universiti Teknologi PETRONAS and Ministry of Education (Higher Education Department) under MyRA Incentive Grant for CO2 Rich Natural Gas Value Chain Program are duly acknowledged.
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b
a
Im
Zn
Figure 1: a) The framework structure of ZIF-8, with tetrahedral Zn ions, bridged by imidazolate (Im) inside the cage represent the pore cavity [18] b) The chemical structure of ZIF-8 [19]
a
b
c
d
e
f
g
h
Figure 2: The physical appearance of a) 6FDA-durene membrane and ZIF-8/6FDA-durene MMMs fabricated at different parameters b) S1 c) S2 d) S3 e) S4 f) S5 g) S6 h) S7
Figure 3: XRD patterns of ZIF-8 and 6FDA-durene membrane
7.3 S7
S6
S5
S4
S3
S2
S1
Figure 4: XRD pattern of ZIF-8/6FDA-durene mixed matrix membranes
a
b
c
d
e
f
g
h
Figure 5: FESEM images of a) ZIF-8 and ZIF-8/6FDA-durene MMMs b) S1 c) S2 d) S3 e) S4 f) S5 g) S6 h) S7
a
b
c
d
.
e
f
g
Figure 6: Cross-section images of ZIF-8/6FDA-durene MMMs at magnification of 100K a) S1 b) S2 c) S3 d) S4 e) S5 f) S6 g) S7 a
c
b
e
f
d
g
Figure 7: EDX mapping for Zn from the cross-section images of ZIF-8/6FDA-durene MMMs a) S1 b) S2 c) S3 d) S4 e) S5 f) S6 g) S7
Figure 8: TGA curve of ZIF-8 and membranes fabricated in the present work (S1-S7)
Figure 9: DSC curve of the membranes obtained in the present work
Figure 10: ATR-FTIR analysis of ZIF-8 and the membranes
Figure 11: CO2 and CH4 permeability of the membranes
S2
S3
S4
S5
S6
S7
Figure 12: CO2/CH4 selectivity of the membranes
Figure 13: CO2 plasticization behavior of pure 6FDA-durene and ZIF-8/6FDA-durene membrane (S1)
Table 1: ZIF-8/6FDA-durene membranes fabricated in the present work by using different fabrication parameters
Sample
Fillers dispersion duration
label
ZIF-8 in DCM
ZIF-8 in 6FDA-durene solution
Total
Priming
Stirring (h)
Sonication (h)
Stirring (h)
Sonication (h)
duration (h)
S1
0.5
0.5
1
1
3
Yes
S2
1
1
2
2
6
Yes
S3
1
1
3
3
8
Yes
S4
2
2
2
2
8
Yes
S5
2
2
3
3
10
Yes
S6
0.5
0.5
1
1
3
No
S7
1
1
2
2
6
No
All dope solution is subjected to the vigorous stirring for 1 h before cast into a Petri dish
Table 2: Pure and mixed gas (50 CO2: 50 CH4) permeation properties of 6FDA-durene and S1 membranes at pressure of 3.5 bar and temperature of 30oC
Membrane
Pure gas permeability
CO2/CH4 ideal
Mixed gas
CO2/CH4
(Barrer)
selectivity
permeability (Barrer)
selectivity
CO2
CH4
6FDA-durene
468.01
66.57
MMM (S1)
693.54
41.92
CO2
CH4
7.03
283.57
62.60
4.53
16.54
320.50
42.10
7.61
41
S1226-086X(16)30306-9 http://dx.doi.org/doi:10.1016/j.jiec.2016.08.030 JIEC 3065
To appear in: Received date: Revised date: Accepted date:
23-6-2016 25-8-2016 26-8-2016
Please cite this article as: Norwahyu Jusoh, Yin Fong Yeong, Cheong Weng Leong, Kok Keong Lau, Azmi M.Shariff, Facile Fabrication of Mixed Matrix Membranes Containing 6FDA-Durene Polyimide and ZIF-8 Nanofillers for CO2 Capture, Journal of Industrial and Engineering Chemistry http://dx.doi.org/10.1016/j.jiec.2016.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile Fabrication of Mixed Matrix Membranes Containing 6FDA-Durene Polyimide and ZIF-8 Nanofillers for CO2 Capture
Norwahyu Jusoh, Yin Fong Yeong*, Cheong Weng Leong, Kok Keong Lau and Azmi M. Shariff Chemical Engineering Department, Universiti Teknologi PETRONAS 32610 Bandar Seri Iskandar, Perak, Malaysia. *Corresponding Author: Yeong Yin Fong, Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. Tel: +605-3687564, Fax: +605-3656176, E-mail: [email protected]
Graphical abstract
Abstract: Membrane separation has been used successfully in numbers of industrial applications especially in CO2 removal from CH4 because it’s involved less energy consumption and low maintenance. The aim of this research is to fabricate ZIF-8/6FDAdurene mixed matrix membranes for enhancement of CO2 capture and consequently to optimize its fabrication method. The results showed that membrane fabricated using total dispersion duration of 3 hours with filler priming procedure demonstrated homogenous distribution of ZIF-8 in 6FDA-durene matrix and improved the separation performance. Therefore, the membrane is potential for the large scale production via the method optimized in this work.
Keywords: Optimization, ZIF-8, 6FDA-durene, CO2 Capture
1.0 INTRODUCTION
Recently, the concerns pertaining natural gas purification have been rising significantly mainly due to the presence of high CO2 content and impurities in the reservoirs. In Southeast Asia, the gas reservoirs that contain CO2 up to 76 mol% have been discovered in Natuna Field, Indonesia [1]. In Malaysia, over 13 trillion cubic feet of natural gas reserves are undeveloped due to high CO2 content up to 87% [2]. At present, natural gas wells with high CO2 contents are undeveloped due to the difficulties in the operation, production and technical activity. Thus, an efficient separation process should be developed to maintain a continuous supply of natural gas demand. The CO2 capture from natural gas can be realized using a number of established technologies in industrial applications, including absorption, adsorption, cryogenic and membrane separations. Among these technologies, membrane is
preferable because it requires small space and weight, low labor intensity and maintenance as well as minimum utility requirements [3].
Membrane separation technology has been continued to progress rapidly due to its economic and environmental advantages over the other separation technologies. Gas separation membranes can be generally classified into three categories; polymeric, inorganic and mixed matrix membranes (MMMs) [4]. Polymeric membranes have been widely utilized in industrial application because it is relatively cheap, flexible and easy to fabricate. However, the separation performance of pure polymeric membrane is suffer from the upper bound limit, where high permeability and high selectivity cannot be simultaneously attained [5]. On the other hand, inorganic membrane offers higher permeability and selectivity with great chemical and thermal stability. However, it is limited by the fabrication cost and mechanical strength of the membrane material [6].
In order to overcome the above aforementioned drawbacks, MMM has been introduced by incorporating fillers into the polymer phase [7]. The separation performance of MMM may surpassed the upper bound line and consequently can be applied for industrial application by combination of fillers and polymer materials. Polymers commonly used for MMM fabrication include cellulose acetate, polycarbonate, polysulfone and polyimides. Polyimide is ultimately used for CO2/CH4 separation compared to other type of polymers due to their good separation performance, high chemical and mechanical stability [8]. Current trend shows that fluorinated polyimides containing 6FDA is attractive for CO2/CH4 separation because of the presence of –C(CF3)2- and bulky methyl group which contributes to the restriction of intrasegmental mobility, interruption of the interchain packing and consequently, resulted in higher gas permeability and gas pair selectivity [9].
The beginning of MMM era have been concentrated on the incorporation of traditional fillers such as zeolites [10-12] and silicas [13, 14] into the commercial type of polymer phase. Notably, the recent years has focused on the incorporation of metal organic frameworks (MOFs) and a subclass of MOF, zeolitic imidazolate frameworks (ZIFs) as the filler in the polymer matrix for CO2 capture [15]. The presence of imidazolate linker in their framework could increase the interfacial properties between filler and polymer matrix and consequently enhance the compatibility between these two materials [16].
Among ZIF materials, zeolitic imidazolate frameworks-8 (ZIF-8) has emerged as an attractive filler for the fabrication of MMM for CO2/CH4 separation. ZIF-8 is built up from zinc (II) cations and 2-methylimidazole anions, giving a sodalite (SOD) zeolite type structure with two times larger of pore size compared to SOD zeolites [17]. Besides, ZIF-8 consists of six-ring β-cages with aperture pore size of 3.4Å. The framework and chemical structure of ZIF-8 is presented in Figure 1, with tetrahedral Zn ions, bridged by imidazolate (Im) inside the cage to represent the pore cavity [18, 19]. The characteristics of ZIF-8 such as high surface area, highly porous open framework structure and great chemical and thermal stability, make it an attractive candidate for gaseous separation, especially for CO2/CH4 separation [20]. Representative examples include the works reported by Bushell et al [21], whose fabricated ZIF-8/PIM-1MMM for CO2, CH4, O2, N2, He and H2 pure gas permeation studies, while later Nordin et al. [22] further investigated the permeation studies of CO2/CH4 and CO2/N2 binary gas mixtures using ZIF-8/polysulfone membranes. Both studies have demonstrated great implications to MMM development, where their findings showed that the incorporation of ZIFs or MOFs in polymer phase enhanced gas separation performance especially for CO2 capture from CH4.
However, most of the studies reported on MMMs were focused solely on the separation performance, concerning with the improvement of permeability and selectivity of the studied MMMs [23-25]. In contrast, the studies in finding the established approaches and the standard techniques for the formation of MMM with minimum defects through the optimization of the fabrication parameters are remained scarce. A great effort in investigating these fabrication parameters in details could contribute a significant impact on the production of high quality MMMs. The ideal MMMs normally were hindered by poor compatibility, phase separation and poor adhesion between the filler and polymer phase, and thus, worsen the thermophysical properties of membrane and caused severe reduction of gas separation performance [26-28]. Furthermore, the fabrication of defect-free MMM is normally requires a strict processing conditions, in which the techniques to prepare MMM is relatively time consuming, cumbersome, and often not economically viable.
Therefore, the present work demonstrates in-depth study to elucidate the basic parameters for fabricating MMM. The effects of dispersion duration of fillers, including stirring and sonication as well as priming and non-priming of fillers procedures were investigated.
The properties of the resultant membranes were characterized by various
analytical tools. In addition, permeation properties of the resultant MMMs in CO2/CH4 separation were studied in order to investigate the effect of fabrication parameters towards the separation performance.
2.0 EXPERIMENTAL METHOD
2.1 Materials Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, > 98% purity, Sigma Aldrich) and 2methylimidazole (Hmim, 98% purity, Sigma Aldrich), methanol (99.8% purity, Merck) were used without further purification. 4, 4’ – (Hexafluoroisopropylidene) diphthalic anhydride
(6FDA, 99% purity, Sigma Aldrich) monomers were purified by vacuum sublimation prior to use. 2, 3, 5, 6- Tetramethyl-p-phenylenediamine (durene diamine, 99% trace metal basis, Sigma Aldrich) monomers were purified by using re-crystallization method in methanol. Nmethyl-2-pyrrolidone (NMP) was purified through vacuum distillation. Propionic anhydride (PA, ≥ 98% purity, Merck), triethylamine (TEA, ≥ 99% purity, Merck), methanol (≥ 99.9% purity, Merck) and dichloromethane (DCM, ≥ 99.8% purity, Sigma Aldrich) solvent were used as received. Gases for permeation experiments, such as carbon dioxide (CO2, 99.99%) and methane (CH4, 99.99%) were purchased from Gas Walker Sdn Bhd and used as received.
2.2 Synthesis of ZIF-8 nanocrystals
ZIF-8 nanofiller was synthesized at room temperature by mixing zinc nitrate hexahydrate (Zn (NO3)2.6H2O) as zinc source and 2-methylimidazole (2-MeIM) as organic ligand in methanol (MeOH) with molar composition of 1 Zn2+: 8 Hmim: 1002 MeOH under stirring for 1 h [29]. The mixture was slowly turned into a cloudy solution and then nanocrystals were separated from the solution by centrifuging at 7800 rpm for 5 minutes. The solid was then washed with methanol for several times and dried in the oven at 60 oC for 24 h prior to use.
2.3 Preparation of 6FDA-durene Polymer
6FDA-durene polymer was synthesized using chemical imidization method reported by Liu et al. [30]. Equal mole of durene-diamine and 6FDA monomers were dissolved in purified NMP. The mixture was stirred for 24 h under nitrogen purge to obtain polyamic acid (PAA) solution. The mole ratio of propionic anhydride (PA) / triethylamine (TEA) to 6FDA of 4:1 were added to the PAA solution for chemical imidization to form polyimide. The polyimide was precipitated in methanol and then washed with methanol for 3 times. The
resultant polymer was dried at 80°C in vacuum oven for 24 h and stored in a desiccator prior to use.
2.4 Fabrication of Pure and ZIF-8/6FDA-durene Mixed Matrix Membrane
6FDA-durene flat sheet membrane was prepared by solvent evaporation method. A 2% w/v solution of polymer was dissolved in DCM before filtered using a 1.0 μm PTFE membrane filter and cast on a Petri dish. The cast film was dried in an oven at 60°C for 24 h followed by another 24 h under vacuum. The oven temperature was then increased from 60°C to 250°C at a heating of 25°C/h before annealed at 250°C for 24 h. On the other hand, MMMs contained 5 wt% of ZIF-8 in 6FDA-durene polymer phase was fabricated using solution blending method by manipulating the fabrication parameters including dispersion of ZIF-8 in DCM under different stirring and sonication durations and priming or non-priming of fillers. First, both solutions of polymer and ZIF-8 were prepared in two separate vials. 5 wt% of ZIF-8 crystals were added into DCM, stirred and sonicated alternately to disperse ZIF-8 in DCM solution. At the same time, 6FDA-durene polymer was stirred in DCM until dissolved and the solution is filtered in order to remove insoluble particles such as dust and impurities. After that, sonication was done in an ultrasonication water bath operating at 120 W and 40 kHz. Then, the ZIF-8 particles were primed by adding 10 wt% of 6FDA-durene polymer solution into the ZIF-8 solution, which it was further stirred and sonicated. After thorough mixing, the remaining bulk polymer solution was added and the mixture was again further stirred and sonicated. The mixture was then stirred vigorously for 1 h before cast into a Petri dish on a leveled glass plate. The resulting membrane was immediately covered with a clean glass plate for slow solvent evaporation in ambient air for overnight. The membrane was then peeled off and dried in an oven at 60°C for 24 h followed by another 24 h under vacuum. Oven temperature was then increased from 60 to 250°C at a heating rate of 25°C/h.
The nascent film was annealed at 250°C for 24 h followed by natural cooled down. Table 1 summarized the MMMs fabricated in the present work and total seven membrane samples were prepared by manipulating the fabrication parameters.
2.5 Characterization of ZIF-8 Particle and Membranes
The crystal structure of the ZIF-8 particle was examined using X-ray Diffraction (XRD), Bruker (USA) D8 Advance diffractometer equipped with CuKα radiation (λ=1.54059Å). The XRD pattern was obtained in 2 theta range of 5-28o at a step size of 0.02 o
. The morphology and crystal size of the ZIF-8 particles were identified using Hitachi
SU8000 field emission scanning electron microscope (FESEM). ZIF-8 particle was placed on top of the FESEM surface holder by using a double-sided carbon tape before analysis. Perkin Elmer Pyris 1 Thermogravimetric analysis (TGA) was used to determine the thermal stability and decomposition temperature of ZIF-8 particles. Samples weighing an average of 15-20 mg were loaded into an alumina pan followed by thermal scanning under air environment in a range of 50°C to 800°C, at heating rate of 10°C min-1. Chemical functionalities of ZIF-8 particle were measured by using Fourier Transform Infra-Red (FTIR), Perkin Elmer One spectrometer through transmittance method. The ZIF-8 particle in powder form was blended with KBr and compressed into the pellet. The analysis was then conducted in a transmittance mode from 650 to 4000 cm-1 using 50 scans at room temperature.
The resultant membranes were characterized by XRD, FESEM, EDX, TGA, DSC and ATR-FTIR. The membranes samples were cut and used for XRD analysis using the same parameters as previously described for ZIF-8 particle analysis. Furthermore, the images of the cross section and the elemental compositions of the resultant membranes were obtained by using FESEM and EDX, respectively. Cross sections of the membranes were prepared by freeze-fracture after immersion in liquid nitrogen for several minutes. Prior to imaging, the
membrane samples were coated with platinum target using Quorum Q150R S sputter coater under vacuum condition for 60s to prevent charging of the sample which could occur during imaging analysis. The elemental compositions of the MMMs and the distribution of the fillers in MMMs were verified using Bruker XFlash Detector 6/30 energy dispersion of X-ray (EDX).
The thermal stability of the membranes was determined by using TGA as previously described in ZIF-8 particle analysis. The measurement was conducted with heating ramp of 10°C rise per minute from 50°C to 800°C under air atmosphere, using 15 mg of sample. The differential scanning calorimetry (DSC, Q2000) was used to evaluate the glass transition temperature (Tg) of membrane samples. The membrane samples were heated under nitrogen environment to 450oC at a heating rate of 10°C/ min during the first cycle in order to remove the thermal history of membrane. Then, the membrane samples were quenched at a rate of 10°C/ min, and reheated at 10°C/ min to complete the second cycle of heating. The Tg of samples were determined from the second cycle. The functional groups in the MMMs were investigated using FTIR spectroscopy operated in the attenuated total reflection mode (ATR) equipped with a diamond crystal. The membrane films were scanned for 50 times for each sample and the scanning was from 650 to 4000 cm-1 at room temperature.
2.6 Gas Permeation Measurements
Single gas permeation and mixed gas separation performance of the membranes were conducted using custom-built gas permeation test rig. The detailed explanation on the testing setup and procedure can be found elsewhere [31]. For each run, at least 2 films, 1 piece from 2 separately cast membranes with the same fabrication parameters, were investigated for the reproducibility test. The permeability of pure CO2 and CH4 gases were measured at 30°C and 3.5 bar and calculated based on the equation (1) as follows [32]:
PA
Vpt
Am ph pl
(1)
Where PA is the permeability of membrane (Barrer), Vp is the permeate flow rate (cm3(STP)/s), t is the thickness of membrane (cm), Am is the membrane area (cm2), ph and pl are the pressure in feed side and permeate side, respectively (cmHg), subscript A is representing CO2 or CH4. The permeability of the membranes is reported in the unit of Barrer (1 Barrer =1×10-10cm3(STP).cm/s.cm2.cmHg).
The ideal selectivity of the membrane can be obtained by dividing permeability of CO2 over permeability of CH4 as shown in equation (2) as follows:
CO
2 / CH 4
PCO2 PCH4
(2)
Where α indicates the ideal selectivity of CO2/CH4 and P is the permeability (Barrer).
Meanwhile, the plasticization behavior of resultant MMMs were studied by intermittently ramped CO2 pure gas pressure from 3.5 to 20 bar. Since the pressure was ramped to the elevated pressure, non-ideal gas condition was considered. The pressure difference across the membrane as described in the equation (1) was changed to the fugacity as the permeation driving force. The fugacity can be calculated using a non-ideal equation of state (EOS), Peng-Robinson EOS where the detailed calculation on fugacity can be found elsewhere [33, 34]. The permeability of CO2 can be determined by the following equation:
PCO2
Vpt
Am h l
where φ is the fugacity of CO2 in the feed stream.
(3)
In addition, mixed gas permeation of MMMs were conducted using 50% CO2 and 50% CH4 binary mixtures at pressure of 3.5 bar and 30°C of temperature. The permeability of components in gas mixture was calculated based on the equations (4-5) as follows:
PCO2
PCH4
V p yCO2 t
Am ph xCO2 p1 yCO2
V p yCH4 t
Am ph xCH4 p1 yCH4
(4)
(5)
Where PCO2 and PCH4 is the permeability of CO2 and CH4, respectively, and x and y refer to the mole fraction of component in feed and retentate side, respectively.
The selectivity for mixed gas measurement can be determined based on equation (6) as follows:
CO
2 / CH 4
yCO2 / yCH4 xCO / xCH 2 4
(6)
3.0 RESULTS AND DISCUSSION
3.1 Characterization of pure 6FDA-durene and mixed matrix membranes
3.1.1 Physical appearance of the membranes
The physical appearance of the pure 6FDA-durene membrane and 5 wt% ZIF8/6FDA-durene MMMs (S1-S7) fabricated using different fabrication parameters are shown in Figure 2. It can be seen from Figure 2 that all the resultant MMMs turn to slightly blur compared to the transparent pure 6FDA-durene membrane. This may be attributed to the dispersion of ZIF-8 particles in polymer matrix and this phenomenon has also been reported
for the other MMMs loaded with different combination of polymer and fillers [35]. However, all the resultant MMMs are still able to bend, flexible, resilient and non-brittle even though they were fabricated using different fabrication parameters.
3.1.2 X-Ray Diffraction (XRD) The XRD patterns for ZIF-8 particles and membranes at 2θ range of 5° to 30° are shown in Figures 3 and 4, respectively. Referring to Figure 3, the XRD pattern for ZIF-8 sample matches with the reported XRD pattern of ZIF-8 reported by Cravillon et al. [36] with the peaks at 2θ = 7.30o, 10.36o, 12.68o, 16.40o and 17.98o. This result confirmed the formation of crystalline ZIF-8 phase. A sharp peak at 2θ of 7.30º was observed in the XRD pattern of the ZIF-8, signifying that a highly crystalline material is successfully obtained. Based on Figure 3 also, the XRD pattern of pure 6FDA-durene demonstrates a broad peak from 8° to 22°, implies the amorphous structure of polymer phase. This observation is correlated well with the reported XRD pattern for 6FDA-durene reported in the literature [16, 37].
On the other hand, the XRD patterns of all MMMs shown in Figure 4 are matched well with the XRD pattern of ZIF-8/6FDA-durene MMM reported in the literature [16]. It was observed that all the membrane samples demonstrate peaks at 2θ values of 7.3°, matching with the characteristic peaks of ZIF-8 crystal. However, the peaks of ZIF-8 crystals at 10.36o, 12.68o, 16.40o and 17.98o are not well pronounced because it is masked by the amorphous phase of 6FDA-durene polymer [38]. Thus, the diffraction patterns of ZIF-8/ 6FDA-durene MMMs verified the presence of ZIF-8 structures and 6FDA-durene phase in the resultant MMM via the intense crystalline peak and the amorphous halo, respectively.
3.1.3 Field Emission Scanning Electron Microscopy (FESEM)
Figures 5 and 6 show the FESEM images of ZIF-8 particles and the resultant MMMs. Meanwhile, Figure 7 displays the EDX mapping of the resultant membranes. Since the ZIF-8 nanoparticles is comprises of zinc element only, the EDX analysis was applied by mapping the presence of Zn element in the membrane in order to further confirm the distribution of ZIF-8 particles in polymer phase.
Based on Figure 5, well-shaped of ZIF-8 with the average particle size of 50 nm is observed. In addition, the FESEM images of all the MMM samples show that the ZIF-8 crystals are encapsulated in 6FDA-durene polymer phase. Referring to the FESEM images of S1 membrane, it is found that the ZIF-8 nanoparticles are well distributed in the polymer phase with the absence of interfacial gaps between the particle and polymer phase. This indicates that a good compatibility between ZIF-8 and 6FDA-durene phase has been achieved. A good dispersion of ZIF-8 particles in the membrane matrix was further confirmed by the uniform dispersion of Zn element in EDX mapping shown in Figure 7(a). However, as the filler dispersion duration increases from 3 h to 6 h (S2), the FESEM images of S2 membrane show the presence of sub-micron particles of ZIF-8 due to the aggregation of ZIF-8 nanoparticles in the polymer phase. In addition, formation of void space between ZIF-8 particles and polymer phase can be observed from the FESEM images. Despite the unselective voids are formed with the presence of sub-micron aggregates in the membrane, it can be seen that the ZIF-8 particles are still homogenously distributed within the 6FDAdurene polymer matrix, as verified in EDX mapping displays in Figure 7(b). The aggregation of nanosized particles to the submicron particles observed in S2 is expected due to the ripening effect which induced by the sonication during membrane fabrication. During sonication process, the acoustic cavitation of liquid which create high pressure and
temperature in the liquid encourages the dissolution of smaller particles and grow to the larger particles [39].
Furthermore, prolonging the total duration of stirring and sonication to 8 h (S3 and S4 membranes) increases the aggregation of ZIF-8 particles up to several sub-microns in size with the evidence of minor agglomeration in polymer matrix. In fact, this result is reasonable since the increase in the stirring and sonication durations generally increases the formation of larger particles due to the ripening effect. Although both of the S3 and S4 were prepared at the same total duration of fillers dispersion, however, S3 was subjected to 2 hours dispersion of ZIF-8 in DCM and 6 hours dispersion of ZIF-8 in 6FDA-durene polymer, whereas S4 was subjected to the 4 hours dispersion of ZIF-8 in DCM and 4 hours dispersion of ZIF-8 in polymer. According to Figures 5 and 6, it can be seen that prolonging the duration of stirring and sonication of ZIF-8 in DCM resulted in larger aggregation of ZIF-8 particles in S4 as compared to the aggregation of ZIF-8 particles in S3. This result might be due to the lower kinetic barrier to Ostwald ripening of ZIF-8 in DCM as compared to the kinetic barrier to Ostwald ripening in the polymer solution [40, 41]. Therefore, enhance the duration of sonication of ZIF-8 in DCM contributes to the larger particles. Meanwhile, referring to the EDX mapping in Figure 7(c) and (d), it can be seen that some of the ZIF-8 particles are migrate and cluster at the surface of S3, whereas a minor agglomeration of ZIF-8 particles in polymer phase can be observed in S4. These results indicate that relatively poor particles distributions of ZIF-8 particles in 6FDA-durene polymer phase were found for S3 and S4.
However, as the duration of fillers dispersion increases to 10 h (S5), obvious agglomeration of the particles in 6FDA-durene polymer phase with the presence of unselective voids and pinholes can be observed in the membrane. This result indicates that the formation of heterogeneity ZIF-8 particles is found in the polymer matrix, which could
consequently create the paths for unselective transport of gases. A poor dispersion of ZIF-8 particles in polymer matrix is further confirmed in Figure 7(e) by EDX-mapping image. Based on the results obtained, the shortest fillers dispersion duration of 3 h produces better homogeneity and blending of ZIF-8 in polymer phase, as compared to the longer stirring and sonication duration.
Besides, Figures 5 and 6 also displayed the FESEM images of the MMMs prepared without using priming procedure at fillers dispersion duration of 3 h (S6) and 6 h (S7). Both samples of S6 and S7 demonstrate poor dispersion of ZIF-8 particles in 6FDA-durene polymer matrix as compared to the samples fabricated using priming procedure. The FESEM images of sample S6 shows the presence of submicron particles with minor agglomeration as compared to sample S1 when subjected to the same total stirring and sonication durations. The EDX-mapping in Figure 7(f) further confirmed the poor dispersion of ZIF-8 particles in the polymer matrix. On the other hand, sample S7 shows severe agglomeration with unselective voids as compared to sample S2. It can be clearly seen from Figure 7(g) that the ZIF-8 particles were heterogeneously dispersed in the polymer phase. This result indicates that the priming procedure is important to assist the distribution of ZIF-8 particle by coating a thin layer of polymer to the surface of ZIF-8 and consequently preventing the formation of ZIF-8 agglomerates that would appear in the resulting membrane [42]. Therefore, in the present work, the optimum mixing duration of ZIF-8 particles in 6FDA-durene polymer phase is 3 h, by including the priming procedure.
3.1.4 Thermogravimetric Analysis (TGA)
The TGA results of ZIF-8 particles and resultant membranes are shown in Figure 8. The degradation temperature of the samples was determined at the 5% weight loss [43]. Based on Figure 8, minor weight loss of ~4wt% from 100°C to 400°C is found in the ZIF-8
crystals TGA curve, which may be contributed from the elimination of residual solvents or moisture molecules trapped in the ZIF-8 [22]. In addition, the framework decomposition of ZIF-8 crystal is occurred in the range of 400°C and 450°C, which is in agreement with the reported literature data [22, 44]. On the other hand, pure 6FDA-durene membrane demonstrates thermal stability up to 500°C, which is in agreement with the thermal decomposition of 6FDA-durene reported by Lin et al [45]. The decomposition of pure 6FDAdurene comprises the first degradation of polymer chain with the elimination of CO2, CH4, CO, and H2 gases and the complete degradation of polymer matrix including carbon and nitrogen [46].
Referring to Figure 8, all the MMMs (S1-S7) demonstrate thermal stability up to ~500°C under air atmosphere. The decomposition of MMMs is contributed to the ZIF-8 nanoparticles framework decomposition and 6FDA-durene matrix. Since the decomposition temperature of ZIF-8 and 6FDA-durene is close to each other, there is no obvious stage of weight loss can be observed. A similar behavior of thermal decomposition temperature of ZIF-8/6FDA-durene MMM was reported by Wijenayake et al.[47]. Furthermore, all the fabricated MMMs (S1-S7) exhibit insignificant changes in the decomposition temperature (~518 - 520°C). This result shows that prolonging the duration of fillers dispersion from 3 h to 10 h with and without priming procedure did not affect the thermal stability of membrane. This is might be due to the similar loading of ZIF-8 nanofillers (5 wt%) incorporated in 6FDA-durene polymer for all the membranes.
3.1.5 Differential Scanning Calorimetry (DSC) Analysis
The DSC results and glass transition temperatures (Tg) of all the membranes are presented in Figure 9. Referring to Figure 9, the Tg of pure 6FDA-durene membrane is 410ºC.
Meanwhile, all MMMs demonstrate higher Tg value of up to 423.50°C as compared to the pure membrane, demonstrating that the presence of ZIF-8 fillers has changed the polymer packing structure of the membrane [16]. As the total duration of stirring and sonication increases, the value of Tg decrease from 423.50°C to 414.96°C (S1-S5), indicating that the rigidity and the pore volume of the membranes are affected [48]. Same results were also obtained for MMMs without the priming procedure where S6 and S7 membranes demonstrated lower Tg as compared to MMM fabricated via priming procedure, indicating that the polymer chain rigidification is reduced.
3.1.6 Attenuated Total Reflection Fourier Transform Infra-Red (ATR-FTIR)
The functional groups presence in ZIF-8 and the resultant membranes are shown in Figure 10. Based on Figure 10, ZIF-8 particles comprises aromatic C-H stretch and aliphatic C-H stretch of the imidazole at 3130 and 2915 cm-1, respectively. The absorption bands at 1580 cm-1 are detected for the C=N stretch. The C-N bonds are characterized by peaks in the region of 1145-1400 cm-1. The characteristic peaks of ZIF-8 sample are consistent with those IR peaks reported in the literature for ZIF-8 structure [49]. Referring to Figure 10, 6FDAdurene membrane exhibits peaks at 1785, 1715, 1352, 1250 and 718 cm-1. The bands at 1715 cm-1 and 1785 cm-1 are attributed to the C=O symmetric and asymmetric stretch of imide group, respectively. The peak at 1352 cm-1 and 1250 cm-1 are assigned to the imide group CN stretch and C-F stretch in CF3 group, respectively. The peak at 718 cm-1 is attributed to the formation of the imide ring or the imide carbonyl group. The characteristic peaks of 6FDAdurene membrane are consistent with those IR peaks reported in the literature for 6FDAdurene structure [30, 45, 50].
Meanwhile, the spectra of MMMs demonstrate the characteristics of both ZIF-8 and 6FDA-durene with no obvious shifts in absorption bands when compared with ZIF-8
nanoparticles and pure 6FDA-durene membrane. This result indicates that a strong chemical interaction between functional group of polymer and ZIF8 particles was not occurred in the resultant MMMs.
3.1.7 Pure gas permeation tests
Figures 11 and 12 show the CO2 and CH4 gas permeabilities and CO2/CH4 selectivity of all the membranes fabricated in the present work. The gas separation performance of the pure 6FDA-durene membrane obtained in this study is lower than those results reported in the literature [16, 51, 52]. This could be due to solvent used in this work for the membrane fabrication.
Different
types
of
solvents
including
dichloromethane
(DCM),
dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and chloroform (CHCl3) have been reported in the literature for the fabrication of 6FDA-durene membrane [16, 37]. These solvents show dissimilar polarity, dielectric constant and dipole moment which can consequently change the properties of membrane and thus, affect the gas separation performance of the resultant membranes [53, 54]. In addition, the lower Tg obtained in this study as compared to the reported data support the lower separation performance observed in this study.
Referring to Figure 11, all the resultant MMMs demonstrate higher CO2 permeability and lower CH4 permeability as compared to pure membrane. The MMMs show CO2 permeability of 693.54 Barrer, 541.24 Barrer, 515.72 Barrer, 520.53 Barrer, 473.54 Barrer, 501.44 Barrer and 489.35 Barrer for S1, S2, S3, S4, S5, S6 and S7, respectively, whereas , for pure membrane, CO2 permeability of 468.01 Barrer is obtained. Besides, based on Figure 12, all the MMMs demonstrate higher CO2/CH4 selectivity as compared to pure membrane, with CO2/CH4 selectivity of only 7.03.
The increment of gas performance of MMM is contributed to the addition of nanoparticles of ZIF-8 in the polymer phase which has interrupted the polymer chain packing and consequently increase the free volume and the diffusion pathways for gas penetration [55]. Furthermore, the combined effects of the pore aperture sizes of ZIF-8 that able to separate smaller gas molecule, CO2 (3.3 Å) from CH4 (3.8 Å) through the interior cavities of ZIF-8 and strong quadrupolar interaction of CO2 with the imidazolate linker in the ZIF-8 particles framework facilitates the transport of gases [56] [22].
In addition, it can be seen from Figures 11 and 12 that the increment of duration of filler dispersion from 3 h (S1) to 10 h (S5) decreases CO2 permeability from 693.54 Barrer to 473.54 Barrer, respectively. On the other hand, the shortest duration of stirring and sonication of 3 h (S1) demonstrates the highest increment of CO2 permeability and CO2/CH4 selectivity. Compared to pure 6FDA-durene membrane, an increase in CO2 permeability of 48%, a reduction of 34% for CH4 permeability and 135% enhancement of CO2/CH4 selectivity are observed for sample S1. The increment of CO2 permeability and CO2/CH4 selectivity of S1 is also supported by the EDX-mapping and FESEM images shown in Figures 5-7, where the excellent dispersion of ZIF-8 particles in the polymer phase and the absence of interfacial gaps between the particle and polymer matrix were attained in sample S1. Besides, as revealed in Figure 9, the highest Tg value obtained for S1 membrane also supports its higher gas separation performance [57]. Moreover, the Tg also can be correlated with the fractional free volume (FFV) where the increment in the intersegmental mobility (Tg) is normally simultaneously accompanied with the enhancement of the intersegmental spacing [58]. Hence, the penetration of gas increases. However, prolong the fillers dispersion duration from 3 h to 10 h resulted in the increment of CH4 permeability from 41.92 Barrer to 65.63 Barrer,
respectively. This might be due to the presence of unselective voids and pronounced agglomeration of ZIF-8 particles in polymer phase and thus contribute to the penetration of larger molecules of CH4 to pass through the membrane matrix.
On the other hand, it is clearly shown in Figures 11 and 12 that membranes prepared at longer duration of fillers dispersion (S2 - S5) exhibit lower CO2 permeability and CO2/CH4 selectivity compared to S1. This result could be due to the presence of unselective voids occupied by the aggregates of ZIF-8 nanoparticles (Figures 5-7). In addition, the reduction of permeability and selectivity might be attributed to the formation of partial pore blockage of inorganic filler by polymer chains [7]. Even though polymer chains are difficult to clog the ZIF-8 pores directly, however the polymer chains may hinder a part of pores by sealing to the particle surfaces [48, 59]. Hence, the gas separation performance reduces.
Meanwhile, the results of pure gas permeation measurements of the MMM fabricated without priming procedure (S6 and S7) can also be observed from Figures 11 and 12. Referring to Figures 11 and 12, both samples S6 and S7 demonstrate the reduction of CO2 permeability and CO2/CH4 selectivity as compared to MMM fabricated with priming procedure (S1 and S2) at the same total duration of stirring and sonication. The reduction of gas separation performance can be explained by the formation of aggregation and agglomeration of the uncoated ZIF-8 particles in the polymer matrix, and thus allow the nonselective permeation of gases through the interstitial spaces [60]. The result obtained was in agreement with the FESEM and EDX-mapping images obtained in the earlier section. In addition, this result again confirmed that the priming procedure is important to provide an initial thin coating on the surface of filler in order to improve the compatibility of particles and polymer matrix. Therefore, in the present work it can be concluded that the shortest
duration of stirring and sonication of 3 h together with the incorporation of priming procedure is the optimum fabrication method for fabricating ZIF-8/6FDA-durene MMM for CO2/CH4 separation performance.
3.1.8 Mixed gas permeation tests and plasticization resistance
Since sample S1 demonstrated the highest pure gas permeation results among the MMMs fabricated in the present work, it was further analyzed for mixed gas permeation test and CO2 plasticization test up to 20 bar. Table 2 shows the gas permeability and selectivity of 6FDA-durene and S1 membranes. As shown in Table 2, both membranes exhibit lower gas separation performance under mixed gas condition compared to pure gas. The reduction in permeability and selectivity in mixed gas condition for both membranes is expected due to the competitive effect between the penetrants permeating through the membrane. Furthermore, the presence of larger molecule of CH4 may blocks the pore aperture of ZIF-8 and consequently inhibits the transport of CO2 through the membranes [61]. In addition, a significant deviation in permeability and selectivity of mixed gas from pure gas might be contributed from the non-ideality of gas phase and gas polarization effect [62].
Figure 13 demonstrates the CO2-induced plasticization behaviours of pure 6FDAdurene and S1 membranes. According to Figure 13, pure 6FDA-durene membrane demonstrates an increment in permeability at 5 bar due to swelling by the condensable CO2. On the other hand, S1 membrane shows plasticization resistance at a CO2 feed pressure of around 10 bar. Therefore, it can be concluded that the S1 MMM demonstrated higher resistance against CO2 plasticization due to the improved molecular interactions between ZIF-8 and polymer matrix.
CONCLUSIONS
In the present work, 5wt%-ZIF-8/6FDA-durene MMMs obtained using different fabricated parameters were characterized using different analytical tools and their performance in CO2/CH4 separation were tested. The structure and morphology of fabricated MMMs were confirmed by the characterization study using XRD, FESEM, EDX, FTIR, TGA and DSC. The separation performance of MMMs was tested in both pure and mixed gas systems including CO2-induced plasticization study. From the results, it was found that MMM fabricated using shortest fillers dispersion duration of 3 h with priming procedure demonstrated better dispersion of particle in polymer phase, absence of the interfacial gaps between ZIF-8 and polymer matrix, the highest value of glass transition temperature, Tg, as well as demonstrated highest value of CO2 permeability and CO2/CH4 selectivity among the membranes. In addition, the membrane also able to increase the CO2 plasticization resistance up to 10 bar as compared to pure 6FDA-durene membrane, with CO2 plasticization pressure of only 5 bar. In summary, the shortest fillers dispersion of 3h with the priming procedure is the optimum fabrication parameters in the present work to yield the improved separation characteristics of MMM, along with enhanced membrane properties. Therefore, a benchmark in fabricating the next generation of MMMs with enhanced characteristics can be anticipated. The successful development of this membrane material can result in a promising way towards mass production of MMM in CO2 capture which beneficial to oil-gas and energy industry.
ACKNOWLEDGEMENTS
The financial and technical supports provided by CO2 Management (MOR) research group, Universiti Teknologi PETRONAS and Ministry of Education (Higher Education Department) under MyRA Incentive Grant for CO2 Rich Natural Gas Value Chain Program are duly acknowledged.
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b
a
Im
Zn
Figure 1: a) The framework structure of ZIF-8, with tetrahedral Zn ions, bridged by imidazolate (Im) inside the cage represent the pore cavity [18] b) The chemical structure of ZIF-8 [19]
a
b
c
d
e
f
g
h
Figure 2: The physical appearance of a) 6FDA-durene membrane and ZIF-8/6FDA-durene MMMs fabricated at different parameters b) S1 c) S2 d) S3 e) S4 f) S5 g) S6 h) S7
Figure 3: XRD patterns of ZIF-8 and 6FDA-durene membrane
7.3 S7
S6
S5
S4
S3
S2
S1
Figure 4: XRD pattern of ZIF-8/6FDA-durene mixed matrix membranes
a
b
c
d
e
f
g
h
Figure 5: FESEM images of a) ZIF-8 and ZIF-8/6FDA-durene MMMs b) S1 c) S2 d) S3 e) S4 f) S5 g) S6 h) S7
a
b
c
d
.
e
f
g
Figure 6: Cross-section images of ZIF-8/6FDA-durene MMMs at magnification of 100K a) S1 b) S2 c) S3 d) S4 e) S5 f) S6 g) S7 a
c
b
e
f
d
g
Figure 7: EDX mapping for Zn from the cross-section images of ZIF-8/6FDA-durene MMMs a) S1 b) S2 c) S3 d) S4 e) S5 f) S6 g) S7
Figure 8: TGA curve of ZIF-8 and membranes fabricated in the present work (S1-S7)
Figure 9: DSC curve of the membranes obtained in the present work
Figure 10: ATR-FTIR analysis of ZIF-8 and the membranes
Figure 11: CO2 and CH4 permeability of the membranes
S2
S3
S4
S5
S6
S7
Figure 12: CO2/CH4 selectivity of the membranes
Figure 13: CO2 plasticization behavior of pure 6FDA-durene and ZIF-8/6FDA-durene membrane (S1)
Table 1: ZIF-8/6FDA-durene membranes fabricated in the present work by using different fabrication parameters
Sample
Fillers dispersion duration
label
ZIF-8 in DCM
ZIF-8 in 6FDA-durene solution
Total
Priming
Stirring (h)
Sonication (h)
Stirring (h)
Sonication (h)
duration (h)
S1
0.5
0.5
1
1
3
Yes
S2
1
1
2
2
6
Yes
S3
1
1
3
3
8
Yes
S4
2
2
2
2
8
Yes
S5
2
2
3
3
10
Yes
S6
0.5
0.5
1
1
3
No
S7
1
1
2
2
6
No
All dope solution is subjected to the vigorous stirring for 1 h before cast into a Petri dish
Table 2: Pure and mixed gas (50 CO2: 50 CH4) permeation properties of 6FDA-durene and S1 membranes at pressure of 3.5 bar and temperature of 30oC
Membrane
Pure gas permeability
CO2/CH4 ideal
Mixed gas
CO2/CH4
(Barrer)
selectivity
permeability (Barrer)
selectivity
CO2
CH4
6FDA-durene
468.01
66.57
MMM (S1)
693.54
41.92
CO2
CH4
7.03
283.57
62.60
4.53
16.54
320.50
42.10
7.61
41