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6FDA-Durene Composite Membranes for CO2 Removal from CH4
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ScienceDirect Materials Today: Proceedings 19 (2019) 1730–1737
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ICCSE 2018
Amine-Functionalized Metal Organic Framework (MOF)/6FDADurene Composite Membranes for CO2 Removal from CH4 Nadia Hartini Suhaimia,b, Yin Fong Yeonga,b*, Norwahyu Jusoha,c and Muhamad Farid Mohd Asria a
Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. CO2 Research Centre (CO2RES), R&D Building, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. c Centre of Contaminant Control (CenCo, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. b
Abstract In this work, composite membranes were fabricated by incorporating amine-functionalized zeolitic imidazolate framework 8 (ZIF-8) into 6FDA-durene polyimide. ZIF-8 particles was functionalized by N-[3-(Dimethoxymethylsilyl)propyl] ethylenediamine (AAPTMS) and N1-(3-Trimethoxysilylpropyl) diethylenetriamine (AEPTMS). Functionalization of ZIF-8 by amine-functional group may possibly enhance the CO2/CH4 separation performances by improving the compatibility between ZIF-8 and 6FDA-durene polymer. The resultant membranes were analysed by using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersion X-ray (EDX). Then, the separation performance test were performed using pure carbon dioxide (CO2) and methane (CH4) gases. The results showed that CO2 permeability of 548 Barrer and CO2/CH4 ideal selectivity of 17 were obtained using 1.0 wt% AAPTMS-ZIF-8/6FDA-durene composite membrane, while 1.0 wt% AEPTMS-ZIF-8/6FDA-durene composite membrane exhibited CO2 permeability of 533 barrer and CO2/CH4 gas pair selectivity of 12. On the other hand, 1.0 wt% ZIF-8/6FDA-durene composite membrane showed CO2 permeability and CO2/CH4 selectivity of 610 barrer and 15, respectively. Therefore, it can be concluded in this work that, selection of another suitable amine functionalization group for the modification of ZIF-8 is needed in later works to further improve the performance of the membrane in CO2 removal from CH4. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018. Keywords: Composite membranes; 6FDA-Durene; metal organic framework (MOF); CO2/CH4 separation
* Corresponding author. Tel.: +0-605-368-7564; fax: +0-605-365-6176 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018.
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1. Introduction Removal of carbon dioxide (CO2) from methane (CH4) are essential to enhance the calorific value of natural gas (NG) [1] and reduce the corrosion of pipelines and equipment [2]. Chemical absorption [3], cryogenic distillation [4], pressure swing adsorption [5] and membrane technology separation [6] are the common technologies reported in the literature for CO2 removal from CH4. However, due to several advantages such as low capital cost, simple operation and low energy demand, membrane technology receive great attention over the years [7]. During the last decade, polymeric membranes have been given massive attention mainly due the ease of fabrication, lower cost and easy to scale up. Nevertheless, polymeric membranes suffers trade-off between permeability and gas pair selectivity [8]. Therefore, researchers pay attention into the development of inorganic membranes due to its superior performance, high thermal and chemical stability [9]. The major constraints in the development of inorganic membranes are the complexity of fabrication procedure and high production cost. Therefore, mixed matrix or composite membranes are introduced by combining polymer and inorganic fillers. Metal organic frameworks (MOFs) is a new class of hybrids materials made from metal ions as connector and organic bridging ligands as linkers [10]. It have great potential as a fillers because of their well-defined, flexible and tunable porous structure [11]. Commonly used MOFs as filler in composite membrane include ZIF-8 [11-16], MIL53 [6, 17-21], MIL-101 [6, 22, 23] and UiO-66 [24, 25]. ZIF-8 received much attention because of its aperture size (0.34 nm) that is close to the kinetic diameter of CO2 (0.33 nm) and CH4 (0.38 nm). ZIF-8 is built from linking of zinc (II) cations and 2-methylimidazole anions[26] which exhibits SOD topology comprised of 1.16 nm cages connected through six-membered windows, 0.34 nm in size [27]. Other features of ZIF-8 such as high sorption capacity due to high surface area and pore volume [13], high thermal and chemical stability [12] besides of proven molecular sieving property, which required for gases separation [26]. Hence, ZIF-8 has been identified as a potential filler for CO2 removal from CH4. Issues raised from incorporation of inorganic filler into polymer matrix such as non-uniform distribution and agglomeration of inorganic filler besides of polymer rigidication, which lead to new approach by functionalized the filler using amine-functional group. Amine-functionalized MOF displayed advantages on the interaction between amino groups and polymer matrix which helps to improve the compatibility between MOF and polymer matrix [28]. Recently, functionalized ZIF-8 with amine group has been explored and reported [11, 29] because basic amine group display strong affinity to acidic gas molecules , particularly CO2 [28]. Nik et al. [10] fabricated MMMs by incorporating UiO-66, MOF-199, NH2-UiO-66 and NH2-MOF-199 into 6FDA-ODA. The increase in CO2/CH4 selectivity while decrease in permeability were found because the presence of amine functional group in the MOF structure rigidified the polymer at the interface of the filler/polymer. In another work, Amedi and Aghajani [11] modified ZIF-8 with APTMS and APTES before incorporated into PEBA. Their results showed that APTES modified ZIF-8 gave better performance due to the proper chain length compare to APTMS modified ZIF-8 which has reduced the selectivity severely. Nordin et al. [29] incorporated aminefunctionalized ZIF-8 into Polysulfone and found that gas selectivity increased 88% compared to pure polysulfone membrane. Incorporation of functionalized MIL-125 (Ti) into Matrimid was reported by Waqas et al. [30]. Addition of 15wt% NH2-MIL-125 (Ti) into Matrimid enhanced selectivity of CO2/CH4 from 44 to 50 because the presence of the amine groups provided extra CO2 demanding sorption sites while offering convenient hydrogen bonding with CO2. From the literature, it has been found that the selection of an appropriate amine-functional group is crucial for the modification of the MOF fillers before embedded into the polymer matrix. Although many studies available for ZIF-8 as a filler in the formation of composite membrane, to the best of our knowledge, the incorporation of amine-functionalization ZIF-8 fillers into 6FDA-durene is hardly found in the literature. Therefore, in the present work, ZIF-8 fillers were modified by using AAPTMS and AEPTMS before incorporated into 6FDA-durene polymer matrix. After characterizing the properties of the resultant fillers and membranes, the permeability over the membranes were tested by using carbon dioxide and methane gases. 2. Experimental 2.1. Materials and Gases 6FDA-durene polyimide synthesized by using two different monomers including 3,6-Diaminodurene (durene diamine, 99% trace metal basis) and 4,4′–(Hexafluoroisopropylidene) diphthalic anhydride (6FDA, 99% purity).
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Durene diamine monomers was purified by using re-crystallization in methanol while 6FDA dianhydride through vacuum sublimation, respectively. Propionic anhydride (PA, ≥98% purity), triethylamine (TEA, ≥99% purity), methanol (CH3OH , ≥99.9% purity) and dichloromethane (DCM, ≥99.8% purity) were used as received. Synthesis of ZIF-8 and amine-functionalized ZIF-8 required zinc nitrate hexahydrate (Zn(NO3)2.6H2O, >98% purity), 2methylimidazole (2-MeIM, 98% purity), N-[3-(Dimethoxymethylsilyl)propyl] ethylenediamine (AAPTMS, ≥97% purity), N1-(3-Trimethoxysilylpropyl) diethylenetriamine (AEPTMS) and methanol (CH3OH, 99.8% purity). All chemicals were purchased from Merck. Pure gases of carbon dioxide (CO2) and methane (CH4) with purity percentage of 99.95% were supplied by Air Product Malaysia Sdn Bhd. 2.2. Synthesis of 6FDA-Durene polyimide 6FDA-durene polyimide was synthesized by two-step method based on the procedure reported by Jusoh et al [15]. First, 6FDA dianhydride and durene diamine with equal number of moles were prepared by solution condensation in NMP. Polyamic acid (PAA) was derived through the stirring of reaction mixture under nitrogen condition for 24 hours. Next, chemical imidization achieved by addition of propionic anhydride and trimethylamine. 2.3. Synthesis of ZIF-8 and Amine-Functionalized ZIF-8 ZIF-8 was synthesized by dissolving Zn(NO3)2.6H2O and 2-MeIM in methanol, separately [11]. Then, the solutions were mixed and stirred for one hour. The ZIF-8 powders were then separated via centrifuge at 7800 rpm for 5 min and thereafter washed few times with methanol and subsequently, dried at 60°C for 24 hours. After that, a mixture of 1 g of ZIF-8 particles, 50 ml of desired amine functionalized agents (AAPTMS and AEPTMS) and 100 ml of methanol were stirred under reflux conditions for 2 hours at 110°C. Amine-functionalized ZIF-8 was then collected by centrifugation and washed three times with methanol. The collected particles were dried at 60 °C for 24 h in the oven. 2.4. Fabrication of Membranes Pure 6FDA-durene known as pristine membrane was fabricated by using solvent evaporation method reported by Jusoh et al,[31]. A 2% w/v polymer was dissolved in DCM before filtered and cast on a petri dish. The resultant membrane was dried in an oven at 60°C for 24 hours and continued another 24 hours under vacuum. Then, the temperature of the oven was increased from 60°C to 250°C at a heating rate of 25°C/hour before subjected to thermal annealing at 250 °C for 24 hours. Composite membranes were fabricated by incorporating 1wt% of ZIF-8 and amine functionalized ZIF-8 into 6FDA-durene using solution blending method. First, 6FDA-durene polymer and filler were dissolved and dispersed in DCM, respectivley. The filler solution was stirred and sonicated alternately for homogeneous dispersion. Then, 10 wt% of 6FDA-durene polymer solution was slowly added into the filler solution through priming technique and the resultant solution was further stirred and sonicated. Next, the remaining 6FDA-durene polymer solution was added into the filler solution and again stirred and sonicated. After the bulk addition, the solution was stirred vigorously for one hour before poured onto petri dish for solvent evaporation. The membrane was peeled off after 24 h and continued to thermal annealing procedure following the heating protocol used for the fabrication of pure membrane. 2.5. Membranes Characterization The crystallinity of ZIF-8 was determined by using X-Ray Diffraction (XRD) analysis using STOE Stadi-P diffractometer. The morphology of the resultant membranes was studied using field emission scanning electron microscope (FESEM, Zeiss Supra 55vp) functioned at 10 kV under vacuum condition. The membranes were cracked in liquid nitrogen (N2) before sputter coated with platinum prior to imaging. On the other hand, the dispersion of filler in the resultant membranes was observed through EDX mapping using energy dispersion X-ray (EDX).
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2.6. Gas Permeation The CO2 and CH4 gas permeability of the resultant membranes was measured, at fixed pressure of 3.5 bar and room temperature. The detailed experimental setup and procedure were described elsewhere [32]. Permeability of CO2 and CH4 is calculated using equation (1) as follows [33]: P
(1)
Where PA is the gas permeability (Barrer), t is thickness of membrane (cm), Vp is the permeate flowrate (cm3(STP)/s), Am is the area of membrane surface (cm2), ph and pl are the pressure of feed and permeate, respectively (cmHg). The CO2/CH4 gas pair selectivity of membrane was calculated by using equation (2) as follows [20]: α
(2)
Where α indicates the CO2/CH4 gas pair selectivity and P is the permeability of gases (Barrer). 3. Results and Discussion 3.1. X-ray diffraction (XRD) Figure 1 shows the XRD pattern of ZIF-8 and amine-functionalized ZIF-8 fillers. The major peaks found in XRD patterns including 2θ= 7.50°, 10.50°, 12.80°, 16.20° and 18.30° are in agreement with those result reported in literature for ZIF-8 particle [15]. Meanwhile, the presence of amine functional group in ZIF-8 did not show significant effect on the crystallinity of ZIF-8. However, the peaks intensity of amine- functionalized ZIF-8 was slightly reduced compared to ZIF-8. This could be due to the space between atomic layers in the crystalline substance, which increased after functionalization of amine group [11]. 7.50
10.50 12.80
16.20 18.30
Fig. 1. XRD patterns of ZIF-8 and amine-functionalized ZIF-8 particles.
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3.2. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) spectroscopy mapping Figure 2 shows the images of FESEM for composite membranes loaded with ZIF-8 and amine-functionalized ZIF-8 fillers. Based on Figure 2, the presence of fillers in polymer matrix was observed. Besides, the fillers are encapsulated in 6FDA-durene polymer. The presence of concentric cavities in the composite membranes demonstrated strong interfacial adhesion between ZIF-8 and amine-functionalized ZIF-8 and 6FDA-durene polymer.
Fig. 2. FESEM images of (a) ZIF-8/6FDA-durene (b) AAPTMS-ZIF-8/6FDA-durene and (c) AEPTMS-ZIF-8/6FDA-durene composite membranes.
In addition, all resultant membranes showed well distribution of fillers without significant agglomeration. The distribution of fillers primarily consist of Zn element were further verified via EDX mapping shown in Figure 3. Referring to Figure 3, it can be seen that Zn element are homogenously dispersed in the membranes with no obvious sign of agglomeration.
(a)
(b)
(c)
Fig. 3. EDX mapping for Zn element from the cross section of (a) ZIF-8/6FDA-durene (b) AAPTMS-ZIF-8/6FDA-durene and (c) AEPTMS-ZIF-8/6FDA-durene composite membranes.
3.3. Permeation Results The CO2 and CH4 permeabilities of the resultant membranes are shown in Figure 4. Based on Figure 4, aminefunctionalized ZIF-8 (AAPTMS and AEPTMS)/6FDA-durene membranes display lower permeabilities than that of ZIF-8/6FDA-durene membrane. Meanwhile, the CO2/CH4 gas pair selectivity of AAPTMS-ZIF-8/6FDA-durene membrane shows the highest value among the membranes fabricated in this work, as shown in Figure 5.
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The CO2 permeability of the resultant membranes decreased from 610 Barrer for ZIF-8/6FDA-durene membrane to 548 and 533 Barrer for AAPTMS-ZIF-8/6FDA-durene and AEPTMS-ZIF-8/6FDA-durene membranes, respectively. The presence of amine-functionalized ZIF-8 particles has caused the polymer chains around the particles become denser due the interaction between amine-functional groups and ZIF-8 particles surface [10, 11], therefore, the gas permeability decreases. Higher CO2 permeability found in ZIF-8/6FDA-durene membrane compared to amine-functionalized ZIF-8 (AAPTMS and AEPTMS)/6FDA-durene membranes could be mainly because of the electrostatic field of ZIF-8 with quadrupole moment of CO2 [14]. The adsorption of CO2 were diminished because the filler pores were partially blocked by amine-functional group [21], and lead to the lower gases permeability. Referring to Figure 5, the CO2/CH4 gas pair selectivity obtained for AAPTMS-ZIF-8/6FDA-durene and AEPTMS-ZIF-8/6FDA-durene membranes are 17 and 12, respectively. Meanwhile, ZIF-8/6FDA-durene exhibits CO2/CH4 ideal selectivity of 15. This result show that the addition of AAPTMS-ZIF-8 fillers in 6FDA-durene has partially diminished the voids between the filler and 6FDA-durene polymer and thus, produced higher gas transport resistance. Overall, CO2 permeability and CO2/CH4 gas pair selectivity attained for amine-functionalized 6FDAdurene membranes did not show significant improvement in gas permeation performance compare to ZIF-8/6FDAdurene membrane.
Fig. 4. CO2 and CH4 permeabilities of the resultant membranes.
Fig. 5. CO2/CH4 ideal selectivity of the resultant membranes
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4. Conclusion Composite membranes containing ZIF-8, AAPTMS-ZIF-8 and AEPTMS-ZIF-8 fillers were fabricated successfully by using solvent evaporation method. FESEM and EDX results displayed that the fillers were homogeneously dispersed in the polymer phase. Overall, the performance of amine-functionalized ZIF-8/6FDAdurene composite membranes in CO2 and CH4 permeations did not show significant improvement on the gas permeation result. This could be due to polymer rigidication around the amine functionalized filler at the polymer/filler interfaces. Thus, the selection of another appropriate amine-functionalization group might be needed in future works in order to improve the separation performances of composite membranes in CO2 removal from CH4. Acknowledgements The financial support provided by Yayasan Universiti Teknologi PETRONAS (YUTP) research grant (Cost center: 0153AA-E68) and the technical support provided by CO2 Research Centre (CO2RES), Institute of Contaminant Management are duly acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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ScienceDirect Materials Today: Proceedings 19 (2019) 1730–1737
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ICCSE 2018
Amine-Functionalized Metal Organic Framework (MOF)/6FDADurene Composite Membranes for CO2 Removal from CH4 Nadia Hartini Suhaimia,b, Yin Fong Yeonga,b*, Norwahyu Jusoha,c and Muhamad Farid Mohd Asria a
Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. CO2 Research Centre (CO2RES), R&D Building, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. c Centre of Contaminant Control (CenCo, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia. b
Abstract In this work, composite membranes were fabricated by incorporating amine-functionalized zeolitic imidazolate framework 8 (ZIF-8) into 6FDA-durene polyimide. ZIF-8 particles was functionalized by N-[3-(Dimethoxymethylsilyl)propyl] ethylenediamine (AAPTMS) and N1-(3-Trimethoxysilylpropyl) diethylenetriamine (AEPTMS). Functionalization of ZIF-8 by amine-functional group may possibly enhance the CO2/CH4 separation performances by improving the compatibility between ZIF-8 and 6FDA-durene polymer. The resultant membranes were analysed by using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersion X-ray (EDX). Then, the separation performance test were performed using pure carbon dioxide (CO2) and methane (CH4) gases. The results showed that CO2 permeability of 548 Barrer and CO2/CH4 ideal selectivity of 17 were obtained using 1.0 wt% AAPTMS-ZIF-8/6FDA-durene composite membrane, while 1.0 wt% AEPTMS-ZIF-8/6FDA-durene composite membrane exhibited CO2 permeability of 533 barrer and CO2/CH4 gas pair selectivity of 12. On the other hand, 1.0 wt% ZIF-8/6FDA-durene composite membrane showed CO2 permeability and CO2/CH4 selectivity of 610 barrer and 15, respectively. Therefore, it can be concluded in this work that, selection of another suitable amine functionalization group for the modification of ZIF-8 is needed in later works to further improve the performance of the membrane in CO2 removal from CH4. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018. Keywords: Composite membranes; 6FDA-Durene; metal organic framework (MOF); CO2/CH4 separation
* Corresponding author. Tel.: +0-605-368-7564; fax: +0-605-365-6176 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Chemical Sciences and Engineering: Advance and New Materials, ICCSE 2018.
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1. Introduction Removal of carbon dioxide (CO2) from methane (CH4) are essential to enhance the calorific value of natural gas (NG) [1] and reduce the corrosion of pipelines and equipment [2]. Chemical absorption [3], cryogenic distillation [4], pressure swing adsorption [5] and membrane technology separation [6] are the common technologies reported in the literature for CO2 removal from CH4. However, due to several advantages such as low capital cost, simple operation and low energy demand, membrane technology receive great attention over the years [7]. During the last decade, polymeric membranes have been given massive attention mainly due the ease of fabrication, lower cost and easy to scale up. Nevertheless, polymeric membranes suffers trade-off between permeability and gas pair selectivity [8]. Therefore, researchers pay attention into the development of inorganic membranes due to its superior performance, high thermal and chemical stability [9]. The major constraints in the development of inorganic membranes are the complexity of fabrication procedure and high production cost. Therefore, mixed matrix or composite membranes are introduced by combining polymer and inorganic fillers. Metal organic frameworks (MOFs) is a new class of hybrids materials made from metal ions as connector and organic bridging ligands as linkers [10]. It have great potential as a fillers because of their well-defined, flexible and tunable porous structure [11]. Commonly used MOFs as filler in composite membrane include ZIF-8 [11-16], MIL53 [6, 17-21], MIL-101 [6, 22, 23] and UiO-66 [24, 25]. ZIF-8 received much attention because of its aperture size (0.34 nm) that is close to the kinetic diameter of CO2 (0.33 nm) and CH4 (0.38 nm). ZIF-8 is built from linking of zinc (II) cations and 2-methylimidazole anions[26] which exhibits SOD topology comprised of 1.16 nm cages connected through six-membered windows, 0.34 nm in size [27]. Other features of ZIF-8 such as high sorption capacity due to high surface area and pore volume [13], high thermal and chemical stability [12] besides of proven molecular sieving property, which required for gases separation [26]. Hence, ZIF-8 has been identified as a potential filler for CO2 removal from CH4. Issues raised from incorporation of inorganic filler into polymer matrix such as non-uniform distribution and agglomeration of inorganic filler besides of polymer rigidication, which lead to new approach by functionalized the filler using amine-functional group. Amine-functionalized MOF displayed advantages on the interaction between amino groups and polymer matrix which helps to improve the compatibility between MOF and polymer matrix [28]. Recently, functionalized ZIF-8 with amine group has been explored and reported [11, 29] because basic amine group display strong affinity to acidic gas molecules , particularly CO2 [28]. Nik et al. [10] fabricated MMMs by incorporating UiO-66, MOF-199, NH2-UiO-66 and NH2-MOF-199 into 6FDA-ODA. The increase in CO2/CH4 selectivity while decrease in permeability were found because the presence of amine functional group in the MOF structure rigidified the polymer at the interface of the filler/polymer. In another work, Amedi and Aghajani [11] modified ZIF-8 with APTMS and APTES before incorporated into PEBA. Their results showed that APTES modified ZIF-8 gave better performance due to the proper chain length compare to APTMS modified ZIF-8 which has reduced the selectivity severely. Nordin et al. [29] incorporated aminefunctionalized ZIF-8 into Polysulfone and found that gas selectivity increased 88% compared to pure polysulfone membrane. Incorporation of functionalized MIL-125 (Ti) into Matrimid was reported by Waqas et al. [30]. Addition of 15wt% NH2-MIL-125 (Ti) into Matrimid enhanced selectivity of CO2/CH4 from 44 to 50 because the presence of the amine groups provided extra CO2 demanding sorption sites while offering convenient hydrogen bonding with CO2. From the literature, it has been found that the selection of an appropriate amine-functional group is crucial for the modification of the MOF fillers before embedded into the polymer matrix. Although many studies available for ZIF-8 as a filler in the formation of composite membrane, to the best of our knowledge, the incorporation of amine-functionalization ZIF-8 fillers into 6FDA-durene is hardly found in the literature. Therefore, in the present work, ZIF-8 fillers were modified by using AAPTMS and AEPTMS before incorporated into 6FDA-durene polymer matrix. After characterizing the properties of the resultant fillers and membranes, the permeability over the membranes were tested by using carbon dioxide and methane gases. 2. Experimental 2.1. Materials and Gases 6FDA-durene polyimide synthesized by using two different monomers including 3,6-Diaminodurene (durene diamine, 99% trace metal basis) and 4,4′–(Hexafluoroisopropylidene) diphthalic anhydride (6FDA, 99% purity).
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Durene diamine monomers was purified by using re-crystallization in methanol while 6FDA dianhydride through vacuum sublimation, respectively. Propionic anhydride (PA, ≥98% purity), triethylamine (TEA, ≥99% purity), methanol (CH3OH , ≥99.9% purity) and dichloromethane (DCM, ≥99.8% purity) were used as received. Synthesis of ZIF-8 and amine-functionalized ZIF-8 required zinc nitrate hexahydrate (Zn(NO3)2.6H2O, >98% purity), 2methylimidazole (2-MeIM, 98% purity), N-[3-(Dimethoxymethylsilyl)propyl] ethylenediamine (AAPTMS, ≥97% purity), N1-(3-Trimethoxysilylpropyl) diethylenetriamine (AEPTMS) and methanol (CH3OH, 99.8% purity). All chemicals were purchased from Merck. Pure gases of carbon dioxide (CO2) and methane (CH4) with purity percentage of 99.95% were supplied by Air Product Malaysia Sdn Bhd. 2.2. Synthesis of 6FDA-Durene polyimide 6FDA-durene polyimide was synthesized by two-step method based on the procedure reported by Jusoh et al [15]. First, 6FDA dianhydride and durene diamine with equal number of moles were prepared by solution condensation in NMP. Polyamic acid (PAA) was derived through the stirring of reaction mixture under nitrogen condition for 24 hours. Next, chemical imidization achieved by addition of propionic anhydride and trimethylamine. 2.3. Synthesis of ZIF-8 and Amine-Functionalized ZIF-8 ZIF-8 was synthesized by dissolving Zn(NO3)2.6H2O and 2-MeIM in methanol, separately [11]. Then, the solutions were mixed and stirred for one hour. The ZIF-8 powders were then separated via centrifuge at 7800 rpm for 5 min and thereafter washed few times with methanol and subsequently, dried at 60°C for 24 hours. After that, a mixture of 1 g of ZIF-8 particles, 50 ml of desired amine functionalized agents (AAPTMS and AEPTMS) and 100 ml of methanol were stirred under reflux conditions for 2 hours at 110°C. Amine-functionalized ZIF-8 was then collected by centrifugation and washed three times with methanol. The collected particles were dried at 60 °C for 24 h in the oven. 2.4. Fabrication of Membranes Pure 6FDA-durene known as pristine membrane was fabricated by using solvent evaporation method reported by Jusoh et al,[31]. A 2% w/v polymer was dissolved in DCM before filtered and cast on a petri dish. The resultant membrane was dried in an oven at 60°C for 24 hours and continued another 24 hours under vacuum. Then, the temperature of the oven was increased from 60°C to 250°C at a heating rate of 25°C/hour before subjected to thermal annealing at 250 °C for 24 hours. Composite membranes were fabricated by incorporating 1wt% of ZIF-8 and amine functionalized ZIF-8 into 6FDA-durene using solution blending method. First, 6FDA-durene polymer and filler were dissolved and dispersed in DCM, respectivley. The filler solution was stirred and sonicated alternately for homogeneous dispersion. Then, 10 wt% of 6FDA-durene polymer solution was slowly added into the filler solution through priming technique and the resultant solution was further stirred and sonicated. Next, the remaining 6FDA-durene polymer solution was added into the filler solution and again stirred and sonicated. After the bulk addition, the solution was stirred vigorously for one hour before poured onto petri dish for solvent evaporation. The membrane was peeled off after 24 h and continued to thermal annealing procedure following the heating protocol used for the fabrication of pure membrane. 2.5. Membranes Characterization The crystallinity of ZIF-8 was determined by using X-Ray Diffraction (XRD) analysis using STOE Stadi-P diffractometer. The morphology of the resultant membranes was studied using field emission scanning electron microscope (FESEM, Zeiss Supra 55vp) functioned at 10 kV under vacuum condition. The membranes were cracked in liquid nitrogen (N2) before sputter coated with platinum prior to imaging. On the other hand, the dispersion of filler in the resultant membranes was observed through EDX mapping using energy dispersion X-ray (EDX).
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2.6. Gas Permeation The CO2 and CH4 gas permeability of the resultant membranes was measured, at fixed pressure of 3.5 bar and room temperature. The detailed experimental setup and procedure were described elsewhere [32]. Permeability of CO2 and CH4 is calculated using equation (1) as follows [33]: P
(1)
Where PA is the gas permeability (Barrer), t is thickness of membrane (cm), Vp is the permeate flowrate (cm3(STP)/s), Am is the area of membrane surface (cm2), ph and pl are the pressure of feed and permeate, respectively (cmHg). The CO2/CH4 gas pair selectivity of membrane was calculated by using equation (2) as follows [20]: α
(2)
Where α indicates the CO2/CH4 gas pair selectivity and P is the permeability of gases (Barrer). 3. Results and Discussion 3.1. X-ray diffraction (XRD) Figure 1 shows the XRD pattern of ZIF-8 and amine-functionalized ZIF-8 fillers. The major peaks found in XRD patterns including 2θ= 7.50°, 10.50°, 12.80°, 16.20° and 18.30° are in agreement with those result reported in literature for ZIF-8 particle [15]. Meanwhile, the presence of amine functional group in ZIF-8 did not show significant effect on the crystallinity of ZIF-8. However, the peaks intensity of amine- functionalized ZIF-8 was slightly reduced compared to ZIF-8. This could be due to the space between atomic layers in the crystalline substance, which increased after functionalization of amine group [11]. 7.50
10.50 12.80
16.20 18.30
Fig. 1. XRD patterns of ZIF-8 and amine-functionalized ZIF-8 particles.
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3.2. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) spectroscopy mapping Figure 2 shows the images of FESEM for composite membranes loaded with ZIF-8 and amine-functionalized ZIF-8 fillers. Based on Figure 2, the presence of fillers in polymer matrix was observed. Besides, the fillers are encapsulated in 6FDA-durene polymer. The presence of concentric cavities in the composite membranes demonstrated strong interfacial adhesion between ZIF-8 and amine-functionalized ZIF-8 and 6FDA-durene polymer.
Fig. 2. FESEM images of (a) ZIF-8/6FDA-durene (b) AAPTMS-ZIF-8/6FDA-durene and (c) AEPTMS-ZIF-8/6FDA-durene composite membranes.
In addition, all resultant membranes showed well distribution of fillers without significant agglomeration. The distribution of fillers primarily consist of Zn element were further verified via EDX mapping shown in Figure 3. Referring to Figure 3, it can be seen that Zn element are homogenously dispersed in the membranes with no obvious sign of agglomeration.
(a)
(b)
(c)
Fig. 3. EDX mapping for Zn element from the cross section of (a) ZIF-8/6FDA-durene (b) AAPTMS-ZIF-8/6FDA-durene and (c) AEPTMS-ZIF-8/6FDA-durene composite membranes.
3.3. Permeation Results The CO2 and CH4 permeabilities of the resultant membranes are shown in Figure 4. Based on Figure 4, aminefunctionalized ZIF-8 (AAPTMS and AEPTMS)/6FDA-durene membranes display lower permeabilities than that of ZIF-8/6FDA-durene membrane. Meanwhile, the CO2/CH4 gas pair selectivity of AAPTMS-ZIF-8/6FDA-durene membrane shows the highest value among the membranes fabricated in this work, as shown in Figure 5.
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The CO2 permeability of the resultant membranes decreased from 610 Barrer for ZIF-8/6FDA-durene membrane to 548 and 533 Barrer for AAPTMS-ZIF-8/6FDA-durene and AEPTMS-ZIF-8/6FDA-durene membranes, respectively. The presence of amine-functionalized ZIF-8 particles has caused the polymer chains around the particles become denser due the interaction between amine-functional groups and ZIF-8 particles surface [10, 11], therefore, the gas permeability decreases. Higher CO2 permeability found in ZIF-8/6FDA-durene membrane compared to amine-functionalized ZIF-8 (AAPTMS and AEPTMS)/6FDA-durene membranes could be mainly because of the electrostatic field of ZIF-8 with quadrupole moment of CO2 [14]. The adsorption of CO2 were diminished because the filler pores were partially blocked by amine-functional group [21], and lead to the lower gases permeability. Referring to Figure 5, the CO2/CH4 gas pair selectivity obtained for AAPTMS-ZIF-8/6FDA-durene and AEPTMS-ZIF-8/6FDA-durene membranes are 17 and 12, respectively. Meanwhile, ZIF-8/6FDA-durene exhibits CO2/CH4 ideal selectivity of 15. This result show that the addition of AAPTMS-ZIF-8 fillers in 6FDA-durene has partially diminished the voids between the filler and 6FDA-durene polymer and thus, produced higher gas transport resistance. Overall, CO2 permeability and CO2/CH4 gas pair selectivity attained for amine-functionalized 6FDAdurene membranes did not show significant improvement in gas permeation performance compare to ZIF-8/6FDAdurene membrane.
Fig. 4. CO2 and CH4 permeabilities of the resultant membranes.
Fig. 5. CO2/CH4 ideal selectivity of the resultant membranes
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4. Conclusion Composite membranes containing ZIF-8, AAPTMS-ZIF-8 and AEPTMS-ZIF-8 fillers were fabricated successfully by using solvent evaporation method. FESEM and EDX results displayed that the fillers were homogeneously dispersed in the polymer phase. Overall, the performance of amine-functionalized ZIF-8/6FDAdurene composite membranes in CO2 and CH4 permeations did not show significant improvement on the gas permeation result. This could be due to polymer rigidication around the amine functionalized filler at the polymer/filler interfaces. Thus, the selection of another appropriate amine-functionalization group might be needed in future works in order to improve the separation performances of composite membranes in CO2 removal from CH4. Acknowledgements The financial support provided by Yayasan Universiti Teknologi PETRONAS (YUTP) research grant (Cost center: 0153AA-E68) and the technical support provided by CO2 Research Centre (CO2RES), Institute of Contaminant Management are duly acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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