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polyimide mixed matrix membranes for gas separation
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Separation behavior of amorphous amino-modified silica nanoparticle/ polyimide mixed matrix membranes for gas separation Chien-Chieh Hu a, b, *, Po-Hsiu Cheng b, Shang-Chih Chou c, Cheng-Lee Lai d, Shu-Hsien Huang e, Hui-An Tsai b, Wei-Song Hung a, b, Kueir-Rarn Lee b a
Graduate Institute of Applied Science and Technology, Applied Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan R&D Center for Membrane Technology, Chung Yuan University, Chung-Li, 32023, Taiwan c Department of Emerging Technology Research, Taiwan Textile Research Institute, New Taipei, 23674, Taiwan d Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, Tainan, 717, Taiwan e Department of Chemical and Materials Engineering, National Ilan University, Yilan, 26047, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous amino-modified silica nano particles Mixed matrix membrane Gas separation Microstructure
In this work, amorphous amino-modified silica nanoparticles (AAMSN) were successfully synthesized via the solgel method and combined with the polyimide (PI) 4,40 -oxydiphthalic anhydride-2,20 -bis(trifluoromethyl) benzidine (ODPA-TFMB) to prepare mixed matrix membranes (MMMs) for gas separation. The physicochemical properties of the AAMSNs were characterized by thermogravimetric analysis, X-ray diffraction, FTIR spectros copy, and scanning electron microscopy, and the CO2 capture and O2 enrichment behaviors of MMMs with different AAMSN contents were compared with those of a control PI membrane. The sorption isotherm and permeation data revealed that addition of AAMSNs not only increases the diffusion coefficient of the resulting membranes but also increases their adsorption coefficient due to the amorphous characteristics of the nano particles and good adhesion between these particles and the PI. The CO2 permeation properties and CO2/N2 selectivity of the MMM with 20 wt% AAMSN were 210.1 barrer and 30.8, respectively, while those of the control membrane were 66.7 barrer and 27.8, respectively. Mixed gas permeation results also showed that the gas permeability of the MMMs improves and their gas selectivity is maintained compared with those of the control PI membrane. The unique structure of the AAMSN/PI membrane makes it an attractive candidate for CO2 capture and O2 enrichment applications.
1. Introduction Tremendous amounts of CO2 are emitted into the atmosphere on account of increased fossil fuel combustion to satisfy current energy demands [1–3]. CO2 is toxic to various ecosystems and human health; it also enhances the greenhouse effect, which results in global warming [4]. CO2 levels in the environment should be reduced to maintain the earth’s environmental sustainable development. Membrane-based CO2 capture and oxygen-enriched combustion techniques are widely used to reduce CO2 levels in the environment. Polymeric membrane was first developed by Loeb and Sourirajan in 1964 to remove CO2 from the atmosphere [5–7]. Unlike traditional sorption techniques, polymeric membrane gas separation technology
offers low energy consumption and avoids the need for desorption. Polymeric membranes also present a variety of advantages, including easily controllable gas transport properties and low cost; however, their low permeability and selectivity are major hindrances to their wider application [8,9]. To address these issues, mixed matrix membranes (MMMs) have been developed for use in gas separation; these mem branes present a number of favorable properties, including excellent physicochemical properties and good stability, permeability, and selectivity [10,11]. MMMs are composed of inorganic particles in a polymeric matrix and have been proposed to provide a solution to the trade-off between the permeability and selectivity of polymer membranes [12]. These mem branes combine the superior permeability or selectivity of inorganic
* Corresponding author. Graduate Institute of Applied Science and Technology, Applied Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan. E-mail address: [email protected] (C.-C. Hu). https://doi.org/10.1016/j.memsci.2019.117542 Received 7 August 2019; Received in revised form 25 September 2019; Accepted 7 October 2019 Available online 9 October 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Chien-Chieh Hu, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117542
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particles and the good processability and mechanical properties of polymeric matrices [13–16]. To date, the development of MMMs has focused on the composition and structural modification of the matrix to improve gas permeability and selectivity [17–22]. Fabrication of MMMs involves the development of inorganic particles with superior gas sep aration properties. An ideal MMM must feature preferential gas trans port through inorganic particles. Several challenges must be overcome to achieve the desired gas separation properties of an ideal MMM. These challenges involve: (i) the homogeneous dispersion of particles in the matrix, (ii) synthesis of a defect-free polymer/inorganic particle inter face, and (iii) selection of appropriate polymer–inorganic particle pairs to obtain good separation properties [23,24]. Modification of particle dispersion can be achieved by controlling particle loading, using ultrafine particles [25], increasing the dope vis cosity [26], and selecting polymer and inorganic particles with the same polarity [27]. Nonselective defects at the interface are formed due to the low adhesion between the polymer and inorganic particles. Thus, increasing the interaction between materials can inhibit non-selective defect formation. Coupling agents have been extensively explored to improve the adhesion between materials [28]. Thermal annealing and polymer matrix plasticization can also reduce the formation of nonse lective defects at the interface. Several studies have extensively reviewed polymers for gas separation membranes, including poly carbonates, polyethersulfones, polyarylates, poly(ether-block-amide)s, and polyimides (PIs). Inorganic particles, such as zeolites, carbon mo lecular sieves, mesoporous materials, nonporous silica, carbon nano tubes, and metal organic frameworks, are among the most common inorganic materials used to prepare MMMs. In this work, we combined PI and amorphous amino-modified silica nanoparticles (AAMSNs) to evaluate the gas separation mechanism of MMMs. Silica nanoparticles are among the additives most frequently used to prepare MMMs. Incorporation of silica nanoparticles into polymer matrices is achieved through two techniques: (1) physical mixing of the preformed silica nanoparticles with the polymer solution and (2) in situ production of silica nanoparticles in the polymer via the sol-gel reaction [29–32]. MMMs obtained through the sol-gel process generally hope to improve both permeability and selectivity of pure polymer membranes. However, the selectivity of MMMs may be lower than that of the pure polymer in some case [33]. Joyl et al. fabricated PI/silica membranes via the sol-gel method by adding tetramethoxysilane to poly(amic acid); the MMM obtained a higher permeability and a lower selectivity compared with those of the pure PI membrane [34]. Kim et al. reported the gas transport properties of poly(amide-6-b-ethylene oxide)/silica MMMs prepared through in situ polymerization of tetraethoxysilane by using the sol-gel method; the membranes exhibited permeabilities and selec tivities higher than those of poly(amide-6-b-ethylene oxide) [31]. In the above investigations, the characteristics of the silica nanoparticles, as well as the interaction between silica nanoparticles and the polymeric matrix, dominated the gas transport behavior of MMMs. During CO2 separation, membranes facilitating transport are well known to exhibit high performance [35–38]. Transport-facilitating membranes could be over the Roberson upper bound due to the selective reaction between carriers in the membrane and CO2 molecules. Carriers with amino functional groups have been identified to be good candidates for the facilitated transport for CO2; thus, amine-containing polymer mem branes are feasible for CO2 capture [39,40]. Herein, the PI 4,40 -oxy diphthalic anhydride-2,20 -bis(trifluoromethyl) benzidine (ODPA-TFMB) was used as a polymeric matrix and combined with AAMSNs to form MMMs. The microstructures of the amorphous silica nanoparticles and MMMs were then investigated in detail. This work assesses the effects of the amorphous structures and functional groups of AAMSNs on the gas separation properties of MMMs.
2. Experimental 2.1. Materials 4,40 -Oxydiphthalic anhydride (ODPA, Chriskev Company) and 2,20 Bis (trifluoromethyl) benzidine (TFMB, Chriskev Company) were used to synthesize ODPA-TFMB polyimide at the Taiwan Textile Research Institute. Analytical-grade 1-methyl-2-pyrrolidone (NMP), N,Ndimethyl acetamide, tetrahydrofuran (THF), and NaOH were obtained from Sigma-Aldrich. 2.2. Preparation of AAMSNs AAMSNs were prepared through a conventional base-catalyzed solgel reaction as follows: Exactly 4.17 g of tetraethyl orthosilicate (TEOS, Sigma-Aldrich) (20 mmol), 0.5 g of NaOH (1 M), and 10.0 mL of DMAc were added to a 100 mL three-necked round-bottom flask connected to a condenser. Then, 1.11 g of (3-aminopropyl)trimethoxysilane (SigmaAldrich) (5 mmol) was added to the TEOS mixture; stirring was per formed for 30 min at 25 � C. The solution was maintained in a N2 gas flow at 70 � C for 3 h under magnetic stirring and allowed to cool. Finally, the AAMSNs were separated from the aqueous mixture by repeated washing and drying for 48 h at room temperature. 2.3. PI synthesis and membrane preparation TFMB was added to NMP in a water bath at room temperature. After complete dissolution, ODPA was added into the diamine solution to react for 4 h and form a poly(amic acid) solution. The molar ratio of diamine to dianhydride was 1:1, and the solid content of the poly(amic acid) solution was 12 wt%. Pyridine and acetic anhydride were then added as chemical catalysts, and the temperature was increased to 120 � C for imidization for 3 h to obtain the PI. The molecular weight and poly dispersity index of ODPA-TFMB polyimide is 101,000 g/mol and 1.8 respectively. AAMSNs were activated at 70 � C in a vacuum oven for 24 h. The activated AAMSNs were dispersed in THF by sonication for 3 h. Once the AAMSNs were well dispersed, ODPA-TFMB polyimide was added to them, and the mixture was stirred at 50 � C for 24 h to obtain a 12 wt% PI casting solution containing homogeneously dispersed AAMSNs. The AAMSN loadings were set to 0 wt%, 3 wt%, 6 wt%, 9 wt%, 12 wt%, 20 wt%, 30 wt%, and 50 wt% based on the PI. The casting solution was casted on a glass plate placed in a chamber with a temperature of 35 � C and relative humidity of 30%–40% using a casting knife to a thickness of 500 μm. The solvent was allowed to evaporate in the chamber for 1 h, and the obtained membranes were kept in a vacuum oven at room temperature for 24 h to remove trace solvents. The membranes were labeled as follows: M denotes the MMM, and Arabic numerals refer to the AAMSN loading. For example, M06 is the MMM containing 6 wt% AAMSNs. A support membrane was prepared using a nonsolvent induced phase inversion method. The casting solution (25 wt% PVDF/NMP) was cast onto a nonwoven polyester fabric by using a 200 μm casting knife. The resulting PVDF support membrane washed thoroughly with DI water and dry in atmosphere before coating. A 3 wt% PI/AAMSN/THF casting solution coated on a PVDF-supported membrane with a 25 μm coating rod to prepare an AAMSN/PI composite MMM. The AAMSN loading of the selective layer was 20 wt% based on the PI. The thickness of the selective layer is about 1.1 μm. 2.4. Characterization of AAMSN and MMMs X-ray diffraction (XRD) was carried out on a D8 Advance diffrac tometer (Bruker) to confirm the amorphous characteristic of the AAMSNs, and thermogravimetric analysis (TGA; Q500, TA Instruments) was utilized to investigate their weight loss. The purging gas used was 2
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N2. A Q800 dynamic mechanical analyzer (TA Instruments) was used to obtain the glass transition temperature (Tg). Morphological studies were accomplished by field-emission scanning electron microscopy (FE-SEM Model: S–4800 N, Hitachi, Japan). The energy-dispersive X-ray micro analysis (EDX) system (SEM-EDX model: S–3000 N, Hitachi, Japan) was employed for analysing the dispersion of AAMSNs in the MMMs. Pore size distribution of the AAMSNs was measured through nitrogen adsorption isotherms by the gas adsorption analyzer (ASAP 2060, Micromeritics, USA). The positron annihilation lifetime (PAL) of the AAMSN/PI membranes was measured by detecting the time difference between γ-rays (1.28 MeV) obtained from the emission of a positron and the subsequent annihilation γ-rays (0.511 MeV). Each PAL spectrum was collected to a fixed total count of 1 � 106. The free-volume radius of the materials was correlated to o-Ps lifetimes using a semi-empirical equa tion [41]. All of the PAL spectra were analyzed by using the PATFIT program. Lifetime distributions were obtained by analyzing the PAL spectrum using MELT software. 2.5. Gas permeation and sorption measurement
Fig. 1. XRD patterns of the AAMSN powders. Inset: XRD patterns of crystalline silica [42].
N2, O2, and CO2 permeabilities were measured by a gas permeability analyzer (Yanaco GTR-11MH) and a gas chromatograph (Yanaco G2700) at 35 oC and 2 atm. Permeability (P) is usually expressed in barrer (10 10 [cm3 (STP) cm/(cm2 s cm Hg)]). The ideal selectivity was calculated using the formula: � αA=B ¼ PA PB (1)
constant weight was maintained despite further increases in tempera ture. The initial weight loss observed (<200 � C) was due to the evapo ration of adsorbed water. Amino group decomposition resulted in a 15% weight loss of the AAMSNs from 200 � C to 700 � C. This result indicates that amino groups make up 15% of the weight of the AAMSNs.
where PA and PB are the permeability coefficients of gases A and B, respectively. A sorption cell with Cahn D200 was used to evaluate the gas equi librium isotherms for N2, O2, and CO2. The samples were tested at 35 � C, and the same pressure was applied as the permeation test. The solubility was calculated using formula S ¼ C/p, where C is the amount of adsorbed gas [cm3 (STP) gas/cm3 membrane] and p is the corresponding pressure. Diffusivity was determined based on the formula: D ¼ P=S
3.2. Characterization of the AAMSN/PI MMMs SEM images of samples containing 0 wt%, 3 wt%, 20 wt%, and 50 wt % AAMSNs are shown in Fig. 2. The micrographs show that the control membrane is dense and has no defects (Fig. 2a and b). In the sample with a low AAMSN content, i.e., M03, the AAMSNs are well dispersed in the PI matrix and exhibit no interfacial defects (Fig. 2c and d). The amino groups of the AAMSNs improve the affinity between the silica and the PI, thereby inhibiting the formation of interfacial defects. Membranes with higher AAMSN contents, i.e., M20 and M50, show AAMSN aggregates in the membranes (Fig. 2e–h). Fig. S3 shows the SEM-EDX silicon mapping results for the AAMSN/PI MMMs, which consists with the SEM results about the AAMSNs dispersion. No obvious interfacial defects are formed in M20 and M50, which means that the loading of AMMSN can increase to high level due to the amino group introduced into silica. However, there is no chemical reaction between the amino groups of silica nano particles with polyimide. Table 1 summarizes the thermal stability and Tg of the control PI membrane and AAMSN/PI MMMs with different AAMSN loadings. Among the membranes tested, the control membrane has the highest decomposition temperature. The decomposition temperature of the AAMSN/PI MMMs decreases with increasing AAMSN loading, which indicates that addition of the nanoparticles reduces the thermal stability of the PI membrane. The decomposition of amino groups in the AAMSNs results in a decrease in decomposition temperature. Compared with that of the control membrane, the Tg of the MMMs increased with increasing AAMSN loading. This increase may be explained by improvements in the interactions between polymer chains and AAMSNs, which can inhibit the movement of the former. The observed increase in Tg confirms that amino modification of silica particles can improve the interfacial adhesion of AAMSN/PI MMMs. Addition of rigid AAMSNs can also reduce the chain movement of the PI and result in higher Tg.
(2)
The gas transport performance of the AAMSN/PI composite MMMs was measured by the constant pressure method (bubble flow meter). Transport experiments were carried out in a 50 � C oven. Here, the dry gas was passed through a humidifier (100 mL of water in a 500 mL stainless steel container) to obtain humidified feed gas. Permeance was expressed as GPU (10 6 [cm3 (STP)/(cm2 s cm Hg)]). 3. Results and discussion 3.1. Characterization of AAMSN In general, combining crystalline silica nanoparticles with polymeric gas separation membranes results in a decrease in permeability but in crease in selectivity because the crystal region is gas impermeable. The permeability and selectivity of a membrane may be simultaneously increased if modified amorphous silica nanoparticles are mixed with polymers to form MMMs. The amorphous structures of the nanoparticles can select gas molecules and allow only specific molecules to pass through. AAMSNs were prepared in this work to improve the gas sorp tion and diffusion properties of silica nanoparticles. Fig. 1 shows the XRD patterns of the crystalline silica and AAMSNs. The XRD patterns of the AAMSNs show a broad peak at 2θ ¼ 21� . Comparing the patterns of the AAMSNs with those of silica crystals (Fig. 1), all of the characteristic peaks of the latter disappeared, which confirms the amorphous nature of the AAMSNs. SEM micrographs show that the particle size of the AAMSNs is less than 100 nm (Fig. S1). The thermal stability of the AAMSNs was investigated by TGA, and the results are shown in Fig. S2. A continuous weight loss was found at 100–700 � C, after which a
3.3. Gas separation properties of AAMSN/PI MMMs The effect of AAMSN loading on the gas separation properties of the AAMSN/PI MMMs was investigated, as shown in Fig. 3. Compared with 3
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Fig. 3. Permeability (a) and selectivity (b) of the control membrane, M03, M06, M09, M12, M20, M30, and M50.
Fig. 2. FE-SEM images of the cross sections and surfaces of pure PI (a, b), M03 (c, d), M20 (e, f), and M50 (g, h).
than that of the control membrane. As M03 features the lowest AAMSN loading and the largest increase in Tg among the membranes studied, the effect of increased chain rigidity by addition of AAMSNs to PI could be concluded to overshadow the effect of pores in the AAMSNs, which result in low permeability. Fig. 3(b) shows the selectivity of the AAMSN/PI MMMs. Similar or slightly increased selectivities were observed at AAMSN contents below 20 wt%, likely because of the in crease in polymer chain rigidity and selective pore formation in the AAMSNs. Amino groups improve interfacial adhesion, which may also explain the selectivities observed. Membrane selectivity dramatically decreased at AAMSN loadings over 20 wt% due to the aggregation of nanoparticles. This phenomenon has been observed in several published reports. The gas separation behaviors of MMMs are controlled by the microstructure of the membranes. The free-volume size distributions of the control membrane shown in Fig. 4 exhibit a broad monodispersed curve. A bimodal dispersion is observed when the AAMSNs are added to the PI. The MMMs show a curve (left side) that is narrower than that of the control membrane. A narrow free-volume size distribution endows membranes with improved gas molecule screening ability, which means the selectivity of the MMMs could be expected to be better than that of the control membrane. A broad distribution curve appears at a larger free volume size (right side) for the MMMs. The control membrane does not have a large free volume similar to that found in the MMMs. The large free volume of MMMs is attributed to the pores of the AAMSNs. The BET result confirms that there are pores exist in the AAMSN (Fig. S4). The pores are within 4–10 Å which is similar to PALS result. A large pore contributes to the high permeability of a membrane. By combining the narrow distribution of smaller free volume and new formation of bigger pore, MMMs could exhibit higher permeability and selectivity compared with the control membrane. Thus, the gas
Table 1 Physical properties of the control PI membrane and AAMSN/PI MMMs. Membrane
Td (� C)
Tg (� C)
Control PI M03 M06 M09 M12 M20 M30 M50
611.8 596.9 587.0 582.2 578.4 570.7 567.9 563.1
243.2 253.5 261.2 263.9 268.8 275.7 277.6 281.8
the control membrane, the AAMSN/PI MMMs showed considerable higher CO2, N2, and O2 permeability (Fig. 3(a)]. The permeability of the AAMSN/PI MMMs smoothly increased with increasing AAMSN loading when the load was less than 30 wt%; by comparison, the permeability of M50 dramatically increased. The Tg of the AAMSN/PI MMMs shows an increasing trend with AAMSN loading (Table 1). Thus, the permeability of the AAMSN/PI MMMs could be expected to decrease due to the presence of more AAMSNs in the PI. Interestingly, the variation in permeability contrasted the trend of Tg. The observed increase in permeability was attributed to the pores present in the AAMSNs; pores provide gas transport pathways and increase membrane permeability. Aggregation of the AAMSNs formed interfacial defects, resulting in the dramatic increase in permeability observed for M50. Extra space formed between the particles when the AAMSNs aggregate. Gases can pass through the extra space easily that is the reason why the permeability dramatically increased. Previous works present similar phenomenon with this work [43–45]. The permeability of M03 was notably lower 4
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Table 2 Permeability (P), selectivity (α), diffusivity (D), and solubility (S) of the control membrane and MMMs at 2 atm and 35 � C. Membrane
PCO2
αCO2/N2
DO2
SO2
αD(CO2/N2)
αS(CO2/N2)
Control PI M03 M20
66.7 54.0 210.1
27.8 28.1 30.8
10.1 8.3 27.6
6.6 6.5 7.6
3.9 4.3 7.1
7.1 6.5 4.3
Membrane
PO2
αO2/N2
DO2
SO2
αD(O2/N2)
αS(O2/N2)
Control PI M03 M20
10.5 8.5 29.5
4.4 4.4 4.3
10.0 6.6 16.3
1.1 1.3 1.8
4.0 4.0 4.2
1.1 1.1 1.0
P: barrer [10 10(cm3) cm cm D: 10 8 cm2 s 1 S: 10 2 cm3 cm 3 cmHg 1.
2
s
1
cmHg 1].
increasing operating pressure; hence, plasticization is a key issue for the industrial use of polymeric membranes. The effect of operating pressure on the CO2/N2 separation performance of the control membrane is summarized in Fig. 6. The pure CO2 permeability of the membrane in creases at operating pressures greater than 5 atm. This finding indicates that the control PI membrane is plasticized by high-pressure CO2. No obvious plasticization effect occurs during high-pressure N2 permeation, and the N2 permeability of the control membrane remains relatively constant, as shown in Fig. 5(a). The plasticization behavior of the AAMSN/PI MMM (M20) was shown in Fig. S5. There is an obvious CO2 plasticization when the pressure above 11 atm. It indicated that the addition of AAMSNs improved the anti-plasticization ability of matrix PI because the AAMSNs reduce chain mobility of PI. The mixed gas (vol ume ratio of CO2 to N2 is 15/85) permeation results showed a trend similar to that of pure gas. The CO2/N2 selectivity decreases and then increases with increasing pressure. The variation of selectivity follows the trend of CO2 permeability, likely due to the change in the perme ability of CO2 with pressure; however, the N2 permeability remains constant. The decreasing lever of nitrogen permeability higher than oxygen permeability, compared Fig. 6(a) and (b), which results in the selectivity for mixed gas over the pure gas (Fig. 6(c)). Table 3 compares the separation performance of the control mem brane and M20 for pure and mixed gas. The pure gas permeability of both membranes is higher than their mixed gas permeability. The permeation competition of mixed gases decreases their permeability. Compared with that for pure gas, the selectivity of both membranes for mixed gas increases, which follows the trade-off description. AAMSN/PI composite MMMs were prepared to evaluate the effect of the amino group of the AAMSNs on the ability of the membranes to facilitate transport. In the presence of H2O in the feed gas, amine carriers can facilitate the permeation of CO2 through amine-containing membranes. The respective permeance and CO2/N2 selectivity of the AAMSN/PI composite MMMs are 96.3 GPU and 1.2 for dry feed stream and 32.1 GPU and 2.0 for wet feed stream. These data indicate that no facilitated
Fig. 4. Free-volume distribution of the control and M20 membranes.
separation performance improving of the MMMs is consistent with the free-volume distribution results obtained from PALS. According to the solution–diffusion model, solubility is an important factor affecting the gas separation performance of a membrane. The O2, N2, and CO2 sorption isotherms of the control membrane and MMMs prepared in this work are shown in Fig. 5. The gas sorption behaviors of the control membrane and MMMs follow the dual-sorption model (solid line). The sorption capacities of the membranes for the different gases show the order CO2 > O2 > N2. The MMMs exhibit higher sorption ca pacity than the control membrane. Addition of AAMSNs to the PI creates more gas sorption spaces in the membrane, which results in a higher sorption capacity. The gas sorption capacity increases with increasing AAMSN loading. Higher AAMSN loadings allow more gas sorption spaces to form in the membrane, thereby increasing its gas sorption capacity. The increase in solubility with AAMSN loading can explain the enhancement in permeability of MMMs by addition of more AAMSNs. Table 2 shows the gas separation factors of the control membrane and MMMs. Addition of a small amount of AAMSNs to M03 obviously decreases the gas diffusion property of the MMM. Decreases in diffu sivity resulted in the lower permeability of M03 compared with that of the control membrane. The strong interaction between amino groups in the AAMSNs and PI inhibited polymeric chain mobility, thereby decreasing diffusivity. More AAMSN contents significantly increased both diffusivity and solubility and dramatically improved the perme ability of M20. During CO2/N2 separation, the diffusivity selectivity increases but the solubility selectivity decreases with increasing AAMSN content. These results indicate that the separation behavior of the MMMs changes from solubility selectivity control to diffusivity selec tivity control as the AAMSN content increases. The permeability of polymeric membranes may increase with
Fig. 5. Sorption isotherms of O2 and N2 (a) and sorption isotherm of CO2 (b) for the control membrane and AAMSN/PI MMMs. 5
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Fig. 6. Effect of operating pressure on pure gas permeability (a), mixed gas permeability (b), and selectivity (c) of the control membrane. Table 3 Separation performance of the control and M20 membranes for pure and mixed gas. Membrane Controlled M20
Pure gas
Mixed gas
PCO2 (barrer)
αCO2/N2
PCO2 (barrer)
αCO2/N2
66.7 210.1
27.8 30.8
45.3 132.4
41.1 40.2
transport occurs in the AAMSN/PI MMMs. A small amount of fixed amino groups and high permeation pressure result in the absence of facilitated transport behavior in the AAMSN/PI MMMs. For wet feed streams, reductions in permeance may be due to the blockage of gas pathways by water molecules in the membrane. To evaluate the potential use of the AAMSN/PI MMMs as gas sepa ration membranes, we compared their permeability/selectivity with those of 2008 Robeson’s upper bound. Fig. 7 shows that addition of AAMSNs allows the gas separation performance of the membranes to approach the upper bound. The greater the amount of AAMSN added, the more satisfactory the gas separation performance obtained. Both permeability and selectivity can be improved by tailoring the compo sition of AAMSN/PI MMMs (e.g., M20). The data indicate that the AAMSN/PI MMMs are candidate materials for CO2 capture.
Fig. 7. Upper bound correlation (2008) for CO2/N2 separation: control mem brane (red), M03 (black), M06 (green), M09 (blue), M12 (light blue), M20 (purple). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
appear to be candidate materials for CO2 capture applications. Declaration of competing interest
4. Conclusion
There is no conflict of interest.
In summary, AAMSNs were successfully synthesized by using the solgel method. ODPA-TFMB polyimide was chosen as the matrix to prepare MMMs via incorporation of different weight percentages of AAMSN. The amorphous characteristics and amino groups of the AAMSNs improved the permeability and selectivity of the resulting membranes. The pores of the AAMSMs also improved membrane permeability, and interactions between the amino groups and polymer chain maintained their selec tivity. M20 exhibited a CO2 permeability of 210.1 barrer with an effective CO2/N2 selectivity of 30.8; by comparison, the pure PI mem brane showed a permeability of 66.7 barrer and an effective CO2/N2 selectivity of 27.8. Taking the results together, the AAMSN/PI MMMs
Acknowledgments The authors sincerely thank the Ministry of Economic Affairs and the Ministry of Science of Taiwan for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117542.
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Separation behavior of amorphous amino-modified silica nanoparticle/ polyimide mixed matrix membranes for gas separation Chien-Chieh Hu a, b, *, Po-Hsiu Cheng b, Shang-Chih Chou c, Cheng-Lee Lai d, Shu-Hsien Huang e, Hui-An Tsai b, Wei-Song Hung a, b, Kueir-Rarn Lee b a
Graduate Institute of Applied Science and Technology, Applied Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan R&D Center for Membrane Technology, Chung Yuan University, Chung-Li, 32023, Taiwan c Department of Emerging Technology Research, Taiwan Textile Research Institute, New Taipei, 23674, Taiwan d Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, Tainan, 717, Taiwan e Department of Chemical and Materials Engineering, National Ilan University, Yilan, 26047, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous amino-modified silica nano particles Mixed matrix membrane Gas separation Microstructure
In this work, amorphous amino-modified silica nanoparticles (AAMSN) were successfully synthesized via the solgel method and combined with the polyimide (PI) 4,40 -oxydiphthalic anhydride-2,20 -bis(trifluoromethyl) benzidine (ODPA-TFMB) to prepare mixed matrix membranes (MMMs) for gas separation. The physicochemical properties of the AAMSNs were characterized by thermogravimetric analysis, X-ray diffraction, FTIR spectros copy, and scanning electron microscopy, and the CO2 capture and O2 enrichment behaviors of MMMs with different AAMSN contents were compared with those of a control PI membrane. The sorption isotherm and permeation data revealed that addition of AAMSNs not only increases the diffusion coefficient of the resulting membranes but also increases their adsorption coefficient due to the amorphous characteristics of the nano particles and good adhesion between these particles and the PI. The CO2 permeation properties and CO2/N2 selectivity of the MMM with 20 wt% AAMSN were 210.1 barrer and 30.8, respectively, while those of the control membrane were 66.7 barrer and 27.8, respectively. Mixed gas permeation results also showed that the gas permeability of the MMMs improves and their gas selectivity is maintained compared with those of the control PI membrane. The unique structure of the AAMSN/PI membrane makes it an attractive candidate for CO2 capture and O2 enrichment applications.
1. Introduction Tremendous amounts of CO2 are emitted into the atmosphere on account of increased fossil fuel combustion to satisfy current energy demands [1–3]. CO2 is toxic to various ecosystems and human health; it also enhances the greenhouse effect, which results in global warming [4]. CO2 levels in the environment should be reduced to maintain the earth’s environmental sustainable development. Membrane-based CO2 capture and oxygen-enriched combustion techniques are widely used to reduce CO2 levels in the environment. Polymeric membrane was first developed by Loeb and Sourirajan in 1964 to remove CO2 from the atmosphere [5–7]. Unlike traditional sorption techniques, polymeric membrane gas separation technology
offers low energy consumption and avoids the need for desorption. Polymeric membranes also present a variety of advantages, including easily controllable gas transport properties and low cost; however, their low permeability and selectivity are major hindrances to their wider application [8,9]. To address these issues, mixed matrix membranes (MMMs) have been developed for use in gas separation; these mem branes present a number of favorable properties, including excellent physicochemical properties and good stability, permeability, and selectivity [10,11]. MMMs are composed of inorganic particles in a polymeric matrix and have been proposed to provide a solution to the trade-off between the permeability and selectivity of polymer membranes [12]. These mem branes combine the superior permeability or selectivity of inorganic
* Corresponding author. Graduate Institute of Applied Science and Technology, Applied Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan. E-mail address: [email protected] (C.-C. Hu). https://doi.org/10.1016/j.memsci.2019.117542 Received 7 August 2019; Received in revised form 25 September 2019; Accepted 7 October 2019 Available online 9 October 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Chien-Chieh Hu, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117542
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particles and the good processability and mechanical properties of polymeric matrices [13–16]. To date, the development of MMMs has focused on the composition and structural modification of the matrix to improve gas permeability and selectivity [17–22]. Fabrication of MMMs involves the development of inorganic particles with superior gas sep aration properties. An ideal MMM must feature preferential gas trans port through inorganic particles. Several challenges must be overcome to achieve the desired gas separation properties of an ideal MMM. These challenges involve: (i) the homogeneous dispersion of particles in the matrix, (ii) synthesis of a defect-free polymer/inorganic particle inter face, and (iii) selection of appropriate polymer–inorganic particle pairs to obtain good separation properties [23,24]. Modification of particle dispersion can be achieved by controlling particle loading, using ultrafine particles [25], increasing the dope vis cosity [26], and selecting polymer and inorganic particles with the same polarity [27]. Nonselective defects at the interface are formed due to the low adhesion between the polymer and inorganic particles. Thus, increasing the interaction between materials can inhibit non-selective defect formation. Coupling agents have been extensively explored to improve the adhesion between materials [28]. Thermal annealing and polymer matrix plasticization can also reduce the formation of nonse lective defects at the interface. Several studies have extensively reviewed polymers for gas separation membranes, including poly carbonates, polyethersulfones, polyarylates, poly(ether-block-amide)s, and polyimides (PIs). Inorganic particles, such as zeolites, carbon mo lecular sieves, mesoporous materials, nonporous silica, carbon nano tubes, and metal organic frameworks, are among the most common inorganic materials used to prepare MMMs. In this work, we combined PI and amorphous amino-modified silica nanoparticles (AAMSNs) to evaluate the gas separation mechanism of MMMs. Silica nanoparticles are among the additives most frequently used to prepare MMMs. Incorporation of silica nanoparticles into polymer matrices is achieved through two techniques: (1) physical mixing of the preformed silica nanoparticles with the polymer solution and (2) in situ production of silica nanoparticles in the polymer via the sol-gel reaction [29–32]. MMMs obtained through the sol-gel process generally hope to improve both permeability and selectivity of pure polymer membranes. However, the selectivity of MMMs may be lower than that of the pure polymer in some case [33]. Joyl et al. fabricated PI/silica membranes via the sol-gel method by adding tetramethoxysilane to poly(amic acid); the MMM obtained a higher permeability and a lower selectivity compared with those of the pure PI membrane [34]. Kim et al. reported the gas transport properties of poly(amide-6-b-ethylene oxide)/silica MMMs prepared through in situ polymerization of tetraethoxysilane by using the sol-gel method; the membranes exhibited permeabilities and selec tivities higher than those of poly(amide-6-b-ethylene oxide) [31]. In the above investigations, the characteristics of the silica nanoparticles, as well as the interaction between silica nanoparticles and the polymeric matrix, dominated the gas transport behavior of MMMs. During CO2 separation, membranes facilitating transport are well known to exhibit high performance [35–38]. Transport-facilitating membranes could be over the Roberson upper bound due to the selective reaction between carriers in the membrane and CO2 molecules. Carriers with amino functional groups have been identified to be good candidates for the facilitated transport for CO2; thus, amine-containing polymer mem branes are feasible for CO2 capture [39,40]. Herein, the PI 4,40 -oxy diphthalic anhydride-2,20 -bis(trifluoromethyl) benzidine (ODPA-TFMB) was used as a polymeric matrix and combined with AAMSNs to form MMMs. The microstructures of the amorphous silica nanoparticles and MMMs were then investigated in detail. This work assesses the effects of the amorphous structures and functional groups of AAMSNs on the gas separation properties of MMMs.
2. Experimental 2.1. Materials 4,40 -Oxydiphthalic anhydride (ODPA, Chriskev Company) and 2,20 Bis (trifluoromethyl) benzidine (TFMB, Chriskev Company) were used to synthesize ODPA-TFMB polyimide at the Taiwan Textile Research Institute. Analytical-grade 1-methyl-2-pyrrolidone (NMP), N,Ndimethyl acetamide, tetrahydrofuran (THF), and NaOH were obtained from Sigma-Aldrich. 2.2. Preparation of AAMSNs AAMSNs were prepared through a conventional base-catalyzed solgel reaction as follows: Exactly 4.17 g of tetraethyl orthosilicate (TEOS, Sigma-Aldrich) (20 mmol), 0.5 g of NaOH (1 M), and 10.0 mL of DMAc were added to a 100 mL three-necked round-bottom flask connected to a condenser. Then, 1.11 g of (3-aminopropyl)trimethoxysilane (SigmaAldrich) (5 mmol) was added to the TEOS mixture; stirring was per formed for 30 min at 25 � C. The solution was maintained in a N2 gas flow at 70 � C for 3 h under magnetic stirring and allowed to cool. Finally, the AAMSNs were separated from the aqueous mixture by repeated washing and drying for 48 h at room temperature. 2.3. PI synthesis and membrane preparation TFMB was added to NMP in a water bath at room temperature. After complete dissolution, ODPA was added into the diamine solution to react for 4 h and form a poly(amic acid) solution. The molar ratio of diamine to dianhydride was 1:1, and the solid content of the poly(amic acid) solution was 12 wt%. Pyridine and acetic anhydride were then added as chemical catalysts, and the temperature was increased to 120 � C for imidization for 3 h to obtain the PI. The molecular weight and poly dispersity index of ODPA-TFMB polyimide is 101,000 g/mol and 1.8 respectively. AAMSNs were activated at 70 � C in a vacuum oven for 24 h. The activated AAMSNs were dispersed in THF by sonication for 3 h. Once the AAMSNs were well dispersed, ODPA-TFMB polyimide was added to them, and the mixture was stirred at 50 � C for 24 h to obtain a 12 wt% PI casting solution containing homogeneously dispersed AAMSNs. The AAMSN loadings were set to 0 wt%, 3 wt%, 6 wt%, 9 wt%, 12 wt%, 20 wt%, 30 wt%, and 50 wt% based on the PI. The casting solution was casted on a glass plate placed in a chamber with a temperature of 35 � C and relative humidity of 30%–40% using a casting knife to a thickness of 500 μm. The solvent was allowed to evaporate in the chamber for 1 h, and the obtained membranes were kept in a vacuum oven at room temperature for 24 h to remove trace solvents. The membranes were labeled as follows: M denotes the MMM, and Arabic numerals refer to the AAMSN loading. For example, M06 is the MMM containing 6 wt% AAMSNs. A support membrane was prepared using a nonsolvent induced phase inversion method. The casting solution (25 wt% PVDF/NMP) was cast onto a nonwoven polyester fabric by using a 200 μm casting knife. The resulting PVDF support membrane washed thoroughly with DI water and dry in atmosphere before coating. A 3 wt% PI/AAMSN/THF casting solution coated on a PVDF-supported membrane with a 25 μm coating rod to prepare an AAMSN/PI composite MMM. The AAMSN loading of the selective layer was 20 wt% based on the PI. The thickness of the selective layer is about 1.1 μm. 2.4. Characterization of AAMSN and MMMs X-ray diffraction (XRD) was carried out on a D8 Advance diffrac tometer (Bruker) to confirm the amorphous characteristic of the AAMSNs, and thermogravimetric analysis (TGA; Q500, TA Instruments) was utilized to investigate their weight loss. The purging gas used was 2
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N2. A Q800 dynamic mechanical analyzer (TA Instruments) was used to obtain the glass transition temperature (Tg). Morphological studies were accomplished by field-emission scanning electron microscopy (FE-SEM Model: S–4800 N, Hitachi, Japan). The energy-dispersive X-ray micro analysis (EDX) system (SEM-EDX model: S–3000 N, Hitachi, Japan) was employed for analysing the dispersion of AAMSNs in the MMMs. Pore size distribution of the AAMSNs was measured through nitrogen adsorption isotherms by the gas adsorption analyzer (ASAP 2060, Micromeritics, USA). The positron annihilation lifetime (PAL) of the AAMSN/PI membranes was measured by detecting the time difference between γ-rays (1.28 MeV) obtained from the emission of a positron and the subsequent annihilation γ-rays (0.511 MeV). Each PAL spectrum was collected to a fixed total count of 1 � 106. The free-volume radius of the materials was correlated to o-Ps lifetimes using a semi-empirical equa tion [41]. All of the PAL spectra were analyzed by using the PATFIT program. Lifetime distributions were obtained by analyzing the PAL spectrum using MELT software. 2.5. Gas permeation and sorption measurement
Fig. 1. XRD patterns of the AAMSN powders. Inset: XRD patterns of crystalline silica [42].
N2, O2, and CO2 permeabilities were measured by a gas permeability analyzer (Yanaco GTR-11MH) and a gas chromatograph (Yanaco G2700) at 35 oC and 2 atm. Permeability (P) is usually expressed in barrer (10 10 [cm3 (STP) cm/(cm2 s cm Hg)]). The ideal selectivity was calculated using the formula: � αA=B ¼ PA PB (1)
constant weight was maintained despite further increases in tempera ture. The initial weight loss observed (<200 � C) was due to the evapo ration of adsorbed water. Amino group decomposition resulted in a 15% weight loss of the AAMSNs from 200 � C to 700 � C. This result indicates that amino groups make up 15% of the weight of the AAMSNs.
where PA and PB are the permeability coefficients of gases A and B, respectively. A sorption cell with Cahn D200 was used to evaluate the gas equi librium isotherms for N2, O2, and CO2. The samples were tested at 35 � C, and the same pressure was applied as the permeation test. The solubility was calculated using formula S ¼ C/p, where C is the amount of adsorbed gas [cm3 (STP) gas/cm3 membrane] and p is the corresponding pressure. Diffusivity was determined based on the formula: D ¼ P=S
3.2. Characterization of the AAMSN/PI MMMs SEM images of samples containing 0 wt%, 3 wt%, 20 wt%, and 50 wt % AAMSNs are shown in Fig. 2. The micrographs show that the control membrane is dense and has no defects (Fig. 2a and b). In the sample with a low AAMSN content, i.e., M03, the AAMSNs are well dispersed in the PI matrix and exhibit no interfacial defects (Fig. 2c and d). The amino groups of the AAMSNs improve the affinity between the silica and the PI, thereby inhibiting the formation of interfacial defects. Membranes with higher AAMSN contents, i.e., M20 and M50, show AAMSN aggregates in the membranes (Fig. 2e–h). Fig. S3 shows the SEM-EDX silicon mapping results for the AAMSN/PI MMMs, which consists with the SEM results about the AAMSNs dispersion. No obvious interfacial defects are formed in M20 and M50, which means that the loading of AMMSN can increase to high level due to the amino group introduced into silica. However, there is no chemical reaction between the amino groups of silica nano particles with polyimide. Table 1 summarizes the thermal stability and Tg of the control PI membrane and AAMSN/PI MMMs with different AAMSN loadings. Among the membranes tested, the control membrane has the highest decomposition temperature. The decomposition temperature of the AAMSN/PI MMMs decreases with increasing AAMSN loading, which indicates that addition of the nanoparticles reduces the thermal stability of the PI membrane. The decomposition of amino groups in the AAMSNs results in a decrease in decomposition temperature. Compared with that of the control membrane, the Tg of the MMMs increased with increasing AAMSN loading. This increase may be explained by improvements in the interactions between polymer chains and AAMSNs, which can inhibit the movement of the former. The observed increase in Tg confirms that amino modification of silica particles can improve the interfacial adhesion of AAMSN/PI MMMs. Addition of rigid AAMSNs can also reduce the chain movement of the PI and result in higher Tg.
(2)
The gas transport performance of the AAMSN/PI composite MMMs was measured by the constant pressure method (bubble flow meter). Transport experiments were carried out in a 50 � C oven. Here, the dry gas was passed through a humidifier (100 mL of water in a 500 mL stainless steel container) to obtain humidified feed gas. Permeance was expressed as GPU (10 6 [cm3 (STP)/(cm2 s cm Hg)]). 3. Results and discussion 3.1. Characterization of AAMSN In general, combining crystalline silica nanoparticles with polymeric gas separation membranes results in a decrease in permeability but in crease in selectivity because the crystal region is gas impermeable. The permeability and selectivity of a membrane may be simultaneously increased if modified amorphous silica nanoparticles are mixed with polymers to form MMMs. The amorphous structures of the nanoparticles can select gas molecules and allow only specific molecules to pass through. AAMSNs were prepared in this work to improve the gas sorp tion and diffusion properties of silica nanoparticles. Fig. 1 shows the XRD patterns of the crystalline silica and AAMSNs. The XRD patterns of the AAMSNs show a broad peak at 2θ ¼ 21� . Comparing the patterns of the AAMSNs with those of silica crystals (Fig. 1), all of the characteristic peaks of the latter disappeared, which confirms the amorphous nature of the AAMSNs. SEM micrographs show that the particle size of the AAMSNs is less than 100 nm (Fig. S1). The thermal stability of the AAMSNs was investigated by TGA, and the results are shown in Fig. S2. A continuous weight loss was found at 100–700 � C, after which a
3.3. Gas separation properties of AAMSN/PI MMMs The effect of AAMSN loading on the gas separation properties of the AAMSN/PI MMMs was investigated, as shown in Fig. 3. Compared with 3
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Fig. 3. Permeability (a) and selectivity (b) of the control membrane, M03, M06, M09, M12, M20, M30, and M50.
Fig. 2. FE-SEM images of the cross sections and surfaces of pure PI (a, b), M03 (c, d), M20 (e, f), and M50 (g, h).
than that of the control membrane. As M03 features the lowest AAMSN loading and the largest increase in Tg among the membranes studied, the effect of increased chain rigidity by addition of AAMSNs to PI could be concluded to overshadow the effect of pores in the AAMSNs, which result in low permeability. Fig. 3(b) shows the selectivity of the AAMSN/PI MMMs. Similar or slightly increased selectivities were observed at AAMSN contents below 20 wt%, likely because of the in crease in polymer chain rigidity and selective pore formation in the AAMSNs. Amino groups improve interfacial adhesion, which may also explain the selectivities observed. Membrane selectivity dramatically decreased at AAMSN loadings over 20 wt% due to the aggregation of nanoparticles. This phenomenon has been observed in several published reports. The gas separation behaviors of MMMs are controlled by the microstructure of the membranes. The free-volume size distributions of the control membrane shown in Fig. 4 exhibit a broad monodispersed curve. A bimodal dispersion is observed when the AAMSNs are added to the PI. The MMMs show a curve (left side) that is narrower than that of the control membrane. A narrow free-volume size distribution endows membranes with improved gas molecule screening ability, which means the selectivity of the MMMs could be expected to be better than that of the control membrane. A broad distribution curve appears at a larger free volume size (right side) for the MMMs. The control membrane does not have a large free volume similar to that found in the MMMs. The large free volume of MMMs is attributed to the pores of the AAMSNs. The BET result confirms that there are pores exist in the AAMSN (Fig. S4). The pores are within 4–10 Å which is similar to PALS result. A large pore contributes to the high permeability of a membrane. By combining the narrow distribution of smaller free volume and new formation of bigger pore, MMMs could exhibit higher permeability and selectivity compared with the control membrane. Thus, the gas
Table 1 Physical properties of the control PI membrane and AAMSN/PI MMMs. Membrane
Td (� C)
Tg (� C)
Control PI M03 M06 M09 M12 M20 M30 M50
611.8 596.9 587.0 582.2 578.4 570.7 567.9 563.1
243.2 253.5 261.2 263.9 268.8 275.7 277.6 281.8
the control membrane, the AAMSN/PI MMMs showed considerable higher CO2, N2, and O2 permeability (Fig. 3(a)]. The permeability of the AAMSN/PI MMMs smoothly increased with increasing AAMSN loading when the load was less than 30 wt%; by comparison, the permeability of M50 dramatically increased. The Tg of the AAMSN/PI MMMs shows an increasing trend with AAMSN loading (Table 1). Thus, the permeability of the AAMSN/PI MMMs could be expected to decrease due to the presence of more AAMSNs in the PI. Interestingly, the variation in permeability contrasted the trend of Tg. The observed increase in permeability was attributed to the pores present in the AAMSNs; pores provide gas transport pathways and increase membrane permeability. Aggregation of the AAMSNs formed interfacial defects, resulting in the dramatic increase in permeability observed for M50. Extra space formed between the particles when the AAMSNs aggregate. Gases can pass through the extra space easily that is the reason why the permeability dramatically increased. Previous works present similar phenomenon with this work [43–45]. The permeability of M03 was notably lower 4
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Table 2 Permeability (P), selectivity (α), diffusivity (D), and solubility (S) of the control membrane and MMMs at 2 atm and 35 � C. Membrane
PCO2
αCO2/N2
DO2
SO2
αD(CO2/N2)
αS(CO2/N2)
Control PI M03 M20
66.7 54.0 210.1
27.8 28.1 30.8
10.1 8.3 27.6
6.6 6.5 7.6
3.9 4.3 7.1
7.1 6.5 4.3
Membrane
PO2
αO2/N2
DO2
SO2
αD(O2/N2)
αS(O2/N2)
Control PI M03 M20
10.5 8.5 29.5
4.4 4.4 4.3
10.0 6.6 16.3
1.1 1.3 1.8
4.0 4.0 4.2
1.1 1.1 1.0
P: barrer [10 10(cm3) cm cm D: 10 8 cm2 s 1 S: 10 2 cm3 cm 3 cmHg 1.
2
s
1
cmHg 1].
increasing operating pressure; hence, plasticization is a key issue for the industrial use of polymeric membranes. The effect of operating pressure on the CO2/N2 separation performance of the control membrane is summarized in Fig. 6. The pure CO2 permeability of the membrane in creases at operating pressures greater than 5 atm. This finding indicates that the control PI membrane is plasticized by high-pressure CO2. No obvious plasticization effect occurs during high-pressure N2 permeation, and the N2 permeability of the control membrane remains relatively constant, as shown in Fig. 5(a). The plasticization behavior of the AAMSN/PI MMM (M20) was shown in Fig. S5. There is an obvious CO2 plasticization when the pressure above 11 atm. It indicated that the addition of AAMSNs improved the anti-plasticization ability of matrix PI because the AAMSNs reduce chain mobility of PI. The mixed gas (vol ume ratio of CO2 to N2 is 15/85) permeation results showed a trend similar to that of pure gas. The CO2/N2 selectivity decreases and then increases with increasing pressure. The variation of selectivity follows the trend of CO2 permeability, likely due to the change in the perme ability of CO2 with pressure; however, the N2 permeability remains constant. The decreasing lever of nitrogen permeability higher than oxygen permeability, compared Fig. 6(a) and (b), which results in the selectivity for mixed gas over the pure gas (Fig. 6(c)). Table 3 compares the separation performance of the control mem brane and M20 for pure and mixed gas. The pure gas permeability of both membranes is higher than their mixed gas permeability. The permeation competition of mixed gases decreases their permeability. Compared with that for pure gas, the selectivity of both membranes for mixed gas increases, which follows the trade-off description. AAMSN/PI composite MMMs were prepared to evaluate the effect of the amino group of the AAMSNs on the ability of the membranes to facilitate transport. In the presence of H2O in the feed gas, amine carriers can facilitate the permeation of CO2 through amine-containing membranes. The respective permeance and CO2/N2 selectivity of the AAMSN/PI composite MMMs are 96.3 GPU and 1.2 for dry feed stream and 32.1 GPU and 2.0 for wet feed stream. These data indicate that no facilitated
Fig. 4. Free-volume distribution of the control and M20 membranes.
separation performance improving of the MMMs is consistent with the free-volume distribution results obtained from PALS. According to the solution–diffusion model, solubility is an important factor affecting the gas separation performance of a membrane. The O2, N2, and CO2 sorption isotherms of the control membrane and MMMs prepared in this work are shown in Fig. 5. The gas sorption behaviors of the control membrane and MMMs follow the dual-sorption model (solid line). The sorption capacities of the membranes for the different gases show the order CO2 > O2 > N2. The MMMs exhibit higher sorption ca pacity than the control membrane. Addition of AAMSNs to the PI creates more gas sorption spaces in the membrane, which results in a higher sorption capacity. The gas sorption capacity increases with increasing AAMSN loading. Higher AAMSN loadings allow more gas sorption spaces to form in the membrane, thereby increasing its gas sorption capacity. The increase in solubility with AAMSN loading can explain the enhancement in permeability of MMMs by addition of more AAMSNs. Table 2 shows the gas separation factors of the control membrane and MMMs. Addition of a small amount of AAMSNs to M03 obviously decreases the gas diffusion property of the MMM. Decreases in diffu sivity resulted in the lower permeability of M03 compared with that of the control membrane. The strong interaction between amino groups in the AAMSNs and PI inhibited polymeric chain mobility, thereby decreasing diffusivity. More AAMSN contents significantly increased both diffusivity and solubility and dramatically improved the perme ability of M20. During CO2/N2 separation, the diffusivity selectivity increases but the solubility selectivity decreases with increasing AAMSN content. These results indicate that the separation behavior of the MMMs changes from solubility selectivity control to diffusivity selec tivity control as the AAMSN content increases. The permeability of polymeric membranes may increase with
Fig. 5. Sorption isotherms of O2 and N2 (a) and sorption isotherm of CO2 (b) for the control membrane and AAMSN/PI MMMs. 5
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Journal of Membrane Science xxx (xxxx) xxx
Fig. 6. Effect of operating pressure on pure gas permeability (a), mixed gas permeability (b), and selectivity (c) of the control membrane. Table 3 Separation performance of the control and M20 membranes for pure and mixed gas. Membrane Controlled M20
Pure gas
Mixed gas
PCO2 (barrer)
αCO2/N2
PCO2 (barrer)
αCO2/N2
66.7 210.1
27.8 30.8
45.3 132.4
41.1 40.2
transport occurs in the AAMSN/PI MMMs. A small amount of fixed amino groups and high permeation pressure result in the absence of facilitated transport behavior in the AAMSN/PI MMMs. For wet feed streams, reductions in permeance may be due to the blockage of gas pathways by water molecules in the membrane. To evaluate the potential use of the AAMSN/PI MMMs as gas sepa ration membranes, we compared their permeability/selectivity with those of 2008 Robeson’s upper bound. Fig. 7 shows that addition of AAMSNs allows the gas separation performance of the membranes to approach the upper bound. The greater the amount of AAMSN added, the more satisfactory the gas separation performance obtained. Both permeability and selectivity can be improved by tailoring the compo sition of AAMSN/PI MMMs (e.g., M20). The data indicate that the AAMSN/PI MMMs are candidate materials for CO2 capture.
Fig. 7. Upper bound correlation (2008) for CO2/N2 separation: control mem brane (red), M03 (black), M06 (green), M09 (blue), M12 (light blue), M20 (purple). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
appear to be candidate materials for CO2 capture applications. Declaration of competing interest
4. Conclusion
There is no conflict of interest.
In summary, AAMSNs were successfully synthesized by using the solgel method. ODPA-TFMB polyimide was chosen as the matrix to prepare MMMs via incorporation of different weight percentages of AAMSN. The amorphous characteristics and amino groups of the AAMSNs improved the permeability and selectivity of the resulting membranes. The pores of the AAMSMs also improved membrane permeability, and interactions between the amino groups and polymer chain maintained their selec tivity. M20 exhibited a CO2 permeability of 210.1 barrer with an effective CO2/N2 selectivity of 30.8; by comparison, the pure PI mem brane showed a permeability of 66.7 barrer and an effective CO2/N2 selectivity of 27.8. Taking the results together, the AAMSN/PI MMMs
Acknowledgments The authors sincerely thank the Ministry of Economic Affairs and the Ministry of Science of Taiwan for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117542.
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