amine-functionalized TiO2 submicrospheres mixed matrix membranes for CO2 separation

amine-functionalized TiO2 submicrospheres mixed matrix membranes for CO2 separation

Journal of Membrane Science 467 (2014) 23–35 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.co...

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Journal of Membrane Science 467 (2014) 23–35

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

SPEEK/amine-functionalized TiO2 submicrospheres mixed matrix membranes for CO2 separation Qingping Xin a,b, Hong Wu a,b,c,n, Zhongyi Jiang a,b, Yifan Li a,b, Shaofei Wang a,b, Qi Li a, Xueqin Li a,b, Xia Lu a, Xingzhong Cao d, Jing Yang d a

Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China d Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 16 January 2014 Received in revised form 24 April 2014 Accepted 28 April 2014 Available online 10 May 2014

Mixed matrix membranes (MMMs) composed of sulfonated poly(ether ether ketone) (SPEEK) and amine-functionalized titania submicrospheres were prepared. TiO2 submicrospheres (  300 nm) were amine-functionalized through a facile two-step method by using dopamine (DA) and polyethyleneimine (PEI) in succession. The resultant TiO2–DA–PEI microspheres were incorporated into SPEEK with a sulfonation degree of 67%. Grafting PEI with abundant amine groups onto the titania fillers remarkably increased the content of facilitated transport sites in the membranes, leading to an increment in both gas permeability and selectivity. High humidity also contributed to the facilitated transport of CO2 via the generation of HCO3 . The highest ideal selectivities of the SPEEK/TiO2–DA–PEI membranes for CO2/CH4 and CO2/N2 were 58 and 64, respectively, with a CO2 permeability of 1629 Barrer. Besides, the mechanical and thermal stabilities of the membranes were also enhanced compared to pure SPEEK membrane. & 2014 Elsevier B.V. All rights reserved.

Keywords: SPEEK Amine-functionalized TiO2 Mixed matrix membranes Facilitated transport CO2 separation

1. Introduction Polymer-based gas separation membranes have already emerged as a relatively realistic platform for use in large scale industries [1–4]. The high efficiency offered by polymeric membranes for gas separation and their beneficial advantages in terms of capital investment, operational cost and energy saving in comparison to the conventional processes have attracted much attention toward the membrane gas separation process. However, the instinct trade-off between permeability and selectivity for polymeric materials becomes one of the major obstacles for further improvement in separation performance and industrial scale-up [1]. Mixed matrix membranes (MMMs) based on a bulk polymer matrix and a dispersed inorganic phase [5–10] have been proved to be an effective approach to overcoming this obstacle. Numerous materials have been used as the inorganic phase in MMMs, such as carbon molecular sieves [11,12], zeolites [13–15], mesoporous materials [16], activated carbons [17], carbon nanotubes [18] and metal-organic frameworks [19,20]. The concept of MMMs combines n Corresponding author at: Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel./fax: þ 86 22 23500086. E-mail address: [email protected] (H. Wu).

http://dx.doi.org/10.1016/j.memsci.2014.04.048 0376-7388/& 2014 Elsevier B.V. All rights reserved.

the advantages of each constituents: high selectivity and desirable mechanical property of the dispersed inorganic fillers and easy processability of the bulk polymers, thus making it possible to surpass the Robeson's upper-bound constraint [21,22]. Facilitated transport is an efficient method commonly used to enhance the CO2 separation performance by the reversible reaction between facilitated transport carriers and CO2 molecules [23–28]. The commonly used facilitated transport membranes can be divided into three categories: (1) immobilized liquid membranes, (2) solvent swollen, fixed-site carrier membranes, and (3) solid electrolyte polymer membranes. Although these membranes have shown very high performance with respect to natural gas and hydrocarbon separation, their mechanical and long-term high-performance stabilities are still unsatisfied [29]. Therefore, the practical application of facilitated transport membranes in natural gas separation (CO2/CH4) is still pending. Incorporation of CO2-facilitated transport fillers (CO2-FTF) into polymeric membranes has been studied recently. Chung et al. [30] investigated the effect of Ag þ ion exchange treatment of zeolite on the CO2/CH4 gas separation performance of PES–zeolite AgA MMMs and found that the CO2 and CH4 permeability decreased with increasing zeolite content due to partial pore blockage and polymer chain rigidification, whereas the CO2/CH4 selectivity

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increased because of the combined effect of the facilitated transport of Ag þ and the molecular sieving of zeolite. Ismail et al. [31] explored the facilitated transport effect of Ag þ ion exchanged halloysite nanotubes (HNTs) in membranes on the CO2/CH4 gas separation performance. Wang et al. [32,33] prepared high performance membranes with CO2-facilitated transport highway constructed by incorporating polyaniline nanoparticles into polyvinylamine matrix for CO2/N2 separation. Kim et al. [34] reported an increased CO2/CH4 selectivity of polysulfone membranes doped with APTES functionalized MCM-41 particles. Filler shape (sheet, tube, spheres, etc.) is also an important factor influencing the dispersion in the mixed matrix membranes and the final separation performance. Coronas et al. [35–39] investigated both micro and submicro spheres, and they found that the use of spherical-shaped fillers led to reduced agglomeration due to minimized contact area between particles. Spherical fillers with high mechanical, thermal stability and ease of chemical functionalization have been incorporated into polymer to increase permeability [16,22]. Sulfonated poly(ether ether ketone) (SPEEK) is a glassy polyelectrolyte and has recently been used as a potential gas separation membrane materials [40–44]. In this study, SPEEK was used as the polymer matrix and amine-functionalized titania submicrospheres were used as the filler to fabricate mixed matrix membranes for gas separation. The membrane morphology, polymer chain rigidity, membrane free volume property, mechanical property and thermal stability were characterized and the separation performance was explored for CO2/CH4 and CO2/N2 mixtures.

Fig. 1. Chemical structure of (a) dopamine, (b) PEI and (c) SPEEK.

2. Experimental 2.1. Chemicals and materials Poly(ether ether ketone) (PEEK) was purchased from Victrex High-performance Materials (Shanghai, China) Co., Ltd. 3-2-(3,4dihydroxyphenyl)ethylamine (Dopamine Fig. 1(a)) was purchased from Yuancheng Technology Development Co., Ltd. (Wuhan, China). Polyethyleneimine (PEI, M.W.¼ 1800, 99%, Fig. 1(b)) was purchased from Gracian chemical technology Co., Ltd. (Chengdu, China). Tetrabutyl titanate, hydrochloric acid, ethylene glycol, sulfuric acid, N,N-Dimethyl acetamide (DMAc) and acetone were of analytical grade and purchased from Tianjin Guangfu Fine Chemical Research Institute(Tianjin, China). Tris(hydroxymethyl) aminomethane (Tris) was purchased from Sigma-Aldrich. Sulfonated poly(ether ether ketone) (SPEEK) (Fig. 1(c)) with different sulfonation degree was prepared by direct sulfonation of PEEK [45]. PEEK was firstly dried in oven at 80 1C for 24 h before sulfonation. Then, the dried PEEK (14 g) in triplicate was gradually dissolved into 100 mL sulfuric acid (H2SO4, 95–98%) in a three-neck flask for about 4.5 h at room temperature, followed by vigorous stirring at 50 1C for 8 h, 10 h and 12 h, respectively. Afterward, the polymer solutions were gradually precipitated into water under mechanical agitation, respectively. Finally, the polymer precipitate was washed several times with deionized water until pH reached neutral and then dried first at room temperature for 24 h and then at 60 1C for another 24 h. The degrees of sulfonation (DS) were determined through acid–base titration method. 2.2. Preparation of amine-functionalized TiO2 submicrospheres and membranes TiO2 submicrospheres (0#) with a particle size of about 300 nm were synthesized by a sol–gel method as described in previous study [46]. Firstly, TBT (0.02 mol) was poured into 100 ml of ethylene glycol under nitrogen atmosphere to prepare the precursor solution. The precursor solution was magnetically stirred

Fig. 2. The grafting mechanism of dopamine and PEI on titania.

for 10 h at room temperature and then added into acetone solution containing 0.3 wt% water. The concentration of TBT in acetone was 0.01 M. For amine-functionalized TiO2, dopamine and PEI were subsequently used as modification reagents (Fig. 2). Firstly, a facile chelation procedure was conducted. 0.5 g TiO2 powders were suspended in 250 ml deionized water under ultrasonic treatment for 2 h to break aggregates, and 1.0 g dopamine was dissolved in 250 ml Tris–HCl (pH¼ 8.5) used as modification reagent solution. The TiO2 suspension was mixed with the same volume of modification reagent, followed by vigorous stirring for 24 h [47]. The polydopamine-coated TiO2 submicrospheres were collected by centrifugation, washed with distilled water until neutral and dried in a vacuum oven at 80 1C for 24 h. Secondly, 0.5 g polydopamine-coated TiO2 subsequently reacted with PEI in 5 wt% aqueous solution via Michael addition or Schiff base reactions at 60 1C for 6 h [48]. The obtained polydopamine-coated titania and the further PEI-grafted titania were designated as TiO2– DA (1#) and TiO2–DA–PEI (2#), respectively. A color change from white (TiO2) to dark gray (TiO2–DA) and then to brown (TiO2–DA– PEI) was observed during the functionalization procedure. SPEEK (0.6 g) was dissolved in DMAc (8 g) at room temperature. A measured amount of fillers was dispersed in DMAc (4 g) and the resulting suspension was added into SPEEK–DMAc solution under vigorously stirring for 24 h to get a fine dispersion of fillers in the polymer solution. After degasification, the mixture was poured on a clean glass plate and heated overnight at 60 1C for

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12 h, followed by annealing at 80 1C for another 12 h. The membranes were peeled off from the glass plate, dried under vacuum at 60 1C for 24 h and designated as SPEEK/0#-X, SPEEK/ 1#-X and SPEEK/2#-X, respectively, where 0#, 1# and 2#, represented the inorganic fillers of TiO2, TiO2–DA and TiO2–DA–PEI, respectively, and X ( ¼5, 10 or 15) was the weight percentage of the filler content relatively to the weight of SPEEK. The amounts of DA and PEI on the TiO2 were determined by TGA. Pure SPEEK membrane was also fabricated for comparison. The thicknesses of the as-prepared membranes were in the range of 50–60 μm. 2.3. Characterization 2.3.1. Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) The morphology of the TiO2, TiO2–DA and TiO2–DA–PEI submicrospheres was characterized by transmission electron microscopy (TEM, JEOL, Tecnai G2 F20). Surface area and pore size distribution of the pristine TiO2 microspheres was measured by nitrogen adsorption–desorption isotherms, which were conducted at 77 K (Micromeritics ASAP2020 V4.0). Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution were calculated from the adsorption and desorption data, respectively. 2.3.2. Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) Fourier transform infrared spectra (FTIR, 4000–400 cm  1) were recorded on a Nicolet MAGNA-IR 560 instrument. The surface chemical composition of the submicrospheres (TiO2, TiO2–DA and TiO2–DA–PEI) was monitored by X-ray photoelectron spectroscopy (XPS) using a PHI 1600 spectrometer with Mg K a radiation for excitation. 2.3.3. Field emission scanning electron microscope (FESEM) The cross-section morphology and dispersion of submicrospheres (TiO2, TiO2–DA and TiO2–DA–PEI) in the membrane samples were examined with a Nanosem 430 field emission scanning electron microscope operated at 10 kV. Membrane samples were cryogenically fractured in liquid nitrogen and then sputtered with a thin layer of gold. 2.3.4. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) To determine the glass transition temperature (Tg) of pure SPEEK and the MMMs, differential scanning calorimetry (DSC) was performed on a 204 F1 NETZSCH. At least 10 mg of each sample was collected in an aluminum sample holder. Samples were preheated under nitrogen from room temperature to 150 1C at 10 1C min  1, then cooled to 90 1C and reheated to 260 1C. The thermal gravimetrical analysis (TGA) was performed on a PerkinElmer TGA 4000. At least 10 mg of each sample was placed into a small aluminum sample holder. The sample was heated to 900 1C at a heating rate of 10 1C min  1 under a constant nitrogen flow of 20 ml min  1.

(the resolution was 201 ps) to investigate the free volume property of the membranes. The positron source-22Na was sandwiched between two pieces of samples with a thickness range of 1.0– 1.2 mm. The spectra with more than one million counts were recorded and then resolved by LT-v9 program. On assumption that the location of o-Ps occurs in a sphere potential well surrounded by an electron layer of a constant thickness Δr (0.1656 nm), radius of free volume cavity (r3) was calculated from the pick-off annihilation lifetime of o-Ps (τ3) by the following semi-empirical equation:     1   1 r 1 2π r 3 τ3 ¼ 1  3 þ ð1Þ sin 2 2π r 3 þ Δr r 3 þ Δr The apparent fractional free volume (FFV) of the equivalent sphere could be calculated by using the following equation: 4 FFV ¼ π r 3 3 I 3 3

ð2Þ

2.3.7. Water uptake and membrane swelling Membranes were dried at 60 1C till constant weight (Wdry, g) and the area (Adry, cm2) of membranes were measured. Then, the weights (Wwet, g) and areas (Awet, cm2) of wet membranes were measured each time after gas permeability test immediately. The water uptake and area swelling were the average of three measurements with an error within 5.0% and calculated based on the following equations (Eqs. (3) and (4)), respectively. Water uptake ð%Þ ¼

W wet  W dry  100 W dry

ð3Þ

Area swelling ð%Þ ¼

Awet  Adry  100 Adry

ð4Þ

2.4. Gas permeation tests Pure gas (CO2, CH4 and N2) and mixed gas (CO2/CH4 (30 vol%: 70 vol%), CO2/N2 (10 vol%:90 vol%)) permeation measurements were conducted at 25 1C based on the conventional constant pressure/variable volume technique (Fig. 3). The gas transport properties of the membrane were measured using a flat-sheet permeation cell which was placed in a thermostat oven to control the experimental temperature. N2 and CH4 were used as the sweep gases for CO2/CH4 feed and CO2/N2 feed, respectively. In a typical measurement, feed gas was introduced into a water bottle (35 1C) to be saturated with water vapor, and then passed through an empty bottle to remove the residual water. Meanwhile, the sweep gas was humidified at room temperature. For comparison, dry-state gas permeation experiments were also conducted, in which case the feed gas and sweep gas were directly introduced into the membrane cell. The gas flow rate was controlled by mass flowmeters.

2.3.5. Mechanical property Mechanical property of the membranes was studied using an Instron Mechanical Tester (Testometric 350 AX). Each sample was cut into 1.0 cm  4.0 cm and examined with an elongation rate of 4 mm min  1 at room temperature. 2.3.6. Free volume property Positron annihilation lifetime spectroscopy (PALS) of the membranes was recorded with an ORTEC fast-fast coincidence system

25

Fig. 3. Schematic diagram of gas permeation apparatus.

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The gases were injected into a gas chromatograph (Agilent 6820 gas chromatography equipped with a thermal conductive detector (TCD)) to determine the composition of permeate. The permeability (Pi, Barrer, 1 Barrer equals 1  10  10 cm3 (STP) cm/(cm2 s cmHg)) of each gas was measured and each set of data was obtained from at least three tests. Pi was determined by the equation: Pi ¼Qil/ΔpiA, where Qi was the volumetric flow rate of gas ‘i’ (cm3/s (SPT)), l was the thickness of the membranes measured by a micrometer calliper (μm). Δpi was the transmembrane pressure difference (cmHg), and A was the effective membrane area, 12.5 cm2. The ideal selectivity and mixed-gas separation factor of gas ‘i and j’ (αi/j) was calculated by: αi/j ¼ Pi/Pj.

3. Results and discussion 3.1. Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) The microstructure of the fillers was determined by TEM and BET analysis as shown in Figs. 4 and S1, respectively. Fig. 4(a) shows the TEM image of the TiO2 submicrospheres with an average particle size of approximately 300 nm. After polydopamine coating (TiO2–DA), the diameter increased by  50 nm (Fig. 4(b)), and no more change in size was observed after further amine functionalization by PEI (TiO2–DA–PEI) (Fig. 4(c)). Moreover, the particle morphology was well maintained after functionalization. The N2 adsorption–desorption isotherm and pore diameter distribution curves of pristine TiO2 microspheres are shown in Fig. S1. The N2 adsorption–desorption isotherm was the type II curve which revealed the non-porous

structure of TiO2 microspheres. The BET data indicated that the surface area of the bare TiO2 particles was 229.7 m2/g. The effect of nonporous fillers on the MMMs is different from porous fillers. Gas molecules cannot permeate through nonporous fillers but can only transport on the filler surface. 3.1.1. Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron (XPS) spectroscopy and thermal gravity analysis (TGA) FTIR, XPS and TGA were employed to analyze the surface composition of the functionalized titania submicrospheres. As shown in Fig. 5(a), compared with the spectrum of pristine TiO2, the characteristic peaks of ethylene glycol from the pristine TiO2 surface at 1080 cm  1, 2868 cm  1 and 2930 cm  1 [49] became weaker after modification. Polydopamine-coated TiO2 and PEIgrafted TiO2 displayed very similar FTIR spectra including the distinct bands at about 1509 cm  1, 1600 cm  1 and 1270 cm  1, which corresponded to the N–H deformation vibration, C ¼ C and C–N stretching vibration in polydopamine, respectively. These characteristic bands suggested that the polydopamine layer was successfully introduced onto the TiO2 surface. XPS was utilized to track the surface compositions of the TiO2 before and after functionalization. Table 1 lists the detailed data from XPS scans on different surfaces. The N1s from polydopamine was about 5.8%, and after the PEI grafting, the content of nitrogen increased to 12.2%. The characteristic signal of N1s at 400 eV was obviously observed compared to those of TiO2–DA and TiO2, indicating the successful grafting of PEI (Fig. 5(b)). In addition, the disappearance of Ti2p species at 458.7 eV could further confirm that the coating layer on the surface of TiO2 was defect-free (Fig. 5(c)). The thermal stability of TiO2–DA and TiO2–DA–PEI analyzed by TGA is shown in

Fig. 4. TEM image of submicrospheres: (a) TiO2; (b) polydopamine-coated TiO2, (TiO2–DA); (c) amine functionalized-TiO2 (TiO2–DA–PEI).

Transmittence

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TiO

2930

27

1080

2868

2

TiO2-DA TiO2-DA-PEI

3500

1600

3000

2500 2000

1270 1509

1500

1000

500

-1

Wave number/cm

Fig. 5. (a) FTIR spectra; (b) XPS spectra of N1s; (c) XPS spectra of Ti2p; (d) TGA of TiO2, TiO2–DA and TiO2–DA–PEI.

Table 1 Main elemental analysis on surface of as-prepared TiO2 and functionalized TiO2 from XPS. Samples

TiO2 TiO2–DA TiO2–DA–PEI

dopamine-functionalized fillers or PEI-grafted fillers led to an improved compatibility between the filler and the polymer matrix which was favorable for eliminating non-selective interface voids.

Element (atom%) C1s

O1s

Ti2p

N1s

75.4 76.4 75.1

23.4 17.8 12.7

1.2 0 0

0 5.8 12.2

Fig. 5(d). The samples exhibited a large weight loss between 50 1C and 195 1C, which was ascribed to desorption of water. The second weight loss stage between 230 1C and 700 1C was attributed to the decomposition of the coating layer, 13.27% and 17.39% for TiO2–DA and TiO2–PEI–DA respectively. The content of grafted PEI was calculated to be 4.12%.

3.1.2. Field emission scanning electron microscope (FESEM) The membrane morphologies were investigated by examining cross section using FESEM shown in Fig. 6. The inorganic particles dispersed well within the membrane with a content of lower than 15%. Compared with the SPEEK/0#-15 membrane doped with unfunctionalized titania, the dispersion of fillers in SPEEK/1#-15 and SPEEK/2#-15 membranes doped respectively with TiO2–DA and TiO2–DA–PEI was modestly improved mainly due to the attractive interactions ( S–O     þ H-HN– and –S–O     þ H-N) existing between the filler and SPEEK matrix. The introduction of

3.1.3. Glass transition temperature (Tg) As shown in Table 2, the embedment of TiO2–DA–PEI in the membranes altered the polymer chain mobility as reflected by the change of glass transition temperature (Tg). Pure SPEEK membrane displayed a Tg of 121.5 1C. The non-functionalized TiO2 based MMMs showed an increase of Tg at different loadings (Table S1), and this increase in Tg was attributed to the increased chain rigidity or stiffness imparted by the addition of fillers [50]. Compared to SPEEK/0# membranes, nearly all the membranes doped with TiO2–DA or TiO2–DA–PEI showed a decreased Tg, indicating a less chain rigidity of these membranes. 3.1.4. Mechanical property and thermal analysis of the membranes Good mechanical and thermal properties are crucial for membranes to guarantee a long lifetime of the gas separation performance. Compared to pure SPEEK membranes, all the as-prepared MMMs showed an increase both in tensile strength and Young's modulus. The pure SPEEK showed a Young's modulus of 542.86 MPa and an elongation at break of 144.0%. For the MMMs, both the tensile strength and Young's modulus increased with the increase of the filler content from 5 wt% to 15 wt%. Especially, Young's modulus increased remarkably from 640 MPa to 1380 MPa, indicating a significant improvement in mechanical strength. As the filler content increased, the elongations at break

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Fig. 6. FESEM images of the cross-section of pure SPEEK and MMMs: (a) SPEEK; (b) SPEEK/0#-5; (c) SPEEK/0#-10; (d) SPEEK/0#-15; (e) SPEEK/1#-5; (f) SPEEK/1#-10; (g) SPEEK/1#-15; (h) SPEEK/2#-5; (i) SPEEK/2#-10; (j) SPEEK/2#-15.

decreased from 144.0% to 118.5%. The increase in tensile strength could be also attributed to increased chain rigidity imparted by the addition of the fillers or improved filler–polymer interactions. The thermal properties of the membranes were investigated by thermogravimetry programmed from 30 1C to 800 1C (Fig. 7). All the membranes exhibited three major weight loss stages. The

initial weight loss around 150 1C was attributed to the evaporation of adsorbed water and residual solvent. The weight loss region starting from about 295 1C was ascribed to the decomposition of the –SO3H groups and the organic layer coated onto TiO2, and then followed by the weight loss from the degradation of the polymer main chains at about 490 1C. A relatively higher degradation

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Table 2 Free volume parameters and Tg of membranes. Membrane

I3 (%)

τ3 (ns)

r3 (nm)

FFV (%)

Tg (1C)

SPEEK SPEEK/0#-15 SPEEK/1#-15 SPEEK/2#-5 SPEEK/2#-10 SPEEK/2#-15

3.9777 0.083 3.7607 0.150 3.7007 0.130 4.1257 0.048 4.5667 0.059 4.4067 0.083

2.071 70.022 2.215 70.034 2.105 70.036 2.07970.020 2.053 70.015 2.112 70.018

0.2917 0.002 0.3047 0.003 0.294 7 0.003 0.292 7 0.002 0.290 7 0.001 0.295 7 0.002

0.4117 0.017 0.4427 0.031 0.394 7 0.026 0.430 7 0.014 0.4667 0.011 0.4747 0.019

121.5 152.0 136.1 124.8 137.8 140.3

60 800

CO2 permeability

90

700

CO2 /CH4 selectivity

70 60 SPEEK/0#-15

50

SPEEK/1#-15 SPEEK/2#-15

40

SPEEK

30

100 200 300 400 500 600 700 800 900 1000 1100 o

Temperature/ C Fig. 7. TGA curves of membranes.

temperature for membranes embedded with functionalized fillers was observed in the last two weight loss stages, mainly due to the degradation of functionalized groups on TiO2 surface. Compared to the pure SPEEK membrane, the intensity of the second peak weakened and the thermal decomposition temperature at the third peak was slightly enhanced for the mixed matrix membranes (Fig. S2). Consequently, the presence of inorganic fillers and amine-functionalized fillers delayed the oxidative degradation of SPEEK main chains, leading to an improvement in membrane thermal stability.

3.1.5. Free volume property of the membranes The free volume property of the membranes was characterized and the parameters are listed in Table 2. The size of the free volume cavity (r3) in the membrane with unfunctionalized TiO2 was enlarged compared to that in pure SPEEK membrane while those in the membranes with TiO2–DA and TiO2–DA–PEI were almost the same as that in pure SPEEK. The fractional free volume (FFV) increased monotonically with the TiO2–DA–PEI contents. This slightly increased free volume cavity size and increased fractional free volume might indicate an increase in the gas permeation without scarifying the separation selectivity much.

3.2. Gas separation performance 3.2.1. Effect of water uptake SPEEK is a glassy polymer and the pure gas permeabilities are very low in dry state. All the membranes were fully humidified before gas separation measurements. Dense SPEEK membranes with different degrees of sulfonation were prepared and pure-gas CO2 permeability, CO2/CH4 and CO2/N2 selectivities were measured at humidified condition as shown in Fig. 8. With the increase in degree of sulfonation, the permeability and selectivity of both gas pairs increased simultaneously. However, membranes with a high degree of sulfonation (higher than 75%) showed too poor mechanical and thermal stability. Consequently, a degree of sulfonation of

50

CO2 /N2 selectivity

600

40

500 30

400 300

Selectivity

Weight (%)

80

CO2 permeability (Barrer)

100

20

200 10 100 0

56

67

75

0

Degree of sulfonation (%) Fig. 8. Pure gas permeability and ideal selectivity for SPEEK membranes with different degrees of sulfonation. Wet membranes were test at 1 bar feed pressure and 25 1C with humidified feed gas and sweep gas.

67% was used in this study to acquire high gas separation performance and good mechanical stability. Time-dependent mixed-gas CO2 separation properties of SPEEK and SPEEK/2#-15 membranes are plotted in Fig. S3. The membrane weight and area swelling became constant in 2 h, and the water uptake and area swelling of the membranes are shown in Fig. 9. Compared to the pristine SPEEK membrane, both the water uptake and the area swelling of the mixed matrix membranes increased. These increases were due to the increased free volume and r3 of the mixed matrix membranes after incorporation of inorganic fillers. Gas separation performance of the membranes is shown in Fig. 10. The water uptake had a tremendous influence on the gas permeability of SPEEK due to the hydrophilic nature of the sulfonated polymer and the accompanying high swelling degree [43]. As a hydrogel with sufficient –SO3 groups, humidified SPEEK exhibited relatively high CO2 selective absorbability which would be beneficial for CO2 transport based on the solution-diffusion mechanism. It can be observed that the separation the performance of some humidified membranes surpassed the Robeson's upper bound line reported in 2008, whereas the separation performance of all the dry membranes fell far below the line. Besides the solution-diffusion mechanism, CO2 transport was also facilitated by the reactions between CO2 and the amine carriers under humidified conditions. From another viewpoint, the permeability was significantly enhanced after humidification for amine-containing membranes, and the content of water uptake made great contribution to CO2 permeability (Fig. 11). As can be seen, CO2 separation performance was strongly dependent on water uptake, which is ubiquitous for the facilitated transport membranes. The presence of water enhanced the solubility of CO2 via acid–base interactions. CO2, as a Lewis acid, could react with the amide functional groups in the humidified membrane, producing HCO3 which facilitated CO2 transport by “hopping” from one site to another along the fixed sites.

3.2.2. Effect of filler content The effect of the filler contents on the separation property of the MMMs for CO2/CH4 and CO2/N2 gas pairs is shown respectively

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in Fig. 12(a) and (b). In humidified SPEEK/2# MMMs, both permeability and selectivity were significantly enhanced after humidification and increased with increasing filler contents. It can be seen that CO2 permeability increased from 565 Barrer for pure SPEEK to 1629 Barrer for SPEEK/2#TiO2–DA–PEI membrane at 15 wt% loading. The ideal CO2/CH4 selectivity and the ideal CO2/N2 selectivity increased from 23 and 38 for pure SPEEK to 58 and 64 for SPEEK/ 2#TiO2–DA–PEI membrane at 15 wt% loading, respectively. The increase in CO2 permeability could be attributed to the following reasons. First, water uptake played an important role in SPEEK/2# MMMs. With the increase of water uptake, both the solubility and the diffusivity of CO2 increased as shown in Fig. 11, and therefore the CO2 permeability increased. Second, the reaction between CO2 and amine carriers was greatly promoted under humidified conditions, the CO2 permeability increased with the increasing amount of amine carriers in the membrane. Third, since the decreased chain rigidity and increased FFV of the SPEEK/2# MMMs compared to SPEEK/0# MMMs, the CO2 permeability increased. It was worth noting that the high FFV in the membranes with high filler content suggested more diffusion pathways for all gases and thus leading to a decreased selectivity. The FFV and r3 of SPEEK/2#-15 membrane were higher than that of pure SPEEK membrane. If the transport of CO2 in the membranes only depended on solution-diffusion model, the selectivity of SPEEK/ 2#-15 should be smaller than or approximately equal to that of pure SPEEK membrane. However, the truth was just the reverse. This phenomenon was attributed to the reversible reaction between amine groups and CO2 which caused an increased driving force for CO2 transport across the SPEEK/2# membranes. It has

been proved that the presence of amine groups on adsorbent surface was able to enhance the adsorbent's affinity toward CO2. As the content of amine groups increased, the conversion of CO2 into HCO3 was more favored which in turn increased the solubility of CO2. Therefore, the high selectivity of SPEEK/2# membrane benefited directly from the presence of numerous amine groups that allowed easier passage of CO2 over CH4 and N2, besides the increased rigidification of polymer chains compared to pure SPEEK. Binary gas mixtures of CO2/CH4 and CO2/N2 were used to evaluate the real separation performance of MMMs. The permeability and mixed gas separation factor are shown in Fig. 13. The separation factor obtained under mixed gas conditions was lower than the ideal selectivity values due to the competition effect of penetrants. 3.2.3. Effect of fillers type The CO2 permeability, CO2/CH4 selectivity and CO2/N2 selectivity of different membrane types were showed by comparing with that of the pure SPEEK membrane (Fig. 14). For the SPEEK/0# membranes loaded with pristine titania, the permeability and selectivity increased with the increase of the filler content up to 10 wt%. The simultaneous increase of both permeability and selectivity suggested that transport properties could be tuned by addition of inorganic phase without generating defects. The addition of TiO2 to SPEEK disrupted the original ordered packing of SPEEK chains, increasing the accessible free volume without introducing cavities that are large enough to promote nonselective 1800

Water uptake (%)

30 25

SPEEK/0# SPEEK/1# SPEEK/2# SPEEK/0# SPEEK/1# SPEEK/2#

18 16 14

20 12

15

10

10

8

5

Area swelling (%)

Water uptake Water uptake Water uptake Area swelling Area swelling Area swelling

35

CO 2 permeability (Barrer)

20

6

0 5

10

1400 1200 1000 800 600 400

4 0

1600

15

10

15

Fig. 9. The water uptake and area swelling properties of the pristine SPEEK membrane and mixed matrix membranes at 25 1C.

30

1000 Upper bond line(2008)

Upper bond line(2008)

CO2 /N 2 selectivity

CO 2 /CH 4 selectivity

25

Fig. 11. Correlations between CO2 permeability and water uptake of membranes. Wet membranes were test at 1 bar feed pressure and 25 1C.

1000

100

Dry membrane

10

Humidified pure SPEEK membrane Humidified SPEEK/0#s MMMs Humidified SPEEK/1#s MMMs

100

Dry membrane

10

Humidified pure SPEEK membrane Humidified SPEEK/0#s MMMs

Upper bond line(1991)

Humidified SPEEK/1#s MMMs Humidified SPEEK/2#s MMMs

Humidified SPEEK/2#s MMMs

1

20

Water uptake (wt%)

Content of filler (%)

1

10

100

1000

CO2 permeability (Barrer)

10000

1

1

10

100

1000

10000

CO2 permeability (Barrer)

Fig. 10. Gas separation performance of membranes. Dry membranes were test at 10 bar feed pressure and 25 1C. Wet membranes were test at 1 bar feed pressure and 25 1C.

Q. Xin et al. / Journal of Membrane Science 467 (2014) 23–35

70

CO2/CH4 SPEEK/0#

1200

60

CO2/CH4 SPEEK/1# CO2/CH4 SPEEK/2#

50

900

40

600

30 300

CO 2 permeability (Barrer)

1500

90

1800

Pure gas CO 2 /CH 4 selectivity

CO 2 permeability (Barrer)

PCO2 SPEEK/0# PCO2 SPEEK/1# PCO2 SPEEK/2#

PCO2 SPEEK/0# PCO2 SPEEK/1# PCO2 SPEEK/2# CO2/N2 SPEEK/0#

1500 1200

80 70

CO2/N2 SPEEK/1# CO2/N2 SPEEK/2#

900

60 50

600

40

300

30

0

20 0

5

10

Pure gas CO 2 /N 2 selectivity

80

1800

31

20

15

0

5

10

15

Content of filler (%)

Content of filler(%)

1200

CO2/CH4 SPEEK/1# CO2/CH4 SPEEK/2#

900

40

600 30 300 20

0 0

5

10

PCO2 SPEEK/0# PCO2 SPEEK/1# PCO2 SPEEK/2# CO2/N2 SPEEK/0# CO2/N2 SPEEK/1# CO2/N2 SPEEK/2#

1200

900

70 60 50

600

40 30

300

20

0

0

15

5

10

Mixed gas CO 2 /N 2 separation factor

CO 2 permeability (Barrer)

50

CO2/CH4 SPEEK/0#

80

1500

CO 2 permeability (Barrer)

60 PCO2 SPEEK/0# PCO2 SPEEK/1# PCO2 SPEEK/2#

1500

Mixed gas CO 2 /CH4 separation factor

Fig. 12. Effect of fillers content on (a) pure gas CO2 permeability and CO2/CH4 ideal selectivity, (b) pure gas CO2 permeability and CO2/N2 ideal selectivity. Wet membranes were test at 1 bar feed pressure and 25 1C with humidified feed gas and sweep gas.

15

Content of filler (%)

Content of filler (%)

Fig. 13. Effect of fillers content on (a) mixed-gas CO2 permeability and CO2/CH4, separation factor, (b) mixed-gas CO2 permeability and CO2/N2 separation factor Wet membranes were test at 1 bar feed pressure and 25 1C with humidified feed gas and sweep gas.

CO2 permeability (Barrer)

1800 5% 10% 15%

1600 1400 1200 1000 800 600 400 200 0

SPEEK

SPEEK/0#

SPEEK/1#

SPEEK/2#

Membrane type 70

50

5% 10% 15%

40 30 20 10 0

5% 10% 15%

60

CO2 /N2 selectivity

CO2 /CH 4 selectivity

60

50 40 30 20 10

SPEEK

SPEEK/0#

SPEEK/1#

Membrane type

SPEEK/2#

0

SPEEK

SPEEK/0#

SPEEK/1#

SPEEK/2#

Membrane type

Fig. 14. The effect of membrane types on (a) CO2 permeability; (b) CO2/CH4, selectivity; (c) CO2/N2 selectivity in pure gases. Permeation tests were performed at 25 1C and 1 bar feed pressure with humidified feed gas and sweep gas.

32

Q. Xin et al. / Journal of Membrane Science 467 (2014) 23–35

flow. Hence, the obtained selectivity was higher than the pure SPEEK. When the TiO2 content was higher than 10 wt%, such as SPEEK/0#-15, the aggregation of the TiO2 became obvious and led to microscopic voids that increased non-selective permeation and decrease of selectivity. For the SPEEK/0# membranes loaded with pristine titania particles, there was no facilitated transport groups in these membranes, therefore, the main separation mechanism obeyed the solution-diffusion mechanism. For SPEEK/1# and SPEEK/2# membranes, CO2-facilitated transport fillers (TiO2–DA and TiO2–DA–PEI) were introduced into the polymer matrix. The amine groups on the titania surface acted as facilitated transport sites helping to transfer CO2 molecules continuously and rapidly in polymer matrix. The CO2 permeability of membranes loaded with TiO2–DA–PEI was higher than that of membranes loaded with TiO2 and TiO2–DA at the same loading content above 5 wt%. Compared to the pure SPEEK membrane, the SPEEK/1#-15 membrane showed an increased CO2/CH4 and CO2/ N2 selectivity of 23% and 35%, respectively, while the SPEEK/2#-15 MMM showed an increased CO2/CH4 and CO2/N2 selectivity of 102% and 65%, respectively. Notably, the selectivity of CO2/CH4 and CO2/N2 in SPEEK/2# membranes increased more remarkably than in SPEEK/0# and SPEEK/1# membranes at the same loading. The analysis of free volume characteristics showed that the free volume cavity and r3 of SPEEK/2#-15 membrane were slightly greater than those of SPEEK/1#-15. However, the selectivity of the SPEEK/2#-15 membrane was obviously greater than that of the SPEEK/1#-15 membrane and this was because the TiO2–DA–PEI with abundant amine groups provided more facilitated sites and increased the probability of collisions between amine and CO2.

Compared to the pure SPEEK membrane, the incorporation of TiO2 showed a substantial enhancement in CO2 permeability and a slight decrease in CO2/CH4 and CO2/N2 selectivity at high loading, which was attributed to the microscopic voids that increased nonselective permeation and decrease of selectivity. The introduction of dopamine-functionalized fillers in MMMs made great contribution to achieving good adhesion and compatibility between filler and polymer matrix as well as the adjustment of polymer chain packing and reducing the formation of adhesion problems such as interface voids or rigidified polymer layer, while these MMMs also showed an increase in both CO2/CH4 and CO2/N2 selectivity. With good dispersion in membranes, the incorporating of the PEIgrafted fillers in membranes showed the most significant enhancements in both CO2/CH4 and CO2/N2 selectivity compared to other type of fillers at the same loading. The increased selectivity was mainly attributed to the increased facilitated transport sites as well as the relatively good filler–polymer interface compatibility in membranes. The increased selectivity was mainly attributed to the increased facilitated transport sites as well as the relatively good filler–polymer interface compatibility in membranes. Consequently, in both SPEEK/1# and SPEEK/2# membranes, the gas separation was mainly controlled by facilitated transport mechanism.

3.2.4. Effect of feed pressure Figs. 15 and S4 show the effect of pressure on humidified gas permeation. Both the CO2 permeability and CO2/CH4 separation factor decreased with the increase of feed pressure for the

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

1600 1400 1200 1000 800 600 400

0

2

4

6

Mixed gas CO 2 /CH 4 separation factor

Mixed gas CO2 permeability (Barrer)

1800 50

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

45 40 35 30 25 0

8

2

4

6

8

Pressure (bar)

Pressure (bar)

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

1400 1200 1000 800 600 400

0

2

4 Pressure (bar)

6

8

Mixed gas CO 2 /N2 separation factor

Mixed gas CO2 permeability (Barrer)

1600

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

80

60

40

0

2

4

6

8

Pressure (bar)

Fig. 15. Effect of pressure on (a) mixed-gas CO2 permeability; (b) mixed-gas CO2/CH4 separation factor and (c) mixed-gas CO2 permeability; (d) mixed-gas CO2/N2 separation factor of membranes. Permeation tests were performed at 25 1C with humidified feed gas and sweep gas.

Q. Xin et al. / Journal of Membrane Science 467 (2014) 23–35

2400 2100

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

1800 1500 1200 900 600 300 295

300

305

in the polymer matrix which indicated a solubility-controlled mechanism.

3.2.5. Effect of operating temperature CO2/N2 (10/90 vol%) mixed gases and CO2/CH4 (30/70 vol%) mixtures were utilized to study the influence of operating temperatures as shown in Fig. 16. Generally, for a glassy polymer, the diffusivity increased with temperature due to increased flexibility of the polymer chains. The CO2, CH4 and N2 permeabilities gradually increased with temperature, while the separation factor of CO2/CH4 and CO2/N2 decreased. Nevertheless, the mixed gas CO2/N2 separation factor still remained as high as 39 at 60 1C,

Mixed gas CH4 permeability (Barrer)

Mixed gas CO2 permeability (Barrer)

SPEEK/2#-15 membrane (Fig. 15(a), (b)), which agreed with the common variation trend of facilitated transport membranes. As the pressure increased, the adsorption capacity of the carriers on the feed side of the membrane became saturated and so the complexation reaction rate stabilized. However, the decline tendency for the as-prepared membrane was not as sharp as that for other conventional facilitated transport polymer membranes. Moreover, the CO2 permeability remained rather high within the whole pressure range of 1–8 bars. For the SPEEK/2#-15 membrane, the CO2 permeability decreased but the CO2/N2 separation factor increased with the increase of feed pressure (Fig. 15(c), (d)). This was due to the weak dependence of CO2 permeability on facilitated transport and the enhanced solubility of CO2 molecules

310

315

320

325

330

335

140 120 100

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

80 60 40 20 295

300

305

Temperature (K)

Mixed gas CO 2 permeability (Barrer)

Mixed gas CO 2 /CH 4 separation factor

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

50 45 40 35 30 25 20 15 300

305

310

315

310

315

320

325

330

335

325

330

335

Temperature (K)

55

10 295

33

320

325

330

2400 2100 1800 1500 1200 900 600 300 295

335

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

300

305

Temperature (K)

310

315

320

Temperature (K)

70 60

Mixed gas CO2 /N 2 separation factor

Mixed gas N 2 permeability (Barrer)

60 80

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

50 40 30 20 10

SPEEK SPEEK/0#-10 SPEEK/1#-15 SPEEK/2#-15

55 50 45 40 35 30 25

0 295

300

305

310

315

320

325

Temperature (K)

330

335

340

20 295

300

305

310

315

320

325

330

335

Temperature (K)

Fig. 16. Effect of operating temperature on (a) CO2 permeability; (b) CH4 permeability; (c) CO2/CH4 separation factor; (d) CO2 permeability; (e) N2 permeability; (f) CO2/N2 separation factor of membranes in mixed feed gases. Permeation tests were performed at 1 bar feed pressure with humidified feed gas and sweep gas.

CO 2 permeability (Barrer)

75 1600 70 1400 65 1200 60 1000 55 800 50 600 45 0

20

40

60

80

100

120

80 1800

CO 2 permeability (Barrer)

80 1800

75 1600 70 1400 65 1200

60

1000

55

800

50

600 0

20

40

Operating time(h)

60

80

100

120

45

Mixed gas CO 2 /N 2 separation factor

Q. Xin et al. / Journal of Membrane Science 467 (2014) 23–35

Mixed gas CO 2 /CH4 separation factor

34

Operating time(h)

Fig. 17. The long-term gas separation performance test of the SPEEK/2#-15 mixed matrix membrane up to 120 h for (a) CO2/CH4 and (b) CO2/N2 mixture at 1 bar feed pressure and 25 1C with humidified feed gas and sweep gas.

much higher than that of pure SPEEK membrane (35) under the identical conditions. The temperature dependence of gas permeability can be further described by using the Arrhenius equation which relates the gas permeability to the operating temperature via the activation energy of permeation (Ep) as expressed by the following equation:   Ep P ¼ P 0 exp  RT

amine groups to the membranes and the resultant membrane showed the most significant increments in both CO2/CH4 and CO2/ N2 selectivity compared to those with unfunctionalized TiO2 or TiO2– DA fillers at the same loading. The increased selectivity was mainly attributed to the increased facilitated transport sites for CO2 as well as the improved filler–polymer interface compatibility. Moreover, the mechanical and thermal stabilities of MMMs were also enhanced compared to the pure SPEEK membrane.

where P is the permeability of the gas, P0 the pre-exponential factor, R the gas constant and T the absolute temperature. The activation energies determined from the plots of permeability vs. reciprocal temperature showed that the permeability coefficients obey the Arrhenius equation. The activation energy of permeation (Ep) calculated from the slope of lnP vs. 1000/T were 24.69, 19.54, 10.68 and 9.30 kJ/mol for SPEEK, SPEEK/0#-10, SPEEK/1#-15 and SPEEK/2#-15 membranes, respectively. The positive values of Ep were attributed to the increase of permeability with the increase of temperature. Ep had a significantly drop in MMMs incorporated with TiO2–DA–PEI at the loading of 15% compared to pristine SPEEK. Therefore, the rate of increase in CO2 permeability as a function of temperature was lower for MMMs compared to pristine SPEEK membrane.

Acknowledgments

3.2.6. Long-term operation stability The long-term operation stability is a vital factor for the industrial application of membrane. Fig. 17 shows the long-term gas separation performance of the SPEEK/2#-15 mixed matrix membrane up to 120 h for CO2/CH4 and CO2/N2 mixture at 1 bar feed pressure and 25 1C. During the entire test period, the CO2 permeability and the separation factors of CO2/CH4 and CO2/N2 remained stable, demonstrating a favorable operation stability.

4. Conclusions Mixed matrix membranes composed of SPEEK and aminefunctionalized TiO2 were prepared and their gas separation performances were investigated. TiO2 microspheres were functionalized with dopamine and polyethyleneimine by a two-step method. As the content of TiO2–DA–PEI in the membrane increased, the CO2 permeability, CO2/CH4 selectivity and CO2/N2 selectivity were all increased. In particular, the SPEEK/2#-15 membrane exhibited high selectivities up to 58 and 64 for CO2/CH4 and CO2/N2 separation, respectively, with a CO2 permeability of 1629 Barrer at 1 bar and 25 1C. This result surpassed Robeson's upper bound reported in 2008. The incorporation of the TiO2–DA–PEI fillers introduced abundant

The authors gratefully acknowledge the financial support from the National High Technology Research and Development Program of China (2012AA03A611), Program for New Century Excellent Talents in University (NCET-10–0623), the National Science Fund for Distinguished Young Scholars (No. 21125627) and the Program of Introducing Talents of Discipline to Universities (B06006).

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