Tuning the performance of CO2 separation membranes by incorporating multifunctional modified silica microspheres into polymer matrix

Journal of Membrane Science 514 (2016) 73–85

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Tuning the performance of CO2 separation membranes by incorporating multifunctional modified silica microspheres into polymer matrix Qingping Xin a, Yuan Zhang d, Yue Shi b, Hui Ye a, Ligang Lin a, Xiaoli Ding a, Yuzhong Zhang a,n, Hong Wu b,c,nn, Zhongyi Jiang b,c a State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China d Tianjin Institute of Pharmaceutical Research, Tianjin 300193, China

art ic l e i nf o

a b s t r a c t

Article history: Received 7 December 2015 Received in revised form 18 April 2016 Accepted 19 April 2016 Available online 26 April 2016

In this study, three types of modified silica microspheres were functionalized with carboxyl, sulfonic acid group and pyridine group via a facile distillation-precipitation polymerization method, respectively. Each type of functionalized silica microspheres was incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate mixed matrix membranes (MMMs) for gas separation. Scanning electron microscopy (SEM) characterization indicated that all the three types of functionalized silica microspheres could disperse homogenously within the SPEEK matrix via tuning the polymer-particle interaction. The incorporation of silica microspheres and sulfonic acid functionalized silica microspheres resulted in increased free volume cavity (r3) in MMMs, whereas the pyridine functionalized silica microspheres led to decreased r3. The relation between r3 and gas diffusion coefficient was revealed: the higher r3 displayed higher gas diffusion coefficient. The functionalized silica microspheres could construct CO2 transport pathways due to the increased CO2 adsorption, imparting MMMs with remarkably enhanced CO2 separation performance. In particular, the pyridine functionalized silica microspheres loaded MMMs showed the optimum gas separation performance with CO2/CH4(N2) selectivity of 64.5 (68.3) and CO2 permeability of 2043 Barrer at loading of 20 wt% in humidified state, surpassing the Robeson upper bound reported in 2008, while the pristine SPEEK membrane exhibited CO2/CH4(N2) selectivity of 26.7 (35.1) and CO2 permeability of 525 Barrer. & 2016 Elsevier B.V. All rights reserved.

Keywords: Mixed matrix membrane Sulfonated poly (ether ether ketone) Multifunctional silica microspheres CO2 separation

1. Introduction CO2 separation has great influence in energy saving, small ecological footprint and environmental sustainability. Among various CO2 separation methods, membrane technology has been paid much attention due to its advantages including high efficiency, low capital and ease of operation [1,2]. Polymeric membrane as one of the main branches of gas separation membranes has been used to simultaneously improve the permeability and selectivity in the past decades. However, the instinct trade-off n

Corresponding author. Corresponding author at: Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail addresses: [email protected] (Y. Zhang), [email protected] (H. Wu). nn

http://dx.doi.org/10.1016/j.memsci.2016.04.046 0376-7388/& 2016 Elsevier B.V. All rights reserved.

between permeability and selectivity for polymeric materials becomes one of the major obstacles for further improvement in gas separation performance. Mixed matrix membranes (MMMs), comprising of a continuous polymer phase and a dispersed filler phase, are fabricated to achieve higher gas separation performance [3–6]. The embedded fillers consist of porous zeolites [7], silica nanoparticles [8,9], nanotubes [10–12], metal-organic frameworks [13–17], graphene oxide [18,19], porous organic framework [20], polymeric submicrospheres [21] and metal oxide [22], etc. MMMs can potentially overcome the trade-off limit and realize a synergistic combination of advantages of polymers and inorganic fillers [3,4]. In order to enhance CO2 separation performance, incorporation of functionalized fillers into polymeric membranes to fabricate MMMs has been studied recently. Ismail et al. [23] fabricated MMMs by incorporating beta-cylcodextrin (beta-CD)-functionalized MWNTs ((f-MWNTs)) into polymer matrix and explored the

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effect of (f-MWNTs) content on the CO2/CH4 gas separation performance. When the content of (f-MWNTs) was 0.7%, the MMMs showed the optimal CO2 separation performance, and CO2/CH4 selectivity increased by 273%. Chung et al. [24] investigated the effect of Ag þ -functionalized zeolite on the CO2/CH4 separation performance of PES/zeolite-Ag þ MMMs, and found that the CO2/ CH4 selectivity increased from 31.4 for pristine polymer to 59.6 at 50 wt% zeolite-Ag þ loading due to the combined effect of the facilitated transport of Ag þ and the molecular sieving effect of zeolite. Wang et al. [25,26] fabricated asymmetric MMMs incorporated with polyaniline nanorods for CO2/N2 separation, and the MMMs showed high CO2 separation performance with CO2 permeability of 3080 GPU and CO2/N2 selectivity of 240. MMMs doped with functionalized MCM-41s were exploited by Kim et al. [27], showing significantly increased CO2/CH4 selectivity. For the well-established fillers, the functional groups are grafted onto the fillers in the form of single dense layer in most cases. In this manner, the content functional groups is limited and the motion of functional group is restricted, both of which are not beneficial to CO2 transport. Monomer polymerization grafting method have triggered considerable interest in virtue of their extraordinary properties such as long chain-length, high functional groups loading and chain motion [28–30], and the monomer polymerization grafting membranes have shown potential in CO2 separation [31–33]. Jiang et al [31] constructed CO2 transport nanochannels in block copolymer (BCP) membrane by the self-assembly of a linear polymer chain, and the membrane exhibited excellent CO2 separation performance. Baker et al [32] investigated the effects of monomer composition on CO2-selective poly(ethylene glycol) (PEG)-containing monomer polymerization grafting membrane, and found the side-chain length of monomer influenced the polymerization rate as well as the permeability, selectivity, and crystallinity of membranes. The smaller chains prevented crystallization, while the presence of longer side chains allowed the membranes to maintain a relative high CO2/H2 selectivity at room temperature. Bruening et al [33] found that cross-linked poly(ethylene glycol dimethacrylate) monomer polymerization grafting membranes exhibited a CO2/CH4 selectivity of about 20, and the more careful selection of monomers could further improve the selectivity of monomer polymerization grafting membranes. In this study, three types of functionalized silica microspheres containing carboxyl, sulfonic acid group and pyridine group were synthesized via a facile distillation–precipitation polymerization method, respectively. Each type of functionalized silica microspheres was incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate MMMs and the CO2 separation performance of MMMs was investigated. The functionalized silica microspheres were expected to construct CO2 transport pathways and improve CO2 solubility selectivity or reactivity selectivity. The microstructure and physicochemical characteristics of functionalized silica microspheres loaded MMMs are investigated by membrane cross-section morphology, fractional free volume, chain rigidity, mechanical property and thermal stability to reveal the relationship between membrane structure and gas separation performance. The gas separation performance of the functionalized silica microspheres loaded MMMs was investigated for CO2/CH4 and CO2/N2 systems.

2. Experimental 2.1. Chemicals and materials s

Poly(ether ether ketone) (Victrex PEEK, grade 381G) was supplied by Nanjing Yuanbang Engineering Plastics Co., Ltd. Sodium-

Fig. 1. Chemical structure of SPEEK.

p-styrenesulfonate (SS), methacrylic acid (MAA) and 4-vinylpyridine (VPy) were supplied by Alfa Aesar and used without further purification. 3-(methacryloxy)-propyltrimethoxysilane (MPS), styrene (St), 2,2ʹ-azoisobutyronitrile (AIBN) and tetraethylorthosilicate (TEOS) were supplied by Aldrich. Acetonitrile, Dimethylformamide (DMF), sulfuric acid and ethanol were supplied from Jingtian Chemistry Co., Ltd. SPEEK is obtained via the post sulfonation of PEEK [34] and the chemical structure of SPEEK is shown in Fig. 1. The PEEK powders (28.0 g) are dispersed into sulfuric acid solution (98 wt%, 200 mL) at room temperature. The reaction mixture is then stirred for 10 h at 50 °C, cooled to room temperature, and then added to water with mechanical agitation. The precipitated SPEEK is washed with excess water until a pH of 7.0 is reached and then dried under vacuum state. The degree of sulfonation of SPEEK polymer is 62% as determined by the titration method. 2.2. Preparation of three types of functionalized silica microspheres Three types of functionalized silica microspheres are synthesized as illustrated in Fig. 2 [35]. Silica microspheres with a diameter of around 180 nm are synthesized according to the Stöber method: TEOS is added to a mixture of ethanol, water and aqueous solution of ammonium hydroxide with vigorous stirring at 25 °C for 24 h. Excess MPS is then added to the silica mixture. After being stirred for another 24 h, the resultant MPS-modified silica particles are purified and dried in a vacuum oven at 50 °C until of a constant weight. The functionalized silica microspheres are synthesized through a facile distillation-precipitation polymerization using MPS-modified silica particles as the seeds. A typical procedure for the synthesis of SiO2-S is as follows: MPS-modified silica (0.30 g), St (0.40 mL), comonomer SS (0.40 g), and initiator AIBN (0.02 g) are dissolved into acetonitrile (80 mL) in a dried one-necked flask (250 mL). The mixture is heated to boiling state and then the solvent is distilled off. After half the acetonitrile is distilled out, the functionalized microspheres are purified by water, followed by several centrifugation cycles with 0.1 M HCl to exchange the Na þ in –SO3Na with H þ until a pH of 7.0 is achieved. Then sulfonic acid functionalized SiO2 microspheres were obtained after being dried in vacuum state at 60 °C, and the functionalized microspheres were designated as SiO2-S. All the same mole quantity of monomers including MAA and VPy, St, initiator AIBN, solvents, reaction temperature and reaction time as the specific conditions were used for the synthesis of functionalized microspheres SiO2-C and SiO2-N, respectively, and both the SiO2-C and SiO2-N functionalized microspheres are only purified by water. 2.3. Preparation of mixed matrix membranes A certain amount of the functionalized microspheres (SiO2-C, SiO2-S or SiO2-N) are treated in DMF (12.0 g) with ultrasonic. Afterwards, 0.6 g SPEEK is added into the above solution and stirred at room temperature for 12 h. The resultant solution is then cast onto a glass plate and dried at 60 °C for 12 h and at 80 °C for 12 h in succession. All the as-prepared membranes were kept in a vacuum oven at 120 °C until the weight of membranes was constant before characterization or gas separation test. The MMMs were designated as SPEEK/SiO2-C-X, SPEEK/SiO2-S-X or SPEEK/SiO2-N-X representing SiO2-C, SiO2-S or SiO2-N as the fillers (X¼ 5, 10, 15 or 20), where X is the weight percentage of the functionalized microspheres to SPEEK.

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75

Fig. 2. Preparation of three types of functionalized silica microspheres: (a) SiO2-C, (b) SiO2-S and (c) SiO2-N.

Fig. 3. TEM images of the particles: (a) SiO2, (b) SiO2-MPS, (c) SiO2-C, (d) SiO2-S and (e) SiO2-N.

2.4. Characterization 2.4.1. Characterizations of three types of functionalized silica microspheres The morphology of three types of functionalized silica microspheres was observed by transmission electron microscopy (TEM,

Tecnai G2 20S-TWIN). Pore volume, size distribution and CO2 adsorption measurement of the SiO2 and functionalized silica microspheres were measured using an ASAP 2020 system. Fourier-transformed infrared (FT-IR) was used to confirm the successful fabrication of functionalized silica microspheres.

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802

Transmittance

SiO2

1735

SiO2-C

1451

Quantity adsorbed (mmol/g)

(a) 954

1646 14471386

SiO2-S

1654 1564

SiO2-N

3 SiO2-N

2 SiO2-S

1

0 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)

500

O1s N1s

Intensity

SiO2-N

C1s Si2sSi2p

SiO2-S

S2p

SiO2-C

100

1000

800 600 400 Binding energy (eV)

200

0

(c)

98

Weight (%)

96 94

SiO2

95.9

SiO2-MPS

92 90

SiO2-C

88

SiO2-S

86

SiO2-N

84

0.2

0.6 0.4 Pressure (bar)

0.8

1.0

Fig. 5. CO2 adsorbed isotherms for functionalized silica microspheres (SiO2-C, SiO2-S and SiO2-N), respectively (measured at 298 K).

(b)

1200

SiO2-C

93.0

2.4.2. Characterizations of membranes The cross-sectional morphology of membranes was observed by scanning electron microscopy (SEM, S-4800). Membrane samples were freeze-fractured in liquid nitrogen and then sputtered with gold before SEM analysis. Differential scanning calorimetry (DSC, DSC 204 F1 NETZSCH instrument, Germany) was carried out to determine the glass transition temperature (Tg) of the membranes. Positron annihilation lifetime spectroscopy (PALS) of the membranes was recorded with an ORTEC fast-fast coincidence system (the resolution was 201 ps) to investigate the free volume characteristics of the membranes, and the thickness of membranes is ∼1.0 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 Eq. (1).

⎛ 1 ⎞ ⎛ 2πr3 ⎞ ⎤−1 1⎡ r ⎟ sin ⎜ τ 3= ⎢ 1− 3 + ⎜ ⎟⎥ ⎝ 2π ⎠ ⎝ r3+∆r ⎠ ⎦ 2 ⎣ r3+∆r

(1)

88.8 87.9 85.9

100 200 300 400 500 600 700 800 900 Temperature (oC)

Fig. 4. (a) FTIR spectra (b) XPS spectra and (c) TGA curves of the functionalized silica microspheres (SiO2-C, SiO2-S and SiO2-N), respectively.

Samples were tested on a Nicolet 6700 instrument with a wavelength range of 4000–400 cm  1. The nitrogen, sulfur and carbon content in the surface layer of three types of functionalized silica microspheres (5–10 nm depth) was determined by X-ray photoelectron spectroscopy (XPS) operated at Axis Ultra DLD (Kratos Analytical Ltd., UK). The thermal stability of three types of functionalized silica microspheres as well as the content of polymer layer in three types of functionalized silica microspheres was explored by thermal gravity analysis (TGA, NETZSCH-TG209 F3 instrument, Germany) with a heat rate of 10 °C min  1 and temperature range of 40–800 °C in N2 atmosphere.

The apparent fractional free volume (FFV) of the equivalent sphere could be calculated by using Eq. (2).

FFV =

4 3 πr3 I3 3

(2)

The thermogravimetric analyses (TGA, NETZSCH TG 209 F3) of membranes were recorded from the 40 °C to 800 °C at a heating rate of 10 °C min  1 in N2 atmosphere. Mechanical property of membranes was evaluated via a universal tensile and compression test systems (Yangzhou Zhongke Jiliang LTD, China). The membrane sample was tailored into a rectangle of 1  4 cm, and the stretching rate employed in this study was 10 mm min  1 at 25 °C. The content of total water, free water and bound water in membranes were measured following the reported procedure [36– 38]. Each membrane was weighed to determine the “humidified” weight (m1, mg) after gas permeation test, and then was heated at 100 °C in a vacuum oven to remove free water. The membranes were reweighed (m2, mg) at this step. After the membranes were dried at 150 °C, they were weighed again to determine their “dried” weight (m0, mg). In this way, the content of total water (Wt, %), free water (Wf, %) and bound water (Wb, %) were obtained by Wt ¼(m1  m0)/m0  100%, Wf ¼(m1  m2)/m0  100%, Wb ¼ (m2  m0)/m0  100%, respectively.

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Fig. 6. SEM images of the cross-sections of (a) pristine SPEEK, (b, c, d) SPEEK/SiO2-20, (e, f, g) SPEEK/SiO2-C-20, (h, i, j) SPEEK/SiO2-S-20 and (k, l, m) SPEEK/SiO2-N-20 membranes.

2.5. Gas permeation experiments Pure gas (CO2, CH4 and N2) and mixed gas (CO2/CH4 (30 vol% CO2), CO2/N2 (10 vol% CO2)) permeation tests were conducted at 25 70.5 °C based on the conventional constant pressure/variable volume technique [39,40]. The gas transport properties of the membrane were measured using a flat-sheet permeation cell which was placed in a thermostat oven to control temperature (Fig. S1). N2 and CH4 were used as the sweep gases for CO2/CH4 and CO2/N2 mixtures, respectively. In a typical measurement, both

feed gas and sweep gas were introduced into a water bottle (35 °C) to be saturated with water vapor. The gas flow rate was controlled by mass flowmeters. The compositions of the feed, retentate and permeate were measured using a gas chromatography (Agilent 6820, equipped with a thermal conductive detector (TCD)). The permeability (Pi, Barrer, and 1 Barrer¼10  10 cm3 (STP) cm/ (cm s cmHg)) of each gas was obtained from the average value of at least three measurements, by using the Eq. (3):

Q. Xin et al. / Journal of Membrane Science 514 (2016) 73–85

Heat flow (mV mg-1)

78

Tg = 155.9 C o

SPEEK

Tg = 157.7 C o

SPEEK/SiO 2-20

Tg = 156.4 C o

SPEEK/SiO 2-C-20

Tg = 154.8 C o

SPEEK/SiO 2-S-20

Tg = 159.2 C o

SPEEK/SiO 2-N-20

100

120

140

160

180

200

o

Temperature ( C) Fig. 7. DSC curves of membranes.

Pi=

Qil ΔPi A

(3) 3

where Qi is the volumetric flow rate of gas ‘i’ (cm /s) at standard temperature and pressure (STP), Δpi is the transmembrane pressure difference (cmHg), and A is the effective membrane area (12.5 cm2). The pure gas ideal selectivity (αi/j) was calculated by Eq. (4):

aij =

pi pj

(4)

The mixed gas separation factor (α

αi*/ j=

* i/j )

was calculated by Eq. (5)

yi /yj xi / x j

(5)

where y and x are the compositions of i and j on the permeate and feed sides, respectively. An apparatus based on the well-known “time-lag” method was utilized to measure the solubility coefficient (S) and diffusivity coefficient (D) of the gases in membranes [41]. Di and Si were calculated by Di ¼l2/(6θi) and Si ¼Pi/Di, respectively, where l is the thickness (μm) of membranes, and θ is the time lag of the gases (s). For each membrane, the Di and Si were tested for three times. In this study, the uncertainties of gas permeability are within 7 5% and selectivity within 78%.

3. Results and discussion 3.1. Characterization of three types of functionalized silica microspheres 3.1.1. TEM and BET Fig. 3 shows the morphology of pristine SiO2 (Fig. 3(a)) and functionalized SiO2 submicrospheres (Fig. 3(b)-(e), SiO2-MPS, SiO2-C, SiO2-S and SiO2-N) probed by TEM. The unmodified SiO2

particles are morphologically identical with an average particle size of 180 nm. Compared with the pristine SiO2 particles, the surfaces of the three types of functionalized silica microspheres become rougher due to the polymerization of organic layers on the particle surface. The N2 adsorption-desorption isotherm of pristine SiO2 microspheres with insert of the pore diameter distribution of the TiO2 particles is shown in the Fig. S1. The results reveal that the N2 adsorption- desorption isotherm is the type II which reveals the nonporous structures of SiO2 microspheres, and the measuring result of BET indicates that the surface area of the pristine SiO2 particles is 118.2 m2/g. Consequently, it can be deduced that almost all of polymer coils on functionalized SiO2 submicrospheres are concentrated on the surface of SiO2 microsphere. The SiO2-MPS and functionalized SiO2 exhibit the similar adsorptiondesorption isotherms with the pristine SiO2, as well as their pore size distributions. The specific surface areas of the SiO2-MPS, SiO2-C, SiO2-S and SiO2-N are estimated 119.2, 121.7, 123.6 and 139.6 m2/g, respectively. The coating layers on the functionalized SiO2 submicrospheres result in an increase in the specific surface area, especially notable on SiO2-N after modification. This increase is attributed to the changes in surface morphology and the formation of the rough surface of functionalized SiO2 submicrospheres [42]. Compared to the pristine SiO2, the functionalized SiO2 submicrospheres can be expected to have a better contact interface with the polymer matrix on account of their larger specific surface area and rougher surface. 3.1.2. FTIR, XPS and TGA The surface composition of the functionalized silica submicrospheres is analyzed by FTIR, XPS and TGA. The surface modification of the silica seeds via the second stage polymerization to incorporate the different functional groups on the silica submicrospheres surface is confirmed further by FTIR spectra as shown in Fig. 4(a). The strong peaks at 802 cm  1 and 954 cm  1 correspond to the asymmetric stretching vibration of Si-O-Si and the stretching vibration of hydroxyl group, respectively. The spectrum of SiO2-C displays two characteristic peaks at 1735 and 1451 cm  1, attributed to the bending of C–O and COO–H of the –CO2H group, respectively [43]. The characteristic peaks in SiO2-S at 1386 cm  1 and 1447 cm  1 are attributed to the O ¼S¼ O stretching vibration bands. The FTIR spectrum of SiO2-N is weak but obvious peaks at 1564 and 1654 cm  1 corresponding to the typical vibration of pyridyl groups [44]. All these results confirm that three types of functionalized silica microspheres are successfully synthesized. XPS is employed to investigate the surface compositions of the SiO2 microspheres before and after functionalization, and the data from XPS scans on different surfaces is listed in Table S1. The content of S2p and N1s from SiO2-S and SiO2-N is about 8.5% and 10.7%, respectively. The characteristic signals of S2p at 167 eV in SiO2-S and N1s at 400 eV in SiO2-N are obviously observed, indicating the successful grafting reaction (Fig. 4(b)). To estimate the amounts of organics layers on the SiO2 surface, TGA is performed (Fig. 4(c)). The three types of functionalized silica microspheres have a similar degradation process composed of two main stages: the weight loss in the temperature range of 30–130 °C is attributed to the release of residual water in particles, and the weight loss

Table 1 Free volume properties of membranes. Membrane

I3 (%)

τ3 (ns)

r3 (nm)

FFV (%)

SPEEK SPEEK-SiO2-20 SPEEK-SiO2-C-20 SPEEK-SiO2-S-20 SPEEK-SiO2-N-20

6.977 0.18 6.65 7 0.15 7.45 7 0.14 6.78 7 0.16 7.247 0.18

1.9797 0.019 2.0477 0.020 1.969 7 0.013 2.028 7 0.018 1.9127 0.017

0.283 70.001 0.289 70.001 0.282 70.001 0.287 70.001 0.27770.001

0.78047 0.0201 0.74467 0.0168 0.8426 7 0.0158 0.7592 7 0.0179 0.81077 0.0201

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Fig. 8. Schematic illustration of the nanoscale morphologies at the polymer-particle interface: Case 0 ( ¼r3–0): morphology of pristine SPEEK membrane; Case I ( Er3–0): morphology of SPEEK/SiO2-C; Case II ( 4r3–0): morphology of SPEEK/SiO2 or SPEEK/SiO2-S; Case III ( or3–0): morphology of SPEEK/SiO2-N.

starting from 300 °C is attributed to the thermal decomposition of organic layers. Based on these data, the weight fraction of monomer polymerization layer on SiO2-C, SiO2-S and SiO2-N particles is about to be 4.2%, 5.1% and 7.1%, respectively. 3.1.3. CO2 adsorption Fig. 5 shows adsorption isotherms of CO2 for functionalized silica microspheres (SiO2-C, SiO2-S and SiO2-N) at 298 K, and the CO2 adsorption capacity of SiO2-C, SiO2-S and SiO2-N is 1.26, 1.78 and 2.67 mmol/g at 1 bar, respectively. As expected in the adsorption isotherms, as pressure increases adsorption capacity increases. As observed, more CO2 molecules are adsorbed in SiO2-N than in SiO2-C and SiO2-S since more CO2 interaction sites are generated after grafting modification which is indicated by the increased specific surface area. This can be important for separation of gases especially for separation of CO2 from N2. Therefore this filler can have a positive effect on separation of CO2 from N2 in MMMs. 3.2. Characterization of membranes 3.2.1. Field emission scanning electron microscope (FESEM) The dispersion of three types of functionalized silica microspheres

within the membranes is probed by SEM as shown in Fig. 6. To tailor the membrane performance effectively and elucidate the influence of the functional groups on the dispersion of the submicrospheres, all three types of functionalized silica microspheres loaded MMMs are characterized at loading of 20 wt%. Pristine silica particles do not disperse very well as shown in Fig. 6(b–d), and this is mainly attributed to the poor interfacial compatibility between SiO2 particles and bulk polymer matrix. After surface modification, due to the presence of the organic shell-layer, the interfacial compatibility is improved. The defect-free polymer/filler interface is the result of the presence of the polymerization layer grafted on the external surface of the SiO2 submicrospheres and the increased surface area as shown by the specific surface area results [42,45]. The increased interaction between the functional groups on modified SiO2 submicrospheres and SPEEK chains further prevents the fillers agglomeration, resulting in a homogenous dispersion of the fillers in the SPEEK matrix. Therefore, the dispersion of the functionalized submicrospheres in MMMs is obviously improved in comparison with pristine SiO2 embedded membrane (Fig. 6(e–m)). In particular, the SPEEK/SiO2-N-20 membrane shows better interface compatibility than SPEEK/SiO2-C-20 or SPEEK/SiO2-S-20 membrane, attributing to generate acid-base pairs (–SO3…H2N–) at SiO2-N–SPEEK interface. The SEM results imply that the submicrospheres can disperse

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Table 2 Pure gas diffusion and solubility coefficients of dry membranes compared with results reported in literatures. Membrane

DCO2a

DCH4a

DN2a

SCO2b

SCH4b

6FDA-ODA 6FDA-ODA/NH2-UiO-66 6FDA-ODA/NH2-MOF-199 s Matrimid s Matrimid -SO3H-MCM-41 Pebax Pebax-PEG-MWCNT PVDF PVDF-NH2-MIL-53(Al)  10% PEI GO-NH2-0.75%/PEI PIM-1 PIM-1/Ti5UiO-66 5 wt% SPEEK SPEEK-MCM-41 SPEEK SPEEK/PEI@MIL-101(Cr)  40 SPEEK SPEEK/GO-DA-Cys-8 SPEEK SPEEK/SiO2-5 SPEEK/SiO2-10 SPEEK SiO2-15 SPEEK/SiO2-20 SPEEK/SiO2-C-5 SPEEK/SiO2-C-10 SPEEK/SiO2-C-15 SPEEK/SiO2-C-20 SPEEK/SiO2-S-5 SPEEK/SiO2-S-10 SPEEK/SiO2-S-15 SPEEK/SiO2-S-20 SPEEK/SiO2-N-5 SPEEK/SiO2-N-10 SPEEK/SiO2-N-15 SPEEK/SiO2-N-20

2.95 3.26 3.60 3.20 5.22 135 402 0.775 0.93 0.53 0.30 7  1011 3.2  1012 4.81 6.73 4.80 6.76 5.00 4.05 5.01 5.12 5.49 7.09 7.36 5.11 5.38 5.89 6.23 5.16 5.58 5.99 6.43 5.11 5.49 5.82 6.09

0.35 0.26 0.31 – – 58 266 0.11 0.13 0.08 0.03 – – – – 1.33 1.82 1.42 0.93 1.42 1.46 1.57 2.91 3.48 1.46 1.52 1.62 1.65 1.47 1.51 1.59 1.67 1.43 1.47 1.51 1.52

5.75 4.20 7.39 – – 48 185 0.15 0.17 – – – – – – 1.90 2.81 1.94 1.38 1.94 1.99 2.03 3.26 4.12 1.97 2.07 2.2 2.25 1.98 2.13 2.18 2.28 1.95 2.01 2.02 2.07

1.05 1.00 1.40 – – 0.5936 1.3815 89.7 1.16 4.50 5.18 4.18  103 3.23  103 3.06 3.64 3.10 5.50 3.11 5.51 3.11 3.24 3.32 3.41 3.54 3.29 3.77 4.11 4.42 3.5 3.94 4.26 4.63 3.47 3.84 4.24 5.38

– – – – – 0.0259 0.0253 28.9 31 1.11 0.35 – – – – 0.41 0.50 0.41 0.49 0.41 0.42 0.42 0.42 0.43 0.42 0.46 0.49 0.51 0.44 0.5 0.5 0.51 0.42 0.44 0.47 0.48

SN2b

DCO2/DCH4

DCO2/DN2

SCO2/SCH4

SCO2/SN2

Refs.

– – – – – 0.1031 0.2128 29.0 32 – – – – – – 0.21 0.27 0.21 0.27 0.23 0.22 0.23 0.24 0.25 0.22 0.24 0.26 0.27 0.24 0.25 0.27 0.28 0.23 0.24 0.26 0.29

8.85 12.54 3.99 – – 0.0281 0.0217 6.86 7.00 6.35 9.74 – – – – 3.61 3.71 3.53 4.35 3.53 3.51 3.50 2.44 2.11 3.50 3.54 3.64 3.78 3.51 3.70 3.77 3.85 3.57 3.73 3.85 4.01

– – – – – 2.33 1.51 5.31 5.47 – – – – – – 2.53 2.41 2.57 2.94 2.58 2.57 2.70 2.17 1.79 2.59 2.60 2.68 2.77 2.61 2.62 2.75 2.82 2.62 2.73 2.88 2.94

– –

5.77 4.20 3.70 – – 2.33 1.58 3.09 3.57 – – – – – – 14.76 20.37 14.76 20.37 13.52 14.73 14.43 14.21 14.16 14.95 15.71 15.81 16.37 14.58 15.76 15.78 16.54 15.09 16.00 16.31 18.55

[46]

– – 5.76 6.49 3.10 3.72 4.06 17.64 – – – – 7.56 11.00 7.56 11.22 7.59 7.71 7.90 8.12 8.23 7.83 8.20 8.39 8.67 7.95 7.88 8.52 9.08 8.26 8.73 9.02 11.21

[47] [12] [48] [49] [50] [41] [15] [19] This This This This This This This This This This This This This This This This This

study study study study study study study study study study study study study study study study study

The errors were all less than 8%. Diffusivity coefficient [cm2/s]  108. Solubility coefficient [cm3(STP)/cm3cmHg]  102.

CO2 permeability (Barrer)

b

(a)

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Fig. 9. (a) Correlations between pure gas CO2 permeability and water uptake in the membranes and (b) correlations between pure gas CO2/CH4, CO2/N2 selectivity and bound water content in membranes.

homogeneously within polymer matrix through appropriate surface modification. 3.2.2. Glass transition temperature (Tg) Fig. 7 displays the Tg of pristine SPEEK membrane and MMMs. The SPEEK/SiO2-20 membrane shows the Tg of 157.7 °C. In comparison, the addition of SiO2-C or SiO2-S decreases the Tg to 156.4 °C and 154.8 °C for SPEEK/SiO2-C-20 or SPEEK/SiO2-S-20 MMMs, respectively. The decreased Tg verifies the generation of

repulsive interactions between SiO2-C (SiO2-S) and SPEEK chains, which promotes the chain motion of SPEEK. Contrary to SiO2-C or SiO2-S, SiO2-N inhibits the chain motion of SPEEK via electrostatic attraction and thus increases Tg to 159.2 °C at loading of 20%. 3.2.3. FFV The pristine SPEEK membrane possesses free volume cavity (r3) with an average radius about 0.283 nm (Table 1), and the free volume cavity of pristine SPEEK was designated r3–0. The size of r3

Q. Xin et al. / Journal of Membrane Science 514 (2016) 73–85

1000

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Upper bond line(2008)

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81

100 Upper bond line(1991)

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Fig. 10. Pure gas separation performance of membranes for (a) CO2/CH4 and (b) CO2/N2 systems, respectively (temperature: 25 °C; pressure: 1 bar).

10

35

8

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7 6

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5 20 4 15

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10 5

2 0

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Total water (wt%)

Fig. 11. (a) Pure gas CO2 permeability and CO2/CH4 selectivity; (b) pure gas CO2 permeability and CO2/N2 selectivity for SiO2, SiO2-C, SiO2-S and SiO2-N based MMMs at 5 wt%, 10 wt%, 15 wt% and 20 wt% loadings (1 bar, 25 °C).

1

Content of filler (%) Fig. 12. The content of total water and bound water in the humidified membranes.

in MMMs implies the important subtle influence of interfacial interaction on the interface morphology. According to the results of r3 in Table 1, it is reasonably proposed three possible models of interface morphology as illustrated in Fig. 8. Case 0 presents the morphology of pristine SPEEK membrane. The repulsive interactions between SiO2-C and SPEEK chains make little contribution to the change of r3 as shown in Case I. Case II corresponds to the interface morphology of SPEEK/SiO2 and SPEEK/SiO2-S membranes, which possess a little larger cavity than pristine SPEEK membrane due to the nonselective defects at the organic-inorganic interface and strong repulsive interactions between SPEEK and particles, respectively. Case III represents the interface morphology of SPEEK/SiO2-N membrane. In this case, the formation of

strong and extensive electrostatic attractive force generates strong stress at the organic-inorganic interface, which remarkably inhibits the mobility of SPEEK chains and leads to dense chain packing, and thus results in significant rigidification near the interfacial region and the decreased r3. In summary, the free volume characteristics of the membrane are strongly dependent on the interfacial interaction between the fillers and polymer matrix. Therefore, the morphology and free volume of the membrane can be tailored by manipulating the interfacial interaction. 3.3. Gas separation performance 3.3.1. Effect of filler content on gas separation performance Diffusion and solubility coefficients of dry membranes are tested at 1.5 bar and 25 °C to have further insight into the role of functionalized silica microspheres in gas separation performance and are compared with results reported in literatures [12, 15, 19, 41, 46–50]. The diffusion coefficients for all gases increase with increasing fillers loadings of membranes (Table 2), which are comparable with or superior to the results reported in literatures. The CO2 diffusion coefficient increases from 5.01  10  8 cm2/s for pristine SPEEK membrane to 6.43  10  8 cm2/s for SPEEK/SiO2-S membrane at the loading of 20 wt%. The SiO2-S loaded MMMs show a considerably larger increase in diffusivity coefficient and CO2/CH4(N2) diffusivity selectivity in comparison with SiO2-C and SiO2-N loaded MMMs. This increase in diffusivity is mainly due to the higher r3. Similar to the diffusivity, almost all three types of MMMs show increased CO2 solubility coefficient and CO2/CH4(N2) solubility selectivity with increased functionalized silica microspheres loading. Moreover, the SiO2-N loaded MMMs show a

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Q. Xin et al. / Journal of Membrane Science 514 (2016) 73–85

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Fig. 13. (a) Mixed gas CO2 permeability and CO2/CH4 (30 vol% CO2) separation factor; (b) mixed gas CO2 permeability and CO2/N2 (10 vol% CO2) separation factor for SiO2, SiO2-C, SiO2-S and SiO2-N based MMMs (1 bar, 25 °C).

3.0

3.1 3.2 1000/T (K-1)

3.3

3.4

100 80 60 40 20 0 2.9

Fig. 14. Temperature on mixed gas separation performance (a) mixed gas CO2 permeability; (b) mixed gas CH4 permeability; (c) mixed gas CO2/CH4 separation factor; (d) mixed gas CO2 permeability; (e) mixed gas N2 permeability and (f) mixed gas CO2/N2 separation factor of pristine SPEEK, SPEEK/SiO2-20, SPEEK/SiO2-C-20, SPEEK/SiO2-S-20 and SPEEK/SiO2-N-20 membranes. Permeation tests were performed at 25 °C with humidified feed gas and sweep gas.

Q. Xin et al. / Journal of Membrane Science 514 (2016) 73–85

higher CO2 solubility and CO2/CH4(N2) solubility selectivity than those of SiO2-C and SiO2-S loaded MMMs. The increased solubility and CO2/CH4(N2) solubility selectivity possibly are attributed to the increased base interaction sites from the highest weight fraction of monomer polymerization layer among the functionalized SiO2 microspheres analyzed by the TGA results for the CO2. This increased solubility coefficient and CO2/CH4(N2) solubility selectivity also illustrate the difference in ideal selectivity among the membranes. As the total water increases due to the increased water swelling as shown in Fig. S3, the CO2 permeability of humidified membrane increases and the CO2 permeability of both the humidified pristine SPEEK membrane and MMMs are shown in Fig. 9(a). Moreover, the selectivity of CO2/CH4(N2) is also proportional to the bound water in humidified pristine SPEEK membrane and MMMs as shown in Fig. 9(b). The SPEEK/SiO2-N MMMs show the highest ideal selectivities of 64.5 and 68.3 for CO2/CH4 and CO2/N2 systems, respectively, with a CO2 permeability of 2043 Barrer, surpassing the Robeson upper bound revised in 2008 (Fig. 10). The gas separation performance of humidified membranes is stable in 2 h (Fig. S4), and the pure gas transport performance of the membranes is shown in Fig. 11. In SiO2-N-doped series MMMs, the increased chain rigidification shown by DSC leads to the increased diffusivity selectivity and may decrease the membrane permeability [51]. However, the SiO2-N-doped series MMMs show an increased CO2 permeability in comparison with SiO2-C-doped and SiO2-S-doped MMMs due to the following three possible reasons. As illustrated by Fig. 12, the increased CO2 permeability is ascribed to the increased total water. Moreover, the increased specific surface area as a favorable parameter facilitates the gas transport [52,53], and the most remarkably increased specific surface area of SiO2-N leads to a more notable improved gas permeability of the resultant SPEEK/SiO2-N membrane. Besides, since the increased CO2 interaction sites of the SPEEK/SiO2-N MMMs compared with other types of MMMs at the same loading of fillers, the CO2 permeability increases. As the content of amine groups increases, the increased CO2 interaction sites from SiO2-N construct CO2 transport pathways in SPEEK/SiO2-N MMMs. All the MMMs loaded with SiO2-C, SiO2-S and SiO2-N show a consistent increase in ideal CO2/CH4 and CO2/N2 selectivities with the increasing of functionalized SiO2 loading up to 20%. In the functionalized SiO2-doped MMMs, the increased selectivities are ascribed to the enhanced solubility of CO2 due to the affinity between CO2 with acid or base sites in comparison with pristine SiO2-doped MMMs. The selectivities of CO2/CH4 and CO2/N2 for the SPEEK/SiO2-N MMMs increase more remarkably than other types of MMMs at the same loadings. The increased selectivities for MMMs doped with SiO2-N are attributed to the following reasons. The increased bound water in SPEEK/SiO2-N MMMs results in an increased CO2/CH4 and CO2/N2 selectivities. Moreover, amine groups from functionalized silica microspheres facilitate CO2 transport in SiO2-N loaded MMMs in humidified sate. Besides, the SPEEK/SiO2-N MMMs show increased diffusivity selectivity due to the increased chain rigidification as shown by DSC results. Consequently, the introduction of SiO2-N into polymer matrix chains forms CO2 pathways due to the increased CO2 adsorption sites indicated by the increased specific surface area, and enables the MMMs to possess more CO2 interaction sites, resulting in the enhanced CO2/CH4(N2) selectivity. 3.3.2. Mixed gas separation performance In order to explore the applicability of MMMs, the mixed gas separation performance of MMMs is investigated. As expected, both the mixed gas CO2 permeability and CO2/CH4(N2) separation factor are slightly lower than those of pure gas (Fig. 13), suggesting the existence of plasticization or competitive sorption effects

83

[54,55]. However, the pressure at 1 bar can not lead to plasticization, and thus the competitive sorption effect is the main reason. 3.3.3. Effect of operating temperature The pristine SPEEK, SPEEK/SiO2-20, SPEEK/SiO2-C-20, SPEEK/SiO2-S-20 and SPEEK/SiO2-N-20 membranes are used to further investigate the effect of operating temperature on gas separation performance, which is varied from 25 °C to 65 °C (Fig. 14). The permeability of CO2 increases with increasing temperature in MMMs as well as in pristine SPEEK membrane. All membranes show decreased CO2 selectivity with the increase of temperature. Nevertheless, the CO2/N2 separation factor is 44 when the temperature is high as 65 °C. The Arrhenius equation is used to explain the relationship between gas permeability and operating temperature using the activation energy of permeability (Ep), and is given by the following expression:

⎛ Ep ⎞ P =P0 exp ⎜ − ⎟ ⎝ RT ⎠

(6)

where P, P0, R and T are the permeability of the gas, the pre-exponential factor, the gas constant and the absolute temperature, respectively. Ep are 9.64, 7.52, 7.49, 7.23 and 6.29 kJ/mol calculated from the slope of lnP vs 1000/T for pristine SPEEK, SPEEK/SiO2-20, SPEEK/SiO2-C-20, SPEEK/SiO2-S-20 and SPEEK/SiO2-N-20 membranes, respectively. The results show that the MMMs have lower activation energies of permeability than the pristine SPEEK membrane. This is attributed to the presence of fillers, leading to the decrease in diffusion through the MMMs. 3.3.4. Comparison with other functionalized filler-doped MMMs Table S3 compares the performance of functionalized fillerdoped MMMs reported in the literatures for CO2 separation with the present study results. The gas separation performance of MMMs is comparable with or superior to results in literatures. The MMMs have shown noteworthy improvement in performance by exhibiting significant separation performance even at high pressure of 10 bar or 65 °C. Moreover, SPEEK/SiO2-N-20 membrane shows a CO2 permeability of 2043 Barrer, CO2/CH4 selectivity of 65.4 and CO2/N2 selectivity of 68.3, respectively, surpassing the Robeson upper bound in 2008.

4. Conclusion In summary, a high performance gas separation membrane was developed by incorporating three types of functionalized silica microspheres into SPEEK polymer. Three types of functionalized SiO2 microspheres were modified via a facile distillation-precipitation polymerization method to graft with carboxyl, sulfonic acid and pyridine, respectively, and then were embedded into SPEEK matrix to fabricate MMMs. The physico-chemical structures and gas separation properties of MMMs were extensively investigated, and the relationship between free volume cavity (r3) and gas diffusion coefficient was obtained: the higher r3 displayed higher gas diffusion coefficient. It was found that amine groups functionalized SiO2 microspheres played at least two roles for enhancing the gas separation performance of SPEEK matrix: (1) generating acid-base pairs (–SO3…H2N–) at polymer-filler interface, which worked as CO2 transport sites; (2) inserting into polymer matrix chains and then forming CO2 pathways with the aid of the amine groups on SiO2 microspheres due to the increased CO2 adsorption. The amine group functionalized SiO2 microspheres loaded MMMs showed the highest ideal selectivities of 64.5 and 68.3 for CO2/CH4 and CO2/N2 systems, respectively, with a CO2 permeability of 2043 Barrer, surpassing trade-off limit in 2008.

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Acknowledgements

References

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21576189, 51503146, and 51373120), China Postdoctoral Science Foundation funded project (2015M581302), State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University) (M1-201501, M2-201504), the National Science Fund for Distinguished Young Scholars (21125627) and the Public Science and Technology Research Funds Projects of Ocean (201305004-5).

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Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2016.04. 046.

Nomenclature Symbols glass transition temperature (°C) free volume radius (nm) τ3 lifetime of o-Ps (ns) intensity of o-Ps component (%) I3 FFV apparent fractional free volume (%) Δr electron layer thickness (nm) m1 the weight of membrane after gas permeation test (mg) m2 the weight of membrane after 100 °C in a vacuum oven to remove free water (mg) m0 the weight of membrane after 150 °C in a vacuum oven to remove bound water (mg) Wt content of total water (%) Wf content of free water (%) Wb content of bound water (%) Pi permeability of each gas (Barrer) Qi volumetric flow rate of gas ‘i’ (cm3/s (SPT)) l thickness of the membranes (μm) Δpi transmembrane pressure difference gas constant (cmHg) αi/j(αi/j*) ideal selectivity (mixed gas separation factor of gas ‘i and j’) D diffusion coefficient ([cm2/s]  108) S solubility coefficient ([cm3(STP)/cm3cmHg]  102) Ep the activation energy of permeability (kJ/mol) Tg r3

Abbreviations TEOS tetraethylorthosilicate silica microspheres SiO2 SiO2-MPS MPS-modified silica microspheres SiO2-C carboxylic acid functionalized SiO2 microspheres SiO2-S sulfonic acid functionalized SiO2 microspheres SiO2-N pyridine functionalized SiO2 microspheres PEEK poly(ether ether ketone) SPEEK sulfonated poly(ether ether ketone) MPS 3-(methacryloxy) propyltrimethoxysilane MAA methacrylic acid StVPy styrene 4-vinylpyridine SS sodium-p-styrenesulfonate AIBN 2-azodiisobutyronitrile

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[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

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