Mixed matrix membranes fabricated by a facile in situ biomimetic mineralization approach for efficient CO2 separation

Author’s Accepted Manuscript Mixed matrix membranes fabricated by a facile in situ biomimetic mineralization approach for efficient CO2 separation Qingping Xin, Yuan Zhang, Tingcheng Huo, Hui Ye, Xiaoli Ding, Ligang Lin, Yuzhong Zhang, Hong Wu, Zhongyi Jiang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30079-5 http://dx.doi.org/10.1016/j.memsci.2016.02.022 MEMSCI14288

To appear in: Journal of Membrane Science Received date: 13 November 2015 Revised date: 17 January 2016 Accepted date: 10 February 2016 Cite this article as: Qingping Xin, Yuan Zhang, Tingcheng Huo, Hui Ye, Xiaoli Ding, Ligang Lin, Yuzhong Zhang, Hong Wu and Zhongyi Jiang, Mixed matrix membranes fabricated by a facile in situ biomimetic mineralization approach for efficient CO2 separation, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.02.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mixed matrix membranes fabricated by a facile in situ biomimetic mineralization approach for efficient CO2 separation Qingping Xina,b, Yuan Zhangd, Tingcheng Huob, Hui Yea, Xiaoli Dinga, Ligang Lina, Yuzhong Zhanga,*, Hong Wub,c*, Zhongyi Jiangb 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

Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin

University, Tianjin 300072, China d

Tianjin Institute of Pharmaceutical Research, Tianjin 300193, China

[email protected]

[email protected] *

State Key Lab of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China.Tel: 86-22-83955807. *

Corresponding author: Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China 1

Abstract

Inspired by the silica formation process mediated by silica deposition vesicle (SDV) in diatoms or sponges, a facile and efficient biomineralization-inspired approach is proposed to in situ fabricate MMMs. Silica nanoparticles are in situ formed within poly(ether-block-amide) (Pebax® 1657) matrix through the controlled biomimetic mineralization using protamine as the inducer. The size of the mineralized silica nanoparticles can be tailored by varying the silicon precursor/protamine ratio. It is found that the confined space within the polymer matrix controls the growth of the mineralized silica nanoparticles and the size of in situ generated mineralized silica nanoparticles is relatively uniform. The mineralized silica shows good interface compatibility with polymer matrix, and the interface interaction can be tuned by silicon precursor/protamine ratio. Effect of the in situ generated mineralized silica particles on the physicochemical structure of membranes is systematically investigated in terms of glass transition temperature (Tg), crystallinity, free volume properties and 29Si nuclear magnetic resonance (NMR). The as-prepared MMMs show simultaneously increased CO2 permeability and CO2/CH4(N2) selectivity. In particular, the Pebax-Pro(Silica)1 mixed matrix membrane displays an optimum gas separation performance with CO2 permeability of 161.5 Barrer and the selectivity of 65.5(82.8) for CO2/CH4(N2) system, surpassing the Robeson upper bound in 2008. Abbreviations: TEOS, tetraethylorthosilicate; Pebax® poly(ether-block-amide),

2

Pro, protamine; Pro(Silica)x, mineralized silica particles with different TEOS/protamine weight ratio

Keywords: Mixed matrix membrane; Protamine; Pebax; Biomimetic mineralization; CO2 separation. Nomenclature

Symbols

Tg

glass transition temperatures (°C)

r3

free volume radius (nm)

τ3

lifetime of o-Ps (ns)

I3

intensity of o-Ps component (%)

FFV

apparent fractional free volume (%)

Δr

electron layer thickness (nm)

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 differencegas constant (cmHg)

3

αi/j

ideal selectivity and 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)

1.

Introduction

Nowadays, climate change has become a growing concern of the world and CO2 is a major greenhouse gas which makes a significant contribution to the global warming. Much attention has been paid to combat this global problem such as improved energy efficiency and CO2 capture [1]. Among the different methods discussed for CO2 capture, membrane separation have received increased attention due to their advantages such as small foot print, high energy efficiency and environmental sustainability [2]. Despite all the investigations and studies, polymeric membrane separation is still restricted by the trade-off limit between gas permeability and selectivity proposed by Robeson [3,4]. The combination of polymeric membrane materials and inorganic particles, which is called mixed matrix membranes (MMMs), has been investigated for gas separation to surpass the trade-off limit [5-15].

Until now, the most commonly used approaches for fabricating MMMs contain physical blending and in situ polymerization. Incorporating inorganic moiety into

4

polymer membrane via in situ sol-gel process is a facile approach in comparison with the physical blending [16,17]. Moreover, for in situ sol-gel process, the organic and inorganic moieties are mixed at the molecular level within the membranes. Lee et al [18] prepared MMMs by doping tetraethoxysilane (TEOS) into poly(amide-6-b-ethylene oxide) (Pebax®) matrix via in situ polymerization using the sol-gel process, and the as-prepared MMMs showed improved CO2 permeability and CO2/N2 selectivity. Copper nanoparticles were successfully prepared using a facile in situ synthetic method involving the ionic liquid by Kang et al [19], and the MMMs showed an improved CO2 separation performance. Chung et al [20] prepared MMMs comprising polydimethylsiloxane (PDMS) and acid-catalyzed 3-glycidyloxypropyltrimethoxysilane (GOTMS) using the in situ sol-gel process. The CO2 permeability of these MMMs can reach 1810 barrer, an improvement of 7.5 folds.

Although in situ sol–gel method is generic and simple, it has difficulty in controlling the reaction rate and the morphology of the inorganic particles [21]. To solve these issues, we are inspired by the formation process of some natural biomaterials, a process by which living organisms generate minerals, biomineralization can receive this goal in a surprisingly facile way [22-38]. Biomimetic mineralization approach will manifest several distinct advantages: rapid, polymorphous mild, inexpensive and green [39,40]. Biomimetic mineralization approach was used to prepare MMMs in the previous study [41-48]. In our previous study, gelatin-silica MMMs were fabricated 5

[41], and mineralized silica nanoparticles with a diameter less than 100 nm were formed homogeneously in gelatin polymer matrix. Quaternary ammonium groups on polymers induced the formation of silica particle in poly(vinyl alcohol) (PVA) matrix by Liu et al [46]. Chitosan (CS) was employed to induce the formation of CdS nanoparticles due to the excellent adsorption capacity by Copello et al. [47], and the CS-Cd2+ complexes formed through the adsorption of the amino on chitosan with Cd2+ ions.

Because pI (isoelectric point) values are directly related to the mineralization rate [49], protamine with the pI value of 10.0 is used as a preferential inducer for mineralization in this study. Inspired by the silica formation process mediated by silica deposition vesicle (SDV) in diatoms or sponges, silica nanoparticles are in-situ formed within poly(ether-block-amide) (Pebax® 1657) bulk matrix through the controlled biomimetic mineralization, using protamine as an inducer. The mineralization ability of the protamine–silica is delicately adjusted by varying the mass ratio of silicon precursor to protamine, mimicking diatom’s mineralization balance. The ways living organisms employed to manipulate multiple interactions are used to fabricate hierarchical structures of MMMs, especially for free volume characteristics. The MMMs with Pebax® 1657 as polymer matrix and mineralized silica nanoparticles as the versatile filler are fabricated to evaluate the CO2 separation performance in this study. The structure of the mineralized silica nanoparticles and its effect on membrane structure is explored. 6

2.

Experimental

2.1. Chemicals and materials Poly (ether-block-amide) (Pebax® 1657) is purchased from Arkema (France) and the chemical structure of Pebax® 1657 is shown in Fig. 1. Protamine sulfate salt from salmon (P4380) and tris(hydroxymethyl) amino methane (Tris) are purchased from Sigma-Aldrich Chemical Company (USA). Tetraethylorthosilicate (TEOS) is obtained from J&K Scientific Ltd. Hydrochloric acid is purchased from Jiangtian Chemical Co. (Tianjin, China). Ethanol is purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). All chemicals are of analytical grade and deionized water is used in the experiment process. Nylon 6 (PA6)

Polyethylene oxide (PEO)

Weight ratio of PA6 to PEO about 40:60

Fig. 1. Chemical structures of Pebax® 1657. 2.2. Preparation of flat sheet membranes The fabrication process of MMMs is elaborated as following. A certain amount of Pebax® 1657 was dissolved in ethanol/water (70/30 wt%) with reflux at 80 oC for 2 h to obtain 3 wt % homogeneous solution. After cooling the solution to ambient temperature, protamine powder was dissolved in 30 mM/L Tris buffer solution followed by acidifying to pH 7.0±0.2 with HCl, and the protamine solution was then divided into six aliquots. Same amounts of Pebax solution (3 wt %) were added into 7

each aliquot (the weight ratio of protamine to Pebax was 1:10). Different amounts of TEOS aqueous solution (30 mM) were added into each aliquot with stirring for 1 h and the TEOS/protamine weight ratio (abbreviated as Pro(Silica)x, where x means the weight ratio of TEOS to protamine ) was precisely controlled at 0, 0.5:1, 1:1, 2:1, 4:1 and 8:1, respectively. The homogeneous solutions were casted onto Teflon plates and then dried under ambient conditions for 24 h. The membranes were further dried in a vacuum oven at 30 oC overnight to remove the residue solvent. Additionally, the previous results suggested that different pretreatments for the membrane could alter membrane structure and gas permeability as reported [50-52]. All the as-prepared membranes were kept in a vacuum oven at 60 oC until the weight of membranes was constant before characterization or gas separation test. The resultant membranes were designated as Pebax–Pro, where silica was absent, or Pebax–Pro(Silica)x, where x means the weight ratio of TEOS to protamine. For comparison, pristine Pebax and Pebax–Pro membranes were also prepared. The thickness of all membranes was controlled within the range of 65–85 μm. 2.3. Characterizations Transmission electron microscopy (TEM) images are observed with a Tecnai G2 20 S-TWIN. The morphological images of the in situ generated protamine-silica nanoparticles are obtained through dropping the casting solution onto the copper mesh. The cross-sectional morphologies of MMMs are characterized by using a field emission scanning electron microscope (FESEM, S-4800) operated at 10 kV. Energy

8

dispersive X-ray spectroscopy (EDS) was used to map the distribution of Si element across the membrane. The crystal structure and intermolecular distances between the intersegmental chains are recorded on a Rigaku D/max 2500 v/pc X-ray diffractiometer (XRD) in the range of 5°–60° at the scan rate of 3 °min−1. The X-rays of 1.5406 Å wavelength are generated by a Cu Kα source．The average d-spacing value of the samples are calculated by using Bragg's law (d= λ/2sinθ). Glass transition temperature (Tg) of the membranes is analyzed by a Netzsch differential scanning calorimetry (DSC) 200F3 calorimeter. The measurements are performed from -100 to 250 °C at the scan rate of 10 °C/min, and nitrogen is used as a purge gas with a flow rate of 20 ml/min. Tg is determined as the midpoint temperature of the transition in the DSC curve. 29

Si solid-state spectrum of selected sample was recorded on a Varian Unity Inova

spectrometer at 300MHz using the magic angle spinning (MAS) technique. The sample was located in a zirconia rotor operating at the spinning rate of 3.5 kHz. The chemical shifts were given with reference to tetramethylsilane (TMS). Positron annihilation lifetime spectroscopy (PALS) of the membranes is recorded with an ORTEC fast-fast coincidence system (the resolution is 201 ps) to investigate the free volume property of the membranes. The positron source-22Na is sandwiched between two pieces of samples with a thickness about 1.0 mm. The spectra with more than one million counts are recorded. On assumption that the location of o-Ps occurs in

9

a sphere potential well surrounded by an electron layer of a constant thickness Δr (Δr = 0.1656 nm): r 1  1   2 r3    3 = 1- 3    sin   2  r3  r  2   r3  r  

1

(1)

where τ3, r3 and π are the o-Ps lifetime (ns), the radius of the free volume element and circumference ratio, respectively.

4 FFV   r33 I 3 3

(2)

where FFV is the apparent fractional free volume, I3 is the intensity of o-Ps component (%). Mechanical property of membranes is evaluated via a universal tensile and compression test systems (Yangzhou Zhongke Jiliang LTD, China). 2.4. Gas permeation tests Pure gas (CO2, CH4 and N2) and mixed gas (CO2/CH4 = 30/70 vol% and CO2/N2 = 10/90 vol%) permeation experiments are conducted at 25 oC based on the conventional constant pressure/variable volume gas permeation system under dry or wet conditions in previous study [39] (Fig.S1). In the permeation experiments, the feed gas is firstly introduced into a water bottle (35 oC) to be saturated with water vapor, and then passed through an empty bottle to remove the residual water. Meanwhile, the sweep gas is humidified at room temperature. The flow rate and composition of sweep gas are recorded every 5 minutes until they no longer varied 10

with time. The compositions of the feed, retentate and permeate are measured using Agilent 6820 gas chromatography equipped with a thermal conductive detector (TCD). For comparison, dry-state gas permeation experiments are also conducted. The permeability (Pi, Barrer, and 1 Barrer = 10-10 cm3 (STP) cm/(cm s cmHg)) of either gas is obtained from the average value of at least twice measurements, by using the equation (3):

Pi 

Qi l Pi A

(3)

where Qi is the volumetric flow rate of gas ‘i’ (cm3/s) at standard temperature and pressure (STP), l was the thickness of the membranes measured by a micrometer calliper (μm), Δpi is the transmembrane pressure difference (cmHg), and A is 12.6 cm2 as the effective membrane area. The CO2/CH4 selectivity (αij) is calculated by equation (4):

 ij 

Pi Pj

(4)

To further illustrate the change in membrane separation performance, diffusivity (D) and solubility (S) coefficients of membranes are measured by the well-known “time-lag” method at 25 °C and 1 bar. The membranes are evacuated for 8 h to remove previously dissolved specie before analysis. For all membranes the gases are tested in the order of N2, CH4 and CO2. All the error range of gas permeability was about ±5%, while the error of gas selectivity was about ±8% from the comprehensive 11

effect of the instrument errors and the test errors. The errors of diffusivity and solubility coefficients of membranes were all less than 10%. 3.

Results and discussion

3.1. TEM of the mineralized silica nanoparticles The morphological images of the in situ generated protamine–silica composites are obtained through dropping the casting solution onto the copper mesh by TEM images as shown in Fig. 2. Few particles can be found when TEOS/protamine =0.5:1 (Fig. 2(c)). However, when TEOS/protamine=2:1, numerous nanoparticles with the diameter of 10–25 nm are observed, indicative of the enhanced mineralized interactions among the mineralized silica nanoparticles (Fig. 2(e)). When TEOS/protamine=8:1, more obvious agglomeration phenomenon is found due to the occurrence of agglomeration of the mineralization particles. In comparison, we observe much larger and less uniform particles when Pro(Silica)2 nanoparticles are generated in polymer-free solvent (Fig. 2(h)). This implies that the confined space within the polymer matrix controls the growth of the nanoparticles. Moreover, as the ratio of the TEOS/protamine increases, the size of mineralized silica nanoparticles becomes larger.

12

(a) Pebax

100 nm (e) Pebax-Pro(Silica)2

100 nm

(b) Pebax-Pro

(c) Pebax-Pro(Silica)0.5 (d) Pebax-Pro(Silica)1

100 nm

100 nm

(f) Pebax-Pro(Silica)4

(g) Pebax-Pro(Silica)8

100 nm

100 nm (h) Pro(Silica)2

100 nm

100 nm

Fig. 2. TEM images of (c)–(g) the in situ generated TEOS/protamine nanoparticles when TEOS/protamine equals to (c) 0.5:1; (d) 1:1; (e) 2:1; (f) 4:1; (g) 8:1 and (h) the Pro(Silica)2 composite generated in the solvent when TEOS/protamine equals to 2:1. 3.2. Characterization of membranes Cross-sectional morphology of the MMMs is probed by SEM (Fig. 3). It clearly indicates that a smooth and dense membrane is formed in pristine Pebax membrane (Fig. 3(a)), and the mineralized silica particles are embedded in the polymer matrix with elaborate homogeneity and dispersion (as shown in Fig. 3(c)–(g)). In addition, the favorable compatibility between the Pebax matrix and the as-prepared mineralized silica inorganic particles facilitate the homogeneous dispersion after mineralization [36]. Owing to the flexible molecular chains, Pebax can self-heal most macro-voids immediately after they are generated in the curing process. No macro-voids can be observed from the SEM cross-sectional images of the membranes.

13

Fig. 3. FESEM cross-section images of the (a) Pebax, (b) Pebax-Pro; (c) Pebax-Pro(Silica)0.5; (d) Pebax-Pro(Silica)1; (e) Pebax-Pro(Silica)2; (f) Pebax-Pro(Silica)4 and (g) Pebax-Pro(Silica)8 membranes. The EDS-mapping reveals the distribution of silica (Si) in Pebax-Pro(Silica)1 MMMs with the distribution of oxygen (O), and carbon (C) as comparison shown in Fig.4. It can be clearly observed that the Si is of uniform distribution in MMMs.

14

Fig. 4. (a) SEM image of Pebax-Pro(Silica)1 MMMs and EDX (b)silica-mapping and (c) oxygen-mapping (d) carbon-mapping images of the Pebax-Pro(Silica)1 MMMs.

The chain rigidity of the membranes, the glass transition temperature (Tg) and melting temperature (Tm) derived from DSC curves (Fig. S3) are presented in Table 1. Compared with pristine Pebax membrane, both Pebax–Pro and Pebax–Pro(Silica)x MMMs display a significantly lower Tg. The highest Tg (–38.2 oC) and melting temperature (Tm) are observed for Pebax–Pro(Silica)1 and Pebax–Pro(Silica)0.5 MMMs, which reveals that the mobility of polymer chains is restrained to the utmost extent for Pebax–Pro(Silica)x MMMs. A broad and strong peak ranging from 10o to 35o is exhibited in WXRD patterns of all membranes, which indicates the irregular packing of the polymer chains (Fig. S4). A sharp peak appears at around 2θ=22.6o, which presents the crystalline region of the PA6 block in polymer. Both Pebax–Pro and Pebax–Pro(Silica)x MMMs display remarkably lower degree of crystallinity (Xc) in comparison with pristine Pebax membrane (Table 1). Both Pebax–Pro and Pebax–Pro(Silica)x MMMs exhibit more amorphous structure in comparison with pristine Pebax membrane, which results in more free volume cavities for gas transport. The average inter-chain spacing (d-spacing) of membranes is shown in Fig. S4, which is obtained by Bragg’s Law. From Fig. S4, the minimum d-spacing is 3.70 nm for Pebax–Pro(Silica)1 mixed

15

matrix membrane, and these decreased crystallinity and d-spacing may lead to an increase in the both permeability and selectivity.

Table 1. Thermal properties of PEO segments for Pebax, Pebax–Pro and Pebax–Pro membranes.

Membrane

Tg (oC)

Tm (oC)

Xc (%)

Pebax

-50.4

23.1

22.4

Pebax–Pro

-44.8

21.7

19.6

Pebax–Pro(Silica)0.5

-41.3

48.1

14.6

Pebax–Pro(Silica)1

-38.2

46.2

14.8

Pebax–Pro(Silica)2

-43.7

22.8

12.1

Pebax–Pro(Silica)4

-46.1

21.4

9.3

Pebax–Pro(Silica)8

-47.7

15.9

5.4

The free volume property including the apparent fractional free volume (FFV) and the size of the free volume cavity (r3) plays an important part in diffusing low 16

molecular weight gases. Free volume elements tested by positron annihilation lifetime spectroscopy (PALS) are used to probe the free volume characteristics in polymer. The PALS results indicate that the r3 in these membranes is slightly changed in the range of 0.316–0.321 nm (Table 2). The FFV of Pebax-Pro(silica)1 mixed matrix membrane is lower than that of pristine Pebax membrane, and the FFV of PebaxPro(Silica)x MMMs is larger than that of pristine Pebax membrane when the ratio of TEOS/protamine is high than 2:1. The FFV of the MMMs is monotonically increased with the TEOS/protamine ratio increasing because the silica nanoparticles destroy the crystalline structure of polymer and loosen polymer chains. Moreover, for Pebax-Pro(Silica)8 mixed matrix membrane, the FFV is increased due to the aggregation of Pro(Silica)8 particles. The increased FFV may contribute to the increase of the gas permeability through membranes.

Table 2. Free volume properties of Pebax, Pebax-Pro and Pebax-Pro(Silica)x membranes.

Membrane

I3a (%)

τ3b (ns)

r3c (nm)

FFVd(%)

Pebax

17.450.24

2.4100.007

0.3200.0005

2.39

Pebax–Pro

16.780.27

2.3730.015

0.3170.001

2.23

Pebax–Pro(Silica)0.5

17.180.28

2.3590.014

0.3160.001

2.27

17

Pebax–Pro(Silica)1

17.060.27

2.3870.014

0.3180.001

2.29

Pebax–Pro(Silica)2

17.450.28

2.3600.012

0.3160.001

2.30

Pebax–Pro(Silica)4

17.970.34

2.4090.013

0.3190.001

2.44

Pebax–Pro(Silica)8

18.490.33

2.4280.013

0.3210.001

2.56

a

Intensity of o-Ps component (%). bo-Ps lifetime (ns). cRadius of the free volume

element. dApparent fractional free volume. The chemical structure of the silica at the atomic scale is studied by 29Si NMR, as shown in Table 3, helping in the analysis of the degree of TEOS condensation induced by protamine. The high proportion of Q3[Si(OSi)3(OH)] and Q4[Si(OSi)4] up to 80∼95% shown by all the membranes demonstrates that well-condensed silica is generated under the induction of protamine. For Pebax–Pro(Silica)1 nanocomposite membrane, the fraction of Q3 and Q4 species is 81.8% (Table 3). However, a large number of mono- and di-substituted groups exists in the Pro(Silica)1 nanoparticles, and these residual -OH groups result in the connection with polymer matrix through hydrogen bonding, leading to homogeneous membranes. Table 3. The results of peak deconvolution in 29Si soild-state NMR.

18

Q2a(-91 to -95 Q3b(-99 to -103 ppm) Q4c(-106 to -120ppm)

Membrane ppm)

Pebax–Pro(Silica)0.5

16.9

28.4

45.2

Pebax–Pro(Silica)1

18.2

32.1

49.7

Pebax–Pro(Silica)2

8.0

16.7

75.3

Pebax–Pro(Silica)4

5.6

12.8

81.6

Pebax–Pro(Silica)8

-

abc

5.7

94.3

Superscript in Q2, Q3 and Q4 referring to the number of –O–Si groups bonded to the

silicon atom.

3.3. Gas separation performance of MMMs

The separation performance of membranes in pure gas are shown in Fig.5. Both Pebax–Pro and Pebax–Pro(Silica)x (when x≤4) membranes show higher CO2/CH4(N2) selectivity in comparison with pristine Pebax membrane. The enhanced selectivity is attributed to the comprehensive effect of increased chain stiffness, the decreased average inter-chain spacing and the decreased free volume cavity size. When ratio of

19

TEOS/protamine exceeds 0.5:1, both CO2 permeability and FFV increase monotonously with TEOS loading. Moreover, the monotonously decreased crystallinity can lead to an increase in the gas permeability. Consequently, the MMMs with high TEOS/protamine ratio (>1:1) show higher CO2 permeability in comparision with pristine Pebax membrane. Particularly, the CO2/CH4(N2) selectivity of Pebax– Pro(Silica)1 membrane is 65.5(82.8), with CO2 permeability of 161.5

Barrer,

surpassing the Robeson upper bound revised in 2008 as shown in Fig. 6. The transport mechanisms of increased gas separation performance in MMMs compared with pristine Pebax®1657 membrane are associated with the membrane structure. The increased permeability is attributed to the increased diffusion coefficient due to the decreased degree of crystallinity and the increased FFV, and the increased selectivity is ascribed to the increased diffusion selectivity, resulting from the increased chain

250

80

200

60

150

40

100 20

50 0

0 Peba Peba Peba Pebax Pebax Pebax Pebax x-Pro x-Pro -Pro -Pro -Pro -Pro x (silic (silica (silic (silic (silica a) a) a) )8 )1 2 4 0 Membrane type

.5

300 (b)

100

CO2 permeability CO2/N2 selectivity

250

80

200

60

150

40

100 20

50 0

0 Peba Peba Peba Pebax Pebax Pebax Pebax x-Pro x-Pro -Pro -Pro -Pro -Pro x (silic (silica (silic (silic (silica a) a) a) ) ) Membrane type

0.5

1

2

Fig. 5. The effect of TEOS/protamine ratio in membrane on (a) CO2/CH4 separation performance (b) CO2/N2 separation performance in pure gas. Permeation tests are 20

Pure gas CO2/N2 selectivity

100

CO2 permeability CO2/CH4 selectivity

CO2 permeability (Barrer)

300 (a)

Pure gas CO2/CH4 selectivity

CO2 permeability (Barrer)

rigidity.

4

8

performed at 30 oC and 2 bar feed pressure.

(a)

1000 Pebax-Pro(Silica)0.5 Pebax-Pro(Silica)1 Pebax-Pro(Silica)2 Pebax-Pro(Silica)4 Pebax-Pro(Silica)8

100

Pristine Pebax membrane Pebax-Pro membrane Pebax-Pro(Silica)x MMMs

1

10

100

Upper bond line(1991)

1000

Upper bond line(2008)

100

10

1

(b)

Upper bond line(2008)

CO2/N2 selectivity

CO2/CH4 selectivity

1000

10000

Pebax-Pro(Silica)0.5 Pebax-Pro(Silica)1 Pebax-Pro(Silica)2 Pebax-Pro(Silica)4 Pebax-Pro(Silica)8

10

1

Pristine Pebax membrane Pebax-Pro MMM Pebax-Pro(Silica)x MMMs

1

10

100

1000

10000 100000

CO2 permeability (Barrer)

CO2 permeability (Barrer)

Fig. 6. Relation between (a) CO2 permeability and ideal CO2/CH4 selectivity (b) CO2

permeability and ideal CO2/N2 selectivity of pristine Pebax, Pebax–Pro and

Pebax–Pro(Silica)x membranes. These data are obtained from pure gas experiments at 30 oC and 2 bar.

In order to have a further research on the role of the mineralized silica particles in gas separation performance, diffusion and solubility coefficients of all membranes are tested (Table 4). The diffusion coefficients for all gases increase as the loading of TEOS increases. The diffusion coefficient of CO2 increases from 1.42×10-6 cm2/s for pristine Pebax membrane to 3.12×10-6 cm2/s for Pebax-Pro(Silica)8 membrane. The Pebax-Pro(Silica)x membranes show a considerably larger increase in diffusion coefficient in comparison with the Pebax-Pro membrane, which is mainly attributed to the decreased degree of crystallinity from PA6 segments in the Pebax- Pro(Silica)x

21

membranes. Moreover, the addition of mineralized silica nanoparticles to Pebax disrupts the original ordered packing of Pebax chains, increasing the accessible free volume fraction without introducing cavities that are large enough to promote nonselective flow. Hence, the obtained permeability is higher than the pristine Pebax membrane. Similar to the diffusion coefficient, the Pebax-Pro(Silica)x membranes show a higher solubility coefficient than the Pebax-Pro membrane. Compared with the pristine Pebax membrane, the membrane loaded with Pro(Silica)1 shows an increased CO2 solubility coefficient by 43%. The increased solubility coefficient mainly attributes to the increased polar interaction sites resulted from the decreased degree of crystallinity in PA6 segments for the quadrupole CO2. This increased solubility also illustrates the explanation behind the difference in ideal selectivity among the membranes.

22

Table 4. Gas diffusivity and solubility coefficients of the membranes (membranes were tested at 2 bar, 25 °C)

DCO

SCO

DN

SN2

a

b

62.

0.7

2.4

21

0

7

62.

0.2

2.4

45

5

0

67.

0.4

2.5

23

3

8

88.

0.7

2.4

91

8

1

85.

0.9

2.8

36

9

2

86.

1.1

3.5

06

4

8

DCH

SCH

DCO2/D

DCO2/

SCO2/S

SCO2/S

CH4

DN2

CH4

N2

3.11

2.04

5.67

25.19

7.22

2.60

5.95

26.02

9.46

2.86

5.98

26.06

5.83

2.24

12.38

36.89

3.81

2.19

10.00

30.27

4.06

2.25

7.68

24.04

Membrane 2

a

2

b

2

4

a

4

b

10.

Pebax 1.43

0.46

98

10.

Pebax-Pro 0.65

Pebax-Pro(Sili ca)0.5

1.23

Pebax-Pro(Sili ca)1

1.75

Pebax-Pro(Sili ca)2

2.17

Pebax-Pro(Sili

0.09

49

11. 0.13

25

7.1 0.30

8

8.5 0.57

4

11. 0.63

ca)4

2.56

21

23

Pebax-Pro(Sili

87.

1.4

2.9

11

2

0

15. 0.91

ca)8

3.12

32

3.43

2.20

5.69

30.03

a.

Diffusivity coefficient [cm2/s] × 106

b.

Solubility coefficient [cm3(STP)/cm3cmHg] × 104

3.4. Mixed gas separation performance In order to explore the applicability of MMMs, both the CO2 permeability and CO2/gas selectivity of mixed gas for CO2/CH4 (30 vol% CO2) and CO2/N2 (10 vol% CO2) systems are investigated (Fig.7). The mixed gas CO2/CH4 permeability and CO2/N2 selectivity of pristine Pebax membrane and MMMs are slightly lower than their corresponding ideal permeability. This decrease in permeability is due to the effect of penetrant competition mainly including competitive sorption and competitive diffusion. However, the mixed gas permeability of Pebax-Pro(Silica)x MMMs for both CO2/CH4 and CO2/N2 mixtures are close to the ideal permeability. Since the incorporation of Pro(Silica)x particles exhibits more amorphous structures, which is anticipated to create more free volume cavities for molecule diffusion, thus diminishing the penetrant competition effect caused by CH4 or N2. Moreover, the kinetic diameters of CO2 (3.30 Å) is significantly smaller than the CH4 (3.80 Å) and N2 (3.64Å) molecules. Consequently, almost equal gas permeability are obtained in both cases of mixed gas permeation.

24

Fig. 7. The effect of TEOS/protamine ratio in membrane on (a) CO2/CH4 separation performance (b) CO2/N2 separation performance in mixed gases. Permeation tests are performed at 30 oC and 2 bar feed pressure. 3.5. The effect of operating pressure As the feed pressure increases from 1 to 10 bar, below the plasticization pressure, both CO2 permeability and CO2/gas selectivity are almost unchanged as shown in Fig. 8. The CO2/CH4 selectivity of Pebax-Pro(Silica)1 mixed matrix membrane keeps above 60, and the high gas separation property results from the good polymer–filler interface compatibility.

25

Mixed gas CO2 permeability (Barrer)

200

(a)

Mixed gas CO2/CH4 selectivity

Mixed gas CO2 permeability (Barrer)

200

CO2/CH4 Pebax Pebax-Pro(Silica)1

150

100

2 (c)

4 6 8 Pressure (bar)

4 6 8 Pressure (bar)

40 20 0

2

(d)

100

100

2

60

120

CO2/N2 Pebax Pebax-Pro(Silica)1

10

CO2/CH4 Pebax Pebax-Pro(Silica)1

80

10

150

50

(b)

100

Mixed gas CO2/N2 selectivity

50

120

4 6 8 Pressure (bar)

10

CO2/N2 Pebax Pebax-Pro(Silica)1

80 60 40 20 0

2

4

6 8 Pressure (bar)

10

Fig. 8. (a) Mixed gas CO2 permeability; (b) mixed gas CO2/CH4 selectivity; (c) mixed gas CO2 permeability and (b) mixed gas CO2/N2 selectivity of pristine Pebax and Pebax-Pro(Silica)1 membranes plotted versus feed pressure at 30 oC. 3.6. Effect of operating temperature The pristine Pebax and Pebax-Pro(Silica)1 membranes are chosen to further study the effect of operating temperature on gas separation performance, which is varied from 25 to 65 oC. Fig. 9 shows that CO2 permeability increases with increasing temperature in Pebax-Pro(Silica)1 membrane as well as in pristine Pebax membrane, and both membranes show decreased CO2 selectivity with the increase of temperature. Nevertheless, the CO2/CH4(N2) selectivity remains as high as 42(53) at 65 oC, which are significantly higher than that of pristine Pebax 19(26) membrane under the same 26

conditions. The Arrhenius equation is used to explain the relationship between the gas permeability and the operating temperature using the activation energy of permeability (Ep), and is given by the following expression:

 E  P  P0exp   p   RT 

(5)

where P denotes the gas permeability, P0 the pre-exponential factor, R the gas constant and T the absolute temperature. The activation energy determined from the plots of permeability vs reciprocal temperature shows that the permeability coefficients follow the Arrhenius equation. Ep calculated from the slope of lnP vs 1000/T are 8.9 and 7.3 kJ/mol for Pebax and Pebax-Pro(Silica)1 membranes, respectively. A positive value of Ep is attributed to the increase of permeability with the increase of temperature. The results show that the Pebax-Pro(Silica)1 membrane has a lower activation energy of permeability than the pristine Pebax membrane. This is attributed to the presence of more amorphous PEO phases, leading to the increase in diffusion through the MMMs.

27

CO2/CH4 Pebax Pebax-Pro(Silica)1

(a)

6.5 6.0 5.5 5.0 4.5

7.0

3.0

3.3

3.4

CO2/N2 Pebax Pebax-Pro(Silica)1

(c)

6.5

Ln PCO2 (Barrer)

3.1 3.2 -1 1000/T (K )

6.0 5.5 5.0 4.5 4.0 2.9

3.0

3.1 3.2 1000/T (K-1)

3.3

120

3.4

CO2/CH4 Pebax Pebax-Pro(Silica)1

(b)

100

Mixed gas CO2/N2 selectivity

4.0 2.9

Mixed gas CO2/CH4 selectivity

Ln PCO2 (Barrer)

7.0

80 60 40 20 0 2.9 120

3.0

(d)

3.1 3.2 -1 1000/T (K )

3.3

3.4

CO2/N2 Pebax Pebax-Pro(Silica)1

100 80 60 40 20 0 2.9

3.0

3.1 3.2 -1 1000/T (K )

3.3

3.4

Fig. 9. Mixed gas separation performance (a) Ln PCO2 (Barrer) for CO2/CH4 mixture; (b) mixed gas CO2/CH4 selectivity; (c) Ln PCO2 (Barrer) for CO2/N2 mixture and (b) mixed gas CO2/N2 selectivity of pristine Pebax and Pebax-Pro(Silica)1 membranes plotted versus temperature at 2 bar. The long-term stability of membrane is a vital factor for the practical application. It is notable that the performance of Pebax-Pro(Silica)1 mixed matrix membrane remains almost unchanged under a 120 h long-term gas separation experiment at 2 bar feed pressure and 25 °C (Fig. 10), demonstrating a preferable operation stability.

28

150

80

100 60 50 40 0

0

20

40

60

80

100

120

(b)

CO2/N2

200

120 100

150

80

100 60 50 40 0

0

20

40

60

80

100

Mixed gas CO2/N2 selectivity

100

CO2 permeability (Barrer)

200

120

Mixed gas CO2/CH4 selectivity

CO2 permeability (Barrer)

CO2/CH4

(a)

120

Operating time(h)

Operating time(h)

Fig. 10. The long-term gas separation performance test of the Pebax-Pro(Silica)1 membrane up to120 h for (a) CO2/CH4 and (b) CO2/N2 mixture at 2 bar feed pressure and 25 oC. 3.7. Comparison with other PEO-based MMMs The CO2 separation performances of some representative Pebax-based MMMs are summarized in Table 5. Obviously, the Pebax-Pro(Silica)1 membrane fabricated by synergy of biomimetic mineralization and in situ method exhibits favorable CO2 separation performance with CO2 permeability 161.5 Barrer, CO2/CH4 selectivity 65.5 and CO2/N2 selectivity 82.8, respectively, surpassing the Robeson upper bound revised in 2008. The membranes fabricated in this study show rather high CO2 permeability and CO2/CH4 selectivity, especially when the feed pressure increases up to 10 bar. Pebax-Pro(Silica)x MMMs fabricated by biomimetic mineralization– inspired show highly attractive for natural gas separation biogas purification and power plant applications.

29

Table 5. Comparison of membrane separation properties of PEO-based MMMs with current work.

Polymer

Fillers

Method

Testi

Thickne

ng

ss (μm)

Pebax

R

condi

(Barr

αCO2/

αCO2/ ef

tions

er)

CH4

N2

.

97

[53]

20 oC, ®

PCO2

0.25∼1

Physical ZIF-7

1657

3.75 atm .052

111

30

blending (dry)

25 oC, 2 Pebax®

Physical

40∼60

atm

32.

ZIF-8 2533

1287 9.0 blending

(humidif

[54] 3

ied)

Pebax® 1657

MWNT

Physical

30 oC,

blending

10 atm

15∼20

329.

-

7

78.

[55]

5

(dry)

Pebax® 1657

MWCNT Physical

25 oC,

80∼10

blending

10 atm

0

30

743

-

10 8

[56]

(dry)

Pebax®

PEI-MC

Physical

25 oC, 1

75∼10

M-41

blending

atm

0

1657

112

25

57

[57]

102

-

72.

[58]

(dry)

Pebax®

SWNT

1657

Physical

21 oC,

blending

2.3 atm

-a

6

(dry)

Pebax®

Zeolite-4

Physical

3.75/7.5

A

blending

atm

1657

-

155

41.

94.

[59]

3

2

-

-

[60]

79

[18]

40.

50.

[61]

0

0

(dry)

25 oC, 1 Pebax

-

®

APTMS

In situ

atm

96.1

1657 (dry)

25 oC, 3 Pebax

-

®

TEOS

In situ

atm

277

1657 (dry)

Pebax® 1657

TEO In situ S

25 oC, 4 atm

31

-

20.0

(dry)

TiO

In situ

-

71

11.

39.

1

4

139

7.2

25

[62]

70∼90

86

74

-

[63]

65∼85

161.

65.

82.

5

5

8

P

Pebax® 4033

TEO

In situ

S

25 oC, 4 atm (dry)

Pebax® 1657

Fe3+–

In situ

DA

30 oC, 10 atm (dry)

Pebax®1

Pro(Si

Biomimetic-min

25 oC,

657

lica)1

eralization+ In

1atm

situ

(dry)

Thi s stud y

25 oC, 10

160.

63.

81.

This

atm (dry)

5

8

6

stud y

65 oC,

0.25∼1

515.

1atm

.052

2

32

8.1

39.

This

89

stud

(dry)

a

y

Thickness of membranes can not be found by readers obviously.

4. Conclusions In summary, the synergy of biomimetic mineralization and in situ process is used to precisely tune MMMs structure for efficient CO2 separation. More specifically, biomimetic mineralization as a facile approach is used to fabricate MMMs with in situ generated silica nanoparticles as the fillers. The silica fillers change polymer chain packing, and the polymer-filler interfacial compatibility can be finely tuned by adjusting the silicon precursor/protamine ratio. When silicon precursor/protamine ratio is ≤ 4:1, the MMMs show strong interfacial interactions, high CO2/CH4 selectivity and moderate CO2 permeability. In particular, the mixed matrix membrane with silicon precursor/protamine ratio 1:1 shows the highest selectivities up to 65.5 and 82.8 for CO2/CH4 and CO2/N2 systems, respectively, with a CO2 permeability of 161.5 Barrer at 2 bar and 25 oC, surpassing the Robeson upper bound in 2008. Moreover, a long-term stability test of mixed matrix membrane within 120 h further indicates the promising gas separation performance.

Acknowledgment The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21576189, 51503146, 51373120), China Postdoctoral 33

Science Foundation funded project (No. 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). References [1] D.L. Gin, R.D. Noble, Designing the next generation of chemical separation membranes, Science 332 (2011) 674–676. [2] Y. Zhang, J. Sunarso, S.M. Liu, R. Wang, Current status and development of membranes for CO2/CH4 separation: a review, Int. J. Greenhouse Gas Control 12 (2013) 84–107. [3] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400. [4] Q. Xin, J. Ouyang, T. Liu, Z. Li, Z. Li, Y. Liu, S. Wang, H. Wu, Z. Jiang, X. Cao, Enhanced interfacial interaction and CO2 separation performance of mixed matrix membrane by incorporating polyethyleneimine-decorated metal-organic frameworks, ACS Appl. Mater. Interfaces 7 (2015) 1065−1077.

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42



Biomineralization-inspired approach is used to fabricate MMMs.



Silica nanoparticles are in situ formed within Pebax® 1657 matrix.



The confined space within the polymer matrix leads to more uniform particle size.



Permeability and selectivity of membranes are elevated in CO2/CH4(N2) systems.



MMMs show improved CO2 separation performance, surpassing trade-off limit.

43