Enhanced permeation through CO2-stable dual-inorganic composite

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Enhanced permeation through CO2-stable dual-inorganic composite membranes with tunable nano-architectured channels Guanying Dong, Xuke Zhang, Yatao Zhang, and Toshinori Tsuru ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00792 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Sustainable Chemistry & Engineering

Enhanced permeation through CO2-stable dual-inorganic composite membranes with tunable nano-architectured channels Guanying Dong†‡#, Xuke Zhang†#, Yatao Zhang*†, Toshinori Tsuru **‡ †

School of Chemical Engineering and Energy, Zhengzhou University, No.100 Science

Avenue, Zhengzhou 450001, China ‡

Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama,

Higashi-Hiroshima 739-8527, Japan

Corresponding authors: Yatao Zhang, E-mail: [email protected]; Toshinori Tsuru, Email: [email protected]

1

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ABSTRACT: In this work, dual-inorganic composite membranes were prepared with outstanding CO2 separation performance by assembling the largely different aspect-ratio nanostructured materials, including the porous reduced graphene oxide (PRG) and modified halloysite nanotubes (mHNTs), into the Pebax®1657 matrix (CO2 permeability: 124 barrer, ideal CO2/N2 selectivity: 118). This unique separation performance stems from the molecular sieving effect of PRG and preferable stacking behavior between PRG and mHNTs, which improves the efficiency of molecular discrimination and decreases the gas transport resistance. In addition, the as-prepared membranes show good chemical stability and nearly 500% improvement in CO2 permeability with wet CO2/N2 gas mixture (1/9, v/v). It is also found that the transition of membrane morphology occurs from the PRG-rich nano-architecture to mHNTs-rich nano-architecture at the mHNTs to PRG mass ratio of 7.5, resulting in an opposite tendency for the CO2/N2 ideal selectivity. Moreover, the as-prepared membranes show great advantage in comparison with the conventional polymeric membranes for carbon capture due to the excellent compaction- and plasticization-resistant behavior. KEYWORDS: Porous reduced graphene oxide; modified halloysite nanotubes; Pebax®1657; nano-architecture; CO2 separation

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INTRODUCTION Membrane technology has been extensively studied since 1960s due to low capital and operating costs, lower energy consumption and ease of operation,1 and it has been industrially applied and become mature, especially for the water treatment.2-4 In contrast to the traditional gas separation technologies, it is also regarded as an efficient alternative in such fields, mainly including oxygen and nitrogen enrichment, hydrogen recovery and hydrocarbon separations. In the past few decades, enormous research effort has been devoted into the application of membrane technology for gas separation, and most of the research work are focused on the investigation of new materials and the development of new membrane structures. Membrane types could be broadly divided into three classes: polymeric membranes, inorganic membranes and mixed matrix membrane (MMMs).5-6 Generally, the polymeric membranes rarely exceed the trade-off limitations between permeability and selectivity,7 and systematically tuning the polymeric chain stiffness and interchain separation are anticipated to improve the intrinsic separation property in the case of solution-diffusion transport mechanism. In the meanwhile, well prepared microporous inorganic membranes, i.e. microporous silica membrane,8 microporous carbon membrane9 and zeolite membranes,10 could offer high gas permeance and selectivity as result of the presence of micropores together with superior thermal resistance and mechanical strength. However, the lack of processability, scaling up and most importantly, fabrication costs are the main hurdle for practical application. In term of 3



these issues, the mixed matrix membranes become predominant due to the collaborative advantage of polymer phase and a dispersed inorganic phase, and the individual drawbacks could be compensated mutually during the membrane preparation. An ideal mixed matrix membrane morphology should fully facilitate the gas transport property of inorganic fillers enabling the preferential gas to transport the dispersed inorganic phase rather than the polymer phase, and recent advances have shifted toward new fillers such as metal organic frameworks (MOFs),11 lamellar inorganic materials12-16 and carbon nanotubes (CNTs).17 A proper match of the polymer and inorganic materials and membrane preparation method could improve the compatibility as well as the separation performance.18 For example, different MOF nanoparticles could be well-dispersed and matched into organosilica networks due to the organic-inorganic hybrid nature through sol-gel method, and the porous organosilica networks as the co-support significantly minimize the interfacial defects after a calcination process.19 However, the present research system on the mixed matrix membranes is usually consisted of the single polymer phase and inorganic phase. Interestingly, we note that the match of two inorganic materials could exert a favorable synergistic effect, and such concept has been repeatedly utilized for water treatment.20-22 Moreover, to the best of our knowledge, very few reports exist in the literature in which a dual-inorganic filler system was adopted for gas separation purpose. Coronas investigated the match of MOFs and zeolites in polysulfone for the first time, the filler synergy gave rise to an improvement in gas separation performance.23 Inspired by this 4


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new strategy, Wu et.al24 fabricated mixed matrix membrane with graphene oxide (GO) and carbon nanotubes (CNTs), permeation result showed an increase in gas permselectivity comparing with those doped with only GO or CNTs. In addition, M. Ba-Shammakh’s group25-26 synergistically integrated the graphene oxide and different ZIFs (ZIF-301 and ZIF-302) into polysulfone to prepare mixed matrix membrane for CO2 separation. Hence, a three-component system (including two types of inorganic filler and one polymer matrix) provides another opportunity for the preparation of desirable MMMs. Even though a considerable effort has been made on the use of graphene or nanotubes in MMMs for gas separation, the suggested combination of modified halloysite nanotubes (mHNTs) and porous reduced graphene oxide (PRG) in the same MMM has not been reported to date. In this work, the aim is the development of Pebax-mHNTs-PRG MMMs. The polymeric matrix, Pebax®1657, is a highly permeable commercial copolymer including 60 wt% poly (ethylene oxide) and 40 wt% polyamide 6. Porous graphene, bearing the nano-scaled pores on the graphene sheet, is supposed to be the ultimate membrane for gas separation. Large computational studies have further demonstrated the highly efficient separation property for gas mixtures.27-28 On the other hand, the double-layer graphene with a few millions of pores had been fabricated by a focused ion beam (FIB), and displayed several orders of magnitude of gas permeance and acceptable selectivity for H2/CO2 gas separation.29 Alternatively, chemical methods could also drill nanopores on the graphene sheet.30-32 In our previous research,33 we had 5



obtained the porous reduced graphene oxide (PRG) with an average pore size of ca.12 nm through a simple chemical treatment on graphene oxide (GO), and incorporating porous reduced graphene oxide into Pebax®1657 matrix indeed improved the gas transport property, as it decreased the gas transport resistance and provided the molecular sieving channels. As for the halloysite nanotubes (HNTs), it is a naturally occurring clay material. HNTs is of high organic solvent tolerance and easily modified advantages. Incorporation of small amount of modified HNTs within polymers to enhance their mechanical properties has received wide interests.34-37 Most importantly, these two materials have different aspect-ratio nanostructures, and the aspect ratio of PRG is much higher than that of HNTs. Thus, it is assumed to construct the 1D nanotubular channel as well as the 2D interlayered channel within the membranes, which is anticipated to enhance the gas separation performance. The resulting membranes proved this speculation and displayed both high selectivity and high gas permeability for CO2/N2 separation. The effect of the mass ratio of mHNTs to PRG on the membrane morphology was also investigated in this work. EXPERIMENTAL METHODS Chemical and materials A natural graphite powder (ca. 45 µm) was purchased from Sinopharm Chemical Regent. Pebax® 1657 was obtained from Shanghai Rongtian Chemical Co., Ltd. The natural halloysite nanotubes with full length of ca 1.5 µm and length/outer diameter ratio of ca 18 was supplied by Henan Xianghu Environmental Protection Technology Co., Ltd (Henan, 6


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China). Concentrated sulfuric acid (H2SO4, 98 wt %), phosphoric acid (H3PO4, 85 wt %), hydrochloric acid (HCl) and hydrogen peroxide (H2O2, 30 wt %) were of analytical grade and purchased from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd (Tianjin, China). Potassium permanganate (KMnO4) and sodium hydroxide (NaOH) were purchased from J&K. Poly(sodium-p-styrenesulfonate) (PSS, MW 70,000) was purchased from Sigma-Aldrich. All chemicals were used as purchased without further purification. The deionized (DI) water was also used in this study. Preparation of porous reduced graphene oxide (PRG) and modified halloysite nanotubes (mHNTs) Porous reduced graphene oxide was obtained after a series of chemical treatments on graphene oxide, which was prepared according to the modified Hummers method. The detailed procedure has been described elsewhere.33 Briefly, GO was mixed at the mass ratio of NaOH/H2O/GO=1/100/0.15. After stirring for 1 h at room temperature and centrifugation, the obtained sediment was re-dispersed in the HCl solution (0.75 wt %) and stirred for another 1 h following by the centrifugation. Finally, it was washed by DI water and acetone and transferred to the vacuum desiccators. The main purpose of the modification on HNTs with PSS was to improve the dispersity because of the decreased ζ potential.38 1 g of PSS was dispersed in 50 mL of deionized water and stirred for 30 min, then 1 g of HNTs was added into the PSS solution and stirred for another 48 h. The excess of PSS was removed by centrifugation and redispersion in DI water for three cycles. The modified HNTs (denoted as mHNTs) was 7



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dried for 24 h in the vacuum oven and grinded before use. Preparation of the mixed matrix membranes Firstly, 0.02g PRG and a certain amount of mHNTs were dispersed into ethanol/H2O (7/3 by weight) and sonicated for 5 h. Subsequently, 3g Pebax® 1657 pellet was dissolved into the mixture under reflux at 80oC for 24h. The total solution weight was 100g and kept constant. Afterwards, 13 mL of the solution was poured into PTFE petri dish (50.26 cm2) with pipette and placed in an enclosed cabinet for drying at room temperature for 24 h. The resulting membranes were finally dried for another 24 h in a vacuum

oven

to

remove

residual

solvents.

Membranes

are

referred

as

Pebax-mHNTs-PRG-X in the following description, where X (X=2.5, 5.0, 7.5, 10.0) indicates the weight ratio of mHNTs to PRG. The pure Pebax®1657 membrane was also prepared by the exactly same procedure but without any addition of fillers. In addition, for the Pebax-mHNTs membranes, the weight ratios of mHNTs to Pebax correspond to that of the Pebax-mHNTs-PRG membranes. Characterization of PRG, mHNTs and membranes X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffractometer using Cu Kα radiation in the range of 5-70o at a step size of 0.02o. The morphologies and structures were characterized using transmission electron microscopy (Tecnai G2 F20 S-Twin) and field-emission scanning electronic microscopy (JSM-6700F). Fastscan mode AFM images were obtained using a FASTSCAN-C probe. For the AFM observations, samples were prepared by depositing one droplet of corresponding 8


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samples or sticking onto a single crystalline Si substrate. The contact angles of water for membranes were confirmed on an optical instrument (OCA 25, dataphysics, Germany) equipped with video capture at room temperature. Fourier Transform Infrared Spectrometer (Thermo-Nicolet Magna 560 IR spectrometer) was used to monitor the variation of characteristic functional groups of samples. Thermogravimetric Analysis (TGA) (DTG-60, Shimadzu) was conducted to evaluate the decomposition behavior of HNTs and mHNTs with a ramping rate of 10oC/min up to 800oC in N2 atmosphere. The specific gas adsorption behaviors of the HNTs and mHNTs were obtained by adsorption experiments at 25oC (ASAP 2420, USA). The samples were degassed under high vacuum at 80oC for 24 h prior to measurement. Gas permeation measurements Gas permeation experiments were carried out with a home-made apparatus with an effective membrane area of 19.6 cm2, as reported in our previous work.39 For single gas (CO2 and N2) test, the feed stream was pressurized, and the downstream was maintained at atmospheric pressure. A soap film bubble flow meter was used to measure the flow rate of the outlet gas. Mixed CO2/N2 permeation (10/90, v/v) was also performed, and the permeate composition were analyzed using a gas chromatograph (Shimadzu, GC-2014). Besides, to illuminate the effect of water vapor on membrane performance, the feed gas was humidified with water activity of 0.6-0.75 (60%-75% RH). Each experimental data point reported here is the average value of 3 samples. The variation in gas permeability during each measurement was found to be less than 6.5%. The 9



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permeability (P) is calculated by the following expression:

∙

(1)

∆∙

In which Q is the gas flux (cm3/s), l is the membrane thickness (µm), ∆p is the trans-membrane pressure difference (cm Hg), and A is the effective membrane area (cm2). The ideal selectivity is:

(2)

In which PCO2 and PN2 are the CO2 and N2 permeability. The selectivity for the gas mixture is:

(3)

In which the y and x represent the molar fractions in the permeate and feed stream. RESULTS AND DISCUSSION Characterization of PRG and mHNTs

Fig. 1 TEM images and a Fastscan mode AFM images on mica surface (1.51.5 µm) of GO (a, b) and PRG (c, d)

10


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In this work, the as-prepared GO displayed obvious wrinkles. The resulting GO particle has a lateral size of 400-500 nm, and the thickness, obtained from the height profile, is 1.1-1.5 nm (Fig. 1a, b). The data is consistent with other literature,40 indicating the formation of single layered GO after sonication. In the meanwhile, the single layered PRG sheet had similar lateral size and thickness (ca. 1.3 nm) with GO. As a result, their aspect ratios are higher than 267. The bumpy height profile could also imply the presence of the nano-pores on graphene sheet (Fig. 1c, d).

Fig. 2 (a) The schematic presentation of the modified HNTs; (b) dispersion of the pristine and modified HNTs dispersion in water after 72h; (c-d) TEM image of HNTs and mHNTs; (e-f) FTIR spectra and TGA analysis of HNTs and mHNTs The pristine HNTs comprise gibbsite octahedral groups (Al-OH) on the internal surface with positive charge and siloxane groups (Si-O-Si) on the external surface with negative charge. Thus, it could be selectively modified by Poly(sodium-p-styrenesulfonate)

11



(PSS), which is served as a water-dispersing agent. The modification process mainly occurs at the internal surface due to the electrostatic effect (Fig. 2a), resulting in the improved dispersity (Fig. 2b). In addition, we noted that the internal diameter decreased slightly from 18.5 nm to 17.3 nm according to the TEM (Fig. 2c, 2d). The successful modification could be further confirmed through FTIR (Fig. 2e), as the new absorbance peaks at 1230 and 602 cm-1 correspond to the stretching vibration of sulfonate group and ring in plane deformation vibration of the PSS, respectively.41 On the other hand, the content of PSS attached on the HNTs could be roughly calculated based on the mass loss (Fig. 2f), which is about 0.019 g (PSS)/g (HNTs). Quantity adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

KD×10 CO2

0.15 0.12

4

KD×10 N2

Selectivity

HNTs(CO2)

(CO2/N2)

HNTs

1.659

0.1073

15.46

mHNTs

1.051

0.0259

40.58

mHNTs(CO2)

0.09 0.06 HNTs(N2)

0.03

mHNTs(N2)

0.00 0

100

200

300

400

500

600

700

800

900

Pressure (mmHg)

Fig. 3 Pure component CO2 and N2 adsorptions on HNTs and mHNTs, both of which employ the single-site Henry models, KD is Henry’s constant (mmolg-1mmHg-1) In the meanwhile, we investigated the gas adsorption behaviors on HNTs and mHNTs, and a typical result of CO2/N2 is shown in Fig. 3. The mHNTs could contribute to 2.62 times higher CO2/N2 selectivity than that could be obtained with HNTs due to the favorable interactions between the sulfonated groups and CO2 molecules.42 However, in 12


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comparison with HNTs, the CO2 adsorption on mHNTs decreases by 36.64%, which may be attributed to the decreased specific area, as shown in Fig. S1, mHNTs show a smaller surface area than the pristine HNTs. Besides, the modification process does not change the original crystalline structure of HNTs (Fig. S2). Pebax-mHNTs-PRG-X membranes characterization

Fig. 4 Cross-sectional SEM images of (a, b) pristine Pebax® 1657 membrane; (c, d) Pebax-PRG membrane (0.02 wt% PRG); (e, f) Pebax-mHNTs membrane (0.15 wt% mHNTs) and (g, h) Pebax-mHNTs-PRG-7.5 membrane To get a better understanding of the transition process of the membrane morphology, the 13



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cross-sectional SEM images of the different membranes with clear contrast are shown in Fig. 4. Firstly, no obvious fracture or defect along the boundaries of interfacial gaps is observed for all membranes, implying the good polymer-filler compatibility. The PRG sheets randomly stack within the membrane matrix (Fig. 4c, 4d), and the mHNTs doped into membranes presents a homogeneous distribution with elusive orientation (Fig. 4e, 4f). In sharp contrast, the coexistence of the PRG and mHNTs displays a totally different membrane morphology, and they very uniformly occupy the membrane cross-section at all depths. The mHNTs are incorporated into PRG sheets, forming the analogous sandwich structure (Fig. 4h).

Fig. 5 Cross-sectional AFM images of Pebax-mHNTs-PRG-7.5 membrane On the other hand, atomic force microscope (AFM) was used to further investigate inner microstructure (Fig. 5). It is found that the mHNTs seems to have the paralleled orientation along the PRG plane, and the mHNTs shows a strongly prevalent orientation at some angles relative to the gas flux direction due to the shear-induced orientation effect between the polymeric matrix and PRG.43 Such phenomenon has also been encountered for other 2D materials within the polymeric matrix.16,

14


44-45

It is also

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possibly caused by the bundling effect. The isolated mHNTs aggregate spontaneously and finally form the small bundles during the drying process.46 Furthermore, the mHNTs distribute more densely with the ratio of mHNTs to PRG ranging from 2.5 to 10 (Fig. S3). Despite the identical PRG loadings in all these membranes (0.88 g/m2), striking difference in the nanostructure is evident. The surface morphology of these membranes was also characterized by AFM (Fig. 6). It proves that the mHNTs could not only be intercalated into the stacked PRG layers, but also attach onto the membrane surfaces. Initially, little mHNTs could be observed on the membrane surface. Afterwards, the membrane morphology changes greatly, and the naked mHNTs become more obvious when the mass ratio of mHNTs to PRG was 7.5 and 10.0, showing extremely good adhesion to the membrane matrix. On the other hand, the agglomeration of the fillers (PRG and/or mHNTs) leads to an increase in surface roughness (Table 1), and the maximum surface roughness occurs in Pebax-mHNTs-PRG-10 membrane with 39.8 nm and 56.1 nm for Ra and Rq, respectively.

15



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Fig. 6 AFM images of the surface of (a) Pebax®1657 membrane; (b) Pebax-PRG membrane; (c-f) Pebax-mHNTs-PRG-X membranes (X=2.5, 5.0, 7.5, 10.0), scan area: 5×5 µm Table 1 Surface roughness parameters of the membranes Membrane

(a)

(b)

(c)

(d)

(e)

(f)

Ra

8.71

29.8

31.2

32.1

36.1

39.8

Rq

11.6

39.4

39.8

41.1

48.8

56.1

According to the FTIR spectra in Fig. 7, the observed adsorption bands at 1090, 1630, 1730 and 3300 cm-1 are the characteristic peaks of Pebax, which belong to C-O-C, H-N-C=O, O-C=O and -N-H- groups. In addition, another two peaks at 1030 (AlOH stretching band) and 3680 cm-1 (Si-O-Si stretching vibrations) reveal the presence of mHNTs on the membrane surface, and the peak intensities become much stronger with 16


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the increase in mHNTs/PRG ratio, indicating the continuous agglomeration process of mHNTs. Thus, according to the AFM and FTIR results, a typical morphologic transformation occurs on the membrane surface by regulating the mHNTs/PRG ratio. (a)

(e)

(b)

(c) (d) (e) 3680

(d)

Intensity

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c) (b) (a)

1730 3300 1630

4000

3500

3000

2500

2000

1030

1090

1500

1000

500

-1

10

20

30

40

50

60

70

2θ (degrees)

Wave number (cm )

Fig. 7 FT-IR spectra (left) and XRD patterns (right) of (a) Pebax® 1657 membrane; (b) Pebax-mHNTs-PRG-2.5; (c) Pebax-mHNTs-PRG-5.0; (d) Pebax-mHNTs-PRG-7.5 and (e) Pebax-mHNTs-PRG-10.0 membranes The XRD patterns of the Pebax® 1657 membrane and the Pebax-mHNTs-PRG-X membranes are also shown in Fig. 7. Pebax® 1657 exhibits semi-crystalline nature with diffraction peaks at 14o (polyether amorphous phase) and 23.7o (crystalline polyamide).47 It is observed that the addition of mHNTs causes the compaction of the polymer matrix, which is common for MMMs48-49. The smaller interchain distance induced by compaction of the polymer matrix is usually reflected in the decreased d spacing and increase in the intensity of the crystalline peak of Pebax®1657, as indicated by the dotted line. Such phenomenon may be attributed to the enhanced interchain hydrogen bonding between the amide blocks due to the inclusion of fillers. However,

17



mHNTs could retain their intrinsic crystalline structure comparing with the pristine mHNTs (Fig.S2). The peak at 12.2o (0.72 nm) corresponds to the characteristic peak (001) of mHNTs, which is related to the dehydrated form.50

Fig. 8 Schematic representation of the formation of the internal structure in polymeric environment based on the interactions among the Pebax®1657, PRG and mHNTs As illustrated in Fig. 8. we put forward some hypotheses, based on the physical-chemical properties of PRG and mHNTs, to illustrate the stacking behavior in polymer. First, the shear-induced orientation effect and/or bundling effect could lead to a strongly prevalent orientation, as mentioned above. Second, the spatial effect induces the paralleled stacking of mHNTs along PRG plane owing to the largely different aspect ratio of PRG and mHNTs, providing the especially dense nanochannels for gas transport. Lastly, both the PRG and the external surface of mHNTs are negatively charged, thus, the electrostatic repulsion prevents the agglomeration, leading to a homogeneous

18


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dispersion of the fillers within the membranes. On the other hand, the mHNTs could act as the excellent spacers and inhibit the PRG sheets from restacking so tightly, reducing the internal gas transport resistance.51 Gas separation performance

α

100

60 50

90

CO2

80

40

70 30

60 50

20

N2

5

(b) Gas Permeability (barrer)

10 0

0 0.00

0.05 0.10 0.15 mHNTs content (wt%)

140 120 100 80 60 40

140

CO2

120 100 80

α

60

4 3

N2

2

40 20

1 0

0.20

0 .0 .5 ax RG -7.5 -10 G-5 G-2 Peb ax-P RG RG -PR -PR Peb s-P s-P NTs NTs HNT HNT -mH -mH m m x x x a x a a a Peb Peb Peb Peb

(c)

(d)

3

200

10

Robeson Upper Bound (2008)

Gas permeability (barrer)

2

10

Pebax Pebax-PRG Pebax-mHNTs-PRG-2.5 Pebax-mHNTs-PRG-5.0 Pebax-mHNTs-PRG-7.5 Pebax-mHNTs-PRG-10

1

10

0

10

1

10

2

10

150

CO2

α

100

1

10

50 0

10

Selectivity(CO2/N2)


110

Ideal selectivity(CO2/N2)

70

120


(a)

Ideal selectivity (CO2/N2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N2 0

2

3

10

G ax .5 .5 5.0 10 Peb bax-PR G-2 -PRG- PRG-7 -PRG-PR s sPe NTs NTs -mHNT mHNT H -mH -m bax x x x a a a Pe Peb Peb Peb

10

CO2 permeability (Barrer)

Fig. 9 The gas permeability of (a) Pebax-mHNTs membranes; (b) Pebax-mHNTs-PRG membranes; (c) Comparison of CO2/N2 separation performance of the as-prepared membranes with the Robeson upper bound (30oC, 0.3 MPa and dry state); (d) gas permeability of Pebax-mHNTs-PRG membranes with dry (open) and wet (solid) CO2/N2 (10/90, v/v) mixture Initially, we prepared the Pebax-mHNTs membranes with different mHNTs loadings and measured the pure gas separation performance (Fig. 9a). Results show that the 19



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permeability of CO2 and N2 increase simultaneously but with significant sacrifice in the CO2/N2 selectivity. The preliminary results indicate that the incorporated mHNTs in the polymeric matrix could provide additional channels to allow gas molecules to transport through the membrane unimpededly. In the meanwhile, though the modification on HNTs improved the dispersion and enhanced the adsorption selectivity for CO2/N2, the inner diameter of mHNTs (13.5±4.1 nm) is slightly lower than the mean free path (λ) of CO2 (18.5 nm) and N2 (20.2 nm) at 30oC and 0.3 MPa,38 calculated from the Eq. (4):

λ

√

(4)

Where KB is the Boltzmann constant (1.3810-23 J·K-1), T is the absolute temperature (K), σ is the gas molecule diameter (m), and P is average pressure inside the membrane (Pa). Thus, it can be deduced that the main gas diffusion mechanism in the mHNTs could be the Knudsen diffusion. Generally, gas transport occurs in the gaseous state but without obvious involvement of adsorption by Knudsen diffusion, because the interaction between diffusing gas molecules and pore wall is negligibly small, resulting in the absence of separation property. Similarly, other HNTs-based MMMs also revealed a low selectivity (15~40) for CO2/N2 separation.52-54 In addition, based on the KJN model predictions,55 the gas permeability of mHNTs should be larger than that of the Pebax®1657 matrix. This led us to adopt a new strategy of combining the mHNTs with PRG for fabricating the three-component MMMs because of the excellent molecular sieving behavior of PRG.33 Permeation results show that the pure CO2 permeability becomes much higher 20


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


than that of pristine Pebax® 1657, Pebax-PRG (0.02 wt% PRG) and Pebax-mHNTs membranes, together with significant enhancement in CO2/N2 ideal selectivity. The gas permeability increases steadily with the increase in mHNTs/PRG ratio, but the ideal gas selectivity presents a firstly increased and then decreased tendency. An optimum mHNTs/PRG ratio of 7.5 was found, and the Pebax-mHNTs-PRG-7.5 membrane shows the highest ideal gas selectivity up to 118 at 30oC and 0.3 MPa (Fig. 9b). Besides, all as-prepared Pebax-mHNTs-PRG membranes approach or exceed the 2008 upper bound for polymeric membranes (Fig. 9c). Again, it not only demonstrates the superior gas separation property of PRG within the polymeric membranes, but also proves the feasibility using a three-component system for MMMs preparation. This can be rationalized by considering how the transport pathways of CO2 and N2 change in the presence of PRG. As mentioned above, the addition of PRG could affect the orientation preference of mHNTs, ranging from the random distribution to certain angles relative to the gas flux direction. As a result, the resulting orientation of mHNTs would shorten the transport pathways of gas molecules. On the other hand, the PRG possesses numerous nanopores on the sheets, which could also decrease the diffusion resistance to a certain degree. However, due to the narrow interlayer spacing of PRG, it is highly possible to obstruct the direct diffusion of larger N2 molecules. Therefore, the N2 molecules preferentially diffuse through the polymer phase rather than through the PRG with higher diffusion resistance. Correspondingly, the N2 molecules are forced to take more tortuous pathways, resulting in low gas permeability. In contrast, the CO2 molecules 21



could transport through the PRG nanosheets with lower diffusion resistance. Moreover, the presence of mHNTs and PRG could cause the distortion of polymer chains and increase the crystalline region (Fig. 7), and this is another explanation for the enhanced CO2/N2 selectivity. To explore the gas separation performance of the Pebax-mHNTs-PRG membranes with mixed-gas feed, permeation tests were carried out with dry and wet CO2/N2 (10/90, v/v) mixture at operating pressure of 0.3 MPa and 30oC, as shown in Fig. 9d. It is observed that gas permeation for dry feed presents the similar behavior in contrast with the pure gas permeation, and the maximum selectivity could be obtained at the mHNTs/PRG ratio of 7.5. However, the presence of competitive sorption leads to reduced gas permeability in the gas mixture. For the wet feed, the gas permeability of CO2 and N2 are improved remarkably, which is several-fold higher than that of the dry feed. It indicates that water vapor strongly influences the gas transport behavior. Considering the hydrophilic property of Pebax® 1657, the water vapor will cause swelling of the polymer, which enhances the mobility of polymer chain and increases gas diffusivity. On the other hand, water molecules usually tend to form clusters in the PEO-based block copolymers, which create additional tortuosity for gas diffusion.56 Nonetheless, the undesired effect of the formation of water clusters could be compensated by establishing other transport channel with mHNTs. As a result, the Pebax-mHNTs-PRG membranes show the highest CO2 permeability of about 640 barrer and CO2/N2 selectivity of 88. 22


Page 22 of 38

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As discussed above, permeation results show that there is a turning point in the CO2/N2 selectivity when the mHNTs/PRG ratio is 7.5, according to which a reasonable mechanism mainly about the transition of membrane morphology is proposed, as shown in Fig. 10. By carefully adjusting the mHNTs/PRG ratio, the membrane morphology can be broadly divided into two types, namely the PRG-rich and mHNTs-rich nano-architectures. In the former case, the PRG is the determining factor for the ultimate gas separation performance by the molecular sieving effect. However, with a further increase in the loading of mHNTs, it will inevitably weaken the desired molecular sieving effect, as most of the gas molecules preferentially penetrate through the inner channels of mHNTs abiding by the Knudsen diffusion mechanism. In addition, higher inorganic filler loadings usually give rise to more defects, especially the interfacial defects at the filler-matrix boundary. As a result, all these two factors bring about the loss of gas selectivity and higher gas permeability. To strictly confirm the turning point, i.e. the corresponding mass ratio of mHNTs to PRG, another two mass ratios (6.0 and 9.0) were adopted, and the gas permeability and selectivity are listed in table 2. The experimental results clearly indicate that the Pebax-mHNTs-PRG-7.5 membranes have the maximum CO2/N2 selectivity and intermediate level of gas permeability. This result provides further support to our previous speculation about the transition of membrane morphology at the mHNTs to PRG mass ratio of 7.5.

23



Page 24 of 38

Fig. 10 Schematic illustration of the possible nano-architectures of the composite membranes with different mHNTs/PRG mass ratio Table 2 Summary of the gas separation performance obtained from Pebax-mHNTs-PRG membranes with the different mass ratio of mHNTs to PRG Membrane

Pebax-mHNTs-PRG

Mass ratio of mHNTs to PRG Membrane thickness (µm) P(MPa) b

P(CO2) (barrer) P(N2) (barrer)b

b

Ideal selectivity (-) a

a

5

6

7.5

9

10

52±2

50±1

54±1

50±1

55±1

0.3

0.3

0.3

0.3

0.3

92.2±2.7

104.3±1.7

123.5±1.4

128.7±2.2

130.3±1.3

0.95±0.00

0.98±0.01

1.05±0.00

1.18±0.00

1.33±0.01

97.1±2.8

106.4±2.8

117.6±1.4

109.1±1.8

98.3±0.9

Error corresponds to the standard deviation from 5 different measurements at different

locations within each membrane using micrometer caliper;

b

Error is shown as the

standard deviation from 3 membranes with the same inorganic fillers loadings Generally, the plasticization can be considered as a weakening effect on the polymer and finally resulting the swollen polymer losses its selectivity at elevated pressure. Such phenomenon has been frequently encountered in the Pebax-based MMMs.57-58 However, 24


Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


our Pebax-mHNTs-PRG membranes display excellent plasticization-resistant behavior to some extent, though the pure gas measurements are not particularly useful for examining the plasticization behavior, as shown in Fig. 11. This method can improve the ideal selectivity as well as an acceptable loss in permeability at elevated pressure. This finding is completely opposite to the general observation for polymeric membranes.59 This is because the existing gas transport channels can restrain the CO2 molecules long-time exposure or contact with the polymer matrix, resulting in the unobvious swelling. In addition, the compaction effect could also occur in the Pebax-based MMMs and reduce the membrane free volume under high pressure.33, 60 While the CO2 permeability keeps nearly constant with increase in the operation pressure. It indicates that the compaction effect could be effectively avoided in this work, which could be attributed to the rigid structure of mHNTs and the electrostatic repulsion between the PRG and mHNTs. Thus, the match of mHNTs and PRG provides an ideal stacking form and ensures stable membrane morphological performance. Long-term stability of gas permeability of Pebax-mHNTs-PRG-7.5 membrane was also examined for up to 120 hours (Fig. 11c). The CO2 permeability was still maintained at 140 barrer after 120 hours at 0.1MPa, and the ideal gas selectivity was almost unchanged, demonstrating the good stability of our Pebax-mHNTs-PRG membranes.

25



160 140 120 100 80 60

10 9 8 7 6 Pebax Pebax-mHNTs-PRG-2.5 Pebax-mHNTs-PRG-5.0 Pebax-mHNTs-PRG-7.5 Pebax-mHNTs-PRG-10.0

4 3 2 1 0.1


0.2

140

0.3 ∆P(MPa)

3 2

0.4

0

120 100 80 60 40

Pebax Pebax-mHNTs-PRG-2.5 Pebax-mHNTs-PRG-5.0 Pebax-mHNTs-PRG-7.5 Pebax-mHNTs-PRG-10

20 0 0.1

(c)

4

1

0

(b)

5

N2 permeability (barrer)

CO2 Permeability (barrer)

(a)

0.2

0.3

0.4

∆P(MPa) 160

120 CO2

120 100

100

α

80

80

60

60

40 4 3 2 1 0

40 20

N2

Ideal selectivity (CO2/N2)

140


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

0 0

20

40

60

80

100

120

Time (h)

Fig. 11 (a, b) gas Permeability and CO2/N2 ideal selectivity of membranes as a function of operation pressure tested at 30°C under the dry state; (c) Long-term operation test of CO2/N2 for Pebax-mHNTs-PRG-7.5 membrane measured at 0.1 MPa and 30oC

Conclusions In conclusion, the CO2 permeability and CO2/N2 selectivity of Pebax®1657 can be significantly enhanced by incorporating a small amount of mHNTs and PRG in a

26


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three-component system. The outstanding gas separation performance derives from the preferable stacking behavior of the fillers, which optimizes the transport pathways of gas molecules and establishes numerous transport channel with lower resistance. Besides,

it

endows

the

Pebax-mHNTs-PRG

membranes

with

extraordinary

plasticization/compaction resistance. Lastly, this strategy provides another alternative way for the graphene-based membrane to break through the current bottleneck for carbon capture and storage (CCS). ASSOCIATED CONTENT Supporting information The supporting information is available: Figure S1 shows Nitrogen sorption isotherms of pristine HNTs and mHNTs. Figure S2 presents the XRD patterns of HNTs and mHNTs. Figure S3 illustrates the cross-sectional SEM images of the Pebax-mHNTs-PRG membranes with different mHNTs/PRG mass ratios AUTHOR INFORMATION Corresponding authors: * E-mail: [email protected] * E-mail: [email protected] Author contributions #

These authors contribute equally to this work.

27



Page 28 of 38

ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Nos. U1704139 and 21376225), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004), Key Science and Technology Program of Henan Province (182102310013), Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002) and Excellent Youth Development Foundation of Zhengzhou University (No. 1421324066). REFERENCES (1) Lonsdale, H. K., The growth of membrane technology. J. Membr. Sci. 1982, 10, 81-181, DOI 10.1016/S0376-7388(00)81408-8. (2) Pendergast, M. M.; Hoek, E. M. V., A review of water treatment membrane nanotechnologies. Energ Environ Sci 2011, 4, 1946-1971, DOI 10.1039/c0ee00541j. (3) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M., Science and technology for water purification in the coming decades. Nature 2008, 452, 301-310, DOI 10.1038/nature06599. (4) Padaki, M.; Surya Murali, R.; Abdullah, M. S.; Misdan, N.; Moslehyani, A.; Kassim, M. A.; Hilal, N.; Ismail, A. F., Membrane technology enhancement in oil–water separation.

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Graphic for manuscript

Synopsis: Both the porous reduced graphene oxide and modified halloysite nanotubes are integrated into the membrane matrix with tunable nano-architectured channel for CO2 separation.

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Enhanced permeation through CO2-stable dual-inorganic composite

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