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Energy, Environmental, and Catalysis Applications

Ultrathin membranes with polymer/nanofiber inter-penetrated structure for high-efficiency liquid separations

Yufan Ji, Guining Chen, Guozhen Liu, Jing Zhao, Gongping Liu, Xuehong Gu, and Wanqin JinACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12445 • Publication Date (Web): 11 Sep 2019

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1

Ultrathin Membranes with Polymer/Nanofiber Inter-penetrated

Structure for High-efficiency Liquid Separations

Yufan Ji, Guining Chen, Guozhen Liu, Jing Zhao*, Gongping Liu, Xuehong Gu, Wanqin

Jin*

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical

Engineering, Nanjing Tech University, 30 Puzhu South Road, Nanjing 211800, P.R. China

*To whom all correspondence should be addressed.

Tel.: +86-25-83172266; fax: +86-25-83172292.

E-mail: ([email protected]) or to W. Jin. ([email protected])

KEYWORDS:

Ultrathin polymer membrane, multi & alternate spin-coating, interface-decoration-layer,

inter-penetrated structure, liquid separation

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ACS Paragon Plus Environment

ACS Applied Materials & Interfaces

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mailto:[email protected]

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ABSTRACT

Ultrathin film composite (UTFC) membrane comprising an ultrathin polymeric active

layer has been extensively explored in gas separation applications benefiting from its

extraordinary permeation flux for high throughput separation. However, the practical

realization of ultrathin active layer in liquid separations is still impeded by the trade-off

effect between membrane thickness (permeation flux) and structural stability (separation

factor). Herein, we report a general multi & alternate spin-coating (MAS) strategy,

collaborating with the interface-decoration-layer of copper hydroxide nanofibers (CHNs)

to obtain ultrathin and robust polymer-based membranes for high-performance liquid

separations. The structural stability arises from the Polydimethylsiloxane (PDMS)/CHNs

inter-penetrated structure, which confers the synergistic effect between PDMS and CHNs

to concurrently resist PDMS swelling and avoid CHNs collapsing, while the ultrathin

thickness is enabled by the sub-10 nm pore size of CHNs layer, the rapid crosslinking

reaction during spin-coating and the small thickness of CHNs layer. As a result, the as-

prepared membrane possesses an exceptional butanol/water separation performance with

the flux of 6.18 kg/m2h and separation factor of 31, far exceeding the state-of-the-art

polymer membranes. The strategy delineated in this work provides a straightforward

method for the design of ultrathin and structurally stable polymer membranes, holding

great potentials for the practical application of high-efficiency separations.

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1. INTRODUCTION

Chemical separations account for a large proportion of the total energy consumption

in the world. In this light, developing energy-efficient separation technologies such as

membrane separation is imperative and of great significance for lowering global energy

cost, carbon emission and pollution.1, 2 The efficiency and the cost-competitiveness of

membrane separation technology essentially lie in the separation performance of

membrane materials.3, 4 Especially, a high permeation flux enables a high-throughput

separation, thus reducing membrane area, endowing a compact membrane equipment and

cutting down the capital cost. In recent years, mixed matrix membranes (MMMs)

containing inorganic fillers and polymeric matrices have been developed, which can

improve permeation flux via tuning free volume and introducing inorganic transport

pathways. 5-10 Theoretically, the permeation flux should be more remarkably influenced by

membrane thickness. As a consequence, fabricating an ultrathin active layer on top of a

porous support has been one of the most attractive research focuses, owing to its potential

to achieve excellent permeation flux. 11-21 However, the preparation and the practical

application of ultrathin active layer especially in liquid separations are still impeded by

some critical issues: 1) the severe solution intrusion into porous support due to the low

viscosity of polymer solution for thinner membrane, which leads to the generation of

defects in active layer; 2) the deteriorated membrane stability in liquid environment due to

the ultrathin thickness, which results in the collapsed membrane structure.22-25

In our previous work, a facile and universal interface-decoration-layer strategy was

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proposed to mitigate the solution intrusion phenomenon via depositing a copper hydroxide

nanofibers (CHNs) layer possessing high porosity, interconnected structure and sub-10-nm

pore size on porous support (Figure S1).26 Ultrathin thin film composite (UTFC)

membranes with the thickness of ~100 nm were obtained, achieving a 2.5-fold higher gas

permeance without the penalty of selectivity. Compared with gas separation, the various

separation systems in liquid environment exert more stringent requirements for membrane

stability due to the stronger permeating molecule-membrane interactions. PDMS is a

benchmark polymeric membrane material showing great potentials for gas separation and

solvent enrichment/recovery from dilute aqueous solution due to its good comprehensive

performance and cost-effective preparation.27-34 Although the PDMS-based membrane

prepared with the above-mentioned interface-decoration-layer strategy has displayed an

excellent gas separation performance, it almost shows no selectivity for organic

solvent/water separation due to the excessive swelling of ultrathin PDMS layer and the

collapsed CHNs network structure upon contacting with water and organic solvent

vapors.35, 36 Through prolonging pre-crosslinking time and increasing PDMS coating

amount, the membrane stability can be effectively improved, yet the consequent thickened

active layer inevitably leads to a lower permeation flux, which means there is a trade-off

phenomenon between membrane thickness (permeation flux) and structural stability

(separation factor).37, 38 So far, the application of ultrathin PDMS membrane in liquid

separations has rarely been reported

To address this issue, in this work, we proposed a novel multi & alternate spin-coating

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(MAS) strategy, collaborating with the CHNs interface-decoration-layer to obtain ultrathin

PDMS membranes for high-performance liquid separation with the butanol recovery from

aqueous solution as a model system. PDMS and cross-linking agent (tetraethyl orthosilicate,

TEOS) solutions were alternately and repeatedly spin-coated on the CHNs layer (Figure 1,

Figure S2). During each cycle of spin-coating, the low-viscosity PDMS solution will

penetrate into the pores of CHNs layer, and then form a crosslinked PDMS structure

wrapping the nanofibers during the subsequent coating of TEOS solution. On one hand,

the rigid CHNs network plays a critical role of physical confinement and interfacial

interlocking in resisting PDMS swelling. On the other hand, the PDMS provides a

protection for CHNs to avoid the direct contact of permeating molecules with CHNs.

Moreover, the thickness of PDMS/CHNs transition layer is limited to <200 nm, while an

ultrathin and defect-free PDMS active layer on top of CHNs layer can be formed due to

the sub-10 nm pore size of CHNs layer and the crosslinking reaction during spin-coating,

leading to a low total thickness. Consequently, the MAS strategy delineated herein realizes

an ultrathin thickness and a stable membrane structure concurrently, overcoming the trade-

off effect and achieving an attractive butanol/water separation performance.

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Figure 1. Schematic illustration of the membrane preparation process.

2. EXPERIMENTAL SECTION

2.1 Materials. PDMS (Mw = 60000) was manufactured from Shanghai Resin Factory

Co., Ltd. (China). N-heptane, tetraethyl orthosilicate (TEOS) and dibutyltin dilaurate

(DBTDL) were received from Sinopharm Chemical Reagent Co. Ltd. (China). Copper

nitrate and aminoethanol were purchased from Kanto Chemical Co. Ltd. (Japan). De-

ionized water generated in the laboratory were used for all the experiments. All chemicals

were of analytical grade and were used throughout without further purification. The

polyacrylonitrile (PAN) ultrafiltration membrane (molecular weight cut-off: 100 kDa) with

the flat-sheet configureuration was provided by Shandong MegaVision Membrane

Technology & Engineering Co. Ltd. (China).

2.2 Preparation of CHNs layer. Copper hydroxide nanofibers were prepared by

mixing an aqueous solution of 1.6 mM aminoethanol and an equivalent volume of 4 mM

copper nitrate solution and leaving it to age for 3 days at 25 oC. 10 ml of the as-synthesized

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CHNs dispersion was diluted to 30 ml with water, and then filtered on a PAN support by

vacuum filtration to obtain CHNs layer with the effective diameter of 4.0 cm.

2.3 Preparation of PDMS UTFC membranes. PDMS was added into n-heptane to

obtain a 10 wt% solution (solution A). The crosslinking agent TEOS (1 wt %) and the

catalyst dibutyltin dilaurate (DBTDL, 0.1 wt %) were dissolved in n-heptane (solution B).

Then the two resulting solutions were stirred at room temperature respectively for 30 min.

A certain amount of PDMS and TEOS–DBTDL solutions were alternately and repeatedly

spin-coated (500 rpm, 15s; 3000 rpm, 1 min) on the surface of CHNs layer to form an

active layer. The solutions were drop wise spin-coated during the 500 rpm stage. The as-

prepared membranes were named as PDMS(X)-Y/CHNs/PAN, where X represents the

total quantity of PDMS dropped on CHNs surface during spin-coating process (mg/cm2),

and Y represents the number of spin cycles. Y=0 refers to the membrane prepared through

single spin-coating of PDMS-TEOS-DBTDL mixed solution (with a pre-crosslinking time

of 36 or 48 h) as reported in the previous work.26 All the membranes were dried in air at

ambient temperature for 24 h, and then in a vacuum oven for 24 h to obtain the final

composite membranes.

2.4 Characterization. Transmission electron microscopy (TEM, JEOL-1010, an

accelerating voltage of 100 kV, JEM, USA) images and electron diffraction patterns were

obtained to observe the morphology and structure of nanofibers. The high-resolution

transmission electron microscopy (HR-TEM, JEOL-2000, an accelerating voltage of 200

kV, JEM, USA) images were also obtained. Morphologies of the as-prepared membranes

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were characterized by scanning electron microscope (FESEM, S4800, Hitachi, Japan) and

atomic force microscopy (AFM, XE-100, Park SYSTEMS, South Korea with non-contact

mode). For membrane thickness measurement, ten different locations in the cross-sectional

SEM images were measured and averaged to obtain the final result. X-ray photoelectron

spectroscopy (XPS, K-Alpha-MAGCIS, Thermo Scientific) depth analysis was performed

to investigate the solution intrusion phenomenon. For each sample, a step length of 15 nm

was employed to detect the elemental composition from membrane surface to interior in

the range of 0–400 nm. The water contact angles on the surface of membranes were tested

by a contact angle drop-meter (A100P, MAIST Vision Inspection & Measurement Co.,

Ltd.).

2.5 Butanol/water separation test. Recently, the emerging inadequacy of oil

resources and the growing demands on sustainable development have promoted the

production of renewable fuels from biomass such as bio-alcohols. Among the varied kinds

of valuable liquid biofuel, bio-butanol is main product of fermentation process. Therefore,

in this work, the butanol recovery from aqueous solution was chosen as a model system to

evaluate the performance of as-prepared PDMS-based membranes in liquid separation. The

butanol/water separation tests were performed with a home-made apparatus. The feed tank

was placed in a thermostat water bath to keep the temperature of the feed solution at a

steady state. The prepared flat-sheet PDMS-CHNs/PAN membrane was installed in a

stainless steel membrane module. The pressure of the permeate side was retained below

300 Pa via a vacuum pump, and the feed solution was sucked into membrane module at

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the flow rate of 60 L h-1 by a peristaltic pump. The permeated vapor was collected using a

cold trap with liquid nitrogen, and at least three samples were analyzed to ensure the

reliability of experimental results. The compositions of the feed and permeate were

analyzed by a gas chromatography (GC-2014, SHIMADZU, Japan). The total permeation

flux (J, kg/m2h) and separation factor (β) can be calculated according to the following

equations

（1） 𝐽 =𝑄

𝐴 ∗ 𝑡

（2） 𝛽 =𝑌𝑏 𝑌𝑤𝑋𝑏 𝑋𝑤

Where Q (kg) is the total mass of permeate collected in t hours, and A (m2) denotes

the effective area of the membrane. X and Y are the mass fractions of water (w) or butanol

(b) in feed and permeate sides, respectively.

2.6 Calculations of permeance (driving force-normalized form of flux) and

activation energy of permeation. The permeance of individual component (𝑃/𝑙) (GPU, 1

GPU=7.501×10-12 m3 (STP)/m2 s pa) and selectivity (α) were calculated by following

equation:

（3） (𝑃/𝑙) 𝑖 = 𝐽𝑖

𝑝𝑖0 ― 𝑝𝑖𝑙=

𝐽𝑖

𝛾𝑖0𝑥𝑖0𝑝𝑠𝑎𝑡𝑖0 ― 𝑝𝑖𝑙

（4） 𝛼 =(𝑃 𝑙)𝑏

(𝑃 𝑙)𝑤

Where 𝐽𝑖 (kg/m2 h) is the permeation flux of component 𝑖; 𝑙 (m) is the thickness of

membrane; p𝑖0, 𝑝𝑖𝑙 (Pa) are the partial pressure of component 𝑖 in the feed side and permeate

side, respectively. 𝑝𝑖𝑙 can be taken the value of 0 for the high vacuum situation in the

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permeate side. 𝛾𝑖0 is the activity coefficient of component 𝑖 in the feed solution; 𝑥𝑖0 is the

mole fraction of component 𝑖 in the feed solution; (Pa) is the saturated vapor pressure 𝑝𝑠𝑎𝑡𝑖0

of pure component 𝑖.

The relationship between operating temperature and permeance usually conforms to

the Arrhenius law:

（5） (𝑃/𝑙) 𝑖 = (𝑃/𝑙)0exp (― 𝐸𝑃

𝑅𝑇 )

Where (𝑃/𝑙)𝑖 , (𝑃/l)0, EP, R and T represent the permeance of component 𝑖, the pre-

exponential factor, the activation energy, gas constant and feed temperature (K),

respectively.

3. RESULTS AND DISCUSSION

3.1. Characterizations of the CHNs and the PDMS-based UTFC membranes. The

nanofibers show a long and straight morphology (Figure 2a), and the width is estimated to

be around 4.2 nm (Figure 2b). On account of the extremely narrow structure, the electron

diffraction pattern is not very clear. However, the three weak halos still can be observed

attributable to the (111)/(002), (130), and (151) diffractions of orthorhombic Cu(OH)2

(Figure 2c).39 With a high-resolution TEM (HR-TEM) characterization on CHNs, the

spacing of 0.25 nm is clearly shown in the Figure 2d, indicating the (111) planes of

orthorhombic Cu(OH)2.

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Figure 2. TEM (a, b), SAED pattern (c) and HR-TEM (d) images of copper hydroxide

nanofibers.

As reported in the previous work, the CHNs layer is conducive to mitigate the solution

intrusion phenomenon, thus favoring the formation of an ultrathin and defect-free active

layer.26 Herein, CHNs/PAN support was prepared via depositing CHNs on PAN surface

with a fixed deposition amount of 0.8 ml/cm2. From the cross-sectional morphology of

CHNs(0.8)/PAN membrane, the CHNs layer shows good homogeneity with the thickness

around 200 nm (Figure S3). Subsequently, the PDMS and TEOS-DBTDL solutions were

alternately and repeatedly spin-coated on CHNs/PAN supports to form a series of PDMS-

based UTFC membranes. Figure 3 shows the surface and cross-section morphologies of

PDMS(X)-1/CHNs/PAN membranes with different PDMS spin-coating amounts. On

account of the low viscosity, PDMS solution will penetrate into the pores of CHNs layer

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during the spin-coating process. After the subsequent spin-coating of TEOS-DBTDL

solution, the covalent reaction occurs between TEOS and PDMS, leading to a crosslinked

PDMS structure surrounding nanofibers. The typical network structure of nanofiber layer

can be apparently observed even in the presence of PDMS active layer (Figure 3a, c) due

to that too little amount of PDMS is insufficient to seal all the pores constructed by the

interlaced nanofibers. With the increasing amount of PDMS solution, the membrane

surface gradually exhibits a visibly dense and defect-free surface morphology, while the

undulating surface arising from the stacked CHNs layer disappears (Figure 3e). The

thickness measurement from the cross-sectional SEM images reveals the same conclusion.

The membranes with PDMS coating amounts of 7 and 10 mg/cm2 possess similar

thicknesses about 200 nm, which is almost equivalent to the thickness of the CHNs layer,

indicating that there is no continuous PDMS active layer on top of CHNs layer.26 When

the PDMS coating amount increases to 17 mg/cm2, the measured thickness is about 259

nm. Combining with the variation of membrane surface morphology, it can be

hypothesized that a PDMS active layer has been formed on top of CHNs layer and the

surface pores of the CHNs layer are covered by PDMS.

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Figure 3. FESEM images of the surface and cross-section of (a, b) PDMS(7)-1/CHNs/PAN;

(c, d) PDMS(10)-1/CHNs/PAN and (e, f) PDMS(17)-1/CHNs/PAN membranes.( The as-

prepared membranes were named as PDMS(X)-Y/CHNs/PAN, where X represents the

total quantity of PDMS in spin-coating solution (mg/cm2), and Y represents the number of

spin cycles.)

To further demonstrate this hypothesis, X-ray photoelectron spectroscopy (XPS)

depth analysis was performed to investigate the variation of elemental composition from

membrane surface to interior. As shown in Figure 4a, according to the change of silicon

(Si) and copper (Cu) elements with depth, the three-layer structure of PDMS(17)-

1/CHNs/PAN composite membrane can be clearly distinguished: (1) PDMS active layer in

the range of 0-75nm, where Si element content is high and constant, and Cu content is zero;

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(2) PDMS/CHNs transition layer in the range of 75-240 nm, where the content of Si starts

to decrease and then reaches a plateau region, while Cu element can be detected and also

reaches a plateau region; (3) PAN support after the etch depth of 240 nm, where both Cu

and Si contents remarkably decrease to approach zero. Comparatively, the PDMS(10)-

1/CHNs/PAN composite membrane shows a totally different phenomenon in the first stage:

in the range of 0-45 nm, the Si content is quite low (5-10 %), while the Cu content is higher

than the plateau region. The CHNs-dominant feature of this region evidences that the

nanopores in CHNs network has not been completely filled by PDMS, leaving some pores

exposed on membrane surface, which proves our assumption. In addition, for PDMS(10)-

1/CHNs/PAN and PDMS(17)-1/CHNs/PAN composite membranes, in the whole range of

the second stage, the phenomenon that Si content remarkably decreases and Cu content

remains quite high hasn’t been observed (for instance, the second stage of PDMS(20)-

0/CHNs/PAN membrane as shown in Figure S5, corresponding to the CHNs layer without

sufficient PDMS penetration). This result indicates that the CHNs layer is wrapped by

PDMS throughout the entire depth, which is conducive to protect CHNs from directly

contacting with permeating molecules and maintain membrane stability in liquid separation

environment. Although the one-cycle alternate spin-coating of PDMS and TEOS-DBTDL

solutions with PDMS coating amount of 10 mg/cm2 couldn’t form a defect-free PDMS

active layer, we obtained a completely different result when we divided the solutions into

6 equal parts and performed alternate spin-coating for 6 cycles (as shown in Figure 4a).

PDMS solution can fully cover the pores in CHNs layer and form a PDMS active layer

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with a thickness about 60 nm. The possible reason is that a cross-linked and solidified

PDMS structure can be formed after each cycle of alternate spin-coating due to the covalent

reaction between TEOS and PDMS, which gradually narrows the pore size, facilitates the

full coverage of the pores in CHNs layers and the formation of a continuous PDMS active

layer. For these membranes, the PDMS/CHNs transition layer also contributes to the

separation process since the CHNs pores are filled with PDMS. Therefore, the effective

membrane thickness should include both the thicknesses of PDMS active layer and

PDMS/CHNs transition layer.

Figure 4. XPS depth analysis results (a) and schematic illustrations of the cross-sectional

structures (b) of PDMS(17)-1/CHNs/PAN, PDMS(10)-1/CHNs/PAN and PDMS(10)-

6/CHNs/PAN membranes.

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The surface hydrophobicity of as-prepared composite membranes was investigated

since higher hydrophobicity would be beneficial for the butanol-preferential adsorption on

the membrane surface. Figure 5 shows the water contact angle values of the PDMS(17)-

1/CHNs/PAN membranes with the TEOS/PDMS mass ratio varying from 5 wt% to 40 wt%.

All the membranes are hydrophobic displaying the water contact angles higher than 90°. It

can be observed that there is an obvious increase from 90.4 o to 121.3 o when the

TEOS/PDMS mass ratio increases from 5 wt% to 20wt% since the intensified crosslinking

reaction consumes more hydroxyl groups on PDMS, thus leading to a higher

hydrophobicity. With the further increase of TEOS/PDMS mass ratio, the crosslinking

reaction between TEOS and PDMS tends to be saturated. The excessive TEOS molecules

are prone to carrying through the self-condensation reaction to generate SiO2 particles (as

shown in the AFM images in Figure S4) 40, 41. The Tyndall effect through PDMS-TEOS-

DBTDL mixed solution with TEOS/PDMS mass ratio of 40 wt% further evidences the

existence of SiO2 particles (Figure 5). The hydrophilic SiO2 particles result in the decrease

of hydrophobicity on the membrane surface.

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Figure 5. Water contact angles of PDMS(17)-1/CHNs/PAN membranes with various mass

ratios of TEOS/PDMS (Inset is the digital image of the Tyndall effect through PDMS-

TEOS-DBTDL mixed solutions with TEOS/PDMS mass ratios of 10 wt% and 40 wt%).

3.2 Effect of preparation condition on separation performance. For PDMS(X)-

1/CHNs/PAN composite membranes, the butanol/water separation factor increases from

2.0 to 31.1 then remains constant while the permeation flux keeps decreasing with the

increase of PDMS coating amount (Figure 6a). As revealed in the XPS depth analysis

(Figure 4a), at a low PDMS coating amount, the nanopores on support surface are partially

exposed, which leads to an ultralow separation factor. With the increase of coating amount,

these nanopores are gradually covered, forming a defect-free PDMS active layer and

achieving the intrinsic separation factor of PDMS membrane for butanol/water separation.

After that, the further increment of PDMS coating amount results in a thicker PDMS active

layer, thus decreasing permeation flux while exerting negligible influence on separation

factor. Therefore, the optimal separation performance can be obtained under the critical

PDMS coating amount of 14~17 mg/cm2 with the highest permeation flux (3.55~3.01

kg/m2h) and the intrinsic PDMS separation factor (~30).

The crosslinking degree of PDMS has a significant impact on the membrane structure

and surface characteristics, thus affecting the membrane separation performance. When the

mass ratio of TEOS/PDMS increases from 5 to 20 wt%, the increase of covalent

crosslinking sites leads to the densification of membrane structure and enhances the mass

transfer resistance of permeating molecules, thereby reducing the flux. Meanwhile, the

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consumption of hydroxyl groups on PDMS during crosslinking reaction improves the

hydrophobicity of membrane, promoting the selective adsorption towards butanol

molecules, and increasing the butanol/water separation factor. As the TEOS/PDMS mass

ratio continues to increase, the crosslinking reaction between TEOS and PDMS gradually

saturates, and SiO2 particles are generated from the self-condensation reaction of excessive

TEOS molecules. The hydrophilic SiO2 particles result in the decrease of hydrophobicity

on the membrane surface, which is not desirable for the preferential adsorption of butanol,

thus giving rise to the reduction of separation factor (Figure 6b). The effect of alternate

spin-coating number on the butanol/water separation performance was also investigated

with the total PDMS coating amount of 10 mg/cm2. The separation factor increases in the

first and then remains nearly unchanged with the increase of spin-coating cycle number,

while the total flux shows a reverse change tendency (Figure 6c). As demonstrated by the

XPS depth analysis, multiple cycles of alternate spin-coating facilitates the coverage of

pores in CHNs layer with PDMS and then leads to a defect-free PDMS active layer due to

the crosslinked and solidified PDMS structure formed in each cycle helps to narrow the

pore size. Therefore, the number of uncovered pores exposed on membrane surface

gradually declines with the increase of cycle number, leading to a decrease of unselective

pores for butanol/water permeation, thus increasing butanol/water separation factor and

decreasing flux until the intrinsic separation factor of PDMS membrane is reached. Since

the total spin-coating amount is same, once a defect-free PDMS layer can be formed, the

increase of spin-coating cycle number exert negligible influence on the separation

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performance. Similarly, the PDMS(17)-Y/CHNs/PAN composite membrane shows a

nearly constant performance with the increase of spin-coating cycle number (Figure S7).

Figure 6. Effect of (a) PDMS coating amount on the separation performance of PDMS(X)-

1/CHNs/PAN composite membrane; (b) TEOS/PDMS mass ratio on the separation

performance of PDMS(17)-1/CHNs/PAN composite membrane; (c) the number of spin

coating cycles on the separation performance of PDMS(10)-Y/CHNs/PAN composite

membrane; (d) Comparison on separation performance of M1 (PDMS(40)-

0/CHNs/PAN), M2 (PDMS(10)-6/CHNs/PAN), M3 (PDMS(17)-1/CHNs/PAN).

Butanol/water separation tests were performed at 40 °C with 1wt% butanol content in the

feed.

To verify the significance of multi & alternate spin-coating strategy in achieving high

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separation performance, the PDMS(X)-0/CHNs/PAN composite membranes were

prepared for comparison via direct spin-coating of PDMS-TEOS-DBTDL mixed solution

(Figure 6d). Furthermore, XPS depth analysis was also performed on the tested membranes

to investigate the evolution of membrane microstructure during butanol/water separation

(Figure 7a). At first, the PDMS(20)-0/CHNs/PAN membrane with a pre-crosslinking time

of 36 h was studied, since it has a ultrathin defect-free PDMS active layer which shows the

highest performance for gas separation.26 However, after 30 min of the separation test, the

membrane structure is damaged and the separation factor is sharply decreased to ~1. This

is mainly due to that the pore penetration in PDMS(20)-0/CHNs/PAN membrane is

inhibited via direct spin coating of mixed solution on CHNs layer (Figure S3), and thus the

CHNs layer is not sufficiently wrapped by PDMS. As a result, the ultrathin PDMS layer is

severely swollen during the butanol/water separation test, while the CHNs network

structure collapses after contacting with permeated vapors, which in turn aggregates the

destruction of PDMS layer (Figure 7b, Figure s6c). Both Si and Cu elements show sharply

fluctuated contents with the depth of PDMS(20)-0/CHNs/PAN membrane after test (Figure

7a), completely different with that before test (Figure S5), while the membrane surface

shows a cracked and defective morphology, sufficiently attesting the above hypothesis. In

contrast, the CHNs in the alternately spin-coated membranes are fully coated by PDMS

and a PDMS/CHNs transition layer is formed. The synergistic effect between PDMS and

CHNs confers high stability: (i) the rigid CHNs network provides mechanical support to

improve the swelling resistance of ultrathin PDMS layer via physical confinement and

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interfacial interlocking; (ii)the PDMS surrounding CHNs acts as a protection layer to avoid

the direct contact of vapors with CHNs and inhibit the collapsing of CHNs network (Figure

7c, Figure S6a, b). As a proof, the XPS depth profiles of PDMS(10)-6/CHNs/PAN and

PDMS(17)-1/CHNs/PAN membranes are exactly maintained after separation test (Figure

4a and Figure 7a). According to the above analysis on deteriorated performance of the

PDMS(20)-0/CHNs/PAN membrane with a pre-crosslinking time of 36 h, there are two

methods to improve the membrane stability: (i) increase the swelling resistance of PDMS

active layer; (ii) protect the CHNs with PDMS through pore penetration. The latter is

achieved with the multi & alternate spin-coating (MAS) strategy in this work. As a

comparison, we also tried the former method to show the superiority of the MAS strategy

via prolonging the pre-crosslinking time to 48 h. It is beneficial to enhance the crosslinking

degree and increased the PDMS coating amount to increase the thickness of active layer.

At the coating amount of 40 mg/cm2, the PDMS(40)-0/CHNs/PAN membrane (M1)

achieves the intrinsic butanol/water separation factor of PDMS membrane and shows an

acceptable stability during the separation test. In this case, the thickness of PDMS active

layer is around 1.5 μm (Figure S8). The much thicker active layer leads to a higher mass

transfer resistance. Therefore, the flux is only 1.45 kg/m2h, which is obviously lower than

that of the PDMS(10)-6/CHNs/PAN (M2) and PDMS(17)-1/CHNs/PAN (M3) membranes

(Figure 6d), confirming that multi & alternate spin-coating of PDMS and TEOS-DBTDL

solutions is an effective approach to achieving high flux. In comparison, the flux of

PDMS(10)-6/CHNs/PAN (M2) is slightly higher than that of PDMS(17)-1/CHNs/PAN

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(M3), which can be attributed to the thinner PDMS active layer in M2 and the resultant

lower mass transfer resistance. This result reveals that the defect-free PDMS active layer

obtained by multi & alternate spin-coating strategy can achieve higher flux with less total

coating amount.

Figure 7. (a) schematic illustration of the cross-sectional structure of PDMS(20)-

0/CHNs/PAN membrane before and after the butanol/water separation test; (b) XPS depth

analysis results of PDMS(17)-1/CHNs/PAN, PDMS(10)-1/CHNs/PAN and PDMS(10)-

6/CHNs/PAN membranes after the butanol/water separation test; (c) schematic illustration

of the cross-sectional structure of PDMS(10)-6/CHNs/PAN membrane before and after the

butanol/water separation test.

3.3 Effect of operation condition on separation performance. 3.3.1 Butanol

content in feed. The butanol content in feed is a key factor influencing the membrane

performance. As shown in Figure 8a, b, with the butanol content in feed varying from 0.5

to 4 wt%, the separation factor decreases from 42.4 to 31.3, and the total flux increases

from 2.70 to 4.14 kg/m2h at the operating temperature of 40 °C. According to the formula

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3, the calculation of separation factor is affected by butanol content in the feed, therefore

the butanol content in permeate is also listed in Figure 8b to analyze the actual influence

on separation efficiency, which shows an increasing tendency from 40.6 to 62.1 wt%. Since

the increase of the butanol content in feed confers different influences on the mass transfer

driving forces of butanol and water molecules, we calculated the permeance (the driving

force-normalized permeation flux) of water and butanol and the butanol/water selectivity

of the prepared membrane as listed in Table S2. With the butanol content in feed increasing,

the butanol permeance elevates marginally, while the water permeance is almost

unchanged, finally realizing an increase in selectivity. The constant water permeance

indicates that the increased butanol content doesn’t contribute to promote the swelling of

PDMS layer, demonstrating the stability of membrane structure. The increase in the butanol

permeance can be ascribed to the promoted butanol adsorption. When the butanol content

in feed is up to 4 wt %, the membrane shows the highest separation performance with the

permeation flux of 4.14 kg/m2h and the butanol content in permeate of 62.1 wt%.

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Figure 8. Effect of the butanol content in feed on separation performance of PDMS(17)-

1/CHNs/PAN composite membrane: (a) flux, (b) separation factor and butanol content in

permeate; (c) Effect of feed temperature on the separation performance of PDMS(17)-

1/CHNs/PAN composite membrane; (d) The Arrhenius plots of water permeance and

butanol permeance.

3.3.2 Operating temperature. The influence of operating temperature on the

separation performance of PDMS(17)-1/CHNs/PAN composite membrane is illustrated in

Figure 8c. With the increase of operating temperature, the flux remarkably increases while

the separation factor slightly decreases. When the temperature is up to 60 oC, the flux

reaches 6.18 kg/m2h and the separation factor is 31. Since temperature will have effects on

different aspects such as mass transfer driving force, membrane structure (molecular

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diffusion) and permeating molecule-membrane interaction (molecular adsorption), the

water and butanol permeance and selectivity are calculated as listed in Table S3 to analyze

the influence of temperature in depth. Furthermore, the apparent activation energies (Ep)

for water and butanol permeation are also calculated by Arrhenius equation (Figure 8d).

Both the butanol permeance and water permeance decrease with increased temperature,

indicating that the increase in the driving force makes a dominant contribution to the

improved flux. The negative activation energies of butanol and water demonstrate that both

the water permeation and butanol permeation through PDMS membrane are adsorption-

controlled processes. The negative effect of increasing temperature on molecular

adsorption is more significant than the positive effect on molecular diffusion. It also reveals

that the membrane structure is not obviously swollen and remains stable at high

temperatures.

3.3.3 Long-term operation test. The long-term operation stability is an important

factor determining the practical application of membranes, which is especially critical for

the composite membranes with a thin active layer on a porous support. Herein, the stability

of PDMS(17)-1/CHNs/PAN membrane was investigated with a continuous butanol/water

separation test up to 200 h (Figure 9a). The total flux and separation factor slightly fluctuate

in the range of 2.61-3.27 kg/m2h and 33.1-38.6, respectively. The desirable stability of as-

prepared composite membrane arises from the inter-penetrated PDMS/CHNs structure

through alternate spin-coating of TEOS and PDMS, indicating a potential application in

bio-alcohol recovery and organic solvent/water separations.

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Figure 9. (a) The long-term separation performance of the PDMS(17)-1/CHNs/PAN

composite membrane (at 40 °C, 1wt% butanol-water mixture with TEOS/PDMS mass ratio

of 20 wt%); (b) Performance comparison of polymer-based membranes in separating 1 wt%

n-butanol aqueous solution..

3.3.4 Comparison with other reported membranes for butanol recovery. PDMS is the

most widely used polymeric membrane material for solvent enrichment/recovery from

dilute aqueous solution. Herein, the performance of recently reported polymer-based

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membranes for butanol/water separation are summarized in Figure 9b and Table S4.42-55 In

addition, the performance of several inorganic membranes also listed in the Table S4.56, 57

Due to the ultrathin and sturdy PDMS layer from the MAS strategy in this work, the

PDMS/CHNs/PAN composite membranes show an excellent flux, which is 2-20 times

higher than that of reported PDMS membranes, and a comparable separation factor. On

basis of the as-prepared high-flux PDMS UTFC membranes, other well-developed

approaches such as incorporating porous nanofillers can be adopted to further optimize the

membrane microstructure and improve the separation performance. Furthermore, the

PDMS/CHNs/PAN membranes even possess a competitive performance when comparing

with the reported polymer-based mixed matrix membranes, demonstrating the great

potential of MAS strategy.

4. CONCLUSIONS

In this work, a novel multi & alternate spin-coating method was explored cooperating

with the CHNs interface-decoration-layer to achieve high-performance PDMS UTFC

membranes for liquid separation. Through the alternate deposition of PDMS and TEOS

solutions on top of CHNs layer, a PDMS/CHNs inter-penetrated structure can be formed,

which enables the robust membrane structure: (i) the rigid CHNs network improves the

swelling resistance of PDMS via physical confinement and strengthens the interfacial

interlocking between the active layer and CHNs layer; (ii) the PDMS wrapping CHNs

provides a barrier to avoid the direct contact of permeating molecules with CHNs and then

protects the CHNs network. In addition, the sub-10 nm pore size of CHNs layer and the

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crosslinking reaction during spin-coating facilitates the formation of an ultrathin and

defect-free PDMS active layer, while the thickness of CHNs layer endows a limitation on

the thickness of PDMS/CHNs transition layer to <200 nm, leading to a low total thickness.

The PDMS/CHNs/PAN membranes in this work show a superior performance with the

flux of 6.18 kg/m2h and the butanol/water separation factor of 31, outperforming the state-

of-the-art polymer-based membranes. The combination of MAS and interface-decoration-

layer strategies proposed here provides a promising approach for the production of ultrathin

membranes for high-efficiency liquid separations.

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ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Schematic representations of the different membrane preparation processes and the PDMS

cross-linking reaction mechanism, FESEM images of substrate and CHNs layer, AFM

images of PDMS(17)-1/CHNs/PAN membranes with various TEOS/PDMS mass ratios,

XPS depth analysis results of PDMS(20)-0/CHNs/PAN membrane, FESEM images of the

membrane surfaces after test; Effect of the number of PDMS coating cycle on the PV

performance, Cross-sectional SEM images of PDMS(40)-0/CHNs/PAN(48h) composite

membrane, Water contact angles of PDMS(17)-1/CHNs/PAN membrane surface.

Permeance and selectivity data, performance comparison of the membrane in this work

with state-of-the-art membranes.

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AUTHOR INFORMATION

Corresponding Author

* [email protected] (Dr. J. Zhao)

* [email protected] (Prof. W. Jin)

Author Contributions

Contribution of each author: Wanqin Jin and Jing Zhao conceived the project. Yufan Ji

prepared the membranes, carried out the characterizations and water/butanol separation

tests. Guining Chen and Guozhen Liu assist me to complete the long-term operation test.

Yufan Ji and Jing Zhao conducted data analysis. Wanqin Jin, Jing Zhao, Gongping Liu and

Xuehong Gu participated in results discussion. All the authors were involved in writing the

manuscript.

ORCID

Jing Zhao: 0000-0002-1423-0291

Wanqin Jin: 0000-0001-8103-4883

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of

China(21728601, 21490585, 21606123), the Natural Science Foundation of Jiangsu

Province (BK20160980), the Innovative Research Team Program by the Ministry of

Education of China (Grant no.IRT_17R54) and the Topnotch Academic Programs Project

of Jiangsu Higher Education Institutions (TAPP).

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P., Highly selective and robust PDMS mixed matrix membranes by embedding two-dimensional ZIF-L for alcohol permselective pervaporation. J. Membr. Sci., 2019, 582, 307-321.50. Liu, G.; Hung, W.-S.; Shen, J.; Li, Q.; Huang, Y.-H.; Jin, W.; Lee, K.-R.; Lai, J.-Y., Mixed matrix membranes with molecular-interaction-driven tunable free volumes for efficient bio-fuel recovery. J. Mater. Chem. A, 2015, 3, 4510-4521.51. Xue, C.; Du, G. Q.; Chen, L. J.; Ren, J. G.; Sun, J. X.; Bai, F. W.; Yang, S. T., A carbon nanotube filled polydimethylsiloxane hybrid membrane for enhanced butanol recovery. Sci. Rep., 2014, 4, 5925.52. Jonquieres, A.; Fane, A., Filled and unfilled composite GFT PDMS membranes for the recovery of butanols from dilute aqueous solutions: influence of alcohol polarity. J. Membr. Sci., 1997, 125, 245-255.53. Lee, J. Y.; Hwang, S. O.; Kim, H.-J.; Hong, D.-Y.; Lee, J. S.; Lee, J.-H., Hydrosilylation-based UV-curable polydimethylsiloxane pervaporation membranes for n-butanol recovery. Sep. Purif. Technol., 2019, 209, 383-391.54. Liu, S.; Liu, G.; Zhao, X.; Jin, W., Hydrophobic-ZIF-71 filled PEBA mixed matrix membranes for recovery of biobutanol via pervaporation. J. Membr. Sci., 2013, 446, 181-188.55. Yen, H. W.; Chen, Z. H.; Yang, I. K., Use of the composite membrane of poly(ether-block-amide) and carbon nanotubes (CNTs) in a pervaporation system incorporated with fermentation for butanol production by Clostridium acetobutylicum. Bioresour. Technol., 2012, 109, 105-109.56. Liu, Q.; Huang, B.; Huang, A., Polydopamine-based superhydrophobic membranes for biofuel recovery. J. Mater. Chem. A, 2013, 1, 11970-11974.57. Shen, D.; Xiao, W.; Yang, J. H.; Chu, N. B.; Lu, J. M.; Yin, D. H.; Wang, J. Q., Synthesis of silicalite-1 membrane with two silicon source by secondary growth method and its pervaporation performance. Sep. Purif. Technol. 2011, 76, 308−315.

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TOC graphic

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TOC

282x92mm (150 x 150 DPI)

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Figure 1. Schematic illustration of the membrane preparation process.

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Figure 2. TEM (a, b), SAED pattern (c) and HR-TEM (d) images of copper hydroxide nanofibers.

125x97mm (220 x 220 DPI)

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Figure 3. FESEM images of the surface and cross-section of (a, b) PDMS(7)-1/CHNs/PAN; (c, d) PDMS(10)-1/CHNs/PAN and (e, f) PDMS(17)-1/CHNs/PAN membranes.( The as-prepared membranes were named as PDMS(X)-Y/CHNs/PAN, where X represents the total quantity of PDMS in spin-coating solution (mg/cm2),

and Y represents the number of spin cycles.)

128x130mm (149 x 149 DPI)

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Figure 4. XPS depth analysis results (a) and schematic illustrations of the cross-sectional structures (b) of PDMS(17)-1/CHNs/PAN, PDMS(10)-1/CHNs/PAN and PDMS(10)-6/CHNs/PAN membranes.

279x190mm (150 x 150 DPI)

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Figure 5. Water contact angles of PDMS(17)-1/CHNs/PAN membranes with various mass ratios of TEOS/PDMS (Inset is the digital image of the Tyndall effect through PDMS-TEOS-DBTDL mixed solutions

with TEOS/PDMS mass ratios of 10 wt% and 40 wt%).

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Figure 6. Effect of (a) PDMS coating amount on the separation performance of PDMS(X)-1/CHNs/PAN composite membrane; (b) TEOS/PDMS mass ratio on the separation performance of PDMS(17)-1/CHNs/PAN composite membrane; (c) the number of spin coating cycles on the separation performance of PDMS(10)-

Y/CHNs/PAN composite membrane; (d) Comparison on separation performance of M1 (PDMS(40)-0/CHNs/PAN), M2 (PDMS(10)-6/CHNs/PAN), M3 (PDMS(17)-1/CHNs/PAN). Butanol/water separation tests

were performed at 40 °C with 1wt% butanol content in the feed.

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Figure 7. (a) schematic illustration of the cross-sectional structure of PDMS(20)-0/CHNs/PAN membrane before and after the butanol/water separation test; (b) XPS depth analysis results of PDMS(17)-

1/CHNs/PAN, PDMS(10)-1/CHNs/PAN and PDMS(10)-6/CHNs/PAN membranes after the butanol/water separation test; (c) schematic illustration of the cross-sectional structure of PDMS(10)-6/CHNs/PAN

membrane before and after the butanol/water separation test.

425x176mm (150 x 150 DPI)

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Figure 8. Effect of the butanol content in feed on separation performance of PDMS(17)-1/CHNs/PAN composite membrane: (a) flux, (b) separation factor and butanol content in permeate; (c) Effect of feed temperature on the separation performance of PDMS(17)-1/CHNs/PAN composite membrane; (d) The

Arrhenius plots of water permeance and butanol permeance.

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Figure 9. (a) The long-term separation performance of the PDMS(17)-1/CHNs/PAN composite membrane (at 40 °C, 1wt% butanol-water mixture with TEOS/PDMS mass ratio of 20 wt%); (b) Performance comparison

of polymer-based membranes in separating 1 wt% n-butanol aqueous solution..

146x190mm (149 x 149 DPI)

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penetrated structure for high-efficiency liquid ...acs paragon plus environment ... (pdms)/chns...

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