Long-Term Stability of PVDF-SiO2-HDTMSComposite Hollow Fiber Membrane For CarbonDioxide Absorption in Gas–Liquid ContactingProcessHonglei Pang  ( panghl@niit.edu.cn )

Nanjing Vocational University of Industry TechnologyYayu Qiu 

Nanjing Vocational College of Information TechnologyWeipeng Sheng 

Zhejiang Xinchai CO., LTD

Research Article

Keywords: Super hydrophobic membrane, Membrane contactor, Carbon dioxide absorption, Hydrophobicmodi�cation

Posted Date: November 12th, 2021

DOI: https://doi.org/10.21203/rs.3.rs-1006466/v1

License: This work is licensed under a Creative Commons Attribution 4.0 International License.  Read Full License

Long-term stability of PVDF-SiO2-HDTMS composite hollow fiber

membrane for carbon dioxide absorption in gas–liquid contacting

process

Honglei Pang a* , Yayu Qiu b*, Weipeng Sheng c

a Nanjing Vocational University of Industry Technology, Nanjing, 210023, PR China

b Nanjing Vocational College of Information Technology, Nanjing, 210023, PR China

c Zhejiang Xinchai CO., LTD, Shaoxing, 312500, PR China

ABSTRACT

To obtain a long-term stable operation of the hollow fiber membrane for using in membrane

contact absorption of carbon dioxide (CO2), hybrid polyvinylidene fluoride-silica-

hexadecyltrimethoxysilane (PVDF-SiO2-HDTMS) membrane were fabricated via the non-solvent

induced phase-inversion method. The surface properties, performance characteristics and long-term

stable operation performance of the prepared membranes were compared and analyzed. The results

show the outer surface of the prepared membranes exhibited superhydrophobicity because of the

formation of rough nano-scale microstructures and the low surface free energy. Due to the addition

of inorganic nanoparticles, the mechanical strength of PVDF-SiO2-HDTMS membranes were

improved. The long-term stable operation experiments were carried out with the inlet gas (CO2/N2

= 19/81, v/v) at a flow rate of 20 mL/min and the absorbent liquid (1 mol/L DEA) at a flow rate of

50 mL/min. And the result showed that the mass transfer flux of PVDF-SiO2-HDTMS membrane

decreased from the initial value of 2.39×10-3 mol/m2s to 2.31×10-3 mol/m2s, which was a decrease

of 3% after 20 days. The main benefit is the addition of inorganic nanoparticles, which have strong

chemical resistance and high hydrophobicity, thereby preventing structural damage and pore wetting

of the membrane. PVDF-SiO2-HDTMS membrane exhibits excellent long-term stable operation

performance of CO2.

Keywords: Super hydrophobic membrane; Membrane contactor; Carbon dioxide absorption;

Hydrophobic modification

*

* Corresponding author.

E-mail address: panghl@niit.edu.cn (H. Pang), qiuyy@njcit.cn(Y., Qiu)

1 Introduction

Biogas is a promising renewable energy source [1]. Usually, it is mainly composed of CH4 (55-

65%), CO2 (30-45%) [2] and other trace gases. In order to use it as a fuel gas, the content of CH4

should be as high as 95%, so the absorption of carbon dioxide (CO2) is of great significance to the

practical application of biogas. Carbon dioxide (CO2) capture from biogas by the hollow fiber

membrane contactor system have been investigated by several researchers. In a membrane contactor

system for CO2 absorption, the mixed gas and the liquid absorbent respectively flow on both sides

of the hollow fiber membrane, and the CO2 in the gas is absorbed by the liquid absorbent after

passing through the hollow fiber membrane. The hollow fiber membrane acts as a non-selective

interface barrier between the liquid phase and the gas phase, which separates the gas phase from the

liquid phase and provides a large gas-liquid contact area. The studies have shown that in the process

of membrane contact absorption, the absorption liquid will enter the membrane pores [3-5], causing

the membrane pores to be wetted. Because the diffusion rate of CO2 in the gas is greater than that

of the liquid phase, it will cause the resistance of diffusion of CO2 in the membrane to be greatly

increased and which resulting in a rapid decrease in the mass transfer flux of CO2. Therefore, the

hollow fiber membrane must be hydrophobic.

In the long-term operation process of membrane contact absorption, it is inevitable that the

membrane will be wetted. On the one hand, it is because the pressure of the liquid phase must be

higher than the pressure of the gas phase during the membrane contact absorption process to avoid

the generation of bubbles. Therefore, the absorbent enters the membrane pores driven by the

transmembrane pressure difference, which will cause the pores to wet; On the other hand, when a

chemical absorbent is used, the surface properties (pore size, porosity, roughness and chemical

composition etc.) of the membrane will change due to the polymer membrane material being

susceptible to erosion by alkaline liquid[6-8], the pore size of the membrane will increase and the

surface contact angle will decrease, so the hydrophobicity of the membrane will decrease. In

addition, during long-term operation, the vapor of the absorption liquid will enter the membrane

pores, and the condensation of water vapor in the membrane pores can also cause the membrane

pores to gradually wet. Therefore, the long-term stability of hollow fibers should be verified to

determine whether they are indeed suitable for use in industry. Therefore, the development of long-

term stable operation of superhydrophobic hollow fiber membranes has become a hot spot in the

research of membrane contact absorption in recent years [9,10].

In our previous studies[15], the hybrid polyvinylidene fluoride-hexadecyltrimethoxysilane

(PVDF-HDTMS) membranes with the super hydrophobic outer surface were prepared through the

simple non-solvent induced phase-inversion method, that because the outer surface of modified

membranes possess low surface free erengy and uneven surface. The PVDF-HDTMS membranes

have the excellent long-term stability which the CO2 mass transfer flux was decreased by only 17%

after 17 days of the long-term contact absorption experiment( 1 mol/L DEA as the absorbent ),but

its long-term stability can still be improved compared with the existing research.

Studies have shown that using special methods to deposit hydrophobic inorganic nanoparticles

on hollow fiber membranes can obtain superhydrophobic hollow fiber membranes with strong long-

term stability [11,12]. Zhang et al. [13] developed a highly hydrophobic organic-inorganic

composite hollow fiber membrane by incorporating a fluorinated silica inorganic (fSiO2)layer on a

polyetherimide (PEI) organic membrane substrate via sol-gel process. The membrane contactor

showed a reasonable stability throughout the 31 days operation with 20% drop of the initial CO2

flux with using a 2M aqueous sodium taurinate solution as the absorbent. This is mainly due to the

fact that the incorporation of the fSiO2 inorganic layer offered high hydrophobicity and protected

the polymeric substrate from the attacks of chemical absorbents, making the membrane a longer

lifespan. Yilin Xu et al. [14] fabricated an inorganic-organic fluorinated titania-silica (fTiO2-

SiO2)/polyvinylidene fluoride (PVDF) composite membrane by forming a superhydrophobic SiO2-

TiO2 inorganic layer on the PVDF membrane substrate via facile in-situ vapor-induced

hydrolyzation method followed by hydrophobic modification. The CO2 absorption flux of fTiO2-

SiO2/PVDF composite hollow fiber membrane decreased by merely 10% over the 31days long-term

test due to its high chemical resistance and hydrophobicity that effectively prevented the PVDF

substrate from corrosion by chemical absorbents of MEA. Therefore, the introduction of

hydrophobic inorganic nanoparticles into the hollow fiber membrane can not only increase the

hydrophobicity of the membrane, but also improve the high chemical resistance of the membrane,

and ultimately the long-term performance of the membrane can be improved.

In this study, we aim to design and fabricate the polyvinylidene fluoride-silica-

hexadecyltrimethoxysilane (PVDF-SiO2-HDTMS) composite membrane via the simple non-solvent

induced phase-inversion method with ammonia water as the non-solvent additive and

dehydrofluorination reagent, and HDTMS as the hydrophobic modifier, and the nano-SiO2 particles

as the inorganic filler. The physical and chemical properties of the prepared PVDF-SiO2-HDTMS

film were characterized, and the long-term stability of CO2 was detected by using DEA as an

absorbent.

2. Experimental

2.1. Materials

PVDF L-6020 in the form of pellet particle was purchased from Solvay Advance Polymers,

USA, and used for the fabrication of the hollow fiber membranes. The reagents of N-methyl-2-

pyrrolidone (NMP, ≥99.0% purity), ammonia water (25–28% purity, pH =12–13), diethanolamine

(DEA, 99.0% purity), and ethanol (≥99.7% purity) were supplied by Chengdu Kelong Inc., China.

HDTMS was supplied by Aladdin Inc., China. Nano-SiO2 particles（The average particle size of is

50 nm，Hydrophilic）were supplied by Shanghai Macklin Biochemical Co., Ltd., China.

2.2 Fabrication of hollow fiber membrane and membrane contactor module[15][19]

In this study, the PVDF-SiO2-HDTMS hollow fiber membranes were fabricated via dry-jet

wet-spinning phase inversion method. The PVDF and SiO2 were dried in a vacuum oven at 70±2°C

for 24h, the dehydrated PVDF polymer particles were added into NMP, ammonia water and SiO2

mixture gradually and stirred by a magnetic stirrer at 60°C until the particles were completely

dissolved, in this process, the solution gradually turned to brown. After the solution was allowed to

stand for 12 h, the HDTMS was added into this solution, and then it was stirred at room temperature

for another 12 h to form a homogenous dope. The dope was degassed under vacuum overnight

before spinning. The compositions of each dope in this study are shown in Table 1. The parameters

of the spinning process are shown in Table 2. The spinning process is shown in Figure 1. The

fabricated hollow fiber membranes were immersed in pure ethanol for 15 min at first after spinning

process, and subsequently stored in water for 3 days to remove NMP and additives, and then

immersed in methanol for 1 day to protect the formed pore. At last, the membranes were kept at

room temperature to evaporate the residual methanol.

Table 1

Polymer dope composition.

Membrane NMP PVDF Ammonia HDTMS SiO2 External Bore liquid

(g) (g) (g) (g) (g) coagulant

PVDF-SiO2-HDTMS 80 20 1 1.5 1.5 Ethanol Distilled Water

PVDF -HDTMS 80 20 1 1.5[15] 0 Ethanol Distilled Water

Table 2

Hollow fiber spinning parameters.

Spinning parameter Value

Dope extrusion rate (ml/min) 4.5

Bore liquid (wt. %) Distilled water

Bore liquid flow rate (ml/min) 1.7

Air gap distance (cm) 0

Spinneret (od/id) (mm) 1.6/0.8

Spinneret wall thickness (mm) 0.3

Spinning dope temperature (℃) 25

Coagulant temperature (℃) 25

Bore liquid temperature (℃) 25

Fig. 1 The preparation device of hollow fiber membrane.

2.3. Characterization

The surface functional groups of the membranes were determined by the attenuated total

reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) on a NEXUS 870, NICOLET,

USA[15]. The ATR-FTIR spectra were collected over a scanning range of 500-4000 cm‒1 with 16

scans over 4.0 cm−1.

The morphology images of the hollow fibers were observed by the scanning electron

microscopy (SEM) instrument Tabletop Microscope FEI Quanta 250 FEG, USA[19]. The cross

section was obtained by fracturing each membrane in liquid nitrogen. The samples were sputtered

with gold for 15 s at 40 mA current before the test [15].

A sessile drop technique using a goniometer (Shanghai Zhongchen Digital Technology

Instrument Corporation Limited, JC2000D1, China) was used to measure the contact angles of the

outer surface of the membranes[19]. For each measurement, 3 μL of pure water or 1M DEA was

pumped out from a syringe. The liquid drop remained on the membrane outer surface for 3 min

before recording, and the result was adopted as the average value of ten measurements for each

sample[15].

The nitrogen (N2) gas permeation test was conducted to obtain the mean pore size and effective

surface porosity. The wettability resistance of the prepared membrane was assessed by

measurements of the critical water entry pressure (CEPw). The measurement method of the nitrogen

(N2) gas permeation and CEPw were referred to literature [15-17].

2.4 CO2 contact absorption experiment

CO2 absorption experiments process and the experimental setup could refer to our previous

study [15]. The corresponding parameters for the experiments are shown in Table 3. Before each

test, the system was operated for at least 30 min to obtain a steady state. The CO2/N2 mixture (19

vol.% CO2) was served as the feed gas flowing through the lumen side of the membranes, and the

aqueous DEA solution of 1 mol/L was employed as the liquid absorbent flowing through the shell

side of the membranes counter-currently. The flowrate of inlet gas was controlled at 20 mL/min by

a gas rotameter, and the flowrate of absorption liquid was controlled by a constant flow pump at 50

mL/min. In order to prevent gas bubbles into the liquid phase, the pressure in the liquid side was

controlled 20 kPa higher than that in the gas side. The concentrations of CO2 in the inlet and outlet

gas were measured by the CO2 detector (MIC-800, Shenzhen Yiyuntian electronics technology Co.

Ltd., China). The equation to calculated CO2 absorption flux of the PVDF membranes referred to

our previous article [15].

Table 3

Characteristics of membrane contactors.

Parameter Value

Module length (mm) 200

Module inner diameter (mm) 8

Effective fiber length (mm) 150

Number of fibers 4

Fiber inner diameter (mm) 0.6-0.7

Fiber outer diameter (mm) 1.1-1.2

Gas-liquid flow

Counter-current flow，gas through the

lumen，liquid through the shell

3. Results and discussion

3.1. Formation mechanism of the PVDF-SiO2-HDTMS membranes

The PVDF-SiO2-HDTMS membranes were fabricated in four steps. The First step The first

step was to introduce oxygen-containing functional groups into PVDF molecules through

defluorination and oxygenation reactions, which had been explained thoroughly in our previous

works[18](Fig. 2). In the second step, the hydroxyl groups on the PVDF chain reacted with most of

the hydroxyl groups on the surface of SiO2, they were connected by -O- bonds. After the reaction

was over, the dope of PVDF-SiO2 is formed. In the three step, the HDTMS was added into the

polymer solution, it was hydrolyzed by the small amount of water in the solution to form silanol,

and then polycondensation occurred between the silanol molecules to form polysiloxane. The

polysiloxane then reacted with the hydroxyl groups on PVDF-SiO2.The mechanism of HDTMS

grafting onto PVDF-SiO2 is shown in Fig. 3, R represents hexadecyl group. Finally, PVDF-SiO2-

HDTMS hollow fiber membranes were fabricated through the non-solvent induced phase-inversion

process.

Fig. 2 The reaction pathways of dehydrofluorination on PVDF chains via alkaline treatment [18].

Si

OCH3

R

OCH3

OCH3 Si

OH

R

OH

OH 3CH3OH

R CH3(CH2)15

Hydrolysis

Si

OH

R

OH

OHnnH2O

SiOH

R

OH

OH

n

Condensation

OH OHSiO2 SiOH

R

OH

OH

nPVDF chains

O

SiO2

O

SiO2

O

SiHO

R

H

x

O

O

SiHO

R

H

y

O

Grafting

Fig. 3 Mechanism of forming the PVDF-SiO2-HDTMS chains.

3.2. Membrane surface chemical structure

4000 3500 3000 2500 2000 1500 1000 500

20

40

60

80

100

120

140

160

180

200

28492917

% R

efle

ctan

ce

Wavenumbers(cm-1)

PVDF-SiO2-HTDMS-1.5

PVDF-HTDMS-1.5

PVDF-PA-8

Fig.4 ATR-FTIR spectra of the membranes.

ATR-FTIR spectra of the fabricated membranes and PVDF-PA-8 membrane (without any

grafting treatment in our previous works[19]) outer layers are presented in Fig. 4. Compared with

the PVDF-PA-8 membrane, two new bands were observed at 2917 and 2849 cm-1 in the ATR-FTIR

spectrum of the PVDF-SiO2-HDTMS and PVDF-HDTMS membrane outer layers, which could be

ascribed to the stretching of methylene groups (CH2) in the HDTMS chain. It shows that HDTMS

has been successfully grafted on the outer surface of PVDF-SiO2-HDTMS film and PVDF-HDTMS

membranes. The absence of Si-O-Si functional group spectra on the outer surface of the PVDF-

SiO2-HDTMS membrane may be caused by the coincidence of its characteristic band with the

characteristic band of PVDF.

3.3 Morphology of a hollow fiber membrane

SEM images of the prepared hollow fiber membranes and PVDF-PA-8 membrane are shown

in Fig. 4. For PVDF, a semi-crystalline polymer, in the process of preparing hollow fiber membranes

by the wet method, the final form of the membrane is mainly formed by liquid-liquid stratification

or solid-liquid stratification. The bath conditions play a decisive role in the membrane

structure[20,21]. In the preparation process of PVDF-SiO2-HDTMS membrane and PVDF-HDTMS

membrane, pure ethanol was used as the external coagulation bath, and water was used as the core

fluid. The PVDF-PA-8 membrane was used water and 80% NMP aqueous solution as the external

coagulation bath and the core fluid respectively. It could be observed in Fig. 2 that the PVDF-SiO2-

HDTMS membrane and PVDF-HDTMS membrane have no outer skin layer but an inner skin layer,

and the outer surface of the PVDF-SiO2-HDTMS membrane and PVDF-HDTMS is mainly

composed of spherulitic structures, while the inner and outer skin structures of PVDF-PA-8 are just

the opposite. This is mainly due to the use of “soft” pure ethanol and 80% NMP aqueous solution

to act as the coagulation bath. In the process of spinning, the exchange rate between solvent and

non-solvent is lower during the phase transfer, which is lower than after the polymer crystallization

process, so solid-liquid stratification accounts for Lead to form a spherulite structure.

Fig. 5 SEM images of the hollow fiber membranes. (a) PVDF-SiO2-HDTMS; (b) PVDF-HDTMS;

(c) PVDF-PA-8

Further observation found that the out surface of PVDF-HDTMS membrane is composed of

honeycomb spherulitic structure(Fig.5b), however, the spherulite structure of the PVDF-SiO2-

HDTMS membrane out surface is covered with lumpy substance materials(Fig.5a), which may be

caused by the accumulation of a large amount of SiO2.

PVDF is hydrophobic, nano SiO2 is hydrophilic, the blending of the two will cause poor

dispersion of SiO2 in the casting solution and the agglomeration of SiO2 will occur, that is because

the interfacial tension between the hydrophobic and hydrophilic inorganic materials [22]. It can be

found from fig.5, the SiO2 distribution on the cross section of the PVDF-SiO2-HDTMS membrane

is uniform, and a large amount of agglomeration is not found. This is due to the introduction of

hydroxyl groups into the PVDF chains under the action of ammonia water [22], which resultes in

the enhanced affinity between PVDF chains and the hydrophilic nanoSiO2 particles.

3.4. Gas permeability and hydrophobicity of the hollow fiber membranes

The gas permeability of the fabricated membranes was inspected by N2 permeation test. The

N2 permeation test of the fabricated membranes and the PVDF-PA-8 membranes are shown in Fig.6.

As shown in Fig. 6, PVDF-PA-8 membrane shows the lowest N2 permeability, which is mainly due

to the large number of sponge-like structures in the cross-section, resulting in greater tortuosity in

the cross-section of the membrane than the other two membranes [23]. The nitrogen permeability

of the membrane decreased because of the addition of inorganic nanoparticles which could blocked

the membrane pores, so the N2 permeability of the PVDF-SiO2-HDTMS membrane was slightly

lower than that of the PVDF-HDTMS membrane. The calculation methods of effective surface

porosity and average pore size have been introduced[24][25]. The values of effective surface

porosity and average pore size of the membranes are shown in Table 4. It could be found that the

surface pore size and surface porosity of PVDF-SiO2-HDTMS and PVDF-HDTMS membranes are

higher than those of PVDF-PA-8, which is because the open membrane structure without outer skin,

which can provide larger pore size and surface porosity. The parameters of CEPW and outer surface

contact angle, which are correlated to the long-term stability of the membranes, are also shown in

Table 4. Compared with PVDF-HDTMS membrane, PVDF-SiO2-HDTMS membrane has

significantly higher CEPW and higher outer surface contact angle. Therefore, PVDF-SiO2-HDTMS

membrane is more conducive to the application of CO2 absorption in membrane contactors.

During the preparation process, the strength of PVDF chain would be decreased due to

defluorination. As a result, the fabricated membranes showed decrease in the mechanical strength.

Therefore, as shown in Table 4, the tensile strength and elongation at break of PVDF-SiO2-HDTMS

and PVDF-HDTMS membranes are lower than those of PVDF-PA-8 membrane. However, the

tensile strength and elongation at break of PVDF-SiO2-HDTMS membrane are higher than that of

PVDF-HDTMS membrane. This is mainly due to the formation of Si-O-Si bonds between SiO2 and

PVDF and between SiO2 and HDTMS, which enhances the molecular bonding force between the

three, thereby increasing the tensile strength and elongation at break of the PVDF-SiO2-HDTMS

membrane.

100 150 200 250 300 350

1.0x10-6

2.0x10-6

3.0x10-6

N2 P

emea

nce

(mo

l/m

2 s

Pa)

Mean Pressure(Pa)

PVDF-PA-8

PVDF-SiO2-HDTMS-1.5

PVDF-HDTMS-1.5

Fig.6 N2 permeance of the hollow fiber membranes

Table 4 Characteristics of the hollow fiber membranes.

Membrane Mean pore

size (nm)

Effective

surface porosity

(m−1)

Elongation

at

break(%)

Tensile

strength

(MPa)

CEPW

(×105)

(Pa)

Contact

angle (◦)

(water)

Contact

angle (◦)

(DEA)

PVDF-SiO2-

HDTMS-1.5

20.21 338.8 25±0.4 3.2±0.3 9±0.5 160±0.2 160±0.2

PVDF-HDTMS-1.5 38.93 404.8 15±0.3 1.7±0.3 7.5±0.5 150.0±0.3 146.5±0.5

PVDF-PA-8 17.40 113.1 46.4±0.9 4.1±0.3 9.0±0.5 78.6±0.4 71.5±0.5

3.5 Long-terms performance of the membranes

A long-term CO2 membrane contact absorption experiment was conducted using 1 mol/L DEA

as the absorbent to investigate its long-term stability under moderate alkaline condition. The

experimental results of PVDF-SiO2-HDTMS, PVDF-HDTMS and PVDF-PA-8 membranes were

shown in Figure 7. The PVDF-PA-8 membrane was completely wetted after 10 days, and the mass

transfer flux was reduced by 31.5%. The CO2 mass transfer flux of PVDF-HDTMS decreased by

17% after 20 days. The PVDF-SiO2-HDTMS membrane showed excellent long-term stability even

in alkaline solution, and the CO2 mass transfer flux was decreased from the initial value of 2.39×10-

3 mol/m2s to 2.31×10-3 mol/m2s and only reduced by 3%.

The anti-wetting ability of PVDF-PA-8 membrane is mainly determined by the large tortuosity

of the membrane cross-section and the small surface pore size. However, its wetting resistance

ability to alkaline solution is poor due to the low hydrophobicity, which leading to its shows poor

long-term stable operation.

PVDF-SiO2-HDTMS membrane and PVDF-HDTMS membrane have similar parameters such

as average pore size, surface effective porosity, surface contact angle, etc., so their anti-wetting

performance and air permeability are similar. In the short-term experiment process, the difference

in the mass transfer flux of the membrane to CO2 was less. However, during long-term operation,

the decline of PVDF-HDTMS membrane is greater than that of PVDF-SiO2-HDTMS membrane,

which may be related to the tensile strength of the membrane, although the tensile strength of PVDF-

HDTMS membrane can be applied to the membrane contact absorption, it is lower than the PVDF-

SiO2-HDTMS membrane, thus leads to alkaline solution erosion during long-term stable operation,

leading to an increase in its decline rate.

0 5 10 15 20

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

PVDF-SiO2-HDTMS-1.5

PVDF-HDTMS-1.5

PVDF-PA-0.8

CO

2 F

lux（

×1

0-3）

DAY

Fig.7 Long-term CO2 absorption performance of the membranes

3.6 The Chemical stability of the composite membrane

In the process of membrane contact absorption, due to the long-term mutual contact between

the membrane and the chemical absorbent, a chemical reaction occurs between the surface of the

membrane and the chemical absorbent, causing the deterioration of the long-term stability of the

membrane and the decreases of CO2 mass transfer flux of membrane. In order to verify the corrosion

resistance of PVDF-SiO2-HDTMS hollow fiber membranes to chemical absorbents, this study used

nitrogen permeation and contact angle experiments to characterize the performance of hollow fiber

membranes after 20 days long-term stable operation.

The nitrogen test results of the three types of membranes after 20 days long-term operation

were shown in Figure 8. Comparing with Figure 6, it was found that the N2 permeability of the

membranes has not changed significantly. The surface pore size and porosity was obtained by

calculation (Table 5), showing that the surface pore size and porosity of the three types of

membranes had changed large, which indicates that MEA did not have much effect on the surface

morphology of the membranes. It could be found the contact angles of the three types of membranes

had decreased after 20 days long-term operation, and the decrease range was PVDF-SiO2-

HDTMS<PVDF-HDTMS<PVDF-PA-8. According to M. Sadoogh et al. [26] studies, the

degradation of PVDF membrane performance may be due to the chemical degradation of the

interface between PVDF and MEA, which leaded to the dehydrogenation and fluorination of the

PVDF surface. Therefore, the reason for the lower drop of the contact angle of PVDF-SiO2-HDTMS

was the surface of PVDF-SiO2-HDTMS membrane grafted with a large amount of

superhydrophobic SiO2 with high hydrophobicity and chemical stability, which effectively

prevented the PVDF substrate from being in the MEA solution. The corrosion in the medium

ensured the long-term stable operation of the membrane. Yilin Xu et al. [14] prepared a

superhydrophobic f-TiO2-SiO2 layer on the outer surface and Yuan Zhang et al. [13] prepared a

superhydrophobic f-SiO2 layer on the outer surface. The same results were also obtained. Therefore,

it can be shown that grafting superhydrophobic metal nanoparticles on the outer surface of the

membrane can effectively prevent the chemical corrosion of PVDF from chemical absorbents and

increase the chemical corrosion resistance of the membrane.

100 150 200 250 300 350

1.0x10-6

2.0x10-6

3.0x10-6

N2 P

emea

nce

(mol/

m2 s

Pa)

Mean Pressure(Pa)

PVDF-PA-8

PVDF-SiO2-HDTMS-1.5

PVDF-HDTMS-1.5

Fig. 8 N2 permeance of the hollow fiber membranes

Table5 Characteristics of the hollow fiber membranes.

Membrane Mean pore size

(nm)

Effective surface

porosity (m−1)

Contact angle (◦)

(water)

Contact angle (◦)

(DEA)

PVDF-SiO2-HDTMS 21.02 324.6 154±0.2 148±0.2

PVDF-HDTMS 40.63 385.8 142.0±0.3 126.2±0.3

PVDF-PA-8 20.66 96.5 58.5±0.5 43.5±0.5

In Table 6, the PVDF-SiO2-HDTMS membrane was compared with other hydrophobic

membranes reported by other research groups. Its carbon dioxide absorption flux and long-term

stability was listed in Table 6. It could be found that the PVDF-SiO2-HDTMS membrane in this

study exhibits competitive performance in terms of CO2 absorption flux as well as long term stability

in cases of moderate alkaline absorbent and absorbent flowrate, and low CO2 concentration.

Table 6

Comparison of CO2 absorption performance in gas-liquid membrane contactor.

Ref. Membrane

types

CO2 Flux

(mol/m2s )

Inlet gas

type

Absorbent

type

Gas flow

rate

Absorbent

flow rate

Contact

angle (◦)

(water)

27

D-TZ-PAN-

20

1.9×10−3 Prue CO2

distilled

water

1500ml/m

in

240ml/min 113±2

28

PVDF+6%S

MM

5.3 ×10−3 Pure CO2

distilled

water

100ml/mi

n

300ml/min 99 ± 1.50

14

PVDF+f-

TiO2-SiO2

8.0×10−3 Pure CO2 1M MEA

Not

reported

0.25m/s

124(dyna

mic)

This

work

PVDF-SiO2-

HDTMS

2.23×10−3 19% CO2 1M MEA 20ml/min 50ml/min 160±0.2

4. Conclusions

In this study, hydrophilic nano SiO2 was used as an inorganic filler to be added to the casting

solution, and ammonia was used as a defluorinating agent to hydroxylate PVDF molecules under

the action of ammonia to generate active sites; add hydrophilic nano SiO2, The hydroxyl group and

the hydroxyl group formed on the PVDF molecule undergo a dehydration reaction to form a cross-

linked PVDF-SiO2 polymer chain; then HDTMS is added to make it hydrophobically modified; the

superhydrophobic PVDF-SiO2 with high strength is prepared by spinning by the NIPS method

HDTMS organic-inorganic composite membrane. The prepared membrane has a maximum CO2

mass transfer flux of 2.39×10-3mol/m2s. After 20 days of membrane contact absorption with 1 mol/L

DEA as the absorbent, the CO2 mass transfer flux of the membrane contactor decreased by only 3%.

Since the surface of PVDF-SiO2-HDTMS film is grafted with a large amount of superhydrophobic

SiO2 with high hydrophobicity and chemical stability, it effectively prevents the corrosion of PVDF

substrate in MEA solution, so PVDF-SiO2-HDTMS is prepared. The film has superhydrophobicity,

the film has the strongest resistance to wetting and chemical stability.

Acknowledgments

The authors gratefully acknowledge the financial supports from the Qing Lan Project of Jiangsu

Province (Nos. 202050220RS005).

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