Preparation of polypropylene based hyperbranched absorbent fibers and the study of their adsorption of CO2

Article abstract of DOI:10.1039/c5ra02182k

Ring-opening copolymerization of aziridine in situ into substrates to introduce polyethyleneimine (PEI) not only required strict experimental conditions but employed the highly toxic monomer aziridine. In this paper, an effective and safe new procedure is

Full text of DOI:10.1039/c5ra02182k

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Preparation of polypropylene based
hyperbranched absorbent fibers and the study of
Cite this: RSC Adv., 2015, 5, 32902
their adsorption of CO2
a
a
ab
and Mengwei Denga
*
Teng Xu, Qinghua Wu, Shuixia Chen
Ring-opening copolymerization of aziridine in situ into substrates to introduce polyethyleneimine (PEI) not
only required strict experimental conditions but employed the highly toxic monomer aziridine. In this paper,
an effective and safe new procedure is developed for hyperbranched structure synthesis by stepwise
growth using N-(2-chloroethyl)-benzaldimine as a monomer. Hyperbranched solid amine fibers for CO2
capture were prepared through co-irradiation grafting copolymerization of polypropylene fibers with
glycidyl methacrylate, followed by amination with ethylenediamine, Hoffman alkylation with N-(2-
chloroethyl)-benzaldimine and then hydrolysis to remove benzaldehyde groups. It was shown that the
adsorption performance of the hyperbranched solid amine fibers G2.0 and G3.0 has been greatly
improved compared with first generation G1.0 fibers, and the adsorption capacities of G2.0 and G3.0
were 5.35 mmol gꢀ1 and 5.53 mmol gꢀ1 at 30 ꢁC, respectively. The amine utilization of G2.0 fibers could
reach 84.1%. These results demonstrate that the branched structure can promote the adsorption
capacity and efficiency greatly due to its low mass transfer resistance of CO2, which is more favorable
than a linear amination reagent.
Received 4th February 2015
Accepted 25th March 2015
DOI: 10.1039/c5ra02182k
www.rsc.org/advances
Much work advocating the use of brous adsorbents for CO2
adsorption has been published by our group,13–16 demonstrating
1. Introduction
that bers have great potential as substrates for CO2 adsorption.
Fibrous adsorbents possess the advantages of large external
surface area, short transit distance, low pressure drops, and
exibility. Yang15 graed allylamine onto polyacrylonitrile bers
and obtained an adsorption ability of 6.22 mmol gꢀ1 PAN-AF
when the graing degree was 60.0 wt%. Zhuang16 graed gly-
cidyl methacrylate onto polypropylene bers, followed by reac-
tion with triethylenetetramine to make a novel kind of solid
amine-containing brous adsorbent and obtained an adsorp-
tion ability of 4.72 mmol gꢀ1. What is more, the adsorbent could
maintain almost constant adsorption behavior within six
recycles.
In 1990, Kim put forward the concept of “hyperbranch”, and
this then became one of the hottest research topics in Polymer
Chemistry.17 Unlike dendritic molecules, hyperbranched mole-
cules do not require uniform structure and high symmetry, thus
they are easy to prepare. The most important property is that
hyperbranched molecules can have many functional groups
that could both improve the hydrophilicity of the material and
increase the number of adsorption sites for use as an adsor-
bent.18,19 Among them, polyethyleneimine (PEI) because of its
high amine content and hyperbranched structure was widely
reported to modify silicon dioxide20,21 and bers13,14 that are
used for gas separation. The in situ ring-opening polymerization
of aziridine into porous silica to introduce PEI for the synthesis
hyperbranched polymers has been conventionally carried out
Global warming is mainly attributed to an increase in atmo-
spheric levels of CO2 which is attributed to the unrestrained
burning of fossil fuels.1 The most effective method to reduce
CO2 emission from the burning of fossil fuels is scrubbing the
ue gas stream with an amine solution. But this brings some
disadvantages such as corrosion, high regeneration energy and
solvent degradation.2,3 To solve these problems, solid amine
adsorbents such as amine-treated MCM-41,4,5 SBA-15,6,7 Zeolite
13X8,9 and activated carbon10,11 have been used, attracting much
attention. The solid amine adsorbents can retain high adsorp-
tion capacities even in the presence of water, compared with
microporous or mesoporous adsorbents without amine modi-
cation. What is more, they can also be regenerated in mild
conditions. The adsorption capacity for CO2 mainly depends on
the amine loading and the pore volume of the porous
substrates. However, increasing amine loading will block the
pore volume of the porous substrate, thus reducing the
adsorption capacity of CO2.6,12 So it is of great signicance to
develop alternative substrates or to improve the amine agents to
increase amine loading without affecting adsorption
performance.
aPCFM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275, PR China. E-mail: cescsx@mail.sysu.edu.cn; Fax: +86-20-
84034027
bMaterials Science Institute, Sun Yat-Sen University, Guangzhou 510275, PR China
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and used for CO2 capture. However, on the one hand, the where W0 and W1 are the weights (g) of the original and graed
approach has some additional disadvantages such as the use of PP ber, respectively.
the highly toxic monomer aziridine and strict experimental
2.2.3 Synthesis of G1.0 ber: PP-GMA-EDA. PP-GMA,
conditions.22,23 On the other hand, Li24 has studied the effect of AlCl3$6H2O and EDA were mixed and added into a 100 mL
the molecular weight of the PEI on the CO2 capture perfor- three neck ask. The reaction was carried out at 105 ꢁC for 6 h.
mance of PEI-nano silica adsorbents and found that a PEI with a Aer reaction, the product bers (G1.0, PP-GMA-EDA) were
high molecular weight leads to unsatisfactory amine utilization washed with deionized water thoroughly to remove the residual
because of its high steric hindrance due to the increased AlCl3 and EDA, then dried at 60 ꢁC for 12 h. The primary or
viscosity. Thus the development of a PEI with low molecular secondary amine contents (n, mmol gꢀ1) of G1.0 bers were
weight and with an accessible structure will be of benet to calculated by the following equation:
improve its adsorption capacity and the amine efficiency.
nðprimary amineÞ ¼ nðsecondary amineÞ
In this study, an effective and safe new procedure is devel-
(2)
1
2
1
2
E
oped for hyperbranched structure synthesis by stepwise growth
using N-(2-chloroethyl)-benzaldimine as monomer. The
¼
ꢂ nðamineÞ ¼
ꢂ
ꢂ 1000
14
a
where E is the mass fraction of nitrogen measured by elemental
analysis.
hyperbranched solid amine bers for CO2 capture were
prepared through co-irradiation graing copolymerization,
followed by amination, Hoffman alkylation and hydrolysis. The
effects of the branched structure on the adsorption perfor-
mance of the ber have been investigated.
2.2.4 Synthesis of G1.5 bers by Hoffman alkylation: PP-
GMA-EDA-CEBI. 1.5 g PP-GMA-EDA (G1.0) ber, 60 mL deion-
ized water and 1.32 g KI were added into a 100 mL three-neck
ask, then 9 g CEBI in 30 mL anhydrous methanol was slowly
dropped into the reaction system. Aer reux of the solution at
60 ꢁC for 12 h the obtained ber (G1.5, PP-GMA-EDA-CEBI) was
washed with excess deionized water and anhydrous ethanol,
2. Experimental
2.1 Materials and reagents
ꢁ
and then dried at 60 C for 12 h.
Glycidyl methacrylate (GMA, Sigma-Aldrich CO) was used aer
the removal of the inhibitor with neutral aluminum oxide.
Polypropylene (PP) bers were provided by Xinshun Special
Fiber Company (Zhongshan, China). Methanol, ethanol, tri-
ethylamine (TEA), hydrochloric acid (HCl), aluminum chloride
hexahydrate (AlCl3$6H2O), magnesium sulfate anhydrous
(MgSO4), benzaldehyde, ammonium ferrous sulfate ((NH4)2-
Fe(SO4)2$6H2O), sodium hydroxide (NaOH), potassium iodide
(KI) and ethylenediamine (EDA) were all purchased from
Guangzhou Chemical Reagents Factory. Diethylenetriamine
(DETA) was purchased from Tianjin Fuchen Chemical Reagents
Factory. 2-Chloroethylamine hydrochloride was purchased from
Aladdin. Ether was provided by Kaixin Chemical Industry Co.
(Hengyang City). All reagents above were analytical grade (AR).
2.2.5 Synthesis of G2.0 bers: PP-GMA-EDA-CEA. The bulky
phenyl groups of CEBI were removed from the G1.5 ber by
mixing the PP-GMA-EDA-CEBI G1.5 Fiber with 4 M HCl (90 mL)
and reacted at 30 ꢁC for 8 h. Then the ber was washed with 1 M
NaOH and with deionized water until the solution turned
neutral. And the resulting product G2.0 ber (PP-GMA-EDA-
CEA) was dried at 60 ꢁC under vacuum. The contents of
primary, secondary, tertiary amines or imine (n, mmol gꢀ1) of
G2.0 bers were calculated by the following equation:
Ai
E
P
nði amineÞ ¼
ꢂ
ꢂ 1000
Ai 14
(3)
i ¼ primary; secondary; tertiary and imine
where E is the mass fraction of nitrogen measured by elemental
analysis. Ai represents the different types of nitrogen peak area
tested by X-ray photoelectron spectrometer.
2.2 Preparation of hyperbranched solid amine bers
2.2.1 Synthesis of N-(2-chloroethyl)-benzaldimine (CEBI).25
To a 250 mL three-neck round bottom ask was added 5.3 g (50
mmol) benzaldehyde, 5.8 g (50 mmol) 2-chloroethylamine
hydrochloride, 10 g MgSO4 and 50 mL anhydrous ether. Then a
mixed solution (5.6 g (55 mmol) TEA and 50 mL anhydrous
ether) was added to the reaction system dropwise over 30 min.
The reaction was carried out at room temperature, and stirred
for 6 hours. Aer reaction, the solvent was removed in vacuo to
obtain N-(2-chloroethyl)-benzaldimine (CEBI).
2.2.2 Synthesis of PP-GMA. PP, GMA and DMF were added
into a 250 mL conical ask, and then subjected to g-ray irra-
diation. Aer irradiation, the graed ber (PP-GMA) was
washed with DMF, and then dried at 60 ꢁC for 12 h. The graing
degree of PP-GMA was calculated using eqn (1):
2.2.6 Synthesis of G2.5 and G3.0 bers. The experimental
conditions used to prepare G2.5 and G3.0 bers were the same
as for the description of G1.5 and G2.0 bers, and the calcula-
tion method for the different types of amine is as for eqn (3).
The preparation scheme of the hyperbranched solid amine
bers is presented in Scheme 1.
2.2.7 Synthesis of G1.0-DETA bers. In order to evaluate
the effect of the structure of amination reagent on the adsorp-
tion, G1.0-DETA bers (PP-GMA-DETA) were synthesized
through using DETA, instead of EDA, to react with G0.5 PP-GMA
bers. DETA is an amination reagent with a longer linear chain.
The experimental conditions used to prepare G1.0-DETA bers
are the same as for the description of G1.0 bers. The structure
of the G1.0-DETA ber is presented in Scheme 2.
In general description, all the modied bers with amino
groups as end groups, G1.0, G2.0 and G3.0, are described as full
W1 ꢀ W0
GGMA
¼
ꢂ 100%
(1)
W0
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Fig. 1 FT-IR spectra of G0, G0.5, G1.0, G1.5, G2.0, G2.5, G3.0 fibers.
(ESCALAB 250, Thermo-VG Scientic) were used to conrm the
structure of the polymers.
A
thermogravimetric analyzer (Netzsch TG-209) was
employed to determine the thermal stability of different full
generation samples. The test was carried out from 100 ꢁC to 600
ꢁC using a heating rate of 10 K minꢀ1
.
2.4 CO2 adsorption experiment
1.0 g of the ber sample was placed into a specially designed
glass tube (F ¼ 13 mm), followed by purging with dry N2
to remove air and excess water. Then a mixture of gases
(CO2 : N2 ¼ 1 : 9 (volume ratio)) with a ow rate of 30 mL minꢀ1
was introduced. The outlet CO2 concentration was determined
by gas chromatography (Techcomp 7900) with a thermal-
conductivity detector.
The adsorption amount was calculated as follows:
ð
Q ¼ t ðCim ꢀ C ÞVdt=22:4W
(4)
Scheme
preparation.
1 Reaction scheme for hyperbranched adsorbent fiber
eff
0
where Q represents the adsorption capacity (mmol CO2 gꢀ1), t is
the adsorption time (min). Cim and Ceff are the inuent and
effluent concentrations of CO2 (vol%), respectively. V is the total
ow rate, 30 mL minꢀ1; W and 22.4 are the weight of sample (g)
and molar volume of gas (mL mmolꢀ1), respectively.
Aer adsorption, the ber was regenerated by purging with
N2 with a ow rate of 30 mL minꢀ1 at 90 C.
ꢁ
3. Result and discussion
3.1 Chemical characterization
Fig. 1 shows the FT-IR spectrum of graed bers at each stage.
Compared with G0 bers, there is a sharp band at 1736 cmꢀ1
,
Scheme 2 Structure of G1.0-DETA fiber.
which is attributed to the C]O stretching vibration of graed
GMA. In addition, a broad and strong absorption band at 3412
cmꢀ1 is attributed to O–H bending vibration. The intensity of
this band at 3412 cmꢀ1 increased aer amination because it is
the overlap of O–H and N–H bending vibrations. The result
above proves that EDA was successfully introduced onto the
surface of the material. And it is found that there was only a
band at 1542 cmꢀ1 for N–H in-plane bending vibration in the
generation bers; and those bers with phenyl groups as the
end groups, G0.5, G1.5, and G2.5, are described as half gener-
ation bers.
2.3 Physical and chemical characterization
Infrared (IR) spectra (Tensor-27 spectrometer), Elemental range from 1550 cmꢀ1 to 1650 cmꢀ1 for the full generation ber
analysis (Elementar, Vario EL), solid state 13C NMR analysis (G1.0, G2.0 and G3.0), but there was another band at 1640 cmꢀ1
(AVANCE AV, Bruker) and X-ray photoelectron spectroscopy for C]N stretching vibration for the half generation (G0.5, G1.5
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Table 1 Element analysis of G0, G0.5, G1.0, G2.0 and G3.0 fibers
Element content (wt%)
Amine content
Fibers
WGa (g per g raw material)
C/%
H/%
N/%
(mmol gꢀ1
)
G0
—
84.30
65.59
50.88
53.57
52.74
13.66
9.02
9.46
9.42
9.34
0.00
0.00
8.46
10.86
12.09
—
—
6.04
7.76
8.64
G0.5
G1.0
G2.0
G3.0
3.12
0.25
0.30b
0.22c
Wp ꢀ Wr
a
WG ¼
, where WG represents the weight gain (g per g raw material), Wr and Wp are the weight of the raw material and product,
Wr
b
respectively. The unknown quantities of Clꢀ would affect the calculation if the raw material refers to G1.5. Thus for the accuracy and
convenience, the raw material in this step refers to G1.0. The raw material in this step refers to G2.0.
c
Table 2 The amine type and content of G1.0, G2.0 and G3.0 fibers
Amine content (mmol gꢀ1
)
Fibers
Total
Primary
Secondary
Tertiary
Imine
G1.0
G2.0
G3.0
6.04
7.76
8.64
3.02
2.58
2.84
3.02
1.89
2.29
—
1.89
2.02
—
1.39
1.48
Fig. 4 The thermogravimetric curve of different generation absorbent
fibers.
analysis, X-ray photoelectron spectroscopy and 13C NMR spectra
were carried out and are presented in Tables 1, 2 and Fig. 2, 3,
respectively. Compared with G0, both the content of carbon and
hydrogen of G0.5 decreased and the calculated graing degree
according to the elemental analysis was 292%, corresponding
well to the result measured from weight gain (312%). Aer
amination with EDA, the nitrogen content of G1.0 reached 6.04
mmol gꢀ1, similar to the data (6.67 mmol gꢀ1) calculated by
weight gain. Furthermore, it was obvious that the nitrogen
content of the full generation increased with increasing
Fig. 2 13C NMR spectra for G2.0 and G3.0.
and G2.5), demonstrating the successful preparation of a
hyperbranched absorbent.
In order to determine the type of nitrogen and whether the
structure was a linear chain or a branched one, elemental
Fig. 3 Typical XPS wide scan spectra for G2.0 and G3.0. The inset shows the characteristic peaks of N 1s at higher resolution.
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Fig. 7 The regeneration performance of G3.0 fiber.
Fig. 5 The cumulative adsorption curves of G1.0, G2.0, G3.0 fibers
(adsorption temperature: 30 ꢁC; N2: 27 mL minꢀ1; CO2: 3 mL minꢀ1).
Fig. 8 FT-IR spectra of fresh and regenerated G3.0 fibers.
Fig. 6 The cumulative adsorption curves of G2.0 and G1.0-DETA
fibers (adsorption temperature: 30 ꢁC; N2: 27 mL minꢀ1; CO2: 3 mL
minꢀ1).
attributed to the difficulty of collection of G2.0 and G3.0 aer
treatment by strong acid and strong base that led to the brit-
tleness of the bers. Moreover, with respect to G2.0 and G3.0,
incomplete hydrolysis which is conrmed by the 13C NMR
spectra where the chemical shis of the benzyl and imine
groups appear at 130 ppm and 140 ppm respectively, gave an
explanation for the lower amine content than expected. In
addition, as shown in Table 2, combining the elemental anal-
ysis data and X-ray photoelectron spectroscopy,26,27 the amount
of the different types of amines was calculated. The presence of
tertiary amine implicated that the branching can happen to
form a hyperbranched polymer.
Table 3 The adsorption capacity and amine utilization efficiency for
CO2 of G1.0, G2.0, G3.0 and G1.0-DETA (adsorption temperature: 30
ꢁC; N2: 27 mL minꢀ1; CO2: 3 mL minꢀ1
)
Alkyl amine
Adsorption capacity
Efficiency
(%)
Sample
(mmol gꢀ1
)
(mmol gꢀ1
)
G1.0
G2.0
G3.0
G1.0-DETA
6.04
6.36
7.15
6.64
4.08
5.35
5.53
3.59
67.5
84.1
77.3
54.1
The results of TG analysis of different generation absorbent
bers are presented in Fig. 4. In the case of PP, tꢁhere was only
generation, proving that the monomer (CEBI) was successfully
introduced onto the surface of the bers. However, the amine
number of G2.0 and G3.0 from weight gain was a little lower
than that from the elemental analysis, which could be
ꢁ
one continuous weight loss from 310 C to 460 C. In the TG
curve of PP-GMA, three distinct weight loss steps were observed
at 200 ꢁC, 260 ꢁC, and 350 ꢁC. The rst weight loss could be
Table 4 Comparison of CO2 adsorption capacity of amine functionalized fibrous sorbents
Temperature
Adsorption capacity
Substrate
Graed monomer
Amine
(ꢁC)
pCO (atm)
(mmol gꢀ1
)
Ref.
2
PP
PP
PP
PP
GMA
GMA
Am
Branched
PEI
PEI
TETA
Allylamine
PEI
30
25
25
30
22
30
0.10
0.10
0.10
0.15
0.15
0.24
5.53
2.51
5.91
4.72
6.22
4.12
This work
14
14
16
15
13
GMA
a
PAN
Glass ber
—
b
—
a
Allylamine was directly graed onto the polyacrylonitrile ber (PAN). b PEI was coated onto glass ber by using epichlorohydrin as cross-linking
agent.
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attributed to decomposition of the homopolymer, and the its adsorption capacity was far below than G3.0’s, further con-
second one between 260 ꢁC and 350 ꢁC was probably related to rming the superiority of our route.
the degradation of GMA, which also indicated that the mono-
In order to evaluate the regeneration performance of G3.0
mer GMA was graed onto the surface of PP. Comparing the bers, 8 cycles of adsorption–desorption were carried out, and
G1.0, G2.0 and G3.0 bers, the thermal stability was G3.0 > G1.0 the results are shown in Fig. 7. Aer 8 cycles, no signicant
> G2.0. As is known to all, increasing chain length would reduce decrease in CO2 adsorption capacity was observed: the adsorp-
the thermal stability. However, G3.0 with the longest chain tion capacity of adsorbent was maintained above 99%.
length showed better stability than the rst two generations, Furthermore, FT-IR spectra of fresh and regenerated G3.0 bers
overcoming this defect.
are presented in Fig. 8. It is obvious that the regenerated bers
exhibit nearly the same spectra as the fresh. All these results
conrmed that G3.0 bers have great regeneration performance
that is of great signicance for practical application.
3.2 Adsorption property
The cumulative adsorption curves are shown in Fig. 5, and the
adsorption capacity and amine utilization efficiency of G1.0,
G2.0, G3.0 bers are shown in Table 3. With the increase of ber 4. Conclusion
generation, the adsorption capacities increased due to the
increase of alkyl amine content, demonstrating the importance
of high amine content.
Polypropylene based hyperbranched absorbent bers for CO2
capture were prepared by co-irradiation graing copolymeriza-
tion of GMA onto PP ber, followed by amination, Hoffman
alkylation and hydrolysis. The present work has shown that the
adsorption performance of G2.0 and G3.0 is greatly been
improved compared with G1.0, and the adsorption capacities of
G2.0 and G3.0 were 5.35 mmol gꢀ1 and 5.53 mmol gꢀ1 at 30 ꢁC,
respectively. In particular, a branched structure can promote
the adsorption capacity and efficiency greatly due to its low
mass transfer resistance of CO2, which is more favorable than
linear amination reagent. Furthermore, hyperbranched absor-
bent bers could be easily and completely regenerated under
mild conditions and are stable in cyclic operations for 8 cycles.
However, the equilibrium adsorption time increased with
the thickening of the adsorption layer which would extend the
diffusion time. Moreover, the efficiency of G3.0 decreased to
77.3% while G2.0’s reached 84.1%, which could be ascribed
to the longer chain length which increases the steric resis-
tance and disfavors the adsorption. This reminds us of the
fact that an appropriate amination reagent which combines
alkyl amine content with lower steric resistance should be
chosen.
For the purpose of studying the effect of a branched structure
on the CO2 adsorption performance, DETA was employed and
immobilized onto PP-GMA ber. Since G2.0 (PP-GMA-EDA-EDA)
and G1.0-DETA (PP-GMA-DETA) bers had similar nitrogen
content, the inuence of amine content on the adsorption
capacity was excluded based on the mechanism in the presence
of water vapor.28,29 In addition, G2.0 (PP-GMA-EDA-EDA) had a
branched structure, and G1.0-DETA (PP-GMA-DETA) had a
linear one. The adsorption capacities and efficiency of G2.0 and
G1.0-DETA were compared. As shown in Fig. 6 and Table 3, in
the early stages, the adsorption rate of G1.0-DETA was almost
the same (up to 20 min) as G2.0’s, and the adsorption capacities
of the two materials were 2 mmol gꢀ1 at that stage. The results
may be attributed to similar numbers of primary amines on the
outer layer. Aer 20 min, the adsorption rate of G1.0-DETA
became slow on account of the higher mass transfer resis-
tance of CO2 into the inner layer for the molecular chain
entanglement. Meanwhile, the molecular chain entanglement
would also prevent the adsorption sites from reacting with CO2.
Both the adsorption capacities and the amine utilization of G2.0
ber were much higher than G1.0-DETA. The above results
demonstrate that the branched structure of G2.0 (PP-GMA-EDA-
EDA) is more favorable for adsorption in the presence of water
than the linear structure of G1.0-DETA.
Acknowledgements
The authors gratefully acknowledge the nancial support
provided by the National Natural Science Foundation of China
(Grant no. 51173211, 51473187).
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G3.0 showed relatively high adsorption efficiency, demon-
strating its potential for practical application. More interest-
ingly, Wu14 has prepared a brous adsorbent (PP-GMA-PEI)
through graing GMA onto PP ber, followed by reaction with
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