DSpace at VNU: Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural Using Ionic Liquids

Materials Transactions, Vol. 56, No. 9 (2015) pp. 1434 to 1440
Special Issue on Nanostructured Functional Materials and Their Applications
© 2015 The Japan Institute of Metals and Materials

Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts
for Direct Conversion of Cellulose
into 5-Hydroxymethylfurfural Using Ionic Liquids
Trung-Dzung Nguyen1, Huy-Du Nguyen2, Phuong-Tung Nguyen1 and Hoang-Duy Nguyen1,3,+
1

Institute of Applied Materials Science, Vietnam Academy of Science and Technology, Hochiminh City, Vietnam
Department of Chemistry, HCM University of Science, Vietnam National University, Hochiminh City, Vietnam
3
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
2

Mesoporous poly(vinylsulfonic-co-divinylbenzene) (VS-DVB) and magnetic polymer (VS-DVB/CoFe2O4) are prepared and used as solid
acidic catalysts to directly transform cellulose into 5-hydroxymethylfurfural (5-HMF). The characteristic and morphology of the polymers were
examined by Fourier transformed infrared spectroscopy, X-ray diffraction, vibrating sample magnetometer, field-emission scanning microscope,
and transmission electron microscopy. The yield of 5-HMF can reach as high as 98% from the dehydration of glucose using CrCl3·6H2O catalyst
in tetrabutylammonium chloride at 120°C for 90 min. Cellulose conversion using the prepared VS-DVB in 1-butyl-3-methyl imidazolium
chloride at 120°C for 180 min showed high yields of 50% glucose and 10% 5-HMF. An enhancement in 5-HMF yield was observed as reaction
time increased. A combination of VS-DVB/CoFe2O4 and CrCl3·6H2O in ionic liquids was employed at optimal conditions for cellulose
conversion. Magnetic catalysts were readily separated from resulting products in the magnetic field, as well as recycled and reused with
negligible loss in activity. Glucose and 5-HMF yields were determined through high-performance liquid chromatography analysis.
[doi:10.2320/matertrans.MA201539]
(Received January 29, 2015; Accepted May 7, 2015; Published June 19, 2015)
Keywords: magnetic acidic-catalysts, cellulose conversion, 5-hydroxymethylfurfural, poly(vinylsulfonic-co-divinylbenzene)

1.

Introduction

The catalytic conversion of biomass into 5-hydroxymethylfurfural, a key renewable chemical for biochemical
and biofuel production, has attracted increased attention
owing because of the rising global demand for energy
and environmental benefits.1,2) Ionic liquids (ILs) have been
employed in transformation processes to form homogeneous
carbohydrate solutions that result in enhancement of breaking
hydrogen and ¢-1,4-glycosidic bonds.3,4) High 5-HMF yield
was prepared from the dehydration of fructose (100% 5-HMF
yield), glucose (60%­80% 5-HMF yield), or sucrose (76% 5HMF yield) using metal halide catalysts in ILs under mild
conditions of 100°C­130°C.5­8) As cellulose is a major
source of glucose and the most abundant photosynthetically
fixed carbon resource in nature, numerous studies have
focused on the direct conversion of cellulose into 5-HMF.
Impressive HMF yields of 48%­54% were obtained from
untreated lignocellulosic biomass and purified cellulose using
N,N-dimethylacetamide-LiCl solvent as a solvent in the
presence of chromium chloride, 1-ethyl-3-methylimidazolium chloride (EMIMC), and HCl acid as co-catalyst at
140°C.9) A single-step process of cellulose conversion into 5HMF catalyzed by a pair of metal chlorides CuCl2­CrCl2
dissolved in EMIMC, showed a 5-HMF yield of 55% at
120°C in 8 h reaction time.10) Another combination of CrCl2­
RuCl3 in EMIMC provided nearly 60% HMF yield at 120°C
in 2 h.11) Zhang and colleagues12) presented the direct
conversion of cellulose into 5-HMF (47% yield) at 120°C
using a catalytic system of CrCl2/zeolite/[BMIM]Cl in
which solid acid zeolite with moderate acidity was employed
to promote cellulose hydrolysis and slow down HMF product
decomposition. The remarkable improvement in 5-HMF
+

Corresponding author, E-mail:

yield of up to 89% by using high loading CrCl2 catalyst in
EMIMC under anhydrous conditions was studied at 120°C in
6 h.13) Recently, CrCl3, a compound with higher environmental stability than that of the strongly reductive CrCl2, was
studied for the 5-HMF production from cellulose in EMIMC
solvent under microwave irradiation, through which a 5HMF yield of 60% was obtained.14,15) The CrCl3 and LiCl
with 1 : 1 molar ratio in BMIMC demonstrated a high 5HMF yield of 62% at 140°C under microwave irradiation for
40 min.16) The direct conversion of cellulose into 5-HMF
(54% yield) using CrCl3 catalyst in BMIMC heated at 150°C
through a conventional water-bath was developed by Qi
et al.7) In recent years, a high 5-HMF yield at approximately
70% was produced from cellulose through an efficient twostep process in which cellulose hydrolysis into glucose was
catalyzed by a strong acidic cation exchange resin through
the gradual addition of water into EMIMC, and then, CrCl3
was used to catalyze hydrolysis products into 5-HMF at
110°C.17) However, solid catalysts should be readily isolated
from solid residues after reaction for the catalysts to be
regenerated and reused in further conversion process.
Magnetic porous silica particles were studied for the efficient
hydrolysis of starch and cellulose into glucose.18) Results
showed that the catalyst can be easily separated from the
reaction system by magnetic force and undergo 3-times
repeated use without significant loss in activity. In this study,
poly(vinylsulfonic-co-divinylbenzene) and magnetic polymers (VS-DVB/CoFe2O4) with meporous structure were
prepared using a reverse micelles method. Effects of the
prepared catalysts and CrCl3·6H2O on the conversion of
cellulose into glucose and 5-HMF in ionic liquids under mild
conditions were studied and discussed. The catalysts

demonstrated high stability and recyclability in the cellulose
conversion process, without reducing activity after several
cycles.


Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural

2.

Experimental Procedure

2.1 Chemicals
D-Glucose (96%, Aldrich), cellulose (microcrystalline
powder, Aldrich), 5-hydroxymethylfurfural (5-HMF 99%,
Aldrich), vinylsulfonic acid sodium salt solution (VS 25%,
Aldrich), divinylbenzene (DVB 86%, Aldrich), sodium
dodecyl sulfate (SDS 99%, Aldrich), hexadecane (HD 99%,
Merck), benzoyl peroxide (BPO 75%, Acros Organics),
polyvinyl alcohol (PVA, MW ³140.000, 87%­89% hydrolyzed, HIMEDIA Co.), sorbitan monooleate (Span 80,
Shanghai Chemical Reagent Co., China), FeCl3 (98%,
Merck), CoCl2·6H2O (97%, Acros Organics), CrCl3·6H2O
(96%, Aldrich), tetrabutylammonium chloride (TBAC 99%,
Aldrich), 1-butyl-3-methyl-imidazolium chloride (BMIC
95%, Aldrich), acetone nitrile (HPLC grade, Scharlau),
NaOH (98.9%, Acros Organics), and H2SO4 (98%),
C2H5OH (99%), CH2Cl2 (99.7%), n-hexane (95%), ethylacetate (99.5%) purchased from Chemsol Co. Vietnam, were
used as received.
Preparation of magnetic polymers (VS-DVB/
CoFe2O4)
CoFe2O4 nanoparticles coated with oleic acid were

synthesized according to the co-precipitation method from
aqueous salt solutions Fe3+ and Co2+ in alkaline medium.19)
DVB-VS polymers were prepared following the reverse
micelles method20) with various mole ratios of VS to DVB
³0.5 : 1, 1 : 1, and 2 : 1. Magnetic DVB-VS polymers were
prepared in the presence of CoFe2O4 nanoparticles. A 1.2
gram mixture of PVA and SDS with weight ratio of 1 : 0.2
was added to 100 mL distilled water containing 5.24 g VS
solution (VS:DVB ³1 : 1) and 0.5 g CoFe2O4 (aqueous
phase). Another solution that includes 1.3 g DVB, 0.2 g BPO
initiator, and 1.0 g span80 was prepared as the oil phase.
Thereafter, the oil phase was dispersed into the aqueous
phase under vigorous stirring and was heated at 75°C for 8 h.
Thereafter, dark brown polymers were separated from the
reaction solution by using an external magnet bar. The
obtained beads was extracted with boiling acetone for 24 h.
Then, 1.0 g of magnetic polymer was ion exchanged by using
a solution of 20 mL dichloromethane and 2 mL H2SO4 (98%)
in 4 h. Finally, the powder was washed with water and
ethanol, and then dried at 60°C in air.

1435

(a)

(b)

2.2

2.3 Characterization

Resulting polymers were characterized via X-ray diffraction (XRD, D2PHASER:Cu-K¡ radiation, Bruker AXS,
Germany). Fourier transform infrared (FTIR) spectra were
recorded using a Bruker Equinox 55 FTIR spectrometer.
Thermogravimetric analysis (Perkin-Elmer TGA7, Model2960, USA) was performed to examine the thermal durability
of polymers. A vibrating sample magnetometer (VSM, EZ11,
Microsene, USA) was used to measure the hysteresis loops
of magnetic catalysts at room temperature. Transmission
electron microscope (TEM, JEOL JEM 1400, Japan) and
field-emission scanning microscope­energy dispersive X-ray
analysis (FESEM-EDX JSM-6700F, JEOL) were employed
to evaluate the size and element component of catalysts. The
N2 adsorption/desorption isotherms of catalysts (degassed at

Fig. 1 (a) HPLC chromatograph plot of 5-HMF solution with various
concentrations; (b) Standard curve of authentic 5-HMF in deionized
water.

170°C for 4 h) were recorded using Quantachrome NOVA
1000e. Surface area was determined using the Barrett­
Emmet­Taller (BET) method within the P/P0 range of 0.05­
0.30. Pore diameter and volume were calculated by the
Barrett­Joyner­Halenda method applied to the adsorption
branch of the isotherm. The acidic site amount (mmol H+
g¹1) of the catalysts was determined using the acid-base
titration method.
2.4 Catalytic tests
2.4.1 Glucose conversion
50 mg glucose and 7.5 mg CrCl3·6H2O (10 mol% with
respect to glucose) were added into a 50 mL glass tube (ACE
Glass Inc., USA) containing 500 mg ionic liquid (BMIC or

TBAC). The mixture was sonicated for 5 min and heated at
the desired temperatures from 30 min to 180 min. Then, the
mixture was immediately cooled down to room temperature.
5-HMF was extracted from the reaction mixture by using
ethyl acetate (5 mL © 2).
Standards and samples of 5-HMF were analyzed at room
temperature by using HPLC Agilent 1100 with a UV detector
(­ = 285 nm) and advanced chromatography technologies
ACE-C18 (150 mm © 4.6 mm, 3.5 µm). A mixture of acetone
nitrile (ACN) and water with a ratio of 5 : 95 in volume was
used as mobile phase at 0.6 mL min¹1 rate. The 5.0 µL
injection volume was employed. As shown in Fig. 1, the


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T.-D. Nguyen, H.-D. Nguyen, P.-T. Nguyen and H.-D. Nguyen

(a)

(a)

(b)

(b)

Fig. 2 (a) HPLC chromatograph plots of 5000 mg L¹1 glucose solution
(dash-dot line), and 5000 mg L¹1 glucose solution in presence of 500 mg
BminCl (solid line), (b) Standard curve of authentic glucose in deionized
water.

calibration curve was obtained from various concentrations
of 5-HMF standard solutions (5.0, 10.0, 25.0, 50.0, 100.0,
and 200.0 mg L¹1). The 5-HMF yield was calculated as
5-HMF yield (%) = (moles of 5-HMF/initial moles of
glucose) © 100.
2.4.2 Cellulose conversion
A 50 mL ACE glass tube containing 500 mg ionic liquid
(BMIC or TBAC) and 50 mg cellulose was sonicated for
15 min. Catalyst was added into the tube. Thereafter, the
mixture was sonicated for 5 min and heated at 110°C­120°C
from 30 to 180 min. After 180 min of cellulose hydrolysis
in accordance with the aforementioned procedure, 7.5 mg
CrCl3·6H2O was added into the reaction to improve the 5HMF yield. The mixture was heated at 120°C in 60 min.
After the reaction, each sample was immediately cooled
down to room temperature. 5-HMF was extracted from the
reaction mixture using ethyl acetate (5 mL © 2). Then, 10 mL
deionized water was poured into the remaining mixture of IL
and byproducts. Magnetic catalysts, which were feasibly
isolated from the magnetic field products, were re-acidified
and reused.
Glucose amount was analyzed by a HPLC system with
an Agilent 1260 RID detector and a LiChrospher NH2
(250 mm © 4.0 mm, 5 µm) column. Acetonitrile/water solution (90/10 V/V) was used as the flowing phase at 1.0
mL min¹1. The glucose concentration range was 500 mg/L­
5000 mg/L. The standard curve of the authentic glucose in

(c)

Fig. 3 (a) FTIR spectra and (b) XRD patterns of (¡) VS-DVB, (¢)

CoFe2O4/OA, and (£) VS-DVB/CoFe2O4; (c) TGA curve of VS-DVB.

water is shown in Fig. 2. Product (glucose or 5-HMF) yields
were calculated as Product yield (%) = ([Product]/[Cellulose]) © 100, in which [Product] was the concentration in
ppm of glucose or 5-HMF obtained from the conversion, and
[Cellulose] was the initial concentration in ppm of cellulose.
3.

Results and Discussions

3.1 Characterizations of magnetic polymers
Figure 3(a) shows the IR spectra of VS-DVB and VS-


Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural

Fig. 4

1437

TEM images of (a) VS-DVB, (b), (c) VS-DVB/CoFe2O4, and (d) CoFe2O4.

DVB/CoFe2O4. The asymmetric (¯as) and symmetric (¯s)
stretching vibrations of ­SO3H groups were observed at
1172 cm¹1 and 1030 cm¹1 reflection bands, respectively. The
peak at 700 cm¹1 was considered because of the C­S
stretching vibration.21) The FTIR spectra of CoFe2O4 coating
olecic acid (CoFe2O4/OA) is also presented in Fig. 3(a). Two
sharp bands at 2922 cm¹1 and 2852 cm¹1 resulted from
the ¯as and ¯s stretching vibrations of ­CHCH2 groups,

respectively. Two bands at 1457 cm¹1 and 1511 cm¹1
corresponded to the ¯as and ¯s stretching vibration bands
of ­COO­ groups, respectively.22) The band at 580 cm¹1
ascribed to the Co/Fe­O stretching vibrations23) is observed
in the spectra of CoFe2O4/OA and VS-DVB/CoFe2O4.
Figure 3(b) depicts the XRD pattern of DVB-VS with a
major peak at 2ª ³ 19°. All diffraction peaks of CoFe2O4
nanoparticles matched well with the database of cubic spinel
magnetite CoFe2O4 JCPDS No. 001-1121. The XRD pattern
of magnetic polymers revealed the characteristic diffraction
peaks of DVB-VS and CoFe2O4. The thermal stability of the
prepared VS-DVB is shown in the TGA curve (Fig. 3(c))
with the major weight loss at 400°C.
The TEM image of the obtained VS-DVB beads with
approximate round shapes and sizes approximately 100­
200 nm in diameter are shown in Fig. 4(a). TEM images of
the VS-DVB/CoFe2O4 samples exhibit magnetic CoFe2O4
nanoparticles as small dark pots into polymer matrix
(Figs. 4(b) and 4(c)). CoFe2O4 nanoparticles with an average

size of 10 nm that match the dark spots of magnetic polymer
samples are observed in Fig. 4(d).
Figure 5(a) shows the elemental components Co, Fe, C, S,
and O as shown in the EDX spectral image of VS-DVB/
CoFe2O4. Figure 5(b) demonstrates the N2 isotherms of the
prepared polymers as type IV with clear hysteresis loop.
Isotherms showed mesoporous materials,23) in addition to the
surface area (SBET), average pore diameter (Dp), and average
pore volume (Vp), which were 332.0 m2 g¹1, 4.2 nm, and
0.44 cm3 g¹1 for VS-DVB, respectively. These SBET, Dp, and

Vp values for VS-DVB/CoFe2O4 were 166.3 m2 g¹1, 3.4 nm,
and 0.33 cm3 g¹1, respectively. Acid site amounts were 0.75,
1.20, 1.28, and 0.95 mmol H+/g for VS-DVB (0.5 : 1), VSDVB (1 : 1), VS-DVB (2 : 1), and VS-DVB (1 : 1)/CoFe2O4,
respectively. Figure 5(c) depicts the magnetization versus
the applied magnetic fields (hysteresis curves) of CoFe2O4,
VS-DVB/CoFe2O4, and spent VS-DVB/CoFe2O4. Permanent magnetization was almost unobserved for these samples,
which suggested that they exhibited superparamagnetic
behavior.24) The saturation magnetization values obtained at
room temperature were 56.14, 37.19, and 25.91 emu g¹1 for
CoFe2O4, VS-DVB/CoFe2O4, and used VS-DVB/CoFe2O4,
respectively.
3.2

Evaluating the catalytic ability of prepared polymers
First, polymers with different acidic strengths were tested


1438

T.-D. Nguyen, H.-D. Nguyen, P.-T. Nguyen and H.-D. Nguyen

(a)

(a)

(b)
(b)

(c)
(c)

Fig. 5 (a) EDX patterns of VS-DVB/CoFe2O4; (b) N2 adsorption­
desorption isotherm of ( ) VS-DVB and ( ) VS-DVB/CoFe2O4; and
(c) Magnetization curves of ( ) CoFe2O4, ( ) VS-DVB/CoFe2O4 and
( ) used VS-DVB/CoFe2O4.

Fig. 6 (a) Cellulose conversion into glucose using 50 mg VS-DVB in
different ionic liquids at 110°C; Cellulose conversion into (b) glucose and
(c) 5-HMF using various VS-DVB contents in BIMC ionic liquid at
120°C.

for cellulose conversion in a BMIC solvent at 110°C for
30 min. Glucose yields were 1.2% and 5.0% by using VSDVB (0.5 : 1) and VS-DVB (1 : 1), respectively. These
results indicated that the cellulose hydrolysis depended
heavily on the acidic strength of the catalyst. Therefore,
VS-DVB (1 : 1) was used for further conversions. Effects of
TBAC and BMIC solvents on the cellulose conversion using
VS-DVB (1 : 1) at 110°C with different reaction times are
shown in Fig. 6(a). Glucose yields gradually increased with
reaction time (30 min­120 min). BMIC was a more effective
solvent for the production of glucose from cellulose
compared with TBAC. Maximum glucose yields were
8.0% and 23.0% at 110°C in 120 min by using TBAC and
BMIC, respectively. Reaction temperature similarly pre-

sented a significant influence on glucose production. Results
are shown in Fig. 6(b). The enhancement in glucose yield of
55.0% was obtained at 120°C in 30 min reaction using BMIC
solvent. Lower glucose yields were reached at increasing
reaction time at high temperature, which can be attributed

to the dehydration of glucose into 5-HMF. As depicted in
Fig. 6(c), the 5-HMF yield increased with the hydrolysis time
of cellulose from 30 min to 180 min. Figure 6(c) similarly
presents the unremarkable effect of VS-DVB content on the
5-HMF yield. In the presence of 50 mg and 75 mg catalysts,
the yield of 5-HMF can reach 8.8% and 8.0% at 120 min, and
10.5% and 8.3% at 180 min with 120°C, respectively. Any
further reaction time increase resulted in lower 5-HMF
yields, which can be due to the condensation of 5-HMF.25)


Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural

1439

low-cost ionic liquid at mild reaction conditions qualify
TBAC as a potential solvent for 5-HMF production from
glucose.
Effects of VS-DVB/CoFe2O4 catalysts on conversion
cellulose into glucose and 5-HMF using TBAC and BMIC
solvents at 120°C for 180 min are presented in Fig. 7(b).
Yields of 28.0% glucose and 1.5% 5-HMF were obtained
using 50 mg VS-DVB/CoFe2O4 in BMIC. However, lower
yields of glucose (7.8%) and 5-HMF (0.1%) were derived
using magnetic catalyst in TBAC. Results highlighted TBAC
as a powerful solvent for glucose conversion contrary to
cellulose conversion. This finding may be due to the weak
solubility of cellulose in TBAC. Figure 7(b) shows the
improved 5-HMF yield when 7.5 mg CrCl3·6H2O was added
into the reaction at 120°C for 60 min as well. Yields of 23.0%

glucose and 5.5% 5-HMF or 14.3% glucose and 1.3% 5HMF were obtained by using BMIC or TBAC, respectively.
Given the efficient catalysis of TBAC and CrCl3·6H2O
systems for the glucose conversion into 5-HMF promoted
the cellulose conversion into glucose, the increase in both
glucose and 5-HMF yields were observed in using TBAC
and VS-DVB/CoFe2O4-Cr (Fig. 7(b)). After the reaction,
VS-DVB/CoFe2O4 was readily isolated from the products
through a magnet. The spent catalyst was regenerated and
reused for a new reaction cycle. Glucose and 5-HMF yields
reached 25% and 1.4% in the second and third cycles,
respectively (Fig. 7(c)). However, the cellulose conversion in
the fourth and fifth cycles was only 50% compared to the
cellulose conversion in the first cycle. The decrease in
production may be due to the trapping of byproducts inside
the porous structure of the catalyst, leading to reduced
activity sites. The saturation magnetization ³25.9 emu g¹1
(as shown in Fig. 5(a)) and porosity at SBET ³ 97.7 m2 g¹1,
VP ³ 0.15 cm3 g¹1, and DP ³ 5.1 nm were retained in the
magnetic catalysts used.

(a)

(b)

(c)

4.

3+

Fig. 7 Glucose conversion into 5-HMF using Cr catalyst in different
ionic liquids at 120°C; (b) Cellulose conversion into glucose and 5-HMF
using VS-DVB/CoFe2O4 and VS-DVB/CoFe2O4-Cr in different ionic
liquids; (c) Reused VS-DVB/CoFe2O4 catalytic performance on the
cellulose conversion in BMIC ionic liquid at 120°C in 180 min.

Therefore, optimal reaction conditions at 120°C in 180 min
were found for the conversion of cellulose into glucose
(50%) and 5-HMF (10.5%) using VS-DVB in the BMIC
solvent.
The dehydration of glucose into 5-HMF in TBAC and
BMIC solvents using CrCl3·6H2O (15 mass% with respect to
glucose) at 120°C are displayed in Fig. 7(a). In this case,
BMIC exhibited a less effective performance than TBAC.
The 5-HMF yields of 26.0% and 6.6% were obtained by
using TBAC and BMIC at 120°C in 10 min, respectively
(Fig. 7(a)). Increases in 5-HMF yields with time reaction
were observed in Fig. 7(a) as well. The 5-HMF yields can
reach up to 12.5% in BMIC in 30 min and 98% in TBAC
during a 90 min reaction. Efficient glucose conversion in a

Conclusion

Mesoporous VS-DVB and magnetic polymer VS-DVB/
CoFe2O4 were prepared by using the reverse micelles
method. Efficient cellulose hydrolysis resulting in 50%
glucose and 10% 5-HMF yields were obtained through VSDVB polymer in BMIC ionic liquid under mild conditions.
The catalytic system of low-toxicity CrCl3·6H2O and
inexpensive ionic liquid TBAC were revealed as excellent
glucose conversion approaches with an impressive HMF

yield of 98% at 120°C in 90 min reaction. In the presence of
VS-DVB/CoFe2O4, cellulose conversion amounted to 28%
glucose and 1.5% 5-HMF yields by using BMIC solvent.
Magnetic catalysts can be reused several times without
obvious deactivation. Furthermore, a significant improvement in 5-HMF yields was observed when CrCl3·6H2O was
added into the cellulose conversion process using magnetic
catalyst and ionic liquid. Results showed VS-DVB/CoFe2O4
as a potential catalyst in directly converting cellulose into
valuable chemicals as glucose and 5-HMF in ionic liquids.
Acknowledgment
This study was funded by the Vietnam National


1440

T.-D. Nguyen, H.-D. Nguyen, P.-T. Nguyen and H.-D. Nguyen

Foundation for Science and Technology Development
(NAFOSTED) under grant number 104.01-2012.50, as well
as the Young Scientists Program of Vietnam Academy of
Science and Technology under grant VAST.ĐLT.07/12-13.
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