Synthesis, characterization and properties of carbon nanotubes microspheres from pyrolysis of polypropylene and maleated polypropylene

Article abstract of DOI:10.1016/j.materresbull.2009.09.007

Microspheres assembled from carbon nanotubes (MCNTs), with the diameters ranging from 5.5 to 7.5 μm, were synthesized by means of pyrolysis of polypropylene and maleated polypropylene in an autoclave. The characterization of structure and morphology was carried out by X-ray diffractometer (XRD), field-emission scanning electron microscopy (FESEM), (high resolution) transmission electron microscope [(HR)TEM)], selected-area electron diffraction (SAED) and Raman spectrum. As a typical morphology, the possible growth process of MCNTs was also investigated and discussed. The results of nitrogen adsorption-desorption indicate that the Brunauer-Emett-Teller (BET) surface area (140.6 m²/g) of the MCNTs obtained at 600 °C is about twice as that (74.5 m²/g) of carbon nanotubes obtained at 700 °C. The results of catalytic experiment show that MCNTs based catalyst has higher catalytic activity than the carbon nanotubes based catalyst for the preparation of methanol and dimethoxy-ethane by oxidation of dimethyl ether.

Full text of DOI:10.1016/j.materresbull.2009.09.007

Materials Research Bulletin 45 (2010) 15–20
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis, characterization and properties of carbon nanotubes microspheres
from pyrolysis of polypropylene and maleated polypropylene
a,b,
b
b
b
Junhao Zhang *, Jin Du , Yitai Qian , Shenglin Xiong
a
School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang Jiangsu 212003, People’s Republic of China
Department of Chemistry, University of Science and Technology of China, Heifei, Anhui 230026, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Microspheres assembled from carbon nanotubes (MCNTs), with the diameters ranging from 5.5 to
Received 18 April 2009
7 mm, were synthesized by means of pyrolysis of polypropylene and maleated polypropylene in an
.5
autoclave. The characterization of structure and morphology was carried out by X-ray diffractometer
XRD), field-emission scanning electron microscopy (FESEM), (high resolution) transmission electron
Received in revised form 2 September 2009
Accepted 11 September 2009
Available online 22 September 2009
(
microscope [(HR)TEM)], selected-area electron diffraction (SAED) and Raman spectrum. As a typical
morphology, the possible growth process of MCNTs was also investigated and discussed. The results of
Keywords:
2
nitrogen adsorption–desorption indicate that the Brunauer–Emett–Teller (BET) surface area (140.6 m /
A. Nanostructures
C. Electron microscopy
C. Raman spectroscopy
D. Catalytic properties
2
g) of the MCNTs obtained at 600 8C is about twice as that (74.5 m /g) of carbon nanotubes obtained at
700 8C. The results of catalytic experiment show that MCNTs based catalyst has higher catalytic activity
than the carbon nanotubes based catalyst for the preparation of methanol and dimethoxy-ethane by
oxidation of dimethyl ether.
ß 2009 Elsevier Ltd. All rights reserved.
1
. Introduction
In recent years, there has been increasing interest in preparation
nanotubes and porous hollow CNTs composite cages were also
produced by a layer-by-layer assembly and templating technique
[13,14]. In addition, sea urchin-like nanostructured carbon spheres
were fabricated from the growth of CNTs bundles on hollow carbon
spheres by CVD with monodispersed iron oxide nanoparticles as
the catalyst [15].
and processing of various nanostructured carbon materials.
Difficulties for functionalizing carbon nanotubes (CNTs) and
limitations in processing methods are main barriers for the pursuit
of these potential applications. For these reasons, new development
to assemble and align CNTs is still a great and fascinating challenge.
To date, synthesis of CNTs is reported via various methods, such as
laser vaporization, arc discharge, pyrolysis, chemical vapor deposi-
tion (CVD) and solvothermal processes [1–7].
The properties of carbon materials are strongly influenced by
their morphologies and structures. Hence, the fabrication of carbon
materials with a particular morphology and well-defined structure
is very important for their various properties-related applications.
Polymeric materials are frequently used as carbon sources for
the preparation of CNTs [16–19]. For example, Tang and co-
workers synthesized multiwalled carbon nanotubes in large
quantities by burning a polypropylene/nickel/montmorillonite
composite in the atmosphere [20–23]. He and co-workers
synthesized carbon nanotubes, chestnut-like carbon nanotube
spheres and carbon nanospheres from catalytic pyrolysis of
polypropylene (PP) using different catalyst precursors [24,25]. In
their methods, it is necessary that catalyst precursors were mixed
with PP and clay, then such nanocomposites were burned to obtain
carbon nanotubes. Otherwise no CNTs were formed. In this article,
we report a simple pyrolysis method to synthesize MCNTs, using
PP as carbon source and maleated polypropylene (MA-PP) as
compatibilizer and carbon source, and ferrocene as catalyst
precursor without pre-mixing all components. The growth
mechanism of MCNTs was investigated by FESEM and proposed
here. The catalytic activities of the MCNTs based catalysts and
CNTs based catalysts have been tested by preparation of methanol
and dimethoxy-ethane (DMET) from dimethyl ether (DME),
showing that the MCNTs based catalysts exhibit higher catalytic
activity than CNTs based catalysts.
5
Hou et al. prepared helically coiled CNTs using Fe(CO) as floating
catalyst precursor [8]. Bajpai and co-workers synthesized large-
scale aligned/micropatterned straight CNTs by pyrolysis of iron(II)
phthalocyanine on the pristine quartz glass plates in a tube furnace
[
9]. Moreover, hollow carbon microspheres were also prepared by
a variety of methods [10–12], and aligned mule-walled carbon
*
Corresponding author at: School of Materials Science and Engineering, Jiangsu
University of Science and Technology, Zhenjiang Jiangsu 212003, People’s Republic
of China. Fax: +86 511 3631760.
E-mail address: jhzhang6@mail.ustc.edu.cn (J. Zhang).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.09.007

1
6
J. Zhang et al. / Materials Research Bulletin 45 (2010) 15–20
2
. Experimental
of the product obtained at 600 8C. Fig. 1b gives their corresponding
À1
Raman spectra, showing two similar Raman bands at 1346 cm (D
À1
All chemicals used in this work, such as ferrocene [Fe(C
5
H
5
)
2
]
band) and 1600 cm (G band). More specifically, the two peaks
2
are A. R. regents from the Shanghai Chemical Factory (China). PP
F401) is supplied as pellets by Yangzi Petrochemical Co. (China).
MA-PP is supplied by Chemical Material Co., Ltd. (China), and the
exhibit an E2g mode of graphite related to the vibration of sp -
(
bonded carbon atoms in a 2D hexagonal lattice (such as in a
grapheme layer) and A1g mode of graphite related to the disorder
features due to the finite particle size effect or lattice distortion of
the graphite crystals, respectively [26–28]. Based on the previous
research results [29] and the present Raman spectra, it can be
rationally speculated that the graphitic layers contained in the
product prepared at 700 8C are more regular than that contained in
the product prepared at 600 8C.
grafting yield of maleic anhydride groups is 4 phr and its M
measured by Gel Permeation Chromatograph (GPC) is 30,000.
In a typical experiment, ferrocene (0.50 g), PP (2.00 g, pellets
with diameters of about 4 mm) and MA-PP (0.50 g, pellets with
diameters of about 4 mm) were added into an autoclave reactor of
w
2
0 mL without pre-mixing the components. The reactors were
sealed and maintained at 600 and 700 8C for 12 h, and then allowed
to cool to room temperature naturally. The dark products were
collected and washed with dilute HCl aqueous solution, ethanol,
and distilled water. The final products were dried in a vacuum at
Typical FESEM images in Fig. 2 indicate that large amounts of
MCNTs and CNTs were produced by pyrolysis of PP and MA-PP at
600 and 700 8C. Fig. 2a displays the MCNTs with diameters ranging
from 5.5 to 7.5
mm, and single microsphere with the diameter of
6
0 8C for 6 h.
TiO /CNTs, TiO
6.3 m is shown in Fig. 2b. The magnified FESEM image in Fig. 2c
m
2
2
/MCNTs, CuO/CNTs and CuO/MCNTs catalysts
give more information of the MCNTs’ surface, which reveals that
these microsphere are composed of CNTs with diameters in the
range of 35–55 nm. Fig. 2d, which is the magnified image of region
1 in Fig. 2c, shows that the obtained products include about 4%
carbon nanoparticles according to the FESEM observation. Fig. 2e,
which is the magnified image of region 2 in Fig. 2c, shows that the
surface of MCNTs is rough. However, at the higher temperature of
700 8C, large amounts of CNTs with length of several micrometers
to tens of micrometers are obtained instead of microspheres, as
presented in Fig. 2f and g. Through careful observation of these
images, the carbon nanotubes have smooth and clean surface with
the diameters ranging from 40 to 70 nm.
were prepared by wet impregnation with an aqueous solution of
cupric nitrate and an ethanol solution of analytical tetrabutyl
titanate to give CuO and TiO
2
content of 10%, respectively. After
1
2 h, the obtained solid samples were firstly dried at 100 8C in air
for 12 h and then calcined at 500 8C in air for 3 h. The obtained
samples were used to test their catalytic properties.
The XRD patterns of the products were recorded on a Rigaku
(Japan) D/max-
gA X-ray diffractometer equipped with graphite
˚
monochromatized Cu K
a radiation (l = 1.54178 A). The Raman
spectrums were investigated at ambient temperature on a Spex
403 Raman spectrometer (Ar ion laser, 514.5 nm). The FESEM
1
images of the products were examined by a field-emission scanning
electron microscope (JEOL-6300F). The TEM and HRTEM images and
the SAED patterns were taken on a JEOL 2010 high resolution
transmission electron microscope at an acceleration voltage of
Fig. 3a is the TEM image of a MCNT with the diameter of about
mm prepared at 600 8C. Fig. 3d shows that the inner diameters of
6
the carbon nanotubes prepared at 700 8C are between 7 and 30 nm,
and the outer diameters are between 40 and 70 nm, as well as few
thinner nanotubes were also observed. Fig. 3b and e displays two
individual nanotubes. From the comparison of the two individual
CNTs displayed in Fig. 3b and e, it is found that the wall of the CNT
obtained at 600 8C is rougher than that of the CNT obtained at
700 8C. To further characterize the wall structure of the as-
synthesized CNTs, more detailed studies are executed by HRTEM.
Fig. 3c and f shows HRTEM images for the wall structures of
nanotubes. The interlayer spacings in the walls are about 0.34 nm,
corresponds to the (0 0 2) distance of graphitic carbon lattice. From
the comparison of Fig. 3c and f, it can be observed that the former
has some stacking faults. The SAED patterns (inset in Fig. 3c and f)
are characteristic of CNTs, and the two elongated arcs and circle
correspond to (0 0 2), (1 0 0) planes of graphite carbon, which are
consistent with the XRD and HRTEM results.
2
00 kV. The nitrogen adsorption–desorption isotherms and textural
properties were determined by Beckman Coulter SA3100.
Catalytic experiments were carried out in a continuous flow
fixed-bed quartz reactor (8 mm i.d.) under atmospheric pressure.
0
.3 g catalysts of 40–60 mesh size were loaded, and the catalysts
were pretreated using the mixed gas of 17% O and 83% He for 1 h
at 350 8C. A feed gas mixture of 42% DME, 8% O and 50% He was
2
2
À1 À1
introduced with a space velocity of 3480 mL g
h
. The effluent
was held at 383 K and analyzed on-line by gas chromatographs.
3
. Results and discussion
As shown in Fig. 1a, XRD patterns of the products prepared at
6
00 and 700 8C for 12 h both have two peaks, which can be indexed
as (0 0 2) and (1 0 0) reflections of hexagonal graphite (JCPDS Card
Files, No. 41-1487). Compared with the (0 0 2) peaks, it indicates
the better crystallinity of the product obtained at 700 8C than that
A series of relevant experiments have been carried out through
altering experimental parameters to investigate the effect of
reaction conditions on the formation of MCNTs. It is obvious that
Fig. 1. (a) XRD patterns of the products obtained at 600 8C and 700 8C for 12 h; (b) Raman spectrums of the products obtained at 600 8C and 700 8C for 12 h.

J. Zhang et al. / Materials Research Bulletin 45 (2010) 15–20
17
Fig. 2. (a and b) Low magnification FESEM images of MCNTs obtained at 600 8C for 12 h; (c) high magnification FESEM images of the surface of MCNTs obtained at 600 8C for
2 h; (d and e) high magnification FESEM images of regions 1 and 2 in (c); (f and g) Different magnification FESEM images of CNTs obtained at 700 8C for 12 h.
1
thereactiontemperatureandtimeplaycriticalrolesintheformation
of MCNTs. Lower temperature than 400 8C cannot initiate the
reactions. When the reaction temperatures are at 400 and 500 8C for
[30]. In our experiment process, PP-MA was used as compatibilizer
to improve the dispersion of Fe(C in the PP matrix [20,31], and
PP and MA-PP may form microspheres containing Fe(C after
the melting temperature of PP. When the PP/MA-PP/Fe(C
composite was heated to 400 8C, Fe(C (decomposition
5 5 2
H )
5 5 2
H )
1
6
2 h, the main products are the amorphous carbon materials. At
00 8C for 12 h, a large scale of MCNTs can be observed in product, as
5 5 2
H )
5 5 2
H )
shown in Fig. 2a. Raising reaction temperature to 700 8C, abundant
CNTs were produced but not assembled into microspheres (see
Fig. 2f). To gain a better understanding of the growth process of the
MCNTs, the time-dependent samples were collected and studied by
FESEM and shown in Fig. 4. After 1, 2 h shown in Fig. 4a and b, the
main products are the solid microspheres. As the reaction time was
increased to 4 h, many white raised dots on the surface of
microspheres are clearly observed, and some dots have openings.
With the extension of reaction time, dots on the surface of
microspheres become more and more. When reaction time reached
temperature: 400 8C [32]) decomposes instantly, giving a large
number of Fe nanoparticles within the microspheres, which acted
as effective and necessary dehydrogenation catalyst in the
experiment. The processes of the degradation of polymers and
the catalytic decomposition of the pyrolytic species around the Fe
particles were endothermic, so the thermal degradation rate of the
sphere surface is faster than that of the interior. In addition, the
pyrolytic species of the surface were thermally degradated to form
carbon layers without catalysts. PP and MA-PP of the interior
pyrolyzes to give pyrolytic species that were sealed in an
autoclave-like microreactor formed by carbon layers. Within the
microreactor, the pyrolytic species were catalyzed to form CNTs on
these Fe nanoparticles. The confinement of the microreactor
resulted in curled and rough CNTs and eventually formed carbon
nanotubes microspheres. However, when the temperature was
higher, for example 700 8C, the pyrolysis rate of polymers might be
8
h, many CNTs grow out from the spheres. CNTs have become the
majorcomponent ofthemicrospheres, whenreactiontimewas10 h.
The mechanisms of formation of MCNTs could be deduced from
the above experimental results (Fig. 4), which is similar to the
report of Ref. [24]. Ma et al. reported that pure PP and PP
nanocomposites formed spheres easily under heating conditions

1
8
J. Zhang et al. / Materials Research Bulletin 45 (2010) 15–20
Fig. 3. (a) TEM image of MCNTs obtained at 600 8C for 12 h; (b) TEM image of a single CNT obtained at 600 8C for 12 h; (c) HRTEM image of the CNT and its SAED pattern (inset)
obtained at 600 8C for 12 h; (d) TEM images of several CNTs obtained at 700 8C for 12 h; (e) TEM images of a single CNT obtained at 700 8C for 12 h; (f) HRTEM image of the CNT
and its SAED pattern (inset) obtained at 700 8C for 12 h.
too fast to result in that the polymer had decomposed completely
before carbon layers formed. So the final product is straight and
dispersed CNTs without the confinement of the carbon layers,
shown in Figs. 2f and 3d.
centers at 44.5 and 4.1 nm, which is more uniform than that of
CNTs. It is noteworthy mentioning that the BET surface area of
the MCNTs is about twice as that of CNTs. According to the
morphology and structure characteristic, the higher BET surface
area of MCNTs can be attributed to the smaller diameters, the
rough surface of CNTs in MCNTs, the more defects, and the
existence of carbon nanoparticles. It is expected that MCNTs
based catalyst has higher activity.
Fig. 5 shows the nitrogen adsorption–desorption isotherms of
the MCNTs and CNTs. The BET surface areas are 140.6 and
2
7
4.5 m /g, respectively. The calculations of Barrett–Joyner–
Halenda (BJH) reveal that the pore-size distribution of MCNTs
Fig. 4. (a–f) FESEM images of the products obtained at 600 8C for 1, 2, 4, 6, 8 and 10 h, respectively.

J. Zhang et al. / Materials Research Bulletin 45 (2010) 15–20
19
Fig. 5. (a) Typical N
2
adsorption–desorption isotherm of MCNTs obtained at 600 8C; (b) pore size distribution of MCNTs obtained at 600 8C; (c) typical N
2
adsorption–
desorption isotherm of CNTs obtained at 700 8C; (d) pore size distribution of CNTs obtained at 700 8C.
In order to investigate the catalytic activity of MCNTs
based catalyst, further experiments were carried out to compare
the catalytic activities of MCNTs and CNTs based catalysts.
for the partial oxidation of dimethyl ether (DME) to DMET, is also
investigated in Fig. 6b. The results demonstrate that DME
conversion and DMET selectivity of CuO(10%)/MCNTs are higher
than those of CuO(10%)/CNTs. The maximum DMET selectivity is
improved from 8.5% to 11.9%, and the DME conversion is enhanced
about 4% at 300 8C. However, under the condition of no catalyst or
MCNTs as catalyst, the DME conversion is lower than that of
CuO(10%)/CNTs or CuO(10%)/MCNTs catalysts; and the DMET
selectivity is zero. The above results indicate that effective
catalysts are necessary, and higher surface area is a very important
factor to improve catalytic activity, which cause the higher
adsorption of DME and prolong the residence time of DME on the
surface of the catalysts. By comparing the catalytic activity of
MCNTs and CNTs based catalysts, the improvement of the
catalytic activity of MCNTs based catalyst can be attributed
mainly to higher BET surface areas.
MCNTs, CuO(10%)/CNTs, CuO(10%)/MCNTs, TiO
2
(10%)/CNTs and
TiO (10%)/MCNTs catalysts were prepared to test the ability of
2
preparing methanol and DMET from DME. Fig. 6a shows the curves
of catalytic performance for the preparation of methanol from
DME under the conditions that without catalyst, MCNTs,
2 2
TiO (10%)/CNTs and TiO (10%)/MCNTs catalysts, respectively.
The results show that the DME conversion and methanol
selectivity are very low in the presence of no catalyst or MCNTs
as catalyst. The DME conversion and methanol selectivity of
2 2
TiO (10%)/MCNTs are higher than those of TiO (10%)/CNTs. The
maximum selectivity of methanol is improved 13% and the DME
conversion is enhanced about 7% at 300 8C. The catalytic
performance of CuO(10%)/CNTs and CuO(10%)/MCNTs catalysts
Fig. 6. (a) Catalytic performance of different catalysts: (1) DME conversion without catalyst, (2) DME conversion of MCNTs catalyst, (3) DME conversion of TiO
4) DME conversion of TiO /MCNTs catalyst, (5) methanol selectivity without catalyst, (6) methanol selectivity of MCNTs catalyst, (7) methanol selectivity of TiO
catalyst and (8) methanol selectivity of TiO /MCNTs catalyst; (b) catalytic performance of different catalysts: (1) DME conversion without catalyst, (2) DME conversion of
2
/CNTs catalyst,
(
2
2
/CNTs
2
MCNTs catalyst, (3) DME conversion of CuO/CNTs catalyst, (4) DME conversion of CuO/MCNTs catalyst, (5) DMET selectivity without catalyst and of MCNTs catalyst, (6) DMET
selectivity of CuO/CNTs catalyst and (7) DMET selectivity of CuO/MCNTs catalyst.

2
0
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