A chemical route from PTFE to amorphous carbon nanospheres in supercritical water

Article abstract of DOI:10.1039/b313733c

Amorphous carbon nanospheres with diameters of 140-200 nm have been synthesized by treating poly(tetrafluoroethylene) in supercritical water at 550°C using Ca(OH)₂ as defluorination reagent.

Full text of DOI:10.1039/b313733c

A chemical route from PTFE to amorphous carbon nanospheres in
supercritical water
Xiaogang Yang, Cun Li, Wei Wang, Baojun Yang, Shuyuan Zhang and Yitai Qian*
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of
China, Hefei 230026, P. R. China. E-mail: ytqian@ustc.edu.cn; Fax: 86 551 3631760
Received (in Cambridge, UK) 30th October 2003, Accepted 10th December 2003
First published as an Advance Article on the web 13th January 2004
Amorphous carbon nanospheres with diameters of 140–200 nm
have been synthesized by treating poly(tetrafluoroethylene) in
supercritical water at 550 °C using Ca(OH)2 as defluorination
reagent.
observed at 20–30°, indicating that low crystalline or amorphous
carbon is produced.
Poly(tetrafluoroethylene) (PTFE) is widely used as a plastic
material for its excellent heat and chemical resistance, low friction
coefficient, and little water absorption etc., partly due to the strong
C–F bond (481 kJ mol21).1 However, the waste PTFE materials
may become persistent solid pollutants which are difficult to
degrade in the environment.2,3 Traditionally, the waste PTFE can
be decomposed via thermal treatment at over 500 °C.4,5 Up to now,
many new defluorination methods have been applied to deal with
PTFE.6–10 For example, Hlavaty and Kavan have reported that
PTFE could react with alkali metals in organic solvents and
generate highly reactive polyynes and metal fluorides.6 Huczko and
his co-workers have developed a self-sustaining reaction for the
defluorination of PTFE using various metals or their alloys.8
However, these reactions either employ organic solvents, or cannot
avoid the serious corrosion of fluorine at high temperatures. In
addition, PTFE powder can be defluorinated into SrF2 and CO by
a mechanochemical solid-reaction method using strontium oxide.11
To reduce the burden on the environment, green methods are
required to deal with PTFE.12
Fig. 1 XRD of CaF2 (a) and amorphous carbon (b) prepared in supercritical
water at 550 °C.
The carbon nanospheres are observed on a JEOL JSM-6700F
field-emission scanning electron microscope (FESEM) and a JEOL
2010 high resolution transmission electron microscope (HRTEM)
with an accelerating voltage of 200 kV. In a typical FESEM image
(Fig. 2a), we can find a large number of carbon spheres with
diameters of 140–200 nm. TEM images (Fig. 2b) also reveal a
Supercritical water treatment is a promising method for the
decomposition of various halogenous organic compounds,13,14
because water has many environmental and technological advan-
tages in the state above its critical point (Tc: 374 °C, Pc: 22.1
Mpa).12,15 Recently, Sato and co-workers have reported that
polyvinyl chloride (PVC) could be decomposed into various
organic compounds by supercritical water treatment.16 Herein, we
report a novel chemical transformation of PTFE in supercritical
water at 550 °C using Ca(OH)2. In this process, PTFE can be
completely transformed into carbon nanospheres, which have
potential applications as reinforcement materials for rubber,17
supports for catalysts,18 and anodes in second lithium ion
batteries.19 Besides, the elemental fluorine could be strongly
absorbed by Ca(OH)2 to form fluorite, CaF2.
Typically, thermal treatment in supercritical water was carried
out in a stainless steel autoclave with 12 mL capacity. 0.5 g
commercial bulk PTFE (Shanghai Shanghua PTFE Material Co.,
Ltd), 0.74 g Ca(OH)2 (Shanghai Chem. Reagent Corp.) and 10 mL
distilled water were mixed in the autoclave. Then the autoclave was
sealed, and maintained at 550 °C for 12 h in an oven. After being
cooled to room temperature, the products were collected and
washed with dilute hydrochloric acid. The sample exhibited two
striking colors (black and white), which indicated that it consisted
of two different substances. Due to the difference of densities, they
were separated by the flotation method, and then filtered off,
washed with pure alcohol respectively. Finally, the black and the
white samples were collected and dried in vacuum at 60 °C for 4
h.
The products are determined by XRD on a Philips X’Pert PRO
SUPER X-ray powder diffractometer using Cu Ka radiation (l =
1.541874 Å). From the XRD pattern of the white product (Fig. 1a),
all the sharp reflection peaks can be indexed to cubic CaF2 with
lattice parameter a = 5.46 Å (JCPDS 35-0816). In the XRD pattern
of the black sample (Fig. 1b), only a broad diffraction peak can be
Fig. 2 (a) is the FESEM image, (b) and (c) are TEM images of amorphous
carbon nanospheres prepared in supercritical water at 550 °C.
342
C h e m . C o m m u n . , 2 0 0 4 , 3 4 2 – 3 4 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

View Article Online
number of carbon nanospheres with sizes of 100–210 nm. And
some striped nanostructures could be observed on the surface of an
individual carbon sphere (Fig. 2c). The selected area electron
diffraction of these carbon nanospheres shows dispersed rings,
indicating that the carbon spheres have low crystalline micro-
graphite structures, which could also be revealed by Raman
scattering (in the next paragraph). Nitrogen sorption isothermal
measurement is conducted on a Coulter Omnisorp 100CX specific
surface analyzer. From the sorption data, the BET surface area of
amorphous carbon nanospheres. The whole reaction could be
formulated as eqn. (3).
–(CF2)–n ? n/2 CF2NCF2
CF2NCF2 + 2 Ca(OH)2 ? 2 CaF2 + C + CO2 + 2 H2O
(1)
(2)
2 –(CF2)–n + 2n Ca(OH)2 ? 2n CaF2 + n C + n CO2 + 2n H2O
(3)
In addition, without any water added in the autoclave, the direct
solid reaction between PTFE and Ca(OH)2 was incomplete. At 550
°C, the water in the autoclave can be regarded as being in the
supercritical state, which can eliminate the limit of transport
between different reactants, and then promote the convertion from
PTFE to amorphous carbon.
In summary, in the supercritical water system at 550 °C, bulk
PTFE materials have been successfully converted into amorphous
carbon nanospheres and the by-product of CaF2, which can be
recycled and have potential applications in industry. The possible
reaction mechanism is also discussed. Supercritical water may play
a key role in the degradation of persistent PTFE wastes. This
method provides an efficient way to deal with waste PTFE
materials. In addition, there is promise in applying this method to
deal with and recycle other stable halogenous polymers.
The authors thank Professor Zhixiang Chen (Institute of Solid
State Physics, Chinese Academy of Sciences, Hefei) for his help in
BET surface area measurements. Financial support from the
National Natural Science Funds and the 973 Projects of China is
gratefully acknowledged.
the carbon nanospheres is about 20.0 m2 g21
.
The crystallinity of the prepared carbon is investigated by Raman
spectrometry on a LABRAM-HR Confocal Laser MicroRaman
Spectrometer using an Ar+ laser with 514.5 nm radiation at room
temperature. In Fig. 3(a), the peak at 1585–1600 cm21 (G-band)
corresponds to an E2g mode of graphite and is related to the
vibration of sp2-bonded carbon atoms in a 2-dimensional graphite
layer.20 Compared with the G-band at 1580 cm21 for the graphitic
carbons,21 the G-band of the products shifts towards a higher
wavenumber due to the less orderly arrangement of the carbon
atoms. The D-band at 1331–1339 cm21 (in Fig. 3(a)) is associated
with vibrations of carbon atoms with dangling bonds at the plane
termination of disordered graphite or glassy carbon. With the
treating temperature increasing, the intensity of the D-band (ID)
decreases and the intensity of the G-band (IG) increases. According
to the relationship La
=
4.4(ID/IG)21 (in nm),20 the small
microcrystalline planar size is 1.4–2.0 nm for the produced carbon
nanospheres.
IR spectra of the samples produced at 400 and 550 °C are shown
in Fig. 3(b) and (c). All the absorption peaks (Fig. 3(b): 1225, 1153,
639, 554 and 503 cm21) are assigned to the vibrations of carbon–
fluorine bonds (–CF2–) in PTFE.22 The absorption peaks at 1640
and 3450 cm21 could be ascribed to the d and n vibrations of water
molecules, respectively. IR spectra of the sample prepared at 550
°C (Fig. 3(c)) show that no absorption peaks can be indexed to
–CF2– or fragments such as C2F4 (1342, 913, 407 cm21), C4F8 (960
Notes and references
1 (a) J. M. Bellon, L. A. Contreras, J. Bujan and A. Carrera-San Martin,
Biomaterials, 1996, 17, 2367; (b) T. G. Vargo, J. A. Gardella, J. M.
Calvert and M. S. Chen, Science, 1993, 262, 1711.
2 J. C. Wittmann and P. Smith, Nature, 1991, 352, 414.
3 G. M. Simon and W. Kaminsky, Polym. Degrad. Stab., 1998, 62, 1.
4 W. Lincoln Hawkins, Polymer Stabilization, Wiley-Interscience, New
York, 1971.
5 D. A. Ellis, S. A. Mabury, J. W. Martin and D. C. G. Muir, Nature, 2001,
412, 321.
6 J. Hlavaty and L. Kavan, Carbon, 1999, 37, 1029.
7 L. Kavan, F. P. Dousek, P. Janda and J. Weber, Chem. Mater., 1999, 11,
329.
8 A. Huczko, H. Lange, G. Chojecki, S. Cudzilo, Y. Q. Zhu, H. W. Kroto
and D. R. M. Walton, J. Phys. Chem. B, 2003, 107, 2519.
9 T. T. Liang, Y. Yamada, N. Yoshizawa, S. Shiraishi and A. Oya, Chem.
Mater., 2001, 13, 2933.
10 A. Yasuda, N. Kawase, T. Matsui, T. Shimidzu, C. Yamaguchi and H.
Matsui, React. Funct. Polym., 1999, 41, 13.
11 Q. W. Zhang, H. Matsumoto and F. Saito, Chem. Lett., 2001, 2, 148.
12 M. Poliakoff, J. M. Fitzpatrick, T. R. Farren and P. T. Anastas, Science,
2002, 297, 807.
cm21 22
or other organic functional groups. This indicates that the
)
degradation of PTFE in the supercritical water could be complete at
550 °C.
Based on the results of the IR spectra (Fig. 3(c)), the
defluorination of PTFE can hardly take place at 400 °C, indicating
that the reaction temperature plays an important role in the
decomposition. To our knowledge, PTFE could degrade to
monomers (96%) at 510 °C with a half life of ~ 30 min (eqn. (1))4
in an isolated atmosphere. Therefore, it is probable that PTFE can
decompose to monomers in our reaction system at 550 °C. A
possible reaction process is proposed as follows. Firstly, the main
chain scission of PTFE takes place as the temperature increases.
Subsequently, at 500–600 °C, the produced monomer C2F4 is
converted into CaF2, carbon clusters and CO2 in the supercritical
alkali Ca(OH)2 fluid (eqn. (2)), following a disproportionation
reaction pathway. Eventually, the carbon clusters grow into
13 P. S. Savage, Chem. Rev., 1999, 99, 603.
14 A. Kubátová, A. J. M. Lagadec and S. B. Hawthorne, Environ. Sci.
Technol., 2002, 36, 1337.
15 J. M. Desimone, Science, 2002, 297, 799.
16 Y. Sato, K. Kato, Y. Takeshita, K. Takahashi and S. Nishi, Jpn. J. Appl.
Phys. 1, 1998, 37, 6270.
17 H. Ismail, P. K. Freakley and E. Sheng, Eur. Polym. J., 1995, 31,
1049.
18 E. Auer, A. Freund, J. Pietsch and T. Tacke, Appl. Catal., 1998, 173,
259.
19 S. Flandrois and B. Simon, Carbon, 1999, 37, 165.
20 T. Jawhari, A. Roidd and J. Casado, Carbon, 1995, 33, 1561.
21 J. S. Qiu, Y. F. Li, Y. P. Wang, C. H. Liang, T. H. Wang and D. H.
Wang, Carbon, 2003, 41, 767.
Fig. 3 Raman shifts of amorphous carbon nanospheres (a); IR spectra of the
samples prepared at 400 °C (b) and 550 °C (c).
22 S. T. Li, E. Arenholz, J. Heitz and D. Bauerle, Appl. Surf. Sci., 1998,
125, 17.
C h e m . C o m m u n . , 2 0 0 4 , 3 4 2 – 3 4 3
343