Synthesis, separation, and characterization of fullerenes and their chlorinated fragments in the glow discharge reaction of chloroform

Article abstract of DOI:10.1021/jp002821w

Fullerenes and various chlorinated carbon clusters were synthesized via the glow discharge reaction using chloroform vapor as starting reactant. High-performance liquid chromatography combined with ultraviolet spectrometry and mass spectrometry (HPLC-UV-M

Full text of DOI:10.1021/jp002821w

1734
J. Phys. Chem. B 2001, 105, 1734-1738
Synthesis, Separation, and Characterization of Fullerenes and Their Chlorinated Fragments
in the Glow Discharge Reaction of Chloroform
Su-Yuan Xie, Rong-Bin Huang, Shun-Liu Deng, La-Jia Yu, and Lan-Sun Zheng*
State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen UniVersity,
Xiamen, China 361005
ReceiVed: August 8, 2000; In Final Form: December 7, 2000
Fullerenes and various chlorinated carbon clusters were synthesized via the glow discharge reaction using
chloroform vapor as starting reactant. High-performance liquid chromatography combined with ultraviolet
spectrometry and mass spectrometry (HPLC-UV-MS) was developed for separation and characterization
of the reaction products, comprising C60 and C70 fullerenes, amorphous carbon, and more than 50 chlorinated
carbon clusters, which molecular formulas were determined from their special isotopic patterns. The formation
of fullerenes and amorphous carbon in the glow discharge reaction of chloroform was found to involve different
systems of chlorinated carbon cluster intermediates. The correlation provides insight into the formation of
fullerenes and the other carbon clusters.
Introduction
Fullerenes can be accessed via thermal reaction of appropriate
carbon sources under various conditions, for instance, arc-
discharge of graphite rods,1 incomplete combustion of benzene
or hydrocarbons,2 pyrolysis of naphthalene,3 and dissociation
of hydrocarbons in thermal plasma.4-7 Efforts to develop
traditional organic methods to synthesize fullerenes have not
achieved success yet, but various bowl-shaped hydrocarbons
have been produced by pyrolytic or nonpyrolytic routes.8-22 In
our prior investigation,23 fullerenes C60 and C70 were produced
from chloroform in a glow discharge reaction, known as a
Figure 1. Scheme of the glow discharge reactors.
typical low-temperature plasma.24 It was found in the experiment
chambers, two glass tubes with diameter of ca. 40 mm and
length of ca. 250 mm. Two pairs of copper pipes, acting as
both electrodes and gas passageway, were mounted to two ends
of each tube. When the vacuum pressure of the tubes was
pumped to less than 0.004 MPa and an alternate voltage of above
10 kV with 25 kHz frequency was applied to the electrodes, a
stable glow discharge would emerge at the gap between the
two electrodes. The glow plasma could be maintained by
adjusting the distance between the electrodes. In the synthetic
experiment, the two reactors, labeled in the figure as reactor I
and reactor II, were connected in series. After reaction for
several hours, soot produced from the discharge reaction was
deposited on the wall of each reactor and then collected
separately from the two reactors, followed by separation and
identification using HPLC-UV-MS.
Effects of various experimental parameters on the glow
plasma were investigated in the experiment. It was found in
the preliminary experiment that glow plasma was very sensitive
to the reaction conditions, such as vacuum of the reactor, feed
rate of the chloroform vapor, and electrical voltage and
frequency. Relatively high vacuum in the reactor was necessary
for the plasma reaction. When the pressure was greater than
0.01 MPa, the plasma could not be maintained. Raising feed
rate of the chloroform vapor would improve the reaction
efficiency but with the cost of disturbing the plasma. In fact,
the plasma would die out when feed rate of the reactant was
higher than 0.6 mL/min. Yield of the soot was also found to
that the fullerenes were accompanied by various fully chlori-
nated fullerene fragments such as perchloroacenaphthylene (C12-
Cl8) and perchlorofluoranthene (C16Cl10), which carbon frame-
works composed of six- and five-membered rings can be
identified as part of the surface of fullerenes. Herein the synthetic
reaction and analysis of the products from the glow plasma
reaction of chloroform are reported in detail. High-performance
liquid chromatography coupled with both ultraviolet spectrom-
etry and mass spectrometry (HPLC-UV-MS) was developed
for separation and characterization of the fullerene products as
well as a series of chlorinated carbon clusters. Special attention
was paid to the correlation between the fullerenes and the
chlorinated carbon clusters formed in the same reaction. The
experimental observation reported herein is not only syntheti-
cally useful but may also provide some important insight into
fullerene formation.
Experimental Section
Chemicals. Solvents used in the experiment were com-
mercially available. Methanol was HPLC grade, and other
solvents were analytical grade. All HPLC solvents were further
distilled and degassed by vacuum filtration over a 0.45 µm
membrane filter prior to use.
Soot Production. The experimental setup is illustrated in
Figure 1. Chloroform was evaporated from a flask into reaction
* Corresponding author. E-mail: lszheng@xmu.edu.cn.
10.1021/jp002821w CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/08/2001

Fullerenes and Their Chlorinated Fragments
J. Phys. Chem. B, Vol. 105, No. 9, 2001 1735
depend on the electrical field applied in the reaction, especially
its frequency. Increase of the frequency generally resulted in
the raise of the soot yield. The optimum conditions used for
the experiment were as follows: vacuum, 0.004 Mpa; feed rate
of the chloroform vapor, 0.3 mL/min; electrical frequency, 25
kHz. Under these conditions, the soot could be produced in a
yield of 3 g/h.
Sample Preparation. Approximately 500 mg of the soot
from the glow discharge reaction of chloroform and 5 mL of
toluene were placed in a glass container with a cap. This
suspension was extracted using ultrasonic bath at room tem-
perature for 1.5 h, allowed to settle for about 2 h, and then
filtered using a 0.45 um pore membrane filter. The solution was
loaded into the instruments for HPLC-UV-MS analysis.
HPLC-UV-MS Analysis. The instruments used for HPLC-
UV-MS analysis were a TSP model P200 HPLC equipped with
a TSP UV 3000 spectrophotometer and a Finnigan LCQ model
mass spectrometer with APCI interface.
The TSP UV 3000 instrument consisted of rapid scan UV
spectrophotometric detection. This technique, involving the
continuous acquisition of UV spectra, is highly sensitive to
characterize fullerenes and the polycyclic carbon clusters, as
peaks eluted from the HPLC column. Absorption spectrum was
recorded over 210-400 nm wavelength range. Scan resolution
and data collection rates of the spectrometer were fixed at 5
nm and 6.7 Hz, respectively.
The Finnigan LCQ mass spectrometer consisted of an
atmospheric pressure chemical ionization (APCI) source and
an ion-trap mass analyzer. The APCI vaporization temperature
was 400 °C. Mass spectra were recorded within the range of
m/z 200-2000 in negative ion mode. Flow rates of sheath gas
and auxiliary gas were 70 and 20 mL/min, respectively.
Capillary temperature and voltage were 200 °C and -28 V,
respectively.
Figure 2. Typical HPLC-UV chromatogram of products collected
in reactor I and recorded at 300 nm wavelength. Peaks are labeled by
numbers, and their molecular formula are suggested from their isotope
distributions in mass spectra as the following: 1, C5HCl5; 2, C58Cl12;
3, C8Cl8; 4, C6Cl6; 5, C8H3Cl5; 6, C14Cl14; 7, C8Cl6; 8, C22Cl14; 9, C10-
Cl8; 10, C12Cl8; 11, C14Cl14; 12, C18Cl14; 13, C14Cl8; 14, C28Cl16; 15,
C20Cl14; 16, C12Cl12; 17, C24Cl14; 18, C14Cl10; 19, C14Cl12; 20, C12Cl8;
21, C30Cl14; 22, C20Cl14; 23, C51Cl12; 24, C22Cl16; 25, C16Cl10; 26, C18C14;
27, C12Cl14; 28, C14Cl8; 29, C30Cl14; 30, C22Cl12; 31, C26Cl14; 32, C16-
Cl10; 33, C26Cl12; 34, C22Cl14; 35, C20Cl14; 36, C50Cl12; 37, C20Cl10; 38,
C20Cl14; 39, C18Cl10; 40, C28Cl10; 41, C60(IV), 42, C32Cl12; 43, C26Cl10;
44, C30Cl10; 45, C50Cl10; 46, C36Cl10; 47, C26Cl10; 48, C60(III); 49, C24-
Cl12; 50, C22Cl12; 51, C60Cl8; 52, C24Cl12; 53, C18Cl10; 54, C34Cl12; 55,
C60(II); 56, C26Cl10; 57, C28Cl10; 58, C60(I); 59, C32Cl10; 60, C28Cl10;
61, C70.
Combination of the instruments was achieved by simply
connecting the outlet of the HPLC detector to the inlet of the
APCI interface of the mass spectrometer. The HPLC-UV data
and the HPLC-MS data were collected by different computer
systems.
The HPLC column selected for analysis was a SUPELCOSIL
LC-18 column with 4.6 mm i.d. and 250 mm in length, packed
with 5 µm octadecylsiyl-bonded silica having a 300 Å pore size.
Nonaqueous reversed-phase HPLC was selected considering the
low volatility and high lipophilicity of fullerenes and chlorinated
carbon clusters. To record UV spectra of the reaction products,
the mobile phase needs to be transparent or have low absorption
in the wavelength region of 210-400 nm. Thus, only a few
solvents, such as methanol, ethanol, acetonitrile, tetrahydrofuran,
hexane, and cyclohexane, could satisfy the requirements. Among
the selections, low-polar solvent was good for the solution of
the products but lack of retention, while some products from
glow plasma of chloroform could not elute out the octadecylsiyl-
bonded silica column thoroughly when polar solvents were
applied. Hence, a binary or multiple mobile phase mixed by
polar and low-polar solvents in a certain ratio might be a better
choice. After testing of various combinations of the mixtures,
it was found that mixed solvents of cyclohexane/ethanol/
methanol could be used as mobile phase for the separation in a
gradient elution mode. The elution procedure of the mobile
phase consisted of a linear increase in cyclohexane concentration
from 0 to 5% in 40 min and from 5% to 35% in the next 180
min and a linear decrease in methanol from 85 to 80% in 40
min and from 80% to 54% in the next 180 min. Ethanol was
premixed with methanol in a 2:11 ratio, which was not varied
Figure 3. UV adsorption spectra of C60 and C70 fullerenes.
during the elution process. Flow rate of the mobile phase was
set at 1.0 mL/min, and sample injection was quantified with a
20 µL loop.
Results and Discussion
Production and Identification of C60 and C70 Fullerenes.
The toluene extract of the products from glow discharge reaction
of chloroform was analyzed by HPLC-UV-MS under the
optimum conditions, and a typical chromatogram of products
collected from reactor I is shown in Figure 2. It could be found
in the chromatogram that the products from the reaction were
quite complicated, but most of them, C60 and C70 fullerenes as
well as various chlorinated carbon clusters, can be separated
under the HPLC analysis condition mentioned above. As shown
in Figure 3, the UV spectra obtained at retention times of 167
and 209 min were virtually identical to those of standard C60
and C70.1,25 Identities of C60 and C70 fullerenes suggested by
the UV spectra were also confirmed by their retention time after
comparing with those of the standard ones and by the mass
distributions of their molecular ions observed in corresponding
mass spectra. The analysis result showed that fullerenes could
be synthesized in the glow discharge reaction of chloroform
under a relatively low-temperature plasma condition.

1736 J. Phys. Chem. B, Vol. 105, No. 9, 2001
Xie et al.
Figure 4. Selected HPLC-MS ion chromatogram of possible C60
isomers in the range of m/z 720-722.
In comparison to what has been achieved by arc discharge
methods,1 fullerene yield by this synthetic reaction was relatively
low. However, the synthesis reported herein was a preliminary
study and various parameters remain to be further optimized
for fullerene production. In addition, the synthetic reaction could
be operated continuously so that the device could be easily
scaled up. Especially, the reaction was performed in a low-
temperature plasma that differs from other synthetic methods,1-7
so the synthetic reaction merits special attention in fullerene
science.
As shown in Figure 4, the selected-ion chromatogram of the
products collected in reactor I in the ranges of m/z 720-722,
besides the dominant peaks corresponding to buckminster-
fullerene, three other 720 amu ions were also observed at the
retention times of 92, 108, and 155 min. These ions are
suggested as novel C60 isomers according to their isotope
distributions in the mass spectra. These mass spectra evidences,
however, might also result from C60 adducts (e.g. with chlorine),
on the basis of the hypothesis that such adducts formed in the
Figure 5. Isotope distributions of selected chlorinated carbon clusters
recorded in mass spectra: (a) C58Cl12 (no. 2); (b) C12Cl8 (no. 20); (c)
C22Cl14 (no. 34); (d) C18Cl10 (no. 39). Simulated distributions of their
molecular ions peaks are inserted at the right of the figures.
-
glow plasma reaction but dissociated to be C60 during the
ionization process. In fact, this evidence had also been observed
in HPLC-MS analysis of products from benzene-oxygen
flames26,27 and microwave plasma reaction of chloroform.28
Unfortunately, the HPLC-MS characterization is insufficient
to determine their structures. Our effort focusing on preparative
separation and structural characterization of these newly ob-
served carbon clusters with 720 amu is in process.
recorded in the experiment. Simulated distributions of their
molecular ions peaks are included in the figures for better
identification. In addition to the molecular ions peaks, ions peaks
with weights 19 less than their molecular weights were also
observed in the figures, and they might result from the clusters
losing a chloride atom and capturing an oxygen anion in the
APCI source.31
Correlation between Fullerenes and Chlorinated Carbon
Clusters. All the products in the reaction, fullerenes, amorphous
carbon, and chlorinated carbon clusters, were created under the
same reaction condition and from the same starting species,
chloroform, a simple molecule composed of a single carbon
atom. It was thus reasonable to conjecture that the small carbon
clusters, which were chlorinated in the reaction, might be the
precursors of larger carbon species such as fullerenes and
amorphous carbons. Most of the small carbon clusters, however,
could not grow up in the reaction. As shown in Figure 2, the
relative abundance of the products is generally reduced with
an increase of their carbon number. In fact, the small carbon
clusters initially produced in the plasma were generally very
reactive. To survive the plasma, their dangling bonds need to
be saturated by radicals such as chlorine atoms which are also
produced in great amount in the glow plasma. The growth of
Production and Identification of Chlorinated Carbon
Clusters. Fullerene formation in the glow discharge reaction
of chloroform is a growth process from small clusters, analogous
to the growth of polycyclic aromatic hydrocarbons.29,30 In fact,
in addition to fullerenes and amorphous carbon, other carbon
clusters with intermediate sizes were also produced in the
reaction, but most of them were chlorine-substituted to eliminate
the dangling bonds. Luckily, mass spectrometry is effective in
analyzing the compositions of these chlorinated products because
the two isotopes of chlorine have their special abundance
distribution: 35Cl:37Cl ) 75.77:24.23. Thus, isotopic distribu-
tions of the compounds containing different numbers of chlorine
atoms can be calculated. By comparison of the simulated
distribution with the recorded mass spectrum, the number of
chlorine atoms and the number of carbon atoms in each
chlorinated carbon cluster can be readily determined, and thus
its molecular formula characterized. As examples, Figure 5
shows typical isotope distributions of selected chlorinated carbon
clusters, C58Cl12, C12Cl8, C22Cl14, and C18Cl10, which were

Fullerenes and Their Chlorinated Fragments
J. Phys. Chem. B, Vol. 105, No. 9, 2001 1737
Figure 7. Growth scheme of fullerene and graphite. C60, graphite, and
chlorinated carbon clusters shown in the figure were the products of
the glow discharge reaction. Molecular formulas labeled in the figure
were characterized by their isotope distributions in corresponding mass
spectra. Structures of the small perchlorinated carbon clusters, such as
C10Cl8, C12Cl8, C16Cl10, and C20Cl10, had been characterized before, and
structures of the large clusters, such as C20Cl14, C28Cl10, C30Cl14, and
C36Cl10, were suggested according to their special compositions, while
their structural identification is still in process.
terization of their structures is still in progress. In these
structures, pentagons are incorperated into their polycyclic
networks and cause curvature so that C-Cl bonds of these
clusters are minimized and generally do not increase with the
number of carbon atoms. Since all these chlorinated clusters
and the fullerenes grew from chloroform in a same reaction,
their formation may follow the same reaction mechanism:
adding C2 or other small carbon particles to the developing
graphite sheet. The dangling bonds on the rim of the carbon
framework were saturated by chlorine atoms, and the closed
fullerenes, once formed, have no open edges and, therefore,
generally do not grow any further. The growth mechanism is
just the one described by the “pentagon road” scheme,33 one of
the schemes that have been proposed for fullerenes formation.
Hence, this system of chlorinated carbon clusters can be
regarded as fullerene precursors or chlorinated intermediates in
fullerenes growth following the scheme of “pentagon road”.
Besides the carbon clusters derived with 8-12 chlorines, the
clusters comprising more chlorine atoms were also produced
during the reaction in a great variety and were found to be more
abundant in products B. In contrast to the clusters described
above, the number of chlorine atoms in this system of clusters
increases with their number of carbons. The compositions
suggested that frameworks of this system of polycyclic carbon
clusters were planar and very likely the fragments of the
graphite. Higher yield of the observed amorphous carbons in
products B, which were in fact the microcrystals of graphite,
supports this hypothesis. A scheme of growth pathways of the
two elementary carbon species is illustrated in Figure 7, but it
should be noted that many structures of the chlorinated carbon
clusters pictured here were speculated on the basis of their
compositions.
Figure 6. HPLC-MS chromatograms of the reaction products: (a)
products A collected in reactor I; (b) products B collected in reactor
II.
most small carbon clusters is thus stopped before they reach
larger aggregates, fullerenes, or amorphous carbons, which have
no dangling bonds at all.
It would be more interesting to distinguish the precursors
leading to the fullerenes and other carbon species. In the
experiment, two reactors, labeled as reactors I and II in Figure
1, were connected in series. The products from reactors I and
II were collected separately and were distinguished as products
A and B, respectively. As shown in Figure 6a,b, the HPLC-
MS chromatograms of products A and B, respectively, the
species forming in the two reactors were similar, but their
relative yields were significantly different. The fullerene yield
in products B was markedly less than that in products A. Further
quantitative analysis indicated that C60 yield in products A was
0.5% and that in products B was less than 0.05%. On the other
hand, in reactor II, production of amorphous carbon, which was
identified by X-ray diffraction analysis, was much higher than
that in reactor I. Similarly, chlorinated carbon clusters with
different compositions were also produced in different yields
in the two reactors. Generally, as shown in Figure 6, the clusters
produced in reactor I tend to consist of less chlorine atoms than
those produced in reactor II. We thus supposed that there might
be two systems of small carbon clusters produced in the
experiment, and they displayed different relative abundances
in the final products grown in the two reactors.
In the glow discharge experiment, the reactions in the two
reactors were carried out under the same vacuum and sharing
the same electrical power source but were fed different starting
reactants. In reactor I, the reactant was chloroform vapor, while
the reactants in reactor II were composed of variety of volatile
chlorinated carbon species formed in the preceding reactor, such
as C4 or C6.34 The experimental evidence suggested the
possibility of designing the proper precursors so as to improve
fullerene yield in the synthetic reaction. Our effort on this
purpose is currently in process.
Different distributions of the chlorinated carbon clusters
produced in the different reactors might be attributed to their
different structures. As shown in Figure 6a, most of chlorinated
carbon clusters with higher abundance produced in reactor I
was composed of 8-12 Cl atoms and even numbers of carbon
atoms. Among them, structures of small clusters, such as C12-
Cl8, Cl16Cl10, and C20Cl10, have been characterized before.23,32
Their carbon frameworks, comprising six- and five-membered
rings, could be identified as part of the surface of buckmin-
sterfullerene. The larger clusters in the series which also
exhibited higher abundance in products A, such as C18Cl10, C28-
Cl10, C36Cl10, and C58Cl12, might be chlorinated fullerene
fragments as well, according to their compositions and the
structures of their hydrocarbon analogues,12-22 though charac-
Acknowledgment. The work was supported by the National
Natural Science Foundation of China (Grant Nos. 2989021 and
No. 20001005).

1738 J. Phys. Chem. B, Vol. 105, No. 9, 2001
Xie et al.
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