An Improved Pyrolytic Route to [60]- and [70]-Fullerene

1
An Improved Pyrolytic Route to 60- and
70-Fullerene
N.R. Conley, J.L. Pearce, and J.J. Lagowski,
Department of Chemistry and Biochemistry, The
University of Texas at Austin, Austin, TX 78712
Telephone and FAX [phone], e-mail
Lagarto2_at_aol.com
Nick Conley
Jennifer Pearce
Prior to the start of an experiment, the
apparatus was flushed with argon for five
minutes. Once this was complete, the argon
flowrate was lowered to approximately 2 ml/min
and the tube furnace was activated. This
decreased flow rate prevented premature pyrolysis
of the precursor while maintaining positive argon
pressure through the system until the tube
furnace reached 1200?C. After the tube furnace
stabilized at 1200 ºC, as indicated by the
built-in thermocouple, the flowrate was increased
to 15 ml/min and the variac controlling the
heating tape was activated to deliver between 50
and 90 volts. Pyrolysis occurred over
approximately 15 minutes and upon completion, the
tube furnace was rapidly cooled to room
temperature with compressed air. By rapidly
decreasing the temperature, potential thermal
destruction of any fullerenes remaining in the
silica tube was avoided. The pyrolysate was
removed from the apparatus by sonication with
toluene. The slurries from the tube and coils
were combined and additional toluene was added to
increase the volume to approximately 100 ml.
This mixture was subject to reflux for thirty
minutes. After reflux, the hot mixture was
gravity-filtered and the excess toluene of the
filtrate was removed under vacuum to give a
saturated solution. These extracts were subject
to mass spectral analysis within 24 hours of
their preparation using a Finnigan Mat TSQ700
instrument the source temperature was
approximately 150?C CH4/3 torr, negative ions
being detected with a chemical ionization
technique.
Abstract
extract from soot produced in the pyrolysis of
1-bromonaphthalene, also contains signals with
m/z ratios and isotopic distributions indicative
of 60- and 70-fullerene these species appear
at more than ten times the intensity than in
Figure 1. Also predominant in Figure 2 is a
species with m/z 447. Based on the isotopic
distribution (M2), it appears to be a
monobrominated species. We believe it may play a
significant role in the pyrolytic formation of
60- and 70-fullerene and are pursuing further
work in its separation and characterization.
The nickel-catalyzed pyrolysis of two fullerene
precursors(1) naphthalene and (2)
1-bromonaphthaleneat 1200 ºC in an argon
atmosphere has been investigated. Fullerenes,
polycyclic aromatic hydrocarbons (PAH), and
polycyclic aromatic brominated species (the
latter obtained in pyrolysis of
1-bromonaphthalene only) were extracted from the
pyrolysates by reflux in toluene. The toluene
extracts were subject to mass spectrometric
analysis using the chemical ionization technique
in the negative mode. Mass spectra are included
with a discussion of the relative fullerene
yields.
Conclusion
Introduction
Fifteen years after the initial discovery of
fullerenes, their high cost continues to present
a significant challenge for many laboratories
interested in fullerene research. Some of these
laboratories have worked around this cost by
setting up their own fullerene reactors and
undertaking the arduous task of separation.
Laboratories with a more limited budget have
simply avoided the field altogether. For this
reason, a great deal of attention has been
focused on the cost-efficient preparation of
fullerenes. Our work indicates that
1-bromonaphthalene is a more suitable fullerene
precursor than naphthalene in contrast to results
reported by Crowley, et al.8 and we believe it
forms the basis of a viable approach to the
continuous production of fullerenes. We are
currently investigating the effects of directly
introducing a halogen into the pyrolysis system
to serve as a hydrogen scavenger.
Following the discovery of buckminsterfullerene
in 19851 and the subsequent publication of the
Krätschmer-Huffmann fullerene synthesis2 in
1990, the first macroscopic preparation of this
novel molecule, an explosion of publications
marked the advent of a new chemistry. Among
these publications, several alternative methods
of fullerene synthesis emerged. In 1991, Howard
et al. identified fullerenes in the mass spectrum
of toluene-extracted soot from hydrocarbon
combustion, triggering several publications on
optimization of fullerene yields in combustion3
and the mechanism by which fullerenes are formed
in flames4-6. In 1993, Taylor et al. described
the pyrolytic conditions in which 60- and
70-fullerene could be produced from
naphthalene7. This work led to the discovery
that corannulene and benzokfluoranthene can
also serve as pyrolytic precursors to
fullerenes8. Most recently, Osterodt et al.
demonstrated that a large variety of hydrocarbons
and cyclopentadienide-metal complexes produce
very small amounts of fullerenes when subject to
pyrolysis9. While few authors agree upon a
mechanism for pyrolytic fullerene formation, all
realize that at some point, carbon-hydrogen bonds
of the hydrocarbon must be broken and new
carbon-carbon bonds must form in their place.
Substantial theoretical and experimental evidence
implicates halonaphthalene derivatives as good
pyrolytic precursors to fullerenes because of the
tendency for homolytic cleavage of carbon-halogen
bonds at high temperatures. The manner and ease
of this cleavage provides several advantages.
First, the naphthalenyl radical may serve as a
site for ring cyclization. Hagen et al. describe
this phenomenon in the pyrolytic synthesis of
bowl-shaped PAH10. Next, the resulting halogen
radical can serve as a hydrogen scavenger to
expedite dehydrogenation of the naphthalene and
larger fullerene intermediates, as observed in
chlorine-doped flames11. Finally, the energy
required to break a carbon-halogen bond
homolytically, especially a carbon-bromine bond
in 1-bromonaphthalene (69.2 kcal/mol)12, is
significantly less than the energy required to
break a carbon-hydrogen bond in naphthalene
(109.5 kcal/mol)13,14 in the same manner.
There is a slight discrepancy in the values
reported for bond dissociation energy in the
formation of a 1-naphthalenyl radical from
naphthalene to account for this, we use the
average value for our arguments. Thus, the
activation barrier for formation of the
1-naphthalenyl radical is 40.3 kcal/mol lower if
a bromonaphthalene derivative is used in place of
naphthalene. The pyrolysis of 1-chloro- and
1-bromonaphthalene has already been carried out
by Crowley et al 8. Based upon peak
intensities in the mass spectrum, this group has
estimated lower fullerene yields in the pyrolysis
of these derivatives compared to naphthalene. We
have chosen to investigate naphthalene and
1-bromonaphthalene as pyrolytic precursors to
60- and 70-fullerene under different
conditions than previously reported7,8.
Results and Discussion
Mass spectrometric analysis of the toluene
extract from soot obtained in the pyrolysis of
naphthalene (Figure 1) suggests the presence of
60- and 70-fullerene, m/z720 and 840
respectively. The observed isotopic
distributions of these species are consistent
with those previously reported15. Under these
conditions however, fullerenes seem to comprise
only a small portion of the product. The mass
spectrum in Figure 2, analysis of the toluene
Acknowledgement
We gratefully and humbly acknowledge the support
of this research by the Robert A. Welch
Foundation.
References
1 Kroto H. W., Heath J. R., OBrien S. C., Curl
R. F., Smalley R. E. Nature 1985318162-163. 2
Krätschmer W., Lamb L. D., Fostiropoulos K.,
Huffmann D. R. Nature 1990347354-358. 3 Howa
rd J. B., McKinnon J. T., Makarovsky Y., Lafleur
A. L., Johnson M. E. Nature 1991352139-141. 4
Ahrens J., Bachmann M., Baum Th., Griesheimer
J., Kovacs R., Weilmünster P., Homann K.-H. Int.
J. Mass Spectrom. Ion Processes
1994138133-148. 5 Bachmann M., Griesheimer
J., Homann K.-H. Chem. Phys. Letters
1994223506- 510. 6 Grieco W. J., Lafleur A.
L., Swallow K. C., Richter H., Taghizadeh K.,
Howard J.B. Fullerenes and PAH in low-pressure
premixed benzene/oxygen flames. Twenty-Seventh
Symposium (International) on Combustion,
Pittsburgh (Pennsylvania, USA) The Combustion
Institute, 1998 1669- 1675. 7 Taylor R.,
Langley G. J., Kroto H. W., Walton D. R. M.
Nature 1993366728- 731. 8 Crowley C., Taylor
R., Kroto H.W., Walton D.R.M., Cheng P., Scott
L.T. Synthetic Metals 19967717-22. 9 Osterod
t J., Zett A., Vögtle F. Tetrahedron
1996524949-4962. 10 Hagen S., Bratcher M. S.,
Erikson M. S., Zimmermann G., Scott L. T. Angew.
Chem. Int. Ed. Engl. 199736(4)406-408. 11 Ric
hter H., Hoffmann E., Doome R., Fonseca A.,
Gilles J.-M., Nagy J.B., Thiry P.A., Vandooren
J., Van Tiggelen P.J. Carbon 199634(6)797-803.
12 Ladacki M., Szwarc M. J. Chem. Phys.
1952201814-1816. 13 Cioslowski J., Liu G.,
Martinov M., Piskorz P., Moncrieff D. J. Am.
Chem. Soc. 19961885261-5264. 14 Reed D. R.,
Kass S.R. J. Mass Spectrom. 200035(4)534-539. 1
5 McElvany Stephen W., Callahan John H. J. Phys.
Chem. 199195(16)6186- 6191.
60fullerene
70-fullerene
Fig. 2 Mass spectrum of toluene extract from
soot produced in the pyrolysis of
1-bromonaphthalene.
Fig.1 Mass spectrum of toluene extract from soot
produced in the pyrolysis of naphthalene.
thermocouple display and probe (inside furnace)
flowmeter
Experimental
glass coils
tube furnace
argon cylinder
Details of the pyrolysis apparatus are reported
elsewhere 8. In a typical experiment, 0.5
grams of nickel powder (2.2-3.0 micron) was first
spread throughout the portion of the silica tube
to be heated. The tube was placed inside of a
tube furnace Thermolyne Type 21100, 40 cm heated
zone and declined at a 25º angle to prevent the
fullerene precursor from condensing at the inlet
of the tube. The precursor (0.5 g) was then
introduced into the well via the inlet of the
tube. A hose through which argon gas flowed was
clamped onto the inlet of the pyrolysis tube and
heating tape Barnstead Thermolyne, 104 W, 61 cm
in length was wrapped around the well up to the
entrance of the tube furnace. The coils were
attached to the tube and submerged in a dry
ice/isopropanol slush to prevent loss of the more
volatile pyrolysate. A toluene bubbler was
placed at the end of the coils.
silica tube
sample well
toluene bubbler
dry ice / isopropanol slush
heating tape
variac
Fig. 3 Set-up of the pyrolysis apparatus.