ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 6, pp. 1031 1034.
Original Russian Text G.G. Shelopin, D.S. Pashkevich, Yu.I. Alekseev, D.A. Mukhortov, V.B. Petrov, V.S. Asovich, 2006, published in Zhurnal
Prikladnoi Khimii, 2006, Vol. 79, No. 6, pp. 1040 1042.
Pleiades Publishing, Inc., 2006.
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Synthesis of Perfluoroalkanes in High-Temperature
Fluorination of Graphite with Fluorine in a Reactor with
a Free-Falling Graphite Bed
G. G. Shelopin, D. S. Pashkevich, Yu. I. Alekseev, D. A. Mukhortov,
V. B. Petrov, and V. S. Asovich
Prikladnaya Khimiya Russian Scientific Center, Federal State Unitary Enterprise, St. Petersburg, Russia
Received November 9, 2005; in final form, February 2006
Abstract Reaction between fluorine and graphite in a reactor with a free-falling bed of graphite was studied
in relation to the temperature in the reaction zone, ratio of the feeding rates of fluorine and graphite, and dilu-
tion of fluorine with an inert gas.
DOI: 10.1134/S1070427206060310
Tetrafluoromethane, hexafluoromethane, octafluoro-
methane, and decafluoromethane are large-tonnage
products used in etching of silicon in semiconductor
industry [1].
the zones of synthesis and thermal decomposition of
carbon polyfluoride [2]. The reactor with an ascending
gas-dust flow also has a number of disadvantages: it
is necessary to use an inert gas (approximately nine
volumes per volume of fluorine) to create as gas-dust
flow; it is virtually impossible to vary the feed rate
of graphite; there occur uncontrollable disruptions of
powder ejection in the device forming the gas-dust
flow.
The present paper reports on a continuation of
previous studies [2, 3], whose main goal was to search
for conditions of carbon fluorination in which a mix-
ture perfluoroalkanes is produced and this mixture
contains hexafluoroethane, octafluoropropane, and
decafluorobutane in commercial amounts.
Therefore, it was suggested to examine the possi-
bility of using a reactor with a free-falling powder
bed to obtain perfluoroalkanes by fluorination of car-
bon, as it has been done in synthesis of sulfur tetra-
fluoride from elements [4]. In this reactor, powdered
graphite is delivered by a screw conveyor into the up-
per part of a hollow vertical tube, within which graph-
ite particles move in the free-fall mode. Fluorine is
also fed into the upper part of the tube. The reaction
between fluorine and graphite (which is present in
a large excess) yields carbon polyfluoride at a com-
plete conversion of fluorine. Thermal decomposition
of carbon polyfluoride is performed in the fixed pow-
der bed in the lower part of the reactor.
A study of graphite fluorination with a mixture of
fluorine and nitrogen (or argon) in a fluidized bed and
in an ascending gas-dust flow [2, 3] led to a conclu-
sion that synthesis of CF4, C2F6, C3F8, and C4F10 at
a temperature of about 600 C occurs by a two-stage
scheme: synthesis of a solid carbon polyfluoride
(CFx)n and thermal decomposition of (CFx)n. The com-
position of the products formed in thermal decomposi-
tion of carbon polyfluoride depends on whether or
not gaseous fluorine is present in the decomposition
zone. An increase in the amount of gaseous fluorine in
the decomposition zone makes smaller the fraction of
fluorocarbons with a longer carbon chain. Therefore,
spatial separation of the zone in which fluorine reacts
with graphite to give carbon polyfluoride and the zone
in which carbon fluoride undergoes thermal decom-
position leads to an increase in the fraction of fluoro-
carbons with the number of carbon atoms greater than
unity in the gaseous reaction products.
The process in a reactor with a free-falling bed
has a number of advantages over that in reactor with
a fluidized bed or a gas-dust flow. The most important
distinctive features of the reactor with a free-falling
bed are as follows: reagents are delivered separately
and in controlled way by a screw conveyor [5], which
should provide stable formation of the gas-dust flow
(falling bed) of prescribed composition; the zones in
In performing the fluorination of graphite in a flu-
idized bed, it is impossible to spatially separate
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