Synthesis of tetrafluoromethane by graphite fluorination with elemental fluorine

Article abstract of DOI:10.1023/B:RJAC.0000024584.69292.6d

Formation of high-temperature inverse wave of the filtration combustion of graphite fixed bed in fluorine was studied. Scientific principles of the industrial process of the tetrafluoromethane synthesis from graphite and fluorine were developed.

Full text of DOI:10.1023/B:RJAC.0000024584.69292.6d

Russian Journal of Applied Chemistry, Vol. 77, No. 1, 2004, pp. 92 97. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 1,
004, pp. 96 101.
Original Russian Text Copyright
2
2004 by Pashkevich, Mukhortov, Petrov, Alekseev, Asovich, and Barabanov.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Synthesis of Tetrafluoromethane by Graphite Fluorination
with Elemental Fluorine
D. S. Pashkevich, D. A. Mukhortov, V. B. Petrov, Yu. I. Alekseev,
V. S. Asovich, and V. G. Barabanov
Prikladnaya Khimiya Russian Scientific Center, Federal State Unitary Enterprise, St. Petersburg, Russia
Received November 29, 2002; in final form, May 2003
Abstract Formation of high-temperature inverse wave of the filtration combustion of graphite fixed bed
in fluorine was studied. Scientific principles of the industrial process of the tetrafluoromethane synthesis
from graphite and fluorine were developed.
Tetrafluoromethane (TFM), CF , is widely used in
the industry for plasma-chemical production of in-
tegrated circuits, as refrigerant, flame inhibitor, etc.
1]. The annual world consumption of TFM is about
000 t today, which determines urgency of the devel-
The TFM formation in the fluorine reaction with
carbon is accompanied by a heat release (932 kJ mol )
4
1
sufficient for graphite heating up to the required tem-
peratures. Powdered graphite has effective heat con-
ductivity of about 10 2 W m 1 K . The filtration
combustion mode is realized when fluorine is passed
through a heated graphite bed. The combustion wave
has inverse structure, since graphite burns out com-
pletely, and all the reaction heat is distributed in the
graphite bed below the reaction zone along the gas
flow direction. In the inverse wave of the filtration
combustion, the zone of fluorine uptake is narrower
than the zone of highly heated graphite. Therefore,
unreacted graphite is subject to prolonged heating
before it appears in the reaction zone, and, as a result,
volatile impurities (e.g., sulfur) are eliminated. This
fact favors preparation of high-purity TFM. It should
be noted that, in the inverse wave of the filtration
combustion, fluorine breakthrough through the reac-
tion zone is excluded, and thus complete fluorine con-
version is provided [12].
[
1
1
opment of its commercial synthesis.
Various methods of TFM preparation are known:
fluorination of methane, its chloro and fluoro deriva-
tives, silicon carbide, and polyfluoroethylene with
elemental fluorine and fluorination of tetrachloro-
methane with various inorganic fluorides [1]. How-
ever, the synthesis from the elements is the most
promising; fluorine is a commercial product, and
electrode graphite, which is characterized by high
purity and appropriate physicomechanical properties,
can be used as a carbon material.
It was found [2 11] that, depending of fluorination
conditions and kind of carbon raw material, the prod-
ucts of fluorine reaction with carbon are liquid, solid,
and gaseous substances:
As mentioned above, at temperatures lower than
C(s) + F (g)
(CF) (s) + CF (g) + C F (g) + C F (g)
n 4 2 6 3 8
2
5
00 C, solid fluorocarbon (CF) is the single reaction
n
product [10, 11]. On heating above 500 C, it decom-
poses to form C F , C F , C F , etc. [10, 11].
+
C F (g) + C F (g) +
(1)
4
10
5 12
2
6
3 8
4 10
Therefore, to obtain pure TFM, the thermal interaction
of the graphite packing in the fluorine-containing area
with heat-bearing surfaces should be excluded. This
can be realized by feeding fluorine directly into the
graphite bed depth.
It was shown in [8, 9] that, at 1000 1500 C, TFM
is the single product of the fluorine reaction with
graphite. At lower temperatures, hexafluoroethane,
octafluoropropane, and higher-molecular-weight per-
fluorinated carbons are also formed. At the tempera-
tures lower than 500 C, fluorographite (CF) is the
However, at the reactor startup, fluorine is (in the
simplest case) fed into the graphite bed at room tem-
perature, and, in the course of formation of the high-
temperature synthesis wave, fluorographite initially
n
main product. At temperatures lower than 100 C,
fluorine does not react with graphite, as indicated
in numerous papers.
1
070-4272/04/7701-0092 2004 MAIK Nauka/Interperiodica

SYNTHESIS OF TETRAFLUOROMETHANE BY GRAPHITE FLUORINATION
accumulates, which then decomposes to form C F ,
93
2
6
C F , C F , etc. In addition, since fluorographite is
3
8
4 10
a thermolabile condensed compound, its accumulation
is undesirable from the viewpoint of safety of the
reactor functioning. Thus, the stage of formation in
the graphite packing with the initial temperature of
Atmosphere
1
0 20 C of a high-temperature wave (1000 1500 K)
in which the synthesis proceeds is the most important
for production of maximally pure CF in an industrial
4
reactor and the process safety.
Absorber (lime)
We found no studies of carbon ignition in fluorine.
Such important characteristics of the industrial process
as the time of ignition delay and time of attainment of
the reactor operating mode ensuring production of
pure TFM (at the content of fluorocarbon impurities
no more than 0.1%) are virtually unknown.
Scheme of the laboratory device for studying carbon
combustion in fluorine.
fluorinating agents, and ground VAZ grade graphite
(TU 48-20-54 84) produced by the MEZ (Moscow
Electrode Plant) Joint-Stock Company and a mixture
of 90% graphite and 10% activated birch charcoal
(ABC), as carbon raw materials.
In this work, we determined the conditions of for-
mation of a high-temperature wave of filtration com-
bustion of graphite or its mixtures with activated birch
charcoal in fluorine and a method for safe and reliable
reactor startup.
The first series of experiments was performed with
VAZ graphite and fluorine. The initial graphite and
graphite already used in high-temperature synthesis of
EXPERIMENTAL
CF were used. The results of measurements of the
4
To study the combustion process, we developed a
laboratory device (see figure).
ignition delay time as influenced by fluorine con-
sumption are listed in Table 1. The nozzle outlet
diameter d and the linear velocity of the fluorine flow
at the nozzle outlet also presented. It should be noted
that the high-temperature reaction zone could not be
formed with spent graphite.
A graphite powder was charged into a cylinder (1)
with an inner diameter of 68 mm and height of
1
64 mm. Fluorine was fed into the reactor through a
thick-walled copper nozzle (2). In the reactor volume
00 mm in diameter, Chromel Alumel thermocouples
7
The results of chromatographic analysis of the reac-
tion gas samples, qualitative monitoring of the fluor-
ine breakthrough, and measurement of temperature as
a function of time from the beginning of fluorine feed-
ing are given in Table 2.
T1 T3 without protective cases were placed. The
distance (along the vertical) between the nozzle outlet
and thermocouple junction was 14 mm, and the dis-
tance between the thermocouple junctions, 30 mm.
In the experiments, the constant fluorine consump-
tion was preset, and samples of the reaction gas for
chromatographic analysis on a katharometer were
taken from a sampler (3) with a syringe. In addition,
the fluorine breakthrough through the sampler (3) was
monitored. A KI aqueous solution was used as an
indicator.
It follows from Tables 1 and 2 that heat accumula-
tion and ignition proceed in the area between thermo-
Table 1. Time of ignition delay tid at different flow rates
WF2 and linear velocities UF2 of fluorine at the nozzle
outlet, d = 0.85 mm
W ,
cm s
F
1
Formation of the high-temperature zone of the syn-
thesis was judged from the thermograms obtained.
The time interval from the beginning of fluorine
introduction to attainment of the temperature of 400 C
near the junction of thermocouple T1 was considered
2
3
Raw material
Graphite
U , m s
t , s
1
F
2
id
40
71
48
79
46
71
26
540
No ignition
2
7
Graphite*
45
26
40
15
as the ignition delay time tid.
1
Graphite+ ABC
Graphite + ABC*
250
No ignition
Fluorine and an F + HF mixture were used as
2
1
This mixture simulates an anodic gas produced by the An-
garsk Electrolysis Chemical Combine.
* After use in high-temperature synthesis of CF .
4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 1 2004

94
PASHKEVICH et al.
Table 2. Composition of the reaction gas and temperatures of thermocouples T1 T3 at various times of the synthesis ts
and fluorine flow rates WF2
T,
C
Content, vol %
W
t ,
s
Breakthrough
F ,
s
Raw material
Graphite
2
3
1
of F2
cm s
T1
36
36
31
29
27
T2
T3
CF4
C F
C F
CO2
C F
3 8
2 6
2 4
40
83
165
140
330
435
540 >1100
623 >1100
735 >1100
825 >1100
42
42
45
44
71
96
168
281
358
369
358
350
37
11.5
80.1
95.6
98.2
99.2
96.6
99.3
99.4
99.6
0.70
3.12
3.11
1.22
0.54
2.68
0.45
0.41
0.25
87.8
15.5
+
+
+
+
+
+
+
+
+
+
+
+
4
4
4
4
4
4
4
4
0
0
0
0
0
0
0
0
7
7
7
1.20
1.27
0.48
0.26
0.81
0.14
0.11
0.18
15
678
637
627
1072
>1200
35
68
112
435
40
45
40
35
26
40
29
23.8
75.7
Graphite*
45
53
45
55
55
55
54
20
22
22
29
29
29
23
25
25
24
21
25
26
26
25
25
24
25
28
26
25
23
22
34
34
34
32
24
67.6
88.1
10.9
11.9
100
100
100
19.2
+
+
+
+
+
+
+
+
+
+
+
45
45
45
45
26
26
26
26
26
26
135
270
360
570
98
210
298
383
495
600
100
100
100
Graphite +
ABC
40
40
120
278
330
405
623
675
57
452
713
748
100
+
+
95.8
90.5
91.6
99.2
99.4
3.28
7.83
5.69
0.55
0.58
0.73
1.65
1.87
0.22
40
40
40
40
0.5
Graphite +
ABC*
40
40
180
255
375
39
35
35
100
100
15.3
+
+
+
40
49.8
34.9
*
After the use in high-temperature synthesis of CF .
4
couples T2 and T3, and, as a result, a counterwave of
the filtration carbon combustion in fluorine is formed.
It propagates upwards the flow, is stabilized in the
region of fluorine outlet from the nozzle, and trans-
forms into the inverse wave. The counterwave of
carbon combustion in fluorine propagates in the filtra-
tion mode; there is no fluorine breakthrough beyond
the reaction front. The wave linear velocity between
thermocouples T2 and T1 is 900 mm h 1 with respect
to the level of 600 C.
fluoride is accumulated, which then decomposes with
formation of gaseous fluoroalkanes containing CF4.
Under the laboratory conditions, after the first 900 s
of the synthesis, the product containing 99.5 vol %
CF was prepared. The main impurities were C F
4
2 6
and C F .
3 8
To control the possibility of shutdown and startup
of the industrial reactor of the TFM synthesis after
cooling the reaction zone, we made experiments with
graphite treated with fluorine in the high-temperature
zone and found that, under laboratory conditions at
the initial temperature of 15 C, such graphite was not
ignited.
The time of ignition delay of the initial graphite is
several hundreds of seconds. In this time, carbon poly-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 1 2004

SYNTHESIS OF TETRAFLUOROMETHANE BY GRAPHITE FLUORINATION
95
Hence, to obtain pure TFM and provide stable Table 3. Time t350 at various flow rates of fluorine WF
2
reactor startup, special measures are required to in-
tensify the ignition.
and ethylene + hydrogen mixture Wmix, d = 0.85 mm
W
cm s
,
W ,
cm s
U ,
m s
t350,
s
mix
F
F
One of the methods of the ignition intensification
is adding certain amount of activated charcoal to the
graphite packing. The charcoal has substantially larger
specific surface area as compared to graphite and
provides higher reaction rate. We performed a series
of experiments with graphite + ABC mixture using
the above method. The results are given in Tables 1
and 2.
Mixture
H + C H
2
3
2
3
1
1
1
13
26
13
26
14
46
23
46
25
188
270
128
173
2
2 4
1
3
C H + C H
4 10
3.7
3
8
1
.8
and 15 vol % ethylene and VAZ graphite (initial and
spent). The nozzle for feeding fluorine and gas fuel
consists of two coaxial tubes. Fluorine was fed
through the axial channel, and a mixture consisting of
hydrogen and ethylene, through the ring channel.
The time of ignition delay for freshly prepared
graphite + ABC mixture was 250 s at fluorine flow
rate of 40 cm s ; there was fluorine breakthrough
through the graphite bed. After 700 s of the synthesis,
3
1
pure CF was not obtained; the hexafluoroethane im-
4
purity was 0.6 vol %. In the case of spent mixture and
The gaseous fuel was fed for 10 s, and then fluor-
ine feeding at a preset flow rate was started. When
the temperature near the thermocouple T2 junction
reached 350 C, feeding the gaseous fuel was stopped,
while feeding fluorine with the preset flow rate was
continued.
3
1
at fluorine flow rate of 15 cm s , no ignition was
observed.
The above facts show that the time of the ignition
delay with ABC as initiator decreases, but the problem
of the startup period (accumulation of thermolabile
carbon polyfluoride under the synthesis conditions) re-
mains. In addition, the spent graphite + ABC mixture,
similarly to pure graphite, has startup characteristics
differing from those of the freshly prepared mixture.
The time intervals between the beginning of fluor-
ine feeding and attainment of 350 C near the thermo-
couple T2 junction, t350, for various flow rates of
fluorine and gas mixture are listed in Table 3. The
compositions of the reaction products after stopping
feeding of the ethylene + hydrogen mixture and meas-
urement of the temperature of the graphite bed are
listed in Table 4.
Using ABC as initiator in an industrial reactor may
have the following substantial disadvantage. In the
case of faster ABC burn-out as compared to graphite,
the ABC in the region of fluorine feeding into the
carbon bed can get exhausted after prolonged reactor
functioning, which worsens the reactor startup param-
eters (time of ignition delay, time of attaining the
steady-state mode).
At the instant of fluorine feeding into the graphite
bed, thermocouple T1 recorded a jump of temperature
from the initial value to more than 1300 C, after
which it burnt down. Thermocouples T2 and T3 re-
corded smooth temperature rise, and after approxi-
mately 5 min, thermocouple T2 recorded the tempera-
ture of 150 300 C.
In this work, we suggest the following method of
the reactor startup. At startup, a gas forming with
fluorine a self-igniting pair is introduced into the
graphite bed coaxially to the fluorine flow. After for-
mation of high-temperature (>1000 K) zone, owing to
heat liberated in oxidation of this gas with fluorine,
feeding of the gas is turned off, and the TFM syn-
thesis is performed.
Table 4 shows that, after stopping the supply of the
gaseous fuel, a stable high-temperature inverse wave
of the CF synthesis is formed in the graphite bed.
4
The time interval from stopping the gaseous fuel feed-
ing to obtaining pure CF (more than 99.9 vol %)
4
Taking into account high reactivity of fluorine,
readily available methane, propane, ethylene, etc. can
be used as a gaseous fuel.
does not exceed 500 s.
No fluorine breakthrough through the graphite bed
was observed. All experiments of this series were per-
formed with graphite that worked in the high-tempera-
ture zone of the synthesis.
Among the above gases, ethylene is the most reac-
tive with respect to fluorine, since in the fluorine
ethylene system a branched chain reaction mechanism
is realized [13]. Therefore, in laboratory trials of this
method, we used a mixture consisting of 85 vol % H2
Thus, the use of gaseous fuel allows stable startup
of the wave reactor of TFM synthesis without ac-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 1 2004

96
PASHKEVICH et al.
Table 4. Composition of the synthesis products and temperatures of thermocouples T1 T3 at various synthesis times ts
after stopping supply of ethylene + hydrogen or propane butane mixture*
T,
C
Content, vol %
C F
ts, s
T1
T2
T3
CF4
C F
CO2
HF
2 6
3 8
H + C H mixture
2
2 4
4
9
57
02
70
00
5
0
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
548
644
822
974
>1100
>1100
207
373
693
1189
29
30
39
51
67
71
26
26
28
30
32
48
68
94
30
97.2
98.8
99.7
99.9
2.8
1.2
0.3
0.1
0.04
0.06
5.52
2.84
0.3
1
2
2
3
99.96
99.94
94.48
97.16
99.91
99.96
99.95
100.0
99.99
99.96
99.97
99.50
99.50
100.0
94.20
99.50
99.90
100.0
88.50
96.70
99.50
99.90
100.0
1
9
5
0
1
2
3
4
6
7
8
1
3
4
2
3
4
6
1
2
3
4
5
50
25
15
65
00
43
40
50
30
20
25
60
72
30
05
25
15
05
55
0.06
0.03
0.03
0.02
0.02
>1100
>1100
>1100
>1100
0.01
0.03
0.01
0.03
4.50
0.50
5.60
0.50
0.10
11.50
3.30
0.50
0.10
C H + C H mixture
3
8
4
10
3
8
50
10
00
60
0
3
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
>1300
589
960
28
29
29
28
44
62
30
31
38
52
80
207
267
402
0.50
0.08
0.03
96.13
99.38
99.75
99.92
99.95
93.14
87.85
98.78
99.29
99.64
99.91
99.94
99.99
0.32
0.04
2.75
0.50
0.23
0.08
0.05
5.42
9.62
0.94
0.56
0.30
0.07
0.05
0.01
1
2
3
3
>1100
>1100
>1100
>1100
489
>1100
>1100
>1100
>1100
>1100
>1100
>1100
3
0
0.77
1.31
0.13
0.09
0.03
0.18
0.24
0.07
0.06
0.03
0.02
0.01
1
2
3
4
7
8
05
40
15
80
50
55
1
065
*
No F breakthrough.
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 1 2004

SYNTHESIS OF TETRAFLUOROMETHANE BY GRAPHITE FLUORINATION
97
cumulation of carbon polyfluoride in the reactor in
the starting period.
the fluorine flow velocity and carbon raw material
history.
Fluorine produced at electrolysis plants contains
impurities of HF, N , and O . To reveal the effect of
HF on the TFM synthesis, we made experiments with
(4) The method of formation of the high-tempera-
ture synthesis zone with a gaseous fuel, which forms
with fluorine a self-igniting pair in the graphite bed,
was suggested and experimentally examined. This
method allows formation of a stable wave of graphite
filtration combustion in fluorine. Propane butane
mixture can be recommended for industrial use.
2
2
an F + 9 vol % HF mixture simulating electrolysis
2
gas produced by the Angarsk electrolysis chemical
combine and found that HF does not affect the start-
ing characteristics and the composition of the syn-
thesis products.
REFERENCES
Propane butane mixture is the most widespread
commercial gaseous fuel. Therefore, we performed
some experiments on its use to obtain high-tempera-
ture zone of TFM synthesis. The scheme of the labora-
tory device and experimental procedure were the same
as above. The results are presented in Tables 3 and 4.
1
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2
3
4
5
Thermocouple T1 at the instant of fluorine feeding
recorded a temperature jump from the initial value to
that higher than 1300 C. The pattern of formation of
the high-temperature zone was similar to that observed
with ethylene hydrogen mixture.
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1
934, vol. 717, pp. 1 19.
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1
The above data show that, under the industrial con-
ditions, startup of the reactor of the TFM synthesis
from electrode graphite and F in the mode of the
2
6. Ruddorff, O. and Ruddorff, G., Z. Anorg. Allg. Chem.,
947, vol. 253, pp. 281 285.
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inverse wave of the filtration combustion is the most
reliable when a gaseous fuel is used. In this case, the
process can be organized so as to minimize accumula-
tion of carbon polyfluoride in the reactor volume in
the time of formation of the high-temperature zone
and ensure the time of attainment of working condi-
tions not exceeding 10 min. The propane butane
mixture can be recommended as the most readily
available.
1
7
8
1
9
. Timofeev, N.V. and Stepanov, V.P., Materialy 1
Mezhvedomstvennoi konferentsii po khimii i tekhnolo-
gii soedinenii ftora (Proc. 1st Interdepartmental Conf.
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pounds), Leningrad: GIPKh, 1973, pp. 23 27.
CONCLUSIONS
1
0. Kita, Y. and Watanabe, N., J. Am. Chem. Soc., 1979,
(
1) A laboratory device was developed, and condi-
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tion combustion in fluorine of graphite and its mix-
tures with activated birch charcoal were studied.
11. Watanabe, N., Koyama, S., and Imoto, H., Bull.
Chem. Soc. Jpn., 1980, vol. 53, pp. 3093 3099.
1
2. Aldushin, A.P. and Merzhanov, A.G., in Rasprostra-
nenie teplovykh voln v geterogennykh sredakh (Prop-
agation of Heat Waves in Heterogeneous Media),
Novosibirsk: Nauka, 1988, pp. 8 52.
(
2) At temperatures higher than 1000 C, tetra-
fluoromethane is the single product of fluorine reac-
tion with graphite.
(
3) At the initial temperature of the bed of graphite
1
3. Semenov, N.N., Khimicheskaya fizika (fizicheskie
osnovy khimicheskikh prevrashchenii) (Chemical
Physics (Physical Principles of Chemical Transforma-
tions)), Moscow: Nauka, 1978.
(
or its mixture with activated birch charcoal) of 20 C,
the ignition process is instable, and formation of the
high-temperature zone of the synthesis depends of
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 1 2004