Kinetics of gaseous fluorination of 1,1,1,2-tetrafluoroethane with elemental fluorine

Article abstract of DOI:10.1016/S0022-1139(98)00332-7

The kinetics and temperature dependence of gaseous fluorination of 1,1,1,2-tetrafluoroethane (TFE) with elemental fluorine have been investigated between 105-148°C. Experiments were conducted in a stationary isothermic regime in copper tubes. Undiluted fluorine was used in a mixture with 1,1,1,2-TFE in concentration from 4% to 10% by volume. Explanations of observed mechanisms are suggested on the basis of the theory of chain reactions.

Full text of DOI:10.1016/S0022-1139(98)00332-7

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Journal of Fluorine Chemistry 96 (1999) 3±5
Kinetics of gaseous ¯uorination of 1,1,1,2-tetra¯uoroethane
with elemental ¯uorine
D.S. Pashkevich*, D.A. Muhortov, E.A. Podpalkina, V.G. Barabanov
Russian Scienti®c Center `Applied Chemistry', 14, Dobrolubov ave., St. Petersburg 197198, Russia
Received 23 September 1997; accepted 26 August 1998
Abstract
The kinetics and temperature dependence of gaseous ¯uorination of 1,1,1,2-tetra¯uoroethane (TFE) with elemental ¯uorine have been
investigated between 105±1488C. Experiments were conducted in a stationary isothermic regime in copper tubes. Undiluted ¯uorine was
used in a mixture with 1,1,1,2-TFE in concentration from 4% to 10% by volume. Explanations of observed mechanisms are suggested on
the basis of the theory of chain reactions. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: 1,1,1,2-Tetra¯uoroethane; Elemental ¯uorine; Chain reactions
1. Introduction
From treatment of the experimental data and heat transfer
calculations it was shown, that in all experiments a sta-
tionary isothermic regime takes place, the temperature over
the tube axis exceeded that of the wall by not more than 48C.
In all experiments the gas ¯ow in the tubes was laminar and a
mode of ideal replacement was realized, as shown by
calculation.
One of methods of production of penta¯uoroethane (PFE)
and hexa¯uoroethane (HFE) is the gaseous ¯uorination of
1,1,1,2-tetra¯uoroethane (TFE) with elemental ¯uorine [1].
However, the literature on the kinetics of gaseous ¯uorina-
tion of ¯uoroalkanes is very limited [2], and the temperature
dependence of these processes has not been studied. In
devising a chemical reactor for these highly exothermic
reactions, a knowledge of the magnitude of the activation
energy is extremely important.
The greatest gas superheating takes place in the experi-
1
ment for 1.2 m tube length, 7 ml s ¯ow rate, the concen-
tration of ¯uorine 8.5% by volume and the temperature of
the tube wall 1488C. The conversion of 1,1,1,2-TFE was
1.3%. The heat generation q in the tube is 1.5 W, the heat
1
transfer coef®cient ꢀ ˆ Nu  ꢁ  d ˆ 3.66  0.03 Â
1
2. Experimental
0.002 ˆ 57 W m 2 K 1, and the ÁT ˆ Tw Tgˆq Â
1
1
ꢀ
 S ˆ 3.58C, where Tw is the temperature of the tube
The mixture of 1,1,1,2-TFE and ¯uorine without inert
diluent was blown through a thermostatically controlled
copper tube of internal diameter 2 mm. The temperature
of the tube wall was varied from 1058C to 1488C. The time
of stay of the mixture at a given temperature was varied by
changing the tube length (1.2±15 m) as well as the ¯ow rate
(3±7 ml s 1). Concentration of ¯uorine was taken from 4%
to 10% by volume. The reacting gas mixtures, extracted at
the output tube, were puri®ed from unreactive ¯uorine by a
lime absorber and then were analyzed using a gas chroma-
tograph (catharometer, silica gel KSS-4 (0.25±0.5 mm) and
vaseline oil, 6 m length and 3 mm internal diameter column,
wall and Tg is the gas temperature on the tube axis.
Before the experiments, the tube surface was passivated
with ¯uorine, and we assumed that the wall was always the
same. In all experiments the quantity of PFE was much more
than HFE, therefore, HFE synthesis was not taken into
account.
3. Results
It is believed that the reactions of gaseous ¯uorination of
¯uoroethanes with elemental ¯uorine are free radical in
nature [3], and that the process of ¯uorination of 1,1,1,2-
TFE in the gas phase
1
508C column temperature and 30 ml min He ¯ow).
*Corresponding author. Fax: +7-812-325-6647; e-mail:
rscac@mail.plus.net
F2 ‡ C2H2F4 ! C2F5 ‡ HF; ÁH ˆ 426 kJ
(1)
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 3 2 - 7

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4
D.S. Pashkevich et al. / Journal of Fluorine Chemistry 96 (1999) 3±5
can be written as
F2 ! 2Fꢀ ; ÁH ˆ 163 kJ
C2H2F4 ‡ Fꢀ ! C2HFꢀ ‡ HF; ÁH ˆ 155 kJ;
4
C2HFꢀ ‡ F2 ! C2HF5 ‡ Fꢀ ; ÁH ˆ 274 kJ
(2)
4
In the reaction mixture a stationary concentration of
¯uorine atoms Fꢀ , determined by the equilibrium ¯uorine
dissociation at a given temperature, is established by ana-
logy with the reaction of chlorination of a hydrogen [4].
Then, according to [5] the activation energy of the process
(1) will be not less than half the energy of dissociation of a
1
¯uorine molecule, and will be not less than 82.5 kJ mol
with the order of the reaction for ¯uorine as 0.5. The
treatment of experimental results was conducted on the
basis of this hypothesis.
Fig. 2. Conversion of 1,1,1,2-TFE to PFE in tubes of internal diameter
2 mm and 4 mm versus time: the temperature in the reaction zone
T ˆ 1198C, initial concentration of fluorine CF ˆ 4.1 vol.%, tube
2
diameter d ˆ 2 mm (&); T ˆ 1198C, CF ˆ 4.5 vol.%, d ˆ 4 mm (~);
2
3.1. Chain generation and influence of the wall
T ˆ 1488C, CF ˆ 3.8 vol.%, d ˆ 2 mm (&); T ˆ 1488C, CF
ˆ
2
2
4.0 vol.%, d ˆ 4 mm (~).
In the ®rst series of experiments the amount of conversion
of 1,1,1,2-TFE to PFE was measured as a function of time.
The results are presented in Fig. 1. The non-stationary part
of the curve is caused by adsorption of ¯uorine atoms on the
tube wall and shows that the wall is an inhibitor of the chain
reaction in the form:
Fꢀ ! ‰FŠ; ‰FŠ ‡ Fꢀ ! F2
where [F] is an adsorbed ¯uorine atom.
All results described below were obtained over the
stationary part of the time dependence of 1,1,1,2-TFE
conversion.
In order to verify the hypothesis (3) some experiments
were conducted using a tube of internal diameter 4 mm, as
shown in Fig. 2. The experimental dependence can be
approximated by straight lines with a correlation coef®cient
not less than 0.95. The reaction rate increases with the tube
diameter con®rming the assumption of the inhibiting action
of the wall.
Analysing the data in Figs. 1 and 2 it is possible to say
that chain generation occurs in the gas phase after thermal
dissociation of ¯uorine molecules to atoms. A heteroge-
neous breaking of the reaction chain takes place on the
surface of the tube wall.
The overall reaction rate (Fig. 2) is proportional to the
tube diameter, from which it follows, that under the con-
ditions of the experiment, for heterogeneous chain breaking
the adsorption of the ¯uorine atom on the surface is the
limiting stage, rather than the diffuse ¯ow, as in the pro-
cesses of photochemical chlorination of hydrogen [5]. If the
diffuse ¯ow is the limiting stage, the rate of reaction should
depend on the diameter to the second power [6].
(3)
3.2. Autocatalytic character of the reaction
A series of experiments were conducted at large ¯uorine
conversions in a tube of 2 mm internal diameter. Results of
these experiments are presented (Fig. 3). The experimental
dependences indicate an autocatalytic (or a branched chain)
character for the process and considering characteristic
times of reaction (10±15 s) it is possible to deduce, that a
degenerate branched chain process occurs [5].
For the observed homogenous reaction an energetic
branched chain could take place. It is believed that in the
gaseous ¯uorination of hydrocarbons hydrogen ¯uoride can
accumulate more than 60% of the reaction heat in the
vibrationally excited molecule [7]. The energetic branching
occurs as a result of formation of vibrationally excited
molecules of hydrogen ¯uoride. For the given reacting
system the following equation can be written
Fig. 1. Conversion of 1,1,1,2-TFE to PFE versus time for different process
conditions: the temperature in the reaction zone T ˆ 1198C, initial
C2H2F4 ‡ Fꢀ ! C2HF4 ‡ HFÃ; HFà ‡ F2 ! HF ‡ 2Fꢀ
concentration of fluorine CF ˆ 7.1 vol.%, time of reaction ꢂ ˆ 0.87 s
2
(*); T ˆ 1338C, CF ˆ 7.6 vol.%, ꢂ ˆ 0.45 s (~); T ˆ 1488C,
2
CF ˆ 8.5 vol.%, ꢂˆ0.75 s (&).
(4)
2

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D.S. Pashkevich et al. / Journal of Fluorine Chemistry 96 (1999) 3±5
5
Fig. 3. Conversion of 1,1,1,2-TFE to PFE versus time: the temperature in
the reaction zone T ˆ 1198C, initial concentration of fluorine
CF ˆ 4.1 vol.% (~); T ˆ 1488C, CF ˆ 6.4 vol.% (&); T ˆ 1058C,
Fig. 4. Conversion of 1,1,1,2-TFE to PFE versus time: the temperature in
the reaction zone T ˆ 1198C, initial concentration of fluorine
CF ˆ 4.1 vol.% (*); T ˆ 1058C, CF ˆ 6.3 vol.% (&); T ˆ 1198C,
2
2
2
2
CF ˆ 10.0 vol.% (*).
CF ˆ 7.1 vol.% (~); T ˆ 1198C, CF ˆ 9.7 vol.% (*); T ˆ 1338C,
2 2
2
2
2
CF ˆ 7.6 vol.% (&); T ˆ 1488C, CF ˆ 8.5 vol.% (~).
where HF* is a vibrationally excited molecule. It is neces-
sary to add Eq. (4) to Eqs. (2) and (3).
The following results are obtained: E ˆ 50 Æ
10 kJ mol 1, k0 ˆ (5.2 Æ 0.6)  10 5 s 1. The values of
activation energy for the reaction obtained are less than
those in model (2), indicating once again the necessity of
considering processes (3) and (4).
3.3. Dependence of order of reaction on fluorine
To determine the order of reaction on ¯uorine, experi-
ments were conducted in which the initial concentration of
1,1,1,2-TFE was much more than the amount of conversion
of 1,1,1,2-TFE. Here, it is possible to neglect the in¯uence
of concentration of 1,1,1,2-TFE on the rate of reaction
(zero-order reaction for 1,1,1,2-TFE).
The results of the experiments are shown in Fig. 4, all
experiments were carried out in a tube of diameter of 2 mm.
From Fig. 4, it can be seen that the time dependences of the
conversion have a linear character. The calculations of
reaction order on ¯uorine were carried out in accordance
with the formula
4. Conclusions
It has been shown, that the process of interaction of
1,1,1,2-TFE and ¯uorine in a copper tube between 1058C
and 1488C has degenerate branched chain character. Thus,
the action centres are formed in the gas phase by thermal
dissociation of ¯uorine. Breaking of the reaction chain
occurs on the wall, the limiting stage of breaking is the
adsorption of ¯uorine atoms on the wall surface. It is likely,
that chain branching occurs because of formation of vibra-
tionally excited hydrogen ¯uoride molecules in the reaction.
The activation energy (50 kJ mol 1), the pre-exponential
multiplier and the order of reaction dependence on ¯uorine
(0.8) have been calculated for the reaction.
ÁÂ
ÁÃ
1
1
1
n ˆ ln W1W2
ln f1f2
where f1, f2 are initial ¯uorine concentrations and W1, W2 are
rates of the reaction. In the calculations, the values of W1,
W2 were used, as obtained at those temperatures. The order
of the reaction obtained on ¯uorine is 0.8, which is a little
more than n ˆ 0.5, which should result from model (2). The
discrepancy could be caused by the in¯uence of processes
(3) and (4) on the equilibrium concentration of the ¯uorine
atoms.
References
[1] D.S. Pashkevich, D.A. Muhortov, Yu.I. Alekseev, G.I. Ryleev,
Process of pentafluoroethane and hexafluoroethane preparation by
gaseous fluorination of 1,1,1,2-tetrafluoroethane with elemental
fluorine. In: Abstracts of the 2nd International Conference,
CTAF'97, St. Petersburg, Russia, 1997, p. 54.
3.4. Activation energy of reaction
[2] C. Moore, I. Smith, J. Chem. Soc., Faraday Trans. 18 (1995) 91.
[3] W. Sheppard, C. Sharts, Organic Fluorine Chemistry, New York,
1969.
From the experimental results, shown in Fig. 4 the acti-
vation energy and the pre-exponential factor of the reaction
were calculated using the model
[4] A.I. Rozlovskiy, Rus. J. Phys. Chem. 28 (1954) 51.
[5] N.N. Semenov, Chemical Physics (Physical Bases of Chemical
Transformations), Science, Moscow, 1978 (in Russian).
[6] N.N. Semenov, Chain Reactions, Goshimtehizdat, Leningrad, 1934
(in Russian).
W ˆ kfn
where n ˆ 0.8 and k is the rate constant of reaction (1).
[7] N.G. Basov, Chemical Lasers, Science, Moscow, 1982 (in Russian).