Synthesis of 1,2-diiodotetrafluoroethane with pyrolysis gas of waste polytetrafluoroethylene as raw material

Article abstract of DOI:10.1039/c3gc36880g

In the present study, waste polytetrafluoroethylene (PTFE) is recycled to generate the monomer tetrafluoroethylene by pyrolysis in vacuum condition. The reaction parameters are investigated; under 500-550°C and 2 kPa, the yields of tetrafluoroethylene can reach 95%. Then the pyrolysis gas is applied without separation as reactant together with iodine to synthesize 1,2- diiodotetrafluoroethane, which is the key intermediate for synthesis of many fluorine-containing compounds. Under the optimal conditions, temperature 150-160°C, pressure 12 atm and ratio of solvent to iodine 0.5-0.6, the conversion of iodine can rise up to 98%. The method of this paper uses waste polytetrafluoroethylene (PTFE) as raw material, which is environmental benign and helpful to prevent white pollution.

Full text of DOI:10.1039/c3gc36880g

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Synthesis of 1,2-diiodotetrafluoroethane with pyrolysis
gas of waste polytetrafluoroethylene as raw material
Cite this: Green Chem., 2013, 15, 1042
Zhu Jing,a Wang Baohe*a and Liu Dongzhib
In the present study, waste polytetrafluoroethylene (PTFE) is recycled to generate the monomer tetra-
fluoroethylene by pyrolysis in vacuum condition. The reaction parameters are investigated; under
500–550 °C and 2 kPa, the yields of tetrafluoroethylene can reach 95%. Then the pyrolysis gas is applied
without separation as reactant together with iodine to synthesize 1,2-diiodotetrafluoroethane, which is
the key intermediate for synthesis of many fluorine-containing compounds. Under the optimal con-
ditions, temperature 150–160 °C, pressure 12 atm and ratio of solvent to iodine 0.5–0.6, the conversion
of iodine can rise up to 98%. The method of this paper uses waste polytetrafluoroethylene (PTFE) as raw
material, which is environmental benign and helpful to prevent white pollution.
Received 24th November 2012,
Accepted 18th February 2013
DOI: 10.1039/c3gc36880g
www.rsc.org/greenchem
synthesized DIPFAs by the telomerization method with tetra-
fluoroethylene and 1,2-diiodotetrafluoroethane; after 4 hours
1. Introduction
α,ω-Diiodoperfluoroalkanes (DIPFAs) are key intermediates1 reaction under 220 °C, the conversion reached 57%, and the
for the synthesis of various fluorochemicals, such as selectivity of 1,4-diiodofluorobutane reached 75%. Tortelli and
fluoropolymers2–4 and fluorosurfactants. Especially, diiodoper- Tonelli14 investigated the effects of the ratio of PTFE to iodine,
fluorobutane is used for the synthesis of perfluorobuta- reaction time and stirring method on the reaction. The total
diene,5,6 which is an important chemical agent used in the yield based on iodine can be more than 96% under the
laser-etching process in the electronics industry, producing optimal reaction conditions.
various high-end electronics.7,8 The earliest reported of syn-
Suzuki et al.15 synthesized DIPFAs by thermal deiodination
thesis of DIPFAs is by Haszeldine in 1951;9 and after that reaction of 1,2-diiodotetrafluoroethane under 250 °C; the con-
much research effort has been focused on this issue because version of 1,2-diiodotetrafluoroethane was 70%. Overall,
of the increasing industrial demand. Dindi and Hagedorn10 thermal deiodination reaction and telomerization are currently
used tetrafluoroethylene and 1,2-diiodotetrafluoroethane to the prevailing methods to synthesize DIPFAs. However, both
produce DIPFAs by telomerization reaction, and the total con- methods need 1,2-diiodotetrafluoroethane as a raw material.
version of 1,2-diiodotetrafluoroethane reached 96% after
Thermal deiodination : ICF2CF2I ! IðC2F4ÞnI þ ðn ꢀ 1ÞI2
12–20 hours reaction at 245 °C and 3.65–3.86 MPa. Bedford
and Baum11 developed a one step route in which tetrafluoro-
Telomerization : ICF2CF2I þ ðn ꢀ 1ÞCF2CF2 ! IðC2F4ÞnI
ethylene and iodine were used to synthesize 1,2-diiodo-
tetrafluoroethane at first, then 1,2-diiodotetrafluoroethane
continued to react with superfluous tetrafluoroethylene. After
22 h reaction under 200–220 °C, the gas in the reaction vessel
was replaced with flash tetrafluoroethylene, and then the reac-
tion kept going for a further 18 h: the total yield of the reaction
was 61.2%. Catalysis telomerization reaction was also devel-
oped,12 using benzoyl peroxide as catalyst; after 26 hours reac-
tion under 80 °C, 650 psig, the mixture of I(C2F4)nI (n = 1, 2, 3,
4) was acquired. Zongli et al.,13 using Cu as catalyst,
Both thermal deiodination or teleomerization use pure tetra-
fluoroethylene with high purity (>99%) as raw material.16
Unfortunately, tetrafluoroethylene is an achromatous, odour-
less vapor at room temperature, and is inflammable and explo-
sive. As a consequence, it is usually stored and transported at
temperature as low as −35 °C, coexisting with triethylamine or
terpene as inhibitor retarder to maintain stability. Such harsh
requirements for storage and transportation has greatly ham-
pered its industrial application.
In this paper, we explored another way to acquire tetrafluoro-
ethylene: pyrolyzing waste polytetrafluoroethylene (PTFE) in
subatmospheric pressure. PTFE is ultra-stable and hardly can
be decomposed by biodegradation. It is one of the sources of
so-called “white pollution”. In terms of recycling the waste
PTFE, there are currently three main recycling methods;17–19
aResearch and Development Center of Petrochemical Technology, Tianjin University,
Weijin Road 92, Tianjin, 300072, PR China
bApplied Chemistry Department of Chemical Engineering College, Tianjin University,
Weijin Road 92, Tianjin, 300072, PR China. E-mail: Wangbh@tju.edu.cn;
Fax: +86-22-27406959; Tel: +86-13803011058
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physical smashing, radiation degradation and pyrolysis. 2.3 Synthesis of DIPFAs
Among these, pyrolysis has the advantages of high conversion
256 g iodine (or mixture of 200 g iodine and 100 g 1,2-diiodo-
tetrafluoroethane) was placed into a 500 ml autoclave made by
Hastelloy; the reactor was cooled to −10 °C and vacuum
applied to the autoclave; then the temperature was raised to
150 °C, and pyrolysis gas fed into the autoclave until the
pressure in the autoclave reached 1.2 MPa, keeping agitation
and gas-feed continuously to maintain the pressure. React pro-
ceeded for half an hour, then feeding was stopped and natural
cooling to room temperature begun. A total of 172 g pyrolysis
gas was added. After venting the unreacted gas in the auto-
clave, 349 g liquid was collected. The product was analyzed by
Agilent 1790 gas chromatography, with SE-54 large caliber
capillary column of 25 m length. The temperature of the capil-
lary column was 100 °C, the temperature of the TCD detector
was 200 °C.
and no constraints for the waste PTFE (it may contain addi-
tives or dyes.). The pyrolysis reported in literature so far have
been under the N2 or a stream of inert gas;20,21 these methods
have relatively high equipment requirements, resulting in
higher investment. In the present study, the pyrolysis of waste
PTFE is carried out in subatmospheric pressure so that the
operation is simpler and the yield of TFE is higher. Moreover,
compared with the need to strictly control the particle size of
the raw material in a fluidized bed reaction, the size and form
of the raw material in vacuum pyrolysis reaction is not
restricted. Recycling waste PTFE as an industrial raw material
is environmentally benign and helpful in reducing the white
pollution.
The pyrolysis gas can react with iodine without separation
to produce 1,2-diiodotetrafluoroethane. The addition reaction
of iodine and tetrafluoroethylene is highly exothermic and has
a high reaction rate, which makes removal of the heat of reac-
tion very difficult. Moreover, the tetrafluoroethylene is prone to
self-polymerization under the reaction conditions owing to its
3. Results and discussion
3.1 Pyrolysis mechanism
active properties, and the self-polymerization can release a It is recognized that the pyrolysis of PTFE is a free radical reac-
large amount of heat that may result in temperature runaway tion.22 The PTFE may break randomly at any position of the
or explosion. By using the pyrolysis gas as raw material, on one molecule chain and thus forms two free radicals; because the
hand, the addition reaction rate decreases with decrease of the C–C bond energy of the free radicals (184 J mol−1) is much less
TFE partial pressure and, on the other hand, octafluorocyclo- than the normal C–C bond energy (359 J mol−1), the C–C bond
butane, which is a radical trapping agent in the gas mixture, of the free radical is easier to break; and thus a large amount
can restrain the self-polymerization of TFE, so that the risk of of difluorocarbene is produced, which is very active and
this reaction is reduced.
dimerizes very quickly into tetrafluoroethylene, which is rela-
tive stable. If the temperature is too low to supply enough free
radicals, PTFE can only be partial pyrolysed. If the temperature
exceeds a certain point, or the pressure is high enough,
difluorocarbene and tetrafluoroethylene will become too active
and likely dimerize to form hexafluoropropylene, and two
molecules of tetrafluoroethylene dimerize forming octafluoro-
cyclobutane. If the temperature exceeds 700 °C, hexafluoro-
propylene will continue to polymerize with difluorocarbene
forming 1-octafluorobutylene, 2-octafluorobutylene, and octa-
fluoroisobutene which is toxic. All the routes are shown in
2. Experimental
2.1 Material
Iodine was commercially supplied by Shanghai Gonghe
Chemical Co., Ltd. with purity greater than 99% in mass
fraction.
2.2 Preparing tetrafluoroethylene
After being smashed, cleaned and dried, 2000 g waste PTFE Scheme 1.
particles were placed into the pyrolysis tank which has
3.1.1 Reaction temperature versus product composition.
diameter of 200 mm and 6000 ml volume. The pressure of the PTFE is chemically stable and the onset of its decomposition
reactor was reduced to 2 kPa, and the tank was heated; the is when the temperature is as high as about 260 °C.22 The reac-
temperature rise was controlled to 200 °C in 30 min. After tion product at low temperature (≤700 °C) includes mainly
the temperature reached 200 °C, the reaction started; pyrolyzed tetrafluoroethylene (TFE), hexafluoropropylene (HFP) and octa-
gas was introduced into the collecting tank, and cooled by fluorocyclobutane (OFCB). Fig. 1 gives the influence of reaction
liquid nitrogen added into the cold trap around the tank. The temperature on the mass fraction of the three compositions in
tank was heated further until the temperature reached 500 °C– the product. As shown in Fig. 1, the mass fraction of tetrafluoro-
550 °C, and this temperature and pressure was maintained to ethylene in the product first increases with the rise of the
react for about 4–6 hours. When the reaction ends, the temp- temperature, but as the temperature exceeds 550 °C the mass
erature of the pyrolysis tank will rise continuously; heating fraction of tetrafluoroethylene declines and the mass fractions
was stopped and the collecting tank weighed; 1960 g gas was of octafluorocyclobutane and hexafluoropropylene increase. It
collected. The collected gas was analyzed by Agilent 1790 gas is known that the pyrolysis of PTFE is a free radical reaction;
chromatography, with PROPARK Q packed column of 2 m at two free radicals can be brought about when the PTFE
length. The temperature of the packed column was 30 °C, the molecule chain breaks at any position. As the C–C bond
temperature of the TCD detector was 160 °C.
energy of the free radicals (184 J mol−1) is much less than the
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Fig. 2 The influence of pressure on the pyrolysis reaction.
therefore, the appropriate pyrolysis temperature of waste PTFE
should be within 500–550 °C, which is lower than the 600 °C–
700 °C reaction temperature of pyrolysis with N2 or steam
medium in fluidized bed reaction.
Scheme 1
3.1.2 Reaction pressure versus product composition. The
influence of reaction pressure on the mass fraction of
the three compositions in the product is indicated in Fig. 2.
The mass fraction of tetrafluoroethylene in the product is rela-
tively large under high vacuum conditions. When the pressure
is below 5 kPa, the mass fraction of tetrafluoroethylene can
reach over 90%; but if the pressure exceeds 5 kPa, the mass
fraction of tetrafluoroethylene decreases sharply, and if the
pressure exceeds 30 kPa, the mass fraction of tetrafluoroethy-
lene is below 50% and the two by-products increase signifi-
cantly. This is probably due to the different escape rates of
pyrolysis gas under different pressures. The by-products OFCB
and HFP can be formed only after tetrafluoroethylene is
formed and activated for the second time. Under lower
pressure, the residence time of pyrolysis gas in the reactor
tank is low and the tetrafluoroethylene gas leaves the pyrolysis
tank immediately before being activated again. We also discov-
ered that under high pressure large amounts of oligomer of
Fig. 1 The influence of temperature to pyrolysis reaction.
normal C–C bond energy (359 J mol−1), the C–C bond of the PTFE came into being together with the mentioned two by-
free radical is broken more easily, and thus a large amount of products, and formed a white wax layer on the wall of the tube,
difluorocarbene is produced, which is very active and and thus the tube is easily blocked. Therefore, with the intent
dimerizes very quickly into tetrafluoroethylene. If the temp- of a high yield of TFE, a reaction pressure under 5 kPa, and
erature is too low to supply enough free radicals, PTFE can even 2 kPa, is needed.
only be partially pyrolysed and ceraceous scrap is left in the
3.1.3 Conversion of PTFE and yield of tetrafluoroethylene.
tank. Especially, when the reaction temperature is lower than Temperature and pressure have an obvious influence on the
420 °C, the reaction rate is very slow and only 20 g tetrafluoro- conversion of PTFE and the yield of tetrafluoroethylene. It is
ethylene is obtained from 100 g PTFE after 1 h reaction. On considered that pressure has a larger influence on the yield of
the contrary, if the temperature exceeds a certain point, tetrafluoroethylene than temperature does, because the yield
difluorocarbene and tetrafluoroethylene will become too active will decrease more sharply as the pressure increases. The
and more likely dimerize mutually forming hexafluoropropy- detailed data are given in Tables 1 and 2. This is mainly
lene, and two molecules of tetrafluoroethylene dimerize because when the temperature is low, the splitting of PTFE is
forming octafluorocyclobutane. In this study, TFE is the target affected by thermodynamic control, and the C–C bond break-
product for synthesis of 1,2-diiodotetrafluoroethane and, age to generate difluorocarbene is the control step. When the
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Table 1 The yield of TFE at different temperatures at 2 kPa
Temperature (°C)
Conversion of PTFE (%)
Yield of TFE (%)
450
500
550
600
650
700
99.1
99.6
99.4
99.3
99.5
99.6
94.24
95.52
95.03
92.95
88.65
85.66
Table 2 The yield of TFE under different pressures at 500 °C
Pressure (kPa)
Conversion of PTFE (%)
Yield of TFE (%)
2
5
10
15
30
40
99.6
99.2
98.9
98.4
97.6
95.3
95.22
89.83
80.65
70.49
46.93
28.68
Fig. 3 The influence of temperature on the reaction of iodine with pyrolysis
gas.
decomposition temperature is sufficiently high, the splitting of
PTFE is affected by kinetic control. If the pressure is increased,
the TFE escape velocity will be reduced and more by-products
appear. Therefore, the yield of TFE is higher when the resi-
dence time of the products is lower, so that there is a higher
yield of TFE in vacuum pyrolysis than when using medium
pyrolysis because the generated gas quickly leaves. In addition,
without a lot of N2 or steam medium to dilute the partial
pressure of TFE at high reaction temperature, all of the energy
is used to heat the raw material for decomposition, so that
there is less energy consumption in vacuum pyrolysis.
3.2 Synthesis of 1,2-diiodotetrafluoroethane
3.2.1 The influence of temperature. The synthesis of 1,2-
diiodotetrafluoroethane is realized by addition reaction of
tetrafluoroethylene and iodine. In this work, the raw material,
tetrafluoroethylene gas, used in the synthesis of 1,2-diiodo-
tetrafluoroethane is the product gas of the pyrolyzed waste PTFE,
which contains small quantities of octafluorocyclobutane and
hexafluoropropylene. The results under different reaction
temperatures with a preset pressure of 12 atm is given in
Fig. 3.
As shown in Fig. 3, the conversion of iodine is rather low
when temperature is 140 °C, which denotes that the reaction
rate is very low, but with the increasing of temperature, the
conversion of iodine leaps sharply to nearly 99%. When the
temperature exceeds 160 °C, the selectivity of 1,2-diiodotetra-
fluoroethane declines; this is due to the product 1,2-diiodo-
tetrafluoroethane continuing to react with tetrafluoroethylene
to generate 1,4-diiodoperfluorobutane when the temperature is
high. As the reaction is exothermic, the temperature of the
autoclave rises obviously in the experiment. Therefore, precau-
tions should be taken to determine the starting temperature
of the reaction to prevent temperature runaway. The proper
temperature of the reaction is 150–160 °C.
Fig. 4 The influence of pressure on the reaction of iodine with pyrolysis gas.
3.2.2 The influence of pressure. The reaction result under
different pressures (from 10–20 atm) is given in Fig. 4, with a
preset reaction temperature of 150 °C. It can be seen from
Fig. 4 that the pressure does not affect obviously the con-
version of iodine and the selectivity of 1,2-diiodotetrafluoro-
ethane, but the reaction time is slightly shortened with the
increasing pressure. Further, under high temperature or
pressure conditions, it is discovered that more tetrafluoroethy-
lene is consumed to form the by-product octafluorocyclo-
butane, which is not desired. So we tend to select a mild
reaction pressure of 12 atm as the proper condition.
3.2.3 The effect of self-solvent. Solvent plays an important
role in many chemical reactions. In the case of the free radical
reactions in this paper, the solvent is the products. The advan-
tages for this choice includes enhancing the contacting of the
reactants, ensuring the supply of the active free radicals, and,
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reaction. As mentioned, this reaction is exothermic, and if the
reaction rate is high a large amount of heat will accumulate
and initiate the polymerization of TFE which will cause
explosion. The octafluorocyclobutane in the pyrolysis gas can
lower the reaction rate and reduce the risk of TFE polymeri-
zation through trapping free radicals, which is desired, and it
does not affect the yields and conversion, so it can be con-
cluded that it is feasible to use the pyrolysis tetrafluoro-
ethylene as the reactant for synthesis of 1,2-
diiodotetrafluoroethane.
4. Conclusion
In this work, waste PTFE was pyrolysed in a subatmospheric
pressure tank to produce the monomer tetrafluoroethylene;
under 500–550 °C and 2 kPa, the yields of tetrafluoroethylene
can reach 95%. The pyrolysis tetrafluoroethylene gas
was applied as reactant together with iodine to synthesize 1,2-
diiodotetrafluoroethane; the reaction parameters were investi-
gated and the optimal conditions are as follows: reaction
temperature: 150–160 °C, pressure: 12 atm; ratio of solvent to
iodine: 0.5–0.6; and the solvent is selected to be 1,2-diiodo-
tetrafluoroethane. The conversion of iodine can rise to 98%.
For comparison, using pure tetrafluoroethylene as reactant was
also investigated; the experiment result indicates that the sub-
stitution of pyrolysis gas for pure tetrafluoroethylene is feasi-
ble. Using the non-separated pyrolysis gas as reaction reagent
can restrain the self-polymerization of TFE, and reduce the
risk of temperature runaway or explosion, while the conversa-
tion of iodine and the yield of 1,2-diiodotetrafluoroethane do
not change. The method in this paper is environmentally
benign and offers another way to recycle waste PTFE, and thus
prevents white pollution.
Fig. 5 The influence of solvent.
more importantly, such a choice does not require any sub-
sequent separation steps and has no other waste. Fig. 5 gives
the transient reaction rate when adding different amounts of
1,2-diiodotetrafluoroethane as solvent into the initial reactant.
It is indicated in Fig. 5 that the reaction rate is higher after
adding solvent, because the solvent is helpful for generating
active free radicals, and the solvent itself can decompose into
free radicals if the temperature is high enough. However, there
is an optimal ratio of solvent to iodine, when the ratio of
solvent to iodine is 0.36, the initial reaction is fastest in our
experiment, and if the ratio exceeds 0.89, the reaction rate is
relatively low. Therefore, the proper ratio of solvent to iodine is
in the range 0.5–0.6.
3.3 Comparison to pure tetrafluoroethylene
We also investigated the reaction with pure tetrafluoroethylene
as reactant for comparison. The pure tetrafluoroethylene was
obtained by distillation of the pyrolysis tetrafluoroethylene and
the purity is as high as 99 wt%. Table 3 gives the result of reac-
tion using pure tetrafluoroethylene under conditions of 150 °C
and 12 atm. Compared to using pyrolysis tetrafluoroethylene
as reactant, the reaction time is greatly shortened, but the
yields and conversion is roughly unchanged. The pyrolysis tetra-
fluoroethylene gas contains octafluorocyclobutane and hexa-
fluoropropylene; octafluorocyclobutane is a trapping agent of
free radicals and hence is inhibitor of the reaction. What
is more, octafluorocyclobutane is also a by-product of the
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