Pyrolyses of chlorodifluoromethane and trifluoromethane in the presence of hydrogen. Mechanism and optimization of reaction conditions

Article abstract of DOI:10.1021/ja0023581

When CHC1F₂ and CHF₃ are subjected to high-temperature, gas-phase flow pyrolysis in the presence of H₂, they are converted, via a free radical chain mechanism, to CH₂F₂, CHF₂CHF₂, and CF₃CH₂F in good yield. Optimal conditions for pyrolysis of CHC1F₂ involve a high conversion (92%) at 650 °C with an observed yield of products = 18, 17, and 28%, respectively, whereas optimal conditions for CHF₃ involve a low conversion (24%) at 775 °C, but a higher yield of products (26, 6, and 39%, respectively).

Full text of DOI:10.1021/ja0023581

J. Am. Chem. Soc. 2001, 123, 6767-6772
6767
Pyrolyses of Chlorodifluoromethane and Trifluoromethane in the
Presence of Hydrogen. Mechanism and Optimization of Reaction
Conditions
Raphaele Romelaer, Virginie Kruger, J. Marshall Baker, and William R. Dolbier, Jr.*
Contribution from the Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200
ReceiVed June 29, 2000
Abstract: When CHClF2 and CHF3 are subjected to high-temperature, gas-phase flow pyrolysis in the presence
of H2, they are converted, via a free radical chain mechanism, to CH2F2, CHF2CHF2, and CF3CH2F in good
yield. Optimal conditions for pyrolysis of CHClF2 involve a high conversion (92%) at 650 °C with an observed
yield of products ) 18, 17, and 28%, respectively, whereas optimal conditions for CHF3 involve a low conversion
(24%) at 775 °C, but a higher yield of products (26, 6, and 39%, respectively).
also comprises a potentially useful source of TFE.7
,10-12
The high-temperature (>600 °C) pyrolysis of chlorodifluo-
romethane (FC-22) comprises the major industrial source of
tetrafluoroethylene (TFE).1 The mechanism of this process has
been demonstrated to involve unimolecular extrusion of mo-
lecular HCl to form difluorocarbene [CF2:], which then dimer-
,2
3
izes to form TFE. The kinetic and thermodynamic parameters
associated with this reaction have been studied extensively, both
experimentally and theoretically,4 and although there remains
ambiguity in some of the values, particularly with respect to
the heat of formation of CF2: (recommended value, -44 kcal/
-8
In an effort to develop broader use of these ready sources of
CF2:, studies of copyrolysis of CHClF2 with hydrogen were
1
3
14
carried out independently by Elf Atochem and DuPont, and
it was discovered that under appropriate conditions this process
leads to effective quenching of TFE formation with resultant
formation of CH2F2 (FC-32), CHF2CHF2 (FC-134), and CF3-
CH2F (FC-134a).
9
mol), the reaction is considered chemically and thermochemi-
cally very well defined, with reversible formation of CF2:
competing with its dimerization to form TFE.
Brief mention of a pyrolytic reaction of CHClF2 with H2 to
form CH2F2 had been made by DiFelice and Ritter in 1994.15
These authors interpreted the CH2F2 formation as deriving from
competitive insertion of CF2: into H2, and no mention was made
of either CHF2CHF2 or CF3CH2F being formed.
The pyrolysis of CHF3 (FC-23), although much less studied
and requiring considerably higher temperatures (>750 °C),
follows essentially the same mechanistic course, and it therefore
Co-pyrolysis of CHF3 (FC-23) with H2 led to similar results.16
In view of the commercial interest in hydrofluorocarbons
(
1) Park, J. D.; Benning, A. F.; Downing, F. B.; Lancery, J. F.;
McHarness, R. C. Ind. Eng. Chem. 1947, 39, 354.
2) Hamilton, J. M., Jr. In AdVances in Fluorine Chemistry; Stacey, M.,
Tatlow, J. C., Sharpe, A. G., Eds.; Butterworth: Washington, 1963; Vol.
, pp 117-180.
3) Hudlicky, M. Chemie der Organischen FluoroVerbindungen; Veb
Deutscher Verlag der Wissenschaften: Berlin, 1960.
(HFC’s) such as CH2F2 and CF3CH2F, a study of the mecha-
nisms of these reactions was initiated in order to determine how
modification of reaction conditions might affect product yields
and selectivities.
(
3
(
(10) Tschuikow-Roux, E.; Marte, J. E. J. Chem. Phys. 1965, 42, 2049-
2056.
(11) Tschuikow-Roux, E. J. Chem. Phys. 1965, 42, 3639-3642.
(12) Politanskii, S. F.; Shevchuk, V. U. Kinet. Catal. 1968, 9, 411-
417.
(13) Schirmann, J.-P.; Hub, S.; Lantz, A. Patent WO 96/25377, 1996.
(14) Manogue, W. H.; Noelke, C. J.; Swearingen, S. H. Patent WO 95/
24369, 1995.
(15) DiFelice, J. J.; Ritter, E. R. Abstracts of Papers, 207th National
Meeting of the American Chemical Society, Spring 1994, San Diego, CA;
American Chemical Society: Washington, DC, 1994; 24-Fuel, pp 158-
162.
(
4) Gozzo, F.; Patrick, C. R. Tetrahedron 1966, 22, 3329-3336.
(5) Edwards, J. W.; Small, P. A. Ind. Eng. Chem. Fundam. 1965, 4,
3
1
1
96-400.
(
6) Barnes, G. R.; Cox, R. A.; Simmons, R. F. J. Chem. Soc. B 1971,
176-1180.
(7) Schug, K. P.; Wagner, H. G.; Zabel, F. Ber. Bunsen-Ges. Phys. Chem.
979, 83, 167-175.
8) Su, M.-C.; Kumaran, S. S.; Lim, K. P.; Michael, J. V.; Wagner, A.
F.; Dixon, D. A.; Kiefer, J. H.; DiFelice, J. J. Phys. Chem. 1996, 100,
(
1
5827-15833.
9) Paulino, J. A.; Squires, R. R. J. Am. Chem. Soc. 1991, 113, 5573-
580.
(
5
(16) Private communication, E. Lacroix and S. Hub.
1
0.1021/ja0023581 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/23/2001

6768 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Romelaer et al.
Chart 1
Experimental Section
Table 1. Relative Response Coefficients (k
GC-FID
i
/kstd) Determined by
2 3 2
Pyrolysis. The pyrolyses of CHClF or CHF in the presence of H
were carried out at atmospheric pressure and isothermally in a
continuous-flow reactor (Chart 1).
i
k /kstd
I
std ) CH
4
std ) CHClF
2
All gas inlets were controlled by the four mass flow meters (Brooks).
CHClF
2
(FC-22)
3.83
n/a
0.73
0.73
2.22
0.73
1
1
Sources of the chemicals were as follows: CHClF
CHF (Elf Atochem, 95%), D (Strate Welding Supply Co., 99.7%),
and H and He (Strate Welding Supply Co., 95-97% and 99.995%).
The gas mixtures passed through the quartz reactor (CHClF
pyrolysis) or the Inconel 600 reactor (CHF pyrolysis), which is heated
2
(Elf Atochem),
CHF
3
(FC-23)
CHF (FC-134)
F (FC-134a)
(FC-32)
dCF (TFE)
2.32
0.19
0.19
0.58
0.19
0.26
3
2
CHF
2
2
2
CF
CH
3
CH
F
2 2
2
2
3
CF
2
2
by a furnace (Applied Test Systems, Series 3210) with three thermo-
couples. Three Omega controllers (CN 76000) (indicated as TRC in
Chart 1) control the temperature of the three-heating-zone furnace. The
temperature inside the reactor is indicated by a thermocouple (Omega,
Type K) with three junctions (indicated as TI in Chart 1).
At the outlet, the gases are passed through a KOH solution (1 M) in
order to neutralize HCl and HF and are dried by anhydrous calcium
sulfate (Drierite). An internal standard, CH
CH
4
Computational Methodology. Density functional theory calculations
were performed using the Gaussian 98 program package. Reactants,
products, intermediates, and transition structures were optimized using
1
7
18
Becke’s hybrid three-parameter functional (B3LYP) and the 6-31G-
(
d) basis set.19 Restricted and unrestricted wave functions were used
4 2
(CHClF pyrolysis) or
for closed- and open-shell species, respectively. Using the same level
of theory, vibrational frequency calculations were performed on all
stationary points to identity transition structures and determine thermal/
zero-point energies. Transition structures were characterized by a single
imaginary frequency. Thermochemical information at temperature T
CHClF (CHF pyrolysis), is then introduced in order to determine
conversion, yields, and carbon balance after reaction and GC analysis.
2
3
The reaction time (t) represents the ratio between the reactor volume
3
3
(
quartz, V ) 150 cm ; Inconel reactor, V ) 100 cm ) and the total
3
-1
flow rate at the reaction temperature (cm ‚s ).
2
0
and P ) 1.00 atm was obtained using frequencies scaled by 0.9804.
The GC analysis of the gas mixture was performed on a HP
chromatograph using the following operating conditions: column, Plot
An intrinsic reaction coordinate (IRC) calculation was performed for
each transition structure to examine the reaction pathway for each
elementary step. Single-point energies were calculated for each structure
and transition state using the B3LYP level of theory, using the
Al
2
O
3
/KCl (Chrompack), 50 m × 0.53 mm, film 10 µm; carrier gas,
N
2
(11 mL/min); temperature, 40 °C (5 min) to 200 °C (at 4 °C/min);
detector, FID, 250 °C; injector temperature, 250 °C. The products were
identified by comparison of their GC retention times and mass spectra
with those of pure samples. Quantitative analyses of the product/
standard ratios were obtained by comparison with mixtures prepared
6
-311+G(2df,2p) basis set.21
Results and Discussion
i
for calibration purposes. The relative response coefficients (k /kstd) of
each compound are given in Table 1.
The pyrolysis of CHClF2, either alone or in the presence of
He, leads to formation of TFE as the major product. When the
reaction is run at 1 atm, at 700 °C, with a contact time of 0.3
s, conversion of CHClF2 is 25%, with TFE being formed in
After the determination of the inlet flow of CHXF
2
and the outlet
flow rates of CHXF and each product, the results given in the tables
2
and represented graphically in the figures were calculated as indicated
below (X ) Cl or F):
9
0% yield along with 10% of other products, the major ones
1
being cyclo-C4F8, HCF2CF2Cl, and H(CF2)3Cl. In no case were
n(CHXF ) - n(CHXF )
2 o
2
i
(
17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
conversion of CHXF )
× 100
2
n(CHXF2)i
n(CHXF2)
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
∆
)
× 100
n(CHXF2)
i
-1
where n(CHXF
)
2 i
2 o
is the inlet molar flow rate (mmol‚h ) and n(CHXF )
-
1
is the outlet molar flow rate (mmol‚h ).
-
1
x[product flow rate (mmol‚h )]
yield (%)
from consumed CHXF2)
)
×
∆
n(CHXF2)
(
(18) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
100
(19) Hariharan, P. C.; Pople, J. A. Theor. Chem. Acta 1973, 28, 213-
2
22.
(
20) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic
Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996.
21) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062-
1065.
where x ) 2 for C
2
1
products and x ) 1 for C products.
(
carbon balance (of consumed CHXF ) )
∑yields of products
2

Pyrolyses of Chlorodifluoromethane and Trifluoromethane
J. Am. Chem. Soc., Vol. 123, No. 28, 2001 6769
Scheme 1. Proposed Free Radical Chain Mechanism for the Thermal Reaction of CHXF2 (X ) Cl or F) with H2
the compounds of interest, CH2F2, HCF2CHF2, or CF3CH2F,
reported as being present in significant amounts in the pyrolysate
when CHClF2 was pyrolyzed in the absence of H2. Likewise,
all studies of the pyrolysis of CHF3 (FC-23) reported a similarly
clean formation of TFE.
In contrast, the co-pyrolysis of CHClF2 and H2 led to
quenching of the formation of TFE, with concomitant formation
of hydrofluorocarbons (HFC’s) CH2F2, HCF2CHF2, and CF3-
CH2F as major products.
Once radicals are produced in the presence of CF2:, TFE,
and H2, reasonable propagating steps can be proposed that will
allow formation of all three of the observed products. Both
CH2F2 and CHF2CHF2 can be formed via the sequential addition
of two hydrogen atoms shown in Scheme 1, but explaining CF3-
CH2F (FC-134a) is another matter. Formation of CF3CH2F
requires a shift of fluorine at some point in the mechanism.
Again, it is tempting to propose that formation of CF3CH2F
derives from insertion of H2 into fluorotrifluoromethylcarbene,
CF3CF:, which could be formed by F-shift rearrangement of
the activated [TFE]*. If formed, CF3CF: should indeed readily
undergo insertion into H2 (activation barrier ∼ 4.7 kcal/mol),
as indicated by our DFT calculations. However, the very large
Mechanistic Hypothesis and Computational Work. Al-
though it is tempting to attribute the formation of CH2F2 to the
simple insertion of CF2: into the H-H bond, it is unlikely that
such an insertion process contributes significantly to CH2F2
formation. Unlike CH2: and even CHF: (which have calculated
barrier heights of 2 and 7 kcal/mol, respectively, for insertion
22,23
into H2),
with a calculated barrier of 34 kcal/mol, the process
of H2 insertion by CF2: does not, in practice, appear able to
24
compete kinetically with its dimerization to TFE, the barrier
for which has been estimated to be 2.8 kcal/mol.8
Instead, the formation of all three HFC products, CH2F2,
CHF2CHF2, and CF3CH2F, is attributed to a free radical chain
process (Scheme 1) that becomes mechanistically accessible only
when the pyrolyses of CHClF2 or CHF3 are carried out in the
presence of H2.
Free radical processes have previously been proposed to
intervene during the pyrolysis of CHClF2, particularly when long
contact times are used, with such processes being proposed to
lead not only to TFE, but also to most of the observed oligomeric
hydrochlorofluorocarbon side products.25
calculated barrier for rearrangement of TFE to CF3CF: (∼68.8
29
kcal/mol activation barrier) makes it unlikely that such a
process can be responsible for the significant amount of CF3-
CH2F formed in the reaction. Instead, we propose that the F-shift
most likely occurs after TFE has picked up its first H atom, at
‚
the CHF2CF2 radical stage:
We propose that initiation of the productive free radical chain
process described in Scheme 1 derives primarily from the
reaction of H2 with “activated” [TFE]*, which is created with
about 70 kcal/mol of excess energy when it is formed by the
combination of two CF2’s. With more excess energy than the
calculated π-bond energy of TFE,2 this [TFE]* species should
act much like a perfluoroalkyl “diradical” when it collides with
an H2 molecule, and it should thus abstract H quite readily.
Activated TFE “diradicals” have previously been proposed as
reactive intermediates in TFE dissociation and CF2: carbene
recombination reactions.2
Reports of fluorine atom shifts in carbon radical systems are
rare, but such rearrangements are apparently feasible.3
0-35
In
6
(
27) Buravtsev, N. N.; Kolbanovsky, Y. A. J. Fluorine Chem. 1999, 96,
35-42.
28) Buravtsev, N. N.; Kolbanovsky, Y. A.; Ovsyannikov, A. A.
MendeleeV Commun. 1994, 190-191.
29) Cramer, C. J.; Hillmyer, M. A. J. Org. Chem. 1999, 64, 4850-
4859.
(30) Kotaka, M.; Sato, S.; Shimokoshi, K. J. Fluorine Chem. 1988, 41,
71-382.
(31) Kotaka, M.; Sato, S.; Shimokoshi, K. J. Fluorine Chem. 1987, 37,
387-396.
(32) Kotaka, M.; Kohida, T.; Sato, S. Z. Naturforsch. 1990, 45b, 721-
722.
(
(
7,28
3
(
22) Sosa, C.; Schlegel, H. B. J. Am. Chem. Soc. 1984, 106, 5847-
5
1
852.
(23) Ignatyev, I. S.; Schaefer, H. F., III. J. Am. Chem. Soc. 1997, 119,
2306-12310.
(24) Battin-Leclerc, F.; Smith, A. P.; Hayman, G. D.; Murrells, T. P. J.
(33) Sung, K.; Tidwell, T. T. J. Org. Chem. 1998, 63, 9690-9697.
Chem. Soc., Faraday Trans. 1996, 92, 3305-3313.
(34) Kovtonyuk, V. N.; Kobrina, L. S.; Volodin, A. M. Russ. Chem.
Bull. 1995, 44, 95-98.
(
(
25) Zhang, Z.; Pollard, R. Int. J. Chem. Kinet. 1995, 27, 1151-1164.
26) Wang, S. Y.; Borden, W. T. J. Am. Chem. Soc. 1989, 111, 7282-
(35) Dolbier, W. R.; Palmer, K.; Koroniak, H.; Zhang, H. Q.; Goedkin,
V. L. J. Am. Chem. Soc. 1991, 113, 1059-1060.
7
283.

6770 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Romelaer et al.
2 2
Table 2. Pyrolysis of CHClF in He and in H at 650 °C
conditions
yield (%)
CF
carbon
balance (%)
He/H
2
/CHClF
2
t (s)
conv (%)
CH
F
2 2
CF
2
dCF
2
3 2
CH F
CHF
2
CHF
2
3
0
0
2/5/1
/5/1
/10/1
3
6
5
90
91
92
4
19
18
43
1
1
6
25
28
2
16
17
55
61
64
The data represented in these figures indicate that, over the
5
0 °C range of temperature studied, there is both a dramatic
increase in the conversion of CHClF2 (from 60% at 600 to 95%
at 650 °C) and a significant inhibition of formation of tetra-
fluoroethylene (from 18% at 600 °C to 1% at 650 °C). HFC
yields rise slightly over this range, remaining just about optimal
at 650 °C (CH2F2, 18%; CHF2CHF2, 16%; and CF3CH2F, 26%
yield), whereas the carbon balance drops slightly. No coking is
observed under these conditions, and most of the unaccounted
mass is believed to correspond to the formation of hydro-
chlorofluorocarbon C3 and C4 byproducts that were not quanti-
fied by our GC analytical process. With 95% conversion and
6
0% overall yield of the hydrofluorocarbons of interest, a
temperature of 650 °C is consequently considered optimal for
selectiVe preparation of HFC’s CH2F2, CHF2CHF2, and
CF3CH2F Via the pyrolysis of CHClF2 in the presence of H2.
Figure 1. Conversion and carbon balance vs temperature during the
pyrolysis of CHClF in H (H /CHClF ) 10, t ) 5 s).
2
2
2
2
(ii) Impact of Dilution with Helium. The effect of changing
the ratio of H2/CHClF2 at optimal temperature, 650 °C, was
examined, as was the impact of diluting the reactants with He.
The data in Table 2 indicate that halving the H2/CHClF2 ratio
from 10 to 5 (only modest lowering of the H2 partial pressure
from 0.9 to 0.83 atm) concentration has little impact on either
the conversion of CHClF2 or the efficiency of product formation.
However, diluting the mixture with a large amount of helium,
while not affecting conversion of CHClF2, modifies drastically
the distribution of the products, diminishing HFC formation and
strongly favoring TFE formation (43% yield). This result
undoubtedly derives from a combination of two factors: (a)
the significant lowering of the partial pressure of H2 (from 0.83
to 0.13 atm), and (b) the collisional deactivation (by He) of the
activated CF2dCF2* formed from combination of two CF2:’s,
both of which would kinetically inhibit the free radical chain
process.
Figure 2. Yields vs temperature during the pyrolysis of CHClF
(H /CHClF ) 19, t ) 5 s).
2
in
H
2
2
2
the accompanying paper, UB3LYP/6-311+G(2df,2p)//UB3LYP/
q
6
-31G(d) calculations provided an estimated barrier (E ) of 29.2
(iii) Effect of a Radical Initiator. Since a free radical chain
36
kcal/mol for this rearrangement. On the basis of these
calculations, we believe that the unimolecular 1,2-fluorine atom
shift of CHF2CF2‚ should be competitive with its second,
bimolecular H atom abstraction.
process is proposed as the mechanism of HFC formation, the
effect of adding a free radical initiator to the reaction flow was
examined. Perfluoroneooctane (BDE ≈ 40 kcal/mol) was chosen
3
7
as initiator, since it should dissociate rapidly into two
Therefore, the free radical chain mechanism, initiated and
facilitated by the presence of H2 as depicted in Scheme 1,
appears to explain the formation of all three product HFC’s.
Pyrolysis of CHClF2 in the Presence of H2. All pyrolyses
of CHClF2 were carried out at atmospheric pressure and
isothermally in a continuous-flow reactor constructed of quartz,
with the goal of determining the effect of temperature, time of
reaction, ratio of H2/CHClF2, and the presence of He diluant
on the yields of the three HFC products and on the conversion
of CHClF2. These results were then compared with those
obtained with CHF3.
perfluoro-tert-butyl radicals at the temperatures being used in
the CHClF2 pyrolyses. Such t-C4F9 radicals should react rapidly
with H2 to initiate the free radical chain process.
‚
Table 3 compares the results of CHClF2/H2 pyrolysis with
and without such initiation. Use of an initiator is seen to enhance
conversion and carbon balance slightly, while enhancing CH2F2
formation, partially at the expense of the C2 HFC products. This
can be understood, within the context of our free radical chain
‚
mechanism, as deriving from generation of H earlier in the
mechanism, when the CF2: moiety should be more prevalent.
(
Under the normal CHClF2/H2 pyrolysis conditions, H‚ would
(i) Effect of the Reaction Temperature. The pyrolysis of
not be formed until [CF2dCF2]* is formed by dimerization of
CHClF2 was carried out between 600 and 650 °C in the presence
of an excess of hydrogen (H2/CHClF2 ) 10, t ) 5 s). The
conversion of CHClF2 as a function of temperature and the
carbon balance, as well as the observed yields of product
formation, are presented in Figures 1 and 2.
CF2:, according to the postulated mechanism.)
(iv) Use of Other Difluorocarbene Sources. To gain further
insight into the CF2: f HFC process from CHClF2, which is
complicated by cogeneration of HCl, three different thermal
CF2: sources that do not generate HCl were used under
(
36) Romelaer, R.; Baker, J. M.; Dolbier, W. R., Jr. J. Am. Chem. Soc.
2
001, 123, 6773-6777.
(37) Tonelli, C.; Tortelli, V. J. Fluorine Chem. 1994, 67, 125-128.

Pyrolyses of Chlorodifluoromethane and Trifluoromethane
J. Am. Chem. Soc., Vol. 123, No. 28, 2001 6771
Table 3. Pyrolysis of CHClF
2
in H
2
2
at 650 °C in the Presence of Initiator, Perfluoroneooctane (5% Molar vs CHClF )
conditions
yield (%)
carbon
2
H /CHClF
2
init
t (s)
conv (%)
CH
F
2 2
CF
2
dCF
2
CF
3
2
CH F
CHF
2
CHF
2
balance (%)
1
1
0/1
0/1
no
yes
4
4
92
96
19
27
1
0
26
24
14
11
60
62
2
Table 4. Production of HFC’s Using Varied Sources of Difluorocarbene: CHClF , Hexafluorocyclopropane (HFCP), Hexafluoropropylene
Oxide (HFPO), and Tetrafluoroethylene (TFE) (H
2
/:CF
2
Source ) 10)
conditions
yield (%)
CF
carbon
balance (%)
:
CF
2
source
t (s)
conv (%)
CH
F
2 2
CF
2
dCF
2
3 2
CH F
CHF
2
CHF
2
CHClF
HFCP
HFPO
TFE
2
5
5
5
5
92
100
100
99
18
35
28
14
1
1
3
n/a
28
26
26
28
17
15
16
16
64
77
73
58
Table 5. Pyrolysis of CHClF
2
in D
2
versus H
2
at 650 °C
yield (%)
conditions
carbon
balance (%)
H /D
2 2
/CHClF
2
t (s)
conv (%)
CH
F
2 2
CF
2
dCF
2
CF
3
2
CH F
CHF
2
CHF
2
1
0
0/0/1
/10/1
5
5
92
87
18
17
1
11
28
21
17
11
64
60
a
a
a
a
a
Partially deuterated products are obtained (see Table 6).
Table 6. D/H Distribution in Products, As Determined by GC/MS
and Yields Obtained from the CHClF Pyrolysis in D (650 °C,
/CHClF ) 10, t ) 5 s)
copyrolysis conditions with H2 at 650 °C. Hexafluorocyclopro-
pane (HFCP) and hexafluoropropylene oxide (HFPO) are
traditional, lower temperature sources of CF2:, with HFCP
2
2
D
2
2
GC/MS distribution molecule deuteration
yield
(%)
38
decomposition starting at 190 °C and HFPO decomposition
compounds
(%)
(%)
3
9,40
at 150 °C.
At 650 °C, complete conversion of these CF2:
CHClF
CDClF
CH2F2
2
20
80
9
45
46
4
27
69
5
31
64
80
2.6
10.4
1.5
7.75
7.8
0.8
5.7
14.5
0.6
3.4
sources is likely. Although TFE is not ordinarily considered a
source of CF2:, it is recognized that TFE formation is reVersible
under the conditions (>650 °C) of CHClF2 pyrolysis, so it was
tested as a potential source of CF2: and HFC’s. Table 4 gives
the results of these experiments.
2
69
83
80
CHDF
CD
2
2 2
F
CF
CF
3
CH
CHDF
2
F
3
Although enhanced carbon balance and CH2F2 yield are
observed for the “pure” CF2:-forming reagents (HFCP and
HFPO), nevertheless all four CF2: sources give remarkably
similar product mixtures. Most notably, the yields of CHF2-
CHF2 and CF3CH2F in each run are virtually identical! HCl
does not, therefore, seem to be playing a determining role in
the free radical chain process. Because CHClF2 does not undergo
thermal conversion to HFC’s in the absence of H2, HCl cannot
be capable of donating an H atom to initiate a free radical chain
process analogous to Scheme 1. This may be because HCl will
preferably undergo a four-center addition reaction to activated
TFE to form CHF2CF2Cl (the reverse of the unimolecular
â-elimination process).41 Of course, that is not to say that HCl
cannot be a source of H atoms once the free radical process
has been initiated by the intervention of H2.
CF CD F
3
2
CHF
CHF
CDF
2
2
2
CHF
CDF
CDF
2
2
2
7.0
TFE yield is another indication of the diminished efficiency of
the chain process in D2.
The observed distribution of deuterium in the compounds of
interest, which was determined by GC/MS analysis for each
compound, is given in Table 6.
Deuterium incorporation into the products as well as into
recovered chlorodifluoromethane is significant. The significant
incorporation of deuterium into recovered chlorodifluoromethane
and the approximately equal amounts of CHDF2 and CD2F2 can
be rationalized by the sequence below:
(v) Use of D2 in Place of H2. When D2 was used in place of
H2, the results in terms of CHClF2 conversion and HFC
production (as shown in Table 5) were remarkably similar to
those obtained with H2. Diminished conversion and C2-HFC
formation are perhaps indicative of an isotope effect in the
propagation steps generating these products,4
2,43
whereas pro-
‚
duction of difluoromethane, which is determined by D addition
to CF2:, is not significantly affected. The observed increase in
Formation of mainly dideuterated C2-HFC’s, CDF2CDF2 and
CF3CD2F (64% and 69%, respectively), is consistent with the
sequential deuterium atom abstractions shown in Scheme 1.
(
38) Birchall, J. M.; Fields, R.; Hazeldine, R. N.; McLean, R. J. J.
Fluorine Chem. 1980, 15, 487-495.
39) Millauer, H.; Schwertfeger, W.; Siegemund, G. Angew. Chem., Int.
Ed. Engl. 1985, 24, 161-179.
40) Krusic, P. J.; Roe, D. C.; Smart, B. E. Isr. J. Chem. 1999, 39, 117-
23.
41) Volkov, G. V.; Barabanov, V. G.; V’yunov, K. A.; Maksimov, B.
N. J. Gen. Chem. USSR (Engl. Transl.) 1990, 60, 1029-1032.
(
(42) At these temperatures, isotope effects should be relatively small.
(
The primary isotope effect, kH/kD, for CF3 radical reacting with H2 versus
4
3
1
D2 can be calculated to have a value of ∼2.1 at 650 °C.
(
(43) Arthur, N. L.; Donchi, K. F.; McDonell, J. A. J. Chem. Soc., Faraday
Trans. 1 1975, 71, 2431.

6772 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Romelaer et al.
Table 7. Pyrolysis of CHF
3
2 2
and of CHClF in H
reagent
CHF
3
CHClF
2
reactor
temperature
pressure
Inconel
775
1
quartz
650
1
(°C)
(atm)
H
2
/reagent
3
4
10
5
92
28
18
17
0
reaction time
conversion
CF
CH
CHF
CH
carbon balance
(s)
(%)
(%)
(%)
(%)
(%)
(%)
21
39
26
6
11
82
3
CH
yield
CHF yield
yield
2
F yield
2 2
F
2
2
4
63
Figure 3. Conversion and carbon balance vs temperature during
pyrolysis of CHF in H (s) and in He (- - -) between 650 and 850 °C
He or H /CHF ) 10, t ) 3 s, Inconel reactor).
Thus, at 775 °C, using a ratio of H2/CHF3 equal to 3 and a
contact time of 4 s, the total conversion is 21%, with the yields
of CF3CH2F, CH2F2, and CHF2CHF2 being 39%, 26%, and 5%
respectively, and the carbon balance being an acceptable 82%.
This compares to the optimal results for CHClF2 at 650 °C,
where the observed yields were 25%, 19%, and 16%, respec-
tively, with 92% conversion.
3
2
(
2
3
An interesting difference in the CHF3 pyrolysis results as
compared to the results obtained using CHClF2 is the relative
lack of CHF2CHF2 as a product in the CHF3 pyrolysis. This
lack of observation of CHF2CHF2 in the CHF3 pyrolyses is
undoubtedly the result of its H2-promoted isomerization to CF3-
CH2F at these high temperatures. This isomerization, which does
not occur thermally in the absence of H2, is the subject of the
3
6
accompanying paper.
Figure 4. Yield vs reaction temperature during the pyrolysis of CHF
in H (T ) 700-850 °C, H /CHF ) 3, t ) 4 s, Inconel reactor).
3
Table 7 presents a comparison of the “optimal” results for
both CHClF2 and CHF3 pyrolysis.
2
2
3
It can be seen that the pyrolysis of CHF3 presents some
advantages, i.e., smaller requirement for hydrogen, improved
carbon balance, and improved yields of CH2F2 and CF3CH2F.
Pyrolysis of CHF3 in the Presence of H2. Although requiring
significantly higher temperatures than the pyrolyses of CHClF2,
the pyrolysis of CHF3 (FC-23) constitutes an equally good
source of difluorocarbene and TFE, and it should therefore also
produce HFC’s when the pyrolysis is carried out in the presence
of H2. Our investigation of the pyrolysis of CHF3 was carried
out in an Inconel 600 reactor, at atmospheric pressure and at
temperatures between 650 and 850 °C.
Initial experiments compared the thermal behavior of CHF3
in the presence of He versus that in the presence of H2 (He or
H2/CHF3 ) 10, reaction time of 3 s). Comparisons of the total
conversion and the carbon balance as a function of the reaction
temperature are presented in Figure 3. Conversion of CHF3 is
the same in He and H2, which suggests a common primary
decomposition pathway for the two processes.
Conclusions
In conclusion, strong evidence, both experimental and
theoretical, has been obtained that allows probable definition
of the mechanism of HFC formation during the pyrolysis of
CHClF2 and CHF3 in the presence of hydrogen. The data that
have been presented are consistent with the free radical chain
process depicted in Scheme 1. It is proposed that unimolecular
formation of CF2: from either CHClF2 or CHF3 is followed by
dimerization of CF2: to form “hot” TFE, which abstracts H
from H2 to initiate a free radical chain process that results in
formation of CH2F2, CHF2CHF2, and CF3CH2F.
However, the carbon balance is much higher in H2, and the
nature of the products is drastically different in the two media.
TFE is the major (almost exclusive) product (60-70% yield)
in He at temperatures between 650 and 800 °C, whereas a
mixture of the usual HFC’s (60-65% total yield) is formed
when CHF3 is pyrolyzed at temperatures from 650 to 775 °C
in a flow of H2. A large amount of methane (∼30% yield) is
also observed in the presence of H2, as a result of wall-catalyzed
hydrogenolysis processes in the Inconel 600 reactor. Figure 4
shows the nature of the product mixtures as a function of
temperature.
Although optimization studies did not result in finding
conditions that favor a single product, it was found that, under
the high-temperature conditions of pyrolysis of CHF3, the
selective formation of two HFC’s, CH2CF2 and CF3CH2F, was
possible.
Acknowledgment. Preliminary work by S. Hub and D.
Guillet, and helpful discussions with S. Hub, E. Lacroix, and J.
P. Schirmann, all of Elf Atochem, are acknowledged with
thanks. Support of this research by Elf Aquitaine, Inc. through
funding of postdoctoral appointments for V.K. and R.R. and
for funding construction of the flow reactor is gratefully
acknowledged.
Further experiments showed that low ratios of H2/CHF3
(
∼3:1) and short reaction times (∼4-5 s) provided optimal
results, in terms of carbon balance and HFC yields. Higher
temperatures favor the formation of increasing amounts of
byproducts, such as methane in the Inconel 600 reactor, but
also of ethane and mono- and difluorobenzene (detected by GC/
MS). Figures depicting these and other optimization experiments
can be found in the Supporting Information.
Supporting Information Available: Computational data
tables and supplementary figures describing thermolysis data
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0023581