Properties of chromium (III) oxides involved in the catalytic gas phase fluorination of CF3CH2Cl

Article abstract of DOI:10.1006/jcat.1997.1806

The fluorination of chromium oxides reduces the chromium species on the surface. A low fluorination by HF does not modify the total amount of reversibly oxidizable chromium sites, but increases the strength of these sites. Mobile dioxygen (measured by isotopic exchange) was required to obtain a good fluorination activity. These dioxygen species would be exchanged by HF which is probably active in the fluorination of CF₃CH₂Cl and would prevent the irreversibly catalyst fluorination.

Full text of DOI:10.1006/jcat.1997.1806

JOURNAL OF CATALYSIS 171, 287–292 (1997)
ARTICLE NO. CA971806
Properties of Chromium (III) Oxides Involved in the Catalytic
Gas Phase Fluorination of CF CH Cl
3
2
S. Brunet, B. Boussand, and D. Martin
Catalyse en chimie organique, URA CNRS 350, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received February 13, 1997; revised May 16, 1997; accepted June 17, 1997
EXPERIMENTAL
The fluorination of chromium oxides reduces the chromium
species on the surface. A low fluorination by HF does not modi- 1. Preparation of the Catalyst
fy the total amount of reversibly oxidizable chromium sites, but
The chromium (III) oxide (9, 13) resulted from the dehy-
increases the strength of these sites. Mobile dioxygen (measured by
isotopic exchange) was required to obtain a good fluorination ac-
tivity. These dioxygen species would be exchanged by HF which is
probably active in the fluorination of CF3CH2Cl and would prevent
dration of chromium hydroxide prepared by the addition
of an ammonia solution (5 M) to a solution of chromium
nitrate (0.5 M). The final pH was equal to 7.5 and the hy-
�
the irreversibly catalyst fluorination.
�c 1997 Academic Press
droxide formed wasstirred continuouslyand heated at 80 C
for 1 h so as to obtain complete precipitation. The solid
obtained was filtered and washed three times with hot dis-
�
tilled water and dried for 16 h in an oven at 90 C. It was
INTRODUCTION
then submitted to a dynamic thermal treatment under ni-
trogen between 300 and 450 C for 8 h. The chromium oxide
�
Chromium (III) oxide is a conventional catalyst for chlo-
rine exchange in chloroalkanes. For example, CF3CH2F
formed was cooled down under the same vector gas.
X-ray measurements revealed that the chromium ox-
ides were amorphous. After their fluorination, a crystalline
phase was observed, exclusively �Cr2O3, alongside amor-
phous structure (new chromium oxifluorine or chromium
fluorine phases were not observed).
(
HFC 134a) is prepared by catalytic gas phase conversion
�
of CF3CH2Cl (1–9). At 380 C by using 4 moles of HF per
mole of CF3CH2Cl, the main reaction is the selective fluori-
nation of CF3CH2Cl corresponding to CF3CH2F formation.
Moreover, competitive elimination of HF gives CF2 == CHCl
which accounts for ca 1.2% of the products but can lead to
the formation ofpolymerswhich cause catalyst deactivation
2
. Chromium Oxide Characterization
(
9, 10):
The adsorption of nitrogen occurred on a sample
100–200 mg) previously degassed at 300 C with a Flow-
�
(
CF3CH2Cl + HF *) CF3CH2F + HCl
CF3CH2Cl *) CF2 == CHCl + HF.
Sorb 2300 (Micromeritics) apparatus.
Temperature programmed reduction and oxydation
(
TPR-TPO) was carried out in a pulse regime by inject-
ing calibrated pulses of hydrogen or oxygen into the cata-
lysts at regular intervals and observing the fraction of the
pulse unconsumed by the solid on a recorder coupled to a
catharometer detector (14). The reactions involved were
These two reactions are thermodynamically limited (see the
Appendix).
Another typical feature of the fluorination reaction is
the role of oxygen. The addition of small amounts of O2
to the feed has been considered to overcome the deacti-
vation of the catalyst, but there is no explanation for this
n
2
Cr On/2 + (n � 3ꢀH2 → Cr2O3 + (n � 3ꢀH2O
TPR1
TPO
x
zCr2O3 + z(x � 3ꢀ/2O2 → 2zCr Ox/2
(
1, 2). We noticed that two factors are essential to obtain
x
good activity: one, the presence of reversibly oxidized sites 2zCr O + z(x � 3ꢀH → zCr O + z(x � 3ꢀH O TPR2
x/2
2
2
3
2
and two, an excess of oxygen in the starting oxide (11, 12).
�
Last the treatment of the oxide with HF at 380 C resulted with n > 3, x � 6 (n and x represent respectively the oxida-
in its reduction as shown by temperature programmed re- tion degree of chromium atom before the first reduction
ductions. All these data prompted us to study the change of and the oxidation degree of chromium atom after the ox-
the chromium (III) oxide during its fluorination and HF/O2 idation) and 0 < z � 1 (z represents the part of chromium
exchange.
reversibly oxidizable after the first reduction).
287
0021-9517/97 $25.00
Copyright �c 1997 by Academic Press
All rights of reproduction in any form reserved.

2
88
BRUNET, BOUSSAND, AND MARTIN
Fluorine titration of chromium oxide was carried out at on oxide surfaces:
the Elf-Atochem Research Center, Pierre-B e´ nite. The cata-
lyst mineralization was carried out in a Parr bomb by reac-
tion with sodium peroxide. Fluorine ions were then titrated
by a potentiometric method with a specific fluoride elec-
trode.
(
a) Homoexchange (or equilibrium) that occurs without
appreciable participation of oxygen of the oxide (Eq. [1]),
18
16
18 16
O2(Gasꢀ + O2(Gasꢀ *) 2 O O(Gasꢀ.
[1]
This reaction occurs according to a mechanism of adsor-
ption–desorption. The rate of exchange of oxygen surface
species of the oxide is negligible. During this homoex-
change, the isotopic oxygen fraction in the gas phase is con-
stant. By using pure isotopic O2 this type of exchange can
not be observed initially.
3
. HF/O2 Exchange
Pulses of HF (or O2) were injected at 380 C at regular
�
intervals using a calibrated loop. The uptake of HF (or O2)
was measured by gas chromatography which detected the
fraction ofthe pulse unconsumed bythe catalyst. In a typical
experiment, pulses of HF (5.9 �mol) were injected every
18
(
b) Simple heteroexchange occurs with the participation
of the only one oxygen of the oxide (Eq. [2]),
5
min. Helium was used as the flowing gas and the mass of
catalyst was 50 mg.
1
8
O2(gasꢀ + O(supꢀ → O O(gasꢀ + 18O(supꢀ.
16
18 16
[2]
The uptake was considered as finished after 3 h. HF was
replaced by O2 and a similar series of injections was carried
out to measure the uptake of dioxygen. To distinguish be-
tween the reversible adsorbed and exchangeable HF or O2,
the procedure was modified, so that the reversible adsorbed
gas on the catalyst surface was swept over by a current of
helium for 3 h. A first adsorption of HF was carried out,
then the helium followed by a second adsorption of HF.
The difference between the two uptakes gave the amount
of reversibly adsorbed HF.
(
c) Multiple heteroexchange occurs with the participa-
tion of two oxygen atoms of the oxide at each step of Eq. [3],
18O2(gasꢀ
+
16
O O
16
*) 16O2(gasꢀ
+
18 18
O O
[3]
(supꢀ
(supꢀ
.
For this multiple exchange on oxide surfaces, there are
two possible mechanisms: isotopic exchange and “place ex-
change.” These two mechanisms differ by the oxygen sur-
face intermediates involved. The isotopic exchange mech-
anism occurs through an associative mechanism with a
four-atomic (O4) surface intermediate. By contrast, the
�
4
. Isotopic Exchange of Oxygen
ads
“
place-exchange” mechanism occurs without requiring a
Isotopic exchange experiments were carried out in a re-
four-atom oxygen intermediate on the surface; this is just
a displacement of a preadsorbed molecule by a gas phase
molecule. This mechanism does not involve the scission of
any O–O bond.
cycle microreactor coupled to a mass spectrometer. The
masses 32, 34, 36 (for the isotopomers) were monitored ev-
ery 10 s. The vacuum connection to the mass spectrometer
was thermoregulated so as to maintain a constant pressure
�
6
of 10 mbar in the ionization chamber, while the total pres-
sure in the reactor was 40 bar for the isothermal isotopic ex-
change and 100 mbar for temperature programmed isotopic
exchange. For isothermal isotopic exchange (ISIE) and for
temperature programmed isotopic exchange (TPIE), the
0
e
Initial rate of exchange. The initial rate of exchange R
being equal to the rate of the disappearance of O from
the gas phase, it was possible therefore to determine R by
Eq. [4],
18
0
e
�
� �
�
2
0
0
oxide sample (5 m ) was submitted in situ to a pretreatment
NA
Vr
Vc
dP
dP
34
0
e
36
�
�
R = �
+
2
+
dt
,
[4]
at 300 C under vacuum pretreatment at 300 C for 30 min,
and cooled down to room temperature under vacuum. A
S �R Tr
Tc
dt
18
0
0
34
1
00-mbar dose of pure O2 was then introduced into the where dP /dt and dP /dt are the initial slopes of the par-
3
6
�
18
16 18
reactor and the temperature increased from 25 to 500 C at tial pressures of O and O O and N is the Avogadro
2
A
�
2
C/min for temperature programmed isotopic exchange number, P is the total pressure, S is the oxide BET area
T
�
� 2
or at constant temperature (380 C) for 2 h for isothermal (m ), R is the gas constant, V and V respectively are
r
c
isotopic exchange. The temperature programmed isotopic the volumes of the heated and unheated zones of the reac-
exchange determines the range of exchange temperature.
tor, and T and T respectively are the temperatures of the
r
c
1
8
The pressure variation of oxygen isotopomers P36 ( O2), heated and unheated zones of the reactor.
1
8
16
16
P34 ( O O), and P32 ( O2) were then recorded every 10 s.
The total pressure (P36 + P34 + P32) remained virtually con-
stant. The number of exchangeable oxygen atoms (Ne) and
Number of exchanged oxygen. The number of oxygen
atoms exchanged at each time N is given by Eq. [5] at each
temperature and calculated after the
t
e
0
e
the initial rate of exchange (R ) were able to be calculated
�
�
t
e
0
g
t
g
from the partial pressure values (15).
N = � � � N ,
[5]
g
According to the literature (16, 22) three types of ex-
0
g
change, described by the following equations could occur where (� ) is the initial atomic fraction of isotopic oxygen

PROPERTIES OF CHROMIUM (III) OXIDES
289
in the gas phase. We can first define the oxygen 18 atomic
TABLE 1
t
fraction in gas phase at each time � as
g
Catalytic Activity and Oxydo-Reducibility of Chromium (III)
Oxide (Prepared by Activation of the Chromium Hydroxide at
380 C under Nitrogen) at Different Degrees of Fluorination
1
2
t
t
p
+ p
34
�
t
g
34
36
�
=
,
t
3
t
t
32
p
+ p + p
6
Degree of fluorination (F/Cr)
0
0.59
1.6
t
18
where N is calculated after the adsorption phase of the O2
2
� 1
)
e
S (BET) (m � g
178
51
0.29
0.33
0.21
1.2
114
51
0.45
0
0.24
2.1
111
25
0.23
0.03
0.28
3.2
�
1
� 1
� 2
which is very fast and does not modify the initial amount of
Activity (mmol � h � g
)
)
18
� 1
O2.
Activity (mmol � h � m
TPR1 (H/Cr)
TPR2 (H/Cr)
5
. Catalytic Activity
�
2
3
TPR2 (H/Cr) � m � 10
�
The fluorination of CF3CH2Cl was carried out at 380 C
under atmospheric pressure in a fixed bed dynamic reactor.
In a first operation, the catalyst was fluorinated in situ for
givesthe change ofthe fluorination activity, ofthe reversible
chromium species (TPR2) over a chromium (III) oxide pre-
2
or 70 h by HF mixed with nitrogen (ratio N2 :HF = 5 :4)
�
at 380 C. CF3CH2Cl was then injected into the reactor in
the presence of HF. The operating conditions of this fluo-
�
pared by dehydration of the hydroxide at 380 C under a
�
nitrogen flow (Cr2O3–N2-380 ) and fluorinated for two dif-
�
rination of CF3CH2Cl were: temperature = 380 C, catalyst
ferent times (2 h and 70 h). When the degree of the fluorina-
tion of the catalyst (F/Cr) increases, the specific area and the
fluorination activity decrease. We can also notice that the
nonprefluorinated chromium (III) oxide and the chromium
weight = 50 mg, contact time = 0.01 s.
HF :CF3CH2Cl :N2 = 4 :1 :5. The same operation was re-
peated with a nonfluorinated catalyst. The catalytic activity
for the CF3CH2F formation was measured both over the
prefluorinated and the nonfluorinated catalyst after a 2 h
reaction with CF3CH2Cl.
(
III) oxide fluorinated 2 h (F/Cr = 0.6) have similar activ-
ity. We observed in a previous paper (9) that the activation
period during the fluorination of CF3CH2Cl was very brief
when no preliminary fluorination of the catalyst was per-
formed. After this period the nonfluorinated catalyst and
the fluorinated (2 h) catalyst had similar catalytic activity. A
preliminary fluorination of the catalyst surface is necessary
for it to be active for the fluorination of CF3CH2Cl.
The products resulting from the reaction were injected
with an automatic sampling valve into a GIRA GC 181 gas
phase chromatograph and analyzed with a flame ionization
detector. The separation was carried out in a capillary col-
umn DB5 (J and W Scientific).
In every case (F/Cr = 0.6 or 1.6), the fluorination of the
RESULTS
catalyst with HF reduces the chromium species on the sur-
During the fluorination of CF3CH2Cl, a fluorination of face (TPR1 = 0), but the total amount of the reversible
the catalyst could be observed which could modify the cata- chromium sites is not modified. However, an increase of
lyst performances. In this paper we study the modifications the amount of reversible oxidized chromium sites per unit
of the catalyst during its fluorination (at different degrees surface can be observe while the intrinsic activity decreases.
of fluorination). We attempt to establish the role played
Figure 1 shows the profile of TPR2 for chromium oxide
by an oxygen catalyst in the fluorination activity. Table 1 with different degrees of fluorination. We observe that
FIG. 1. Profiles of the temperature of the second reduction (TPR2) with the degree of fluorination of the chromium oxide (prepared by activation
�
of the chromium hydroxide at 380 C under nitrogen).

2
90
BRUNET, BOUSSAND, AND MARTIN
FIG. 2. Dioxygen exchange by simple and multiple heteroexchange over chromium (III) oxide (prepared by activation of the chromium hydroxide
�
at 380 C under nitrogen) at different temperatures.
�
the nonfluorinated catalyst and the fluorinated catalyst (III) oxide (Cr2O3–N2-380 C). After the two first cycles,
(
F/Cr = 0.6) show similar behavior. The maximum re- HF/O2 ratio (which represents HF–O2 exchange) is repro-
duction is obtained at a temperature of approximatively ducible with ca 60% of reactants irreversibly adsorbed.
�
3
80 C. The reduction peak of the chromium oxide with the This means that the adsorption of HF makes the adsorp-
F/Cr = 1.6 ratio is larger than the others (corresponding tion of O2 possible and vice versa. This exchange involves a
to higher reduction temperatures). Chromium species are O2/HF ratio approximately equal to 2. This value is too high
more difficult to reduce when the F/Cr ratio increases.
for any surface reaction as the one with H2 and O2 (tem-
perature programmed reductions and temperatures pro-
grammed oxidations) to occur.
Isotopic Exchange of Oxygen Atom
The mobility of the oxygen atom over chromium oxides
prepared by the activation of the hydroxide chromium at
DISCUSSION
�
different temperatures (300–450 C) was measured by iso-
topic exchange 1 O/ O. The catalytic activity depends on
6
18
A detailed description of the catalyst surface for the flu-
orination of CF3CH2Cl is not yet possible. However, the
results obtained in this work (the fluorination of the catalyst
the chemical properties of the oxides catalysts (9, 11, 12).
16
Figure 2 gives the number of O exchanged within a
�
2
50–450 C temperature range. The oxygen exchange be-
�
gan at a temperature around 300 C and the two mech-
anisms (simple and multiple heteroexchange) could be
observed. However, the main reaction was the multiple het-
eroexchange. The maximum difference between the two
mechanism was observed at 380 C.
Table 2 gives the rate of exchange R and the number of
TABLE 2
Isotopic Exchange of Dioxygen and Catalytic Activity for
Chromium (III) Oxides Prepared by Activation of the Chromium
Hydroxide at Different Temperatures
�
0
e
�
Activation temperature ( C)
300
350
380
400
450
�
atoms exchanged (Ne) at 380 C, for chromium oxides acti-
2
� 1
)
S (BET) (m � g
235
33
0.14
199
44
0.22
178
55
0.31
177
54
0.31
86
35
0.41
vated at different temperatures. The exchange rate and the
number of oxygen atom exchanged for the multiheteroex-
change reaction were more significant than with the simple
heteroexchange reaction. These results are similar what-
ever the temperature of activation.
�
1
� 1
)
Activity (mmol � h � g
� 2
)
�
1
Activity (mmol � h � m
Initial exchange rate
�
2
(
�mol � m � min� 1):
16
O2
O
0.4
0.3
0.7
0.5
0.4
0.9
0.6
0.3
0.9
0.6
0.3
0.9
0.7
0.2
0.9
1
8
16O
O2/HF Exchange
Total
N exchange (�mol � m 2):
�
The chromium (III) oxide is reduced after its fluorina-
tion by HF (TPR1 = 0) which could correspond to a lost
of dioxygen. Table 3 gives the amounts of reversible and
1
1
6
8
O2
19
25
19
23
18
O
16O
8
27
10
35
10
29
7
30
13
31
Total
�
exchangeable HF and O2 adsorbed, at 380 C on chromium

PROPERTIES OF CHROMIUM (III) OXIDES
291
and the role of the oxygen) may be used qualitatively. The
TABLE 4
fluorination of the chromium (III) oxide (Cr2O3–N2-380)
decreases its specific surface which could correspond to a
partial �Cr2O3 crystallized phase and Cr–F bond involved
Comparison between the Uptake of HF and of O2 Measured dur-
ing the First Cycles of Pulses and the Uptake of H2 and of O2 Mea-
sured during TPR and TPO
in CrF3 formation determined by XPS (9). Total fluorina-
tion of the catalyst would lead to a CrF3 catalyst structure
which is not active for the fluorination reaction (9). For a
weak F/Cr ratio (0.6 and 1.6) the total amount of reversibly
Catalyst
Cr2O3
–
2
N -380
�
2
3
3
TPR1 (H/Cr) � m � 10
1.9
0.67
1.2
� 2
3
TPO (O/Cr) � m � 10
�
2
oxidable sites is practically constant. However, the nature TPR2 (H/Cr) � m � 10
or the strength of these sites are modified when the degree
� 2
2
3
1
st cycle of HF (HF/Cr) � m � 10
0.39
1.7
0.32
�
3
of fluorination increases. In fact, chromium (III) oxide with 1st cycle of O (O/Cr) � m � 10
2
�
2
3
high F/Cr = 1.6 ratio requires a higher temperature of re- 2nd cycle of HF (HF/Cr) � m � 10
duction and this catalyst has a smaller activity. The presence
of the �Cr2O3 crystallized phase and Cr–F bond involved
in CrF3 formation would lead to chromium species more
difficult to reduce (13). The most active catalysts for the flu- could be adsorbed on the catalyst surface without oxida-
orination of CF3CH2Cl are those which contain chromium tion of the chromium atom. Table 4 gives the comparison
atoms easily reduced by hydrogen (lower temperature of between the uptakes of O2 and of HF during the exchange
reduction).
and the uptakes of O2 and of H2 during the oxidation and
The mobility of oxygen was carried out by isotopic ex- reduction of the chromium (III) oxide. The HF uptake is
change. Two mechanism of exchange can be observed: the smaller than that of H2 uptake. HF reduces only a part of
simple and the multiple heteroexchange, the main reac- the sites reductible by H2. But the O2 uptake is more signif-
tion being the multiple heteroexchange. The temperature icant after the reduction of HF than after H2. The hydrogen
of the activation of the hydroxide chromium does not mod- fluoride would modify the catalyst surface and would create
ify the catalytic activity and the number of oxygen atoms oxygen adsorption sites.
exchanged.
We have shown that two types of HF are present at the
Dioxygen-fluoride hydrogen exchange, measured by catalyst surface; the first type is desorbed only by inert gas,
pulse chromatography can be observed. A O2/HF ratio whereas the second type is stable in the presence of in-
equal to approximately 2 was calculated but we have no ex- ert gas and probably participates in the O2/HF exchange
planation for this. This ratio could not solely be explained and in the CF3CH2Cl fluorination. These two types of HF
by the oxidation by O2 and by the reduction by HF of the lead to a third type of HF which fluorinates irreversibly the
III
catalyst. The O2/HF ratios, correspondingto a reversible ad- catalyst (by the formation of Cr
–
F bond) and causes its
sorption or an O2/HF exchange, are the same. The dioxygen deactivation (9, 23). These three types of HF are related
and fluorinate the catalyst progressively (23).
active fluorine
exchangeable by O2
TABLE 3
III
weak HF *)
→ Cr –F
O2 and HF Uptake Reversible and Exchangeable over Chromium
Oxide (Prepared by Activation of the Chromium Hydroxide at
The presence of oxygen, measured by O2/HF exchange and
by isotopic exchange, maintains the presence of the second
type of HF (labile) without an irreversible fluorination of
the catalyst. In fact, the presence of oxygen slows down the
degree of fluorination of the catalyst.
�
3
80 C under Nitrogen)
Reversible adsorption O2/HF exchange
Total
adsorbed
O2/HF
ratio
O2/HF
ratio
(
�mol)
(�mol) (% )
(�mol)
1
1
2
2
3
3
st cycle of HF
46
30
66
17
41
17
41
65
65
45
43
41
41
16
2.2
3.9
2.4
2.4
2.4
2.3
1.7
2.6
2.3
2.5
APPENDIX
st cycle of O2
nd cycle of HF
nd cycle of O2
rd cycle of HF
rd cycle of O2
102
38
36
21
54
24
59
�
By using 4 moles of HF per mole of CF3CH2Cl, at 380 C,
the CF3CH2Cl transformation involves two reactions, the
fluorination reaction and the dehydrofluorination, which
are thermodynamically limited. The value of the free en-
thalpy (1RG ), of the formation enthalpy (1RH ), and of
the equilibrium constant (Kp) for each reaction were cal-
culated with the theorical values (24) and are reported in
Table 5.
95
�
�
41
100
�
Note. T = 380 C, mCr2O3 = 50 mg.

2
92
BRUNET, BOUSSAND, AND MARTIN
7. WO Patent 92/10481 (Du Pont De Nemours), 1991.
TABLE 5
8
9
. EUR Patent 0546883 A1 (Elf Atochem), 1993.
. Brunet, S., Requieme, B., Colnay, E., Barrault, J., and Blanchard, M.,
Appl. Catal. B 5, 305 (1995).
Thermodynamic Calculated Values for the Reactions
of CF3CH2Cl in an Open System at 653 K
1
0. Brunet, S., Boussand, B., and Barrault, J., Stud. Surf. Sci. Catal. 101,
379 (1996).
�
�
� 1
1
RG
1RH
Product
(molar% )
�
3
Reaction
Fluorination
(KJ mol 1) (KJ mol
)
Kp � 10
11. Barrault, J., Brunet, S., Requieme, B., and Blanchard, M., J. Chem.
Soc. Chem. Commun., 374 (1993).
27.7
34.4
29.7
6.1
CF3CH2F
(14.2)
CF2 == CHCl
12. Brunet, S., Requieme, B., Matouba, E., Barrault, J., and Blanchard,
M., J. Catal. 152, 70 (1995).
(
Cl/F exchange)
Dehydrofluorination
131.9
1.2
13. Burwell, R. C., Jr., Haller, G. L., Taylor, K. C., and Read, J. F., Adv.
Catal. 20, 29 (1969).
(
–HF)
(1.2)
1
1
1
1
1
1
4. Duprez, D., J. Chim. Phys. 80, 487 (1983).
5. Martin, D., and Duprez, D., J. Chim. Phys. 100, 9429 (1996).
6. Winter, E. R. S., J. Chem. Soc. 1, 2889 (1968).
7. Winter, E. R. S., Adv. Catal. 10, 186 (1958).
8. Boreskov, G. K., Adv. Catal. 15, 285 (1964).
9. Boreskov, G. K., and Muzykantov, V. S., Ann. N.Y. Acad. Sci. 213, 137
ACKNOWLEDGMENT
We thank the Elf-Atochem company for financial support.
(
1973).
20. Novakova, J., Catal. Rev. 4, 77 (1970).
REFERENCES
21. Muzykantov, V. S., Popovskii, V. V., and Boreskov, G. K., Kinet. Catal.
1
2
3
4
5
6
. Manzer, L. E., Catal. Today 13, 13 (1992).
. Manzer, L. E., Science 249, 31 (1990).
. Kohne, A., and Kemnitz, E., J. Fluorine Chem. 75, 103 (1995).
. Jpn Patent, J.P. 03 261731 (Daikin), 1992.
. EUR Patent 0449617, A2 (Imperial Chemical Industries), 1991.
. EUR Patent 0331991 (Du Pont De Nemours), 1989.
5, 624 (1964).
22. Kemnitz, E., Menz, D. K., Stocker, C., and Olesch, T., Thermochim.
Acta 225, 119 (1993).
23. Kijowski, J., Webb, G., and Winfied, J., Appl. Catal. 27, 181 (1986).
24. Stull, R., Wesrum, E. F., Jr., and Sinke, G. C., “The Thermodynamics
of Organic Compounds,” Wiley, New York, 1969.