JOURNAL OF CATALYSIS 164, 378–386 (1996)
ARTICLE NO. 0394
Catalytic Hydrodechlorination of 1 ,1 -Dichlorotetrafluoroethane
by Pd/Al O
2
3
1
2
Zbigniew Karpinski, Kintu Early, and Julie L. d’Itri
Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Received February 28, 1996; revised September 4, 1996; accepted September 5, 1996
tion of CF3CFCl2 to yield CF3CFH2, a replacement used
Palladium supported on � -alumina displays high activity for the in automobile air-conditioning units (2). Another, the hy-
hydrodechlorination of 1,1-dichlorotetrafluoroethane. High H2 par- drodechlorination of CF ClCFCl , produces CF == CFH, a
2
2
2
tial pressures are needed to avoid deactivation, and steady state
monomer used in the production of Teflon-like polymers
4). Both of these reactions require selective cleavage of
is obtained after � 5 h time on stream. Under these conditions
(
(
H2/CFC feed ratio = 20) the reaction is zero order in H2 partial
C–Cl bonds which in turn requires highly selective catalysts.
Numerousexamplesof hydrodechlorination catalystsare
described in the literature (5–9), many of which emphasize
the superior performance of palladium supported on vari-
ous high surface area materials (2, 6, 10, 11). Several fac-
tors have been identified which influence the activity and
pressure and positive (0.65) order in 1,1-dichlorotetrafluoroethane
partial pressure. Three main products are formed: 1,1,1,2-tetra-
fluoroethane, 1-chloro-1,2,2,2-tetrafluoroethane, and 1,1,1-tri-
fluoroethane, with approximately 85% selectivity toward the de-
sired CF3CFH2. The apparent activation energies associated with
the formation of each product range from 52 to 68 kJ/mol. All three
major products have a nonzero rate of formation in the limit of zero selectivity of hydrodechlorination catalysts. These factors
conversion, the implication of which is that all are primary prod- include support material, metal particle size, and reaction
ucts. The kinetics results are consistent with a reaction mechanism conditions. Coq et al. observed that increasing the Pd par-
involving a carbene intermediate. Variation of the temperature at
which the catalyst is prereduced from 300 to 600 C results in an
ticle size from 1.5 to 8 nm resulted in higher activity for
CF2Cl2 conversion of CF2H2 (11). Further, Pd supported
on AlF3 was found to have higher initial activity than Pd
supported on either carbon or Al2O3. However, the cata-
lytic activity of Pd supported on Al2O3 has been shown to
increase as a function of time on stream for the conversion
of CF2Cl2. As methane was observed as a product, Coq et al.
suggested that the hydrogenolysis of C–F bonds results in
�
increase in particle size from 11–53 nm and in an increase in the
�
1
hydrodechlorination TOF from 2.3 to 5.0 s
.
�c 1996 Academic Press, Inc.
INTRODUCTION
Chlorofluorocarbons (CFCs) are industrially important
compounds which are extremely stable. Because of their
stability, CFCs emitted into the atmosphere do not react
before reaching the stratosphere. Once in the stratosphere
the CFCs photodissociate, producing Cl radicals which par-
ticipate in ozone depletion reactions (1). This damaging be-
havior has resulted in the phase-out of their production and
the creation of a market for alternative compounds (2). The
development of CFC alternatives has focused on nonchlo-
rinated compounds which have similar physical properties
but will break down in the lower atmosphere. One such
class of alternatives is the hydrofluorocarbons (HFCs) (3).
While there are a number of synthetic routes for the pro-
duction of HFCs, one of the simplest methods involves
the selective hydrodechlorination of CFCs. One poten-
tially important industrial process is the hydrodechlorina-
surface fluorine species (11, 12, 17). These fluorine atoms
were subsequently incorporated into the Al2O3 lattice, thus
transforming it, at least partially, into the more active AlF3
support. Finally, the reaction conditions also influence ac-
tivity in the hydrodechlorination of CF2Cl2. For CFC/H2
feed ratios in the range 0–3, the activity goes through a
maximum (11).
Far less fundamental information is available on the
hydrodechlorination of chlorofluoroethanes. Clear differ-
ences between the ethanes and methanes are illustrated
by the study of CF3CFHCl hydrodechlorination by Kellner
and Rao (10). In contrast to the studies by Coq et al. with
CF2Cl2 (11, 12), very high (nearly 100% ) selectivity toward
CF3CFH2 was achieved with palladium supported either on
AlF3 or fluorinated Al2O3. Important differences in reac-
tivity have also been observed among the different ethanes.
Gervasutti et al. found that over a Pd/C catalyst CF3CFCl2
is much more readily hydrodechlorinated than its structural
isomer, CF ClCF Cl (6).
1
On leave from the Institute of Physical Chemistry, Polish Academy
of Sciences.
2
To whom correspondence should be addressed.
2
2
378
0021-9517/96 $18.00
Copyright �c 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.