A kinetic and mechanistic study of the Cl/F exchange reaction of CCl3F, CCl2F2, and CClF3 with prefluorided chromia

Article abstract of DOI:10.1021/jp0205259

A kinetic and mechanistic study of the Cl/F exchange reaction of CCl₃F, CCl₂F₂, and CClF₃ with prefluorided chromia was conducted. The activation energies of 53 and 54 kJ/mol for the simultaneous mono- and bi-exchange of the chlorine atoms of CCl₃F with the surface fluoride ions of a prefluorided chromia were lower than that found for CCl₄ (65 kJ/mol). This was due to the stronger C-F bond formed in CCl₂F₂ and CClF₃, the C-Cl bond strengths in CCl₄ and CCl₃F were roughly the same. Mono-exchange occurred through a four-center intermediate in which the breaking of a C-Cl bond is compensated for by the formation of a C-F bond. Bi-exchange occurred by the CCl³F adsorption by the bonding of two Cl atoms to two appropriately spaced Cr ions on the CrF surface. Mono-exchange only was observed in the reaction of CCl₂F₂, the activation energy for which was 84 kJ/mol. This higher activation energy was due to the higher C-Cl (～ 318 kJ/mol) bond strength in CCl₂F₂ than in CCl₃F (D(C-Cl) = 305 kJ/mol). The absence of the occurrence of bi-exchange producing CF₄ was due to the instantaneous formation of CClF₃ on the intermediate, which was adsorbed through two chlorine atom where bi-exchange should have been possible. No exchange of the C-Cl bond of CClF₃ with the surface CrF was observed.

Full text of DOI:10.1021/jp0205259

J. Phys. Chem. B 2002, 106, 9567-9575
9567
A Kinetic and Mechanistic Study of the Cl/F Exchange Reaction of CCl3F, CCl2F2, and
CClF3 with Prefluorided Chromia
A. Farrokhnia, B. Sakakini, and K. C. Waugh*
Department of Chemistry, Faraday Building, UMIST, PO Box 88, Manchester M60 1QD, U.K.
ReceiVed: February 26, 2002; In Final Form: June 17, 2002
Chromium oxide has been fluorided to several monolayers depth by interaction with HF at 625 K. The
interaction of CCl3F and CCl2F2 with these surface chromium fluoride layers has been studied by temperature-
programmed reaction. In the case of the reaction of CCl3F, mono- and bi-exchange occurs with the surface
monolayer fluoride, forming CCl2F2 and CClF3 simultaneously at a peak maximum temperature of 520 K.
Exchange with the subsurface fluoride layers occurs at a peak maximum temperature of 680 K, as the fluoride
ions from the bulk migrate to the surface and chloride ions on the surface migrate inward. In both temperature
regions, mono-exchange predominates by a factor of about 102. Line-shape analysis of the temperature
dependence of the rates of production of CCl2F2 and CClF3 gave activation energies of 53 and 54 kJ mol-1
for exchange with the surface fluoride, so the difference in rates must be the need for special dimer-type
surface sites for the bi-exchange. The activation energy for exchange of F ions evolving from the bulk calculated
by line-shape analysis is 83 kJ mol-1 for mono-exchange and 100 kJ mol-1 for bi-exchange. These higher
values than those obtained by reaction with the surface fluoride ion derive from the added activation energy
for migration of the fluoride ion from the bulk to the surface.
Introduction
This mechanism (reactions 4-8) is based on our previously
published work on the fluorination of CCl4 in which we showed
The Montreal protocol in 1989 banned production and use
of chlorofluorocarbons (CFCs) because of their ability to destroy
the ozone layer. This is thought to be accomplished by the
following chain reaction (reactions 1-3), which leads to the
net destruction of O3 and O• atoms.
that the fluorination reaction proceeds by the exchange of the
chlorine atom of the CCl4 with the F atom/ion of the fluorided
chromium (usually surface CrF3).1,2 The role of the HF was
considered to be to refluoride the surface of the CrF3, which
had been partially chlorided (reaction 6).
In the study reported in this paper, we have determined the
kinetics and mechanism of the fluorination of CCl3F and CCl2F2
over a prefluorided Cr2O3 (CrF3) as models for reactions 5 and
7, respectively, of the above mechanism. We use these model
systems to avoid complications of reactions involving chlorine/
fluorine exchange on the second C atom.
RC-Cl
9
hν8 RC• + Cl•
(1)
(2)
Cl• + O3 f ClO + O2
ClO + O• f Cl + O2
(3)
Experimental Section
The CFCs have been replaced by hydrofluorocarbons (HFAs),
which are not sufficiently stable to rise through the atmosphere
to the ozone layer. One of the HFA replacements is 1,1,1,2-
tetrafluoroethane (134a), which is produced by reaction of 1,1,2-
trichloroethene with HF over a prefluorided chromia catalyst.
The reaction is considered to proceed according to the following
mechanism.
Apparatus. The microreactor and gas-handling system have
been previously reported.1 The microreactor is a U-shaped (30
cm long, 0.6 cm i.d.) monel microreactor tube, which was
connected via a heated capillary to an on-line, computer-
interrogated, mass spectrometer (Hiden Analytical, Warrington,
England). It is used for temperature-programmed reaction (TPR),
temperature-programmed desorption (TPD) in which 16 masses
can be followed with temperature/time and in-situ surface area
measurements.
Catalyst. The catalyst that was used in this study is gel
chromium oxide. The preparation of this catalyst has been
previously described. The calcined chromium(III) oxide was
crushed and sieved into particles size of 300 to 350 µm. The
total surface area of the catalyst was determined to be 200 m2
g-1 by applying the BET method, using N2, adsorption at 77
K. X-ray diffraction (XRD) of the powder showed that the
material contained only amorphous chromium oxide.
CrF3
CCl2dCHCl + HF
CFCl2CH2Cl + CrF3 f CF2ClCH2Cl (132a) + CrF2Cl (5)
CrF2Cl + HF f CrF3 + HCl (6)
8 CFCl2CH2Cl (131a)
(4)
CF2ClCH2Cl + CrF3 f CF3CH2Cl (133a) + CrF2Cl (7)
CF3CH2Cl + CrF3 f CF3CH2F (134a) + CrF2Cl (8)
Catalyst Pretreatment. The chromium(III) oxide catalyst
(∼0.2 g) was loaded into the microreactor tube, and before any
* To whom correspondence should be addressed. Tel: 44 (0) 161 200
4503. Fax: 44 (0) 161 200 4430. E-mail: ken.waugh@umist.ac.uk.
10.1021/jp0205259 CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/24/2002

9568 J. Phys. Chem. B, Vol. 106, No. 37, 2002
Farrokhnia et al.
Figure 1. Temperature-programmed reaction of CCl3F over HF-pretreated Cr2O3.
measurement and reaction, it was heated under helium (25 cm3
min-1, 101 kPa) to 623 K and was left at this temperature for
30 min to remove any adsorbed water. The catalyst was then
fluorinated in situ by 10% HF in He (25 cm3 min-1, 101 kPa)
for approximately 90 min at 623 K. The total amount of HF
passed was always 200 cm3 or 8.2 × 10-3 mol or 2.5 × 1022
fluorine atoms per gram of catalyst. The surface area of the
fluorided chromia was measured in situ to be 103 m2 g-1, so
assuming unit reaction probability of the HF with the oxide,
this corresponds to a coverage of the oxide with fluoride ions
of 1.9 × 1017 ions cm-2, which, if the assumption is correct,
means that the surface and several layers of the bulk of the
Cr2O3 have been fluorinated. The catalyst was then sealed,
removed from the fluorinating unit, and connected to the mass
spectrometer.
Results and Discussion
The Temperature-Programmed Reaction of CCl3F over
Fluorided Cr2O3. The temperature-programmed reaction of
CCl3F on fluorided Cr2O3 is shown in Figure 1. A CCl3F mixture
(∼4% CCl3F, 101 kPa) was produced by bubbling He (25 cm3
min-1, 101 kPa) through liquid CCl3F held in a slush bath of
Figure 2. The temperature dependence of the rate production and curve
fit (a) for CCl2F2, m/z ) 85 and (b) for CClF3, m/z ) 69. The solid
line is the experimental data; the dashed line is the curve fitting to the
data.
liquid nitrogen/chlorobenzene at 228 K. Initially, the CCl3F/
He mixture was flowed over the catalyst for 12 min at 296 K
to ensure adsorption equilibrium, after which the temperature
was increased linearly from 296 to 773 K at a rate of 5 K min-1
,
following masses m/z ) 117 (CCl4), m/z ) 101 (CCl3F), m/z )
85 (CCl2F2), and m/z ) 69 (CClF3) on the mass spectrometer.
The temperature-programmed reaction profile so produced is
shown in Figure 1.
Several points can be made by inspection of the Figure 1.
The initial increase in CCl3F concentration between 300 and
350 K derives from the desorption of physisorbed CCl3F from
the surface of the catalyst. The remaining profile can be divided
into two regions: (i) the low-temperature region, where the ion-
exchange reaction between CCl3F (m/z ) 101) and the catalyst,
CrF3, begins at around 400 K, and (ii) the high-temperature
region, where the bulk to surface fluorine/chlorine ion-exchange
reaction begins at about 600 K.
the kinetics of the surface exchange reaction and to determine
the kinetics of the exchange reaction of CCl3F with a F ion on
the surface, which had migrated there from the bulk. The
temperature dependence of the rate of production of CClF3 (with
the curve fitting) is shown in Figure 2b. It is clear from these
figures that mono-exchange (producing CCl2F2) and bi-exchange
(producing CClF3) occur simultaneously. Simultaneous mono-
and bi-exchange had been observed previously with CCl4 as
the reactant.1
Evaluation of the Detailed Energetics of the Process by
Line-Shape Analysis of Figure 1. The Heat of Adsorption of
CCl3F. The temperature dependence of the CCl3F line shape in
the temperature range 300-350 K in Figure 1 contains data
for the measurement of the heat of adsorption of CCl3F. Figure
3 shows an expanded version of the profile of the temperature
Figure 2a shows the temperature dependence of the rate of
production of CCl2F2 with the curve fitting used to determine

Cl/F Exchange Reaction
J. Phys. Chem. B, Vol. 106, No. 37, 2002 9569
Figure 3. A profile of temperature dependence of CCl3F desorption from fluorided Cr2O3.
dependence of the rate of desorption of CCl3F. If the rate of
desorption is given by
exchange reaction occurs between chemisorbed CCl3F and
surface F-. (The loss of gas-phase CCl3F upon the production
of CCl2F2 shows the chemisorbed material to be replenished
from the gas phase.) Assuming the reaction to be first-order in
CCl3F, the reaction is described by eq 12 and the rate of
production of CCl2F2 is given by eq 13.
-d[CCl3F]
-E/(RT)
) A
e
[CCl3F]
(9)
ex
dt
where [CCl3F] is the amount of CCl3F that is desorbed in units
of mol cm-3, Aex and E are the A-factor and desorption
activation energy, which in this physisorption process is equal
to ∆Ha, the heat of adsorption. The rate of desorption is
proportional to the height, h, of the mass spectrometer response,
and the amount of [CCl3F] adsorbed is proportional to the area,
A, of the CCl3F peak. Equation 9 can be rewritten as
CCl3F + F-(s) f CCl2F2 + Cl-
(12)
(13)
(s)
d[CCl2F2]
) A e-E/(RT) [CCl3F][F-
]
(s)
dt
where [CCl3F], [CCl2F2], and [F-(s)] are the concentrations of
each species in units of mol cm-3 and Aex and E are the A-factor
and activation energy, respectively, for the exchange reaction.
Figure 5 shows the rate of production of CCl2F2. The concentra-
tions of CCl3F and CCl2F2 are given by the height of the mass
spectrometer response. Each of these species is multiplied by
its respective calibration constant, that is, [CCl3F] in mol cm-3
) k1h1 and [CCl2F2] in mol cm-3 ) k2h2 and the surface F-
ion concentration is given by the area, A, under the CCl2F2 peak
so that eq 13 can be rewritten as
k1h ) Aex e-E/(RT) k2A
(10)
or
k2
h
A
E
ln ) lnAex + ln
-
(11)
( )
k1 RT
where k1 and k2 are calibration constants.
A plot of ln(h/A) versus 1/T gives the desorption activation
energy. The integration of the area was taken to 339 K, because
this was the temperature at which a minimum in the rate of
desorption of CCl3F was observed. The desorption activation
energy of CCl3F is obtained by plotting ln(h/A) versus 1/T in
the temperature range 293-303 K for the line shape shown in
Figure 3, which gives a straight line is shown in Figure 4. A
value of 34 kJ mol-1 is obtained for desorption activation
energy, which is characteristic of physical adsorption.3
fk2h2 ) Aex e-E/(RT) k1h1A
(14)
or
h2
E
k1
ln
) ln(Aex ) - ex + ln
(15)
(
)
( )
h1A
RT
fk2
where f is the flow rate in cm3 s-1
.
The ActiVation Energies for the Formation of CCl2F2 and
CClF3. The activation energies for the interaction of chemi-
sorbed CCl3F with surface and bulk F- ions of the prefluorided
Cr2O3 forming CCl2F2 and CClF3 can be obtained by line-shape
analysis of the temperature dependence of the CCl2F2 peaks at
520 and 684 K and of the CCl3F peaks at 520 and 680 K. The
line-shape analysis technique for calculation of the activation
energies of the above reactions is as follows. The halogen
A plot of ln(h2/(h1A)) versus 1/T gives the exchange activation
energy. The surface exchange activation energy for the formation
of CCl2F2 is obtained by plotting ln(h2/(h1A)) versus 1/T in the
temperature range 400-490 K for the deconvoluted [CCl2F2]
line shape shown in Figure 5. The Arrhenius plot for the
determination of the surface exchange activation energy is
shown in Figure 6 from which a value of 53 kJ mol-1 is obtained
for the reaction of an adsorbed CCl3F molecule with a surface

9570 J. Phys. Chem. B, Vol. 106, No. 37, 2002
Farrokhnia et al.
Figure 4. Arrhenius plot of ln(h/A) versus 1/T for determination of desorption activation energy.
Figure 5. Temperature dependence of the rate of production of CCl2F2.
TABLE 1: The Amounts of CCl2F2 and CClF3 Formed in
the Low-Temperature Region (at 520 K) and the
High-Temperature Region (at 680 K) by
F- ion. Justification of the assumption that the 520 K CCl2F2
peak results from the exchange of CCl3F with a surface F- and
the assumption that the 684 K peak results from exchange with
a bulk F- ion is found by calculation of the amounts of F-
exchanged in each peak; see below and Table 1.
Temperature-Programmed Reaction of CCl3F over
Fluorided Chromia Catalyst
surface F
reaction peak
(520 K)
exchange reaction with a F
ion that had migrated from
the bulk to the surface
An identical analysis of the temperature dependence of the
rate of production of CCl2F2 in the temperature range 640-
666 K gave an activation energy of 83 kJ mol-1 for the surface
exchange reaction between chemisorbed CCl3F and a F- ion
that had evolved at the surface by diffusion from the bulk of
the fluorided chromia. The Arrhenius plot for this reaction is
shown in Figure 7. The same analysis was used for the
temperature dependence of the rate of formation of CClF3 from
which a value of 54 kJ mol-1 was obtained for the surface
exchange reaction and 100 kJ mol-1 for exchange with F- ions
emanating at the surface from the bulk. The nearly identical
values for the activation energy for the mono- and bi-exchange
CCl2F2
amount
2.03 × 1020 molecule or
5.2 × 1020 molecule or
4.1 × 1021 molecule g-1
99.8%
1.6 × 1021 molecule g-1
selectivity 98.7%
CClF3
amount
3.3 × 1018 molecule or
4.9 × 1019 molecule or
3.9 × 1020 molecule g-1
0.2%
2.6 × 1019 molecule g-1
selectivity 1.3%
suggests that a similar type of intermediate is involved in both
reactions; see later.

Cl/F Exchange Reaction
J. Phys. Chem. B, Vol. 106, No. 37, 2002 9571
Figure 6. Arrhenius plot of ln[h2/(h1A)] versus 1/T for the determination of the activation energy of CCl3F with surface F- to form CCl2F2.
Figure 7. Arrhenius plot of ln[h2/(h1A)] versus 1/T for the determination of the activation energy for the exchange reaction (to form CCl2F2)
between CCl3F and F- ions evolving at the surface from the bulk of the fluorided chromium oxide.
Table 1 lists the amount of CCl2F2 and CClF3 by both the
surface exchange reaction and by exchange of F- ion that had
migrated to the surface from the bulk. The total surface fluoride,
which removed from the catalyst in the first two peaks, was
1.7 × 1021 atoms g-1 (0.127 g catalyst, total surface area 103
m2 g-1), while the amount in the higher temperature peaks was
4.9 × 1021 fluorine ions g-1. The total amount of fluorine
removed from the catalyst in the higher temperature region
constitutes 4.74 × 1015 ions cm-2. This is greater than five
monolayers of fluorine confirming that the higher temperature
product peaks derive from F- in bulk CrF3 migrating to the
surface.
is probably a function of the low surface coverage of ap-
propriately spaced Cr3+ sites for bidentate adsorption of the
CCl3F.
A final point of note is that the total amount of fluorine in
the 520 K CCl2F2 and CClF3 peaks, which is considered to be
the surface exchange reaction, is 2.08 × 1020 atoms, corre-
sponding to a surface coverage of 1.59 × 1015 ions cm-2 or a
fluorine ion area of 6.3 Å2 or a fluorine ion radius of 1.4 Å.
This latter value accords well with the quoted value of the
fluorine ion radius of 1.33 Å.4 This results gives added credence
to the suggestion that the 520 K peaks derive from exchange
of the surface F- species.
The product percentage ratio of CCl2F2/CClF3 in the first two
peaks was 98.7:1.3 and in the second two peaks was 99.8:0.2,
which clearly shows that the reaction is highly selective for
mono-exchange producing CCl2F2. The low selectivity to CClF3
The activation energy for mono chloro/fluoro exchange of
CCl4 is 65 kJ mol-1.1 This is slightly higher than the value of
53 kJ mol-1 found here for CCl3F. Our previous paper1
postulated that the ion exchange reaction proceeds by a

9572 J. Phys. Chem. B, Vol. 106, No. 37, 2002
Farrokhnia et al.
TABLE 2: C-Cl and C-F Bond Dissociation Energies (kJ
mol-1) in Chlorofluoromethane4
X
X-CF3
X-CClF2
X-CCl2F
X-CCl3
F
Cl
546
360.7
514.6
318
460
305
426
306
Langmuir-Hinshelwood mechanism through a four-center inter-
mediate. The structure of four-center intermediate that we
consider is likely to be involved is shown in intermediate 1.
Figure 8. A cut-away model of the (001) face of CrF3 with a CCl3F
molecule adsorbed on it for bi-exchange reaction together with the
product CClF3 and the catalyst after the reaction.
The exchange reactions of both CCl4 and CCl3F require the
breaking of a C-Cl and a Cr-F bond and, at the same time,
the formation of a C-F and a Cr-Cl bond. As a result, during
the exchange process, the formation of the reaction intermediate
depends on the strengths of the bonds being formed or broken.
The bond strengths of C-F and C-Cl bonds in different
chlorofluoromethanes are reported in Table 2.4
and the smaller chromium ions are shown to scale. The CCl3F
molecule can adsorb with two of the Cl atoms interacting with
two exposed Cr3+ ions. This will allow simultaneous abstraction
of the Cl atoms from the adsorbed CCl3F molecule and
substitution of them by the F of the surface.
Because the bond strengths of Cr-F and Cr-Cl are similar
(∼356 kJ mol-1), the driving force for the reaction is the
formation of the stronger C-F bond. The fluorination of CCl4
to form CCl3F requires 306 kJ mol-1 of energy to dissociate
the Cl-CCl3 bond, which is more than compensated for by the
formation of the F-CCl3 bond (D(C-F) ) 426 kJ mol-1), eq
16. On the other hand, in the fluorination of CCl3F to form
CCl2F2, dissociation of the Cl-CCl2F bond requires 305 kJ
mol-1, while the formation of the F-CCl2F bond releases 460
kJ mol-1, eq 17.
Another point of note in relation to Figure 1 is the production
of CCl4 from CCl3F. This reaction would occur through a four-
center intermediate (intermediate 3) resulting from the reaction
of chemisorbed CCl3F with partially chlorided catalyst (CrF2-
Cl) shown in Figure 8.
CrF3
Cl 306 CCl3
8 F 426 CCl3
Ea ) 65 kJ mol-1 (16)
Ea ) 53 kJ mol-1 (17)
CrF3
Cl 305 CCl2F
8 F 460 CCl2F
Temperature-Programmed Reaction of CCl2F2 over HF
Prefluorided Cr2O3. The temperature-programmed reaction of
CCl2F2 over prefluorided chromium(III) oxide is shown in
Figure 9. The catalyst is pretreated with HF, using 10% HF in
He (25 cm2 min-1, 101 kPa) for approximately 1 h at 623 K.
The reactant gas, 5% CCl2F2 in He (25 cm3 min-1, 101 kPa),
is prepared by choosing the appropriate flow rates of both
CCl2F2 and helium using flow meters. The temperature-
programmed reaction profile (Figure 9) is obtained by passing
CCl2F2 (5% in He) continuously over the catalyst while raising
Therefore because of the formation of the stronger C-F bond
in the fluorination of CCl3F, the exchange activation energy of
CCl3F is smaller than that of CCl4.
As is clear from Figure 2a,b, the production of CCl2F2 and
CClF3 from CCl3F occurs simultaneously. Because the temper-
ature dependences of the rates of production of CCl2F2 and
CClF3 are identical, it is highly unlikely that the production of
CClF3 derives from the sequential fluorination of CCl2F2. Were
it to have been sequential, a small delay in the rate of production
of CClF3 would have been expected. The intermediate that
would allow bi-exchange, is a “bi-four-center” intermediate
(intermediate 2).
the temperature from ambient to 773 K at 5 K min-1
.
Only CClF3 is observed. No CF4 is detected. Therefore,
whereas mono- and bi-exchange were observed with CCl4 and
CCl3F as the reactants, bi-exchange is not observed with CCl2F2
as the reactant. Because the availability of the appropriately
spaced two Cr3+ ions on the surface CrF3 in the catalyst is the
same and because the distance between the Cl atoms on CCl2F2
will be the same as that between two Cl atoms on CCl4 and
CCl3F, the lack of observation of bi-exchange cannot be
structural. It must therefore be energetic.
It could be a function of the reaction dynamics. If the first
Cl/F exchange occurs fractionally before the second, then in
the case of the reaction of CCl2F2, the first product will be
CCl3F, the C-Cl bond strength of which is 361 kJ mol-1. This
high C-Cl bond strength probably prevents the near-simulta-
The adsorption of CCl3F on the 001 surface of CrF3 for the
bi-exchange reaction is shown in Figure 8. The Cl- and F- ions

Cl/F Exchange Reaction
J. Phys. Chem. B, Vol. 106, No. 37, 2002 9573
Figure 9. Temperature-programmed reaction of CCl2F2 over HF-pretreated Cr2O3.
Figure 10. Temperature dependence of the rate production of CClF3.
neous bi-exchange from occurring. It also suggests that, whereas
in the cases of the bi-exchange of CCl4 and CCl3F we had
argued that bi-exchange was a concerted and simultaneous
process, it is probable that the first exchange occurs fractionally
earlier (say 10-10 s) than the second. (It should be noted that
no exchange reaction is observed with CClF3 as reactant (see
later). This observation supports the C-Cl bond-strength
argument.)
the temperature dependence of the two product peaks at 570
and 600 K. The line-shape analysis technique for calculation
of the activation energies of the above reactions is exactly the
same one that was used for the CCl3F and CCl4 reaction. The
crude deconvolution of the CClF3 reaction profile used to
calculate the exchange activation energies is shown in Figure
10. Again the reaction is assumed to be first-order in CCl2F2
(eq 18), and the rate of production of CClF3 is given by eq 19.
Another point to note in relation to Figure 9 is the production
of CCl3F from CCl2F2. It is being suggested here that the
chlorination reaction of CCl2F2 to CCl3F occurs with the same
mechanism as that involved in the fluorination reaction. Here,
however, it can be seen from Figure 9 that production of CClF3
only occurs after some significant production of CClF3 is
observed, which produces a considerably chlorided surface.
The ActiVation Energies for the Formation of CCl3F and
CClF3. The activation energies for the interaction of chemi-
sorbed CCl2F2 with surface and bulk F- ions of the fluorided
Cr2O3 to form CClF3 can be obtained by line-shape analysis of
CCl2F2 + F-(s) f CClF3 + Cl-
(18)
(19)
(s)
d[CClF3]
) Aex e-E/(RT) [CCl2F2][F-
]
(s)
dt
where [CCl2F2], [CClF3], and [F-(s)] are the concentration of
each species in units of mol cm-3 and Aex and E are the A-factor
and activation energy, respectively, for the exchange reaction.
The concentrations of CCl2F2 and CClF3 are given by the height
of the mass spectrometer response.

9574 J. Phys. Chem. B, Vol. 106, No. 37, 2002
Farrokhnia et al.
Figure 11. Arrhenius plot of ln[h2/(h1A)] versus 1/T for the determination of the activation energy of CCl2F2 with surface F- to form CClF3.
Figure 12. Temperature-programmed reaction of CClF3 over prefluorided Cr2O3.
As explained in the previous equation, a plot of ln(h2/(h1A))
versus 1/T gives the exchange activation energy. The surface
exchange activation energy for the formation of CClF3 is
obtained by plotting ln(h2/(h1A)) versus 1/T in the temperature
range 550-575 K for the deconvoluted [CClF3] line shape
shown in Figure 10. The Arrhenius plot for the determination
of the surface exchange activation energy is shown in Figure
11 from which a value of 84 kJ mol-1 is obtained for the surface
F/Cl exchange and 110 kJ mol-1 for the exchange with F- ions
emanating at the surface from the bulk.
possibility of substitution of the chlorine atom of CClF3 with a
F- atom. For this purpose, first the reaction of CClF3 was
investigated over fluoride chromia. Figure 12 is the temperature-
programmed reaction mass spectrum obtained by flowing CClF3
in He (5% CClF3, 25 cm3 min-1, 101 kPa) continuously over
the catalyst (CrF3), while raising the temperature from ambient
to 773 K at a heating rate of 5 K min-1
.
Figure 12 clearly shows that no fluorination, chlorination, or
decomposition reaction occurs. This is in agreement with the
findings of Kemnitz et al.5 who have reported that fluorination
of CClF3 by CrF3 does not occur. As explained earlier for our
absence of the observation of the formation of CF4 from CCl2F2,
the value of the C-Cl bond strength of the CCl3F molecule
(361 kJ mol-1) results in too high an activation energy for the
exchange reaction.
Second, another TPR was conducted over CrCl3 to investigate
the possibility of the chlorination reaction on this catalyst. The
chromium(III) oxide (0.2 g) was loaded in the microreactor; it
was heated under He (25 cm3 min-1, 101 kPa) to 623 K and
was left at this temperature for 45 min and then was chlorided
by 4% CCl4 in He (25 cm3 min-1, 101 kPa).
As Figure 10 clearly shows the mono-exchange reaction
occurs at around 650 K. This is higher than mono-exchange
reaction of CCl4 and CCl3F so the activation energy is expected
to be higher, as we have found.
CrF3
Cl 318 CClF2
8 F 515 CClF2
Ea ) 84 kJ mol-1 (20)
The C-Cl bond in CCl2F2 is 318 kJ mol-1, which is higher
than that in CCl3F and CCl4 (∼305 kJ mol-1), and although
this is conpensenated for by a higher C-F bond strength (D(C-
F) in CCl2F2 ) 515 kJ mol-1), the net effect is a higher
activation energy for the exchange reaction.
Figure 13 shows the temperature-programmed reaction of
CClF3 over CrCl3 from room temperature to 773 K. There was
no indication of any reaction. Therefore, the CClF3 does not
Temperature-Programmed Reaction of CClF3 over CrF3
and CrCl3. The final part of this study is to investigate the

Cl/F Exchange Reaction
J. Phys. Chem. B, Vol. 106, No. 37, 2002 9575
Figure 13. Temperature-programmed reaction of CClF3 over CrCl3.
react on the CrCl3 catalyst either. This is probably because this
molecule has the highest C-F bond strength, and the exchange
of the fluorine atom by a chlorine atom is likely to have a high
activation energy.
CF4 is thought to be due to the instantaneous formation of CClF3
on the intermediate, which is adsorbed through two chlorine
atom where bi-exchange should have been possible. The C-Cl
bond strength in CClF3 is 361 kJ mol-1, this high value
prohibiting bi-exchange. This result suggests that, whereas the
results of bi-exchange of CCl2F2 suggest simultaneous two atom
exchange, the detailed dynamics occur by one fluorine/chlorine
exchange occurring with CCl2F2 being held in a bidentate mode
on the Cr-F surface. The second exchange occurs immediately
afterward without desorption of the reaction.
No exchange of the C-Cl bond of CClF3 with the surface
CrF was observed. This is probably due to the high bond
strength of the C-Cl bond (∼361 kJ mol-1) in this molecule.
The mechanism presented here accounts for the formation
of the fluorohydrocarbon (134a) by reaction of 1,1,2-trichloro-
ethene with HF over a prefluorided Cr2O3. Exchange of the
chlorine atoms of the adsorbed chlorofluorocarbon occurs with
the surface fluoride ions, the resulting surface chloride ion being
replaced by exchange with the HF.
Conclusion
The activation energies of 53 and 54 kJ mol-1 for the
simultaneous mono- and bi-exchange of the chlorine atoms of
CCl3F with the surface fluoride ions of a prefluorided chromia
are lower than that found for CCl4 (65 kJ mol-1). It is probable
that this is due to the stronger C-F bond formed in CCl2F2
and CClF3, the C-Cl bond strengths in CCl4 and CCl3F being
roughly the same.
It is suggested that mono-exchange occurs through a four-
center intermediate in which the breaking of a C-Cl bond is
compensated for by the formation of a C-F bond. Bi-exchange
is thought to occur by the adsorption of CCl3F by the bonding
of two Cl atoms to two appropriately spaced Cr ions on the
CrF surface.
Mono-exchange predominates by a factor of 102 even though
the activation energies are nearly the same. It is suggested that
this is due to the scarcity of specific sites on the CrF surface
for the adsorption of CCl3F through two chlorine atoms to two
surface Cr ions.
References and Notes
(1) Farrokhnia, A.; Sakakini, B.; Waugh, K. C. J. Catal. 1998, 174,
219.
(2) Farrokhnia, A.; Sakakini, B.; Waugh, K. C. Catal. Lett. 2001, 76
(3-4), 241.
(3) Gasser, R. P. H. An Introduction to Chemisorption and Catalysis
by Metals; Clarendon Press: Oxford, U.K., 1985.
(4) West, R. C., Ed. Handbook of Chemistry and Physics, 67th ed.;
CRC Press: Boca Raton, FL, 1987.
(5) Kemnitz, E.; Hansen, G.; Heb, A.; Kohne, A. J. Mol. Catal. 1992,
77, 193.
Mono-exchange only is observed in the reaction of CCl2F2,
the activation energy for which is 84 kJ mol-1. This higher
activation energy is due to the higher C-Cl (∼318 kJ mol-1
)
bond strength in CCl2F2 than in CCl3F (D(C-Cl) ) 305 kJ
mol-1). The absence of the occurrence of bi-exchange producing