FTIR spectroscopic and reaction kinetics study of the interaction of CF3CFCl2 with γ-Al2O3

Article abstract of DOI:10.1021/jp993088f

The interaction of CF₃CFCl₂ with γ-Al₂O₃ has been investigated by a combination of reaction kinetics experiments and FTIR spectroscopic studies. The reaction of fresh γ-Al₂O₃ with CF₃CFCl₂ at 573 K resulted in the fluorination of the γ-Al₂O₃ surface. The reaction also resulted in dehydroxylation of the γ-Al₂O₃ and an increase in both the number and strength of Lewis acid sites on the surface. The fluorinated γ-Al₂O₃ was catalytically active for the disproportionation of CF₃CFCl₂ to CF₃CCl₃ and CF₃CF₂Cl at 353 K. Adsorption of CF₃CFCl₂ on γ-Al₂O₃ at temperatures between 298 and 523 K resulted in the formation of surface trifluoroacetate species, which were stable up to 573 K. These species are likely to be the intermediates for the complete oxidation of CF₃CFCl₂ to CO and CO₂. The transformation of γ-Al₂O₃ as a result of the CF₃-CFCl₂ adsorption and reaction is discussed.

Full text of DOI:10.1021/jp993088f

J. Phys. Chem. B 2000, 104, 1277-1284
1277
FTIR Spectroscopic and Reaction Kinetics Study of the Interaction of CF3CFCl2 with
γ-Al2O3
Subodh S. Deshmukh, Vladimir I. Kovalchuk, Victor Yu. Borovkov, and Julie L. d’Itri*
Department of Chemical Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261
ReceiVed: August 31, 1999; In Final Form: December 2, 1999
The interaction of CF3CFCl2 with γ-Al2O3 has been investigated by a combination of reaction kinetics
experiments and FTIR spectroscopic studies. The reaction of fresh γ-Al2O3 with CF3CFCl2 at 573 K resulted
in the fluorination of the γ-Al2O3 surface. The reaction also resulted in dehydroxylation of the γ-Al2O3 and
an increase in both the number and strength of Lewis acid sites on the surface. The fluorinated γ-Al2O3 was
catalytically active for the disproportionation of CF3CFCl2 to CF3CCl3 and CF3CF2Cl at 353 K. Adsorption
of CF3CFCl2 on γ-Al2O3 at temperatures between 298 and 523 K resulted in the formation of surface
trifluoroacetate species, which were stable up to 573 K. These species are likely to be the intermediates for
the complete oxidation of CF3CFCl2 to CO and CO2. The transformation of γ-Al2O3 as a result of the CF3-
CFCl2 adsorption and reaction is discussed.
Introduction
volumetric sorption analyzer (Micromeritics ASAP 2010). X-ray
diffraction (XRD) patterns of the samples were obtained with
Metal oxides and metal halides are an important class of
materials to convert halogenated molecules. These materials
catalyze the oxidative destruction of halocarbons.1,2 When a
noble metal is supported on aluminum oxides and aluminum
fluoride, the hydrodechlorination of chlorofluorocarbons (CFCs)
to hydrofluorocarbons (HFCs) readily proceeds at temperatures
above 450 K.3-5 For these reactions, catalyst activity and
selectivity are influenced by the type and stability of the support
material.5,6 Furthermore, previous investigations have shown that
metal oxides and fluorides catalyze disproportionation/isomer-
ization reactions of chlorofluorocarbons (CFCs) and hydro-
fluorocarbons (HFCs).7-10 However, a molecular-level descrip-
tion of halocarbon conversions on oxides is still lacking.
In the present work, we have conducted FTIR spectroscopic
studies of CF3CFCl2 adsorption along with reaction kinetics and
characterization experiments to understand the mechanism
γ-Al2O3 fluorination by the CFC. We report that γ-Al2O3 is
converted during reaction with CF3CFCl2 into a partially
fluorinated material that retains a surface area above 200 m2/g.
This is in contrast to previous reports that concluded that the
complete fluorination of γ-Al2O3 to low surface area AlF3 occurs
as a result of reaction with CFCs.11,12 Furthermore, surface
species information is provided along with a mechanistic
description of reaction pathways for the disproportionation of
CF3CFCl2 catalyzed by fluorinated γ-Al2O3 at 353 K.
a Philips XPERT diffractometer using a standard Cu KR
radiation source. Energy-dispersive X-ray analysis (EDX) on
powdered samples was performed using a EDAX detector
equipped with Super-Ultrathin (SUTH) window and attached
to a Philips XL 30 scanning electron microscope (SEM).
3. Infrared Spectroscopy. A Mattson Research Series II
FTIR spectrometer equipped with a MCT detector was used in
the transmission mode for infrared measurements. Spectra were
collected in the 400-4000 cm-1 range. Approximately 400
scans with a nominal resolution of 4 cm-1 were collected and
averaged to produce a spectrum with high signal-to-noise ratio.
The evacuated cell (10-6 Torr) was used as the background for
all measurements.
All adsorption experiments were conducted in a vacuum
system with a base pressure of 10-6 Torr. A stainless steel
dosing chamber equipped with two leak valves was used to
admit gas into the IR cell. The chamber was also equipped with
two pressure manometers (MKS instruments) with a pressure
range of 10-7-0.1 Torr and 0.01-1000 Torr, respectively. The
placement of the pressure manometers allowed for direct
measurement of the gas pressure in the IR cell.
The infrared cell consisted of a quartz tube equipped with
Teflon stopcocks for the gas inlet, outlet ports, and a vacuum
port. Potassium bromide windows were attached to the lower
portion of the cell, which was in the path of the IR beam. The
upper portion was equipped with a furnace for the in situ
pretreatment of the catalyst. The catalyst wafer was held within
a quartz sample holder inside the quartz tube and could be raised
into the heated zone of the cell by means of a magnetic bar.
The heated zone temperature was measured with a thermocouple
in contact with the outside wall of the cell at a distance of
approximately 11 mm from the wafer and controlled using a
temperature controller (Omega, model CN2011). Independent
measurements indicated that the temperature difference between
the center of the cell and the external thermocouple was within
(5 K.
Experimental Section
1. Materials. The materials used for this study were γ-Al2O3
(Vista B), AlF3 (Strem Chemicals, purity > 99%), CF3CFCl2
(Dupont, > 99.5%), and CF3COCl (Aldrich, > 98%). Indepen-
dent GC analysis of the CF3CFCl2 showed that CF2Cl2 (<0.1%)
was the only detectable impurity. Helium (Praxair, > 99.999%),
10% O2/He (Liquid Carbonic, > 99.95%), and CO (Praxair, >
99.99%) were used without further purification.
2. Characterization. The sample surface area was determined
by N2 physisorption according to the BET method using a
A self-supporting γ-Al2O3 wafer was prepared from a
powdered sample (<60 mesh) that was pressed in a hydraulic
* To whom correspondence should be addressed.
10.1021/jp993088f CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/26/2000

1278 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Deshmukh et al.
press at 830 atm for 3 min. The pressure was gradually relieved
in 1 min. In situ pretreatment of the sample was performed by
treating the wafer in a 10% O2/He flow (25 mL/min) and heating
from 298 to 773 K at 8 K/min and holding the sample at 773
K for 60 min. Then, the sample was evacuated at 773 K for 60
min and cooled to 298 K under vacuum. The sample thus
obtained is referred to as “fresh” γ-Al2O3 in the remainder of
this paper. A “used” γ-Al2O3 sample was obtained from fresh
γ-Al2O3 by the following in situ treatment. The fresh γ-Al2O3
wafer was treated with 10 Torr of CF3CFCl2 at 573 K overnight
followed by evacuation of the adsorbed species at 673 K and
subsequent treatment of the sample in 10% O2/He as described
above.
Before admitting the CFC, a spectrum of the pretreated wafer
was recorded at room temperature and 10-6 Torr pressure. Then
for a typical adsorption experiment, 1-5 Torr of CF3CFCl2 or
CF3COCl was introduced in the cell at room temperature. The
spectra of the strongly bound adsorbed species were obtained
by heating the wafer to the desired temperature (298-523 K)
in the presence of the gas phase followed by evacuation of the
cell to < 10-6 Torr at 298 K to remove the gas phase and the
weakly bound species. The wafer was then lowered into the
beam and the spectrum recorded at 298 K. The stability of the
adsorbed species formed by adsorption of CF3CFCl2 was
investigated by monitoring the decay in the intensity of the bands
associated with the adsorbed species while heating the wafer at
523-673 K under continuous evacuation after the adsorption
step described above.
Figure 1. Rate of conversion of CF3CFCl2 to halogenated products
with TOS at 573 K over γ-Al2O3 (9) and AlF3 (b).
Carbon monoxide adsorption was carried out by introducing
10 Torr of CO at 298 K into the cell containing the γ-Al2O3
wafer. Prior to CO adsorption, the γ-Al2O3 was exposed to CF3-
CFCl2 at different temperatures for 15 min (as described above)
followed by evacuation of the CFC gas phase (<10-6 Torr) at
298 K.
The formation of gas-phase reaction products, due to the
interaction of CF3CFCl2 or CF3COCl with γ-Al2O3, was
monitored by recording the gas-phase spectra as follows. The
reactant (1-5 Torr) was introduced into the IR cell containing
the pretreated sample at 298 K and the spectrum of the gas-
phase was recorded after 10 min. The temperature of the sample
was then increased stepwise from 298 to 593 K. The spectrum
of the gas phase in the cell was recorded at each temperature
after holding the wafer at that temperature for 10 min.
4. Kinetics Experiments. The CF3CFCl2 reaction was
conducted at atmospheric pressure in a stainless steel flow
reaction system described elsewhere.13 Prior to reaction, the
sample was treated in flowing 10% O2/He (25 mL/min) while
heating from 298 to 773 K at the rate of 8 K/min and held at
773 K for 60 min. This was followed by purging the sample in
He for 60 min at 773 K. The sample was then cooled in flowing
He to the reaction temperature (573 K). For a standard reaction,
0.1-0.2 g of sample was used and the total flow of the reactant
mixture was 30 mL/min, consisting of CF3CFCl2 (2 mL/min)
and He as the balance.
Figure 2. Selectivity toward CF3CCl3 (circles) and CF3CF2Cl (squares)
with TOS at 573 K over γ-Al2O3 (filled symbols) and AlF3 (open
symbols).
increased monotonically reaching a maximum value of 0.0009
µmol m-2 s-1 after 9 h. During the next 5 h of TOS, the activity
change was less than 5%. The activity loss exhibited by fresh
AlF3 was <5% during the first 10 h of TOS. Compared to the
activity of the γ-Al2O3 after 10 h TOS, AlF3 was a factor of 10
more active.
Figure 2 shows the change in the product selectivity with
TOS of the CF3CFCl2 reaction on γ-Al2O3 and AlF3 at 573 K.
With γ-Al2O3, the two major reaction products were CF3CCl3
and CF3CF2Cl accounting for >95% of all halogen-containing
products. Trifluoromethane and CF2Cl-CFCl2 (together <5%)
were the only other halogen-containing products observed. The
formation of CO and CO2 during the reaction was also detected
using the GC-MS, but they were not quantified.
Results
1. Kinetics of CF3CFCl2 Reaction over γ-Al2O3 and AlF3.
Figure 1 shows the change in reaction rate (expressed per unit
surface area of the fresh catalyst) with time on stream (TOS)
for the conversion of CF3CFCl2 over γ-Al2O3 and AlF3 at 573
K. The activity of the fresh γ-Al2O3 changed dramatically during
the first 10 h of TOS. Specifically, the initial activity (0.0022
µmol m-2 s-1) decreased by 75% during the first hour and
reached a minimum between 1 and 3 h TOS. The rate then
At early TOS, CF3CCl3 was the major product formed on
γ-Al2O3 (selectivity > 90%). With increasing TOS, the selectiv-
ity toward CF3CCl3 monotonically decreased while that of CF3-
CF2Cl monotonically increased. After approximately 9 h TOS,
the selectivity ratio of the two major products reached a constant
value of 1:1, and remained unchanged thereafter. In the course
of reaction, the total concentration of the other products changed
insignificantly.

Interaction of CF3CFCl2 with γ-Al2O3
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1279
TABLE 1: Fundamental Gas-Phase Stretching Vibrations of CF3CFCl2, CF3CCl3, and CF3CF2Cl
CF3CFCl2
this work
CF3CF2Cl
this work
CF3CCl3
this work
mode
ref 37
mode
ref 37
mode
ref 37
sym CF3
asym CF3
C-F
1295
1232
1109
942
896
736
1295
1232
1110
943
897
735
sym CF3
asym CF3
sym CF2
asym CF2
C-C
1349
1240
1183
1129
982
1351
1241
1185
1133
982
sym CF3
asym CF3
C-C
asym C-Cl
sym C-Cl
1249
1223
907
858
713
1255
1227
909
859
714
C-C
asym C-Cl
sym C-Cl
C-Cl
762
762
With AlF3, the selectivity ratio of CF3CCl3 to CF3CF2Cl was
∼1:1 at early TOS and remained constant throughout the course
of the reaction. The results presented in Figure 2 demonstrate
that the product selectivity of CF3CFCl2 conversion over fresh
γ-Al2O3 differed markedly from that over AlF3.
2. FTIR Study of CF3CFCl2 Conversion under Static
Conditions. The identification of gas-phase products formed
by the interaction of CF3CFCl2 with a fresh or used γ-Al2O3
wafer at elevated temperatures was based on the analysis of
gas-phase IR spectra of individual CFCs. The position of the
absorption bands and their assignments are presented in Table
1.
After contact of CF3CFCl2 with fresh γ-Al2O3 at temperatures
lower than 473 K, the gas-phase spectra were the same as the
spectrum of gaseous CF3CFCl2 (Figure 3A). At higher contact
temperatures, the band intensities characteristic of CF3CFCl2
decreased while new absorption bands with maxima at 855 and
1220 cm-1 appeared. After contact of CF3CFCl2 with fresh
γ-Al2O3 at 593 K, the spectrum of gas-phase species consisted
of a superimposition of the spectra of gaseous CF3CF2Cl and
CF3CCl3 with comparable intensities (Figure 3B). Contact of
CF3CFCl2 with γ-Al2O3 at 593 K for longer times resulted in
the gradual development of IR bands characteristic of gas-phase
CO, CO2, and HCl (not shown).
Conversion of CF3CFCl2 over used γ-Al2O3 occurred at
significantly lower temperatures than that over fresh γ-Al2O3.
Figure 4 shows the gas phase spectrum after contact of CF3-
CFCl2 for 10 min with a used γ-Al2O3 wafer at 353 K. (As in
the previous case, the initial IR cell CF3CFCl2 pressure was 3
Torr and the wafer temperature was 298 K). The bands
corresponding to gaseous CF3CFCl2 were of very low intensity.
Further, the spectrum is well approximated by the superimposi-
tion of the spectra of gaseous CF3CF2Cl and CF3CCl3 with the
concentration ratio close to 1:1 (Figure 4b).
3. Surface Characterization of Fresh and Used Aluminas.
Table 2 shows the change in the surface area of the γ-Al2O3
and AlF3 samples as a result of the CF3CFCl2 reaction at 573
K. Exposure of fresh γ-Al2O3 to a He flow (30 mL/min) at 573
K for 15 h did not change the BET surface area (297 m2/g). In
contrast, the fresh γ-Al2O3 BET surface area decreased by ca.
20%, to 241 m2/g, after 6 h of reaction with CF3CFCl2. The
reaction with CF3CFCl2 for longer times (up to 15 h) did not
further decrease the surface area significantly. The surface area
of the AlF3 (47 m2/g) did not change during the reaction with
CF3CFCl2 for 15 h.
X-ray diffraction (XRD) patterns of fresh and used γ-Al2O3
(CF3CFCl2 reaction at 573 K for 15 h) were similar and
possessed only characteristic lines of the γ-Al2O3 phase.
Similarly, fresh and used AlF3 (also after CF3CFCl2 reaction at
573 K for 15 h) had the same XRD patterns, characteristic of
AlF3. In addition to Al and O peaks detected in fresh γ-Al2O3,
energy-dispersive X-ray spectroscopy (EDX) of used γ-Al2O3
revealed the presence of fluorine, but no chlorine.
Figure 3. Infrared spectra of gas phase after the exposure of the wafer
of fresh γ-Al2O3 to 3 Torr of CF3CFCl2 at different temperatures (A)
and deconvolution of the spectrum (e) into CF3CFCl2 and CF3CCl3
constituents (B).
three absorption bands with maxima at 3680, 3723, and 3770
cm-1. These bands have been previously assigned to isolated
hydroxyls coordinated to three octahedral Al3+ cations, to a
tetrahedral and to an octahedral Al3+ cation and to a single
tetrahedral Al3+ cation, respectively.14,15
The changes of the hydroxyl group vibration intensity after
γ-Al2O3 treatment with CF3CFCl2 are most clearly seen in the
difference spectrum (Figure 5b), in which the spectrum of the
fresh γ-Al2O3 was subtracted. The intensity of the hydroxyl
vibration bands did not change after room-temperature contact
of CF3CFCl2 with γ-Al2O3 followed by evacuation of phys-
isorbed molecules. However, after a 10 min exposure of CF3-
CFCl2 at 523 K and evacuation of physisorbed molecules at
Figure 5a shows the OH-group spectrum of a fresh γ-Al2O3
sample preheated in a 10% O2/He flow at 773 K. It consists of

1280 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Deshmukh et al.
TABLE 3: Summary of Positions and Intensities of the
Band of CO Adsorbed at 298 K on γ-Al2O3 Exposed to
CF3CFCl2 in Different Conditions
no.
sample
ν
CO, cm-1 ICO, au
1
2
fresh γ-Al2O3
sample 1 after chemisorption of
CF3CFCl2 at 298 K
2202
2203
0.026
0.021
3
sample 2 after chemisorption of
CF3CFCl2 at 473 K
2204
0.015
4
5
sample 3 following evacuation at 673 K
sample 4 after chemisorption of
CF3CFCl2 at 523 K
2207
2212
0.016
0.014
6
7
sample 5 following evacuation of at 673
sample 6 after chemisorption of CF3CFCl2
at 573 K followed by treatment in
O2/He at 773 K
2213
2225
0.047
0.052
single tetrahedral Al3+ cation and perturb those hydroxyls via
hydrogen bonding.
Chemisorption of CF3CFCl2 not only perturbed the OH
groups but also resulted in a negative band at 1050 cm-1 in the
difference spectrum (Figure 5b). This band has been previously
assigned to an Al-O vibrational mode localized in strained
surface groups such as [Alδ+‚‚‚Oδ-sAl].15 Thus, it appears that
these groups participate in CF3CFCl2 chemisorption. More
severe treatment of γ-Al2O3 with CF3CFCl2 resulted in an almost
complete disappearance of surface OH groups and a more
pronounced negative peak in the range of 1000-1100 cm-1
(Figure 5c).
Lewis acidity, an important characteristic of γ-Al2O3, is
usually associated with coordinatively unsaturated Al3+ cations
formed during high-temperature γ-Al2O3 surface dehydroxyla-
tion. The FTIR spectra of CO adsorbed on fresh γ-Al2O3 and
γ-Al2O3 pretreated with CF3CFCl2 under different conditions,
consisted of a single band corresponding to CdO bond
stretching vibrations of molecules adsorbed on coordinatively
unsaturated Al3+ cations. Table 3 shows the main characteristics
of these spectra, such as band position and intensity.
Figure 4. Infrared spectra of gas phase after the exposure of the wafer
of used γ-Al2O3 to 3 Torr of CF3CFCl2 at 353 K (curve a) and the
deconvolution of the spectrum into CF3CFCl2, CF3CF2Cl, and CF3-
CCl3 constituents (curves b).
TABLE 2: BET Surface Area of γ-Al2O3 and AlF3 Samples
during the Course of Reaction with CF3CFCl2
material
time on stream, h
BET surface area, m2/g
γ-Al2O3
fresh
2.0
6.0
298
260
241
248
235
47
9.5
15.0
fresh
15.0
AlF3
48
Room-temperature CF3CFCl2 adsorption on fresh γ-Al2O3 did
not affect the 2202 cm-1 CO absorption band. Preadsorption
of CF3CFCl2 at 473 K followed by CO adsorption also did not
influence the position of the CO band but decreased its intensity
slightly. However, adsorption of CO after evacuation at 673 K
following CF3CFCl2 adsorption at 298 and 523 K resulted in a
blue shift of the band maximum of 3 and 5 cm-1, respectively.
Further, the band intensity increased by a factor of ∼3.
Treatment of a sample with CF3CFCl2 preadsorbed at 573 K
in flowing 10% O2/He mixture at 773 K did not change the
band intensity of the adsorbed CO but increased its maximum
frequency from 2202 to 2225 cm-1. This shift is attributed to
the increase of electron pair donation from the coordinated CO
molecule 5σ orbital to the empty Al3+ cation 3p orbital.
Therefore, it reflects an increase of electron-accepting properties
of Al3+ cations or their Lewis acidity. It should be noted that
CF3CFCl2 chemisorption had little influence on the adsorbed
CO band intensities. This suggests that chemisorbed species
formed upon CF3CFCl2 adsorption did not block the γ-Al2O3
surface Lewis acidic sites.
Figure 5. IR spectrum of (a) fresh γ-Al2O3 after oxidation at 773 K,
(b) γ-Al2O3 wafer on exposure to CF3CFCl2 at 523 K followed by
evacuation of gas phase at 298 K (referenced to the fresh γ-Al2O3,
spectrum a) and (c) γ-Al2O3 pellet following exposure to CF3CFCl2 at
673 K and treatment in 10% O2/He at 773 K (referenced to the fresh
γ-Al2O3, spectrum a).
4. FTIR study of CF3CFCl2 Adsorption on γ-Al2O3. Figure
6 shows the FTIR spectra of chemisorbed species formed on
the surface of fresh and used γ-Al2O3, due to CF3CFCl2
adsorption at 473 K. The fresh γ-Al2O3 surface species spectrum
(bottom spectrum) consisted of six absorption bands with
maxima at 1666, 1500, 1425, 1243, 1215, and 1189 cm-1. The
IR spectrum of chemisorbed species formed upon adsorption
of CF3CFCl2 under the same conditions on used γ-Al2O3 (top
room temperature, the band at 3770 cm-1 disappeared while
the bands at 3680 and 3723 cm-1 remained. The disappearance
of the narrow band at 3770 cm-1 was accompanied by the
development of a broad band centered at 3560 cm-1, charac-
teristic of hydrogen bonded OH-groups. Some of the chemi-
sorbed species, resulting from CF3CFCl2 adsorption at 523 K,
are probably localized near the OH-groups coordinated to a

Interaction of CF3CFCl2 with γ-Al2O3
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1281
Figure 8. IR spectra of chemisorbed species (a) after adsorption of
2.2 Torr of CF3COCl at 373 K and (b) after adsorption of 3 Torr of
CF3CFCl2 at 473 K on fresh γ-Al2O3 followed by evacuation of the
gas phase at 298 K in each case.
Figure 6. Infrared spectra of chemisorbed species formed after
adsorption of 3 Torr of CF3CFCl2 at 473 K followed by evacuation of
the gas phase at 298 K, on the surface of fresh γ-Al2O3 (bottom curve)
and used γ-Al2O3 (top curve).
tion temperature to 543 K resulted in a gradual increase in the
concentration of chemisorbed species on the surfaces of both
fresh and used γ-Al2O3. However, at the same CF3CFCl2
adsorption temperature, the concentration of chemisorbed spe-
cies on the used γ-Al2O3 was much lower than on the fresh
γ-Al2O3. The increased surface species concentration with
increasing adsorption temperature indicated that the chemisorp-
tion of CF3CFCl2 is activated for both fresh and used γ-Al2O3.
A spectrum of chemisorbed species similar to that of CF3-
CFCl2 adsorption on fresh γ-Al2O3 was obtained by adsorption
of CF3COCl molecules on γ-Al2O3. Figure 8 shows the spectra
of chemisorbed species on fresh γ-Al2O3 after CF3COCl
adsorption at 373 K (spectrum a) and after CF3CFCl2 adsorption
at 473 K (spectrum b). With the exception of the low intensity
band at 1425 cm-1 (for the adsorbed CF3CFCl2), both spectra
consisted of bands with the same maximum positions but with
significantly different intensities. The intensity of IR bands due
to the surface species resulting from CF3COCl chemisorption
were ∼4 times more intense than those resulting from CF3CFCl2
adsorption, despite the lower adsorption temperature of CF3-
COCl.
Figure 7. Change in intensity of the IR band at 1215 cm-1 of surface
species formed on chemisorption of CF3CFCl2 on fresh (9) and used
(b) γ-Al2O3 as a function of adsorption temperature.
Discussion
spectrum) also consisted of six absorption bands (1653, 1500,
1425, 1245, 1215, and 1189 cm-1). However, the band
intensities associated with the used γ-Al2O3 was lower compared
to the band intensities associated with the fresh γ-Al2O3. The
primary differences between spectra of chemisorbed species on
fresh and used aluminas was the shift of the band at 1666 to
1553 cm-1 with the broadening of the latter. On both fresh and
used γ-Al2O3, the intensity of the absorption bands gradually
decreased in a vacuum at 553 K while the ratio between the
intensities of all the bands remained constant. This indicates
that the bands belong to one type of surface species. A GC-MS
analysis of the gases that evolved during heating in a vacuum
at 573 K of γ-Al2O3 (with CF3CFCl2 preadsorbed at 473 K)
established that the only products resulting from decomposition
of chemisorbed surface species were CO and CO2.
The literature provides several examples demonstrating that
halocarbons halogenate γ-Al2O3 at elevated temperatures,11,12
and the present research shows that CF3CFCl2 is no exception.
However, transformation of the oxide creates new sites, both
active and selective for disproportionation of CF3CFCl2. The
results of this investigation provide the framework for under-
standing the nature of the active site for CF3CFCl2 dispropor-
tionation and the crucial mechanistic aspects of the reaction.
1. The Nature of Surface Species Formed upon CF3CFCl2
Chemisorption on γ-Al2O3. The similarity of the FTIR spectra
of chemisorbed species formed during adsorption of either CF3-
CFCl2 or CF3COCl (Figure 8) provides insight into the nature
of those species. One could expect that during CF3COCl
chemisorption on γ-Al2O3, the surface species formed are similar
to those produced from acetic acid decomposition on metal
The concentration of chemisorbed species formed on the
surface of fresh and used aluminas was dependent on the CF3-
CFCl2 adsorption temperature (Figure 7). Increasing the adsorp-
16
oxides. It is known that adsorption of acetic acid on TiO2 or
MgO17-19 results in the formation of surface acetate species

1282 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Deshmukh et al.
TABLE 4: Fundamental Stretching Vibrations of Gaseous Trifluoroacetyl Chloride, Solid Aluminum Trifluoroacetate, and
Surface Trifluoroacetate Species
this work
ref 23
ref 31
ref 29
ref 38
CF3CFCl2
adsorption on
fresh γ-Al2O3
CF3CFCl2
adsorption on
used γ-Al2O3
CF3COCl
adsorption on
fresh γ-Al2O3
CF3CFdCF2
adsorption
on TiO2
CF3COCF3
adsorption
on TiO2
(C2F5)2O
adsorption
on Al2O3
gaseous
CF3COCl
(CF3CO2)3Al
(solid)
vibration
ν
ν
ν
ν
ν
asym O-C-O
sym O-C-O
sym CF3
asym CF3
asym CF3
1666
1500
1243
1215
1189
1653
1425
1245
1215
1189
unidentate
1666
1501
1245
1215
1189
1670
1485
1205
1160
1050
unidentate
1734
1470
1580
1480
1700
1491
1248
1810
1274
1239
1202
n/a
1258
1204a
1124
bonding
bridging
bridging
unidentate
bidentate
a Assigned to δCO bending vibration.
than 200 cm-1, while a ∆νas-s between 100 and 200 cm-1
indicates a bridging structure.26,27 The analysis of the frequency
difference between asymmetric and symmetric C-O bond
stretching vibrations of the O-C-O fragment was also used
to determine the structure of the adsorbed acetate and fluoro-
acetate species. The following examples illustrate these assign-
ments. First, the surface species formed upon adsorption of
ethanol on γ-Al2O3 was assigned to a bidentate acetate based
on the value of ∆νas-s ) 106 cm-1 28
. Second, for the adsorption
of hexafluoroacetone on TiO2, an assignment of bidentate
trifluoroacetate species was made based on ∆νas-s ≈ 100 cm-1 29
.
Third, for the adsorption of hexafluoroacetone on γ-Al2O3, a
∆νas-s ) 193 cm-1 led to an assignment of bridge-bonded
formate.30 Finally, a ∆νas-s ) 264 cm-1 for the adsorption of
CF3CFdCF2 on TiO2 resulted in the assignment of a unidentate
trifluoroacetate.31
Figure 9. Possible structures of surface trifluoroacetate species.
and hydroxyl groups. In both cases, the IR spectrum of the
chemisorbed species contained two absorption bands in the
1300-1600 cm-1 region. These bands are assigned to the
symmetric and asymmetric C-O bond stretching vibrations of
the O-C-O fragment of surface metal acetates based on a
comparison to the IR spectra of different solid metal acetates
which consist of two absorption bands in the 1350-1600 cm-1
range attributed to the C-O stretching vibrations of O-C-O
fragments.20,21 For metal trifluoroacetates, those two C-O bond
stretching vibrations are shifted to higher frequences (1400-
1750 cm-1).22-24 For all acetates, including trifluoroacetates,
the asymmetric C-O bond stretching vibration frequency is
higher than the symmetric frequency. The chemisorbed CF3-
CFCl2 high-frequency IR bands (centered at 1666 and 1500
cm-1 for fresh γ-Al2O3 and 1653 and 1425 cm-1 for used
γ-Al2O3) are therefore assigned to the asymmetric and sym-
metric C-O bond stretching vibrations of the O-C-O fragment
of surface aluminum acetates.25 The 1243-1245 cm-1 band is
ascribed to the symmetric stretching vibration of the CF3 group,
while the bands at 1215 and 1189 cm-1 are assigned to the
nondegenerate asymmetric vibrations of CF3. Such an assign-
ment of low-frequency bands is quite reasonable. Indeed, in
different CF3 compounds in which the CF3 group is attached to
a carbon atom (e.g., acetates), the symmetric C-F bond
stretching vibration frequency is higher than that of asymmetric
vibration.23 Further, for gaseous trifluoroacetyl chloride (CF3-
COCl), with a geometric structure analogous to monodentate
acetate, the asymmetric C-F bond stretching vibration is
nondegenerate due to distortion of the CF3- group symmetry
from C3V (see Table 4).
In the present investigation, the frequency difference between
the asymmetric and symmetric C-O bond stretching vibrations
of the acetate species O-C-O fragment, formed upon CF3-
CFCl2 chemisorption on fresh and used γ-Al2O3, is 166 and
228 cm-1, respectively. The frequency difference for acetate
species on the surface of the used γ-Al2O3 (∆νas-s) is higher
than 200 cm-1, allowing one to conclude that the surface acetate
has a unidentate structure. The lower frequency difference
between asymmetric and symmetric C-O bond stretching
vibrations of O-C-O fragments of surface acetates for the fresh
γ-Al2O3 suggests a bridging structure.
It is known that hydrogen bonding of acetates with the surface
OH groups of γ-Al2O3 can decrease the difference in frequency
between asymmetric and symmetric stretching vibrations of
O-C-O fragments in surface acetates. For example, small
values of ∆νas-s. ) 120-130 cm-1 for unidentate acetates
formed on MgO upon adsorption of acetic acid were explained
by hydrogen bonding between neighboring surface acetates.17
However, the possibility of hydrogen bonding of OH-groups
of γ-Al2O3 with surface acetates appears unlikely, because the
OH-groups are the most reactive and sterically accessible groups
on the surface of γ-Al2O3. Thus, the OH-groups are consumed
immediately during reaction with CF3CFCl2 to form HCl and,
probably, a CF3CFCl-O-Ald species that transforms into the
acetate. The HCl either desorbs into the gas phase or interacts
with Al-O-Al fragments on the surface to form adjacent
dAl-Cl and HO-Ald groups. The latter case is confirmed
by the development in the IR spectrum of a broad absorption
band at 3560 cm-1, characteristic of hydrogen bonded OH
groups (Figure 5b).
Figure 9 shows the possible structures of the acetate species
that may form on metal oxide surfaces. Analysis of the
frequency difference between asymmetric and symmetric C-O
bond stretching vibrations of O-C-O fragment (∆νas-s) has
been used to determine the structure of acetates.20,25 In general,
a frequency difference of less than 100 cm-1 is characteristic
of a bidentate acetate coordinated to a single metal cation. For
unidentate acetate, the frequency difference is usually greater
Table 4 shows the characteristic IR absorption bands and
possible band assignments for the surface trifluoroacetate
species, formed upon adsorption of various fluorine compounds
onto oxides.

Interaction of CF3CFCl2 with γ-Al2O3
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1283
2. Pathways to γ-Al2O3 Fluorination. The fluorination of
γ-Al2O3 by CF3CFCl2 involves its chemisorption to form a
surface aluminum trifluoroacetate by abstraction of F and Cl
atoms from the -CFCl2 group. Then, the surface Cl atoms are
substituted for fluorine by an exchange reaction with CF3CFCl2
to form the chlorine-enriched product CF3CCl3:
localized to strained groups of the Alδ+‚‚‚Oδ-Al type. Similar
sites seems to be responsible for the thermal decomposition of
aluminum trifluoroacetates on the surface of γ-Al2O3.
Evidence for the involvement of these “strained” sites in the
decomposition of surface acetates comes from the results
presented in spectra b and c of Figure 5. This figure shows the
disappearance of the IR absorption band in the 1000-1100 cm-1
range in the spectrum of the γ-Al2O3 after CF3CFCl2 adsorption.
The disappearance of a similar band at 1034 cm-1, after
adsorption of CCl4 on a highly dehydroxylated γ-Al2O3, was
explained by the depletion of Al3+-O2- cluster sites due to
formation of monodentate carbonate ions.10
Clads + CF3CFCl2 f CF3CCl3 + Fads
(1)
The final fluorination step is the decomposition of the
trifluoroacetate species forming only CO and CO2. The follow-
ing general scheme represents the overall fluorination process:
3. Low-Temperature Activity of γ-Al2O3 in Halogen
Exchange Reaction. According to FTIR spectroscopic studies
under static conditions, the halogen exchange reaction
3CF3CFCl2 + Al2O3 f
2AlF3 + 2CF3CCl3 + CO + CO2 (2)
2CF3CFCl2 f CF3CCl3 + CF3CF2Cl
(5)
A strong decrease in OH group concentration was observed
in the regenerated sample after treatment with CF3CFCl2 at high
temperature (Figure 5c). This observation, combined with the
appearance of gaseous HCl during prolonged heating of fresh
γ-Al2O3 in CF3CFCl2 at 573 K indicates that partial removal
of Cl from the surface proceeds through HCl elimination from
interacting adjacent Al-Cl and Al-OH groups.
The formation of surface trifluoroacetate species proceeds via
abstraction of all halogen atoms X (X ) Cl or F) from the
-CFCl2 group of CF3CFCl2 and attachment of surface oxygen
atom of γ-Al2O3 to carbon atom of chemisorbed substance:
catalyzed by the used γ-Al2O3 proceeds rapidly at 353 K, while
with the fresh γ-Al2O3 the transformation of CF3CFCl2 into CF3-
CCl3 was detected only at temperatures higher than 473 K. The
low-temperature activity of the used γ-Al2O3 for the halogen
exchange reaction is connected with the fluorination of γ-Al2O3
during pretreatment with CF3CFCl2 at elevated temperatures.
According to Peri, fluorination of γ-Al2O3 leads to a strengthen-
ing of the surface Lewis acid sites, coordinatively unsaturated
Al3+ cations.33 It was also observed that an increase in the Lewis
acidity of oxides results from an interaction with CFCs.34,35
Corma et al. concluded that for γ-Al2O3 the total number of
Lewis sites decreases with increasing fluorine content, but the
concentration of strong Lewis acidic sites exhibits a maximum
for fluorine concentrations between 2% and 4%.36 Our FTIR
spectroscopic study results of CO adsorption on γ-Al2O3
samples pretreated with CF3CFCl2 also demonstrate the strength-
ening of Lewis acidic sites (coordinatively unsaturated Al3+
cations) upon CF3CFCl2 chemisorption, especially after thermal
decomposition of chemisorbed species. The IR spectra band
intensity of the coordinated CO molecules, after high-temper-
ature vacuum treatment with preadsorbed CF3CFCl2 at 523 K,
provide a measure of the number of Lewis acidic sites. The
observed increase in Lewis acidic sites is attributed to removal
of surface chlorine anions by formation of gaseous HCl.
Accordingly, it is suggested that the coordinatively unsatur-
ated Al3+ cations surrounded by F- anions on the fluorinated
γ-Al2O3 surface participate in low-temperature halogen ex-
change reactions between CF3CFCl2 and the halogenated
surface. Indeed, Coulson et al.7 reported that the formation of
the disproportionation products follows a monomolecular
scheme involving halogen exchange with the surface Lewis acid
sites. Similar to exchange reactions of CCl4 with prefluorinated
chromia,35 a four-center intermediate for chloro-fluoro ex-
change in CF3CFCl2 on fluorinated γ-Al2O3 surface is proposed.
The resulting Al-X pairs act as active sites for high-temperature
halogen exchange reactions between the γ-Al2O3 surface and
CF3CFCl2 to form the chlorine enriched CF3CCl3. In this case,
the fluorine atom abstraction from the -CFCl2 group of CF3-
CFCl2 probably becomes thermodynamically more favorable
because of the higher bond energy of Al-F compared to the
Al-Cl bond.
Only adsorbates containing two carbons are observed. This
suggests that the limiting step of trifluoroacetate decomposition
is the carbon-carbon bond cleavage with simultaneous CO
molecule abstraction. As a result, trifluoromethoxy groups are
formed that rapidly convert into fluoroformates, the decomposi-
tion of which leads to CO2 evolution and further γ-Al2O3
fluorination.
The properties of surface Al3+ cation-O2- anion pair sites
is markedly different for γ-Al2O3 samples treated at 773 K than
those treated at lower temperatures.32 According to XPS results
for Al and O binding energies of γ-Al2O3, new strong acid-
base cluster sites (consisting of Al3+ and O2- ions) are formed
Conclusions
The CF3CFCl2 reaction over γ-Al2O3 leads to a modification
of the γ-Al2O3 surface due to fluorination. The modified surface
possesses sites that readily catalyze the disproportionation of
CF3CFCl2 at 353 K. FTIR spectroscopic studies show the
when the sample is dehydroxylated at and above 773 K.32
A
characteristic feature in the IR spectrum is an absorption band
at ∼1050 cm-1 corresponding to an Al-O vibrational mode

1284 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Deshmukh et al.
(16) Griffiths, D. M.; Rochester, C. H. J. Chem. Soc., Faraday 1 1977,
73, 1988.
formation of unidentate and bridge-bonded trifluoroacetate
species as a result of adsorption of CF3CFCl2 on the surface of
γ-Al2O3 at 298-573 K. More importantly, the decomposition
of trifluoroacetates results in formation of CO and CO2 and the
fluorination of the γ-Al2O3 surface. Such catalytically active
fluorinated γ-Al2O3 possesses increased Lewis acidity as
measured by FTIR spectroscopy using CO adsorption.
(17) Xu, C.; Koel, B. E.; J. Chem. Phys. 1995, 102, 8158.
(18) Peng, X. D.; Barteau, M. A. Langmuir 1991, 7, 1426.
(19) Peng, X. D.; Barteau, M. A. Catal. Lett. 1990, 7, 395.
(20) Edwards, D. A.; Hayward, R. N. Can. J. Chem. 1968, 46, 3443.
(21) Ito, K.; Bernstein, H. J. Can. J. Chem. 1956, 34, 170.
(22) Fuson, N.; Josien, M. L.; Lawson, J. R. J. Chem. Phys. 1952, 20,
1627.
(23) Weiblen, D. G. The Infrared Spectra of Fluorocarbons and Related
Compounds. In Fluorine Chemistry; Simons, J. H., Ed.; Academic Press:
New York, 1954; Vol. II.
Acknowledgment. Funding from National Science Founda-
tion (CTS-9502090 and CTS-9509580) is gratefully acknowl-
edged.
(24) Spinner, E. J. Chem. Soc. 1964, 4217.
(25) Bands at 1653 and 1425 cm-1 of different intensity are present in
the spectra of both fresh and used samples. In addition, the band at 1666
cm-1 for the fresh sample has low wavenumber shoulder, and the band at
1653 cm-1 for the used sample is broad and may include the band at 1666
cm-1. Thus, two types of acetates form on the surface of alumina during
CF3CFCl2 adsorption with signature bands at 1666 and 1500 cm-1 dominate
for fresh γ-Al2O3 and at 1653 and 1425 cm-1 for the used γ-Al2O3,
respectively.
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