Catalytic Oxidation of Dichloromethane on γ-Al2O3: A Combined Flow and Infrared Spectroscopic Study

Article abstract of DOI:10.1006/jcat.1998.2274

When γ-Al₂O₃ is applied as a support for noble metals in the catalytic oxidation of dichloromethane, the reaction primarily takes place on the alumina. In our flow study, dichloromethane was completely converted at 600 K on γ-Al₂O₃, CO being a main product, especially at higher temperatures. Methyl chloride was observed in substantial amounts as an intermediate product. The formation of CH₃Cl, a reduction product formed under oxidative conditions, has been described before, but no satisfactory explanation has been given. To this end, reaction between alumina and CH₂Cl₂ has been studied by FTIR spectroscopy as well. The spectrum of the adsorbed species at 523 K was clearly different from that at 298 K. Apparently, dichloromethane reacted with surface hydroxyl groups, which disappeared. Experiments with CD₂Cl₂ enabled the identification of the surface species. The bands found could be attributed to surface methoxy groups and to adsorbed formate. With these species the products found in the flow experiments could be explained: the formates are converted into CO and the methoxy groups can react with HCl to form CH₃Cl. The first step, reaction of the adsorbed dichloromethane with a surface hydroxyl group, yields a chloromethoxy species. This leads to a chemisorbed formaldehyde analogue, which appears to disproportionate due to some hydride shift, to form the observed methoxy and formate groups. The Cl from the CH₂Cl₂ is released as HCl and/or reacts with the alumina to form aluminum chlorides. The observed rate enhancing effect of added water in the gas stream can be explained by reaction with chlorinated alumina and restoration of surface hydroxyl groups, the active sites in the reaction with dichloromethane.

Full text of DOI:10.1006/jcat.1998.2274

JOURNAL OF CATALYSIS 180, 153–160 (1998)
ARTICLE NO. CA982274
Catalytic Oxidation of Dichloromethane on � -Al O : A Combined Flow
2
3
and Infrared Spectroscopic Study
�
�
�
Ruud W. van den Brink, Peter Mulder, Robert Louw, Gilles Sinquin,† Corinne Petit,†
,1
and Jean-Paul Hindermann†
�
Center for Chemistry and the Environment, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands;
and †Laboratoire d’Etude de la R e´ activit e´ Catalytique, des Surfaces et Interfaces, ECPM-UMR CNRS-ULP 7515,
1
, rue Blaise Pascal, 67008 Strasbourg, France
Received May 26, 1998; revised August 14, 1998; accepted August 26, 1998
rine. Both metal-oxide and noble metal-based catalysts are
When � -Al2O3 is applied as a support for noble metals in the used for this process. Generally, chromium oxide is found
catalytic oxidation of dichloromethane, the reaction primarily takes to be the most active metal oxide for the catalytic combus-
place on the alumina. In our flow study, dichloromethane was com-
pletely converted at 600 K on � -Al2O3, CO being a main product,
especially at higher temperatures. Methyl chloride was observed
in substantial amounts as an intermediate product. The formation
of CH3Cl, a reduction product formed under oxidative conditions,
drocarbon combustion (8–11). Formation of noble metal
has been described before, but no satisfactory explanation has been
given. To this end, reaction between alumina and CH2Cl2 has been
studied by FTIR spectroscopy as well. The spectrum of the adsorbed
species at 523 K was clearly different from that at 298 K. Appar-
tion of chlorinated hydrocarbons (5, 6). However, it can
form rather volatile and very toxic chromium oxychlorides,
which eventually leads to deactivation (7).
Noble metals are active catalysts in chlorinated hy-
(
oxy)chlorides is observed (12), but these are less suscep-
tible to volatilization. Interaction of Pt and Pd with chlo-
rine is observed to promote the formation of highly chlo-
ently, dichloromethanereacted with surfacehydroxylgroups, which rinated by-products, e.g., polychlorinated benzenes from
disappeared. Experiments with CD2Cl2 enabled the identification chlorobenzene (13, 14). The support material plays an im-
of the surface species. The bands found could be attributed to sur- portant role in the destruction mechanism. Noble metal
face methoxy groups and to adsorbed formate. With these species
the products found in the flow experiments could be explained: the
formates are converted into CO and the methoxy groups can react
with HCl to form CH3Cl.
The first step, reaction of the adsorbed dichloromethane with a
ward chlorinated methanes for Pt supported on TiO2 and
surface hydroxyl group, yields a chloromethoxy species. This leads
to a chemisorbed formaldehyde analogue, which appears to dispro-
portionate due to some hydride shift, to form the observed methoxy
and formate groups. The Cl from the CH2Cl2 is released as HCl
catalysts for catalytic oxidation of chlorinated compounds
are supported on various materials, e.g., zeolites (13) and
TiO2 (15), but most frequently � -Al2O3 is used. Whindawi
and Zhang observed a completely different reactivity to-
Al2O3, respectively (15). The reaction of dichloromethane
(
CH2Cl2) is particularly interesting in this light. The rate of
CH2Cl2 conversion for Pt on � -Al2O3 and plain � -Al2O3 is
and/or reacts with the alumina to form aluminum chlorides. The exactly the same (16). The only difference is the product dis-
observed rate enhancing effect of added water in the gas stream tribution. With a Pt containing catalyst at 723 K CH2Cl2 is
can be explained by reaction with chlorinated alumina and restora- converted into CO , while on alumina alone CO isthe major
2
tion of surface hydroxyl groups, the active sites in the reaction with
carbon containing product. In both cases, methyl chloride
dichloromethane.
�c 1998 Academic Press
(
CH3Cl) is observed as a by-product (15, 17). Formation of
CH3Cl, formally a reduction product, is rather unexpected
in an oxidative environment. The decomposition of CH2Cl2
on � -Al2O3 has been studied by Haber et al., using IR spec-
troscopy (17). They suggest a mechanism for methyl chlo-
1
. INTRODUCTION
Catalytic oxidation is one of the emerging technologies to
reduce the release of chlorinated compounds into the envi-
ronment (1–4). Catalysts for this process should be selective
to HCl, carbon dioxide, and water, and resistant to chlo-
�
ride formation in which a hydride (H ) from the surface
reacts with an adsorbed dichloromethane molecule. How-
ever, conceptually a hydride shift from an hydroxyl group
seems highly unlikely.
Therefore, we have studied the surface reaction of
CH2Cl2 on � -Al2O3 by means of flow techniques and FTIR
spectroscopy to elucidate the ruling mechanism(s).
1
To whom correspondence should be addressed.
153
0021-9517/98 $25.00
Copyright �c 1998 by Academic Press
All rights of reproduction in any form reserved.

1
54
VAN DEN BRINK ET AL.
2
. METHODS
which consisted of a cell with KBr windows. The disc was
mounted into the cell and then was lifted into an oven. Sub-
sequently, the system was evacuated at ambient tempera-
2
.1. Catalyst
�
5
ture ( p = 10 mbar), and the alumina was heated slowly
(5 K/min) to 623 K and left at this temperature for 1 h,
to eliminate adsorbed water and part of the OH groups.
According to Kn o¨ zinger and Ratnasamy (18), this treat-
The surface area of the � -Al2O3 (Rh oˆ ne Poulenc, Crys-
2
tal structure confirmed by X-ray diffraction) is 206 m /g,
as measured by liquid nitrogen adsorption. Particle size
distribution in the flow experiments was between 125 and
14
� 2
ment leaves about 6 � 10 OH-groups cm on the alumina
2
50 �m.
surface.
All spectra were recorded at room temperature on a
Nicolet 5DXC FTIR spectrometer averaging 32 scans with
a 2 cm resolution and analyzed using OMNIC software. A
2
.2. Flow Experiments
Gases were introduced by electronic mass flow con-
� 1
trollers (Brooks 5850 E). Teflon tubing was used to prevent
metal wall mediated reactions. All tubes and the injection
valves of the GC were heated to prevent condensation of
products.
1
3-point smoothing was performed. The spectrum recorded
after the thermal treatment (Fig. 3a) was used for subtrac-
tion as a blank (subtraction factor = 1). Next, 2.4 mbar of
dichloromethane or dichloromethane-d2 (CD2Cl2, Aldrich)
was introduced into the cell at 298 K, and after 10 min
spectra were recorded of both the gas phase and the disc.
Subsequently, the disc was lifted from the IR beam and a
spectrum of the gas phase in the cell was taken. The spec-
tra shown for the adsorbed dichloromethane at 298 K were
obtained by subtracting the spectrum of the blank catalyst
and of the gas phase (subtraction factor = 1). Next, the disc
was lifted into the oven and maintained at 523 K for 30 min;
this period was sufficient to arrive at stationary state. After
evacuation the pellet was reintroduced into the beam and
after cooling a spectrum was recorded, showing the peaks
of the adsorbed species on the catalyst.
For each experiment 0.30 g (� 0.6 ml) of the catalyst
was placed in a Pyrex microreactor (internal diameter =
5
5
mm, bed height about 30 mm). The total gas flow was
0 ml/min: 1000 ppm CH2Cl2, 89% Helium, 10% oxygen
(
Air Liquide, 99.9% ), and 1% water. A stream of 1000 ppm
dichloromethane (Carlo Erba, >99% ) was obtained by
addition of a known amount of CH2Cl2 to a gas bottle,
which was pressurized with helium (Air Liquide). Water
was added to the gas stream by bubbling He through an
impinger.
The effluent gases were analyzed by two on-line GCs
(
Varian Star 3400CX) provided with a FID detector. The
first GC was equipped with a packed column (Hayesep D)
and a methanizer to analyze CO and CO2. The analysis was
calibrated by injecting a known amount of CO2. The sec-
ond GC-FID was equipped with a capillary column (Restek
RTX) and used for analysis of organic compounds. Con-
version was calculated using the surface area of the CH2Cl2
peak, relative to the one obtained by bypassing the reac-
tor to provide a blank value. The amount of CH3Cl was
calculated from the CH2Cl2 peak, using a relative response
factor of 0.9. A Fisons QTMD mass spectrometer operating
at 70 eV was also connected to the setup, to allow on-line
real-time-resolved measurements.
3
. RESULTS
3
.1. Flow Experiments
First, the � -Al2O3 was heated in He/O2 to 573 K, then
the reactor was bypassed and the flows of CH2Cl2, He, O2,
and water were adjusted. The flow was allowed to stabilize,
until the MS signal of CH2Cl2 (mass 49, which gives a higher
signal than the molecular ion 84) was constant. Then the
flow was switched over the reactor again (time = 0 min)
and the MS signals are followed as shown in Fig. 1.
A typical experiment started by heating the catalyst un-
der a He/O2 flow to 623 K. After 1 h to allow the flow to
stabilize, CH2Cl2 was added and the effluent of the reactor
was monitored by the mass spectrometer. About 3 h is nec-
essary to reach a stationary state. These conditions were
maintained overnight, whereafter the catalyst was cooled
to 423 K, and conversion and products were analyzed by
GC. Subsequently the temperature was raised (5 K/min, in
steps of 25 K), and after a stabilization time of about 20 min,
effluent gases were analyzed.
2
.3. FTIR Measurements
FIG. 1. Initial behavior of dichloromethane on � -Al2O3 at T = 573 K.
Conditions: 0.30 g � -Al2O3, flow = 50 ml/min, 1000 ppm dichloromethane,
About 10 mg of the catalyst was pelleted as a disc 12 mm
1
0% O2, 1% H2O, and 89% He. At time = 0 min, the catalyst was exposed
in diameter. The spectra were taken in a vacuum device, to the reaction mixture.

CATALYTIC OXIDATION OF DICHLOROMETHANE ON � -Al2O3
155
When the catalyst is exposed to dichloromethane at
73 K, the mass 49 signal decreases rapidly to zero. Just
5
after the CH2Cl2 containing flow is switched from the by-
pass to the reactor a high-intensity mass 28 is observed.
This is probably due to the N2 signal of air which enters the
MS during switching the bypass valve. After about 10 min
the mass 28 signal becomes stable, and we assume that this
corresponds to CO. During a period of about 25 min, CO
is the only compound leaving the reactor. Then the CO
signal decreases, mass 50 (CH3Cl) appears, and mass 49
reappears. After 10 min the signal intensities become sta-
+
ble. At this level about one-third of the mass 49 (CH2Cl )
signal is due to fragmentation of CH3Cl; the rest of the
signal comes from CH2Cl2 and corresponds to a conver-
sion of 92% . Hydrochloric acid (mass 36, not shown in the
graph) appears only after several hours, probably after the
�
-Al2O3 surface is saturated. This product mixture remains
stable overnight. In similar experiments, Haber et al. (17)
observed oscillations in the conversion of dichloromethane,
while CH3Cl production remained constant. We could not
reproduce these results, but conditions were not exactly the
same and Haber et al. used a different brand of � -Al2O3.
FIG. 3. FTIR spectra of the O–H stretching frequency range showing
the consumption of surface hydroxyl groups. (a) Alumina heated at 623 K.
b) After addition of 2.4 mbar of CH2Cl2 at 298 K for 10 min, and after
subtraction of the gas phase and of the blank spectrum (spectrum a) (this
(
Figure 2 shows the conversion curve of dichloromethane applies also to the b spectra in the other figures). (c) Upon reaction of
CH2Cl2 at 523 K for 30 min, after evacuation of the gas phase and after
subtraction of the blank spectrum (this applies also to the c spectra in the
other figures).
and intermediate products formed over � -Al2O3. At higher
temperatures CO is the major product and small amounts
of CO2 are observed. During the course of the experiment
(
9
which lasts about 30 h) the carbon balance decreases from
5% at 598 K to 60% at 773 K. Furthermore, the catalyst
verted). Compared to MS analysis, conversion at 573 K is
slightly lower: 88% .
becomes darker, which indicates deposition of carbona-
ceous material. This may entail deactivation of the alu-
mina. Between 473 and 773 K, CH3Cl is observed, reach-
ing its maximum production at around 600 K (40% based
on dichloromethane input, which then is completely con-
In these experiments1% of steam waspresent in the feed.
In the catalytic combustion of chlorinated hydrocarbons
water may promote formation of HCl rather than Cl2 (10).
Furthermore, addition of steam improves conversion: the
temperature of 50% conversion is 543 K in the presence
of water, but increases to 593 K in the absence of water.
Product composition was comparable: methyl chloride is
also formed as an intermediate, albeit in a slightly reduced
amount (31% at 600 K).
3
.2. FTIR Spectroscopic Results
Figure 3 shows the O–H stretching frequency range of
the � -Al2O3 disc pretreated at 623 K before reaction with
CH2Cl2 (spectrum 3a). Distinct types of surface hydroxyl
are observed, which can be ascribed, following Kn o¨ zinger
and Ratnasamy (18), to OH groups attached to one or more
�
1
Al ions. The large tail below 3600 cm indicates that some
hydrogen-bonded OH groups and strongly adsorbed water
are still present on the surface. When CH2Cl2 is introduced
at 298 K, and spectrum 3a is subtracted, negative features at
�
1
3
767, 3727, and 3675 cm are observed (spectrum 3b). This
indicates interaction of the adsorbed dichloromethane with
surface OH groups. The broad peak at lower wavenumbers
becomes larger, so probably dichloromethane forms hydro-
gen bonds with the surface hydroxyl groups.
FIG. 2. Flow results: relative amounts of dichloromethane and pro-
ducts against temperature. Conditions: 0.30 g � -Al2O3, flow = 50 ml/min,
1
000 ppm dichloromethane, 10% O2, 1% H2O, and 89% He.

1
56
VAN DEN BRINK ET AL.
FIG. 4. FTIR spectra showing the C–H stretching vibration range.
a) Gas phase spectrum of dichloromethane. (b) Adsorption of CH2Cl2 at
FIG. 5. C–D stretching vibration range. (a) Gas phase spectrum of
dichloromethane-d2. (b) Adsorption of CD2Cl2 at 298 K. (c) Reaction of
CD2Cl2 at 523 K.
(
2
98 K. (c) Reaction of CH2Cl2 at 523 K.
Spectrum 3c shows the �(O–H) region after reaction of
CH2Cl2 at 523 K and after removal of the gas phase. The
negative peaks are more prominent than in the spectrum at
trum is obtained, with peaks at 1591, 1468, 1393, 1380, and
�
1
1
091 cm
In Fig. 7 the spectra of gas phase CD2Cl2 (7a) and CD2Cl2
.
2
98 K. The large band of hydrogen bonded hydroxyl groups
adsorbed at 298K in the C–D deformation and C–O stretch-
ing frequency range are shown. Spectrum 7c refers to this
region after reaction of dichloromethane-d2 at 523 K. Some
peaks show only a small shift compared to spectrum 6c,
while other peaks are shifted to a larger extent.
is shifted to lower frequency, indicating that a different type
of hydrogen bonding is present at 523 K than at 298 K.
In Fig. 4 the C–H stretching region is shown. The spec-
trum of gas phase CH2Cl2 (spectrum 4a) shows a strong
�
1
doublet at 3005/2989 cm . Adsorption of dichloromethane
at 298 K (spectrum 4b) yields two major bands at 3056 and
�
1
2
980 cm . After reaction of dichloromethane at 523 K the
�
1
adsorption at 3056 cm has entirely disappeared, indicat-
ing that at 523 K different species are present on the surface
than at 298 K. Major peaks are located at � 2965, 2921, and
�
1
2
853 cm .
To facilitate attribution of the peaks, the reaction of
CD2Cl2 was also studied. In Fig. 5 the C–D stretching region
is shown. Compared to CH2Cl2, the doublet in the gas phase
�
1
spectrum (5a) is shifted to 2212/2198 cm . In spectrum 5b
of the CD2Cl2 adsorbed at 298 K, two peaks are observed
�
1
at 2305 and 2197 cm . Again, the picture after reaction at
5
23 K is more complex. Important features are seen at 2213
�
1
and 2080 cm
.
At lower frequencies, the gas phase spectrum of dichloro-
�
1
methane (Fig. 6a) shows peaks at 1276/1260 cm . In spectra
of adsorbed species on Al2O3, it is not possible to identify
�
1
peaks below � 1000 cm due to the strong Al–O signals, so
C–Cl vibrations cannot be seen. In spectrum 6b of CH2Cl2
�
1
adsorbed at 298 K, the peaks around 1270 cm remain,
although the intensities change. After dichloromethane
FIG. 6. C–O stretching and C–H bending vibration ranges. (a) Gas
phase spectrum of dichloromethane. (b) Adsorption of CH2Cl2 at 298 K.
had reacted at 523 K (6c), a completely different spec- (c) Reaction of CH2Cl2 at 523 K.

CATALYTIC OXIDATION OF DICHLOROMETHANE ON � -Al2O3
157
7
c) permits the attribution of the bands to adsorbed biden-
tate formates and monodentate and bidentate methoxy
species (see Table 1). In Figs. 6 and 7, bands at 1591 (1586 for
�
1
CD2Cl2), 1380 (1350), and 1091 (1061) cm are shifted only
slightly for the hydrogenated and deuterated molecules.
�
1
Bands at 1592 and 1380 cm have also been observed by
Haber et al. (17), who attributed these to deformation vibra-
tions of CH2 of dichloromethane adsorbed on Al2O3. How-
ever, our results with deuterated dichloromethane show
that this assignment is not correct, since the bands should
be shifted to lower frequencies.
Peaks which are not shifted much likely arise from C–O
�
1
stretch vibrations. The bands at 1591 and 1380 cm are
near those observed for the asymmetric and symmetric
stretch vibrations of COO of bidentate formates (19–21).
These bands were also observed by Greenler in his study
of methanol adsorption on alumina (22). The shift of the
�
1
�
s(COO) peak (30 cm ) is larger than that of �as(COO)
�
1
(
5 cm ), because for the asymmetric stretch the hydro-
FIG. 7. C–O stretching and C–D bending vibration ranges. (a) Gas
phase spectrum of dichloromethane-d2. (b) Adsorption of CD2Cl2 at gen (or deuterium) atom moves in phase with the carbon
�
1
2
98 K. (c) Reaction of CD2Cl2 at 523 K.
atom (22). The band at 1091 (1061) cm can be due to the
(C–O) vibration of a mono- or bidentate methoxy species,
although Beebe et al. (23) found the peak at higher fre-
�
�
1
quency (1055 cm ). The shift upon deuteration with
0 cm to lower frequency is in accordance with the shift
4
. DISCUSSION
�
1
3
4
.1. Assignment of Peaks and Comparison to Literature
of this band on some other catalysts (24, 25).
The band at 1468 and the very small feature at 1420 cm
�
1
The peaks in the spectrum of the OH groups (Fig. 3a) can
(
Fig. 4c), situated in the frequency area of carbonates, can-
be ascribed in accordance with Kn o¨ zinger and Ratnasamy
not be ascribed to oxygenated species since they are shifted
�
�
1
(
18): the small feature at 3789 cm stems from the Ib-type
1
OH group. The signal at 3764 cm is due to the Ia species,
attached to a tetrahedral coordinated Al3 ion. This group
+
showsthe largest interaction with CH2Cl2 adsorbed at 298K
TABLE 1
�
1
(
spectrum 3b). The bands at 3726 and 3675 cm can be
Assignments of Vibrational Modes for Methoxy and Formate
Species and Comparison to Literature
assigned to the type IIa and III OH groups, respectively.
These latter two OH groups show only little interaction
with dichloromethane adsorbed at 298 K. At 523 K all OH
groups react with CH2Cl2.
The spectrum after dichloromethane adsorption at 298 K
shows some differences compared to the gas phase spec-
Comparison to literaturea,b
This study a,b
adsorbed
523 K
CH3O-
monodentate
CH3O-
bidentate
Formate
bidentatee
c
d
Mode
trum. The asymmetric C–H stretch (Fig. 4) is affected more �as(CH3)
2963 (2213)
2921 (2143)
2884 (n.o.)
2853 (2080)
1591 (1586)
1468 (1123)
1420 (1089)
1393 (1035)
1380 (1350)
1091 (1061)
2960 (2255)
2850 (2089)
�
(CH3)
�(H–C)
s(CH3)
as(COO)
s(CH3)
2921 (2152)
than the symmetric one (the former shift from 3005 to
�
1
� 1
2870
3
056 cm , while the latter shift from 2989 to 2980 cm ).
�
Likewise, in the spectrum of deuterated dichloromethane
�
1597 (1587)
�
1
(
Fig. 5), �as(CD2) is shifted from 2212 to 2305 cm , while
�
1475 (1119)
1420 (1086)
�
s(CD2) remains the same. The shift in the asymmetric �as(CH3)
�
(H-C)
s(COO)
CO
1394 (1034)
stretch vibration is larger in the case of the deuterated
dichloromethane. This can be understood when taking into
account that at 298 K hydroxyl groups are involved in hy-
drogen bonding (spectrum 3b). If a hydrogen from CH2Cl2
interacts with an OH group, the shift in the stretch vibration
is different from when a deuterium from CD2Cl2 interacts
with an OH group.
�
�
1055 (1067)
Note. n.o., not observed.
a
b
c
d
e
� 1
.
Frequencies in cm
For CH2Cl2, in parentheses for CD2Cl2.
From adsorption of CH3Cl on alumina (23).
From methoxy species on CeO2 (41).
Comparison of the spectra after reaction of CH2Cl2 with
From reactions of methanol on alumina (22) and studies on sodium
�
-Al2O3 at 523 K (Figs. 4c and 6c) and CD2Cl2 (Figs. 5c and formate (19).

1
58
VAN DEN BRINK ET AL.
in the experiment with CD2Cl2 (Fig. 5c). They must there-
fore arise from species containing hydrogen or deuterium.
Both peaks are also found by Beebe et al. upon reaction
of methyl chloride with alumina (23). We assign the peaks
�
1
at 1468 (1123) and 1420 (1089) cm to the asymmetric
and symmetric bending modes of surface methoxy groups,
respectively. As well as some other bands in the �(C-H) re-
gion, these bands were not observed by Haber et al. (17).
SCHEME 2
This could be due to our higher CH2Cl2 concentration, olite is CO, as in our experiments. Analogously, it is docu-
which yielded a higher surface coverage, which promotes mented that in the aqueous phase dichloromethane reacts
the formation of monodentate methoxy species (vide in- with water by a S 2 substitution reaction to produce HCl
N
�
1
fra). The band at 1393 cm can be attributed to the de- and chloromethanol (CH ClOH) (31), which is equivalent
2
formation of the C–H of the formate species. The shift of to the species observed by Krawietz (30). The latter product
�
1
this band to 1035 cm in the experiment with deuterated decomposes to HCl and formaldehyde. Under our condi-
dichloromethane is in accordance with Greenler’s results tions surface OH groups can play the role of water. Indeed,
(
22). The C–H stretch of the formate has been observed as hydroxyl groups are consumed in the reaction (Fig. 3). Free
�
1
a very small band at 2884 cm
.
formaldehyde is not observed in either flow or FTIR exper-
Ramanchandran et al. (26) observed an additional band iments. Probably no free formaldehyde is formed, mean-
�
1
at 1790 cm after adsorption of CH2Cl2 on a cobalt- ing that species (1) is converted into some chemisorbed
exchanged Y zeolite. They attributed this band to a CHOCl formaldehyde analogue, for example, (2) and/or (3), the
surface species, referring to a publication by Fevrier et al. latter involving formaldehyde sorbed at an alumina Lewis
(
27) in which this species is only a speculation. This peak acid site.
is indeed in the area of �(C == O) of acyl chlorides (28).
However, they also observe bands at 2976 and 2850 cm
The chloride displaced in the first and second stages of
�
1
3+
,
Scheme 1 can react with surface Al , giving an aluminum
which they ascribe to the �as and �s vibrations of the CH2 chloride species, or pick up a proton to form HCl. HCl is
in dichloromethane. This is at variance with our gas phase known to react with alumina under our conditions (32), so
spectra. Possibly, Ramanchandran et al. (26) had obtained probablythe surface of� -Al O becomespartlychlorinated
2
3
�
1
some gas phase methyl formate (�(C == O) = 1768 cm
in the process.
It is clear that a disproportionation—involving some hy-
(
29)). However, this hypothesis should be confirmed by
tests with CD2Cl2. This band has not been observed in our dride shift—is required to arrive from species like (2) and
study of adsorption on alumina, but this could be due to the (3), at the observed formate (4), and methoxy groups (5).
difference in catalyst and operating conditions.
This type of conversion is known as the Cannizzaro reac-
tion. It is usually described as taking place in solution under
strongly alkaline conditions (33), but is also known to pro-
ceed with acid catalysis (34). Alumina is a more acidic than
a basic catalyst (35), so an acid-type Cannizzaro reaction is
4
.2. Mechanism
In the previous section was shown that two kinds of
species originate from reaction of dichloromethane with � -
alumina at 523K:surface methoxygroupsand adsorbed for-
mate. A first step in the mechanism leading to these species
could be the displacement of a chlorine of dichloromethane
more likely. In (sulfuric acid) solution, formaldehyde can
+
be converted into the hydroxycarbenium ion, HO–CH ,
2
which is stated to accept a hydride from hydrated formalde-
hyde, HO–CH2–OH (34). In our case species (3) and (2),
respectively, can react likewise, as illustrated in Scheme 2.
Closely related to the Cannizzaro reactions is the acid-
catalyzed Tishchenko reaction: two formaldehyde entities
give a methyl formate molecule. The latter product was not
seen in our IR study or our flow experiments, so we tend to
rule out this pathway.
on the alumina surface. Krawietz et al. (30) observed in
solid state 13C-NMR on a Zn-Y zeolite that 13C-labeled
dichloromethane can interact with a surface OH group to
form a chloromethoxy species (species (1) in Scheme 1).
This species is stable at 298 K, but decomposes with dealu-
mination of the zeolite at 423 K as observed by Al-NMR.
The main product of decomposition of CH2Cl2 on this ze-
27
Haber et al. (17) have proposed that the formation of
CH3Cl occurs by a hydride transfer from the surface to an
adsorbed dichloromethane molecule. From their IR spec-
tra they concluded that at 523 K CH2Cl2 is coordinated
with the chlorine atoms to O2 ions on the alumina. Our
experiments with deuterated dichloromethane show that
this proposal is not correct. Furthermore, according to the
authors hydrogen would be provided as a hydride by the
�
SCHEME 1

CATALYTIC OXIDATION OF DICHLOROMETHANE ON � -Al2O3
159
surface hydroxyl groups. This is against the known rela-
tive electronegativity of oxygen and hydrogen, and there-
fore must be discarded. Moreover, in the case of deuterated
dichloromethane, if hydrogen originates from the surface,
the formed methyl chloride should be CD2HCl. How-
ever, the surface methoxy species we observed were CD3O
species, and not CD2HO species. Characteristic adsorption
�
1
bands of the latter species at 2177 and 2131 cm (36) were
not found. Therefore, hydride transfer from the surface OH
groups does not occur. Direct hydride transfer to adsorbed
CH2Cl2 might be possible, when a species like (2) is con-
sidered as the hydride donor. This would yield surface for-
mate and gas phase methyl chloride. Reaction of gas phase
methyl chloride with alumina would yield methoxy groups
SCHEME 3
subsequent CH3Cl formation. Analogously, due to the high
surface coverage, not all dichloromethane can react and
CH2Cl2 reappears in the MS spectrum. The initial selectiv-
ity to CO can be explained by thermal decomposition of
isolated chemisorbed formaldehyde.
At higher temperatures species (2) and/or (3) could be
decomposed rapidly and desorb more easily, making dis-
proportionation less likely. At these temperatures methyl
chloride, if still intermediately formed, may also be decom-
posed on � -Al2O3 (23). This is in agreement with the flow
experiments, where CH3Cl is not observed as a product at
higher temperatures. Interestingly, following this scheme,
oxygen is not necessary for the production of both ma-
jor products, CO and CH3Cl. Indeed, Bond and Rosa (39)
found that conversion of CH2Cl2 also takes place around
(
1
23). But the characteristic vibrations of CH3Cl (2968 and
355 cm (37)) were not observed in the gas phase after
�
1
reaction of CH2Cl2 at 523 K. And even if it were present
in very small amounts, the large excess of CH2Cl2 present
in the gas phase makes CH3Cl as an important source for
surface methoxy groups very unlikely.
It is of interest to compare the behavior of CH2Cl2 with
that of CCl4, as studied by Clet et al. (32). On hydroxylated
alumina it first reactswith a surface hydroxylgroup, forming
a trichloromethoxygroup and HCl, which then reacts with a
second surface hydroxyl group to form phosgene (COCl2)
and HCl. This compound is more stable than formaldehyde
and is indeed observed by Clet. They also studied dehydrox-
ylated alumina; phosgene is formed as well, but aluminum
chlorides are formed instead of HCl.
7
00 K in an atmosphere of only N2 and water.
As stated above, alumina will be chlorinated during the
course of the reaction. Reaction with water can regenerate
surface hydroxyl groups, as depicted in Scheme 3. Accord-
ing to Clet et al. (32), in the absence of water, chlorinated
alumina might react with O2, forming aluminum-oxide
bondsand Cl2. But thisisnot probable under our conditions,
as alumina is a very poor “Deacon” catalyst, releasing Cl2
only above 1000 K (40).
From our flow studies water is known to improve the rate
of conversion of dichloromethane. The fact that the pro-
duct distribution is the same in the presence and absence
of water indicates that an identical mechanism is operative.
The role of water is primarily to increase the concentration
of surface hydroxyl groups and to regenerate these from
Al–Cl entities (39). When the catalyst was first exposed
to the reaction mixture at 573 K, HCl was only observed
after several hours, indicating that the regeneration of the
chlorinated alumina surface is not easy.
Dai et al. (38) studied the decomposition CF2Cl2 on alu-
mina dehydroxylated at 1000 K, which leaves only few OH
groups. This dehydroxylated alumina has very active coor-
dinatively unsaturated Al3 sites, which are able to disso-
ciate CF2Cl2 to form carbonates already at temperatures
even below 300 K. When the alumina was pretreated at
only 623 K (which is the dehydroxylation temperature we
use), no reactivity was found toward CF2Cl2 at these low
reaction temperatures. So in our case, in the presence of
+
surface hydroxyl groups, the very active Al3 sites are not
+
accessible.
With the observed surface species, the products found in
the flow experiments, CO and CH3Cl, can be explained. The
adsorbed formate species decomposes to form CO or, to a
lesser extent, is oxidized to CO2. The reaction of the sur-
face methoxy species with formed hydrochloric acid would
provide the methyl chloride observed.
5
. CONCLUSIONS
The formation of the methoxy species requires a certain
surface concentration of the chemisorbed formaldehyde,
Dichloromethane reacts with � -Al O at moderate tem-
2
3
species (2) and (3). This could explain the observation that peratures, to form CO and CO as end products. Methyl
2
in Fig. 1 methyl chloride is formed only after a certain in- chloride, a reduction product, is also formed; up to 40% of
duction period. In the beginning the surface concentration CH Cl is converted into CH Cl at around 600 K. The pres-
2
2
3
of species (2) and (3) is too low for disproportionation ence of mono- and bidentate methoxy groups and biden-
Scheme 2). After a certain time the surface concentration tate formates on the � -Al O surface has been confirmed
(
2
3
is high enough to allow formation of methoxy groups and by using deuterated dichloromethane in FTIR experiments.

1
60
VAN DEN BRINK ET AL.
These groups are all fully deuterated, proving that hydro-
gen on the surface (from the hydroxyl groups) is not in-
volved in the mechanism. The proposed first step is reaction
with a surface hydroxyl group, consonant with the observa-
tion that the IR bands of the surface OH groups disappear
upon reaction with CH2Cl2. Thisresultsin a chloromethoxyl
species which gives chemisorbed formaldehyde (species
4. Sinquin, S., Libs, S., Petit, C., Hindermann, J. P., and Kiennemann,
A., in “Proceedings, 11th International Congress of Catalysis,” p. 264.
Baltimore, 1996.
5
6
7
. Petrosius, S. C., Drago, R. S., Young, V., and Grunewald, G. C., J. Am.
Chem. Soc. 115, 6131 (1993).
. Agarwal, S. K., Spivey, J. J., and Butt, J. B., Appl. Catal. A 82, 259
(1992).
. Griesbaum, K., H o¨ nicke, D., Haas, A., Schwerdtner, F., and Khemili,
A., in “Proceedings of the 213th National Meeting,” p. 175. American
Chemical Society, San Francisco, 1997.
(
2) and (3) in Scheme 1). Analagous to the acid-catalyzed
Cannizzaro reaction, species (2) and (3) could dispropor-
tionate on the surface, yielding surface methoxy groups (4)
and adsorbed formate species (5).
The surface formates can react further to CO or CO2. The
methoxy species can react with HCl to form methylchloride.
A striking aspect of the proposed mechanism is that first
both chlorines of CH2Cl2 are removed and then Cl re-enters
8. Bond, G. C., and Sadeghi, N., J. Appl. Chem. Biotechnol. 25, 241
(
1975).
9. Mendyka, B., Musialik-Piotrowska, A., and Syczewska, K., Catal.
Today 11, 597 (1992).
0. M u¨ ller, H., Deller, K., Despeyroux, B., Peldszus, E., Kammerhofer, P.,
K u¨ hn, W., Spielmannleitner, R., and Str o¨ ger, M., Catal. Today 17, 383
1
1
(
1993).
1. Windawi, H., and Wyatt, M., Platinum Metals Rev. 37, 186 (1993).
by nucleophilic displacement on a surface methoxy species. 12. Lieske, H., Lietz, G., Spindler, H., and V o¨ lter, J., J. Catal. 81, 8 (1983).
1
1
3. Becker, L., and F o¨ rster, H., J. Catal. 170, 200 (1997).
4. van den Brink, R. W., Louw, R., and Mulder, P., Appl. Catal. B 16, 219
The mechanism explains both product formation in the flow
experiments and the observations after first exposure of
alumina to CH2Cl2 (Fig. 1). After an induction period, in
which a surface concentration sufficient for the reaction of
adsorbed species is built up, CH3Cl starts to appear.
The addition of water to the feed was found to increase
the rate, by increasing the number of active sites (i.e., hy-
droxyl groups) on the surface. Water also regenerates the
Al–Cl entities, formed either directly by reaction of di-
chloromethane or by reaction of HCl.
(
1998).
1
1
5. Windawi, H., and Zhang, Z. C., Catal. Today 30, 99 (1996).
6. Feijen-Jeurissen, M. M. R., van Winden, E. C., and Nieuwenhuys,
B. E., in preparation.
17. Haber, J., Machej, T., Derewinski, M., Janik, R., Krysciak, J., and
Sadowska, H., Z. Phys. Chem. 197, 97 (1996).
1
1
2
8. Kn o¨ zinger, H., and Ratnasamy, P., Catal. Rev. Sci. Eng. 17, 31 (1978).
9. Newman, R., J. Chem. Phys. 20, 1663 (1952).
0. Fonteyne, R., Naturwissenschaften 31, 441 (1943).
21. Schubert, M. M., Gasteiger, H. A., and Behm, R. J., J. Catal. 172, 256
1997).
(
A disadvantage of alumina as a catalyst for the oxida-
tion of dichloromethane is that CO is formed, while CO2 is
preferred, and that it will lose its activity by deposition of
carbonaceous material. The catalyst can be much improved
2
2
2. Greenler, R. G., J. Chem. Phys. 37, 2094 (1962).
3. Beebe, T. P., Crowell, J. E., and Yates, J. T., J. Phys. Chem. 92, 1296
(
1988).
24. Groff, R. P., J. Catal. 86, 215 (1984).
by the addition of a small amount of Pt (or presumably any 25. Bradley, D. C., Mehrota, R. C., and Gaur, D. P., in “Metal Alkoxides,”
p. 116. Academic Press, London, 1978.
6. Ramachandran, B., Greene, H. L., and Chatterjee, S., Appl. Catal. B
other metal active in CO oxidation). It has been shown that
2
Pt/Al2O3 can convert dichloromethane with 100% selectiv-
ity to CO2, although the rate of conversion of CH2Cl2 does
not increase (16).
8, 157 (1996).
2
2
2
3
7. Fevrier, D., Mignon, P., and Vernet, J. L., J. Catal. 50, 390 (1977).
8. Young, R. P., Can. J. Chem. 47, 2237 (1969).
9. Wilhushurst, J. K., J. Mol. Spectrosc., 201 (1957).
0. Krawietz, T. R., Goguen, P. W., and Haw, J. F., Catal. Lett. 42, 41 (1996).
ACKNOWLEDGMENTS
31. Roberts, J., and Caserio, M. C., in “Basic Principles of Organic Chem-
istry,” p. 330. Bejamin, New York, 1964.
This work was performed at the L.E.R.C.S.I. in Strasbourg and was 32. Clet, G., Goupul, J. M., and Cornet, D., Bull. Soc. Chim. Fr. 134, 223
made possible by a grant from the Netherlands Organization for Scientific
Research (NWO). The authors thank Professor Alain Kiennemann for
his hospitality and useful discussions, and Suzanne Libs and Jean Cressly
for their experimental assistance. This work is part of a research program
in the European Union Framework Climate and Environment, Contract
EV5V-CT94-0530.
(1997).
33. March, J., “Advanced Organic Chemistry: Reactions, Mechanisms,
and Structure.” McGraw–Hill, New York, 1968.
34. Nenitzescu, C. D., in “Carbobium Ions” (G. A. Olah and P. von R.
Schleger, Eds.), p. 463. Wiley, New York, 1970.
35. Morterra, C., and Magnacca, G., Catal. Today 27, 497 (1996).
3
3
6. Morrow, B. A., J. Chem. Soc. Faraday. Trans. 2 70, 1527 (1974).
7. Crowell, J. E., Beebe, T. P., and Yates, J. T., J. Chem. Phys. 87, 3668
(1987).
REFERENCES
38. Dai, Q., Robinson, G. N., and Freedman, A., J. Phys. Chem. B 101,
1
2
3
. Freidel, I. M., Frost, A. C., Herbert, K. J., Meyer, F. S., and Summers,
J. C., Catal. Today 17, 367 (1993).
. Matros, Y. S., Bunimovich, G. A., Patterson, S. E., and Meyer, S. F., 40. Hisham, M. W. M., and Benson, S. W., J. Phys. Chem. 99, 6194 (1995).
Catal. Today 27, 307 (1996).
4940 (1997).
39. Bond, G. C., and Roza Calzadilla, F., Catal. Lett. 39, 261 (1996).
41. Demri, D., Chateau, L., Hindermann, J. P., Kiennemann, A., and
. Spivey, J. J., Ind. Eng. Chem. Res. 26, 2165 (1987).
Bettahar, M. M., J. Mol. Catal. A 104, 237 (1996).