On the mechanism of catalytic oxidation of CH2Cl2 on γ-Al2O3 and its oscillatory behaviour

Article abstract of DOI:10.1524/zpch.1996.197.part_1_2.097

Adsorption of CH₂Cl₂ and its oxidation with dioxygen have been investigated on pure γ-Al₂O₃ by means of infrared spectroscopy and gas chromatographic analysis. Infrared spectroscopy showed that CH₂Cl₂ may be adsorbed on the alumina surface through chlorine ions either on two adjacent exposed Al ions at the (111) plane of Al₂O₃, or on the isoloated Lewis acid sites. The oxidation of CH₂Cl₂ at temperatures below 450°C yields an intermediate product CH₃Cl with 50% of selectivity whereas the other half of the CH₂Cl₂ amount is totally oxidized to CO₂ and HCl. At temperatures higher than 450°C practically total conversion of CH₂Cl₂ to CO₂ and HCl takes place. Selfsustained oscillations of the CH₂Cl₂ conversion in a broad range are observed when water vapour is added to the feed. The mechanism explaining the nature of the activity of alumina in the CH₂Cl₂ oxidation and selectivity to CH₃Cl as well as a possible reason of oscillations of the conversion are proposed. by R. Oldenbourg Verlag, 1996.

Full text of DOI:10.1524/zpch.1996.197.part_1_2.097

Zeitschrift für Physikalische Chemie, Bd. 197, S. 97-112 (1996)
by R. Oldenbourg Verlag, München 1996
©
On the Mechanism of
Oxidation
Catalytic
of CH2C12 on
-
\203
and its
Behaviour
Oscillatory
Tadeusz
and Halina Sadowska
Mirostaw
Robert
Janik,
By Jerzy Haber*,
Joanna
Machej,
Derewinski,
Krysciak
Institute of Catalysis and Surface
Polish Academy of Sciences,
Chemistry
ul. Niezapominajek, PL-30239 Kraków, Poland
Dedicated to Gerhard Erti on the occasion
of his
60th
birthday
(Received May 14, 1996)
oxidation
spectroscopy of
I
Chlorinated
IAlumina
I
I reaction
Catalytic
hydrocarbons
IR
Al203
surface Oscillatory
Adsorption of CH2C1, and its oxidation with dioxygen have been investigated on pure
y-Al20, by means of infrared spectroscopy and gas chromatographic analysis. Infrared
spectroscopy showed that CH,C12 may be adsorbed on the alumina surface through chlo-
rine ions either on two
Al ions at the (111)
of ALO,, or on the
plane
adjacent exposed
isoloated Lewis acid sites. The oxidation of CH2C12 at temperatures below 450°C yields
an intermediate product CH,C1 with 50% of selectivity whereas the other half of the
CH2C12 amount is totally oxidized to CO, and HC1. At temperatures higher than 450°C
total conversion of CH,C1, to CO, and HC1 takes place. Selfsustained oscil-
practically
lations of the CH,C1, conversion in
added to the feed. The mechanism
a
broad range are observed when water vapour is
the nature of the
of alumina in the
explaining
CH,C1 as well as
activity
reason of oscillations of
CH,C1, oxidation and
the conversion are proposed.
selectivity to
a
possible
Adsorption von CtLCl, und seine Oxydation mit Sauerstoff auf reinem y-Al20, als Kata-
IR
und
untersucht.
lysator wurden mit
Spektroskopie
gaschromatografischer Analyse
Analyse der „in situ"-IR-Spektren zeigte, daB CH,C1, auf der Oberflàche des A120, durch
Chlor-Atome entweder auf zwei auf die (111) Kristallflâche des A1203 nebeneinander
liegenden exponierten Al-Ionen, oder auf den isolierten Lewissaure-Zentren, adsorbiert
werden kann. Die
von CH,C1, bei
unter 450 °C
ein Zwi-
Oxydation
Temperaturen
gibt
schenprodukt CH,C1 mit 50% Selektivitàt, wàhrend die zweite Hàlfte von CH,C1, zu CO,
und HC1 volloxidiert wurde. Bei über 450 °C findet
eine volle
Temperaturen
Umwandlung von CH,C1, zu C02 und HO statt. Wenn Wasserdampf in den Reaktionsge-
mischstrom
wurde, sind selbsttreibende Oszillationen von CH,CI,-Konversion
praktisch
eingefuhrt
im breiten Bereich zu beobachten. Der Mechanismus zur
der Natúr von ALO,
möglichen
Erklarung
Aktivitát in CH,C1,
von Oszillationen der CH,C12-Umwandlung wurden diskutiert.
und Selektivitàt zu CH,C1, sowie die
Ursachen
Oxydation
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98
J. Haber, T.
M. Derewinski, R. Janik, J.
Krysciak and
H. Sadowska
Machej,
1.
Introduction
Removal of
dation is
from
oxi-
halogenated hydrocarbons
flue-gases by catalytic
more and more attention because of its
control [1], The main
recently receiving
growing
in
is total conver-
importance
pollution
requirement
sion of chlorine contained in the
molecule, into HC1, and oxidation
organic
of the
the combustion of
into C02 and H,0 [2 5], Whereas the mechanism of
organic parta
molecule is
cleavage
understood, little is
hydrocarbon
partially
known about the mechanism of the
of C-Cl bond and the forma-
tion of HC1. Both metals and transition metal oxides have been tried as
and
considerable attention is
such as A1203, Ti02, Si02 and their mixtures [7—9], Some re-
to classical acid
catalysts [2—6],
recently
paid
catalysts
searchers claim the
of acid sites, other
that other
importance
emphasize
of the solid may be more
[4, 8], It seemed therefore
parameters
important
of interest to undertake
a
more detailed
of the mechanism of oxidation
study
of chlorinated
A1203 as
it has been
hydrocarbons using
catalyst, although
found not to be the best
because it contains well defined acid sites
catalyst,
of Lewis
and its surface
be
characterized
type
properties may
easily
by
spectroscopic techniques [11],
Destruction of CH2C12 was selected as the
model reaction as the H:C1 ratio enables the total conversion of Cl into
HC1 without the presence of other reagents.
2
.
Experimental
2.1 Materials
Alumina was
in
of aluminum
was dried at 120°C for
dissolved
hrs and then
prepared by hydrolysis
isopropoxide
2
isopropanol. Resulting precipitate
calcined at 550°C for 24 hrs.
2.2 Methods
diffraction (XRD)
were obtained with
Ni-filtered Cu K„ radiation. It has been found that the
a
DRON-3.0 dif-
X-ray
patterns
fractometer
using
XRD
of alumina was characteristic of
pattern
}'-Al203.
The BET surface area was determined from
nitrogen adsorption at
77 K.
The
specific surface
area of the
was 210
sample
m2/g.
was measured in
reactor of inner diameter 10 mm. The volume of
ml. The oxidation of
a
conventional flow
Catalytic activity
apparatus
with
bed was
a
equipped
glass
5
chloride was studied at
curves. Some
catalytic
méthylène
different
in order to determine
temperatures
light-off
experi-
ments were done after
each
a
certain time-on-stream at 250°C and 300 °C. Before
catalytic experiment samples were standarized at 450°C for 1 h.
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On the Mechanism of Catalytic Oxidation of CH2C12 on y-Al20,
99
chloride was introduced into the hot stream of air in order to
Méthylène
allow its
The concentration of
chloride in air
experiments were
rapid evaporation.
méthylène
was 2.5
of air. G.H.S.V. was 10,000
h
'. Some
g/Nm3
done
air saturated with water vapour (24 Tr) at 25 °C.
using
chloride
and HC1 were the
of the
an "on-
Methyl
(CH3C1), C02,
chloride oxidation. CH3C1 and CH2C12 were
only products
méthylène
analyzed by
line"
(SRI 86 10A) with DCQF-1 column. HC1 was
gas chromatograph
absorbed in water
a
volume of the reaction The
gas.
by passing
given
amount of HC1 was determined from the
of HC1 solution.
conductivity
FTIR
were
in the transmition mode at room
spectra
acquired
tempera-
ture
alumina
a
Nicolet 800
The resolution was 2
cm-1.
A
thin
by using
spectrometer.
was used.
pellet
(12
mg/cm2)
Before
of
for 0.5
0~6Tr. Alumina
(Fluka AG) alumina was
outgassed
at
adsorption
pyridine
room
h
followed
(57min) to 450°C under
temperature
by heating up
1
was activated at 450°C for
1
h and then
pressure
pellet
cooled to room
ried out for 15 min. Excess
of
Torr) was car-
temperature. Adsorption
pyridine (several
was evacuated for 1
was then heated
h and FTIR
pyridine
spec-
trum was
Alumina
(10°/min) to 150°C,
registered.
pellet
up
1
h
under continu-
2
50°C, 350°C or 450°C and kept at each temperature for
ous evacuation. From each
alumina
pellet was
cooled to room
temperature
for
FTIR
temperature
acquiring
spectra.
CH2C12 was adsorbed
on alumina
pellet activated, as
described
above,
from
The
a
stream of
air
10
g
of CH2C12/Nm3 at 250°C for 0.5 h.
dry
containing
was then cooled to room
in air and
under
pyridine.
sample
temperature
outgassed
5
1
0
Tr. Alumina
of adsorbed
in such
way was exposed
to
pressure
The
pretreated
were
as described above.
spectra
pyridine
registered
CH2C12, in presence of water vapour (24 Tr), was adsorbed on stand-
ardized alumina
at 250°C for 0.5 h. Then, the
was cooled to
pellets
sample
5
1
50°C in moist air and evacuated under
10 Tr.
was
pressure
at
Pyridine
adsorbed at 150°C followed
150°C,
250
350°C or
°C,
by desorption
450°C. FTIR
were
after
the
pellet to
room tem-
spectra
registered,
cooling
from each temperature.
perature
3.
Results and discussion
The
of the initial and
activities of alumina in oxi-
dependence
steady-state
dation of CH2C12 as
a
function of the reaction
It can be seen that alumina deactivates
is shown in
temperature
1.
strongly reaching,
however,
Fig.
stable
after
temperature.
a
certain time-on-stream. The
of deactivation
activity
degree
on
At
alumina exhibits stable ac-
depends
tivity.
high temperature
of alumina with
to CH3C1 and
2. CH3C1 and
Fig.
Steady-state activity
together
selectivity
HC1 as
a
function of the reaction
is shown in
temperature
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100
J. Haber, T. Machej, M. Derewiiíski, R. Janik, J. Krysciak and H. Sadowska
00
1
90
80
70
60
5
0
0
4
3
0
0
-after
steady- state
3
min
2
10
o
0
50
100
150
200
250
300
350
400
450
Temperature, °C
Fig. 1. CH2C12 conversion as
a
function of temperature for undoped alumina.
100
150
200
250
300
350
400
450
500
Temperature, °C
2
.
CH2C12 conversion and
to CH,CI and HC1 in the
steady state as a
Fig.
selectivity
function of temperature.
alumina.
Undoped
HCl are the
It follows from
reaction
Cl2 is not detected.
only chlorine-containing
products.
that most of HCl remains adsorbed on alumina sur-
lower than 350°C. All HCl in the reaction
2
Fig.
face at the
is desorbed from the surface at the
temperatures
produced
400°C. At the same time
temperature
CH3C1 decreases
abruptly.
the
selectivity to
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On the Mechanism of Catalytic Oxidation of CH2C12 on
;
'-Al20,
101
100
90
8
0
0
7
3
00 °C
50 °C
200 °C
6
5
4
0
0
0
2
V.
¿
3
0
0
2
1
0
—-1
- . ___
r--*
»—» »—·—-·-
0
10
20
30
40
50
60
70
80
90
Time on stream, min
3
.
The variation of CH2C12 conversion and selectivity to CH,Cl as
a
function of
the
Fig.
time-on-stream at different
alumina.
temperatures. Undoped
The
of the alumina
and
is shown in
temperatures. However,
dependence
activity
selectivity to CH,C1 on
time-on-stream at different
that the initial
3. It can be seen
it decreases
temperatures
Fig.
is
at all
activity
high
at 200 and 250°C. CH3C1 is not observed in the reaction
strongly
products
with
appears
at the very
of the reaction at 250°C and 300°C but it
beginning
time-on-stream in these
HC1 is
with
temperatures.
produced
poor selectivity
(
Fig. 2). 4
the variation of the
on the time-on-stream for alumina at 250°C in the
in the feed. It can be seen that the alumina
to
CH,C1 and
Fig.
presents
activity, selectivity
of
yield
CH,C1
presence
of water
after
vapour
activity,
initial decrease,
to oscillate and so does
to CH,C1. Yield
remains stable.
begins
selectivity
of the
of CH3C1 after decrease at the
beginning
reaction,
HCl is not observed in
products.
shows that A1203 obtained in our
After
has the
X-ray analysis
experiments
structure of defected
at 450°C for
1 h
spinel y-Al203.
dehydroxylation
under the
of 10"6 Torr
5
different kinds of OH
were formed
5. Two of them,
pressure
at the surface as indicated
groups
the IR
shown in
by
spectra
Fig.
prevail,
are visible as shoulders at 3755 cm"', 3772 cm"1 and 3790 cm"'. Some of
the surface OH
as indicated
the
with
3682 cm"' and 3731 cm"1,
other three
stretching frequencies
are
groups
broad band with maximum at about 3570 cm"'. These results are in
with the
Saad et al. [12].
hydrogen-bonded,
by
presence
of
a
a
of
very good agreement
spectra
)'-Al203 quoted by
Recent studies of the structure of aluminas, obtained
oxidation of
by
aluminium in an
mixture, carried out with the
y-alumina have
very pure
of HREM, showed that small
argon-oxygen
help
of
almost
particles
spherical
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102
J. Haber, T. Machej, M. Derewinski, R. Janik, J. Krysciak and H. Sadowska
80
0
'
0
20
40
60
80
100
120
140
Time on stream, min
Fig. 4. The variation of CH2C12 conversion, selectivity
to CH,C1 and
presence
yield
of
CH3C1
as
a
function of time-on-stream at 250 °C in the
of water
in the feed
vapour
(24 Torr).
and the
are facetted so that
shape
exposed crystal planes
predominant
hexagonally
preferentially
surfaces are of the (111)
on which
atoms are
type,
oxygen
arranged [13].
Also the (110)
planes
found
previously
to be
in the case of
[11] consist of
small (111) facetts.
exposed
y-alumina
many
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On the Mechanism of
Oxidation of CHUCU on
103
Catalytic
}'-Al20,
Fig. 6. A) (lll)-face of alumina spinel lattice, -layer; B) (lll)-face of alumina spinel
lattice, B-layer.
of the
of OH
observed here with those sum-
Comparison
frequencies
groups
marized
[11] shows that
of our
by Knozinger
dehydroxylation
preparation
left at the surface
OH
groups
located in the centre of three
mainly
exposed
notation,
and tetra-
of OH
coordinated Al ions
III in
octahedrally
(type
Knözinger's
exposed octahedrally
Other
3
682 cur1) and OH
located between
groups
coordinated Al ions
Ha, 3731
hedrally
groups
(type
cmr').
types
are
in small
A
model of the
present
quantities only.
A1203 surface,
studied in our
On
be thus
as shown in
6.
experiments, may
to
proposed
intensity
clearly
Fig.
1
the
This is
of the band at 3682 cm
visible from
exposure
pyridine vapour
remains
in
practically unchanged.
Fig. 7A,
which difference spectra are illustrated,
after
of
registered
adsorption
pyri-
dine at room
and its
at different
It
temperature
desorption
temperatures.
be seen that
interacts with all OH
at the surface
may
except
through
pyridine
groups present
those
its aromatic electron
for the band at 3680 cm"1,
forming H
the
appearance
bonds
of the
responsible
as indicated
system,
by
broad band with maximum at 3560 cm"1. This is consistent with the results
illustrated in
um ions and the
Lewis sites formed
Lewis acid sites are characterized
7A and which show the absence of the bands of
of the bands of
Fig.
pyridin-
bound to
presence
pyridine coordinatively
Al ions. It
be noticed that two
of
by exposed
may
types
doublets of
present,
by
coordinatively
bound
pyridine at
1450/1456 and 1617/1623 cm"1
[12].
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104
J. Haber, T.
M. Derewinski, R. Janik, J.
Krysciak and
H. Sadowska
Machej,
A
4
000
3 50
3700
3SS0
34 00
32S0
ufiVENUMBEH
3100
29S0
2 00
2650
un
1800
17S0
1700
1650
1600
ISSO
WfiVENUnBER
1500
1
SO
IHOO
1350
Fig. 7. The difference IR spectra of A1:0, after adsorption of pyridine at room tempera-
ture and its desorption at different temperatures. The spectrum of , ,
before
registered
B) deformation vi-
the experiment has been subtracted. A)
vibration
range,
stretching
bration
range.
When
reactor over
CH,C12 is passed
at different
the
temperatures through
catalytic
as the
the conversion
to be observed
y-Al20,
catalyst,
begins
at 150°C and initial conversion of 100% is reached at 300°C (solid line in
Fig. 1).
However the
initial 100% conversion
ed after about 20 min.
catalyst undergoes
deactivation so that at 300°C the
to below 80% when the
state is attain-
drops
steady
with 100% conversion is achieved
Steady-state
only
at 400°C
line in
The
products obtained in the
steady-state
(dotted
Fig. 1).
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On the Mechanism of
Oxidation of CH2CI2 on
105
Catalytic
)'-Al20,
as
a
function of the reaction
are
in
2. It may be
temperature
presented
Fig.
seen that at low
and desorbed:
CH3C1 is formed as the intermediate
temperatures
product
CH2C12 -> CH,C1
+
HC1
for this reaction to occur is
surface
Hydrogen, required
initially supplied by
OH
to CH3C1 amounts to about 50%, the other half of
groups. Selectivity
the initial amount of CH2C12 introduced into the reactor
oxidized
being
totally to
C02 and HC1:
+
->
+
HC1.
CH2C12
02
C02
HC1 is
sorb
held at the surface at lower
and
begins to de-
strongly
temperature
than 450°C
at 350°C. At
total
only
temperatures higher
practically
conversion of CH2C12 to C02 and HC1 takes
place.
Let us discuss in more details the mechanism of the transformations of
CH2C12 in the presence of oxygen. When CH2C12 is introduced on the cata-
at lower
(200°C), about 60% of it disappears, but no
lyst
temperature
prod-
ucts except CO, are observed
3). In the first minutes of the reaction
(Fig.
the decrease of the conversion occurs and after 80 min the
becomes
catalyst
deactivated. At 250°C, and even more at 300°C, CH,C1
to
totally
begins
as
a
after first few minutes of the reaction and its amount
appear
attains
product
a
certain constant level of the
steady-state, corresponding to
20% or
due to
80% conversion
respectively.
The deactivation of the
at low
is
catalyst
temperature
apparently
the
of both CH2C12 and HC1. Indeed,
in
strong adsorption
experiments,
which CH2C12 was adsorbed at 250 °C in situ in the IR cell showed that it
interacts with the surface.
and after
8
shows the IR
on the surface of
of CH2C12
strongly
Fig.
spectra
A1203.
in the
It be
may
gas phase
adsorption
seen that the
and of the
of the
modes r(CH2) at about 1270 cm"'
frequencies
rocking
modes
at about 1465 cm"1
[14]
are shifted to
bending
<5(CH2)
values.
in the
of
fre-
that
adja-
considerably higher
Simultaneously,
range
may
stretching
a
new
at 2940 cm"1
It
be
quencies
adsorption
cent
strong peak
appears.
suggested
of CH2C12 takes
as
a
two-sites
two
place
process involving
Al ions at the (111)
of A1203. Two chlorine atoms of
exposed
plane
CH2C12 would occupy
the
vacancies above Al ions.
oxygen
o2-
Cl
Cl
o2-
AP+
AP+
Scheme 1.
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106
J. Haber, T.
M. Derewiiíski, R. Janik, J.
Krysciak and
H. Sadowska
Machej,
8
.
The difference IR spectrum of CH2C12 after adsorption on alumina and
Fig.
at 250 °C for
for comparison. A)
outgassing
CH2C12 is
range.
5
min (the spectrum of A120, subtracted). The spectrum of
pure
B) deformation vibration
vibration
given
stretching
range,
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On the Mechanism of
Oxidation of CH2C1, on
107
Catalytic
y-Al203
This
ions
the two
atoms of the tetrahedron near
oxide
brings
hydrogen
neighbour
above the surface and enables their
interaction to form
with
exposed
strong
OH groups, which in turn will interact
by hydrogen bonding
existing
surface OH groups. Indeed, the difference
of A1203 after
adsorption
spectra
of
in
9
show that all OH bands are involved in the
CH2C12 presented
interaction with the adsorbed CH2C12 and
OH
Fig.
a
band of
hydrogen bonded
huge
groups appears.
If two-sites
even at full
of CH2C12 is assumed at the surface of A1203,
adsorption
certain number of isolated sites must be left unoccu-
In order to check this
a
coverage
conclusion,
the after
in situ in the IR cell and IR
sample
of
A1203
adsorption
pied.
of CH2C12 was
to
exposed
pyridine
spectra
were
after
at 150°C, 250°C, and 450°C
9A and
registered
outgassing
(Fig.
B). It may be
seen that considerable amount of
is adsorbed at the
pyridine
of
appearance
acid Lewis sites as indicated
the
doublet at
by
strong
1
450 cm~71455 cm"1, and
at 1494 cm"' and 1625 cm1. It is evident
peaks
that two
of Lewis sites of different acid
exist at the surface.
at 1450 cm"'
types
strength
From those, which are
for the
is
at
responsible
peak
pyridine
desorbed
at 250 °C, whereas on those related to the
already
peak
1
455 cm1, it is retained even after
at 450°C when the adsorbed
heating
CH2C12 has already been removed from the surface as the result of its reac-
tion to form CO, and HC1. It may be concluded that the isolated sites left
after
of CH2C12 show strong Lewis
unoccupied
The
adsorption
acidity.
isolated Lewis acid sites at the surface may adsorb not
unoccupied
but
also be involved in another
of
only pyridine,
may
possible type
adsorp-
tion of CH2C12,
through a
a
of this tetrahedral molecule
namely single-site adsorption
chlorine
:
apex
Cl
Al3+
Scheme 2.
If an OH
group present at
is
of the
the
next to the adsorbed molecule,
position
take
in the
in the
of
rearrangement
complex maya
place, consisting
splitting
the Cl-C bond and formation of H—C bond,
resulting
desorption
of CH3CI. This model finds its confirmation in the results of IR
spectro-
studies.
it
be seen from
9A that after
scopic
Namely,
may
Fig.
adsorption
of CH2C12 on A1203 at 250 °C four
in the
vibration
peaks appear
stretching
Two
ones at 3008 cm"' and 2920 cm"' are
due to
range.
strong
obviously
the vas and vs vibrations in the CH2
of adsorbed CH2C12 [14]. Two
group
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108
J. Haber, T. Machej, M. Derewinski, R. Janik, J. Krysciak and H. Sadowska
HOOO
3 50
3700
3S50
3100
URVENUMBER
3250
3100
29S0
2000
2650
Fig. 9. The difference IR spectra of CH2C12 adsorbed on , , (a) and pyridine adsorbed
on A120, covered with CH2C12 and desorbed at different temperatures (b). The
spectrum
of Al2Oj registered before the experiment has been substracted. A) stretching vibration
B) deformation vibration
range,
range.
other
at 2854 cm"1 and 2960 cm"1
be
to the
peaks
may
assigned
stretching
vibration in the CH3
of CH,C1 formed at the surface as the result of
group
the transformation
Scheme 2.
represented by
It is
that the
to 250°C as could be
of these two
increases on
because of the
noteworthy
intensity
peaks
raising
the
temperature
expected
increasing
rate of the attack of
en on the adsorbed CH2C12
hydrog
species.
shown
After
of CH3C1 formed in the
Scheme
desorption
process
chlorine atoms.
by
2,
hy-
the surface is left
from surface
poisoned by
has been consumed in this
Hydrogen
so that no more
droxyl groups
process
hydrogen
is available to react with chlorine and desorb in form of HC1. The surface
with chlorine is determined
the ratio of the rates of reactions
coverage
by
represented by Scheme 1
and 2. Indeed,
Fig. 2 shows that
HC1
in
appears
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On the Mechanism of
Oxidation of CH2C12 on
109
Catalytic
)>-AI20,
the
in
amount
when the amount of CH3C1 de-
only
products
increasing
creases so that at 500 °C
in the feed
the whole amount of chlorine introduced
products.
practically
appears as
The situation,
HC1 in the
in
3
when water
of oscillations
chlorine-con-
represented
Fig. changes dramatically
is added to the feed and an
vapour
of the conversion
taining compounds
are the initial
interesting phenomenon
This is shown in
4. The
appears.
found in the
Fig.
only
formed in the course of this
products
and
The
of the latter
experiment
remains constant, whereas the amount of CH2C1, oscillates in
the oscillations of its
reagent CH2C12
CH,C1.
yield
a
broad
range
and hence the oscillations of
indicating
consumption
the
of CH3C1. It may be assumed that the reaction
selectivity
represented
Scheme
2
+
a
:
by
-> CH3C1
+
+
Os
CH2C12
at
OHs
Cls
constant rate, its
constant, and that in the
proceeds
of water
in an
be
yield being
presence
CH2C12
vapour some
oscillatory way
other
process proceeds parallely consuming
without
HC1 or Cl2. The lack of HC1
producing
may
pre-
explained by assuming that hydroxylation
of the surface
by
water
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110
J. Haber, T. Machej, M. Derewiiíski, R. Janik, J. Krysciak and H. Sadowska
1800
1750
1700
1650
1600
1550
UflVENUMBEB
1500
1450
IHOO
1350
1
0. The difference IR spectra of A1203 exposed to: a) the
of CH2C12, b)
a
Fig.
vapour
to
mixture of CH2C12 and 24 Torr H20, both followed by
and evacua-
has been sub-
exposure
before the
pyridine
tion at 250 °C. The spectrum of A120,
stracted.
registered
experiment
vents the Scheme
1
to operate because basic oxide ions become inaccessible
bonded water.
for
from the CH2C12 due to
hydrogen
blocking by hydrogen
further transformation is the extrac-
In such circumstances the
only possible
tion of Al ions from the lattice
known to be stable volatile
of
aluminium chlorides,
by desorption
alkyl
[15]. Some confirmation of this
compounds
Fig.
A1203 first to of CH2C12, or the
vapour
comes from the in situ IR measurements.
10 shows the IR
hypothesis
after
spectra registered
exposing
mixture of
of CH2C12 and 24 Torr of H20, in both cases followed
vapours
to
at room
and then evacuation at 250 °C
by exposure
pyridine
temperature
for
1
1
h. It
be seen that in the
of water the
at 1382,
peaks
may
presence
393 and 1596 cm"1
to adsorbed CH2C12 are less intense. Two
assigned
of further transformations
be
Either the surface
AF+ ions must recon-
possibilities
may
envisaged.
of adsorbed CH2C12 and
layer composed
underlying
struct before
of
aluminium chloride can take
or at
of the
desorption
alkyl
place,
2
50°C the
removes the (111)
of
desorption
facetting
undergo
(110) planes
lattice
and these
must
reconstruction back to
of new molecules of CH2C12 can take
spinel
111)
exposed
planes
facets before the
(
adsorption
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On the Mechanism of
Oxidation of CH2C12 on
111
Catalytic
)»-Al,0,
0
50
100
150
200
250
300
350
400
450
500
Temperature, °C
Fig. 11. CH2C12 conversion and selectivity
to CH,C1 in the
steady state
in the
presence
of water
in the feed as
a
function of temperature.
vapour
The
reason of the
systems [16].
of such reconstruction in both alternatives
may
be the
described for other
place.
necessity
oscillatory behaviour
of the
system, as
At
such reconstruction is
and the oscillator
higher temperatures
rapid
behaviour
Instead,
a
state is established
which total conversion of CH2C12 is observed, whereas in the
11) in
disappears.
steady
(Fig.
products only
CH3C1 is present and only at temperatures below 450°C. Simultaneously,
the
of AIO OH was observed on the walls in the cool part of the
deposition
reactor.
Acknowledgement
This work was financed
KBN Warsaw, within the research
the State Committee for Scientific Research,
by
project no 2
303 016 06.
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J. Haber, T.
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