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Kinetics and mechanisms in the ammoxidation of toluene over a V2O5 catalyst. Part 2: Non-selective reactions
Catalysis Today, 3 (1988) 223-234 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
KINETICS
AND
MECHANISMS
PART 2: NON-SELECTIVE
JONATHAN
C. OTAMIRI
IN THE AMMOXIDATION
OF TOLUENE
OVER
A V2OS
CATALYST.
REACTIONS
and ARNE ANDERSSON
of Chemical Technology, Chemical P.O. Box 124, S-221 00 Lund (Sweden)
Center,
Department nology,
223
Lund
Institute
of
Tech-
ABSTRACT In the amnoxidation of toluene over a V 0 catalyst, the variation of initial rates with partial pressures of oxygen, t&8 ene and ammonia was determined for formation of carbon oxides. Dependencies obtained were analyzed and expressed in are initial products terms of rate equations. It was found that CO and CO formed at different sites, none of which are involved ? n selective reactions. The mechanisms derived for the formation of the two oxides have many features in common. In each mechanism there are two parallel routes originating from the same active site, which is suggested to be an ensemble exposing vanadium ions and electrophilic oxygen species. One of the routes proceeds without participation of ammonia, while in the other route ammonia is adsorbed. In both routes, the rate-determining step can be considered to be one of the steps in a stepwise reoxidation process. The rates for formation of carbon oxides decrease strongly with increasing partial pressure of ammonia, which is due to a combined effect of introduction of a new reaction pathway and competitive adsorption between oxygen and ammonia.
INTRODUCTION Knowledge oxides,
about
reaction
is of great
importance
that need to be answered products
of selective
intermediate,
which
iii) whether the answers question carbon
how
oxides
ii),
it would
tive
iii),
to
oxidation is common
reduce
are formed
readsorption
the solution
for alternatives
through different reactions, we would
and amnoxidation, for both
i) and iii)
intermediates,
to know
selective
is to block,
or if different want
of
of
route.
these
Questions
products.
stabilize
to solving
In the
case
it is necessary
of
to know,
If the latter are
0920-5861/88/$04.20 0 1988 Elsevier Science Publishers B.V.
If
alternative For alterna-
route.
Further-
if the same site,
both in non-selective
sites
the general products.
the intermediate. the parallel
different
and
found
to the problem would
or prevent,
is involved
from
routes,
Once we have
non-selective
i), a solution
sites are involved.
if these
and non-selective
reaction
we have to make efforts formation
to somehow
catalysts.
carbon
ii) if they are formed from an
selective
in a parallel
the
especially
are i) if carbon oxides are formed
due to alternative
be necessary
products,
for the design of industrial
to these questions,
of
to non-selective
in this regard,
they are formed
be to prevent
more,
paths
and selective
is the case,
localized
at
the
then
same
or
224 different
crystal
structure
faces.
sensitivity
reviewed
of metal
(ref. 1). Another
oxidation
In ammoxidation
catalysts,
question
of alkyl
oxide-based
investigations
of
treating
oxides
frequently mediate
our interest a field
interest
to the so-called
that
is whether
formed
recently CO2
has been
is formed
from
nitrile a
of solid state chemistry of olefines
investigation
catalyst
that
However,
of
(ref. 13) data
with respect
and
the
to corresponding
have been reported. over
and directly
a V205
kinetic
from
aromatics
for carbon
(refs. 11, 12) anoxidation
oxides
were
obtained
inter-
showing
the
oxygen species
is lacking.
of
that It has
a common
investigation
and the role of electrophilic
selective
Ito and Sano
catalyst,
from m-xylene.
and CO2 are formed
thorough
nitriles,
used (refs. Z-6), and some kinetic
of m-xylene
via m-tolunitrile
reported
in degradation
pyridines
carbon oxides formation
(refs. 8-10).
importance
and
are frequently
in the ammoxidation
are
been
benzenes
catalysts
(ref. 7) concluded
tailed
oxide
turns
of CO.
vanadium
carbon
The last question
toluene
In a de-
over
a
V205
and are here analyzed
to the above considerations.
METHODS Preparation described
of
catalyst,
elsewhere
catalytic
measurements
and
product
analysis
are
(ref. 13).
RESULTS Influence
of partial
pressure
The rate dependencies partial
pressure
of ammonia
three temperatures.
the rates
pressure. easily
A
are
presented
Figure 3 shows
of asnnonia at 4OO'C. that
of oxygen
of CO2 and CO on partial
Comparison
partial
order
be linearized
the dependencies
of rates obtained
at low pressure
of ammonia
dependency
by plotting
pressure
is
inverse
of oxygen. Thus, the rate expressions
of oxygen
for a high
in Figs. 1 and 2, respectively, obtained
at a low pressure
in the separate
are much noticeable
higher in
than
all
rate as a function
series those
cases,
(11
and
CO
are
amnonia products. with
of partial
influence
has
pressure
of partial
presented strong
in
increase
in
pressures
of ammonia
and toluene.
of ammonia pressure
Figs. 4
influence
At low pressures
further
can
pressure
are of the form
where a and B might depend on partial
The
shows
at high
which
of inverse
r = aPo/(l + BPo)
Influence
for
on
of ammonia
and the
of ammonia, partial
5,
on rates
respectively.
rates
for
of
As
formation
the rates diminish
pressure
for formation
ammonia,
could of
be
total
of CO2 noticed,
oxidation
more than a half, but the
rates
gradually
won
0%
2
4 2
P -
012.
'00 x B 008.
004.
10 PO2
20
30
CkPa)
Fig. 1. Effect of partial pressure of oxygen on the rate for gormation of CO at n 34O'C; q 370 C; and ~85~"",l~. PT = 0.765 kPa and PA =
.
Fig. 2. Effect of partial pressure of oxygen on th& rate for,formation of CO at A 340 C; P 370 C; and A 400 C. PT = 0.765 kPa and PA = 2.855 kPa.
.
2.4.
Fig. 3. Rates for formation of CO 0, and CO A, as a function 8f parti.E1 pressure of oxygen at 400 C. PT = 0.765 kPa and PA = 0.485 kPa. 0.8
10
20 PO,_,CkPa)
30
2 2
4 qUH3
and remain
that ammonia
constant
influences
From the nature can be represented
6
(kPa)
CkPa)
Fig. 5. Influence of partial pressure of ammonia on the &ate for fgrmation of 60 at A 340 C; P 370 C; and A 400 C. Pl = 0.765 kPa and PO = 11.485 kPa.
Fig. 4. Influence of partial pressure of ammonia on the rate for fogmation of fi02 at m 340 C; 0 370 C; and 0 400 C. PT = 0.765 kPa and PO = 11.485 kPa.
decrease
4 h,.,s
6
at high pressures
the formation
of ammonia.
It is also apparent
of CO more than that of C02.
of the dependencies
observed,
it can be concluded
that they
as follows:
r = (y t 6PA)/(1 + EP~)
(2)
where y, 6, and E might depend on pressures Considering oxides
exhibit
high pressures
Figs, l-3, invariance of ammonia,
r = (koPo + kBPAPo)/(l where
it with
is
seen
that
respect
in which
the partial
pressure
toluene
of
carbon
of oxygen at low and
(3)
In all the series rates
pressure
formation
+ kyPA + k&PAP0 + kEPo)
of toluene
the
for
of eqns. 1 and 2 gives (ref. 14)
pressure
that
rates
to partial
of partial
found
the
hence, a combination
ka, . . . . k. might be real constants
Influence
of oxygen and toluene.
for formation
within
the
range
or depend upon pressure
pressure
of CO2
of toluene was varied,
and CO were
investigated.
of toluene.
Typical
independent results
it was
of partial obtained
are
227 shown
in Figs. 6 and 7. However, r = $P.+(l
the nature pressure.
+ wPT),
Considering
the
thus it can be concluded be used to describe
“i 0
since
fact that
the general
the rates
Values procedures are
also
Arrhenius draw
the
scribes
0.4
rate expressions
o-3
ToL
of effective described the
by eqn. 3 can
04
0
1.2
0.8
1.2
PTOL (kPd
plots.
Fig. 7. Rate dependency on partial pressure of tgluene for CO Cl, and CO A, at 400 C. PO = 11.465 kPa and PA = 0.485 kPa.
values
presented
of eqn. 3 calculated
are given
of apparent
The
curves
constants
above
values
in Tables
activation
of constants
observed
energies,
given
in Figs. l-7.
for CO2 and CO using
1 and 2, respectively.
in Tables
It is seen
on partial
that
were
the
Included
obtained
1 and 2 were
from
used
that eqn. 3 perfectly
to de-
pressures.
for total oxidation the mechanism features
CO2 and CO. However, 1 and 2 are
each separate Referring in a
at zero
investigated,
constants
Figure 8 shows
Tables
to zero
of the form given
(kPa)
by eqn. 3. The general both
be equal
is of
all data obtained.
the dependencies
Mechanisms
must
dependencies
oPT>>l under the conditions
that
Fig. 6. Rate for formation of CO versus pal;tial pressure of tolue i; 8 at u 340 C;p 370 C; and 0 400 C. = 11.485 kPa and PA = 2.855 kPa. pO Effective
form of these
to the dependencies
of the mechanism
are the same for formation
of
constants
Included
in
of eqn. 3 and those
of
the values
the relations
mechanistic
that corresponds
of kinetic
between
the constants
step.
to Fig. 8, S, is a site which
specific
oxygen species.
differ.
expressed
configuration.
Some
of
In what can be considered
comprises
these
cations
a number are
of vanadium
covered
with
ions
active
to be the first step of the mechanism,
228 TABLE 1 Values
of effective
constants
and apparent
activation
energies
for formation
of
co2. Temperatures
Keff
('C)
E
340
370
400
(kcaY!Xole)
0.6361
1.635
5.340
27.8
0.1868
0.8753
3.420
39.7
ka z klOST
(mole/m2*min.kPa
k0 * k20KNST
(mole/m2.min.kPa2
kY= KN
(kPa-')
6.222
5.660
4.572
-4.6
ks 2 k20KN/k21
(kPav2 x 10')
0.7150
1.257
1.975
13.9
kc * klo’kll
(kPa-' x 102)
1.149
2.220
4.320
18.3
x 107) x 107)
TABLE 2 Values
of effective
constants
and apparent
activation
energies
for formation
of
co. K eff
Temperatures
('C)
E app (kcal/mole)
340
370
400
0.3259
0.6928
1.712
21.9
x 115') 6.341
6.586
6.838
2.0
ka = klOST
(mole/m'.min*kPa
k6 = k20KNST
(mole/m2.min.kPa2
ky = KN
(kPa-')
9.191
6,773
3.740
-13.5
ks = k20KN/k21
(kPam2 x 10')
4.191
2.712
1.302
-15.9
kc = klo’kll
(kPa-' x 102)
4.561
4,004
3.480
toluene by
a
formed, ates,
is strongly
adsorbed
rate-determining which
through
step
reduced
Sv. Then,
conditions
of
gaseous
oxygen
a
intermediate
oxide
is present
competition
An
step is followed I2
is
into new intermedi-
and water.
After
13, which
14, which
complete is more
dissociates
in a
of Sv.
of intermediate between
11, there is, under
oxidation
at the same site. Adsorption
15, which
-4.0
intermediate
in a state,
is adsorbed,
in the formation
of ammonia
Il. This
oxygen.
is transformed
of carbon
configuration
arnnoxidation,
of
gaseous
of toluene and formation
adsorption
in formation
an intermediate
of equilibria
formation
slow step resulting
After adsorption
reversible
with
the surface
than
relatively
forming involving
a series
Ii, accompanied
combustion,
x 107)
in a rate-determining
at
site
of ammonia step
I1
and
results
reacts
with
229 oxygen
to
oxide,
water
ammonia
form
16. Through
and
can
new
be HCN,
then be reoxidized secutive
steps,
a
of
series
Ij.
intermediates,
N2 and back
to the original adsorption
comprising
Products
oxides.
nitrogen
16 reacts
equilibria,
The
originally
reduced
Sv. This
site,
to give
of molecular
resulting
site
formed,
can occur
oxygen,
carbon from
17, can
in some
18, followed
con-
by its
dissociation.
Fig. 8. Reaction
mechanisms
for total oxidation
of toluene over V205.
DISCUSSION Fractional conversion formed
conversions
of toluene
in parallel
found
obtained that
Wachs
in combined
of
silver-cerium
formed
as a function
of total
CO and CO2 are initial
products
can be drawn considering
the rate
products
the
over
in a consecutive
carbon
However,
suggested
that
can
react
oxides in
ammoxidation
it was
formed of
observed
that carbon
In the case of ammoxidation
aldehyde
in
the
of the oxiSaleh from
the
and
direct
toluene that
by
it was
formed
catalyst,
primarily
100 %, indicating
that carbon to
when
in a study
a V205/Ti02 are
the
Also
is supported
(ref. 15), where
degradation
oxides. over
(ref. 17),
was almost
reaction.
has been frequently alternatively
vanadium
catalyst
to benzonitrile
against
anhydride
that
reactant.
vanadate
and TPD experiments
stable
to phthalic
(ref. 16) concluded
selectivity
pulse is
of 3-picoline
of o-xylene
oxidation
that benzonitrile,
routes. The same conclusion
nicotinonitrile
ammoxidation dation
showed
to various
see eqn. 3 and Part 1 (ref. 13). Such a conclusion
expressions, results
of toluene
over
a
initial
oxides
are
of aromatics,
it
oxides are formed from an intermediate and
nitrile
(refs. 8-10,
18).
In a
230 discussion
of reaction
(ref. 19) dation
expresses
proceeds
the
do not
in the oxidation
opinion
from a common
investigations concluded
networks
that
support
the
that the same surface
in the formation for formation
selective
intermediate.
oxides
clearly
(refs. 11,
species, course
20)
has
results
in
reactions
selective
catalysts
has
potential
(ref. 22) measurements.
of
0;
been
species
experimentally
show
with
benzene,
that
are
of
oxidation.
In
total
(ref. 12) and
o-xylene
ogies,
found
it was
dride, both
respectively,
making
at
planes
use
of
the
could
investigations,
situated
(ref. 24)
that
and
that
oxygen
species.
suggest
that
planes
the sites
and
over
active
of
electrophilic
oxidation
and
ESR
O-
the
and
with
(010).
It was that
to this
results
of
oxidation
the
(010)
it
combustion,
Due to the fact that the vanadium-oxygen
(ref. 12),
by
exposes
(ref. 25), plete,
it can be expected
i.e.
depends
both
upon
Therefore, species,
vanadium
temperature,
number
and
partial
Sv can be considered whose
that
ions
the surface
coverage
oxygen
species
reasonable
located
at
pressure
of
oxygen,
to be an ensemble
in principle
exposed. and
of vanadium
is determined
by
the
size
to
(loo), oxygen
are relatively
of oxygen
are
02-
electrophilic
seems
distances
In be
plane
S,, are
planes. to
accommodate
described,
anhy-
concluded
demonstrated
plane
morphol-
phthalic
(010)
were
the
3-picoline
different
to the frequency total
of
interaction
of
and
V205
surface
composition
ammoxidation
catalysts
on
the reaction
of such an
of nicotinonitrile
for total
species
(ref. 21)
The
products
in the
of 02- species
(OOl), and (h01) types of plane. All of these planes expose electrophilic species.
at
oxygen
occurs
presence
0;
using
of
calculations,
the
total
o-xylene.
V205
in
to
are formed
et al. (ref. 23) studied
studies
perpendicular
Considering
whenever
existence
formation
perpendicular
products
whereas
the final
active
bond-strength
species
that
be correlated
centers
these
(amm)oxidation.
toluene,
indicated,
those
that
intermediates,
It was
derived
established
products
intermediate.
involved
Gasymov
desorption
oxi-
of the present
was
at the surface,
The
as non-selective
of a common
of hydrocarbons,
oxidation.
Andersson
(ref. 13). The rate expressions
concluded,
i.e. 0; and O-, appear
of catalytic
S.L.T.
the results
site, via different
sites that do not take part in selective Haber
as well
However,
existence
of nitrile and aldehyde
of carbon
of toluene,
long
is not comThe
coverage
reaction
rates.
ions and oxygen of
the
toluene
that
in both
molecule. the
In
parallel
formation routes
of
carbon
the interaction
oxides, between
see
Fig. 8,
gaseous
it was
oxygen
or Ig, was the rate-determining
step. II was obtained
and
both
I5 from
ensemble gaseous ism.
the adsorption
of
toluene
Sv. Such a type of rate-determining oxygen
reacts with adsorbed
Alternatively,
this
step
can
species be
seen
and
found
and an intermediate,
from adsorption
ammonia
according as
a
of toluene
at a partly
step does not necessarily
oxidized mean that
to an Eley-Rideal
part
of
I1
reoxidation
mechanof
the
231 surface. steps.
The
reoxidation
First, and
vacancy, species
by
then,
negligible
concentration lar oxygen
and
of oxygen
due
quently, step,
to
the
the
these
step,
however,
change
monoatomic
vacancy.
As
steps,
will
a competitive
hindrance
caused now
number
oxygen
long
as
the
in Fig. 8, can be
will
presented.
appear
be a relatively
where
the
large
of molecular
be even slower.
if they occur
derived,
Therefore,
because
at oxygen for
reoxi-
last
Conse-
reoxidation
is a slow step, and a these
steps constitute
indicated
as two slow consecutive
the form of rate expression
13+14+Sv
molecule.
in the
oxygen
In Fig. 8 they have been
i.e.
available
toluene
that
of molecu-
of toluene
vacancies
the
slow process.
route,
adsorption
of
by
in each
is so low,
the adsorption will
indicated
a situation
to the last two steps
step.
even
two
is so low that, even if adsorption
of vacancies
dissociation
two consecutive
ion, i.e. an oxygen
the form of rate expression
its dissociation
the
rate-determining
oxygen
finally,
decreases
steric number
into
not specifically
process,
vacancies
11+12 and 15+16,
consecutive
dissociate
neighbouring
involved,
corresponds
drastically
to comprise
at a vanadium
is high, both of these steps are fast and the concen-
fast,
After
a
can
and do not affect
is still
17+18+Sv.
vacancies dation
with
reoxidation
situation
is adsorbed
molecule
of vacancies
In a step-wise
This
this
of intermediates
considered
can be considered
molecule
interacting
concentration trations
process
an oxygen
as
steps,
at steady
a concerted
this will
state their
not
rates
must be equal. It is not rate-limiting molecule
step
in
of toluene
species. oxygen
surprising
to find total
requires
A consequence
in the reaction
that
the
oxidation, no less
of this mixture
fact
reoxidation
process
especially,
than
since
11 to 18 moles
is, that
feed is depleted
of gaseous
oxygen,
then the rate for formation
will
decrease.
Indeed,
this has been observed
immediately
toluene and toluene Considering
derivatives
the
active
species.
ensemble
Haber
formed,
of
reagents,
attack
electron
density,
Such
electrophilic
an
complexes, aromatic higher
which
both
the
because
are
addition
organic
temperatures
undergo
are
molecule
of toluene,
results
intermediates
appropriate
ions
information they
above, and
gaseous
rate.
If the
of carbon oxides using
in
the
available
rapid
highly
the aromatic formation
oxidation.
a
oxygen
reactive.
are strongly
region
of
ring
of
process.
are at first formed.
total
through
on the nature of
and
which
in the
in the degradation
anhydrides
proceeds
it seems clear that electrophilic
unstable
that 0; and O- species,
i.e. in the case
compounds,
oxygen
in experiments
to carbon oxides
vanadium
is not much
probably
(ref. 20) has concluded
trophilic
at
there
of toluene
Ii and Ij. As discussed
consists
Additionally,
intermediates
noted
one
(refs. 26, 27).
Fig. 8, combustion
series of intermediates
to have
a high reaction
the
of
of monoatomic
it is necessary
in order to observe
constitutes
combustion
Suvorov
elec-
its highest is attacked.
peroxo
or
epoxy
In the case Then,
of
these may
(ref. 28)
also
232 considers
formation
as possible
of unstable
in the degradation
bility
of
formed
in the series
a
reaction
the process,
lar oxygen directly
naked
between
oxygen,
ions
of
the
the
in the mechanism competition
sense that once toluene
reoxidation
is completed.
process
at vanadium
to form carbon oxygen
decrease
species.
oxides.
toluene
discussion
radicals,
stages
of
can be considered that molecu-
which
can be formed
competitive
can be expected
and
oxygen
the latter
several
quinone
During
possibilities
and
Germain
adsorbed
diimine
possibility
to occur. This
in
the
is slowed down. before
In the
can be coordinately of -NH2,
=NH, and
the transformation
exist
concerning
(refs. 18, benzene
participation
is adsorbed
the existence
(refs. 13, 31).
adsorption
is considered
It is known that ammonia
Simon
Another
a
the rate of reoxidation
of toluene,
to produce
32)
can
which
is that adsorbed
of
the fate
have
species
intermediates,
4 and 5 show that the rates for formation
when
(ref. 10)
the
partial
observed
V-Ti-0 catalyst. for
the
of
However, combustion
of
ammonia
was
is caused
philic
ammonia in the
who concluded
suggested react with
then degradate
ammonia
reacts with
nitrile
and
due
a
to
carbon
oxides,
Furthermore,
through
When
Also, an electronic
the
pressure
of oxygen
toluene
over
that
of
bonds,
a
the
strong
intermediate.
on the rates of ammonia
ammonia
al.
intermedi-
and ammonia
for the formation
effect on vanadium-oxygen
et
and the concentration
adsorption
strongly
Cavalli
initial
of ammonia
the
adsorption
new intermediates
oxides
of
suggested
of
the effect
factors.
of competitive
of carbon increased.
ammoxidation
stabilization
investigation,
species.
pathway
is
that there was a common
of both the rate of reoxidation
oxygen
reaction
of
behaviour
by several
the existence
in a decrease
species,
same
in the present
creased,
pressure
These authors,
formations
effect
The
intermediates
to form N2, N20, and NO (refs. 33, 34).
Figures
ate
the
At later
In the route without
of ammonia,
suggested
a dealkylation
ammonia
and ammonia
(refs. 29, 30). Also
IS into products, ammonia
that after adsorbed
ions
has been
intermediate of adsorbed
of
it is also possible
ensemble,
presented.
is adsorbed,
participation
species
active
between
the route including
-HNOH
to previous However,
In Fig. 8, the possi-
each
indicated.
reacts with active hydrocarbon
the ring of toluene,
fact is reflected
adsorbed
and
and alkylquinons
(ref. 28).
vanadium
of ammonia,
oxygen has been
according
of the catalyst.
of alkylphenols
of alkylbenzenes.
molecular
of transformation
such a reaction
as intermediates
process
between
a step in reoxidation
At
intermediates
for
is inresults
of electro-
introduces
a
new
of carbon
oxides.
induced by adsorbed
ammonia
cannot be excluded. discussion
mechanisms expressions at different
for
so far
carbon
derived sites,
has
oxide
clearly
been
concerned
formations.
with
However,
a general
treatment
it is a fact
that
show that CO and CO2 at low conversions
but also that these
sites have many
features
of the
the rate
are formed
in common.
It
233 is quite
certain
conclude
about
mentioned of
in addition
a reaction O-
species
11).
can
Once
their
involving
be
the first
rate of
oxygen
considered
with
surface.
of
Above
oxygen
is most
likely.
0;
species
species.
must
in the
relative
V-V distances, shorter
0;
Looking
these do not involve
is not
the
of O-
from
can be 0;
surface,
species will
it seems
factor
(ref.
reasonable is more
determining
vanadium-vanadium
0;
depend
rate of dissociation.
then
on
If that
probable
the rate of
distances
at
the
that on V205, carbon oxides are formed at (loo), of V-V distances
and that CO is formed at e.g. (101)
species
CO2
species
CO formation
be
it is formed
CO results
of
that CO2 is formed at (100) and (001) planes
V-V bonds.
formation,
As
0; species
to their
Consequently,
the
oxygen
process
will
per each atom
that
while
formation
to the latter,
be
of water.
of adsorbed
to fully
possibilities
to suggest
These
is high. A crucial
it was concluded
a few
in order
two atoms of oxygen
participating,
(OOl), and (h01) types of plane. Consideration the suggestion,
only
reoxidation
the fate
is large compared
CO2
species
toluene
necessary
in formation
intermediates
is adsorbed,
are
reasonable
In the
the rate of dissociation
dissociation
consumed it seems
monoatomic
rate of reaction
formation when
to that
respectively.
toluene
Therefore,
of CO2 consumes
from CO,
with diatomic
species,
investigations
differences.
consecutively
in a reaction
and
their
further
here. The formation
carbon
formed
that
at Fig. 8, a consequence
dissociates
intermediates
only
in the
with adsorbed
(ref. 25) admits
having
rather
long
planes, which have relatively is that
steps
14+S,
toluene
in the case of CO2 and
18+Sv,
because
fragments.
ACKNOWLEOGMENT The Swedish
authors
gratefully
Board for Technical
acknowledge Development
financial
support
from
the
National
(STU).
REFERENCES
6 L 9 10 11
J.C. Volta and J.L. Portefaix, Appl. Catal., 18 (1985) l-32. Y. Murakami, M. Niwa, T,, Hattori, S. Osawa, I. Igushi and H. Ando, J. Catal., 49 (1977) 83-91. A. Andersson and S.T. Lundin, J. Catal., 58 (1979) 383-395. M. Niwa, H. Ando and Y. Murakami, J. Catal., 70 (1981) 1-13. A. Andersson and S.L.T. Andersson, in R.K. Grasselli and J.F. Brazdil (Editors), ACS Symposium Series, American Chemical Society, Washington, O.C., 1985, Vol. 279, pp. 121-142. P. Cavalli, F. Cavani, I. Manenti, F. Trifiro, Ind. Eng. Chem. Res., 26 (1987) 639-647. M. Ito and K. Sano, Bull. Chem. Sot. Japan, 40 (1967) 1307-1314. R. Prasad and A.K. Kar, Ind. Eng. Chem. Process Des. Oev., 15 (1976) 170175. A. Oas and A.K. Kar, J. Chem. Techn. Biotechnol., 29 (1979) 487-498. P. Cavalli, F. Cavani, I. Manenti, F. Trifiro and M. El-Sawi, Ind. Eng. Chem. Res., 26 (1987) 804-810. J. Haber, in J.P. Bonnelle, B. Delmon and E. Derouane (Editors), Surface Properties and Catalysis by Non-Metals, Reidel, Dordrecht, 1983, pp. l-45.
234 12 A. Andersson, J.-O. Bovin and P. Walter, J. Catal., 98 (1986) 204-220. 13 J.C. Otamiri and A. Andersson, Catal. Today, this volume (1988) preceding article. 14 R. Schmid and V.N. Sapunov, Non-Formal Kinetics, Verlag Chemie, Weinheim, 1982. 15 A. Andersson, R. Wallenberg, S.T. Lundin and J.-O. Bovin, in Proc. 8th Int. Congr. on Catalysis, Berlin, F.R.G., July 2-6, 1984, Verlag Chemie, Weinheim, 1984, Vol. 5, pp. 381-392. 16 R.Y. Saleh and I.E. Wachs, Appl. Catal., 31 (1987) 87-98. 17 P.J. van den Berg, K. van der Wiele and J.J.J. den Ridder, in Proc. 8th Int. Congr. on Catalysis, Berlin, F.R.G., July 2-6, 1984, Verlag Chemie, Weinheim, 1984, Vol. 5, pp. 393-404. G. Simon and J.-E. Germain, Bull. Sot. Chim. France, 11-12 (1975) 2617-2621. :: S.L.T. Andersson, J. Catal., 98 (1986) 138-149. 20 J. Haber, in Proc. 8th Int. Congr. on Catalysis, Berlin, F.R.G., July 2-6, 1984, Verlag Chemie, Weinheim, 1984, Vol. 1, pp. 85-111. 21 V.A. Shvets, V.M. Vorotinzev and V.B. Kazansky, J. Catal., 15 (1969) 214. 22 B. Grzybowska, V. Barbaux and J.-P. Bonnelle, J. Chem. Research (M), (1981) 0650-0663. 23 A.M. Gasymov, V.A. Shvets and V.B. Kazansky, Kinet. Katal., 23 (1982) 951-954. M. Gasior and T. Machej, J. Catal., 83 (1983) 472-476. ;: H.G. Bachmann, F.R. Ahmed and W.H. Barnes, 2. Kristallogr., 115 (1961) 110-131. 26 L.N. Raevskaya and Yu.1. Pyatnitskii, React. Kinet. Catal. Lett., 26 (1984) ._...-_ l/j-111.
27 ;: :: 32 33 34
A.B.. Guseinov, E.A. Mamedov, Yu.D. Pankrat'ev and R.G. Rizeav, Kinet Katal.. 26 (1985) 146-150. B.V. Suvorov, Int. Chem. Eng., 8 (1968) 588-615. Yu.V. Belokopytov, K.M. Kholyavenko and S.V. Gerei, J. Catal., 60 (19791 l-7. G. Busca, Langmuir, 2 (1986) 577-582. A.B. Guseinov, E.A. Mamedov and R.G. Rizeav, React. Kinet. Catal. Lett., 27 (1985) 371-374. G. Simon and J.-E. Germain, Bull. Sot. Chim. France Pt. 1, 3-4 (1980) 149-155. N.I. Il'chenko and G.I. Golodets, J. Catal., 39 (1975) 57-72. Y. Kosaki, A Miyamoto and Y. Murakami, Bull. Chem. Sot. Japan, 52 (1979) 617-618.
KINETICS
AND
MECHANISMS
PART 2: NON-SELECTIVE
JONATHAN
C. OTAMIRI
IN THE AMMOXIDATION
OF TOLUENE
OVER
A V2OS
CATALYST.
REACTIONS
and ARNE ANDERSSON
of Chemical Technology, Chemical P.O. Box 124, S-221 00 Lund (Sweden)
Center,
Department nology,
223
Lund
Institute
of
Tech-
ABSTRACT In the amnoxidation of toluene over a V 0 catalyst, the variation of initial rates with partial pressures of oxygen, t&8 ene and ammonia was determined for formation of carbon oxides. Dependencies obtained were analyzed and expressed in are initial products terms of rate equations. It was found that CO and CO formed at different sites, none of which are involved ? n selective reactions. The mechanisms derived for the formation of the two oxides have many features in common. In each mechanism there are two parallel routes originating from the same active site, which is suggested to be an ensemble exposing vanadium ions and electrophilic oxygen species. One of the routes proceeds without participation of ammonia, while in the other route ammonia is adsorbed. In both routes, the rate-determining step can be considered to be one of the steps in a stepwise reoxidation process. The rates for formation of carbon oxides decrease strongly with increasing partial pressure of ammonia, which is due to a combined effect of introduction of a new reaction pathway and competitive adsorption between oxygen and ammonia.
INTRODUCTION Knowledge oxides,
about
reaction
is of great
importance
that need to be answered products
of selective
intermediate,
which
iii) whether the answers question carbon
how
oxides
ii),
it would
tive
iii),
to
oxidation is common
reduce
are formed
readsorption
the solution
for alternatives
through different reactions, we would
and amnoxidation, for both
i) and iii)
intermediates,
to know
selective
is to block,
or if different want
of
of
route.
these
Questions
products.
stabilize
to solving
In the
case
it is necessary
of
to know,
If the latter are
0920-5861/88/$04.20 0 1988 Elsevier Science Publishers B.V.
If
alternative For alterna-
route.
Further-
if the same site,
both in non-selective
sites
the general products.
the intermediate. the parallel
different
and
found
to the problem would
or prevent,
is involved
from
routes,
Once we have
non-selective
i), a solution
sites are involved.
if these
and non-selective
reaction
we have to make efforts formation
to somehow
catalysts.
carbon
ii) if they are formed from an
selective
in a parallel
the
especially
are i) if carbon oxides are formed
due to alternative
be necessary
products,
for the design of industrial
to these questions,
of
to non-selective
in this regard,
they are formed
be to prevent
more,
paths
and selective
is the case,
localized
at
the
then
same
or
224 different
crystal
structure
faces.
sensitivity
reviewed
of metal
(ref. 1). Another
oxidation
In ammoxidation
catalysts,
question
of alkyl
oxide-based
investigations
of
treating
oxides
frequently mediate
our interest a field
interest
to the so-called
that
is whether
formed
recently CO2
has been
is formed
from
nitrile a
of solid state chemistry of olefines
investigation
catalyst
that
However,
of
(ref. 13) data
with respect
and
the
to corresponding
have been reported. over
and directly
a V205
kinetic
from
aromatics
for carbon
(refs. 11, 12) anoxidation
oxides
were
obtained
inter-
showing
the
oxygen species
is lacking.
of
that It has
a common
investigation
and the role of electrophilic
selective
Ito and Sano
catalyst,
from m-xylene.
and CO2 are formed
thorough
nitriles,
used (refs. Z-6), and some kinetic
of m-xylene
via m-tolunitrile
reported
in degradation
pyridines
carbon oxides formation
(refs. 8-10).
importance
and
are frequently
in the ammoxidation
are
been
benzenes
catalysts
(ref. 7) concluded
tailed
oxide
turns
of CO.
vanadium
carbon
The last question
toluene
In a de-
over
a
V205
and are here analyzed
to the above considerations.
METHODS Preparation described
of
catalyst,
elsewhere
catalytic
measurements
and
product
analysis
are
(ref. 13).
RESULTS Influence
of partial
pressure
The rate dependencies partial
pressure
of ammonia
three temperatures.
the rates
pressure. easily
A
are
presented
Figure 3 shows
of asnnonia at 4OO'C. that
of oxygen
of CO2 and CO on partial
Comparison
partial
order
be linearized
the dependencies
of rates obtained
at low pressure
of ammonia
dependency
by plotting
pressure
is
inverse
of oxygen. Thus, the rate expressions
of oxygen
for a high
in Figs. 1 and 2, respectively, obtained
at a low pressure
in the separate
are much noticeable
higher in
than
all
rate as a function
series those
cases,
(11
and
CO
are
amnonia products. with
of partial
influence
has
pressure
of partial
presented strong
in
increase
in
pressures
of ammonia
and toluene.
of ammonia pressure
Figs. 4
influence
At low pressures
further
can
pressure
are of the form
where a and B might depend on partial
The
shows
at high
which
of inverse
r = aPo/(l + BPo)
Influence
for
on
of ammonia
and the
of ammonia, partial
5,
on rates
respectively.
rates
for
of
As
formation
the rates diminish
pressure
for formation
ammonia,
could of
be
total
of CO2 noticed,
oxidation
more than a half, but the
rates
gradually
won
0%
2
4 2
P -
012.
'00 x B 008.
004.
10 PO2
20
30
CkPa)
Fig. 1. Effect of partial pressure of oxygen on the rate for gormation of CO at n 34O'C; q 370 C; and ~85~"",l~. PT = 0.765 kPa and PA =
.
Fig. 2. Effect of partial pressure of oxygen on th& rate for,formation of CO at A 340 C; P 370 C; and A 400 C. PT = 0.765 kPa and PA = 2.855 kPa.
.
2.4.
Fig. 3. Rates for formation of CO 0, and CO A, as a function 8f parti.E1 pressure of oxygen at 400 C. PT = 0.765 kPa and PA = 0.485 kPa. 0.8
10
20 PO,_,CkPa)
30
2 2
4 qUH3
and remain
that ammonia
constant
influences
From the nature can be represented
6
(kPa)
CkPa)
Fig. 5. Influence of partial pressure of ammonia on the &ate for fgrmation of 60 at A 340 C; P 370 C; and A 400 C. Pl = 0.765 kPa and PO = 11.485 kPa.
Fig. 4. Influence of partial pressure of ammonia on the rate for fogmation of fi02 at m 340 C; 0 370 C; and 0 400 C. PT = 0.765 kPa and PO = 11.485 kPa.
decrease
4 h,.,s
6
at high pressures
the formation
of ammonia.
It is also apparent
of CO more than that of C02.
of the dependencies
observed,
it can be concluded
that they
as follows:
r = (y t 6PA)/(1 + EP~)
(2)
where y, 6, and E might depend on pressures Considering oxides
exhibit
high pressures
Figs, l-3, invariance of ammonia,
r = (koPo + kBPAPo)/(l where
it with
is
seen
that
respect
in which
the partial
pressure
toluene
of
carbon
of oxygen at low and
(3)
In all the series rates
pressure
formation
+ kyPA + k&PAP0 + kEPo)
of toluene
the
for
of eqns. 1 and 2 gives (ref. 14)
pressure
that
rates
to partial
of partial
found
the
hence, a combination
ka, . . . . k. might be real constants
Influence
of oxygen and toluene.
for formation
within
the
range
or depend upon pressure
pressure
of CO2
of toluene was varied,
and CO were
investigated.
of toluene.
Typical
independent results
it was
of partial obtained
are
227 shown
in Figs. 6 and 7. However, r = $P.+(l
the nature pressure.
+ wPT),
Considering
the
thus it can be concluded be used to describe
“i 0
since
fact that
the general
the rates
Values procedures are
also
Arrhenius draw
the
scribes
0.4
rate expressions
o-3
ToL
of effective described the
by eqn. 3 can
04
0
1.2
0.8
1.2
PTOL (kPd
plots.
Fig. 7. Rate dependency on partial pressure of tgluene for CO Cl, and CO A, at 400 C. PO = 11.465 kPa and PA = 0.485 kPa.
values
presented
of eqn. 3 calculated
are given
of apparent
The
curves
constants
above
values
in Tables
activation
of constants
observed
energies,
given
in Figs. l-7.
for CO2 and CO using
1 and 2, respectively.
in Tables
It is seen
on partial
that
were
the
Included
obtained
1 and 2 were
from
used
that eqn. 3 perfectly
to de-
pressures.
for total oxidation the mechanism features
CO2 and CO. However, 1 and 2 are
each separate Referring in a
at zero
investigated,
constants
Figure 8 shows
Tables
to zero
of the form given
(kPa)
by eqn. 3. The general both
be equal
is of
all data obtained.
the dependencies
Mechanisms
must
dependencies
oPT>>l under the conditions
that
Fig. 6. Rate for formation of CO versus pal;tial pressure of tolue i; 8 at u 340 C;p 370 C; and 0 400 C. = 11.485 kPa and PA = 2.855 kPa. pO Effective
form of these
to the dependencies
of the mechanism
are the same for formation
of
constants
Included
in
of eqn. 3 and those
of
the values
the relations
mechanistic
that corresponds
of kinetic
between
the constants
step.
to Fig. 8, S, is a site which
specific
oxygen species.
differ.
expressed
configuration.
Some
of
In what can be considered
comprises
these
cations
a number are
of vanadium
covered
with
ions
active
to be the first step of the mechanism,
228 TABLE 1 Values
of effective
constants
and apparent
activation
energies
for formation
of
co2. Temperatures
Keff
('C)
E
340
370
400
(kcaY!Xole)
0.6361
1.635
5.340
27.8
0.1868
0.8753
3.420
39.7
ka z klOST
(mole/m2*min.kPa
k0 * k20KNST
(mole/m2.min.kPa2
kY= KN
(kPa-')
6.222
5.660
4.572
-4.6
ks 2 k20KN/k21
(kPav2 x 10')
0.7150
1.257
1.975
13.9
kc * klo’kll
(kPa-' x 102)
1.149
2.220
4.320
18.3
x 107) x 107)
TABLE 2 Values
of effective
constants
and apparent
activation
energies
for formation
of
co. K eff
Temperatures
('C)
E app (kcal/mole)
340
370
400
0.3259
0.6928
1.712
21.9
x 115') 6.341
6.586
6.838
2.0
ka = klOST
(mole/m'.min*kPa
k6 = k20KNST
(mole/m2.min.kPa2
ky = KN
(kPa-')
9.191
6,773
3.740
-13.5
ks = k20KN/k21
(kPam2 x 10')
4.191
2.712
1.302
-15.9
kc = klo’kll
(kPa-' x 102)
4.561
4,004
3.480
toluene by
a
formed, ates,
is strongly
adsorbed
rate-determining which
through
step
reduced
Sv. Then,
conditions
of
gaseous
oxygen
a
intermediate
oxide
is present
competition
An
step is followed I2
is
into new intermedi-
and water.
After
13, which
14, which
complete is more
dissociates
in a
of Sv.
of intermediate between
11, there is, under
oxidation
at the same site. Adsorption
15, which
-4.0
intermediate
in a state,
is adsorbed,
in the formation
of ammonia
Il. This
oxygen.
is transformed
of carbon
configuration
arnnoxidation,
of
gaseous
of toluene and formation
adsorption
in formation
an intermediate
of equilibria
formation
slow step resulting
After adsorption
reversible
with
the surface
than
relatively
forming involving
a series
Ii, accompanied
combustion,
x 107)
in a rate-determining
at
site
of ammonia step
I1
and
results
reacts
with
229 oxygen
to
oxide,
water
ammonia
form
16. Through
and
can
new
be HCN,
then be reoxidized secutive
steps,
a
of
series
Ij.
intermediates,
N2 and back
to the original adsorption
comprising
Products
oxides.
nitrogen
16 reacts
equilibria,
The
originally
reduced
Sv. This
site,
to give
of molecular
resulting
site
formed,
can occur
oxygen,
carbon from
17, can
in some
18, followed
con-
by its
dissociation.
Fig. 8. Reaction
mechanisms
for total oxidation
of toluene over V205.
DISCUSSION Fractional conversion formed
conversions
of toluene
in parallel
found
obtained that
Wachs
in combined
of
silver-cerium
formed
as a function
of total
CO and CO2 are initial
products
can be drawn considering
the rate
products
the
over
in a consecutive
carbon
However,
suggested
that
can
react
oxides in
ammoxidation
it was
formed of
observed
that carbon
In the case of ammoxidation
aldehyde
in
the
of the oxiSaleh from
the
and
direct
toluene that
by
it was
formed
catalyst,
primarily
100 %, indicating
that carbon to
when
in a study
a V205/Ti02 are
the
Also
is supported
(ref. 15), where
degradation
oxides. over
(ref. 17),
was almost
reaction.
has been frequently alternatively
vanadium
catalyst
to benzonitrile
against
anhydride
that
reactant.
vanadate
and TPD experiments
stable
to phthalic
(ref. 16) concluded
selectivity
pulse is
of 3-picoline
of o-xylene
oxidation
that benzonitrile,
routes. The same conclusion
nicotinonitrile
ammoxidation dation
showed
to various
see eqn. 3 and Part 1 (ref. 13). Such a conclusion
expressions, results
of toluene
over
a
initial
oxides
are
of aromatics,
it
oxides are formed from an intermediate and
nitrile
(refs. 8-10,
18).
In a
230 discussion
of reaction
(ref. 19) dation
expresses
proceeds
the
do not
in the oxidation
opinion
from a common
investigations concluded
networks
that
support
the
that the same surface
in the formation for formation
selective
intermediate.
oxides
clearly
(refs. 11,
species, course
20)
has
results
in
reactions
selective
catalysts
has
potential
(ref. 22) measurements.
of
0;
been
species
experimentally
show
with
benzene,
that
are
of
oxidation.
In
total
(ref. 12) and
o-xylene
ogies,
found
it was
dride, both
respectively,
making
at
planes
use
of
the
could
investigations,
situated
(ref. 24)
that
and
that
oxygen
species.
suggest
that
planes
the sites
and
over
active
of
electrophilic
oxidation
and
ESR
O-
the
and
with
(010).
It was that
to this
results
of
oxidation
the
(010)
it
combustion,
Due to the fact that the vanadium-oxygen
(ref. 12),
by
exposes
(ref. 25), plete,
it can be expected
i.e.
depends
both
upon
Therefore, species,
vanadium
temperature,
number
and
partial
Sv can be considered whose
that
ions
the surface
coverage
oxygen
species
reasonable
located
at
pressure
of
oxygen,
to be an ensemble
in principle
exposed. and
of vanadium
is determined
by
the
size
to
(loo), oxygen
are relatively
of oxygen
are
02-
electrophilic
seems
distances
In be
plane
S,, are
planes. to
accommodate
described,
anhy-
concluded
demonstrated
plane
morphol-
phthalic
(010)
were
the
3-picoline
different
to the frequency total
of
interaction
of
and
V205
surface
composition
ammoxidation
catalysts
on
the reaction
of such an
of nicotinonitrile
for total
species
(ref. 21)
The
products
in the
of 02- species
(OOl), and (h01) types of plane. All of these planes expose electrophilic species.
at
oxygen
occurs
presence
0;
using
of
calculations,
the
total
o-xylene.
V205
in
to
are formed
et al. (ref. 23) studied
studies
perpendicular
Considering
whenever
existence
formation
perpendicular
products
whereas
the final
active
bond-strength
species
that
be correlated
centers
these
(amm)oxidation.
toluene,
indicated,
those
that
intermediates,
It was
derived
established
products
intermediate.
involved
Gasymov
desorption
oxi-
of the present
was
at the surface,
The
as non-selective
of a common
of hydrocarbons,
oxidation.
Andersson
(ref. 13). The rate expressions
concluded,
i.e. 0; and O-, appear
of catalytic
S.L.T.
the results
site, via different
sites that do not take part in selective Haber
as well
However,
existence
of nitrile and aldehyde
of carbon
of toluene,
long
is not comThe
coverage
reaction
rates.
ions and oxygen of
the
toluene
that
in both
molecule. the
In
parallel
formation routes
of
carbon
the interaction
oxides, between
see
Fig. 8,
gaseous
it was
oxygen
or Ig, was the rate-determining
step. II was obtained
and
both
I5 from
ensemble gaseous ism.
the adsorption
of
toluene
Sv. Such a type of rate-determining oxygen
reacts with adsorbed
Alternatively,
this
step
can
species be
seen
and
found
and an intermediate,
from adsorption
ammonia
according as
a
of toluene
at a partly
step does not necessarily
oxidized mean that
to an Eley-Rideal
part
of
I1
reoxidation
mechanof
the
231 surface. steps.
The
reoxidation
First, and
vacancy, species
by
then,
negligible
concentration lar oxygen
and
of oxygen
due
quently, step,
to
the
the
these
step,
however,
change
monoatomic
vacancy.
As
steps,
will
a competitive
hindrance
caused now
number
oxygen
long
as
the
in Fig. 8, can be
will
presented.
appear
be a relatively
where
the
large
of molecular
be even slower.
if they occur
derived,
Therefore,
because
at oxygen for
reoxi-
last
Conse-
reoxidation
is a slow step, and a these
steps constitute
indicated
as two slow consecutive
the form of rate expression
13+14+Sv
molecule.
in the
oxygen
In Fig. 8 they have been
i.e.
available
toluene
that
of molecu-
of toluene
vacancies
the
slow process.
route,
adsorption
of
by
in each
is so low,
the adsorption will
indicated
a situation
to the last two steps
step.
even
two
is so low that, even if adsorption
of vacancies
dissociation
two consecutive
ion, i.e. an oxygen
the form of rate expression
its dissociation
the
rate-determining
oxygen
finally,
decreases
steric number
into
not specifically
process,
vacancies
11+12 and 15+16,
consecutive
dissociate
neighbouring
involved,
corresponds
drastically
to comprise
at a vanadium
is high, both of these steps are fast and the concen-
fast,
After
a
can
and do not affect
is still
17+18+Sv.
vacancies dation
with
reoxidation
situation
is adsorbed
molecule
of vacancies
In a step-wise
This
this
of intermediates
considered
can be considered
molecule
interacting
concentration trations
process
an oxygen
as
steps,
at steady
a concerted
this will
state their
not
rates
must be equal. It is not rate-limiting molecule
step
in
of toluene
species. oxygen
surprising
to find total
requires
A consequence
in the reaction
that
the
oxidation, no less
of this mixture
fact
reoxidation
process
especially,
than
since
11 to 18 moles
is, that
feed is depleted
of gaseous
oxygen,
then the rate for formation
will
decrease.
Indeed,
this has been observed
immediately
toluene and toluene Considering
derivatives
the
active
species.
ensemble
Haber
formed,
of
reagents,
attack
electron
density,
Such
electrophilic
an
complexes, aromatic higher
which
both
the
because
are
addition
organic
temperatures
undergo
are
molecule
of toluene,
results
intermediates
appropriate
ions
information they
above, and
gaseous
rate.
If the
of carbon oxides using
in
the
available
rapid
highly
the aromatic formation
oxidation.
a
oxygen
reactive.
are strongly
region
of
ring
of
process.
are at first formed.
total
through
on the nature of
and
which
in the
in the degradation
anhydrides
proceeds
it seems clear that electrophilic
unstable
that 0; and O- species,
i.e. in the case
compounds,
oxygen
in experiments
to carbon oxides
vanadium
is not much
probably
(ref. 20) has concluded
trophilic
at
there
of toluene
Ii and Ij. As discussed
consists
Additionally,
intermediates
noted
one
(refs. 26, 27).
Fig. 8, combustion
series of intermediates
to have
a high reaction
the
of
of monoatomic
it is necessary
in order to observe
constitutes
combustion
Suvorov
elec-
its highest is attacked.
peroxo
or
epoxy
In the case Then,
of
these may
(ref. 28)
also
232 considers
formation
as possible
of unstable
in the degradation
bility
of
formed
in the series
a
reaction
the process,
lar oxygen directly
naked
between
oxygen,
ions
of
the
the
in the mechanism competition
sense that once toluene
reoxidation
is completed.
process
at vanadium
to form carbon oxygen
decrease
species.
oxides.
toluene
discussion
radicals,
stages
of
can be considered that molecu-
which
can be formed
competitive
can be expected
and
oxygen
the latter
several
quinone
During
possibilities
and
Germain
adsorbed
diimine
possibility
to occur. This
in
the
is slowed down. before
In the
can be coordinately of -NH2,
=NH, and
the transformation
exist
concerning
(refs. 18, benzene
participation
is adsorbed
the existence
(refs. 13, 31).
adsorption
is considered
It is known that ammonia
Simon
Another
a
the rate of reoxidation
of toluene,
to produce
32)
can
which
is that adsorbed
of
the fate
have
species
intermediates,
4 and 5 show that the rates for formation
when
(ref. 10)
the
partial
observed
V-Ti-0 catalyst. for
the
of
However, combustion
of
ammonia
was
is caused
philic
ammonia in the
who concluded
suggested react with
then degradate
ammonia
reacts with
nitrile
and
due
a
to
carbon
oxides,
Furthermore,
through
When
Also, an electronic
the
pressure
of oxygen
toluene
over
that
of
bonds,
a
the
strong
intermediate.
on the rates of ammonia
ammonia
al.
intermedi-
and ammonia
for the formation
effect on vanadium-oxygen
et
and the concentration
adsorption
strongly
Cavalli
initial
of ammonia
the
adsorption
new intermediates
oxides
of
suggested
of
the effect
factors.
of competitive
of carbon increased.
ammoxidation
stabilization
investigation,
species.
pathway
is
that there was a common
of both the rate of reoxidation
oxygen
reaction
of
behaviour
by several
the existence
in a decrease
species,
same
in the present
creased,
pressure
These authors,
formations
effect
The
intermediates
to form N2, N20, and NO (refs. 33, 34).
Figures
ate
the
At later
In the route without
of ammonia,
suggested
a dealkylation
ammonia
and ammonia
(refs. 29, 30). Also
IS into products, ammonia
that after adsorbed
ions
has been
intermediate of adsorbed
of
it is also possible
ensemble,
presented.
is adsorbed,
participation
species
active
between
the route including
-HNOH
to previous However,
In Fig. 8, the possi-
each
indicated.
reacts with active hydrocarbon
the ring of toluene,
fact is reflected
adsorbed
and
and alkylquinons
(ref. 28).
vanadium
of ammonia,
oxygen has been
according
of the catalyst.
of alkylphenols
of alkylbenzenes.
molecular
of transformation
such a reaction
as intermediates
process
between
a step in reoxidation
At
intermediates
for
is inresults
of electro-
introduces
a
new
of carbon
oxides.
induced by adsorbed
ammonia
cannot be excluded. discussion
mechanisms expressions at different
for
so far
carbon
derived sites,
has
oxide
clearly
been
concerned
formations.
with
However,
a general
treatment
it is a fact
that
show that CO and CO2 at low conversions
but also that these
sites have many
features
of the
the rate
are formed
in common.
It
233 is quite
certain
conclude
about
mentioned of
in addition
a reaction O-
species
11).
can
Once
their
involving
be
the first
rate of
oxygen
considered
with
surface.
of
Above
oxygen
is most
likely.
0;
species
species.
must
in the
relative
V-V distances, shorter
0;
Looking
these do not involve
is not
the
of O-
from
can be 0;
surface,
species will
it seems
factor
(ref.
reasonable is more
determining
vanadium-vanadium
0;
depend
rate of dissociation.
then
on
If that
probable
the rate of
distances
at
the
that on V205, carbon oxides are formed at (loo), of V-V distances
and that CO is formed at e.g. (101)
species
CO2
species
CO formation
be
it is formed
CO results
of
that CO2 is formed at (100) and (001) planes
V-V bonds.
formation,
As
0; species
to their
Consequently,
the
oxygen
process
will
per each atom
that
while
formation
to the latter,
be
of water.
of adsorbed
to fully
possibilities
to suggest
These
is high. A crucial
it was concluded
a few
in order
two atoms of oxygen
participating,
(OOl), and (h01) types of plane. Consideration the suggestion,
only
reoxidation
the fate
is large compared
CO2
species
toluene
necessary
in formation
intermediates
is adsorbed,
are
reasonable
In the
the rate of dissociation
dissociation
consumed it seems
monoatomic
rate of reaction
formation when
to that
respectively.
toluene
Therefore,
of CO2 consumes
from CO,
with diatomic
species,
investigations
differences.
consecutively
in a reaction
and
their
further
here. The formation
carbon
formed
that
at Fig. 8, a consequence
dissociates
intermediates
only
in the
with adsorbed
(ref. 25) admits
having
rather
long
planes, which have relatively is that
steps
14+S,
toluene
in the case of CO2 and
18+Sv,
because
fragments.
ACKNOWLEOGMENT The Swedish
authors
gratefully
Board for Technical
acknowledge Development
financial
support
from
the
National
(STU).
REFERENCES
6 L 9 10 11
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