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magnesia catalyst
23
Applied Catalysis, 12 (1984) 23-34 Elsevier Science Publishers B.V., _4msterdam
- Printed
KINETICS
OF ETHYLBENZENE
DEHYDROGENATION
MAGNESIA
CATALYST
G.V. SHAKHNOVICH,
OXIDATIVE
I.P. BELOMESTNYKH,
in The Netherlands
TO STYRENE
N.V. NEKRASOV,
M.M.
OVER A VANADIA/
KOSTYUKOVSKY,
and
S.L. KIPERMANa Zelinsky
Institute
Leninsky
Prospect
of Organic 47, Moscow
aTo whom corresoondence
(Received
V-334,
should
20 September
Academy
Chemistry,
of Sciences
of the USSR,
USSR.
be addressed.
1983, accepted
26 April
1984)
ABSTRACT Kinetics of vapor-phase ethylbenzene oxidative dehydrogenation have been studied at 420 to 550°C in a gradientless reactor in the presence of a vanadia/magnesia catalyst. Kinetic equations have been obtained for this reaction in the direction of styrene and CO2 formation which considers the effect of the reaction medium on the surface state of the catalyst. A process scheme is proposed. This scheme suggests that dehydrogenation involves the interaction of adsorbed ethylbenzene with lattice oxygen whereas the ion-radicals of oxygen adsorbed on the catalyst surface are active in the total oxidation.
INTRODUCTION Although
the development
ethylbenzene kinetics
of catalysts
is the subject
of this reaction
ics of simultaneously
of a number
[l-4], while
occurring
for the oxidative
no studies
reactions
dehydrogenation
of
only a few of them discuss
of papers,
have been reported
of ethylbenzene
oxidative
the
on the kinetdehydrogenat-
ion and total oxidation. This paper deals with
the kinetic
behaviour
of both oxidative
and total oxidation
reactions
over a vanadia/magnesia
ion of the reaction
mechanism
is based on the data obtained
gation.
These
rogenation
include
[I], and their further
as the observation
EXPERIMENTAL Reaction
the intermediate
regarding
products
transformation
the oxygen
species
dehydrogenation
catalyst
[S]. The descript-
in the earlier
of ethylbenzene into carbon
oxidative
oxides
on the vanadia
investidehyd-
[6] as well
catalysts
[7,8].
METHODS kinetics
were
studied
in a small volume
gradientless
pulsating
reactor
c91. The products bons and oxygen
were analysed containing
by chromatography
compounds
UV-spectrophotometer
(Specord
0166-9834/84/$03.00
@ 1984 Elsevier
while
UV VIS). Science
for the concentration
the aqueous
COz,CO, Publishers
methane, B.V.
of hydrocar-
layer was analysed ethylene
on a
and oxygen
were
24 G hdd
iii
\_
0.5,
0
_
=
y
0
a
01
0.4 0.2 0.3
I 0.2 0.1
i
0.02
0.1
0‘ 01
::-
FIGURE (0)
1
Variations
deposited
20
carbon
the gaseous
(styrene
characterized
oxide,
residual
by oxidizing
benzaldehyde,
by the constants
corresponding
moisture
out using a sample
and zinc nitrate.
content
dried and activated
CO
- x2
and oxygen-containing
acetophenone) to those
reported
in the literature.
of a zinc oxide modified by impregnating
After drying,
com-
used in this paper were
the resulting
of about 40 to 50%) was extruded,
in an air stream
of products
them to CO2.
styrene
(92% MgO, 7% V205 and l?, ZnO) prepared vanadate
- x, (e),
In some runs the amount
components.
phenol,
nesia catalyst with ammonium
to styrene
with time at 500°C and VC8U,,, = 220 h- i; .
pure ethylbenzene,
All runs were carried
mill)
60
were determined
The chromatographically pounds
50
of ethylbenzene
- G m)
reaction
on the catalyst
40
30
of conversion
and of deposited
found among
IO
vanadia/magmagnesia paste
the granules
for two hours at 18O"C,
(with
were
two hours at 35O"C,
and 3 hours at 500 to 550°C. Specific catalyst surface determined from nitrogen adsorption by the BET method 3 -1 2 -1 , g , the pore volume (measured by mercury porosimetry) was 0.7 cm p was 100m n the pore radius was predominantly 200 - 500 A. In order uted with
to maintain
quartz
with quartz Whether
isothermal
homogeneous
the catalyst
by returning
at 5OO"C,
volume
was dil-
was also filled
oxidation.
activity
to the conditions
lyst was reactivated
the bed, the catalyst
along
ratio of 1:2. The free reactor
in a volume
to avoid
conditions
was constant
accepted
each series
after
as standard.
first by air/stream
Between
mixture
of runs was checked
the runs, the cata-
for 10 minutes
and
then by air for one hour. Initial conversion
(PO) and current to styrene
and w 2, respectively, coefficient
(x,) and carbon the accumulation
$), and selectivity
of the reaction
RESULTS
(P) partial
products
after
pressures
dioxide
of the components,
(x2). their
rate w2 being
(S) were calculated drawing
ethylbenzene
accumulation
calculated
from the analytical
up the material
rates
considering
(w, the
results
balance.
AND DISCUSSION
The experimental
conditions
were as follows:
temperature
range 420 to 55O"C,
25
X 0.2
FIGURE
2
Effect
accumulation 52O"C,
of ethylbenzene
rates at various
0.3
0.4
0.5
0.6
conversion
on styrene
- w, (a) and CO2 - w2 (b)
temperatures:
(1) 46O"C,
(2) 48O"C,
(3) 5OO"C,
(4)
(5) 55O'C.
a)
w,.106(mol/gej
b)
W2.106(mo1/gj~ w,.106(aol/gs)
I.4 ml 0.6
3.0
FIGURE (0)
3
Influence
and CO2 (V)
starting
7.0
space
rates;
(Pc8hlo
of styrene
5OO"C,
velocities
(a) and oxygen
a grain
size of 0.25-0.5
reactor
was gradientless.
mixture
diluted
with
(b) on styrene
190 to 1100 h-', initial
(VQ+,~)
partial
) 4.2 to 11.2 kPa, oxygen (PE,) 4.4 to 9.6 kPa, compounds
added
reaction
and carbon
kPa, 0.2 to 0.5 kPa and up to 1.0 kPa, respectively, lyst with
9.0
x = 0.4.
kPa, those of specially
oxygen-containing
2.0
I.0
t,.o(kPa)
pressures
accumulation
10.0-80.0
(P&,h8),
6.0
of partial
of ethylbenzene
(Pi20)
styrene
5.0
ethylbenzene
pressures steam
4.0
to 2-3 mm. Under
Most of the kinetic
steam since carbon
products,
dioxide
over
(P&J~)
1.3 to 6.2
1 to 5 ml of the cata-
these conditions,
runs were carried
deposition
namely
the pulsating
out with reaction
on the catalyst
was negligible
in this case. In the absence these amounts
of the catalysts
were considered
The experiments rate of ethylbenzene
ethylbenzene
in calculating
with varying demonstrated
grain
conversion
the reaction
size and linear
the absence
did not exceed
velocity
of inhibition
89 and
rates. at a constant
effects
flow
due to volume
26
0.2
0.1
FIGURE
4
46O'C
Dependence
(0).
FIGURE
5
480°C
0.4
of ethylbenzene
(o,,
Conversion
in the gas phase.
0.3
5OO'C
520°C
and selectivity
(v).
550'C
as the function conditions:
0.7
0.6
conversion
(n,,
to styrene
Experimental
0.5
on temperature;
(A).
of time in the absence -1 . VC8H,o = 600 h
of oxygen
5OO'C,
and pore diffusion. Figure
1 shows the experimental
beainning
of the run, conversion
CO2 tends
to increase
results to styrene
with progressively
on testing
higher
is 0.5 h or below even at low ethylbenzene
Steady
state will
of time without Liquid
more
to
The relaxation
space velocities will function
made up at least 908 of the total
for the main fraction (benzene
x-phenylethyl
and the catalyst
In the
conversion
(100 to 200 h-l). for a long period
reactivation.
products
destruction
stability.
while
coke formation.
period
then be reached
catalyst
tends to decline
alcohol
of the liquid and toluene)
products.
and mild oxidation
and acetophenone)
conversion.
In addition
were detected
Styrene
the products
(styrene
oxide,
accounts
of oxidative
benzaldehyde,
in small quantities
(not
than 3";).
The aqueous ethylbenzene)
condensate
contains
and the gaseous
only phenol
products
eratures, CO (up to 1%). The product -1 or less based on carbon. g
contain
deposited
(not more than 0.02"1 based on CO2 (up to 12':) and, at higher on the catalyst
amounted
temp-
to 0.5 mg
The complete
analysis
ing over this catalyst to styrene
and, to a negligible
resulting
in the formation
ducts tend to proceed Figures2a
curves"
ial mixture
The increasing creases
slightly
oxidation
styrene
accumulation
rate remains
on the catalyst position
tends
on the catalyst not represent
virtually
presented
scheme in Figure
ivity is independent
catalyst
[II].
ii1 1lly5,
3a) decreases
of
by at
into the init-
w, values
contribution
compounds
rate becomes
and in-
of styrene
The oxygen-containing
are added
considerably
the same, and the amount
lower,
of products
compounds
compounds
suggests
CO2
deposited
undergo
decom-
their occurrence
and that their further
with ethylbenzene of ethylbenzene
reaction
of temperature
conversions
(styreneproceeds
conversion
process
indicates
can
scheme
it was earl-
to parallel-
on selectivity with a possible
of the reaction
is presented
rate of styrene
is comparablewiththatobtained
that the energies
select-
of acti-
are similar.
in the course
formation
rate decreased
over this
on Figure
5. Only styrene
is equal
to 3.8 10m6 mol
in the presence
formation
C mixture
according
of conversion
pathways
of gas phase oxygen Initial
14
[IZ]. The fact that the reaction
possibly
in its different
of ethylbenzene
of oxygen
dramatically
(6.2 10e6 mol
due to reduct-
ions.
When performing urrence
reaction
The data on the effect
at 4 kPa 02). Styrene
9
slopes
inhibited
dioxide
the
in this case.
4 also point to a parallel
in this case.
g-' s-',which
The concave
is possibly
due to a greater
drops
in small quantities
of consecutive
in the absence
is formed
(Figure
of the oxygen-containing
for the reaction
The change
against
slow steps of the process.
contribution
vation
pro-
conditions.
ier shown that total oxidation
minor
[IO]).
(up to 2":) oxygen-containing
On the basis of experiments
consecutive
curves'
pressure
the main
to increase.
surface
oxidation
products
of up to IO' carbon
selectivity
under the reaction
A high reactivity
the reactions
and incomplete
that the reaction
of w2 probably
quantities mixture,
reaction;
rates of the reaction
("conversion
partial
the values
When even minor
occurr-
of ethylbenzene
the rates of the two reactions.
rate. The process
into the starting
destruction
The addition
does not affect
that the reactions
dehydrogenation
lower rates.
indicate
least one of the products.
showed
total oxidation
of oxidative
with still
conversion
"conversion
products oxidative
extent,
and 2b show accumulation
total ethylbenzene these
of the reaction
are predominantly
a kinetic
of the following
analysis,
account
was taken of the simultaneous
occ-
reactions:
I.
C6H5C2H5
+ 0.5 O2 = C6H5C2H3
II.
C6H5C2H5
t 10.5 O2 = 8 CO2 + 5 H20
III.
C6H5C2H3
t 10 02 = 8 CO2 + H20
+ H20 (I)
Since the styrene lation
is subject
to further
in the experiment
conversions,
are expressed
I, II and III,
and rIII in the directions
rI' rII
w1 =r
formed
rates measured
in terms
the product
accumu-
of reaction
rates,
respectively,
by the equations
I - rIII
w2 = rII + rIII w=r
(2)
I + rII
where w = w
+ w - ethylbenzene consumption rate. 12 An increase in current partial oxygen pressures
actions
I-III.
rations
in the reaction
It is seen from Figure mixture
3b, however,
lead to higher
results
in higher
that enhanced
rates of w
while
2
rates of re-
oxygen
concent-
w, values
chanoe
less markedly. A rise of initial most experiments
partial
ethylbenzene
were carried
pressures
out) at a constant
above
oxygen
8-10 kPa (at which
values
and steam to ethylbenzene
ratios
Go2 =
’ H20 = p"H20'P~8H10
P"c2'Poz8H,0'
does not result catalyst
in the increase
is completely
An attempt variable
to linearize
reaction
ful. That
covered
mixture
indicates
The stationary
on the catalyst
form as follows
that probably
the temperature
dependence
composition
activity
yield
molecules,
a nonpower
value.
indicates
of reaction
in the Arrhenius
from of the kinetic
level of the catalyst
ably lower than the initial mixture
of styrene by reacting
equation
was unsuccess-
[13].
to styrene
formation
This may be due to the action
and the kinetic
rates at an in-
coordinates
equation
that the
is consider-
of the reaction
can be expressed
in the general
[14]
w = f (Pi) m (Pj)
where ponent
(3)
f and q are functions partial
pressures
ion of its activity The equation ed (Table
fied scheme to styrene catalyst
the dependence
for the constant
under the influence
catalyst
routes
implies
to directions
is based on the assumption proceeds
lattice.
according
to redox
This mechanism
proceeds I-III
that conversion reaction
follows
rate on com-
and for the variat-
medium,
from a proposed
that the reaction
corresponding
of reaction
composition
of the reaction
of this type can be obtained
1). The scheme
stoichiometric
representing
respectively.
process
scheme
in three
(equations
propos-
independent
1). This simpli-
of ethylbenzene
mechanism
involving
from the experimental
oxygen
results
of the
which
showed
29
TABLE
1
Process
scheme
for ethylbenzene
oxidative
dehydrogenation
Step
Stoichiometric
Nos.
Steps
numbers following
1
02+z”=z’o*-
2
Z’02_ t Z”
3
z'o- + Z"
4
C8H,g
=
zz’o-
the routes
T
yr
0.5
10.5
10
0.5
0.5
1
0
0
1
0
0
0
0
0
0
1
0
0
1
0
= Z'O2_ + Z'
t Z' = C8H,0Z' 2-
for steps
_.I
5
C8H,0Z'
+ Z'O
6
C8H8Z"
= c*v* + Z”
7
C8H,&
+ Z'O2
8
C8H,000Z'
9
C8H80Z'
+ Z'O2
= C7H80Z'
+ CO2 + Z"
0
1
0
10
C7H80Z'
+ Z'O2
= C7H6OZ'
+ H20 + Z'O
0
1
0
11
C8H8 + Z' = C8H8Z'
0
0
1
12
C8H8Z'
0
0
1
13
C8H800Z'
0
0
i
14
C8H60Z'
t Z'O2 = C7H60Z'
0
0
I
15
C7H60Z'
+ Z'O
0
1
1
16
C6H6Z'
+ 22'02-
0
1
1
17
C5H40Z'+
0
1
1
18
C4H40Z'
19
= C8H8Z"
+ H20 + Z"
= C8H,000Z'
= C8H80Z'
+ Z'O2
+ Z"
+ H20
= C6H6Z'
Z'02
+ Z"
+ H20
= C8H800Z'
= C8H60Z'
1
+ CO2 + Z" + CO2 + Z"
= C5H40Z' = C4H40Z'
+ H20 + CO2 + 22" t CO2 t Z"
+ Z'02
= C4H403Z'
+ Z"
0
1
1
C4H403Z'
+ Z'O
= C4H203Z'
+ H20 + Z"
0
1
1
20
C4H203Z'
+ 2Z'02
= X02
+ C2H202Z'
0
1
1
21
C2H202Z'
+ Z'02
+ Z'O
= 2C02 + H20 + 32"
0
1
1
22
H20 t 7' = Z'H20
0
0
0
where
Z' - active Z"
center
containing
- active center containing
+ Z'O- + Z"
an oxidated a reduced
vanadium vanadium
ion (V5'); ion (V4').
30 the possibility the absence
of ethylbenzene
of oxygen
data 1151 which
in the gas phase,
point to the presence
Similar
mechanisms
carbons
are postulated
radical
forms of oxygen,
The assumption formed
for the oxidative in 116,171.
is supported
ium, or under favorable of
the catalyst
scheme,
steps
being accompanied
by a change
presence
vapour
of water
of hydrocarbons
11). An interaction
forms of oxyqen
for the initiation
oxidation
1) that carbon
dioxide
with the reaction
of active
ion-
reaction.
adsorbed
is med-
ion-radical
l-3 correspond to the appearance of ion-radical 2as 0 to the catalyst lattice, this process
reoxidation
of adsorbed
state of the vanadium
ethylbenzene
of adsorbed
styrene
ions. The
the V 4+ z V5+ interconversions,
may be assumed
on the centers
by the interaction
containing
to proceed rapidly. Ad5+ . a V ion (steps 4 and
with the lattice oxygen (step 5) 4+ ions. The formation and water and V
of ethylbenzene
and styrene
with
ion-radical
(steps 7-10 and 12-21).
5, 7 and 12 are taken to be slow and irreversible;
the other
steps are
to be fast.
Let us now express
the reaction
of the reaction
medium
(stoichiometric
numbers
rI
(Figure
surface. of hydro-
that the adsorbed
has been contacted
in the oxidative
occurs
in the formation
of CO2 is caused
taken
and total oxidation
suqqested
in
by the spectroscopic
in the total
is known 1191 to favour
so that the steps of catalyst
Steps
dehydrogenation
by the observation
oxygen species and oxygen return
results
it is also supported
of Vst and V4+ ions on the catalyst
The mechanisms
conditions
over this catalyst
oxygen [183.
In the suggested
sorption
dehydropenation
02 and Cl-, are involved
only some time after
species
oxidative
=r
=
5
rates for routes
I-III,
neqlecting
in terms of the rates
on the catalyst,
of these steps for each other
the effect
of steps 5, 7 and
route are equal
12
to zero):
k; -‘C8H,0~02 M2O
rII
k;PC8H,0P02
=r
7 M
(5)
') LU
ki 2~C8Hg002 = r12 =
rIII
where
M = z aiPi,Pi
adsorption
- the fugacities
coefficients
on the most
k; and ki2 - rate constants adsorption Polanyi
coefficients
relation
of components reactive
for steps
between
containing
the reaction
styrene
and oxygen,
the activation
this coefficient
proceeds
in the adsorbed
sites of the catalyst
5, 7 and 12, respectively,
of ethylbenzene,
coefficient
step [20] or a factor In our case,
(6)
M2=
most
probably
energy
layer;
ai -
surface; including
ki, also
and CL - Brbnsted-
and heat effect
of the
(1 2 3 2 0). at complete
coverage
of the
catalyst erence
surface.
between
erent meaning
(4)-(6)
of the constants
Depending steady
In equations
on the surface
states
u is forma lly equal
react ions on the uniform
and nonun iform
of Z' and Z"
1 respectively, z" '
centers.
and their
is reduced
to diff-
[Zl].
concentrations
of ethylbenzene
tend to set in on the surface
concentration
to unity and the diff-
surface
These
invariable
which
and oxygen
are characterized
concentrations
different
by different
are denoted
total concentrations
L, that
as lz, and is
lz' + lz" = L
(7)
Once each of these stationary centers
and oxidation
ibrium with
where
respectively,
From equations
1z’
=
1
would
the reduction
by the lattice
be equal,
oxygen
rate of Z'
(which
is in equil-
that is
0.5 , = i*Po z II 2
z~, and y2 are permanent
ation,
is established,
centers
the gas phase oxygen)
1 "
xlpC H 810
states
rate of Z"
values
of the catalyst
(7) and
containing
constants
of reduction
and reoxid-
surface.
(8) we obtain
(9)
Pc8Hl 0 +X p A
“2 where
Y, = x,/x2
Since the bimolecular
interaction
of surface
steps the rates of each step in the eauations While
I:, enters
variations considered.
the expressions
in the reaction
is assumed
for the rates of these steps,
mixture
It is therefore
components
(4)-(5) are proportional
composition
possible
according
to express
factor
in the slow to I:,.
its changes
to equation
due to
(9) were not
: (Pj) in equation
(3) as
follows:
o (Pj)
=
1 +
[ where
k - proportionality
ponding
x
(IO)
1
Pc8H,0 2 0.5
p0* coefficient
which
is further
introduced
into the corres-
constants.
Considering
adsorption
equilibria
in steps
1-4, 11 and 22 and substituting,
corresponding
partial
the expressions with regard
pressures
for styrene
to the effect
kIPC8H,0 w, =
Pcoa5 2
for fugacities
of the surface
and CO2 accumulation
of the reaction
system
layer, we obtain
rates from equations
(2) to (10)
on the catalyst:
kIIIPC8H8p02
i-
12
(’ + klPC8H,0
+ k2P
(II)
'C H 8 10 ) k3P02)(l+x C8H8 + cl.5 pO,
w2 =
(kIIPC H 810
kI
+
(' + klPC8H,0
IIpC8H,,)PO
2
+ k2PC8H8
+ k3P02)(1tx[
pc ,&‘I’31 1
po;
where
kI, kII, kIII and k
The unity
in the first
steam concentration
k3 are constants. 1' k2' parentheses of each denominator
which
is suggested
If Pc,~,,,!P~;~ <
will
least squares
In equations
of the reaction
medium
(11) and (12), instead
steady
by calculating
The calculations
in the range
of taking
the denominator
from 0 to 3; in addition
showed
kIII
. IO-* e -7'oo/T
= 6.55.10m4
= 4.55
e -7050
. lob3
k, = 2.31 . 10W5 e
that,
the experimental
powers
equal
the PO2 powers
data are best described
form with the constants
having
mol/g
/T mol/g
s kPale5
s kPa*
e -7550/Tmo1,g 6300/Tkpa-1
;
s kpa2
;
; k2 = 6.72. 10m5 e 6300/Tkpa-l;
2500 k3 = 2.62 . 10T3 e
data by
of all constants
values:
kII
can be neg-
state of the catalyst.
to were
by
each of them to zero.
ions (11) and (12) in the above
kI = 4.91
to ethylbenzene.
experimental
in the range from 0 to 3 as well as the values
means of equating
the adsorbed
on the catalyst
with a single
was specified
considers
and in excess
technique.
2, its value was varied also varied
to be constant
then proceed
The form of the equations nonlinear
(12)
2
1
'T kPa-';
x = 0.4 . IO-' kPa -Oa5.
by equat-
the following
33 The root-mean-square is 15.7',.and equal mental
to 35.6', and 39.6%, error
of about
The above model For example,
Cortes
dehydrogenation of styrene
and Seoane
oxidation
of different
postulated
be independent
investigation, ration
was
equations
proved
In the present the process
of oxygen
the adsorbed
concentrations
atoms.
to describe
in total
envisages
in the gas phase.
of reaction
as the desired
changed
and
mechanism
is suggested
of ethylbenzene
the kinetic
in [21.
and rapid oxid-
observed.
as the oxygen data by simple
oxygen
rate In our concentpower
describing
range of parameters.
that takes account in the catalyst
of the fact
lattice,
whereas
oxidation.
is governed
The equation
rates on the oxygen
equations
for a wide
the participation
ratio
the V5+/V4+
by styrene
suggests
In that case the reaction
the experimental
mechanism
the surface
participates
dependence
does not consider
Another
and this was really
and total oxidation
to a specific
mechanism
for ethylbenz-
too.
involves
states,
model
catalysts.
dehydrogenation
effect.
of oxygen
adsorption
hydrogen
work we were able to obtain
corresponds
The suggested ent oxidative
in the presence
the rates of both reactions
unsuccessful
oxygen
the experi-
both for oxidative
in this work
in oxidative
inhibitions
concentration
An attempt
that dehydrogenation
with
for other
equation
developed
forms of oxygen
the dissociative
rate to styrene
The equation
for w, and w2
they found that high concentrations
model
the styrene
adsorbed
however,
varied.
Although
of oxygen
rate, the appropriate
dehydrogenation
ation of the resultant
earlier
two site Langmuir
the same species
the kinetic
and considers
for ethylbenzene
rates for w, and w2
deviation
is in fair agreement
suggested
[I] proposed
the reaction
By contrast
the participation
would
from the models
that implies
inhibits
which
reaction
single
15':.
and total oxidation.
this phenomenon.
The authors
of the calculated (with maximum
respectively)
differs
ene dehydrooenation
total
deviation
18.4?., respectively
of vanadium
by oxygen
considers
concentration
ions in differ-
and ethylbenzene
the different
patterns
and on process
for
inhibition
product.
REFERENCES 1 2 3 4 5
6 7 8 9 IG 11
A. Cortes and J.L. Seoane, J. Catalysis, 34 (1974) 7. P.N. Degannes and D.M. Ruthven, Canadian 3. Chem. Eng., 57 (1979) 627. Y. Murakami, K. Iwayama, H. Uchida, T. Hattori and T. Tagawa, J. Catalysis, 71 (1981) 257. G. RUSSO, S. rrescitelli and P. Ciambelli, Chem. Ind., 55 (1973) 627. I.P. Belomestnykh, O.K. Bogdanova and G.V. Shakhnovich, A Catalyst for Oxidative Dehydrogenation of Ethyl Benzene to Styrene, USSR Inventor's Certificate No. 522.851, Bull. Izobr. No. 28 (1976). G.V. Isagulyants, I.P. Belomestnykh, G.V. Shakhnovich, N.N. Rozhdestvenskaya, L.L. Kovalenko and A.A. Greish, Neftekhimiya, 21 (1981) 365. P. Batist, C. Kapteijns, B. Lippens and G. Schuit, J. Catal!,sis, 7 (1967) 33. V.A. Phenin, V.A. Shvets and V.B. Kazansky, Dokl. AN SSSR. Z52 (1980) 1427. V.I. Timoshenko, R.A. Buyanov and 0.1. Proshin, Kinet. Katal., IO (1969) 681. S.G. Bashkirova and S.L. Kiperman, Kinet. Katal., 11 (1970) 631. G.V. Isagulyants, I.P. Belomestnykh, A.A. Greish, G.V. Shakhnovich and O.K.
34
12 13 14 15 16
19
20 21
Bogdanova, Neftekhimiya, 18 (1978) 34. S.L. Kiperman, Kinet. K&al., 22 (1981) 30. N.I. Koltsov and S.L. Kiperman, Journal of the Research Institute for Catalysis, Hokkaido University, 26 11978) 85. G.K. Boreskov. Kinet. Katal.. 13 (1972) 543. N.S. Kozlov, i.A. Skrigan, G:V. Mdlorava, L.N. Kupcha, G.V. Shakhnovich and I.P. Belomestnykh, Izv. AN BSSR, Ser. Khim. Nauk., 1983, No. 3, p. 10. J. Hanuza, B. Jesowska-Thzebiatowska and W. Oganowski, Bull. Acad.Polon. Sci., Ser. Sci. Chim., No.11 (1977) 899. J. Haber and B. Grzybowska, J. Catalysis, 28 (1973) 489. K.N. Spiridonov arid O.V. Krylov, Problems of Kinetics and Catalysis, v.16, P.17, Nauka, Moscow, 1975. R.V. Prudnikov and V.F. Kiselev, Kinet. 1.1. Burbulavicus, Y.A. Zarifyants, Katal., 12 (1971) 258. M.I. Temkin, Advances in Catalysis a. Related Subjects, 28 (1979) 173. S.L. Kiperman, The Foundations of Chemical Kinetics. Kinetic Problems in Heterogeneous Catalysis, Chimiya, Moscow, 1979.
Applied Catalysis, 12 (1984) 23-34 Elsevier Science Publishers B.V., _4msterdam
- Printed
KINETICS
OF ETHYLBENZENE
DEHYDROGENATION
MAGNESIA
CATALYST
G.V. SHAKHNOVICH,
OXIDATIVE
I.P. BELOMESTNYKH,
in The Netherlands
TO STYRENE
N.V. NEKRASOV,
M.M.
OVER A VANADIA/
KOSTYUKOVSKY,
and
S.L. KIPERMANa Zelinsky
Institute
Leninsky
Prospect
of Organic 47, Moscow
aTo whom corresoondence
(Received
V-334,
should
20 September
Academy
Chemistry,
of Sciences
of the USSR,
USSR.
be addressed.
1983, accepted
26 April
1984)
ABSTRACT Kinetics of vapor-phase ethylbenzene oxidative dehydrogenation have been studied at 420 to 550°C in a gradientless reactor in the presence of a vanadia/magnesia catalyst. Kinetic equations have been obtained for this reaction in the direction of styrene and CO2 formation which considers the effect of the reaction medium on the surface state of the catalyst. A process scheme is proposed. This scheme suggests that dehydrogenation involves the interaction of adsorbed ethylbenzene with lattice oxygen whereas the ion-radicals of oxygen adsorbed on the catalyst surface are active in the total oxidation.
INTRODUCTION Although
the development
ethylbenzene kinetics
of catalysts
is the subject
of this reaction
ics of simultaneously
of a number
[l-4], while
occurring
for the oxidative
no studies
reactions
dehydrogenation
of
only a few of them discuss
of papers,
have been reported
of ethylbenzene
oxidative
the
on the kinetdehydrogenat-
ion and total oxidation. This paper deals with
the kinetic
behaviour
of both oxidative
and total oxidation
reactions
over a vanadia/magnesia
ion of the reaction
mechanism
is based on the data obtained
gation.
These
rogenation
include
[I], and their further
as the observation
EXPERIMENTAL Reaction
the intermediate
regarding
products
transformation
the oxygen
species
dehydrogenation
catalyst
[S]. The descript-
in the earlier
of ethylbenzene into carbon
oxidative
oxides
on the vanadia
investidehyd-
[6] as well
catalysts
[7,8].
METHODS kinetics
were
studied
in a small volume
gradientless
pulsating
reactor
c91. The products bons and oxygen
were analysed containing
by chromatography
compounds
UV-spectrophotometer
(Specord
0166-9834/84/$03.00
@ 1984 Elsevier
while
UV VIS). Science
for the concentration
the aqueous
COz,CO, Publishers
methane, B.V.
of hydrocar-
layer was analysed ethylene
on a
and oxygen
were
24 G hdd
iii
\_
0.5,
0
_
=
y
0
a
01
0.4 0.2 0.3
I 0.2 0.1
i
0.02
0.1
0‘ 01
::-
FIGURE (0)
1
Variations
deposited
20
carbon
the gaseous
(styrene
characterized
oxide,
residual
by oxidizing
benzaldehyde,
by the constants
corresponding
moisture
out using a sample
and zinc nitrate.
content
dried and activated
CO
- x2
and oxygen-containing
acetophenone) to those
reported
in the literature.
of a zinc oxide modified by impregnating
After drying,
com-
used in this paper were
the resulting
of about 40 to 50%) was extruded,
in an air stream
of products
them to CO2.
styrene
(92% MgO, 7% V205 and l?, ZnO) prepared vanadate
- x, (e),
In some runs the amount
components.
phenol,
nesia catalyst with ammonium
to styrene
with time at 500°C and VC8U,,, = 220 h- i; .
pure ethylbenzene,
All runs were carried
mill)
60
were determined
The chromatographically pounds
50
of ethylbenzene
- G m)
reaction
on the catalyst
40
30
of conversion
and of deposited
found among
IO
vanadia/magmagnesia paste
the granules
for two hours at 18O"C,
(with
were
two hours at 35O"C,
and 3 hours at 500 to 550°C. Specific catalyst surface determined from nitrogen adsorption by the BET method 3 -1 2 -1 , g , the pore volume (measured by mercury porosimetry) was 0.7 cm p was 100m n the pore radius was predominantly 200 - 500 A. In order uted with
to maintain
quartz
with quartz Whether
isothermal
homogeneous
the catalyst
by returning
at 5OO"C,
volume
was dil-
was also filled
oxidation.
activity
to the conditions
lyst was reactivated
the bed, the catalyst
along
ratio of 1:2. The free reactor
in a volume
to avoid
conditions
was constant
accepted
each series
after
as standard.
first by air/stream
Between
mixture
of runs was checked
the runs, the cata-
for 10 minutes
and
then by air for one hour. Initial conversion
(PO) and current to styrene
and w 2, respectively, coefficient
(x,) and carbon the accumulation
$), and selectivity
of the reaction
RESULTS
(P) partial
products
after
pressures
dioxide
of the components,
(x2). their
rate w2 being
(S) were calculated drawing
ethylbenzene
accumulation
calculated
from the analytical
up the material
rates
considering
(w, the
results
balance.
AND DISCUSSION
The experimental
conditions
were as follows:
temperature
range 420 to 55O"C,
25
X 0.2
FIGURE
2
Effect
accumulation 52O"C,
of ethylbenzene
rates at various
0.3
0.4
0.5
0.6
conversion
on styrene
- w, (a) and CO2 - w2 (b)
temperatures:
(1) 46O"C,
(2) 48O"C,
(3) 5OO"C,
(4)
(5) 55O'C.
a)
w,.106(mol/gej
b)
W2.106(mo1/gj~ w,.106(aol/gs)
I.4 ml 0.6
3.0
FIGURE (0)
3
Influence
and CO2 (V)
starting
7.0
space
rates;
(Pc8hlo
of styrene
5OO"C,
velocities
(a) and oxygen
a grain
size of 0.25-0.5
reactor
was gradientless.
mixture
diluted
with
(b) on styrene
190 to 1100 h-', initial
(VQ+,~)
partial
) 4.2 to 11.2 kPa, oxygen (PE,) 4.4 to 9.6 kPa, compounds
added
reaction
and carbon
kPa, 0.2 to 0.5 kPa and up to 1.0 kPa, respectively, lyst with
9.0
x = 0.4.
kPa, those of specially
oxygen-containing
2.0
I.0
t,.o(kPa)
pressures
accumulation
10.0-80.0
(P&,h8),
6.0
of partial
of ethylbenzene
(Pi20)
styrene
5.0
ethylbenzene
pressures steam
4.0
to 2-3 mm. Under
Most of the kinetic
steam since carbon
products,
dioxide
over
(P&J~)
1.3 to 6.2
1 to 5 ml of the cata-
these conditions,
runs were carried
deposition
namely
the pulsating
out with reaction
on the catalyst
was negligible
in this case. In the absence these amounts
of the catalysts
were considered
The experiments rate of ethylbenzene
ethylbenzene
in calculating
with varying demonstrated
grain
conversion
the reaction
size and linear
the absence
did not exceed
velocity
of inhibition
89 and
rates. at a constant
effects
flow
due to volume
26
0.2
0.1
FIGURE
4
46O'C
Dependence
(0).
FIGURE
5
480°C
0.4
of ethylbenzene
(o,,
Conversion
in the gas phase.
0.3
5OO'C
520°C
and selectivity
(v).
550'C
as the function conditions:
0.7
0.6
conversion
(n,,
to styrene
Experimental
0.5
on temperature;
(A).
of time in the absence -1 . VC8H,o = 600 h
of oxygen
5OO'C,
and pore diffusion. Figure
1 shows the experimental
beainning
of the run, conversion
CO2 tends
to increase
results to styrene
with progressively
on testing
higher
is 0.5 h or below even at low ethylbenzene
Steady
state will
of time without Liquid
more
to
The relaxation
space velocities will function
made up at least 908 of the total
for the main fraction (benzene
x-phenylethyl
and the catalyst
In the
conversion
(100 to 200 h-l). for a long period
reactivation.
products
destruction
stability.
while
coke formation.
period
then be reached
catalyst
tends to decline
alcohol
of the liquid and toluene)
products.
and mild oxidation
and acetophenone)
conversion.
In addition
were detected
Styrene
the products
(styrene
oxide,
accounts
of oxidative
benzaldehyde,
in small quantities
(not
than 3";).
The aqueous ethylbenzene)
condensate
contains
and the gaseous
only phenol
products
eratures, CO (up to 1%). The product -1 or less based on carbon. g
contain
deposited
(not more than 0.02"1 based on CO2 (up to 12':) and, at higher on the catalyst
amounted
temp-
to 0.5 mg
The complete
analysis
ing over this catalyst to styrene
and, to a negligible
resulting
in the formation
ducts tend to proceed Figures2a
curves"
ial mixture
The increasing creases
slightly
oxidation
styrene
accumulation
rate remains
on the catalyst position
tends
on the catalyst not represent
virtually
presented
scheme in Figure
ivity is independent
catalyst
[II].
ii1 1lly5,
3a) decreases
of
by at
into the init-
w, values
contribution
compounds
rate becomes
and in-
of styrene
The oxygen-containing
are added
considerably
the same, and the amount
lower,
of products
compounds
compounds
suggests
CO2
deposited
undergo
decom-
their occurrence
and that their further
with ethylbenzene of ethylbenzene
reaction
of temperature
conversions
(styreneproceeds
conversion
process
indicates
can
scheme
it was earl-
to parallel-
on selectivity with a possible
of the reaction
is presented
rate of styrene
is comparablewiththatobtained
that the energies
select-
of acti-
are similar.
in the course
formation
rate decreased
over this
on Figure
5. Only styrene
is equal
to 3.8 10m6 mol
in the presence
formation
C mixture
according
of conversion
pathways
of gas phase oxygen Initial
14
[IZ]. The fact that the reaction
possibly
in its different
of ethylbenzene
of oxygen
dramatically
(6.2 10e6 mol
due to reduct-
ions.
When performing urrence
reaction
The data on the effect
at 4 kPa 02). Styrene
9
slopes
inhibited
dioxide
the
in this case.
4 also point to a parallel
in this case.
g-' s-',which
The concave
is possibly
due to a greater
drops
in small quantities
of consecutive
in the absence
is formed
(Figure
of the oxygen-containing
for the reaction
The change
against
slow steps of the process.
contribution
vation
pro-
conditions.
ier shown that total oxidation
minor
[IO]).
(up to 2":) oxygen-containing
On the basis of experiments
consecutive
curves'
pressure
the main
to increase.
surface
oxidation
products
of up to IO' carbon
selectivity
under the reaction
A high reactivity
the reactions
and incomplete
that the reaction
of w2 probably
quantities mixture,
reaction;
rates of the reaction
("conversion
partial
the values
When even minor
occurr-
of ethylbenzene
the rates of the two reactions.
rate. The process
into the starting
destruction
The addition
does not affect
that the reactions
dehydrogenation
lower rates.
indicate
least one of the products.
showed
total oxidation
of oxidative
with still
conversion
"conversion
products oxidative
extent,
and 2b show accumulation
total ethylbenzene these
of the reaction
are predominantly
a kinetic
of the following
analysis,
account
was taken of the simultaneous
occ-
reactions:
I.
C6H5C2H5
+ 0.5 O2 = C6H5C2H3
II.
C6H5C2H5
t 10.5 O2 = 8 CO2 + 5 H20
III.
C6H5C2H3
t 10 02 = 8 CO2 + H20
+ H20 (I)
Since the styrene lation
is subject
to further
in the experiment
conversions,
are expressed
I, II and III,
and rIII in the directions
rI' rII
w1 =r
formed
rates measured
in terms
the product
accumu-
of reaction
rates,
respectively,
by the equations
I - rIII
w2 = rII + rIII w=r
(2)
I + rII
where w = w
+ w - ethylbenzene consumption rate. 12 An increase in current partial oxygen pressures
actions
I-III.
rations
in the reaction
It is seen from Figure mixture
3b, however,
lead to higher
results
in higher
that enhanced
rates of w
while
2
rates of re-
oxygen
concent-
w, values
chanoe
less markedly. A rise of initial most experiments
partial
ethylbenzene
were carried
pressures
out) at a constant
above
oxygen
8-10 kPa (at which
values
and steam to ethylbenzene
ratios
Go2 =
’ H20 = p"H20'P~8H10
P"c2'Poz8H,0'
does not result catalyst
in the increase
is completely
An attempt variable
to linearize
reaction
ful. That
covered
mixture
indicates
The stationary
on the catalyst
form as follows
that probably
the temperature
dependence
composition
activity
yield
molecules,
a nonpower
value.
indicates
of reaction
in the Arrhenius
from of the kinetic
level of the catalyst
ably lower than the initial mixture
of styrene by reacting
equation
was unsuccess-
[13].
to styrene
formation
This may be due to the action
and the kinetic
rates at an in-
coordinates
equation
that the
is consider-
of the reaction
can be expressed
in the general
[14]
w = f (Pi) m (Pj)
where ponent
(3)
f and q are functions partial
pressures
ion of its activity The equation ed (Table
fied scheme to styrene catalyst
the dependence
for the constant
under the influence
catalyst
routes
implies
to directions
is based on the assumption proceeds
lattice.
according
to redox
This mechanism
proceeds I-III
that conversion reaction
follows
rate on com-
and for the variat-
medium,
from a proposed
that the reaction
corresponding
of reaction
composition
of the reaction
of this type can be obtained
1). The scheme
stoichiometric
representing
respectively.
process
scheme
in three
(equations
propos-
independent
1). This simpli-
of ethylbenzene
mechanism
involving
from the experimental
oxygen
results
of the
which
showed
29
TABLE
1
Process
scheme
for ethylbenzene
oxidative
dehydrogenation
Step
Stoichiometric
Nos.
Steps
numbers following
1
02+z”=z’o*-
2
Z’02_ t Z”
3
z'o- + Z"
4
C8H,g
=
zz’o-
the routes
T
yr
0.5
10.5
10
0.5
0.5
1
0
0
1
0
0
0
0
0
0
1
0
0
1
0
= Z'O2_ + Z'
t Z' = C8H,0Z' 2-
for steps
_.I
5
C8H,0Z'
+ Z'O
6
C8H8Z"
= c*v* + Z”
7
C8H,&
+ Z'O2
8
C8H,000Z'
9
C8H80Z'
+ Z'O2
= C7H80Z'
+ CO2 + Z"
0
1
0
10
C7H80Z'
+ Z'O2
= C7H6OZ'
+ H20 + Z'O
0
1
0
11
C8H8 + Z' = C8H8Z'
0
0
1
12
C8H8Z'
0
0
1
13
C8H800Z'
0
0
i
14
C8H60Z'
t Z'O2 = C7H60Z'
0
0
I
15
C7H60Z'
+ Z'O
0
1
1
16
C6H6Z'
+ 22'02-
0
1
1
17
C5H40Z'+
0
1
1
18
C4H40Z'
19
= C8H8Z"
+ H20 + Z"
= C8H,000Z'
= C8H80Z'
+ Z'O2
+ Z"
+ H20
= C6H6Z'
Z'02
+ Z"
+ H20
= C8H800Z'
= C8H60Z'
1
+ CO2 + Z" + CO2 + Z"
= C5H40Z' = C4H40Z'
+ H20 + CO2 + 22" t CO2 t Z"
+ Z'02
= C4H403Z'
+ Z"
0
1
1
C4H403Z'
+ Z'O
= C4H203Z'
+ H20 + Z"
0
1
1
20
C4H203Z'
+ 2Z'02
= X02
+ C2H202Z'
0
1
1
21
C2H202Z'
+ Z'02
+ Z'O
= 2C02 + H20 + 32"
0
1
1
22
H20 t 7' = Z'H20
0
0
0
where
Z' - active Z"
center
containing
- active center containing
+ Z'O- + Z"
an oxidated a reduced
vanadium vanadium
ion (V5'); ion (V4').
30 the possibility the absence
of ethylbenzene
of oxygen
data 1151 which
in the gas phase,
point to the presence
Similar
mechanisms
carbons
are postulated
radical
forms of oxygen,
The assumption formed
for the oxidative in 116,171.
is supported
ium, or under favorable of
the catalyst
scheme,
steps
being accompanied
by a change
presence
vapour
of water
of hydrocarbons
11). An interaction
forms of oxyqen
for the initiation
oxidation
1) that carbon
dioxide
with the reaction
of active
ion-
reaction.
adsorbed
is med-
ion-radical
l-3 correspond to the appearance of ion-radical 2as 0 to the catalyst lattice, this process
reoxidation
of adsorbed
state of the vanadium
ethylbenzene
of adsorbed
styrene
ions. The
the V 4+ z V5+ interconversions,
may be assumed
on the centers
by the interaction
containing
to proceed rapidly. Ad5+ . a V ion (steps 4 and
with the lattice oxygen (step 5) 4+ ions. The formation and water and V
of ethylbenzene
and styrene
with
ion-radical
(steps 7-10 and 12-21).
5, 7 and 12 are taken to be slow and irreversible;
the other
steps are
to be fast.
Let us now express
the reaction
of the reaction
medium
(stoichiometric
numbers
rI
(Figure
surface. of hydro-
that the adsorbed
has been contacted
in the oxidative
occurs
in the formation
of CO2 is caused
taken
and total oxidation
suqqested
in
by the spectroscopic
in the total
is known 1191 to favour
so that the steps of catalyst
Steps
dehydrogenation
by the observation
oxygen species and oxygen return
results
it is also supported
of Vst and V4+ ions on the catalyst
The mechanisms
conditions
over this catalyst
oxygen [183.
In the suggested
sorption
dehydropenation
02 and Cl-, are involved
only some time after
species
oxidative
=r
=
5
rates for routes
I-III,
neqlecting
in terms of the rates
on the catalyst,
of these steps for each other
the effect
of steps 5, 7 and
route are equal
12
to zero):
k; -‘C8H,0~02 M2O
rII
k;PC8H,0P02
=r
7 M
(5)
') LU
ki 2~C8Hg002 = r12 =
rIII
where
M = z aiPi,Pi
adsorption
- the fugacities
coefficients
on the most
k; and ki2 - rate constants adsorption Polanyi
coefficients
relation
of components reactive
for steps
between
containing
the reaction
styrene
and oxygen,
the activation
this coefficient
proceeds
in the adsorbed
sites of the catalyst
5, 7 and 12, respectively,
of ethylbenzene,
coefficient
step [20] or a factor In our case,
(6)
M2=
most
probably
energy
layer;
ai -
surface; including
ki, also
and CL - Brbnsted-
and heat effect
of the
(1 2 3 2 0). at complete
coverage
of the
catalyst erence
surface.
between
erent meaning
(4)-(6)
of the constants
Depending steady
In equations
on the surface
states
u is forma lly equal
react ions on the uniform
and nonun iform
of Z' and Z"
1 respectively, z" '
centers.
and their
is reduced
to diff-
[Zl].
concentrations
of ethylbenzene
tend to set in on the surface
concentration
to unity and the diff-
surface
These
invariable
which
and oxygen
are characterized
concentrations
different
by different
are denoted
total concentrations
L, that
as lz, and is
lz' + lz" = L
(7)
Once each of these stationary centers
and oxidation
ibrium with
where
respectively,
From equations
1z’
=
1
would
the reduction
by the lattice
be equal,
oxygen
rate of Z'
(which
is in equil-
that is
0.5 , = i*Po z II 2
z~, and y2 are permanent
ation,
is established,
centers
the gas phase oxygen)
1 "
xlpC H 810
states
rate of Z"
values
of the catalyst
(7) and
containing
constants
of reduction
and reoxid-
surface.
(8) we obtain
(9)
Pc8Hl 0 +X p A
“2 where
Y, = x,/x2
Since the bimolecular
interaction
of surface
steps the rates of each step in the eauations While
I:, enters
variations considered.
the expressions
in the reaction
is assumed
for the rates of these steps,
mixture
It is therefore
components
(4)-(5) are proportional
composition
possible
according
to express
factor
in the slow to I:,.
its changes
to equation
due to
(9) were not
: (Pj) in equation
(3) as
follows:
o (Pj)
=
1 +
[ where
k - proportionality
ponding
x
(IO)
1
Pc8H,0 2 0.5
p0* coefficient
which
is further
introduced
into the corres-
constants.
Considering
adsorption
equilibria
in steps
1-4, 11 and 22 and substituting,
corresponding
partial
the expressions with regard
pressures
for styrene
to the effect
kIPC8H,0 w, =
Pcoa5 2
for fugacities
of the surface
and CO2 accumulation
of the reaction
system
layer, we obtain
rates from equations
(2) to (10)
on the catalyst:
kIIIPC8H8p02
i-
12
(’ + klPC8H,0
+ k2P
(II)
'C H 8 10 ) k3P02)(l+x C8H8 + cl.5 pO,
w2 =
(kIIPC H 810
kI
+
(' + klPC8H,0
IIpC8H,,)PO
2
+ k2PC8H8
+ k3P02)(1tx[
pc ,&‘I’31 1
po;
where
kI, kII, kIII and k
The unity
in the first
steam concentration
k3 are constants. 1' k2' parentheses of each denominator
which
is suggested
If Pc,~,,,!P~;~ <
will
least squares
In equations
of the reaction
medium
(11) and (12), instead
steady
by calculating
The calculations
in the range
of taking
the denominator
from 0 to 3; in addition
showed
kIII
. IO-* e -7'oo/T
= 6.55.10m4
= 4.55
e -7050
. lob3
k, = 2.31 . 10W5 e
that,
the experimental
powers
equal
the PO2 powers
data are best described
form with the constants
having
mol/g
/T mol/g
s kPale5
s kPa*
e -7550/Tmo1,g 6300/Tkpa-1
;
s kpa2
;
; k2 = 6.72. 10m5 e 6300/Tkpa-l;
2500 k3 = 2.62 . 10T3 e
data by
of all constants
values:
kII
can be neg-
state of the catalyst.
to were
by
each of them to zero.
ions (11) and (12) in the above
kI = 4.91
to ethylbenzene.
experimental
in the range from 0 to 3 as well as the values
means of equating
the adsorbed
on the catalyst
with a single
was specified
considers
and in excess
technique.
2, its value was varied also varied
to be constant
then proceed
The form of the equations nonlinear
(12)
2
1
'T kPa-';
x = 0.4 . IO-' kPa -Oa5.
by equat-
the following
33 The root-mean-square is 15.7',.and equal mental
to 35.6', and 39.6%, error
of about
The above model For example,
Cortes
dehydrogenation of styrene
and Seoane
oxidation
of different
postulated
be independent
investigation, ration
was
equations
proved
In the present the process
of oxygen
the adsorbed
concentrations
atoms.
to describe
in total
envisages
in the gas phase.
of reaction
as the desired
changed
and
mechanism
is suggested
of ethylbenzene
the kinetic
in [21.
and rapid oxid-
observed.
as the oxygen data by simple
oxygen
rate In our concentpower
describing
range of parameters.
that takes account in the catalyst
of the fact
lattice,
whereas
oxidation.
is governed
The equation
rates on the oxygen
equations
for a wide
the participation
ratio
the V5+/V4+
by styrene
suggests
In that case the reaction
the experimental
mechanism
the surface
participates
dependence
does not consider
Another
and this was really
and total oxidation
to a specific
mechanism
for ethylbenz-
too.
involves
states,
model
catalysts.
dehydrogenation
effect.
of oxygen
adsorption
hydrogen
work we were able to obtain
corresponds
The suggested ent oxidative
in the presence
the rates of both reactions
unsuccessful
oxygen
the experi-
both for oxidative
in this work
in oxidative
inhibitions
concentration
An attempt
that dehydrogenation
with
for other
equation
developed
forms of oxygen
the dissociative
rate to styrene
The equation
for w, and w2
they found that high concentrations
model
the styrene
adsorbed
however,
varied.
Although
of oxygen
rate, the appropriate
dehydrogenation
ation of the resultant
earlier
two site Langmuir
the same species
the kinetic
and considers
for ethylbenzene
rates for w, and w2
deviation
is in fair agreement
suggested
[I] proposed
the reaction
By contrast
the participation
would
from the models
that implies
inhibits
which
reaction
single
15':.
and total oxidation.
this phenomenon.
The authors
of the calculated (with maximum
respectively)
differs
ene dehydrooenation
total
deviation
18.4?., respectively
of vanadium
by oxygen
considers
concentration
ions in differ-
and ethylbenzene
the different
patterns
and on process
for
inhibition
product.
REFERENCES 1 2 3 4 5
6 7 8 9 IG 11
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