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TiO2 (anatase) catalysts
Applied Catalysis, 31(1987) 87-98 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
REACTION
NETWORK
AND KINETICS
OF 0-XYLENE
V,CJ,/TiO2 (ANATASE)
CATALYSTS
Ramzi Y. SALEH and
Israel E. WACHS*
Intermediates 08801,
Technology
Division,
OXIDATION
Exxon Chemical
87
TO PHTHALIC
Company,
ANHYDRIDE
Annandale,
OVER
New Jersey
U.S.A.
*Department
of Chemical
Bethlehem,
(Received
PA 18015,
13 August
Engineering,
Whitaker
Laboratory
#5, Lehigh
University,
U.S.A.
1986, accepted
22 December
1986).
ABSTRACT The reaction network of o-xylene oxidation to phthalic anhydride over V205/Ti02 (anatase) catalysts was determined in the present investigation. The detailed oxvlene conversion/oroduct selectivitv data obtained demonstrate that ohthalic anhydride is not produced from the d4rect oxidation of o-xylene over V205/Ti02 (anatase) catalysts, but indirectly via a series of consecutive reaction steps involving o-tolualdehyde and phthaiide intermediates. The maleic anhydride byproduct is produced from the over-oxidation of phthalic anhydride. The influence of the two reactive sites in V205/Ti02 (anatase) catalysts, surface vanadium oxide monolayer and V705 crystallites, upon the o-xylene oxidation reaction were obtained for moderate am6unt.s of vanadium oxide by varying the crystalline V2O5 content of the catalysts. The initial reaction step of the oartial oxidation of o-xvlene to o-tolualdehyde occurs almost exclusively over the surface vanadium oxide-monolayer. The o-tolualdehyde intermediate is directly converted to phthalic anhydride primarily over the surface vanadium oxide monolayer and to phthalide over both the surface vanadium oxide monolayer and the V205 crystallites. The partial oxidation of phthalide to phthalic anhydride mostly occurs over the monolayer of surface vanadium oxide. The over-oxidation of phthalic anhydride to maleic anhydride also occurs primarily over the monolayer of surface vanadium oxide. The non-selective formation of the combustion products, CO and CO2, primarily occurs from the direct oxidation of o-xylene over the monolayer of surface vanadium oxide. The turnover rate for o-xylene oxidation over V205/Ti02(anatase) catalvsts is determined for the first time, and found to be ~1 x low3 molecules site-1 s-1 for the experimental conditions used in the present study. The apparent activation energy for the conversion of o-xylene to all products is found to be %32 kcal mole-l.
INTRODUCTION The complex V205/Ti02
reaction
(anatase)
network
catalysts
[l-7]. A generalized
reaction
the probable
pathways
by different
investigators
propose
scheme
is presented
that the desired
of reaction
of o-xylene
for o-xylene
in Figure
are also shown phthalic
I,2 and 3, or 1 and 9) [2,4,6],
oxidation
1. The various 1 [l-7].
product
and phthalide
and other
to phthalic
of several
in Figure
anhydride
steps via o-tolualdehyde
0166-9834/87/$03.50
oxidation
has been the subject
anhydride
recent
which
includes
reaction
steps
can only be formed
0 1987 Elsevier Science Publishers B.V.
all proposed
Some investigators
intermediates
investigators
over
investigations
propose
by a series
(reaction
steps
that phthalic
88
0 Maleic Anhydride
0 Maleic
Reaction Investigators
1
Herten & Froment
& Foster
Yabrov & lvanov Skrzypek et al.
FIGURE
1
Generalized
(anatase)
4
5
X
x
x
x
x x
x x
x x
x
x
x
x
x
x
reaction
catalysts
3
X
Van Hove 8 Blanchard Calderbank et al. Wainwright
2
x
Steps
6
7
8
9
10
11
x
x
x
x
x
x
x
x
x x
x X
x x
x x
x
X
x
x
x
x
X
for o-xylene
reaction
12
X
x
network
and the various
Anhydride
x
oxidation
steps proposed
x
over V205/Ti02 by different
investi-
gators.
anhydride
can be formed
step 6) [1,3,5,7]. of the maleic direct
directly
Similar
anhydride
results
conditions. containing of these
(reaction
different
kinds of promoters
studies were
also performed
reactors
where
the observed
results.
In order
the oxidation
(anatase)
reaction
steps
catalysts,
difficulty
performed
transfer
catalysts Several
in commercial
may have affected
issues we decided over unpromoted
isothermal
or
experimental
V205/Ti02
conditions
limitations
anhydride
and under
with
types of carriers.
some of these
to phthalic
a carrier,
1,2,3,4)
in comparing
under very different
were
under non-isothermal
to resolve
of o-xylene without
(reaction
and different
heat and mass
(reaction
for the formation
is that they were performed studies
of o-xylene
have been proposed
step 12). The major
Some of these reaction
size tubular
examine
paths series
byproduct:
one step reaction
the above
in one step from the oxidation
reaction
reaction
to
V,O,/TiO2 conditions.
EXPERIMENTAL The unsupported
V205/Ti02
described
[8]. The TiO2
possessed
a surface
contain
(anatase)
(anatase)
catalysts
was obtained
were
prepared
from Mobay
area of Q9 m * g-'. The Ti02
(anatase)
as previously
Corporation, support
and
was found
0.15 wt% K, 0.10 wt% P, 0.10 wt% Al and 0.16 wt% Si as determined
absorption.
The V205/Ti02
in an aqueous
solution
excess water
was allowed
(anatase)
of oxalic
catalysts
to evaporate
dried at 110°C and calcined
were prepared
acid and impregnating
in oxygen
at %65"C. for 2 h
by dissolving
the titania
The catalysts at 450°C.
to
by atomic V205
support.
The
were subsequently
89 The performance xylene
of the V205/Ti02
was examined
were examined
(anatase)
in the reactor
for this reaction
catalysts
unit previously
with
1.25 mole%
velocity
of 2760 h-' and between
320 and 370°C.
obtained
by varying
temperature.
passed
through
the reaction
an o-xylene
bath. A slip stream multicolumn
analyzed
concentration
immersed
in a molten
effluent
was analyzed
reaction
products.
tolualdehyde, including present
during o.d.,
analyzed
maleic
anhydride
three
was always
anhydride, and benzoic
startup
and overnight
the center. throughout
catalyst
steel)
(corresponding
The remaining
inserted
volume
and 2 cm3 at the bottom). inert with respect
was filled
Blank
to o-xylene
oxidation
effluent
was The
with N2
located
(0.5 in.
at
the temperature
to the salt bath temperature was packed having
the reactor
with 2 cm3 of a particle
This catalyst
the length
with 3 mm glass
runs showed
they were
The reactor
monitored
(anatase)
along
products,
blanketed
in. thermowell
with 8 cm3 of 0.5 mm glass beads. profile
and the
feed analyses.
was usually
The reactor
to 1.96 g of V205/Ti02
to give an isothermal
reactor
the reactor
in the thermowell
the desired
anhydride,
because
the feed was being analyzed.
from the bottom.
of 0.4 - 0.7 mm) diluted
phthalic
ignored
by several
The
of the reactor
Other
5%. The reactor
while
After
conversions
were
on-line
to the reactor
CD, CO2 and water.
At each temperature
water
detector.
acid, were
bed. The feed was preheated
the reactor
ratio was found
observed
was fitted with a 0.125
A thermocouple the catalyst
and entered
within
components.
for o-xylene
products
rate was
by a calibrated
bath. A slip stream
to five times followed
balance
316 stainless
RESULTS
reaction
were
flow
controlled
the feed was diverted
Hi Tech)
in very small quantities.
typically carbon
The main
in conversion
conductivity
for all gases and organic
(DuPont
[8]. All catalysts
in a temperature
by the gas chromatograph
phthalide,
Changes
with a thermal
of o-
in air, at a space
feed was analyzed
equipped
was established, salt
citraconic
described
o-xylene
Air at the desired
immersed
of the o-xylenelair
gas chromatograph
gas chromatograph o-xylene
generator
for the oxidation
diameter
dilution
of the catalyst
beads
(2 cm3 at the top
walls
and beads to be
at the temperature
range
bed.
investigated.
AND DISCUSSION
The product
selectivity
7% V205/Ti02(anatase) 2. The conversion (see Figure the major product.
was varied
6). At very
reaction
Tar formation
phthalide
and maleic
by changing
approaching
very rapidly
bed
zero % conversion,
over V205/Ti02
and the phthalic
of o-xylene
to ~20%,
in Figure
of the catalyst
COx (CO2 and CO), and a tar by-
of o-xylene
are not present
is increased
decreases
of o-xylene,
for an unsupported
is presented
by Bond et al. [6]. In this low conversion
zero % as the conversion
the tar byproduct
conversion
conditions
the temperature
are o-tolualdehyde,
anhydride
of o-xylene
of o-xylene
isothermal
at low conversions
was also reported
vity approaches
under
low conversions
products
catalysts
conversion
as a function
catalyst
approaches
the selectivity
with o-xylene
(anatase) region
anhydride
selecti-
zero. As the
to o-tolualdehyde
conversion,
and
and the selecti-
90 90
8
I
I
,
,
,
,
I
,
7°hV~0d’TiO~ (Anatase)
90 -
O-Xylene Conversion, O? FIGURE
2
The product
V20S/Pi02(anatase)
vity to phthalic behavior
anhydride
of the o-xylene
tolualdehyde, oxidation, secondary Further
which
in the phthalide
and decreases
selectivity
formation.
However,
anhydride
production
with
tolualdehyde.
consistent
with
in a parallel of phthalide
increases
anhydride
anhydride formation
probably
the behavior [9,10].
reaction
results partial expected
The phthalide
reaction
path will be provided
varied).
The constant
conversion
where stream.
oxidation
at very
products
for a series
anhydride
selectivity
towards
high conversions
COx as a function
anhydride.
conversion
is
consecutive
is probably
evidence
below when the V205 content
found
from o-tolualde-
can be produced
(additional
that of o-
catalysts
to o-xylene
however,
is not present,
of phthalic
of first-order
intermediate,
for phthalic
oxidation
produced
that
anhydride
suggests
over V20S/Ti02
the over-oxidation
of o-xylene
suggests
phthalide This
towards
The maximum
responsible
from the partial
is only observed
step since phthalic
at low conversions
phthalide.
intermediate
oxidation
are the
reactant.
the selectivity
towards
are both directly
from
of o-xylene
anhydride
that leads to phthalic
in the product directly
products
The
that o-
from the o-xylene
o-xylene
is not the only
of o-tolualdehyde
of the o-xylene
steps
increasing
can be produced
and most
The response
of o-xylene
the selectivity
is present
and phthalic
hyde [Z]. Maleic of o-xylene
for 7%
conversion.
suggest
and maleic
since at very low conversions,
Studies
that phthalide
directly
in the reaction
phthalide
anhydride
anhydride
phthalide
are not derived
is an intermediate
phthalic
conversion
with o-xylene
with conversion
are the primary
anhydride,
in the conversion
anhydride,
some phthalic
of o-xylene
increases
products
COx and the tar byproduct
increase
phthalide
as a function
and phthalide
reaction
and that phthalic products
phthalic
reaction
selectivity
catalyst.
produced
independently
for this parallel
of the catalyst of o-xylene
is
conversion
91 Tar +
CO,+H,O
Maleic Anhydride
4 Phthalide
FIGURE 3
Reaction
V205/Ti02
(anatase)
suggests
that almost
little
network
all the COx is produced from the partially
by the significantly
tolualdehyde
over V205/Ti02
with supported
V205/Ti02
and by varying
conversion
The detailed present
anhydride,
V205/Ti02 steps.
o-xylene
investigation
maleic
results
3 for the oxidation catalysts.
based on their employed
experimental
by these authors,
of o-xylene conditions conditions. and Ivanov
over V205/Ti02 minimized
[5], at Skrzypek
directly
integral
fixed
fluidized
catalyst
regime
phthalic residence
An examination
et al. [7] suggest
microreactors
operating
[5]. It is, therefore,
anhydride
of o-xylene
that reaction
anhydride
over
over V205/Ti02
article with the
of the experimental
apparatus
to study the oxidation
products
These
[6]
that their experimental and non-isothermal et al. [33, Yarbov anhydride
studies,
catalyst-basket close
review
reaction
[Z] and Bond and Konig
that phthalic
reactors
that these
and maleic
anhydride
at low conversions
time of the partial
oxidation
products
can be
however,
to the continuous
not surprising
reaction
network
are also consistent
[I], Calderbank
of o-xylene.
spinning
in the
as well as
the most probable
suggests
time of the reaction
[1,7],
anhydride,
in the literature,
and Froment
conditions,
in the reactor.
in their comprehensive
findings.
from the oxidation
bed reactors
agree with
catalysts
were obtained
data obtained
study suggest
and Blanchard
of Herten
results
oxidation
by Vanhove
(anatase)
is subof o-
via a series of consecutive
[4]. These findings
as described
and that very
This
non-isothermal
to phthalic
findings
and Foster
residence
The results
produced
reactor
proposed
from the direct
of o-xylene
synthesis
kinetic
that phthalic
but indirectly
by Wainwright
networks
over
for the oxidation
selectivity
demonstrates
The present
from o-xylene
time of the gases
of the present
(anatase)
reaction
anhydride
intermediates.
[2]. Similar
conversion/product
clearly
anhydride
directly
oxidized
under moderately
via residence
in Figure
proposed
to phthalic
selectivities
catalysts
catalysts,
catalysts,
The kinetic
network
higher
is not produced
(anatase)
on phthalic
oxidation
catalysts.
COx is produced
stantiated
for o-xylene
stirred
studies
of o-xylene.
in the spinning
used
[33 and tank
observed The long
catalyst-basket
92
g 2
60
5
55 I
CATALYST COMPOSITION e 1.9% V,O,ITiO, (A) q 3 % V,06/Ti0, (A) l 7 % V,O,/TiO* (A) 0 21 % V,OJTiO, (A)
404
65
65
70
75
80
85
90
95
100
0-Xylene Conversion FIGURE
4
Influence
G8-oxygenate
of V205 content
(o-tolualdehyde,
[S] and fluidized
microreactor
intermediate
products
on the walls
of these reactors
presence
of external
conducted
in integral
be to further diffusion
convert
gradients
[I,71 are most maleic
(anatase) Recent
anhydride
gradients
in highly
the partial
characterization
studies
in V205/Ti02
(anatase)
(350-575°C)
[6,8,11-181.
These
the properties
crystallites
vanadium
(anatase)
catalysts
[IO]. Reaction
and reactant
The
concentrations
such as o-xylene effect
The presence integral
oxidation
oxidation, would
of external
fixed
of phthalic
o-xylene
oxide
support.
Thus,
vanadium
bed reactors
anhydride
and/or
over V205/Ti02
corresponds
of surface
increases
vanadium
V205/Ti02
oxide
small
oxide
content
the amount
oxide
study,
and crystalline
a monolayer
on the Ti02
of V205 are also
of the surface
vanadium
of V205/Ti02
(anatase)
oxide
of
[8]. Below mono-
are present
of vanadium
of
as small
area of the TiOp
crystallites
to the monolayer
modifies
a monolayer as well
(anatase)
species
oxide
temperatures
(anatase)
by forming
and the surface
coverage,
the vanadium
that Ti02
to the Ti02 support
to %1.9%
in addition
showed
of vanadium
at moderate
used in the present
vanadium
Above monolayer
above monolayer
amount
support
that two types
calcined
oxide phase
oxide content
(anatase)
increasing
have revealed catalysts
coordinated
only the surface
on the support
species.
species
the vanadium
For the Ti02
layer coverage,
present
oxide
of V205. The relative
V205 depends,on
surface
during
investigations
of the supported
vanadium
support.
products.
employing
of these
catalysts.
are present
surface
anhydride
[IO], and the resulting
for the presence
at low conversions
conversion
this situation.
reactions,
oxidation
responsible
exacerbate
upon the
selectivity.
in the further
in temperature
fixed bed reactors
catalysts
anhydride)
and/or maleic
exothermic
in the investigations
likely
anhydride
result
could further
diffusion
(anatase)
and phthalic
[5] would
to phthalic
are known to be present
of V205/Ti02
phthalide
present
oxide
as VZ05
93
Tolualdehyde
Phthalide
0 A
. .
1.9% V205/Ti02 (A)
Catalyst Composition
0
l
0
.
7% V2051Ti02 (A) 21% V105/Ti02 (A)
3% V205/Ti02 (A)
I
f 50
60
70
80
o-Xylene Conversion FIGURE
5
Influence
o-tolualdehyde
crystallites.
of V205 content
and phthalide
The previous
oxide was the active possesses
a higher
of o-xylene complete
because
of surface
the exposed
of the reaction
the catalytic
activity during
of this phase.
o-xylene
oxidation
vities
at partial
dramatic
drop
monolayer surface
conversion
coverage, vanadium
greater
complete
coverage
catalytic conversion conditions. sample,
of o-xylene Significant
decrease
kinetically underlying
significant surface
(anatase)
is approached amounts
anhydride
affect
selectivity)
area and poor catalytic
catalysts exhibit
Below monolayer improvement
is shown
very
in Figure
similar
coverage
there
suggests
cover
is a above
that the
all the exposed
titania
contents
are necessary
to assure
titania
sites.
The differences
in the
become
somewhat
more pronounced
due to the increased
of crystalline performance
oxide monolayer
severity
V205, 21% V205/Ti02
because possess
as complete
of the reaction (anatase)
the V205 c%-ystallites become a lower
[8,13,18].
4.
selecti-
in selectivity
(anatase),
selectivity
oxide
and intrinsically
vanadium
amounts
upon the C8-oxygenate
does not effectively
of these samples
Moderate
do not significantly
and phthalic
catalysts
higher vanadium
the catalytic
deposition).
surface
than 1.9% V205/Ti02
of the few remaining
performance
carbon
catalysts
[81. The slight
oxide monolayer
sites and that slightly
(combustion
of o-xylene.
in selectivity
inferior
reactions
of V205 content
(anatase)
very
in undesirable
conversion
over V205/Ti02
The 1.9%, 3% and 7% V205/Ti02
exhibit
less than a
selectivity)
of the low effective
The effect
possessing
however,
and that it
V205 for the oxidation
anhydride
(anatase)
(o-xylene
because
catalysts
vanadium
and phthalic
and possibly
in V205/Ti02
performance
upon the
of hydrocarbons
than crystalline
oxide,
conversion
sites result
intermediates
of such catalysts
(anatase)
vanadium
(o-xylene titania
V205
catalysts
that the surface
oxidation
and selectivity
V205/Ti02
performance
(anatase)
also demonstrated
site for the partial
[8,13,18].
of crystalline
studies
activity
monolayer
catalytic
of V205/Ti02
selectivities.
selectivity
than the
94 Additional
insights
phthalic
anhydride
reaction
studies
of V205 content in Figure
into the reaction
over V,O,/TiO,
5 as a function
addition (anatase).
These
are being compared
a monolayer
is slightly
of surface
to phthalic
anhydride
is also sensitive
for V,O,/TiO2
vanadium
mediate
was not observed
phthalide
vanadium
and phthalide
of o-xylene.
vanadium amounts
monolayer,
oxide monolayer,
phthalide
(anatase)
of phthalide
selectivity
V205 may be related
to the slightly
(anatase)
catalyst.
identical
o-toIua\dehyde
suggests
that in the presence
reaction
step is enhanced
relative
reaction
step. The strong
dependence
content
is probably
produced
V20,-/Ti02 (anatase) V205 content monolayer
catalysts.
(anatase)
conversions
its formation
is primarily
The slight oxide
but differing
oxide for moderate The maleic
is also independent associated
of the 21%
samples
o-xylene
amounts
anhydride
to phthalic
upon the V205
with the monolayer
intermediate
oxidation
constant
associated
selectivity
the
V205
observed
and suggests
of surface
over
with the with
of crystalline
of V205 content
This
anhydride
that the phthalide
is fairly
exhibit
selectivity.
selectivity
is primarily
in
from 7% to 21%
to phthalic
step during
much higher
decrease
the o-tolualdehyde
statement
containing oxide
exhibit
selectivity
of the phthalide
for
to the
vanadium
(anatase)
phthalide
and its
was detected
content
to the o-tolualdehyde
reaction
and these
The catalysts
(anatase),
lower $-oxygenate
The CO, selectivity
catalysts.
high o-xylene
stream.
that COx formation
vanadium
however,
(anatase).
the vanadium
the earlier
of
inter-
support
V205 in addition
to the surface
of V205 crystallites
in a parallel
and suggests
of surface
in V205/Ti02
supports
a monolayer
catalyst,
intermediate,
The 3% and 7% V205/Ti02
selectivity,
crystalline
the phthalide
of the titania
of crystalline
in the product
intermediates
Bond and Konig did detect
and 21% V205/Ti02
V205/Ti02
crystalline
approximately
nature
3% V205/Ti02
upon increasing
selectivity
than unsupported,
(anatase),
V205 in addition
of crystalline
in
since the catalysts
The phthalide
oxide monolayer
to the specific
small amounts
7% V205/Ti02
concentrations
catalysts
over the range examined.
[6]. The phthalide
containing
species,
V205
of V 0 crystallites, and 25 are in agreement with the obser-
containing
with their vanadium
impurities
the catalyst
oxide
crystalline
to the content
1.9% V205/TiO2
species,
may be related
associated
surface
oxide
formation
differences
(anatase)
[19]. For the catalyst
surface
of o-xylene,
lower for the
with V 0 content. These observations 25 made by Chandrasekharan and Calderbank that the reaction
V205 catalysts
higher
At a given conversion
containing
increases vation
from
3%, 7% and 21% V 0 /TiO 25 2 that the crystallites slightly retard the con-
at the same conversion
stream
are less evident
to
oxide monolayer,
suggest
of' o-tolua'ldehyde
in the product
oxidation
are also obtained
stream
than the catalysts
vanadium
results
conversion.
in the product
approximately
(anatase),
to the surface
version
of o-xylene
selectivity
containing
1.9% V205/Ti02
of o-xylene
catalysts
in which the V 0 content of the catalyst is varied. The influence 25 upon the o-tolualdehyde and phthalide selectivities are presented
the o-tolualdehyde catalyst
network
(anatase)
at that
vanadium
oxide.
95 TABLE
1
0-xylene
oxidation
moderate
amounts
Reaction
step
o-xylene
+
reaction
steps and their corresponding
of crystalline
V20S in V205/Ti02
reactive
(anatase)
Most Probable
+
o-tolualdehyde
-f phthalide
phthalic
anhydride
reactive
sites
Surface
vanadium
Surface
vanadium
oxide monolayer
Surface
vanadium
oxide monolayer
o-tolualdehyde
o-tolualdehyde
sites for
catalysts.
oxide monolayer
and V205 crystallites phthalide phthalic
-f phthalic anhydride
anhydride
-f maleic and COx
o-xylene
+
COx
1
21
*0-
ii! s!
10
E,I
- 32 22 I kcalfmole , 1.68
1.66
plot for o-xylene
study
reveals
the o-xylene
oxidation
the surface
vanadium
the surface to either slightly
of a-xylene
of o-xylene
vanadium
retard
the conversion
with temperature
in V205/Ti02
network.
vanadium
of o-xylene
reaction
can be performed
however,
occurs
The o-tolualdehyde
or phthalide
[8,13].
oxide monolayer
[8,13],
for the 7%
(anatase)
The initial
or the V206 crystallites
to o-tolualdehyde
oxide monolayer.
anhydride
1.78
1.76
x IO-3,“K
to o-tolualdehyde
of the surface
for the conversion
1.74
conversion
reaction
oxide monolayer
activity
phthalic
-
-
1.72
that the two sites
oxidation
the partial
oxidation
2 (Anatase)
\,
1.70
catalyst.
crystallites
1
\
(anatase)
partial
1
30-
Arrhenius
higher
oxide monolayer
40-
6
cantly
oxide monolayer
vanadium
%
V205/Ti02
The above
vanadium
Surface
50-
l/Temp.
affect
Surface
’
1.64
FIGURE
oxide monolayer
7%V205/T10 .
:\’
60
z s^ ._ r? : E 0 is
vanadium
1
‘9000 80 70
Surface
anhydride
almost
suggests
by either The signifi-
that the
exclusively
intermediate
and enhance
step of
than the V205
[Z]. The V205 crystallites
of o-tolualdehyde
catalysts
over
is converted appear
the relative
to
formation
96 TABLE
2
Apparent Ti02
activation
(anatase)
energies
for the conversion
to products
over v205/
catalysts.
Investigators
Herten
of o-xylene
Apparent -1 mole
and Froment
(1968)
Calderbank
et al. (1977)
Wainwright
and Hoffmman
activation
energies/kcal
16 26 at 370-440°C 14 at 440-550°C
Bond and Konig Skrzypek
(1977)
27
(1982)
27
et al.
(1985)
20-26
Saleh and Wachs
(1986)
32
of phthalide
compared
tolualdehyde
to phthalic
vanadium
to phthalic anhydride
oxide monolayer
is associated
anhydride. appears
and the partial
with both the surface intermediate
this step apparently
can be performed
layer or the V205 crystallites. vanadium
that the partial mostly
associated
oxidation vanadium version
and V205 content of o-xylene
The above
reaction
reaction
are listed
in Table
An Arrhenius
suggests
V205
(anatase)
Previously
with
produced
vanadium
the over-
o-xylene
for moderate
The origin
not well
corresponding
con-
from the direct
oxide
catalysts.
is currently
is
with the surface
of this
understood.
reactive
sites
kinetics
to all reaction
activation
catalysts
gradients
oxidation,
to yield
lower [IO]. The somewhat
apparent
(anatase)
Diffusion
such as o-xylene
of o-xylene
products
catalyst is presented in Figure 6. An apparent -1 1s determined for the conversion of o-xylene reported
over V205/Ti02
the intrinsic
(anatase)
anhydride
1.
13-27 kcal mole-'.
reactions
of surface
area
suggests
Similarly,
is associated
and
oxide mono-
surface
to phthalic
COx selectivity
probable
anhydride
vanadium
and effective
that COx is primarily
steps and their most
and the V205 crystal-
oxide monolayer.
anhydride
unfortunately,
to phthalide
to V205 crystallites
intermediate
constant
in V205/Ti02
step,
of 32 f 2 kcal mole
of o-xylene from
The fairly
plot for the conversion
the 7% V205/Ti02
this catalyst,
vanadium
to maleic
over the monolayer
of crystalline
non-selective
energy
anhydride
oxide monolayer.
activity
of o-
over the surface
to phthalic
the surface
compared
of the phthalide
with the surface
of phthalic
oxidation amounts
oxidation
oxidized
by either
The higher
occur
oxidation
of o-tolualdehyde
oxide monolayer
is further
oxide monolayer
the partial
to primarily
oxidation
vanadium
lites. The phthalide
of the surface
Thus,
energies
are listed
and these gradients
apparent
lower apparent
activation activation
activation over
for the conversion
in Table
can be present
for
2 and range
in highly
exothermic
have been shown to mask
energies energies
that are significantly for o-xylene
conversion
97 reported suggest
by Herten
that diffusion
used by these however,
gradients
investigators.
energies.
lower apparent
non-isothermal
discussion
to minimize
conditions
residence
(references
energies,
27-32
absence
of phthalic
anhydride
and/or maleic
over V205/Ti02
Turnover
rates
(anatase)
(the number
are routinely
measured
chemisorption
techniques
atoms
between
chemisorption
techniques,
and the number The number
of exposed containing
supported
vanadium
information
vanadium
ly over the monolayer. 20
V205/Ti02
however,
report
and
high and the of o-
by metals.
and V205/Ti02
for supported
The total number
catalysts
over V205/Ti02
molecule
of vanadium
sites
conditions
known.
metal
oxide
the turnover
occurs
rates
[211. The us to determine
(anatase)
is given by the monolayer
sites for our experimental
oxides
in the case of
above also allow
of the o-xylene
has
are not generally
et al. determined
of o-xylene
an easy
The selective
do not work with metal
presented
oxide sites
of surface
and consequently
can be determined
conditions
selective
the number
and have been applied Murakami
site per second) because
rate is that it permits
of catalysis
over V205/A1203
since the oxidation
ponds to %2.5 x 10
to determine
usually
rate for the oxidation
oxide
per active
oxide catalysts
vanadium
study)
catalysts
investigators,
in metal
catalysts.
of reactions
of active
reacted
unfortunately,
oxide monolayers oxide
The kinetic products
at low conversions
sites
and experimental
the turnover The number
anhydride
metallic
to the progress
sites,
over V205/Ti02
network).
, for o-xylene conversion
of using the turnover
of exposed
catalysts
for a number
of molecules
the work of different
contributed
somewhat and/or
catalysts.
have been developed
significantly
-1
apparent
anhydride
time of the reaction
kcal mole
for many different
[20]. The advantage
comparison
of o-xylene
4,6 and the present
activation
systems
that reported
that phthalic
on reaction
apparent
xylene
in the reactor
some of the determined
also claimed
et al. [7]
in some of these catalysts,
the same investigators
at low conversions
(see earlier
that appeared
present
influenced
energies
were present
catalysts
et al. [3] and Skrzpek
may have been present
The promoters
Interestingly,
activation
anhydride
(anatase) studies
[I], Calderbank
may also have partially
activation
maleic
and Froment
catalysts.
of surface almost
in the monolayer of 1.96 gms of
exclusivecorres-
1.9%
(anatase).
The total amount of o-xylene converted for our experimental 17 corresponds to 3 x IO o-xylene molecules/set at 60% conversion which
conditions is achieved
at 325°C (see Figure 6). From these figures one obtains a turnover -3 -1 -1 o-xylene molecules site for the present experimental s
rate of $1 x 10 conditions.
The present
rate for o-xylene rates measured well
with
for
study
oxidation the
the turnover
reports
the first
over V205/TiO2
supported
vanadium
rates measured
determination
(anatase) oxide
for metal
of the turnover
catalysts.
The turnover
monolayer
catalysts
catalysts
[20].
agree
very
98 ACKNOWLEDGEMENTS The assistance catalysts,
of C.C. Chersich
and P. Chiasson
in the preparation
in obtaining
the catalytic
of the V205/TiO2
(anatase)
data are greatly
appreciated.
REFERENCES Herten and G.F. Froment, Ind.Eng. Chem., Process Des. Dev., 1 (1968) 516. D. Vanhove and M. Blanchard, Bull. Sot. Chim. (Paris) (1971) 3291; J. Catal., 36 (1975) 6. P.H. Calderbank, K. Chandrasekharan and C. Fumagalli, Chem. Eng. Sci., 32 (1977) 1435. M.S. Wainwright and N.R.Forster, Catal. Rev.-Sci. Eng., 19 (1979) 211; M.S. II, H.M. Wainwright and T.W. Hoffman, in Chemical Reaction Engineering, Hulbert, Ed., American Chemical Society, Washington, DC. (1974). A.A. Yabrov and A.A. Ivanov, React. Kinet. Catal. Lett., 14 (1980) 347. G.C. Bond and P. Konig, J. Catal., 77 (1982) 309. J. Skrzypek, M. Grzesik, M. Galantowicz and J. Solinski, Chem. Enp. Sci., 40 (1985) 611. I.E.Wachs, R.Y. Saleh, S.S. Chan and C.C. Chersich, Appl. Catal., 15 (1985) 339. Englewood Cliffs, M. Boudart, Kinetics of Chemical Processes, Prentice-Hall,
J.
5 6 7 8 9 10
NJ (1968). J.J. Carberry,
Chemical and Catalytic Reaction Engineering, McGraw-Hill, New York (1976). 11 G.C. Bond and K. Bruckman, Faraday Discuss., 72 (1981) 235. 12 A.J. Van Hengstum, J.G. Van Ommen, H. Bosch and P.J. Gellings, Appl. Catal., 8 (1983) 369. 13 M. Gasior, I. Gasior and B. Grzybowska, Appl. Catal., IO (1984) 87. 14 F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijn, J. Medema, V.H.J. de Beer and P.J. Gellings, J. Phys. Chem., 84 (1980) 2783. R. Kozlowski, R.F. Pettifer and J.M. Thomas, J. Phys. Chem., 87 (1983) 5176. is5 I.E. Wachs, S.S. Chan and R.Y. Saleh, J. Catal., 91 (1985) 366. 17 I.E. Wachs, S.S. Chan and C.C. Chersich, in Reactivity of Solids, Ed. P. Barret and L.C. Dufour, p. 1047, Elsevier, Amsterdam (1985). R.Y. Saleh, I.E. Wachs, S.S. Chan and C.C. Chersich, J. Catal., 98 (1986) 102. 1: K. Chandrasekharan and P.H. Calderbank, Chem. Eng. Sci., 35 (1980) 1523. 20 M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, NJ (1984). 21 Y. Murakami, M. Inomato, A. Miyamoto and K. Mori, Proc. 7th Int. Congr. Catal. Tokyo, (1980) 1344.
REACTION
NETWORK
AND KINETICS
OF 0-XYLENE
V,CJ,/TiO2 (ANATASE)
CATALYSTS
Ramzi Y. SALEH and
Israel E. WACHS*
Intermediates 08801,
Technology
Division,
OXIDATION
Exxon Chemical
87
TO PHTHALIC
Company,
ANHYDRIDE
Annandale,
OVER
New Jersey
U.S.A.
*Department
of Chemical
Bethlehem,
(Received
PA 18015,
13 August
Engineering,
Whitaker
Laboratory
#5, Lehigh
University,
U.S.A.
1986, accepted
22 December
1986).
ABSTRACT The reaction network of o-xylene oxidation to phthalic anhydride over V205/Ti02 (anatase) catalysts was determined in the present investigation. The detailed oxvlene conversion/oroduct selectivitv data obtained demonstrate that ohthalic anhydride is not produced from the d4rect oxidation of o-xylene over V205/Ti02 (anatase) catalysts, but indirectly via a series of consecutive reaction steps involving o-tolualdehyde and phthaiide intermediates. The maleic anhydride byproduct is produced from the over-oxidation of phthalic anhydride. The influence of the two reactive sites in V205/Ti02 (anatase) catalysts, surface vanadium oxide monolayer and V705 crystallites, upon the o-xylene oxidation reaction were obtained for moderate am6unt.s of vanadium oxide by varying the crystalline V2O5 content of the catalysts. The initial reaction step of the oartial oxidation of o-xvlene to o-tolualdehyde occurs almost exclusively over the surface vanadium oxide-monolayer. The o-tolualdehyde intermediate is directly converted to phthalic anhydride primarily over the surface vanadium oxide monolayer and to phthalide over both the surface vanadium oxide monolayer and the V205 crystallites. The partial oxidation of phthalide to phthalic anhydride mostly occurs over the monolayer of surface vanadium oxide. The over-oxidation of phthalic anhydride to maleic anhydride also occurs primarily over the monolayer of surface vanadium oxide. The non-selective formation of the combustion products, CO and CO2, primarily occurs from the direct oxidation of o-xylene over the monolayer of surface vanadium oxide. The turnover rate for o-xylene oxidation over V205/Ti02(anatase) catalvsts is determined for the first time, and found to be ~1 x low3 molecules site-1 s-1 for the experimental conditions used in the present study. The apparent activation energy for the conversion of o-xylene to all products is found to be %32 kcal mole-l.
INTRODUCTION The complex V205/Ti02
reaction
(anatase)
network
catalysts
[l-7]. A generalized
reaction
the probable
pathways
by different
investigators
propose
scheme
is presented
that the desired
of reaction
of o-xylene
for o-xylene
in Figure
are also shown phthalic
I,2 and 3, or 1 and 9) [2,4,6],
oxidation
1. The various 1 [l-7].
product
and phthalide
and other
to phthalic
of several
in Figure
anhydride
steps via o-tolualdehyde
0166-9834/87/$03.50
oxidation
has been the subject
anhydride
recent
which
includes
reaction
steps
can only be formed
0 1987 Elsevier Science Publishers B.V.
all proposed
Some investigators
intermediates
investigators
over
investigations
propose
by a series
(reaction
steps
that phthalic
88
0 Maleic Anhydride
0 Maleic
Reaction Investigators
1
Herten & Froment
& Foster
Yabrov & lvanov Skrzypek et al.
FIGURE
1
Generalized
(anatase)
4
5
X
x
x
x
x x
x x
x x
x
x
x
x
x
x
reaction
catalysts
3
X
Van Hove 8 Blanchard Calderbank et al. Wainwright
2
x
Steps
6
7
8
9
10
11
x
x
x
x
x
x
x
x
x x
x X
x x
x x
x
X
x
x
x
x
X
for o-xylene
reaction
12
X
x
network
and the various
Anhydride
x
oxidation
steps proposed
x
over V205/Ti02 by different
investi-
gators.
anhydride
can be formed
step 6) [1,3,5,7]. of the maleic direct
directly
Similar
anhydride
results
conditions. containing of these
(reaction
different
kinds of promoters
studies were
also performed
reactors
where
the observed
results.
In order
the oxidation
(anatase)
reaction
steps
catalysts,
difficulty
performed
transfer
catalysts Several
in commercial
may have affected
issues we decided over unpromoted
isothermal
or
experimental
V205/Ti02
conditions
limitations
anhydride
and under
with
types of carriers.
some of these
to phthalic
a carrier,
1,2,3,4)
in comparing
under very different
were
under non-isothermal
to resolve
of o-xylene without
(reaction
and different
heat and mass
(reaction
for the formation
is that they were performed studies
of o-xylene
have been proposed
step 12). The major
Some of these reaction
size tubular
examine
paths series
byproduct:
one step reaction
the above
in one step from the oxidation
reaction
reaction
to
V,O,/TiO2 conditions.
EXPERIMENTAL The unsupported
V205/Ti02
described
[8]. The TiO2
possessed
a surface
contain
(anatase)
(anatase)
catalysts
was obtained
were
prepared
from Mobay
area of Q9 m * g-'. The Ti02
(anatase)
as previously
Corporation, support
and
was found
0.15 wt% K, 0.10 wt% P, 0.10 wt% Al and 0.16 wt% Si as determined
absorption.
The V205/Ti02
in an aqueous
solution
excess water
was allowed
(anatase)
of oxalic
catalysts
to evaporate
dried at 110°C and calcined
were prepared
acid and impregnating
in oxygen
at %65"C. for 2 h
by dissolving
the titania
The catalysts at 450°C.
to
by atomic V205
support.
The
were subsequently
89 The performance xylene
of the V205/Ti02
was examined
were examined
(anatase)
in the reactor
for this reaction
catalysts
unit previously
with
1.25 mole%
velocity
of 2760 h-' and between
320 and 370°C.
obtained
by varying
temperature.
passed
through
the reaction
an o-xylene
bath. A slip stream multicolumn
analyzed
concentration
immersed
in a molten
effluent
was analyzed
reaction
products.
tolualdehyde, including present
during o.d.,
analyzed
maleic
anhydride
three
was always
anhydride, and benzoic
startup
and overnight
the center. throughout
catalyst
steel)
(corresponding
The remaining
inserted
volume
and 2 cm3 at the bottom). inert with respect
was filled
Blank
to o-xylene
oxidation
effluent
was The
with N2
located
(0.5 in.
at
the temperature
to the salt bath temperature was packed having
the reactor
with 2 cm3 of a particle
This catalyst
the length
with 3 mm glass
runs showed
they were
The reactor
monitored
(anatase)
along
products,
blanketed
in. thermowell
with 8 cm3 of 0.5 mm glass beads. profile
and the
feed analyses.
was usually
The reactor
to 1.96 g of V205/Ti02
to give an isothermal
reactor
the reactor
in the thermowell
the desired
anhydride,
because
the feed was being analyzed.
from the bottom.
of 0.4 - 0.7 mm) diluted
phthalic
ignored
by several
The
of the reactor
Other
5%. The reactor
while
After
conversions
were
on-line
to the reactor
CD, CO2 and water.
At each temperature
water
detector.
acid, were
bed. The feed was preheated
the reactor
ratio was found
observed
was fitted with a 0.125
A thermocouple the catalyst
and entered
within
components.
for o-xylene
products
rate was
by a calibrated
bath. A slip stream
to five times followed
balance
316 stainless
RESULTS
reaction
were
flow
controlled
the feed was diverted
Hi Tech)
in very small quantities.
typically carbon
The main
in conversion
conductivity
for all gases and organic
(DuPont
[8]. All catalysts
in a temperature
by the gas chromatograph
phthalide,
Changes
with a thermal
of o-
in air, at a space
feed was analyzed
equipped
was established, salt
citraconic
described
o-xylene
Air at the desired
immersed
of the o-xylenelair
gas chromatograph
gas chromatograph o-xylene
generator
for the oxidation
diameter
dilution
of the catalyst
beads
(2 cm3 at the top
walls
and beads to be
at the temperature
range
bed.
investigated.
AND DISCUSSION
The product
selectivity
7% V205/Ti02(anatase) 2. The conversion (see Figure the major product.
was varied
6). At very
reaction
Tar formation
phthalide
and maleic
by changing
approaching
very rapidly
bed
zero % conversion,
over V205/Ti02
and the phthalic
of o-xylene
to ~20%,
in Figure
of the catalyst
COx (CO2 and CO), and a tar by-
of o-xylene
are not present
is increased
decreases
of o-xylene,
for an unsupported
is presented
by Bond et al. [6]. In this low conversion
zero % as the conversion
the tar byproduct
conversion
conditions
the temperature
are o-tolualdehyde,
anhydride
of o-xylene
of o-xylene
isothermal
at low conversions
was also reported
vity approaches
under
low conversions
products
catalysts
conversion
as a function
catalyst
approaches
the selectivity
with o-xylene
(anatase) region
anhydride
selecti-
zero. As the
to o-tolualdehyde
conversion,
and
and the selecti-
90 90
8
I
I
,
,
,
,
I
,
7°hV~0d’TiO~ (Anatase)
90 -
O-Xylene Conversion, O? FIGURE
2
The product
V20S/Pi02(anatase)
vity to phthalic behavior
anhydride
of the o-xylene
tolualdehyde, oxidation, secondary Further
which
in the phthalide
and decreases
selectivity
formation.
However,
anhydride
production
with
tolualdehyde.
consistent
with
in a parallel of phthalide
increases
anhydride
anhydride formation
probably
the behavior [9,10].
reaction
results partial expected
The phthalide
reaction
path will be provided
varied).
The constant
conversion
where stream.
oxidation
at very
products
for a series
anhydride
selectivity
towards
high conversions
COx as a function
anhydride.
conversion
is
consecutive
is probably
evidence
below when the V205 content
found
from o-tolualde-
can be produced
(additional
that of o-
catalysts
to o-xylene
however,
is not present,
of phthalic
of first-order
intermediate,
for phthalic
oxidation
produced
that
anhydride
suggests
over V20S/Ti02
the over-oxidation
of o-xylene
suggests
phthalide This
towards
The maximum
responsible
from the partial
is only observed
step since phthalic
at low conversions
phthalide.
intermediate
oxidation
are the
reactant.
the selectivity
towards
are both directly
from
of o-xylene
anhydride
that leads to phthalic
in the product directly
products
The
that o-
from the o-xylene
o-xylene
is not the only
of o-tolualdehyde
of the o-xylene
steps
increasing
can be produced
and most
The response
of o-xylene
the selectivity
is present
and phthalic
hyde [Z]. Maleic of o-xylene
for 7%
conversion.
suggest
and maleic
since at very low conversions,
Studies
that phthalide
directly
in the reaction
phthalide
anhydride
anhydride
phthalide
are not derived
is an intermediate
phthalic
conversion
with o-xylene
with conversion
are the primary
anhydride,
in the conversion
anhydride,
some phthalic
of o-xylene
increases
products
COx and the tar byproduct
increase
phthalide
as a function
and phthalide
reaction
and that phthalic products
phthalic
reaction
selectivity
catalyst.
produced
independently
for this parallel
of the catalyst of o-xylene
is
conversion
91 Tar +
CO,+H,O
Maleic Anhydride
4 Phthalide
FIGURE 3
Reaction
V205/Ti02
(anatase)
suggests
that almost
little
network
all the COx is produced from the partially
by the significantly
tolualdehyde
over V205/Ti02
with supported
V205/Ti02
and by varying
conversion
The detailed present
anhydride,
V205/Ti02 steps.
o-xylene
investigation
maleic
results
3 for the oxidation catalysts.
based on their employed
experimental
by these authors,
of o-xylene conditions conditions. and Ivanov
over V205/Ti02 minimized
[5], at Skrzypek
directly
integral
fixed
fluidized
catalyst
regime
phthalic residence
An examination
et al. [7] suggest
microreactors
operating
[5]. It is, therefore,
anhydride
of o-xylene
that reaction
anhydride
over
over V205/Ti02
article with the
of the experimental
apparatus
to study the oxidation
products
These
[6]
that their experimental and non-isothermal et al. [33, Yarbov anhydride
studies,
catalyst-basket close
review
reaction
[Z] and Bond and Konig
that phthalic
reactors
that these
and maleic
anhydride
at low conversions
time of the partial
oxidation
products
can be
however,
to the continuous
not surprising
reaction
network
are also consistent
[I], Calderbank
of o-xylene.
spinning
in the
as well as
the most probable
suggests
time of the reaction
[1,7],
anhydride,
in the literature,
and Froment
conditions,
in the reactor.
in their comprehensive
findings.
from the oxidation
bed reactors
agree with
catalysts
were obtained
data obtained
study suggest
and Blanchard
of Herten
results
oxidation
by Vanhove
(anatase)
is subof o-
via a series of consecutive
[4]. These findings
as described
and that very
This
non-isothermal
to phthalic
findings
and Foster
residence
The results
produced
reactor
proposed
from the direct
of o-xylene
synthesis
kinetic
that phthalic
but indirectly
by Wainwright
networks
over
for the oxidation
selectivity
demonstrates
The present
from o-xylene
time of the gases
of the present
(anatase)
reaction
anhydride
intermediates.
[2]. Similar
conversion/product
clearly
anhydride
directly
oxidized
under moderately
via residence
in Figure
proposed
to phthalic
selectivities
catalysts
catalysts,
catalysts,
The kinetic
network
higher
is not produced
(anatase)
on phthalic
oxidation
catalysts.
COx is produced
stantiated
for o-xylene
stirred
studies
of o-xylene.
in the spinning
used
[33 and tank
observed The long
catalyst-basket
92
g 2
60
5
55 I
CATALYST COMPOSITION e 1.9% V,O,ITiO, (A) q 3 % V,06/Ti0, (A) l 7 % V,O,/TiO* (A) 0 21 % V,OJTiO, (A)
404
65
65
70
75
80
85
90
95
100
0-Xylene Conversion FIGURE
4
Influence
G8-oxygenate
of V205 content
(o-tolualdehyde,
[S] and fluidized
microreactor
intermediate
products
on the walls
of these reactors
presence
of external
conducted
in integral
be to further diffusion
convert
gradients
[I,71 are most maleic
(anatase) Recent
anhydride
gradients
in highly
the partial
characterization
studies
in V205/Ti02
(anatase)
(350-575°C)
[6,8,11-181.
These
the properties
crystallites
vanadium
(anatase)
catalysts
[IO]. Reaction
and reactant
The
concentrations
such as o-xylene effect
The presence integral
oxidation
oxidation, would
of external
fixed
of phthalic
o-xylene
oxide
support.
Thus,
vanadium
bed reactors
anhydride
and/or
over V205/Ti02
corresponds
of surface
increases
vanadium
V205/Ti02
oxide
small
oxide
content
the amount
oxide
study,
and crystalline
a monolayer
on the Ti02
of V205 are also
of the surface
vanadium
of V205/Ti02
(anatase)
oxide
of
[8]. Below mono-
are present
of vanadium
of
as small
area of the TiOp
crystallites
to the monolayer
modifies
a monolayer as well
(anatase)
species
oxide
temperatures
(anatase)
by forming
and the surface
coverage,
the vanadium
that Ti02
to the Ti02 support
to %1.9%
in addition
showed
of vanadium
at moderate
used in the present
vanadium
Above monolayer
above monolayer
amount
support
that two types
calcined
oxide phase
oxide content
(anatase)
increasing
have revealed catalysts
coordinated
only the surface
on the support
species.
species
the vanadium
For the Ti02
layer coverage,
present
oxide
of V205. The relative
V205 depends,on
surface
during
investigations
of the supported
vanadium
support.
products.
employing
of these
catalysts.
are present
surface
anhydride
[IO], and the resulting
for the presence
at low conversions
conversion
this situation.
reactions,
oxidation
responsible
exacerbate
upon the
selectivity.
in the further
in temperature
fixed bed reactors
catalysts
anhydride)
and/or maleic
exothermic
in the investigations
likely
anhydride
result
could further
diffusion
(anatase)
and phthalic
[5] would
to phthalic
are known to be present
of V205/Ti02
phthalide
present
oxide
as VZ05
93
Tolualdehyde
Phthalide
0 A
. .
1.9% V205/Ti02 (A)
Catalyst Composition
0
l
0
.
7% V2051Ti02 (A) 21% V105/Ti02 (A)
3% V205/Ti02 (A)
I
f 50
60
70
80
o-Xylene Conversion FIGURE
5
Influence
o-tolualdehyde
crystallites.
of V205 content
and phthalide
The previous
oxide was the active possesses
a higher
of o-xylene complete
because
of surface
the exposed
of the reaction
the catalytic
activity during
of this phase.
o-xylene
oxidation
vities
at partial
dramatic
drop
monolayer surface
conversion
coverage, vanadium
greater
complete
coverage
catalytic conversion conditions. sample,
of o-xylene Significant
decrease
kinetically underlying
significant surface
(anatase)
is approached amounts
anhydride
affect
selectivity)
area and poor catalytic
catalysts exhibit
Below monolayer improvement
is shown
very
in Figure
similar
coverage
there
suggests
cover
is a above
that the
all the exposed
titania
contents
are necessary
to assure
titania
sites.
The differences
in the
become
somewhat
more pronounced
due to the increased
of crystalline performance
oxide monolayer
severity
V205, 21% V205/Ti02
because possess
as complete
of the reaction (anatase)
the V205 c%-ystallites become a lower
[8,13,18].
4.
selecti-
in selectivity
(anatase),
selectivity
oxide
and intrinsically
vanadium
amounts
upon the C8-oxygenate
does not effectively
of these samples
Moderate
do not significantly
and phthalic
catalysts
higher vanadium
the catalytic
deposition).
surface
than 1.9% V205/Ti02
of the few remaining
performance
carbon
catalysts
[81. The slight
oxide monolayer
sites and that slightly
(combustion
of o-xylene.
in selectivity
inferior
reactions
of V205 content
(anatase)
very
in undesirable
conversion
over V205/Ti02
The 1.9%, 3% and 7% V205/Ti02
exhibit
less than a
selectivity)
of the low effective
The effect
possessing
however,
and that it
V205 for the oxidation
anhydride
(anatase)
(o-xylene
because
catalysts
vanadium
and phthalic
and possibly
in V205/Ti02
performance
upon the
of hydrocarbons
than crystalline
oxide,
conversion
sites result
intermediates
of such catalysts
(anatase)
vanadium
(o-xylene titania
V205
catalysts
that the surface
oxidation
and selectivity
V205/Ti02
performance
(anatase)
also demonstrated
site for the partial
[8,13,18].
of crystalline
studies
activity
monolayer
catalytic
of V205/Ti02
selectivities.
selectivity
than the
94 Additional
insights
phthalic
anhydride
reaction
studies
of V205 content in Figure
into the reaction
over V,O,/TiO,
5 as a function
addition (anatase).
These
are being compared
a monolayer
is slightly
of surface
to phthalic
anhydride
is also sensitive
for V,O,/TiO2
vanadium
mediate
was not observed
phthalide
vanadium
and phthalide
of o-xylene.
vanadium amounts
monolayer,
oxide monolayer,
phthalide
(anatase)
of phthalide
selectivity
V205 may be related
to the slightly
(anatase)
catalyst.
identical
o-toIua\dehyde
suggests
that in the presence
reaction
step is enhanced
relative
reaction
step. The strong
dependence
content
is probably
produced
V20,-/Ti02 (anatase) V205 content monolayer
catalysts.
(anatase)
conversions
its formation
is primarily
The slight oxide
but differing
oxide for moderate The maleic
is also independent associated
of the 21%
samples
o-xylene
amounts
anhydride
to phthalic
upon the V205
with the monolayer
intermediate
oxidation
constant
associated
selectivity
the
V205
observed
and suggests
of surface
over
with the with
of crystalline
of V205 content
This
anhydride
that the phthalide
is fairly
exhibit
selectivity.
selectivity
is primarily
in
from 7% to 21%
to phthalic
step during
much higher
decrease
the o-tolualdehyde
statement
containing oxide
exhibit
selectivity
of the phthalide
for
to the
vanadium
(anatase)
phthalide
and its
was detected
content
to the o-tolualdehyde
reaction
and these
The catalysts
(anatase),
lower $-oxygenate
The CO, selectivity
catalysts.
high o-xylene
stream.
that COx formation
vanadium
however,
(anatase).
the vanadium
the earlier
of
inter-
support
V205 in addition
to the surface
of V205 crystallites
in a parallel
and suggests
of surface
in V205/Ti02
supports
a monolayer
catalyst,
intermediate,
The 3% and 7% V205/Ti02
selectivity,
crystalline
the phthalide
of the titania
of crystalline
in the product
intermediates
Bond and Konig did detect
and 21% V205/Ti02
V205/Ti02
crystalline
approximately
nature
3% V205/Ti02
upon increasing
selectivity
than unsupported,
(anatase),
V205 in addition
of crystalline
in
since the catalysts
The phthalide
oxide monolayer
to the specific
small amounts
7% V205/Ti02
concentrations
catalysts
over the range examined.
[6]. The phthalide
containing
species,
V205
of V 0 crystallites, and 25 are in agreement with the obser-
containing
with their vanadium
impurities
the catalyst
oxide
crystalline
to the content
1.9% V205/TiO2
species,
may be related
associated
surface
oxide
formation
differences
(anatase)
[19]. For the catalyst
surface
of o-xylene,
lower for the
with V 0 content. These observations 25 made by Chandrasekharan and Calderbank that the reaction
V205 catalysts
higher
At a given conversion
containing
increases vation
from
3%, 7% and 21% V 0 /TiO 25 2 that the crystallites slightly retard the con-
at the same conversion
stream
are less evident
to
oxide monolayer,
suggest
of' o-tolua'ldehyde
in the product
oxidation
are also obtained
stream
than the catalysts
vanadium
results
conversion.
in the product
approximately
(anatase),
to the surface
version
of o-xylene
selectivity
containing
1.9% V205/Ti02
of o-xylene
catalysts
in which the V 0 content of the catalyst is varied. The influence 25 upon the o-tolualdehyde and phthalide selectivities are presented
the o-tolualdehyde catalyst
network
(anatase)
at that
vanadium
oxide.
95 TABLE
1
0-xylene
oxidation
moderate
amounts
Reaction
step
o-xylene
+
reaction
steps and their corresponding
of crystalline
V20S in V205/Ti02
reactive
(anatase)
Most Probable
+
o-tolualdehyde
-f phthalide
phthalic
anhydride
reactive
sites
Surface
vanadium
Surface
vanadium
oxide monolayer
Surface
vanadium
oxide monolayer
o-tolualdehyde
o-tolualdehyde
sites for
catalysts.
oxide monolayer
and V205 crystallites phthalide phthalic
-f phthalic anhydride
anhydride
-f maleic and COx
o-xylene
+
COx
1
21
*0-
ii! s!
10
E,I
- 32 22 I kcalfmole , 1.68
1.66
plot for o-xylene
study
reveals
the o-xylene
oxidation
the surface
vanadium
the surface to either slightly
of a-xylene
of o-xylene
vanadium
retard
the conversion
with temperature
in V205/Ti02
network.
vanadium
of o-xylene
reaction
can be performed
however,
occurs
The o-tolualdehyde
or phthalide
[8,13].
oxide monolayer
[8,13],
for the 7%
(anatase)
The initial
or the V206 crystallites
to o-tolualdehyde
oxide monolayer.
anhydride
1.78
1.76
x IO-3,“K
to o-tolualdehyde
of the surface
for the conversion
1.74
conversion
reaction
oxide monolayer
activity
phthalic
-
-
1.72
that the two sites
oxidation
the partial
oxidation
2 (Anatase)
\,
1.70
catalyst.
crystallites
1
\
(anatase)
partial
1
30-
Arrhenius
higher
oxide monolayer
40-
6
cantly
oxide monolayer
vanadium
%
V205/Ti02
The above
vanadium
Surface
50-
l/Temp.
affect
Surface
’
1.64
FIGURE
oxide monolayer
7%V205/T10 .
:\’
60
z s^ ._ r? : E 0 is
vanadium
1
‘9000 80 70
Surface
anhydride
almost
suggests
by either The signifi-
that the
exclusively
intermediate
and enhance
step of
than the V205
[Z]. The V205 crystallites
of o-tolualdehyde
catalysts
over
is converted appear
the relative
to
formation
96 TABLE
2
Apparent Ti02
activation
(anatase)
energies
for the conversion
to products
over v205/
catalysts.
Investigators
Herten
of o-xylene
Apparent -1 mole
and Froment
(1968)
Calderbank
et al. (1977)
Wainwright
and Hoffmman
activation
energies/kcal
16 26 at 370-440°C 14 at 440-550°C
Bond and Konig Skrzypek
(1977)
27
(1982)
27
et al.
(1985)
20-26
Saleh and Wachs
(1986)
32
of phthalide
compared
tolualdehyde
to phthalic
vanadium
to phthalic anhydride
oxide monolayer
is associated
anhydride. appears
and the partial
with both the surface intermediate
this step apparently
can be performed
layer or the V205 crystallites. vanadium
that the partial mostly
associated
oxidation vanadium version
and V205 content of o-xylene
The above
reaction
reaction
are listed
in Table
An Arrhenius
suggests
V205
(anatase)
Previously
with
produced
vanadium
the over-
o-xylene
for moderate
The origin
not well
corresponding
con-
from the direct
oxide
catalysts.
is currently
is
with the surface
of this
understood.
reactive
sites
kinetics
to all reaction
activation
catalysts
gradients
oxidation,
to yield
lower [IO]. The somewhat
apparent
(anatase)
Diffusion
such as o-xylene
of o-xylene
products
catalyst is presented in Figure 6. An apparent -1 1s determined for the conversion of o-xylene reported
over V205/Ti02
the intrinsic
(anatase)
anhydride
1.
13-27 kcal mole-'.
reactions
of surface
area
suggests
Similarly,
is associated
and
oxide mono-
surface
to phthalic
COx selectivity
probable
anhydride
vanadium
and effective
that COx is primarily
steps and their most
and the V205 crystal-
oxide monolayer.
anhydride
unfortunately,
to phthalide
to V205 crystallites
intermediate
constant
in V205/Ti02
step,
of 32 f 2 kcal mole
of o-xylene from
The fairly
plot for the conversion
the 7% V205/Ti02
this catalyst,
vanadium
to maleic
over the monolayer
of crystalline
non-selective
energy
anhydride
oxide monolayer.
activity
of o-
over the surface
to phthalic
the surface
compared
of the phthalide
with the surface
of phthalic
oxidation amounts
oxidation
oxidized
by either
The higher
occur
oxidation
of o-tolualdehyde
oxide monolayer
is further
oxide monolayer
the partial
to primarily
oxidation
vanadium
lites. The phthalide
of the surface
Thus,
energies
are listed
and these gradients
apparent
lower apparent
activation activation
activation over
for the conversion
in Table
can be present
for
2 and range
in highly
exothermic
have been shown to mask
energies energies
that are significantly for o-xylene
conversion
97 reported suggest
by Herten
that diffusion
used by these however,
gradients
investigators.
energies.
lower apparent
non-isothermal
discussion
to minimize
conditions
residence
(references
energies,
27-32
absence
of phthalic
anhydride
and/or maleic
over V205/Ti02
Turnover
rates
(anatase)
(the number
are routinely
measured
chemisorption
techniques
atoms
between
chemisorption
techniques,
and the number The number
of exposed containing
supported
vanadium
information
vanadium
ly over the monolayer. 20
V205/Ti02
however,
report
and
high and the of o-
by metals.
and V205/Ti02
for supported
The total number
catalysts
over V205/Ti02
molecule
of vanadium
sites
conditions
known.
metal
oxide
the turnover
occurs
rates
[211. The us to determine
(anatase)
is given by the monolayer
sites for our experimental
oxides
in the case of
above also allow
of the o-xylene
has
are not generally
et al. determined
of o-xylene
an easy
The selective
do not work with metal
presented
oxide sites
of surface
and consequently
can be determined
conditions
selective
the number
and have been applied Murakami
site per second) because
rate is that it permits
of catalysis
over V205/A1203
since the oxidation
ponds to %2.5 x 10
to determine
usually
rate for the oxidation
oxide
per active
oxide catalysts
vanadium
study)
catalysts
investigators,
in metal
catalysts.
of reactions
of active
reacted
unfortunately,
oxide monolayers oxide
The kinetic products
at low conversions
sites
and experimental
the turnover The number
anhydride
metallic
to the progress
sites,
over V205/Ti02
network).
, for o-xylene conversion
of using the turnover
of exposed
catalysts
for a number
of molecules
the work of different
contributed
somewhat and/or
catalysts.
have been developed
significantly
-1
apparent
anhydride
time of the reaction
kcal mole
for many different
[20]. The advantage
comparison
of o-xylene
4,6 and the present
activation
systems
that reported
that phthalic
on reaction
apparent
xylene
in the reactor
some of the determined
also claimed
et al. [7]
in some of these catalysts,
the same investigators
at low conversions
(see earlier
that appeared
present
influenced
energies
were present
catalysts
et al. [3] and Skrzpek
may have been present
The promoters
Interestingly,
activation
anhydride
(anatase) studies
[I], Calderbank
may also have partially
activation
maleic
and Froment
catalysts.
of surface almost
in the monolayer of 1.96 gms of
exclusivecorres-
1.9%
(anatase).
The total amount of o-xylene converted for our experimental 17 corresponds to 3 x IO o-xylene molecules/set at 60% conversion which
conditions is achieved
at 325°C (see Figure 6). From these figures one obtains a turnover -3 -1 -1 o-xylene molecules site for the present experimental s
rate of $1 x 10 conditions.
The present
rate for o-xylene rates measured well
with
for
study
oxidation the
the turnover
reports
the first
over V205/TiO2
supported
vanadium
rates measured
determination
(anatase) oxide
for metal
of the turnover
catalysts.
The turnover
monolayer
catalysts
catalysts
[20].
agree
very
98 ACKNOWLEDGEMENTS The assistance catalysts,
of C.C. Chersich
and P. Chiasson
in the preparation
in obtaining
the catalytic
of the V205/TiO2
(anatase)
data are greatly
appreciated.
REFERENCES Herten and G.F. Froment, Ind.Eng. Chem., Process Des. Dev., 1 (1968) 516. D. Vanhove and M. Blanchard, Bull. Sot. Chim. (Paris) (1971) 3291; J. Catal., 36 (1975) 6. P.H. Calderbank, K. Chandrasekharan and C. Fumagalli, Chem. Eng. Sci., 32 (1977) 1435. M.S. Wainwright and N.R.Forster, Catal. Rev.-Sci. Eng., 19 (1979) 211; M.S. II, H.M. Wainwright and T.W. Hoffman, in Chemical Reaction Engineering, Hulbert, Ed., American Chemical Society, Washington, DC. (1974). A.A. Yabrov and A.A. Ivanov, React. Kinet. Catal. Lett., 14 (1980) 347. G.C. Bond and P. Konig, J. Catal., 77 (1982) 309. J. Skrzypek, M. Grzesik, M. Galantowicz and J. Solinski, Chem. Enp. Sci., 40 (1985) 611. I.E.Wachs, R.Y. Saleh, S.S. Chan and C.C. Chersich, Appl. Catal., 15 (1985) 339. Englewood Cliffs, M. Boudart, Kinetics of Chemical Processes, Prentice-Hall,
J.
5 6 7 8 9 10
NJ (1968). J.J. Carberry,
Chemical and Catalytic Reaction Engineering, McGraw-Hill, New York (1976). 11 G.C. Bond and K. Bruckman, Faraday Discuss., 72 (1981) 235. 12 A.J. Van Hengstum, J.G. Van Ommen, H. Bosch and P.J. Gellings, Appl. Catal., 8 (1983) 369. 13 M. Gasior, I. Gasior and B. Grzybowska, Appl. Catal., IO (1984) 87. 14 F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijn, J. Medema, V.H.J. de Beer and P.J. Gellings, J. Phys. Chem., 84 (1980) 2783. R. Kozlowski, R.F. Pettifer and J.M. Thomas, J. Phys. Chem., 87 (1983) 5176. is5 I.E. Wachs, S.S. Chan and R.Y. Saleh, J. Catal., 91 (1985) 366. 17 I.E. Wachs, S.S. Chan and C.C. Chersich, in Reactivity of Solids, Ed. P. Barret and L.C. Dufour, p. 1047, Elsevier, Amsterdam (1985). R.Y. Saleh, I.E. Wachs, S.S. Chan and C.C. Chersich, J. Catal., 98 (1986) 102. 1: K. Chandrasekharan and P.H. Calderbank, Chem. Eng. Sci., 35 (1980) 1523. 20 M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, NJ (1984). 21 Y. Murakami, M. Inomato, A. Miyamoto and K. Mori, Proc. 7th Int. Congr. Catal. Tokyo, (1980) 1344.