Reaction scheme of o-xylene oxidation on vanadia catalyst

Article abstract of DOI:10.1016/j.apcata.2011.03.006

The oxidation of o-xylene (OX) to phthalic anhydride (PA) is one of the important industrial processes based on selective oxidation reactions. However, the fundamental understanding of the by-product formation is still an open task. By using a sample port pilot plant, a detailed investigation was conducted for the first time of the by-product formation at different operation conditions. Several hitherto unknown intermediates could unambiguously be identified. The combination of process conditions and by-product formation enables the discussion of a new improved reaction scheme for the catalytic oxidation of o-xylene. The reaction path from OX to PA is commonly described by a rake mechanism consisting of a number of parallel and serial reactions. Tolualdehyde and phthalide are seen as the main intermediates. The most important by-products are maleic anhydride (MA), CO and CO₂. The reaction paths towards these by-products are widely unknown. Several gas phase components, such as toluquinone and 2,3-dimethyl-p-benzoquinone not reported yet in the current literature, were observed for the first time in this study. Most of these previously unknown components are reaction intermediates, which later do not form the desired reaction product, PA.

Full text of DOI:10.1016/j.apcata.2011.03.006

Applied Catalysis A: General 398 (2011) 37–43
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Reaction scheme of o-xylene oxidation on vanadia catalyst
Robert Marxa, Hans-Jörg Wölka, Gerhard Mestla,∗, Thomas Turekb
a Süd-Chemie AG, Lenbachplatz 6, D-80333 Munich, Germany
b Technische Universität Clausthal, Institut für Chemische Verfahrenstechnik, Leibnitzstrasse 17, D-38678 Clausthal-Zellerfeld, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of o-xylene (OX) to phthalic anhydride (PA) is one of the important industrial processes
based on selective oxidation reactions. However, the fundamental understanding of the by-product for-
mation is still an open task. By using a sample port pilot plant, a detailed investigation was conducted for
the first time of the by-product formation at different operation conditions. Several hitherto unknown
intermediates could unambiguously be identified. The combination of process conditions and by-product
formation enables the discussion of a new improved reaction scheme for the catalytic oxidation of
o-xylene.
Received 24 January 2011
Received in revised form 3 March 2011
Accepted 3 March 2011
Available online 10 March 2011
In honor of Helmut Knözingers 75th
birthday.
The reaction path from OX to PA is commonly described by a rake mechanism consisting of a number
of parallel and serial reactions. Tolualdehyde and phthalide are seen as the main intermediates. The
most important by-products are maleic anhydride (MA), CO and CO2. The reaction paths towards these
by-products are widely unknown.
Several gas phase components, such as toluquinone and 2,3-dimethyl-p-benzoquinone not reported
yet in the current literature, were observed for the first time in this study. Most of these previously
unknown components are reaction intermediates, which later do not form the desired reaction product,
PA.
Keywords:
Phthalic anhydride
Vanadia
Partial oxidation
Mechanism
Kinetics
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
last decades the development went more and more towards the
application of egg-shell catalysts with an inactive ceramic carrier,
With an annual production of 4.5 million tons in 2005, phthalic
anhydride is a significant commodity in chemical industry with
applications in the manufacture of phthalate plasticizers, phthalo-
cyanine dyes, polyester resins and numerous fine chemicals [1].
Historically, the feedstock for phthalic anhydride production was
naphthalene, which has gradually been replaced by o-xylene
throughout the past 50 years.
In the industrial process, the oxidation of o-xylene is conducted
in fixed bed tubular reactors with up to 30,000 tubes. Reaction
temperatures range from 300 to 450 ◦C with o-xylene feed con-
centrations between 0.5 vol% and 1.8 vol% in air. The reaction is
conducted at nearly atmospheric pressure and the cooling temper-
ature, typically adjusted by a molten saltbath, ranges from 340 ◦C
to 390 ◦C [2,3].
which the active mass is fixed to [6]. In addition, the industrial cat-
alytic systems were gradually modified by adjusting the catalytic
behavior dependent on the axial position in the reactor tube. Nowa-
days, the industrial catalyst consists of up to four catalyst layers
with optimized activities and selectivities. Modern catalytic sys-
tems for this reaction, such as the Süd-Chemie PHTHALIMAXTM
benchmark, allow molar selectivities of up to 83%. Considering the
quantities produced on an industrial scale, even an increase of 1%
in selectivity has a substantial economic effect.
In the course of this reaction 12 bonds need to be broken and
12 new bonds are formed [7]. Consequently, the reaction does not
comprise only a single step, but passes through a number of inter-
mediates. In addition, by-products such as maleic anhydride (MA),
CO and CO2 account for the loss in selectivity. However, in spite
of the industrial importance of this process and numerous studies
[8–16] in this respect, the reaction scheme still lacks a number of
linking elements. Especially the formation of non-selective oxida-
tion products is yet not well understood. In this communication any
oxidation product, which cannot be converted to PA in a subsequent
reaction path is considered non-selective.
Catalysts most widely applied for the reaction of both naph-
thalene and o-xylene consist of vanadium and titanium oxides.
Historically, vanadium oxide catalysts were applied as bulk cat-
alysts [4,5] and loaded to industrial reactors as extrudates. This
resulted in poor selectivities and limited catalyst lifetimes. In the
The by-product formation is not only crucial in terms of selec-
tivity, i.e. PA yield, but also in terms of plant product quality.
Although the largest fraction of annual PA production is utilized for
∗
Corresponding author. Tel.: +49 8061 4903825; fax: +49 8061 4903704.
E-mail address: gerhard.mestl@sud-chemie.com (G. Mestl).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.03.006

38
R. Marx et al. / Applied Catalysis A: General 398 (2011) 37–43
While the focus of Bond [7] lay on the investigation of the
Nomenclature
reaction mechanism on the catalyst surface, the reaction scheme
suggested consists of a rake mechanism with the main intermedi-
ates detectible in the gas phase (paths 1 and 5–8 in Fig. 1) along
with surface species thereof.
AAc
BA
BAc
BQ
BZ
acetic acid (C2H4O2)
benzaldehyde (C7H6O1)
benzoic acid (C7H6O2)
benzoquinone (C6H4O2)
benzene (C6H6)
Saleh and Wachs [11] have conducted a study of the reac-
tion network based on conversion selectivity profiles obtained
by controlling different reaction temperatures. According to their
experimental results, MA is formed mainly by oxidation of PA (path
15 in Fig. 1). Possible intermediates in this path are not indicated.
The phthalic anhydride formation path in o-xylene oxidation is
quite well described and experimentally backed. In contrast, the
formation paths of by-products such as benzoic acid, maleic anhy-
dride, CO and CO2 and especially their sources are widely unknown.
This communication is aimed towards giving more insight in
the oxidation paths of o-xylene and its intermediates, especially
considering by-product formation.
CA
citraconic anhydride (C5H4O3)
DMBQ 2,3-dimethyl-p-benzoquinone (C8H8O2)
DMMA 2,3-dimethyl maleic anhydride (C6H6O3)
HQ
MA
oX
hydroquinone (C6H6O2)
maleic anhydride (C4H2O3)
o-xylene (C8H10
)
PA
PAld
PH
phthalic anhydride (C8H4O3)
phthalaldehyde (C8H6O2)
phthalide (C8H6O2)
phenol (C6H6O)
PL
TA
tolualdehyde (C8H8O)
toluic acid (C8H8O2)
toluene (C7H8)
2. Experimental
TAc
TOL
TQ
2.1. Reactor
toluquinone (C7H6O2)
The reaction was carried out in a continuous, pilot scale, sin-
gle tube fixed bed sample port reactor, supplied by MAN/DWE.
With a length of 4 m and an inner tube diameter of 25 mm, reac-
tor dimensions are equivalent to industrial conditions. The cooling
temperature is adjusted by means of a molten salt bath, which is
stirred to ensure isothermicity and mixing. Flow rates of air are
measured by mass flow controllers with thermal measurement
principle (Brooks), while the organic feed stream is controlled by a
Coriolis-type mass flow controller (Brooks). Flow measurement of
the liquid stream is therefore independent of its composition. The
organic reaction feed is vaporized by a preheated hot air flow in a
separate self-constructed evaporator. Mixing of the feed gas stream
is ensured by an inert bed prior to the catalyst bed. In analogy to
the industrial process, the reaction product, crude PA is collected
in switch condensers, which are operated in parallel with typical
cycle times of 48 h. Cooling agent is water during operation at 50 ◦C,
while the condensers can be heated with steam at 160 ◦C in order
to melt and recover the product.
the production of PVC, a number of other applications especially
also in pharmaceutical industry, require higher product purities.
The knowledge of by-product formation paths can also help in this
respect, since any impurity that is not formed, later does not need
to be removed from the final product.
The majority of studies engaged with identifying the o-xylene
reaction scheme have been conducted with small lab-scale appara-
tuses, which are limited in the concentrations of intermediates and
by-products produced. The pilot-scale sample port reactor devel-
oped and applied in this research in combination with an industrial
catalyst offers promising opportunities in this respect. Both theo-
retical and experimental approaches have been made in order to
better understand oxidation paths of o-xylene.
It is commonly accepted that tolualdehyde (TA) and phthalide
(PH) are the main intermediates in the selective o-xylene (oX) oxi-
dation to phthalic anhydride (PA).
Reaction temperatures are measured by means of a multiposi-
tion thermocouple positioned in a 3 mm thermo-well in the center
of the reactor tube (Fig. 2).
Bernardini et al. [8,17,18] succeeded in directly oxidizing all
intermediates and by-products previously identified in the o-
xylene oxidation in presence of a vanadium oxide catalyst. Selective
oxidation products include PA, PH, toluic acid (TAc), TA and methyl-
benzylalcohol. Also non-selective oxidation to CO, CO2 and MA
is observed. On the same vanadia catalyst, TAc oxidation yields
MA, citraconic anhydride (CA) and benzoic acid (BAc), which are
commonly known impurities in crude PA (reaction paths 12–14 in
Fig. 1).
Blanchard and Vanhove [19] have studied the reaction mech-
anism by radioactive tracing of methyl groups of o-xylene. Apart
from intermediates and by-products mentioned above, dimethyl-
maleic anhydride was identified. Due to lacking radioactivity of
MA, they concluded that MA formation, no matter from which
source occurs by oxidative attack of the aromatic ring. A theoretical
MA formation path via quinones (benzoquinone (BQ), toluquinone
(TQ) and 2,3-dimethyl-p-benzoquinone (DMBQ)) is postulated in
analogy to benzene [20–22] and toluene oxidation [22,23] paths
observed on vanadia catalyst.
The 14 sample ports (including reactor inlet and outlet) are con-
nected to an analysis station where both the organic compounds
and the remaining gas phase compositions are analyzed online. The
transfer lines to the analysis station are heated by a heat exchanger
using oil as heating medium at a temperature of 250 ◦C. A potential
homogeneous reaction in the sampling lines at this temperature
was investigated through several tests without catalyst filling and
is regularly checked by closely analyzing the gas composition at
the reactor inlet. O-xylene conversions in these tests are inferior
to 1% with CO and CO2 as only products. The carbon balance was
regularly closed with deviation of 1–2%.
Analysis of organic compounds is conducted by a standard gas
chromatograph (Agilent 6820N) using a capillary column (Zebron
ZB-5, 60 m). After the sample loop for the gas chromatograph, the
sample stream is passed through a series of condensers to clean it
from heavy components. Concentrations of total oxidation prod-
ucts (CO and CO2) as well as oxygen in the remaining gas phase
are analyzed in an infrared analyzer (Emerson NGA2000) with a
paramagnetic channel for oxygen.
Recently, Ballarini et al. [12] reported benzoic acid, phthalic acid
(PAc) and phthalaldehyde (PAld) as intermediates or by-products
in o-xylene oxidation. The involvement of PAld and PAc in the reac-
tion scheme was investigated by feeding PAld dissolved in toluene
comparing the product spectrum to that of toluene oxidation. A
secondary reaction path towards PA, paths 9–11 in Fig. 1, can be
thereby derived.
2.2. Identification of intermediates
In order to identify unknown intermediates, samples of the
organic composition of the gas stream were taken at different

R. Marx et al. / Applied Catalysis A: General 398 (2011) 37–43
39
Fig. 1. Reaction scheme of o-xylene oxidation with experimentally confirmed reaction paths as reported in the literature, compare to sources [7–12].
sample ports by means of a cooling finger suspended in a cold trap
with a mixture of isopropanol and dry ice. The gas sample is passed
through cooled acetone or pentanone, in which the organic com-
pounds within the gas stream are dissolved. Samples are analyzed
offline by a standard GC (5890II, Hewlett Packard) equipped with a
mass spectrometer (5971A, Hewlett Packard). Mass spectral iden-
tification was carried out applying corresponding data supplied in
the NIST database [24].
which can be considered the most selective of catalysts currently
available on the market. It consists of V2O5 supported on TiO2 as
an eggshell catalyst on an inert carrier. Four different catalyst lay-
ers were filled in the reactor. The total length of the catalyst bed
applied in this investigation was 350 cm. The catalyst bed is not
diluted with inert material.
Calcination and formation of the catalyst were conducted
according to the procedures described in the literature [28,29].
2.3. Catalyst
2.4. Dosage experiments
The catalyst applied in this investigation is the industrial
PHTHALIMAXTM S4 catalyst supplied by Süd-Chemie [6,25–27],
In order to determine oxidation paths of each of the interme-
diates, a series of dosage experiments was conducted. In each of
Fig. 2. Simplified flowsheet of the fixed bed sample port reactor set-up.

40
R. Marx et al. / Applied Catalysis A: General 398 (2011) 37–43
Fig. 4. Conversion selectivity plot of o-xylene oxidation where TA (♦) has a pro-
file of a primary intermediate, PH (×) that of a secondary intermediate and PA (ꢀ)
the profile of a final product; CO (᭹) and CO2 (ꢁ) are produced mainly in parallel
reactions to the selective oxidation.
Fig. 3. Typical temperature profile recorded during the equilibration phase.
these experiments a given intermediate was dissolved in o-xylene
and the organic solution was fed to the reactor. The distinction of
o-xylene and intermediate selectivities was conducted by compar-
ing the concentration profiles of the respective dosage experiment
with those of a reference experiment conducted with only o-xylene
feed at the same total organic inlet concentration just prior to the
dosage experiment.
Total concentration of the organics within the feed gas ranged
from 0.5 to 1.5 mole%, with an air flowrate of constant 4 N m3/h
while the cooling temperature was controlled to values between
340 ◦C and 390 ◦C. The pilot reactor was conducted at non-
isothermal conditions which resulted hotspots of up to 465 ◦C.
Inlet concentrations of intermediates were up to 10 mole% in
o-xylene for those components which have good solubility in
o-xylene and the respective solubility limit for those with lower sol-
ubility. Higher intermediate concentrations proved to significantly
change the temperature profiles, interfering with the comparability
of reference and dosage experiments. Intermediate concentrations
between 2% and 10% allowed quantification by means of product
selectivities with respect to the intermediate under scrutiny. Gen-
erally, the concentrations of the additionally dosed components
in the feed gas stream were kept within the range of their maxi-
mum intermediate concentrations during the industrial o-xylene
oxidation process.
of 2.3% at nearly full conversion, leading to the conclusion that
it is a final product, which is produced in a consecutive reaction.
According to its selectivity profile with a maximum at about 20%
conversion, TAc is also a secondary intermediate.
BAc selectivity shows a very interesting selectivity profile. It
reaches a maximum at lower conversions to subsequently decrease.
However, at conversions above 60%, BAc selectivity begins again to
rise. This leads to the conclusion, that BAc is produced via multiple
reaction paths.
The selectivity profile of CA shows a broad maximum where
it remains nearly constant at conversions between 20% and 80%.
However, at high conversions, the CA selectivity decreases to reach
a value close to zero at full conversion.
Beyond the literature known by-products and intermedi-
ates, acetic acid (AAc), toluene (TOL), TQ and 2,3-dimethyl-
p-benzoquinone (DMBQ) were identified as intermediates of
o-xylene oxidation, which have previously never been reported of
in the literature on this reaction.
DMBQ has a high selectivity at very low conversions, which first
sharply and then only slightly decreases to reach zero at full con-
version. This component is therefore a primary intermediate to
non-selective o-xylene oxidation.
3. Results and discussion
3.1. Intermediates
After complete activation the catalyst, a typical hotspot profile
as shown in Fig. 3 can be observed during steady-state operation
of the pilot reactor. Depending on the actual operation conditions,
hotspots form of up to 465 ◦C.
A typical conversion selectivity plot of the main components
recorded in o-xylene oxidation is depicted in Fig. 4. The PA selec-
tivity rises with conversion to reach a value above 80% at 95%
conversion. TA has a high selectivity at low conversions which then
decreases to zero at nearly full conversion. The shape of the PH
selectivity profile, showing a distinct maximum, can be attributed
to the fact that it is a secondary intermediate [30]. Both CO and
CO2 selectivities remain nearly constant throughout the course of
the reaction. However, they slightly rise to reach a lumped value of
about 15 mole% at high conversions.
Fig. 5. Conversion selectivity plot of the components with low concentrations;
dimethyl-p-benzoquinone (×) yields the run of a primary intermediate, TAc (ꢀ),
TQ (ꢂ) and CA (ꢁ) that of secondary intermediates and MA (᭹) the run of a final
product; the run of the BAc profile (♦) shows two maximums.
Selectivities of components with lower concentrations are
shown in Fig. 5. The most important component in this respect is
MA. Its selectivity rises with o-xylene conversion to reach a value

R. Marx et al. / Applied Catalysis A: General 398 (2011) 37–43
41
Fig. 7. Selectivities of toluene oxidation (I); BAc (ꢀ) and BQ (ꢃ) are intermediates
while CO2 (ꢁ), CO (᭹) and MA (ꢂ) are final products in toluene oxidation; about 25%
of toluene is converted to MA, while 65% is totally oxidized to CO and CO2.
Fig. 6. Reaction scheme of toluene oxidation as reported by Andersson [23].
selectivities in toluene oxidation are especially BAc, but also BQ.
AAc can be determined as final product (Fig. 8), while TA and
CA show minor intermediate selectivities. Additional components
detected in traces by GC/MS analysis are benzaldehyde (BA), ben-
zene (Bz) and phenol (PL).
In order to further refine and confirm the reaction scheme, the
main intermediates of toluene oxidation, BAc, TQ, BQ and CA were
separately added to the reactor feed.
The TQ selectivity shows the typical run shape of a secondary
intermediate. The maximum in selectivity is reached at an o-
xylene conversion of 40%. At higher conversions it is completely
consumed.
Small traces of toluene can also be detected as an impurity in
the o-xylene feed. However, the concentrations of toluene rise sig-
nificantly along the reactor length and decrease again to zero at
high o-xylene conversions. Taking into account the repeatedly high
concentrations of toluene oxidation products such as toluquinone,
benzoic acid or citraconic anhydride, it is safe to conclude that
toluene is an intermediate of o-xylene oxidation.
3.3. Supplementary dosage experiments
The main products of BAc oxidation on said catalytic system
are MA and CO2 while BQ shows selectivities of an intermedi-
ate component. Additional products identified in traces comprise
hydroquinone, phenol and benzene.
The general comportment of the product and intermediate
selectivities has been reproduced in over 100 measurements at
different operating conditions.
Products of BQ oxidation are MA, CO and CO2. Intermediates
could not be established. GC/MS analysis of reaction products also
did not yield any unexpected components specific to BQ oxidation.
TQ oxidation products comprise CO, CO2, MA and CA, with CA
showing the concentration profile of an intermediate. In addition,
AAc showed increasing selectivities at nearly complete TQ conver-
sion. Selectivities towards BQ could not be observed. Also GC/MS
analysis of the gas sample did not yield any unexpected compo-
nents.
In order to identify oxidation paths of the different reaction
intermediates, a series of dosage experiments was conducted. The
catalyst applied for this investigation, was completely activated
according to the above method and has gone through several weeks
of steady state operation at the design inlet concentration of 1.5%
o-xylene in air.
3.2. Toluene dosage experiments
Toluene seems to be a key intermediate in identifying forma-
tion paths of the by-products from overoxidation like BAc and MA.
Although the selectivities to toluene are very small, a number of
further intermediates, such as TQ, CA or BAc which themselves
have significant selectivities are reported of in the literature as
products or intermediates of toluene oxidation on vanadia catalyst
[22,23,31].
Fig. 6 shows a toluene oxidation scheme based on experimental
data as reported by Andersson [23]. Toluene accordingly decom-
poses via two possible paths, either by a direct oxidative attack
of the aromatic ring (path 8 in Fig. 6) or by an attack of the
methyl group (path 1) and subsequent oxidation of the aromatic
ring. The former path yields CA as product with TQ as an inter-
mediate, while the latter leads to MA through a series of different
intermediates.
The oxidation of toluene added to the feed over the applied
catalyst yields results which are in good accordance with the
presented literature data. The main final selectivities (Fig. 7) of
toluene are MA, CO and CO2. Components with high intermediate
Fig. 8. Selectivities of toluene oxidation (II); toluquinone (ꢀ) and citraconic anhy-
dride (×) have intermediate selectivities while acetic acid (ꢂ) is a final product.

42
R. Marx et al. / Applied Catalysis A: General 398 (2011) 37–43
Table 1
Summary of the results of all dosage experiments. I stands for intermediate, P for product and T for traces of the respective component found in the oxidation of the
intermediate.
Tac
PAld
PAc
PH
PA
DMBQ
IDMMA
AAc
TOL
BA
BAc
BZ
PL
HQ
BQ
MA
TQ
CA
CO
CO2
o-Xylene
Tolualdehyde
Toluic acid
I
I
I
I
I
I
I
I
I
–
–
–
–
–
P
P
P
–
–
–
–
–
I
I
I
–
I
I
–
I
I
I
I
–
–
–
–
–
–
T
T
–
–
I
–
T
T
T
T
–
–
–
–
–
–
T
T
–
–
–
–
–
–
T
T
–
T
–
I
P
P
–
P
P
P
P
–
I
I
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
I
–
–
–
–
–
I
–
–
I
–
–
–
–
–
–
I
–
I
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Toluene
Benzoic acid
Toluquinone
Benzoquinone
Citraconic anhydride
I
–
–
–
–
I
–
–
–
CA appears to be comparably stable and is oxidized only in minor
amounts. Oxidation products of CA are CO, CO2 and AAc. On the
applied catalyst, MA is not formed from CA oxidation.
3.4. Reaction scheme
This spectrum of by-products and intermediates can be divided
into two different reaction schemes. The first describes the oxida-
tion of o-xylene with a selective reaction path to PA together with
the non-selective paths to MA via DMBQ and to CO and CO2. The
selective oxidation of TA occurs via TAc and PH or PAld and PAc as
intermediates. TA is converted to PA with relatively high selectiv-
ity. However, both TA and TAc have non-selective paths to toluene
(Fig. 9).
In a next step, it was attempted to identify the source of toluene
production. Bernardini et al. [8] reported on MA selectivities in
TAc oxidation. Also, it seems likely that the acid group of TAc
is abstracted to give toluene. However, a dosage experiment of
TAc unambiguously revealed that main products are PA and PH.
While selectivities to other components such as COx or MA could
not clearly be identified, measured toluene concentrations in the
dosage experiment are significantly higher than in the reference
experiment.
TA is another possible source of toluene, which in dosage exper-
iments yields PA with a final selectivity of around 90%. However,
at full tolualdehyde conversion, MA exhibits a selectivity of about
1.6%. Combined CO and CO2 selectivities range around the same
value. Intermediates with high maximum selectivities are toluic
acid and phthalide, which account for the selective path to PA.
Toluene can be detected with a smaller selectivity of an inter-
mediate. The oxidation products of toluene, BQ, TQ and BAc, too
show significant intermediate selectivities while traces of benzene,
phenol and benzaldehyde can be identified by GC/MS analysis of
intermediate gas samples.
The second reaction scheme consists of the toluene oxidation
paths. A large number of intermediates lead to the formation of
COx, MA and CA.
At certain operating conditions, the PA selectivity profile shows
a clear maximum, while the BAc selectivity reveals a second maxi-
mum, and the selectivities to MA, CO and CO2 increase. PA oxidation
on vanadia catalyst has been investigated by Volfson [32], who con-
cludes that MA and CO2 are its oxidation products. However PA
oxidation on metal oxide catalysts is reported to yield benzoic acid
as well [33]. In consequence, a possible PA oxidation path is most
likely the decarboxylation to BAc, which then further decomposes
to MA and COx.
In general, two main mechanisms can be observed. The first is
the nucleophilic oxidative attack of the methyl group, the other the
electrophilic attack of the aromatic ring. Once the side chain is oxi-
dized, the attack of the aromatic ring is prevented, unless one side
chain is completely consumed. In a similar way, once the aromatic
Consequently, it can be concluded that both TAc and TA can
be sources of toluene. Its main source however appears to be
TA. The results of all dosage experiments are summarized in
Table 1.
Fig. 9. Experimentally confirmed reaction scheme of o-xylene oxidation.

R. Marx et al. / Applied Catalysis A: General 398 (2011) 37–43
43
structure of the ring is cracked, the oxidation of the side chains
Acknowledgement
becomes less likely.
In analogy to the formation of MA from TQ, it is assumed
that DMBQ is an intermediate to MA formation directly from
o-xylene. In addition in analogy to CA, dimethyl-maleic anhy-
dride (DMMA), which only occurs in minor concentrations, is a
rather stable product which may be oxidized to AAc but not to
MA.
The authors would like to thank Mr. Bernd Mischke of
“Chromatographie und Service” for conducting the GC/MS mea-
surements.
Appendix A. Supplementary data
Some of the components observed in higher quantities in dosage
experiments, are only detected in traces in the o-xylene oxidation
process. Consequently, several consecutive reactions, e.g. the oxi-
dation route of benzene in Fig. 9, appear to have high intrinsic
reaction rates and desorption of the surface intermediates is not
favored.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.03.006.
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4. Conclusions
The reaction scheme of o-xylene oxidation on industrial
vanadia–titania catalyst was refined especially considering the
reaction paths leading to undesired by-products such as maleic
anhydride and benzoic acid. Maleic anhydride formation occurs via
three different reaction paths. The main path to maleic anhydride
is the direct oxidation of o-xylene with DMBQ as an interme-
diate. A second formation path leads via a larger number of
identified intermediates, where toluene, which is produced by
oxidation of tolualdehyde and toluic acid, intermediates in selec-
tive o-xylene oxidation to phthalic anhydride, appears to be
a key component. Toluene reacts to maleic anhydride via dif-
ferent reaction paths. Finally, also phthalic anhydride can be
oxidized to maleic anhydride, where benzoic acid appears to be an
intermediate.
Rather than being a final product, benzoic acid apparently serves
as an intermediate for different paths leading to maleic anhydride.
The main sources of benzoic acid are toluene and phthalic anhy-
dride.
There are multiple sources for COx. However, the intermedi-
ates leading to maleic anhydride are more likely to be completely
oxidized than the selective intermediates leading to phthalic
anhydride. Also, in the course of the oxidation of any interme-
diate to maleic anhydride, substantial amounts CO and CO2 are
formed.
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The quantitative results presented in this communication
enable an improved understanding of the oxidation paths
of o-xylene on vanadia–titania catalyst and will be used
to derive
network.
a detailed kinetic understanding of the reaction
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