New silver- and vanadium-containing multimetal oxides for oxidation of aromatic hydrocarbons

Article abstract of DOI:10.1016/j.cattod.2010.05.022

Carboxylic acids and carboxylic anhydrides such as benzoic acid, maleic anhydride, phthalic anhydride or pyromellitic anhydride are produced industrially by catalytic gas-phase oxidation of aromatic hydrocarbons such as benzene, o-xylene, naphthalene, toluene or durene in fixed-bed multitube reactors. Although the processes for the oxidation of o-xylene and/or naphthalene to phthalic anhydride have been studied very intensively for decades, there still is a need for improved catalysts with a selectivity to phthalic anhydride which is higher than 82 mol%. We have found a novel multimetal oxide catalyst of the formula Ag_a-bM_bV ₂O_x which allow significantly higher phthalic anhydride yield, since the new silver vanadium oxide catalysts can oxidize o-xylene significantly more selectively to form phthalic anhydride or intermediates than by using catalysts of the vanadium oxide/anatase type. By combining the new silver vanadates with conventional V₂O₅/TiO₂ catalysts, a PA yield of more than 84 mol% could be reached.

Full text of DOI:10.1016/j.cattod.2010.05.022

Catalysis Today 157 (2010) 339–344
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
New silver- and vanadium-containing multimetal oxides for oxidation of
aromatic hydrocarbons
∗
Frank Rosowski , Stefan Altwasser, Cornelia Katharina Dobner, Sebastian Storck,
Jürgen Zühlke, Hartmut Hibst
BASF SE, 67056 Ludwigshafen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 29 June 2010
Carboxylic acids and carboxylic anhydrides such as benzoic acid, maleic anhydride, phthalic anhydride
or pyromellitic anhydride are produced industrially by catalytic gas-phase oxidation of aromatic hydro-
carbons such as benzene, o-xylene, naphthalene, toluene or durene in fixed-bed multitube reactors.
Although the processes for the oxidation of o-xylene and/or naphthalene to phthalic anhydride have
been studied very intensively for decades, there still is a need for improved catalysts with a selectivity
to phthalic anhydride which is higher than 82 mol%. We have found a novel multimetal oxide catalyst of
the formula Aga−bMbV2Ox which allow significantly higher phthalic anhydride yield, since the new silver
vanadium oxide catalysts can oxidize o-xylene significantly more selectively to form phthalic anhydride
or intermediates than by using catalysts of the vanadium oxide/anatase type. By combining the new silver
vanadates with conventional V2O5/TiO2 catalysts, a PA yield of more than 84 mol% could be reached.
Keywords:
Silver vanadate
Oxidation of aromatic hydrocarbons
o-xylene
Phthalic anhydride
Benzaldehyde
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
oxidation and it is possible to achieve initial yields of 112–115 g
PA/100 g o-xylene on an industrial scale. This is equivalent to a
molar yield of 80–82%. Review articles concerning the state of the
art and the reaction mechanism can be found in [3,4].
The commercially utilized PA-catalyst is a shell catalyst with the
active mass being coated on hollow cylinders that exhibit diame-
ters ranging from 6 to 10 mm. The layer is about 150–200 ␮m thick
and accounts for about 5–14 wt.%. The carrier material is composed
of non-porous steatite or SiC exhibiting a low pressure loss geom-
etry and is inert under PA production conditions. The active mass
The side chain oxidation of alkyl aromatics is one of the most
important processes in the production of industrial intermedi-
ates. The oxyfunctionalized aromatics are then further reacted to
higher value products. The most important of these products on an
industrial scale with regards to world production are terephthalic
acid, phthalic anhydride and benzoic acid. Products manufactured
on a smaller scale include the higher priced aldehydes such as
benzaldehyde and terephthalaldehyde [1]. Phthalic anhydride and
pyromellitic anhydride are used in the production of plasticizers,
alkyd- and polyester-resins, phthalocyanin dyes and a variety of
fine chemicals. Benzoic acid finds use as an intermediate in the dye
and perfume industries and as a raw material for the production
of phenol. Benzaldehyde is used as an aroma compound (bitter
almond) and as a starting material in a variety of aromatic and
flavoring product syntheses.
consists predominantly of TiO -particles with a diameter of 50 nm
2
that are coated with vanadium oxide. The vanadium content within
the active mass ranges from 4 to 10 wt.%. A complete coverage of
a 1 m2 TiO2 surface (monolayer) is achieved with 0.12 wt.% V O5
2
without the formation of V O5-crystallites. The BET surface of the
2
2
TiO2 carrier ranges from 6 to 25 m /g so that the surface of the
TiO -particles is fully covered by multiple V O5 monolayers. The
2
2
Phthalic anhydride (PA) has been commercially produced con-
tinuously since 1872 when BASF developed the naphthalene
oxidation process. The break-through that led to commercial pro-
duction of PA was the development of the gas-phase oxidation of
naphthalene or o-xylene over a vanadium oxide catalyst [2]. Start-
ing in 1960, naphthalene was replaced by o-xylene as single feed
source. Today, 87% of the world PA capacity is produced by o-xylene
use of TiO2 in its anatase modification is essential for high selec-
tivities. Alkaline, phosphorous and Sb components, as well as some
transition metal complexes, are used in small amounts as reaction
promoters.
The PA process is carried out almost exclusively in fixed-bed
multitube reactors cooled by a molten salt, e.g., developed by
Deggendorfer Werft und Eisenbau GmbH (DWE) [5]. The heat
formed by the strongly exothermic reaction (1109 kJ/mol) is dis-
sipated via the salt-bath and further utilized for steam production.
Each reactor contains between 3000 and 25,000 tubes. The reac-
tor tubes have a diameter of about 21–25 mm and are up to 4 m
∗ Corresponding author. Tel.: +49 621 60 54480.
E-mail address: frank.rosowski@basf.com (F. Rosowski).
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.05.022

3
40
F. Rosowski et al. / Catalysis Today 157 (2010) 339–344
◦
long. The PA production occurs in the explosible region, where
each reaction tube is dosed with 2.4–4.2 Nm /h air and the xylene
and 40 wt.% glycerol at 20 C in a coating drum over a period of
3
20 min were subsequently dried. The weight of the catalytically
active composition applied in this way, determined on a sample of
the precatalyst obtained, was 18 wt.% after heat treatment at 450 C
for 1 h, based on the total weight of the finished catalyst. The forma-
tion procedure was accomplished by a two-step procedure, starting
with a reduction of the carbon content in nitrogen atmosphere at ca.
concentrations of up to 2.5 vol.%. Although the processes for the oxi-
dation of o-xylene and/or naphthalene to phthalic anhydride have
been studied very intensively for decades, there still is a need for
improved catalysts with a selectivity to phthalic anhydride which
is higher than 82 mol% [6,7].
◦
◦
Mixed oxides of silver and vanadium having an atomic ratio of
Ag/V < 1 are known as silver vanadium oxide bronzes [8]. Gener-
ally, such semiconducting or metallically conductive oxidic solids
preferably crystallize in layer or tunnel structures in which part of
150 C in the first step and a adjustment of the vanadium oxidation
stage at 450–500 C in an oxygen-containing atmosphere.
◦
2.1.3. Preparation of the V/Ti catalysts
the vanadium is present as V4 embedded in the [V O5]∞ host lat-
+
The V O5/TiO2 two-layer reference catalyst (catalysts A and B)
was prepared by conventional techniques. 1400 g of steatite rings
having an external diameter of 8 mm, a length of 6 mm and a wall
2
2
tice. Usually, these compounds are prepared by joint melting of
the starting materials silver or silver nitrate and V O5 at about
2
◦
◦
7
50 C, giving a multiphase mixture which, owing to the prepa-
thickness of 1.6 mm were heated to 160 C in a coating drum and,
ration method, has a low BET surface area. When such silver
vanadium oxide bronzes are used as oxidation catalysts for the
selective oxidation of o-xylene or naphthalene and toluene [9],
in all cases the activity, selectivity and desired product yield are
unsatisfactory. In the case of o-xylene, PA selectivities of ca. 55% at
ca. 75 mol% o-xylene conversion were obtained. For toluene oxida-
tion, even at only 30% conversion, the combined benzaldehyde and
benzoic selectivity is 62 mol% only.
together with 13.8 g of an organic binder comprising a copoly-
mer of acrylic acid–maleic acid (weight ratio 75/25), sprayed with
a suspension comprising 466 g of anatase having a BET surface
2
area of 21 m /g, 67.2 g of vanadyl oxalate, 14.4 g of antimony tri-
oxide, 3.15 g of ammonium hydrogenphosphate, 2.87 g of cesium
sulphate, 721 g of water and 149 g of formamide. The catalyti-
cally active composition applied in this way comprised, on average,
0.16 wt.% of phosphorous (calculated as P), 7.5 wt.% of vanadium
(calculated as V O5), 3.2 wt.% of antimony (calculated as Sb O ),
It was an objective of the present work to provide novel silver-
and vanadium-containing catalysts and also processes for produc-
ing such catalysts [10]. We have found a novel multimetal oxide
2
2
3
0.40 wt.% of cesium (calculated as Cs) and 88.74 wt.% of titanium
dioxide (catalyst A).
◦
catalyst of the formula Ag M V Ox, wherein M is a metal selected
The coated catalyst obtained in this way was heated to 160 C
a−b
b
2
from the group consisting of Li, Na, K, Rb, Cs, Tl, Mg, Ca, Sr, Ba, Cu,
Zn, Cd, Pb, Cr, Au, Al, Ce, Fe, Co, Ni and/or Mo, a = 0.3–1.9, b = 0–0.5,
c = 0–20 and x is a number determined by the valence and amount of
elements different from oxygen. The novel catalysts were evaluated
both for o-xylene and for toluene oxidation.
in a coating drum and, together with 14 g of an organic binder
comprising a copolymer of acrylic acid–maleic acid (weight ratio
75/25), sprayed with a suspension comprising 502 g of anatase hav-
2
ing a BET surface area of 21 m /g, 35.8 g of vanadyl oxalate, 2.87 g of
ammonium hydrogenphosphate, 2.87 g of cesium sulphate, 720 g of
water and 198 g of formamide. The catalytically active composition
applied in this way comprised, on average, 4 wt.% of vanadium (cal-
2
2
2
. Experimental
culated as V O5), 0.4 wt.% of cesium (calculated as Cs) and 88.8 wt.%
2
of titanium dioxide. The weight of the layers applied was 9.3 wt.%
of the total weight of the finished catalyst (catalyst B).
.1. Catalyst preparation
.1.1. Preparation of silver vanadate precursor powder
2.2. Characterization
Synthesis of Ce0.02Ag0.71V Ox: 102 g V O5 (0.56 mol) were
2
2
◦
added whilst stirring to 7 L of deionised water at 60 C. The
suspension was admixed with an aqueous solution of 4.94 g of
CeNO ·6H O (0.011 mol, Aldrich, 99% purity). An aqueous solution
The BET surface areas were determined by measuring 5 points
◦
between p/p = 0.06–0.20 after activating the samples at 200 C at
0
3
2
a pressure below 0.1 mbar. X-ray powder diffraction patterns were
recorded by means of a D 5000 diffractometer from Siemens using
Cu K␣ radiation (40 kV, 30 mA). The diffractometer was equipped
with an automatic primary and secondary diaphragm system and
a secondary monochromator and scintillation detector.
Transmission electron microscopy images were obtained on a
electron microscope Tecnai F-20 (FEI, Eindhoven/The Netherlands)
operated at an accelerating voltage of 200 kV. The samples were
prepared by embedding the powder into a resin and ultramicro-
toming into slices.
of 68 g of AgNO (0.398 mol) in 1 L of water was added to the result-
3
ing orange suspension while continuing to stir. The temperature of
◦
the suspension obtained was subsequently increased to 90 C over a
period of 2 h and the mixture was stirred for 24 h temperature. The
dark brown suspension obtained was then cooled and spray dried
◦ ◦
inlet temperature (air) = 350 C, outlet temperature (air) = 110 C).
(
2.1.2. Coating of the silver vanadate catalysts
The multimetal oxides were used as precursors for preparing
coated catalysts on either steatite (magnesium silicate) spheres or
on steatite rings. For the coating of silver vanadates on steatite
spheres, 300 g of the spheres having a diameter from 3.5 to 4 mm
were coated with 40 g of the powder and 4.4 g of oxalic acid with
addition of 35.3 g of a mixture of 60 wt.% water and 40 wt.% glycerol
The oxidation state of the vanadium was determined by poten-
tiometric titration. For the determination, in each case from 200
to 300 mg of the sample are added under an argon atmosphere of
15 mL of 50% strength sulphuric acid and 85% strength phosphoric
acid and are dissolved with heating. The solution is then transferred
to a titration vessel which is equipped with two Pt electrodes. The
◦
at 20 C in a coating drum over a period of 20 min were subse-
◦
quently dried. The weight of the catalytically active composition
applied in this way, determined on a sample of the precatalyst
titrations are conducted in each case at 80 C. First, a titration is
carried out with 0.1 M potassium permanganate solution. If two
steps are obtained in the potentiometric curve, the vanadium was
present in an average oxidation state from +3 to less than +4. If only
one step is obtained, the vanadium was present in an oxidation step
from +4 to less than +5.
◦
obtained, was 10 wt.% after heat treatment at 400 C for 1 h, based
on the total weight of the finished catalyst. For the coating of sil-
ver vanadates on steatite rings, 350 g of steatite rings having an
external diameter of 7 mm, a length of 3 mm and a wall thick-
ness of 1.5 mm were coated with 84.4 g of the powder and 9.4 g
of oxalic acid with addition of 66.7 g of a mixture of 60 wt.% water
In the first mentioned case (two steps), the solution contains
no V5+, i.e., all of the vanadium was detected titrimetrically. The

F. Rosowski et al. / Catalysis Today 157 (2010) 339–344
341
amount of V3+ and V4+ is calculated from the consumption of
.1 M permanganate solution and the position of the two steps. The
diameter of 25 mm. To regulate the temperature, the iron tube was
3
0
surrounded by a salt melt. 4.0 standard m /h of air with a loading
of 80 g of 98.5% wt.% o-xylene per standard m3 of air were passed
through the tube from the top downward.
weighted average then gives the average oxidation state. In the sec-
ond mentioned case (one step), the amount of V4+ can be calculated
from the consumption of 0.1 M potassium permanganate solution.
By then reducing all of the V5 in the resulting solution with a 0.1 M
ammonium iron(II) sulphate solution and further oxidation with
+
3. Results and discussion
0.1 M potassium permanganate solution, the total amount of vana-
3.1. Kinetic aspects and approach
dium can be calculated. The difference between the total amount of
vanadium and the amount of V4+ gives the amount of V originally
present. The weighted average then gives the average oxidation
state.
The equipment for the in situ XRD measurements is described in
detail in [11] and consists of three parts: (1) the dosing unit for the
gaseous and liquid reactants, (2) the XRD diffractometer equipped
with the in situ reaction chamber, and (3) the online GC analytical
part. Dosing of toluene and air was done as described in Section 3.1.
The gaseous feed consisting of toluene and air and was preheated to
5+
It is well established that the synthesis of PA proceeds via
the intermediates toluic aldehyde, toluic acid and phthalide, as
schematically shown in Fig. 1. Studies into the reaction mecha-
nism and kinetics can be found in [12–18]. PA selectivity is lost
by the total oxidation of o-xylene to CO and CO , which predomi-
2
nantly occurs in the first step of the reaction scheme. By using the
intermediates as reactor feed it was found by the authors that con-
version into PA can be achieved with very high selectivities (>93%).
From these experiments it is clear that the total PA selectivity can
be increased by increasing the conversion of o-xylene to any of
the above-mentioned intermediates. Experiments to this respect
are ongoing and are based on a two-stage process with upstream
liquid phase oxidation [19].
4
73 K and introduced directly into the in situ chamber. The in situ
XRD chamber was a commercially available Paar high-temperature
chamber which was modified with a special sample holder having a
5
mL catalyst bed volume and a modified heating system. The ther-
mocouple was embedded in the catalyst bed. The temperature was
controlled within ± 2 K. The catalyst was granulated into particles
from 0.2 to 0.4 mm in size.
Besides the continuous improvement of conventional catalyst
systems, the challenge lies in the development of new catalysts
that allow the highly selective conversion of xylene to the reac-
tion intermediates. Ideally, this catalyst would be placed as the first
layer in a reactor with conventional V/Ti catalyst layers and exhibit
2
2
.3. Catalyst testing
◦
working temperatures around 350–380 C. In a preferred embodi-
.3.1. Single-layer tests
ment of the process for partial oxidation, o-xylene is first reacted
over a bed of the catalyst, which is a carrier coated with the silver
vanadium oxide, to convert it mainly into phthalic anhydride and
other oxidation intermediates such as o-toluic aldehyde, o-toluic
acid and phthalide. For toluene oxidation, the challenge is similar,
i.e., activating the methyl group in a selective way. The novel cat-
alysts were, therefore, evaluated both for o-xylene and for toluene
oxidation.
For the single-layer tests, the catalysts (120–140 g coated
spheres) were placed into an iron tube reactor having a length
of 80 cm and an internal diameter of 15 mm. The preheated o-
xylene/air mixture (360 L (S.T.P.)/h air) with o-xylene loadings of
3
4
0–50 g/m (S.T.P.) were passed through the reactor. In the case of
toluene oxidation, the preheated toluene/air/steam mixture (360 L
(
4
S.T.P.)/h air and 30–40 L (S.T.P.)/h steam) with toluene loadings of
0–50 g/m (S.T.P.) were passed through the reactor. The reactor
3
outlet gas composition was determined by gas chromatography or
by an infrared gas analyzer.
3.2. Preparation and characterization of silver vanadates
The basis for the new high performance catalyst for o-xylene
oxidation to intermediate oxidation products like toluic aldehyde,
toluic acid or phthalide, is a new preparation route resulting in
2
.3.2. Multi-layer tests
The combination of the silver vanadate catalyst with conven-
2
tional V/Ti catalysts was done in the following way: 80 cm of the
coated silver vanadate rings at the reactor inlet, 140 cm of cata-
lyst B as middle layer and 80 cm of catalyst A at the reactor outlet
were introduced into a 385 cm long iron tube having an internal
silver vanadate precursors with BET surface areas of ca. 60 m /g.
The new preparation route is rather simple and straight forward
and described in detail in Section 2. The X-ray powder pattern of
the prepared silver vanadium oxides shows a new diffraction pat-
Fig. 1. Reaction scheme of the PA synthesis (selectivity data measured by the authors).

3
42
F. Rosowski et al. / Catalysis Today 157 (2010) 339–344
Fig. 4. In situ XRD study of the silver vanadate catalyst.
3.4. In situ XRD investigations
Fig. 2. X-ray powder diffraction pattern of the silver vanadate catalyst precursor.
In order to study the transformation process of the catalyst
precursor to the final catalyst in more detail, in situ XRD investiga-
tions were done. A precursor sample was, therefore, heated in air
from room temperature to 330 C. The gas atmosphere was, then,
tern which does not correspond to known silver vanadium oxide
bronzes [20] (see Fig. 2). This means that the low-temperature
synthesis leads to a new crystal structure. Potentiometric titration
showed that the oxidation state of the vanadium in the powder
obtained is predominantly +5 while a very small amount of the
vanadium (less than 5 at.%) is in the oxidation state of +4. The new
material crystallized in a fibrous morphology with fibre diameters
of up to ca. 1 ␮m with typical ratios of fibre diameter to fibre
length of below 0.6. The TEM image in Fig. 3 shows double layers
of vanadium oxide octahedra with intermediate silver layers.
◦
changed from air to a toluene/air mixture and the temperature
◦
was raised stepwise to 370 C. During the heating of the sample,
X-ray powder diffractions patterns were measured. Fig. 4 shows
selected XRD patterns of that study. The XRD at room temperature
corresponds to the XRD of the catalyst precursor already shown
◦
in Fig. 2. After heating in air to 330 C, significant changes in the
XRD pattern can already be seen (Fig. 4). The phase transformation
was completed after heating the sample in a toluene/air mixture to
◦
3
70 C. The diffraction pattern of the catalyst precursor is not visibly
3.3. Catalytic testing of the silver vanadates in o-xylene oxidation
2
5
any more in that XRD, while the pattern of the ␦-Ag0.7V O phase
(
single-layer tests)
has formed. The measurements under reaction conditions have
shown that under reducing conditions V4+-containing ␦-Ag0.7V O5
2
This new silver vanadate was tested as catalyst precursor (Fig. 3).
is formed. The crystal structure of this phase is known [20]. The
At the first day on stream, such a material almost completely burns
␦-Ag0.7V O is organized in a layer structure with double layers of
2
5
the o-xylene feed to CO and CO . The test was not stopped but con-
VO6 octahedra and single Ag layers. Statistically, the Ag positions
are about 70% occupied.
2
tinued for a week. During that week, a formation process occurred
during which the silver vanadate reached a stable state and the
total oxidation tendency continuously decreased. This behaviour
is completely different compared to a conventional V/Ti catalyst,
which reaches the final selectivity already after 2 days.
3.5. Combination of the new silver vanadates with V/Ti catalysts
for o-xylene oxidation (multi-layer tests)
The silver vanadate catalyst shows an extremely low COx selec-
tivity. The PA selectivity is ca. 55%. Besides, 10% phthalide, 2%
o-toluic acid and 24% o-toluic aldehyde are formed. The total C8
oxygenate selectivity is ca. 91%. At these reaction conditions (silver
vanadate on spheres, 325 C) the o-xylene conversion is 30% and can
only be increased with concomitantly decreasing the C8 oxygenate
selectivity significantly (e.g., 88.6 mol% C8 oxygenate selectivity at
As already shown in Fig. 1, the intermediates can be converted
into PA with very high selectivities (>93%). Based on that knowl-
edge, we have combined properties of the new silver vanadate
catalyst with conventional V/Ti catalysts (see Fig. 5). State of the
art catalysts for oxidation of o-xylene to phthalic anhydride consist
of two to four different V/Ti catalysts in one reactor. A typical setup
is the combination of one so-called selective catalyst layer at the
reactor inlet (high selectivity at partial conversion, e.g., catalyst B)
and one active catalyst layer at the reactor outlet (higher catalyst
activity in order to ensure high o-xylene conversion and conver-
sion of intermediate oxidation products to PA, e.g., catalyst A). For
an further increase in PA yield, such a conventional 2-layer setup
was complemented by an additional catalyst layer, which can be
◦
5
5% o-xylene conversion). For comparison, silver vanadate catalysts
described in [9], show C8 oxygenate selectivities of ca. 78 mol% at
similar o-xylene conversion of ca. 30%.
Fig. 3. Silver vanadium oxide catalyst with crystalline structure.
Fig. 5. Combination of two different catalyst systems.

F. Rosowski et al. / Catalysis Today 157 (2010) 339–344
343
operated at comparably low conversion, but high selectivity, the
new silver vanadate catalyst. In order to do so, it was necessary to
optimize various parameters: length of the silver vanadate cata-
lyst layer, number and length of the V/Ti catalyst layers, variation
of the chemical composition of the V/Ti catalyst layers, operation
point (reaction temperature) and start-up conditions. One success-
ful way to adapt all these parameters is the following: 80 cm of the
coated silver vanadate rings at the reactor inlet, 140 cm of catalyst B
as middle layer and 80 cm of catalyst A at the reactor outlet. In order
to start-up such a catalyst with a load of 30 g/Nm3 it was neces-
sary to use a formation procedure to adjust the carbon content and
the vanadium oxidation state of the silver vanadate catalyst (vana-
dium oxidation state = 4.68, dark green colour). Upon doing so, the
extremely low COx selectivities of the silver vanadate catalyst can
be used from the first day on stream.
phase oxidation. It is possible to increase the selectivity via doping,
for example with Sb-oxides, whereby benzaldehyde is formed only
at low temperatures in low conversions. Renken et al. have pub-
lished comprehensive studies into the reaction kinetics and the
nature of the active sites [21]. The formation of benzaldehyde in
high selectivities is possible by isolation of the active vanadium
oxide centers, for example via integration into microporous silica
[18]. The conversions achieved by such processes are, however, far
from industrial application.
There are further examples of V-containing systems that are
able to produce benzaldehyde with selectivities of 30–80% with
low conversions. Of particular interest are the Ag-containing vana-
dium oxide systems that show different redox properties to the
vanadium oxide catalysts and that are able to form either benzalde-
hyde alone or benzaldehyde and benzoic acid in higher conversions
with high product selectivities. When such silver vanadium oxide
bronzes are used as oxidation catalysts for the selective oxidation
of toluene [9], in all cases the activity of the catalyst is compara-
bly low due to the low BET surface area of the catalyst precursor of
Upon optimizing all these parameters, the new catalyst con-
figuration showed PA yields up to 84.2 mol% at o-xylene loadings
3
3
of 80 g/Nm and 4 Nm /h air flow. Compared to conventional V/Ti
catalyst technology, this a significant increase of PA yield. From
an ecological perspective, the higher PA yield correlated with a
decreased CO2 emission of ca. 50 kg of carbon dioxide per ton of
produced PA.
2
◦
ca. 0.5 m /g. 30% toluene conversion was achieved at 400 C with a
combined benzaldehyde and benzoic selectivity of 62%. By our new
preparation route, catalyst precursors with significantly higher BET
2
surface areas of up to 60 m /g can be synthesized resulting in a
3.6. Upscaling and technical application
significantly higher catalyst activity. 30% toluene conversion could
be reached at 340 C already with a combined benzaldehyde and
◦
As a next step, the lab scale catalyst preparation was scaled up
benzoic selectivity of 76%.
to production scale. The most difficult production step is the silver
vanadate crystallization. After a stirring time of several hours, the
silver vanadate formation starts and is typically completed after
ca. 12 h. Upon formation of the silver vanadate, the viscosity of the
suspension increases first and then decreases again afterwards with
longer stirring times. The morphology of the particles at the end of
the crystallization step is fibrous. In order to ensure the handling
of the suspension during the viscosity changes, the content of solid
material in the suspension was limited.
During catalyst development, both activity and selectivity of the
silver vanadate catalysts were increased further. Upon doping the
silver vanadate catalyst, 44% toluene conversion could be reached at
◦
◦
340 C instead of 360 C without doping. Besides, benzaldehyde and
benzoic acid selectivity could be improved from 71.3 to 78.8 mol%.
One disadvantage is that, at higher conversions, benzoic acid is pro-
duced as a byproduct alongside the more valuable benzaldehyde (at
40% toluene conversion, the benzaldehyde to benzoic acid ratio is ca
1:1). The limited influence affected on the ratio of products by vary-
ing parameters such as conversion, dwell time and the steam partial
pressure means that the industrial application of such a process is
improbable at the moment.
A combination of silver vanadate catalyst with V/Ti catalysts is
used on commercial scale in the mean time. BASF SE has filed a
series of patents on silver vanadate catalysts for o-xylene oxida-
tion to PA, both concerning the synthesis of the silver vanadates
and their application in o-xylene oxidation in the oxidation of aro-
matic hydrocarbons, especially for combination with conventional
V O5/TiO catalysts [10,21–25].
4. Conclusions
2
2
A new silver- and vanadium-containing multimetal oxide for the
oxidation of aromatic hydrocarbons was identified and its catalytic
performance for the phthalic anhydride synthesis from o-xylene
and the toluene oxidation to benzoic acid and benzaldehyde was
demonstrated. The preparation route of the new catalytic mate-
rial is simple and was scaled up to production scale. The formation
procedure of the catalyst was analyzed. Besides, the material was
chemically modified. Upon combination of the new silver vana-
3
.7. Oxidation of toluene to benzoic acid and benzaldehyde
The processes of DOW, Snia Viscosa and Amoco for the oxidation
of toluene are known [1]. The processes of DOW and Snia Viscosa
involve liquid phase air oxidation over cobalt catalysts with partial
conversion. The Amoco process, on the other hand, applies a liquid
phase air oxidation process over cobalt catalysts in the presence of
bromine as co-catalyst with full conversion. It is possible to produce
benzoic acid with a selectivity of ca. 90 mol%.
Benzaldehyde is produced as a byproduct during the oxida-
tion of toluene with an approximate yield of 7–8% with respect
to benzoic acid. It is possible to vary the ratio of benzaldehdye pro-
duced during the manufacture of benzoic acid. Benzaldehyde can
alternatively be produced via the hydrolysis of benzylidene chlo-
ride. It is also possible to produce benzoic acid and benzaldehyde
via heterogeneously catalyzed gas-phase oxidation. The selectivi-
ties achieved in such processes are, however, significantly lower.
date catalyst with conventional V O5/TiO2 catalysts we were able
2
to develop a new catalyst generation for o-xylene oxidation to
phthalic anhydride, which is already applied commercially. Silver
vanadate catalysts can also be applied for toluene oxidation to ben-
zoic acid and benzaldehyde. Currently, the industrial application of
such a gas-phase toluene oxidation process is improbable. In con-
clusion, the new silver vanadate catalysts are a new development
platform for a mature field of activity in heterogeneous oxidation
catalysis.
Recently, research into the V O5–TiO2 PA catalyst system has been
a particular focus of scientific literature.
2
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