Oxidation of acetaldehyde and propionaldehyde on a V2O 5-TiO2 catalyst

Article abstract of DOI:10.1524/zpch.2006.220.12.1575

The partial oxidation of acetaldehyde and propionaldehyde on a TiO ₂ supported VO_x catalyst in presence of water vapour was investigated in the temperature range from 120 to 280°C. The selective oxidation to the appropriate carboxylic acids depended on the kind of aldehyde and the reaction temperature. An oxidative cleavage to lower carboxylic acids and CO_x was also found. Furthermore, the kinetic model for the process of oxidation of acetaldehyde and propionaldehyde over the V ₂O₅-TiO₂ catalyst was developed. by Oldenbourg Wissenschaftsverlag.

Full text of DOI:10.1524/zpch.2006.220.12.1575

Z. Phys. Chem. 220 (2006) 1575–1588 / DOI 10.1524/zpch.2006.220.12.1575
by Oldenbourg Wissenschaftsverlag, München
©
Oxidation of Acetaldehyde and
Propionaldehyde on a V O -TiO Catalyst
2
5
2
1
, ∗
1
2
1
By W. Y. Suprun , T. Machold , H.-K. Schädlich , and H. Papp
1
Institut für Technische Chemie, Universität Leipzig, Linnèstr. 3, D-04103 Leipzig,
Germany
Institut für Chemische Verfahrenstechnik, Technische Universität Clausthal,
Leibnizstr. 17, D-38678, Clausthal-Zellerfeld, Germany
2
(Received July 20, 2006; accepted September 18, 2006)
VO -TiO Catalyst / Partial Oxidation / Acetaldehyde / Propionaldehyde /
x
2
Kinetics Modelling
2 x
The partial oxidation of acetaldehyde and propionaldehyde on a TiO supported VO
catalyst in presence of water vapour was investigated in the temperature range from 120 to
◦
2
80 C. The selective oxidation to the appropriate carboxylic acids depended on the kind
of aldehyde and the reaction temperature. An oxidative cleavage to lower carboxylic acids
and CO was also found. Furthermore, the kinetic model for the process of oxidation of
acetaldehyde and propionaldehyde over the V -TiO catalyst was developed.
x
2
O
5
2
1. Introduction
The vanadium-titanium oxide system has found a lot of applications in nu-
merous partial oxidation processes including the selective oxidation [1–5] and
oxidative dehydrogenation (ODH) [6, 7] of C –C hydrocarbons. These oxides
2
4
are reported to be selective catalysts for the oxidation of n-butenes to acetic
acid, formerly by the Hüls AG [8] and recently by the Consortium für elek-
trochemische Industrie GmbH [9]. The presence of about 20 vol. % of water
vapour in the feed has been shown to enhance the yields of acetic acid [9–11].
Kaneko et al. [3] and Seiyama et al. [11] developed fundamental de-
tails about the formation of acetic acid by catalytic oxidation of n-butene on
V O containing catalysts. Both groups proposed the formation of a carbo-
2
5
cation as the first step. According to Kaneko et al. [3], acetic acid is formed
by reaction of oxygen with the carbocation leading to acetaldehyde further
oxidized to acetic acid. On the contrary, Seiyama et al. [11] suggested that
*
Corresponding author. E-mail: suprun@chemie.uni-leipzig.de

1
576
W. Y. Suprun et al.
water reacts with the carbocation to form 2-butanol, followed by the oxi-
dation to 2-butanone and its oxidative cleavage to acetic acid and acetalde-
hyde.
Furthermore, propionic acid has also been reported as a product of the
1
-butene oxidation on a VO containing catalyst [3, 9–12]. The formation of
x
propionic acid was envisioned via the formation of propionaldehyde by asym-
metric C–C scission of the chemisorbed 1-butene [12]. The C , C aldehydes
2
3
and their further oxidation to carboxylic acids and/or to CO products were
x
also observed in the ODH of C , C hydrocarbons [4, 5]. However, the yields of
2
3
C –C carboxylic acids were usually low (about 0.5–2.0%) and the total oxida-
1
3
◦
tion was favoured at temperatures above 300 C and in absence of water vapour.
According to Centi et al. [13], the selectivity to carboxylic acids by oxidation
and/or ODH of hydrocarbons should depend on the reactivity of the interme-
diate C , C aldehyde species. Nevertheless, not much information is available
2
3
for catalytic gas phase oxidation of C –C aldehydes.
2
3
A preliminary study on the oxidation of propionaldehyde on VO contain-
x
ing catalysts was carried out by Slavucha et al. [14]. The selectivity to propi-
◦
onic acid was found to be ∼ 60% on a VO -MgO catalyst in the 250–300 C
x
temperature range in the presence of 20 vol. % water vapour. Ai et al. [15]
investigated the partial oxidation of propionaldehyde on vanadium phosphate
◦
and iron phosphate at 400 C in the presence of water. Propionic acid was
formed with the 20% and 40% selectivity on P/FeO and P/VO , respectively.
y
x
Quaranta et al. [16] reported on the partial oxidation of ethanol to acetalde-
hyde on VO containing catalysts. The selectivity to acetic acid on V O /TiO ,
x
2
5
2
◦
V O /Al O and V O /SiO catalysts between 150–250 C were about 2–5%.
2
5
2
3
2
5
2
Erdöhelyi and Solymosi [17] and Iwamoto et al. [18] investigated the ODH of
ethane on supported V O catalysts in presence of 20 vol. % water from 400 to
2
5
◦
5
50 C. The selectivity to ethylene was about 40% and to acetaldehyde about
4%; carboxylic acids could not be detected. Evnin et al. [19] reported for the
oxidation of ethylene on Pd/V O catalysts in presence of water vapour be-
2
5
◦
tween 110–180 C the selectivities up to 65% and 25% for acetaldehyde and
acetic acid, respectively. At high temperature the total oxidations of acetalde-
hyde are favoured.
It can be concluded that the selectivity to carboxylic acids during the ox-
idation of 1-butene and the ODH of hydrocarbons depends not only on the
reactivity of 2-butanol or 2-butanone, but also on the reactivity of the inter-
mediate C and C aldehyde species. Therefore, further investigations on the
2
3
oxidative activity of C –C aldehydes are necessary for a better understand-
2
3
ing of the mechanism of the formation of carboxylic acids on VO containing
x
catalysts during the catalytic oxidation of n-butenes, the ODH of C and C hy-
2
3
drocarbons and the selective oxidation of ethanol. The present work deals with
the role of temperature in the oxidation of acetaldehyde and propionaldehyde
to different oxidation products on a VO -TiO catalyst in presence of water
x
2
vapour.

Oxidation of Acetaldehyde and Propionaldehyde on a V
2
O
5
-TiO
2
Catalyst
1577
2. Experimental
2.1 Catalyst preparation and characterization
The VO -TiO catalyst was prepared by spray-drying of a slurry of TiO -
x
2
2
Anatase (Millenium GmbH) and V O . The catalyst synthesis is described in
2
5
◦
detail elsewhere [9, 10]. After calcination at 450 C for 5 h, the sample was
pressed and sieved. A fraction of 0.10–0.31 mm was used for the catalytic
measurements. The catalyst contained 6.1 wt. % of vanadium and was charac-
terised by BET surface area, temperature programmed desorption of ammonia
and oxygen, temperature programmed reduction with hydrogen and X-ray pho-
toelectron spectroscopy (XPS). The detailed results of the catalyst characteri-
2
zation are described elsewhere [12, 20]. The BET surface area was ca. 95 m /g
and the average pore size was ca. 90 Å.
2.2 Catalytic reactions
The oxidation of aldehydes was performed in a catalytic test apparatus
equipped with a continuous-flow quartz reactor (length: 29.0 cm; inner diam-
eter: 0.6 cm), mass flow controllers for the feed gas and a liquid-flow evapo-
rator to add water and the aldehydes to the feed. The catalytic reactions were
◦
carried out at atmospheric pressure in the temperature range from 120 to 280 C
using 200 mg of the catalyst mixed with 200 mg of corundum. The gaseous
−1
hourly space velocity (GHSV) was about 10000 h . The feed gas consisted
of 0.27 vol. % acetaldehyde or 0.14 vol. % propionaldehyde, 3.5 vol. % oxygen
and 15.0 vol. % water vapour and helium (rest). Before the catalytic reac-
◦
tion, the catalyst sample was activated at 300 C for about 12 h in a flow
of 3.5 vol. % oxygen in helium. The reaction system was found to stabilise
in 3 h. Therefore, all data in this work were measured after the initial time
of 3 h.
The product gas was analysed by online mass spectrometry and infrared
gas analysis. The mass spectrometer used was an Omnistar GSD 300 QMS 200
(1–200 amu) by Pfeiffer Vacuum. The gas passed to the mass spectrometer via
a gas stream selector (GSS 300) which was kept at 393 K and time depending
spectra were recorded for m/e values of 46 (formic acid), 60 (acetic acid), 74
(
propionic acid). CO and CO were determined by online FTIR-analysis (IR
2
gas analyzer 1301; INNOVA). A detailed description of the apparatus is given
in [20].
3. Results and discussion
3.1 Catalytic reactions
Oxidation of Acetaldehyde. Acetic acid, formic acid, CO and CO were de-
2
tected as main reaction products during the oxidation of acetaldehyde. The

1
578
W. Y. Suprun et al.
Fig. 1. Conversion of acetaldehyde and yields of main oxidation products as function of
reaction temperature in the oxidation of acetaldehyde on the VO
ence of water vapour.
x 2
-TiO catalyst in pres-
dependence of acetaldehyde conversion and the yield of the oxidation products
on temperature are shown in Fig. 1. The oxidation of acetaldehyde started at
◦
temperatures as low as 120 C with acetic acid as the major product.
With increasing reaction temperature the conversion of acetaldehyde and
◦
the yield of CO increased. At temperatures above 260 C a nearly complete
x
conversion was observed. The maximum yield of acetic acid (about 72%) was
◦
reached at reaction temperatures of about 200 C. The yield of formic acid is
significantly lower than that of acetic acid. The maximum yield of formic acid
is in the same temperature range as for acetic acid. The decrease in the acetic
acid and formic acid yield beyond this temperature can be attributed to the
formation of the total oxidation products. This indicates that acetaldehyde is
a very reactive compound and undergoes selective oxidation to acetic acid even
at low reaction temperatures. Furthermore, small amounts of ethyl acetate and
◦
methane were detected. The yield of methane and ethyl acetate at 180 C was
about 0.5 and 0.3%, respectively.
Oxidation of Propionaldehyde. The products of the oxidation of propionalde-
hyde were propionic acid, acetic acid, formic acid and CO . The propionalde-
x
hyde conversion and the yield of the oxidation products in dependence of
temperature are illustrated in Fig. 2. The conversion of propionaldehyde and
the composition of the reaction product gas varied with increasing reaction
temperature (Fig. 2a). The maximum yield of acetic acid (30–35%) was ob-
◦
tained between 180 and 220 C (Fig. 2b).
The maximum yield of formic acid and propionic acid was found to be
◦
about 20% and 14% at 140 C, respectively. Compared to the oxidation of ac-
etaldehyde the maximum yield of acetic acid was only half as much but the

Oxidation of Acetaldehyde and Propionaldehyde on a V
2
O
5
-TiO
2
Catalyst
1579
Fig. 2. Conversion of propionaldehyde and yields of CO
x
(a); yields of C
1
–C
3
carboxylic
acids (b) as a function of reaction temperature in the oxidation of propionaldehyde on cat-
alyst VO -TiO in presence of water vapour.
x
2
maximum yield of formic acid was twice as much. Contrary to the oxidation
of propionaldehyde on P/VO catalysts [15], the formation of acrolein was
x
◦
negligible (0.5%). At the temperature region of 220–280 C, small amounts
of ethylene were detected. The yield of ethylene at 220 and 280 C was about
◦
0
.1 and 0.6%, respectively. It was observed that the yield of ethylene increased
◦
drastically with increasing temperature. At 320 C the yield of ethylene was
about 15%.
Comparison of the product formation from acetaldehyde and propionaldehyde.
In order to compare the oxidation behaviour of two aldehydes, the selectiv-
ities to the oxidation products were calculated at various temperatures. The
variation of selectivities for carboxylic acids and CO from oxidation of ac-
x
etaldehyde and propionaldehyde with temperature are depicted in Fig. 3.

1
580
W. Y. Suprun et al.
Fig. 3. Dependence of the selectivity to CO and CO
b) on temperature during the catalytic oxidation of acetaldehyde and propionaldehyde on
catalyst VO -TiO . Reactions conditions: see Figs. 1 and 2.
2 1 3
(a) and C –C carboxylic acids
(
x
2
The selectivity for the catalytic oxidation of acetaldehyde and propionalde-
hyde to the corresponding carboxylic acids at temperatures in the range of
◦
1
20–180 C was observed to be about 80% and 20%, respectively. At tempera-
◦
tures above 220 C the selectivity to acetic acid from acetaldehyde decreased
rapidly, whereas the selectivity to CO increased. At 280 C, the selectivity to
◦
x
acetic acid was only 10%, and the selectivity to CO was about 80–85%.
x
The main oxidation products during oxidation of propionaldehyde at lower
temperature were acetic acid and formic acid. Contrary to the oxidation of
acetaldehyde, the maximum selectivity to propionic acid during oxidation of
◦
propionaldehyde was only 20–15% between 120–160 C. Moreover, the se-
lectivity to formic acid and CO in this temperature region is higher during
x
oxidation of propionaldehyde compared to acetaldehyde.
Therefore, acetaldehyde appears to be oxidized to acetic acid more selec-
tively than propionaldehyde to propionic acid. The low selectivity for propionic
◦
acid and also the formation of formic acid and CO even at 120 C sug-
x
gests a higher lability of propionaldehyde towards the oxidative cleavage. The
propionaldehyde is more susceptible to the C–C bond cleavage compared to
acetaldehyde due to difference in activity of the C–H bonds of CH or CH
2
3
group in the both aldehydes. It can be expected that the secondary C–H bond

Oxidation of Acetaldehyde and Propionaldehyde on a V
2
O
5
-TiO
2
Catalyst
1581
Fig. 4. The possible reaction pathways for the formation of oxidations products at the cata-
lytic oxidation of acetaldehyde (a) and propionaldehyde (b).
on propionaldehyde will be more activated and attacked on the surface of VOx-
catalyst as compared to the primary C–H bond of acetaldehyde
3.2 Kinetic modelling of the reactions pathway
According to the literature [13, 21] the formation of the carboxylic acid from
the aldehyde proceeds via selective oxidation of the C–H bond of the carbonyl
group by active lattice oxygen present on the surface of metal oxides (insertion
of lattice oxygen). Total oxidation products, namely CO and formic acid, can
x
be formed directly from the aldehyde by the decomposition and/or oxidative
cleavage of intermediate species (formate, acetate, or propionate).
In order to appropriate the correct reaction pathways and the rate of for-
mation of each oxidation product, a modelling of the reaction was carried out.
Possible reactions describing the catalytic oxidation of acetaldehyde and pro-
pionaldehyde are shown in Fig. 4. The series-parallel reaction network was
considered for this modelling. The formation of different products was based
on the following assumptions: (a) formic acid, CO and CO can be formed
2
from starting aldehydes and from the carboxylic acids; (b) propionic acid can
be formed only from propionaldehyde; (c) acetic acid can be formed from ac-
etaldehyde and also from oxidative cleavage of propionic acid.

1
582
W. Y. Suprun et al.
For the modelling of oxidation process of both acetaldehyde and propi-
onaldehyde, the experimental data for product distribution at different tempera-
tures were used. Figure 4 summarises the possible reaction pathways for the
oxidation of acetaldehyde and propionaldehyde.
Seven reaction products are possible for the oxidation of acetaldehyde,
namely CO, CO , formic acid, acetic acid, acetaldehyde, O and H O. Never-
2
2
2
theless, modelling reveals that the minimum of four reactions must take place
for generation of the above-mentioned reaction products. Similarly to acetalde-
hyde, for the oxidation of propionaldehyde eight reaction products (CO, CO2,
formic acid, acetic acid, propionic acid, propionaldehyde, O and H O) can
2
2
be obtained by five key reactions. The range of element-species matrix is 3
for propionaldehyde oxidation reaction. Hence, the least of 5 keys reactions
of independent linear key reactions are necessary for the description of the
stoichiometric modelling.
From Fig. 4, a fixed set of reactions was selected to simulate the experimen-
tal data at the different temperatures. A one-dimensional pseudo-homogeneous
plug-flow reactor model was used to describe the steady state under isother-
mal conditions. The pseudo-homogeneous mass balance for the key species, in
terms of molar concentration, can be stated as a differential equation with an
initial condition:
dci
dz
qi
= u
,
dci
dz
where c (atz = 0) = c (measured); i = 1, . . . , 5; and
= the change in
i
io
concentration over the axial coordinate z of the reactor.
The concentration of the linearly dependent compounds was calculated
by stoichiometric relation with the numerical procedure of non-linear regres-
sion analogue to [22]. A fourth-order Runge–Kutta–Mearson algorithm [23]
was adapted for the simultaneous step length integration of all differential
equations. For each temperature, partial mass balance equations with respect
to the axial coordinate z were solved numerically and combined to calculate
the concentration at the end of the catalyst bed. The reaction models were
discriminated by the value of the sum-of-squared differences of model and ex-
perimental values.
9
3
ꢀ
ꢀ
2
(
(c −w )g) ,
(1)
i,j
i,j
i=1 j=1
3
where c = concentration (mol/m ) of the species i at temperature j; g =
ij
weight of the sum-of-squared of all species; and w = experimental values.
ij
The Simplex method of Nedler–Mead [24] was used to minimise the sum-of-
squares by variation of the values of the frequency pre-factor and the activation
energy.

Oxidation of Acetaldehyde and Propionaldehyde on a V
2
O
5
-TiO
2
Catalyst
1583
Table 1. Optimal frequency factors and activation energy of the best schemes for the oxi-
dation of acetaldehyde.
Nr.
reaction
kinetical equation
lg k
o
i
E (kJ/mol)
k
1
c
AcAl
c
O2
1
2
3
CH
CH
CH
3
3
3
CHO + 0.5O
CHO + 2.5O
CHO + 1.5O
2
2
2
→ CH
→ 2CO
→ 2HCOOH
3
COOH
r
r
r
1
2
3
=
=
=
28.76
26.48
24.81
97.0
98.7
88.6
1
+k
c
6
c
CO
k
2
AcAl
c
O2
2
+ 2H
2
O
O
1
+k
c
6
c
CO
k
3
AcAl
c
O2
1
+k
6
c
CO
4
5
CH
CH
3
3
COOH + O
COOH + 2O
2
→ 2CO + 2H
2
O
r
r
4
5
= k
= k
4
c
AcOH
c
O2
O2
27.15
30.79
113.0
133.1
2
→ 2CO + 2H
2
2
5
c
AcOH
c
ads
CO
des
CO
K
6
Sorption of CO
k
6
= K
9.05
0.1
Oxidation of Acetaldehyde. The reaction rates for the formation of acetic acid,
formic acid and CO from acetaldehyde are found to be of first order with
2
respect to acetaldehyde and oxygen (Table 1). Similarly, the reaction rate for
oxidative cleavage of acetic acid to CO and CO is also found to be of first
2
order, with respect to the concentration of acetic acid and oxygen.
The dependence of the above-mentioned reaction rates on the oxygen con-
centration suggests that the reoxidation of the VO /TiO catalyst by oxygen
x
2
from the gas phase might be the rate limiting step for the overall oxidation
process. The kinetic equations for the formation of C , C acid and CO con-
1
2
2
−1
tain term (1+k c ) , where k is the sorption rate coefficient for CO. This
6
CO
6
indicates that the rate for oxidation of acetaldehyde in the reactions 1, 2,
and 3 (Table 1) is dependent on the sorption behaviour of CO on the cata-
lyst surface. The selective oxidation of acetaldehyde (k ) to acetic acid and
1
their direct combustion to CO (k ) has similar activation energy of about
2
2
98 kJ/mol. It suggests that these two reactions proceed parallel and concur-
rently at the catalyst surface. Compared to that, the activation energy for the
oxidative cleavage of acetic acid to CO and CO is 113 and 133 kJ/mol, re-
2
spectively. This indicates that combustion of acetic acid must be favoured at
higher temperatures. The variation in the ratio of the reaction constants for the
formation of CO from acetaldehyde and acetic acid and for formation of CO
2
2
◦
and CO from acetic acid in the temperature range of 120–280 C is summarised
in Fig. 5.
It is evident from the plot that, at low temperatures, CO is primarily
2
formed from direct oxidation of acetaldehyde. Further, it is evident with the
sharp increase in the value of the k /k ratio at higher temperatures that CO
5
4
2

1
584
W. Y. Suprun et al.
Fig. 5. Dependencies of the ratio of the reaction constants for the formation of CO
acetaldehyde and acetic acid (k /k ), and for formation of CO and CO from acetic acid
/k ) on the reaction temperature.
2
from
2
5
2
(k
5
4
Table 2. Optimal frequency factors and activation energy of the best schemes for the oxi-
dation of propionaldehyde.
Nr.
1
reaction
kinetical
equation
lg k
o
i
E
(kJ/mol)
k
1
c
PrAl
c
O2
C
2
H
5
CHO + 2.5O
CHO + 0.5O
2
2
→ 3HCOOH
→ C COOH
COOH + HCOOH
r
1
= 1
27.60
88.7
+k
7
c
PrAl
k
2
c
PrAl
c
O2
2
3
C
C
2
2
H
H
5
5
2
H
5
r
r
2
3
= 1
24.08
10.59
75.1
28.9
+k
7
c
PrAl
COOH + O
2
→ CH
3
= k
3
c
PrOH O2
c
4
5
6
HCOOH → CO + H
CH → 2CO + 2H
COOH + 2O
HCOOH + 0.5O → CO + H
2
O
r
r
r
4
5
6
= k
= k
= k
4
5
6
c
c
c
FA
10.99
111.0
27.76
36.1
480
3
2
2
O
AcOH O2
c
2
2
2
O
FA
c
O2
75.9
ads
PrAl
K
7
Sorption of propionaldehyde (PrAl)
k
7
= K
12.21
42.1
des
PrAl
is formed predominantly from the total oxidation of acetic acid under these
reaction conditions.
Oxidation of propionaldehyde. The overall reaction can be proposed to involve
the following six reactions. The reactions and the corresponding kinetical equa-
tions are listed in Table 2. This table also summarises the calculated values of
the apparent frequency factors and the activation energy for optimal reaction

Oxidation of Acetaldehyde and Propionaldehyde on a V
2
O
5
-TiO
2
Catalyst
1585
Fig. 6. Dependencies of ratio of the reaction constants for the formation of propionic acid,
acetic acid, and formic acid and for their oxidative cleavage on the reaction temperature.
for oxidation of propionaldehyde. The selective oxidation of propionaldehyde
to propionic acid, oxidative cleavage of propionaldehyde to formic acid and
total oxidation of formic acid to CO (reactions 1, 2 and 6; see Table 2) proceed
2
with comparable activation energies of about 75–88 kJ/mol. The oxidative
cleavage of propionic acid to acetic acid and formic acid (reaction 3, see
Table 2) is also found to proceed with even lower activation energy of about
29 kJ/mol.
Thus, the comparable values of activation parameters and frequency fac-
tors explain the observed lower selectivity for propionic acid during oxidation
of propionaldehyde on the VO -TiO catalyst. At the same time, the oxida-
x
2
tive cleavage of acetic acid to CO is limited by the high activation energy
x
of 480 kJ/mol for reaction 5. Therefore, the ratio of acetic acid production to

1
586
W. Y. Suprun et al.
Table 3. Ratio of sum of the reaction constants for the oxidation of acetaldehyde (AcAl)
and propionaladehyde (PrAl) on VO
x
-TiO
2
at various temperatures.
◦
T ( C)
120
140
160
180 200 220
240
260
280
(
k
1
+k
2
)
PrAl
AcAl
8.72 6.95
5.69 4.76 4.06 3.52
3.11 2.78 2.50
(k
1
+k +k
2
3
)
its consumption is very high at low temperatures. Hence, promoting its highly
◦
selective formation at temperatures lower than 200 C.
Additional information about reaction behaviour of propionaldehyde can
be obtained by analysing the reaction constants for the formation of oxygenates
at various temperatures. The comparison of ratio of the reaction constants for
the formation of propionic acid (k ), acetic acid (k ), and formic acid (k , k )
2
3
1
3
with respect to that for the oxidative cleavage of propionic acid (k ), at different
3
temperatures is depicted in Fig. 6.
◦
It can be seen from Fig. 6a that at 120 C the constant for the formation of
propionic acid (k ) is twice as high as that for the formation of formic acid (k ).
2
1
◦
◦
With the temperature increase from 120 C to 280 C the ratio k /k decreased
2
1
and the ratio k /k increased. This explains the improved selectivity observed
2
3
for the formation of formic acid at higher temperatures. In Fig. 6b the curve
representing the ratio between reaction rates for formation and consumption of
formic acid
k +k
1
3
6
,
k +k
4
◦
rises sharply with the increase in temperature beyond 200 C. This indicates
that the formation of formic acid is favoured at high temperatures. Formic acid
can be formed in two reaction ways: (i) directly from propionaldehyde (k ) and
1
(
ii) via the oxidative cleavage of propionic acid (k ). Since the ratio of reaction
3
rates for formation of formic acid via oxidative cleavage of propionic acid to its
overall consumption
k3
k +k6
4
decreases with temperature, it can be inferred that the selective formation of
formic acid from the direct oxidation of propionaldehyde is the favoured step
◦
at temperatures above 200 C.
Oxidation activity of acetaldehyde and propionaldehyde. In order to com-
pare the activity of propionaldehyde and acetaldehyde for their oxidation on
the VO -TiO catalyst, the ratio of the reaction constants for their different
x
2

Oxidation of Acetaldehyde and Propionaldehyde on a V
2
O
5
-TiO
2
Catalyst
1587
reactions (as mentioned in Tables 1 and 2) was evaluated in detail for the
◦
temperature range of 120–280 C. These data are summarised in Table 3. The
ratio of the reaction constants decreased with increasing temperature indicating
higher oxidation activity of propionaldehyde for the given reaction condi-
tions, especially at lower temperatures. The activation energies for the direct
oxidation of acetaldhyde (reactions 1, 2, and 3) given in Table 1 are about
10 kJ/mol higher compared to the activation energies for the direct oxidation
of propionaldehyde (reactions 1 and 2) given in Table 2. From these results
it can be concluded that the oxidation of acetaldehyde is favored at higher
temperatures.
4. Conclusion
The oxidation of acetaldehyde and propionaldehyde follows two main routes:
1) selective oxidation to respective carboxylic acids, and (2) oxidative cleav-
(
age to lower carboxylic acids. The first route was favoured for both aldehydes
under low temperature conditions, whereas the second route was favoured for
◦
temperatures above 200 C. Though the routes followed remained the same,
the following differences were observed in the oxidations pathways of propi-
onaldehyde and acetaldehyde:
i) While the formation of CO from oxidation of acetaldehyde was found
2
to proceed mainly through the total oxidation of acetaldehyde, the main re-
action route for the formation of CO from propionaldehyde was through the
2
oxidative cleavage of C , C and C carboxylic acids.
1
2
3
ii) In the case of the oxidation of propionaldehyde the rate for the con-
sumption of aldehyde was found to depend on terms related to concentration
of aldehyde and oxygen and on term describing sorption behaviour of propi-
onaldehyde. On the other hand, for oxidation of acetaldehyde the reaction rate
was found to depend on terms related to the concentration of aldehyde and
oxygen and the sorption’s term for carbon monoxide.
The kinetic model confirmed the consecutive formation of C –C carboxylic
1
3
acids. CO and CO may be produced directly via decomposition of aldehy-
2
des on the catalyst surface and via total oxidation of carboxylic acids. The
low selectivity for the formation of propionic acid from propionaldehyde can
be attributed to the low stability of propionic acid and high activity of propi-
onaldehyde towards the oxidative cleavage. These results are also relevant for
explaining the low selectivity for carboxylic compounds during the ODH of
◦
C –C hydrocarbons at temperatures above 300 C.
2
4
Acknowledgement
The authors wish to acknowledge the Deutsche Forschungsgemeinschaft for
their financial support (PA: 194/15).

1
588
W. Y. Suprun et al.
References
1
. B. M. Weckhuysen and D. E. Keller, Catal. Today. 78 (2003) 25.
2. H. Miyata, M. Kohno, T. Ono, T. Ohno, and F. Hatayama, J. Chem. Soc., Faraday
Trans. 1 85 (1989) 3663.
3. K. Kaneko, T. Koyama, and S. Wada, Bull. Jpn. Petr. Inst. 16 (1974) 17.
4. M. E. Harlin, V. M. Niemi, and A. O. Krause, J. Catal. 195 (2002) 67.
5. J. M. Lopez Niento, J. Soler, P. Conception, J. Herguido, M. Menendez, and J. San-
tamaria, J. Catal. 185 (1999) 324.
6
7
. W. Slinkard and P. B. DeGroot, J. Catal. 68(2) (1981) 423.
. G. Busca, V. Lorenzelli, G. Oliveri, and G. Ramis, Stud. Surf. Sci. Catal. 82 (1994)
2
53.
8
. R. Brockhaus, Chem. Ing. Techn. 38 (1966) 1039.
9
. Ch. Rüdinger and H.-J. Eberle, Eur. Pat. Appl. EP 960874 (1999) A1 19991201.
1
1
1
0. U. Bentrup, A. Brückner, C. Rüdinger, and H.-J. Eberle, Appl. Catal. A: General.
69 (2004) 237.
1. T. Seiyama, K. Nita, T. Maehara, N. Yamazone, and Y. Takita, J. Catal. 49 (1977)
64.
2
1
2. T. Machold, W. Suprun, H. Papp, H.-J. Eberle, and C. Rüdinger, Proceedings of the
DGMK/SCI-Conference “Oxidation and Funktionalization” Tagungsbericht 2005-
2
, Milan, Italy (2005).
1
3. G. Centi, F. Cavani, and F. Trifirò, Selective Oxidation by Heterogeneous Catalysis.
Kluwer Academic/Plenum Publishers, New York (2001), pp. 431–437.
4. J. Svalucha, L. Tichy, and J. Machek, Catal. Letter 2 (1989) 257.
1
1
5. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Bull. Chem. Soc. Japan. 67
(
1994) 551.
1
6. N. E. Quaranta, J. Soria, V. Cortes Corberan, and J. L. G. Fierro, J. Catal. 171 (1997)
1.
1
1
1
2
7. A. Erdöhely and M. Solymosi, J. Catal. 123 (1990) 3122.
8. M. Iwamoto, T. Taga, and S. Kagawa, Chem. Lett. (1982) 1469.
9. B. Evnin, J. A. Rabo, and P. H. Kasai, J. Catal. 30(1) (1973) 109.
0. W. Ya. Suprun, D. P. Sabde, H.-K. Schädlich, B. Kubias, and H. Papp, Appl. Catal.
A: General. 289 (2005) 66.
2
2
2
2
1. K. Krauß, A. Drochner, M. Fehlings, J. Kunert, and H. Vogel, J. Molec. Catal. A:
Chemical. 162 (2000) 413.
2. K. Schädlich, W. Suprun, H.-K. Schädlich, and H. Papp, Z. Phys. Chem. 218 (2004)
1011.
3. C. W. Gear, Numerical Initial Value Problems in Ordinary Differential Equations.
Prentice Hall (1971).
4. J. A. Nedler and R. Mead, Comp. J. 7 (1965) 308.