Oxidative and non-oxidative degradation of C1-C3 carboxylic acids over V2O5/TiO2 and MoVTeNb oxides: A comparative study

Article abstract of DOI:10.1016/j.apcata.2013.06.018

Carboxylic acids are valuable chemical products, which can be produced efficiently by catalytic oxidation of various organic compounds using vanadium-based catalysts. Degradation resistance of carboxylic acids under applied reaction conditions is an important factor influencing selectivity of such reactions. This paper addresses the oxidative and non-oxidative gas phase degradation of formic, acetic and propionic acids over V₂O ₅/TiO₂ and MoVTeNb mixed oxides. Formic acid has been found to undergo mainly acid catalyzed decarbonylation, regardless the presence or absence of oxygen in the feed. Both acetic and propionic acids underwent oxidation (in aerobic conditions) through one-carbon degradation steps until CO and CO₂ were produced. The mechanistic aspects of these transformations are discussed. The key intermediacy of 2-(hydro)peroxy carboxylic acids is suggested, based on the nature of primary reaction products. The catalytic activity of MoVTeNb mixed oxides toward oxidative and non-oxidative degradation of acetic and propionic acids appeared notably lower compared to that of V₂O₅/TiO₂. This may be linked to the enhanced efficiency of the former catalyst in selective oxidation of organic substrates to carboxylic acids.

Full text of DOI:10.1016/j.apcata.2013.06.018

Applied Catalysis A: General 466 (2013) 45–50
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Oxidative and non-oxidative degradation of C1–C3 carboxylic acids
over V2O5/TiO2 and MoVTeNb oxides: A comparative study
Vladimir I. Soboleva,∗, Konstantin Yu. Koltunova,b,∗∗
a Boreskov Institute of Catalysis, Russian Academy of Science, Pr. Akademika Lavrentieva, 5, Novosibirsk 630090, Russia
b Novosibirsk State University, Pirogova, 2, Novosibirsk 630090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2013
Received in revised form 3 June 2013
Accepted 17 June 2013
Available online 25 June 2013
Carboxylic acids are valuable chemical products, which can be produced efficiently by catalytic oxida-
tion of various organic compounds using vanadium-based catalysts. Degradation resistance of carboxylic
acids under applied reaction conditions is an important factor influencing selectivity of such reactions.
This paper addresses the oxidative and non-oxidative gas phase degradation of formic, acetic and pro-
pionic acids over V2O5/TiO2 and MoVTeNb mixed oxides. Formic acid has been found to undergo mainly
acid catalyzed decarbonylation, regardless the presence or absence of oxygen in the feed. Both acetic and
propionic acids underwent oxidation (in aerobic conditions) through one-carbon degradation steps until
CO and CO2 were produced. The mechanistic aspects of these transformations are discussed. The key
intermediacy of 2-(hydro)peroxy carboxylic acids is suggested, based on the nature of primary reaction
products. The catalytic activity of MoVTeNb mixed oxides toward oxidative and non-oxidative degrada-
tion of acetic and propionic acids appeared notably lower compared to that of V2O5/TiO2. This may be
linked to the enhanced efficiency of the former catalyst in selective oxidation of organic substrates to
carboxylic acids.
Keywords:
V2O5/TiO2
Mo–V–Te–Nb mixed oxides
Formic acid
Acetic acid
Propionic acid
Degradation
Oxidative degradation
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Unfortunately, the concerned synthetic approach to carboxylic
acids often suffers from overoxidation to COx, which is usually
Carboxylic acids are important bulk chemicals. In principle, they
can be obtained by catalytic aerobic oxidation of various organic
compounds, such as aldehydes, alcohols, hydrocarbons and oth-
ers. It is relevant in this connection that vanadium-based catalysts
proved to be extraordinary efficient in large variety of oxidation
reactions, including production of carboxylic acids [1]. For exam-
ple, formaldehyde is readily oxidized to formic acid over V2O5/TiO2
[2]. Ethanol oxidation gives acetic acid in high yields when the
same catalyst is used [3–5]. The incorporation of special additives
and promoters can stabilize a certain crystal structure of vana-
dium oxide, which determines the catalytic activity [6,7]. Thus,
three-component MoVNb oxides supported on TiO2 as well as
bulk four-component MoVTeNb mixed oxides [8,9] have shown
the best selectivity and productivity in ethanol (as well ethane and
ethylene) oxidation to acetic acid [10–12]. Furthermore, butenes,
2-butanone and 2-butanol underwent oxidative scission in the
presence of V2O5/TiO2 to give again acetic acid [13–15].
regarded as nonselective oxidation of starting materials or inter-
mediate compounds. However, insufficient resistance of a target
carboxylic acid should also be taken into account as an impor-
tant factor influencing selectivity of these reactions. The catalytic
decomposition of a number of aliphatic carboxylic acids upon var-
ious reaction conditions has been extensively studied [16,17]. In
particular, it was shown recently that aqueous formic, acetic and
propionic acids underwent degradation over a technical 6 wt%
V2O5/TiO2 catalyst [18]. This offered an incentive to a systematic
study on behavior of carboxylic acids in the presence of various
vanadia catalysts. In this paper, we focused our investigation on
activity of typical (but distinct) representatives of widely used
vanadium catalysts, such as V2O5/TiO2 and MoVTeNb mixed oxides,
toward degradation of formic, acetic and propionic acids under
oxidative and non-oxidative conditions. The catalytic activity of
TiO2 support was also tested for the purpose of comparison (see
also Ref. [17]).
2. Experimental
∗
Corresponding author. Tel.: +7 383 326 9765; fax: +7 383 330 8056.
Corresponding author at: Boreskov Institute of Catalysis, Russian Academy of
∗∗
A 20 wt% V2O5/TiO2 sample (SBET = 111 m2 g−1) designated as
V2O5/TiO2 was obtained by impregnating TiO2 (anatase; specific
surface area, 350 m2 g−1; Alfa Aesar, Germany) with an aqueous
solution of vanadyl oxalate prepared using >99.6% V2O5 (Reakhim,
Science, Pr. Akademika Lavrentieva, 5, Novosibirsk 630090, Russia.
Tel.: +7 383 326 9765; fax: +7 383 330 8056.
E-mail addresses: visobo@catalysis.ru (V.I. Sobolev), koltunov@catalysis.ru
(K.Yu. Koltunov).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.018

46
V.I. Sobolev, K.Yu. Koltunov / Applied Catalysis A: General 466 (2013) 45–50
3
2
1
3
a
b
2
1
0
3
0
3
100
150
200
100
150
200
c
d
2
1
2
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0
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0
3
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150
200
100
150
200
e
f
2
1
2
1
0
0
100
150
200
250
300 100
150
200
250
300
T / oC
T / oC
Fig. 1. Formic acid degradation over TiO2 (a and b); V2O5/TiO2 (c and d) and MoVTeNb (e and f) as a function of temperature: concentration of formic acid ( ), CO ( ) and
CO2 (ꢀ). Feed compositions: 2.5 vol% HCOOH, 20.0 vol% O2 and He balance (a, c, e); 2.5 vol% HCOOH and He balance (b, d, f); GHSV = 3600 h−1
.
Russia) and >97% oxalic acid (Reakhim, Russia), followed by drying
in air at 110 ◦C for 24 h and calcination in an air flow (50 mL min−1
measured inside the reactor using a thermocouple inserted into
the catalyst bed. The heating rate was 2 K min−1. During the cat-
alytic runs, gas samples were analyzed periodically by integrated
online gas chromatography (GC, gas chromatograph Tsvet-500;
30 m × 0.32 mm monolithic poly(divinylbenzene) capillary col-
umn; 170 ◦C) with a flame-ionization detector for determination
of organics. For determination of CO and CO2, they were prelimi-
narily separated on a 1.5 m × 3 mm steel column filled with Porapak
Q at 20 ◦C followed by methanation.
)
for 4 h at 400 ◦C. The resulting catalyst was characterized by XRD
analysis, IR and Raman spectroscopy, temperature-programmed
reduction with hydrogen, and differential dissolution [2]. Vana-
dium in V2O5/TiO2 is represented by both V2O5 bulk phase (mixture
of amorphous and crystalline V2O5) and monolayer polymeric VOx
species [2].
The procedure for preparing and classifying the sample of
mixed MoVTeNb oxides, designated here as MoVTeNb, was
described elsewhere [19]. A sample of nominal composition
Mo1V0.3Te0.23Nb0.12Ox (obtained by spray drying at 190 ◦C) was
calcined in He flow at 600 ◦C to ensure the formation of the tar-
geted active phase. After calcination, specific surface area of the
sample was 7.3 m2 g−1; the molar ratio Te/Mo decreased from 0.23
to 0.16. Thus, the final chemical composition of the sample was
Mo1V0.3Te0.12Nb0.12Ox [19]. The catalytic properties of MoVNbTe
are related to the presence of two crystalline phases: orthorhombic
M1 and hexagonal M2, which act as a bifunctional catalyst. The
M2 phase consists of a well crystallized MoVTe oxide, and M1 is
composed of MoVNbTe crystals with a minor Te content [8,9,19].
The temperature programmed gas-phase reactions were per-
formed in a quartz tube flow reactor with an internal diameter
of 6 mm using shaped catalyst granules (d = 0.25–0.5 mm; 1 mL,
ca. 500 mg). The reactive gas mixture (60 mL min−1) was fed into
the reactor filled with the catalyst; the reactor was operated
under atmospheric pressure. The reaction temperature ( 1 ◦C) was
Prior to the kinetic measurements, the catalysts were activated
in the reactor at 400 ◦C in a flow of O2/He (1:3) for 1 h. The reactor
was then cooled to 100 ◦C and the feed was switched at this tem-
perature to the reactive gas mixture for 20–30 min in order to reach
steady state initial conditions.
3. Results and discussion
3.1. Reactivity of formic acid
At first, temperature-dependent behavior of formic acid in
aerobic and anaerobic conditions was studied. The catalytic perfor-
mance of TiO2, V2O5/TiO2 and MoVTeNb samples in a single-pass
flow reactor system is summarized in Fig. 1. As it can be seen, CO
is the main product of the formic acid degradation for each cat-
alyst regardless of the presence or absence of oxygen. Moreover,
oxygen does not influence the rate of these reactions (see Suppor-
ting information, Fig. 1S). It is evident that formic acid undergoes

V.I. Sobolev, K.Yu. Koltunov / Applied Catalysis A: General 466 (2013) 45–50
47
100
Scheme 1. Mechanism of the acid-catalyzed decarbonylation of formic acid.
80
60
40
20
0
100
75
50
25
0
1
3
2
4
100
150
200
250
T / oC
Fig. 3. Ethanol oxidation over V2O5/TiO2: conversion of ethanol ( ), selectivities
to acetaldehyde ( ), acetic acid ( ), CO ( ) and CO2 (ꢀ). Feed composition: 2 vol%
100
200
300
400
EtOH, 18 vol% O2 and He balance, GHSV = 3600 h−1
.
o
T /
C
Fig. 2. Formic acid conversion over TiO2 (1); V2O5/TiO2 (2), MoVTeNb (3) and with-
and MoVTeNb. According to the data in Fig. 4, MoVTeNb ensures
best resistanceof aceticacid, whichis stablerightup to 250 ◦Cunder
aerobic conditions and practically does not react in the absence of
oxygen. This can be part of the reason for the extraordinary catalytic
efficiency of MoVTeNb in ethanol oxidation to acetic acid [11]. In
contrast, V2O5/TiO2 is notably more active toward undesirable oxi-
dation of acetic acid, which is in agreement with the diminished
efficiency of this catalyst in the acetic acid production [3–5].
It is significant that the main products of the acetic acid oxi-
dation are CO and CO2 in about 1:1 ratio on both V2O5/TiO2 and
MoVTeNb catalysts (Fig. 4). The equimolar generation of carbon
oxides can be explained by intermediate producing of formic acid,
which then undergoes decarbonylation (Scheme 2). In our experi-
ments we did not observe the intermediate formic acid, although
it was produced with 5–20% selectivity at 175–275 ◦C upon oxida-
tion of aqueous acetic acid over V2O5/TiO2 [18]. This discrepancy is
explained by relatively facile decarbonylation of formic acid in our
case, providing low content of water in the feed (see above).
Unlike the vanadium-containing catalysts, TiO2 alone evokes
another type of reactivity of acetic acid. Under anaerobic condi-
tions acetic acid converts into equimolar mixture of acetone and
CO2 (Fig. 4b). This is in accordance with the literature data. Simi-
lar reaction is known as a synthetic approach to obtain acetone via
thermolysis of acetates [25]. Moreover, the analogous conversion of
acetic acid was carried out catalytically over various metal oxides,
including TiO2 [17,26,27]. When oxygen is present in the feed, the
produced acetone undergoes oxidation, especially at temperatures
above 250 ◦C (Fig. 4a). In addition to this reaction pathway, direct
oxidation of acetic acid to CO2 and CO (i.e. HCOOH) also takes place.
As a final result, a mixture of CO and CO2 (predominant product) is
formed (Scheme 3).
out catalyst (4) as a function of temperature. Feed composition: 2.5 vol% HCOOH and
He balance, GHSV = 3600 h−1
.
facile acid-catalyzed decarbonylation to give CO in agreement
with generally accepted reaction mechanism (Scheme 1) [20–22].
Fig. 2 combines temperature dependent profiles for the formic acid
decomposition catalyzed by TiO2, V2O5/TiO2 and MoVTeNb in the
absence of oxygen. The catalytic activity of V2O5/TiO2 is somewhat
higher compared to that of TiO2, whereas MoVTeNb is notably less
active. Very similar results were obtained also for aerobic condi-
tions (see Supporting information, Fig. 2S). Obviously, the catalytic
activity in the formic acid degradation reflects the acidity of the
catalysts upon the reaction conditions [21,23].
According to the literature, formic acid showed comparatively
higher resistance in the presence of technical V2O5/TiO2 [18]. This,
however, was most likely due to a high content of water (16.5 vol%)
in the feed [18]. Indeed, the suppression effect of water vapor on
decarbonylation of formic acid over V2O5/TiO2 was unambiguously
shown by kinetic and in situ FT-IR studies [24]. It seems reasonable
that water as a stronger base (compared to formic acid) competes
efficiently for adsorption in the acid sites and therefore prevents
the acid-catalyzed reaction.
A negligible evolution of CO2 during degradation of formic
acid over V2O5/TiO2 and MoVTeNb (Fig. 1) can be attributed to a
reversible production of formates and their partial cleavage on the
catalyst surface [21,24].
3.2. Reactivity of acetic acid
Initially, we have checked the activity of V2O5/TiO2 sample in
ethanol oxidation to acetic acid (Fig. 3) [5]. In particular, Fig. 3
shows temperature-dependent selectivities to acetic acid, CO and
CO2 at complete ethanol conversion, which is achieved to 140 ◦C. It
is apparent that carbon oxides are produced starting from the very
beginning of the acetic acid generation. Noteworthy is the ratio of
CO and CO2, which is about 1:1 being quite constant right up to
250 ◦C. Moreover, in a separate experiment, we observed that CO
is not oxidized to CO2 over V2O5/TiO2 up to 300 ◦C. Therefore, one
may propose direct oxidation of acetic acid to carbon oxides simi-
lar to that for oxidation of aqueous acetic acid over V2O5/TiO2 [18].
Indeed, further experiments on the acetic acid resistance (at condi-
tions applied to ethanol oxidation) supported such an assumption.
Fig. 4 shows temperature-dependent behavior of acetic acid in aer-
obic and anaerobic conditions in the presence of TiO2, V2O5/TiO2
Notably that acetone is almost not generated on V2O5/TiO2
(Fig. 4c and d). This is due to an exhaustive coverage of the support
by vanadium oxide (see Section 2 and Ref. [2]).
Fig. 5 combines temperature dependent profiles for the acetic
acid degradation under oxidation conditions catalyzed by TiO2,
Scheme 2. Oxidative degradation of acetic acid over V2O5/TiO2 and MoVTeNb
mixed oxides.

48
V.I. Sobolev, K.Yu. Koltunov / Applied Catalysis A: General 466 (2013) 45–50
b
a
1,2
0,8
0,4
0,0
1,2
0,8
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0,0
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200
250
300
150
200
250
300
1,2
0,8
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0,0
1,2
0,8
0,4
0,0
c
d
100
150
200
250
300
100
150
200
250
300
1,2
0,8
0,4
0,0
1,2
0,8
0,4
0,0
f
e
150
200
250
300
350
400
150
200
250
/ oC
300
350
T / oC
T
Fig. 4. Acetic acid degradation over TiO2 (a and b); V2O5/TiO2 (c and d) and MoVTeNb (e and f) as a function of temperature: concentration of acetic acid ( ), CO ( ), CO2
(ꢀ), and acetone ( ). Feed compositions: 1 vol% CH3COOH, 20 vol% O2 and He balance (a, c, e); 1 vol% CH3COOH and He balance (b, d, f); GHSV = 3600 h−1
.
V2O5/TiO2 and MoVTeNb. Surprisingly, TiO2 support revealed
significant catalytic activity, which is even higher than that of
MoVTeNb.
3.3. Reactivity of propionic acid
According to the literature, aqueous propionic acid is oxidized
over V2O5/TiO2; the main products are acetic acid, formic acid
and carbon oxides [18]. We have studied reactivity of propionic
acid on V2O5/TiO2 and MoVTeNb in the presence and absence
of oxygen (Fig. 6). Likewise acetic acid, it is notably more reac-
tive on V2O5/TiO2 than on MoVTeNb (Fig. 6). In general, propionic
acid is found less resistant than acetic acid (see, for comparison,
Fig. 7).
Noteworthy is the formation of diethyl ketone on V2O5/TiO2 at
temperatures above 300 ◦C under anaerobic conditions (Fig. 6b).
This reaction was shown earlier to occur on TiO2 surface [17]
and is analogous to acetone production from acetic acid (see
above).
Under oxidation conditions propionic acid converts to a set of
products (Fig. 6a and c). Among the primary products appeared
to be acetaldehyde, CO2 and acetic acid, which are observed from
the beginning of the reaction. However, the early appearance of
acetic acid may be related to immediate oxidation of acetaldehyde.
Finally, a mixture of CO and CO2 is formed at temperatures above
250 and 380 ◦C for V2O5/TiO2 and MoVTeNb, respectively.
Scheme 3. Oxidative degradation of acetic acid over TiO2.
100
75
50
25
0
3
2
1
150
200
250
T / oC
300
350
400
Fig. 5. Acetic acid conversion over TiO2 (1); V2O5/TiO2 (2) and MoVTeNb (3) as a
function of temperature. Feed composition: 1 vol% CH3COOH, 20 vol% O2 and He
balance, GHSV = 3600 h−1
.

V.I. Sobolev, K.Yu. Koltunov / Applied Catalysis A: General 466 (2013) 45–50
49
0,4
a
b
0,6
0,4
0,2
0,0
0,6
0,2
0,0
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150
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300
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150
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250
300
350
0,4
0,2
0,0
c
d
0,4
0,2
0,0
150
200
250
T / oC
300
350
200
250
300
T / oC
350
Fig. 6. Propionic acid degradation over V2O5/TiO2 (a and b) and MoVTeNb (c and d) as a function of temperature: concentration of propionic acid ( ), acetic acid ( ),
acetaldehyde (ꢁ), CO ( ), CO2 (ꢀ) and diethyl ketone ( ). Feed compositions: 0.4 vol% C2H5COOH, 20 vol% O2 and He balance (a and c); 0.4 vol% C2H5COOH and He balance
(b and d); GHSV = 3600 h−1
.
3.4. Proposed mechanism for the carboxylic acids oxidation
on vanadium oxide as a result of dissociative adsorption), under-
goes initially hydrogen atom transfer (HAT) [32–34] upon action
of vanadium oxides to give radicals 1. The same radicals were sug-
gested earlier as key intermediates in oxidative reactions of acetic
and propionic acids in the presence of Mn(III) acetate [35]. Reaction
of 1 with O2 will give peroxy radicals 2, which can undergo immedi-
ate thermal decomposition to yield corresponding aldehyde RCHO
and CO2 [31] or give rise to 2-hydroperoxy acid 3 (Scheme 4, Path-
way 1). The latter can easily transform to RCHO and CO2 as a
result of acid-catalyzed scission [29]. Note that the transient for-
mation of acetaldehyde, proved experimentally for the propionic
acid oxidation (Fig. 6a and c), strongly supports the suggested
mechanism. In case of the acetic acid oxidation, the intermediate
production of formaldehyde is anticipated. However, formalde-
hyde is readily oxidized to formic acid and therefore not observed
It seems very likely that oxidation of a linear carboxylic acid
over V2O5/TiO2 and MoVTeNb follows a general mechanism includ-
ing one-carbon degradation steps as long as formic acid (or CO) is
produced. Worth to note, that there are not too many reactions sus-
ceptible to yield Cn−1 aldehydes upon degradation of Cn carboxylic
acids. Literature precedents exist for conversions of 2-hydroxy and
2-hydroperoxy carboxylic acids to aldehydes accompanied by CO
[28] and CO2 [29–31] elimination, respectively. Since CO2 is the de
facto initial co-product of oxidative degradation for both propio-
nic and acetic acids, the intermediacy of 2-hydroperoxy carboxylic
acids may be surmised.
A hypothetical reaction mechanism for the catalytic oxidation
of acetic and propionic acids is depicted in Scheme 4. The adsorbed
carboxylic acid, RCH2COOH (or corresponding carboxylate, formed
[2]. Alternatively, radicals
1 may undergo electron transfer
100
75
3
1
2
50
25
0
100
150
200
250
300
T / oC
Fig. 7. Catalytic performance of V2O5/TiO2 as a function of temperature: conversions
of formic acid (1), acetic acid (2) and propionic acid (3). Feed compositions: 2.5 vol%
HCOOH (or 1 vol% CH3COOH, or 0.4 vol% C2H5COOH), 20 vol% O2 and He balance,
Scheme 4. Proposed mechanism of one-carbon oxidative degradation of carboxylic
acids over V2O5/TiO2 and MoVTeNb mixed oxides.
GHSV = 3600 h−1
.

50
V.I. Sobolev, K.Yu. Koltunov / Applied Catalysis A: General 466 (2013) 45–50
to give carbocations 4 (Scheme 4, Pathway 2). The electron trans-
fer seems energetically unfavorable, especially when R = H [36], as
the carboxy-carbocations are highly destabilized species. However,
formation of 4 (R = CH3) is more probable. Reaction of this ion with
water will lead to producing of lactic acid (5, R = CH3). The lat-
ter can undergo acid-catalyzed decomposition into acetaldehyde
and CO [28]. Also, lactic acid may be oxidized to pyruvic acid (6,
R = CH3) followed by acid-catalyzed decomposition to give equimo-
lar mixture of acetic acid and CO [37,38]. As a result, the increased
overall production of CO could happen. Indeed, in case of propionic
acid oxidative degradation, the actual ratio of CO to CO2 exceeds
the expected 1:2 ratio assumed by combination of Scheme 2 and
Pathway 1 of Scheme 4.
The proposed mechanism of reaction is in agreement with the
relatively facile oxidation of propionic acid, which is due to easier
formation of 1, when R = CH3, because of significant stabilization
of such radicals by methyl group [39]. It should be noted also that
radicals 1 can be produced additionally as a result of HO• attack
on carboxylic acid [40]. The hydroxyl radicals could be derived
from 2 according to Scheme 4, and the process might be partially
autocatalytic.
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In summary, degradation resistance of formic, acetic and pro-
pionic acids in the presence of TiO2, V2O5/TiO2 and MoVTeNb
mixed oxides was investigated. Formic acid appeared to be the most
unstable of the carboxylic acids due to comparatively facile, acid-
catalyzed decarbonylation, which occurred regardless of aerobic or
anaerobic conditions. Acetic acid, in its turn, showed notably higher
stability than propionic acid.
Unlike V2O5/TiO2, MoVTeNb mixed oxides ensure more resis-
tance both of acetic and propionic acids toward oxidative and
non-oxidative reactions. This could be a reason for the extraor-
dinary selective ethanol oxidation to acetic acid catalyzed by
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Acknowledgments
The authors thank Dr. G.Y. Popova, Dr. E.V. Ischenko and Dr.
V.M. Bondareva (Boreskov Institute of Catalysis) for supplying and
characterizing samples of the examined catalysts.
Appendix A. Supplementary data
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Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2013.
06.018.