Reaction products and pathways in the selective oxidation of C 2-C4 alkanes on MoVTeNb mixed oxide catalysts

Article abstract of DOI:10.1016/j.cattod.2010.01.046

The catalytic properties of MoVTeNbO catalysts during the selective oxidation of short chain alkanes and olefins (C₂-C₄) have been comparatively studied. The main reaction products have been: ethylene from ethane, acrylic acid from propane, maleic anhydride from n-butane and methacrolein from isobutane. FTIR studies of the adsorption of the main reaction products, i.e. olefins and aldehydes, over MoVTeNbO catalyst has been carried out. Accordingly, the reaction pathway is explained on the basis of the characteristics of the alkane fed, the stability and reactivity of both the intermediates and the reaction products, and the nature of the catalytic sites involved in each reaction.

Full text of DOI:10.1016/j.cattod.2010.01.046

Catalysis Today 157 (2010) 291–296
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Reaction products and pathways in the selective oxidation of C –C alkanes on
2
4
MoVTeNb mixed oxide catalysts
a,
b
a
a
b
b
J.M. L o´ pez Nieto *, B. Solsona , P. Concepci o´ n , F. Ivars , A. Dejoz , M.I. V a´ zquez
a
Instituto de Tecnolog ı´ a Qu ı´ mica (UPV-CSIC), Campus de la Universidad Polit e´ cnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain
Dep. Ingenier ı´ a Qu ı´ mica, c/ Dr. Moliner, Universidad de Valencia, 46100 Burjassot, Spain
b
A R T I C L E I N F O
A B S T R A C T
Article history:
The catalytic properties of MoVTeNbO catalysts during the selective oxidation of short chain alkanes and
Available online 3 March 2010
2 4
olefins (C –C ) have been comparatively studied. The main reaction products have been: ethylene from
ethane, acrylic acid from propane, maleic anhydride from n-butane and methacrolein from isobutane.
FTIR studies of the adsorption of the main reaction products, i.e. olefins and aldehydes, over MoVTeNbO
catalyst has been carried out. Accordingly, the reaction pathway is explained on the basis of the
characteristics of the alkane fed, the stability and reactivity of both the intermediates and the reaction
products, and the nature of the catalytic sites involved in each reaction.
Keywords:
MoVTeNb mixed oxides
Selective oxidation of alkanes
In situ FTIR spectra
ß 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Lower alkanes are interesting raw materials in the production
ethylene [16], relatively selective in the oxidation of n-butane to
maleic anhydride [17] but presenting a low selectivity in the
partial oxidation of isobutane [18].
of olefin and/or oxygen containing products by partial oxidation
reactions [1–10]. However, at the present, only the partial
oxidation of n-butane to maleic anhydride over VPO catalysts
has achieved commercial operation [5].
On the other hand, the comparison of the catalytic behaviour of
different alkanes can contribute to the understanding of the
catalytic performance of catalysts. This is the case of comparative
studies on the partial oxidation of lower alkanes over VPO [6],
niobium and pyridine-exchanged heteropolyacids [7] catalysts,
Typically, the most efficient MoVTe(Sb)NbO catalysts present
generally two crystalline phases [14–26]: (i) an orthorhombic
(AO)2ꢀ2x(A
0 < x <1), the so-called M1 phase; and (ii) an orthorhombically
distorted (A O)M 19 phase (A = Te or Sb and M = Mo, V, Nb), the
so-called M2 phase. In addition, TeMo 16 (or Sb Mo10 ),
(V,Nb) 14 and/or tetragonal bronzes can be also present
2 x 20
O) M O56 (A = Te or Sb and M = Mo, V, Nb; with
2
6
O
5
O
4
O
x
x
Mo5ꢀxO
depending on the catalyst preparation procedure [22]. Pure M1
phase based catalysts can be obtained by hydrothermal synthesis
[13,14,16,22–25] or by using post-synthesis treatment [26].
In this paper it will be presented a comparative study on the
but also on the oxidative dehydrogenation of C
containing catalysts [8–10].
2 4
–C alkanes over V-
However, Mo-V-O based catalysts appear as the most important
catalysts in alkane activation [11–26]. MoVNbO catalysts were
reported as active and selective in the ODH of ethane [11], although
2 4
partial oxidation of C –C alkanes over a MoVTeNbO mixed metal
oxide catalyst, prepared hydrothermally, and presenting the M1
phase. It will be shown that the nature of the reaction products and
the corresponding intermediates are very different depending on
the alkane fed. According to these results different reaction
pathways will be proposed.
3 4
they show a poor selectivity in the ODH of C –C alkanes [12].
However, Te- or Sb-containing Mo-V-O catalysts, prepared
hydrothermally, were reported as active and relatively selective
in both the ODH of ethane and the partial oxidation of propane
[
13,14].
Ushikubo et al. proposed in the early 1990s that MoVTe(Sb)NbO
2. Experimental
mixed metal oxides could be an extremely efficient catalytic
system for the selective oxidation of propane to acrylic acid [15].
More recently, these multicomponent catalysts have been reported
to be very selective in the oxidative dehydrogenation of ethane to
MoVTeNb oxidic bronze catalyst, with a Mo/V/Te/Nb atomic
ratio of 1/0.24/0.24/0.13, was prepared hydrothermally from an
aqueous solution containing ammonium heptamolybdate tetra-
hydrate, vanadyl sulphate, telluric acid and niobium oxalate,
according to the procedure previously proposed [22]. The solid
2
obtained was then heat-treated in flowing N at 600 8C for 2 h.
*
Corresponding author. Fax: +34 963877809.
Chemical analyses of the catalysts were carried out by atomic
absorption spectroscopy (in the case of Mo, V and Te elements) or
E-mail address: jmlopez@itq.upv.es (J.M. L o´ pez Nieto).
0920-5861/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.01.046

2
92
J.M. L o´ pez Nieto et al. / Catalysis Today 157 (2010) 291–296
inductively coupled plasma atomic emission spectroscopy, ICP (for
Nb element). X-ray diffraction patterns (XRD) were collected using
a
Phillips X’Pert diffractometer equipped with
monochromator, operating at 40 kV and 45 mA, and employing
nickel-filtered CuK radiation ( = 0.1542 nm).
a graphite
a
l
IR spectra of adsorbed olefins have been recorded with a Nicolet
Nexus spectrometer using a conventional quartz IR cell connected
to a vacuum dosing system. Prior to olefin adsorption the sample
was evacuated at 250 8C for 2 h and cooled down to room
ꢀ6
temperature (r.t.) in vacuum (10 mbar). Olefins (30 mbar) have
been adsorbed at 25 8C and IR spectra have been collected at r.t.
and increasing temperatures. In the co-adsorption experiments,
2
0 mbar of O
2
has been co-adsorbed on the previous evacuated
samples.
The catalytic experiments were performed in a fixed-bed quartz
tubular reactor (i.d. 12 mm; length 400 mm) under atmospheric
pressure. The reaction temperature studied was in the 360–400 8C
range. The feed consisted of a mixture of alkane, oxygen, and
helium with a molar ratio of 4/8/88. The amount of catalyst was
varied from 0.1 to 3 g in order to obtain several contact times.
Reactant and products were analyzed by gas chromatography
Fig. 1. Variation of the selectivity to the main reaction products, i.e. olefin and O-
containing products (OCP), achieved during the oxidation of C –C alkanes over a
2
4
MoVTeNbO catalyst at 400 8C and an alkane conversion of ca. 25%. O-containing
products: acrylic acid (AA); maleic anhydride (MA); methacrolein (MAC).
˚
using two packed columns: (i) molecular sieve 5 A (2.5 m) and (ii)
PorapackQ (3 m). Blank runs showed no conversion in the
temperature range studied.
3 4
were observed. However, in the case of C –C alkanes, the nature of
the main partial oxidation products changes depending on the
alkane fed. Thus, acrylic acid and propene (from propane), maleic
anhydride and butadiene (from n-butane) and methacrolein (from
isobutane) were mainly observed (Fig. 1). We must notice that the
sum of the selectivities to the main partial oxidation products (i.e.
olefin and O-containing products) strongly depends on the alkane
fed and decreases according to the following trend (Fig. 1):
ethane > propane > n-butane > isobutane. On the other hand, the
presence of olefins in the reaction products decreases also from
ethane to isobutane. In this way, isobutene was practically not
observed during the oxidation of isobutane in all the isobutane
conversion range studied (from 5% to 25% isobutane conversion).
Fig. 2 shows the variation of the yield to the main reaction
3
. Results and discussion
The XRD pattern of the catalyst is characterized by Bragg
reflections at 2u = 6.6, 7.7, 8.9, 10.7, 22.1, 27.2, 29.2, 35.48,
indicating the presence of M1 phase [22]. However, the presence
of M2 as a minority phase cannot be ruled out. On the other hand,
we must indicate that no changes in the XRD patterns of catalyst
after the catalytic tests were observed.
3.1. Catalytic properties in the partial oxidation of C
2 4
–C alkanes and
olefins
products achieved during the oxidation of C
and a contact time, W/F, of 75 gcat h mol
2
–C
4
olefins at 380 8C
(Fig. 2 a) and the
variation of the selectivity to the partial oxidation products
achieved during the oxidation of C –C olefins at 380 8C and an
Firstly, it must be mentioned that the catalytic activity for the
alkane conversion depends on the nature of the alkane and, as
expected, the larger is the alkane the higher is the reactivity, with
ethane and propane being the less reactive alkanes. Regarding the
selectivity to reaction products, very different patterns have been
observed. Fig. 1 shows the selectivity to the main partial oxidation
olefinꢀ
1
2
4
olefin isoconversion of 20% (Fig. 2b). According to these results, it
can be concluded that the same O-containing products were
achieved from alkanes (Fig. 1) and from the corresponding olefins
2 4
products achieved during the partial oxidation of the C –C alkanes
(
Fig. 2a), although the selectivity to the main partial oxidation
at 400 8C and an alkane conversion of ca. 25%. In the oxidation of
ethane, ethylene was the only partial oxidation product, with a
selectivity to ethylene of ca. 95%, meanwhile no aldehydes or acids
products observed during the partial oxidation of olefins was
higher than those achieved from the corresponding alkanes.
Fig. 2. (a) Variation of the yield of the main reaction products achieved during the oxidation of C
2
–C
4
olefins over a MoVTeNbO catalyst at 380 8C and a contact time, W/F, of
–C olefins over a MoVTeNbO catalyst at 380 8C
ꢀ1
7
5 gcat h mololefin . (b) Variation of the selectivity to the main partial oxidation products achieved during the oxidation of C
2
4
and an olefin conversion of ca. 20% (different contact times were employed). Reaction products: acrolein (ACR); acrylic acid (AA); butadiene (BD); maleic anhydride (MA);
methacrolein (MAC); acetic acid (AcA).

J.M. L o´ pez Nieto et al. / Catalysis Today 157 (2010) 291–296
293
ꢀ
1
In the case of ethylene no oxygen containing products different
from CO were observed. In addition, we must inform that the
1413 cm are evident (Fig. 3B, spectrum e). These bands involves
ꢀ1
x
species with a carbonyl group (IR band at 1780 cm ), and because
of the very high (C=O) frequency it could be assigned to an
adsorbed lactone like specie or cyclic anhydride compound,
formed by over-oxidation of surface species [32].
The absence of IR bands after isobutene adsorption could
tentatively be explained by the formation of a carbocation type
intermediate specie weakly adsorbed on the catalyst surface. In
fact, a high reactivity of isobutene and its tendency to cationic
polymerization catalyzed by Lewis or Br o¨ nsted acid sites has been
previously reported [33].
catalyst studied here presents a very low conversion in ethylene
oxidation (Fig. 2a), which explains the high selectivity to ethylene
observed during the oxidation of ethane. In this way we have
calculated the ratio between the reaction rates of the olefin and
alkane oxidation. Meanwhile the reaction rate for ethane is much
n
higher than the rate for ethylene (r
–C hydrocarbons, the reactivity of the olefins is remarkably
higher than that of the alkanes, with rolefin/ralkane of ca. 24
ðr =r Þ, 31 ðr1C4 =rnC4 Þ and 38 ðr =riC4H10 Þ.
C
2¼=r
C
< 0:1). In the case of the
2
C
3
4
C
H
C
H
H
H
10
iC H
4 8
3
6
3
8
8
Finally, we must indicate that there is a correlation between the
The adsorption of ethylene on MoVTeNbO catalyst is shown in
Fig. 3C. Ethoxy species are formed at 25 8C as evidenced by the IR
nature of partial oxidation products and the reactivity of the olefin.
Thus, the higher is the amount of the products directly formed by
an allylic mechanism (i.e. acrolein, butadiene and methacrolein)
the lower is the amount of acid and anhydrides. An increase in the
reactivity of the olefin has been reported to favour a higher
reduction of the catalyst surface [27]. Thus, one could expect that
the reduction degree of the catalysts during the oxidation of
isobutene [28] must be higher than during the propene oxidation
ꢀ
1
band at 1057 cm (Fig. 3C, spectrum g). Increasing the tempera-
ture no IR bands are observed until 250 8C, probably due to the
desorption of adsorbed species. However, a broad IR band at
ꢀ
1
1550 cm
starts to appear together with another band at
ꢀ1
1789 cm (Fig. 3C, spectrum h) at 250 8C. Both bands increase
2
in intensity at 250 8C after O co-adsorption and could be assigned
to oxalate species formed due to the oxidation of re-adsorbed
species (Fig. 3C, spectrum i). However, no bands related to
carboxylate and formiate species were observed.
[
29,30] favouring a lower extension of the consecutive conversion
to acid or anhydrides.
Fig. 4 shows the spectra of acrolein (Fig. 4A) and methacrolein
(Fig. 4B) adsorption on MoVTeNbO catalyst at increasing evacua-
tion temperature. IR bands at 1721, 1605, 1460, 1441 and
3
.2. FTIR study on the adsorption of olefins and aldehydes
ꢀ1
Fig. 3 shows the main adsorbed species observed by FTIR of
1198 cm
are observed after evacuation at 25 8C (Fig. 4A,
propene (Fig. 3A), isobutene (Fig. 3B) or ethylene (Fig. 3C) adsorbed
over the MoVTeNbO catalyst. The appearance of bands at
spectrum b), which are related to adsorbed acrolein species.
Increasing the temperature to 60 8C and 150 8C a new broad band
ꢀ
1
ꢀ1
ꢀ1
1
493 cm (at 100 8C) and 1292 cm (at 150 8C) after adsorption
at 1545 cm is evidenced. This band can be assigned to acrylate
of propene can be related to -allylic intermediate and acrylate
p
species [34], which are considered as precursors of acrylic acid.
species respectively, which can be considered as intermediates in
the formation of acrolein and acrylic acid [31,32]. In addition to
these, it is also observed the presence of bands at 1554 and
In the case of methacrolein adsorption at 25 8C, IR bands at
ꢀ1
1708, 1606, 1460 and 1382 cm
are observed associated to
adsorbed methacrolein species (Fig. 4B, spectrum b). These bands
remain after heating at 100 8C, though less intense, and disappear
completely at 150 8C due to the desorption of the adsorbed surface
species (methacrolein is already detected in the gas phase).
Increasing temperature to 250-300 8C new IR bands at 1784, 1606,
ꢀ
1
1
194 cm (at 100 8C, related to propene interacting with Lewis
ꢀ1
acid sites via enolate species) and at 1482 and 1436 cm (at 150
and 200 8C related to carboxylate species), which are the
precursors of the formation of carbon oxides [32].
ꢀ1
The IR spectra of isobutene adsorbed on the MoVTeNbO catalyst
does not show any IR band in the temperature interval between 25
1549 cm are observed. These bands are due to over-oxidation
ꢀ1
products: cyclic anhydride type species (IR band at 1784 cm ),
ꢀ1
and 250 8C (Fig. 3B, spectrum d). However after O
2
co-adsorption at
and carboxylates (IR band at 1549 cm ). The formation of cyclic
anhydride species has been also observed by other authors after
2
50 8C the appearance of broad IR bands at 1780, 1593 and
Fig. 3. Main adsorbed intermediates observed by FTIR of propene, isobutene or ethylene adsorbed over MoVTeNbO catalyst: (A) propene adsorption: (a, b and c) after
adsorption and evacuated at 25 8C, 100 8C and 250 8C, respectively. (B) Isobutene adsorption: (d) after adsorption and evacuated at 250 8C, (e) after co-adsorption of O at
00 8C. (C) Ethylene adsorption: (f) prior to adsorption, (g and h) after adsorption and evacuated at 25 8C and 250 8C, respectively, (i) after co-adsorption of O at 250 8C.
2
3
2

2
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J.M. L o´ pez Nieto et al. / Catalysis Today 157 (2010) 291–296
Fig. 4. Main adsorbed intermediates observed by FTIR of acrolein and methacrolein adsorbed over MoVTeNbO catalyst: (A) acrolein and O
2
co-adsorption: (a) prior to
adsorption, (b, c and d) after adsorption, evacuated at 25 8C, 60 8C and 150 8C, respectively. Methacrolein and O
adsorption at 25 8C, 100 8C, 150 8C and 250 8C, respectively.
2
co-adsorption: (a) prior to adsorption, (b, c, d and e) after
methacrolein adsorption on Mo-based heteropolycompounds [35].
Thus, in agreement to previous results [36], from the IR spectra
desorption of methacrolein to the gas phase is evidenced hindering
further oxidation to methacrylic acid.
Accordingly, partial oxidation of acrolein to acrylic acid is
favoured even at low temperature on MoVTeNbO catalysts, while
the partial oxidation of methacrolein to the corresponding
methacrylic acid is not favoured, which is in good agreement to
catalytic results.
formed in one type of active sites (probably V–O–Mo pairs in the
heptagonal channels), then is desorbed and subsequently adsorbed
in a second type of sites (probably Te–O–Mo pairs in hexagonal
channels]) and finally transformed into acrylic acid in Mo–O–
V(Nb) pairs [37,3]. The adsorption/desorption experiments of
propene indicate the formation of an allylic intermediate from
propene, which is finally transformed to acrylic acid. However,
acrolein is not observed in our catalytic tests, suggesting that it is
easily transformed during the reaction without any apparent
desorption in agreement to the adsorption/desorption experi-
ments carried out feeding acrolein. On the other hand, we must
indicate that propene has been proposed as the first reaction
product, although the efficiency of these type of catalysts changes
clearly depending on the catalyst composition and the crystalline
phases present [3,14,15b,37].
3.3. General remarks
2 4
The results presented here show that C –C alkanes can be
selectively activated over a MoVTeNbO catalyst. In all cases, the
formation of the corresponding olefin seems to be the first step in
the selective oxidative activation of lower alkanes. However, the
adsorption/desorption behaviour of the olefinic intermediates and
also the amount of olefin observed during the catalytic oxidation of
lower alkanes change depending on the length of the alkane. In
2 4
addition, the FTIR spectra of C –C olefins show that the activation
mechanism and the nature of the intermediates change. Thus, low
interaction is observed when ethylene is adsorbed over MoV-
TeNbO catalyst, while the rest of olefins present a low interaction
with the catalyst.
In the case of ethane, ethylene has been the main partial
oxidation product (with a selectivity higher than 90% at an ethane
conversion of 50%). Since no adsorbed species have been observed
during the adsorption/desorption experiments, it has been
concluded that this catalyst is very effective in the transformation
of ethane to ethylene. This is also in agreement to the low reactivity
of ethylene over this catalyst.
Propene is mainly formed at low propane conversion and then
is selectively transformed to acrylic acid at higher propane
conversion. Thus, consecutive reactions (two steps) on the
catalysts surface can be proposed in agreement to previously
reported mechanisms [3,14,15b,31,37] (Fig. 5a). Propene is initially
Fig. 5. Reaction steps in the propane (a) and isobutane (b) oxidation.

J.M. L o´ pez Nieto et al. / Catalysis Today 157 (2010) 291–296
295
In the case of C
4
-alkanes the formation of olefins at low alkane
basis of the different stability of the corresponding acids during the
reaction depending on the characteristics of catalysts [42]. Thus, it
has been noticed that while the oxidation of acrolein on mixed
oxide (Mo, V, W) catalysts shows an exceptionally high selectivity
with respect to acrylic acid, the selectivity of the analogous
oxidation of methacrolein is only moderate [43]. This has been
explained due to a considerably lower oxidation rate of metha-
crolein compared to that of acrolein, while in the undesired
subsequent oxidation of carboxylic acids, methacrylic acid is of the
same order of magnitude as the oxidation of acrylic acid [43a].
However, it has been proposed that a moderate increase of
methacrylic acid selectivity could be reached over MoVW-based
catalysts by optimizing the Mo/V ratio and/or the incorporation of
promoters [43,44].
conversion is low for n-butane and very low in the case of
isobutane (Fig. 1), indicating that other aspects must be considered
in these two cases.
It has been observed that 2-butene and butadiene are the
primary partial oxidation products observed during the 1-butene
oxidation on a MoVTeNbO catalyst [17]. This suggests that, both
acid (isomerisation of monoolefins) and redox (oxidative dehy-
drogenation of monoolefins to butadiene) sites are present in the
catalyst. Since butadiene is formed from monoolefins by an allylic
mechanism, a similar reaction network than that suggested for
propane oxidation can be proposed for n-butane oxidation
Fig. 5b), although maleic anhydride rather than acrylic acid is
the main partial oxidation product. Indeed, the adsorption of 1-
butene on a similar catalyst leads to the appearance of IR bands
related to butadiene (band at 1568 cm ) and enolic type species
bands at 1521, 1176, 1168 cm ) at an adsorption temperature of
(
5
As in Mo O14 [39], the M1 phase in MoVTeNbO catalysts is built
ꢀ
1
from center-occupied pentagonal rings that are linked together by
corner-sharing octahedral sites [19,25]. Thus, one could expect to
have a similar behavior in the oxidation of acrolein to acrylic acid
ꢀ
1
(
1
1
00 8C, while furan species (IR band at 1527, 1472, 1442, 1294,
ꢀ1
125 and 1091 cm ) are observed by increasing adsorption
co-adsorption at this temperature
150 8C) has no effect on the spectra.
In the case of isobutane oxidation, isobutene is practically not
5
on (MoVW) O14-based catalysts and on M1-containing MoV-
temperature [17]. However, O
2
TeNbO catalysts. In this way, the results presented here confirm
the relatively low consecutive reaction of acrylic acid to carbons
oxides [15–26] and the higher degradation of methacrolein [18]
over MoVTe-based catalysts.
Finally, a low reactivity of ethylene on MoVTeNbO catalyst is
observed, which is an anomalous behaviour if compared to the
other olefins studied. However, the low reactivity of ethylene is
probably due to the fact that vinyl hydrogen rather than allyl
hydrogen is only present. So, the MoVTeNbO catalyst has a high
(
observed even at low isobutane conversion. This suggests that
isobutene, if formed, is rapidly transformed into adsorbed species
which should be then transformed into methacrolein. However, a
fast combustion is observed at high isobutane conversion rather
than the formation of methacrylic acid (Fig. 5b). The absence of IR
bands after isobutene adsorption could tentatively be explained by
the formation of a carbocation type intermediate species weakly
adsorbed on the catalysts surface as a consequence of the high
reactivity of isobutene [33].
We must indicate that a different behaviour has been observed
after isobutene adsorption on VPO [32]. Accordingly, different
reaction mechanism and therefore selectivity to the main reaction
products can be proposed for VPO and MoVTeNbO-based catalysts
due to the different acid characteristics of catalysts.
3 4
facility to abstract allyl hydrogen (as in C –C olefins forming
partial reaction products by an allylic oxidation mechanism) but no
to abstract vinyl hydrogen (favouring no consecutive transforma-
tion of ethylene). This favours the achievement of high yield to
ethylene during the oxidation of ethane.
4. Conclusions
On the other hand, the nature of the partial oxidation products
depends also on the length of alkanes and olefins as reported on
other catalytic systems [5–26]. These results are in good
agreement to those previously proposed by Costine and Hodnett
MoVTeNbO mixed metal oxides present interesting catalytic
properties in the oxidation of ethane, propane, n-butane and
isobutane, yielding valuable reaction products. However the
nature of the reaction products obtained strongly depends on
the alkane fed. These results can be explained on the basis of (i) the
characteristics of the active sites of MoVTeNb catalysts, (ii) the
nature of the alkane fed and (iii) the stability of the intermediate
and reaction products obtained.
[
38], in which ethylene, acrylic acid and maleic anhydride/
butadiene are the most favoured reaction products during the
oxidation of ethane, propane and n-butane, respectively. In the
case of the isobutane oxidation, methacrolein and methacrylic acid
are not apparently stable reaction products although higher
selectivity to the aldehyde than to the acid is expected. In fact,
the oxidation of isobutane leads to the lowest yields to partial
oxidation products in comparison with the oxidation of other light
alkanes.
In our case, acrylic acid and methacrolein are the main partial
oxidation products achieved during the partial oxidation of
propane and isobutane, respectively. The absence of IR bands
associated to adsorbed methacrolein could be related to a fast
surface desorption avoiding secondary oxidation toward
methacrylic acid. Moreover, the mechanism and characteristics
of industrial catalysts used in the partial oxidation of correspond-
ing aldehydes could help to understand the differences observed in
the nature of O-containing partial oxidation products during the
oxidation of propane or isobutane. Thus, different catalysts are
MoVTeNbO-based catalysts present active sites of different
nature in a single matrix [11–26]. They have active sites for the
2 4
selective oxidative activation of C –C alkanes. In addition, they
have active and relatively selective sites for partial oxidation of
olefins (except in the case of ethylene, in which a very low activity
was observed). However, different reaction products are observed
2 4
during the partial oxidation of C –C alkanes, which can be
explained by the presence of different intermediates depending on
the number of carbon atoms in the alkanes/olefins. Thus, propene
is clearly observed as a primary product during the oxidation of
propane [14,15,31,36,37], although it is then selectively trans-
formed to acrylic acid via the formation of acrolein as an
intermediate. Moreover, butenes or isobutene are only observed
as traces during the oxidation of n-butane or isobutane,
4
respectively. This suggests that these C -olefins easily react to
used industrially for the oxidation of acrolein ((MoVW)
5
O
14-type
form partial oxidation products (butadiene and methacrolein) by
an allylic mechanism. However, due to the different reactivity of
the hydrocarbons fed, the nature of the partial oxidation products
is quite different. Butadiene is selectively transformed to maleic
anhydride (from both butane and 1-butene), while deep oxidation
of methacrolein is favoured at high conversion from isobutane
although it is relatively stable from isobutene.
oxides) [39] or methacrolein (heteropolyacid based catalysts) [40]
to the corresponding unsaturated carboxylic acid, while similar
catalysts can be used for the partial oxidation of propene or
isobutene to the corresponding unsaturated aldehydes [41]. The
use of different types of catalysts for the partial oxidation of C
aldehydes to the corresponding acids has been explained on the
3 4
–C

2
96
J.M. L o´ pez Nieto et al. / Catalysis Today 157 (2010) 291–296
[
[
16] P. Botella, E. Garc ı´ a-Gonz a´ lez, A. Dejoz, J.M. L o´ pez Nieto, M.I. V a´ zquez, J. Gonz a´ lez-
Calbet, J. Catal. 225 (2004) 428.
17] B. Solsona, F. Ivars, P. Concepci o´ n, J.M. L o´ pez Nieto, J. Catal. 250 (2007) 129.
According to this, a reaction network for propane and isobutane
oxidation can be proposed (Fig. 5) in which the detection of
reaction intermediates and the nature of the final reaction
products depends on the more or less facility in the desorption
of the corresponding reaction intermediates. It is clear that these
aspects are clearly related to the nature and number of active sites
in the catalyst. In addition, the different facility in the desorption of
intermediates could determine the nature of the final reaction
products, which is related to the structure of catalyst.
[18] (a) J. Guan, S. Wu, H. Wang, Sh. Jing, G. Wang, K. Zhen, Q. Kan, J. Catal. 251 (2007)
54;
b) Ji. Guan, Ch. Xu, B. Liu, Y. Yang, Y. Ma, Q. Kan, Catal. Lett. 126 (2008) 301.
3
(
[
19] J.M.M. Millet, H. Roussel, A. Pigamo, J.L. Dubois, J.C. Jumas, Appl. Catal. A: Gen. 232
(2002) 77.
[20] R.K. Grasselli, J.D. Burrington, D.J. Buttrey, P. DeSanto, C.G. Lugmair, A.F. Volpe, T.
Weingand, Top. Catal. 23 (2003) 23;
P. DeSanto, D.J. Buttrey, R.K. Grasselli, C.G. Lugmair, A.F. Volpe, B.H. Toby, T. Vogt,
Zeits. F. Krist 219 (2004) 152.
[
[
21] H. Tsuji, Y. Koyasu, J. Am. Chem. Soc. 124 (2002) 5608.
22] P. Botella, E. Garc ı´ a-Gonz a´ lez, J.M. L o´ pez Nieto, J.M. Gonz a´ lez-Calbet, Solid State
Sci. 7 (2005) 507.
Acknowledgements
[
[
[
23] A. Celaya Sanfiz, T.W. Hansen, A. Sakthivel, A. Trunschke, R. Schlogl, A. Knoester,
H.H. Brongersma, M.H. Looi, S.B.A. Hamid, J. Catal. 258 (2008) 35.
24] N.R. Shiju, X. Liang, A.W. Weimer, Ch. Liang, Sh. Dai, V.V. Guliants, J. Am. Chem.
Soc. 130 (2008) 5850.
This work was supported by DGICYT in Spain (CTQ2006-09358/
BQU). J.M.L.N., P.C. and F.I. also thank to the European Union for FP6
Integrated Project TOPCOMBI (Project No. NMP2-CT2005-515792).
25] (a) T. Katou, D. Vitry, W. Ueda, Catal. Today 91–92 (2004) 237;
(b) H. Murayama, D. Vitry, W. Ueda, G. Fuchs, M. Anne, J.L. Dubois, Appl. Catal. A
References
318 (2007) 137.
[
26] B. Deniau, G. Bergeret, B. Jouguet, J.L. Dubois, J.M.M. Millet, Top. Catal. 50 (2008)
3
3.
[
[
1] F. Cavani, N. Ballarini, S. Luciani, Top. Catal. 52 (2009) 935.
[
[
27] P. Botella, B. Solsona, J.M. L o´ pez Nieto, Catal. Today 141 (2009) 311.
28] J.C. Vedrine, G. Coudurier, M. Forissier, J.C. Volta, Mater. Chem. Phys. 13 (1985)
2] J.-P. Lange, Sustainable strategies for the upgrading of natural gas: fundamentals,
challenges, and opportunities, in: E.G. Derouane, V. Parmon, F. Lemos, F.R.
Ribeiro (Eds.), NATO Sciences Series, vol. 191, Springer, The Netherlands, 2005, pp.
365.
[
[
[
[
[
[
29] K. Br u¨ ckman, J. Haber, T. Wiltowski, J. Catal. 106 (1987) 188.
5
1–83 (Chapter 3).
30] V.M. Zhiznevskii, L.I. Ivanyik, E. Dvonchova, Kinet. Catal. 25 (1984) 1281.
31] P. Concepci o´ n, P. Botella, J.M. L o´ pez Nieto, Appl. Catal. A 278 (2004) 45.
32] G. Busca, G. Ramis, V. Lorenzelli, J. Mol. Catal. 55 (1989) 1.
33] B. Ledo, V. Rives, V. S a´ nchez-Escribano, G. Busca, Catal. Lett. 18 (1993) 329.
34] K. Krauss, A. Drochner, M. Fehlings, J. Kunert, H. Vogel, J. Mol. Catal. A: Chem. 162
[
3] R.K. Grasselli, J.D. Burrington, D.J. Buttrey, P. DeSanto, C.G. Lugmair, A.F. Volpe, T.
Weingand, Top. Catal. 23 (2003) 5.
4] J.M. L o´ pez Nieto, Top. Catal. 41 (2006) 3.
[
[
5] N. Ballarini, F. Cavani, C. Cortelli, S. Ligi, F. Pierelli, F. Trifir o` , C. Fumagalli, G.
Mazzoni, T. Monti, Top. Catal. 38 (2006) 147.
(2000) 413.
[
6] (a) G. Centi, J.M. L o´ pez Nieto, F. Ungarelly, F. Trifir o´ , Catal. Lett. 4 (1990) 309;
[
[
[
35] G. Busca, F. Cavani, E. Etienne, E. Finnocchio, A. Galli, G. Selleri, F. Trifir o´ , J. Mol.
Catal. A: Chem. 114 (1996) 343.
36] U. Bentrup, A. Br u¨ ckner, M. Kant, S. Kolf, U. Dingerdissen, S. Jansen, D. Masch-
meyer, H. Siegert, H.W. Zanthoff, Oil Gas Eur. Mag. 32 (3) (2006) 145.
37] P. Botella, J.M. L o´ pez Nieto, B. Solsona, A. Mifsud, F. Marquez, J. Catal. 209 (2002)
(b) G. Centi, Catal. Lett. 22 (1993) 53.
[
[
[
7] J.H. Holles, C.J. Dillon, J.A. Labinger, M.E. Davis, J. Catal. 218 (2003) 42.
8] T. Blasco, J.M. L o´ pez Nieto, Appl. Catal. A 157 (1997) 117.
9] B. Grzybowska-Swierkosz, Appl. Catal. A 157 (1997) 409.
[
[
[
10] F. Cavani, N. Ballarini, A. Cericola, Catal. Today 127 (2007) 113.
11] E.M. Thorsteinson, T.P. Wilson, F.G. Young, P.H. Kasei, J. Catal. 52 (1978) 351.
12] A.A. Adesina, N.W. Cant, A. Saberi-Moghaddam, C.H.L. Szeto, D.L. Trim, J. Chem.
Technol. Biotechnol. 72 (1998) 19.
13] W. Ueda, N.F. Chen, K. Oshihara, Chem. Commun. (1999) 517.
14] (a) W. Ueda, K. Oshihara, Appl. Catal. A 200 (2000) 135;
4
45.
[
[
[
[
[
38] A. Costine, B.K. Hodnett, Appl. Catal. A 290 (2005) 9.
39] G. Mestl, Top. Catal. 38 (2006) 69.
40] M. Misono, N. Nojiri, Appl. Catal. 64 (1990) 1.
41] N. Nojiri, Y. Sakai, Y. Watanabe, Catal. Rev. Sci. Eng. 37 (1995) 145.
42] B. Stein, J.C. Petzoldt, H. Bohnke, J. Gaube, Stud. Surf. Sci. Catal. 130B (2000)
[
[
(
(
b) W. Ueda, K. Oshihara, D. Vitry, T. Hisano, Y. Kayashima, Catal. Surv. Jpn. 6
2002) 33.
1
745.
43] (a) H. B o¨ hnke, J. Gaube, J. Petzoldt, Ind. Eng. Chem. Res. 45 (2006) 8801;
b) H. B o¨ hnke, J. Gaube, J. Petzoldt, Ind. Eng. Chem. Res. 45 (2006) 8794.
44] S.A. Schunk, A. Sundermann, H. Hibst, Catal. Today 137 (2008) 36.
[
[
[
15] (a) T. Ushikubo, H. Nakamura, Y. Koyasu, S. Wajiki, US Patent 5,380,933 (1995).
b) T. Ushikubo, K. Oshima, A. Kayou, M. Hatano, Stud. Surf. Sci. Catal. 112 (1997)
73.
(
(
4