Selective propane oxidation over MoVSbO catalysts. On the preparation, characterization and catalytic behavior of M1 phase

Article abstract of DOI:10.1016/j.jcat.2008.11.021

Nb-free (SbO)₂M₂₀O₅₆ catalysts (M = Mo, V) presenting pure M1 phase have been prepared by a post-synthesis treatment with hydrogen peroxide of a heat-treated MoVSbO mixed metal oxide catalyst previously prepared by hydrothermal method. The characterization of catalysts and their results for propane oxidation suggest that the optimization in the preparation of the M1 phase depends strongly on the washing procedure. The optimal removing of Sb species formed during post-synthesis treatment can explain the improvement in the catalytic activity; while the better selectivity to acrylic acid of the catalysts obtained by post-synthesis treatment can be explained by the elimination of M2 phase and the modification of the M1 phase crystals surface. The importance of M1 phase in the catalytic performance during the selective propane oxidation over Nb-free Mo-V-Sb based catalysts is also discussed.

Full text of DOI:10.1016/j.jcat.2008.11.021

Journal of Catalysis 262 (2009) 35–43
Contents lists available at ScienceDirect
Journal of Catalysis
www.elsevier.com/locate/jcat
Selective propane oxidation over MoVSbO catalysts. On the preparation,
characterization and catalytic behavior of M1 phase
Francisco Ivars a, Benjamín Solsona b, Enrique Rodríguez-Castellón c, José M. López Nieto a,∗
a
Instituto Universitario Mixto de Tecnología Química (UPV-CSIC), Avda. Los Naranjos s/n, 46022 Valencia, Spain
Departamento de Ingeniería Química, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Spain
Departmento Química Inorgánica, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Nb-free (SbO)2M20O56 catalysts (M = Mo, V) presenting pure M1 phase have been prepared by a
post-synthesis treatment with hydrogen peroxide of a heat-treated MoVSbO mixed metal oxide catalyst
previously prepared by hydrothermal method. The characterization of catalysts and their results for
propane oxidation suggest that the optimization in the preparation of the M1 phase depends strongly
on the washing procedure. The optimal removing of Sb species formed during post-synthesis treatment
can explain the improvement in the catalytic activity; while the better selectivity to acrylic acid of the
catalysts obtained by post-synthesis treatment can be explained by the elimination of M2 phase and the
modification of the M1 phase crystals surface. The importance of M1 phase in the catalytic performance
during the selective propane oxidation over Nb-free Mo–V–Sb based catalysts is also discussed.
© 2008 Elsevier Inc. All rights reserved.
Received 1 August 2008
Revised 21 November 2008
Accepted 21 November 2008
Available online 24 December 2008
Keywords:
Hydrothermal synthesis
Post-synthesis treatment
Mo–V–Sb mixed oxides
M1 phase
Propane oxidation
1. Introduction
(i) (SbO)2M20O56 (with M = Mo, V, Nb) as the M1-like phase [26],
and (ii) (Sb2O)M6O19 (with M = Mo, V, Nb) as the M2-like
phase [26].
The functionalization of light alkanes is a challenging matter
since they present a comparatively low price and the exploration
of new catalytic systems for the oxidation of short chain alkanes in
gas phase has special interest from both fundamental and indus-
trial point of view [1–5]. However, only since mid 90’s promising
catalytic results began to emerge. This is the case of Mo–V–Te(Sb)–
Nb–O mixed oxides, initially reported by Mitsubishi [6], which
are the most effective catalysts in the (amm)oxidation of propane
[6–8] and in the oxidative dehydrogenation (ODH) of ethane to
ethylene [9–11], presenting, moreover, an interesting selectivity to
partial oxidation products during the oxidation of n-butane [12].
Typically, MoVTeNb-based catalysts present as main crystalline
phases [8,13–25]: (i) orthorhombic (TeO)2M20O56 (M = Mo, V, Nb),
also called M1; and (ii) orthorhombically distorted HTB-type phase
Te0.33MO3.33 (M = Mo, V, Nb), also called M2. In addition, other
phases as TeMo5O16 or (V, Nb)-containing Mo5O14 , or TTB-bronzes
can also be observed depending on the catalyst preparation proce-
dure [16].
The presence of Nb ions in both Te- and Sb-containing catalysts
[6–34] favors an improvement in the selective oxidation reactions.
This effect has been clearly explained by a better stability of acrylic
acid on Nb-containing catalysts [15], as a consequence of the low
number/strength of acid sites in these catalysts [34].
On the other hand, a synergetic effect for propane oxidation
between M1 and M2 phases has been proposed for MoVTeNbO
catalysts [24,25], which is not clear for the corresponding Sb-
containing catalysts [25]. This different behavior between Te- and
Sb-containing catalysts could be due to the different performance
of their corresponding M2 phases [25,30,35]. Moreover, and al-
though it has been demonstrated that the incorporation of potas-
sium with the elimination of acid sites improves the formation of
acrylic acid [30,31], the role of the M1 and M2 phases in Nb-free
MoVSbO catalysts is still under discussion.
Pure M1 phase can be obtained directly by hydrothermal syn-
thesis [19,21–23] or by a post-synthesis treatment of the corre-
sponding mixed metal oxides [25,36,37], although in most of these
cases, the preparation of the M1 phase takes place when Nb is
present in the catalysts.
Mo–V–Sb-based catalysts have also been reported as efficient
catalysts in the oxidation of propane to acrylic acid [18,23,25–32]
and in the ODH of ethane [33] most of them presenting crys-
talline phases similar to those observed in Te-containing catalysts:
In this paper we present a comparative study on the selective
oxidation of propane to acylic acid over Nb-free MoVSbO cata-
lysts presenting pure and mixed M1 and M2 phases. The mixed
phases of MoVSbO were prepared hydrothermally, while the pure
M1 phase was obtained by a post-synthesis treatment with an
Corresponding author. Fax: +34 963877809.
E-mail address: jmlopez@itq.upv.es (J.M. López Nieto).
*
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.11.021

36
F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
Table 1
Physico-chemical characteristics of MoVSb mixed metal oxide catalysts.
Catalyst
EDX
composition
Mo/V/Sb
SBET
(m2/g)
NH3-TPD
Acid sitesa
(μmol/g)
Phases detected
by XRDc
b
Tmax
( C)
◦
(μmol/m2)
A-1
A-2
A-3
1/0.27/0.28
1/0.26/0.17
1/0.24/0.16
20.6
18.2
15.8
154
156
129
8
8
8
212
190
182
M1, Sb-rich mixed oxide
M1, Sb-rich mixed oxide
M1
HT-1
HT-2
1/0.27/0.15
1/0.35/0.50
13.2
3.4
320
3.7
24
1
212
165
M1, M2
M2
A-3W
0.02–0.01/0/1
–
–
–
–
Sb-rich mixed oxide
a
◦
Amount of ammonia chemisorbed at 100 C during the NH3-TPD experiment, per gram or per surface area of catalyst (measured in standard temperature and pressure).
Tmax, temperature of maximum desorption observed in the NH3-TPD experiments.
b
c
M1 phase = (SbO)2M20O56; M2 phase = (Sb2O)M6O19 (M = Mo, V); Sb-rich mixed oxide = Mo0.01–0.02SbO1.9−3.3·xH2O (determined by EDS) which presents XRD pattern
very similar to that of a phase previously reported, Mo0.32SbO3.3·xH2O (JCPDS: 44-0054).
aqueous solution of hydrogen peroxide [25]. Finally, the role of
M1 and M2 phases in MoVSbO mixed oxide catalysts during the
selective oxidation of propane to acrylic acid as well as the impor-
tance of the procedure used in the preparation of M1 phase are
discussed.
Transmission electron microscopy images were obtained on a
Philips CM10 electron microscope operating at 100 kV. The sam-
ples were prepared by dispersing the powder products in pure
ethanol and transferred to carbon-coated copper grids.
Infrared spectra were recorded at room temperature in the
−1
300–3900 cm
region with a Nicolet 205xB spectrophotometer,
−1
equipped with a Data Station, at a spectral resolution of 1 cm
and accumulations of 128 scans. The pellets were prepared with
20 mg of sample mixed with 100 mg of dry KBr and pressed into
a disk.
2. Experimental
2.1. Catalysts preparation
Temperature programmed desorption of ammonia (NH3-TPD)
experiments were carried out on a TPD/2900 apparatus from Mi-
cromeritics. 300 mg of sample was pre-treated in an Ar stream at
450 C for 1 h. Ammonia was chemisorbed by pulses at 100 C un-
til equilibrium was reached. Then, the sample was fluxed with He
A MoVSbO mixed metal oxide, containing both M1 and M2
phases, has been prepared hydrothermally from a gel with vanadyl
sulfate, antimony sulfate, ammonium heptamolybdate and water
with a Mo/V/Sb atomic ratio of 1/0.18/0.15 according to the proce-
dure previously reported [30]. The gel obtained was introduced in
◦
◦
◦
◦
stream for 15 min, prior to increase the temperature up to 500 C
Teflon-line stainless steel autoclave at 175 C for 48 h. The result-
−1
◦
in a helium stream of 100 ml min
and using a heating rate of
ing solid was filtered, washed and dried at 100 C for 12 h. Finally,
10 C min−1. The NH3 desorption was monitored with a thermal
conductivity detector (TCD) and a mass-spectrometer. The mea-
sures were at standard pressure and temperature.
◦
◦
the solid was heat-treated at 600 C for 2 h in flowing N2. This
sample will be named as HT-1.
Pure M1-like catalysts were prepared by washing the HT-1 sam-
ple according to the following procedure: the solid was treated at
room temperature with an aqueous solution of 15 wt% hydrogen
peroxide for 5 h under vigorous stirring, filtered and washed with
X-ray photoelectron spectra were collected using a Physical
Electronics PHI 5700 spectrometer with non-monochromatic MgK
α
radiation (300 W, 15 kV, 1253.6 eV) for the analysis of photo-
electronic signals of C 1s, O 1s, V 2p, Mo 3d, and Sb 3d and
with a multi-channel detector. Spectra of powdered samples were
recorded with the constant pass energy values at 29.35 eV, using
a 720 μm diameter analysis area. Under these conditions, the Au
4 f7/2 line was recorded with 1.16 eV FWHM at a binding energy
of 84.0 eV. The spectrometer energy scale was calibrated by using
the Cu 2p3/2, Ag 3d5/2 and Au 4 f7/2 photoelectron lines at 932.7,
368.3 and 84.0 eV, respectively. During data processing of the XPS
spectra, binding energy values were referenced to the C 1s peak
(284.8 eV) from the adventitious contamination layer. The PHI AC-
CESS ESCA-V6.0 F software package was used for acquisition and
data analysis. A Shirley-type background was subtracted from the
signals. Recorded spectra were always fitted using Gauss–Lorentz
curves, in order to determine the binding energy of the different
element core levels more accurately. The error in BE was estimated
to be ca. 0.1 eV. Short acquisition time of 10 min was used to ex-
amine C 1s, V 2p regions in order to avoid, as much as possible,
photo-reduction of V5+ species. Satellite subtraction was always
performed to study the V 2p region.
distilled water. Three different samples were achieved by washing
−1
treated sample HT-1 with 1, 2 or 3 cycles (100 mlH O gcat. each cy-
2
◦
cle), and the resulting solids were heated at 500 C for 2 h in N2
stream. These catalysts have been named as A-1, A-2 and A-3, re-
spectively. In addition, the turbid mother liquids obtained in each
case after filtering and washing, were separately centrifuged in or-
der to recover the solid in suspension. These solids were dried at
◦
100 C and named as A-1W, A-2W and A-3W, respectively.
For comparison, pure M2-like catalysts were also prepared
hydrothermally by using a gel with a Mo/V/Sb atomic ratio of
◦
1/0.5/0.5. The sample heat-treated at 600 C in N2 will be named
as HT-2. The characteristics of catalysts are presented in Table 1.
2.2. Catalysts characterization
BET specific surface areas were measured on a Micromeritics
TriStar 3000 instrument, with adsorption of N2 at 77.300 K.
X-ray diffraction patterns (XRD) were collected using a Philips
X’Pert diffractometer equipped with a graphite monochromator,
operating at 40 kV and 45 mA and employing nickel-filtered CuK
α
2.3. Catalytic tests
radiation (λ = 0.1542 nm).
Scanning electron microscopy (SEM) and energy dispersive
X-ray spectrometry microanalysis (EDX) were performed on
a
The catalytic experiments were carried out at atmospheric pres-
sure, in the 340–400 C temperature range, using a fixed bed
quartz tubular reactor (i.d. 20 mm; length 400 mm). Catalyst sam-
ples were introduced in the reactor diluted with silicon carbide in
◦
JEOL JSM 6300 LINK ISIS instrument. The quantitative EDX anal-
yses were performed using an Oxford LINK ISIS System with the
SEMQUANT program, which introduces the ZAF correction.

F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
37
Fig. 1. XRD patterns of MoVSbO based catalysts: (a) A-1; (b) A-2; (c) A-3; (d) HT-1; (e) HT-2. Symbols: (!) (SbO)2M20O56; (2) (Sb2O)M6O19
.
order to keep a constant volume in the catalytic bed. The feed con-
sisted in a mixture of C3H8/O2/He/H2O or C3H6/O2/He/H2O with
molar ratios of 4/8/58/30 or 1.7/6.8/76.5/15, respectively. Reactants
and products were analyzed by online gas chromatography using
two packed columns: (i) Molecular sieve 5 Å (3 m); and (ii) Pora-
pak Q (3 m). Blank runs showed no conversion in the temperature
range studied [19,30].
3. Results and discussion
3.1. Characterization results
Table 1 shows the physico-chemical characteristics of catalysts.
M1-like catalysts, i.e. A-series, present surface areas from 15 to 21
m2 g−1, with the less washed sample (A-1 catalyst) showing the
Fig. 2. XRD patterns of A-1 and A-3 catalysts, and AW3 (solid obtained from mother
liquids of washing A-3 catalyst). Dashed lines are the characteristic diffraction peaks
of Mo0.32SbO3.3·xH2O (JCPDS: 44-0054).
highest surface area. We must indicate that sample HT-1 (mixed
−1
M1 and M2 phases) presents a BET surface area of 13.2 m2 g
,
whereas sample HT-2 (M2-like phase) presents the lowest surface
area (below 4 m2 g−1).
In order to determine the nature of these new peaks, the solids
recovered from the washing mother liquids obtained after wash-
ing the A-series solids have been also studied by XRD. It must be
indicated that these solids (A-1W, A-2W and A-3W) presented a
similar XRD pattern, although the amount of solid recovered in-
creased with the number of washing cycles.
The Mo/V/Sb atomic ratios of catalysts determined by SEM-EDX
microanalyses are also presented in Table 1. The EDX results for
HT-1 and HT-2 catalysts were very close to those previously re-
ported for similar catalysts [30]. Moreover, the composition of cat-
alysts of the A-series strongly depends on the washing conditions.
Thus, the samples washed during three cycles (A-3 catalyst) pre-
sented the lowest Sb/Mo ratio, while the sample washed during
one cycle (A-1 catalyst) presented the highest Sb/Mo ratio. Thus,
the Sb-enrichment observed for sample A-1 decreases in the sam-
ple washed during three cycles (sample A-3).
XRD patterns of A-series catalysts are presented in Fig. 1. For
comparison, the XRD pattern of sample HT-1 (showing the char-
acteristic peaks of both (SbO)2M20O56 and (Sb2O)M6O19 crystalline
phases [26,30]), and sample HT-2 (presenting only the character-
istic peaks of (Sb2O)M6O19 crystallites [26,30]) have been also in-
cluded.
In the case of A-series catalysts, (SbO)2M20O56 was the crys-
talline phase mainly observed (Fig. 1, patterns a, b and c). Thus,
we can conclude that the post-synthesis treatment with an aque-
ous solution of hydrogen peroxide leads to the selective removal
of the M2 phase from MoVSbO catalysts containing M1 and M2
phase mixtures. However, additional diffraction peaks have been
observed in XRD patterns of these catalysts and especially in the
less washed sample, i.e. A-1 catalyst (Fig. 1, pattern a).
Fig. 2 compares the XRD pattern of the solid A-3W, with
that of A-3 and A-1 catalysts. Accordingly, it can be concluded
that the appearance of small reflections at 2θ = 14.9, 28.6, 29.9,
◦
34.7, 37.9, 45.9, 49.9, 52.4, 59.3 in catalyst A-1 corresponds to
the presence of a Sb-rich mixed oxide phase with a structure
similar to Mo0.32SbO3.3·xH2O (JCPDS: 44-0054) but with a Sb
loading higher, as it has been observed by SEM/EDX analysis:
Mo0.01−0.02SbO1.9−3.3·xH2O. This Sb-rich phase is mainly observed
in the solid recovered (A-3W) from mother liquids of washing A-3
(Fig. 2). Thus, the amount of Mo0.01–0.02SbO1.9–3.3·xH2O on the
surface of M1 phase crystals of A-series catalysts, is in good agree-
ment with the Sb/Mo ratio observed by EDX in A-1 to A-3 samples.
The lower the number of washing cycles the higher is the Sb/Mo
ratio, being the catalyst A-1 the one with the highest Sb/Mo molar
ratio.
On the other hand, we must indicate that the unit cell pa-
rameters of phase M1, i.e. a (nm) = 21.022; b (nm) = 26.666;
c (nm) = 4.012, in the A-3 catalyst are in good agreement to those
previously reported for Te- or Sb-samples presenting M1 phase

38
F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
Fig. 3. TEM micrographs of MoVSbO catalysts: (a) HT-1; (b) HT-2.
Fig. 4. TEM micrographs of MoVSbO catalysts: (a) A-1; (b) A-2; and (c) A-3. For comparison, TEM micrography of A-3W (centrifuged solid obtained from the mother liquids
of washing sample A-3) is also included (d).
[13,14,26,38]. The unit cell parameters for M1 phase of A-1 and
A-2 catalysts resulted to be almost identical to those of A-3 cata-
lyst.
The TEM micrographs of catalysts are presented in Figs. 3 and 4.
The TEM image of sample HT-1 (Fig. 3a) is characterized by the
presence of both needle-like crystals (with length between 500 nm
and 2 μm) and compact crystals (with length between 100 nm and
200 nm). The sample HT-2 (Fig. 3b) only presents very compact
crystals (with length between 300 nm and 1500 nm) correspond-
ing to M2 phase crystals, as observed by XRD.
Fig. 4 shows some TEM pictures of catalysts A-1, A-2 and A-3.
For comparison, TEM micrography of A-3W (centrifuged solid ob-

F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
39
Table 2
Surface composition and binding energies of Mo 3d5/2, V 2p3/2 and Sb 3d3/2 in
MoVSbO catalysts.
Catalyst
Binding energies (eV) and FWHMa
Surface composition
Mo 3d5/2
V 2p3/2
Sb 3d3/2
Mo
V
Sb
A-1
A-2
A-3
232.5
(2.10)
232.5
(2.11)
232.6
(2.12)
516.6
(2.01)
516.6
(1.63)
516.8
(1.74)
540.6
(1.80)
540.5
(1.77)
540.6
(1.69)
1.00
0.20
1.00
1.00
1.00
0.15
0.12
0.68
0.31
HT-1
HT-2
232.9
(2.11)
232.9
(2.05)
516.7
(1.80)
516.6
(1.75)
539.8
(1.68)
540.1
(1.90)
1.00
1.00
0.18
0.18
0.10
0.49
A-3W
232.9
(3.17)
540.8 (2.4)
539.6 (2.2)
0.02
–
1.00
–
a
Binding energies at peak maximum and peak half widths (FWHM) of the V
2p3/2, Mo 3d5/2 and Sb 3d3/2 core level lines in the XPS spectra of the catalysts. In
parenthesis peak half widths (FWHM) of each element.
presence of these bands seems to be characteristic of M2-like crys-
tals.
The IR spectra of A-1, A-2 and A-3 samples show the most char-
−1
Fig. 5. FT-IR spectra of MoVSbO based catalysts: (a) A-1; (b) A-2; (c) A-3; (d) HT-1;
(e) HT-2.
acteristic bands at 918, 870, 805, 715, 650 and 602 cm
(Fig. 5,
spectra a to c, respectively). Since IR spectrum of sample HT-1
(with M1 and M2 phases) presents the same bands of A-series
tained from the mother liquids of washing sample A-3) is also
included (Fig. 4d).
samples in addition to those related to M2 phase, it can be con-
−1
cluded that bands at 918, 870, 805, 715, 650 and 602 cm
are
Samples A-1 to A-3 present needle-like shape crystals (with
length between 500 nm and 2 μm), corresponding to the or-
thorhombic (SbO)2M20O56 phase (observed by XRD), which suggest
that this phase shows a preferential growth in the c axis direction
[0, 0, 1]. However, we must inform the presence of nanoparticles
(diameter of particle within range 20–30 nm, with a spherical
morphology) surrounding the former structure, whose amount de-
creases when increasing the number of washing cycles. Accord-
ingly, A-1 catalyst shows the highest amount of these nanospheres
over M1-like crystals surface (Fig. 4a), while these ones are almost
absent in A-3 catalyst (Fig. 4c).
related to the presence of M1 phase. The intense bands at 924
−1
or 918 cm
(from M2 or M1 crystals, respectively) are likely re-
lated to the M–Ot bond (M = Mo, V) (Ot, terminal oxygen) [39,
40], whilst bands at 870, 805, 715 and 650 cm−1, observed in
M1-containing catalysts, are very likely related to antisymmetric
vibrations of Mo–O–X (X = Mo, Sb) bridging bonds [39,40]. The
−1
band at 750 cm
could be assigned to asymmetric stretching
Mo–Ob bonds (Ob, oxygen is not coordinating Sb inside hexagonal
−1
channel), while the band at 455 cm
corresponds to asymmetric
stretching M–Oa–M (M = Mo, V) (Oa, oxygen is coordinating Sb
inside hexagonal channel) [40], in M2-like phases.
TEM micrographs of the solid A-3W (Fig. 4d), recovered from
washing mother liquids of sample A-3, show the same morphology
that those nanoparticles surrounding the M1 crystals in samples
with a lower number of washing cycles, i.e. A-1 and A-2. According
to the XRD and SEM/EDX results (Fig. 1 and Table 1), it can be con-
cluded that Mo0.01−0.02SbO1.9−3.3·xH2O nanospheres are deposited
on the surface of M1 phase crystals during the post-synthesis
treatment when the catalyst is not washed enough. This fact can
explain the results shown in Table 1, in which the BET surface
area of washed samples decreases when increasing the number of
washing cycles, probably as a consequence of the elimination of
Sb-rich nanospheres supported over M1 phase crystals. In addition,
these results can also explain the higher Sb/Mo ratio of A-1 cata-
lyst compared to HT-1, since the stoichiometries of the nanoparti-
cles (Mo0.01−0.02SbO1.9−3.3·xH2O) present a Sb-loading around 150–
300 and 500–1000 times higher than M2 and M1 phases, respec-
tively. As can be seen in Fig. 4a, the nanospheres recovering the
M1 crystallite have a relative important contribution respect to the
size of the M1 crystallite.
On the other hand, the absorption bands in the region of 650–
−1
550 cm
at 650 cm
Sb–O while the band at 560 cm
are characteristics of antimony oxides [41]. So, the band
−1
can be also assigned for stretching bending of O–
−1
can be assigned to asymmetric
stretching Sb–O–M2 (M = Mo, V) [42].
Binding energies at peak maximum (BEs) and peak half widths
(FWHM) of the V 2p3/2, Mo 3d5/2 and Sb 3d3/2 core level lines
in the XPS spectra of the catalysts, as well as the surface metal
atomic ratio determined from XPS, are shown in Table 2. Sample
HT-2 presents a Sb/Mo molar ratio higher than sample HT-1, which
is in agreement to both, the EDX analysis and the stoichiometries
of the crystalline phases, i.e. (SbO)2M20O56 and/or (Sb2O)M6O19
present in these samples.
,
A
Sb-enrichment on the surface of catalysts can be con-
cluded from the XPS results obtained for samples of A-series,
although Sb/Mo ratio decreases when increasing the number
of washing cycles. This is in agreement with the other char-
acterization results shown here and confirms the presence of
(Mo0.01−0.02SbO1.9−3.3·xH2O) nanoparticles on the surface of M1
phase in these catalysts as a consequence of the elimination of
M2 phase during the treatment with an aqueous solution of hy-
drogen peroxide. Thus, it appears that the presence of Sb species
strongly depends on the number of washing cycles. However, also
in the sample in which it is not possible to observe the presence of
the Sb-rich nanoparticles (as occurs in sample A-3), an important
contamination of Sb on the surface of M1 phase can be observed
(Table 2).
Fig. 5 shows the infrared spectra of A-series samples. For com-
parison, the IR spectra of samples HT-1 and HT-2 have also been
included.
The IR spectrum of HT-2 catalyst is characterized by the pres-
ence of an intense band at 924 cm−1, two broad bands at 750
−1
and 560 cm−1, and a low intense band at 455 cm
(Fig. 5, spec-
trum e). This spectrum is very similar to that previously reported
for a MoVTeO sample presenting pure M2 phase [35]. Thus, the

40
F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
Fig. 7. TPD-NH3 profiles of catalysts A-1, A-3, HT-1 and HT-2.
The NH3-TPD results are summarized in Table 1, while the NH3-
Fig. 6. XPS spectra of Sb 3d3/2 in MoVSbO based catalysts: (a) A-1; (b) A-2; (c) A-3;
(d) HT-1; (e) HT-2.
TPD profiles are shown in Fig. 7. According to these results sample
−1
HT-1 presents the highest number of acid sites (320 μmolNH3 g
,
which corresponds to 24 μmolNH3 m−2); while a drastic reduction
of the number of acid sites can be inferred in the case of samples
obtained by post-synthesis treatment, i.e. A-series. This important
On the other hand, a modification of the oxidation state of an-
timony is also observed in catalysts prepared by the post-synthesis
treatment. Thus, the samples prepared hydrothermally and heated
in N2 flow (i.e. HT-1 and HT-2) present mainly Sb3+ species on
the catalyst surface with binding energies for Sb 3d3/2 at ca.
540.0 eV, while samples prepared by post-synthesis treatment (A-
series) present mainly Sb5+ species on the catalyst surface with
binding energies for Sb 3d3/2 at ca. 540.6 eV (see Fig. 6) [27,47].
In addition, in Table 2 are included the XPS results obtained
for A-3W sample (representative of A-W-series solids). These
results confirm the homogeneous stoichiometry of the Sb-rich
nanospheres, since it agrees with the composition obtained by
EDX. Moreover, it must be commented that a deconvolution of
Sb 3d3/2 signal shows a mixed oxidation state of Sb (77% of Sb3+
and 23% of Sb5+ with binding energies at ca. 539.6 and 540.8 eV,
respectively) in the A-3W solid.
decrease in the number of acid sites of washed samples, especially
−2
when considering the surface area of catalysts (ca. 8 μmolNH3 m
)
(Table 1), suggests that the post-synthesis treatment favors a dras-
tic elimination of acid sites on the surface of the catalyst. However,
it is difficult at the moment indicating if this drastic reduction of
the number of acid sites is related to the elimination of M2 phase
or is a consequence of modifications on the surface of M1-phase
during the post-synthesis treatment.
It must be informed that the temperature of maximum desorp-
tion is remarkably lower in the A-series than in the HT-1 catalyst
(Table 1 and Fig. 7). This means that the strength of acid sites de-
creases after the post-synthesis treatment with aqueous solution
of H2O2. Moreover, differences among A-series catalysts are also
observed, decreasing the acid strength as the number of washing
cycles increases. In this way, A-3 catalyst presents both the lowest
amount and the lowest strength of acid sites.
On the other hand, the sample HT-2 presents the lowest num-
ber of acid sites (3.7 μmolNH3 g−1), probably as a consequence
of both the low surface area and the low specific acidity (ca.
1 μmolNH3 m−2). It must be indicated that the average particle size
of M2 crystals is much higher in HT-2 sample (M2 phase alone)
than in HT-1 (a mixture of M1 and M2 crystals).
In the case of Mo and V, no apparent modifications of their cor-
responding oxidation states have been observed between samples
obtained hydrothermally or those achieved after the post-synthesis
treatment with an aqueous solution of hydrogen peroxide. The V
2p3/2 core level signal appears at 516.6 eV in almost all the studied
samples. This value is ∼0.4 eV lower than that generally reported
for pure V2O5. However, some authors report a binding energy of
516.6 eV for vanadium (V) [43,44], while binding energies at 516.3,
515.7, 515.6 and 515.4 eV have been reported for the V 2p3/2 sig-
nal in V6O13, VO2, V2O4 and V2O3, respectively [43]. In the case of
a phase mixture V6Mo4O25–MoO3–As2O3, a value of 516.6 eV for
the V 2p3/2 signal was reported [45]. We must indicate that the
oxidation state of vanadium atoms in V6O13 is formally 4.33 and
the shift to V(V) is only 0.3 eV. V6O13 is slowly formed by heating
in vacuum from V2O5 [46], but the reduction of V(V) does not oc-
3.2. Catalytic results
Table 3 shows the catalytic results obtained during the propane
oxidation over Mo–V–Sb–O catalysts. Acrylic acid, acetic acid,
propylene and carbon oxides were the main reaction products de-
tected in the oxidation of propane. Traces of acrolein, acetaldehyde
and acetone were also identified.
HT-1 catalyst (containing both M1 and M2 phases) presents
high activity in propane oxidation but low selectivity to acrylic
acid, while HT-2 catalyst (presenting pure M2 phase) is inactive
in the propane oxidation.
Strong differences have been observed in the case of samples
obtained by post-synthesis treatment (A-series catalysts) depend-
ing on the number of washing cycles. Fig. 8 shows the variation of
the propane conversion with the reaction temperature over these
catalysts. For comparison the results obtained over HT-1 catalysts
◦
cur in He atmosphere up to temperatures of 600 C [44]. It must
be noted that a salt of V4+ was used as a precursor for the synthe-
sis of these catalysts and the presence of V(IV) as minority cannot
be completely ruled out.
On the other hand, the chemical shift between Mo(VI) and
Mo(V) is 1.1 eV. This implies that Mo unequivocally is present in
all our samples mainly as Mo(VI) (see Table 2) [44]. However, the
observed broadening of the Mo 3d5/2 signal could suggest some
minority presence of Mo cations with an oxidation state lower
than +6.

F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
41
Table 3
Oxidation of propane at 380 C over MoVSb-based catalysts.
◦
Conversiona
(%)
Selectivityb (%)
Acrylic acid
Areal reaction
ratec
d
Catalyst
STYAA
Acetic acid
Propylene
Acetone
CO
CO2
A-1
A-2
A-3
8.5
17.1
23.8
18.0
38.7
56.1
6.7
6.0
4.9
23.2
13.9
11.1
1.2
0.8
1.4
27.1
18.9
13.1
23.7
21.8
13.2
9.4
20.7
33.1
5.5
23.8
48.1
HT-1
HT-2
25.4
< 0.2
13.4
13.8
8.8
1.1
29.4
33.5
41.9
12.3
a
◦
−1
) .
3H8
Propane conversion at reaction temperature of 380 C, and a contact time, W /F , of 200 gcat. h (molC
b
c
Selectivity to reaction products (in some experiments acetaldehyde and acrolein were also observed but with selectivity below 0.2%).
−1
−2
Areal reaction rate expressed in 104 gC
h
m
(SBET data in Table 1).
3H8
−1
Rate of formation of acrylic acid (AA) per unit mass of catalyst and per unit time, STYAA, in gAA h−1 kgcat.
.
d
Fig. 8. Variation of propane conversion with the reaction temperature obtained dur-
ing the oxidation of propane over MoVSbO based catalysts: A-1 ); A-2 );
(
(
A-3 ( ); HT-1 (!). Reaction conditions: C3H8/O2/He/H2O molar ratio of 4/8/58/30,
−1
W /F = 200 gcat. h molC
.
3H8
are also included. The catalytic activity for propane oxidation in-
creases when increasing the number of washing cycles, with cata-
lysts A-3 and HT-1 presenting similar catalytic activity for propane
oxidation.
Fig. 9 shows the variation of the selectivity to acrylic acid
(Fig. 9a) and acetic acid (Fig. 9b) with the propane conversion
obtained at 380 C on pure and mixed phases. For post-treated
Fig. 9. Variation of the selectivity to acrylic acid (a) and acetic acid (b) with the
propane conversion obtained during the oxidation of propane over MoVSbO based
◦
samples, the selectivity to acrylic acid increases when increasing
the number of washing cycles, while the selectivity to acetic acid
presents an opposite trend to that observed for the selectivity to
acrylic acid. We must indicate that HT-1 catalyst has been the most
selective catalyst to acetic acid and the less selective to acrylic acid.
Accordingly, pure orthorhombic M1-containing catalysts are
very active and selective in the oxidation of propane to acrylic
acid, although both the activity for propane oxidation and the se-
lectivity to acrylic acid strongly depend on the number of washing
cycles after the post-synthesis treatment with aqueous solution of
hydrogen peroxide. In this way, it is interesting to remark that A-3
catalyst presents selectivity to acrylic acid of ca. 57% at a propane
catalysts. Reaction conditions in the text; symbols: A-1 ( ); A-2 ( ); A-3 ( );
HT-1 (!).
Moreover, it must be considered that this catalyst (A-3) shows a
STYAA similar to those achieved over MoVTeNb catalysts [19].
Table 4 shows comparatively the catalytic results obtained dur-
ing the oxidation of propylene over MoVSbO catalysts. Acrylic acid,
acetic acid, acrolein, acetone and carbon oxides were the main
reaction products, although traces of acetaldehyde were also ob-
served.
As observed in propane oxidation, the post-synthesis treated
samples, i.e. A-series, show also high activity and selectivity to
acrylic acid during the oxidation of propylene. In this way, both
propylene conversion and the areal reaction rate increase with the
number of washing cycles. However, although following the same
trend, low differences in the selectivity to acrylic acid have been
observed for A-series catalysts.
◦
conversion of 30% (at reaction temperature of 380 C and contact
−1
time, W /F , of 200 gcat. h molC ).
3H8
◦
Table 3 presents also the reaction rates (obtained at 380 C) per
unit surface area for untreated and post-synthesis treated catalysts.
The areal reaction rate for A-series catalysts increases as increas-
ing the number of washing cycles, although these ones are lower
than that observed for sample HT-1. However, if we consider the
rate of formation of acrylic acid per unit mass of catalyst and
per unit time, STYAA (space time yield), it can be seen that the
higher acrylic acid formation is obtained during the propane oxida-
tion over the M1-pure based catalysts, especially over A-3 sample.
−1
At this point we must inform that a STYAA of 275 gAA h
−1 kgcat.
is achieved during the selective oxidation of propylene over A-3
sample.
Moreover, it must be indicated that HT-1 catalyst (containing
both M1 and M2 phases) presents high activity in propylene oxi-
dation and a relatively high selectivity to acrylic acid, meanwhile

42
F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
Table 4
Oxidation of propylene on MoVSb-based catalysts.
W /Fa
Selectivityc (%)
Areal reaction
rated
STYAA
e
Conversionb
(%)
Catalyst
Acrylic acid
Acetic acid
Acetone
CO
CO2
A-1
A-2
A-3
150
150
150
27.5
66.4
74.8
68.8
73.3
76.7
0.3
3.9
2.1
8.3
4.5
1.0
18.1
12.6
13.7
4.3
5.5
6.5
37.4
102
133
90.8
234
275
HT-1
HT-2
150
480
65.8
36.7
38.1
0
15.0
27.9
8.7
15.9
26.4
42.6
11.9
12.5
140
94.4
120
0
a
−1
Contact time, W /F , in gcat. h (molC
Propylene conversion at 380 C.
)
.
3H6
b
c
◦
Selectivity to reaction products (acetaldehyde and acrolein were observed in some experiments although with selectivity lower than 0.5%).
−1
d
◦
−2
m
Areal reaction rate at 380 C expressed in 104 gC
h
(SBET data in Table 1).
3H6
−1
Rate of formation of acrylic acid (AA) at 380 C per unit mass of catalyst and per unit time, STYAA, in gAA h−1 kgcat.
.
e
◦
HT-2 catalyst (containing only the M2 phase) presents a low activ-
ity in propylene oxidation and is unselective in the formation of
acrylic acid. So it is clear that the elimination of the M2 phases
could have a positive effect on the catalytic behavior of these cat-
alysts.
sites for alkane activation favoring a decrease in the catalytic ac-
tivity. If this is so, it is not surprising that increasing the num-
ber of washing cycles we can decrease the amount of both the
(Mo0.01−0.02SbO1.9−3.3·xH2O) nanospheres and leached antimony
species deposited over catalyst surface, increasing thus the alkane
conversion.
As it has been already indicated, the oxidation state of anti-
mony species on the surface of solids changes during the post-
synthesis treatment with aqueous solution of hydrogen peroxide.
Thus Sb3+ is only observed in samples HT-1 and HT-2 (Table 2) in
agreement to previous results [30], while Sb5+ is mainly observed
in catalysts of A-series. Although the best catalytic performance of
post-synthesis treated samples (containing pure M1 phase) could
be also related to the higher oxidation state of antimony in the
catalysts; a similar improvement is obtained on potassium doped
MoVSbO catalysts, and no important variation of the oxidation
state of antimony species on the catalysts surface is observed [30].
In this way, Millet et al. have proposed that the Sb atoms in M1
phase have a high versatility and its oxidation state can change
independently of the catalytic behavior [48].
Finally, we must comment that the influence of the post-
synthesis treatment on the catalytic behavior of MoVSbO is dif-
ferent to that recently observed over MoVTeNbO catalysts [25]. In
our case, the post-synthesis treatment with aqueous H2O2 solu-
tion improves the selectivity to acrylic acid, with a relatively low
loss of the catalytic activity in the best case, A-3 catalyst. In a dif-
ferent way, the post-synthesis treatment with an aqueous H2O2
solution of MoVTeNbO catalysts is reported to favor an increase in
the catalytic activity without changes in the selectivity to acrylic
acid [25].
3.3. On the role of M1 and M2 phase in MoVSbO mixed oxide catalysts
It is known that the M2 phase, in both Nb-free and Nb-
containing Mo–V–Te–O catalysts, is inactive in propane oxidation
but it is active and selective in the partial oxidation of propylene
to acrolein or acrylic acid, respectively [35]. This could explain a
possible synergism with M1 phase during the propane partial oxi-
dation over MoVTeNbO catalysts [24,25]. Accordingly, the presence
of the M2 phases could have a positive influence on the catalytic
behavior of these catalysts.
In the case of Nb-free Mo–V–Sb–O catalysts the role of M2
phase is not clear and seems to be quite different, since according
to the results presented here, this phase shows a poor selectivity to
partial oxidation products during the propene oxidation (Table 4).
Thus, one first approach to explain the poor results of MoVSbO
catalysts containing both M1 and M2 phases could be related to a
negative influence of M2 phase (in an opposite trend to that pro-
posed in the case of MoVTeO) catalysts. However we can neither
discard that these poor results achieved over sample HT-1 could be
related to the worse catalytic performance of M1 phase by itself. In
fact, the better selectivity to acrylic acid of pure M1 phase based
catalysts seems to be associated to the changes on the surface of
this phase as a consequence of the post-synthesis treatment with
aqueous H2O2 solution. Thus, it has been evidenced that the elim-
ination and the modification of acid sites observed by NH3-TPD
on the catalyst surface of A-series, probably due to surface Sb en-
richment (as shown by XPS results), favors a higher formation of
acrylic acid. In the same way, it is known that the incorporation
of K or Nb in MoVSbO mixed oxides favors both, an elimination of
the surface acid sites and an improvement of the catalytic behavior
[15,23,25,30,31].
According to the characterization results presented here, three
important modifications on the surface of the catalysts of the A-
series are observed if compared to sample HT-1: (i) a drastic de-
crease in the number of acid sites, (ii) a Sb-enrichment of the
catalyst surface, and (iii) a modification of the oxidation state of
antimony on the catalyst surface. Thus, the changes observed in
samples obtained by post-synthesis treatment can be related to
the elimination of acid sites by leached antimony species (obtained
from the M2 phase decomposition during the post-synthesis treat-
ment) on the catalysts surface. However, the optimal amount of
Sb on the catalyst surface should have a limit. This limit must
be related to the number of acid sites on the catalyst surface,
while an excess of antimony species could also block the active
4. Conclusions
At the present paper highly active and selective Nb-free MoVSb
catalysts have been prepared by a post-synthesis procedure: heat-
treated MoVSb solids, previously obtained hydrothermally, are
treated with aqueous H2O2 solutions. Using these catalysts, yields
to acrylic acid of ca. 30% have been attained, which represents an
important improvement compared to those previously reported on
MoVSb mixed oxides [27–31,34]. However, the catalytic behavior
of these catalysts strongly depends on the washing procedure.
The characterization results of catalysts prepared by a post-
synthesis treatment suggest that the M2 phase is completely re-
moved, and M1 phase is mainly present. However, in samples
poorly washed, Sb-rich nanoparticles (Mo0.01−0.02SbO1.9−3.3·xH2O)
on the surface of M1 crystals are also observed.
The results presented here demonstrate that the role of the M2-
type crystalline phase in MoVSbO catalysts is completely different
to that previously proposed for MoVTeNbO catalysts. In the case of
Sb-based catalysts, the M2 phase presents a poor selectivity to par-
tial oxidation products during the oxidation of propene. However,

F. Ivars et al. / Journal of Catalysis 262 (2009) 35–43
43
this aspect cannot explain completely the poor catalytic results ob-
tained during the selective oxidation of propane to acrylic acid
over Nb-free Mo–V–Sb–O catalysts.
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in our case the catalytic improvement is obtained through an
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Moreover, we consider the results presented in this work are
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The authors thank the Spanish CICYT for financial support
(Projects NAN2004-09267-C03-02 and CTQ2006-09358/BQU) and
the technical support of the Microscopy Department of Universi-
dad Politécnica de Valencia (Spain).
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