Spectroscopic surface characterization of MoVNbTe nanostructured catalysts for the partial oxidation of propane

Article abstract of DOI:10.1016/j.cattod.2011.12.035

The main objective of the present contribution is to analyze the effect of composition, coverage and Te doping in the surface composition and catalytic behavior of supported-nanoscaled MoVNb(Te)O catalysts. Different spectroscopy techniques combine to aff

Full text of DOI:10.1016/j.cattod.2011.12.035

Catalysis Today 187 (2012) 195–200
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Spectroscopic surface characterization of MoVNbTe nanostructured catalysts for
the partial oxidation of propane
Ricardo López-Medinaa,b, Izabela Sobczak , Hanna Golinska-Mazwa , Maria Ziolek , Miguel A. Ba n˜ ares
b
b
b
a
c,∗
,
M. Olga Guerrero-Pérez
a
Catalytic Spectroscopy Laboratory, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, E-29049 Madrid, Spain
Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland
Departamento de Ingeniería Química, Universidad de Málaga, E-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:
Received 17 October 2011
Received in revised form 1 December 2011
Accepted 12 December 2011
Available online 9 February 2012
The main objective of the present contribution is to analyze the effect of composition, coverage and
Te doping in the surface composition and catalytic behavior of supported-nanoscaled MoVNb(Te)O cata-
lysts. Different spectroscopy techniques combine to afford such objective. The composition of the surface
species has been studied by Raman and UV–Vis spectroscopies, whereas it has been performed several
pyridine adsorption experiments to quantify the number of acid sites on the surface of catalysts. The
nature of reaction intermediates and the capability of the samples to adsorb them have been studied
by FTIR spectroscopy combined with the adsorption of propene. The results have shown that tellurium
induces several changes in the structure and catalytic properties of MoVNb catalytic materials, improving
the reaction yield to the desired acrylic acid during the propane oxidation reaction.
Keywords:
Propane
Acrylic acid
Nanostructured-catalysts
MoVNbTeO
FTIR
© 2012 Elsevier B.V. All rights reserved.
Raman
1
. Introduction
A general trend in the design of catalytic materials nowadays
MoVNb(Te)O catalysts, by the use of different spectroscopic tech-
niques, such as FTIR, UV–Vis and Raman spectroscopy. By this
manner, it is possible to identify the surface species (by Raman and
UV–Vis spectroscopies), the acid sites in each catalyst (determined
from the FTIR spectra of pyridine adsorption) and the nature of reac-
tion intermediates (determined form the FTIR spectra of propene
adsorption), and all this information can be correlated with the cat-
alytic test during the oxidation of propane into acrylic acid. The
interest of this reaction is because acrylic acid is a useful interme-
diate that is currently obtained thought a two-steps process from
propylene. Propane partial oxidation would replace the alkene as
starting material, delivering acrylic acid in a single step, since actu-
ally the acrylic acid production is carried out in a two steps process,
the first one is propylene to acrolein, and the subsequent transfor-
mation of acrolein to acrylic acid is the second one.
is based on nanostructured catalysts, since they present valuable
advantages from both industrial and academic points of view [1].
The active phases can be stabilized by a support versus syntheriza-
tion on a non-expensive material that acts as support [2]. Thus, the
amount of active phase required for a satisfactory catalytic perfor-
mance, can be dismissed, and, in addition, the use of nano-scaled
oxide phases maximizes the surface-to-volume ratio allowing a
better insight on the nature of the active phase. Following this
approach, in our group we have reported the synthesis and catalytic
behavior of nanoscaled SbVO4 [3] and VPO [4,5] based catalysts,
and, recently, we have reported a very promising results with the
use of nanoscaled MoVNb(Te)O catalysts for both propane oxida-
tion [6] and amoxidation [7] reactions.
Since the understanding of the factors that determine the sur-
face reactivity is necessary to design new and more effective
catalytic processes; the aim of the present contribution is to analyze
the effect of composition, coverage and tellurium doping on the sur-
face composition and catalytic behavior of these novel nanoscaled
2
. Experimental
The Mo Nb Te oxide catalysts were prepared using the
V
slurry method. An aqueous solution of ammonium heptamolybdate
tetrahydrate (NH ) Mo7O ·4H O (Sigma–Aldrich) was added to
4
6
24
2
an aqueous solution of NH VO (Sigma–Aldrich), heating and
4
3
◦
stirring at 80 C for 50 min; then, an aqueous solution of ammo-
nium niobium soluble complex (Niobium Products) and tellurium
acid Te(OH)6 (Sigma–Aldrich) was added using tartaric acid as
∗ Corresponding author.
E-mail address: oguerrero@uma.es (M.O. Guerrero-Pérez).
0
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.12.035

1
96
R. López-Medina et al. / Catalysis Today 187 (2012) 195–200
complexing agent. This solution was kept under continuous stirring
gas-chromatograph equipped with flame ionization and thermal-
conductivity detectors. The accuracy of the analytical determina-
tions was checked for each test by verification that the carbon
balance (based on the propane converted) was within the cumu-
lative mean error of the determinations (± 10%). The catalytic tests
were made using 0.2 g of powder sample with particle dimensions
in the 0.25–0.125 mm range. The axial temperature profile was
monitored by a thermocouple sliding inside a quartz tube inserted
into the catalytic bed. Tests were made using the following reaction
◦
at 80 C for 60 min until a wet gel was obtained; and then ␥-Al O
2
3
support (Sasol Puralox SCCa-5/200) was added. The ␥-Al O sup-
2
3
2
port presented a BET surface area of 193 m /g. The water excess was
removed in a rotatory evaporator at 80 C and in a reduced pressure
of 10–40 mmHg. The resulting solid was dried at 120 C for 24 h
and then calcined at 600 C for 2 h at a rate of 5 C/min either in N
◦
◦
◦
◦
2
atmosphere. The catalysts were named as CMo5V Nb0.5(Te0.5) with
4
a final Mo/V/Te/Nb atomic ratio of 0.5–0.6/0.4–0.3/0.05–1/0.05;
and they were prepared in order to have a total Mo + V + Nb + Te
coverage (C) on alumina of 4, 8 or 12 atoms per nm2 of support.
Specific surface areas were measured on a Micromeritics ASAP-
feed composition (% volume): 20.4% O , 12.5% propane and 15.9%
2
steam in helium. The total flow rate was 40 ml/min, correspond-
−
1
ing to 4800 h gas hourly space velocity (GHSV). The quantity of
catalyst and total flow were determined in order to avoid inter-
nal and external diffusion limitations. Yields and selectivities in
products were determined on the basis of the moles of propane
feed and products, considering the number of carbon atoms in each
molecule.
2
000 apparatus. Prior to the adsorption experiments, samples were
◦
outgassed at 140 C for 2 h. Raman spectra were collected in a
Renishaw System 1000 spectrometer equipped with green laser
(
Spectra Physics, ꢀ = 514 nm, power 19 mW, 1 mW on sample) and
◦
a cooled CCD detector (−73 C). The spectral resolution was ca.
−
1
3
3
2
cm and spectrum acquisition consisted of 10 accumulations of
0 s. The spectra were obtained under dehydrated conditions (ca.
◦
00 C) in a hot stage (Linkam TS-1500).
3. Results and discussion
UV–Vis spectra were recorded on a Varian-Cary 300 Scan UV–Vis
spectrophotometer with an integrated sphere CA-30I. Catalysts in
the form of powders were placed into a cell equipped with a quartz
window. The Kubelka–Munk function (F(R)) was used to convert
reflectance measurements into equivalent absorption spectra using
the reflectance of SPECTRALON as a reference.
Fig. 1 shows the Raman spectra of all the catalysts obtained
under dehydrated conditions. Just with a first view, it can be
observed that there are some important differences in the shape
of the spectra in Fig. 1A and B, suggesting that the differences
in the surface structure of the catalysts without (Fig. 1A) and
with (Fig. 1B) tellurium, even when a small amount of tellurium
has been added. The spectra of the samples without tellurium
Surface properties of the materials prepared were studied by
in situ FT-IR spectroscopy of pyridine adsorbed. Self-supporting
−2
−1
pellets of around 15 mg cm were prepared and located in a clas-
sical glass cell connected to a vacuum-adsorption apparatus for
in situ experiments. IR spectra were recorded at room temperature
are characterized by a broad signal near 800 cm , related with
the presence of a rutile structure and another broad signal near
−
1
1000 cm characteristic of the stretching modes of Mo O and/or
O bonds, respectively [16]. When both the total coverage as well
as the molybdenum content is increased, the bands near 993 and
−
1
with a Bruker Vector 22 FT-IR spectrometer (resolution 4 cm ).
V
◦
All the samples were activated in situ at 30 C for 3 h under vac-
−
1
uum before adsorption of pyridine. Pyridine (Sigma–Aldrich) was
817 cm become sharper and more intense, and also appears a
◦
◦
−1
adsorbed at 30 C and evacuated five times at 30 C for 30 min. After
each evacuation the FTIR spectra were registered. The IR spectra of
the activated samples were subtracted from those recorded after
the adsorption of pyridine. The reported spectra are the results of
this subtraction, and were used to calculate the number of acid sites
of each type, with the equation [8–11]:
band near 285 cm , the presence of this three bands indicate the
presence of MoO3 phase [6]. Thus, the presence of MoO3 oxide
is not detected when tellurium is present. Tellurium favors the
presence of mixed oxides, thus, Fig. 1B shows a broad signal from
−
1
800 to 870 cm , characteristics of rutile mixed phases. The spec-
trum of 12Mo V Nb0.5Te0.5 presents in addition, signals near 870
6
3
−
1
and 885 cm ; that can be related with the presence of tellurium-
containing samples, the band near 870 cm
ALCd
εLm
ABCd
εBm
−1
nT =
+
can be assigned to
the orthorhombic Te M O57 phase (M = Mo, V, Nb), called M1
2
20
−
1
where:
[17,18], whereas the signal near 880–890 cm
can be assigned
to the stretching mode of bridging Mo–O–M (M = Mo, V) bonds of
nT = total number of micromoles of pyridine per gram of sample
g).
AL, AB = integrated absorbance of FTIR bands due to base adsorbed
highly distorted Te M20O57 on alumina [19].
2
(
Fig. 2 shows the UV–Vis DR spectra of the catalysts. A strong
signal between 250 and 400 nm is assigned to ligand–metal charge
transfer (LMCT) transitions involving primarily the charge transfer
−
1
at Lewis acid sites (L) or at Bronsted acid sites (B) (cm ).
2
6+
5+
Cs = cross-sectional area of pressed disc (cm ).
from oxygen ligands to Mo or V metal cations, in addition to
Nb and Te
5+
4+
m = mass of the pressed disc (g).
[20,21]. This band extends up to 600 nm when tel-
εL, εB = is the molar absorption coefficient for base at Lewis (L)
lurium is added to the catalysts (Fig. 2B), suggesting the presence of
reduced Mo cations [22–24]. The catalysts without tellurium with a
high coverage (Fig. 2Ac and f) show a broad absorption at ∼600 nm
that is associated with V4+ cations [25–27].
−1
−1
sites or at Bronsted (B) sites (␮mol cm). εL = 1.5 ␮mol cm;
−
1
εB = 1.8 ␮mol cm [12–15].
Propene adsorption and co-adsorption with oxygen were per-
formed with the Vertex 70 (Bruker) spectrometer (resolution
Lewis acid (LAS) and BrØnsted acid centres (BAS) were detected
in the FT-IR spectra after pyridine adsorption at 30 C and des-
◦
−
1
−1
4
cm ). The pressed wafers of the materials (around 15 mg cm
)
orption at the same temperature. According to literature, the
interaction of pyridine with Lewis acid sites leads to the appear-
◦
were placed in the vacuum cell and activated at 30 C for 3 h. The
procedure was as follows: (i) propene adsorption at room temper-
ature (RT) and heating at 50 C for 30 min (ii) evacuation at RT for
−
1
ance of bands at ca. 1450 and ∼1610 cm [28–31]. The intensity of
the first one is related to the number of LAS, whereas the position
of the second band characterizes the strength of LAS. Adsorption
◦
6
0 min (iii) propene adsorption at RT followed by O admission and
2
◦
−1
heating at 50 C for 30 min (iv) evacuation at RT for 30 min.
Activity measurements were performed using a conventional
micro-reactor (9 mm ID) with minimized void volume. The feed
stream and effluents of the reactor were analyzed by an on-line
of pyridine on BrØnsted acid sites gives a band at ∼1550 cm and
−
1
two others in the 1620–1640 cm range. Moreover, the bands at
−
1
1445 and 1596 cm , originate from pyridine hydrogen bonded to
surface hydroxyls [29]. Figs. 3 and 4 show the FTIR spectra obtained

R. López-Medina et al. / Catalysis Today 187 (2012) 195–200
197
◦
Fig. 1. In situ Raman spectra of dehydrated samples (200 C in 50 ml/min of air flow) of A: Te-free catalysts; (a) 4Mo5V4Nb1, (b) 8Mo5V4Nb1, (c) 12Mo5V4Nb1, (d)
4
Mo6V3Nb1, (e) 8Mo6V3Nb1, (f) 12Mo6V3Nb1, and B: Te-containing catalysts; (a) 4Mo5V4Nb0.5Te0.5, (b) 8Mo5V4Nb0.5Te0.5, (c) 12Mo5V4Nb0.5Te0.5, (d) 4Mo6V3Nb0.5Te0.5,
(
e) 8Mo6V3Nb0.5Te0.5, (f) 12Mo6V3Nb0.5Te0.5.
Fig. 2. UV–Vis DRS spectrum of A: Te-free catalysts; (a) 4Mo5V4Nb1, (b) 8Mo5V4Nb1, (c) 12Mo5V4Nb1, (d) 4Mo6V3Nb1, (e) 8Mo6V3Nb1, (f) 12Mo6V3Nb1, and B: Te-containing
catalysts; (a) 4Mo5V4Nb0.5Te0.5, (b) 8Mo5V4Nb0.5Te0.5, (c) 12Mo5V4Nb0.5Te0.5, (d) 4Mo6V3Nb0.5Te0.5, (e) 8Mo6V3Nb0.5Te0.5, (f) 12Mo6V3Nb0.5Te0.5.
◦
after pyridine adsorption at 30 C for catalysts with and without
pronounced for samples containing tellurium (Figs. 3B and 4B). The
spectra of Tellurium containing catalysts exhibit noise characteris-
tic for not fully evacuated water. It suggests that for these materials
water is more strongly held and evacuation at room temperature,
even 150 min, does not remove water totally. The significant
difference in the spectra of catalysts with and without Tellurium
one can find in the 1445–1450 cm range. Pyridine adsorbed on
Te-containing samples gives rise to the formation of two IR bands
originating from chemisorption on two different Lewis acid sites,
−
1
tellurium. The bands at 1615, 1490, and 1446 cm
assigned to the vibrational modes of Lewis site coordinated pyri-
dine. The bands at 1640, 1490 and 1538 cm
pyridinium ion bonded to a Brønsted site. Thus the band around
490 is associated to both Brønsted and Lewis acid sites. The band
have been
−
1
correspond to a
1
−1
−1
at 1575 cm corresponds to physisorbed pyridine. Another band
−1
at 1595 cm is assigned to hydrogen-bounded pyridine (together
with a shoulder at ∼1437 cm−1) [32]. This band seems to be more
◦
◦
Fig. 3. IR spectra after pyridine adsorption at 30 C of A: Te-free catalyst (8Mo5V4Nb1) and B: Te-containing catalyst (8Mo5V4Nb0.5Te0.5); (a) adsorption at 30 C, (b) desorption
◦
at 30 C, 30 min, (c) 60 min, (d) 90 min, (e) 120 min and (f) 150 min.

1
98
R. López-Medina et al. / Catalysis Today 187 (2012) 195–200
◦
◦
Fig. 4. IR spectra after pyridine adsorption at 30 C of A: Te-free catalysts (8Mo6V3Nb1) and B: Te-containing catalysts (8Mo6V3Nb0.5Te0.5); (a) adsorption at 30 C, (b)
◦
desorption at 30 C, 30 min, (c) 60 min, (d) 90 min, (e) 120 min and (f) 150 min.
contrary to pyridine adsorbed on Te-free catalysts generating only
one IR band in this range. This difference is especially well visible
in Fig. 3. The detected presence of two different LAS centres on the
surface of Te-containing catalysts can be caused by the presence
of mixed phases identified by Raman (Fig. 1) and UV–Vis (Fig. 2)
spectra.
Fig. 5 shows the FTIR spectra obtained during the propene
and oxygen adsorption experiments. After propene adsorption, the
characteristic bands of propene adsorbed appear at 1439, 1368
−
1
(asymmetric and symmetric CH3 deformations) and 1095 cm
(isopropoxide species) on MoVNb(Te) [34]. These bands are visible
after evacuation, indicative that propene remains retained on the
surface of the catalyst. The admission of oxygen to these samples
Table 1 shows the relative acidity of the catalysts with and with-
−
1
−1
out tellurium, expressed as area under the peak at ∼1445 cm
results in additional bands near 1440 cm , characteristic of car-
−
1
normalized by surface unit in the spectra recorded after des-
orption at 30 C for 3 h. It can be observed that the acidity of
boxylate compounds, and at 1298 cm , assigned to acrylate type
compounds [35,36].
◦
the surface of the catalysts change upon tellurium incorpora-
tion. In the case of samples without tellurium, the major part of
acid sites is Brønsted, and their number increases as the cover-
age is increased, since they are associated with the oxide phase
Fig. 6 shows the subtracted FTIR spectra after propene adsorp-
tion and after admission of oxygen. Fig. 6A and C corresponds
to the catalysts without tellurium, whereas Fig. 6B and D to the
Te-containing samples. It is important to stress that the shape of
spectra is totally different indicating that a small amount of tel-
lurium is able to change the structure of the surface of the catalysts
as well as the adsorption properties, and, subsequently, the cat-
alytic properties. The bands are more intense in the case of catalysts
without tellurium, which are those that presented a higher amount
of acid sites. Thus, it confirms that the intermediates are held on
the surface of catalysts strongly in the absence of Te, and such
feature could decrease the activity of the samples. The band at
[
24]. The number of LAS decreases with coverage since the alu-
mina sites are titrated when the coverage is increased. In the
case of catalysts containing tellurium, the maximum number of
2
acid sites is detected for intermediate coverage (8 atoms/nm ),
although the presence of tellurium clearly decreases the num-
ber of acid sites. Thus, the changes in the structure of catalysts
detected by both Raman and UV–Vis spectroscopies is also reflected
in the changes detected in the surface acidity of the samples. The
addition of Te induces the formation of mixed phases contain-
ing Te, such as M1, identified by Raman spectroscopy (Fig. 1),
and also the presence of reduced Mo species identified by UV–Vis
−
1
1631–1636 cm due to C C stretching [37], together with other
−
1
intense bands at 1452 and 1368 cm , associated to the asymmet-
ric and symmetric CH3 deformation are observed after propene
adsorption at room temperature. When the oxygen is added, and
specially in the case of the Te-containing samples, the intensity
(
Fig. 2), this changes in the structure inhibit the formation of
MoO , since in the samples without Te this oxide has been iden-
3
−
1
tify by Raman spectroscopy (Fig. 1), and the number of acid
sites when Te is present decreases. The formation of MoO3 is
also possible under reaction conditions, even when such oxide
is not present in the fresh catalysts, and it has been demon-
strated that the presence of Te in the catalyst formulation can
inhibit the formation of such oxide under reaction conditions
of the band at 1452 cm increases, together with an increase of
−
1
intensity of the bands at 1456, 1465, 1368–1372 cm . These bands
can be assigned to a mixture of acetate ions and formate ions
[34,38]. The 1441 and 1368–1386 cm IR bands correspond to the
−
1
asymmetric and symmetric CH deformations, while the band at
3
−
1
1095 cm is due to isopropoxide species [39]. The bands at 1440
and 1321 cm can be related also to carboxylic-type compounds
−
1
[
33].
Table 1
BET surface area, compositions determined by ICP, and number of LAS and BAS sites.
Catalysts
Atomic ratio ICP
BET area
Number of
Number of
BAS × 10
Catalysts
Atomic ratio ICP BET area
Number of
LAS × 10
Number of
2
LASa × 10
18
a
18
2
a
18
BASa × 10
18
(
m /g)
(m /g)
4Mo5V4Nb0.5Te0.5
8Mo5V4Nb0.5Te0.5
12Mo5V4Nb0.5Te0.5
4Mo6V3Nb0.5Te0.5
8Mo6V3Nb0.5Te0.5
12Mo6V3Nb0.5Te0.5
Mo5.5V4.0Nb0.29Te0.21
Mo5.0V4.5Nb0.23Te0.27
Mo5.4V4.2Nb0.20Te0.30
Mo6.4V3.0Nb0.30Te0.20
Mo6.5V3.0Nb0.25Te0.25
Mo6.5V3.0Nb0.21Te0.29
169
150
123
169
149
125
3.20
4.01
1.19
5.73
3.73
1.18
0.144
0.869
0.435
0.509
1.21
4Mo5V4Nb1
8Mo5V4Nb1
12Mo5V4Nb1 Mo5.4V4.2Nb0.4
4Mo6V3Nb1
8Mo6V3Nb1
12Mo6V3Nb1 Mo6.4V3.2Nb0.4
Mo5.5V4.0Nb0.4
Mo5.0V4.5Nb0.5
172
157
128
170
153
132
4.28
2.22
1.84
3.99
2.01
1.96
6.05
10.5
11.9
6.39
9.18
9.39
Mo6.5V3.1Nb0.4
Mo6.7V3.0Nb0.4
0.518
a
Number of LAS (Lewis acid sites) and BAS (Brønsted acid sites) calculated in the catalysts on the basis of FTIR bands observed after desorption of pyridine expressed as
area under the peak at ∼1445 cm− and ∼1538 cm , normalized by surface unit in the spectra recorded after desorption at 30 C.
1
−1
◦

R. López-Medina et al. / Catalysis Today 187 (2012) 195–200
199
◦
Fig. 5. FTIR spectra of C3H6 and O2 coadsorption and desorption at room temperature and 50 C, complete process.
[
40]. Thus, this results show the capability of the samples to adsorb
in order to yield the desired acrylic acid; although it is not possi-
ble to make a definitive assignment of the intermediate adsorbed
oxygenated compounds.
The catalytic results during the propane oxidation into acrylic
acid are shown in Table 2. The yield to acrylic acid is higher in
propene, which is the main intermediate during the propane partial
oxidation [41]. The presence of IR bands characteristic for different
oxygenates indicates that the catalysts are able to transform the
adsorbed propene by inserting the oxygen atom into its molecule,
Fig. 6. FTIR spectra after C3H6 adsorption and next O2 admission in A: (a–b) 4Mo6V3Nb1, (c–d) 8Mo6V3Nb1, (e–f) 12 Mo6V3Nb1; B: (a–b) 4Mo6V3Nb0.5Te0.5, (c–d)
8
1
Mo6V3Nb0.5Te0.5, (e–f) 12 Mo6V3Nb0.5Te0.5; C: (a–b) 4Mo5V4Nb1, (c–d) 8Mo5V4Nb1, (e–f) 12 Mo5V4Nb1 and (D) (a–b) 4Mo5V4Nb0.5Te0.5, (c–d) 8Mo5V4Nb0.5Te0.5, (e–f)
2 Mo5V4Nb0.5Te0.5.

2
00
R. López-Medina et al. / Catalysis Today 187 (2012) 195–200
Table 2
Propane conversion, selectivity to main products, and yield to acrylic acid obtained during the propane partial oxidation, reaction conditions: total flow 40 ml/min feed gas
− ◦
1
(
vol.%) C3/O2/H2O/He = 12.5/20.4/15.9/51.2, 200 mg, GHSV = 4800 h , T = 400 C.
Catalysts
Conversion
Acrylic Acid
Acrolein
Propylene
COx
Yield to acrylic acid
4
8
1
4
8
1
4
8
1
4
8
1
Mo5V4Nb1
Mo5V4Nb1
2Mo5V4Nb1
Mo6V3Nb1
Mo6V3Nb1
2Mo6V3Nb1
Mo5V4Nb0.5Te0.5
Mo5V4Nb0.5Te0.5
2Mo5V4Nb0.5Te0.5
Mo6V3Nb0.5Te0.5
Mo6V3Nb0.5Te0.5
2Mo6V3Nb0.5Te0.5
11.6
47.8
31.8
3.14
10.5
36.7
27.5
33.0
51.0
29.2
32.0
55.7
30.4
35.2
22.8
30
48.6
14.5
37.0
42.5
44.7
33.0
44.3
47.0
16.2
28.4
11.8
25.4
17.7
17.7
20.4
20.2
9.5
23.7
5.3
9.3
19.5
31.1
54.2
5.7
3.5
16.9
7.3
1
5.1
30.9
22.3
5.6
30.7
20.7
12.4
40.4
25.8
16.9
3.8
60.5
11.3
17.5
31.5
13.0
11.6
13.0
5.3
10.1
14.0
22.7
9.6
14.2
26.2
16.8
17.9
21.6
the case of Te-containing samples, especially those with higher
coverages, and, in this case, the best results are reported for the
catalysts with higher vanadium contents (8Mo V Nb0.5Te0.5 and
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(
Spain) and ICyTDF (México) for his PhD program fellowship and
&
COST Action D36 for the financial support (STSM) during his stay
at AMU (Poland). The authors express their thanks to Olaf Torno
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(
Sasol Germany GmbH) for providing alumina support.