V- or Mo-modified niobium catalysts for glycerin conversion reactions in the presence of H2O2

Article abstract of DOI:10.1016/j.apcata.2012.07.041

In the present work, amorphous niobium oxides with 5% (w/w) vanadium or molybdenum isomorphically substituted into the material structure were synthesized. The materials were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that vanadium and molybdenum were incorporated into the niobium oxide structure with concomitant changes in the morphologic and catalytic properties. Catalytic studies on the conversion of residual glycerin generated during biodiesel production in the presence of H₂O₂ showed enhanced efficiency of ether formation with the vanadium-containing material. The NbV/ethanol/glycerin/250 °C system was found to convert 80% of glycerin. Experiment-planning studies in conjunction with the analysis of the surface response via gas chromatography-mass spectrometry (GC-MS) aided in the determination of the optimal conditions. The catalytic tests monitored by GC-MS showed the formation a mixture of ethers obtained by the condensation of glycerol. Furthermore, it was observed that the modification of the catalyst directs the formation of larger molecules such as ketone mainly in the catalyst containing vanadium.

Full text of DOI:10.1016/j.apcata.2012.07.041

Applied Catalysis A: General 443–444 (2012) 153–160
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
V- or Mo-modified niobium catalysts for glycerin conversion reactions in the
presence of H O
2
2
a
a
b
c
Juber P. Souza , Thaís Melo , Marcone A.L. de Oliveira , Roberto M. Paniago ,
d
a,∗
Patterson P. de Souza , Luiz C.A. Oliveira
a
Departamento de Química, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Campus Pampulha, 31270-901 Belo Horizonte, MG, Brazil
Departamento de Química, Universidade Federal de Juiz de Fora, Rua José Louren c¸ o Kelmer, s/n – Campus Universitário Bairro São Pedro, 36036-900 Juiz de Fora, MG, Brazil
Departamento de Física, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Campus Pampulha, 31270-901 Belo Horizonte, MG, Brazil
Departamento de Química, Centro Federal de Educa c¸ ão Tecnológica de Minas Gerais, CEFET-MG, Av. Amazonas 5253, 30421-169 Belo Horizonte, MG, Brazil
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2012
Received in revised form 27 July 2012
Accepted 29 July 2012
Available online 11 August 2012
In the present work, amorphous niobium oxides with 5% (w/w) vanadium or molybdenum isomorphi-
cally substituted into the material structure were synthesized. The materials were characterized by X-ray
diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled
with energy-dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), and
X-ray photoelectron spectroscopy (XPS). The results show that vanadium and molybdenum were incor-
porated into the niobium oxide structure with concomitant changes in the morphologic and catalytic
properties. Catalytic studies on the conversion of residual glycerin generated during biodiesel production
in the presence of H2O2 showed enhanced efficiency of ether formation with the vanadium-containing
Keywords:
Doped niobia
Glycerol conversion
Heterogeneous catalysis
Oxidation process
◦
material. The NbV/ethanol/glycerin/250 C system was found to convert 80% of glycerin. Experiment-
planning studies in conjunction with the analysis of the surface response via gas chromatography–mass
spectrometry (GC–MS) aided in the determination of the optimal conditions. The catalytic tests monitored
by GC–MS showed the formation a mixture of ethers obtained by the condensation of glycerol. Further-
more, it was observed that the modification of the catalyst directs the formation of larger molecules such
as ketone mainly in the catalyst containing vanadium.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
and alcohols [5,6]. In this context, the use of niobium oxide cata-
lysts is an alternative heterogeneous catalyst for the conversion of
glycerin into ethers and esters, which are in great demand in the
oil industry [7]. Our group has recently published a study in which
synthetic niobium oxides were modified to bear oxidizing groups
on their surface; the modified niobium oxides were subsequently
used as catalysts in the conversion of glycerin into ethers with
C6–C9 length chains [8]. The isomorphic incorporation of transition
elements into the niobium oxide structure was found to increase
its catalytic properties [9]. The association of the niobium oxide
with other metals, such as vanadium or molybdenum, may gener-
ate materials with high catalytic activity toward the oxidation of
organic compounds [7,10]. In fact, some transition elements with
high oxidation states, such as niobium, vanadium, and molybde-
num, can form strongly oxidizing species upon reaction with H O .
Biodiesel is defined as a mono-alkyl-ester-based fuel derived
from long-chain fatty acids obtained from renewable sources
such as vegetable oils [1]. Transesterification is the most com-
monly utilized method for the conversion of oils and fat from
vegetable sources into biodiesel. The transesterification reaction
can be performed using different catalytic systems (acid, alkaline,
or enzymatic), different alcohol/vegetable oil ratios, and differ-
ent temperatures [2]. Biodiesel has been garnering considerable
interest in the past few years as a sustainable alternative to
petroleum-based diesel with less impact on the environment [1–3].
The production of biodiesel in an alkaline medium generates glyc-
erin as one of the main by-products, which, as the worldwide
production of biodiesel increases, may lead to a drastic reduction in
the market price of this product [3,4]. One alternative to this prob-
lem consists of transforming residual glycerin into synthetic gas
compounds, methane, or other products, such as acrolein, ethers,
2
2
The highly reactive oxidative species formed – commonly referred
to as peroxometalates – are effective in transferring oxygen in oxi-
dation reactions [11].
The present work was aimed at the synthesis of vanadium- and
molybdenum-doped niobium oxides and at the evaluation, through
experiment planning, of the influence of the presence of a doping
metal, the influence of different solvents (H2O, tert-butyl alcohol,
∗ Corresponding author. Tel.: +55 31 3409 7550; fax: +55 31 3409 5700.
E-mail address: luizoliveira@qui.ufmg.br (L.C.A. Oliveira).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.07.041

1
54
J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
Table 1
Contrasts coefficients, factors, levels, and responses.
Issues
Mean
Contrast coefficients
Responses % conversion
2
1
2
2
2
3
X1
X2
X3
X
X
X
X1X2
X1X3
X2X3
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
−1
1
−1
−1
1
1
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
1
−1
−1
1
0
0
0
0
0
0
0
0
0
0
1
−1
−1
1
0
0
0
0
0
0
0
0
0
0
1
−1
−1
1
0
0
16
19
13
16
12
22
64
80
14
23
56
68
19
19
21
−1
1
0
−1
1
−1
−1
1
−1
1
1
0
0
0
0
0
0
0
−1
−1
−1
1
1
0
1
1
1
1
1
1
1
−1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
X1 – Solvent (%, v/v): (−1) tert-butyl alcohol 10%; (0) water 10%; (+1) ethanol 10%.
X2 – Catalysts: (−1) pure Nb; (0) NbV; (+1) NbMo.
◦
X3 – Temperature ( C): (−1) 150; (0) 200; (+1) 250.
or EtOH), and the influence of temperature in the conversion of
residual glycerin from biodiesel production. The conversion of glyc-
erin and the formation of its products were detected using gas
chromatography–mass spectrometry (GC–MS).
FTS 3000) FTIR spectrometer and processed with IRDM (IR data
manager) software using the KBr pellet technique. The powder
XRD data were obtained in a Rigaku model Geigerflex using Cu K␣
radiation scanning from 2 to 75 at a scan rate of 4 min . A scan-
ning electron microscope (SEM) manufactured by JEOL Ltd., was
used. SEM was coupled with EDS/INCA 350 (energy dispersive X-ray
analyzer) manufactured by Oxford Instruments. The transmission
electron microscopy (TEM) images of all samples were taken with
a JEOL transmission electron microscope model JEM 2000EXII.
◦
◦
−1
2
. Materials and methods
2.1. Preparation and characterization of the catalysts
Niobia (Pure Nb) was prepared by slow dropping of an
−
1
1
mol L
NaOH solution in a 500 mL Teflon beaker contain-
−1
2.2. Catalytic tests
ing 100 mL 0.26 mol L solution of NH [NbO(C O )(H O)](H O)n,
4
2
4
2
2
kindly donated by CBMM – Companhia Brasileira de Metalurgia e
Minera c¸ ão (Araxá, state of Minas Gerais, Brazil) at 70 C under vig-
◦
The catalytic tests were performed at 150, 200 and 250 C, using
0 mL of glycerol (1.15 mol L ) and 10 mg of catalyst in the pres-
◦
−
1
2
orous stirring. The solids obtained were washed with distilled water
until neutral pH. The Mo (NbMo) and V (NbV) substituted niobia
ence of H O2 (1.00 mL) in a batch reactor for 4 h at pH = 6. The
2
−
1
products of the reaction were analyzed by GC–MS (Agilent). The
percent conversion of glycerol was investigated by integrating the
peak of glycerol to the total ion content (TIC), obtained before and
after the reaction with the catalyst. The parameters for the GC–MS
were prepared from NH [NbO(C O )–(H O)](H O)n (1.7 mol L
)
4
2
4
2
2
−
1
and Na MoO ·2H O or Na VO solutions (0.09 mol L ) by precip-
2
4
2
3
4
itation with concentrated ammonium hydroxide. The precipitates
◦
were washed with water, dried at 70 C for 12 h. The treatment
◦
analysis were as follows: injector temperature 200 C, injection
with hydrogen peroxide was performed treating the previously
synthesized niobium oxide (300 mg) with 8 mL of aqueous hydro-
gen peroxide 30% (v/v) and 80 mL of water for 60 min. After these
contact times the solids were washed with distilled water and oven
−
1
volume 1 mL, flow 1.3 mL min and Column HP-5 (5% polimetil-
fenil siloxane). A heating curve was established at 3 C min , from
0 C to 150 C. Table 1 presents all details about the experimental
◦
−1
◦
◦
9
◦
conditions of the catalytic tests.
dried for 12 h at 70 C. Spectra were recorded in a Digilab(Excalibur,
(b)
(a)
pure Nb
Pure Nb
NbV
NbMo
NbMo
1
272
8
95
6
45
NbV
60
1
402
3
160
1696
1
0
20
30
40
50
3500
3000
2500
2000
1500
1000
500
Wavenumber/ cm-1
2
θ/ °
Fig. 1. X-ray diffractograms (a) and FTIR spectra (b) of pure Nb, NbMo and NbV catalysts.

J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
155
O 1s
(
A)
(B)
Nb 3d
Pure Nb
NbV
530
O 1s
Nb
Pure Nb
NbV
3p 3p
1
3
Nb3s
NbMo
C1s
C1s
533
Pure Nb
NbV
Na Auger
Na Auger
NbMo
Mo3d
NbMo
800
600
400
200
0
542 540 538 536 534 532 530 528 526 524 522
Binding Energy / eV
Binding energy/ eV
Fig. 2. XPS analyses of pure Nb, NbMo and NbV catalysts.
Fig. 3. SEM images of pure Nb, NbV and NbMo catalysts.

1
56
J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
2.3. Design of experiment (DOE) approach
In order to understand the hole of each relevant variable in
doped niobia as catalyst 33 Box–Behnken designs was performed
12]. Thus, an experimental design set containing three factors (sol-
[
vent percentage, catalyze and temperature) and three levels, that
is low (−1), central (0) and high (+1) with triplicate in central
point was carried out. In the present case were studied two vari-
ables quantitative (solvent percentage and temperature) and one
qualitative (catalyze). Other variables such as reaction time were
maintained fixed. Table 1 shows the contrast coefficients, factors,
levels, and responses obtained. The statistical analysis involving
®
DOE were performed in Microsoft Office Excel 2007 software.
3
. Results and discussion
3.1. Catalyst characterization
The results obtained by X-ray diffraction (XRD) indicate the for-
mation of an amorphous material, and no reflections consistent
with the formation of crystalline phases were observed. Addi-
tionally, the presence of molybdenum and vanadium was not
observed, which suggests the incorporation of these elements
into the niobium oxide structure. These results were somewhat
expected because the co-precipitation method used in the syn-
thesis (without material calcination) does not induce long-range
structural organization, which is a characteristic of crystalline
materials [13].
The FTIR spectra of V- and Mo-doped niobium oxides (Fig. 1b)
exhibit the characteristic bands of amorphous niobium oxide [14].
The O H bond stretching bands are evident, especially the one cen-
−
1
tered at 3160 cm , which is assigned to the stretching of Nb OH
−
1
internal hydroxyl bonds. The band centered at 1696 cm is char-
acteristic of the angular deformation of water molecules adsorbed
onto the surface of the catalyst. Absorption bands centered at
402 and 1272 cm are associated with the vibration modes of
the niobium precursor. The bands located between 500 cm and
−
1
1
−
1
−
1
9
50 cm are consistent with angular vibrations of the O–Nb–O,
O–V–O and O Mo O bonds; a broad band centered at approx.
− −1
1
6
45 cm with a shoulder at 895 cm is characteristic of an amor-
phous material, the presence of which is in agreement with the XRD
results.
The catalysts were further characterized using X-ray photoelec-
tron spectroscopy (XPS) (Fig. 2). Fig. 2A, which shows the general
spectrum of the catalysts, exhibits signals assigned to Nb3d, Nb3p1,
and Nb3p3, plus typical O1s signals assigned to niobium oxides [15].
In the figure inset, the zoomed-in section details the Mo3d (NbMo)
signal, but not the V (NbV) one, which might be due to a low concen-
tration of vanadium at the surface (less than 1%). Furthermore, all of
the doped material spectra contained signals for sodium, an impu-
rity. Fig. 2B shows an interesting result: the zoomed-in portion of
Fig. 4. EDS microanalyses of pure Nb, NbV and NbMo catalysts.
EDS analyses (Fig. 4) confirm the presence of vanadium and
molybdenum in concentrations of 0.4% and 0.6% (m/m), respec-
tively. Some impurities were detected, mostly sodium and carbon,
which had also been observed in the XPS spectra.
3.2. Glycerin conversion: kinetics study
the O1s region reveals, in addition to the typical oxide (O2 ) sig-
nal centered at 530 eV, a shoulder at approximately 533 eV, which
is attributed to the peroxometalate group formed at the surface of
−
An early kinetic study of glycerin conversion was performed
using the catalysts in the presence of H O . The profiles of the
2
2
H O -treated catalysts [15]. These signals are most evident in the
kinetics curves are shown in Fig. 5.
2
2
spectra of the NbV-doped materials, but also appear in those of the
pure Nb materials.
The vanadium-containing catalyst, NbV, showed the largest con-
version of glycerin in the time interval of the study. After 360 min,
the NbV, NbMo, and pure Nb catalysts had converted 92, 90, and
69% of the glycerin, respectively. The result obtained for the NbV
catalyst is attributed to the presence of a large number of highly
oxidative groups at the surface, i.e., peroxo groups, as observed in
the XPS spectra. In fact, other authors have already noted that vana-
The morphology of the materials (Fig. 3) was characterized using
scanning electron microscopy (SEM). EDS chemical analyses were
performed in conjunction with the SEM micrographs (Fig. 4). The
morphologies of the pure Nb and NbMo, as shown in the SEM
images, are similar: both are characterized by a smooth surface
with few rough spots. By contrast, the vanadium-modified sample
dium compounds react more readily with H O2 to form reactive
2
(
NbV)exhibiteda different morphology, where particle aggregation
species that can transfer oxygen to organic substrates [10]. Fig. 5b
shows a schematic representation of the entire process involving
led to a rougher surface.

J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
157
Fig. 5. Glycerin conversion kinetics using the Mo- and V-modified catalysts at
◦
2
50 C. Glycerin 90% (v/v); solvent: H2O (a). Schematic representation of the peroxo
group generation and glycerin conversion (b).
the reaction with H O2 that leads to the formation of a peroxo-
2
vanadate reactive group at the surface of the catalyst and to the
conversion of glycerin. We emphasize that, for studies where sol-
vent and temperature were varied, a reaction time of 4 h was chosen
because conversion rates are suitably high in this time period, as
demonstrated during the kinetics study (Fig. 5a).
3.3. Factorial design analysis
Factorial design is a very useful tool to obtain deeper informa-
tion about the relationships among the studied variables and to
establish the optimum experimental conditions presenting more
comprehensive understanding of the investigated system through
simultaneous evaluation of variables combined with a reduced
Fig. 6. Surface response to the 33 Box–Behnken design studies: A, pure Nb oxide; B,
NbV; C, NbMo.
3
number of experiments. In the present case, for the 3 Box–Behnken
design with triplicate in central point, the variables considered
were solvent percentage added in glycerin, catalyst and temper-
ature according to shown in Table 1. Analyzing Table 1 under a
general optical, it is observed that high response (glycerin con-
version) was obtained through experiment 8 (1,0,1), that is, in
Analyzing the coefficient model within 99% confidence inter-
val, just the coefficient X3 (temperature) was significant. On other
hand, the factors solvent and catalyst in average did not present
relevant effects within interval investigated. Since the model did
not present lack of fit for 99% significant interval, three sur-
face responses considering variable X2 fixed in levels −1 (A, pure
Nb), 0 (B, NbV) and +1 (C, NbMo) were considered in order to
observe the effect of the different catalyst in relation to tem-
perature and solvent in different levels. Through the surfaces it
is possible to observer that in average the response tends to
increase when temperature is fixed in high level for glycerin
added to ethanol 10%, independently to the catalyst used. How-
ever, by surfaces was possible also to see that pure niobia presents
lower glycerin conversion in comparison with niobia doped with
metal. Nevertheless, the higher response obtained was achieved
◦
condition containing ethanol 10%, NbV and 250 C, while the lower
response was obtained in experiment 5 (−1,0,−1), that is, tert-butyl
◦
1
0%, NbV and 150 C. After the calculations the following model was
obtained:
2
2
yˆ = 19.7 + 4.00x + 1.88x + 24.63x + 0.29x − 3.96x2
1
2
3
1
(
± 0.67)
(± 0.41)
(± 0.41)
(± 0.41)
(
± 0.60)
(± 0.60)
2
+
24.54x + 0.00x x + 1.50x x + 0.75x x
3
1
2
1
3
2
3
(
± 0.60)
(± 0.58)
(± 0.58)
(± 0.58)

1
58
J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
Fig. 7. Glycerol chromatograms (A), detailed pure glycerol (B) and detailed cyclic ethers (C). Evaluation of the influence of solvents: EtOH (black line) and t-But alcohol (blue
line). Amount of ether formed at different reaction times (D). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
the article.)
when NbV was used as catalyst. Finally, the surface response
Fig. 6) modeling study indicates that the increases of tempera-
The reactions with higher rates of conversion under the experi-
mental conditions were analyzed in further detail. For this purpose,
chromatograms for the products of the conversion reactions
with the highest rates were obtained and are summarized in
Table 1.
The chromatogram in Fig. 7A–C describes the NbV-catalyzed
conversion reactions using ethanol and tert-butyl alcohol as sol-
vents. As evident from the results in Table 1, the conversion
rate is higher when ethanol is used, probably because of a more
(
ture will improve the response. Thus, the results suggesting the
possibility of the increases in the glycerin conversion taking into
account experiment containing as conditions: ethanol 10%, NbV
◦
and temperature above to 250 C. However, it is necessary to
know to what extent the temperature increases will be chemi-
cal significance, that is, will be practicable to glycerin conversion
process.

J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
159
Fig. 8. Glycerol chromatogram – treatment with pure Nb (solid line) and NbMo doped (dash line); solvent: H2O.
Glycerol
HO
OH
OH
Condensed Phase
Volatile Phase
15%
8
5%
OH
OH
Allyl Alcohol
O
O
O
O
(
25% )
HO
HO
OH
O
p-Dioxane-2,5-dimethanol
1,4-Dioxane-2,6-dimethanol
35% )
(
O
HO
O
Butadione
OH
(
08% )
OH
OH
O
Dyglycerol
10% )
(
Pentanone
11% )
(
Others Products and Tryglycerol
55% )
Others Products
(
( 56% )
Fig. 9. Scheme showing the products obtained after glycerol conversion.

1
60
J.P. Souza et al. / Applied Catalysis A: General 443–444 (2012) 153–160
favorable reaction with the shorter-chain alcohol. However, the
branched-chain alcohol showed a higher selectivity for ethers.
Ether production from the large volume of residual glycerin from
biodiesel production is considered an added-value product as the
worldwide production of this biofuel increases [5]. The detail in
Fig. 7B shows the difference in conversion capacity of glycerin
and to further product formation in the form of cyclic ethers.
In fact, the reaction may involve dehydration steps and a sim-
ilar process like Mars Van Krevelen mechanism [8] that takes
place in two steps: a reaction between the niobia surface and the
glycerol, in which the latter is oxidized and the former reduced,
and morphological changes to the catalysts. Their treatment with
H O appears to promote the formation of highly reactive groups at
the surface, which are most prevalent on the vanadium-containing
material, and these reaction groups generated the peroxovanadate
species.
Based on experiment planning in conjunction with GC–MS
analysis, neither the presence of the doping metals nor the use
of different solvents influenced the glycerin conversion rate. An
increase in temperature was found to aid in the oxidation of resid-
ual glycerol to cyclic ethers, which can be used in the oil industry
as fuel additives.
2
2
followed by the reaction of the reduced oxide with added H O2 to
2
restore the initial state (Fig. 5b). The presence of vanadium seems
to facilitate the formation of groups peroxos by the decomposi-
tion of H O compared with pure niobia and Mo-doped catalyst.
Based on peak area on the chromatogram the mixtures of cyclic
ether correspond to approximately 35% from glycerol conver-
sion in the presence of ethanol as solvent after 1 h of reaction
Acknowledgment
2
2
Pro-Reitoria de Pesquisa da Universidade Federal de Minas
Gerais, PETROBRAS, FAPEMIG, CNPq, CAPES.
References
(
Fig. 7D).
In addition, multidimensional NMR spectroscopy (do not shown
[
[
1] J.M. Marchetti, Energy Convers. Manage. 52 (2011) 3227–3233.
2] A. Demirbas, Energy Convers. Manage. 49 (2008) 125–130.
here) and CG-MS confirmed that the while the condensed phase is
composed of ethers, esters among other classes of organic com-
pounds, the carbonyl compounds found in the volatile phase are
mostly ketones.
The chromatograms in Fig. 8, which characterize the products
of the reactions catalyzed by pure Nb and NbMo using water as
solvent (the other reactions with higher conversion rates), show
different rates for each material: 56 and 68% for Nb and NbMo,
respectively. Both reactions produced cyclical ethers as the main
product. The structures of such compounds are shown in Fig. 8.
Fig. 9 displays a general scheme showing the products formed by
glycerol conversion.
[3] A. Gog, M. Roman, M. Tosa, C. Paizs, F.D. Irimie, Renew. Energy 39 (2012) 10–16.
[4] M. Stelmachowski, Ecol. Chem. Eng. S 18 (2011) 9–30.
[5] C.J.A. Mota, C.X.A. da Silva, V.L.C. Goncalves, Quim. Nova 32 (2009) 639–648.
[6] X.D. Wang, M.S. Li, M.H. Wang, H. Wang, S.R. Li, S.P. Wang, X.B. Ma, Fuel 88
(2009) 2148–2153.
[
7] L.C.A. Oliveira, T.C. Ramalho, E.F. Souza, M. Goncalves, D.Q.L. Oliveira, M.C.
Pereira, J.D. Fabris, Appl. Catal. B: Environ. 83 (2008) 169–176.
8] L.C.A. Oliveira, M.F. Portilho, H.A. Taroco, A.C. Silva, P.P. Souza, Appl. Catal. B:
Environ. 117 (2012) 7.
[
[9] L.C.A. Oliveira, M. Goncalves, M.C. Guerreiro, T.C. Ramalho, J.D. Fabris, M.C.
Pereira, K. Sapag, Appl. Catal. A: Gen. 316 (2007) 117–124.
10] F. Difuria, G. Modena, R. Curci, S.J. Bachofer, J.O. Edwards, M. Pomerantz, J. Mol.
Catal. 14 (1982) 219–229.
[
[11] A.C. Silva, R.M. Cepera, M.C. Pereira, D.Q. Lima, J.D. Fabris, L.C.A. Oliveira, Appl.
Catal. B: Environ. 107 (2011) 237–244.
[
12] P. Lauriol-Garbey, G. Postole, S. Loridant, A. Auroux, V. Belliere-Baca, P. Rey,
J.M.M. Millet, Appl. Catal. B: Environ. 106 (2011) 94–102.
4
. Conclusion
[
13] L.C.A. Oliveira, T.C. Ramalho, M. Goncalves, F. Cereda, K.T. Carvalho, M.S. Naz-
zarro, K. Sapag, Chem. Phys. Lett. 446 (2007) 133–137.
In the present work, niobium compounds that contain Mo and
V as doping elements in the niobium oxide structure were synthe-
sized. The insertion of these elements induced significant structural
[
[
14] M. Goncalves, L.C.A. Oliveira, M.C. Guerreiro, Quim. Nova 31 (2008) 518–522.
15] T.C. Ramalho, L.C.A. Oliveira, K.T.G. Carvalho, E.F. Souza, E.F.F. da Cunha, M.
Nazzaro, J. Mater. Sci. 43 (2008) 5982–5988.