Oxidation of terpenic alcohols with hydrogen peroxide promoted by Nb2O5 obtained by microwave-assisted hydrothermal method

Article abstract of DOI:10.1016/j.mcat.2020.110941

The present work describes the synthesis of niobium oxides by microwave-assisted hydrothermal method and their evaluation as a solid catalyst in oxidation reactions of terpenic alcohols with hydrogen peroxide. Effects of main parameters of synthesis were assessed and all the prepared catalysts were characterized by physical adsorption/ desorption analyses of nitrogen, infrared and Raman spectroscopies, scanning electron microscopy and powder X-rays diffraction analyses. The strength and number of acidic sites of the catalysts were determined by potentiometric titration. Morphological and structural characterization corroborate with the activity and selectivity achieved by the niobium oxides. The reusability of the catalyst was evaluated. The impacts of main reaction variables such as temperature, catalyst, and oxidant load were assessed. Niobium oxide demonstrated to be an effective catalyst, selectively converting the nerol (model molecule) to epoxide and aldehyde. Oxidation of various terpenic alcohols was investigated. Only geraniol and nerol were selectively epoxidized, suggesting a hydroxyl group assisted reaction. Although being also allylic alcohol, linalool was unreactive toward epoxidation due to the presence of a methyl group at the same carbon atom than the hydroxy group. The use of an environmentally cheap friendly oxidant (H₂O₂) and efficient solid catalyst (Nb₂O₅) are positive aspects of this process.

Full text of DOI:10.1016/j.mcat.2020.110941

Molecular Catalysis 489 (2020) 110941
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Oxidation of terpenic alcohols with hydrogen peroxide promoted by Nb O5
2
obtained by microwave-assisted hydrothermal method
a
b
b
Daniel Carreira Batalha , Natália Hadler Marins , Ricardo Marques e Silva ,
b
c
a,
Neftalí Lenin Villarreal Carreño , Humberto Vieira Fajardo , Márcio José da Silva *
Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais, 36570‑900, Brazil
Graduate Program in Materials Science and Engineering, Technology Development Center, Federal University of Pelotas, Pelotas, Rio Grande Do Sul, 96010-610, Brazil
Chemistry Department, Institute of Exact and Biological Sciences, Federal University of Ouro Preto, Ouro Preto, Minas Gerais, 35400-000, Brazil
a
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
The present work describes the synthesis of niobium oxides by microwave-assisted hydrothermal method and
their evaluation as a solid catalyst in oxidation reactions of terpenic alcohols with hydrogen peroxide. Effects of
main parameters of synthesis were assessed and all the prepared catalysts were characterized by physical ad-
sorption/ desorption analyses of nitrogen, infrared and Raman spectroscopies, scanning electron microscopy and
powder X-rays diffraction analyses. The strength and number of acidic sites of the catalysts were determined by
potentiometric titration. Morphological and structural characterization corroborate with the activity and se-
lectivity achieved by the niobium oxides. The reusability of the catalyst was evaluated. The impacts of main
reaction variables such as temperature, catalyst, and oxidant load were assessed. Niobium oxide demonstrated to
be an effective catalyst, selectively converting the nerol (model molecule) to epoxide and aldehyde. Oxidation of
various terpenic alcohols was investigated. Only geraniol and nerol were selectively epoxidized, suggesting a
hydroxyl group assisted reaction. Although being also allylic alcohol, linalool was unreactive toward epoxidation
due to the presence of a methyl group at the same carbon atom than the hydroxy group. The use of an en-
Niobium oxide
Heterogeneous catalysis
Hydrogen peroxide
Nerol
vironmentally cheap friendly oxidant (H
process.
2 2 2 5
O ) and efficient solid catalyst (Nb O ) are positive aspects of this
Introduction
group) may hamper the control of the reaction selectivity [13].
Green oxidants as hydrogen peroxide are very appealing; similarly
to the molecular oxygen, it generates water as an only by-product.
However, unlikely molecular oxygen, it has the advantage to be liquid,
and not flammable at room conditions. Moreover, it is soluble in water
and polar solvents, safe handling, and it is a cost-effective and atom-
efficient oxidant [14,15]. When used in the presence of solid catalysts,
hydrogen peroxide can interact with the catalyst surface and induce
The conversion of abundant and renewable raw materials to high
value-added chemicals assumed great importance due to economic and
environmental reasons [1–3]. Fine chemicals, pharmaceutical, and
fragrance industries are each time more dependent on sustainable
processes that use friendly reactants, recyclable catalysts, and in-
expensive feedstocks [4,5].
Among the natural origin feedstock, monoterpenes deserve to be
highlighted due to wide application as an ingredient of perfumes, fla-
vors, and food, all motivated by their attractive organoleptic properties
homo- or heterolytic cleavage of OeO bond in H
erate different oxidative species [16,17].
2 2
O , which may gen-
The direct oxidation of olefin or alcohol by hydrogen peroxide is
always kinetically unfavored and impeded by the high energy barrier of
the oxygen transfer reaction from oxidant to the substrate [18,19].
Thus, several catalysts that facilitate the oxidation providing a low-
energy pathway, have been proposed. In special, various metal oxides
or salt-based catalysts have demonstrated to be active in either oxida-
tion or epoxidation reactions [20–22].
[
6–8]. Terpenic alcohol derivatives as epoxides are building block in
synthesis of chiral compounds, while carbonyl products as aldehydes
and ketones are valuable ingredients of perfume, whose obtention per
pass through oxidative processes where clean oxidant and solid cata-
lysts are highly desirable [9–12]. However, the presence of two oxi-
dizable sites in terpenic alcohols (i.e., the double bond and a hydroxyl
⁎
Corresponding author at: Chemistry Department, Federal University of Viçosa, Campus Universitário, Avenue P.H. Rolfs, s/n, Viçosa, Minas Gerais State, CEP:
36570-000, Brazil.
E-mail address: silvamj2003@ufv.br (M.J. da Silva).
https://doi.org/10.1016/j.mcat.2020.110941
Received 11 January 2020; Received in revised form 17 March 2020; Accepted 30 March 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
2 5
Scheme 1. Nb O synthesis by microwave-assisted hydrothermal method.
The use of a heterogeneous catalyst leads to a cleaner process, since
it reduces the waste generation, allows the recovery and the reuse of the
catalyst, and diminish the cost of the processes. Different strategies
have been proposed to obtain efficient heterogeneous catalysts in
epoxidation or oxidation reactions of functionalized olefins with hy-
such as temperature, oxidant, and catalyst loads were also investigated.
Special attention was paid to describe the reaction mechanism. The
reaction scope was also extended to other terpenic alcohols.
Experimental
2
drogen peroxide. For a long time, Ti/SiO catalysts have been preferred
in the epoxidation of terpenic alcohols [23]. Mixed oxides, solid-sup-
ported metals, framework-substituted molecular sieves, and modified
mesoporous silica are also examples of catalysts active on these reac-
tions [24–26].
Nevertheless, in reactions that use heterogenized homogeneous
catalysts, the leaching provoked by the high polarity of the reactants
Catalyst preparation
Nb
2
O
5
was synthesized by microwave-assisted hydrothermal
method as previously described [38,39]. Ammonium niobium oxalate
was dispersed in distilled water under stirring for 30 min. Following,
hydrogen peroxide was dropped in the solution and stirred for 30 min.
The final solution was poured into a Teflon autoclave and placed in a
microwave oven (Electrolux, MEF41, Brazil). The catalysts were syn-
thesized at 433 K for different times (2, 4, and 8 h) under the constant
pressure of 85 psi. After cooling, the precipitate was washed with ab-
solute ethanol (LabSynth, Brazil), and dried in an oven (DeLeo, A5SE,
Brazil) at 323 K for 24 h under air atmosphere. The Scheme 1 below
shows briefly the niobium oxides synthesis used as catalysts.
(
i.e., aqueous hydrogen peroxide) is always an obstacle that may
compromise the reuse of the catalyst. Therefore, the development of a
stable, recyclable, and truly heterogeneous catalyst, which achieves
epoxidation rates and selectivity likewise the homogeneous counter-
parts is still a challenge to be overcome. Recently, to circumvent the
leaching of the active phase in solid catalysts, we have synthesized
insoluble Keggin heteropolyacid salts, which were successfully tested as
catalysts in terpenic alcohol oxidation reactions with hydrogen per-
oxide [27].
Catalyst characterization
The use of solid metal oxide as the catalyst excludes the possibility
of leaching. Niobium oxide has desirable properties to be used as a
heterogeneous catalyst. It has a high surface area and good thermal
Textural properties of niobium oxides were obtained at 77 K in a
NOVA 1200 Quantachrome equipment. The Brunauer-Emmett-Teller
equation was applied to the isotherms of adsorption/ desorption of N ,
2
2 5
stability. Moreover, Nb O has Brønsted and Lewis acidic sites in their
structure, being potentially a bifunctional catalyst [28]. Niobium-con-
taining catalysts can be prepared in several routes such as sol-gel,
grafting, hydrothermal, coprecipitation, or polymeric precursor
methods [29–32]. The treatment of niobium catalysts with hydrogen
peroxide is a strategy that improves their activity in oxidation reactions
which provided the surface area of the solids. Diameter and pore vo-
lume distribution was calculated according to the DFT method. The
morphologies of the catalysts were investigated by scanning electron
microscopy (SEM) using a microscope SSX-550, SHIMADZU, Japan.
Infrared spectroscopy analyses were performed in KBr pellets using
an ABB Bomen MB 3000 FTIR spectrometer (Quebec, Canada) equipped
with ZnSe optics and a deuterated triglycine sulfate (DTGS) detector set
[
33].
Due to these features, niobium oxide has been an efficient catalyst in
different oxidation reactions. Nanoparticles of niobium oxide were
synthesized through microwave-assisted method and used as catalysts
in aniline oxidation reactions with hydrogen peroxide in liquid phase
−1
−1
at a 4 cm
XRD pattern of the Nb
tion (Shimadzu, XRD 6000, Japan), using a diffractometer with CuK
resolution and wavenumber range of 400−4000 cm
.
2 5
O
powders was analyzed by X-ray diffrac-
α
[
34]. The same reaction was also efficiently performed when Al
2 3
O /
radiation (λ = 1.5418 Å), and a scan range of 20−70°. Crystallite sizes
were calculated using the Scherrer equation:
AlNbO were applied as the catalyst [35]. Cerium-chromium-niobium
4
oxides were successfully evaluated in processes to deep oxidation of
dichloroethane, demonstrating have potential industrial application
K. λ
L =
β. cosθ
(1)
[
36]. Conversely, the high acidity of niobium oxides makes them effi-
cient and reusable acid catalysts in xylose dehydration processes to
furfural production [37].
Where λ is the X-ray wavelength in nanometer (nm), β is the peak
width of the diffraction at half maximum height resulting from small
In this work, niobium oxides were prepared by microwave-assisted
hydrothermal method and evaluated in oxidation reactions of terpenic
alcohols with hydrogen peroxide. The impacts of the temperature of
synthesis on the physical and structural properties of niobium oxides
were assessed. The solid catalysts were characterized by FT-IR and
Raman spectroscopy analyses, physical adsorption/ desorption of ni-
trogen, thermogravimetry and powder XRD analyses. Acidity properties
were determined by n-butylamine potentiometric titration. The nerol
oxidation was studied in detail. The effects of main reaction parameters
crystallite size in radians, and
shape, normally taken as 0.89.
K
is a constant related to crystallite
Raman spectra were recorded in a Horiba/Jobin-Yvon Labram-HR
spectrometer equipped with He-Ne laser (laser excitation lines of 632.8
nm and 18 mW of power) and an Olympus microscope. The analyses
were carried out with an acquisition time of 5 s with 10 accumulations
−1
in the region of 60 to 1200 cm
and a 100x microscope objective.
The amount and strength of the acidic sites of niobium oxides were
determined by n-butylamine potentiometric titration using a BEL
2

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
−1
potentiometer (model W3B) with a glass electrode. Typically, 50 mg of
[41,42]. The broad band at 3600−3200 cm
regions refers to the
Nb
2 5 3
O was suspended in CH CN (ca. 30 mL) and magnetically stirred
vibration modes of hydrogen-bonded hydroxyl groups. The presence of
−1
for 3 h. Posteriorly, the suspension was titrated with slow addition of
portions of n-butylamine (ca. 0.025 mol L ) until that the electrode
potential remained stable after the addition of the titrant again.
bands at 1700 cm
reinforces the Nb-linked OH- groups.
−
1
It is noteworthy that the intensities of the bands in this last region
varied according to the synthesis time, suggesting that shorter synthesis
times favored a higher presence of the hydroxyl groups. The ability of
2 2
these groups to bind to H O molecules generating different reactive
oxygen species may improve the activity of these niobium oxides in
oxidation reactions [41,43].
Fig. 3 represents the X-rays diffractograms of the three synthesized
catalysts. The literature describes the existence of four different phases
Catalytic tests
Oxidation reactions were carried out in liquid phase using a glass
reactor fitted (ca. 50 mL) with a reflux condenser and sampling septum,
in a glycerin batch under magnetic stir and heating. Typically, H
2 2(aq)
O
2
for Nb O
5
: Nb
2
O
5
2
.nH O (amorphous), TT-Nb
2 5 2 5
O (hexagonal), T-Nb O
(
ca. 35 wt. %) and nerol (1.375 mmol) were dissolved in CH CN (ca. 10
3
(
orthorhombic) and H-Nb O
2 5
(monoclinic) [44]. The diffractograms
mL) and heated to 333 K. The addition of niobium started the reaction,
which proceeded for 8 h.
revealed by XRD analyses indicate the peaks are related to the hex-
agonal phase (JCPDS No. 28-0317) [39]. The broad peak noticed at 26°
is assigned to the amorphous phase due to the use of hydrated starting
materials and water. The presence of weak peaks is due to the low
temperature used in the synthesis, which influenced their crystallinity
Aliquots were collected at regular time intervals. Before the ana-
lysis, the aliquots were centrifuged, and the supernatant was analyzed
on a GC-2010 Plus Shimadzu Gas Chromatograph with a flame ioni-
zation detector (FID), equipped with an auto-injector AOC-20i, and
fitted with a capillary column Rtx®-Wax (30 m x 0.25 mmID x0.25 μm).
The chromatographic conditions were initial temperature (353 K/ 3
min), heating rate (10 K / min until 493 K), which was kept constant 3
min. Injector and detector temperatures were 523 K and 523 K, re-
spectively. Nitrogen was the carrier gas (1.2 ml / min).
[
45,46]. The Nb
sizes of 12.7 nm, 14.8 nm, and 15.1 nm, respectively.
Fig. 4 presents the Raman spectra of the three Nb
found that they had distinct profiles, indicating the presence of more
2 5
O samples of 2 h, 4 h, and 8 h exhibited crystallite
2 5
O samples. It was
−1
than one phase in each material. Bands in the region of 200−300 cm
were assigned to the stretching of Nb–O–Nb bonds [46].
The absorption bands in the 570−770 cm
symmetrical stretching modes of the Nb–O bonds (NbO
NbO ) [46,47]. For the two samples synthesized in shorter time, 2 and 4
−
1
region refer to the
Results and discussion
6
, NbO , and
7
8
Catalysts characterization
h, the single band in this region may indicate the greater presence of the
amorphous phase. For the catalyst synthesized for 8 h, this band has a
Table 1 shows the surface area, volume, and pore diameter values
quantified by the BET and DFT methods of Nb O catalysts.
2 5
−1
small shoulder closer to 700 cm , which may indicate the greater
formation of the orthorhombic phase due to longer synthesis time [48].
This indicates that the time of synthesis affected the proportion be-
tween these two phases.
An increase in the time of synthesis resulted in an improvement of
surface area and a higher volume of pores. Conversely, pores diameters
were lower when longer times were used. This effect was also verified
by us in a previous work [39]. We can consider that each rod that
compose this material is composed of several rods with smaller dia-
meters. The synthesis time influences the rods growth; if a greater time
of synthesis is used, larger will be the size of the stem. As a result, may
occurs an increase in surface area and pore volume [39].
The broad band more pronounced in the spectra of catalysts syn-
−1
thesized at 2 and 4 h, present in the 900-1000 cm
region, were as-
signed to the symmetrical and asymmetrical stretching of terminal
bonds Nb = O, which is associated with Lewis acid sites [47]. This
stretching band is related to the crystallinity degree of the sample; the
more intense is the band, the lower is the crystallinity and more acid is
the material [47–49]. This same phenomenon is little noticed for the
sample synthesized for 8 h, confirming the higher crystallinity and may
indicate a lower number of Lewis acid sites for this material.
2 5
The catalyst Nb O - 2 h presented the lowest values of surface area
and pore volume but showed a larger value in its diameter, above 30 Ǻ,
characteristic of a mesoporous material. In contrast, the two other
oxides showed an increasing on the surface area and pore volume when
the synthesis time was increased. Since pore diameter was reduced to
values lower than 20 Ǻ, we can conclude that micropores were formed
in the materials [38,39].
2 5
The lower crystallinity degree of the Nb O - 2 h catalyst can be
verified by the XRD graph. Comparing the results shown by XRD and IR,
it should be noted the shorter is the catalyst synthesis times, the lower is
−
the crystallinity and more intense is the OH grouping bands in IR
2
Fig. 1 shows the physical adsorption/desorption isotherms of N and
spectra. According to Ziolek et al, the activity for niobium oxides varies
according to its crystallinity, being a parameter proportional to the
number of hydroxyl groups present on the catalyst surface [17,43].
Fig. 5 shows the potentiometric titration curves of three synthesized
niobium oxides. The initial electrode potential (Ei) indicates the max-
imum acidity strength and the total number of acidic sites being pro-
vided by the inflection value of titration curve (ca. mEq n-butylamine /
g catalyst), which was obtained from the minimum of the first deriva-
tive of each titration curve. The strength of the sites can be classified
according to the scale: Ei > 100 mV (very strong), 0 < Ei < 100 mV
pore diameter distributions. According to IUPAC recommendations, the
isotherms of the synthesized salts can be classified as type V, char-
acteristic of microporous and mesoporous materials.
The hysteresis observed in the isotherm curves can be classified as
H3 type, which is typical of plate-shaped particle aggregates giving rise
to slit pores [40]. According to the pore diameter distribution curve
profiles, the niobium oxides synthesized presented diameters in the
range of micro and mesoporous.
−1
FT-IR spectra presented bands at approximately 1100 cm
were assigned to the asymmetric stretching of Nb–O–Nb bonds (Fig. 2)
that
(
strong), -100 < Ei < 0 mV (weak) e Ei < -100 mV (very weak) [50].
All the niobium catalysts presented very strong acidic sites as de-
Table 1
monstrated by the high Ei values showed in Fig. 5, which was higher
than 100 mV. Moreover, as higher the synthesis time, lower was the
acidity strength. The number of total acidic sites (ca. mEq of n-buty-
a
Characterization of the catalysts through the physical adsorption of nitrogen .
2
3
Catalyst
S
BET (m /g)
V
DFT (cm /g)
D (Ǻ)
lamine/ g of catalyst) followed the order 0.95, 0.45, and 0.40 for Nb
2 h, Nb - 4 h and Nb - 8 h oxides, respectively. Therefore, the
solid Nb - 2 h showed both the strongest acid sites and the greatest
2 5
O
Nb
Nb
Nb
2
2
2
O
O
O
5
5
5
- 2 h
- 4 h
- 8 h
17.4
107.4
215.9
0.035
0.095
0.340
31.7
16.9
16.9
-
2
O
O
2 5
5
2 5
O
number of total acidic sites.
a
S
BET = surface area; VDFT = cumulative pore volume; D = pore diameter.
It is important to highlight that the results of potentiometric
3

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
2
Fig. 1. Isotherms of physical adsorption/desorption of N and pore diameter distribution for niobium oxides.
2 5
Fig. 2. FT-IR spectra obtained from Nb O catalysts.
titration reinforce those provided by IR spectra, which evidenced that
the Nb - 2 h material has the highest number of Brønsted acidic
sites. According to the IR spectra of niobium oxide catalysts, the
2 5
O
−1
stretching bands at 1700 cm
wavenumber indicates the presence of
the Brønsted acid sites. These vibration bands are correlated with the
bonds between Nb and OH- groups [39]. The intensity of this band is
linked to the number of Brønsted acid sites present in the catalyst
surface. As can be seen, the Nb
2
O
5
- 2 h catalyst presented the highest
2 5
Fig. 3. Powder XRD patterns of Nb O synthesized at different times.
number of acid sites (ca. 095) and the more intense vibration band at
-
1
1
700 cm wavenumber (see Fig. 2).
4

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
support this conclusion (Table 1).
Catalytic tests
An initial screening of niobium catalysts
The catalytic performance of synthesized niobium oxides was as-
sessed in the nerol oxidation reaction with H . Fig. 7 displays the
2 2
O
kinetic curves and selectivity achieved after 8 h of reaction, following
conditions selected from the literature [32].
Although an excess of hydrogen peroxide has been used (ca. 2:1 in
relation to nerol), only a poor conversion (ca. 5 %) was obtained in the
absence of a catalyst (omitted by simplification). Conversely, in the
niobium oxide-catalyzed reactions the nerol was selectively converted
to oxidation products (Scheme 2).
2 5
The Nb O - 2 h catalyst was much more efficient than other syn-
thesized oxides. The highest conversion (ca. 82 %) and selectivity to-
ward nerol epoxide (ca. 94 %) was achieved after 8 h of reaction.
Notably, these results were superior that achieved by niobium oxides
prepared by the peroxidation method [52]. Even though nerol epoxide
has been the major product in all the reactions, the selectivity toward
carbonyl products (i.e., 1b and 2a) was gradually improved when nio-
bium catalysts were synthesized at longer times. Conversely, the
terminal double bond of the nerol remained untouchable along with the
reaction, probably due to the steric hindrance triggered by two methyl
groups.
Fig. 4. Raman spectra of the three different synthesized niobium.
A comparison of the characterization data of niobium oxides and
their catalytic activity leads to a conclusion that the surface properties
and acidity of niobium oxides played an essential role in this reaction.
2 5
The most acidic catalyst (Nb O - 2 h; Fig. 5) with the lowest surface
area (Table 1) was the most efficient, achieving the highest conversion.
On the other hand, catalysts synthesized at longer periods showed a
different selectivity; while the selectivity of aldehydes increased, the
selectivity of epoxide products was reduced, which may be assigned to a
modification on the catalyst surface.
Balker et al reported that the alcohols are preferentially oxidized by
hydrogen peroxide to carbonyl products when Ti silicate (i.e., TS-1) is
the catalyst [26]. However, Tatsumi et al. described that this reaction is
suppressed if allylic hydroxyl groups are present and, consequently,
epoxide becomes the major product [53]. Investigating the geraniol
Fig. 5. Potentiometric titration curves with n-butylamine of the niobium
oxides.
2 2
epoxidation with H O over TS-1 catalysts, those authors demonstrated
that the epoxidation of allylic alcohols is a hydroxy-assisted reaction.
Therefore, based on the literature and experimental data obtained we
propose that on niobium-catalyzed oxidation of nerol involves an in-
termediate as depicted in Scheme 3.
The literature describes the oxidation of geraniol by hydrogen
peroxide in tungsten, titanium, or vanadium-catalyzed reactions; on
these processes, steps such as the coordination of metal oxide catalyst to
the olefin double bond, as well as the peroxidation of the metal are
always involved [54]. In general, after the peroxidation step, a three-
centered ring intermediate involving the metal oxide and an oxygen
atom of peroxide is formed; however, it was omitted in Scheme 3
aiming to do cleaner the reaction. Only the peroxide group (i.e., OOH)
bonded to the metal center was depicted herein.
Fig. 6 shows SEM micrographs of synthesized niobium oxides. The
Nb O particles were moderately aggregated and exhibited an ap-
2 5
proximately spherical shape, regardless of the synthesis times. Although
this aspect it is not totally clear on the SEM images, previous works
showed that this material presented a spherical shape [39].
The morphologies varied with an increase of hydrothermal treat-
ment. At longer synthesis times, materials roughness increases.
Although the SEM images are unable to provide information about the
pores size, we have found that the formation of particles with larger
surface area and high pore volume was obtained when a longer
synthesis time was used [39,51]. The BET analyses and porosimetry
2 5 2 5 2 5
Fig. 6. SEM micrographs at x15000 magnification of Nb O - 2 h (A), Nb O - 4 h (B), and Nb O - 8 h (C) catalysts.
5

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
a.
Fig. 7. Effect of Nb
2
O
5
catalyst treatment time on the kinetic curves (a), conversion and products selectivity (b) of nerol oxidation with H
2 2
O
a
Reaction conditions: nerol (1.375 mmol), H (2.750 mmol), temperature (333 K), catalyst (4 mol %), CH CN (10 mL)
2
O
2
3
2 5
Scheme 2. Oxidation products of nerol obtained in Nb O -catalyzed reactions.
The change in chemoselectivity can be attributed to the coordina-
tion of the nerol on the surface of Nb . It is very important to
highlight that analogously to the described by Adams et al and Kumar
et al., we are supposing that the transition state of the epoxidation of
nerol involves the coordination of the allylic hydroxyl group to the Nb
active site; it means that the double bond interacts with one peroxidic
oxygen atom, not the niobium oxide-oxygen atom [23,55].
2
O
5
The peroxidation step of the niobium catalyst and the interaction
with the organic substrate is attributed to the different species of active
sites present in the niobium catalyst. Lewis acidic sites and Brønsted
acidic sites allow the simultaneous adsorption of the oxidant and the
organic substrate even in the presence of water [56].
Scheme 3. Nb
2
O
5 2 2
-catalyzed hydroxy-assisted epoxidation of nerol with H O adapted from [26].
6

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
Fig. 8. Effect of catalyst load on the kinetic curves (a), conversion and products selectivity (b) of the nerol oxidation with H
Reaction conditions: nerol (1.375 mmol), H O (2.750 mmol), temperature (333 K), CH CN (10 mL)
2 2 3
2
O
2
over Nb
2
O
5
- 2 ha.
a
We can conclude that once the Nb
Brønsted acidity (see IR spectra and acidity measurements, Figs. 2 and
), the interaction of nerol hydroxyl group with the peroxidized catalyst
2
O
5
- 2 h presented a higher
conversion was obtained using a higher oxidant load. However, this
effect was only noticed when the reactions were performed with 4 mol
% of catalyst.
5
is more favorable, therefore, epoxide was the major product (Fig. 7).
Conversely, when the catalysts have lower Brønsted acid sites number
The presence of a higher load of oxidant also resulted in an increase
in the amount of water in the solution. This aspect changed the che-
moselectivity of the reaction, although nerol epoxide has continued to
be the main product. The water benefited the isomerization of nerol to
(
i.e., because they were synthesized in 4 or 8 h), the epoxide selectivity
is lowered, favoring the formation of the neraldehyde (Nb
h, Fig. 7).
2 5
O - 4 and 8
2 2
geraniol, which was also formed when the proportion of H O : nerol
XRD also demonstrated that the Nb
lowest crystallinity among the three materials. These characteristics
positively contributed to improving the interaction of the H and
nerol with the catalyst surface [17,57]. The Nb - 2 h catalyst was
2
O
5
- 2 h catalyst presented the
was 1:2 and 1:3. There was an increase in the selectivity for ger-
anialdehyde. However, the nerol epoxide selectivity was close to 90 %.
No significant amount of geraniol epoxide was detected along with the
reactions.
2 2
O
2 5
O
selected to study the effect of main reaction parameters due to its better
performance.
Effects of reaction temperature
The impacts of temperature on the kinetic curves, conversion, and
selectivity of nerol oxidation were assessed using the optimized con-
ditions (ca. 2:1 oxidant: substrate ratio, 4 mol % of catalyst) (Fig. 10).
When the reactions were carried out at different temperatures, an
induction period at the process beginning was noticed, mainly at the
lowest temperatures. We suppose that at low temperatures, the catalyst
peroxidation step is less favored. Consequently, a minimum amount of
substrate is consumed within the initial period of reaction. This effect
was being suppressed when higher temperatures were used. On the
other hand, the reaction selectivity remained almost the same; at the
end of the reactions, nerol epoxide was obtained always with a se-
lectivity close to 94 %.
Effects of catalyst load
The impacts of catalyst load were evaluated keeping constant the
other variables in the reaction. Fig. 8 shows the kinetic curves, con-
version, and selectivity achieved in reactions with a load of catalyst
ranging from 2 to 8 mol %.
Although no shown in Fig. 8, a control experiment was carried out
without the presence of the catalyst. When nerol and hydrogen per-
oxide were heated and stirred during 8 h of reaction, no oxidation
product or epoxide was detected. It is a guarantee that the formation of
2 5
these compounds depends on the presence of the Nb O -2 h catalyst.
Conversely, while the reactions were performed in the presence of
the niobium oxide catalyst, nerol epoxide and the carbonylic products
were obtained, with the highest selectivity toward epoxide.
With 4 mol % of catalyst load, the reaction achieved 82 % of con-
version. When we performed reactions with higher loads of catalyst, no
significant difference was observed in the conversions, which varied
from 82 to 87 %. This enhancement of conversion is a consequence of a
higher acidic sites number, feasible when a higher concentration is
used. Therefore, lower catalysts loading were used (ca. 2.0 – 0.5 mol
Effects of terpenic alcohol structure
2 5
The catalytic activity of Nb O - 2 h was also tested on the oxidation
of different terpene alcohols. Fig. 11 displays all the terpenic alcohols
investigated; allylic acyclic primary alcohols (nerol and geraniol),
allylic acyclic tertiary alcohol (linalool) or cyclic (α-terpineol) were
selected. It is important to note that although the hydroxyl group of
tertiary alcohols is not oxidizable (i.e., linalool and α-terpineol), these
substrates have double bonds which may be epoxidized.
Among the evaluated alcohols, only nerol and their geometric
isomer geraniol were selectively oxidized, providing their epoxides as
main products. Therefore, only the selectivity of these three alcohols is
displayed in Fig. 12. The double bond of α-terpineol remained un-
touchable and no epoxide was detected. This result is reinforcing that
the epoxidation is a hydroxy assisted-reaction.
%). At this range of catalyst load, the reaction conversions were gra-
dually diminished. Notwithstanding, the reaction selectivity remained
basically unaltered; nerol epoxide was always the main product (ca. 94
%), and aldehydes of nerol and geraniol were the secondary products. It
is evidence that selectivity depends on the nature of the catalyst.
Effects of oxidant: substrate ratio
The results for Nb
amounts of oxidant are presented in Fig. 9. An improvement in the
2
O
5
- 2 h catalyst in reaction with different
Although linalool has an allylic hydroxyl group likewise geraniol
and nerol, it was not epoxidized. It can be attributed to the hysteric
7

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
Fig. 9. Effect of substrate: oxidant ratio on the kinetic curves (a), conversion and products selectivity (b) of the nerol oxidation by H
2
O
2
in the presence of Nb
2
O
5
- 2 h
me-
a.
catalyst
a
3
Reaction conditions: Nerol (1.375 mmol), temperature (333 K), catalyst (2 mol %), CH CN (10 mL)
hindrance provoked by the methyl group, which is bonded to the same
carbon atom that the hydroxyl group. On the other hand, these three
allylic alcohols have another double bond disubstituted that due to the
absence of an allylic hydroxy group and a high steric hindrance re-
mained intact.
3 2
those achieved in oxidation reactions carried out over WO -SiO
soporous catalysts, the conversions were higher even at a lower tem-
perature [25].
The reaction selectivity showed that the oxidation of the hydroxy
group of geraniol to aldehyde was easier than nerol. This higher se-
lectivity for aldehyde was a consequence of a lower formation of ep-
oxide; possibly, the formation of the transition state that led to the
epoxide more favourably for nerol than geraniol. It is well established
that geraniol has a trans-ol and nerol a cis-ol configuration around the
trisubstituted carbon-carbon double bond. Consequently, the interac-
tion of niobium with the hydroxyl group of nerol is more effective.
Although the selectivity obtained herein have been comparable to
2 5
Recovery and reuse of Nb O - 2 h catalyst
The catalyst was easily recovered from the reaction medium by
centrifugation. High recovery rates were achieved (ca. 95 %). Although
the conversion did not reach 40 %, the reaction maintained the high
selectivity for nerol epoxide (Fig. 13).
To try understanding the reason for the drop in the activity of
2 5
Nb O -2 h catalyst after it has been reused, we carried out analyses of
FT-IR spectroscopy and XRD patterns and compared them with the data
obtained from the fresh catalyst.
The IR spectra (Fig. 14) showed a decrease in the intensity of the
2 2 2 5
Fig. 10. Effects of temperature on the kinetic curves (a), conversion and products selectivity (b) of the nerol oxidation with H O over Nb O
- 2 h catalyst.a.
a
Reaction conditions: nerol (1.375 mmol), H
2
O
2
3
(2.750 mmol), catalyst (4 mol %), CH CN (10 mL)
8

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
Fig. 11. Structure of terpenic alcohols.
a.
2
Fig. 12. Kinetic curves (a), conversion and products selectivity (b) of Nb
2
O
5
- 2 h-catalyzed terpenic alcohols oxidation by H
2
O
a
Reaction conditions: terpenic alcohol (1.375 mmol), H
2
O
2
(2.750 mmol), temperature (333 K), catalyst (4 mol %), CH
3
CN (10 mL)
Fig. 13. Conversion (a) and selectivity (b) of nerol oxidation reactions with H
2
O
2
in the presence of Nb
2
O
5
- 2 h (fresh and reused). The black bar on b graphic
a.
represents the conversion at 8 h of reaction
a
Reaction conditions: Nerol (1.375 mmol), H
2
O
2
(2.750 mmol), temperature (333 K), catalyst (4 mol %), CH CN (10 mL)
3
9

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
2 5
Fig. 14. FT-IR spectra obtained from Nb O - 2 h fresh and reused catalysts.
hydroxyl groups present on the catalyst surface; with less hydroxyl
groups, the catalyst has lower activity [17,41].
Therefore, we can conclude that the main change that occurred on
2 5
the Nb O -2 h catalyst after it has been recovered and reused was the
lower availability of hydroxyl groups on their surface. The loss of sur-
face hydroxyl groups suggests that the catalyst also had their strength
acidity reduced. Moreover, the interaction with the hydrogen peroxide
oxidant may be also compromised, triggering a significant decrease in
the activity of the reused catalyst in subsequent nerol oxidation reac-
tions. Literature has described that the hydroxyl groups may have the
2 2
ability to bind to H O molecules generating different reactive oxygen
species, which may favor the activity of these niobium oxides in oxi-
dation reactions [40,42].
Finally, an additional test was carried to verify the possible leaching
of catalyst could be also has contributed to their deactivation. The
niobium oxide was stirred and heated in the presence of the oxidant in
solution without the presence of the nerol. After the removal of solid
catalyst through hot filtration, nerol and oxidant were added to the
supernatant and the reaction monitored during 8 h. No conversion was
noticed as well as no oxidation product was detected. It is a guarantee
2 5
that the Nb O -catalyzed reaction was truly heterogeneous. Besides the
high recovery rate of catalyst achieved in the reuse, this evidence as-
sures that the catalyst is almost insoluble in the reaction.
Conclusions
The microwave-assisted hydrothermal method was efficient to
produce niobium oxides with controlled morphological properties,
which demonstrated to be effective catalysts for the selective epoxida-
tion of allylic terpenic alcohols with hydrogen peroxide. Reactions were
conveniently carried out in the presence of a solid catalyst (i.e., Nb
h), and hydrogen peroxide, an inexpensive and green oxidant.
Notwithstanding its lowest surface area, the most active catalyst
was Nb - 2 h, a consequence of the highest amount of strongest
2 5
O -
2
2 5
O
acidic sites, which favored the interaction of the catalyst surface with
terpenic alcohol. XRD patterns, IR and Raman spectra obtained from
niobium oxides assisted in the interpretation of catalytic test results
through the acidity presented by the materials.
Fig. 15. XRD patterns obtained from the Nb
2
O
5
- 2 h fresh and reused catalysts.
Likewise described for tungsten or titanium catalyzed reactions, the
epoxidation of allylic terpenic alcohols with H
2 2
O is a hydroxy-assisted
−
band regarding the binding of Nb to the OH groups, at wavenumbers
reaction. Therefore, the Nb - 2 h catalyst that had a higher number
2 5
O
-
1
1
700 cm , after the catalyst has been reused. On the basis of the lit-
erature, it is correct to state that it is suggestive that there are less OH
groups on the catalyst surface [41]
On the other hand, the analysis of the XRD patterns revealed an
increase in crystallinity for the reused catalyst (Fig. 15). According to
Ziolek et al, an increase of this parameter is directly related to the loss
of Brønsted acidic site was the most active. The reactivity of other
terpenic alcohols was also evaluated. Linalool, another allylic alcohol
was unreactive, the consequence of high steric hindrance of the tertiary
hydroxy group. The double bond of α-terpineol was not epoxidized,
possibly due to the absence of an allylic hydroxy group. The stereo-
chemistry of nerol makes it more reactive than geraniol, consequently,
higher conversion and selectivity toward epoxide was achieved. The
reusability of catalysts was investigated. High recovery rates (ca. 95 %)
−
−
of availability of OH groups on the catalyst surface [17]. The catalytic
activity of the niobium oxides varies according to the number of
10

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
were obtained. After recovery, the solid catalyst was reused without
loss of selectivity, however, there was a significant decrease of con-
version. Infrared spectroscopy analysis and XRD patterns of the reused
catalyst showed that there was a loss of hydroxyl groups on the catalyst
surface, which compromised their activity.
peroxide and t-Butyl hydroperoxide: reaction pathway analysis, Mol. Catal. 481
(
2018) 110234, , https://doi.org/10.1016/j.mcat.2018.11.005.
[
[
17] M. Ziolek, I. Sobczak, P. Decyk, K. Sobanska, P. Pietrzyk, Z. Sojka, Search for re-
active intermediates in catalytic oxidation with hydrogen peroxide over amorphous
niobium(V) and tantalum(V) oxides, Appl. Catal. B 164 (2015) 288–296, https://
doi.org/10.1016/j.apcatb.2014.09.024.
18] R. Noyori, M. Aoki, K. Sato, Green oxidation with aqueous hydrogen peroxide,
Chem. Commun. 16 (2003) 1977–1986, https://doi.org/10.1039/B303160H.
[19] W. Zhao, Y. Zhang, B. Ma, Y. Ding, W. Qiu, Oxidation of alcohols with hydrogen
peroxide in water catalyzed by recyclable Keggin-type tungstoborate catalyst, Catal.
Commun. 11 (2010) 527–531, https://doi.org/10.1016/j.catcom.2009.12.010.
20] C.B. Vilanculo, M.J. da Silva, S.O. Ferreira, M.G. Teixeira, A rare oxidation of
camphene to acid and aldehyde in the presence of Lacunar Keggin heteropoly salts,
Mol. Catal. 478 (2019) 110589, , https://doi.org/10.1016/j.mcat.2019.110589.
21] D.E. De Vos, B.F. Sels, P.A. Jacobs, Practical heterogeneous catalysts for epoxide
production, Adv. Synth. Catal. 345 (2003) 457–473, https://doi.org/10.1002/adsc.
Credit author statement
I, Marcio Jose da Silva, corresponding author of this paper, state
that:
The corresponding author is responsible for ensuring that the de-
scriptions are accurate and agreed by all authors.
Authors equally contributed in the paper.
[
[
[
[
2
00390051.
22] C. Wang, H. Yamamoto, Tungsten-catalyzed asymmetric epoxidation of allylic and
homoallylic alcohols with hydrogen peroxide, J. Am. Chem. Soc. 136 (2014)
Declaration of Competing Interest
1
222–1225, https://doi.org/10.1021/ja411379e.
23] R. Kumar, G.C.G. Pais, B. Pandey, P. Kumar, Hydroxy-assisted chemo- and stereo-
selective epoxidation catalysed by a titanium silicate molecular sieve (TS-1)/H
system, J. Chem. Soc. Chem. Commun. 13 (1995) 1315–1316, https://doi.org/10.
039/C39950001315.
2 2
O
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
1
[24] N. Marin-Astorga, J.J. Martinez, G. Borda, J. Cubillos, D.N. Suarez, H. Rojas,
Control of the chemoselectivity in the oxidation of geraniol over lanthanum, tita-
nium and niobium catalysts supported on mesoporous silica MCM-41, Top. Catal.
Acknowledgements
5
5 (2012) 620–624, https://doi.org/10.1007/s11244-012-9840-0.
[
25] F. Somma, G. Strukul, Oxidation of geraniol and other substituted olefins with
hydrogen peroxide using mesoporous, sol–gel-made tungsten oxide–silica mixed
oxide catalysts, J. Catal. 227 (2004) 344–351, https://doi.org/10.1016/j.jcat.2004.
The authors thank the Federal University of Viçosa, the PPGMQ-
MG, and the development agencies CNPq and FAPEMIG. This study was
financed in part by the Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior – Brasil (CAPES) – Finance Code 001.
0
7.006.
[26] M. Dusi, T. Mallat, A. Baiker, Epoxidation of functionalized olefins over solid cat-
alysts, Catal. Rev. 42 (2000) 213–278, https://doi.org/10.1081/CR-100100262.
[
27] M.J. Da Silva, P.H.S. Andrade, S.O. Ferreira, C.B. Vilanculo, C.M. Oliveira,
Monolacunary K SiW11 39-Catalyzed terpenic alcohols oxidation with hydrogen
peroxide, Catal. Lett. 148 (2018) 2516–2527, https://doi.org/10.1007/s10562-
18-2434-0.
28] E.L. Moreno, K. Rajagopal, Challenges of catalysis acidity in solids, Quím. Nova 32
2009) 538–542, https://doi.org/10.1590/S0100-40422009000200044.
29] A. Esteves, L.C.A. Oliveira, T.C. Ramalho, M. Goncalves, A.S. Anastacio,
H.W.P. Carvalho, New materials based on modified synthetic Nb as photo-
8
O
References
0
[
[
[
1] F. Cavani, N. Ballarini, S. Luciani, Catalysis for society: towards improved process
efficiency in catalytic selective oxidations, Top. Catal. 52 (2009) 935–947, https://
doi.org/10.1007/s11244-009-9244-y.
(
2 5
O
[
[
[
[
2] W. Schwab, C. Fuchs, F. Huang, Transformation of terpenes into fine chemicals, Eur.
J. Lipid Sci. Technol. 115 (2013) 3–8, https://doi.org/10.1002/ejlt.201200157.
3] P. Gallezot, Catalytic routes from renewables to fine chemicals, Catal. Today 121
catalyst for oxidation of organic contaminants, Catal. Commun. 10 (2008) 330–332,
https://doi.org/10.1016/j.catcom.2008.09.012.
30] A.C. Silva, D.Q.L. Oliveira, L.C.A. Oliveira, A.S. Anastácio, T.C. Ramalho,
J.H. Lopes, H.W.P. Carvalho, C.E.R. Torres, Nb-containing hematites Fe2−xNb O :
x 3
2 2
the role of Nb on the reactivity in presence of the H O or ultraviolet light, Appl.
Catal. A Gen. 357 (2009) 79–84, https://doi.org/10.1016/j.apcata.2009.01.014.
31] C. Tiozzo, C. Bisio, F. Carniato, M. Guidotti, Grafted non-ordered niobium-silica
materials: versatile catalysts for the selective epoxidation of various unsaturated
fine chemicals, Catal. Today 235 (2014) 49–57, https://doi.org/10.1016/j.cattod.
[
[
[
(2007) 76–91, https://doi.org/10.1016/j.cattod.2006.11.019.
4] M. Besson, P. Gallezot, C. Pinel, Conversion of biomass into chemicals over metal
catalysts, Chem. Rev. 114 (2013) 1827–1870, https://doi.org/10.1021/cr4002269.
5] P.T. Anastas, M.M. Kirchhoff, Origins, current status, and future challenges of green
chemistry, Acc. Chem. Res. 35 (2002) 686–694, https://doi.org/10.1021/
ar010065m.
5
+
[
6] L.A.S. Viana, G.R.N. Da Silva, M.J. Da Silva, A highly selective Na
oxidation of terpenic alcohols by hydrogen peroxide, Catal. Lett. 148 (2018)
74–382, https://doi.org/10.1007/s10562-017-2246-7.
7] K.A.D. Swift, Catalytic transformations of the major terpene feedstocks, Top. Catal.
7 (2004) 143–155, https://doi.org/10.1023/B:TOCA.0000013549.60930.da.
2 4
WO -Catalyzed
2
014.02.027.
32] C. Tiozzo, C. Palumbo, R. Psaro, C. Bisio, F. Carniato, A. Gervasini, P. Carniti,
M. Guidotti, The stability of niobium-silica catalysts in repeated liquid-phase,
epoxidation tests: a comparative evaluation of in-framework and grafted mixed
oxides, Inorg. Chim. Acta Rev. 431 (2015) 190–196, https://doi.org/10.1016/j.ica.
3
[
[
2
8] E.J. Lenardao, G.V. Botteselle, F. De Azambuja, G. Perin, R.G. Jacob, Citronellal as
key compound in organic synthesis, Tetrahedron 63 (2007) 6671–6712, https://
doi.org/10.1016/j.tet.2007.03.159.
2
015.01.048.
[
[
33] J.V. Coelho, L.C.A. Oliveira, F.C.C. Moura, P.P. De Souza, C.A. Silva, K.B. Batista,
M.J. Da Silva, β-pinene oxidation by hydrogen peroxide catalyzed by modified
niobium-MCM, Appl. Catal. A Gen. 419 (2012) 215–220, https://doi.org/10.1016/
j.apcata.2012.01.032.
34] W.M. Ventura, D.C. Batalha, H.V. Fajardo, J.G. Taylor, N.H. Marins,
B.S. Noremberg, T. Tańskic, N.L.V. Carreño, Low temperature liquid phase catalytic
oxidation of aniline promoted by niobium pentoxide micro and nanoparticles,
Catal. Commun. 99 (2017) 135–140, https://doi.org/10.1016/j.catcom.2017.06.
[
9] S. Serra, D. De Simeis, Two complementary synthetic approaches to the en-
antiomeric forms of the chiral building block (2, 6, 6-Trimethyltetrahydro-2H-
pyran-2-yl) methanol: application to the stereospecific preparation of the natural
flavor linaloyl oxide, Catalysts 8 (2018) 362, https://doi.org/10.3390/
catal8090362.
[
[
[
[
10] J.L.F. Monteiro, C.O. Veloso, Catalytic conversion of terpenes into fine chemicals,
Top. Catal. 27 (2004) 169–180, https://doi.org/10.1023/B:TOCA.0000013551.
0
04.
9
9872.8d.
[
35] D.C. Batalha, S.C. Luz, J.G. Taylor, H.V. Fajardo, B.S. Noremberg, I.J.S. Cherubin,
11] M.J. Da Silva, D.A.M. Ayala, Unravelling transition metal-catalyzed terpenic al-
cohol esterification: a straightforward process for the synthesis of fragrances, Catal.
Sci. Technol. 6 (2016) 3197–3207, https://doi.org/10.1039/C5CY01538C.
12] Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang, Y. Yang, Recent advances in hetero-
geneous selective oxidation catalysis for sustainable chemistry, Chem. Soc. Rev. 43
R.M. Silva, M.R.F. Gonçalves, C.P. Bergmann, A. Valentini, N.L.V. Carreño,
Application of Al O /AlNbO in the oxidation of aniline to azoxybenzene, Chem.
2 3 4
Pap. 74 (2020) 543–553, https://doi.org/10.1007/s11696-019-00897-5.
[
36] J. Wan, P. Yang, G. Xiaolin, R. Zhou, Investigation on the structure-activity re-
lationship of Nb
elimination, Mol. Catal. 470 (2019) 75–81, https://doi.org/10.1016/j.mcat.2019.
3.010.
2 5
37] N.K. Gupta, A. Fukuoka, K. Nakajima, Amorphous Nb O as a selective and reusable
2 5 2 x 2 5
O promoting CeO -CrO -Nb O catalysts for 1,2-dichloroethane
(2014) 3480–3524, https://doi.org/10.1039/C3CS60282F.
13] L.A.F. Azevedo, L. Menini, E.V. Gusevskaya, Functionalization of the naturally oc-
curring linalool and nerol by the palladium catalyzed oxidation of their trisub-
stituted olefinic bonds, J. Mol. Catal. A Chem. 426 (2017) 429–434, https://doi.
org/10.1016/j.molcata.2016.07.033.
0
[
[
catalyst for furfural production from xylose in biphasic water and toluene, ACS
Catal. 7 (2017) 2430–2436, https://doi.org/10.1021/acscatal.6b03682.
38] N.H. Marins, C.T. Meereis, R.M. Silva, C.P. Ruas, A.S. Takimi, N.L. Carreño,
F.A. Ogliari, Radiopaque dental adhesive with addition of niobium pentoxide na-
noparticles, Polym. Bull. Berl. (Berl) 75 (2018) 2301–2314, https://doi.org/10.
[14] S. Sakaguchi, Y. Nishiyama, Y. Ishii, Selective oxidation of monoterpenes with
hydrogen peroxide catalyzed by peroxotungstophosphate (PCWP), J. Org. Chem. 61
(1996) 5307–5311, https://doi.org/10.1021/jo960275q.
[
15] J. Piera, J. Baeckvall, Catalytic oxidation of organic substrates by molecular oxygen
and hydrogen peroxide by multistep electron transfer—a biomimetic approach,
Angew. Chem., Int. Ed. 47 (2008) 3506–3523, https://doi.org/10.1002/anie.
1
007/s00289-017-2150-8.
[
39] W.M. Ventura, D.C. Batalha, H.V. Fajardo, J.G. Taylor, N.H. Marins,
B.S. Noremberg, T. Tańskic, N.L.V. Carreño, Low temperature liquid phase catalytic
oxidation of aniline promoted by niobium pentoxide micro and nanoparticles,
Catal. Commun. 99 (2017) 135–140, https://doi.org/10.1016/j.catcom.2017.06.
2
00700604.
[16] V.M. Vaschetti, A.L. Cánepa, D. Barrera, K. Sapag, G.A. Eimer, S.G. Casuscelli,
Limonene oxyfunctionalization over Cu-modified silicates employing hydrogen
0
04.
11

D. Carreira Batalha, et al.
Molecular Catalysis 489 (2020) 110941
on the properties of mesoporous niobium oxide, Mater. Chem. Phys. 110 (2008)
387–392, https://doi.org/10.1016/j.matchemphys.2008.02.027.
[
40] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F.R. Reinoso, J. Rouquerol,
K.S. Sing, Physisorption of gases, with special reference to the evaluation of surface
area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87
[50] L.R. Pizzio, M.N. Blanco, A contribution to the physicochemical characterization of
nonstoichiometric salts of tungstosilicic acid, Microporous Mesoporous Mater. 103
(2007) 40–47, https://doi.org/10.1016/j.micromeso.2007.01.036.
[51] N.H. Marins, B.E.J. Lee, R.M. Silva, A. Raghavan, N.L.V. Carreno, K. Grandfield,
Niobium pentoxide and hydroxyapatite particle loaded electrospun poly-
caprolactone/gelatin membranes for bone tissue engineering, Colloids Surf. B
Biointerfaces 182 (2019) 110386, https://doi.org/10.1016/j.colsurfb.2019.
110386.
(
2015) 1051–1069, https://doi.org/10.1515/pac-2014-1117.
41] T. Athar, A. Hashmi, A. Al-Hajry, Z.A. Ansari, S.G. Ansari, One-pot synthesis and
characterization of Nb nanopowder, J. Nanosci. Nanotechnol. 12 (2012)
922–7926, https://doi.org/10.1166/jnn.2012.6645.
42] D.C. Castro, R.P. Cavalcante, J. Jorge, M.A. Martines, L. Oliveira, G.A. Casagrande,
A. Machulek Jr, Synthesis and characterization of mesoporous Nb and its ap-
[
2
O
5
7
[
2 5
O
plication for photocatalytic degradation of the herbicide methylviologen, J. Braz.
Chem. Soc. 27 (2016) 303–313, https://doi.org/10.5935/0103-5053.20150244.
[52] J. Cubillos, J. Martínez, H. Rojas, N. Marín-Astorga, Oxidation of geraniol using
niobia modified with hydrogen peroxide, Rev. Fac. Ing., Univ. Antioquia 91 (2019)
106–112, https://doi.org/10.17533/udea.redin.n91a10.
[53] T. Tatsumi, M. Yako, M. Nakamura, Y. Yuhara, H. Tominaga, Effect of alkene
structure on selectivity in the oxidation of unsaturated alcohols with titanium si-
licalite-1 catalyst, J. Mol. Catal. A Chem. 78 (1993) L41–L45.
[54] M. Aminia, M.M. Haghdoostb, M. Bagherzadeh, Monomeric and dimeric oxido–-
peroxido tungsten(VI) complexes in catalytic and stoichiometric epoxidation,
Coord. Chem. Rev. 268 (2014) 83–100, https://doi.org/10.1016/j.ccr.2014.01.035.
[55] W. Adam, A. Corma, T.I. Reddy, M. Renz, Diastereoselective catalytic epoxidation
of chiral allylic alcohols by the TS-1 and Ti-β zeolites: evidence for a hydrogen-
bonded, peroxy-type loaded complex as oxidizing species, J. Org. Chem. 62 (1997)
3631–3637, https://doi.org/10.1021/jo970364i.
[56] I.W.C.E. Arends, R.A. Sheldon, Activities and stabilities of heterogeneous catalysts
in selective liquid phase oxidations: recent developments, Appl. Catal. A Gen. 212
(2001) 175–187, https://doi.org/10.1016/S0926-860X(00)00855-3.
[57] L.C.A. Oliveira, N.T. Costa, J.R. Pliego Jr, A.C. Silva, P.P. Souza, P.S.O. Patrício,
Amphiphilic niobium oxyhydroxide as a hybrid catalyst for sulfur removal from fuel
in a biphasic system, Appl. Catal. B 147 (2014) 43–48, https://doi.org/10.1016/j.
apcatb.2013.08.003.
[
[
[
43] M. Ziolek, I. Sobczak, P. Decyk, L. Wolski, The ability of Nb
generate active oxygen in contact with hydrogen peroxide, Catal. Commun. 37
2013) 85–91, https://doi.org/10.1016/j.catcom.2013.03.032.
2 5 2 5
O and Ta O to
(
44] R. Brayner, F. Bozon-Verduraz, Niobium pentoxide prepared by soft chemical
routes: morphology, structure, defects and quantum size effect, Phys. Chem. Chem.
Phys. 5 (2003) 1457–1466, https://doi.org/10.1039/B210055J.
45] R. Turco, A. Aronnec, P. Carniti, A. Gervasini, L. Minieric, P. Pernicec, R. Tessera,
R. Vitiello, M. Di Serio, Influence of preparation methods and structure of niobium
oxide-based catalysts in the epoxidation reaction, Catal. Today 254 (2015) 99–103,
https://doi.org/10.1016/j.cattod.2014.11.033.
46] I. Nowak, M. Ziolek, Niobium compounds: preparation, characterization, and ap-
plication in heterogeneous catalysis, Chem. Rev. 99 (1999) 3603–3624, https://doi.
org/10.1021/cr9800208.
[
[
2 5
47] O.F. Lopes, E.C. Paris, C. Ribeiro, Synthesis of Nb O nanoparticles through the
oxidant peroxide method applied to organic pollutant photodegradation: a me-
chanistic study, Appl. Catal. B 144 (2014) 800–808, https://doi.org/10.1016/j.
apcatb.2013.08.031.
[
48] S.M. Maurer, E.I. Ko, Structural and acidic characterization of niobia aerogels, J.
Catal. 135 (1992) 125–134, https://doi.org/10.1016/0021-9517(92)90274-L.
49] S.K.N. Ayudhya, A. Soottitantawat, P. Praserthdam, C. Satayaprasert, Effect of aging
[
12