A novel hydrofobic niobium oxyhydroxide as catalyst: Selective cyclohexene oxidation to epoxide

Article abstract of DOI:10.1016/j.apcata.2013.01.007

A surface modified niobium oxyhydroxide catalyst was utilized for the selective liquid-phase oxidation of cyclohexene to epoxide in the presence of hydrogen peroxide. After surface modification with a surfactant (cetiltrimetilamonium bromide -CTAB), the very active niobium oxyhydroxide (NbO₂OH), presented hydrophobic characteristics and good activity for the oxidation of cyclohexene over the heterogeneous catalyst. Furthermore, the hydrophobic material presented a high selectivity toward the formation of the epoxide species. The mass spectrometry analysis of the reaction showed that epoxide species was produced with a selectivity of 60% using 0.2 mL of hydrogen peroxide, with 65% of conversion. The results strongly suggest that the reaction involves oxidizing species generated after the reaction with H ₂O₂. The surfactant anchored over the niobium catalyst promotes a better interaction with the nonpolar substrate.

Full text of DOI:10.1016/j.apcata.2013.01.007

Applied Catalysis A: General 454 (2013) 88–92
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A novel hydrofobic niobium oxyhydroxide as catalyst: Selective
cyclohexene oxidation to epoxide
Poliane Chagasa, Henrique S. Oliveiraa, Raquel Mambrinia, Mireille Le Hyaricb,
Mauro V. de Almeidab, Luiz C.A. Oliveiraa,∗
a Department of Chemistry, Federal University of Minas Gerais, Av. Antonio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil
b Department of Chemistry, Federal University of Juiz de Fora, 36033-600, Juiz de Fora, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
A surface modified niobium oxyhydroxide catalyst was utilized for the selective liquid-phase oxidation
of cyclohexene to epoxide in the presence of hydrogen peroxide. After surface modification with a surfac-
tant (cetiltrimetilamonium bromide -CTAB), the very active niobium oxyhydroxide (NbO2OH), presented
hydrophobic characteristics and good activity for the oxidation of cyclohexene over the heterogeneous
catalyst. Furthermore, the hydrophobic material presented a high selectivity toward the formation of
the epoxide species. The mass spectrometry analysis of the reaction showed that epoxide species was
produced with a selectivity of 60% using 0.2 mL of hydrogen peroxide, with 65% of conversion. The results
strongly suggest that the reaction involves oxidizing species generated after the reaction with H2O2. The
surfactant anchored over the niobium catalyst promotes a better interaction with the nonpolar substrate.
Received 5 October 2012
Received in revised form 2 January 2013
Accepted 5 January 2013
Available online xxx
Keywords:
Niobium oxyhydroxide
Hydrogen peroxide
Cyclohexene
Epoxide
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
catalysis [10,12]. The oxidation of cyclohexene over heterogeneous
niobium catalysts has been studied by the Ziolek’s group [16,17].
Many homogeneous and heterogeneous catalysts for the selec-
tive activation of C H bond (oxidation) have been developed in the
last years [1–8]. Among them, niobium compounds have been the
objects of an increasing interest as peroxo complexes for the oxi-
dation of organic compounds and as precursors for the preparation
of niobium oxides [9–11].
Niobium compounds are of great interest in heterogeneous
catalysis, due to their promoter and support effects for other metals,
increasing the catalytic activity and the stability of the catalysts.
Recently, the properties of niobia compounds as active species
have been explored by several authors [11,12]. The formation of
peroxo species improves the oxidation properties of the niobium
catalyst, due to their ability to release active oxygen, promoting
the oxidations of different inorganic or organic substrates [13–15].
Not only niobium compounds, but also peroxo complexes of vana-
dium, molybdenum and tantalum have attracted attention due to
their properties to donate active oxygen able to oxidize organic
compounds in liquid phase reactions [9]. However most of the
works involving the use of niobium reagents for the oxidation of
organic compounds in liquid phase take place via homogeneous
In a recent work they showed that H2O2 can react with amorphous
Nb2O5 to produce a yellow reactive material, typical of metal per-
oxo species. We described in a previous work [11] a similar result
using a synthetic niobia catalyst obtained by precipitation with
sodium hydroxide. Parida et al. [18] described a hybrid titanium
complex as an excellent catalyst for epoxidation of cyclohexene
using hydrogen peroxide as oxidant. However, the reactions were
relatively slow, and, comparing to our work, a higher temperature
was necessary to perform the oxidation. These problems, however,
have been overcome by the development of a new hydrophobic
niobium catalyst [19]. Hybrid catalysts offer the possibility of mod-
ulating their surface hydropholicity, increasing the diffusion of the
organic compounds on the catalyst surface. In fact, Morandin et al.
[20] reported that the increase of the hydrophobicity of zirconia-
silica mixed oxides lead to an improvement of the specific activity
of the catalysts by three to four times, and allows a better con-
trol of the efficiency with which hydrogen peroxide is consumed
in the oxidation reaction. Recently, a new class of niobium com-
pounds was developed by pretreatment of a synthetic niobia with
H2O2 to generate a modified catalyst [11,21]. The synthetic niobium
oxyhydroxide obtained in that work can be modified by surfac-
tant treatment generating a new catalyst bearing hydrophobic tail
on the surface. The partial hydrophobization of the catalyst pro-
vides it the possibility to react with compounds present in polar or
nonpolar solvent (amphiphilic properties).
∗
Corresponding author. Tel.: +55 31 3409 7550; fax: +55 31 34095700.
E-mail address: luizoliveira@qui.ufmg.br (L.C.A. Oliveira).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.01.007

P. Chagas et al. / Applied Catalysis A: General 454 (2013) 88–92
89
on the catalyst as well as pictures of the catalysts (S2 and S3)
partitioned between water and hexane.
The thermal gravimetric analyses (TG) of the materials were
made in an SDT-simultaneous TGA-DTA model TA, under N2 atmo-
sphere with heating rate of 10 ◦C min−1 up to 800 ◦C.
The acidic properties of the solids were determined by
temperature-programmed desorption using ammonia as probe
molecule. The NH3-TPD (Temperature Programmed desorption)
profiles were obtained using a Chembet-3000 (Quantachrome)
equipment (TCD detector, current of 150 mA, attenuation of 32).
About 0.2 g of each solid was treated at 400 ◦C for 50 min in a contin-
uous flow of helium (80 mL/min) prior to adsorption of NH3 at 50 ◦C.
After the sample has been purged, the TPD profiles were obtained
at a heating rate of 10◦/min up to 700 ◦C.
The resonance Raman spectra was adquired using a (SENTERRA
at ꢀ = 532 nm, power 0.2 mW, objective 50×, integration time 30 s,
4 coadditions and 2 mm laser aperture). The zeta potential variation
(PCZ) of aqueous suspensions of the powder as a function of pH was
measured by electrophoretic mobility using a Zetasizer 3000 (from
M/s Malvern Instruments Ltd., UK) equipment.
The oxidation reactions of cyclohexene were performed using a
mixture of 2 mmol of the reactant, 50 mg of the catalyst in acetoni-
trile (20 mL) and 0.1 mL of aqueous hydrogen peroxide 30% at 25 ◦C.
The cyclohexene conversion was monitored by UV (Spectrovision),
with scan between 250 and 800 nm and GC–MS (Hewlett-Packard
5890 Series II with MSD 5971A).
Fig. 1. Schematic illustration of the incorporation of hydrophobic tail over niobium
oxyhydroxide and pictures of the hydrophilic catalyst and hydrophobic niobium
oxyhydroxide partitioned between water and hexane.
In order to give some contribution to elucidate the chemistry
involved in the interaction between niobium species and H2O2, we
studied in the present work the use of niobium oxyhydroxide and
hydrophobic niobium oxyhydroxide (prepared in the presence of
CTAB) as heterogeneous catalysts for the oxidation of cyclohexene
in the presence of hydrogen peroxide. We observed that the pre-
vious treatment of the synthetic niobia with H2O2 improved the
catalytic activity of the material. An interesting result relative to
selectivity for epoxide species was found by anchoring a surfactant
on the catalyst surface.
3. Results and discussion
3.1. Catalytic tests
3.1.1. UV–vis spectroscopy analyses
The kinetic of the conversion of cyclohexene over synthetic nio-
bium oxyhydroxide, monitored by UV–vis spectroscopy is shown in
Fig. 2a. The results clearly show that the surface modified catalyst
(S3) was more efficient than the hydrophilic catalyst (S2), allowing
a conversion of approximately 80% after 180 min of reaction, while
S2 lead to only 15% of conversion.
Furthermore, to verify the absence of heat and mass transfer
limitations, the Koros-Nowak test [22] was employed. This test was
carried out with different amounts of catalyst (15, 30, and 50 e
100 mg) as shown in Fig. 2b. The similarity of the conversion in
all the systems indicates that there are no heat or mass transfer
limitations in our measurements.
2. Experimental
The hydrophilic catalyst (S2) was prepared treating
NH4[NbO(C2O4)2(H2O)] (H2O)n with NaOH (50 mL, 1 mol L−1),
followed by heating at 60 ◦C for 72 h [8]. In the case of hydropho-
bic catalyst (S3) 15 g of CTAB (cetiltrimetilamonium bromide
–
C19H42BrN) previously dissolved in 105 mL of a mixture
water/butanol (80:25) were added. The solid was then filtered,
washed with distilled water and dried at 60 ◦C for 12 h. Fig. 1
presents a scheme of the incorporation of the hydrophobic tails
Fig. 2. Kinetic of the reaction of cyclohexene oxidation with hydrogen peroxide monitored by UV–vis spectroscopy (a), and the oxidation with different amounts of catalyst
(b) (T = 25 ◦C; 1.0 mL of H2O2; 1 h of reaction).

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P. Chagas et al. / Applied Catalysis A: General 454 (2013) 88–92
Table 1
Reactions of cyclohexene with hydrophylic (S2) and hydrophobic (S3) niobia at 25 ◦C for 1 h.
Reaction
Conversion (%)
Seletivity (%)
2-Cyclohexenol
Cyclohexanol
2-Cyclohexen-1-one
1,2-Cyclohexanodiol
Cyclohexene oxide
No catalyst (only H2O2)
8
18
5
55
65
–
9
–
6
–
1
–
3
–
–
4
12
–
–
8
2
9
–
4
7
–
1
–
28
60
S2a
S3 (without H2O2)
S3a
S3b
a
VH2O2 = 0.1 mL;
VH2O2 = 0.2 mL; time of reaction = 1 h.
b
3.1.2. GC–MS analyses
[11]. In Fig. 3b a schematic representation of the entire process is
depicted showing the reaction of H2O2 that leads to the formation
of a peroxoniobate reactive group at the surface of the catalyst and
to the conversion of cyclohexene.
In order to identify the species formed during the reaction, the
products were analyzed by GC–MS (Table 1). The material treated
with the surfactant presented a higher conversion of the cyclo-
hexene when compared to other catalysts under mild conditions.
The results show that the reaction of cyclohexene (0.978 mmol)
over the hydrophobic catalyst (S3) in the presence 0.1 mL of H2O2
(2.94 mmol) gave 55% of conversion (28% selectivity to cyclohex-
ene epoxide) after 1 h of reaction. The same reaction with 0.2 mL
of H2O2 lead to a better result: a conversion of 65% was obtained
with a high selectivity toward the cyclohexene epoxide (60%). These
results are better than those found in the literature. For exam-
ple, the homogenous Nb-catalysts sodium tetraperoxo-niobate was
selective for the formation of the epoxide (73% selectivity), but the
rate of conversion was very low (7%) [7]. Ziolek et al. [16] obtained
72% conversion of cyclohexene when Nb was incorporated into
mesoporous silicate, however, using pure Nb2O5 and H2O2 the con-
version was only 49%.
The presence of the hydrophobic tails on the surface of the
catalyst lead to a much higher conversion of cyclohexene when
compared to the hydrophilic catalyst S2 (respectively 55% and
18% using 0.1 mL H2O2). This may be due to the increased area of
contact between the catalyst S3 and the nonpolar cyclohexene. Fur-
thermore, the presence of the hydrophobic part over the catalyst
decreases the capacity of oxidation of the material, which permits
the formation of cyclohexene epoxide without further reaction pro-
cess such as the formation of diol, since the peroxo groups are active
in the opening of ring [17].
The results suggest that the functionalization occurs in the
surface OH groups attached to niobium. This fact decreases the
formation of peroxos groups (oxidant groups) resulting from the
reaction of these OH groups with the H2O2 added (Fig. 3b). Thus,
due to the prevention of formation of peroxo group by anchoring of
a surfactant group, the material reduces its oxidative ability becom-
ing more selective for partial oxidation products. Furthermore, one
evidence that leaching of organic groups does not occur is that after
several reuses the material continues with hydrophobic properties,
since it keeps itself in the interface between a polar and the other
nonpolar solvents, as shown in Fig. 4a. Additionally, leaching tests
were performed and the results were negligible conversion.Several
reuse tests were conducted to ensure the stability of the catalyst.
After each reaction, the catalyst was recovered by centrifugation
and weighed to check a possible loss of mass with the recovery
process. The cyclohexene conversion results are shown in Fig. 4b.
It can be observed that after six reuses the cyclohexene conversion
remained very similar, which proves the stability of the catalyst. It
is also interesting to note that the material after reuse stays with
the hydrophobic property because it stays at the interface of polar
and nonpolar liquids, as shown in Fig. 4b.
3.2. Characterization of the materials
No leaching problems with the catalysts were revealed in the
experiments. Thus, the S2 and S3 materials can be used as truly
heterogeneous oxidative catalysts for the epoxidation process.
The influence of the hydrogen peroxide amount on the rate of
conversion was studied and the results are displayed in Fig. 3a.
Clearly, the increase of hydrogen peroxide favors the cyclohexene
conversion, which can be explained by the formation of peroxo-
niobate from the reaction between H2O2 and the surface niobium
The acidic properties of the materials were investigated by
temperature-programmed desorption using a mmonia as probe
molecule. TPD-NH3 profiles are shown in Fig. 5a. In both materials,
S2 and S3, adsorbed ammonia began to desorb at a temperature
above 250 ◦C and exhibited the peaks centered at nearly 320 ◦C.
The relatively low desorption temperature indicates the presence
of weak acid sites on the surface of S2 sample. The S3 material
exhibited similar NH3-TPD profiles and less acidity than S2, and
Fig. 3. Influence of H2O2 amount in ciclohexene conversion (a) and a schematic illustration of the entire process involving the formation of a peroxoniobate reactive group
and conversion of cyclohexene.

P. Chagas et al. / Applied Catalysis A: General 454 (2013) 88–92
91
Fig. 4. Picture of catalyst in the interface of a biphasic system (hexane/water) after six reuses (a) and reuses tests of the catalysts (b) (m = 50 mg; Vol H2O2 = 0.2 mL; Vol
cyclohexene = 10 mL; 60 min of reaction).
Fig. 5. TPD-NH3 profile of the S2 and S3 samples (a). The pH dependence of zeta potentials for S2 and S3 (b).
Fig. 6. TG analyses of the catalysts S2 and S3 under oxygen atmosphere.
a decrease of approximately 32% of the acid sites was observed
by incorporating the surfactant. This suggests that the surfac-
tant molecules are bound to the surface by substituting hydroxyl
groups.
The surface changes of the materials were studied by zeta poten-
tial measurements. The variation of zeta potential with pH for the
aqueous suspensions is shown in Fig. 5b. The zeta potential was
found to be approximately pH 0.59 and pH 1.71 for S2 and S3,

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P. Chagas et al. / Applied Catalysis A: General 454 (2013) 88–92
niobium oxyhydroxide prepared in the presence of CTAB surfac-
tant as an efficient heterogeneous catalyst. The presence of the
hydrophobic moiety was verified by TG measurements. These novel
results describe, for the first time, the use of a synthetic niobium
oxide in liquid phase at low temperature (25 ◦C) in the presence of
hydrogen peroxide without metal impregnation. Furthermore, the
catalyst can be reused without deactivation, which is an advantage
compared to others catalytic systems.
Acknowledgments
This work was supported by the CNPq, CBMM (Brazil), and
FAPEMIG.
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4. Conclusion
High conversions can be achieved for the oxidation of cyclo-
hexene to epoxide specie with high selectivity, using a synthetic