Low temperature liquid phase catalytic oxidation of aniline promoted by niobium pentoxide micro and nanoparticles

Article abstract of DOI:10.1016/j.catcom.2017.06.004

A microwave-assisted hydrothermal method was employed to synthesis Nb₂O₅ (niobium pentoxide) nanoparticles and the effects of synthesis conditions on the physical-chemical properties were evaluated. The catalytic activity of the prepared samples in the liquid phase oxidation of aniline with aqueous hydrogen peroxide as oxidizing agent was also studied. The nanoparticles showed hexagonal structure and rounded shape covered by nanoneedles. The results evidenced high catalytic activity with total conversion of aniline at ambient condition. Aniline conversion and product selectivities depended on the experimental parameters, particularly the oxidizing agent concentration, the nature of the solvent, type of the catalyst and reaction time.

Full text of DOI:10.1016/j.catcom.2017.06.004

Accepted Manuscript
Low temperature liquid phase catalytic oxidation of aniline
promoted by niobium pentoxide micro and nanoparticles
Wellington M. Ventura, Daniel C. Batalha, Humberto V. Fajardo,
Jason G. Taylor, Natália H. Marins, Bruno S. Noremberg, Tomasz
Tański, Neftalí L.V. Carreño
PII:
DOI:
Reference:
S1566-7367(17)30235-2
doi: 10.1016/j.catcom.2017.06.004
CATCOM 5070
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
3 April 2017
1 June 2017
8 June 2017
Please cite this article as: Wellington M. Ventura, Daniel C. Batalha, Humberto V. Fajardo,
Jason G. Taylor, Natália H. Marins, Bruno S. Noremberg, Tomasz Tański, Neftalí L.V.
Carreño , Low temperature liquid phase catalytic oxidation of aniline promoted by
niobium pentoxide micro and nanoparticles. The address for the corresponding author was
captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi:
10.1016/j.catcom.2017.06.004
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ACCEPTED MANUSCRIPT
Low temperature liquid phase catalytic oxidation of aniline promoted by niobium
pentoxide micro and nanoparticles
Wellington M. Venturaa, Daniel C. Batalhaa, Humberto V. Fajardoa,*, Jason G. Taylora,
Natália H. Marinsb, Bruno S. Norembergb, Tomasz Tańskic,d, Neftalí L. V. Carreñob,**
a
Chemistry Department, Institute of Exact and Biological Sciences, Federal University
of Ouro Preto, 35400-000, Ouro Preto, MG, Brazil
b
Graduate Program in Materials Science and Engineering, Technology Development
Center, Federal University of Pelotas, 96010-000, Pelotas, RS, Brazil
c
Institute of Engineering Materials and Biomaterials, Silesian University of
Technology, 44-100, Gliwice, Poland
d
Center for Nanotechnology, Silesian University of Technology, 44-100, Gliwice,
Poland
* E-mail Corresponding author: hfajardo@iceb.ufop.br
** E-mail Corresponding author: neftali@ufpel.edu.br
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Abstract
A microwave-assisted hydrothermal method was employed to synthesis Nb2O5
(niobium pentoxide) nanoparticles and the effects of synthesis conditions on the
physical-chemical properties were evaluated. The catalytic activity of the prepared
samples in the liquid phase oxidation of aniline with aqueous hydrogen peroxide as
oxidizing agent was also studied. The nanoparticles showed hexagonal structure and
rounded shape covered by nanoneedles. The results evidenced high catalytic activity
with total conversion of aniline at ambient condition. Aniline conversion and product
selectivities depended on the experimental parameters, particularly the oxidizing agent
concentration, the nature of the solvent, type of the catalyst and reaction time.
Keywords: niobium pentoxide, aniline, oxidation, hydrogen peroxide, nitrobenzene,
phenylhydroxylamine.
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1. Introduction
The oxidation of aniline to its corresponding derivatives, such as
phenylhydroxylamine, nitrosobenzene, nitrobenzene, azobenzene and azoxybenzene,
has been investigated over the past few decades due to the potential applications of
these products in several processes important to the chemical and pharmaceutical
industries. These oxidations products are valuable intermediates in industry and are
used to produce pharmaceuticals, dyes, reducing agents, polymer stabilisers, food
additives and as liquid crystals in electronic displays [1–5]. Selective oxidation of
aromatic amines is one of the most challenging reactions for heterogeneous catalysis.
Achieving a highly selective catalytic process with reasonable substrate conversion is
hampered by competitive over-oxidation and condensation reactions (Scheme 1).
NH2
NHOH
[O]
NO
NO2
[O]
[O]
aniline
N-phenylhydroxyamine nitrosobenzene nitrobenzene
NHOH
NO
N
N
condensation
+
O
azoxybenzene
NH2
NHOH
N
N
condensation
+
azobenzene
Scheme 1. Illustrative scheme of main possible products of aniline oxidation.
Hydrogen peroxide is a reasonable environmentally friendly oxidant for
satisfying the ideals of green chemistry given that it does not produce any toxic waste.
Methodologies utilizing heterogeneous catalysts and hydrogen peroxide have been
developed for the oxidation of aromatic amines. Examples include: rhenium,
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molybdenum, cobalt oxide, titanium silicate, Au/TiO2, Ag/WO3, CuCr2O4, dimeric
[Ln4(H2O)6(b-GeW10O38)2]12- anions, titanium silicalite-1, titanium (IV) oxide and Cu–
CeO2 [6-16]. Although varying degrees of success in terms of substrate conversion and
selectivity’s have been achieved, there is clearly still a demand for reusable and efficient
heterogeneous catalysts for the selective oxidation of anilines. There is precedence for
the application of niobium catalysts in oxidative processes that employ hydrogen
peroxide as oxidant. Most examples involve the epoxidation of alkenes [17], but
oxidation of dithioacetals, benzylalcohols, naphthols and sulfides [17-21] have also
been reported. Reddy and Sayari demonstrated that vanadium was capable of catalyzing
the oxidation of amines in the presence of hydrogen peroxide to afford the
corresponding imines and oximes under mild conditions [22]. Given that no examples
for the oxidation of aniline by niobium catalysis were encountered in the literature and
that niobium bears some similarity in electronic structure and catalytic behavior in
oxidation-reduction reactions to vanadium, a niobium catalyzed oxidation of aniline
under heterogeneous conditions is described herein for the first time. Niobium
pentoxide (Nb2O5) exhibits good chemical stability, nontoxicity and commercial
availability [23]. This oxide has been reported as a promising material for application in
many fields due to its properties, such as high specific surface area and strength of
surface acid sites. Nb2O5 presents band gap values between 3.4 and 4.2 eV [24], which
is another promising property for its application.
2. Experimental
2.1 Catalyst preparation
Nb2O5 nanoparticles were synthesized by following a method reported
previously [25], meanwhile microwave radiation was used to accelerate the synthesis. In
this synthesis was used ammonium niobium oxalate (NH4[NbO(C2O4)2(H2O)2].nH2O),
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supplied by Companhia Brasileira de Metalurgia e Mineração (CBMM, Brazil),
hydrogen peroxide 30% in vol. (LabSynth, Brazil), and distilled water. 2.0 g of
NH4[NbO(C2O4)2(H2O)2].nH2O were dispersed in 100 mL of distilled water. After the
homogenization, hydrogen peroxide (10 mol/mol) was added. The solution was poured
into a container of polytetrafluoroethylene, sealed and placed into a microwave oven
(Electrolux, MEF41, Brazil), then heated to 160 °C under different times (15, 30 and 60
minutes). After the cooling until room temperature, the precipitate was collected and
washed three times with absolute ethanol (LabSynth, Brazil) and then dried in an oven
(DeLeo, A5SE, Brazil) for 24 hours at 50°C. The powders were denominated
Nb2O5#15, Nb2O5#30 and Nb2O5#60, according to the synthesis time.
2.2 Catalyst characterization
The Nb2O5 powder was analyzed by: X-ray diffraction (XRD; 6000, Shimadzu,
Japan) with Cu Kα radiation (λ = 1.5418 Å). Raman analysis (green line ion laser,
514.53 nm, Horiba, LABRAM-HR, Japan). Transmission Electron Microscope (JEOL,
JEM 1400, Japan). Field Emission Scanning Electron Microscope (Zeiss, Supra 35,
Germany). Brunauer-Emmett-Teller (BET) method using a Surface Area Analyzer
(BEL Japan Inc., BELSORP-mini, Japan).
The acidity of the catalysts was determined by titration method. 20 mL of a
sodium hydroxide aqueous solution (0.01 mol.L-1) was added to the catalyst (10 mg).
The mixture was stirred for 24 hours at room temperature. Upon separation by
centrifugation, the supernatant was then titrated with hydrochloric acid (0.05 mol.L-1)
aqueous solution using phenolphthalein as indicator. The quantity of acid sites with in
the catalysts was determined by: number of acid sites = [initial quantity of NaOH added
(moles) – quantity of HCl consumed (moles)] x Avogadro constant.
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2.3 Catalytic tests
The liquid phase oxidation of aniline was carried out in a one necked round
bottom flask (10 mL batch reactor) maintained at 25 °C on a hotplate stirrer with an oil
bath. To the flask was added catalyst (10 mg), specific solvent (3 mL), aniline (0.10
mL) and finally the drop wise addition of H2O2 (0.15-1.0 mL of a 35% aq. solution) and
the reaction allowed to stir at 25 °C for 3-24 hours. Regarding the solvent, water,
chloroform, ethanol, acetone, tetrahydrofuran and acetonitrile were evaluated. Upon
completion, the reaction was quenched by the addition of a saturated aqueous solution
of NaHCO3 (10 mL) and then taken up in to a separatory funnel and extracted with
chloroform (2 x 10 mL), the organic extractions were combined, dried over anhydrous
sodium sulphate, filtered and made up to 25 mL in a volumetric flask with chloroform.
This solution was injected in to the GC for analysis. Analysis was performed on Gas
Chromatograph (Shimadzu, GC-2014) connected with Rtx-Wax capillary column and
flame ionization detector (FID). Aniline conversion and product formation were
quantified with calibration curves which were obtained by manually injecting authentic
samples of known concentrations. Conversion of aniline was calculated by: [moles of
reactant reacted] / [initial moles of reactant used] x 100 and selectivity of products were
calculated by: [total moles of product formed]/[the sum of total moles of all oxidation
products formed] x100. The obtained products were also confirmed by GC-MS
(Shimadzu, GCMS-QP2010).
3. Results and discussion
3.1 Characterization of the catalysts
In this study, three groups of Nb2O5 nanoparticles were synthesized and each
group was differentiated by the exposure time during the microwave-assisted
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hydrothermal synthesis (MHS). The synthesized nanoparticles were also compared to
commercially available microparticles (Nb2O5#c) and amorphous Nb2O5 (Nb2O5#a).
The XRD patterns of samples are shown in Fig. 1. The peaks of the synthesized Nb2O5
could be indexed to the hexagonal phase (JCPDS No. 28-0317) [26,27]. The peaks of
the Nb2O5#c could be attributed to the orthorhombic phase (JCPDS No. 27-1003)
[26,28]. The broad peak of the synthesized Nb2O5 formed at 26° is relative to the
hydrated niobium oxide. The weak bands present in the synthesized Nb2O5 may be due
to the low temperature used in the synthesis, which may have influenced its
crystallinity. The formation of more crystalline materials can be managed by increasing
the synthesis temperature [29]. The Nb2O5#15, Nb2O5#30, and Nb2O5#60 samples
exhibited crystallographic orientation of 8 nm, 10 nm, and 12 nm, respectively, while
the Nb2O5#c sample is microcrystalline. XRD confirms that Nb2O5#a is amorphous.
The Raman spectra of the Nb2O5 samples are shown in Fig. 1. The Raman shift
presented typical chemical groups of Nb2O5 compounds. The spectra of the
pseudohexagonal (TT phase) and orthorhombic (phase T) phases have almost identical
structures [30]. Broad bands between 650 and 750 cm-1 are assigned to the symmetric
stretching mode of the Nb-O polyhedra (presence of disordered NbO6, NbO7, and
NbO8) [24,31]. The vibration mode at 226 cm−1 is assigned to the stretching of the Nb-
O-Nb bonds. Synthesized Nb2O5 has a broad peak at 649 cm−1, which is characteristic
of hydrated niobium oxide (Nb2O5·nH2O), and the poorly defined shoulder at 712 cm−1
is characteristic of the orthorhombic phase. This indicates that the sample has higher
proportion of Nb2O5·nH2O than the orthorhombic phase [29]. The commercial Nb2O5
shows a transition from the orthorhombic phase (phase T) to the monoclinic phase
-1
(phase H) due to the emergence of a weak peak at 902 cm and the average peak at 996
-1
cm [31].
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Morphology of the samples is shown in Fig. 2. Synthesized Nb2O5 showed shape
of nanoneedles aggregates in 3D nanostructures in the form of nanoflowers [32].
Commercial Nb2O5 showed irregular and large particles. Fig. 3 shows that the
synthesized Nb2O5#30 has homogeneous particle size and average diameter of 85.83
nm.
The N2-physisorption results are shown in Table 1. As observed, the synthetized
Nb2O5 samples have greater specific surface area and pore volume than Nb2O5#c. For
Nb2O5#60, the specific surface area was much higher (151.6 m2/g) than Nb2O5#c (6.3
m2/g). Also, the increasing of the synthesis time contributed to the formation of particles
with larger surface area and higher pore volume.
The amount of acid sites of the Nb2O5 samples was determined by the titration
method in aqueous solutions (Table 1). The Nb2O5#c catalyst had 30.1x1020 H+ sites/g,
which was less than that of the other Nb2O5 samples which presented values in the
range of 74.3 x1020-91.3x1020 H+ sites/g [33,34].
3.2 Catalytic reactions
In order to investigate the catalytic behavior of the Nb2O5, liquid phase aniline
oxidation reactions were carried out under different conditions. Control experiments
were conducted without catalyst under the test conditions. No consumption of aniline
was observed in all cases, indicating the necessity of the catalyst for the process. It
could be observed that Nb2O5 were very active and the results revealed that aniline
conversion and product selectivity’s were influenced by the nature of the catalyst and
experimental conditions employed (Figures
4
and 5). Nitrobenzene and
phenylhydroxylamine were detected as the major products, nitrosobenzene as a minor
product whereas only traces of azobenzene were observed, for which the selectivity was
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always less than 2%. Fig. 4 presents the catalytic activity of Nb2O5 samples in the
aniline oxidation reaction, using water as solvent. All catalysts practically promoted the
complete conversion of aniline, however significant differences in product selectivity’s
could be observed. The selectivity towards nitrobenzene decreased and the formation of
phenylhydroxylamine increased when the specific surface area and pore volume within
the prepared Nb2O5 nanoparticles catalysts were greater. Nb2O5#15 catalyst provided
the highest selectivity towards nitrobenzene (71.1%) and the lowest selectivity to
phenylhydroxylamine (28.3%). It is interesting to observe that Nb2O5#c catalyst had an
opposite trend presenting a poor selectivity to nitrobenzene (25.7%). This catalyst
possesses irregular and larger particles indicating that the size and form could influence
the reactivity of the catalyst toward the advanced oxidation product (nitrobenzene) since
its surface sites can be less accessible toward aniline molecules owing to their lower
surface/volume ratio compared to that of the Nb2O5 synthesized nanoparticles [11,12].
Furthermore, in this study both crystalline and amorphous niobium oxide compounds as
well as compounds with intermediate states between the two extremes were evaluated in
order to determine if these properties affect catalytic performance. Although Nb2O5#c
demonstrated high catalytic activity, Nb2O5#a was significantly more selective towards
nitrobenzene (58.2%). These findings are similar to the results described in an important
study carried out by Ziolek and Pietrzyk [35,36] which demonstrated the high ability of
the amorphous Nb2O5 to form superoxo, peroxo, and hydroxyl reactive oxygen species
upon contact with hydrogen peroxide in the epoxidation and oxidation reactions.
Differences in the interaction between amorphous and crystalline Nb2O5 with H2O2
were dependent on the level of surface hydroxylation. The more effective interaction of
amorphous Nb2O5 with H2O2 could be attributed to the higher content of free hydroxyl
groups present on the catalyst surface (niobic acid) which is comparatively not as
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readily available on crystalline Nb2O5. According to Ziolek and Pietrzyk, amorphous
Nb2O5 is more efficient in generating peroxo radicals and so exhibits high ability to
form radical peroxo species which are the source of active oxygen species. Thus, since
the peroxo groups replace the acid group on the surface [34], from the data presented in
Table 1 one can speculate that the lower hydroxylation level of Nb2O5#c surface gives
rise to decrease in surface concentration of those peroxo species [34-36]. Ziolek and
Pietrzyk also suggest that amorphous Nb2O5 is beneficial since they act as a sponge for
−•
trapping O2 and O22−, shifting the electroprotic equilibrium towards •OH radical
formation. The greater efficiency of amorphous niobium oxides to generate these
species may be conducive to faster consumption of aniline and also limiting the
possibility of under oxidation, leading to higher conversions to nitrobenzene.
The effect that the amount of hydrogen peroxide had on conversion and product
distribution are presented in Fig. 4. At the lowest H2O2 amount (0.15 mL), 70.8% of
aniline conversion was achieved and the reaction proceeds via the formation mainly of
phenylhydroxylamine and nitrosobenzene, with 88.1% phenylhydroxylamine, 9.6%
nitrosobenzene and 2.3% nitrobenzene selectivities, respectively. Interestingly when
increasing the H2O2 amount up to 0.50 mL, complete aniline conversion was achieved
and phenylhydroxylamine selectivity dropped to 41.8% with a considerable increase in
the nitrobenzene formation (58.2%). On increasing the H2O2 amount up to 1.0 mL,
aniline conversion rate remained almost the same, however the catalyst was very
selective toward nitrobenzene formation (92.5%). This indicates that the oxidation of
phenylhydroxylamine to nitrobenzene was favored due to the higher amount of
oxidizing agent present in the reaction mixture.
It is known that the catalyst performances depend on the nature of the solvent
used in these reactions [14]. The Nb2O5#30 catalyst was used to investigate the effect of
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different solvents on the aniline oxidation reactions (Fig. 5). In almost all cases, the
solvents used were able to reach aniline conversion above 90%, except with ethanol, in
which the aniline conversion was, around 48.4%. The selectivity of the products varied
according to the solvent employed. Using ethanol as solvent, only phenylhydroxylamine
was obtained indicating that phenylhydroxylamine oxidation to nitrosobenzene and
further to nitrobenzene is probably inhibited in ethanol solvent. In the presence of
acetone and THF, phenylhydroxylamine selectivity was high (92.8% and 94.3%,
respectively) and less nitrobenzene being formed (5.4% and 4.3%, respectively). On the
other hand, selectivities to nitrobenzene and nitrosobenzene in the ranges of 20-30%
and 11-13%, respectively, in both chloroform and acetonitrile were observed. The
amount of nitrosobenzene is found to be very similar in these two solvents. It is
interesting to observe that low amounts of azobenzene were detected and
nitrosobenzene was not detected in the final products when acetone and THF were used
as solvents. This result suggests that most probably, phenylhydroxylamine reacted with
unreacted aniline leading to the formation of azobenzene [9].
Fig. 5 presents the formation of the products in the course of the reaction over
Nb2O5#60 catalyst, with 0.5 mL of H2O2 and acetonitrile as solvent.
Phenylhydroxylamine, which can be obtained by direct oxidation of aniline, was formed
in high conversion at the early stages of the reaction and slowly disappeared with time,
achieving 41.8% of selectivity after 24 hours of reaction. Concomitantly, the amount of
nitrosobenzene and nitrobenzene increased continuously with time. At the end of the
run (24h), phenylhydroxylamine and nitrobenzene were the only products indicating
that initially formed nitrosobenzene from phenylhydroxylamine is converted to
nitrobenzene. Furthermore, the oxidation of nitrosobenzene to nitrobenzene is
significantly faster than the conversion of phenylhydroxylamine to nitrosobenzene.
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4. Conclusions
In this study we have investigated the effect of various parameters on the liquid
phase oxidation of aniline over synthesized Nb2O5 nanoparticles and commercial Nb2O5
microparticles. The influence of the oxidant amount, nature of solvents and reaction
time on the oxidation of aniline was evaluated. To our knowledge, the use of Nb2O5 has
not yet been reported to promote this reaction. It should be highlighted that the results
showed that the prepared catalysts could be very active in the proposed reaction even at
mild reaction conditions (ambient temperature and pressure). The process could be
made selective towards the formation of phenylhydroxylamine when employing ethanol
as solvent and utilizing smaller amounts of H2O2 or favour the formation of
nitrobenzene in the presence of excess H2O2 and acetonitrile as solvent.
Acknowledgments
This work was supported by the CAPES/PROCAD 2013/2998/2014, UFOP, CNPq and
FAPEMIG. Nb2O5#c and Nb2O5#a samples were kindly supplied by Companhia
Brasileira de Metalurgia e Mineração (CBMM, Brazil).
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Figures
Figure 1. XRD patterns and Raman spectra of Nb2O5 catalysts.
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Figure 2. TEM images of Nb2O5 catalysts.
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Figure 3. FE-SEM image and histogram of size distribution of Nb2O5#30 catalyst.
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Figure 4. Aniline oxidation catalyzed by different Nb2O5 catalysts (with 0.5 mL of
H2O2 and water as solvent) and effect of H2O2 volume on oxidation of aniline over
Nb2O5#60 catalyst (with acetonitrile as solvent).
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Figure 5. Effect of solvent on oxidation of aniline over Nb2O5#30 catalyst (with 0.5 mL
of H2O2) and effect of reaction time on oxidation of aniline over Nb2O5#60 catalyst
(with 0.5 mL of H2O2 and acetonitrile as solvent).
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Table
Table 1. Textural properties measured by N2-physisorption and acid sites amount
determined by a titration method, for the Nb2O5 catalysts.
S
V
(cm3/g)
D
Acid sites
(H+ sites/g)
Catalyst
(m2/g)
(Å)
Nb2O5#c
Nb2O5#60
Nb2O5#30
Nb2O5#15
Nb2O5#a
6.3
151.6
109.4
97.3
-
0.003
0.074
0.053
0.047
-
20.84
19.43
19.43
19.28
-
30.1x1020
91.3x1020
74.3x1020
90.3x1020
75.2x1020
S: specific surface area; V: pore volume; D: pore diameter.
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Graphical abstract
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Highlights
 Low temperature liquid phase catalytic oxidation of aniline
 Hydrogen peroxide is a milder oxidant
 Selective oxidation of aniline
 Niobium mineral
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