Efficient and Selective Oxidation of Aromatic Amines to Azoxy Derivatives over Aluminium and Gallium Oxide Catalysts with Nanorod Morphology

Article abstract of DOI:10.1002/cctc.201901378

Aluminium oxide and gallium oxide nanorods were identified as highly efficient heterogeneous catalysts for the selective oxidation of aromatic amines to azoxy compounds using hydrogen peroxide as environmentally friendly oxidant. This is the first report of the selective oxidation of aromatic amines to their azoxy derivatives without using transition metal catalysts. Among the tested transition-metal-free oxides, gallium oxide nanorods with small dimensions (9–52 nm length and 3–5 nm width) and fully accessible, high surface area (225 m² g^?1) displayed the best catalytic performance in terms of substrate versatility, activity and azoxybenzene selectivity. Furthermore, the catalyst loading, hydrogen peroxide type (aqueous or anhydrous), and the amount of solvent were tuned to optimise the catalytic performance, which allowed reaching almost full selectivity (98 %) towards azoxybenzene at high aniline conversion (94 %). Reusability tests showed that the gallium oxide nanorod catalyst can be recycled in consecutive runs with complete retention of the original activity and selectivity.

Full text of DOI:10.1002/cctc.201901378

Accepted Article
Title: Efficient and selective oxidation of aromatic amines to azoxy
derivatives over aluminium and gallium oxide catalysts with
nanorod morphology
Authors: Bhawan Singh, Dalmo Mandelli, and Paolo Pescarmona
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Efficient and selective oxidation of aromatic amines to azoxy derivatives
over aluminium and gallium oxide catalysts with nanorod morphology
Bhawan Singh,a,b,c Dalmo Mandelli,c Paolo P. Pescarmonaa,*
Abstract: Aluminium oxide and gallium oxide nanorods were
identified as highly efficient heterogeneous catalysts for the selective
oxidation of aromatic amines to azoxy compounds using hydrogen
peroxide as environmentally friendly oxidant. This is the first report
for the selective oxidation of aromatic amines to their azoxy
derivatives without using transition metal catalysts. Among the
tested transition-metal-free oxides, gallium oxide nanorods with
small dimensions (9-52 nm length and 3-5 nm width) and fully
accessible, high surface area (225 m2 g-1) displayed the best
catalytic performance in terms of substrate versatility, activity and
azoxybenzene selectivity. Furthermore, the catalyst loading,
hydrogen peroxide type (aqueous or anhydrous), and the amount of
solvent were tuned to optimise the catalytic performance, which
allowed reaching almost full selectivity (98 %) towards
azoxybenzene at high aniline conversion (94 %). Reusability tests
showed that the gallium oxide nanorod catalyst can be recycled in
consecutive runs with complete retention of the original activity and
selectivity.
alkyl groups are used in liquid crystal displays[12] and therapeutic
drugs.[13,14]
Conventionally, azoxy compounds are synthesised either from
the oxidation of amines or from the reduction of nitro compounds.
In both cases, the formation of the azoxy derivatives is
accomplished through the condensation of intermediate reaction
products (Scheme 1). Due to the possible formation of different
reaction products, it is quite challenging to control the reaction
towards the selective formation of the azoxy product.
Additionally, mild reaction conditions (i.e. low temperature and
atmospheric pressure) and the use of green solvents and
oxidants or reductants are preferable. For this purpose, the
identification of an efficient and reusable catalyst is of the utmost
importance.
Aniline
Phenylhydroxylamine
Nitrosobenzene
Nitrobenzene
Introduction
The selective oxidation of aromatic amines into O-containing
derivatives such as hydroxylamine, nitroso, nitro, oxime, azo and
azoxy compounds by using a sustainable chemical process has
a great fundamental as well as practical interest.[1–4] Among this
class of products, azoxy compounds have received considerable
research attention owing to their versatile applications as dyes
and pigments, reducing agents, analytical reagents, food
additives, chemical stabilisers and polymer inhibitors.[5–9] In
addition, azoxy compounds are used as precursors for Wallach
rearrangement, which offers a straightforward path for the
synthesis of azo compounds with one aromatic ring substituted
by a hydroxyl group in the aromatic para-position.[10,11] Moreover,
some derivatives of azoxy compounds containing both aryl and
Azobenzene
Azoxybenzene
Scheme 1. Reaction pathways involved in the synthesis of azoxy compounds
Until now, numerous transition metals based homogeneous and
heterogeneous catalysts have been developed for the selective
oxidation of amines into azoxy compounds by using H2O2 or tert-
butyl hydrogen peroxide (TBHP) as the oxidant. Homogeneous
catalysts based on transition metals, such as tetrastearyl
tetratitanate,[15] ruthenium chloride,[16] and polyoxometalates[17]
have been reported to be active and selective in the oxidation of
aromatic amines into azoxy compounds. Recently, the reaction
could be achieved also with 2,2,2-trifluoroacetophenone as
organo-catalyst using hydrogen peroxide as the oxidant.[18]
However, all these homogeneous systems suffer from the
difficult separability and reusability of the catalyst. In order to
[a]
Dr. B. Singh, Prof. Dr. P. P. Pescarmona
Chemical Engineering Group, Engineering and Technology Institute
Groningen (ENTEG)
University of Groningen
overcome this issue,
a variety of transition-metal-based
Nijenborgh 4, 9747 AG Groningen, The Netherlands
E-mail: p.p.pescarmona@rug.nl
heterogeneous catalysts including microporous and mesoporous
titanosilicates (e.g. TS-1, Ti-Beta, Ti-ZSM-48, Ti-MCM-41, Ti-
HMS, ETS-10, Ti-PILC),[3,19–23] transition metal oxides (such as
TiO2,[24,25] LiNbO3,[26] CuCr2O3 spinel[27] and mesoporous CoO-
[b]
[c]
Dr. B. Singh
Centre for Surface Chemistry and Catalysis, KU Leuven
Celestijnenlaan 200F, 3001 Heverlee, Belgium
Dr. B. Singh, Prof. Dr. Dalmo Mandelli
Centre of Natural and Human Sciences
Federal University of ABC
SiO2[28]) and supported metal or metal oxide nanoparticles (such
[29]
as Ag on WO3,
Ag on Fe2O3,[30] MnO2 on graphene oxide[31]
166 Santa Adélia Street, Bangu, Santo André, SP 09210-170, Brazil
and Cu on CeO2[32]) have been employed for the oxidation of
aromatic amines into azoxybenzene derivatives. Although all
these heterogeneous catalytic systems are active in the
selective oxidation of amines to azoxy derivatives, they still
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201901378
ChemCatChem
suffer from one or more drawbacks, such as the use of
Recent reports showed that aluminium oxide and gallium oxide
with nanorod morphology are highly active, selective and stable
heterogeneous catalyst that are able to activate hydrogen
peroxide towards the epoxidation of alkenes.[36,40] This feature
has the potential to be exploited in other oxidation reactions
involving the activation of hydrogen peroxide. In this context, we
studied a set of Al and Ga oxides with different morphologies
and crystal structures as transition-metal-free heterogeneous
catalysts for the oxidation of aromatic amines to azoxy
derivatives using hydrogen peroxide as an environmentally
friendly oxidant. Al2O3 and Ga2O3 nanorods were prepared
according to an organic-template-free protocol developed by our
group and their catalytic performance was compared to the two
most common polymorphs of gallium oxide, i.e. γ-Ga2O3 (surface
area: 155 m2/g, see Fig. S1 for X-ray diffraction and the N2-
physisorption data) and β-Ga2O3 (surface area: 13 m2/g, see Fig.
S2 for X-ray diffraction and the N2-physisorption data), and to
two commercial aluminium oxides: Al2O3 activated, neutral
Brockmann I (particle size ~150 mesh, surface area: 155 m2/g),
γ-Al2O3 nanopowder (particle size <50 nm, surface area: 171
m2/g). The structure and textural properties of Al2O3-NR, e.g.
surface area, nanorod morphology and presence of γ-Al2O3
phase are in line with our recent report (Fig. S3 for selected
characterisation data).[40] In this work, we improved the synthesis
protocol of Ga2O3-NR, reaching a higher specific surface area
(225 m2/g) and tuning the size of the nanorods. TEM analysis
confirmed that the material presents the rod-like morphology
with length varying from 9 to 52 nm and width between 3 and 5
nm (Fig. 1 A). The X-ray diffractogram displays two major broad
diffraction peak at 2θ = 35.54˚and 61.92˚ (Fig.1 B), which match
well with the expected pattern attributed to the ε-Ga2O3
polymorph.[36] The N2-adsorption-desorption isotherms of Ga2O3-
NR present the type II isotherm with H3 hysteresis loop over the
relative pressure range of 0.62- 0.97 (Fig. 1 C), attributed to the
packing of the nanorods leading to the formation of slit-shaped
interparticle voids. Additional characterisation data (FT-IR
spectrum) can be found in the Supporting Information (Fig. S4).
Initially, the Al and Ga oxides were screened as heterogeneous
catalysts for the oxidation of aniline with 50 wt % aqueous
expensive transition metals that may be prone to leaching,
diffusion limitations, complex and tedious catalyst synthesis,
inefficient recyclability and the use of high loading of catalyst. In
this respect, the development of an efficient, transition-metal-
free heterogeneous catalyst that can operate with an
environmentally benign oxidant as H2O2 in the selective
oxidation of aromatic amines to azoxy derivatives is an attractive
and timely goal.
A class of transition-metal-free heterogeneous catalysts that has
attracted research interest for the selective oxidation of
unsaturated compounds to the corresponding epoxides in the
last two decades consists of materials containing Ga or Al active
sites, either as oxides (Al2O3 or Ga2O3)[33–36] or as
metallosilicates (Ga-MCM-41, Al-MCM-41).[37,38] These catalysts
operate with hydrogen peroxide as an environmentally friendly
oxidant under relatively mild reaction conditions. In addition, they
also have shown to be active and selective catalysts towards the
formation of alkyl lactates from triose sugars.[39] Recently, our
group discovered a template-free method to prepare Al2O3 and
Ga2O3 nanorods (NR) with high specific surface area and a high
population of accessible acid sites that act as catalytic sites for
activating hydrogen peroxide.[36,40] Here, we report for the first
time the catalytic performance of these aluminium and gallium
oxides as heterogeneous catalysts for the selective oxidation of
aromatic amines to azoxy derivatives with hydrogen peroxide as
the oxidant. We reasoned that Al2O3-NR and Ga2O3-NR could be
suitable catalysts for this reaction not only owing to their ability
to activate hydrogen peroxide but also because their acid sites
could catalyse the condensation of the phenhydroxylamine and
nitrosobenzene intermediates leading to the formation of
azoxybenzene (Scheme 1). The observed promising activity of
these catalysts was further optimised by screening and tuning (i)
the catalyst loading, (ii) the hydrogen peroxide source (aqueous
or anhydrous), and (iii) the use and amount of solvent. For the
optimum catalysts identified in this work (Ga2O3-NR), the
versatility in the conversion of different substituted anilines and
the reusability were demonstrated.
Results and Discussion
Figure 1. Characterisation of Ga2O3-NR: (A) TEM images, (B) XRD pattern, and (C) N2 adsorption-desorption isotherms.
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10.1002/cctc.201901378
ChemCatChem
hydrogen peroxide as the oxidant in an environmentally
approximately 0.63 acid sites per nm2.[36] With this figure in mind,
the turnover number (TON) of Ga2O3-NR in the oxidation of
aniline to azoxybenzene can be estimated to be 208. The
differences in selectivity observed between the tested catalysts
can be rationalised considering that the number and strength of
Lewis and Brønsted acid sites on the surface of aluminium and
gallium oxides depend not only on the intrinsic differences
between the two metals, but also on the synthesis method of the
materials, which determine surface area and morphology and
acceptable and inexpensive aprotic solvent as ethyl acetate
(Table 1). In the absence of catalyst, almost no conversion of
aniline was observed (Entry 1, Table 1). Among the tested Al
and Ga oxides, the ε-Ga2O3 polymorph in the form of small
nanorods (Ga2O3-NR) showed the highest aniline conversion
(49 % in 4 h, Table 1, Entry 7) with 92 % selectivity towards
azoxybenzene. Azobenzene, which is an industrially relevant
molecule used in the production of dyes, food additives,
pigments and therapeutic agents, was observed as the only side
product.[5,41] This catalytic activity is markedly superior compared
to that of Al2O3-NR and γ-Ga2O3, which showed moderate
conversion of aniline (26 and 21 %, respectively, see Entries 4
and 6 in Table 1), with azoxybenzene as the main product. On
the other hand, both types of commercial Al2O3 (Table 1, Entry 2
and 3), displayed significantly lower activity and nitrobenzene
was formed as a major product besides azoxybenzene and
azobenzene, which indicates the further oxidation of the
nitrosobenzene intermediate (Scheme 1). β-Ga2O3, which has
the lowest surface area, was virtually inactive in this reaction
(Table 1, Entry 5).
thus also the surface density of accessible coordinatively
40]
In this context, the higher selectivity
unsaturated sites.[36,
towards nitrobenzene observed with the two commercial
alumina indicates that these materials are relatively poor in acid
sites that are able to catalyse the condensation reaction
between the reaction intermediates (phenylhydroxylamine and
nitrosobenzene, see Scheme 1), which leads to their further
oxidation into nitrobenzene.
100
80
60
Aniline Conversion
Table 1. Catalytic performance of Al and Ga oxides in the oxidation of
Azobenzene Selectivity
Azoxybenzene Selectivity
aniline using 50 wt % aqueous H2O2.
40
Aniline Conversion ( leaching test)
20
0
0
4
8
12
16
20
24
28
Reaction Time (h)
Conv.
Surface
area
Sel.(%)
(%)
Entry
Catalyst
(m2/g)
I
II
-
III
-
IV
-
Figure 2. Conversion of aniline, selectivity of azo and azoxybenzene, and
leaching test with 50 wt % aqueous H2O2 as a function of reaction time
over Ga2O3-NR. The reaction conditions are the same as those in Table 1.
1
No catalyst
-
≤ 1
2
Al2O3-activated, neutral
Brockmann-1
155
11
28
34
39
3
4
5
6
7
Al2O3-nanopowder
171
376
13
14
26
≤ 1
21
49
38
86
-
37
14
-
25
-
Al2O3-NR
β-Ga2O3
γ-Ga2O3
The catalytic performance of Ga2O3-NR in terms of activity and
selectivity of the desired products for the oxidation of aniline was
further investigated as a function of reaction time (Fig. 2). The
conversion of aniline gradually reached 92 % in 20 h, after which
only a slight increase in aniline conversion was observed (94 %
in 26 h), suggesting the almost full consumption of hydrogen
peroxide. This was confirmed by the negligible amount of H2O2
found in the reaction mixture after 26 h, as determined by
titration with a 0.1 M CeIV solution. This indicates that a fraction
of H2O2 disproportionated into H2O and O2. Based on these data,
a 70 % efficiency of the utilisation of H2O2 in the oxidation of
aniline was achieved with the Ga2O3-NR catalyst. Azoxybenzene
was the main product at all stages of the reaction and its
selectivity increased rapidly in the first 4 h of reaction, after
which it slowly reached nearly full selectivity (98 %) after 26 h.
Remarkably, azobenzene was the only one side product and its
-
155
225
98
92
2
-
Ga2O3-NR
8
-
Reaction conditions: aniline (2 mmol), anisole (1 mmol, as GC internal
standard), 50 wt % H2O2 (4 mmol), catalyst (20 mg), ethyl acetate (4 g) as
solvent, 80 °C, 4 h.
These differences in activity and selectivity can be correlated to
the reaction mechanism, which involves both oxidation and
condensation reactions (Scheme 1). The higher activity of
Ga2O3-NR is attributed to the high, accessible surface area and
to the large surface density of coordinatively unsaturated sites
acting as acid sites that are able to activate hydrogen peroxide.
Both features stem from the nanorod morphology of this
material.[36] Previous work showed that Ga2O3-NR contains
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10.1002/cctc.201901378
ChemCatChem
selectivity decreased as a function of reaction time, suggesting
The formation of the corresponding azo-derivatives was
observed as the only minor side product. The behaviour was
different when
the further oxidation of azobenzene to azoxybenzene (Scheme
1). No products of the over-oxidation of aniline such as
nitrosobenzene and nitrobenzene were observed during the
course of reaction, whereas these side-products are commonly
observed with transition-metal-containing catalysts.[3,21,22]
Table 2: Oxidation of aromatic amines into their azoxy derivatives with 50 wt%
H2O2 catalysed by Ga2O3-NR.
Azoxy
product
Sel. (%)
Product
(Azoxybenzene)
Conv.
(%)
Entry Amines
Catalyst
1
Ga2O3-NR
49
58
92
96
O
N
N
Ga2O3-NR
2
3
Ga2O3-NR
Ga2O3-NR
54
22
97
93
4
5
6
Figure 3: Recycling test of Ga2O3-NR as catalyst. Reaction conditions:
aniline (2 mmol), anisole (1 mmol, as GC internal standard), 50 wt %
H2O2 (4 mmol), Ga2O3-NR catalyst (20 mg), ethyl acetate (4 g) as solvent,
80 °C, 4 h.
Ga2O3-NR
Ga2O3-NR
42
49
37
95
Besides achieving promising catalytic activity and selectivity, it is
crucial to demonstrate the truly heterogeneous nature and the
recyclability of the catalyst as these features are essential in
view of possible industrial application. For this purpose, a hot
filtration leaching test was performed (Fig.2, dotted line). The
catalyst was removed by hot filtration after 1 h of reaction when
the conversion of aniline was 26 %. The filtrate was allowed to
react for further 19 h reaching a final conversion of 28 %. This
increase in aniline conversion after 20 h of reaction when
compared to the 92 % conversion obtained after 20 h in the
presence of Ga2O3-NR (Fig. 2) indicates that no or negligible
leaching of active species occurs proving the truly
heterogeneous nature of the Ga2O3-NR catalyst. The
recyclability of Ga2O3-NR was evaluated in several consecutive
runs. The colour of Ga2O3-NR was slightly changed from white
to light brown at the end of the first catalytic test, which indicates
the adsorption of organic species on the surface of the catalyst.
In order to remove these organic species, the catalyst was
washed at 80 ᴼC with ethyl acetate and ethanol. By following
this procedure, the activity and selectivity of Ga2O3-NR were fully
retained in four consecutive runs (Fig. 3). This is in line with our
previous results in the epoxidation of alkenes.[36]
TS-1
2
96
7
Reaction conditions: amine (2 mmol), anisole (GC internal standard, 1
mmol), 50 wt% aqueous H2O2 (4 mmol), catalyst (20 mg), ethyl acetate (4g)
as solvent, 4 h, 80 °C.
4-methoxy aniline (para-anisidine) was used as substrate (Entry
5, Table 2). In such case, a 42% conversion was achieved but
the main product was the azo-derivative (52 % selectivity), with
the other two products being the azoxy-derivative (38
%
selectivity) and 4-amino-4’-methoxydiphenyl amine (10%
selectivity). The different selectivity in the conversion of 4-
methoxy aniline compared to other substituted anilines can be
ascribed to the increased electron density on the -NH2 group in
the 4-methoxy aniline due to the resonance effect of the
methoxy group. For what concerns the trend in activity, the
aromatic amines containing an electron donating group (Entries
2, 6, Table 2) or a weakly electron withdrawing group (Entry 3,
Table 2) were more effectively converted into the corresponding
The versatility of the Ga2O3-NR catalyst was tested in the
oxidation of a range of substituted aromatic amines containing
electron donor or acceptor groups on the phenyl ring (Table 2).
Under the employed reaction conditions, most substituted
amines were effectively and selectively (≥ 92 %) converted into
the corresponding azoxy-derivatives (Entries 2-4 and 6, Table 2).
azoxy-derivatives compared to those containing
a strong
electron withdrawing group as -NO2 (Entry 4, Table 2). No steric
effects were observed (compare Entry 1 and 6 in Table 2). It is
worth noting that the open structure of Ga2O3-NR allows
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10.1002/cctc.201901378
ChemCatChem
catalysing the conversion of bulky substrates, while
a
improvement in the conversion of aniline and selectivity of
azoxybenzene were observed as results of reducing the amount
of water. The conversion of aniline increased from 49 to 62 %,
and the selectivity of azoxybenzene from 92 to 98 %, by using
70 wt % aqueous H2O2 compared to 50 wt % aqueous H2O2. The
catalytic performance did not improve further with the use of
nearly anhydrous H2O2 in ethyl acetate, indicating that the
Ga2O3-NR can operate efficiently also in the presence of some
water.
benchmark catalyst as TS-1 showed very poor activity in the
conversion of 2-ethyl aniline due to diffusion limitations of this
reactant in the micropores (d = 0.56 nm)[42] of the zeolite
(compare Entries 6 and 7, Table 2).
With the purpose of fine-tuning of the performance of the Ga2O3-
NR catalyst, we investigated the effect of the catalyst loading, of
the H2O2 source and of the amount of solvent on the conversion
of aniline. The influence of the catalyst loading was studied over
5 mg, 10 mg, and 20 mg of Ga2O3-NR per mmol of aniline using
50 wt % aqueous H2O2 as the oxidant (Fig. 4). The conversion of
aniline and the selectivity towards azoxybenzene increase with
the catalyst loading, reaching 70 % aniline conversion with 94 %
azoxybenzene selectivity after 4 h reaction at 80 °C when
employing 20 mg of Ga2O3-NR loading per mmol of aniline.
Aniline Conversion
Azoxybenzene Selectivity
Azobenzene Selectivity
100
80
60
40
20
0
%
Aniline Conversion
Azoxybenzene Selectivity
Azobenzene Selectivity
100
80
60
40
20
0
50 % (aq.)
70 % (aq.)
H2O2
24.5 % (anhyd.)
%
Figure 5. Influence of the hydrogen peroxide source on the conversion of
aniline and on the selectivity towards azo and azoxybenzene over Ga2O3-
NR. Reaction conditions: aniline (2 mmol), anisole (1 mmol, as GC internal
standard), H2O2 (4 mmol), Ga2O3-NR catalyst (20 mg), ethyl acetate (4 g)
as solvent, 80 °C, 4 h.
5
10
Catalyst loading (mg/mmol of aniline)
20
According to the principles of green chemistry, benign solvents
or ideally no solvent should be employed in catalytic reactions.
Besides ethyl acetate, which was selected as a green solvent in
analogy to previous studies of epoxidation reactions catalysed
by Al2O3-NR[40] and Ga2O3-NR,[36] we tested dioxolane as
alternative solvent but achieved lower activity and selectivity
(Table S1). Further studies aimed at investigating if the reaction
could be carried out with no or a minimum amount of solvent.
This was studied with the Ga2O3-NR catalyst using 24.5 wt%
anhydrous H2O2 (in ethyl acetate) as oxidant. Decreasing the
amount of ethyl acetate led to a gradual increase in the
conversion of aniline, while retaining very high selectivity
towards azoxybenzene in all cases (Table 3). The conversion of
aniline obtained without using extra solvent (93 %, Entry 4,
Table 3) was significantly higher compared to the reaction with
the amount of ethyl acetate used in the rest of this work (Entry 1,
Table 3). These results indicate that with the Ga2O3-NR catalyst
the use of a solvent is detrimental, most likely as a consequence
of a dilution effect that decreases the interaction between aniline
and H2O2 on the surface of Ga2O3-NR. When considering the
option of not adding a solvent, it should be taken into account
that these experiments were carried out with anhydrous
hydrogen peroxide, which forms a single liquid phase with
aniline, whereas with aqueous hydrogen peroxide a biphasic
Figure 4. Influence of Ga2O3-NR catalyst loading on the conversion of
aniline and on the selectivity towards of azo and azoxybenzene. Reaction
conditions: 10 mg, 20 mg, and 40 mg of Ga2O3-NR, aniline (2 mmol),
anisole (1 mmol, as GC internal standard), 50 wt % H2O2 (4 mmol), ethyl
acetate (4 g) as solvent, 80 °C, 4 h.
Next, the effect of the H2O2 source was investigated. Hydrogen
peroxide is commercially available in aqueous solution with
different concentrations. The water supplied with the aqueous
H2O2 and that formed during the reaction due to the utilisation or
decomposition of H2O2, can influence the catalyst activity and
selectivity. For example, it is well-documented that water is
detrimental to the catalytic performance of Al2O3 in the liquid
phase oxidation of alkenes with H2O2.[33,43–45] This effect has
been attributed to the absorption of H2O on the catalyst surface,
which hinders the access of H2O2 to the active sites.
Considering the similarities between aluminium and gallium
oxide,[36,45] it is worthwhile to investigate the influence of water
on the oxidation of aniline over Ga2O3-NR. Therefore, we
performed the oxidation of aniline by using three sources of
H2O2: 50 wt % aqueous, 70 wt % aqueous, and 24.5 wt % of
nearly anhydrous H2O2 in ethyl acetate (Fig. 5). Significant
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10.1002/cctc.201901378
ChemCatChem
liquid system would be formed with aniline in the absence of a
nitrosobenzene (step 4) through heterolytic cleavage of the O-O
bond. The acid sites at the surface of Ga2O3-NR are also
expected to catalyse the subsequent formation of azoxybenzene
through the condensation of the phenylhydroxylamine and
nitrosobenzene intermediates (step 5).
solvent. In order to explore further the catalytic activity of Ga2O3-
NR without adding a solvent, we tested the catalyst at lower
temperature. Notably, Ga2O3-NR displayed significant activity
also at 50 and even at 25 °C (Entry 4 and 5, Table 3), with only
a
minor decrease in selectivity towards azoxybenzene.
Moreover, the activity and selectivity of the Ga2O3-NR were fully
retained upon recycling also under these conditions (Entry 7,
Table 3), in line with the results obtained with solvent (Figure 3).
Table 3: Effect of solvent nature and amount on the conversion of
aniline and selectivity of azo and azoxybenzene with 24.5 wt % H2O2
(anhydrous, in ethyl acetate) over Ga2O3-NR.
Ethyl
acetate
(g)
T
Azobenzene
Sel. (%)
Azoxybenzene
Sel.(%)
Entry
1
(°C)
Conv.(%)
61
4
80
2
98
2
3
2
80
80
80
50
25
50
50
63
69
93
47
22
46
46
2.2
1.9
3
98
98
97
96
94
95
96
1
4
No solvent
No solvent
No solvent
No solvent
No solvent
5
4
6
6
7a
8b
5
4
Reaction conditions: aniline (2 mmol), anisole (1 mmol) as GC internal
standard, 24.5 wt % anhydrous H2O2 in ethyl acetate (4 mmol), Ga2O3-NR
(20 mg), 4h. a With reused catalyst. b In the presence of TEMPO (1 mol %
relative to H2O2) as radical scavenger.
Scheme 2. Proposed catalytic cycle for the oxidation of aniline with
H2O2 over Ga2O3-NR.
Conclusions
Generally, the catalytic activation of hydrogen peroxide can
occur through the formation of metal (hydro)peroxo species
followed by heterolytic cleavage of the O-O bond, or follow a
homolytic pathway involving the formation of hydroxyl radicals
(˙OH).[46] It has been extensively reported that the epoxidation of
alkenes with H2O2 over Al2O2 or Ga2O3 catalysts proceeds
through the formation of hydroperoxo species.[36,40,47] In order to
gain more insight in the way in which Ga2O3-NR activates H2O2
towards the oxidation of aniline, a control experiment was
performed in the presence of TEMPO (1 mol% to H2O2) as a
redical scavenger. No difference was observed in the conversion
of aniline and in the selectivity towards azoxybenzene with or
without adding TEMPO (compare entry 5 and 8, Table 3). This
result indicates that the catalytic mechanism over Ga2O3-NR
does not involve radical species, thus ruling out homolytic
cleavage of the peroxide bond. On the basis of these
observations, a catalytic mechanism for the oxidation of aniline
over Ga2O3-NR can be proposed (Scheme 2). Ga-OH groups
located at the surface of the nanorods activate H2O2 through the
formation of surface gallium hydroperoxide (Ga-OOH) species
A set of Al and Ga oxides with different morphologies and crystal
structures were studied as transition-metal-free heterogeneous
catalysts for the oxidation of aromatic amines to azoxy
derivatives using the environmentally friendly H2O2 as the
oxidant. Gallium oxide nanorods displayed the most promising
catalytic performance compared to the other transition-metal-
free oxides, achieving 49 % conversion of aniline with 92 %
selectivity towards azoxybenzene after 4 h of reaction at 80 °C,
and reaching 94% conversion and nearly complete selectivity
towards azoxybenzene (98%) at longer reaction times. The
versatility of Ga2O3-NR was demonstrated in the conversion of
substituted and bulky aromatic amines into their corresponding
azoxy derivatives. Recycling tests demonstrated that the catalyst
could be efficiently reused in consecutive catalytic cycles. The
excellent catalytic results obtained with Ga2O3-NR are ascribed
to the combination of high surface area, open structure and
large surface density of coordinatively unsaturated sites acting
as acid sites that stem from the nanorod morphology.
Additionally, the activity of Ga2O3-NR could be substantially
improved by using 70 % aqueous or 24.5 wt% anhydrous
hydrogen peroxide instead of 50 % aqueous H2O2 as the oxidant
(steps
1 and 3), which are able to oxidise aniline to
phenylhydroxylamine (step 2) and phenylhydroxylamine to
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10.1002/cctc.201901378
ChemCatChem
and by minimising the amount of solvent used in the catalytic
gallium chloride under stirring at room temperature. As a result, a dark
brown solution was formed while HCl gas evolved. The solution was
stirred for 10 min and then 17.23 g of 2-butanol were added dropwise
using a pressure-equalising dropping funnel over 30 min. The solution
was stirred for further 1 h to obtain a homogeneous solution with a dark
orange-brown colour. Next, a homogeneous solution of 2.94 g of milli-Q
water and 7.23 g of 2-butanol were added dropwise using a pressure-
tests. Under these optimised conditions, Ga2O3-NR was active
also under very mild conditions (25 or 50°C). A test in the
presence of a radical scavenger demonstrated that the reaction
proceeds through the heterolytic cleavage of the activated
peroxide species. In summary, this work introduces an effective
and attractive catalytic system for the synthesis of (substituted)
azoxy compounds and at the same time represents an advance
in the exploration of the scope of applications of gallium oxide
nanorods.[36],[48]
equalising dropping funnel over
1 h and stirred for 3 h at room
temperature. Finally, the resultant solution was stirred for 23 h at 70 °C
and aged for 3 days at room temperature, yielding the gallium oxide
nanorods in the form of a white solid precipitate. The obtained solid was
separated by centrifugation (4500 rpm) and thoroughly washed with
absolute ethanol (45 ml x 5 times) and then dried at 80 °C for overnight.
A typical yield of the gallium oxide nanorods was 18 %, assuming a
product with the formula of Ga2O3.
Experimental Section
Materials
γ-Ga2O3 was synthesised according to a procedure from the literature.[49]
Briefly, 3.0 g of high-purity gallium nitrate (Strem Chemicals) were mixed
in 50 ml of ethanol. Then, a solution of aqueous ammonia (25 wt %)
diluted in ethanol (50 vol %) was added dropwise under continuous
stirring at room temperature until no further precipitation was observed.
The resulting precipitate was filtered, thoroughly washed with ethanol,
and dried in vacuum at 60 °C for overnight. Finally, the white solid was
calcined in air at 500 °C for 1 h with a heating rate of 3 °C min-1, leading
to a typical yield of approximately 96 %, assuming the product formula of
Ga2O3.
Anhydrous granular gallium chloride (GaCl3, 99.999%) was purchased
from Strem Chemicals Inc. Aluminium-tri-sec-butoxide (Al(OBus)3, 97%),
aniline (99.5%), anisole (99 %), 50 wt% aqueous H2O2 were obtained
from Sigma-Aldrich. The substituted anilines and 2-butanol (99%) were
obtained from Acros Chemicals. Aqueous ammonia (25 wt%), ethanol,
ethyl acetate (99.95%) were purchased from Boom chemicals. β-Ga2O3
(≥ 99.99 % trace metal basis); γ-Al2O3 activated, neutral, Brockmann I;
and γ-Al2O3 nanopowder were purchased from Sigma-Aldrich. 70 wt %
aqueous H2O2 was provided by Peróxidos do Brasil (Solvay) in Brazil, for
academic research. 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO,
Sigma-Aldrich) was used as radical scavenger.
Catalyst Characterisation
Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8
Advance diffractometer with a Cu Kα (λ= 0.15406 nm) radiation with a
beam voltage of 40 kV and a beam current of 40 mA. The measurements
were carried out in the region of 2θ = 10 to 80 ° with a scanning rate of
1.0 °/min, and the International Centre for Diffraction Data (ICDD) was
employed to assign the XRD patterns. N2-sorption experiments were
performed using a Micromeritics ASAP-2020 instrument using N2 as
adsorbate at -196 °C. The Brunauer–Emmett–Teller (BET) method was
used to calculate the specific surface area. Prior to the measurement, the
samples were degassed under reduced pressure at 120 °C for 12 h
(5 °C/min). Fourier transform infrared (FT-IR) spectra were measured on
an IRTracer-100 spectrometer by averaging 32 scans with a spatial
resolution of 4 cm-1.Transmission electron microscopy (TEM) images
were recorded on the Philips CM12 microscope operating at 120 kV.
Prior to the measurement, the samples were suspended in ethanol by
sonication for 5 min and then deposited onto a holey carbon film on a Cu
grid.
Catalyst Synthesis
Aluminium oxide nanorods (Al2O3-NR) were synthesised according to the
protocol developed by some of us and reported in detail recently.[40] In a
typical synthesis, aluminium tri-sec-butoxide (1.23 g,
5 mmol) was
weighed in a 25 ml round bottle flask, then 2-butanol (1.60 g) was added
dropwise within 10 min. The obtained clear solution was stirred for 30
min. After that, a solution of ammonium hydroxide (25 wt% aqueous) and
absolute ethanol (1:1 v/v, 1.6 ml) was added dropwise over 30 min. The
resulting white gel was stirred for 1.5 h and then heated at 70 °C for 23 h.
Then, the obtained material was aged for 3 days at ambient temperature
while stirring. Finally, the white precipitate was collected by centrifugation,
thoroughly washed with absolute ethanol, and dried overnight in an oven
at 80 °C in air. Finally, the powder was calcined in air at 400 °C for 10 h
with
a heating rate of 3 a typical yield of
°C min-1, leading to
approximately 82 %, assuming the product formula of Al2O3.
Gallium oxide nanorods were prepared according to a modified version of
a method previously reported by some of us.[36] This new protocol allows
a precise control of the morphology and size of the nanorods, which is
achieved by the adjustment of the addition of H2O and 2-butanol. The
required amount of anhydrous granular gallium chloride (99.999%) was
weighed in a glove box under N2 atmosphere because of its highly
hygroscopic and deliquescent nature. In a typical synthesis, 4.80 g of 2-
Catalytic tests
The catalytic tests for the conversion of aromatic amines into their azoxy
derivatives were carried out in a 48-well parallel reaction block that
allows the individual magnetic stirring of each well. In a typical reaction
procedure, 20 mg of the catalyst, 4 g of ethyl acetate, and 4 mmol of
aqueous hydrogen peroxide were mixed in a 10 ml glass vial. Then, a
solution of 2 mmol of amines and 1 mmol of anisole as GC internal
standard was added to the mixture. The obtained liquid is monophasic.
butanol were added dropwise using
a pressure-equalising dropping
funnel within 5 min into a 50 ml round bottom flask containing 15 mmol of
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10.1002/cctc.201901378
ChemCatChem
The vials were capped with aluminium crimp seals with PTFE septa and
protocol was repeated again two times. Thereafter, the sample was
transferred into a 50 ml centrifuge tube and dispersed in 40 ml of
absolute ethanol. Then, the sample was centrifuged for 20 min at 4200
rpm to deposit the catalyst and the supernatant ethanol solution was
removed. The washing procedure was repeated two times. Finally, the
catalyst was dried overnight at 110 ⁰C and reused for further catalytic
tests. This described washing protocol was repeated for each recycling
test. All recyclability tests were performed in triplicate and average values
for conversion and selectivity were reported (the standard deviation in the
conversion of aniline and selectivity towards azoxybenzene was 3 %).
placed each in a different position in the 48-well parallel reaction block.
Then, each reaction mixture was stirred vigorously (900 rpm) at 80 °C for
4 h. Selected catalytic tests were performed in duplicate or in triplicate. In
such cases, the mean values for conversion and selectivity was reported
(the standard deviation in the conversion of aniline and selectivity
towards azoxybenzene was 2 %). Three types of hydrogen peroxide
were employed in these tests (always using the same molar ratio
between H2O2 and aniline): 50 wt% aqueous solution, 70 wt% aqueous
solution and 24.5 wt% anhydrous in ethyl acetate. It should be noted that
the concentration of hydrogen peroxide was almost the same (ca. 3 wt%
of the reaction mixture) in all the catalytic tests. The catalytic tests with
70 wt% aqueous H2O2 were performed in Brazil. 24.5 wt% anhydrous
H2O2 in ethyl acetate was prepared by removing water from the
homogeneous solution of 20 ml of 70 wt% aqueous H2O2 and 188 ml of
ethyl acetate by means of a Dean-Stark trap at 110 °C. Special attention
should be dedicated to safety during this preparation, because
anhydrous hydrogen peroxide in flammable organic solvents should be
considered as dangerous and explosive.[50] Therefore, the preparation
was carried out in a fume hood with a well ventilated preparation set-up
in which the Dean-Stark trap was connected with a water condenser with
open lid to release the oxygen generated by hydrogen peroxide
decomposition and the possible explosive gas mixtures of solvent and
oxygen. The concentration of H2O2 was determined by titration of the
solution with a 0.1 M solution of Ce(SO4)2.
Leaching tests were performed under the general reaction conditions
employed for the catalytic tests (vide supra). The catalyst was removed
from the reaction mixture by centrifugation after 1 h reaction at 80 °C. A
small aliquot of the filtrate was analysed by GC and the rest was allowed
to react at 80 °C. An aliquot of the reaction solution was taken at the time
intervals of 4 h and 20 h and analysed by GC.
Acknowledgements
The authors acknowledge sponsoring in the frame of project
G0D9313N from FWO (Flemish Science Foundation, Belgium)
and of the related project 490043/2013-5 from CNPq (National
Council for Scientific and Technological Development, Brazil).
We thank Dr. Marc Stuart for assistance in the TEM
measurements. We thank Yehan Tao for useful discussion. We
also thank Leon Rohrbach, and Jan Henk Marsman for technical
and analytical support.
Conversion and selectivity were determined by gas chromatography
(GC) on an Agilent 7890 B gas chromatograph, equipped with DB-5
fused silica column (15 m; 0.32 mm) and low thermal module (LTM) or
on an Agilent 7890A gas chromatograph, equipped with an HP 5 capillary
column (30 m; 0.25 mm). The products were identified by comparison
with standard solutions or by gas chromatography-mass spectroscopy
analysis (GC-MS) on an Agilent Hewlett-Packard-HP 6890 gas
chromatograph coupled with an Agilent Hewlett-Packard 5973 MSD
mass spectrometer, equipped with Rxi®-5 Sil MS capillary column (30 m;
0.25 mm). Since 2 mol of amine are required for the formation of 1 mol of
the corresponding azo and azoxy-derivatives, the conversion of aromatic
amines and selectivity of azo and azoxy derivatives were calculated
using the following equations:
Keywords: Transition-metal-free oxide, aniline oxidation,
azoxybenzene, hydrogen peroxide, nanorods
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Gallium oxide nanorods with small
dimensions (9-52 nm length and 3-5
nm width) and fully accessible, high
surface area (225 m2 g-1) displayed
high activity and selectivity as
heterogeneous catalyst for the
oxidation of aromatic amines to their
azoxy derivatives with hydrogen
peroxide as environmentally benign
oxidant.
Bhawan Singh, Dalmo Mandelli, Paolo
P. Pescarmona*
Page No. – Page No.
Efficient and selective oxidation of
aromatic amines to azoxy
derivatives over transition metal-
free oxide catalysts with nanorods
morphology
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