Novel molybdenum–cerium based heterogeneous catalysts for efficient oxidative coupling of benzylamines under eco-friendly conditions

Article abstract of DOI:10.1007/s11164-015-2332-7

Abstract: This work demonstrates that MoO_x/CeO₂–ZrO₂ solid acid acts as a promising catalyst in the solvent-free oxidative-coupling of benzylamine into dibenzylimine with O₂ as the oxidant. The benzylamine conversions were found to be ~27, 51, 74, and 99?% for CeO₂, CeO₂–ZrO₂, MoO_x/CeO₂, and MoO_x/CeO₂–ZrO₂ catalysts, respectively. The MoO_x/CeO₂–ZrO₂ catalyst also efficiently converts various substituted- and secondary-benzylamines under solvent-free conditions with O₂ resulting in moderate to excellent yields. Remarkably, a high selectivity of ~99.6–99.9?% towards the dibenzylimine product was found for all reaction conditions. The outstanding performance of MoO_x/CeO₂–ZrO₂ catalyst was attributed to high specific surface area, improved structural defects, and superior acid properties. Graphical Abstract: [Figure not available: see fulltext.] The MoO_x/CeO₂–ZrO₂ catalyst show an outstanding performance in the oxidation of benzylamine and its derivatives, which is attributed to abundant acidic sites, large number of oxygen vacancies, and superior BET surface area.

Full text of DOI:10.1007/s11164-015-2332-7

Res Chem Intermed
DOI 10.1007/s11164-015-2332-7
Novel molybdenum–cerium based heterogeneous
catalysts for efficient oxidative coupling
of benzylamines under eco-friendly conditions
1
2
Agolu Rangaswamy
1
Bolla Govinda Rao
•
Putla Sudarsanam
Benjaram M. Reddy
•
1
•
Received: 28 August 2015 / Accepted: 20 October 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract This work demonstrates that MoO /CeO –ZrO solid acid acts as a
x
2
2
promising catalyst in the solvent-free oxidative-coupling of benzylamine into diben-
zylimine with O as the oxidant. The benzylamine conversions were found to be *27,
2
5
respectively. The MoO /CeO –ZrO catalyst also efficiently converts various substi-
1, 74, and 99 % for CeO , CeO –ZrO , MoO /CeO , and MoO /CeO –ZrO catalysts,
2 2 2 x 2 x 2 2
x
2
2
tuted- and secondary-benzylamines under solvent-free conditions with O resulting in
2
moderate to excellent yields. Remarkably, a high selectivity of *99.6–99.9 % towards
the dibenzylimine product was found for all reaction conditions. The outstanding per-
formance of MoO /CeO –ZrO catalyst was attributed to high specific surface area,
x
2
2
improved structural defects, and superior acid properties.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2332-7)
contains supplementary material, which is available to authorized users.
&
Benjaram M. Reddy
bmreddy@iict.res.in; mreddyb@yahoo.com
1
2
Inorganic and Physical Chemistry Division, CSIR – Indian Institute of Chemical Technology,
Uppal Road, Hyderabad 500 007, India
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied
Sciences, RMIT University, Melbourne, VIC 3001, Australia
123

A. Rangaswamy et al.
Graphical Abstract
The MoO /CeO –ZrO catalyst show an outstanding performance in the oxidation of
x
2
2
benzylamine and its derivatives, which is attributed to abundant acidic sites, large
number of oxygen vacancies, and superior BET surface area.
Keywords Benzylamine oxidation Á Molecular oxygen Á Molybdenum Á Cerium Á
Zirconium
Introduction
The oxidative coupling of amines to imines is an important organic transformation
in chemical synthesis. Imines are crucial building blocks for the synthesis of dyes,
fragrances, fungicides, agrochemicals, oxazolidines, pharmaceuticals, nitrones,
amides, chiral amines, hydroxyamines, anti-inflammatory agents, lipoxygenase
inhibitors, anti-cancer agents, polymer compounds, and biologically active
compounds [1–14]. Additionally, imines are key intermediates in a number of
reactions, such as addition, reduction, aziridination, b-lactamization, cyclization,
condensations, and aza-Diels–Alder reactions [4]. The oxidative coupling of amines
is traditionally performed with stoichiometric amounts of high valent inorganic
oxidants, such as chromate or permanganate, as well as with organic oxidants, like
2
-iodoxybenzoic acid and N-tert-butylphenylsulfinimidoylchloride [15–18]. The use
of these oxidants leads to the generation of large amounts of metal/organic waste,
which is undesirable from the viewpoint of the 12 Green Chemistry Principles. On
the other hand, oxidative coupling of amines using O as the oxidant has received
2
tremendous attention in recent years [14, 19]. O is an abundant and green oxidant,
2
and gives only water as the by-product in the oxidation reactions [4, 20].
Several catalysts based on Pd [1, 11, 21, 22], Pt [23], and Au [10, 24] have been
reported for the oxidation of amines. The requirement of large catalyst quantities
and the expensive nature of these metals limit their applicability in the chemical
industry. It has been recently demonstrated that ceria-based materials act as efficient
catalysts in the selective oxidative coupling of various benzylamines [4]. The
properties, such as high surface area, enriched structural defects, and importantly,
abundant acid properties are found to be the key factors for the observed catalytic
1
23

Novel molybdenum–cerium based heterogeneous catalysts for…
efficiency of ceria-based catalysts. It is therefore believed that, by modifying the
properties of ceria-based materials, their catalytic efficiency can be improved for the
oxidative coupling of amines [25, 26].
In this work, we have synthesized MoO /CeO and MoO /CeO –ZrO catalysts along
x
2
x
2
2
with the CeO and CeO –ZrO using facile preparation methods, and their catalytic
2
2
2
efficiency was evaluated for the oxidation of benzylamine using O under solvent-free
2
conditions. MoO based materials are widely used catalysts in several reactions because of
x
their strong acid properties [27–29]. For this reason, MoO was selected as a promoter in
x
this study. Much attention has been paid to correlate the physicochemical properties of the
catalysts with their catalytic efficiency for the benzylamine oxidation.
Experimental
Catalyst preparation
The CeO –ZrO (1:1, mole ratio) binary oxide was prepared by a simple and
2
2
economical coprecipitation method. The desired amounts of Ce(NO ) Á6H O
3
3
2
(
dissolved separately in double distilled water and mixed together under mild stirring
Aldrich, AR grade) and ZrO(NO ) ÁXH O (Fluka, AR grade) precursors were
3
2
2
conditions. Aq. NH solution (2.5 w/w%) was added slowly to the above solution
3
until the pH of the solution reached *8.5. The resulting yellowish precipitates were
filtered off and washed with double distilled water several times to remove the anion
impurities. Finally, the obtained cake was dried at 393 K for 12 h and calcined at
7
73 K for 5 h in an air atmosphere.
To synthesize 10 wt% MoO supported on CeO –ZrO , the required quantity of
3
2
2
CeO –ZrO was dispersed in deionised water under stirring conditions at room
2
2
temperature. The requisite amounts of (NH ) Mo O Á4H O were dissolved in
4
6
7
24
2
deionised water and mixed with CeO –ZrO solution under stirring conditions. The
2
2
remaining procedure follows the same as the above. Pure CeO and MoO /CeO are
2
x
2
also prepared using the same procedure.
Catalyst characterization
The physicochemical characterization of the catalysts was undertaken using X-ray
diffraction (XRD), Raman spectroscopy, X-ray photo electron spectroscopy (XPS),
Barrett–Joyner–Halenda (BJH) pore size distribution, Brunauer–Emmett–Teller
(
BET) surface area, and NH -temperature programmed desorption (TPD) tech-
3
niques. The detailed procedure related to characterization methods used in this work
is presented in the supporting information.
Catalytic activity
Catalytic performance of the samples was studied for the oxidation of benzylamine
with O as the oxidant under solvent-free conditions. Reactions were conducted in a
2
1
0 mL round bottom flask by taking 0.2 mmol of substrate and 100 mg of catalyst
123

A. Rangaswamy et al.
-
under the O flow (20 mL min ). After completion of the reaction, the catalyst and
1
2
the products were separated by centrifugation. The liquid products were confirmed
by GC–MS equipped with a DB-5 capillary column and a flame ionization detector
(
(
FID). Samples were taken periodically and analyzed by GC equipped with a BP-20
wax) capillary column and a FID.
Results and discussion
Characterization studies
The XRD patterns of the samples are shown in Fig. 1. As shown in Fig. 1, the XRD
patterns of the samples can be indexed to (111), (200), (220), (311), (222), and (400)
reflections, which indicates the presence of fluorite cubic structured ceria in the
synthesized samples [30–36]. Interestingly, no XRD peaks related to MoO , t-ZrO ,
3
2
and m-ZrO2 phases were found for CeO –ZrO , MoO /CeO , and MoO /
2
2
3
2
3
CeO –ZrO samples. This observation may be due to either the amorphous nature
2
2
of the respective metal oxides or incorporation of Zr and Mo into the CeO lattice.
2
The average crystallite sizes of CeO , CeO –ZrO , MoO /CeO and MoOx/
2
2
2
2
x
CeO –ZrO samples were found to be *8.9, 4.7, 6.5, and 3.9 nm, respectively
2
2
(
Table 1). It is obvious that the addition of Mo and Zr to CeO leads to a decrease in
2
its crystallite size. This interesting observation reveals the existence of a strong
interaction between the Ce and Mo/Zr in the final oxide matrix.
Raman analysis is an efficient method to understand the structural defects in the
metal oxides because of its sensitive nature towards the M–O vibration of the metal
oxides. Figure 2 shows Raman spectra of the samples. All the samples show a
-
1
prominent Raman peak at around 460 cm , which corresponds to the F2g active
mode of fluorite structured ceria [4, 37, 38]. This observation well supports the XRD
results (Fig. 1). As observed in XRD studies, here also no Raman bands
corresponding to MoO3 and ZrO phases were found for CeO –ZrO , MoO /
2
2
2
x
CeO , and MoO /CeO –ZrO samples. It is interesting to note that only the MoO /
x
2
x
2
2
Fig. 1 Powder XRD patterns
of CeO , CeO –ZrO , MoO
CeO and MoO /CeO –ZrO
catalysts
MoO /CeO -ZrO
2
x
2
2
2
2
x
/
2
x
2
2
MoO /CeO2
x
CeO -ZrO2
2
CeO2
70
2
0
30
40
2
50
60
o
Theta ( )
1
23

Novel molybdenum–cerium based heterogeneous catalysts for…
2
Table 1 BET surface area (S), average crystallite size (D), pore volume (V), and pore size (P) of CeO ,
CeO
2
–ZrO
2
, MoO /CeO
x
2
and MoO
x
/CeO
2
–ZrO
2
catalysts
2
S (m g
-1
a
3 -1 b
V (cm g )
b
Sample
)
D (nm)
P (nm)
CeO
2
2
41
82
50
87
8.9
4.7
6.5
3.9
0.113
0.079
0.083
0.084
9.82
3.38
4.37
3.69
CeO
–ZrO
2
MoO
MoO
a
x
/CeO
/CeO
2
x
2
–ZrO
2
From XRD spectra
b
From BJH analysis
Fig. 2 Raman spectra of
F2g Band
CeO
CeO
catalysts
2
, CeO
and MoO
2
–ZrO
2
, MoO
x
/
2
x
/CeO
2
–ZrO
2
MoO /CeO -ZrO
x
2
2
Oxygen vacancy
band
MoO /CeO2
x
CeO -ZrO2
2
CeO2
2
00
300
400
500
600
700
-
1
Raman shift (cm )
-
1
CeO –ZrO sample exhibits a broad Raman band centred at *642 cm . This band
2
2
reveals the presence of structural defects, such as oxygen vacancies in the MoOx/
CeO –ZrO sample [4, 28]. The formation of oxygen vacancies in MoOx/
2
2
CeO –ZrO catalyst can be explained by combined effect of two factors: (1) doping
2
2
of Zr into the CeO lattice, which can lead to generation of oxygen vacancies due to
2
4?
4?
the difference in the ionic radii of dopant Zr (0.084 nm) and host Ce (0.111 nm)
and (2) the existence of interaction between the MoO and CeO –ZrO , which can
influence the Ce–O bond to create the oxygen vacancies in the CeO lattice.
x
2
2
2
The obtained N adsorption–desorption isotherms showed that all the samples
2
exhibit type 4 isotherms with a H hysteresis loop (Fig. 3) [39–41]. This observation
1
reveals the mesoporous nature of the samples. The achieved pore diameters are
3
-1
0.113, 0.079, 0.083, and 0.084 cm g for CeO , CeO –ZrO , MoO /CeO and
2 2 2 x 2
*
MoO /CeO –ZrO catalysts, respectively (Table 1). As well, the pore volumes of
x
2
2
the CeO , CeO –ZrO , MoO /CeO and MoO /CeO –ZrO catalysts are *9.82,
2
2
2
x
2
x
2
2
3
.38, 4.37, and 3.69 nm, respectively (Table 1). Interestingly, the BET surface area
of the cerium oxide was significantly improved after the addition of Mo and Zr, in
line with the achieved crystallite sizes (Table 1). The BET surface areas are found
123

A. Rangaswamy et al.
70
60
50
40
30
20
10
60
CeO2
MoO /CeO2
3
50
40
30
20
0.2 0.4 0.6 0.8 1.0
0.2 0.4 0.6 0.8 1.0
7
6
5
4
3
2
0
0
0
0
0
0
60
50
CeO -ZrO2
MoO /CeO -ZrO
3 2 2
2
40
30
20
0
.2 0.4 0.6 0.8 1.0
0.2 0.4 0.6 0.8 1.0
0
Relative pressure (p/p )
Fig. 3 N
catalysts
2
adsorption–desorption isotherms of CeO
2
, CeO
2
–ZrO , MoO
2
x
/CeO
2
and MoO
x
/CeO
2
–ZrO
2
2
-1
to be *41, 82, 50, and 87 m g , respectively, for CeO , CeO –ZrO , MoO /CeO
2
2
2
x
2
and MoO /CeO –ZrO catalysts.
Figure 4 shows HRTEM images of CeO , CeO –ZrO , MoO /CeO and MoO /
x
2
2
2
2
2
x
2
x
CeO –ZrO catalysts. It can see from HRTEM images that the synthesized catalysts
2
2
are highly crystalline materials in the nanoscale range. The lattice fringes for all the
samples are clearly visible with an estimated d-spacing of 0.31 nm, which is
attributed to CeO (111) plane [4]. Careful analysis of these figures disclosed that
2
the particle sizes of CeO and CeO –ZrO were in the range of 6–12 nm. A careful
2
2
2
observation of MoO /CeO and MoO /CeO –ZrO samples revealed the existence
x
2
x
2
2
of highly dispersed MoO species with much smaller sized particles (*1–1.5 nm)
3
on the surface of the supports.
XPS studies were performed to find the oxidation states of elements present in the
synthesized catalysts. Figure 5 shows Ce 3d core level XP spectra of the prepared
samples. It can be noted from Fig. 5 that the Ce 3d XP spectra are complex and
composed of several peaks [37, 42, 43]. Hence, as per the literature, the peaks are
labelled with u and v, which correspond to Ce 3d5/2 and Ce 3d3/2 contributions,
0
0
3?
respectively. The peaks denoted by u , v , u and v represent the Ce and the bands
0
0
0
0
000
00
000
4?
labelled with u, u , u , v, v , and v indicate the Ce [44, 45]. This observation
3
?
4?
confirms the coexistence of both Ce and Ce species in the synthesized samples.
Interestingly, the XPS spectrum of the cerium oxide was significantly shifted to
lower binding energies after the addition of Zr and Mo. This finding reveals that the
Zr and Mo influence the chemical environment of ceria. The Mo 3d XP core level
spectra of the MoO /CeO and MoO /CeO –ZrO samples show two bands at about
x
2
x
2
2
1
23

Novel molybdenum–cerium based heterogeneous catalysts for…
CeO
2
2 2
CeO -ZrO
d-spacing = 0.31 nm
CeO (111)
d-spacing = 0.31 nm
CeO (111)
2
2
MoO
x
/CeO
2
MoO /CeO -ZrO
x
2
2
d-spacing = 0.31 nm
d-spacing = 0.31 nm
CeO2 (111)
2
CeO (111)
MoO
3
MoO
3
2 2 2 x 2 x 2 2
Fig. 4 HRTEM images of CeO , CeO –ZrO , MoO /CeO and MoO /CeO –ZrO catalysts
Fig. 5 Ce 3d XP spectra of
MoO /CeO -ZrO
2
Ce 3d
x
2
CeO
CeO
2
, CeO
and MoO
2
–ZrO
/CeO
2
, MoO
x
/
2
x
2
–ZrO
2
x
MoO /CeO2
catalysts
CeO -ZrO2
2
v
/
v
///
u
u0
CeO2
v
/
u
v0
///
u
//
u
//
v
880
890
900
910
920
Binding Energy (eV)
*232.20 (3d5/2) and 235.32 eV (3d3/2) (Fig. S1 of supplementary material). The
presence of these binding energies indicates the existence of Mo in ?6 oxidation
state [27]. The achieved Zr 3d XP spectra of CeO –ZrO and MoO /CeO –ZrO
2
2
2
x
2
catalysts show two bands at around 181.65 (3d5/2) and 184.03 eV (3d3/2), indicating
4
?
the presence of Zr ions (Fig. S2 of supplementary material). A careful observation
of O 1s spectra of the samples indicate the presence of two bands, corresponding to
different oxygen species (Fig. S3 of the supplementary material). The band appears
123

A. Rangaswamy et al.
Fig. 6 NH
MoO /CeO
CeO –ZrO
3
-TPD profiles of
x
2
and MoO
catalysts
x
/
2
2
MoO /CeO -ZrO
2
3
2
MoO /CeO
3
2
200
400
600
800
Temperature (K)
at a lower binding energy (*529.5 eV) indicates lattice oxygen [46–49]. The band
at the higher binding energy (532.2 eV) can be assigned to surface hydroxyl groups.
The NH -TPD analysis was performed to estimate the acidic properties of the
3
catalysts because of the highly sensitive nature of NH towards the acidic sites
3
present on the surface of the catalysts [4, 27, 48, 49]. Figure 6 shows the NH -TPD
3
profiles of the samples. The achieved profiles can be defined as low-temperature
(LT) and high-temperature (HT) regions, corresponding to before and after 673 K,
respectively. The LT region indicates the release of NH from the weak acid sites,
3
whereas the HT region reveals desorption of NH from the strong acidic sites
3
present on the catalyst surface. It is noted from Fig. 6 that the MoO /CeO –ZrO
x
2
2
2
sample shows a higher intensity HT peak compared with that of the MoO /CeO
x
sample. This significant result confirms the presence of more amounts of strong
acidic sites in the MoO /CeO –ZrO sample.
x
2
2
Catalytic activity studies
Benzylamine was chosen as a model substrate to perform the oxidative coupling of
amines over CeO , CeO –ZrO , MoO /CeO and MoO /CeO –ZrO catalysts. The
reaction was performed at 383 K for 6 h using O as the oxidant under solvent-free
2
2
2
x
2
x
2
2
2
conditions. The achieved results are shown in Fig. 7. The catalytic efficiency of
ceria was remarkably improved after the addition of Zr and Mo. Amongst them, the
MoO /CeO –ZrO catalyst exhibited the highest benzylamine conversion, which
x
2
2
indicates the co-promoting effect of Mo and Zr on the catalytic efficiency of CeO2.
The achieved benzylamine conversions are *99, 74, 51, and 27 % for the MoOx/
CeO –ZrO , MoO /CeO , CeO –ZrO and CeO catalysts, respectively (Fig. 7). On
2
2
x
2
2
2
2
the other hand, low conversion of benzylamine (8 %) was found under blank
conditions, which indicate the necessity of a catalyst in the oxidative coupling of
benzylamine. Remarkably, a high selectivity of dibenzylimine (99.6–99.9 %) was
found, along with negligible amounts of benzonitrile (0.1–0.4 %) for all the
catalysts. This observation indicates the importance of the present study in selective
oxidation catalysis.
1
23

Novel molybdenum–cerium based heterogeneous catalysts for…
Fig. 7 Aerobic oxidation of benzylamine over CeO
MoO /CeO –ZrO (MCZ) catalysts. Reaction conditions: benzylamine (0.2 mmol), time (6 h),
temperature (383 K), catalyst amount (0.1 g), and O
2 2 2 x 2
(C), CeO –ZrO (CZ), MoO /CeO (MC) and
x
2
2
-
1
2
bubbling rate (20 mL min )
NH2
N
MoO /CeO -ZrO
x
2
2
O , Solvent-free
R
2
R
R
Benzylamine
Dibenzylimine
9.6-99.9%
9
R = H, Cl, F, Me, OMe
O2
Selectivity
NH3
δ
H N
2
N
O2
NH
HN
R
R
R
R
Benzonitrile
.1-0.4%
Selectivity
NH2
Aminal
Intermediate
0
R
MoO /CeO -ZrO
2
x
2
x 2 2
Scheme 1 Possible reaction mechanism for the aerobic oxidation of benzylamine over MoO /CeO –ZrO
catalyst
A possible mechanism for the aerobic oxidation of benzylamine to dibenzylimine
is presented in Scheme 1 [4, 9, 24, 39, 50, 51]. As shown in Scheme 1, an imine
intermediate (Ph-CH=NH) can be formed through the oxidative dehydrogenation of
benzylamine. This imine intermediate (Ph-CH=NH) has a net positivity charge, thus
there is a high possibility of the attack of available benzylamine (PhCH NH ) on
2
2
imine intermediate due to the presence of a lone pair of electrons on the nitrogen
atom of the benzylamine. Then, the dibenzylimine product can be formed through
the aminal formation followed by release of NH . As stated previously, a small
3
amount of benzonitrile (0.1–0.4 %) was found in this study, which is due to the
consecutive oxidative dehydrogenation of the imine intermediate (Scheme 1). It was
123

A. Rangaswamy et al.
x 2 2
Fig. 8 Effect of reaction time on the oxidation of benzylamine with MoO /CeO –ZrO catalyst.
Reaction conditions: benzylamine (0.2 mmol), temperature (383 K), catalyst amount (0.1 g), and O
)
2
-1
bubbling rate (20 mL min
found that the acid catalysts exhibit higher efficiency in the amine oxidation through
the activation of the imine intermediate as shown in Scheme 1 [4, 50, 51]. It is a
well-known fact in the literature that high surface area catalysts exhibit a high
catalytic performance due to the availability of more active sites on the catalyst
surface. The presence of oxygen vacancies especially in MoO /CeO –ZrO catalyst
x
2
2
(
Fig. 2) is an additional advantage because the gas phase O can be activated on
2
these oxygen vacancies, which can play a key role in the oxidation of benzylamine.
Therefore, the superior performance of MoO /CeO –ZrO catalyst can be attributed
x
2
2
to the improved physicochemical properties, such as a high BET surface area
Table 1), abundant oxygen vacancy defects (Fig. 2), and high concentration of
acidic sites (Fig. 6).
(
The effect of the amount of catalyst on the reaction was studied with the MoOx/
CeO –ZrO catalyst (Fig. S4 of supplementary material). The employed reaction
conditions are as follows: benzylamine (0.2 mmol), temperature (383 K), reaction
2
2
-
1
time (6 h), and O bubbling rate (20 mL min ). It was found that the conversion of
2
benzylamine gradually increases with the increase in the amount of catalyst . This
observation could be attributed to the availability of more active sites at higher
catalyst amounts. The benzylamine conversions were found to be *28, 54, 70, 89,
and 99 %, respectively, for 20, 40, 60, 80, and 100 mg of the catalyst. Interestingly,
there was no considerable variation in the selectivity of imine product
(99.6–99.9 %) with the variation in the catalyst amount. Figure 8 shows the effect
of reaction time on benzylamine oxidation over MoO /CeO –ZrO catalyst.
Reactions were performed using 0.2 mmol of benzylamine at 383 K with 100 mg
x
2
2
-
of catalyst amount and O bubbling rate of 20 mL min . The conversion of
1
2
benzylamine was increased rapidly with the increase in the reaction time. The
achieved benzylamine conversions for 1, 2, 3, 4, 5, and 6 reaction times were 15, 28,
6
5, 79, 95, and 99 %, respectively. It is noted that the selectivity of the imine
product (*99.6–99.9 %) almost remains the same at all reaction times, along with
negligible amounts of nitrile by-product (0.1–0.4 %).
1
23

Novel molybdenum–cerium based heterogeneous catalysts for…
Table 2 Oxidative self-coupling of substituted benzylamines with MoO /CeO
x
2
–ZrO
2
catalyst
NH2
MoO /CeO -ZrO
x 2 2
N
N
R
O2
R
R
R
Nitrile
R = Cl, F, Me, OMe
Imine
Serial no.
Substrate
Time (h)
Amine conversion (%)
Selectivity (%)
Imine
99.8
Nitrile
1
2
6
6
99
96
0.2
0.2
NH2
NH2
99.8
99.9
99.7
99.7
99.9
99.6
99.9
NH2
Cl
3
4
5
6
7
8
6
6
6
6
6
6
91
90
98
89
99
93
0.1
0.3
0.3
0.1
0.4
0.1
Cl
NH2
F
NH2
Me
NH2
Me
NH2
MeO
NH2
OMe
Reaction conditions: temperature (383 K), time (6 h), amine (0.2 mmol), catalyst amount (0.1 g), and O
2
-1
bubbling rate (20 mL min
)
We have further explored the catalytic efficiency of MoO /CeO –ZrO catalyst
x
2
2
for the oxidative coupling of various benzylamines bearing different substituent
groups such as, F, Cl, Me, and OMe. The results obtained are presented in Table 2.
Excellent amine conversions were found for all the substituted benzylamines
irrespective of the nature of substituent present on the phenyl ring of benzylamine.
On the other hand, steric hindrance nature of the substituent groups was found to
play a role in the oxidation of benzylamine. For example, a higher conversion for
para-substituted amines was found compared with that of meta- and ortho-
substituted amines (Table 2, entries 3, 6, 8). The selectivity of the imine and nitrile
123

A. Rangaswamy et al.
x 2 2
Table 3 Oxidation of various secondary benzylamines with MoO /CeO –ZrO catalyst
Serial no.
1
Amine
Conversion (%)
98
Selectivity (%)
N
H
N
(
100)
2
3
85
79
NH
N
(
100)
(
100)
NH
N
Reaction conditions: temperature (383 K), time (6 h), amine (0.2 mmol), catalyst mount (0.1 g), and O
)
2
-1
bubbling rate (20 mL min
was found to be *99.6–99.9 and 0.1–0.4 % for all the substituted benzylamines,
respectively.
We have also investigated the oxidation of different secondary amines with
MoO /CeO –ZrO catalyst (Table 3). The amine conversion was found to be low
x
2
2
for all the secondary amines compared with that of the primary benzylamines
Fig. 7; Table 2). This observation clearly indicates the role of steric hindrance in
(
the oxidation of secondary benzylamines. Interestingly, 100 % selectivity towards
the imine product was found for all the secondary benzylamines. This observation is
attributed to the lack of hydrogen attached to the N-benzylamine to give the nitrile
product. Overall, the MoO /CeO –ZrO catalyst can be efficiently used for a
x
2
2
number of oxidation reactions under eco-friendly conditions, such as O as the
2
oxidant and without the use of solvent.
We have finally studied the reusability efficiency of MoO /CeO –ZrO catalyst
x
2
2
for the oxidation of benzylamine using O under solvent-free conditions. The results
2
obtained are presented in Fig. 9. The catalytic reaction parameters were fixed as
x 2 2
Fig. 9 Reusability of MoO /CeO –ZrO catalyst for benzylamine oxidation. Reaction conditions:
benzylamine (0.2 mmol), temperature (383 K), reaction time (6 h), catalyst amount (0.1 g), and O
)
2
-1
bubbling rate (20 mL min
1
23

Novel molybdenum–cerium based heterogeneous catalysts for…
follows: benzylamine (0.2 mmol), catalyst amount (100 mg), O bubbling rate
2
-
20 mL min ), reaction time (6 h), and temperature (383 K). After completion of
1
(
each cycle, the solid catalyst was recovered from the reaction mixture by means of
centrifugation, washed several times with ethanol to remove reaction gradients and
then dried at 423 K for 2 h. It was found that the MoO /CeO –ZrO catalyst can be
x
2
2
efficiently used up to five cycles without any significant variation in the
benzylamine conversion. The achieved amine conversions are *99, 97, 96, 94,
and 89 % for the first, second, third, fourth, and fifth cycles, respectively. As well,
no considerable variation in the selectivity of imine product was observed with the
repeated use of the catalyst. This thorough activity investigation reveals that the
MoO /CeO –ZrO solid acid is a versatile catalyst for the oxidation of various
x
2
2
benzylamines under eco-friendly reaction conditions.
Conclusions
In summary, the MoO /CeO –ZrO catalyst was found to exhibit an outstanding
2
x
2
catalytic performance for the oxidation of benzylamine under eco-friendly reaction
conditions. High surface area, abundant structural defects, and large amounts of
acidic sites were found to be key factors for superior performance of the MoOx/
CeO –ZrO catalyst. The conversion of benzylamine is highly dependent on the
2
2
catalyst amount and reaction time. It was found that the steric hindrance of the
groups attached to benzylamine plays a key role, especially in the oxidation of
secondary benzylamines. It is therefore suggested that the catalytic efficiency of
ceria-based materials for the solvent-free aerobic oxidation reactions can be
improved by modifying their unique textural, structural, and acid properties.
Acknowledgments A. R and B. G thank the Council of Scientific and Industrial Research (CSIR), New
Delhi for research fellowships. Financial support for this project was received from the Department of
Science and Technology, New Delhi, under the SERB Scheme (SB/S1/PC-106/2012).
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