Reusable α-MoO3 nanobelts catalyzes the green and heterogeneous condensation of 1,2-diamines with carbonyl compounds

Article abstract of DOI:10.1039/c3nj00058c

Crystalline nanobelts of α-MoO₃ have been obtained successfully using novel and safe agents through a simple and safe sol-gel method (polymerizing-complexing) and characterized by XRD, FT-IR, Raman spectroscopies, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and the temperature programmed desorption (TPD) of ammonia. An orthorhombic lattice system for nano-MoO₃ was established. The HRTEM images showed that the nanobelt morphology of α-MoO₃ mostly ranged from 20-70 nm in width and 200-400 nm in length. The ammonia TPD profile demonstrated strong acidic sites. The spectral and analysis data confirmed the effectiveness of the method for the preparation of α-MoO₃ nanobelts by prevention of grain growth or agglomeration of the particles. The nanostructured MoO₃ exhibited a high efficiency in catalyzing the condensation reaction of various 1,2-diamine and carbonyl compounds for synthesis of heterocyclic compounds. The recovery of the title heterogenous nanocatalyst was easy and efficient and its catalytic activity was strikingly different from the bulk material.

Full text of DOI:10.1039/c3nj00058c

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Reusable a-MoO3 nanobelts catalyzes the green and
heterogeneous condensation of 1,2-diamines with
carbonyl compounds†
Cite this: DOI: 10.1039/c3nj00058c
Maasoumeh Jafarpour,* Abdolreza Rezaeifard,* Mahboube Ghahramaninezhad
and Tooba Tabibi
Crystalline nanobelts of a-MoO3 have been obtained successfully using novel and safe agents through a
simple and safe sol–gel method (polymerizing–complexing) and characterized by XRD, FT-IR, Raman
spectroscopies, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM)
and the temperature programmed desorption (TPD) of ammonia. An orthorhombic lattice system for nano-
MoO3 was established. The HRTEM images showed that the nanobelt morphology of a-MoO3 mostly ranged
from 20–70 nm in width and 200–400 nm in length. The ammonia TPD profile demonstrated strong acidic
sites. The spectral and analysis data confirmed the effectiveness of the method for the preparation of a-MoO3
nanobelts by prevention of grain growth or agglomeration of the particles. The nanostructured MoO3
exhibited a high efficiency in catalyzing the condensation reaction of various 1,2-diamine and carbonyl
compounds for synthesis of heterocyclic compounds. The recovery of the title heterogenous nanocatalyst was
easy and efficient and its catalytic activity was strikingly different from the bulk material.
Received (in Porto Alegre, Brazil)
14th January 2013,
Accepted 26th March 2013
DOI: 10.1039/c3nj00058c
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vanadium, cobalt, or aluminum, Mo-based catalysts can be
active and selective for many reactions. Selective oxidation,17–27
1. Introduction
hydrogenation and dehydrogenation,28–35 are a few examples of
the important applications of these catalysts.
Nanoparticles have been the subject of considerable interest
because of their special properties, resulting from the nano-
scale regime, such as a large surface-to-volume ratio, and
increased surface reactivity as compared to that of the bulk
material. This enables their use in catalysis, as well as in
mechanical, electronic, and optical applications. Nanostructured
oxides, compared to their bulk counterparts, can exhibit unexpected
However, the low cost preparation of metal oxide nanoparticles at
the industrial scale is a challenge in material production. So, the use
of cheap materials, simple fabrication processes and suitable con-
ditions of the synthesis are the main requirements for this process.
Therefore, the study of the influence of various parameters such
as initial solution combination, time and temperature of heat
treatments, and the type of complexing and polymerizing agents
is very important for nanopowder production.
Indeed, the PC (polymerizing–complexing) sol–gel method
provides a more homogeneous fine powder than any other
technique, with the immobilization of metal–chelate complexes
and the formation of stable metal complexes via increasing the
polymerization.
catalysis. Some examples are MgO,1–5 CeO2,6 TiO2,7 and WO3.
8
To the best of our knowledge, there has been limited reports on
utilizing nanostructured MoO3 in a catalytic reaction.9 Most reports
have been devoted to material synthesis. Regarding the surface
science, research on the structure of bulk MoO3 with perfect crystal-
line phase by microscopy,10 spectroscopy,11,12 and calculations13–16
has provided significant insight into the atomic surface chemistry
that governs the catalytic activity.
Much research has focused on Mo-based complex oxide
catalysts. In combination with other elements, such as bismuth,
The first step in this investigation is the preparation and
characterization of a-MoO3 nanobelts. This nanocatalyst can be
easily prepared using a PC sol–gel method under safe conditions.
The spectral and analysis data confirmed the effectiveness of the
method for preparation of a-MoO3 nanobelts by preventing of
grain growth or agglomeration of the particles.
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414, Iran. E-mail: mjafarpour@birjand.ac.ir,
rrezaeifard@birjand.ac.ir, rrezaeifard@gmail.com; Fax: +98 561 2502515;
Tel: +98 561 2502516
The significant importance of a-MoO3 nanobelts in catalytic
organic reactions, which has been limited to a few reports,
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj00058c
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The progress of the reactions was monitored by TLC using
silica-gel SIL G/UV 254 plates and also by GC on a Shimadzu
GC-16A instrument using a 25 m CBP1-S25 (0.32 mm ID, 0.5 mm
coating) capillary column. NMR spectra were recorded on
Brucker Avance DPX 250 MHz and 400 MHz instruments. Mass
spectra were recorded on a Shimadzu GC-MS-QP 5050A.
2.2. Catalyst preparation
First, an initial solution consisting of (NH4)6Mo7O24ꢁ4H2O
(0.003 mol) and distilled water was prepared. Then citric acid
(0.126 mol) and ethylene glycol (0.126 mol) were subsequently
added to this solution, and the resulting mixture was stirred
and the solids dissolved at 80 1C for 30 min until a completely
clear solution was obtained. The obtained solution was refluxed
at 100 1C for 12 h. During refluxing the solution turned into a
metal–citrate homogeneous complex with a slight color change
from clear to light blue. After cooling, in order to bring about
the required chemical reactions for the development of the
polymerization and evaporation of the solvent, the sol was
further slowly heated at 80 1C for 3 h in an open bath until a
black-blue wet gel was obtained. During continuous heating
at this temperature, the polymerization between citric acid,
ethylene glycol and the complexes occurs and ultimately the sol
became more viscous as a wet gel. In the final step of the sol–gel
process, the wet gel was fully dried by direct heating on a hot
plate at 120 1C for 3 h. The resultant product was a black-blue
powder, which was then calcined in a furnace at 600 1C for 6 h
at a rate 10 1C minꢀ1. Finally MoO3 nanocrystals with a light
green color were obtained (Fig. 1).
Scheme 1 Synthesis of heterocyclic compounds in the presence of a-MoO3
nanobelts.
attracted our attention for use in the development of a practical
synthesis of heterocyclic molecules of biological interest, which
is part of our ongoing interest.36–43 We have explored the
possibility of synthesizing quinoxaline derivatives, pyrido pyrazines
and benzimidazole derivatives using a-MoO3 nanobelts as a
practical, environmentally benign and reusable heterogeneous
catalyst (Scheme 1).
2.3. General procedure for the synthesis of quinoxaline
derivatives and pyrido pyrazines in the presence of a-MoO3
nanobelts in ethanol
At first, a mixture of 1,2-dicarbonyl (1 mmol) and nano-MoO3
(2 mol%, 0.002 g) in ethanol (0.5–0.65 mL) was stirred at 25 1C
for 2 min. Then, 1,2-diamine (1.5 mmol) was added to the
2. Experimental
2.1. General remarks
All chemicals were purchased from Merck or Fluka Chemical reaction mixture and stirred at 60 1C for the appropriate
Companies and used as received. Power X-ray diffraction (XRD) reaction time. The progress of the reaction was monitored by
was performed on a Bruker D8-advance X-ray diffractometer TLC or GC. After completion of the reaction, the precipitated
with Cu Ka (l = 1.54178 Å) radiation. The FT-IR spectra were products were dissolved by adding ethanol, and the nano-MoO3
recorded on a NICOLET system 800 beam splitter KBr SCAL = was separated by centrifuging followed by decantation (ethanol,
800 using a resolution in the range of 400–4000 cm. Raman 3 ꢂ 5 mL). Most of product was precipitated purely by concen-
spectra were recorded on a Brucker’s senterra (2009) model with trating and cooling of the ethanolic solution.
a spectral range of 200–3500 cmꢀ1 and a laser wavenumber of
The remaining product was then isolated from the excess of
785 nm. Thermogravimetric analysis (TGA) of the nanopowders 1,2-diamine by plate chromatography eluted with n-hexane–
was performed in air using a Shimadzu 50 instrument. SEM EtOAc (10 : 2). Structural assignment of the product was based
1
micrographs were obtained by SEM instrumentation (SEM, on its H NMR, 13C NMR and MS spectra (ESI†).
XL-30 FEG SEM, Philips, at 20 kV), TEM images were obtained
by TEM instrumentation (Philips CM 10).
The temperature-programmed desorption of NH3 adsorbed
on the catalyst (NH3-TPD) was carried out in a conventional flow
2.4. General procedure for the synthesis of benzimidazole
derivatives in the presence of a-MoO3 nanobelts under solvent-
free conditions
system equipped with a thermal conductivity detector (TCD).
To a mixture of aldehyde (1 mmol) and nano-MoO3 (2 mol%,
The sample was pretreated in nitrogen at 550 1C for 1 h 0.002 g) was added 1,2-diamine (1.2 mmol). The reaction
before adsorbing NH3. The NH3-TPD experiments were carried mixture was stirred at 25 1C for 1 min. Then, tert-butyl hydro-
out at 100–600 1C in a flow of dry He (19 mL minꢀ1). The rate of peroxide [TBHP] (2 mmol) was added to the reaction mixture and
heating was 10 1C minꢀ1
.
stirred at 50 1C for the appropriate reaction time. The progress of
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synthesis of the corresponding benzimidazole under solvent-free
conditions, according to the procedure mentioned in Section 2.4.
3. Results and discussion
3.1. Characterization of the catalyst
XRD was employed to exam the crystal structure of the prepared
MoO3. As shown in Fig. 2, all of the diffraction peaks can be
indexed to orthorhombic-phase MoO3 (JCPDS no. 35-0609, a =
0.3963 nm, b = 1.3856 nm, c = 0.3697 nm, space group Pbnm).
The strong diffraction peaks of MoO3 appear at 12.80, 25.80,
39.1, 58.90 and 67.401 2y, which correspond to the (020), (040),
(060), (081) and (010) planes, respectively. These sharp diffraction
peaks indicated the good crystallizability of the prepared MoO3
nanobelts.
Fig. 3 shows the Raman spectra of the MoO3 nanobelts,
centered at 665, 818, and 994 cmꢀ1. This suggests that the
lattice structure of the MoO3 nanobelts has pure orthorhombic
symmetry.44,45 The strongest peak appeared at 818 cmꢀ1, which
represents the Ag mode corresponding to the stretching of the
Mo–O–Mo bond along the axis in the crystal structure of MoO3,
which means that its polarizability is the highest.
Fig. 1 The flow chart of preparation of nanobelts of a-MoO3 by sol–gel process.
the reaction was monitored by TLC or GC. After completion of
the reaction, ethylacetate (2–4 mL) was added to the mixture.
After adding the solvent, the product was precipitated. Both
precipitated products and the solid nano-MoO3 were separated
by centrifuging followed by decantation (ethanol, 3 ꢂ 5 mL).
[The products that remained in the liquid phase were then
isolated from the excess of 1,2-diamine by plate chromatography
eluted with n-hexane–EtOAc (10: 3)]. By addition of ethanol to the
solid phase, products were dissolved and isolated from the solid
nano-MoO3 by centrifuging followed by decantation (ethanol, 3 ꢂ
5 mL). Evaporation of the solvent afforded the desired product in
high purity.
Fig. 2 XRD pattern of the a-MoO3 nanobelts.
Structural assignment of the product was based on its
1H NMR, 13C NMR and MS spectra (ESI†).
2.5. Reusability of the catalyst
At first, a mixture of 4,40-dimethoxybenzil (1 mmol) and nano-
MoO3 (2 mol%, 0.002 g) in ethanol (0.5 mL) was stirred at 25 1C
for 2 min. Then, o-phenylenediamine (1.5 mmol) was added to
the reaction mixture and stirred at 60 1C for the appropriate
reaction time. The progress of the reaction was monitored by
TLC or GC. After completion of the reaction, the precipitated
products were dissolved by adding ethanol, and the nano-MoO3
was separated by centrifuging followed by decantation (ethanol,
3 ꢂ 5 mL). The isolated solid phase (nano-MoO3) was dried
under reduced pressure and reused for five runs. Catalyst
recovery was also tested in the condensation of o-phenylene-
diamine (1 mmol) and 4-chlorobenzaldehyde (1.2 mmol) for
Fig. 3 Raman spectra of the a-MoO3 nanobelts.
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Fig. 4 FT-IR patterns of the a-MoO3 nanobelts.
Fig. 6 The TGA of the a-MoO3 nanobelts.
This structure was further supported by the FT-IR spectra.
The a-MoO3 nanobelts exhibit three main vibration modes in
the 500–1000 cmꢀ1(Fig. 4).
High resolution transmission electron microscopy (HRTEM)
observations clearly revealed the nanobelt morphology of the pre-
pared a-MoO3, which was 20–70 nm in width and 200–400 nm in
length (Fig. 5A and B). This result was further supported by the SEM
image (Fig. 5C).
Thermogravimetric analysis (TGA) of nanopowders was per-
formed in air. The plot of the weight loss against temperature
showed three distinctive peaks at 340, 480, and about 548 1C
(Fig. 6). These results on a-MoO3 powder showed that at high
annealing temperatures, especially above 600 1C, all the organic
additives were removed from the powders and crystalline a-MoO3
nanoparticles were formed.
For further analysis of the synthesized catalyst, an ammonia-
TPD experiment was carried out to determine the acid strength
of the catalyst. According to the literature,46 three ranges of the
NH3 desorption temperature should be taken into account:
Fig. 7 The NH3-TPD of the a-MoO3 nanobelts.
293–473, 473–623, and 623–823 K and these correspond to
weak, intermediate, and strong acid centers, respectively. As
shown in Fig. 7, the profile of the NH3 TPD of the prepared
MoO3 nanobelts shows a broad band in the range of 450–550 1C
(723–823 K). No NH3-desorption peak was observed at lower
temperatures. So, the presented TPD profile demonstrates the
strong acid centers in the a-MoO3 nanobelts in comparison
with that reported for the bulk counterpart.47
3.2. Catalytic activity of the a-MoO3 nanobelts
The catalytic potential of the a-MoO3 nanobelts was evaluated
in condensation reactions. The condensation reactions of
various 1,2-diamines and 1,2-dicarbonyl compounds, for the
synthesis of quinoxaline derivatives and pyrido pyrazines, and
also the condensation reactions of various 1,2-diamines and
aldehydes for the synthesis of benzimidazole derivatives in the
presence of the title heterogeneous catalyst have been
investigated.
3.2.1. Synthesis of quinoxaline derivatives and pyrido pyrazines.
In the family of biologically active heterocyclic templates,
quinoxalines and pyrido pyrazines have their own identity.
These compounds play a key role and have great synthetic
potentials for applications in academia and in many aspects of
pharmaceutical and medicinal chemistry.48–59
Fig. 5 HRTEM (A and B) and SEM (C) images of the a-MoO3 nanobelts.
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Blank experiments were performed for the condensation of conduct the condensation of o-phenylenediamine (1.2 mmol)
o-phenylenediamine (1.5 mmol) and 4,40-dimethoxybenzil and 4-chlorobenzaldehyde (1 mmol) using 2 mmol TBHP under
(1 mmol), as a model reaction under neat conditions, which solvent-free conditions at 50 1C. Under the optimized reaction
gave 45% yield (NMR) after 7 h. After that, the influence of the conditions, we generally obtained 2-substituted products in 90%
solvent nature on this reaction at room temperature was yield within 20 min (Scheme 1).
investigated. From the evaluation of different solvents, EtOH,
The low yield of the condensation product obtained in the
which is a standard ‘‘green’’ medium, was found to be the best above mentioned reaction in the presence of bulk MoO3 once
solvent in terms of yield and selectivity, nevertheless, the result again established the superior catalytic activity of the nano
was not satisfactory from a methodological point of view and structured MoO3.
the desired product was formed only in 58% yield after 3 h.
To further demonstrate the generality and scope of this
Then, the optimized reaction conditions were investigated method, we examined the condensation reactions of various
on the same condensation reaction in the presence of nano- structurally diverse 1,2-diamines and aldehydes for the synth-
MoO3 with respect to temperature, time, and the molar ratio of esis of 2-substituted benzimidazoles (Table 2).
catalyst to the substrate in ethanol (0.5 mL). We found that
2 mol% of nanocatalyst was sufficient to obtain the desired yields depend on the substituents introduced on the substrates.
quinoxaline in 95% yield within 10 min at 60 1C. For instance, the condensation reaction of a diamine bearing a
As can be seen in Table 2, the reaction times and the product
It should be mentioned that, when the nano-MoO3 was strong electron-withdrawing group (Table 2, entry 13) or an
replaced by its bulk counterpart, the reaction yield was reduced aldehyde substituted with a electron-donating group (Table 2,
close to that observed in ethanol under catalyst-free conditions. entries 2, 5–7) proceeded slowly.
The enhanced acid strength of the nano-structured MoO3
(Fig. 7) may be a good reason for its improved catalytic activity aliphatic aldehydes and sterically hindered aldehydes were also
compared to that of the bulk. proceeded well under these mild conditions. The presented
It is noteworthy to mention that heteroaryl aldehydes,
The optimized reaction conditions were applied for various method has the ability to tolerate other functional groups on
aryl 1,2-diamines and 1,2-dicarbonyl compounds. The yields of the aryl aldehyde, such as hydroxyl, methoxy, halo and nitro.
the products were highly dependent on the electronic demands
3.2.3. Recovery of the catalyst. The excellent yields of the
of the substrate used. For instance, the condensation reaction quinoxaline derivatives, pyrido pyrazines and benzimidazole
of a diamine bearing a strong electron-withdrawing group derivatives obtained using these new catalytic methods demon-
(Table 1, entries 3,5,7,9 and 11) or a dicarbonyl compound strated the high stability and catalytic activity of prepared
substituted with a strong electron-donating group (Table 1, a-MoO3 nanobelts. This was further supported by evaluation
entries 4,5 and 13) proceeded slowly.
of the recovery potential of the catalyst in the condensation of
Also, when we applied similar reaction conditions to the various 1,2-diamines and carbonyl compounds for synthesis
condensation of 5-bromo-2,3-diaminopyridine and a variety of of heterocyclic compounds under different conditions, as
1,2-diketones for synthesis of pyrido[2,3b]pyrazines, the desired mentioned in Tables 1 and 2.
products were isolated in high yields (Table 1, entries 12–16).
Recovery of the nano-MoO3 catalyst was easy and efficient.
As expected, the condensation reactions of 5-bromo-2,3-diamino- For this reason, the condensation reaction of o-phenylenediamine
pyridine with dicarbonyl compounds required longer reaction and 4,40-dimethoxybenzil in ethanol at 60 1C in the presence
times than that of the aryl 1,2-diamines.
of nano-MoO3 (2 mol%) was studied. When the reaction was
3.2.2. Synthesis of benzimidazole derivatives. To establish complete, the precipitated products were dissolved by adding
the generality and scope of the present methodology, this work was ethanol and nano-MoO3 was separated by centrifuging followed
further extended to the condensation of various 1,2-diamines and by decantation (ethanol, 3 ꢂ 5 ml). The isolated solid phase
aldehydes, which were expected to produce the benzimidazole (nano-MoO3) was then dried under reduced pressure and reused
derivatives.
for five runs without any noticeable drop in the product yield and
Substituted benzimidazole derivatives have found commercial its catalytic activity. The same results were obtained for the
applications in several therapeutic areas such as in antitumor,60 catalyst recovery in condensation of o-phenylenediamine (1 mmol)
antimicrobial,61 and anti-inflammatory62 agents as well as in and 4-chlorobenzaldehyde (1.2 mmol) for synthesis of the corre-
antihelminthic agents in veterinarian medicine.63 Furthermore, sponding benzimidazole under solvent-free conditions. Therefore,
benzimidazoles are very important intermediates in organic reac- the title methodologies are environmentally friendly, cost effective
tions64 and can also act as ligands to transition metals for modeling and industrially important because of the catalyst reuse and the
biological systems.65 Therefore, the preparation of benzimidazoles use of safe reaction media. These advantages of the high yielding
has gained considerable attention in recent years.66–69
condensation methods offer ready scalability. For example, the
At the beginning of this work, the condensation of o-phenyl- condensation of o-phenylenediamine and benzyl (or o-phenyl-
enediamine (1 mmol) and 4-chlorobenzaldehyde (1.2 mmol) enediamine with benzaldehyde) in a 20 mmol scale reaction in
was considered as a model reaction.
the presence of the nanocatalyst was investigated. As expected, the
Our studies on the different reaction conditions (solvent, reaction proceeded similarly to smaller scale, and the desired
temperature, oxidants and catalyst loading) showed that 2 mol% corresponding products were obtained in the same isolated yields
of nano-MoO3 was an adequate amount of the catalyst to smoothly and reaction times.
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Table 1 Synthesis of quinoxaline derivatives and pyrido pyrazines in the presence of a-MoO3 nanobeltsa
Entry
Diamine R1
1,2-Diketone R2
Product R1, R2
Time (min)
Isolated yieldb (%)
1
2
3
4
5
H
H
H
H
OCH3
OCH3
H, H
3
4
120
10
120
95
96
80
95
o10
CH3
NO2
H
CH3, H
NO2, H
H, OCH3
NO2, OCH3
NO2
6
7
H
2
95
97
NO2
15
8
H
10
120
5
97
85
95
9
NO2
10
H
11
12
NO2
4 h
30
94
98
H
13
14
15
OCH3
5 h
92
96
95
5
30
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Table 1 (continued )
Entry
Diamine R1
1,2-Diketone R2
Product R1, R2
Time (min)
10
Isolated yieldb (%)
16
98
a
b
The reactions were run at 60 1C and the molar ratio of 1,2-dicarbonyl–1,2-diamine–MoO3 was 100 : 150 : 2 in 0.5–0.65 mL ethanol. All products
were identified by their spectroscopic data.40
Table 2 Synthesis of 2-substituted benzimidazoles in the presence of a-MoO3 nanobeltsa
Entry
1
Diamine R
H
Aldehyde
X = H
Product
Time (min)
30
Isolated yieldb (%)
93
2
3
4
5
6
H
H
H
H
H
X = 4-Me
X = 2-Cl
40
30
20
76
25
90
80
90
85
75
X = 4-Cl
X = 4-OMe
X = 2-OH
7
8
H
H
X = 4-OH
X = 4-NO2
60
15
94
85
9
H
45
80
10
11
H
H
15
15
84
90
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Table 2 (continued )
Entry
Diamine R
Aldehyde
Product
Time (min)
Isolated yieldb (%)
12
13
H
15
80
85
NO2
X = 4-Cl
300
14
Me
Me
20
20
93
94
15
X = 4-Cl
a
The reactions were run at 50 1C and the molar ratio of aldehyde–1,2-diamine–TBHP–MoO3 was 100 : 120 : 200 : 2 under solvent-free conditions.
b
All products were identified by their spectroscopic data.70,71
6 S. Carrettin, P. Concepcion, A. Corma, J. M. L. Nieto and
V. F. Puntes, Angew. Chem., 2004, 116, 2592 (Angew. Chem.,
Int. Ed., 2004, 43, 2538).
4. Conclusions
a-MoO3 nanobelts, of 20–70 nm in width and 200–400 nm in
length, can be easily prepared using a PC sol–gel method under
safe conditions. The spectral and analysis data confirmed the
effectiveness of the method for the preparation of a-MoO3
nanobelts by preventing the grain growth or agglomeration
of the particles. The a-MoO3 nanobelts, as an effective and
reusable heterogeneous catalyst, can provide an environmentally
friendly alternative for the heterogeneous synthesis of quinoxaline
derivatives, pyrido pyrazines and benzimidazole derivatives under
mild conditions.
7 S. Yurdakal, G. Palmisano, V. Loddo, V. Augugliaro and
L. Palmisano, J. Am. Chem. Soc., 2008, 130, 1568–1569.
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Acknowledgements
Support for this work by the Research Council of the University
of Birjand is highly appreciated.
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