Magnetically recyclable magnetite-ceria (Nanocat-Fe-Ce) nanocatalyst-applications in multicomponent reactions under benign conditions

Article abstract of DOI:10.1039/c3gc40375k

A novel magnetite nanoparticle-supported ceria catalyst (Nanocat-Fe-Ce) has been successfully prepared by a simple impregnation method and was characterized by XRD, SIMS, FEG-SEM-EDS, and TEM. The exact nature of Nanocat-Fe-Ce was confirmed by X-ray photoelectron spectroscopy and it is noted that CeO₂ nanoparticles are supported on magnetite, with evidence of secondary ion mass spectrometry. Catalytic activity of the nano-catalyst was explored for the synthesis of dihydropyridines under benign conditions; a greener protocol is described that provides a simple and efficient method for the synthesis of functionalized 1,4-dihydropyridines using a recyclable nanocatalyst. Notably, 5.22 mol% of the catalyst is sufficient to catalyze the multicomponent reaction in ethanolic medium at room temperature. Importantly, the catalyst could be easily separated from the reaction mixture by using an external magnet and recycled several times without loss of activity.

Full text of DOI:10.1039/c3gc40375k

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Magnetically recyclable magnetite–ceria (Nanocat-Fe-Ce)
nanocatalyst – applications in multicomponent
reactions under benign conditions†
Cite this: DOI: 10.1039/c3gc40375k
Manoj B. Gawande,*a Vasco D. B. Bonifácio,a Rajender S. Varma,b
Isabel D. Nogueira,c Nenad Bundaleski,d C. Amjad A. Ghumman,d
Orlando M. N. D. Teodorod and Paula S. Branco*a
A novel magnetite nanoparticle-supported ceria catalyst (Nanocat-Fe-Ce) has been successfully prepared
by a simple impregnation method and was characterized by XRD, SIMS, FEG-SEM-EDS, and TEM. The
exact nature of Nanocat-Fe-Ce was confirmed by X-ray photoelectron spectroscopy and it is noted that
CeO2 nanoparticles are supported on magnetite, with evidence of secondary ion mass spectrometry. Cata-
lytic activity of the nano-catalyst was explored for the synthesis of dihydropyridines under benign con-
ditions; a greener protocol is described that provides a simple and efficient method for the synthesis of
functionalized 1,4-dihydropyridines using a recyclable nanocatalyst. Notably, 5.22 mol% of the catalyst is
sufficient to catalyze the multicomponent reaction in ethanolic medium at room temperature. Impor-
tantly, the catalyst could be easily separated from the reaction mixture by using an external magnet and
recycled several times without loss of activity.
Received 21st February 2013,
Accepted 20th March 2013
DOI: 10.1039/c3gc40375k
www.rsc.org/greenchem
consumption of energy for heating and cooling of reactions;
the performance of reactions at ambient temperature or using
Introduction
Green chemistry has made a major impact on several fronts alternative and selective energy systems may be a viable
encompassing various key features such as the use of sustain- option.3a
able magnetic nanomaterials, catalyst-free aqueous and
In recent years, nano-catalysts have been introduced to
solvent-free organic reactions, and the material consumption, address the sustainability issues3b and have emerged as an
preferably the use of bio-renewable resources, etc.1 Addition- alternative approach for the improvement of many significant
ally, various other alternative solvents such as polyethylene organic reactions.4 These advances have opened the door for
glycol (PEG), biodiesel byproduct, glycerol, and deep eutectic the design of novel nanocatalysts for specific applications in
solvents (DESs) are now employed for greener reactions and synthetic chemistry. These innovations led to the evolution of
other sustainable applications.2 Environmentally benign and sustainable catalysts with excellent activity, selectivity, separ-
sustainably enhanced protocols for organic synthesis are being ation from the reaction mixture, and recyclability without
introduced with the improvement in atom economy. Among losing their activity.5–8 Magnetic nanoparticle-based materials
other factors, a major adverse effect on the environment is the (MNPs) are ideal supports for the immobilization of catalysts
since they can be re-collected by using an external magnet.
This enables convenient purification avoiding conventional fil-
aREQUIMTE, Department of Chemistry, Faculdade de Ciências e Tecnologia,
Universidade Nova de Lisboa, 2829-516 Caparica, Lisbon, Portugal.
E-mail: m.gawande@fct.unl.pt, mbgawande@yahoo.co.in, paula.branco@fct.unl.pt;
Fax: +351 212948550; Tel: +351 212948300
bSustainable Technology Division, National Risk Management Research Laboratory,
US Environmental Protection Agency, MS 443, 26 West Martin Luther King Drive,
Cincinnati, Ohio 45268, USA
cInstituto de Ciência e Engenharia de Materiais e Superfícies, IST, Lisbon, Portugal
dCentre for Physics and Technological Research (CeFITec), Department of Physics,
Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516
Caparica, Portugal
tration or centrifugation processes. Several metals have been
successfully used for this purpose, including, ruthenium,8
cobalt,9 ruthenium–cobalt,10 palladium,11 and nickel.12 Mag-
netic nanoparticles of Fe3O4 have recently emerged as a versa-
tile support for immobilization, since they are inexpensive,
non-toxic, chemically stable, and easily prepared from low-cost
precursors.13 Most importantly, Fe3O4–MNP-supported cata-
lysts have been shown to be highly useful supports for asym-
metric Michael addition in water,14 enantioselective
acylations,15 Ullman-type,16 Suzuki, Sonogashira, Stille,17 and
Suzuki–Miyaura18 coupling reactions. In metal catalyze
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3gc40375k
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reactions, ceria is an important and promising material for cata-
lysis and environmental and energy applications, due to its
extraordinary thermal and chemical stability.19 A few examples
of mixed oxides (Fe2O3–CeO2) have appeared, namely in dye
degradation,20 oxidations,21 and synthesis of carbonyl com-
pounds.22 In view of the importance of this research area,
reviews on nano-magnetite-supported catalysts and sustain-
able protocols are now available.4g,13n,23 It is an important and
urgent need to design magnetically recyclable nano-catalysts,
to reduce their cost of preparation, and to show their efficacy
in benign reaction media.
Multicomponent reactions (MCR) are the most powerful
tool for the construction of complex structures in a single
step.24a Despite the significant importance of MCR in the syn-
thesis of biologically active Hantzsch 1,4-dihydropyridines,24b,c
its synthesis using “free” nano-γ-Fe2O3 25 and nano-CeO2 26 cata-
lysts has been only recently reported. In continuation of our
Fig. 1 XRD patterns of Fe3O4 and Nanocat-Fe-Ce MNPs.
research on sustainable protocols, heterogeneous catalysis and are discernible. EDS confirms the presence of Ce metal in the
nanomaterials,27–29 herein, we report the multicomponent syn- nanocatalyst (see the spectrum in Fig. 2d).
thesis of di- and tetrahydropyridines using a facile, efficient,
TOF-SIMS analysis provides a highly efficient elemental and
recyclable and reusable ferrite-CeO2 nanocatalyst (Nanocat-Fe- molecular composition of the top surfaces (<1 nm) with
Ce).
minimum surface damage. Therefore, TOF-SIMS in positive
mode was used to confirm the presence of CeO2 over the
ferrite surface.
Fig. 3 shows the mass spectra in the m/z range of 0–150
(top) and 135–335 (bottom), with the characteristic ions of
Fe3O4 and CeO2 observed at m/z 56, 73, 140, 156, 157, 228, 312,
328, and 329, corresponding to Fe+, FeOH+, Ce+, CeO+, CeOH+,
Results and discussion
Nanocat-Fe-Ce was synthesized using a simple wet impreg-
nation protocol (Scheme 1) and fully characterized by X-ray
diffraction (XRD), inductive coupled plasma-atomic emission
spectroscopy (ICP-AES), transmission electron microscopy
(TEM), secondary ion mass spectrometry (SIMS), X-ray photo-
electron spectroscopy (XPS) and a field-emission gun scanning
electron microscope-electron dispersive spectrometry (FEG-SE-
M-EDS). The XRD spectra of Fe3O4 and Nanocat-Fe-Ce nano-
particles are depicted in Fig. 1. The XRD spectrum of Fe3O4
clearly matches with the literature data from the Joint Commit-
tee on Powder Diffraction System (JCPDS 79-0419). The crystal-
lite size of the Nanocat-Fe-Ce catalyst, determined by using the
Debye–Scherrer equation, was found to be 21 nm. Due to the
low percentage of Ce (7.41% by ICP-AES analysis) the peaks of
CeO2 are not detectable in the XRD spectrum.
FeCeO2 , Ce2O2+, Ce2O3 , and Ce2O3H+, respectively. Sodium
(Na+, m/z = 23) is present in the spectrum as a contamination.
The sodium adducts with CeO2 are also observed at m/z = 195
+
+
(NaCeO2 ) and at m/z = 196 (NaCeO2H+). The SIMS results
+
clearly indicate that CeO2 is present on the surface of the
catalyst.
In order to know the exact nature of Ce metal in the cata-
lyst, XPS measurements were performed (Fig. 4). Besides the
signals corresponding to Fe, O and Ce, the signal for carbon
was also observed, which is an expected impurity. The position
of the carbon 1s line is shifted 0.6 eV towards higher binding
energies with respect to the position characteristic of adventi-
tious carbon (285.0 eV) due to charging effects. The binding
energy axis was shifted accordingly in order to compensate the
sample charging.
The characteristic XPS peak of Ce (Ce 3d doublet) is shown
in the spectrum of Fig. 4. The position of the main Ce 3d5/2
line, its satellite peaks (S1 and S2), and the main Ce 3d3/2 line
are assigned in the figure. The satellite S2 is particularly
intense and partially overlaps with the 3d3/2 line. The two
main oxide phases of Ce (CeO2 and Ce2O3) can be clearly identi-
fied due to different positions and intensities of their shake
up satellite peaks. The general shapes of the measured Ce 3d
line, including the presence of two satellite peaks, indicate
that Ce is present at the surface as Ce(IV) (CeO2). Indeed, the
main 3d5/2 line should be at 882.5 eV, its first satellite S1 is at
889.5 eV and the second one S2, which is more intense, is at
898.5 eV.30 Additionally, Ce 3d3/2 should be at 901.3 eV.31 The
The TEM and SEM images of Fe3O4 and Fe3O4–CeO2 MNPs
are shown in Fig. 2a–c. Notably, the particles are observed in
the nano range sizes (20–35 nm). The clustering tendencies
formerly observed (Fig. 2c) notwithstanding, small groups of
equiaxial particles, almost entirely detached from each other,
Scheme 1 Synthesis of Nanocat-Fe-Ce by wet impregnation.
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Fig. 2 Characterization of the MNPs: (a) TEM image of Fe3O4; (b) TEM image of Nanocat-Fe-Ce; (c) SEM image of Nanocat-Fe-Ce; (d) EDS profile of Nanocat-Fe-Ce.
Fig. 4 The Ce 3d line taken from the Nanocat-Fe-Ce powder. The expected
Fig. 3 TOF-SIMS spectra of Nanocat-Fe-Ce MNPs (positive mode): top spec-
trum m/z range 0–150, and bottom spectrum m/z range 135–355.
positions of the main Ce 3d5/2 line, its satellite peaks (S1 and S2) and the main
Ce 3d3/2 line in the case of CeO2 compound30,31 are marked by arrows.
positions of peaks are in good agreement with the literature, ESI†). The low yield of 1a and the formation of 1a′ could be
which further supports the suggested phase identification. attributed to the low solubility of the reactants in water
The catalytic activity of Nanocat-Fe-Ce was investigated for (Table 1, entry 5). When ethanol was used excellent selectivity
the synthesis of di- and tetrahydropyridines using an MCR and yield were obtained (Table 1, entry 6). Although not exactly
approach. For a model reaction, the reaction between benz- the same, a related work for the synthesis of dihydropyridines
aldehyde, methyl acetoacetate, and 5,5-dimethyl-1,3-cyclohexa- has been described at elevated temperature.32
nedione was chosen at room temperature and in the presence
Further scope of the reaction for the synthesis of dihydro-
of benign solvents (Table 1). Magnetite (Fe3O4) did not show pyridines (Table 2) was investigated using Nanocat-Fe-Ce and
any catalytic activity under the optimized conditions (Table 1, ethanol as the solvent. Notably, substrates having electron-
entry 2). When ceric ammonium nitrate (CAN) was used as a donating or withdrawing groups led to the formation of pro-
catalyst the Hantzsch product 1a was obtained only in moder- ducts and the reactions proceeded smoothly in short reaction
ate yield (Table 1, entry 3). When water was used as a solvent, times with yields in the range of 83–95%. Moreover, Nanocat-
a side product was observed (bis-adduct 1a′, 2,2′-(phenyl- Fe-Ce is highly robust and can be recycled and reused several
methylene)-bis(3-hydroxy-5,5-dimethylcyclohex-2-enone), see times without any significant loss of the catalytic activity.
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Table 1 Optimization of reaction conditions for the synthesis of 1,4-
dihydropyridinesa
Entry
Solvent
Catalyst
Time [h]
Yield [%]
1
2
3
4
5
6
EtOH
EtOH
EtOH
None
H2O
No-catalyst
Fe3O4
CAN
Fe3O4–CeO2
Fe3O4–CeO2
Fe3O4–CeO2
2
2
1.5
2
2
Trace
Trace
60
Fig. 5 Reusability of the magnetic catalyst Nanocat-Fe-Ce.
45
45b
93
EtOH
0.33
In order to prove that the reaction is heterogeneous, a stan-
dard leaching experiment was conducted by a hot filtration
method. Benzaldehyde, methyl acetoacetate, and dimedone
were allowed to react for 3–4 minutes in the presence of
Nanocat-Fe-Ce at room temperature, in ethanol. The filtered
reaction mixture was then stirred without a catalyst for 12 h;
no formation of the corresponding product was observed, indi-
cating that no homogeneous catalyst was involved. Inductively
coupled plasma atomic emission spectra (ICP-AES) analysis of
the filtrate (hot) revealed the absence of Fe and Ce species in
the filtrate. The excellent catalytic activity and stability of
Nanocat-Fe-Ce encouraged us to explore their catalytic activity
for the functionalized tetrahydropyridines, as they are impor-
tant building blocks for abundant natural products, pharma-
a Reaction conditions: benzaldehyde (1 mmol), methyl acetoacetate
(1 mmol), ammonium acetate (1.5 mmol), 5,5-dimethyl-1,3-
cyclohexanedione (1 mmol), solvent (2 mL), catalyst (100 mg, 5.22 mol
%
Ce). b A bis-adduct was also isolated in 25% yield (see the
Experimental section). CAN = ceric ammonium nitrate.
Table 2 Nanocat-Fe-Ce catalyze multicomponent synthesis of 1,4-
dihydropyridinesa
ceutical products, and
a variety of biologically active
Entry
R1
R2
Compound
Time [min] Yield [%] compounds;33 the development of methodologies for the syn-
thesis of tetrahydropyridine derivatives under sustainable reac-
1
2
3
4
5
6
7
8
H
H
4-CN
4-Br
3,4-(OMe)2
3-OH
4-OMe
3-OH, 4-OMe
4-Br
3,4-(OPh)2
4-CN
4-OMe
Me
Et
Et
Et
Et
Et
Et
Et
Me
Et
1a
1b
1c
1d
1e
1f
1g
1h
1i
20
20
35
35
20
25
25
25
35
25
35
20
93
95
92
90
92
83
90
86
92
85
92
95
tion conditions is a desirable pursuit and requires further
attention.
The multicomponent reactions between 4-methoxybenz-
aldehyde, anilines, and methyl acetoacetate under benign con-
ditions provided the desired compound 5 in 82% yield
(Scheme 2).
In summary, a highly versatile magnetic nanocatalyst has
been developed by coating magnetite with cerium oxide nano-
particles. The catalyst used in the multicomponent synthesis
of functionalized 1,4 dihydropyridines was easily recovered
from the reaction mixture by a simple and convenient mag-
netic separation of products using an external magnet. Func-
tionalized tetrahydropyridines were synthesized in good yield
9
10
11
12
1j
1l
1m
Me
Me
a Reaction conditions: aldehyde (1 mmol), acetoacetate (1 mmol),
ammonium acetate (1.5 mmol), 5,5-dimethyl-1,3-cyclohexanedione
(1 mmol), Nanocat-Fe-Ce (100 mg, 5.22 mol% Ce), ethanol (2 mL),
room temperature.
The stability of Nanocat-Fe-Ce and its activity were investi-
gated in recycling experiments for the reaction between benz-
aldehyde, methyl acetoacetate, and dimedone under the
optimized conditions reported in Table 1 (entry 6). After each
cycle, the catalyst was separated magnetically, washed with
ethanol, dried at 50 °C under vacuum to remove residual sol-
vents, and used for the next cycle. The yields of the reaction vs.
the number of runs are shown in Fig. 5.
Using this procedure, Nanocat-Fe-Ce MNPs could be reused
up to six times without any significant loss of the initial cataly-
tic activity.
Scheme 2 Synthesis of methyl 2,6-bis(4-methoxyphenyl)-1-phenyl-4-(phenyl-
amino)-1,2,5,6-tetrahydropyridine-3-carboxylate (5).
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under benign reaction conditions. This greener magnetic
nanocatalyst could be used for other significant organic reac-
Acknowledgements
tions and transformations. Further explorations of similar pro- This work has been supported by Fundação para a Ciência e a
tocols are underway in our laboratory.
Tecnologia through grant PEst-C/EQB/LA0006/2011 and PEst-
OE/FIS/UI0068/2011. M. B. Gawande also thanks the PRAXIS
program for the award of a research fellowship (SFRH/BPD/
64934/2009).
Experimental
Preparation of Fe3O4 MNPs29a–d
FeCl3·6H2O (5.4 g) and urea (3.6 g) were dissolved in water
(200 mL) at 85 to 90 °C for 2 h. The solution turned brown. To
the resultant reaction mixture cooled to room temperature was
added FeSO4·7H2O (2.8 g) and then 0.1 M NaOH until pH 10.
The molar ratio of Fe(III) to Fe(II) in the above system was
nearly 2.00. The obtained hydroxides were treated by ultra-
sound in a sealed flask at 30 to 35 °C for 30 min. After ageing
for 5 h, the obtained black powder (Fe3O4) was washed, and
dried under vacuum.
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(1 mmol), and Nanocatalyst-Fe-Ce (100 mg, 5.22 mol% Ce) in
ethanol (2 mL) were stirred at room temperature. After com-
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