Synthesis and heterogeneous catalytic activity of covalently immobilized hexamine cation as a magnetically-recoverable nanocatalyst

Article abstract of DOI:10.1007/s13738-015-0652-6

Functionalized magnetic core-shell nanoparticles Fe₃O₄-SiO₂-HMTA are prepared by co-precipitation method and characterized by SEM, TEM, FT-IR, XRD, and VSM. The particles are spherical with an average size of approximately 48 nm. The catalytic activity of these nanoparticles was tested in solvent-free synthesis of coumarin derivatives. The catalyst was readily recycled by the use of an external magnetic field and can be reused four times without significant loss of activity or mass.

Full text of DOI:10.1007/s13738-015-0652-6

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0652-6
ORIGINAL PAPER
Synthesis and heterogeneous catalytic activity of covalently
immobilized hexamine cation as a magnetically‑recoverable
nanocatalyst
Sahar Kangari1 · Issa Yavari1 · Bakhshali Maasoumi2
Received: 10 December 2014 / Accepted: 25 April 2015
© Iranian Chemical Society 2015
Abstract Functionalized magnetic core–shell nanopar-
ticles Fe3O4-SiO2-HMTA are prepared by co-precipitation
method and characterized by SEM, TEM, FT-IR, XRD, and
VSM. The particles are spherical with an average size of
approximately 48 nm. The catalytic activity of these nano-
particles was tested in solvent-free synthesis of coumarin
derivatives. The catalyst was readily recycled by the use
of an external magnetic field and can be reused four times
without significant loss of activity or mass.
Introduction
The significant advancement in nanotechnology has led to
applications of magnetic nanoparticles (MNPs) in cataly-
sis, environmental protection, energy storage, and biomedi-
cine [1, 2]. The catalytic application of MNPs has attracted
increasing attention owing to easy separation from the
reaction system by external magnetic field rather than fil-
tration or centrifugation [3]. Uncoated MNPs tend to form
aggregates [4]. Silica is often employed as coating material
over the surface of iron oxide NPs. The nonmagnetic shell
of silica can prevent MNPs from aggregating and agglom-
erating. The highly porous surface of silica materials has
reactive silanol groups that allow covalent immobilization
of organic functional groups for the formation of organic–
inorganic hybrid nanomaterials [5].
Graphical abstract
O
O
Fe3O4-SiO2-HMTA
Solvent-free, heat
+
OMe
R
OH
R
O
O
15 Examples; 88-96% Yield
An organic–inorganic hybrid nanomaterial consists of
an inorganic support with organic functionalities covalently
connected to the framework. Organic–inorganic hybrid
nanomaterials can be designed at a molecular level to per-
form catalysis, adsorption, separation, and drug delivery
[6, 7]. Recently, several magnetic organic–inorganic hybrid
nanomaterials have been used as heterogeneous catalysts
in bromination, nitrile hydration, and oxidation reactions
[8–10].
Keywords Magnetically retrievable · Grafting ·
Silica · Nano-hybrid · Coumarin synthesis · Hexamine ·
Solvent-free
Herein, we report that the Fe3O4-SiO2-HMTA organic–
inorganic hybrid nanomaterial can be used as a heteroge-
neous catalyst and surface modification was carried out by
grafting HMTA groups and chloride via formation of cova-
lent bonds to the particle surface. This catalyst for solvent-
free synthesis of coumarin derivatives by condensation of
phenolic compounds with methyl acetoacetate and could be
readily separated from solution via application of an exter-
nal magnet, allowing straightforward recovery and reuse
[11–16].
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-015-0652-6) contains supplementary
material, which is available to authorized users.
* Issa Yavari
yavarisa@modares.ac.ir
1
Department of Chemistry, Science and Research Branch,
Islamic Azad University, Tehran, Iran
2
Department of Chemistry, Payame Noor University,
Tabriz, Iran
1 3

J IRAN CHEM SOC
washed with 50 % aqueous ethanol, and dried under vac-
uum at 80 °C.
Experimental
Materials and methods
Synthesis of citrate‑coated Fe3O4 NPs
Iron (III) chloride hexahydrate (FeCl3·6H2O), iron (II)
chloride tetrahydrate (FeCl2.4H2O), ammonium hydroxide
solution (NH3.H2O, 25 %), tetraethylorthosilicate (TEOS),
3-chloropropyltrimethoxysilane (CPTMS), hexamethylene-
tetramine (hexamine, HMTA) [17], methyl acetoacetate,
phenolic compounds, and solvents were obtained from
Merck.
To obtain the well-dispersed Fe3O4 NPs, the prepared
MNPs were added to citric acid (10 mL, 0.1 M) with under
sonication for 30 min. The reaction was maintained for
12 h at room temperature, and the product was washed sev-
eral times with water. The obtained citrate-coated MNPs
(CMNPs) were separated with external magnetic field. Cit-
ric acid was used as a coating agent for colloidal stabiliza-
tion of MNPs in aqueous solution and to prevent aggrega-
tion of MNPs [18].
Preparation of the catalyst
Synthesis of magnetite NPs
Synthesis of Fe3O4‑SiO2 core–shell NPs
Magnetite NPs were prepared by chemical co-precipitation
method under alkaline condition [18]. Molar ratio of Fe2+
and Fe3+ salts was maintained at 1:2. In a typical synthe-
sis, FeCl3·6H2O (5.4 g, 20 mmol) and FeCl2·4H2O (3.9 g,
20 mmol) were dissolved in water (25 mL) with vigorous
stirring. The solution was stirred under N2 at 80 °C for 1 h.
Then, NH4OH (10 mL) was added and stirred for 2 h. The
reaction mixture was cooled to room temperature, and the
black powder was collected by an external magnetic field,
The Fe3O4-SiO2 core–shell NPs were prepared by the
growth of a silica layer [19]. Thus, the obtained CMNPs
(1.0 g) were added to 10 % aqueous EtOH (20 mL) and
the mixture sonicated for 30 min. Under continuous
stirring, ammonia solution (5 mL) and tetraethoxyor-
thosilicate (TEOS, 5 mL) were consecutively added to
the reaction mixture. The resulting mixture was stirred
under N2 atmosphere for 12 h at room temperature.
Scheme 1 Preparation of Fe3O4-SiO2-HMTA
1 3

J IRAN CHEM SOC
Fig. 1 Powder XRD patterns of a Fe3O4 and b Fe3O4-SiO2-HMTA
Synthesis of Fe3O4‑SiO2‑HMTA
To a solution of hexamine (1.40 g, 10 mmol) in CHCl3 (7 mL),
was added Fe3O4-SiO2-CPTMS nanoparticles and the suspen-
sion was refluxed for 24 h. Then, the brown solid was washed
by CHCl3 and dried under vacuum at 80 °C for 2 h.
Catalyst characterization
Scan electron microscope (SEM) images were obtained
using a LEO 1430VP instrument. Transmission electron
microscopy (TEM) image was acquired using a Philips
EM 208, at 100 kV. FT-IR spectra of the materials were
recorded over the range of 400–4000 cm−1 region by using
a Bruker Tensor 27 FT-IR spectrometer, using a KBr disc
made by 1 % sample in 200 mg of spectroscopic-grade
KBr. Powder X-ray diffraction (XRD) patterns were
recorder in the range of 2°–70° on a Siemens D5000 X-ray
Diffractometer, using CuKα radiation (λ = 1.5418 Å) at
30 kV and 30 mA. The magnetic studies were carried out
on a VSM lakeshore 7307 vibrating sample magnetometer.
Fig. 2 The TEM image of Fe3O4-SiO2-HMTA
Then, the core–shell NPs were separated by a magnet to
eliminate the homogeneous silica nucleus and washed
with ethanol.
Typical procedure for the synthesis of coumarin
derivatives
Synthesis of Fe3O4‑SiO2‑CPTMS
A mixture of phenolic compound (1 mmol), methyl ace-
toacetate (0.1 g, 1 mmol) and Fe3O4-SiO2-HMTA nanoma-
terial (10 % mol) was stirred at 100 °C for 20 min. Then,
the reaction was allowed to cool to room temperature. After
completion of reaction, as monitored by TLC, the mixture
was diluted with EtOH (2 mL). The catalyst was separated
by an external magnet and the product was purified by
recrystallization from EtOH.
The obtained MNPs powder were dispersed in MeOH
(100 mL), toluene (2 mL), was added to the solution and
sonicated for 30 min, then, 3-cloropropyltrimethoxysilane
(CPTMS, 2 mL) was added dropwise. After stirring for
48 h under N2, the mixture was centrifuged, the precipitate
was washed with MeOH (3 × 10 mL), and dried under vac-
uum at 80 °C for 2 h [19].
1 3

J IRAN CHEM SOC
Fig. 3 The SEM images and
narrow distribution size
diagrams of a Fe3O4, b Fe3O4-
SiO2 and c Fe3O4-SiO2-HMTA
of MNPs at physiological condition. Coating the magnetic
nanoparticles with the silica matrix should be a good solu-
tion. Furthermore, the silica hydroxyl groups (Si–OH) were
modified by CPTMS groups with covalent bonds. Finally,
HMTA was immobilized on the nanoparticle with a cova-
lent bond between silane groups and HMTA.
Results and discussion
Preparation and characterization of the catalyst
The schematic presentation for preparation of Fe3O4-SiO2-
HMTA organic–inorganic hybrid nanomaterial is illustrated
in Scheme 1. The magnetic nanoparticles were easily pre-
pared via the co-precipitation method. Citric acid was used
as a coating agent, covalently attached on the MNPs sur-
face, and made stable colloidal dispersion [15]. The choice
of citric acid over other commonly available coating agents
is to control the hydrodynamic size and prevent aggregation
Structural and compositional characterization
The XRD powder diffraction patterns of Fe3O4 and Fe3O4-
SiO2 modified by hexamine are shown in Fig. 1. All of
the observed diffraction peaks are indexed by the cubic
1 3

J IRAN CHEM SOC
structure of Fe3O4 (PDF: 00-011-0614). As can be seen,
the Fe3O4-SiO2 modified by hexamine, the XRD patterns of
Fe3O4 (Fig. 1a) and Fe3O4-SiO2-HMTA (Fig. 1b) organic–
inorganic hybrid nanomaterial were in agreement with that
of the standard Fe3O4 structure indicated that these parti-
cles have phase stability and the structural integrity was
preserved. The diffraction peaks at 2θ = 29.9, 35.4, 43.1,
53.4, 57.3, 62.4 and 74.1 that correspond to the (2 2 0),
(3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) of
Fe3O4 crystalline structure. A broad diffraction peak near
2θ = 12° is observed in the Fe3O4-SiO2-HMTA organic–
inorganic hybrid nanomaterial. This may be attributed to
the amorphous SiO2.
uniform structures. Moreover, as it is shown in Fig. 3c,
the Fe3O4-SiO2-HMTA organic–inorganic hybrid nano-
material has core–shell structure, even after surface
modification of Fe3O4-SiO2 NPs with HMTA groups.
The mean radii in Fig. 3a are 22.5, 44.6, and 48.7 nm,
respectively.
The magnetic properties of Fe3O4 were characterized by
vibrating sample magnetometer (VSM). The typical room
temperature magnetization curves bare Fe3O4 (Fig. 4a)
and Fe3O4-SiO2-HMTA (Fig. 4a) are shown. The magnetic
saturation values of the Fe3O4 and Fe3O4-SiO2-HMTA
were 59 and 32 emu/g, respectively. Lower magnetic sat-
uration of later nanoparticles could be owing to the exist-
ence of nonmagnetic materials on the surface of magnetic
nanoparticles.
The size of the prepared catalyst was characterized by
TEM, which shows the Fe3O4-SiO2-HMTA Fig. 2.
To have a better visual insight into the morphology
of Fe3O4, Fe3O4-SiO2, Fe3O4-SiO2-HMTA and compos-
ite structural changes, scanning electron microscopy
images are observed. The diagrams adjacent to Fig. 3a
illustrate narrow size distribution of NPs. Figure 3b
clearly shows the Fe3O4 and Fe3O4-SiO2 NPs with
The FT-IR spectra of (a) Fe3O4, (b) Fe3O4-SiO2, and
(c) Fe3O4-SiO2-HMTA are shown in Fig. 5. The absorp-
tion band at 3438 cm−1 in the FT-IR spectrum of Fe3O4
nanoparticles (Fig. 5a) is attributed to the stretching vibra-
tions of –OH groups absorbed by Fe3O4 nanoparticles.
The bands at 1613 and 565 cm−1 are assigned to the −
OH bending and Fe–O bond vibration, respectively. The
broad high-intensity band at 1062 cm−1 of Fe3O4-SiO2
(Fig. 5b) is assigned to the Si–O–Si vibration. The bands
at 799 and 455 cm−1 are assigned to the Si–O–Si sym-
metric stretch and O–Si–O bending modes, respectively.
The bands at 3425, 1630 cm−1 correspond to the stretch-
ing and bending vibrations of Si–OH [19]. Characteristic
bonds at 3000, 2925, 1459, and 1374 cm−1 for the organic
groups on Fe3O4-SiO2 and C–N+ stretching at 1615 cm−1
,
O
O
Fe3O4-SiO2-HMTA
Solvent-free, heat
+
OMe
R
OH
R
O
O
Fig. 4 Magnetization curves obtained by VSM at room temperature
for (a) Fe3O4 MNPs, (b) Fe3O4-SiO2-HMTA
Scheme 2 Preparation of coumarins from phenolic compounds and
methyl acetoacetate in the presence of Fe3O4-SiO2-HMTA
Fig. 5 FT-IR spectra of a
Fe3O4, b Fe3O4-SiO2, and c
Fe3O4-SiO2-HMTA
1 3

J IRAN CHEM SOC
Scheme 3 A plausible mechanism for the synthesis of coumarin on Fe3O4-SiO2-HMTA surface
C–N stretching bonds are at 1009–1233 cm−1 are shown in
Fig. 5c.
with those reported in literature. The reactions went on
well to afford products in good to high yields and short
times.
Application of Fe3O4‑SiO2‑HMTA as nanomagnetic
catalyst for synthesis of coumarin derivatives
The superiority of the present protocol over reported
methods can be seen by comparing our results with those
of some recently reported procedures, as shown in Table 2.
The synthesis of 4-methyl-2H-chromen-2-one was used as
a model reaction and the comparison is in terms of mol %
of the catalysts, temperature, reaction time, and percentage
yields.
The catalytic activity of Fe3O4-SiO2-HMTA organic–inor-
ganic hybrid nanomaterial was used as an appropriate het-
erogeneous acidic catalyst for solvent-free synthesis of
coumarin by condensation of phenolic compounds with
methyl acetoacetate (Scheme 2).
A plausible mechanism for the synthesis of coumarin on
Fe3O4-SiO2 modified by hexamine surface is depicted in
Scheme 3.
In order to generalize the scope of reaction, a variety
of phenols was subjected for reaction with methyl ace-
toacetate under the solvent-free optimized reaction con-
ditions, and the results are presented in Table 1. The
spectral data and melting points are in good agreement
Catalyst stability and reusability
The stability of Fe3O4-SiO2-HMTA was tested on the reac-
tion of phenol and methyl acetoacetate. The catalyst was
readily recovered by simple magnetic decantation, washed
with EtOH and dried at 60 °C for 2 h. The recycled catalyst
was used four times with 1–3 % loss of activity or mass
(see Table 3).
1 3

J IRAN CHEM SOC
Table 1 Synthesis of coumarin derivatives catalyzed by Fe3O4-SiO2-HMTA
Entry
Ar-OH
Product
Time (min)
Yield (%)
96
MP (oC)
Observed
Reported (references)
79–81 [20]
1
Phenol
12
77–82
O
O
2
Catechol
10
90
163–165
161–165 [21]
O
O
OH
3
4
5
Resorcinol
Hydroquinone
m-Cresol
15
20
15
93
91
90
184–187
241–244
125–129
182–184 [22]
241–242 [20]
130–131 [23]
O
O
O
O
HO
HO
O
O
6
7
p-Cresol
17
15
90
89
152–156
157–160
150–153 [23]
158–160 [22]
O
O
3-Methoxyphenol
O
O
O
MeO
MeO
8
9
4-Methoxyphenol
2-Methylresorcinol
15
15
88
95
246–248
133–137
240–242 [15]
137–139 [24]
O
O
O
HO
1 3

J IRAN CHEM SOC
Table 1 continued
Entry
Ar-OH
Product
Time (min)
12
Yield (%)
89
MP (oC)
Observed
140–143
Reported (references)
141–144 [24]
10
5-Methylresorcinol
O
O
HO
11
12
3,5-Dimethylphenol
1-Naphthol
15
20
92
89
87–89
88–90 [23]
O
O
153–157
154–156 [22]
O
O
13
2‑Naphthol
20
89
181–185
182–183 [15]
O
O
14
15
Benzene-1,2,3-triol
Benzene-1,3,5-triol
15
14
93
90
240–243
283–285
238–243 [22]
289–290 [21]
O
O
O
O
HO
HO
OH
OH
Table 2 Comparison of the results obtained in this work for 4-methyl-2H-chromen-2-one to those reported by other groups
Entry Catalyst
Reaction conditions
Time
Yield (%) References
1
2
3
4
5
6
7
8
9
Sulphated zirconia
[BMIM] [Tf2N], FeCl3
Neat/80 °C
Neat/70 °C
24 h
10 h
52
77
Rodríguez-Domínguez and Krisch [15]
Karimi and Behzadnia [25]
Valizadeh et al. [26]
H3PO4 imidazolium dihydrogenphosphate MW heating oven/140 W 25 min 78
KAl(SO4)2·12H2O
Yb(OTf)3
Neat/80 °C
Neat/85 °C
90 °C
2 h
1 h
90
91
Dabiri et al. [27]
Wang et al. [28]
Nano-Fe3O4
50 min 59
50 min 63
50 min 93
12 min 96
Nasseri and Sadeghzadeh [29]
Nasseri and Sadeghzadeh [29]
Nasseri and Sadeghzadeh [29]
This work
Nano γ-Fe2O3
90 °C
Fe3O4-DABCO MNP
Fe3O4-SiO2-HMTA
Solvent-free
Solvent-free
1 3

J IRAN CHEM SOC
Table 3 Recycling of Fe
by condensation reaction of phenol and methyl acetoacetate
3O4-SiO2-HMTA for synthesis of coumarin
7. S. Pramanik, C. Zheng, T.J. Emge, J. Li, J. Am. Chem. Soc. 133,
4153–4155 (2011)
8. H. Erdemi, A. Baykal, E. Karaoğlu, M.S. Toprak, Mat. Res. Bull.
Cycle
Time (min)
Yield (%)
47, 2193–2199 (2012)
9. V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara, J.M.
Basset, Chem. Rev. 111, 3036–3075 (2011)
1
2
3
4
12
12
15
20
96
92
89
88
10. A. Rostami, Y. Navasi, D. Moradi, A. Ghorbani-Choghamarani,
Cat. Commun. 43, 16–20 (2014)
11. I. Yavari, R. Hekmat-Shoar, A. Zonouzi, Tetrahedron Lett. 39,
2391–2396 (1998)
12. A. Shaabani, R. Ghadari, A. Rahmati, A.H. Rezayan, J. Iran.
Chem. Soc. 6, 710–716 (2009)
13. G. Brufola, F. Fringuelli, O. Piermatti, F. Pizzo, Heterocycles 43,
1257–1266 (1996)
Conclusions
14. S.K. De, R.A. Gibbs, Synthesis, (8), 1231–1233 (2005)
15. J.C. Rodríguez-Domínguez, G. Kirsch, Synthesis, (11), 1895–
1899 (2006)
16. M.S. Manhas, S.N. Ganguly, S. Mukherjee, A.K. Jain, A.K.
Bose, Tetrahedron Lett. 47, 2423–2426 (2005)
17. N. Kaur, D. Kishore, J. Iran. Chem. Soc. 10, 1193–1228 (2013)
18. S. Srivastava, R. Awasthi, N.S. Gajbhiye, V. Agarwal, A. Singh,
A. Yadav, R.K. Gupt, J. Colloid Interface Sci. 359, 104–111
(2011)
In summary, we have shown that functionalized magnetic
core–shell NPs Fe3O4-SiO2-HMTA, prepared by co-pre-
cipitation method, is an effective and recyclable catalyst
for the synthesis of coumarin derivatives from phenolic
compounds and methyl acetoacetate, under solvent-free
conditions. Mild reaction conditions, high activity, easy
preparation and reusability of the catalyst make the present
protocol a green alternative.
19. Y. Yao, Sh Miao, Sh Yu, L.P. Ma, H. Sun, Sh Wang, J. Colloid
Interface Sci. 379, 20–27 (2012)
20. L.L. Woods, J. Sapp, J. Org. Chem. 27, 3703–3705 (1962)
21. F. Chavan, B. Madje, J. Bharad, M. Ubale, M. Ware, M. Shin-
gare, N. Shinde, Bull. Cat. Soc. India 7, 41–45 (2008)
22. S.B. Patil, R.P. Bhat, V.P. Raje, S.D. Samant, Synth. Commun.
36, 525–531 (2006)
23. A. Russel, J.R. Frye, Org. Synth. 21, 22–27 (1941)
24. A.S.R. Anjaneyulu, L.R. Row, C.S. Krishna, C. Srinivasulu, Cur-
rent Sci. 375, 13–18 (1968)
References
1. M. Faraji, Y. Yamini, M. Rezaee, J. Iran. Chem. Soc. 7, 1–37
(2010)
2. G.H. Imanzadeh, A. Zare, A. Khalafi-Nezhad, A. Hasaninejad,
A.R. Moosavi-Zare, A. Parhami, J. Iran. Chem. Soc. 4, 467–475
(2007)
25. B. Karimi, H. Behzadnia, Cat. Commun. 12, 1432–1436 (2011)
26. H. Valizadeh, F.M. Kordi, H. Gholipur, M. Amiri, Phosphorus,
Sulfur. Silicon Relat. Elem. 184, 3075 (2009)
3. F. Shirini, M. Abedini, J. Nanosci, Nanotechnol. 13, 4838–4860
(2013)
27. M. Dabiri, M. Baghbanzadeh, S. Kiani, Y. Vakilzadeh, Monatsh.
4. S. Pagoti, S. Surana, A. Chauhan, B. Parasar, J. Dash, Catal. Sci.
Chem. 138, 997 (2007)
Technol. 3, 584–588 (2013)
5. S. Chen, J. Feng, X. Guo, J. Hong, W. Ding, Mat. Lett. 59, 985–
988 (2005)
6. A.J. Lan, K.H. Li, H.H. Wu, D.H. Olson, T.J. Emge, W. Ki, M.C.
Hong, J. Li, Angew. Chem. Int. Ed. 48, 2334–2338 (2009)
28. L. Wang, J. Xia, H. Tian, C. Qian, Y. Ma, Ind. J. Chem. 42B,
2097 (2003)
29. M.A. Nasseri, S.M. Sadeghzadeh, J. Iran. Chem. Soc. 11, 27
(2014)
1 3