Phosphotungstic acid supported on silica-coated CoFe2O4 nanoparticles: An efficient and magnetically-recoverable catalyst for N-formylation of amines under solvent-free conditions

Article abstract of DOI:10.1016/j.molcata.2015.05.009

Phosphotungstic acid (PTA) was rapidly and efficiently anchored on the surface of silica coated cobalt ferrite core to obtain a new three-component CoFe₂O₄@SiO₂-PTA nanocomposite. The resultant composite was characterized by various techniques, including FT-IR, XRD, SEM, EDX, ICP-AES, and VSM. The as-synthesized nanocomposite was examined as catalyst for N-formylation of various amines under solvent-free conditions at room temperature. Due to the presence of magnetic core in this catalyst, it can be easily separated from the reaction media by applying an external magnetic field. The isolated catalyst can be reused for at least five successive times without significant leaching or loss of its high catalytic activity.

Full text of DOI:10.1016/j.molcata.2015.05.009

Accepted Manuscript
Title: Phosphotungstic acid supported on silica-coated
CoFe2O4 nanoparticles: An efficient and
magnetically-recoverable catalyst for N-formylation of amines
under solvent-free conditions
Author: M. Kooti E. Nasiri
PII:
S1381-1169(15)00182-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.05.009
MOLCAA 9495
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
21-12-2014
29-4-2015
14-5-2015
Please cite this article as: M.Kooti, E.Nasiri, Phosphotungstic acid supported on silica-
coated CoFe2O4 nanoparticles: An efficient and magnetically-recoverable catalyst for
N-formylation of amines under solvent-free conditions, Journal of Molecular Catalysis
A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.05.009
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Phosphotungstic acid supported on silica-coated CoFe2O4 nanoparticles:
An efficient and magnetically-recoverable catalyst for N-formylation of
amines under solvent-free conditions
M. Kooti* and E. Nasiri
Department of Chemistry, Shahid Chamran University, Ahvaz, Iran
*m_kooti@scu.ac.ir
Graphical abstract
Highlights
 Phosphotungstic acid was anchored on CoFe2O4@SiO2.
 This composite was used as catalyst in N-formylation reactions.
 The N-fomylation reactions were carried out under solventless conditions.
 The catalyst can be recovered by applying a permanent magnet.
Abstract
1

Phosphotungstic acid (PTA) was rapidly and efficiently anchored on the surface of silica
coated cobalt ferrite core to obtain a new three-component CoFe2O4@SiO2-PTA
nanocomposite. The resultant composite was characterized by various techniques, including
FT-IR, XRD, SEM, EDX,ICP-AES, and VSM. The as-synthesized nanocomposite was
examined as catalyst for N-formylation of various amines under solvent-free conditions at
room temperature. Due to the presence of magnetic core in this catalyst, it can be easily
separated from the reaction media by applying an external magnetic field. The isolated
catalyst can be reused for at least five successive times without significant leaching or loss of
its high catalytic activity.
Keywords: Phosphotungstic acid; Aniline; Nanocomposite; Cobalt ferrite; N-formylation.
1. Introduction
Heteropolyacids, with the molecular formula of [XM12O40]n-, where X = P5+, As5+, Si4+, or
Ge4+ and M = Mo6+, or W6+, have been extensively used as catalysts for various organic
reactions due to their unique physicochemical properties[1-6]. Among the well- known
Keggin types of these solid acids, phosphotungstic acid (PTA), H3PW12O40, has been the
target catalyst and received considerable attention in the last decade because of its super
strong Brønsted acidity as well as its ease of preparation [7-9]. It is well documented that
immobilization of PTA on a suitable support can eliminate its encountered major drawbacks,
namely, (i) very low surface area, and (ii) instability in polar solvents. In fact, most of the
acidic protons of the bulk phosphotungstic acid are in the interior of the solid which are
inaccessible for catalysis of non-polar hydrocarbon reactions. Supported PTA, however, has a
2

greater number of surface acid sites than its bulk component and therefore presents higher
catalytic activity [10-13].
Various materials, especially mesoporous type, have been used as support for PTA,
such as, silica, activated carbon, zeolites, alumina, zirconia, titaniaand etc. [14-18]. Out of
these materials, silica (SiO2) is the most appealing support for PTA due to its relative ease of
preparation, hydrophilic nature, non-basic character, physical and chemical stability and good
biocompatibility [19,20]. While PTA supported on silica shows high catalytic efficiency and
can be recycled more readily than its homogeneous counterpart, the tedious recovery
procedure via filtration or centrifugation and the inevitable loss of the solid catalyst in the
separation process still limited its application especially for the small nanosized particles.
Introducing magnetic nanoparticles (MNPs) into the PTA supported silica can provide a
means for efficient separation of the catalyst from reaction media. Therefore, MNPs based
catalysts have been recently receiving growing attention, as they can be easily separated from
the reaction mixture by applying a simple permanent magnet [21-24].The approach of
magnetic separation, taking advantage of MNPs, is typically more effective than filtration or
centrifugation as it prevents loss of the catalyst into the environment.
Although there are many types of MNPs, spinel ferrites with the general formula of
MFe2O4 (M is a divalent metal like Co2+, Ni2+ and etc.) show interesting properties and have
potential applications in various fields [25-27]. Among several spinel ferrites, cobalt ferrite
(CoFe2O4) has gained a great deal of attention because of its high thermal stability, moderate
saturation magnetization, remarkable chemical stability and mechanical hardness [28,29].
Coating of CoFe2O4 nanoparticles with silica to obtain CoFe2O4@SiO2 composite, will
achieve further stabilization of these MNPs as well as prevents their agglomeration in
solution. The outer mesoporous silica shell of CoFe2O4@SiO2 composite, with core-shell
3

structure, makes it suitable for multifunctional surface modification such as catalyst
immobilization [30-34].
Phosphotungstic acid can catalyze many organic reactions requiring acid sites. One of
these reactions is N-formylation of amines, which generally leads to the formation of
formamides. This is an important organic reaction since foramides are intermediates in the
synthesis of various heterocyclic medicinally useful compounds. Although several methods
have been so far used for achieving formylation of amines, most of these methods suffer from
some disadvantages, including strict anhydrous conditions, low yields, long reaction times,
high reaction temperature, expensive and toxic reagents, use of harmful solvents, and etc.
[35-40].
In continuation of our previous works on the development of non-toxic, low cost, eco-
friendly and easily recyclable catalysts [41,42], we herein report the preparation of a new
three-component CoFe2O4@SiO2-PTA nanocomposite via a straightforward procedure. The
as-synthesized composite is consisted of a magnetically responsive CoFe2O4 core, silica as
inner shell and PTA as outer shell. A pre-synthesized CoFe2O4 sample was first prepared
which was then coated with silica layer and finally PTA was anchored onto the surface of the
silica shell. After preparation and characterization of the new CoFe2O4@SiO2-PTA
composite, it was used as efficient reusable, and green catalyst for some N-formylation
reactions under solvent-free conditions (see Scheme 1).Within the limit of our knowledge,
this is the first example of using PTA supported on silica-coated CoFe2O4 as efficient
catalyst for N-formylation process.
4

H
CHO
NH2
N
O
CoFe2O4@SiO2-PTA
Solvent-free, r.t
+
HO
H
Scheme 1. N-formylation of amine in the presence of CoFe2O4@SiO2-PTA.
2. Experimental
2.1. Materials and methods
All the chemicals were of analytical grade and used without further purification.
Manipulation and reactions were carried out in air without the protection of inert gas.
Phosphotungstic acid was synthesized according to the literature [43]. Fourier transform
infrared (FT-IR) spectra were obtained using a FT BOMEM MB102 spectrophotometer. X-
ray diffraction (XRD) patterns of the synthesized samples were taken with a Philips X-ray
diffractometer (model PW1840) over a 2θ range from 10 to 80° using Cu Kα radiation
(λ=1.54056 Å). The FESEM images were obtained using a Hitachi Japan S4160 scanning
electron microscope. The magnetic properties of the fabricated CoFe2O4@SiO2-PTA
composite as well as other samples were studied using vibrating sample magnetometer
(VSM) of Meghnatis DaghighKavir Company.
2.2. Synthesis of CoFe2O4@SiO2-PTA nanocomposite
Cobalt ferrite MNPs were prepared using the method reported by Maaz et al. with some
minor modification [44]. In our procedure of synthesizing CoFe2O4 nanoparticles, olive oil
was used as surfactant instead of oleic acid. No heat treatment is required in this method and
the ferrite can be easily and rapidly separated from reaction mixture using a permanent
magnet. Coating of CoFe2O4 nanoparticles with a layer of silica was achieved through Stöber
method [45], which includes hydrolysis of tetraethylorthosilicate (TEOS) in the presence of
NH3 to obtain the two-component CoFe2O4@SiO2 composite. The latter composite was
5

collected by applying a permanent magnet and was finely powdered after being dried. The
surface of the as-fabricated CoFe2O4@SiO2composite was then anchored with PTA via a
simple procedure. In this procedure, a round-bottom flask was charged with ethanol-water
(80:20 ratio) solution containing 1.0 g of PTA and the solution was stirred at room
temperature for 1 h. Into this solution, 1.0 g of CoFe2O4@SiO2 was dispersed and stirring was
continued for additional 3 h. The resulting final solid was separated by magnetic decantation,
washed with cold absolute ethanol and dried in an oven at 100 °C for 2 h. The amount of
PTA in CoFe2O4@SiO2-PTA composite was found to be 0.0875 mmol g-1, as determined by
ICP-AES analysis. The amount of H+ in the CoFe2O4@SiO2-PPA, determined by acid-base
titration, was 0.10 mmol/g. The step-by-step fabrication of the composite (CoFe2O4@SiO2-
PTA) is shown in Scheme 2.
Scheme 2. Step - by- step synthesis of CoFe2O4@SiO2-PTA nanocomposite.
2.3.General procedure for N-formylation reactions catalyzed by CoFe2O4@SiO2-PTA
under solvent-free conditions
A mixture of aniline (1.0 mmol), formic acid (1.0 mmol), and CoFe2O4@SiO2-PTA (0.05 g)
was stirred at room temperature. The progress of the reaction was continuously monitored by
TLC. After 30 m period of time and upon completion of the reaction, as indicated by TLC
tests, ethyl acetate was added to the reaction mixture. The catalyst was magnetically fixed at
6

the side wall of the reactor and the liquid phase was decanted. The catalyst was then washed
with ethyl acetate and dried to be reused in a new N-formylation reaction. The isolated
organic phase was washed with a saturated solution of NaHCO3 and dried over anhydrous
Na2SO4. The solvent, ethyl acetate, was removed by a rotary evaporator and the resulted
residue was then subjected to recrystallization to obtain pure formamide product with 95%
yield. Characterization of the N-formylated products, as well as determination of the yields,
in this and other similar reactions were achieved by GC analysis. To further confirm the
identity of the products, their physical and FT-IR spectral data were compared with those
reported in the literature.
The obtained products from selected reactions of aniline derivatives with formic acid
1
were also characterized by H NMR and GC-MS techniques. The obtained analysis data,
which are given below, clearly confirmed the structure and formula of the expected
formamide products.
Selected spectroscopic date
N-Phenyl formamide( Entry 1)
1H NMR (CDCl3, 250 MHz): δ 7.1, 7.5 (m; cis and trans, para, meta and ortho Ar-H), 7.8 (s,
broad, cis, NH), 8.3 (s, cis, CHO), 8.7 (d, trans, CHO), 8.8 (s, broad, trans, NH).
EI-MS: m/z = 121.2(M+).
N-(4-Nitrophenyl) formamide (Entry 4)
1H NMR (CDCl3, 250 MHz: δ 6.57–8.24 (m, Ar-H), 10.83 (s, CHO, trans), 10.74 (s, broad,
trans, NH), 9.04 (s, broad, NH, cis), 8.40 (s, CHO, cis).
N-(4-cyanophenyl) formamide (Entry 5)
1H NMR (CDCl3, 250 MHz: δ 10.83 (s, CHO, trans), 10.74 (s, broad, trans, NH), 9.04 (s,
broad, NH, cis), 8.40 (s, CHO, cis), 6.57-8.24 (m, Ar-H). EI-MS: m/z = 146.1(M+).
7

N-(4-Methoxyphenyl) formamide (Entry 8)
1H NMR (CDCl3, 250 MHz): δ6.86-6.91 (m, Ar-H), δ 8.47-8.51(d, CHO, cis), 8.34 (s, CHO,
trans), 7.43-7.46 (d, NH, trans), 7.01-7.04 (d, NH, cis), 3.80 (S, CH3).
N-(2,5-Dimethylphenyl)formamide (Entry 9)
EI-MS: m/z = 149.5(M+).
N-benzyl formamide (Entry 10)
EI-MS: m/z = 135.3(M+).
N,N'-benzene-1,2-diyldiformamide (Entry 11)
EI-MS: m/z = 164.4(M+).
3. Results and discussion
3.1.Characterizationof CoFe2O4@SiO2-PTA composite
The nanocompositeCoFe2O4@SiO2-PTA was fabricated via a straightforward three-step
procedure. Cobalt ferrite, as magnetic core of the composite, was made by co-precipitation
method which was then coated with a thin layer of silica. In the final step PTA was
immobilized onto the surface of the silica shell.
The as-synthesized nanocomposite was characterized by various techniques. The FT-
IR spectra of all synthesized samples along with pure PTA are presented in Fig. 1. As it is
seen in Fig. 1-B, the FT-IR spectrum of CoFe2O4@SiO2-PTA contains all the characteristic
peaks of its three components, i.e. CoFe2O4, SiO2 and PTA. Two peaks at 425 cm-1 and 588
cm-1 are attributed to Fe-O stretching in the tetrahedral and octahedral sites of CoFe2O4
respectively [46]. The bands located at 1091 cm-1 and 798 cm-1,in the spectrum of
CoFe2O4@SiO2 composite, are ascribed to the symmetrical and asymmetrical vibration
8

modes of the Si–O–Si bonds in SiO2 ( see Fig.1-B) [47]. Both these peaks, with some shift
due to the presence of PTA, are clearly seen in Fig. 1-B of CoFe2O4@SiO2-PTA.Twoother
bands at 890 cm-1 and 959 cm-1 are also observed in the spectrum of this composite, which
are respectively attributed to W–O–W and W=O stretching modes of PTA [48]. In all these
samples the band centered at about 1630 cm-1 is assigned to the H–O–H bending vibration of
the absorbed water.
Fig. 1. FT-IR spectra of PTA(A), CoFe2O4@SiO2-PTA(B), CoFe2O4@SiO2(C) and CoFe2O4(D).
The XRD patterns of CoFe2O4,CoFe2O4@SiO2,CoFe2O4@SiO2-PTA, and free PTA
are displayed in Fig. 2. The diffraction peaks related to Bragg’s reflections from (2 2 0), (3 1
9

1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes correspond to the standard spinel structure of
CoFe2O4(JCPDS card no. 22-1086) with a space group of Fd3m (see Fig. 2-A). The
crystallite size of CoFe2O4 nanoparticles was about 18 nm, as measured by using the well-
known Debye-Scherrer formula [49]. The XRD patterns of CoFe2O4 core, after coating with
silica and anchoring with PTA (Fig.2-B, and 2-C), are more or less the same as pristine
CoFe2O4revealing that the spinel cubic structure of the ferrite is retained. In the XRD patterns
of the as-made CoFe2O4@SiO2-PTA composite, however, no crystalline phase for the
immobilized PTA was observed which implied that PTA is homogeneously distributed onto
the support in a non-crystalline form or that crystallites are too small. The later specificity is
of importance for the catalytic properties of the synthesized composite as accessibility of the
active species on the surface would be enhanced [50,51]. The lack of observing the peaks of
PTA in the XRD of CoFe2O4@SiO2-PTA may be also due to the change in its crystal
structure after immobilization.
Fig. 2. XRD patterns of CoFe2O4(A), CoFe2O4@SiO2(B), CoFe2O4@SiO2-PTA(C) and PTA(D).
10

The SEM images of CoFe2O4, CoFe2O4@SiO2 and CoFe2O4@SiO2-PTA are depicted
in Fig. 3. As it is clearly seen, the CoFe2O4 and CoFe2O4@SiO2 particles are well-resolved
Fig. 3. SEM image of CoFe2O4 (A), CoFe2O4@SiO2 (B), CoFe2O4@SiO2-PTA(C).
and have spherical shapes (Fig. 3-A and 3-B). The PTA particles in the SEM of
CoFe2O4@SiO2-PTA are seen as white spots randomly inlaid on the surface of
11

CoFe2O4@SiO2 (see Fig. 3-C). In order to further confirm the composition of the as-
fabricated CoFe2O4@SiO2-PTA composite, energy dispersive X-Ray (EDX) analysis was
conducted. The EDX spectrum, which is displayed in Fig. 4, clearly reveals the presence of
all the expected main elements (O, Si, P, Fe, Co and W) comprising this three-component
composite. The surface area of CoFe2O4@SiO2-PTA was also determined using the nitrogen
gas adsorption (BET) method and a value of 6 m2 g−1 was found for this composite.
5000
O K
SiK
W M
4000
3000
2000
1000
0
FeL
AuM
AuM
P K
FeK
CoK
CoL
W Ll
P K
FeK
AuLl
W L
AuL
W L
CoK
keV
0
5
10
Fig. 4. EDX spectrum of CoFe2O4@SiO2-PTA nanocomposite.
Magnetic measurements for CoFe2O4, CoFe2O4@SiO2 and CoFe2O4@SiO2-PTA
samples were carried out using a vibrating sample magnetometer (VSM) with a peak field of
8 kOe and their hysteresis curves are shown in Fig. 5. It is clear from the hysteresis loops that
the saturation magnetization (Ms) of CoFe2O4, CoFe2O4@SiO2 and CoFe2O4@SiO2-PTA are
61.05, 26.22 and 16.60 emu/g, respectively. The decrease in mass saturation magnetization in
the last two composites can be attributed to the contribution of the non-magnetic silica shell
and functionalized phosphotungstic acid. In fact, silica shell which surrounds the magnetic
CoFe2O4 cores, will prevent them from approaching each other and therefore decreases the
interactions between these cores [52,53]. Although the value of Ms for CoFe2O4 core is
evidently decreased after coating with silica and anchoring with PTA, the CoFe2O4@SiO2-
12

PTA composite, can still be efficiently and easily separated from solution with the help of an
external magnetic force.
Fig. 5.Hysteresis loops of CoFe2O4 (A), CoFe2O4@SiO2 (B) and CoFe2O4@SiO2-PTA (C).
3.2. Evaluation of catalytic activity of CoFe2O4@SiO2-PTA in N-formylation reactions
The reaction of aniline and formic acid was chosen as a model of N-formylation reaction. In
an experiment, this reaction was carried out in the absence of CoFe2O4@SiO2-PTA under
solvent-free conditions. No change in the starting materials was observed after stirring the
mixture at room temperature for 3 h. However, addition of a catalytic amount of
13

CoFe2O4@SiO2-PTA to this mixture has rapidly induced N-formylation to produce
formamide in high yield. This clearly reveals that this reaction can only take place in the
presence of catalyst. In order to further investigate the scope and limitations of the as-
prepared catalyst, different aniline derivatives and formic acid were subjected to N-
formylation reaction under the same conditions and the obtained results are summarized in
Table 1.
It is evident from the data in Table 1 that all examined reactions proceeded rapidly
and cleanly, irrespective of the substituent types. The conversions were nearly completed
within 30-60 min at room temperature and no undesirable side-reactions were observed. The
difference in the yields of the corresponding N-formamides for the examined aniline
derivatives is not significant. However, the observed minor differences in the obtained yields
can be attributed to the change in aniline substituents i.e., anilines with electron-donating
substituents, such as CH3 and OCH3 (Table 1; entries 8 and 9) proceed somewhat more
efficiently than those having electron-withdrawing groups like Cl or NO2 (Table 1; entries 4
and 6). The observed results of this work are in accordance with other previously reported
finding [54, 55]. Interestingly, no reaction was observed with phenols (Table 1; entries 15and
16) using the same conditions applied for N-formylation reactions. Moreover, when a
molecule containing both hydroxyl and amino group was treated with formic acid, under the
present reaction conditions, only N-formylated product was obtained and the OH group
remained intact (Table 1; entry 14). This fact that no O-formylation was observed for phenols
and aminophenol clearly conform the chemoselectivity of these reactions. It was found that the
order of addition of the reactants on the catalyst has no effect on the N-formylation reactions.
14

Table 1 N-formylation of some primary amines using CoFe2O4@SiO2-PTA as catalysta.
Entry
1
Amine
Productb
Time(min)
30
Yields (%)c
95
H
NH2
N
CHO
H
2
3
35
35
92
85
NH2
N
CHO
Br
Br
H
NH2
N
CHO
Cl
Cl
H
4
5
60
50
45
89
50
85
O2N
NC
NH2
O2N
N
CHO
H
NH2
NC
N
CHO
CHO
Cl
Cl
H
6
7
NH2
N
O
O
H
N
NH2
60
25
88
99
CHO
O
O
H
8
9
H3CO
NH2
H3CO
N
CHO
Me
Me
H
35
93
NH2
N
N
CHO
H
Me
Me
NH2
10
11
35
60
91
CHO
H
NH2
N
CHO
88d
H
NH2
N
CHO
15

Table 1(Continued)
Entry
12
Amine
Productb
Time(min)
45
Yields (%)c
90
H
NH2
N
CHO
H
N
H
NH2
13
14
60
35
97
88
O
H
H3C
NH2
H3C
N
O
H
OH
OH
15
16
OH
-
-
-
-
-
-
OH
a Aromatic or aliphatic amine (1 mmol), formic acid (1 mmol), and CoFe2O4@SiO2-PTA(0.05 g) at rt.
b All the products were identified by spectral data IR with those of authentic samples.
c Isolated yields
d amine: formic acid (1:2 mmol)
Our finding, that the –NH2 group is preferred in these types of reactions than –OH, is in
agreement with other previously reported works [39,40].This can be attributed to
nucleophilicities of these functional groups. The -NH2 group is obviously stronger
nucleophile than -OH and therefore no O-fomylation reaction is expected.
The effectiveness of the as-synthesized CoFe3O4@SiO2-PTA catalyst in N-
formylation reactions is due to presence of PTA in this composite. To further prove this idea,
each of CoFe3O4, CoFe3O4@SiO2 and neat PTA were examined separately for N-formyltion
reactions. As seen in Table 2, neither of CoFe3O4 or CoFe3O4@SiO2 has shown any catalytic
effect on this reaction; whereas neat PTA was found to be as effective catalyst as
CoFe3O4@SiO2-PTA.The difficulty in recovery and recycling of pristine PTA, however,
limits its application as catalyst for these types of reactions.
16

Table 2 Comparative study of Catalysts for N-formylation reaction
Entry
Catalysts
CoFe3O4
Time (min)
Yield%
1
2
3
4
120
120
15
-
CoFe3O4@SiO2
H3PW12O40
-
91
95
CoFe3O4@SiO2- PTA
30
In order to illustrate the merit of the present catalytic system, we compared the results
of CoFe2O4@SiO2-PTA in the N-formylation of aniline with formic acid to other reported
protocols (see Table 3).Our newly synthesized three-component nanocomposite is an equally
or more efficient catalyst than previously reported ones with respect to reaction time and
reaction conditions. The superiority of the introduced catalyst in the present work will be
more obvious if its magnetic property is taken into account. This magnetically-responsive
catalyst can be readily separated from solution by applying an external magnetic field.
Table 3 Comparison of efficiency of various conditions in the N-formylated of aniline.
Entry
catalyst
Time (min)
Temperature (°C)
Yield%
96
Ref.
57
58
36
56
a
1
2
3
4
5
HClO4-SiO2
15
45
10
7
room
room
70
bTiO2-P25
ZnCl2
99.2
96
Silica sulfuric acid
50-60
Room
99
CoFe3O4@SiO2- PTA
30
95
a)This work. baniline:formic acid ratio 3:1, in CH3CN.
17

3.3. Recycling of the catalyst
The reusability of the catalysts is crucial in the practical application and has to be considered
in heterogeneous process. Therefore, reusability of the CoFe2O4@SiO2-PTA catalyst was
investigated. For this purpose, the same model reaction was again examined under optimized
conditions. After completion of the N-formylation of aniline, the catalyst was collected by an
external magnet, followed by washing it for three times with ethyl acetate. After drying the
isolated catalyst at 100 ◦C for 2 h, it was reused in a new reaction. As shown in Fig 6, the
recovered catalyst could be reused for at least five successive times without any significant
loss of its activity. The FT-IR spectrum of the recovered catalyst showed no change after it
has been used for four successive times (see Fig.7). Furthermore, the amount of PTA was
measured in the recycled catalyst by ICP-AES analysis. This measurement showed that the
PTA content of the recycled CoFe2O4@SiO2-PTA catalyst, after 5 runs, is 0.0823 mmolg-1 .
This indicates the loss of only 6% of the original PTA in the composite.
Leaching test can further confirm the heterogeneous nature of as-fabricated
catalyst. Therefore, reaction of aniline and formic acid was carried out in the presence of
CoFe2O4@SiO2-PTA for 15 min under the same used conditions. At this point, the catalyst
was removed by applying a permanent magnet and the residue was then allowed to react in
the absence of catalyst. No significant conversion of the reactants to the N-formylated
product was observed after 3 h of stirring the mixture. These observations clearly rule out the
simple contribution of leached homogeneous catalyst in the reaction. The obtained results
showed that the designed magnetically-responsive catalyst is economic, easily made and
environmentally friendly because of its low leaching after at least five cycles.
18

Fig. 6 Reusability of CoFe2O4@SiO2-PTA catalyst in the model reaction
Fig. 7 FT-IR spectra of catalyst: fresh (A) and recycled catalyst after first to fourth runs (B-D)..
19

3.4. Proposed mechanism for N-formylation of aniline
Based on the previously reported studies [58-61] and our observations in the course of this
reaction, the following mechanism, which is consisted of two pathways, can be proposed (see
Scheme 3). According to this mechanism; in path 1 the carbonyl group of formic acid is first
activated by coordination to PTA on the surface of CoFe2O4@SiO2-PTA catalyst to produce
intermediate (v). In fact, formic acid is much weaker BrØnsted acid (Ka=1.77×10-4) than PTA
(super-strong acid) and acts as a base in contact with PTA. The activated carbonyl center of
intermediate (V) subsequently reacts with the –NH2 group of aniline to give intermediate (VI)
followed by a proton transfer to the catalyst yielding the corresponding formamide (VII). In
path 2, on the hand, both formic acid and aniline are coordinated to PTA to form
intermediate (III) which then produces intermediate (IV). The later undergoes some
rearrangement and proton transfer to give the final N-formylated product.
20

NH2
O
O
C
H
Path 1
+
+
H
OH
H
HO
H3PW12O40
(I)
H2PW12O40
(II)
H3PW12O40
Path 2
(V)
(II)
HO
H
2
1
N
H
O
H
H
H
O
H
H2PW12O40
H2PW12O40
+
NH2
H
HO
H2PW12O40
(III)
(VI)
-H2O
2
+
N
H
H
HO
H
O
1
4
3
O
H
-
H
N
H
H3PW12O40
H2PW12O40
-H2O
+
(VII)
(IV)
Scheme 3. Proposed mechanism for N-formylation of aniline on the surface of CoFe2O4@SiO2-PTA catalyst
21

Conclusion
In this study, a new magnetically-responsive composite, CoFe2O4@SiO2-PTA, has been
prepared via a simple and rapid procedure. This catalyst was used in some N-formylation
reactions which gave high yields of formamides in short reaction times. This chemoselective
catalytic reaction was performed under solvent-free conditions so no harmful volatile solvent
is released into the environment and can be considered as "Green procedure". This new
catalyst has a magnetic core and therefore can be readily separated from the reaction mixture
to be reused for several times with high efficiency.
Acknowledgment
The authors wish to acknowledge the support of this work (grant No.1393) provided by the
Research Council of Shahid Chamran University, Ahvaz, Iran.
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Figure Captions
Scheme 1. N-formylation of amine in the presence of CoFe2O4@SiO2-PTA.
Scheme 2. Step - by- step synthesis of CoFe2O4@SiO2-PTA nanocomposite.
Scheme 3. Proposed mechanism for N-formylation of aniline on the surface of
CoFe2O4@SiO2-PTA catalyst.
Fig. 1. FT-IR spectra of PTA (A), CoFe2O4@SiO2-PTA(B), CoFe2O4@SiO2(C) and
CoFe2O4(D).
Fig. 2. XRD patterns of CoFe2O4 (A), CoFe2O4@SiO2(B), CoFe2O4@SiO2-PTA(C) and
PTA(D).
Fig. 3. SEM image of CoFe2O4 (A), CoFe2O4@SiO2(B), CoFe2O4@SiO2-PTA(C).
Fig. 4. EDX spectrum of CoFe2O4@SiO2-PTA nanocomposite.
Fig. 5.Hysteresis loops of CoFe2O4(A), CoFe2O4@SiO2(B) and CoFe2O4@SiO2-PTA(C).
26

Fig. 6 Reusability of CoFe2O4@SiO2-PTA catalyst in the model reaction.
Fig. 7 FT-IR spectra of catalyst: fresh (A) and recycled catalyst after first to fourth runs (B-D).
Table Captions
Table 1 N-formylation of some primary amines using CoFe2O4@SiO2-PTA as a catalysta.
Table 2 Comparative study of Catalysts for N-formylation reaction.
Table 3 Comparison of efficiency of various conditions in the N-formylated of aniline.
27