H3PW12O40 anchored on graphene-grafted silica-coated MnFe2O4 as magnetic catalyst for Mannich reaction

Article abstract of DOI:10.1007/s13738-018-1292-4

In this study, a three-component nanocomposite consisted of graphene, manganese ferrite and phosphotungstic acid (PTA) has been prepared. This composite, which is designated as Graphene/MnFe₂O₄@PTA, was synthesized through anchoring of PTA–imidazolium ionic liquid on magnetic graphene sheets. The structural and magnetic properties of the fabricated nanocomposite were studied by employing FT-IR, SEM, EDX, TEM, ICP, VSM, P-XRD and BET techniques. The synthesized magnetic nanocomposite was examined as an efficient and recyclable acidic catalyst for Mannich reaction under solvent-free conditions. The products of this reaction, which are an important class of potentially bioactive compounds, were obtained with good to excellent yields, and the catalyst could be readily recycled without any significant loss of its activity.

Full text of DOI:10.1007/s13738-018-1292-4

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1292-4
ORIGINAL PAPER
H3PW12O40 anchored on graphene‑grafted silica‑coated MnFe2O4
as magnetic catalyst for Mannich reaction
M. Kooti1 · F. Kooshki1 · E. Nasiri1 · A. Naghdi Sedeh1
Received: 21 March 2017 / Accepted: 5 January 2018
© Iranian Chemical Society 2018
Abstract
In this study, a three-component nanocomposite consisted of graphene, manganese ferrite and phosphotungstic acid (PTA)
has been prepared. This composite, which is designated as Graphene/MnFe2O4@PTA, was synthesized through anchoring
of PTA–imidazolium ionic liquid on magnetic graphene sheets. The structural and magnetic properties of the fabricated
nanocomposite were studied by employing FT-IR, SEM, EDX, TEM, ICP, VSM, P-XRD and BET techniques. The syn-
thesized magnetic nanocomposite was examined as an efcient and recyclable acidic catalyst for Mannich reaction under
solvent-free conditions. The products of this reaction, which are an important class of potentially bioactive compounds, were
obtained with good to excellent yields, and the catalyst could be readily recycled without any signifcant loss of its activity.
Keywords Nanocatalyst · Mannich reaction · Phosphotungstic acid · Graphene · Manganese ferrite
Introduction
increase the specifc surface area as well as its acidity; both
of which lead to the enhancement of the catalytic activity.
Recently, magnetic nanoparticles (MNPs) have appeared
as interesting support materials for catalyst immobilization.
This is due to their high stability, ease of synthesis and func-
tionalization, high surface area, low toxicity and above all
their facile separation by applying magnetic feld. There are
many types of MNPs but spinel ferrites with the general
formula of MFe2O4 (M is a divalent metal like Co2+, Ni2+,
etc.) exhibit attractive properties and have many potential
applications in various felds. Among the well-known spinel
ferrites, manganese ferrite (MnFe2O4) has received increas-
ing attention for its remarkable magnetic properties, low
coercivity, moderate saturation magnetization, chemical sta-
bility, high permeability and mechanical hardness [11–19].
On the other hand, graphene, one of the thinnest materials
with a two-dimensional carbon sheet structure has inspired
enormous interest in diverse felds including chemistry. The
extraordinary properties of graphene make it a favorable
material for wide applications [20–22]. Moreover, combin-
ing of graphene with a magnetic material, such as MnFe2O4,
will provide an excellent means for separation of graphene
composites via magnetic decantation as well as reducing
the agglomeration tendency of graphene sheets [23–27].
Although immobilization of homogenous catalyst can be
achieved directly on the surface of MNPs, coating of MNPs
with mesoporous silica modifed by ionic liquid seems to
Heteropoly acids (HPAs) especially those of tungsten,
molybdenum and vanadium have attracted a great deal of
interest due to their high acidity and favorable redox prop-
erties. Among the various HPAs, phosphotungstic acid,
H3PW12O40, has been widely studied as homogeneous cat-
alyst for many valuable organic reactions. Although bulk
PTA has shown good activity in most homogeneous catalytic
systems, it commonly sufers from some serious drawbacks,
mainly high solubility in polar solvents and low surface area.
The high solubility of PTA in polar solvents obstructs its
separation from the product which can lead to economic and
environmental problems. However, recent studies have dem-
onstrated that immobilization of PTA on an inert support,
via forming ionic liquid, opens a novel strategy for extending
the applicability of this catalyst [1–10]. Anchoring of PTA
on support materials through ionic liquids will signifcantly
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-018-1292-4) contains
supplementary material, which is available to authorized users.
* M. Kooti
m_kooti@scu.ac.ir
1
Department of Chemistry, Shahid Chamran University
of Ahvaz, Ahvaz, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
be more promising for attractive applications in the feld of
heterogeneous catalysis [28–32].
were performed with a Shimadzu GC-16A instrument using
a 2-m column packed with siliconDC-200 or Carbowax
20 m.
In the present study, we have prepared a multi-component
nanocomposite, Graphene/MnFe2O4@PTA, which is con-
sisted of phosphotungstic acid anchored on silica-coated
Graphene/MnFe2O4. For the synthesis of this nanocompos-
ite, Graphene/MnFe2O4 was frst coated with silica to obtain
Graphene/MnFe2O4@SiO2 which then modifed with meth-
ylimidazolium chloride. PTA was subsequently grafted on
the silica shell of the latter composite via ion exchange and
the formation of PTA–Imid ionic liquid. The as-prepared
nanocomposite was then used as catalyst for the three-
component coupling Mannich-type reactions with alde-
hydes, amines, and ketone substrates. The products which
obtained from these condensations reactions, Mannich bases
or β-amino carbonyl compounds, have wide applications in
industry, pharmacology and organic synthesis. The conven-
tional and even some of the recently developed catalysts for
the classical Mannich reactions sufer from some disadvan-
tages and limitations such as, low yields, high cost, tedious
method of separation, long reaction times and use of toxic
solvents [33–41]. Therefore, developing of eco-friendly pro-
cedures for Mannich reaction in which the catalyst can be
easily separated and recycled and the need for organic sol-
vents is eliminated are of high value. Our introduced catalyst
system for Mannich reaction is a one-pot procedure achieved
in free-solvent conditions, and the catalyst can be readily
separated by an external magnetic force to prevent any con-
tamination to the environment.
Preparation of Graphene/MnFe2O4@PTA
The graphene/MnFe2O4 composite was synthesized by dis-
persing of 0.1 g of graphene into 20 mL of deionized water
with sonication for 30 min. Then, MnFe2O4 nanoparticles
were deposited onto graphene sheets using co-precipitation
method. The obtained nanocomposite was collected by
applying a permanent magnet, washed with hot water–eth-
anol and fnely powdered after being dried in an oven at
100 °C for 4 h. The Graphene/MnFe2O4 nanocomposite was
coated with silica through Stober method [42]. PTA was sub-
sequently immobilized on Graphene/MnFe2O4 by our previ-
ously reported method [43]. In the fnal fabricated compos-
ite, PTA is attached to the silica coat through imidazolium
linker (see Scheme 1). The amount of PTA in this nano-
composite was found to be 0.0583 mmol g−1, as determined
by ICP-AES analysis. The amount of H+ in the composite,
measured by acid–base titration, was 0.175 mmol g−1.
General procedure for Mannich reactions catalyzed
by Graphene/MnFe2O4@PTA
A mixture of benzaldehyde (2.5 mmol, 0.26 g), aniline
(2.5 mmol, 0.23 g) and cyclohexanone (2.5 mmol, 0.24 g) with
catalytic amount of Graphene/MnFe2O4@PTA (0.1 g) was
ground using a mortar and pestle. The progress of this con-
densation reaction was continuously monitored by TLC. Ethyl
acetate (10 mL) was fnally added to the reaction mixture, and
the nanocatalyst was magnetically separated and washed with
ethyl acetate and dried to be reused in a new Mannich reaction.
The left liquid was washed with saturated aqueous sodium
chloride solution (5 mL) and water (10 mL). The organic layer,
which contains the product of Mannich reaction, was separated
Experimental section
Materials and methods
All the reagents were of analytical grade and used as
received unless specifed otherwise. The products of Man-
nich reactions were characterized by comparison of their
1
physical data, such as FT-IR and H NMR spectra, with
those of known samples. NMR spectra were recorded on
a Bruker Advance DPX 400 MHz instrument spectrometer
using TMS as internal standard. FT-IR spectra were obtained
using a FT BOMEM MB102 spectrophotometer. X-ray dif-
fraction (XRD) patterns were taken with a Philips X-ray dif-
fractometer (model PW1840) over a 2θ range from 10° to
80° using Cu Kα radiation (λ = 1.54,056 Å). The FESEM
images were obtained using a Hitachi Japan S4160 scan-
ning electron microscope. The magnetic properties of the
samples were studied using vibrating sample magnetometer
(VSM) of Meghnatis Daghigh Kavir Company. The tung-
sten content of the Graphene/MnFe2O4@PTA nanocatalyst
was determined using an ICP-AES instrument (HORIBA
Jobin–Yvon, Longjumeau Cedex, France). GC experiments
Scheme 1 Structure of Graphene/MnFe2O4@PTA
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Journal of the Iranian Chemical Society
and dried over anhydrous Na2SO4 followed by evaporating of
the solvent under rotary evaporator. The left residue was then
recrystallized in ethanol to give pure β-amino carbonyl com-
pound product with 97% yield. Determination of the yields in
this and other similar reactions were performed by GC analy-
sis. All the obtained products from these reactions are known
compounds and hence were easily characterized on the basis of
their physical and FT-IR spectral data and by comparison with
those reported in the literature [38–40]. The products obtained
from a number of selected reactions of aniline and benzalde-
hyde derivatives with cyclohexanone were also further charac-
terized by 1HNMR and GC–MS techniques. The physical and
spectral data of the selected products are given below.
1.51–1.63 (m, 2H), 1.87–1.91 (m, 4H), 2.30–2.40 (m,
2H), 2.75–2.78 (m, 1H), 3.32 (br S, 1H), 3.90 (S, 3H,
OCH3),5.06–5.09 (d, 1H), 6.44–6.52 (m, 2H), 6.81–6.90
(m, 1H), 6.92–6.99 (m, 2H), 7.11–7.18 (m, 2H), 7.34–7.38
(m, 2H). EI–MS: m/z = 297.1(M+).
2‑(Phenyl (p‑toluidino) methyl) cyclohexanone (Table 1,
entry 18)
IR (KBr) ʋmax/cm−1: 3369, 1714, 1613, 1519, 1452, 1274,
1143, 698. 1HNMR (250 MHz, CDCl3, Me4Si) δ: 1.53–2.02
(m, 6H), 2.15–2.48(m, 1H), 2.23 (s, 3H, CH3), 2.51–2.56 (m,
1H), 2.81–2.95 (m, 1H),4.20 (br s, 1H), 6.43–6.62 (m, 1H),
6.84–6.88 (m, 1H), 6.91–6.94 (d, 1H, J = 7.5), 7.16–7.40
(m, 5H), 7.43–7.50 (m, 1H). EI–MS: m/z = 309.2 (M+).
Selected spectroscopic data
2‑(Phenyl (phenylamino) methyl) cyclohexanone (Table 1,
entry 1)
2‑(Phenyl (4‑dimethylaminophenyl (amino methyl)
cyclohexanone (Table 1, entry 8)
IR (KBr) ʋmax/cm−1:3330, 1699, 1598, 1492, 1282, 753.
1HNMR (250 MHz, CDCl3, Me4Si) δ: 1.17–1.21 (m, 1H),
1.49–1.51 (m, 2H), 1.69–1.71 (m, 1H), 1.80–1.82 (m, 2H),
2.29–2.42 (m, 2H), 2.70–2.73 (m, 1H), 4.71–4.78 (m, 1H),
6.04–6.07 (d, 1H, J = 8), 6.39–6.44 (m, H), 6.51–6.54 (d,
2H, J = 8), 6.90–6.96 (m, 2H), 7.13–7.18 (m, 1H), 7.23–7.29
(m, 2H), 7.39–7.42 (d, 2H). EI–MS: m/z = 279.1 (M+).
EI–MS: m/z = 322.2(M+).
Results and discussion
Characterization of Graphene/MnFe2O4@PTA
nanocomposite
2‑(Phenyl (2‑chloro phenylamino) methyl) cyclohexanone
(Table 1, entry 11)
In order to characterize the as-synthesized nanocatalyst and
confrming immobilization of PTA, FT-IR spectroscopy was
utilized. Figure 1 shows the FT-IR spectra of Graphene/
MnFe2O4 (A), Graphene/MnFe2O4@PTA (B) and PTA
(C). The FT-IR spectrum of Graphene/MnFe2O4 (Fig. 1a)
shows two peaks at 463 and 582 cm−1 which are assigned
to Fe–O stretching in the octahedral and tetrahedral sites of
MnFe2O4, respectively. In the FT-IR spectrum of the Gra-
phene/MnFe2O4@PTA composite, two peaks at 1097 and
798 cm−1 are observed which are related to stretching and
bending vibration modes of Si–O–Si bonds in the silica coat.
Two other bands at 933 and 959 cm−1 are also observed in
the spectrum of this nanocatalyst, which are, respectively,
attributed to W–O–W and W=O stretching modes of PTA
[43, 44]. In all these samples, the band at about 1637 cm−1 is
due to the H–O–H bending vibration of the absorbed water.
Some of the characteristic bonds of PTA overlap with vibra-
tion modes of SiO2. The two peaks of MnFe2O4 are clearly
seen in Fig. 1b of Graphene/MnFe2O4@PTA, indicating the
presence of this magnetic spinel ferrite in the composite.
The crystalline structure of the as-prepared Graphene/
MnFe2O4@PTA was determined by X-ray powder difrac-
tion (XRD). The XRD patterns of MnFe2O4, graphene, Gra-
phene/MnFe2O4@PTA, and PTA are presented in Fig. 2. The
observed sharp and strong peaks in the XRD patterns of all
samples confrm that the materials are highly crystalline.
IR (KBr) ʋmax/cm−1:3389, 1702, 1590, 1504, 1282, 1126,
750, 698. 1HNMR (250 MHz, DMSO, Me4Si) δ: 1.48–2.01
(m, 6H), 2.31–2.36 (m, 2H), 2.90–2.99 (m, 1H),3.39 (br, S,
1H) 4.67–4.69 (m, 1H), 6.48–6.54 (m, 2H), 6.90–6.96 (m,
1H), 7.15–7.20 (m, 2H), 7.24–7.30 (m, 2H), 7.35–7.38 (d,
2H, J = 7.5).
2‑(2‑Nitro phenyl (phenylamino) methyl) cyclohexanone
(Table 1, entry 5)
IR (KBr) ʋmax/cm−1: 3389, 1699, 1603, 1506, 1526, 1445,
1
1289, 1128,763. HNMR (250 MHz, CDCl3, Me4Si) δ:
1.24–1.28 (m, 1H), 1.45- 1.69–2.07 (m, 5H), 1.96–2.07 (m,
1H), 2.23–2.48 (m, 1H), 2.90–2.96 (m, 1H), 5.16–5.22 (m,
1H), 6.12–6.15 (m, 1H), 6.42–6.48 (m, 3H), 6.92–6.98 (m,
2H), 7.42–7.48 (m, 1H), 7.61–7.67 (m, 1H), 7.73–7.77 (d,
1H, J = 10), 7.81–7.84 (d, 1H, J = 7.5).
2‑(2‑Methoxy phenyl (phenylamino) methyl)
cyclohexanone (Table 1, entry 10)
IR (KBr) ʋmax/cm−1:3353, 1700, 1600, 1516, 1496, 1284,
1
1106, 759, 697. HNMR (250 MHz, DMSO, Me4Si) δ:
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Journal of the Iranian Chemical Society
Table 1 Mannich reaction of benzaldehyde and aniline derivatives with cyclohexanone using Graphene/MnFe2O4@PTA as nanocatalyst in sol-
vent-free conditions
Entry
Aldehyde
Amine
ketone
Producta
Time(min) Yield M.P(°C) Ref.
(%)
CHO
NH2
O
1
30
97
128
49
O
HN
CHO
NH2
NH2
NH2
O
O
O
2
3
4
45
95
135
52
O
HN
Cl
Cl
CHO
40
40
95
96
oil
oil
-
-
O
HN
Cl
Cl
CHO
O
HN
NO2
NO2
CHO
NH2
NH2
NH2
NH2
O
O
O
O
No2
5
6
7
8
40
45
45
40
97
96
81
96
154
167
Oil
54
55
-
O
HN
NO2
CHO
O
HN
F
F
CHO
O
HN
OH
OH
CHO
161
56
O
HN
N
H3C
CH3
CH3
N
CH3
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Journal of the Iranian Chemical Society
Table 1 (continued)
CHO
NH2
O
9
50
45
115
53
O
HN
CH3
CH3
CHO
NH2
O
10
OCH3
35
50
96
65
133
57
O
HN
OCH3
CHO
NH2
O
Cl
11
Cl
-
-
O
HN
CHO
NH2
O
O
O
O
O
Br
12
50
50
40
30
50
94
80
78
94
71
97
135
-
58
58
-
O
O
O
HN
Br
CHO
NH2
Br
13
HN
Cl
Br
Cl
CHO
NH2
Cl
14
Cl
HN
NO2
NO2
CH3
CHO
NH2
15
No2
155
56
O
HN
NO2
CH3
CHO
NH2
Cl
16
Cl
-
-
O
HN
Cl
Cl
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Journal of the Iranian Chemical Society
Table 1 (continued)
CHO
NH2
O
O
O
O
O
CH3
17
45
40
50
50
50
92
92
61
45
46
120
117
Oil
-
35
37
-
O
HN
CH3
NH2
Cl
Cl
CHO
CH3
18
19
20
21
O
HN
CH3
CHO
NH2
H3C
CH3
CH3
O
O
O
HN
CH3
CHO
NH2
CH3
-
HN
CH3
CH3
CH3
CHO
NH2
Cl
-
-
HN
Cl
CH3
CH3
aProducts were characterized using NMR, IR and Mass compared with the reported data
The main peaks at 2θ = 30.1°, 36.0°, 43.5°, 57.5°, 57.3° and
63.1° corresponding to refecting planes (2 2 0), (3 1 1), (4
0 0), (4 2 2) and (3 3 3), respectively, belong to MnFe2O4
structure. The same patterns are observed in the XRD of
Graphene/MnFe2O4@PTA nanocomposite. This reveals that
MnFe2O4 retained its structure in the synthesized compos-
ites. Bragg’s refection at 2θ = 15.2° corresponds to lattice
planes of bare PTA which is the main peak of phospho-
tungstic acid. Also, the main peak of graphene is observed
at 2θ = 12° but this peak becomes much less intense after
graphene being coated with other components. Furthermore,
the difraction peaks of PTA and graphene in Graphene/
MnFe2O4@PTA composite were shifted to lower 2θ com-
pared with the peaks of their pure phases. This observation
indicates some change in the MnFe2O4 lattice constants
which is due to the presence of other samples interacting
with the ferrite.
SEM images of Graphene/MnFe2O4 (A) and Graphene/
MnFe2O4@PTA (B) are depicted in Fig. 3. As it is clearly
seen, the MnFe2O4 nanoparticles are well resolved and
have spherical shapes in this two-component composite
(Fig. 3a). The PTA particles in the SEM image of Graphene/
MnFe2O4@PTA are seen as white spots randomly inlaid on
the surface of silica shell in Fig. 3b.
The composition of the as-fabricated Graphene/
MnFe2O4@PTA nanocomposite was further afrmed by
EDX analysis. The EDX spectrum of this nanocomposite is
shown in Fig. 4. The EDX analysis indicates the presence
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Journal of the Iranian Chemical Society
of all main elements, i.e., Mn, Fe, Si, C, N, O, W and P
comprising the nanocomposite (see Fig. 4).
The magnetic properties of Graphene/MnFe2O4 and
Graphene/MnFe2O4@PTA were determined by a vibrating
sample magnetometer (VSM), and the magnetic hysteresis
loops are shown in Fig. 5. The VSM curve for Graphene/
MnFe2O4@PTA shows that the value of saturation magneti-
zation (Ms) is 17.35 emu g−1 which is much less than the Ms
value of the two-component Graphene/MnFe2O4 composite
(43.53 emu g−1) (Table 1). The decrease in Ms may be due to
the entrapment of MnFe2O4 with the nonmagnetic silica and
PTA shells. However, the magnetization of the Graphene/
MnFe2O4@PTA is still high enough to ensure its magnetic
separation.
The surface area, pore volume and pore size distribution
of the prepared Graphene/MnFe2O4@PTA nanocompos-
ite were also determined through BET and BJH methods
from adsorption of nitrogen at 77 K on a micromeritics
gemini surface area analyzer. The Mean pore diameter and
the surface area of the nanocomposite are 20.68 nm and
6.35 m2 g−1, respectively (see Fig. 6).
Evaluation of catalytic activity of Graphene/
MnFe2O4@PTA
Fig. 1 FT-IR spectra of Graphene/MnFe2O4 (A), Graphene/
MnFe2O4@PTA (B) and PTA (C)
We are reporting an efcient method for performing one-pot
Mannich reaction of aldehydes, amines and cyclohexanone
in the presence of Graphene/MnFe2O4@PTA as catalyst. In
an experiment, the starting materials, aldehydes, amine and
cyclohexanone were ground in the absence of Graphene/
MnFe2O4@PTA under solventless conditions for 40 min but
no change in the starting materials was observed accord-
ing to TLC monitoring. However, on addition of catalytic
amount of this nanocomposite to the reaction mixture under
above-mentioned conditions, the reaction proceeded in short
period of time, and β-amino carbonyl compounds were pro-
duced in high yields. This result confrmed that the conden-
sation of these substrates can only take place in the presence
of the catalyst. In order to further explore the scope and
limitations of this catalytic system, diferent aldehydes and
amines were tested for the Mannich reaction with cyclohex-
anone under optimized conditions. The obtained results of
various reactions are presented in Table 2. As it is seen in
Table 2, aromatic aldehydes bearing diferent groups, such
as Cl, F, NO2, OH, OCH3 and Me2N lead to the desired prod-
ucts in good to excellent yields (entries 2–10 and 13–17).
The para methyl-substituted benzaldehyde gave moderate
yields (entries 6, 9, 20 and 21). The ortho-substituted aro-
matic amines were found to be less reactive in these con-
densation reactions because of large steric hindrance efect
(entries 11, 14, 16, 19 and 21). These results are in agree-
ment with the other previously reported fndings [45–51].
Fig. 2 XRD patterns of Graphene/MnFe2O4@PTA (A), MnFe2O4
(B), Graphene (C) and PTA (D)
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Journal of the Iranian Chemical Society
Fig. 3 Graphene/MnFe2O4 (a) and Graphene/MnFe2O4@PTA (b)
Separate experiments for Mannich reaction in the same
conditions were performed in the presence of other samples
instead of Graphene/MnFe2O4@PTA. As seen in Table 2,
none of the examined materials show catalytic activity for
this reaction except free PTA but with lower yield compared
to Graphene/MnFe2O4@PTA.
In Table 3, the catalytic activity of the as-prepared nano-
catalyst was compared with other recently studied catalysts
for Mannich reaction. The data show that the catalyst of
the present work is superior to the other reported catalysts
with respect to reaction time and reaction conditions. The
superiority of our introduced nanocatalyst will be more evi-
dent when its magnetic property is taken into account which
provides a facile and efcient separation for the catalyst.
Fig. 4 EDX spectrum of Graphene/MnFe2O4@PTA
Fig. 5 Hysteresis loops of Graphene/MnFe2O4 (a), Graphene/MnFe2O4@PTA (b)
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Journal of the Iranian Chemical Society
Fig. 6 Nitrogen adsorption–desorption (a) and pore size distributions (b) isotherms of Graphene/MnFe2O4@PTA
Recycling and reusing of the catalyst
Table 2 Comparative study of various materials for Mannich reaction
Entry
Samples
Yield (%)
Facile catalyst recovery is an important advantage of any
heterogeneous catalyst. In order to test the as-made catalyst
reusability, Mannich reaction was performed in the pres-
ence of catalytic amount of Graphene/MnFe2O4@PTA under
optimized reaction conditions. After completion of the reac-
tion, the catalyst was isolated by applying an external mag-
netic force, 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 with fresh substrates. The reus-
ability of this catalyst is presented in Table 4, which indi-
cates that the recovered catalyst could be reused for at least
fve successive times with almost the same catalytic activity.
The amount of PTA was measured in the recycled nano-
catalyst after 5 runs by ICP-AES analysis. This measure-
ment showed that the PTA content of the recycled Graphene/
MnFe2O4@PTA, after 5 runs, was 0.0567 mmol g−1. This
shows the loss of only 2.74% of the original PTA amount
in the catalyst. Leaching of the catalyst in these reactions is
therefore not signifcant and practically has no detectable
efect on its catalytic activity.
1
2
3
4
5
6
MnFe2O4
–
–
–
–
89
97
Graphene/MnFe2O4
Graphene/MnFe2O4@SiO2
Graphene/MnFe2O4@SiO2@Imid-Cl
Neat PTA
Graphene/MnFe2O4@PTA
Table 3 Comparison of efciency of the Graphene/MnFe2O4@PTA
with other catalysts in Mannich reaction (all at room temperature)
Entry Catalyst
Time(min) Yield (%) References
1
2
3
4
5
6
7
8
SiCl4
60–180
120
180
45
60
30–200
60
97
93
92
95
97
90
90
97
45
46
47
48
49
50
51
Fe3O4-systeine MNPs
SO42−/TiO2
MmT/Ps-SO3H
ZSM-5-SO3H
ZnI2
I2
Graphene/MnFe2O4@
30
This work
PTA
Table 4 Reusability of
Graphene/MnFe2O4@PTA
nanocatalyst
Run
Fresh
97
1
2
3
4
5
Yield (%)
96
96
96
95
93
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Journal of the Iranian Chemical Society
Proposed mechanism for Mannich reaction
Conclusion
Based on the previously reported study [35] and our obser-
vations in the course of this reaction, the following mecha-
nism can be proposed. In Scheme 2, a proposed mechanism
for Mannich reaction is shown, which is consisted of three
successive steps. In the frst step, the carbonyl group of
benzaldehyde is activated by PTA located on the surface
of Graphene/MnFe2O4@PTA nanocatalyst (intermediate
I). A nucleophilic attack of aniline, through its lone pair of
electrons, to the carbonyl group of benzaldehyde will subse-
quently occur to produce intermediate (II). This intermedi-
ate passes through some rearrangement, transfer of proton
and dehydration to give intermediate (III). In the second
step or simultaneous with step (I), cyclohexanone is acti-
vated by the nanocatalyst and converted to enol form (IV).
In the third and fnal step, the latter enol form attacks the
iminium ion at carbon adjacent to positively charged nitro-
gen to give the expected β-aminocarbonyl compound (V).
Phosphotungstic acid in the form of ionic liquid pair with
imidazolium was immobilized on the surface of silica-coated
graphene–MnFe2O4. The as-made nanocatalyst showed a
promising green route with high catalytic activity for Man-
nich reactions. A solventless one-pot approach has been
successfully applied for the three-component reactions of
an aldehyde, an amine and a ketone to give high yield of
β-amino carbonyl compound. This solventless catalytic
system can certainly contribute to a safer environment and
green technology for the facile preparation of β-amino car-
bonyl compounds. This protocol ofers several advantages
including high yield, clean reaction media, short reaction
time and simple recycling process of the catalyst. The Man-
nich reaction has important applications since the products
of this reaction, i.e., Mannich bases or β-aminocarbonyl
Scheme 2 Proposed mechanism for Mannich reaction with Graphene/MnFe2O4@PTA
1 3

Journal of the Iranian Chemical Society
26. X. Yan, J. Chen, Q. Xue, P. Miele, Microporous Mesoporous
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Acknowledgements The authors wish to acknowledge the support of
this work (Grant No. 1395) provided by the Research Council of Shahid
Chamran University of Ahvaz, Iran.
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