Supported sulfonic acid on magnetic nanoparticles used as a reusable catalyst for rapid synthesis of α-aminophosphonates

Article abstract of DOI:10.1080/15533174.2016.1186096

Sulfonic acid was anchored on the surface of silica-coated cobalt ferrite core to obtain a new magnetically separable catalyst. The resultant composite was characterized by various techniques, including FT-IR, X-ray diffraction, FESEM, energy-dispersive X-ray, TEM, TGA, and vibrating sample magnetometer. The immobilized sulfonic acid was shown to be an efficient heterogeneous catalyst for the synthesis of a-aminophosphonates under solvent-free conditions at room temperature. The catalyst is readily recovered by simple magnetic decantation and can be recycled 7 times with no significant loss of catalytic activity.

Full text of DOI:10.1080/15533174.2016.1186096

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Supported Sulfonic Acid on Magnetic Nanoparticles
Used as a Reusable Catalyst for Rapid Synthesis of α-Aminophosphonates
Mozhgan Afsharia,*, Maryam Gorjizadeha, Mohammad Nasehb
aDepartment of Chemistry, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran
bDepartment of Chemistry, Dezful Branch, Islamic Azad University, Dezful, Iran
Mozhgan Afshari , Corresponding author, Department of Chemistry, Shoushtar Branch, Islamic
Azad University, Shoushtar, Iran, 64517-41117, Iran, Tel.: +98 9166161157; Fax:
+986126231300, E-mail address: m.afshari@iau-shoushtar.ac.ir
Abstract
Sulfonic acid was anchored on the surface of silica coated cobalt ferrite core to obtain a new
magnetically separable catalyst. The resultant composite was characterized by various
techniques, including FT-IR, XRD, FESEM, EDX, TEM, TGA, and VSM. The immobilized
sulfonic acid was shown to be an efficient heterogeneous catalyst for the synthesis of α-
aminophosphonates under solvent free conditions at room temperature. The catalyst is readily
recovered by simple magnetic decantation and can be recycled 7 times with no significant loss of
catalytic activity.
Keywords
Cobalt ferrite, α-Aminophosphonates, Magnetic nanoparticles, Surface functionalization,
Sulfonic acid
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Introdution
Recently magnetic nanoparticles (MNPs) widely have employed as a support for homogenous
catalysts. A striking feature of the MNPs-supported catalysts is that they can be easily separated
from the reaction mixture by external magnetic field and reused [1-3]. Additionally, the
nanoparticle-based catalysts not only show superior catalytic activities to their corresponding
bulk material but also a high degree of chemical stability, and they do not swell in organic
solvents [4].
Among the different MNPs as the core magnetic supports, cobalt ferrite nanoparticles were
chosen for use as magnetic supports since they have moderate saturation magnetization,
inexpensive, high chemical stability and mechanical strength [5–7].
On the other hand, sulfonic acid catalysts have been widely used in various industries; however,
it is often difficult to separate the final product after the reaction is completed. Thus magnetic
nanoparticles can be as attractive candidates for supporting of sulfonic acid catalysts [8, 9].
Sulfonic acid functionalized magnetic nanoparticles as recyclable strong solid acid catalyst open
up new avenue to introduce an amazing and efficient system for facilitating catalyst recovery in
different organic reactions. This catalyst allows the combination of well known procedures for
catalyst heterogenization with techniques for magnetic separation.
Although many procedure have been performed to immobilize acid moiety on magnetic
nanoparticles, we mentioned only some reports have appeared in the literature about sulfonic
acid supported on MNPs. Firstly the use of silica coated cobalt ferrite nanoparticles for the
immobilization of sulfonic acid was reported by Jones et al. [8]. They evaluated acid catalysts in
test reactions in terms of activity and recyclability and compared with commercially available
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heterogeneous acidic resins and homogeneous sulfonic acids. In another report, Sulfonic acid
functionalized silica coated magnetite nanoparticles was prepared by chlorosulfonic acid and
used as an efficient catalyst for the synthesis of a variety of tetraketone derivatives via the
Knoevenagel condensation and Michael addition reactions [10]. In this study high conversions
were obtained and the catalysts could be easily separated from the reaction solution by applying
an external magnetic field and recycled several times. Esterification reaction of fatty alcohols
was studied by another researcher group using an organosulfonic acid functionalized silica
coated magnetic nanoparticles with dual functionality comprised of both hydrophobicity and
acidity properties [11].
The developing of one-pot multicomponent reactions (MCRs) and improving the known MCRs
have attracted considerable attention in recent years [12]. One of these MCRs is the preparation
of α-aminophosphonates derivatives. α-Aminophosphonates as a kind of natural amino acid
analogs occupy an important place and reveal diverse and interesting biological and biochemical
properties [13–15]. One-pot syntheses of α-aminophosphonates have been carried out by multi
component condensation of an aldehyde, an amine, and a phosphite in the presence of various
catalysts such as Lewis acids [16–20], Brønsted acids [21, 22], solid acids [23,24], rare metal
salts [25], or metal oxides [26] over recent decades. However, some of these methods are not
environmentally friendly and suffer from one or more disadvantages, such as harsh conditions,
long reaction times, and frequently low yields, toxicity and recovery and reusability of the
catalyst. In view of these, the developments of clean processes and utilizing ecofriendly and
green heterogeneous catalysts which can be simply recycled at the end of reactions are still
relevant.
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The demand for environmentally benign procedure with heterogeneous and reusable catalyst,
promoted us to develop a safe alternate method for the preparation of α-aminophosphonates. In
pursuit of this goal we wish to explore a straight forward synthesis of α-aminophosphonate via
the one pot three component reaction under solvent free conditions using a new sulfonic acid
moiety grafted on the surface of silica coated cobalt ferrite (CoFe2O4@SiO2-RSR-SO3H) as an
efficient magnetic powerful solid acid catalyst with good stability (Scheme 1).
Experimental
General
All the chemicals were of analytical grade and used without further purification. . X-ray
diffraction (XRD) patterns of the synthesized samples were taken with a Philips X-ray
diffractometer (model PW1840) using Cu Kα radiation (λ = 1.54056 Å). FT-IR spectra were
obtained using BOMEM MB-Series 1998 FT-IR spectrometer. The FESEM images were
obtained using a Hitachi Japan S4160 scanning electron microscope. The TEM images were
recorded using a Leo 912 AB transmission electron microscope operated at a 100 keV. The
magnetic properties of the fabricated CoFe2O4@SiO2-RSR-SO3H composite as well as other
samples were studied using vibrating sample magnetometer (VSM) of Meghnatis Daghigh Kavir
Company, Iran. NMR spectra were recorded in CDCl3 on a Bruker Advanced DPX 400 MHz
spectrometer.
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Synthesis of sulfonic acid supported on silica coated cobalt ferrite
Cobalt ferrite MNPs were prepared using the method reported by Maaz et al. [27]. Coating of
CoFe2O4 nanoparticles with a layer of silica was achieved through Stöber method [28-30]. A one
pot procedure was used to synthesize sulfonic acid supported on CoFe2O4@SiO2 as follow. At
first, a mixture of 3-meraptopropyltrimehoxy silane (5.9 ml, 32 mmol) and 1, 3-propane sultone
(3.25 ml, 32 mmol) was heated at 110 ˚C for 24 h with continuous stirring under N2 atmosphere.
The product of this step was dissolved in 25 mL of toluene followed by addition of
CoFe2O4@SiO2 (1.1 g) and the mixture was refluxed for 24 h. finally the produced sulfonic acid
silica coated magnetic nanoparticles (CoFe2O4@SiO2-RSR-SO3H) were magnetically separated
and washed twice with toluene and ether. A schematic representation of CoFe2O4@SiO2-RSR-
SO3H synthesis is shown in Scheme 2.
Measuring of Sulfonic acid loadings
Sulfonic acid loadings of the catalyst (mmol of sulfonic acid per g of CoFe2O4@SiO2-RSR-
SO3H) were determined by the back titration method [31]. 0.5 g of the sample was soaked
overnight in 50 ml of distilled water containing 5ml of 0.1 N NaOH to exchange sodium ions
with the protons in the nano catalyst. Back-titration was accomplished by titrating the remaining
NaOH in solution with 0.1 MHCl solutions.
General procedure of Catalytic α-aminophosphonates synthesis
To a mixture of an aldehyde (1 mmol), aniline (1 mmol), triethyl phosphate (1.2 mmol) was
added 0.05 g CoFe2O4@SiO2-RSR-SO3Hcatalyst and the mixture was stirred for an appropriate
time at room temperature. After completion of the reaction (monitored by TLC), CH3Cl was
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added to dilute the reaction mixture and the organic layer was simply decanted by means of an
external magnet. The isolated solution was purified on a silica-gel plate to obtain pure product.
The products were identified by FT-IR and 1H NMR matches with the literature.
Results and discussion
Characterization of CoFe2O4@SiO2-RSR-SO3H
As described in the experimental section, the catalyst was prepared by straightforward one pot
procedure and characterized by various techniques. The FT-IR spectrum of CoFe2O4(a) ,
CoFe2O4@SiO2 (b) and CoFe2O4@SiO2-RSR-SO3H (c), shows the characteristic peaks of Fe-O
at 589 cm-1 and an intense broad peak at 1000–1250 cm−1 in Fig. 1b and 1c corresponded to the
Si–O stretch in the amorphous silica shell [32] (Fig. 1). The O–H stretch and vibration of
surface hydroxyl groups and physisorbed water were present as broad peaks at 3000–3700 cm−1
and a sharper peak at 1640 cm−1, respectively. The presence of the anchored alkyl groups in
CoFe2O4@SiO2-RSR-SO3H is confirmed by the aliphatic weak C- H stretching vibrations
appearing at 2943 cm−1. Evidence of the surface functionalization was difficult to observe due to
the low loadings of SO3H and overlapping of peaks corresponding to Si–O–Si, Fe–O–Si, and
symmetric SO2 stretching [8, 33].
Figure 2 illustrates the XRD pattern of CoFe2O4, CoFe2O4@SiO2 and CoFe2O4@SiO2-RSR-
SO3H, The peaks are compatible with pure CoFe2O4 phase (JCPDS PDF #221086), indicating
the retention of cubic reverse spinel structure of CoFe2O4 during coating and functionalization.
The XRD pattern of Si-MNPs and CoFe2O4@SiO2-RSR-SO3H (Fig. 2b and 2c) shows an
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obvious diffusion peak at 2θ =20-28º that appeared because due to the existence of amorphous
silica in the shell of CoFe2O4.
In order to investigate the crystalline nature and morphological features of the catalyst, scanning
electron microscopy (SEM). SEM image (Fig.3) of the catalyst shows that CoFe2O4@SiO2-RSR-
SO3H nanoparticles are nearly spherical and the average size of these nanoparticles is in the
range of 30 nm. In these cases, the TEM image (Fig.4) provide more accurate information on the
particle size and morphology of MNPs, therefore, these image was investigated. TEM
observations clearly reveal the core–shell structure of the nanoparticles.
In order to further confirm the composition of the as-fabricated CoFe2O4@SiO2-RSR-SO3H
composite, energy dispersive X-ray (EDX) analysis was conducted. The EDX spectrum, which is
displayed in Fig. 5, clearly reveals the presence of all the expected main elements (O, Si, Fe, Co,
and S) comprising this composite.
The magnetic properties of CoFe2O4, CoFe2O4@SiO2 and CoFe2O4@SiO2-RSR-SO3H were
studied by a vibrating sample magnetometer (VSM) at 300 K with a peak field of 10 kOe and
their hysteresis curves are presented in Fig. 4. It could be seen from the loops in Fig. 4 that
saturation magnetization (MS) of CoFe2O4, CoFe2O4@SiO2 and CoFe2O4@SiO2-RSR-SO3H are
59.42, 38.93 and 31.16 emu g-1 respectively. The decrease in mass saturation magnetization may
be ascribed to the contribution of the non-magnetic silica shell and functionalized groups [34].
However, the as- fabricated nanocomposite, can still be efficiently and easily separated from
solution with the help of an external magnetic force.
Thermogravimetric analysis (TGA) of CoFe2O4@SiO2-RSR-SO3H catalyst was performed to
prove its thermal stability. There are two weight loss steps in TGA curve of CoFe2O4@SiO2-
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RSR-SO3H catalyst (Fig. 7).The first mass loss of 4.19 % (between 30 to 192 ◦C) may be due to
removal of surface adsorbed water of the catalyst. The loss of weight at temperatures higher than
192 ◦C could be mainly attributed to the evaporation and subsequent decomposition of RSR-
SO3H groups. The TGA analysis of the as-prepared catalyst showed that the sulfonic acid moiety
loading approximately 0.48 mmol/g. It is worth mentioning that this value of loading show
agreement with the values calculated by back-titration (0.50 mmol/g). In addition, elemental
analysis showed 3.25 % S (= 0.51 mmol/g SO3H) that confirmed catalyst loading obtained from
TGA and back-titration.
α-Aminophosphonates synthesis
At the outset, the three-component reaction of benzaldehyde, aniline, and thriethylphosphite was
investigated in various conditions at room temperature. Initially, we have focused our attention
on the effect of the solvent on model reaction; we have studied the effect of different solvents
such as CH3CN, CH2Cl2, and EtOH in the presence of CoFe2O4@SiO2-RSR-SO3H as catalyst.
But solvent -free approach showed better results (see Table 1). Secondly, to investigate the effect
of SO3H moiety on this catalysis reaction, we have also examined CoFe2O4@SiO2 in similar
reaction in the separate experiment. As seen in Table 1, CoFe2O4@SiO2 showed the weak
catalytic activity for the α-aminophosphonates synthesis.
Then, this condensation reaction with various aldehydes, amines, and triethylphosphite in the
presence of CoFe2O4@SiO2-RSR-SO3H as the catalyst was explored under the optimized
reaction conditions described above, and the results are presented in Table 2.
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It can easily be seen that this one-pot, three component condensation completed within 10–40
min, and the products were isolated in good to excellent yields. For aromatic aldehydes carrying
either electron-donation or electron withdrawing substituents could afford good yields of α-
aminophosphonates. The lower yields obtained for furfural and 2-amino pyridine are probably
due to the lower reactivity of the heterocyclic substrates.
A plausible mechanism for the one-pot, three-component condensation reaction of aldehydes,
amines and triethyl phosphite is believed to involve formation of an activated imine by supported
sulfonic acid so that addition of the phosphite is facilitated to give a phosphonium intermediate,
which then undergoes reaction with the water to give the α-aminophosphonate and ethanol as
shown in Scheme 3 [35].
A comparison of the efficacy of CoFe2O4@SiO2-RSR-SO3H catalyst with some of those reported
in the literature is presented in Table 3.
Catalyst recycling
The nanocatalyst reusability was also investigated. For this purpose, the same model reaction
was again studied under the optimized conditions. After the completion of the first reaction, the
product was diluted with CH3Cl and the solution was removed by magnetic decantation. The left
used catalyst was washed with methanol and CHCl3 and dried at 60 °C for 30 min prior to the
next run. A new reaction was then conducted with fresh reactants under the same reaction
conditions. The results showed the yield of α-aminophosphonates dropped from 92 % to 83%
after seven runs (see Table 4). Elemental analysis of the recovered catalyst demonstrated no
change in amount of S content after recycle it for seven times. Thus, the slight decrease in the
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activity could be attributed to mainly unavoidable loss of the catalyst during the process of
collection.
Conclusions
Sulfonic acid supported on silica coated CoFe2O4 magnetic nanoparticles is successfully
achieved by a one pot synthetic procedure which is confirmed with XRD, FT-IR, TGA, VSM,
TEM, FESEM and EDX techniques. The immobilized sulfonic acid was shown environmentally
safe heterogeneous solid acid catalyst for synthesis of α-aminophosphonates under solvent- free
conditions at room temperature. This method offers several advantages including short reaction
time, good to excellent product yields, simple work-up, ease of separation of the magnetic
catalyst, as well as the ability to tolerate a wide variety of substitutions in the reagents. Moreover
catalyst could be recovered from the reaction mixture by compartmentation with the aid of an
external magnet and could be reused seven times with no significant loss of catalytic activity.
Acknowledgment
The authors wish to acknowledge the support of this work by the Research Council of, Shoushtar
Branch, Islamic Azad University, Shoushtar, Iran.
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Table 1. Effect of various catalysts and solvents on the synthesis of α -aminophosphonates
Entry
Catalyst
Solvent
Time (min)
Yieldb(%)
1
2
3
4
5
CoFe2O4@SiO2-RSR-SO3H
CH3CN
25
30
86
88
70
92
30
CoFe2O4@SiO2-RSR-SO3H CH2Cl2,
CoFe2O4@SiO2-RSR-SO3H
CoFe2O4@SiO2-RSR-SO3H
CoFe2O4@SiO2
EtOH
Neat
Neat
30
10
120
a All reaction conditions: benzaldehyde(1 mmol), aniline (1 mmol), P(OEt)3 (1.2 mmol), Catalyst
(0.05 g) at room temperature
b Isolated yield
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Table 2. Synthesis of diversified α-aminophosphonates in the presence of CoFe2O4@SiO2-RSR-
SO3H a
Time
Entry
Aldehyde
R1(Amine)
Yieldb (%)
(min)
10
25
30
25
25
25
20
25
25
20
15
15
10
20
20
15
30
2
3
R=H
H
2-Br
4-F
92
95
86
89
94
90
92
90
90
91
89
92
95
95
95
93
76
R=H
4
R=H
5
R=H
4-OMe
H
6
R= 2-Br
R= 2-Br
R= 3-OMe
R= 3-OMe
R= 3-OMe
R= 4-OMe
R= 4-OMe
R= 4-OMe
R= 4-NO2
R= 2-Br
R= 2-Br
R= 4-OMe
R= 4-NO2
7
4-Br
H
8
9
4-Br
4-F
10
11
12
13
14
15
16
17
18
4-Br
4-F
2-Br
H
4-F
4-OMe
H
2-amino
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pyridine
19
20
furfural
furfural
H
30
40
85
70
2-amino
pyridine
a All reaction condition: aldehydes (1 mmol), amine (1 mmol), P(OEt)3 (1.2 mmol) and 0.05 g of
CoFe2O4@SiO2-RSR-SO3H at room temperature
b Isolated yields
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Table 3. Comparison of CoFe2O4@SiO2-RSR-SO3H with other catalysts reported in the
literature for the one-pot three component coupling reaction of benzaldehyde, aniline,and triethyl
phosphitea
Time
(min)
45
Yieldb
(%)
63
Entry
Catalyst
Ref.
1
2
H2SO4
[36]
[37]
SiO2@Al2O3
60
93
Fe3O4@SiO2- poly(amidoamine)-
SO3H
3
25
91
[36]
4
5
6
CoFe2O4@SiO2-Imid/H3PW12O40
Fe3O4@(CH2)3NSO3H
Fe3O4@SiO2-2mimid-SO3H
15
60
20
95
78
94
[7]
[38]
[39]
This
work
7
CoFe2O4@SiO2-RSR-SO3H
10
92
a Reaction conditions: solvent-free and room temperature
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Table 4. Catalyst recycling experiments for α-aminophosphonates synthesis
Cycle
1st 2st 3st 4st 5st 6st 7st
92 90 90 88 86 85 83
Yeild %
a All reaction condition: aldehydes (1 mmol), amine (1 mmol), P(OEt)3 (1.2 mmol) and 0.05 g of
CoFe2O4@SiO2-RSR-SO3H at room temperature
b Isolated yields
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Fig.1. FT-IR spectra of (a) CoFe2O4, (b) CoFe2O4@SiO2 and (c) CoFe2O4@SiO2-RSR-SO3H
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Fig. 2. XRD pattern of (a) CoFe2O4, (b) CoFe2O4@SiO2 and (c) CoFe2O4@SiO2-RSR- SO3H
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Fig. 3. FESEM image of the CoFe2O4@SiO2-RSR- SO3H catalyst
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Fig. 4. TEM image of the CoFe2O4@SiO2-RSR- SO3H catalyst
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Fig. 5. EDX spectrum of CoFe2O4@SiO2-RSR- SO3H catalyst
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Fig. 6. Hysteresis loops of (a) CoFe2O4, (b) CoFe2O4@SiO2 and (c) CoFe2O4@SiO2-RSR- SO3H
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Fig. 7. TGA of CoFe2O4@SiO2-RSR- SO3H
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Scheme1. CoFe2O4@SiO2-RSR-HSO3 catalyzed synthesis of α-aminophosphonates
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Scheme 2. Schematic representation of the formation of CoFe2O4@SiO2-RSR- SO3H catalyst
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Scheme 3. Proposed mechanism for the synthesis of α-aminophosphonates catalyzed by
CoFe2O4@SiO2-RSR-HSO3
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