A highly reactive and magnetic recyclable catalyst based on silver nanoparticles supported on ferrite for N-monoalkylation of amines with alcohols

Article abstract of DOI:10.1002/aoc.3720

Fe₃O₄@SiO₂-Ag catalyst was found to be highly active and selective in the N-alkylation of amines with a variety of aromatic and linear alcohols. The heterogeneous nature of the Fe₃O₄@SiO₂-Ag catalyst allows easy recovery and regeneration by applying an external magnet for six subsequent reaction cycles. The prepared catalyst was characterized using electron microscopy techniques, X-ray diffraction, vibrating sample magnetometry and atomic absorption spectroscopy.

Full text of DOI:10.1002/aoc.3720

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Received: 19 October 2016
DOI 10.1002/aoc.3720
Revised: 26 November 2016
Accepted: 29 November 2016
F U L L PA P E R
A highly reactive and magnetic recyclable catalyst based on silver
nanoparticles supported on ferrite for N‐monoalkylation of amines
with alcohols
Ahmad Bayat1
| Mehdi Shakourian‐Fard2 | Peyman Nouri1 |
Mohammad Mahmoodi Hashemi1
1 Department of Chemistry, Sharif University of
Fe3O4@SiO2‐Ag catalyst was found to be highly active and selective in the N‐alkyl-
Technology, PO Box 11465‐9516, Tehran, Iran
2 Department of Chemical Engineering, Birjand
University of Technology, POBox 97175/569
Birjand, Iran
ation of amines with a variety of aromatic and linear alcohols. The heterogeneous
nature of the Fe3O4@SiO2‐Ag catalyst allows easy recovery and regeneration by
applying an external magnet for six subsequent reaction cycles. The prepared cata-
lyst was characterized using electron microscopy techniques, X‐ray diffraction,
vibrating sample magnetometry and atomic absorption spectroscopy.
Correspondence
Ahmad Bayat, Department of Chemistry, Sharif
University of Technology, PO Box 11465‐9516,
Tehran, Iran.
Email: bayat@ch.sharif.ir
KEYWORDS
Mehdi Shakourian‐Fard, Department of Chemical
Engineering, Birjand University of Technology,
PO Box 97175/569, Birjand, Iran.
alcohol, green chemistry, magnetic nanoparticles, N‐alkylation of amines, silver
nanoparticles
Email: shakourian@birjandut.ac.ir
1
|
INTRODUCTION
One way to overcome these drawbacks is the use of an
inexpensive, recoverable and reusable catalyst to accomplish
The N‐alkylation of amines is considered to be a very impor-
tant reaction in synthetic organic chemistry. This process
allows access to higher amines that are widely used as fun-
damental materials, additives, dyes and biologically active
compounds.[1–4] N‐alkylamines are typically synthesized
using alkylating agents having a good leaving group such
as halide, tosylate, mesylate, triflate, etc. However, this pro-
cedure can be problematic due to over‐alkylation and the
toxic nature of many alkyl halides and related alkylating
agents.[5]
The use of alcohols instead of alkyl halides to obtain N‐
alkylamines is attractive because it produces only water as a
by‐product and does not need special equipment. A variety
of transition metal complexes such as those of ruthenium,
iridium, rhodium, platinum, gold, nickel, aluminium, copper,
iron and silver are known to be good catalysts for the N‐alkyl-
ation of amines with alcohols.[6–16]
the reaction. An immobilized catalytic system on the large
surface area of a solid carrier can solve the problems of
homogeneous systems. Although there are several reports of
the N‐alkylation process using heterogeneous catalysts, most
of them require high pressure and reaction temperature and
are limited in their substrate scopes.[20–24] Therefore, the
development of efficient heterogeneous catalysts for N‐alkyl-
ation of amines with alcohols is still a challenge.
As part of our continuing effort[25–29] to synthesize mag-
netic heterogeneous catalytic systems for various organic
transformations, we report a magnetic recyclable heteroge-
neous catalyst based on silver nanoparticles supported on fer-
rite for one‐pot selective N‐monoalkylation of amines with
alcohols. To the best of our knowledge, silver nanoparticles
supported on Fe3O4 magnetic nanoparticles have not been
reported so far for N‐monoalkylation of amines with alcohols.
Moreover, the use of the magnetic heterogeneous catalytic
systems enables separation from the reaction mixture using
an external magnet and reuse of the catalyst itself. This cata-
lyst system also shows exceptionally high activity for N‐
alkylation of amines with various alcohols.
Unfortunately, the recovery and reuse of expensive cata-
lysts is difficult and the indispensable use of co‐catalysts such
as stabilizing ligands is unavoidable for most of the known
homogeneous catalysts.[17–19]
Appl Organometal Chem 2017;e3720.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3720

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BAYAT ET AL.
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RESULTS AND DISCUSSION
characteristic peaks for Ag nanoparticles (red peaks) are also
found at 38.45°, 44.43°, 64.67°, 77.48° and 81.56° which
correspond to (111), (200), (220), (311) and (222) planes
of Ag nanoparticles, respectively, in accordance with the
JCPDS card number. These peaks indicate the presence of
Ag nanoparticles in the SMNP‐Ag catalyst.[31] A compari-
son between XRD patterns of SMNP and the SMNP‐Ag cat-
alyst indicates that the main peaks in the XRD patterns do
not change after deposition of Ag nanoparticles, indicating
retention of crystalline structure during their deposition.
In order to investigate the morphology and particle size
of the SMNP‐Ag catalyst, scanning electron microscopy
(SEM) and transition electron microscopy (TEM) images
of the catalyst were obtained, as shown in Figure 2(a)
and (b), respectively. These images show that the size of
the SMNP‐Ag nanoparticles is between 20 and 50 nm,
and the nanoparticles are also semi‐spherically uniform.
Energy‐dispersive X‐ray spectroscopy (EDS) analysis of
the SMNP‐Ag catalyst confirms the presence of Si, Fe,
O, and Ag atoms in the catalyst structure (Figure 2c).
The remaining weak peaks are due to the presence of a thin
layer of gold on the sample surface. In addition, the
amount of Ag deposited on the SMNP nanoparticles was
measured using atomic absorption spectroscopy (AAS)
2.1 | Preparation and Characterization of
Fe3O4@SiO2‐Ag (SMNP‐Ag) Catalyst
In the first step, bare Fe3O4 nanoparticles were synthesized
via a conventional co‐precipitation method using a solution
of iron salts (FeCl3⋅6H2O and FeCl2⋅4H2O) (Scheme 1). In
order to protect the bare Fe3O4 nanoparticles from oxidation
and agglomeration, silica shell (SiO2) was used as an inert
and stabilizer material for coating the Fe3O4 nanoparticles.
Then, Ag nanoparticles were deposited on the Fe3O4@SiO2
(SMNP) core–shell via reduction of AgNO3 solution by
NaBH4 at room temperature to generate the SMNP‐Ag cata-
lyst (Scheme 1). Finally, the SMNP‐Ag catalyst was charac-
terized using various techniques.
The crystalline nature and surface state of SMNP‐Ag
catalyst were studied using powder X‐ray diffraction
(XRD), as shown in Figure 1. The peaks at 2θ values of
30.4°, 35.8°, 43.7°, 53.8°, 57.3° and 63.0° in the XRD pat-
terns of SMNP and SMNP‐Ag catalyst correspond to (220),
(311), (400), (422), (551) and (440) diffraction planes of
Fe3O4 nanoparticles, respectively. These peaks match well
with the standard XRD data of the Joint Committee on Pow-
der Diffraction Standards (JCPDS) card number and reveal a
cubic inverse spinel structure for Fe3O4 in SMNP and
SMNP‐Ag catalyst.[30] As seen in Figure 1, five
analysis at about 0.192 mmol g−1
.
Magnetic measurements of Fe3O4 nanoparticles and the
SMNP‐Ag catalyst were conducted using a vibrating
SCHEME
1
Synthesis of magnetically recoverable heterogeneous
nanocatalyst Fe3O4@SiO2‐Ag (SMNP‐Ag)
FIGURE 2 (a) SEM and (b) TEM images, and (c) EDS analysis of SMNP‐
FIGURE 1 XRD patterns of Fe3O4@SiO2 and SMNP‐Ag catalyst
Ag catalyst

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BAYAT ET AL.
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TABLE 2 Optimization of amount of SMNP‐Ag catalysta
Yield (%)b
4a 5a
Entry
Catalyst (mg)
Conversion (%)
3a
1
2
3
4
5
6
7
8
—
—
—
5
—
—
4
—
—
1
—
—
—
<1
<1
—
1
AgNO3 (85)
Fe3O4 (100)
SMNP‐Ag (2.6)
SMNP‐Ag (5.2)
SMNP‐Ag (26)
SMNP‐Ag (52)
SMNP‐Ag (104)
67
82
93
98
>99
64
78
89
92
93
2
3
4
5
6
<1
aReaction conditions: aniline (1 mmol), benzyl alcohol (1.5 mmol), KOH (20 mol
%), toluene (5 ml) at 100 °C for 18 h.
bGC yield.
FIGURE 3 Magnetization curves of (a) Fe3O4 and (b) SMNP‐Ag catalyst
sample magnetometer at room temperature. As seen in
Figure 3, the magnetization curves exhibit no hysteresis
loop which demonstrates superparamagnetic characteristics.
Figure 3 shows that the deposition of Ag nanoparticles and
the coating of the silica shell cause a decrease of the mag-
netization of the Fe3O4 nanoparticles, although the SMNP‐
Ag magnetic material can still be collected under an
applied magnetic field.
investigated. It is found that toluene is the best solvent for
the N‐alkylation reaction (Table 1, entry 4).
To study the effect of base on the catalytic activity of the
SMNP‐Ag catalyst, various bases were examined. In the
absence of base, the desired amine product is formed in only
7% (Table 1, entry 13). This observation indicates that the
presence of base is helpful in deprotonation of primary alco-
hol to alkoxide ion and also facilitates the removal of a
hydride ion from it. It is found that KOH as base not only
shows good conversion of amine but also provides good
selectivity for the desired amine product.
After choosing the best solvent and base, the effect of the
amount of the SMNP‐Ag catalyst on the N‐alkylation reac-
tion was investigated. As evident from Table 2, in the absence
of the SMNP‐Ag catalyst (entry 1), the desired amine is not
2.2 | Optimized Reaction Conditions
In order to determine the best reaction conditions, the reac-
tion of aniline with benzyl alcohol was selected as a model
reaction. The effect of various solvents on the reaction was
TABLE 1 Effect of base and solvent on N‐alkylation of aniline with benzyl alcohola
Yield (%)b
4a
Entry
Solvent
Base
Conversion of 1a (%)
3a
5a
1
H2O
KOH
KOH
No reaction
0
6
0
1
0
0
2
DMF
7
3
EtOH
KOH
29
98
23
75
22
68
88
26
42
37
8
21
92
16
75
15
56
81
20
34
30
7
6
0
4
Toluene
Dioxane
o‐Xylene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
KOH
5
1
5
KOH
6
<1
<1
<1
2
6
KOH
1
7
LiOH
NaOH
K2CO3
Na2CO3
t‐BuOK
K3PO4
—
6
8
10
6
9
2
10
11
12
13
5
<1
<1
<1
0
7
6
1
aReaction conditions: aniline (1 mmol), benzyl alcohol (1.5 mmol), SMNP‐Ag catalyst (1 mol%, 52 mg), base (20 mol%), solvent (5 ml) at 100 °C for 18 h.
bGC yield.

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BAYAT ET AL.
produced. The N‐alkylation of aniline under similar condi-
tions using AgNO3 as catalyst does not lead to any product
formation. Also, the use of Fe3O4 results in the formation
of secondary amine with conversion of 4%. Our results show
that increasing the amount of SMNP‐Ag catalyst from
0.05 mol% (2.6 mg) to 1 mol% (52 mg) leads to an increase
of conversion of aniline from 67 to 98%. When more catalyst
(2 mol%, 104 mg) is used, the conversion of aniline and yield
of the secondary amine do not change markedly. Therefore,
1 mol% of SMNP‐Ag catalyst is sufficient for N‐alkylation
of amines using alcohols. Once the best reaction conditions
were determined, the scope of the method was investigated
for the N‐alkylation of various amines.
The scope of N‐alkylation of amines with alcohols in
the presence of the SMNP‐Ag catalyst is summarized in
Table 3. As seen from entries 1–4, the reaction of aniline
with benzyl alcohols containing electron‐donating or
electron‐withdrawing substituents proceeds to give good
yields. The reaction of aniline derivatives with benzyl alco-
hol (entries 5 and 6) also proceeds efficiently. Table 3 also
shows that the catalyst system is not limited to aromatic
alcohols, and aliphatic alcohols can also react with amines
to form higher amines in excellent yield. Alcohols with the
least reactivity, such as fully saturated alcohols, were also
tested in this reaction (Table 3). Secondary alcohols are
challenging alkylating agents compared with primary alco-
hols using hydrogen borrowing strategy because the dehy-
drogenation of secondary alcohols to the corresponding
ketones is more difficult than that of primary alcohols. In
addition, aliphatic alcohols (entries 8 and 10) and second-
ary alcohols (entry 9) require more time to achieve higher
yields of the products.
The efficiency of the SMNP‐Ag catalyst in the N‐alkyl-
ation reaction of aniline with benzyl alcohol was compared
with that of other catalyst systems reported in the literature
(Table 4). One of the advantages of our catalyst is that it
can be easily removed from the reaction mixture by apply-
ing an external magnetic field. Thus, this property prevents
the loss of the SMNP‐Ag catalyst, although other methods
(entries 1–3 and 5) use filtration for separation of the cat-
alyst systems from the reaction mixture. Moreover, our
magnetically recoverable nanocatalyst is not limited to aro-
matic amines and can also be used for aliphatic amines,
although other catalysts (entries 1, 2, 4 and 5) are used
TABLE 3 Scope of catalytic activity of SMNP‐Ag catalyst in N‐alkylation of amines with various alcoholsa
Entry
R1
R2
Time (h)
Conversion (%)
Yield (%)b
1
4‐MePhCH2
4‐BrPhCH2
4‐ClPhCH2
4‐MeOPhCH2
PhCH2
Ph
Ph
Ph
Ph
17
24
24
14
20
24
18
30
30
36
18
24
20
>99
98
95
97
97
98
96
90
95
93
95
93
94
85
95
2
3
96
4
>99
>99
>99
>99
97
5
4‐MePh
4‐ClPh
PhCh2
Ph
6
PhCH2
7
PhCH2
8
PhC3H6
9
PhCHCH3
C8H17
Ph
98
10
11
12
13
Ph
96
PhCH2
─C2H4OC2H4─
C6H13
97
PhCH2
89
PhCH2
─C5H10
─
98
aReaction conditions: amine (1 mmol), alcohol (1.5 mmol), SMNP‐Ag catalyst (1 mol%, 52 mg), KOH (20 mol%), toluene (5 ml) at 100 °C for 18 h.
bGC yield.
TABLE 4 Comparison of activity of various catalysts in N‐alkylation of aniline with benzyl alcohol
Entry
Catalytic system
Conditions
Yield (%)
Ref.
[1]
1
2
3
4
5
6
7
Fe‐Phthalocyanine
Ru(OH)x/Al2O3
Au/TiO2‐VS
Toluene, N2, NaOtBu, 100 °C, 12 h
132 °C, 11 h, mesitylene
86
98
92
99
94
95
92
[8]
[11]
[32]
[33]
[16]
120 °C, toluene, N2, 14 h
Cu‐AcTp@Am‐Si‐Fe3O4
Ag/Al2O3
No solvent, KOH, 100 °C, 10 h, air
FeCl3⋅6H2O, o‐xylene, reflux, 24 h
Cs2CO3, toluene, 100 °C, 12 h
KOH, toluene, reflux, 18 h
Ag@polypyrrole
SMNP‐Ag
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