Sulfonated core-shell magnetic nanoparticle (Fe3O4@SiO2@PrSO3H) as a highly active and durable protonic acid catalyst; Synthesis of coumarin derivatives through pechmann reaction

Article abstract of DOI:10.1002/cctc.201402547

Sulfonic acid supported silica coated magnetic nanoparticles (Fe₃O₄@SiO₂@PrSO₃H), was prepared by using low cost precursors and a facile immobilization technique. The final catalyst, which was characterized by XRD, FT-IR, vibrating sample magnetometer (VSM), TEM, and TGA techniques, was found to be an efficient and environmentally benign solid acid for the Pechmann condensation of substituted phenols with ethyl acetoacetate leading to the formation of coumarin derivatives. After the reaction, the catalyst could be effortlessly separated by external magnet and reused for 22 consecutive runs, without any significant loss in catalytic efficiency. The catalytic system presented offers a reusable strategy for the efficient synthesis of coumarin, simplicity in operation, and a green reaction profile by avoiding toxic conventional catalysts and solvents. Green earth and blue sky: Herein, we wish to disclose a simple bench top procedure for the synthesis of sulfonated core-shell magnetic nanoparticles (SMNPs) (Fe₃O₄@SiO₂@PrSO₃H) and discuss its performance as a very strong solid acid in the Pechmann condensation reaction.

Full text of DOI:10.1002/cctc.201402547

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DOI: 10.1002/cctc.201402547
Sulfonated Core-Shell Magnetic Nanoparticle
(Fe3O4@SiO2@PrSO3H) as a Highly Active and Durable
Protonic Acid Catalyst; Synthesis of Coumarin Derivatives
through Pechmann Reaction
Farhad Kabiri Esfahani,*[a] Daryoush Zareyee,*[b] and Reza Yousefi[c]
Sulfonic acid supported silica coated magnetic nanoparticles
(Fe3O4@SiO2@PrSO3H), was prepared by using low cost precur-
sors and a facile immobilization technique. The final catalyst,
which was characterized by XRD, FT-IR, vibrating sample mag-
netometer (VSM), TEM, and TGA techniques, was found to be
an efficient and environmentally benign solid acid for the
Pechmann condensation of substituted phenols with ethyl ace-
toacetate leading to the formation of coumarin derivatives.
After the reaction, the catalyst could be effortlessly separated
by external magnet and reused for 22 consecutive runs, with-
out any significant loss in catalytic efficiency. The catalytic
system presented offers a reusable strategy for the efficient
synthesis of coumarin, simplicity in operation, and a green re-
action profile by avoiding toxic conventional catalysts and sol-
vents.
tion of the products.[2–16] However, a major impediment to
such progress is the lack of a solid acid that is as stable, active,
and inexpensive as sulfuric acid. A perfect solid acid material
for the catalytic applications should have high stability and nu-
merous strong protonic acid sites. Therefore, developing
a facile and effective approach to immobilization sulfuric acid
on a solid support is would be beneficial. In this regard, many
support materials are often used for immobilization of sulfuric
acid such as mesoporous silica,[17–23] amorphous silica,[24] meso-
porous carbon,[25] amorphous carbon,[26–32] and polymers.[33–38]
These supports can be separated by conventional separation
techniques such as centrifugation and filtration. Recently, mag-
netic nanomaterials have emerged as alternatives to conven-
tional materials as readily available, robust, high surface area
heterogeneous catalyst supports.[39–42] Notably, one of the in-
teresting features of magnetically supported catalysts is that
they can be easily recovered with an external magnet.[43–50]
Herein, we report a simple bench-top synthesis of sulfonat-
A large amount of liquid sulfuric acid is used annually as a cata-
lyst in the chemical industry for the production of important
chemicals, such as esters, alcohols, ethers, and various starting
materials for polymers and resins.[1] Although this catalyst is
used in critical chemical processes, it suffers from several draw-
backs, such as the large amount of waste produced by neutral-
ization of H2SO4, troublesome separation of sulfates (from neu-
tralization processing), and purification of the product, which
also involves substantial energy and material use. Consequent-
ly, owing to increasingly stringent environmental standards
and economic pressures, significant attention has been direct-
ed toward the use of solid acid catalysts to achieve effective
separation of catalyst, waste reduction, and simplified purifica-
ed
core-shell
magnetic
nanoparticles
(SMNPs)
(Fe3O4@SiO2@PrSO3H) and discuss their performance as a very
strong solid acid in the Pechmann condensation reaction. Silica
coated Fe3O4 nanoparticles were prepared by a known proce-
dure, which utilizes cheap starting materials.[51–53] The resulting
silica-coated magnetic nanoparticles were then allowed to
react under vigorous stirring with an appropriate concentra-
tion of (3-mercaptopropyl)trimethoxysilane to give mercapto-
propyl-functionalized silica-coated magnetic nanoparticles
(Fe3O4@SiO2@PrSH). To this end, the obtained mercaptopropyl-
coated Fe3O4 were oxidized to the corresponding sulfonic acid
derivative using H2O2 as the oxidant (Scheme 1).
To confirm the synthesis of the Fe3O4 magnetic nanoparti-
cles, XRD analysis was performed (Figure 1). As can be seen in
Figure 1, the nanomagnets exhibit six well resolved peaks that
are indexable as the (200), (311), (400), (422), (511), and
(440) that exactly match with the standard Fe3O4 sample
(JCPDS file No. 19-0629). The size of Fe3O4 magnetic nanoparti-
cles was estimated to be approximately 40 nm by employing
the Debye–Scherrer equation from XRD analysis. Inspection of
the catalyst TEM images indicates magnetic nanoparticles with
a size distribution of 30–60 nm (average ꢀ40 nm), a value
which is in good agreement with the diameter estimated from
the XRD spectra (Figure 2).
[a] Dr. F. K. Esfahani
Department of Chemistry, Faculty of Sciences
University of Zanjan
Zanjan 45371-38791 (Iran)
E-mail: f.kabiri@znu.ac.ir
[b] Dr. D. Zareyee
Department of Chemistry, Qaemshahr Branch
Islamic Azad University
Qaemshahr (Iran)
E-mail: zareyee@gmail.com
[c] R. Yousefi
Department of Chemistry, Science and Research Branch
Islamic Azad University
The FT-IR spectrum of SMNPs shows peaks that are charac-
teristic of SO3H-functionalized magnetic nanoparticles
(Fe3O4@SiO2@PrSO3H), which differ from that of the silica-
Mazandaran (Iran)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402547.
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Scheme 1. Synthesis of sulfonated core-shell magnetic nanoparticles (SMNPs, Fe3O4@SiO2@PrSO3H).
Figure 3. FT-IR spectrum of A) Fe3O4@SiO2 B) Fe3O4@SiO2@PrSO3H.
to the OH groups on the core-shell magnetic nanoparticle sur-
face after the modification and sulfonation (Figure 3B). The in-
crease in the intensity of this band suggests that there are
more OH groups over the surface of final catalyst in compari-
son with the spectra of the unfunctionalized catalyst (Fig-
ure 3A). This analysis corroborated the successful anchoring of
the SO3H groups on the surface of magnetic nanoparticles
(Figure 3).
Figure 1. Powder XRD patterns of Fe3O4.
To investigate the magnetic properties of the naked Fe3O4
nanoparticles and SMNPs, the vibrating sample magnetometer
(VSM) was performed at room temperature and applied mag-
netic field up to 10000 Oe. The magnetic hysteresis loop
showed ferromagnetic property for both of samples. The mag-
netic saturation of naked Fe3O4 nanoparticles, silica coated
magnetic nanoparticles and SMNPs, are 55 emugÀ1,
43 emugÀ1, and 40 emugÀ1, respectively (Figure 4). This re-
duced magnetic strength is attributed to the silica shells
coated on magnetic nanoparticles.
The loading of the SO3H groups onto the magnetic nanopar-
ticles can be calculated from thermogravimetric analysis (TGA)
(Supporting Information), ion exchange pH analysis and ele-
mental analysis, which confirmed a loading of approximately
0.9 mmolgÀ1.
Figure 2. TEM image of the Fe3O4@SiO2@PrSO3H. Scale bar=100 nm.
coated magnetic nanoparticles (Fe3O4@SiO2). The antisymmet-
ric stretching vibrations peaks of Si-O-Si bond in the silica shell
are shown around 1084 cmÀ1. Furthermore, the presence of
alkyl groups in the Fe3O4@SiO2@SO3H is confirmed by the ali-
phatic weak CÀH stretching vibrations at 2923 cmÀ1 (Fig-
ure 3B). Absorbance bands at 3100–3700 cmÀ1 were attributed
In continuation of our studies on the catalytic properties of
heterogeneous acid catalysts,[20–23,25] the catalytic activity of
SMNPs was tested against the Pechmann condensation reac-
tion. We chose this reaction for two reasons: 1) the Pechmann
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Table 1. Comparison of activity of some homogeneous and heterogene-
ous sulfonic acid catalysts in Pechmann condensation of resorcinol with
ethyl acetoacetate.[a]
Entry
Catalyst
Time [min]
Yield [%][b]
1
2
3
4
5
6
7
8
Fe3O4@SiO2@PrSO3H
none
H2SO4
25
25
25
25
60
20
420
240
96
N.R.
10
10
90
95
88
63
p-TSOH
SBA-15-Ph-PrSO3H[c]
CMK-5-SO3H[d]
mordenite[e]
zeolite BEA[f]
Figure 4. Magnetic hysteresis curves of A) Fe3O4, B) Fe3O4@SiO2,
C) Fe3O4@SiO2@PrSO3H.
[a] Reaction condition: resorcinol (1 mmol), ethyl acetoacetate (1 mmol),
catalyst (ꢀ1.6 mol%) at 1308C. [b] Isolated yields. [c] ꢀ7 mol% of SBA-
15-Ph-PrSO3H was used. [d] 3 mol% of CMK-5-SO3H was used. [e] Reaction
condition: resorcinol (10 mmol), ethyl acetoacetate (10 mmol), 608C, cata-
lysts (0.3 g), ultrasound.[62] [f] Reaction condition: resorcinol (8.5 mmol),
ethyl acetoacetate (10 mmol), catalyst (0.2 g), solvent PhNO2, 1308C.[63]
reaction is usually performed in high temperature, and hence
the stability and reusability of the catalyst can be investigated
under this harsh reaction conditions; 2) Coumarins as the prod-
ucts of this reaction are extremely valuable in the range from
additive in foods, perfumes, cosmetics, pharmaceuticals, and in
the preparation of insecticides,[54] optical brighteners,[55] and
dispersed fluorescent and tunable laser dyes.[56] Additionally,
some coumarins have varied bioactivities, such as, inhibitory of
platelet aggregation,[57] antibacterial,[58] anticancer,[59] and inhib-
itory of HIV-1 protease.[60] Coumarins also act as intermediate
for the synthesis of furocoumarins, chromones, coumarones,
and 2-acylresorcinols.[61] These wide-ranging properties mean
that coumarins are very interesting to chemists.
substrates with electron-donating groups para to the site of
electrophilic substitution afforded good yields of correspond-
ing coumarin derivatives (Table 2, entries 1–5). Interestingly, 3-
methoxyphenol showed no detectable demethylation under
the given conditions (Table 2, entry 3) and 3-aminophenol re-
acted to provide the amino coumarin in good yield with high
chemoselectivity (Table 2, entry 5). Pyrogallol, phloroglucinol,
orcinol, 3,5-dimethyl phenol, and 2,6-dihydroxy toluene
(Table 2, entries 6–10) also gave excellent yields in shorter reac-
tion times. However, 1-naphthol and 2-naphthol required
longer reaction times, owing to the presence of another
phenyl rings (Table 2, entries 11–12).
In this sense, the feasibility of the reaction was initially ex-
amined with resorcinol and ethyl acetoacetate. At the outset,
we found that, when a mixture of resorcinol (1 mmol), ethyl
acetoacetate (1 mmol), and
a small amount of SMNPs
(1.6 mol% of SO3H groups), at 1308C was stirred for 25 min,
the corresponding coumarin was obtained in a 96% yield
(Table 1, entry 1). As expected, reaction did not proceed with-
out catalyst (Table 1, entry 2). For a better comparison of the
catalytic performance of Fe3O4@SiO2@PrSO3H, a set of individu-
al catalytic experiments were conducted by employing H2SO4,
p-TSOH, SBA-15-Ph-PrSO3H, and CMK-5-SO3H under the same
reaction condition and compared with zeolites under different
conditions (Table 1, entries 3–8). It can be clearly seen that
Fe3O4@SiO2@PrSO3H with lower catalyst loading exhibits higher
activity in Pechmann coupling reaction of resorcinol and ethyl
acetoacetate under the same reaction condition. The most im-
portant issue that should be considered for practical applica-
tions of heterogeneous systems is the easy recovery of catalyst
from the reaction. In compared with other heterogeneous cat-
alysts (Table 1), SMNPs can recover more easily from reaction
mixture with an external magnet and reused for several times
without decrease in catalytic activity.
Our attention next turned to the recycling performance of
the catalyst in the condensation of resorcinol with ethyl ace-
toacetate. After the completion of the first run, the catalyst
was effortlessly separated from the reaction medium by an ex-
ternal magnet. As shown in Figure 5, the recovered catalyst
could be directly reused in twenty two successive runs without
significant loss of activity.
Encouraged by these results and having established that
Fe3O4@SiO2@PrSO3H constitutes an active catalyst system for
von Pechmann condensation, we then investigated substrate
generality of substituted phenols and ethyl acetoacetate under
optimized conditions (Table 2). Resorcinol, hydroquinone, and
Figure 5. Recyclability of the catalyst for the Pechmann condensation of re-
sorcinol.
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aration and solvent-free reaction conditions add to the versatil-
ity of this method. Further study to catalytic applications of
this system especially for bio-diesel formation is under way in
our group.
Table 2. Synthesis of coumarins by Fe3O4@SiO2@PrSO3H.
Entry Substrate
Product[a]
Time [min] Yield [%][b,c]
1
25
30
35
35
30
96
90
88
87
93
Experimental Section
2
3
4
5
Chemicals were either prepared in our laboratories or were pur-
chased from Alderich or Fluka chemical companies. Reaction moni-
toring was achieved by TLC on silica-gel polygram SILG/UV 254
1
plates. The products were characterized by H and 13C NMR spec-
troscopy by using Bruker 250 or 400 MHz apparatus. X-ray diffrac-
tion (XRD) patterns were recorded by an Xpert-Pro, X-ray diffrac-
tometer using CuKa radiation. A transmission electron microscope,
TEM (Philips CM-10), was also used to obtain TEM images.
Synthesis of magnetic nanoparticles (Fe3O4): In a typical prepara-
tion procedure, FeCl2 (5.4 g) and FeCl3 (2 g) was dissolved in aque-
ous hydrochloride acid (2m, 25 mL) at room temperature and soni-
cated until the salts dissolved completely. Subsequently, aqueous
ammonia (25%, 40 mL) was added slowly over 20 min to the mix-
ture. The resultant solution was stirred for 30 min under argon at-
mosphere at room temperature. The Fe3O4 nanoparticles were
then separated by external magnet and thoroughly washed with
distilled water and ethanol and dried under vacuum to obtain the
final product. Also, the Fe3O4 nanoparticles could be re-suspended
in ethanol and stored in the refrigerator to use.
6
25
97
7
8
9
20
25
30
98
94
90
Synthesis of silica-coated magnetic nanoparticles: The resulting
Fe3O4 was then dispersed in deionized water (6 mL) and ethanol
(35 mL) and gently sonicated for 15 min to obtain a homogeneous
dispersion of magnetic nanoparticles. Subsequently tetraethyl or-
thosilicate (TEOS, 1.5 mL) was added slowly to the mixture and so-
nicated for 10 min. and then aqueous ammonia (10%, 1.4 mL) was
added slowly over 10 min under mechanical stirrer. The mixture
was heated at 408C for 12 h. To this end, the silica coated magnet-
ic nanoparticles (Fe3O4@SiO2) were separated by an external
magnet and washed three times with ethanol and dried under
vacuum.
10
3
89
Synthesis of the mercaptopropyl silica-coated magnetic nano-
particles: The resulting Fe3O4@SiO2 (10 g) was then dispersed in
toluene (200 mL) and gently sonicated for 30 min to produce a ho-
mogeneously mixed solution. After that, (3-mercaptopropyl)trime-
thoxysilane (2.5 mL) was added under mechanical stirring and the
mixture was slowly heated to 1058C and kept at this temperature
for 20 h. The final material was separated by an external magnet
and washed three times with distilled water and ethanol and dried
under vacuum. The concentrated product stored in a refrigerator
to use.
11
12
50
45
91
88
[a] Reaction condition: substrates (1 mmol), ethyl acetoacetate (1 mmol),
Fe3O4@SiO2@PrSO3H (1.6 mol%),1308C. [b] All products were character-
ized by comparison of their 1H and 13C NMR spectra with those of authen-
tic samples. [c] Isolated yields after recrystallization.
Synthesis of sulfonic acid supported on the silica coated mag-
netic nanoparticles (SMNPs): The resulting MMNPs were then dis-
persed in hydrogen peroxide (30%, 100 mL) and the mixture was
stirred vigorously for 24 h under room temperature. After 12 h, sul-
furic acid (6m, 200 mL) was added slowly to the mixture. The final
catalyst was separated by an external magnet and washed several
times with distilled water and ethanol and dried under vacuum.
The concentrated product could be stored in a refrigerator to use.
In conclusion, sulfonated core-shell magnetic nanoparticles
(Fe3O4@SiO2@PrSO3H) was established as a simple and highly
recyclable solid acid, which can be prepared by low cost pre-
cursors
and
facile
immobilization
technique.
Fe3O4@SiO2@PrSO3H was an effective catalyst for the Pech-
mann condensation reaction of a neat mixture of phenols with
ethyl acetoacetate leading to the formation of excellent yields
of coumarin derivatives at 1308C. The excellent catalytic ca-
pacity and outstanding recyclability of catalyst, the simple sep-
General procedure for the Pechmann condensation: In a round
bottom flask, SMNPs (0.018 g, 1.6 mol%) was added to the mixture
of phenolic compounds (1 mmol) and ethylacetoacetate (1 mmol)
at 1308C and the reaction mixture stirred for the appropriate time.
The progress of reaction was monitored by TLC (eluent, n-hexane/
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Keywords: condensation
catalysis · nanoparticles · solid acid
·
coumarin
·
heterogeneous
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Published online on && &&, 0000
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ChemCatChem 0000, 00, 1 – 5
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5
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These are not the final page numbers!

COMMUNICATIONS
F. K. Esfahani,* D. Zareyee,* R. Yousefi
Green earth and blue sky: Herein, we
wish to disclose a simple bench top
procedure for the synthesis of sulfonat-
ed core-shell magnetic nanoparticles
(SMNPs) (Fe3O4@SiO2@PrSO3H) and dis-
cuss its performance as a very strong
solid acid in the Pechmann condensa-
tion reaction.
&& – &&
Sulfonated Core-Shell Magnetic
Nanoparticle (Fe3O4@SiO2@PrSO3H) as
a Highly Active and Durable Protonic
Acid Catalyst; Synthesis of Coumarin
Derivatives through Pechmann
Reaction
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!