TiO2-coated magnetite nanoparticle-supported sulfonic acid as a new, efficient, magnetically separable and reusable heterogeneous solid acid catalyst for multicomponent reactions

Article abstract of DOI:10.1039/c5ra06515a

TiO₂-coated magnetite nanoparticle-supported sulfonic acid (nano-Fe₃O₄-TiO₂-SO₃H (n-FTSA)) is synthesized by immobilizing -SO₃H groups on the surface of nano-Fe₃O₄-TiO₂. This catalyst can be isolated readily after completion of the reaction by an external magnetite field. The obtained results demonstrate the use of coated magnetite nanoparticles as an excellent new support for the facile recovery of sulfonic acid catalysts. The newly synthesized heterogeneous solid acid is characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FE-SEM), a vibrating sample magnetometer (VSM), FT-IR spectroscopy, thermal gravimetric analysis (TGA), Hammett acidity function and pH analysis. Significantly, the as-prepared n-FTSA exhibits a high catalytic activity for the synthesis of heterocyclic compounds such as 1,8-dioxo-decahydroacridine, 1,8-dioxo-octahydroxantene, hexahydroquinoline and polyhydroquinoline derivatives in multicomponent reactions. The magnetically separated n-FTSA can be magnetically separated and reused for several times without significant loss of activity. This confirms that the sulfonic acid groups have high stability. TiO₂-coated magnetite nanoparticle-supported sulfonic acid has advantages such as low cost and toxicity, ease of preparation, magnetic separation, high stability, reusability and operational simplicity. This journal is

Full text of DOI:10.1039/c5ra06515a

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TiO2-coated magnetite nanoparticle-supported
sulfonic acid as a new, efficient, magnetically
separable and reusable heterogeneous solid acid
catalyst for multicomponent reactions†
Cite this: RSC Adv., 2015, 5, 45974
*
Ali Amoozadeh, Sanaz Golian and Salman Rahmani
TiO2-coated magnetite nanoparticle-supported sulfonic acid (nano-Fe3O4–TiO2–SO3H (n-FTSA)) is
synthesized by immobilizing –SO3H groups on the surface of nano-Fe3O4–TiO2. This catalyst can be
isolated readily after completion of the reaction by an external magnetite field. The obtained results
demonstrate the use of coated magnetite nanoparticles as an excellent new support for the facile
recovery of sulfonic acid catalysts. The newly synthesized heterogeneous solid acid is characterized by
X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron
microscopy (FE-SEM), a vibrating sample magnetometer (VSM), FT-IR spectroscopy, thermal gravimetric
analysis (TGA), Hammett acidity function and pH analysis. Significantly, the as-prepared n-FTSA exhibits a
high catalytic activity for the synthesis of heterocyclic compounds such as 1,8-dioxo-decahydroacridine,
1,8-dioxo-octahydroxantene, hexahydroquinoline and polyhydroquinoline derivatives in multicomponent
reactions. The magnetically separated n-FTSA can be magnetically separated and reused for several times
without significant loss of activity. This confirms that the sulfonic acid groups have high stability. TiO2-
coated magnetite nanoparticle-supported sulfonic acid has advantages such as low cost and toxicity, ease
of preparation, magnetic separation, high stability, reusability and operational simplicity.
Received 11th April 2015
Accepted 12th May 2015
DOI: 10.1039/c5ra06515a
www.rsc.org/advances
impossible. This problem is overcome by using magnetite
nanoparticle as heterogeneous supports, which can be readily
1. Introduction
isolated from the reaction mixture in the presence of an external
magnetite eld.22–37 An intrinsic disadvantage of bare magnetite
nanoparticles is magnetite aggregation38,39 that outside surface
coating by metal oxides reduces lower this problem40 and can be
improved catalytic activity of the magnetite nanoparticles.
Among the many metal oxides, titanium dioxide is attractive
because of extra ordinary features and different applications in
gas sensing,41,42 catalysis,43 photocatalysis44–46 etc.
The synthesis of surface-modied magnetite nanoparticles is
one of the major areas of current research in the eld of catal-
ysis.47–53 The magnetite solid acid nanocatalysts have emerged
as efficient and powerful in organic reactions because of high
activity, easy separation, minimization of environmental waste
and clean reaction products without contamination of active
metals.54 Based on the mentioned facts and in continuation of
our last efforts and studies about synthesis of heterogeneous
solid acid nanocatalysts55,56 and their applications in organic
reaction,57–59 we decided to introduce a magnetically recoverable
solid acid catalyst. For this purpose, we have designed and
synthesized TiO2-coated magnetite nanoparticles sulfonic acid
(n-FTSA) and used it for some multicomponent reactions. As
sulfonation with chlorosulfonic acid is a convenient, fast and
efficient method for heterogenization of homogeneous
Very oen the most exclusive components in chemical reactions
are catalysts. Sulfuric, nitric, hydrochloric and phosphoric acid
are the most important and commonly used homogeneous acid
catalysts in organic reactions and industrial processes.1 Among
these, sulfuric acid is not a recyclable and environmentally
benign material. Corrosive properties and inefficient separation
from reaction mixtures leads to a huge waste of energy and
production of large amounts of waste products. In spite of the
aforementioned drawbacks, it is essential for chemical indus-
tries, too.2 Recently, heterogenization of homogeneous catalysts
by their immobilization on the surface of various solid supports
has attracted great interest from the environmental and
economic points of view.2–16 In this regard, metal oxide nano-
particle supports are attractive due to their large surface area-to-
volume.17–21 As a result, nanoparticles have high catalyst loading
capacity which leads to the improved performance. The active
sites on their surfaces are easily available by reactants. Separa-
tion of the nanoparticles by conventional means is almost
Department of Chemistry, Semnan University, Semnan, 35131-19111, Iran. E-mail:
aamozadeh@semnan.ac.ir; Fax: +98 233 3354110; Tel: +98 233 3366177
† Electronic supplementary information (ESI) available: FE-SEM, VSM, TGA,
reusability of the n-FTSA, 1H NMR and 13C NMR spectra. See DOI:
10.1039/c5ra06515a
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catalysts55,60–64 that has absorbed much attention aer Zolgol's
report,65 and as we have already successfully synthesized and
reported nano-TiO2–SO3H (n-TSA) as an excellent heteroge-
neous nanocatalyst for different organic reactions,55 we have
decided to react the nano-Fe3O4–TiO2 react with chlorosulfonic
acid to produce nano-Fe3O4–TiO2-supported sulfonic acid
(nano-Fe3O4–TiO2–SO3H).
The new synthesized nano-Fe3O4–TiO2–SO3H catalyst has
been successfully used for some multicomponent one-pot
reactions to synthesis different heterocyclic compounds such
as 1,8-dioxo-decahydroacridine, 1,8-dioxo-octahydroxantene,
hexahydroquinoline and polyhydroquinoline derivatives. This
novel nanocatalyst can be magnetically separated and recycled
several runs without appreciable loss of its catalytic activity.
2. Results and discussion
Fig. 1 X-ray diffraction pattern of (a) the nano-Fe3O4/TiO2 and (b)
nano-Fe3O4/TiO2–SO3H.
The supported sulfonic acid analogue was prepared by the
concise route outlined in Scheme 1.
Fe3O4/TiO2 core–shell nanoparticles were prepared accord-
ing to the reported procedure.66
around 22.75 nm. The interplanar spacing (d) was calculated by
Bragg's equation using the peak at 35.48ꢀ and 35.61ꢀ for nano-
Fe3O4/TiO2 and nano-Fe3O4/TiO2–SO3H, respectively. Dd ¼ d(a)
2.1. Characterization of nano-Fe3O4/TiO2–SO3H
2.1.1. X-ray diffraction spectra. Fig. 1 shows the X-ray
diffraction patterns of the synthesized nano-Fe3O4/TiO2 core–
shell and the acidic modied nano-Fe3O4/TiO2–SO3H (n-FTSA).
Fig. 1a shows the XRD pattern of nano-Fe3O4/TiO2. The
following signals at (220), (311), (400), (511) and (440) in Fig. 1a
planes conrm that the main formed phase is a cubic Fe3O4,
which conforms with the JCPD 79-0417 standard.
However, there are two other phases including SiO2 with
orthorhombic crystal structure (JCPDS: 44-1394) and TiO2 with
anatase crystal structure which conforms by (105), (211), (116),
(220), (512) and (224) diffraction peaks (JCPDS: 89-4921). The
crystal size of the nano-Fe3O4/TiO2 powder was also determined
from X-ray pattern using the Scherrer formula given as t ¼ 0.9l/
B1/2 cos q, that t is the average crystal size, l the X-ray wave-
˚
ꢁ d(b) ¼ 0.253 ꢁ 0.238 ¼ 0.015 A. So according to the data, there
is a contraction in the unit cell.
2.1.2. Energy-dispersive X-ray spectroscopy (EDX). The
presence of Fe, O, Si and Ti atoms was observed in the EDX
spectrum (Fig. 2).
2.1.3. Field emission scanning electron microscopy.
According to the FE-SEM image of nano-Fe3O4/TiO2, it was
found that the morphology of the synthesized nano material
has a sphere like structures. The size distribution and
morphology of nanoparticles are almost homogeneous and
diameter sizes obtained nanoparticles are about 50–60 nm.
The FE-SEM image of nano-Fe3O4/TiO2–SO3H shows that by
modifying nano-Fe3O4/TiO2 with SO3H, the morphology of the
obtained materials has been maintained particle structures.
However the size distribution is nearly non-homogeneous.
˚
length used (1.54 A), B1/2 the angular line width at half
maximum intensity and q the Bragg's angle. The average crystal
size of the nano-Fe3O4–TiO2 powder for 2q ¼ 35.48ꢀ is calculated
to be around 23 nm.
Fig. 1b shows the XRD pattern of nano-Fe3O4/TiO2–SO3H. It
shows that with modifying nano-Fe3O4/TiO2, the XRD pattern
shows a higher noise/signal ratio indicating that the material
has an acidic agent on their surface. The average crystal size of
nano-Fe3O4/TiO2–SO3H for 2q ¼ 35.61ꢀ is calculated to be
Scheme 1 Preparation of nano-Fe3O4/TiO2-supported sulfonic acid.
Fig. 2 EDX spectrum of nano-Fe3O4/TiO2.
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Since the size distribution of nano-Fe3O4/TiO2 is not homo-
geneous, and there are small and large particles together in the
structure, so there are two kinds of particles sizes. The diameter
size for the small particles is about 40–60 nm and for the large
particles is about 100–150 nm.
2.1.4. Vibrating sample magnetometer. The magnetite
properties of the Fe3O4/TiO2 nanoparticles and nano-Fe3O4/
TiO2–SO3H have been measured by use of the vibrating sample
magnetometer. The magnetization curves measured at room
temperature showed that n-FTSA was super paramagnetic and
had magnetization saturation value of 26.31 emu gꢁ1. The
number is lower than that of TiO2-coated magnetite nano-
particles (about 50 emu gꢁ1). The decrease in saturation
magnetization aer being coated is because of the immobili-
zation of non-magnetite sulfonic acid groups on the surface of
magnetite nanoparticles.
Fig. 4 Absorption spectra of (a) 4-nitroaniline (indicator) and (b) nano-
Fe3O4/TiO2–SO3H (catalyst) in CCl4.
Nonetheless, the magnetization value is enough for common
magnetite separation. As can be seen, n-FTSA was readily
dispersible in reactive system and can be easily separated with a
small magnet near the tube.
2.1.5. FT-IR spectra. The FT-IR spectra of nano-Fe3O4–TiO2
and nano-Fe3O4–TiO2–SO3H are shown in Fig. 3. The O–H stretch
and vibration of surface hydroxyl groups and physically adsorbed
water were present as broad peaks at 3000–3600 cmꢁ1 and a
sharper peaks at 1628 cmꢁ1 (a) and 1632 cmꢁ1 (b), respectively.
We assign the absorption band appears at 979 and 1080 cmꢁ1 (a),
997 and 1092 cmꢁ1 (b) to a combination of Ti–O–Si and Si–OH
vibrations.67–69
corresponds to the dehydroxylation of nano-Fe3O4/TiO2.71 The
TGA curve of n-FTSA shows three regions corresponding to
different mass lose ranges. The rst region is below 100 ꢀC that
displayed a mass loss (3 wt%) that was attributable to the loss of
trapped water from the catalyst. A mass loss of approximately
22.37% weight occurred between 100 and 540 ꢀC that is related
to the slow mass loss of SO3H groups. Finally, a mass loss of
approximately 15.46% weight occurred between 540 and 644 ꢀC
that is related to the sudden mass loss of SO3H groups.70,72,73 In
addition, from the TGA, it is understood that n-FTSA has a great
ꢀ
thermal stability (until 200 C) conrming that it can be safely
As respects, the O¼S¼O asymmetric and symmetric
stretching vibration and S–O stretching vibration of the sulfonic
groups (–SO3H) are appeared in the absorption range 1150–
1250, 1010–1100 and 650–850 cmꢁ1, respectively.70 Stretching
vibrations of TiO2 were observed in the absorption range of
sulfonic groups so these bands have been hidden (Fig. 3b).
2.1.6. Thermo gravimetric analysis. According to the TGA
curve of nano-Fe3O4/TiO2 a mass loss (2 wt%) below 100 ꢀC
which corresponds to the loss of the physically adsorbed water.
used in organic reactions at temperatures in the range of
80–180 C.
2.1.7. Surface acidity studies. The Hammett acidity func-
tion (H0) can effectively express the acidity strength of an acid in
organic solvents.74 It can be calculated using the following
equation:
ꢀ
H0 ¼ pK(I)aq + log([I]s/[IH+]s).
ꢀ
Here, ‘I’ represents the indicator base (mainly substituted
nitroanilines) and [I]s and [IH+]s are respectively the molar
concentrations of the unprotonated and protonated forms of
the indicator. The pK(I)aq values are already known (for example
the pK(I)aq value of 4-nitroaniline is 0.99) and can be obtained
from many references. According to the Lambert–Beer law, the
value of [I]s/[IH+]s can be determined and calculated using the
UV-visible spectrum. In the present experiment, 4-nitroaniline
was chosen as the basic indicator and CCl4 was chosen as the
Also, there is a slight weight loss (1 wt%) between 100–440 C
and weight loss (5 wt%) between 440 and 800 ꢀC, which possibly
Table 1 Calculation of Hammett acidity function (H0) of n-FTSAa
Entry
Catalyst
Amax
[I]s (%)
[IH+]s (%)
H0
1
2
—
n-FTSA
2.417
1.987
100
82.64
0
—
1.75
17.36
a
Condition for UV-visible spectrum measurement: solvent, CCl4;
indicator, 4-nitroaniline (pK(I)aq
¼
0.99), 1.44
ꢂ
10ꢁ4 mol Lꢁ1
;
catalyst, n-FTSA (10 mg), 25 ꢀC.
Fig. 3 The FT-IR spectra of (a) the nano-Fe3O4/TiO2 and (b) nano-
Fe3O4/TiO2–SO3H.
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Table 3 Effect of temperature on the n-FTSA catalyzed synthesis of
tetramethyl-9-phenyl-hexahydroacridine-dionea
Entry
Temperature [ꢀC]
Yieldb [%]
1
2
3
4
5
80
90
100
110
120
70
70
85
95
80
Scheme 2 Synthesis of 1,8-dioxodecahydroacridine derivatives cata-
lyzed by n-FTSA.
solvent because of its aprotic nature. The maximal absorbance
of the unprotonated form of 4-nitroaniline was observed at
330 nm in CCl4. As Fig. 4 shows, the absorbance of the unpro-
tonated form of the indicator in n-FTSA was weak as compared
to the sample of the indicator in CCl4, which indicated that the
indicator was partially in the form of [IH+].
The obtained results are listed in Table 1, which shows the
acidity strength of nano-Fe3O4–TiO2–SO3H. These results of the
Hammett acidity function (H0) also conrm the synthesis of
new catalyst with a good density of acid sites (–SO3H groups) on
the surface of n-FTSA (Table 1).
a
Reaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol),
b
NH4OAc (2.5 mmol), n-FTSA (0.01 g), solvent free. Refers to isolated
yield.
On the basis of the information obtained from the above
mentioned studies, in this context, we synthesized nano-Fe3O4/
TiO2-supported sulfonic acid (nano-Fe3O4/TiO2–SO3H) with
highly density of sulfonic acid groups (–SO3H) and introduced
as a novel, highly efficient and strong, stable, magnetically
separable, reusable and inexpensive heterogeneous solid acid.
The catalytic activity of new nanocatalyst was examined in
multicomponent reactions for the synthesis of heterocyclic
compounds such as 1,8-dioxo-decahydroacridine, 1,8-dioxo-
octahydroxanthene, polyhydroquinoline and hexahydroquino-
line derivatives. The structures of the some nal products were
well characterized by using spectral (IR, 1H NMR, 13C NMR)
data.
2.2.1. Synthesis of 1,8-dioxo-decahydroacridine deriva-
tives. In the rst glance, in order to initially optimize the reac-
tion conditions and identity of the best amount of catalyst,
benzaldehyde 1, cyclic-diketone 2a and ammonium acetate 3
were stirred under solvent free conditions in 110 ꢀC (Scheme 2).
By screening different amounts of the n-FTSA and
comparing with other sources of iron and titanium, we found
out that the product 4a could be obtained in yields ranging from
The mmol of H+ per gram of catalyst (5 mmol gꢁ1 of n-FTSA)
was determined by the titration of 0.1 g of sample with a stan-
dard solution of NaOH (0.1 N). For this purpose, the surface
acidic protons of n-FTSA (100 mg) were ion-exchanged with a
saturated solution of NaCl (10 mL) by sonication. This process
was repeated twice more, yielding 30 mL of proton-exchanged
brine solution. Therefore, to determine the loading of acid
sites on the synthesized catalyst, the obtained solution was
titrated by NaOH (0.1 M) solution in presence of phenol red
indicator solution or pH meter.
2.2. Investigating of nano-Fe3O4–TiO2–SO3H in one-pot
multicomponent reactions
In continuation of our studies on the development of solid acids
as efficient, inexpensive and also environmentally benign
methodologies for organic reactions,57–59 we have recently
reported nano-titania-supported sulfonic acid,55 nano-WO3-
supported sulfonic acid56 as novel and heterogeneous nano-
catalyst. Also chromium trioxide-supported sulfonic acid was
studied where the results were also very satisfactory.
Table
acridinesa
4 n-FTSA catalyzed synthesis of 1,8-dioxo-decahydro-
R1]R2
Ar
Product
Time (min)
Yieldb [%]
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
Ph
4a
4b
4c
4d
4e
4f
4g
4h
4i
5a
5b
5c
5d
5e
5f
50
40
45
25
25
50
55
60
60
50
40
45
45
60
55
95
95
94
96
95
90
90
92
93
90
94
94
94
92
90
Table 2 Optimization of catalyst amount for the synthesis of tetra-
p-Cl–C6H4
o-Cl–C6H4
p-O2N–C6H4
m-O2N–C6H4
p-(CH3)2N–C6H4
p-HO–C6H4
p-H3C–C6H4
p-H3CO–C6H4
Ph
m-O2N–C6H4
p-Cl–C6H4
p-Br–C6H4
p-H3C–C6H4
p-HO–C6H4
methyl-9-phenyl-hexahydroacridine-dionea
Entry
Catalyst (g)
Yieldb [%]
1
2
3
4
5
6
7
8
9
—
8
24
43
28
49
50
60
95
95
75
Bulk-TiO2
Nano-TiO2
Bulk-Fe2O3
Nano-Fe2O3
Nano-Fe3O4
Nano-Fe3O4–TiO2
n-FTSA (0.015)
n-FTSA (0.01)
n-FTSA (0.005)
H
H
H
H
H
10
a
Reaction conditions: aromatic aldehyde (1 mmol), cyclic di-ketone (2
a
Reaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol), mmol), NH4OAc (2.5 mmol), n-FTSA (0.01 g) under solvent free
b
NH4OAc (2.5 mmol), solvent free, 110 ꢀC. Refers to isolated yield.
conditions at 110 ꢀC. b Yields refer to isolated products.
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Table 5 Reuse of n-FTSA in the synthesis of tetramethyl-9-phenyl-
hexahydroacridine-dionea
Run
1
2
3
4
5
Yieldb [%]
95
95
94
93
93
a
Scheme 4 Synthesis of polyhydroquinoline derivatives catalyzed by n-
FTSA.
Benzaldehyde 1 (1 mmol), dimedone 2a (2 mmol), NH4OAc 3 (2.5
mmol), solvent free conditions at 110 ꢀC. b Isolated yields.
halogens on the aromatic ring) and cyclic 1,3-di-ketone
compounds. The obtained results are summarized in Table 4.
Because of ferromagnetic features, the separation and reuse
of n-FTSA is very facile. In this regard, the catalyst was washed
with hot ethanol and acetone twice and then was dried in the
open air. The recycled catalyst could be reused for at least ve
times with no appreciable decrease in yield of product 4a
(Table 5).
2.2.2. Synthesis of 1,8-dioxo-octahydroxanthene deriva-
tives. The prepared n-FTSA has been tested for the synthesis of
1,8-dioxo-octahydroxanthene derivatives under optimal condi-
tions by condensation between aromatic aldehydes 1 (1 mmol)
and cyclic di-ketones 2 (2 mmol) (Scheme 3). The results are
recorded in Table 6.
Scheme
catalyzed by n-FTSA.
3 Synthesis of 1,8-dioxo-octahydroxanthene derivatives
60 to 95%. The best yield was obtained by using 0.01 g of n-FTSA
(Table 2, entry 9) and is a more suitable option than nano-
Fe3O4–TiO2 (Table 2, entry 7). Also, the obtained results of other
catalysts showed low yields of product 4a. The greater catalytic
activity of n-FTSA was most likely related to the SO3H groups of
the catalyst, which could provide efficient acidic sites.
In the next step, to study the effect of temperature was
investigated. The results showed that the best temperature was
110 ꢀC (Table 3, entry 4) and the obtained yield remained
2.2.3. Synthesis of polyhydoquinoline derivatives. The
efficiency of obtained n-FTSA has been tested by the synthesis of
polyhydroquinoline derivatives under optimal conditions by
condensation between benzaldehyde
1 (1 mmol), ethyl-
acetoacetate 9 (1 mmol), cyclic di-ketones 2a–c (1 mmol) and
ammonium acetate 3 (2.5 mmol) (Scheme 4). The results are
summarized in Table 7.
2.2.4. Synthesis of hexahydroquinoline derivatives. The
catalytic activity of n-FTSA has been examined for the synthesis
of hexahydroquinoline derivatives under optimal conditions
with condensation benzaldehyde 1 (1 mmol), cyclic di-ketones 2
ꢀ
unchanged by increasing the temperature to 120 C.
Aer optimizing the conditions (n-FTSA (0.01 g) at 110 ꢀC
under solvent free conditions), the scope of method and catalytic
activity of new nanocatalyst was successfully studied by using
various aromatic aldehydes (including aldehydes with electron-
releasing substituents, electron-withdrawing substituents and
Table 7 n-FTSA catalyzed synthesis of polyhydroquinolinesa
Table 6 n-FTSA catalyzed synthesis of xanthenesa
R1
R2
Ar
Product
Time (min)
Yieldb [%]
R1
R2
Ar
Product
Time (min)
Yieldb [%]
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
Ph
10a
10b
10c
10d
10e
10f
10
5
9
5
7
15
15
20
13
54
5
17
15
15
5
97
97
97
95
95
96
97
92
96
94
91
97
89
95
96
97
p-O2N–C6H4
m-O2N–C6H4
p-Cl–C6H4
o-Cl–C6H4
p-HO–C6H4
p-H3C–C6H4
p-(CH3)2N–C6H4
Ph
p-O2N–C6H4
p-Cl–C6H4
p-H3C–C6H4
p-HO–C6H4
Ph
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
Ph
Ph
ph
Ph
Ph
6a
6b
6c
6d
6e
6f
6g
7a
7b
7c
7d
8a
8b
8c
8d
50
40
42
45
57
55
55
52
45
42
52
52
55
55
47
90
89
91
88
82
85
86
88
89
86
84
86
85
85
87
p-Cl–C6H4
p-O2N–C6H4
m-O2N–C6H4
p-HO–C6H4
p-H3C–C6H4
p-H3CO–C6H4
Ph
p-O2N–C6H4
p-Br–C6H4
p-H3C–C6H4
Ph
10g
10f
11a
11b
11c
11d
11e
12a
12b
12c
H
H
H
Ph
Ph
Ph
H
H
H
p-Cl–C6H4
p-H3C–C6H4
p-H3CO–C6H4
p-Cl–C6H4
p-H3CO–C6H4
17
a
Reaction conditions: aromatic aldehyde (1 mmol), cyclic di-ketone (1
a
Reaction conditions: aromatic aldehyde (1 mmol), cyclic di-ketone (2 mmol), NH4OAc (2.5 mmol), ethylacetoacetate (1 mmol), n-FTSA (0.01
b
mmol), n-FTSA (0.01 g) under solvent free conditions at 110 ꢀC. g) under solvent free conditions at 80 ꢀC. Yields refer to isolated
b
Yields refer to isolated products.
products.
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Germany) X-ray diffractometer using Cu-Ka radiation of wave-
˚
length 1.54 A. The EDX characterization of the catalyst was
performed using a Mira 3-XMU scanning electron microscope
equipped with an energy dispersive X-ray spectrometer oper-
ating. A particle size study of n-FTSA sample was carried out
using Philips XL30 eld emission scanning electron microscope
(FE-SEM) (Royal Philips Electronics, Amsterdam, The Nether-
lands) instrument operating at 10 kV. The sample was mounted
on a double-sided adhesive carbon disk and sputter-coated with
a thin layer of gold to prevent sample charging problems.
Thermo gravimetric analyses (TGA) were conducted on a Du
Pont 2000 thermal analysis apparatus under air atmosphere at a
heating rate of 5 ꢀC minꢁ1. The magnetite measurement was
carried out in a vibrating sample magnetometer (VSM) (4 inch,
Daghigh Meghnatis Kashan Co., Kashan, Iran) at room
temperature.
Scheme 5 Synthesis of hexahydroquinoline derivatives catalyzed by
n-FTSA.
Table 8 n-FTSA catalyzed synthesis of hexahydroquinolinesa
R1]R2
Ar
Product
Time (min)
Yieldb [%]
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
Ph
14a
14b
14c
14d
14e
14f
14g
14h
15a
15b
15c
15d
15e
15f
10
5
5
7
7
15
15
15
12
11
12
7
9
13
14
92
95
91
95
94
90
89
90
92
93
92
94
91
89
88
p-Cl–C6H4
o-Cl–C6H4
p-Br–C6H4
m-O2N–C6H4
p-H3C–C6H4
p-(CH3)2N–C6H4
p-H3CO–C6H4
Ph
p-Cl–C6H4
o-Cl–C6H4
p-O2N–C6H4
m-O2N–C6H4
p-H3C–C6H4
p-H3CO–C6H4
3.2. Preparation of iron oxide magnetite nanoparticles
Both FeCl2 (2 g) and FeCl3 (5.4 g) were dissolved in aqueous
hydrochloric acid (2 M, 25 mL), and the air in the mixture was
eliminated using a pump. The mixture was stirred for 10 min
under nitrogen followed by the addition of aqueous ammonia
(28%, 40 mL) with stirring. Aer 1 h, the generated iron oxide
nanoparticles were rinsed with deionized water two or three
times.
H
H
15g
a
Reaction conditions: aromatic aldehyde (1 mmol), cyclic di-ketone (1
mmol), NH4OAc (2.5 mmol), malononitrile (1 mmol) n-FTSA (0.01 g)
b
under solvent free conditions at 80 ꢀC. Yields refer to isolated
3.3. Preparation of titania-coated iron oxide (nano-Fe3O4/
TiO2) magnetite nanoparticles
products.
The nanoparticles generated above (0.2 g) were rinsed with
ethanol three times and then resuspended in ethanol (40 mL)
under sonication for 1 h. Aqueous ammonia (4.5 mL), deionized
water (3.75 mL), and TEOS (0.1 mL) were added in sequence to
the suspension. The mixture was sonicated for 1 h followed by
vortex mixing for another 8 h. The silica modied nanoparticles
were isolated by magnetite separation and were rinsed with
ethanol two or three times. The nanoparticles were resuspended
in ethanol (40 mL) followed by reuxing at 60 ꢀC for 12 h. Aer
rinsing with deionized water, the generated nanoparticles were
resuspended in deionized water (40 mL) and the suspension was
acidied with nitric acid (0.5 M, 0.22 mL)/deionized water
(124.78 mL) solution. The suspension was heated at 60 ꢀC, and a
solution containing titanium isopropoxide (15 mL) and 2-prop-
anol (11.985 mL) was slowly added to the mixture under stirring
for 6 h. TEOS, titanium isopropoxide, and DMF are toxic. They
should be handled with care by preparing the reagents in a hood
and wearing gloves if necessary. The generated nano-Fe3O4/TiO2
magnetite nanoparticles were rinsed with deionized water twice.
Direct contact of the nanoparticles with the hands should be
avoided as the nanoparticles have photocatalytic activity.
(1 mmol), malononitrile 13 (1 mmol) and ammonium acetate 3
(2.5 mmol) (Scheme 5). The results are listed in Table 8.
2.3. Reusability of the n-FTSA
For practical applications of this heterogeneous solid acid
catalyst, the level of reusability was also tested. The recycled
catalyst could be reused for at least ve times with no appre-
ciable decrease in yield. It signied that the nature of the
catalyst remains intact aer each run and –SO3H moiety was
tightly anchored with nano-Fe3O4/TiO2, most probably through
a covalent linkage.
3. Experimental section
3.1. General remarks
Chemicals were purchased from the Merck chemical compa-
nies. Nano-Fe3O4/TiO2 was prepared with the reported
method.66 Fourier transform infrared spectroscopy (FTIR) was
recorded on a Shimadzu 8400s spectrometer using KBr pressed
powder discs. 1H NMR and 13C NMR spectra which were
recorded on Bruker Advance Spectrometer 400 & 500 MHz using
3.4. Preparation of the nano-Fe3O4/TiO2–SO3H (n-FTSA)
CDCl3-d and DMSO-d6 as solvent. The chemical shis are A suction ask equipped with a constant-pressure dropping
expressed in parts per million (ppm) and tetramethylsilane funnel and a gas inlet tube for conducting HCl gas over an
(TMS) was used as an internal reference. Wide-angle X-ray adsorbing solution (i.e., water) was used, charged with the
diffraction (XRD) spectrum for the n-FTSA powder sample was nano-Fe3O4/TiO2 (0.2 g) in dry CH2Cl2 (10 mL). Then chlor-
obtained using a Siemens D5000 (Siemens AG, Munich, osulfonic acid (0.05 mL) was added drop wisely over a period of
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30 min at room temperature. (Caution: a highly corrosive and
water absorbant. Be careful when using this liquid. Protective
gloves, protective clothing and eye and face protection equip-
ment are needed.)
3.8. General procedure for the synthesis of
hexahydroquinoline derivatives
In a typical experiment, a mixture of different aromatic alde-
hydes 1 (1 mmol), cyclic di-ketones 2 (1 mmol), malononitrile 13
(1 mmol), ammonium acetate 3 (2.5 mmol) as the nitrogen
source and n-FTSA (0.01 g) were added in a glass reactor under
solvent free conditions at 80 ꢀC were stirred for an appropriate
time. The progress of the reaction was monitored by TLC, aer
completion of the reaction, hot ethanol was added to the reac-
tion ask. The catalyst was isolated by external magnetite eld
and remained. Solution was cooled to room temperature and
crude product was recrystallized from ethanol to yield pure
hexahydroquinoline derivatives.
HCl gas immediately evolved from the reaction vessel. Stir-
ring was continued until HCl evolution was seized. Aer the
addition was completed, the mixture was shaken for 30 min. A
powder of nano-Fe3O4/TiO2 supported sulfonic acid was
obtained. Then, the CH2Cl2 was removed under reduced pres-
sure and the solid powder was washed with ethanol (10 mL) and
ꢀ
dried at 70 C.
The prepared n-FTSA stored in vacuum desiccator over
ꢀ
anhydrous silica gel, then, was dried in 120 C for 6 hours.
3.9. General procedure for recycling of n-FTSA
3.5. General procedure for the synthesis of 1,8-dioxo-
decahydroacridine derivatives
Recyclability of nano-Fe3O4–TiO2-supported sulfonic acid
(0.01 g) was tested for the synthesis of tetramethyl-9-phenyl-
hexahydroacridine-dione between benzaldehyde 1 (1 mmol),
dimedone 2a (2 mmol), NH4OAc 3 (2.5 mmol), under solvent
free conditions for 50 min at 110 ꢀC. Aer completion of the
reaction, the heterogeneous catalyst was easily separated from
the production mixture by external magnetite eld, washed with
ethanol (2 ꢂ 15 mL), acetone (2 ꢂ 10 mL) to remove the organic
compounds and dried in oven to be used in the next cycles. In
every run, the yield of product was performed in constant time.
This process was repeated ve more times, affording the desired
product in good yields, with undiminishing efficiency.
In a typical experiment, a mixture of different aromatic alde-
hydes 1 (1 mmol), cyclic di-ketones 2 (2 mmol) and ammonium
acetate 3 (2.5 mmol) as the nitrogen source and n-FTSA (0.01 g)
were added in a glass reactor under solvent free conditions at
110 ꢀC were stirred for an appropriate time. The progress of the
reaction was monitored by TLC, aer completion of the reac-
tion, hot ethanol was added to the reaction ask. The catalyst
was isolated by external magnetite eld. Solution was cooled to
room temperature and crude product was recrystallized from
ethanol to yield pure acridine derivatives.
4. Conclusion
3.6. General procedure for the synthesis of 1,8-dioxo-
octahydroxanthene derivatives
In summary, for the rst time, we have reported nano-Fe3O4–
TiO2-supported sulfonic acid (nano-Fe3O4–TiO2–SO3H) as a
high-performance heterogeneous solid acid nanocatalyst for the
In a typical experiment, a mixture of different aromatic alde-
hydes 1 (1 mmol), cyclic di-ketones 2 (2 mmol) and n-FTSA
(0.01 g) were added in a 25 mL round bottomed ask under
solvent free conditions at 110 ꢀC were stirred for an appropriate
time. The progress of the reaction was monitored by TLC, aer
completion of the reaction, hot ethanol was added to the reac-
tion ask. The catalyst was isolated by external magnetite eld
and remained. Solution was cooled to room temperature and
crude product was recrystallized from ethanol to yield pure 1,8-
dioxo-octahydroxanthene derivatives.
synthesis
of
1,8-dioxo-decahydroacridine,
1,8-dioxo-
octahydroxantene, polyhydroquinoline and hexahydroquino-
line derivatives. High catalytic activity, reusability, easy
magnetically separability and environmental acceptability as
compared to liquid acid catalyst make it a powerful solid acid
catalyst for various organic reactions.
Acknowledgements
We gratefully acknowledge the Faculty of chemistry of Semnan
University for supporting this work.
3.7. General procedure for the synthesis of
polyhydroquinoline derivatives
In a typical experiment, a mixture of different aromatic alde-
hydes 1 (1 mmol), cyclic di-ketones 2 (1 mmol), ethyl-
acetoacetate 9 (1 mmol), ammonium acetate 3 (2.5 mmol) as the
nitrogen source and n-FTSA (0.01 g) were added in a glass
reactor under solvent free conditions at 80 ꢀC were stirred for an
appropriate time. The progress of the reaction was monitored
by TLC, aer completion of the reaction, hot ethanol was added
to the reaction ask. The catalyst was isolated by external
magnetite eld. Solution was cooled to room temperature and
crude product was recrystallized from ethanol to yield pure
polyhydroquinoline derivatives.
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