Nano titania-supported sulfonic acid: An efficient and reusable catalyst for a range of organic reactions under solvent free conditions

Article abstract of DOI:10.1016/j.catcom.2014.07.002

Nano titania-supported sulfonic acid (n-TSA) has been easily prepared from the reaction of nano titania (titanium oxide) with chlorosulfonic acid as sulfonating agent and characterized by the FT-IR spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The catalytic activity of n-TSA was investigated in the synthesis of important organic derivatives such as pyrimidones, benzothiazoles and chalcones. All of the reactions are very fast and the yields are good to excellent. The catalyst was easily separated and reused for several runs without appreciable loss of its catalytic activity.

Full text of DOI:10.1016/j.catcom.2014.07.002

Catalysis Communications 56 (2014) 184–188
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Nano titania-supported sulfonic acid: An efficient and reusable catalyst
for a range of organic reactions under solvent free conditions
⁎
⁎
Salman Rahmani, Ali Amoozadeh , Eskandar Kolvari
Department of Chemistry, Semnan University, Semnan 35131-19111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Nano titania-supported sulfonic acid (n-TSA) has been easily prepared from the reaction of nano titania (titanium
oxide) with chlorosulfonic acid as sulfonating agent and characterized by the FT-IR spectroscopy, scanning elec-
tron microscopy (SEM), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The catalytic activity of
n-TSA was investigated in the synthesis of important organic derivatives such as pyrimidones, benzothiazoles
and chalcones. All of the reactions are very fast and the yields are good to excellent. The catalyst was easily sep-
arated and reused for several runs without appreciable loss of its catalytic activity.
Received 26 February 2014
Received in revised form 30 June 2014
Accepted 1 July 2014
Available online 9 July 2014
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Nano titania-supported sulfonic acid
Heterogeneous solid acid catalyst
Solvent free
Green chemistry
1. Introduction
of different compounds [2,14,15] that has attracted much attention after
Zolfigol's report on the preparation of silica sulfuric acid [16].
In accordance with the principles of green chemistry, the design of
easily separable, reusable, non-toxic, low cost, and insoluble acidic cat-
alysts has become an important area of research in chemistry [1]. In
this context, the use of nanoparticles as heterogeneous catalysts has
attracted considerable attention because of the interesting structural
features and high levels of catalytic activity associated with these mate-
rials [2]. Furthermore, nanoparticle materials are considered to be a
bridge between homogeneous and heterogeneous catalysts [3]. The ap-
plication of nanoparticles has extended to almost all possible fields of
science ranging from medicine to rocketry.
2. Experimental section
2.1. Characterization methods of nano-TiO2–SO3H (n-TSA)
2.1.1. X-ray diffraction spectrum
Wide angle X-ray diffraction spectrum for the n-TSA powder sample
was obtained utilizing a Bruker D8 X-ray diffractometer using Cu-Ka
radiation of wavelength 1.54 Å.
Among various nanoparticles, nano TiO2 has been widely investi-
gated over the past few decades due to its multiple potential applica-
tions such as catalytic activity for esterification [4], catalyst support
for proton exchange membrane fuel cells [5], sono catalytic degrada-
tion of methyl parathion [6], photodecomposition of methylene-blue
by highly dispersed on Ag [7], catalyst for rhodamine B degradation
[8], synthesis of dibenzo[a,j]xanthenes [9], polyhydroquinoline [10],
1,8-dioxooctahydroxanthenes [11], crosslinking of cotton cellulose
with succinic acid under UV [12] and synthesis of β-acetamido ketones
[13].
2.1.2. Scanning electron microscopy
The particle size study of n-TSA samples was carried out using BAL-
TEC SCD 005 scanning electron microscope 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.
2.1.3. Infrared spectra
The n-TSA sample was mixed with KBr powder and compressed into
pellet, wherein, the n-TSA powder was evenly dispersed. Fourier
transform infrared spectrum was recorded on Shimadzu FT-IR 8400
instrument.
Based on the above, we decided to improve the catalytic properties of
nano-TiO2 by reacting it with chlorosulfonic acid to produce nano-titania
sulfonic acid (n-TSA) (Scheme 1). Sulfonation with chlorosulfonic acid is
a convenient, fast and efficient method for Brønsted acid fictionalization
2.1.4. Thermo gravimetric analyses
Thermo gravimetric analyses (TGA) were conducted on a Du Pont
2000 thermal analysis apparatus under air atmosphere at a heating
rate of 5 °C/min.
⁎
Corresponding authors.
E-mail addresses: aamozadeh@semnan.ac.ir (A. Amoozadeh), kolvari@semnan.ac.ir
(E. Kolvari).
http://dx.doi.org/10.1016/j.catcom.2014.07.002
1566-7367/© 2014 Elsevier B.V. All rights reserved.

S. Rahmani et al. / Catalysis Communications 56 (2014) 184–188
185
described in the literature [17]. For this purpose, the surface acidic
protons of nano-TiO2–SO3H (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 solution obtained was titrated with NaOH
(0.1 M) solution in presence of phenol red indicator solution or by
using a pH meter.
Scheme 1. Preparation of n-TSA.
2.3. General procedure for synthesis of pyrimidinone derivatives
In a typical experiment, aromatic aldehyde (1 mmol), cyclo-
pentanone (1 mmol), urea or thiourea (1.2 mmol) and n-TSA (0.032
g) under solvent free conditions were taken in a 25 mL round bottomed
flask and was stirred at 70 °C for an appropriate time. The reaction
mixture was cooled, eluted with hot ethanol (5 mL) and was centri-
fuged and filtrated to collect the formed precipitate. The crude product
was recrystallized from ethanol to yield pure pyrimidinone derivatives.
2.4. General procedure for preparation of benzothiazole derivatives
In
a typical experiment, aromatic aldehyde (1 mmol), 2-
aminothiophenol(1 mmol) and n-TSA (0.032 g) under solvent free
conditions were taken in a 25 mL round bottomed flask and were stirred
at 70 °C for an appropriate time. The reaction mixture was cooled, elut-
ed with hot ethanol (5 mL) and was filtrated to collect the formed pre-
cipitate. The crude product was recrystallized from ethanol to yield pure
benzothiazole derivatives.
Fig. 1. The X-ray diffraction patterns of the (a) nano-TiO2 powder, (b) n-TSA.
2.5. General procedure for preparation of chalcone derivatives
2.2. Preparation of nano-TiO2–SO3H (n-TSA)
In a typical experiment, aromatic aldehyde (1 mmol), acetophenone
(1 mmol) and n-TSA (0.032 g) under solvent free conditions were taken
in a 25 mL round bottomed flask and were stirred at 70 °C for an appro-
priate time. The reaction mixture was cooled, eluted with hot ethanol
(5 mL) and was filtrated to collect the precipitate. The crude product
was recrystallized from ethanol to yield pure chalcone derivatives.
A suspension of titania nanoparticles (4 g) in dry CH2Cl2 (20 mL) was
added to a suction flask that was equipped with a constant-pressure
dropping funnel and a gas inlet tube for conducting HCl gas over an
adsorbing solution (i.e., water). Then chlorosulfonic acid (CAUTION: a
highly corrosive and water-absorbent. Be careful when using this liquid.
Protective gloves, protective clothing and eye and face protection equip-
ment are also needed.) (1 mL, 15 mmol) was added dropwisely over a
period of 30 min at room temperature while the mixture was stirred
slowly in an ice bath. HCl gas immediately evolved from the reaction
vessel. Stirring was continued until HCl evolution was seized. After the
addition was completed, the mixture was stirred for 30 min. A white
solid of nano titania-supported sulfonic acid was obtained. Then, the
CH2Cl2 was removed under reduced pressure and the solid powder
was washed with ethanol (10 mL) and dried at 70 °C.
3. Results and discussion
3.1. Characterization of nano-TiO2–SO3H
3.1.1. X-ray diffraction spectra
The nano-TiO2 powder can be easily synthesized by a hydrothermal
method as reported in the literature [18]. Fig. 1a shows the XRD patterns
of nano-TiO2 powder before modification. The following signals at
(101), (103), (004), (112), (200), (105), (211), (204), (220), (215)
and (224) in Fig. 1a planes confirm that only the anatase crystal phase
The n-TSA was stored in vacuum desiccator over anhydrous silica
gel. Then, it was dried at 120 °C for 6 h. The mmol of H+ per gram of
catalyst (4.5 mmol/g of n-TSA) was determined by the methodology
Fig. 2. The SEM image of the (a) nano-TiO2 powder, (b) n-TSA.

186
S. Rahmani et al. / Catalysis Communications 56 (2014) 184–188
Fig. 5. Absorption spectra of (a) 4-nitroaniline (indicator) and (b) n-TSA (catalyst) in CCl4.
(Fig. 3a). In Fig. 3b, the absorption range in 1176–1284 and 1006–
1088 cm−1 was certified the O_S_O asymmetric and symmetric
stretching modes lies respectively and the S\O stretching mode lies
in 675–852 cm−1 showing the presence of sulfonic acid functional
group which is consistent with the reported IR spectra for \SO3H [20].
Fig. 3. The FT-IR spectra of (a) nano-TiO2 powder and (b) n-TSA.
is formed, which conforms with the JCPD 89-4921 standard. The
estimated crystalline size of titanium dioxide nanoparticles was also
determined from X-ray line broad using the Debye–Scherrer formula
given as t = 0.9λ/B1/2cosθ, where t is the average crystalline size, λ is
the X-ray wavelength used (1.54 Å), B is the angular line width at half
maximum intensity and θ is the Bragg's angle. The average size of the
nano-TiO2 powder for 2θ = 25.303° is estimated to be around
21.88 nm. Fig. 1b illustrates XRD patterns of the samples of modified
nano TiO2. As shown in Fig. 1b, the main peaks of modified nano TiO2
are the same as nano TiO2 (Fig. 1a) i.e. the main phase of the XRD
pattern has not been changed and it indicates that chlorosulfonic acid
has not influenced the crystallinity phase of TiO2. As it was expected,
the reaction between chlorosulfonic acid and nano-TiO2 for modifica-
tion by SO3H groups was carried out on the surface of nano-TiO2 and
the crystallinity of nano-TiO2 has not been changed. This fact is illustrat-
ed by comparison between XRD pattern of nano-TiO2 and n-TSA.
3.1.4. Thermo gravimetric analysis
Thermo gravimetric analysis (TGA) of n-TSA in comparison with
nano TiO2 is shown in Fig. 4. The TGA curve of TiO2 (Fig. 4a) displays a
weight loss (4 wt.%) below 100 °C which corresponds to the loss of
the physically adsorbed water. Also, there is a slight weight loss
(1 wt.%) between 100 °C and 800 °C, which possibly corresponds to
the dehydroxylation of TiO2 as has already been reported in the litera-
ture for similar case [21].
In the TGA curve of n-TSA (Fig. 4b), three regions corresponding to
different mass lose ranges exist. The first region is below 150 °C
that displayed a mass loss that was attributable to the loss of trapped
water from the catalyst. A mass loss of approximately 5% weight oc-
curred between 150 and 400 °C that is related to the slow mass loss of
SO3H groups. Finally, a mass loss of approximately 10% weight occurred
between 400 and 570 °C that is related to the sudden mass loss of SO3H
groups [20,22,23]. Also, from the TGA, it is understood that n-TiO2–SO3H
has a greater thermal stability (until 300 °C) confirming that it can be
safely used in organic reactions at temperatures in the range of 80–
150 °C.
3.1.2. Scanning electron microscopy
The scanning electron microscopy (SEM) analysis of nano TiO2
powder reveals the spherical nano TiO2 powder with average size
20–30 nm (Fig. 2a). As shown, the SEM analysis of n-TSA reveals
the spherical n-TSA powder with average size 35–60 nm (Fig. 2b).
3.1.5. Surface acidity studies
3.1.3. FT-IR spectra
The Hammett acidity function (H0) can effectively express the
acidity strength of an acid in organic solvents [24]. It can be calculated
using the following equation:
The FT-IR spectra of nano TiO2 powder and n-TSA are shown in Fig. 3.
The absorbance bands at around 3200–3500 cm−1 were attributed to
the adsorbed water in nano TiO2 (Fig. 3a) and n-TSA (Fig. 3b). The
broad intense band below 1200 cm−1 is due to Ti\O\Ti vibration
which is consistent with the reported IR spectra for nano TiO2 [19]
ꢀ
h
i ꢁ
H0 ¼ pKðIÞaq þ log ½Iꢀs= IHþ
:
s
‘I’ represents the indicator base 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–Beerlaw, the value of
[I]s/[IH+
] can be determined and calculated using the UV–visible
s
spectrum. In the present experiment, 4-nitroaniline was chosen as the
Table 1
Calculation of Hammett acidity function (H0) of n-TSA.
Entry
Catalyst
Amax
[I]s (%)
[IH+
0
]
s (%)
H0
1
2
–
1.83
0.473
100
25.35
–
n-TSA
74.65
1.12
Condition for UV–visible spectrum measurement: solvent, CCl4; indicator, 4-nitroaniline
(pK(I)aq = 0.99), 1.44 ∗ 10−4 mol/L; catalyst, n-TSA (20 mg), 25 °C.
Fig. 4. TGA curve of (a) nano-TiO2 and (b) n-TSA.

S. Rahmani et al. / Catalysis Communications 56 (2014) 184–188
187
Scheme 3. Synthesis of benzothiazole derivatives.
Scheme 2. Synthesis of pyrimidinone derivatives.
the products were identified by the comparison of the melting point
data with those reported in literature [28,29].
basic indicator and CCl4 was chosen as the solvent because of its
aprotic nature. The maximal absorbance of the unprotonated form
of 4-nitroaniline was observed at 329 nm in CCl4. As Fig. 5 shows, the ab-
sorbance of the unprotonated form of the indicator in n-TiO2–SO3H 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 results obtained
are listed in Table 1, which shows the acidity strength of n-TiO2–SO3H.
Reusability of the catalyst was checked by recovered n-TSA and
reused for a five consecutive reactions and obtained the yield 90–95%.
It indicates that n-TSA does not lose its activity and can be recycled.
3.2.2. Cyclo condensation for synthesis benzothiazole derivatives
The efficiency of n-TSA for the synthesis of benzothiazoles
(Scheme 3) was compared by other titanium dioxide base such as
unmodified nano TiO2, unmodified bulk TiO2 and bulk-modified TiO2.
The best yield achieved by n-TSA (90% yield), whereas bulk-modified
TiO2 provides 65% yield. Synthesis of benzothiazole derivatives was
also studied (Table 3). All the products were identified by the compari-
son of the melting point data with those reported in literature [30–32].
The recyclability of the catalyst was tested with five consecutive
synthesis of benzothiazole by using recovered n-TSA and the desired
product was obtained in 88–90% yield.
3.2. Using nano-TiO2–SO3H as a new heterogeneous solid acid nano
catalyst in some organic reaction
In the present study, to justify the efficiency of the new catalyst
and in continuation of the studies on developing inexpensive and
environmentally benign methodologies for synthesis of heterocyclic
compounds and organic reactions [25,26], it has been decided to
investigate the possibility of synthesizing some of organic reactions
by one-pot condensation reaction strategy in presence of n-TSA
under solvent free conditions.
3.2.3. Cyclo condensation for synthesis chalcone derivatives
To determine the productivity of n-TSA for synthesis of chalcones
(Scheme 4), it was compared with the titanium dioxide base. Obtained
results show that n-TSA (88% yield) is a better option in comparison to
the other titanium dioxide base (0–62% yield). Synthesis of different
chalcone derivatives were also studied (Table 4). All the products
were identified by the comparison of the melting point data with
those reported in literature [33–35].
3.2.1. Biginelli reaction for synthesis of pyrimidinone derivatives
In this study, to justify the efficiency of the new catalyst for synthesis
of pyrimidinones (Scheme 2), it was compared by some other titanium
dioxide base such as nano TiO2, bulk TiO2 and modified bulk TiO2
(prepared by reported procedure [27]) which gave the desired product
in 25–78% yield, respectively. The results obtained indicate clearly that
n-TSA performed well to give the desired product within 0.75 h in 91%
yield at 90 °C in solvent free condition. The greater catalytic activity of
n-TSA was most likely related to the SO3H groups of the catalyst,
which could provide efficient acidic sites.
The recyclability of the catalyst was tested with five consecutive
synthesis of chalcone by using recovered n-TSA and the desired product
was obtained in 86–88% yield.
The achieved results of optimization of other conditions showed that
the best yield was obtained at 70 °C and under solvent free conditions.
Synthesis of pyrimidinone derivatives was also studied (Table 2). All
4. Conclusion
In summary, nano titania-supported sulfonic acid was efficiently
used as a heterogeneous catalyst for synthesis of pyrimidinones,
benzothiazoles and chalcones under solvent-free conditions. The
catalyst was reused for 5 consecutive cycles with consistent activity.
The excellent catalytic performance, the easy preparation and
Table 2
Synthesis of pyrimidinone derivatives by n-TSAa.
Entry
Ar
X
Product
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
3a: \C6H5
O
O
O
O
O
O
O
O
O
S
S
S
S
S
S
S
S
S
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
0.75
1.5
1.25
1.25
1.25
1
95
86
84
83
89
94
93
83
82
96
86
92
86
89
91
93
79
91
87
Table 3
3b: 2–Cl–C6H4
3c: 4–Cl–C6H4
3f: 4–F–C6H4
3g: 4–Br–C6H4
3h: 4–Me–C6H4
3i: 4–MeO–C6H4
3k: 3–O2N–C6H4
3m: 2-Naphthyl
3a: \C6H5
3b: 2–Cl–C6H4
3c: 4–Cl–C6H4
3f: 4–F–C6H4
3g: 4–Br–C6H4
3h: 4–Me–C6H4
3i: 4–MeO–C6H4
3k: 3–O2N–C6H4
3l: 4–O2N–C6H4
3m: 2-Naphthyl
Synthesis of benzothiazole derivatives by n-TSAa.
Entry
Ar
Product
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
3a: \C6H5
6a
6b
6c
6d
6e
6f
30
50
40
20
15
15
20
45
25
20
15
10
20
20
90
84
86
82
83
86
86
90
93
81
86
89
90
79
1
3b: 2–Cl–C6H4
3c: 4–Cl–C6H4
3d: 2–OH–C6H4
3e: 4–OH–C6H4
3f: 4–F–C6H4
2.5
1.5
1
9
10
11
12
13
14
15
16
17
18
19
2
1.5
1.6
1.4
1
1
3.5
3
3g: 4–Br–C6H4
3h: 4–Me–C6H4
3i: 4–OMe–C6H4
3j: 2–NO2–C6H4
3k: 3–NO2–C6H4
3l: 4–NO2–C6H4
3m: 2-Naphtyl
3n: Furyl
6g
6h
6i
6j
6k
6l
9
10
11
12
13
14
6m
6n
S
1.5
Bold values indicate that the 3a-3m identify different aldehydes and 4a-4s identify the
pyrimidinone derivatives.
Bold values indicate that 3a-3m identify different aldehydes, 7a-7c identify different
acetophenones and 8a-8q identify the chalcone derivatives.
a
a
Reaction conditions: aromatic aldehyde (1 mmol), cyclopentanone (1 mmol), urea or
Reaction condition: aldehyde (1 mmol), 2-aminothiophenol (1 mmol) and catalyst
thiourea (1.2 mmol) in solvent-free at 70 °C.
(0.032 g) in solvent free condition at 70 °C.
b
b
Isolated yield.
Isolated yield.

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S. Rahmani et al. / Catalysis Communications 56 (2014) 184–188
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4
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8
3a: \C6H5
7a
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3c: 4–Cl–C6H4
3d: 2–OH–C6H4
3e: 4–OH–C6H4
3f: 4–F–C6H4
3g: 4–Br–C6H4
3h: 4–Me–C6H4
3i: 4–OMe–C6H4
3j: 2–NO2–C6H4
3k: 3–NO2–C6H4
3l: 4–NO2–C6H4
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a
Reaction condition: different aromatic aldehydes (1 mmol), different acetophenones
(1 mmol) and catalysts (0.032 g) in solvent free condition at 70 °C.
b
Isolated yield.
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We gratefully acknowledge the Department of Chemistry of Semnan
University for supporting this work. The authors wish to thank Dr. Ben
Buckley (Chemistry Department, Loughborough University; Loughbor-
ough, UK) for language editing of this article.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.07.002.