Nano-sulfated titania (TiO2/SO2-4) as a new solid acid catalyst for Friedel-Crafts acylation and Beckman rearrangement in solvent-free conditions

Article abstract of DOI:10.1080/17415993.2011.600313

The present work describes a novel application of nano-sulfated titania (nano-ST) as a solid acid catalyst for Friedel-Crafts acylation and Beckmann rearrangement. A nano-ST catalyst has been synthesized and characterized using various techniques such as XRD, scanning electron morphology, transmission electron microscopy, FT-IR, measurement of the specific surface area by Brunauer-Emmett-Teller theory and thermal analysis. The influences of various reaction parameters such as reaction temperature, molar ratio of reactants, reaction time and solvent effects have been investigated. [image omitted].

Full text of DOI:10.1080/17415993.2011.600313

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Nano-sulfated titania (TiO ) as a new
solid acid catalyst for Friedel–Crafts
acylation and Beckman rearrangement
in solvent-free conditions
a
a
Mona Hosseini-Sarvari & Ensieh Safary
a
Department of Chemistry, Shiraz University, Shiraz, 71454, Iran
Version of record first published: 27 Jul 2011.
To cite this article: Mona Hosseini-Sarvari & Ensieh Safary (2011): Nano-sulfated titania (TiO ) as
a new solid acid catalyst for Friedel–Crafts acylation and Beckman rearrangement in solvent-free
conditions, Journal of Sulfur Chemistry, 32:5, 463-473
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Journal of Sulfur Chemistry
Vol. 32, No. 5, October 2011, 463–473
2
4
−
Nano-sulfated titania (TiO /SO ) as a new solid acid catalyst
2
for Friedel–Crafts acylation and Beckman rearrangement in
solvent-free conditions
Mona Hosseini-Sarvari* and Ensieh Safary
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
(Received 9 May 2011; final version received 22 June 2011)
The present work describes a novel application of nano-sulfated titania (nano-ST) as a solid acid catalyst
for Friedel–Crafts acylation and Beckmann rearrangement. A nano-ST catalyst has been synthesized and
characterized using various techniques such as XRD, scanning electron morphology, transmission electron
microscopy, FT-IR, measurement of the specific surface area by Brunauer–Emmett–Teller theory and
thermal analysis. The influences of various reaction parameters such as reaction temperature, molar ratio
of reactants, reaction time and solvent effects have been investigated.
1
(
R CO) O
O
2
Friedel–Crafts acylation
R1
Ar-H
Ar
-
2
Nano– TiO /SO4
O
2
H
8
0 nm
R3
N
R2 R3
R2
Beckmann rearrangement
O
Keywords: nano-sulfated titania; TiO2; Beckmann rearrangement; Friedel–Crafts acylation; solvent-free
condition
1
.
Introduction
Manufacturing protocols can be made economically greener and more sustainable, by designing
and by the vigilant use of catalysts, which reduces the chemical waste harmful to human health and
the environment. In this regard, nano-sulfated oxides showed major impacts on catalysis and many
other areas (1–3). These inexpensive materials are accessible via a simple synthesis and they can
be easily tuned by post-synthetic surface modifications. Among these materials, sulfated titania
is considered as a potential alternative catalyst to environmentally harmful mineral acids such as
HF and H2SO4. Sulfated titania as a strong solid acid has been found to exhibit a high catalytic
*
Corresponding author. Email: hossaini@shirazu.ac.ir
ISSN 1741-5993 print/ISSN 1741-6000 online
2011 Taylor & Francis
©
http://dx.doi.org/10.1080/17415993.2011.600313
http://www.tandfonline.com

4
64 M. Hosseini-Sarvari and E. Safary
activity for several reactions and much interest has been devoted to the nature of acidity of these
surfaces. Sulfated titania has gained attention due to its unusual ability to initiate acid-catalyzed
reactions at low temperatures (4, 5). Therefore, the use of sulfated titania, especially nano-sized
sulfated titania, has emerged as a feasible substitute to conventional materials as a robust, active,
high-surface-area catalyst support (6, 7). Recently, nano-sulfated titania (nano-ST) has become
a very significant area of research and the preparation, characterization and its application in the
synthesis of organic molecules have attracted worldwide attention (8–15). Although extensive
amounts of research have been carried out exploring nano-ST, the following aspects still remain
unexplored: (i) benign solvent-free protocol without using any organic solvent; (ii) use of actual
catalytic amounts of catalysts. As part of our programme designed to develop greener and more
sustainable pathways for organic transformation and nanomaterials, we report herein a simple
and efficient synthesis of nano-ST by a sol–gel method, and its application as a good catalyst for
solvent-free Friedel–Crafts acylation and Beckmann rearrangement.
2
2
.
Results and discussion
.1. Design and synthesis of nano-ST
The first step in this project was the selection of method for the preparation of nano-ST.We decided
to explore the sol–gel method, since it is presently a widely accepted method for the preparation of
such materials (16). The advantages of the sol–gel process in general are high purity, homogeneity
and low temperature. For a low-temperature process, there is a reduced loss of volatile components
and thus the process is more environmental-friendly. Therefore, the nano-ST was prepared in high
yield and characterized using the sol–gel method as shown in Figure 1. Anchoring of the sulfonyl
group on the surface of nano-ST was examined by FT-IR spectroscopy (Figure 2). Four bands in
−
1
the region between 1221 and 993 cm attributed to vibrational modes of bidentate sulfate ions
−
1
can be seen (17, 18). The bands at 1047 and 993 cm are assigned to asymmetric and symmetric
−
1
S · · · O stretching vibrations, respectively (17). At 1221 and 1138 cm , two bands appear due to
asymmetric and symmetric stretching of S=O vibrations, respectively (17); meanwhile, the band
−
1
located at 1402 cm has been related to the S=O stretching vibration from the sulfate ion on the
surface of the solid.
The crystalline structures of the nano-ST were determined by XRD (Figure 3(a)).Transmission
electron microscopy (TEM) analysis (Figure 3(b)) and scanning electron morphology (SEM)
analysis(Figure 3(c))showeduniform-sizedparticleswithasphericalmorphologywithanaverage
size ranging from 40 to 50 nm. This is comparable to the crystallite size (40 nm) calculated from
the XRD spectrum using the Scherer formula (19) for the full-width at half-maximum of the
(
θ = 110) reflection.
In thermograms shown in Figure 4, the thermal behavior of nano-sulfated TiO2 was compared
with that of pure TiO2. According to the thermogram (Figure 4), a significant decrease in the
◦
weight percentage of the sulfated TiO2 at temperatures ∼100 C is related to the desorbed ion of
water molecules from the catalyst substrate. This was evaluated to be ∼0.7% according to the TG
1. stirred to obtain sol
1. drying at 110
0
C
nano sulfated Titania
(TiO /SO -) powder
2 4
Ti(OC
HNO
4
H
3
9
)
4
gel+H SO
2 4
2
2. dried at 60° to get gel for 4 days
2. calcination at 500°C
Figure 1. Preparation process of nano-ST powder.

Journal of Sulfur Chemistry 465
Figure 2. FT-IR spectrum of nano-sulfated TiO2.
◦
analysis. In addition, the sharp decrease in the weight percentage at temperatures around 290 C
is due to the decomposition of sulfuric acid and the formation of sulfur dioxide. According to
the thermogram, the amount of sulfuric acid functionalized on TiO2 support is evaluated to be
∼
5.8%.
We have also determined the pH of sulfated TiO2. For this purpose, the same amount of
pure TiO2 or sulfated TiO2 (∼0.02 g) was sonicated in side bottles containing 5 ml H2O. Under
these similar conditions, it was observed that there was a significant difference between the
pH values. This difference was evaluated to be ∼5.13 unit pH, revealing the capability of the
synthesized catalyst to act as a suitable proton-donating agent during the synthesis of organic
compounds. Also, in this study, conductometric analysis was used to evaluate the acidity of
the synthesized catalyst. In accordance with the conductometric results, the pKa is evaluated to
be 1.911, which is in good agreement with the acidic strength of sulfuric acid (pKa2 = 2.00).
According to the Hammett-type acidity functions (H0), the amount of acidity was estimated to
be ∼1.916.
The nitrogen adsorption isotherm of the synthesized sulfated titania was also compared with
◦
pure titania at 25 C using the home-made TG system as shown in Figure 5. As clearly shown in
accordance with the N2 adsorption isotherms, a significant increase to ∼0.8% was determined for
titania during the sulfating process. In addition, according to the N2 adsorption isotherms, there
2
−1
is also a significant increase to ∼218 m g in the active surface area of the titania during the
sulfating process.
O
nano sulfated titania (0.015 mol%)
1
Ar-H + (R CO)
O
R1
2
Ar
5
0oC, Solvent-free
1
2
3
1
R : Ac, Bu, Ph
Scheme 1. Friedel–Crafts acylation catalyzed by nano sulfated titania.

4
66 M. Hosseini-Sarvari and E. Safary
Figure 3. (a) XRD: A, anatase; R, rutile; (b) TEM and (c) SEM image of nano-ST.
2
.2. Friedel–Crafts acylation using nano-ST in solvent-free condition
Aromatic ketones are principle compounds in the organic chemical industry. Friedel–Crafts-type
acylations of aromatic compounds have been generally utilized for the preparation of aromatic
ketones (20). The scope of Friedel–Crafts reactions is extremely wide and forms a large part of
the more general field of electrophilic reactions. A diverse range of clean protocols have been
developed by using homogenous and heterogeneous catalysts such as zeolites, heteropoly acids,
etc. (21–27). However, most of the methods involve the use of excess amount of catalyst, toxic
organic solvents and tedious workup and cannot be considered as real green protocols.Although in
many methods using acyl halides as the acylating agent has wide applicability, it has a drawback

Journal of Sulfur Chemistry 467
Figure 4. Thermograms revealing the thermal stability of (a) pure TiO2 and (b) sulfated TiO2.
Figure 5. Nitrogen adsorption isotherms of (a) pure TiO2 and (b) sulfated TiO2.
Table 1. Optimization of reaction conditions.a
OMe
O
nano ST
+
Ac O
2
solvent-free
MeO
◦
Yield (%)b
Entry
Temperature ( C)
Amount of catalyst (mol%)
Time (h)
1
2
3
4
5
Room temperature
0.015
0.015
0.015
0.005
0.022
24
0.5
0.5
24
40
90
90
Trace
90
50
80
50
50
0.5
Notes: aConditions: 1 mmol of anisole, 1 mmol of acetic anhydride, solvent-free. bIsolated yield.
that equimolar amounts of hydrogen halides are concomitantly formed. Hence, we decided to
explore nano-ST in a catalytic amount for Friedel–Crafts acylation using an anhydride as the
acylating agent (Scheme 1).
Reaction conditions were optimized for the Friedel–Crafts reaction using anisole and acetic
anhydride as substrates and nano-ST as the catalyst under solvent-free conditions (Table 1). The
◦
reaction was first conducted at room temperature (25 C) and a poor conversion was observed
(
◦
◦
Entry 1). Upon increasing the reaction temperature to 50 C and 80 C, a good conversion was

4
68 M. Hosseini-Sarvari and E. Safary
achieved (Entries 2 and 3). For the determination of the effective amount of catalyst, the reaction
was carried out with a lower amount of the catalyst (0.005 mol%, 0.07 g); however, a trace amount
of the product was formed or the incomplete conversion of the starting materials was observed
even after 24 h (Entry 4). An excellent yield (90%) was observed with 0.015 mol% (0.2 g) of
nano-ST. Increasing the amount of catalyst to 0.022 mol% provided no significant increase in
conversion (Entry 5). It is interesting to note that acylation of anisole afforded exclusively the
para-isomer in 100%, indicating a high selectivity with this catalyst.
The Friedel–Crafts acylation catalyzed by nano-ST (0.015 mol%) was explored with a variety
of aromatic compounds to evaluate the scope of this method. The results summarized in Table 2
show that this novel method affords aromatic ketones in good to excellent yields.
According to Table 2, the reactions are remarkably clean, and no chromatographic separations
are necessary to get the spectra-pure compounds except in a few cases (Entries 3, 4 and 9) where
some starting materials remained. This methodology shows an excellent regioselectivity for the
para-substituted product. It is interesting to note that acylation of alkyl-substituted benzenes
such as toluene and ethyl benzene is more difficult, and some of the methods reported in the
literature are not successful when applied to these substrates or gave poor yields of ketone (28,29).
Under our catalytic condition, acylation of toluene and ethyl benzene gave the corresponding
ketones in excellent regioselectivity and yields (Entries 6 and 7). Also, 2-methoxy naphthalene
and anthracene underwent acylation with high regioselectivity in excellent yields (Entries 8 and 9).
This procedure can also be satisfactorily used for the acylation of ferrocene (Entry 10), producing
the corresponding ferrocenyl methyl ketone in excellent yield (90%). This procedure is also good
enough for the acylation of heterocyclic compounds such as furan, pyrrole and thiophene (Table 2,
Entries 11–13), producing the corresponding 2-acylated products in excellent yields. The Friedel–
Crafts acylation of anisole with benzoyl chloride on a 100 mmol scale proceeded just as well as
the 1 mmol reaction.
The Friedel–Crafts acylation of anisole with different anhydrides was also investigated in the
presence of nano-ST. These reactions were also regioselective, producing only the para-acyl
products in 95% and 90% yields, respectively (Entries 14 and 15).
2
.3. Beckmann rearrangement catalyzed by nano-ST
Beckmannrearrangements(30), amostimportantandusefultransformationinsyntheticchemistry
(
31), are generally effected starting from an oxime that is derived from the corresponding ketone
or aldehyde. Recently, several efficient methods have been developed to both generate the oxime
in situ and effect its Beckmann reaction (32, 33). In fact, we have recently reported an improved
oxime synthesis and Beckmann rearrangement employing metal oxides and acid catalysts (34–
3
7). Thus, herein, we decided to extend the exploration of nano-ST for the one-pot Beckmann
rearrangement (Scheme 2).
To optimize the reaction conditions in order to find out the activity of nano-ST as a general Beck-
mann rearrangement catalyst, we chose benzaldehyde as a representative substrate and treated
it with hydroxyl amine hydrochloride under solvent-free conditions. The Beckmann rearrange-
◦
ment was complete in 5 h at 120 C using 0.022 mol% catalyst to give benzanilide in 90% yield.
The scope and efficiency of this solvent-free approach were explored for the reaction of various
aldehydes and ketones (Table 3). It is obvious that excellent yields were obtained for most of the
aldehydes and ketones under solvent-free conditions for 4–6 h with a catalytic amount of nano-ST
(
0.022 mol%).
Aromatic aldehydes with both electron-donating and -withdrawing substituents in ortho, meta
and para positions were converted into the corresponding amides in good yields. Aldehydes gave
only the primary amides. We have also found that in the presence of nano-ST, a 0:100 E/Z ratio

Journal of Sulfur Chemistry 469
Table 2. Friedel–Crafts acylation of aromatic compounds catalyzed by nano-TiO2/SO2
−
4
◦
(
0.015 mol%) under solvent-free conditions at 50 C.
a
Entry
1
R1
Product
OMe
Time (h)
0.5
Yield (%)
CH3
95 (first use)
93 (second use)
90 (third use)
0.5
0.75
O
3
a
2
3
CH3
CH3
Me
2
85
Me
O
3
b
75b
Me
4
Me
O
3
c
4
5
6
7
8
CH3
CH3
CH3
CH3
CH3
MeO
0.5
70
95
OMe
Me
O
3
d
Me
1
O
O
O
Me
3
e
f
90b
90b
60
Me
Et
3
3
5.5
3
g
O
4
OMe
3h
9
CH3
O
6.5
60
3
i
(
Continued)

4
70 M. Hosseini-Sarvari and E. Safary
Table 2. Continued
a
Entry
R1
Product
Time (h)
3
Yield (%)
10
CH3
90
O
Fe
3
j
1
1
1
1
1
2
3
4
CH3
CH3
CH3
Bu
40
30
40
0.5
95
95
O
S
O
3
k
O
3
l
90
N
O
H
3
m
OMe
95b
O
3n
15
Ph
OMe
5
90b
Ph
O
3o
Notes: Isolated yield. bOnly the para isomer was obtained and the selectivity was determined by 1H-NMR.
a
O
H
Nano sulfated titania (0.022 mol%)
R3
N
R2
R3
R2
0
1
20 C, Solvent-free, NH OH.HCl
2
O
4
5
Scheme 2. Beckmann rearrangement catalyzed by nano sulfated titania.
of the corresponding aldoximes (OH syn to aryl) were obtained. The Beckmann rearrangement
is generally suggested to proceed through anti-migration, wherein the Z forms of oximes are
expected to give the corresponding amides. Furthermore, the Beckmann rearrangement of cyclic
ketones such as cyclohexanone, cyclopentanone and cycloheptanone was also investigated. We
found that our catalytic system is applicable to cyclic ketones (Entries 8–10). The Beckmann
rearrangement of cyclohexanone (50 mmol) was scaled-up and a yield of ε-caprolactam (80%)
was obtained with nano-ST (0.075 mol%).
In order to investigate the recovery and reuse of the catalyst, after the reaction was completed,
the mixture was diluted with EtOAc and nano-sulfated TiO2 was removed by centrifugation. The
◦
◦
catalyst after drying in an oven at 100 C and reactivated in a furnace at 400 C for 2 h was reused

Journal of Sulfur Chemistry 471
Table 3. Nano-ST (0.022 mol%) catalyzed Beckmann rearrangement in sol-
◦
vent-free condition at 120 C.
Entry
Substrate
Product
Time (h)
Yield (%)a
1
2
3
4
5
6
7
C6H5CHO
C6H5CONH2
4-ClC6H4CONH2
3-ClC6H4CONH2
2-ClC6H4CONH2
4- NO2H4CONH2
4-OMeC6H4CONH2
4-MeC6H4 CONH2
5
90
90
80
90
90
90
80
4-ClC6H4CHO
3-ClC6H4CHO
2-ClC6H4CHO
4-NO2C6H4CHO
4-OMeC6H4CHO
4-MeC6H4CHO
5.5
6.5
5.5
4
5
5.5
O
8
Cyclohexanone
6
80
NH
O
9
Cyclopentanone
Cycloheptanone
6
6
80
80
NH
O
1
1
1
0
1
2
NH
H
N
Acetophenone
5
5
80
80
O
O
O
N
H
N
Note: aIsolated yields.
N
for the same reaction. Table 2 (Entry 1) presents the results of a recycle efficiency study of the
catalyst. The sulfated TiO2 had been reused three times without loss in activity.
3
. Conclusion
In summary, a novel use of nano-ST was developed. The catalyst showed an excellent activ-
ity for Friedel–Crafts acylations and Beckmann rearrangements. The advantages include the
high yields; the ability to use catalytic amounts of catalyst, simple operation and work-up,
excellent chemoselectivity, solvent-free conditions and the ability to scale-up the procedure.
Further studies will be focussed on exploring more versatile and environ mentally benign cat-
alytic Friedel–Crafts acylation and Beckmann rearrangement systems and clarifying the catalytic
mechanism.
4
. Experimental
Chemicals were purchased from Fluka, Merck, B.D.H. and Aldrich Chemical Companies. TLC
Monitoring: silica gel Polygrams SIL G/UV 254 plates. GC Monitoring: Shimadzu gas chro-
matograph GC-14A, flame ionization detector 3 m glass column packed with DC-200 stationary

4
72 M. Hosseini-Sarvari and E. Safary
phase; N2 as the carrier gas. IR spectra: Perkin-Elmer 781 or Nicolet Impact-400-D FT-IR spec-
−
1
trophotometers; in cm NMR spectra: Bruker-DPX-250 MHz instrument; δ in ppm rel. to SiMe4.
MS: in m/z (rel%).
4
.1. Preparation of the catalyst
Sulfated-TiO2 nanopowder was prepared by the sol–gel method (38). Titanium isobutoxide (98%
Aldrich) was used as the source of TiO2. An amount, 14.3 ml, of Ti (OC4H9)4 was hydrolyzed
in 150 ml water containing 1.25 ml nitric acid (65% Merck). Precipitates formed were stirred
continuously at room temperature for 2 h to form a highly dispersed sol, and then the sol was
◦
−1
concentrated and dried at 60 C. Sulfation was done using 0.5 M sulfuric acid solution (2 g ml
◦
◦
of the hydroxide). The samples, after 2 h drying at 110 C, were calcined for 5 h at 500 C.
4
.2. Friedel–Crafts acylation using nano-ST
To asolutionof aromatic compound (1 mmol) and anhydride(1.5 mmol), 0.2 g (0.015 mol%) ofthe
◦
nano-ST was added and the reaction mixture was stirred magnetically at 50 C. After completion
of the reaction, (monitored by TLC), the reaction mixture was diluted with EtOAc and centrifuged
to remove the catalyst. Then the organic layer was washed with an aqueous solution of sodium
bicarbonate, the organic layer was concentrated on a rotary evaporator under reduced pressure
to give the corresponding ketone. Whenever required, the products were purified by column
chromatography (silica gel) using petroleum ether–ethyl acetate as the eluent to afford the pure
ketone.
4
.3. Beckmann rearrangement using nano-ST
To a solution of aldehyde or ketone (1 mmol) and hydroxyl amine hydrochloride (1.5 mmol),
0
1
.3 g (0.022 mol%) of nano-ST was added and the reaction mixture was stirred magnetically at
20 C. After completion of the reaction (monitored by TLC), the reaction mixture was diluted
◦
with EtOAc and centrifuged to remove the catalyst. Then the organic layer was washed with
an aqueous solution of sodium bicarbonate and the organic layer was concentrated on a rotary
evaporator under reduced pressure to give the corresponding amide.
Acknowledgement
We gratefully acknowledge the support of this work by the Shiraz University Research Council.
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