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M. Hosseini-Sarvari, E. Sodagar / C. R. Chimie 16 (2013) 229–238
Nano sulfated-TiO2
activityofthissolidsulfatedcatalyst ismorethanhalfthatof
a liquid sulfuric acid catalyst. However, the esterification
reaction needed to be carried out under reflux at 80 8C, a
fatty acid:ethanol molar ratio1:10 wereused, and the weigh
of the used catalyst was 2%. Therefore, a new family of
catalyst, in order to show high activity, and recyclable is
required. Nano sulfated-TiO2 used in the present study is
found to have large surface area(218m2/g), compared to the
bulky TiO2 and previous prepared sulfated titania reported
in the literature. Also, significantly higher catalytic activity
for nano sulfated-TiO2 could be attributed to its stronger
surface acidity (pKa is evaluated to 1.911, which is in good
agreement with acidic strength of sulfuric acid pKa2 = 2.00)
and nanometres sized and large surface area compared to
the previously studied catalysts.
In general, sulfated-TiO2 has low toxicity and shows no
evidence of carcinogenicity. Therefore, sulfated-TiO2 has
attracted great interest as potential green catalyst because
green catalysts require not only high catalytic activity and
atom efficiently, but also low toxicity, low cost and ease of
handling. With this in mind, in the present study, we report
the synthesis of new nano-sized sulfated titania by the sol-
gel method and its catalytic activity for esterification of
fatty acids with various alcohols. The studies were
performed in a very simple media, under solvent-free
condition. In addition, we have conducted XRD, FT-IR, SEM,
TEM, and other various techniques to determine the
properties of the catalyst [17].
R(Ar)CO2H + R'OH
R(Ar)CO2R'
Solvent-free,80 oC
Scheme 1.
the reactions were carried out with lower amount of the
catalyst (0.005 mol %), either trace amounts of product
were formed or incomplete conversion of the starting
materials to the product was observed after 8 h at 80 8C.
Excellent (90%) conversion to n-butyl-benzoate (3a) after
4.5 h took place with (0.011 mol %) of nano sulfated-TiO2,
at 80 8C. By increasing the amount of catalyst to 0.022 mol%
the yield not only increased but also decreased. Thus, the
effective/convenient catalyst loading was found to be
0.011 mol% (Table 1, entry 3). A brief screening of solvents
showed that water, CH2Cl2, and EtOH, were less effective
than toluene, MeCN, and THF solvents system (entries 5–
10). So because of toxicity and volatile nature of organic
solvents, the design of solvent-less catalytic reaction has
received tremendous attention in present study in the area
of the green synthesis.
Using the optimized reaction conditions, from Table 1
and in order to extended the scope of the reaction as a
general and practical procedure for esterification, we
carried out the reaction of a series of aryl, heteroaryl,
aliphatic, and cycloalkane carboxylic acid with equimolar
amount of primary/secondary short and long chain
aliphatic, allylic, propargylic, and cycloalkyl alcohols under
solvent-free conditions at 80 8C in the presence of nano
sulfated-TiO2 (0.011 mol%) (Scheme 1). The reactions were
completed after 2–10 h affording good to excellent yields
of the corresponding esters (Table 2). No competitive side
reactions of acid sensitive substrate were observed.
As shown in Table 2, various aromatic and aliphatic
carboxylic acids as well as heterocyclic with different
alcohols converted to the corresponding esters in good to
high yields. In most of cases, the products obtained after
the usual workup was pure (spectral data) and did not
require additional efforts of purification. Wherever re-
quired, the purification was performed by column
chromatography. In the case of esterification of benzoic
acids and its derivatives (as the carbonyl carbon of
aromatic carboxylic acid is less electrophilic compared
to fatty acids, and aliphatic ones) with various alcohols, the
esters were obtained in good to excellent yields (entries 1–
12). The efforts were made for the evaluation of mildness
and efficiency of the nano sulfated-TiO2 for the esterifica-
tion of the electron-donating and electron-withdrawing
substituted benzoic acid with n-octanol under the same
reaction conditions (entries 8–10). The desired esters 3h–j
were obtained in 70, 60, and 50% yields, respectively.
2. Results and discussions
The first step in this concept was the selection of
method for the preparation of nano sulfated titania (nano
ST). We decided to explore the sol-gel method, since it is
presently a widely accepted method for the preparation of
such materials [18]. The advantages of the sol-gel process
in general are high purity, homogeneity and low tempera-
ture. For low temperature processes, there is a reduced loss
of volatile components and thus the process is more
environmentally friendly. Recently, we reported the
preparation and characterization of nano ST and investi-
gated its application in the amidation of fatty acids [17].
Therefore, herein, prepared nano ST (by using sol-gel
method as shown in Fig. 1) was described as a practical,
inexpensive, and environmentally benign catalyst for the
esterification of free fatty acids in high yields.
At first, efforts were made to optimize the reaction
conditions using nano sulfated-TiO2 as an efficient catalyst
for direct esterification of various carboxylic acids. For the
determination of the effective amount of nano sulfated-
TiO2, the reaction of benzoic acid (1a) with n-butanol (2a)
was considered as the model (Scheme 1, Table 1). When
1. stirred to obtain sol
1. drying at 1100C
(TiO2/SO42-) powder
2.calcination at 5000C
nano sulfated Titania
Ti(OC4H9)4
HNO3
gel+H2SO4
2.dried at 600 to get gel
for 4 days
Fig. 1. Preparation process of nano sulfated titania powder.
Hosseini-Sarvari, Mona
