Solvent effects in esterification of phthalic anhydride on sulfated titania-based solid acid

Article abstract of DOI:10.1080/17415993.2011.617446

Esterification of phthalic anhydride with 2-octanol has been studied in the presence of sulfated titania. This study shows that sulfated titania has strong catalytic effects on esterification reactions. The catalyst was characterized with x-ray diffraction, FT-IR, Brunauer Emmett Teller, SEM and thermogravimetric methods. The effect of the type and volume of solvent has been investigated. Three solvents were used in this work: acetonitrile, dimethyl sulfoxide (DMSO) and an ionic liquid, 2-hydroxy ethylammonium formate. The results show that addition of various solvents has significant effects on the reaction rate of phthalic anhydride. DMSO and 2-hydroxy ethylammonium formate enhance the reaction rate but acetonitrile reduces it.

Full text of DOI:10.1080/17415993.2011.617446

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Solvent effects in esterification of
phthalic anhydride on sulfated titania-
based solid acid
Mohmmad Khodadadi-Moghaddam a , Farshid Salimi a , Leila
Jamshidi a , Nasrin Jafari a & Hajar Sahebalzamani a
a Departments of Chemistry, Faculty of Science, Ardabil Branch ,
Islamic Azad University , Ardabil , Iran
Published online: 14 Nov 2011.
To cite this article: Mohmmad Khodadadi-Moghaddam , Farshid Salimi , Leila Jamshidi ,
Nasrin Jafari & Hajar Sahebalzamani (2011) Solvent effects in esterification of phthalic
anhydride on sulfated titania-based solid acid, Journal of Sulfur Chemistry, 32:6, 533-541, DOI:
10.1080/17415993.2011.617446
To link to this article: http://dx.doi.org/10.1080/17415993.2011.617446
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Journal of Sulfur Chemistry
Vol. 32, No. 6, December 2011, 533–541
Solvent effects in esterification of phthalic anhydride
on sulfated titania-based solid acid
Mohmmad Khodadadi-Moghaddam, Farshid Salimi*, Leila Jamshidi, Nasrin Jafari
and Hajar Sahebalzamani
Departments of Chemistry, Faculty of Science, Ardabil Branch, Islamic Azad University, Ardabil, Iran
(Received 19 March 2011; final version received 22 August 2011)
Esterification of phthalic anhydride with 2-octanol has been studied in the presence of sulfated titania. This
study shows that sulfated titania has strong catalytic effects on esterification reactions. The catalyst was
characterized with x-ray diffraction, FT-IR, Brunauer Emmett Teller, SEM and thermogravimetric meth-
ods. The effect of the type and volume of solvent has been investigated. Three solvents were used in this
work: acetonitrile, dimethyl sulfoxide (DMSO) and an ionic liquid, 2-hydroxy ethylammonium formate.
The results show that addition of various solvents has significant effects on the reaction rate of phthalic
anhydride. DMSO and 2-hydroxy ethylammonium formate enhance the reaction rate but acetonitrile
reduces it.
Keywords: solvent effects; esterification; sulfated titania; phthalic anhydride; 2-octanol
1. Introduction
Selective esterification of bifunctional carboxylic acids with alcohols is a useful organic synthesis
reaction. The esters thus obtained are utilized to prepare fine chemicals used in the synthesis
of drugs, food preservatives, plasticizers, pharmaceuticals, solvents, perfumes and cosmetics
(1–3). Generally, octyl esters of phthalic anhydride are prepared under liquid-phase conditions
by refluxing the reactants (carboxylic acid and alcohol) in the presence of small amounts of
concentrated H2SO4, HCl, POCl3 or sulfonic acids as a catalyst. Although many effective and
reliable methods for the preparation of aromatic esters exist, there is still a need for research toward
*Corresponding author. Email: salimif@yahoo.com
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2011 Taylor & Francis
http://dx.doi.org/10.1080/17415993.2011.617446
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534 M. Khodadadi-Moghaddam et al.
finding eco-friendly and economically viable processes. The use of the above-mentioned catalysts
is undesirable from the environmental point of view as these chemicals are corrosive and generally
encounter the problems of handling and transportation. Moreover, reusability of the catalysts
cannot be expected. Hence, there are numerous global efforts to replace these environmentally
harmful chemicals with more eco-friendly and less expensive catalysts. The use of heterogeneous
catalysts for liquid-phase synthesis has a lot of benefits such as easy separation of product from the
catalyst with simple filtration, easy recovery and reuse of the catalyst and avoidance of corrosion.
Solid acids such as zeolites, oxides, aluminophosphates and their modified forms such as sul-
fatedoxideshavebeenextensivelystudiedaspossiblealternativestoconventionalLewis/Bronsted
acid catalysts (4–9). Acidity of oxides are found to increase on sulfate ion treatment (10) and that
of zeolites by protonation (11). The increase in acidity is due to an increase in the number and
strengths of acid sites. In several reactions, it is observed that the yield and selectivity of a product
depend not only on the concentration but also on the strength of the acid sites.
It has been observed that the solid acids such as modified zeolites and oxides catalyze very
efficiently esterification and alkylation reactions (12–17). Solid super acids such as SO24−/TiO2
(H0 < −11.93), which can be easily prepared, is sufficiently stable at the elevated reaction tem-
peratures and regenerated conveniently. Different kinds of solid super acids and the field in which
they are used have been developed. They have been used in isomerization, alkylation, acylation
polymerization, oligomerization esterification and oxidation processes. (18–21). SO24−/MxOy
(M = Ti, Zr, Fe) is a useful catalyst for the esterification of the reaction of sebacic acid with
ethanol and phthalic anhydride with 2-octanol. Dioctyl phthalate, the octyl ester of phthalic anhy-
dride and diethyl sebacate are important chemicals which are used in polymer industries as a
plasticizer and in detergent industries, respectively.
Room-temperature ionic liquids (RTILs) have recently gained increasing attention as environ-
mentally benign alternatives to conventional organic solvents in a variety of synthetic (22, 23),
catalytic (24) and electrochemical applications (25, 26), which is a result of their unique physical
and chemical properties and the relative ease with which these properties can be fine-tuned by
altering the cationic or anionic moieties comprising the RTILs (27, 28). They continue to excite
interest for a number of reasons: the first arises from the claim that ionic liquids are environ-
mentally benign solvents, particularly because they have very low vapor pressures under ambient
conditions (29, 30). Second, that they might provide improved reactivities in a number of chemical
processes (31, 32). Third, the fact that a wide range of cations and anions can be employed giving
chemists the potential to design the solvent with specific properties that make the description of
ionic liquids as “designer solvents” very appropriate (33, 34). Furthermore, they readily dissolve
many organic, inorganic and organometallic compounds. ILs thus have the potential to act as
high-performance green solvents for organic synthesis. Products are mostly separated from the
reaction mixture via extraction with a solvent or via distillation.
The present investigation is aimed to find out the catalytic activity and the effect of sulfate ion
treatment of titania in reactions between phthalic anhydride and 2-octanol. The results obtained
are very encouraging in the sense that solid acids show not only high selectivity but also show
high activity toward the formation of desired products (35).
2. Result and discussion
2.1. X-ray diffraction study
The average crystallite size L of the particle was determined by a x-ray diffraction (XRD) line
broadening technique using the Scherrer equation, L = 0.94 λ/b · cos θ, in which λ is the wave-
length of the X-ray and b is the relative peak broadening. b was calculated as b2 = be2xp − br2ef
,

Journal of Sulfur Chemistry 535
where bexp and bref are half-widths observed on a given sample and on a reference material, respec-
tively. For the sample calcined at 500 ◦C, the orthorhombic TiOSO4 phase appears therewith the
anatase phase in the XRD patterns of the catalysts. Using the Scherrer equation, one can see that
the average crystallite size L of the particles for the anatase phase is approximately 93 nm, and
for the orthorhombic TiOSO4 phase, it is approximately 84 nm for the sample calcined at 500 ◦C.
It is well known that the crystallinity is dependent on the calcination temperatures. The crystallite
size increases when the calcination temperature increases.
A clear relationship between sulfate content and crystallite size cannot be obtained in this work.
However, it has been reported that the crystallite size increases in the presence of the sulfate,
phosphate and tungstenate ions, SO42−, PO43− and WO23−. These ions could possibly interact with
TiO2 particles and cause the growth of the particle. Even a very small amount of these ions is
responsible for this effect (35–39).
2.2. FT-IR spectra
=
In order to identify the sulfate S O species anchored to TiO2, the characteristic FT-IR transmit-
tance band around 1380–1360 cm−1 was followed.These transmittance bands are assigned to S O
=
◦
= =
and O S O stretching vibrations (40). The FT-IR spectra of sulfated titania calcined at 300 C
and exposed to ambient conditions for 24 h are shown in Figure 1. The evolution of the 1360 cm−1
=
band is shown as a function of the temperature. This band corresponds to (TiO)3–S O and to
=
(TiO)2–SO2 asymmetric vibrations (41–44). It can be seen that the S O band appears around
1320 cm−1 in the sample observed after ambient exposure. This band is shifted to 1369 cm−1 when
the temperature of the sample reaches 400 ◦C (45). In our case, because the sample was exposed
=
=
to a wet atmosphere, the S O species coordinates with water and hence the S O band appears
at lower frequency as an S–O vibration band (46, 47). When the temperature of the sample was
raised, the adsorbed water was eliminated and the band is then shifted to a higher frequency.
Figure 1. FT-IR spectrum of sulfated titania.

536 M. Khodadadi-Moghaddam et al.
2.3. SEM study
Figure 2 shows the SEM image of sulfated titania. One can see in this image that the TiO2 particles
are nearly spherical and almost all of them have diameter below 300 nm. This image shows that
the addition of sulfate ion on the surface of titania does not agglomerate the nanoparticles.
2.4. Thermogravimetric study
Figure 3 shows the thermal gravimetric curves of the sulfated titania nanoparticles. In Figure 3a,
one can see one broad peak at 50–200 ◦C and one major peak at 500–550 ◦C. The first peak (at
50–200 ◦C) depends on the water lost from the catalyst. Water can be adsorbed physically and
chemically on the sulfated titania surface. The adsorption energy of physically adsorbed water
is less than that of the chemically adsorbed water. Thus, we see a broad peak for water loss in a
thermogravimetric curve. This curve indicates that physically adsorbed water begins to desorb at
temperatures around 50–100 ◦C and immediately chemically adsorbed water begins to disorb at
temperatures 100–200 ◦C.
A major peak at 500–550 ◦C is depending on the desorption of sulfate groups from the sulfated
titania surface. The sulfate ion was adsorbed on TiO2 with a strong chemical bond and thus was
desorbed at high temperatures.
Figure 3b shows that approximately 2% of the weight of the catalyst was lost up to 400 ◦C, which
shows the weight percent of the water. But, approximately 10% weight lost around 500–550 ◦C
indicates that the weight percent of the sulfate ion on the TiO2 surface is 10%.
2.5. Surface area measurement
The Brunauer Emmett Teller (BET)-specific surface area of the support titania and calcined
samples at 500 ◦C was 5 m2/g.
Figure 2. SEM image of sulfated titania.

Journal of Sulfur Chemistry 537
Figure 3. Thermal gravimetric curve of sulfated titania nanoparticles.
2.6. Solvent effects on reaction rate
Figure 4 shows the dependence of the pseudo-first-order reaction rate constants of the ester-
ification reaction vs. the volume of acetonitrile. One can see in this figure that reaction rate
constants decrease continuously with the increase in the solvent volume from 0 to 3 ml. The
dipolarity/polarizibility parameter (π∗) of acetonitrile is 0.75, and the reactants (2-octanol and
phthalicanhydride)weresolubleinthissolvent.Thus, theadditionofacetonitrileremovesreagents
from the surface of the catalyst and reaction rate constants decrease with the increase in the solvent
volume.
Figure 4. Solvent effects on pseudo-first-order rate constants of esterification of phthalic anhydride in acetonitrile in
various solvent volumes.

538 M. Khodadadi-Moghaddam et al.
Figure 5 shows the dependence of pseudo-first-order reaction rate constants vs. the addition
of dimethyl sulfoxide (DMSO). In this figure, one sees the opposite behavior. The rate constants
increase with the increase in the solvent volume. The dipolarity/polarizibility parameter (π∗) of
DMSO is 1, and this solvent is more polar than acetonitrile. In this case, increasing the volume of
the polar solvent decreases the solubility of reactants in the solvent, and the tendency of reactants
to adhere to the surface of the catalyst was increased. This increases the coverage of the catalyst
surface with reactants and thus the reaction rate was increased.
Figure 6 shows the dependence of the reaction rate constants on the volume of the ionic liquid
(2-hydroxyethylammonium formate). The behavior of the system is close to that of the DMSO
solvent. The dipolarity/polarizibility parameter (π∗) of the ionic liquid is greater than DMSO and
is 1.15; thus, as one can expect, the increase in the solvent decreases the solubility of reactants.
Figure 5. Solvent effects on pseudo-first-order rate constants of esterification of phthalic anhydride in DMSO in various
solvent volumes.
Figure 6. Solvent effects on pseudo-first-order rate constants of esterification of phthalic anhydride in ionic liquid in
various solvent volumes.

Journal of Sulfur Chemistry 539
Thus, the reactant concentrations increase on the surface of the catalyst and then the reaction rate
was increased.
2.7. Catalyst recycling
Figure 7 shows the effect of the times of catalyst recycling on the reaction yield in 15 min. This
figure clearly shows that five times recycling and the reuse of the catalyst do not have negative
effects on the reaction yield. This behavior indicates that the sulfate group leaching from the
catalyst surface and other physical and chemical damages were negligible in this catalyst at least
for five times recycling.
2.8. Catalyst efficiency
Table 1 shows the effect of catalyst on the reaction rate of the esterification reaction. One can
see in Table 1 that in the case of acetonitrile, sulfated titania has a weaker catalytic effect in
comparison with the conventional inorganic acid catalyst (sulfuric acid), but in DMSO and
2-hydroxyethylammonium formate, sulfated titania has a strong catalytic effect than sulfuric
acid. As mentioned in Section 2.6, the solubility of reactants in acetonitrile is better than that in
DMSO and ionic liquid. Thus, in acetonitrile, a homogeneous catalyst (sulfuric acid) was better
Figure 7. Effects of catalyst recycling of reaction yield in 15 min.
Table 1. Catalyst effects on the reaction rate of esterification of phthalic anhydride
and 2-octanol in various solvents.
Pseudo-first-order reaction
Solvent
Catalyst
rate constant (s−1 g−1
)
Acetonitrile
Acetonitrile
Acetonitrile
DMSO
DMSO
DMSO
Ionic liquid
Ionic liquid
Ionic liquid
No catalyst
0.00310
0.01367
0.00247
0.00514
0.01911
0.03468
0.01043
0.03178
0.04214
Sulfuric acid
Sulfated titania
No catalyst
Sulfuric acid
Sulfated titania
No catalyst
Sulfuric acid
Sulfated titania

540 M. Khodadadi-Moghaddam et al.
than a heterogeneous catalyst; in contrast, in DMSO and ionic liquid, the solubility of reactants
is weak, and thus the tendency of reactants to adhere to the surface of the heterogeneous catalyst
is more than acetonitrile. As a result, in these solvents, the heterogeneous catalyst works better
than the conventional homogeneous acid catalyst.
3. Conclusions
The pseudo-first-order reaction rate constants of the esterification of phthalic anhydride with
2-octanol were obtained at the reflux condition in three solvents (acetonitrile, DMSO and
2-hydroxyethylammonium formate). The results show that with increasing dipolarity/polar-
izibility parameters (π∗) of the solvents, one can see the increase in reaction rate constants.
As an explanation, we think that the increase in solvent polarity can decrease the solubility of
the reactants in the solvent, and thus, the concentration of the reactants on the catalyst surface
was increased. The increase in the reactants on the catalyst surface results in the increase in the
reaction rate.
4. Experimental
Phthalic anhydride, titania and sulfuric acid were purchased from Merck. 2-Octanol was pur-
chased from RIEDEL-DE HAEN and used without further purification. The reactor consisted of
a 50 ml two-necked Pyrex flask fitted with a reflux condenser and a stopper. The temperature was
controlled with a glycerin bath. In a typical kinetic run, 0.5 g of acid was mixed with 4.5 ml of
alcohol, 0.02 g of catalyst and sufficient amount of solvent (in each run varies between 0.5 and
3 ml) in a 50 ml flask and heated to the boiling point of the mixture. SO24−/TiO2 was prepared
by immersing TiO2 in 1 N H2SO4 solution for 30 min and then decanted and dried from 80 ◦C to
150 ◦C. The resultant solid was calcinated at 500 ◦C and powdered. The catalyst was characterized
with XRD, FT-IR and BET methods. The reaction mixture was analyzed by the titration method.
The unconverted acid was determined by standardized 0.1 N NaOH.
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