Esterification of salicylic acid using Br?nsted acidic ionic liquid based on Keggin heteropoly acid

Article abstract of DOI:10.1080/15533174.2013.768639

For the first time, a newwater stable Br?nsted acidic ionic liquid based on Keggin heteroployacid (HPA) was used as environmentally benign catalytic medium in the esterfication of salicylic acid with aliphatic alcohols, C_nH_2n+1OH (n = 1-5) and benzylic alcohols, RC₆H₄CH₂OH (R = H, NO₂, OCH₃). This ionic liquid (IL) afforded excellent yield in both thermal conditions and microwave irradiation. Maximum yields were observed under microwave irradiation. Different reaction runs were conducted by varying the reaction parameters such as molar ratio of reactants, weight of the IL, and reaction period in order to optimize the reaction. The IL was easily recovered and reused many times. No significant loss in catalytic activity was observed on recycling.

Full text of DOI:10.1080/15533174.2013.768639

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Esterification of Salicylic Acid Using Brønsted Acidic
Ionic Liquid based on Keggin Heteropoly Acid
Fatemeh F. Bamoharram a , Majid M. Heravi b , Javad Ebrahimi a & Nilofar Tavakoli-Hoseini a
a Department of Chemistry , Mashhad Branch, Islamic Azad University , Mashhad , I. R. Iran
b Department of Chemistry , School of Science, Alzahra University , Vank , Tehran , I. R. Iran
Accepted author version posted online: 08 Aug 2013.Published online: 27 Sep 2013.
To cite this article: Fatemeh F. Bamoharram , Majid M. Heravi , Javad Ebrahimi & Nilofar Tavakoli-Hoseini (2014)
Esterification of Salicylic Acid Using Brønsted Acidic Ionic Liquid based on Keggin Heteropoly Acid, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:1, 36-42, DOI: 10.1080/15533174.2013.768639
To link to this article: http://dx.doi.org/10.1080/15533174.2013.768639
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:36–42, 2014
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.768639
Esterification of Salicylic Acid Using Brønsted Acidic Ionic
Liquid based on Keggin Heteropoly Acid
Fatemeh F. Bamoharram,1 Majid M. Heravi,2 Javad Ebrahimi,1
and Nilofar Tavakoli-Hoseini1
1Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, I. R. Iran
2Department of Chemistry, School of Science, Alzahra University, Vank, Tehran, I. R. Iran
reaction mixture biphasic, thus combining the advantages of
homogeneous and heterogeneous catalysis.
For the first time, a new water stable Brønsted acidic ionic liquid
based on Keggin heteroployacid (HPA) was used as environmen-
tally benign catalytic medium in the esterfication of salicylic acid
with aliphatic alcohols, CnH2n+1OH (n = 1-5) and benzylic alco-
hols, RC6H4CH2OH (R = H, NO2, OCH3). This ionic liquid (IL)
afforded excellent yield in both thermal conditions and microwave
irradiation. Maximum yields were observed under microwave ir-
radiation. Different reaction runs were conducted by varying the
reaction parameters such as molar ratio of reactants, weight of the
IL, and reaction period in order to optimize the reaction. The IL
was easily recovered and reused many times. No significant loss in
catalytic activity was observed on recycling.
Thus, number of ILs grows daily and there is many examples
of different ways in which ILs have been applied in catalysis (i.e.
as the catalyst, cocatalyst, or catalyst activator) as the source of
a new ligand for a catalytic metal center, or just as the solvent
for the reactions.[13]
In general, ILs consists of a salt where one or both the ions
are large, and the cation has a low degree of symmetry. These
factors tend to reduce the lattice energy of the crystalline form
of the salt, and hence lower the melting point.[14] ILs come
in two main categories, namely simple salts (made of a single
anion and cation) and binary ILs (salts where an equilibrium is
involved).
Keywords Brønsted-acidic ionic liquid, esterification, heteropoly-
acid, Keggin, microwave, salicylic acid
The cations are generally bulky, asymmetric ammonium or
phosphonium salts, or heteroatomics, with low symmetry, weak
intermolecular interactions, and low charge densities.
The anions can be organic or inorganic and the combinations
of the different cations and anions can dramatically change
the physicochemical properties of the ILs.[15] The most typi-
cal anions are: Cl−, Br−, [BF4]−, [SbF6]−, [AlCl4]−, [AuCl4]−,
INTRODUCTION
The search for catalytic processes involving environmentally
benign catalysts has attracted much attention. In this area, ionic
liquids (ILs) have been generating increasing interest over the
last decade,[1] due to their particular properties[2] and applica-
tions in organic synthesis,[3] catalysis,[4] biocatalysis,[5] liquid-
liquid separations,[6] extraction and dissolution processes,[7–9]
nanomaterials synthesis,[10] polymerization reactions,[11] and
electrochemistry.[12] Generally, they have investigated as green
solvents and catalysts in different organic reactions.[13] They are
made up at least two components including an anion and a cation
and their merit lies in their special physical-chemical properties,
which can be tuned by varying either the anion, cation, or its
substitution pattern. By this means, an IL with optimal proper-
ties for a given application can be designed. In catalysis, this
feature can be favorably exploited by rendering a homogeneous
[NO3], [NO2]−, [SO4]2−
,
[AcO]−, [Tf]−, [N(OTf)2]−,
[CF3CO2]−, [CF3SO3]−, [PhCOO]−, [C(CN)2]−, [RSO4]−,
[OTs]−, and [SCN]−.
Although a variety of anions have been used, the application
of heteropolyacids (HPAs) has been largely overlooked and very
limited.
One of the most important ILs, is SO3-H-functionalized
IL. For synthesis of these ILs, first a N-alkylimidazol (or N-
alkylpyrrolidin, pyridine) is reacted with 1,4-butan(or propan)
sultone to obtain a solid zwitterion. This zwitterion is then proto-
nated with a Brønsted acid such as sulfuric acid and phosphoric
acid to obtain corresponding IL. Although the mentioned IL
shows satisfactory behavior, it needs hazardous acids as starting
materials.
To the best of our knowledge, protonation of such zwitterions
with HPAs has not been reported in the literature.
The principles of green chemistry have been introduced to
eliminate or reduce the use of hazardous materials such as
Received 20 October 2011; accepted 16 January 2013.
Address correspondence to Fatemeh F. Bamoharram, Department
of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad,
I. R. Iran. E-mail: abamoharram@yahoo.com
36

ESTERIFICATION USING IONIC LIQUID
37
FIG. 1. The structure of the Keggin heteropoly anion [XM12O40 n−
figure available online).
]
in polyhedral (left), ball-and-stick (middle) and space-filling (right) representations (color
H2SO4 or H3PO4 in chemical processes. Cleaner technologies EXPERIMENTAL
could bepossibleby makinguseof environmentallyfriendlyma-
Chemicals and Instruments
terials involving the use of solid acids. It was shown that HPAs
in the solid state are pure Brønsted acids and stronger acids than
conventional acids such as SiO2-Al2O3, H3PO4, HNO3, H2SO4,
HX, and HY zeolites.[16,17] These compounds have several ad-
vantages that make them economically and environmentally at-
tractive.[18–22] These solid acids are green with respect to corro-
siveness, safety, quantity of waste, and separability. Moreover,
while some of the acid catalysts such as HCl, HNO3, and H2SO4,
can produce chlorated, nitrated, and sulfated by-products, HPAs
do not produce any of these byproducts. HPAs have been ex-
tensively studied as green acid catalysts for many reactions and
found industrial applications in several processes.[23] Many re-
actions catalyzed by Brønsted and Lewis acids, but proceeded
more effectively under milder conditions with greater selec-
tively, better yields, and shorter reaction times in the presence
of HPAs.
All of the chemicals were commercially available.
IR spectra were obtained with a Brucker 500 scientific spec-
trometer. GLC analysis was performed on a Pu 4500 gas chro-
matograph with FID detector. Microwave (MW) irradiation was
performed in MILESTONE ONE APC-55E.
Preparation of IL
Pyridine (0.2 mol) and 1,4-butane sultone (0.2 mol) were
charged into a 100 mL round-bottom flask. Then, the mixture
was stirred at 40◦C for 10 h. The white solid zwitterion was
washed repeatedly with ether to remove non-ionic residues and
dried in vacuum.[39] Then, H3PW12O40, was added and the mix-
ture stirred for 20 min at 60◦C to form the IL.
Catalytic Test
The utility of the IL was established in the esterification of
salicylic with alcohols. In a typical reaction benzyl alcohol,
salicylic acid and IL were taken in a 50 mL two-necked round
bottom flask and the reaction mixture was stirred and refluxed
for 3 h. At room temperature reactions were carried out with
stirring for 6 h.
At regular intervals, Karl Fisher titration was performed for
determination of produced water. The products were charac-
terized by comparison of their spectroscopic data with those
of authentic samples. Yields were determined by GC. Under
MW irradiation a solution of salicylic acid, alcohol, and IL was
irradiated for 2 min.
Among HPAs, the application of Keggin structures have been
extensively studied.[24–27] The Keggin structure with formula of
[XM12O40]n−, is shown in Figure 1.
In pursuing these studies and in continuation of our previous
works on the applications of Keggin HPAs[28–30] and ILs[31–33]
in the synthesis of organic compounds, in the present work for
the first time we have used a Brønsted acidic IL based on Keg-
gin HPA, for the esterification of salicylic acid with different
alcohols, in both thermal conditions and microwave irradiation.
Currently many reports are available in the literature on esteri-
fication using ILs.[34–38]
To the best of our knowledge there is no report with HPA-
based ILs in esterification reactions.
The role of this IL will be to provide a medium for the catalyst
and solubility for starting materials and products. Such combi-
nation of catalyst with medium has been found to be a viable
alternative approach towards modern synthetic chemistry, es-
pecially when considering the growing environmental demands
being placed on chemical processes.
RESULTS AND DISCUSSION
The esterification of salicylic acid with aliphatic alcohols,
CnH2n+1OH (n = 1-5) and benzylic alcohols, RC6H4CH2OH
(R = H, NO2, OCH3) was carried out in the presence of
a new and green IL based on Keggin HPA with formula of
[XM12O40]3−
.

38
F. F. BAMOHARRAM ET AL.
FIG. 2. (A) IR spectrum of the Keggin, H3 [PW12 O40]. (B) IR spectrum of IL formed via interaction of Keggin with 1.

ESTERIFICATION USING IONIC LIQUID
39
For preparation of this IL, first, pyridine was reacted with 1,4-
butansultone to give solid zwitterions (1). Interestingly, when we
mixed two solids including (1) and H3PW12O40, a liquid formed
after 20 min at 60◦C. It is necessary to be mentioned that pro-
tonating of zwitterions (1) with H2SO4 as Brønsted acids need
longer reaction time (6 h) and higher temperature (80◦C).[37–39]
Therefore, H3PW12O40 not only reduced the time, but also the
temperature. Figure 2 shows the IR spectra of H3PW12O40 and
prepared IL.
The [XM12O40] consists of one XO4 tetrahedron (X = SiIV,
GeIV, PV. . .) surrounded by four M3O13 sets (M = WVI, MoVI)
linked together through oxygen atoms (named M-Ob-M).The
XO4 tetrahedron is assumed to vibrate almost independently
from the rest of the anion. This assumption is particularly valu-
able for X = PV. The symmetric and asymmetric stretching
of the different kinds of M-O bonds are observed in the fol-
lowing spectral regions: M-Od bonds (960–1000 cm−1), M-Ob-
M bridges (inter bridges between corner-sharing octahedral,
850–890 cm−1) and M-Oc-M bridges (intra bridges between
edge-sharing octahedra, 760–800 cm−1). Only the M-Od stretch-
ing can be considered as pure vibrations: the stretching involving
Ob or Oc atoms present some bend character.[40,41]
FIG. 3. Esterification of salicylic acid with benzyl alcohol with various molar
ratio of acid:alcohol (solvent free) (color figure available online).
In order to check the amount of the IL we examined different
amounts of used IL.
This attempt was performed with different amounts of IL
(0.02–0.1 g) in molar ratio of 1:5 for salicylic acid: alcohol. The
data are shown in Figure 4.
In our study, the IR spectrum of the formed IL (Figure 2b)
exhibits the characteristic frequencies of the Keggin structure
in the range 600–1100 cm−1. Compared with the initial Keggin
structure (Figure 2a), the bands arising from the HPA change
obviously either in intensity or position. Comparing the IR spec-
trum of the IL with the IR spectrum of H3[PW12O40] (Figure 2),
the vibrational band of the M-Oc-M of the Keggin is split from
793.88 into two bands, including 728.73 and 820.20, due to
difference in the M-Oc-M bonds. The vibrational band of the
P-O is split from 1080.22 into two bands, including 1040.96
and 1079.43. This means that, the inter bridges between edge-
sharing octahedra (760–800 cm−1) and the P-O stretchings have
been affected. These results indicate that many of the bonds
were strengthened and the others were weakened. It means that
the HPA has a interaction with (1).
Our studies showed that under these conditions, the reaction
yields were affected by changing the amount of the IL.
The yield increases up to 0.1 g and any further increasing for
amount of the IL did not have any effect on the reaction yields.
Under optimum conditions, the esterification reactions were
carried out in different temperatures. The results are shown in
Figure 5.
The maximum yield of ester is reached at reflux temperature.
This was expected, since increasing the temperature is appar-
ently favorable for the acceleration of the forward reaction.
The yield is lower when the reactions are carried out at room
temperature.
Under the best conditions (0.1 g IL, 3 h, molar ratio 1:5, and
refluxtemperature) wehave expendedour study andinvestigated
the application of the IL for esterification of salicylic acid with
the other alcohols (Figure 6).
It is suggested that, the Keggin HPA as a Brønsted acid gen-
erate acidic IL from its zwitterion to continuously and the pro-
duced IL acid, provides a medium for the catalyst and solubility
for starting materials and products in esterification reaction.
Our findings show that this IL is very active in these reactions.
Thermal Conditions
Initial attempts to optimize the esfterification of salicylic acid
were performed with benzyl alcohol.
The mole ratio of salicylic acid to alcohol was varied as 1:1
to 1:6. The results are shown in Figure 3. All of the reactions
were carried out in reflux conditions and 3 h. This figure shows
the dependence of the molar ratio in the esterification reaction.
As we can see, the yield increases with that of the molar
ratio up to 1:5. Any further increasing of the molar ratio does
not have any appreciable effect on the reaction yield. By using
some excess of acid the results were the same.
FIG. 4. Esterification of salicylic acid with benzyl alcohol using various
amounts of the IL (solvent free) (color figure available online).

40
F. F. BAMOHARRAM ET AL.
Green chemistry is defined as the approach to synthesis,
processing, and use of chemicals that reduces risks to humans
and the environment.
Green chemistry has been one of the keywords when MW
and especially ILs have been introduced. MW and ILs are green
chemistry by themselves. Thus, there is enough room for new
works to this field. The value of MW is that they usually reduce
reaction time by efficient heating and ease the path to high
temperature reactions.
Anyway, despite many reports in literature, there is no report
for MW-assisted reactions using HPA-based ILs.
FIG. 5. Esterification of salicylic acid with benzyl alcohol in different tem-
peratures (solvent free) (color figure available online).
This work has been carried out with green chemistry princi-
ples in mind.
In this section we wish to report our preliminary results on
the application of this IL in an MW-assisted reaction. Under
optimum conditions, we investigated applicability of this IL for
highly efficient esterification of salicylic acid with alcohols. The
results are shown in Figure 7. As can be seen, this IL afforded
excellent yields.
The two fundamental mechanisms for transferring energy
from microwaves to the substance are dipole rotation and ionic
conduction. Dipole rotation is an interaction in which polar
molecules try to align themselves with the rapidly changing
electric field of the microwave. Ionic conduction mechanism
Microwave-Assisted Esterification Reactions
During our on going study in thermal conditions using this
IL, we interested in the combination of this IL with MW-assisted
reaction. ILs consisting entirely of ions could absorb the MW
energy efficiently.
Moreover, the risk of an explosion caused by rapid increase
of vapor pressure of the solvent can be avoided, since ILs have
no measurable vapor pressure.
FIG. 6. Comparison between esterification of salicylic acid with alcohols at room temperature and reflux conditions (solvent free) (color figure available online).

ESTERIFICATION USING IONIC LIQUID
41
FIG. 7. Yields of esterification reactions in the presence of the IL, under microwave irradiation (solvent free) (color figure available online).
consists in the instantaneous superheating of the ionic substance existing procedures and provides great promise toward further
due to the ionic motion generated by the electric field. When useful applications. This process will pave a way for large-scale
the temperature increases, the transfer of energy becomes more applications of synthesis of other esters. Further applications for
efficient.
Due to their ionic character, ILs absorb microwave irradia-
other reaction systems are currently under investigation.
tion extremely well and transfer energy quickly by ionic conduc-
tion.[42] Thus, can diminish reaction time, avoid side products,
increase yield and simplify the course of reactions for combi-
natorial chemistry.[43]
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