Functionalized dicationic ionic liquids: Green and efficient alternatives for catalysts in phthalate plasticizers preparation

Article abstract of DOI:10.1007/s12039-016-1127-8

Two highly acidic, imidazolium-based, functionalized dicationic ionic liquids (FDCILs) were synthesized and characterized by FTIR, ¹H NMR and¹³C NMR. The synthesized FDCILs were used as efficient and green catalysts in the synthesis of phthalate plasticizers through esterification of phthalic anhydride (PhA) with ethanol, n-propanol and n-butanol. Among these two FDCILs, (dimethyl-4-sulfobutyl-ammonium) 1,2-ethan-1-methyl-imidazolium-sulfonic acid hydrogen sulfate performed better. The catalytic activity of FDCIL is related to the density of acidic groups on it and the length of the carbon chain in the cationic part. The influences of the reaction temperature, catalyst dosage, and molar ratio of phthalic anhydride to alcohol on the esterification reaction were investigated. The reusability of the catalyst in these reactions was studied too. The diester phthalates were obtained up to 98.8% yield. The products can be separated easily by decantation from the reaction mixture. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1127-8

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1127-8
Functionalized dicationic ionic liquids: Green and efficient alternatives
for catalysts in phthalate plasticizers preparation
NEGAR ZEKRI, REZA FAREGHI-ALAMDARI∗ and ZAHRA KHODARAHMI
Faculty of Chemistry and Chemical Engineering, Malek-Ashtar University of Technology,
Tehran, 16765-3454, I. R. Iran
e-mail: reza_fareghi@yahoo.com
MS received 9 January 2016; revised 22 April 2016; accepted 14 June 2016
Abstract. Two highly acidic, imidazolium-based, functionalized dicationic ionic liquids (FDCILs) were syn-
thesized and characterized by FTIR, 1H NMR and 13C NMR. The synthesized FDCILs were used as efficient
and green catalysts in the synthesis of phthalate plasticizers through esterification of phthalic anhydride (PhA)
with ethanol, n-propanol and n-butanol. Among these two FDCILs, (dimethyl-4-sulfobutyl-ammonium) 1,2-
ethan-1-methyl-imidazolium-sulfonic acid hydrogen sulfate performed better. The catalytic activity of FDCIL
is related to the density of acidic groups on it and the length of the carbon chain in the cationic part. The
influences of the reaction temperature, catalyst dosage, and molar ratio of phthalic anhydride to alcohol on the
esterification reaction were investigated. The reusability of the catalyst in these reactions was studied too. The
diester phthalates were obtained up to 98.8% yield. The products can be separated easily by decantation from
the reaction mixture.
Keywords. Acidic dicationic ionic liquids; highly acidic catalysts; esterification; phthalate plasticizers.
1. Introduction
some suffer from disadvantages such as limited acces-
sibility of the matrix-bound catalyst, rapid deactivation
and high molecular weight/active-site ratios.9 Therefore,
development of an efficient, highly selective, environ-
mentally benign and easy method using novel catalysts
for preparation of phthalate esters will be interesting.
Among the variety of catalysts, ionic liquids (ILs)
have the advantages of liquid and solid phase together.11
Task-specific ionic liquids (TSILs) have received great
attention over the past few decades performing as cata-
lyst and solvent in organic chemistry due to their unique
properties such as environmentally friendly nature, non-
volatility, high polarity, and good thermal and chemi-
cal stabilities.12 16 TSIL can be defined as association
of cation and anion, to which a functional group is
covalently bonded that confers the assembly a specific
task. In general, TSILs have been used as substitutes
for conventional homogenous and heterogeneous acid
catalysts.17
A new approach using alkyl sulfonated functional-
ized ILs has drawn more efforts due to their acidic prop-
erties and water solubility. Recently some imidazolium
and pyridinium based acidic ILs were used as catalyst to pre-
pare phthalate plasticizers.9,18 Meanwhile, multifunctional
ILs especially dicationic ILs (DCILs) are attracting
more attention as alternative reaction media in green
chemistry than conventional mono-cationic ILs.19,20
The DCIL contains two head groups and aliphatic chain
Phthalate plasticizers are the main plasticizers used
as softening agents in various industrial applications.1
These compounds are mainly used as plasticizers for
cellulosic resins and some vinyl ester resins, PVC and
nitrocellulose lacquers.2 Diethyl, dibutyl and dioctyl
phthalates are the most common plasticizers used in
the formulation of double-based solid propellants.3,4
Phthalates are manufactured by esterification reaction
between corresponding acids/ anhydrides and alcohols
in the presence of acidic catalysts.5 Various catalysts
have been reported for this reaction. Sulfuric acid has
been mainly used in traditional and industrial esteri-
fication of phthalates.2 Many heterogeneous catalysts
such as heteropoly acids,6 zirconium titanium phos-
phate,7 and zeolites8 are used as catalysts for esterifica-
tion of phthalic anhydride. However, the use of sulfuric
acid causes some problems such as formation of by-
products, corrosion, loss of catalyst, toxicity, tedious
work-up and environmental problems.9 Heteroge-
neous catalysts can give some benefits; for example,
easy work-up, catalyst reusability and avoidance of
corrosion.10 Despite of the benefits of these catalysts,
∗For correspondence

Negar Zekri et al.
linked by either a rigid or flexible spacer.21 Functional-
2. Experimental
ization ability of DCILs were explored to design their
structures with respect to cations, anions and length of
their linker chains in between cations.22
2.1 Materials and apparatus
All reagents and solvents were purchased from Merck
and Fluka chemical companies. The newly synthesized
FDCILs were characterized by 1H NMR and 13C NMR
on a Bruker DRX - 300 spectrometer. Chemical shifts
are reported relative to TMS as an internal standard.
The solvents used were DMSO and D2O. FT-IR spec-
tra were obtained on a Nicolet 800 instrument. Ele-
mental analyses were obtained using a Heraeous CHN
analyzer. Melting points were determined using melt-
ing point IA 8103 apparatus. Quantitative product anal-
ysis was conducted by gas chromatography on Hewlett
Packard HP-5890 instrument equipped with HP-1 col-
umn (30 m long, 0.5 mm diameter), and flame ioniza-
tion detector (FDI), using N2 as carrier gas at a flow rate
of 2 mL. min−1.
A few DCILs were used as catalysts in the esterifica-
tion reaction. Basic binuclear functional ILs were syn-
thesized and used as catalyst in the preparation of
biodiesel through transesterification from cottonseed
oil,23 and esterification of organic acids with ethanol
was carried out in the presence of a dicationic ionic
liquid.24 A group of imidazolium-based DCILs have
been used for esterification of alcohols by carboxylic
acids.25 In these reactions, researchers have focused on
the synthesis and application of geminal DCILs (same
cationic counterparts). Recently, we synthesized dual
dicyanamide-based ionic liquids consisting of coupled
ammonium–imidazolium cationic structures and also
unsymmetrical dicationic azide functionalized ionic li-
quids and studied their physical and thermal proper-
ties.26,27 Continuing our research activities in the field
of synthesis, characterization and application of ionic
liquids,26 32 we decided to synthesize the geminal and
unsymmetrical (different cationic counterparts) DCILs
with traditional counter anion, HSO−4 . The catalytic
activities of the synthesized FDCILs were tested for
esterification of phthalic anhydride with ethanol, n-
propanol and n-butanol (Scheme 1). The structures of
FDCILs used in this study are shown in Figure 1.
2.2 Preparation of functionalized dicationic ionic
liquids (FDCILs)
2.2a Synthesis of 4-((2-Hydroxyethyl) dimethyl azanyl)
butan-1-sulfate(1):2-(Dimethyl amino)-ethanol(10.1 mL,
100 mmol) was dissolved in tolueneunder vigorous stirring.
10.2 mL (100 mmol) of 1,4-butane sulfonate was poured
slowly in an ice bath. After the pouring was finished, the
O
O
FDCILs 1-2
OR
H2O
O
ROH
Toluene,
OR
O
O
R = ethyl, n-propyl, n-butyl
Scheme 1. Esterification of phthalic anhydride with alcohols using FDCIL1 and 2.
HO3S
SO3H
HO3S
N
N
N
N
N
N
N
-
-
2HSO4
2HSO4
(FDCIL1)
(FDCIL2)
FDCIL1: [(Dimethyl-4-sulfobutyl-ammonium)-1,2-ethane-1H-imidazolium sulfonic acid] hydrogen sulfate.
FDCIL2: 3,3’-(1,2-Ethanediyl)bis[1-(4-sulfobutyl)-1H-imidazolium sulfonic acid] hydrogen sulfate.
Figure 1. Structures of FDCILs used in this study. FDCIL1: [(Dimethyl-4-sulfobutyl-ammonium)-1,2-
ethane-1H-imidazolium sulfonic acid] hydrogen sulfate. FDCIL2: 3,3’-(1,2-Ethanediyl)bis[1-(4-sulfo-
butyl)-1H-imidazolium sulfonic acid] hydrogen sulfate.

Efficient protocol for preparation of phthalate plasticizers
mixture was slowly heated up to 111◦C and refluxed were mixed and stirred magnetically at 60◦C until imi-
for 12 h. Then, the reaction mixture was filtered to get dazole was dissolved. Subsequently, 0.8 mL (10 mmol)
the white precipitate. The precipitate was washed thrice of 1,2-dibromoethane was added dropwise into the mix-
with diethyl ether and dried at 80◦C for 5 h, giving (1) ture at 60◦C and stirred continuously for 2 h. After that,
as a white powder (M.p. 74◦C, yield 93.8%).
the reaction mixture was dumped into cold water, and
settled for 3 h to form (4) as a white solid product (M.p.
84◦C, yield 94.8%).
2.2b Synthesis of 4-((2-chloroethyl)dimethylazanyl)
butan-1-sulfate (2): 16.1 g (70 mmol) of (1) was dis-
solved in chloroform under vigorous stirring and thionyl
chloride (5 mL, 70 mmol) was poured slowly to the
reaction mixture in an ice bath. The mixture was slowly
heated up to 62◦C and refluxed for 12 h. Then it was
washed with chloroform thrice and heated at 50◦C for
2 h, giving (2) as a light yellow viscous liquid (yield
94.7%).
2.2f Synthesis of 3, 3’-(1, 2-ethanediyl) bis [1-(4-sulfo-
butyl)-1H-Imidazoliumdisulfate (5): 2.64 g (0.02 mol)
of (6) was mixed with 4.09 mL (0.04 mol) 1, 4-butane
sulfonate under stirring at room temperature for 72 h.
After solidification of the mass, the product was washed
three times with diethyl ether and dried at 120◦C for
2 h, giving (5) as a white powder (M.p. 124◦C, yield
96.3%).
2.2c Synthesis of (dimethyl-4-sulfobutyl-ammonium)1,2-
ethan-1 methyl-imidazolium sulfate (3): 3.0 g (12 mmol)
of (2) and 1-methyl imidazole (1.0 g, 12 mmol) were
dissolved in acetonitrile under vigorous stirring and
refluxed for 12 h. Then, the reaction mixture was fil-
tered to get the white precipitate. The precipitate was
washed thrice with diethyl ether and dried at 80◦C for
5 h, giving (3) as a white powder (M.p. 109◦C, yield
94.5%).
2.2g Synthesis of 3,3’-(1,2-ethanediyl)bis[1-(4-sulfo-
butyl)-1H-imidazolium hydrogen sulfate (FDCIL2):
2 g (5 mmol) of (5) was dissolved in methanol and 0.5 mL
(10 mmol) of sulfuric acid 98% was added slowly. The
mixture was heated up to 60◦C and stirred for 6 h. The
reaction mixture was washed with diethyl ether and
toluene three times and heated at 120◦C for 1 h, giving
FDCIL2. Compound FDCIL2: Light yellow viscous
1
liquid; yield 95.3%; H NMR (300 MHz, D2O, 25◦C,
2.2d Synthesis of (dimethyl-4-sulfobutyl-ammonium)1,
2-ethan-1-methyl-imidazolium-sulfonic acid hydrogen
sulfate (FDCIL1): 2.56 g (10 mmol) of (3) was dis-
solved in methanol and sulfuric acid 98% (10.6 mL,
20 mmol) was poured slowly. The mixture was heated
up to 60◦C and stirred for 12 h. Then, the reaction mix-
ture was washed thrice with diethyl ether and heated
at 70◦C for 1 h, giving FDCIL1. Compound FDCIL1:
TMS, δ ppm) 1.39-1.456 (m, 4H, HSO3CH2CH2CH2),
1.63-1.95 (m, 4H, HSO3CH2CH2), 3.11 (t, J = 5.8 Hz,
4H, HSO3CH2), 3.33 (s, 4H, Im(CH2)2Im), 4.33 (t, 4H,
J = 5.4 Hz, HSO3CH2CH2CH2CH2), 768 (d, J = 6.9
Hz, 4H, HIm), 8.36 (s, 2H, HIm); 13CNMR (75 MHz,
D2O, 25◦C, TMS, δ ppm) 23.69, 24.08, 49.06, 49.87,
54.63, 121.07, 121.17, 123.69; FT-IR (KBr) ν/cm−1:
3422, 3140, 2939, 1638, 1448, 1350, 1302, 1188, 1066,
784, 603; Anal. Calcd. for C16H30O14N4S4 Calcd. C,
30.48; H, 4.76; N, 8.89% Found C, 30.36; H, 4.80; N,
8.92%.
1
Light yellow viscous liquid; yield 96.3%; H NMR
(300 MHz, D2O, 25◦C, TMS, δ ppm) 1.07-1.13 (m, 2H,
HSO3CH2CH2CH2), 1.24-1.26 (m, 2H, HSO3CH2C
H2), 2.29 (t, J = 7.48 Hz, 2H, HSO3CH2CH2CH2CH2),
2.47 (s, 6H, N(CH3)2), 2.66 (s, 3H, Im-CH3), 2.70-2.76
(m, 2H, HSO3CH2), 3.04 (t, J = 6.0 Hz, 2H, N(CH3)2
CH2CH2Im), 3.30 (t, J = 6.0 Hz, 2H, N(CH3)2CH2
CH2Im), 6.75 (s, 1H, HIm), 6.76(s, 1H, HIm), 7.94 (s,
1H, HIm)13CNMR (75 MHz, D2O, 25◦C, TMS, δ ppm)
20.10, 20.21, 34.69, 49.24, 50.40, 54.55,63.61, 64.12,
118.72, 118.76, 122.33; FT-IR (KBr) ν/cm−1: 2953,
1636, 1474, 1230, 1166, 1150, 881, 591; Anal. Calcd.
for C12H27O11N3S3 Calcd. C, 29.69; H, 5.56; N, 8.65%
Found C, 29.64; H, 5.63; N, 8.81%.
2.3 General procedure for the synthesis of phthalate
plasticizers with FDCIL1,2
Weighed amounts of phthalic anhydride, alcohol (etha-
nol, n-propanol or n-butanol), ionic liquid and toluene
(as solvent) were mixed in a flask equipped with a reflux
condenser, dean-stark and magnetic stirring apparatus.
The reaction mixture was refluxed until completion
of the reaction. Completion of the reaction was mon-
itored by TLC (EtOAc/n-hexane 2:5). After comple-
2.2e Synthesis of 1, 1’-(1, 2- ethanediyl) bis-1H- imi- tion of the reaction, the mixture was washed with ethyl
dazolium (4): Imidazole (1.36 g, 20 mmol), sodium hy- acetate to separate the catalyst. Then, solvents were
droxide (0.4 g, 10 mmol) and dimethyl sulfoxide 20 mL removed under reduced pressure at 90◦C and the residue

Negar Zekri et al.
Table 1. The pH value of FDCILs in ethanol at concen-
tration of 1 mmol.L−1 at 298 K.
was purified by column chromatography to afford pure
products.
Ionic Liquid
pH value
FDCIL1
1.09
FDCIL2
3. Results and discussion
1.22
In this study, highly acidic FDCILs were synthesized
with high yields using multistep synthetic route as
shown in Scheme 2. The structure of these ionic liq-
uids were elucidated by NMR and FT-IR spectroscopy
as well as elemental analysis. The acidity of FDCILs
was determined in ethanol (Table 1). Two kinds of
FDCILs of this study showed strong acidity. This is
the main reason why they could efficiently promote the
esterification reaction.
alcohols with FDCILs1-2 as catalyst under optimized
conditions.
3.1 Comparison the efficiency of the newly
synthesized FDCIL1-2 in the esterification reaction
of phthalic anhydride with ethanol
Evaluation of the efficiency of synthesized FDCILs in
In order to optimize the esterification reaction condi- the esterification of phthalic anhydride with ethanol
tions, effect of some parameters such as reaction tem- (Scheme 3) revealed that FDCIL1 is an effective cata-
perature, catalyst dosage and molar ratio of phthalic lyst for the esterification reaction. Table 2, entries 1-2
anhydride to alcohol on the synthesis of phthalate give the yield values of the esterification of phthalic
diesters were studied. After that, phthalate diesters anhydride with ethanol in the presence of FDCIL1-2.
were synthesized using phthalic anhydride and various These results suggested that the performance of
i) Synthesis of FDCIL1
SOCl2
O3S
OH
O
OH
Toluene
O3S
N
N
Cl
S
N
CHCl3
reflux,12 h
O
O
reflux,12 h
(1)
(2)
C2H3N
SO3
O3S
N
Cl
N
NCH3
H3CN
N
N
Cl-
reflux,12 h
(2)
(3)
H2SO4
SO3H
SO3
N
H3CN
N
N
H3CN
N
CH3OH
55°C,12 h
-
Cl-
2HSO4
(3)
ii) Synthesis of FDCIL2
(FDCIL1)
DMSO
N
NH
N
N
Br
Br
2HBr
2
N
N
12 h, 80°C
(4)
O
2
S
SO3
N
O
N
O
O3S
N
N
N
N
N
N
72 h, r.t.
(4)
(5)
SO3
2H2SO4
CH3OH
SO3H
O3S
N
N
HO3S
N
N
N
N
N
N
-
2HSO4
55°C,12 h
(FDCIL2)
(5)
Scheme 2. Multistep synthetic route of FDCILs 1-2.

Efficient protocol for preparation of phthalate plasticizers
O
O
FDCILs 1-2
Toluene
OC2H5
OC2H5
H2O
O
C2H5OH
O
O
Phthalic anhydride
Diethyl phthalate
Scheme 3. Esterification of phthalic anhydride with ethanol using FDCILs 1-2.
Table 2. Results of esterification of phthalic anhydride with different alcohols in the presence of FDCILs.
PhA: alcohol: FDCIL
Entry Catalyst
Alcohol
molar ratio
T (◦C) Time (h) aYield (%)
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
1
FDCIL1
FDCIL2
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
FDCIL1
1:2:0.2
1:2:0.2
110
110
r.t.
6.5
8
57.3
50.5
0
2
3
1:2:0.2
10
10
10
10
10
8
4
1:2:0.2
40
0
5
1:2:0.2
55
3.5
6
1:2:0.2
70
12.7
17.2
28.3
37.5
72.3
77.4
77.4
80.6
96.4
96.4
41.3
96.4
96.4
96.4
98.1
98.8
7
1:2:0.2
80
8
1:2:0.2
90
9
1:2:0.2
100
115
120
130
120
120
120
120
120
120
120
120
120
6.5
4
10
11
12
13
14
15
16
17
17
18
19
20
1:2:0.2
1:2:0.2
4
1:2:0.2
4
1:2.2:0.2
1:2.5:0.2
1:3:0.2
4
3
3
1:2.5:0.05
1:2.5:0.1
1:2.5:0.4
1:2.5:0.1
1:2.5:0.1
1:2.5:0.1
3
3
3
3
OH
OH
3
1.5
HO
b
OH
OH
21
FDCIL1
1:2.5:0.1
120
2
-
b
HO
22
23
24
25
FDCIL1
FDCIL2
FDCIL2
FDCIL2
1:2.5:0.1
1:2.5:0.1
1:2.5:0.1
1:2.5:0.1
120
120
120
120
2
3
-
OH
OH
OH
91.6
92.7
95.1
3
1.5
aYields estimated by GC.
b Polymer product.

Negar Zekri et al.
FDCILs is dependent upon the character of the cation increase the rate of esterification. In addition, the opti-
side chain (low lipophilic character of the FDCIL1 mized reaction temperature is higher than the boiling
cation). Moreover, the pH values of these catalysts point of ethanol (78◦C), therefore some of it vapor-
showed that the acidity of FDCIL1 is higher than ized during the reaction. The influence of rectants molar
FDCIL2 (Table 1). So, as expected, FDCIL1 performs ratio was studied at 120◦C, the reaction conditions
better.
and yields obtained are shown in Table 2, entries 11,
For optimization purpose, various reaction condi- 13–15. The results showed that increasing the molar
tions for the synthesis of phthalate diesters were tested ratio of phthalic anhydride to ethanol, increases the
using FDCIL1 as catalyst (Table 2, entries 1, 3–18).
reaction yield. The highest achieved yield was 96.4%
with phthalic anhydride to ethanol molar ratio of 1:2.5
(Table 2, entry 14). Further increase of this molar ratio
did not increase the product yield probably because
phthalic anhydride and FDCIL1 became too diluted
with excess ethanol.
3.2 Effect of the reaction temperature on
esterification of phthalic anhydride
Effect of the reaction temperature on esterification
of phthalic anhydride with ethanol was studied. The
results of this study are shown in Table 2, entries 1, 3-
12. It is clear from this Table that when the reaction
was carried out at room temperature, phthalic anhydride
was not converted into diethyl phthalate, even after 10 h.
Esterification yield increased with a rise in temperature,
reaching a maximum of 77.4% at 120◦C (Table 2, entry
11). As can be seen from Table 2, entry12, increas-
ing the temperature above 120◦C did not lead to any
improvement. So, the optimum reaction temperature is
120◦C (Figure 2).
3.4 Effect of catalyst concentration on esterification
of phthalic anhydride
To determine the optimum concentration of the cata-
lyst (FDCIL1) in the esterification reaction of phthalic
anhydride with ethanol, the reaction was done at 120◦C
with phthalic anhydride: ethanol molar ratio of 1:2.5
and different amounts of FDCIL1. As shown in Table 2,
entries 14, 16–18, by increasing the mole ratio of
phthalic anhydride: FDCIL1 from 0.05 to 0.1, the yield
of diethyl phthalate increased. But further increase in
amount of the catalyst did not improve the reaction
yield.
3.3 Effect of the molar ratio of reactants on
esterification of phthalic anhydride
Generally an excess amount of ethanol is necessary
for esterification of phthalic anhydride; because, it can
3.5 Reusability of FDCIL1 in esterification of
phthalic anhydride
In order to examine the reusability of FDCIL1 after
completion of the reaction, ethyl acetate (2 × 5 mL)
was added. The reaction mixture was divided into two
Figure 2. Effect of the reaction temperature on the ester-
ification of phthalic anhydride with ethanol. PA: Ethanol: Figure 3. Reusability of FDCIL1 in the esterification reac-
FDCIL1 molar ratios is 1: 2: 0.2 and the solvent is toluene.
tion of phthalic anhydride with ethanol.

Efficient protocol for preparation of phthalate plasticizers
layers. The upper phase contained the products and the We also tested the esterification reaction of phthalic
lower phase the FDCIL1. After further extractions (3 × anhydride with diols such as 1,4-butandiol and 1,5-
5 mL ethyl acetate) and phase separation, the FDCIL1 pentanediol (Table 2, entries 21, 22). As we expected
was dried under vacuum for 5 h at 70◦C. The results of and as reported by other researchers,34,35 related polyes-
the reusability of FDCIL1 are shown in Figure 3, indi- ters were produced.
cating no major decrease in the product yield even after
7 runs.
3.7 Comparison the efficiency of FDCILs with other
reported catalysts in the esterification of phthalic
anhydride
3.6 Esterification of phthalic anhydride with various
alcohols using FDCIL1-2
Comparison of the efficiency of FDCILs with other cat-
alytic systems in the esterification reaction of phthalic
anhydride with n-butanol was studied too (Table 3).
According to Table 3, the catalysts of this study, espe-
cially FDCIL1, are more effective than other reported
catalysts for the esterification reaction.
Esterification of phtalic anhydride were carried out with
ethanol, n-propanol and n-butanol in the presence of
FDCIL1-2, and the diester product was isolated con-
veniently in high yield. Table 2 (entries 18–20, 23–25)
shows the results of these reactions. As we explained
above, FDCIL1 showed higher efficiency in the syn-
thesis of phthalate esters (Table 2, entries 18–20). It is
noteworthy to mention that the immiscibility of the
phthalate diester with the FDCIL facilitated the reac-
3.8 Esterification of maleic and succinic anhydrides
with ethanol in the presence of FDCIL1
tion equilibrium shifting to the product side. The same To investigate the generality and scope of this method,
result was observed in another report.33
the optimized reaction conditions was extended to other
The esterification reaction that began as a homoge- anhydrides such as maleic and succinic anhydrides
neous process, ended as biphasic mixture which facili- (Table 4). Considering Table 4, it is clear that FDCIL1
tated the separation of phthalate esters from the reaction is an effective catalyst for the esterification reaction of
mixture.
these anhydrides too.
Table 3. Comparison of the activity of some catalysts in the esterification reaction of phthalic anhydride with n-butanol.
PhA: n-Butanol
molar ratio
Entry
Catalyst
T (◦C) Time (h) PhA conv. (%) Yield (%)
Ref.
36
6
1
2
3
4
5
6
7
8
Amberlyst-36
H3PW12O40
H14NaP5W29MoO110
[emim]BF4
Phosphinite ionic liquids
[HSO3−pmim] pTSA
Sulfamic acid
1:10
1:2
1:2
1:2.5
1:2.1
1:2.5
1:4
1:2.5
1:2
1:2.5
1:2.5
120
110
110
120–130
100
120–130
113–115
115
–
3
3
7
3.5
7
4
10
4
1.5
1.5
39
77
100
62
78
98
81.1
–
35
100
100
–
–
–
–
–
–
–
62.4
–
6
9
37
9
38
39
6
Amorphous zirconium titanium phosphate
p−Toluene sulfonic acid
FDCIL1
9
10
11
110
120
120
98.8
95.1
This work
This work
FDCIL2
Table 4. Results of esterification of maleic and succinic anhydrides with ethanol in
the presence of FDCIL1a.
Entry
Anhydride
Time (h)
Conv. (%)
Yield (%)
1
2
Maleic anhydride
Succinic anhydride
3
3.5
100
100
94.2
91.5
a
Reaction conditions: molar ratios of anhydride/ ethanol/ catalyst =1: 2.5: 0.1, T =
120◦C.

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5962
Two novel, highly acidic imidazolium based FDCILs
with HSO−4 counter ion were synthesized, character-
ized and used as green and efficient catalysts for the
esterification reaction of phthalic anhydride with vari-
ous alcohols. The best results were obtained in the pres-
ence of FDCIL1, due to the higher acidity and low
lipophilic character of the cationic side chain of this
catalyst. Some valuable advantages of this method are
high yields of the products, short reaction times, easy
work up, not producing monoester by-product, reusabil-
ity, non-toxicity, non-corrosive reagents, highly acidity
of catalysts and less amounts of alcohol compared to
some of the reported methods.
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1
tion of the compounds using H NMR and 13C NMR
spectra are given in the supporting information. Sup-
plementary information is available at www.ias.ac.in/
chemsci.
Acknowledgements
We gratefully acknowledge the financial support by
Malek-Ashtar University of Technology.
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