Synthesis and characterization of a new hydroxyl functionalized diacidic ionic liquid as catalyst for the preparation of diester plasticizers

Article abstract of DOI:10.1016/j.molliq.2016.11.121

Two new functionalized diacidic ionic liquids (FDAILs) including hydroxyl functionalized diacidic ionic liquid (HFDAIL) and sulfonated diacidic ionic liquid (SFDAIL) were synthesized and characterized by ¹HNMR, ¹³CNMR and FT-IR. The catalytic activities of these FDAILs were examined in esterification reaction of anhydrides with some alcohols to give corresponding dialkyl plasticizers under solvent-free conditions. The results indicate that HFDAIL, as hydroxyl-bearing catalyst, show better catalytic performance. Under the optimum conditions, using HFDAIL, the conversion of phthalic anhydride was high and diester plasticizers were obtained with good to excellent yields in the presence of only 10?mol% of ionic liquid. All the produced diesters could be easily recovered due to their immiscibility with the ionic liquid. Recycling experiments suggests that these ionic liquids can be reused several times without remarkable loss in their catalytic activity.

Full text of DOI:10.1016/j.molliq.2016.11.121

Journal of Molecular Liquids 227 (2017) 153–160
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and characterization of a new hydroxyl functionalized diacidic
ionic liquid as catalyst for the preparation of diester plasticizers
a
b
Reza Fareghi-Alamdari a, , Mehri Nadiri Niri , Hassan Hazarkhani
⁎
a
Faculty of Chemistry and Chemical Engineering, Malek-Ashtar University of Technology, Tehran, Iran
Center for Research and Science Education, Organization for Educational Research and Planning, Tehran, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2016
Received in revised form 20 October 2016
Accepted 29 November 2016
Available online 02 December 2016
Two new functionalized diacidic ionic liquids (FDAILs) including hydroxyl functionalized diacidic ionic liquid
(HFDAIL) and sulfonated diacidic ionic liquid (SFDAIL) were synthesized and characterized by 1HNMR,
13CNMR and FT-IR. The catalytic activities of these FDAILs were examined in esterification reaction of anhydrides
with some alcohols to give corresponding dialkyl plasticizers under solvent-free conditions. The results indicate
that HFDAIL, as hydroxyl-bearing catalyst, show better catalytic performance. Under the optimum conditions,
using HFDAIL, the conversion of phthalic anhydride was high and diester plasticizers were obtained with good
to excellent yields in the presence of only 10 mol% of ionic liquid. All the produced diesters could be easily recov-
ered due to their immiscibility with the ionic liquid. Recycling experiments suggests that these ionic liquids can
be reused several times without remarkable loss in their catalytic activity.
Keywords:
Hydroxyl functionalized diacidic ionic liquid
Sulfonated diacidic ionic liquid
Esterification reaction
Plasticizer
© 2016 Elsevier B.V. All rights reserved.
Solvent-free conditions
1. Introduction
electrochemistry [21]. In the recent years, researchers are focused on syn-
thesis of functionalized ionic liquids (FILs), in order to obtain better char-
Plasticizers are almost a class of oligomers which are used in formula-
tion of explosive fuels and leads to improvement in mechanical proper-
ties, softening polymeric tissue and flexibility [1–5]. One of the most
popular plasticizers in double-based propellants are ester plasticizers
such as diethyl, dibutyl and dioctyl phthalates, which are a class of the
neutral plasticizers. Ester plasticizers are widely used in non-military in-
dustries, e.g. in formulation of PVC and cellullosic resins to manufacture
of plastic products such as tube, blood bag and etc. [6–8]. The catalysts
used in ester plasticizers synthesis process can be both homogeneous
and heterogeneous. Most commonly used catalysts in traditional methods
include sulfuric acid and solid acids [9–12]. The sulfuric acid cannot be
reused and also has other disadvantages such as equipment corrosion,
more byproducts production, tedious workup process and environmental
problems. Furthermore, it is very difficult to separate and reuse the min-
eral acidic catalysts from the reaction mixture [13–16]. Solid acid catalysts
[17–19] have capability to overcome many of the above mentioned de-
fects. However, the use of solid acids in this reaction has many disadvan-
tages including easy deactivation and high mass transfer resistance,
which limit their efficiency [20]. Along this line, there is a need to expand
an environmentally friendly and economic method for synthesis of ester
plasticizers using esterification process.
acteristics. Cole et al. [22] reported a pioneering work on sulfonic FIL with
strong Brønsted acidity. Many progress have been achieved in this field
and divers FIL for special applications have been reported [23,24].
Among them there are many studies on the use of FIL in different esteri-
fication processes [25,26]. However, to the best of our knowledge, there is
no report on the synthesis of plasticizers using ionic liquids of this study.
In continuation of our works on synthesis and application of ionic
liquids in the organic synthesis [27–30], herein, we would like to intro-
duce two novel functionalized diacidic ionic liquids to be used in syn-
thesis of diester plasticizers (Scheme 1). We have chosen ILs with
these functional groups as catalysts for esterification reactions because:
(1) they are immiscible with a number of organic solvents, (2) they are
soluble in a wide range of inorganic and organic materials, (3) high hy-
drophilicity and acidity of the hydroxyl functionalized catalyst and (4)
They can be recycled for several times. The structures of ionic liquids
used in this study are shown in Fig. 1.
2. Experimental
2.1. Material and methods
On the other hand, the room temperature ionic liquids due to their
tunable physical and chemical properties have attracted the attention of
scientists in various fields such as synthesis, catalysis, separation and
All solvents and chemicals were commercially available and used
without further purification and purchased from Merck and Fluka chem-
ical companies. NMR spectra were recorded on a Bruker DRX - 500 spec-
trometer in DMSO or D2O and calibrated with tetramethylsilane (TMS) as
the internal reference. IR measurements were performed on a Nicolet 800
⁎
Corresponding author.
E-mail address: reza_fareghi@yahoo.com (R. Fareghi-Alamdari).
http://dx.doi.org/10.1016/j.molliq.2016.11.121
0167-7322/© 2016 Elsevier B.V. All rights reserved.

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R. Fareghi-Alamdari et al. / Journal of Molecular Liquids 227 (2017) 153–160
O
O
FDAIL
OR
OR
+
H2O
H2O
H2O
ROH
ROH
O
O
O
O
RO
OR
FDAIL
+
O
O
O
O
O
O
FDAIL
HO
RO
ROH
+
+
OH
OR
O
O
R = Methyl, Ethyl, n- Propyl, n-Butyl, 2- Ethylhexyl, 2- Methoxyethyl, Allyl
Scheme 1. Esterification of phthalic and maleic anhydrides and succinic acid with alcohols using synthesized FDAILs.
instrument using KBr or liquid film. Quantitative product analysis was
conducted by gas chromatography on Hewlett Packard HP-5890 instru-
ment equipped with HP-1 column (30 m long, 0.5 mm diameter), and
flame ionization detector (FDI), using N2 as carrier gas at a flow rate of
do this, 0.8 g IL-Br salt (2 mmol) was solved in acetonitrile. Then
0.5 mL chlorosulphonic acid (8 mmol) was dropped slowly at room
temperature. After the dropping was finished, the mixture was stirred
for 4 h at the same temperature. The reaction mixture was filtered to
get the yellow precipitate. It was washed with toluene three times
and dried under vacuum at 100 °C for 1 h, giving SFDAIL as a yellow
solid (yield: 97.5%). 1H NMR (500 MHz, DMSO-d6): δ (ppm) = 3.33
(s, 4H; CH2N), 3.47 (s, 6H; CH3), 4.04 (s, 4H; CH2OH), 5.33 (s, 2H;
SO3H), 7.06 (d, J = 5 Hz, 4H; CH _CH), 8.30 (s, 2H; NCHN). 13C NMR
(125 MHz, DMSO-d6): δ (ppm) = 36.19, 43.83, 48.27, 63.89, 123.88,
124.26, 137.79. FT-IR (KBr, ν/cm−1): 587.3, 782.5, 1019.1, 1251.7,
1458.7, 1584.9, 1709.1, 3160.1.
2 mL·min−1
.
2.2. Preparation of hydroxyl and sulfonic acid functionalized diacidic ionic
liquids
2.2.1. Preparation of 2,2-bis ((3-methylimidazolidin-1-yl) methyl) pro-
pane-1,3-diol bromide salt (IL-Br)
A mixture of 0.52 g 1-methylimidazole (6.25 mmol, 0.5 mL) and 0.65 g
2,2-bis (bromomethyl)-1,3-propanediol (2.5 mmol) was heated at 150 °C
for 8 h under magnetic stirring. After cooling to room temperature, the
obtained solid was washed three times with acetonitrile and the residue
was dried in an oven at 100 °C for 2 h, giving IL-Br as a white powder
(yield: 95.25%, melting point = 150 °C). 1H NMR (500 MHz, DMSO-d6):
δ (ppm) = 3.24 (s, 4H; CH2N), 3.84 (s, 6H; CH3), 4.31 (s, 4H; CH2OH),
4.72 (s, 2H; OH), 7.42 (dd, J = 10 Hz, J = 5 Hz, 4H; CH _CH), 8.72 (s,
2H; NCHN). 13C NMR (125 MHz, DMSO-d6): δ (ppm) = 35.94, 45.21,
48.58, 58.48, 123.72, 123.86, 137.38. FT-IR (KBr, ν/cm−1): 620.8, 770.7,
829.8, 1060.5, 1164.9, 1450.8, 1573.1, 1760.4, 1655.9, 2935.4, 3260.7.
2.2.3. Preparation of 3,3′-(2,2-bis (hydroxymethyl) propane-1,3-diyl) bis
(1-methyl-1H-imidazole-3-ium) hydrogen sulfate (HFDAIL)
The hydroxyl functionalized ionic liquid was prepared by anion ex-
change of IL-Br salt. 0.5 g (1.2 mmol) of IL-Br salt, obtained in the first
stage, was dissolved in methanol (30 mL) under stirring. After that the
system was slowly heated up to 60 °C in an oil bath. Then 0.13 mL sul-
furic acid (2.4 mmol) was dropped and stirred for 24 h. Upon comple-
tion of the reaction, the solvent was evaporated under vacuum. The
resultant material was washed with dichloromethane (3 × 5 mL) and
then dried under vacuum at 100 °C for 2 h to afford HFDAIL as yellow
viscous oil liquid at 95% yield. 1H NMR (500 MHz, DMSO-d6): δ
(ppm) = 3.20 (s, 4H; CH2N), 3.81 (s, 6H; CH3), 4.27 (s, 4H; CH2OH),
4.74 (s, 2H; OH), 7.39 (dd, J = 10 Hz, J = 5 Hz, 4H; CH _CH), 8.69 (s,
2.2.2. Preparation of 3,3′-(2,2-bis ((sulfoxy) methyl) propane-1,3-diyl) bis
(1-methyl-1H-imidazole-3-ium) bromide (SFDAIL)
The sulfonic acid functionalized ionic liquid, was prepared by nucle-
ophilic substitution reaction of IL-Br salt with chlorosulphonic acid. To
(SFDAIL): 3,3'- (2,2-Bis ((sulfoxy) methyl) propane-1,3-diyl) bis (1-methyl-1H-imidazole-3-ium) bromide
(HFDAIL): 3,3'- (2,2-Bis (hydroxymethyl) propane-1,3-diyl) bis (1-methyl-1H-imidazole-3-ium) hydrogen sulfate
Fig. 1. Structures of FDAILs used in this study.

R. Fareghi-Alamdari et al. / Journal of Molecular Liquids 227 (2017) 153–160
155
2H; NCHN). 13C NMR (125 MHz, DMSO-d6): δ (ppm) = 36.73, 44.97,
3. Results and discussion
49.63, 59.53, 124.31, 124.58, 138.73. FT-IR (KBr, ν/cm−1): 589.8,
776.2, 879.9, 1053.6, 1222.7, 1458.4, 1572.1, 1641.8, 2509.7, 3095.5,
3328.5.
In this study, ionic liquids were prepared by two-step synthetic pro-
tocol (Scheme 2). At the first step, IL-Br salt was prepared by nucleophil-
ic substitution reaction of 1-methylimidazole and 2,2-bis
(bromomethyl)-1,3-propanediol. Then the resulting ionic liquid (IL-
Br) was transformed into the target compounds by using
chlorosulphonic acid and concentrated sulphuric acid as sulfonating
and anion exchange agents to afford SFDAIL and HFDAIL respectively.
The acidity of the synthesized ionic liquids was determined by acid-
base titration against a standard NaOH solution with phenolphthalein as
indicator. The results are shown in Table 1. As indicated in this Table,
the synthesized ionic liquids especially HFDAIL, showed high acidity.
Therefore we tested their catalytic activity in the esterification reactions.
In order to optimize the reaction conditions, the esterification of phthalic
anhydride with butanol was examined and the influence of reaction tem-
perature, molar ratio of catalyst and molar ratio of phthalic anhydride to
alcohol were studied. The results are shown in Figs. 2 and 3 and Table 2.
2.3. Acidity measurement of the catalyst
To measure the acidity of ionic liquids, first of all the NaOH solution
was standardized. Dried potassium hydrogen phthalate (0.03 g) was
dissolved in 25 mL distilled water and titrated the 0.05 N NaOH solution
(the titrant, 50 mL) using 3 drops of phenolphthalein as the indicator.
Then the acidity of the synthesized ionic liquids (0.03 g ionic liquid in
50 mL distilled water) were determined by acid-base titration against
the standard NaOH solution.
2.4. Synthesis of dibutyl phthalate as a typical procedure for esterification
reaction of phthalic anhydride with butanol
The typical procedure for esterification of phthalic anhydride with
butanol is as follows: phthalic anhydride (1 mmol, 0.15 g), butanol
(5 mmol, 0.46 mL, 0.37 g) and HFDAIL as catalyst (10 mol% to phthalic
anhydride, 0.05 g) were charged into a 50 mL round bottom flask with
a dean-stark apparatus, reflux condenser and a magnetic stirrer. Then
the mixture was stirred at 125 °C for 7 h. The completion of reaction
was monitored by TLC using (EtOAC/Hexane 2:8) as eluent. After com-
pletion of the reaction as indicated by TLC, the product (dibutyl phthal-
ate) was separated simply by extraction with ethyl acetate (3 × 5 mL)
and ethyl acetate was evaporated under vacuum to afford desired prod-
uct as yellow oil liquid at 95% yield (0/26 g, boiling point = 340 °C). The
obtained product dried under vacuum at 70 °C for 5 h. Viscous ionic liq-
uid could be reused after removal of water under vacuum at 80 °C for 5 h
without any disposal.
3.1. Optimizing the reaction conditions of synthesis of phthalate plasticizers
by the novel synthesized ionic liquids
3.1.1. Investigation of the role of catalyst in the esterification reaction of
phthalic anhydride with butanol
The results of our experiments showed that the catalytic activity of
HFDAIL, for esterification of phthalic anhydride with butanol, was re-
markably better than SFDAIL (Table 1). This may be attributed to the
more acidity of HFDAIL and hydrophilic property of hydroxyl groups at-
tached to the cation. Two hydroxyl groups of this catalyst, can absorb
the produced water and thus enhance the reaction rate and product yield.
Therefore, initial studies demonstrated that HFDAIL is more efficient
catalytic system for this transformation. In this way, next optimization
studies were performed using HFDAIL.
Scheme 2. Two-step synthetic route of hydroxyl and sulfonic acid functionalized diacidic ionic liquids.

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R. Fareghi-Alamdari et al. / Journal of Molecular Liquids 227 (2017) 153–160
Table 1
Comparison the acidity and performance of SFDAIL and HFDAIL in the esterification reaction of phthalic anhydride with butanola.
Entry
1
Catalyst
H+ (mmol/g)b
1.0667
Time (h)
29
Yield (%)
30
2
2.266
7
60
a
Reaction conditions: The molar ratio of butanol to phthalic anhydride to catalyst = 5:1:0.1, reaction temperature = 75 °C.
The concentration of ILs is 1 mmol·L−1 in water.
b
3.1.2. Effect of the reaction temperature on esterification reaction
The effect of temperature was studied for the model reaction
(phthalic anhydride with butanol) and results are summarized in Fig.
2. Based on these results, no reaction product occurs at 25 °C. The
yield of product was enhanced by increasing the reaction temperature
and at temperatures less than 100 °C water evaporation rate is low, so
the equilibrium reaction returns to the reactants. It seems that by
reaching the reaction temperature to the boiling point of water, the
yield of product significantly improved.
It was observed that at higher temperatures, alcohol was quickly
evaporated and hence rising temperature above 125 °C does not affect
the reaction yield. The optimum reaction temperature for this synthesis
process is selected as 125 °C.
3.1.3. Effect of the molar ratio of reactants on esterification reaction
Increasing the amount of butanol promotes the reaction equilibrium
toward the product and this is a suitable way to enhance the rate of es-
terification. As shown in Fig. 3, the reaction yield is enhanced by increas-
ing the molar ratio of butanol to phthalic anhydride. No much
improvement in reaction yield was observed for molar ratio of phthalic
anhydride to butanol 1:7.5. This observation is attributed to the de-
crease in the concentration of the catalyst. The optimum molar ratio of
phthalic anhydride to butanol is suggested to be 1:5.
Fig. 2. Results of esterification of phthalic anhydride with butanol at different
temperatures with (HFDAIL) as catalyst. The molar ratio of butanol to phthalic
anhydride to catalyst = 5:1:0.1.
3.1.4. Effect of amount of catalyst on esterification reaction
To investigate the optimum amount of HFDAIL, different quantities
of the catalyst were tested. The yield of product was improved with in-
creasing the catalyst loading (Table 2, entries 1, 2). The best result was
obtained in the presence of 10 mol% of catalyst (Table 2, entry 2). Higher
Table 2
Results of esterification of phthalic anhydride with butanol in the presence of different ra-
tios of HFDAIL to phthalic anhydridea.
Entry
Catalyst (mol%)
Time (h)
Yield (%)
1
2
3
4
5
10
7
8
80
95
55
50
10
15
20
7
Fig. 3. Results of esterification of phthalic anhydride with different amounts of butanol in
the presence of HFDAIL as catalyst. The molar ratio of HFDAIL to phthalic anhydride =
1:10, reaction temperature = 125 °C.
a
Reaction coditions: The molar ratio of phthalic anhydride to butanol = 1:5 reaction
temperature = 125 °C.

R. Fareghi-Alamdari et al. / Journal of Molecular Liquids 227 (2017) 153–160
157
Table 3
Results of dialkyl phthalate, maleate and succinate diesters synthesis using different alcohols in presence of HFDAIL as catalysta.
Entry
1
Anhydride or Acid
Alcohol
CH3OH
Time (h)
24
Yield (%)
40
50
80
95
100
60
60
2
3
4
5
6
7
24
10
7
2
6
6
8
9
4
2
95
9
98
10
100
11
12
10
2
90
100
13
14
3
4
100
–
15
4
–
a
Reaction coditions: Reflux, anhydride or acid: alcohol: HFDAIL = 1:5:0.1.
molar ratios of the catalyst will not further improve in the reaction
(Table 2, entries 3, 4).
Succinic acid compared with phthalic and maleic anhydride has fewer
steric hinderance in the reaction center (carbonyl carbon) and afforded
better results in the presence of 1-butanol and allyl alcohol (Table 3, en-
tries 12, 13). The produced esters were immiscible with HFDAIL and
were separated by extraction with EtOAc. It should be mentioned that
HFDAIL catalyst was reusable after removal of water.
3.2. Esterification reaction of succinic acid, phthalic and maleic anhydrides
with various alcohols using HFDAIL
In order to study the generality of the protocol in esterification process
for synthesis of diverse diester plasticizers, maleic, phthalic anhydride and
succinic acid were reacted with different alcohols and results are summa-
rized in the Table 3. Results show that in esterification reaction of phthalic
and maleic anhydrides with alcohols, esterification of long alkyl chain al-
cohols are very satisfactory (Table 3, entries 1–5 and 8–10). However allyl
alcohol and 2-methoxy ethanol did not act same as previous entries.
3.3. Reusability of HFDAIL in esterification of phthalic anhydride
The reusability of HFDAIL was also evaluated in the esterification of
phthalic anhydride with butanol (Fig. 4). As shown, HFDAIL could be
recycled and reused for 7 runs without significant loss in its catalytic
activity.

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3.4. Comparison of the efficiency of synthesized ionic liquids with reported
catalysts in the esterification of phthalic anhydride.
Performance of the catalysts of this work in esterification reaction
has been compared with other reported catalysts (Table 4). The data
in Table 4 show that the catalytic activity of catalysts in this work, espe-
cially HFDAIL, was better than the other catalytic systems even H2SO4
(Table 4, entry 8) for the esterification reaction. Because these catalysts
have high acidity and moreover, HFDAIL has two hydroxyl groups,
which can absorb the produced water by hydrogen bonding (Scheme
4). So, the esterification reaction equilibrium will shift to the right and
the product yield will increase.
3.5. Reaction mechanism
The proposed reaction mechanism of phthalic anhydride with alco-
hol is shown in Scheme 5. As shown, first, the carbonyl group is activat-
ed by adsorption of H+ from the catalyst (HFDAIL). In the next step,
Fig. 4. Reusability of HFDAIL in the esterification reaction of phthalic anhydride with
butanol under optimized reaction conditions.
Table 4
Comparison of the efficiency of FDAILs of this work with other reported catalysts in the esterification of phthalic anhydride with butanol.
Entry
1
Catalyst
The molar ratio of PhA/BuOH
1:2.5
T (°C)
Time (h)
1.5
PhA conv. (%)
98
Yield (%)
Ref.
[31]
120–135
–
2
3
4
H6P2W18O62
H4Si MoO40
1:2
1:2
1:2.5
110
110
120–135
4
4
12
91
49
63
–
–
–
[32]
[32]
[31]
5
6
7
8
9
Phosphinite ionic liquids
Sulfamic acid
Amorphous zirconium titanium phosphate
H2SO4
1:2.1
1:2.5
1:2.5
1:5
100
130–180
115
125
125
3.5
4
10
4
78
84
–
93
100
–
–
62.4
84
98
[34]
[33]
[35]
This work
This work
1:5
4
10
1:5
125
8
100
70
This work
N
N
N
N
HSO4
HSO4
O
OH
OH
O
OR
OR
HFDAIL
H
ROH
OH
O
O
H
O
Scheme 3. Adsorption of water by HFDAIL in esterification reaction.

R. Fareghi-Alamdari et al. / Journal of Molecular Liquids 227 (2017) 153–160
159
H
OH
O
ROH
O
O
OR
OH
O
O
O
START
O
OR
OH
N
N
O
O
O
N
N
O
OR
OR
HSO4
HSO4
OR
OH
O
OH
ROH
O
HO
HO
[HFDAIL]
O
OR
OR
H2O
OR
OH
H
o
O
HO
H
R
RO
HO
RO
OH
O
OR
OH2
OR
O
OH
Scheme 4. The proposed reaction mechanism of the synthesis of phthalates from phethalic anhydride in the presence of HFDAIL.
nucleophilic attack of alcohol on the activated carbonyl group is done.
Finally, by nucleophilic attack of the second alcohol and releasing H2O,
diester is synthesized. As no monomer molecule is synthesized in this
reaction; we can conclude that the first nucleophilic attack step is
done very fast. It is noteworthy to mention that, the presence of OH
groups in the structure of the catalyst, accelerate the esterification reac-
tion due to the formation of hydrogen bonds and adsorption of the re-
leased water molecules.
Engineering of the Malek-Ashtar University of Technology, for her help-
ful comments.
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Acknowledgements
The financial support of the Malek-Ashtar University of Technology
(MUT) through the project is gratefully acknowledged. We would like
to thank Dr. Negar Zekri from Department of Chemistry and Chemical

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