Sulphonic Acid-Functionalized Benzimidazolium Based Poly Ionic Liquid Catalyzed Esterification of Levulinic Acid

Article abstract of DOI:10.1007/s10562-017-2284-1

Abstract: Esters of levulinic acid (LA) are bio-based compounds having widespread applications and have the remarkable potential to be blended with commercial diesel or gasoline. In this study, a sulphonic acid-functionalized benzimidazolium based poly ionic liquid (SAFBPIL) catalyst was prepared and characterized by NMR, FT-IR, elemental analysis (C, H, N, S), TGA and BET surface area. Br?nsted acidity was determined by anion-exchange/neutralization titration method and was found to be 5.55?mmol/g. The heterogeneous ionic liquid catalyst showed an excellent thermal stability up to 270?°C. Experiments were carried at different temperatures, LA to ethanol molar ratios, catalyst loadings and duration of the reaction in a batch reactor. The highest conversion was obtained at 70?°C using an initial LA:ethanol molar ratio of 1:10 and a catalyst loading of 15?wt% in 9?h. It was observed that ethyl levulinate (EL) could be effectively obtained over SAFBPIL with a selectivity higher than 99.5%. This is due to more acidic sites exhibited on SAFBPIL catalyst under optimized conditions. Herein, we are reporting for the first time an esterification of LA to EL using a new SAFBPIL as a catalyst. The catalyst was recycled for five runs without significant loss of catalytic activity.

Full text of DOI:10.1007/s10562-017-2284-1

Catalysis Letters
https://doi.org/10.1007/s10562-017-2284-1
Sulphonic Acid-Functionalized Benzimidazolium Based Poly Ionic
Liquid Catalyzed Esterification of Levulinic Acid
Avinash Ganesh Khiratkar1 · Kamlesh Rudreshwar Balinge1 · Manikandan Krishnamurthy1 · K. K. Cheralathan1 ·
Dipesh S. Patle2 · Vishal Singh2 · Sanyam Arora2 · Pundlik Rambhau Bhagat1
Received: 3 November 2017 / Accepted: 17 December 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
Esters of levulinic acid (LA) are bio-based compounds having widespread applications and have the remarkable potential to
be blended with commercial diesel or gasoline. In this study, a sulphonic acid-functionalized benzimidazolium based poly
ionic liquid (SAFBPIL) catalyst was prepared and characterized by NMR, FT-IR, elemental analysis (C, H, N, S), TGA and
BET surface area. Brønsted acidity was determined by anion-exchange/neutralization titration method and was found to
be 5.55 mmol/g. The heterogeneous ionic liquid catalyst showed an excellent thermal stability up to 270 °C. Experiments
were carried at different temperatures, LA to ethanol molar ratios, catalyst loadings and duration of the reaction in a batch
reactor. The highest conversion was obtained at 70 °C using an initial LA:ethanol molar ratio of 1:10 and a catalyst loading
of 15 wt% in 9 h. It was observed that ethyl levulinate (EL) could be effectively obtained over SAFBPIL with a selectivity
higher than 99.5%. This is due to more acidic sites exhibited on SAFBPIL catalyst under optimized conditions. Herein, we
are reporting for the first time an esterification of LA to EL using a new SAFBPIL as a catalyst. The catalyst was recycled
for five runs without significant loss of catalytic activity.
Graphical Abstract
Keywords Sulphonic acid-functionalized · Levulinic acid · Benzimidazolium · Poly ionic liquid · Esterification · Brønsted
acid
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-017-2284-1) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0123456789)
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A. G. Khiratkar et al.
groups [40] and silicas [41, 42]. For example, Wang et al.
[43] had reported recyclable green solid catalysts for
esterifications from solid organic–inorganic hybrid mate-
rials by combining polystyrene functionalized IL-forming
organic cations with Keggin-structured heteropolyanions.
A novel and environmentally benign heterogeneous cata-
lytic method were developed [44] for the direct conversion
of carbohydrates biomass into methyl levulinate (ML) as
biofuels and/or high-value chemicals. The heterogeneous
catalyst, (SO42−/TiO2) was easily recovered by filtration
and reused for the next run after calcination. In the same
year, an effective catalytic protocol was identified for the
practicality of converting glucose to EL using a series of
sulphated solid acid catalysts [30]. Bokade and Dharne
[45] have described a promising HPA supported on acid-
treated clay montmorillonite (K10) for the synthesis of
n-butyl levulinate by esterification of LA with n-butanol.
The catalyst was found to be the most effective with 97%
LA conversion and 100% selectivity to n-butyl levulinate.
The synthesis of ethyl levulinate (EL) from the esterifi-
cation of LA with ethanol was achieved under batch condi-
tions at 353 K using a bio-glycerol derived carbon –SO3H
catalyst [46]. The reusable catalyst assigned high catalytic
performance to Brønsted acidity of the –SO3H groups and
also supports the esterification of LA with other alcohols.
Another research group [47] tested ZrO2/SBA-15 catalysts
with different ZrO2 loadings for the esterification of LA in a
fixed-bed reactor in vapor phase conditions at atmospheric
pressure. The catalyst with 7 wt% ZrO2 securing more num-
ber of moderate acidic sites showed 100% conversion of
LA and 96% selectivity to ML. Zheng and his co-workers
[48] fabricated a novel catalyst by immobilizing 12-sili-
cotungstic acid (H4SiW12O40, HSiW) in the mesoporous
molecular sieves MCM-41 using an open synthesis method
without hydrothermal process. The HSiW/MCM-41 cata-
lysts maintained hexagonal mesoporous features of MCM-41
and the usual Keggin structure of bulk HSiW and displayed
admirable catalytic activity with 81.2% methyl oleate with
recyclability.
1 Introduction
Even today, researchers are continuing to devote more
attention on sustainable methods for the production
of fuels, fuel additives and chemicals from renewable
resources due to the depletion of non-renewable resources.
In this respect, lignocelluloses have been recognized as
valuable resources for fuels, fuel additives and chemicals
[1, 2]. Out of various specialty chemicals derived from the
hydrolysis of cellulose, LA is given major significance due
to its broad range of applications in flavoring agents, plas-
ticizers, solvents and fragrances etc. as well as in the pro-
duction of chemicals, fuels and fuel additives [3, 4]. LA
containing both highly reactive functional groups [5] has
become an inexpensive renewable resource in recent times
for the synthesis of many value added chemicals such as
1,4-pentanediol, 2-methyl tetrahydrofuran-valerolactone,
γ-Valerolactone [6–8] angelica lactone (α, β), δ-amine LA,
acrylic acid, β-acetyl acrylic acid and levulinate esters [9,
10]. These valuable compounds can be obtained by differ-
ent methods like esterification, oxidation, hydrogenation
and reductive amination etc. In particular, esters of LA
are valuable compounds which are used as solvents, plas-
ticizers and fuel additives. Moreover, EL is suitable due
to its diesel miscibility. In order to reduce the dependence
on petroleum-derived fuels, researchers are concentrating
on the esterification of LA by using various acid catalysts
[11–16]. The alkyl levulinate has been usually achieved
by inorganic acids like H2SO4 [17–21] or homogeneous
Brønsted acid catalysts [22–24].
Likewise, the literature on butyl levulinate (BL) synthe-
sis is sparse: first attempts at synthesis were undertaken
with homogeneous catalysts [25] and a kinetic model for
the esterification of LA with butanol (BuOH) was pro-
posed by Bart et al. [22]. Some effort has been made in
BL synthesis directly from cellulose with homogeneous
catalysts. The main drawbacks associated with the use of
these acids are their corrosiveness, lack of reusability, neu-
tralization and difficulty in the separation after the reac-
tion. Therefore, the replacements of homogeneous with
heterogeneous catalysts are beneficial due to ease of pro-
cessing after the reaction, simple separation and recycla-
bility. A number of heterogeneous catalytic systems have
been reported in the literature [26–30] and although the
sulphated catalysts were found to be efficient, leaching of
the catalyst was noticed.
Recently, liquid-phase synthesis of BL was described [49]
by using a set of 0.8% acidic ion exchange catalyst resins
at 80 °C and 2.5 MPa in a batch reactor with an initial AL/
BuOH molar ratio of 1:3. It was observed that BL could
be effectively attained over ion-exchange resins with selec-
tivity greater than 99.5%. Very recently, a series of metal-
zeolite hybrids was obtained by impregnation and deposi-
tion–precipitation methods and characterized with various
techniques [34]. Acid–base bifunctional zirconia-zeolite
hybrid ZrY6(0.5) with high stability and porosity demon-
strated superior activity towards the production of ML from
carbohydrates. Moreover, the catalyst could be reused for at
least five times with stable conversion rates and ML yields.
Shagufta et al. [50] have stated strong evidence in their
Among the various heterogeneous catalysts, solid acid
catalysts have been tested including supported heteropo-
lyacid (HPA) [31], metal organic framework [32], zeolites
[33], metal-based catalyst [34–37], zirconia [38], graphene
oxide based [19], sulfated carbon nanotubes [39], star-
bon mesoporous materials functionalized with sulfonated
1 3

Sulphonic Acid-Functionalized Benzimidazolium Based Poly Ionic Liquid Catalyzed…
review about esterification and transesterification of alco-
hols with carboxylic acids using several heterogeneous sul-
fonic acid-functionalized solid acid catalysts in the field of
organic transformation. These testified catalysts are ecologi-
cally benevolent and cleaner than traditional homogeneous
catalysts. The scope of these catalysts makes the industrial
procedures easier and cleaner by an increasing recyclabil-
ity; surface area and sulfonic acid content. A novel Brønsted
acidic polymeric hybrid catalyst was invented by coupling
task-specifically designed SO3H-functionalized polymeric
ionic liquid with Keggin-structured heteropolyanion [51].
The heterogeneous polymeric hybrid was shown to be a
highly competent solid catalyst for numerous esterification
reactions contributing to the advantages of easy retrieval and
steady recycle. Among a variety of heterogeneous Brønsted
acidic catalysts, polymer supported ionic liquids consisting
of sulfonic acid groups are of great interest [52].
n-Nonane (Internal standard) was obtained from TCI chemi-
cals (India) PVT Ltd. Sodium hydride (60% suspension in
paraffin oil), benzimidazole, benzyl chloride and sulphuric
acid (98%) were purchased from S D Fine chemicals India.
Ethanol used was procured from Changshu Hongsheng fine
chemical Co., Ltd. Other solvents used were of commercial
grade and used as supplied unless it is specified.
2.2 Instrumentation
NMR spectra were recorded in DMSO-d6 and CDCl3 on
Bruker spectrometer operating at 400 MHz with TMS as
an internal standard. The TGA analysis of the SAFBPIL
catalyst was obtained in the range of 30–800 °C with an
increase in temperature at the rate of 10 °C min−1 in a N2
atmosphere on TG/DTA thermoanalyzer SII, 7200 (Seiko,
Japan). FT-IR spectra were recorded on IR affinity-1 Shi-
madzu FT-IR spectrophotometer using KBr pellet method.
EL in the reaction products was analyzed on a GC (Shi-
madzu GC-2010 plus instrument) equipped with a column
model Rtx-1 (crossbond 100% dimethylpolysiloxane) with
a flame ionization detector. Elemental analysis (C, H, N and
S) was performed on the Elementar Vario EL III model.
Brunauer–Emmett–Teller (BET) surface area was meas-
ured at the temperature of 77 K liquid N2 using a Quanta
chrome nova-1000 instrument analyzer. Specific surface
area, average pore diameter and pore volume are measured
using N2 adsorption–desorption isotherms. Before analysis,
the sample was degassed at 250 °C for 5 h under vacuum
for the removal of moisture and unwanted adsorbents from
the sample.
Conversely, to the best of our knowledge very few works
reported the applications of polymer-supported IL catalysts
for biofuel production from esterification reaction. In our
latest work [53] we have reported the catalytic performance
of polymer-supported benzimidazolium based ionic liquid
catalyst for the synthesis of 3,4-dihydropyrimidine-2(1H)-
one/thione derivatives. Presently, commercially available
heterogeneous Brønsted acid catalysts mainly include ion-
exchange resin and ZSM-5. The ion-exchange resin is not
durable and long-lasting at high temperature due to its rela-
tively poor thermal stability. Esterification of the carboxyl
group is comparatively sluggish and needs initiation either
by high temperature or by a catalyst to attain equilibrium
conversion in a practical time. Owing to an existence of
these complications, it is particularly significant and neces-
sary to cultivate an environmentally benevolent catalyst with
high activity for the direct production of alkyl levulinates.
Accordingly, this paper is dedicated to the synthesis of sul-
phonic acid functionalized poly ionic liquid and to evaluate
the catalytic activity in liquid phase synthesis of EL by the
reaction of LA with ethanol in a batch process. Moreover,
the effects of various process parameters comprising cata-
lyst loading, reaction time, reaction temperature and LA to
ethanol molar ratio as well as catalyst reusability towards the
efficiency of reaction were investigated.
2.3 Synthesis of Sulphonic Acid Functionalized Ben-
zimidazolium Based Poly Ionic Liquid (SAFBPIL)
2.3.1 Synthesis of 1-(4-Vinylbenzyl)-1H-benzimidazole (1)
1-(4-Vinylbenzyl)-1H-benzimidazole was synthesized
according to our previously reported procedure [54]. The
detailed procedure and the corresponding spectral data are
given in the supplementary information (Yield: 9.36 g,
80%).
2.3.2 Synthesis of 3-Benzyl-1-(4-vinylbenzyl)-1H-benzimi-
dazolium chloride (2)
2 Experimental
2.1 Materials
In 250 mL two neck round bottom flask 1-(4-vinylbenzyl)-
1H-benzimidazole (11.7 g, 50 mmol) and acetonitrile
(150 mL) were allowed to stir for 10 min at 80 °C and benzyl
chloride (6.93 g, 55 mmol) was added drop-wise under stir-
ring and continued for 48 h. After completion of the reaction,
the mixture was cooled to room temperature, and the dirty
white solid compound 2 formed was separated by removal of
4-Vinylbenzyl chloride (90%) containing 500 ppm of tert-
butyl catechol was obtained from Sigma-Aldrich India. Lev-
ulinic acid (LA) 98% and Azobisisobutyronitrile (AIBN)
(98%) were obtained from Avra synthesis (India) PVT
Ltd. AIBN was recrystallized from methanol before use.
1 3

A. G. Khiratkar et al.
acetonitrile with a rotary evaporator. The separated product
was washed repeatedly with ethyl acetate to get white amor-
phous powder of 3-benzyl-1-(4-vinylbenzyl)-1H-benzimi-
dazolium chloride, filtered and dried in an oven at 50 °C for
10 h (Yield: 15 g, 83.33%). 1H NMR (400 MHz, DMSO-d6)
δ: 10.38 (s, 1H), 7.98 (d, 2H), 7.63 (d, 2H), 7.53 (s, 5H),
7.42 (d, 3H), 6.76 (dd, 1H), 5.88 (s, 1H), 5.83 (s, 4H), 5.30
(d, 1H). 13C NMR (100 MHz, DMSO-d6) δ: 142.82, 137.49,
135.86, 133.95, 133.34, 130.98, 128.91, 128.66, 128.23,
126.69, 126.60, 155.20, 113.98, 49.92, 49.70.
2.3.4 Synthesis of SAFBPIL (4) by Sulphonation
of Poly[3-benzyl-1-(4-vinylbenzyl)-1H-benzimidazo-
lium chloride] (3)
Synthesis of SAFBPIL was carried out by adapting the
method proposed by Kore and Shrivastava [55]. In 100 mL
round bottom flask, poly(3-benzyl-1-(4-vinylbenzyl)-
1H-benzimidazolium chloride) (3) (10 g) was suspended in
water and stirred for half an hour at ice-cold condition. To
this, an excess of H2SO4 (16.66 g, 170 mmol) was added
drop-wise and stirred for half an hour. The resulting reaction
mixture was then stirred at 70–80 °C for 12 h. The yellowish
SAFBPIL solid was obtained by washing viscous reaction
mass with deionized water several times followed by drying
overnight at 50 °C for 12 h (Scheme 1). The formation of
SAFBPIL catalyst was confirmed by negative silver nitrate
2.3.3 Synthesis of Poly(of 3-ben-
zyl-1-(4-vinylbenzyl)-1H-benzimidazolium chloride)
(3)
3-Benzyl-1-(4-vinylbenzyl)-1H-benzimidazolium chloride
(2) (12 g, 33.33 mmol) was suspended in 120 mL mixture
of acetonitrile and methanol (10:2) followed by charging of
4 mol% AIBN (0.328 g, 2 mmol). The polymerization was
carried out at 70 °C for 48 h under N2 atmosphere. After this
period, the solvents were removed by rotary evaporator and
the resultant viscous yellow liquid dried in vacuum for half
an hour. It is further dried in an oven at 50 °C for 12 h to
get yellowish crystalline polymer. Yield: (10 g, 87.5%). The
NMR spectrum of compound 3 is given in supplementary
1
test for chloride anion (Fig. S11). H NMR (400 MHz,
DMSO-d6) δ: 10.38 (s, 1H), 7.98 (d, 2H), 7.63 (d, 2H),
7.53 (s, 5H), 7.42 (d, 3H), 6.76 (dd, 1H), 5.88 (s, 1H), 5.83
(s, 4H), 5.30 (d, 1H). 13C NMR (100 MHz, DMSO-d6) δ:
142.82, 137.49, 135.86, 133.95, 133.34, 130.98, 128.91,
128.66, 128.23, 126.69, 126.60, 155.20, 113.98, 49.92,
49.70. Anal. Calcd for C22H28N2O7S2 (%): C, 56.37, H, 5.30,
N, 5.26, S, 12.04. Found (%): C, 58.31, H, 5.75, N, 6.32, S,
7.30.
1
information (Fig. S4). H NMR (400 MHz, DMSO-d6) δ:
10.38 (s, 1H), 7.98 (d, 2H), 7.63 (d, 2H), 7.53 (s, 5H), 7.42
(d, 3H), 6.76 (dd, 1H), 5.88 (s, 1H), 5.83 (s, 4H), 5.30 (d,
1H). 13C NMR (100 MHz, DMSO-d6) δ: 142.82, 137.49,
135.86, 133.95, 133.34, 130.98, 128.91, 128.66, 128.23,
126.69, 126.60, 155.20, 113.98, 49.92, 49.70.
2.4 Esterification of LA with Ethanol
LA esterification was carried out in a two-neck round-
bottom flask with a reflux condenser kept at 70 °C for the
specified period of time. The catalytic assessments were
Scheme 1 Schematic repre-
Cl
sentation of the synthesis of
H
N
ACN, 80 o
48 h
C
sulphonic acid functionalized
NaH, THF
50 oC, 48 h
poly ionic liquid catalyst
N
Cl
N
N
N
N
Cl
(2)
(1)
ACN:MeOH
(10:2)
70 oC,
AIBN
48 h, N2 atm.
n
n
Excess H2SO4
Cl
N
HSO4
SO3H
N
70-80 oC, 12 h
N
N
(3)
(4)
1 3

Sulphonic Acid-Functionalized Benzimidazolium Based Poly Ionic Liquid Catalyzed…
performed by using LA (5 mmol) and ethanol (50 mmol)
(LA:ethanol molar ratio 1:10) under stirring at 500 rpm and
15 wt% (87 mg) of catalyst (Scheme 2).
TGA analysis. The presence of olefinic protons in compound
1 and 2 observed between 5.74 and 6.70 and 5.30–6.76 ppm
respectively. However, 1H NMR of compound 3 revealed a
complete disappearance of these signals corresponding to
the olefinic protons as shown in Fig. 1 and the appearance of
the broad signal between 0.77 and 1.43 ppm due to the poly-
meric backbone of aliphatic protons (compound 3). Justifica-
tion of the monomer ratio by 1H NMR was yet difficult at
this step since the polymer (polystyrene) backbone protons
overlapped with other signals in the spectrum. The broad-
Upon completion of reaction, the mixture was allowed to
cool to room temperature. The heterogeneous catalyst was
separated and n-nonane was added to the reaction mixture as
an internal standard. EL formed in the reaction was analyzed
on a GC (Shimadzu GC-2010 plus instrument) equipped
with a column model Rtx-1 (crossbond 100% dimethylpo-
lysiloxane) with dimensions of 30.0 m×0.32 mm×0.5 μm
and a flame ionization detector operating at max prog. temp.
350 °C. The carrier gas was nitrogen with a flow rate of
0.5 mL/min. The conversion of LA was calculated based on
the following formula,
1
ness of H NMR spectrum of compound 4 confirmed the
sulphonation of compound 3 and also the peak at 3.75 ppm
in SAFBPIL exhibited due to –OH of sulphonic acid.
The FT-IR spectrum of SAFBPIL (Fig. 2C) showed a
broad band of 3600–3300 cm−1 which is ascribed to –OH
groups of –SO3H and water molecules adsorbed on the
surface. The broadness of band at 3412 cm−1 indicated the
presence of –OH [56] group in the sample (Fig. 2c) while in
FT-IR spectra of Fig. 2a, b, the peak corresponding to –OH
does not exist. The intense peaks at 2926 and 2968 cm−1 are
characteristic to aliphatic –C–H and 3120 cm−1 is attributed
to the aromatic=C–H group in the spectrum of SAFBPIL.
The region around 574, 1016 and 1182 cm−1 relates to the
S–O symmetric vibration and S=O asymmetric and symmet-
ric stretching frequency of –SO3H group separately [57, 58].
Initial area of LA − Final area of LA
Conversion of LA (%) =
× 100
Initial area of LA
3 Results and Discussion
3.1 Characterization of SAFBPIL
The synthesized SAFBPIL catalyst was confirmed by various
characterization techniques such as NMR, FT-IR, BET and
Scheme 2 Esterification of LA
with ethanol using SAFBPIL
catalyst
Fig. 1 1H NMR overlay of com-
pound 1, 2, 3 and 4 (SAFBPIL)
1 3

A. G. Khiratkar et al.
0.0020
0.0015
0.0010
0.0005
0.0000
0
5
10
15
20
25
Pore diameter (nm)
Fig. 4 Pore diameter and pore volume of SAFBPIL
Fig. 2 Comparative FT-IR spectra of (a) compound 2, (b) compound
3 and (c) compound 4 SAFBPIL
the aqueous solution was filtered and titrated with 0.15 wt%
standardized NaOH solution [60]. The acidity of SAFB-
PIL was found to be 5.55 mmol/g which is higher than the
reported heterogeneous acidic catalyst [61].
Specific surface area and pore size distribution of the
SAFBPIL was measured using adsorption and desorption
isotherm of nitrogen molecules. The BET specific surface
area of synthesized SAFBPIL was found to be 2.75 m²/g
(Fig. 3). The obtained specific surface area was compara-
tively greater than that of the reported method [53]. The total
pore volume and average pore diameter of SAFBPIL were
0.004 cm3/g and 3.53 nm respectively (Fig. 4).
3.3 Optimization of Reaction Parameters
3.3.1 Effect of Catalyst Loading
The influence of catalyst loading on esterification was stud-
ied by changing the amount of SAFBPIL catalyst from 5
to 20 wt%. Initially, esterification was performed without
catalyst that yielded around 26% conversion of the LA. The
result indicated that the rate of esterification of LA was sig-
nificantly influenced by the amount of catalyst and the opti-
mum dose was observed at 15 wt% of the catalyst with 99%
conversion. An increase in the amount of catalyst provided
more sulphonic acid sites on the catalyst surface for accel-
erating the esterification which in turn enhanced the rate of
reaction significantly (Fig. 5).
3.2 Determination of Acidity of SAFBPIL
It is well known that the sulphonic acid functionalized ionic
liquids containing HSO4− counter anion enhances the Brøn-
sted acidity of the catalyst [59]. Therefore we have synthe-
sized the SAFBPIL and the acidic site was determined by
using an ion-exchange/neutralization titration technique. In
this method, 300 mg of SAFBPIL was dried at 105–110 °C
for 1 h. The dried catalyst was suspended in 10 mL volume
of a 20 wt% NaCl solution in a conical flask and kept under
agitation at 150 rpm, for 24 h on a mechanical shaker. Then,
3.3.2 Effect of Reaction Time
3.0
2.5
2.0
1.5
1.0
0.5
The effect of reaction time on the esterification of LA with
ethanol is illustrated in (Fig. 6). The rate of conversion of LA
and selective formation of EL as the main product increases
with time up to 9 h. At this stage, the rate of conversion
remains more or less persistent till 24 h under the given
reaction conditions. The liquid phase esterification reaction
covers the surface of the solid oxide that leads to effective
use of catalyst [50].
3.3.3 Effect of Temperature
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P )
o
The influence of temperature on the rate of esterification
and selectivity of formation of EL was investigated by var-
ying temperature from 40 to 80 °C. Temperature is vital
Fig. 3 BET isotherm of SAFBPIL
1 3

Sulphonic Acid-Functionalized Benzimidazolium Based Poly Ionic Liquid Catalyzed…
110
100
90
80
70
60
50
40
30
20
100
90
80
70
60
50
40
30
20
40
50
60
70
°C
80
-
5
1o
15
20
Temperature
(
)
Amount of catalyst (wt%)
Fig. 7 Effect of temperature on LA conversion with ethanol over
SAFBPIL catalyst. Reaction conditions: dose 15 wt%, time 9 h and
LA:ethanol molar ratio (1:10)
Fig. 5 Effect of amount of catalyst on LA conversion with ethanol
over SAFBPIL catalyst. Reaction conditions: temperature 70 °C: time
24 h, LA: ethanol molar ratio (1:10)
100
90
80
70
60
50
40
100
80
60
40
20
0
5
10
15
Time (h)
20
25
1:6
1:8
1:10
1:12
1:14
LA:EtOH molar ratio
Fig. 6 Effect of reaction time on LA conversion with ethanol over
SAFBPIL catalyst. Reaction conditions: amount of catalyst 15 wt%,
temperature 70 °C and LA:ethanol molar ratio (1:10)
Fig. 8 Effect of LA to ethanol molar ratio on LA conversion with eth-
anol over SAFBPIL catalyst. Reaction conditions: dose 15 wt%, time
9 h and temperature 70 °C
with respect to rate of reaction, product conversion and
dissemination of heat energy. It was observed that raising
the temperature signifies positive impact on esterification
of LA using SAFBPIL catalyst. The conversion of LA to
EL improved from 27 to 99% as the temperature increased
from 40 to 70 °C (Fig. 7). This is due to the fact that higher
temperature provides more energy for the reactant molecules
to overcome activation energy. However, a further increase
in temperature has no significant effect on the esterification.
produce more EL. The gradual increase in the conversion
of LA to EL with respect to variation in the LA to ethanol
molar ratio is shown in Fig. 8. The ratio was varied from 1:6
to 1:14. The results illustrated that the highest conversion
(99%) of LA to EL was obtained at 1:10 molar ratio. On
the other hand, as the molar ratio was increased further to
1:14, no substantial progress was observed. Additionally, we
have performed the esterification reaction using isopropyl
alcohol and 1-pentanol under optimized conditions. How-
ever, isopropyl levulinate obtained with 16% conversion and
1-pentanol did not give satisfactory conversion.
3.3.4 Effect of LA to Ethanol Molar Ratio
Thus, it was found that an excess ethanol feed was pro-
ductive to a certain extent. The formation of EL dropped
from 99 to 97% which may be affected by excessively eth-
anol-diluted LA. This does not favor the reaction; conse-
quently, conversion of LA to EL has been dropped [38]. The
formation of EL was confirmed by 1H NMR and 13C NMR
The molar ratio of LA to ethanol is one of the most impor-
tant parameters that affect the esterification reaction. Stoi-
chiometrically, 1 mol of ethanol is required for each mole
of LA but in practice, the higher molar ratio is utilized in
order to drive the reaction towards good performance and
1 3

A. G. Khiratkar et al.
spectra (Figs. 9, 10). The plausible mechanism of esterifica-
tion of LA using SAFBPIL is given in the Scheme 3.
catalysts are employed. To examine the catalyst reusabil-
ity, the separated SAFBPIL catalyst was washed with ethyl
acetate (3 × 10 mL), filtered and dried in an oven for 12 h
and reuse for the next run. It was observed that less sig-
nificant changes in the conversion of LA to EL after each
cycle and the catalyst was reused for five successive runs
(Fig. 11). Whereas in normal esterification using H2SO4, we
could not recover and reuse the catalyst hence the present
3.3.5 Recyclability and Stability of the SAFBPIL
The reaction between LA and ethanol was carried out to
investigate the recyclability and deactivation of the hetero-
geneous catalyst which is equally essential when supported
Fig. 9 1H NMR spectrum of EL
Signature SIF VIT VELLORE
EL
Current Data Parameters
NAME
Dr.PDK190517
EXPNO
41
PROCNO
1
F2 - Acquisition Parameters
Date_
Time
INSTRUM
PROBHD
PULPROG
TD
20170520
1.08 h
spect
Z108618_0505 (
zg30
65536
CDCl3
16
SOLVENT
NS
DS
2
SWH
8012.820 Hz
FIDRES
0.244532 Hz
4.0894465 sec
19.37
AQ
RG
DW
62.400 usec
6.50 usec
299.9 K
DE
TE
D1
1.00000000 sec
1
TD0
SFO1
NUC1
P1
400.2604716 MHz
1H
14.25 usec
PLW1
14.00000000 W
F2 - Processing parameters
SI
65536
400.2580090 MHz
EM
SF
WDW
SSB
LB
0
0
0.30 Hz
1.00
GB
PC
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
ppm
Fig. 10 13C NMR spectrum
Signature SIF VIT VELLORE
EL
of EL
Current Data Parameters
NAME
Dr.PDK190517
EXPNO
42
PROCNO
1
F2 - Acquisition Parameters
Date_
Time
INSTRUM
PROBHD
PULPROG
TD
20170520
1.38 h
spect
Z108618_0505 (
zgpg30
65536
SOLVENT
CDCl3
NS
512
DS
4
SWH
24038.461 Hz
0.733596 Hz
1.3631488 sec
175.97
FIDRES
AQ
RG
DW
20.800 usec
6.50 usec
300.5 K
DE
TE
D1
2.00000000 sec
0.03000000 sec
1
D11
TD0
SFO1
NUC1
P1
100.6550186 MHz
13C
9.80 usec
58.00000000 W
400.2596010 MHz
1H
PLW1
SFO2
NUC2
CPDPRG[2
PCPD2
PLW2
PLW12
PLW13
waltz16
90.00 usec
14.00000000 W
0.35097000 W
0.17654000 W
F2 - Processing parameters
SI
32768
100.6449569 MHz
EM
SF
WDW
SSB
LB
0
0
1.00 Hz
1.40
GB
PC
200
180
160
140
120
100
80
60
40
20
0
ppm
1 3

Sulphonic Acid-Functionalized Benzimidazolium Based Poly Ionic Liquid Catalyzed…
Scheme 3 A plausible mecha-
nism for esterification of LA
to EL
LA
EL
99
120
90
60
30
0
98
C
100
95
Compound 2
92
88
Compound 3
B
Compound 4 (SAFBPIL)
80
60
40
20
A
0
1
2
3
4
5
Runs
0
200
400
600
800
Temperature (oC)
Fig. 11 Recyclability of SAFBPIL after esterification at 70 °C
Fig. 12 TGA spectra of (a) compound 2, (b) compound 3 and (c)
compound 4 (SAFBPIL)
methodology is advantageous over the normal esterification
method. Additionally, sulphuric acid is corrosive to nature.
Further, to test leaching of the catalyst, we have per-
formed Sheldon’s hot filtration test [62] which includes fil-
tration of the catalyst part way through a reaction of LA and
ethanol under optimized conditions, followed by a continu-
ation of the reaction with the filtrate. It was observed that
the reaction stopping completely after filtration confirmed
the inactive nature of filtrate. Accordingly, the hot filtration
test can be taken as definite evidence that leaching of active
catalyst did not occur during esterification.
temperature range (up to 100 °C) relates to the removal of
moisture content of sample while the weight loss (45.20%)
from 200 to 402 °C may be due to decomposition of ben-
zyl group on SAFBPIL. The final weight loss of 31.60%
was observed from 400 to 680 °C would correspond to the
decomposition of poly[1-(4-vinylbenzyl)-1H-benzimida-
zole] in the SAFBPIL. Since, a decrease in weight of the
SAFBPIL commenced from 270 °C, it could conclude that
the polymer is stable up to 270 °C. Additionally, thermogram
of ionic liquid monomer (compound 2) and poly ionic liquid,
prior to sulphonation (compound 3) have been studied. In
the thermogram of compound 2, the initial weight loss was
observed up to 203 °C (Fig. 12a) while thermal stability
of the compound 3 was noticed up to 232 °C (Fig. 12b).
From the thermogram (Fig. 12c) it could be recognized that
thermal stability of the SAFBPIL is more compared to the
To determine the thermal stability of catalyst, ther-
mogravimetric analysis (TGA) of the compound 2, 3 and
4 (SAFBPIL) was conducted in the range of 30–800 °C
with an increase in temperature at the rate of 10 °C min−1
in a N2 atmosphere. The TGA thermogram of SAFBPIL
(Fig. 12c) displayed a continuous decrease in weight from
100 to 800 °C. The initial weight loss (9.056%) at a lower
1 3

A. G. Khiratkar et al.
ionic liquid monomer (compound 2) and poly ionic liquid
(compound 3).
biofuels/oxygenated fuel additives was confirmed by blend-
ing 7.5 and 10% EL with real diesel sample and found to
acquire comparable properties as those of the real diesel.
Therefore, SAFBPIL catalyst can be considered as a com-
petent catalyst for the esterification of LA giving a substitute
of inorganic acid to the synthesis of EL.
The catalytic activity of SAFBPIL towards esterification
of LA to EL is better than or comparable with the literature
reports (Table 1). Moreover, the reactivity of SAFBPIL has
been compared with commercially available acidic Dowex®
50WX2 hydrogen form (Table 1, entry 1) and Amberlyst-15
(Table 1, entry 2). It was found that 84% of EL obtained
using optimized conditions by Dowex catalyst.
Acknowledgements We gratefully acknowledge SIF DST-VIT-
FIST, VIT-Vellore for providing NMR, GC–MS, FT-IR. The
authors also acknowledge the financial help provided by DST-SERB
(ECR/2016/001866) and ‘VIT SEED GRANT’, VIT-Vellore for car-
rying out this research work. The authors express gratitude to “Smart
Materials Laboratory for Bio-sensing and Catalysis” for providing
basic facilities.
To verify the potential application of EL as biofuels/
oxygenated fuel additives [3], we have blended diesel sam-
ples with 7.5 and 10% EL and compared the properties with
the real diesel sample. Among these, the cetane number,
kinematic viscosity @ 40 °C, calorific values (MJ/Kg) and
density of the blended samples were found to be comparable
with those of the real diesel sample while the flash point and
fire point of the blended samples were found to be better
with respect to the real sample (Table S1).
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Affiliations
Avinash Ganesh Khiratkar1 · Kamlesh Rudreshwar Balinge1 · Manikandan Krishnamurthy1 · K. K. Cheralathan1 ·
Dipesh S. Patle2 · Vishal Singh2 · Sanyam Arora2 · Pundlik Rambhau Bhagat1
2
* Pundlik Rambhau Bhagat
Department of Chemical Engineering, School
of Civil and Chemical Engineering, VIT, Vellore,
Tamil Nadu 632 014, India
drprbhagat111@gmail.com
1
Department of Chemistry, School of Advanced Sciences,
VIT, Vellore, Tamil Nadu 632 014, India
1 3