A novel hydrogen-bonded silica-supported acidic ionic liquid: An efficient, recyclable and selective heterogeneous catalyst for the synthesis of diesters

Article abstract of DOI:10.1007/s12039-018-1454-z

Abstract: In this study, two novel acidic ionic liquids, including a hydroxyl functionalized diacidic ionic liquid [HFDAIL] and a sulfonated diacidic ionic liquid [SFDAIL], were prepared and immobilized on the surface of silica nanoparticles (SNPs) via hydrogen bonding. The materials were characterized by FT-IR, NMR, SEM, nitrogen physisorption measurement, TGA and acid-base titration. The catalytic activity of the prepared catalysts was investigated in the synthesis of phthalate, maleate and succinate diesters under solvent-free conditions. It was found that nanosilica@[HFDAIL] with higher availability of acidic sites and higher hydrophilicity was more efficient compared to the nanosilica@[SFDAIL]. Notably, nanosilica@[HFDAIL] catalyst has also demonstrated excellent selectivity for the diester product while the monoester product was predominant in the case of nanosilica@[SFDAIL] even after prolonged reaction time or higher catalyst loading. In addition, the nanosilica@[HFDAIL] catalyst could be separated by simple filtration and reused several times without any significant loss of catalytic performance, but a remarkable decrease in activity was observed for nanosilica@[SFDAIL] in the next runs. GRAPHICAL ABSTRACT?: SYNOPSIS Two novel acidic ionic liquids, including a hydroxyl functionalized diacidic ionic liquid [HFDAIL] and a sulfonated diacidic ionic liquid [SFDAIL], were prepared and immobilized on the surface of silica nanoparticles via hydrogen bonding. The catalytic activity of the catalysts was investigated in the synthesis of diesters under solvent-free conditions.

Full text of DOI:10.1007/s12039-018-1454-z

J. Chem. Sci. (2018) 130:48
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1454-z
REGULAR ARTICLE
A novel hydrogen-bonded silica-supported acidic ionic liquid: An
efficient, recyclable and selective heterogeneous catalyst for the
synthesis of diesters
REZA FAREGHI-ALAMDARIa,∗, MEHRI NADIRI NIRIa and HASSAN HAZARKHANIb
aFaculty of Chemistry and Chemical Engineering, Malek-Ashtar University of Technology, Tehran, Iran
bCenter for Research and Science Education, Organization for Educational Research and Planning, Tehran, Iran
E-mail: reza_fareghi@yahoo.com
MS received 12 August 2017; revised 22 March 2018; accepted 22 March 2018
Abstract. In this study, two novel acidic ionic liquids, including a hydroxyl functionalized diacidic ionic
liquid [HFDAIL] and a sulfonated diacidic ionic liquid [SFDAIL], were prepared and immobilized on the
surface of silica nanoparticles (SNPs) via hydrogen bonding. The materials were characterized by FT-IR,
NMR, SEM, nitrogen physisorption measurement, TGA and acid-base titration. The catalytic activity of the
prepared catalysts was investigated in the synthesis of phthalate, maleate and succinate diesters under solvent-
free conditions. It was found that nanosilica@[HFDAIL] with higher availability of acidic sites and higher
hydrophilicity was more efficient compared to the nanosilica@[SFDAIL]. Notably, nanosilica@[HFDAIL]
catalyst has also demonstrated excellent selectivity for the diester product while the monoester product was
predominant in the case of nanosilica@[SFDAIL] even after prolonged reaction time or higher catalyst loading.
In addition, the nanosilica@[HFDAIL] catalyst could be separated by simple filtration and reused several times
without any significant loss of catalytic performance, but a remarkable decrease in activity was observed for
nanosilica@[SFDAIL] in the next runs.
Keywords. Supported ionic liquids; hydrogen bonding; phthalate diesters; selectivity; diesterification.
1. Introduction
these systems suffer from some limitations such as steric
hindrance of bulky support and its mass transfer prob-
lem, low interaction of reactants with active sites and
low catalyst distribution in the reaction media as well
as the leaching of active sites during the course of the
reaction. Consequently, there is much room to develop
more improved catalytic systems for esterification reac-
tion which may integrate the interesting features of
both homogeneous and heterogeneous systems.6–8 In
this context, the immobilization of acidic molecules on
the high surface area nono-sized supports is one of the
interesting research directions along the avoiding deter-
rent effect of bulky supports. Also, along this research
line, acidic ionic liquids as a key sub-class of ionic
liquid-tagged catalysts, with organic skeleton and thus
improvedinteractionwithsubstratesaswellasenhanced
acidity, haveattractedenormousattentioninrecentyears
in the acid catalyzed organic transformation.
Aliphatic and aromatic esters important organic com-
pounds, which are generally used as solvents, perfumes
and also as precursors for pharmaceuticals, agrochem-
icals and fine chemicals.1 Particularly, there has been
increasing interest for the synthesis of diester product
which is employed as plasticizers in many industrials
applications. Usually, esters are obtained by the esterifi-
cation of alcohols with carboxylic acids2 in the presence
of inorganic liquid acids,3 solid acids4 or bioenzymes5
as conventional catalysts. However, the application of
conventional homogeneous acid catalysts in the syn-
thesis of esters has been restricted today because of
the difficulty in the waste neutralization, separations,
reactor corrosion, and reusability. On the other hand,
the use of solid acid catalysts in this reaction has typ-
ically resulted in lower catalytic performance because
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1454-z) contains
supplementary material, which is available to authorized users.

48 Page 2 of 13
J. Chem. Sci. (2018) 130:48
Scheme 1. Esterification of phthalic anhydride with alcohols using synthesized supported ionic liquids.
The room temperature ionic liquids (RTILs) are envi- achievements using these systems in terms of activity,
ronmentally friendly solvents and catalysts, and have the main challenge of this transformation is the selec-
adjustablephysicalandchemicalproperties. Thesecom- tive formation of diester product which has not been
pounds have received broad attention of researchers addressed properly in the previous studies; almost all
from various fields such as synthesis, catalysis, sepa- catalytic systems gave monoester compound as the final
ration and electrochemistry.9–13 Their favorable prop- product. Due to importance and various applications of
erties, such as high thermal stability, negligible vapor phthalate diesters in different industries and in continu-
pressure, tunable acidity and strong dissolvability pro- ation of our studies on the synthesis and application of
vide good opportunities to ionic liquids for specific various ionic liquids,29–35 herein we report the synthesis
purposes.14–18 However, many of ionic liquids are rel- of two novel supported ILs, [HFDAIL] and [SFDAIL],
atively expensive or toxic thus employing the ionic viahydrogenbondingimmobilizationandcomparetheir
liquids in an unrecoverable form results in crucial eco- catalytic performance in the preparation of phthalate
nomic and environmental shortcomings. In addition, the diesters under solvent-free conditions (Scheme 1).
high viscosity of the ILs can induce mass transfer limi-
tations. If the chemical reaction is fast, only a minor part
of the IL contributes to the reaction, known as “diffusion
2. Experimental
layer”.19 In addition, product separation and catalyst
recovery were proven to be difficult in homogeneous
2.1 Material and methods
reactions.20 Therefore, the heterogenization of ILs on a
solid support is highly desirable and the concept of sup-
ported ionic liquid catalysis (SILC)21 has been recently
developed to combine the advantages of ILs and recov-
erable acid catalysts.22–26 Among the varied approaches
developed for the immobilization of ILs on a solid sup-
port, covalent immobilization has been considered as the
major method in the literature since it provides a versa-
tile and strong anchoring of IL to support thus avoids the
leaching of IL under reaction conditions. In this context,
less attention has been paid to other chemical immobi-
lization methods such as hydrogen bonding. Here, we
tried to develop novel supported acidic IL systems by
emphasizing on hydrogen bonding immobilization. We
believe that immobilization of IL via hydrogen bonding
may increase the accessibility of active sites and facili-
tate the immobilization process since it can be achieved
under milder reaction condition.
In recent years, the application of various immobi-
lized ionic liquids in esterification reaction has received
more consideration. In 2011, Miao et al., reported the
use of acidic IL on silica gel for esterification.27 In
2013, Shao et al., used an immobilized Brønsted acidic
IL catalyst on chloromethyl polystyrene grafted silica
gel for esterification reaction.28 Despite the remarkable
All the solvents and chemicals were commercially purchased
from Merck and Fluka and used without further purification.
IRmeasurementswereperformedonaNicolet800instrument
1
using KBr or liquid film. H NMR spectra were recorded
on a Bruker DRX500 (500 MHz) and 13C NMR spec-
tra on Bruker DRX500 (125 MHz) spectrometer in DMSO
and D2O, respectively, and calibrated with tetramethylsi-
lane (TMS) as the internal reference. Quantitative product
analysis was conducted by gas chromatography on Hewlett
Packard HP-5890 instrument 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 2 mL min−1
.
Scanning electron microscope (SEM) images of the samples
were examined on aphilips XL-30 ESEM instrument. The
N2 adsorption/desorption isotherms were measured using a
Micrometrics Tristar II 3020 instrument. The nanosilica sup-
port (Fumed silica S5130 purchased from Sigma Chemical
Co.) was dried at 250 ◦C for 6 h before use. All the solvents
used were strictly dried according to standard operations and
stored over 4 Å molecular sieves.
2.2 Preparation of supported IL catalysts
2.2a Preparation of hydroxyl and sulfonic acid func-
tionalized diacidic ionic liquids: 2.2.1a Preparation

J. Chem. Sci. (2018) 130:48
Page 3 of 13 48
of 2,2-bis ((3-methylimidazolidin-1-yl) methyl) propane-1,3- was dispersed in dry methanol (15 mL). The [HFDAIL] ionic
diol bromide salt (IL-Br): A mixture of 1-methylimidazole liquid (0.25 g) was dissolved in hot methanol (10 mL) and
(6.25 mmol, 0.5 mL) and 0.65 g of 2,2-bis (bromomethyl)- addedslowlyintothepreparedmixture. Afterthat, themixture
1,3-propanediol (2.5 mmol) was heated at 150 ◦C for 8 h was stirred at room temperature for 24 h. Upon completion
under magnetic stirring. After cooling to room temperature, of the process, the solvent (methanol) was evaporated under
the obtained solid was washed three times with acetonitrile reduced pressure. Finally, the resulting solid was washed with
◦
and the residue was dried in an oven at 100 C for 2 h, ◦giv- acetone (2 ×10 mL) and dried under vacuum at 70 ◦C for 5 h
ing IL-Br as a white powder (yield: 95.25 %, M.p.: 150 C). togivenanosilica-supportedhydroxyl-functionalizeddiacidic
1H NMR (DMSO-d6): δ 3.24 (4H, s, CH2N), 3.84 (6H, s, ionic liquid (nanosilica@[HFDAIL]) as a white powder.
CH3), 4.31 (4H, s, CH2OH), 4.72 (2H, s, OH), 7.42 (4H,
dd, J = 10 Hz, J = 5 Hz, CH = CH) and 8.72 (2H, s,
2.2c Immobilization of [SFDAIL] onto the silica nano-
NCHN) ppm; 13C NMR (DMSO-d6): δ 35.94, 45.21, 48.58,
particles: Silica nanoparticles were dried in an oven at
250 ◦C for 6 h. In a round bottom flask, 1.5 g of nanosil-
ica was dispersed in dry methanol (15 mL). The [SFDAIL]
ionic liquid (0.5 g) was dissolved in hot methanol (10 mL) and
addedslowlyintothepreparedmixture. Afterthat, themixture
was stirred at room temperature for 24 h. Upon completion
of the process, the solvent (methanol) was evaporated under
reduced pressure. Finally, the resulting solid was washed with
acetone (2 ×10 mL) and dried under vacuum at 70 ◦C for 5 h
to give nanosilica supported sulfonated diacidic ionic liquid
(nanosilica@[SFDAIL]) as a white powder.
ν
58.48, 123.72, 123.86 and 137.38 ppm; FT-IR (KBr)
:
max
621, 771, 830, 1061, 1165, 1451, 1573, 1760, 1656, 2935 and
3261 cm−1
.
2.2.1bPreparationof3,3’-(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 nucleophilic substitution reaction of IL-Br salt
with chlorosulphonic acid. To do this, 0.8 g of IL-Br salt
(2 mmol) was dissolved in acetonitrile. Then 0.5 mL of
chlorosulphonic acid (8 mmol) was added slowly at room
temperature. Then, the mixture was stirred for 4 h at the same
temperature. The reaction mixture was filtered to give yellow
precipitate. It was washed with toluene three times and dried
under vacuum at 100 ◦C for 1 h, giving SFDAIL as a yellow
2.3 Typical procedure for the esterification reaction
with nanosilica@[HFDAIL]
1
The typical procedure for esterification of phthalic anhy-
dride with butanol as an example, was performed as follows:
phthalic anhydride (1 mmol, 0.15 g), butanol (5 mmol, 0.46
mL, 0.37 g) and nanosilica@[HFDAIL] as catalyst (10 mol%
to phthalic anhydride, 0.05 g) were poured into a 50 mL round
bottom flask equipped with a dean-stark apparatus, reflux con-
denser and a magnetic stirrer. Then the mixture was stirred
at 125 ◦C for 8 h. The completion of reaction was monitored
by TLC using (EtOAC/Hexane 2:8) as eluent. After comple-
tion of the reaction as indicated by TLC, ethyl acetate was
added to dissolve the product and then the reaction mixture
was filtered to separate the catalyst from the reaction mixture.
Then, ethyl acetate was evaporated under vacuum to afford
the desired product as yellow oil liquid at 92% yield (0.25
g, B.p.: 340 ◦C). The obtained product (dibutyl phthalate)
solid (yield: 97.5%). H NMR (DMSO-d6): δ 3.33 (4H, s,
CH2N), 3.47 (6H, s, CH3), 4.04 (4H, s, CH2OH), 5.33 (2H,
s, SO3H), 7.06 (4H, d, J = 5 Hz, CH = CH) and 8.30 (2H, s,
NCHN) ppm; 13C NMR (DMSO-d6): δ 36.19, 43.83, 48.27,
ν
63.89, 123.88, 124.26 and 137.79 ppm; FT-IR (KBr)
:
max
587, 783, 1019, 1252, 1459, 1585, 1709 and 3160 cm−1
.
2.2.1.cPreparationof3,3’-(2,2-bis(hydroxymethyl)propane-
1,3-diyl) bis (1-methyl-1H-imidazole-3-ium) hydrogen sulfate
[HFDAIL]: The hydroxyl functionalized ionic liquid was pre-
pared by anion exchange 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 of
sulfuric acid (2.4 mmol) was added drop-wise and stirred for
24 h. Upon completion of the reaction, the solvent was evapo-
rated 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
◦
was subjected to drying under vacuum at 70 C for 5 h. The
solid catalyst was washed with acetone to remove the residual
product and dried under vacuum at 70 ◦C.
1
in 95% yield. H NMR (DMSO-d6): δ 3.20 (4H, s, CH2N),
3.81 (6H, s, CH3), 4.27 (4H, s, CH2OH), 4.74 (2H, s, OH),
7.39 (4H, dd, J = 10 Hz, J = 5 Hz, CH = CH) and 8.69
(2H, s, NCHN) ppm; 13C NMR (DMSO-d6): δ 36.73, 44.97,
49.63, 59.53, 124.31, 124.58 and 138.73 ppm; FT-IR (KBr)
3. Results and Discussion
3.1 Catalyst characterization
νmax
: 590, 776, 880, 1054, 1223, 1458, 1572, 1642, 2510,
For the preparation of supported acidic ILs, [HFDAIL]
and [SFDAIL] were initially synthesized using read-
ily available or pre-synthesized precursors. Then, the
prepared acidic ILs were immobilized onto the nanosil-
ica via hydrogen bonding of either sulfonic acid or
3096 and 3329 cm−1
.
2.2b Immobilization of [HFDAIL] onto the silica
nanoparticles: Silica nanoparticles were dried in an oven
at 250 ◦C for 6 h. In a round bottom flask, 0.75 g of nanosilica hydroxyl group in ILs with surface hydroxyl group of

48 Page 4 of 13
J. Chem. Sci. (2018) 130:48
amount of H+ loading in the catalysts are 1 mmol/g and
0.5 mmol/g, respectively.
3.1a FT-IR: To confirm the immobilization of ILs
on the nanosilica support, FT-IR spectra were recorded
(Figure 1). The characteristic peaks of pure ionic liquid
[SFDAIL], around 3160 cm−1, 1709 cm−1, 1584 cm−1
and 1458 cm−1 were clearly observed in Figure 1a,
which are attributed to the O-H stretching vibration
of the −SO3H group and C=N, C=C stretching vibra-
tions of the imidazole ring, respectively. The peaks at
1251 cm−1 and 1186 cm−1 were assigned to the S=O
asymmetric and symmetric stretching vibrations of the
−SO3H group. In the FT-IR spectra of the nanosilica
sample (Figure 1b), the Si-O stretching vibration of
the nanosilica could be observed as a strong peak at
Scheme 2. The prepared supported acidic ionic liquids via
hydrogen bonding.
nanosilica in methanol for 24 h at room temperature to 808 cm−1 and 1101 cm−1. Moreover, the peak centered
give nanosilica@[HFDAIL] and nanosilica@[SFDAIL] at 456 cm−1 was due to Si-O-Si bending vibration and
(Scheme 2).
After synthesis and immobilization of acidic ILs on Si-OH groups and adsorbed water in nanosilica.
the surface, the structure of the prepared catalysts were In the FT-IR spectra of nanosilica-supported acidic
a broad feature was seen at 3400 cm−1 belonging to the
analyzed by various techniques. Determining density of ILs (Figure 1c and 1d), apart from the main peaks
acidic groups in the nanosilica@[HFDAIL] and nanosil- of nanosilica, additional peaks attributed for the func-
ica@[SFDAIL] by titration with NaOH, showed that the tional groups of the [SFDAIL] and [HFDAIL] were also
Figure 1. FT-IR spectra of (a) pure ionic liquid [SFDAIL], (b) nanosilica, (c) nanosilica@[SFDAIL] and
(d) nanosilica@[HFDAIL]. The spectra are vertically shifted for clarity.

J. Chem. Sci. (2018) 130:48
Page 5 of 13 48
Figure 2. SEM images of (a) nanosilica, (b) nanosilica [HFDAIL] and (c) anosilica@[SFDAIL].
observed, but, as expected, the intensity of the peaks respectively. Furthermore, TG analysis revealed that
decreased significantly. This observation clearly con- despite the immobilization of ionic liquid on the support
firms that the both ILs have successfully supported onto via hydrogen bonding which is weaker than covalent
the surface of the nanosilica. As shown in Figures 1c bonding, ionic liquids are◦remarkably stable on the solid
and1d, theintensityofpeakcenteredatabout3400 cm−1 support up to about 200 C. Furthermore, as indicated
(corresponding to the Si-OH band and adsorbed water in DTG curves (Figure S1 a” and b”), maximum weight
◦
in nanosilica) in the nanosilica@[HFDAIL] is relatively loss in both materials is centered about 380 C. This
higher than that of nanosilica@[SFDAIL]. This obser- observation strongly confirms that major content of IL
◦
vation might be responsible for the higher hydrophilicity is still stable on the support up to 380 C (Figure S1 in
of nanosilica@[HFDAIL] than nanosilica@[SFDAIL]. Supplementary Information).
3.1d N2 adsorption-desorption: The specific surface
3.1b SEM: The shape and surface morphology of the
area and average pore diameter of the prepared materials
samples were characterized by SEM technique. SEM
were studied by the N2 adsorption-desorption isotherms
images for typical nanosilica samples before and after
(Figure 3). Results showed that both samples pure
ILs supporting are shown in Figure 2. It should be noted
nanosilica and nanosilica@[HFDAIL] possess type-IV
that the morphological homogeneity is maintained after
isotherms with a H3 hysteresis loop centered at rela-
supporting of ILs which demonstrates that the particles
tive pressure of about 0.4–0.8, confirming the ordered
of nanosilica have a good mechanical stability during
mesoporous structure of the materials (Figures 3a, 3b).
the supporting process. The SEM images also reveal
Also, according to BJH calculation, pore diameter for
that nanosilica support contains nanoparticles with the
the nanosilica and nanosilica@[HFDAIL] was 20 and
size of less than 50 nm (Figure 2a). Comparing the
18 nm, respectively, corroborating the mesoporous and
images of Figure 2a with 2b and 2c shows that no obvi-
regular pore structure of the materials (Figures 3c, 3d).
ous aggregation of the silica nanoparticles was detected
The data obtained from N2 adsorption-desorption exper-
after immobilization of ILs.
iments of pure nanosilica and nanosilica@[HFDAIL]
are summarized in Table 1, in which the Brunauer–
3.1c TG: TG analysis was conducted in the range
Emmett–Teller (BET) surface area of the supported
ionic liquid on nanosilica was found to be remarkably
lower than that of corresponding mesoporous nanosilica
(Figures 3e, 3f). These data indicate that the ionic liquid
was successfully immobilized onto the nanosilica.
◦
of 25−900 C under nitrogen atmosphere to investi-
gate the thermal stability and organic content of the
catalysts. As shown in Figure S1 (a and b) in Sup-
plementary Information, both materials demonstrated
remarkable weight loss above 200 ◦C, which corre-
sponds to the supported ILs on nanosilica. Loading of
ILs (wt%) was calculated as shown in Figure S1 in
Supplementary Information. Based on TG curves (Fig-
ure S1 a’ and b’), it was concluded that the amount of
3.2 Catalytic activity of supported ILs in
esterification reaction of different alcohols with
anhydrides or succinic acid
loaded ILs on the solid support is about 29% and 33% In order to evaluate the catalytic activity of the prepared
for nanosilica@[HFDAIL] and nanosilica@[SFDAIL], catalysts, the esterification of n-butanol with phthalic

48 Page 6 of 13
J. Chem. Sci. (2018) 130:48
Figure 3. N2 adsorption/desorption isotherms of (a) pure nanosilica and (b) nanosilica@[HFDAIL], BJH
plots of (c) pure nanosilica and (d) nanosilica@[HFDAIL], BET plots of (e) pure nanosilica and (f) nanosil-
ica@[HFDAIL].
anhydride was tested with nanosilica@[HFDAIL] and while the nanosilica@[HFDAIL] demonstrated com-
nanosilica@[SFDAIL] under solvent-free condition at plete conversion and excellent selectivity for diester
◦
120 C. As can be seen in Table 2, entries 1 and 2, compound, lower conversion (even after prolonged

J. Chem. Sci. (2018) 130:48
Page 7 of 13 48
Table 1. Results of N2 adsorption-desorption mea-
surements of nanosilica and nanosilica@[HFDAIL].
expelled from the channels until it is converted to the
morehydrophobicdiesterproduct. Notably, thecatalytic
performance of nanosilica@[HFDAIL] was compara-
ble, even higher, than that of homogeneous HFDAIL in
both terms of activity and selectivity (Table 2, Entries
1 and 4). This may be a strong evidence to the fact
that, being the counter ion of IL, acidic groups in
the nanosilica@[HFDAIL] is completely exposed and
available to the substrates like a homogeneous catalytic
system.
Samples
SBET (m2g−1 a
)
D (nm)b
Pure nanosilica
nanosilica@[HFDAIL]
399
13
20
18
aBET surface area calculated using N2 adsorption-
desorption measurements; bMean pore diameter cal-
culated using BJH method from adsorption branch of
N2 adsorption-desorption isotherm.
In the next step, to obtain appropriate conditions for
esterification of phthalic anhydride with alcohols, effect
of the reaction temperature and time, amount of catalyst
and molar ratio of phthalic anhydride to butanol were
investigated in the presence of nanosilica@[HFDAIL]
astheselectivecatalyst. TheresultsareshowninTable3.
As shown in Table 3, the optimum conditions is 125 ◦C,
10 mol% catalyst and molar ratio of phthalic anhydride
to butanol 1:5 (Table 3, entry 5).
In order to evaluate the scope and limitations in the
catalytic performance of nanosilica@[HFDAIL], ester-
ification reaction of different alcohols with phthalic and
maleic anhydrides as well as succinic acid was carried
out. The results are summarized in Table 4. Generally, it
could be found that nanosilica@[HFDAIL] showed very
high catalytic performance for esterification reactions
of a wide range of substrates and in all cases the diester
product was selectively formed. As shown in Table 4,
good to excellent yields ranging from 56 to 100% with
perfect conversion (100%) of phthalic or maleic anhy-
dride and succinic acid were obtained in all systems
under investigation. According to Table 4 (entries 3 to
5), esterification of long alkyl chain alcohols was very
reaction time) and higher selectivity for monoester prod-
uct was obtained in the case of nanosilica@[SFDAIL],
using 10 mol% of each catalyst under other same reac-
tion conditions. Surprisingly, increasing the catalyst
loadingto20mol%inthecaseofnanosilica@[SFDAIL]
resulted in complete conversion in the reaction within
15 h, but the monoester compound was still the major
product in these circumstances (Table 2, Entry 3). The
higher catalytic activity as well as improved selec-
tivity toward the diester compound in the case of
nanosilica@[HFDAIL] might be attributed to both its
higher availability of acidic sites (as counter ion of
IL) and its higher hydrophilicity, compared to the
nanosilica@[SFDAIL] catalyst. Considering the higher
hydrophilicity of nanosilica@[HFDAIL] when com-
pared with nanosilica@[SFDAIL] as well as relatively
higher hydrophobicity of diester product compared
to monoester one, we hypothesize that the retention
time of monoester compound in the pore surface of
nanosilica@[HFDAIL], where the catalytic sites are
located, is longer; thus, the monoester compound is not
Table 2. Comparison of catalytic performance of ILs/nanosilica in the esterification reaction of phthalic anhydride with
butanol.a
Entry
Catalyst
Catalyst
(mol %)
PhA Conv.
(%)
Time (h)
Yield (%)
A
B
1
2
3
4
nanosilica@[HFDAIL]
nanosilica@[SFDAIL]
nanosilica@[SFDAIL]
[HFDAIL]
10
10
20
10
100
65
100
100
8
24
15
8
8
92
12
33
75
88
67
25
aReaction coditions: The molar ratio of butanol to phthalic anhydride = 5:1; T = 125 ◦C.

48 Page 8 of 13
J. Chem. Sci. (2018) 130:48
Table 3. Optimization of reaction conditionsa for the esterification reaction of
phthalic anhydride with butanol in the presence of nanosilica@[HFDAIL].
Entry Catalyst (mol%) (PA:BuOH) Temperature (◦C) Time (h) Yield (%)b
1
2
3
4
5
6
7
8
10
10
10
10
10
10
5
15
10
10
10
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:2
25
50
75
24
24
10
10
8
2
9
8
10
9
0
3
30
70
92
63
75
90
70
80
90
100
125
150
125
125
125
125
125
9
10
11
1:3.5
1:6.5
8
aReaction conditions: reaction was carried out under solvent-free conditions; bYields
refer to the diester products (GC yield).
satisfactory due to their high boiling points. Esterifi- esterification of phthalic anhydride and butanol as a
cation reactions of 2-methoxy ethanol and allyl alcohol model reaction. To do this, the nanosilica@[HFDAIL]
showed good results too (Table 4, entries 6 and 7). Ester- catalyst was simply filtered from the reaction mixture
ification of methanol and ethanol (entries 1 and 2) gave and checked for the next run. As can be seen in Fig-
lower yields compared with other alcohols due to their ure 4, the recovered catalyst could be reused for at least
lower boiling points. Esterification reaction of maleic 7 times without significant loss of catalytic activity. The
anhydride with alcohols also provided very satisfactory successful recovery of the catalyst from such reaction
results (Table 4, entries 8 and 9). Since succinic acid accomplishing under harsh condition, despite the immo-
has lower steric hindrance compared with phthalic and bilization of IL via hydrogen bonding, highlights the
maleic anhydrides in the reaction center (carbonyl car- notion that the developed immobilization approach in
bon), thus it demonstrated better results in esterification this study not only provides high catalytic activity of
reaction (Table 4, entries 10 and 11). Satisfied with these supported acidic groups, like its homogeneous counter-
significant achievements, we then checked the ability part, but it also ensures the stability and recoverability of
of the catalyst for esterification of butandiol and 1,2,6- the catalyst under the described reaction conditions. For
hexanetriol with PA. However, no product was achieved comparison purpose, we also checked the recoverability
in these reactions even at elevated temperatures (Table 4, of the other catalyst, nanosilica@[SFDAIL], in the same
entries 12 and 13). It seems that a polymer was pro- transformation. Unexpectedly, we found that the activity
duced instead of the cyclic diester in these cases. Again, of this catalyst was significantly dropped in the subse-
for comparison purposes, we checked the esterification quent runs under the same reaction conditions. Although
reactions of phthalic anhydride with 2-ethylhexyl alco- the exact reason for this observation is unclear for us,
hol or butanol in the presence of [HFDAIL] instead of we think that the catalytic and immobilization sites in
nanosilica@[HFDAIL] as the catalyst. These reactions this catalyst being same is responsible for loss of its
indicated 100% and 95% yield of diester, respectively, performance.
with 100% conversion of phthalic anhydride. Thus, it
Finally, the catalytic performance of nanosilica
is significant that the nanosilica@[HFDAIL] exhibited @[HFDAIL] was compared with previous literature
similar catalytic activity as its homogeneous counter- reports about application of various ionic liquids for
part.
esterification reaction. As can be seen in Table 5,
the nanosilica@[HFDAIL] demonstrated comparable or
better, in most cases, catalytic activity than the other cat-
alysts. Lower catalyst loading, higher selectivity toward
3.3 Reusability of the catalyst
Since the recoverability of the supported catalyst is the diester product and, most importantly, the recover-
very important issue from both economic and envi- ability and stability of nanosilica@[HFDAIL] are the
ronmental viewpoints, we next checked the reusabil- main advantages of this system in comparison with
ity of the nanosilica@[HFDAIL] catalyst in the previous catalysts.

J. Chem. Sci. (2018) 130:48
Page 9 of 13 48
Table 4. Esterification reaction of anhydride and succinic acid with different alcohols catalyzed by nanosilica@[HFDAIL].a
Entry
1
Anhydride or Acid
Alcohol
CH3OH
Temperature (◦C)
65
Time (h)
24
Yield (%)b
56
2
3
EtOH
78
98
24
10
70
82
4
5
118
180
8
3
92
100
6
97
7
72
7
8
125
180
6
1
68
100
9
97
8
2
95
10
118
100
11
12
97
6
5
100
-
235

48 Page 10 of 13
J. Chem. Sci. (2018) 130:48
Table 4. (contd.)
Entry
Anhydride or Acid
Alcohol
Temperature (◦C)
Time (h)
5
Yield (%)b
-
13
178
aReaction coditions: anhydrides or acid: alcohol: nanosilica@[HFDAIL] = 1:5:0.1; bYields refer to the diester products (GC
yield).
4. Conclusions
In summary, we developed a novel approach in sup-
ported ionic liquid catalysis by design of new sup-
ported ionic liquids via hydrogen bonding. Using this
strategy, two novel acidic ionic liquids including a
hydroxyl functionalized diacidic ionic liquid [HFDAIL]
and a sulfonated diacidic ionic liquid [SFDAIL], were
prepared and immobilized on the surface of silica
nanoparticles (SNPs) via hydrogen bonding. The cat-
alytic activity of the prepared catalysts was inves-
tigated in the synthesis of phthalate, maleate and
succinate diesters under solvent-free conditions. It
for esterification of phthalic anhydride with butanol under was found that nanosilica@[HFDAIL] with higher
optimized reaction condition.
Figure 4. Reusability of nanosilica@[HFDAIL] catalyst
availability of acidic sites and higher hydrophilicity
Table 5. Comparison of catalytic activity of nanosilica@[HFDAIL] with other reported catalysts in various esterification
reactions.
Entry
1a
Catalyst
Catalyst
(mol%)
(Acid:Alcohol)
1:4
T (◦C)
130
Time (h) Conv.
(%)
Yield
(%)
Ref.
36
11
3
96.4
-
O
EtO
37
38
2
3
20
5
1:2
80
4
-
70
-
S
N
N
O
O
S
O
EtO
O
1:2.5
120-130
1.5
98
N
O
N
HO
O
S
OSO3H
39
40
4
Excess
17
1:2
1:4
100
60
3.5
4
-
-
78
74
5a

J. Chem. Sci. (2018) 130:48
Table 5. (contd.)
Page 11 of 13 48
Entry
Catalyst
Catalyst
(mol%)
(Acid:Alcohol) T (◦C) Time (h) Conv.
(%)
Yield
(%)
Ref.
41
6
15
1:2.5
115-125
1.5
95
-
42
43
7
8
Excess
Excess
1:2
1:2
110
80
4
>99
50
-
18
99
9
10
nanosilica@[HFDAIL]
nanosilica@[SFDAIL]
10
10
1:5
1:5
125
125
8
24
100
80
92
12
This work
This work
aMonoester was formed as the main product.
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