Diacidic ionic liquid supported on magnetic-silica nanocomposite: a novel, stable, and reusable catalyst for selective diester production

Article abstract of DOI:10.1007/s13738-018-1450-8

Abstract: Supported diacidic ionic liquid on magnetic silica nanoparticles (SDAIL@magnetic nanoSiO₂) was successfully prepared through a multi-step approach. 2,2- bis ((3- methylimidazolidin-1-yl) methyl) propane- 1,3- diol bromide salt was immobilized onto the surface of magnetic silica nanoparticles via covalent bonding to prepare a novel powerful acidic catalyst. The synthesized catalyst was characterized by FT-IR, SEM, TGA, VSM, N₂ adsorption–desorption measurements and acid-base titration. The catalytic activity of the prepared SDAIL@magnetic nanoSiO₂ was investigated for the selective diesterification of alcohols by phthalic anhydride to afford corresponding dialkyl plasticizers under solvent-free conditions. The nature of two acidic counter anions as well as the presence of Lewis acidic species (Fe₃O₄) on the magnetic nanosilica and high surface area of the nanosilica influenced the behavior of the catalyst. Surperisingly, the high acidic character of the catalyst facilitates the reaction with a short reaction time. Furthermore, TG analysis strongly demonstrates that major content of IL is still stable on the support up to 290?°C, so catalyst has a good thermal stability. Under the optimized conditions, the conversion of phthalic anhydride was 100% and diester plasticizers were obtained with excellent yields (80–100%). The SDAIL@magnetic nanoSiO₂ catalyst showed a good reusability and could be easily separated from the reaction mixture using an external magnet thanks to its superparamagnetic behavior and reused for several runs without significant activity loss. An important advantage of the SDAIL@magnetic nanoSiO₂ was its high-hydrophilicity resulted in excellent selectivity towards the formation of only diesters which are commonly used plasticizers in different industries. Graphical abstract [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-018-1450-8

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1450-8
ORIGINAL PAPER
Diacidic ionic liquid supported on magnetic-silica nanocomposite:
a novel, stable, and reusable catalyst for selective diester production
Reza Fareghi‑Alamdari1 · Mehri Nadiri Niri1 · Hassan Hazarkhani2 · Negar Zekri1
Received: 8 January 2018 / Accepted: 9 July 2018
© Iranian Chemical Society 2018
Abstract
Abstract Supported diacidic ionic liquid on magnetic silica nanoparticles (SDAIL@magnetic nanoSiO2) was successfully
prepared through a multi-step approach. 2,2- bis ((3- methylimidazolidin-1-yl) methyl) propane- 1,3- diol bromide salt
was immobilized onto the surface of magnetic silica nanoparticles via covalent bonding to prepare a novel powerful acidic
catalyst. The synthesized catalyst was characterized by FT-IR, SEM, TGA, VSM, N2 adsorption–desorption measurements
and acid-base titration. The catalytic activity of the prepared SDAIL@magnetic nanoSiO2 was investigated for the selective
diesterification of alcohols by phthalic anhydride to afford corresponding dialkyl plasticizers under solvent-free conditions.
The nature of two acidic counter anions as well as the presence of Lewis acidic species (Fe3O4) on the magnetic nanosilica
and high surface area of the nanosilica influenced the behavior of the catalyst. Surperisingly, the high acidic character of the
catalyst facilitates the reaction with a short reaction time. Furthermore, TG analysis strongly demonstrates that major content
of IL is still stable on the support up to 290 °C, so catalyst has a good thermal stability. Under the optimized conditions,
the conversion of phthalic anhydride was 100% and diester plasticizers were obtained with excellent yields (80–100%). The
SDAIL@magnetic nanoSiO2 catalyst showed a good reusability and could be easily separated from the reaction mixture
using an external magnet thanks to its superparamagnetic behavior and reused for several runs without significant activity
loss. An important advantage of the SDAIL@magnetic nanoSiO2 was its high-hydrophilicity resulted in excellent selectivity
towards the formation of only diesters which are commonly used plasticizers in different industries.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-018-1450-8) 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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Graphical abstract
Keywords Magnetic silica nanoparticles · Selective diesterification · Stability · High-hydrophilicity · Plasticizer
of fine chemicals [5], drugs [6], plasticizers [7, 8], chi-
ral-auxiliaries [9], agrochemicals [10]. Different cata-
lysts have been used to promote the synthesis of diesters
[11–13]; most of these protocols need a high loading of
expensive and nonrecoverable catalysts and long reaction
time in many instances. In addition, the selectivity of the
catalyst towards the synthesis of diesters is not always
satisfactory.
Introduction
Plasticizers are low molecular weight materials added to
polymeric materials, such as paints, plastics or adhesives
to improve their original properties [1]. They decrease
the glass transition temperature of the polymer and make
materials more flexible [2]. One of the most popular groups
of plasticizers is ester plasticizers which are a class of the
neutral plasticizers. Additionally, the most common ester
plasticizers are diesters such as diethyl, dibutyl, and dioctyl
phthalates, which are used in formulation of double-based
propellants [3]. As shown in Fig. 1, the main function of
these compounds is to locate between polymer molecules
and increase polymer free volume, consequently, polymer
chains easily slip on each other. Therefore, the plasticizers
make the polymer soft and pliable by reducing the viscosity
and glass transition temperature of polymer [4].
Selectivity of catalyst is also an important factor in
esterification processes of anhydrides or double car-
boxylic acids with alcohols to synthesize their relative
diesters. The selective esterification using mesoporous Si-
MCM-41, clay and alumina supported metal Lewis acid,
such as InCl3 and GaCl3, as catalyst have been reported
[14–16]. However, use of these materials as catalyst, has
two main problems. First of all the main product of the
esterification reaction is monoester not diester, and the
second problem is that these metal species are toxic and/
or expensive. Hence, there is still a need to develop a suit-
able method for the selective diesterification of anhydrides
using more efficient catalysts.
In other words, diesters are important organic com-
pounds which are used as an intermediate in the synthesis
Because of the inherent properties of magnetic nanopar-
ticles, such as biocompatibility, easy recovery by magnetic
separation, thermal stability against degradation, reaction
rates closer to homogeneous counterparts, large surface
area, high loading of active sites as well as low toxicity
and price, these materials have emerged as a useful group
of heterogeneous catalysts [17–22]. These catalysts have
been widely applied as magnetically separable catalysts
Fig. 1 Interaction of plasticizer and polymer
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Scheme 1 Synthesis of diesters
from different alcohols in the
presence of SDAIL@magnetic
nanoSiO2 catalyst
Catalyst
in organic reactions [23–32]. However, limited reports are
available on ionic liquid supported MNPs as recyclable
catalysts for esterification reaction.
Also the hydrophilic surface of the prepared catalyst
indeed provides a means of departure of diester from the
catalyst surface as soon as it is produced, thus under appro-
priate conditions no monoester compound is formed.
With a view of these considerations, and as part of a
continuing effort in our laboratory towards the synthe-
sis and application of ionic liquids in the organic syn-
thesis [33–36], and as ionic liquids show high-selectivity
in organic reactions [37–40], herein, we wish to dis-
close a new magnetic highly acidic and reusable cata-
lyst (SDAIL@magnetic nanoSiO2) for diesterification of
phthalic and maleic anhydrides as well as succinic acid
under solvent-free conditions. On the other hand, a hydro-
phil and efficient catalytic system with suitable surface
properties for the selective production of diesters has been
developed in this study (Scheme 1).
Experimental section
Chemicals and instruments
All chemicals and solvents were purchased from Merck and
Fluka chemical companies. Melting point was determined
1
using melting point IA 8103 apparatus. H NMR and 13C
NMR spectra were recorded in DMSO-(d6) and D2O sol-
vents using Brucker-Avance 500 and 125 MHz spectrom-
eter, respectively, and TMS as an internal standard. FT-IR
spectroscopy (KBr pellets in the range of 400–4000 cm−1
with a Nicolet-860 spectrometer) was employed for char-
acterization of the catalyst and synthesized compounds.
The N2 adsorption/desorption isotherms were measured
We believe that double site for immobilization and at the
same time double catalysis site in the structure of our pre-
pared catalyst can improve its efficiency in both terms of
stability and activity.
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using a Micrometrics Tristar II 3020 instrument. Thermal
gravimetric analysis (TGA) was recorded using a thermal
analysis instrument Perkin Elmer (Pyris Diamond model).
In this method, runs were carried out in 40 µL alumina pans
and under an inert atmosphere of nitrogen at a flow rate of
15 mL min−1. Initial mass introduced in the pan was set
at 10 1 mg. Dynamic measurements were conducted at
25–600 °C. The magnetic measurement of sample was car-
ried out in a vibrating sample magnetometer (VSM) (4 in.,
Daghigh Meghnatis Kashan Co., Kashan, Iran) at room tem-
perature. The scanning electron micrographs (SEM) were
obtained on a Philips XL-30 ESEM instrument and trans-
mission electron microscopy (TEM) analysis was recorded
on a Philips CM-10 instrument with an accelerating voltage
of 100 kV. Elemental analysis for C, H, and N were per-
formed using a Heraeus CHN Rapid analyzer. 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 ioniza-
tion detector (FDI), using N2 as carrier gas at a flow rate of
2 mL min−1. The reaction monitoring was accomplished by
TLC on silica gel PolyGram SILG/UV254 plates. The nano-
silica support (Fumed silica S5130 purchased from Sigma
chemical company) was dried at 250 °C for 6 h before use.
All solvents were strictly dried according to standard opera-
tions and stored over 4A° molecular sieves.
Synthesis of Fe3O4 supported silica nanoparticles (magnetic
silica nanoparticles)
Magnetite (Fe3O4) nanoparticles were prepared according
to the reported method in the literature [42] in a chemical
co-precipitation step of ferric and ferrous ions in an alka-
line solution. An aqueous solution of FeCl3·4H2O (2.0 g in
10 mL deionized water) was mixed with an aqueous solu-
tion of FeCl2·6H2O (0.8 g in 10 mL deionized water). The
mixture was stirred vigorously at 80 °C for 30 min, and then
this solution was dropped to the 250 mL round bottom flask
containing 100 mL of ammonium hydroxide (25% aqueous
solution) and the pH value of the solution was adjusted to
9. The reaction was further stirred for another 2 h at 80 °C
to afford Fe3O4 nanoparticles as a black precipitate. After
completion of the reaction, the black precipitate (Fe3O4) was
collected with an external magnet under the reaction flask.
The obtained Fe3O4 NPs were washed using deionized water
and dried at 80 °C in an oven under vacuum.
To synthesis of Fe3O4 supported silica nanoparticles
(Fe3O4 @ SiO2), silica nanoparticles were dried in an oven
at 250 °C for 6 h before using. 0.8 g of dried silica nanopar-
ticles were dispersed in dry methanol (10 mL), then 0.2 g of
Fe3O4 NPs were added to this solution. The resultant mixture
was allowed to stirr at room temperature for 24 h, then the
solvent was evaporated with rotary under reduced pressure
to give Fe3O4 @ SiO2. Finally, the obtained material was
dried at 80 °C under vacuum.
Preparation of supported diacidic ionic liquid
on magnetic silica nanoparticles (SDAIL@magnetic
nanoSiO2)
Preparation of magnetic nanosilica chloride
A multi-step methodology was employed for the synthesis
of catalyst in this study as described below:
The methodology used to obtain magnetic nanosilica chlo-
ride was previously reported in the literature [43]. Magnetic
silica nanoparticles (0.4 g) were placed in an oven at 200 °C
for 1 h to dry it. Thionyl chloride (5 mL) was then dropped
slowly over 15 min and the reaction continued at room tem-
perature for 1 h with stirring. Then the mixture was refluxed
for about 3 h at 70 °C in an oil bath. The resulting compound
was placed in a vacuum desiccator for 24 h to obtain the yel-
low powder of magnetic nanosilica chloride.
Preparation of 2,2- bis ((3- methylimidazolidin-1-yl) methyl)
propane- 1,3- diol bromide salt (IL-Br)
The IL-Br salt was synthesized according to our previ-
ous work [41]. 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 tempera-
ture, the obtained solid was washed three times with ace-
tonitrile and the residue was dried in an oven at 100 °C for
2 h, giving IL-Br as a white powder (yield: 95%, melting
point = 150 °C). FT-IR (KBr) νmax = 620.8, 770.7, 829.8,
1060.5, 1164.9, 1450.8, 1573.1, 1760.4, 1655.9, 2935.4,
3260.7 cm−1. 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.
Preparation of Fe3O4 @ SiO2-IL-Br
2,2- bis ((3- methylimidazolidin-1-yl) methyl) propane- 1,3-
diol bromide salt (IL-Br) (2 mmol, 0.85 g) was added to the
round bottom flask containing magnetic nanosilica chloride
(0.5 g) and NaH (4 mmol, 0.01 g) in 25 mL of dry toluene
and the mixture was refluxed for 24 h with stirring. The
resulting solid was filtered out, washed twice with dry tolu-
ene and dried at 80 °C for 5 h in an oven under vacuum to
give Fe3O4 @ SiO2-IL-Br.
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Preparation of SDAIL@magnetic nanoSiO2
Spectral and analytical data of some products
The SDAIL@magnetic nanoSiO2 was synthesized by
anion exchange of Fe3O4 @ SiO2-IL-Br. 0.74 g of Fe3O4
@ SiO2-IL-Br was dispersed in methanol (30 mL) under
stirring. Then 0.23 mL sulfuric acid (4 mmol) was dropped
slowly and the mixture was refluxed for 24 h at 60 °C. After
completion of the reaction, the solvent was evaporated under
vacuum. The resulting material was washed with dichlo-
romethane (3×5 mL) and dried in an oven at 150 °C for 2 h
to afford SDAIL@magnetic nanoSiO2 as a brown powder.
bis (2- methoxyethyl) phthalate
FT-IR (KBr) νmax = 745, 1281, 1448, 1600, 1726, 2884,
3080 cm−1. 1HNMR (100 MHz, CDCl3): δ (ppm) 3.41 (s,
6H, CH3–O), 3.70 (t, J = 1 Hz, 4H, O-CH2-CH2-O), 4.47
(t, J = 2 Hz, 4H, O-CH2-CH2-O), 7.54 (d, J = 2 Hz, 2H,
CH = CH), 7.75 (d, J = 1 Hz, 2H, CH = CH). Anal. Cald.
for C14H18O6 (282.29): C, 59.51; H, 6.38. Found: C, 59.53;
H, 6.42.
General procedure for diesterification of alcohols
by SDAIL@magnetic nanoSiO2
Diallyl phthalate
FT-IR (KBr) νmax = 745, 1123, 1271, 1449, 1601, 1728,
2926, 3083 cm−1. 1HNMR (100 MHz, CDCl3): δ (ppm) 4.83
(d, J=1 Hz, 4H, CH2-CH=CH2), 5.33 (d, J=7 Hz, 2H,
CH2-CH=CH2), 5.45 (d, J=4 Hz, 2H, CH2-CH=CH2), 6.05
(m, 2H, CH2-CH=CH2), 7.59 (d, J=5 Hz, 2H, CH=CH),
7.79 (m, 2H, CH=CH). Anal. Cald. for C14H14O4 (246.26):
C, 68.22; H, 5.69. Found: C, 68.25; H, 5.71.
Weighed amounts of anhydrides or acid, alcohol and cata-
lyst, were added to round bottom flask with a dean–stark
apparatus, reflux condenser and a magnetic stirrer. The
esterification reaction was typically carried out for 2–10 h at
the desired temperature with vigorous stirring. The catalyst
could be easily recovered from the reaction mixture using
an external magnet due to its magnetic property without any
disposal. Then the product was separated simply by decan-
tation with ethyl acetate. Ethyl acetate was removed under
reduced pressure to afford pure desired product.
Dipropyl phthalate
FT-IR (KBr) νmax = 743, 1124, 1275, 1464, 1721, 2968,
3071 cm−1. 1HNMR (100 MHz, CDCl3): δ (ppm) 1.13 (t,
J = 2 Hz, 6H, CH3-), 1.94 (m, 4H, CH3-CH2-CH2), 4.07
(t, J=2 Hz, 4H, CH3-CH2-CH2-O), 7.70 (d, J=2 Hz, 2H,
CH=CH). Anal. Cald. for C14H18O4 (250.29): C, 67.12; H,
7.19. Found: C, 67.16; H, 7.22.
Typical procedure for diesterification of 2‑
methoxyethanol by SDAIL@magnetic nanoSiO2
as catalyst
The required amounts of phthalic anhydride (1 mmol,
0.15 g), 2- methoxyethanol (5 mmol, 0.39 mL, 0.38 g) and
SDAIL@magnetic nanoSiO2 as catalyst (10 wt% to phthalic
anhydride, 0.01 g) were added successively into a 50 mL
round bottom flask with a dean–stark apparatus, reflux con-
denser and a magnetic stirrer. The esterification reaction was
allowed to proceed at 130 °C for.8 h under vigorous stirring.
The completion of reaction was monitored by TLC using
(EtOAC/Hexane 2:8) as eluent. After completion of the reac-
tion, the catalyst was recovered from the reaction mixture
using an external magnet and subjected to drying under high
vacuum at 70 ◦C for 6 h to remove water, leaving behind
the SDAIL@magnetic nanoSiO2 (recovery 98%) which was
recycled. Then, the product (bis (2- methoxyethyl) phtha-
late) was separated simply by extraction with ethyl acetate
(3×5 mL) and ethyl acetate was evaporated under vacuum
to afford desired product as colorless oily liquid at 98% yield
(0/25 g, boiling point=340 °C). The obtained product dried
under vacuum at 70 °C for 6 h.
Results and discussion
Immobilization of ionic liquid onto the surface
of magnetic support via covalent bonding
The ionic liquid can be bounded or supported on a solid sup-
port surface via two distinct approaches. The first approach
involves the simple adsorption of the ionic liquid onto the
surface of the solid support (physical bonding) whereas the
second approach involves the covalent attachment of ionic
liquid to the solid support surface (chemical bonding). The
former has a serious drawback of leaching of ionic liquid
during vigorous reaction conditions. Thus, the chemical
bonding has more advantageous for catalytic applications.
In this approach, the ionic liquid could be covalently
bonded to the solid support surface via the cation or anion.
SDAIL@magnetic nanoSiO2 as catalyst was prepared based
on the following procedure (Scheme 2). At first, the precur-
sor ionic liquid was prepared by nucleophilic substitution
reaction of 1- methylimidazole and 2,2- bis (bromomethyl)-
1,3- propanediol. Then magnetic nanosilica chloride easily
synthesized via nucleophilic substitution reaction between
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Scheme 2 Multi- step route
for the synthesis of SDAIL@
magnetic nanoSiO2
(a)
(b)
Fig. 2 FT-IR spectra of (a) pure ionic liquid, (b) magnetic nanosilica chloride, and (c) SDAIL@magnetic nanoSiO2
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Journal of the Iranian Chemical Society
magnetic silica nanoparticles and thionyl chloride. After
the grafting of the ionic liquid to the magnetic nanosilica
chloride through covalent bonds, the obtained compound
converted to the SDAIL@magnetic nanoSiO2 by an anion
exchange step with sulfuric acid. The catalytic activity of
this catalyst was investigated in the synthesis of diesters.
Characterization of SDAIL@magnetic nanoSiO2
catalyst
The prepared magnetic catalyst was characterized using
different techniques such as scanning electron microscopy
(SEM), transmission electron microscopy (TEM), FT-IR,
N2 adsorption/desorption measurement, thermal gravimetric
analysis, and vibrating sample magnetometer.
The amount of H+ in the catalyst, which was determined
by acid–base titration against a standard NaOH solution with
phenolphthalein as indicator, was obtained 6.16 mmol/g.
FT-IR was employed to witness the successful function-
alization of magnetic nanosilica chloride with acidic ionic
liquid and to characterize organic functionalities. Figure 2
presents the FT-IR spectra of pure ionic liquid (a), magnetic
nanosilica chloride (b) and SDAIL@magnetic nanoSiO2
(c). As shown in Fig. 2a, the characteristic bonds of pure
ionic liquid (IL-Br) around 3265, 3063, 2935, 1655 and
1065 cm−1 attribute to the O–H, =C–H, –C–H, C=N and
C–N stretching vibrations. The peaks at 1450 and 1573 cm−1
correspond to the C=C stretching vibrations of the imida-
zole ring and the strong bond at 1164 cm−1 assigns to the
C–O stretching vibration also. For magnetic nanosilica chlo-
ride (Fig. 2b), the bonds at 463 and 1618 cm−1 represent the
stretching vibrations of the Fe–O bond in Fe3O4 [44]. The
strong band at 1101 cm−1 belongs to the Si–O stretching
vibration and the broad peak at 3399 cm−1 corresponds to
the stretching vibration of Si–OH group. In the FT-IR spec-
tra of SDAIL@magnetic nanoSiO2 (Fig. 2c), apart from the
original peaks of magnetic nanosilica chloride, additional
peaks attributed to the functional groups of the IL-Br also
Fig. 4 Magnetization curve of SDAIL@magnetic nanoSiO2
observed, but the intensity of the peaks decreased notably.
Therefore, the above results prove that the IL-Br ionic liquid
was successfully bonded with magnetic nanosilica chloride.
The surface morphology and nanocomposite nature of
SDAIL@magnetic nanoSiO2 were characterized by the scan-
ning electron microscopy (SEM) and transmission electron
microscopy (TEM) techniques, respectively. SEM images
for typical nanosilica samples before and after IL supporting
are shown in Fig. 3a, b. The SEM images reveal that nano-
silica support contains nanoparticles with the average size
of 40 nm (Fig. 3a). As shown in Fig. 3b, SDAIL@magnetic
nanoSiO2 catalyst has uniform and spherical morphology
and the average size of the obtained particles is 48 nm. Com-
paring the images of Fig. 3a, b shows that the morphological
homogeneity is maintained after supporting of IL and no
obvious aggregation of the silica nanoparticles was detected
after immobilization of IL which demonstrates that the par-
ticles of nanosilica have a good mechanical stability during
the supporting process.
Fig. 3 SEM images of a nanosilica b SDAIL@magnetic nanoSiO2 and c TEM image of SDAIL@magnetic nanoSiO2
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TEM observations clearly reveal the nanocompos-
ite structure of the SDAIL@magnetic nanoSiO2 catalyst
(Fig. 3c). On the other hand, black iron particles (Fe3O4)
were distributed sporadically on spherical nanosized silica
particles.
magnetic nanosilica. So, the amount of organic moiety is
about 57% against the total solid catalyst. Additionally, the
TGA curve shows that the maximum weight loss of the ionic
liquid occurs at 290 °C. Therefore, major content of IL is
still stable on the support up to 290 °C and the adhesion of
iron particles and ionic liquid on the surface of the nano-
silica has been done well.
Magnetic measurement for SDAIL@magnetic nanoSiO2
was performed by vibrating sample magnetometer (VSM)
technique. Its hysteresis curve is presented in Fig. 4. It can
be seen from the curve that the saturation magnetization
of SDAIL@magnetic nanoSiO2 is 9.8 emu/g. The high
magnetic property of this catalyst is very beneficial in het-
erogeneous catalysis as it is easily recoverable by external
magnetic field.
The corresponding textural parameters such as surface
area and pore diameter were determined by Brunauer-
Emmet-Teller (BET) technique (Fig. 6; Table 1). The results
obtained from N2 adsorption–desorption experiments of
pure nanosilica and SDAIL@magnetic nanoSiO2 showed
that both samples pure nanosilica and SDAIL@magnetic
nanoSiO2 possess type-IV isotherms with H3 hysteresis loop
centered at relative pressure of about 0.4–0.8 confirming the
mesoporous structure of the materials (Fig. 6a, b). Moreo-
ver, the BJH calculation of the SDAIL@magnetic nanoSiO2
(Table 1, entry 2) shows, the pore diameter was decreased
after supporting of the ionic liquid on the surface of nano-
silica. Additionally, the BJH diagrams of the samples con-
firm the mesoporous and regular structure of the materials
(Fig. 6c, d).
The thermal stability of SDAIL@magnetic nanoSiO2 was
determined by TG analysis through heating from room tem-
perature to 600 °C under nitrogen flow (Fig. 5). Three steps
of weight loss are exhibited in the thermogram of catalyst.
The TGA curve indicates an initial weight loss of about 18%
at the temperature range of 25–210 °C, which is due to the
desorption of hydrogen bonded water molecule both on the
surface and in the pores of the particles. This high-absorbed
water demonstrated that the catalyst surface has hydro-
philic character to some extent. This property may make
it suitable when its surface is exposed to the hydrophilic
reactants. The second weight loss of approximately 41% at
210–348 °C, weight loss of almost 13% at 348–465 °C and
in the final step, the weight loss of about 4% appeared at
the range of 465–595 °C can be ascribed to decomposition
of covalently bonded ionic liquid from the surface of the
BET surface area measurements indicated that, compared
with nanosilica (Table 1, entry 1) the surface area of the
SDAIL@magnetic nanoSiO2 (Table 1, entry 2, Fig. 6e, f)
was significantly decreased from 399 to 48 m2 g−1. Pre-
sumably, the loading of ionic liquid on the surface of the
nanosilica led to this obvious decrease. In other words, the
reduction of the surface area verified that the immobilization
Fig. 5 The TGA curve of SDAIL@magnetic nanoSiO2
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Fig. 6 N2 adsorption/desorption
isotherms of a pure nano-
silica and b SDAIL@magnetic
nanoSiO2, BJH plots of c pure
nanosilica and d SDAIL@mag-
netic nanoSiO2, BET plots of e
pure nanosilica and f SDAIL@
magnetic nanoSiO2
of ionic liquid onto the surface of the nanosilica has been
successful.
Table 1 Textural parameters of nanosilica and SDAIL@magnetic
nanoSiO2
Entry
Sample
BET surface
areaa
BJH desorption
average pore
widthb
Optimization of esterification reaction conditions
To optimize esterification reaction conditions, we exam-
ined the effects of reaction temperature, amount of the
catalyst and molar ratio of phthalic anhydride to alcohol
in the esterification reaction of phthalic anhydride with
butanol in the presence of SDAIL@magnetic nanoSiO2
without any solvent. The results are summarized in
1
2
Nanosilica
399
48
20
17
SDAIL@magnetic
nanoSiO2
aBET surface area (m2 g−1
bAverage pore width (nm)
)
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Table 2 Optimization of reaction conditions for the esterifica-
tion reaction of phthalic anhydride with butanol in the presence of
SDAIL@magnetic nanoSiO2 as catalyst
Table 2. As can be seen in this Table, excellent yields
of diester products were afforded by SDAIL@magnetic
nanoSiO2 catalyst but the best result was obtained under
optimized reaction conditions for this esterification reac-
tion which are 10 wt% of catalyst to phthalic anhydride,
molar ratio of reactants 1:5 and 125 °C for 2 h. The fol-
lowing is a detailed description of optimized conditions.
In examining the effect of temperature, yield of the
reaction was low at 25 °C (Table 2, entry 1). The yield
of products were enhanced by increasing the reaction
temperature and at temperatures less than 100 °C water
evaporation rates are low, so the equilibrium reactions
return toward the reactants. It seems that by reaching the
reaction temperatures to the boiling point of water, the
yield of products improved (Table 2, entries 2–4). It was
observed that at higher temperatures, alcohol was quickly
evaporated and hence by rising temperature above 125 °C
the reaction yield decreased (Table 2, entry 6). So, the
optimum reaction temperature for this synthesis process
was 125 °C (Table 2, entry 5).
Entry Catalyst Molar ratio Tempera- Time (h) Yielda,b (%)
(wt %)
of reactantsa ture (˚C)
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
5
15
10
10
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:2
1:3.5
25
50
75
100
125
150
125
125
125
125
24
10
9
5
2
2
5
4
7
10
60
95
100
100
80
97
97
80
97
7
Reaction coditions: reactions were carried out under solvent-free con-
ditions
aConversion of phthalic anhydride was 100% in all cases (except for
the entries 1 and 2)
bYields refer to the diester products
A suitable way to enhance the rate of esterification reac-
tion is increasing the amount of alcohol which promotes
the reaction equilibrium toward the product. As shown in
Table 2, the reaction yield is enhanced by increasing the
molar ratio of butanol to phthalic anhydride and the opti-
mum molar ratio of phthalic anhydride to butanol is sug-
gested to be 1:5 (Table 2, entries 5, 9, 10).
under solvent-free conditions were obtained. As shown
in this Table, values of turn over number (TON) and turn
over frequency (TOF) were calculated in the presence of
SDAIL@magnetic nanoSiO2 for different diesters. Accord-
ing to Table 3 (entries 1–5) esterification of long alkyl chain
alcohols was very satisfactory due to their high-boiling
points and lower-boiling point alcohols have lower amounts
of TON and TOF. Allyl alcohol and 2- methoxy ethanol led
to excellent yields too (entries 6, 7). As shown in Table 3,
the highest values of TON and TOF were obtained for bis
(2-ethylhexyl) succinate (entry 8). Due to the resonance of
the double bond with the carbonyl group in phthalic and
To investigate the optimum amount of SDAIL@magnetic
nanoSiO2, different quantities of the catalyst were exam-
ined. The yield of products was improved with increasing
the catalyst loading (Table 2, entries 5,7). The best result
was obtained in the presence of 10 wt% of catalyst (Table 2,
entry 5). Higher molar ratios of the catalyst will not improve
the yield of reaction (Table 2, entry 8).
maleic anhydride, the electron density is displaced toward
carbonyl carbon, while succinic acid does not have such a
conjugated double bond. Therefore, carbonyl carbon has a
higher positive charge and had better result in esterification
reaction (Table 3, entries 8, 9). The above results show that
SDAIL@magnetic nanoSiO2 has excellent selectivity for
diester products.
Catalytic activity of SDAIL@magnetic nanoSiO2
for esterification reaction
The catalytic potential of SDAIL@magnetic nanoSiO2 was
investigated for the synthesis of diester plasticizers via the
esterification reaction (Table 3). It is clear that SDAIL@
magnetic nanoSiO2 has very high-catalytic activity for
esterification reactions and excellent yields ranging from
80 to 100% with perfect conversion of phthalic or maleic
anhydride and succinic acid (100%) in short reaction times
Then, we investigated the ability of the catalyst for esteri-
fication of butandiol and 1, 2, 6-hexanetriol with phthalic
anhydride. No product was obtained in these reactions
1 3

Journal of the Iranian Chemical Society
Table 3 Synthesis of diester plasticizers in the presence of SDAIL@magnetic nanoSiO2 as catalyst
Entry
Anhydride
or acid
Alcohol
Temperature
º(C)
Time
(h)
Yielda (%)
TONb/TOFc (h-1)
Measured b.p. (ºC)/L
b.p. (ºC) [Refs.]
O
CH3OH
1
65
78
10
8
80
15.5/1.6
285/284 [45]
O
O
O
2
3
4
5
6
7
8
95
95
21.1/2.6
23.8/3.4
25.8/12.9
27.9/27.9
23.4/4.7
27.6/3.5
28.5/57
296/295 [45]
HO
O
O
O
HO
98
7
321/Not reported
340/339-340 [45]
387/385 [45]
O
O
O
118
180
97
2
100
100
95
HO
O
O
O
1
HO
O
CH2CH3
O
O
HO
5
293 (New compound)
330 (New compound)
360/Not reported
O
O
O
O
125
180
8
98
HO
O
O
O
0.5
100
HO
HO
OH
O
CH2CH3
9
118
235
3
5
100
–
23.24/7.9
–
282/Not reported
–
HO
O
O
O
O
OH
10
HO
O
O
O
OH
11
178
5
–
–
–
OH
HO
O
O
Reaction coditions, anhydrides or acid: alcohol = 1:5, catalyst was 10 wt% to anhydrides or acid
aYields refer to the diester products (GC yield)
bTON Turn over number, the amount of diester formed (g)\the amount of the catalyst (g)
cTOF Turn over frequency, the amount of diester formed (g)\(the amount of the catalyst (g) × time (h))
1 3

Journal of the Iranian Chemical Society
Table 4 Comparison of the efficiency of the catalyst at this work with other reported catalysts in the diesterification reactions
Entry
Catalyst
Acid or
Alcohol
Amount
Acid:alc
ohol
T
Time
(h)
PhA
conv.
(%)
Yielda
(%)
Ref.
[46]
anhydride
of catalyst
(°C)
Ph2PO
1
Phthalic
anhydride
Butanol
Excess
1:2
1:2.5
1:2
100
3.5
1.5
4
Not
reported
78
N
N
Cl
2
Phthalic
anhydride
2-Ethyl-1-
hexanol
15 mol%
115–
125
95
Not
reported
[8]
O
S
N
O
S
O
HO
HO
O
O
O
S
3
Succinic
acid
Methanol
20 mol%
80
Not
reported
70
[47]
EtO
N
N
O
O
S
EtO
O
O
NH+
BF4
4b
5c
Oxalic acid
1-Octanol
Butanol
2 mL
1:2
110
4
> 99
Not
[48]
-
N
reported
H6P2W18O62
1:2
110
3
91
Not
reported
[49]
Phthalic
anhydride
0.058
mol%
Amorphous zirconium titanium
phosphate
1:2.5
115
10
Not
reported
62.4
6d
Phthalic
anhydride
Butanol
0.2 g
[50]
OH
N
OH
7
Phthalic
anhydride
Butanol
10 mol%
1:5
125
7
100
95
[41]
HSO4
HSO4
N
N
N
(HFDAIL)
SO3H SO3H
8
Phthalic
anhydride
Butanol
10 mol%
1:5
125
8
100
70
[41]
O
O
Br-
Br-
N
N
N
N
(SFDAIL)
9
SDAIL@magnetic nanoSiO2
Phthalic
anhydride
Butanol
10 wt%
1:5
125
2
100
100
[This
work]
aYields refer to the diester products
bn (Oxalic acid) = 0.02 mol
cThe dosage of the catalyst was mol percentage based on phthalic anhydride and mol of phthalic anhydride was 0.3
dn (Phthalic anhydride) = 0.025 mol
1 3

Journal of the Iranian Chemical Society
(Table 3, entries 10 and 11). It seems that a polymer pro-
duced instead of the cyclic diester.
Comparison of the efficiency of SDAIL@
magnetic nanoSiO2 with other reported catalysts
in the esterification reaction
A comparative study on the catalytic performance of
SDAIL@magnetic nanoSiO2 with the other reported cata-
lysts in the literature as well as the other synthesized cata-
lysts in our previous work for the synthesis of diesters is
shown in Table 4. As can be seen in this Table, the cata-
lytic activity of SDAIL@magnetic nanoSiO2 is much better
than the other catalysts. The highest yield of diester without
contamination by monoester compound was obtained in the
presence of the catalyst at this work. It is noteworthy that
this result was obtained in the shortest reaction time and
with an appropriate amount of the catalyst compared to other
catalytic systems.
Fig. 8 FT-IR spectrum for the comparison of the fresh catalyst and
the eight times reused catalyst
Reusability of SDAIL@magnetic nanoSiO2
in esterification of phthalic anhydride with butanol
The reusability of the SDAIL@magnetic nanoSiO2 catalyst
was also studied in the diesterification of phthalic anhydride
and butanol as a model reaction. The catalyst was recovered
from the reaction mixture by an external magnet, washed
with dichloromethane and reused for the next runs with
consistent activity even after eight catalytic cycles (Fig. 7).
The successful recovery of the catalyst from such reaction
accomplishing under harsh condition, highlights the notion
that the developed immobilization approach in this study
not only provides high-catalytic activity of supported acidic
groups, but it also ensure the stability and recoverability of
the catalyst under the described reaction conditions. This
might be attributed to the anchoring of acidic ionic liquid to
the surface via two hydroxyl group.
The high polar nature of ionic liquid containing two
HSO−4 acidic sites and high surface area of the magnetic
nanosilica possessing Lewis acidic species (Fe3O4), coop-
erate with each other to increase the efficiency of the cat-
alyst. Moreover, the high-hydrophilicity of the catalyst,
arised from the presence of HSO−4 anions on it, as well as
higher hydrophobicity of diester product compared with
monoester one, cause the diester expell from the surface of
the catalyst, but the monoester remains on the surface of
the catalyst to convert to diester compound. So, excellent
selectivity for diester product was observed in the pres-
ence of SDAIL@magnetic nanoSiO2. It is noteworthy that
the synthesized ionic liquids in our previous work showed
excellent results in the synthesis of diesters. However, in
the present work, grafting of ionic liquid on the solid sup-
port reduced the reaction time and increased selectivity for
diester. As a result, the catalytic activity of the catalyst in
this work is much better than the other catalytic systems
for the esterification reaction.
Fig. 7 Reusability of SDAIL@magnetic nanoSiO2 for diesterification
Fig. 9 SEM image of eight times reused SDAIL@magnetic nanoSiO2
of phthalic anhydride with butanol at optimized reaction condition
catalyst
1 3

Journal of the Iranian Chemical Society
Scheme 3 The proposed reaction mechanism of the synthesis of diesters from phthalic anhydride in the presence of SDAIL@magnetic
nanoSiO2
O
O
SDAIL@magnetic nanoSiO2
O
O
H2O
O
+
OH
solvent-free, reflux
O
O
The recovered catalyst after eight runs had no obvious
change in structure, referring to the FT-IR spectrum in
comparison with the fresh one (Fig. 8). In other words, this
figure shows the presence of the ionic liquid in the catalyst
after eight runs.
first, the carbonyl group is activated by adsorption of H+
from the catalyst. In the next step, by nucleophilic attack of
alcohol on the activated carbonyl group, monoester product
is synthesized. The obtained product (monoester) was then
subjected to esterification reaction by the addition of second
mole of alcohol. Due to the hydrophilicity of catalyst and
hydrophobicity of diester than monoester, the catalyst was
highly selective towards diester and diester products were
obtained with excellent yields (80–100%).
A SEM observation of the recovered catalyst was also
made (Fig. 9). By comparing Fig. 3b with Fig. 9, it is evident
that there was no obvious change in the morphology of the
reused catalyst after eight runs.
Moreover, the stability of the reused SDAIL@magnetic
nanoSiO2 was examined by acid–base titration after eight
runs. Only a slight loading amount loss of ionic liquid was
observed after eight runs. The loading amount of ionic liquid
in the nanocatalyst was obtained 5.99 mmol/g. We can con-
clude that no remarkable leaching of the ionic liquid from
the surface of the nanosilica was occured.
Conclusion
In conclusion, we have developed a new magnetic nanosilica
supported diacidic ionic liquid catalyst. The catalyst showed
extraordinary catalytic activity in the esterification reaction
of anhydrides and succinic acid with different alcohols to
give corresponding diesters under solvent-free conditions. It
is believed that the high-acidity of the ionic liquid together
with Lewis acidity of Fe3O4 and high surface area of nano-
particles, might increase the activity of the synthesized
catalyst. Furthermore, high-hydrophilicity of the catalyst as
well as more hydrophilicity of monoester than diester led to
excellent selectivity towards diester. The present study offers
some other advantages such as easy recovery of the catalyst
The results of the SEM technique, FT-IR spectra and
acid–base titration revealed that the catalyst is stable under
these reaction conditions.
Reaction mechanism
The suggested reaction mechanism of phthalic anhydride
with alcohol in the presence of SDAIL@magnetic nanoSiO2
as an acidic catalyst is shown in Scheme 3. As it shown,
1 3

Journal of the Iranian Chemical Society
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operational simplicity and short reaction times. Moreover,
the catalyst could be reused for at least 8 runs without any
significant loss of its activity.
Acknowledgements The authors are grateful to Malek-Ashtar Univer-
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Affiliations
Reza Fareghi‑Alamdari1 · Mehri Nadiri Niri1 · Hassan Hazarkhani2 · Negar Zekri1
2
* Reza Fareghi-Alamdari
Center for Research and Science Education, Organization
for Educational Research and Planning, Tehran, Iran
reza_fareghi@yahoo.com
1
Faculty of Chemistry and Chemical Engineering, Malek-
Ashtar University of Technology, Tehran, Iran
1 3