Novel SO3H-functionalized polyoxometalate-based ionic liquids as highly efficient catalysts for esterification reaction

Article abstract of DOI:10.1016/j.molstruc.2019.04.027

Three novel heteropolyanion-based Br?nsted acidic ionic liquids (BAILs), butane mono sulfoacid-functionalized 1,10-phenanthrolinum, butane mono and bis sulfoacid-functionalized 1,4-dimethylpiperazinium salts of phosphortungstate catalyst (PhBs₁-PW, [PipBs₁]₃-PW and [PipBs₂]₃-(PW)₂) were synthesized and well characterized with FTIR, ¹H and ¹³C NMR, Electro-Spray Ionization Mass Spectrometry (ESI-MS), Elemental analysis (CHNS), EDX and TG analysis techniques. The esterification reactions of monocarboxylic acids with monohydric alcohols were carried out using these catalysts. The introduced catalysts present a self-separation performance after reaction, which can be easily recovered and quite steadily reused as confirmed by six-run recycling test. Moreover, bis sulfoacid-functionalized 1,4-dimethylpiperazinium salt of phosphortungstate showed the best catalytic performance among the prepared catalysts for the esterification reaction.

Full text of DOI:10.1016/j.molstruc.2019.04.027

Accepted Manuscript
Novel SO H-functionalized polyoxometalate-based ionic liquids as highly efficient
3
catalysts for esterification reaction
Mosadegh Keshavarz, Nasir Iravani, Abolfath Parhami
PII:
S0022-2860(19)30423-5
DOI:
https://doi.org/10.1016/j.molstruc.2019.04.027
MOLSTR 26399
Reference:
To appear in: Journal of Molecular Structure
Received Date: 13 February 2019
Revised Date: 25 March 2019
Accepted Date: 5 April 2019
Please cite this article as: M. Keshavarz, N. Iravani, A. Parhami, Novel SO H-functionalized
3
polyoxometalate-based ionic liquids as highly efficient catalysts for esterification reaction, Journal of
Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.04.027.
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ACCEPTED MANUSCRIPT
Novel SO H-functionalized polyoxometalate-based ionic liquids as highly
3
efficient catalysts for esterification reaction
1
∗
2
3
Mosadegh Keshavarz , Nasir Iravani and Abolfath Parhami
1
2
3
Department of Chemistry, Faculty of Gas and Petroleum, Yasouj University, Gachsaran, Iran
Department of Chemistry, Gachsaran Branch, Islamic Azad University, Gachsaran, Iran
Department of Chemistry, Payame Noor University, 19395-4697 Tehran, I. R. of Iran
Abstract
Three novel heteropolyanion-based Brønsted acidic ionic liquids (BAILs), butane mono sulfoacid-
functionalized 1,10-phenanthrolinum, butane mono and bis sulfoacid-functionalized 1,4-
dimethylpiperazinium salts of phosphortungstate catalyst (PhBs -PW, [PipBs ] -PW and [PipBs ] -(PW) )
1
1 3
2 3
2
1
13
were synthesized and well characterized with FTIR, H and C NMR, Electro-Spray Ionization Mass
Spectrometry (ESI-MS), Elemental analysis (CHNS), EDX and TG analysis techniques. The esterification
reactions of monocarboxylic acids with monohydric alcohols were carried out using these catalysts. The
introduced catalysts present a self-separation performance after reaction, which can be easily recovered
and quite steadily reused as confirmed by six-run recycling test. Moreover, bis sulfoacid-functionalized
1
,4-dimethylpiperazinium salt of phosphortungstate showed the best catalytic performance among the
prepared catalysts for the esterification reaction.
Keywords: 1,10-Phenanthroline; 1,4-dimethylpiperazine; 1,4-Butane Sultone; Heteropoly Acid; Ionic
Liquid Catalyst; Reusable Catalyst; Esterification Reaction.
∗
Mosadegh Keshavarz; mail: chem.mosadegh@gmail.com; Zip-code: 75918-74831
1

ACCEPTED MANUSCRIPT
1
.
Introduction
Since the past decade, ionic liquids (ILs) have received much attention as effective and eco-friendly
reaction media due to their unique properties such as negligible vapor pressure, good thermal stability,
high ion conductivity, simple functionality and ease of recovery/reuse processes [1,2]. Nevertheless of
their environmental compatibility, the role of ILs as solvent almost is not cost-effective and not entirely
satisfactory in most cases because the present systems still suffer from high content of ILs (20–300
mol%) needed as the reaction media, relatively long reaction time, and the need to remove by-product
from the IL phase in the IL recycling experiment. For this reason, a new window at the ILs was opened
with targeted applications. The term “task-specific IL” refers to the non-solvent applications of the ILs
such as catalyst in the organic synthesis [3–5]. Probably, the most significant subcategory of the task-
specific ILs is referred to as BAIL. High thermal stability, high acidity accompanied by a negligible
release of hazardous gases, simple separation and reusability for several times are among the advantages
of the BAIL [6,7].
Heteropolyacids (HPAs) are complex protic acids with self-specific structural and physicochemical
properties [8–11]. HPAs are highly soluble in polar solvents and ultra-strong Brønsted acidity welcomes
them to the solid superacids classification [12,13]. The investigations have showed that in HOAc solution,
HPAs act like monobasic acids due to the low degree of dissociation [14].
HPAs have versatile properties such as strong acidity, good thermal stability, and excellent oxidizing
capability. These together with the stable and relatively nontoxic characteristics rendering HPA widely
appreciated as environmentally benign catalysts for practical replacement of hazardous conventional
liquid acids used in industrial processes. However, the unmodified HPAs as catalysts suffer from several
disadvantages, such as poor solubility, low selectivity and difficulty in recovery. Moreover, in non-polar
medium, the HPAs are not soluble and marginally active due to low accessibility to their active sites.
Furthermore, bulk HPAs are soluble in polar media that leads in difficulty of their recovery. It has been
found that when HPAs are immobilized on suitable supports with large surface area, the catalytic activity
of well-dispersed HPAs could be improved and simple separation of the supported catalysts from the
reaction media and products would be easily accessible [15,16].
Unfortunately, the use of supported HPA catalysts is not recommended in polar solvents (e.g., H O or
2
HOAc), because of their high solubility in these solvents which in turn leads to leaching of HPA from the
solid support and compromises the above mentioned advantages of the supported HPA catalyst. Thus, to
facilitate easy separation of the products or recovery and recycling of the HPA catalyst, an innovative
design of a homogeneous catalyst would be desirable.
2

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To rich this goal, a new family of BAIL, which was composed of POM polyoxometalate () anions and
appropriate organic cations, has attracted intense interest. BAILs-cation salts of HPAs usually have high
melting points due to the large volume and high valence of HPA-anions[17]. Up to now, a series of novel
HPA-IL catalysts have been prepared and their catalytic activities have been investigated in various
organic transformations [18-24].
Herein, we wish to develop the preparation, characterization and investigation the catalytic activity of
a new series of ILs-cation phosphotungstate based on 1,4-dimethyl piperazine and 1,10-phenantroline for
the esterification reaction.
2
Experimental
All materials were purchased commercially from Fluka Chemical Corp. and Merck & Co., Inc. and
1
were used without further purification. H NMR spectra of samples were recorded at a Bruker Advanced
DPX 400-MHz spectrometer. Fourier transform infrared (FT-IR) spectra were obtained as potassium
-1
bromide pellets in the range400–4000 cm using an AVATAR 370 Thermo Nicolet spectrophotometer.
Thermogravimetric analyses (TGA) were performed using a Du Pont 2000 thermal analysis apparatus at
-
1
heating rate 5
̊
C min from 30 to 800
̊
C under air atmosphere. The sample weight was ca. 10 mg and
C with 10 C/min using alumina sample holders.
was heated from room temperature up to 600
̊
̊
2
.1.1 Preparation of 4-(1,10-phenanthrolin-1-ium-1-yl)butane-1-sulfonate (PhBs1)
1
,10-Phenanthroline (10 mmol) and 1,4-butane sultone (20 mmol) were dissolved in anhydrous
toluene (10 mL), with a stirring at 75 C for 24 h under nitrogen atmosphere. The resulting white
̊
precipitate (PhBs ) was filtered and washed with diethyl ether and then dried in vacuum (Scheme 1, 1a).
1
A 87% yield of white solid product was obtained. (ESI-MS) m/z Calcd for PhBs : 316.37; Found: 316.08.
1
Anal. Calcd for C16
H
16
N
O
2 3
S (316.37): C, 60.74; H, 5.09; N, 8.86. Found: C, 60.79; H 5.1; N, 8.91%.
1
The spectral data for PhBs : H NMR (400 MHz, DMSO, TMS):
δ
1.16 (quintet, 2H, J = 7.6 Hz), 2.16
1
(
quintet, 2H, J = 7.6 Hz), 2.46 (t, 2H, J = 7.6 Hz), 5.96 (t, 2H, J = 7.6 Hz), 8.1 (m, 1H), 8.40-847 (m, 3H),
1
3
8
5
2
.82 (m, 1H), 9.34 (m, 1H), 9.41 (m, 1H), 9.66 (m, 1H). C NMR (400 MHz, DMSO):
1.9, 56.8, 121.1, 121.3, 124.2, 128.1, 128.4, 128.8, 136.3, 138.2, 143.2, 146.9, 150.2, 199.1.
.1.2 Preparation of 4-(1,4-dimethylpiperazin-1-ium-1-yl)butane-1-sulfonate (PipBs
δ 22.4, 27.7, 50.7,
1
)
A round bottom flask containing a magnet was charged with 1,4-dimethylpiperazine (10 mmol) and
,4-butane sultone (15 mmol) and stirred for 4 h under nitrogen atmosphere at room temperature. The
1
resulting white precipitate was filtered and washed with diethyl ether and then dried in vacuum which was
named as PipBs
1
(Scheme 2, 2a). A 95% yield of white solid product was obtained.
3

ACCEPTED MANUSCRIPT
1
The spectral data for PipBs
1
: H NMR (400 MHz, D
2
O, TMS):
δ
1.65-1.80 (m, 2H), 1.81-1.97 (m,
1
3
2
H), 2.28 (s, 3H), 2.66-2.86 (m, 4H), 2.88 (t, 3H, J= 6.8 Hz), 3.02 (s, 3H), 3.20-3.50 (m, 6 H). C NMR
(
400 MHz, D O): 20.3, 22.1, 31.20, 46.7, 50.5, 54.0, 56.5, 58.1. Anal. Calcd for C H N O S: C, 47.98;
δ
2
10 22
2
3
H, 8.86; N, 11.19; S, 12.81. Found: 47.95; H, 8.90; N, 11.21; S, 12.83.
2
1
.1.3 Preparation of 1,4-dimethylpiperazine-1,4-diium-1,4-diyl)bis(butane-1-sulfonate) (PipBs2)
A round bottom flask containing a magnet was charged with 1,4-dimethylpiperazine (10 mmol) and
,4-butane sultone (30 mmol) and stirred for 2h under nitrogen atmosphere at 60 C̊ . The resulting white
precipitate was filtered and washed with diethyl ether and then dried in vacuum which was named as
PipBs (Scheme 2, 2b). A 90% yield of white solid product was obtained.
2
1
The spectral data for PipBs : H NMR (400 MHz, D O, TMS):
δ 1.65-1.82 (m, 4H), 1.82-2.35 (m,
2
2
4
H), 2.90 (t, 4H, J= 8 Hz), 3.22 (s, 3H), 2.24 (s, 3H), 3.30-4.50 (m, 12H). (ESI-MS) m/z Calcd for
PipBs : 386.15; Found: 306.12. Anal. Calcd for C H N O S : C, 43.50; H, 7.82; N, 7.25; S, 16.59%.
2
14 30
2
6 2
Found: C, 43.40; H, 7.88; N, 7.23; S, 16.63%
+
3-
+
3-
2+
3-
2
.2 Preparation of the [PhBs
1
]
3
PW12O
40 , [PipBs
1
]
3
PW12O
40 and [PipBs
2
]
3
(PW12O
40
)
2
catalysts
PhBs
1 1
(18 mmol) or PipBs was dissolved in 25 ml of distillated water and added to a round bottom
flask containing a magnet and 25 ml of an aqueous solution of H PW O (6 mmol). Then, the mixture
3
12 40
was stirred at room temperature for 24 h. Finally, water was removed in vacuum to give
+
3-
+
3-
2+
3-
[
PhBs ] PW O
or [PipBs ] PW O
as solids. For the preparation of [PipBs ] (PW O )2
2 3 12 40
1
3
12 40
1
3
12 40
catalyst 12 mmol of H
3 2
PW12O40 and 18 mmol of PipBs were used with the same procedure. To simplify
the catalyst names we used [PhBs ] -PW (1c), [PipBs ] -PW (2c) and [PipBs ] -(PW) (2d) in the text.
1
3
1 3
2 3
2
The acid strength of the salts was measured with Hammett indicators (-3.0 < H < 0.8), including crystal
0
violet (pKa = 0.8), dicinnamalacetone (pKa = -3.0), and anthraquinone (pKa = -8.20). The prepared
1
13
catalysts were characterized with FT-IR, HNMR, CNMR, TG and EDX analysis.
+
3-
The spectral data for [PhBs ] PW O (1c): FTIR (KBr): 3428.8, 3074, 2925.5, 1629.5, 1527.3,
1
3
12 40
1
467.5, 1434.8, 1351.8, 1230.3, 1153.2, 1078, 1035.6, 977.7, 894.8, 809.9, 715.4, 593.9, 511. Positive-
+
1
ion electro-spray ionization mass spectrometry: (ESI-MS) m/z calc. 316.09, found. 316.2 (PhBs ).
Negative-ion electro-spray ionization mass spectrometry: (ESI-MS) m/z calc. 959.1, found. 959.5
3−
(
PW12O40 ).
+
3-
12 40
2+
3-
The spectral data (FTIR (KBr) for [PipBs ] PW O (2c) and [PipBs ] (PW O )2 (2d) catalysts:
1
3
2
3
12 40
+
3-
12 40
-1
FTIR (KBr) for [PipBs ] PW O : 1507, 1217, 1079, 1039, 977, 895, 801, 508 cm . FTIR (KBr) for
1
3
2+
3-
-1
[
PipBs ] (PW O ) : 1473, 1217, 1079, 977, 895, 802, 517 cm .
2 3 12 40 2
4

ACCEPTED MANUSCRIPT
2
.3 Typical procedure for the [PhBs
reaction
1 3 1 3 2 3 2
] -PW, [PipBs ] -PW and [PipBs ] -(PW) catalyzed esterification
In a typical esterification reaction procedure, a round-bottomed flask with a water segregator was
charged with acetic acid (1.80 g, 30 mmol), n-butanol (2.67 g, 36 mmol), and optimized amount of each
catalyst (0.25 g of [PhBs ] -PW 0.25 g of [PipBs ] -PW and 0.15 g of [PipBs ] -(PW) ). The resulting
1
3
,
1 3
2 3
2
solution was heated under reflux condition at 110 ̊C for 1.5 h with vigorous stirring, see Scheme 3. Then,
the reaction mixture was cooled to room temperature. The top liquid layer was simply separated by
decantation (or filtration) and analyzed using the gas chromatography. For other esterification reactions,
the test was carried out accordingly. Exceptionally, for esterification of acetic acid with ethanol,
cyclohexane was added as the water-carrying agent.
3
. Results and Discussion
1 3 1 3 2 3 2
.1 Preparation of [PhBs ] -PW (1c), [PipBs ] -PW (2c) and [PipBs ] -(PW) (2d) catalysts
3
The representation routes followed to prepare the novel introduced BAILs-PW are depicted in scheme
1
and 2. At the first, in a clean simple two steps procedure [PhBs ] -PW (Scheme 1, 1c) catalyst was
1
3
obtained (Scheme 1). Typically, 1,10-phenanthroline and 1,4-butane sultone were mixed and heated in
dry toluene. After completion of the prcipitation, the mixture was filtered and the white pricipitate was
washed sereval times with acetone and dried which was named as Phenanthrolinum-Butane sulfonate
(
PhBs ). Then, PhBs was added to an aquas solution of H PW O and stirred for 24 h. Finally, water
1 1 3 12 40
was evaporated under vacuum to give [PhBs ] -PW which was completly soluble in water and was
1
3
insoluble or very low soluble in aprotic solvents. It is worth to note that the attempts to graft two
sulfonate linker to 1,10-phenanthroline using various equivalents of 1,10-phenanthroline and 1,4-butane
sultone under different conditions were not successful. This fail is probably because of the increased
steric hindrance in the double linkers structure (PhBs , Scheme 1, 1b) and also probably because of the
2
disterbing effect of the inhainced positive charge on the ring aromaticity that force 1,10-phenanthroline to
be resistant against the intrance of the second linkage. In contrast, both of the mono linker structure and
double linkers structure of 1,4-dimethylpipirazine were successfuly obtaind by using different
experimental conditions. To achieve the mono linker structure (PipBs , Scheme 2, 2a) , 1,4-
1
dimethylpiperazine and 1,4-butane sultone were reacted in 1:1.5 molar ratio at room temperature for 12 h
while for the preparation of PipBs
2
(Scheme 2, 2b), 1:3 molar ratio of 1,4-dimethylpiperazine and 1,4-
butane sultone were reacted at 60 C
̊
for 24 h.
5

ACCEPTED MANUSCRIPT
1 3
Scheme1. Preparation of [PhBs ] -PW
Scheme2. Preparation of [PipBs ] -PW and [PipBs ] -(PW)
2
1
3
2 3
3
.2 Characterization of the catalysts
.2.1 IR spectra
3
The FT-IR technique is quite useful to identify or conform the structural and bonding changes in the
Keggin unit present in the prepared BAILs-PW comparing to that of the neat H PW O . FT-IR spectra
3
12 40
of [PhBs ] -PW catalyst, neat H PW O and pristine 1,10-phenanthroline are illustrated in Fig. 1. In the
1
3
3
12 40
FT-IR spectrum of 1,10-phenanthroline strong bands are observed in two frequency regions, namely from
6

ACCEPTED MANUSCRIPT
–1
-1
-1
-1
7
00 to 900 cm and from 1400 to 1650 cm . The strong bands at 738 cm and 854 cm have been
assigned to the out of the plane motion of the hydrogen atoms on the heterocyclic rings, and to the
hydrogen on the center ring respectively. Most intense and characteristic bands in the vibrational
-
1
spectrum of 1,10-phenanthroline appear in the region 1400 to 1600 cm . All of these bands involve C —
C and C — N stretching vibrations. Four other bands can be assigned to C — C and C — N vibrations,
two of which give rise to symmetric and two anti-symmetric in plane ring deformation frequencies (Fig.
1
8
a). FT-IR spectrum of PWA shows specific absorption bands correspond to the Keggin structure at 802,
-1
91, 979 and 1079 cm which are assigned to the stretching vibrations ʋas(W–Oe–W), ʋas(W–Oc–W),
ʋas(W–Ot) and ʋas(P–O*), respectively (Fig. 1b). From the FT-IR spectrum of [PhBs ] -PW (Fig. 1c), in
1 3
spite of the slight shift of four featured peaks positions, are appeared distinctively (800, 894, 977, 1078
-
1
cm ) which indicates that the Keggin structure of H
3
PW12O40 was well reserved after the protons in the
H PW O were substituted by the organic cation. On the other hand, S=O stretching vibrations at 1153
3
12 40
−
1
and 1230 cm were observed to confirm the presence of sulfonic group in [PhBs ] -PW catalyst. Finally,
1
3
the characteristic peaks of pristine 1,10-phenanthroline are obviously presented in the FT-IR spectrum of
PhBs -PW.These observations are clear proofs for the combination of organic cation (PhBs ) with
[
]
1 3
1
H
3
PW12O
40 via ionic interactions to produce a new catalyst of organic PWA salt.
7

ACCEPTED MANUSCRIPT
3 1 3
Fig. 1 FT-IR spectrum of pristine 1,10-Phenanthroline (a), H PW12O40 (b) and [PhBs ] -PW catalyst (c)
8

ACCEPTED MANUSCRIPT
1 3 2 3 2 3
Fig. 2 FT-IR spectrum of pure 1,4-dimethylpiperazine (a), [PipBs ] -PW (b) [PipBs ] -(PW) (c) H PW12O40 (d)
9

ACCEPTED MANUSCRIPT
The FT-IR spectra of pure 1,4-dimethylpiperazine, [PipBs
. Again, herein distinctive featured peaks of H 40 are clearly observable in the FT-IR spectra of
PipBs ] -PW and [PipBs ] -(PW) that confirm the successful preparation of these new BAILs-PW
1 3 2 3 2
] -PW and [PipBs ] -(PW) are shown in Fig.
2
3
PW12O
[
1
3
2 3
2
catalysts.
3
.2.2 NMR spectra of PhBs , PipBs and PipBs
2
1
1
1
1
In the HNMR spectrum of PhBs four featured peaks positions are appeared in the aliphatic region
which are assigned to the eight hydrogens of butane sulfonate linker attached to one of the two 1,10-
phenanthrolinum nitrogens which strongly confirms the successful preparation of organic PhBs (Please
1
+
see S1). The two triplet peaks (J = 7.6 Hz) give rise to CH -N (more deshielded
δ
5.96 ppm) and CH -S
2
2
+
(
δ
2.46 ppm), two quintet peaks (J = 7.6 Hz) refer to the two middle methylene groups (N -CH -CH -
2
2
2 2
CH -CH -S) of butane sulfonate linker (See S1b). The existence of butane sulfonate linker was
1
3
additionally confirmed in C-NMR spectrum when four carbon peaks were observed in the aliphatic
region of PhBs . Eight aromatic hydrogens of 1,10-phenanthrolinum were found in the aromatic region
1
which have been slightly more deshielded than aromatic hydrogen of pure 1,10-phenanthroline (See S1c).
Comparison of the hydrogen integrations in the aromatic and aliphatic regions approves the mono grafted
butane sulfonate (PhBs ) and confirms the unsuccessful preparation of PhBs .
1
2
1
Fig. 3 HNMR (400 MHz, D
2
O) spectrum of PipBs
1
2
(blue) and PipBs (green)
1
0

ACCEPTED MANUSCRIPT
The identification of PipBs
1
from PipBs
2
structure was simply confirmed with the comparsion of the
1
1
chemical shifts of N-CH
3
groups from their HNMR spectra. In HNMR spectrum of PipBs , more
1
+
deshielded N -CH group is appeared as a singlet peak at 3.02 ppm while N-CH group is appeared as a
3
3
1
singlet peak at 2.28 ppm. The HNMR spectrum of PipBs shows two singlet peaks for both deshielded
2
+
N -CH groups at 3.22 and 3.24 ppm which indicated that the chemical environment around these groups
3
2
are not completly chemical equivalent groups (Fig. 3). In another word, the structure of PipBs is not
completly symetrical or two methyl groups ocuppy almost different steric poistions. On the base of
1
HNMR principles, one of them has axial conformation which is more shielded (3.22 ppm) and another
1
3
has equiterial conformation (3.24 ppm) [25]. From the C-NMR spectrum of PipBs (see S2), the
1
+
chemical shift of deshielded N -CH group was observed at 54 ppm while N-CH was seen at 46.7 ppm.
3
3
3
.2.1 EDX analysis
The EDX spectra of the fresh and reused catalysts show that the [PhBs
1 3 1 3
] -PW, [PipBs ] -PW and
[
PipBs ] -(PW) have organometalic composition. The EDX spectra of the fresh catalysts confirm the
2
3
2
existence of butane sulfonate and phosphotangestate in the structure of the catalysts which is another
proof for the successful preparation of target BAILs-PW catalysts. The EDX spectra of the fresh and
1 3
reused [PhBs ] -PW catalyst after 6 runs for the esterification reaction between acetic acid and n-butanol
were prepared (See S3). The comparison of the elements and their weight percent in the fresh and reused
catalyst are almost the same which indicating that the structure, composition and catalytic activity of
[
PhBs ] -PW catalyst is remained with negligible changes after 8 runs (in agreement with negligible
1 3
product yield differences during 6 runs in Fig. 4).
3
.2.2 TG analysis
The TG analysis of three BAILs-PW catalysts indicated that all these catalysts are thermally stable up
to 300 °C nearby. For example, [PipBs ] -(PW) catalyst shows an endothermic peak of the DSC curve
2
3
2
emerged at 160 °C which refers to the loss of water (mass loss 1%). After this step, three step by step
+
exothermic peaks above 300 °C (320, 475 and 600 °C) are arisen. Most probably, [PipBs
2
] cation is
decomposed in the first and second steps at 325 °C and 475 °C, then in continue, [PW12
O
40] anion is
decompounded at 600 °C nearby (See S4).
3
.3 Optimization of the esterification reaction condition catalyzed by [PhBs ] -PW, [PipBs ] -PW
1 3 1 3
2 3 2
and [PipBs ] -(PW)
After synthesis and characterization of polyoxometalate-based ionic liquid salts, their catalytic role
and activity were explored for the esterification reaction. In order to optimize the reaction conditions, the
esterification reaction of acetic acid with n-butanol over various amounts of reactants and catalysts, at
1
1

ACCEPTED MANUSCRIPT
different temperatures was selected as model reaction (Table 1). Under neat condition at 110 C
considerable ester product was observed even after 4 h (entry 1). By using PipBs as catalyst, the reaction
did not progressed again but when 0.2 g of pure H PW O was added at 110 C, a heterogeneous solid–
̊ , no
2
̊
3
12 40
liquid biphasic system was observed and the reaction was progressed which after 2 h, 78% yield of the
desired ester was obtained on the base of GC analysis (entries 2 and 3). When 0.1 g of [PhBs ] -PW or
1
3
[
PipBs
clear that the existence of acidic hydrogen is critical for the esterification reaction. By using each of
PhBs ] -PW, [PipBs ] -PW or [PipBs ] -(PW) as catalyst below 100 C, a heterogeneous solid–liquid
1 3
] -PW was used as catalyst, the target ester was obtained (entries 4 and 5). From entries 1-5, it is
[
̊
1
3
1 3
2 3
2
biphasic system was observed due to the insolubility of the catalysts in monocarboxylic acid and
monohydric alcohol reactants. The catalysts became liquid when the reaction mixtures were heated to
1
10 ̊ C with stirring however made heterogeneous liquid-liquid systems with the reactants but dissolvable
with produced water. By increasing the molar ratio of alcohol to acid up to 1.2 and increasing amount of
the [PhBs ] -PW, [PipBs ] -PW catalysts from 0.1 g to 0.25 g, the reaction times were decreased and
1
3
1 3
stopped at 1.5 h while the yields of the reactions were improved (entries 7-13). The maximum yield
97%) of the ester was obtained when 0.15 g of [PipBs -(PW) was used as catalyst (entry 14). Larger
amounts of the catalysts did not lead to shorter reaction time or higher yield of the produced ester (entries
5-19). By further enhancing the molar ratio beyond 1.2, acetic acid became diluted in the presence of
(
]
2 3
2
1
excess amount of n-butanol and led to slight decrease in the obtained yield of target ester (entries 20-22).
Consequently, 1.2 molar ratio of n-butanol to acetic acid was selected as the optimized molar ratio.
Finally, by comparison of the obtained yields and the loading account of using [PhBs
PW and [PipBs -(PW) catalysts, [PipBs -(PW) was selected as catalyst of choice to extend its
generality in esterification reaction of various carboxylic acid and alcohols (entry 14).
1 3 1 3
] -PW, [PipBs ] -
]
2 3
2
2
]
3
2
3
.4 Extend the generality of the catalytic activity
Under optimized reaction conditions, the esterification reaction of monocarboxylic acids with
monohydric alcohols in the presence of the superior [PipBs ] -(PW) catalyst was performed as shown in
2
3
2
scheme 3 and table 2. From table 2, it can be clearly seen that good to excellent yields with complete
selectivity for corresponding esters (100%) were achieved in all cases. The prolonged carbon chains in
reactants did not significantly affect the high yields for esters (6c, 6g and 6j). However, reactants with
benzyl group more or less lowered the yields (6d, 6e and 6k).
For all [PhBs
1 3 1 3 2 3 2
] -PW, [PipBs ] -PW and [PipBs ] -(PW) as catalyst in esterification reaction, after the
mixtures were cooled to room temperature, the catalysts dissolved in the produced water precipitated at
1
2

ACCEPTED MANUSCRIPT
the bottom layer as gelatinous solid, and the produced ester with unconverted reactants were at the upper
liquid layer.
Table 1. Optimization of esterification reaction of acetic acid and n-butanol in the presence of prepared catalysts
[
a, b]
Entry
Molar ratio of ŔOH/RCOOH
T [ ͦC]
Catalyst (g)
Time(h)
Yield(℅)
1
1
1
10
10
10
neat
1
2
3
4
5
6
7
1
1
4
4
2
3
3
3
3
Trace
Trace
78
PipBs (0.2 g)
2
H
3
PW12
O40 (0.2 g)
1
8
0
[PhBs ] -PW (0.1 g)
1
1 3
60
1
1
1
00
10
10
[PipBs ] -PW(0.1 g)
1 3
1
70
[PipBs
1 3
] -PW (0.2 g)
1
90
[PhBs ] -PW (0.1 g)
1
3
1.2
85
1
1
10
10
[PipBs ] -(PW) (0.1 g)
2 3 2
8
9
1.2
1.2
1.2
1.2
1.2
3
2
2
2
90
85
87
91
[PhBs ] -PW (0.2 g)
1
3
1
1
10
10
[PipBs ] -PW (0.2 g)
1
1
0
1
1 3
[PipBs ] -(PW) (0.15 g)
2
3
2
1
10
[PhBs ] -PW (0.25 g)
1 3
1
1
1
1
1
1
1
1
2
2
2
2
3
4
5
6
7
8
9
0
1
2
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
90
93
97
96
97
96
92
91
92
89
86
1
1
1
1
1
1
.2
.2
.2
.2
.2
.2
110
110
110
110
110
110
[PipBs ] -PW (0.25 g)
1 3
2 3 2
[PipBs ] -(PW) (0.15 g)
[PipBs ] -(PW) (0.2 g)
2 3
2
[PipBs
2
]
3
-(PW)
2
(0.3 g)
[PipBs ] -(PW) (0.5 g)
2 3
2
[PipBs
1
]
3
-PW (0.3 g)
1
10
[PhBs ] -PW (0.4 g)
1 3
1.2
1
1
1
.3
.4
.6
110
110
110
2 3 2
[PipBs ] -(PW) (0.15 g)
[PipBs ] -(PW) (0.15 g)
2 3
2
[PipBs
2
]
3
-(PW)
2
(0.15 g)
a
b
Obtained from GC analysis, Using water segregator
2 3 2
Scheme 3. Esterification reaction catalyzed by [PipBs ] -(PW)
1
3

ACCEPTED MANUSCRIPT
Table 2. Esterification reaction of different carboxylic acids and alcohols in the presence of [PipBs
2
]
3
-(PW)
2
as catalyst
[
a]
Entry
RCOOH (4)
ŔOH (5)
[PipBs
2
]
3
-(PW)
2
Time(h)
Yield(℅)
Selectivity(%)
6
6
a
Acetic acid
Acetic acid
Acetic acid
Acetic acid
Benzoic acid
Lactic acid
n-Butanol
Ethanol
0.15 g
0.15 g
0.15 g
0.15 g
0.15 g
0.15 g
1.5
1.5
2
94
96
93
87
89
87
100
100
100
100
100
100
b
6
c
Dodecanol
Benzyl alcohol
n-Butanol
Ethanol
6
d
2.5
1.5
2
6
e
6
f
6
6
g
Mandelic acid
Lauric acid
Dodecanol
n-Butanol
Benzyl alcohol
n-Butanol
Ethanol
0.15 g
0.15 g
0.15 g
0.15 g
0.15 g
0.15 g
2
1.5
2
90
92
87
92
88
96
100
100
100
100
100
100
h
6
i
Lactic acid
6
j
Mandelic acid
Benzoic acid
Lactic acid
2
6
k
1.5
1.5
6
l
Ethanol
a
Obtained from GC analysis
3
.4 Recoverability and Reusability of the catalysts
As mentioned above, after completion of the esterification reaction and cooling the mixtures down
slowly, the catalyst is appeared at the bottom layer as gelatinous solid in the produced water. Thus, in all
cases, the solid catalyst could be conveniently recovered simply by decantation (or filtration) and easily
recycled into the next run. To study the recycling and reusability performance of the introduced BAILs-
PW, the esterification of acetic acid with n-butanol was selected as model reaction. The esterification
reaction was checked for six repeated runs (fresh + five consecutive runs) using each of [PhBs
1 3
] -PW,
[
PipBs -PW and [PipBs -(PW) as catalyst. The yield of the desired ester using these catalysts for six
1
]
3
2
]
3
2
repeated runs is represented (Fig. 4). It is obvious that the yield difference between the first and 6th runs
is not considerable for all of the introduced catalysts by taking into account the weight loss in the
operation for recovering the catalysts. These observations indicate the still very slow deactivation for the
1 3
catalysts. Additionally, the EDX analysis of [PhBs ] -PW after six runs (see S3b) demonstrates a quite
stable structure and very low leaching accounts of the catalyst elements after the repeated use of the
catalyst indicating that the catalyst efficiency is almost completely maintained during 6 consecutive runs
in agreement with the recoverability/reusability study data.
1
4

ACCEPTED MANUSCRIPT
9
9
9
9
8
8
8
8
8
6
4
2
0
8
6
4
2
0
PipBs2-PW
PipBs1-PW
PhBs-PW
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
+
Fig. 4 Recycling of [PhBs]
3
PW, [PipBs
1
]
3
-PW and [PipBs
2
]
3
-(PW)
2
for the esterification reaction of acetic acid with n-butanol (6a). Conditions:
+
Molar ratio of n-butanol/CH
3
COOH = 1.2, loading of [PhBs]
3
PW, [PipBs -PW=0.25 g and [PipBs -(PW) =0.15 g, T=110 C, t=1.5 h
1
]
3
2
]
3
2
̊
4
Conclusion
In conclusion, a new series of BAILs with the phosphotungstate anion was prepared and well
characterized. The catalytic activity and reusability of the novel introduced acidic catalysts was examined
for the esterification reaction of monocarboxylic acids with monohydric alcohols under optimized
2 3 2
condition. It was found that [PipBs ] -(PW) is the superior catalyst for the esterification reaction. The
catalysts showed excellent activity and was simply separable and reusable up to 6 runs. The EDX analysis
and the recoverability/reusability study of the introduced catalysts over repeated runs proved quite stable
structures with almost completely retained efficiency of catalysts. Other applications of these BAILs are
under investigation in our lab.
List of abbriviations
S: Supplementary file
POM: polyoxometalate
BAIL: Brønsted Acidic Ionic Liquid
PhBs : 4-(1,10-phenanthrolin-1-ium-1-yl)butane-1-sulfonate
1
1
5

ACCEPTED MANUSCRIPT
PipBs
1
: 4-(1,4-dimethylpiperazin-1-ium-1-yl)butane-1-sulfonate
PipBs : 1,4-dimethylpiperazine-1,4-diium-1,4-diyl)bis(butane-1-sulfonate)
2
BAILs-PW: Brønsted acidic ionic liquids with phosphotungstate anion
3
-
[
[
[
PhBs ] -PW: [4-(1,10-phenanthrolium-1-yl)butane-1-sulfonate] PW O
1 3 3 12 40
3
-
PipBs ] -PW: [4-(1,4-dimethylpiperazin-1-ium-1-yl)butane-1-sulfonate] PW O
12 40
1
3
3
3-
12 40
PipBs ] -(PW) : [1,4-dimethylpiperazine-1,4-diium-1,4-diyl)bis(butane-1-sulfonate)] (PW O )2
2
3
2
3
Acknowledgement
The authors gratefully acknowledge to the Research Council of Yasouj University for financial
support of this work.
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7

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Novel SO H-functionalized polyoxometalate-based ionic liquids as highly
3
efficient catalysts for esterification reaction
1
∗
2
Mosadegh Keshavarz , Nasir Iravani and Abolfath Parhami
Highlights
ꢀ
Three classes of novel SO H-functionalized polyoxometalate-based ionic liquids were
3
synthesized
ꢀ
The esterification reactions were efficiently catalyzed in the presence of introduced ionic
liquids
ꢀ
ꢀ
ꢀ
The catalysts were easily separable and reusable up to six runs
The procedure is simple and green
Very low leaching of the polyoxometalate anion was observed
∗
Mosadegh Keshavarz; mail: chem.mosadegh@gmail.com; Zip-code: 75918-74831