Catalytic synthesis of chloroacetates with thermoregulated ionic liquids based on vanadium-substituted polyoxometalate

Article abstract of DOI:10.1039/c8ra10659b

A series of polyoxometalate-based ionic liquid (POM-IL) catalysts with functional sulfonic acid groups, [TEAPS]_3+nPW_12-nV_nO₄₀ (n = 1, 2, 3) were synthesized and characterized by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectrophotometry (FT-IR), UV-Vis spectrophotometry (UV), potentiometric titration and thermogravimetry-differential scanning calorimetry (TG-DSC). The catalytic ability and reusability of the POM-IL catalysts were evaluated on esterification of chloroacetic acid and n-amyl alcohol. The optimum reaction conditions, 0.2 g of the catalyst amount, 10 mL of water carrier, 140 °C of reaction temperature, and 1.2/1 of the molar ratio of alcohol/acid, were obtained by an orthogonal test. [TEAPS]₅PW₁₀V₂O₄₀ was found to be the best active catalyst with an esterification rate of 98.75% and could be reused five times without significant decrease in activity. The ionic liquid acted as a temperature-responsive catalyst, forming a homogeneous mixture with the reactants at reaction temperature, and could be precipitated and separated from products when the reaction ends at ambient temperature. Therefore, an environmentally friendly and highly efficient approach for the synthesis of chloroacetates is provided.

Full text of DOI:10.1039/c8ra10659b

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Catalytic synthesis of chloroacetates with
thermoregulated ionic liquids based on vanadium-
substituted polyoxometalate
Cite this: RSC Adv., 2019, 9, 8404
Jingsen Yan,a Zeqing Wang,a Yongsheng E,a Fengwei He,a Danfeng Zhanga
ab
*
and Qingyin Wu
A series of polyoxometalate-based ionic liquid (POM-IL) catalysts with functional sulfonic acid groups,
[TEAPS]3+nPW12ꢀnVnO40 (n ¼ 1, 2, 3) were synthesized and characterized by nuclear magnetic resonance
spectroscopy (NMR), Fourier transform infrared spectrophotometry (FT-IR), UV-Vis spectrophotometry
(UV), potentiometric titration and thermogravimetry-differential scanning calorimetry (TG-DSC). The
catalytic ability and reusability of the POM-IL catalysts were evaluated on esterification of chloroacetic
acid and n-amyl alcohol. The optimum reaction conditions, 0.2 g of the catalyst amount, 10 mL of water
carrier, 140 ꢁC of reaction temperature, and 1.2/1 of the molar ratio of alcohol/acid, were obtained by an
orthogonal test. [TEAPS]5PW10V2O40 was found to be the best active catalyst with an esterification rate
of 98.75% and could be reused five times without significant decrease in activity. The ionic liquid acted
as a temperature-responsive catalyst, forming a homogeneous mixture with the reactants at reaction
temperature, and could be precipitated and separated from products when the reaction ends at ambient
temperature. Therefore, an environmentally friendly and highly efficient approach for the synthesis of
chloroacetates is provided.
Received 31st December 2018
Accepted 9th February 2019
DOI: 10.1039/c8ra10659b
rsc.li/rsc-advances
Cole et al. rstly synthesized Brønsted acidic ILs that bear an
alkyl sulfonic acid group in imidazolium cations or triphenyl-
phosphine cations.17 These Brønsted acidic ionic liquids
1. Introduction
Chloroacetates are important organic synthesis intermediates
in medicine, pesticides, plasticizers, and preparation of the
esters is mostly achieved through acid-catalyzed reaction
processes with mineral acids, such as H2SO4, HCl, H3PO4.1–3
However, these acids as homogenous catalysts are hardly
separated and reused. Furthermore, these acids are highly
corrosive and need to be neutralized at the end of the esteri-
cation reaction.4–6 Thus, various heterogeneous catalysts, such
as polyoxometalates (POM), acidic resins, solid superacids have
been explored as alternatives.7–9 However, these catalysts also
have some disadvantages such as easy deactivation and high
mass transfer resistance, which limit their application.
In response to both the advantages and disadvantages of
homogeneous and heterogeneous catalysts, phase-transfer
catalysts and temperature regulated catalysts have attracted
much attention.10,11 Among them ionic liquids (ILs) have been
regarded as green reaction media or promise catalyst owing to
their excellent thermal stability, outstanding solubility, negli-
gible volatility, diversied structure and physiochemical prop-
erties.12–14 Cations and anions of ILs can be fashioned to bind
specic functional group for specic chemical applications.15,16
exhibited temperature-controlled liquid–solid separation as
solvent/catalysts for Fischer esterication and the pinacol
rearrangement. Then, there were many articles on the func-
tionalized ILs used in different esterication, the esterication
system is homogeneous at reaction temperature and heteroge-
neous at the end of reaction.18–20 However the ionic liquid
catalysts is also not an excellent candidate owing to high
content of ionic liquid needed in the reaction and relatively long
reaction time.
In order to resolve this problem, the design and synthesis of
polyoxometalate-based ionic liquid catalysts (POM-ILs) have
been applied in esterication reaction.21–24 However, the regular
anion in POM-ILs is binary polyoxometalate rather than ternary,
only a few reports focus on ternary polyoxometalate. In fact,
ternary polyoxometalates usually show more perfect properties
than binary ones such as reversible thermal response and
electrochemical performance.25–28 Herein, a series of novel pol-
yoxometalate ionic liquid catalysts were prepared by using an
organic ammonium, 1-(3-sulfonic group) graed triethylamine
(TEAPS) and vanadium-substituted ternary heteropolyacids.
The catalysts have good activity and reusability in esterication
system owing to their acidity and thermoregulated property.
Their structures, activities in esterication, reversible thermal
aSchool of Biomedical & Chemical Engineering, Liaoning Institute of Science and
Technology, Benxi 117004, Liaoning, P. R. China. E-mail: qywu@zju.edu.cn
bDepartment of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China
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responsive properties and reaction mechanism are also temperature. Water was rstly evaporated in a water bath at
investigated.
50 ꢁC and then removed under vacuum at 60 ꢁC to give the nal
products. The obtained [TEAPS]4PW11VO40, [TEAPS]5PW10V2O40
and [TEAPS]6PW9V3O40 were pale yellow powder, yellow quasi
solid and brown gel-type solid, and their melting points were
2. Experimental section
2.1 Instrumentation and reagents
ꢁ
ꢁ
ꢁ
determined to be 145.0 C, 105.4 C and 88.1 C, respectively.
1H NMR and 13C NMR spectra were recorded on a Bruker
AVANCE 500 MHz spectrometer in D2O. Solid-state 31P MAS
NMR spectra was carried out on a Varian 400 InnityPlus
spectrometer at relaxation delay of 4 s. Infrared spectra was
measured by Beijing Beifen-Ruili WQF-510A FT/IR spectrometer
during the wavenumber range 400–4000 cmꢀ1 using KBr pellet.
UV absorption spectra was monitored by a Beijing Persee Spe-
cord TU-1901 UV-Vis spectrophotometer during scanning range
190–400 nm. X-ray powder diffraction analysis was conducted
on a BRUKER D8 ADVANCE X-ray diffractometer using a Cu
tube operated at 40 kV and 40 mA in the range of 2q ¼ 4–40ꢁ at
a rate of 0.02ꢁ sꢀ1. The thermal stability of samples was inves-
tigated on a SHIMADZU thermal analyser in the range of 25–
550 ꢁC at a rate of 10 ꢁC minꢀ1. The reaction products were
measured by Agilent 1200 high performance liquid chroma-
tography (HPLC). Melting point of samples was measured using
WRS-1B digital microscopic melting point apparatus. The
acidity of samples was determined using potentiometric titra-
tion. For the potentiometric titration, 0.1 g of POM-ILs solid was
dissolved in menthol (25 mL). The solution was titrated with
0.01 mol Lꢀ1 solution of n-butylamine in acetonitrile. The
potential variation was measured by Shanghai Leici pHS-2F
digital acidity meter using pH composite electrode.
2.3 Procedure for esterication reactions
The typical esterication reaction process was as follows:
chloroacetic acid (0.16 mmol), 1-pentanol (0.192 mmol), POM-
ILs catalysts (0.5 g) and benzene (10 mL) were added into
a three-necked ask with a water segregator. The reaction was
reuxed at 140 ꢁC with stirring until the water separated in the
water segregator no longer increases. The products were ana-
lysed by HPLC with external standard method. The determi-
nation conditions were as follows: Agilent ZORBAX XDB-C18
column (4.6 ꢂ 150 mm), UV absorbance detector at 213 nm, and
the mobile phase was ethanol and water (65 : 35, v/v), ow rate
at 1.0 mL minꢀ1 and column temperature at 20 C. Esterica-
tion rate was calculated by conversion of chloroacetic as follows:
ꢁ
ðC0 ꢀ CRÞ
Y ¼
ꢂ 100
C0
wherein C0 was initial content of chloroacetic acid and CR was
chloroacetic acid content at the end of reaction.
3. Results and discussion
3.1 NMR analysis
Fig. 2 shows the 1H NMR spectrum (Fig. 2a), 13C NMR (Fig. 2b)
All the chemicals were of analytical grade and used without
further purication.
and 31P MAS NMR of [TEAPS]5PW10V2O40. The H NMR chem-
1
ical shi values (ppm) are: d 1.33 (t, 9H), 2.15 (m, 2H), 2.98 (t,
2H), 3.17 (t, 2H), and 3.36 (m, 6H), corresponding to the
structural formula ([(C2H5)3N(CH2)3SO3H]PW10V2O40). Peaks at
1.33 ppm and 3.36 ppm correspond to methyl and methylene in
ethyl group, respectively. Peaks at 2.15 ppm, 2.98 ppm and
3.17 ppm correspond to three methylene linking to N and S
atoms.21 The 13C NMR chemical shi values (ppm) for
[TEAPS]5PW10V2O40 are: d 9.61, d 20.16, d 50.08, d 55.77, and
d 57.75, representing carbon atoms in ve different chemical
environments, respectively. For the solid 31P MAS NMR spectra
(Fig. 2c), the sharp resonance at ꢀ15.4 ppm was assigned to the
chemical shi of phosphorus, and the peak value is a little more
negative compared to the corresponding signal from HPW(dp ¼
ꢀ15.0 ppm).33 It may be a result of the interaction between the
Keggin tungstophosphate anion and the V5+ ion aer the
synthesis of [TEAPS]5PW10V2O40. The results indicated that the
heteropoly acid and TEAPS were successfully assembled.
2.2 Synthesis of the POM-ILs catalysts
Synthesis procedure of the POM-ILs was expressed in the
following schematic Fig. 1. TEAPS was synthesized according to
the literature,21 and the detailed procedure was as follows: 1,3-
propane sultone (0.066 mol) and triethylamine (0.066 mol) were
dissolved in acetone(13 mL). Then they were mixed and stirred
at 50 ꢁC for 12 h. A white precipitate (TEAPS) was ltered,
washed with acetone three times, and dried at 60 ꢁC in
a vacuum. A series of polyoxometalate (POM), H4[PW11VO40],
H5[PW10V2O40], H6[PW9V3O40] and the POM-ILs were synthe-
sized according to the literatures.29–32 The synthesis method of
POM-ILs catalysts are as follows: the pre-synthesized TEAPS and
POM, H4[PW11VO40], H5[PW10V2O40], H6[PW9V3O40] were taken
in 4 : 1, 5 : 1 and 6 : 1 molar ratio to give one mole of
[TEAPS]4PW11VO40, [TEAPS]5PW10V2O40, and [TEAPS]6PW9V3-
O40. TEAPS was added into an aqueous solution of various POM,
respectively. The mixture was stirred for 12 h at room
3.2 FTIR and UV analysis
Fig. 3 shows FT-IR spectra of H5[PW10V2O40], fresh [TEAPS]5-
PW10V2O40 and recycled [TEAPS]5PW10V2O40. The wavenumber
of major bands of the compounds are given in the Table 1. As
seen from Fig. 3 and Table 1, there were four characteristic
vibrational bands which were assigned to nas(P–Oa), nas(M]Od),
nas(M–Ob–M) and nas(M–Oc–M), (M]W, V) in the region
Fig. 1 Scheme of POM-ILs synthesis procedure.
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Fig. 3 FT-IR spectra of H5[PW10V2O40] (a), fresh [TEAPS]5PW10V2O40
(b) and recycled [TEAPS]5PW10V2O40 (c).
decomposition. At the same time they maintained structure of
sulfonic-functionalized triethylamine cation when TEAPS was
added. This further proved the heteropoly acid and TEAPS were
successfully assembled. Compared with the fresh catalyst, the
infrared spectrum of the recycled catalyst was almost
unchanged, indicating that Keggin-type structure of the catalyst
remained stable aer catalytic reaction.
The UV absorption spectrum of these compounds are shown
in Fig. 4. As shown Fig. 4, we can nd that the absorption bands
of [TEAPS]5PW10V2O40, fresh [TEAPS]5PW10V2O40 and recycled
[TEAPS]5PW10V2O40 appeared at about 191 nm and 252 nm. The
absorption bands were attributed to the charge transfer
between oxygen and coordinate metal.30,34 This can further
provide an evidence that Keggin-type structure was still present
in fresh [TEAPS]5PW10V2O40 and recycled [TEAPS]5PW10V2O40
.
3.3 XRD analysis
The phase and structure of the heteropoly compounds were
further identied, as shown in Fig. 5. It is clear that the XRD
pattern of [TEAPS]5PW10V2O40 was distinctly different from that
of the pure heteropoly acid. For H5[PW10V2O40], there were
typical and strong diffraction peaks in the range of 2q ¼ 7–12ꢁ,
18–24ꢁ, 24–36ꢁ, which revealed its crystalline state. The strong
diffraction peaks at 7–12ꢁ can be considered as the POM anion
structure, they were also observed in the XRD pattern of
Fig. 2 1H NMR (a), 13C NMR (b) and 31P MAS NMR spectra of
[TEAPS]5PW10V2O40
.
between 700 and 1100 cmꢀ1. Those four well-known charac- [TEAPS]5PW10V2O40 in the small angle area, suggesting that the
teristic bands proved the presence of Keggin-type heteropoly POM anion structure existed in the hybrid material.30,34
anion.34,35
However, [TEAPS]5PW10V2O40 exhibited a broad diffraction
Compared with pure heteropolyacids, the M–Od and P–Oa peak in the range of 2q ¼ 12–40ꢁ, which revealed its amorphous
vibration frequencies of POM-ILs were decreased when sulfo- state, which was consistent with the gel-type phase at room
group graed ammonium ions were added to POM. This temperature. It is reported that an organized layer-type struc-
phenomenon could be explained that the electrostatic between ture existed in this series of compounds, and the weak
the organic cation and heteropolyacid anions were weakened as connections of the layers leaded to the appearance of smectic
the anion–anion distance increases.34 Furthermore, some other gel-type phase.30,34
characteristic peaks at 1160 cmꢀ1, 2977 cmꢀ1 were assigned to
nS]O and nC–H, respectively.34 Since the characteristic peaks still
existed, we can infer that these compounds still retained
3.4 TG-DSC analysis
TG-DSC curves of typical samples, H5PW10V2O40 and [TEAPS]5-
PW10V2O40, were shown in Fig. 6, which displayed three steps
Keggin-type
structure
of
heteropolyanion
without
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Table 1 The IR spectrum of compounds at 1100–700 cmꢀ1
Vibrations/cmꢀ1
PW10V2
Fresh [TEAPS]5PW10V2O40
Recycled [TEAPS]5PW10V2O40
P–Oa stretching
M]Od stretching
M–Ob–M stretching
M–Oc–M stretching
S]O bending
1074
975
883
794
—
1066
968
888
1064
964
890
804
800
1160
2977
1486
1158
2981
1489
–CH2 stretching
–CH2 scissoring
—
—
Fig. 4 UV spectra of H5[PW10V2O40] (a), fresh [TEAPS]5PW10V2O40 (b)
and recycled [TEAPS]5PW10V2O40 (c).
Fig. 6 TG-DSC plots of H5PW10V2O40 (a) and [TEAPS]5PW10V2O40 (b).
3.5 Determination of acidity of POM-ILs catalysts
Esterication is a typical acid-catalyzed reaction and so it is
necessary to measure the acidity of catalysts. The acidity of
POM-ILs were determined by potentiometric titration with n-
butylamine. Potentiometric titration curves of different POM-
ILs were shown in Fig. 7. The initial electrode potential (Ei) or
pH before the titration demonstrates the maximum strength of
the acid sites. At the end of titration, the value from which the
plateau is reached (mmol amine per g catalyst) indicates the
total number of acid sites.36,37 The acid strength of heteropoly
acid can be judged by initial titration potential (Ei). It is dened
as very strong acid (Ei > 100 mV), strong acid (0 mV < Ei < 100
mV), and weak acid (ꢀ100 mV < Ei < 0 mV).37 As shown in Fig. 7,
Fig. 5 XRD patterns of H5PW10V2O40 and [TEAPS]5PW10V2O40
.
ꢁ
ꢁ
weight loss for the H5PW10V2O40 at 144.7 C, 220–400 C and
400–550 ꢁC. The weight loss below 400 ꢁC corresponded to loss
of surface adsorption water and protonated water. The primary
weight loss between 400–550 ꢁC corresponded to the loss of
structure water and decomposition of Keggin-type structure.
For [TEAPS]5PW10V2O40, there was an obvious endothermic
peak at 105.4 ꢁC, which corresponded to its melting point. The
large weight loss beginning at 350 ꢁC was due to the decom-
position of organic cation, suggesting that [TEAPS]5PW10V2O40
the initial Ei value of all POM-ILs catalysts, [TEAPS]4PW11VO40
¼
341 mV (pH ¼ 1.25), [TEAPS]5PW10V2O40 ¼ 263 mV (pH ¼ 2.56),
[TEAPS]6PW9V3O40 ¼ 229 mV (pH ¼ 3.13), which meant that
acid strength of POM-ILs catalysts are very strong acid. The
was stable below 350 C.30
ꢁ
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catalytic activity of conventional catalyst, such as sulfuric acid,
presented unsatisfactory esterication rate of 87.47% (entry 2).
Although serial heteropolyacid catalysts exhibited high esteri-
cation rate (nearly 95%, entry 3–5), it is hardly separated from
the product.36 However, it is noted that POM-ILs catalysts not
only could be separated from the product conveniently but also
exhibited notable catalytic activity (95.3–98.75%, entry 6–8).
Esterication reaction mechanism of chloroacetic acid using
[TEAPS]5PW10V2O40 catalyst was proposed in Fig. 9. Carbonyl of
chloroacetic acid was protonated by hydrogen ions which were
provided by functional group TEAPS in the ionic liquid catalysts
and n-pentyl alcohol attack carbocation of carbonyl in chloro-
acetic acid. The ester was generated by dehydrate process.
We measured the acidity of POM-ILs by potentiometric
titration with n-butylamine to analyze any relationship between
acidic properties and catalytic activity. As previous mentioned,
Fig. 7 Potentiometric titration curves of different POM-ILs.
the order of acid strength was as follows: [TEAPS]4PW11VO40
[TEAPS]5PW10V2O40 > [TEAPS]6PW9V3O40. The number of acid
sites was in the following order: [TEAPS]5PW10V2O40
[TEAPS]4PW11VO40 > [TEAPS]6PW9V3O40
>
order of acid strength was as follows: [TEAPS]4PW11VO40
[TEAPS]5PW10V2O40 > [TEAPS]6PW9V3O40, whose order is in
accordance with their initial pH values. The number of acid
>
>
.
sites was in the following order: [TEAPS]5PW10V2O40
[TEAPS]4PW11VO40 > [TEAPS]6PW9V3O40
>
As esterication reaction is a typical acid-catalyzed mecha-
nism, generally, the higher acid strength of catalysts, the higher
catalytic activities. However, the esterication rate of
[TEAPS]4PW11VO40 was less than [TEAPS]5PW10V2O40 or
[TEAPS]6PW9V3O40. This result could be attributed to their
different melting point and solubility in reaction system.
[TEAPS]4PW11VO40 catalyst has higher melting point (145.0 ꢁC)
and partly dissolved in the reactants at reaction temperature,
and that it was more likely to result in lower activity. Another
reason may be resulted from its less number of acid sites. From
the result of esterication reaction over various catalysts and
their phenomenon, it is assumed that the sulfonic-
functionalized triethylamine cation provides the acid site
responsible for the high activity in esterication, and the static
interaction between organic cation and polyoxometalate anion
is crucial to temperature responsive characteristics, which is
responsible for the solid–liquid–solid phase transformation
and catalyst separation.
The effect of various reaction parameters, including the
amount of catalyst, water carrier volume, reaction temperature,
the ratio of alcohol to acid, were investigated by an orthogonal
experiment method using (TEAPS)5PW10V2O40 as catalyst. The
above-mentioned four reaction parameters and three orthog-
onal levels were reasonably designed using L9(34) orthogonal
table. The effect of every experiment factor and level on esteri-
cation rate was shown in Table 3. Range analysis showed that
the reaction temperature was the dominant factor, next were
water carrier volume, the amount of catalyst and the ratio of
alcohol to acid in descending order. The optimized solution was
that the amount of catalyst was 0.2 g, water carrier was 10 mL,
reaction temperature was 140 ꢁC, and the ratio of alcohol to acid
.
3.6 Analysis of catalytic esterication reaction process
Conventional homogenous catalyst for esterication is difficult
to be separated and reused from product. However, [TEAPS]5-
PW10V2O40 catalyst can easily overcome the above-mentioned
problems due to a characteristic of temperature-controlled
liquid–solid separation. Its reaction process with temperature
change was shown in Fig. 8. At the beginning of the reaction,
the catalyst was undissolved in the reactant mixture and formed
a heterogeneous system (Fig. 8a). At reaction temperature, the
catalyst was dissolved in the mixture, the system transferred
from heterogeneous to homogeneous and became yellow owing
to the colour of catalyst itself (Fig. 8b). At the end of the reac-
tion, with temperature decreased, the produced colourless ester
was in the upper level of the resulting mixture, and the catalyst
gradually precipitated at bottom (Fig. 8c). Therefore the catalyst
could be easily recycled from the products by simple ltration.
Table 2 listed the results of esterication reaction over
various catalysts. The esterication of chloroacetic acid with n-
pentyl alcohol system can be self-catalytic and reach only 75%
of esterication rate because of its strong acidity (entry 1). The
Fig. 8 Photographs of the esterification of chloroacetic acid and n- was 1.2 : 1.
pentyl alcohol over [TEAPS]5PW10V2O40 catalyst. (a) [TEAPS]5PW10V2-
O40 (light yellow solid at bottom), chloroacetic acid (white solid in the
middle), and n-pentyl alcohol (liquid in the upper level) before reac-
tion; (b) homogeneous yellow mixture during the reaction; (c) the
catalyst has precipitated at bottom, the colourless product is in the
upper level at the end of the reaction.
3.7 Reusability of [TEAPS]5PW10V2O40 catalyst
As the catalyst could be easily recovered by simple ltration at
the end of the reaction, the catalyst without any regeneration
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Table 2 Results of esterification reaction over various catalystsa
Esterication
Entry
Catalyst
Phenomenon
rate (%)
1
2
3
4
5
6
7
8
Without catalyst
H2SO4
H4PW11VO40
H5PW10V2O40
H6PW9V3O40
[TEAPS]4PW11VO40
[TEAPS]5PW10V2O40
[TEAPS]6PW9V3O40
Homogenous1,2
75.00
87.47
94.52
94.63
95.00
95.30
98.75
97.54
Homogenous1,2
Homogenous1,2
Homogenous1,2
Homogenous1,2
Partly dissolved1, phase seperation2
Homogenous1, phase separation2
Homogenous1, phase separation2
a
1, 2 represent the catalyst state during reaction and aer reaction, respectively.
Fig. 10 Reusability of [TEAPS]5PW10V2O40 catalyst on the esterifica-
tion reaction.
was reused for the next run. The reusability of [TEAPS]5PW10
-
Fig. 9 Esterification reaction mechanism of chloroacetic acid using
V2O40 in the esterication reaction system was shown in the
Fig. 10. The esterication rate decreased from 98.75% for the
rst run to 90.97% for the h run. The IR and UV spectra for
the recycled catalyst was well consistent with the fresh catalyst
(see Fig. 3 and 4), which meant that structure of the catalyst was
stable. Thus, the slight decrease in catalytic activity result from
the slight loss of catalyst during dissolution process.
[TEAPS]5PW10V2O40 catalyst.
Table
3 Orthogonal experimental scheme and results of chlor-
a
oacetates catalyzed by (TEAPS)5PW10V2O40
Esterication
rate (%)
No.
A (g)
B (mL)
C (ꢁC)
D
1
2
3
4
5
6
7
8
1(0.1)
1(0.1)
1(0.1)
2(0.2)
2(0.2)
2(0.2)
3(0.3)
3(0.3)
3(0.3)
266.29
272.43
264.43
8.00
1(5)
1(100)
2(120)
3(140)
2(120)
3(140)
1(100)
3(140)
1(100)
2(120)
250.03
267.94
285.18
35.15
1(1.1)
2(1.2)
3(1.3)
3(1.3)
1(1.1)
2(1.2)
2(1.2)
3(1.3)
1(1.1)
265.02
270.52
267.61
5.5
80.21
91.82
94.26
89.09
97.78
85.56
93.14
84.26
87.03
4. Conclusions
2(10)
3(15)
1(5)
2(10)
3(15)
1(5)
2(10)
3(15)
262.44
273.86
266.85
11.42
A series of thermo-regulated ionic liquids catalysts were
synthesized by ionic self-assembly method with sulfonic-
functionalized triethylamine cation and vanadium-substituted
heteropolyacids. The ionic liquid, [TEAPS]5PW10V2O40 exhibi-
ted the highest catalytic activity with esterication rate of
98.75% under the optimized conditions. The sulfonic-
functionalized triethylamine cation provided the acid site
responsible for the high activity in esterication, and the static
interaction between organic cation and polyoxometalate anion
led to temperature responsive characteristics, which was
responsible for the solid–liquid–solid phase transformation
and separation. The catalyst could be easily recycled by simple
ltration and reused without signicant decrease.
9
K1
K2
K3
R
Factor order: C > B > A > D
Optimal solution: A2B2C3D2
a
A, B, C, D represent the amount of catalyst, water carrier volume,
reaction temperature, the ratio of alcohol to acid, respectively. The
numbers in brackets represent orthogonal level order, K1, K2, K3
represent the sum of the experimental results at each level,
respectively. R represents range.
Conflicts of interest
There are no conicts to declare.
This journal is © The Royal Society of Chemistry 2019
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19 M. Bonamico, A. Medici, C. Chiesa and F. Carpino, Green
Chem., 2012, 14, 1736–1742.
Acknowledgements
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The study was nancially supported by the key projects of
Liaoning Provincial Natural Science Foundation of China (No.
20170540475, 201602404 & 20180550114), the Zhejiang Provin-
cial Natural Science Foundation of China (No. LY18B010001)
and PhD Research Startup Foundation of Liaoning Institute of
Science and Technology (No. 1810B08 & 1810B07).
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