Robust acidic pseudo-ionic liquid catalyst with self-separation ability for esterification and acetalization

Article abstract of DOI:10.1007/s11696-019-00693-1

The novel acidic pseudo-ionic liquid catalyst with self-separation ability has been synthesized through the quaternization of triphenylphosphine and the acidification with silicotungstic acid. The pseudo-IL showed high activities for the esterification with average conversions over 90%. The pseudo-IL showed even higher activities for acetalization than traditional sulfuric acid. The homogeneous catalytic process benefited the mass transfer efficiency. The pseudo-IL separated from the reaction mixture automatically after reactions, which was superior to other IL catalysts. The high catalytic activities, easy reusability and high stability were the key properties of the novel catalyst, which hold great potential for green chemical processes.

Full text of DOI:10.1007/s11696-019-00693-1

Chemical Papers
https://doi.org/10.1007/s11696-019-00693-1
ORIGINAL PAPER
Robust acidic pseudo‑ionic liquid catalyst with self‑separation ability
for esterifcation and acetalization
Yingxia Shi1 · Xuezheng Liang1
Received: 27 June 2018 / Accepted: 21 January 2019
© Institute of Chemistry, Slovak Academy of Sciences 2019
Abstract
The novel acidic pseudo-ionic liquid catalyst with self-separation ability has been synthesized through the quaternization of
triphenylphosphine and the acidifcation with silicotungstic acid. The pseudo-IL showed high activities for the esterifcation
with average conversions over 90%. The pseudo-IL showed even higher activities for acetalization than traditional sulfuric
acid. The homogeneous catalytic process benefted the mass transfer efciency. The pseudo-IL separated from the reaction
mixture automatically after reactions, which was superior to other IL catalysts. The high catalytic activities, easy reusability
and high stability were the key properties of the novel catalyst, which hold great potential for green chemical processes.
Keywords Acidic pseudo-ionic liquid · Self-separation · Esterifcation · Acetalization
Introduction
suitable supports became the good choice (Miao et al. 2011;
Rashinkar et al. 2011). However, the expensive coupling
reagents were needed to bond IL moieties onto supports.
The acidic ILs were copolymerized with divinylbenzene
to form the porous copolymer in our previous work (Liang
2013a, b, c; Liang et al. 2013). However, heterogeneous pro-
cess added the mass transfer efciency, which reduced the
catalytic activities. The ideal ILs-catalyzed process was the
homogenous catalysis and heterogeneous recovery. During
the catalytic process, ILs mixed well in the reaction system
and formed the homogeneous phase. The active sites and the
reactants interacted smoothly, which benefted the catalytic
activities. After reactions, the catalysts could separate from
the reaction mixture, which benefted their recovery. ILs
with lower critical solution temperature (LCST) in reaction
system were very useful for catalysts recovery. The ILs sepa-
rated from the reaction system at lower temperature, while
the mixture became homogeneous at higher reaction tem-
perature. The poly(ethylene glycol)-functionalized ILs were
widely used in organic synthesis due to the easy separation
from reaction mixture (Pauline et al. 2010; Li et al. 2010).
The poly (ethylene glycol) moieties acted as the solvent at
relatively high temperature during the reaction process and
separated automatically after reaction. The polyoxometalate-
based ILs were reported as the reaction-induced self-sep-
aration catalysts for esterifcation (Leng et al. 2009). The
development of novel ILs with self-separation ability was
important for their wide application in chemical industry.
Ionic liquids (ILs) received wide attention due to the excel-
lent properties such as high conductivity, negligible volatile
pressure, wide electrochemical window and strong dissolu-
tion ability (Greaves and Drummond 2008). The functional
groups could be easily introduced into the ILs structure,
which were widely used in chemical synthesis (Wasser-
scheid and Keim 2000; Sheldon 2001; Zhao et al. 2002;
Dupont et al. 2002). Sulfonic acid groups-functionalized
ILs showed even higher activities than those homogeneous
catalysts such as sulfuric acid (Cole et al. 2002; Sugimura
et al. 2007; Da Silveira Neto et al. 2004). Although these
acidic ILs were very efcient for esterifcation, acetaliza-
tion, alkylation and so on, the ILs sufered from the dif-
fculty in recovery. The large ILs usage and high solubility
with reaction mixture caused the recovery difculty, which
also added the product purifcation complexity especially in
the polar compounds-involved reactions such as esterifca-
tion and acetalization. Various methods were developed to
solve the above problems. The immobilization of ILs onto
* Xuezheng Liang
liangxuezheng@126.com
Yingxia Shi
376130344@qq.com
1
School of Chemistry and Chemical Engineering, Shaoxing
University, Shaoxing 312000, China
Vol.:(0123456789)
1 3

Chemical Papers
Poly phosphonium ILs showed the adjustable temperature-
sensitive LCST-type phase behavior (Men et al. 2012). To
enhance the self-separation ability, here the novel silicotung-
stic acid(SW)-based phosphonium pseudo-IL with both pol-
yoxometalate and phosphonium was present (Scheme 1). To
facilitate the catalyst recovery, the bulky triphenylphosphine
was used to form the pseudo-IL with higher melting point.
After reactions, the novel pseudo-IL was separated automati-
cally, which made the catalyst recovery quite simple. The
pseudo-IL showed very high activities for the esterifcation
and acetalization with high reusability.
(%)=[1−(fnal acidity)/(initial acidity)]×100%. Here, the
acidity from acidic pseudo-IL was excluded. For fnal and
initial acidity, the acidity from the pseudo-IL was subtracted.
The products were confrmed using GC–MS analysis. After
reaction, the IL was separated from the reaction mixture by
cooling the temperature, which made the catalyst recovery
quite simple.
The procedure for acetalization
The mixture of aldehyde or ketone (0.1 mol), diol (0.1 mol),
cyclohexane (10 mL) and the pseudo-IL (0.05 g) was
magnetically stirred in a 50-mL three-necked flask. A
Dean–Stark apparatus was used to remove the water continu-
ously from the reaction mixture. The reaction was monitored
by GC analysis with small samples from reaction mixture at
half-an-hour intervals. On completion, the IL was separated
automatically from the reaction system by cooling, which
made the catalyst recovery quite simple.
Experimental
All organic reagents were commercial products of the high-
est purity available (>98%) and used for the reactions with-
out further purifcation.
Synthesis of the novel acidic pseudo‑IL
Triphenylphosphine (2.62 g, 0.01 mol), 1,3-propane sul-
fonate (1.22 g, 0.01 mol) and 10 mL tetrahydrofuran were
mixed and stirred magnetically for 24 h at room tempera-
ture. Then, a white solid zwitterion was formed. The white
solid zwitterion was fltrated and washed repeatedly with
ether. After dried in vacuum (110 °C, 1.33 Pa), the white
solid zwitterion was obtained in high yield (95%). Sili-
cotungstic acid (7.195 g, 0.0025 mol) and the zwitterion
(3.84 g, 0.01 mol) were dissolved in 10-mL methanol. The
mixture was evaporated at 130 °C for 8 h on rotary evapo-
rators to form the acidic pseudo-IL. 1H NMR for the zwit-
terion (400 MHz, DMSO, TMS): δ 2.02 (m, 2H), 2.92 (t,
J=7.6 Hz, 2H), 3.36 (t, JH–H =7.2 Hz, 2H), 7.58 (m, 15H).
IR(KBr): 1052 cm−1 and 940 cm−1 (–SO3H), 1160 cm−1
(C–P), 3400 cm−1 (O–H).
Results and discussion
The characterization of the pseudo‑IL
The pseudo-IL showed the melting point of 125 2 °C,
which accounted for the bulky triphenylphosphonium cation
and silicotungstate anion. The bulky ions would improve
the phase separation behaviors of the pseudo-IL (Leng et al.
2009; Men et al. 2012). The TG analysis gave the decom-
posed temperature over 250 °C, which showed high thermal
stability. The result indicated that the pseudo-IL could be
used in high-temperature reactions.
The FT-IR spectrum of the pseudo-IL showed the strong
OH (3400 cm−1) and S=O (1020 cm−1) absorbability, which
confrmed the sulfonic acid groups (Fig. 1). The adsorp-
tion at 1630 cm−1 was assigned to the bending vibration
of hydroxyl groups, which further confrmed that abundant
hydroxyl groups were contained in pseudo-IL. The C–H
adsorption at 2930 and 2850 cm−1 were overlapped by the
strong OH adsorption, which represented the alkyl group in
pseudo-IL. The peaks at 1019 cm−1 (Si–O–Si), 980 cm−1
(W–Od terminal), 924 cm−1 (W–Ob–W edge shared), and
788 cm−1 (W–Oc–W corner shared) corresponded to the
primary structure [SiW12O40]4− of the pseudo-IL (Isahak
et al. 2014).
The procedure for the esterifcation
Acidic pseudo-IL (0.05 g) was loaded into a 50-mL three-
necked fask with a magnetic stirrer, and a refux condenser.
A Dean–Stark apparatus was used to remove the water
continuously from the reaction mixture. n-Butanol (14.8 g,
0.2 mol) and aliphatic acid (with –OH/–COOH molar
ratio=1.5:1) were added and reaction was carried out under
refux condition. The reaction progress was monitored by the
acid–base titration. The conversion of the reaction was calcu-
lated on the acidity using the following equation: Conversion
The XRD pattern of the pseudo-IL is illustrated in Fig. 2.
The characteristic sharp Bragg peaks for Keggin-type
Scheme 1 The synthetic route
THF
R.T.(25 oC), 72 h
SW
of the novel acidic pseudo-IL
Ph3P
SO3H
O
Ph3P
SO3
+
Ph3P
SW
S
O
O
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Chemical Papers
7000
6000
5000
4000
3000
2000
1000
44
42
40
38
36
34
Binding energy(eV)
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers(cm )
Fig. 3 The W 4f XPS spectrum of the pseudo-IL
Table 1 Optimization of the esterifcation condition
Fig. 1 The FT-IR spectrum of the pseudo-IL
Entry
Catalyst/g
Molar ratio (–
COOH:–OH)
Reaction Conversion/%a
time/h
1
2
3
4
5
6
7
8
0.3
0.3
0.3
0.3
0.5
0.7
0.5
0.5
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.5
1:1.7
2
3
4
5
4
4
4
4
91.3
95.2
97.5
97.4
98.5
98.3
99.2
98.9
Reaction condition: n-butanol 14.8 g (0.2 mol), refuxed for certain
time
20
40
60
80
2Theta degree/o
a spin orbit doublet at 36.9 eV (W 4f7/2 component), which
accounted for the bigger area of the total spectra, and a sec-
ond doublet at 39.0 eV (W 4f5/2 component), accounting for
the amorphous structure.
Fig. 2 The XRD pattern of the pseudo-IL
[SiW12O40]4− could be observed in pseudo-IL. The new
broad Bragg peak was detected at 2θ= 10.0°, which indi-
cated the amorphous structure. The results were very close
to the theoretical size of a primary structural unit of IL–poly-
oxometalates ionic hybrid (Chen et al. 2014). The pseudo-IL
owned the solid structure, which kept some Keggin-type
structure of the silicotungstate anion. The strong interaction
between ions formed some amorphous structures.
The catalytic activities for the esterifcation
Initially, the esterification between tartaric acid and
n-butanol using novel acidic pseudo-IL was investigated to
optimize the reaction condition (Table 1). The novel acidic
pseudo-IL catalyst showed high activity for the reaction
with the optimal conversion over 99%. Here, n-butanol was
used as both reactant and water-removal reagent. To assure
the continuous refux condition, butanol amount was fxed
as 14.8 g (0.2 mol). The novel pseudo-IL showed high
activity for esterifcation with the conversion over 90%
after 2 h (entry 1). The conversion increased with time
and reached the peak value after 4 h (entries 1–4). In addi-
tion, the molar ratio of –COOH and –OH was investigated
The W 4f XPS spectrum recorded from the pseudo-IL
(Fig. 3) was composed of the spin–orbit doublet with bind-
ing energies for the W 4f7/2 and W 4f5/2, respectively.
These values are typical of the presence of W (VI) (Li et al.
2007). The W 4f spectrum was not well resolved. The W 4f
XPS also ftted on the basis of two diferent W contributions:
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Chemical Papers
(entries 6–8). The optimal molar ratio was 1:1.5 with the
highest conversion of 99.2%. The pseudo-IL amount was
also very important, which determined active sites amount.
The conversion increased with pseudo-IL amount due to
more active sites. However, the pseudo-IL could catalyze
both the forward and backward reactions. The conversion
decreased instead when the pseudo-IL amount increased to
0.7 g (entry 6). Therefore, the optimal reaction condition
was selected as entry 7.
1240 cm−1 (C–O); ESI–MS (m/z): 361, 259, 185, 129, 57,
41.
Entry 5, butyl lactate, boiling point: 185–186 °C; den-
sity: 0.984 g/mL; nD20: 1.421; IR(KBr): 3400 cm−1 (O–H),
2988 cm−1 and 2840 cm−1 (C–H), 1760 cm−1 (C=O),
1360 cm−1 (C–O); ESI–MS (m/z): 145, 85, 57, 45, 41, 29.
Entry 6, butyl benzoate, boiling point: 250 °C; den-
sity: 1.01 g/mL; n2D0: 1.498; IR(KBr): 3210 cm−1 (Ar–H),
2990 cm−1 and 2870 cm−1 (C–H), 1720 cm−1 (C=O),
1240 cm−1 (C–O); ESI–MS (m/z): 179, 123, 105, 77, 56,
51, 41, 29.
Then, various aliphatic acids were applied to investigate
the activity of the novel pseudo-IL (Table 2). The results
further confrmed the high activities of the pseudo-IL. Citric
acid with three carboxyl acid groups showed high reactivity
with the conversion over 98% (entries 1–2). Furthermore,
the pseudo-IL could separate from the reaction mixture
automatically. The pseudo-IL dissolved in the reaction mix-
ture at high temperature during the reaction process, which
resulted in high activity. After reaction, the pseudo-IL sepa-
rated from the reaction mixture, which made the IL recov-
ery quite simple. Compared to butanol, propanol showed
a little lower conversion of 97.8% due to the lower refux-
ing temperature (entry 4). Besides the hydroxyl acids, the
aromatic acids were also investigated. Benzoic acid showed
lower conversion of 87.6% for the conjugation efect of the
aromatic ring (entry 6). The conversion further decreased to
84.3% when the electron-donating hydroxyl group attached
onto the aromatic ring (entry 7). The steric hindrance and
H bond were also the reasons for lower reactivity. Cinnamic
acid showed much higher conversion for the weaker conju-
gation efect and lower steric hindrance (entries 8, 9). For
long-carbon-chain fatty acids, the conversion was also high
(entries 10–13). The conversion decreased with the carbon
chain due to higher steric hindrance and lower acidity. As
to polar methanol, the conversion was quite low (entries 13,
14). The low boiling point of methanol greatly limited the
reaction temperature. The reaction carried out at 140 °C in
an autoclave gave the high conversion of 98.7%. Further-
more, the pseudo-IL dissolved in the reaction mixture due
to the high polarity of methanol, which caused the difculty
in the pseudo-IL separation. The novel pseudo-IL gave high
reusability in most reactions. The pseudo-IL separated from
the reaction mixture after cooling to room temperature with
the total the recycled yield over 90%. After recycling the
pseudo-IL catalysts, the reaction mixture was neutralized
using 5 wt% NaHCO3 aqueous solution to remove the unre-
acted acid reactants. The alcohols were removed using the
evaporation under reduced pressure. The isolated yields were
quite similar to the conversion, which further confrmed the
high activities of the pseudo-IL.
Entry 7, butyl salicylate, boiling point: 260 °C; den-
sity: 1.08 g/mL; n2D0: 1.512; IR(KBr): 3650 cm−1 (O–H),
3170 cm−1 (Ar–H), 2980 cm−1 and 2889 cm−1 (C–H),
1750 cm−1 (C=O), 1230 cm−1 (C–O); ESI–MS (m/z): 194,
138, 120, 92, 64, 41, 29.
Entry 11, butyl laurate, boiling point: 194 °C; density:
0.86 g/mL; n2D0: 1.435; IR(KBr): 2978 cm−1 and 2876 cm−1
(C–H), 1737 cm−1 (C=O), 1280 cm−1 (C–O); ESI–MS
(m/z): 258, 201, 183, 129, 116, 73, 56, 41, 29.
Entry 12, butyl stearate, boiling point: 223 °C; density:
0.86 g/mL; n2D0: 1.445; IR(KBr): 2925 cm−1 and 2854 cm−1
(C–H), 1740 cm−1 (C=O), 1244 cm−1 (C–O); ESI–MS
(m/z): 340, 285, 267, 185, 129, 116, 73, 56, 43, 29.
Entry 14, methyl stearate, boiling point: 181–182 °C
(533.2 Pa); density: 0.850 g/mL; n2D0: 1.436; IR(KBr):
2963 cm−1 and 2850 cm−1 (C–H), 1743 cm−1 (C=O),
1264 cm−1 (C–O); ESI–MS (m/z): 298, 267, 199, 143, 129,
87, 74, 55, 43, 29.
The catalytic activities for acetalization
Besides esterifcation, the acetalization was also performed
using the pseudo-IL catalyst (Table 3). All carbonyl com-
pounds were successfully transformed to acetals or ketals
with high yields. Aliphatic aldehydes were almost com-
pletely conversed under the reaction condition (entries
1–10). The reactivity decreased with the carbon chain due
to higher steric hindrance. For isobutyraldehyde, the yield
decreased to 98% with longer reaction time (entries 7, 8).
Cyclohexanone owned high yield for ketalization with dif-
ferent diols (entries 11–13). The carbonyl group in cyclohex-
anone was fxed by the ring, which made the nucleophilic
attack quite easily. Butanone gave relatively low yields for
high steric hindrance (entries 14–15). The aromatic alde-
hydes, such as benzaldehyde, could also be smoothly acetal-
ized with high yields (entries 16–23). The conjugation efect
of carbonyl groups and aromatic ring reduced the reactiv-
ity. Therefore, benzaldehyde showed relatively low yield of
94% (entry 18). The electron-donating methyl group fur-
ther decreased the eletrophilicity of carbonyl group, which
resulted in lower yield of 90% (entry 21). For p-chlorobenza-
ldehyde, the yield increased due to the electron-withdrawing
The characterization of selected compounds. Entry 1,
tributyl citrate, boiling point: 234 °C (17 mmHg); den-
sity: 1.043 g/mL; nD20: 1.445; IR(KBr): 3600 cm−1 (O–H),
2980 cm−1 and 2870 cm−1 (C–H), 1765 cm−1 (C=O),
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Chemical Papers
Table 2 The catalytic activities
for diferent acids and alcohols
Entry
1
Acids
Alcohol
Recyclablea Conversion/% b Yield/% c
OH
HOOC
HOOC
COOH
COOH
n-butanol
Yes
Yes
99.0
98.5
97.5
96.0
COOH
OH
2
3
n-propanol
n-propanol
COOH
OH
COOH
Yes
98.2
96.1
HOOC
OH
OH
OH
4
5
6
n-propanol
n-butanol
n-butanol
Yes
Yes
Yes
97.6
98.5
87.6
95.8
96.5
84.6
COOH
COOH
COOH
COOH
OH
7
n-butanol
Yes
84.3
82.3
COOH
8
9
n-propanol
n-butanol
Yes
Yes
95.5
98.3
92.5
96.7
COOH
n-butanol
n-butanol
10
11
Yes
Yes
95.3
94.5
94.3
92.5
7
COOH
COOH
COOH
10
n-butanol
12
13
14
Yes
No
No
92.5
80.0
78.5
90.8
87.8
75.2
16
7
methanol
methanol
COOH
COOH
16
aReaction condition: alcohol (0.2 mol), acid (–COOH:–OH=1.5), pseudo-IL 0.5 g, refuxed for certain
time. The reaction was carried out under the optimal condition in Table 1. Recyclable means that the
pseudo-IL could be separated from the reaction mixture automatically with the recycled amount over 90%
bThe reaction was monitored by the acid–base titration. The conversion was calculated on the acidity
cYield was the isolated yield
1 3

Chemical Papers
Table 3 Preparation of various
acetals from the corresponding
carbonyl compounds
Entry
Substrate
Diols
1,2-ethanediol
Reaction time/h Yield/% a, b
CHO
1
2
3
4
5
6
0.5
0.5
1.0
1.0
1.0
1.5
99
99
99
99
99
98
CHO
1,2-propanediol
1,3-propanediol
1,2-ethanediol
1,2-propanediol
1,3-propanediol
1,2-ethanediol
CHO
CHO
CHO
CHO
CHO
7
8
1.5
2.0
98
98
1,3-propanediol
CHO
1,2-ethanediol
1,2-propanediol
1,2-ethanediol
9
1.5
1.5
99
99
6
6
CHO
10
CHO
Cl
Cl
Cl
CHO
11
12
13
14
15
16
17
18
0.5
1.0
1.5
1.5
1.5
1.5
1.5
2.0
99
99
99
97
95
97
96
94
1,2-propanediol
1,3-propanediol
1,2-ethanediol
1,3-propanediol
1,2-ethanediol
1,2-propanediol
1,3-propanediol
1,2-ethanediol
CHO
CHO
CHO
Cl
Cl
CHO
Cl
Cl
Cl
CHO
CHO
CHO
Cl
Cl
Cl
CHO
19
20
21
22
23
2.0
2.5
3.0
1.5
2.5
95
93
90
98
92
1,2-propanediol
1,3-propanediol
1,2-ethanediol
1,3-propanediol
CHO
CHO
CHO
CHO
Cl
Cl
1 3

Chemical Papers
Table 3 (continued)
aAll reactions were carried out under Dean–Stark conditions: pseudo-IL (0.05 g), diols (0.15 mol), car-
bonyl compound (0.1 mol) and cyclohexane (10 ml), refuxed
bThe yield was taken by GC according to carbonyl compounds. In all cases, the corresponding products
were exclusively obtained
100
80
60
40
20
0
Table 4 The comparison of diferent catalysts
Catalyst
Catalyst
Reaction
time/h
Yield/ %a, b
amount/mg
Novel IL
[SO3H-PPh3P][HSO4]
H2SO4
H4[W12SiO40]
[MIMPS]3PW12O40
Amberlyst-15
50
75
50
160
70
2.0
2.5
3.0
3.5
2.5
3.5
94
90
88
87
91
83
0.5
aAll reactions were carried out under Dean–Stark conditions: benza-
ldehyde (0.1 mol), 1,3-propanediol (0.15 mol), cyclohexane (10 ml),
refuxed
bThe yield was taken by GC according to carbonyl compounds. In all
cases, the corresponding products were exclusively obtained
1
2
3
4
5
6
7
8
Run
Fig. 4 The recycled activity of the novel pseudo-IL
phosphonium cations and SW anions and resulted in high
stability.
efect of chlorine (entries 22, 23). 1,2-Ethanediol, 1,2-pro-
panediol and 1,3-propanediol all interacted well with car-
bonyl groups. Ethanediol showed the highest yield due to
lowest steric hindrance. 1,2-Propanediol gave even higher
yield than that of 1,3-propanediol, which might be caused
by the higher nucleophilicity of secondary hydroxyl group
in 1,2-propanediol.
Comparative study on the catalytic activities
of diferent catalysts
A comparative study on the catalytic activities of the
pseudo-IL with other catalysts was carried out (Table 4).
The acetalization of benzaldehyde and 1,3-propanediol
was used as the model reaction. The phosphonium IL with
HSO4− gave relatively high yield of 90%, which was even
higher than sulfuric acid (88%). IL owned higher afnity to
the organic reactants due to the organic phosphonium cation.
Although sulfuric acid had higher acidity, the acetalization
was the typical reversible reaction. The strong acidic sites
of sulfuric acid could also catalyze the reverse reaction
and resulted in lower yield. Silicotungstic acid (SW) also
gave the comparable yield to sulfuric acid (87%). Although
the amount of SW was much higher than sulfuric acid, the
acidic sites were still much lower due to the large molecular
weight. Furthermore, the acid strength of SW was lower
than sulfuric acid. The Keggin structure of SW ofered high
afnity to reactants, which caused the enrichment efect of
reactants and resulted in high activity (Isahak et al. 2014).
These catalysts showed high activity for the reaction, but
the recovery of these acid catalysts was quite difcult. Sul-
furic acid and SW were totally dissolved with the reaction
mixture. The phosphonium IL was partially separated from
the reaction mixture. Some IL layer was formed after the
reaction mixture was settled for 2 days. Over 80% IL was
dissolved in the reaction mixture. For novel pseudo-IL, the
catalytic activity was the highest with even lowest catalyst
The recycled activity of the catalyst
After reactions, the novel pseudo-IL could be automatically
separated from the reaction mixture by cooling the tempera-
ture, which made the pseudo-IL recovery quite simple. The
recycled activities of pseudo-IL for the acetalization of ben-
zaldehyde and 1,3-propanediol were investigated (Fig. 4).
The result showed that the pseudo-IL owned high stabil-
ity with the activity almost remained the same (93%) after
eight times. For the [MIMPS]3PW12O40 (Leng et al. 2009),
the recycled activities decreased from 95.5 to 84.5% after
four runs. The novel pseudo-IL owned relative large molecu-
lar structure, which resulted in higher melting point. The
pseudo-IL formed the solid state during the cooling process
after reaction. The reaction mixture was investigated by ICP
and no S and W elements were detected. This indicated that
little pseudo-IL was contained in the reaction mixture. The
acidity of the recycled pseudo-IL was 0.906 mmol/g (fresh
pseudo-IL 0.91 mmol/g), which further confrmed the high
stability of the pseudo-IL. The bulky anion SW owned four
negative charges, which enhanced the interaction between
1 3

Chemical Papers
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amount. The organic phosphonium cation held high afn-
ity to reactants and the silicotungstate anion enhanced the
enrichment efect. As a result, the novel pseudo-IL held the
highest activity. In addition, the pseudo-IL could be easily
recovered after reactions, which was another advantage over
other catalysts. For the heterogeneous Amberlyst-15 (acidity
0.8 mmol/g) owned much lower activity. The heterogeneous
catalytic process added the mass transfer difculty, which
greatly afected the catalytic activity. The novel pseudo-IL
owned the homogeneous catalytic process at high tempera-
ture, which confrmed the high activity. Furthermore, the
auto-separation after reaction ofered great convenience
of the pseudo-IL, which was another advantage over other
acid catalysts. Also, the [MIMPS]3PW12O40 (Leng et al.
2009), which showed high activity for esterifcation, was
used here. [MIMPS]3PW12O40 gave high yield for acetali-
zation of benzaldehyde and 1,3-propanediol with the yield
of 91%. However, the yield was still lower than the novel
pseudo-IL. Moreover, the pseudo-IL showed higher phase
separation ability with the recycled amount over 95%, while
[MIMPS]3PW12O40 only showed the recycled yield of 80%.
This result further confrmed that the bulky organic phos-
phonium cation also attached much importance to the cata-
lytic activity and reusability.
Da Silveira Neto BA, Ebeling G, Goncalves RS, Gozzo FC, Eberlin
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Dupont J, Souza RF, Suarez PAZ (2002) Ionic liquid (molten salt)
phase organometallic catalysis. Chem Rev 102:3667–3692. https
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Greaves TL, Drummond CJ (2008) Protic ionic liquids: properties and
applications. Chem Rev 108:206–237. https://doi.org/10.1021/
cr068040u
Isahak W, Ramli Z, Ismail M, Yarmo M (2014) Highly selective glyc-
erol esterifcation over silicotungstic acid nanoparticles on ionic
liquid catalyst. Ind Eng Chem Res 53:10285–10293. https://doi.
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Liang XZ (2013a) Synthesis of biodiesel from waste oil under mild
conditions using novel acidic ionic liquid immobilization on poly
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Conclusion
The robust acidic pseudo-IL with self-separation ability
has been synthesized through the quaternization of triph-
enylphosphine and propane sultone and the acidifcation
with silicotungstic acid. The novel pseudo-IL showed the
homogeneous catalytic process and automatically separation
ability after reactions. The pseudo-IL showed high activities
for the esterifcation and acetalization. The organic phospho-
nium cation and silicotungstate anion with some Keggin-
type structure benefted the catalytic activities, which ofered
even higher yield than sulfuric acid and [SO3H-PPh3P]
[HSO4]. The catalyst owned the special advantage of auto-
separation after reactions, which made the novel catalyst
hold great potential for the replacement of the homogeneous
catalysts in green chemical processes.
Liang XZ (2013b) Novel efcient procedure for biodiesel synthesis
from waste oils using solid acidic ionic liquid polymer as the
catalyst. Ind Eng Chem Res 52(2013):6894–6900. https://doi.
org/10.1021/ie303564b
Liang XZ (2013c) Novel acidic ionic liquid polymer for biodiesel syn-
thesis from waste oils. Appl Catal A Gen 455:206–210. https://
doi.org/10.1016/j.apcata.2013.01.036
Liang XZ, Xiao HQ, Qi CZ (2013) Efcient procedure for biodiesel
synthesis from waste oils using novel solid acidic ionic liquid
polymer as catalysts. Fuel Process Technol 110:109–113. https://
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(2012)
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Acknowledgements The work was supported by Public Welfare Pro-
ject from Science and technology department of Zhejiang province
no. 2017C31109 and National Undergraduate Training Program for
Innovation and Entrepreneurship no. 201610349004.
Pauline P, Clarence C, Jean M, Lucy P, Francisco G, Frederic L,
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glycol)-ionic liquid and its application in peptide synthesis. Chem
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