Preparation and Application of Silica Films Supported Imidazolium-Based Ionic Liquid as Efficient and Recyclable Catalysts for Benzoin Condensations

Article abstract of DOI:10.1007/s10562-019-03030-z

Abstract: Two silica films -immobilized imidazolium-based ionic liquids (TMICl @silica films) were prepared, characterized and utilized as efficient catalysts for the benzoin reaction. Combined characterization results from FT-IR, elemental analysis, N₂ adsorption–desorption isotherms and SEM, suggested that the imidazolium-based ionic liquids were successfully immobilized on the silica films. Moreover, the catalytic performance tests demonstrated that silica films immobilized imidazolium-based ionic liquids (ILs) exhibited excellent activity for the benzoin reactions of aromatic aldehydes. The influence of catalyst concentration, temperature and reaction duration on the catalytic activity were investigated by employing 0.7TMICl @silica film(catalyst C) as the catalyst. The results also showed that the benzoin condensations of aromatic aldehydes could give desired products with satisfactory yields under the optimized conditions. Additionally, the catalyst can be effortlessly separated by filtration and reused more than five times without significant loss of activity. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-019-03030-z

Catalysis Letters
https://doi.org/10.1007/s10562-019-03030-z
Preparation and Application of Silica Films Supported
Imidazolium‑Based Ionic Liquid as Efcient and Recyclable Catalysts
for Benzoin Condensations
Cunwei Qian1 · Changsheng Yao2 · Liujun Yang1 · Bo Yang1 · Shuchun Liu1 · Zongtang Liu1
Received: 20 July 2019 / Accepted: 2 November 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Two silica flms -immobilized imidazolium-based ionic liquids (TMICl @silica flms) were prepared, characterized and
utilized as efcient catalysts for the benzoin reaction. Combined characterization results from FT-IR, elemental analysis, N2
adsorption–desorption isotherms and SEM, suggested that the imidazolium-based ionic liquids were successfully immobi-
lized on the silica flms. Moreover, the catalytic performance tests demonstrated that silica flms immobilized imidazolium-
based ionic liquids (ILs) exhibited excellent activity for the benzoin reactions of aromatic aldehydes. The infuence of catalyst
concentration, temperature and reaction duration on the catalytic activity were investigated by employing 0.7TMICl @
silica flm(catalyst C) as the catalyst. The results also showed that the benzoin condensations of aromatic aldehydes could
give desired products with satisfactory yields under the optimized conditions. Additionally, the catalyst can be efortlessly
separated by fltration and reused more than fve times without signifcant loss of activity.
Graphic Abstract
Catalyst C:
O
O
O
O
H
130oC, CH3OH, NaOH
Ar, Catalyst C, 10h
Ar
Si
N
Ar
N
2
Ar
O
HO
Cl
Wide substrate variety Easy separation and recycle
Good potential application
0.7TMICl@ silica films (catalyst C) exhibited robust catalytic performance in the benzoin reaction
Keywords Silica flms · Immobilized · Imidazolium-based ionic liquids · Catalysis · Benzoin reaction
1 Introduction
α-Hydroxyketones (acyloins) are valuable intermediates for
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-03030-z) contains
supplementary material, which is available to authorized users.
the synthesis of a wide range of natural and unnatural com-
pounds [1]. Thus the efcient synthesis of α-hydroxyketones
has been pursued for decades [2–8]. In its traditional form,
only aromatic aldehydes or glyoxals could be coupled using
cyanide anion as a catalyst [9, 10]. However, the toxicity
of cyanide anion and fnite suitable substrates limit the
large-scale industrial application of the reaction. In 1943,
Ugai et al. [11] discovered that VB1 could promote benzoin
condensation reaction with the aid of alkali, which avoided
the involvement of highly tox cyanide as catalyst. Further
studies showed that thiazolium salt in VB1 was converted
into nucleophilic N-heterocyclic carbene (NHCs) under
the action of alkali, which was the actually active catalyst.
* Cunwei Qian
qiancunwei@163.com
* Zongtang Liu
liuzt@yctu.edu.cn
1
School of Chemical and Environmental Engineering,
Yancheng Teachers College, Yancheng 224051, Jiangsu,
China
2
Jiangsu Key Lab of Green Synthetic Chemistry
for Functional Materials, School of Chemistry and Materials
Science, Jiangsu Normal University, Xuzhou 221116,
Jiangsu, China
Vol.:(0123456789)
1 3

C. Qian et al.
Accordingly, considerable attention has been paid to the use
of NHCs as organocatalysts [12, 13]. Many NHCs derived
from triazolium or thiazolium salts have been identifed as
efcient catalysis for benzoin condensation reaction [14, 15].
Although NHCs catalysts have made great achievements in
benzoin condensation as organic catalysts, these homogene-
ous catalysts tend to cause difculties in catalyst recovery
and product purifcation. Moreover, because the reactants
and ionic liquids are often immiscible and can not be fully
contacted each other, the catalytic efciency of the catalyst
is poor, resulting in a large amount of catalyst used. To suc-
cessfully conquer these issues, the development of NHCs-
based heterogeneous catalysts is considered as the most
efective alternative solution. Furthermore, when immo-
bilized on a suitable support having a large surface area,
the welldispersed NHCs facilitate not only high catalytic
activity but also facile separation of catalyst from reaction
products [16–20]. Several groups have reported that sup-
ported imidazolium/thiazolium salt pre-catalysts have been
prepared and used to catalyze various reactions, such as the
benzoin reactions and alcohol etherifcation, and that the
pre-catalysts could be readily recycled and reused many
times [21–28]. Although these supported imidazolium salt
pre-catalysts could catalyze benzoin reactions and be recy-
cled, their wide application is limited due to the inherent
shortcomings of organic compounds, such as poor thermal
stability, low substrate adaptability, etc. On the other hand,
silicon-based materials have become a research hotspot due
to their large surface area and good thermal stability. Among
numerous silicon-based materials, silica flms have better
structural stability and emerges as a promising candidate
for the catalyst support [29]. When ionic liquid pre-cata-
lysts were dispersed and immobilized on silica flms, the
surface area of ionic liquid pre-catalyst increases. This will
enable the ionic liquid pre-catalyst to fully contact with the
reactants and improve the catalytic efciency. On account
of the above reasons, we designed and prepared two easily
prepared silica flms immobilized imidazolium ionic liquids
pre-catalysts and applied them to benzoin reaction of varied
aromatic aldehydes. In order to compare the efects of difer-
ent supports on the activity of catalysts, we also synthesized
SBA-15 immobilized imidazolium salt pre catalyst accord-
ing to the literature [20].
Cl
N
H3CO
H3CO
H3CO
H3CO
N
Si
OCH3
Cl
+
Si
OCH3
N
CH3
N
CH3
a
Scheme 1 Schematic illustrations of synthesis procedures for TMICl
OH OH OH
Si Si Si
O
O
O
OCH3
H3CO Si
OCH3
Cl
N
Si
N
N
N
Cl
Scheme 2 Schematic illustrations of synthesis procedures for
TMICl@ silica flms
was dissolved in the aqueous solution containing P123 and
oxalic acid. The mixture was stirred at 40 °C for 1 h and a
clear solution with partially hydrolyzed silica yielded. The
reaction mixture was coated on glass slides, and then these
glass plate was placed in an oven at 40 °C overnight to form
a flm. Silica flms (Catalyst A) were obtained by remov-
ing P123 and oxalic acid and drying. 0.3TMICl @silica
flm (Catalysts B) and 0.7TMICl@ silica flm (Catalyst C)
were synthesized by using of TEOS, P123, oxalic acid and
1-methyl-3-trimethoxysilylpropylimidazol-ium chloride as
raw materials according to the previous method of preparing
catalyst A. The diference between catalyst B and catalyst
C is that more ionic liquids are used to prepare catalyst C
than catalyst B. At last, the synthesis of TMICl@ SBA-15
(Catalyst D) was carried out by frst dispersing dehydrated
SBA-15 in toluene, followed by adding TMICl to the SBA-
15 suspension under inert (N2 gas) atmosphere.
These as-prepared catalysts have been characterized by
FT-IR, EA, BET and SEM. FT-IR spectra of silica flms
(catalyst A), 0.3TMICl@silica films (catalyst B) and
0.7TMICl@ silica flms (catalyst C) are illustrated in Fig. 1.
As shown in Fig. 1, the absorption bands at about 457, 797
and 1072 cm−1 were characteristic of SiO2 [30, 31], whereas
the bands at around 1640 and 3431 cm−1 were attributed to
the stretching frequency of Si–OH and physisorbed water
[30, 32]. For 0.3TMICl @silica films and 0.7TMICl@
silica flms, the characteristic peaks were found at around
1570 and 1456 cm−1, which can be assigned to the C=N
and C=C stretching vibrations of the imidazole ring [30,
33]. In comparison with silica flms(A), 0.3TMICl@silica
flms (B) and 0.7TMICl@ silica flms (C) exhibited the same
characteristic bands such as the Si–O–Si vibrations (797 and
1072 cm−1) and the stretching vibration of –OH (around
1640, 3440 cm−1). In addition, the absorption peaks at 2985,
2924, 1456 and 1383 cm−1 are the stretching and bending
vibration peaks of C-H bonds of methyl and methylene. The
N2 adsorption–desorption isotherms and pore size distribu-
tions of silica flms(A), 0.3TMICl@ silica flms (B) and
2 Results and Discussion
The schematic procedure for the preparation of catalysts is
shown in Schemes 1 and 2. At frst, 1-methyl-3-tri-meth-
oxysilylpropyl-imidazolium chloride (TMICl) was syn-
thesized by mixing 3-chloropropyltrimethoxysilane with
1-methylimidazole under Ar atmosphere at 120 °C under
stirring condition. Then, tetraethoxysilane (TEOS, AR)
1 3

Preparation and Application of Silica Films Supported Imidazolium-Based Ionic Liquid as…
2.0
1452
Catalyst C
Catalyst B
Catalyst A
2984
2985
catalyst A
797
1576
1383
1.5
1.0
0.5
0.0
1641
467
catalyst C
1577
3423
3431
797
1647
1456
1357
457
1088
catalyst B
797
1636
3445
1072
1076
0
2
4
6
8
10 12 14 16 18 20
D/nm
455
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber/cm
Fig. 3 Pore size distribution of catalyst A, B and C
Fig. 1 FT-IR spectra of Catalyst A, B and C
Table 1 BET surface properties of catalyst A, B and C
Sample
BET surface
Total pore volume Average
600
500
400
300
area (m2/g)
(cm3/g)
pore width
(nm)
Catalyst C
A
B
C
630.96
567.79
537.90
0.3140
0.3866
0.6059
1.99
2.28
6.35
Catalyst B
200
about 1.7 and 2.6 nm, and that of 0.7TMICl@silica flms(C)
is between 2.3 and 10. The reason for these diferences is
that the amount of TMICl ionic liquid used in the process
of preparing these materials is diferent. The surface area,
pore size and pore volume of the samples were calculated
by the BET equation and shown in Table 1. According to
the data in Table 1, it can be seen that the specifc surface
of the synthesized flm material decreases slightly with the
addition of TMICl and the increase of its dosage. On the
contrary, the average pore size and the pore volume increase
with the addition of TMICl and the increase of its dosage.
The imidazolium salt loading levels of 0.7TMICl@ silica
flms(C) and 0.3TMICl@ silica flms(B) were determined
by elemental analysis, were 0.93 and 0.46 mmol g−1, respec-
tively. The morphology of catalyst A, B and C were also
studied by use of SEM. As shown in Fig. 4a–c, the prepared
solid catalysts displayed lamellar structure. Compared with
the catalyst A, Catalysts B and C have porous structure due
to the addition of ionic liquids. Finally, TMICl @ SBA-
15(catalyst D) was characterized by IR, SEM and elemental
analysis. FT-IR spectra of SBA-15 and catalyst D are showed
in Fig. 5. Compared with the infrared spectra of SBA-15 and
TMICl @ SBA-15, in the infrared spectra of TMICl @ SBA-
15, the absorption peaks at 2980, 1574 and 1383 cm−1 are
the characteristic absorption peaks of imidazolium salt and
100
0
Catalyst A
0.0
0.2
0.4
0.6
P/P0
0.8
1.0
Fig. 2 N2 Adsorption–desorption isotherms of catalyst A, B and C
0.7TMICl@ silica flms (C) are illustrated in Figs. 2 and
3. The isotherms of 0.7TMICl@silica flms (C) are typical
type IV isotherms according to IUPAC classifcations, and
the hysteresis loops of H3 of 0.7TMICl@ silica flms are
the characteristics of mesoporous structure under the pres-
sure of p/p0=0.45–1.0 [34]. The isotherms of silica flms
and 0.3TMICl@silica flms show typical type I isotherms
according to IUPAC classifcations. However, the isotherm
of 0.3TMICl@silica flms has a very small H4 hysteresis
loop under the pressure of p/p0=0.1–0.8, which indicates
that silica flms have microporous structure characteristics,
while 0.3TMICl@silica flms has mixed characteristics of
microporous structure and mesoporous structure. These con-
clusions are also confrmed by the pore size distribution.
The pore size distribution shows that the pore size of silica
flms is about 1.7 nm, that of 0.3TMICl@ silica flms(B) is
1 3

C. Qian et al.
Table 2 The benzoin reaction of benzaldehyde: optimization of the
reaction conditions
O
H
O
T, Solvent, Base
Ar, Catalyst, t
2
HO
Catalyst A = silica films
B = 0.3TMICl@silica films
C = 0.7TMICl@silica films
D = TMICl@SBA -15
Entry
Catalyst
Solvent
Base
Yield (%)a
1b
2
3
4
5
6
7
8
9
10
11c
12d
13e
14f
15g
16h
17i
A
B
C
D
C
C
C
C
C
C
C
C
C
C
C
C
C
CH3OH
CH3OH
CH3OH
CH3OH
THF
CH3CN
Dioxane
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
DBU
K2CO3
NaOAc
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
0
64
88
70
21
0
15
9
0
Fig. 4 SEM images of catalyst A, B, C and D
SBA-15
1384
1384
805
1629
Catalyst D
3428
464
0
67
31
80
61
73
76
86
2980
1631
1083
1068
1574
806
3399
461
4000
3500
3000
2500
2000
-1
1500
1000
500
Unless otherwise stated, General reaction conditions:: 2.0 mmol
of benzaldehyde, 0.1 mmol of imidazolium salt, 3 mL of solvent,
0.8 mmol of base, 130 °C oil bath under Ar atmosphere for 10 h
Wavenumber/cm
aIsolated yield
Fig. 5 FT-IR spectra of catalyst D and SBA-15
b0.1 g catalyst A, [imidazolium salt]=0 mol%
c110 °C oil bath
d80 °C oil bath
methylene, which indicates that the imidazolium salt ionic
liquid has been immobilized onto the SBA-15. The imidazo-
lium salt loading level of TMICl @ SBA-15 was determined
by elemental analysis, was 0.11 mmol g−1. The morphology
of TMICl@SBA-15 (catalyst D) was also studied by SEM.
As shown in Fig. 4d, catalyst D showed a wheatlike mor-
phology similar to the SBA-15, in agreement with the earlier
literature [35], with the irregular surface and diferent sizes.
After the characterization of the catalysts, the catalytic
activities of these catalysts were examined in the benzoin
reaction. The investigation was initiated by using the ben-
zoin reaction of benzaldehyde as a model reaction. Ini-
tially, we briefy evaluated the catalytic efciency of dif-
ferent catalysts (A=silica flm, B=0.3TMICl@silica flm,
C = 0.7TMICl@ silica flm and D= TMICl @SBA-15) for
the benzoin reaction of benzaldehyde. As shown in Table 2
(entries 1–4), catalyst C showed better catalytic potency
e140 °C oil bath
f0.5 mmol of NaOH was used in benzoin reactions
g1 mmol of NaOH was used in benzoin reactions
hReaction time was changed to 8 h
iReaction time was changed to 14 h
than catalyst A, B and D for the benzoin reaction of ben-
zaldehyde. This was related to the proper pore size of cata-
lyst C. The surface pore size of catalyst C ranges from
2 to 10 nm, which facilitates the adsorption of substrate
molecules and promotes the reaction. A subsequent solvent
screen showed CH3OH was proved to be a far superior sol-
vent with respect to the rate of benzoin formation of those
tested (Table 2, entries 3 vs. 5–7). This may be attributed
to the good solubility of NaOH in methanol. To know the
1 3

Preparation and Application of Silica Films Supported Imidazolium-Based Ionic Liquid as…
efect of bases on the outcome of the reaction, diferent
bases were employed in the benzoin reaction and it was
observed that NaOH was the most efective base (Table 2,
entries 3 and 8–10). Therefore NaOH was used in subse-
quent reactions as the base. As described in Table 2, lower
or higher temperature resulted in decrease of yields and
130 °C was the appropriate temperature (Table 2, entries
3 and 11–13). The experimental results also show that
the amount of NaOH has a signifcant efect on the yield
of the reaction. The optimal amount of base used here
was 0.4 equiv., and more or less alkali led to decrease of
yields (Table 2, entries 3 and 14–15). Finally, we investi-
gate the efect of time on the reaction. The data in Table 2
indicated that the best yield was obtained when the reac-
tion was extended to 10 h. Hower, if the reaction time is
prolonged, the yield will not increase(Table 2, entries 3
and 16–17). It was that the benzoin reaction of benzalde-
hyde gave the best yield in the presence of 0.7TMICl@
silica flm (Catalyst C, 5 mmol% imidazolium salt) and
40 mmol % of NaOH as base in CH3OH at 130 °C under
an Ar atmosphere for 10 h.
reduced pressure and reused in the next run under the same
conditions without any further treatment.
3 Conclusions
To sum up, two silica flms-supported imidazolium salts
were successfully prepared, characterized and utilized in
catalyzing the benzoin condensation reaction to give the
α-hydroxyl ketone. On the basis of the characterization
results, the ionic liquid (TMICl) were proved to be immo-
bilized successfully onto the silica flms. Moreover, silica
flms-supported imidazolium salt pre-catalysts showed good
catalytic efciency for the preparation of α-hydroxy ketones.
Catalyst C (0.7TMICl @ silica flms) displayed the best cata-
lytic efciency for benzoin reaction of aldehyde. In addition,
the catalyst can simply be separated and reused for fve times
without signifcant decrease in its catalytic activity. Further
applications of this catalyst to other transformations are cur-
rently under investigation.
The scope of benzoin reaction was demonstrated by using
various aromatic aldehyde under the optimized conditions.
The results are listed in Table 3. We were delighted to fnd
that our catalytic system was capable of tolerating relatively
diferent aromatic aldehydes under the optimized condi-
tions. As is evident from Table 3, the benzoin condensa-
tion of various substituted aldehydes including electron rich
derivatives such as CH3 or OCH3 were accomplished with
no complications, generally resulting in the desired product
in good yields (56–80%). As it can be observed, substitu-
ent on the aromatic ring, such as CH3, OCH3 and Cl had
a considerable efect on the yield. The best yield of 87%
was obtained by using benzaldehyde as a substrate. On the
other hand, no desired product was observed in the case of
4-nitro-benzaldehyde as previously recorded by Xia et al.
[36]. As expected, alkyl aldehyde gave lower yield as com-
pared to aryl aldehyde. For n-butylaldehyde, there are no
acyloin products, but α,β-unsaturated aldehyde was obtained
in moderate yield.
3.1 Experimental Section
3.1.1 General Information
Reagents were obtained from commercial suppliers and
used without further purifcation unless otherwise noted.
All the reactions were carried out in reaction tube under
argon atmosphere. Thin layer chromatography (TLC) was
performed using Qingdao silica gel (silica gel GF254).
1
TLC plates were analyzed by ultraviolet (UV) light. H
NMR and 13C NMR spectra were measured in Bruker AV
400 MHz using CDCl3 or CD3OD as solvent and TMS as
internal standard. Melting points were measured with an
X-5 micro-melting point apparatus and are uncorrected. Fou-
rier transform infrared (FT-IR) spectra of all samples were
recorded using a Bruker Vertex 80. The BET sorption of N2
was determined using Micromeritics ASAP2010 equipment
(Micromeritics Instrument Co., Ltd., Norcross, USA). The
C, N and H amount contents of these catalysts was estimated
by elemental microanalysis technique using a Vario EL cube
elemental analyzer (Elementar Analysensysteme Co., Ltd.,
Hanau, Germany), and the loading amount of ionic liquids
on the silica flms was calculated by the content of N ele-
ment. Morphologies and these catalysts were investigated by
a Nova NanoSEM 450 scanning electron microscopy (SEM).
From environmental and economic points of view, the
stability and sustained activity of the catalyst are of great
importance. To explore the reusability of 0.7TMICl@ silica
flms (catalyst C), the benzoin reaction of benzaldehyde was
chosen as a model reaction. As shown in Fig. 6, catalyst C
can be recycled up to fve times with no appreciable decrease
in the yields. The slight decrease in the catalytic activity
may be reasonably attributed to the negligible breakage of
the supported ionic liquids from the surface of the silica
flms during the benzoin reaction processes. In each cycle,
the catalyst was separated from the reaction mixture by fl-
tration, washed with methanol (3–5 mL), then dried under
3.1.2 Preparation of Silica Films (A)
First, 1.224 g P123, 50 mL distilled water and 4.48 g oxalic
acid were added to 250 mL round bottom fask, stirred for
1 3

C. Qian et al.
Table 3 0.7TMICl @ silica flm (C)—catalyzed benzoin reaction
O
H
130oC, CH3OH, NaOH
Ar, Catalyst C, 10h
Ar
Ar
Ar
2
O
HO
Yieldb, %
72
Entry
Substrate
CHO
Product
CH3
O
1
2
HO
H3C
CH3
CHO
O
H3
C
CH3
78
HO
CH3
CHO
CH3
O
H3
C
H3
CH3
HO
C
3
4
71
CH3
CH3
Cl
O
CHO
74
Cl
HO
Cl
Br
CHO
O
Br
5
76
HO
Br
CHO
O
(H3C)3
C
C(CH3)
3
6
7
80
63
HO
C(CH3)3
CHO
O
HO
CHO
OCH3
O
8
9
56
70
HO
H3CO
OCH3
CHO
O
(H3C)2HC
CH(CH3
)
2
HO
CH(CH3)2
CHO
O
O2N
NO2
10
11
0
HO
NO2
CHO
CHO
57
Unless otherwise stated, general reaction conditions: 2.0 mmol of aldehyde, 0.1 mmol of imidazolium salt, 0.8 mmol of NaOH, 3 mL of CH3OH
at 130 °C under Ar atmosphere for 10 h
bIsolated yield
1 3

Preparation and Application of Silica Films Supported Imidazolium-Based Ionic Liquid as…
2 h at 40 °C, then 5 mL TEOS was added to fask, and stirred
for 1 h. After stirring, the reaction mixture was coated on
glass slides (20 mm × 75 mm), and then these glass plate
was placed in an oven at 40 °C overnight to form a flm. The
precipitate was scraped down and washed three times with
a mixture of deionized water and ethanol (1:1) and ethanol
to remove oxalic acid, and dried under reduced pressure to
obtain a white solid. At last, silica flms (catalyst A) were
obtained by removing P123 from white solids by use of Sox-
hlet extraction using the 150 mL mixture of ethanol and
was placed on in an oven at 40 °C for 12 h. Two kinds of
white thin flms were obtained. Subsequently, the thin flms
were scraped and washed three times with a mixture of
water and ethanol (1:1) and ethanol to remove oxalic acid
and excess ionic liquids, and then dried. Thereafter, two
obtained solid powders were extracted in a Soxhlet apparatus
using the 150 mL mixed solvent of ethanol and hydrochlo-
ric acid (V CH3OH: V HCl =4:1) as solvent to remove P123.
0.3TMICl@ silica flms(B) and 0.7TMICl@silica flms (C)
were obtained, as showed in Scheme 2.
hydrochloric acid (VCH OH ∶ VHCl:VHCl =4:1) as solvent.
3
3.1.5 Preparation of TMICl@SBA‑15 (Catalyst D)
3.1.3 Synthesis
of 1‑Methyl‑3‑Trimeth‑Oxyysilylpropyl‑Imidazolium
Chloride
Referring to the literature [20], the method of synthesizing
TMICl@SBA-15 (catalyst D) is as follows: The synthesis
of TMICl@ SBA-15 was carried out by frst dispersing
1.7 g of dehydrated SBA-15 (pre-treated at 150 °C for 6 h
under vacuum) in 100 mL of toluene, followed by adding
0.19 g of TMICl to the SBA-15 suspension under inert (N2
gas) atmosphere. Subsequently, the mixture was stirred and
refuxed at 80 °C for 24 h, followed by washing with tolu-
ene and water, fltered, then dried overnight to obtained the
fnal solid product, which was denoted as TMICl@SBA-15
(catalyst D).
The imidazolium-based ionic liquid was synthesized by mix-
ing 18.6 mL of 3-chloropropyltrimethoxysilane with 8 mL of
1-methylimidazole in an inert (Ar gas) atmosphere at 120 °C
for 24 h while under stirring condition [37]. The reaction
mixture was then cooled to room temperature (25 °C) to
obtain 1-methyl-3-trimethoxysilyl- propyl-imidazolium
chloride (TMICl) ionic liquid, as illustrated in Scheme 1.
3.1.4 Preparation of 0.3TMICl@ Silica Films(B)
and 0.7TMICl@ Silica Films (C)
3.1.6 General Procedure for the Reduction Reaction
Firstly, 1.224 g P123, 50 mL distilled water and 4.48 g oxalic
acid were added to two 250 mL fasks, respectively, and
stirred for 2 h at 40 °C. Then, 5 mL TEOS was added to
the two fasks, respectively. At the same time, 0.30 g and
0.70 g TMICl were added to the two fasks respectively.
After stirring for 1 h, the reaction mixture was coated on
glass slides(20 mm × 75 mm), and then these glass slides
In a typical experiment, 0.331 g ([imidazolium
salt] = 0.223 mmol) of catalyst C, 4 mL of CH3OH and
0.455 mL (4.46 mmol) of benzaldehyde were added to a
Schlenk tube. The reaction mixture was stirred for 10 h at
130 °C under argon. The mixture was allowed to cool down
to room temperature, and the catalyst was obtained by fl-
tration and washed with 5 mL CH3OH. The organic layer
was collected, then removed in vacuo, and the residue was
purifed by column chromatography on silica gel to aford
the desired product. The recycled catalyst was cleaned with
dry CH3OH and dried at 40 °C twice.
80
60
40
20
0
3.1.7 Thermal Filtration Experiment
Thermal fltration experiment is usually an important test
method to detect the heterogeneity of catalyst [38]. The het-
erogeneity of 0.7TMICl@ silica flms(C) was studied by
using the benzoin reaction of benzaldehyde as template.
Firstly, the catalyst was removed by fltration after the reac-
tion proceeded at 135 °C for 2 h, and the fltrate was fur-
ther heated for 8 h under the same conditions. GC analysis
showed that 31% yield was obtained after 2 h of reaction and
30% yield was obtained after 10 h of reaction. Clearly, no
appreciable reaction took place after removal of the catalyst.
The above experimental results distinctly showed that no
active species leached into the liquid phase and provides
1
2
3
4
5
Number of run
Fig. 6 Reusability of catalyst C in the benzoin reaction of benzalde-
hyde
1 3

C. Qian et al.
18. Balaraju M, Nikhitha P, Jagadeeswaraiah K, Srilath K, Sai Prasad
PS, Lingaiah N (2010) Fuel Process Technol 91:249
19. Jagadeeswaraiah K, Balaraju M, Sai Prasad PS, Lingaiah N (2010)
Appl Catal A 386:166
direct evidence for the heterogeneity of the present catalyst
system.
Acknowledgements National Natural Science Foundation of China
(41877118); Natural Science Foundation of Jiangsu Province of China
(BK20181479); Natural Science Foundation of the Jiangsu Higher
Education Institutions of China (17KJB610013).
20. Hung CT, Liu LL, Wang JJ, Wu PH, Wang CB, Tsai TC, Liu SB
(2019) Catal Today 327:10
21. Giacalone F, Gruttadauria M (2016) ChemCatChem 8:664
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25. Bortolini O, Cavazzini A, Dambruoso P, Giovannini PP, Caciolli
L, Massi A, Pacifco S, Ragnoa D (2013) Green Chem 15:2981
26. Bivona LA, Quertinmont F, Beejapur HA, Giacalone F, Buaki-
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357:800
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no confict of
interest.
27. Kuzmicz D, Coupillaud P, Men Y, Vignolle J, Vendraminetto G,
Ambrogi M, Taton D, Yuan J (2014) Polymer 55:3423
28. Pinaud J, Vignolle J, Gnanou Y, Taton D (2011) Macromolecules
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