Imine-bridged periodic mesoporous organosilica as stable high-activity catalytic for Knoevenagel reaction in aqueous medium

Article abstract of DOI:10.1007/s11164-019-03781-9

An imine-functionalized mesoporous solid base catalyst (BA@BE-PMO) was prepared by template agent-directed self-assembly condensation of bis[3-(triethoxysilyl)propyl]amine and 1,2-bis(triethoxysilyl)ethane in acid solution. The imine groups with catalytic

Full text of DOI:10.1007/s11164-019-03781-9

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03781-9
Imine‑bridged periodic mesoporous organosilica as stable
high‑activity catalytic for Knoevenagel reaction in aqueous
medium
Zechun Sun1 · Yiran Wang2 · Zhiruo Zhang1 · Fengxia Zhu1,3 · Pusu Zhao1 ·
Guisheng Li3 · Fengfeng Shao3 · Jiahui Rui1
Received: 20 December 2018 / Accepted: 18 February 2019
© Springer Nature B.V. 2019
Abstract
An imine-functionalized mesoporous solid base catalyst (BA@BE-PMO) was pre-
pared by template agent-directed self-assembly condensation of bis[3-(triethoxysilyl)
propyl]amine and 1,2-bis(triethoxysilyl)ethane in acid solution. The imine groups
with catalytic activity were integrally embedded into mesopore walls of as-made
BA@BE-PMO. In Knoevenagel reactions in aqueous medium, the BA@BE-PMO
catalyst exhibited better catalytic activity than imine-functionalized SBA-15 catalyst
synthesized using the traditional co-condensation method, which can be attributed
to the pore surface with strong hydrophobicity originating from –CH2CH2– group
fragments incorporated into pore walls. The strong hydrophobicity of the surface
facilitates adsorption and difusion of organic compounds on the catalyst surface in
reactions in aqueous medium. Moreover, it exhibited comparable catalytic activity
to dipropylamine homogeneous base catalyst because of the uniform dispersion of
imine group active sites. The BA@BE-PMO catalyst could also be recovered and
reused in up to fve runs without signifcant loss in activity without any negative
environmental impact.
Keywords Imine-bridged PMO · Solid base catalyst · Knoevenagel reaction in
aqueous medium · Green catalysis
* Fengxia Zhu
zhufengxia501@163.com
* Pusu Zhao
zhaopusu@163.com
* Guisheng Li
liguisheng@shnu.edu.cn
Extended author information available on the last page of the article
Vol.:(0123456789)
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Z. Sun et al.
Introduction
The Knoevenagel reaction between activated methylene compounds and aldehyde
is a very efcient and atom-economic method for C=C bond formation in organic
chemistry [1–4]. This reaction has been applied in many important areas of organic
synthesis, such as synthesis of chemical reaction intermediates, perfume prepara-
tion, pharmaceutical industry, etc. [5, 6]. Traditionally, a homogeneous catalyst and
organic solvent have been applied in such processes, but this approach is not envi-
ronmentally friendly according to the principles of green chemistry. Development of
safer, more efcient, and more environmentally benign chemical methods is a main
demand of humanity, particularly at the start of the third millennium [7].
Sheldond believed that “The best solvent is solvent-free. Water is the best choice
if a solvent is really needed” [8]. Water is regarded as a green solvent because it is
safe, nontoxic, readily available, and nonfammable [9, 10]. Recently, application of
various organocatalysts for reactions in aqueous medium has been reported, having
the potential to reduce pollution of the environment by organic solvents [11–14].
Although homogeneous organocatalysts exhibit high catalytic activity and selectiv-
ity, their use will decrease resources and pollute the environment owing to their dif-
fcult separation. It is thus imperative to seek a kind of catalyst that can overcome
these disadvantages and achieve environmentally benign synthesis. Recently, dif-
ferent sorts of heterogeneous solid catalyst have been synthesized for Knoevenagel
condensation reactions in aqueous medium. A few examples include l-lysine-func-
tionalized polyacrylonitrile fber [15], amino-functionalized ionic liquid [16], amine
supported on silica gel [17], amino- and Schif base supported on mesoporous sil-
ica [18], etc. [19–23]. Among these supports, mesoporous silica with high surface
area and nanometer-sized pores has attracted widespread attention for application in
catalysts [18, 24, 25], adsorbents [26, 27], and sensor devices [28, 29]. Previously,
we reported the binding of amine groups onto the pore wall surface of mesoporous
silica using a co-condensation method, which results in signifcantly improved
catalytic activity due to the enhanced dispersion of active sites and reduced limita-
tion by difusion [30]. However, the weak hydrophobicity of such catalyst surfaces
does not favor adsorption and difusion of organic molecules, particularly during
organic reactions in aqueous medium. Results obtained for organometal immobi-
lized ordered mesoporous silica containing organic groups in the framework have
confrmed that functionalization of mesoporous silica with organic groups can
enhance the hydrophobicity of the catalyst surface compared with mesoporous sil-
ica without organic groups [30, 31]. Periodic mesoporous organosilica (PMO) with
organic fragments embedded in the silica walls represents a useful solution to these
problems. PMO materials are a kind of organic–inorganic hybrid silica containing
a highly uniform distribution of organic groups in an organosilica framework, hav-
ing attracted much interest recently for application in catalysts for use in aqueous
medium owing to their stronger surface hydrophobicity compared with traditional
inorganic silica materials [32–35].
We report herein a novel method for preparation of imine-functionalized PMO
(BA@BE-PMO) with high nitrogen content and highly ordered two-dimensional
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Imine-bridged periodic mesoporous organosilica as stable…
mesoporous channels under acid conditions. The nitrogen content in the BA@
BE-PMO sample can be adjusted to 4.1 wt% without disrupting its periodic
ordered structure. The as-made BA@BE-PMO was used as a heterogeneous solid
organic base catalyst, showing comparable catalytic activity to the homogeneous
catalyst dipropylamine in Knoevenagel reactions in aqueous medium. Moreover,
the BA@BE-PMO catalyst could be recycled and used for more than fve times
without a signifcant decrease in its catalytic activity, which can ultimately reduce
pollution by organic agents and production cost, indicating promise for use in
industrial applications.
Experimental
Materials
Bis[3-(triethoxysilyl)propyl]amine (BTPA) and Pluronic P123 (EO20PO70EO20,
M=5800) were obtained from Gelest Inc. 1,2-Bis(triethoxysilyl)ethane (BTEE) was
obtained from Sigma-Aldrich Company Ltd. Tetraethylorthosilicate (TEOS), con-
centrated hydrochloric acid (37 %), ethanol, aromatic aldehyde, ethyl cyanoacetate,
and other chemical reagents used were analytically pure and supplied by Sinopharm
Chemical Reagent Co. Ltd. The water used in the experiments was deionized water.
Preparation of BA@BE‑PMO
BA@BE-PMO catalyst was prepared according to the series of reactions illus-
trated in Scheme 1. The typical process was as follows: P123 (0.50 g) was added to
0.50 mol/L HCl solution (16 mL) containing KCl (1.5 g) at 313 K. After it dissolved
completely, BTEE (2.5–2.5x mmol) was added dropwise to the above solution under
stirring. The resulting emulsion was stirring continued at 313 K for 2 h. The reac-
tion emulsion was then kept under stirring at 313 K for 24 h, after which BTPA
(2.5xmmol) was introduced into the emulsion; x refers to the molar ratio of BTPA
to all silane, defned as [BTPA/(BTPA+BTEE)]. Finally, the emulsion was trans-
ferred to a reaction vessel and placed in an oven for another 24 h at 373 K. Finally,
white solid powder was obtained by decompression fltration, washing with deion-
ized water, and drying at 373 K for 8 h. The white powder was then extracted with
acidic ethanol to remove templates and other organic compounds at 353 K for 24 h,
yielding BA@BE-PMO.
Scheme 1 Schematic illustration of preparation route for BA@BE-PMO
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Z. Sun et al.
The above obtained powder was added into a certain amount of saturated Na2CO3
aqueous solution and stirred for 24 h at ambient temperature to deprotonate the
imine-bridged groups. The thus-obtained materials were washed with copious
deionized water until the flter liquor was neutral. The fnal materials are denoted as
BA@BE-PMO-x. For comparison with the catalytic activity of the BA@BE-PMO
catalyst, imine-bridged mesoporous silica SBA-15 catalyst, denoted as BA@SBA-
15, was also prepared, using the traditional method of co-condensation of BTPA and
TEOS. Firstly, P123 (0.50 g) was dissolved in 0.88 mol/L HCl (34 mL) at 313 K.
Then, TEOS (3.0 mol) was introduced into the above solution. When the TEOS had
hydrolyzed thoroughly at 313 K, BTPA (1.0 mol) was added into the above solution.
The solution was then kept under rapid stirring at 313 K for 20 h and treated hydro-
thermally at 373 K for 24 h. Finally, white solid was obtained by fltration then dry-
ing at 373 K in an oven. The template agent and other adsorbed organic substances
were removed, and deprotonated using the above-described methods.
Characterization of catalysts
X-ray difraction (XRD) patterns of diferent samples were measured using a
Rigaku D/Max-RB difractometer with Cu Kα radiation. Transmission electron
microscopy (TEM) images were obtained using a JEM2011 microscope operating
at 200 kV. Nitrogen adsorption–desorption isotherms were obtained at 77 K on a
Quantachrome NOVA 4000e analyzer. Fourier-transform infrared (FTIR) spec-
troscopy was carried out using a Magna 550 infrared spectroscope (Nicolet Com-
pany) using the KBr compression method. Specifc surface area (SBET) values were
obtained based on the multipoint Brunauer–Emmett–Teller (BET) model. The aver-
age pore diameter (DP) was calculated from the adsorption isotherms using the Bar-
rett–Joyner–Halenda (BJH) model. Solid nuclear magnetic resonance (NMR) spec-
tra were obtained using a Bruker DRX-400 NMR spectrometer. X-ray photoelectron
spectroscopy (XPS) was carried out using a PerkinElmer PHI 5000C ESCA to ana-
lyze the surface electronic states of the samples. All binding energy values were
corrected using the C 1s line at 284.6 eV for reference. The nitrogen content of the
BA@BE-PMO and BA@SBA-15 samples was determined using an Element Vario
EL III analyzer. All samples were fully combusted in pure oxygen atmosphere at
1273 K.
Activity testing
Knoevenagel condensation between aldehyde and ethyl cyanoacetate in water
(Scheme 2) was selected as a model reaction to assess the catalytic performance
of the catalysts. Aromatic aldehyde (1.0 mmol), ethyl cyanoacetate (1.2 mmol),
deionized water (3.0 mL), a catalyst with a certain nitrogen content, and 200 μL
internal standard (n-nonane) were added into a round-bottomed fask loaded with a
condenser tube. Reaction completion was monitored by gas chromatography (GC)
analysis at certain reaction time intervals. After reaction completion, the reaction
mixture was extracted with ethyl acetate after cooling to ambient temperature. The
1 3

Imine-bridged periodic mesoporous organosilica as stable…
Scheme 2 Chemical equation for Knoevenagel reaction in aqueous medium
extracted organic mixture was analyzed by gas chromatography (Shimadzu GC-
17A) equipped with a fame ionization detector (FID) and a capillary column (DB-
5) containing 95 % dimethyl-5 % diphenylpolysiloxane with nitrogen as carrier gas.
The chromatograph column temperature was varied from 363 to 473 K at rate of
10 K/min. The conversion of each reaction was calculated based on aromatic alde-
hyde, since excess ethyl cyanoacetate was used in the reaction. Activity data were
obtained by repeating each experiment more than three times, achieving accuracy in
the range of 5 %.
To investigate the recyclability of the as-made catalyst, the solid catalysts were
collected by centrifugation and fltration respectively after each run to reaction com-
pletion. To remove remaining reactants, the catalysts were separated by washing
with plenty of deionized water and ethanol, and drying in a vacuum oven at 353 K
for 8 h. Finally, the catalysts were reused in the Knoevenagel reaction using fresh
solvent and reactants with the same reaction temperature and time.
Results and discussion
Catalyst characteristics
The small-angle XRD patterns (Fig. 1) confrmed that all the materials showed a
stronger difraction peak near 2θ=0.9°, characteristic of (100) plane, and another
two weak peaks at around 2θ=1.5° and 1.7°, corresponding to difraction from
(110) and (200) planes. These difraction pattern results reveal that all the samples
possessed two-dimensional p6mm hexagonal ordered mesoporous structure [36].
The main difraction peak gradually weakened while the two weak difraction peaks
at large angle gradually vanished as the imine content was increased from BA@
BE-PMO-20 to BA@BE-PMO-60 with molar fraction of BTPA of 60 %. Such a
decrease in the degree of order of the mesoporous structure may occur because use
of a larger amount of BTPA might interfere with the self-assembly process of the
template agent [37–39]. Figure 2 further shows that, after removal of the template
agent, the ordered mesoporous material BA@BE-PMO-40 exhibited a sudden capil-
lary condensation step of nitrogen at p/p0 =0.45–0.55, which is representative of a
type IV isotherms, indicating uniform mesopores [40]. TEM images (Fig. 3) con-
frmed the ordered mesoporous structure of the BA@BE-PMO-40 material.
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Z. Sun et al.
BA@BE-PMO-10
BA@BE-PMO-20
BA@BE-PMO-30
BA@BE-PMO-40
BA@BE-PMO-50
BA@BE-PMO-60
1.0
1.5
2.0
2.5
3.0
3.5
4.0
2θ
/degree
Fig. 1 Small-angle XRD patterns of diferent BA@BE-PMO samples
1.0
800
0.8
0.6
700
0.4
600
0.2
0.0
500
3
4
5
6
7
8
9
10
Pore Diameter/ nm
400
300
200
100
0.0
0.2
0.4
0.6
0.8
1.0
P/P
0
Fig. 2 Nitrogen adsorption–desorption isotherms and pore size distribution (inset) of BA@BE-PMO-40
sample
The FT-IR spectrum of the BA@BE-PMO-40 sample (Fig. 4) displayed absorb-
ance signals at 1418, 1278, 1150, 776, and 702 cm−1, confrming that some
organic moieties were retained. The strong peaks at around 1027 cm−1 indicate
silicon–oxygen bond formation [41, 42]. The absorption bands near 3400 and
1650 cm−1, respectively, could correspond to nitrogen–hydrogen bonds [43, 44],
while the relatively wider peaks at 1150 and 776 cm−1 are indicative of antisym-
metric stretching vibration of carbon–nitrogen–carbon bonds and wagging vibra-
tion of nitrogen–hydrogen bonds, respectively. The peak at 694 cm−1 correspond-
ing to secondary amine was also clearly observed in each spectrum [18–20]. These
1 3

Imine-bridged periodic mesoporous organosilica as stable…
Fig. 3 TEM images of BA@BE-PMO-40
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4 FT-IR spectrum of BA@BE-PMO-40
FT-IR spectral data demonstrate that all the functional group components were fully
bonded to the framework of the hybrid organosilica material BA@BE-PMO-40.
The solid-state NMR spectra further revealed organic groups such as
CH2–CH2– and –NH– embedded in the framework of the BA@BE-PMO-40 mate-
rial. As shown in Fig. 5, the 29Si magic-angle spinning (MAS) NMR resonant
spectrum presented two distinct resonant peaks downfeld at around δ=−67 and
−59 ppm, belonging to T2 and T3 of silicon sites, respectively, where T2 refers to
≡C–Si(OH)(OSi)2) and T3 indicates ≡C–Si(OSi)3) [45–47]. The weak shoulder res-
onant peak at δ=−50 ppm may be attributable to silicon site of T1, where T1 refers
1 3

Z. Sun et al.
T2
29Si MAS NMR
13C MAS NMR
4.2
(a)
(b)
9.2
20
42
0
-20
-40
-60
-80
-100
-120
60
40
20
0
-20
δ
(ppm)
Chemical shift (ppm)
Fig. 5 29Si MAS NMR and 13C MAS NMR results for BA@BE-PMO-40
to ≡CSi(OH)2(OSi). The lack of obvious signals ascribable to Qm sites, where Qm
refers to Si–(OSi)n–(OH)4−n, n=2–4) from δ=−90 to δ=−110 ppm confrms that
all the Si atoms of the organosilica were covalently linked with carbon atoms with-
out breaking during the co-condensation process [48]. Additionally, the 13C MAS
NMR resonant spectrum exhibited the dominant resonance peak at δ=4.2 ppm, cor-
responding to –CH2CH2– from BTEE [49–51]. The other obvious peaks at δ=9.0,
21, and 42 ppm are assigned to three C atoms bonding to imine group, namely
–Si–(CH2)3–NH–(CH2)3–Si– [39, 52]. These results confrm that the imine groups
were successfully incorporated into the organosilica framework. The results of both
the NMR and IR characterizations demonstrate that the organic functional groups
were present in the BA@BE-PMO-40 sample.
The XPS results (Fig. 6) exhibited peaks at binding energy of −100.9, −284.2,
−399.5, and −532.6 eV, assigned to Si 2p, C 1s, N 1s, and O 1s ionization pro-
cesses, respectively [53, 54]. The N 1s spectrum (Fig. 6) showed an intense peak
O
1s
N1s
(a)
(b)
399.4 eV
BA@BE-PMO-40
BA@BE-PMO-40
dipropylamine
C
1s
Si
2p
N
1s
404
402
400
398
396
394
392
100
200
300
400
500
600
Binding Energy (eV)
Binding Energy (eV)
Fig. 6 XPS spectra of diferent samples
1 3

Imine-bridged periodic mesoporous organosilica as stable…
at binding energy of 399.5 eV, demonstrating presence of secondary amine groups
[53, 55]. Compared with dipropylamine (CH3CH2CH2NHCH2CH2CH3), no evident
shift in binding energy was found, confrming that the imine-bridged silanes (BTPA)
were directly immobilized into the framework of the mesoporous organosilica with-
out damaging the chemical microenvironment of nitrogen atom.
Catalytic performance
Knoevenagel reactions in aqueous medium should be carried out carefully, because
they contain solid, organic, and inorganic phases. First, the reaction conditions were
optimized by investigating the efects of the reaction time, amount of catalyst, and
type of catalyst on the conversion and yield of the reaction. As presented in Table 1,
besides the remaining ethyl cyanoacetate and aldehyde, the only product was moni-
tored by GC–mass spectrometry (MS) analysis, indicating that the selectivity of the
catalyst reached nearly 100 % under various conditions.
Efect of reaction time and amount of catalyst
From Fig. 7a, it can be seen that the conversion of benzaldehyde gradually increased
with prolongation of the reaction time from 1 to 6 h. The yield reached 96 % after
reaction for 5 h. However, the yield did not reach 100 % even after prolonging for
another 1 h, revealing that the best reaction time is 5 h. As shown in Fig. 7b, the
product yield increased from 54 to 96 % when the amount of catalyst was raised
from 0.010 to 0.040 mmol, which could be due to an increase of alkaline active
sites. However, further increase of the amount of catalyst did not further improve the
conversion rate of the reaction. This can probably be attributed to a decrease of the
solubility of the catalyst in water solvent with increase of its dose [56].
Table 1 Catalytic performance of diferent catalysts in Knoevenagel reaction in aqueous medium
Entry
Catalyst
Nitrogen content
(wt%)
R
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
9
Dipropylamine
/
H
H
H
H
H
H
H
OCH3
99
89
92
98
91
82
89
87
99
99
89
92
98
91
82
89
87
99
BA@BE-PMO-20
BA@BE-PMO-30
BA@BE-PMO-40
BA@BE-PMO-50
BA@BE-PMO-60
BA@SBA-15
1.3
2.2
2.7
3.3
4.1
2.5
2.7
2.7
BA@BE-PMO-40
BA@BE-PMO-40
NO2
Reaction conditions: catalyst; aromatic aldehyde (1.0 mmol), ethyl cyanoacetate (1.2 mmol); 3.0 mL
H2O; T=373 K; t=5 h
1 3

Z. Sun et al.
100
90
80
70
60
50
40
30
20
10
0
(b)
(a)
100
80
60
40
20
0.01
0.02
0.03
0.04
0.05
0
1
2
3
4
5
6
7
Reaction time (h)
The mass of the catalysts (g)
Fig. 7 Efect of reaction time (a) and amount of BA@BE-PMO-40 (b) on reaction yield
Efect of diferent catalysts
The catalytic efciency of the diferent catalysts in the Knoevenagel reaction in
aqueous medium is presented in Table 1. For the series of BA@BE-PMO catalysts,
the catalytic performance frst increased and then reduced with increasing nitrogen
content in BA@BE-PMO. The BA@BE-PMO-20 and BA@BE-PMO-30 catalysts
with low nitrogen content displayed poor catalytic activity owing to the compara-
tively greater distance between adjacent catalytic active sites, i.e., nitrogen atoms.
The Knoevenagel reaction requires two kinds of organic reactant substrate to adsorb
on adjacent catalytic active sites, i.e., nitrogen atoms (Scheme 3). Therefore, greater
distance between adjacent nitrogen atoms in BA@BE-PMO will decrease their syn-
ergetic efect and thereby the conversion. However, the BA@BE-PMO-50 and BA@
BE-PMO-60 catalysts with very high nitrogen contents also exhibited low activity,
which can probably be attributed to the relatively strong steric hindrance between
adjacent nitrogen atoms, which does not favor adsorption and difusion of organic
substances onto the active sites on the surface of the catalyst. BA@BE-PMO-40
was thus confrmed to be the optimal catalyst, showing catalytic performance
Scheme 3 Schematic illustration of base-catalyzed mechanism for Knoevenagel condensation reaction
1 3

Imine-bridged periodic mesoporous organosilica as stable…
comparable to the dipropylamine organocatalyst. Table 1 also compares the cata-
lytic activity of catalysts with diferent hydrophobicity but similar nitrogen content.
The BA@BE-PMO-40 catalyst resulted in higher conversion and yield than BA@
SBA-15 due to the –CH2CH2– groups embedded in the organosilica framework,
enhancing the hydrophobicity of the catalyst surface, which helps organic reactant
substrates to difuse and adsorb onto active sites on the surface of the catalyst [57].
To investigate the universality of the as-formed catalyst, use of BA@BE-PMO-40
was also examined in Knoevenagel condensation reactions between ethyl cyanoac-
etate and diferent aromatic aldehydes containing electron-withdrawing or electron-
donating groups in water medium. As shown by the results presented in Table 1, the
properties of the substituent on the benzene ring had an important efect on the cata-
lytic efciency of BA@BE-PMO-40. Aromatic aldehydes containing an electron-
withdrawing group gave higher conversions in the reaction compared with those
containing an electron-donating group. p-Nitrobenzaldehyde containing an electron-
withdrawing substituent (–NO2) was propitious for the Knoevenagel reaction, pro-
viding a product yield of 99 %. In contrast, benzaldehyde containing electron-donat-
ing groups (–OCH3) resulted in lower yield (87 %). These results are consistent with
the mechanism for the Knoevenagel reaction shown in Scheme 3. First, the active
methylene hydrogen of ethyl cyanoacetate is captured by a basic group (–NH–) of
the BA@BE-PMO catalyst to form carbon anion. The carbon anion then attacks
the carbonyl carbon atom of aldehyde to produce an oxygen anion intermediate,
which captures a hydrogen ion and loses a molecule of water to form the product.
According to this reaction mechanism, it can be seen that the nucleophilic addition
of the carbonyl carbon becomes easier when there are electron-withdrawing groups
attached to the benzene ring. In contrast, electron-donating groups on the aromatic
ring are not conducive to the nucleophilic addition reaction [58].
To confrm that active sites immobilized on the organosilica caused the cata-
lytic efect rather than detached organocatalyst, the following experiment proposed
by Sheldon et al. [59] was performed: When the reaction had been carried out for
3 h, at which point the conversion had reached about 60 %, the BA@BE-PMO-40
solid powder catalyst was centrifuged and fltered out, and the reaction conditions
(reaction temperature and stirring speed) were continued with the obtained fltrate
for another 4 h. The results showed that the conversion of benzaldehyde did not
change signifcantly, revealing that the catalytic activity resulted from the BA@BE-
PMO-40. It is thus reasonable to conclude that the BA@BE-PMO-40 catalyst was a
kind of heterogeneous catalyst.
Recyclability of catalyst
These excellent catalytic activity results motivated us to study the signifcant
issue of the endurance of the BA@BE-PMO-40 catalyst. As shown in Fig. 8, the
BA@BE-PMO-40 catalyst could be easily recovered and reused for fve times
without a large decrease in its catalytic activity in the Knoevenagel reaction in
aqueous medium. Based on elemental analysis, the nitrogen content in the reused
BA@BE-PMO-40 catalyst was not evidently reduced, further confrming that
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Z. Sun et al.
100
80
60
40
20
0
1
2
3
4
5
Recycled Times
Fig. 8 Durability results of BA@BE-PMO-40 catalyst in Knoevenagel condensation reaction in aqueous
medium
embedding the imine groups into the organosilica pore walls could efectively
prevent their leaching. Moreover, TEM and small-angle XRD analysis (Fig. 9)
confrmed that, after being used fve times, the BA@BE-PMO-40 catalyst still
possessed ordered mesoporous channels, revealing excellent hydrothermal
stability.
Fig. 9 XRD pattern and TEM
image (inset) of BA@BE-
PMO-40 catalyst after recycling
fve times
1
2
3
4
2 Theta Degree
1 3

Imine-bridged periodic mesoporous organosilica as stable…
Conclusions
We report BA@BE-PMO as a new type of solid base catalyst with imine groups
integrally embedded into the mesoporous walls of a PMO support. In Knoeve-
nagel reactions in aqueous medium, the catalyst displayed high catalytic activ-
ity owing to a reduction of the difusion limit. In addition, the catalyst not only
showed high catalytic activity comparable to the homogeneous dipropylamine
catalyst, but could also be conveniently recovered and recycled for more than
fve times without a signifcant reduction in its catalytic activity, which can be
ascribed to the preservation of the chemical microenvironment of the nitrogen
atoms at active sites. These results demonstrate that such materials could have
important potential practical applications in the area of organic synthesis. Moreo-
ver, this new approach could be extended to synthesize other heterogeneous solid
organocatalysts.
Acknowledgements This work was supported by the National Natural Science Foundation of China
(51872109, 21677099, 21876112), Natural Science Foundation of Jiangsu Provincial Department of
Education (17KJA150002, 15KJA150003), and College Students’ Practical Innovation Project of Jiangsu
Province (201810323009Z).
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Afliations
Zechun Sun1 · Yiran Wang2 · Zhiruo Zhang1 · Fengxia Zhu1,3 · Pusu Zhao1 ·
Guisheng Li3 · Fengfeng Shao3 · Jiahui Rui1
1
School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huai’an 223300,
China
2
School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
1 3

Imine-bridged periodic mesoporous organosilica as stable…
3
Education Ministry Key and International Joint Lab of Resource Chemistry, Shanghai Key
Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234,
People’s Republic of China
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