Controllable acid-base bifunctionalized mesoporous silica: Highly efficient catalyst for solvent-free Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.catcom.2011.08.004

A controllable acid-base bifunctionalized mesoporous catalyst with acidic sites and basic sites in adjacent arrangements was prepared via an in situ cleavage of sulfonamide bond on synthetic process. During Knoevenagel condensation reaction of aromatic al

Full text of DOI:10.1016/j.catcom.2011.08.004

Catalysis Communications 15 (2011) 10–14
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Controllable acid-base bifunctionalized mesoporous silica: Highly efficient catalyst
for solvent-free Knoevenagel condensation reaction
⁎
Yin Peng, Jianyao Wang, Jie Long, Guohua Liu
The Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Normal University, Shanghai 200234, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2011
Received in revised form 9 July 2011
Accepted 1 August 2011
Available online 10 August 2011
A controllable acid-base bifunctionalized mesoporous catalyst with acidic sites and basic sites in adjacent
arrangements was prepared via an in situ cleavage of sulfonamide bond on synthetic process. During
Knoevenagel condensation reaction of aromatic aldehydes and ethyl cyanoacetate under microwave
irradiation in solid media, the mesoporous bifunctionalized catalyst Me-A/B-SBA-15 exhibited higher
catalytic activity than those of the corresponding amine-functionalized catalyst and the randomly-arranged
acid-base catalyst, showing obvious acid-base cooperativity.
Keywords:
© 2011 Elsevier B.V. All rights reserved.
Heterogeneous catalysis
Mesoporous materials
Supported catalysts
C–C coupling
1. Introduction
Kubota group [18] reported that an organic cationic-MCM-41
composite showed higher activity than that of organic cation due to
Development of well-defined catalytic sites and exploration of
cooperative catalysis have attracted a great deal of interest due to
their high efficiency in organic synthetic chemistry [1]. One common
strategy to study cooperative catalysis is to employ a supporting
material [2], in which different functionalities can be assembled onto
this material and well-defined catalytic microenvironment can be
controlled. Such a strategy does not only solve the problem of catalyst
recycling, but also does improve the catalytic activity via cooperative
catalysis of different functionalities. Mesoporous silica as a kind of
ideal supporting materials shows some salient features [3]. Besides
general advantages of large surface area and regular pore structure,
some mesoporous silica prepared by a co-condensation method can
allow to load different types of functionalities and to control their
uniform distribution onto pore surfaces [4,5]. Recently, some of
mesoporous materials have been used successfully to explore
bifunctional cooperativity in a variety of catalytic reactions [6–14].
Knoevenagel condensation as an important carbon\carbon bond
forming reaction is commonly used to evaluate both organic [15] and
inorganic [16] basic catalysis. In particular, some demonstrations
about acid-base cooperativity have also been appeared in the
literatures. The pioneering work reported by Hein group [17] found
that there was an acid-base cooperativity when a weakly basic resin
with an additive of acetic acid was employed as a bifunctional catalyst.
acid-base cooperativity. Recently, some mesoporous silicas for the
investigation of acid-base cooperativity have also been explored.
Angeletti group [19] expatiated a silanol act as an acidic site and 1,8-
bis(dimethylaminonaphthalene) functional group immobilized onto
MCM-41 material act as a basic site in acid-base cooperativity. In
particular, Shanks group and Katz [20,21] explored cooperative
primary amine mechanism of acid-base cooperativity through
employing an aminopropyl-functionalized SBA-15 mesoporous silica.
Although the fruitful achievements have been obtained in this
research field, however, these bifunctional groups onto supports
distribute randomly and lack of a continuous arrangement of acidic
and basic sites. There is a question whether bifunctional groups onto
materials do reflect truly the nature of cooperative catalysis.
Therefore, an innovative strategy to control independent acid and
base sites in adjacent arrangement and to match equivalent mole
relationship onto materials is helpful to elucidate the nature of acid-
base cooperativity.
Recently, we have reported a series of mesoporous catalysts and
their applications in catalytic processes [22,23]. Herein we develop a
synthesis of mesoporous acid-base bifunctionalized catalyst via a co-
condensation method, and apply it to microwave-promoted Knoeve-
nagel condensation reaction in solid medium. The key feature is that
arylsulfonated amide silica resource is separated on an in situ synthetic
process to keep acid and base sites in adjacent arrangements onto
mesoporous materials. The research focuses on as follows: (1) construc-
tion of a mesoporous well-defined acid-base bifunctionalized catalyst;
(2) investigation of synergistical effect of acid-base cooperativity in
Knoevenagel condensation reaction.
⁎
Corresponding author at: The Key Laboratory of Resource Chemistry of Ministry of
Education, Shanghai Normal University, Shanghai, 200234, PR China. Tel.: +86 21
64321819; fax: +86 21 64322511.
E-mail address: ghliu@shnu.edu.cn (G. Liu).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.08.004

Y. Peng et al. / Catalysis Communications 15 (2011) 10–14
11
2. Experimental
Scheme 1. Firstly, the reaction 3-aminopropyltrimethoxysilane (1) of
2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (2) using Et3N as a
basic reagent gave N-trimethoxysilylpropyl-(4-trimethoxysilylethyl)
benzenesulfonamide (3) with 56% isolated yield (See supporting
information). The condensation N-trimethoxysilylpropyl-(4-tri-
methoxysilylethyl)benzenesulfonamide (3) with Si(OEt)4 was then
carried out to afford A/B-SBA-15 (4) as a white powder (See supporting
information) according to reported method [23]. Finally, after the
protection of silicon-hydroxyl groups of A/B-SBA-15 (4) with trimethy-
silyl groups [24], the resulting rude products were treated with excess
potassium tert-butoxide followed by control of pH to afford catalyst Me-
A/B-SBA-15 (5) as a paler white powder. In order to clarify when
cleavage of sulfonamide silicaresourceoccurred, thecontrolexperiment
using the parent N-propyl-toluenesulfonamide as a starting material
was carried out. The result showed that N-propyl-toluenesulfonamide
was rather stable in 2 N hydrochloric acid at 40 °C, even no cleavage
occurred in refluxed condition. This demonstrated no cleavage occurred
on prepared process of A/B-SBA-15 (4). However, quantitative cleavage
products were obtained when 10 equivalent of t-BuOK was used as a
cleavage reagent [25]. This behavior indicated that the cleavage of
sulfonamide silica resource occurred on last step, resulting in an
adjacent arrangement of an acidic site and a basic site onto materials.
Elemental analysis of Me-A/B-SBA-15 (5) calculated from mass% of N
(0.747%) and S (1.707%) further confirmed that the mole ratio of sulfur
to nitrogen was 1:1. The thermal gravimetric datum (see Fig. S3 in
supporting information) indicated 32% organic molecules were loaded
onto the mesoporous materials, which was consistent with the data of
Elemental analysis.
2.1. Preparation of acid-base bifunctionalized catalyst Me-A/B-SBA-15 (5)
Under argon atmosphere, a suspension of A/B-SBA-15 (4) (1.00 g)
and HMDS [(CH3)3Si)2 N] (5 mL, 0.025 mol) in 25 mL of dry THF was
stirred overnight. The mixture was filtered through filter paper and
rinsed with excess acetone. The collecting solids were then suspended
again in 20 mL dry THF and 0.74 g of t-BuOK (6.70 mmol) was added.
The mixture was allowed to reflux for 6 h. After cooling to room
temperature, the pH value of suspension was adjusted to 7.0 using
acetic acid. Finally, the suspension was filtered and rinsed with excess
ethanol. The collecting solids were dried under vacuum at 60 °C for
6 h to afford Me-A/B-SBA-15 (5) (1.06 g) in the form of a paler white
powder. IR (KBr) cm−1: 3043 (w), 2962 (w), 2898 (w), 1597 (w),
1088 (s), 947 (m), 801 (w), 557 (w), 465 (m); Elemental analysis (%):
C 24.58, H 4.176, N 0.747, S 1.707; dpore: 5.4 nm; SBET: 532 m2/g; 29Si
MAS NMR (79.5 MHz): Q4 (δ=−110.4 ppm), Q3 (δ=−101.3 ppm),
T3 (δ=−68.6 ppm); 13C CP MAS NMR (100.6 MHz): 127.7, 56.9, 41.6,
27.4, 20.3, 15.1, 8.1, 0.5 ppm.
2.2. Characterizations
The X-ray powder diffraction (XRD) experiments were carried out
on a Rigaku D/Max-RB diffractometer with Cu Kα radiation.
Transmission electron microscopy (TEM) studies were performed
on a JEOL JEM2010 electron microscope, operated at an acceleration
voltage of 200 kV. Fourier transform infrared (FTIR) spectra were
collected with a Nicolet Magna 550 spectrometer by using the KBr
method. Nitrogen adsorption isotherms were measured at 77 K with a
Quantachrome Nova 4000 analyzer. The samples were measured after
being outgassed at 423 K overnight. Pore size distributions were
calculated by using the BJH model. The specific surface areas (SBET) of
samples were determined from the linear parts of BET plots (p/
p0 =0.05–1.00). Solid-state 29Si MAS NMR and 13C CP MAS NMR
spectra were recorded at 79.5 and 100.6 MHz, respectively, using a
Bruker AV-400 spectrometer.
3.2. Structural and morphological properties of A/B-SBA-15 (4) and Me-
A/B-SBA-15 (5)
The incorporation of organic acid and base groups onto the
mesoporous materials could be confirmed by solid-state NMR spectra.
As shown in Fig. 1(a), the 29Si MAS NMR spectra of A/B-SBA-15 (4) and
Me-A/B-SBA-15 (5) showed two groups of signals with four oxygen
neighbors (Q-type species) originated from TEOS and with three oxygen
neighbors (T-type species) derived from silylether groups. Typical
isomer shift values were −91.5/−101.5/−110 ppm for Q2/Q3/Q4
signals (Q2{(HO)2Si(OSi)2}, Q3 {(HO)Si(OSi)3}, Q4 {Si(OSi)4}) and
−48.5/−58.5/−67.5 ppm for T1/T2/T3 signals (T1{R(HO)2SiOSi}, T2 {R
(HO)Si(OSi)2}, T3 {RSi(OSi)3}) [26]. A/B-SBA-15 (4) gave a medium Q4
(−112.2 ppm), a strong Q3 (−102.8 ppm), a weak Q2 (−92.5 ppm), a
medium T3 (−67.4 ppm) and a weak T2 (−58.9 ppm) peak. Me-A/B-
SBA-15 (5) presented a strong Q4 (−110.4 ppm), a medium Q3
(−101.3 ppm) and a weak T3 (−68.6 ppm) peak. As compared with
A/B-SBA-15 (4), the enhanced Q4 signal and disappeared Q2 signal in
Me-A/B-SBA-15 (5) suggested that the catalyst 5 possessed mainly
network structure of {Si(OSi)4 and(HO)Si(OSi)3} while the enhanced T3
signal and the disappeared T2 signal indicated the formation of {RSi
(OSi)3} (R = organic acid and base groups) as a part of wall in
mesoporous structure. The 13C CP/MAS NMR spectra displayed the
peaks at 127.6 (C-Ph), 68.9 (−OCH2CH3), 41.6 (−CH2NH2), 27.4
(−CH2Ph), 20.3 (−CH2CH2NH2), 15.1 (−OCH2CH3), 8.1 (−CH2Si),
0.5 (−CH3Si) ppm in the Me-A/B-SBA-15 (5) and 127.6 (C–Ph), 69.6
(−OCH2CH3), 42.1 (−CH2NH2), 27.7 (−CH2Ph), 20.4 (−CH2CH2NH2),
14.8 (−OCH2CH3), 8.3 (−CH2Si) ppm in the A/B-SBA-15 (4),
corresponding to aromatic and aliphatic carbon atoms as marked in
Fig. 1(b).
2.3. Catalytic reaction
A typical procedure was as follows: Aldehyde (2.0 mmol) and
ethyl cyanoacetate (0.24 mL, 2.20 mmol) and Me-A/B-SBA-15 (5)
(60.0 mg, 32.0 μmol, based on Elemental analysis) were added to a
thick walled Pyrex tube. When the addition was complete, the tube
was positioned in a MAS-2 single mode cavity microwave with a
water-cooled condenser from Sineo Microwave Chemistry Technol-
ogy (China) Co. LTD, adjusting the reaction temperature button at
100 °C and producing continuous irradiation at 2.45 GHz. The mixture
was irradiated in 700 W for 10 min. After being cooled down to room
temperature, 2.0 mL of ethyl acetate was then added and the mixture
was filtrated. The organic layer was dried over Na2SO4 and
concentrated. The residue was further purified by flash column
chromatography on silica gel (eluent: ethyl acetate/hexane=1:1) to
afford a mixture of aldehydes and the corresponding product, which
was used to determine conversion and selectivity via a comparison of
concentration of start materials using a HPLC analysis with a UV–vis
detector containing a shim-pack VP-ODS column ( Φ 4.6×150 mm)
(refer to Fig. S5 in the supporting information).
3. Results and discussion
The powder XRD patterns (Fig. 2) revealed that both A/B-SBA-15
(4) and Me-A/B-SBA-15 (5) exhibited one similar intense d100
diffraction and two similar weak d110 and d200 diffractions, implying
that the ordered dimensional-hexagonal mesostructure (p6mm)
observed in pure SBA-15 [22] could be well preserved after the co-
condensation and the protection [27]. The TEM morphologies further
confirmed that both A/B-SBA-15 (4) and Me-A/B-SBA-15 (5) had
3.1. Synthesis of the acid-base bifunctionalized catalyst Me-A/B-SBA-15
(5)
The mesoporous bifunctionalized catalyst, abbreviated as Me-A/B-
SBA-15 (5), was prepared by a co-condensation approach as shown in

12
Y. Peng et al. / Catalysis Communications 15 (2011) 10–14
Scheme 1. Synthesis of the acid-base bifunctionalized catalyst 5.
well-ordered mesostructures with the dimensional-hexagonal ar-
rangements as shown in Fig. 3. Nitrogen adsorption–desorption
isotherms of A/B-SBA-15 (4) and Me-A/B-SBA-15 (5) (Fig. 4)
exhibited typical IV type nitrogen adsorption–desorption isotherms
with H1 hysteresis loop and a visible step at P/P0 =0.50–0.85,
corresponding to capillary condensation of nitrogen in mesopores.
In comparison with A/B-SBA-15 (4), Me-A/B-SBA-15 (5) presented a
decrease in the nanopore size, surface area, and pore volume. This
behavior suggested that the occupation of trimethysilyl groups onto
the pore channels made the pores narrower, resulting in an increase of
the wall thickness [22].
Fig. 1. NMR spectra of 4–5. (a) 29Si MAS NMR and (b) 13C CP MAS NMR.
Fig. 2. Powder XRD patterns of 4–5.

Y. Peng et al. / Catalysis Communications 15 (2011) 10–14
13
Fig. 3. TEM images of 4–5 viewed along [100] and [001] directions.
3.3. Microwave-promoted Knoevenagel condensation of aromatic
aldehydes and ethyl cyanoacetate
corresponding products with high yields. No by-products and the
corresponding isomers were observed confirmed by HPLC, in which
only one well-separated HPLC peak was observed. The highly catalytic
efficiency observed here should be due to the acid-base cooperativity
discussed below [20].
Microwave-assisted catalysis as a convenient method [28] can
accelerate obviously catalytic reactions from hours or days to minutes,
which “solvent-free” catalytic processes can match the need of green
environment [29]. Recently, employing both strategies for Knoeve-
nagel condensation reaction of aromatic aldehydes and ethyl
cyanoacetate had appeared in the literature [30–32]. Shown in
Table 1 was Knoevenagel condensation reaction of aromatic alde-
hydes and ethyl cyanoacetate performed under microwave radiation
in solid medium, where for each reaction a mixture of each aromatic
aldehyde and 1.1 equivalent of ethyl cyanoacetate and a certain
amount catalyst was irradiated for 10 min at 100°C under ambient
pressure using a power setting of 700 W. When the catalyst Me-A/B-
SBA-15 (5) with the same loading-amount amino functionality was
employed according to the literature [20,21], it was found that ethyl
(E)-2-cyano-3-phenyl-2-propenoate was obtained quantitatively.
GC–MS analysis showed that only one well-separated GC peak was
observed, suggesting that only E isomer of the products was formed
[33]. Such a reaction yield was obviously higher than those without
the microwave irradiation [20,21,30,31], even higher than that with
the microwave irradiation [32]. As shown in entry 1, when the mole
ratio of catalyst/substrate was down to 1.6%, the catalyst Me-A/B-SBA-
15 (5) still gave the corresponding product with 91.0% conversion. On
the basis of the about excellent result, the catalyst Me-A/B-SBA-15 (5)
was further investigated using a variety of aromatic aldehydes as
substrates (entries 2–8). In general, a variety of aromatic aldehydes
were reacted smoothly with ethyl cyanoacetate to afford the
To gain better insight into the effect of acid-base cooperativity, three
control experiments were carried out using unsilylated NH2-SBA-15,
unsilylated A/B-SBA-15 (4) and silylated Me-NH2-SBA-15 as catalysts
under the similar reaction conditions when 4-methylbenzaldehyde was
chosen as a substrate. It was found that the reaction using unsilylated
NH2-SBA-15 and A/B-SBA-15 (4) as catalysts gave ethyl (E)-2-cyano-3-
(4-methyl)phenyl-2-propenoate with high conversion as nearly same
as the catalyst Me-A/B-SBA-15 (5) (entries 9–10 versus entry 7), while
that of using silylated Me-NH2-SBA-15 as a catalyst afforded the
corresponding product with 55.8% conversion (entry 11). High activities
in the former suggested that both NH2-SBA-15 and A/B-SBA-15 (4) had
an acid-base cooperativity, in which silanol onto NH2-SBA-15 [20,21] or
sulfonic acid group onto A/B-SBA-15 (4) acted as an acidic site in
cooperative catalysis. The lower activity in the latter indicated that the
absence of silanol as an acidic site located on close proximity of basic site
resulted in a loss of acid-base cooperativity, which only underwent basic
catalysis. Such a behavior could be further proved by the physical
mixture of Me-NH2-SBA-15 plus SO3H-SBA-15 as a bifunctionalized
catalyst in the literatures and our experimental result (entry 12) [34,35].
In order to further prove truly cooperative catalysis in Me-A/B-SBA-15
(5), the control experiment employing silylated NH2-SBA-15/SO3H-
SBA-15 (a random distribution of acidic and basic sites onto materials)
[14] as an acid-base bifunctionalized catalyst was also carried out under
the similar reaction conditions. It was found that only 82.4% conversion
was obtained (entry 13). Apparently, the silylated NH2-SBA-15/SO3H-
SBA-15 presented a lower reactivity than Me-A/B-SBA-15 (5) and
higher reactivity than Me-NH2-SBA-15. Such a result suggested that
there was the part of acid-base cooperativity, in which the parts of
amino groups and sulfonic groups were in adjacent arrangements
because the acid and base functionalities were randomly-arranged on
mesoporous materials.
4. Conclusion
In conclusion, we supplied a facile approach to prepare the
mesoporous silica-supported acid-base bifunctionalized catalyst Me-
A/B-SBA-15 (5). The bifunctionalized catalyst exhibited excellent
catalytic activity in solvent-free Knoevenagel condensation under
microwave irradiation, which was obviously higher than that of the
corresponding amine-functionalized catalyst Me-NH2-SBA-15, even
than that of the randomly-arranged acid-base catalyst. Such behavior
was originated from an acid-base cooperativity, in which a sulfonic
acid group could activate substrate and a primary amine group could
catalyze Knoevenagel condensation reaction.
Fig. 4. Nitrogen adsorption–desorption isotherms of 4–5.

14
Y. Peng et al. / Catalysis Communications 15 (2011) 10–14
Table 1
Microwave-promoted Knoevenagel condensation of aromatic aldehydes and ethyl cyanoacetate.a
O
COOEt
CN
CN
WM
H
+
R
R
COOEt
O
O
O
O
O
O
OH
O
O
NO2
Cl
H
H
H
H
H
H
H
H
H3C
O2N
H3CO
Cl
7
8
9
10
11
12
13
14
Entry
Substrate
Catalyst
Conversion (%)b
Yield (%)c
1
2
3
4
5
6
7
8
7
8
9
5
5
5
5
5
5
5
5
4
91.0
97.1
96.5
99.5
98.2
92.2
97.9
91.6
95.5
94.6
55.8
86
92
91
94
93
89
93
87
93
91
50
10
11
12
13
14
13
13
13
13
13
9
10
11
12
13
NH2-SBA-15
Me-NH2-SBA-15
Physical mixtured
49.5
82.4
47
d
d
Bifunctional catalyste
79
e
e
a
Reactions were carried out using a MAS-2 single mode cavity microwave with a water-cooled condenser (continuous irradiation: 2.45 GHz, power: 700 W). Reaction conditions:
catalyst (60.0 mg, 32.0 μmol, based on Elemental analysis), aromatic aldehyde (2.0 mmol), ethyl cyanoacetate (2.2 mmol), reaction time 10 min, and reaction temperature 100 °C.
b
HPLC conversion.
c
Isolated yield (1H-NMR Spectra of products refer to supporting information in Fig. S4).
d
Data was obtained using physical mixture of Me-NH2-SBA-15 plus SO3H-SBA-15 [14] as a catalyst.
Data were obtained using the randomly-arranged silylated NH2-SBA-15/SO3H-SBA-15 [14] as an acid-base bifunctional catalyst.
e
[13] S. Shylesh, A. Wagener, A. Seifert, S. Ernst, W.R. Thiel, Angew. Chem. Int. Ed. 49
Acknowledgements
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(2007) 136.
[16] A.P. Wight, M.E. Davis, Chem. Rev. 102 (2002) 3589.
[17] R.W. Hein, M.J. Astle, J.R. Shelton, J. Org. Chem. 26 (1961) 4874.
[18] Y. Kubota, Y. Sugi, T. Tatsumi, Catal. Surv. Asia 11 (2007) 158.
[19] E. Angeletti, C. Canepa, G. Martinetti, P. Venturello, J. Chem. Soc., Perkin Trans. 1
(1989) 105.
We are grateful to China National Natural Science Foundation
(20673072), Shanghai Sciences and Technologies Development Fund
(10dj1400103 and 10jc1412300) for financial support.
Appendix A. Supplementary data
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Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.08.004.
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