Synthesis of a core-shell-shell structured acid-base bifunctional mesoporous silica nanoreactor (MS-SO3H@MS@MS-NH2) and its application in tandem catalysis

Article abstract of DOI:10.1039/c3ta13185h

An efficient and facile method was designed to produce an acid-base bifunctional mesoporous silica nanoreactor (MS-SO₃H@MS@MS-NH ₂) with acid sites on the inner core, unfunctionalized silica as the neutral zone shell and basic sites on the outer shell. Such acid core-neutral zone-base shell structure design resulted in excellent spatial separation of the hostile functionalities and was better than that of the core-shell structure. The spatial order of acid sites and basic sites in the mesoporous silica nanosphere offered a designated diffusion pathway that was favorable for reaction species, while the mesoporous structure endowed the material with the confinement effect. As a result, the core-shell-shell structured mesoporous silica nanosphere was an excellent bifunctional catalyst for one-pot cascade reaction sequences.

Full text of DOI:10.1039/c3ta13185h

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DOI: 10.1039/C3TA13185H
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ARTICLE TYPE
Synthesis of core-shell-shell structured acid-base bifunctional
mesoporous silica nanoreactor (MS-SO H@MS@MS-NH ) and its
3
2
application in tandem catalysis
Ping Li, Chang-Yan Cao, Hua Liu, Yu Yu and Wei-Guo Song*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
An efficient and facile method was designed to produce acidꢀbase bifunctional mesoporous silica
nanoreactor (MSꢀSO H@MS@MSꢀNH ) with acid sites on the inner core, unfunctionalized silica as
3
2
neutral zone shell and basic sites on the outer shell. Such acid coreꢀneutral zoneꢀbase shell structure
design resulted in excellent spatial separation of the hostile functionalities and was better than that of the
coreꢀshell structure. The spatial order of acid sites and basic sites in the mesoporous silica nanosphere
offered a designated diffusion pathway that was favorable for reaction species; while the mesoporous
structure endowed the material with confinement effect. As a result, the coreꢀshellꢀshell structured
mesoporous silica nanosphere was an excellent bifunctional catalyst for oneꢀpot cascade reaction
sequences.
1
0
1
5
site isolation of the two antagonistic functionalities and a
designated diffusion pathway for reaction species, we have
previously reported a coreꢀshell structured mesoporous silica
nanosphere with the acid sites in the inner core and the basic sites
1
. Introduction
Multifunctional heterogeneous catalyst, with two or more
functionalities integrated on one solid platform, has attracted a
great deal of research interest, as it can efficiently catalyze oneꢀ
pot multistep cascade reaction sequences requiring different
catalytic reactive sites to perform different reaction steps in the
sequences. Application of multifunctional catalystꢀparticipated
cascade reactions in the field of complex organic synthesis is
particularly appealing, due to the lowꢀcost, stepꢀsaving and
environmentꢀfriendly of this strategy. However, to design and
fabricate a multifunctional catalyst with excellent catalytic
performance, precise control over the spatial distribution and the
relative concentrations of different active sites is crucial and not
an easy task. This is especially the case for mutually hostile
33
5
5
6
6
0
5
0
5
in the outer shell using a twoꢀpot multistep preparation route,
which was quite complicated to spatially control the location of
acid sites and base sites. Simplifying the preparation route was
necessary for practical catalyst.
2
2
3
3
4
4
0
5
0
5
0
5
1
ꢀ3
In this study, we develop an efficient and facile method to
produce coreꢀshellꢀshell structured acidꢀbase bifunctional
mesoporous silica nanoreactor (MSꢀSO H@MS@MSꢀNH ),
3
2
which has acid sites on the inner core, unfunctionalized silica as
neutral zone shell and basic sites on the outer shell. Such acid
coreꢀneutral zoneꢀbase shell structure design resulted in excellent
spatial separation of the hostile functionalities and was better than
that of the coreꢀshell structure. The spatial order of acid sites and
basic sites in the mesoporous silica nanosphere offered a
designated diffusion pathway that was favorable for reaction
species; while the mesoporous structure endowed the material
4
ꢀ10
functionalities such as acid and base groups.
Besides, tedious
and complicated preparation as well as relatively low activity and
selectivity is still an obstacle for their practical applications.
Mesoporous structured silica material, owing to its high
surface area, uniform and ordered porosity and tunable
morphology, has gained intensive research attention.
introduction of additional functionalities, the physical and
chemical properties of mesoporous silica can be modified
with confinement effect. As
a result, the coreꢀshellꢀshell
structured mesoporous silica nanosphere was an excellent
bifunctional catalyst for oneꢀpot cascade reaction sequences.
1
1ꢀ14
Through
2. Experimental section
1
5ꢀ26
conveniently.
Mesoporous silica is regarded as a desirable
solid support for heterogeneous catalysis and rapid development
2.1. Materials and reagents
2
7ꢀ32
has been made in its functionalization.
However, to better
7
0
Toluene, paraꢀxylene, ethanol, tetraethyl orthosilicate (TEOS),
diꢀtertꢀbutyl dicarbonate, ammonia solution (25 wt%) and
hydrogen peroxide solution(35 wt%) were purchased from
Beijing Chemical Reagent Corporation. (3ꢀmercaptopropyl)
regulate of the surface properties of mesoporous silica so as to
design more sophisticated materials, selective immobilization of
different functionalities at different locations of mesoporous
silica with a simple and straightforward route is needed.
trimethoxysilane
(APTMS),
(MPTMS),
benzaldehyde
aminopropyltrimethoxysilane
dimethyl acetal, 4ꢀ
In an effort to obtain acidꢀbase bifunctional catalyst with good
75
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DOI: 10.1039/C3TA13185H
methoxybenzaldehyde dimethyl acetal, ethyl cyanoacetate,
cetyltrimethylammonium bromide (CTAB) were bought from
Alfa Aesar Corporation. 4ꢀbromobenzaldehyde dimethyl acetal, 60 synthesis strategy of MS@MSꢀNH
organosilane precursor APTMS.
2.3.5. Synthesis of MS
Pure mesoporous silica (MS) material was prepared via the same
while without addition of
2
4
ꢀchlorobenzaldehyde dimethyl acetal, 4ꢀpropylbenzaldehyde
5
dimethyl acetal, 4ꢀmethylbenzaldehyde dimethyl acetal were
purchased from J&K Corporation. All chemicals were used as
received without any further purification.
2
.4. Catalytic activity test
Oneꢀpot deacetalizationꢀKnoevenagel condensation reaction. A
homogeneous reaction mixture composed of toluene (3 mL),
acetal (0.5 mmol), ethyl cyanoacetate (1 mmol), paraꢀxylene (0.5
mmol) was placed in a glass reactor. Then 40 mg of solid catalyst
was added. The resulting mixture was vigorously stirred at 80 °C
for 0.5 h, and the reaction was stopped by cooling to room
temperature. The catalyst was separated via centrifugation, and
the reaction filtrate was analyzed by GC (Agilent 6890N) with
capillary column (DBꢀ5, 30.0 m × 320 ꢁm × 0.25 ꢁm) and FID,
and the products were further confirmed by GCꢀMS
(SHIMADZU, GCMSꢀQP 2010S) with capillary column (DBꢀ5
ms, 30.0 m × 320 ꢁm × 0.25 ꢁm).
2
.2. Synthesis of MS-SO H@MS@MS-NH
3 2
6
5
MSꢀSO H@MS@MSꢀNH was fabricated via
condensation method. First, 2.6 mL of TEOS and 220 ꢁL of
MPTMS was added sequentially into the homogeneous solution
composed of H O (38 mL), ethanol (10 mL), CTAB (1.5 g) and
2
After continually stirring for 1 h, TEOS (200 ꢁL) was added to
create an intermediate shell in order to isolate the inner core shell
from the outer shell. The resulting suspension was stirred for an
additional 1 h. Then a mixture of TEOS (500 ꢁL) and APTMS
a facile coꢀ
3
2
1
1
2
2
0
5
0
5
2
70
5 wt% NH ꢂH O (160 ꢁL) under vigorous stirring at 60 °C.
3 2
(400 ꢁL) was added. The solid material MSꢀSH@MS@MSꢀNH2
75
2.5. Characterization
was obtained after CTAB removal via ethanol extraction. The
amino groups were protected by mixing MSꢀSH@MS@MSꢀNH2
with diꢀtertꢀbutyl dicarbonate in ethanol overnight at room
temperature, giving the sample MSꢀSH@MS@MSꢀNHBoc. The
thiol group was oxidized to the sulfonic acid group by hydrogen
peroxide solution (35 wt%) for 12 h at room temperature,
obtaining MSꢀSO H@MS@MSꢀNHBoc. The deprotection of
NHBoc group was performed by the thermal treatment of MSꢀ
SO H@MS@MSꢀNHBoc at 150 °C for 10 h under vacuum.
The microscopic features of the samples were obtained by
scanning
electron
microscopy
(SEM,
JEOLꢀ6701F).
Transmission electron microscopy (TEM) was conducted on a
Tecnai G2 F20 UꢀTWIN field emission transmission electron
microscope operated at 200 kV. EDX analysis was obtained with
an EDAX system. Nitrogen adsorption–desorption measurement
was performed on Quantachrome Autosorb ASꢀ1 at 77 K. For
pore size and pore volume determination, a nonꢀlocal density
functional theory (NLDFT) equilibrium model of N2 on silica
was used. The smallꢀangle XRD measurement was carried out on
Rigaku D/maxꢀ2400 diffractometer equipped with a secondary
graphite monochromator with CuKα radiation (wavelengths λ =
0.154 nm). Data was collected in a stepꢀscan mode in the range of
1ꢀ6 degree with stepꢀwidth of 0.02 and speed of 1 degree /min.
80
3
3
2
2
.3. Synthesis of the control samples
85
.3.1. Synthesis of MS-SO H@MS-NH
3
2
30
MSꢀSO H@MSꢀNH was synthesized through the same
3 2
preparation method of MSꢀSO H@MS@MSꢀNH , but without
3
2
the unfunctionalized silica intermediate shell construction step.
.3.2. Synthesis of MS-SO H-NH
29
13
2
90 Si and C solidꢀstate NMR analysis were carried out on an
3
2
2
9
Mixed silica precursors containing TEOS (3.3 mL), MPTMS
(220 ꢁL) and APTMSꢀBoc (400 ꢁL) was added into the uniform
suspension composed of CTAB (1.5 g), H O (38 mL), ethanol
AVANCE III 400 spectrometer operating at 79.30 MHz for Si
13
35
and 100.38 MHz for C equipped with a 4 mm rotor spun at 5
kHz. Elemental analysis was performed on the elemental analyzer
(Flash EA 1112).
2
(10 mL) and 25 wt% NH ꢂH O (160 ꢁL) under vigorous stirring
3 2
at 60 °C, and the asꢀobtained solution was stirred for another 3 h.
The following steps, that is, template removal, amino group
deprotection and thiol group oxidation were conducted via the
same synthesis route of MSꢀSO H@MS@MSꢀNH .
9
5
3. Results and discussion
40
45
50
The preparation route was basically a combination of the
renowned Stöber method and the coꢀcondensation protocol.
Through sequential and stepwise addition of silica precursors in
the same reaction vessel, spatialꢀselective functionalization of
3
2
3
4, 35
2
.3.3. Synthesis of MS-SO H@MS
3
A mixture of TEOS (2.6 mL) and MPTMS (220 ꢁL) was added
into the homogeneous mixed solution composed of H O (38 mL),
2
1
00 specific locations in the mesoporous silica nanospheres was
efficiently achieved, i.e. acid functionalities on the inner core,
basic functionalities on the outer shell and unfunctionalized silica
as neutral zone in the middle. Scheme 1 briefly illustrated the
overall process for the synthesis of such coreꢀshellꢀshell
105 structured bifunctional mesoporous silica nanospheres. First, thiol
group functionized mesoporous silica nanosphere (MSꢀSH) as
inner core was constructed via the coꢀcondensation between
MPTMS and TEOS in the presence of CTAB. In the next step,
the intermediate shell of unfunctionalized silica was created by
ethanol (10 mL), CTAB (1.5 g) and 25 wt% NH ꢂH O (160 ꢁL) at
3
2
6
0 °C and vigorous stirred about 1 h. Then 1.1 mL of TEOS was
added. After CTAB removal via solvent extraction, the thiol
group was converted to the sulfonic acid group by H O oxidation
2
2
as described in the synthesis of MSꢀSO H@MS@MSꢀNH .
3
2
2.3.4. Synthesis of MS@MS-NH2
1
.5 g of CTAB and 160 ꢁL of NH ꢂH O (25 wt%) were dissolved
3
2
in a mixed solution containing H O (38 mL) and ethanol (10 mL),
2
3
.02 mL of TEOS was added and stirred at 60 °C for about 2 h.
Then TEOS (500 ꢁL) and APTMS (400 ꢁL) were added and
55
vigorously stirred for another 1 h. The final material was obtained 110 addition pure TEOS at specific time during the synthesis. Then a
after CTAB removal via ethanol extraction and dried at 60 °C
under vacuum.
mixture of APTMS and TEOS was added to the reaction system
to grow amino group modified mesoporous silica outer shell.
2
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DOI: 10.1039/C3TA13185H
During the synthesis, the change in the particle sizes of the
sample was monitored. As shown in Figure 1, the particle grew
gradually with the sequential addition of silica precursors. After
the completion of hydrolysis and condensation, the template
CTAB was removed via ethanol extraction, the thiol group was
converted sulfonic acid group via oxidation, obtaining the final
bifunctional material MSꢀSO H@MS@MSꢀNH . Besides, the
5
3
2
thickness of the each shell, the overall particle size, and the
concentrations of the acid sites/basic sites on the MSꢀ
SO H@MS@MSꢀNH can be conveniently regulated by
1
0
3
2
adjusting the preparation parameters.
Fig. 2 (a) SEM image, (b, c) TEM images, (d) STEM image and (e) sulfur,
(f) nitrogen EDX elemental mappings of coreꢀshellꢀshell structured MSꢀ
35
SO H@MS@MSꢀNH
3
2
.
From the SEM and TEM images in Figure 2a and 2b, MSꢀ
SO H@MS@MSꢀNH2 was uniform and monodispersed
3
nanospheres with an average particle size in the range of 120 to
1
40 nm. The winding mesopores were observed throughout the
4
4
5
5
0
5
0
5
particle from the high magnification TEM image (Figure 2c),
indicating the highly mesoporous structure of the sample. The
STEM image (Figure 2d), Combined with the sulfur and nitrogen
EDX elemental mappings (Figure 2e and 2f) confirmed that the ꢀ
SO H group was located in the inner core, and the ꢀNH group
Scheme 1 Synthesis procedure for acidꢀbase bifunctional coreꢀshellꢀshell
structured MSꢀSO H@MS@MSꢀNH .
3 2
3
2
located in the outer shell. Controlled addition of different
functional groups at different stages of particle growth was the
key in this method. In addition, the particle size of the coreꢀshellꢀ
shell structured MSꢀSO H@MS@MSꢀNH can be also regulated.
3
2
Through adjusting the ratios of different precursors in the
synthesis mixture, changing the composition of solvent as well as
reaction temperature during synthesis, a series of MSꢀ
1
5
Fig. 1 TEM images of (a) MSꢀSH, (b) MSꢀSH@MS and (c) MSꢀ
SH@MS@MSꢀNH
SO H@MS@MSꢀNH2 with different densities of acid/basic
3
2
.
reactive sites, various particle sizes were obtained (for
experimental details see Section S1 in ESI), as shown in Figure
S2.
For comparison, a series of control samples with comparable
acid site and/or basic site concentration, that is, bifunctional
2
2
3
0
5
0
material MSꢀSO H@MSꢀNH
mesoporous silica shell, bifunctional MSꢀSO HꢀNH nanosphere
with random distributed basic and acid catalytic reactive sites,
monofunctional catalyst MSꢀSO H@MS and MSꢀSO H@MSꢀ
without sandwiched pure
3
2
3
2
3
3
NH , were also prepared.
2
The intermediate and the final bifunctional material MSꢀ
SO H@MS@MSꢀNH2 were systematically characterized by
3
scanning electron microscopy (SEM), transmission electron
microscopy
(TEM),
EDX
analysis,
smallꢀangle
nitrogen
Xꢀray
adsorption/desorption
diffraction (SAXRD), C and Si solidꢀstate NMR spectroscopy
and elemental analysis.
measurement,
1
3
29
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DOI: 10.1039/C3TA13185H
of mesostructural ordering in MSꢀSO H@MS@MSꢀNH was
3
2
30
largely equivalent to that of the precursor sample, further
confirmed the wellꢀmaintained structural integrity of the
materials during the functionalization process.
Fig. 3 (a) N
distributions of MSꢀSH@MS@MSꢀNH
NHBoc (curve b), MSꢀSO H@MS@MSꢀNHBoc (curve c) and MSꢀ
SO H@MS@MSꢀNH (curve d).
2
adsorptionꢀdesorption isotherms and (b) NLDFT pore size
2
(curve a), MSꢀSH@MS@MSꢀ
3
5
3
2
2
9
Fig. 5 Si solidꢀstate NMR spectra of (a) MSꢀSH@MS@MSꢀNH
2
, (b)
2
Table 1 Structural parameters from N adsorptionꢀdesorption isotherms of
the samples.
35
MSꢀSH@MS@MSꢀNHBoc, and (c) MSꢀSO H@MS@MSꢀNH
3
2
.
Sample
BET surface Pore volume NLDFT pore
Solidꢀstate NMR spectroscopy provided convincing
spectroscopic evidences for the presence of the desired
2
3
area (m /g)
(cm /g)
size (nm)
MSꢀSH@MS@MSꢀNH
2
989.7
992.1
991.3
995.7
0.61
0.60
0.60
0.62
2.58
2.58
2.58
3.18
36ꢀ38
29
functional groups.
Figure 5 showed the Si solidꢀstate NMR
MSꢀSH@MS@MSꢀNHBoc
MSꢀSO H@MS@MSꢀNHBoc
MSꢀSO H@MS@MSꢀNH
spectra of the samples, three signals at ꢀ90, ꢀ100 ppm and ꢀ109
3
2
3
4
3
2
40 ppm were attributed to the Q , Q and Q peaks. The presence of
2
3
T and T peaks, which assigned to the organosiloxane species of
RꢀSi(OSi) (OH) and RꢀSi(OSi) confirmed the successful
incorporation of the organic silane moieties in the silica wall
The N sorption measurement was performed on the samples
MSꢀSH@MS@MSꢀNH2,
SO H@MS@MSꢀNHBoc and MSꢀSO H@MS@MSꢀNH . As
2
,
3
2
10
15
20
MSꢀSH@MS@MSꢀNHBoc,
MSꢀ
39
structure.
3
3
2
shown in Figure 3, all the materials displayed typical type IV
isotherms, with capillary condensation in low relative pressure
region (0.2 < P/P0 < 0.4), demonstrating the characteristic
mesoporous structure. All of them exhibited high BET surface
area, large total pore volume and narrow NLDFT pore size
distribution, as summarized in Table 1. The structural property of
MSꢀSO H@MS@MSꢀNH was comparable to the initial sample
3
2
MSꢀSH@MS@MSꢀNH , confirming the good stability of the
2
material during the whole chemical treatment.
45
Fig. 4 Smallꢀangle XRD patterns of (a) MSꢀSH@MS@MSꢀNH
SH@MS@MSꢀNHBoc, (c) MSꢀSO H@MS@MSꢀNHBoc and (d) MSꢀ
SO H@MS@MSꢀNH
2
, (b) MSꢀ
3
3
2
.
25
The mesoporous structures was investigated by SAXRD
analysis, a large and broad reflection indexed to (100) reflection
can be observed for the four samples (Figure 4), indicating the
mesopores in the nanosphere were relatively ordered. The degree
4
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DOI: 10.1039/C3TA13185H
Table 2 Oneꢀpot acetal hydrolysisꢀKnoevenagel cascade reaction
a
35
sequence catalyzed by various catalysts.
Entry
Catalyst
Conv. of Yield of B1 Yield of C1
A1 (%)
100
(%)
0
0
19.6
25.0
trace
17.0
0
(%)
≈ 100
95.2
40.4
0
trace
67.0
0
1
2
3
4
5
6
7
MSꢀSO
3
H@MS@MSꢀNH
2
MSꢀSO
MSꢀSO
MSꢀSO
MS@MSꢀNH
physical mixture
MS
3
H@MSꢀNH
HꢀNH
H@MS
2
95.2
60.0
25.0
trace
84.0
0
3
2
3
2
a
Reaction conditions: A1 (0.5 mmol), ethyl cyanoacetate (1.0 mmol),
toluene (3 mL), catalyst (40 mg), reaction temperature = 80 °C, reaction
1
3
Fig.6 C solidꢀstate NMR spectra of (a) MSꢀSH@MS@MSꢀNH
2
, (b)
time = 0.5 h. Conversions and yields were determined using GC data.
MSꢀSH@MS@MSꢀNHBoc, and (c) MSꢀSO H@MS@MSꢀNH . The
asterisked peaks denote residual surfactant and methoxy/ethoxy groups.
3
2
b
3 2
Physical mixture composed of MSꢀSO H@MS and MS@MSꢀNH .
40
The catalytic property of coreꢀshellꢀshell structured MSꢀ
5
Moreover, the introduction of the desired organic functional
groups in the mesoporous silica framework could be
SO H@MS@MSꢀNH2 as acidꢀbase bifunctional heterogeneous
3
catalyst was firstly investigated in a oneꢀpot tandem reaction
sequence to convert benzaldehyde dimethyl acetal into
benzylidene ethyl cyanoacetate (reaction 1). This reaction
sequence was composed of two separate steps: the hydrolysis of
the acetal by the acid sites and the subsequent Knoevenagel
condensation by the basic sites. The catalytic performance of a
series of control samples were also tested for comparison and the
results were summarized in Table 2. The cascade reaction was
1
3
13
characterized by the C solidꢀstate NMR spectroscopy. The
C
solidꢀstate NMR spectrum of MSꢀSH@MS@MSꢀNH in Figure
2
6
a confirmed the successful grafting of the mercaptopropyl and
4
5
3
9
1
1
2
2
3
0
5
0
5
0
aminopropyl groups on the silica surface. The resonances at
2
ppm (carbonyl resonance) were observed in the C NMR spectra
of MSꢀSH@MS@MSꢀNHBoc (Figure 6b), proving the
3
7.08 (CH resonance of tertꢀbutyl), 78.59 (C sp ꢀO), and 156.90
3
1
3
40
successful protection of the amino groups by (Boc) O. In the
2
50 carried out successfully with 100% conversion and nearly 100%
13
3
C solidꢀstate NMR spectrum of MSꢀSO H@MS@MSꢀNH2
yield using the MSꢀSO H@MS@MSꢀNH2 catalyst (Table 2,
3
(Figure 6c), the resonances corresponding to the Boc group
entry 1). While the control sample MSꢀSO H@MSꢀNH (without
3
2
disappeared, and the signals associated with the sulfonic acid
propyl and aminopropyl groups remained, indicating that desired
acid and basic reactive sites were located on the silica
nanospheres.
the intermediate unfunctionalized mesoporous silica neutral zone)
showed relatively lower catalytic activity (Table 2, entry 2),
suggesting that the sandwiched neutral zone in MSꢀ
55
SO H@MS@MSꢀNH2 provided an additional benefit to fully
3
Elemental analysis was also performed and the result
demonstrated that the content of sulfonic acid groups and amino
groups in the sample MSꢀSO H@MS@MSꢀNH were 0.50
isolate the acid/base sites.
In the case of MSꢀSO HꢀNH , where the acid sites and base
3
2
3
2
sites were mixed in the same region, the catalytic activity and
mmol/g and 0.80 mmol/g, respectively.
60 selectivity were much lower (Table 2, entry 3), likely due to a
certain degree of mutual neutralization between of the two hostile
sites as a result of the random distribution of the organic
functionalities in the mesoporous silica, as well as lack of the
designated diffusion pathway for reaction species. As for the
In addition, a series of characterizations (SEM, TEM, SAXRD,
nitrogen adsorption/desorption measurement and elemental
analysis) were also performed on the control samples, MSꢀ
SO H@MS, MS@MSꢀNH2, MSꢀSO H@MSꢀNH and MSꢀ
3
3
2
SO HꢀNH (detailed characterization information were listed in 65 monofunctional catalysts with similar coreꢀshell structure, neither
3
2
Figure S3, S4 and Table S1). According to the results, the control
samples had comparable physicochemical parameters and similar
amount of the acid sites and/or basic sites.
MSꢀSO H@MS nor MS@MSꢀNH could convert the reactant A1
3 2
into the desired product C1 (Table 2, entries 4 and 5). Besides,
the physical mixture of MSꢀSO H@MS and MS@MSꢀNH with
3
2
comparable active sites concentrations showed less ability (Table
2, entry 6), as both the activity and the selectivity were
significantly lower than those of the bifunctional MSꢀ
SO H@MS@MSꢀNH catalyst, demonstrating the advantage of
70
3
2
the coreꢀshellꢀshell structure design with confined reaction
microenvironment provided by the mesopores.
75
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DOI: 10.1039/C3TA13185H
Table 3 Oneꢀpot deprotectionꢀKnoevenagel cascade reactions towards
different substrates using MSꢀSO H@MS@MSꢀNH
as catalyst.a
Acknowledgements
3
2
We thank the financial supports from the National Basic
Research Program of China (2009CB930400), National Natural
Science Foundation of China (NSFC 21273244, 21121063), and
55 the Chinese Academy of Sciences (KJCX2ꢀYWꢀN41).
Conv. of A
%)
Yield of B
Yield of C
(%)
Entry
R
(
(%)
0
0
0
0
0
0
1
2
3
4
5
6
H
OCH
100
100
100
100
100
100
≈100
≈100
≈100
≈100
≈100
≈100
3
CH
nꢀC
3
3
H
7
Notes and references
Cl
Br
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key
Laboratory of Molecular Nanostructures and Nanotechnology, Institute
of Chemistry, Chinese Academy of Sciences, Beijing,100190, P. R. China.
a
Reaction conditions: A (0.5 mmol), ethyl cyanoacetate (1.0 mmol),
toluene (3 mL), catalyst (40 mg), reaction temperature = 80 °C, reaction
time = 0.5 h. Conversions and yields were determined using GC data.
6
6
7
7
0
5
0
5
E-mail: wsong@iccas.ac.cn; Fax: (+86) 10-62557908
Electronic Supplementary Information (ESI) available: characterization
of catalyst. See DOI: 10.1039/b000000x/
5
†
More reactions were tested using coreꢀshellꢀshell structured
catalyst MSꢀSO H@MS@MSꢀNH . A variety of acetals were
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examined, as shown in Table 3. In the presence of the MSꢀ
SO H@MS@MSꢀNH material, aromatic acetals, either with
3
2
4
10
15
20
25
30
35
electronꢀdeficient or electronꢀrich groups, could be converted to
the corresponding target products with excellent activity and
selectivity. The results indicated that due to good site isolation of
the incompatible active sites by the coreꢀshellꢀshell structure
construction, designated direction of mass transportation
originating from the rational arrangement of the locations of
active sites, as well as the enrichment and confinement effect
provided by the mesoporous structure, MSꢀSO H@MS@MSꢀ
NH2 served as an efficient bifunctional catalyst for acidꢀbase
tandem catalysis.
3
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was investigated. The catalyst can be retrieved via centrifugation 80 11. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge,
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image of the recovered catalyst in Figure S6 confirmed that the 85 13. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F.
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amount of the organic functionalities on the silica support were
also remained the same. Besides, N2 adsorptionꢀdesorption
measurement was performed on the reused catalyst MSꢀ
SO H@MS@MSꢀNH (Figure S7). The BET surface area and
pore volume were slightly reduced (833.5 m /g and 0.52 cm /g,
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90
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1
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95
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. Conclusions
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00
In
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structured
MSꢀ
2
2
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a
3
2
straightforward sequential coꢀcondensation method. The strategy
40
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2
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15 28. A. Kuschel, M. Drescher, T. Kuschel and S. Polarz, Chem. Mater.,
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