Synthesis, characterization, and catalytic properties of two zwitterionic hybrid SBA-15 mesoporous silicas

Article abstract of DOI:10.1002/ejic.201200498

Zwitterionic hybrid SBA-15 mesoporous silica materials containing both amine and sulfonic acid groups have been synthesized through cocondensation from as-synthesized zwitterionic siloxane precursors (ZS) with tetraethoxysilane (TEOS). Characterization by XRD, TEM, and N₂ adsorption-desorption measurements show that the as-synthesized materials have ordered mesostructures. Analysis with UV, ²⁹Si/¹³C cross-polarization magic angle spinning (CP-MAS) NMR, and X-ray photelectron spectroscopy (XPS) and elemental analysis demonstrates that the zwitterionic parts have been successfully introduced into the silica framework, and the organic contents of 15 % hybrid samples are over 1.0 mmol/g. The presence of a secondary organoamine group enables the hybrid material to be further alkalized to achieve the desired basic properties. The materials were used as catalysts for the aldol condensation of acetone with 4-nitrobenzaldehyde. With a good catalytic performance, the catalyst could be easily separated, recycled, and reactivated by secondary basification. Copyright

Full text of DOI:10.1002/ejic.201200498

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DOI: 10.1002/ejic.201200498
Synthesis, Characterization, and Catalytic Properties of Two Zwitterionic
Hybrid SBA-15 Mesoporous Silicas
Miao Yang,*[a] Feifei Xu,[a] Chang Wang,[a] Xiumei Liu,[a] Peifang Yan,[a] Peipei Li,[a] and
Urs Welz-Biermann[a]
Keywords: Zwitterions / Mesoporous materials / Organic–inorganic hybrid composites / Heterogeneous catalysis / Aldol
reactions
Zwitterionic hybrid SBA-15 mesoporous silica materials con-
successfully introduced into the silica framework, and the or-
taining both amine and sulfonic acid groups have been syn- ganic contents of 15% hybrid samples are over 1.0 mmol/g.
thesized through cocondensation from as-synthesized zwit-
terionic siloxane precursors (ZS) with tetraethoxysilane
(TEOS). Characterization by XRD, TEM, and N2 adsorption–
desorption measurements show that the as-synthesized ma-
terials have ordered mesostructures. Analysis with UV, 29Si/
13C cross-polarization magic angle spinning (CP-MAS) NMR,
and X-ray photelectron spectroscopy (XPS) and elemental
analysis demonstrates that the zwitterionic parts have been
The presence of a secondary organoamine group enables the
hybrid material to be further alkalized to achieve the desired
basic properties. The materials were used as catalysts for the
aldol condensation of acetone with 4-nitrobenzaldehyde.
With a good catalytic performance, the catalyst could be eas-
ily separated, recycled, and reactivated by secondary basifi-
cation.
(2-aminoethylamino)ethylamino]propyl group, to catalyze
various reactions that involve carbonyl activation.[16] Davis
et al. reported the formation of bifunctionalized mesopo-
rous SBA-15 containing a primary amine and different acid
centers including benzenesulfonic acid, phosphoric, and
carboxylic acid.[17–18] Asefa et al. prepared an efficient
mesoporous catalyst containing spatially distributed organ-
oamine and silanol groups by one-pot postgrafting.[19–20]
Kan et al. synthesized acid–base bifunctional mesoporous
materials by controlling steric hindrance to prevent interac-
tions between acid and base.[13] Although much effort has
been made, precise control of the molar ratio and distribu-
tion of the acid and base centers to avoid their neutraliza-
tion is still a big challenge.
Zwitterions are neutral molecules with a positive and a
negative electrical charge at different locations within the
molecule. Their latent acidity (or basicity) could be ob-
tained by combining 1:1 molar quantities of zwitterions
with acids (or bases) possessing sufficiently low pKa (or
pKb) to convert the acidic (or basic) groups into the corre-
sponding acids (or bases). Examples are known, such as
SO3H-functionalized ionic liquids, which were prepared
through the acidification of zwitterions containing alkane-
sulfonic groups by various strong donor acids.[21–22]
Furthermore, CO2-philic ionic liquid materials were pre-
pared by alkalizing zwitterions with quaternary ammonium
hydroxides.[23] Therefore, it is very interesting to introduce
zwitterions into mesoporous silica materials; this means
that acidic and basic seeds with a fixed molar ratio and
position can be introduced into the silica framework. After
Introduction
Ordered mesoporous silica materials, such as the repre-
sentative MCM-41[1] and SBA-15,[2] with stable mesostruc-
tures, large surface areas, and size-tailored mesopores on
the nanometer scale have been widely investigated and em-
ployed in many fields. Organically functionalized mesopo-
rous silicas, in particular, have received considerable atten-
tion because various organic functional groups enlarge their
practical applications to catalysis, separation, sensor, and
drug delivery.[3–4] Mesoporous silicas functionalized with
acidic groups, such as sulfonic acid, Nafion, and carboxyl
groups, show high activity and selectivity for classical acid-
promoted organic reactions, such as esterification, Friedel–
Crafts acylation, and olefin polymerization.[5–9] Various N-
group-modified mesoporous silicas have been used in some
base-catalyzed reactions including the Heck reaction, Kno-
evenagel condensation, and Michael addition.[10–12] Re-
cently, base- and acid-bifunctional mesoporous catalysts
were developed and received great attention because a co-
operative catalytic system constructed from general acid
and base groups is desired for some carbon–carbon bond-
formation reactions.[13–15] Lin et al. demonstrated that a ge-
neral acid group, the ureidopropyl group, can activate sub-
strates in cooperation with a general base group, the 3-[2-
[a] China Ionic Liquid Laboratory, Dalian Institute of Chemical
Physics, Chinese Academy of Science
Zhongshan Road 457, 207 Group, Dalian 116023, P. R. China
Fax: +86-411-84379992
E-mail: yangmiao@dicp.ac.cn
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that, the immobilization of acidic or basic ionic liquids be-
The synthesis of zwitterionic hybrid SBA-15 mesoporous
comes possible by acidification or alkalization of the zwit- silica was realized by hydrolysis–polycondensation from the
terionic hybrid mesoporous compounds. Research on sup- as-synthesized ZS (SNSA or SSHI) with tetraethoxysilane
ported task-specific ionic liquids on ordered mesoporous (TEOS) under acidic conditions with the block copolymer
materials is quite rare,[24–25] though some reports on ionic Pluronic P123 (P123) as a template. The synthetic details
liquid immobilization on silica-based carriers have been are described in the Experimental Section. It should be
published.[26–30]
noted that the feeding sequence and prehydrolysis time of
Recently, zwitterionic mesoporous SBA-15 has been de- TEOS were two key factors for the formation of the prod-
veloped as a good ultralow-fouling material.[31] The pres- ucts. When ZS was added before or together with TEOS,
ence of –COOH and –NH2 groups offers the material high the sample obtained was disordered SiO2. The presence of
resistance to nonspecific protein adsorption. In this work, ZS might perturb or destroy the formation and self-as-
we report the direct synthesis of two new zwitterionic func- sembly of surfactant aggregates, therefore, a full prehydroly-
tionalized mesoporous SBA-15 materials that contain si- sis treatment of TEOS is necessary.[32] During the synthesis,
multaneous amine and sulfonic acid groups. Their meso- it took 4 h to achieve an equilibrium of TEOS in solution
structures were well characterized, and their catalytic prop- at 40 °C, and then a certain amount of ZS was added into
erties were studied in an aldol reaction of acetone with 4- the solution, and the molar composition of the initial gel
nitrobenzaldehyde as a model.
was
(1–x)-
TEOS/xZS/0.017P123/5.82HCl/156.7H2O, where x = 0.05,
0.10, 0.15, and 0.20. After further continuous stirring at 40
°C for 20 h, the resulting reaction mixture was heated to
100 °C for 24 h. The surfactant was removed by heating to
reflux in 95% ethanol for 24 h. The final products were de-
noted as SBA-15-SNSA (100x%) and SBA-15-SSHI
(100x%), where x stands for the molar ratio of ZS/(TEOS
+ ZS) in the initial gel.
Results and Discussion
Synthesis and Characterization
Two zwitterionic siloxane (ZS) precursors with N-(3-tri-
ethoxysilylpropyl)-N-(3-sulfopropyl)amine (SNSA) and 1-
(3-triethoxysilylpropyl)-3-(3-sulfopropyl)-4,5-dihydro-imid-
azole (SSHI) were first prepared. The synthetic route to the
two compounds is shown in Schemes 1 and 2, and the prep-
aration and characterization details are described in the Ex-
perimental Section.
In order to achieve the desired acidity or basicity, both
acidification and alkalization of the zwitterionic hybrid
SBA-15 mesoporous silica were tried. However, the experi-
mental results showed that only alkalization of SBA-15-
SNSA materials was successful due to the existence of
+
NH2 groups (Scheme 3). The treated samples were named
as SBA-15-SNSA-B. Acidification of SBA-15-SNSA and
SBA-15-SSHI by various acids with different strengths were
unsuccessful although the supposed route was reasonable
(see Scheme 4). The reason for this is still under investiga-
tion.
Scheme 1. Synthesis of SNSA.
Scheme 3. Basification process for SBA-15-SNSA-B.
The structural properties of SBA-15-SNSA and SBA-15-
SSHI were investigated by XRD, TEM, and N2 adsorp-
tion–desorption experiments.
Figures 1 and 2 show the XRD profiles of the corre-
sponding samples; most of the compounds have similar pat-
terns that are typical of SBA-15 materials. The diffraction
peak intensity of the 20% hybrid compounds greatly de-
clines or even disappears, which suggests that the amount
of organosilane loading is limited below 20%. The XRD
pattern of SBA-15-SNSA-B (5%) reveals that the alkaliza-
tion process has little effect on the mesoscopic order.
Scheme 2. Synthesis of SSHI.
2
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Two Zwitterionic Hybrid SBA-15 Mesoporous Silicas
This might be due to the longer assembly time during the
synthetic process. Moreover, all samples contain well-or-
dered hexagonal arrays of mesopores with 1D channels, in-
dicating a 2D hexagonal (P6mm) mesostructure. The basifi-
cation treatment does not have a distinct influence on the
morphology and structure of the sample.
Scheme 4. Supposed acidification process for SBA-15-SNSA (a)
and SBA-15-SSHI-A (b).
Figure 1. XRD patterns of SBA-15-SNSA.
Figure 3. TEM images of SBA-15-SNSA (10%) (a and b), SBA-
15-SNSA-B (10%) (c and d), and SBA-15-SSHI (10%) (e and f).
Figure 4 illustrates the N2 adsorption–desorption iso-
therms and the pore size distribution curves of as-synthe-
sized SBA-15, SBA-15-SNSA (10%), SBA-15-SNSA-B
(10%), and SBA-15-SSHI (10%). All isotherms are type IV
with H1 hysteresis loops, which indicates the presence of
mesopores and a narrow pore size distribution. The surface
area, pore size, and pore volume data of the samples are
collected in Table 1 and are comparable with those of other
rod-like pure and functionalized SBA-15 materials.[13,33]
The surface areas of the zwitterionic hybrid SBA-15 are
clearly reduced, and that of the alkalized sample decreases
even more, possibly due to the presence of tetraethylammo-
nium cations attached by electrostatic interactions. The sur-
Figure 2. XRD patterns of SBA-15-SSHI.
The TEM images of SBA-15-SNSA (10%) and SBA-15- face area of SBA-15-SSHI (10%) is smaller than that of
SSHI (10%) (Figure 3) show that the materials have rod- SBA-15-SNSA (10%), possibly because the molecular vol-
like morphology, which is thermodynamically supported. ume of SSHI is larger than that of SNSA.
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Figure 4. N2 adsorption–desorption isotherm (left) and pore size distribution by the Barrett–Joyner–Halenda (BJH) method with adsorp-
tion branch (right) of SBA-15, SBA-15-SNSA (10%), SBA-15-SNSA-B (10%), and SBA-15-SSHI (10%).
Table 1. Structural and textural parameters of selected samples.
content of SBA-15-SNSA (15%) and SBA-15-SSHI (15%)
was 4.20 and 3.43 wt.-%, respectively, which indicate that
the ZS precursor loading was around 1.3 mmol/g and
1.1 mmol/g, respectively.
Sample
SBET
Pore size
[nm]
Vpore
[m2/g]
[cm3/g]
SBA-15[a]
SBA-15-SNSA (10%)
SBA-15-SNSA-B (10%)
SBA-15-SSHI (10%)
399
274
215
227
9.6
7.7
7.7
7.7
0.83
0.54
0.43
0.50
The incorporation of the organic entities into the silica
network was monitored by using 29Si/13C CP-MAS NMR
spectroscopy. The 13C CP-MAS NMR spectrum of the as-
synthesized SNSA precursor (Figure 6, a) contains four
main resonance signals at 58.9, 49.9, 22.9, and 19.0 ppm.
The low field peaks at 58.9 and 49.9 ppm correspond to the
carbon atoms linked directly to S, N, and O atoms (C3, C4,
C6, and C7, inset in Figure 6).[10,35] The signals located at
around 20 ppm could be attributed to the carbon atoms far
away from electron withdrawing groups, including C1, C2,
C5, and C8 (inset in Figure 6).[6] Figure 6 (b) shows the 13C
CP-MAS NMR spectrum of SBA-15-SNSA (15%). All sig-
nals found in the spectrum in Figure 6 (a) can be also found
in that in Figure 6 (b) with a slight chemical shift difference,
which illustrates the successful incorporation of SNSA ZS
into the silica structure. The decrease in the relative inten-
sity of the peaks at δ = 58 and 18 ppm is due to the hydroly-
sis of ethoxy groups. The solid-state 13C CP-MAS NMR
spectra of the as-synthesized SSHI precursor and SBA-15-
SSHI (15%) are shown in Figure 7. Similarly, the chemical
shifts at ca. 159.3 (C4), 57.6 and 49.2 (C3, C5, C6, C7, C9,
and C10), 19.7, and 11.3 (C1, C2, C8, and C11) ppm in the
spectrum in Figure 7 (a) appeared in that in Figure 7 (b)
with some small deviations.[8] This provides clear evidence
that the SSHI precursor is indeed introduced into the struc-
ture of the SBA-15 materials.
[a] SBA-15 was synthesized with the same procedure as SBA-15-
SNSA (10%) without any organic silica source.
The incorporation of ZS precursor was also charac-
terized by using UV/Vis and 29Si/13C CP-MAS NMR spec-
troscopy and elemental analysis.
UV/Vis spectroscopy is sensitive to unsaturated bonds
and can therefore be used to check the existence of ZS
SSHI.[34] Pure siliceous SBA-15 does not have any UV/Vis
absorption in the range from 200 to 600 nm. However, both
ZS SSHI and SBA-15-SSHI samples exhibit strong peaks
at around 241 nm, which confirms the presence of the SSHI
precursor in the hybrid SBA-15 mesoporous structure (Fig-
ure 5).
The inclusion of the organic moieties in the silica frame-
work was also confirmed by the 29Si CP-MAS NMR spec-
tra (Figure 8) of SBA-15-SNSA (15%) and SBA-15-SSHI
(15%). Both spectra show two overlapping signals in the
region from –100 to –110 ppm, ascribable to the Q3
(–100 ppm) and Q4 (–110 ppm) bands of Si(OH)(OSi)3 and
Si(OSi)4 silicate species. In addition to these low field sig-
nals, the samples show peaks at around –66 ppm, which can
Figure 5. UV/Vis spectra of ZS SSHI and SBA-15-SSHI (5%).
The amount of incorporated ZS could be assessed by ele- be assigned to T3 groups [T3 = RSi(OSi)3], demonstrating
mental analysis assuming that the sulfur content is repre- that the organosiloxanes are condensed as a part of the sil-
sentative of the organic precursor. The experimental sulfur ica framework.[6]
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Two Zwitterionic Hybrid SBA-15 Mesoporous Silicas
amine groups are ca. 1.5 eV higher.[36–37] Thus, the presence
of two peaks at 399.6 and 401.8 eV suggests a coexistence
of –NH– and –NH2–+ in the hybrid SBA-15 material. A S2p
peak was observed at 168.4 eV, which is typical for sulfur in
SO3H groups.[38] It is believed that a proton vibrates be-
tween the amine and sulfonic groups to keep a balance.
Figure 6. Solid-state 13C CP-MAS NMR spectra of ZS SNSA (a)
and SBA-15-SNSA (15%) (b).
Figure 9. N1s XPS pattern of SBA-15-SNSA (10%).
Catalytic Properties
Figure 7. Solid-state 13C CP MAS NMR spectra of ZS SSHI (a)
and SBA-15-SSHI (15%) (b).
A material with accessible sulfonic acid and amine sites
should exhibit cooperative catalytic behavior for aldol reac-
tions, in which the regeneration of the active intermediate
nucleophilic enolate species relies on the rate of proton ex-
change between the acid and base groups.[17–18] The cata-
lytic activities of the free host and as-synthesized function-
alized materials were tested in an aldol reaction (Scheme 5).
The reaction conditions and catalytic results are shown in
Table 2. SBA-15-SNSA-B (5%) gave the best outcome in
terms of the conversion, and SBA-15-SNSA-B (15%) gave
the best selectivity to aldol products. The high converstion
and the selectivity of alcohol products over alkene products
is comparable with those of other reported work.[17–19] On
the contrary, the samples of basified pure SBA-15 and
SBA-15-SNSA (10%) had negligible catalytic abilities,
which indicates that the catalytic activity came from the
supported alkalized zwitterion. The recyclability and reus-
ability of SBA-15-SNSA-B were also examined. The results
show that the catalyst could be recycled and used at least
three times without much deactivation. In addition, the
used catalysts could be reactivated by a second alkalization,
Figure 8. Solid-state 29Si CP MAS NMR spectra of SBA-15-
SNSA-15% (a) and SBA-15-SSHI-15% (b).
XPS measurements for SBA-15-SNSA (10%) were taken and shows unchanged catalytic ability for the reaction, see
to investigate the status of ZS SNSA. The N1s peak, as Table 2 entries 6, 7, and 8.
shown in Figure 9, has a rather asymmetrical shape, which
indicates that more than one component is present. The
broad N1s line profiles could be easily fitted to two peaks
at 399.6 and 401.8 eV by deconvolution in the region 396–
406 eV using XPS Peak Fit software. According to previous
reports, the standard binding energy of free amine groups
falls in the region 399–401 eV and those of the protonated Scheme 5. Aldol reaction.
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Table 2. Aldol reaction of 4-nitrobenzaldehyde with acetone.[a]
Typically, TPDIm (5.0 mL, 0.018 mol) and γ-propanesultone
(2.8 g, 0.023 mol) were charged directly into a 50 mL flask in an
ice bath. The mixture was stirred until γ-propanesultone dissolved
completely and then it was heated to 40 °C for 0.5 h. The product
was washed with ethyl acetate several times and then dried in vacuo
at 80 °C for 24 h. The final product was a white powder. 1H NMR
(400 MHz, D2O): δ = 0.75 (t, J = 8.4 Hz, 2 H), 1.26 (m, 9 H, 3ϫ
CH3), 1.77 (m, 2 H), 2.12 (m, 2 H), 2.96 (t, J = 7.2 Hz, 2 H), 3.50
(t, J = 6.4 Hz, 2 H), 3.65 (t, J = 6.8 Hz, 2 H), 3.93 (m, 6 H, 3ϫ
CH2), 3.99 (s, 2ϫ 1 H), 8.16 (s, 1 H) ppm. 13C NMR (101 MHz,
D2O): δ = 8.0, 16.5, 19.9, 47.1, 47.6, 49.6, 57.1, 157.1 ppm. ESI-
MS: m/z = 396.2 [C15H32N2O6SSi].
Entry Catalyst A [%] B [%] Conv. [%]
1
2
3
4
5
6
7
8
SBA-15-SNSA-B (5%)
59.2
65.0
54.9
7.7
14.1
7.4
5.7
0
0.4
13.7
8.0
12.1
73.3
72.4
60.6
7.7
SBA-15-SNSA-B (10%)
SBA-15-SNSA-B (15%)
SBA-15-B[b]
SBA-15-SNSA (10%)
SBA-15-SNSA-B (5%)-1[c]
SBA-15-SNSA-B (5%)-2[d]
SBA-15-SNSA-B (5%)-3[e]
1.7
2.1
63.1
54.4
65.1
76.8
62.4
77.2
[a] Reaction conditions: 0.076 g of 4-nitrobenzaldehyde, 0.1 g of
catalyst, 5 mL of acetone, 5 mL of hexane, 50 °C, 20 h. The conver-
sion was determined through 1H NMR spectroscopic analysis with
tetrahydrofuran (THF) as the internal standard. [b] Basified pure
SBA-15 sample. [c] Catalyst used for the second time. [d] Catalyst
used for the third time. [e] Renewed SBA-15-SNSA-B (5%)-2 used.
Synthesis of Zwitterionic SBA-15: In a typical synthetic process,
P123 (0.4 g) was dissolved with stirring in H2O (10 mL), followed
by the addition of HCl solution (2 mL, 36 wt.-%). The solution
was heated to 40 °C before adding TEOS, and NaF (0.008 g) was
added to promote TEOS prehydrolysis. After the solution was
equilibrated at 40 °C for 4 h, a certain amount of ZS (SNSA or
SSHI) was added to the solution. The final mixture was stirred at
40 °C for 20 h, and then transferred into a Teflon-lined stainless
steel autoclave and aged at 100 °C for 24 h under static conditions.
The solid product was collected by filtration and then dried in air.
The template was removed from the as-synthesized materials by
heating to reflux in 95% ethanol for 24 h (1.0 g of sample per
100 mL of ethanol). The solid was recovered by filtration and re-
peatedly washed with water and then dried at 60 °C in air.
Conclusions
Two new zwitterionic hybrid SBA-15 mesoporous silicas
have been successfully synthesized through cocondensation
from an as-synthesized zwitterionic siloxane precursor with
tetraethoxysilane in the presence of P123 as a template.
Catalytic properties can be obtained by an alkalization pro-
cess, which gave good conversion and reusability in an aldol
reaction. This work provides a new route to synthesize acid-
or base-functionalized mesoporous silica with a fixed distri-
Basification of Zwitterionic SBA-15: A typical basification process
bution of organic groups. More work on the design and was as follows: An SBA-15-SNSA sample (0.3 g) was stirred in a
TEAOH solution (25 mL, 0.02 mol/L) for 0.5 h. After that, the so-
lid product was recovered by filtration and repeatedly washed with
water until the washing water was neutral, and then the sample was
dried in air.
synthesis of zwitterionic siloxane precursors and their hy-
brid mesoporous materials is underway.
Experimental Section
Characterization: XRD patterns were recorded with a PANalytical
X’Pert PRO diffractometer with Cu-Kα radiation (λ = 1.5406 Å)
operating at 40 mA and 40 kV. Transmission electron microscopy
(TEM) was performed with a FEI Tecnai G2 Spirit Transmission
Electron Microscope. The nitrogen adsorption–desorption experi-
ments were performed at 77 K with a QUADRASORB SI auto-
mated surface area and pore size analyzer. Samples were degassed
at 100 °C for 8 h prior to the measurements. The BET surface area
was calculated from the adsorption data in the relative pressure P/
P0 range from 0.05 to 0.3. The pore size distributions were deter-
mined from the adsorption branches using the Barret–Joyner–
Halenda (BJH) method. The pore volume was estimated at relative
pressure P/P0 close to 1.0. UV/Vis spectra were collected with a
HIMADZU UV-2550 spectrometer. The 13C (100.5 MHz) and 29Si
(79.4 MHz) NMR spectra with cross-polarization magic-angle-
spinning (CP-MAS) were obtained with a Bruker DRX-400 spec-
trometer. The experimental parameters were as follows: for 13C CP-
MAS NMR experiments, 4 kHz spin rate, 2 s pulse delay, 2200
scans; for 29Si CP-MAS NMR experiments, 4 kHz spin rate, 3 s
pulse delay, 500 scans. Elemental analysis was performed with an
Elementar Vario EL III. XPS analysis was conducted with a VG
ESCALAB MK2 spectrometer. Charging effects were corrected by
calibrating the spectra to the binding energy values of the Si2p peak
at 103.4 eV.
Chemicals and Materials: γ-Aminopropyltriethoxysilane (APTES)
was purchased from Jingzhou Jianghan Fine Chemical Co. and
used as received. γ-Propanesultone was obtained from Wuhan
Fengfan Chemical Co. and was distilled before use. Triblock copol-
ymer Pluronic P123 {HO(CH2CH2O)20[CH2CH(CH3)O]70-
(CH2CH2O)20H, Mav = 5800} and N-[3-(triethoxysilyl)propyl]-4,5-
dihydroimidazole (TPDIm) were purchased from Aldrich Co. and
used in the reactions without further purification. All solvents, such
as THF and ethyl acetate, were analytical reagent grade and used
as received.
Preparation of Precursors
Zwitterionic Siloxane Precursor (ZS) N-(3-Sulfopropyl)-N-(3-trieth-
oxysilylpropyl)amine (SNSA): SNSA was synthesized from an alk-
ylation reaction between APTES and γ-propanesultone (Scheme 1).
Typically, γ-propanesultone (18.0 g, 0.15 mol) was first dissolved in
THF (30 mL), and then APTES (34.0 mL, 0.15 mol) was added
dropwise under a N2 atmosphere. The mixture was heated to reflux
at 60 °C for 8 h. After the removal of THF in vacuo, the product
was washed with ethyl acetate several times to remove the residues
and then dried in vacuo at 80 °C for 24 h. The final product was a
1
white powder. H NMR (400 MHz, D2O): δ = 0.82 (t, J = 7.6 Hz,
2 H), 1.24 (m, 9 H, 3 ϫ CH3), 1.78 (m, 2 H), 2.13 (m, 2 H), 3.01
(t, J = 7.6 Hz, 2 H), 3.07 (t, J = 7.6 Hz, 2 H), 3.20 (t, J = 8.4 Hz,
2 H), 3.90 (m, 6 H, 3ϫ CH2) ppm. 13C NMR (101 MHz, D2O): δ
= 6.7, 7.9, 17.7, 19.6, 21.8, 46.7, 48.4, 50.2, 60.0 ppm. ESI-MS: m/z
= 343.1 [C12H29NO6SSi].
Aldol Reaction: Aldol catalytic reactions were carried out with the
following general procedure: the catalyst (0.1 g) was added to a
mixture of 4-nitrobenzaldehyde (0.076 g, 0.5 mmol), acetone
(5 mL), and hexane (5 mL), and then the reaction flask was sealed
ZS 3-(3-Sulfopropyl)-1-(3-triethoxysilylpropyl)-4,5-dihydroimidazole under a N2 atmosphere and heated at 50 °C for 20 h. After the
(SSHI): The synthetic route for SSHI is illustrated in Scheme 2. reaction, the organic solvent was removed in vacuo, and the prod-
6
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Two Zwitterionic Hybrid SBA-15 Mesoporous Silicas
uct was analyzed by 1H NMR spectroscopy in CDCl3 with THF
as an internal standard. Recyclability tests were performed after
the catalyst was washed with acetone and dried at room tempera-
ture. To regenerate activity, the used catalyst was basified by the
same alkalization process after the third recycle test.
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Published Online: ᭿
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/KAP1
Date: 20-08-12 15:51:52
Pages: 8
M. Yang et al
FULL PAPER
Mesoporous Materials
Two new zwitterionic hybrid SBA-15 meso-
porous silicas have been successfully syn-
thesized through cocondensation from an
as-synthesized zwitterionic siloxane precur-
sor with tetraethoxysilane. This work pro-
vides a possible route to synthesize bifunc-
tionalized mesoporous silica with a fixed
distribution of acid and base centers.
M. Yang,* F. Xu, C. Wang, X. Liu,
P. Yan, P. Li, U. Welz-Biermann ....... 1–8
Synthesis, Characterization, and Catalytic
Properties of Two Zwitterionic Hybrid
SBA-15 Mesoporous Silicas
Keywords: Zwitterions / Mesoporous ma-
terials / Organic–inorganic hybrid com-
posites / Heterogeneous catalysis / Aldol re-
actions
8
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