Synthesis, characterization and catalytic activity of acid-base bifunctional materials through protection of amino groups

Article abstract of DOI:10.1016/j.materresbull.2011.12.001

Acid-base bifunctional mesoporous material SO₃H-SBA-15-NH ₂ was successfully synthesized under low acidic medium through protection of amino groups. X-ray diffraction (XRD), N₂ adsorption-desorption, transmission electron micrographs (TEM), back titration, ¹³C magic-angle spinning (MAS) NMR and ²⁹Si magic-angle spinning (MAS) NMR were employed to characterize the synthesized materials. The obtained bifunctional material was tested for aldol condensation reaction between acetone and 4-nitrobenzaldehyde. Compared with monofunctional catalysts of SO₃H-SBA-15 and SBA-15-NH₂, the bifunctional sample of SO₃H-SBA-15-NH₂ containing amine and sulfonic acid groups exhibited excellent acid-basic properties, which make it possess high activity for the aldol condensation.

Full text of DOI:10.1016/j.materresbull.2011.12.001

Materials Research Bulletin 47 (2012) 768–773
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis, characterization and catalytic activity of acid–base bifunctional
materials through protection of amino groups
a,
Yanqiu Shao a,b, Heng Liu a, Xiaofang Yu a, Jingqi Guan a, , Qiubin Kan
*
*
a College of Chemistry, Jilin University, Changchun 130023, PR China
b College of Chemistry, Mudanjiang Normal University, Mudanjiang 157012, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Acid–base bifunctional mesoporous material SO3H-SBA-15-NH2 was successfully synthesized under low
acidic medium through protection of amino groups. X-ray diffraction (XRD), N2 adsorption–desorption,
transmission electron micrographs (TEM), back titration, 13C magic-angle spinning (MAS) NMR and 29Si
magic-angle spinning (MAS) NMR were employed to characterize the synthesized materials. The
obtained bifunctional material was tested for aldol condensation reaction between acetone and 4-
nitrobenzaldehyde. Compared with monofunctional catalysts of SO3H-SBA-15 and SBA-15-NH2, the
bifunctional sample of SO3H-SBA-15-NH2 containing amine and sulfonic acid groups exhibited excellent
acid–basic properties, which make it possess high activity for the aldol condensation.
ß 2011 Elsevier Ltd. All rights reserved.
Received 11 February 2011
Received in revised form 7 November 2011
Accepted 3 December 2011
Available online 9 December 2011
Keywords:
B. Chemical synthesis
C. Electron microscopy
C. X-ray diffraction
D. Catalytic properties
1. Introduction
containing sulfonic acid and thiol exhibited a cooperative effect
in a condensation reaction [20]. Dufaud and Davis reported a
Since the discovery of ordered mesoporous M41S [1,2], a variety
of ordered mesoporous materials have been synthesized using a
surfactant templating method. Surface-modified mesoporous
materials with different active sites have been extensively
investigated in recent years for catalysis [3–6], separation [7,8],
chemical sensing [9], and nanoscience [10,11]. Many monofunc-
tionalized SBA-15 mesoporous silica catalysts have been prepared
[12–21], which have unique properties alone or in combination
with other functional groups. However, some catalytic reactions
need multifunctionality of active sites with cooperative effect, e.g.
the combination of acid and base properties [22–26]. Previous
work on bifunctional solid-catalyst synthesis mainly focuses on
the use of metal centers with acid sites [27–31]. Recently, much
effort has been devoted on the combination of organic functional
groups, bifunctionalized mesoporous silica nanosphere materials
with ureidopropyl group and 3-[2-(2-aminoethylamino)ethyla-
mino]-propyl group [27], amine functional groups and thiols
[30,31]. Katz et al. [18,32] reported the synthesis of imprinted
amines in bulk silica by using the protection of carbamate groups,
which was commonly used as protecting agents for amino groups
in synthetic chemistry. Zeidan et al. reported that SBA-15
multifunctional heterogeneous SBA-15 containing primary amine
and sulfonic acid [19]. However, the equilibrium between acid and
base is likely very important in the catalytic reaction process.
Therefore, Zeidan et al. investigated the effect of the balance
between acid and base groups by varying the pKa of the acidic
groups [33]. Alauzun et al. [34] reported the synthesis of
bifunctional mesoporous materials containing two antagonist
functions, an acidic group in the framework and a basic one in the
channel pores. Our group has reported two acid–base bifunctional
mesoporous materials benzyl-APS-S-SBA-15 (APS: (3-trimethox-
ysilanyl-propyl)-amine) and anthracyl-APS-S-SBA-15 by control-
ling steric hindrance [35]. Very recently, Thiel et al. described the
synthesis of various bifunctional acid–base materials for co-
operative catalytic reactions [36].
Among the well-known mesoporous materials, SBA-15 has
relatively good hydrothermal stability [37], hexagonal arrays of
uniform pores, high special surface area and large pore volume.
However, the inherent lack of active sites, such as acid and basic
sites of adequate strength, has severely restricted its application
in industry. It is still a challenge for its surface functionalization. In
this paper, we report
a method to synthesize acid–base
bifunctional materials under low acidic medium through protec-
tion of amino groups. The method is important to prevent
interaction between acid and base, and generate an efficient
catalyst. The catalytic activity of the materials in aldol condensa-
tion reaction between acetone and 4-nitrobenzaldehyde was
investigated.
* Corresponding authors at: Jiefang road 2519, Changchun 130021, PR China.
Tel.: +86 431 88499140; fax: +86 431 88499140.
E-mail addresses: guanjq@jlu.edu.cn (J. Guan), qkan@mail.jlu.edu.cn,
catalysischina@yahoo.com.cn (Q. Kan).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.12.001

Y. Shao et al. / Materials Research Bulletin 47 (2012) 768–773
769
Scheme 1. Synthesis of NH2-SBA-15-SO3H.
2. Experimental
Deprotection amino group was completely achieved by thermal
treatment of NHBoc-SBA-15-SO3H at 185 8C under vacuum for 4 h.
The resulting bifunctional mesoporous silica contained both
aminopropyl groups and sulfonic acid groups, named NH2-SBA-
15-SO3H (Scheme 1).
2.1. Chemicals and materials
Pluronic123 (EO20PO70EO20) (Aldrich), HCl (A.R), 4-nitroben-
zaldehyde (Acros), acetone (A.R), tetraethyorthosilicate (Aldrich),
3-aminopropyltrimethoxysilane (APTMS) (Aldrich), ethanol (A.R),
3-mercaptopropyltrimethoxysilane (MTPMS) (Aldrich), di-tert-
butyl dicarbonate (Acros) were commercially available and used
as received.
2.3. Characterization
N2 adsorption–desorption isotherms were obtained on
a
Micromeritics ASAP 2020 system at liquid N2 temperature
(À196 8C). Before measurements, the samples were outgassed at
100 8C for 24 h. The specific surface area was calculated by using
the Brunauer–Emmett–Teller (BET) method and the pore size
distributions were measured by using Barrett–Joyner–Halenda
(BJH) analyze from the adsorption curve of the isotherms. Powder
X-ray diffraction patterns (XRD) were collected using a Siemens
2.2. Sample preparation
2.2.1. Synthesis of 3-tert-
butyloxycarbonylaminopropyltrimethoxysilane (NHBoc, I)
I was prepared by mixing 3-aminopropyltrimethoxysilane
(APTMS) (0.77 mL, 4.1 mmol) and di-tert-butyl dicarbonate
(BOC) (1.005 g, 4.6 mmol) in 30 mL of anhydrous ethanol [38].
The mixture was stirred overnight at room temperature. Subse-
quently, the additional solvent was removed under vacuum. The
residual liquid was distilled to afford 1.2 g of I (3 mmol).
D5005 (0.28/min) with Cu Ka radiation (40 kV, 30 mA). Transmis-
sion electron microscope (TEM) was performed on a Hitachi H-
8100 electron microscope, operating at 200 kV. The solid-state 29Si
MAS NMR spectra were measured using a Varian infinity-400
spectrometer with a frequency of 59.63 MHz and 7.5 mm zirconia
rotors spun at 4 kHz. 13C CP/MAS NMR spectra were measured
2.2.2. Synthesis of bifunctional materials
using a Bruker AM-300 spectrometer with a frequency of
1.02 g of EO20PO70EO20 (P123) was dissolved in 40 g of distilled
water, Then, 5.05 mL of TEOS (prehydrolysis 50 min in pH = 4.0)
was injected. The mixture was allowed to stir at 40 8C for 40 min.
Subsequently, premixing of 0.6 g I and 0.33 mL 3-mercaptopro-
pyltrimethoxysilane (MTPMS) was added in to the system. The
reaction mixture was stirred at 40 8C for 20 h, and then heated to
100 8C for 24 h in oven. The resulting solid was cooled to room
temperature, and rinsed with excess H2O until the slurry reached a
neutral pH value. Next, the solid was allowed to dry overnight on a
filter. Then the dried solid was extracted with EtOH (400 mL/g) by
refluxing for 48 h to remove additional P123. The obtained solid
was washed with copious amount of water until a neutral pH value
reached. Finally, these solid was dried at 80 8C under vacuum for
12 h, and NHBoc-SBA-15-SH was obtained [20].
The NHBoc-SBA-15-SH was washed and treated with H2O2
solution at room temperature for 3 h. Then the diluted sulfuric acid
solution was added and the reaction was kept for 30 min, the SH
group on NHBoc-SBA-15-SH was totally oxided into SO3H. The
product was washed with copious amount of water until a neutral
pH value reached, and then dried overnight to obtain a dry white
solid termed NHBoc-SBA-15-SO3H.
75.48 MHz. Elemental analyses (EA) of sulfur and nitrogen were
performed on a VarioEL CHN elemental analyzer. A back titration
method was used to measure the amount of acid–base centers of
the materials according to the modified procedure in literature
[35]. 0.1 g of the sample was added to a conical beaker, and then
10 mL 0.01 M HCl (or NaOH) solution was added. The mixture was
stirred at room temperature for half an hour; then, the mixture was
filtered and rinsed repeatedly for four times with 25 mL of distilled
water. The resulting filtrate was titrated with 0.01 M NaOH (or
HCl) solution using phenolphthalein as indicator.
2.4. Catalytic experiments
The aldol condensation reaction was performed in a flask under
N2. First, 4-nitrobenzaldehyde (76 mg, 0.5 mmol) was dissolved in
acetone (10 mL). Then catalytic reaction was carried out over
0.05 mmol the total amount of amine and/or sulfonic acid
functional catalyst at 50 8C for 20 h. Then filtration was used to
remove the solid catalyst. After washing with acetone and
chloroform, the resulting product was analyzed by 1H NMR in
CDCl3 [32].

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Y. Shao et al. / Materials Research Bulletin 47 (2012) 768–773
3. Results and discussion
3.1. Characterization of the prepared samples
Fig. 1 shows the XRD patterns of SH-SBA-15-NHBoc, SO3H-SBA-
15-NHBoc and SO3H-SBA-15-NH2. All samples showed a promi-
nent peak at 2u = 0.88. However, the intensity of the diffraction
patterns of SO3H-SBA-15-NH2 was much larger than that of SH-
SBA-15-NHBoc. The decrease in the intensity of the (1 1 0) and
(2 0 0) peaks for SH-SBA-15-NHBoc and SO3H-SBA-15-NHBoc was
observed, which indicates that
a decreased pore structural
ordering due to the multi-step synthesis procedures.
The TEM images of sample SO3H-SBA-15-NH2 are shown in
Fig. 2. Parallel straight pores and hexagonally arrayed pores were
clearly observed, which can be ascribed to the (1 1 0) and (1 0 0)
directions of p6 mm phase, respectively. TEM images provide
evidence for that the structural ordering of SO3H-SBA-15-NH2
remains intact after the organic modifications. These results,
together with the XRD patterns, confirmed the formation of the
highly ordered mesostructure.
The N2 adsorption–desorption isotherms and pore size
distributions of SH-SBA-15-NHBoc, SO3H-SBA-15-NHBoc and
SO3H-SBA-15-NH2 are shown in Fig. 3. It can be seen that the
samples displayed a type IV isotherm with H1 hysteresis and a
sharp increase in volume adsorbed at P/P0 ꢀ 0.72, indicating that
the ordered mesoporous structure of SBA-15 was well remained.
The structural properties of the different mesoporous materials are
listed in Table 1. It can be found that the specific surface area
gradually decreases from SO3H-SBA-15-NH2 to SO3H-SBA-15-
NHBoc and SH-SBA-15-NHBoc. The pore diameter increases from
7.6 nm for SH-SBA-15-NHBoc to 8.3 nm for SO3H-SBA-15-NH2.
The incorporation of carbamate groups in the mesoporous
materials and transformation of carbamate groups to the
corresponding primary amine were confirmed by solid-state 13C
Fig. 1. XRD patterns of the synthesized samples: (a) SH-SBA-15-NHBoc, (b) SO3H-
SBA-15-NHBoc, and (c) SO3H-SBA-15-NH2.
MAS NMR (Fig. 4). The 13C CP/MAS NMR spectrum of SO3H-SBA-
NHBoc (Fig. 4a) demonstrates that the carbamate group remains
intact as shown by the signals at 25.95 ppm (CH3 resonances of
tert-butyl), 158.00 ppm (carbonyl resonances) and additional
signals attributed to the propyl spacer [39]. The resonances
associated with the carbamate protecting group (25.95 and
158.00 ppm) disappeared after thermal treatment under N2 flow,
indicating that the carbamate deprotection can be successfully
achieved by thermal treatment.
Solid-state 29Si MAS NMR spectroscopy has been proven to
be the most useful technique for providing chemical information
regarding the condensation of organosiloxane. As shown in
Fig. 5, the peaks with chemical shift at À106.9, and À111.9 ppm
were attributed to the Q3 and Q4 bands of Si(OH)(OSi)3,
and Si(OSi)4 silicate species, respectively. In addition, the
Fig. 2. TEM images of sample SO3H-SBA-15-NH2: (a) in direction perpendicular to pore axis and (b) in direction of pore axis.
Fig. 3. N2 adsorption isotherm and pore size distributions of samples: (a) SH-SBA-15-NHBoc, (b) SO3H-SBA-15-NHBoc, and (c) SO3H-SBA-15-NH2.

Y. Shao et al. / Materials Research Bulletin 47 (2012) 768–773
771
Fig. 4. Solid-state 13C MAS NMR of samples: (a) SO3H-SBA-15-NHBoc and (b) SO3H-SBA-15-NH2.
Fig. 5. Solid-state 29Si MAS NMR of samples SO3H-SBA-15-NH2.
resonance peaks from À50 to À70 ppm can be attributed to
Tm (Tm = RSi(OSi)3(OH)3, m = 1–3), confirming that the silane
organic moieties are incorporated as a part of the silica wall
structure and a significant fraction of the surfactant is still present
even after the surfactant extraction as well as multi step synthesis
procedures [40,41].
Quantification of the functional group loaded in SBA-15
samples was performed for the bifunctional materials using
elemental analysis (CHNS). The results of CHNS elemental analysis
indicate that there is 0.249 mmol/g of sulfoacid and 0.233 mmol/g
of NH2 for SO3H-SBA-15-NH2.
Table 1
Pore structural properties of functionalized samples.
Samples
BET surface
area (m2 gÀ1
Pore sizea
(nm)
Pore vol.
)
(cm3 gÀ1
)
SH-SBA-15-NHBoc
SO3H-SBA-15-NHBoc
SO3H-SBA-15-NH2
264.7
385.0
433.0
7.6
8.1
8.3
0.48
0.59
0.62
a
Calculated by using the BJH model on the adsorption branch of the isotherms.

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Y. Shao et al. / Materials Research Bulletin 47 (2012) 768–773
Table 2
Test of catalysts.
Entry
Catalyst
Conv. (%)a
Isolated yield (%)
A
B
1
2
3
4
5
6
SBA-15
0
0
0
SBA-15-SO3H
SBA-15-NH2
6.5
42.8
6
0.5
9.1
6.1
4.2
5.8
33.7
18.2
43
SO3H-SBA-15-NHBoc
SBA-15-A/SBA-15-Bb
SO3H-SBA-15-NH2
24.3
47.2
75.2
69.4
a
Total conversion. Yields determined through 1H NMR spectroscopic analysis with THF as the internal standard.
1:1 Mixture of sulfonic acid-functionalized SBA-15 (SBA-15-A) and amine-functionalized SBA-15 (SBA-15-B).
b
To further confirm the elemental analysis, back titration was
described. XRD and TEM data provide evidence that the
functionalized materials keep a 2D-hexagonal mesoporous frame-
work. Back titration, 13C MAS NMR and 29Si MAS NMR confirm that
the amine and sulfonic acid are successfully incorporated into the
mesoporous SBA-15. Compared with SBA-15-SO3H and SBA-15-
NH2, bifunctional material SO3H-SBA-15-NH2 displays higher
catalytic performance, and shows excellent conversion for the
aldol condensation reaction. These results indicate that the acid–
base cooperation effect should be involved in the catalytic reaction
and efficient acid/base active sites might increase during the
protection of amino group for prevention of the interaction
between acid and base in the bifunctional material.
performed for the materials. It was found that the SO3H-SBA-15-
NHBoc sample gave a loading of 0.2734 mmol/g of sulfoacid and
0.08309 mmol/g of NH2. In addition, the sample of carbamate
deprotection gave a loading of 0.2457 mmol/g of NH2. The results
are in good agreement with the elemental analysis results.
3.2. Catalytic properties
The materials were used in the aldol condensation between
acetone and 4-nitrobenzaldehyde. The numerical results of this
reaction over the catalysts are summarized in Table 2. The aldol
condensation of 4-nitrobenzaldehyde with acetone was reported to
be catalyzed by acid, base and bifunctionalized acid/base catalysts.
The reaction results showed that there was almost no conversion for
pure silica SBA-15 (Table 2, entry 1). Meanwhile, sulfonic acid
functionalized SBA-15 showed relatively low conversion of 6.5%
(Table 2, entry 2). The conversion for amine functionalized SBA-15
was 42.8% (Table 2, entry 3). The use of bifunctionalized material
through protection amino group gave the conversion of 24.3% (Table
2, entry 4), which indicates that there is weak basicity in the
material. However, the bifunctionalized deprotected sample of
SO3H-SBA-15-NH2 showed the significant conversion of 75.2%
(Table 2, entry 6), which was higher than the monofunctional
catalysts of SO3H-SBA-15 and SBA-15-NH2. Moreover, a physical
mixture of acid-functionalized SBA-15 and amine functionalized
SBA-15(Table1, entry5)showedanintermediatelevelofconversion
that was lower than the bifunctionalized acid–base catalyst. In
addition, a slightly increased catalytic activity for SO3H-SBA-15-
NHBoc sample is noted in Table 2, entry 4 with compared with SBA-
15-SO3H (entry 2), suggesting that a very small amount of amine
groups may be not protected with BOC. These results indicated that
the acid–base cooperation effect should be involved in the catalytic
reaction. In addition, the efficient acid/base active sites might
increase due to protection of amino groups from the interaction
between acid and base.
Acknowledgments
This work was supported by Jilin province (20090591 and
201105006), Jilin University (450060445017), Specialized Re-
search Fund for the Doctoral Program of Higher Education
(20100061120083) and Doctoral Research Fund of Mudanjiang
Normal University (MSB201001).
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