Synthesis of hexagonal mesoporous silicates functionalized with amino groups in the pore channels by a co-condensation approach

Article abstract of DOI:10.1039/c5ra24597d

Mesoporous silicates functionalized with amino groups in the pore channels have been made by the co-condensation of tetraethoxyl siloxide (TEOS) with precursors of P-Si through a triblock copolymer-templated sol-gel process under acidic conditions. Poly(alkylene oxide) block copolymer (P123) was eluted by ethanol extraction and template molecules were removed by refluxing the materials in a mixture of DMSO and water. The resulting materials were characterized in detail by FT-IR, XRD, TEM and N₂ adsorption, in order to study the effect of the precursors on the mesoscopic order and pore structure. Evidence of amino groups located in the pore channels was shown through variation of pore size and BET surface area after the amino groups were coupled with benzaldehyde, and TEM images of materials after staining with RuO₄. Finally, a comparative study of the catalytic performance of materials SBA-Am-10 and SBA-T-10, obtained by two methods, revealed that the catalyst synthesized by our method gave rapid reaction speed and high yields of flavanone by the Claisen-Schmidt condensation of benzaldehyde and 2′-hydroxyacetophenone.

Full text of DOI:10.1039/c5ra24597d

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ARTICLE
Synthesis of hexagonal mesoporous silicates with amino groups
functionalized on the pore channels by a co-condensation
approach
Received 00th January 20xx,
Accepted 00th January 20xx
Yunping Li,ab‡ Wei Xiong,ab‡ Chun Wang,a* Bo Songac and Guolin Zhanga
DOI: 10.1039/x0xx00000x
Mesoporous silicates with amino groups functionalized on the pore channels have been made by co-condensation of
tetraethoxyl siloxide (TEOS) with precursors of P-Si through a triblock copolymer-templated sol-gel proess under acidic
comdition. Poly(alkylene oxide) block copolymer(P123) was eluted by ethanol extraction and template molicules were
removed by refluxing the materials in a mixture of DMSO and water. The resulted materials were characterized in detail by
means of FT-IR, XRD, TEM and N2 adsorption, in order to study the effect of precursors on their mesoscopic order and pore
structure. Evidence of amino groups located on the pore channels was found through variation of pore size and BET
surface after amino groups coupled with benzaldehyde and TEM images of materials after stained by RuO4. Finally,
comparative study on catalytic performance of materials SBA-Am-10 and SBA-T-10, obtained by two methods revealed
that catalyst synthesized by our method gave rapid speed and higher yield to flavanone from Claisen-Schmidt condensatio-
n of benzaldehyde and 2’-hydroxyacetphenone.
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1 Introduction
mesoporous materials are perhaps the most difficult to synthesis
using a direct synthesis approach in the presence of non-ionic
Amino-functionalized mesoporous materials have received
considerable attention in recent years among the variety of organo-
functionalized mesoporous materials synthesized through the direct
synthesis route. It has been found to be effective in base catalyzed
reactions1-5 or further post-synthesis functionalization.6,7 However,
most of the work was on the modification of small mesopores of
MCM-type,8 which were synthesized under basic conditions. Highly
ordered mesoporous materials with large pores (>4.5 nm) have
recently attracted particular interest from catalytical view. The large
pore diameters of the channels facilitate the molecular diffusion
and accessible to catalytical sites and improve the catalytical
efficiency. On the other hand, the well-defined channels also have
confined effect on the catalytic selectivity. In several investigations,
confinement of the catalyst in the mesoporous solid improved the
activity compared to attachment to amorphous9 or microporous
silica,10 due to enhanced selectivity in a steric homogeneous
environment. It is generally admitted that antagonistic effects at
the mesoscale result in the existence of an optimal catalytic
efficiency depending on pore size.11
surfactants as structure directing agents under acidic condition.
Prehydrolysis method probably was the most efficient approach to
obtain well-ordered materials, in which the silica source, usually
TEOS or sodium silicate (Na2SiO3), were pre-hydrolysed for certain
time before the functional monomers were added. In order to gain
well-ordered mesoporous structure, the optimal prehydrolysis time
of TEOS was 2 h.12 In this case, an intact silicate framework was
formed before the addition of (3-aminopropyl)triethoxysilane
(APTES), which could minimize the effect of the protonated amino
groups on the self-organization of the surfactants. Protonated
species can cross-link with the alkoxysilane precursor during the
condensation step and as a consequence the organosilanes are
eventually deposited both in the pore channels and inside the walls
of the mesoporous materials.13 As a result, amino groups were
buried in the silica matrix and on both inner and outer surfaces of
the mesopores.14 This is not desired in point of confined catalysis.
Another commonly adapted method to obtain ordered amino-
functionalized mesoporous silicates was to use inorganic salts as
additives to strengthen the intereaction between the silicate
species and the surfactant hydrophilic head groups.15 However, the
accessibility of the amino groups in the resulted material was
remained unknown.
However, well-ordered large pore amino-functionalized
a.Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041,
China. E-mail: wangchun@cib.ac.cn
In our previous research to prepare molecular imprinted
mesoporous silicates (MIMS) for 2-naphthol by a surfactant
directed sol-gel process, we found that instead of imprinting the
template in the matrix, the material turned out to be a periodic
mesoporous silicate with inner amino functionalized mesoporous
pores.16 We rationalized that this was due to the mono-
functionalized organic template tends to interact with the
b.University of Chinese Academy of Sciences, Beijing 100049, China
c. College of Architecture and Environment, Sichuan University, Chengdu 610065,
China
. These authors contributed equally to this work.
‡
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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2.2 Instrumentation
hydrophobic core of the surfactant micelle during sol-gel process.
This enlightened us to use aromatic groups as protecting groups for
amino in order to obtain well-ordered inner pore amino
functionalized mesoporous materials.
The infrared spectra were recorded with Perkin-Elmer
system 2000 FT-IR spectrometer. Transmission electron
microscopy (TEM) images were obtained on a Tecnai-G2-F20
electron microscope operating at 200 kV. X-Ray diffraction
(XRD) patterns were obtained at room temperature using
instrument equipped with a Cu kα X-ray source. The content of
total amino groups in the samples was determined by the CHN
elemental analysis, which was performed using a PerkinElmer
2400 Elemental Analyzer. Solid-state cross-polarization (CP)
magic-angel spinning (MAS) nuclear magnetic resonance (NMR)
analysis was carried out on a Bruker AV-400 spectrometer at
79.49 MHz for 29Si and 100.61 MHz for 13C. The 29Si MAS NMR
spectra were measured at 60 s repetition delay and 3 μs pulse
width. The 13C CP MAS NMR spectra were measured at 2 s
Synthesis of functionalized mesoporous materials through
protecting group strategies were used by several research groups.
Schuths groups synthesized cyano (-CN) functionalized SBA-15 and
further hydrolysed into carboxyl (-COOH).17 Corrius groups reported
the use of N-tert-butyloxycarbamate (-Boc) as protecting group for
preparation of amino functionalized SBA-15. 18 Despite the difficulty
in preparing Boc-protected monomers, the ordering of the resulted
mesoporous materials were poorly preserved.
The structure and contents of organo-functionalized
mesoporous materials synthesized from co-condensation of
TEOS and functional monomer in the presence of surfactant,
depends strongly on the nature of the functional monomers. In
fact, the strong hydrophobic interaction between phenyl
groups and the hydrophobic core of the pluronic type
surfactants was theoretical investigated by A. Patti et al. using
lattice Monte Carlo simulations. They observed when the
hybrid precursor was sufficiently hydrophobic, it could act as a
co-surfactant, swell the core of the surfactant liquid crystal,
and lead to structures with smaller interfacial curvature. On
the other hand, if the hybrid precursor acted as a co-solvent it
would solubilize the surfactant leading to the destruction of
1
repetition delay, 2 ms contact time and 2.8 μs H 90° pulse. All
chemical shifts were referenced to tetramethylsilane (TMS).
Nitrogen adsorption-desorption isotherms were measured at
77K using a HYA2010-B2 system. The Barret-Joyner-Halenda
(BJH) pore size distribution was calculated from the desorption
branch of the isotherm.
2.3 Synthesis of materials
The synthesis of aromatic protecting triethoxysilane
precursors (APTPs) was achieved referring to the method in ref
19
20. Aromatic moieties (Ph, Np, Bp, Am) were reacted with
the preformed liquid crystal. We visualize that is possible to
stoichiometric amount of IPTES using triethylamine as catalyst
in dried THF at 65oC until the reaction was totally carried out.
The products were obtained by evaporation of solvents and
purified by column chromatography. The products synthesized
obtain highly ordered organo-functionalized mesoporous materials
with accessible organic groups on the pore channels, by carefully
selecting functional monomers with suitable solubility and
hydrophobicity.
with Ph, Np, Bp, Am were identified as Ph-Si, Np-Si, Bp-Si, Am-
In this paper, we reported the synthesis of amino-
functionalized mesoporous material with several aromatic
groups as protecting groups, and compared with materials
synthesized by pre-hydrolysis method. The major features of
this work are comparison of products from both methods with
respect to efficiencies of the synthetic routes, precursor
effects, the locations of functional groups in the mesoporous
products, and the catalytical behaviors in solid state catalyzed
synthesis of flavanone.
Si respectively. For comparison, Boc protecting triethoxysilane
precursor synthesized by DbD was obtained according to the
method in ref 18 and named as Boc-Si
.
Amino-functionalized SBA-
following procedure:
P-x materials were synthesized by
Throughout this paper, we abbreviate the amino-
functionalized mesoporous silica materials as SBA- -x or SBA-
-x (where is Ph Np Bp Am or Boc denotes APTES and x
P
T
P
,
,
,
, T
is 3, 5, 7, 10, 15 or 20, which is calculated from the molar
percentage of [precursor/(precursor + TEOS)]. P-Si denotes
triethoxysilane precursors and x denotes the molar percentage
of P-Si or APTES in the total silane monomer used in the co-
condensation reaction.
2 Experimental
2.1 Chemicals and Materials
Phenol (Ph), 2-naphthol (Np), 4-benzylphenol (Bp), 9-
anthracenemethanol (Am), di-tert-butyl dicarbonate (DbD
)
P-Si functionalized SBA-15 material was synthesized under
and TEOS were purchased from Gracia Chemical Technology
Co., Ltd. Chengdu. 3-isocyanatopropyltriethoxysilane (IPTES),
APTES and Pluronic P123 (EO20PO70EO20, MW= 5800 Da) were
purchased from Sigma Chemical Co., St. Louis. All other
acidic conditions from a P123/TEOS/P-Si/HCl/H2O mixture with
a molar composition of 0.0172/1/x%/6/208. Typically, P123
(2.0 g) was fully dissolved in 75 ml of 1.6 M HCl. To the solution
was added a stoichiometric amount of P-Si, which was pre-
dissolved by 1 ml THF. After the pre-hydrolysis of P-Si, TEOS
(4.45 ml) was added to the mixture and stirred up to 24 h at
40oC. Then the mixture was transferred into an autoclave
aging at 100oC for 24 h. After cooling down to room
temperature, the resultant particles were isolated by filtration
and rinsed with water, and named as SBA-
extracted by Soxhlet extraction with ethanol for 24 h, the
materials with P123 removed was designated as SBA- -x-PR.
P-x-as. P123 was
chemicals used were of analytical grade. All solvents and
reagents were used without further purification except THF
(dried).
P
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By refluxing the particles in a mixture of DMSO and water (v/v,
40/1) at 160oC for 6 h, the carbamate bond was cleaved and
Two kinds of amine functionalized SBA-15 (SBA-
T-10 and
P
SBA-Am-10) were selected for kinetic study, the reaction was
was removed, except that Boc was removed by acidic carried out in a two necked flask equipped with a reflux
hydrolysis. After filtration, the materials were rinsed with condenser and a magnetic stir bar. Typically, 6 mmol (612 μL)
DMSO and ethanol successively, and vacuum dried at 60oC for benzaldehyde
(
A
)
and
) were stirred in 25 ml DMSO, 250 mg
-10 (or SBA-Am-10) was added into the solution. The
4
mmol (485 μL) 2'-
24 h. The pre-hydrolysis time of P-Si were 1.5 h. Pure SBA-15 hydroxyacetophenone (
was prepared by the similar way without functional group of SBA-
adding. For comparison, SBA- -x was synthesized by adding mixture was heated to 140 C under stirring for 14 h. To get
APTES after 2 h pre-hydrolysis of TEOS
B
T
o
T
.
the kinetic curve, the reaction was monitored every two hours
by withdrawing an aliquot of solution from the flask. The
2.4 Chemical component, texture and structural samples were filtered and the filtrates were analyzed using an
characterizations Agilent HPLC equipped with an Agilent C18 column (4.6 × 250
The nitrogen content, corresponding to the amino groups mm, 5 μm) to determine the yields of flavanone (Fl).
incorporated, was measured by elementary analysis. Texture
properties were determined from N2 adsorption/desorption 2.6.2 Intramolecular Michael addition of 2’-hydroxychalcone
and the specific surface areas were evaluated by the Brunauer- catalyzed by SBA-T-10 and SBA-Am-10
Emmett-Teller (BET) method, pore size distribution profiles
The reaction was carried out in a round-bottom flask equipped
were calculated on the desorption branch of the isotherms with a reflux condenser and a magnetic stir bar. Typically, 20 mg
using the Barrett-Joyner-Halenda (BJH) method. Information SBA-T-10 (or SBA-Am-10) was suspended in 2.0 ml DMSO, 0.036
about mesoscopic order can be obtained by TEM and mmol (8.0mg) 2’-hydroxychalcone (Ch) was added into the
o
crystallographic form can be observed by small angel X-ray suspension. The mixture was heated to 140 C under stirred for 4h.
diffraction (SXRD) approach.
After cooling to room temperature, the mixture was filtered and
1
the filtrate was analysed by H NMR. The conversion of Ch and the
2.5 Determination of the location of aminopropyl groups
yield of Fl were calculated from the relative volume of C2-H on Fl
2.5.1 Condensation reaction of the amine groups with and β-H of the carbonyl on Ch. Control experiments were
benzaldehyde conducted in the presence of blank SBA-15, with and without DMSO,
Two kinds of solid amino functionalized SBA-15 (SBA-Am-10, or without added solvent and catalyst under otherwise same
0.25 g and SBA- -10, 0.25 g) were dispersed in anhydrous conditions as described above. Each reaction was repeated for tree
T
toluene, an excess of benzaldehyde (2 equiv. of benzaldehyde times and the average analysed volume vs time was plotted in Fig. 6.
per NH2 moiety) and 100 μl glacial acetic acid was added. The
reaction was carried out under reflux for 12 h, then the 3 Results and discussion
resulted materials, denoted as SBA-Am-imine and SBA-
T-
3.1 Synthesis of materials
imine, were filtered and wash with adequate toluene and
The aryl protected functional monomers were synthesized
ethanol to eliminate the unreacted benzaldehyde, after dried according to published procedure.20 Aromatic hydroxyl
at 80oC for 6 h
，the changes of pore diameters, BET surfaces moieties were reacted with a stoichiometric amount of IPTES,
and pore volumes were determined by nitrogen adsorption respectively, using triethylamine as catalyst in dried THF at
65oC for 24 h. The products were obtained by evaporation of
measurements.
solvents, and identified as P-Si (Scheme 1, a)). For comparison,
2.5.2 Characterization by combination of staining technique Boc protected functional monomer (Boc-Si) was prepared by
and TEM
Following a general method14 for RuO4 staining SBA-15,
SBA-Am-10 and SBA- -10 have been exposed to the vapor of
following Cprriu et al’s method (Scheme 1, b)). 18
The amino-functionalized mesoporous materials (SBA-P-x)
were prepared by a co-condensation of trialkoxysilanes (P-Si
)
T
and TEOS through a triblock copolymer-templated sol-gel
process16 with a slight modification. P-Si was added into the
surfactant/HCl solution and allowed to pre-hydrolysis for 1.5 h
before addition of TEOS, to let the aromatic groups fully
interacted with the surfactants micelle. Surfactant was
0.5 % RuO4 (aq.) for 15 min to stain amino groups. Then the
stained samples were characterized by Transmission electron
micrographs on a Tecnai-G2-F20 electron microscope working
at 200 kV.
2.6 Catalytic reaction
OEt
OEt
a)
OEt
OEt
O
H
In order to remove the residue Cl- ions and to neutralize the
protonated amine groups, the functionalized SBA-15 materials
were dispersed into 100 ml of 0.1 M methanol solution of
TMAOH at room temperature for 40 min before the reactions.
The resulted solids were recovered by filtration, washed with
methanol, and finally dried at 100oC for 12 h.
Et3N
C
O
N
Si
OEt
P
OH
N
Si
P
THF, 60°C
OEt
O
P-Si
OH
,
IPTES
OH
OH
OH
P
OH =
,
,
b)
O
OEt
ethanol
R.T.
OEt
OEt
O
O
O
Si
Si
H2N
N
O
OEt
O
O
H
OEt
OEt
Boc-Si
2.6.1 Kinetic study of flavanone synthesis catalyzed by SBA-T-
10 and SBA-Am-10
DbD
APTES
Scheme 1 Synthesis of the aryl protected functional monomers.
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removed by ethanol extraction. To remove the aromatic
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protecting groups (P), the carbamate bond was cleaved by
heating the materials in DMSO/H2O at 160oC, while the one
synthesized by Boc-Si was heated in aqueous solution of HCl.
After filtration and dried at 60oC for 24 h, a series of amino
functionalized materials can be obtained.
SBA-
hydrolysis of TEOS 12
way as SBA-
-x, then, the materials were dried at 60oC for 24 h
T
-x were synthesized by adding APTES after 2 h pre-
.
The surfactant was removed via the same
P
for the further analysis.
3.2 FT-IR characterization
In order to confirm that the P-Si was incorporated in the
structure and the successful cleavage of the carbamate bond,
the directly synthesized material (SBA-Am-10-as), after
surfactant removal (SBA-Am-10-PR) and after protecting group
removal (SBA-Am-10) were characterized by Fourier transform
infrared spectroscopy (FT-IR, Fig. 1). In all three materials, the
clear bands around 1070 and 1220 cm-1 indicate that
condensed silica networks are formed (Si-O-Si, asymmetric
vibration). For the SBA-Am-10-as, absorbance peaks
corresponding to C-H stretching and C-H deformation
vibrations appear in the range of 2850-2900 cm-1 , which were
due to CH2 absorption of P123. This peak is reduced on SBA-
Am-10-PR, indicating that the surfactant was almost removed
upon extraction with ethanol. The characteristic bands of
carbamate C=O stretching vibration around 1710 cm-1 are
clearly visible for SBA-Am-10-as and SBA-Am-10-PR, indicating
the existence of P-Si. After ethanol extraction and DMSO
Fig. 1 FT-IR spectra of SBA-Am-10-as, SBA-Am-10-PR and SBA-Am-10.
treatment, the 1710 cm-1 peak is disappeared (Fig.1, SBA-Am
-
10), indicating the cleavage of carbamate. An increasing
intensity of the peak around 1640 cm-1 could be assigned to
NH2 bending, which was overlapped with an OH bending of
absorbed water.21The relatively weak peak at 1503 cm-1
Fig. 2 13C CP MAS NMR spectrum : (a) SBA-Am-10-PR and (b) SBA-Am-10..
+
corresponding to the symmetric –NH3 bending,22 further
proves the successful recovering of the amino groups from
removal of the protecting group.
3.3 13C and 29Si CP MAS NMR
The incorporation of amino moieties in the mesoporous
materials was confirmed by solid-state NMR spectroscopy. The 13C
CP MAS NMR spectrum of SBA-Am-10-PR (Fig. 2a ) demonstrates
that Am-Si precursor was incorporated as shown by the signals at
56.7 ppm of CH2, 157.1 ppm of carbamate C=O resonances, 124.2-
132.6 ppm of anthracene ring and three additional signals (42.2,
21.3 and 9.1 ppm) are attributed to the propyl spacer of Am-Si. The
remove of protecting group by the treatment of DMSO/H2O is
clearly reflected in Fig. 2b by the disappearance of signals (56.7,
157.1 and 124.2-132.6 ppm) while the signals (42.7, 21.5 and 9.3
ppm) of the propyl spacer were retained. The 29Si MAS NMR
spectrum of SBA-Am-10 (Fig. 3) exhibits two sets of signals
attributed to T (C-SiO3, -55 to -68 ppm) and Q (SiO4, -90 to -120 ppm)
environments. The presence of T units gives further evidence of the
existence of CH2-Si groups. These data could further support the
Fig.3 29Si MAS NMR spectrum of SBA-Am-10.
3.4 Elemental analysis
To further confirm the incorporation of aminopropyl groups,
the N contents of the samples were analyzed by elementary
analysis. The results were represented in Table 1. Percentages
of functional monomer used and integrated, indicate that 81%
to 89% of added functional monomers were integrated in the
final materials. We also found that the time of functional
monomer pre-hydrolysis would significantly affect the N
content in the final materials (Data not shown in Table 1). The
optimal pre-hydrolysis time was between 1 and 1.5 h. When
pre-hydrolysis time was less than 1 h, only 6-7% of functional
monomers were contained, prolonging hydrolysis time greater
conclusion
that aminopropyl moieties
were
successfully
incorporated into the mesoporous material SBA-Am-10 and the
protecting groups were successfully removed.
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than 1.5 h mwould not increase the functional monomer and
resulted in less ordered materials.
3.5 Small angel X-ray diffraction measurements
The SXRD patterns of materials prepared from different
precursors, SBA-P-10 (P=Ph, Bp, Np, Am, T) were represented
in Fig. 4(a) and the effect of amount of aminopropyl contents
from 5-20% were demonstrated by SBA-Am-X (X=5-20%, Fig.
3(b)). The SXRD patterns of materials in Fig. 4 (a) all show an
intense peak of d (100) and two weak peaks of d (110) and d
(200), which are characteristics of a 2-d hexagonally ordered
(p6mm) structure.23-25 However, all the amino functionalized
materials were with lower ordering than the blank SBA-15,
which could be attributed to the disturbance of the organic
precursors to the ionic interaction between the silicate species
and the surfactant hydrophilic head groups.
The structural ordering is very sensitive to the aromatic
groups used in the aromatic group protected precursor P-Si
and the way P-Si and TEOS are mixed. Well-ordered hexagonal
mesoporous materials can be obtained with amino contents
up to 20% for SBA-Np-x and SBA-Am-x (Only the later set is
shown in Fig. 4 (b)). The intensity of the reflection declined as
the proportion of functional monomer (x) increasing,
suggesting that less ordered materials were resulted. However,
for materials prepared with Bp-Si and Ph-Si, the long rang
ordering were lost when x reached 10%. The peak intensity
attributed to d 100 reflection was decreased with the order:
SBA-Np-10>SBA-Am-10->SBA-Bp-10>-SBA-T-10>SBA-Ph-10.
When P-Si was pre-mixed with TEOS before being added
into the surfactant solution as usual doing,26 the resulting
materials were amorphous. However, when P-Si was added
into the surfactant solution 1 h earlier than TEOS, well ordered
hexagonal structures were formed.
The varying intensity of d (100) in Fig. 4 (a) may be due to
different interaction strengths between non-polar aromatic
groups and the hydrophobic core of the surfactant micelle,
which depends on the solubility, size and rigidity of the
aromatic moieties.19When such hydrophobic interaction is
strong enough, it would draw the organic precursors further
into the micelles, leading to the formation of well-ordered
mesopores (In the case of Np-Si and Am-Si), otherwise,
resulting in disordered mesopores (In the case of Bp-Si and Ph-
Si). The addition of organoalkoxysilane prior to TEOS into the
surfactant solution would favor the interaction between the
Fig. 4 XRD patterns of prepared materials a) SBA
with different APTPs addition, b) SBA-Am x with different concentrations of Am-
Si
-P-5, P=Am, Np, Bp and Ph
-
.
terminal organic moiety of the organoalkoxysilane and the
core of the surfactant, therefore promote the formation of
well-ordered meso-structures. Although pre-hydrolysis of
TEOS can also get well-ordered mesoporous silicates by
formation of an intact silica framework, which could shell the
disruptive interactions of organic monomers with the micelles,
but consequently the organic monomers were distributed in
the silica walls. The reduced intensity of the d100 peak of SBA-
Table 1 Amino contents incorporated calculated from elementary analysis
Percentage of
Amino
N content
(%)
functional precursor
Samples
group
Am-10 relative to SBA-T-10 (Fig. 4 (a), could be due to the
useda
integratedb
(mmol g-1)
organic content present in the pores.
SBA-Np-10
SBA-Am-10
SBA-Bp-10
SBA-Ph-10
SBA-T-10
SBA-15
1.36
1.46
1.34
1.31
1.47
—
0.97
1.04
0.96
0.93
1.05
10%
10%
10%
10%
10%
—
8.2%
8.8%
8.3%
8.1%
8.9%
—
It is worth to mention that with Boc-Si as precursor, no
ordered material was obtained.
3.6 Nitrogen adsorption/desorption characterization
Nitrogen adsorption/desorption isotherms of all obtained
materials except SBA-Ph-10 (Fig 5 (a)), exhibited typical type IV
isotherms according to IUPAC classification. 27 The Brunauer-
Emmett-Teller (BET) surface area, pore size, and pore volume
—
b
a
Molar ratio of P-Si/TEOS as added; Molar ratio of P/SiO2 calculated from the
N contents of elementary analysis.
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Table 2 Texture data of the amine-functionalized SBA-15 prepared from different
protecting groups and by pre-hydrolysis
Nitrogen Adsorption
Sample
Pore size
(nm)
Pore volume
(cc/g)
SBET (m2/g)
SBA-Np-10
SBA-Am-10
SBA-Bp-10
SBA-Ph-10
SBA-T-10
SBA-15
7.16
6.58
6.10
4.22
5.58
6.56
439.625
571.566
422.182
498.411
545.577
689.486
0.945411
0.941257
0.446348
0.464072
0.760954
1.096177
are listed in Table 2. The pore sizes of SBA-Bp-10 and SBA-Ph
-
10 were smaller than the blank SBA-15, probably due to the
co-solvents effects of Bp and Ph to the surfactant. On the
other hand, the pore sizes of SBA-Np-10 and SBA-Am-10 were
larger than SBA-15, due to the strong interactions between
these aromatic groups and the hydrophobic cores of the
surfactants (co-surfactants effects), causing the swelling of the
hydrophobic cores. Pore size distributions calculated from
desorption branch of nitrogen adsorption isotherm are
presented in Fig. 5 (b). Fig 5 (a) depicts that pure silica and SBA-
Np-10, SBA-Am-10 show similar high porosity and well-defined
adsorption. The sharp steep increases in the adsorption at P/P0 =
0.7-0.8 implied that the materials of SBA-Np-10 and SBA-Am-10
possess large pore sizes with narrow distribution. This was
conformed further by pore size distribution (Fig. 5 (b)). SBA-T-10
shows a less well-defined hysteresis loop than SBA-Np-10 and
SBA-Am-10, SBA-Bp-10 and SBA-Ph-10 display poorer defined
capillary condensation steps and lower porosity (Table 2, pore
volume). SBA-Bp-10 and SBA-Ph-10 lose mesoporosity might result
from the incorporated micelles being more poorly ordered. The
decrease in surface area of SBA-Np-10 and SBA-Am-10 comparing
with SBA-15 (Table 2), probably due to the strong interactions
between the aromatic groups in the hydrophobic-hydrophilic
palisade region of surfactant micelle, which could prevent the
hydrophilic head of the surfactants reaching into the pore walls to
form micropores.
Fig. 5 (a) Nitrogen adsorption/desorption isotherms of SBA-P-10, (b) BJH pore
size distribution of SBA-P-10.
The pore size of materials made with different aromatic
protecting groups are in the order: SBA-Np-10>SBA-Am-10>SBA-T-
10.
samples.
The possible interaction of silanol groups of pure non-
functionalized SBA-15 silica with the staining agent was
checked for control. In order to giving a better view of the
details, we enlarged the magnification of TEM images for
3.7 Transmission electron microscopy (TEM)
The mesoscopic order and symmetry inferred from XRD data
can be further confirmed by TEM (Fig. 6). As can been seen, all
materials except SBA-Ph-10 (not shown), show excellent
hexagonal ordering of mesopores in a uniform longitudinal
pore channel, characteristic of SBA-15 materials
stained SBA-15, SBA-Am-10 and SBA-T-10. (Fig. 6 (a’), (d’) and
(e’)). Fig. 6(a) and (a’) show the TEM micrographs for SBA-15
before and after the staining treatment with RuO4. As can be
seen, the bright and dark regions corresponding to pore
cavities and pore walls, respectively. The contrasts are the
same in both cases, that is, bright and dark areas correspond
to the same regions (pore cavities and pore walls respectively)
regardless of the treatment of RuO4. The results indicate that
RuO4 does not interact with the silanol groups of the silica
surface under the staining conditions used in this work.
The distributions of amino groups of materials made from
TEOS prehydrolysis method (SBA-T-10) and through aromatic
protecting group path (SBA-Am-10), were demnstrated by the
selective complexation of RuO4 over amino groups and its
subsequent reduction to RuO2. The sample treatment was
similar with the method reported by Rual Sanz. 14 The high
electronic density of this heavy metal makes the electron
beam pass through it difficult when the sample is analysed by
TEM. Thus, a specific darkening can be assigned to RuO2 and,
as a consequence, to the position of amino groups in the
TEM image of the stained SBA-T-10 (Fig. 6 (e’) exhibits a
number of black spots across the walls, assigned to ruthenium
species fixed over amino groups homogeneously distributed in
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the sample, including pores and walls. This was not the case of could be an evidence that amino groups are not detectable by
SBA-Am-10 (Fig. 6 (d’)).. The dark areas of RuO2 were mainly TEM if the staining step is not performed.
found within the channels for the stained sample, while the
3.8 Determination of the location aminopropyl groups by
pore walls remained bright. This could be an evidence of
condensation the amino-functionalized materials with
amino groups located mostly on the pore surfaces of the
benzaldehyde
nesosilicates. It is also clear that the amino-functionalized
To further verify the location of amino groups of materials
samples of SBA-Am-10 and SBA-T-10 present a distribution of
made from different methods (SBA-
benzaldehyde( ) was used to form imine with amino groups in
the materials (Scheme 2). If amino groups locate at the pore
surface of the nesosilicates, the introduction of into the pore
T-10 and SBA-Am-10),
pore cavities (bright) and pore walls (dark) similar to the silica
SBA-15 before staining with RuO4 (Fig. 6 (a), (d), and (e)). This
A
A
by formation of imine with amino groups on the pore surface
would reduce the pore size, or otherwise, the pore size would
be remained unchangeable.
The mesoporous properties of SBA-
before and after treatment obtained by nitrogen adsorption
experiments, are presented in Table 3. It is interesting that the
pore size of SBA- -10 had reduced slightly from 5.58 to 5.47
nm, while BET surface and pore volume of SBA- -10 did not
change significantly after treatment of . In contrast, the pore
T-10 and SBA-Am-10
A
T
T
A
size of SBA-Am-10 had been reduced from 6.58 to 5.78 nm,
and pore volume from 0.941 to 0.773 cc/g. These could be
explained by the different location of amino groups in SAB-T-
10 and SBA-Am-10. Since a TEOS pre-hydrolysis process was
used for synthesis of SBA-T-10, in which amino groups were
distributed both in the pore channels (minor) and inside the
walls of the resulting materials, little
A could be attached on
pore surface. The changes in pore size, BET surface and pore
volume are ignorable. On the other hand, amino groups are
mostly located on the pore surface of SBA-Am-10, they are
available to react with
A, resulting in pore size and pore
volume changes. These results are in consistent with what
observed in TEM stain images.
3.9 Kinetic study of flavanone synthesis
Amino-functionalized SBA-15 type material prepared by
TEOS pre-hydrolysis method, has been found to be an effective
base catalyst for the synthesis of flavanone(Fl).22,28,29 Xueguang
Wang et al. found that the yield of Fl from and 2’-
hydroxyacetophenone(B) catalyzed by amino functionalized
SBA-15 was the highest under solvent free condition, and
DMSO was the best solvent for the reaction. For operative
purpose, we use DMSO as solvent to study the kinetic behavior
of the reaction catalyzed by two types of functionalized
mesoporous materials. Two materials SBA-Am-10 and SBA-T-
10 had comparable loading of amine groups, based on the
elementary analysis (Table I), were chosen to catalyze the
reaction of
A
and B (Scheme 3). The kinetic behaviors of
the reaction were monitored by HPLC analysis.
The total yield of Fl catalysed by SBA-Am-10 was about 20%
Fig. 6 TEM images of (a) SBA-15, (a’) SBA-15-stained, (b) SBA-Bp-10, (c) SBA-Np
-
-
Scheme 2 Condensation reaction of the amine function with A.
10, (d) SBA-Am-10, (d’) SBA-Am 10-stained, (e) SBA-T-10, (e’) SBA-T-10-stained.
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Table 3 Texture properties of materials obtained with two methods after treated with A
Before reaction
After reaction
Samples
Pore size
(nm)
BET surface
(m2/g)
Pore volume
(cc/g)
Pore size
(nm)
BET surface
(m2/g)
Pore volume
(cc/g)
0.735
0.773
SBA-T-10
5.58
6.58
545.6
571.6
0.761
0.941
5.47
5.78
539.8
550.8
SBA-Am-10
in terms of converting Ch into Fl. In other words, the
cyclization does not related with the distribution of amino
groups in the materials.
To verify if the mesopores were responsible for the
cyclization, blank SBA-15 with/without DMSO were used for
the same reaction. The selectivity to Fl were also similar with
those of amino functionalized materials. Surprisingly, when no
solid catalyst was added in DMSO solution of Ch, the selectivity
of Fl was also 66%. When Ch alone was heated to 140 oC for 4h,
only 9% of Fl was detected. The results revealed that DMSO,
amino-functionalized SBA-15 and none functionalized SBA-15
can catalyze the cyclization of Ch. Comparing data in Table 4
and Table 5, we could conclude that amino-functionalized SBA-
15 was an effective catalyst for the Claisen-Schmidt
Scheme 3 synthesis of Ch and Fl catalysed by amino-functionalized mesoporous
silicates.
higher than that of SBA-T-10 (Fig. 7 and Table 4). It was also
about 2 h faster to reach the highest volume of yield (Fig.
7).The results could be interpreted as amino groups of SBA-
Am-10 are more accessible than those of SBA-
catalytical synthesis of Fl from and
Since two steps were involved in the synthesis of Fl from
Claisen-Schmidt condensation of and , followed by
T-10 for the
A
B.
A
B
cyclization of Ch to Fl. To investigate the catalytic performance
of the materials, we checked the cyclization of Ch in the
presence and absence of catalyst and solvent (Scheme 4). The
conversion of Ch in the presence of catalysts, DMSO and no
solvent and without materials at 140 for 4 h are shown in
Table 5.
OH
O
Conditions
O
O
Fl
Ch
Scheme
4
Reaction of Ch.
As can been seen from Fig. 8, the two curves are almost
overlapped, indicating that both of the materials were efficient
Fig. 8 Kinetic conversion of Ch to Fl
Table 5 Cyclization of Ch over 20mg materials or without materials at 140 ℃
for 4 h
Fig. 7 Kinetic curves of yield of Fl from Claisen-Schmidt condensation of A and B
catalyzed by SBA-Am-10 and SBA-T-10, respectively.
Entry
Solvent
Catalyst
Yield of Fl (%)
1
2
3
4
5
6
7
8
DMSO
--
SBA-Am-10
SBA-Am-10
SBA-T-10
SBA-T-10
SBA-15
SBA-15
--
64a
70b
62a
71b
69a
61b
66c
9d
Table 4 The catalytic performance of materials in the condensation of A and B
with DMSO at 140 oC for 8 h a
DMSO
--
Entry
Catalyst
Conversion of A (%)
Yield of Fl b (%)
DMSO
--
1
2
3
4
SBA-Am-10
SBA-T-10
SBA-15
--
69
53
<5
--
44
33
<3
--
DMSO
--
--
a The yield of Fl were analyzed by NMR. Reaction conditions: 0.036mmol Ch, 2mL
a Reaction conditions: 6 mmol
-10 or SBA-Am-10.b The yield of Fl were analyzed by HPLC.
A; 4 mmol B; 2 ml DMSO; 0.25g SBA-15,SBA-
b
DMSO. free-solvent. c without materials. d solvent-free and without materials.
T
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condensation. Mesoporous silicats made with aryl-protecting 16 G. T. Luo, Y. P. Li, A. Wang, Q. Lin, G. L. Zhang and C. Wang,
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5
T
A
,
A
B.
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Acknowledgements
This work was financially supported by National Natural
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Graphical Abstract
Synthesis of hexagonal mesoporous silicates with amino groups functionalized on
the pore channels by a co-condensation approach
Yunping Li,ab Wei Xiong,ab Chun Wang,a,* Bo Songac and Guolin Zhanga