direct synthesis of thiol-ligands-functionalized sba-15: effect of 3-mercaptopropyltrimethoxysilane...
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Materials Letters 59 (2
Direct synthesis of thiol-ligands-functionalized SBA-15: Effect of
3-mercaptopropyltrimethoxysilane concentration on pore structure
Qi Wei, Zuoren Nie*, Yali Hao, Zengxiang Chen, Jingxia Zou, Wei Wang
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, China
Received 5 January 2005; accepted 17 June 2005
Available online 20 July 2005
Abstract
Mesoporous thiol-functionalized SBA-15 silicas have been directly synthesized by co-condensation of tetraethyl orthosilicate and 3-
mercaptopropyltrimethoxysilane with triblock copolymer poly(ethylene glycol)-B-poly(propylene glycol)-B-Poly(ethylene glycol) as
structure-directing agent under hydrothermal condition. Mesoporous structure was obtained after the surfactant removal by Soxhlet ethanol
extraction. These materials have been characterized by means of powder X-ray diffraction, nitrogen sorption, transmission electron
microscopy, thermogravimetry analysis, elemental analysis and solid state 29Si nuclear magnetic resonance. The effect of 3-
mercaptopropyltrimethoxysilane concentration in the initial mixture on the pore structure of functionalized SBA-15, including pore
ordering, surface area, pore size and pore volume, is investigated in detail. In order to functionalize the SBA-15 silicas without a significant
change of pore structure, the molar concentration of 3-mercaptopropyltrimethoxysilane should be limited to less than 20%.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Mesoporous; Thiol groups; Functionalized SBA-15; Pore ordering; Pore structure
1. Introduction
Ordered mesoporous silica-based materials have attracted
much attention due to their large surface area and well-
defined pore size easily tuned by choosing different supra-
molecular surfactants as structure-directing agents. These
materials may find promising application in catalysis,
sensing and separation, as well as environmental remedia-
tion. Recently, considerable efforts have been devoted on
the application of mesoporous silicas as adsorbents to
remove toxic heavy metal cations or organic pollutants
from wastewater [1–5]. Thiol ligands, acting as biting sites
for heavy metal cations, have been incorporated into
mesoporous silica by either postsynthesis grafting or co-
condensation of functional organosilane with tetraethyl
orthosilicate (TEOS) [6–9]. It is proved that thiolated
mesoprous silicas exhibit high complexation affinity to
0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2005.06.034
* Corresponding author. Tel./fax: +86 10 67391536.
E-mail address: zrnie@bjut.edu.cn (Z. Nie).
mercury and other metal cations. Liu et al. reported that the
mercury concentration in aqueous solution could be reduced
by thiol monolayers on MCM-41 to below U.S. Environ-
mental Protection Agency elemental limits for hazardous
wastes [6]. Similar results for functionalized HMS, MSU
have also been achieved by Mercier et al. [1,7,10,11] and
Corrius et al. [12]. In contrast to the extensive investigation
on functionalized HMS, MSU and MCM-41, less attention
has been paid, to the best of our knowledge, to the surface
modification of SBA-15 with nonionic surfactant as
template, although SBA-15 is more hydrothermally stable
because of its more regular structure and much thicker pore
wall. Among the limited literature related to surface
modification of SBA-15 [8,9,13], most of them have
concentrated on the interaction between the heavy metal
ions and the functional groups. For example, Liu et al.
functionalized SBA-15 with thiol and amino groups by
postsynthesis route and tested them for Hg2+, Zn2+ and other
metal cations adsorption [9]. The thiol-grafted SBA-15
exhibited high complexation affinity to mercury, while the
aminated SBA-15 showed a high binding ability to copper,
005) 3611 – 3615
Q. Wei et al. / Materials Letters 59 (2005) 3611–36153612
zinc, chromium, and nickel cations. Unfortunately, little
attention has been focused on the parameters that may affect
the pore structure of SBA-15 during direct surface
modification. In the present paper, thiol-ligands-functional-
ized SBA-15 was prepared by co-condensation of 3-
mercaptopropyltrimethoxysilane (MPTMS) and TEOS
under suitable condition. The evolution of the pore structure
with the concentration of functional organosilane in initial
mixture was investigated in detail by means of X-ray
diffraction (XRD), transmission electron microscopy
100
210
200
110
x5
210
200
100
x5
(c)
(e)
(a)
0 1 2 3 4 5 6
20011
0
100
x5
2Theta/deg.
0 1 2 3 4 5 62Theta/deg.
0
0 1 2 32Theta/
Fig. 1. XRD spectra of the thiol-functionalized SBA-15 with different molar ratio
20%. The region where (110), (200) and (210) peaks appear is scaled 3 or 5 tim
(TEM), nitrogen adsorption, thermogravimetry (TGA) and
solid state 29Si nuclear magnetic resonance (NMR).
2. Experimental
2.1. Materials synthesis
The synthesis of SBA-15 material was performed
according to the following procedure with a molar ratio of
20011
0
100
x3
210
200
110
100
x5
(b)
(d)
0 1 2 3 4 5 62Theta/deg.
1 2 3 4 5 62Theta/deg.
4 5 6deg.
s of MPTMS/(MPTMS+TEOS): (a) 0%, (b) 5%, (c) 10%, (d) 15%, and (e)
es.
Table 1
Textural data of the thiol-functionalized SBA-15 with different molar ratios of MPTMS/(MPTMS+TEOS)
MPTMS/(MPTMS+TEOS)
(mol%)
Surface area
(m2 g�1)
Pore volume
(m3 g�1)
Pore diameter
(nm)
d100(nm)
Cell parameter
(nm)
Wall thickness
(nm)
0 733.4 0.96 6.2 9.4 10.9 4.7
5 699.0 0.89 6.1 10.0 11.5 5.4
10 643.2 0.76 5.5 10.2 11.8 6.3
15 433.5 0.52 4.7 10.8 12.5 7.8
20 170.0 0.23 3.4 11.0 12.7 9.3
Fig. 2. TEM image of the thiol-functionalized SBA-15 with different molar
ratios of MPTMS/(MPTMS+TEOS): (a) 0% and (b) 20%.
Q. Wei et al. / Materials Letters 59 (2005) 3611–3615 3613
TEOS ( Acros, 98% ) :Surfactant :HCl :H2O of 1 :0.017 :
5.854 :162.681. Triblock copolymer poly(ethylene glycol)-
B-poly(propylene glycol)-B-poly(ethylene gylcol), referred
to as P123 (Aldrich, 100%), was used as surfactant template.
Organosilane 3-mercaptopropyltrimethoxysilane (Aldrich,
95%), as the source of thiol groups, was introduced into
the mixture with a MPTMS/(MPTMS+TEOS) molar ratio
ranging from 5% to 20%. A certain amount of P123 was
dissolved in a mixture of water and 2 M hydrochloric acid
(36–38%) aqueous solution with strong stirring at 40 -Cand then TEOS was added dropwise into the mixture,
followed by the addition of MPTMS 4 h later. After stirring
for another 20 h, the mixture was moved into Teflon-lined
autoclaves and aged for 24 h at 100 -C. The product was
filtered and air-dried, followed by surfactant removal by
Soxhlet extraction with ethanol for 24 h. The final material
was obtained after drying at 60 -C atmosphere overnight.
2.2. Materials characterization
The XRD measurement was performed on Rigaku
Dmax/2000 diffractometer with a resolution of 0.02- and
scanning speed of 0.5-/min using Cu Ka radiation. N2
adsorption was measured with Micromeritics ASAP 2020
at �196 -C. Before analysis, the samples were first
degassed at 110 -C for 5 h. The surface area was
calculated according to BET equation at a relative pressure
ranging from 0.05 to 0.20 and the pore size distribution
was obtained from the desorption branch of isotherms
using BJH approach. The pore volume was obtained by
the amount adsorbed at saturated pressure. The morphol-
ogy of mesoporous SBA-15 was observed by transmission
electron microscopy (JEOL JEM-2010). The samples were
dispersed in acetone until a suspension was obtained and a
drop of the suspension was deposited and dried on a Cu
grid. A low-exposure technique was used to reduce the
effect of beam damage and sample drift. A Dupont
thermoanalyzer (1090B) was used for thermogravimetry
analysis at a heating rate of 10 -C/min in air. An elemental
analyzer (Elementar Vario EL) was used for the determi-
nation of sulfur concentration in the extracted products.
The solid state 29Si NMR measurement was performed on
a Bruker AV300 spectrometer operating at a frequency of
59.62 MHz with the following experimental conditions:
magic-angle spinning at 5 kHz; k/2 pulse, 7 As; a
repetition delay of 600 s; 200 scans. The chemical shift
is referenced to tetramethylsilane.
3. Results and discussion
Fig. 1 depicts X-ray diffraction spectra of the functionalized
SBA-15 with different molar ratios of MPTMS/(MPTMS+TEOS).
The region where (110), (200) and (210) peaks appear is scaled 3
or 5 times. All the samples have a single intensive reflection at 2hangle around 0.8- as is the case for typical SBA-15 materials and
the reflection is generally related to a regular pore size and an
ordered pore arrangement [14]. For the pure SBA-15 material, two
additional well-resolved peaks corresponding to the higher order-
ing (110) and (200) reflections are also observed, which is
consistent with the XRD pattern of a well-ordered hexagonal
structure (P6mm). However, the (110) and (200) reflections
decrease gradually in intensity as the relative amount of MPTMS
in the initial mixture increases, which indicates a decrease of the
0.0 0.2 0.4 0.6 0.8 1.00
100
200
300
400
500
600
700
0% 5% 10% 15% 20%
Qua
ntity
ads
orbe
d/cm
3 g-1
ST
P
Relative pressure
Fig. 3. Isotherms of the thiol-functionalized SBA-15 with different molar
ratios of MPTMS/(MPTMS+TEOS).
0 100 200 300 400 500 600 700 800
60
70
80
90
100
(c)
(b)
(a)
Wei
ght l
oss/
%
Temperature/°C
Fig. 5. TGA measurements of different samples. (a) Pure SBA-15 materials
after Soxhlet extraction, (b) pure SBA-15 materials before Soxhlet
extraction, and (c) thiol-functionalized SBA-15 with a MPTMS/
(MPTMS+TEOS) molar ratio of 20%, after Soxhlet extraction.
Q. Wei et al. / Materials Letters 59 (2005) 3611–36153614
higher ordering. A gradual increase of d100 spacing from 9.4 to
11.2 nm and the cell parameter from 10.9 to 12.7 nm is also
observed, as shown in Table 1. This pore structure can be further
confirmed in the transmission electron microscopy images (Fig. 2).
The pure SBA-15 material shows a typical hexagonal pore array
(Fig. 2a). In addition to the hexagonal structure, an additional more
disordered wormhole-motif pore structure, however, is also
observed in the sample with a MPTMS concentration of 20%
(Fig. 2b). The wormhole framework seems to be contrary to the
ordered pore structure as indicated in the XRD results. The
abnormal micrograph in Fig. 2b might be interpreted as the
following: while the smaller particle at the lower left is well aligned
to the electron beam and shows perfect resolution, the larger particle
at the upper right is somewhat tilted and does not give the perfect
resolution in all directions. Our result is in good agreement with the
work of Margolese et al. [13], in which functionalized SBA-15
materials of good quality can be obtained even for relatively high
amounts of mercaptopropyltrimethoxysilane in the reaction mixture
if TEOS is allowed to prehydrolyze for a few hours.
1 10 100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0% 5% 10% 15% 20%
Por
e vo
lum
e/cm
3 g-1
nm-1
Pore diameter/nm
Fig. 4. Pore size distribution of the thiol-functionalized SBA-15 with
different molar ratio of MPTMS/(MPTMS+TEOS).
The isotherms and pore size distribution (PSD) of the
functionalized SBA-15 are shown in Figs. 3 and 4, respectively.
It can be seen from Fig. 3 that all the samples exhibit type IV
isotherms with apparent hysteresis loop, indicative of the
existence of defined mesopores in the frameworks. The adsorbed
amount at the saturated pressure decreases as the concentration of
MPTMS increases, indicating that the more content of MPTMS is
incorporated, the lower porosity may be obtained. The capillary
nitrogen condensation step shifts gradually to lower vapor
pressure with the higher MPTMS content, which is related to
the decrease of the mesopore diameter. The sharp peaks of PSD
(Fig. 4) emerge at a pore diameter ranging from 3.4 to 6.2 nm and
shift to lower pore diameter, dependent on the relative content of
MPTMS, which is consistent with what is implied in the
isotherms. As shown in Table 1, the surface area decreases
dramatically from 733 to 170 m2 g�1 and pore volume from 0.96
to 0.23 cm3 g�1 as the content of MPTMS increases from 0% to
20%. The evolution of pore structure with the addition of thiol
groups can be described as following. The methoxy terminal of
MPTMS may co-condense with TEOS to form inorganic frame-
work and the mercaptopropyl branch may react with the hydro-
philic groups of P123 template via H-bonding to form an
organized structure. Upon removal of P123 template, the
mercaptopropyl chains may rearrange to act as linked organic
functionalities protruding from the inorganic walls into the pore
channels [15,16], thus resulting in the occupation of space inside
the pore and the gradual increase of pore wall thickness. The more
MPTMS content is introduced, the more pore space may be
packed and the smaller pore diameter can be obtained. The
Table 2
Elemental analysis result of the thiol-functionalized SBA-15 with different
molar ratios of MPTMS/(MPTMS+TEOS)
Sample (initial mole percentage) Extracted producta
MPTMS 10% 5.77
MPTMS 20% 13.70
a Mole percentage estimated by elemental analysis.
0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200
Q4
Q3
Q2
T3
T220%
10%
0%
Chemical shift/ppm
Fig. 6. Solid state 29Si MAS NMR of the thiol-functionalized SBA-15 with
different molar ratios of MPTMS/(MPTMS+TEOS).
Q. Wei et al. / Materials Letters 59 (2005) 3611–3615 3615
evolution of pore structure might also result from the co-surfactant
effect of MPTMS, which interacts with the templates and reduces
the diameter of the micelles [17].
The introduction of thiol groups into SBA-15 is confirmed by
the results of TGA and elemental analysis. As seen from Fig. 5,
for the as-prepared pure SBA-15 materials, a weight loss of about
2 wt.% is observed at a temperature lower than 180 -C, which is
due to the evaporation of adsorbed water, and the further weight
loss (about 35 wt.%) at temperatures higher than 180 -C is
attributed to the surfactant decomposition. For the extracted SBA-
15, there is still a weight loss of about 15 wt.% at temperatures
higher than 210 -C, which indicates that Soxhlet treatment cannot
remove all the surfactants. The extracted functionalized samples
lost weights in two steps: one at 215 -C (approximately 10%),
which is corresponding to the removal of the residual copolymer,
and the other at 315 -C (approximately 22–25%), which is due to
the loss of thiol groups. The result of elemental analysis indicates
the existence of thiol groups in the functionalized materials, but
the MPTMS mole percentage estimated by elemental analysis is
only half of the MPTMS content in the initial mixture (Table 2),
which might result from the effect of the surfactants left after the
Soxhlet treatment.
The incorporation of functional group into the mesoporous
SBA-15 materials is further supported by solid state 29Si MAS
NMR spectroscopy (Fig. 6). The three peaks at �110, �102 and
�92 ppm are attributed to silicon groups with different chemical
environments Q4[Si(SiO)4], Q3[Si(SiO)3OH] and Q2[Si(SiO)2(OH)2] in the mesoporous silica matrix, respectively. In addition
to these signals, two additional lines related to T3[Si(SiO)3R] and
T2[Si(SiO)2ROH] groups, where R is referred to as functional
group—C3H7SH, can be observed at the chemical shift of �66 and
�55 ppm, respectively, for the functionalized materials. The
intensity of T groups increases with the concentration of MPTMS
in the initial composition, indicating that the more MPTMS is
added into the mixture, the more functional groups can be
incorporated into mesoporous silica materials. In general, as high
as 20% thiol group has been introduced into the pore surface of
SBA-15, however, the incorporation is completed at the expense of
the integrity of pore structure. Further study is necessarily required
to explore a method to functionalize the SBA-15 material without a
significant loss of porosity.
4. Conclusion
The functional thiol groups were introduced into the pore
surface of SBA-15 silicas by the co-condensation of 3-
mercaptopropyltrimethoxysilane and tetraethyl orthosilicate.
The functionalized SBA-15 materials with an ordered pore
arrangement can be obtained even for as high as 20 mol% 3-
mercaptopropyltrimethoxysilane in the reaction mixture and
the additive leads to an increase of d100 spacing and cell
parameter. The addition of thiol groups also results in a
decrease in both surface area and pore size, as well as pore
volume, of the functionalized SBA-15 silicas. The porosity
decreases dramatically when the 3-mercaptopropyltrime-
thoxysilane concentration reaches 20 mol%, with a loss of
76% for both the surface area and pore volume. In order to
functionalize the SBA-15 silicas without a significant change
of pore structure, the molar concentration of 3-mercapto-
propyltrimethoxysilane should be limited to less than 20%.
Acknowledgements
This work was financially supported by the Doctoral
Foundation of Beijing University of Technology (granted
No.KZ0902200378).
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