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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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