Synthesis of large-pore urea-bridged periodic mesoporous organosilicas

Article abstract of DOI:10.1002/asia.200800383

In this article we report the successful synthesis of a new class of periodic mesoporous organosilicas (PMOs) with a urea-bridged organosilica precursor under acid-catalyzed and inorganic-salt-assisted conditions. The large-pore hybrid materials have an o

Full text of DOI:10.1002/asia.200800383

DOI: 10.1002/asia.200800383
Synthesis of Large-Pore Urea-Bridged Periodic Mesoporous Organosilicas
Ling Gao,[a] Feng Wei,[a] Yu Zhou,[a] Xiao Xing Fan,[b] Ying Wang,*[b] and Jian Hua Zhu*[a]
Abstract: In this article we report the successful synthesis of a new class of period-
ic mesoporous organosilicas (PMOs) with a urea-bridged organosilica precursor
under acid-catalyzed and inorganic-salt-assisted conditions. The large-pore hybrid
Keywords: mesoporous materials ·
materials have an ordered mesostructure with uniform pore size distributions, ex-
NMR spectroscopy · organosilicas ·
cellent thick framework walls, thermal stability, and specific functional groups. The
urea derivatives · X-ray diffraction
composition of the mesoporous organosilicas was characterized by FTIR spectros-
copy, 13C cross-polarization magic-angle spinning (CP MAS) NMR spectroscopy,
and 29Si MAS NMR spectroscopy.
Introduction
are still mainly limited to a few types of alkane,[11–13]
alkene,[14,15] phenyl,[16] and aromatic bridging functional
groups in the organosilane precursors.[17,18] Therefore the
synthesis of mesoporous materials by heteroatom-containing
organosilica precursors is rarely reported. Hossain and Mer-
cier had synthesized a ligand group BTMSEN (1,4-bis-[3-
(tirmethoxysilyl)propyl]ethylenediamine) instead of the
more commonly used monosiliylated ethylenediamine, but
when the molar ratio BTMSEN/(BTMSEN+TEOS) in the
organosilica source reached 0.05, the resulting mesostructure
was not well-ordered.[19] Jaroniec et al.[20–22] have incorporat-
ed trimethoxysilyl-functionalized isocyanurate (ICS-Si) into
mesoporous silicas using ICS-Si and tetraethyl orthosilicate
(TEOS) as coprecursors. The highest content of ICS groups
incorporated was less than 25 mol% to obtain well-ordered
mesostructured materials, and the frameworks of such mate-
rials mainly consisted of SiO2. Although the development of
PMOs has led to great interest in materials science owing to
their specific composition and function, the synthesis of
PMOs with a large and flexible organic bridging group in
the mesoporous wall still remains a challenge facing chem-
ists.[23]
Since its discovery in 1999,[1] the surfactant-mediated synthe-
sis of periodic mesoporous organosilicas (PMOs) by hydrol-
ysis and condensation of bridged organosilane (R’O)3-Si-R-
Si-(R’O)3 as precursor has attracted more and more atten-
tion. Unlike the conventional mesoporous silica, not only
the surface properties of PMOs (hydrophilicity/hydropho-
bicity, interaction with guest molecules, etc.) but also their
physical properties, such as stabilities,[2] can be facilely ad-
justed through changing the nature of the organic groups in
the hybrid mesoporous silsesquioxanes.[3–5] Also unlike sur-
face-functionalized mesoporous silica, the organic moieties
in PMOs are built directly in the walls of the channels.[6]
The resulting materials have uniform pores, higher stability,
and the controllable morphologies, thus having a greater po-
tential application as novel catalysts,[7] adsorbents,[8] enzyme
immobilizers, and hosts for nanocluster synthesis.[9]
Although significant advances in the synthesis of PMOs
have been made,[10] up to now the organic spacer R groups
[a] L. Gao, F. Wei, Y. Zhou, J. H. Zhu
Key Laboratory of Mesoscopic Chemistry of MOE
College of Chemistry and Chemistry Engineering
Nanjing University
Herein we report novel PMOs prepared by using a large
organosilicon molecule, N,N’-bis[3-(triethoxysilyl)propyl]-
urea,
(C2H5O)3Si-(CH2)3-NH-CO-NH-(CH2)3-SiACTHUNGTREN(NUG OC2H5)3
(Mw =460), which was synthesized by ourselves and is de-
noted as A-I, as a new organosilica precursor incorporated
into the silica framework. As depicted in Scheme 1, the core
of this bridging group is a urea group that has been integrat-
ed with two triethoxysilyl groups through flexible propyl
chains by a cross-linking reaction. Since it is possible to in-
troduce a high percentage of these chelating groups into the
Nanjing 210093 (China)
Fax : (+86)25-83317761
E-mail: jhzhu@nju.edu.cn
[b] X. X. Fan, Y. Wang
Eco-materials and Renewable Energy Research Center (ERERC)
Nanjing University
Nanjing 210093 (China)
E-mail: wangy@nju.edu.cn
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Scheme 1. The structure of A-I.
silica framework through the synthesis, the resulting materi-
als are expected to have high affinity toward heavy-metal
ions, thus making this kind of PMO materials very attrac-
tive.[21]
Results and Discussion
Structure, Porosity, and Molecular Ordering
Figure 1 depicts the powder X-ray diffraction patterns of the
surfactant-extracted samples prepared with different propor-
tions of A-I in silica source and named as AIn, where n is
the silicon molar percent of 2A-I/(2A-I+TEOS) in the ini-
tial sol mixture. Three peaks were observed on the patterns
of AI0, AI10, and AI30 samples, which could be indexed as
the (100), (110), and (200) reflections of a hexagonal sym-
metry lattice. Samples AI40 and AI50 also had the (100) dif-
fraction peak, though the other two peaks were weakened.
These results indicate the presence of a periodic arrange-
ment of channels in a hexagonal geometry.[1] Nonetheless,
the intensities of the peaks decline as the ratio of A-I to
TEOS in the silica source increases. XRD patterns of AI70
exhibit a broad diffraction peak while no diffraction peak
can be observed when the organosilica precursor A-I is used
as the exclusive silica source. Besides, when the concentra-
tion of A-I increases in synthetic raw materials, the first dif-
fraction peak of the resulting composite gradually shifts to a
lower-angle region, implying the expansion of the unit cell,
as seen in Table 1. The intense (100) peak also shows the
Figure 1. Low-angle XRD patterns of the solvent-extracted AIn samples.
Table 1. Physicochemical properties of urea-bridged mesoporous organo-
silica synthesized with different A-I/TEOS ratio.
[a]
[b]
[c]
[d]
[e]
Sample d100
[nm]
a0
[nm]
SBET
[m2 gꢀ1
Vp
DBJH
[nm]
Wall thick-
ness[f] [nm]
]
[cm3 gꢀ1
]
PMS
AI10
AI30
AI50
AI10-c
AI30-c
AI50-c
9.87
10.16
10.59
11.49
8.59
11.40 880
11.73 572
12.22 502
13.27 265
9.91 491
8.94 472
9.17 157
1.146
0.800
0.693
0.403
0.758
0.550
0.242
7.24
7.16
7.19
7.21
7.28
5.34
6.15
4.16
4.57
5.03
6.06
2.63
3.60
3.02
7.74
7.94
p
[a] d100 is the dACTHUNGENRT(NUG 100) spacing. [b] a0 is the cell parameter a0 =2d100/ 3).
[c] SBET is the BET surface area. [d] Vp is the total pore volume. [e] Pore
diameter DBJH is obtained from the peak positions of the BJH pore size
distribution curves calculated from the adsorption branches of the iso-
therms. [f] Wall thickness=a0ꢀDBJH
.
Abstract in Chinese:
larger lattice spacing corresponding to the 2D hexagonal
unit cell parameter a0.
TEM images of AI30 (Figure 3) clearly show hexagonal
sets of lattice fringes as well as parallel fringes, which corre-
spond to side-on projections of the hexagonal mesostruc-
ture, confirming the presence of a 2D hexagonal mesopo-
rous structure in AI30 samples. Furthermore, the pore
center-to-center distance of about 10 nm correlates well with
the lattice parameter observed in the XRD measurements.
Figure 2 provides a comparison of nitrogen adsorption
isotherms for the AIn series of PMOs. For composites AIn
(n=10, 30, 50) and the PMS (the periodic mesoporous silica
SBA-15 synthesized for comparison), these isotherms are
type IV with steep capillary condensation/evaporation steps
and H1 hysteresis loops, characteristic of SBA-15-type mes-
oporous silica. Table 1 lists the BET specific surface area,
volume of fine pores (i.e., pores below 4 nm), volume of pri-
mary mesopores, and mesopore diameter of PMS and its
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Chem. Asian J. 2009, 4, 587 – 593

Large-Pore Urea-Bridged Periodic Mesoporous Organosilicas
Figure 3. TEM images of the surfactant-free AI30 material.
lary condensation/evaporation step in the isotherm of the
AI50 sample becomes less sharp, and the desorption branch
shows a slight tail extending toward a lower relative pres-
sure of P/P0 =0.45, which indicates a gradual deterioration
of the mesoporous structure. This is expected because of the
geometrical constrictions to accommodate a large number
of bulky A-I bridging groups in the mesopore walls. More-
over, adsorption isotherms of the AI30 and AI50 samples
do not level off at relative pressures above 0.8, but instead
rise, and this rising intensifies with increasing concentration
of A-I, which reflects a gradual development of textural po-
rosity, that is, secondary disordered mesopores. Note that
the pore size distribution of AI50 contains a broader pore
range indicating the presence of newly formed larger meso-
pores owing to the penetration of some organic groups into
the silica wall.[26] A-I has a large molecular size, and the
bulk organic groups occupy more space of the mesopore
walls in the case that a large amount of the guest is incorpo-
rated in the composite, which no doubt results in the reduc-
tion of pore volume (see Table 1). The wall thickness was
calculated by subtracting the BJH pore diameter from the
lattice constant determined in XRD tests, and the growing
wall thickness of the AIn sample may be due to the fact
that a large amount of the organic moiety is exposed on the
pore wall. A confirmation of this inference comes from
PMS, which has the thinnest walls (4.14 nm) because it is
self-assembled through charge matching between P123 and
a silicate species. Based on these results, it is safe to con-
clude that precursors with more organic content generally
form thicker walls, so as to counterbalance charge owing to
their inherently lower charge-to-volume ratios.[27]
Figure 4 presents the SEM images of the AI30 sample. As
is evident in Figure 4a, the PMO sample of AI30 mainly
consists of monodisperse nanoparticles with relatively uni-
form particle size distributions of about 0.4 mm (mean sizes).
At high magnification (Figure 4b), it is seen that the surface
of cubic or short cylindrical particles are more corrugated.
Many factors, such as the pH value, the surfactant/silica
ratio, the hydrolysis, and the condensation rate of silica pre-
cursor[28,23] and hydrophilicity/hydrophobicity of the organo-
silanes are always the important elements in controlling the
curvature of mesoporous silica. In this study, the trend of
developing nanoparticle morphology is believed to be simi-
lar to those reported by Huh et al.[29] They investigated orga-
Figure 2. N2 adsorption–desorption isotherm (a; solid symbols: adsorp-
tion data; open symbols: desorption data) and the pore size distribution
(b) of AIn samples.
hybrid analogues. The corresponding pore size distributions
of AI30 and AI10 (Figure 2b), evaluated by the BJH
method, are as narrow as that of PMS, indicating high uni-
formity of ordered mesopores in these composites.[24] The
TEM image of MI30 further confirms the hexagonal ar-
rangement of the mesopore throughout the sample
(Figure 3), which is consistent with the results of XRD and
N2 sorption isotherms. As seen from Figure 2, the sorption
isotherm and the corresponding pore size distribution
change gradually for the PMO samples from PMS to AI50.
The BET surface area and pore volume decrease with in-
creasing concentration of the A-I bridging groups, while the
pore sizes do not change much (about 7.2 nm), indicating
the presence of uniform mesopores.[25] In addition, the capil-
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589

J. H. Zhu et al.
FULL PAPERS
surfactant residues, and their weak intensity indicates the re-
moval of most surfactant by EtOH extraction.
Figure 6 shows the 29Si magic-angle spinning (MAS) and
13C cross-polarization MAS (CP MAS) solid-state NMR
spectra of AI30. The 29Si NMR spectrum of AI30 shows
both Q and T sites, as expected. In the range between ꢀ90
and ꢀ150 ppm, the prominent signal at ꢀ107.1 ppm and two
Figure 4. SEM images of the surfactant-free AI30 PMO material; a) low
magnification; b) enlargement showing shape of the particles.
nosilanes possessing different hydrophilic groups and con-
cluded that organosilanes with hydrophilic groups were not
favored by the interaction with the surfactant molecules, in-
hibiting the formation of long micelles and reducing their
tendency toward side-on condensation.[29] Thus, the amount
of hydrophilic precursor introduced in the synthesis may be
enough to disturb the side-on growth mechanism. The A-I
organosilica precursor used here has a hydrophilic urea
group, so the synthesis follows the mechanism to give small
nanoparticles.
Composition Information
Figure 5 displays the FTIR spectra of the AIn samples. The
vibration peaks at 2984, 2928, and 2875 cmꢀ1 are assigned to
the CH stretching vibrations of -CH2CH2CH2- groups, and
the band at 1660 cmꢀ1 is attributed to the C=O stretching vi-
bration of the urea group.[30] The peaks at 1495 cmꢀ1 togeth-
er with 692 cmꢀ1 result from the presence of the bending vi-
ꢀ1
ꢀ
bration of N H, while the band at 1442 cm is the charac-
Figure 6. Solid-state 13C CP MAS NMR (a) and 29Si MAS NMR (b) spec-
tra of surfactant-free AI30 sample.
teristic of an -N-C- group. As evidenced in Figure 5, the in-
tensity of 1660–1456 cmꢀ1 bands increases gradually from
the AI10 to the AI50 sample, which signifies the increased
concentration of urea groups in the resultant materials. The
weak signals at 1348 and 1377 cmꢀ1 originate from the P123
shoulders at ꢀ100.3 and ꢀ91.5 ppm can be assigned to the
resonances of Si
G
ACHTUNGTRENNUNG(OSi)3
(3Q, d=ꢀ100.3 ppm), and (OH)2Si
(OSi)2 (2Q, d=
ACHTUNGTRENNUNG
ꢀ91.5 ppm), implying a high degree of TEOS cross-linking
under the employed synthetic conditions. The signals origi-
nating from silicon bridged by the organic group can be
found in the range ꢀ45 to ꢀ90 ppm, thus confirming the in-
corporation of organic groups inside the framework.[31–33]
The signals at ꢀ61.3 and ꢀ65.7 ppm can be assigned to the
silicon
resonances
of
partially
condensed
2T
3
[(SiO)2(OH)SiC] and fully condensed T [(SiO)3SiC] frame-
work sites, respectively,[34] confirming both the presence of
organic moieties inside the silica framework and the high
degree of condensation of the silanol groups. Based on the
n
normalized peak areas, the ratio of T/
N
to be about 0.28, which is consistent with the A-I concentra-
tion in the initial gel mixture. The resonances at 9.5, 22.7,
42.2, and 160.5 ppm in the 13C CP MAS NMR spectrum of
1
2
3
4
AI30 can be attributed to the C, C, C, and C carbon spe-
2
3
2
1
Figure 5. FTIR spectra of the surfactant-free AIn PMO materials.
cies of Si-1CH2 CH2 CH2-NH-4CO-NH-3CH2 CH2 CH2-Si,[29]
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Large-Pore Urea-Bridged Periodic Mesoporous Organosilicas
confirming the maintenance of the organic groups during
the synthesis and extraction processes.
sharp endothermic peak emerges (Figure 7a). If the test is
carried out in nitrogen, the organic groups begin to decom-
pose when the temperature is high enough, so it shows an
evidently exothermic peak in the DSC curves (Figure 7b).
The thermogravimetric analysis is employed to indicate
whether the material has high thermal stability; hence if the
analysis performed in air or nitrogen stream, the result will
be more comprehensive. These results indicate the surfac-
tant-extracted AI50 sample exhibited an excellent thermal
stability without mass loss up to 573 K both in a flow of air
and N2 (Figure 7b).
Thermal and Hydrothermal Stability
Figure 7 provides the thermogravimetry and differential
scanning calorimetry (TG-DCS) curves of the AI50 sample
in air as well as in a nitrogen flow. A weight loss of less than
5% was generally observed below 373 K, caused by desorp-
To examine the effect of thermal degradation of urea and
propyl bridging groups on the mesostructure, a series of ex-
tracted samples (AI10, AI30, and AI50) was calcined in air
at 823 K for 5 h and then investigated by powder X-ray dif-
fraction and nitrogen sorption (Figure 8). As demonstrated
in the figure, thermal removal of organic bridging groups
did not lead to the collapse of the mesoporous structure and
even retained its ordered nature; however, a substantial re-
duction in the pore volume, pore width, and surface area
was observed (Table 1), which is not surprising as a result of
the structure shrinkage and a complete elimination of bulky
organic bridging groups from the PMO framework caused
by calcination of the sample in a flow of air.[31] The high sta-
bility of AIn PMO mesoporous materials is very important
for their practical application.
Conclusions
Urea-bridged mesoporous organosilicas have been success-
fully synthesized by a one-step co-condensation of TEOS
and A-I precursor. The resulting PMOs composites have
well-ordered 2D hexagonal mesoporous structure, thick
framework walls, cubic morphology of nanoparticles, and ex-
cellent thermally stability. Incorporation of organic groups
in the pore wall of mesoporous organosilica is fully charac-
terized by FTIR and solid-state NMR techniques. Through
this new synthetic route it is feasible to introduce specific
functional groups with high percentage into the silica frame-
work. The resulting hybrid materials are expected to have
various new potential applications than the conventional sili-
ceous mesoporous materials.
Figure 7. TG-DSC analysis of the surfactant-free AI50 composite in (a)
air and (b) nitrogen flow.
tion of physisorbed and chemisorbed water and ethanol, and
by condensation of terminal silanol or ethoxysilane groups.
In air, the urea and propyl species stay in the mesoporous
network up to 573 K. The lack of any substantial weight loss
in the range of 473–623 K indicated the successful removal
of surfactant through HCl/ethanol solution extraction (Fig-
ure 7a).[21] In nitrogen, at 573 K a sharp decomposition step
occurred, aside of the desorption of physisorbed water be-
tween 293 K and 373 K. It is clear that the TG profiles are
analogous but the DSC profiles are totally different, similar
to what was reported in literature.[31] This phenomenon is
caused by the different atmosphere. If the experiment is per-
formed in air, the peak in the DSC curves is caused by the
reaction of organic groups and oxygen in the air; therefore a
Experimental Section
Chemicals and Reagents: All reagents were analytical grade and used as
purchased without further purification. Triblock copolymer poly(ethyle-
neoxide)-poly(propyleneoxide)-poly(ethyleneoxide)
((EO)20(PO)70(EO)20, Pluronic 123; MW, 5800) was purchased from
Sigma–Aldrich (USA). Tetraethyl orthosilicate (TEOS), (3-isocyanato-
propyl)triethoxysilane (IPTES), (aminopropyl)triethoxysilane (APTES),
and other reagents were obtained from Nanjing Chemical Reagent Com-
pany (China).
Preparation of bridge molecules (A-I) with two kinds of cross-linking re-
agents: A typical procedure for the preparation of A-I was as follows:
IPTES (4.95 g, 20 mmol) was first dissolved in acetone by stirring, and
APTES (4.40 g, 20 mmol) was then added to the solution dropwise. The
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mesoporous silica was also synthesized by the same procedure with
TEOS as the exclusive silica source, and named as PMS.
Characterization: The 1H NMR experiment was performed on a set of
Bruker DPX-300 spectrometers at 298 K, and CDCl3 was used as the sol-
vent. All chemical shifts were reported in the standard d notation of
ppm; positive chemical shifts were denoted to higher frequency from the
given reference. X-ray diffraction (XRD) patterns of samples were re-
corded on an ARLXTRA diffractometer with CuKa radiation in the 2q
ranges from 0.58 to 58 or from 58 to 808. TEM analysis was performed on
a FEI Tecnai G2 20 S-TWIN electron microscope operating at 200 kV.
The nitrogen adsorption–desorption experiments were performed at 77 K
on
a Micromeritics ASAP 2020 system. Prior to measurement, the
sample was degassed at 373 K for at least 4 h. The BET specific surface
areas were calculated using adsorption data in a relative pressure range
of P/P0 =0.04–0.2. The pore size distribution curves were derived from
the adsorption branch using the Barrett–Joyner–Halenda (BJH) method,
while the total pore volumes were estimated from the amounts adsorbed
at a relative pressure (P/P0) of 0.99. The FTIR spectrum was obtained on
a Bruker Vector 22 FTIR spectrometer, and the sample was mixed with
KBr at a ratio of 3:97 (w/w) and then pressed as a thin disk for test. Ther-
mogravimetric and differential scanning calorimetric analysis (TG-DSC)
of the extracted samples was performed on a NETZSCH STA 449C ap-
paratus in air or nitrogen from 293 to 1273 K with a heating rate of
10 Kminꢀ1. Particle morphology of these materials was determined by
scanning electron microscopy (SEM) using a Hitachi S4800 FE-SEM
system with 10 kV accelerating voltage and 0.005 nA of beam current for
imaging. The solid-state 29Si MAS NMR and 13C CP MAS NMR spectra
were collected on a Bruker AVANCE-300 NMR spectrometer with the
following experimental parameters: for 13C CP MAS NMR experiments,
7 kHz spin rate, 1 s pulse delay, 3.8 ms contact time, 10000 scans; for 29Si
MAS NMR experiments, 7 kHz spin rate, 10 s pulse delay, 3000 scans.
Tetramethylsilane was used as a reference.
Acknowledgements
Financial support from the National Basic Research Program of China
(2007CB613301) along with Grant 2008AA06Z327 from the 863 Program
of the Ministry of Science and Technology of China, NSF of China
(20773061, 20873059, and 20871067) and from the scientific research
foundation of graduate school and Analysis Center of Nanjing University
is gratefully acknowledged.
Figure 8. Low-angle XRD patterns (a) and N2 adsorption–desorption iso-
therm (b; solid symbols: adsorption data; open symbols: desorption data)
of the calcined AIn (n=10, 30, 50) composites.
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Synthesis of urea-bridged PMO: Pluronic P123 (2.0 g, 0.35 mmol) was
dissolved in a solution of 80.0 g of 1 n HCl and stirred for 24 h at room
temperature. Sodium chloride (9.36 g, 0.16 mol) was added, and the solu-
tion was continually stirred for another 6 h. A pre-prepared mixture of
A-I and TEOS was then added to the above solution under vigorous stir-
ring. After a few minutes, a pale-yellow precipitate began to form. The
reaction mixture was stirred for 24 h, and the suspension was aged at
373 K for another 24 h under static conditions.
A fine pale-yellow
powder was collected by filtration and was washed with water. Templates
were removed from the as-synthesized materials by refluxing 1.0 g of the
sample in 200 mL of ethanol containing 1.5 g of concentrated aqueous
HCl solution for 24 h. The resulting materials are denoted as AIn (n=10,
30, 40, 50, 70, and 100, where n is the silicon molar percent of 2A-I/[2A-
I+TEOS] in the initial sol mixture). For comparison, SBA-15 periodic
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Received: October 7, 2008
Revised: November 24, 2008
Published online: January 7, 2009
Chem. Asian J. 2009, 4, 587 – 593
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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