General synthesis of mesoporous spheres of metal oxides and phosphates

Article abstract of DOI:10.1021/ja029964b

Monodisperse and high-surface-area mesoporous inorganic spheres of various compositions including metal oxides, mixed oxides, and metal phosphates are prepared by templating mesoporous carbon spheres which are replicated from spherical mesoporous silica.

Full text of DOI:10.1021/ja029964b

Published on Web 04/04/2003
General Synthesis of Mesoporous Spheres of Metal Oxides and Phosphates
Angang Dong, Nan Ren, Yi Tang,* Yajun Wang, Yahong Zhang, Weiming Hua, and Zi Gao
Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Department of Chemistry,
Fudan UniVersity, Shanghai 200433, P. R. China
Received December 30, 2002; E-mail: ytang@fudan.ac.cn
Table 1. Precursors and Properties of the Representative
Mesoporous Inorganic Spheres
Mesoporous materials with variable compositions, controllable
morphologies, and tunable wall structures are of particular interest
for catalysis, adsorption, and applications as hosts for nanomaterials
synthesis.1-9 Recently, the preparation of spherical mesoporous
particles in the micrometer- and submicrometer-size range has
received much attention because of their broad applications in
chromatography, bioseparation, and nanotechnology.4-8 On the basis
of a modified Sto¨ber reaction process, monodisperse mesoporous
SiO2 spheres with tunable particle size have been synthesized by
Unger’s group.4a However, up to now, there are still no general
methods to prepare mesoporous spheres built of nonsiliceous
inorganic materials.5-8
Recently, ordered mesoporous carbon, which is an inverse replica
templated from mesoporous silica by a nanocasting route,9 has
attracted increasing attention, due to its potential applications as
catalyst supports, adsorbents, and other advanced materials.9b More
recently, mesoporous carbon such as CMK-39c has been successfully
employed as further template to achieve the reversible replication
of ordered mesoporous silica.10 Additionally, some mesoporous
oxides have also been templated from activated carbons.11 These
may open a new possibility to synthesize mesoporous materials
for the rigid framework of carbon templates different from the
commonly used surfactant templates.1-5
d
e
wall structure
(by XRD)
SBET
(m /g)
w
V
samplea
precursorb
TIP
(nm)
(cm /g)
2
3
TiO2
anatase
158
143
96
442
455
117
228
474
6
7
6
4
4
4
5
5
0.30
0.34
0.14
0.61
0.33
0.14
0.32
0.60
TiO2
TIP
anatase/rutile
ZrO2
ZrCl4
T/Mc
Al2O3
Ti2Si3Oy
Ti2ZrOy
ZrP
Al(OsBu)3
TIP/APS
TIP/ ZrCl4
ZrCl4/TEP
AlCl3/TEP
γ-Al2O3
amorphous
amorphous
amorphous
amorphous
AlP
a ZrP and AlP are zirconium phosphate and aluminum phosphate,
respectively. b TIP, APS, and TEP are titanium isopropoxide, (3-amino-
propyl)triethoxysilane, and triethylphosphate, respectively. c T and M are
the tetragonal and monoclinic phases of solid mesoporous ZrO2 spheres,
respectively. d w is the pore size calculated from nitrogen data based on
the BJH model of the desorption branch. e V is the pore volume.
Here, we present a versatile procedure based on the two-step
nanocasting route to prepare monodisperse and high-surface-area
mesoporous spheres of tailored compositions, as well as controllable
crystalline phases and defined morphologies. To begin, uniform
mesoporous silica spheres prepared by Unger’s method4a were used
as the template for the formation of mesoporous carbon spheres
by the nanocasting technique.9a Mesoporous inorganic spheres were
then obtained by templating the specific precursor with carbon
spheres. Typically, the pore system of carbon template was
infiltrated with the desired precursor such as alkoxides or inorganic
salts (dissolved in organic solvent) by wet impregnation. After
removing the redundant precursor by centrifugation, the hydrolysis
of precursor was initiated by exposure to the moisture in air. The
mesoporous inorganic spheres were finally obtained by crystalliza-
tion or polymerization of inorganic species at 600 °C (or 950 °C
for rutile-type titania formation) in nitrogen for 6 h, coupled with
removal of carbon at 550 °C in air for 8 h. The carbon content
after calcination was ca. 0.7%, determined by elemental analysis.
Because of the excellent structure-directing ability of carbon
template resulting from its rigid framework and high chemical
stability, various mesoporous spheres including single- and multi-
component metal oxides and phosphates can be easily prepared by
choosing appropriate precursors, as we have done here (Table 1).
Another important feature of carbon is its high thermal stability in
inert atmosphere; therefore, not only can inorganic species be highly
crystallized or polymerized but their crystalline phases may also
be tailored by controlling crystallization temperatures. In addition,
it is interesting to find that either solid or hollow spheres can be
Figure 1. TEM images and ED patterns (inset) of mesoporous anatase-
type (a) and anatase/rutile-type (b) TiO2 spheres.
produced in some cases by changing the polarity of the precursor
to match the hydrophobic character of carbon.
The transmission electron microscopy (TEM) image of meso-
porous anatase-type TiO2 spheres (Figure 1a) prepared from
titanium isopropoxide shows that the discrete and uniform spherical
morphology and the disordered pore structure of the original SiO2
spheres4a are well preserved in the final product after two steps of
replication. The TiO2 sphere size (500-600 nm) is somewhat
smaller than that of carbon spheres (800-900 nm), resulting from
the shrinkage after the removal of carbon. The electron diffraction
(ED) pattern (Figure 1a, inset) verifies that the pore walls of TiO2
spheres are composed of crystalline anatase, which is further
confirmed by X-ray diffraction (XRD) analysis. The size of anatase
nanocrystals is determined as 10 nm by TEM at high magnification.
If the crystalline temperature of titania was increased to 950 °C in
nitrogen, mesoporous anatase/rutile bicrystalline TiO2 spheres
(Figure 1b) were generated after the removal of carbon. The weight
fraction of rutile phase calculated from the XRD integrated
intensities of rutile (110) and anatase (101) peaks12 is 60%. The
shape of the N2 adsorption/desorption isotherms of both anatase-
and anatase/rutile-type TiO2 spheres present the characters of their
mesoporosity, and the Brunauer-Emmett-Teller (BET) surface
areas are both at about 150 m2/g.
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10.1021/ja029964b CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. TEM images of solid (a) and hollow (b) mesoporous ZrO2
spheres.
Figure 4. TEM image (a) and 31P NMR spectrum (b) of mesoporous
zirconium phosphate spheres.
source, respectively. The 31P NMR spectrum of mesoporous ZrP
spheres in Figure 4b shows two peaks at -20.7 and -25.2 ppm,
corresponding to tetrahedral phosphorus of connectivity 3 and 4
[P(OZr)3OH and P(OZr)4], respectively. This result is in agreement
with the previously reported data of mesoporous ZrP after
calcination.3a
In conclusion, we have adopted a two-step nanocasting route to
prepare monodisperse mesoporous inorganic spheres with pore walls
of variable compositions and controllable crystalline phases. In
addition, hollow mesoporous spheres can be obtained in some cases,
depending on the polarity of the precursor. These novel mesoporous
inorganic spheres are expected to have a variety of applications in
chromatography, catalysis, and nanotechnology.
Figure 3. (a) TEM image and ED pattern (inset) and (b) N2 adsorption/
desorption isotherms of mesoporous γ-Al2O3 spheres.
The polarity of the precursor had a great effect on the final
product morphologies due to the hydrophobic nature of carbon
template. For example, in the preparation of mesoporous ZrO2
spheres, when ZrCl4 was dissolved in a mixture of ethanol and
cyclohexane (1:1, v/v) as the initial precursor, the resulting ZrO2
spheres exhibited a solid structure (Figure 2a) similar to that of
TiO2 spheres, whereas using the ethanol solution of ZrCl4 as
precursor, only hollow mesoporous ZrO2 spheres (Figure 2b) were
obtained. This result indicated that it was difficult to infiltrate the
inner parts of the carbon spheres with the ethanol solution of ZrCl4,
which was somewhat hydrophilic, resulting in hollow ZrO2 spheres
after calcination. On the contrary, the precursor became very
hydrophobic with the addition of cyclohexane and therefore could
fill all the parts of carbon template to lead to solid ZrO2 spheres.
Different from TiO2 and ZrO2 spheres which are both composed
of particulate nanocrystals, mesoporous Al2O3 spheres (Figure 3a)
prepared from aluminum sec-butoxide are composed of interesting
lath-shaped nanoparticles. The XRD pattern exhibits reflections
consistent with γ-Al2O3. A similar phenomenon was also found
by Zhang et al.2b in the preparation of mesoporous MSU-γ Al2O3
by hydrolysis of aluminum salts in the presence of nonionic
surfactant. The N2 adsorption/desorption isotherms in Figure 3b
indicate the mesoporosity of γ-Al2O3 spheres, although their pore
size distribution is broader than that of TiO2 spheres, probably due
to their special nanoparticle morphology. The BET surface area of
γ-Al2O3 spheres is 442 m2/g, much larger than the reported data of
both conventional γ-Al2O3 (185-250 m2/g) and MSU-γ Al2O3
(299-370 m2/g).2b
Acknowledgment. This work is supported by the NSFC
(20273016, 20233030), the SNPC (0249 nm028), and the Major
State Basic Research Development Program (2000077500).
Supporting Information Available: Figure S1, S2, S3, S4, S5,
and S6 (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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In addition to single-component, this two-step procedure can be
readily extendible to multicomponent mesoporous oxides and metal
phosphates using the homogeneously mixed precursors (Table 1).
The obtained mesoporous spheres of mixed oxides are generally
amorphous, indicating that the two components are distributed
uniformly in the wall.2a Figure 4a shows the TEM image of
mesoporous zirconium phosphate (ZrP) spheres prepared using
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