Mesoporous zirconia obtained with anionic templates

Article abstract of DOI:10.1039/a607438c

Mesoporous zirconia is synthesized with anionic templates.

Full text of DOI:10.1039/a607438c

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Mesoporous zirconia obtained with anionic templates
a
a
c
b
a
G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J. J. Fripiat*
a
Department of Chemistry and Laboratory for Surface Studies, and b Advanced Analysis Facility, University of
Wisconsin-Milwaukee, PO Box 413, Milwaukee, WI 53211, USA
c
Materials Department, Instituto Polytechnico Nacional, Zacatenco, Lindavista, CP 07300, Mexico, DF, Mexico
Mesoporous zirconia is synthesized with anionic templates.
elimination of this category of surfactants could be easier. The
preparation was as follows. The source of zirconium was a 70%
m/m solution of zirconium propoxide in propanol. The two
The success of Mobil scientists in the surfactant-assisted
1,2
synthesis of mesoporous molecular sieves with hexagonal or
cubic ordering of the pore system and high temperature stability
has been followed by an enormous amount of research
worldwide. An array of cationic surfactants (such as quaternary
alkylammonium) with chains containing, typically, twelve
carbon atoms interact with an inorganic anionic or cationic
oligomer (or polymer) to form a solid in which small-angle
X-ray scattering reveals the existence of an ordered system of
pores.
‘straight alkyl chain’ surfactants were: C12
H25OP(OH)
2
(M = 266), abbreviated P12 and C12 25OSO Na (M = 288),
H
3
abbreviated S12, respectively. The P12 solution was left at pH
= 3, namely, the original pH of the solution. The initial pH of
the S12 solution was adjusted by HCl at about 3. The propanolic
zirconium alkoxide solution was added slowly to the surfactant
solution until the molar ratio S/Zr was 0.16. The mixture was
transferred into a hydrothermal bomb where it was heated
between 90 and 120 °C for about 12 h. The products of the
reaction, called, in short, either P12 or S12 solids, were dried at
140 °C or calcined at 500 °C.
The basic principles of these syntheses have been reviewed
3
by Huo et al. who have distinguished the interaction between
cationic or anionic surfactant with anionic or cationic metal
oligomers on the basis of studies involving a large variety of
metals other than silicon. Three kinds of structures have been
characterized by their X-ray patterns, namely, hexagonal,
lamellar and cubic. A high spacing reflection is easily observed
in the range 2q 2–4°, whereas the other Bragg reflections, on
which the differentiation between the structures is based, are
more difficult to observe because of their intrinsic lower
intensity and the broadening due to disorder. Removal of the
surfactant does not necessarily keep the hexagonal or cubic
structure intact, as desirable for producing high-area solids.
Low-angle X-ray scattering in the range 2q 0.8–10° was
obtained with a Scintag Q–Q X-ray diffractometer (Cu-Ka)
fitted with a solid-state detector and the following set of slits:
0.5, 0.3, 0.2 and 0.07°. For medium-angle X-ray scattering
(2–70°), the slits were 3, 2, 0.5 and 0.3°. Fig. 1 shows a
diffraction line corresponding to spacing at about 47 Å after
drying at 140 °C. After calcination at 500 °C, the spacing
generally increases, but sometimes, as observed for the S12
solid, the ordering disappears.
2
The N adsorption–desorption isotherms were obtained with
an automated physisorption instrument operated in a static
mode (Omnisorp 100, Coulter Co.). All measurements were
performed after outgassing at 150 °C under vacuum down to a
4
5
The incorporation of titanium and of zirconium in hexago-
nal mesoporous silica has been proposed to circumvent the lack
of stability of mesoporous titania or zirconia. However,
hexagonally packed transition-metal mesoporous molecular
sieves of pure niobium, titanium or tantalum oxide have been
synthesized with octadecylamine by Antonelli and Ying.6 The
preparation of pure mesoporous zirconia with surface area of ca.
2
4
residual pressure better than 10 Torr. During calcination in air
at 500 °C, all organics were burned as proved by the analysis of
residual carbon (Table 1). The typical shape expected for
mesoporous solids is observed in Fig. 2(a), namely, for the S12
and P12 solids dried at 140 °C. The t-plot specific surface area
,7
2
21
2
3
00 m g at 500 °C has been achieved by Knowles and
is in excess of 500 m . A broad maximum in the porosity
8
Hudson using tetramethyl alkylammonium surfactants and
zirconyl chloride. Starting from Zr sulfate or propoxide,
Sch u¨ th9 and Ciesla et al. have obtained a hexagonal
mesoporous zirconia of a quality corresponding to that of the
silica analogue using alkyl tetramethylammonium surfactants
as templates. Reddy and Sayari11 obtained either hexagonal or
lamellar phases of templated zirconia using either quaternary
distribution function dV/dr vs. r, the radius of the pore, is
observed at 2r ca. 35 Å. As shown in Table 1, for these solids
over 70% of the porous volume is in pores with diameters
between 10 and 100 Å, even in P12 calcined at 500 °C, in spite
of a ca. 50% loss in mesoporosity [Fig. 2(b)]. The qualitative
agreement between the small-angle X-ray scattering data is
10
ammonium surfactants or acidified primary alkylamine, re-
spectively. Sachtler and coworkers12 have used a primary
(a)
(b)
acidified (C16) alkylamine. The ordering of the pores was far
from perfect but the sulfation of the ‘as-synthesized precursor’
remarkably stabilizes the structure. After sulfation the X-ray
diagram characteristic of tetragonal ZrO
00 °C.
Except in this latter work, the formation of structurally
ordered’ walls has not drawn much attention. Since the stability
2
appears at about
6
‘
47
appears to be driven by particle size,1
3,14
the
of tetragonal ZrO
2
onset of crystallization of the wall in the monoclinic phase could
give information on the thickness of the walls between pores. At
54
5
00 °C, particles with diameters > 16 nm undergo the tetragonal
2
4
6
8
10
θ / °
2
4
6
8
10
15
to monoclinic transformation.
2
Here, we report on the synthesis of mesoporous zirconia
using two anionic surfactants. As far as we know such synthesis
has not been achieved up to now and it may be that the
Fig. 1 Small-angle XRD reflections for the P12 (—) and S12 (---) solids (a)
dried at 140 °C or (b) calcined at 500 °C
Chem. Commun., 1997
491

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Table 1 Chemical and physical characteristics of mesoporous zirconias
obtained from templating with P12 or S12 surfactants. The solids are dried at
1
40 °C or calcined at 500 °C. S/Zr: surfactant to zirconium molar ratio.
21
2
Porous volume (ml liquid N g ) in pores with diameter, f < 10 Å or
between 10 and 100 Å. N
2
desorption branch is used
Pore volume
%
Pore volume
S/Zr
f < 10 10 @ f @ 100 10 @ f @ 100
P12, 140
S12, 140
P12, 500
S12, 500
0.12
0.13
0
0.007
0.028
0.006
0.001
0.538
0.577
0.225
0.093
89.0
74.2
81.3
37.5
0
4
3
2
00
00
00
(
a)
150
(b)
Fig. 4 HRTEM picture of mesoporous zirconia obtained with P12
template
100
and S12 solids calcined at 500 °C provides some insight into the
sintering observed for S12, since in this sample the phase
transformation from a tetragonal to monoclinic structure is
evident while P12 is at the onset of crystallization. The thickness
of the walls between pseudo-hexagons increases from approx-
imately 10 Å to ca. 20 Å (HRTEM). 31P NMR spectroscopy
shows that the phosphate remains in the solid after calcination
at 500 °C.
50
V
100
0
0
0
.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
P / Po
Scanning electron microscopy revealed that P12 and S12
consist mainly of a collection of polygonal particles, of diameter
4–10 mm. Spherical particles in this range of dimensions appear
upon sintering S12 at 500 °C.
Fig. 2 N
2
adsorption–desorption isotherms for mesoporous zirconia
obtained from templating with P12 (—) and S12(---); (a) samples dried at
40 °C or (b) samples calcined at 500 °C
1
Thus, it may be concluded that mesoporous zirconia can be
prepared by hydrolysing the zirconium propoxide in the
presence of anionic surfactants (alkyl-phosphate and -sulfate)
and subsequent ageing.
(
a)
0.294 nm
(b)
The partial support of DOE grant DE-FG02-90 ER1430 is
gratefully acknowledged.
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(
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Received, 1st November 1996; Com. 6/07438C
492
Chem. Commun., 1997