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Rare earth stabilization of mesoporous alumina molecular sieves assembled
1
1
through an N I pathway
Wenzhong Zhang and Thomas J. Pinnavaia*†
Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing,
MI 48824-1322, USA
The incorporation of 1.0–5.0 mol% Ce3+ or La3+ ions in
MSU-X alumina molecular sieves, prepared through an
N I assembly pathway, dramatically improves their ther-
mal stability without altering the mesopore size or the
wormhole channel motif.
The molar compositions of the above reaction mixtures were
as follows:
1
1
3+
3+
s
0.05 Ce (or 0.01 La ):1.0 Al(Bu O)
(or 0.20 Pluronic):3.0 H O:15.5 Bu OH
where the non-ionic surfactants are Tergitol 15-S-12 (Union
Carbide) with the formula C15 12OH (E is a polyethylene
oxide segment) and Pluronic P65 or P123 (BASF) block
copolymers with the respective compositions E19 19 and
20 (P is a isopropylene oxide segment).
3
:0.40 Tergitol
s
2
®
33
H E
®
Following the supramolecular assembly of M41S mesoporous
molecular sieves in 1992,1 there have been relatively few
30
P E
2
reports of mesostructured aluminas. Davis and coworkers have
20 69
E P E
1
prepared porous aluminas (ca. 20 Å pore diameters) by the
hydrolysis of aluminium alkoxides in the presence of a
carboxylate surfactant as the structure director. The assembly
pathway involved S–I complexation reaction between the
surfactant (S) and the inorganic reagent (I), as judged by the
presence of IR bands characteristic of chelating carboxylate
groups. Yada et al.3 reported the preparation of hexagonal
Essentially all of the N surfactant could be removed from
the alumina mesostructures by extraction with hot ethanol. For
convenience, however, the as-synthesized products were freed
of surfactant and prepared directly for N adsorption studies in
2
one step by calcining at 500 °C for 6.0 h. Under the calcination
conditions the pure alumina mesostructures assembled from the
three N1 surfactants begin to collapse, as evidenced by a
broadening of the one-line diffraction pattern and a decrease in
the surface area and pore volume. Upon the introduction of 1–5
mol% rare earth ions, however, the thermal stability of the
framework is greatly improved, as judged by substantial
increases in surface areas and liquid pore volume (see below).
2
+
alumina mesostructures by electrostatic S I assembly of
dodecyl sulfate surfactants and aluminium nitrate. However, the
mesostructures were not stable to surfactant removal. In
contrast, we have obtained mesoporous alumina molecular
1
1
sieves, denoted MSU-X, by N I assembly of electrically
1
neutral polyethylene oxide surfactants (N ) and an aluminium
2
N adsorption–desorption isotherms for alumina molecular
1
4,5
3+
3+
alkoxide as the inorganic precursor (I ). These materials
exhibited wormhole channel motifs and BJH pore diameters
that can be extended beyond 100 Å, depending on the surfactant
size.
sieves doped with 5.0% Ce and 1.0% La are shown in Fig.
1. The positions of the pore filling steps in the adsorption curves
0
shift to higher P/P values with increasing surfactant size, as
expected for pore structures formed by a supramolecular
assembly process. The desorption hystereses signify some
One limitation of MSU-X alumina molecular sieves is the
loss of surface area and porosity when they are heated above
5
00 °C. The potential applications of these materials in catalysis
1200
and other materials areas could be greatly extended by
improving their thermal stability. One possible approach to
improving the thermal stability of a metastable alumina is to
dope the oxide framework with rare earth cations. For instance,
the incorporation of rare earths into g-alumina (pseudo-
boehmite) and other transition aluminas is known to stabilize
these metastable phases against sintering and conversion to
a-alumina (corundum). Two stabilization mechanisms have
been proposed, namely, the formation of a surface rare earth
Doped alumina-500 °C
1000
800
5%Ce, Tergitol 15-S-12
6
3+
aluminate phase and the simple replacement of Al by rare
earth ions in the pseudoboemite structure, which reduces the
lability of the oxide matrix. Analogous mechanisms might also
600
7
be effective in stabilizing the non-crystalline (amorphous)
framework walls of MSU-X alumina molecular sieves against
collapse at elevated temperatures. In the present work, we
demonstrate that MSU-X aluminas indeed are stabilized by
doping with rare earth metal ions.
400
1%La, Pluronic P65
P65 = E19P30E19
200
The incorporation of Ce3 or La into MSU-X aluminas was
accomplished by first dissolving the corresponding rare earth
nitrate in a solution of the non-ionic surfactant in warm butanol.
The solution was cooled to room temperature and then
aluminium sec-butoxide was added with stirring. After an
additional 1 h of stirring at ambient temperature, a dilute
solution of water in sec-butanol was added dropwise. The
reaction vessel was then placed in a reciprocating shaker bath at
+
3+
1%La, Pluronic P123
P123 = E20P69E20
0
0.0
0.2
0.4
P / P0
0.6
0.8
1.0
2
Fig. 1 N adsorption–desorption isotherms of rare earth-stabilized MSU-X
alumina molecular sieves assembled in the presence of the non-ionic
surfactants Tergitol 15-S-12, Pluronic P65, and Pluronic P123 as structure
directors and calcined at 500 °C. The BJH pore sizes obtained from the
desorption isotherms are included for comparison.
4
5 °C for a period of 40 h. Recovery of the as-synthesized
reaction products was achieved by filtration and air drying.
Chem. Commun., 1998
1185
Zhang, Wenzhong
