Hierarchical design of mixed metal oxides: novel macroporous VPO
phases
Moises A. Carreon and Vadim V. Guliants*
Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0171 USA.
E-mail: Vadim.Guliants@uc.edu
Received (in Cambridge, UK) 5th April 2001, Accepted 20th June 2001
First published as an Advance Article on the web 19th July 2001
Macroporous vanadium–phosphorus–oxide phases (macro-
VPO) displaying ordered 0.2–0.4 mm pores and unprece-
dented high surface areas (44–75 m2 g21) have been
synthesized using colloidal arrays of polystyrene spheres as
a template.
array, where it reacted with a phosphorus source and condensed
into a macro-VPO framework upon drying. It was found that the
initial surface treatment with a phosphorus source was critical
for the nucleation and growth of macro-VPO phases in the voids
of the PS sphere arrays. In all experiments the P/V molar ratio
was 1+1, which provides the optimal surface and bulk
composition for selective oxidation of n-butane.3 The PS
spheres were removed from as-synthesized macro-VPO by
either calcination in air at 723 K or Soxhlet extraction using a
mixture of acetone and tetrahydrofuran (1:1 volume ratio).
Typical synthesis conditions, crystallographic phases deter-
mined by powder XRD (Siemens D-500) and the BET surface
areas (Micromeritics Gemini 2360) for selected calcined and
Soxhlet-extracted macro-VPO phases are given in Table 1.
The ordered pore structure of the macro-VPO phases after the
template removal is evident in Fig. 2. Interconnected pores
appear as dark spots (ca. 0.2 mm diameter) inside spherical 0.4
mm cavities left after the removal of the PS spheres. The walls
of the macropore structure are formed by the (VO)2P2O7
nanocrystals. The wall thickness was estimated from the SEM
images to be ca. 90 nm. The average nanocrystal size
determined using the Scherrer formula10 was ca. 20 nm, which
indicated that the wall was only four crystals thick. Relatively
large size of the nanocrystal building blocks explains the
Mixed metal oxides possess promising catalytic properties for
the selective oxidation of lower alkanes (C2–C5).1 For example,
Mo–V–Nb and Sb–V oxides are catalytically active in selective
oxidation of ethane and propane2 and propane ammoxidation
and vanadium–phosphorus–oxides (VPOs) are selective in the
oxidation of n-butane to maleic anhydride.3
The conventional synthesis methods, both wet chemistry and
solid-state, offer limited control over the phase, bulk and
surface compositions of mixed metal oxides, preferential
exposure of active and selective surface planes, surface areas
and pore architectures, which define their catalytic properties in
selective oxidation of lower alkanes. There is a critical need for
novel routes of assembling hierarchically designed mixed metal
oxides, which display remarkable ordering on micro- ( < 3 nm
for the surface region structure and composition), nano- (3–100
nm for the bulk and phase compositions) and macro- ( > 100 nm
for pore architectures) scales.
Macroscale-templated synthesis of nanocrystalline mixed
metal oxides represents an attractive approach for the synthesis
of hierarchically designed catalytic materials. Several single-
element macroporous oxides with very interesting structural
properties have been synthesized by self-assembly using
colloidal sphere templates.4–9 However, macroporous mixed
metal oxides for applications in oxidation catalysis have not, as
yet, been reported. We report here the first successful example
of a hierarchically designed macroporous mixed vanadium–
phosphorus–oxide (macro-VPO) with desirable structural and
compositional properties for selective oxidation of n-butane.
Macro-VPO phases were assembled using close-packed
arrays of colloidal polystyrene (PS) spheres (0.4 mm diameter)
as a template. Monodispersed PS spheres were synthesized by
an emulsion polymerization process described elsewhere.7 The
ordered arrays of PS spheres were obtained by either centrifuga-
tion or filtration of PS sphere suspensions. Fig. 1 shows the
SEM image (Hitachi, Model S-3200N) of colloidal PS spheres
used as the template.
In a typical synthesis, an array of PS spheres was first
impregnated with a phosphoric or phosphorous acid solution in
ethanol. Then a solution of a vanadium-(IV) or -( ) source in
V
Fig. 1 SEM image of the synthesized monodispersed polystyrene spheres
used as templates for the formation of macroporous VPO.
ethanol or isobutyl alcohol was introduced into the voids of the
Table 1 Selected properties of macroporous VPOs
Specific surface
VPO sources
General description
area/m2 g21
Crystalline phase
VO[CHO(CH3)2]3, H3PO3
VO[CHO(CH3)2]3, H3PO3
V2O5, H3PO3
V2O5, H3PO3
V2O5, H3PO4
Calcined in air at 723 K
Soxhlet extracted
Calcined in air at 723 K
Soxhlet extracted
64
50
41
75
44
VOPO4·2H2O
VOPO4·2H2O
VOPO4·2H2O
VOHPO4·4H2O/b-VOHPO4·2H2O
(VO)2P2O7
Calcined in air at 723 K
1438
Chem. Commun., 2001, 1438–1439
DOI: 10.1039/b103131g
This journal is © The Royal Society of Chemistry 2001
Carreon
