Crystalline Mo3VOx mixed-metal-oxide catalyst with trigonal symmetry

Article abstract of DOI:10.1002/anie.200603923

Outstanding catalytic activity for the selective oxidation of acrolein (see picture) is observed with a crystalline metal oxide, Mo₃VO _x (x ≤ 11.1). The catalyst is synthesized from a solution containing pentagonal units of {Mo(Mo₅O₂₇)} (blue), which further react with molybdenum and vanadium species (red) to form a 3D metal oxide. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200603923

Angewandte
Chemie
DOI: 10.1002/anie.200603923
Polyoxometalates
Crystalline Mo VO Mixed-Metal-Oxide Catalyst with Trigonal
3
x
Symmetry**
Masahiro Sadakane,* Nobufumi Watanabe, Tomokazu Katou, Yoshinobu Nodasaka, and
Wataru Ueda*
About a quarter of all organic products are synthesized by
selective partial oxidation and continuous efforts have been
made to develop high-performance catalysts that improve
conversion and selectivity. The majority of catalysts are metal
oxides with either molybdenum or vanadium as one of the key
orthorhombic Mo VO , trigonal Mo VO showed outstanding
3 x 3 x
catalytic activity for the selective oxidation of acrolein, which
indicates the importance of these structures for this catalytic
activity.
Both orthorhombic and trigonal Mo VO mixed-metal
3
x
[
1]
elements. Recently, much attention has been paid to
orthorhombic MoVTe(Sb)NbO mixed-metal-oxide catalysts
developed by Mitsubishi Chemicals, which have the best
performance in propane oxidation to acrylic acid and
oxides were synthesized from a reaction mixture of ammo-
nium heptamolybdate, (NH ) Mo O ·4H O, and vanadyl
4
6
7
24
2
sulfate, VOSO ·nH O (Mo:V= 4:1), in H O under hydro-
4
2
2
thermal conditions. By controlling the pH value of the
precursor solution, the orthorhombic or trigonal Mo VO
[
2]
ammoxidation to acrylonitrile. Although a fully analyzed
crystal structure of the catalyst has not yet been reported, it
was shown that this complex mixed-metal oxide has a layered,
orthorhombic structure with a slab comprising 6- and 7-rings
of {MO } octahedra and pentagonal {(M)M O } units with a
3
x
was selectively synthesized (Scheme 1). The crude materials
contained amorphous phase as a by-product, which was
removed by washing the products with aqueous oxalic acid.
The ratio of Mo:V atoms was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES)
and was about 3:1for both materials.
6
5
27
{
{
MO } pentagonal bipyramidal unit and five edge-sharing
7
[
3]
MO } octahedra (M is a metal atom; Figure 1a).
6
In the course of our investigation to design a catalyst for
selective oxidation, we succeeded in synthesizing an ortho-
rhombic mixed-metal oxide that contains Mo and V atoms,
with and without other elements (Te, Sb, or Nb) under
[
4]
hydrothermal conditions. Herein, we describe the synthesis
and structural characterization of a novel crystalline Mo VOx
3
mixed-metal oxide (trigonal, Figure 1b x ꢀ 11.1), which con-
tains the same building units (6- and 7-rings and a pentagonal
unit) as orthorhombic Mo VO but in different ratios. Like
3
x
[*] Dr. M. Sadakane, N. Watanabe, T. Katou, Prof. W. Ueda
Catalysis Research Center
Hokkaido University
N-21, W-10, Sapporo, 001-0021 (Japan)
Fax: (+81)11-706-9164
E-mail: sadakane@cat.hokudai.ac.jp
ueda@cat.hokudai.ac.jp
Y. Nodasaka
Laboratory of Electron Microscopy
Graduate School of Dental Medicine
Hokkaido University
Sapporo (Japan)
[**] Supported in part by Grants-in-Aid for Scientific Research “B” (No:
16360396), “Grant for Research” of The Japan Petroleum Institute,
and “Nanotechnology Support Project” of the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT), Japan. We
thank Dr. T. Nagai, Dr. Y. Matsui (High Voltage Electron Microscopy
Station, National Institute for Materials Science), K. Sugawara, and
Prof. N. Sakaguchi (High Voltage Electron Microscope Laboratory,
Center for Advanced Research of Energy Conversion Materials,
Hokkaido University) for supporting TEM measurements and
Nippon Kayaku Corp. for ICP-AES analysis and acrolein oxidation
test.
Figure 1. Polyhedral presentation of the structures of a) orthorhombic
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
and b) trigonal Mo VO . (The basic structure of MoVTe(Sb)NbO is the
3
x
same as Mo VO .)
3
x
Angew. Chem. Int. Ed. 2007, 46, 1493 –1496
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Scheme 1. Synthesis of orthorhombic and trigonal Mo VO .
3
x
Figure 3. HRTEM images and selected-area electron diffraction (SAED)
patterns (insets) of a) orthorhombic and b) trigonal Mo VO viewed
3
x
along the [001] direction as well as the corresponding simulated
contrast for c) orthorhombic Mo VO calculated for a crystal thickness
3
x
close to 24 nm and a defocus value Df=ꢂ130 nm and d) trigonal
Mo VO calculated for a crystal thickness close to 20 nm and a defocus
3
x
value Df=ꢂ155 nm. L and S indicate large and small spots, respec-
tively.
electron beam, three large white spots that correspond to a 7-
ring and two small spots that correspond to a 6-ring were
found per unit cell (Figure 3b). The pentagonal unit is
surrounded by three 7- and two 6-rings to produce the
structure presented in Figure 1b.
This trigonal structure was confirmed by analyzing the
powder XRD data by the Rietveld method. Its XRD pattern
(Figure 2) was simulated with lattice parameters a =
Figure 2. Observed (dots), calculated (solid line), and difference
pattern (solid line, bottom) from the Rietveld analysis of the XRD data
for trigonal Mo VO at room temperature, space group P3. Bragg
3
x
reflections are indicated by tick marks.
2
1.433(3), c = 4.0045(18) Š, space group P3 (No. 143), which
converged reasonably (Rwp = 0.1223) although Rietveld
[
5]
Indexing the Bragg reflections of the powder X-ray
refinement with other deduced models did not converge.
diffraction (XRD) pattern (Figure 2) of trigonal Mo VOx
From these data, there is no doubt that the proposed structure
is reasonable.
3
revealed lattice parameters of a ꢁ b ꢁ 21Š, c ꢁ 4 Š, a = b =
9
08, and g = 1208. Similar to orthorhombic Mo VO , trigonal
Orthorhombic and trigonal Mo VO were synthesized by
3
x
3
x
Mo VO has a layered structure with a repetition along the
mixing a solution of ammonium heptamolybdate with vanadyl
sulfate under hydrothermal conditions to give a dark-violet
solution. To elucidate the synthesis mechanism, Raman
spectra of the precursor solution before the hydrothermal
reaction (Figure 4, top, curves a and b) were compared with
those of various polyoxomolybdate solutions. The character-
3
x
c axis of 4.0 Š, which corresponds to the MꢂOꢂM separation.
It is evident from TEM and SEM observations that the a–b
planes are oriented perpendicular to the long axis of the
rodlike crystal, which coincides with the c direction (Fig-
ure S1in the Supporting Information).
ꢂ1
The trigonal structure was resolved by comparing its high-
resolution (HR) TEM image with that of orthorhombic
Mo VO . Figure 3 shows HRTEM images along the [001]
istic Raman peaks at 1000–700 cm are very similar to those
of the polyoxomolybdates that contain the pentagonal
[
6]
[7]
[8]
{(Mo)Mo } unit; those are Mo V , Mo132
,
and Mo V ,
57 6
3
x
5
72 30
zone axis of both materials. Two kinds of white spots are
present, large (L) and small (S; Figure 3). In the case of
orthorhombic Mo VO , the large spots correspond to 7-rings
shown in Figure 4, curves c, e, and d, respectively.
The UV/Vis spectrum of the precursor solution is similar
to that of Mo V (Figure 4, bottom) and a band arising from
3
x
72 30
IV
VI [6]
and the small spots correspond to 6-rings of octahedral {MO6}
units (Figure S2 in the Supporting Information). The pentag-
onal rings are surrounded by three 7- and two 6-rings. The
structure of trigonal Mo VO is assumed to be the same.
intervalence charge transfer (IVCT; V !Mo ) at around
510 nm is present in both spectra. The Mo V polyoxo-
7
2
30
VI
VI
molybdate has 12 pentagonal {(Mo )Mo 5} units connected
by 30 vanadium(IV) centers. These results indicate that the
[
6]
3
x
Although the spots are slightly uneven because of a small
misorientation of the crystal with respect to the incident
pentagonal {(Mo)Mo } unit is present in the precursor
solution before the hydrothermal reaction takes place. The
5
1
494
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Angewandte
Chemie
Table 1: Oxidation of acrolein over Mo VO catalysts to acrylic acid (AA),
3
x
[
a]
acetic acid (AcA), and CO/CO2.
Catalyst
BET Surface
Conversion [%]
Selectivity [%]
2
ꢂ1
area [m g
]
AA AcA CO/
CO2
trigonal
15.6
14.9
99.7
99.4
90.5 3.2 6.3
93.6 1.5 4.9
Mo VOx
3
orthogonal
Mo VOx
3
[a] Reaction conditions: 0.45 g of catalyst with 0.05 g of SiC; gas
composition: acrolein/O /H O/N =4.8/7.6/26.9/60.6; total flow:
2
2
2
ꢂ1
1
03.3 mLmin , reaction temperature: 463 K. Both catalysts were
calcined at 673 K under a nitrogen flow before the reaction.
and pentagonal columns is important for high oxidation
activity. Moreover, crude Mo VO was less active (at 503 K:
3
x
[
4c]
9
5
6.4% conversion and 94.2% selectivity to acrylic acid; at
23 K: 99.8% conversion and 90.9% selectivity to acrylic
acid) than the purified material. The trigonal or orthorhombic
structures of Mo VO may be the true catalytically active
3
x
species in previously reported Mo–V catalysts.
Experimental Section
Materials: All chemicals were reagent-grade and used as supplied and
distilled water that was made in the laboratory was used throughout.
Preparation of trigonal Mo VO : (NH ) Mo O ·4H O (8.82 g,
Figure 4. Top: Raman spectra of Mo–V solutions. a) Mo:V=4:1,
cMo =0.21m, pH 3.3; b ) Mo:V=4:1, cMo =0.21m, pH 2.2; c) aqueous
Mo V (1 g per 30 mL); d) aqueous Mo V solution (1 g per 30 mL);
3
x
4
6
7
24
2
7
2
30
57 6
50 mmol) dissolved in water (120 mL) was mixed at 298 K with
e) aqueous Mo132 solution (1 g per 30 mL). Bottom: UV/Vis spectra of
an Mo–V solution (Mo:V=4:1, cMo =0.42m, pH 3.3; solid line) and an
aqueous Mo V solution (dotted line).
VOSO ·nH O (3.28 g, n = 5.4, 12.5 mmol) dissolved in water
4
2
(120 mL). The resulting solution was stirred for 10 min then trans-
7
2 30
ferred to a 300-mL teflon liner of a stainless-steal autoclave. The
pH value of the solution was adjusted to 2.2 with dilute sulfuric acid
(
2 mL of concentrated sulfuric acid in 20 mL of water). The reaction
reaction of the pentagonal unit with other molybdenum and
mixture was purged with nitrogen for 10 min then heated at 448 K for
20 h. A gray solid (about 5 g) was isolated from the reaction mixture
by filtration, washed with water, and dried at 353 K overnight. The
crude trigonal Mo VO was purified by treatment with oxalic acid:
3
vanadium species produces orthorombic or trigonal Mo VOx
depending on the pH value. The pH value is known to be one
of the most important factors in producing a giant polyoxo-
3
x
[
9]
molybdate. These discrete polyoxomolybdate units have
been prepared at temperatures ranging from room temper-
ature to 363 K by the reaction of pentagonal units with
dry solid (4.0 g) was added to an aqueous solution of oxalic acid
0.4m, 100 mL) and this mixture was stirred at 333 K for 30 min. The
(
solid was isolated from the suspension by filtration, washed with
water, and dried at 353 K overnight (0.8 g).
Characterization: Powder XRD patterns were recorded on a
diffractometer (Rigaku, RINT Ultima + ) with CuKa radiation.
Rietveld simulation was performed using RIETAN-2000. Surface
areas were determined by the Brunauer–Emmett–Teller (BET)
method. Prior to the sorption measurements, the samples were
degassed under vacuum at 573 K for 2 h. ICP-AES (inductively
coupled atomic absorption spectroscopy) was carried out with a
VISTA-PRO apparatus (Varian). Scanning electron microscopy
[
9]
molybdenum and other metal oxides. Under hydrothermal
conditions, the pentagonal unit assembled further into a
complex mixed-metal oxide.
[
11]
As summarized in Table 1, the trigonal Mo VO catalyst
3
x
has outstanding catalytic activity for the oxidation of acrolein
into acrylic acid, with a conversion of around 100% at 463 K
and selectivity to acrylic acid of 90% (yield 90%). The
orthorhombic Mo VO catalyst shows similarly high catalytic
3
x
(
SEM) was performed on a JSM-7400F (JEOL). Transmission
activity. This oxidation is known as the second process in
acrylic acid production. Acrylic acid (3000000 tons per year,
worldwide) is manufactured by catalytic oxidation in a two-
step process, which involves the oxidation of propene to
acrolein and then to acrylic acid. The catalytic activities of our
catalysts are significantly higher than those of Mo–V-based
oxide catalysts reported in patents and papers, which need
higher temperatures (usually more than 500 K) to obtain
electron microscopy (TEM) images were obtained on JEM-ARM-
1300 and JEM-2000FX (JEOL) electron microscopes at Hokkaido
University. Specimens were embedded in TAAB Epon 812 Resin and
ultrathin sections were cut with a diamond knife. A Raman study of
aqueous solutions at room temperature was performed on a System
2000R NIR FT-Raman spectrometer (PERKIN ELMER). Nd:YAG
laser was used to supply 300 mW at 1064 nm.
Received: September 25, 2006
Revised: November 20, 2006
Published online: January 15, 2007
[
10]
similar activity.
These results indicate that the layered
structure comprising 6- and 7-rings of metal-oxide octahedra
Angew. Chem. Int. Ed. 2007, 46, 1493 –1496
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1495

Communications
Keywords: heterogeneous catalysis · molybdenum · oxidation ·
polyoxometalates · vanadium
the following reasons: 1) the Raman pattern of the precursor
.
solution was very similar to that of the Mo V cluster, in which
72 30
{
(Mo)Mo } units are connected by V atoms; 2) all pentagonal
5
units reported in polyoxometalates comprise only Mo, even in
mixed polyoxomolybdates (Mo V , Mo Fe , and Mo V );
7
2
30
72
30
57
6
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1
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