Hydrothermal Synthesis of a New Mo-V-O Complex Metal Oxide and Its Catalytic Activity for the Oxidation of Propane

Article abstract of DOI:10.1246/cl.2003.1028

Preparation of a new orthorhombic Mo-V-O catalyst was succeeded by hydrothermal synthesis for the first time. This orthorhombic structure showed high activity for propane oxidation. Selectivity to acrylic acid dramatically increased by the introduction of Te into the orthorhombic Mo-V-O catalyst.

Full text of DOI:10.1246/cl.2003.1028

1
028
Chemistry Letters Vol.32, No.11 (2003)
Hydrothermal Synthesis of A New Mo–V–O Complex Metal Oxide and Its Catalytic Activity
for The Oxidation of Propane
ꢀ
Tomokazu Katou, Damien Vitry, and Wataru Ueda
Catalysis Research Center, Hokkaido University, N11-W10, Sapporo 060-0811
(Received August 1, 2003;CL-030707)
Preparation of a new orthorhombic Mo–V–O catalyst was
succeeded by hydrothermal synthesis for the first time. This or-
thorhombic structure showed high activity for propane oxida-
tion. Selectivity to acrylic acid dramatically increased by the in-
troduction of Te into the orthorhombic Mo–V–O catalyst.
Mo–V–Te(Sb)–M–O type complex metal oxides have at-
tracted increasing interest because of their high catalytic activity
for partial oxidation of light alkanes. These metal oxides have
1
been reported to assume a layered (orthorhombic) structure with
a slab consisting in 5,6,7-membered rings of octahedral molyb-
2
denum oxides. This structural material gives characteristic
ꢁ
peaks at 6.6, 7.9, 9.0, and 22.2 in powder-XRD pattern (Cu
Kꢀ). The Mo–V–Te–Nb–O mixed metal oxide reported by Mi-
tsubishi Chemical Co. has the structure and shows excellent cat-
alytic performance both in the ammoxidation and oxidation of
propane to acrylonitrile and acrylic acid, respectively (more than
Figure 2. XRD patterns (Cu Kꢀ) of the Mo–V–(Te)–O catalysts. (a) Cat-A-
fresh, (b) Cat-A-1, (c) Cat-A-2, (d) Cat-A-3, (e) Cat-B-fresh, (f) Cat-B-1, (g)
Cat-B-2, (h) Cat-C-fresh, (i) Cat-C-1. Symbols: orthorhombic Mo–V–O or
Mo–V–Te–O phase ( ), (V0:07Mo0:93)5O14 ( ), V0:95Mo0:97O5 ( ), MoO3
1a,3
50% yields). We have reported that the Mo–V–Te–O system
(
).
having the same structure could be prepared under hydrothermal
conditions and gave high selectivity to acrylic acid in the selec-
tive oxidation of propane.
Recently we succeeded in preparation of a ternary Mo–V–O
metal oxide which also has the same orthorhombic structure as
the above catalysts (Figure 1). We report here the hydrothermal
synthesis of the Mo–V–O catalyst and emphasize the structure
effect on the oxidation activity and the role of each element in
the Mo–V–Te–O catalyst in the selective oxidation of propane
to acrylic acid.
as on the wall. The obtained gray solid was separated from the
ꢁ
1
b,4–6
solution, washed with distilled water, and dried at 80 C over
night. The obtained material was abbreviated as Cat-A.
Figure 2a shows the XRD patterns of the hydrothermally
synthesized Mo–V–O catalyst before heat-treatment (Cat-A-
fresh). Cat-A-fresh had three characteristic diffraction peaks
ꢁ
ꢁ
ꢁ
(
2ꢁ ¼ 6:6, 7.9, and 9.0 ) at angle region lower than 10 and
sharp peaks at 22 and 45 . It was ascertained by ICP analysis that
Cat-A contained each metal cation with a composition of
Mo1V0:34Ox (Table 1). No other elements were detected beside
Mo and V. Reasonably high phase purity could be ascertained
by SEM analysis in which prism-shaped crystalline was ob-
served as a major particle, although a ramp can be seen addition-
ally in the low angle region of the XRD (Figure 2a). The ob-
served XRD pattern was exactly the same as that of the Mo–
8.82 g of (NH4)6Mo7O24 was dissolved in 120 mL of dis-
tilled water. Separately, an aqueous solution of vanadium was
prepared by dissolving 3.28 g of hydrated VOSO4 in 120 mL
ꢁ
of distilled water. The two solutions were mixed at 20 C and
stirred for 10 min before being introduced into a stainless steel
autoclave equipped with a 300 mL-Teflon inner tube. The con-
centration of Mo in the mixed solution (dark purple liquid)
7
V–Te–Nb–O catalysts as well as of the Mo–V–Te(Sb)–O cata-
The XRD pattern was nicely simulated by using or-
1
b,4–6,8
ꢃ1
lysts.
was 0.2 molꢂL . Hydrothermal reaction of the liquid (mole ra-
ꢀ
ꢁ
thorhombic structure with lattice parameter (a ¼ 21:19 A,
tio: Mo/V = 1/0.25) was carried out at 175 C for 20 h. After
20 h solids were yielded in the bottom of the autoclave as well
ꢀ
ꢀ
b ¼ 26:57 A, c ¼ 4:006 A). It is reported that there are many
9
structural variants in Mo–V–O system and it can be recognized
that the present structural material is a new variant. Additionally,
it should be pointed out that Te or Sb is unnecessary for the for-
mation of the orthorhombic structure in Mo–V–O, although it
has been thought that the structure formation needs Te or Sb.
Under the present synthetic procedure, however, the yield of
the orthorhombic structural material was low less than 10 wt %,
because we noticed that the yield was strongly dependent on
many preparative factors like pH, element concentration, synthe-
sis duration, and so on. Particularly, the element concentration
was highly effective;when the preparation solution contained
ꢃ1
high Mo in concentration, for example, 0.6 molꢂL with the
Figure 1. Structural model of the orthorhombic phase of the Mo–V–O. Mo and
V are located in the octahedral positions.
same Mo/V ratio as Cat-A, we obtained a similarly black-col-
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.11 (2003)
1029
Table 1. Propane oxidation over Mo–V–(Te)–O catalystsa
2
ꢃ1
Selectivity/%c
Catalysts
SBET/m ꢂg
Bulk composition
Reaction
ꢁ
C3H8 conv./%
(
Mo/V/Te)b
1/0.34/0
—
temp./ C
AA
5.7
6.0
6.4
3.2
1.1
49.3
PEN
6.8
6.0
19.9
14.7
30.8
10.4
AcA
19.0
21.1
24.9
29.5
23.9
15.1
Ace
0.4
0.5
1.1
0.9
1.2
2.7
CO2
29.1
28.5
19.1
18.8
15.0
10.2
CO
39.0
37.9
28.6
32.9
28.0
12.3
Cat-A-1
Cat-A-2
Cat-A-3
Cat-B-1
Cat-B-2
6.1
5.9
9.3
10.3
7.6
5.8
350
345
346
349
343
354
24.0
26.0
10.8
12.4
5.9
—
1/0.29/0
—
1/0.44/0.1
Cat-C-1
25.6
a
ꢃ1
b
Catalyst weight = 0.5 g;feed flow rate = 20 mL ꢂmin , feed composition (mol%), C3H8:O2:H2O:N2 = 6.5:10.0:45.0:38.5. Bulk composition was deter-
c
mined by ICP analysis. AA = acrylic acid, PEN = propene, AcA = acetic acid, Ace = acetone.
2
ꢃ1
ored solid with one broad diffraction peak at the low angle re-
gion instead of the characteristic three peaks (Figure 2e), though
the same diffraction peaks at 22 and 45 were observed. The ob-
was smaller than that of Cat-B-1 (10.3 m ꢂg ). It is clear from
this result that the orthorhombic structure has intrinsically high
catalytic oxidation activity. This conclusion is supported by
the following results: The activity of Cat-A-3, in which the or-
thorhombic structure has been decomposed by increasing calci-
ꢁ
tained material was abbreviated as Cat-B. Its bulk composition
was determined to be Mo1V0:29Ox (Table 1) which was interest-
ingly quite resembled to that of the orthorhombic material.
Based on the observed broad XRD pattern and elemental analy-
sis, we speculate a disorder in the arrangement of Mo and V giv-
ing the broad peak at the low angle region. We have already
ꢁ
nation temperature (500 to 600 C), was less than half of that of
mono-phasic orthorhombic Mo–V–O (Cat-A-1). Moreover, Cat-
B-2 in which the main phase was (V0:07Mo0:93)5O14 type struc-
ture showed very poor activity.
In the case of Mo–V–Te–O catalyst, air calcination before
N2 calcination can improve the selectivity to acrylic acid from
propane as described before. In the case of Mo–V–O catalyst,
however, this air calcination hardly influenced the catalytic per-
formance (Cat-A-2) as well as the XRD patterns. Thus, air cal-
cination is an unnecessary treatment in the case of orthorhombic
1
0
reported the similar disordered structure previously. The main
structural difference between Cat-A and Cat-B is, therefore,
whether the slab plane is ordered or not.
Thermal stabilities of the above materials were tested by
XRD and the results are shown in Figure 2. Tellurium-contain-
ing catalyst (Mo6V3Te1Ox, abbreviated as Cat-C), which was ef-
6
ꢁ
fective for selective oxidation from propane to acrylic acid, was
also tested for comparison. The preparative procedure was
Mo–V–O and N2 calcination at 500 C is enough to attain the
catalyst performance.
5
,6
reported previously. Cat-A and Cat-B were heat-treated in
The most striking point in the present work is that the ortho-
rhombic Mo–V–O (Cat-A-1) and the orthorhombic Mo–V–Te–
O (Cat-C-1) was greatly different in the catalytic selectivity.
The propane conversion over Cat-A-1 was about the same of that
of Cat-C-1, but as for the selectivity to acrylic acid, Cat-A-1 was
only a tenth part of Cat-C-1. Accordingly, it is perceived that the
effect of Te is not only thermal stabilization, but also the im-
provement of the selectivity to acrylic acid from propane.
The catalytic function of orthorhombic Mo–V–Te–O is now
clear;intrinsic catalytic oxidation activity comes from Mo and V
oxides but these elements have to be organized in a ordered
structure, and the selectivity derives from the third element, like
Te or Sb, which has to be located in the above ordered structure
without disturbing the structure. In this respect the element ar-
rangement shown in Figure 1 seems to be one of the most suit-
able structures.
ꢃ1
ꢁ
N2 stream (50 mLꢂmin ) at 500 C (Cat-A-1 and Cat-B-1) or
ꢁ
600 C (Cat-A-3 and Cat-B-2) for 2 h. Since Cat-C is normally
calcined in the condition (in air at 280 C for 2 h and in N2 at
600 C for 2 h, Cat-C-1) for achieving high selectivity to acrylic
acid, Cat-A was also calcined in the similar condition (in air at
ꢁ
ꢁ
ꢁ
ꢁ
2
80 C for 2 h and in N2 at 500 C for 2 h) for comparison
Cat-A-2).
As can be seen in Figure 2, Cat-A and Cat-B was thermally
(
ꢁ
stable in N2 up to 500 C and the stability (Cat-A-2) was not ap-
preciably affected by air treatment unless the heat-treatment
ꢁ
ꢁ
temperature in N2 exceeds 500 C. When the temperature of cal-
cination increased up to 600 C, many other phases, such as
V0:95Mo0:97O5 [JCPDS 77-0649], (V0:07Mo0:93)5O14 [JCPDS
31-1437], and MoO3 [JCPDS 76-1003] (for only Cat-B-2), were
detected in the both cases (Figure 2d for Cat-A-3 and Figure 2g
for Cat-B-2). However, it should be noted that the orthorhombic
phase still partially remained in Cat-A-3 but the broad peaks
were completely disappeared in Cat-B-2. On the other hand,
Cat-C maintained the mono-phasic state even after the heat-
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ꢁ
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2
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(
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2
ꢃ1
)
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Published on the web (Advance View) October 13, 2003;DOI 10.1246/cl.2003.1028