V–Zr–P oxide catalysts for highly selective oxidation of propane to acrylic acid
Yi-Fan Han, Huai-Ming Wang, Hua Cheng and Jing-Fa Deng*
Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China.
E-mail: jfdeng@srcap.stc.sh.cn
Received (in Cambridge, UK) 6th January 1999, Accepted 19th February 1999
V–Zr–P oxide catalysts have been prepared and exhibited
high selectivity in the oxidation of propane to acrylic acid.
The selective oxidation of lower alkanes to other chemicals is
attracting attention for economic reasons and their availability.
The best-known oxidation of lower alkanes is the selective
oxidation of n-butane to maleic anhydride over vanadium
1
phosphorus oxide catalysts (VPO). Moreover, oxidation of
2–4
5
6
propane to propylene, acrolein and acrylonitrile has been
widely developed. It was found first by Ai7 that a VPO based
catalytic system could directly oxidize propane to acrylic acid
,8
effectively. The V
Nb , Sb , SiO
2
O
5
–P –X
2
O
5
n m n m 3 2
O (X O = SO , TeO ,
2
O
3
2
O
3
2
and B O ) type catalysts have been tested
2 3
and the catalytic performance in this process is clearly
improved. However, the reaction carried out on VPO catalysts
at relatively high temperature (400 °C) not only results in a low
selectivity to acrylic acid, but also leads to serious formation of
coke on the catalyst surface and shortage of lifetime. To solve
these problems, we have reported the use of a titania–silica
9
xerogel supported VPO catalyst in this reaction which showed
Fig. 1 XRD patterns of V–Zr–P oxide catalysts; (a) (VO) P O , (b)
VOPO .
4
2
2
7
highly selective oxidation of propane to acrylic and acetic acid
at low temperature (300 °C). So far, the yield and selectivity to
acrylic acid for all developed catalysts are too low to be applied
at commercial level. For this reason, we sought to develop a new
catalyst that possesses good performance at low temperature in
order to give high yield and selectivity to acrylic acid without
coking.
when Zr:V was raised to 1.0, indicating that (VO)
2
P
2
O
7
starts
to become disordered. No distinct XRD line can be found for
ZrPO, suggesting that ZrPO is an amorphous material. In
addition, no zirconium phosphate was evidenced for any V–Zr–
P oxides, implying that ZrPO can only disperse into the
In our study, it was demonstrated that V–Zr–P (Zr:V = 0.5)
oxide catalyst showed significant high selectivity and yield to
acrylic acid at 340 °C, and had the potential for practical use.
V–Zr–P oxide catalysts were prepared by the following
structure of VPO or form a solid solution with (VO)
2 2 7
P O . It is
also interesting to observe that the VOPO phase disappears as
4
Zr:V is increased to 0.25, which suggests that its formation has
been suppressed.
2 5 2 2
procedure: a mixture of V O and ZrOCl ·H O in stoichiome-
All catalysts employed in this study were tested for the
selective oxidation of propane to acrylic acid. The optimal
results summarized in Table 1 show that ZrPO is inert to this
reaction. For VPO catalyst, the yield and selectivity to acrylic
acid are 11.2 and 48.1%, respectively. With the addition of Zr,
the yield and selectivity increase sisgnificantly in comparison
with the VPO catalyst. As Zr:V is changed from 0.125 to 0.5,
the selectivity increases from 70.0 to 81.0% and the yield
increases from 13.5 to 14.8%. With continuous increasing ratio
of Zr:V to 1.0, the activity and selectivity start to decrease.
It can be seen in Fig. 2, that the temperature for achieving
maximum selectivity shifts from 400 (VPO) to 340 °C (V–Zr–P
trical atomic ratio (Zr:V) was reduced by refluxing in a solution
of isobutanol (20 ml)–benzyl alcohol (10 ml) for 12 h; a black
blue or gray suspended precipitate formed. Then, an appropriate
3 4
amount of 85% H PO [atomic ratio P:(Zr + V) = 1.0] was
added to the solution, which was refluxed for 6 h to give a light
blue–green suspended precipitate and a black–blue solution.
The precipitate was filtered off and the obtained paste was dried
in an oven at 120 °C overnight. The resulting precursor was
ground and sieved to obtain a 40–60 mesh size portion. VPO
(
V:P = 1.0) and ZrPO (Zr : P = 1.0) catalysts were also
prepared by the same procedure to enable comparison. The
activation of precursor and oxidation of propane were carried
out in a continuous tubular flow fixed-bed microreactor. The
precursor of the V–Zr–P oxide catalyst (1.0 ml) was packed into
a stainless steel reactor (id: 6.0 mm, length: 20 cm) and the
temperature was raised to 773 K at a rate of 20 K min in a
mixture of air–propane–water vapor (75.6:1.2:23.2) at a rate of
2
Table 1 The performance of V–Zr–P oxide catalystsa
2
1
V:Zr
Conv.
Yield
Sel.
atomic ratio T/°C
(mol%)
(mol%)
(mol%)
2
1
0 ml min for 12 h. The sample was then cooled to the
reaction temperature within 6 h.
The X-ray diffraction patterns for all the catalysts are in
Fig. 1. Lines at 2q = 23.1, 28.4 and 29.9° are attributed to
1:1
340
340
360
380
400
400
16.1
17.5
18.4
18.5
23.0
—
12.7
14.8
14.2
13.5
11.2
—
70.3
81.0
71.0
70.0
48.1
—
1
1
1
:0.5
:0.25
:0.125
(
VO)
attributed to VOPO
VPO catalyst is mainly constituted of (VO)
2 2 7
VOPO . The X-ray line due to (VO) P O
2
P
2
O
7
, and those at 2q = 22.0, 26.0 and 28.9° are
, respectively.10 It can be seen that the bare
with some
broadened and
VPO
ZrPO
4
2 2 7
P O
a
Reaction conditions: GHSV = 1000 h21, feed gas = air–propane–
4
water = 73.4:3.2:23.4, time = 50 h, cat. 1.0 g; analysis: on-line gas
chromatograph.
diminished with increase of the atomic ratio of Zr:V in the V–
Zr–P oxides. Only small broad diffraction peaks were detected
Chem. Commun., 1999, 521–522
521
Han, Yi-Fan
