Bismuth-modified vanadyl pyrophosphate catalysts
Y.H. Taufiq-Yap
a,
Ã
, K.P. Tan
a
, K.C. Waugh
b
, M.Z. Hussein
a
, I. Ramli
a
, and M.B. Abdul Rahman
a
a
Department of Chemistry, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
b
Department of Chemistry, University of Manchester Institute of Science and Technology, POBox 88, Manchester, M60 1QD, United Kingdom
Received 14 November 2002; accepted 1 May 2003
A1% bismuth-doped VPO catalyst was prepared by refluxing BiðNO
3
Þ
3
and VOPO
4
Á 2H
2
O in isobutanol. The incorporation of
Bi into the VPO lattice lowered the overall V oxidation state from 4.24 to 4.08. It also lowered both the peak maximum temperature
for the desorption of oxygen from the lattice from 1001 K (undoped) to 964 K with a shoulder at 912 K and the peak maxima for H
2
temperature-programed reduction from 863, 1011 and 1143 K (undoped) to 798, 906 and 1151 K. The total oxygen desorbed from the
Bi-doped catalyst was only one-fourth that of the undoped catalyst, while the amount of oxygen removed by TPR was roughly the
same for both catalysts. These results suggest that in anaerobic oxidation, the Bi-doped catalyst will have roughly the same activity as
in undoped catalyst in C
4
hydrocarbon oxidation but will have a higher selectivity to products such as olefins and maleic anhydride.
KEY WORDS: vanadyl pyrophosphate; promoter; bismuth; butane oxidation; maleic anhydride.
1. Introduction
Vanadium phosphorus oxide (VPO) is the commer-
cial catalyst for the selective oxidation of butane to
maleic anhydride. It is, so far, the only successful
industrial process utilizing alkanes [1,2]. It is considered
that the main active phase in catalysts stabilized under
reaction conditions is ðVOÞ
2
P
2
O
7
, which is formed
topotactically from the precursor VOHPO
4
Á 0:5H
2
O
that can be prepared by a number of methods [3]. One of
the methods of preparation is by the reduction of
VOPO
4
Á 2H
2
O, the catalyst resulting from which is
found to be particular effective for n-butane oxidation.
Catalytic performance may be improved by adding
specific doping agents to the VPO composition. The
nature, the location and the role of these dopants have
been previously reviewed [4]. Several different metals
have been used as the modifier, and data has been
published on their influence on the yield and selectivity
of formation of maleic anhydride, and on the reaction
rate of these catalysts [5,6]. A somewhat recent
publication was concerned not only with general aspects
of the reaction of modified VPMO catalysts (where M
stands for metal promoter) but also with the effect of
modifiers on the structure and morphology of the
catalysts in optimizing selective butane oxidation to
maleic anhydride [7].
In general, promoters only constitute a small part of
the overall composition of VPO catalysts [8]. Bismuth
(Bi) has been used as a promoter by some researchers [9–
11], showing an increase in selectivity to MA upon the
addition of Bi. For the most advantageous performance
properties, it is preferred that the total atomic ratio of Bi
to the proportion of vanadium should be in the range of
0.001 : 1 to 0.2 : 1, preferably 0.005 : 1 to 0.07 : 1 [10].
Furthermore, X-ray diffraction (XRD) studies have
shown that the Bi-doped materials are predominantly
ðVOÞ
2
P
2
O
7
with a minor amount of BiPO
4
[12].
A bismuth additive was introduced into VPO catalyst
as bismuth chloride, but a yield of only 37% was found
with this catalyst [8]. Haber et al. [13] have studied the
effect of bismuth additive introduced into the VPO
precursor, which was prepared in a classical organic
medium. The Bi-doped catalyst gave an n-butane
conversion of 76% and selectivity of 68% after the
catalytic tests. A VPBiO catalyst is also used in
n-pentane partial oxidation [14].
In this paper, the physical and chemical modifications
induced in a vanadyl pyrophosphate catalyst will be
examined.
2. Experimental
2.1. Preparation of doped vanadyl pyrophsophate
catalysts
The undoped VOHPO
4
Á 0:5H
2
O precursor was pre-
pared via VOPO
4
Á 2H
2
O (denoted as VPD method).
V
2
O
5
(12.0 g from Fisher Scientific) and ortho-phospho-
ric acid H
3
PO
4
(115 g, 85% from Fisher) were refluxed in
water (24-mL H
2
O=g solid) for 24 h. The resulting
VOPO
4
Á 2H
2
O was recovered by filtration and washed
with water and finally identified by XRD. This solid
(4 g) was then refluxed with isobutanol (80 mL from
BDH) for 21 h, and the resulting solid was recovered by
filtration and dried in air (383 K, 16 h).
Ã
To whom correspondence should be addressed.
Catalysis Letters Vol. 89, Nos. 1–2, July 2003 ( # 2003) 87
1011-372X/03/0700–0087/0 # 2003 Plenum Publishing Corporation