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Fluidized bed reactors as an option to avoid the well-known hot spot
temperatures. However, owing to their low mechanical resistance, un-
supported iron molybdates are not suitable for this configuration.
Currently, research is focused on finding a suitable catalyst. Until now,
low surface area silica seemed to be the best support, but the activity of
this supported iron molybdate is much lower than that of the un-
supported material.
3
The replacement of part of the Mo by other
elements (such as V, Sb, P, Sn and Mn) could probably lead to more
active supported catalysts. Nevertheless, there is a need for more re-
search in this area.
d
n
4
r
4
n
g
|
3
8.2.3 New Classes of Catalysts for Methanol Oxidation
Vanadium oxides have redox properties that make them very interesting
catalysts for oxidation reactions, especially in the highest oxidation states
(V
2
O
5
). For instance, when using bare powdered V
2
O
5
at 50% methanol
conversion the main product is formaldehyde, whereas at higher conversion
methylformate is the main product.
8
Different kinds of vanadates have been
extensively investigated by Wachs et al.;
9-11
more specifically, in a patent
12
they claimed that bulk metal vanadates possess higher activity than the
traditional MoO
3
-Fe
2
(MoO
4
)
3
catalysts for methanol selective oxidation,
thus making it possible to use them at lower temperatures while main-
taining selectivity and also avoiding the problems related to catalyst
deactivation by volatilization. Among the catalysts tested, those that
performed better (at methanol conversion higher than 96%) were CrVO
4
,
NbVO
5
and Ni
3
(VO
4
)
2
with 90.4, 90.0 and 94.0% selectivity to formaldehyde,
respectively (at 300 1C and a methanol-oxygen-helium feed gas composition
of 6 : 13 : 81 mol%).
In another investigation by Wachs and co-workers
13
on methanol
oxidation over bulk FeVO
4
and supported 4% V
2
O
5
-a-Fe
2
O
3
, it was found
that both surfaces are similar and consist of VO
x
species which are the
catalytically active sites. The study was performed with temperature-
programmed IR spectroscopy comparing the individual oxides, supports and
the bulk catalyst while methanol was being adsorbed. They found that in all
cases the species CH
3
OH* and CH
3
O* are present on the catalyst surface
with the exception of the acidic a-Fe
2
O
3
, yielding dimethyl ether (DME).
Instead, for V
2
O
5
, FeVO
4
and supported 4% V
2
O
5
-a-Fe
2
O
3
, both surface
intermediates gave rise mainly to HCHO (see Scheme 8.1). The results were
confirmed with HR-TEM images showing an amorphous VO
x
-enriched layer
of
.
1 nm at the outer surface of the bulk FeVO
4
catalysts and also by the in-
creased V : Fe atomic ratio in the surface of the FeVO
4
particles found in the
elementalanalysisbyelectronenergylossspectroscopy(EELS).
Recently, a catalyst formulation containing V and Sb supported on silica
was patented by Wacker Chemie,
14
claiming a high yield for methanol oxi-
dation to formaldehyde (92,1%), which is of great importance in moving
towards fluidized bed technology.
B
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