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V
2
O
5
FeVO
4
V
2
O
5
/Fe
2
O
3
CH
3
HO
O
CH
3
Formaldehyde
d
n
4
r
4
n
g
|
3
O V O V O V O
CH
3
HO
Fe
2
O
3
O
CH
3
Methanol and DME
O Fe O Fe O Fe O
Scheme 8.1
Surface intermediate species detected on different catalysts when
adsorbing methanol at 100 1C and their respective products released
at higher temperature.
13
8.2.4 New Reactions and Applications of Methanol
Gas-Phase Oxidation
8.2.4.1 Dimethoxymethane Synthesis
Dimethoxymethane (DMM) is generally synthesized after the oxidation of
methanol to formaldehyde by using an acidic catalyst and additional
methanol to perform acetalization (Scheme 8.2). Among the most commonly
used catalysts are ion-exchange resins (which may degrade and cause clog-
ging of the reactor) and inorganic acids (such as sulfuric acid, with all its
related complications such as corrosion, environmental pollution, the need
for neutralization of waste water and irreversible loss of the acid).
Recently, DMM has gained importance as a green solvent, fuel additive
and precursor of the so-called POMM [poly(oxymethylene dimethyl ether)],
which is a good candidate to replace methanol in fuel cells, with lower
toxicity and higher eciency.
15
However, to make it attractive from the
economical point of view, research in progress is aiming to produce it in a
single step directly from methanol.
Among the catalysts used in the attempt to synthesize DMM directly, the
most successful ones include bulk and supported Keggin structures
16
[H
4
PVMo
11
O
40
-SiO
2
(DMM selectivity of 58.1% at a methanol conversion
of 68.2%)], RuO
2
-Al
2
O
3
17
(DMM selectivity of 66.8% at a methanol con-
version of only 20%), Re oxide and Re-based mixed oxides supported on
TiO
2
,V
2
O
5
,ZrO
2
,Fe
2
O
3
and Al
2
O
3
18
(maximum DMM selectivity between 60
and 93% for a methanol conversion of 15-60%) and also (SO
4
)
2-
-V
2
O
5
-
TiO
2
19
(with a DMM selectivity of 90% for 64% methanol conversion).
However, the catalytic system with the best productivity for this reaction
appears to be industrial Fe-Mo (
.
4.6 kg
DMM
h
1
kg
cat
1
at 280 1Cata
methanol conversion of 56% and a DMM selectivity of 90%) as reported
recently by Dumeignil et al.
20,21
What really makes the difference and directs
the reaction towards DMM production is mainly the use of a feed rich in
B
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