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In-Depth Information
O
O
O
-
H
2
,-C
O
1/2 O
2
O
2
CO
2
OH
OH
-H
2
O
OH
O
2
HO
OH
d
n
4
r
4
n
g
|
3
-H
2
O
1/2 O
2
-H
2
O
O
OH
HO
O
O
ketals and heavy compounds (aromatics)
Scheme 8.6 Main reactions occurring in glycerol oxidehydration to acrylic acid.
3-hydroxypropanal and hydroxyacetone; the latter can be further converted
into acetaldehyde and oxidized to acetic acid, whereas the former dehydrates
to acrolein. However, through a retroaldol reaction, 3-hydroxypropanal can
lead to acetaldehyde and formaldehyde. Furthermore, aldehydes can react
with unconverted glycerol to form cyclic ether products. In fact, one key issue
in the aim of limiting the formation of by-products is to operate under
conditions at which no residual glycerol originates along the catalytic bed.
113
Regarding gas-phase glycerol ammoxidation, Guerrero-P´rez and
Ba˜ares
122
reported that, by using a V-Nb-Sb mixed oxide with the rutile
structure, it was possible to convert glycerol directly into acrylonitrile, with
58.3% selectivity at 82.6% conversion. An alternative two-bed approach
was proposed by Dumeignil and colleagues;
109,123
here, glycerol is first
dehydrated to acrolein on to a WO
3
/TiO
2
catalyst, while in the subsequent
bed, acrolein is ammoxidized with a V-Fe-Sb-O catalyst. The same group
also proposed an innovative process from allyl alcohol, which can also be
obtained from glycerol, by using a rutile/iron antimonate catalyst; under
optimized reaction conditions, a yield of acrylonitrile as high as 83% was
obtained.
124
.
8.5 Gas-Phase Photocatalytic Oxidation of Alcohols
In recent years, gas-phase photocatalytic oxidation (PCO) has gained im-
portance as a green technology in obtaining partial oxidation products of
alcohols under mild conditions. This kind of process uses semiconductor
materials as photocatalysts, the most important and widely applied being
titanium dioxide (TiO
2
) owing to its high photoactivity, low cost, low toxicity
and good chemical and thermal stability. It is, however, a UV light absorber,
but recently important advances have been achieved, making it possible to
activate it with visible light.
125
The use of photocatalysis for the abatement of pollutants (including
alcohols) by complete oxidation has been the subject of many research
studies and reviews. For instance, the study of the complete oxidation of
ethanol using SiO
2
-TiO
2
composites,
126
UO
2
(NO
3
)
2
-TiO
2
,
127
CdS/TiO
2
128
and
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