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conjunction with microwave heating resulted in dramatic reductions in re-
action time, which decreased from hours to minutes. Similar results were
obtained by Garc
´
a-Mart
´
nez and co-workers when using iron oxide nano-
particles (0.5-1.2 wt%) on MCM-41 type silica materials.
158
d
n
4
r
4
n
g
|
1
1.6 Photochemistry in Oxidation Reactions
Photocatalysis is a branch of chemistry that exploits light radiation to
overcome the energy barriers of chemical reactions.
159
Solar light is a re-
newable source of energy but only in the last century has it been analyzed as
a potential motor for chemistry. Photochemical processes between light and
matter are now being studied to become both supplementary and comple-
mentary to traditional reactions. So far, the most studied applications for
photocatalysis have been the destruction of pollutants in water
160-162
and
water splitting.
163
Nevertheless, photocatalysis has recently been used for the oxidation re-
actions of both aliphatic and aromatic alcohols and a review on the trans-
formation of alcohols by heterogeneous photocatalysts was published fairly
recently.
164
Pristine and doped titania have been mainly used as photocatalysts, owing
to their reliability, stability under irradiation and low cost, although differ-
ent semiconductor and insulator solids have also been used. Recently,
Augugliaro and co-workers
165
investigated the UV photo-oxidation of piper-
onyl alcohol in water using a TiO
2
suspension. All the TiO
2
samples, pre-
pared by the hydrolysis of TiCl
4
or from TiOSO
4
precursor - less crystalline
and more hydroxylated than commercial titania - exhibited higher select-
ivity, but their reaction rate was higher in the presence of commercial
Degussa P25. This behavior was in accordance with previous results
obtained for the selective oxidation of other alcohols to the corresponding
aldehydes.
166,167
Other products formed, in addition to piperonal, were
carbonic anhydride and 1,3-bis[3,4-(methylenedioxy)benzyl] ether
(Scheme 1.21); traces of this last compound were found only starting from
the highest concentration of alcohol.
The use of heterogeneous photocatalysis for selective chemical transfor-
mations of biomass-derived compounds was reviewed recently.
168
The
catalytic selective photo-oxidation of biomass was reported to provide vari-
ous platform molecules such as levulinic, 2,5-furandicarboxylic, succinic,
.
O
O
CO
2
O
O
OH
O
O
O
O
CO
2
CO
2
O
O
Trace
Scheme 1.21 Main reaction pathways for piperonyl alcohol oxidation.
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