90
M. Ziolek et al. / Catalysis Communications 37 (2013) 85–91
Scheme 1. General reaction pathway of glycerol oxidation.
(
hydrated niobia) [30] and isolated Nb\OH hydroxyls interact easily
making amorphous oxide active in glycerol oxidation.
Crystalline oxides do not interact with hydrogen peroxide toward
the formation of radical peroxo species.
2 2
with H O
Crystalline niobia is not covered by hydroxyls like amorphous mate-
rial and oxygen located in the crystalline structure is too stable for
It has been proved that peroxo radical species formed in the inter-
action of H
the oxidation of glycerol to glycolic acid with hydrogen peroxide.
The radicals formed on amorphous Ta surface treated with hy-
2 2 2 5
O with amorphous Nb O are the active intermediates in
the interaction with H
gests the role of peroxo species (Nb(O)
ol. As was mentioned before peroxo species are present on the Nb
surface after treatment with H at the reaction temperature (Fig.
S1 SD). Peroxo species takes part in the dehydrogenation of glycerol
H-abstraction from one of the primary hydroxyl groups) toward
glycerolate being the intermediate in products formation [34]. Selec-
tivity to glycolic acid is determined by the presence of H which is
responsible for oxidative C\C bond cleaving. It is important to stress
that the selectivity to glycolic acid on amorphous Nb is much
higher than that shown in [36] for gold modified carbon and TiO
2
O
2
. The activity of amorphous sample sug-
2
) in the conversion of glycer-
2 5
O
2
O
5
drogen peroxide are poorly active in the oxidation of glycerol.
2 2
O
Acknowledgments
(
National Science Centre in Cracow, Poland (Grant No. 2011/01/B/
ST5/00847) is acknowledged for the financial support of this work.
The authors acknowledge Prof. Z. Sojka for valuable discussion. Thanks
are also due to CBMM Company (Brazil) for supplying hydrous amor-
phous Nb O .
2 2
O
2 5
O
2
2
5
(
ca. 40–50%). This makes amorphous Nb
for e.g. gold in glycerol oxidation with H
In contrast, the amorphous Ta
the conversion) in glycerol oxidation with H
the fact that peroxo species were not observed on the surface of this
oxide after H treatment. The activity of crystalline Ta is a little
bit higher than on amorphous Ta because this catalyst possesses
acidic hydroxyl groups which can interact with H
2
O
5
very promising support
toward glycolic acid.
2
O
2
Appendix A. Supplementary data
2
O
5
is almost inactive (only ca. 1% of
in accordance with
O
2 2
O
2 2
2 5
O
2
O
5
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4
. Conclusions
Amorphous Nb
2 5
O contacted with hydrogen peroxide exhibits
[
[
high ability to form radical peroxo species which are the source of ac-
tive oxygen allowing the conversion of glycerol to glycolic acid.
[
2 5
Amorphous Ta O contacted with hydrogen peroxide also interacts
with the oxidant toward radicals (unidentified yet) which are formed
on the surface.
[