T. Förster et al. / Journal of Catalysis 283 (2011) 25–33
33
presence of Mn3+, the O–O bond is preferably cleaved heterolytical-
ly (see Scheme 3 upper route) resulting in an alkoxide intermedi-
ate, which is subsequently transformed to 2-methylbenzyl
oxidation catalysis. The possibility to subtly tailor the polarity of
the pore walls and the potential to include specific bonding sites
into these walls open up a highly flexible strategy for catalyst
design.
2+
alcohol. The higher selectivity of Co toward o-tolualdehyde
2
+
attributed to Co favoring homolytic bond scission (see Scheme 3
lower route). This is supported by the observation that in the case
of Co2+ a higher maximum concentration of 2-methylbenzyl hydro-
Acknowledgments
3
+
peroxide is produced during the reaction compared to Mn (2-
methylbenzyl hydroperoxide yield 0.3 vs. 0.08 mol%, see Fig. 6).
Note that the observation of hydroperoxide can directly be related
to the presence of radicals in the reaction solution, as it can only be
formed via a radical pathway.
The authors thank the German Bundesministerium für Bildung
und Forschung (BMBF) for partial financial support of the pre-
sented work (Project No. 03X2007A). We are very grateful to Dipl.
Ing. M. Neukamm for conducting AAS measurements, Dipl. Ing. X.
Hecht for operational support, and S. Pedron for experimental help.
Furthermore, the authors acknowledge fruitful discussions in the
framework of the network of excellence IDECAT.
The qualitative and quantitative stability of the initiator/cata-
lyst materials are unequivocally proven by the combination of
assessing the concentrations of metal cations (i.e., the constant ele-
2
+
mental composition of the polysiloxanes and the absence of Co in
the liquid phase after reaction) as well as by spectroscopic charac-
terization of their interaction with probe molecules and reactants.
In this respect, note that the CO concentration on used catalysts
was identical to that on the fresh materials provided that the
accessibility of the transition metal cations was assured. UV/vis
spectroscopy in turn gives evidence that the Co species after reac-
tion are identical to those before the reaction indicating no change
in the overall oxidation state.
Appendix A. Supplementary material
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