Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

6.4 Discussion
would need to be formed prior to product formation. While indeed DMF oxidation is the cause of the
pronounced solvent effect, a link between conversion and formed peroxides could not be established
in the present case. Subsequent leaching of the MOF to release catalytically active Co species is also
not responsible for the initial increase in the reaction rate. The reaction was shown to be primarily
heterogeneous and pretreatment of the MOF in DMF did not shorten the induction although
leaching could be found. Further in situ EPR investigations suggested that the formation of Co‐O2*
species is not at the origin of the induction phase since these are already present in fresh STA‐12(Co).
Also the formation of a steady‐state radical concentration could at least not be corroborated as a
cause for the induction and reaction. Note that it cannot be excluded that the radical concentration
was below the limit of detection by EPR. Thus, the primary reason for the induction phase appears to
be the formation of benzaldehyde promoting the reaction. The exact role of benzaldehyde is unclear
at this point but its influence is not similar to the Mukaiyama epoxidation since only minute amounts
of benzoic acid were detected and the mass balance was closed for the epoxidation of the stilbene
isomers. Since no reaction was observed in the beginning of the reaction, benzaldehyde might
activate the MOF catalyst and increase the availability of active sites. Benzaldehyde concentration as
a limiting factor for the catalyst activity might serve as an explanation for the small influence of the
amount of MOF used on the catalytic reaction but it might also be conceivable that mass transport
limitations dominate the reaction at high catalyst loadings. Clearly, further studies are necessary to
elucidate the effect of benzaldehyde. With respect to the actual epoxidizing agent three species are
conceivable from which free peroxides found in high concentrations can be excluded. Reaction of Co II
with O2 can result in two different species namely Co‐superoxo species and (mostly) binuclear Co‐
peroxo species [53]. These species are interrelated as Co III ‐O2* ‐ can (possibly reversibly) bind to Co II
with one available coordination site as shown for Co complexes in water [54‐57]. Binuclear Co‐
peroxo complexes also form in DMF [58]. In the cited studies, the formation of peroxo‐species was
favored over the formation of superoxo‐species. From the MOF structure, the formation of peroxides
cannot be excluded. Typical Co‐Co distances in binuclear Co‐peroxo complexes are around 4.5 Å [58,
59] similar to Co‐Co distances in the MOF of 4.87 Å. Still, the formation of Co‐peroxo species would
require some rearrangement in the ligand sphere around the active site, potentially induced by
benzaldehyde. Co‐superoxo species were found both in untreated STA‐12(Co) and under in situ
conditions by EPR hence their formation should not cause an induction phase. μ‐Peroxo complexes
are diamagnetic [59] and homogeneous complexes were found to be active in oxidation reactions
[60]. Peroxides with electron‐deficient oxygen as in percarboxylic acids are effective epoxidizing
agents which is also plausible for Co‐peroxides. The very low activity of STA‐12(Co,Ni) might indeed
be interpreted by a synergistic effect of adjacent Co sites. Due to the 1:3 ratio of Co:Ni enough Co
single sites should be available for an appreciable catalytic activity while the amount of adjacent Co‐
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