Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

6.3 Results
Figure 6‐8: Influence of temperature (a), oxygen flow (b), catalyst amount (c) and reactant concentration (d)
on the epoxidation of (E)‐stilbene. Reaction conditions: 2.0 mmol (E)‐stilbene, 100 mg biphenyl, 30 mL DMF,
50 mL min ‐1 O2, 2.0 mg STA‐12(Co), 100 °C, 10 h reaction time; variations: (a) temperature: 60‐120 °C; (b)
oxygen flow 0‐100 mL min ‐1 , reaction time 6 h; (c) Substrate to catalyst ratio 30‐600; (d) DMF 20‐40 mL (■)
(E)‐stilbene conversion; (Δ) epoxide selectivity.
high oxygen availability is crucial. The reaction rate did not increase steadily with the amount of
catalyst (Figure 6‐8c) but rather exhibited an optimal cobalt to substrate ratio of 0.34 mol‐% (80 %
conversion after 10 h) whereas a higher cobalt amountdid not significantly influence the conversion
under the chosen conditions as also found previously [17] and might indicate that the reaction is
limited by mass transfer. Also the substrate concentration had a peculiar influence on the catalytic
process (Figure 6‐8d). With a higher initial concentration of the reactants, a higher conversion was
found after 10 h keeping the catalyst/substrate ratio constant, i.e. using different solvent amounts. In
this way, also the solvent‐to‐catalyst ratio changes. This effect was, however, only seen for long
reaction times and variation in the concentration via the solvent amount. Varying the amount of
substrate leaving the catalyst and solvent amount the same, the reaction rate was almost not
sensitive to the substrate concentration at sufficiently high concentrations (Figure 6‐9) as also
observed with the homogeneous Co catalyst (vide supra). Thus, the heterogeneous reaction rate is
(initially) (I) not depending on the substrate concentration, and (II) at the same time steadily
165