Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 6
Figure 6‐7: Comparison of the catalytic epoxidation of (E)‐ and (Z)‐stilbene with homogeneous cobalt
complexes; (■) (E)‐stilbene conversion (□) trans‐stilbene oxide selectivity from (E)‐stilbene; (●) (Z)‐stilbene
conversion (○) trans‐stilbene oxide selectivity from (Z)‐stilbene; reaction conditions: 2.0 mmol stilbene, 100
mg biphenyl, 30 mL DMF, 50 mL min ‐1 O2, 2.4 mg Co(acac)3.
6.3.4 Influence of reaction parameters
As a next step the effects of temperature, catalyst amount, substrate concentration and oxygen flow
rate were studied. Higher temperatures enhanced the reaction rate (Figure 6‐8a). At 60 °C the
conversion was only 7 % after 10 h while at 120 °C the reaction was almost complete (95 %
conversion). Interestingly, also the selectivity to the epoxide was lowest at 60 °C (48 %). Low epoxide
selectivity was on the other hand always obtained at low conversions. Above 80 °C the temperature
had little influence on the epoxide selectivity being close to 90 %. The reaction rate was highly
dependent on the oxygen flow rate (Figure 6‐8b). This is interesting, since the overall reaction rate is
small and the consumption of oxygen should be overcompensated already by small gas flows.
Epoxidations with gold catalysts proceed at similar rates [6] and were conducted in an open reaction
vessel without any additional oxygen flow. In the present study, faster removal of volatile catalyst
poisons such as amines (vide infra) might be the cause of a beneficial high gas flow. Consequently,
reaction mixtures became considerably darker after 12 h when the flow was low potentially due to
amine oxidation products. This was, however, not the sole reason for the observed effect. Using air
instead of pure oxygen with the same flow rate resulted in smaller conversions. After 6 h the
conversion was 8 % with an air flow of 50 mL/min compared to 38 % with 50 mL/min oxygen. Thus,
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