Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 6
Figure 6‐9: Conversion (in mmol)‐time profile at different substrate/catalyst ratios. Reaction conditions: 2.0/
4.0 mmol (E)‐stilbene, 100 mg biphenyl, 30 mL DMF, 50 mL min ‐1 O2, 2.0 mg STA‐12(Co), 100 °C.
increasing.The choice of solvent played a vital role. Toluene did not yield any epoxidation products
while mixtures of toluene and DMF exhibited some catalytic activity. N,N‐Dimethylacetamide (DMAc)
as another amide solvent also proved to be a suitable solvent affording an even higher reaction rate
(Table 6‐2, entry 1,2). With similar conditions as used previously, full conversion (97 %) was achieved
already after 8.5 h compared to 12 h with DMF. On the other hand, the selectivity to trans‐stilbene
oxide was markedly lower, i.e. around 60 % compared to approximately 90 % with DMF. The
generally higher selectivity to benzaldehyde in DMAc might explain the higher reactivity in this
solvent (vide infra). No reaction was observed in pure (E)‐stilbene at 130 °C in contrast to a Co II
exchanged heteropoly acid catalyst tested in the oxidation of styrene to benzaldehyde [46]. Likewise
no reaction was observed in a 1:1 mixture of DMF and water the latter thus being an inhibitor in high
concentrations.
6.3.5 Amines as inhibitors
In a closed reactor system (i.e. an autoclave), the selectivity and conversion dropped significantly
even though relatively high oxygen pressures were applied (Table 6‐2, entry 3) resulting in a high
oxygen availability which had proved beneficial in experiments performed in an open reactor.
Reference [23] also states a lower conversion observed in closed reactors while other studies
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