Styrene oxidation was investigated using manganese(III) salen complexes as catalysts in homogeneous media and encapsulated in NaX and NaY zeolites using tert-butylhydroperoxide as the oxygen source. The oxidation of styrene led to formation of benzaldehyde, styrene oxide and phenylacetaldehyde, with minor amounts of phenyl-1,2-ethane-diol; some polymer formation was also observed. In homogeneous media, both complexes with ligands functionalized with methoxyl groups showed similar styrene conversions and chemoselectivities in styrene epoxide and benzaldehyde. Upon immobilization, [Mn(3-MeOsalen)]@X showed the highest catalytic activity at 60 β’ C and atmospheric pressure, and the highest chemoselectivities in styrene epoxide and benzaldehyde. No leaching of the encapsulated metal complexes was observed during the catalytic reactions and support for the entrapment of metal complexes inside the zeolite cages and not on their external surfaces was provided by nitrogen adsorption analyses and SEM micrographs.
The reaction mechanism was investigated using the most active heterogeneous catalyst, [Mn(3-MeOsalen)]@X, and the influence of various reaction parameters, such as substrate, catalyst concentration and temperature, on alkene conversion and styrene epoxide yield have been studied. Furthermore, the homogeneous oxidation of styrene using [Mn(3-MeOsalen)CH 3 COO] was monitored by electronic spectroscopy, the results suggesting a free radical mechanism involving a peroxomanganese species as the catalyst active intermediate.