(658d) In Situ ATR-IR Studies On the Mechanism of Limonene Epoxidation Over PW-Amberlite

PW-Amberlite is a promising catalyst for limonene epoxidation with aqueous hydrogen peroxide and acetonitrile as solvent; limonene conversion and limonene epoxide selectivity higher than 80% and 90%, respectively, have been reported. The mechanistic pathway proposed for limonene epoxidation on PW-Amberlite, includes the interaction of H₂O₂ with peroxophosphotungstate species to form polyperoxophosphotungstate active sites, where limonene is adsorbed and reaction takes place. The catalytic cycle is completed with reaction products desorption. In situ attenuated total reflection (ATR) IR spectroscopy is used in this contribution to follow the time course reaction. Limonene IR spectrum has a characteristic band signal in the 910-880 cm^-1 range that is attributed to the =CH and =CH₂ out of the plane bending vibrations. It was observed that its intensity decreased with time during reaction. Bands in the 1145 - 970 cm^-1 region are attributed to oxygen interaction with phosphorous, and to P-O-C symmetric and asymmetric stretching vibrations [5]. The bands between 960 - 800 cm^-1 have been assigned to oxygen interactions with tungsten [5]. Given that IR signals of peroxophosphotungstate from catalyst samples and IR signals of solvent and water or aqueous hydrogen peroxide were subtracted as background, the observed peaks appear to correspond to interactions of polyperoxophosphotungstate species with limonene. Weak multiple signals in the 880-840 cm^-1 region are due to the epoxy group vibration of limonene epoxide formation, while those between 840-820 cm^-1 are tentatively attributed to W=O and O-O interactions related to reaction intermediates. The spectroscopic measurements are in accord with the previously proposed mechanistic pathway and kinetic expressions that have been derived for this system.