Benzene formation during the oxidation of cyclohexene in the presence and absence of thiophene over a supported palladium catalyst

The catalytic oxidations of cyclohexene and thiophene, singly and together, have been studied over a Pd/CeO₂/Al₂O₃ monolith catalyst. On a freshly reduced sample, the oxidation of cyclohexene on its own commences below 100°C with benzene as the major product over a considerable temperature range, even when O₂ is in large excess. Benzene is still produced when cyclohexene and thiophene are co-oxidised and the reaction exhibits oscillations, associated with accumulation and removal of sulfur, at high conversions. The oxidation of thiophene is self-poisoned to some degree and sulfur accumulated during it considerably reduces the activity of the catalyst for the subsequent oxidation of cyclohexene. However, the maximum yield of benzene remains high.