Dehydrogenation of cyclohexene over carbon deposited on alumina

Abstract

The dehydrogenation of cyclohexene over carbon deposited on alumina, C/Al₂O₃, which was prepared by contacting hydrocarbons such as cyclohexane, cyclohexene and cyclohexanone with alumina surface at ≥823 K, was investigated under non-oxidative conditions. The deposited carbon was identified as condensed aromatic hydrocarbon with graphite-like structure in the results of XRD and ¹³C NMR measurements. It was found that the dehydrogenation activity was generated on alumina by carbon deposition: the dehydrogenation of cyclohexene proceeded over C/Al₂O₃ at 773 K, while the isomerization to methylcyclopentenes proceeded over pure alumina without carbon. The selectivity to dehydrogenated products such as benzene and 1,3-cyclohexadiene was maximized at a specific carbon content. The disproportionation of cyclohexene into benzene and cyclohexane proceeded over C/Al₂O₃ above the specific carbon content. Unpaired electrons observed in the EPR measurement are related to the ratio of dehydrogenation and disproportionation, whereas the dehydrogenation activity is not related with the unpaired electron content.