Partial oxidation of ethane and ethylene in the presence and absence of (sup 13)C labeled methane on reducible and non-reducible oxide catalysts

摘要

The most widely acceptable mechanism for activation of methane appears to be the reaction of methane from the gas phase with surface oxygen, which abstracts hydrogen from methane to form methyl radicals. The methyl radicals are released into the gas phase and form ethane. However, the reaction pathways for activation of ethane and ethylene are not well established. The relative H-abstraction rates for hydroxyl radicals in the gas phase for C(sub 2)H(sub 6)/CH(sub 4) and C(sub 2)H(sub 4)/PH(sub 4) are reported by McCarty (1992). From these data and suggestion from Lunsford that the C-H bond strength in ethylene is greater than methane, the relative gas-phase reactivity can be estimated to be in the order of C(sub 2)H(sub 6) > CH(sub 4) > C(sub 2)H(sub 4). However, this order was not observed in this study, and it has been reported only for LiCl/MnO(sub x) catalyst, indicating that the H-abstraction is not the only pathway for ethane and ethylene activation. Likewise, depending on the catalyst and the experimental conditions, C-C and C=C bonds are also attacked by the active centers on the catalyst and by the active species in the gas phase. Therefore, the relative reactivity of ethane and ethylene compared to methane appears to strongly depend on the partial pressures of reactants and the type of catalysts used. The relative reactivities are in the order of ethylene > ethane (much gt) methane for reactions in the gas phase and on the catalysts containing reducible oxides of transition metals such as Ca/Ni/K. However, this order changed to ethane > ethylene (much gt) methane for a non-reducible catalyst such as sodium promoted Sm(sub 2)O(sub 3). Oxidation of ethane and ethylene in the presence of methane and catalyst show that methane and ethylene, depending on their partial pressures, compete for active centers, and neither formaldehyde nor ethylene oxide were detected in the presence of methane and the catalyst.