Gas-phase studies of alkane oxidation by transition-metal oxides. Selective oxidation by CrO⁺

Abstract

The gas-phase reactions of CrO⁺ with alkanes have been studied by using ion beam reactive scattering techniques. CrO⁺ undergoes facile reactions with alkanes larger than methane. CrO+ selectively oxidizes ethane to form ethanol. In addition to the possibility of alcohol formation, reactions with larger alkanes are more complex, yielding products in which dehydrogenation and loss of alkenes and alkanes occur. In reactions with cyclic alkanes, cyclopropane and cyclobutane yield products characteristic of C-C bond cleavage. In contrast, reactions with cyclopentane and cyclohexane mainly involve dehydrogenation and elimination of H₂O. A series of hydrogen abstraction reactions are examined to determine the bond dissociation energy D⁰(CrO⁺-H) = 89 ± 5 kcal mol⁻¹. This bond energy has implications for the reaction mechanisms of CrO⁺ with alkanes, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add across the Cr⁺-O bond as an initial step. This is supported by an examination of the reactions of Cr⁺ with alcohols.