Controlling Selectivity in Alkyl Oxidation with Oxygen Coverage: The Reactions of Ethyl and 2-Propyl Iodide on Oxygen-Covered Rh(111)

Article abstract of DOI:10.1021/j100031a022

The oxidation of alkyl iodides on oxygen-covered Rh(111) has been studied using temperature-programmed reaction and high-resolution electron energy loss spectroscopies.Ethyl and 2-propyl iodide are selectively oxidized to acetaldehyde and acetone with selectivities of ca. 62percent and ca. 40percent, respectively, on Rh(111)-p(2 x 1)-O (θ = 0.5).Formation of ethene and propene are competing pathways.No CO or CO2 is formed at this oxygen coverage.C-I bond breakage in the intact alkyl iodides is proposed to be the rate-determining step in the formation of both the alkenes and the oxygenates.The resulting alkyl either rapidly dehydrogenates, eliminating the alkene, or adds oxygen, forming a transient alkoxide which subsequently eliminates the aldehyde or ketone via dehydrogenation.The selectivities for the different reaction pathways depend strongly on the oxygen coverage.On clean Rh(111), nonselective decomposition to H2 and adsorbed carbon is the dominant pathway.At moderate oxygen coverages (θ < 0.3), CO, CO2, alkene and alkane formation predominate.The strong dependence of the product distributions on the oxygen coverage is attributed to the multifunctionaI role of oxygen on Rh(111).Oxygen inhibits dehydrogenation such that selective β-H elimination and oxygen addition are enhanced at the expense of nonselective dehydrogenation leading to CO and CO2.Carbon-iodine bond cleavage is also retarded by oxygen so that the resulting alkyl radical rapidly reacts to products at high oxygen coverage, allowing for direct oxygen addition to the alkyls.The strong dependence of product distribution on oxygen coverage has important implications for alkane oxidation on Rh catalysts and is in excellent agreement with recent studies of alkane oxidation over rhodium monoliths.Our results suggest that the oxygen coverage can be used to manipulate product distributions in alkane oxidation so as to enhance direct partial oxidation.