Partial Oxidation of Olefins: Conversion of Isobutene to tert-Butyl Alcohol on Oxygen-Covered Rh(111)

Article abstract of DOI:10.1021/j100179a043

The selective oxidation reactions of isobutene on oxygen-covered Rh(111) have been investigated by temperature-programmed reaction and X-ray photoelectron spectroscopies.Isobutene is selectively oxidized to tert-butyl alcohol by atomic oxygen on Rh(111) with coverages in the range of 0.3-0.5.Desorption of isobutene and combustion to carbon monoxide, carbon dioxide, and water are competing processes.One C-O, one C-H and one O-H bond are formed during the oxidation of isobutene to tert-butyl alcohol.No reversible C-H bond activation and, specifically, no allylic C-H bond breaking is induced in the isobutene that reacts to produce tert-butyl alcohol.We propose that oxygen directly adds to the 2-carbon of isobutene followed by C-H bond formation to afford tert-butoxide at ca. 250 K.The C-H bond breaking of tert-butoxide and the other hydrocarbon fragments is proposed to be the rate-limiting step for the evolution of tert-butyl alcohol, isobutene, and water at ca. 370 K.Nonselective dehydrogenation occurs at the clean Rh sites of the oxygen-covered surfaces at ca. 250 K and involves both methylenic and allylic C-H bond breaking to provide a source of hydrogen for tert-butoxide formation.The product yields and selectivity depend on the oxygen coverage, and a maximum tert-butyl alcohol yield is observed on Rh(111) with an oxygen coverage of ca. 0.4 monolayers.Intermediate oxygen coverages optimize the requirements for C-O addition, without dehydrogenation, and some dehydrogenation to produce a source of adsorbed hydrogen.Isobutene oxidation on Rh(111) is dramatically different from that on Ag or metal oxides, in particular, because the oxygen on Rh(111) does not serve as a Bronsted base.