PARTIAL OXIDATION OF OLEFINS - CONVERSION OF ISOBUTENE TO TERT-BUTYL ALCOHOL ON OXYGEN-COVERED RH(111)

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 approximately 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 approximately 370 K. Nonselective dehydrogenation occurs at the clean Rh sites of the oxygen-covered surfaces at approximately 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 approximately 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.