The Mechanism of the Selective Hydrogenation of 1,3-Butadiene on Copper Surfaces

The hydrogenation of 1,3-butadiene on copper surfaces was studied in detail. A highly selective formation of 1-butene was observed in the range of temperatures from 27 to 76 °C; the initial rate of the formation was well expressed in terms of the partial pressures of hydrogen, ph and butadiene, ph, as v=kph⁄(1+Kpb)2 in the pressure range of 5–100 Torr. The reaction with D2 gave butene-d0 and -d1 as the main products, and the butadiene was partly exchanged with D2 during the hydrogenation. The microwave spectroscopic analysis revealed that the 1-butene produced consisted of CH2=CHCH2CH2D, CH2=CHCHDCH3, and CHD=CHCH2CH3, without even a trace of CH2=CDCH2CH3. Therefore, it was concluded that the reaction proceeds via the isobutenyl surface intermediate and that the dissociative adsorption of hydrogen is rate-determining. The structural analysis of the surface suggested that the stable lattice planes, such as (111), (100)3, and (110), are responsible for the reaction. The origin of the high 1-butene selectivity was discussed on the basis of the surface geometry and the proposed mechanism.