Nature and reactivity of intermediates in hydrogenation of buta-1,3-diene catalyzed by cobalt and palladium-gold alloys

The gas-phase hydrogenation of buta-1,3-diene has been studied in a static system using alumina-supported cobalt, a series of cobalt powders, and pumice-supported palladium-gold alloys as catalysts. But-1-ene and but-2-enes, which were formed by 1,2- and 1,4-addition of hydrogen respectively to adsorbed diene, were initial products under all conditions, and yields of n-butane were either very low or zero. The distributions of butenes were very dependent on the physical nature of the cobalt catalysts. Well-sintered metal, consisting of large particles, gave selective formation of trans-but-2-ene, whereas, cobalt prepared in a finely divided state at lower temperatures was more active per unit surface area, but gave high yields of but-1-ene and much lower trans to cis ratios in the but-2-enes. These variations are attributed to differences in the catalytic properties of different crystal faces and the change in reaction mechanism is believed to be mainly due to a change in the ease of formation and stability of 1-methyl-π-allyl intermediates.Stable π-allylic complexes are formed readily on the surface of palladium and various amounts of gold in the alloys have a small effect on the mechanism of buta-1,3-diene hydrogenation. The but-1-ene yields and activation energies exhibited sharp maxima which coincided in the 60–75 % gold range and an explanation in terms of variations in the nature of the palladium d-orbitals is attempted. The temperature dependencies of the product distributions were influenced between 80 and 130°C by the β- to α-phase change of the hydrides of palladium and palladium-rich alloys and the effect is discussed.