Catalytic hydrogenation in the liquid phase .1. Hydrogenation of isoprene catalysed by palladium, palladium-gold and palladium-silver catalysts

The hydrogenation of isoprene (2-methyl-1,3-butadiene) proceeds at 273 K in EtOH solvent in the presence of Pd catalysts to give the three possible methylbutene isomers with high selectivity (greater than or equal to 98%): their proportions, which remain constant until all the isoprene has reacted, are approximately 2-methyl-2-butene, 50%; 3-methyl-1-butene and 2-methyl-1-butene, each 25%. However especially with Pd/CaCO3, Pd/BaSO4, and Pd/C the proportion of the 2-methyl-2-butene is significantly less than 50% and those of the others correspondingly greater; with Pd-Au/SiO2 made by reduction with N2H4, the proportion of 2-methyl-2-butene decreases with increasing concentration of the Group 11 element. Monoalkene selectivities are described by a simple reaction model based on two isomeric half-hydrogenated pi-allylic radicals; the terminal carbon atoms of the delocalised pi-bond carry charges which depend on the number of CH3-substituents, and the relative chances of adding an H atom at each position, which determine product composition, are affected by the charge on the H atom. Increasing Au or AE concentration is the bimetallic series increases its negative charge.