Hydrogenation of conjugated dienes over ZrO2 by H2 and cyclohexadiene

Hydrogenation of 1,3-butadiene and 2-methyl-1,3-butadiene and equilibration reaction of H₂D₂ were carried out over ZrO₂ using H₂ and Cyclohexadiene as hydrogen sources. Reaction rates were measured by changing the activation temperature of the catalyst. While the hydrogenation with H₂ and H₂D₂ equilibration reaction gave an optimum activity at 600 °C, another optimum was obtained for transfer hydrogenation at 800 °C. Hence it is concluded that the sites responsible for the transfer hydrogenation are not the same as those which catalyze hydrogenation with H₂ and H₂D₂ equilibration. Product distributions in n-butenes and methylated butenes were compared on ZrO₂, ThO₂, La₂O₃, and MgO and were also compared in direct hydrogenation with H₂ and transfer hydrogenation with Cyclohexadiene. Assuming an ionic intermediate, selectivity changes in the monoolefins produced over different catalysts and by different hydrogen sources were interpreted in terms of the variation of the anionic character of the intermediate and the shift of anionic to neutral or cationic intermediate, respectively.