Hydrogenation of conjugated dienes and isomerization of butenes on CdO, Co3O4 and Cr2O3
Abstract
Deuteration of buta-1,3-diene, 2-methylbuta-1,3-diene and penta-1,3-diene selectively yielded but-2-ene, 2-methylbut-2-ene and 1,4-[2H2]pent-2-ene on CdO and Co3O4, and but-1-ene, 2-methylbut-1-ene and 4,5-[2H2]pent-2-ene on Cr2O3, respectively. These results imply that the selectivity for 1,2- and 1,4-hydrogen addition is maintained in hydrogenation of conjugated dienes, i.e.1,4-addition on CdO and Co3O4, and 1,2-addition on Cr2O3. The hydrogenation of buta-1,3-diene using a 1:1 mixture of H2 and D2 gave [2H2]but-1-ene in larger amounts than the [2H0]isomer on Cr2O3. An isotope effect smaller than unity was interpreted by preferential adsorption of D2 on the Cr2O3 surface. Formation of [2H1]butene was dominant in deuteration of buta-1,3-diene, indicating that residual hydrogen participates in the reaction; the amount corresponded to 7 times as much as that of chemisorbed hydrogen at room temperature. Co-isomerization of [2H0] and [2H8]but-1-ene yielded, selectively, [2H0] but-2-ene on CdO. Hydrogenation of buta-1,3-diene with a 1:1:1 mixture of H2, HD and D2 also gave [2H0] butene isomers selectively on CdO. These results infer that one intermediate species for hydrogenation of buta-1,3-diene is closely related to that for isomerization of butenes, and these reactions occur through a π-allylic anion intermediate on CdO.