The role of Pd~(2+)/Pd~0 in hydrogenation by [Pd(2-pymo)_2]_n: An X-ray absorption and IR spectroscopic study

摘要

The metal-organic framework (MOF) [Pd(2-pymo)_2]_n (2-pymo=2-pyrimidinolate) was used as catalyst in the hydrogenation of 1-octene. During catalytic hydrogenation, the changes at the metal nodes and linkers of the MOF were investigated by in situ X-ray absorption spectroscopy (XAS) and IR spectroscopy. With the help of extended X-ray absorption fine structure and X-ray absorption near edge structure data, Quick-XAS, and IR spectroscopy, detailed insights into the catalytic relevance of Pd~(2+)/Pd~0 in the hydrogenation of 1-octene could be achieved. Shortly after exposure of the catalyst to H_2 and simultaneously with the hydrogenation of 1-octene, the aromatic rings of the linker molecules are hydrogenated rapidly. Up to this point, the MOF structure remained intact. After completion of linker hydrogenation, the linkers were also protonated. When half of the linker molecules were protonated, the onset of reduction of the Pd~(2+) centers to Pd~0 was observed and the hydrogenation activity decreased, followed by fast reduction of the palladium centers and collapse of the MOF structure. Major fractions of Pd0 are only observed when the hydrogenation of 1-octene is almost finished. Consequently, the Pd~(2+) nodes of the MOF [Pd(2-pymo)_2]_n are identified as active centers in the hydrogenation of 1-octene. Ex and in situ X-ray absorption and IR spectroscopy revealed that the active centers in hydrogenation of olefins by metal-organic frameworks (MOFs) [Pd(2-pymo)_2]_n (see figure) are Pd~(2+). Deactivation of the catalyst is caused by Pd ~0 formation following hydrogenation of the linker molecules, which leads to decomposition of the MOF structure.