Investigations on stability and reusability of [Pd(2-pymo)2]n as hydrogenation catalyst

The MOF [Pd(2-pymo)2]n (2-pymo = 2-pyrimidinolate) was used in the liquid-phase hydrogenation of 1-octene and cyclododecene. Up to a reaction time of 4 h, reactant shape selectivity could be observed, i.e. 1-octene was hydrogenated, but not the more bulky molecule cyclododecene. However, if the catalyst was exposed to hydrogen for more than 4 h, also the hydrogenation of cyclododecene took place. Visually a color change of the catalyst was observed and by XRD it was found that Pd0 nanoparticles were formed and that the MOF [Pd(2-pymo)2]n was transformed into a new crystalline phase, where the Pd0 seemed to be accessible. Additionally, Pd0 was leached out and was available for hydrogenation. When reusing the MOF after reaction times less than 1 h, the reaction rate increased with every reuse. We assume that even in the first 4 h Pd0 was formed, but could not be detected by XRD. In-situ XRD measurements exposing the MOF to hydrogen in the gas phase showed that the MOF was much more stable in gas phase hydrogenation, because the MOF structure remained intact even when Pd0 was formed.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Pd in MOF [Pd(2-pymo)2]n was reduced to Pd0 during hydrogenation reactions. ► Shape-selective hydrogenation of 1-octene took place in the first 4 h of reaction time. ► After leaching out the formed Pd0 nanoparticles, cyclododecene was hydrogenated. ► Formation of Pd0 was followed by in-situ XRD experiments in a hydrogen atmosphere. ► Protonation of the linker during hydrogenation was shown by solid-state NMR.