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Influence of 1āButene Adsorption on the Dimerization Activity of Single Metal Cations on UiO-66 Nodes
Grafting metal cations to missing linker defect sites
in zirconium-based
metalāorganic frameworks, such as UiO-66, produces a uniquely
well-defined and homotopic catalytically active site. We present here
the synthesis and characterization of a group of UiO-66-supported
metal catalysts, M-UiO-66 (M = Ni, Co, Cu, and Cr), for the catalytic
dimerization of alkenes. The hydrogenādeuterium exchange via
deuterium oxide adsorption followed by infrared spectroscopy showed
that the last molecular water ligand desorbs from the sites after
evacuation at 300 Ā°C leading to M(OH)-UiO-66 structures. Adsorption
of 1-butene is studied using calorimetry and density functional theory
techniques to characterize the interactions of the alkene with metal
cation sites that are found active for alkene oligomerization. For
the most active Ni-UiO-66, the removal of molecular water from the
active site significantly increases the 1-butene
adsorption enthalpy and almost doubles the catalytic activity for
1-butene dimerization in comparison to the presence of water ligands.
Other M-UiO-66 (M = Co, Cu, and Cr) exhibit 1ā3 orders of magnitude
lower catalytic activities compared to Ni-UiO-66. The catalytic activities
correlate linearly with the Gibbs free energy of 1-butene adsorption.
Density functional theory calculations probing the CosseeāArlman
mechanism for all metals support the differences in activity, providing
a molecular level understanding of the metal site as the active center
for 1-butene dimerization