2 research outputs found

    2‑Propanol Dehydration on the Nodes of the Metal–Organic Framework UiO-66: Distinguishing Catalytic Sites for Formation of Propene and Di-isopropyl Ether

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    2-Propanol dehydration was used as a test reaction to probe the catalytic properties of metal–organic framework (MOF) UiO-66. Experiments were performed with a flow reactor operated at atmospheric pressure and 510 K, showing (a) how the catalytic activity increased and then decreased, depending on the nature of ligands on the Zr6O8 MOF nodes (such as formate, acetate, hydroxyl, or alkoxy groups); and (b) how the selectivity changed with changing node ligands, which were characterized by IR spectroscopy, 1H NMR spectroscopy of digested MOF samples, and other techniques. The selectivity is sensitive to the node ligand composition, with the dehydration reaction initially facilitated by the removal of adventitious node formate and acetate ligands formed in the MOF synthesis and concomitant formation of node OH ligands from water formed in the catalysis. Node pair sites consisting of a node Zr-μ1-OH site and a neighboring node zirconium vacancy site are inferred to be active for propene formation. The ether formation rate increased with an increasing density of node 2-propoxy ligands, leading to the suggestion that these ligands at a paired zirconium defect site react with adjacent 2-propanol molecules to form di-isopropyl ether in a bimolecular nucleophilic substitution mechanism. These results show how the selectivity of UiO-66 can be modulated simply by changing the node ligands though postsynthetic modifications, without changing the node motif, oxidation state of the node metal atoms, pore structure, MOF topology, or linker chemistry

    Beating Heterogeneity of Single-Site Catalysts: MgO-Supported Iridium Complexes

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    Catalysts consisting of isolated metal atoms on oxide supports have attracted wide attention because they offer unique catalytic properties, but their structures remain largely unknown because the metals are bonded at various, heterogeneous surface sites. Now, by using highly crystalline MgO as a support for metal sites made from a mononuclear organoiridium precursor and investigating the surface species with X-ray absorption spectroscopy, atomic resolution electron microscopy, and electronic structure theory, we have differentiated among the MgO surface sites for iridium bonding. The results demonstrate the contrasting structures and catalytic properties of samples, even including those incorporating iridium at loadings as low as 0.01 wt % and showing that the latter are nearly ideal in the sense of having almost all the Ir atoms at equivalent surface sites, with each Ir atom bonded to three oxygen atoms of the MgO surface. These supported molecular catalysts are modeled accurately with density functional theory. The results open the door to the precise synthesis of families of single-site catalysts
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