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
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
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