In this article, we describe the development of a new aerobic C−H oxidation methodology catalyzed by a precious metal-free LaMnO 3 perovskite catalyst. Molecular oxygen is used as the sole oxidant in this approach, obviating the need for other expensive and/or environmentally hazardous stoichiometric oxidants. The electronic and structural properties of the LaMnO 3 catalysts were systematically optimized, and a reductive pretreatment protocol was proved to be essential for acquiring the observed high catalytic activities. It is demonstrated that this newly developed method was extremely effective for the oxidation of alkylarenes to ketones as well as for the oxidative dimerization of 2-naphthol to 1,1-binaphthyl-2,2-diol (BINOL), a particularly important scaffold for asymmetric catalysis. Detailed spectroscopic and mechanistic studies provided valuable insights into the structural aspects of the active catalyst and the reaction mechanism.
Fundamental understanding
of catalytic deactivation phenomena such
as sulfur poisoning occurring on metal/metal-oxide interfaces is essential
for the development of high-performance heterogeneous catalysts with
extended lifetimes. Unambiguous identification of catalytic poisoning
species requires experimental methods simultaneously delivering accurate
information regarding adsorption sites and adsorption geometries of
adsorbates with nanometer-scale spatial resolution, as well as their
detailed chemical structure and surface functional groups. However,
to date, it has not been possible to study catalytic sulfur poisoning
of metal/metal-oxide interfaces at the nanometer scale without sacrificing
chemical definition. Here, we demonstrate that near-field nano-infrared
spectroscopy can effectively identify the chemical nature, adsorption
sites, and adsorption geometries of sulfur-based catalytic poisons
on a Pd(nanodisk)/Al2O3 (thin-film) planar model
catalyst surface at the nanometer scale. The current results reveal
striking variations in the nature of sulfate species from one nanoparticle
to another, vast alterations of sulfur poisoning on a single Pd nanoparticle
as well as at the assortment of sulfate species at the active metal–metal-oxide
support interfacial sites. These findings provide critical molecular-level
insights crucial for the development of long-lifetime precious metal
catalysts resistant toward deactivation by sulfur.
Two-dimensional (2D)
bimetallic Ni
x
Mn1–x
(OH)
y
layered double
hydroxide (LDH) nanostructures were synthesized
and optimized as a remarkably active catalytic platform for low-temperature
aerobic C–H bond activation in alkylarenes and partial oxidation
of alcohols using a wide substrate (i.e., reactant)
and diverse solvent scope. The Ni
x
Mn1–x
(OH)y structure consists
of nonprecious and earth-abundant metals that can effectively operate
at low catalyst loadings, requiring only molecular oxygen as the stoichiometric
oxidant. Structurally diverse alkylarenes as well as primary and secondary
alcohols were shown to be competent substrates where oxidation products
were obtained in excellent yields (93–99%). Comprehensive catalyst
structural characterization via XRD, ATR-IR, TEM, EDX, XPS, BET, and
TGA indicated that the ultimately optimized Ni0.6Mn0.4(OH)
y
-9S catalyst
possessed not only particular rotational faults in its β-Ni0.6Mn0.4(OH)
y
domains but also distinct α/β-Ni0.6Mn0.4(OH)
y
interstratification
disorders, in addition to a relatively high specific surface area
of 125 m2/g, a 2D platelet morphology, and an average Mn
oxidation state of +3.5, suggesting the presence of both Mn3+ and Mn4+ species in its structure working in a synergistic
fashion with the Ni2+/x+ cations (the
latter is justified by the lack of catalytic activity in the monometallic
LDH catalysts Ni(OH)2 and Mn(OH)2). Kinetic
isotope effect studies carried out in the fluorene oxidation reaction
(k
H
/k
D = 5.7)
revealed that the rate-determining step of the catalytic oxidation
reaction directly involved the scission of a C–H bond. Moreover,
the optimized catalyst was demonstrated to be reusable through the
application of a regeneration protocol, which can redeem the full
initial activity of the carbon-poisoned spent catalyst in the fluorene
oxidation reaction.
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