Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed
Oxidative Dehydrogenation of Cyclohexane
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Abstract
Alkane
oxidative dehydrogenation (ODH) over supported redox active
metal oxides is highly sensitive to support identity, but the underlying
cause of support effects has not been well-established. Here, we provide
evidence that charge transfer between the support and active oxide
phase impacts the rates of C–H bond abstraction and CO<sub>X</sub> formation pathways in the oxidative dehydrogenation of cyclohexane
over supported copper oxide catalysts. The surface Lewis acid strength
of nine metal oxide supports is quantified by alizarin dye intramolecular
charge transfer shifts and compared with supported copper oxide d–d
transition energies to determine the relationship between support
Lewis acid strength and copper oxide electronic properties. Model
cyclohexane ODH reaction studies show that selectivity to C<sub>6</sub> products increases with increasing support Lewis acid strength,
with selectivities to benzene and cyclohexene over combustion products
at zero conversion increasing from 20% over nucleophilic Cu/MgO to
over 90% over the more Lewis acidic Cu/Nb<sub>2</sub>O<sub>5</sub> and Cu/Ta<sub>2</sub>O<sub>5</sub>. This is ascribed to a linear
relationship between the amount of electron density on the copper
oxide valence states as described by Cu d–d transition energy
and the ratio of rates of C–H bond abstraction and CO<sub>X</sub> formation pathways. This approach to quantifying support Lewis acid
strength and applying it as a key catalytic descriptor of support
effects is a useful tool to enable rational design of next-generation
oxidative dehydrogenation catalysts