Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed Oxidative Dehydrogenation of Cyclohexane

摘要

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 COX 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-6 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/Nb2O5 and Cu/Ta2O5. 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 COX 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.