(391h) Catalytic Roles of the Vanadium-Oxygen Coordination Sphere in Alkanol Oxidative Dehydrogenation

Alkanol oxidative dehydrogenation (ODH) reactions occur on the surface redox sites of metal oxide catalysts, such as supported vanadium oxide (VO_x). Activation enthalpies for the kinetically relevant C-H scission step depend on the vanadium-lattice oxygen (V-O_L) site pair hydrogen atom addition energy (HAE). However, V-O_L in different coordination environments, including anchoring (V-O-support), bridging (V-O-V), and vicinal (V=O) configurations,^[1] differ in their HAE, with values that are difficult to estimate because typical assessments by density functional theory (DFT) cannot accurately capture the complex catalyst surface. To address this limitation, here, we develop experimental scaling relations using H₂ and O₂ activation probe reactions to quantify the HAE of various V-O_L sites, verified by DFT calculations, and illustrate the consequences of this varying HAE on the alkanol C-H scission rates. As the support reducibility increases, for VO_x on SiO₂, Al₂O₃, CeO₂, TiO₂, and ZrO₂, V-O_L HAE values at the VO_x-support interface decrease by 100 kJ mol^-1; consequently, methanol ODH turnover rates per vanadium increase over 2000-fold at 423 K, with elementary C-H bond activation enthalpies that decrease by 60 kJ mol^-1. For V-O-V and V=O environments, the HAE values are experimentally accessible by O₂ uptakes on reduced catalysts and increase by 15 kJ mol^-1 as the most reactive lattice oxygen atoms are removed. These findings are valid for a wide range of alkanols (methanol, ethanol, 2-butanol, cyclohexanol, and benzyl alcohol), where their C-H bond dissociation energies dictate their activation enthalpies. With both H₂ and O₂ activation as proxies for the HAE, we have enabled a new way to assess the redox strength of structurally complex metal oxide catalysts and confirmed the crucial role of the VO_x-support interface for alkanol ODH reactions.

[1] Broomhead, W. T.; Tian, W.; Herrera, J. E.; Chin, Y.-H. ACS Catalysis 2022, 12, 11801–11820.