Thermochemical Interconnectivities of Active Sites on Metal Oxides and their Catalytic Consequences in C−O Formation and C−H Scission Chemistry
Surfaces of transition metal oxides consist of metal cations (Mn+) and oxygen anions (O2−), each with distinct catalytic functions—the coordinated protons on lattice O-atoms act as Brønsted acid sites (H+), the coordinatively unsaturated cationic metal centers function as Lewis acid sites (Mδ+−Oδ−), and the anionic lattice O-atoms serve as redox sites (O*). These active sites coexist and collectively catalyze both parallel and sequential reactions during the upgrading of light alkanols and alkanes into value-added chemicals, yet their intrinsic thermochemical and electronic interconnectivities, along with the resulting kinetic correlations of reactions that they catalyze, have not been quantitatively examined and established. In this thesis, alkanol dehydration (DEH) and oxidative dehydrogenation (ODH), the two primary reaction pathways during alkanol conversions, together with alkane ODH, are utilized as model reactions in combined kinetic assessments, chemical titrations, spectroscopic interrogations, theoretical calculations, and thermochemical analysis, to investigate the thermochemical and electronic interconnectivities between two Brønsted sites, redox and Brønsted sites, redox and Lewis sites, and two redox sites. We establish their interconnectivities using a library of structured and unstructured model catalysts, including Brønsted acidic tungsten-based polyoxometalate acid clusters (POM), Brønsted acidic and redox active molybdenum-based POM clusters, and Lewis acidic and redox active CoyMoOx oxides, where their electronic structures are systematically perturbed by either introducing counter cations (e.g., Na+, K+, Cu2+, Mg2+, or Al3+) into POM clusters, varying the Co-to-Mo atomic ratio (y, 0−1) of CoyMoOx oxides, or adjusting oxygen chemical potentials on their surfaces during catalysis by changing oxidant identity (O2 or CO2). Mechanistically, alkanol DEH, encompassing both intermolecular and intramolecular pathways, occurs via the transfer of a bound proton from catalytic surfaces to adsorbed alkanol species at Brønsted sites or of a negatively charged ⊝OH group from adsorbed alkanol species to Lewis acid centers (Mδ+) in Lewis acid-base pairs to evolve their respective transition states. Concurrently, alkanol and alkane ODH requires the transfer of an H-atom, equivalent to the transfer of a proton and an electron together, from reactant fragments to reductive O-centers to evolve their transition states. These distinct thermochemical events establish the deprotonation energy (DPE) of Brønsted sites, the electron affinity (EA) of Lewis sites, and the H-atom addition energy (HAE) of redox sites as kinetic descriptors of Brønsted acid, Lewis acid, and redox site catalyzed reactions. The shared proton transfer step, occurred in opposite directions at redox and Brønsted acid chemistry, results in negative correlations between HAE and DPE, whereas the shared electron transfer step, occurred in the same direction at redox and Lewis acid chemistry, results in positive correlations between HAE and −EA. These thermochemical correlations lead to contrasting kinetic correlations in activation enthalpies for redox and Brønsted acid catalysis and for redox and Lewis acid catalysis on bifunctional metal oxides, within a thermochemical-kinetic framework, derived from transition state theory and Born-Haber thermochemical cycle. Electron donation from counter cations into Brønsted acidic POM clusters modulates the electrostatic interactions between the remaining protons and conjugate POM anions, thereby altering the DPE values, while that from departing O-atoms into the metal centers of Co0.5MoOx catalysts, i.e., the formation of O-vacancies, affects the electronic structures of these catalysts, thereby influencing the HAE values of adjacent redox active O-atoms. These modulations underscore the electronic correlations between two Brønsted sites, and two redox sites on the same catalytic surface, respectively. The fundamental knowledge regarding the interconnectivities among diverse active sites on transition metal oxide surfaces, in terms of their thermochemical, electronic, and kinetic properties, provides a framework for rationalizing turnover rate couplings in acid and redox catalysis. This insight could also guide the design of catalysts with multiple catalytic functions, with active sites at atomistic proximity from each other, for optimizing reaction rates and product distributions.
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