(43a) First Principles Based Microkinetic Interrogation of Ethylene Oligomerization and Hydrogenation Potential of Transition Metal Ions Supported on Silica

Group 4 metal (M⁴⁺ = Zr, Ti, Hf) ions anchored on silica supports are reported to be active for ethylene coupling. Density functional theory (DFT) calculations of M⁴⁺ ions anchored on silica rationalize observed activity. While Group 4 ions can be substituted isovalently into silica, lower-valent metal ions (M³⁺, M²⁺) substitution may produce more diversity into the reactive site along with the introduction of proximal BrØnsted acid-like silanol groups. Here, we use atomistic models of metal sites built on (111) surface of ß-cristobalite silica polymorph to contrast ethylene oligomerization, hydrogenation, and dehydrogenation pathways found for Group 4 ions with those on similarly constructed Ga³⁺, Zn²⁺, Co²⁺, Ni²⁺models selected to probe the roles of charge and of partially filled d orbitals on reaction pathways, including Cossee-Arlman, metallacycle, and proton transfer mechanisms proposed in the literature. We apply microkinetic models parameterized on DFT based rate constants to infer reactivity under experimentally relevant conditions and compare results with experimental observations. Degree of rate control analysis shows that the steps controlling the rates vary across these metal ions. This work aids to understand trends across metal ions and factors governing their activity towards hydrocarbon conversion reactions relevant to today's economy based on oil and shale gas.