(356j) Sensitivity of Ethylene Oligomerization Rates and Selectivities to the Nature of Metal Ion on Siliceous Supports By First Principles and Microkinetic Interrogation

Research Interests in exploring kinetic and thermodynamic parameters that influence performance of a catalyst towards particular products. Building kinetic models for determining reactor dynamics in the process. Learing to apply techniques such as machine learning to expedite the screening process across material electronics and composition and arrive at relationships governing the experimental design and performance.

Group 4 metal (M⁴⁺= Zr, Ti, Hf) ions anchored on siliceous supports are reported to be active for ethylene coupling. Computational analysis using density functional theory (DFT) calculations rationalize the observed activity of M ions anchored on silica. While M⁴⁺ ions can be substituted isovalently into Si⁴⁺ of the silica model, lower-valent metal ions (M³⁺, M²⁺) substitution may produce more diversity into the reactive site due to introduction of proximal Br∅nsted acid-like silanol groups. Here, we use atomistic models of isolated metal sites built on (111) surface of β-cristobalite silica to contrast ethylene oligomerization, hydrogenation to ethane and other dehydrogenation pathways found for Group 4 ions with those on Ga³⁺, Zn²⁺, Co²⁺, Ni²⁺ site models. This combination is selected to probe the roles of charge and of partially filled d-orbitals on reaction pathways, including Cossee-Arlman, metallacycle, and proton transfer cycles proposed in the literature. We apply microkinetic models parameterized on DFT based rate constants to infer reactivity under relevant operating conditions and compare results with experimental observations. Degree of rate control analysis shows that the steps controlling the rates vary across these metal ions. The work is directed towards understanding trends across metal ions and factors governing their activity in hydrocarbon conversion reactions that upgrade products obtained during oil and shale gas processing.