(574d) Interrogation of Single-Site, Oxide-Supported Metal Hydrides for Ethylene Oligomerization By Density Functional Theory

Metal ions anchored on oxide supports have been reported to catalyze C-C coupling of olefins. The extent of chain growth appears to depend on metal ion identity and the nature of the support. Here we provide molecular level insights into the catalytic performance of site-isolated metal ions on silica and silica-alumina for ethylene oligomerization based on periodic supercell density functional theory (DFT) models. We construct representative models of metal ions anchored on (111) and (001) surfaces of ß-cristobalite, a silica polymorph, as well as an Al³⁺-doped variant that introduces Brønsted acid sites proximal to transition metal. We compute potential energy surfaces for oligomerization pathways, revealing that oligomerization can occur through hydride intermediates and a Cossee-Arlman propagation step on both two- and three-fold oxide-anchored ions. We apply a reduced microkinetic model to predict turnover frequencies (TOF) and average degree of polymerization (DOP) across metal and site types. We show that the silica anchored group-4 metal hydrides are likely to be active for oligomerization at conditions of interest, the DOP in particular is expected to be much less on anchored Ti⁴⁺ than on Zr⁴⁺ or Hf⁴⁺, and the DOP is modified by proximal acid sites. These results rationalize experimental observations that ethylene oligomerization over Zr-hydrides is more effective on a silica-alumina support than on silica alone. Results further highlight the relevant local environmental features that influence oligomerization pathways relevant to lower-valent ions.