Significance The Phillips catalyst—CrO x /SiO 2 —produces 40–50% of global high-density polyethylene, yet several fundamental mechanistic controversies surround this catalyst. What is the oxidation state and nuclearity of the active Cr sites? How is the first Cr–C bond formed? How does the polymer propagate and regulate its molecular weight? Here we show through combined experimental (infrared, ultraviolet-visible, X-ray near edge absorption spectroscopy, and extended X-ray absorption fine structures) and density functional theory modeling approaches that mononuclear tricoordinate Cr(III) sites immobilized on silica polymerize ethylene by the classical Cossee–Arlman mechanism. Initiation (C–H bond activation) and polymer molecular weight regulation (the microreverse of C–H activation) are controlled by proton transfer steps.

Silica-supported tantalum hydride, (SiO)2Ta-H (1), proves to be the first single-site catalyst for the direct non-oxidative coupling transformation of methane into ethane and hydrogen at moderate temperatures, with a high selectivity (>98%). The reaction likely involves the tantalum-methyl-methylidene species as a key intermediate, where the methyl ligand can migrate onto the tantalum-methylidene affording the tantalum-ethyl.