New Mechanistic Understanding of Ethylene Polymerization with Isolated d8 Metal Atoms in Zeolites Via Precise Molecular Synthesis and Operando Spectroscopy

Zeolite-supported transition metals (single atoms, clusters) represent an important class of materials with uses in chemical industry, emissions control, and as model systems to derive structure–function properties in catalysis. Such species show activity for the conversion of ethylene to butenes, where activity is enhanced by adding hydrogen, which stimulates the formation of a metal hydride species. [1,2] However, there existed a lack of detailed studies identifying the intermediates through which such a chemical transformation occurred. Multiple studies sought to reveal the underlying mechanism for this transformation, but the experimental observation of such intermediate species remained elusive for the last 50 years. [3] In this work, we synthesized uniform d⁸ metal Ir(I) and Ni(II) species supported on zeolites [5] to operando-spectroscopically observe well-resolved metal ligand transitions and provide unique insight on the mechanism for ethylene polymerization in the absence of hydrogen [4]. We found that the oxidative addition of C₂H₄ to the metal center occurs with the formation of a d⁶ metal vinyl-hydride species via the abstraction of hydrogen from ethylene, explaining the initiation of the olefin-polymerization cycle on d⁸ M(I/II) sites in the absence of pre-existing M–H bonds, contrary to commonly accepted view of heterolytic C-H bond dissociation based on DFT. Post-reaction characterization of the materials reveals that the active metal cations remain site-isolated whereas deactivation occurs due to the formation of carbonaceous deposits on the zeolites.

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