Despite over 50 years of research, no studies have captured the intermediates of ethylene di-/oligomerization on metal ions on any solid support. Due to the non-uniformity of oxide-supported metal species and inability to elucidate which sites are the active sites for ethylene dimerization, many contradictory conclusions have been reached. We now solve this mystery for d8 metal ions on a heterogeneous catalyst by creating uniform arrays of d8 metal ions in H-FAU and H-BEA zeolites. We extensively characterize them spectroscopically and microscopically to prove the uniform nature of anchored metal ions. The oxidative addition of C2H4 to the metal center occurs with the formation of a d6 metal vinyl hydride, explaining the initiation of the Cossee-Arlman cycle on d8 M(I/II) sites in the absence of pre-existing M-H bonds as we show spectroscopically. The as-formed vinyl hydrides then further react with ethylene forming vinyl ethyl species, from which upon thermal tretament C4 products evolve. Post-reaction characterization of the two materials reveals that the active metal cations remain site-isolated whereas deactivation occurs due to the formation of carbonaceous deposits on the zeolites. Our findings have significant implications for the molecular level understanding of ethylene conversion.

On the mechansim of ethylene di- and oligomerization on uniform d8 metal ions in zeolites

ACS Fall 2021

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