(314b) Rationalizing Ethylene Oligomerization on Single Site Ga3+ Catalysts on Amorphous Silica: A First Principles Study Combined with Microkinetic Modeling and Experiments

The heterogeneous-catalytic production of short linear alpha olefins (LAOs) from light alkenes is of great technological interest, as they form key ingredients in plastics and fuels production.¹ Transition metal ion catalysts supported on porous oxides have generally been found to suffer from either low activity or poor selectivity to LAOs,¹ but we have recently determined that amorphous silica-supported single-site Ga³⁺ ions selectively catalyze olefin oligomerization at 250°C and 1 atm.² To unravel the molecular factors that control this reactivity, however, a comprehensive theoretical analysis to elucidate the link between site diversity on amorphous silica and reaction pathway selectivity, combined with rigorous X-ray absorption characterization, is required. Here, we develop multiple single-site, amorphous silica-supported Ga³⁺ models using periodic density functional theory and determine free energies of the corresponding ethylene oligomerization pathways. Two representative sites are exhaustively analyzed: a stretched three-coordinated site (✱) and a constrained four-coordinated site (#), where extra coordination stems from a Ga-hydroxyl interaction. At 250°C, although the # site may complete the ethylene oligomerization cycle through proton transfer, the site is not likely to control the reactivity due to a high ethylene insertion barrier. The ✱ site, however, enables a more favorable energy landscape, where ethylene insertion (1.6 eV) and β-hydride transfer (0.8 eV) occur, forming a Ga-ethyl species. The site activation process is followed by Cossee-Arlman cycle, which involves ethylene insertion and β-hydride transfer to reform the Ga-alkyl moiety. The comparison between barriers of β-hydride transfer (1.0 eV) and ethylene insertion (1.8 eV) suggests that the ✱ site favors ethylene oligomerization over polymerization. Microkinetic modeling confirms that the less populated ✱ sites are responsible for oligomerization, and a degree of rate control analysis suggests that ethylene insertion is rate-determining.

¹ A. Finiels et al., CatalSciTechnol. 2014; 4: 2412.

² N.J. LiBretto et al., NatComms, 2021; accepted.