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

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

Authors 

Xu, Y. - Presenter, Purdue University
LiBretto, N., Purdue University
Miller, J. T., Purdue University
Greeley, J., Purdue University
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.