First Principles Analysis of Ethylene Oligomerization on Single-site Ga3+ Catalysts Supported on Amorphous Silica

Amorphous, single site, silica-supported main group metal catalysts have recently been found to promote olefin oligomerization with high activity at moderate temperatures and pressures (~250°C and 1 atm). Herein, we explore the molecular-level relationship between active site structures and the associated oligomerization mechanisms by developing amorphous, silica-supported Ga3+ models from periodic, first-principles calculations. Representative Ga3+ sites, including three- and four-coordinated geometries, are tested for multiple ethylene oligomerization pathways. We show that the three-coordinated Ga3+ site promotes oligomerization through a facile initiation process that generates a Ga-alkyl intermediate, followed by a Ga-alkyl-centered Cossee-Arlman mechanism. The strained geometry of a three-coordinated site enables a favorable free energy landscape with a kinetically accessible ethylene insertion transition state (1.7 eV) and a previously unreported β-hydride transfer step (1.0 eV) to terminate further C-C bond formation. This result, in turn, suggests that Ga3+ does not favor polymerization chemistry, while microkinetic modeling confirms that ethylene insertion is the rate-determining step. The study demonstrates promising flexibility of main group ions for hydrocarbon transformations and, more generally, highlights the importance of the local geometry of metal ions on amorphous oxides in determining catalytic properties.

Deep Reaction Network Exploration at a Heterogeneous Catalytic Interface

Characterizing the reaction energies and reaction barriers of complex reaction networks is central to catalyst development and optimization. Nevertheless, heterogeneous catalytic surfaces pose several unique challenges to automatic reaction network characterization, including large system sizes and open-ended reactant lists, that make ad hoc network construction and characterization the current state-of-the-art. Here we show how automated algorithms for exploring and characterizing reaction networks can be adapted to the constraints of heterogeneous systems using ethylene oligomerization on silica-supported single site Ga3+ catalysts as a model system. Using only graph-based rules for exploring the network and elementary constraints based on activation energy and system size for identifying network terminations, a comprehensive reaction network was generated for this system and validated against standard methods. The automated algorithm (re)discovers the classic Cossee-Arlman mechanism for this system that is hypothesized to drive major product formation while remarkably also predicting several new pathways for producing alkanes and coke precursors. This demonstration represents the largest heterogeneous catalyst (more than 50 atoms, with an open-ended pool of reactants) to be characterized using a quantum chemistry-based automated reaction method.