How does the SiO2-supported nickel catalyst improve the ethylene polymerization process? Insights from DFT studies

Polymerization of ethylene plays a pivotal role in the production of polyethylene, a widely used polymer. Efficient catalysts are crucial for enhancing the polymerization process and improving polymer properties. In this study, we employ density functional theory (DFT) calculations to investigate the influence of the SiO2 surface by linker approach on the reactivity of ethylene polymerization catalyzed by the Ni-alpha-imine ketone-type complex. Our results unveil valuable insights into molecular interactions and mechanistic aspects driving the improved polymerization process in good agreement with experimental findings. We identify strong hydrogen bonds as the key factor facilitating the favorable binding of the molecular catalyst with the SiO2 surface, involving a chem-isorption process. Furthermore, we find that chain initiation is a rate-determining step for the molecular catalyst, whereas chain propagation is rate-determining for the supported catalyst, aligning with experimental differences in ethylene polymerization activity. Energy decomposition analysis reveals the dominant roles of electrostatic and orbital interactions in stabilizing both catalysts. Notably, the chemisorbed catalyst exhibits enhanced catalyst-ethylene interaction, while steric effects primarily impact the reactivity of the homogeneous catalyst. The supported catalyst exhibits a higher propagation barrier due to a late transition state. Additionally, the heterogeneous catalyst demonstrates superior ethylene uptake and thermal stability across a range of temperatures in agreement with the experimental data. These findings offer guidance for future experimental designs, emphasizing the importance of heterogenization and catalyst-ethylene interaction for enhancing ethylene polymerization processes.