Cr(II)/SiO2 Phillips catalysts for olefin polymerization: How calcination and support morphology control site accessibility and polymer microstructure

This work presents a comparative spectroscopic and catalytic study of two Cr/SiO2 Phillips-type catalysts supported on industrial-grade Grace-Davison-955 and high-surface-area AGC-DM-302 silicas. By combining DR UV–Vis-NIR and FT-IR spectroscopy of adsorbed CO with in-situ olefin polymerization experiments, we investigated how calcination temperature and support morphology influence the formation, accessibility, and reactivity of Cr(II) active sites. Spectroscopic data reveal that increasing calcination temperature enhances the number of coordinatively unsaturated Cr(II) sites without significantly altering their speciation. While both supports exhibit similar Cr(II)A/Cr(II)B distributions, the Grace-based catalyst shows a lower density of accessible sites. Nevertheless, Cr/Grace outperforms Cr/AGC in ethylene polymerization under standard slurry-phase conditions. This is attributed to its larger pores and greater fragmentation propensity, which enhance monomer diffusion and polymer chain escape. Cr/AGC, on the other hand, produces polyethylene with higher molecular weight and crystallinity, consistent with the confinement effect suppressing chain termination. In the spectroscopic analysis, the low active site density, small primary particle size, and mild conditions of Cr/AGC hinder chain mobility and crystallization, resulting in an amorphous product. These findings underscore the regime-dependent nature of polymer growth and highlight the value of integrating spectroscopic characterization with catalytic testing to fully understand how support morphology and thermal history shape catalyst performance and polymer architecture.