Exploring the support modification effects on a CrOx/VOx/SiO2 bimetallic catalyst for bimodal UHMWPE/HDPE in-reactor blends: experimental and theoretical approaches

Abstract

Support modification is a simple but effective method for enhancing the performance of olefin polymerization catalysts, which is often correlated with enhanced acidity. Here, following this approach, a bimetallic CrO_x/VO_x/silica catalyst for producing UHMWPE/HDPE in-reactor blends was modified with alumina, sulfate, and alumina/sulfate. The performance of all the modified catalysts was promoted notably, with ethylene polymerization activities about twice that of the unmodified catalyst. Similarly, NH₃-TPD and FTIR-pyridine spectra confirmed their enhanced acidity. Theoretical studies emphasized the importance of the Lewis acidic Al³⁺ sites, which induced geometric and electronic rearrangement of the active site, thus reducing steric hindrance and improving LUMO reactivity. Sulfation further strengthened Al³⁺ acidity, intensifying the rearrangement and lowering the ethylene insertion barrier. Contrary to expectations, the performance of the alumina/sulfate-modified catalyst—with much stronger Brønsted and Lewis acidity—was only moderately promoted. XPS, DRS UV-vis, elemental analysis, and DFT results indicate that, while modification increases the metal-center electron deficiency and facilitates ethylene insertion, the stronger acidity also weakens the anchoring of the Cr(VI) and V(V) active precursors and promotes the formation of inactive Cr(III) and V(III) species. This effect was more pronounced for the superacidic alumina/sulfate modifier, partially counteracting its promotional effect.