Exploring the Support Modification Effects over the CrOx/VOx/SiO2 Bimetallic Catalyst for Bimodal UHMWPE/HDPE In-reactor Blends: A View from Experimental and Theoretical Approaches

Abstract

Support modification is a simple but effective method to promote olefin polymerization catalysts, which is often correlated with enhanced acidity. Here, following this rule, the bimetallic catalyst CrOx/VOx/silica for UHMWPE/HDPE in-reactor blends was modified by alumina, sulfate, and alumina/sulfate. All modified catalysts were promoted notably, with ethylene polymerization activities about twice that of the unmodified one. Similarly, NH3-TPD and FTIR-pyridine confirmed their enhanced acidity. Theoretical studies emphasized the importance of Lewis acidic Al³⁺ site, which induced geometric and electronic rearrangement of the active site, thus reducing steric hindrance and improving LUMO reactivity. Sulfation further strengthened Al³⁺ acidity, exacerbating rearrangement and lowering the insertion barrier. Contrary to expectations, 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 improves metal center electron deficiency and facilitates ethylene insertion, stronger acidity also weakens the anchoring of Cr(VI) and V(V) active precursors and promotes inactive Cr(III) and V(III) species. This effect was more pronounced for the superacidic alumina/sulfate modifier, partially counteracting its promotional effect.