High-resolution spectroscopy (XPS, 1^H MAS solid-state NMR) and DFT investigations into Ti-modified Phillips CrO_x/SiO_2 catalysts

Ti-modified Phillips catalyst is a most important industrial catalyst widely used in ethylene polymerization, but the mechanism still remains mysterious. In this work, Ti-modified Phillips catalysts were characterized by high-resolution X-ray photoelectron spectroscopy (XPS) and 1^H magic-angle-spin solid-state nuclear magnetic resonance (1^H MAS solid-state NMR) combined with density functional investigations on the effects of Ti-modification on promotion of polymerization activity, and regulation of microstructures of the polymer chains. XPS data revealed Ti-modification caused increase of electron-deficiency and photo-stability of the surface chromate species. 1^H NMR provided the first direct evidence of surface residual Ti–OH groups. Modeling results rationalized well the effects of Ti-modification on promotion of polymerization activity, extension of molecular weight distribution (MWD) to lower MW region, improvement of the distribution of inserted co-monomer and enhancement of 2,1-insertion in regioselectivity of Phillips catalysts. It was the first time the Ti-modification on Phillips catalysts were theoretically elucidated