Multinuclear solid-state NMR study of the coordinative nature of alkylaluminum cocatalyst on Phillips CrO x/SiO 2 catalyst

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopy was used to investigate the coordinative states of surface Al species on various triethylaluminum (TEA)-modified Phillips CrO x /SiO 2 catalysts under different Al/Cr molar ratios. 1H and 27Al MAS NMR spectra clearly demonstrated that the existing states of surface Al species in TEA modified catalysts strongly depended on the concentration of TEA and on the calcination temperature used during the catalyst preparation process. 1H MAS NMR spectra of TEA-modified Phillips CrO x /SiO 2 catalysts calcined at three different temperatures exhibited similar trends in peak shifts with increasing Al/Cr molar ratios, but the sensitivity dependence of variation in chemical shift of the main peaks on Al/Cr molar ratios increases with increasing of calcinations temperature. This increased sensitivity might have been due to the relatively low amount of residual hydroxyl groups present on the silica support after catalyst calcination. 27Al MAS NMR spectra showed the presence of three different coordination states (6-, 5-, and 4-coordinated Al species) in the TEA-modified Phillips catalysts. At relatively low Al/Cr molar ratios, the 6-coordinated Al species was the dominant species observed in the catalysts. However, the 4-coordinated Al species became dominant at relatively high Al/Cr ratios for TEA-modified catalysts calcined at 600 and 800 °C, because the increase in TEA concentration might have decreased the amount of oxygen atoms surrounding each Al species. After a saturation state of Al species directly coordinated on the catalyst or silica surface reached, residual TEA coordinated with the Al species already coordinated on the catalyst or silica surface, thus increasing the coordination number of the Al species. Consequently, the 6-coordinated Al species became predominant again for the TEA-modified catalysts calcined at 600 and 800 °C.