SEARCH

SEARCH BY CITATION

Keywords:

  • co-based catalyst;
  • 1, 3-Butadiene polymerization;
  • x-ray absorption spectroscopy

Abstract

Activation process and active site of Co-based Ziegler-Natta catalysts were studied by using synchrotron X-ray absorption spectroscopic technique and an optimum model of the cobalt active site was proposed with density functional theory (DFT) calculations. In the X-ray absorption near edge structure (XANES) spectrum of Co(2-ethylhexanoate)2, the preedge peak, 1s [RIGHTWARDS ARROW] 3d transition at 7707.8 eV and the K-edge peak, 1s [RIGHTWARDS ARROW] 4p transition at 7719.2 eV were observed. Both the intense K-edge and the weak preedge peaks indicate that Co(2-ethylhexanoate)2 possesses high Oh symmetry and the cobalt exists in a divalent state. Upon addition of methylaluminoxane (MAO), some changes in peak positions and intensities were observed. The preedge peak was a little shifted to 7708.2 eV with increased intensity, and the K-edge peak was shifted to a lower energy, 7717.2 eV with decreased intensity. This indicates that the oxidation state of cobalt still mainly remained +2 and the coordination geometry was altered from Oh to Td resulting from Co 3d and 4p orbital mixing with a loss of Oh symmetry. In the XANES spectrum of a mixture of Co(2-ethylhexanoate)2, MAO and t-BuCl, the K-edge peak at 7715.7 eV was observed. The orbital mixing of 4p cobalt and 2p chlorine orbitals results in a low energy transition. In 1,3-butadiene polymerization, this is attributable to the coordination of chlorine to cobalt, which is believed to facilitate cis-configuration and increasing reactivity. B(C6F5)3 was employed as Lewis acid, but no significant interaction with cobalt was found in the XANES spectrum of a mixture of Co(2-ethylhexanoate)2, Al(iBu)math image and B(C6F5)3, which is similar to the XANES of a mixture of Co(2-ethylhexanoate)2 and Al(iBu)3. B(C6F5)3 as Lewis acid does not greatly disturb the coordination symmetry of cobalt nor influence the oxidation state. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009