Study of activation process and active site of co‐based catalyst for 1, 3‐butadiene polymerization using X‐ray absorption spectroscopy and DFT calculations

Abstract

AbstractActivation 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 → 3d transition at 7707.8 eV and the K‐edge peak, 1s → 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) 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