CATALYTIC MECHANISM OF TERNARY RARE-EARTH-MATAL COORDINATION CATALYSTS IN THE COPOLYMERIZATION OF CARBON DIOXIDE AND EPOXIDE

Ternary rare earth metal coordination catalysts prepared from Y(CCl3 COO)(3), glycerin, and ZnEt2, were confirmed to be effective for the copolymerization of carbon dioxide and various epoxides. Whereas their precise modification was hampered by the lack of insight into active centers. Here an X-ray pholoelectron spectroscopy (XPS) technique was employed to study the interaction of various components, especially the central metal atoms in these catalysts. Copolymerization experiments with typical conditions were also carried out to investigate the correlation between the binding energy (BE) of central metals and the catalytic activity. The BE of Zn2p3/2 in Y (CCl3 COO)(3)-glycerin-ZnEt2 ternary catalyst was tested to be 0.3 eV higher than that in glycerin-ZnEt2 binary catalyst, while the BE of 01s increased by 0.4 eV; On the contrary, the BE of Y3d5/2 in the ternary catalyst system decreased by 1.4 eV than that in Y(CCl3COO)(3) while Cl2p decreased by 1.6 eV. he BE shifts showed that the rare-earth-metal compound has assuredly coordinated with zinc-oxygen bond in the ternary catalyst. Active center for the copolymerization is the coordinating activated zinc-oxygen bond, and rare-earth-metal compounds shift the electron distribution of active center by a conjectured "double metal bridged coordinating" mode. Electrons around Zn partially flow to Y via coordinating, resulting in decrease of electron cloud density and increase of the Lewis acidity around catalytic centre. That makes the activated zinc-oxygen bond more liable to be attacked by carbon dioxide and epoxide monomer, thereby it enhances the catalytic activity. As a result, the activity of Y (CCl3 COO), based ternary catalyst reached 563.0 g polymer/(mol Zn h) in the copolymerization, which is 23.6% higher than the activity of the binary catalyst system without rare-earth-metal compounds. Yttrium compounds with various anionic groups were also investigated to study the influences of Lewis acidity on central site. A consistent relationship was found among the pK(a) of ligands in yttrium compounds, BE of the coordinated catalysts and catalytic activities.