|
Physical Society Colloquium
Correlation between Catalyst Surface Structure and Polypropylene properties Seong H. Kim
University of California at Berkeley The titanium chloride based Ziegler-Natta catalytic system is extensively used in industry for polyethylene and polypropylene production. However, many fundamental aspects of the heterogeneous catalytic process still remain controversial. The barriers for a molecular-level understanding of the Ziegler-Natta catalyst system were the complexity of the high-surface-area catalyst, the extreme sensitivity of the catalyst to oxygen and moisture, and the lack of direct experimental methods detecting the active site structures at the catalyst surface. The utilization of surface science techniques to the Ziegler-Natta polymerization system enabled to circumvent these barriers and provided clear insights into the correlation between the surface properties of the catalyst and its polymerization behavior. Two types of thin films TiClx/MgCl2 and TiCly/Au were fabricated on an inert gold substrate, using chemical vapor deposition methods, to mimic the high-yield catalysts of MgCl2-supported TiCl4 and TiCl3-based catalysts, respectively. Once activated by reactions with triethylaluminum (AlEt3) vapor, both catalysts showed polymerization activities comparable to the high-surface-area industrial catalysts. The surface analyses of these model catalysts revealed that both catalyst surfaces were terminated with chlorine; but each type had a different distribution of two surface structures: a (001) basal plane structure of these halide crystallites and a non-basal plane structure. Comparison of the surface structure of the model catalysts and the tacticity of the polypropylene product indicated that the basal plane structure produces atactic polypropylene and the non-basal plane structure produces isotactic polypropylene. The titanium oxidation states did not appear to be an important factor in determining the tacticity of the polypropylene.
Monday, February 12th 2001, 15:00 |