Ethylene polymerization over transition-metal supported catalysts. II. Cr on zeolite, silica, and charcoal: characterization and activity studies

Cr catalysts supported on silica, zeolite NaY, and charcoal were prepared with two different methods. They were characterized and examined in the polymerization of ethene. Cyclic voltammetry, electron spin resonance, X-ray photoelectron spectroscopy and silicon-29 magic-angle spinning nuclear magnetic resonance spectroscopy were used to characterize them, demonstrating that Cr is not in a single oxidation state, that the distribution among the various oxidation states depends on the history of the sample, and that even for a single oxidation state a variety of different environments can occur. In the polymerization of ethene, the supported Cr catalysts exhibited activity values varying from 10³ to 10⁵ gPE · molCr^-1 · [M]^-1 · h^-1, depending on the choice of the support and on the method by which the Cr is placed on it. Silica seemed to be the most efficient support for Cr for this reaction. However, the zeolite-supported catalysts also showed reasonable activity values (A_p ≅ 10⁴ units) and presented the advantage of having a strong interaction between the Cr and the support, which may prevent leaching of the cation into the surrounding environment.