Heterolytic Activation of C-H Bonds on CrIII-O Surface Sites Is a Key Step in Catalytic Polymerization of Ethylene and Dehydrogenation of Propane

We describe the reactivity of well-defined :chromium silicates toward ethylene and propane The initial Motivation for this study was to obtain a molecular Understanding of the Phillips polymerization catalyst. The Phillips catalyst contains reduced,chromium sites on silica and catalyzes the polymerization of ethylene without activators or a preformed Cr-C bond. Cr-II sites are commonly proposed active sites in this catalyst. We synthesized and characterized well-defined chromium(II) silicates and found that these materials, slightly contaminated with a minor amount of Cr-III sites, have poor polymerization activity and few active sites. In contrast, chromium(Ill) silicates have 1 order of magnitude higher activity. The chromium(III) silicates initiate polymerization by the activation of a C-H bond of ethylene. Density functional theory analysis of this process showed that the C-H bond activation Step is heterolytic and corresponds to a sigma-bond metathesis type process. The same well-defined chromium(TIT) silicate catalyzes the dehydrogenation of propane at elevated temperatures with activities similar to those of a related industrial chromium-based catalyst. This reaction also involves a key heterolytic C-H bond activation step similar to that described for ethylene but With a significantly higher energy barrier. The higher energy barrier is consistent with the higher pK(a) of the C-H bond in propane compared to the C-H bond in ethylene. In both cases, the rate-determining step is the heterolytic C-H bond activation.