Heterolytic Activation of C–H Bonds on Cr^III–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(III) 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 σ-bond metathesis type process. The same well-defined chromium­(III) 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 p<i>K</i>_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