Strain effect and dual initiation pathway in CrIII/SiO2 polymerization catalysts from amorphous periodic models

• Realistic amorphous models were constructed for CrIII single sites on a highly dehydroxylated SiO2 surface.
• The strain of the support plays a critical role in the initiation of ethylene polymerization.
• For more strained sites the insertion of ethylene into the CrO bond is the preferred pathway of ethylene polymerization.
• For less strained sites the CH activation of ethylene and the ethylene insertion into the CrO bond are competing pathways.

Heterolytic CH bond activation of ethylene on CrIIIO has been proposed as the initial step in olefin polymerization on (SiO)3CrIII active sites of molecularly-defined analogues of the Phillips catalyst. Here, by using realistic amorphous periodic models that account for structural complexity, strain, and active site heterogeneity of well-defined silica-supported CrIII-based polymerization catalysts, we show that this activation step is significantly favored due to the strain present on highly dehydroxylated silica. Furthermore, we find that initiation by insertion of ethylene into the CrO bond is even more favorable, especially for more strained sites, while both mechanisms can compete for less strained and thereby less active sites. Our results suggest a competing dual pathway for ethylene polymerization on CrIII sites and are consistent with a distribution of active sites, the experimentally observed broad distribution of polymer molecular weight, and the increased polymerization activity upon high-temperature calcination in Phillips catalysts.

Figure optionsDownload high-quality image (85 K)Download as PowerPoint slide