Operando Molecular Spectroscopy During Ethylene Polymerization by Supported CrOx/SiO2 Catalysts: Active Sites, Reaction Intermediates, and Structure-Activity Relationship

Time-resolved operando molecular spectroscopy was applied during ethylene polymerization by supported CrOx/SiO2 catalysts to investigate the structure-activity relationships for this important industrial catalytic reaction. A combination of spectroscopic techniques (Raman, UV-Vis, XAS, DRIFTS and TPSR) during ethylene polymerization allows for the first time to monitor the molecular events taking place during activation of supported CrOx/SiO2 catalysts by ethylene and establishment of the structure-activity relationships for this reaction. Based on complementary DFT computational studies, a new initiation mechanism for ethylene polymerization is proposed. During reaction, the initial surface Cr+6Ox sites reduce to Cr+3 sites to form Cr-(CH2)(2)CH=CH2 and Cr-CH=CH2 reaction intermediates with the latter representing the catalytic active site.