Susannah Scott

Self-Activating Heterogeneous Catalysts for Olefin Transformation

Heterogeneous, silica-supported catalysts are used in the production of a large fraction of the world’s HDPE. Despite extensive research, the structures of the active sites and the origin of the first metal-carbon s-bonds which insert ethylene remain unknown. The Union Carbide catalyst, Cp₂Cr, is proposed to have an active site described as [(ºSiO)Cr(Cp)R], while the active site of Phillips catalyst is often suggested to be [(ºSiO)₂CrR]. Model catalysts prepared by reaction of [Cp*Cr(CH₂SiMe₃)₂] and [Cr{(CH(SiMe₃)₂}₃] with the surface hydroxyl groups of silica gave [(ºSiO)CrCp*(CH₂SiMe₃)] and [(≡SiO)₂Cr(CH(SiMe₃)₂)], regardless of the silica pretreatment temperature (200 or 500 °C). The silica-supported catalysts spontaneously initiate ethylene polymerization with no induction period. The activities and activation barriers are comparable to those for the industrial catalysts, demonstrating the kinetic competence of both models. The surface organometallic catalysts are much more uniform than the conventional heterogeneous systems and much more stable than the homogeneous analogs.

The question of how the Phillips catalyst precursor, the silica-supported chromate [(ºSiO)2CrO2], is converted to [(ºSiO)₂CrR] was investigated. Experiments conducted on the CO-reduced catalyst suggest that disproportionation of ethylene leads to [(ºSiO)₂CrCH=CH₂] initiating sites and ethyl radicals via Cr-C bond homolysis. Computational (DFT) analysis suggests that flexibility in the amorphous silica framework allows ethylene activation with a lower Cr coordination number as well as homolytic cleavage with a higher Cr coordination number.

The synthesis and characterization of well-defined single-site surface organochromium complexes with pre-installed Cr-C s-bonds can provide molecular-level insight into the function of the industrial catalysts. For the well-defined surface organometallic catalysts, the initiation mechanism is more complicated than expected. Analysis of very low-pressure ethylene oligomerization products revealed products characteristic of olefin metathesis involving the initiating ligands, suggesting the spontaneous transformation of the Cr alkyl sites into metathesis-active Cr alkylidene sites prior to the onset of oligomerization. This insight may eventually serve to unify the self-activating behaviors of all three group 6 transition metals, of which Cr is known for catalytic olefin polymerization, while Mo and W are known for catalytic olefin metathesis.