Theoretical Models of Active Sites: General Considerations and Application to the Study of Phillips-Type Cr/Silica Catalysts for Ethylene Polymerization

Abstract

A review. Quantum chem. cluster-model approxns. are briefly reviewed as tools for studying local properties of solid surfaces. In its mol.-cluster version, this technique is applied to Phillips-type Cr/silica catalysts for ethylene polymn. and used to discuss spectroscopic and catalytic properties of low-valent chromium sites. The characterization part includes d-d electronic transitions as well as IR vibrational spectra of adsorbed carbonyl species. Comparison to exptl. spectra facilitates identification of surface species. Next, cluster models are used to investigate if and how polymn. may be initiated and sustained at the most topical of the proposed active sites. Two of the chromium sites studied here show promising activity for ethylene polymn. [on SciFinder(R)]