Experimental and Molecular Simulational Study on Phillips CrOx/SiO2 Catalysts for Their Active Center and Polymerization Mechanisms
|School||East China University of Science and Technology|
|Keywords||Phillips CrOx/SiO2 catalyst Geminal silanols high-resolution solid state NMR density functional theory|
Nowadays, Phillips CrOx/SiO2 catalysts are industrially important in ethylene polymerization. In order to clarify the active center and polymerization mechanism, we use high-resolution solid state 1H MAS NMR and 29Si CP/MAS NMR allow the identification and comparison the geminal and single silanols on silica gels, and the silica gel supported CrOx catalysts subject to pretreatment temperatures. It suggests that 1) chromium species may react with geminal silanols even at 120℃; 2) the grafting of Cr species may obstruct the removal of single (isolated and vicinal) silanols at higher teamperatures. According to this conclusion, we establish five plausible molecular models for modeling chain initiating, chain increasing and transfer in ethylene polymerization by density functional theory (DFT). The chain initiating is calculated on the basis of metallacycle mechanism, spin surface crossing occurs on the minimum energy crossing points (MECPs) between quintuplet energy surface and triplet energy surface. The chain increasing and transfer are calculated by cossee mechanism. There are two possible ways for chain transfer. By comparison of five models, silanols in the catalyst system was found to be disadvantage influence to the chain initiating and advantage influence to the chain increasing and transfer. To the best of our knowledge, this is the first time the geminal and single silanols on Phillips catalysts were experimental and theoretically elucidated.