Investigation On The Active Sites And Ethylene Polymerization Mechanisms Of Phillips Catalyst

Phillips CrOx/SiO2catalyst, one of the most important industrial catalysts for ethylene polymerization, is responsible for40-50%of the world production of high density polyethylene, as well as linear low density polyethylene. Compared with its commercial success, academic research still lags far behind, especially for the understandings of the active sites and polymerization mechanisms. In this study, heterogeneous models for Phillips catalyst have been established to investigate the correlation between the coordination environment of the active sites and polymerization behavior via the combination of experiments and theoretical simulation by density functional theory.First of all, theoretical calculations have been adopted to achieve a basic understanding of the effect of formaldehyde desorption on the active site formation and transformation during the induction period of the Phillips CrⅡOx/SiO2catalyst by density functional theory. The coordination of formaldehyde on CrⅡOx. surface is stable due to the high coordinative unsaturation of CrⅡ and lone-paired electrons of O in formaldehyde. No reaction can be initiated over the cluster model coordinated with two formaldehyde molecules owing to steric hindrance. The first reaction over cluster models, on which either one or no formaldehyde molecule is adsorbed, follows the metallacyclic mechanism into chromacyclopentane. Subsequent dimerization to1-butene and metathesis to propylene/ethylene are more favorable over the cluster model adsorbed with one formaldehyde molecule. Only after a complete desorption of formaldehyde does further ring expansion to chromacycloheptane followed by1-hexene formation become preferential. Spin crossing from quintet diethylene-Cr" complex to triplet chromacyclopentane with a spin acceleration effect is revealed.Secondly, theoretical calculations have been adopted to achieve a basic understanding of the effect of surface fluorination or titanium-modification of silica support on the ethylene initiation during the induction period of the Phillips CrⅡOx/SiO2catalyst adsorbed with different amounts of formaldehyde molecules. The electron deficiency of CrⅡOx. surface is enhanced and the coordination between formaldehyde and CrⅡOx. surface is more stable after surface fluorination or titanium-modification. It is demonstrated that no reaction can be initiated over cluster models adsorbed with two formaldehyde molecules on account of steric hindrance. For cluster models adsorbed with one formaldehyde molecule, ethylene dimerization to1-butene and metathesis to ethylene/propylene via the chromacyclobutane intermediate takes place and fluorination/titanium-modification of the silica support shows minor influence on both reactions. After a complete desorption of formaldehyde molecules, further ring expansion to chromacycloheptane occurs and the surface fluorination/titanium-modification of the silica support shows improvement on this process. Fluorination of the silica support is unfavorable to the ring-opening of chromacycloheptane to give1-hexene, while titanium-modification shows a positive effect to this process. It is also demonstrated that fluorination/titanium-modification shows positive effect on the chain propagation over the models of CrⅢ-alkyl active sites during chain propagation.Finally, compositionally and structurally uniform heterogeneous Phillips model catalysts have been prepared via the ambient temperature reaction of CrO2Cl2with silica pretreated at either500or800℃, followed by mild heating and CO reduction at300℃to obtain the corresponding CrⅡOx surface sites. CrⅡ grafted onto silica pretreated at800℃shows higher polymerization activity than that of500℃at the same reaction condition. The resulting structures have been investigated by IR spectroscopy of adsorbed CO combined with X-ray absorption spectroscopy and compared with model structures predicted by ONIOM calculations (B3PW91*/BS1:B3PW91*/STO-3G). The IR spectra suggest that there are two distinct and non-interconverting CrⅡOx. surface sites whose relative amounts differ on the two types of catalysts. The CO frequencies for the two model structures,(=SiO)2CrⅡ(CO)2and (=SiO)2(=Si2O)CrⅡ(CO), are calculated via the cluster models cut from the (100) crystal face of β-cristobalite. The calculated results agree well with experimental data. The X-ray absorption spectra reveal that the CO-free CrⅡOx. surface sites are coordinated by additional siloxane ligands, which might influence their propensity to initiate ethylene polymerization. Experimental observations have been rationalized well and deeper understandings on the active sites and polymerization mechanisms of Phillips catalyst have been achieved by this study.