Investigation On The Reduction Mechanism Of Chromium Center Of Phillips Catalyst

Polyethylene(PE)is the largest and most demanding polyolefin product in the market.Due to its unique mechanical,chemical and physical properties,it has been widely used in many fields such as industry,agriculture and military.Phillips catalyst is an important catalyst for ethylene polymerization and produces about 30 million tons of high-density polyethylene(HDPE),which accounts for 50%of the world's annual production.However,it is necessary to modify the traditional Phillips catalyst for improving the catalytic performance and obtaining high value-added and multi-functional polyethylene products.Compared with the flourishing development of Phillips catalysts in industry,the active center of Phillips catalyst and the chain initiation mechanism have not been uniformly recognized in the academic community.The theoretical simulation method can avoid the difficulties in the experimental method,and explore the reduction mechanism and polymerization mechanism of Phillips catalyst at the atom level.In this study,the chrome-siloxane six-element ring model was used to study the reduction mechanism of Phillips catalyst and the effect of fluorine modification on the reduction reaction via the theoretical calculation method by density functional theory.The main content includes the following two aspects:(1)The reduction mechanism of the hexavalent chromium center of Phillips Catalyst by CH2CH2,HCHO and CO were studied.There were five different reaction paths of CH2CH2 reduces Phillips catalyst hexavalent chromium center.The Cr(?)in the Phillips catalyst reduced by CH2CH2 to the Cr(?)center with the formaldehyde or methyl formate by-product;the Cr(?)in the Phillips catalyst reduced by CH2CH2 to Cr(?)center with the epoxy ethane or acetaldehyde by-product;the Cr(?)in the Phillips catalyst reduced by CH2CH2 to tetravalent Cr=C carbene active center with the formaldehyde by-product.The Cr(?)in the Phillips catalyst reduced by CO to the Cr(?)center with the CO2 by-product.The Cr(?)in the Phillips catalyst reduced by HCHO to the Cr(?)center with CO2/CO/H2O by-products.Considering the spin-crossing,every reduction of the hexavalent chromium center in Phillips catalyst has an optimal path.The lowest energy barrier for the reaction of the Cr(?)was the reduction by CO to the Cr(?)center.The second lowest one was reduced by CH2CH2 to the Cr(?)center with the by-product formaldehyde.The reaction reduced by CH2CH2 to tetravalent Cr-C carbene active center with formaldehyde had the highest energy barrier,which unlikely occurred.(2)The DFT theoretical calculations have been adopted to investigate the effect of fluorine modification on the reduction of hexavalent chromium in Phillips catalyst by CH2CH2,HCHO,and CO.The increase of the number of fluorine atoms in the model led to the increase of charge of NBO on Cr,indicating the inhancement of the electron deficiency of Cr.As the number of fluorine atoms increased,the bond angle O-Cr-O decreased.After the fluorine modification,the energy barrier of the rate-determining step of the optimal pathway of the Cr(?)in the Phillips catalyst was reduced by CH2CH2 to the Cr(?)center and obtain methyl formate is increased,which is not conducive to the occurrence of this reduction reaction.Except for that,all the energy barrier of the rate-determining step of the optimal pathway of the Cr(?)in the Phillips catalyst was decreased after the fluorine modification.The lowest energy barrier for the reaction of the Cr(VI)was the reduction by CH2CH2 to the Cr(?)center with the by-product formaldehyde.The second lowest one was reduced by CO to the Cr(?)center with the by-product CO2.The reaction reduced by CH2CH2 to the Cr(?)center with the methyl formate had the highest energy barrier,which unlikely occurred.