The Modification Of Phillips Cr-based Polyethylene Catalyst And Polymerization Mechanism Study

Polyethylene(PE)is one of the most widely used resins in the world and widely used in daily life and industry.Phillips Cr-based catalyst is one of the most important catalysts for PE production,however,it is necessary to modify the traditional Phillips catalyst for improving the catalytic performance and the product property.Additionally,the modification of Phillips catalyst increases the difficulty of investigating the polymerization mechanism.Through a combination of the experimental study and the theoretical study,we are able to understand the mechanism of the ethylene polymerization over the Phillips catalyst and the modified Phillips catalysts.Therefore,the catalyst development and innovation has been achieved on basis of our mechanistic understanding.Different active sites models for Phillips Cr-based polyethylene catalyst were designed and used for investigating the chain growth mechanism in the condition of polymerization with metal alkyl as cocatalyst.The results of density functional theory(DFT)calculation showed that the energy barrier was the lowest when the ethylene inserted into Cr(?)-C bond of 4d species,and the second lowest one was Cr(?)species.The coexistence of Cr(?)and Cr(?)species might be responsible for the activity of Phillips catalyst in conditions polymerization with metal alkyl as cocatalyst.The Cr(?)species is stable while Cr(?)species is unstable and could be affected by coordination of the silica surface hydroxyl,therefore,the activity gradually decreases and even disappears.Different active sites models for silica supported Mo-based polyethylene catalyst were designed and used for investigating the chain growth mechanism in the condition of polymerization with metal alkyl as cocatalyst.The energy barrier of ethylene insertion was the lowest over Mo(?)sites,which might be recognized as the active sites in the silica-supported monometallic Mo catalyst.The ethylene insertion of Mo(?)and Cr(?)/Cr(?)sites in the Mo-modified Phillips catalyst was investigated and the Mo-modification increased the energy barrier for ethylene insertion over Cr(?)and Cr(?)sites,which explained well the lower polyethylene activity of Mo-modified Phillips catalyst.The monovanadium catalyst(V/SiO2),unmodified Phillips catalyst(600Cr/SiO2)and vanadium modified Phillips catalyst(Cr/V-SiO2)were prepared and investigated using combined experimental and computational methods.The catalysts were slightly damaged by magnetic stirring and calcination in preparation process.The characterization of V/SiO2,600Cr/SiO2,Cr/V-SiO2 by X-ray photoelectron spectroscopy(XPS)showed which showed an interaction between the chromium and vanadium components on the silica support.The results of ethylene polymerization experiments conducted at higher pressure(0.4 MPa)showed that the ethylene polymerization activity of V/SiO2 was much lower than that of Phillips catalyst,and the activity of Cr/V-SiO2 was higher than that of 600Cr/SiO2.The viscosity average molecular weight(Mn)of polyethylene obtained via V/SiO2 was much larger than that of Phillips catalyst and the M? of polyethylene obtained by Cr/V-SiO2 was increased.The Mn increased with the increasing of cocatalyst dosage.The V(VI)sites and V(?)sites might be recognized as the active sites in the silica-supported monometallic V catalyst according to the DFT modeling results.The ability of the vanadium sites on V/SiO2 models to coordinate with ethylene was much lower than that of chromium sites on the Phillips catalyst models.The DFT results explained that the activity of monometallic vanadium catalyst was lower than that of Phillips catalyst.The calculation results of ethylene insertion for Cr-V catalysts showed that the coordination and insertion of ethylene on Cr sites were promoted by vanadium-modification and a series of V active sites with low insertion energy barriers were formed,which gave an explanation to the increase of polyethylene activity over vanadium modified Phillips catalyst.The fluoride modified Phillips catalyst(Cr/F-SiO2)and unmodified Phillips catalyst(500Cr/SiO2)were investigated using combined experimental and computational methods.The effects of cocatalyst concentration,polymerization temperature,1-hexene and hydrogen on polymerization activity,kinetics,and product performance characteristics were considered.The characterization results over catalysts showed that the catalysts were broken in the preparation process but the specific surface area,pore volume and structures were still good enough for polymerization.The addition of fluoride to the Phillips Cr/SiO2 catalyst increased the activity of the catalyst calcined at low temperature,as well as the molecular weight(Mw)of produced polyethylene.The activity of 500Cr/SiO2 and Cr/F-SiO2 decreased with the increasing of the amount of cocatalyst.The characterization results of HT-13C NMR showed that the 1-hexene insertion of Cr/F-SiO2 was higher than that of 500Cr/SiO2.The calculations showed that the difference of Gibbs free energy barriers between the chain transfer and the chain propagation increased after the introduction of the fluoride onto the silica surface,which was in accordance with the increased Mw of the polyethylene produced by the modified Phillips catalyst.The computations also supported that the activity of the ethylene/1-hexene copolymerization increased after the introduction of the fluoride,although the modification had little effect on the regioselectivity of the ethylene/1-hexene copolymerization.Moreover,the copolymers produced by the F-modified Phillips catalyst showed more short chain branches in the high molecular weight fraction and significant enhancement of the environmental stress crack resistance(ESCR),which can also be explained on the basis of the DFT calculations.