Yuwei Fang

Theoretical Study of the Monomer Reaction Mechanism on Phillips CrOx/SiO2 Catalyst Using Density Functional Theory (DFT) and Paired Interacting Orbitals (PIO) Methods

The importance of Phillips CrOx/SiO2 catalyst could still be reflected from its 7 million tons of world HDPE production. However, the activation mechanism of this catalyst by ethylene in terms of active sites formation still remains unclear. In this work, the paired interacting orbital (PIO) method combined with density functional theory (DFT) calculation was applied for the theoretical studies on the intermolecular orbital interactions between ethylene monomer and a molecular model of surface monochromate species under six typical intermolecular geometric orientations (GO-1∼GO-6). Both DFT and PIO results indicated that GO-2 orientation should be the most preferential orientation for the reaction between ethylene monomer and monochromate species. Within GO-2 orientation the ethylene monomer preferentially approaches to the surface monochromate Cr(VI) species in a symmetric orientation relative to the two carbon atoms of ethylene from the upper site between the two double-bonded oxo-atoms of surface monochromate species. The electronic and orbital origin of the GO-2 orientation was elucidated in terms of its low energy increase due to lower repulsive interaction and in-phase overlap of molecular orbital interaction in-between the intermolecular frontier region.

Theoretical investigation of novel silsesquioxane-supported phillips-type catalyst by density functional theory (DFT) method

In spite of great commercial importance of the Phillips CrOx/SiO2 catalyst and long term research efforts, the precise physicochemical nature of active sites and polymerization mechanisms still remains unclear. The difficulties in a clear mechanistic understanding of this catalyst mainly come from the complexity of the surface chemistry of the amorphous silica gel support. In this work, novel silsesquioxane-supported Phillips Cr catalysts are utilized as realistic models of the industrial catalyst for theoretical investigation using the density functional theory (DFT) method in order to elucidate the effects of surface chemistry of silica gel in terms of supporting of chromium compounds and fluorination of the silica surface on the catalytic properties of the Phillips catalyst. Both qualitative and quantitative aspects with respect to various electronic properties and thermodynamic characteristics of the model catalysts were achieved. The future prospects of a state-of-the-art catalyst design and mechanistic approaches for the heterogeneous SiO2-supported Phillips catalyst has been demonstrated.

Various activation procedures of phillips catalyst for ethylene polymerization

Nowadays, the Phillips CrOx/SiO2 catalyst is still attracting interest from both industrial and academic fields owing to its unique characteristics for HDPE production. Compared with other industrial catalysts for ethylene polymerization, the Phillips catalyst can be activated by ethylene monomer, CO or Al-alkyl cocatalyst after a simple calcination process (thermal activation). In this work, a brief review of our recent new understanding on various activation procedures, including thermal activation, monomer activation, and CO activation, on industrial Phillips catalyst was presented. A new initiation mechanism, ethylene metathesis mechanism, was proposed according to some experimental evidence during the induction period when ethylene monomer was used to activate the catalyst. Such an ethylene metathesis mechanism was also indirectly confirmed in CO-prereduced Phillips catalyst. The formation of short chain branches in polymer can be rationalized well by this newly proposed unique mechanism during ethylene homo-and copolymerization with hexene-1 using CO-prereduced Phillips catalyst in the presence of triethylaluminum cocatalyst.

Plausible mechanism for the formation and transformation of active sites on novel phillips type catalyst with new organo-siloxane ligand

A novel Phillips type catalyst with chiral organo-siloxane ligand was investigated for ethylene polymerization in the presence of triethylaluminum (TEA) cocatalyst. Plausible mechanism of the formation and transformation of active sites was proposed to rationalize its unique polymerization behavior.