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

Abstract

The importance of Phillips CrO_x/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.

Keywords:

• Phillips CrOx/SiO2 catalyst
• Monochromate site
• Ethylene
• Chromic acid
• Density functional theory (DFT)
• Paired interacting orbitals (PIO)

Acknowledgements

The authors are grateful to Dr Akinobu Shiga, LUMMOX Research Laboratory, Japan for his valuable advices and kind help for this work.