DFT and PIO study of the influences of Mo valance state and surface hydroxyl on supported-MoOx catalysts for ethylene polymerization

Abstract

Phillips catalyst (CrOx/SiO2) is an important industrial catalyst for ethylene polymerization, but the highly toxic chromium may contaminate the environment and do harm to human's health. Supported-MoOx catalyst with low toxicity has potential to replace Phillips catalyst if its catalytic performance could be improved. In this work, models of molybdenum active sites with different valence states (5+, 4+, 3+, 2+) supported on Al2O3 and SiO2, respectively, were established to investigate the effects of the valence states and surface hydroxyl on the catalyst activity using the combination of density functional theory (DFT) and paired interacting orbitals (PIO) methods. DFT results showed that supported Mo2+ center had the lowest energy barrier of ethylene insertion and thus possessed the highest activity for ethylene polymerization. PIO method additionally elucidated the orbital interaction and electron transformation between Mo2+ center and ethylene monomer. Hydroxyl on the support surface could poison the active center by coordination with Mo center. It had been demonstrated that pre-reduction of hexa-valent molybdenum into lower valence state +2 and elimination of surface hydroxyl groups of support were the key factors to obtain highly efficient ethylene polymerization catalyst. The molecular modeling results of this work provided theoretical basis for further experimental developments of green and highly efficient supported Mo-based polyethylene catalysts.