Surface physico-chemical state of CO-prereduced Phillips CrOx/SiO2 catalyst and unique polymerization behavior in the presence of Al-alkyl cocatalyst

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Abstract

In this work, a CO-prereduced Phillips CrOx/SiO2 catalyst (PC600/CO) was characterized by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption with mass spectrometer (TPD-MS) in comparison with a calcined catalyst (PC600). It was found that surface chromate Cr(VI) species had not been completely reduced into Cr(II) species, meanwhile, CO and CO2 still strongly adsorbed on the PC600/CO catalyst. Subsequently, the unique real-time profiles of ethylene polymerization rate using PC600/CO in the presence of TEA indicated the existence of two types of active sites: the first type derived from the desorption of CO or CO2 from Cr(II) species by alkyl-Al and featured with fast activation, higher activity and fast decay; the second type generated from the further reduction of residual chromate Cr(VI) species by alkyl-Al and featured with slow activation, lower activity and slow decay.

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