III International Conference

PP-II-96ethylene. Active centers contain the surface organo-chromium compounds with chromiumions in low oxidation states (lower than Cr(VI)).Surface chromates (the compounds of Cr(VI)), formed after high-temperature activationof Phillips catalysts are rather easily reduced at interaction with different compounds(ethylene, carbon monoxide, aluminiumorganic compounds and etc.) Nowadays, there is ageneral agreement to ascribe the Cr(II) species as precursor of active site at ethylenepolymerization. We have studied the influence of chromium content (0.07-5% wt. of Cr) onthe state of surface chromium species and activity at ethylene polymerization ofCrO 3 /SiO 2 (800) catalyst. According the DRIFTS and DRS data CrO 3 interact with silica,dehydroxylated at 800 o C, with formation of di- and polychromates. The Cr 2 O 3 phase isformed also in the CrO 3 /SiO 2 (800) catalysts with Cr content more than 1% wt. The catalystactivity at ethylene polymerization strongly increases with decreasing the chromium amount.We investigated the CrO 3 /SiO 2 (800) catalysts (0.07, 1.03 and 1.56 % wt. of Cr) reducedby CO. According DRIFT (adsorption of CO) and UV-vis spectroscopy dinuclear andpolynuclear Cr(II) and Cr(III) ions are formed in the CrO 3 /SiO 2 (800) catalysts after reducingwith CO. The ligand’s surround of Cr(II) ions is the main factor affecting the catalystsperformance at ethylene polymerization.References1. E.Groppo, C.Lamberti, S.Bordiga, G.Spoto, A.Zecchina, Chem.Rev. 105(2005)115-183.2. V.N.Panchenko, V.A.Zakharov, E.A.Paukshtis, Applied Catalysis A: General 313(2006)130-136.338