III International Conference

PP-II-96DRIFTS AND UV-VIS STUDY OF PHILLIPS ETHYLENE POLYMERIZATIONCATALYSTSPanchenko V.N., Zakharov V.A., Paukshtis E.A.Boreskov Institute of Catalysis SB RAS, Novosibirsk, RussiaE-mail: panchenko@catalysis.nsk.suFormation of the CrO 3 /SiO 2 catalyst prepared by the reaction of CrO 3 vapor with silicapre-dehydroxylated at 250-800 °C was studied by the DRIFT and DRS methods. Thereactions of CrO 3 with the silica dehydroxylated at 250°, 400°C and 800 °C yielded,respectively, monocromates; mono- and dichromates; polychromates. According DRIFT(adsorption of CO) and UV-vis spectroscopy dinuclear and polynuclear Cr(II) and Cr(III) ionsare formed in the CrO 3 /SiO 2 (800) catalysts (0.07, 1.03 and 1.56 % wt. of Cr) after reducingwith CO. The ligand’s surround of Cr(II) ions is the main factor affecting the catalystsperformance at ethylene polymerization.Chromium-oxide catalysts (Phillips catalysts) for ethylene polymerization are knownsince the late 1950s. Although the catalysts have been studied intensively, numerousquestions regarding the structure and composition of the surface chromium species,mechanisms by which they convert to active centers of polymerization, structure andcomposition of these active centers remain open and are still discussed in literature [1, 2].Phillips catalysts are usually prepared by impregnating of silica with an aqueous solutionof a chromium compound, for example, CrO 3 , chromium acetate, chromium nitrate, etc.During subsequent activation in a flow of dry oxygen or air at 400-850 °C, Cr(VI) species ofvarious composition are assumed to form on the catalyst surface. We have studied theformation of the CrO 3 /SiO 2 catalyst prepared by the reaction of CrO 3 vapor with silica predehydroxylatedat 250-800 °C was studied by the DRIFT and UV-VIS spectroscopy. It wasfound that CrO 3 reacts both with terminal and hydrogen-bonded Si-OH groups of silica at250 °C to produce various chromate species and water. The latter is removed from the catalystsurface by vacuumation at 250°C. It was found the structure and composition of the surfacespecies depends on silica dehydroxylation temperature. Monocromates; mono- anddichromates; polychromates are formed after the interaction of CrO 3 with the silicadehydroxylated at 250°, 400 °C and 800 °C yields, respectively. The catalyst activity atethylene polymerization strongly increases with increasing dehydroxylation temperature,especially in the range of 250-400 °C. The results obtained prove di- and polychromates to bethe active components of the Phillips type chromium-oxide catalysts from which the activecenters are formed at interaction of the active component with co-catalyst (AlEt 3 ) and337