Characterisation of Cr–silica polymerisation catalysts

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Abstract
The Cr–silica catalyst system is commercially important in the polymerisation of ethene. The nature of the chromium species present on the support following activation, as well as the nature of the polymerisation active oxidation state, have been investigated extensively but results have not always been conclusive. This investigation attempts to identify the nature of the surface species and the oxidation state of the chromium on both the activated and hydrogen-reduced samples. Temperature-programmed reduction was employed to identify the number of surface species on the support. These species were identified with the aid of EPR and UV–VIS diffuse reflectance spectroscopy. The surfaces of the activated Cr–silica catalysts were generally covered by chromate-like surface species, accompanied by dichromate-like species and chromium aggregates, the proportions of which were dependent upon chromium loading and activation conditions. These samples were mostly oxidised to CrVI during activation with traces of CrIII remaining after the treatment. Samples which were reduced in hydrogen indicated that reduction of CrVI to CrIII had occurred, but traces of CrVI always remained. The reduction process was accompanied by an increase in the amount of aggregation of the chromium.

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