Abstract
IN AN investigation of the polymerization of ethylene on a chromium oxide catalyst [1], we came to the conclusion tha t the activity of the latter is produced by the action of the reacting substance ethylene. The object of the present work was to confirm this conclusion with the aid of electron paramagnetic resonance (ESR) and to at tempt to discover the nature of the active centres of the polymerization catalyst. There are a number of papers in the literature in which the ESR method has been used to establish the valency stale of chromium in chromic oxide catalysts on various supports [2-7]. Studying chromium-aluminium (CrOa/A12Oa) catalysts in the initial state and when partially reduced by hydrogen, Pecherskaya, Kazanskii, and Voyevodskii [2, 3] found two lines in the ESR spectra: a line with a width of 500-1000 oersted and a g factor of 1.97, and a narrow line with a width of 30-45 oersted and the same g factor. The authors associated the broad line with microcrystals of Cr203 [4] and assigned the narrow line to Cr +5 ions. In the work of Cossee and Van Reijen [6], the ESI~ spectra of chromium oxide catalysts which had been subjected to mild reduction were found to have one broad resonance line (ca. 600 oersted, g factor 1.97) with a narrow line (line width ca. 40 oersted, g factor 1.97) superposed on it, which the authors assigned to compounds of the ions Cr +4 and Cr +5, respectively. O'Reilly and MacIver [7] also considered the narrow line in the ESR spectra to be due to Cr +5 ions. All these investigations are of great interest; however, it would be extremely important to connect the ESR spectral data with the catalytic activity of the catalysts studied. We prepared chromium oxide catalysts by impregnating aluminosilicate with an aqueous solution of chromic anhydride, with subsequent activation. Two portions of catalyst were prepared. One was act ivated in a current of dry air at 500 ° for 4 hours, and the other in vacuum at 350 ° for the same
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