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The organic products of the reaction between Cr(VI)/silica and ethylene, propene, 2-pentene, 1-hexene, or cyclohexene at temperatures between 25 and 150 °C were investigated to understand the formation of chromium sites active for polymerization. A wide assortment of products were obtained, either directly or through ligand displacement or extraction, which depended on the specific olefin and the conditions. Hydrocarbon products were released under conditions relevant to commercial operation and indicated both polymerization and acid catalysis: short olefin oligomers (from polymerization), alkanes (possibly from hydride transfer), internal olefins (from double bond shift) and isoalkylbenzenes (from skeletal isomerization and alkylation of the diluent benzene). Aldehydes, ketones, and alcohols were found in polar organic extraction media, some the result of aldol condensations in the extraction medium. Surface carboxylates, evident by in situ IR spectroscopy, could be detached from the catalyst only through hydrolysis. The high threshold to desorb oxygenates implies that the chromium sites retain self-generated ligands. The nature of these ligands depends on the reductant, and, consequently, the result may be an active or an inactive site. Consistently, reduction of Cr(VI)/silica by various olefins resulted in catalysts with different polymerization behavior, and addition of external ligands to Cr(II)/silica led to a poisoned or to an active catalyst, depending on the ligand. |
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